Phenothiazine and phenothiazine sulfone derivatives: AIE, HTMs for doping free fluorescent and multiple-resonance TADF OLEDs

Twisted organic compounds play a significant role in the development of solid-state fluorescent materials and have gained substantial attention owing to their potential applications in optoelectronic devices. The organic luminogens, i.e., derivatives of phenothiazine (PTZ)3, phenothiazine and phenothiazine sulfone (PTZO2-(PTZ)2), phenothiazine and tetraphenyl ethene (PTZ-(TPE)2), and phenothiazine sulfone and tetraphenyl ethene (PTZO2-(TPE)2), were synthesized by the Pd-catalyzed Suzuki cross-coupling reaction. The incorporation of two phenothiazine units in (PTZ)3 and PTZO2-(PTZ)2 and two tetraphenylethylene units in PTZ-(TPE)2 and PTZO2-(TPE)2 on the vinylic carbon atom produces steric crowding leading to non-planarity, which is reflected in enhanced emission in the solid state. The photophysical and electroluminescence properties of (PTZ)3, PTZO2-(PTZ)2, PTZ-(TPE)2, and PTZO2-(TPE)2 are reported. In the solid state, all the derivatives displayed impressive values of both hole mobilities (mu h) and photoluminescence quantum yields (PLQYs). Ideally, the film of one compound was characterized by mu h higher than 1 x 10-4 cm2 V-1 s-1 at an electric field of 5 x 105 V cm-1 and a PLQY of 40%. These doping-free fluorescent emitters played a role in organic light-emitting diodes, which showed an external quantum efficiency (EQE) of up to 4.33%. These diodes also had low efficiency roll-offs, which were near zero at 1000 cd m-2. Usage of hole-transporting layers (HTLs) of the synthesized compounds resulted in the improved efficiency of blue OLEDs based on multiple-resonance thermally activated delayed fluorescence, which showed EQEs of up to 22%. These results are compared to those of reference devices with the same device structures based on commercial HTLs. Twisted organic compounds play a significant role in the development of solid-state fluorescent materials and have gained substantial attention owing to their potential applications in optoelectronic devices.