Modular synthesis of zwitterionic, xanthene bridged, low twist angle chromophores with high hyperpolarizability†
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/Mw figure of merit (FOM) of 121 × 10−48 esu, where μ is the calculated dipole moment and Mw 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.