Vapochromism is a type of chemical sensing response in which a material reversibly absorbs a vapor and undergoes a color change. In this dissertation, the first examples of vapochromic simple salts (c.f., double salts) of [Pt(2,6-bis(N-methylbenzimidazoly-2-yl)pyridine)Cl]+(M) are described. A major finding from this research is that the vapochromic properties (selectivity, color, vapor capacity, response time) of these materials are readily modulated by changing the counter anion, thus allowing for preparation of a wide range of materials with different vapochromic properties. For example, M(Cl−) changes from orange to red in response to vapors of acetonitrile, methanol and chloroform, whereas M(PF6−) changes from orange to purple in response to acetonitrile but does not respond to methanol or chloroform.
As solids, M(X−) (X = Cl−, PF6−, NO3−, CF3SO3−, BF4−) salts undergo color changes, from orange to red or purple upon exposure to various organic vapors, such as methanol, ethanol, acetonitrile, chloroform, nitromethane and methylamine. In the absence of vapor, four of the salts give rise to excimeric emissions, in which the excitation is delocalized over the 2,6-bis(N-methylbenzimidazoly-2-yl ligands of two adjacent complexes with strong ligand⋯ligand interactions. The triflate salts gives rise to emission characteristic of a lowest MMLCT (metal-metal-to-ligand charge transfer) state, which arises from significant interaction between adjacent Pt centers. The temperature dependence of the solid-state emission spectra of these complexes after exposure to organic vapors is indicative of formation of a lowest MMLCT (metal-metal-to-ligand charge transfer) state. This conclusion is supported by diffuse reflectance spectra which show that vapor exposure results in new absorbtion bands at longer wavelengths. Investigations of a series of salts with alkyl substituents on the tridentate ligand suggest that ligand substitution reduces vapochromic activity. However, the salts exhibit even greater range of color, including yellow, orange, red, blue, green and violet. The temperature effects on the spectroscopy are similar and indicate that, depending on the salt, the emission originates from either a “unimolecular” ligand-centered state, an excimeric state or a MMLCT state. Overall, the accumulated data are consistent with the hypothesis that vapor absorption is accompanied by changes in packing of the cations that result in increased intermolecular Pt⋯Pt interactions. In support of this notion, the crystal structure of the orange form of M(PF6)·DMF reveals that the cations adopt a columnar stacking arrangement with short interplanar spacings (3.35, 3.39 Å), but relatively long Pt⋯Pt separations (4.336(2), 4.565(2) Å). The crystals are vapochromic, changing from orange to violet upon exposure to acetonitrile vapor. The DMF solvate molecules reside in channels perpendicular to the columnar stacks of metal complexes, suggesting a possible conduit for diffusion of vapors in and out of the crystals.
To better understand the electron-donor properties of the tridentate ligand, two additional complexes [Pt(2,6-bis(N-methylbenzimidazoly-2-yl)pyridine)Z](PF6) (Z=Ph, C≡CPh) were synthesized and characterized. Electrochemical studies, as well as UV-visible absorption and emission spectroscopy, suggest that 2,6-bis(N-methylbenzimidazoly-2-yl)pyridine is a stronger π acceptor than 2,2':6',2"-terpyridine, which is expected to enhance intermolecular interactions. Interestingly, the phenylacetylide complex sensitizes formation of 1O2 in acetonitrile.
Finally, Pt(2,2'-bipyridine)(1,4,7-trithiacyclononane)2+ was shown to undergo nearly reversible oxidation in 4:1 water:acetonitrile with 0.1 M LiCl as the supporting electrolyte. This behavior contrasts with the irreversible oxidation that is observed when “innocent” electrolytes are present. The accumulated data are consistent with reversible oxidation of the platinum(II) complex to form a six-coordinate platinum(IV) adduct.