Ultrafast kinetics of fluorescence decay of aqueous solutions of styryl dye derivatives and their complexes with cucurbit[7]uril

The ultrafast relaxations of electronically excited states of styryl dyes, viz., 4-[(E)-2-(3,4-dimethoxyphenyl)- 1-ethylpyridinium] perchlorate (1) and 1-(3-ammoniumpropyl)-4-[(E)-2-(3,4-dimethoxyphenyl) vinyl]pyridinium diperchlorate (2), and their inclusion complexes with cucurbit[7]uril (СВ[7]) have been studied in aqueous solutions as a function of the fluorescence wavelength by femtosecond laser-induced fluorescence spectroscopy.For all dyes and their complexes, the fluorescence decay curves are fitted satisfactorily by the sum of two and three exponentials. The relaxation of electronically excited states in the blue wing of fluorescence band contains a component with a short decay time of about 100–200 fs and the red wing displays a rising component with a characteristic time of the same order of magnitude. In addition, the fluorescence decay includes a component with a characteristic time of about 2 ps, which is independent of complexation and fluorescence wavelength. The addition of СВ[7] to aqueous solutions of dyes 1 and 2 results in an increase in the longer relaxation times from ≈50 ps for the free dyes to 100–150 ps for the bound ones. The extra positive charge of dication 2 has almost no effect on the photophysical properties compared to cation 1, which agrees with quantum chemical calculations of the structures of inclusion complexes. A considerable increase in the fluorescence lifetime can be attributed to the increase in the potential energy barrier between the excited state of dye and the conical region on the potential energy curve, which is assumed to be caused by the displacement of the dye cation from the equilibrium state deep into the cavity within the first several picoseconds after laser pulse excitation.