Azobenzenes are widely used as light-responsive molecules in creating functional materials for applications ranging from non-linear optics to biotechnology. The light-sensitivity arises from the fully reversible light driven trans-cis-trans isomerization upon which the molecules exhibit large spectral and geometrical changes. Azobenzenes are often used as building blocks in self-assembling supramolecular materials. The self-assembly may strongly affect the isomerization dynamics of azobenzenes, especially the thermal half-life of the metastable cis-isomer. We show that the isomerization dynamics can be utilized to create a highly sensitive, optically readable relative humidity sensor.
The optically active molecules, i.e. hydroxyazobenzene derivatives, are embedded into a poly(4‑vinylpyridine) matrix, where they are supramolecularly bound to the polymer chains. Environmental humidity causes intrinsic changes in the thermal isomerization mechanism of the hydroxyazobenzene molecules, and this leads to a large change in the cis-isomer lifetime. The cis-lifetime decreases exponentially up to 3 orders of magnitude with the change from 0 to 100 %RH. The lifetimes are stable and highly reproducible, which allows a high accuracy of the sensor. The supramolecular concept allows to embed a high concentration of the probe molecules into the polymer, while retaining amorphous structure within the thin film. This allows a thin sensing layer and a fast response to changes of relative humidity. Our new humidity sensing concept is fully integrable with optical fibres and by optimising the materials, it may be extended to sensing of also other hydrogen-bonding gases.
Arri Priimagi, Mikko Poutanen, and Olli Ikkala, "Optical humidity sensing based on azobenzene photoswitching (Conference Presentation)," Proc. SPIE 10529, Organic Photonic Materials and Devices XX, 105290Y (Presented at SPIE OPTO: February 01, 2018; Published: 15 March 2018); https://doi.org/10.1117/12.2287085.5752200750001.
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