Twisted pi-electron system electrooptic chromophores : structural and electronic consequences of relaxing twist-inducing nonbonded repulsions

The synthesis, structural and spectroscopic characterization, and nonlinear optical response properties of a "slightly" twisted zwitterionic 4-quinopyran electrooptic chromophore FMC, 2-{4-[1-(2-propylheptyl)-1H-pyridine- 4-ylidene]cyclohexa-2,5-dienylidene}malononitrile, are reported. X-ray diffraction data and density functional theory (DFT) minimized geometries confirm that deletion of the four o-, o′-, o″-, and o‴-methyl groups in the parent chromophore TMC-2, 2-{4-[3,5-dimethyl-1-(2-propylheptyl)- 1H-pyridin-4-ylidene]-3,5-dimethylcyclohexa-2,5-dienylidene}malononitrile, relaxes the arene-arene twist angle from 89.6 to 9.0°. These geometrical changes result in a significantly increased contribution of the quinoidal structure to the molecular ground state of FMC (versus TMC-2), reduced solvatochromic shifts in the optical spectra, and a diminished electric-field-induced second-harmonic (EFISH) generation derived molecular hyperpolarizability (μβ = -2340 × 10-48 esu of DFMC, the dendrimer derivative of FMC, vs -24000 × 10-48 esu of TMC-2) in CH2Cl2 at 1907 nm. Pulsed field gradient spin-echo (PGSE) NMR spectroscopy and EFISH indicate that the levels of FMC aggregation in solution are comparable to those of TMC-2 (monomers and dimers) in CH2Cl2 solution. B3LYP and INDO/S computation of chromophore molecular structure, aggregation, and hyperpolarizability trends are in good agreement with experiment.