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

Zarkadis, A. K. (EN)

Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας (EL)
Zarkadis, A. K. (EN)

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))]. (EN)

laser flash-photolysis (EN)

Πανεπιστήμιο Ιωαννίνων (EL)
University of Ioannina (EN)

Photochemical & Photobiological Sciences (EN)

2005

<Go to ISI>://000229398300005

The Royal Society of Chemistry (EN)



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