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Photochromics meet Organic Electronics: a Novel Generation of Functional Materials and Interfaces

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Photochromic systems are capable of undergoing efficient and reversible photochemical reactions, i.e. to switch between two or more (meta)stable isomers featuring markedly different properties. Such bi- or multi-stable building blocks can then be employed to translate an incoming stimulus such as light into a macroscopic property change of the materials.[1]

In particular, azobenzenes (AZO) are known to undergo reversible photoinduced isomerization between trans and cis form which can exhibit different optical and electrical properties. We demonstrate that a photochromic bi-stable AZObenzene-Self-Assembled Monolayer (AZO-SAM) chemisorbed on Au electrodes can mediate the injection through the variation of the tunneling barrier across the SAM making it possible to modulate reversibly the charge injection at the Au electrodes/semiconductor interface in an organic transistor. The observed result is in full agreement with previous I-V characterizations of the AZO -SAM when incorporated in two terminal junctions as studied by C-AFM2 and Hg-drop based measurements,[3] demonstrating that the switching effect of the transport properties relies on the difference in tunneling barrier thickness. Wettability properties of the AZO -SAM/Au (in its trans and cis isomer) were characterized by contact-angle, and its work function variation was measured by means of macroscopic Kelvin Probe.

Diarylethenes (DAEs)[4] are another class of photochromic systems which are very popular scaffolds in molecular electronics5 because their open and closed isomers feature different HOMO and LUMO levels depending on the specific irradiation wavelength (UV or white light). Hence other properties such as absorption, emission as well as redox characteristics can be phototuned as well. Promising electrical current switching properties achieved by combining DAEs and organic semiconductors in blends were theoretically predicted.[6] The blend between DAE derivatives featuring different energy levels with an organic semiconducting polymer such as P3HT was used as a bi-component film forming the electroactive layer of organic thin-film transistors (OTFTs).Taking into account the hole transport levels of P3HT , we designed and synthesized a DAE molecule, DAE _1, featuring different ionisation energy (IE) in its open and closed form with respect to the IE of P3HT . To explore the role of different energy levels in the two different DAE isomers with respect to the IE of P3HT we have extended our study to DAE _2 with higher IE of both isomers.

In our devices the source-drain current through the channel can be therefore gated both electrically (through gate control), like in a conventional OFET , and optically through photochemical control exerted either in the charge injection or transport level . Such a proof of concept is instrumental to the field of organic electronics which searches for solutions to integrate new and more functionalities in a device.

References [1] M.-M. Russew, S. Hecht, Adv. Mater. 2010, 22, 3348. [2] J. M. Mativetsky, G. Pace, M. Elbing, M. A. Rampi, M. Mayor, P. Samorì, J. Am. Chem. Soc. 2008, 130, 9192. [3] V. Ferri, M. Elbing, G. Pace, M. D. Dickey, M. Zharnikov, P. Samorì, M. Mayor, M. A. Rampi, Angew. Chem. Int. Ed. 2008, 47, 3407. [4] M. Irie, Chem. Rev. 2000, 100, 1683 (2000). [5] A. J. Kronemeijer et al., Adv. Mater. 2008, 20, 1467. [6] F. L. E. Jakobsson et al., J. Phys. Chem. C 2009 , 113, 18396.

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