Mechanism of the photodissociation of 4-diphenyl( trimethylsilyl)methyl-N,N-dimethylaniline

On irradiation in hexane (248- and 308-nm laser light) 4-diphenyl(trimethyl silyl)methyl-N,N-dimethylaniline, undergoes photodissociation of the C-Si b ond giving 4-N,N-dimethylamino-triphenylmethyl radical, 3(.) (lambda(max) a t 343 and 403 nm), in very high quantum yield (Phi = 0.92). The interventio n of the triplet state of 2 (lambda(max) at 515 nm) is clearly demonstrated through quenching experiments with 2,3-dimethylbuta-1,3-diene, styrene, an d methyl methacrylate using nanosecond laser flash photolysis (LFP). The fo rmation of 3(.) is further demonstrated using EPR spectroscopy. The detecti on of the S-1 state of 2 was achieved using 266-nm picosecond LFP, and its lifetime was found to be 1400 ps. in agreement with the fluorescence lifeti me (tau(f) = 1500 ps, Phi(f) = 0.085). The S-1 state is converted almost ex clusively to the T-1 state (Phi T = 0.92). In polar solvents such as MeCN, 2 undergoes (1) photoionization to its radical cation 2(.+), and (2) photod issociation of the C-Si bond, giving radical 3(.) as before in hexane. The formation of 2(.+) occurs through a two-photon process. Radical cation 2(.) does not fragment further, as would be expected, to 3(.) via a nucleophil e(MeCN)assisted C-Si bond cleavage but regenerates the parent compound 2. O bviously, the bulkiness of the triphenylmethyl group prevents interaction o f 2(.+) with the solvent (MeCN) and transfer to it of the electrofugal grou p Me3Si+. The above results of the laser flash photolysis are supported by pulse radiolysis, fluorescence measurements, and product analysis.