Conjugated Oligomers and Polymers of cis- and trans-Platinum(II)-para- and ortho-bis(ethynylbenzene)quinone Diimine

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Abstract

Three conjugated polymers of the type (−spacer−C≡C−PtL2−C≡C−)n, with spacer = para- and ortho-bis(diphenyl(tetramethoxy)quinone diimine) and L = PEt3, were synthesized as the cis- and trans-isomers about the Pt. The three combinations (spacer/PtL2Cl2), ortho/trans, para/trans, and para/cis led to polydispersed polymers, which upon fractionation provided more monodispersed materials with Mn ranging from 3600 to 32400 (GPC). The spacer ortho-bis(diphenylethynyl)tetramethoxyquinone diimine reacts with cis-PtL2Cl2 to form small oligomers and a chelate cyclic complex (7), which was used as a model compound. All materials along with the aryl−C≡C−PtL2− C≡C−aryl models (L = PEt3; aryl = C6H5, 2,4,6-C6H2Me3; 2,4,5-C6H2Me3) were studied by 1H and 31P NMR, IR, UV–vis, TGA, DSC, luminescence spectroscopy, photophysics, and cyclic voltammetry. The UV–vis spectra exhibit an intramolecular low-energy and low-intensity charge transfer band (CT) assigned to π(Ph−C≡C) (for organics) and π/dxy((Ph−C≡C)2Pt) (for organometallics) to the π*(quinone diimine), as demonstrated by comparison with a corresponding amine derivative (i.e., no CT band) and corroborated by DFT and TDDFT. The polymers and 7 are not luminescent at room temperature (solid and solution). However in 2MeTHF at 77 K, the polymers bearing the combination ortho/trans and para/cis as well as the model compound 7 exhibit higher energy phosphorescence (Tn → S0) originating from the ππ*/dxy((Ph−C≡C)2Pt) excited states. A correlation between the structural parameters (angle made by the PtP2(C≡C)2 vs the aryl planes, angle made by the aryl and the quinone diimine planes, molecular weight) with the calculated oscillator strength, absorptivities, and the observed position of the lowest energy absorption bands is demonstrated. Finally, upper energy delayed fluorescence (Sn → S0) was detected for the polymers and 7 at 77 K and was assigned to T1−T1 interactions via aggregation as its intensity varies with the concentration.

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