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

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 (ZnPc)-Zn-1* 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 technique; the presence of ZnPc+center dot 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, photoelectrochemical 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.