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

Abstract

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 S₂ 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 S₂ state occurred. Global analysis of the spectroscopy data revealed an ultrafast (∼15 fs) and large (>130 nm) red shift of the S₂ ESA spectrum together with the well-known S₂ → S₁ IC (∼190 fs) and Cars → BChl c EET (∼120 fs). The S₂ 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 S₂ dynamics, we have tentatively proposed two alternative schemes. The first scheme assumed the formation of a vibrational wavepacket in the S₂ state, the motion of which caused a dynamical red shift of the S₂ ESA spectrum. The second scheme assumed the presence of two potential minima in the S₂ state and incoherent energy transfer between them.