Photo-responsivity Of Self-assembled Monolayers Of Stiff Stilbene Derivatives

Stimuli-responsive surface can alter their properties in response to changes inenvironment or an external stimulus. A lot of stimuli can be utilized to control propertiesof surfaces, including temperature, light, pH, electric, and chemicals, etc. Among them,light is promising because of the fast response times of photochemical process, and itprovides a clean, noninvasive, and tunable energy input. Control of the wettability can beachieved by a chemical modification of the surface free energy and thus changing theaffinity of liquid to the surface. Light-activated molecular switches and motors open a newway to produce surfaces with controlled wettability. A number of approaches have beendeveloped in order to enhance the wettability contrast between the two states of theazobenzene-and spiropyran-functionalized surfaces, such as using mixed layers,increasing surfaces roughness, and using polymer-based surfaces. However, it is still achallenge to control wettability of light-responsive self-assembled monolayers(SAMs) onflat substrates.Stiff stibene is a light-responsive molecule with the characteristics of planarity,thermal stability and large difference in configuration of isomers. We have designed andsynthesized a stiff stilbene derivative (S5). We fabricated cis-S5and trans-S5SAMs on flatgold surface, whose water contact angles were67.8°and66.3°,respectively. In order toenhance the wettability contrast between the two states of SAMs, we connectedhydantoic acid to S5to obtain another stiff stilbene derivative (S7). We fabricated cis-S7and trans-S7SAMs on flat gold surface, whose contact angles were63.8°and56.4°,respectively. As a result, we got self-assembled monolayers with differentwettability at two states. There was no obvious change of the water contact angle afterirradiating cis-S7SAM with UV light (392nm). This is possibly owing to no enough space for isomerism. We fabricated a mixed SAM of cis-S7/n-C4H9SH so as to reduce sterichindrance. It was found that its wettability changed from70.8°to60.4°as a result ofUV (392nm) irradiation, and there was no obvious change of the water contact angleafter irradiating the SAM with UV light (352nm). Thus, the mixed SAM of cis-S7/n-C4H9SHwas unidirectional photo-responsive.