Quantum chemical DFT and spectroscopic study of a push-pull chromophore for second-order nonlinear optics containing bithiophene as the electron relay

We report on a density functional theory and spectroscopic study of a pi-conjugated push-pull chromophore with second order nonlinearities, 5-[1,3-bis(dicyanomethylidene)indan-2-ylidenemethyl]-5'-[(E)-4-N,N-dimethylaminobenzylidene)methyl]-2,2'-bithiophene. The effects of the intramolecular charge transfer (ICT) from the donor to the acceptor groups on the molecular geometry and atomic charge distribution of this NLO-phore are derived from its DFT//B3LYP/6-31G* minimum-energy structure and Natural Population Analysis. The optical absorptions of this chromophore in the UV-Vis spectral region are analyzed with the help of TDDFT//B3LYP/6-31G* calculations of one-electron vertical excitations and of the DFT//B3LYP/6-31G* topologies of the relevant molecular orbitals. The main infrared and Raman features of the NLO-phore are also analyzed by performing DFr//B3LYP/3-21G* vibrational calculations. The great resemblance, in peak positions and relative intensities, between the infrared and Raman spectra of this compound reveals that it really behaves as an efficient push-pull system. (C) 2004 Elsevier B.V. All rights reserved.