Photocatalytic water oxidation by persulfate (Na₂S₂O₈) with [Ru(bpy)₃]²⁺ (bpy=2,2′-bipyridine) as a photocatalyst provides a standard protocol to study the catalytic reactivity of water oxidation catalysts. The yield of evolved oxygen per persulfate is regarded as a good index for the catalytic reactivity because the oxidation of bpy of [Ru(bpy)₃]²⁺ and organic ligands of catalysts competes with the catalytic water oxidation. A variety of metal complexes act as catalysts in the photocatalytic water oxidation by persulfate with [Ru(bpy)₃]²⁺ as a photocatalyst. Herein, the catalytic mechanisms are discussed for homogeneous water oxidation catalysis. Some metal complexes are converted to metal oxide or hydroxide nanoparticles during the photocatalytic water oxidation by persulfate, acting as precursors for the actual catalysts. The catalytic reactivity of various metal oxides is compared based on the yield of evolved oxygen and turnover frequency. A heteropolynuclear cyanide complex is the best catalyst reported so far for the photocatalytic water oxidation by persulfate and [Ru(bpy)₃]²⁺, affording 100 % yield of O₂ per persulfate.

One-dimensional supramolecular columnar phases composed of porphyrins (electron donor: D) and benzo[ghi]perylenetriimides (electron acceptor: A) through triple hydrogen bonds have been successfully constructed to perform sequential light-harvesting and electron-transfer processes. A series of benzo[ghi]peryleneimide derivatives have been synthesized to examine the substituent effects such as imide and nitrile groups on the spectroscopic and electrochemical properties. Then, formation of the 1:1 supramolecular complex between zinc porphyrin and benzo[ghi]perylenetriimide derivatives through triple hydrogen bonds was confirmed by Job's plot of ¹H NMR titration. Next, the one-dimensional supramolecular nanoarrays were successfully prepared in a mixed solvent. X-ray diffraction (XRD) measurement suggested that these nanoarrays contained one-dimensional columnar phases composed of stacked donor and acceptor layers. Finally, femtosecond transient absorption and electron spin resonance (ESR) measurements clearly indicated that photoinduced electron transfer occurred via the singlet excited states in the supramolecular columns.

A coupled light-harvesting antenna–charge-separation system, consisting of self-assembled zinc chlorophyll derivatives that incorporate an electron-accepting unit, is reported. The cyclic tetramers that incorporated an electron acceptor were constructed by the co-assembly of a pyridine-appended zinc chlorophyll derivative, ZnPy, and a zinc chlorophyll derivative further decorated with a fullerene unit, ZnPyC₆₀. Comprehensive steady-state and time-resolved spectroscopic studies were conducted for the individual tetramers of ZnPy and ZnPyC₆₀ as well as their co-tetramers. Intra-assembly singlet energy transfer was confirmed by singlet–singlet annihilation in the ZnPy tetramer. Electron transfer from the singlet chlorin unit to the fullerene unit was clearly demonstrated by the transient absorption of the fullerene radical anion in the ZnPyC₆₀ tetramer. Finally, with the co-tetramer, a coupled light-harvesting and charge-separation system with practically 100 % quantum efficiency was demonstrated.

Recent developments in the one-step thermal and photochemical catalytic hydroxylation of benzene to phenol using various oxidants are described, focusing on the catalytic mechanisms. Hydroxylation of benzene with hydrogen peroxide is catalyzed by homogeneous metal complexes and heterogeneous metal oxides to produce phenol when high-valent metal-oxo complexes are reactive species The selective hydroxylation of benzene to phenol is achieved by using mesoporous silica-alumina as the catalyst support, which prohibits further oxidation of phenol due to the strongly acidic sites. Photocatalytic oxidation of benzene with dioxygen and water is made possible by using organic photocatalysts via photoinduced electron transfer from benzene to the excited states of the photocatalysts, followed by the hydroxylation of the benzene radical cation with water to yield phenol selectively. The further oxidation of phenol is prohibited because of the fast back electron transfer from the one-electron reduced species of the photocatalyst to the phenol radical cation, whereas back electron transfer from the one-electron reduced species to the benzene radical cation is slowed down because of the large driving force of the back electron transfer, which is in the Marcus inverted region.

The energetics and photodynamics of carbonaceous molecular bearings with discrete molecular structures were investigated. A series of supramolecular bearings comprising belt-persistent tubular cycloarylene and fullerene molecules accepted photonic stimuli to afford charge-separated species via a photoinduced electron transfer process. The energy conversion processes associated with the photoexcitation, however, differed depending on the molecular structure. A π-lengthened tubular molecule allowed for the emergence of an intermediary triplet excited state at the bearing, which should lead to an energy conversion to thermal energy. On the other hand, low-lying charge-separated species induced by an endohedral lithium ion in fullerene enabled back electron transfer processes to occur without involving triplet excited species. The structure–photodynamics relationship was analyzed in terms of the Marcus theory to reveal a large electronic coupling in this dynamic supramolecular system.

Lithium-ion-encapsulated fullerene (Li⁺@C₆₀) exhibits greatly enhanced reactivity in photoinduced electron-transfer reduction with electron donors compared with pristine C₆₀. The enhanced reactivity of Li⁺@C₆₀ results from the more positive one-electron reduction potential of Li⁺@C₆₀ (+0.14 V versus a standard calomel electrode (SCE)) than that of C₆₀ (−0.43 V versus SCE), whereas the reorganization energy of electron transfer of Li⁺@C₆₀ (1.01 eV) becomes larger than that of C₆₀ (0.73 eV) because of the change in electrostatic interactions of encapsulated Li⁺ upon electron transfer. Li⁺@C₆₀ can form strong supramolecular complexes with various anionic electron donors through electrostatic interactions. Li⁺@C₆₀ can also form strong supramolecular π complexes with various electron donors, such as cyclic porphyrin dimers, corannulene, and crown ether fused monopyrrolotetrathiafulvalenes. Photoinduced electron transfer from electron donors to Li⁺@C₆₀ afforded long-lived charge-separated states of supramolecular complexes between electron donors and Li⁺@C₆₀. A photoelectrochemical solar cell composed of supramolecular nanoclusters of Li⁺@C₆₀ and zinc sulfonated meso-tetraphenylporphyrin exhibits significant enhancement in the photoelectrochemical performance than that of the reference system containing only a single component.

Redox-inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal–oxygen intermediates, such as metal–oxo and metal–peroxo complexes. The mechanisms of the oxidative CH bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)–oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)₃ have been unified as rate-determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)₃) by iron(III)–oxo complexes. All logarithms of the observed second-order rate constants of Lewis acid-promoted oxidative CH bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)–oxo complexes exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)₃ to the metal–oxo moiety has been confirmed for Mn^IV–oxo complexes. The enhancement of the electron-transfer reactivity of metal–oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox-inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)–peroxo complexes, resulting in acceleration of the electron-transfer reduction but deceleration of the electron-transfer oxidation. Such a control on the reactivity of metal–oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca²⁺ in the oxidation of water to dioxygen by the oxygen-evolving complex in photosystem II.

Redox-inactive metal ions play important roles in tuning chemical properties of metal–oxygen intermediates. Herein we report the effect of water molecules on the redox properties of a nonheme iron(III)–peroxo complex binding redox-inactive metal ions. The coordination of two water molecules to a Zn²⁺ ion in (TMC)Fe^III-(O₂)-Zn(CF₃SO₃)₂ (1-Zn²⁺) decreases the Lewis acidity of the Zn²⁺ ion, resulting in the decrease of the one-electron oxidation and reduction potentials of 1-Zn²⁺. This further changes the reactivities of 1-Zn²⁺ in oxidation and reduction reactions; no reaction occurred upon addition of an oxidant (e.g., cerium(IV) ammonium nitrate (CAN)) to 1-Zn²⁺, whereas 1-Zn²⁺ coordinating two water molecules, (TMC)Fe^III-(O₂)-Zn(CF₃SO₃)₂-(OH₂)₂ [1-Zn²⁺-(OH₂)₂], releases the O₂ unit in the oxidation reaction. In the reduction reactions, 1-Zn²⁺ was converted to its corresponding iron(IV)–oxo species upon addition of a reductant (e.g., a ferrocene derivative), whereas such a reaction occurred at a much slower rate in the case of 1-Zn²⁺-(OH₂)₂. The present results provide the first biomimetic example showing that water molecules at the active sites of metalloenzymes may participate in tuning the redox properties of metal–oxygen intermediates.

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent-free hydroxylation of benzene derivatives with electron-withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3-dichloro-5,6-dicyanohydroquinone (DDQH₂) in the presence of water under deaerated conditions. In the presence of dioxygen and tert-butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto- and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH-adduct radicals. On the other hand, DDQ^.− produced by the photoinduced electron transfer from benzene derivatives reacts with the OH-adduct radicals to yield the corresponding phenol derivatives and DDQH₂. DDQ is recovered by the reaction of DDQH₂ with tert-butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.

Recent developments in thermal and photochemical water oxidation by using homogeneous and heterogeneous catalysts is described together with the conversion of the homogeneous catalysts into heterogeneous catalysts during the course of water oxidation. The use of homogeneous catalysts is advantageous in the elucidation of detailed catalytic mechanisms including the detection of active intermediates for water oxidation. In contrast, heterogeneous catalysts are advantageous for practical applications, because of their high catalytic activity and the ease with which they can be separated by filtration. However, it is quite difficult to identify the active intermediates on the surfaces of heterogeneous catalysts, and therefore, the heterogeneous catalytic mechanism of water oxidation has not been clarified. Although investigations on homogeneous and heterogeneous catalysts for water oxidation have been performed rather independently, the link between homogeneous and heterogeneous catalysts is becoming more important for the development of efficient WOCs. This microreview focuses on factors to determine if the actual catalysts for water oxidation are homogeneous or heterogeneous depending on the conditions under which the catalysts are used. Ligand oxidation of homogeneous catalysts sometimes results in dissociation of the ligands to form nanoparticles, which act as much more efficient catalysts for water oxidation.

