Ultra-High-Response, Multiply Twisted Electro-optic Chromophores: Influence of π‑System Elongation and Interplanar Torsion on Hyperpolarizability

The systematic synthesis, structural, optical spectroscopic, and second-order nonlinear optical (NLO) characterization of a series of donor–acceptor poly-arylene chromo­phores which have heretofore unachieved π-extension and substantial twisting from planarity, are reported: specifically, two-ring 2TTMC, dicyano­(4-(3,5-dimethyl-1-(2-propyl­heptyl)­pyridin-1-ium-4-yl)-3-methyl­phenyl)­methanide; three-ring 3TTMC, dicyano­(4′-(3,5-dimethyl-1-(2-propyl­heptyl)­pyridin-1-ium-4-yl)-2,2′,3′,5′,6′-penta­methyl­[1,1′-biphenyl]-4-yl)­methanide; and four-ring 4TTMC, dicyano­(4″-(3,5-dimethyl-1-(2-propyl­heptyl)­pyridin-1-ium-4-yl)-2,2′,3″,6,6′-penta­methyl­[1,1′:4′,1″-terphenyl]-4-yl)­methanide. Single-crystal X-ray diffraction, DFT-optimized geometries, and B3LYP/INDO-SOS analysis identify three key features underlying the very large NLO response: (1) For ring catenation of three or greater, sterically enforced π-system twists are only essential near the chromo­phore donor and acceptor sites to ensure large NLO responses. (2) For synthetic efficiency, deletion of one ortho-methyl group from o,o′,o″,o‴-tetra­methyl­biaryl junctures, only slightly relaxes the biaryl twist angle from 89.6° to ∼80°. (3) Increased arylene catenation from two to three to four rings (2TTMC→ 3TTMC → 4TTMC) greatly enhances NLO response, zwitterionic charge localization, and thus the ground-state dipole moment, consistent with the contracted antiparallel solid-state π–π stacking distances of 8.665 → 7.883 → 7.361 Å, respectively. This supports zwitterionic ground states in these chromo­phores as do significant optical spectroscopic solvato­chromic shifts, with aryl–aryl twisting turning on significant intra-subfragment absorption. Computed molecular hyper­polariz­abilities (μβ) approach an unprecedented 900 000 × 10–48 esu, while estimated chromo­phore figures of merit, μβvec/Mw, approach 1500 × 10–48 esu, 1.5 times larger than the highest known values for twisted chromo­phores and >33 times larger than that of planar donor–acceptor chromo­phores.