Triplet-vs.singlet-state imposed photochemistry. The role of substituent effects on the photo-Fries and photodissociation reaction of triphenylmethyl silanes

The photochemistry of three structurally very similar triphenylmethylsilanes 1, 2, 3 [p-X - C6H4 - CPh2- SiMe3: X = PhCO, 1; H, 2; Ph(OCH2CH2O) C, 3] is described by means of 248 and 308 nm nanosecond laser. ash photolysis (ns-LFP), femtosecond LFP, EPR spectroscopy, emission spectroscopy (fluorescence, phosphorescence), ns-pulse radiolysis (ns-PR), photoproduct analysis studies in MeCN, and X-ray crystallographic analysis of the two key-compounds 1 and 2. The photochemical behavior of 1, 2 and 3 is discussed and compared with that of a fourth one, 4, bearing on the p-position an amino group ( X = Me2N) and whose detailed photochemistry we reported earlier (J. Org. Chem., 2000, 65, 4274 - 4280). Silane 1 undergoes on irradiation with 248 and 308 nm laser light a fast photodissociation of the C - Si bond giving the p-(benzoyl) triphenylmethyl radical ( 1(center dot)) with a rate constant of k(diss) = 3 x 10(7) s(-1). The formation of 1(center dot) is a one-quantum process and takes place via the carbonyl triplet excited state with high quantum yield (Phi(rad) = 0.9); the intervention of the triplet state is clearly demonstrated through the phosphorescence spectrum and quenching experiments with ferrocene (k(q) = 9.3 x 10(9) M-1 s(-1)), Et3N ( 1.1 x 10(9) M-1 s(-1)), and styrene (3.1 x 10(9) M-1 s(-1)) giving quenching rate constants very similar to those of benzophenone. For comparative reasons radical 1(center dot) was generated independently from p-( benzoyl) triphenylmethyl bromide via pulse radiolysis in THF and its absorption coefficient at lambda(max) = 340 nm was determined (epsilon = 27770 M-1 cm(-1)). We found thus that the p-PhCO-derivative 1 behaves similar to the p- Me2N one 4 ( the latter giving the p-( dimethylamino) triphenylmethyl radical with Phi(rad) = 0.9), irrespective of their completely different ground state electronic properties. In contrast, compounds 2, 3 that bear only the aromatic chromophore give by laser or lamp irradiation both, (i) radical products [ Ph3Ccenter dot and p-Ph(OCH2CH2O)C-C6H4 -C((center dot)) Ph-2, respectively] after dissociation of the central C - Si bond ( Phi(rad) = 0.16), and ( ii) persistent photo-Fries rearrangement products ( of the type of 5-methylidene-6-trimethylsilyl-1,3-cyclohexadiene) absorbing at 300 - 450 nm and arising from a 1,3-shift of the SiMe3 group from the benzylic to the ortho-position of the aromatic ring (Phi approximate to 0.85 for 2). Using fs-LFP on 2 we showed that the S-1 state recorded at 100 fs after the pulse decays on a time scale of 500 fs giving Ph3C center dot through C - Si bond dissociation. In a second step and within the next 10 ps trityl radicals either escape from the solvent cage ( the quantum yield of Ph3C center dot formation Phi(rad) = 0.16 was measured with ns- LFP), or undergo in-cage recombination to photo-Fries products. Thus, singlet excited states (S-1) of the aromatic organosilanes ( 2, 3) prefer photo-Fries rearrangement products, while triplet excited states ( 1, 4) favor free radicals. Both reactions proceed via a common primary photodissociation step ( C - Si bond homolysis) and differentiate obviously in the multipliciy of the resulting geminate radical pairs; singlet radical pairs give preferably photo-Fries products following an in-cage recombination, while triplet radical pairs escape the solvent cage ( MeCN). The results demonstrate the crucial role which is played by the chromophore which prescribes in a sense, ( i) the multiplicity of the intervening excited state and consequently that of the resulting geminate radical pair, and ( ii) the dominant reaction path to be followed: the benzophenone-and anilino-chromophore present in silanes 1 and 4, respectively, impose effective intersystem crossing transitions (k(isc) = 10(11) s(-1) and 6 x 10(8) s(-1), respectively) leading to triplet states and finally to free radical products, while the phenyl chromophore in 2 and 3, possessing ineffective isc (k(isc) = 6 x 10(6) s(-1)) leads to photo-Fries product formation via the energetic high lying S-1 state [approximate to 443 kJ mol(-1) (106 kcal mol(-1))]. Photochemical & Photobiological Sciences