Two sets of perylenediimide-[60]fullerene dyads PDI-C₆₀ connected through 1,2,3-triazole units have been synthesized and characterized. The cyclic dyad PDICl₄-C₆₀ has four chlorine atoms in the 1,6,7,12-PDI positions, whereas the cyclic dyad PDIPip₂-C₆₀ has two piperidine units in the 1,7-PDI positions. On the other hand, PDICl₄-C₆₀ and PDIPip₂-C₆₀ dyads were synthesized as linear counterparts with the same substitution pattern. Also, a C₆₀-PDICl₄-C₆₀ triad has been prepared. A small interaction between C₆₀ and PDI moieties in the ground state was detected by UV/vis and electrochemical measurements in both PDI-C₆₀ cyclic systems. The occurrence of photoinduced energy-transfer processes between PDI and C₆₀ units was confirmed by time-resolved emission and transient absorption techniques. Femtosecond laser flash photolysis showed energy transfer from ¹PDI* to C₆₀ followed by intersystem crossing from ¹C₆₀* to ³C₆₀* in the case of the PDICl₄-C₆₀ systems. The energy transfer rate for PDICl₄-C₆₀ in the cyclic dyad and the triad is one order of magnitude faster than that in PDICl₄-C₆₀ in the linear dyad. The fast energy transfer rate together with the enhanced molar absorption coefficient by perylenediimide functionalization will be highly beneficial for applications as acceptors in polymer solar cells. On the other hand, no electron transfer from the donor PDIpip₂ units to the acceptor C₆₀ moiety was detected in the case of PDIPip₂-C₆₀ dyads.

Lithium-ion-encapsulated [6,6]-phenyl-C₆₁-butyric acid methyl ester fullerene (Li⁺@PCBM) was utilized to construct supramolecules with sulfonated meso-tetraphenylporphyrins (MTPPS⁴⁻; M=Zn, H₂) in polar benzonitrile. The association constants were determined to be 1.8×10⁵ M⁻¹ for ZnTPPS⁴⁻/Li⁺@PCBM and 6.2×10⁴ M⁻¹ for H₂TPPS⁴⁻/Li⁺@PCBM. From the electrochemical analyses, the energies of the charge-separated (CS) states were estimated to be 0.69 eV for ZnTPPS⁴⁻/Li⁺@PCBM and 1.00 eV for H₂TPPS⁴⁻/Li⁺@PCBM. Upon photoexcitation of the porphyrin moieties of MTPPS⁴⁻/Li⁺@PCBM, photoinduced electron transfer occurred to produce the CS states. The lifetimes of the CS states were 560 μs for ZnTPPS⁴⁻/Li⁺@PCBM and 450 μs for H₂TPPS⁴⁻/Li⁺@PCBM. The spin states of the CS states were determined to be triplet by electron paramagnetic resonance spectroscopy measurements at 4 K. The reorganization energies (λ) and electronic coupling term (V) for back electron transfer (BET) were determined from the temperature dependence of k_BET to be λ=0.36 eV and V=8.5×10⁻³ cm⁻¹ for ZnTPPS⁴⁻/Li⁺@PCBM and λ=0.62 eV and V=7.9×10⁻³ cm⁻¹ for H₂TPPS⁴⁻/Li⁺@PCBM based on the Marcus theory of nonadiabatic electron transfer. Such small V values are the result of a small orbital interaction between the MTPPS⁴⁻ and Li⁺@PCBM moieties. These small V values and spin-forbidden charge recombination afford a long-lived CS state.

Various molecular and supramolecular systems have been synthesized and characterized recently to mimic the functions of photosynthesis, in which solar energy conversion is achieved. Artificial photosynthesis consists of light-harvesting and charge-separation processes together with catalytic units of water oxidation and reduction. Among the organic molecules, derivatives of BF₂-chelated dipyrromethene (BODIPY), “porphyrin’s little sister”, have been widely used in constructing these artificial photosynthetic models due to their unique properties. In these photosynthetic models, BODIPYs act as not only excellent antenna molecules, but also as electron-donor and -acceptor molecules in both the covalently linked molecular and supramolecular systems formed by axial coordination, hydrogen bonding, or crown ether complexation. The relationships between the structures and photochemical reactivities of these novel molecular and supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. Femto- and nanosecond transient absorption and photoelectrochemical techniques have been employed in these studies to give clear evidence for the occurrence of energy- and electron-transfer reactions and to determine their rates and efficiencies.

A porphyrin–flavin-linked dyad and its zinc and palladium complexes (MPorFl: 2M, M=2 H, Zn, and Pd) were newly synthesized and the X-ray crystal structure of 2Pd was determined. The photodynamics of 2M were examined by femto- and nanosecond laser flash photolysis measurements. Photoinduced electron transfer (ET) in 2H₂ occurred from the singlet excited state of the porphyrin moiety (H₂Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state ¹(H₂Por^.+Fl^.−), which decayed through back ET (BET) to form ³[H₂Por]*Fl with rate constants of 1.2×10¹⁰ and 1.2×10⁹ s⁻¹, respectively. Similarly, photoinduced ET in 2Pd afforded the singlet CS state, which decayed through BET to form ³[PdPor]*Fl with rate constants of 2.1×10¹¹ and 6.0×10¹⁰ s⁻¹, respectively. The rate constant of photoinduced ET and BET of 2M were related to the ET and BET driving forces by using the Marcus theory of ET. One and two Sc³⁺ ions bind to the flavin moiety to form the FlSc³⁺ and Fl(Sc³⁺)₂ complexes with binding constants of K₁=2.2×10⁵ M⁻¹ and K₂=1.8×10³ M⁻¹, respectively. Other metal ions, such as Y³⁺, Zn²⁺, and Mg²⁺, form only 1:1 complexes with flavin. In contrast to 2M and the 1:1 complexes with metal ions, which afforded the short-lived singlet CS state, photoinduced ET in 2Pd⋅⋅⋅Sc³⁺ complexes afforded the triplet CS state (³[PdPor^.+Fl^.−(Sc³⁺)₂]), which exhibited a remarkably long lifetime of τ=110 ms (k_BET=9.1 s⁻¹).

Inclusion complexes of benzo- and dithiabenzo-crown ether functionalized monopyrrolotetrathiafulvalene (MPTTF) molecules were formed with Li⁺@C₆₀ (1⋅Li⁺@C₆₀ and 2⋅Li⁺@C₆₀). The strong complexation has been quantified by high binding constants that exceed 10⁶ M⁻¹ obtained by UV/Vis titrations in benzonitrile (PhCN) at room temperature. On the basis of DFT studies at the B3LYP/6-311G(d,p) level, the orbital interactions between the crown ether moieties and the π surface of the fullerene together with the endohedral Li⁺ have a crucial role in robust complex formation. Interestingly, complexation of Li⁺@C₆₀ with crown ethers accelerates the intersystem crossing upon photoexcitation of the complex, thereby yielding ³(Li⁺@C₆₀)*, when no charge separation by means of ¹Li⁺@C₆₀* occurs. Photoinduced charge separation by means of ³Li⁺@C₆₀* with lifetimes of 135 and 120 μs for 1⋅Li⁺@C₆₀ and 2⋅Li⁺@C₆₀, respectively, and quantum yields of 0.82 in PhCN have been observed by utilizing time-resolved transient absorption spectroscopy and then confirmed by electron paramagnetic resonance measurements at 4 K. The difference in crown ether structures affects the binding constant and the rates of photoinduced electron-transfer events in the corresponding complex.

Donor–acceptor distance, orientation, and photoexcitation wavelength are key factors in governing the efficiency and mechanism of electron-transfer reactions both in natural and synthetic systems. Although distance and orientation effects have been successfully demonstrated in simple donor–acceptor dyads, revealing excitation-wavelength-dependent photochemical properties demands multimodular, photosynthetic-reaction-center model compounds. Here, we successfully demonstrate donor– acceptor excitation-wavelength-dependent, ultrafast charge separation and charge recombination in newly synthesized, novel tetrads featuring bisferrocene, BF₂-chelated azadipyrromethene, and fullerene entities. The tetrads synthesized using multistep synthetic procedure revealed characteristic optical, redox, and photo reactivities of the individual components and featured “closely” and “distantly” positioned donor–acceptor systems. The near-IR-emitting BF₂-chelated azadipyrromethene acted as a photosensitizing electron acceptor along with fullerene, while the ferrocene entities acted as electron donors. Both tetrads revealed excitation-wavelength-dependent, photoinduced, electron-transfer events as probed by femtosecond transient absorption spectroscopy. That is, formation of the Fc⁺–ADP–C₆₀^.− charge-separated state upon C₆₀ excitation, and Fc⁺–ADP^.−–C₆₀ formation upon ADP excitation is demonstrated.

