Photosynthetic model systems

Investigation of a bacteriochlorin-containing pentad array for panchromatic light-harvesting and charge separation

A new pentad array designed to exhibit panchromatic absorption and charge separation has been synthesized and characterized. The array is composed of a triad panchromatic absorber (a bis(perylene-monoimide)-porphyrin) to which are appended an electron acceptor (perylene-diimide) and an electron donor/hole acceptor (bacteriochlorin) in a crossbar arrangement. The motivation for incorporation of the bacteriochlorin versus a free-base or zinc chlorin utilized in prior constructs was to facilitate hole transfer to this terminal unit and thereby achieve a higher yield of charge separation across the array. The intense S₀ → S₁ (Q_y) band of the bacteriochlorin also enhances absorption in the near-infrared spectral region. Due to synthetic constraints, a phenylethyne linker was used to join the bacteriochlorin to the core porphyrin of the panchromatic triad rather than the diphenylethyne linker employed for the prior chlorin-containing pentads. Static and time-resolved photophysical studies reveal enhanced excited-state quenching for the pentad in benzonitrile and dimethyl sulfoxide compared to the prior chlorin-containing analogues. Success was only partial, however, as a long-lived charge separated state was not observed despite the improved energetics for the final ground-state hole/electron-shift reaction. The apparent reason is more facile competing charge-recombination due to the shorter bacteriochlorin - porphyrin linker that increases electronic coupling for this process. The studies highlight design criteria for balancing panchromatic absorption and long-lived charge separation in molecular architectures for solar-energy conversion.

Awakening a Molecular Mummy: The Inter-and Intramolecular Photochemistry of Pyromellitic Diimides with Alkyl Carboxylates

Pyromellitic acid diimides are not as chemically unreactive as conjecturable (and presupposed) from their numerous applications as electron acceptor units or electron carriers in molecular donor-acceptor dyads or triads. Similar to the corresponding phthalimides, electronically excited pyromellitic diimides oxidize alkyl carboxylates in aqueous solution via intermolecular electron transfer (PET) processes, which eventually results in radical-radical combination products, e.g., the benzylation product 6 from N,N'-dimethyl pyromellitic diimide 5. The analogous product 7 was formed with pivalic acid as tert-butyl radical source. One additional product 8 was isolated from alkylation/dearomatization and multiple radical additions, respectively, after prolonged irradiation. In intramolecular versions, from N-carboxyalkylated pyromellitic diimides 9a-e (C1 to C5-spaced), degradation processes were detected, e.g., the cyclization products 10 from the GABA substrate 9c. In sharp contrast to phthalimide photochemistry, the green pyromellitic diimide radical anion was detected here by UV-vis absorption (λabs = 720 nm), EPR (from 9d), and NMR spectroscopy for several intramolecular electron transfer examples. Only the yellow 1,4-quinodial structure is formed from intermolecular PET, which was deduced from the absorption spectra (λabs = 440 nm) and the subsequent chemistry. The pyromellitimide radical anion lives for hours at room temperature in the dark, but is further degraded under photochemical reaction conditions.

Charge-separation in panchromatic, vertically positioned bis(donor styryl)BODIPY-aluminum(iii) porphyrin-fullerene supramolecular triads

