Fungal lignin peroxidase does not produce the veratryl alcohol cation radical as a diffusible ligninolytic oxidant.

Peroxidases are considered essential agents of lignin degradation by white-rot basidiomycetes. However, low-molecular-weight oxidants likely have a primary role in lignin breakdown because many of these fungi delignify wood before its porosity has sufficiently increased for enzymes to infiltrate. It has been proposed that lignin peroxidases (LPs, EC 1.11.1.14) fulfill this role by oxidizing the secreted fungal metabolite veratryl alcohol (VA) to its aryl cation radical (VA+•), releasing it to act as a one-electron lignin oxidant within woody plant cell walls. Here, we attached the fluorescent oxidant sensor BODIPY 581/591 throughout beads with a nominal porosity of 6 kDa and assessed whether peroxidase-generated aryl cation radical systems could oxidize the beads. As positive control, we used the 1,2,4,5-tetramethoxybenzene (TMB) cation radical, generated from TMB by horseradish peroxidase. This control oxidized the beads to depths that increased with the amount of oxidant supplied, ultimately resulting in completely oxidized beads. A reaction-diffusion computer model yielded oxidation profiles that were within the 95% confidence intervals for the data. By contrast, bead oxidation caused by VA and the LPA isozyme of Phanerochaete chrysosporium was confined to a shallow shell of LP-accessible volume at the bead surface, regardless of how much oxidant was supplied. This finding contrasted with the modeling results, which showed that if the LP/VA system were to release VA+•, it would oxidize the bead interiors. We conclude that LPA releases insignificant quantities of VA+• and that a different mechanism produces small ligninolytic oxidants during white rot.

Tuning Electronic Structure, Redox, and Photophysical Properties in Asymmetric NIR-Absorbing Organometallic BODIPYs.

Stepwise modification of the methyl groups at the α positions of BODIPY 1 was used for preparation of a series of mono- (2, 4, and 6) and diferrocene (3) substituted donor-acceptor dyads in which the organometallic substituents are fully conjugated with the BODIPY π system. All donor-acceptor complexes have strong absorption in the NIR region and quenched steady-state fluorescence, which can be partially restored upon oxidation of organometallic group(s). X-ray crystallography of complexes 2-4 and 6 confirms the nearly coplanar arrangement of the ferrocene groups and the BODIPY π system. Redox properties of the target systems were studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the first oxidation process in all dyads is ferrocene centered, while the separation between the first and the second ferrocene-centered oxidation potentials in diferrocenyl-containing dyad 3 is ∼150 mV. The density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to investigate the electronic structure as well as explain the UV-vis and redox properties of organometallic compounds 2-4 and 6. TDDFT calculations allow for assignment of the charge-transfer and π → π* transitions in the target compounds. The excited state dynamics of the parent BODIPY 1 and dyads 2-4 and 6 were investigated using time-resolved transient spectroscopy. In all organometallic dyads 2-4 and 6 the initially excited state is rapidly quenched by electron transfer from the ferrocene ligand. The lifetime of the charge-separated state was found to be between 136 and 260 ps and demonstrates a systematic dependence on the electronic structure and geometry of BODIPYs 2-4 and 6.

Synthesis and charge-transfer dynamics in a ferrocene-containing organoboryl aza-BODIPY donor-acceptor triad with boron as the hub.

A N,N'-bis(ferroceneacetylene)boryl complex of 3,3'-diphenylazadiisoindolylmethene was synthesized by the reaction of an N,N'-difluoroboryl complex of 3,3'-diphenylazadiisoindolylmethene and ferroceneacetylene magnesium bromide. The novel diiron complex was characterized by a variety of spectroscopic techniques, electrochemistry, and ultrafast time-resolved methods. Spectroscopy and redox behavior was correlated with the density functional theory (DFT) and time-dependent DFT calculations. An unexpected degree of coupling between the two Fc ligands was observed. Despite a lack of conjugation between the donor and acceptor, the complex undergoes very rapid (τ = 1.7 ± 0.1 ps) photoinduced intramolecular charge separation followed by subpicosecond charge recombination to form a triplet state with a lifetime of 4.8 ± 0.1 µs.

A supramolecular tetrad featuring covalently linked ferrocene-zinc porphyrin-BODIPY coordinated to fullerene: a charge stabilizing, photosynthetic antenna-reaction center mimic.