A robust one-compartment H₂O₂ fuel cell, which operates without membranes at room temperature, has been constructed by using a series of polynuclear cyanide complexes that contain Fe, Co, Mn, and Cr as cathodes, in sharp contrast to conventional H₂ and MeOH fuel cells, which require membranes and high temperatures. A high open-circuit potential of 0.68 V was achieved by using Fe₃[{Co^III(CN)₆}₂] on a carbon cloth as the cathode and a Ni mesh as the anode of a H₂O₂ fuel cell by using an aqueous solution of H₂O₂ (0.30  M, pH 3) with a maximum power density of 0.45 mW cm⁻². The open-circuit potential and maximum power density of the H₂O₂ fuel cell were further increased to 0.78 V and 1.2 mW cm⁻², respectively, by operation under these conditions at pH 1. No catalytic activity of Co₃[{Co^III(CN)₆}₂] and Co₃[{Fe^III(CN)₆}₂] towards H₂O₂ reduction suggests that the N-bound Fe ions are active species for H₂O₂ reduction. H₂O₂ fuel cells that used Fe₃[{Mn^III(CN)₆}₂] and Fe₃[{Cr^III(CN)₆}₂] as the cathode exhibited lower performance compared with that using Fe₃[{Co^III(CN)₆}₂] as a cathode, because ligand isomerization of Fe₃[{M^III(CN)₆}₂] into (FeM₂)[{Fe^II(CN)₆}₂] (M=Cr or Mn) occurred to form inactive FeC bonds under ambient conditions, whereas no ligand isomerization of Fe₃[{Co^III(CN)₆}₂] occurred under the same reaction conditions. The importance of stable Fe²⁺N bonds was further indicated by the high performance of the H₂O₂ fuel cells with Fe₃[{Ir^III(CN)₆}₂] and Fe₃[{Rh^III(CN)₆}₂], which also contained stable Fe²⁺N bonds. The stable Fe²⁺N bonds in Fe₃[{Co^III(CN)₆}₂], which lead to high activity for the electrocatalytic reduction of H₂O₂, allow Fe₃[{Co^III(CN)₆}₂] to act as a superior cathode in one-compartment H₂O₂ fuel cells.

An efficient functional mimic of the photosynthetic antenna-reaction center has been designed and synthesized. The model contains a near-infrared-absorbing aza-boron-dipyrromethene (ADP) that is connected to a monostyryl boron-dipyrromethene (BDP) by a click reaction and to a fullerene (C₆₀) using the Prato reaction. The intramolecular photoinduced energy and electron-transfer processes of this triad as well as the corresponding dyads BDP-ADP and ADP-C₆₀ have been studied with steady-state and time-resolved absorption and fluorescence spectroscopic methods in benzonitrile. Upon excitation, the BDP moiety of the triad is significantly quenched due to energy transfer to the ADP core, which subsequently transfers an electron to the fullerene unit. Cyclic and differential pulse voltammetric studies have revealed the redox states of the components, which allow estimation of the energies of the charge-separated states. Such calculations show that electron transfer from the singlet excited ADP (¹ADP*) to C₆₀ yielding ADP^.+-C₆₀^.− is energetically favorable. By using femtosecond laser flash photolysis, concrete evidence has been obtained for the occurrence of energy transfer from ¹BDP* to ADP in the dyad BDP-ADP and electron transfer from ¹ADP* to C₆₀ in the dyad ADP-C₆₀. Sequential energy and electron transfer have also been clearly observed in the triad BDP-ADP-C₆₀. By monitoring the rise of ADP emission, it has been found that the rate of energy transfer is fast (≈10¹¹ s⁻¹). The dynamics of electron transfer through ¹ADP* has also been studied by monitoring the formation of C₆₀ radical anion at 1000 nm. A fast charge-separation process from ¹ADP* to C₆₀ has been detected, which gives the relatively long-lived BDP-ADP^.+C₆₀^.− with a lifetime of 1.47 ns. As shown by nanosecond transient absorption measurements, the charge-separated state decays slowly to populate mainly the triplet state of ADP before returning to the ground state. These findings show that the dyads BDP-ADP and ADP-C₆₀, and the triad BDP-ADP-C₆₀ are interesting artificial analogues that can mimic the antenna and reaction center of the natural photosynthetic systems.

A novel multimodular donor–acceptor polyad featuring zinc porphyrin, fullerene, ferrocene, and triphenylamine entities was designed, synthesized, and studied as a charge-stabilizing, photosynthetic-antenna/reaction-center mimic. The ferrocene and fullerene entities, covalently linked to the porphyrin ring, were distantly separated to accomplish the charge-separation/hole-migration events leading to the creation of a long-lived charge-separated state. The geometry and electronic structures of the newly synthesized compound was deduced by B3LYP/3-21G(*) optimization, while the energy levels for different photochemical events was established using data from the optical absorption and emission, and electrochemical studies. Excitation of the triphenylamine entities revealed singlet-singlet energy transfer to the appended zinc porphyrin. As predicted from the energy levels, photoinduced electron transfer from both the singlet and triplet excited states of the zinc porphyrin to fullerene followed by subsequent hole migration involving ferrocene was witnessed from the transient absorption studies. The charge-separated state persisted for about 8.5 μs and was governed by the distance between the final charge-transfer product, that is, a species involving a ferrocenium cation and a fullerene radical anion, with additional influence from the charge-stabilizing triphenylamine entities located on the zinc-porphyrin macrocycle.

Nanomaterials with various shapes and sizes have been developed to mimic functions of photosynthesis in which solar energy conversion is achieved by using nanosized proteins with controlled shapes and sizes. Artificial photosynthesis consists of light-harvesting and charge-separation processes together with catalytic units of water oxidation and reduction. Nanosized mesoporous silica–alumina was utilized to encapsulate organic charge-separation molecules inside the nanospace to elongate the lifetimes of the charge-separated states, as observed in the photosynthetic reaction centers. Metal nanoparticles with controlled shapes and sizes have also been utilized as efficient catalysts for photocatalytic hydrogen evolution from water with reductants by using electron donor–acceptor organic molecules as photocatalysts. The control of the shape and size of metal nanoparticles plays a very important role in achieving high catalytic performance in catalytic hydrogen evolution in water reduction and also in catalytic oxygen evolution in water oxidation.

Manganese(V)–oxo–porphyrins are produced by the electron-transfer oxidation of manganese–porphyrins with tris(2,2′-bipyridine)ruthenium(III) ([Ru(bpy)₃]³⁺; 2 equiv) in acetonitrile (CH₃CN) containing water. The rate constants of the electron-transfer oxidation of manganese–porphyrins have been determined and evaluated in light of the Marcus theory of electron transfer. Addition of [Ru(bpy)₃]³⁺ to a solution of olefins (styrene and cyclohexene) in CH₃CN containing water in the presence of a catalytic amount of manganese–porphyrins afforded epoxides, diols, and aldehydes efficiently. Epoxides were converted to the corresponding diols by hydrolysis, and were further oxidized to the corresponding aldehydes. The turnover numbers vary significantly depending on the type of manganese–porphyrin used owing to the difference in their oxidation potentials and the steric bulkiness of the ligand. Ethylbenzene was also oxidized to 1-phenylethanol using manganese–porphyrins as electron-transfer catalysts. The oxygen source in the substrate oxygenation was confirmed to be water by using ¹⁸O-labeled water. The rate constant of the reaction of the manganese(V)–oxo species with cyclohexene was determined directly under single-turnover conditions by monitoring the increase in absorbance attributable to the manganese(III) species produced in the reaction with cyclohexene. It has been shown that the rate-determining step in the catalytic electron-transfer oxygenation of cyclohexene is electron transfer from [Ru(bpy)₃]³⁺ to the manganese–porphyrins.

In this paper, nanosecond laser flash photolysis has been used to investigate the influence of metal ions on the kinetics of radical cations of a range of carotenoids (astaxanthin (ASTA), canthaxanthin (CAN), and β-carotene (β-CAR)) and various electron donors (1,4-diphenyl-1,3-butadiene (14DPB), 1,6-diphenyl-1,3,5-hexatriene (16DPH), 4-methoxy-trans-stilbene (4 MeOSt), and trans-stilbene (trans-St)) in benzonitrile. Radical cations have been generated by means of photosensitized electron-transfer (ET) using 1,4-dicyanonaphthalene (14DCN) and biphenyl (BP). The kinetic decay of CAR^.+ shows a strong dependence on the identity of the examined metal ion. For example, whereas NaClO₄ has a weak effect on the kinetics of CAR^.+, Ni(ClO₄)₂ causes a strong retardation of the decay of CAR^.+. It is also interesting to note that Mn²⁺, which is a biologically relevant metal ion, shows the strongest effect of all the investigated metal ions (e.g., in the presence of Mn²⁺ ions, the half-life (t_1/2) of CAN^.+ (t_1/2>90 ms) is more than three orders of magnitude higher than in the absence of the metal ions (t_1/2≈16 μs)). Furthermore, the influence of metal-ion and oxygen concentrations on the kinetics of CAR^.+ reveals their pronounced effect on the kinetic decay of CAR^.+. However, these remarkable effects are greatly diminished if either oxygen or metal ions are removed from the investigated solutions. Therefore, it can be concluded that oxygen and metal ions interact cooperatively to induce the observed substantial effects on the stabilities of CAR^.+. These results are the first direct observation of the major role of oxygen in the stabilization of radical cations, and they support the earlier mechanism proposed by Astruc et al. for the role of oxygen in the inhibition of cage reactions. On the basis of these results, the factors that affect the stability of radical cations are discussed and the mechanism that shows the role of oxygen and metal ions in the enhancement of radical-cation stability is described.

Photoinduced electron transfer was studied in self-assembled donor–acceptor dyads, formed by axial coordination of pyridine appended with naphthalenediimide (NDI) to zinc naphthalocyanine (ZnNc). The NDI-py:ZnNc (1) and NDI(CH₂)₂-py:ZnNc (2) self-assembled dyads absorb light over a wide region of the UV/Vis/near infrared (NIR) spectrum. The formation constants of the dyads 1 and 2 in toluene were found to be 2.5×10⁴ and 2.2×10⁴ M⁻¹, respectively, from the steady-state absorption and emission measurements, suggesting moderately stable complex formation. Fluorescence quenching was observed upon the coordination of the pyridine-appended NDI to ZnNc in toluene. The energy-level diagram derived from electrochemical and optical data suggests that exergonic charge separation through the singlet state of ZnNc (¹ZnNc*) provides the main quenching pathway. Clear evidence for charge separation from the singlet state of ZnNc to NDI was provided by femtosecond laser photolysis measurements of the characteristic absorption bands of the ZnNc radical cation in the NIR region at 960 nm and the NDI radical anion in the visible region. The rates of charge-separation of 1 and 2 were found to be 2.2×10¹⁰ and 4.4×10⁹ s⁻¹, respectively, indicating fast and efficient charge separation (CS). The rates of charge recombination (CR) and the lifetimes of the charge-separated states were found to be 8.50×10⁸ s⁻¹ (1.2 ns) for 1 and 1.90×10⁸ s⁻¹ (5.3 ns) for 2. These values indicate that the rates of the CS and CR processes decrease as the length of the spacer increases. Their absorption over a wide portion of the solar spectrum and the high ratio of the CS/CR rates suggests that the self-assembled NDI-py:ZnNc and NDI(CH₂)₂-py:ZnNc dyads are useful as photosynthetic models.

A ferrocene–distyryl BODIPY dyad and a ferrocene–distyryl BODIPY–C₆₀ triad are synthesized and characterized. Upon photoexcitation at the distyryl BODIPY unit, these arrays undergo photoinduced electron transfer to form the corresponding charge-separated species. Based on their redox potentials, determined by cyclic voltammetry, the direction of the charge separation and the energies of these states are revealed. Femtosecond transient spectroscopic studies reveal that a fast charge separation (k_CS=1.0×10¹⁰ s⁻¹) occurs for both the ferrocene–distyryl BODIPY dyad and the ferrocene–distyryl BODIPY–C₆₀ triad, but that a relatively slow charge recombination is observed only for the triad. The lifetime of the charge-separated state is 500 ps. Charge recombination of the dyad and triad leads to population of the triplet excited sate of ferrocene and the ground state, respectively.

Donor–bridge–acceptor triad (Por-2TV-C₆₀) and tetrad molecules ((Por)₂-2TV-C₆₀), which incorporated C₆₀ and one or two porphyrin molecules that were covalently linked through a phenylethynyl-oligothienylenevinylene bridge, were synthesized. Their photodynamics were investigated by fluorescence measurements, and by femto- and nanosecond laser flash photolysis. First, photoinduced energy transfer from the porphyrin to the C₆₀ moiety occurred rather than electron transfer, followed by electron transfer from the oligothienylenevinylene to the singlet excited state of the C₆₀ moiety to produce the radical cation of oligothienylenevinylene and the radical anion of C₆₀. Then, back-electron transfer occurred to afford the triplet excited state of the oligothienylenevinylene moiety rather than the ground state. Thus, the porphyrin units in (Por)-2TV-C₆₀ and (Por)₂-2TV-C₆₀ acted as efficient photosensitizers for the charge separation between oligothienylenevinylene and C₆₀.

A molecular dyad and triad, comprised of a known photosensitizer, BF₂-chelated dipyrromethane (BDP), covalently linked to its structural analog and near-IR emitting sensitizer, BF₂-chelated tetraarylazadipyrromethane (ADP), have been newly synthesized and the photoinduced energy and electron transfer were examined by femtosecond and nanosecond laser flash photolysis. The structural integrity of the newly synthesized compounds has been established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular-clip-type structure for the triad, in which the BDP and ADP entities are separated by about 14 Å with a dihedral angle between the fluorophores of around 70°. Differential pulse voltammetry studies have revealed the redox states, allowing estimation of the energies of the charge-separated states. Such calculations revealed a charge separation from the singlet excited BDP (¹BDP*) to ADP (BDP^.+-ADP^.−) to be energetically favorable in nonpolar toluene and in polar benzonitrile. In addition, the excitation transfer from the singlet BDP to ADP is also envisioned due to good spectral overlap of the BDP emission and ADP absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from ¹BDP* to ADP (in benzonitrile and toluene) and electron transfer from BDP to ¹ADP* (in benzonitrile, but not in toluene). The kinetic study of energy transfer was measured by monitoring the rise of the ADP emission and revealed fast energy transfer (ca. 10¹¹ s⁻¹) in these molecular systems. The kinetics of electron transfer via ¹ADP*, measured by monitoring the decay of the singlet ADP at λ=820 nm, revealed a relatively fast charge-separation process from BDP to ¹ADP*. These findings suggest the potential of the examined ADP–BDP molecules to be efficient photosynthetic antenna and reaction center models.

The four-electron reduction of dioxygen by decamethylferrocene (Fc*) to water is efficiently catalyzed by a binuclear copper(II) complex (1) and a mononuclear copper(II) complex (2) in the presence of trifluoroacetic acid in acetone at 298 K. Fast electron transfer from Fc* to 1 and 2 affords the corresponding Cu^I complexes, which react at low temperature (193 K) with dioxygen to afford the η²:η²-peroxo dicopper(II) (3) and bis-μ-oxo dicopper(III) (4) intermediates, respectively. The rate constants for electron transfer from Fc* and octamethylferrocene (Me₈Fc) to 1 as well as electron transfer from Fc* and Me₈Fc to 3 were determined at various temperatures, leading to activation enthalpies and entropies. The activation entropies of electron transfer from Fc* and Me₈Fc to 1 were determined to be close to zero, as expected for outer-sphere electron-transfer reactions without formation of any intermediates. For electron transfer from Fc* and Me₈Fc to 3, the activation entropies were also found to be close to zero. Such agreement indicates that the η²:η²-peroxo complex (3) is directly reduced by Fc* rather than via the conversion to the corresponding bis-μ-oxo complex, followed by the electron-transfer reduction by Fc* leading to the four-electron reduction of dioxygen to water. The bis-μ-oxo species (4) is reduced by Fc* with a much faster rate than the η²:η²-peroxo complex (3), but this also leads to the four-electron reduction of dioxygen to water.

New multi-modular donor–acceptor conjugates featuring zinc porphyrin (ZnP), catechol-chelated boron dipyrrin (BDP), triphenylamine (TPA) and fullerene (C₆₀), or naphthalenediimide (NDI) have been newly designed and synthesized as photosynthetic antenna and reaction-center mimics. The X-ray structure of triphenylamine-BDP is also reported. The wide-band capturing polyad revealed ultrafast energy-transfer (k_ENT=1.0×10¹² s⁻¹) from the singlet excited BDP to the covalently linked ZnP owing to close proximity and favorable orientation of the entities. Introducing either fullerene or naphthalenediimide electron acceptors to the TPA-BDP-ZnP triad through metal–ligand axial coordination resulted in electron donor–acceptor polyads whose structures were revealed by spectroscopic, electrochemical and computational studies. Excitation of the electron donor, zinc porphyrin resulted in rapid electron-transfer to coordinated fullerene or naphthalenediimide yielding charge separated ion-pair species. The measured electron transfer rate constants from femtosecond transient spectral technique in non-polar toluene were in the range of 5.0×10⁹–3.5×10¹⁰ s⁻¹. Stabilization of the charge-separated state in these multi-modular donor–acceptor polyads is also observed to certain level.

The catalytic activity of Pt nanoparticles (PtNPs) with different sizes and shapes was investigated in a photocatalytic hydrogen-evolution system composed of the 9-mesityl-10-methylacridinium ion (Acr⁺–Mes: photocatalyst) and dihydronicotinamide adenine dinucleotide (NADH: electron donor), based on rates of hydrogen evolution and electron transfer from one-electron-reduced species of Acr⁺–Mes (Acr^.–Mes) to PtNPs. Cubic PtNPs with a diameter of (6.3±0.6) nm exhibited the maximum catalytic activity. The observed hydrogen-evolution rate was virtually the same as the rate of electron transfer from Acr^.–Mes to PtNPs. The rate constant of electron transfer (k_et) increased linearly with increasing proton concentration. When H⁺ was replaced by D⁺, the inverse kinetic isotope effect was observed for the electron-transfer rate constant (k_et(H)/k_et(D)=0.47). The linear dependence of k_et on proton concentration together with the observed inverse kinetic isotope effect suggests that proton-coupled electron transfer from Acr^.–Mes to PtNPs to form the PtH bond is the rate-determining step for catalytic hydrogen evolution. When FeNPs were used instead of PtNPs, hydrogen evolution was also observed, although the hydrogen-evolution efficiency was significantly lower than that of PtNPs because of the much slower electron transfer from Acr^.–Mes to FeNPs.

A series of molecular triads, composed of closely positioned boron dipyrrin–fullerene units, covalently linked to either an electron donor (donor¹–acceptor¹–acceptor²-type triads) or an energy donor (antenna–donor¹–acceptor¹-type triads) was synthesized and photoinduced energy/electron transfer leading to stabilization of the charge-separated state was demonstrated by using femtosecond and nanosecond transient spectroscopic techniques. The structures of the newly synthesized triads were visualized by DFT calculations, whereas the energies of the excited states were determined from spectral and electrochemical studies. In the case of the antenna–donor¹–acceptor¹-type triads, excitation of the antenna moiety results in efficient energy transfer to the boron dipyrrin entity. The singlet-excited boron dipyrrin thus generated, undergoes subsequent energy and electron transfer to fullerene to produce a boron dipyrrin radical cation and a fullerene radical anion as charge-separated species. Stabilization of the charge-separated state in these molecular triads was observed to some extent.

