Wide class of organic compounds studying at 2-mm waveband EPR is conducting polymers in which non-linear carriers, solitons and polarons, transfer the charges (see Ref.1). The right Figure demonstrates how increses spectral resolution of experimental EPR spectrum of paramagnetic centers with anisotropic g-factor in semiconducting poly(tetrathiafulvalen) (PTTF) at the increse of the electron precession frequency. Besides, the sensitivity of the method increases considerably due to higher both the energy distance between ground and exited states and the filling factor of MW cavity at millimeter wavebands.
One of the main advantages of the millimeter EPR technique is the saturation of paramagnetic centers at comparatively low microwave polarizing field. This comes from to exponential dependence of number of excited spins on the registration frequency. The next Figure shows how change a shape of in-phase and p/2-out-of-phase terms of 2-mm dispersion spectra of solitons in cis- and trans-polyacetylene at the increase of microwave field in the series 1 ® 3. From the amplitudes of these terms the spin-lattice (T₁) and spin-spin (T₂) relaxation times of charge carriers can be determined separately (see Ref.2).
Using an appropriate spectral density functions and relaxation parameters obtained, the intersite dynamics of different spins and also microconductivity due to mobility of these carriers can be determined. In the right Figure the rates of the polaron diffusion along D_1D and between D_3D polymer chains in poly(bis-alkylthioacetylene) and an appropriate polymer conductivities due to such motion are presented as function of temperature. From the analysis of conductivity vs. temperature the mechanism of charge transport can be determined. For the comparison, the conductivity calculated in frames of polaron interaction with lattice phonons and intersite spin activation hopping are shown respectively by yellow and magenta lines (see Ref.3).
The interaction between spin charge carriers should affect electronic properties of organic polymer composites with different spin ensembles. It can be realized, e.g., in multispin bulk heterojunctions formed by emeraldine salt form of polyaniline (PANI-ES) with regioregular poly(3-alkylthiophene) modified with [6,6]-phenyl-C₆₁-butanoic acid methyl ester (P3AT:PCBM) shown in the left Figure. The main magnetic parameters of polarons stabilized on chains of both polymers were shown to be governed by their Q1D and Q3D hopping in crystalline domains and between such domains, respectively. These parameters were described in terms of the exchange interaction of polarons P₁⁺ and P₂⁺ with g-factors lying near g-factor of free electron (2.00232) hopping along neighboring solitary PANI-ES and P3AT chains, respectively. This deepens overlapping of wave functions of polarons in both polymer matrices and provides the increase in energy barrier required for crossing bulk heterojunctions. This allows handling the effective electronic properties of the composite for the creation of components of molecular electronics and spintronics. If then one such composite illuminate by visible light, two paramagnetic centers appear, namely additional polarons P₂⁺ (holes) and PC₆₁BM^- anion radicals with g=1.9998 (see Ref.4). The photoresponce of the P3AT:PCBM sub-composition depends on the photon energy as it is shown in the Figure. Electron relaxation of these paramagnetic centers differ sufficiently, therefore one can separately determine spin-lattice and spin-spin relaxation times and then dynamics parameters of charge carriers in all spin reservoirs of the PANI-ES/P3AT:PCBM composite, namely polaron Q1D and Q3D hopping, fullerene pseudo-rotation round own main molecular axis shown in the scheme presented as well as slow macromolecular librations modulating Q1D charge transfer. Unambiguously, these molecular and electronic processes in this system should correlate. Charge carriers possess spin, so such processes can be studied by direct EPR spectroscopy, especially at millimeter wavebands, accompanied with appropriate methods. This allows to study separately all the processes realized in the system, so then to determine complex correlations of electron transport and macromolecular dynamics. The data obtained for multispin polymer composites can open new horizon for the further handling of their electronic properties and creation of flexible and scalable organic molecular devices with spin-assisted electronic properties. The correlations established between dynamics, electronic and structural parameters of these systems can be used for controllable synthesis of various organic spintronic devices with optimal properties. Since coherent spin dynamics in organic semiconductors is anisotropic, it is important to obtain complex correlations of anisotropic electron transport and spin dynamics for the further design of progressive elements of molecular electronics and spintronics.
2-mm waveband EPR allowed us to analyze quite completely the effect of hydration degree of different biological systems on dynamic properties and polarity of microenvironment of introduced spin probes (see Ref.5). The Figure displays as an example 2-mm waveband EPR spectra of one of spin probe introduced into a-chymotrypsin registered at low and high temperature limits as well as the temperature change of main spectral components' positions and widths with temperature. With the temperature increase above 200 K EPR spectral components of micellar system are broadened and shifted to g_iso. At temperatures exceeding 260 K the spectral components merge into a singlet with the width of about 20 G, attributed to radicals, rotating with correlation time ca. 10^-8 sec. A weaker and almost isotropic triplet spectrum of radicals with smaller (ca. 10^-10 sec) correlation time of rotation is overlapping this singlet. The dynamics of spin probe and its microenvironment in micellar systems was estimated analyzing X and Z spectral component to be described as activation rotating diffusion. It was shown from the analysis of EPR spectra of micellar systems with different number of the water molecules that in the presence of protein the increase of water content leads to the increase of probe mobility and the decrease of activation energy in inverted micelles. Such compensation dependencies evidence for the cooperative mechanism of water and protein macromolecule effect on micelle structure, polarity, conformation and molecular dynamics of microenvironment of the spin probe, arranged not less than by 0.6 nm from the surface interface.
In some cases the steady-state saturation method allows to detect an appearance in the system under study spins with close g-factor, line shape and width but with different relaxation parameters. In the left Figure the 2-mm waveband EPR spectrum of laser-irradiated fullerene triphenilamine complex is shown by blue line. If we register p/2-out-of-phase term of 2-mm dispersion spectrum and then analyze its first derivative, two paramagnetic centers with single and double lines are detected. The total spectrum obtained by such a procedure is shown by dashed red line. Because p/2-out-of-phase dispersion spectra strongly depend on the relaxation times, this means that doublet should be attributed to radical pairs existing in the laser-irradiated complex as well (see Ref.6). It should be noted that the analysis of the p/2-out-of-phase dispersion spectrum shape of paramagnetic center with anisotropic magnetic parameters takes in principle a possibility to determine superslow motion of spin situated on i.e. macromolecule and, therefore, the dynamics parameters of own microenvironment.
It was revealed that the single crystals of some ion-radical salts being placed into liquids can reversibly be oriented by strong magnetic field (~ 5 T) at 2-mm waveband EPR. This effect leads to a shift of their EPR line to lower fields. We use this effect for the study of different macro-properties of viscous condensed systems. The Figure shows the sum 2-mm EPR spectra of spin macroprobe, (dibenzotetrathiafulvalene)₃PtBr₆, single crystal of size ~10⁴ mm³ (marked by the symbol g_p), and spin microprobe, 2,2,6,6-tetramethyl-3-phenylethyl-4-oxopiperidinyl-1-oxyl nitroxide radical, in somewhat frozen (1) and melting (2) model system. Analyzing both the broadening of microprobe’s EPR spectrum and the shift of macroprobe’s EPR line at wide temperature range one can determine the dynamics of the probes and their environment and various structural transformations in the system under study. The Figure demonstrates as an example how changes the micro- (open points) and macroviscosity (fulled points calculated for some macroprobes of different size) with temperature (see Ref.7).