Synthesis and cis-trans kinetics of an azobenzene-based molecular switch for light-responsive silica surfaces

Abstract

Light-responsive materials design is an attractive research field that stimulates demand for the synthesis of novel molecular photoswitches. The novel photoswitches should be capable of being attached to a desired surface in order to provide it with light-responsive properties. The switchback to its ground state is expected to be gradual, with the possibility of acceleration. This study presents a comprehensive description of a chemical route to a molecular photoswitch based on an azobenzene derivative with a trifluoromethoxy group and a long alkyl chain. The photoswitch is capable of modifying submicrometer silica particles and silicon wafers as models of curved and flat surfaces, respectively. The thermal relaxation times of the synthesized azobenzene were studied in three different solvents: ethanol, chloroform, and toluene. The results reveal a significantly delayed relaxation process lasting for several days. This observation is found to be related to the polarity of the solvents used. To characterize the extensive thermal relaxation process, a methodology including the use of white-light pulses to accelerate cis-trans isomerization is suggested. The white light spectrum used in pulses affects thermal relaxation but does not alter the relaxation time extrapolated to zero. The density of the photoswitch molecules attached to particles varies from 0.7 to 1.7 molecules/nm2 as the initial amount of azobenzene in the reaction increases. Surface-attached photoswitches demonstrate reversible cis-trans isomerization behavior when exposed to UV and white light irradiation. A 14° contact angle change is observed when flat surfaces modified with photoswitches are irradiated with UV light.