Excited states of metalloporphyrins

A meaningful discussion of porphyrin photochemistry would require an in-depth description of the excited states of metalloporphyrin systems and of energy transfer which is outside the scope of the present Chapter. Porphyrin photochemistry is also the basis of photosynthesis which, in itself, is a massive area of current research endeavor. Readers are therefore directed to authoritative reviews on these aspects (B-75MI30709, B-75MI30710). 

Excited states of metalloporphyrins have absorption features in the 600-900-nm range that can be used to identify the particular type of transient state that is present. Empirical differences exist for the individual n X n, n d, n ), and (d, d) excited states. For the (d, n ) state there is an extra relatively intense band in the 650-700 nm range that is not observed in the spectra of the n) state. Discrimination between the (tc, n) and (ti, n) excited states can also be made. The spectra of the (tc, n) state show distinct absorption peaks in the 600-900-nm region, whereas the n) shows a broad featureless absorption across this region. For metalloporphyrins that have the (d, d) excited state, a diagnostic feature is that the excited state does not generally absorb in the 600-900-nm range of the electronic spectrum. 

Systematic research of two-quantum excitation of tetrapyrrolic pigments (2,3) has made it evident that cooperative triplet-triplet excitation of metalloporphyrin upper electronic states successfully competes with the processes of stepwise two-quantum excitation. This made us pay greater attention to porphyrin TTA since high quantum yields of intersystem crossing to triplet Tj states is characteristic of these types of compounds. (It should be mentioned that interest in photoprocesses in excited states of tetrapyrrolic pigments has been greatly increased by development of solar energy converters based on metalloporphyrins (4,5)). 

Judging from the large absorption cross section obtained, the transition is expected to be strongly allowed in the Q-band region. The first excited singlet state (Q state) of metalloporphyrins has E. symmetry. Hence our finding of the strongly allowed transi-... 

The electron transfer proceeded with the participation of the triplet excited state of Chi. The discovery of the reversible reaction of chlorophyll photoreduction served as a stimulus for starting systematic research on photochemical redox reactions of chlorophyll and its synthetic analogues, i.e. various metalloporphyrins. 

Taking advantage of the fact that the triplet state of metalloporphyrins with tetrabenzopor-phyrin (H2TBP or H2I, fig. 10) lies around 12 500 cm-1, a team from Minsk succeeded in sensitizing Ybm luminescence in solutions of [Yb(TBP)L], where L = Cl or acac (acetyl-acetonate), in several solvents like benzene, dimethylformamide (dmf), pyridine, quinoline or a mixture of octane and benzene (Kachura et al., 1974). Energy transfer from the porphyrin chromophore was ascertained by the excitation spectrum of the Ybm luminescence being identical to the absorption spectrum of the complex. In benzene, the quantum yield of... 

The Gibbs free energy of electron transfer is determined by the formal electron transfer potentials associated with the excited state of the metalloporphyrin ( p, ) and the redox couple in the organic phase... 

Calculations on the Excited States of Porphyrin, Hydroporphyrins, Tetrazaporphyrins, and Metalloporphyrins. 

In addition to direct one photon excitation in the Soret band, the Sj state of metalloporphyrin can also be excited by either coherent or sequential two-photon absorption in the visible. The early work of Stelmakh and Tsvirko showed that the S2 state can be populated sequentially either directly via prompt two-photon absorption at high laser fluence + hv S, + hv Sj Sj Sq + /tv... 

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