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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, nalkanes, n-alkylnitriles and n-alcohols. The photoisomerisation activation barrier E,, 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 4000bar) it follows, however, that the photoisomerisation rate constant k,, decreases by a factor of 2.0 for a 3.3 fold viscosity increase, resulting in a fractional viscosity dependence, k,,=q -', with a = 0.61. When the increase of the permittivity with pressure is taken into account, a becomes 0.53. From the observation of a linear relationship between the rotational relaxation time rrof and the viscosity, t / q o t = q -I.', 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 a< 1 is not due to a nonlinear viscosity behaviour.

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The dynamic behavior of a novel polyunsaturated allylic borane, 2,4,6-heptatrienyldipropylborane, 5, was studied by NMR spectroscopy. It was found that four geometric isomers 5a-d interconvert reversibly via successive 1,3 sigmatropic shifts of boron (BPr2 group). Due to the thermal 1,7-hydrogen shift occurring in the minor Z,Z-isomer, 5d, the allylic borane 5 is converted stereoselectively and reversibly into the vinylic (E,Z,Z)-1,3,5-heptatrienyldipropylborane, 8. The final equilibrium mixture contains 92% 8 and 8% 5. The effective activation energy of the conversion of 5 into 8 and the activation energy for the reverse reaction were found to be 79.8 and 87.1 kJ/mol, respectively; the preexponential factors proved to be the same, 4~1 0~ s-1.Reaction of 5 with cyclopentanone gives the trans-homoallylic alcohol 15a, while 8 reacts with cyclopentanone yielding (Z,Z)-1,3,5-heptatriene, 18.