Intramolecular charge transfer, photoisomerisation and rotational reorientation of trans-4-dimethylamino-4'-cyanostilbene in liquid solution

In the excited singlet state of trans-4-dimethylamino-4'-cyanostilbene (DCS) ultrafast intramolecular charge transfer (ICT) takes place followed by trans-cis photoisomerisation from a highly polar charge transfer (CT) state. The photoisomerisation of DCS is studied as a function of temperature in three homologous solvent classes, n-alkanes, n-alkylnitriles and n-alcohols. The photoisomerisation activation barrier E(tc) increases with viscosity for DCS in n-alkane solvents, whereas in the polar n-alkylnitriles and n-alcohols the separate influence of viscosity and polarity on the isomerisation reaction cannot be disentangled. The activation barrier for the trans-cis photoisomerisation is considerably larger in acetonitrile and methanol, highly polar solvents of low viscosity, than in the nonpolar n-heptane, due to the strongly polar character of the CT state as compared to the transition state of the isomerisation reaction. This polarity trend is reversed in solvents with the lowest polarity and the highest viscosity of the homologous series of the n-alkylnitriles and the n-alcohols. Simultaneous operation of polarity, viscosity and temperature as factors changing the potential energy surfaces, leads to isoviscosity plots that appear to indicate that the trans-cis photoisomerisation of DCS does not depend on solvent viscosity. From measurements of fluorescence decay times as a function of pressure (to 4000 bar) it follows, however, that the photoisomerisation rate constant k(tc) decreases by a factor of 2.0 for a 3.3 fold viscosity increase, resulting in a fractional viscosity dependence, k(tc) similar or equal to eta(-alpha), with alpha = 0.61. When the increase of the permittivity with pressure is taken into account, alpha becomes 0.53. From the observation of a linear relationship between the rotational relaxation time tau(tot) and the viscosity, 1/tau(tot) similar or equal to eta(-1.0), it is concluded that the Stokes-Einstein-Debye equation holds for the overall rotation of the DCS molecule, showing that viscosity is a good measure of solvent friction and that the fractional viscosity dependence with alpha < 1 is not due to a nonlinear viscosity behaviour.