Luminescent Pt(2,6-bis(N-methylbenzimidazol-2-yl)pyridine)X+: a comparison with the spectroscopic and electrochemical properties of Pt(tpy)X+ (X = Cl, CCPh, Ph, or CH3).

A series of platinum(ii) pincer complexes of the formula Pt(mbzimpy)X+, 1(a-d), (mbzimpy = 2,6-bis(N-methylbenzimidazol-2-yl)pyridine; X = Cl; (a), CCPh; (b), Ph; (c), or CH3; (d), CCPh = phenylacetylide, and Ph = Phenyl) have been synthesized and characterized. Electronic absorption and emission, as well as electrochemical properties of these compounds, have been investigated. Pt(tpy)X+ analogs (tpy = 2,2';6'2''-terpyridine), 2(a-d), have also been investigated and compared. Electrochemistry shows that 1 and 2 analogs undergo two chemically reversible one-electron reduction processes that are shifted cathodically along the a < b < c < d series. Notably, these reductions occur at slightly higher negative potentials in the case of 1. The absorption spectra of 1 and 2 in acetonitrile exhibit ligand-centered (1LC) transitions (ε ≈ 104 M-1 cm-1) in the UV region and metal-to-ligand-charge transfer (1MLCT) transitions (ε ≈ 103 M-1 cm-1) in the visible region. The corresponding visible bands of 1b and 2b have been assigned to 1(LLCT/MLCT) mixed state (LLCT: ligand-to-ligand-charge transfer). The preceding 1LC and 1MLCT transitions of 1 occur at lower energies than that of 2. These 1LC transitions have distinctly been blue-shifted along a < c < d in 2, but occur at nearly identical energies in 1. Conversely, 1MLCT transitions are red-shifted along a < c < d in both the analogs. The 77 K glassy solutions of 1 and 2 exhibit an intense vibronically-structured emission band at λmax(0-0) in the 470-560 nm range. This band is red-shifted along b < a ≤ c < d in 1 and along a ≤ d ≈ c ≪ b in 2. The main character of these emissions is assigned to 3LLCT emissive state in 1b and 2b, whereas to 3LC in the rest of the compounds. Relative stabilization of these spin-forbidden emissive states is discussed by invoking configuration mixing with the higher-lying 3MLCT state.

Structure of a crystalline vapochromic platinum(II) salt.

Square-planar cations of the orange form of [Pt(Me2bzimpy)Cl](PF6) x DMF [Me2bzimpy = 2,6-bis(N-methylbenzimidazol-2-yl)pyridine] stack along the b axis in a head-to-tail arrangement with short interplanar spacings (3.35 and 3.39 A). Long intermolecular Pt...Pt contacts [4.336(2) and 4.565(2) A] and comparatively short Me2bzimpy...Me2bzimpy distances are consistent with spectroscopic measurements for orange salts of Pt(Me2bzimpy)Cl+. The DMF solvent molecules line channels parallel to c, which may provide a conduit for vapor absorption. The crystals are vapochromic, changing from orange to violet upon exposure to acetonitrile vapor. The changes in spectroscopic properties accompanying vapor absorption are consistent with changes in intermolecular interactions between complexes.

A new class of platinum (II) vapochromic salts.

Luminescent chloride and hexaflurophosphate salts of Pt(Me2bzimpy)Cl+ (Me2bzimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine) are reported. As solids, both compounds are vapochromic, undergoing pronounced and reversible changes of color and emission in the presence of volatile organic compounds. The chloride salt responds to vapors of methanol, chloroform, ethanol, and acetonitrile, undergoing a distinct change in color from yellow to red within seconds. The PF6- salt responded selectively to acetonitrile vapor, changing from yellow to violet while sorbing 1.0 +/- 0.1 equiv. For either salt, leaving vapor-exposed samples in air for several days or heating for several minutes restored the original color. UV-visible absorption spectra and solid-state room temperature and 77 K emission spectra are reported, and the accumulated data are consistent with a decrease in Pt...Pt separation accompanying vapor sorption.