High-Pressure Effects on an Octa-Hydrated Curium Complex: An Experimental and Theoretical Investigation

An octa-hydrated curium compound [Cm(H₂O)₈](Hdtp)(dtp)·H₂O (Cm1, H₂dtp = 2,3-di(tetrazol-5-yl)pyrazine) along with its lanthanide analogues [Ln(H₂O)₈](Hdtp)(dtp)·H₂O (Ln1, Ln³⁺ = La³⁺-Nd³⁺, Sm³⁺-Lu³⁺) were synthesized and characterized using single crystal X-ray diffraction and spectroscopic methods. Bond length analysis of ^VIIICm(III)-OH₂ (where ^VIII refers to the coordination number) was compared to ^VIIILn(III)-OH₂ (Ln³⁺ = Nd³⁺ and Sm³⁺), indicating similar ^VIIIM(III)-OH₂ bond lengths because of their similar eight-coordinate ionic radii of these ^VIIIM(III) cations. Owing to the reduced coordination number, the ^VIIICm(III)-OH₂ bond lengths were shorter than previously reported ^IXCm(III)-OH₂ bonds in [Cm(H₂O)₉](CF₃SO₃)₃. The octa-aquo complexes were also characterized by solid-state UV-vis-NIR spectroscopy in addition to variable-temperature and variable-pressure photoluminescence. Variable-pressure absorption spectra of Cm1 were compared with Ln1 and show that the Cm(III) f → f transitions have a stronger dependence on pressure than that observed in Ln1 (Ln³⁺ = Nd³⁺ and Sm³⁺). The experimental and computational analyses reveal that the monotonic decrease in the computed energy difference between the ground state and the first excited state corresponds to the observed red shift of the photoluminescence peak. This is accompanied by a gradual reduction in the average Cm(III)-OH₂ bond length and a delocalization of spin densities, alongside an intensified interaction involving the 5f orbitals under increasing pressure. These changes accommodate the new geometry and collectively modify the energy landscape, resulting in peak broadening and quenching.