The excitation energy transfer in three photosynthetic organism samples, Bacteriochlorophyll a-protein from Prosthecochloris aestuarii, and Chl/P700~60 and Chl/P700~200 from spinach, have been investigated by pump-probe ultrafast spectroscopy. The isotropic photobleaching profiles were best described by two exponential decay components in Bchl a-protein and Chl/P700~60, and three exponential decay components in Chl/P700~200. The experimental results from the three samples show that nonrandom chromophore orientations exist and Sauer's "pebble mosaic" model is an appropriate one for excitation transfer in these samples. The polarized pump-probe transients have been analyzed in terms of an exciton hopping model that incorporates the known geometry of the Bchl a-protein;The crystal and molecular structures of four metalloporphyrins have been determined by X-ray diffraction and molecular mechanics. The results are summarized as follows: (H[subscript]2O)MgT(OME)PP.HCCl[subscript]3: monoclinic, a = 15.966(5), b = 9.192(1), c = 14.882(4) A, and [beta] = 100.38(2)°; space group I2; R = 0.068, R[subscript] w = 0.070. Intermolecular hydrogen bonding results in the formation of two-dimensional infinite polymers; (cis-(ala)[subscript]2DPE) Ni(II).ChCl[subscript]3.CH[subscript]3OH: monoclinic, a = 14.195(5), b = 12.175(6), c = 28.071(1), and [beta] = 98.97(3)°; space group P2[subscript]1/n; R = 0.085, and R[subscript] w = 0.089. The average Ni-N distance is 1.92 A and N-Ni-N bond angles range from 88.1(4)° to 92.3(4)° indicating that the Ni environment is essentially square planar; (Ni(DPE)) -(py)[subscript]2.2CHCL[subscript]3: triclinic, a = 14.306(4), b = 14.719(5), c = 14.296(5) A, [alpha] = 94.86°, [beta] = 98.97(3)° and [gamma] = 63.452°; space group P1; R = 0.049, and R[subscript] w = 0.064. The average Ni-N distance is 1.92 A and N-Ni-N bond angles range from 88.5(1)° to 91.9(1)°. The distance between the two terminal nitrogen atoms is 11.4 A; (Ni(DPE)) -(py)[subscript]2.H[subscript]2O: monoclinic, a = 14.240(2), b = 25.418(4), c = 14.725(5) A, and [beta] = 96.38(3)°; space group P2[subscript]1/n; R = 0.081, and R[subscript] w = 0.107. The average Ni-N distance and N-Ni-N bond angle are 1.93 A and 90.2°, respectively. The distance between the two terminal nitrogen atoms is 7.6 A.