Time- and Temperature-Resolved Triplet Dynamics in Tungsten Iodide Clusters

This study comprehensively investigates the excited-state dynamics of two tungsten iodide prototype clusters, [(W6I8)I6]2– and [(W6I8)(TFA)6]2– (TFA = trifluoroacetate), utilizing a combination of ultrafast transient absorption spectroscopy from 200 fs up to 400 μs and temperature-dependent emission spectroscopy from 4 to 340 K. Both clusters exhibit rapid intersystem crossing occurring within 6 ps, populating triplet states that subsequently deactivate through emission or dynamical bimolecular quenching involving molecular oxygen. The temperature-dependent emission behavior aligns well with a group-theoretical spin-sublevel model, indicating three distinct emissive sublevels. However, contrary to previous findings in molybdenum-based clusters, no additional splitting of the lowest triplet states was observed experimentally. Time-dependent density functional theory calculations highlight substantial excited-state geometrical distortions, suggesting limitations in sole group-theoretical descriptions. Instead, we propose a relativistic model with three thermally accessible excited-state geometries, each presenting three triplet sublevels.