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.

本文报道了一系列供体-受体聚亚芳基发色团（donor–acceptor poly-arylene chromophore）的系统合成、结构表征、光学光谱表征及二阶非线性光学（second-order nonlinear optical, NLO）表征；该类发色团具备此前尚未实现的π扩展程度与显著偏离平面的扭曲结构。具体而言，包含双环结构的2TTMC，即二氰基(4-(3,5-二甲基-1-(2-丙基庚基)吡啶-1-鎓-4-基)-3-甲基苯基)甲醇化物；三环结构的3TTMC，即二氰基(4′-(3,5-二甲基-1-(2-丙基庚基)吡啶-1-鎓-4-基)-2,2′,3′,5′,6′-五甲基-[1,1′-联苯基]-4-基)甲醇化物；以及四环结构的4TTMC，即二氰基(4″-(3,5-二甲基-1-(2-丙基庚基)吡啶-1-鎓-4-基)-2,2′,3″,6,6′-五甲基-[1,1′:4′,1″-三联苯基]-4-基)甲醇化物。通过单晶X射线衍射（single-crystal X-ray diffraction）、密度泛函理论（Density Functional Theory, DFT）优化的几何结构以及B3LYP/INDO-SOS分析，明确了超大NLO响应背后的三个关键特征：（1）当环连接数达到3个及以上时，空间位阻诱导的π体系扭曲仅需在发色团的供体与受体位点附近存在，即可保证获得大的NLO响应；（2）为提升合成效率，从o,o′,o″,o‴-四甲基联芳基连接位点移除一个邻位甲基，仅会将联芳基扭转角从89.6°轻微松弛至约80°；（3）亚芳基环连接数从2增加至3再到4（2TTMC→3TTMC→4TTMC），可显著增强NLO响应与两性离子电荷定域化程度，进而增大基态偶极矩，这与固态中收缩的反平行π-π堆积距离（分别为8.665 →7.883 →7.361 Å）相符。上述结果佐证了该类发色团存在两性离子基态，这一点也得到了显著的光学溶剂化变色位移的支持；同时芳基-芳基扭转会激活显著的片段内吸收过程。计算得到的分子超极化率（molecular hyperpolarizability，μβ）可达前所未有的900 000 ×10–48 esu，而估算的发色团品质因数（figure of merit）μβvec/Mw可达1500 ×10–48 esu，较已知扭曲发色团的最高值高出1.5倍，较平面供体-受体发色团高出33倍以上。