Dynamics of structural relaxation of stilbene 3 in solution, cetyltrimethylammonium bromide micelle, and bis(2-ethylhexyl) sulfosuccinate reverse micelle.

The relaxation dynamics of the excited states of stilbene 3 in various solvents, confined environment cetyltrimethylammonium bromide (CTAB) micelle, and water/bis(2-ethylhexyl) sulfosuccinate (AOT)/hexane reverse micelle are investigated. In the time-resolved decay curves of fluorescence measured in solution at excitation wavelength 256 or 266 nm, stilbene 3 underwent initially an ultrafast internal conversion to the S2 state or was directly excited at the S2 then via a conformational relaxation with time constant 1.1-4.6 ps. This relaxation process displays a linear dependence on solvent viscosity. Slow relaxation in deuterated methanol and water is explained that the vibrational energy is efficiently coupled to the librational modes of solvent. The conformational relaxation rate decreases slightly in the CTAB micelle but is greatly hindered in the AOT reverse micelle with small cavities. According to the results of anisotropy measurements, in both CTAB and AOT reverse micelles with a cavity diameter as large as ∼7 nm, the dynamics of molecular rotation remain significantly hindered.