Light-Induced Magnetostructural Anomalies in a Polymer Chain Complex of Cu(hfac)2 with tert-Butylpyrazolylnitroxides

We report the study of light-induced magnetostructural anomalies in a polymer chain complex of Cu(hfac)2 (hfac = hexafluoroacetylacetonate) with an unusual acyclic tert-butylpyrazolylnitroxide radical (Ltert(Me)) using EPR. This complex ([Cu(hfac)2Ltert(Me)]n) belongs to the family of thermo- and photoswitchable molecular magnets "breathing crystals". Compared to previously studied breathing crystals with nitronyl nitroxides, [Cu(hfac)2Ltert(Me)]n shows much weaker absorption bands in the visible spectral region and therefore is superior for optical manipulation of the spin states. Illumination with light (λ ≈ 540 nm) at cryogenic temperatures leads to formation of a metastable weakly coupled spin state, which relaxes to the ground strongly coupled spin state on a time scale of hours. These phenomena are in many aspects similar to the light-induced excited spin state trapping (LIESST) well-known for spin-crossover compounds. Remarkably, the photoinduced spin state in [Cu(hfac)2Ltert(Me)]n is metastable at temperatures up to TLIESST ≈ 60 K, which is a significant improvement compared to that of previously studied breathing crystals with nitronyl nitroxides (TLIESST ≈ 20 K). We describe LIESST-like behavior observed in [Cu(hfac)2Ltert(Me)]n and discuss possible reasons for the increased stability of the photoinduced spin state.