Supramolecular complex of a fused zinc phthalocyanine–zinc porphyrin dyad assembled by two imidazole-C₆₀units: ultrafast photoevents

Abstract

A new zinc phthalocyanine-zinc porphyrin dyad (ZnPc-ZnP) fused through a pyrazine ring has been synthesized as a receptor for imidazole-substituted C₆₀ (C₆₀Im) electron acceptor. Self-assembly via metal-ligand axial coordination and the pertinent association constants in solution were determined by ¹H-NMR, UV-Vis and fluorescence titration experiments at room temperature. The designed host was able to bind up to two C₆₀Im electron acceptor guest molecules to yield C₆₀Im:ZnPc-ZnP:ImC₆₀ donor-acceptor supramolecular complex. The spectral data showed that the two binding sites behave independently with binding constants similar in magnitude. Steady-state fluorescence studies were indicative of an efficient singlet-singlet energy transfer from zinc porphyrin to zinc phthalocyanine within the fused dyad. Accordingly, the transient absorption studies covering a wide timescale of femto-to-milli seconds revealed ultrafast energy transfer from ¹ZnP* to ZnPc (k_EnT ∼ 10¹² s^-1) in the fused dyad. Further, a photo induced electron transfer was observed in the supramolecularly assembled C₆₀Im:ZnPc-ZnP:ImC₆₀ donor-acceptor complex leading to charge separated states, which persisted for about 200 ns.