Photoresponsive luminescent single-molecule magnets based on dysprosium–anthracene complexes: regulating the de-dimerization temperature of the photocycloaddition product by co-ligand

Lanthanide-anthracene complexes that can undergo reversible photocycloaddition reaction are attractive for the development of photoresponsive luminescent single-molecule magnets (SMMs). However, how to regulate the de-dimerization temperature of <i>in situ</i> formed dianthracenes remains an open question. Here, we report two binuclear dysprosium-anthracene complexes [Dy₂(SCN)₄(L)₂(depma)₂(DEPP)₂] (1) and [Dy₂(SCN)₄(L^Me)₂(depma)₂(DEPP)₂] (2), where L is 2,6-dimethoxyphenol, L^Me is 4-methyl-2,6-dimethoxyphenol, depma is 9-diethyl-phosphonomethylanthracene, and DEPP is diethylpropylphosphonate. Both undergo single-crystal-to-single-crystal (SC-SC) photocycloaddition reactions to form the 1D coordination polymers [Dy₂(SCN)₄L₂(depma₂)(DEPP)₂] _<i>n</i> (1UV) and [Dy₂(SCN)₄(L^Me)₂(depma₂)(DEPP)₂] _<i>n</i> (2UV), where depma₂ is photo-dimerized depma, concomitant with changes in photoluminescence and magnetic properties. Interestingly, the de-dimerization temperatures of 1UV (80 °C) and 2UV (71 °C) are much lower than those of the known lanthanide-dianthracene complexes (≥100 °C). We found that the stability of <i>in situ</i> formed dianthracene depends largely on the spacing and slip angle of the anthracene pair in the original complex, with spacing being more important, and these parameters can be modulated by choosing suitable co-ligands. In addition, we investigated the kinetics of the photocycloaddition reaction of 1 at different temperatures and found that the rate of the reaction reached a maximum at the temperature of complete de-dimerization.