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

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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 kdiss = 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 (Φrad = 0.9); the intervention of the triplet state is clearly demonstrated through the phosphorescence spectrum and quenching experiments with ferrocene (kq = 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 λmax = 340 nm was determined (e = 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 Φrad = 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 (Φrad = 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 Φrad = 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 (kisc = 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 (kisc = 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)].

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Authors and Affiliations

  1. Department of Chemistry, University of Ioannina, 451 10, Ioannina, Greece

    Antonios K. Zarkadis, Vassilios Georgakilas, Gerasimos P. Perdikomatis, Stavroula Skoulika & Michael G. Siskos

  2. Institut für Physikalische und Theoretische Chemie, Technische Universität München, 85748, Garching, Germany

    Anton Trifonov & Gagik G. Gurzadyan

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  1. Antonios K. Zarkadis
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Correspondence to Antonios K. Zarkadis.

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† Electronic supplementary information (ESI) available: Fig. A–D, Table A, absorption coefficient of 6. See http://www.rsc.org/suppdata/pp/b5/b502089a/

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Zarkadis, A.K., Georgakilas, V., Perdikomatis, G.P. et al. Triplet- vs. singlet-state imposed photochemistry. The role of substituent effects on the photo-Fries and photodissociation reaction of triphenylmethyl silanes. Photochem Photobiol Sci 4, 469–480 (2005). https://doi.org/10.1039/b502089a

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