The photochemistry of three structurally very similar triphenylmethylsilanes 1 , 2 , 3 [ p -X–C_6H_4–CPh_2-SiMe_3: X = PhCO, 1 ; H, 2 ; Ph(OCH_2CH_2O)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 = Me_2N) 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 × 10^7 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 × 10^9 M^−1 s^−1), Et_3N (1.1 × 10^9 M^−1 s^−1), and styrene (3.1 × 10^9 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 ( e = 27770 M^−1 cm^−1). We found thus that the p -PhCO-derivative 1 behaves similar to the p -Me_2N 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 [Ph_3C· and p -Ph(OCH_2CH_2O)C–C_6H_4–C(·)Ph_2, 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 SiMe_3 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 S_1 state recorded at 100 fs after the pulse decays on a time scale of 500 fs giving Ph_3C· 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 Ph_3C· formation Φ _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 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 = 10^11 s^−1 and 6 × 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 × 10^6 s^−1) leads to photo-Fries product formation via the energetic high lying S_1 state [ ≈ 443 kJ mol^−1 (106 kcal mol^−1)].

The rotation motion of a larger substituent of an aromatic ring is accompanied by the electron density fluctuation of the highest occupied molecular orbitals. For benzyltrimethylsilanes p-R-3-C6H4-(sic)-CR1-R2(SiMe3) [R-3 = H,PhCO; R-1, R-2 = H,H; H,Me; Me,Me; H,Ph; Ph,Ph] the silyl containing substituent rotates on the axis between the aromatic moiety and the benzylic carbon as indicated in the formula. In the course of this rotation a diversity of various conformers is passed. For the sake of simplicity, we reduce this diversity to two (extreme) borderline structures, one with the substituent in plane with the aromatic ring (transient), and one where the substituent is twisted by 90degrees (stable structure). In the very rapid and non-hindered electron transfer from the conformer mixture to n-butyl chloride parent radical cations, the singlet ground state conformers are converted into two kinds of radical cations. The cations tending to the planar type are stable whereas those of the twisted type dissociate immediately into benzyl type radicals (p-R-3-C6H4-(CR1R2)-R-.) and trimethylsilyl cations. The above mentioned product pattern and quantum-chemical calculations on characteristic parameters of the ground state conformers and the product cations, show that the electron transfer occurs completely unhindered after diffusional encounter, i.e. after each approach of the reactants. Therefore the statistics of the rotation conformers mixture is reflected by the pattern and ratio of the product transients such as metastable radical cations and radicals derived from a dissociative radical cation. Up to now this is the only case where a diffusion-controlled electron transfer reaction proceeding in the nanosecond scale is influenced by intramolecular rotation motions within the electron donor.

Molecular oscillations are accompanied with electron density fluctuations mainly of the higher occupied orbitals. This is particularly marked for rotational motions of large substituents of aromatic molecules. Hence, the extremely rapid and non-adiabatic free electron transfer from benzyltrimethylsilanes to n-butyl chloride parent radical cations affords the existence of different short-living molecular conformers. This is indicated by a characteristic pattern of transient products including metastable radical cations and fragment radicals derived from a dissociative and very unstable ionisation product.

The synthesis of p-acetyl and p-benzoyl derivatives of various benzylsilanes (p-R1CO–C6H4–CR2R3–SiMe3: R1, R2, R3=Me, H, H; Me, H, Ph; Me, H, Me; Me, Me, Me; Ph, H, H; Ph, H, Ph; Ph, H, Me; Ph, Me, Me), xanthenes (3-benzoyl and 3,6-dibenzoyl), and 9-Me3Si-xanthenes (3-benzoyl, 3,6-dibenzoyl, 3-acetyl and 3,6-diacetyl) using Friedel–Crafts reactions and typical Lewis acid catalysts (AlCl3, FeCl3, ZnCl2) is described. Desilylation side reactions caused by AlCl3 become significant in substrates with weak carbon–silicon bonds (e.g. Ph2CH–SiMe3), and in the case of Ph3C–SiMe3 only the desilylation product Ph3CH was isolated.