Although organic p-type cathode materials with high redox potential (>3.5 V vs. Li/Li⁺) are sustainable alternatives to transition metal oxide cathodes for lithium-ion batteries, only a limited number of these materials have been investigated to date. Therefore, the discovery of new p-type redox centers is essential for further development of successful organic cathodes. Herein, we report phenoxazine (PXZ) as a new p-type redox center for high-voltage cathode materials. Negligible structural reorganization of this PXZ center facilitates a kinetically faster electrochemical pathway, leading to a narrow voltage plateau, full utilization of the capacity, and superior rate capability in a new PXZ-based cathode material, PXZ trimer (3PXZ). The 3PXZ cathode delivered a specific capacity of 112 mA h g⁻¹ at 1C with a high average discharge voltage of 3.7 V vs. Li/Li⁺ in a Li-organic cell; moreover, even at a high rate of 20C, 73% capacity retention (76 mA h g⁻¹) was achieved. In addition, a 3PXZ composite with mesoporous carbon CMK-3 exhibited a capacity of 100 mA h g⁻¹ with high stability, losing only 0.044% capacity per cycle over 500 cycles at 5C. As 3PXZ outperforms most reported p-type cathodes in terms of both rate capability and stability, we suggest the adoption of the PXZ unit as a novel and promising redox center for high-performance and sustainable energy storage systems.

Thermally activated delayed fluorescence (TADF) has recently become an extensively investigated phenomenon due to its high potential for application in organic optoelectronics. Currently, there is still lack of a model describing correctly basic photophysical parameters of organic TADF emitters. This article presents such a photophysical model describing the rates of intersystem crossing (ISC), reverse ISC (rISC), and radiative deactivation in various media and emphasizing key importance of molecular vibrations on the example of a popular TADF dye 9,10-dihydro-9,9-dimethyl-10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-acridine (DMAC-TRZ). The presented experimental and theoretical investigations prove that ISC and rISC can occur efficiently between the singlet and triplet states of the same charge-transfer nature (¹CT and ³CT, respectively). In emitters with the orthogonal donor and acceptor fragments, such spin-forbidden ¹CT ↔ ³CT transitions are activated by molecular vibrations. Namely, the change of dihedral angle between the donor and the acceptor affords reasonable spin–orbit coupling, which together with a small energy gap and reorganization energy enable ¹CT ↔ ³CT transition rates reaching 1 × 10⁷ s^–1. Evidence of direct ¹CT ↔ ³CT spin-flip and negligible role of a second triplet state, widely believed as a key parameter in the design of (r)ISC materials, change significantly the current understanding of TADF mechanism. In authors’ opinion, photophysics, and molecular design principles of TADF emitters should be revised considering the importance of vibrationally enhanced ¹CT ↔ ³CT transitions.

Hybrid organic emitters containing abundant heavy atoms (HAs) are of rapidly growing interest for organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF). As substitutes of expensive and toxic heavy-metals, HAs can accelerate reverse intersystem crossing (rISC), whose low rate is specifically troublesome in blue all-organic TADF materials. Unfortunately, due to complex photophysics even qualitative predictions of the HA effect on organic emitters and hence molecular design are currently very challenging. Here, we report a photoluminescence data analysis approach for investigations of the TADF mechanism based on empirical spin–orbit coupling constants. Using this approach, heavy-halogen derivatives of a popular blue TADF emitter are investigated to understand the mechanism of the HA effect and molecular design fundamentals of high-efficiency TADF materials bearing HAs. Different HA effects on direct and reverse ISC were revealed. We thus conclude that to improve TADF in organic emitters (i) HA should participate in low-energy molecular vibrations which activate rISC between the lowest excited triplet and singlet states of the charge-transfer (CT) character and (ii) ISC should be minimized through a large energy gap between the ¹CT-state and HA-affected locally-excited (³LE) triplet state.