title: Synthesis, structure, photoluminescence and photoreactivity of 2,3-diphenyl-4-neopentyl-1-silacyclobut-2-enes.
abstract: A series of six 2,3-diphenyl-4-neopentyl-1-silacyclobut-2-enes with different 1,1-substituents has been prepared and characterized by single-crystal X-ray crystallography. These compounds possess a cis-stilbene-like chromophore involving also the four-membered ring, and exhibit a photophysical behavior similar to that of previously reported 1,2-diphenyl-cycloalkenes. This chromophore system is confirmed by a theoretical investigation of the electronic structure and excitation spectra. The absorption and photoluminescence of selected derivatives were studied in solution, as solid powder samples, and in doped-polymer thin films. In well-dissolved solution, the silacyclobutenes show only very weak fluorescence emission (quantum yield approximately 0.1%), due to competition with photochemical and non-radiative photophysical relaxation. When the solubility is degraded in a poor (aqueous) solvent, the formation of nanoscale aggregates leads to a significant enhancement factor in the emission intensity, due to the suppression of the photoreactivity in the more rigid molecular environment, although the quantum yield still remains below a few percent. In the solid-state, however, photoreactivity is completely suppressed leading to fluorescence quantum efficiencies of 8-23% depending on the 1,1-substituents, which demonstrates these compounds' potential as chromophores for condensed-phase luminescence applications. Two dominant competing photochemical reactions have been identified in solution (for excitation in the lowest-energy absorption band, >260 nm), which are analogous to related (sila-)cyclobutenes and stilbenoids. The first involves ring-opening due to cleavage of the 1,4-Si-C bond to form metastable silabutadienes, which was confirmed by isolating the stereospecifically formed allylsilane which results from a secondary reaction with trapping agents such as methanol or water. The second photochemical reaction involves ring closure of the 2,3-diphenyl substructure to form a dihydrophenanthrene analogue, which was confirmed by isolating the phenanthrene derivative that results following subsequent hydrogen abstraction in the presence of oxygen. Measurements of the silacyclobutenes in doped-polymer thin films reveal a spectroscopic behavior ranging from that in solution to the nano-aggregate case as the silacyclobutene dopant concentration is increased.
authors.affiliation: Institut für Anorganische Chemie, Johann-Wolfgang-Goethe-Universität, Max-von-Laue-Str. 7, 60439 Frankfurt am Main (Germany), Fax: (+49) 69-798-29188.
authors.forename: Duanchao
authors.initials: D
authors.lastname: Yan
authors.affiliation: Physikalisches Institut, Johann-Wolfgang-Goethe-Universität, 60054 Frankfurt am Main (Germany).
authors.forename: Mark D
authors.initials: MD
authors.lastname: Thomson
authors.affiliation: Institut für Anorganische Chemie, Johann-Wolfgang-Goethe-Universität, Max-von-Laue-Str. 7, 60439 Frankfurt am Main (Germany), Fax: (+49) 69-798-29188.
authors.forename: Michael
authors.initials: M
authors.lastname: Backer
authors.affiliation: Institut für Anorganische Chemie, Johann-Wolfgang-Goethe-Universität, Max-von-Laue-Str. 7, 60439 Frankfurt am Main (Germany), Fax: (+49) 69-798-29188.
authors.forename: Michael
authors.initials: M
authors.lastname: Bolte
authors.affiliation: Physikalisches Institut, Johann-Wolfgang-Goethe-Universität, 60054 Frankfurt am Main (Germany).
authors.forename: Robert
authors.initials: R
authors.lastname: Hahn
authors.affiliation: Frankfurt Institute for Advanced Studies (FIAS), Johann Wolfgang Goethe-Universität, 60438 Frankfurt am Main (Germany).
authors.forename: Robert
authors.initials: R
authors.lastname: Berger
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei (Taiwan).
authors.forename: Wunshain
authors.initials: W
authors.lastname: Fann
authors.affiliation: Physikalisches Institut, Johann-Wolfgang-Goethe-Universität, 60054 Frankfurt am Main (Germany).
authors.forename: Hartmut G
authors.initials: HG
authors.lastname: Roskos
authors.affiliation: Institut für Anorganische Chemie, Johann-Wolfgang-Goethe-Universität, Max-von-Laue-Str. 7, 60439 Frankfurt am Main (Germany), Fax: (+49) 69-798-29188.
authors.forename: Norbert
authors.initials: N
authors.lastname: Auner
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