Christopher L. Exstrom, Christina A. Garretson, and Bradley D. Miller. University of Nebraska at Kearney, Kearney, NE
The solution-phase MLCT solvatochromism of Fe(diimine)2(CN)2 complexes is well-known. However, our recent work indicates that solid-vapor intermolecular interactions trigger “vapochromic” behavior in Fe(phen)2(CN)2 (phen = 1,10-phenanthroline), Fe(bipy)2(CN)2 (bipy = 2,2'-bipyridine) and the novel Fe(dtbbpy)2(CN)2 (dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). In the solid state, these complexes reversibly sorb alcohol vapors, exhibiting dramatic shifts in MLCT absorption energies and CN stretching frequencies in the process. Films cast on filter paper show immediate purple-to-pink color changes upon immersion in an alcohol vapor chamber. FTIR spectra of films cast on ATR crystals exhibit shifts of 5-20 cm-1 in symmetric and asymmetric CN stretching modes. Degrees of UV-vis and IR vapochromic shifts correspond to the volatility of the alcohol vapor source (CF3CH2OH > CH3OH > CH3CH2OH > CH3CH2CH2OH) and the bulk and flexibility of the diimine ligand (dtbbpy > bipy > phen). The vapor sorption phenomena do not appear to be simple "dissolving" of Fe complex units by alcohol molecules. IR absorption peaks do not lose intensity upon sorption, and post-sorption peak profiles suggest that the degree of crystallinity changes very little from the "dry" state. This is the first report of solid-state vapochromism by metal complexes that does not appear to involve metal-metal stacking interactions associated with the creation of additional room in the lattice to accommodate guest vapor molecules.
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