Discovery of S···C≡N Intramolecular Bonding in a Thiophenylcyanoacrylate-Based Dye: Realizing Charge Transfer Pathways and Dye···TiO2 Anchoring Characteristics for Dye-Sensitized Solar Cells

Jacqueline M Cole; Martin A Blood-Forsythe; Tze-Chia Lin; Philip Pattison; Yun Gong; Álvaro Vázquez-Mayagoitia; Paul G Waddell; Lei Zhang; Nagatoshi Koumura; Shogo Mori

doi:10.1021/acsami.7b03522

Discovery of S···C≡N Intramolecular Bonding in a Thiophenylcyanoacrylate-Based Dye: Realizing Charge Transfer Pathways and Dye···TiO₂ Anchoring Characteristics for Dye-Sensitized Solar Cells

ACS Appl Mater Interfaces. 2017 Aug 9;9(31):25952-25961. doi: 10.1021/acsami.7b03522. Epub 2017 Jul 25.

Authors

Jacqueline M Cole^{1

2

3

4}, Martin A Blood-Forsythe¹, Tze-Chia Lin¹, Philip Pattison⁵, Yun Gong¹, Álvaro Vázquez-Mayagoitia³, Paul G Waddell^{1

6}, Lei Zhang¹, Nagatoshi Koumura⁷, Shogo Mori⁸

Affiliations

¹ Cavendish Laboratory, Department of Physics, University of Cambridge , J. J. Thomson Avenue, Cambridge, CB3 0HE, United Kingdom.
² ISIS Neutron and Muon Source, STFC Rutherford Appleton Laboratory , Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0QX, United Kingdom.
³ Argonne National Laboratory , 9700 S. Cass Avenue, Argonne, Illinois 60439, United States.
⁴ Department of Chemical Engineering and Biotechnology, University of Cambridge , West Cambridge Site, Philippa Fawcett Drive, Cambridge, CB3 0FS, United Kingdom.
⁵ Swiss Norwegian Beamlines, European Synchrotron Radiation Facility , F-38000 Grenoble, France.
⁶ Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, Lucas Heights NSW 2234, Australia.
⁷ National Institute of Advanced Industrial Science and Technology , 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
⁸ Division of Chemistry and Materials, Shinshu University, Faculty of Textile Science and Technology , Ueda, Nagano 3868567, Japan.

PMID: 28692246
DOI: 10.1021/acsami.7b03522

Abstract

Donor-π-acceptor dyes containing thiophenyl π-conjugated units and cyanoacrylate acceptor groups are among the best-performing organic chromophores used in dye-sensitized solar cell (DSC) applications. Yet, the molecular origins of their high photovoltaic output have remained unclear until now. This synchrotron-based X-ray diffraction study elucidates these origins for the high-performance thiophenylcyanoacrylate-based dye MK-2 (7.7% DSC device efficiency) and its molecular building block, MK-44. The crystal structures of MK-2 and MK-44 are both determined, while a high-resolution charge-density mapping of the smaller molecule was also possible, enabling the nature of its bonding to be detailed. A strong S···C≡N intramolecular interaction is discovered, which bears a bond critical point, thus proving that this interaction should be formally classified as a chemical bond. A topological analysis of the π-conjugated portion of MK-44 shows that this S···C≡N bonding underpins the highly efficient intramolecular charge transfer (ICT) in thiophenylcyanoacrylate dyes. This manifests as two bipartite ICT pathways bearing carboxylate and nitrile end points. In turn, these pathways dictate a preferred COO/CN anchoring mode for the dye as it adsorbs onto TiO₂ surfaces, to form the dye···TiO₂ interface that constitutes the DSC working electrode. These results corroborate a recent proposal that all cyanoacrylate groups anchor onto TiO₂ in this COO/CN binding configuration. Conformational analysis of the MK-44 and MK-2 crystal structures reveals that this S···C≡N bonding will persist in MK-2. Accordingly, this newly discovered bond affords a rational explanation for the attractive photovoltaic properties of MK-2. More generally, this study provides the first unequivocal evidence for an S···C≡N interaction, confirming previous speculative assignments of such interactions in other compounds.

Keywords: S···C≡N intramolecular bond; charge-density mapping; crystal structures of MK-2 and MK-44; cyanoacrylate anchoring mode; dye-sensitized solar cell; dye···TiO2 interface; intramolecular charge transfer; molecular design.