A highly efficient functional mimic of the photosynthetic antenna-reaction-center complexes has been designed and synthesized. The model contains a zinc(II) porphyrin (ZnP) core, which is connected to three boron dipyrromethene (BDP) units by click chemistry, and to a C₆₀ moiety using the Prato procedure. The compound has been characterized using various spectroscopic methods. The intramolecular photoinduced processes of this pentad have also been studied in detail with steady-state and time-resolved absorption and emission spectroscopic methods, both in polar benzonitrile and nonpolar toluene. The BDP units serve as the antennae, which upon excitation undergo singlet–singlet energy transfer to the porphyrin core. This is then followed by an efficient electron transfer to the C₆₀ moiety, resulting in the formation of the singlet charge-separated state (BDP)₃–ZnP^.+–C₆₀^.−, which has a lifetime of 476 and 1000 ps in benzonitrile and toluene, respectively. Interestingly, a slow charge-recombination process (=2.6×10⁶ s⁻¹) and a long-lived triplet charge-separated state (=385 ns) were detected in polar benzonitrile by nanosecond transient measurements.

A trefoil-like porphyrin trimer linked by triphenylamine (TPA-TPZn₃) was synthesized. A three-electron oxidation of TPA-TPZn₃ forms a radical trication (TPA-TPZn₃³⁺), in which each porphyrin ring undergoes a one-electron oxidation. The radical trication TPA-TPZn₃³⁺ spontaneously dimerizes to afford (TPA-TPZn₃)₂⁶⁺ in CH₂Cl₂. The characteristic charge-resonance band due to the charge delocalization over the π system of (TPA-TPZn₃)₂⁶⁺ was observed in the NIR region. The initial oxidation potential of TPA-TPZn₃ is negatively shifted relative to that of the corresponding monomer porphyrin, which results from the stabilization of the oxidized state of TPA-TPZn₃ associated with the dimerization. The thermodynamic parameters (i.e., ΔH, ΔS, and ΔG) for the formation of (TPA-TPZn₃)₂⁶⁺ were determined by measuring Vis/NIR spectra at various temperatures. The formation constant of (TPA-TPZn₃)₂⁶⁺ is significantly larger than that of the radical cation dimer of the corresponding monomer porphyrin (e.g., over 2000-fold at 233 K). The electronic states were investigated using EPR spectroscopic analysis. The greatly enhanced dimerization of TPA-TPZn₃³⁺ results from multiple π-bond formation between the porphyrin radical cations.

Zinc bis-porphyrin molecular tweezers composed of a N₄ spacer bound through pyridyl units to the meso position of porphyrins were synthesized, and the tweezers are closed by the coordination of a copper(II) ion inside the spacer ligand. The effect of the π–π interaction between the porphyrin rings in the closed conformation on the absorption spectra of multi-electron oxidized species and the reduction potentials were clarified by chemical and electrochemical oxidation of the closed form of the zinc bis-porphyrin molecular tweezers in comparison with the open form without copper(II) ion and the corresponding porphyrin monomer. The shifts in redox potentials and absorption spectrum of the porphyrin dication indicate a strong electronic interaction between the two oxidized porphyrins in the closed form, whereas there is little interaction between them in the neutral form. The dynamics of copper(II) ion coordination and subsequent electron transfer was examined by using a stopped-flow UV/Vis spectroscopic technique. It was confirmed that coordination of copper(II) occurs prior to electron-transfer oxidation of the closed form of the zinc bis-porphyrin molecular tweezers.

The rate constants of intermolecular photoinduced electron transfer from triplet excited states of metalloporphyrins to a series of p-benzoquinone derivatives in benzonitrile were determined to examine the effects of the driving force, the metal, and the conformational distortion of the porphyrin ring on the reorganization energies (λ) of electron transfer by laser flash photolysis. The λ values were evaluated from the determined rate constants on the basis of the Marcus theory of electron transfer. The λ values of planar metalloporphyrins, [Al(TPP)(PhCOO)] and [Zn(TPP)] (TPP²⁻=tetraphenylporphyrin dianion), are approximately the same, but they are 0.27 eV smaller than those of the corresponding nonplanar (saddle-distorted) metalloporphyrins [Al(DPP)(PhCOO)] and [Zn(DPP)] (DPP²⁻=dodecaphenylporphyrin dianion) when they are compared for the same driving force of photoinduced electron transfer. The axial ligand PhCOO⁻ of [Al(TPP)]⁺ and [Al(DPP)]⁺ was replaced by anthraquinone-2-carboxylate (AqCOO⁻) to afford the electron donor–acceptor complexes [Al(TPP)(AqCOO)] and [Al(DPP)(AqCOO)], respectively. The X-ray crystal structure of [Al(TPP)(AqCOO)] revealed strong coordination of AqCOO⁻ to the Al³⁺ ion of [Al(TPP)]⁺ and the existence of π–π interactions between AqCOO⁻ and the porphyrin ring. In the case of the saddle-distorted [Al(DPP)(AqCOO)], however, the AqCOO⁻ moiety is nearly perpendicular to the porphyrin ring. The photodynamics of intracomplex photoinduced electron transfer from the singlet excited state of [Al(TPP)]⁺ and [Al(DPP)]⁺ to the AqCOO⁻ moiety were also examined in comparison with the intermolecular photoinduced electron-transfer reactions, and the determined rate constants were evaluated in light of the Marcus theory of electron transfer to reveal that the electron transfer is adiabatic in each case.

Three new bisperylenebisimide–silicon phthalocyanine triads [(PBI)₂–SiPcs 1, 2, and 3] connected with either rigid or flexible bridges were synthesized and characterized. A new synthetic approach to connect SiPc and PBI moieties through click chemistry produced triad 3 with an 80 % yield. In (PBI)₂–SiPc 1, PBI and SiPc are orthogonal and were connected with a rigid connector; triads 2 and 3 bear flexible aliphatic bridges, resulting in a tilted (2) or nearly parallel arrangement (3) of PBI and SiPc. Photoinduced intramolecular processes in these (PBI)₂–SiPcs were studied and the results are compared with those of the reference compounds SiPc-ref and PBI-ref. The occurrence of electron-transfer processes between the SiPc and PBI units was confirmed by time-resolved emission and transient absorption techniques. Charge-separated (CS) states with lifetimes of 0.91, 1.3 and 2.0 ns for triads 1, 2, and 3, respectively, were detected using femtosecond laser flash photolysis. Upon the addition of Mg(ClO₄)₂, an increase in the lifetime of the CS states to 59, 110 and 200 μs was observed for triads (PBI)₂–SiPcs 1, 2, and 3, respectively. The energy of the CS state (SiPc^.+–PDI^.−/Mg²⁺) is lower than the energy of both silicon phthalocyanine (³SiPc*–PDI) and perylenebisimide (SiPc–³PDI*) triplet excited states, which decelerates the metal ion-decoupled electron-transfer process for charge recombination to the ground state, thus increasing the lifetime of the CS state. The photophysics of the three triads demonstrate the importance of the rigidity of the spacer and the orientation between donor and acceptor units.

The pterin-coordinated ruthenium complex, [Ru^II(dmdmp)(tpa)]⁺ (1) (Hdmdmp=N,N-dimethyl-6,7-dimethylpterin, tpa=tris(2-pyridylmethyl)amine), undergoes photochromic isomerization efficiently. The isomeric complex (2) was fully characterized to reveal an apparent 180° pseudorotation of the pterin ligand. Photoirradiation to the solution of 1 in acetone with incident light at 460 nm resulted in dissociation of one pyridylmethyl arm of the tpa ligand from the Ru^II center to give an intermediate complex, [Ru(dmdmp)(tpa)(acetone)]²⁺ (I), accompanied by structural change and the coordination of a solvent molecule to occupy the vacant site. The quantum yield (ϕ) of this photoreaction was determined to be 0.87 %. The subsequent thermal process from intermediate I affords an isomeric complex 2, as a result of the rotation of the dmdmp²⁻ ligand and the recoordination of the pyridyl group through structural change. The thermal process obeyed first-order kinetics, and the rate constant at 298 K was determined to be 5.83×10⁻⁵ s⁻¹. The activation parameters were determined to be ΔH^≠=81.8 kJ mol⁻¹ and ΔS^≠=−49.8 J mol⁻¹ K⁻¹. The negative ΔS^≠ value indicates that this reaction involves a seven-coordinate complex in the transition state (i.e., an interchange associative mechanism). The most unique point of this reaction is that the recoordination of the photodissociated pyridylmethyl group occurs only from the direction to give isomer 2, without going back to starting complex 1, and thus the reaction proceeds with 100 % conversion efficiency. Upon heating a solution of 2 in acetonitrile, isomer 2 turned back into starting complex 1. The backward reaction is highly dependent on the solvent: isomer 2 is quite stable and hard to return to 1 in acetone; however, 2 was converted to 1 smoothly by heating in acetonitrile. The activation parameters for the first-order process in acetonitrile were determined to be ΔH^≠=59.2 kJ mol⁻¹ and ΔS^≠=−147.4 kJ mol⁻¹ K⁻¹. The largely negative ΔS^≠ value suggests the involvement of a seven-coordinate species with the strongly coordinated acetonitrile molecule in the transition state. Thus, the strength of the coordination of the solvent molecule to the Ru^II center is a determinant factor in the photoisomerization of the Ru^II–pterin complex.

A novel distyryl BODIPY–fullerene dyad is prepared. Upon excitation at the distyryl BODIPY moiety, the dyad undergoes photoinduced electron transfer to give a charge-separated state with lifetimes of 476 ps and 730 ps in polar (benzonitrile) and nonpolar (toluene) solvents, respectively. Transient absorption measurements show the formation of the triplet excited state of distyryl BODIPY in the dyad, which is populated from charge-recombination processes in both solvents.

