Modular synthesis of zwitterionic, xanthene bridged, low twist angle chromophores with high hyperpolarizability

Abstract

Push–pull chromophores exhibit useful electro-optic (EO) properties that have the potential to significantly improve non-linear EO applications ranging from telecommunications to quantum informatics. Compounds with large hyperpolarizability values (β) that are stable and have easily modifiable functional groups are needed to advance organic-based EO systems. In this report, theoretical and experimental methods were used to investigate the design, synthesis, and application of a new class of push–pull chromophores for their use as a guest in guest–host EO polymer composites intended for non-linear EO applications. The chromophores combine the known intramolecular charge transfer benefits of increasing electron localization among π-conjugated zwitterionic structures with the aim of enhancing their β without the necessity of large dihedral angles. The class of chromophores reported here include a xanthene unit between a dicyanomethanide donor and pyridinium accepter. This arrangement increases charge separation due to the central sp³ hybridization and enables modular aliphatic substitutions while maintaining concise, high yielding synthetic steps. Chromophore design selection was guided by high-throughput quantum chemical Density Functional Theory calculations to predict β and dipole moment (μ) a priori. Out of 230 molecular candidates considered, we present the results for three chromophores: dicyano(9,9-dibutyl-7-(1-(2-ethylhexyl)pyridin-1-ium-4-yl)-9H-xanthen-2-yl)methanide (E1), dicyano(4-(9,9-dibutyl-7-(1-2-ethylhexyl)pyridine-1-ium-4-yl)-9H-xanthen-2-yl)methanide (E2a), and (4-(9,9-bis(2-ethylhexyl)-7-(1-(2-ethylhexyl)pyridine-1-ium-4-yl)-9H-xanthen-2-yl)phenyl)dicyanomethanide (E2b). While bulk EO coefficients were low (<2 pm V⁻¹), perhaps due to strong aggregation, the results demonstrate exceptional experimental Hyper Raleigh Scattering (HRS) β values. Eb2, for example has an HRS β value of 1650 ± 150 × 10⁻³⁰ esu at 900 nm in acetonitrile, which corresponds to μβ of 86 000 × 10⁻⁴⁸ esu and a μβ/M_w figure of merit (FOM) of 121 × 10⁻⁴⁸ esu, where μ is the calculated dipole moment and M_w is the chromophore's molecular weight. The chromophores are moderately stable in ambient conditions, their size and shape are easily modifiable, and they exhibit strong negative solvatochromism with solvent polarity.