Nonlinear optical properties of organic crystals with hydrogen-bonded molecular units: The case of urea

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Abstract
The validity of simpler additive models, accounting for nonlinear optical properties of van der Waals crystals in terms of independent molecular contributions is questionable in the case of more associated molecular structures such as examplified by the hydrogen-bonded crystal lattice of urea. A number of one-electron properties of single isolated urea molecules are first considered by means of a finite-field perturbated LCAO Hartree–Fock model implemented at both semiempirical (INDO) and ab initio levels of approximation with different basis set extensions. Charge distribution, dipole (μ), multipoles, linear (α), quadratic (β), and cubic hyperpolarizabilities γ are computed and analyzed. The α tensor is shown to be quasi-isotropic in the molecular plane while the β tensor exhibits a stronger anisotropy and a prevailing βzzz coefficient (z is the molecular symmetry axis). The negative value of the product βzzzμz is interpreted by a decrease of the dominant excited state dipole as compared to that of the ground state dipole. A mutually consistent field approach coupled with a finite-field dipolar perturbation is developed at the LCAO Hartree–Fock ab initio level to account for the influence of the crystalline environment on molecular properties. Coulombian point-charge potentials are located around each molecule of the crystal, at specific atomic sites taking part in the lattice hydrogen bonding. The translational invariance of the crystalline charge distribution in the presence of the external field is ensured by a fast convergent iterative procedure. This model evidences, in the case of urea, a stronger anisotropic influence of the local crystalline environment on the lowest order nonlinear susceptibility (βzzz going from −44 to −74 a.u.) than on the polarizability (αzz, going from 27 to 31 a.u.). This noticeable enhancement of ‖βzzz‖ is linked to a possible cooperative influence of intramolecular charge-transfer and intermolecular bonding in the general case of a linear chain of hydrogen-bonded charge-transfer molecules interacting through the donor and acceptor groups.