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

## TL;DR

This paper uses advanced computational methods to study the vibrational properties of squaraine dyes, important for solar cell applications.

## Contribution

The study introduces a refined computational framework using VPT2 for accurate interpretation of vibrational spectra in squaraine dyes.

## Key findings

- VPT2 calculations are essential for interpreting vibrational bands in the 1100–1650 cm–1 region of squaraine dyes.
- Low-frequency modes and hydrogen-bonding dynamics play a critical role in vibrational features of symmetric squaraines.
- The approach offers a transferable method for spectroscopic analysis of functional chromophores in energy applications.

## Abstract

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 N,N-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.