Light-induced spin transitions in copper-nitroxide-based switchable molecular magnets: insights from periodic DFT+U calculations

Abstract

The electronic structure and magnetic interactions of three representative members of the breathing crystal Cu(hfac)2LR family, mainly Cu(hfac)2LPr (1), Cu(hfac)2LBu·0.5C8H18 (2) and Cu(hfac)2LBu·0.5C8H10 (3), have been analyzed by means of periodic plane-wave based DFT+U calculations. These copper(II)-nitroxide based molecular magnets display promising thermally and optically induced switchable behaviour, similar to the spin-crossover and light-induced excited spin state trapping phenomena. The calculations confirm the presence of temperaturedependent exchange interaction within the spin triads formed by the three-spin nitronyl nitroxide-copper(II)-nitronyl nitroxide units, in line with the changes observed in the effective magnetic moment when temperature increases. Moreover, they quantify the interchain interaction mediated by the terminal nitroxide group of two spin triads in neighbour polymer chains. This interaction competes with the exchange interaction within the spin triads at high temperature and introduces 1D exchange channels that do not coincide with the polymeric chains. The density of states reveal that the low-lying conduction states potentially involved in the UV-vis transitions are mainly located on the nitronyl-nitroxide radicals, the hfac groups and the Cu atoms. Then, the density of states is almost independent of the solvent and the R group substituting the pyrazole moiety, which suggests the possibility of light-induced spin switching for other members of the breathing crystal family. The band at 500 nm characterizing the strongly-coupled low-temperature phase can be ascribed to a ligand-to-metal charge transfer transition between the nitronyl-nitroxide and the Cu bands. This study appears to be fundamental in order to properly understand the optical properties of these switching molecular magnets.