ABSTRACT
This chapter aims to provide an overview on the spectroscopic properties and photophysics of both symmetrical and unsymmetrical squaraines. It highlights the effects of substituent, solvent, and ambient temperature on these properties. The perturbation enhances vibronic coupling, leading to vibronic fine structures in both absorption and emission spectra. Vibronic couplings are enhanced and vibrational fine structures are observable in both absorption and fluorescence spectra. In toluene, the fluorescence emissions from the squaraine and the complex can be isolated by carefully choosing the measuring wavelengths. The lifetime of the γ-emission is solvent-sensitive also, indicating that solvent molecules do play a role in the radiationless decay process. In Tetrahydrofuran, the lifetimes of the β- and γ-emissions are insensitive to the ambient temperature. The spectral data so far indicate that the spectral characteristics of unsymmetrical squaraines are different from those of symmetrical and pseudo-unsymmetrical squaraines in terms of the origin of the multiple emissions.
