Rational design of near-infrared fluorophores with a phenolic D-A type structure and construction of a fluorescent probe for cysteine imaging

Fluorescent probes with near-infrared (NIR) emission have attracted great attention in biological applications because of the low background interference and deep penetration depth. Therefore, as an important component of the fluorescent probe, the development of NIR fluorophores with desired properties is of vital significance. Herein, four donor-pi-acceptor (D-pi-A) type fluorophores with simple structures using a hydroxyl group as the D, naphthalene or benzene as the pi-bridge, and dicyanoisophorone as the A were synthesized and systemically studied. Their photophysical properties were regulated by the variety of the pi-bridge and the relative position of the D and A. Naphthalene-bridged fluorophores (HNTC-2,6, HNTC-1,2 and HNTC-1,4) were found to demonstrate longer-wavelength emission than benzene-bridged one (HBTC-1,4). Compared with HNTC-2,6, HNTC-1,2 and HNTC-1,4 displayed a surprisingly large red-shift in fluorescence with peaks above 710 nm in PBS/DMSO. Both experimental measurements and theoretical calculations confirmed that they deprotonated in PBS/DMSO ascribed to the better molecular conjugation degree and hence showed an enhanced intramolecular charge transfer process. Then, HNTC-1,4 was utilized to build a probe ANTC to test its ability as the signal reporting group. ANTC revealed good sensing performance with NIR emission toward cysteine (Cys) which is a universal intracellular nonprotein biothiol. Furthermore, ANTC was successfully applied to map the endogenous Cys in living cells and in vivo. The structure-property relationships of the fluorophores summarized here, the example of constructing a cysteine probe, and the corresponding bio-applications were all supposed to be instructive to the development of other new NIR fluorescent probes with diverse functions.