Quadratic nonlinear properties of N-(4-nitrophenyl)-L-prolinol and of a newly engineered molecular compound N-(4-nitrophenyl)-N-methylaminoacetonitrile: a comparative study

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Abstract
A new molecular engineering strategy is proposed that favors the packing of charge-transfer conjugated molecules to enhance their crystalline quadratic nonlinear efficiency. A highly polar substituent is grafted to an achiral molecule at a position remote from the donor–acceptor π-electron conjugated system. Dipolar interaction forces will act mainly toward the antiparallel coupling of local dipoles, while the remaining nonlinear portion of the molecule is freed, under other influences, to set up a noncentrosymmetric and possibly optimal structure. Among other 4-nitroanilinelike compounds, N-(4-nitrophenyl)-N-methylaminoacetonitrile (NPAN) exemplifies this new approach and is shown to have a powder second-harmonic generation efficiency of the same order as that of N-(4-nitrophenyl)-L-prolinol (NPP), i.e., more than 2 orders of magnitude above that of urea. The nonlinearity of both molecules (vector part of the β tensor projected along the dipole moment) has been measured by use of electric-field induced second-harmonic (EFISH) generation in solution at 1.06 μm. The nonlinearity of the NPAN molecule is roughly half that of NPP, but the transparency range of NPAN is significantly increased toward the UV compared with that of NPP. Two theoretical models, based, respectively, on a finite-field perturbation of the Hartree–Fock equations and on a sum-over-states expansion of tensor β both at a semiempirical level of approximation, are used to compute the coefficients of the first-order hyperpolarizability of NPP and NPAN. A two-level quantum model is used to account for frequency dispersion, and theoretical crystalline coefficients are obtained from an oriented-gas description of the crystal. Theoretical molecular polarizabilities are in satisfactory agreement with the EFISH experimental results. The experimental crystalline nonlinearity of NPP is also well accounted for by calculations, while the optimized nonlinear coefficient dZYY of crystalline NPAN is predicted to be of the order of 140 × 10−9 esu, coming close to that of NPP.