A novel photosynthetic-antenna-reaction-center model compound, comprised of BF2 -chelated dipyrromethene (BODIPY) as an energy-harvesting antenna, zinc porphyrin (ZnP) as the primary electron donor, ferrocene (Fc) as a hole-shifting agent, and phenylimidazole-functionalized fulleropyrrolidine (C60 Im) as an electron acceptor, has been synthesized and characterized. Optical absorption and emission, computational structure optimization, and cyclic voltammetry studies were systematically performed to establish the role of each entity in the multistep photochemical reactions. The energy-level diagram established from optical and redox data helped identifying different photochemical events. Selective excitation of BODIPY resulted in efficient singlet energy transfer to the ZnP entity. Ultrafast electron transfer from the (1) ZnP* (formed either as a result of singlet-singlet energy transfer or direct excitation) or (1) C60 * of the coordinated fullerene resulting into the formation of the Fc-(C60 (.) (-) Im:ZnP(.) (+) )-BODIPY radical ion pair was witnessed by femtosecond transient absorption studies. Subsequent hole migration to the ferrocene entity resulted in the Fc(+) -(C60 (.) (+) Im:ZnP)-BODIPY radical ion pair that persisted for 7-15 µs, depending upon the solvent conditions and contributions from the triplet excited states of ZnP and ImC60 , as revealed by the nanosecond transient spectral studies. Better utilization of light energy in generating the long-lived charge-separated state with the help of the present "antenna-reaction-center" model system has been successfully demonstrated.

Redox and photoinduced electron-transfer properties in short distance organoboryl ferrocene-subphthalocyanine dyads.

Reaction between ferrocene lithium or ethynylferrocene magnesium bromide and (chloro)boronsubphthalocyanine leads to formation of ferrocene- (2) and ethynylferrocene- (3) containing subphthalocyanine dyads with a direct organometallic B-C bond. New donor-acceptor dyads were characterized using UV-vis and magnetic circular dichroism (MCD) spectroscopies, NMR method, and X-ray crystallography. Redox potentials of the rigid donor-acceptor dyads 2 and 3 were studied using the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches and compared to the parent subphthalocyanine 1 and conformationally flexible subphthalocyanine ferrocenenylmethoxide (4) and ferrocenyl carboxylate (5) dyads reported earlier. It was found that the first oxidation process in dyads 2 and 3 is ferrocene-centered, while the first reduction as well as the second oxidation are centered at the subphthalocyanine ligand. Density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to probe the electronic structures and explain the UV-vis and MCD spectra of complexes 1-5. DFT-PCM calculations suggest that the LUMO, LUMO+1, and HOMO-3 in new dyads 2 and 3 are centered at the subphthalocyanine ligand, while the HOMO to HOMO-2 in both dyads are predominantly ferrocene-centered. TDDFT-PCM calculations on compounds 1-5 are indicative of the π → π* transitions dominance in their UV-vis spectra, which is consistent with the experimental data. The excited state dynamics of the parent subphthalocyanine 1 and dyads 2-5 were investigated using time-resolved transient spectroscopy. In the dyads 2-5, the initially excited state is rapidly (<2 ps) quenched by electron transfer from the ferrocene ligand. The lifetime of the charge transfer state demonstrates a systematic dependence on the structure of the bridge between the subphthalocyanine and ferrocene.

Synthesis, redox properties, and electronic coupling in the diferrocene aza-dipyrromethene and azaBODIPY donor-acceptor dyad with direct ferrocene-α-pyrrole bond.

3,3'-Diferrocenylazadipyrromethene (3) and corresponding difluoroboryl (azaBODIPY) complex (4) were synthesized in several steps from ferrocenecarbaldehyde, following the well-explored chalcone-type synthetic approach. The novel diiron complexes, in which ferrocene groups are directly connected to the α-pyrrolic positions were characterized by a variety of spectroscopic techniques, electrochemistry, spectroelectrochemistry, and X-ray crystallography, while their electronic structure, redox properties, and UV-vis spectra were correlated with the density functional theory (DFT) and time-dependent DFT calculations.

Syntheses and excitation transfer studies of near-orthogonal free-base porphyrin-ruthenium phthalocyanine dyads and pentad.

