LASER FLASH-PHOTOLYSIS STUDIES ON HYDROGEN-ATOM ABSTRACTION FROM PHENOL BY TRIPLET NAPHTHOQUINONES IN ACETONITRILE

By means of ns laser flash photolysis, the absorption spectra and molar absorption coefficients (epsilon) of the 1,4-naphthosemiquinone radical (NQH(.)) and 2,3-dimethyl-1,4-naphthosemiquinone radical (DMNQH(.)) have been determined to be 8200 dm(3) mol(-1) cm(-1) at 365 nm and 7100 dm(3) mol(-1) cm(-1) at 368 nm in acetonitrile (MeCN) and MeCN-H2O (4:1 v/v) at 295 K. On the basis of the determined absorption spectra and epsilon values, hydrogen atom abstraction (HA) by triplet 1,4-naphthoquinone (NQ) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) from phenol (PhOH) in MeCN was studied by laser photolysis techniques. It was found that HA by triplet NQ and DMNQ ((3)NQ* and (3)DMNQ*) from PhOH occurred in a collision process with quenching rate constants, k(q) = 8.6 x 10(9) and 5.5 x 10(8) dm(3) mol(-1) s(-1), respectively. On the basis of the obtained values of k(q) and quantum yields (Phi(HA)), the efficiencies (psi(HA)) for HA by both (3)NQ* and (3)DMNQ* from PhOH were obtained to be unity. The rate constants (k(HA)) for HA of (3)NQ* and (3)DMNA* were determined to be 8.6 x 10(9) and 5.5 x 10(8) dm(3) mol(-1) s(-1), respectively. The difference in k(HA) may be derived from (1) the steric hindrance by the methyl groups of DMNQ and (2) the degree of (3)(pi,pi*) character mixed in with the (3)(n,pi*) of NA and DMNQ. The triplet-triplet absorption spectra of NQ and DMNQ were identified, and their absolute molar absorption coefficients (epsilon(lambda)(T-T)) were determined to be 8200 dm(3) mol(-1) cm(-1) at 365 nm and 5200 dm(3) mol(-1) cm(-1) at 368 nm, respectively, in MeCN on the basis of the quantum yields of intersystem crossing (Phi(isc); 0.74 for NQ, 0.98 for DMNQ) evaluated by thermal lensing techniques. The deactivation mechanism of (3)NQ* and (3)DMNQ* in the absence of PhOH was found to be serf-quenching by NQ and DMNQ in MeCN with the rate constants, k(SQ) = 9.7 x 10(8) and 1.2 x 10(8) dm(3) mol(-1) s(-1), respectively.