Solutions of the basic form of polyaniline in m cresol were studied by ESR and optical spectroscopy in the visible region. m Cresol can slowly (during one month) protonate polyaniline. For the first time characteristic features of spin crossover were found: sharp changes in the magnetic susceptibility and the ESR line width of polyaniline at ~200 and 250 K, a smooth decrease in the susceptibility and absorption with the temperature increase from 293 to 423 K, and the temperature hysteresis. The temperature induced structural rear rangements of polyaniline are caused, most likely, by singlet triplet transitions in relatively short sections of the polymer chain. The model of short sections permits to explain the origin of the temperature independent part of susceptibility. Quantum chemical calculations for the aniline dimers and tetramers describe correctly the singlet triplet splitting value, thermo chromism, and HFS constants in the spectrum of polyaniline.The term "spin crossover" (SC) or "spin transition" is used for the description of characteristic phenomena in samples, where transitions between the high and low spin states of the centers of the sample occur upon the temperature change or under pressure or irradiation and lead to the structural rearrangement of the sample.Most systems manifesting SC contain iron(II). 1,2 The systems containing no iron but including, for example, Cu atoms and nitroxy radicals are also known. 3 The energy of low spin states of the centers is usually lower than that of the high spin states and, hence, the fraction of the low spin states increases with the temperature decrease. The geometry of the low spin and high spin centers is differ ent and, therefore, strains accelerating the conversion of the high spin to low spin states increase with an increase in the fraction of the low spin states in the samples, and the phase transition occurs at some temperature. The sharpness of the transition is determined by the degree of cooperativity of the system and, as a rule, the temperature hysteresis is observed. Investigation of the nature of this phenomenon, including establishing conditions for the appearance of cooperativity and the mechanism of an irradiation effect and search for new systems manifest ing SC is the matter of numerous studies. The high sensi tivity of parameters of the systems under study, especially absorption in the visible region and magnetic susceptibil ity, to slight changes in the temperature, pressure, and irradiation makes it possible to use them as sensors, dis plays, and information storage devices. To increase the sharpness of the transition, the centers are bound through ligands into polymer structures; however, no true poly mers exhibiting SC are known. The SC term is applied to both solid state systems with sharp temperature induced transitions and solutions with smooth transitions reflect ing the change in the Boltzmann populations of spin states. The systems manifesting SC are rather few, because they need to fulfill several conditions; in particular, t...

Temperature dependences of the magnetic susceptibility of solutions and powders of polyaniline synthesized by oxidative polymerization using two methods were measured by ESR in the temperature range from 123 to 423 K. The dependences observed can be described by the integral of susceptibility of the polymer fragments in the triplet state over the singlet-triplet splitting from E 1 to E 2 with constant weight. The susceptibility of the fragments was accepted to obey the Bleaney-Bowers equation. The most part of the experimental dependences can be presented as the sum of the temperature independent susceptibility and the susceptibility obeying the Curie law. The both susceptibilities are described in a single manner at E 1 < 0. In some cases, the comparison of the calculated and experimental dependences makes it possible to determine the length of the fragments L. The conditions of polymer synthesis, heating, and water vapors affect the E 1 and E 2 values. A similar analysis can be applied to other conducting polymers. , and many other, are interesting due to their unusual physical prop erties and a possibility of their diverse practical use. From this point of view, the most attention was given to studies of luminescence and conductivity. Magnetic properties are of a special interest, being tightly related to the nature of charge carriers and to fine features of the polymer structure.The frequently observed experimental linear temperature dependence of the product of the magnetic susceptibility by the temperaturemakes it possible to divide the susceptibility into two com ponents: the temperature independent part χ P and the part obeying the Curie law χ = C/T (see, e.g., data for polyaniline, 1-3 polythiophene, 3 and polypyrrole 4 ). The origin of these two components is usually explained in terms of the "metallic" model, according to which pow ders and films of doped conducting polymers are highly ordered metallic regions immersed into amorphous re gions (see, e.g., Refs 1-3 and references cited therein). Paramagnetism and conductivity of conducting polymers are considered to appear after doping, i.e., when charges and unpaired electrons are formed in the polymers. Due to the spatial periodicity of the metallic regions, the energy spectrum of electrons has a zone character. The upper zone is partially filled, resulting in the appearance of such properties as high electron conductivity and the temperature independent Pauli susceptibility. Distortions in the lattice periodicity induce a minor number of local ized electrons, viz., polarons with the spin 1/2, whose susceptibility obeys the Curie law. However, some experimental facts does not obey this scheme. It is natural to expect that the ESR lines of the localized spins and metallic regions have different widths but, in the most cases, the ESR lines of the conducting polymers exhibit no superposition of the lines with differ ent widths. As a rule, the shape of the ESR lines in the line center is close to the Lorentzian one, and the descent on the wings is faster. ...

Solutions of polyaniline in m cresol with and without camphorsulfonic acid (CSA), as well as films cast from these solutions were studied by ESR spectroscopy at 133-423 K and by optical spectroscopy in the range λ = 350-1100 nm. An analysis of the optical and ESR spectra shows that in the solutions and films without CSA polyaniline is fully doped but the conductiv ity of these films is low (~10 -8 S cm -1 ; cf. 100 S cm -1 for the films with CSA). Compared with the CSA containing samples, the samples without CSA are characterized by broader ESR lines and higher contribution of the Curie spins to the magnetic susceptibility. These facts indicate a weak aggregation of polyaniline chains without CSA, which leads to low conductivity. A formula was proposed, which describes the temperature dependence of the polyaniline ESR linewidth and allows the interchain distance and the mobility of electrons moving along polymer chains to be determined. The conductivity of polyaniline films is affected by moderate heating (363-388 K) of the films and solutions from which the films were cast. It was found that the interchain distances correlate with the conductivity of the films and with the broadening of their ESR lines caused by the effect of O 2 .Polyaniline films cast from polyaniline solutions in m cresol containing camphorsulfonic acid (CSA) addi tives have been a subject of intensive research because they exhibit recordly high conductivities (see, e.g., Refs 1-7 and references cited therein). It is accepted that CSA favors transformation of polyaniline into a protonated conducting form and that m cresol changes the confor mation of the polyaniline molecules from a coil to ex tended chain. Extended chains are characterized by closer packing and the film conductivity increases due to fast inter chain electron transfer.Protonation of the basic form of polyaniline leads to the appearance of paramagnetic polarons.Because of spatial periodicity of the "polaron" lattice the electron energy spectrum has a band structure. The upper band is partially filled, which is responsible for the high electronic conductivity and temperature indepen dent Pauli susceptibility. The conformation of polyaniline molecules can be affected by counterions X -and solvent molecules. Theoretical analysis 8 showed that high solu bility and extended conformation of the polyaniline molecules in solution in the m cresol-CSA system is due to the formation of a specific complex polyani line-m cresol-CSA owing to combination of the stack ing of phenyl groups, hydrogen bonding, and electrostatic interaction.Conducting polymers including polyaniline are called quasi 1D metals, because their properties strongly de pend on the interchain interaction. It was theoretically shown that the existence of true 1D metals is impossible, because any violation of periodicity leads to localization Base Salt m Cresol CSA