A new set of [Cu(phen)2]+ based rotaxanes, featuring [60]-fullerene as an electron acceptor and a variety of electron donating moieties, namely zinc porphyrin (ZnP), zinc phthalocyanine (ZnPc) and ferrocene (Fc), has been synthesized and fully characterized with respect to electrochemical and photophysical properties. The assembly of the rotaxanes has been achieved using a slight variation of our previously reported synthetic strategy that combines the Cu(I)-catalyzed azide–alkyne cycloaddition reaction (the “click” or CuAAC reaction) with Sauvage's metal-template protocol. To underline our results, complementary model rotaxanes and catenanes have been prepared using the same strategy and their electrochemistry and photo-induced processes have been investigated. Insights into excited state interactions have been afforded from steady state and time resolved emission spectroscopy as well as transient absorption spectroscopy. It has been found that photo-excitation of the present rotaxanes triggers a cascade of multi-step energy and electron transfer events that ultimately leads to remarkably long-lived charge separated states featuring one-electron reduced C60 radical anion (C60˙−) and either one-electron oxidized porphyrin (ZnP˙+) or one-electron oxidized ferrocene (Fc˙+) with lifetimes up to 61 microseconds. In addition, shorter-lived charge separated states involving one-electron oxidized copper complexes ([Cu(phen)2]2+ (τ < 100 ns)), one-electron oxidized zinc phthalocyanine (ZnPc˙+; τ = 380–560 ns), or ZnP˙+ (τ = 2.3–8.4 μs), and C60˙− have been identified as intermediates during the sequence. Detailed energy diagrams illustrate the sequence and rate constants of the photophysical events occurring with the mechanically-linked chromophores. This work pioneers the exploration of mechanically-linked systems as platforms to position three distinct chromophores, which are able to absorb light over a very wide range of the visible region, triggering a cascade of short-range energy and electron transfer processes to afford long-lived charge separated states.

A new series of nanoscale electron donor–acceptor systems with [2]catenane architectures has been synthesized, incorporating magnesium porphyrin (MgP) or free base porphyrin (H2P) as electron donor and C60 as electron acceptor, surrounding a central tetrahedral Cu(I)-1,10-phenanthroline (phen) complex. Model catenated compounds incorporating only one or none of these photoactive moieties were also prepared. The synthesis involved the use of Sauvage's metal template protocol in combination with the 1,3-dipolar cycloaddition of azides and alkynes (“click chemistry”), as in other recent reports from our laboratories. Ground state electron interactions between the individual constituents was probed using electrochemistry and UV-vis absorption spectroscopy, while events occurring following photoexcitation in tetrahydrofuran (under both aerobic and anaerobic conditions) at various wavelengths were followed by means of time-resolved transient absorption and emission spectroscopies on the femtosecond and nanosecond time scales, respectively, complemented by measurements of quantum yields for generation of singlet oxygen. From similar studies with model catenates containing one or neither of the chromophores, the events following photoexcitation could be elucidated. The results were compared with those previously reported for analogous catenates based on zinc porphyrin (ZnP). It was determined that a series of energy transfer (EnT) and electron transfer (ET) processes take place in the present catenates, ultimately generating long-distance charge separated (CS) states involving oxidized porphyrin and reduced C60 moieties, with lifetimes ranging from 400 to 1060 nanoseconds. Shorter lived short-distance CS states possessing oxidized copper complexes and reduced C60, with lifetimes ranging from 15 to 60 ns, were formed en route to the long-distance CS states. The dynamics of the ET processes were analyzed in terms of their thermodynamic driving forces. It was clear that intramolecular back ET was occurring in the inverted region of the Marcus parabola correlating rates and driving forces for electron transfer processes. In addition, evidence for triplet excited states as a product of either incomplete ET or back ET was found. The differences in behavior of the three catenates upon photoexcitation are analyzed in terms of the energy levels of the various intermediate states and the driving forces for EnT and ET processes.

