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.
ABSTRACT
Starting from a silicon dichloro substituted silole and a silacyclobutene, a series of new organosilicon-based spiro compounds was synthesized by using standard organometallic reaction procedures. The spiro compounds that combine two organic photoactive subunits at one silicon center were fully characterized by the usual analytical and spectroscopic methods, which include molecular structure determination by single-crystal X-ray analysis. Photoluminescence spectra of the compounds were recorded in the solid state and also as dilute solutions in THF. Interpretation of the spectra revealed that photoluminescence in this series of compounds originated from the stilbene or its vinylogue subunits. Different linkages of these groups to the silicon atoms (cyclic or open structures, four- or five-membered cycles) strongly affected both the excitation and the emission spectra, which show different emission maxima depending on the state of the sample (solid state or in solution) and the wavelength of light used for excitation. Thus, owing to their optoelectronic properties these compounds might be useful tools for the design of sensitive sensor materials and of optical switches.
