Synthesis and characterization of N-carbazole end-capped oligofluorene-thiophenes
Graphical abstract
Introduction
Over the past few years, π-conjugated organic materials have been the subject of great interest due to the increasing development of potentially active components for a wide range of electronic and optoelectronic devices.1, 2 The simplicity in the modification of chemical structure, solubility, and optical properties of the organic materials makes them superior to those based on inorganic substances in both manufacturing costs of the devices and improving some technological aspects. In particular, oligothiophenes have shown excellent conductivity, electroluminescent behavior, and characteristics as advanced materials for organic field effect transistors (OFETs)3, 4 and organic light-emitting diodes (OLEDs).5, 6 With π-conjugated small molecules or oligomers, not only can they be obtained in high-purity and well-defined structures, but also their optical, electrochemical, and thermal properties can be easily tuned to the desired properties by changing the structure, e.g. solubilizing chains, end-capping groups, insertion of various functional groups, and changing the oligomer lengths. Oligothiophenes with different terminal α,α′ substituents such as aldehyde,7 diphenylamine,8 cyclophane,9 pyrene,10 bis(4-methylphenyl)aminophenyl,11 cyclohexen,12 ethylenedithio,13 and phenyl14 groups are a growing synthetic interest. Recently, a series of oligothiophenes terminated with fluorene moieties15, 16 have also been synthesized using palladium catalyzed Suzuki and Stille coupling reactions or nickel-catalyzed reductive dimerization. These materials exhibited interesting optical and electrochemical properties and have been shown to act as potential light-emitting materials in OLEDs and active components in OFETs. Carbazole has strong absorption in the near-UV region and a low redox potential. The electrochemical and spectroscopic properties of carbazole and its derivative have been extensively investigated.17 Chemically, carbazole can be easily functionalized at its 3-,6- or 9-positions and covalently linked to other molecular moieties.18, 19 Due to its unique optical, electrical, and chemical properties, carbazole has been used widely as a functional building block or substituent in the construction of organic molecules for use as light-emitting and hole-transporting layers in OLED devices,20, 21, 22, 23, 24, 25, 26, 27 as host materials for electrophosphorescent applications,26, 28 and as active components in solar cells.29, 30 Moreover, the thermal stability and glassy state durability of the organic molecules were found to be significantly improved upon incorporation of a carbazole moiety into the structure.31, 32 To our best knowledge, there are no reports on N-carbazole end-capped oligofluorene-thiophenes, the synthesis and physical properties of which still remain to be explored. Therefore, in this paper, we report a successful synthesis of a series of new N-carbazole end-capped oligofluorene-thiophenes with one, two, three, and four thiophene rings by the use of either nickel-catalyzed reductive dimerization or palladium catalyzed cross-coupling reaction. Their basic optical, electrochemical, and thermal properties have been investigated with the aim of understanding the structure–physical property relationships and developing novel molecular organic materials.
Section snippets
Synthesis and characterization
Scheme 1 illustrates the synthetic approach to the target oligomers 5, 8, 9, 12, and 15. The n-hexyl substituents were introduced on the C-9 position of fluorene ring to increase the solubility. In this synthetic sequence, the key intermediate to the carbazole end-capped oligofluorene-thiophenes 5, 8, 9, and 12 was 2-bromo-7-(carbazol-9-yl)-9,9-bis-n-hexylfluorene (4), the preparation of which we have reported previously. Earlier, we synthesized this compound by a Cu-mediated Ullmann
Conclusion
We have presented the synthesis of a series of new N-carbazole end-capped oligofluorene-thiophenes with one, two, three, and four thiophene units via a nickel-catalyzed reductive or palladium catalyzed coupling reactions of the corresponding bromo oligofluorene-thiophenes and their properties were investigated. The presence of carbazole and 9,9-bis-n-hexylfluorene rings at both ends of the oligomers has a significant effect on improving the material solubility and thermal stability. The optical
General procedures
Tetrahydrofuran (THF) was refluxed with sodium and benzophenone and distilled. Nitrobenzene was distilled from calcium hydride under high vacuum prior to use. Dichloromethane for cyclic voltammetry (CV) measurements was washed with concd H2SO4 and distilled twice from calcium hydride. All reagents and solvents were purchased from Aldrich, Acros, Fluka or Thai Suppliers and used as received unless otherwise stated. 1H and 13C NMR spectra were recorded on a Brüker AVANCE 300 MHz spectrometer using
Acknowledgements
This work was supported financially by the National Research Council of Thailand (NRCT) and Ubon Ratchathani University. We thank the Chulabhorn Research Institute (CRI) of Thailand for HRMS measurements.
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