Phenothiazine-based TADF emitters with dual conformations for single-molecule white OLEDs

Abstract

Donor–π–acceptor (D–π–A) type phenothiazine derivatives, exhibiting quasi-axial and quasi-equatorial conformers with distinct photophysical properties, serve as tunable, multifunctional materials for applications in sensors and optoelectronic devices. Herein, we report two phenothiazine based D–π–A emitters, NTPH and NTPCF, incorporating phenothiazine as the donor and nicotinonitrile as the acceptor, designed as stimuli-responsive functional materials with switchable conformers exhibiting differential optical properties. Single-crystal analyses show that phenothiazine adopts a quasi-axial conformation in NTPCF, while NTPH exhibits polymorphism with quasi-axial (NTPH-B) and quasi-equatorial (NTPH-O) conformers. The quasi-axial conformer of NTPCF exhibits green emission with a fluorescence maximum at 545 nm, while NTPH shows cyan emission with a fluorescence maximum at 463 nm and a shoulder band at around 516 nm. The equatorial conformer of NTPH has a characteristic TADF orange emission with a maximum of 585 nm. Theoretical studies highlight that singlet-triplet energy gap ΔE_ST is significantly influenced by the molecular conformation and the low energy barriers (<3 kcal mol⁻¹) between the axial and equatorial conformers enable dynamic conformational flexibility at room temperature. Mechano-responsive changes in the emission spectra further support the availability of energetically close, quasi-axial and quasi-equatorial conformers, enabling the tuning of TADF in the solid state. NTPCF shows high-contrast, reversible cyan-to-orange mechanochromism (Δλ_em ∼ 135 nm), suitable for anticounterfeiting, while NTPH exhibits irreversible blue-to-orange switching. Solution-processed orange organic light-emitting diodes (OLEDs) based on the neat film of NTPCF achieve a high luminance of 8400 cd m⁻². Additionally, NTPCF-based single-molecule white OLEDs were developed by varying the doping concentration, yielding a cool white OLED with CIE coordinates of (0.33, 0.37), a CRI of 70, and an EQE of 3.36% and a warm white OLED with CIE coordinates of (0.39, 0.45) and an EQE of 4.25%.