Design of Dual-Emitting Nonaromatic Fluorescent Polymers through Thermal Processing of l-Glutamic Acid and l-Lysine

Fluorescence emission of proteins containing aromatic groups and conjugated bonds is generally associated with light absorption in the ultraviolet range, around 185-320 nm. Photoluminescence in nonaromatic biopolymers, however, has also been observed in amyloid-like structures and polymers derived from l-lysine and glycine. Here, we show, for the first time, that branched polymers obtained through thermal copolymerization of two nonaromatic amino acids, l-lysine and l-glutamic acid, exhibit two-color centers with relative absorptions in the visible range. Thermal homopolymerization of l-lysine or l-glutamic acid gives rise to the formation of branched polyglutamic acid and polylysine with a single fluorescence emission peaking at around 450 nm. The coreaction of the two amino acids produces instead a branched peptide-like polymer with a new emission centered at around 380 nm. The structures of the copolymers were studied by differential scanning calorimetry, in situ temperature-resolved FTIR, NMR, and TEM spectroscopy techniques. The optical properties were investigated by UV-vis and fluorescence spectroscopy. The double emission can be correlated with two different intramolecular charge transfer processes between the polymer backbone and the oppositely charged moieties of the two precursor side chains, Lys and Glu, which are at the origin of near-UV fluorescence.