Exploring the Triplet Spin Dynamics of the Charge-Transfer Co-crystal Phenazine/1,2,4,5-Tetracyanobenzene for Potential Use in Organic Maser Gain Media

Charge-transfer co-crystals have garnered interest for use in organic electronics and photovoltaics, where their ease of growth by solution/vapor deposition and flexibility in choice of constituent molecules allows tailoring of their band gaps and triplet spin dynamics. Here, we report on the triplet spin dynamics of one such charge-transfer co-crystal phenazine/1,2,4,5-tetracyanobenzene (PNZ/TCNB), as investigated using algorithm-assisted continuous-wave electron paramagnetic resonance and transient powder electron paramagnetic resonance (EPR) at X-band, as well as transient EPR at zero field (ZF-trEPR). The zero-field splitting parameters are |D| = 2118 ± 2 MHz and |E| = 297 ± 1 MHz, with initial triplet sublevel populations Px/Py/Pz being 0.764:0.127:0.109. The triplet spin dynamics were found to have decay times in the sub-microsecond regime. Furthermore, with the aid of a Q-boosted dielectric cavity, PNZ/TCNB can output a maser burst at 2412 MHz at room temperature and zero field under 445 nm pulsed excitation. This demonstrates that it is feasible for PNZ/TCNB to mase if a cavity with a sufficiently high-quality factor is provided and opens up avenues toward investigating other charge-transfer co-crystals with similar triplet spin dynamics as potential maser gain media at room temperature and zero field.