The photo-Fries rearrangement of 9-trimethylsilyl substituted xanthenes

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dc.contributor.authorSiskos, M. G.en
dc.contributor.authorZarkadis, A. K.en
dc.contributor.authorGritzapis, P. S.en
dc.contributor.authorBrede, O.en
dc.contributor.authorHermann, R.en
dc.contributor.authorMelissas, V. S.en
dc.contributor.authorGurzadyan, G. G.en
dc.contributor.authorTriantafyllou, A. S.en
dc.contributor.authorGeorgakilas, V.en
dc.date.accessioned2015-11-24T16:50:57Z
dc.date.available2015-11-24T16:50:57Z
dc.identifier.issn1010-6030-
dc.identifier.urihttps://olympias.lib.uoi.gr/jspui/handle/123456789/9664
dc.rightsDefault Licence-
dc.subjectphoto-fries rearrangementen
dc.subjecttrimethylsilyl xanthenesen
dc.subjectlaser flash photolysisen
dc.subjectfluorescenceen
dc.subjectab initioen
dc.subjectbond-dissociation enthalpiesen
dc.subjectelectron-transferen
dc.subjectaryldisilane photochemistryen
dc.subjectorganosilicon compoundsen
dc.subjectradical cationsen
dc.subjectexcited-stateen
dc.subjectmechanismen
dc.subjectlaseren
dc.subjectspectroscopyen
dc.subjectresonanceen
dc.titleThe photo-Fries rearrangement of 9-trimethylsilyl substituted xanthenesen
heal.abstractIn 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
heal.accesscampus-
heal.fullTextAvailabilityTRUE-
heal.identifier.primaryDOI 10.1016/j.jphotochem.2006.01.006-
heal.identifier.secondary<Go to ISI>://000239712300003-
heal.identifier.secondaryhttp://ac.els-cdn.com/S1010603006000098/1-s2.0-S1010603006000098-main.pdf?_tid=f71a4ede-356f-11e3-80e0-00000aacb35d&acdnat=1381824255_7bce31deaa4ad238357f8616fc9d9325-
heal.journalNameJournal of Photochemistry and Photobiology a-Chemistryen
heal.journalTypepeer reviewed-
heal.languageen-
heal.publicationDate2006-
heal.publisherElsevieren
heal.recordProviderΠανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείαςel
heal.typejournalArticle-
heal.type.elΆρθρο Περιοδικούel
heal.type.enJournal articleen

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