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Belt-Shaped π-Systems: Relating Geometry to Electronic Structure in a 2 Six-Porphyrin Nanoring

Johannes K. Sprafke, D. V. Kondratuk, M. Wykes, A. L. Thompson, M. Hoffmann, R. Drevinskas, W. Chen, Ch. Keong Yong, J. Karnbratt, J. E. Bullock, M. Malfois, M. R. Wasielewski, B. Albinsson, L. M. Herz, D. Zigmantas, D. Beljonne, and H. L. Anderson, New York (2011)

Linear π-conjugated oligomers have been widely investi- 18 gated, but the behavior of the corresponding cyclic oligomers is poorly 19 understood, despite the recent synthesis of π-conjugated macrocycles such 20 as [n]cycloparaphenylenes and cyclo[n]thiophenes. Here we present an 21 efficient template-directed synthesis of a π-conjugated butadiyne-linked 22 cyclic porphyrin hexamer directly from the monomer. Small-angle X-ray 23 scattering data show that this nanoring is shape-persistent in solution, even 24 without its template, whereas the linear porphyrin hexamer is relatively 25 flexible. The crystal structure of the nanoringtemplate complex shows that most of the strain is localized in the acetylenes; the 26 porphyrin units are slightly curved, but the zinc coordination sphere is undistorted. The electrochemistry, absorption, and 27 fluorescence spectra indicate that the HOMOLUMO gap of the nanoring is less than that of the linear hexamer and less than that 28 of the corresponding polymer. The nanoring exhibits six one-electron reductions and six one-electron oxidations, most of which are 29 well resolved. Ultrafast fluorescence anisotropy measurements show that absorption of light generates an excited state that is 30 delocalized over the whole π-system within a time of less than 0.5 ps. The fluorescence spectrum is amazingly structured and red- 31 shifted. A similar, but less dramatic, red-shift has been reported in the fluorescence spectra of cycloparaphenylenes and was 32 attributed to a high exciton binding energy; however the exciton binding energy of the porphyrin nanoring is similar to those of 33 linear oligomers. Quantum-chemical excited state calculations show that the fluorescence spectrum of the nanoring can be fully 34 explained in terms of vibronic HerzbergTeller (HT) intensity borrowing.

Partners : UMons, Chalmers

Place of Publication : New York

Date of Publication : 2011/09/22

Additional Data : | J. Am. Chem. Soc. 2011, 133, 17262–17273

Link to the online version of the article.