Three, broad band capturing, vertically aligned supramolecular triads, R2-BDP-AlPorF3←Im-C60 [R = H, styryl (C2H2-Ph), C2H2-TPA (TPA = triphenylamine); ← = coordinate bond], have been constructed using BODIPY derivative (BDP, BDP-Ph2 or BDP-TPA2), 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(iii) porphyrin (AlPorF3) and fullerene (C60) entities. The C60 and BDP units are bound to the Al center on the opposite faces of the porphyrin: the BDP derivative through a covalent axial bond using a benzoate spacer and the C60 through a coordination bond via an appended imidazole. Owing to the bis-styryl functionality on BDP, the constructed dyads and triads exhibited panchromatic light capture. Due to the diverse absorption and redox properties of the selected entities, it was possible to demonstrate excitation wavelength dependent photochemical events. In the case of the BDP-AlPorF3 dyad, selective excitation of BDP resulted in singlet-singlet energy transfer to AlPorF3 (kEnT = 1.0 × 1010 s-1). On the other hand, excitation of the AlPorF3 entity in the BDP-AlPorF3←Im-C60 triad revealed charge separation leading to the BDP-(AlPorF3)˙+-(C60)˙- charge separated state (kCS = 2.43 × 109 s-1). In the case of the Ph2-BDP-AlPorF3 dyad, energy transfer from 1AlPorF3* to 1(Ph2-BDP)* was witnessed (kEnT = 1.0 × 1010 s-1); however, upon assembling the supramolecular triad, (Ph2-BDP)-AlPorF3←Im-C60, electron transfer from 1AlPorF3* to C60 (kCS = 3.35 × 109 s-1), followed by hole shift (kHS = 1.00 × 109 s-1) to Ph2-BDP, was witnessed. Finally, in the case of the TPA2-BDP-AlPorF3←Im-C60 triad, only electron transfer leading to the (TPA2-BDP)˙+-AlPorF3←Im-(C60)˙- charge separated state, and no energy transfer, was observed. The facile oxidation of Ph2-BDP and TPA2-BDP compared to AlPorF3 in the latter two triads facilitated charge separation through either an electron migration or hole transfer mechanism depending on the initial excitation. The charge-separated states in these triads persisted for about 20 ns.

Introduction

Primordial life forms on earth comprised oxygen-sensitive organisms: the anaerobic fermenters and cyanobacteria, which released oxygen as a metabolic by-product, causing the oxygen levels in the atmosphere to rise Benzie (Eur J Nutr 39:53–61, 2000 [1]), Halliwell (Free Radic Res 31:261–272, 1999 [2]).

Induced polarization restricts the conformational distribution of a light-harvesting molecular triad in the ground state

Free energy surface of the light-harvesting triad employing a non-polarizable force field (NFF) and a polarizable force field (PFF) shows that induced polarization limits the motion of rotation about chemical bonds as well as bending at the porphyrin, which are prominent using the NFF, thus limiting the conformational space of the triad.

Axially assembled photosynthetic reaction center mimics composed of tetrathiafulvalene, aluminum(III) porphyrin and fullerene entities

The distance dependence of sequential electron transfer has been studied in six, vertical, linear supramolecular triads, (TTF-Ph(n)-py → AlPor-Ph(m)-C60, n = 0, 1 and m = 1, 2, 3), constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities. The C60 and TTF units are bound to the Al center on opposite faces of the porphyrin; the C60 through a covalent axial bond using a benzoate spacer, and the TTF through a coordination bond via an appended pyridine. Time-resolved optical and EPR spectroscopic methods and computational studies are used to demonstrate that excitation of the porphyrin leads to step-wise, sequential electron transfer (ET) between TTF and C60, and to study the electron transfer rates and exchange coupling between the components of the triads as a function of the bridge lengths. Femtosecond transient absorption studies show that the rates of charge separation, k(CS) are in the range of 10(9)-10(11) s(-1), depending on the length of the bridges. The lifetimes of the charge-separated state TTF˙(+)-C₆₀˙⁻ obtained from transient absorbance experiments and the singlet lifetimes of the radical pairs obtained by time-resolved EPR are in good agreement with each other and range from 60-130 ns in the triads. The time-resolved EPR data also show that population of the triplet sublevels of the charge-separated state in the presence of a magnetic field leads to much longer lifetimes of >1 μs. The data show that a modest stabilization of the charge separation lifetime occurs in the triads. The attenuation factor β = 0.36 Å(-1) obtained from the exchange coupling values between TTF˙(+) and C₆₀˙⁻ is consistent with values reported in the literature for oligophenylene bridged TTF-C60 conjugates. The singlet charge recombination lifetime shows a much weaker dependence on the distance between the donor and acceptor, suggesting that a simple superexchange model is not sufficient to describe the back reaction.

Synthesis and photophysical properties of octa-substituted phthalocyaninato oxotitanium(IV) derivatives

The synthesis, spectral and photophysical properties including fluorescence quenching of the following octa-substituted oxotitanium phthalocyanines are reported: 2,3,9,10,16,17,23,24-octaphenoxyphthalocyaninato titanium(IV) oxide, 2,3,9,10,16,17,23,24-[octakis(4-t-butylphenoxyphthalocyaninato)]titanium(IV) oxide, 2,3,9,10,16,17,23,24-{octakis[(4-benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide and 2,3,9,10,16,17,23,24-octaphenylthiophthalocyaninato titanium(IV) oxide. The complexes are characterized by1H NMR, IR and UV-vis spectroscopies. Their photophysical properties are presented where moderate fluorescence quantum yields (0.14-0.19) and lifetimes were determined. Varied triplet quantum yields were obtained and the triplet lifetimes (40-100 μs) were short.

