Femtosecond fluorescence studies of two-dimensional dynamics in photoexcited Michlerʹs ketones

Femtosecond fluorescence upconversion experiments are reported for two diphenyl ketone dye molecules, the unbridged Michlerʹs ketone [4,4′-bis(N,N-dimethylamino)-benzophenone] and its bridged derivative compound 3,6-bis(dimethylamino)-10,10-dimethylanthrone (BMK). For Michlerʹs ketone (MK), in alcoholic solution, anomalously fast dynamic Stokes shifts and fluorescence decay transients are observed. It is discussed that phenyl-group twisting as well as solvation determine the excited-state relaxation dynamics of MK. In the case of BMK, twisting is absent and the excited-state dynamics is determined solely by (fast) initial solvation followed by population decay to the ground state.