Enhanced fluorescence and thermal sensitivity of polyethylenimine modified by Michael addition

Polyethylenimine (PEI) with enhanced fluorescence and thermal sensitivity was achieved by Michael addition of divinylsulfone (DVS) and N-isopropylacrylamide (NIPAm) respectively. The fluorescence enhancement is quantitatively studied by tailoring the amount of DVS, the medium pH, the substituting acrylates, the substrate polymers with different type amine groups, and the substrate molecular weights. The results suggested that different amine groups affected the performance of fluorescence with quantum yields varied from 0.340 for primary amine (NH2-) to 0.090 for tertiary amine (N(-C)3), and further to 0.049 for secondary amine (-NH-). It was also found that the fluorescence enhancement was attributed to the specific molecular structure of DVS-substituted product. The fluorescence moiety is believed to involve a proton-transfer process and have O2-dependent and pH-sensitive fluorescence properties. From the current study, it can be expected that the accurate prediction of the O2-dependent fluorescence would be dependent on molecular orbital calculation of the small amine molecules (oxidized with O2) and their DVS-substituted products.