A new far-red naphthorhodamine dye: Synthesis, fluorescent probe and bioimaging applications

• A new π-conjugation extended naphthorhodamine dye 1 with a carboxylic acid-functional group was designed and synthesized.
• The single crystal of 1 was obtained and characterized.
• The photochemical properties of the dye were investigated in different solvent polarity.
• The dye was successfully used to construct a new fluorescent probe and a biomarker.

A new extended-π-conjugated naphthorhodamine dye, 9′-diethylamino-2′-hydroxyl-benzo[a]fluoran (1) with a carboxylic acid-functional group was designed and synthesized. Compared to Rhodamine B, the emission wavelength of 1 was extended to ca. 600 nm, and its fluorescence properties could be controlled by the key carboxylic acid-functional group. By taking advantage of 1, we synthesized a new far-red bioimaging dye 2-ethyl-9′-diethylimino-2′-hydroxyl- benzo[a]fluoran (2) by esterification of 1, which could selectively stain mitochondria. Moreover, encouraged by the spirocyclization switching mechanism platform, we successfully constructed a new fluorescent probe 9′-diethylamino-2′-hydroxyl-benzo[a]fluoran hydrazine (3), the hydrazide derivative of 1, for Hg2+ with good selectivity. These results suggest that the new naphthorhodamine dye could be used as a platform to construct fluorescent probes and bioimaging reagents.

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