Time-resolved two-photon spectroscopy of carotenoids

Carotenoid radical formation after multi-photon excitation of 8'-apo-β-carotenal

Carotenoids containing a conjugated CO group exhibit complex excited-state dynamics that are influenced by solvent polarity due to the involvement of an intramolecular charge transfer (ICT) state. Our study explores the excited-state behavior of 8'-apo-β-carotenal under multi-photon excitation conditions. Using near-infrared (1300 nm) multi-photon excitation, we observe the formation of a cation radical of 8'-apo-β-carotenal, a process distinct from those following one-photon visible or UV excitation. Our findings suggest that this radical formation results from multi-photon excitation involving a higher-lying dark state, supported by intensity-dependent experiments. This work demonstrates that radical generation is a characteristic of this higher excited state and is not produced during relaxation from the S1/ICT state. The results open new pathways for understanding carotenoid radical formation mechanisms under intense multi-photon excitation.

Understanding Carotenoid Dynamics via the Vibronic Energy Relaxation Approach

Carotenoids are an integral part of natural photosynthetic complexes, with tasks ranging from light harvesting to photoprotection. Their underlying energy deactivation network of optically dark and bright excited states is extremely efficient: after excitation of light with up to 2.5 eV of photon energy, the system relaxes back to ground state on a time scale of a few picoseconds. In this article, we summarize how a model based on the vibrational energy relaxation approach (VERA) explains the main characteristics of relaxation dynamics after one-photon excitation with special emphasis on the so-called S* state. Lineshapes after two-photon excitation are beyond the current model of VERA. We outline this future line of research in our article. In terms of experimental method development, we discuss which techniques are needed to better describe energy dissipation effects in carotenoids and within the first solvation shell.

Transient Absorption of Chlorophylls and Carotenoids after Two-Photon Excitation of LHCII

Femtosecond transient absorption spectroscopy following two-photon excitation (2PE) is used to determine the contributions of carotenoids and chlorophylls to the 2PE signals in the main plant light-harvesting complex (LHCII). For 2PE, excitation at 1210 and 1300 nm was used, being within the known 2PE profile of LHCII. At both excitation wavelengths, the transient absorption spectra exhibit a shape characteristic of excited chlorophylls with only a minor contribution from carotenoids. We compare the 2PE data measured for LHCII with those obtained from 2PE of a lutein/chlorophyll a mixture in acetone. We estimate that although the 2PE cross section of a single carotenoid in acetone is ∼1.7 times larger than that of a Chl a, due to the 1:3.5 carotenoid/Chl ratio in LHCII, only one-third of the absorbed 2PE photons excite carotenoids in LHCII in the 1200-1300 nm range.