Spin-crossover cobalt(II) complexes exhibiting temperature- and concentration-dependent optical changes in solution

This work investigated the spin states of the cobalt(II) complexes [Co(L1)₂](X)₂ (1·X; L1 = 4'-(4-N,N'-diphenylaminophenyl)-2,2':6',2''-terpyridine, X = PF₆, BPh₄) and [Co(L2)₂](X)₂ (2·X; L2 = 4'-(4-N,N'-dimethylaminophenyl)-2,2':6',2''-terpyridine, X = PF₆, BPh₄) in the solid state and in solution. In the solid state, 1·PF and 2·PF, both containing smaller PF₆^- counter anions, showed gradual spin-crossover. In contrast, 1·BPh and 2·BPh remained in the high-spin state over the temperature range of 5-400 K due to a lower degree of molecular cooperativity. Each of the cobalt(II) complexes exhibited effects of temperature and concentration on their absorption spectra that were related to the spin states in various organic solvents. This work provides new insights into the spectroscopic properties resulting from the spin states of cobalt(II) complexes in solution.