The solvent-dependent isomerization dynamics of 4-(dimethylamino)azobenzene (DMAAB) studied by subpicosecond pump–probe spectroscopy

The photoisomerization dynamics of 4-(dimethylamino)azobenzene (DMAAB) are studied using subpicosecond pump–probe spectroscopy. Following photoexcitation at 400 nm, resonant with the lowest-lying π–π* transition, excited-state internal conversion occurs with a time-constant of ∼0.8 ps in acetone and ethanol, and ∼1.4 ps in fluorotrichloromethane. Internal conversion is followed by ground-state vibrational relaxation on the ∼10 ps timescale. It is proposed that the solvent dependence of the excited-state decay reflects the solvent-polarity dependence of the excited-to-ground-state energy gap. The results presented here, in combination with earlier work, suggest that rapid isomerization following photoexcitation into the lowest-energy electronic state is a general feature of azobenzene photochemistry.