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

Abstract

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.

Impact of position of electron withdrawing cyano groups on nonlinear optical properties of centrosymmetric donor‐π‐acceptor system

“…In such (D‐A‐π‐A‐D) molecules TPA cross‐sections σ 2 = 890 GM at 790 nm (1.57 eV) for the dibutylamino‐terminated and σ 2 = 730 GM at 825 nm (1.50 eV) for the corresponding biphenylamino‐terminated α‐CN‐2PV were obtained . To gain nevertheless access to the TPA properties of both compounds and to relate them to the findings of Pond et al, we performed quantum chemical calculations that were shown earlier to reliably predict the trends among 2 substituted pheneylene vinylene chromophores . The calculated TPA cross‐sections and the associated excited state energies are given in Table .…”

Design and preparation of novel Diels–Alder crosslinking polymer and its application in NLO materials

“…So, researchers have paid more and more attentions to organic EO materials in the past decades. And a large amount of novel organic EO materials with excellent EO activity has been emerged [6][7][8][9][10]. Else, the development of novel EO devices including organic EO modulator, optical switch, terahertz detectors and so on is also very fast.…”

Novel poly(aryl ether ketone) with electro-optic chromophore side chains for light modulators

“…This resulted in better film-forming ability, higher electro-optics coefficients and better long-term stability. Quantum chemical simulations of the second order polymers have been reported in the past two decades, it is very difficult to find organic second order nonlinear optical materials that have both large electro optic coefficients and long-term stability [13][14][15][16][17][18]. Due to the large EO coefficients of about 42-78 pm/V (all the EO parameters are given at wavelength 1.3 µm) at low chromophore loading density (10 wt%), chromophore CLDs have been widely studied, but their thermal decomposition temperature is very low (~180 °C) [53].…”