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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 (Institutionen för kemi- och bioteknik, Fysikalisk kemi) ; Joseph E. Bullock ; Marc Malfois ; Michael R. Wasielewski ; Bo Albinsson (Institutionen för kemi- och bioteknik, Fysikalisk kemi) ; Laura M. Herz ; Donatas Zigmantas ; David Beljonne ; Harry L. Anderson
Journal of the American Chemical Society (0002-7863). Vol. 133 (2011), 43, p. 17262-17273.
[Artikel, refereegranskad vetenskaplig]

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.



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Denna post skapades 2011-12-25. Senast ändrad 2012-01-05.
CPL Pubid: 150932

 

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Institutioner (Chalmers)

Institutionen för kemi- och bioteknik, Fysikalisk kemi (2005-2014)

Ämnesområden

Nanovetenskap och nanoteknik
Hållbar utveckling
Kemi

Chalmers infrastruktur