Low Temperature Structural Studies of Photochemical Reactions. Angus Mackinnon^a, Judith A. K. Howard^a, Peter Wan^b, ^aChemistry Dept., University of Durham, South Rd., DURHAM, DH1 3LE, U.K.; ^bDept. of Chemistry, University of Victoria, Victoria, British Columbia, Canada

There have been very few studies of biphenyl derivatives involving their different conformations in ground state and excited state. When a biphenyl derivative is in its ground state it will, in general, have a twisted conformation in the solid state[1], with the dihedral angle in the range between 10-40 degrees. This angle will increase when there are ortho substituents on either or both of the rings, typically to between 50-80 degrees [1,3]. In the excited state these systems tend to have an inherent twisting motion towards planarity, this is thought to be the result of increased pi-system overlap between the two benzene rings[2,3]. The crystal structures of several biphenyl derivatives have been characterised at low temperature, 100-150 K, to determine the differences between packing in the solid state of the ground and excited states. We can see how much of a factor the increased symmetry of the excited state has to the packing of the structure. This will be discussed in the poster.

1 - Brock, C. P.; Minton, R. P. j. Am. Chem. Soc. 1989, 111, 4586.

2 - Takei, Y.; Yammaguchi, T.;'Osamura, Y.; Fuke, K.; Kaya, K. J. Chem. Phys. 1988, 92, 577.

3 - Huang, C. G.; Beveridge, K. A.; Wan, P. J, Am. Chem. Soc., 1991, 113, 7676.