This Perspective traces the career of the author in the area of mechanistic organic photochemistry from the primitive state of this field in the late 1950s and early 1960s until its maturity as a highly sophisticated discipline at the present time. The paper focuses on early studies carried out by the author and his associates to delineate mechanisms of basic types of photochemical reactions of small organic molecules to studies in recent years involving photoinduced electron-transfer processes occurring in nanoscale artificial photosynthetic systems in ultrashort time domains. The important role that serendipitous events played in directing key career decisions and events is emphasized. The important ways that developments in instrumentation influenced the choices, possibilities, and accomplishments of performing research in photochemistry between 1960 and the present time are emphasized. Acknowledgment is given by the author to the many people who contributed directly and indirectly to the course that his career has taken over more than half a century.

The effect of molecular topology and conformation on the dynamics of photoinduced electron transfer (ET) processes has been studied in interlocked electron donor–acceptor systems, specifically rotaxanes with zinc(II)-tetraphenylporphyrin (ZnP) as the electron donor and [60]fullerene (C60) as the electron acceptor. Formation or cleavage of coordinative bonds was used to induce major topological and conformational changes in the interlocked architecture. In the first approach, the tweezer-like structure created by the two ZnP stopper groups on the thread was used as a recognition site for complexation of 1,4-diazabicyclo[2.2.2]octane (DABCO), which creates a bridge between the two ZnP moieties of the rotaxane, generating a catenane structure. The photoinduced processes in the DABCO-complexed (ZnP)2-[2]catenate-C60 system were compared with those of the (ZnP)2-rotaxane-C60 precursor and the previously reported ZnP-[2]catenate-C60. Steady-state emission and transient absorption studies showed that a similar multistep ET pathway emerged for rotaxanes and catenanes upon photoexcitation at various wavelengths, ultimately resulting in a long-lived ZnP•+/C60•– charge-separated radical pair (CSRP) state. However, the decay kinetics of the CSRP states clearly reflect the topological differences between the rotaxane, the catenate, and DABCO-complexed-catenate architectures. The lifetime of the long-distance ZnP•+–[Cu(I)phen2]+–C60•– CSRP state is more than four times longer in 3 (1.03 μs) than in 1 (0.24 μs) and approaches that in catenate 2 (1.1 μs). The results clearly showed that creation of a catenane from a rotaxane topology inhibits the charge recombination process. In a second approach, when the Cu(I) ion used as the template to assemble the (ZnP)2–[Cu(I)phen2]+–C60 rotaxane was removed, it was evident that a major structural change had occurred. since charge separation between the chromophores was no longer observed upon photoexcitation in nonpolar as well as in polar solvents. Only ZnP and C60 triplet excited states were observed upon laser excitation of the Cu-free rotaxane. These results highlight the critical importance of the central Cu(I) ion for long-range ET processes in these nanoscale interlocked electron donor–acceptor systems.Keywords: catenanes; electron transfer; fullerene; porphyrin; rotaxanes;

Aromatic triazoles have been frequently used as π-conjugated linkers in intramolecular electron transfer processes. To gain a deeper understanding of the electron-mediating function of triazoles, we have synthesized a family of new triazole-based electron donor–acceptor conjugates. We have connected zinc(II)porphyrins and fullerenes through a central triazole moiety—(ZnP–Tri–C60)—each with a single change in their connection through the linker. An extensive photophysical and computational investigation reveals that the electron transfer dynamics—charge separation and charge recombination—in the different ZnP–Tri–C60 conjugates reflect a significant influence of the connectivity at the triazole linker. Except for the m4m-ZnP–Tri–C6017, the conjugates exhibit through-bond photoinduced electron transfer with varying rate constants. Since the through-bond distance is nearly the same for all the synthesized ZnP–Tri–C60 conjugates, the variation in charge separation and charge recombination dynamics is mainly associated with the electronic properties of the conjugates, including orbital energies, electron affinity, and the energies of the excited states. The changes of the electronic couplings are, in turn, a consequence of the different connectivity patterns at the triazole moieties.

