Fig. 4. Spin echo decay for PDI-ph-trityl, PDI-biph-trityl, and PDI-biph-eTEMPO measured at the Q-band in frozen toluene solution at 80 K in the dark (a) and after photoexcitation at 535 nm (b). The spin coherence times (Tm), obtained from a stretched exponential fit, are indicated. The coherence time of the quartet state was measured at a field position corresponding to the |mS = +1/2〉 ↔ |mS = −1/2〉 transition. Any residual dark state signal was suppressed during the quartet state Tm measurements by application of a pre-saturation pulse.
