Discovery of S···C≡N Intramolecular Bonding in a Thiophenylcyanoacrylate-Based Dye: Realizing Charge Transfer Pathways and Dye···TiO ₂ Anchoring Characteristics for Dye-Sensitized Solar Cells

Journal Article · Tue Jul 25 04:00:00 EDT 2017 · ACS Applied Materials and Interfaces

DOI:https://doi.org/10.1021/acsami.7b03522· OSTI ID:1427527

Cole, Jacqueline M. ^[1]; Blood-Forsythe, Martin A. ^[2]; Lin, Tze-Chia ^[2]; Pattison, Philip ^[3]; Gong, Yun ^[2]; Vázquez-Mayagoitia, Álvaro ^[4]; Waddell, Paul G. ^[5]; Zhang, Lei ^[2]; Koumura, Nagatoshi ^[6]; Mori, Shogo ^[7]

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

Donor-pi-acceptor dyes containing thiophenyl pi-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 center dot center dot center dot C equivalent to 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 pi-conjugated portion of MK-44 shows that this S center dot center dot center dot C equivalent to 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 center dot center dot center dot C equivalent to 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 center dot center dot center dot C equivalent to N interaction, confirming previous speculative assignments of such interactions in other compounds.

• Research Organization: Argonne National Laboratory (ANL)
• Sponsoring Organization: USDOE Office of Science - Office of Basic Energy Sciences; Australian Nuclear Science and Technology Organisation; Argonne National Laboratory - Argonne Leadership Computing Facility
• DOE Contract Number: AC02-06CH11357
• OSTI ID: 1427527
• Journal Information: ACS Applied Materials and Interfaces, Journal Name: ACS Applied Materials and Interfaces Journal Issue: 31 Vol. 9; ISSN 1944-8244
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

S⋯CºN intramolecular bond
charge-density mapping
crystal structure of MK-2
cyanoacrylate anchoring mode
dye-sensitized solar cell
dye⋯TiO2 interface
intramolecular charge transfer
molecular design