Reversible switching of the Au(111) work function by near infrared irradiation with a bistable SAM based on a radical donor–acceptor dyad

Abstract

We describe the modification of the work function (WF) of Au(111) upon deposition of self-assembled monolayers (SAMs) with two donor–acceptor (D–A) systems, one based on a ferrocene-polychlorotriphenylmethyl radical (Fc–PTM) dyad and another on its non-radical dyad analogue. Kelvin Probe Force Microscopy (KPFM) has been used to measure the changes in the Contact Potential Difference (CPD) between the tip and the SAM under application of a cycling sweep of direct current (DC) voltage bias. These measurements showed that both SAMs exhibit a hysteretic behaviour in their WF changes. Interestingly, the hysteresis loop of the radical SAM is notably reduced when irradiated with NIR light, which we attribute 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. Consequently, under NIR irradiation the WF hysteresis is almost quenched and the WF value of the functionalized gold surface is significantly shifted by +250 mV recovering their original values when the irradiation is suppressed. 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 value and the WF hysteresis of the non-radical SAM does not change upon NIR irradiation since this SAM does not display bistability.