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Synthesis, Structure, Photoluminescence and Photoreactivity of 2,3‐Diphenyl‐4‐neopentyl‐1‐silacyclobut‐2‐enes
Author(s) -
Yan Duanchao,
Thomson Mark D.,
Backer Michael,
Bolte Michael,
Hahn Robert,
Berger Robert,
Fann Wunshain,
Roskos Hartmut G.,
Auner Norbert
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200901323
Subject(s) - chromophore , quantum yield , photoluminescence , photochemistry , chemistry , absorption (acoustics) , luminescence , absorption spectroscopy , fluorescence , excited state , solubility , materials science , physics , composite material , optoelectronics , quantum mechanics , nuclear physics
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 ∼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‐SiC 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.