Comparative study on the structural, optical, and electrochemical properties of bithiophene-fused benzo[c]phospholes.
Three types of bithiophene-fused benzo[c]phospholes were successfully prepared by Ti(II)-mediated cyclization of the corresponding dialkynylated bithiophene derivatives as a key step. Each sigma(3)-phosphorus center of the benzo[c]phosphole subunits was readily transformed into sigma(4)-phosphorus center by Au coordination or oxygenation. In addition, the bithiophene subunit was functionalized at the alpha,alpha'-carbon atoms by Pd-catalyzed cross-coupling reactions with heteroarylmetals and by an S(N)Ar reaction with hexafluorobenzene. The experimentally observed results (NMR spectroscopy, X-ray analysis, UV/Vis absorption/fluorescence spectroscopy, and cyclic/differential-pulse voltammetry) have revealed that the structural, optical, and electrochemical properties of the bithiophene-fused benzo[c]phospholes vary considerably depending on the pi-conjugation modes at the bithiophene subunits and the substituents of the heterocyclopentadiene components. The appropriately ring-annulated sigma(3)-P derivatives and sigma(4)-P-AuCl complexes were found to emit fluorescence in the orange-red region, and the sigma(4)-P-oxo derivatives proved to undergo reversible one-electron reduction at -1.4 to -1.8 V (vs ferrocene/ferrocenium). These results indicate that the bithiophene-fused benzo[c]phospholes possess narrow HOMO-LUMO gaps and low-lying LUMOs, which was confirmed by density functional theory calculations of their model compounds. The time-of-flight measurement of an ITO/benzo[c]phosphole/Al device showed that the electron mobility in the P-oxo derivative is one-order higher than that in Alq(3) at low electric fields. The present study demonstrates that the arene-fused benzo[c]phosphole skeleton could be a highly promising platform for the construction of a new class of phosphole-based optoelectrochemical materials.
Matano
Yoshihiro
Y
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. matano@scl.kyoto-u.ac.jp
Miyajima
Tooru
T
Fukushima
Tatsuya
T
Kaji
Hironori
H
Kimura
Yoshifumi
Y
Imahori
Hiroshi
H
D018360:Crystallography, X-Ray; D003500:Cyclization; D004563:Electrochemistry; D004583:Electrons; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D009943:Organophosphorus Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D013237:Stereoisomerism; D013876:Thiophenes / Q000737:chemistry*
D009943:Organophosphorus Compounds; D013876:Thiophenes