Six star-shaped oligomers containing triphenylamine (D1–D3) and benzene unit (D4–D6) as cores have been synthesized by Wittig condensation or Heck coupling reaction using aromatic aldehydes and triphenylphosphonium salts or aromatic halogenated compounds with vinyl triphenylamine. All oligomers have well-defined molecular structure and high purity. Characterization of the oligomers was made by FT-IR, 1H-NMR spectroscopy, UV-Vis, and fluorescence spectroscopy. The electrochemical behavior was studied by cyclic voltammetry (CV). The cyclic voltammograms have revealed that oligomers undergo quasireversible or irreversible redox processes. The irreversible process is associated with electrochemical polymerization of oligomers by dimerization of unsubstituted triphenylamine groups. Thermal characterization was accomplished by TGA and DSC methods and evidenced that all oligomers were stable materials until 250°C and have formed stable molecular glasses after first heating scan. 1. Introduction Conjugated oligomers with linear, highly branched and dendrimer structures form an important class of electro- and photoactive materials, investigated both in academic and industrial laboratories [1–9]. These architectures have advantages to offer molecules with a well-defined form and structure, a high chemical purity, and degree of order, being characterized by a polydispersity degree of one. It is important to note that the purity of materials is vital for the long-term stability of optoelectronic devices. Dendrimers can be obtained using laborious step-by-step synthesis in a convergent or divergent methodology [10]. The advantage of using small conjugated compounds is based on the possibility of tuning their photophysical properties by changing the chemical structure, for example, by introduction of side substituents, end-capping groups, insertion of certain specific functional groups, and by changing the oligomer length. Moreover, conjugated oligomers are used as model compounds for conducting polymers since their monodispersity, defectless structure, and better supramolecular organization in the solid state facilitate their experimental and theoretical investigations. The real interests for conjugated oligomers emerge also from interesting application such as active components in organic electronic or electrochemical devices, such as organic light emitting diodes (OLEDs) [11–15], photovoltaic cells [16, 17], optical power limiting [18], and field-effect transistors [19]. One of the important challenging goals of the conjugated oligomer chemistry is to develop new
References
[1]
T. A. Skotheim and J. R. Reynolds, Handbook of Conducting Polymers, CRC Press, New York, NY, USA, 3rd edition, 2007.
[2]
A. G. MacDiarmid, “Synthetic metals: a novel role for organic polymers (Nobel lecture),” Angewandte Chemie, International Edition, vol. 40, pp. 2581–2590, 2001.
[3]
A. J. Heeger, “Semiconducting and metallic polymers: the fourth generation of polymeric materials (Nobel lecture),” Angewandte Chemie, International Edition, vol. 40, pp. 2591–2611, 2001.
[4]
J. Heinze, “Electronically conducting polymers,” Topics in Current Chemistry, vol. 152, pp. 1–47, 1990.
[5]
S. J. Higgins, “Conjugated polymers incorporating pendant functional groups-synthesis and characterization,” Chemical Society Reviews, vol. 26, no. 4, pp. 247–257, 1997.
[6]
T. Noda, H. Ogawa, N. Noma, and Y. Shirota, “Organic light-emitting diodes using a novel family of amorphous molecular materials containing an oligothiophene moiety as colour-tunable emitting materials,” Journal of Materials Chemistry, vol. 9, no. 9, pp. 2177–2181, 1999.
[7]
T. Otsubo, Y. Aso, and K. Takimiya, “Functional oligothiophenes as advanced molecular electronic materials,” Journal of Materials Chemistry, vol. 12, no. 9, pp. 2565–2575, 2002.
[8]
H. Nakanishi, Y. Aso, and T. Otsubo, “The longest class of oligothiophenes,” Synthetic Metals, vol. 101, no. 1–3, pp. 604–605, 1999.
[9]
D. Sek, E. Grabiec, H. Janeczek et al., “Structure-properties relationship of linear and star-shaped imines with triphenylamine moieties as hole-transporting materials,” Optical Materials, vol. 32, no. 11, pp. 1514–1525, 2010.
