A re-examination of spin–orbit coupling in the triplet state of the primary donor in photosynthetic reaction centers

Abstract

Abstract Recent high-field EPR studies have revealed important differences between the g -matrices of the photoexcited triplet and the cation radical states of the primary donor P in bacterial photosynthetic reaction centers. One of the more unusual differences is the temperature dependence in the g -tensor of the triplet state 3 P, which contrasts with the temperature independence that has been observed in the g -tensor of the cation state P + . This difference leads us to suggest that intersystem (singlet–triplet) spin–orbit coupling could play a significant role in determining the electronic structure of the triplet state, 3 P. Such coupling may also be important for explaining other poorly understood characteristics of 3 P, including the temperature dependence of its zero-field splitting D , and the reduction of D relative to the triplet state in monomeric bacteriochlorophyll. We present a survey of ZFS data from a wide variety of bacteriochlorophyll species that supports this idea. Implications of this model for the electronic structure of the primary donor are discussed.