Joy Joseph

EPR study of polarons in a conducting polymer with nondegenerate ground states: Alkali metal complexes of poly (p‐phenylene) and phenylene oligomers

EPR measurements are used to characterize electronic states relevant for carrier transport in alkali metal doped poly(p‐phenylene), PPP, fully deuterated poly(p‐phenylene), DPPP, and phenylene oligomers. Observed spin concentrations per carbon are at least one decade higher than the Curie spin concentration for Na‐doped polyacetylene. The number of these spins, which likely corresponds to polarons (mobile radical anions), is much less than the amount of alkali metal dopant, suggesting that much of the charge on the polymer chains is in bipolarons (spinless dianions). Relevant to the interaction between spins on the polymer chain and the metal cations, the observed g values are close to the free electron value and do not substantially vary with the donor dopant, temperature, or the molecular weight of the phenylene chain. Although the spin‐orbit effect on g values is small, room temperature linewidth tends to increase with increasing atomic number of dopant—suggesting some interaction, albeit a smaller mag...

EPR study of acceptor doped p‐terphenyl crystals: The oriented radical cation precursor for a conducting polymera)

Upon reacting gaseous SO3 or AsF5 with single crystals of p‐terphenyl, the resulting π‐radical cation is displaced from its original molecular position by a rotation of 64° about the rod axis and a tilt of 42° toward the c* crystal axis. Such action causes an eventual destruction of the crystal. This observation suggests that polymerization of p‐terphenyl occurs between monomers related by a 21 symmetry axis.

Temperature dependent electron spin echo studies of polarons in donor- and acceptor-doped poly(p-phenylene): Structural studies

Abstract Electron spin echo (ESE) measurements of donor-doped (Li, Na, K and Cs) and acceptor-doped (AsF 5 ) poly(p-phenylene), PPP, and fully deuterated PPP samples predict a temperature independent EPR linewidth equal to less than 0.65 gauss that decreases with increasing conductivity. In contrast, EPR linewidths either decrease or increase with decreasing temperature, are dependent on dopant and always exhibit a linewidth either equal to or larger than that predicted from ESE measurements. Deuteration studies indicate that rapid spin exchange is present. Analysis of these results suggest that an exchange exists between isolated radicals in equilibrium with polarons and bipolarons with the equilibrium in favor of bipolarons at 4 K.

Epr Study of Polarons In a Conducting Polymer With Nondegen-Erate Ground States: asF5 Complexes of Poly(p-phenylene)

Abstract EPR measurements of temperature-dependent susceptibility and linewidth are reported for poly(p-phenylene) as a function of exposure time to dopant. Most of the charge is in bipolarons, rather than in the spin-associated polarons. Observed decreases in high temperature spin concentration as a function of doping time and post-doping anneal likely arise from both polaron-polaron reaction and polaron ionization to form bipolarons. The susceptibility of all the investigated AsF5-doped samples strongly deviate from Curie-Weiss behavior at low temperatures. The observed temperature dependence can be explained by the low-temperature condensation of isolated polarons to form intermolecular polaron pairs in singlet ground states. The interaction between spins is antiferro-magnetic and the energy of the singlet-state polaron pair relative to two isolated polarons (-2.4 to -4.8 meV) is similar to that for the n-doped polymer (-2.4 to -2.7 meV).

EPR study of arene radical cation salt crystals

Abstract Single-crystal EPR studies of the charge transfer salts grown by the electrochemical oxidation of p -terphenyl (TP) and p -quaterphenyl (QP) in the presence of PF 6 − at −25 °C and +5 °C respectively, are reported. The TP salt exhibits an angular-dependent EPR linewidth varying between 70 and 125 mG at −48 °C and a g tensor of 2.00312, 2.00230 and 2.00206, while the QP salt exhibits an angular-dependent linewidth varying between 150 and 220 mG and a g tensor of 2.00310, 2.00217 and 2.00217, suggesting a pseudo reorientation about the chain axis, a feature not previously detected in other arene salt crystals.

Electron spin echo studies of donor-doped poly(p-phenylene) and its oligomers

Abstract Electron spin echo (ESE) measurements for T1 and Tm were obtained from 4–300 K for potassium and lithium-doped poly(p-phenylene), (PPP), and potassiumdoped p-sexiphenyl (6o) and p-terphenyl (3o). At 300 K, T1=Tm for K-PPP; T1=2Tm for K-6o and T1=10Tm for K-3o at the highest possible concentration of donor dopant. T1 and Tm are temperature independent between 4 and 50 K and vary by a factor of less than four between 4 and 300 K. The temperature independence of T1 and Tm, and T1=Tm is attributed to exchange narrowing beotween anions located on neighboring chains with r<10 A and not to charge carriers of conductivity.

E.P.R. study of potassium-doped poly(p-perfluorophenylene): A conducting polymer with non-degenerate ground states

Abstract Samples of poly(p-perfluorophenylene) heavily doped with potassium (K-FPPP) give rise to a conductivity that is slightly larger than that of the analogously doped poly(p-phenylene), K-PPP, but the unpaired spin concentration is decreased by an order of magnitude. Even so, the measured e.p.r. susceptibilities have a temperature dependence that suggests equilibrium between separated polaron defects and the ground state bipolarons, similar to that observed for K-PPP.

The oriented C2Cl6− radical in irradiated hexachloroethane single crystals: An EPR and molecular-orbital study

Abstract The C 2 Cl 6 − radical has been detected by EPR methods in X-ray irradiated C 2 Cl 6 crystals at 77 K. The radical, which decays rapidly upon exposure to visible light at 77 K, retains the staggered configuration of C 2 Cl 6 and exhibits a structure in which the directions of the largest chlorine hyperfine tensor component and smallest component of the g -tensor occur close to the CC bond direction. In contrast, an INDO calculation predicts that the largest unpaired electron density occurs along the CCl bond direction and the CCl bond length increases by 0.4 A upon electron addition.

An Epr Study of Alkali-Metal-Doped Poly(P-Phenylene) and P-Phenylene Oligomers

Abstract Poly(p-phenylene), PPP, and fully-deuterated poly(p-phenylene), DPPP, were heavily doped with lithium, sodium, potassium, rubidium, cesium, and successively with potassium then lithium. Inert atmosphere conditions using alkali naphthalides in THF were used. The spin susceptibility of each of these samples was not Curie-Weiss over the entire temperature range 7 to 300 K. The EPR linewidths at 300 K of donor-doped PPP were dependent on spinorbit coupling with the alkali metal, varying from a peak-to-peak width at 300 K of 0.5 G for the lithium-doped sample to 5.5 G for the cesium-doped sample. A small decrease in linewidths was noted upon doping of DPPP, these same linewidths equaling 0.35 and 4.5 G. In contrast, the linewidths of Li, K. and Rb graphite complexes are much larger. The linewidth for potassium-doped PPP decreased from 1.9 G at 300 K to 0.45 G at 7 K while the linewidth for potassium-doped p-terphenyl, p-quaterphenyl, and p-sexiphenyl increased. The concentration of unpaired spins at 3...