Cobalt(II) Porphyrins Supported on Crosslinked Polymer Matrix as Model Compounds

Polymer-bound cobalt(II) porphyrins were studied for their dioxygen—binding capacity. Tetra—aminoporphyrins were anchored on a divinylbenzene (DVB)-crosslinked chloromethyl polystyrene network. The crosslinked, solid polymers were swelled in chloroform and the swollen polymers were used for the entire studies. Ortho-, meta- and para-substituted porphyrin systems were developed by adjusting the bonding position with the help of suitably substituted aminoporphyrins. The products were characterized by chemical and spectroscopic methods. Cobalt(II) complexes of polymeric porphyrins were synthesized and characterized by electronic and ESR spectral methods. The spectra gave evidence for the systematic variation of electronic properties in ortho, meta and para compounds and for the dioxygen-binding capacity of cobalt complexes. These results are discussed.

A Porphyrin-Anthracene Supramolecular System Assembled via Complementary Nucleic Acid Base Pairing

Covalently linked porphyrin–adenine (meso-5(4-(9-(2-oxyethyl)adenine)phenyl)-10,15,20-tritolylporphyrin, 1) and anthracene–thymine (1-(9-methylanthracene)thymine, 2) conjugates have been synthesized and fully characterized by elemental analysis, FAB mass, UV-vis, 1 H NMR, fluorescence and cyclic voltammetric methods. Detailed 1 H NMR studies reveal that 1 and 2 self-assemble in CDCl 3 solutions at 293 ± 3 K to form a two-point hydrogen-bonded, bichromophoric, supramolecular system 3 with a binding constant of 47 ± 5 M−1 and that both Hoogsteen- and Watson–Crick-type A–T assemblies exist in solution under these experimental conditions. Spectral and electrochemical data point out the possibility of occurrence of both energy and electron transfer reactions from the singlet excited state of 2 to 1 in the ensemble 3. The singlet state activity of the ensemble 3 has been probed mainly by the time-resolved fluorescence method and the results have been discussed in the light of those obtained earlier on similar ‘non-covalently’ or covalently bound bichromophoric systems.

Increasing the Yield of Photoinduced Charge Separation through Parallel Electron Transfer Pathways

A strategy for increasing the yield of long-lived photoinduced charge separation in artificial photosynthetic reaction centers which is based on multiple electron transfer pathways operating in parallel has been investigated. Excitation of the porphyrin moiety of a carotenoid ( C )–porphyrin ( P )–naphthoquinone (Q) molecular triad leads to the formation of a charge-separated state C ·+– P – Q ·− with an overall quantum yield of 0.044 in benzonitrile solution. Photoinduced electron transfer from the porphyrin first excited singlet state gives C – P ·+– Q ·− with a quantum yield of ~1.0. However, electron transfer from the carotenoid to the porphyrin radical cation to form the final state does not compete well with charge recombination of C – P ·+– Q ·−, reducing the yield. The related pentad C 3– P – Q features carotenoid, porphyrin and quinone moieties closely related to those in the triad. Excitation of this molecule gives a C ·+– P ( C 2)– Q ·− state with a quantum yield of 0.073. The enhanced yield is ascribed to the fact that three electron donation pathways operating in parallel compete with charge recombination. The yield does not increase by the statistically predicted factor of three owing to small differences in thermodynamic driving force between the two compounds.

Photosensitization of Thin SnO2 Nanocrystalline Semiconductor Film Electrodes with Metalloporphyrins and Alkyl-substituted Metalloporphyrins

Modest fill factors (~0.2) and efficiencies for sensitized photocurrent generation are observed with porphyrins adsorbed to saturation on a nanocrystalline SnO 2 thin film employed as the working electrode in a photoelectrochemical cell. No dye aggregation is observed at the metal oxide/adsorbate interface, and no advantage in the photosensitization efficiency is seen with two porphyrins that exhibit a stable liquid crystalline phase over another porphyrin that does not.

