Stepwise charge separation and charge recombination in ferrocene-meso,meso-linked porphyrin dimer-fullerene triad

A meso,meso-linked porphyrin dimer [(ZnP)(2)] as a light-harvesting chromophore has been incorporated into a photosynthetic multistep electron-transfer model for the first time, including ferrocene (Fc), as an electron donor and fullerene (C(60)) as an electron acceptor to construct the ferrocene-meso,meso-linked porphyrin dimer-fullerene system (Fc-(ZnP)(2)-C(60)). Photoirradiation of Fc-(ZnP)(2)-C(60) results in photoinduced electron transfer from the singlet excited state of the porphyrin dimer [(1)(ZnP)(2)] to the C(60) moiety to produce the porphyrin dimer radical cation-C(60) radical anion pair, Fc-(ZnP)(2)(*+)-C(60)(*-). In competition with the back electron transfer from C(60)(*-) to (ZnP)(2)(*+) to the ground state, an electron transfer from Fc to (ZnP)(2)(*+) occurs to give the final charge-separated (CS) state, that is, Fc(+)-(ZnP)(2)-C(60)(*-), which is detected as the transient absorption spectra by the laser flash photolysis. The quantum yield of formation of the final CS state is determined as 0.80 in benzonitrile. The final CS state decays obeying first-order kinetics with a lifetime of 19 micros in benzonitrile at 295 K. The activation energy for the charge recombination (CR) process is determined as 0.15 eV in benzonitrile, which is much larger than the value expected from the direct CR process to the ground state. This value is rather comparable to the energy difference between the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)) and the final CS state (Fc(+)-(ZnP)(2)-C(60)(*-)). This indicates that the back electron transfer to the ground state occurs via the reversed stepwise processes,that is, a rate-limiting electron transfer from (ZnP)(2) to Fc(+) to give the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)), followed by a fast electron transfer from C(60)(*-) to (ZnP)(2)(*+) to regenerate the ground state, Fc-(ZnP)(2)-C(60). This is in sharp contrast with the extremely slow direct CR process of bacteriochlorophyll dimer radical cation-quinone radical anion pair in bacterial reaction centers.     

Control of photoinduced energy- and electron-transfer steps in zinc porphyrin-oligothiophene-fullerene linked triads with solvent polarity

The dramatic changes of the lifetimes of the charge-separated (CS) states were confirmed in zinc porphyrin (ZnP)-oligothiophene (nT)-fullerene (C(60)) linked triads (ZnP-nT-C(60)) with the solvent polarity. After the selective excitation of the ZnP moiety of ZnP-nT-C(60), an energy transfer took place from the (1)ZnP moiety to the C(60) moiety, generating ZnP-nT-(1)C(60). In polar solvents, the CS process also took place directly via the (1)ZnP moiety, generating ZnP(*+)-nT-C(60)(*-), as well as the energy transfer to the C(60) moiety. After this energy transfer, an indirect CS process took place from the (1)C(60) moiety. In the less polar solvent anisole, the radical cation (hole) of ZnP(*+)-nT-C(60)(*-) shifted to the nT moiety; thus, the nT moiety behaves as a cation trapper, and the rates of the hole shift were evaluated to be in the order of 10(8) s(-1); then, the final CS states ZnP-nT(*+)-C(60)(*-) were lasting for 6-7 mus. In the medium polar solvent o-dichlorobenzene (o-DCB), ZnP-nT(*+)-C(60)(*-) and ZnP(*+)-nT-C(60)(*-) were present as an equilibrium, because both states have almost the same thermodynamic stability. This equilibrium resulted in quite long lifetimes of the CS states (450-910 mus) in o-DCB. In the more polar benzonitrile, the generation of ZnP-nT(*+)-C(60)(*-) was confirmed with apparent short lifetimes (0.6-0.8 mus), which can be explained by the fast hole shift to more stable ZnP(*+)-nT-C(60)(*-) followed by the faster charge recombination. It was revealed that the relation between the energy levels of two CS states, which strongly depend on the solvent polarity, causes dramatic changes of the lifetimes of the CS states in ZnP-nT-C(60); that is, the most appropriate solvents for the long-lived CS state are intermediately polar solvents such as o-DCB. Compared with our previous data for H(2)P-nT-C(60), in which H(2)P is free-base porphyrin, the lifetimes of the CS states of ZnP-nT-C(60) are approximately 30 times longer than those in o-DCB.     

