Fluorescence emission studies of an azlactone derivative embedded in polymer films: An optical sensor for pH measurements

Abstract

Azlactone molecules can be protonated in acidic media, and the process of protonation is irreversible due to ring opening in solutions. A new fully reversible pH sensor has been developed 4-( p- N, N-dimethylamino phenyl methylene)-2-phenyl-5-oxazolone (DPO), an azlactone derivative, embedded in a plasticized PVC polymer film. The sensor membrane is completely transparent and exhibits pH induced color change which is capable of measuring of pH in the range 1–7 and has a p K a value of 3.4. In agreement with crystal phase studies, DPO displays enhanced fluorescence emission quantum yield, Q f=0.52, and fluorescence emission lifetime, τ f =1.35 ns , in immobilized PVC film matrix, compared to Q f≅0.03 and τ f =0.02–0.03 ns in solutions. Stokes shift decreased to 46 nm in plasticized PVC film, with respect to Stokes shift of 81 nm in acetonitrile solution. Singlet energy, E s =60.4 kcal mol −1 , of DPO in PVC film, which is 1.1–1.4 kcal mol −1 lower with respect to solutions, is taken as evidence of lessened singlet–triplet crossings in excited state of DPO in immobilized phase, and result in enhanced fluorescence emissions.