Artificial molecular machines represent a growing field of nanoscience and nanotechnology. Stimulated by chemical reagents, electricity, or light, artificial molecular machines exhibit precisely controlled motion at the molecular level; with this ability molecular machines have the potential to make significant impacts in numerous engineering applications. Compared with molecular machines powered by chemical or electrical energy, light-driven molecular machines have several advantages: light can be switched much faster, work without producing chemical waste, and be used for dual purposes-inducing (writing) as well as detecting (reading) molecular motions. The following issues are significant for light-driven artificial molecular machines in the following aspects: their chemical structures, motion mechanisms, assembly and characterization on solid-state surfaces. Applications in different fields of nanotechnology such as molecular electronics, nano-electro-mechanical systems (NEMS), nanophotonics, and nanomedicine are envisaged.
The paper studies the molecular-level active control of localized surface plasmon resonances (LSPRs) of Au nanodisk
arrays with molecular machines. Two types of molecular
machines - azobenzene and rotaxane - have been demonstrated
to enable the reversible tuning of the LSPRs via the controlled mechanical movements. Azobenzene molecules have the
property of trans-cis photoisomerization and enable the photo-induced nematic (N)-isotropic (I) phase transition of the
liquid crystals (LCs) that contain the molecules as dopant. The phase transition of the azobenzene-doped LCs causes the
refractive-index difference of the LCs, resulting in the reversible peak shift of the LSPRs of the embedded Au nanodisks
due to the sensitivity of the LSPRs to the disks' surroundings' refractive index. Au nanodisk array, coated with
rotaxanes, switches its LSPRs reversibly when it is exposed to chemical oxidants and reductants alternatively. The
correlation between the peak shift of the LSPRs and the chemically driven mechanical movement of rotaxanes is
supported by control experiments and a time-dependent density functional theory (TDDFT)-based, microscopic model.
Artificial molecular motors have recently attracted considerable interest from the nanoscience and nanoengineering
community. These molecular-scale systems utilize a 'bottom-up' technology centered around the design and
manipulation of molecular assemblies, and are potentially capable of delivering efficient actuations at dramatically
reduced length scales when compared to traditional microscale actuators. When stimulated by light, electricity, or
chemical reagents, a group of artificial molecular motors called bistable rotaxanes - which are composed of mutually recognizable
and intercommunicating ring and dumbbell-shaped components - experience relative internal motions of
their components just like the moving parts of macroscopic machines.
Bistable rotaxanes' ability to precisely and cooperatively control mechanical motions at the molecular level reveals the
potential of engineering systems that operate with the same elegance, efficiency, and complexity as biological motors
function within the human body. We are in a process of developing a new class of bistable rotaxane-based
electroactive/photoactive biomimetic muscles with unprecedented performance (strain: 40-60%, operating frequency: up
to 1 MHz, energy density: ~50 J/cm<sup>3</sup>, multi-stimuli: chemical, electricity, light). As a substantial step towards this longterm
objective, we have proven, for the first time, that rotaxanes are mechanically switchable in condensed phases on
solid substrates. We have further developed a rotaxane-powered microcantilever actuator utilizing an integrated
approach that combines "bottom-up" assembly of molecular functionality with "top-down" micro/nano fabrication. By
harnessing the nanoscale mechanical motion from artificial molecular machines and eliciting a nanomechanical response
in a microscale device, this system mimics natural skeletal muscle and provides a key component for the development of
nanoelectromechanical system (NEMS).