Covalently Linked Tetraphenylporphyrin Trimers and Tetramers and their Coordination Behaviour

Several covalently linked tetraphenyl porphyrin trimers and tetramers bearing ethylene oxide bridges originating from the m and p positions of adjacent and/or opposite meso-aryl groups have been synthesized and characterized by fast atom bombardment mass spectroscopy, 1 H NMR, optical absorption and emission spectroscopies. The fully and partially metallated zinc(II) and copper(II) derivatives have been prepared and their photophysical properties are outlined. The steady-state and time-resolved fluorescence studies of the heterotrimers indicate efficient energy transfer from the ZnP to the H 2 P unit. The efficiency and rate of energy transfer are dependent on the number of free-base units in the hetero-oligomers and the geometrical position of the porphyrin units. The heterotrimers consisting of one, two and three zinc(II) centres bind 4,4″-bipyridyl in different modes as revealed by the nature of binding curves. The bipyridyl ligand functions as a chelate in the complexes of biszinc(II) porphyrin trimers while two bipyridyls bind to trizinc(II) porphyrin trimers with one ligand functioning as a chelate and another exhibiting monodentate coordination behaviour. The folding of two ZnP units is clearly seen in the complexes of bis and triszinc(II) trimers with bipyridyl, which is substantiated by molecular mechanics calculations. Axial ligation studies with trimers show that the preorganization of porphyrin hosts is necessary for recognition of the guest. The binding of bipyridyl to the trimers is followed by global conformational changes and this behaviour is analogous to activated complex binding in induced-fit enzymes.

Photoinduced electron transfer in supramolecules composed of porphyrin/phthalocyanine and nanocarbon materials

Photoinduced electron transfer in supramolecules composed of porphyrin/phthalocyanine and nanocarbon materials such as fullerenes, single-walled carbon nanotubes, and single-walled carbon nanohorns have been reviewed. With the aid of highly efficient visible-light harvesting porphyrin/phthalocyanine, the photosensitized electron transfer takes place from the photoexcited porphyrin/phthalocyanine to fullerene, which acts as a strong electron acceptor. In the case of nanocarbon materials such as single-walled carbon nanotubes and nanohorns, they may act as electron-trapping sites. From the holes and electrons generated on porphyrin/phthalocyanine-nanocarbons, electron pooling takes place at the strong and stable electron trapper (viologen dication) in solution.

10,15-di(4-pyridyl)-5,20-di(4-tolyl)-21-thiaporphyrin as a building block for porphyrin coordination arrays

10,15-di(4-pyridyl)-5,20-di(4-tolyl)-21-thiaporphyrin, ( SDPyDTP ) H , was prepared by condensation of 2,5-bis(4-tolylhydroxymethyl)thiophene, pyrrole and 4-pyridinecarboxaldehyde in boiling propionic acid. The synthesis introduced two pyridyl substituents at two defined (opposite to thiophene) meso positions of the porphyrin periphery. The self-assembly of the angular 10,15-dipyridyl-21-thiaporphyrin modules with cis square-planar diphosphineplatinum(II) complex leads to a cyclic rhomboid dimer [ Pt ( DPPP )( SDPyDTP ) H ]2( OTf )4. The molecule acquires butterfly geometry. The 1 H NMR studies confirmed the π-π stacking of the pyridyl ring with the equatorial phenyl rings of the phosphine fragment. The conformational equilibrium, interchanging the phenyl ring positions and affording the mixture of seven conformers in solution, has been considered. 1 H NMR> spectroscopy was applied to identify oligomeric species, constructed by coordination of ( SDPyDTP ) H to the paramagnetic nickel(II) complex 5,10,15,20-tetra(4-tolyl)-21-thiaporphyrin, ( STTP ) Ni II Cl . ( SDPyDTP ) H acts as a mono- or bidentate ligand coordinating by meso-pyridyl substituents. Using the paramagnetically shifted resonances as an unambiguous spectroscopic probe, 1 H NMR spectroscopy readily discriminated between five- {[( STTP ) Ni II ](( SDPyDTP ) H )} and six-coordinate {[( STTP ) Ni II](( SDPyDTP ) H )2} oligomeric subunits. The applicability of 10,15-di(4-pyridyl)-5,20-di(4-tolyl)-21-thiaporphyrin as a suitable building block in construction of larger molecules was confirmed. The new route to modify the porphyrin coordination arrays, which preserves the overall architecture but modifies intrinsic chemical properties, using heteroporphyrin was demonstrated.

