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 11 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(y) 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 I 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 similar to200 cm(-1). (2) The Q(x) transition of Pheo(1) is at 544.4 nm with an absorption bandwidth of similar to200 cm(-1). (3) Within the pentamer model four of the five Qy states are delocalized over both the D-1 and D2 branches. The delocalization results in significant narrowing (similar to40%) 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 similar to80 cm(-1)) is equivalent to that of similar to1.8 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.