Unprecedented Large Hyperpolarizability of Twisted Chromophores in Polar Media

Twisted
intramolecular charge transfer (TICT) chromophores exhibit
uniquely large second-order optical nonlinearities (μβ). 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 μβ. 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°.
The most twisted, B2TMC-2, exhibits a large μβvec = −26,000 × 10–48 esu
(at 1907 nm) in dilute nonpolar CH2Cl2 solution,
which is maintained in polar DMF (μβvec= −20,370 × 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 μβ. We show that increasingly disrupted conjugation
is correlated with increased μβ even
at intermediate twist angles. The excellent performance and reduced
aggregation in polar solvents opens new avenues for bridging microscopic
and macroscopic chromophore performance.