Steric Hindrance of Photoswitching in Self-AssembledMonolayers of Azobenzene and Alkane Thiols.

Surface-bound azobenzenesexhibit reversible photoswitching viatrans–cis photoisomerization and have been proposed for a varietyof applications such as photowritable optical media, liquid crystaldisplays, molecular electronics, and smart wetting surfaces. We reporta novel synthetic route using simple protection chemistry to formazobenzene-functionalized SAMs on gold and present a mechanistic studyof the molecular order, orientation, and conformation in these self-assembledmonolayers (SAMs). We use vibrational sum-frequency generation (VSFG)to characterize their vibrational modes, molecular orientation, andphotoisomerization kinetics. Trans–cis conformational changeof azobenzene leads to the change in the orientation of the nitrilemarker group detected by VSFG. Mixed SAMs of azobenzene and alkanethiols are used to investigate the steric hindrance effects. While100% azobenzene SAMs do not exhibit photoisomerization due to tightpacking, we observe reversible switching (>10 cycles) in mixedSAMswith only 34% and 50% of alkane thiol spacers. [ABSTRACT FROM AUTHOR]