Supramolecular Donor-Acceptor Assembly Derived from Tetracarbazole-Zinc Phthalocyanine Coordinated to Fullerene: Design, Synthesis, Photochemical, and Photoeiectrochemical Studies

摘要

A functional photosynthetic antenna-reaction center mimicking donor-acceptor polyad has been newly designed, synthesized, and characterized. The polyad was comprised of four entities of carbazole covalently linked to the macrocycle periphery of a zinc phthalocyanine (ZnPc). Efficient singlet excitation transfer from the carbazole to zinc phthalocyanine has been witnessed from the emission studies. Axial coordination of phenylimidazole-functionalized fulleropyrrolidine to ZnPc served as an electron acceptor in the polyad. Optical absorption and emission along with computational studies revealed stable complex formation wherein the evaluated binding constant K was 7.7 ± 0.2 X 10~5 M~(-1), an order of magnitude higher than that observed earlier for similar complexes due to the electronic effect induced by the carbazole entities. From the free-energy calculations, photoinduced electron transfer from the ~1ZnPc* to fullerene within the polyad was established to be an exothermic process. Kinetics of charge separation, k_(CS) monitored by time-resolved emission was found to be 2.8 x 10~9 s~(-1), indicating a relatively fast charge-separation process. The electron-transfer products were characterized by nanosecond transient absorption spectroscopic technique; the presence of ZnPc~(+·) radical cation at 890 nm and fulleropyrrolidine anion radical at 1000 nm was clear from this study. The kinetics of charge recombination, k_(CR), evaluated from the decay of either of the radical ions, was found to be 6.25 ± 0.2 x 10~7 s~(-1), revealing the persistence of the radical ion-pair species to some extent. Further, photoeiectrochemical studies, performed by constructing photocells by electrophoretic deposition of the studied polyad on nanocrystalline SnO2 modified surface, revealed a higher value of incident photon-to-current conversion efficiency covering the wide visible-near IR spectral region and good on-off switchability. Better charge injection from the excited polyad to the conduction band of the semiconductor was evident from the electrochemical impedance spectral measurements of electron recombination resistance calculations.