Electron Donor-Functionalized Pyrenes with Amplified Spontaneous Emission for Violet–Blue Electroluminescent Devices Beyond the Spin Statistical Limit

Abstract

Direct electrically pumped organic lasers remain inaccessible to date, due to an interplay of different adverse effects. The most important of these effects are insufficient charge carrier mobility in the organic material, accumulation of triplet excitons upon charge injection, and absorption from the electrodes in the device. While triplet state management can be achieved using molecules that recycle triplet states into emissive singlet states, these molecules rarely support amplified emission. Pyrene derivatives not only show excellent charge transport properties, but their rigid π-conjugated structure also entails excellent electro- and photoluminescence efficiency. Pyrenes exhibit very low-lying first excited triplet states, rendering this class of molecules interesting for ultrafast upper-level reverse intersystem crossing. This process counteracts the triplet accumulation in organic lasers. Here, pyrenes are functionalized with electron-rich moieties of different donor strengths. Through comprehensive spectroscopic and quantum chemical analysis, the character of the charge transfer excited state is correlated with optical properties, excited state lifetimes, amplified spontaneous emission, and triplet recycling. Two donor functionalized pyrenes are reported with violet–blue emission that overcome the spin-statistical limit of conventional organic emitters with spin factors ηST of up to 0.43 and low thresholds for amplified spontaneous emission Eth down to 1.73 µJ cm−2.