Dynamics of the hole-capture processes in biphenyl and poly(4-vinylbiphenyl): A direct ab initio trajectory study

Direct ab initio trajectory calculations have been applied to the ionization (i.e., hole-capture) processes of biphenyl (Bp) in order to shed light on hole-capture processes of Bp and poly(4-vinylbiphenyl) (PVB). The static ab initio calculations at several levels of theory showed that the neutral Bp has a nonplanar structure with a twist angle between benzene rings in the range phi = 38-50degrees. The potential energy curve for the twist rotation of benzene rings was shallow for Bp. This twist angle was changed to 18-21degrees in biphenyl cation (Bp(+)), which has a more planar structure and a more tight potential shape than those of Bp. Full dimensional direct ab initio trajectory calculations showed that the structure of Bp is spontaneously changed after the ionization: the nonplanar structure was changed to planar one after 120 fs, and then the twist angle vibrated between -40degrees and +40degrees with a time period of about 500 fs. The C-C bond distance in the connection site between two benzene rings was immediately shortened after the hole capture of Bp, and it vibrated in the range 1.375-1.481 Angstrom. Direct PM3 dynamics calculations for the model compound composed of two-monomer units of PVB indicated that a hole is localized on one of the biphenyl groups at time zero, and then it is delocalized over the two side-groups after the structural relaxation. The mechanism of hole capture in Bp and in PVB was discussed on the basis of theoretical results.