Coumarin-based benzilmonohydrazone as a new proton-sensitive fluorescence dye: synthesis and investigation of photophysical and acidochromic properties

Coumarin-based chromogenic and fluorogenic dye 4 was designed and synthesized. The photophysical properties of 4 were determined in solvents with different polarities. Protonation affinity was determined by adding trifluoroacetic acid (TFA) to a dichloromethane solution of dye 4 utilizing the UV-Vis and fluorescence titration methods. The protonation region was investigated by using the $^{1}$H NMR method. Additionally, DFT and TDDFT studies were performed to support the structural and absorption spectral properties of 4.