Light-Up of Rhodamine Hydrazide to Generate Emissive Initiator for Polymerization and to Afford Photochromic Polypeptide Metal Complex

Ring-opening polymerization (ROP) of cyclic peptide monomer of γ-propargyl-l-glutamate N-carboxyanhydride (PLG–NCA) was originally initiated by non-emissive, ring-close rhodamine 6G hydrazide (R-C). However, instantaneously after adding PLG–NCA to R-C, the spirolactam ring of R-C was opened by PLG–NCA, rendering emissive, ring-open R-O to initiate ROP of PLG–NCA. The emissive R-O moiety therefore produced fluorescent R–PLG with aggregation-induced emission (AIE) properties. Moreover, R–PLG was found to exhibit photochromic properties with good fatigue resistance and long lifetime when forming metal complexes with Sn(II) and Fe(III). In the dark, irradiated metal complexes slowly (~50 min) restored to the initial state. This research provides foundation for the development of new photochromic materials with long lifetime.