Oxygen quenching of n pi* triplet phenyl ketones: Local excitation and local deactivation

We have studied the charge-transfer-induced deactivation of n pi* excited t riplet states of benzophenone derivatives by O-2((3)Sigma (-)(g)) and the c harge-transfer-induced deactivation of O-2((1)Delta (g)) by ground-state be nzophenone derivatives in CH2Cl2 and CCl4. The rate constants for both proc esses are described by Marcus electron-transfer theory, and are compared wi th the respective data for a series of biphenyl and naphthalene derivatives , the triplet states of which have mr configuration. The results demonstrat e that deactivation of the locally excited n pi* triplets occurs by local c harge-transfer and non-charge-transfer interactions of the oxygen molecule with the ketone carbonyl group, Relatively large intramolecular reorganizat ion energies show that this quenching process involves large geometry chang es in the benzophenone molecule, which are related to favorable Franck-Cond on factors for the deactivation of ketone-oxygen complexes to the ground-st ate molecules. This leads to large rate constants in the triplet channel, w hich are responsible for the low efficiencies O-2((1)Delta (g)) formation o bserved with n pi* excited ketones. Compared with the deactivation of pi pi * triplets, the non-charge-transfer process is largely enhanced, and charge -transfer interactions are less important. The deactivation of singlet oxyg en by ground-state benzophenone derivatives proceeds via interactions of O- 2((1)Delta (g)) with the Ph rings.