The nature of excited-state absorption in polymethine and squarylium molecules

Abstract

Subpicosecond transient absorption measurements were performed for several polymethine and squarylium dyes in ethanol solution and a polymeric host over the spectral range 400-1500 nm. A variety of nonlinear effects including saturable absorption, reverse saturable absorption, and gain were observed and analyzed. We observe strong excited-state absorption (ESA) in all dyes in the range 450-600 nm. We also report the first prediction and observation of additional ESA bands in the near-infrared range. The predictions were based on quantum chemical calculations and the ESA experiments were performed with femtosecond pump-continuum probe techniques. For polymethine dye 2-[2-[3-[(1,3-dihydro-3,3-dimethyl-1-phenyl-2H-indol-2-ylidene) ethylidene]-2-phenyl-1-cyclohexen-1-yl]ethenyl]-3,3dimethyl-1-phenylindolium perchorate, an additional ESA band was detected near 1250 nm, and for squarylium dye 1,3-Bis-[(1,3-dihydro-1-butyl-3,3-dimetyl-2H-benzo[e]indol-2-ylidene)methyl]squaraine, two additional ESA bands were found around 870-and 1380-nm, respectively. To further study the nature of these transitions, the steady-state excitation anisotropy was also studied and compared with predictions. The relationship between ESA spectra of organic dyes and their molecular structure is discussed.

Comparison of nonlinear absorption in three similar dyes: Polymethine, squaraine and tetraone

“…4(2 0 ), are also in agreement with theoretical findings. We note that weak ESA bands around 1380 and 870 nm for SD 2243 in ethanol were experimentally observed in our earlier work in good agreement with theoretical predictions [15].…”

“…A description of this technique can be found in Refs. [15,16]. Briefly, the output from one of the independently tunable OPGs/OPAs is set to the absorption maximum wavelength for the pump.…”

Superfluorescent Squaraine with Efficient Two‐Photon Absorption and High Photostability

“…It can be assumed from these data and the corresponding 2PA spectra that primarily the central squaraine core and two phenyl rings of 1 determine the characteristics of its electronic structure. The second transient absorption band with a wavelength of approximately 600 – 720 nm with negative Δ D (2) is adequately strong and determined primarily by optical gain processes in 1 49. The phenomenon of saturable absorption50 is also possible in this spectral range, but the corresponding estimated values of saturable absorption were much smaller than the experimentally observed negative Δ D .…”

The Solitonic Nature of the Electronic Structure of the Ions of Linear Conjugated Systems

“…This was confirmed by appropriate quantum-chemical modelling [58,158]. The possible destruction of the symmetry and the simultaneous existence of two components, symmetrical and unsymmetrical, are also indicated by the splitting of the bands in the reabsorption spectra, corresponding to electronic transitions from the excited state (transient absorption spectra) [161][162][163][164], the destruction of the perpendicularity of polarization of the highest electronic transitions during measurement of the anisotropy in the fluorescence excitation spectra [165], or the generation of a second harmonic by solutions of symmetrical thiacyanines [158]. Information on the electronic structure of conjugated molecules in the excited state and its dependence on the molecular topology (chain length, evenness of the number of p centers, the nature of the terminal groups) is necessary during investigation of the generation characteristics of conjugated compounds and, primarily, of ionic polymethine dyes [140,141,166].…”