First hyperpolarizability of donor-acceptor substituted aromatic backbone molecular systems: Effect of dihedral angle and excitation energy

Abstract

Charge transfer molecular systems that contain two fused benzene rings as the core have been studied for their contribution to the first order hyperpolarizability β. Two identical donor groups are attached through the core to two cyanide groups that act as acceptors. Calculation of β has been done using ab-initio HF and semi-empirical AM1 Hamiltonian. The effect of twist (dihedral) angle θ on β and HOMO-LUMO energy gap has been analyzed using both the methods. Inverse relationship between HOMO-LUMO energy gap and β is observed from their variation with the dihedral angle only in the case of AM1. It is clear from the study that a planar geometry is not an essential condition for a large value of β. Dispersion plots show the first excitation energy values for all the molecules using AM1 and HF/6–31+G(d,p). These plots show a dip in the first excitation peak for AM1. This feature is also observed in the frequency-dependent β calculated by the same method for the equivalent single-core molecules. The first excitation energy values with AM1 calculations are comparable to HF/6–31G(d) but different from HF/6–31+G(d,p). However, the calculation of β using ab-initio Hartree-Fock method does not show the double peak feature, thus providing a clue about the robustness of the model and the influence of basis set in calculating β.