Reaction conditions have been developed for the preparation of a series of rotaxanes and catenanes with appended electron-donor and electron-acceptor ([60]fullerene) subunits using a straightforward one-pot procedure based on Cu(I)-template synthesis and “click” chemistry.

A new class of [2]catenanes containing zinc(II)−porphyrin (ZnP) and/or [60]fullerene (C60) as appended groups has been prepared. A complete description of the convergent synthetic approach based on Cu(I) template methodology and “click” 1,3-dipolar cycloaddition chemistry is described. This new electron donor−acceptor catenane family has been subjected to extensive spectroscopic, computational, electrochemical and photophysical studies. 1H NMR spectroscopy and computational analysis have revealed that the ZnP−C60−[2]catenane adopts an extended conformation with the chromophores as far as possible from each other. A detailed photophysical investigation has revealed that upon irradiation the ZnP singlet excited state initially transfers energy to the (phenanthroline)2−Cu(I) complex core, producing a metal-to-ligand charge transfer (MLCT) excited state, which in turn transfers an electron to the C60 group, generating the ZnP−[Cu(phen)2]2+−C60•− charge-separated state. A further charge shift from the [Cu(phen)2]2+ complex to the ZnP subunit, competitive with decay to the ground state, leads to the isoenergetic long distance ZnP•+−[Cu(phen)2]+−C60•− charge-separated radical pair state, which slowly decays back to the ground state on the microsecond time scale. The slow rate of back-electron transfer indicates that in this interlocked system, as in previously studied covalently linked ZnP−C60 hybrid materials, this process occurs in the Marcus-inverted region.

Two protocols based on Cu(I) template synthesis and “click” reactions for the synthesis of functionalized [2]catenanes are described. A straightforward procedure, involving high dilution conditions at high temperatures (70 °C), affords [2]catenanes bearing two identical peripheral groups in high yields. For the preparation of non-symmetrically functionalized [2]catenanes, a second milder and simpler protocol was developed, generalizing and extending the method. The introduction of peripheral functional groups into the catenane structure opens up possibilities of using [2]catenanes as building blocks for preparation of even more complex structures.

A series of [2]rotaxane materials, in which [60]fullerene is linked to a macrocycle and ferrocene (Fc) moieties are placed at the termini of a thread, both of which possess a central Cu(I)−1,10-phenanthroline [Cu(phen)2]+ complex, were synthesized by self-assembly using Sauvage metal template methodology. Two types of threads were constructed, one with terminal ester linkages, and a second with terminal 1,2,3-triazole linkages derived from Cu(I)-catalyzed “click” 1,3-cycloaddition reactions. Model compounds lacking the fullerene moiety were prepared in an analogous manner. The ability of the interlocked Fc−[Cu(phen)2]+−C60 hybrids to undergo electron transfer upon photoexcitation in benzonitrile, dichloromethane, and ortho-dichlorobenzene was investigated by means of time-resolved fluorescence and transient absorption spectroscopy, using excitation wavelengths directed at the fullerene and [Cu(phen)2]+ subunits. The energies of the electronic excited states and charge separated (CS) states that might be formed upon photoexcitation were determined from spectroscopic and electrochemical data. These studies showed that MLCT excited states of the copper complex in the fullerenerotaxanes were quenched by electron transfer to the fullerene in benzonitrile, resulting in charge separated states with oxidized copper and reduced fullerene moieties, (Fc)2−[Cu(phen)2]2+−C60•−. Even though electron transfer from Fc to the oxidized copper complex is predicted to be exergonic by 0.16 to 0.20 eV, no unequivocal evidence in support of such a process was obtained. The conclusion that Fc plays no role in the photoinduced processes in our systems rests on the lack of enhancement of the lifetime of the charge separated state, as measured by decay of C60•− at ∼1000 nm, since one-electron oxidized Fc is very difficult to detect spectroscopically in the 500−800 nm spectral region.

