Femtosecond excited-state dynamics in chlorosomal carotenoids of the photosynthetic bacterium revealed by near infrared pump-probe spectroscopy

• Published in: Physical chemistry chemical physics : PCCP 2021-06, Vol.23 (22), p.12761-1277
• Main Authors: Yakovlev, Andrei G, Taisova, Alexandra S, Fetisova, Zoya G
• Format: Article
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d1cp00927c

In photosynthetic green bacteria, chlorosomes provide light harvesting with high efficiency. Chlorosomal carotenoids (Cars) participate in light harvesting together with the main pigment, bacteriochlorophyll (BChl) c / d / e . In the present work, we studied the excited-state dynamics in Cars from Chloroflexus ( Cfx .) aurantiacus chlorosomes by near infrared pump-probe spectroscopy with 25 fs temporal resolution at room temperature. The S 2 state of Cars was excited at a wavelength of ∼520 nm, and the absorption changes were probed at 860-1000 nm where the excited state absorption (ESA) of the Cars S 2 state occurred. Global analysis of the spectroscopy data revealed an ultrafast (∼15 fs) and large (>130 nm) red shift of the S 2 ESA spectrum together with the well-known S 2 → S 1 IC (∼190 fs) and Cars → BChl c EET (∼120 fs). The S 2 lifetime was found to be ∼74 fs. Our findings are in line with earlier results on the excited-state dynamics in Cars in vitro . To explain the extremely fast S 2 dynamics, we have tentatively proposed two alternative schemes. The first scheme assumed the formation of a vibrational wavepacket in the S 2 state, the motion of which caused a dynamical red shift of the S 2 ESA spectrum. The second scheme assumed the presence of two potential minima in the S 2 state and incoherent energy transfer between them. In Chloroflexus aurantiacus chlorosomes, extremely fast dynamics occurs in the S 2 excited state of carotenoids.