The molecular photophysics of chlorophyll : a study of its triplet state

The lowest excited triplet state T ₀ of chlorophyll a , chlorophyll b , bacteriochlorophyll and corresponding pheophytins has been studied by magnetic resonance and optical spectroscopy. Zero field splitting (US) parameters D and E, populating rates, and depopulating rate constants for the three triplet spin states of these pigments have been obtained. The magnitude of the triplet state parameters for the various pigments has been correlated with the differences in molecular structure, especially with the extent and shape of the π-electron cloud. In particular it has been shown that effects of molecular structure on the ZFS parameter D and on the energy of T ₀ can be accounted for by Gouterman's 4-orbital model when configuration interaction (CI) in T ₀ and the lowest excited singlet state S ₁ is of comparable magnitude. Furthermore it has been found that the effect of substitution of the central Mg ²⁺- ion by two protons is essentially different from the effects of changes in the structure of the outer porphyrin ring.The considerable increase of the mean triplet decay constant on substitution of Mg ²⁺by 2H ⁺cannot be accounted for by the energy-gap law. The presence of occupied non-bonding orbitals, as well as of N-H groups inducing additional promoting vibrational modes, are assumed to be responsible for the large increase of the non-radiative decay rate. There is experimental evidence that both effects are important.The relatively stable and highly symmetric compound Mg-tetrabenzoporphin (MgTBP) has been found to be a good model compound for chlorophylls w.r.t. the effects of ligation on the lowest excited singlet states, but not for the triplet state T ₀ . Some special properties of MgTBP are reported and discussed, i.e. the S ₂ -fluorescence and the splitting of the lowest excited states S ₁ and T ₀ by Jahn-Teller coupling.