The photo-Fries rearrangement of 9-trimethylsilyl substituted xanthenes

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 (Φfl = 0.03 for xanthene, 0.008 for 1 and <10−4 for 2 in MeCN) and the fluorescence lifetimes (τfl = 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 kcal/mol for 2 in MeCN). Ab initio calculations predict a ‘roof-like’ structure for 1 with folding angles 30° for the S0 and 20° 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-1H-xanthen-1-yl)-silane, 2CHT) are formed within the 20 ns laser pulse and show absorption spectra peaking up at 410 and 403 nm, respectively. Additionally, small amounts of the corresponding 9-xanthyl radicals were detected as a result of the CSi bond rupture. Using ps-laser flash photolysis (266 nm laser, MeCN) we observed a broad absorption spectrum peaking up at 960 nm 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 (S1 → Sn absorption). We assume the S1 state as the origin of the photo-Fries rearrangement, giving via CSi bond dissociation a singlet geminal radical pair (9-xanthyl radical + SiMe3). In the next step, the radical pair undergoes predominantly in-cage recombination to the persistent photo-Fries intermediates with high quantum yields (Φ1CHT = 0.70 and Φ2CHT = 0.50), while to a lesser extent it escapes the solvent cage (Φ = 0.30 for 1) and undergoes typical free radical reactions (e.g., scavenging with O2). The estimation of the above quantum yields was possible only after determination of the absorption coefficients (ɛ) of the photo-Fries intermediates [ɛ1CHT(410 nm) = 16,800 M−1 cm−1 and ɛ2CHT(403 nm) = 15,900 M−1 cm−1] using three independent methods; this represents the first example in the literature.