Azobenzene-linked porphyrin-fullerene dyads

A new group of porphyrin-fullerene dyads with an azobenzene linker was synthesized, and the photochemical and photophysical properties of these materials were investigated using steady-state and time-resolved spectroscopic methods. The electrochemical properties of these compounds were also studied in detail. The synthesis involved oxidative heterocoupling of free base tris-aryl-p-aminophenyl porphyrins with a p-aminophenylacetal, followed by deprotection to give the aldehyde, and finally Prato 1,3-dipolar azomethineylide cycloaddition to C60. The corresponding Zn(II)-porphyrin (ZnP) dyads were made by treating the free base dyads with zinc acetate. The final dyads were characterized by their 1H NMR, mass, and UV-vis spectra. 3He NMR was used to determine if the products are a mixture of cis and trans stereoisomers, or a single isomer. The data are most consistent with the isolation of only a single configurational isomer, assigned to the trans (E) configuration. The ground-state UV-vis spectra are virtually a superimposition of the spectral features of the individual components, indicating there is no interaction of the fullerene (F) and porphyrin (H2P/ZnP) moieties in the ground state. This conclusion is supported by the electrochemical data. The steady-state and time-resolved fluorescence spectra indicate that the porphyrin fluorescence in the dyads is very strongly quenched at room temperature in the three solvents studied: toluene, tetrahydrofuran (THF), and benzonitrile (BzCN). The fluorescence lifetimes of the dyads in all solvents are sharply reduced compared to those of H2P and ZnP standards. In toluene, the lifetimes of the free base dyads are 600-790 ps compared to 10.1 ns for the standard, while in THF and BzCN the dyad lifetimes are less than 100 ps. For the ZnP dyads, the fluorescence lifetimes were 10-170 ps vs 2.1-2.2 ns for the ZnP references. The mechanism of the fluorescence quenching was established using time-resolved transient absorption spectroscopy. In toluene, the quenching process is singlet-singlet energy transfer (k approximately 10(11) s-1) to give C60 singlet excited states which decay with a lifetime of 1.2 ns to give very long-lived C60 triplet states. In THF and BzCN, quenching of porphyrin singlet states occurs at a similar rate, but now by electron transfer, to give charge-separated radical pair (CSRP) states, which show transient absorption spectra very similar to those reported for other H2P-C60 and ZnP-C60 dyad systems. The lifetimes of the CSRP states are in the range 145-435 ns in THF, much shorter than for related systems with amide, alkyne, silyl, and hydrogen-bonded linkers. Thus, both forward and back electron transfer is facilitated by the azobenzene linker. Nonetheless, the charge recombination is 3-4 orders of magnitude slower than charge separation, demonstrating that for these types of donor-acceptor systems back electron transfer is occurring in the Marcus inverted region.     

Long-lived charge-separated state generated in a ferrocene-meso,meso-linked porphyrin trimer-fullerene pentad with a high quantum yield

A meso,meso-linked porphyrin trimer, (ZnP)3, as a light-harvesting chromophore, has been incorporated for the first time into a photosynthetic multistep electron-transfer model including ferrocene (Fc) as an electron donor and fullerene (C60) as an electron acceptor, to construct the ferrocene-meso,meso-linked porphyrin trimer-fullerene system Fc-(ZnP)3-C60. Photoirradiation of Fc-(ZnP)3-C60 results in photoinduced electron transfer from both the singlet and triplet excited states of the porphyrin trimer, 1(ZnP)3* and 3(ZnP)3*, to the C60 moiety to produce the porphyrin trimer radical cation-C60 radical anion pair, Fc-(ZnP)3*+-C60*-. Subsequent formation of the final charge-separated state Fc+-(ZnP)3-C60*- was confirmed by the transient absorption spectra observed by pico- and nanosecond time-resolved laser flash photolysis. The final charge-separated state decays, obeying first-order kinetics, with a long lifetime (0.53 s in DMF at 163 K) that is comparable with that of the natural bacterial photosynthetic reaction center. More importantly, the quantum yield of formation of the final charge-separated state (0.83 in benzonitrile) remains high, despite the large separation distance between the Fc+ and C60*- moieties. Such a high quantum yield results from efficient charge separation through the porphyrin trimer, whereas a slow charge recombination is associated with the localized porphyrin radical cation in the porphyrin trimer. The light-harvesting efficiency in the visible region has also been much improved in Fc-(ZnP)3-C60 because of exciton coupling in the porphyrin trimer as well as an increase in the number of porphyrins.     

