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

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• The photophysical behavior of carotenoids is complex, with the assignment of low-lying excited states, particularly the S* feature, remaining elusive.
• Understanding carotenoid excited states is crucial for interpreting their roles in photosynthesis and light harvesting.

Purpose of the Study:

• To elucidate the assignment of low-lying excited states in carotenoids and the origin of the S* feature.
• To investigate the dynamic interplay between the bright 1¹B_u⁺ and dark 2¹A_g^- states.
• To provide a new framework for interpreting experimental observations in carotenoid photophysics.

Main Methods:

• Employed mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) for electronic structure calculations.
• Utilized nonadiabatic molecular dynamics (NAMD) to simulate excited-state dynamics and internal conversion processes.
• Performed excited-state absorption (ESA) simulations to analyze transient spectral features.

Main Results:

• Demonstrated geometry-dependent reordering of the 1¹B_u⁺ and 2¹A_g^- states in polyenes and lutein.
• Revealed an ultrafast dynamic interconversion equilibrium between the 1¹B_u⁺ and 2¹A_g^- states, indicating their coexistence.
• Reassigned the origin of transient absorption bands, attributing the low-energy band to residual 1¹B_u⁺ population and the S* feature to the 2¹A_g^- state.

Conclusions:

• The study resolves puzzling experimental observations by reinterpreting the assignment of excited states and their dynamics.
• The established geometry-sensitive state ordering and coexistence model offer a new perspective on carotenoid photophysics.
• This work provides a refined framework for understanding fundamental photophysical processes in carotenoids and related systems.