The relationship between nonlinear susceptibilities and hyperpolarizabilities defined using different conventions is reexamined. In previous work [Willetts et al., J. Chem. Phys. 97, 7590 (1992)], relations between different conventions for microscopic hyperpolarizabilities have been derived, but the application of the corresponding conversion factors led to several inconsistencies. It is shown that different conventions for macroscopic susceptibilities have to be taken into account, too, in order to arrive at consistently comparable values. The complete set of conversion factors between several conventions are given for second harmonic generation, electric field induced second harmonic (EFISH) generation, and third harmonic generation. As an illustration, experimental EFISH and hyper-Rayleigh scattering results of p-nitroaniline are compared with each other and with recent results of ab initio computations including solvation effects. Several problems in the comparison of computational and experimental values are also discussed.

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Macroscopic first-, second-, and third-order susceptibilities of the urea crystal are calculated using static and frequency-dependent ab initio molecular (hyper)polarizabilities at the self-consistent field (SCF) and second-order-Mo/ller–Plesset (MP2) levels. Environmental effects are taken into account using a rigorous local-field theory and are compared with the anisotropic Lorentz field factor approximation. The electric field arising from the permanent dipoles of the surrounding molecules is explicitly taken into account using a self-consistent approach. The dipole moment and the first hyperpolarizability are particularly strongly dependent on this field, but the crystal structure leads to a fortuitous cancellation of the field effect on the second-order susceptibility. The experimental linear susceptibility is accurately reproduced, while differences remain for the quadratic susceptibility. Dispersion curves for the first-order susceptibility, and results for quadratic electrooptic effect (QEO), electric-field-induced second-harmonic generation (EFISH), and third-harmonic generation (THG) experiments are predicted. The (hyper)polarizabilities of a linear dimer of urea molecules are calculated and used to estimate the effect of hydrogen bonding on the susceptibilities, which proves to be small. Semiempirically calculated (hyper)polarizabilities methods yield unreliable results for the susceptibilities compared with those from the ab initio method. This deficiency can be overcome by recourse to additional experimental data.

Using a wide variety of quantum-chemical methods we have analyzed in detail the linear and non-linear optical properties of [60]fullerene-chromophore dyads of different electron-donor character. The dyads are composed of [60]fullerene covalently linked with 2,1,3-benzothiadiazole and carbazole derivatives. Linear scaling calculations of molecular (hyper)polarizabilities were performed using wave function theory as well as density functional theory (DFT). Within the former approach, we used both semiempirical (PM3) and ab initio (Hartree-Fock and second-order Møller-Plesset perturbation theory) methods. Within the latter approach only the recently proposed long-range (LRC) schemes successfully avoid a large overshoot in the value obtained for the first hyperpolarizability (β). Calculations on model fullerene derivatives establish a connection between this overshoot and the electron-donating capability of the substituent. Substitution of 2,1,3-benzothiadiazole by the triphenylamine group significantly increases the electronic first and second hyperpolarizabilities as well as the two-photon absorption cross section. For [60]fullerene-chromophore dyads we have, additionally, observed that the double harmonic vibrational contribution to the static beta is much larger than its electronic counterpart. The same is true for the dc-Pockels β as compared to the static electronic value, although the vibrational term is reduced in magnitude; for the intensity-dependent refractive index the vibrational and electronic terms are comparable. A nuclear relaxation treatment of vibrational anharmonicity for a model fulleropyrrolidine molecule yields a first-order contribution that is substantially more important than the double harmonic term for the static β.

