ELECTROCHEMICALLY DEPOSITED POLYTHIOPHENE .2. DRY AND WET RELAXATION

Polythiophene (PT) freshly electrodeposited at 1.9 V vs 0.01 M Ag+/Ag from 0.1 M thiophene + 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB)/acetonitrile is not in equilibrium either as an isolated phase or when it is in contact with the solution it was deposited from. Consequently, spontaneous relaxation processes take place during and after growth until equilibrium conditions are established. These simultaneous processes happen at very different speeds. Relaxation of the PT film begins already during its growth, electroneutrality of its bulk being ensured by the fast uptake of anions. If the fresh PT is quickly washed and then kept dry and electrically isolated, its electron work function decreases. A further decrease in work function can be measured if dry relaxation is followed with wet relaxation in the supporting electrolyte. If the freshly prepared PT is kept in the dark in the solution where it was deposited from, the electrode potential of PT decays slowly down to about -450 mV. White light causes the potential to jump fast up to 140-160 mV, while when PT is put in the dark again the potential drops down to about -450 mV again. This light effect diminishes in the order of blue > green > red light. If the fresh PT film is washed and then relaxed in a supporting electrolyte without thiophene, the final potential is only 30-200 mV, and the observed light effect is smaller. Parallel changes in color (increase in the reflectance of red light) and an increase in real impedance can be recorded during relaxation. Similar changes in color and impedance can be measured when the electrode potential is cycled between 1.2 and 0 V. The spontaneous relaxation phenomena are caused by many factors, including the slow reduction of PT by certain impurities (e.g., water and, if present, thiophene). Its stable final state depends on the environment where it is kept (dry or wet, in monomer or in other solutions, in the presence or absence of water and oxygen). Poly(3-methylthiophene) (PMT) electrodeposited at 1.3 V from 0.1 M 3-methylthiophene + 0.1 M TBATFB/acetonitrile exhibited a behavior similar to that of PT with respect to all types of experiments, except for the very fast potential decay of PT from 1.9 V to about 0.8 V during potential relaxation, a part of which was necessarily missing in the case of PMT. © 1990 American Chemical Society.