New organic chromophores for metal complexation: investigations into the synthesis and photophysics of thioindigo diimines, azaDIMEs, and their metal complexes

Date

2018-04-30

Authors

Boice, Geneviève Nicole

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Abstract

The synthesis and comprehensive characterization of diamine and diimine derivatives of thioindigo are reported. X-ray crystal structures demonstrate a planar structure for the diimine derivatives and a twisted conformation for the diamines. The diamine compounds absorb in the UV (λmax 324 nm - 328 nm), and exhibit moderate fluorescence (ΦF = 0.25, 0.045). A transient triplet state is observed in laser flash photolysis (LFP) experiments, with lifetimes an order of magnitude longer than those of the triplet state of thioindigo. The diimine compounds absorb at longer wavelengths than the diamines (λmax 495 nm - 510 nm), but are still slightly blue-shifted from thioindigo. The diimines have molar extinction coefficients 17 – 70% higher than thioindigo. The diimine compounds are not emissive, and LFP studies show transient species with microsecond lifetimes. The transient absorption spectra and quenching experiments of the diimines are consistent with trans-cis isomerisation about the central double bond. Mono- and diruthenium hexafluoroacetylacetonate (hfac) complexes of thioindigo-N,Nʹ-diphenyldiimine have been prepared. The monoruthenium complex was isolated as a racemic mixture and the diruthenium complexes were isolated as the meso (ΔΛ) and rac (ΔΔ and ΛΛ) diastereomers. Extensive structural characterization of the compounds revealed intrinsic diastereomeric differences in the X-ray crystal structures, cyclic voltammograms, and NMR spectra. Variable temperature NMR experiments demonstrated that the rac diastereomer undergoes conformational exchange with a rate constant of 8700 sec-1 at 298 K, a behavior that is not observed in the meso diastereomer. Ground state optical properties of the complexes were examined, showing that all the complexes possess metal-to-ligand charge transfer (MLCT) absorption bands in the near-infrared (λmax 689 nm – 783 nm). The compounds do not display photoluminescence in room temperature solution-phase experiments or in experiments at 77 K. Ultrafast transient absorption spectroscopy measurements revealed excited states with picosecond lifetimes. Unexpectedly, the transient absorption measurements revealed differences in the transient spectra and disparate time constants for the excited state decay of the diastereomers, which are linked to the conformational changes observed in the NMR experiments. Investigations into the synthesis of azaDIMEs and azaDicarbazolyls are described. Examination of the Buchwald-Hartwig amination produced reaction conditions that enabled preparation of amino-diindoles. Oxidation of the amino-diindoles to azaDIMEs was complicated by concomitant oligomerization of the substrates. Substitution of the reactive positions of the amino-diindole afforded increased stability towards oxidative oligomerization. Scalable synthetic routes to azaDicarbazolyl precursors were identified and optimized, and preparation of amino and azaDicarbazolyl compounds was explored.

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Keywords

metal complexes, thioindigo diimines, azaDIMEs, photophysics, organic chromophores

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