A surface confined yttrium(III) bis-phthalocyaninato complex: a colourful switch controlled by electrons
In the last 20 years huge efforts have been devoted to studying and developing organic molecules for electronic applications. Molecules can be, in principle, synthesized in mass production at a relatively low cost and, by chemical design, their properties can be tuned. In order to fabricate devices, molecules are typically supported on inorganic substrates (mainly metals or metal-oxides), facilitating their manipulation and the possibility to direct the application of external stimuli on them. One common route is the fabrication of self-assembled monolayers (SAMs) which is focused on the use of molecules with a specific functional group that spontaneously bonds to the surface.1,2 Electrochemical molecular switches are a particularly appealing class of molecular devices where electroactive molecules are switched reversibly between different redox states triggered by an electrical signal.3 Optical, magnetic, electrical or chemical outputs can be used to read the state of the switch.4–12 Most of the reported examples are based on bi-stable molecules where the two accessible redox states can be visualized as 1's or 0's mimicking the terminology employed in the binary logic system which is the basis of current memory devices. However, it is known that the fabrication of devices with a higher number of states would facilitate the processing of higher memory densities.13,14 Despite this interest, only a few examples based on electroactive SAMs that can present three or more states have been reported to date.13–15 Most of these systems take advantage of the different optical absorption levels that the distinct redox states exhibit at determined wavelengths as the readout mechanism.16–18 Double and triple-decker phthalocyanine lanthanide complexes are potential building blocks for the fabrication of electrochemical switches owing to their rich electrochemistry that allows an easy access to a range of oxidation states centred on the ligands.19–21 Lindsey and Bocian demonstrated with Eu phthalocyanine triple-decker SAMs that four available redox states could be accessed electrochemically.4 In solution and in thin films it is widely known that the reduction and oxidation processes in these materials are accompanied by significant changes in their optical absorption spectra.22–24 This prompted us to explore the possibility to exploit this property as the output of a surface confined switch based on a double-decker phthalocyanine lanthanide complex. In this work, a ternary switchable SAM of a bis-phthalocyaninato–Y(III) complex has been prepared. By the application of a low bias voltage, three redox states have been accessed and clearly identified using optical absorption spectroscopy. Remarkably, each state shows characteristic absorption bands giving complementary colours. The SAMs are revealed to be very robust and stable upon the application of more than 100 switching cycles.
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