Conformational equilibrium and specific solute–solvent interactions of 9,9′-bianthryl in its lowest electronically excited singlet state

Abstract

We have investigated the influence of solvents on the properties of electronically excited states of 9,9′-bianthryl by studying the transient dielectric loss, the lifetime and quantum yield of fluorescence, the transient optical absorption and the transient electrical conductance of the solution. We arrive at the following conclusions. The short-wavelength fluorescence observed for the solution in apolar solvents is not emitted by a state with excitation localized on one of the anthracene rings but by a polar charge-transfer state. Fluorescence of solutions in alkanes arises from two states in which the molecular conformation is not the same; the rate of interconversion between these conformations is faster than 10¹⁰ s^–1 and the difference in free energy is 4 × 10³ J mol^–1. The long-wavelength fluorescence of the solutions in polar and some apolar solvents is emitted by solute-solvent exciplexes of the first excited singlet state and solvent molecules such as benzene, 1,4-dioxane and acetonitrile. In solutions with strongly polar solvents such as acetonitrile the solute-solvent exciplexes are precursors of ions.