Geometry-Sensitive State Ordering and Internal Conversion Equilibrium of Polyenes and Carotenoids

The photophysics of carotenoids has long been obscured by the elusive assignment of their low-lying excited states, particularly the origin of the so-called S* feature. Using mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) combined with nonadiabatic molecular dynamics (NAMD), we uncover geometry-dependent reordering of the bright 1¹B_u⁺ and dark 2¹A_g^- states in both polyenes and lutein. Upon slight excited-state geometric relaxation, the 1¹B_u⁺ state initially lies below the 2¹A_g^- state. Subsequent BLA-driven internal conversion then drives the system toward the 2¹A_g^- minimum region, where the energetic ordering is reversed and the 2¹A_g^- state becomes lower than the 1¹B_u⁺ state. NAMD trajectories of lutein further capture the ultrafast dynamic interconversion equilibrium between 1¹B_u⁺ and 2¹A_g^-, indicating their coexistence during the early stages of photoexcitation. Within this framework, excited-state absorption (ESA) simulations of lutein indicate that the intense low-energy transient band, traditionally assigned to S₁ with the 2¹A_g^- character, instead arises from a minor residual population with the 1¹B_u⁺ character, whereas the weaker, higher energy band previously assigned to S* originates from the global-minimum 2¹A_g,min^- structure and is dominated by the 2¹A_g^- character. This reinterpretation naturally resolves several puzzling experimental observations. Thus, the geometry-sensitive state ordering and the coexistence model established through internal conversion equilibrium open a new avenue for understanding the fundamental features of these systems and provide a fresh framework for interpreting experimental observations.