Luminescent Metal‐Organic Frameworks for Selectively Sensing Nitric Oxide in an Aqueous Solution and in Living Cells

Cu²⁺‐based metal‐organic framework (Cu? TCA ) (H₃ TCA = tricarboxytriphenyl amine) having triphenylamine emitters was assembled and structurally characterized. Cu? TCA features a three‐dimensional porous structure consolidated by the well‐established Cu₂(O₂CR)₄ paddlewheel units with volume of the cavities approximately 4000 nm³. Having paramagnetic Cu²⁺ ions to quench the luminescence of triphenylamine, Cu? TCA only exhibited very weak emission at 430 nm; upon the addition of NO up to 0.1 mM , the luminescence was recovered directly and provided about 700‐fold fluorescent enhancement. The luminescence detection exhibited high selectivity – other reactive species present in biological systems, including H₂O₂, NO₃^?, NO₂^?, ONOO^?, ClO^? and ¹O₂, did not interfere with the NO detection. The brightness of the emission of Cu? TCA also led to its successful application in the biological imaging of NO in living cells. As a comparison, lanthanide metal‐organic framework Eu? TCA having triphenylamine emitters and characteristic europium emitters was also assembled. Eu? TCA exhibited ratiometric fluorescent responses towards NO with the europium luminescence maintained as the internal standard and the triphenylamine emission exhibited more than 1000‐fold enhancement.