Long‐Lived Room‐Temperature Phosphorescence of Coronene in Zeolitic Imidazolate Framework ZIF‐8

Abstract

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...

Liesegang Banding for Controlled Size and Growth of Zeolitic‐Imidazolate Frameworks

“…Their importance is due to their porosity, high crystallinity, and chemical and thermal stability . They are used in various applications including separation and gas storage, catalysis, encapsulation of small molecules and drug delivery . Therefore, the control of the size and morphology of their crystals is considered an essential endeavor .…”

Molecular‐Barrier‐Enhanced Aromatic Fluorophores in Cocrystals with Unity Quantum Efficiency

“…3.10 eV and ca. 2.76 eV, respectively, and they have been widely used as active materials in optics . Octafluoronaphthalene (OFN) has a higher HOMO–LUMO energy gap (ca.…”

Hydrogen‐Bonded Organic Aromatic Frameworks for Ultralong Phosphorescence by Intralayer π–π Interactions

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Ultralong organic phosphorescence (UOP) based on metal-free porous materials is rarely reported owing to rapid nonradiative transition under ambient conditions.Inthis study,h ydrogen-bonded organic aromatic frameworks (HOAFs) with different pore sizes were constructed through strong intralayer p-p interactions to enable ultralong phosphorescence in metal-free porous materials under ambient conditions for the first time.I mpressively,y ellowU OP with al ifetime of 79.8 ms observed for PhTCz-1 lasted for several seconds upon ceasing the excitation. [6] Thel uminescence emitted from these materials can be divided into fluorescence and phosphorescence.O wing to the contribution of aromatic ligands and metal components for facilitating spin-orbital coupling (SOC) in MOFs, [7] both fluorescence and phosphorescence can be obtained from such porous materials. This result not only outlines aprinciple for the design of new HOFs with high thermal stability,but also expands the scope of metal-free luminescent materials with the property of UOP.

Luminescent porous organic materials,which mainly include metal-organic frameworks (MOFs), [1] covalent organic frameworks (COFs), [2] and hydrogen-bonded organic frameworks (HOFs), [3] have been experiencing exponential growth because of their broad applications ranging from chemical sensing [1b,c, 2a,d,e, 4] and bioimaging [1e, 5] to lighting [1d] and 3D optical storage.

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