Accessing blue-room-temperature phosphorescence from pyridine-fused extended coumarins

Coumarin derivatives have potential applications in sensing, laser dyes, fluorescent probes, and dye−sensitized solar cells due to their unique photophysical properties. However, the realization of room-temperature phosphorescence (RTP) under ambient conditions in coumarin derivatives remains unexplored. Extended coumarins can therefore be used to achieve RTP under ambient conditions. Here, we report two pyridine-fused coumarins (CPP, CPPCz); CPPCz contains a 9-phenyl carbazole unit that is covalently attached to the 5-position of CPP via a C–C single bond. Spectroscopic studies and quantum chemistry calculations reveal that CPPCz exhibits local emission (LE) and charge transfer (CT) emission in solution, while only LE emission is realized in CPP. In films, both compounds show fluorescence and blue-RTP under ambient conditions due to the low singlet-triplet gap (0.07–0.14 eV). The photoluminescence quantum yields are found to be 29–43%. Phosphorescence quantum yield was found to be 1.3–3%. This design principle reveals a method to understand RTP in extended coumarins.