Triplet electron transfer and spin polarization in a palladium porphyrin-fullerene conjugate.

Transient electron paramagnetic resonance (TREPR) spectroscopy is used to investigate the pathway and dynamics of electron transfer in a palladium porphyrin-fullerene donor-acceptor conjugate. The heavy Pd atom in the porphyrin greatly enhances the rate of intersystem crossing and as a result, electron transfer from the porphyrin to fullerene occurs via the porphyrin triplet state. The sign of the polarization pattern of the radical pair generated by the electron transfer is opposite in benzonitrile and the liquid crystal 5CB. This difference is the result of a change in sign of the spin-spin coupling, which allows the values of the dipolar and exchange couplings between the electrons in the charge-separated state to be estimated. In addition to the radical pair, signals from the fullerene triplet state are also observed. The polarization of the fullerene triplet state inverts with time, while the radical pair signal decays to a multiplet pattern that persists for times longer than the spin-lattice relaxation time. A kinetic model, developed to explain these effects, reveals that forward and reverse electron transfer between the charge-separated state and the fullerene takes place. This process, combined with singlet recombination of the radical pair accounts for the inversion of the fullerene triplet state polarization and the long-lived multiplet polarization of the radical pair.

Photoinduced charge separation in wide-band capturing, multi-modular bis(donor styryl)BODIPY-fullerene systems.

A new series of multi-modular donor-acceptor systems capable of exhibiting photoinduced charge separation have been designed, synthesized and characterized using various techniques. In this series, the electron donor was a BF2-chelated dipyrromethene (BODIPY) appended with two styryl linkers carrying two electron rich triphenylamine or phenothiazine entities. Fulleropyrrolidine linked at the meso-position of the BODIPY ring served as an electron acceptor. As a result of extended conjugation and multiple electroactive chromophore entities, the bis-styryl BODIPY revealed absorbance and emission well-into the near-infrared region covering a 300-850 nm spectral range. Using redox, computational, absorbance and emission data, an energy level diagram was constructed that helped in envisioning the different photochemical events. Spectral evidence for photoinduced charge separation in these systems was established from femtosecond and nanosecond transient absorption studies. The measured rate constants indicated fast charge separation and relatively slow charge recombination revealing their usefulness in light energy harvesting and optoelectronic device building applications. The bis(donor styryl)BODIPY-fullerene systems populated BODIPY triplet excited states during the process of charge recombination.

A High-Energy Charge-Separated State of 1.70†eV from a High-Potential Donor-Acceptor Dyad: A Catalyst for Energy-Demanding Photochemical Reactions.

A high potential donor-acceptor dyad composed of zinc porphyrin bearing three meso-pentafluorophenyl substituents covalently linked to C60 , as a novel dyad capable of generating charge-separated states of high energy (potential) has been developed. The calculated energy of the charge-separated state was found to be 1.70 eV, the highest reported for a covalently linked porphyrin-fullerene dyad. Intramolecular photoinduced electron transfer leading to charge-separated states of appreciable lifetimes in polar and nonpolar solvents has been established from studies involving femto- to nanosecond transient absorption techniques. The high energy stored in the form of charge-separated states along with its persistence of about 50-60 ns makes this dyad a potential electron-transporting catalyst to carry out energy-demanding photochemical reactions. This type of high-energy harvesting dyad is expected to open new research in the areas of artificial photosynthesis especially producing energy (potential) demanding light-to-fuel products.

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.

Controlling electron and energy transfer paths by selective excitation in a zinc porphyrin-BODIPY-C60 multi-modular triad.

A multi-modular donor-acceptor triad composed of zinc porphyrin, BF2-chelated dipyrromethene (BODIPY), and C60 was newly synthesized, with the BODIPY entity at the central position. Using absorbance and emission spectral, electrochemical redox, and computational optimization results, energy level diagrams for the ZnP-BODIPY dyad and ZnP-BODIPY-C60 triad were constructed to envision the different photochemical events upon selective excitation of the BODIPY and ZnP entities. By transient absorption spectral studies covering a wide femtosecond-to-millisecond time scale, evidence for the different photochemical events and their kinetic information was secured. Efficient singlet-singlet energy transfer from 1BODIPY* to ZnP with a rate constant kENT = 1.7 × 1010 s-1 in toluene was observed in the case of the ZnP-BODIPY dyad. Interestingly, in the case of the ZnP-BODIPY-C60 triad, the selective excitation of ZnP resulted in electron transfer leading to the formation of the ZnP˙+-BODIPY-C60˙- charge-separated state. Owing to the distal separation of the radical cation and radical anion species (edge-to-edge distance of 18.7 Å), the radical ion-pair persisted for microseconds. By contrast, the selective excitation of BODIPY resulted in an ultrafast energy transfer to yield ZnP-BODIPY-1C60* as the major product. The 1C60* populated the low-lying 3C60* via intersystem crossing prior to returning to the ground state. The present study successfully demonstrates the importance of supramolecular geometry and selection of excitation wavelength in regulating the different photoprocesses.

Modulating the generation of long-lived charge separated states exclusively from the triplet excited states in palladium porphyrin-fullerene conjugates.

This study demonstrates molecular engineering of a series of donor-acceptor systems to allow control of the lifetime and initial spin multiplicity of the charge-separated state. By tuning the rate of intersystem crossing (ISC) and the donor-acceptor distance, electron transfer can be made to occur exclusively from the triplet excited state of the electron donor resulting in long-lived charge separation. To achieve this, three new palladium porphyrin-fullerene donor-acceptor systems were synthesized. The heavy Pd atom enhances the rate of ISC in the porphyrin and the rates of electron and energy transfer are modulated by varying the redox potential of the porphyrin and the porphyrin-fullerene distance. In the case of the meso-tris(tolyl)porphyrinato palladium(ii)-fulleropyrrolidine, the donor-acceptor distance is relatively long (13.1 Å) and the driving force for electron transfer is low. As a result, excitation of the porphyrin leads to rapid ISC followed by triplet-triplet energy transfer to fullerene. When the fullerene is bound directly to the porphyrin shortening the donor-acceptor distance to 2.6 Å electron transfer from the singlet excited palladium porphyrin leading to the generation of a short-lived charge separated state is the main process. Finally, when the palladium porphyrin is substituted with three electron rich triphenylamine entities, the lower oxidation potential of the porphyrin and appropriate donor-acceptor distance (∼13 Å), lead to electron transfer exclusively from the triplet excited state of palladium porphyrin with high quantum yield. The results show that when electron transfer occurs from the triplet state, its increased lifetime allows the distance between the donor and acceptor to be increased which results in a longer lifetime for the charge separated state.

Tuning Interfacial Electron Transfer in Nanostructured Cuprous Oxide Photoelectrochemical Cells with Charge-Selective Molecular Coatings.

The coating of nanostructured films of cuprous oxide with electroactive molecules strongly affects their photoelectrochemical performance in nonaqueous photocells, with photocurrent density increased up to an order of magnitude relative to bare cuprous oxide films or almost completely suppressed, depending on the choice of molecular adsorbant. Among adsorbants that enhance photocurrent, a strong variance of photoelectrochemical behavior is observed with changes in the molecular structure of the sensitizer, associated with differences in the reorganization energy and molecular size, which are interpreted to enhance forward electron transport and impede electrolyte/photocathode recombination, respectively. These results demonstrate that nanostructured cuprous oxide is a promising cathode material for p-type dye-sensitized solar cells.