An excitonic pentamer model (an adaptation of the multimer model; Durrant et al. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 4798) is proposed for the core Q y states of the photosystem II reaction center (PSII RC). The core chlorins consist of four chlorophyll a molecules (P 1 , P 2 , Chl 1 , Chl 2 ) and two pheophytin a molecules (Pheo 1 , Pheo 2 ). In the pentamer model Pheo 2 on the inactive D 2 branch is, for all intents and purposes, decoupled from the other five chlorins. This model is the result of theoretical simulations of several types of spectra obtained at liquid helium temperatures in the Q y region and the Pheo Q x region of the absorption spectrum. They include bleaching spectra obtained by reduction of Pheo 2 with dithionite (in the dark) and reduction of the active Pheo 1 with dithionite and white light illumination, triplet bottleneck hole spectra, and femtosecond pump-probe spectra (from S. R. Greenfield et al. J. Phys. Chem. B 1999, 103, 8364). The model structure of Svensson et al. of PSII RC (Biochemistry 1996, 35, 14486) and the recent X-ray RC structure of Zouni et al. (Nature 2001, 409, 739) were used to construct hexamer excitonic Hamiltonians. Both Hamiltonians, with uncorrelated site excitation energy disorder taken into account, yield similar results and acceptable fits to the spectra but only if Pheo 2 is decoupled. Such decoupling would require a significant weakening of the Pheo 2-Chl 2 interaction predicted by the RC structures. Possible reasons for weakening are given. Our findings include the following: (1) The localized Q x /Q y transitions of Pheo 2 are at 541.2 and 668.3 nm with absorption bandwidths of ∼200 cm -1 . (2) The Q x transition of Pheo 1 is at 544.4 nm with an absorption bandwidth of ∼200 cm -1 . ( 3) Within the pentamer model four of the five Q y states are delocalized over both the D 1 and D 2 branches. The delocalization results in significant narrowing (∼40%) of inhomogeneous spectral broadening that stems from the width of the site (chlorin) excitation energy distribution functions. (4) The contributions of P 1 and P 2 to the lowest energy (primary donor, P680*) state are, on average, the largest although the contributions from the other three chlorins are significant. (5) The triplet state associated with the bottleneck spectrum appears to be localized on Chl 1 (or P 2 ). ( 6) The combined absorption dipole strength of Pheo 1 associated with the two lowest energy and strongly absorbing states (separated by only ∼80 cm -1 ) is equivalent to that of ∼1.8 monomer Pheo molecules. This finding provides a plausible explanation for the results of Greenfield et al. The paper ends with discussion of the nature of P680* and the triplet state(s) formed by charge recombination of the primary radical ion pair.

Nonphotochemical hole burning (NPHB) at low temperatures of the electronic absorption bands of molecular chromophores imbedded in amorphous solids (glasses and polymers) and in proteins is a striking manifestation of configurational tunneling triggered by electronic excitation. The current mechanism of NPHB has it due to a hierarchy of relaxation events that begin in the outer shell and involve the intrinsic two-level systems (TLS int ) of the glass and terminate in the inner shell where the rate determining step involving the extrinsic TLS (TLS ext ) occurs. The TLS correspond to asymmetric intermolecular double well potentials. The TLS ext are associated with the chromophore and the inner shell of solvent molecules. The TLS int are intimately associated with the excess free volume of glasses. Their tunneling leads to diffusion of excess free volume. Results for Al-phthalocyanine tetrasulfonate (APT) in hyperquenched glassy water (HGW) and ethanol (HGE) films that provide strong support for the critical role of excess free volume are discussed. Hole spectra of APT/HGW obtained over eight decades of burn fluence reveal that the current mechanism needs to be modified to include multilevel extrinsic systems (MLS ext ) in order to explain why the antihole ("photoproduct" absorption) lies to the blue of the burn frequency for sufficiently high burn fluences, an intriguing up-conversion process. The spectra also reveal, for the first time, that the zero-phonon hole (ZPH) profile is non-Lorentzian. This is shown to be a natural consequence of the interplay between the three distributions that result in dispersive hole growth kinetics. They are associated with the tunnel parameter λ of the TLS ext , the angle R between the laser polarization and transition dipole, and off-resonant absorption of the zero-phonon line (the ω distribution). Theoretical simulations of hole growth data for APT/HGW obtained over six decades of burn fluence show that the λ distribution is of primary importance, describing well the first 80% of the saturated burn. The paper ends with an application of NPHB combined with high pressure and external electric (Stark) fields to the critically important "red" antenna states of photosystem I. The addition of pressure and Stark fields enhances the already impressive selectivity of NPHB. The results show that the linear pressure shift, permanent dipole moment change, and linear electron-phonon coupling are correlated. Of particular importance is that these properties can be used to identify states which involve interacting chlorophyll molecules that possess significant charge transfer character because of electron-exchange coupling. The results also show that the site distribution functions of the antenna states are largely uncorrelated, consistent with the findings for previously studied complexes. This is important because the absence of correlation means that the electronic energy gaps of donor and acceptor states are distributed which, in turn, means that the kinetics can be dispersive under certain ...