Molecular Origins of the Nonlinear Optical Responses of a Series of α‑(X-2-Pyridylamino)‑<i>o</i>‑cresol Chromophores from Concerted X‑ray Diffraction, Hyper-Rayleigh Scattering, and <i>Ab Initio</i> Calculations

Faster transmission rates are desperately needed in the telecommunications industry, given that global demand for signal transmission already outstrips supply. New organic nonlinear optical (NLO) materials offer a prospective solution for the telecommunications industry given that they exhibit shorter optical response times compared to their inorganic counterparts. α-(X-2-Pyridylamino)-<i>o</i>-cresol-type motifs with their donor−π–acceptor (D−π–A) architecture and high level of π-conjugation are expected to afford high levels of intramolecular charge transfer (ICT). Such ICT phenomena will contribute to these types of molecules having faster optical response times and large (hyper)­polarizabilities. The linear and second-order NLO properties of three new α-(X-2-pyridylamino)-<i>o</i>-cresol motifs (X = 4-CH<sub>3</sub> (<b>1</b>); 5-Cl (<b>2</b>); 3,5-Cl (<b>3</b>)) were hence evaluated. NLO structure–function relationships were established in terms of their ICT characteristics and molecular hyperpolarizabilities (β). Given the typically large errors (10–30%) that are associated with determinations of β coefficients, three independent methods were used to enumerate β: (i) direct optical measurements using hyper-Rayleigh scattering, (ii) <i>ab initio</i> density functional theory (DFT) calculations, and (iii) X-ray wavefunction refinement (XWR). In the third case, a pseudoexperimental wavefunction of the compounds under investigation is afforded, from which its molecular properties are derived. The β values obtained from these three different methods are subsequently compared and assessed in terms of the effects of phase state on each chromophore on the NLO response. In terms of a materials-centered scientific interpretation, these β determinations reveal that compound <b>3</b> is the most promising candidate for use as an NLO chromophore owing to the presence of its withdrawing 3,5-chloro substituents. More generally, the correlation of the molecular structures of these α-(X-2-pyridylamino)-<i>o</i>-cresol chemicals to their linear and nonlinear optical properties extends the growing knowledge base of molecular design rules for organic NLO materials and specifically highlights the implications of ICT between two moieties in an NLO chromophore, causing a diminutive effect upon their NLO responses. This augments already well-established structure–property rules for the molecular engineering of NLO chromophores and points toward the ultimate goal of being able to tailor-make organic molecules to suit a given NLO device application.