Synthesis and Photophysics of Thioindigo Diimines and Related Compounds

Abstract

We report the synthesis and comprehensive characterization of diamine and diimine derivatives of the fluorescent compound thioindigo. Diamines 1 were obtained by metal-mediated amine condensation reaction with thioindigo. Oxidation of the products of the coupling reaction provided the diimines 2. X-ray crystal structures, cyclic voltammetry, and spectral and photophysical data of the compounds are presented. X-ray crystal structures demonstrate a planar structure for the diimine derivatives and a twisted conformation for the diamines. The diamine compounds 1 absorb in the UV (λmax 324-328 nm), significantly blue-shifted from the absorption spectrum of thioindigo. Diamines 1 exhibit moderate fluorescence (ΦF = 0.25, 0.045). A transient triplet state is observed in laser flash photolysis (LFP) experiments of 1, with lifetimes 1 order of magnitude longer than those of thioindigo. The diimine compounds 2 absorb at longer wavelengths (λmax 495-510 nm) than the diamines but are still slightly blue-shifted from thioindigo, with molar extinction coefficients 17-70% higher compared to thioindigo. The diimine compounds are not emissive, and LFP studies indicate transient species with microsecond lifetimes. Quenching experiments and transient absorption spectra are consistent with trans-cis isomerization.

Molecular photoswitches in aqueous environments

“…234 For this reason its (photophysical) characteristics and substitution effects were the subject of several experimental and theoretical studies. 232,[234][235][236] The negative photochromism 237 of thioindigo was observed already around 80 years ago. 221,238,239 The molecule could be switched in both direction with visible light showing good PSDs and QYs (PSD E-Z = 63%Z, PSD Z-E = 74%E for 6,6 0 -diisopropoxythioindigo in epoxy resin; F E-Z = 0.20, F Z-E = 0.38 for 6,6 0 -di-isopropoxythioindigo in toluene).…”

Game of Isomers: Bifurcation in the Catalytic Formation of Bis[1]benzothieno[1,4]thiazines with Conformation-Dependent Electronic Properties

“…In the course of investigating the surprising regioselectivity, an additional side product was isolated as red single crystals, which was then identified by X-ray diffraction as thioindigo- N , N ′-di­(4- tert -butylphenyl)­diimine ( 5l ) (Figure F) . Thioindigo diimines are rarely described in literature, and only two examples have been synthesized to date utilizing a tedious synthesis starting from thioindigo …”

Diastereomerically Differentiated Excited State Behavior in Ruthenium(II) Hexafluoroacetylacetonate Complexes of Diphenyl Thioindigo Diimine

“…To this end, our initial research focused on preparing derivatives of thioindigo that would be amenable to metal coordination. These synthetic efforts resulted in the preparation of thioindigo diimines: N-substituted diimine derivatives of thioindigo . Thioindigo diimines are potentially interesting as redox-active ligands for metal complexes.…”

“…The thioindigo diimines contain two such sets of potentially chelating sites, making it possible for the molecule to serve as a bridging ligand in bimetallic complexes. Like the parent thioindigo compound, thioindigo diimines absorb relatively low energy visible light despite being small molecules with limited conjugation length. , Upon metal coordination, this may lead to absorption in the near-infrared (NIR) spectral region. − Comprehensive photophysical studies of metal complexes of thioindigo diimines may provide new fundamental knowledge of the excited state behavior of transition metal complexes, leading to the design of new ligand systems with advantageous light absorption properties.…”