Multielectrochromism of redox states of thin electropolymerised films of poly(3-dodecylpyrrole) involving a black coloured state

• Multi-electrochromism in thin films of an alkylpyrrole derivative was observed.
• Polymer film switches between yellow, violet and black.
• Layer thicknes optimization yields reasonable contrast switching electrochromic films.
• Colouration is related to polymer doping states and to prevalent charge carriers.

Thin films of poly(3-dodecylpyrrole) have been found to yield multi-electrochromic characteristics. Optimisation of the polymer electrodeposition process enabled fabrication of layers exhibiting three distinct colourations at different oxidation levels. Coupled EPR-UV-Vis-NIR spectroelectrochemistry enabled correlation of these colour changes with the formation and decay of polaron and bipolaron charge carriers, offering an insight into the mechanism of doping/dedoping processes taking place in this polymer. The ionisation potential of the polymer was found to be very low, with valence band edge located at -3.7 eV, explaining the relative oxidation ease of the polymer and accounting for an ample number of spins present in the electrochemically dedoped polymer film. Maximum optical contrast of up to 15% and a visible light range averaged one of 10% was observed. Dynamic colour switching contrast of 10% is attainable at 5 s switching time, while switching steps as short as 0.5 s afford 6% contrast value. The fully oxidised polymer features broad UV-Vis absorption, making the system an interesting candidate for transmissive-to-black electrochromic devices.

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