Conformational dependence of linear and nonlinear molecular optical properties by ab initio methods: the case of oligo-p-phenylenes Original Research Article

Molecular properties of oligo-para-phenylenes are determined mostly by the combined effect of π electron delocalization and intra-ring confinement. In order to study their relative role, we have carried out an ab initio study on biphenyl and terphenyl. Energies of formation, geometrical parameters, vibrational spectra and molecular hyperpolarizabilities are studied (using the 3-21G basis) as function of the torsional angle between two adjacent rings and the number of rings in the molecule. Relaxation contribution to first- and second-order hyperpolarizabilities (βr and γr, respectively) are obtained with a vibrational method from computed frequencies, infrared and Raman intensities. Raman lines of Я-containing normal modes progressively weaken when the systems are distorted. Similarly the component of γr along the chain axis (γzzzzr) and the orientation average 〈γr〉 decrease. For planar molecules, Raman intensities and molecular hyperpolarizabilities show a superlinear dependence upon the number of rings in the chain, while when adjacent rings lie on perpendicular planes an almost linear dependence is obtained. The optical properties of the para-phenylenes studied can be predicted by the simple addition of the corresponding values for the repeating unit (benzene) only in the conformationally distorted case. This fact suggests that in the planar case the effects of π electron delocalization are much stronger than those of the intra-ring confinement potential. The comparison with experimental determinations of 〈γr〉 offers clear evidence that in solution oligo-p-phenylenes are highly conformationally distorted. An improvement of molecular properties is thus predicted if torsions about inter-ring bonds are hindered by proper substitutions or bridging.