A new series of molecular dyads and pentad featuring free-base porphyrin and ruthenium phthalocyanine have been synthesized and characterized. The synthetic strategy involved reacting free-base porphyrin functionalized with one or four entities of phenylimidazole at the meso position of the porphyrin ring with ruthenium carbonyl phthalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor-acceptor polyads (dyad and pentad) is investigated in nonpolar toluene and polar benzonitrile solvents using both steady-state and time-resolved emission techniques. Electrochemical and computational studies suggested that the photoinduced electron transfer is a thermodynamically unfavorable process in nonpolar media but may take place in a polar environment. Selective excitation of the donor, free-base porphyrin entity, resulted in efficient excitation transfer to the acceptor, ruthenium phthalocyanine, and the position of imidazole linkage on the free-base porphyrin could be used to tune the rates of excitation transfer. The singlet excited Ru phthalocyanine thus formed instantly relaxed to the triplet state via intersystem crossing prior to returning to the ground state. Kinetics of energy transfer (k(ENT)) was monitored by performing transient absorption and emission measurements using pump-probe and up-conversion techniques in toluene, respectively, and modeled using a Förster-type energy transfer mechanism. Such studies revealed the experimental k(ENT) values on the order of 10(10)-10(11) s(-1), which readily agreed with the theoretically estimated values. Interestingly, in polar benzonitrile solvent, additional charge transfer interactions in the case of dyads but not in the case of pentad, presumably due to the geometry/orientation consideration, were observed.

Diameter-sorted SWCNT-porphyrin and SWCNT-phthalocyanine conjugates for light-energy harvesting.

A non-covalent double-decker binding strategy is employed to construct functional supramolecular single-wall carbon nanotubes (SWCNT)-tetrapyrrole hybrids capable of undergoing photoinduced electron transfer and performing direct conversion of light into electricity. To accomplish this, two semiconducting SWCNTs of different diameters (6,5 and 7,6) were modified via π-π stacking of pyrene functionalized with an alkyl ammonium cation (PyrNH(3)(+)). Such modified nanotubes were subsequently assembled via dipole-cation binding of zinc porphyrin with one (1) or four benzo-18-crown-6 cavities (2) or phthalocyanine with four benzo-18-crown-6 cavities at the ring periphery (3), employed as visible-light photosensitizers. Upon charactering the conjugates using TEM and optical techniques, electron transfer via photoexcited zinc porphyrin and phthalocyanine was investigated using time-resolved emission and transient absorption techniques. Higher charge-separation efficiency is established for SWCNT(7,6) with a narrow band gap than the thin SWCNT(6,5) with a wide band gap. Photoelectrochemical studies using FTO/SnO(2) electrodes modified with these donor-acceptor conjugates unanimously demonstrated the ability of these conjugates to convert light energy into electricity. The photocurrent generation followed the trend observed for charge separation, that is, incident-photon-to-current efficiency (IPCE) of a maximum of 12 % is achieved for photocells with FTO/SnO(2)/SWCNT(7,6)/PyrNH(3)(+):1.

Near unity photon-to-electron conversion efficiency of photoelectrochemical cells built on cationic water-soluble porphyrins electrostatically decorated onto thin-film nanocrystalline SnO2 surface.

Thin transparent SnO(2) films have been surface modified with cationic water-soluble porphyrins for photoelectrochemical investigations. Free-base and zinc(II) derivatives of three types of cationic water-soluble porphyrins, (P)M, viz., tetrakis(N-methylpyridyl)porphyrin chloride, (TMPyP)M, tetrakis(trimethylanilinium)porphyrin chloride, (TAP)M, and tetrakis(4'-N-methylimidazolyl-phenyl)porphyrin iodide, (TMIP)M, (M = 2H or Zn) are employed. The negative surface charge and the porous structure of SnO(2) facilitated binding of positively charged porphyrins via electrostatic interactions, in addition to strong electronic interactions in the case of (TMPyP)M binding to nanocrystalline SnO(2). The SnO(2)-porphyrin binding in solution was probed by absorption spectroscopy which yielded apparent binding constants in the range of 1.5-2.6 × 10(4) M(-1). Both steady-state and time-resolved fluorescence studies revealed quenching of porphyrin emission upon binding to SnO(2) in water suggesting electron injection from singlet excited porphyrin to SnO(2) conduction band. Addition of LiClO(4) weakened the ion-paired porphyrin-SnO(2) binding as revealed by reversible emission changes. Over 80% of the quenched fluorescence was recovered in the case of (TMPyP)M and (TAP)M compounds but not for (TMIP)M suggesting stronger binding of the latter to SnO(2) surface. Photoelectrochemical studies performed on FTO/SnO(2)/(P)M electrodes revealed incident photon-to-current conversion efficiencies (IPCE) up to 91% at the peak maxima for the SnO(2)-dye modified electrodes, with very good on-off switchability. The high IPCE values have been attributed to the strong electrostatic and electronic interactions between the dye, (TMPyP)M and SnO(2) nanoparticles that would facilitate better charge injection from the excited porphyrin to the conduction band of the semiconductor. Electrochemical impedance spectral measurements of electron recombination resistance calculations were supportive of this assignment.

