Ultrafast internal conversion in 132-OHNi-bacteriochlorophyll in reaction centers of Rhodobacter sphaeroides R26

Abstract The excited state lifetime of 13 2 -OHNi-bacteriochlorophyll (NiBChl) is investigated by fluorescence quenching studies after incorporating the Ni pigment into the photosynthetic reaction center of Rhodobacter sphaeroides R26, replacing there the two monomeric bacteriochlorophylls B A and B B . In contrast to the native reaction center, the excitation spectrum of the primary donor fluorescence shows a drastic reduction of excitation energy transfer from the two NiBChls at the B A, B sites to P. This effect is attributed to ultrafast internal conversion within the manifold of vibronic states of NiBChl starting from its lowest excited π−π ∗ singlet state. Based on ultrafast (100 fs) excitation energy transfer from this excited state to the primary donor, the lifetime of the singlet excited NiBChl is estimated to be shorter than 40 fs, favoured by the energetic proximity of a lower lying excited singlet state 1 B 1g of the central metal. Due to the large depression of the 1 Q y (NiBChl) band in the fluorescence excitation spectrum, the upper exciton component of the special pair can be unambiguously located at 814 nm, while in absorption spectra this weak transition is buried under the strong bands of B A and B B .