Ultrafast excitation transfer and charge stabilization in a newly assembled photosynthetic antenna-reaction center mimic composed of boron dipyrrin, zinc porphyrin and fullerene

A self-assembled supramolecular triad as a model to mimic the light-induced events of the photosynthetic antenna-reaction center, that is, ultrafast excitation transfer followed by electron transfer ultimately generating a long-lived charge-separated state, has been accomplished. Boron dipyrrin (BDP), zinc porphyrin (ZnP) and fullerene (C(60)), respectively, constitute the energy donor, electron donor and electron acceptor segments of the antenna-reaction center imitation. Unlike in the previous models, the BDP entity was placed between the electron donor, ZnP and electron acceptor, C(60) entities. For the construction, benzo-18-crown-6 functionalized BDP was synthesized and subsequently reacted with 3,4-dihydroxyphenyl functionalized ZnP through the central boron atom to form the crown-BDP-ZnP dyad. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity of the dyad via ion-dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational and electrochemical methods. Steady-state fluorescence and excitation studies revealed the occurrence of energy transfer upon selective excitation of the BDP in the dyad. Further studies involving the pump-probe technique revealed excitation transfer from the (1)BDP* to ZnP to occur in about 7 ps, much faster than that reported for other systems in this series of triads, as a consequence of shorter distance between the entities. Upon forming the supramolecular triad by self-assembling fullerene, the (1)ZnP(*) produced by direct excitation or by energy transfer mechanism resulted in an initial electron transfer to the BDP entity. The charge recombination resulted in the population of the triplet excited state of C(60), from where additional electron transfer occurred to produce C(60)(?-):crown-BDP-ZnP(?+) ion pair as the final charge-separated species. Nanosecond transient absorption studies revealed the lifetime of the charge-separated state to be ~100 μs, the longest ever reported for this type of antenna-reaction center mimics, indicating better charge stabilization as a result of the different disposition of the entities of the supramolecular triad.     

Mimicking photosynthetic antenna-reaction-center complexes with a (boron dipyrromethene)3-porphyrin-C60 pentad

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 C(60) 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 C(60) moiety, resulting in the formation of the singlet charge-separated state (BDP)(3)-ZnP(·+) -C(60)(·-) , which has a lifetime of 476 and 1000 ps in benzonitrile and toluene, respectively. Interestingly, a slow charge-recombination process (k(CR)(t)=2.6×10(6) s(-1)) and a long-lived triplet charge-separated state (τ(CS)(T)=385 ns) were detected in polar benzonitrile by nanosecond transient measurements.     

Synthesis and photoinduced electron transfer processes of rotaxanes bearing [60]fullerene and zinc porphyrin: effects of interlocked structure and length of axle with porphyrins

Three rotaxanes, with axles with two zinc porphyrins (ZnPs) at both ends penetrating into a necklace pending a C60 moiety, were synthesized with varying interlocked structures and axle lengths. The intra-rotaxane photoinduced electron transfer processes between the spatially positioned C60 and ZnP in rotaxanes were investigated. Charge-separated (CS) states (ZnP*+, C60*-)rotaxane are formed via the excited singlet state of ZnP (1ZnP*) to the C60 moiety in solvents such as benzonitrile, THF, and toluene. The rate constants and quantum yields of charge separation via 1ZnP decrease with axle length, but they are insensitive to solvent polarity. When the axle becomes long, charge separation takes place via the excited triplet state of ZnP (3ZnP*). The lifetime of the CS state increases with axle length from 180 to 650 ns at room temperature. The small activation energies of charge recombination were evaluated by temperature dependence of electron-transfer rate constants, probably reflecting through-space electron transfer in the rotaxane structures.     

