5778 J. Phys. Chem. 1994, 98, zyxwvu 5778-5783 15000 zyxwvu - Time Resolution of Electronic Transitions of Photosynthetic Reaction Centers in the Infrared IC.) , IC.) zyxwv ,,er’’ -__. --.--e It) G. C. Walker,+*t S. Maiti,* B. R. Cowen,’ C. C. Maser,* P. L. Dutton,* and R. M. Hochstrasser’*t Department of Chemistry and The Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pennsylvania I9104 Received: November 12, 1993; In Final Form: March 16, 1994” Electronic transitions of the special pair excited state, P*, and of its positive ion, P+, have been identified by transient infrared spectroscopy in the 170&2000-~m-~ region. The P* transition is present immediately (ca. 300 fs) after light absorption and decays with time constant 3.4 ps. The P+ transition appears with time constant 3.4 ps. The transition dipoles of these transitions are both measured to have a squared projection of 0.63 onto the direction of the ground state to Qv- (870 nm) transition. This is interpreted to imply that both P* and P+ transitions have dipoles along the line joining the centroids of charge of the two bacteriochlorophylls (BChl) composing the dimer. The P* transition is assigned as an interexciton transition brought about by mixing of exciton and charge-separated states. The P+ transition is assigned as a transition between the symmetric and antisymmetric combination of the localized hole states of the dimer. The results are compared with theoretical calculations, static FTIR, and Stark effect measurements on the reaction center. While the results are in qualitative agreement with recent theoretical calculations, better agreement requires a larger admixture of charge resonance states in the QU- state than is found in most calculations. Introduction In bacterial photosynthesis the primary event is a photoinduced charge separation that takes place in a transmembrane pigment- protein complex known as the reaction center (RC). The central part consists of two protein subunits L and M, related by approximate C2 symmetry, interfaced at a bacteriochlorophyll dimer (P).l Absorption of a near-IR photon (ca. 870 nm) leads to the formation of the singlet excited state P*, which donates an electron to a bacteriopheophytin (BPhL) on the L side to form the positively charged dimer state P+ with a time constant of -3.5 ps and with a quantum efficiency of >0.98.2 Despite the near C2 symmetry, there is negligible electron transfer to BPhM. Understanding the remarkable features of this fast, efficient, and unidirectional electron transfer requires an understanding of the nature of the electronic structure of the cofactors and in particular of the excited state P* and the ionic state P+. The dominant contribution to the P* state undoubtedly comes from the lower zyxwvutsrqpon (Qy-) of the two excitonic states (Qv- and Qy+) that arise from the interaction of the individual zyxwvutsrq Qy states of the two monomer constituents.3 It was proposed’ll that the P* state also contains a significantadmixture of intradimer charge transfer states. Wave functions of the electronic states of P obtained from the most recent calculationsi2 do show that the P* state can be reasonably well represented by a linear combination of the excitonic (Qy- and Qy+) and charge-transfer (PL+PM-and PL-PM+) states. Spectroscopic measurements of thedipole moment change associated with the formation of the P* state’s support the substantial charge-transfer character of the P* state. To verify the extent to which the zero-order excitonic and charge-transfer states are mixed requires direct probing of the low-energy transitions of P*. Unfortunately, there are many overlapping electronic states associated with the multitude of cofactors in the RC, making it difficult to obtain unambiguous information about these eigenstatesfrom the ground-statevisible/ near-infrared (VIS/NIR) spectrum at ambient temperatures. These transitions have been probed by low-temperature hole- * To whom correspondence should be addressed. t Department of Chemistry. f The Johnson Research Foundation. 8 Current address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260. Abstract published in Advance ACS Absfracfs, April 15, 1994. burning experiments which have led to an assignment of the ground- to excited-state transition energies of a number of transitions of the RC (see refs 9 and 34 for review). The charge- transfer character of the lowest excited state of the RC has also been directly addressed by Stark and VIS/NIR spectros- ~opy.’~J~J~ However, the energies and transition dipole moments of excited-state transitions, which should contain a wealth of information regarding the specificnature of the individual excited states, have not been explored. IR methods now have time resolution and sensitivity comparable to the optical measurements and permit studies not only of transient vibrational states but also of low-energy electronic transitions. We report here the first experimental observation of transitions between the excited states of P in the 5-6-Km wavelength region. It is also important to characterize the eigenstates of P+ for understanding the charge-transfer and recombination (or lack thereof) dynamics. A wide absorption band centered at 2600 cm-1 observed in the static FTIR difference spectra of RC’s has recently been assigned to an electronic transition of the P+ state.15 In our work, the time evolution of this spectral band has been I , , 0 1994 American Chemical Society