Role of Pyridine\nNitrogen Position on the Moisture\nSensitivity of Organic Emitters

Moisture-sensitive fluorescent emitters are a class of\nsmart materials\nthat can change their emission behavior upon exposure to water. In\nthis study, we have synthesized two highly fluorescent organic emitters,\n4BPy-PTA and 2BPy-PTA, and showed how moisture sensitivity can be\nenhanced by molecular design modification. Owing to the different\nnitrogen atom positions in the acceptor units, the emitters show different\ndegrees of moisture sensitivity. Upon moisture exposure, both emitters\nchange their emission color from greenish-yellow to blue, but a larger\nshift was witnessed in 4BPy-PTA (81 nm) than in 2BPy-PTA (68 nm).\nMoisture exposure enhances the photoluminescence quantum yield (PLQY)\nof 4BPy-PTA from 37 to 48%, whereas it suppresses the PLQY of 2BPy-PTA\nfrom 59 to 15%. A shorter moisture sensing time, large emission color\nshift, and enhanced PLQY make 4BPy-PTA a better moisture-sensitive\nmaterial than 2BPy-PTA. Interestingly, the emission colors of the\nemitters can be completely regained by heating and partially by applying\nmechanical force to the moisture-exposed solids. In addition, these\nemitters also show mechanochromic luminescence behavior with a completely\nreversible emission color switch between blue and green.