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7 March 2014 Two-photon solvatochromism of 4-dimethylamino-4'-nitrostilbene (DANS)
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Variation of two-photon absorption (2PA) of organic chromophores as a function of the environment such as different solvent polarity has important implications for practical applications of nonlinear optical (NLO) materials but could be also used as a sensitive probe of local inter- and intra-molecular interactions. We report measurement of the 2PA spectrum in the broad wavelength range 700-1000 nm of a benchmark NLO chromophore 4-Dimethylamino-4’-nitrostilbene (DANS) in a series of solvents with varying polarity by using a femtosecond nonlinear-optical transmission method with a nearly-collimated laser beam. The 2PA peak of DANS shifts systematically from 834 nm in a low dielectric constant solvent isobutyl isobutyrate (dielectric constant = 4) to 892 nm in a highly polar solvent DMSO (dielectric constant = 48), which quantitatively follows the corresponding solvatochromic shift of the S0→S1 transition peak in the linear absorption spectrum, and indicates that DANS has a significant ground state permanent electric dipole moment that interacts with the solvent environment. The corresponding change of the S0→S1 transition peak 2PA cross section in solvents with different polarity is quite large, ranging from 160 GM in isobutyl acetate to 240 GM in 1,3-Dioxolane. However, this variation exhibits no direct correlation with the solvent polarity. This effect may be tentatively attributed to the change of the excited state permanent electric dipole moment due to the instantaneous S0→S1 transition. Our results confirm that accurate measurement of 2PA spectra may serve as a probe of local molecular-level interactions.
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Geoffrey Wicks, Aleksander Rebane, and Mikhail Drobizhev "Two-photon solvatochromism of 4-dimethylamino-4'-nitrostilbene (DANS)", Proc. SPIE 8983, Organic Photonic Materials and Devices XVI, 89830R (7 March 2014);

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