Photoprocesses, intersystem crossing

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 photochemical reactions and intersystem crossings do not take place in the upper singlet states, then the general kinetic pattern of the photoprocesses occurring during absorption of light quanta can be expressed as follows ... 

The photochemistry of the carboxyl group has attracted only limited attention because it absorbs only at wavelengths well below 250 nm. The principle primary photoprocess is known to be either homolytic cleavage in the case of undissociated acids [for example, the dissociation energy (Dc c) in CH3—COOH is 385 kJ mol ] or heterolytic cleavage in the case of carboxylate anions, followed by a photodecarboxylation step (Scheme 6.143).322,1025-1027 The latter process may involve the formation of a radical cation precursor. The photoreactions of aliphatic derivatives usually proceed via an excited singlet state (the intersystem crossing efficiency in substituted acetic acids is low) and they are inefficient ([Pg.331]

In very rare cases in which photoionization is the only photoprocess, the absorption observed in a nitrous oxide-saturated solution will be that of the cation radical. However, generally, other processes such as intersystem crossing also occur in parallel. In the presence of oxygen, the hydrated electron and the triplet will be scavenged at diffusion-controlled rates, and the absorption observed in the oxygenated solution will be due to the cation radical. Under these conditions, the cation radical spectrum is easily determined. The molar absorption coefficient of the cation radical can also be calculated using the hydrated electron as an internal standard. The molar absorption coefficient for sulphacetamide cation radical was determined in this manner (Land et al 1982) and later confirmed by pulse radiolysis (see Section 12.2.2.6). 

PCB generally exhibit low fluorescence quantum yields (22), but phosphoresce efficiently ( jL) due to high Intersystem crossing yields from the singlet to the triplet excited states. Dechlorination of the rneta- and para- Isomers may Involve exclmer formation (29). In condensed PCB films the major photoprocesses probably Include e transfer reactions leading to both dechlorination and polymerization. 

These are commonly observed in transient absorption spec troscopy, and make it difficult to analyze one-photon photoprocesses of polymers. An exceptional case involves studies on polymers having carbonyl groups reported by Schnabel et al. (42). No distinct contribution of intrapolymer S2 -Si annihilation was observed, which was ascribed to a small value of the molar extinction coefficient at the laser wavelength and to fast intersystem crossing. Simultaneous as well as successive two-photon processes might be suppressed compared to one-photon processes. On the other hand, studies on primary photoprocesses of polymers with pendant aromatic groups can be performed only by examining excitation intensity effects in detail. This has been demonstrated clearly not only by our experiments described here, but also by recent reports by Webber et al. (43). 

---