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

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 pK(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, tau (f) = 1.35 ns, in immobilized PVC film matrix, compared to Q(f) congruent to 0.03 and tau (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. (C) 2000 Elsevier Science B.V All rights reserved.