Belt-Shaped pi-Systems: Relating Geometry to Electronic Structure in a Six-Porphyrin Nanoring

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Linear pi-conjugated oligomers have been widely investigated, but the behavior of the corresponding cyclic oligomers is poorly understood, despite the recent synthesis of pi-conjugated macrocycles such as [n]cycloparaphenylenes and cyclo[n]thiophenes. Here we present an efficient template-directed synthesis of a pi-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 pi-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.


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

Ämnesklassifikation (UKÄ) – OBLIGATORISK

  • Atom- och molekylfysik och optik
Sidor (från-till)17262-17273
TidskriftJournal of the American Chemical Society
StatusPublished - 2011
Peer review utfördJa