Vapochromic properties of a Zinc(II) salen-type Schiff-base complex for detection of volatile organic compounds

Volatile Organic Compounds (VOCs) are organic compounds, in liquid or solid form, which, owing to their high vapor pressure, can easily evaporate at room temperature. Some VOCs, used and produced from many manufacturing industries can be hazardous to human-health, and are becoming a major cause of air and ground-water pollution [1]. In this context, the development of simple, low-cost and fast chemosensors, is needed for the direct monitoring of VOCs in specific environments. Among them, vapochromic molecular materials based on Zn(II) salen-type Schiff-base complexes can represent promising and straightforward chemosensors. In fact, their Lewis acidic character allows to form stable penta-coordinated adducts with VOCs having Lewis basicity. This phenomenon leads to significant color changes with respect to pristine zinc complexes, allowing in-situ detection of volatile organic compounds [2]. In this contribution, we explored the stimuli-responsive chromic properties of a substituted Zn(salen)-type complex derivative from the 2,3-diaminomaleonitrile, Zn(salmal), with 5,5´-tert-butyl- bulky substituents on the salicylidene rings [3]. This complex exhibits distinct thermo- and vapochromic properties (Figure 1). In particular, the anhydrous complex, and related cast films on glass or paper substrates, achieved by solvent evaporation under anhydrous conditions from its tetrahydrofuran solutions, shows a marked vapochromism if exposed to vapors of various VOCs. Interestingly, such vapochromic behavior involves well-defined optical absorptions and distinct naked-eye color changes, also allowing to discriminate primary aliphatic amines. This contribution has been partially funded by European Union (NextGeneration EU), through the MUR-PNRR project SAMOTHRACE (ECS00000022).