Nonlinear optics. A semiempirical study of organic chromophores

Many donor-acceptor substituted organic compounds show internal charge sepa ration upon excitation. Within this class, the following compounds are stud ied as suitable candidates for molecules with high second-order nonlinear o ptical (NLO) response: 4-N,N-dimethylamino-benzonitrile (DMABN), 6-propanoy l-2-(dimethylamino)-naphthalene (PRODAN), and various 4-(4-N,N-dimethylamin o-phenyl)-1,3-diphenyl-pyrazolo[3,4-b]quinolines (DMA-DPPQ). The configurat ion interaction method with single and double excitations (CISD) is used in combination with the sum over states (SOS) approach within the semiempiric al AM I parametrization to calculate the vector component of the hyperpolar izability along the dipole axis beta (mu). The influence of changing molecu lar geometry on the second-order nonlinear properties is calculated for var ious intramolecular relaxation coordinates. Solvent effects are explicitly studied for DMABN by a self-consistent reaction field (SCRF) model revealin g an increasing NLO-effect with increasing solvent polarity. The hyperpolar izability is discussed in relationship to the charge transfer properties of the excited state. The calculated values are in good agreement with experi mental data for both DMABN and PRODAN. DMA-DPPQ is found to be a good NLO c hromophore only when increasing either the donor or acceptor strength. Its nonlinear response can be drastically increased by an additional strong ele ctron acceptor group, e.g. a NO2-group, at the primary acceptor moiety. (C) 2001 Elsevier Science B.V. All rights reserved.