The rate of Qx→Qy relaxation in bacteriochlorophylls of reaction centers from Rhodobacter sphaeroides determined by kinetics of the ultrafast carotenoid bandshift

Transient absorption changes induced by excitation of isolated reaction centers (RCs) from Rhodobacter sphaeroides with 600 nm laser pulses of 20 fs (full width at half maximum) were monitored in the wavelength region of 420–560 nm. The spectral features of the spectrum obtained are characteristic for an electrochromic band shift of the single carotenoid (Car) molecule spheroidene, which is an integral constituent of these RCs. This effect is assigned to an electrochromic bandshift of Car due to the local electric field of the dipole moment formed by electronic excitation of bacteriochlorophyll (BChl) molecule(s) in the neighborhood of Car. Based on the known distances between the pigments, the monomeric BChl (B_B) in the inactive B-branch is inferred to dominate this effect. The excitation of B_B at 600 nm leads to a transition into the S₂ state (Q_x band), which is followed by rapid internal conversion to the S₁ state (Q_y band), thus leading to a change of strength and orientation of the dipole moment, i.e., of the electric field acting on the Car molecule. Therefore, the time course of the electrochromic bandshift reflects the rate of the internal conversion from S₂ to S₁ of B_B. The evaluation of the kinetics leads to a value of 30 fs for this relaxation process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.