Role of hydrogen in ground and excited state studies of 2-aryl-3-hydroxychromenones in different solvents

Radical scavenging activity (%SC) of three 2-heteroaryl 3-hydroxy chromenones (3-HCs) relative to 3-hydroxy flavones (3-HF) has been determined, using 2,2-diphenyl-1-picrylhydrazyl (DPPH) as a radical scavenger both in methanol (MeOH) and acetonitrile (ACN) as solvents. Among the three 3-HCs, 2-(furan-2-yl)-3-hydroxy-4 H-chromen-4-one (FHC), 3-hydroxy-2-(thiophene-2-yl)-4H-chromen-4-one (THC), and 3-hydroxy-2-(pyrrol-2-yl)-4H-chromen-4-one (PHC), the last 3-HC was included in our earlier reported studies on absorption, emission, and excitation spectra. Detailed studies on the excited state intramolecular proton transfer (ESIPT) on PHC relative to other three 3-HCs have been given. Order of %SCs is PHC > FHC > THC > 3-HF in MeOH and PHC > FHC ≈ THC > 3-HF in ACN, which are similar to that of intensity ratio of normal and tautomeric forms (I_N*/I_T*). Both %SC and I_N*/I_T* depend upon the potential of hydrogen of 3-hydroxy in 3-HCs in their ground and excited states, respectively. It has been found that for PHC, I_N*/I_T* and %SC are distinctly high compared with other chromenones. It is concluded that the determination of %SC can be as important as I_N*/I_T* for these. (author)