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

Abstract

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 P x /P y /P z 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.

Enhanced quantum sensing with room-temperature solid-state masers

“…Our oscillator is derived from the high–Purcell factor STO resonator used in ( 27 ) by driving it in positive active feedback ( 34 ), which leads to a much slower damping and thus a substantial reduction of the resonator linewidth from ~180 kHz ( 35 ) to ~8 kHz (see section S2). The narrower linewidth implying a boosted quality factor Q ( 36 ) (section S2) results in a lower masing threshold ( P threshold ) in our setup based on the correlation P threshold ∝1/ Q ( 27 ). In addition, our detection circuit with the microwave oscillator also offers more accurate frequency selection and better spectral resolution for measuring the magnetic resonance features.…”

Spin Hyperpolarization in Modern Magnetic Resonance

“…Electron spin polarization produced upon photoexcitation of a 1:1 cocrystal of phenazine and 1,2,4,5-tetracyanobenzene (PNZ/TCNB) at 445 nm was sufficient to generate maser oscillations at 2412 MHz in a high-Q MW cavity. 627 Hyperpolarization techniques based on the same principles as ONP and tDNP are also applied to paramagnetic color centers associated with lattice vacancies in certain materials. In particular, in silicon carbide (SiC) a silicon vacancy and an adjacent carbon vacancy constitute a defect with a triplet electronic ground state.…”

“…One approach to overcome this limitation is the use of charge-transfer materials comprising molecular cocrystals, in which the fraction of triplet-generating molecules is 50%, i.e., 2 orders of magnitude higher than in conventional t DNP samples. Electron spin polarization produced upon photoexcitation of a 1:1 cocrystal of phenazine and 1,2,4,5-tetracyanobenzene (PNZ/TCNB) at 445 nm was sufficient to generate maser oscillations at 2412 MHz in a high-Q MW cavity …”

N-heteroacenes as an organic gain medium for room temperature masers

“…This compensated for radiative loses from the resonator cavity and resulted in an artificially increased QL. [54] By adjusting the attenuation of the circuit, the QL of the maser cavity could be willingly increased until circuit self-oscillation, which generally occurred when QL ≥ 220,000 between 1755 and 1765 MHz. For less intense maser bursts, the QL required adjustment closer to the self-oscillation threshold.…”