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Issue 28, 2011
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Unsymmetrical diimine chelation to M(II) (M = Zn, Cd, Pd): atropisomerism, pi–pi stacking and photoluminescence

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Three types of atropisomeric unsymmetrical diimine complexes, tetrahedral (LRϕ)MX2 (M = Zn, Cd; X = Cl, Br; R = Me, CMe3, OH, OMe, Cl; 1a–k, type-I), tetrahedral (LMe2ϕ)ZnBr2 (2, type-II) and square planar (LOHϕ)PdCl2 (3, type-III) with different photoluminescence properties, have been reported (LRϕ = (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 dichloromethanetoluene 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 (λex = 320–380 nm; λem = 440–485 nm) are red shifted in the solid (λex = 390–455 nm; λem = 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 πdiimine and πdiimine* orbitals of the LRϕ over the stacked layers shift the λext and λem 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.

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

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Publication details

The article was received on 18 Feb 2011, accepted on 28 Apr 2011 and first published on 17 Jun 2011

Article type: Paper
DOI: 10.1039/C1DT10270B
Citation: Dalton Trans., 2011,40, 7375-7384
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    Unsymmetrical diimine chelation to M(II) (M = Zn, Cd, Pd): atropisomerism, pi–pi stacking and photoluminescence

    A. Saha Roy, P. Saha, P. Mitra, S. Sundar Maity, S. Ghosh and P. Ghosh, Dalton Trans., 2011, 40, 7375
    DOI: 10.1039/C1DT10270B

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