Signatures of a Conical Intersection in Two-Dimensional Spectra of a Red-Absorbing Squaraine Dye

Squaraine dyes are promising for dye-sensitized solar cells (DSSCs) due to their strong absorption in the red and near-infrared regions. However, their ultrafast photophysical behavior remains poorly understood. Using two-dimensional electronic spectroscopy (2DES) and ab initio computations, we reveal signatures of a conical intersection (CI) governing excited-state dynamics in a prototypical squaraine dye. This sub-200 fs decay pathway may hinder squaraine performance as dye-sensitizers. Our results provide experimental evidence of a sloped CI landscape and elucidate the role of at least two vibrational modes strongly coupled to electronic degrees of freedom. The passage through a CI emerges as a key relaxation mechanism, shedding light on the complex photophysics of these dyes.