To address and overcome the difficulties associated with the increased reactivity and susceptibility of blue emitters to deactivation pathways arising from the high-lying triplet excited states, we have successfully demonstrated an innovative strategy of harvesting triplet emission via the “thermally stimulated delayed phosphorescence” mechanism, where thermal up-conversion of excitons from the lower-energy triplet excited states (T1) to higher-energy triplet excited states (T1′) are observed to generate blue emission. The lower-lying T1 excited state could serve as a mediator to populate the emissive T1′ state by up-conversion via reverse internal conversion, which could enhance the photoluminescence quantum yield by over 20-folds. Organic light-emitting devices with respectable external quantum efficiencies of up to 7.7% and sky-blue emission with CIE coordinates of (0.17, 0.37) have been realized. The operational stability for the device based on complex 1 has also been explored, and the device is found to show fairly respectable lifetime. This work opens up a new avenue to the design and synthesis of blue phosphorescent emitters.
The photophysical properties of as eries of Tshaped coinage d 10 metal complexes,s upportedb yabis-(mesoionic carbene)carbazolide( CNC) pincerl igand,a re explored. The series includes ar are new example of at ri-dentateT -shaped Ag I complex. Post-complexation modification of the Au I complex provides access to al inear cationic Au I complex following ligand alkylation,o rt he first example of ac ationic square planar Au III ÀFc omplex from electrophilic attack on the metal centre. Emissions ranging from blue (Cu I )t oo range (Ag I )a re obtained, with variable contributionso ft hermally-dependentf luorescencea nd phosphorescence to the observed photoluminescence. Green emissions are observed for all three gold complexes (neutralT -shaped Au I ,c ationic linear Au I and square planar cationic Au III ). The higher quantum yield and longer decay lifetimeo ft he linear gold(I) complex are indicative of increased phosphorescence contribution.In the development of efficient organic light emitting devices (OLEDs), basic requirements of phosphorescente mitters include high externalq uantum efficiencies (EQE) of the emission, coupled with appropriater adiative lifetimes in the ordero fm icroseconds to facilitate the intersystem crossing (ISC) from the triplet to the singlet state. [1] These specifications have been amply met using iridium(III) and platinum(II) emitters, [2] with an increasing number of reports detailing the utility of the lesser explored gold(III) complexes. Indeed, CNC- [3] and CCN- [4] cyclometallated gold(III) complexes excelling in emissioncolor tuning, solubility and thermals tability have recently yielded OLEDsw ith very high EQEs and similarly long device operationalh alf-lifetimes. [4b] Lowero xidation state gold(I), and the other d 10 coinage metals, copper(I) and silver(I), have also been the focus of concurrent investigationsi nto their use in OLEDs. One of the more successful design strategieso nt his front employs the linear bonding geometry of carbene-metal-amides (CMAs),i nw hich all three d 10 metals (Cu I ,A g I and Au I ), have similarly accomplished excellent EQE performance and/or high brightnessO LED operation, notably employing carbazolide derivativesa st he donor amidep artnersw ith the acceptor carbenes. [5] These CMAsc an display particularly short (ns) emission lifetimesi nt hermally assisted delayed fluorescence (TADF), basedo nt he rapid triplet-to-singlet ISC, unlike the heavya tom (metal) phosphorescente mitters relying on spinorbit coupling. [1b, 6] The dependency of photoluminescence on the coordination geometry of d 10 coinage metal complexes is well-known, [7] for example, 3-coordinate trigonal planar copper(I)c omplexes showed tunable behavior from pure phosphorescence to TADF dependingo nt he carbene-metal-amine dihedral angles. [7c] Extending 3-coordinate systemsb eyondt rigonal planar geometries to ag round state Jahn Teller-distorted T-shape has been an early theoretical target for photophysical tuning of the singlet-triplet gap. [8] However,t he availab...
A new class of pyrazine-based carbazole-containing gold(III) complexes featuring thermally stimulated delayed phosphorescence (TSDP) properties has been designed and synthesized. The emission colors are found to be sensitive to the coordinating atom of the carbazolyl ligands at the gold(III) center, with emission energies spanning from green to red. The efficiency of TSDP can be enhanced by lowering the polarity of the solvent, as supported by the variable-temperature emission and computational studies. Interestingly, a significant spectral shift in electroluminescence with the change of Commission Internationale de L’Eclairage (CIE) coordinates from (0.35, 0.60) to (0.44, 0.54) has been achieved by simply changing the host material from CBP to TmPyPB. Solution-processable organic light-emitting devices (OLEDs) have also been fabricated, with maximum current efficiencies of up to 22.4 cd A–1 and maximum external quantum efficiencies (EQEs) approaching 7.0%. A higher current efficiency of 35.1 cd A–1 and EQE of 10.7% can be achieved for the vacuum-deposited device based on 1, representing the first demonstration of pyrazine-based tridentate ligand-containing gold(III) complexes as phosphorescent material for OLED application.
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