Vibrational signature of 11B +u and hot 21A −g 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 1¹B⁺_u state (conventionally S₂), which rapidly decays to the optically forbidden 2¹A⁻_g 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 S_m), 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 CC 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.