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Issue 20, 2013
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Two-photon absorption spectra of a near-infrared 2-azaazulene polymethine dye: solvation and ground-state symmetry breaking

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Abstract

Polymethine dyes (PDs) with absorption bands in the near-infrared region undergo symmetry breaking in polar solvents. To investigate how symmetry breaking affects nonlinear optical responses of PDs, an extensive and challenging experimental characterization of a cationic 2-azaazulene polymethine dye, including linear absorption, fluorescence, two-photon absorption and excited-state absorption, has been performed in two solvents with different polarity. Based on this extensive set of experimental data, a three-electronic-state model, accounting for the coupling of electronic degrees of freedom to molecular vibrations and polar solvation, has been reliably parameterized and validated for this dye, fully rationalizing optical spectra in terms of spectral position, intensities and bandshapes. In low-polarity solvents where the dye is mainly in its symmetric form, a nominally forbidden two-photon absorption band is observed, due to a vibronic activation mechanism. Inhomogeneous broadening plays a major role in polar solvents: absorption spectra represent the weighted sum of contributions from states with a variable amount of symmetry breaking, leading to a complex evolution of linear and nonlinear optical spectra with solvent polarity. In more polar solvents, the dominant role of the asymmetric form leads to the activation of two-photon absorption as a result of the symmetry lowering. The subtle interplay between the two mechanisms for two-photon absorption activation, vibronic coupling and polar solvation, can be fully accounted for within the proposed microscopic model allowing a detailed interpretation of the optical spectra of PDs.

Graphical abstract: Two-photon absorption spectra of a near-infrared 2-azaazulene polymethine dye: solvation and ground-state symmetry breaking

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Publication details

The article was received on 22 Feb 2013, accepted on 19 Mar 2013 and first published on 05 Apr 2013


Article type: Paper
DOI: 10.1039/C3CP50811K
Citation: Phys. Chem. Chem. Phys., 2013,15, 7666-7678
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    Two-photon absorption spectra of a near-infrared 2-azaazulene polymethine dye: solvation and ground-state symmetry breaking

    H. Hu, O. V. Przhonska, F. Terenziani, A. Painelli, D. Fishman, T. R. Ensley, M. Reichert, S. Webster, J. L. Bricks, A. D. Kachkovski, D. J. Hagan and E. W. Van Stryland, Phys. Chem. Chem. Phys., 2013, 15, 7666
    DOI: 10.1039/C3CP50811K

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