Figure 2.

When excitation is transferred from a donor bacteriochlorophyll (BChl) to an acceptor BChl, the Stokes shift between the emission and absorption spectra causes an imperfect energy overlap, as shown in Case 1 (see filled area illustrating overlap J_DA). This results in a reduced rate of excitation transfer. The reaction center can counter the reduced overlap by introducing a second acceptor BChl that is strongly coupled to the first, forming the special pair (SP). Strong coupling, accounted for by an interaction energy of V = 500 cm⁻¹ (V is determined in Ref. 50), coherently spreads excitation between the two acceptor SP BChls, shifting also the SP exciton energies from the single BChl excited state energy E to energies ε₋ and ε₊. This shift alters the absorption spectrum, as shown in Case 2, and accordingly increases the overlap, J_DA, between emission (green line) and absorption (blue line) spectra (see filled area). Furthermore, due to the anti-parallel orientation of the SP BChl’s transition dipole moments, the lower energy exciton state $\widetilde{|-\rangle }$ attains a transition dipole moment of ${\mathbf{d}}_A=\sqrt{2}\mathbf{d}$. The combination of better spectral overlap J_DA and a stronger d_A value increases the rate of excitation transfer for Case 2 over that of Case 1.