Electron Paramagnetic Resonance Studies of Donor-Acceptor Salts in Polymer Media
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Electron Paramagnetic Resonance Studies of Donor-Acceptor Salts in Polymer Media YANG-CHENG FANN AND SUSAN ANN JANSEN Department of Chemistry and Materials Science, Temple University, Philadelphia, Pa. 19122 ABSTRACT: Charge transfer complexes, their inherent electrical conductivity, magnetic properties and donor-acceptor redox relationships have been a focus of much research in the last several years. One direction has been in tYe 2design of memory devices and applications in molecular electronics. • Our work has focused on analysis of such processes in a polymeric medium. Polycarbonate films of 7,7',8,8'-Tetracyanoquinodimethane (TCNQ) and o-tolidine (o-T) complexes were studied by EPR spectroscopy from 100-300K. EPR spectra and magnetic susceptibility of the dispersed charge-transfer complexes are presented and compared with the pristine materials. These studies were carried out as a function of donor/acceptor stoichiometry and concentration within the polymer matrix. Saturation studies show significant differences as the composition varies. In addition the g-tensor and linewidth are strongly dependent on temperature and composition.
EXPERIMENTAL: The materials studied are described in the table I below: Table I. o-T:TCNQ 1:1 1:2 2:1
Formulation for each Salt Pristine 1% by weight in polycarbonate 10% by weight in polycarbonate
Cyclic voltammetry, CV, (IBM EC-225) and electron paramagnetic resonance, EPR, (Bruker ER-200) and optical microscopy were used to characterize the pristine and dispersed samples. The structures for o-tolidine and tetracyanoquinodimethan are shown below:
TCNO
Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
162
RESULTS AND DISCUSSION: The CV studies have shown that the complexation of the o-T and TCNQ provides a multi-redox chemical system which can undergo a quasi-reversible four electron oxidation. Figure 1 shows the CV trace of the the pristine 1:1 complex in a 1:1 solvent mixture of THF and acetonitrile. Table II shows the redox potentials measured for the complexes listed above which 3 are similar to those previously reported for simple complexes of TCNQ. Figure 1.
CV trace for o-T\TCNQ measured at a scan rate of 50 mv/sec, using a glass carbon electrode in 0.1 M tetramethylammonium tetrafluoroborate. A Pt electrode was used as the reference.
15 AA
Table II.
4..;0V
.
-13 v
Redox Potentials in volts for o-T/TCNQ Complexes. E,
E2
TCNQ o-Tolidine
-0.28 0.18
TCNQ-oT=1:1 TCNQ-oT=1:2 TCNQ-oT=2:1
-0.71 -0.80 -0.76
E3
E4_
-0.88 0.43
-
-
-0.15 -0.18 -0.16
0.19 0.16 0.17
0.41 0.34 0.36
EPR studies have shown the presence of two distinct magnetic species. These were resolved by a saturation/population study. The species can be described as (1)a fixed nitrogen radical and (2) an exchange coupled magnetic species. The former saturates easily and is not identifiable at high microwave power levels. The other species shows magneti4 exchange as observed in many charge transfer complexes based on TCNQ and is attributed to formation of a charge tranfer
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