Exciplex Formation in Complexes between Cyclophane Hosts and Aromatic Guests: Evidence That the Ground-State Complexes Exist in More Than One Distinct Geometry

Abstract

Several naphthalene derivatives and fluorene form exciplexes with cyclophane 1 when the host−guest complexes are photoexcited. The electron-rich aromatic units of host 1 act as electron donors (D), while the aromatic guests serve as electron acceptors (A). A linear dependence between exciplex fluorescence energy (hνemax) and the difference between the oxidation potential of the host and the reduction potential of the guest (EDox − EAred) has been established for the exciplexes observed with fluorene, naphthalene, and 1-fluoronaphthalene, in accord with data from previous studies in which triethylamine is the donor with these and other aromatic acceptors. Points for N-acetyl-2-naphthylalanine, 1-methylnaphthalene, 1-chloronaphthalene, and 2-chloronaphthalene deviate from the line, indicating reduced exciplex stability resulting from unfavorable steric interactions between host and guest. The guests with the highest reduction potentials, 1- and 2-cyanonaphthalene, exhibit extensive fluorescence quenching with little or no observable exciplex fluorescence, suggesting efficient formation of solvent-separated radical-ion pairs. Time-resolved fluorescence experiments reveal that the exciplexes arise via two independent pathways, one of which occurs very rapidly. The data indicate that the ground-state host−guest complexes exist at equilibrium in at least two distinct geometries.