Belt-shaped π-systems: Relating geometry to electronic structure in a six-porphyrin nanoring

Johannes K. Sprafke, Dmitry V. Kondratuk, Michael Wykes, Amber L. Thompson, Markus Hoffmann, Rokas Drevinskas, Wei Hsin Chen, Chaw Keong Yong, Joakim Kärnbratt, Joseph E. Bullock, Marc Malfois, Michael R. Wasielewski, Bo Albinsson, Laura M. Herz, Donatas Zigmantas, David Beljonne, Harry L. Anderson

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Abstract

Linear π-conjugated oligomers have been widely investigated, but the behavior of the corresponding cyclic oligomers is poorly understood, despite the recent synthesis of π-conjugated macrocycles such as [n]cycloparaphenylenes and cyclo[n]thiophenes. Here we present an efficient template-directed synthesis of a π-conjugated butadiyne-linked cyclic porphyrin hexamer directly from the monomer. Small-angle X-ray scattering data show that this nanoring is shape-persistent in solution, even without its template, whereas the linear porphyrin hexamer is relatively flexible. The crystal structure of the nanoring-template complex shows that most of the strain is localized in the acetylenes; the porphyrin units are slightly curved, but the zinc coordination sphere is undistorted. The electrochemistry, absorption, and fluorescence spectra indicate that the HOMO-LUMO gap of the nanoring is less than that of the linear hexamer and less than that of the corresponding polymer. The nanoring exhibits six one-electron reductions and six one-electron oxidations, most of which are well resolved. Ultrafast fluorescence anisotropy measurements show that absorption of light generates an excited state that is delocalized over the whole π-system within a time of less than 0.5 ps. The fluorescence spectrum is amazingly structured and red-shifted. A similar, but less dramatic, red-shift has been reported in the fluorescence spectra of cycloparaphenylenes and was attributed to a high exciton binding energy; however the exciton binding energy of the porphyrin nanoring is similar to those of linear oligomers. Quantum-chemical excited state calculations show that the fluorescence spectrum of the nanoring can be fully explained in terms of vibronic Herzberg-Teller (HT) intensity borrowing.

Original languageEnglish
Pages (from-to)17262-17273
Number of pages12
JournalJournal of the American Chemical Society
Volume133
Issue number43
DOIs
Publication statusPublished - Nov 2 2011

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ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Sprafke, J. K., Kondratuk, D. V., Wykes, M., Thompson, A. L., Hoffmann, M., Drevinskas, R., Chen, W. H., Yong, C. K., Kärnbratt, J., Bullock, J. E., Malfois, M., Wasielewski, M. R., Albinsson, B., Herz, L. M., Zigmantas, D., Beljonne, D., & Anderson, H. L. (2011). Belt-shaped π-systems: Relating geometry to electronic structure in a six-porphyrin nanoring. Journal of the American Chemical Society, 133(43), 17262-17273. https://doi.org/10.1021/ja2045919