Highly luminescent polymer single crystals with anisotropic optical and electronic properties from appropriately designed, defect-free regioregular polythiophenes
Final Report Abstract
Poly(2´,5´-dioctyl-3-phenylthiophene) PDOPT exhibits an intriguing solid state structure which allows to study fundamental structure function relationships in the absence of π-π-interactions. Until the beginning of this project, PDOPT could only be synthesized using oxidative coupling with FeCl3, leading to relatively high but still limited regioregularity. The remaining defects did not allow for the reproducible formation of ordered thin films. This can be understood on the basis that structure formation in PDOPT is only governed by side chain crystallization and not – in opposite to P3HT – by π-π interactions. Thus, regio-defects in PDOPT are stronger defects compared to P3HT where for example single tail-to-tail defects can be incorporated into the crystal lattice. Hence, a certain number of defects hinders crystallization and structure formation much stronger in PDOPT than in P3HT. To access highly regioregular PDOPT, we have designed and synthesized new nickel catalysts for the first time, and have used Kumada catalyst transfer polycondensation (KCTP) successfully. The sterically demanding 2,5-dioctylphenyl side chain did not allow the usage of common, commercially available nickel catalysts to polymerize 2-bromo-5-chloromagnesio-(2´,5´-dioctyl)-3-phenylthiophene. Therefore, new nickel catalysts with bidentate P,N ligands were developed and investigated. Alternatively, direct arylation polycondensation (DAP) was used. This method allows to use simple monomer structures and streamlines synthesis. However, synthesis of PDOPT via DAP was limited by dehalogenation and thus only low molecular weight materials could be obtained. The best conditions to synthesize highly regioregular PDOPT with controlled molar masses therefore involves KCTP and catalyst 12. We have further analyzed the crystallization of PDOPT in films either by isothermal crystallization or by epitaxial crystallization. Using the latter method, a structural model could be determined by the Brinkmann group. In this model, PDOPT backbones are fully planarized despite the absence of stabilizing π-π interactions. This planarization within a crystal in which π-π interactions are absent is a speciality of PDOPT and caused by the sterically demanding side chains which surround and insulate the backbones from other. This effect can already been observed in single chains as probed by single molecule experiments carried out in the Hildner group. These aspects render PDOPT unique and further demonstrate the importance of side chains as well as side chain engineering for tailoring opto-electronic properties and finally the performance of organic electronic devices. Initial measurements of charge carrier mobility extracted from transistor devices indicate low mobility values, which is not surprising as π-π interactions are absent. Electrical measurements should only then be reconsidered if large crystalline structures can be prepared on electrode structures, which has so far be unsuccessful. The determination of absolute degrees of crystallinity of PDOPT samples remains to be done. A value of the enthalpy of melting for a 100% crystalline sample as a reference is not available. First solid state NMR experiments (collaboration with M. Hansen, U Münster) have been carried out on PDOPT to distinguish between amorphous and crystalline side chains, and to provide reference samples for DSC experiments with known degrees of crystallinity.
Publications
- Anisotropic Photophysical Properties of Highly Aligned Crystalline Structures of a Bulky Substituted Poly(thiophene). ACS Macro Lett. 2014, 3, 881–885
Y. Wang, B. Heck, D. Schiefer, J. O. Agumba, M. Sommer, T. Wen, G. Reiter
(See online at https://doi.org/10.1021/mz500411c) - Nickel Catalyst with a Hybrid P, N Ligand for Kumada Catalyst Transfer Polycondensation of Sterically Hindered Thiophenes. ACS Macro Lett. 2014, 3, 617–621
D. Schiefer, T. Wen, Y. Wang, P. Goursot, H. Komber, R. Hanselmann, P. Braunstein, G. Reiter, M. Sommer
(See online at https://doi.org/10.1021/mz500282j) - Highly Oriented and Crystalline Films of a Phenyl‐Substituted Polythiophene Prepared by Epitaxy: Structural Model and Influence of Molecular Weight. Macromolecules 2016, 49, 3452–3462
A. Hamidi‐Sakr, D. Schiefer, S. Covindarassou, L. Biniek, M. Sommer, M. Brinkmann
(See online at https://doi.org/10.1021/acs.macromol.6b00495) - Poly(3‐(2,5‐dioctylphenyl)thiophene) Synthesized by Direct Arylation Polycondensation: End Groups, Defects, and Crystallinity. Macromolecules 2016, 49, 7230–7237
D. Schiefer, H. Komber, F. Mugwanga Keheze, S. Kunz, R. Hanselmann, G. Reiter, M. Sommer
(See online at https://doi.org/10.1021/acs.macromol.6b01795) - Signatures of Melting and Recrystallization of a Bulky Substituted Poly(thiophene) Identified by Optical Spectroscopy. Macromolecules, 2017, 50 (17), 6829‐6839
F. Keheze, D. Raithel, T. Wu, D. Schiefer, M. Sommer, R. Hildner, G. Reiter
(See online at https://doi.org/10.1021/acs.macromol.7b01080) - Balancing steric and electronic effects of bidentate, mixed P,N ligands to control Kumada catalyst transfer polycondensation of a sterically hindered thiophene. Polym. Chem., 2018, 9, 3398‐3405
S. Hameury, C. Gourlaouen, M. Sommer
(See online at https://doi.org/10.1039/c8py00452h) - Direct observation of backbone planarization via side‐chain alignment in single bulky‐ substituted polythiophenes. PNAS, 2018, 201719303
D. Raithel, L. Simine, S. Pickel, K. Schötz, F. Panzer, S. Baderschneider, D. Schiefer, R. Lohwasser, J. Köhler, M. Thelakkat, M. Sommer, A. Köhler, P. J. Rossky and R. Hildner
(See online at https://doi.org/10.1073/pnas.1719303115)
