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, μβ_vec/M_w, 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.