Poly redox switching

A first example is represented by the Mn(III)/Mn(II) redox switch. The complexes of Mn(II) and Mn(III) with the water-soluble tetraphenylsulpho-nate porphyrin (TPPS, Chart 13) display significantly different ri values at low magnetic field strength (lower than 1 MHz), but very similar values at the fields currently used in the clinical practice (> 10 MHz) (141). However, the longer electronic relaxation rates of the Mn(II) complex makes its relaxivity dependent on the rotational mobility of the chelate. In fact, upon interacting with a poly-p-cyclodextrin, a 4-fold enhancement of the relaxivity of [Mn(H)-TPPS(H20)2] at 20 MHz has been detected, whereas little effect has been observed for the Mn(III)-complex. The ability of the Mn(II)/Mn(III)... 

Figure 5.67 Cyclic voltammogram and frequency potential plot for [Os(bpy)2(PVP)ioCI ]CI in 0.1 M p-toluene sulfonic acid. The potential scan rate was 1 mV s with a surface coverage of 2.0 x 10-8 mol cm-2. A/ is frequency change. Reprinted from. Electroanal. Chem., 389, A. P. Clarke, J. G. Vos, A. R. Hillman and A. Glidle, Overall redox switching characteristics of osmium-containing poly(4-vinylpyridine) films immersed in aqueous p-toluene sulfonic acid, 129-140, Copyright (1995), with permission from Elsevier Science...

Our published studies [18, 19] of poly(thionine) redox switching showed that, in acetic acid buffer, the leuco (reduced) form (L) is oxidized, losing two electrons and two protons, to generate the oxidised form (7 ). Furthermore, acetic acid is coordinated only to the 7-form so that reduction causes 7 to lose an acetic acid molecule. However, reduction of 7 to L follows a different path than the reverse of the oxidation path. Water transfer is also involved in the redox cycle. We can (in simplified form) write the overall redox switching process as ... 

Fig. 13.4. Current vs. potential, charge vs potential and mass change vs potential curves for redox switching of a poly(thionine) film (r = 7.7 nmol cm-2) in aqueous O.i mol dm-3 acetic acid (pH = 2.9). The electrode was subject to a cyclic potential scan at 5 mV s l. (Adapted from Ref. [18] with permission.)...

Fig. 13.5. 3D axes showing elementary steps associated with poly(thionine) redox switching in aqueous acetic acid buffer coupled electron/ion (proton) transfer, solvation and acetic... 

The related fully sulfonated, self-doped polymer poly(2-methoxyaniline-5-sulfonic acid) (PMAS 9) may be prepared under normal atmospheric pressure by the oxidation of 2-methoxyaniline-5-sulfonic acid (MAS) monomer with aqueous (NH4)2S208 in the presence of ammonia or pyridine (to permit dissolution of the MAS monomer).141 The polymerization pH was therefore >3.5. Subsequent studies showed that the product consisted of two fractions a major fraction with Mw of ca. 10,000 Da whose electrical conductivity and spectroscopic and redox switching properties were consistent with a PAn emeraldine salt, as well as a nonconducting, electroinactive oligomer (Mw ca. 2,000 Da).143 144 Pure samples of each of these materials can be obtained using cross-flow dialysis.145... 

A rather neat example of the manipulation of film electroneutrality maintenance mechanism is provided by the redox behavior of poly(xylylviologen) (PXV) films, in which the nominal counterion is poly(styrenesulfonate) (PSS). In this case, the counterion is effectively immobile, and the question arises as to whether the PXV and PSS ion charges compensate each other or are separately compensated by (other) electrolyte ions. This will clearly have implications for the possible sources and sinks of ionic charge available to satisfy electroneutrality upon PXV redox switching, and the EQCM is ideally placed to make the distinction. It was found that... 

Sotzing, G.A. and J.R. Reynolds. 1995. Poly[tra s-bis(3,4-ethlyenedioxythiophene)vinylene] A low band-gap polymer with rapid redox switching capabilities between conducting transmissive and insulating absorptive states. J Chem Soc Chem Commun 703. 

Krishna, V., Y.-H. Ho, and K. Rajeshwar. 1991. Luminescence probe and voltammetry study of ion transport during redox switching of poly(pyrrole) thin films. J Am Chem Soc 113 3325. 

Andrade, E.M., RV. Molina, M.I. Florit, and D. Posadas. 2000. Volume changes of poly(2-methy-laniline) upon redox switching—anion and relaxation effects. Electrochem Solid-State Lett 3 504. 

The same electrochemical ion exchange process makes the polymer change its volume, so that polymer films shrink and swell following oxidation and reduction. This property suggested their use as actuators. In fact if poly(3-octylthiophene) [344] and PP [345] are electrodeposited onto a gold-coated polyethylene strip, redox switching induces bending of the strip. 

Plots similar in shape to those illustrated in Fig. 1.29 have been obtained experimentally for a number of conducting polymers, such as poly(pyrrole) and poly(aniline). The currents passed during redox switching are usually quite large, and therefore experiments have been... 

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