Electron paramagnetic resonance (EPR) spectra of the reduced quinone-iron acceptor complex in reaction centers were measured in a variety of environments and compared with spectra calculated from a theoretical model. Spectra were obtained at microwave frequencies of 1, 9, and 35 GHz and at temperatures from 1.4 to 30 K. The spectra are characterized by a broad absorption peak centered at g = 1.8 with wings extending from g approximately equal to 5 to g less than 0.8. The peak is split with the low-field component increasing in amplitude with temperature. The theoretical model is based on a spin Hamiltonian, in which the reduced quinone, Q-, interacts magnetically with Fe2+. In this model the ground manifold of the interacting Q-Fe2+ system has two lowest doublets that are separated by approximately 3 K. Both perturbation analyses and exact numerical calculations were used to show how the observed spectrum arises from these two doublets. The following spin Hamiltonian parameters optimized the agreement between simulated and observed spectra: the electronic g tensor gFe, x = 2.16, gFe, y = 2.27, gFez = 2.04, the crystal field parameters D = 7.60 K and E/D = 0.25, and the antiferromagnetic magnetic interaction tensor, Jx = -0.13 K, Jy = -0.58 K, Jz = -0.58 K. The model accounts well for the g value (1.8) of the broad peak, the observed splitting of the peak, the high and low g value wings, and the observed temperature dependence of the shape of the spectra. The structural implications of the value of the magnetic interaction, J, and the influence of the environment on the spin Hamiltonian parameters are discussed. The similarity of spectra and relaxation times observed from the primary and secondary acceptor complexes Q-AFe2+ and Fe2+Q-B leads to the conclusion that the Fe2+ is approximately equidistant from QA and QB.

We have measured the static magnetization of unreduced and reduced reaction centers that vary in their quinone content. Measurements were performed in the temperature range 0.7 degrees K less than T less than 200 degrees K and magnetic fields of up to 10 kG. The electronic g-value, crystal field parameters D, E, and the exchange interaction, J, between the quinone spin and Fe2+ were determined using the spin Hamiltonian formalism. The effective moment mu eff/Fe2+ of both reduced and unreduced samples were determined to be 5.35 +/- 0.15 Bohr magnetons. This shows, in agreement with previous findings, that Fe2+ does not change its valence state when the reaction centers are reduced. Typical values of D congruent to +5 cm-1 and E/D congruent to 0.27 are consistent with Fe being in an octahedral environment with rhombic distortion. The values of D and E were approximately the same for reaction centers having one and two quinones. These findings imply that quinone is most likely not a ligand of Fe. The Fe2+ and the spin on the quinone in reduced reaction centers were found to be coupled with an exchange interaction 0 less than /J/ less than 1 cm-1. The validity of the spin Hamiltonian was checked by using an orbital Hamiltonian to calculate energy levels of the 25 states of the S = 2, L = 2 manifold and comparing the magnetization of the lowest five states with those obtained from the spin Hamiltonian. Using the orbital Hamiltonian, we calculated the position of the first excited quintet state to be 340 cm-1 above the ground state quintet. This is in good agreement with the temperature dependence of the quadrupole splitting as determined by Mossbauer spectroscopy.

Trans and cis (CH)x were chemically doped with FeCl 3 using nitromethane solutions as well as by using the vapor pressure of solid FeCI 3 • Electrical conductivities were measured vs temperature for all samples. Room temperature conductivities of approximately 1 ()() mho/ cm were achieved. Thermoelectric power was also measured as a function of temperature. The room temperature thermoelectric power starts at + 850 P, V /K for the dilutely doped samples and decreases to + 12 p,V /K for the more heavily doped samples which exhibit metallic thermoelectric power, proportional to the absolute temperature.

Samples of trans-polyacetylene, (CH)x, were doped with the magnetic ion, FeCl−4, by immersion in nitromethane solutions of FeCl3. The resulting dopant levels ranged over two orders of magnitude. ESR spectra and spin–lattice relaxation times, T1, were measured for the undoped polymer and lightly doped polymer over the temperature range 1.5–120 K. The ESR spectra of trans-(CH)x lightly doped (<1 mol % Fe−4 ) with FeCl−4 exhibit a narrow (ΔH≊5–10 G at 4.2 K) g=2.003 signal which decreases in relative amplitude with increasing dopant level, and broad signals at g=2 (ΔH≊500 G), which increases with increasing dopant level, and at g=4.3 (ΔH≊200 G). The narrow signal is the intrinsic trans-(CH)x signal; the broad signals are attributable to iron. T1 for the narrow signal is approximately one-half of the value of undoped trans-(CH)x at 4.2 K. dc susceptibilities for all of the doped samples were measured over the temperature range of 1.5–300 K. The magnetically doped samples all exhibit Curie law behavior, and an analysis of the magnetization data yields an effective moment, Jeff, equal to 2.77, close to the value 5/2 expected for Fe(III).