Two new supramolecular architectures based on zinc phthalocyanine (Pc) and imidazolyl-substituted perylenediimide (PDI), ZnPc/DImPDI/ZnPc 1 and ZnPc/ImPDI 2, have been prepared. A strong electron-donor, 8, which contained eight tert-octylphenoxy groups was synthesized to ensure high solubility, thereby reducing aggregation in solution and providing σ-donor features while avoiding regioisomeric mixtures. Also, PDI units were functionalized with tert-octylphenoxy groups at the bay positions, which provide solubility to avoid aggregation in solution, together with one and two imidazole moieties in the amide position, 6 and 4, respectively, to be able to strongly coordinate with the ZnPc complex. Supramolecular complexation studies by ¹H NMR spectroscopy and ESI-MS demonstrate a high coordinative binding constant between imidazole-substituted 4 or 6 and 8. The same results were confirmed by UV/Vis and fluorescence titration studies. UV/Vis titration studies revealed the formation of a 1:1 complex ZnPc/ImPDI 2 for the systems 8 and 6 and a 2:1 complex ZnPc/DImPDI/ZnPc 1 for the interaction of 8 and 4. The binding constant in both cases was determined to be on the order of 10⁵ M⁻¹. Femtosecond laser flash photolysis measurements provided a direct proof of the charge-separated state within both supramolecular assemblies by observing the transient absorption band at 820 nm due to the zinc phthalocyanine radical cation. The lifetimes of charge-separated states are (9.8±3) ns for triad 1 and (3±1) ns for dyad 2. As far as we know, this is the first time that a radical ion pair has been detected in a supramolecular assembled ZnPc–PDI system and has obtained the longest lifetime of a charge-separated state published for ZnPc–PDI assemblies.

Se han preparado dos nuevos sistemas supramoleculares basados en ftalocianina de zinc y en perilenobisimida sustituida con grupos imidazolilo, ZnPc/DImPDI/ZnPc 1 y ZnPc/ImPDI 2. Se ha sintetizado la ftalocianina 8 con ocho grupos terc-octilfenoxilo con objeto de aumentar la solubilidad, reducir la agregación en disolución, introducir grupos dadores de electrones y, por último, evitar mezclas de regioisómeros. Además, las 6 y 4 también se han funcionalizado con grupos terc-octilfenoxilo en las posiciones bahía, los cuales les confieren solubilidad e impiden la agregación en disolución, y con una o dos unidades de imidazol en la posición imida, respectivamente, con objeto de coordinar fuertemente la ZnPc. Los estudios de complejación supramolecular llevados a cabo por ¹H-RMN y MS-ESI han demostrado una constante de complejación muy alta entre las 4 o 6 sustituidas con imidazol y la 8. Los mismos resultados han sido confirmados mediante estudios de valoración realizados por UV-vis y fluorescencia. Los estudios de valoración realizados mediante UV-vis indican la formación de un complejo 1:1 ZnPc/ImPDI 2 para los sistemas 8 y 6, y un complejo 2:1 ZnPc/DImPDI/ZnPc 1 para los sistemas 8 y 4. Las constantes de complejación en los dos son del orden de 10⁵ M⁻¹. En ambos sistemas supramoleculares también ha sido posible determinar el estado de separación de cargas mediante medidas de fotólisis por destello láser, observando la banda de absorción transiente a 820 nm debida al catión radical de la ftalocianina de zinc. Los tiempos de vida de los estados de separación de carga son (9.8±3) ns para la tríada 1 y (3±1) ns para la díada 2. Según nuestros datos, esta es la primera vez que ha sido detectada la existencia de un par ión radical en un sistema de ZnPc–PDI ensamblado de manera supramolecular, obteniendo el tiempo de vida más largo, conocido hasta la fecha, de un estado de separación de cargas en un sistema ZnPc–PDI.

Spectroscopic, redox, computational, and electron transfer reactions of the covalently linked zinc porphyrin–triphenylamine–fulleropyrrolidine system are investigated in solvents of varying polarity. An appreciable interaction between triphenylamine and the porphyrin π system is revealed by steady-state absorption and emission, redox, and computational studies. Free-energy calculations suggest that the light-induced processes via the singlet-excited porphyrin are exothermic in benzonitrile, dichlorobenzene, toluene, and benzene. The occurrence of fast and efficient charge-separation processes (≈10¹² s⁻¹) via the singlet-excited porphyrin is confirmed by femtosecond transient absorption measurements in solvents with dielectric constants ranging from 25.2 (benzonitrile) to 2.2 (benzene). The rates of the charge separation processes are much less solvent-dependent, which suggests that the charge-separation processes occur at the top region of the Marcus parabola. The lifetimes of the singlet radical-ion pair (70–3000 ps at room temperature) decrease substantially in more polar solvents, which suggests that the charge-recombination process is occurring in the Marcus inverted region. Interestingly, by utilizing the nanosecond transient absorption spectral technique we can obtain clear evidence about the existence of triplet radical-ion pairs with relatively long lifetimes of 0.71 μs (in benzonitrile) and 2.2 μs (in o-dichlorobenzene), but not in toluene and benzene due to energetic considerations. From the point of view of mechanistic information, the synthesized zinc porphyrin–triphenylamine–fulleropyrrolidine system has the advantage that both the lifetimes of the singlet and triplet radical-ion pair can be determined.

Novel conglomerates consisting of saddle-distorted Sn^IV(DPP) (H₂DPP=dodecaphenylporphyrin) complexes and μ₃-O-centered and carboxylato-bridged trinuclear Ru^III clusters linked by pyridine carboxylates were synthesized and characterized. Sn^IV–DPP complexes with Cl⁻, OH⁻, and 3- and 4-pyridine carboxylates ligands were characterized by spectroscopic methods and X-ray crystallography. Reactions of [Sn(DPP)(pyridinecarboxylato)₂] with trinuclear Ru^III clusters gave novel conglomerates in moderate yields. The conglomerates are stable in solution as demonstrated by ¹H NMR and electrospray ionization mass spectrometry (ESI-MS) measurements, which show consistent spectra with those expected from their structures, and also by electrochemical measurements, which exhibit reversible multistep redox processes. This stability stems from the saddle distortion of the DPP²⁻ ligand to enhance the Lewis acidity of the Sn^IV center that strengthens the axial coordination of the linker. The fast intramolecular photoinduced electron transfer from the Sn^IV(DPP) unit to trinuclear Ru^III clusters, affording the electron-transfer (ET) state {Sn(DPP^.+)–Ru^IIRu^III₂}, was observed by femtosecond laser flash photolysis. The lifetimes of ET states of the conglomerates were determined to be in the range 98–446 ps, depending on the clusters and energies of the ET states. The reorganization energy of the electron transfer was determined to be 0.58±0.08 eV in light of the Marcus theory of electron transfer. The rate constants of both the photoinduced electron transfer and the back electron transfer in the conglomerates fall in the Marcus inverted region due to the small reorganization energy of electron transfer.

The effects of axial ligands on electron-transfer and proton-coupled electron-transfer reactions of mononuclear nonheme oxoiron(IV) complexes were investigated by using [Fe^IV(O)(tmc)(X)]ⁿ⁺ (1-X) with various axial ligands, in which tmc is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is CH₃CN (1-NCCH₃), CF₃COO⁻ (1-OOCCF₃), or N₃⁻ (1-N₃), and ferrocene derivatives as electron donors. As the binding strength of the axial ligands increases, the one-electron reduction potentials of 1-X (E_red, V vs. saturated calomel electrode (SCE)) are more negatively shifted by the binding of the more electron-donating axial ligands in the order of 1-NCCH₃ (0.39) > 1-OOCCF₃ (0.13) > 1-N₃ (−0.05 V). Rate constants of electron transfer from ferrocene derivatives to 1-X were analyzed in light of the Marcus theory of electron transfer to determine reorganization energies (λ) of electron transfer. The λ values decrease in the order of 1-NCCH₃ (2.37) > 1-OOCCF₃ (2.12) > 1-N₃ (1.97 eV). Thus, the electron-transfer reduction becomes less favorable thermodynamically but more favorable kinetically with increasing donor ability of the axial ligands. The net effect of the axial ligands is the deceleration of the electron-transfer rate in the order of 1-NCCH₃ > 1-OOCCF₃ > 1-N₃. In sharp contrast to this, the rates of the proton-coupled electron-transfer reactions of 1-X are markedly accelerated in the presence of an acid in the opposite order: 1-NCCH₃ < 1-OOCCF₃ < 1-N₃. Such contrasting effects of the axial ligands on the electron-transfer and proton-coupled electron-transfer reactions of nonheme oxoiron(IV) complexes are discussed in light of the counterintuitive reactivity patterns observed in the oxo transfer and hydrogen-atom abstraction reactions by nonheme oxoiron(IV) complexes (Sastri et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 19 181–19 186).

We report here the formation of a long-lived charge-separated state of a self-assembled donor–acceptor tetrad, formed by axial coordination of a fulleropyrrolidine appended with an imidazole coordinating ligand (C₆₀Im) to the zinc center of a subphthalocyanine–triphenylamine–zinc porphyrin (SubPc–TPA–ZnP), as a charge-stabilizing antenna reaction center mimic in toluene. The subphthalocyanine and triphenylamine entities, with their high-energy singlet states, act as an energy-transferring antenna unit to produce a singlet zinc porphyrin. The formation constant for the self-assembled tetrad was determined to be 1.0×10⁴ M⁻¹, suggesting a moderately stable complex formation. The geometric and electronic structures of the covalently linked SubPc–TPA–ZnP triad and self-assembled SubPc–TPA–ZnP:C₆₀Im tetrad were examined by using an ab initio B3LYP/6-31G method. The majority of the highest occupied frontier molecular orbital was found over the ZnP and TPA entities, whereas the lowest unoccupied molecular orbital was located over the fullerene entity, suggesting the formation of the radical-ion pair (SubPc–TPA–ZnP^.+:C₆₀Im^.−). The redox measurements revealed that the energy level of the radical-ion pair in toluene is located lower than that of the singlet and triplet states of the zinc porphyrin and fullerene entities. The femtosecond transient absorption measurements revealed fast charge separation from the singlet porphyrin to the coordinated C₆₀Im with a lifetime of 1.1 ns. Interestingly, slow charge recombination (1.6×10⁵ s⁻¹) and the long lifetime of the charge-separated state (6.6 μs) were obtained in toluene by utilizing the nanosecond transient measurements.

