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

Summary, in English

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.

• Author: 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
• Department/s: Chemical Physics
• Publishing year: 2011
• Language: English
• Pages: 17262-17273
• Publication/Series: Journal of the American Chemical Society
• Volume: 133
• Issue: 43
• Links: http://dx.doi.org/10.1021/ja2045919
• Document type: Journal article
• Publisher: The American Chemical Society (ACS)
• Topic: Atom and Molecular Physics and Optics
• Status: Published
• ISBN/ISSN/Other: ISSN: 1520-5126