Diameter dependent electron transfer in supramolecular nanohybrids of (6,5)- or (7,6)-enriched semiconducting SWCNT as donors and fullerene as acceptor.

Diameter dependent electron donor behavior of (6,5)- and (7,6)-enriched semiconducting SWCNT is proved by constructing supramolecular nanohybrids with pyrene functionalized electron acceptor, fullerene, and the subsequent photocatalytic/ photoelectrochemical processes.

Photochemical charge separation in supramolecular phthalocyanine-multifullerene conjugates assembled by crown ether-alkyl ammonium cation interactions.

Self-assembled phthalocyanine-multifullerene donor-acceptor conjugates have been formed by crown ether-ammonium cation dipole-ion binding strategy to probe the photochemical charge separation. To achieve this, phthalocyanine is functionalized to possess four 18-crown-6 moieties on the macrocycle periphery, whereas fullerene is functionalized to possess an alkyl ammonium cation of short and long chain lengths. Stable donor-acceptor conjugates accommodating multifullerene entities have been obtained by the crown ether-ammonium cation inclusion complexation. From the efficient fluorescence quenching of the zinc phthalocyanine by the bound fullerene entities, the rate constants of charge separation are evaluated to be slightly larger for closely held via shorter alkyl chain length fullerene, which are also larger compared to the earlier reported analogous zinc porphyrin-multifullerene conjugate. Nanosecond transient absorption studies yielded spectral signatures corresponding to both the phthalocyanine radical cation and fullerene radical anion at the same time, providing evidence of light-induced electron transfer within the conjugates. The evaluated lifetimes of the radical ion pairs in the present phthalocyanine-fullerene conjugates are found to be hundreds of nanoseconds and are much longer compared to the earlier reported conjugate of zinc porphyrin analogue, revealing higher possible usage of the generated radical ion pairs.

Sensitive efficiency of photoinduced electron transfer to band gaps of semiconductive single-walled carbon nanotubes with supramolecularly attached zinc porphyrin bearing pyrene glues.

Photoinduced electron transfer in self-assembled single-walled carbon nanotube (SWNT)/zinc porphyrin (ZnP) hybrids utilizing (7,6)- and (6,5)-enriched SWNTs has been investigated. Toward this, first, zinc porphyrin was covalently functionalized to possess four pyrene entities (ZnP(pyr)(4)). Exfoliation of the semiconducting nanotube bundles occurred due to pi-pi-type interactions with the pyrene and porphyrin entities in organic solvents. The nanohybrids thus formed were isolated and characterized by TEM, UV-visible-near-IR, and Raman spectroscopy. Free-energy calculations suggested the possibility of electron transfer in both the (7,6)- and (6,5)-possessing ZnP(pyr)(4)/SWNT nanohybrids. Accordingly, fluorescence studies revealed efficient quenching of the singlet excited state of ZnP in the nanohybrids, originating from the charge separation, as confirmed by observation of a ZnP pi-cation radical in transient absorption spectra. The rates of charge separation were found to be slightly higher for (7,6)-SWNT-derived hybrids compared to the (6,5)-SWNT-derived hybrids. Charge recombination revealed an opposite effect, indicating that the (7,6)-SWNTs are slightly better for charge stabilization compared to the (6,5)-SWNTs. The present nanohybrids were further utilized to photochemically reduce the hexyl viologen dication in the presence of a sacrificial electron donor in an electron-pooling experiment, offering additional proof for the occurrence of photoinduced charge separation and potential utilization of these materials in light-energy-harvesting applications. Finally, solar cells constructed using the ZnP/SWNT hybrids revealed higher efficiency for the ZnP(pyr)(4)/(7,6)-SWNT hybrid with narrower nanotube band gap compared with the ZnP(pyr)(4)/(6,5)-SWNT having a relatively wider band gap.

Molecularly imprinted poly[bis(2,2'-bithienyl)methane] film with built-in molecular recognition sites for a piezoelectric microgravimetry chemosensor for selective determination of dopamine.

