Dicarbazolyldicyanobenzenes as Thermally Activated Delayed Fluorescence Emitters: Effect of Substitution Position on Photoluminescent and Electroluminescent Properties

Abstract

We demonstrate two efficient blue-green thermally activated delayed fluorescence (TADF) compounds comprising a dimeric phenylcarbazole and four cyano substituents on the phenyl rings. A comparison of the 2,6-dicyano-substituted derivative (26IPNDCz) with the 3,5-dicyano-substituted derivative (35IPNDCz) shows that 26IPNDCz provides a larger dihedral angle and a lower decrease in the energy difference between the first singlet and triplet excited states (¦E ST ) and the TADF lifetime. An organic light-emitting diode based on 26IPNDCz afforded an external quantum efficiency of 10% with reduced efficiency roll-off.Organic light-emitting diodes (OLEDs) have attracted much interest recently because of their potential application in flat-panel displays and light sources. In OLEDs containing only fluorescent dyes, the internal quantum efficiency (IQE) is limited to 25% because of the exciton branching ratio of singlet states. Phosphorescent OLEDs containing noble-metal-based phosphors can harvest the remaining 75% triplet excitons and can thus achieve 100% internal quantum efficiency.1,2 However, the drawback of phosphorescent OLED materials is their cost. Recent progress in OLEDs development has focused on lowcost solutions to improve the device efficiency. A promising way is to harvest triplet excitons in fluorescent OLEDs by triplettriplet annihilation (TTA) or thermally activated delayed fluorescence (TADF). TTA comprises intermolecular fusion, in which one singlet and one ground state can be produced from two triplet states. Thus, the upper IQE limit of OLEDs made using TTA can increase to 63% (0.25 + 0.75 © 0.5). Assuming that the optical outcoupling is 0.20, the maximum external quantum efficiency (EQE) is estimated to be 13%. 3 In 2009, the EQE of TTA devices was reported to be 11%, which is very close to the theoretical limit.3 On the other hand, triplets can also be converted to singlets by reverse intersystem crossing (RISC) under thermal activation when the energy difference between the first singlet (S 1 ) and triplet (T 1 ) excited states (¦E ST ) is small enough. 4,5 In 2012, our group demonstrated that an OLED with a green TADF emitter composed of a carbazole donors (D) and a dicyanobenzene acceptor (A) can achieve an EQE as high as 19%, corresponding to an IQE of nearly 100%. 6 At the same time, pure blue and orange-red TADF OLEDs have also achieved high EQEs of 10% and 17.5%, respectively. 7a,7b However, their EQE roll-off at high current density is rather significant because of their long TADF lifetimes. We, thus, investigated the photophysical mechanism of TADF emitters in detail and further demonstrate molecular design principles.According to the Boltzmann statistics, a small ¦E ST is essential for the efficient thermally activated repopulation of the emissive S 1 state; this also governs the TADF lifetime. To achieve a small ¦E ST , a small overlap between the occupied and unoccupied orbitals involved in the excitation is necessary, CT in some cases.9,10 A comparison with the carbazolyldicyanobenz...

Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

“…Taking dimeric phenylcarbazole as an example, the HOMO distribution is dispersed onto the phenylcarbazole unit, which is significantly larger than a single carbazole group. Through calculation, 3,5‐IPNDCz and 3,6‐IPNDCz exhibit clearly more extension of HOMO distributions than originally reported similar single carbazole‐containing TADF system . The Δ E ST is reduced to 0.14 and 0.06 eV, respectively, and the devices fabricated with these two emitters both show decent performances.…”

“…The strategy of utilizing cyano‐substituted phenyl‐ or heterocycle‐rings has been proven to be quite useful in creating efficient TADF materials during the past few years. A series of TADF molecules based on two substitution positions of cyano‐units was reported by Adachi et al., with 3, 3′‐bicarbazolyl as EDG (3,5‐IPNDCz, 2,6‐IPNDCz) (Figure b) . The substitution positions of cyano‐groups evidently affects the dihedral angles between EDG and EWG, consequently resulting in distinct energy differences between S 1 and T 1 .…”

“…As eries of TADF moleculesb ased on two substitution positions of cyanounits was reported by Adachie tal.,w ith 3, 3'-bicarbazolyl as EDG (3,2, ( Figure 2b). [28] The substitution positions of cyano-groups evidently affects the dihedral angles between EDG and EWG, consequently resulting in distinct energy differences between S 1 and T 1 .T he 3,5-IPPNDCz has smaller steric hindrance compared to 2,6-IPNDCz,w ith twist angles between the cyano-substitutedp henylr ings and bicarbazolyl-unit of 698 for 2,6-IPNDCz and 508 for 3,5-IPNDCz,r espectively.Aslightly larger HOMO and LUMO separation degree wase xpected accordingt ot heoretical calculation, subsequently leading to smaller DE ST for 2,6-IPNDCz (0.06 eV), and also slightly better device performances with the EQE max data of 9.6 %f or 2,6-IPNDCz and 9.2% for 3,5-IPNDCz-based OLEDs, respectively,u nder the same device configuration. Adachi et al also reported at ri-cyano substituted benzene as EWG for efficient TADF design in 2015, [29] with 100 %i nternal quantum efficiency and an excellent EQE of 21.4 %f or aT ADF emitter, 3DPA3CN ( Figure 2c).…”

Recent progress of green thermally activated delayed fluorescent emitters

“…The dicyanobenzene and 3,3-bicarbazole-based emitters 5,5 -(9H,9 H-[3,3 -bicarbazole]-9,9 -diyl)diisophthalonitrile (35IPNDCz) and 2,2 -(9H,9 H-[3,3 -bicarbazole]-9,9 -diyl)diisophthalonitrile (26IPNDCz) were also green TADF emitters [29]. The effect of the CN position on E ST and the delayed fluorescence lifetime of the TADF emitters was studied.…”

Thermally Activated Delayed Fluorescence Emitter with a Symmetric Acceptor-Donor-Acceptor Structure

“…The 3,3'-bicarbazolebased group of CzX acts as an electron-donating moiety and the two 9H-xanthen-9-one-based ones act as electron-accepting moieties. Both 3,3′-bicarbazole [19][20][21][22][23][24] and 9H-xanthen-9-one [25][26][27][28] units have been used in various other TADF emitters.…”