Tail-on' and 'Tail-off' binding approach to probe the proximity effects on the physico-chemical properties of porphyrin-fullerene conjugates

摘要

A new approach of probing proximity effects in porphyrin-fullerene dyads by using an axial ligand coordination controlled 'tail-on' and tail-off' binding mechanism is explained. Two methods, viz., temperature variation and axial ligand replacement are utilized to control the porphyrin-fullerene proximity. The measured electrochemical redox potentials and UV-visible absorption spectra reveal little or no ground state interactions between C{sub}60 spheroid and porphyrin π-system. The proximity effects on the photoinduced charge separation and charge recombination is probed by both steady-state and time-resolved fluorescence techniques. It is observed that in the 'tail-off' form, the charge-separation efficiency changes to some extent in comparison with the results obtained for the 'tail-on' form suggesting the presence of some through space interactions between singlet excited zinc porphyrin and C{sub}60 moiety in the 'tail-off' form. The charge separation rates and efficiencies are evaluated from the fluorescence lifetime studies. The charge-separation via the singlet-excited states of zinc porphyrin in the studied dyads is also confirmed by the quick rise-decay of the anion radical of the C{sub}60 moiety within 20 ns. Furthermore, a long-lived ion-pair with lifetime of about 1000 ns is also observed in the investigated zinc porphyrin-C{sub}60 dyads.