Rational Design of a Red-Emissive Fluorophore with AIE and ESIPT Characteristics and Its Application in Light-Up Sensing of Esterase

The
development of red fluorophores with efficient solid-state
emission is still challenging. Herein, a red fluorophore 1 with aggregation-induced emission (AIE) and excited-state intramolecular
proton transfer (ESIPT) characteristics is rationally designed and
facilely synthesized by attaching an electron-donor diethylamine and
an electron-acceptor maleonitrile group to salicyladazine. In contrast
to many red fluorophores which undergo serious aggregation-caused
quenching (ACQ), compound 1 emits bright red fluorescence
(λem = 650 nm, ΦF = 24.3%) in the
solid state with a large Stokes shift of 174 nm. Interestingly, control
compounds 2 and 3, which have similar structures
as 1, exhibit obvious aggregation-caused quenching (ACQ)
characteristics. The difference in the crystal structures of 1, 2, and 3 reveals that the interplanar
spacing among molecules plays a decisive role in realizing the AIE
characteristics of 1. Moreover, when the hydroxyl group
of 1 was substituted by an esterase reactive acetoxyl,
a fluorescence light-up probe 4 was developed for sensing
of esterase based on the selective reaction between 4 and esterase to generate the AIE and ESIPT active molecule 1. The linear range for in vitro quantification of esterase
is 0.01–0.15 U/mL with a detection limit of 0.005 U/mL. Probe 4 was also successfully applied to image esterase in mitochondria
of living cells.