Further motivation for selecting the compounds wasprovided bya report that DBT, DBF, and CBZ exhibit long-lived solid-state phosphorescence,49 a useful phenomenon that is rare in molecularmaterials. Recently, the unusual emissive behavior of crystallinesamples of DBT, DBF, CBZ, and related compounds has been attributedto inadvertent contamination by structurally analogous dopants.38 These observations show that low levels of structurallyrelated impurities resulting from mixed crystallization can have majoreffects on the properties of ordered solids. Such impurities cannotnecessarily be eliminated by repeated crystallizations and are bestavoided by choosing routes of synthesis that do not produce them.
To create mixed-crystal models that allow sufficiently smallincrements in composition and that minimize artifacts, such as thosearising from the effect of periodic boundary conditions on the randommixing of components, 32-molecule supercells based on the Z = 4 crystal structures of DBT and DBF were built. Foreach composition, randomly chosen molecules in the supercells of thehost compound were replaced by isostructural imposters, with theiratomic positions overlaid on those of the host as closely as possible.To probe the possible effect of placing imposters in alternative sitesin the supercells, 40 distinct configurations were assayed for eachcomposition, differing in which randomly selected molecules were replaced.Our study has focused on a small set of structurally related compoundsthat have been reported to crystallize in only two space groups, andone of them (P21/n) is a subgroup of the other (Pnma); nevertheless, the computational methodology we use toassess the feasibility of mixed crystallization is not limited tothe study of compounds crystallizing in specific space groups. Inthe cases of DBT, DBF, and their analogues, replacing molecules insupercells was straightforward because the molecular shapes are closelysimilar. In applying the method to sets of molecules that are moredissimilar, we plan to add a step after molecular replacement, inwhich clashes between molecules are detected and relieved by approachesused in our CSP methods.57 In such ways,the approach we have followed promises to be generally useful.
Banwell Molecular Spectroscopy.pdf
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