N1-aryl-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one as convenient platform to design high photostable and long-lived dyad fluorophore with potential application in live-cell imaging

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The design of long-lived and high photostable fluorescent probes with high quantum yields (QYs) is of great significance for biological applications. It favors the monitoring of individual molecular events for extended period of time without side-effect consequences into cell. From a series of N1-aryl-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-ones, we found that their photophysical properties were successfully tuned through the modulation of the intramolecular charge-transfer (ICT), generating the largest lifetimes, highest QYs and photostability for those fluorophores with a high CT character. The ICT was primarily controlled by incorporation of methoxy group at 7-position and secondarily by electron-deficient N1-arene at 1-position of benzo[b] [1,8] naphthyridin-4(1H)-one core. The trifluoromethyl at 2-position seems to strength the ICT because it reduced gap energies and increase dipole moments in their derivatives compared to 2-methyl or -hydrogen analogues. Importantly, from the 7-methoxy derivatives, two high photo-stable (15- to 17-fold more than fluorescein) and long-lived (9.75–14.20 ns) fluorophores with acceptable QYs and HOMO-LUMOs were identified. In particular, the fluorophore 3k showed a low cytotoxicity in J774.1A macrophage, pH insensibility and appropriate lipophilicity, which in combination with their good photophysical properties favored the application of the 7-methoxy dyes for bioimaging, giving blue-fluorescent macrophages in relatively short time. Then, the N1-aryl-2-(trifluoromethyl)benzo[b] [1,8] naphthyridin-4(1H)-one by its molecular extension and planarity emerges as an interesting platform to generate long-lived and highly photostable dyes and, the key point is to favor the strength of the internal ICT mechanism through the incorporation of appropriate donor (D) and acceptor (A) groups along the D-A chain.