Spectroscopic Exploration of Squaraine Dyes: Molecular Characterization of Fundamental, Combination, and Overtone Bands

A comprehensive vibrational analysis of squaraine dyes, a relevant class of molecules for dye-sensitized solar cell devices, is presented here. Exploiting density functional theory (DFT) in conjunction with second-order vibrational perturbation theory (VPT2), fundamental, overtone, and combination vibrational bands are computed and analyzed, comparing them directly to experimental infrared and Raman spectra. Our results unequivocally demonstrate that VPT2 calculations are mandatory for accurately interpreting the experiments, particularly in the 1100-1650 cm^-1 region, where anharmonic effects such as frequency shifts, intensity redistribution, and mode couplings are most prominent. Only going beyond harmonic treatment, we were able to undoubtedly identify peculiar vibrational features among symmetric <i>N</i>,<i>N</i>-disubstituted squaraines and highlight the critical role of low-frequency modes and intramolecular hydrogen-bonding dynamics. These findings provide a refined framework for interpreting coherent vibrational phenomena in squaraine-based molecular systems, offering a transferable computational approach for the spectroscopic characterization of functional chromophores in energy and photonic applications.