A series of bithiophene derivatives bearing an intramolecular monosilanylene or disilanylene bridge between the β,β‘-positions were synthesized, and their properties were
investigated. UV spectral and cyclic voltammetric analyses of the silicon-bridged bithiophenes
indicated that they have lower lying LUMOs, relative to those for bithiophene and methylene
bridged bithiophenes, probably due to σ*−π* interaction between the silicon atom(s) and
bithiophene π-orbitals, in good agreement with the results of theoretical calculations using
simplified model compounds based on RHF/6-31G. The silicon-bridged bithiophenes exhibit
high electron-transporting properties, and triple-layer-type electroluminescent (EL) devices,
using the silicon-bridged bithiophenes, tris(8-quinolinolato)aluminum(III) complex (Alq), and
N,N‘-diphenyl-N,N‘-di-m-tolylbiphenyl-4,4‘-diamine (TPD) as the electron-transporting,
emitting, and hole-transporting layers, respectively, emitted strong EL.
Poly [4,4'-(tetramethyldisilanylene)biphenylylene] (la), poly [4,4'-(tetraethyldisilanylene)biphenylylene] (lb), and poly[4,4'-( 1,2-dimethyl-1,2-diphenyldisilanylene)biphenylylene] (lc) were prepared by the condensation reaction of the respective 4,4'-bis(chlorosilyl)biphenyls with sodium. Poly[4,biphenylylene] (Id), poly [4,4'-(methylphenylsilylene)biphenylylene] (le), and poly[3,3'-(dimethylsilylene)biphenylylene] (If) were obtained from the nickel-catalyzed dehalogenative coupling of the products formed from the reaction of the respective bis(bromophenyl)-substituted silanes with magnesium. Poly[(disilanylene)terphenylylenes] 2a-c and poly[(disilanylene)quaterphenylylenes] 3a-c were also synthesized by the similar nickel-catalyzed dehalogenative coupling reactions of di-Grignard reagents prepared from the respective bis(4-bromophenyl)-substituted disilanes, with p-dibromobenzene and 4,4'-dibromobiphenyl, respectively. The disilanylene-substituted polymers la-c, 2a-c, and 3a-c were found to be photoactive. Irradiation of the thin solid films prepared from these polymers in air resulted in scission of the Si-Si bonds, leading to the formation of degradation products including silanol and siloxy units. Poly[(silylene)biphenylylenes] Id-f were found to be inert toward UV irradiation. Photochemical behavior of the disilanylene-substituted polymers was also investigated in solution, and photoactivities of these polymers were found to decrease with increasing the degree of an extension of the -electron system. The results of crystal orbital calculations for poly[(disilanylene)oligophenylylenes] have also been reported.
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