Azobenzenes are photoresponsive molecules capable of switching between trans and cis isomers, making them versatile photoswitches with applications spanning photobiology, energy storage, and more. However, translating laboratory discoveries to market-ready products remains a challenge. This study focuses on hydroxyazobenzenes, whose isomerisation kinetics exhibit significant humidity sensitivity, a property exploited in optical humidity sensors. The research builds upon a previous demonstration of humidity sensing using hydroxyazobenzene-containing polymer thin films, addressing critical questions regarding the impact of azobenzene structure, polymer matrix, and azobenzene concentration on sensor performance and long-term reliability. The core concept involves the thermal isomerisation kinetics of tautomerizable azobenzenes in polymer matrices, which, through calibrated curves, can be translated into relative humidity (RH) readings at a known temperature. The goal is to establish design principles for enhancing the performance and commercial viability of these sensors by understanding the factors influencing humidity sensitivity. This includes addressing the accuracy and working range limitations observed in earlier prototype devices.

The literature review highlights the extensive use of azobenzenes as photoswitches due to their significant isomerization-induced molecular motions and modifiable properties via substitution on the phenyl rings. Near-quantitative photoisomerization and control over cis isomer stability are achieved through strategic substitutions. The paper acknowledges the influence of environmental factors (temperature, polarity, concentration, pressure) on azobenzene isomerization kinetics, citing the example of hydroxyazobenzenes exhibiting drastically different thermal isomerisation rates in polar versus non-polar solvents. This environmental sensitivity provides a dynamic range for kinetics manipulation, useful in applications like gated photoresponsive systems and lifetime-based sensing. Prior work demonstrated the feasibility of optical humidity sensing using hydroxyazobenzene in polymer thin films, attributed to hydrogen bond-assisted azo-hydrazone tautomerism. However, open questions remained regarding the dependency on azobenzene structure, polymer matrix, and sensor reliability for commercial applications.

The sensing concept relies on the readily measurable differences in the absorption spectra of azobenzene's trans and cis isomers. Thermal relaxation kinetics are determined by measuring absorption at a fixed wavelength post-photoexcitation. A stretched exponential model better describes the thermal isomerisation kinetics in the solid phase compared to the first-order kinetics observed in solutions. The study examines a small library of tautomerizable azobenzenes, including hydroxyazobenzenes, hydroxy-heteroazoarenes, and aminoazobenzenes, in various polymer matrices (poly(4-vinylpyridine) (P4VP), poly(methyl methacrylate) (PMMA), ethyl cellulose (EC)) with varying water absorption properties. The effect of the azobenzene content on both water absorption and isomerisation kinetics is also evaluated. The humidity sensitivity is quantified using the relation k(RH) = k₀e^(vRH), where k(RH) is the rate constant at a given RH, k₀ is the rate constant at 0% RH, and v describes the humidity sensitivity. Film fabrication involved spin coating solutions of azobenzene and polymer onto glass substrates. Absorption spectra and thermal isomerisation kinetics were measured using UV-vis spectroscopy and a humidity-controlled chamber with fiber-optic spectrometer. Density functional theory (DFT) calculations were employed to investigate the isomerisation pathways and their energies in THF. Water absorption isotherms were measured using quartz crystal microbalance with dissipation (QCM-D) and dynamic vapor sorption (DVS) techniques.

The study confirms that azo-hydrazone tautomerization is essential for humidity sensitivity in hydroxyazobenzenes. All investigated 4-hydroxyazobenzenes, regardless of the 4'-substituent, demonstrated significant humidity sensitivity (4–6 orders of magnitude dynamic range). 4'-substitution and heteroaromaticity influence the isomerisation rate at a given RH. Electron-donating groups slow down isomerisation, while electron-withdrawing groups accelerate it. 4-Aminoazobenzenes exhibit weaker humidity sensitivity compared to 4-hydroxyazobenzenes. Carboxylic acid substitution in 4-dimethylaminoazobenzenes leads to even higher humidity sensitivity than 4-hydroxy substitution. DFT calculations revealed that the humidity sensitivity is governed by the ease of tautomerization to hydrazone, while the overall isomerisation rate depends on transition state energies. The polymer matrix significantly impacts humidity sensitivity, with P4VP (high water absorption) resulting in higher sensitivity than PMMA (low water absorption) and EC (moderate absorption). A strong correlation between the polymer's water absorption isotherm and the humidity sensitivity of the isomerisation rate is observed. Increasing azobenzene content reduces water absorption and decreases humidity sensitivity, likely due to reduced water-azobenzene interactions and increased intermolecular azobenzene interactions. The interplay of water molecules and hydroxy groups from azobenzenes influences the overall isomerisation kinetics.

The findings directly address the research question by establishing the key factors governing humidity-dependent azobenzene isomerisation kinetics. The ability to tune the isomerisation kinetics through azobenzene structure, polymer selection, and azobenzene concentration offers valuable design strategies for optical humidity sensors. The importance of azo-hydrazone tautomerism for humidity sensitivity is clearly demonstrated. The observation that the isomerisation rate is also affected by azobenzene concentration highlights the interplay between intermolecular interactions and water absorption. Understanding this complex interplay is crucial for optimizing sensor performance, especially concerning accuracy and response time. The study's results pave the way for designing sensors with improved characteristics based on a nuanced understanding of material properties and their interactions.

This research offers three strategies for controlling the humidity sensitivity of azobenzene isomerization kinetics: selecting the appropriate azobenzene, choosing a suitable polymer matrix, and controlling the azobenzene concentration. The study provides a detailed understanding of the underlying mechanisms, emphasizing the critical roles of tautomerization, polymer water absorption, and intermolecular interactions. Future research should focus on developing polymers with tailored water absorption characteristics (linear isotherms and fast water absorption/desorption) and exploring novel azobenzene derivatives for enhanced sensitivity and performance. This will pave the way for more accurate, reliable, and commercially viable optical humidity sensors based on this technology.

The study's scope is limited to a specific set of azobenzenes and polymer matrices. While the findings provide valuable insights, it's important to note that the observed trends might not be universally applicable to all azobenzene-polymer combinations. The experimental investigation was also limited to a specific humidity range, potentially impacting the generalizability of the observed exponential relationships between humidity and isomerisation rates. Further research exploring a broader range of materials and conditions is necessary for comprehensive validation and optimization.