Porphyrins excited state parameters

Table 9. Singlet excited state parameters of hetero-atom substituted porphyrins and their dications (value in parentheses are red shifts for dications)...

The phosphorescence decay kinetics of the triplet excited states of CuP molecules (Fig. 14) is adequately described by Eq. (16). Using this equation one can obtain the values of the parameter p = (Tra /2) In2 veT from the initial non-exponential part of the phosphorescence decay curves and the values of t = l/ k, i.e. the characteristic time of phosphorescence decay, from the final exponential part. Then the data on the dependence of the quantum yield of CuP phosphorescence on the concentration of C(N02)4 have been used to estimate the effective radii of electron tunneling from triplet excited copper porphyrins to C(N02)4 within the time x R, = (ac/2) In vet (Table 3). In doing so, the quenching of CuP luminescence by electron abstraction was assumed to be the only process leading to a decrease in the quantum yield of CuP phosphorescence in the presence of C(N02)4. From Table 3 an electron is seen to tunnel, within the lifetime of triplet excited states x at 10-4s, from CuP particles to C(N02)4 molecules over the distance R, 11 A. Further, the parameter vc and ae for different porphyrins were estimated from the values of (3, Rt, and x. These values are also cited in Table 3. 

Low-spin Fe(iii) porphyrins have been the subject of a number of studies. (638-650) The favourably short electronic spin-lattice relaxation time and appreciable anisotropic magnetic properties of low-spin Fe(iii) make it highly suited for NMR studies. Horrocks and Greenberg (638) have shown that both contact and dipolar shifts vary linearly with inverse temperature and have assessed the importance of second-order Zeeman (SOZ) effects and thermal population of excited states when evaluating the dipolar shifts in such systems. Estimation of dipolar shifts directly from g-tensor anisotropy without allowing for SOZ effects can lead to errors of up to 30% in either direction. Appreciable population of the excited orbital state(s) produces temperature dependent hyperfine splitting parameters. Such an explanation has been used to explain deviations between the measured and calculated shifts in bis-(l-methylimidazole) (641) and pyridine complexes (642) of ferriporphyrins. In the former complexes the contact shifts are considered to involve directly delocalized 7r-spin density... 

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