Modelling charge transport of discotic liquid-crystalline triindoles: the role of peripheral substitution

We have performed a multiscale approach to study the influence of peripheral substitution in the semiconducting properties of discotic liquid-crystalline triindoles. Charge carrier mobility as high as 1.4 cm V s was experimentally reported for triindoles substituted with alkynyl chains on the periphery (Gómez-Lor et al. Angew. Chem., Int. Ed., 2011, 50, 7399-7402). In this work, our goal is to get a deeper understanding of both the molecular electronic structure and microscopic factors affecting the charge transport properties in triindoles as a function of the spacer group connecting the central cores with the external alkyl chains (i.e., alkyne or phenyl spacers groups). To this end, we first perform Quantum Mechanical (QM) calculations to assess how the peripheral substitution affects the electronic structure and the internal reorganization energy. Secondly, boxes of stacked molecules were built and relaxed through molecular dynamics to obtain realistic structures. Conformational analysis and calculations of transfer integrals for closed neighbours were performed. Our results show that the insertion of ethynyl spacers between the central aromatic core and the flexible peripheral chains results in lower reorganization energies and enhanced intermolecular order within the stacks with a preferred cofacial 60° staggered conformation, which would result in high charge-carrier mobilities in good agreement with the experimental data. This work allows a deeper understanding of charge carrier mobility in columnar phases, linking the structural order at the molecular level to the property of interest, i.e. the charge carrier mobility. We hope that this understanding will improve the design of systems at the supramolecular level aiming at obtaining a more defined conducting channel, higher mobility and smaller fluctuations within the column.
D.A.S.F. gratefully acknowledges the financial support from CNPq, grants 304020/2016-8 and 407682/2013-9, and FAP-DF grants 0193.001.062/2015 and 193.001.284/2016. ML thanks SeRC (Swedish e-Science Research Center) for funding and SNIC (Swedish National Infrastructure for Computing) for providing computer resources (project ID snic2015-1-420). Research at University of Malaga was supported by MINECO (CTQ2015-66897-P) and Junta de Andalucia (P09-4708). BGL acknowledges the financial support from MINECO (CTQ2016-78557-R) and Comunidad de Madrid S2013/MIT.