Ultrafast Photophysics of Regioisomeric Triphenylamine Dyes Having Dipolar D-π-A-A and Octupolar D-(π-A-A)3 Character with a Benzothiazole Unit in Matched or Mismatched Orientation.

Benzothiazole is among prominent electron-withdrawing heteroarene moieties used in a variety of π-conjugated molecules. Its relative orientation with respect to the principal dipole vector(s) of chromophores derived thereof is crucial, affecting photophysical and nonlinear optical properties. Here we compare the photophysics and ultrafast dynamics of dipolar and octupolar molecules comprising a triphenylamine electron-donating core, ethynylene π-conjugated linker(s) and benzothiazole acceptor(s) having the matched or mismatched orientation (with respect to the direction of intramolecular charge transfer), while a carbaldehyde group is attached as an auxiliary acceptor. Among chromophores without the auxiliary acceptor, stronger fluorescence solvatochromism and faster excited state dynamics are exhibited for the derivatives with the mismatched geometry. On the contrary, introduction of the auxiliary acceptor to the benzothiazole unit enhances the intramolecular charge transfer ICT (featuring ultrafast dynamics of the excited state) for the matched geometry. The data confirm the crucial role of the relative orientation of asymmetric heteroaromatic unit (regioisomeric effect) in dipolar as well as in multipolar molecules in tuning linear and nonlinear optical properties as well as excited state dynamics.

Octupolar molecules for nonlinear optics: from molecular design to crystals and films with large second-harmonic generation.

We summarize the nonlinear optical (NLO) properties of octupolar molecules, crystals, and films developed in our laboratory. We present the design strategy, structure-property relationship, and second-order NLO properties of 1,3,5-trinitro- and 1,3,5-tricyano-2,4,6-tris(p-diethylaminostyryl)benzene (TTB) derivatives, TTB crystals, and films prepared by free-casting TTB in poly(methyl methacrylate) (PMMA). The first hyperpolarizability of TTB was fivefold larger than that of the dipolar analogue. Moreover, the TTB crystal showed unprecedentedly large second-harmonic generation (SHG). While TTB crystal films (20 wt% TTB/PMMA) on various substrates showed appreciable SHG values, the cylinder film exhibited much larger SHG values and large electro-optic (EO) coefficients. The large SHG values and EO coefficients, as well as the high thermal stability of the cylinder film, will make it a potential candidate for NLO device applications.

l-Histidinium thiocyanurate: Experimental and theoretical studies of a new nonlinear optical material.

A new organic compound, l-histidinium thiocyanurate thiocyanuric acid dihydrate, has been synthesized and characterized by single crystal X-ray diffraction, infrared spectroscopy and nonlinear optical measurements. The efficiency of the second-harmonic generation was evaluated with the Kurtz and Perry powder method at a fundamental wavelength of 1064nm. By using the experimental structure, the molecular first hyperpolarizability tensor was determined with Hartree-Fock and density functional theory methods. The second-order susceptibility tensor of the crystal was evaluated using the oriented gas model with the Lorenz-Lorentz and the Wortmann-Bishop local-field corrections.

Structural and nonlinear optical studies of a salt with an octupolar chromophore: Guanidinium cyclopropanecarboxylate.

A new organic compound, guanidinium cyclopropanecarboxylate, has been synthesized and characterized by single crystal X-ray diffraction, infrared spectroscopy and nonlinear optical measurements. The infrared spectrum was calculated with density functional theory (DFT). The second-order NLO response was evaluated with the Kurtz and Perry powder method. From the molecular structure, the molecular hyperpolarizability tensor was determined with Hartree-Fock and DFT methods. The second-order susceptibility tensor of the crystal was evaluated by the summation of the effective hyperpolarizability tensors calculated for the asymmetric unit surrounded by ESP-derived charges.