This is the first part of a study of the local field effects on (non)linear optical susceptibilities of solutions of para-nitroaniline (pNA) in three different solvents, cyclohexane (CH), 1,4-dioxane (DI), and tetrahydrofuran (THF), using a discrete molecular representation of the condensed phase. To account for dipolar and quadrupolar effects, the latter of which are especially important for DI solution, all the electric properties necessary to compute the local fields and local field gradients in quadrupolar approximation as well as the dipolar hyperpolarizabilities for the four molecules are computed, including frequency dispersion and vibrational contributions to the dipolar properties. The convergence of the perturbation treatment for the pure vibrational (PV) contributions is examined by comparison of the values obtained at the lowest order with those of partially computed second order in mechanical and electrical anharmonicity. For pNA, for which previous computations of the hyperpolarizabilities have generally found poor agreement with experimental results, a thorough investigation of the effects of solvent-induced geometry changes, dynamic and static correlation, frequency dispersion, and classical thermal averaging over the torsional modes of the substituent groups and the inversion mode of the amino group on the dipolar properties is carried out. Computations using self-consistent continuum reaction field models show that the amino group is substantially less pyramidalized in polar solvents than in the gas phase. With all the effects taken into account, reasonable agreement with the experimental electric-field induced second harmonic generation (EFISH) result on pNA vapor of Kaatz, Donley, and Shelton is obtained.

The electric field poling process of nonlinear optical chromophores embedded in an amorphous polymer matrix was studied using molecular dynamics (MD) simulations. Three systems were considered, consisting of a poly(methyl methacrylate) matrix doped with the following chromophores:  N,N-dimethyl-p-nitroaniline (DPNA), 4-(dimethylamino)-4‘-nitrostilbene (DMANS), and N,N‘-di-n-propyl-2,4-dinitro-1,5-diaminobenzene (DPDNDAB). The cooling process in the presence of a poling electric field was simulated at constant NPT conditions using simulated annealing. The rotational dynamics of the dopants was investigated in the unpoled and poled states above T g and in the poled state below T g. The short-time behavior with respect to the back-relaxation to the unpoled state following removal of the poling field was examined for the systems below T g and was found to deviate from the single-exponential model. The electric field effects, during and following poling, were examined by computing the angle between the dipole moment of the chromophores and the external electric field. MD simulations at temperatures in the vicinity of T g revealed that during the simulated phase transition from the liquid state to the glassy structure the degree of alignment remained constant. The dependence of back-relaxation to the unpoled glassy state on the chromophores was investigated. DPNA molecules were found to be in closer proximity to the side groups than to the backbone units of the polymer at both temperatures, in contrast to DMANS at both temperatures and to DPDNDAB in the glassy state. The radial distribution functions for all systems are typical of amorphous structures. The reorientation of chromophores exhibits a higher degree of correlation with the facile motion of the PMMA side groups than with the configurational motion along the polymer backbone. The degree of chromophore alignment depends on its size and distance from the side groups of the polymer.

The electronic and vibrational contributions to the dipole moment, polarizabilities, and first hyperpolarizabilities of HArF are reported. These have been computed by using a series of systematically built basis sets and a hierarchy of computational methods. HArF has a very large first hyperpolarizability along the z axis. This has been rationalized by invoking the difference in the electronic structure between the ground and the first excited state. The argon fluorohydride has been recently derived and characterized. The present study provides complementary data for the understanding of the electronic structure of this interesting argon derivative.

Electronic structures of 9,10-anthrylene dimers and trimers analogous to 9,9‘-bianthryl (BA) have been investigated by means of steady-state fluorescence and transient absorption spectroscopy and dipole moment evaluation based on the solvatochromic fluorescence shifts and integrated electrooptical emission measurements (IEOEM) in various solvents. Formation of the charge-separation (CS) state in the excited state was strongly dependent on the substituent group:  unsubstituted and n-hexyl-substituted anthrylenes exhibited almost the same behavior compared to BA, whereas the tert-butyl-substituted dimer and trimer relaxed into an excitonic state rather than the CS state even in a polar solvent. The lower possibility of the CS state for the tert-butyl-substituted anthrylenes was mainly ascribed to the smaller solvation energy because of the larger effective Onsager cavity due to bulky tert-butyl groups. The limitation of a simple continuum model using Onsager's reaction field was also discussed in relation to the solvent-induced electronic structure change in the excited state of anthrylenes.