At 90 K, the EPR spectrum of Cu24(L1)24 exhibits an anisotropic fine structure, which at Q-band frequency (34 GHz), consists of four transitions falling at around 500 mT, 700 mT, 950 mT and 1350 mT (Fig. black trace, 3b). The observed fine structure was attributed to an excited S = 1 triplet composed of pairs of antiferromagnetically coupled Cu(II) ions (3d , S = 1/2) inside PW units, and can be understood as the intra-PW interaction of Cu(II) centers . An additional isotropic signal was detected at around 1080 mT, which increases in intensity as the temperature is raised (Supplementary Fig. ). This signal results from the interdinuclear exchange 5 interaction between adjacent PW units, which leads to the collapse of the anisotropic signal of isolated Cu(II) pairs, so it can be understood as the inter-PW interaction (Fig. green trace and Supplementary Fig. ) . All copper-containing frameworks we analyzed also contain a weak signal around 1170 mT, which remained observable below 90K (Supplementary Fig. ), when the Cu2 PWs are in their EPR-silent S = 0 ground state. This signal originates from monomeric Cu(II) centers generated by framework defects, and is analyzed in detail in the SI (Supplementary Figs. 69-72) . Unlike Cu24(L1)24, the mixed-metal samples PdxCu1-x-kofo-1 exhibit the typical EPR powder pattern of monomeric Cu(II) ions over the whole temperature range, which confirms the presence of Pd(II) in the form of PdCu mixed-metal PWs. With increasing Pd content in PdCu-kofo-1 (Fig. ), Cu2 PWs (S = 1) are gradually replaced by PdCu PWs for which no magnetic coupling between Cu(II) (S = 1/2) and Pd(II) (S = 0) takes place (Fig. the inter-PW S = 1 signal, we defined an interaction score (IS), which is dependent on the temperature, the M:Cu ratio, and the metal distribution within the framework (Fig. ). The IS score indicates the contribution (in %) of the simulated S = 1 inter-PW signal and reaches zero for an isolated Cu2 PW (see SI for derivation and discussion). For a series of MoCu-kofo-2 (Fig. ) and RhCu-kofo-2 (Fig. ) samples in which the measurement temperature and mixing level remained constant, the IS score decreased with increasing Mo or Rh content because Cu2 PWs in the MOPs were increasingly diluted by the presence of either Mo2 or Rh2 PWs. Notably, the experimentally determined IS values of Mo0.58Cu0.42-kofo-2 and Rh0.53Cu0.47-kofo-2 are close to zero, which implies that Mo/Rh PWs 5 effectively dampen inter-PW communication, as would be expected for an alternating arrangement of Mo2/Rh2 and Cu2 PWs. In the MOP crystal, adjacent cages only interact weakly, but in MOF structures, Cu2 PWs within adjacent cages are linked via L2. For L2 linkers that permit appreciable electronic communication between cages, a notable difference in IS score for MOPs and MOFs of the same mixing level was thus expected (Fig. ). A comparison of Mo0.25Cu0.75-kofo-x (x = 2, 3, 4, Fig. ) or Rh0.25Cu0.75-kofo-x (x = 2, 3, 4, Supplementary Fig. ) shows that kofo-2 and kofo-3 samples could be clearly differentiated based on their IS scores. For MOF samples, all three mixing levels could be clearly distinguished (Fig. and Supplementary Fig. ) due to a gradual increase in the IS score as the Cu2 PWs become increasingly clustered.