An afterglow organic light-emitting diode (OLED) that displays electroluminescence with long transient decay after it is turned off is demonstrated. This OLED exhibits blue and green dual emission originating from fluorescence and phosphorescence, respectively. A phosphorescence lifetime of 4.3 s is achieved.

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1015 wileyonlinelibrary.com COMMUNICATION for all existing host matrices. Limited pore size is a problem for cyclodextrins, low solubility and dispersibility of guest emitters limits PMMA and hydroxyl-steroids, and aggregation of guest emitters over time is common. Here, we propose metal-organic frameworks (MOFs) as host materials to suppress nonradiative decay processes because of their rigid porous structure and pore structures that can trap and stabilize emitter molecules. MOFs are crystalline materials composed of metal ions and organic ligands and are attracting considerable interest because of their thermal and chemical stability and ability to encapsulate a wide variety of guest species in their cavities, including gases, [ 12-14 ] nanopar-ticles, [ 15-17 ] enzymes, [ 18,19 ] and organic molecules. [ 20-22 ] The individual pores of MOFs can isolate guest emitters and restrict molecular motion by their rigid frameworks. There are a few reports describing the suppression of molecular motion or concentration quenching in MOFs, such as the inhibition of cis-trans isomerization of stilbene, [ 23 ] amplifi cation of fl uores-cence quantum yield, [ 24 ] and demonstration of a two-photon pumped laser. [ 25 ] However, these are not realizing long-lived organic triplet excitons and related phosphorescence of guest molecules. Although a number of phosphorescent MOFs are known, most of these emit only at low temperatures (77 K), indicating that nonradiative deactivation is dominant at room temperature. [ 26,27 ] In contrast, in all of the MOFs that exhibit room-temperature phosphorescence, the well-known heavy-atom effect is used. Heavy atoms such as I, Br, Pb, and Ir [ 28-34 ] incorporated in the structure of the framework itself or encapsulated within the pores enhance the rate of radiative decay from the triplet state k phos , rather than reducing the rate of nonradiative deactivation k nr. As a result, the phosphorescence lifetime of these MOFs is quite short (10 −3-10 −6 s), signifi cantly less than the lifetime desired for many applications. Moreover, since the origin of the photoluminescence is not a pure guest emitter but a metal-coordinated ligand or an exciplex between ligand and guest molecule, careful design of both the linker and MOF is necessary to achieve effi cient room-temperature phosphorescence. In this work, we demonstrate that long-lived emission from triplet excitons can be achieved even at high temperature by encapsulating organic emitter in MOFs. In addition , we show that thermally activated delayed fl uorescence (TADF) [ 35 ] can be achieved from MOF-encapsulated emitters at temperatures >300 K. TADF is an effi cient process to harvest triplet excitons for light emission especially in OLEDs; however, it requires careful molecular design of the emitter to enable thermal up-conversion from triplet exciton to singlet exciton. Our method realizes TADF from common aromatic molecules without any chemical modifi cation. As a proof of concept we used the well-known long-lived phosphorescent An abili...

Long-lived triplet excitons on organic molecules easily deactivate at room temperature because of the presence of thermally activated nonradiative pathways. This study demonstrates long-lived phosphorescence at room temperature resulting from suppression of the nonradiative deactivation of triplet excitons in conventional organic semiconducting host-guest systems. The nonradiative deactivation pathway strongly depends on the triplet energy gap between the guest emitting molecules and the host matrices. The triplet energy gap required to confine the long-lived triplet excitons (≈0.5 eV) is much larger than that of conventional host-guest systems for phosphorescent emitters. By effectively confining the triplet excitons, this study demonstrates long-lived room-temperature phosphorescence under optical and electrical excitation.

Excited states of emissive organic molecules undergo various kinds of quenching phenomena such as vibration-coupled quenching depending on their environmental conditions. Because bright singlet excitons in purely organic molecules exhibiting thermally activated delayed fluorescence (TADF) can access dark triplet excited states, photogenerated singlet excitons can decay nonradiatively through both singlet and triplet excited states. Here, we investigated nonradiative decay behavior, including internal and external exciton quenching processes, of various types of TADF materials in solution. Under air-saturated conditions, both the lowest singlet and triplet excited states of almost all of the TADF materials showed oxygen quenching. We considered the effect of oxygen quenching for both spin states to develop a method for determination of the triplet contribution to the total photoluminescence quantum yield from the transient photoluminescence profiles. Furthermore, we observed a clear energy gap law for the internal nonradiative processes.