title: Probing flexibility in porphyrin-based molecular wires using double electron electron resonance.
abstract: A series of butadiyne-linked zinc porphyrin oligomers, with one, two, three, and four porphyrin units and lengths of up to 75 A, have been spin-labeled at both ends with stable nitroxide TEMPO radicals. The pulsed EPR technique of double electron electron resonance (DEER) was used to probe the distribution of intramolecular end-to-end distances, under a range of conditions. DEER measurements were carried out at 50 K in two types of dilute solution glasses: deutero-toluene (with 10% deutero-pyridine) and deutero-o-terphenyl (with 5% 4-benzyl pyridine). The complexes of the porphyrin oligomers with monodentate ligands (pyridine or 4-benzyl pyridine) principally adopt linear conformations. Nonlinear conformations are less populated in the lower glass-transition temperature solvent. When the oligomers bind star-shaped multidentate ligands, they are forced to bend into nonlinear geometries, and the experimental end-to-end distances for these complexes match those from molecular mechanics calculations. Our results show that porphyrin-based molecular wires are shape-persistent, and yet that their shapes can deformed by binding to multivalent ligands. Self-assembled ladder-shaped 2:2 complexes were also investigated to illustrate the scope of DEER measurements for providing structural information on synthetic noncovalent nanostructures.
authors.affiliation: Centre for Advanced Electron Spin Resonance, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom. janet.lovett@path.ox.ac.uk
authors.forename: Janet E
authors.initials: JE
authors.lastname: Lovett
authors.forename: Markus
authors.initials: M
authors.lastname: Hoffmann
authors.forename: Arjen
authors.initials: A
authors.lastname: Cnossen
authors.forename: Alexander T J
authors.initials: AT
authors.lastname: Shutter
authors.forename: Hannah J
authors.initials: HJ
authors.lastname: Hogben
authors.forename: John E
authors.initials: JE
authors.lastname: Warren
authors.forename: Sofia I
authors.initials: SI
authors.lastname: Pascu
authors.forename: Christopher W M
authors.initials: CW
authors.lastname: Kay
authors.forename: Christiane R
authors.initials: CR
authors.lastname: Timmel
authors.forename: Harry L
authors.initials: HL
authors.lastname: Anderson
mesh_terms: D003497:Cyclic N-Oxides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D015335:Molecular Probes / Q000737:chemistry; D015394:Molecular Structure; D053770:Nanowires / Q000737:chemistry*; D011166:Porphyrins / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D003497:Cyclic N-Oxides; D015335:Molecular Probes; D011166:Porphyrins; C003959:TEMPO