Intramolecular exciplex and cation-mediated charge-transfer fluorescence in a podand carrying terminal electron donor and acceptor groups

Abstract

A podand carrying terminal electron donor and acceptor groups (DPA) has been synthesized. There is no intramolecular interaction between the two chromophores in the ground state. Selective excitation of the acceptor moiety, and a very weak and structureless exciplex emission was observed in benzene. In acetonitrile, only quenching of the monomer fluorescence occurred. In the presence of calcium ions conformational modification through complexation with the non-cyclic poly(oxyethylene) chain resulted in the formation of a charge-transfer (CT) complex in the ground state. Selective excitation of the acceptor moiety (380 nm) or at the CT band (340 nm) gave rise to intramolecular excited CT complex emission. The similarity in the fluorescence maxima exhibited by the exciplex and the excited CT complex implies that the electronic structures of their fluorescent states are very similar. The slight difference in the fluorescence quantum yields and decay rate constants between the exciplex and excited CT complex suggests that they have different geometries. Exciplex fluorescence was also observed in γ-CD, depending on the γ-CD concentration.