Bruno Källebring

Exciton splitting in the spectra of covalently linked porphyrins

Abstract Absorption spectra are obtained for bis- and penta-porphyrin derivatives of meso-tetra-phenylporphyrins in which the porphyrin units are connected via 1,4 substituted benzene bridges. The results are interpreted with the help of the CNDO/S model and the Forster model using transition charges or dipoles of the monomer. An energy splitting of about 10 nm occurs in the Soret band. The polarization directions for the dimer were examined using the method of stretched polymer films. The energy splitting, intensities and polarization of the Soret band are explained by the transition dipole model. Dual fluorescence, indicating the existence of an emitting charge transfer state, was not found.

Singlet spectra of chromophores in the reaction center of rhodopseudomonas viridis as calculated by the CNDO/S method

Abstract The CNDO/S method is used to calculate spectra of monomers and pairs in the reaction center of Rhodopswdomonas vindis . The special pair is a strongly coupled dimer and its spectrum cannot be described by exciton coupling theory. The S 1 and S 2 states are both suggested to be in the long-wavelength region above 900 nm. The S 2 and S 3 states are CI mixtures of Q x - and Q y -type transitions. The electronic coupling between the special pair and the BPh L monomer is small and therefore it seems reasonable, as suggested by others, that the accessory BChl L monomer is involved either in the formation of the S 1 state of the special pair or as a very short-lived intermediate in the electron-transfer process.

A theoretical study of the effect of charge recombination on the transfer and trapping of excitation energy in photosynthesis

Abstract Photosynthetic organisms absorb solar energy using chlorophyll-containing antennas and convert it to chemical energy in the form of charge-separated radical pairs. Experimental studies of photosystem 2 from plants and cyanobacteria are often interpreted in terms of a model in which the excited antenna is in equilibrium with the charge-separated states. Assigning a single state for the excited antenna is a simplification since it is known that the excitation energy migrates over the antenna for some time until it is trapped in special reaction centers where charge separation takes place. In the present work, existing random-walk models for excitation-energy transfer are extended to a case where charge recombination is allowed for. The treatment is based on Laplace and discrete Fourier transform techniques. It is shown that the simple equilibrium model approximates the situation well, provided that the overall trapping and detrapping rates are appropriately scaled to the rate constants for charge separation and recombination in the reaction center. Expressions for the quantum yields of charge separation and of fluorescence are derived and the consistency between the present random-walk model and the general notion of antenna entropy is demonstrated.

First Example of Ultrafast Photoisomerisation-Like Photophysics for Tetrapyrrol Systems: Ethylene-Bridged Porphyrin Dimers

It is commonly accepted that porphyrin molecules are sufficiently rigid structures having relatively large (~15500 cm-1) ∆E(S1-S0) energy gap. As a result, if there is no quenching effect of central metal atom, S1→S0 radiationless decay (internal conversion) has only minor contribution, if any, to the S1 state deactivation. Sometimes, however, one can find evidences in literature that (metal-free) symmetric close-spaced porphyrin dimers, used as models of photosynthetic structures, exhibit quenched S1 state, in spite of quite normal long-lived photophysics of initial monomers. This effect usually either remained unexplained or was accounted for by a participation of some “accepting intermolecular vibration mode” in the S1 state deactivation [1]. We believe, however, that, due to steric hindrances, close-spaced tetrapyrrol dimers can have strongly distorted macrocycles, and after excitation, structural relaxation can occur resulting in decrease of S1-S0 energy gap and corresponding increase of S1→S0 internal conversion rate. Recently synthesized [2] ethylene-bridged porphyrin dimers, EBPDs, of side-to-side geometry (see Fig.1) can be considered as a striking example of such unknown earlier for tetrapyrrol systems photoisomerisation-like photophysics.

Theoretical Investigations of Reaction Center Chromophores

The primary electron donor (P or “special pair”) in the bacterial reaction center (RC) is a dimer of bacterio-chlorophyll (BCh1). The experimental spectrum of P is red-shifted compared to BCh1 in solution. This is due to interactions between the two BChls and to interactions between P and the surrounding protein. Explanations of the special properties of P have been offered by several investigators [1–8]. Recent hole-burning [9] and electric-field [10] studies suggest that the first excited state of P has charge-transfer character. In our earlier calculations [11], using preliminary coordinates where P was planar, we could not see any localization of the orbitals on to either BChl. With the refined coordinates P was found to be non-planar. We have now investigated the effects of this non-planarity, as well as those of the axial histidine ligands, on spectra, orbital distributions and transition-dipole orientations.