Photodissociation of N-Arylmethylanilines: A Laser Flash Photolysis, Fluorescence, and Product Analysis Study(1).
Increased phenylation in the molecular series PhCH(2)NHPh (1), Ph(2)CHNHPh (2), and Ph(3)CNHPh(3) has two important consequences on the photophysical/photochemical behavior: (i) decrease in the fluorescence quantum yields (cyclohexane), Phi(f) = 0.115, 0.063, 0.001 (lambda(exc) = 254 nm) and 0.164, 0.089, 0.019 (lambda(exc) = 290 nm), respectively, and (ii) increase in the quantum yield (MeCN) of the photodissociation products PhCH(2)(*), Ph(2)CH(*), and Ph(3)C(*), Phi(radical) = 0.16, 0.25, 0.65 (lambda(exc) = 248 nm) and (not measured), 0.18, 0.29 (lambda(exc) = 308 nm), respectively. As the C-N bond progressively weakens in the series 1, 2, 3 (bond dissociation enthalpy: 52, 48, 39 kcal/mol, respectively), the C-N fission channel becomes obviously more favorable and competes effectively with fluorescence. The involved intermediates PhCH(2)(*), Ph(2)CH(*), Ph(3)C(*), and PhNH(*) were identified using laser flash photolysis (248 and 308 nm). Product analysis (lamp irradiation) gives as main products aniline and (i) 1,1-diphenylethane and o- and p-benzylaniline for 1, (ii) 1,1,2,2-tetraphenylethane for 2, (iii) Ph(3)CH and 9-Ph-fluorene for 3; all these compounds are formed from the above radicals through coupling or H-abstraction reactions.