Dipole moments of singlet and triplet excited states of 4-N,N-dimethylaminobenzonitrile and related compounds and of their solute—solvent exciplexes

Abstract

Changes in dielectric loss at microwave frequencies resulting from UV laser excitation of solutions of 4-N,N-dimethylaminobenzonitrile and some of its derivatives in cyclohexane and in 1,4-dioxane have been studied. The changes are due to differences between the dipole moments of the solute in the ground and excited states. Exciplex formation with 1,4-dioxane leads to a reduction of the excited-state dipole moment both where the amino group is restricted to be coplanar and where it is non-coplanar with the aromatic ring of the solute. We did not find indications of large molecular polarizabilities of DMABN which could govern its fluorescence behaviour and we have to reject the suggestion (TICT theory) that twisting of the amino group causes a large increase of the dipole moment of DMABN in the S 1 state.