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 1 1 B u + and dark 2 1 A g – states in both polyenes and lutein. Upon slight excited-state geometric relaxation, the 1 1 B u + state initially lies below the 2 1 A g – state. Subsequent BLA-driven internal conversion then drives the system toward the 2 1 A g – minimum region, where the energetic ordering is reversed and the 2 1 A g – state becomes lower than the 1 1 B u + state. NAMD trajectories of lutein further capture the ultrafast dynamic interconversion equilibrium between 1 1 B u + and 2 1 A 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 S 1 with the 2 1 A g – character, instead arises from a minor residual population with the 1 1 B u + character, whereas the weaker, higher energy band previously assigned to S* originates from the global-minimum 2 1 A g,min – structure and is dominated by the 2 1 A 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.