Molecular binding of fullerenes, C₆₀ and C₇₀, with the Zn^II complex of a monomeric ring-fused porphyrin derivative (2-py) as a host molecule, which has a concave π-conjugated surface, has been confirmed spectroscopically. The structures of associated complexes composed of fullerenes and 2-py were explicitly established by X-ray diffraction analysis. The fullerenes in the 2:1 complexes, which consist of two 2-py molecules and one fullerene molecule, are fully covered by the concave surfaces of the two 2-py molecules in the crystal structure. In contrast, in the crystal structure of the 1:1 complex consisting of one 2-py molecule and one C₆₀ molecule, the C₆₀ molecule formed a π–π stacked pair with a C₆₀ molecule in the neighboring complex using a partial surface, which was uncovered by the 2-py molecule. Additionally, the molecular size of fullerene adopted significantly affects the ¹H NMR spectral changes and the redox properties of 2-py upon the molecular binding.

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).

The thermal and photochemical reactions of a newly synthesized complex, [Ru^II(TPA)(tpphz)]²⁺ (1; TPA=tris(2-pyridylmethyl)amine, tpphz=tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h: 2′′′,3′′′-j]phenazine), and its derivatives have been investigated. Heating a solution of complex 1 (closed form) and its derivatives in MeCN caused the partial dissociation of one pyridylmethyl moiety of the TPA ligand and the resulting vacant site on the Ru^II center was occupied by a molecule of MeCN from the solvent to give a dissociated complex, [Ru^II(η³-TPA)(tpphz)(MeCN)]²⁺ (1′, open form), and its derivatives, respectively, in quantitative yields. The thermal dissociation reactions were investigated on the basis of kinetics analysis, which indicated that the reactions proceeded through a seven-coordinate transition state. Although the backwards reaction was induced by photoirradiation of the MLCT absorption bands, the photoreaction of complex 1′ reached a photostationary state between complexes 1 and 1′ and, hence, the recovery of complex 1 from complex 1′ was 67 %. Upon protonation of complex 1 at the vacant site of the tpphz ligand, the efficiency of the photoinduced recovery of complex 1+H⁺ from complex 1′+H⁺ improved to 83 %. In contrast, dinuclear μ-tpphz complexes 2 and 3, which contained the Ru^II(TPA)(tpphz) unit and either a Ru^II(bpy)₂ or Pd^IICl₂ moiety on the other coordination edge of the tpphz ligand, exhibited 100 % photoconversion from their open forms into their closed forms (2′2 and 3′3). These results are the first examples of the complete photochromic structural change of a transition-metal complex, as represented by complete interconversion between its open and closed forms. Scrutinization by performing optical and electrochemical measurements allowed us to propose a rationale for how metal coordination at the vacant site of the tpphz ligand improves the efficiency of photoconversion from the open form into the closed form. It is essential to lower the energy level of the triplet metal-to-ligand charge-transfer excited state (³MLCT*) of the closed form relative to that of the triplet metal-centered excited state (³MC*) by metal coordination. This energy-level manipulation hinders the transition from the ³MLCT* state into the ³MC* state in the closed form to block the partial photodissociation of the TPA ligand.

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.

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.

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.