Ultrafast excited state dynamics of trans-[4-(4′-dimethylaminostyryl) ] pyridine N-oxide in solution: Femtosecond fluorescence up-conversion and theoretical calculations

The results of the steady-state and time-resolved fluorescence-spectroscopy measurements and DFT calculations for trans-[4-(4′-dimethylaminostyryl)] pyridine N-oxide (trans-DPO) in various solvents are presented. These results are similar to those reported in the literature for trans-4-(dimethylamino)- 4′-cyanostilbene (DCS) where the S₁ emissive state shows the charge-transfer (CT) nature. Alcohol solvents, however, have aroused our particular interest because hydrogen-bonded complexes are formed between them and the trans-DPO molecule. They demonstrate a stronger CT character of the lower lying excited state and a larger separation between the first and the second absorption band than in the free trans-DPO molecule. The different effects found in time-dependent femtosecond up-conversion (performed for several emission wavelengths in dioxane and methanol solution over the time range 0-40 ps) were assigned to the hydrogen bond assisted charge transfer process in trans-DPO in methanol, to the excited-state solvation dynamics and mainly to solvent relaxation of trans-DPO dissolved in dioxane and methanol.