Belt-shaped π-systems: relating geometry to electronic structure in a six-porphyrin nanoring
Journal
Journal of the American Chemical Society
Journal Volume
133
Journal Issue
43
Pages
17262
Date Issued
2011-11-02
Author(s)
Sprafke, Johannes K
Kondratuk, Dmitry V
Wykes, Michael
Thompson, Amber L
Hoffmann, Markus
Drevinskas, Rokas
Chen, Wei-Hsin
Kärnbratt, Joakim
Bullock, Joseph E
Malfois, Marc
Wasielewski, Michael R
Albinsson, Bo
Herz, Laura M
Zigmantas, Donatas
Beljonne, David
Anderson, Harry L
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.
Subjects
CONJUGATED PORPHYRIN OLIGOMERS; DENSITY-FUNCTIONAL THEORY; ABSORPTION CROSS-SECTION; X-RAY SOLUTION; CRYSTAL-STRUCTURES; BUILDING-BLOCKS; EXCITED-STATE; BIOLOGICAL MACROMOLECULES; PERSISTENT MACROCYCLE; NANOMETER REGIME
Publisher
AMER CHEMICAL SOC
Type
journal article