Phosphoric acid-catalyzed synthesis of fused pyridocoumarins and their acidochromic applications

Abstract

Pyridocoumarins are well known to exhibit various potent applications in optoelectronics, fluorescent probes, and sensors due to their unique photophysical properties. A concise and effective approach has been demonstrated to synthesize pyridocoumarins via a relatively inexpensive phosphoric acid-catalyzed Povarov reaction and their photophysical properties has also been investigated. The structures of all the synthesized compounds were well characterized by using 1H NMR, 13C NMR, IR, HRMS, and single crystal X-ray structure analysis. Detailed spectroscopic studies revealed that all the synthesized pyridocoumarins displayed acidochromic properties in solutions, which could be utilized as acid-base vapor sensors, and anti-counterfeiting agents. The detection limit was also calculated for all the synthesized acid sensors, which were found to be in the range of 41–103.8 ppm (parts per million). DFT and TDDFT studies were also performed to understand the protonation sites and the electronic transitions involved in these compounds. HOMO–LUMO energy gap of the synthesized pyridocoumarins ranges from 3.50 to 3.90 eV.