Isc

oh oh

H-shift

H-shift

In Scheme 2 a reaction mechanism consistent with the properties of the observed intermediates is presented. Upon decay of the lowest triplet, a transient with 4 jlls lifetime, main absorption at 360 nm and a weak tail at ca. 580 nm, pH insensitive in the range 6-12, was assigned to a decarboxylated triplet anion. This species, in contrast to the KPF case, does not show the acid-base properties of an oxygen centred anion. Its decay rate has a temperature dependence consistent with a spin forbidden process and can be attributed to ISC. The residual absorption has a maximum at 370 nm and intensity which depends on pH in the range 6-10. This suggests that protonation occurs in the ground state at the oxygen site with formation of an enol from which the final product is formed by formal intramolecular H-shift. Accordingly, the rate constant of the last step, ~ 2 x 103 s"1 at pH 6.5, is much lower in deuterated buffer 60 s"1). This mechanism is further supported by the results of quantum mechanical calculations of the charge distribution in the triplet anion, which indicate that the negative charge is rather delocalised with significant fractions located on the oxygen atom and on the carbon centres of the phenyl substituent.

In the (3-CD-SPF inclusion complex a decrease of the ISC efficiency occurs, consistent with a change in the relative position of the ISC-coupled 'n,7i* and 37t,7t* states, determined by the lower environmental polarity of the carbonyl group. The lowest triplet of the complex exhibits self-quenching and oxygen quenching rate parameters one order of magnitude lower than those of the free molecule, thus making the photodegradation to be rather independent of external agents. Finally, the temperature dependence of the triplet decay rate is consistent with the opening of the photochemical channels (decarboxylation and addition to CD) from the T2 (n,7t*) state.12

The transients in TPA are similar to those observed in SPF. The lowest triplet (^max = 600 nm, lifetime ca. 0.8 jis) is of n,n* nature and is characterised by a thermally activated decay. The energy barrier (ca. 7-10 kcal/mol) corresponds to that of decarboxylation and fairly well to the energy separation between Tjfa.rc*) and T2 (n,rc*). As already suggested in the case of SPF, a direct involvement of T2 in the reaction could be hypothesised.11 With a rate constant equal to that of the decay of the lowest triplet a pH sensitive further transient is formed. This intermediate is characterised by a protonation equilibrium with pKa -8.2 and by a slightly temperature dependent decay, consistent with the occurrence of an activated ISC process. It was identified as an oxygen centred triplet anion, undergoing ISC to a ground state anion. Evolution to the final product involves complex kinetics with submillisecond and millisecond components.

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