The photo-Fries rearrangement of 9-trimethylsilyl substituted xanthenes

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The photo-Fries rearrangement of 9-trimethylsilyl substituted xanthenes (EN)

Siskos, M. G. (EN)

Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας (EL)
Siskos, M. G. (EN)

In this paper, the absorption and fluorescence spectra of xanthene, 9-trimethylsilyl xanthene (1) and 9-methyl-9-trimethylsilyl xanthene (2) are presented and discussed with the help of CNDO/S and ab initio calculations. introduction of the trimethylsilyl group at the 9-position of the xanthene molecule considerably shifts the short-wavelength band (246 nm) to the red and reduces the fluorescence quantum yields (Phi(fl) = 0.03 for xanthene, 0.008 for 1 and < 10(-4) for 2 in MeCN) and the fluorescence lifetimes (tau(fl) = 7.4 ns for xanthene, 220 ps for 1 and < 100 ps for 2 in MeCN), while it does not affect seriously the long-wavelength band and the singlet excited state energies (Es = 97.7 kcal/mol for xanthene, 94.5 kcal/mol for 1 and 97.4 kcaVmoI for 2 in MeCN). Ab initio calculations predict a 'roof-like' structure for 1 with folding angles 30 degrees for the So and 20 degrees for the S1 state. Laser (248, 266 and 308 nm) and lamp photolysis (254 nm) of 1 and 2 (MeCN or cyclohexane) results in [1,3]-trimethylsilyl rearrangement into the ortho-position of the xanthene moiety in the sense of a photo-Fries type reaction. The corresponding photo-Fries intermediates (exocyclic cyclohexatrienes: trimethyl-(1H-xanthen-1-yl)-silane, 1CHT and trimethyl-(9-methyl-lH-xanthen-1-yl)-silane, 2CHT) are formed within the 20 ns laser pulse and show absorption spectra peaking up at 4 10 and 403 nm, respectively. Additionally, small amounts of the corresponding 9-xanthyl radicals were detected as a result of the C-Si bond rupture. Using ps-laser flash photolysis (266 nm laser, MeCN) we observed a broad absorption spectrum peaking up at 960 mn and decaying monoexponentially with a lifetime of 130 ps, close to the measured fluorescence lifetime. We assigned therefore this transient to the singlet excited state of 1 (S-1 -> S-n, absorption). We assume the S, state as the origin of the photo-Fries rearrangement, giving via C-Si bond dissociation a singlet gerninal radical pair (9-xanthyl radical(+) (SiMe3)-Si-center dot)- In the next step, the radical pair undergoes predominantly in-cage recombination to the persistent photo-Fries intermediates with high quantum yields (Phi(1CHT) = 0.70 and Phi(2CHT) = 0.50), while to a lesser extent it escapes the solvent cage (Phi = 0.30 for 1) and undergoes typical free radical reactions (e.g., scavenging with O-2). The estimation of the above quantum yields was possible only after determination of the absorption coefficients (8) of the photo-Fries intermediates [epsilon(ICHT)(410 nm) = 16,800 m(-1) cm(-1) and epsilon(2CHT)(403 nm) = 15,900 W cm(-1) using three independent methods; this represents the first example in the literature. (c) 2006 Elsevier B.V All rights reserved. (EN)

photo-fries rearrangement (EN)

Journal of Photochemistry and Photobiology a-Chemistry (EN)

Αγγλική γλώσσα


Elsevier (EN)

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