Journal of the American Chemical Society, Vol.140, No.28, 8746-8755, 2018
Unprecedented Large Hyperpolarizability of Twisted Chromophores in Polar Media
Twisted intramolecular charge transfer (TICT) chromophores exhibit uniquely large second-order optical non-linearities (mu beta). However, their promise as electro-optic (E-O) materials is yet untapped, reflecting a strong tendency to aggregate in low-polarity media, leading to a dramatic fall in mu beta. Until now, TICT chromophores in deaggregating polar solvents suffered decreased response due to polarity-driven changes in electronic structure. Here we report a new series of benzimidazolium-based TICT chromophores with interaryl torsional angles in the range of 64-77 degrees. The most twisted, B2TMC-2, exhibits a large mu beta(vec) = -26,000 X 10(-48) esu (at 1907 nm) in dilute nonpolar CH2Cl2 solution, which is maintained in polar DMF (mu beta(vec) = -20,370 X 10(-48) esu) as measured by DC electric field-induced second harmonic generation (EFISH). Sterically enforced interaryl torsional angles are confirmed by single-crystal X-ray diffraction and solution phase Nuclear Overhauser Effect (NOE) NMR, and spectroscopic characterization reveals a zwitterionic/aromatic ground state electronic structure associated with the high mu beta. We show that increasingly disrupted conjugation is correlated with increased mu beta even at intermediate twist angles. The excellent performance and reduced aggregation in polar solvents opens new avenues for bridging microscopic and macroscopic chromophore performance.