Highly Coupled Heterobicycle-Fused Porphyrin Dimers: Excitonic Coupling and Charge Separation with Coordinated Fullerene, C60

Porphyrin dimers have been widely explored and studied owing to their importance in photosynthetic systems. A vast variety of dimers linked by different groups and at different angles have been synthesized and studied; however, the means by which to synthesize rigidly fused porphyrins with direct conjugation of the chromophores remains limited. Such a class of porphyrins may possess interesting properties that unconjugated or stacked dimers may not exhibit. In this study, bisbenzimidazole-fused porphyrin dimers and their mono- and bis-zinc derivatives are synthesized and characterized. As a consequence of excitonic coupling, these dimers exhibit a split Soret band irrespective of the metal ion in the porphyrin cavity. Steady-state fluorescence and excitation spectra followed by femtosecond transient absorption spectral studies of the heterometallated dimer, (free-base and zinc porphyrin) reveals the occurrence of efficient singlet-singlet energy transfer (>95 % efficiency and rate constant >10¹²  s^-1 ) within the dyad. Further, donor-acceptor conjugates were formed by metal-ligand axial coordination of phenyl imidazole functionalized C₆₀ and were characterized by a variety of physicochemical techniques. Excited state charge separation from both singlet and triplet excited states of ZnP in the conjugates has been established. The lifetime of the final charge-separated state was in the 30-40 μs range revealing charge stabilization. Interestingly, no charge separation in the conjugate derived from the heterometallated dimer was observed wherein excitation transfer dominated the process. The present study brings out the importance of the rigid π-spacer connecting porphyrin dimers in governing the energy and electron transfer events when coupled with an electron acceptor.

Aromatic heterobicycle-fused porphyrins: impact on aromaticity and excited state electron transfer leading to long-lived charge separation

A new synthetic method to fuse benzo[4,5]imidazo[2,1-a]isoindole to the porphyrin periphery at the β,β-positions has been developed, and its impact on the aromaticity and electronic structures is investigated. Reactivity investigation of the fused benzoimidazo-isoindole component reveals fluorescence quenching of a zinc porphyrin (AMIm-2) upon treatment with a Brønsted acid. The reaction of the zinc porphyrin (AMIm-2) with methyl iodide initiated a new organic transformation, resulting in the ring-opening of isoindole with the formation of an aldehyde and dimethylation of the benzoimidazo component. The fused benzoimidazo-isoindole component acted as a good ligand to bind platinum(ii), forming novel homobimetallic and heterobimetallic porphyrin complexes. The fusion of benzoimidazo-isoindole on the porphyrin ring resulted in bathochromically shifted absorptions and emissions, reflecting the extended conjugation of the porphyrin π-system. Time-resolved emission and transient absorption spectroscopy revealed stable excited state species of the benzoimidazo-isoindole fused porphyrins. Zinc porphyrin AMIm-2 promoted excited state electron transfer upon coordinating with an electron acceptor, C60, generating a long-lived charge-separated state, in the order of 37.4 μs. The formation of the exceptionally long-lived charge-separated state is attributed to the involvement of both singlet and triplet excited states of AMIm-2, which is rarely reported in porphyrins.