Effects of metal ions on photoinduced electron transfer in zinc porphyrin-naphthalenediimide linked systems

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)(3)) or lutetium triflate (Lu(OTf)(3)), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc(3+) was confirmed by the appearance of absorption bands due to ZnP(.) (+) and NIm(.) (-) in the absence of metal ions and of those due to ZnP(.) (+) and the NIm(.) (-)/Sc(3+) complex in the presence of Sc(3+) in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0 x 10(-3) M Sc(3+) (14 micros for ZnP-NIm, 8.3 micros for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 micros for ZnP-NIm, 0.33 micros for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from (1)ZnP(*) to NIm in the presence of Sc(3+) occurs without involvement of the metal ion to produce ZnP(.) (+)-NIm(.) (-), followed by complexation with Sc(3+) to afford the ZnP(.) (+)-NIm(.) (-)/Sc(3+) complex. The one-electron reduction potential (E(red)) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k(ET)) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.     

Structural and electron-transfer characteristics of carbon-tethered porphyrin monolayers on Si(100)

Structural and electron-transfer characteristics are reported for two classes of zinc porphyrin monolayers attached to Si(100) surfaces via Si-C bonds. One class, designated ZnP(CH(2))(n)- (n = 2-4), contains an alkyl linker appended to the meso-position of the porphyrin, with the nonlinking substituents being p-tolyl groups. The other, designated ZnPPh(CH(2))(n)- (n = 0-3), contains a phenyl or phenylalkyl linker appended to the meso-position of the porphyrin, with the nonlinking substituents being mesityl groups. Both classes of zinc porphyrin monolayers on Si(100) were examined using Fourier transform infrared spectroscopy and various electrochemical methods. The studies reveal the following: (1) The structural and electron-transfer characteristics of the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- monolayers are generally similar to those of monolayers formed from porphyrins with analogous linkers, but anchored with an O, a S, or a Se atom. (2) The ZnP(CH(2))(n)-, ZnPPh-, and ZnPPhCH(2)- monolayers exhibit lower saturation coverages and have their porphyrin ring more tilted with respect to the surface normal than the ZnPPh(CH(2))(2)- and ZnPPh(CH(2))(3)- monolayers. (3) The electron-transfer rates for both the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- classes of monolayers monotonically decrease as the length of the linker increases. (4) For all the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- monolayers, both electron-transfer rates and charge-dissipation rates decrease monotonically as the surface coverage increases. Collectively, the studies reported herein provide a detailed picture of how the linker type influences the structural and electron-transfer characteristics of these general classes of monolayers.     

Energy and electron transfer in beta-alkynyl-linked porphyrin-[60]fullerene dyads

Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.     

Photoinduced energy and electron-transfer processes in porphyrin-perylene bisimide symmetric triads

The photophysics of two symmetric triads, (ZnP)2PBI and (H2P)2PBI, made of two zinc or free-base porphyrins covalently attached to a central perylene bisimide unit has been investigated in dichloromethane and in toluene. The solvent has been shown to affect not only quantitatively but also qualitatively the photophysical behavior. A variety of intercomponent processes (singlet energy transfer, triplet energy transfer, photoinduced charge separation, and recombination) have been time-resolved using a combination of emission spectroscopy and femtosecond and nanosecond time-resolved absorption techniques yielding a very detailed picture of the photophysics of these systems. The singlet excited state of the lowest energy chromophore (perylene bisimide in the case of (ZnP)2PBI, porphyrin in the case of (H2P)2PBI) is always quantitatively populated, besides by direct light absorption, by ultrafast singlet energy transfer (few picosecond time constant) from the higher energy chromophore. In dichloromethane, the lowest excited singlet state is efficiently quenched by electron transfer leading to a charge-separated state where the porphyrin is oxidized and the perylene bisimide is reduced. The systems then go back to the ground state by charge recombination. The four charge separation and recombination processes observed for (ZnP)2PBI and (H2P)2PBI in dichloromethane take place in the sub-nanosecond time scale. They obey standard free-energy correlations with charge separation lying in the normal regime and charge recombination in the Marcus inverted region. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted leading to sharp changes in photophysical mechanism. With (ZnP)2PBI, the electron-transfer quenching is still fast, but charge recombination takes place now in the nanosecond time scale and to triplet state products rather than to the ground state. Triplet-triplet energy transfer from the porphyrin to the perylene bisimide is also involved in the subsequent deactivation of the triplet manifold to the ground state. With (H2P)2PBI, on the other hand, the driving force for charge separation is too small for electron-transfer quenching, and the deactivation of the porphyrin excited singlet takes place via intersystem crossing to the triplet followed by triplet energy transfer to the perylene bisimide and final decay to the ground state.     

