Synthesis, optical properties, acid-base vapochromism and anti-counterfeiting of novel π-extended pyridine fused coumarins

Coumarin and its derivatives have potential applications in sensing, laser dyes, fluorescent probes, and dye sensitized solar cells due to their unique photophysical properties. However, synthesis and studies of linear pyridine-fused-π-extended coumarins remained unexplored. Here, we synthesised two π-expanded coumarin derivatives (PC1, PC2) in which molecular backbone is linearly extended with pyridyl ring where 5,7-positions are substituted with aryl groups. Single crystal X-ray structure analysis confirmed the molecular geometry. We found in spectroscopic studies that both coumarins in solutions show fluorescence via predominant radiative decay of charge transfer (CT) state due to rotational motion associated with the molecules. In addition, detailed spectroscopic analysis of both coumarins in presence of acid and base revealed the acidochromic property in solutions. Based on the acid-base sensing property of PC1, we could show its use as solid-state acid-base vapor sensor and anti-counterfeiting agent.