Self-assembly of chiral-at-end diketopyrrolopyrroles: symmetry dependent solution and film optical activity and photovoltaic performanace

Hume, Paul A.; Monks, James P.; Pop, Flavia; Davies, E. Stephen; MacKenzie, Roderick C.I.; Amabilino, David B.

Abstract

Chiral thiophene-diketopyrrolopyrrole derivatives have been synthesised to investigate the potential of stereochemistry and symmetry as a means of modulating properties by influencing self-assembly of these purely organic materials. In particular, derivatives of diketopyrrolopyrrole were employed because of their proven interest as dyes, especially for organic solar cells. The natural product myrtenal was used as the source of stereochemistry, introduced through a Kröhnke reaction of a thiophenebearing pyridinium salt and diketopyrrolopyrroles were prepared through Suzuki coupling with this chiral moiety at one end only as well as at both ends. Absorption spectroscopy and electrochemistry confirmed the potential suitability of the compounds for photovoltaic devices. The nanostructures formed by the compounds have been probed with circular dichroism spectroscopy in solution and in films. It is shown that a chiral C2 symmetric molecule assembles in solution giving a strong circular dichroic signal while as a film this optical activity is nulled, whereas an asymmetric homologue is most optically active as a thin film. The X-ray crystal structure of the asymmetric compound shows a polar order of the molecules that might explain this observation. The lack of optical activity in solution is very likely a result of the high solubility of the compound. The results reaffirm the sensitivity of circular dichroism spectroscopy to inter-chromophore organisation, whereas absorption spectroscopy in the visible region reveals only slight changes to the bands. The differing order in the compounds also affects their performance in bulk heterojunction photovoltaic devices. Atomic force microscopy of the blended thin films with the fullerene derivative usually employed (PC61BM) showed that smooth and well mixed films were achieved, with the conditions required during spin coating depending greatly on the derivative, because of their differing solubility. The apparently better performance of the symmetrical compound (although with very low efficiency) is probably a result of the alignment of the molecules inferred by the circular dichroism experiments, whereas the asymmetric compound presumably adopts a twisted supramolecular organisation.

Citation

Hume, P. A., Monks, J. P., Pop, F., Davies, E. S., MacKenzie, R. C., & Amabilino, D. B. (2018). Self-assembly of chiral-at-end diketopyrrolopyrroles: symmetry dependent solution and film optical activity and photovoltaic performanace. Chemistry - A European Journal, 24(54), 14461-14469. https://doi.org/10.1002/chem.201802610

• Journal Article Type: Article
• Acceptance Date: Jul 26, 2018
• Online Publication Date: Jul 26, 2018
• Publication Date: Sep 25, 2018
• Deposit Date: Oct 1, 2018
• Publicly Available Date: Jul 27, 2019
• Journal: Chemistry - A European Journal
• Print ISSN: 0947-6539
• Electronic ISSN: 1521-3765
• Publisher: Wiley
• Peer Reviewed: Peer Reviewed
• Volume: 24
• Issue: 54
• Pages: 14461-14469
• DOI: https://doi.org/10.1002/chem.201802610
• Public URL: https://nottingham-repository.worktribe.com/output/1140211
• Publisher URL: https://onlinelibrary.wiley.com/doi/abs/10.1002/chem.201802610
• Additional Information: This is the pre-peer reviewed version of the following article: P. A. Hume, J. P. Monks, F. Pop, E. S. Davies, R. C. I. MacKenzie, D. B. Amabilino, Chem. Eur. J. 2018, which has been published in final form at https://doi.org/10.1002/chem.201802610. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions