Although (E)‐4‐(2‐(4‐(dicyanomethylene)‐4H‐chromen‐2‐yl)vinyl)phenolate anion (DCPO−) has recently emerged as a potential near infrared (NIR) biosensor signaling unit, the pKa value of its conjugate acid is relatively high (∼9); this will lead to relatively low concentrations of DCPO− under physiological conditions and, hence, unsatisfactory sensitivity of DCPO−‐based bio‐probes. By difluoro‐substitution on DCPO−, we have exploited a new fluorophore of o‐FDCPO− whose conjugate acid has a much lower pKa value of 7.42. Meanwhile, o‐FDCPO− is NIR emissive with λem=693 nm and has a 0.76‐fold higher fluorescence efficiency than DCPO−. The significant superiority of o‐FDCPO− over DCPO− in sensitivity for NIR biosensor applications was confirmed by comparative studies on two HNO probes, namely o‐FDCPO‐P and DCPO‐P, which bear signaling units of o‐FDCPO− and DCPO−, respectively. Moreover, o‐FDCPO‐P has been demonstrated to be a high‐performance HNO probe with high selectivity, high sensitivity (detection limit: 50 nm), and a rapid response, together with a two‐photon NIR‐excitation imaging capability.
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