Critical Role of Non-classical Intermolecular Hydrogen Bonding in Affecting the π–π Stacking and Nonlinear Optical Properties of Tricyanofuran-Based Push–Pull Heptamethines

Currently, there is immense interest in the chemistry of push–pull heptamethines containing a strong acceptor of 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF). This class of compounds is important as second-order nonlinear optical (NLO) chromophores for electro-optic (EO) applications. The crystal packing of TCF-based NLO chromophores is generally dominated by intermolecular π–π stacking and non-classical C–H···N(cyano) hydrogen bonding interactions, considerably limiting the bulk NLO properties of chromophores in solid films. In this paper, we report a series of TCF-based push–pull heptamethines bearing the substituent of 4,5-diphenyl-oxazol-2-ylsulfanyl group at the central position of the π-conjugated bridge. Crystallographic studies show a very uncommon crystal packing mode of one-dimensional chain structure with vanished C–H···N(cyano) contacts and negligible π–π stacking. Such reduced intermolecular interactions and fine-tuned bond length alternation lead to NLO performance breakthrough of these chromophores in poled polymeric films, including much better EO coefficients up to 465% in improvement, higher polar order, and larger molecular hyperpolarizabilities (up to 4,135 × 10^–30 esu at 1304 nm), over the results from the simple chloro-substituted heptamethines. Our study indicates that the interplay of steric effect, non-classical C–H···N hydrogen bonding, and intermolecular π–π stacking is critically important in optimizing the supramolecular assembly and NLO properties of TCF-based heptamethines for photonic applications.