Photoinduced electron transfer (ET) of a series of aromatic electron donors (D) to the singlet or triplet excited state of a flavin analogue (10-methylisoalloxazine: MeFl) and intermolecular back electron transfer (BET) from MeFl^.− to D^.+ in benzonitrile (PhCN) has been investigated in light of the Marcus theory of ET. The rate constants of intermolecular photoinduced ET (k_et) from D to the singlet excited state (¹MeFl*) and the triplet excited state (³MeFl*) were determined by fluorescence quenching and enhanced decay rates of triplet–triplet (T–T) absorption by the presence of D, respectively. The k_et values increase with an increase in the ET driving force to reach the diffusion-limit value that remains constant with a further increase in the ET driving force. Nanosecond laser flash photolysis was performed to determine the rate constants of intermolecular BET (k_bet) from MeFl^.− to D^.+ in PhCN. In contrast to the case of k_et, the driving force dependence of k_bet shows a pronounced decrease towards the highly exothermic region. The reorganization energy (λ) of intermolecular BET is determined to be 0.68 eV by applying the Marcus equation in the inverted region, where the k_bet value decreases with increasing the BET driving force. The slowest BET was observed for BET from MeFl^.− to N,N-dimethylaniline radical cation (DMA^.+) with the k_bet value of 3.5×10⁶ M⁻¹ s⁻¹, which is 1600 times smaller than the diffusion rate constant in PhCN (5.6×10⁹ M⁻¹ s⁻¹). Then, DMA was linked to the 10-position of isoalloxazine to synthesize a DMA–flavin linked dyad (10-[4′-(N,N-dimethylamino)phenyl]–isoalloxazine: DMA–Fl). Photoexcitation of DMA–Fl results in photoinduced ET from the DMA moiety to the singlet excited state of Fl moiety to form the charge-separated (CS) state (DMA^.+–Fl^.−) that has an extremely long lifetime (2.1 ms) in PhCN at 298 K.

Electron donor (D) substituted 3-ethoxycarbonylcoumarin (CM) derivatives [D–CM: D=4-diphenylaminophenyl (DPA), 4-diethylaminophenyl (DEA), 4-dimethylaminophenyl (DMA), and 2-methyl-4-dimethylaminophenyl (MeDMA)] are synthesized and characterized. Photoinduced electron transfer (ET) from the D moiety to the acceptor (CM) and back electron transfer (BET) are investigated by femtosecond and nanosecond laser flash photolysis measurements. Femtosecond laser excitation at 355 nm of a deaerated acetonitrile (MeCN) solution of D–CM shows generation of the singlet charge-separated (CS) state [¹(D^.+–CM^.−)] by ET from D to the singlet excited state of the CM moiety (¹CM*), and this is followed by rapid decay within 3 ns to afford the triplet excited state (D–³CM*). Nanosecond laser excitation of a deaerated MeCN solution of D–CM results in formation of the triplet CS state by ET from D to ³CM*. The quantum yield of formation of the triplet CS state [³(DPA^.+–CM^.−)] in the presence of iodobenzene (PhI) in deaerated MeCN increases with increasing concentration of PhI to reach 27 % at 0.5 M PhI. The triplet CS state decays by bimolecular BET because of the long CS lifetime by unimolecular BET. Formation of the long-lived triplet CS state was confirmed by electron spin resonance (ESR) measurements. The photorobust nature of DPA–CM is demonstrated by multiple laser pulse excitation (>1000 times) at 355 nm. The photoinduced ET and BET rate constants of a series of D–CM are thoroughly analyzed in light of the Marcus theory of electron transfer.

A series of (porphyrinato)zinc(II) compounds were synthesized with use of 5,10,15,20-tetraphenylporphyrin (H₂TPP), 2,3,5,10,12,13,15,20-octaphenylporphyrin (H₂OPP), and 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H₂DPP). Those compounds form complexes with aniline, pyridine, and 3- and 4-aminopyridines as axial ligands. X-ray crystallography was performed on the complexes with 3-aminopyridine (3-AP) and 4-aminopyridine (4-AP) as axial ligands. 3-Aminopyridine was revealed to bind through the amino group to the Zn(OPP), exhibiting intermolecular π–π interaction between 3-AP and one of the pyrrole rings and intermolecular NH–π interactions of the coordinated amino group with two β-phenyl groups of an adjacent molecule. In solution, the aminopyridines form a single species at ambient temperature and are assumed to have pyridine coordination through the aromatic pyridine nitrogen atom. Variable-temperature NMR spectroscopy in CD₂Cl₂ indicates that two different species exist at lower temperatures, suggesting that amino-bound complexes of 3-AP can be formed as a metastable species in solution, which is stabilized in the crystal as a result of noncovalent interactions. The binding constants of aminopyridines to the three kinds of (porphyrinato)zinc complexes reveal enhancement of the axial ligation by virtue of the distortion of the porphyrin ring. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A series of porphyrins, M[TCPP-Et₄] [M = Zn (1), Cu (2), and Ni (3); Et = CH₂CH₃; TCPP = meso-tetra(4-carboxyphenyl)porphyrin], M[TCPP-Me₄] [M = Zn (4), Cu (5), and Co (6); Me = CH₃], and two nonmetalated compounds, TCPP-Et₄ (7) and TCPP-Me₄·H₂O (8), were synthesized by solvothermal reactions and characterized by single-crystal X-ray diffraction. Compounds 1–3 feature an isolated structure with a planar macrocycle and an embedded metal-ion coordinating to four pyrrole nitrogen atoms. Compound 4 is characterized as a two-dimensional coordination polymer, and the zinc ion coordinates to four nitrogen atoms and two oxygen atoms. Compound 4 possesses a large void space (361 ų), which corresponds to 14 % of the unit-cell volume. Compounds 5 and 6 are characteristic of an isolated motif with a four-coordinate metal ion and a saddle-distorted nonplanar porphyrin macrocycle. Nonmetalated compounds 7 and 8 also show an isolated structure with a planar macrocycle. For compounds 1–3 and 7, the TCPP is esterified with ethanol, while for compounds 4–6 and 8, the TCPP is esterified with methanol. The molecules in 1 and 6–8 are interconnected by hydrogen bonds and π–π interactions to yield 3D supramolecular networks, while in 2–5, 2D supramolecular motifs are formed. The reaction mechanism was explored. Esterification plays an important role in changing the properties of the compounds as well as in the formation of different structural motifs and supramolecular networks. The UV/Vis, FTIR, fluorescence, phosphorescence, and MALDI-TOF MS spectra, quantum yields, luminescence lifetimes, and cyclic voltammograms were also studied in detail.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The site of electron-transfer reduction of AuPQ⁺ (PQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quino-xalino[2, 3−b′]porphyrin) and AuQPQ⁺ (QPQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]porphyrin) is changed from the Au^III center to the quinoxaline part of the PQ macrocycle in the presence of Sc³⁺ in benzonitrile because of strong binding of Sc³⁺ to the two nitrogen atoms of the quinoxaline moiety. Strong binding of Sc³⁺ to the corresponding nitrogen atoms on the quinoxaline unit of ZnPQ also occurs for the neutral form. The effects of Sc³⁺ on the photodynamics of an electron donor–acceptor compound containing a linked Zn^II and Au^III porphyrin ([ZnPQ–AuPQ]PF₆) have been examined by femto- and nanosecond laser flash photolysis measurements. The observed transient absorption bands at 630 and 670 nm after laser pulse irradiation in the absence of Sc³⁺ in benzonitrile are assigned to the charge-shifted (CS) state (ZnPQ^.+–AuPQ). The CS state decays through back electron transfer (BET) to the ground state rather than to the triplet excited state. The BET rate was determined from the disappearance of the absorption band due to the CS state. The decay of the CS state obeys first-order kinetics. The CS lifetime was determined to be 250 ps in benzonitrile. Addition of Sc³⁺ to a solution of ZnPQ–AuPQ⁺ in benzonitrile caused a drastic lengthening of the CS lifetime that was determined to be 430 ns, a value 1700 times longer than the 250 ps lifetime measured in the absence of Sc³⁺. Such remarkable prolongation of the CS lifetime in the presence of Sc³⁺ results from a change in the site of electron transfer from the Au^III center to the quinoxaline part of the PQ macrocycle when Sc³⁺ binds to the quinoxaline moiety, which decelerate BET due to a large reorganization energy of electron transfer. The change in the site of electron transfer was confirmed by ESR measurements, redox potentials, and UV/Vis spectra of the singly reduced products.

