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.

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.