Electronic responses of donor acceptor substituted twisted biphenyls

Donor-acceptor biphenyl derivatives are particularly interesting model compounds presenting a charge transferabsorption band because the extent of conjugation, and therefore the amount of charge transfer between the substituents,may be varied by a controllable structural feature, namely the torsion angle between the two phenyl rings. This featurehas become even more interesting since chromophores of strong zwitterionic character, synthesized with a twist of nearly90°, have been shown to exhibit unprecedented quadratic responses [1], over an order of magnitude above that of thebest conventional push-pull chromophores. In this context, we have investigated both experimentally and theoretically,two biphenyl based systems with varying inter-aryl angles: a nitro-piperidinyl series [2] of conventional push-pullcharacter and a pyridinium-phenoxide series [3] of zwitterionic character. The results agree qualitatively with semiempiricalsimulations based on the AM1 Hamiltonian [4] used with the COSMO solvation model [5]. For the first series,the decrease in quadratic response with increasing dihedral angle indicates that oscillator strength loss is the dominantfactor. In the second series, the corresponding increase in quadratic response points to the change in dipole moment uponexcitation as the leading effect. Here, we will analyze to what extent the more ab initio electronic structure calculationsbased on the density functional theory may provide more quantitative results in spite of the problems they face in thedescription of charge transfer systems.