with higher-lying triplet states. [9] TADF behavior can be observed if the energy gap between T 1 and S 1 (ΔE ST ) is sufficiently small. This requires minimization of the electron exchange interaction, which is typically achieved by spatial separation of a material's frontier molecular orbitals (HOMO and LUMO). The ability to productively harvest triplet excitons in allorganic systems has unlocked many new applications for TADF and RTP materials, including photocatalysts, [10][11][12][13] luminescent sensors, [14][15][16][17][18] tools for information encryption, [19][20][21][22][23] bioimaging probes, [24][25][26][27] and emitters for electroluminescent devices.Despite their advantages, many donoracceptor (D-A) TADF compounds exhibit poor photostability and low color purity, hindering their practical use. Triarylamines in particular are often the cause of poor stability in these systems, despite being the most common π-electron donors used in TADF materials today. The cleavage of C(sp 2 )N(sp 2 ) bonds is a common degradation mechanism in triarylamine-containing organic semiconductors, [28] and few triphenylamine radical cations are known to be stable. [29] As a result, the identification of new electron donors with improved photophysical properties remains a critical need.Rigidified electron donors such as hexamethylazatriangulene (HMAT) have recently been used to give TADF materials with improved photoluminescence quantum yields, narrower emission spectra, higher two-photon cross sections, and improved photostability. [30][31][32][33] These outcomes are achieved by reducing vibrational and rotational degrees of freedom, giving improved properties that are almost universally desirable in optoelectronics. This approach was described by Brédas in 2017 by combining triazine (TRZ) as a weak acceptor with the HMAT donor, giving the green emitter HMAT-TRZ (Figure 1). [33] In spite of a small D-A dihedral angle and a large ΔE ST (0.38 eV), this material displays TADF in doped poly(methylmethacrylate) (PMMA) films. Several more coplanar D-A TADF materials have since been reported, exhibiting enhanced PLQY, color purity, and molar absorptivity as a consequence of molecular rigidity. [31,32,34,35] These advances motivated us to seek new rigidified moieties for the design of high-performance TADF and RTP materials. In 2019, Lacquai and Kato simultaneously described an S,C,C-bridged triphenylamine (SMAT), and demonstrated Structural constraint is an emerging strategy for improving the photoluminescence quantum yield, molar absorptivity, and color purity of luminescent materials. In this report, emitters displaying thermally activated delayed fluorescence (TADF) are achieved for the first time using the planarized donor sulfidotetramethyazatriangulene (SMAT). Compared to non-planarized phenothiazine (PTZ) donors, the enhanced rigidity of SMAT contributes to improved color purity and photoluminescence quantum yield, with up to 90% quantum yield observed for donor-acceptor compounds in toluene solution. Planarized SMAT-base...