This project is the product of a three-month collaboration with the Max Planck Institute for Polymer Research (Mainz, Germany). Highly ordered two-dimensional colloid monolayers have been used to mimic biological structural color, to control wetting and surface chemistry, and to engineer plasmonic constructs, among many other applications.

 

Here, we engineered a binary monolayer containing two distinct particle sizes: a larger template colloid and a smaller interstitial particle with photoswitchable properties. The optical properties of the photochrome (cis-1,2-dicyano-1,2-bis-(2,4,5-trimethyl-3- thienyl)ethene, CMTE) and fluorophore (d N-(2,6-diisopropylphenyl)- perylene-3,4-dicarboximide, PMI) were analyzed in solution and encased in polymeric colloids.

 

Using miniemulsion polymerization, the photochrome/fluorophore pair was encapsulated in colloids composed of either polystyrene (PS) or PS and poly(butyl acrylate) (PBA). PS and PBA were compared to understand whether the relative rigidity (PS) or softness (PBA) of the polymer matrix restricts the photoswitching process. 

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We showed that the fluorescence emission of  PMI can be turned off and on using UV and VIS light, respectively.

When the particles are irradiated with UV light, CMTE undergoes a conformational change to a ring-closed form that absorbs light of PMI's emission wavelength (system is switched off). Upon irradiation with VIS light, the photochrome reversibly isomerizes to the thermodynamically favorable ring-open form (system is switched on).

Photoswitching of this system was shown to be reversible and highly thermally stable in the polymer matrix. 

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The self-assembly of the smaller photoswitchable particles with larger PS colloids into a binary monolayer raft was shown to have a high degree of ordering. This high crystallinity translates to selective activation of specific particles using light.

 

We demonstrated that custom light patterns on the monolayers could be both added and reversibly erased using light. Thus, this simple system can be used as a model for data storage or light-erasable barcoding. 

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The project culminated in a publication in Nanoscale. 

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