Switching crystals off and on again

Kyoto University researchers have demonstrated an on/off switching behaviour in a coordination polymer crystal. Their study has been published in the journal Angewandte Chemie.

Coordination polymer crystals are inorganic and organic hybrid materials that are known for their structural and functional diversity and their ability to conduct protons. Proton conduction is a form of electrical conduction in which positive hydrogen ions (H+) carry the charge instead of electrons — a process which plays a key role in powering photosynthesis in plants and could be used to develop better fuel cells.

Led by Satoshi Horike and Susumu Kitagawa, the researchers synthesised a coordination polymer (CP) by reacting zinc oxide, phosphoric acid and imidazole in ethyl alcohol at room temperature. The CP was then melted and triflic acid was added. The resultant mixture was then cooled and recrystallised. This ‘acid doping’ of the CP significantly enhanced its proton conductivity.

The team melted their original CP again and instead added the photoacid pyranine — a molecule that becomes more acidic upon absorption of light. After cooling the material, its now recrystallised form was exposed to light and its proton conductivity improved. When the light was turned off, its conductivity decreased and returned to its original state. This change could be switched on and off over several consecutive cycles of light exposure.

Acid doping of the CP resulted in minimal structural change with overall enhancement of proton conductivity. Doping the CP with photoacid gave the researchers on-demand external control of the ionic current in the material.

The researchers claimed that theirs is the first demonstration of utilisation of the melting state for CP functionalisation. The strategy could also potentially be extended to synthesise a new class of proton-conducting solids that can be used in non-volatile memory technologies, ionics-based transistors and light-induced ionic/electric current circuits.