Thermally activated delayed fluorescence (TADF) has recently expanded in scope largely as a result of applications in OLEDs. Numerous molecules have been investigated to promote high quantum yield singlet fluorescence at room temperature. Exciting results have been obtained from heavy metal free molecules and potential applications in optoelectronics appear promising. However, tuning the photophysical properties of organic molecules remains challenging and requires resource and time consuming synthetic effort. It has been recently demonstrated that triplet excitons can be efficiently harvested from inorganic semiconductor nanocrystals. Extraction of triplet exciton energy from these nanomaterials occurs very efficiently through a Dexter-like mechanism with quantum yields approaching unity. The net result is similar to intersystem crossing observed in organic molecules; here the “singlet state” resides on CdSe and the “triplet state” on the molecular adsorbate. Thus, the combination of nanoparticle and molecular triplet acceptor can be considered as a united entity and not as isolated species. Therefore, the equivalent of reverse intersystem crossing observed in molecular TADF would be manifested as reverse triplet energy transfer from the molecular triplet acceptor to CdSe. This behavior thermally repopulates the initial CdSe excited state providing the opportunity for photoluminescence emission from the CdSe “bright state” through thermally activated delayed photoluminescence (TADPL). This exact behavior was observed in 1-pyrenecarboxylic acid (PCA) functionalized CdSe nanocrystals for the first time and the TADPL lifetime directly correlated to the energy difference between the bandgap of the CdSe material, tuned through systematic variation in size, and the triplet energy of PCA. The TADPL lifetime ranged from a few hundreds of nanoseconds to the millisecond regime at room temperature without significant changes in quantum yield.

Thermally activated delayed photoluminescence from pyrenyl decorated CdSe quantum dots

254th National Meeting (2017)

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