A piezoelectric microgravimetry (PM) chemosensor, featuring a film of molecularly imprinted polymer (MIP) of poly[bis(2,2'-bithienyl)methane] bearing either a 3,4-dihydroxyphenyl or benzo-18-crown-6 substituent, for selective determination of dopamine was devised and tested. A Pt/quartz resonator and a dopamine-templated MIP film, deposited by electropolymerization onto an underlayer of poly(bithiophene), served as the transducer and recognition element of the chemosensor, respectively. The UV-vis spectroscopic and XPS as well as electrochemical measurements verified completeness of the dopamine template extraction with a strong base solution. The extraction-generated molecular cavities featured recognition sites that served selective dopamine analyte binding. The SECM imaging substantiated the permeability characteristics of the template-free MIP film. The dopamine analyte was determined under FIA conditions with the PM detection. The lower limit of detection was 10nM dopamine at favorable conditions involving the 35 µL/min carrier solution flow rate and the injected sample volume of 1 mL. The sensitivity of the chemosensor increased almost fivefold when the poly(bithiophene) film coated Pt/quartz electrode was used instead of the bare Pt/quartz electrode as the substrate for deposition of the MIP film. The chemosensor successfully discriminated dopamine from structural and functional analogues, such as 2-phenylethylamine, histamine, and ascorbic acid. The optimum mean thickness of the MIP film was ∼220 nm.

NOR and AND logic gates based on supramolecular porphyrin-fullerene conjugates.

Supramolecular porphyrin-fullerene conjugates are employed to build NOR and AND logic gates. A crown ether appended zinc porphyrin and an imidazole or an alkyl ammonium functionalized fullerene are utilized. Quenching of zinc porphyrin fluorescence upon addition of binding-site-selective fullerene inputs forms the design basis of the NOR logic gate, while replacing the fullerene entities from the porphyrin-fullerene conjugate and restoring the original fluorescence by chemical inputs form the design basis of the AND logic gate.

Ultrafast singlet-singlet energy transfer in self-assembled via metal-ligand axial coordination of free-base porphyrin-zinc phthalocyanine and free-base porphyrin-zinc naphthalocyanine dyads.

Singlet-singlet energy transfer in self-assembled via axial coordination of imidazole-appended (at different positions of one of the meso-phenyl entities) free-base tetraphenylporphyrin, H(2)PIm, to either zinc phthalocyanine, ZnPc, or zinc naphthalocyanine, ZnNc, dyads is investigated in noncoordinating solvents, o-dichlorobenzene and toluene, using both steady-state and time-resolved transient absorption techniques. The newly formed supramolecular dyads were fully characterized by spectroscopic, computational, and electrochemical methods. The binding constants measured from optical absorption spectral data were found to be in the range of 10(4)-10(5) M(-1) for the 1:1 dyads, suggesting fairly stable complex formation. Electrochemical and computational studies suggested that photoinduced electron transfer is a thermodynamically unfavorable process when free-base porphyrin is excited in these dyads. Selective excitation of the donor free-base porphyrin entity was possible in both types of dyads formed by either of the ZnPc or ZnNc energy acceptors. Efficient singlet-singlet energy transfer was observed in these dyads, and the position of imidazole linkage on the free-base porphyrin entity, although flexible, seems to have some control over the overall efficiency of excited energy transfer process. Kinetics of energy transfer was monitored by performing transient absorption measurements using both up-conversion and pump-probe techniques. Such studies revealed ultrafast singlet-singlet energy transfer in the studied dyads with time constants on the order of 2-25 ps depending upon the type of the dyad.

Photosynthetic antenna-reaction center mimicry: sequential energy- and electron transfer in a self-assembled supramolecular triad composed of boron dipyrrin, zinc porphyrin and fullerene.

A self-assembled supramolecular triad, a model to mimic the photochemical events of photosynthetic antenna-reaction center, viz., sequential energy and electron transfer, has been newly constructed and studied. Boron dipyrrin, zinc porphyrin, and fullerene respectively constitute the energy donor, electron donor, and electron acceptor segments of the antenna-reaction center mimicry. For the construction, first, boron dipyrrin was covalently attached to a zinc porphyrin entity bearing a benzo-18-crown-6 host segment at the opposite end of the porphyrin ring. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity via ion-dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational, and electrochemical methods. Selective excitation of the boron dipyrrin moiety in the dyad resulted in energy transfer over 97% efficiency creating singlet excited zinc porphyrin. The rate of energy transfer from the decay measurements of time-correlated singlet photon counting (TCSPC) and up-conversion techniques agreed well with that obtained by the pump-probe technique and revealed efficient photoinduced energy transfer in the dyad (time constant in the order of 10-60 ps depending upon the conformer). Upon forming the supramolecular triad by self-assembling fullerene, the excited zinc porphyrin resulted in electron transfer to the coordinated fullerene yielding a charge-separated state, thus mimicking the antenna-reaction center functionalities of photosynthesis. Nanosecond transient absorption studies yielded a lifetime of the charge-separated state to be 23 micros indicating charge stabilization in the supramolecular triad. The present supramolecular system represents a successful model to mimic the rather complex "combined antenna-reaction center" events of photosynthesis.

