Fast Delayed Emission in New Pyridazine-Based Compounds

Abstract

Three novel donor-acceptor molecules comprising the underexplored pyridazine (Pydz) acceptor moiety have been synthesized and their structural, electrochemical and photophysical properties thoroughly characterized. Combining Pydz with two phenoxazine donor units linked via a phenyl bridge in a meta configuration (dPXZMePydz) leads to high reverse intersystem crossing rate kRISC = 3.9 · 106 s−1 and fast thermally activated delayed fluorescence (TADF) with <500 ns delayed emission lifetime. Efficient triplet harvesting via the TADF mechanism is demonstrated in OLEDs using dPXZMePydz as the emitter but does not occur for compounds bearing weaker donor units.

Derivatives of Pyridazine with Phenoxazine and 9,9-Dimethyl-9,10-dihydroacridine Donor Moieties Exhibiting Thermally Activated Delayed Fluorescence

“…The calculated HOMO levels confirm the slightly stronger electron-donating strength of phenoxazine compared to that of the 9,9-dimethyl-9,10-dihydroacridine moiety. The levels of LUMO are in good agreement with the LUMO values of previously reported pyridazine-based host compounds [ 29 ].…”

“…The possibility of spin being flipped in the organic material without insertion of heavy atoms into the structure is realized because k RISC is deeply dependent on the triplet excitonic Bohr radius, which enhances k RISC by several orders of magnitude more than the rate constant of ISC ( k ISC ) ( Table 3 ) [ 11 ]. The energy level of 2.29 eV is attributed to 3 CT of 2PO-PYD , similar to previously reported phenoxazine-based TADF compounds [ 28 , 29 , 31 ]. The presence of 3 LE is anticipated with an energy level close to that of 1 CT state, i.e., of ca.…”

“…They were reported as host materials for OLEDs and liquid-crystalline materials, and as phosphorescent iridium complexes [ 25 , 26 , 27 , 28 ]. A series of pyridazine-based TADF emitters have been described, where the combination of the pyridazine moiety with phenoxazine resulted in an emitter with a photoluminescence quantum efficiency of 10.9% with an estimated high RISC rate k RISC of 3.9 × 10 6 s −1 and a small singlet–triplet splitting value of 86 meV [ 29 ]. The OLED based on this emitter demonstrated external quantum efficiency of over 5.8%, confirming triplet utilization in the device via TADF [ 29 ].…”

“…A series of pyridazine-based TADF emitters have been described, where the combination of the pyridazine moiety with phenoxazine resulted in an emitter with a photoluminescence quantum efficiency of 10.9% with an estimated high RISC rate k RISC of 3.9 × 10 6 s −1 and a small singlet–triplet splitting value of 86 meV [ 29 ]. The OLED based on this emitter demonstrated external quantum efficiency of over 5.8%, confirming triplet utilization in the device via TADF [ 29 ]. Pyridazine is not only more electron-accepting, but also more polar than pyridine, which is beneficial for its use as an electron acceptor in the design of D-A-type materials [ 28 ].…”

Diazine-based thermally activated delayed fluorescence chromophores

“…They have been designed by Zysman-Colman, Samuel and coworkers in the prospect to develop TADF emitters (compounds 49, Chart 20). 100 The carbazole and dimethylacridan derivatives 49a and 49b do not exhibit significant TADF emission in a DPEPO host material. This is not surprising as compound 49a is not emissive at all and only a weak delayed fluorescence signal is observed for 49b.…”