Optical detection of amine vapors using ZnTriad porphyrin thin films

• A tripodal porphyrin (ZnTriad) has been used for the optical detection of amines.
• The tripodal structure of the porphyrin reduces its aggregation in films.
• Exposure of ZnTriad films to amines led to fast, intense and reversible optical changes.
• ZnTriad films exhibited selective responses to the set of amines.
• The films showed ability to discriminate amines regarding their size and/or shape.

In this work, we have used a tripodal porphyrin, 1,3-di[5-(3-hydroxyphenyl)-10,15,20-(triphenyl)porphyrin]-2-(5-(3-hydroxyphenyl)-10,15,20-(triphenyl) porphyrin)-2-methylpropane Zn(II) (ZnTriad), to detect amines through UV–vis spectroscopy. We have also compared ZnTriad to its single unit, Zn(II) 5,10,15,20-tetraphenylporphyrin (ZnTPP) to assess the influence of the branched structure on the molecular aggregation and sensing capabilities. The comparison between tripodal ZnTriad and monomeric ZnTPP has shown that this multiporphyrin system can reduce the aggregation of porphyrins in films prepared by spin coating. ZnTPP deposition on glass resulted in completely unusable films after less than 1 h from their preparation due to extreme aggregation, while ZnTriad showed excellent stability. The gas-sensing properties of ZnTriad films have been analyzed through their exposure to butylamine as testing gas, featuring intense and reversible spectral changes in the form of shift of their Soret band to longer wavelengths and increase of its maximum absorbance. The sensor also shows fast response and repeatability after consecutive exposure-recovery cycles. In order to test if the geometry of ZnTriad facilitates the selective detection of amines regarding their size and shape, we have chosen a total of five primary amines, being three of them linear or one-dimensional with increasing length (propylamine, butylamine and hexylamine), one planar or two-dimensional (aniline) and one three-dimensional (tert-butylamine). The exposure of ZnTriad films to these analytes has resulted in selective responses toward the amine vapors. The linear amines have produced the fastest and more intense spectral changes on ZnTriad films, followed by tert-butylamine and aniline. Overall, films made with ZnTriad have shown great stability, preventing porphyrin aggregation, and ability to discriminate amines regarding their size and/or shape. Therefore, this porphyrin would be an excellent candidate for its inclusion in multisensor arrays.