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 electrooptic (EO) applications. The crystal packing of TCF-based NLO chromophores is generally dominated by intermolecular pi-pi 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 it-conjugated bridge. Crystallographic studies show a very uncommon crystal packing mode of one-dimensional chain structure with vanished C-H center dot center dot center dot N(cyano) contacts and negligible pi-pi 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 X 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 center dot center dot center dot N hydrogen bonding, and intermolecular pi-pi stacking is critically important in optimizing the supramolecular assembly and NLO properties of TCF-based heptamethines for photonic applications.

Automated summary

This study introduces a novel class of push-pull heptamethines based on TCF, which exhibit unique crystal packing and NLO properties. The optimized supramolecular assembly of these compounds, through a combination of steric effects, non-classical C-H center dot center dot center dot N hydrogen bonding, and intermolecular pi-pi stacking, leads to significantly improved EO coefficients and larger molecular hyperpolarizabilities.