Design and Synthesis of New Circularly Polarized Thermally Activated Delayed Fluorescence Emitters
Abstract
This work describes the first thermally activated delayed fluorescence material enabling circularly polarized light emission through chiral perturbation. These new molecular architectures obtained through a scalable one-pot sequential synthetic procedure at room temperature (83% yield) display high quantum yield (up to 74%) and circularly polarized luminescence with an absolute luminescence dissymmetry factor, |g lum |, of 1.3 × 10 −3. These chiral molecules have been used as an emissive dopant in an organic light emitting diode exhibiting external quantum efficiency as high as 9.1%. T he discovery of efficient thermally activated delayed fluorescence (TADF) materials and small organic molecules enabling circularly polarized luminescence emission (CPL-SOMs) is crucial for the development of future optical and photonic devices. 1,2 In TADF emitters both singlet and triplet excitons can be harvested for light emission by a reverse intersystem crossing process thanks to a small energy gap between their singlet and triplet states (ΔE ST). This property has recently motivated numerous research works because of the theoretical possibility to develop organic light emitting diodes (OLEDs) with maximum efficiency. 3 The conception of CPL-SOMs is also a great challenge for organic chemists. Currently, only a small number of CPL-SOMs display high performance both in terms of quantum yield (ϕ F) and luminescence dissymmetry factor (g lum). 4 Moreover, such enantiopure compounds are usually obtained after numerous synthetic steps or require enantiomeric separation through preparative HPLC, restricting their potential application because of lack of cost-effectiveness. 5 As a consequence, rapid, easy, and flexible access to more performant CPL-SOMs is required in order to unlock their tremendous technological potential. 6 In the context of OLED devices, CPL emitters are appealing in order to decrease the energy loss arising from the required use of a polarizer and a quarter-wave plate for the attenuation of the external light reflection (50% of the light emitted is absorbed by the polarizer for standard molecules). 7 For CPL-SOMs, TADF properties can be advantageous for their overall photophysical properties (high quantum yield, long fluorescence lifetime). In other words, the design of new molecular architectures presenting both TADF and CPL emission properties can be considered as a cornerstone for enhancement of the performances of different types of devices (optical displays, optical storage and processing systems, spintronics-based devices). Hirata et al. have recently described a molecule exhibiting both TADF and CPL emission. 8 Their elegant design relies on the introduction of a chiral carbon center sandwiched between a donor and an acceptor moiety. Their approach consists in the synthesis of a racemic mixture followed by separation of both enantiomers using chiral preparative HPLC. However, although typical |g lum | values were measured (1.1 × 10 −3), this molecule has a very moderate fluorescence quantum yield (ϕ F = 4% in toluene), and its use as an emissive dopant in an OLED has not been reported. Here, we describe the design of a new TADF material enabling CPL through chiral perturbation by a tethered chiral unit (Figure 1). Obtained via a one-pot sequential