Light-induced energy transfer followed by electron transfer in axially co-ordinated benzothiazole tethered zinc porphyrin-fullero[C60/C70]pyrrolidine triads

Benzothiazole (BTZ)-zinc porphyrin (ZnP) dyads, Dyad-1 and Dyad-2 connected together with two different spacers, ester and ethoxy esters, were synthesized and light induced energy and electron transfer events were investigated. Within these dyads, due to the spectral overlap of the BTZ emission with the ZnP absorption, a selective photoexcitation of BTZ at 325 nm resulted in the photo-induced energy transfer (PEnT) from 1BTZ* to ZnP displaying the quenching of the BTZ emission followed by the concurrent appearance of the ZnP emission at 600 and 650 nm suggesting the formation of the( 1)ZnP* i.e. (1)BTZ*-ZnP -> BTZ-(ZnP)-Zn-1*. When the dyads are titrated with imidazole appended fullero[C-60/C-70]pyrrolidines, four supramolecular triads, involving the axial co-ordination of the imidazole to the zinc center of the ZnP, were formed and the assembly formation was systematically monitored by the optical absorption technique. Cyclic voltammetry and the density functional theory calculations have revealed that, in these triads, the zinc porphyrin acts as an electron donor and fullerene moiety as the electron acceptor. Steady state fluorescence studies revealed that, upon selective excitation of the ZnP moiety at 550 nm, the emission of ZnP at 600 and 650 nm was quenched revealing the occurrence of photo-induced electron transfer (PET) from 1ZnP* to fullerene moiety leading to the formation of charge separated state i.e. BTZ-(ZnP)-Zn-1* : (ImC(60/70))-> BTZ-ZnP+center dot:(ImC(60/70))(-center dot). More importantly, when the supramolecular triads were excited at 325 nm, the wavelength at which the BTZ absorbs predominantly, the emission of the BTZ moiety which was quenched due to PEnT from (1)BTZ* to ZnP followed by the PET from (ZnP)-Zn-1* to fullerene indicates the probability of occurrence of (1)BTZ*-ZnP:(ImC(60/70)) -> BTZ-(ZnP)-Zn-1*: (ImC(60/70)) -> BTZ-ZnP+center dot:(ImC(60/70))(-center dot).