Vibrational signature of 1Bu+ and hot 2Ag– excited states of carotenoids revisited by femtosecond stimulated Raman spectroscopy

Abstract

The significance of carotenoids in biological systems cannot be overstated. Their functionality largely arises from unique excited-state dynamics, where photon absorption promotes the molecule to the optically allowed 1Bu+ state (conventionally S₂), which rapidly decays to the optically forbidden 2Ag– state (S₁). While the vibrational signature of the S₁ state is well established, that of the initial S₂ state has remained elusive. In this work, we identify a consistent S₂-state vibrational signature across five different carotenoids and explain the challenges in isolating this signature. Resonance conditions in the near infrared region cause the S₂ signal to appear as a complex superposition of contributions from stimulated emission to the ground state, excited-state absorption (S₂ to Sn), and vibrationally inverted S₁ signals. This results in a mix of positive and negative features that are not trivial to disentangle. After careful analysis, we isolated the genuine S₂ signature, characterized by a C=C stretch near 1600 cm⁻¹ and a C–C stretch near 1350 cm⁻¹. The remaining signals originate from the ground or S₁ states but are virtually inseparable from the FSRS signal on S₂-state timescales. Our findings resolve a long-standing spectroscopic challenge and clarify the early excited-state dynamics of carotenoids.