Electron paramagnetic resonance saturation characteristics of pristine and doped polyacetylenes

Abstract

Electron paramagnetic resonance saturation experiments have been carried out on undoped polyacetylene and on the doped polymer as a function of dopant concentration. The relaxation times of undoped trans-(CH)x show considerable variability from sample to sample, Tr = (2.7 ± 1.7) X 10"5 s and T2 = (7.8 ± 1.0) X 10~8 s, suggesting sensitivity to trace impurities. Exposure to air reduces Tx to 8.1 X 10"6 s and T2 to 6.6 X 10"8 s; this effect is reversible. The use of radioassay techniques with 125I made it possible to prepare and characterize samples of known dopant concentration at the level of parts per million (ppm). There are three distinct regimes which characterize the effect of iodine concentration on the EPR characteristics of trans-(CHly)x. In the region 3 X 10"6 < y < 10"3, T1 decreases with increasing y, while T2 is unaffected. The neutral soliton concentration, [S•], remains constant in this region. Dilute doping results in conversion of S• to a spinless positive soliton S+ and/or direct oxidation of (CH)X to create S+S+ pairs. In the intermediate concentration regime, 10~3 < y < 10"2, both 7\ and T2 decrease with y, and [S•] decreases as y"3•7, suggesting that the dopant is predominantly in the material as I3" with the remainder as I5". When y exceeds 1CT3, there Eire one or more dopant molecules per (CH)X chain and the number of neutral solitons is significantly reduced.In the heavily doped samples, the EPR has a Dysonian line shape until it vanishes for y > 2 X 10~2. The observation of changes in relaxation times even at the dopant level of ppm implies that the dopant ions are randomly distributed throughout the polymer and that the solitons are highly mobile. On the other hand, for cis-(CH)x, doping with iodine from y = 3.3 X 10~5 to 3.9 X 10"4 does not significantly affect 7\ or T2, for pristine cis-(CH)x, Tl = (5.4 ± 0.9) X 10"5 s and T2 = (1.0 ± 0.07) X 10"8 s. Therefore, the solitons in cis-(CH)Z have low diffusivity. Very slowly doped trans-[CH(AsF6);y]x also displays three characteristic regions. In the dilute regime, Tb T2, [S•], and the AZ/pp dependence on Hx are not significantly different from those of pristine trans-(CH)x. Above y > 6 X 10"3, [S-] first decreases with y and then the EPR intensity increases rapidly as the Pauli susceptibility makes its contribution. In the transitional regime, 6 X 10"3 < y < 5 X 10"2, the resonance could not be saturated with the power available, implying Einomalously short rekixation times (Tf), whereas saturation was again possible in the heavily doped metallic limit.

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“…This relatively short T sl value found is similar to previous reports for undoped polymers that have been exposed to air at ambient temperature. 21 EPR spectra of lm samples being recorded under alternative instrumental conditions (see Methods) provide the same EPR patterns as their initial powders (spectra not shown). Fig.…”

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“…where Pw is the power level and a is the proportionality constant. 28 Increased dopant uptake results in shortened Ti values (Table IV), indicating increased coupling between the spin system and dopant nuclei. The tighter the coupling of the spin system and the lattice, the shorter 7\…”

Electron spin resonance study of radical formation in cis-poly(phenylacetylene)/ferric acetylacetonate-triethylaluminum (PPA/Fe-Al). 1. Procedure for line-shape and saturation analysis

“…The significance of fit of the line-shape model can be established by defining a 100(1 -a)% confidence limit contour about the global minimum20 r = l(ap)2 + ( )2] /2; = arctan ( / ) (11) to give for the dispersion, D, and absorption, V ( ; ) = ^^Ers, V(ap;ar) = -Ere (12) where the functions Ere and Ers follow the matrix relation…”

“…In view of the complexities involved it is not surprising that very few detailed saturation treatments have been reported for these materials. 11,12 To handle the complexities peculiar to conjugated polymer spectra in general and to interpret the electron spin + Present address: Department of Chemistry, Rochester Institute of Technology, Rochester, NY 14623. 0024-9297/90/2223-2198$02.50/0 responance (ESR) spectra of cts-poly(phenylacetylene) (cts-PPA) in particular, we propose a new line-shape and saturation treatment.…”