Triplet ground state structural comparisons

We can conclude from these thermodynamic considerations that it is possible to estimate the redox potentials of excited molecules, if we know the equilibrium redox potentials for the molecules in the ground state, as well for reduction as for oxidation, and add or subtract from these redox potentials the excitation energy AE of the lowest singlet or triplet state. For most dye molecules the reduction redox potential is experimentally more easily accessible than the oxidation redox potential. In such cases we have found that an estimation can be made by assuming that the ionisation energy of the dye molecule in crystalline state is similar to the ionisation energy in a polar solvent and gives an approximate value for the absolute redox potential. Such estimations are especially useful for a comparison of molecules with similar structure. 

The analysis of the data of PS I gave quite accurate information on the distance of the spin centres (25.4 0.3 A)301 that compared well with the crystal structure data.68 A problem is the extended it-spin density distribution in the donor and acceptor. For a solid comparison a centre of gravity for the spin must be calculated from experimental or theoretical spin density distributions of the two radicals. Similar data with almost unaltered distances were obtained for PS I with other quinones substituted into the Ai site.147-302This work has been extended to other electron acceptors,303 which show a larger heterogeneity in distances. It has been shown that the lifetime of the RP can also be measured and can even be controlled in the experiments by an additional mw pulse prior to the 2-pulse echo sequence.302 This pulse transfers population to triplet levels which cannot directly recombine to the singlet ground state. This has earlier been shown for the bRC.304,305 The OOP-ESEEM technique has also been applied to various mutants of PS I to characterize them by the measured distances between fixed donor and variable acceptors.254 256-263-264... 

Our TRIR data are qualitatively consistent with the simple resonance structures in Figure 2.2. As indicated in Table 2.1, we analyzed our results by correlating triplet IR bands with ground state vibrational modes. Of course, vibrational modes in the triplet states are not exactly analogous to those in the ground state and, as revealed by our observed isotopic shifts, are also not pure, localized modes. In addition, these isotopic studies indicate that the triplet modes for TtTt 1 are not identical to those for nru 2. Nonetheless, a comparison with ground state modes is informative. 

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