Porphyrins intersystem crossing

Figure 3 Type I and type II photooxidation processes with a porphyrin sensitizer illustrated with a modified Jablonski diagram. (S0 = ground singlet state, Si = first excited singlet state, S2 = second excited singlet state, T,i— ground triplet state, Ti = first excited triplet state, i.s.c. — intersystem crossing.)...

For the photodiagnostic use of these compounds, a high quantum yield of fluorescence, r, is desirable. The metal complexes of the common first-row transition metals are not suitable, because they show very low 4>f values. On the other hand, porphyrin complexes of d° and d10 elements show appreciable fluorescence, although generally less than that of the metal-free compounds, presumably because of the heavy-atom effect (e.g., TPP ZnTPP, Table 5). The further operation of the heavy-atom effect, which increases the rate of intersystem crossing (/cisc) by... 

Intersystem crossing results in the formation of the excited triplet state of the porphyrin ... 

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)). 

If the intersystem crossing rate with diamagnetic iron porphyrins is slow then the observed correspondence in relaxation times between S=0 and S=2 iron(II) porphyrins may not necessarily indicate that the relaxation pathways are the same. 

The macrocycles of conformationally strained porphyrins, and, in particular, 2,3,7,8,12,13,17,18-octaalkyl-5,10,15,20-tetraarylporphrins, tend to adopt nonplanar conformations. This has dramatic consequences in singlet and triplet lifetimes. Increasing the macrocycle distortion with the introduction of four or more ethyl groups in adjacent P positions decreases triplet lifetimes to submicrosecond values and compromises This factor adds to the decrease of due to decrease of the singlet state lifetime, and intersystems crossing and relatively short-lived triplet states (69), which are inappropriate for PDT. 

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