Intramolecular photoinduced electron-transfer processes in buta-1,3-diynyl-benzene-linked porphyrin-fullerene dyad

Intramolecular electron-transfer process of porphyrin-fullerene dyad linked by phenyl buta-1,3-diynyl-phenyl unit ( ZnP - sp - C 60) was studied by laser flash photolysis. Picosecond fluorescence lifetime and transient absorption measurements revealed that photoinduced charge-separation takes place via the excited singlet state (1 ZnP *) with the rate constant of (1-2) × 1010 s −1. For the charge recombination, about a half of the radical-ion pair decayed quickly with 2.9 × 109 s −1 as evaluated from picosecond transient absorption measurements, whereas the remaining half was long-lived with slow decay (1.6 × 106 s −1) as estimated from nanosecond transient absorption measurements. The lifetime of the radical-ion pair of ZnP - sp - C 60 was longer than those of directly connected dyads with a buta-1,3-diynyl bridge and buta-1,3-diynyl-phenyl bridge by the insertion of an extra phenyl group in addition to the pyrrodino ring.

Photocurrent generation from Ru(bpy)3(2+) immobilized on phospholipid/alkanethiol hybrid bilayers

A new photocurrent-generation system based on Ru(bpy)3(2+) (bpy=2,2'-bipyridine) tethered on phospholipid/alkanethiol hybrid bilayers in aqueous media is reported. The construction of such a system is straightforward. First, a self-assembled monolayer (SAM) of alkanethiol is formed on gold, and separately, liposomes containing Ru(bpy)3(2+)-conjugated dioleoylphosphoethanolamine (DOPE) are prepared by extrusion. Subsequent exposure of the Ru(bpy)3(2+)-containing liposome solution to the preformed SAM induces the addition of a monolayer of phospholipids on top of the SAM and thereby the immobilization of a Ru(bpy)3(2+) layer on the gold electrode. Either anodic or cathodic photocurrent generation can be obtained, when ascorbate (anodic) or methyl violgen/oxygen (cathodic) is used as a sacrificial electron donor/acceptor, respectively. Light conversion quantum efficiencies of 0.84% (anodic) and 0.21% (cathodic) were obtained under blue light (lambda=470+/-20 nm) irradiation. The photocurrent-generation and electron-transfer mechanisms of this new system as well as its potential usefulness in fundamental photoconversion studies are discussed.

Energy- and hole-transfer dynamics in oxidized porphyrin dyads

The mechanisms and dynamics of quenching of a photoexcited free base porphyrin (Fb*) covalently linked to a nearby oxidized zinc porphyrin (Zn(+)) have been investigated in a set of five dyads using time-resolved absorption spectroscopy. The dyads include porphyrins joined at the meso-positions by a diphenylethyne linker or a diarylethyne linker with 2,6-dimethyl substitution on either one or both of the aryl rings. Another dyad is linked at the beta-pyrrole positions of the porphyrins via a diphenylethyne linker. The type of linker and attachment site modulate the interporphyrin through-bond electronic coupling via steric hindrance (porphyrin-linker orbital overlap) and attachment motif (porphyrin electron density at the connection site). For each ZnFb dyad, the zinc porphyrin is selectively electrochemically oxidized (to produce Zn(+)Fb), the free base porphyrin is selectively excited with a 130 fs flash (to produce Zn(+)Fb*), and the subsequent dynamics monitored. The Zn(+)Fb* excited state has a lifetime of approximately 3 to approximately 30 ps (depending on the linker steric hindrance and attachment site) and decays by parallel excited-state energy- and hole-transfer pathways. The relative yields of the two channels depend on a number of factors including the linker-mediated through-bond electronic coupling and a modest (< or =20%) Forster through-space contribution for the energy-transfer route. One product of Zn(+)Fb* decay is the metastable ground-state ZnFb(+), which decays to the Zn(+)Fb preflash state by ground-state hole transfer with a linker-dependent rate constant of (20 ps)(-1) to (150 ps)(-1). Collectively, these results provide a detailed understanding of the mechanism and dynamics of quenching of excited porphyrins by nearby oxidized sites, as well as the dynamics of ground-state hole transfer between nonequivalent porphyrins (Zn and Fb). The findings also lay the foundation for the study of ground-state hole transfer between identical porphyrins (e.g., Zn/Zn, Fb/Fb) in larger multiporphyrin arrays wherein a hole is selectively placed via electrochemical oxidation.

