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 only 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 could be useful to counteract the triplet accumulation in organic lasers. Here, we functionalize pyrenes with electron-rich moieties of different donor strength. Through comprehensive spectroscopic and quantum chemical analysis, we correlate the nature of the excited state with the optical properties, excited state lifetimes, amplified spontaneous emission, and triplet recycling. We report a donor functionalized pyrene that overcomes the spin-statistical limit of conventional organic emitters with a spin factor ηST of up to 0.43 and low threshold Eth for amplified spontaneous emission down to 1.73 µJ cm-2.