Fig. 3. Thermal relaxation experiments for the identification of the K- and L-state spin system.

(A) ¹³C-¹³C PDSD spectrum of ¹³C_10–18-KR2 in the dark (bottom). Upon illumination at 110 K (middle), two sets of C15-14 and C14-12 cross peaks occur in addition to the residual dark state resonances (x). Under thermal relaxation (top), only one set of these cross peaks remains and is assigned to the L-state (◊). The disappearing resonances belong to the K-state (•). (B) ¹³C DQF spectra of ¹³C_14,15-KR2. Upon illumination at 110 K, C15 shifts, while C14 splits into two resonances. The latter disappears upon thermal relaxation, which confirms the assignment of both resonances to K and L. The residual dark state contribution has been subtracted from the pure light-induced spectra. (C) Same experiment as in (B) but on ¹³C_12,13,20-KR2. Here, C20 shifts, and C12 shows a pronounced splitting, which reduces to just one peak after thermal relaxation, confirming here the resonance assignments to K- and L-states as well. See table S1 for chemical shift. An extensive temperate scan is shown in fig. S2.