Photodissociation of N-arylmethylanilines: A laser flash photolysis, fluorescence, and product analysis study

Increased phenylation in the molecular series PhCH2NHPh (1), Ph2CHNHPh (2), and Ph3CNHPh (3) has two important consequences on the photophysical/photo chemical behavior: (i) decrease in the fluorescence quantum yields (cyclohe xane), 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 quan tum yield (MeCN) of the photodissociation products PhCH2., Ph2CH., and Ph3C ., 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 progres sively weakens in the series 1, 2, 3 (bond dissociation enthalpy: 52, 48, 3 9 kcal/mol, respectively), the C-N fission channel becomes obviously more f avorable and competes effectively with fluorescence. The involved intermedi ates PhCH2., Ph2CH., Ph3C., and PhNH.. were identified using laser flash ph otolysis (248 and 308 nm). Product analysis (lamp irradiation) gives as mai n products aniline and (i) 1,1-diphenylethane and o- and p-benzylaniline fo r 1, (ii) 1,1,2,2-tetraphenylethane for 2, (iii) Ph3CH and 9-Ph-fluorene fo r 3; all these compounds are formed from the above radicals through couplin g or H-abstraction reactions.