A new π-conjugated copolymer, namely, poly{cyanofluore-alt-[5-(N,N′-diphenylamino)phenylenevinylene]} ((CNF–TPA)_n), was synthesized by condensation polymerization of 2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)diacetonitrile and 5-(N,N′-diphenylamino)benzene-1,3-dicarbaldehyde by using the Knoevenagel reaction. By design, diphenylamine, alkylfluorene and poly(p-phenylenevinylene) linkages were combined to form a (CNF–TPA)_n copolymer which exhibits high thermal stability and glass-transition temperature. Photodynamic measurements in polar benzonitrile indicate fast and efficient photoinduced electron transfer (≈10¹¹ s⁻¹) from triphenylamine (TPA) to cyanofluorene (CNF) to produce the long-lived charge-separated state (90 μs). The finding that the charge-recombination process of (CNF^.−–TPA^.+)_n is much slower than the charge separation in polar benzonitrile suggests a potential application in molecular-level electronic and optoelectronic devices.

High-valent manganese(IV or V)–oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated Mn^V– and Mn^IV–oxo porphyrins in basic aqueous solution and investigated their reactivities in CH bond activation of hydrocarbons. We now report that Mn^V– and Mn^IV–oxo porphyrins are capable of activating CH bonds of alkylaromatics, with the reactivity order of Mn^V–oxo>Mn^IV–oxo; the reactivity of a Mn^V–oxo complex is 150 times greater than that of a Mn^IV–oxo complex in the oxidation of xanthene. The CH bond activation of alkylaromatics by the Mn^V– and Mn^IV–oxo porphyrins is proposed to occur through a hydrogen-atom abstraction, based on the observations of a good linear correlation between the reaction rates and the CH bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of Mn^IV–oxo porphyrins to Mn^V–oxo and Mn^III porphyrins is not a feasible pathway in basic aqueous solution and that Mn^IV–oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The CH bond activation of alkylaromatics by Mn^V– and Mn^IV–oxo species proceeds through a one-electron process, in which a Mn^IV–-oxo porphyrin is formed as a product in the CH bond activation by a Mn^V–oxo porphyrin, followed by a further reaction of the Mn^IV–oxo porphyrin with substrates that results in the formation of a Mn^III porphyrin complex. This result is in contrast to the oxidation of sulfides by the Mn^V–oxo porphyrin, in which the oxidation of thioanisole by the Mn^V–oxo complex produces the starting Mn^III porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the Mn^V–oxo species occurs by means of a two-electron oxidation process. In contrast, a Mn^IV–oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.

Cup-shaped nanocarbons (CNC) generated by the electron-transfer reduction of cup-stacked carbon nanotubes have been functionalized with porphyrins (H₂P) as light-capturing chromophores. The resulting donor–acceptor nanohybrid has been characterized by thermogravimetric analysis (TGA), Raman and IR spectroscopy, transmission electron microscopy, elemental analysis, and UV/Vis spectroscopy. The weight of the porphyrins attached to the cup-shaped nanocarbons was determined as 20 % by TGA and elemental analysis. The UV/Vis absorption spectrum of CNC(H₂P)_n in DMF agrees well with that obtained by the superposition of reference porphyrin (ref-H₂P) and cup-shaped nanocarbons. The photoexcitation of the CNC(H₂P)_n nanohybrid results in formation of the charge-separated (CS) state to attain the longest CS lifetime (0.64±0.01 ms) ever reported for donor–acceptor nanohybrids, which may arise from efficient electron migration following the charge separation. The formation of a radical ion pair was detected directly by electron spin resonance (ESR) measurements under photoirradiation of CNC(H₂P)_n with a high-pressure mercury lamp in frozen DMF at 153 K. The observed ESR signal at g=2.0044 agrees with that of ref-H₂P^.+ produced by one-electron oxidation with [Ru(bpy)₃]³⁺ in deaerated CHCl₃, indicating the formation of H₂P^.+. The electron-acceptor ability of the reference CNC compound (ref-CNC) was also examined by the electron-transfer reduction of ref-CNC by a series of semiquinone radical anions.

Recent developments in photocatalytic hydrogen production by using artificial photosynthesis systems is described, together with those in hydrogen storage through the fixation of CO₂ with H₂. Hydrogen can be stored in the form of formic acid, which can be converted back to H₂ in the presence of an appropriate catalyst. Electron donor–acceptor dyads are utilized as efficient photocatalysts to reduce methyl viologen (MV²⁺) by NADH (β-nicotinamide adenine dinucleotide, reduced form) analogues to produce the methyl violgen radical cation that acts as an electron mediator for the production of hydrogen. Porphyrin-monolayer-protected gold clusters that enhance the light harvesting efficiency can also be used for the photocatalytic reduction of methyl viologen by NADH analogues. The use of a simple electron donor–acceptor dyad, the 9-mesityl-10-methylacridinium ion (Acr⁺–Mes), enables the construction of a highly efficient photocatalytic hydrogen-evolution system without an electron mediator such as MV²⁺, with poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt–PVP) and NADH as a hydrogen-evolution catalyst and an electron donor, respectively. Hydrogen thus produced can be stored in the form of formic acid (liquid) by fixation of CO₂ with H₂ in water by using ruthenium aqua complexes [Ru^II(η⁶-C₆Me₆)(L)(OH₂)]²⁺ [L = 2,2′-bipyridine (bpy), 4,4′-dimethoxy-2,2′-bipyridine (4,4′-OMe-bpy)] and iridium aqua complexes [Ir^IIICp*(L)(OH₂)]²⁺ (Cp* = η⁵-C₅Me₅, L = bpy, 4,4′-OMe-bpy) as catalysts at pH 3.0. Catalytic systems for the decomposition of HCOOH to H₂ are also described. The combination of photocatalytic hydrogen generation with the catalytic fixation of CO₂ with H₂ and the decomposition of HCOOH back to H₂ provides an excellent system for cutting CO₂ emission.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The cover picture shows the photocatalytic production of hydrogen and the interconversion between hydrogen and formic acid with the use of catalysts. These processes have become important in the search for alternative energy resources that are renewable, in order to combat global environmental problems. Hydrogen is a clean energy source, and therefore its production and storage is a key to the future. Details are presented in the Microreview by S. Fukuzumi on p. 1351 ff.

A mechanistic study on the Nieuwland catalysis for dimerization of acetylene is performed by detecting copper–acetylene and copper–monovinylacetylene π-complexes and also by examining the kinetics under virtually the same reaction conditions employed in the industrial process. An efficient H/D exchange occurs between acetylene and protons in the Nieuwland catalytic system. Addition of a coordinating ligand to the conventional Nieuwland catalytic system results in improvement of the catalytic activity and selectivity for the acetylene dimerization. The kinetic analysis including the kinetic deuterium isotope effect provides valuable insight into the Nieuwland catalytic mechanism of the dimerization of acetylene. Copyright © 2008 John Wiley & Sons, Ltd.

Deuterium kinetic isotope effects were observed in the dimerization of acetylene with a Nieuwland catalyst in an aqueous media, where the copper–acetylene and copper–monovinylacetylene (MVA) π-complexes were detected as intermediates by ¹H NMR. The copper–acetylene π-complex is responsible for an efficient H/D exchange between acetylene and proton (or deuteron) of water, which occurs rapidly with a Nieuwland catalyst. The deuterium kinetic isotope effects and the rapid H/D exchange reaction together with the detection of the copper–acetylene and copper–MVA π-complexes provide deeper insight into the catalytic mechanism of the Nieuwland catalysis for the dimerization of acetylene. Copyright © 2008 John Wiley & Sons, Ltd.

Formic acid (HCOOH) decomposes efficiently to afford H₂ and CO₂ selectively in the presence of a catalytic amount of a water-soluble rhodium aqua complex, [Rh^III(Cp*)(bpy)(H₂O)]²⁺(Cp*=pentamethylcyclopentadienyl, bpy=2,2′-bipyridine) in aqueous solution at 298 K. No CO was produced in this catalytic decomposition of HCOOH. The decomposition rate reached a maximum value at pH 3.8. No deterioration of the catalyst was observed during the catalytic decomposition of HCOOH, and the catalytic activity remained the same for the repeated addition of HCOOH. The rhodium-hydride complex was detected as the catalytic active species that undergoes efficient H/D exchange with water. When the catalytic decomposition of HCOOH was performed in D₂O, D₂ was produced selectively. Such an efficient H/D exchange and the observation of a deuterium kinetic isotope effect in the catalytic decomposition of DCOOH in H₂O provide valuable mechanistic insight into this efficient and selective decomposition process.

The accelerating effect of Sc³⁺ on the electron-transfer (ET) reduction of the p-benzoquinone derivative 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) by 10,10′-dimethyl-9,9′-biacridine ((AcrH)₂) at 233 K changes to a decelerating effect with increasing reaction temperature; the observed second-order rate constant k_et decreases with increasing Sc³⁺ concentration at high concentrations of Sc³⁺ at 298 K. At 263 K the k_et value remains constant with increasing Sc³⁺ concentration. Such a remarkable difference with regard to dependence of k_et on [Sc³⁺] between low and high temperatures results from the difference in relative activity of two ET pathways that depend on temperature, one of which affords 1:1 complex TolSQ^.−–Sc³⁺, and the other 1:2 complex TolSQ^.−–(Sc³⁺)₂ with additional binding of Sc³⁺ to TolSQ^.−–Sc³⁺. The formation of TolSQ^.−–Sc³⁺ and TolSQ^.−–(Sc³⁺)₂ complexes was confirmed by EPR spectroscopy in the ET reduction of TolSQ in the presence of low and high concentrations of Sc³⁺, respectively. The effects of metal ions on other ET reactions of quinones to afford 1:1 and 1:2 complexes between semiquinone radical anions and metal ions are also reported. The ET pathway affording the 1:2 complexes has smaller activation enthalpies ΔH^≠ and more negative activation entropies ΔS^≠ because of stronger binding of metal ions and more restricted geometries of the ET transition states as compared with the ET pathway to afford the 1:1 complexes.