Phase stability trend in linear oligothiophene materials


J. Costa1, C. Lima1, L. Gomes2 and L. Santos1

1CIQ, Departamento de Quimica e Bioquimica, Faculdade de Ciencias da Universidade do Porto, Porto, Portugal
2CIAGEB, Faculdade de Ciencias da Saude, Universidade Fernando Pessoa, Porto, Portugal

Keywords: vapour pressures, fusion, sublimation, DSC, Knudsen effusion, phase diagram
property: phase stability, vapour pressures, enthalpies and entropies of phase transition
material: oligothiophenes, organic semiconductors

The experimental determination of the properties values associated with thermodynamic equilibrium is essential for the interpretation and understanding of the structural and phase stability for organic compounds, which is a key knowledge to the development of new organic semiconductors and materials. The energetic and structural characterization of organic semiconductor compounds is a recent research target in our laboratory [1, 2]. This work involves the thermodynamic and structural study of a series of the α-polythiophene oligomer materials. Oligothiophenes are aromatic polyheterocyclic compounds and the volatility and phase stability along an homologous series of these organic semiconductor materials was evaluated. The effect of the successive increase of thiophene rings in the thermodynamic properties related to the solid-liquid and solid-gas equilibria was explored. The studied compounds were α-quaterthiophene (α-4T), α-quinquethiophene (α-5T) and α-sexithiophene (α-6T). For each of these the vapour pressures at different temperatures, using the Knudsen effusion method incorporated with a quartz crystal microbalance were measured [3]. Accordingly, the standard molar enthalpies, entropies and Gibbs energies of sublimation were determined. The fusion temperatures and the standard molar enthalpies of fusion were determined in a power compensated differential scanning calorimeter. An even-odd effect was observed in some thermodynamic parameters indicating that the even-numbered oligomers have higher stability in their crystalline phase.

Thanks are due to Fundação para a Ciencia e Tecnologia (FCT) and Programa Operacional Ciencia e Inovaçao 2010 (POCI 2010) for the financial support to Project POCI/QUI/61873/2004, supported by the European Community Fund FEDER. J.C.S. Costa and C.F.R.A.C. Lima also thanks FCT and the European Social Fund (ESF) under the third Community Support Framework (CSF) for the award of Ph.D. Research Grants SFRH/BD/74367/2010 and SRFH/BD/29394/2006, respectively.

References
  1. M.A.A. Rocha, C.F.A.C. Lima and L.M.N.B.F. Santos, J. Chem. Therm. 40, 1458 (2008)

  2. J.C.S. Costa, C.F.R.A.C. Lima, M.A.A. Rocha, L.R. Gomes and L.M.N.B.F. Santos, J. Chem. Therm. 43, 133 (2011)

  3. L.M.N.B.F. Santos, L.M.S.S. Lima, C.F.A.C. Lima, F.D. Magalhães, M.C. Torres, B. Schröder and M.A.V. Ribeiro da Silva, J. Chem. Therm. (2011) doi: 10.1016/j.jct.2010.12022

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