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