Multiphoton Harvesting in an Angular Carbazole-Containing Zn(II)-Coordinated Random Copolymer Mediated by Twisted Intramolecular Charge Transfer State

Multiphoton excited fluorescent probes with highly emissive, photostable, low cytotoxic properties are very important for photodynamic therapy, sensing, and bioimaging, etc., even though still challenging. Here, we report the synthesis and spectroscopic studies of two statistical Zn(II)-coordinated copolymers containing different donor types and the same acceptor type (a dithienylbenzothiadiazole-based ditopic terpyridine ligand), aiming to achieving efficient multiphoton harvesting systems. Our results indicate that an angular carbazole-based ditopic terpyridine ligand donor shows a strong tendency to form a twisted intramolecular charge transfer (TICT) state. Taking advantage of the large multiphoton absorption coefficient in the donor and efficient Forster resonance energy transfer (FRET) mediated by TICT state, efficiently enhanced fluorescence from the acceptor under two- and even three-photon excitation is consequently achieved. In contrast, for a linear carbazole-based ditopic terpyridine ligand donor, the enhanced multiphoton excited fluorescence from the acceptor originates from reabsorption effect instead of FRET. For the first time, we have reported the multiphoton harvesting properties of metal-organic complexes, especially stressing the crucial role of TICT state in multiphoton excited FRET, which sheds light on how to design efficient multiphoton harvesting systems in general.