title: Belt-shaped π-systems: relating geometry to electronic structure in a six-porphyrin nanoring.
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.
authors.affiliation: Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Oxford OX1 3TA, United Kingdom.
authors.forename: Johannes K
authors.initials: JK
authors.lastname: Sprafke
authors.forename: Dmitry V
authors.initials: DV
authors.lastname: Kondratuk
authors.forename: Michael
authors.initials: M
authors.lastname: Wykes
authors.forename: Amber L
authors.initials: AL
authors.lastname: Thompson
authors.forename: Markus
authors.initials: M
authors.lastname: Hoffmann
authors.forename: Rokas
authors.initials: R
authors.lastname: Drevinskas
authors.forename: Wei-Hsin
authors.initials: WH
authors.lastname: Chen
authors.forename: Chaw Keong
authors.initials: CK
authors.lastname: Yong
authors.forename: Joakim
authors.initials: J
authors.lastname: Kärnbratt
authors.forename: Joseph E
authors.initials: JE
authors.lastname: Bullock
authors.forename: Marc
authors.initials: M
authors.lastname: Malfois
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
authors.forename: Bo
authors.initials: B
authors.lastname: Albinsson
authors.forename: Laura M
authors.initials: LM
authors.lastname: Herz
authors.forename: Donatas
authors.initials: D
authors.lastname: Zigmantas
authors.forename: David
authors.initials: D
authors.lastname: Beljonne
authors.forename: Harry L
authors.initials: HL
authors.lastname: Anderson
mesh_terms: D004583:Electrons; D008958:Models, Molecular; D015394:Molecular Structure; D049329:Nanostructures / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry; D011166:Porphyrins / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D011166:Porphyrins