Journal of Physical Chemistry A, Vol.106, No.10, 2164-2172, 2002
Solvation dynamics of the excited 1,2-(p-cyano-p'-methoxydiphenyl)-ethyne
Solvation dynamics of 1,2-(p-cyano-p'-methoxydiphenyl)-ethyne (CMPE) in various solvents were investigated by observing its time-resolved fluorescence and transient absorption in the picosecond regime. Anisotropy decays of CMPE in most of the solvents examined were apparently single-exponential, except for n-butanol, in which the decay was nonexponential. Dipole moment of CMPE in the excited state (mu(e)), limiting anisotropy (A(0)) and rotational diffusion coefficient (D-0) were obtained from the anisotropy decays with the theoretical expression based on continuum model combined with a nonlinear least-squares's method. The values of mu(e) were linearly dependent on the solvent polarity except for benzene (9.9 D), and were 3.4 D in diethyl ether, 5.7 D in tetrahydrofuran, 13 D in n-butanol, 17 D in acetonitrile and 42 D in dimethyl sulfoxide. From transient absorption spectral measurements, it was demonstrated that the CT states appeared with time less than 10 ps in acetonitrile, 10 ps in methanol and 20 ps, in dioxane after pulsed excitation, but not in n-hexane. These results reveal that the dipole moment of CMPE in the excited state is dependent on the solvent polarity. The values of D-0 in benzene and in polar solvents with short relaxation times were always less than those calculated with stick or slip model, which suggests that rotational volume of the excited CMPE increased by the solvation. Time-resolved Stokes shift and anisotropy of the fluorescence of CMPE in n-butanol were simultaneously analyzed to obtain mu(e) = 14 D, mu(g) (the dipole moment in the ground state) = 1.5 D and D-0 = 0.94 ns(-1).