Interpretation of a low-lying excited state of the reaction center of Rb. sphaeroides as a double triplet

The recently observed transient absorption of the lowest excited state of the special pair P∗ at 2710 cm−1 is assigned as a singlet which arises from the coupling of the two lowest triplets from the two dimer halves. INDO calculations are used to predict its intensity. The analogy of the coupling mechanism to the trip-doublet spectrum from P + is shown and the influence of the double triplet on the Stark effect of P∗ is investigated.