6.1 Molecular switch designs and mechanisms

Molecular switches are the building blocks of molecular electronics, allowing us to control electrical signals at the nanoscale. These tiny devices can change their properties in response to stimuli like light or electricity, opening up new possibilities for computing and sensors.

Understanding how molecular switches work is key to designing better electronic components. We'll look at different switch designs, from simple molecules that change shape to complex systems that mimic biological processes, and explore how they can be used in real-world applications.

Molecular Switching Mechanisms

Conformational Changes and Isomerization

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  Ion-mediated conformational switches - Chemical Science (RSC Publishing) DOI:10.1039/C4SC03525A View original

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  Photo-isomerization and light-modulated aggregation behavior of azobenzene-based ionic liquids ... View original

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• Conformational changes involve the rearrangement of atoms in a molecule without breaking or forming new bonds
  • Can be triggered by various stimuli such as light, heat, or electric fields
  • Example: azobenzene molecules undergo cis-trans isomerization upon exposure to UV light
• Isomerization is a type of conformational change where the connectivity of atoms remains the same, but their spatial arrangement changes
  • Can occur through rotation around single bonds or the inversion of chiral centers
  • Example: stilbene molecules can switch between cis and trans isomers upon irradiation with light
• Tautomerization involves the rapid interconversion between two or more structural isomers called tautomers
  • Occurs through the migration of a proton or other group within the molecule
  • Example: porphyrin molecules can undergo tautomerization between different protonation states, affecting their electronic properties

Stimuli-Responsive Molecules

• Stimuli-responsive molecules change their properties or conformation in response to external stimuli
• Can respond to various stimuli such as light, pH, temperature, electric or magnetic fields, and chemical species
• Photochromic molecules change color or opacity upon exposure to light of specific wavelengths
  • Example: spiropyran molecules undergo a ring-opening reaction when exposed to UV light, resulting in a color change
• pH-responsive molecules change their properties in response to changes in the acidity or basicity of their environment
  • Example: pH-sensitive polymers can swell or shrink depending on the pH, making them useful for controlled drug delivery
• Thermochromic molecules change color in response to temperature changes
  • Example: leuco dyes exhibit a reversible color change when heated or cooled

Switch Properties

Bistability and On/Off States

• Bistability refers to the ability of a molecular switch to exist in two distinct stable states
  • Each state corresponds to a different conformation, electronic configuration, or chemical structure
  • The two states are often referred to as the "on" and "off" states, representing different properties or functions
• On/off states can be distinguished by various properties such as optical absorption, fluorescence, conductivity, or reactivity
  • Example: a molecular switch based on a donor-acceptor system can have a low-conductance "off" state and a high-conductance "on" state

Reversibility and Switching Kinetics

• Reversibility is a crucial property of molecular switches, allowing them to switch back and forth between states multiple times
  • The switching process should be reversible without significant degradation of the molecule
  • Example: diarylethene derivatives can undergo reversible photochromic switching between open and closed forms
• Switching kinetics describe the rate at which a molecular switch transitions between states
  • Fast switching kinetics are desirable for applications requiring rapid response times
  • Factors influencing switching kinetics include the nature of the stimuli, molecular structure, and environmental conditions
  • Example: the switching speed of azobenzene derivatives can be tuned by modifying their molecular structure

Molecular Actuators

Molecular Machines and Artificial Muscles

• Molecular actuators are molecules or molecular assemblies that convert energy into mechanical motion
• Can be designed to perform various functions such as rotation, translation, or bending in response to stimuli
• Molecular machines are complex molecular actuators that mimic the behavior of macroscopic machines
  • Example: rotaxanes and catenanes can act as molecular shuttles, switches, or motors
• Artificial muscles are materials that exhibit reversible contraction or expansion, similar to biological muscles
  • Can be based on stimuli-responsive polymers or molecular actuators
  • Example: polymer gels containing light-responsive molecules can undergo reversible volume changes, mimicking muscle contraction and expansion