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.