Crystalline Metal‐Organic Materials with Thermally Activated Delayed Fluorescence

Abstract

intersystem crossing (RISC) from the lowest triplet (T 1 ) excited state to the lowest singlet (S 1 ) excited state. The singlet excitons generated in this way from triplet excitons can then decay radiatively to the electronic ground state (S 0 ). [18,19] To utilize both singlet and triplet excitons for enlarging electroluminescence efficiency in organic light-emitting diodes (OLEDs), TADF materials were applied as an emissive layer. In 2009, the first TADF-OLED device was realized based on a Sn(IV)porphyrin complex by Adachi and coworkers [31] In 2011, the real breakthrough of TADF-OLEDs was achieved by using organic molecules. [32] Thereafter, Adachi and co-workers developed a series of purely organic TADF emitters, [33] and achieved internal quantum efficiencies approaching 100%, which hence woke the extensive research interests on TADF materials. [17,20,34] Moreover, the TADF-active organo-copper cluster was also reported for OLED applications with an external quantum efficiency (EQE) of 11% by Matthias. [35] These works demonstrate that both singlet and triplet excitons can be harvested in TADF-OLEDs achieving high performance compared to the fluorescence-emitter-based OLEDs.Great progress has been made on TADF-based metal-organic emitters. [18,19,21,24,25,29,[36][37][38] The transitions metals atoms could support metal-to-ligand charge transfer (MLCT) due to the participation of d electrons in TADF emitters; [21] the metal atoms in the main group could provide favorable coordination modes and/or rigidity for the organic moieties in TADF emitters. [29] Some metal-organic TADF emitters have demonstrated excellent performance in OLED devices and continuously new metal-containing TADF emitters are reported. Currently, small-molecule metal-organic complexes are involved in metals atoms including transitions metal Cu(I), Ag(I), Au(I)/(III), Zn(II), Zr(IV), W(VI), and main group metals Alkali (Li + , Na + , K + , Rb + , Cs + ); ligand-protected metal clusters included Cu(I) clusters, Ag(I) clusters, and Au(0, I) clusters; TADF-based metal-organic coordination polymers have Zr(IV), Zn(II), Ag(I), Cu(I). Although there are some review articles concerning small-molecule metal complexes that exhibiting TADF emission, [18,19,21,24,25,29,[36][37][38] they mainly focus on a or several certain types of metal atoms, [27] or pay attention to the performance of lighting devices. [21] In this review, we aimed to give a full view of emitters containing metal atoms and displaying TADF in the crystalline state or solid-state, which are expected to encourage readers to design more excellent such materials for multifunctions besides electroluminescence.In this review, the detailed requirements for TADF will be discussed in Section 2. Three categories of metal-organic TADF Thermally activated delayed fluorescence (TADF) properties of crystalline metal-organic materials, mainly including small-molecule metal-organic complexes, organic ligand-protected metal clusters, and metal-organic coordination polymers are summarized here. The c...

Thermally activated delayed fluorescence in an optically accessed soft matter environment

“…Sample temperature is known to affect (a) the phosphorescence quantum yield (QY Phos ) of the used dye; (b) the quenching constant(s); (c) the solubility of oxygen; (d) the diffusion of oxygen; and (e) the ability for singlet–triplet and triplet–singlet transitions. 37,38 On the other hand, even the presence of a small (in the range of ppm O 2 ) amount of molecular oxygen will transform the T-response of an optically excited triplet ensemble in a non-predictable manner. 47…”

“…Sample temperature is known to affect (a) the phosphorescence quantum yield (QY Phos ) of the used dye; (b) the quenching constant(s); (c) the solubility of oxygen; (d) the diffusion of oxygen; and (e) the ability for singlet-triplet and triplet-singlet transitions. 37,38 On the other hand, even the presence of a small (in the range of ppm O 2 ) amount of molecular oxygen will transform the T-response of an optically excited triplet ensemble in a non-predictable manner. 47 Thus, to fully demonstrate the substantial advantages of the TTA-UC process used as a T-sensing tool, creation of concepts for effective oxygen diffusion suppression, as well as protection against the subsequent photo oxidation, caused by the generated reactive singlet oxygen is a decisive requirement.…”

“…The intramolecular relaxation processes occur on a time-scale of a few picoseconds. 35,36 Therefore, in the TADF process only thermally equilibrated electronic states 37 participate leading to a multi-microsecond decay time for the observed delayed fluorescence. The diffusion controlled processes (in a soft matter environment) of triplet-triplet annihilation (TTA, time scale of n  100 ms) and the triplet-triplet energy transfer 38 (TTT, time scale of ms) predetermine the decay time of the upconverted delayed emitter fluorescence (time scale of n Â100 ms).…”

QM/MM Studies on Thermally Activated Delayed Fluorescence of a Dicopper Complex in the Solid State

“…Plenty of emitters have been explored to identify efficient radiative decay routes and high luminescent quantum yields. − As the first generation of OLEDs, fluorescent agents are the most common and simplest emitters for generating bright light with a short lifetime. Depending on the narrow band gap and easy modification of the chromophores, realizing clear and multicolor emission is no longer a conundrum . However, singlet excitons are three times less than the triplet counterparts according to charge-recombination spin statistics; thus, only singlet excitons can be used in fluorescence materials, which limits the theoretical limit of the internal quantum efficiency to 25% .…”

“…Depending on the narrow band gap and easy modification of the chromophores, realizing clear and multicolor emission is no longer a conundrum. 5 However, singlet excitons are three times less than the triplet counterparts according to chargerecombination spin statistics; thus, only singlet excitons can be used in fluorescence materials, which limits the theoretical limit of the internal quantum efficiency to 25%. 6 In this regard, phosphorescent emitters, named second-generation OLED materials, have been discovered to overcome this shortcoming of fluorescence emitters.…”