Artificial photosynthetic reaction centers: mimicking sequential electron and triplet-energy transfer.

An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylporphyrin (P), and a bis(heptafluoropropyl)porphyrin (P(F)), C-P-P(F) , and the related triad in which the central porphyrin has been metalated to give C-P(Zn)-P(F) have been synthesized and characterized by transient spectroscopy. These triads are models for amphipathic triads having a carboxylate group attached to the P(F) moiety; they are designed to carry out redox processes across lipid bilayers. Triad C-P-P(F) undergoes rapid singlet-singlet energy transfer between the porphyrin moieties, so that their excited states are in equilibrium. In benzonitrile, photoinduced electron transfer from the first excited singlet state of P and hole transfer from the first excited singlet state of P(F) yield the initial charge-separated state C-P(.) (+)-P(F) (.) (-). Subsequent hole transfer to the carotenoid moiety generates the final charge-separated state C(.) (+)-P-P(F) (.) (-), which has a lifetime of 1.1 mus and is formed with a quantum yield of 0.24. In triad C-P(Zn)-P(F) energy transfer from the P(Zn) excited singlet to the P(F) moiety yields C-P(Zn)-(1)P(F) . A series of electron-transfer reactions analogous to those observed in C-P-P(F) generates C(.) (+)-P(Zn)-P(F) (.) (-), which has a lifetime of 750 ns and is formed with a quantum yield of 0.25. Flash photolysis experiments in liposomes containing an amphipathic version of C-P(Zn)-P(F) demonstrate that the added driving force for photoinduced electron transfer in the metalated triad is useful for promoting electron transfer in the low-dielectric environment of artificial biological membranes. In argon-saturated toluene solutions of C-P-P(F) and C-P(Zn)-P(F) , charge separation is not observed and a considerable yield of triplet species is generated upon excitation of the porphyrin moieties. In both triads triplet energy localized in the P(F) moiety is channeled to the carotenoid chromophore by a triplet energy-transfer relay mechanism. Certain photophysical characteristics of these triads, including the sequential electron transfer and the triplet energy-transfer relay mechanism, are reminiscent of those observed in natural reaction centers of photosynthetic bacteria.     

Exciplex intermediates in photoinduced electron transfer of porphyrin-fullerene dyads

The photoinduced electron transfer in differently linked zinc porphyrin-fullerene dyads and their free-base porphyrin analogues was studied in polar and nonpolar solvents with femto- to nanosecond absorption and emission spectroscopies. A new intermediate state, different from the locally excited (LE) chromophores and the complete charge-separated (CCS) state, was observed. It was identified as an exciplex. The exciplex preceded the CCS state in polar benzonitrile and the excited singlet state of fullerene in nonpolar toluene. The behavior of the dyads was modeled by using a common kinetic scheme involving equilibria between the exciplex and LE chromophores. The scheme is suitable for all the studied porphyrin-fullerene compounds. The rates of reaction steps depended on the type of linkage between the moieties. The scheme and Marcus theory were applied to calculate electronic couplings for sequential reactions, and consistent results were obtained.     

Photoinduced hydrogen evolution with water soluble bisviologen linked zinc porphyrin and hydrogenase

Water soluble bisviologen linked zinc porphyrin (ZnP(C₄V_AC₄V_B)₄) was synthesized and characterized. The quenching processes of the photoexcited singlet state and triplet state of ZnP(C₄V_AC₄V_B)₄ were measured by using fluorescence lifetime and laser flash photolysis. The photoexcited singlet state of the zinc porphyrin was quenched by the bonded bisviologen. Photoinduced hydrogen evolution with ZnP(C₄V_AC₄V_B)₄ and hydrogenase was observed under steady state irradiation.     

Dyads and triads containing perylenetetracarboxylic diimide and porphyrin: efficient photoinduced electron transfer elicited via both excited singlet states

