Impact of Ion‐Pairing Effects on Linear and Nonlinear Photophysical Properties of Polymethine Dyes**

Abstract

The two-photon absorption (2PA) and photophysics of heptamethine dyes featuring cyanine or dipolar electronic structures have been compared for the first time. The perfectly delocalized cyanine system is classically characterized by a two-photon transition matching the vibronic component of its lower energy absorption band. The dipolar species is generated by ion-pairing with a hard counterion in a non-dissociating solvent and displays significant modifications oft he optical properties, including a significant hypochromic shift of absorption, weaker emission and 2PA matching the lower energy transition, thus revealing symmetry breaking within the polymethine electronic structure.

Near-infrared dyes for two-photon absorption in the short-wavelength infrared: strategies towards optical power limiting

“…[109][110][111][112] Note that several parameters are also able to promote the stabilization of blue-shifted polyene-like (i.e. dipolar) structures, including the solvent polarity, 113,114 ionpairing effects [115][116][117][118][119] or substitution of electron-donating moieties at the centre of the polymethine chain. [120][121][122] The design of polymethines is a foolproof strategy to reach NIR absorption with strong extinction coefficients, thus the chemistry and optical properties of polymethines has thrilled scientists for decades.…”

“…The impact of ion-pairing occurring in nonpolar solvent (toluene) between a polymethine analogue of 83 and a small bromine anion was recently investigated and reveals a slight blue-shift of the 2PA maximum and a lower σ2PA value with 310 GM at 1400 nm, mainly due to the loss of the cyanine electronic structure for a dipolar polyene-like one. 119 Dissymmetrical heptamethine 84 exhibits a profound modification of its electronic structure from cyanine to dipole upon decreasing the solvent polarity. This equilibrium implies a broadening of the 1PA spectrum, thus this merocyanine shows a broad 2PA from 1200 to 1700 nm with a maximum of 790 GM at 1445 nm.…”

A Peierls Transition in Long Polymethine Molecular Wires: Evolution of Molecular Geometry and Single-Molecule Conductance

“…Thus, A and B become degenerate bond-shift tautomers rather than resonance structures. In cationic cyanines, this symmetry-breaking transition is sensitive to the nature of the counterion, , the polarity of the medium, , and the structures of the end groups, , and it is associated with dramatic changes in the absorption spectra, − molecular polarizability, and nonlinear optical properties. − …”

Counterion Pairing Effects on a Flavylium Heptamethine Dye†

“…In DCM, indolenine cyanine dyes showed similar absorption spectra regardless of counterion (Br vs B(C 6 F 5 ) 4 ), similar to what was observed for Flav7. Changing the solvent to PhMe showed broadened and weaker absorption spectra for the small counterion Br (6) (39). On the other hand, the larger counterion (B(C 6 F 5 ) 4 ), showed no broadening of absorption spectra or major changes in photophysical properties.…”

“…The ion-pair relationship can be explored by observing whether anion exchange has an impact on symmetry breaking. In indolenine-based cyanine dyes, the relationship between anion and dye scaffold is crucial to whether fluorophores maintain a symmetric, fully delocalized state (27,39,41). For example, in nonpolar solvents where dye and anion are more closely associated, anion size has been shown to impact the symmetry of polymethine dyes as observed by a broadened absorption spectra, larger full width half maxima (FWHM), decreased Φ F and ε (27).…”