Molecular origins of the high-performance nonlinear optical susceptibility in a phenolic polyene chromophore: electron density distributions, hydrogen bonding, and ab initio calculations

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American Chemical Society
The molecular and supramolecular origins of the superior nonlinear optical (NLO) properties observed in the organic phenolic triene material, OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile), are presented. The molecular charge-transfer distribution is topographically mapped, demonstrating that a uniformly delocalized passive electronic medium facilitates the charge-transfer between the phenolic electron donor and the cyano electron acceptors which lie at opposite ends of the molecule. Its ability to act as a "push-pull" pi-conjugated molecule is quantified, relative to similar materials, by supporting empirical calculations; these include bond-length alternation and harmonic-oscillator stabilization energy (HOSE) tests. Such tests, together with frontier molecular orbital considerations, reveal that OH1 can exist readily in its aromatic (neutral) or quinoidal (charge-separated) state, thereby overcoming the "nonlinearity-thermal stability trade-off". The HOSE calculation also reveals a correlation between the quinoidal resonance contribution to the overall structure of OH1 and the UV-vis absorption peak wavelength in the wider family of configurationally locked polyene framework materials. Solid-state tensorial coefficients of the molecular dipole, polarizability, and the first hyperpolarizability for OH1 are derived from the first-, second-, and third-order electronic moments of the experimental charge-density distribution. The overall solid-state molecular dipole moment is compared with those from gas-phase calculations, revealing that crystal field effects are very significant in OH1. The solid-state hyperpolarizability derived from this charge-density study affords good agreement with gas-phase calculations as well as optical measurements based on hyper-Rayleigh scattering (HRS) and electric-field-induced second harmonic (EFISH) generation. This lends support to the further use of charge-density studies to calculate solid-state hyperpolarizability coefficients in other organic NLO materials. Finally, this charge-density study is also employed to provide an advanced classification of hydrogen bonds in OH1, which requires more stringent criteria than those from conventional structure analysis. As a result, only the strongest OH center dot center NC interaction is so classified as a true hydrogen bond. Indeed, it is this electrostatic interaction that influences the molecular charge transfer: the other four, weaker, nonbonded contacts nonetheless affect the crystal packing. Overall, the establishment of these structure?property relationships lays a blueprint for designing further, more NLO efficient, materials in this industrially leading organic family of compounds. © 2013, American Chemical Society.
Polyenes, Neutron diffraction, Scattering, Crystals, Electron density, Hydrogen, Aromatics
Lin, T. C., Cole, J. M., Higginbotham, A. P., Edwards, A. J., Piltz, R. O., Pérez-Moreno, J., Seo, J. Y., Lee, S. C., Clays, K., & Kwon, O. P. (2013). Molecular origins of the high-performance nonlinear optical susceptibility in a phenolic polyene chromophore: electron density distributions, hydrogen bonding, and ab Initio Calculations. Journal of Physical Chemistry C, 117(18), 9416-9430. doi:10.1021/jp400648q