Synthesis, characterizations, and photophysical properties of new photoactive dyads and triads containing perylenetetracarboxylic diimide (PIm) and porphyrin (free-base porphyrin (H(2)P) and zinc porphyrin (ZnP)), in which both entities were connected with a short ether bond, were examined with the aim of using these systems for molecular photonics. The porphyrin(P)-PIm systems absorbed strongly across the visible region, which greatly matched the solar spectrum. The geometric and electronic structures of the dyads and triads were probed using density function theory method at the B3LYP/3-21G level. It was revealed that the majority of the highest-occupied molecular orbital was located on the porphyrin entity, while the lowest-unoccupied molecular orbitals were entirely on the PIm entity. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as transient-absorption techniques in polar solvent benzonitrile. Upon excitation of the P (H(2)P and ZnP) moieties, efficient fluorescence quenching of the P moiety was observed, suggesting that the main quenching paths involved charge separation from the excited singlet porphyrin ((1)P) to the PIm moiety. Upon excitation of the PIm moiety, fluorescence quenching of the (1)PIm moiety was also observed. The nanosecond transience of spectra in near-IR region revealed the charge separation process from the P moieties to the PIm moiety via their excited singlet states. The lifetimes of the charge-separated states were evaluated to be 7-14 ns, depending on the solvent polarity. Photosensitized electron mediation systems were also revealed in the presence of methyl viologen and sacrificial electron donor.     

Long-lived, charge-shift states in heterometallic, porphyrin-based dendrimers formed via click chemistry

A series of multiporphyrin clusters has been synthesized and characterized in which there exists a logical gradient for either energy or electron transfer between the porphyrins. A central free-base porphyrin (FbP), for example, is equipped with peripheral zinc(II) porphyrins (ZnP) which act as ancillary light harvesters and transfer excitation energy to the FbP under visible light illumination. Additional energy-transfer steps occur at the triplet level, and the series is expanded by including magnesium(II) porphyrins and/or tin(IV) porphyrins as chromophores. Light-induced electron transfer is made possible by incorporating a gold(III) porphyrin (AuP(+)) into the array. Although interesting by themselves, these clusters serve as control compounds by which to understand the photophysical processes occurring within a three-stage dendrimer comprising an AuP(+) core, a second layer formed from four FbP units, and an outer layer containing 12 ZnP residues. Here, illumination into a peripheral ZnP leads to highly efficient electronic energy transfer to FbP, followed by charge transfer to the central AuP(+). Charge recombination within the resultant charge-shift state is intercepted by secondary hole transfer to the ZnP, which occurs with a quantum yield of around 20%. The final charge-shift state survives for some microseconds in fluid solution at room temperature.     

A functionalized noncovalent macrocyclic multiporphyrin assembly from a dizinc(II) bis-porphyrin receptor and a free-base dipyridylporphyrin

The bis-porphyrin system ZnP(2), in which two zinc porphyrins are connected by a phenanthroline linker in an oblique fashion, acts as a bifunctional receptor towards the complexation of free-base meso-5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cis DPyP). In solution, NMR spectroscopy evidenced quantitative formation of the tris-porphyrin macrocyclic assembly ZnP(2)(4'-cis DPyP), in which the two fragments are held together by two axial 4'-N(pyridyl)-Zn interactions. The remarkable stability of the edifice (an association constant of about 6x10(8) M(-1) was determined by UV/Vis absorption and emission titration experiments in toluene) is due to the almost perfect geometrical match between the two interacting units. The macrocycle was crystallized and studied by X-ray diffraction, which confirmed the excellent complementarity of the two components. Photoinduced energy transfer from the singlet excited state of the zinc porphyrin chromophores to the free-base porphyrin occurs with an efficiency of 98 % (k(en)=2x10(10) s(-1) in toluene, ambient temperature) with a mechanism consistent with a dipole-dipole process with a low orientation factor.     

Effect of axial ligation or pi-pi-type interactions on photochemical charge stabilization in "two-point" bound supramolecular porphyrin-fullerene conjugates

