Heterocyclic biphenyl-based fluorochrome sensor for rapid hydrazine detection: design, synthesis, single crystal XRD, and DFT studies†

Abstract

As part of our continuous research focused on enhancing sensing technologies, this article presents a series of ground-breaking fluorochromes that feature a biphenyl scaffold. Novel fluorochrome sensors are developed with various heterocyclic aldehydes via Claisen–Schmidt condensation. This condensation is performed using KOH and pyrrolidine as catalysts to provide two different methods with competitive studies. The obtained results show that KOH is a rapid catalyst (2–3 h; 71–80%), while pyrrolidine is an effective catalyst (5–6 h, 85–95%). The structures of the prepared fluorochromes are characterized using various spectral techniques and single crystal XRD. The photophysical properties of these fluorochromes are investigated using UV-vis and Fluorescence spectrophotometry in different solvent systems. Density functional theory (DFT) calculations are carried out and have a good correlation with experimental results. The obtained results for absorption, photoluminescence, and their theoretical correlation suggest that the prepared fluorochromes can be optimized for applications in optoelectronics, sensing, and bioimaging. Fluorochrome 3g, which exhibits the highest Stokes shift (129 nm) and photoluminescence (QY 0.87), is used to demonstrate the detection of hydrazine in actual water, soil, and air samples. The fluorochromes are inherently colored compounds and exhibit good photoluminescence, which is significantly quenched when hydrazine is added in very small quantities. The disappearance of the color and quenching of the photoluminescence signal are attributed to the formation of hydrodiazole via cyclization with hydrazine. A strong linear relationship for detecting hydrazine is observed over the concentration range of 1–5 μM in methanol. The limit of detection (LOD) for hydrazine is observed to be 1.1 μM with 5 μM 3g. Moreover, the color change of the fluorochrome solution from yellow to colorless can be observed by the naked eye, indicating that these fluorochromes can also be used as a colorimetric sensors for detecting hydrazine at very low concentration. Fluorochrome 3g was evaluated for its real-time detection ability over a pH range of 4–10, showing excellent efficiency in selectively detecting hydrazine among interfering analytes, and in soil and water samples. A probable mechanism for the detection of hydrazine is also established via spectral study. Additionally, this study describes a straightforward cost-effective probe-coated paper sheet for the detection of hydrazine in the environment and gives further hope for its commercial applications.