Physicochemical and Photophysical Studies on Porphyrin-Based Donor−Acceptor Systems: Effect of Redox Potentials on Ultrafast Electron-Transfer Dynamics

We report new polychromophoric complexes, where different porphyrin (P) derivatives are covalently coupled to a redox active Mo center, MoL*(NO)Cl(X) (L* is the face-capping tridentate ligand tris(3,5-dimethylpyrazolyl) hydroborate and X is a phenoxide/pyridyl/amido derivative of porphyrin). The luminescence quantum yields of the bichromophoric systems (1, 2, and 5) were found to be an order of magnitude less than those of their respective porphyrin precursors. Transient absorption measurements revealed the formation of the porphyrin radical cation species (P(.)(+)) and photoinduced electron transfer from the porphyrin moiety to the respective Mo center in 1, 2, and 5. Electrochemical studies showed that the reduction potentials of the acceptor Mo centers in a newly synthesized pyridyl derivative (2; E(1/2)[Mo(I/0)] = approximately -1.4 V vs Ag/AgCl) and previously reported phenoxy- (1; E(1/2)[Mo(II/I)] = approximately -0.3 V vs Ag/AgCl) and amido- (3; E(1/2)[Mo(II/I)] = approximately -0.82 V vs Ag/AgCl) derivatives were varied over a wide range. Thus, studies with these complexes permitted us to correlate the probable effect of this potential gradient on the electron-transfer dynamics. Time-resolved absorption studies, following excitation at the Soret band of the porphyrin fragment in complexes 1, 2, and 5, established that forward electron transfer took place biexponentially from both S2 and S1 states of the porphyrin center to the Mo moiety with time constants 150-250 fs and 8-20 ps, respectively. In the case of MoL*(NO)ClX (where X is pyridine derivative 2), the high reduction potential for the MoI/0 couple allowed electron transfer solely from the S2 state of the porphyrin center. Time constants for the charge recombination process for all complexes were found to be 150-300 ps. Further, electrochemical and EPR studies with the trichromophoric complexes (3 and 4) revealed that the orthogonal orientation of the peripheral phenoxy/pyridyl rings negated the possibility of any electronic interaction between two paramagnetic Mo centers in the ground state and thereby the spin exchange, which otherwise was observed for related Mo complexes when two Mo centers are separated by a polyene system with comparable or larger separation distances.

Energy and Electron Transfer in β-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 Processes in a Tricomponent Molecule Consisting of Diphenylaminofluorene−Dicyanoethylene−Methano[60]fullerene

Photoinduced intramolecular processes in a tricomponent molecule C60(>(CN)2-DPAF), consisting of an electron-accepting methano[60]fullerene moiety (C60>) covalently bound to an electron-donating diphenylaminofluorene (DPAF) unit via a bridging dicyanoethylenyl group [(CN)2], were investigated in comparison with (CN)2-DPAF. On the basis of the molecular orbital calculations, the lowest charge-separated state of C60(>(CN)2-DPAF) is suggested to be C60*-(>(CN)2-DPAF*+) with the negative charge localized on the fullerene cage, while the upper state is C60(>(CN)2*--DPAF*+). The excited-state events of C60(>(CN)2-DPAF) were monitored by both time-resolved emission and nanosecond transient absorption techniques. In both nonpolar and polar solvents, the excited charge-transfer state decayed mainly through initial energy-transfer process to the C60 moiety yielding the corresponding 1C60, from which charge separation took place leading to the formation of C60*-(>(CN)2-DPAF*+) in a fast rate and high efficiency. In addition, multistep charge separation from C60(>(CN)2*--DPAF*+) to C60*-(>(CN)2-DPAF*+) may be possible with the excitation of charge-transfer band. The lifetimes of C60*-(>(CN)2-DPAF*+) are longer than the previously reported methano[60]fullerene-diphenylaminofluorene C60(>(C=O)-DPAF) with the C60 and DPAF moieties linked by a methanoketo group. These findings suggest an important role of dicyanoethylenyl group as an electron mediating bridge in C60(>(CN)2-DPAF).