The photo-Fries rearrangement of 9-trimethylsilyl substituted xanthenes

Abstract

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 ( Φ fl = 0.03 for xanthene, 0.008 for 1 and <10 −4 for 2 in MeCN) and the fluorescence lifetimes ( τ 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 ( E S = 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 S 0 and 20° for the S 1 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-(1 H-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 C Si 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 (S 1 → S n absorption). We assume the S 1 state as the origin of the photo-Fries rearrangement, giving via C Si bond dissociation a singlet geminal radical pair (9-xanthyl radical + SiMe 3). In the next step, the radical pair undergoes predominantly in-cage recombination to the persistent photo-Fries intermediates with high quantum yields ( Φ 1CHT = 0.70 and Φ 2CHT = 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 O 2). The estimation of the above quantum yields was possible only after determination of the absorption coefficients ( ɛ) of the photo-Fries intermediates [ ɛ 1CHT (410 nm) = 16,800 M −1 cm −1 and ɛ 2CHT (403 nm) = 15,900 M −1 cm −1] using three independent methods; this represents the first example in the literature.