The one- and two-photon properties of free base porphin, free base porphin dianion, and the 2,4-substituted diformyl and divinyl analogs of these molecules are studied using a semiempirical SCF-MO formalism (CNDO-π-SCF-MO-PSDCI) including extensive single and double configuration interaction. Strongly two-photon allowed states are predicted to lie in the Soret region as well as in the region between the Soret and visible bands. A number of the two-photon allowed states in the Soret region are predicted to have two-photon absorptivities exceeding 100×10−50 cm4 s molecule−1 photon−1. The calculations indicate that the visible (Q) states are well characterized by the four orbital model, whereas the Soret (B) states contain significant contributions from configurations comprised of other orbitals. The inclusion of extensive double configuration interaction significantly reduces the Soret-visible (B–Q) splitting, increases the Qx–Qy splitting, and yields calculated oscillator strengths for the Q bands in better agreement with experiment than values calculated using single CI alone. The effects of conjugation into the porphyrin macrocycle are predicted to be more significant than inductive effects on macrocycle π orbitals due to substituent polarity. The 〈Qx‖r‖S0〉 and 〈Qy‖r‖S0〉 transition moment vectors are predicted to lie approximately through adjacent pyrrole rings in 2- and 4-monoformyl free base porphin dianions and approximately through adjacent methine bridges in 2,4-diformyl free base porphin dianion.

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In this paper we characterize the mechanistic roles of the crystalline purple membrane (PM) lattice, the earliest bacteriorhodopsin (BR) photocycle intermediates, and divalent cations in the conversion of PM to laser-induced blue membrane (LIBM; lambda(max)= 605 nm) upon irradiation with intense 532 nm pulses by contrasting the photoconversion of PM with that of monomeric BR solubilized in reduced Triton X-100 detergent. Monomeric BR forms a previously unreported colorless monomer photoproduct which lacks a chromophore band in the visible region but manifests a new band centered near 360 nm similar to the 360 nm band in LIBM. The 360 nm band in both LIBM and colorless monomer originates from a Schiff base-reduced retinyl chromophore which remains covalently linked to bacterioopsin. Both the PM-->LIBM and monomer-->colorless monomer photoconversions are mediated by similar biphotonic mechanisms, indicating that the photochemistry is localized within single BR monomers and is not influenced by BR-BR interactions. The excessively large two-photon absorptivities (> or =10(6) cm(4) s molecule(-1) photon(-1)) of these photoconversions, the temporal and spectral characteristics of pulses which generate LIBM in high yield, and an action spectrum for the PM-->LIBM photoconversion all indicate that the PM-->LIBM and Mon-->CMon photoconversions are both mediated by a sequential biphotonic mechanism in which is the intermediate which absorbs the second photon. The purple-->blue color change results from subsequent conformational perturbations of the PM lattice which induce the removal of Ca(2+) and Mg(2+) ions from the PM surface.