C O M M U N I C A T I O N S
Figure 2. ESR spectra (9.07 GHz) of (a) 3, (b) 4, (c) 5, and (d) 6 in chloroform (blue) and ethyl acetate (red). The spectra were measured at room
temperature at a concentration of 0.3 mM. Intensity was normalized by equating double integrated values.
CIF format. This material is available free of charge via the Internet at
the observed ESR spectra consisted of three lines centered at g )
2.005-2.006, which are typical of nitroxide radicals. In ethyl
acetate, line broadening was observed for 3-5, indicating that spin-
spin interactions were present between the two nitroxide radicals.15
This interaction is assumed to originate from the close proximity
of the spin labels within the folded chains. The degree of the
broadening was the largest for oligomer 4, in which there are five
repeating units between spin labels. Line broadening was not
observed in singly labeled oligomer 6. This excludes the possibility
of intermolecular aggregation or any effect of folding as being
responsible for the broadening.
These results indicate that the spin labels are spatially closer in
the folded structure of 4, as compared to the oligomers 3 or 5. The
experimental findings can be explained by considering that six
repeating units make one helical turn.16 Because the oligomers are
meta-connected phenylene groups, a hexagonal helical structure
made of six repeating units should have a maximum intramolecular
overlap with the least amount of bond angle and torsion strain.
In conclusion, we have designed and synthesized phenylene
ethynylene oligomers which were selectively spin labeled at two
sites. The measurement and analysis of the ESR spectra revealed
that one helical turn, within the folded structure, consists of six
repeating units. The use of spin labeling methods provides a
powerful way to answer fundamental questions about the confor-
mational structure of chain molecules in solution. Attempts to gain
more detailed quantitative information are currently underway.
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Acknowledgment. ESR spectra were measured at the Illinois
EPR research center with the help of Prof. Mark J. Nilges. This
work was supported by the National Science Foundation under
Grant No. 00-91931 and the U.S. Department of Energy, Division
of Materials Sciences, under Award No. DEFG02-91ER45439,
through the Frederick Seitz Materials Research Laboratory at the
University of Illinois at Urbana-Champaign. K.M. acknowledges
the support of JSPS Postdoctral Fellowships for Research Abroad.
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(14) We used ethyl acetate because acetonitrile has too high of a dielectric
constant to measure the spectrum under the same conditions used with
chloroform.
(15) We performed quantitative spin integration experiments using DPPH as
a reference (0.1 mM in chloroform). The spin content per molecule in
oligomers 3-5 was confirmed to be 2.0 within experimental error.
(16) Both the exchange interaction and the dipolar interaction can contribute
to the line broadening. Because the two interactions are both contributing
to the observed line broadening, quantitative analysis is difficult in this
case.
Supporting Information Available: Experimental procedure of the
syntheses of 2-6, UV-vis spectra of 3-6, ESR integrated spectra of
3-6, and crystallographic data of 2 (PDF). Crystallographic data in
JA027437M
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