Over the past two years, the field of molecular nonlinear optics has benefited from both upstream rejuvenation (the emergence of new molecular engineering guidelines) and downstream application-oriented breakthroughs, helping to bring the field closer to industrial developments. In the fundamentals areas, traditional perspectives based on the paranitroaniline ‘molecular diode’ template are being enlarged to encompass three-dimensional multicharge transfer ‘multipolar’ systems with dipolarless ‘octupoles’, demanding that earlier models and experiments be thoroughly revisited. Furthermore, the relevance of polar structures primarily targeted towards quadratic nonlimear optics has been extended to cubic all-optical processes based on a cascaded sequence of quadratic processes. In the application domain, progress in crystal growth is best illustrated by the demonstration of low threshold pulsed near-IR optical parametric oscillators based on stable organic or organomineral crystals. Functionalized polymers have confirmed their current supremacy as organic optoelectronic device materials with the reports of low voltage electro-optic modulation in integrated waveguide structures and high modulation bandwidth.

Status and perspective for molecular nonlinear optics: from crystals to polymers and fundamentals to applications

The relationships between molecular structure and the nonlinear optical phenomenon second–harmonic generation (SHG) are discussed. New–found relationships built up from basic structural axioms that were deduced in the 1970s and 1980s are the particular focus of this article, using structural results from X–ray and neutron–diffraction studies. The molecular and supramolecular manifestations of the SHG effect are borne out, although ways to optimize the effect on the molecular scale feature predominantly, since control of SHG on the supramolecular scale remains difficult given present limitations. The use of a variety of templates to generate head–to–tail oriented host–guest species thereby bypassing such limitations is described. The paper concludes with a look ahead at next generation ‘octupolar’ SHG–active compounds, the prediction of new series of SHG–active compounds via data–mining computational procedures, and developments in diffraction technology that may enable structural movies of a molecule to be captured during the SHG process. A practical assessment of the viability of organic SHG materials for industrial application is reviewed with a positive outcome, thus indicating a promising future for organic SHG materials.

Organic materials for second-harmonic generation: advances in relating structure to function.

Although reliable determination of the molecular dipole moment from experimental charge density analyses on molecular crystals is a challenging undertaking, these values are becoming increasingly common experimental results. We collate all known experimental determinations and use this database to identify broad trends in the dipole moment enhancements implied by these measurements as well as outliers for which enhancements are pronounced. Compelling evidence emerges that molecular dipole moments from X-ray diffraction data can provide a wealth of information on the change in the molecular charge distribution that results from crystal formation. Most importantly, these experiments are unrivalled in their potential to provide this information in such detail and deserve to be exploited to a much greater extent. The considerable number of experimental determinations now available has enabled us to pinpoint those studies that merit further attention, either because they point unequivocally to a considerable enhancement in the crystal (of 50 % or more), or because the experimental determinations suggest enhancements of 100 % or more—much larger than independent theoretical estimates. In both cases further detailed experimental and theoretical studies are indicated.

Dipole Moment Enhancement in Molecular Crystals from X-ray Diffraction Data

Although it was proposed some time ago that (hyper)polarizabilities might be estimated from the results of x-ray charge density refinements, early results were unconvincing. In this work we show that the one particle density obtained from the usual multipole refinement model does not contain sufficient information to determine these response properties and instead pursue the “constrained wave function” approach of fitting to x-ray structure factors. Simplified sum-over-states expressions are derived for determining the dipole polarizability from these wave functions, and these clearly show that the earlier work ignored important two-electron expectation values for the dipole polarizability, and two- and three-electron terms for β, etc. Correction factors for the simplified sum-over-states polarizability tensors from the constrained wave function are obtained by calibration against coupled Hartree-Fock ab initio results to yield in-crystal effective polarizability tensors. Results obtained for benzene, ure...

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Effective molecular polarizabilities and crystal refractive indices estimated from x-ray diffraction data

The use of dipole lattice sums to estimate electric fields and dipole moment enhancement in molecular crystals

The study of the electron density and the determination of the dipolar moment of the molecule in the crystalline compound N-(4-nitrophenyl)-L-prolinol have been completed using X-ray and neutron diffraction. The deformation maps and the calculations of the atomic net charges demonstrate the character of the donor-acceptor couple (DA) linked to the phenyl transmitter. The dipolar molecular moments obtained by several methods have similar values, except the method of kappa refinement. The kappa refinement appears inadequate to solve the phase problem. Accurate knowledge of the first molecular moment reduces the ambiguity on the quadratic polarizability coefficient β, measured from electric field-induced second-harmonic generation (EFISHG).The coefficients of the cubic moment (third-order semi-invariant) of the electronic distribution in the crystal state are compared with those of the quadratic hyperpolarizability obtained by theoretical calculations. An estimate of relevant coefficients in octupolar-type material would provide guidance to the choice of systems for non-linear optics. Crystal data: C 11 H 14 N 2 O 3 , M r , = 222.2, monoclinic, P2 1 , Z = 2, F(000) = 236, T = 122 K, a = 5.152 (4), b = 14.790 (3), c = 7.134 (2) A, β = 106.14 (4)°, V = 522.18 A 3 , D x = 1.41 Mg mm -3 at 122 K, μ = 0.084 mm -1 for λ = 0.7107 A

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A. Guelzim

Papers

Electron density study by X-ray and neutron diffraction of an NLO compound: N-(4-nitrophenyl)-l-prolinol. Description of quadratic hyperpolarizability