Electron Transfer Studies of High Potential Zinc Porphyrin–Fullerene Supramolecular Dyads

Abstract

The ability of high oxidation potential zinc porphyrins acting as electron donors in photoinduced electron-transfer reactions is investigated. Donor−acceptor dyads were assembled via metal−ligand axial coordination of either pyridine or phenylimidazole functionalized fulleropyrrolidine with zinc porphyrin functionalized with different numbers of halogen substituents on the meso-aryl rings. Optical absorption studies on complex formation revealed relatively higher binding constants. Efficient quenching of fluorescence was observed for the newly assembled dyads, revealing their ability to undergo photoinduced events. Differential pulse voltammetry studies were performed to understand the structure−activity relationships with respect to the electron deficient nature of the porphyrins and to utilize these data to estimate free-energy change for charge-separation and charge-recombination processes. The absolute value of free-energy change for charge separation was found to be lower for halogenated porphyrins with higher oxidation potentials expecting to form high-energy radical ion pairs. Using femtosecond transient techniques, evidence for charge separation and kinetics of charge separation and recombination were obtained in toluene. The kinetic data obtained by analyzing the time profiles of the radical ions revealed occurrence of ultrafast charge separation and relatively slower charge recombination processes in the dyads. Notably, electron-transfer rates did not exactly follow the trends predicted based on Marcus theory of electron transfer. Donor− acceptor geometry and populating the triplet excited states of the sensitizers during charge recombination are considered to be possible reasons for this behavior.

Design and photochemical study of supramolecular donor–acceptor systems assembled via metal–ligand axial coordination

“…In this context, a series of high potential metallated porphyrins and phthalocyanines have been designed to study the photoinduced electron transfer processes. Supramolecular dyads comprised of high potential zinc porphyrins and zinc phthalocyanines having various numbers of electron withdrawing halogen atoms, such as fluorine and chlorine, at the periphery in conjunction with coordinated C 60 Im have been investigated [121][122]. The oxidation of the metalloporphyrins and metallophthalocyanines was harder due to the presence of halogen atoms at the ring periphery that ultimately involved in generating high energy charge-separated states in the dyads.…”

“…[121] reported donor-acceptor systems comprised of high oxidation potential ZnPs and C 60 functionalized with imidazole and pyridine groups, self-assembled via metal-ligand axial coordination. Four derivatives of zinc metallated high potential porphyrins were utilized in this study whose structures are shown in Fig.…”

One-pot synthesis of manganese porphyrin covalently functionalized graphene oxide for enhanced photocatalytic hydrogen evolution

“…Herein the MnTPP moiety serves as a light-harvesting photosensitizer and GO in the composite acts as an excellent electron accepter and mediator to adjust electron transfer. More importantly, the manganese porphyrin with high oxidation potential can react with sacrificial reagents rapidly, thus, effectively enhancing electron-hole separation [33][34][35]. Under UV-vis light irradiation, the as-prepared GO-MnTPP demonstrated excellent stability and remarkable enhanced photocatalytic performance for water reduction to produce hydrogen.…”

Photoinduced energy and electron transfer in rubrene–benzoquinone and rubrene–porphyrin systems

“…In an idealized system, all of the chromophores are chemically bound or they are located in close proximity of one other [7]. Because reorganization energy decreases with the increase in relative donor-acceptor separation, such molecular systems are prepared by linking the synthetic chromophores covalently with a short bridge into molecular arrays [8,9], dendrimers [10], and dyads [11][12][13], placing the donor moiety in close proximity to the acceptor moiety and reducing the energy barrier for the charge transfer process. In this case, charge transfer and separation are expected to occur on the femtosecond (fs) and picosecond (ps) time scales.…”