Pyrene-based phenothiazine–5–oxides for volatile acids, temperature, latent fingerprints detection and their data encryption and anti–counterfeiting ink applications

Pyrene, a strong electron donor, is well known for its exceptional fluorescence properties in solution; however, the planar, conjugated structure of pyrene can lead to excimer formation through cofacial stacking in the aggregated state, which is detrimental to its use in material applications. Herein, we have designed and synthesized pyrene-phenothiazine– 5 –oxide donor-acceptor conjugates in which the conformational distortion due to the butterfly–shaped geometry of phenothiazine– 5 –oxide and steric effects from the substituents reduce the possibility of excimer formation. Single crystal X-ray diffraction (SCXRD) analysis confirmed the non–planar geometry of these compounds. Multiple weak intermolecular interactions, including H–bonds and C–H … π, were also observed in the crystal lattice. All compounds are emissive in both solution and solid state and exhibit very high molar absorptivity ( ε = 85000 mol −1 dm 3 cm −1 ). These molecules also showed sensitivity to solvent polarity, aggregation, and external stimuli. Computational studies showed that the solvent stabilization of the excited state played an important role in the photophysical response of these compounds. The compounds could detect volatile acids (TFA and HNO 3 ) at low concentrations (LoD = 1.17 ppm for TFA and 0.35 ppm for HNO 3 ) through quenching of emission; these changes were reversible upon treatment with triethylamine. Additionally, superparamagnetic behavior due to the formation of a phenothiazine radical cation upon acid treatment was studied. Further, a systematic variation in emission intensity was observed over a wide temperature range (0 to −140 °C) in these compounds, which show their potential as sensitive fluorescent probes for low temperatures. The practical applications of these compounds in fingerprint detection, anti–counterfeiting writing, and information encryption are also discussed.