On the precision and accuracy of structural analysis of light-induced metastable states

Abstract

Bragg diffraction data were collected on single crystals of the spin-crossover complex [Fe(phen)2(NCS)2] in its low-spin and light-induced metastable high-spin states. Experimental variables included the temperature (32 and 15 K), the X-ray source (sealed tube and synchrotron), and the time interval between laser light excitation of the sample (λ = 647 nm). From a comparison of the structural parameters refined, it is shown that photo-crystallographic measurements suffer significantly and systematically from bias if the probed sample contains residual ground-state species, resulting from an incomplete photo-conversion or a significant metastable- to ground-state relaxation. It follows that a 4% population of species in a different spin state affects the Fe—N bond lengths by more than three standard deviations, and the FeN6polyhedron volume by as much as seven standard deviations, while the mean atomic position misfit exceeds 0.005 Å.