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

Abstract

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 flash 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˙) with a rate constant of k_diss = 3 × 10⁷ s⁻¹. The formation of 1˙ is a one-quantum process and takes place via the carbonyl triplet excited state with high quantum yield (Φ_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 × 10⁹ M⁻¹ s⁻¹), Et₃N (1.1 × 10⁹ M⁻¹ s⁻¹), and styrene (3.1 × 10⁹ M⁻¹ s⁻¹) giving quenching rate constants very similar to those of benzophenone. For comparative reasons radical 1˙ was generated independently from p-(benzoyl)triphenylmethyl bromide via pulse radiolysis in THF and its absorption coefficient at λ_max = 340 nm was determined (ε = 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 Φ_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 [Ph₃C˙ and p-Ph(OCH₂CH₂O)C–C₆H₄–C(˙)Ph₂, respectively] after dissociation of the central C–Si bond (Φ_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 (Φ ≈ 0.85 for 2). Using fs-LFP on 2 we showed that the S₁ state recorded at 100 fs after the pulse decays on a time scale of 500 fs giving Ph₃C˙ 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˙ formation Φ_rad = 0.16 was measured with ns-LFP), or undergo in-cage recombination to photo-Fries products. Thus, singlet excited states (S₁) 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 multiplicity 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 = 10¹¹ s⁻¹ and 6 × 10⁸ 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 × 10⁶ s⁻¹) leads to photo-Fries product formation via the energetic high lying S₁ state [ ≈ 443 kJ mol⁻¹ (106 kcal mol⁻¹)].