Ultrafast internal conversion in 132-OHNi-bacteriochlorophyll in reaction centers of Rhodobacter sphaeroides R26 Original Research Article

The excited state lifetime of 132-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 BA and BB. 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 BA, 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 1B1g of the central metal. Due to the large depression of the 1Qy(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 BA and BB.