Nonlinear optics. A semiempirical study of organic chromophores

Many donor–acceptor substituted organic compounds show internal charge separation upon excitation. Within this class, the following compounds are studied as suitable candidates for molecules with high second-order nonlinear optical (NLO) response: 4-N,N-dimethylamino-benzonitrile (DMABN), 6-propanoyl-2-(dimethylamino)-naphthalene (PRODAN), and various 4-(4-N,N-dimethylamino-phenyl)-1,3-diphenyl-pyrazolo[3,4-b]quinolines (DMA-DPPQ). The configuration interaction method with single and double excitations (CISD) is used in combination with the sum over states (SOS) approach within the semiempirical AM1 parametrization to calculate the vector component of the hyperpolarizability along the dipole axis βμ. The influence of changing molecular geometry on the second-order nonlinear properties is calculated for various intramolecular relaxation coordinates. Solvent effects are explicitly studied for DMABN by a self-consistent reaction field (SCRF) model revealing an increasing NLO-effect with increasing solvent polarity. The hyperpolarizability is discussed in relationship to the charge transfer properties of the excited state. The calculated values are in good agreement with experimental data for both DMABN and PRODAN. DMA-DPPQ is found to be a good NLO chromophore only when increasing either the donor or acceptor strength. Its nonlinear response can be drastically increased by an additional strong electron acceptor group, e.g. a NO2-group, at the primary acceptor moiety.