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, serves 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 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) 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 neat film of NTPCF achieve a high luminance of 8400 cd/m². Additionally, NTPCF-based single-molecule white OLEDs were developed by varying the doping concentration, yielding a cool white OLED with CIE (0.33, 0.37), CRI of 70, and EQE of 3.36%, as well as a warm white OLED with CIE (0.39, 0.45) and EQE of 4.25%.