Photosynthetic reaction center mimicry: low reorganization energy driven charge stabilization in self-assembled cofacial zinc phthalocyanine dimer-fullerene conjugate.

By employing well-defined self-assembly methods, a biomimetic bacterial photosynthetic reaction center complex has been constructed, and photoinduced electron transfer originating in this supramolecular donor-acceptor conjugate has been investigated. The biomimetic model of the bacterial "special pair" donor, a cofacial zinc phthalocyanine dimer, was formed via potassium ion induced dimerization of 4,5,4',5',4'', 5'',4''',5'''-zinc tetrakis(1,4,7,10,13-pentaoxatridecamethylene)phthalocyanine. The dimer was subsequently self-assembled with functionalized fullerenes via "two-point" binding involving axial coordination and crown ether-alkyl ammonium cation complexation to form the donor-acceptor pair, mimicking the noncovalently bound entities of the bacterial photosynthetic reaction center. The adopted self-assembly methodology yielded a supramolecular complex of higher stability with defined geometry and orientation as revealed by the binding constant and computational optimized structure. Unlike the previously reported porphyrin analog, the present phthalocyanine macrocycle based model system exhibited superior electron-transfer properties including formation of a long-lived charge-separated state, a key step of the photosynthetic light energy conversion process. Detailed analysis of the kinetic data in light of the Marcus theory of electron transfer revealed that small reorganization energy of the relatively rigid phthalocyanine is primarily responsible for slower charge-recombination process. The importance of the cofacial dimer in stabilizing the charge-separated state is borne out in the present all-supramolecular "reaction center" donor-acceptor mimic.

Metal quinolinolate-fullerene(s) donor-acceptor complexes: evidence for organic LED molecules acting as electron donors in photoinduced electron-transfer reactions.

Tris(quinolinolate)aluminum(III) (AlQ3) is the most widely used molecule in organic light-emitting devices. There exists a strong demand for understanding the photochemical and photophysical events originating from this class of molecules. This paper provides the first report on the electron donor ability of MQ(n) (M = Al or Zn for n = 3 or 2) complexes covalently connected to a well-known electron acceptor, fullerene. To accomplish this, fullerene was functionalized with 8-hydroxyquinoline at different ligand positions and their corresponding zinc(II) and aluminum(III) complexes were formed in situ. The weakly fluorescent metal quinolinolate-fullerene complexes formed a new class of donor-acceptor conjugates. The stoichiometry and structure of the newly formed metal quinolinolate-fullerene complexes were established from various spectroscopic methods including matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and computational density functional theory studies. Electrochemical studies involving free-energy calculations suggested the possibility of photoinduced electron transfer from excited metal-quinolinolate complex to the appended fullerene entity. Femtosecond transient absorption studies confirmed such a claim and analysis of the kinetic data allowed us to establish the different photophysical events in sufficient detail. The novel features of this class of donor-acceptor conjugates include faster charge recombination compared to charge separation and decay of the charge-separated state to populate the low-lying fullerene triplet state in competition with direct charge recombination to the ground state.

Face-to-face pacman-type porphyrin-fullerene dyads: design, synthesis, charge-transfer interactions, and photophysical studies.

Pacman-type face-to-face zinc-porphyrin-fullerene dyads have been newly synthesized and studied. Owing to the close proximity of the donor and acceptor entities, strong pi-pi intramolecular interactions between the porphyrin and fullerene entities resulted in modulating the spectral and electrochemical properties of the dyads. New absorption and emission bands that correspond to the charge-transfer interactions were observed in the near-IR region. Time-resolved transient absorption studies revealed efficient photoinduced electron transfer from the singlet excited porphyrin to the fullerene entity. The rate constants for photoinduced electron transfer are analyzed in terms of the Marcus theory of electron transfer, which afforded a large electron coupling matrix element (V=140 cm(-1)) for the face-to-face dyads. As a consequence of the large charge-recombination driving force in the Marcus inverted region, a relatively long lifetime of the charge-separated state has been achieved.