| Abstract
| - Laser flash photolysis of a series of bichromophoric compounds 1−12 containing the 2-benzoylthiophene (BT) and phenol (PhOH) or indole (InH) moieties has been used to determine the possiblegeometrical effects in the intramolecular quenching of triplet excited ketones, resulting in formalhydrogen abstraction. The results are compared with those obtained in the intermolecular process.In both cases, substitution either at the thienyl or the phenyl moiety has a marked influence onthe photoreactivity. Time-resolved experiments showed that the rate constants for bimolecularquenching by phenol and indole of 2-benzoylthiophene substituted at the thienyl 5-position werelower than those for BT substituted at the phenyl p-position, which agrees with the higher energyfound for the excited triplet state of the latter compounds. However, the rate constant for hydrogenabstraction in the bichromophoric compounds by the π,π* triplet state of the derivatives with thespacer linked to the thienyl 5-position are higher than those of their regioisomers. These resultsindicate a possible geometry-dependence in the intramolecular quenching process. Theoretical DFTstudies have been carried out in order to estimate the optimum conformation for hydrogenabstraction in two pairs of phenolic and indolic bichromophoric regioisomers. The energy profilefor photoactivation/deactivation of the aromatic ketone and the structures of the triplet states andbiradicals involved in the process have been determined. The observed regiodifferentiation in theexperimental studies is consistent with a dependence of the rate constant on orbital overlap betweenthe carbonyl oxygen and the X−H bonds.
|
