Symmetric azobenzene-substituted diketopyrrolopyrroles dyes as acid-base switchable molecular-probe for colorimetric paper-based sensors

A novel synthetic approach for the preparation of azobenzene-substituted diketopyrrolopyrroles (DPPs) has been developed and optimized. The dyes were synthesized through a pseudo-Stobbe condensation reaction starting from appropriate benzonitriles, followed by azo coupling. The presence of electron-donating groups (EDGs), such as amino groups, in the benzonitrile leads to the formation of the DPP structural isomer (isoDPP) due to acid-base equilibria that can influence the cyclization mechanism. The resulting dyes exhibited significant pH-dependent chromatic shift, with behaviour that can be predicted on the basis of the aromatic substituents. In particular, the system functionalized with the phenolic group (2a) exhibits the most prominent chromatic response (Δλ = 90 nm (5.84·103 cm−1)), making it ideal for use in paper-based sensors. Chromatic switching mechanism has been proposed and confirmed by NMR spectroscopy and Time-Dependent Density Functional Theory (TD-DFT) calculations in gas and solution phases. The sensors, fabricated via dip-coating techniques, demonstrate high functionality, stability, and reversible colour changes even after multiple cycles of exposure to acidic (HCl) and basic (NH3) conditions in both liquid and gas environments. These results highlighted how the azobenzene-DPP derivatives can be used as a promising platform for developing advanced halochromic sensors with potential applications in environmental and industrial monitoring.