[2 + 2] Photochemical modulation of the Dy(iii) single-molecule magnet: opposite influence on the energy barrier and relaxation time

Abstract

The photochemical [2 + 2] reaction between two coordinated bpe (bpe = 1,2-bis(4-pyridyl)ethene) ligands in a new Dy(III)-based single-molecule magnet (SMM) was successfully performed. Under UV irradiation, each pair of coordinated bpe ligands from two adjacent cationic motifs in the complex (Hbpe)₂[Dy(bpe)(H₂O)(4-pyO)(NO₃)(SCN)₃]SCN (1) successfully underwent a complete [2 + 2] dimerization, leading to a dimeric cyclobutyl product, (Hbpe)₂(H₂tpcb)[Dy₂(tpcb)₂(H₂O)₂(4-pyO)₂(NO₃)₂(SCN)₆](SCN)₂ (2, tpcb = tetrakis(4-pyridyl)cyclobutane, 4-pyO = 4-(1H)-pyridone), through single-crystal-to-single-crystal (SCSC) transformation. As revealed by alternating-current (ac) susceptibility measurements, the subtle changes of the coordination environment around Dy(III) ions result in the enhancement of the effective energy barrier from 1 (U_eff = 153.8 K and 201.9 K under zero and 1500 Oe dc field respectively) to 2 (U_eff = 205.5 K and 234.5 K under zero and 1500 Oe dc field respectively). Oppositely, the magnetic relaxation time at low temperatures under the field of 1500 Oe is obviously shortened after irradiation due to the variation of the dominant Raman process.