Modulating the acceptor structure of dicyanopyridine based TADF emitters: Nearly 30% external quantum efficiency and suppression on efficiency roll-off in OLED
Graphical abstract
Introduction
Organic light emitting diodes (OLEDs) have experienced constant and rapid development in recent years. The discovery of phosphorescent emitters endues the “non-emissive” triplet excitons with fast radiative decay capabilities resulting from enhanced spin-orbital coupling via the introduction of heavy metal elements [1], [2]. The recent development of thermally activated delayed fluorescent (TADF) emitters may bring OLEDs into a new era since they allow conversion of triplet excitons to singlet ones through reverse intersystem crossing (RISC) process via minimizing the energy difference (ΔEST), thus, harvesting 100% electro-generated excitons [3]. Lots of existing reports have validated their advantages in highly efficient OLEDs [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Especially, the recent achievements of ultra-high external quantum efficiency (EQE) of 38% [12] in blue region and narrow electroluminescent (EL) emission spectrum (18 nm of full width at half maximum for pure organic compound) with EQE of 34.4% [13] further demonstrate their potential.
To reduce efficiency roll-off at high brightness [14], triplet and singlet excitons should be converted to photons as fast as possible. The triplet excitons can be consumed via RISC process. Since the ΔEST value is related to the overlap of frontier molecular orbitals [15], a clear separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is needed. To guarantee efficient conversion of singlet excitons, the high photoluminescent quantum yield (PLQY), the radiative decay rate (kr) of the corresponding S1 should be sufficiently high. However, the prerequisite parameter that related to PLQY and kr, oscillator strength (f) [16], is directly proportional to the overlap of HOMO-LUMO [17], [18]. Therefore, theoretically, a small ΔEST value will encumber the PLQY and kr [16]. The contradiction between both fast RISC and high PLQY/kr requires careful selection of donor-acceptor (D-A) building blocks. Interestingly, molecules possessing both fast RISC and high PLQY/kr has been experimentally presented in several molecules such as TDBA-DI [12], 4CzIPN [3], NAI-DPAC [19], BPPZ-PXZ [6], etc. Detailed investigation on the relationship between structure and properties would be necessary to reveal the intrinsic character and valuable to guide the development of more practical materials.
The cyano (CN) group gas has been extensively employed in the early design of TADF emitters and still exhibits huge potential currently due to its strong electron-withdrawing properties and short conjugation length [3], [20], [21], [22], [23]. One early example is 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) [3], a CN-benzene derived TADF emitter with EQE as high as 31.2% [24]. The similar CN-pyridine group also show excellent abilities as acceptor [25], [26], [27], [28], [29], [30]. Very recently, we have reported 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-dimethylpyridine-3,5-dicarbonitrile (Me-DMAC) with maximum EQE of 25.8% [31]. Other CN substituents, e.g. CN-pyrimidine [28], CN-phenanthrene [32], etc. performs well as CN-benzene and CN-pyridine does. In a word, CN-constructing TADF emitters would be a suitable research target to get into the insights.
With all the aforementioned aspects, we reported three CN-pyridine based D-A structured molecules, namely 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (Ph-DMAC), 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-di(naphthalen-1-yl)pyridine-3,5-dicarbonitrile (Na-DMAC) and 4′-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-[3,2′:6′,3″-terpyridine]-3′,5′-dicarbonitrile (3Py-DMAC), with similar skeleton but minor different substitutes in their acceptor units. Theoretical simulation and photophysical investigation indicate the favorable PLQY can be achieved by rigid structures and reduction of extra vibration. And the effective RISC process is established via highly twisted D-A conformation. Finally, excellent EQE of 29.1% are accomplished through careful adjustment of the carrier balance in device together with intrinsic better photophysical property of Ph-DMAC. Slow efficiency roll-off is revealed for 3Py-DMAC based device attributing to its shorter delayed lifetime and large ratio of delayed component.
Section snippets
Molecular design and synthesis
As shown in Fig. 1, all the three compounds were constructed with D-π-A structures. 9,9-dimethyl-9,10-dihydro-acridine (DMAC) were chosen as donor, while 3,5-diCN-pyridine was chosen as the acceptor backbone to favor a twist conformation due to large steric hinderance induced by adjacent CN groups. A benzene ring is used as a spacer to further separate the distribution of frontier molecular orbitals. Such a design will also ensure an efficient charge transfer (CT) transition. To systematically
Conclusion
In this work, we have provided a comprehensive investigation on the correlation between structure, TADF feature and EL performance. Three TADF emitters, namely Ph-DMAC, Na-DMAC and 3Py-DMAC with similar D-A skeletons and different side substitutes on acceptors were developed for verification. Theoretical simulation predicts the existence of plenty intramolecular interactions such as van de Waals interaction and large hinderance effects managed a rigid geometry, leading to the suppression of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51803125 and 91833304), Shenzhen Science and Technology Program (KQTD20170330110107046) and Science and Technology Innovation Commission of Shenzhen (JCYJ20180507182244027). We thank the Instrumental Analysis Center of Shenzhen University for analytical support.
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