Effects of Molecular Doping by Hydroquinones on Morphology and Optical Properties of Polyaniline

Abstract

Thin films of the emeraldine base (EB) form of polyaniline containing different forms of hydroquinones (hydroquinone, HQ1; tetrafluorohydroquinone, HQ2; tetrahydroxy-1,4,-quinone, HQ3) were cast from the solutions of EB/HQ1,2,3 in N-methylpyrrolidinone (NMP). Their properties were studied using optical micrography, UV−vis, and infrared absorption. It was found that hydroquinones act as molecular dopants of PANI by donating protons from their OH groups to the imine centers of the polymer and thus provoking a conversion of EB toward the emeraldine salt (ES). The extent of this conversion process is determined by the concentration of HQs and their ability to transform into the anion. Other important parameters are the temperature of drying and the time of storage of the liquid substance before casting. Because of the presence of at least two OH groups in HQ molecules, they can react with two or more imine centers of the polymer, thus bridging different polymer chains and provoking their ordering. Drying of the macroscopically ordered EB/HQ1,2,3 liquid allows us to fix this ordering and to observe the nucleation and growth of spherulite crystals. Thus, HQ molecules play a double role in determining the structure and properties of emeraldine base. They, on one hand, produce protonation of the imine bonds of the polymer, similar to the mechanism of acid anion doping, and on the other hand, they operate as agents, causing the reorientation of conformational planes in polymeric chains and their mutual ordering.

Anodic polymerization of 1-amino-2-methyl-9,10-anthraquinone in mixed solvent

“…5C, the peak current (I) obtained at À0.62 V increased slightly and a new peak assigned as (XI) appeared at À0.4 V. It is expected that quinone moieties may undergo a kind of interactions with Ar-NH groups, where both are present in the polymer chain. A similar kind of interactions was described for 1,4-benzoquinone molecules, in the bulk, with redox sites of polyaniline [18][19][20]. On the other hand couples V/VI and VII/VIII appeared in the positive side may be related to the redox process of the polymer chain, i.e.…”

Impedance Study of the Charge-Transfer Interaction Between the Polyaniline Chain and Redox Pair in Solution

“…Our recent optical study confirmed that in mixed EBϩHqui(s) composites prepared from N-methylpyrrolidone, the EB chains are partially protonated and the negative charged particles from Hqui(s) are formed simultaneously. 9 Therefore, active centers on the polymer chains play an important role as sites for elec-trical exchange with the environment (electrolyte, solvent, or gas). If polymers are used as electrodes, these centers guide the electrochemical processes.…”

“…Acid-like doping of PANI-EB by several substituted hydroquinones in solid-state conditions has been confirmed by electrical, Fourier transform infrared (FTIR), and UV-vis measurements. 9 Finally, polymer-molecular systems may possess new physical properties not found in traditional acid doped polyaniline.…”

Molecular doping principles in organic electronics: fundamentals and recent progress

“…Initial studies on p-type molecular dopants were mostly related to the investigation of quinone molecules 18,19) (i.e. conjugated molecules derived from aromatic molecules with a cyclic dione structure), owing to the unsaturated aromatic ring structure.…”