An easy one-pot procedure to synthesize rotaxanes bearing electron donors and C60 is described. The straightforward strategy, based on copper(I)-templated synthesis and “click” chemistry, proved to be very efficient and versatile, allowing the preparation of porphyrin- and ferrocene-stoppered fullerene−rotaxanes in high yields. As revealed by NMR analysis and computational studies, the highly flexible porphyrin−fullerene rotaxane can assume different conformations, which are most likely driven by attractive interactions between porphyrin and fullerene moieties.

Thorough examination of a previous report on the high solubility of carbon nanotubes in refluxing aniline reveals that the reported optical properties of the “nanotube” solution are due to aniline decomposition products, rather than functionalized nanotubes. No appreciable dissolution of single-wall carbon nanotubes can be achieved by this method, although some solubilization of multiwall nanotubes has been confirmed by SEM and TEM analyses.

Computational studies have been performed on a variety of C60–porphyrin dyads, a class of donor–acceptor materials which have been a subject of considerable attention in recent years. Molecular modelling studies were carried out to clarify the relationship between molecular topology and experimentally determined rates of intramolecular electron and energy transfer in these systems. The systems studied include doubly linked cyclophane-like C60–porphyrin dyads, where structural variations were made computationally on the porphyrin and linker portions, as well as dyads with flexible polyether and rigid steroid linkers. The molecular modelling studies involved building and minimising structures of the various fullerene–porphyrin dyads, followed by molecular dynamics to find the equilibrium and lowest energy conformations. The study confirmed that attractive van der Waals interactions between porphyrin and C60 moieties cause these dyads to adopt unusual conformations in which these groups are in close proximity, often in orientations which are not readily predictable from conventional structural representations. The implications of these computational data for the design of fullerene–porphyrin dyads with specific properties in the context of electron and energy transfer processes are discussed.

Spectroscopic, electrochemical and computational data show that C60 and a highly phenylated s-triazine derivative form a stable supramolecular complex at micromolar concentrations in solution at ambient temperatures, due to strong van der Waals attraction between their complementary surfaces.

A new set of [Cu(phen)2]+ based rotaxanes, featuring [60]-fullerene as an electron acceptor and a variety of electron donating moieties, namely zinc porphyrin (ZnP), zinc phthalocyanine (ZnPc) and ferrocene (Fc), has been synthesized and fully characterized with respect to electrochemical and photophysical properties. The assembly of the rotaxanes has been achieved using a slight variation of our previously reported synthetic strategy that combines the Cu(I)-catalyzed azide–alkyne cycloaddition reaction (the “click” or CuAAC reaction) with Sauvage's metal-template protocol. To underline our results, complementary model rotaxanes and catenanes have been prepared using the same strategy and their electrochemistry and photo-induced processes have been investigated. Insights into excited state interactions have been afforded from steady state and time resolved emission spectroscopy as well as transient absorption spectroscopy. It has been found that photo-excitation of the present rotaxanes triggers a cascade of multi-step energy and electron transfer events that ultimately leads to remarkably long-lived charge separated states featuring one-electron reduced C60 radical anion (C60˙−) and either one-electron oxidized porphyrin (ZnP˙+) or one-electron oxidized ferrocene (Fc˙+) with lifetimes up to 61 microseconds. In addition, shorter-lived charge separated states involving one-electron oxidized copper complexes ([Cu(phen)2]2+ (τ < 100 ns)), one-electron oxidized zinc phthalocyanine (ZnPc˙+; τ = 380–560 ns), or ZnP˙+ (τ = 2.3–8.4 μs), and C60˙− have been identified as intermediates during the sequence. Detailed energy diagrams illustrate the sequence and rate constants of the photophysical events occurring with the mechanically-linked chromophores. This work pioneers the exploration of mechanically-linked systems as platforms to position three distinct chromophores, which are able to absorb light over a very wide range of the visible region, triggering a cascade of short-range energy and electron transfer processes to afford long-lived charge separated states.

Reaction conditions have been developed for the preparation of a series of rotaxanes and catenanes with appended electron-donor and electron-acceptor ([60]fullerene) subunits using a straightforward one-pot procedure based on Cu(I)-template synthesis and “click” chemistry.