Controlling wettability by light: illuminating the molecular mechanism

Area of Science:

• Surface Science
• Photochemistry
• Materials Chemistry

Background:

• Organic monolayers with photoactive groups offer light-switchable surface properties.
• Azobenzene chromophores enable photo-induced cis-trans isomerization, altering molecular orientation and surface characteristics.

Purpose of the Study:

• To investigate the microscopic origins of light-induced wettability changes in azobenzene-containing monolayers.
• To correlate molecular orientation and dipole moment changes with macroscopic surface energy variations.

Main Methods:

• Langmuir-Blodgett monolayer deposition of a diphenyl-diazene copolymer.
• Spectroscopic techniques (Polarized UV-Vis, FTIR) for orientational order.
• Microscopy and ellipsometry (AFM, Imaging Ellipsometry) for surface morphology.
• Contact angle and surface potential measurements for wettability and polar ordering.
• Computational chemistry (semi-empirical, ab-initio) for molecular properties.

Main Results:

• Photo-induced cis-trans isomerization of the azobenzene chromophore was confirmed.
• Changes in the projection of the molecular dipole moment onto the surface normal were identified as the key factor.
• Isomerization-induced dipole moment changes directly correlate with observed shifts in surface energy and wettability.

Conclusions:

• The microscopic mechanism for light-controlled wettability involves azobenzene isomerization and subsequent dipole moment reorientation.
• Understanding this mechanism allows for the rational design of molecules to maximize photo-responsive wetting contrasts.