[10]
D. A. Tomalia, A. M. Naylor, and W. A. Goddard, “Starbust dendrimers: molecular-level control of size, shape, surface chemistry, topology, and flexibility from atoms to macroscopic matter,” Angewandte Chemie, International Edition, vol. 29, no. 2, pp. 138–175, 1990.
[11]
A. Kraft, A. C. Grimsdale, and A. B. Holmes, “Electroluminescent conjugated polymers—seeing polymers in a new light,” Angewandte Chemie, International Edition, vol. 37, no. 4, pp. 402–428, 1998.
[12]
B. W. D'Andrade and S. R. Forrest, “White organic light-emitting devices for solid-state lighting,” Advanced Materials, vol. 16, no. 18, pp. 1585–1595, 2004.
[13]
S. A. Jenekhe, “Excited-state complexes of conjugated polymers,” Advanced Materials, vol. 7, no. 3, pp. 309–311, 1995.
[14]
J. S. Miller, “Conducting polymers—materials of commerce,” Advanced Materials, vol. 5, no. 9, pp. 671–676, 1993.
[15]
R. L. Carroll and C. B. Gorman, “The genesis of molecular electronics,” Angewandte Chemie, International Edition, vol. 41, no. 23, pp. 4378–4400, 2002.
[16]
G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, “Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions,” Science, vol. 270, no. 5243, pp. 1789–1791, 1995.
[17]
Y. J. Cheng, S. H. Yang, and C. S. Hsu, “Synthesis of conjugated polymers for organic solar cell applications,” Chemical Reviews, vol. 109, no. 11, pp. 5868–5923, 2009.
[18]
J. D. Bhawalkar, G. S. He, and P. N. Prasad, “Nonlinear multiphoton processes in organic and polymeric materials,” Reports on Progress in Physics, vol. 59, no. 9, pp. 1041–1070, 1996.
[19]
J. H. Schon, A. Dodabalapur, C. Kloc, and B. Batlogg, “A light-emitting field-effect transistor,” Science, vol. 290, no. 5493, pp. 963–965, 2000.
[20]
J. Louie, J. F. Hartwig, and A. F. Fry, “Discrete high molecular weight triarylamine dendrimers prepared by palladium-catalyzed amination,” Journal of the American Chemical Society, vol. 119, no. 48, pp. 11695–11696, 1997.
[21]
B. O'Regan and M. Gr？tzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films,” Nature, vol. 353, no. 6346, pp. 737–740, 1991.
[22]
M. Zheng, F. Bai, and D. J. Zhu, “New light emitting materials: alternating copolymers with hole transport and emitting chromophores,” Journal of Applied Polymer Science, vol. 74, no. 14, pp. 3351–3358, 1999.
[23]
G. Lai, X. R. Bu, J. Santos, and E. A. Mintz, “Reinvestigation of the Vilsmeier-Haack formylation of triphenylamine,” Synlett, vol. 1997, no. 11, pp. 1275–1276, 1997.
[24]
T. Mallegol, S. Gmouh, M. A. A. Meziane, M. Blanchard-Desce, and O. Mongin, “Practical and efficient synthesis of tris(4-formylphenyl)amine, a key building block in materials chemistry,” Synthesis, no. 11, pp. 1771–1774, 2005.
[25]
M. Grigoras and L. Stafie, “Synthesis and characterization of linear, branched and hyperbranched triphenylamine-based polyazomethines,” Designed Monomers and Polymers, vol. 12, no. 2, pp. 177–196, 2009.
[26]
L. Vacareanu and M. Grigoras, “Synthesis and electrochemical characterization of new linear conjugated arylamine copolymers,” High Performance Polymers, vol. 23, no. 2, pp. 112–124, 2011.
[27]
Y. Kuwabara, H. Ogawa, H. Inada, and Y. Shirota, “Thermally stable multilayered organic electroluminescent devices using novel starburst molecules, 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) and 4,4′,4″-tris(3-methylphenylphenyl-amino)triphenylamine (m-MTDATA), as hole-transport materials,” Advanced Materials, vol. 6, no. 9, pp. 677–679, 1994.
