In organic electronic devices, charge injection barriers at metal-organic interfaces can be tuned by modifying the work function (WF) of metallic electrodes using self-assembled monolayers (SAMs) of polar molecules. An interesting example of polar molecule is offered by donor–acceptor (D–A) dyads based on ferrocene (Fc) as electron-donor unit and a polychlorotriphenylmethyl radical as electron-acceptor unit, connected by a π-conjugated vinylene bridge. The magnitude of the shift in the charge injection barriers for this D–A systems is estimated by means of surface potential measurements performed by Kelvin probe force microscopy (KPFM). The experimental data has been rationalized by density functional theory calculations, which evidence the importance of presenting not only high molecular dipole moments but also low polarizabilities along the direction normal to the substrate to achieve high work function (WF) shifts of metals upon SAM formation.
Based on these findings, we have described the modification of the WF of Au(111) upon deposition of self-assembled monolayers (SAMs) of the two donor–acceptor (D–A) systems, the radical (Fc–PTM) dyad and its non-radical analogue. Interestingly, the WF of the radical SAM is significantly shifted by +250 when irradiated with NIR light recovering their original values when the irradiation is suppressed. This phenomena, is attributed to the bistable nature of this SAM in which neutral radical dyad molecules are excited into a zwitterionic state following a light driven intramolecular charge transfer (ICT) from the Fc unit to the PTM radical unit. Remarkable is the large WF shift attained, one of the highest values reported in the literature, and the unprecedented fact that it is achieved under irradiation in the IR region due to an intramolecular electronic reorganization. In contrast, the WF of the non-radical SAM does not change upon NIR irradiation since this SAM does not display bistability.
Effect of the Molecular Polarizability of SAMs on the Work Function Modification of Gold: Closed- versus Open-Shell Donor–Acceptor SAMs
Valentin Diez-Cabanes, Deyana Morales, Manuel Souto, Markos Paradinas, Francesca Delchiaro, Anna Painelli, Carmen Ocal, David Cornil, Jérôme Cornil, Jaume Veciana, Imma Ratera
Advanced Materials Technologies 4, 1800152, 2019
Reversible switching of the Au(111) work function by near infrared irradiation with a bistable SAM based on a radical donor-acceptor dyad
Valentin Diez-Cabanes, Andrés Gomez, Manuel Souto, Nerea Gonzalez-Pato, Jérôme Cornil, Jaume Veciana, Imma Ratera
Journal of Materials Chemistry C 7, 7418, 2019
Figure: Reversible switching of gold WF by NIR irradiation using SAMs of D-A radicals.