Tetrapyrrole quenching

A few examples to render tetrapyrrolic compounds less phototoxic can be found in the hterature. In one approach, carotenoid structures were employed for the synthesis of some carotenoporphyrin derivatives [92-94]. Figure 8 shows two stuctures by way of example. Due to similar photophysical properties of the two structural components, the excited triplet state of the porphyrin is quenched by the carotenoid moiety, thus inhibiting the formation of singlet oxygen, while its fluorescence capabilities are still preserved. Biodistribution studies revealed enhanced uptake into tumour tissue [39,93,95]. However, microscopy studies have shown that such compounds are associated with connective tissues in the tumors rather than with cancerous cells indicating low specificities for mahgnant transformation [96]. 

Information regarding the solution conformation of 13 was derived from the pyropheophorbide ring current induced shifts in the resonance positions of the carotenoid and quinone moieties. These two species were found to be extended away from the tetrapyrrole, rather than folded back across it. The absorption spectrum of 13 was essentially identical to the sum of the spectra of model compounds. The pyropheophorbide fluorescence, however, was strongly quenched by the addition of the quinone. This implies the formation of a C-Phe -Q state via photoinitiated electron transfer from the pyropheophorbide singlet state, as was observed for C-P-Q triads (see Figure 4). Excitation of the molecule in dichloromethane solution at 207 K with a 590 nm laser pulse led to the observation of a carotenoid radical cation transient absorption. Thus, the C-Phe -Q " state can go on via an electron transfer step analogous to step 4 in Figure 4 to yield a final C -Phe-Q state. This state had a lifetime of 120 ns. The quantum yield at 207 K was 0.04. At ambient temperatures, the lifetime of the carotenoid radical cation dropped to about SO ns, and the quantum yield could not be determined accurately because of the convolution of the decay into the instrument response function. 

Transient absorption and fluorescence experiments on 1 and 4 and the model carotenoid pigments 3 and 6 yielded convincing results about the energy transfer pathway (Fig 1) (A.F. Moore et al, unpublished). In 1, the lifetime of Sj was measured by fluorescence upconversion to be 45 fs, whereas the Sj lifetime of model carotenoid 3 is 160fs.TheS, lifetime was 8 ps in both 1 and 3. Moreover, the S, rise time for tetrapyrrole 2 was found to be 62 fs by fluorescence upconversion. Therefore, only the carotenoid 83 state was quenched by the attached tetrapyrrole. The observation that the time constant associated with the decay of the energy donor(carotenoid 83) matches... 

Dyad 4 illustrates that both pathways can be active. In initial experiments on dyad 4 the attached tetrapyrrole was found to quench the carotenoid S3 state of6 from 95 to 28 fs and its S, level from 12 to 9 ps. The rise of the 8, level of tetrapyrrole 5 as measured by fluorescence upconversion required a major exponential component (74%) of 41 fs- and a minor component (26%) of4 ps-. While the match between the 9 ps decay of the carotenoid 8, and the 4 ps rise component of the tetrapyrrole 8, is only qualitative, these prelimenary experiments do provide evidence that both states can be energy donors. 

What is the role of energy or electron transfer in the quenching of tetrapyrrole fluorescence by carotenoids In other model studies the redox levels of a porphyrin-carotenoid dyad have been shown to influence the quenching mechanism to the extent that electron transfer from the carotenoid to the excited porphyrin was shown to occur (Hermant et al., 1993). However, in a series of carotenoid-porphyrin dyads in which the number of conjugated carbon-carbon double bonds in the carotenoid moiety was systematically increased from 7 to 11, quenching... 

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