Flexible diphenylsulfone versus rigid dibenzothiophene-dioxide as acceptor moieties in donor-acceptor-donor TADF emitters for highly efficient OLEDs

Highlights

• tert-Butylacridinyl-sybstitued flexible diphenyl sulfones and rigid dibenzothiophene-dioxide were synthesized.

• Glass transition temperatures of the compounds ranged from 92 to 131 °C.

• Their singlet-triplet energy gaps ranged from 0.01 to 0.03 eV.

• Para-disubstituted diphenylsulfone showed electron and hole mobilities of 10⁻⁵ and 1.3 × 10⁻⁴ cm²V⁻¹s⁻¹.

• OLED with para-substituted diphenylsulfone as emitter showed external quantum efficiency of 24.1%.

Abstract

Flexible versus rigid molecular structures of donor-acceptor-donor type compounds are investigated with respect to efficiency of thermally activated delayed fluorescence (TADF) by theoretical and experimental approaches. Three highly efficient TADF emitters based on flexible diphenylsulfone and rigid dibenzothiophene-dioxide as acceptor units and di-tert-butyldimethyldihydroacridine as donor moiety were designed and synthesized. Despite they showed similar singlet-triplet splitting (0.01–0.02 eV) and high photoluminescence quantum yields in appropriate hosts, maximum external quantum efficiencies as different as 24.1 and 15.9/19.4% were obtained for organic light emitting devices based on these emitters with, respectively, flexible and rigid molecular structures. The high efficiency of the light-emitting compounds with the flexible molecular structure could be traced to the bi-configurational nature of the lowest singlet and triplet states resulting in higher spin-orbit coupling than for molecules with rigid structures. All derivatives showed bipolar charge transport character. High device efficiency with electron mobility of 3 × 10⁻⁵ cm²V⁻¹s⁻¹ and hole mobility of 1.3 × 10⁻⁴ cm²V⁻¹s⁻¹ at the electric field of 5 × 10⁵ Vcm⁻¹ was recorded for the layer of para-disubstituted diphenylsulfone with flexible molecular structure. This TADF emitter showed an excellent performance in the organic light emitting device, exhibiting a maximum current efficiency, power efficiency, and external quantum efficiency of 61.1 cdA^-1, 64.0 lmW⁻¹, and 24.1%, respectively.

Introduction

Organic light-emitting diodes (OLEDs) attract nowadays a great deal of attention for display and lighting applications [[1], [2], [3], [4], [5]]. The expanding OLED industry is on constant pursuit of new materials to level up the device performance, especially for blue emitter-based devices. Phosphorescence and thermally activated delayed fluorescence (TADF) are two well-known emission mechanisms which make it possible to achieve theoretical 100% internal quantum efficiency of OLEDs are [[6], [7], [8], [9]]. Particularly, the latter mechanism has been much at focus as it does not require noble metal atoms in the molecular design [10]. In TADF based OLEDs under electrical pump, the statistical 75% triplet excitons up-convert to the singlet fluorescent energy through reverse intersystem crossing (rISC). The rISC efficiency depends on several factors, most crucially on the energy difference (ΔE_ST) between the lowest singlet excited state (S₁) and triplet excited state (T₁), which also affects the overall device performance [11]. ΔE_ST can be flexibly adjusted by molecular design combining different donor and acceptor moieties [12,13]. Specially designed linear donor-acceptor-donor (D-A-D) compounds are thus widely used as highly efficient TADF emitters in OLEDs with state-of-art maximum external quantum efficiency [[14], [15], [16], [44][45]]. Such TADF emitters have been synthesized using various donating moieties among of which the acridine unit is one showing excellent performance [17,18]. It has been shown that modification of hosts by tert-butyl units can lead to the blocking of energy-loss pathways of TADF emitters distributed in the host leading to enhancement of OLED efficiency [19]. “Full-exciton radiation” was achieved in TADF OLEDs using diphenylphosphine oxide as the secondary acceptor linked to tert-butylcarbazoles [20]. Direct attachment of tert-butyl groups to donating carbazole of D-A-D TADF emitters made it possible to increase device performance [21]. tert-Butylcarbazole moieties are widely utilized in TADF emitters [22]. There are also examples of usage of the tert-butyl substituted acridinyl moiety donor unit in the design of TADF molecules [23]. The selection of appropriate acceptor is evidently also a crucial point in the search for highly efficient TADF emitters. However, there is no established strategy so far for this selection.

In 2014 Adachi et al. described derivative of diphenyl sulfone and dimethylacridine with para-linkage (DMAC-DPS), which exhibited pronounced TADF in OLED which showed maximum EQE of 19.5% [24]. It was later reported that DMAC-DPS-based device emitting at 477 nm demonstrated EQE of 22.9% [25]. TADF emitter having the similar structure as DMAC-DPS but different accepting unit (dibenzothiophene-dioxide) was employed in green OLEDs (CIE 1931 color index varied from (0.27,0.52) to (0.34,0.56)) which showed EQE of 15.4% of EQE [26]. TADF based devices with the similar CIE coordinates were earlier reported. OLEDs based on 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-3-methoxyphenyl)-9,9-dimethyl-9,10-dihydroacridine (1mAcTrz) (0.19, 0.40) and 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (DMAC-TRZ) (0.24, 0.51) showed EQE_max of 17.7% [27]. OLED based on 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DTAO) (0.30, 0.55) showed EQE_max of 14.3% [15b]. Thus earlier reported devices with the similar CIE coordinates showed inferior efficiencies compared those of OLEDs based on compounds 1–3.

Aiming to investigate the effect of rigidity of the acceptor on performance of TADF emitters in OLEDs, flexible diphenyl sulfone and rigid dibenzothiophene dioxide were selected as acceptor moieties in this work for the design of new TADF emitters containing tert-butylacridinyl group as upgraded donor unit.

We here find that both the rigidity of the accepting units and tert-butyl modification of the donating units are significantly reflected on the efficiencies of the developed OLEDs. Electroluminescent devices with state-of-art maximum power efficiency of 64 lm·W⁻¹ and maximum external quantum efficiency of 24.1% without outcoupling are demonstrated. In the present work we have presented the comparative study of three emitters that demonstrated the different performance in OLEDs depending on the acceptor nature. Our experimental observations are supported by the high-level theoretical calculations accounting for the spin-orbit coupling matrix elements between the frontier S₁ and T₁ excited states. The anomalously high SOC matrix element between S₁ and T₁ excited states for compound 1 (0.65 cm⁻¹ vs. only 0.06 and 0.01 cm⁻¹ for compounds 2 and 3, respectively) is explained in this manuscript for the first time based on the rotation of p-AO on O atom of SO₂ group upon rISC transition.