We apply in situ surface-enhanced Raman spectroscopy (SERS) to probe the reversible photoswitching of azobenzene-functionalized molecules inserted in self-assembled monolayers that serve as controlled nanoscale environments. Nanohole arrays are fabricated in Au thin films to enable SERS measurements associated with excitation of surface plasmons. A series of SERS spectra are recorded for azobenzene upon cycling exposure to UV (365 nm) and blue (450 nm) light. Experimental spectra match theoretical calculations. On the basis of both the simulations and the experimental data analysis, SERS provides quantitative information on the reversible photoswitching of azobenzene in controlled nanoscale environments.

Molecular switches and motors respond structurally, electronically, optically, and/or mechanically to external stimuli, testing and potentially enabling extreme miniaturization of optoelectronic devices, nanoelectromechanical systems, and medical devices. The assembly of motors and switches on surfaces makes it possible both to measure the properties of individual molecules as they relate to their environment and to couple function between assembled molecules. In this review, we discuss recent progress in assembling molecular switches and motors on surfaces, measuring static and dynamic structures, understanding switching mechanisms, and constructing functional molecular materials and devices. As demonstrative examples, we choose a representative molecule from three commonly studied classes including molecular switches, photochromic molecules, and mechanically interlocked molecules. We conclude by offering perspectives on the future of molecular switches and motors on surfaces.

Stimuli-responsive molecules are key building blocks of functional molecular materials and devices. These molecules can operate in a range of environments. A molecule's local environment will dictate its conformation, reactivity, and function; by controlling the local environment we can ultimately develop interfaces of individual molecules with the macroscopic environment. By isolating molecules in well-defined environments, we are able to obtain both accurate measurements and precise control. We exploit defect sites in self-assembled monolayers (SAMs) to direct the functional molecules into precise locations, providing a basis for the measurements and engineering of functional molecular systems. The structure and functional moieties of the SAM can be tuned to control not only the intermolecular interactions but also molecule-substrate interactions, resulting in extraction or control of desired molecular functions. Herein, we report our progress toward the assembly and measurements of photoresponsive molecules and their precise assemblies in SAM matrices.

We identify and control the photoreaction paths of self-assembled monolayers (SAMs) of thiolate-linked anthracene phenylethynyl molecules on Au substrate surfaces, and study the effects of nanoscale morphology of substrates on regioselective photoreactions. Two types of morphologies, atomically flat and curved, are produced on Au surfaces by controlling substrate structure and metal deposition. We employ surface-enhanced Raman spectroscopy (SERS), combined with Raman mode analyses using density functional theory, to identify the different photoreaction paths and to track the photoreaction kinetics and efficiencies of molecules in monolayers. The SAMs on curved surfaces exhibit dramatically lower regioselective photoreaction kinetics and efficiencies than those on atomically flat surfaces. This result is attributed to the increased intermolecular distances and variable orientations on the curved surfaces. Better understanding of the morphological effects of substrates will enable control of nanoparticle functionalization in ligand exchange in targeted delivery of therapeutics and theranostics and in catalysis.

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