Formation of a supramolecular porphyrin-spacer-acceptor ternary complex and intracomplex electron transfer

A new ditopic spacer, N-propylisonicotinamidine, has been developed to assemble a ternary supramolecular complex, zinc-tetraphenylporphyrin (ZnTPP)-spacer acceptor (3,4-dinitrobenzoic acid), via an axial coordination of the spacer to ZnTPP and a salt bridge interaction between the spacer and acceptor. Formation of this ternary complex was probed by ¹H HMR and UV-vis spectroscopy. The ternary complex is formed with an overall association constant of 1.9 × 10⁸ M^-2 in dichloromethane. The fluorescence is nearly completely quenched by the electron acceptor in the ternary complex as confirmed by the steady-state fluorescence measurements. The triplet excited state of ZnTPP is also quenched by the electron acceptor in the ternary complex. The ternary complex is also formed in acetonitrile, where fluorescence quenching is observed with an intracomplex quenching rate constant of 1.9 × 10⁹ s^-1. Photoinduced intracomplex electron transfer is invoked as a likely mechanism of the fluorescence quenching and triplet bleaching.