[28]
K. M. Yeh, C. C. Lee, and Y. Chen, “Poly(4-vinyltriphenylamine): optical, electrochemical properties and its new application as a host material of green phosphorescent Ir(ppy)3 dopant,” Synthetic Metals, vol. 158, no. 14, pp. 565–571, 2008.
[29]
H. Saito, S. Ukai, S. Iwatsuki, T. Itoh, and M. Kubo, “Synthesis of soluble poly(arylenevinylene)s carrying various heterocycles as arylene units,” Macromolecules, vol. 28, no. 24, pp. 8363–8367, 1995.
[30]
X. Zhang, X. Yu, Y. Sun et al., “Synthesis and nonlinear optical properties of two new two-photon initiators: triphenylamine derivatives,” Optical Materials, vol. 28, no. 12, pp. 1366–1371, 2006.
[31]
E. Campaigne and W. L. Archer, “The use of dimethylformamide as a formylation reagent,” Journal of the American Chemical Society, vol. 75, no. 4, pp. 989–991, 1953.
[32]
F. H. C. Stewart, “The Preparation of some surface active alcohols containing the anthracene nucleus,” Australian Journal of Chemistry, vol. 13, no. 4, pp. 478–487, 1960.
[33]
R. Sander, V. Stuempflen, J. H. Wendorff, and A. Greiner, “Synthesis, properties, and guest-host systems of triphenylamine-based oligo(arylenevinylene)s: advanced materials for LED applications,” Macromolecules, vol. 29, no. 24, pp. 7705–7708, 1996.
[34]
X. Zhang, X. Yu, J. Yao, and M. Jiang, “Synthesis and nonlinear optical properties of two three-branched two-photon polymerization initiators,” Synthetic Metals, vol. 158, no. 21–24, pp. 964–968, 2008.
[35]
M. J. Plater and T. Jackson, “Polyaromatic amines. Part 3: synthesis of poly(diarylamino)styrenes and related compounds,” Tetrahedron, vol. 59, no. 25, pp. 4673–4685, 2003.
[36]
A. Heller, “Organic liquid scintillators. VI. Substituted distyrylbenzenes: scintillation properties and spectra of absorption and fluorescence,” The Journal of Chemical Physics, vol. 40, no. 10, pp. 2839–2850, 1964.
[37]
H. Y. Wang, G. Chen, X. P. Xu, H. Chen, and S. J. Ji, “The synthesis and photophysical properties of novel poly(diarylamino) styrenes,” Dyes and Pigments, vol. 88, no. 3, pp. 358–365, 2011.
[38]
Y. Jiang, J. Y. Wang, Y. Ma, Y. X. Cui, Q. F. Zhou, and J. Pei, “Large rigid blue-emitting π-conjugated stilbenoid-based dendrimers: synthesis and properties,” Organic Letters, vol. 8, no. 19, pp. 4287–4290, 2006.
[39]
E. T. Seo, R. F. Nelson, J. M. Fritsch, L. S. Marcoux, D. W. Leedy, and R. N. Adams, “Anodic oxidation pathways of aromatic amines. Electrochemical and electron paramagnetic resonance studies,” Journal of the American Chemical Society, vol. 88, no. 15, pp. 3498–3503, 1966.
[40]
S. C. Creason, J. Wheeler, and R. F. Nelson, “Electrochemical and spectroscopic studies of cation radicals. I. Coupling rates of 4-substituted triphenylaminium ions,” Journal of Organic Chemistry, vol. 37, no. 26, pp. 4440–4446, 1972.
[41]
K. Y. Chiu, T. X. Su, J. H. Li, T. H. Lin, G. S. Liou, and S. H. Cheng, “Novel trends of electrochemical oxidation of amino-substituted triphenylamine derivatives,” Journal of Electroanalytical Chemistry, vol. 575, no. 1, pp. 95–101, 2005.
[42]
L. Vacareanu and M. Grigoras, “Electrochemical characterization of arylene vinylene oligomers containing triphenylamine and carbazole units,” Journal of Applied Electrochemistry, vol. 40, no. 11, pp. 1967–1975, 2010.
