Blue Electrofluorescence Resulting from Exergonic Harvesting of Triplet Excitons
Journal
Advanced Optical Materials
Date
2019.06.11
Abstract
Advances in electroluminscence demand the development of emissive molecules capable of utilizing all of the electrogenerated excitons. We here designed and evaluated a molecular strategy to harvest the excitons without relying on late transition metals or thermal activation. The key process deployed was an exergonic, El？Sayed rule-allowed reverse intersystem crossing (ES-rISC) from the triplet n-pi* transition state to the singlet pi-pi* transition state, followed by fluorescent decay of the latter. The spontaneity and quantum mechanically allowed nature of this process enabled an rapid harvesting of excitons. To demonstrate the strategy, we created a series of fluorescent molecules consisting of a 9,10-diphenylanthracene (DPA) core and different carbonyl n-pi* units (SY compounds). The compounds displayed a strong blue fluorescence emission with high photoluminescence quantum yields of up to 0.93. Multi-layer organic electroluminescence devices containing the SY compounds exhibited maximum external quantum efficiencies greater than that obtained from a singlet-exciton-only control device. This improvement was due to their capacity to utilize triplet excitons for fluorescence emission. This explanation was supported unambiguously by the observation of a linear correlation between the exciton-utilization efficiency and the contribution of ES-rISC. Control experiments excluded the involvement of the other electroluminescence processes, including p-polarized
light and P- and E-type delayed fluorescence. Our studies demonstrated that exergonic ES-rISC can serve as a promising pathway for creating inexpensive and tractable organic dopants for high-efficiency electroluminescence.