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

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 CH 3 ; ( 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 ( 1 LC) transitions ( 10 4 M −1 cm −1 ) in the UV region and metal-to-ligand-charge transfer ( 1 MLCT) transitions ( 10 3 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 1 LC and 1 MLCT transitions of 1 occur at lower energies than that of 2 . These 1 LC transitions have distinctly been blue-shifted along a < c < d in 2 , but occur at nearly identical energies in 1 . Conversely, 1 MLCT 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 λ max (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 3 LLCT emissive state in 1b and 2b , whereas to 3 LC in the rest of the compounds. Relative stabilization of these spin-forbidden emissive states is discussed by invoking configuration mixing with the higher-lying 3 MLCT state. Spectroscopic and electrochemical properties of photoluminescent Pt(2,6-bis( N -methylbenzimidazol-2-yl)pyridine)X + have been discussed and contrasted with that of the archetypal Pt(tpy)X + for better understanding of their electronic structures.