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

Abstract

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 (μ_h) and photoluminescence quantum yields (PLQYs). Ideally, the film of one compound was characterized by μ_h higher than 1 × 10⁻⁴ cm² V⁻¹ s⁻¹ at an electric field of 5 × 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.