Two types of structurally well-defined, self-assembled zinc porphyrin-fullerene conjugates were formed by "two-point" binding strategies to probe the effect of axial ligation or pi-pi-type interactions on the photochemical charge stabilization in the supramolecular dyads. To achieve this, meso-tetraphenylporphyrin was functionalized to possess one or four [18]crown-6 moieties at different locations on the porphyrin macrocycle while fullerene was functionalized to possess an alkyl ammonium cation, and a pyridine or phenyl entities. As a result of the crown ether-ammonium cation complexation, and zinc-pyridine coordination or pi-pi-type interactions, stable zinc porphyrin-fullerene conjugates with defined distance and orientation were formed. Evidence for the zinc-pyridine complexation or pi-pi-type interactions was obtained from the spectral and computational studies. Steady-state and time-resolved emission studies revealed efficient quenching of the zinc-porphyrin singlet excited state in these dyads, and the measured rates of charge separation, k(CS) were found to be slightly better in the case of the dyads held by axial coordination and crown ether-cation complexation. Nanosecond transient absorption studies provided evidence for the electron transfer reactions, and these studies also revealed charge stabilization in these dyads. The lifetimes of the radical ion pairs were found to depend upon the type of porphyrins utilized to form the dyads, that is, porphyrin possessing the crown ether moiety at the ortho position of one of the phenyl rings yielded prolonged charge stabilized states. Addition of pyridine to the supramolecular dyads eliminated the zinc-pyridine coordination or pi-pi-type interactions of the "two-point" bound systems due to the formation of a new zinc-pyridine axial bond thus giving a unique opportunity to probe the effect of axial coordination or pi-pi interactions on k(CS) and k(CR). Under these conditions, the measured electron transfer rates revealed faster k(CS) and slower k(CR) as compared to those obtained in the absence of added pyridine. The evaluated lifetimes of the radical ion-pairs were found to be hundreds of nanoseconds and were longer in the presence of pyridine.     

Photoinduced electron transfer in multiporphyrinic interlocked structures: the effect of copper(I) coordination in the central site

Photoinduced processes have been determined in a [2]catenane containing a zinc(II) porphyrin, a gold(III) porphyrin, and two free phenanthroline binding sites, Zn-Au(+), and in the corresponding copper(I) phenanthroline complex, Zn-Cu(+)-Au(+). In acetonitrile solution Zn-Au(+) is present in two different conformations: an extended one, L, which accounts for 40 % of the total, and a compact one, S. In the L conformation, the electron transfer from the excited state of the Zn porphyrin to the gold-porphyrin unit (k = 1.3x10(9) s(-1)) is followed by a slow recombination (k = 8.3x10(7) s(-1)) to the ground state. The processes in the S conformation cannot be clearly resolved but a charge-separated (CS) state is rapidly formed and decays with a lifetime on the order of fifty picoseconds. In the catenate Zn-Cu(+)-Au(+), the zinc-porphyrin excited state initially transfers energy to the Cu(I)-phenantholine unit, producing a metal-to-ligand charge-transfer (MLCT) excited state localized on the copper complex with a rate k = 1.4x10(9) s(-1). From this excited state the transfer of an electron to the gold-porphyrin unit takes place, producing the CS state Zn-Cu(2+)-Au(.), which decays with a lifetime of 10 ns. The results are discussed in comparison with the closely related [2]rotaxane, in which a further charge shift from the copper center to the zinc-porphyrin unit leads to the fully CS state. Even in the absence of such full charge separation, it is shown that the lifetimes of the CS states are increased by a factor of about 2-2.5 over those of the corresponding rotaxanes.     

Probing the donor-acceptor proximity on the physicochemical properties of porphyrin-fullerene dyads: "tail-on" and "tail-off" binding approach.

A new approach of probing proximity effects in porphyrin-fullerene dyads by using an axial ligand coordination controlled "tail-on" and "tail-off" binding mechanism is reported. In the newly synthesized porphyrin-fullerene dyads for this purpose, the donor-acceptor proximity is controlled either by temperature variation or by an axial ligand replacement method. In o-dichlorobenzene, 0.1 M (TBA)ClO(4), the synthesized zincporphyrin-fullerene dyads exhibit seven one-electron reversible redox reactions within the accessible potential window of the solvent and the measured electrochemical redox potentials and UV-visible absorption spectra reveal little or no ground-state interactions between the C(60) spheroid and porphyrin pi-system. The proximity effects on the photoinduced charge separation and charge recombination are probed by both steady-state and time-resolved fluorescence techniques. It is observed that in the "tail-off" form the charge-separation efficiency changes to some extent in comparison with the results obtained for the "tail-on" form, suggesting the presence of some through-space interactions between the singlet excited zinc porphyrin and the C(60) moiety in the "tail-off" form. The charge separation rates and efficiencies are evaluated from the fluorescence lifetime studies. The charge separation via the singlet excited states of zinc porphyrin in the studied dyads is also confirmed by the quick rise-decay of the anion radical of the C(60) moiety within 20 ns. Furthermore, a long-lived ion pair with lifetime of about 1000 ns is also observed in the investigated zinc porphyrin-C(60) dyads.