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

Squarainedyes arepromising for dye-sensitizedsolar cells (DSSCs)due to their strong absorption in the redandnearinfrared regions. However, their ultrafast photophysical behavior remains poorlyunderstood.Using two-dimensional electronic spectroscopy(2DES)andabinitiocomputations,werevealsignaturesofa conical intersection (CI) governing excited-state dynamics in a prototypicalsquarainedye.Thissub-200fsdecaypathwaymayhinder squaraine performance as dye-sensitizers. Our results provide experimental evidence of a slopedCI landscape and elucidate the roleof at least twovibrationalmodes stronglycoupledtoelectronic degrees of freedom. The passage through aCI emerges as a key relaxationmechanism, sheddinglightonthecomplexphotophysicsof thesedyes.