312624-93-4Relevant academic research and scientific papers
Flexibility of shape-persistent molecular building blocks composed of p-phenylene and ethynylene units
Jeschke, Gunnar,Sajid, Muhammad,Schulte, Miriam,Ramezanian, Navid,Volkov, Aleksei,Zimmermann, Herbert,Godt, Adelheid
supporting information; experimental part, p. 10107 - 10117 (2010/10/04)
Ethynylene and ρ-phenylene are frequently employed constitutional units in constructing the backbone of nanoscopic molecules with specific shape and mechanical or electronic function. How well these properties are defined depends on the flexibility of the backbone, which can be characterized via the end-to-end distance distribution. This distribution is accessible by pulse electron paramagnetic resonance (EPR) distance measurements between spin labels that are attached at the backbone. Four sets of oligomers with different sequences of ρ-phenylene and ethynylene units and different spin labels were prepared using polar tagging as a tool for simple isolation of the targeted compounds. By variation of backbone length, of the sequence of ρ-phenylene and ethynylene units, and of the spin labels a consistent coarse-grained model for backbone flexibility of oligo(ρ-phenyleneethynylene)s and oligo(ρ-phenylenebutadiynylene)s is obtained. The relation of this harmonic segmented chain model to the worm-like chain model for shape-persistent polymers and to atomistic molecular dynamics simulations is discussed. Oligo(ρ-phenylene butadiynylene)s are found to be more flexible than oligo(ρ-phenyleneethynylene)s, but only slightly so. The end-to-end distance distribution measured in a glassy state of the solvent at a temperature of 50 K is found to depend on the glass transition temperature of the solvent. In the range between 91 and 373 K this dependence is in quantitative agreement with expectations for flexibility due to harmonic bending. For the persistence lengths at 298 K our data predict values of (13.8 ± 1.5) nm for poly(ρ-phenyleneethynylene)s and of (11.8 ± 1.5) nm for poly(ρ-phenylenebutadiynylene)s.
EPR probes with well-defined, long distances between two or three unpaired electrons
Godt,Franzen,Veit,Enkelmann,Pannier,Jeschke
, p. 7575 - 7582 (2007/10/03)
The synthesis of rod- and star-shaped compounds carrying two or three spin labels as end groups is described. The unpaired electrons are 2.8-5.1 nm apart from each other. The shape-persistent scaffolds were obtained through Pd-Cu-catalyzed alkynyl-aryl coupling and Pd-Cu-catalyzed, alkyne dimerization in the presence of oxygen using p-phenyleneethynylene as the basic shape-persistent building block. The spin label 1-oxyl-2,2,5,5-tetramethylpyrroline-3-carboxylic acid (4) was attached through esterification of the terminal phenolic OH groups of the scaffold.
