Unsymmetrical diimine chelation to M(ii) (M = Zn, Cd, Pd): atropisomerism, pi–pi stacking and photoluminescence

Abstract

Three types of atropisomeric unsymmetrical diimine complexes, tetrahedral (L_R^ϕ)MX₂ (M = Zn, Cd; X = Cl, Br; R = Me, CMe₃, OH, OMe, Cl; 1a–k, type-I), tetrahedral (L_Me2^ϕ)ZnBr₂ (2, type-II) and square planar (L_OH^ϕ)PdCl₂ (3, type-III) with different photoluminescence properties, have been reported (L_R^ϕ = (E)-4-R-N-(pyridine-2-ylmethylene)aniline; ϕ = dihedral angle between the diimine unit including the pyridine ring and the phenyl ring planes). In crystals, ϕ = 0° for type-I, 90° for type-II and 63° for type-III atropisomers have been confirmed by single crystal X-ray structure determinations of 1c, 1e, 2 and 3·H₂O isomers. Optimizations of geometries in methanol have established ϕ = 28–32° for type-I, 90.83° for type-II and 43.44° for type-III isomers. In solids, type-I atropisomers with ϕ = 0, behave as conjugated 14πe systems facilitating π–π stacking and are brightly luminescent at room temperature while type-II and type-III isomers in solid and type-I isomers in solutions are more like non-conjugated 8πe + 6πe systems and non-emissive. Frozen glasses of acetonitrile, methanol and dichloromethane–toluene mixture at 77 K of type-I isomers are emissive and display structured excitation and emission spectra for R = Me, CMe₃, OMe species. Excitation and emission maxima of frozen glasses (λ_ex = 320–380 nm; λ_em = 440–485 nm) are red shifted in the solid (λ_ex = 390–455 nm; λ_em = 470–550 nm). TD–DFT calculations on 1b, 1d, 1f and stacked (1b)₂ isomers and luminescence lifetime measurements have elucidated that an excited ¹ILCT state has been the origin of emission of the type-I isomers and delocalizations of the photoactive π_diimine and π_diimine^* orbitals of the L_R^ϕ over the stacked layers shift the λ_ext and λ_em of solids to lower energies than those in frozen glasses. The trends of diimine ligand based electron transfer events of the complexes in DMF have been investigated by cyclic voltammetry at 298 K.