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.

Boron Difluoride Curcuminoid Fluorophores with Enhanced Two‐Photon Excited Fluorescence Emission and Versatile Living‐Cell Imaging Properties

“…Indeed, the more TP‐active transition is the S 0 –S 2 transition due to its stronger CT character compared to the cyanine‐like S 0 –S 1 transition. However, and similarly to the polymethine derivatives and to the curcumin 8 ‐H, the lowest‐energy transition is not completely TPA forbidden and a weak σ TPA value of approximately 10–40 GM in that wavelength range was measured for the dyes 5 ‐BF 2 , 6 ‐BF 2 , and 7 ‐BF 2 in dichloromethane. Those low‐energy TPA transitions could not be observed in compound 1 ‐BF 2 , 2 ‐BF 2 , 3 ‐BF 2 , and 4 ‐BF 2 because the OPA is too shifted in the red region to be monitored with our TPEF setup…”

Modeling Infrared Spectral Dynamics upon Symmetry Breaking of a Photo-Excited Quadrupolar Dye

“…This model could reproduce the effects of ES-SB on one-and two-photon absorption spectra in quadrupolar and octupolar donor-acceptor chromophores. [39][40][41][42]48,49 More recently, a two-level model describing the effect of SB on the vibrational spectrum of a quadrupolar dye with an exact solution has been derived. 50 This model was able to successfully reproduce the IR absorp-tion spectrum in the −C≡N stretching region of a A-π-D-π-A dye after ES-SB in dipolar solvents of varying dielectric constant.…”

Unsymmetrical Relaxation Paths of the Excited States in Cyanine Dyes Detected by Time-Resolved Fluorescence: Polymethinic and Polyenic Forms

“…7,8 Comprehensive spectral and quantum-chemical investigations have helped to establish that this phenomenon is associated with coexisting of two different forms of the dye having the long chain: with symmetrical and unsymmetrical charge distributions along the polymethine chain. 4,[7][8][9][10][11][12][13] Such mobility of the electronic charge distribution has been explained by a soliton conception. [7][8][9][10][11][12][13][14][15][16][17] According to this conception, the charge in the collective -electron systems produces a solitonic wave of partial charges, so that the charges at neighboring carbon atoms of the polymethine chain alternate considerably.…”