title: Two-photon absorption spectra of a near-infrared 2-azaazulene polymethine dye: solvation and ground-state symmetry breaking.
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.
authors.affiliation: CREOL & FPCE, The College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA.
authors.forename: Honghua
authors.initials: H
authors.lastname: Hu
authors.forename: Olga V
authors.initials: OV
authors.lastname: Przhonska
authors.forename: Francesca
authors.initials: F
authors.lastname: Terenziani
authors.forename: Anna
authors.initials: A
authors.lastname: Painelli
authors.forename: Dmitry
authors.initials: D
authors.lastname: Fishman
authors.forename: Trenton R
authors.initials: TR
authors.lastname: Ensley
authors.forename: Matthew
authors.initials: M
authors.lastname: Reichert
authors.forename: Scott
authors.initials: S
authors.lastname: Webster
authors.forename: Julia L
authors.initials: JL
authors.lastname: Bricks
authors.forename: Alexey D
authors.initials: AD
authors.lastname: Kachkovski
authors.forename: David J
authors.initials: DJ
authors.lastname: Hagan
authors.forename: Eric W
authors.initials: EW
authors.lastname: Van Stryland
mesh_terms: D001372:Aza Compounds / Q000737:chemistry*; D052176:Azulenes / Q000737:chemistry*; D005456:Fluorescent Dyes / Q000737:chemistry*; D007211:Indoles / Q000737:chemistry*; D015394:Molecular Structure; D017785:Photons; D011789:Quantum Theory; D012995:Solubility; D013057:Spectrum Analysis
chemical_list: D001372:Aza Compounds; D052176:Azulenes; D005456:Fluorescent Dyes; D007211:Indoles; C098209:VPM chloride