Molecular Origins of the High-Performance Nonlinear Optical Susceptibility in a Phenolic Polyene Chromophore: Electron Density Distributions, Hydrogen Bonding, and ab Initio Calculations

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”
π-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···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.