Stereoselective synthesis of heterocyclic tetraphenylethylene analogues with configuration-dependent solid-state luminescence

Abstract

While nowadays ubiquitous in a variety of optoelectronic applications, fluorophores displaying aggregation induced emission (AIE) and in particular those constructed around the tetraphenylethylene (TPE) core suffer severe limitations. In particular, it has been reported in many instances that stereoconfiguration around the central double bond may severely impact the solid-state luminescence properties (maximal emission wavelength and fluorescence quantum yield). Stereoselective synthesis of extended TPE cores remains challenging, and separation of diastereoisomer mixtures is generally tedious. In this paper, we introduce ditriazolostilbene moities (DTS) as an alternative to TPE. DTS offers two significant advantages over its TPE counterpart: firstly, a fully stereoselective synthesis of the (E)-isomer, and secondly, the use of a copper-catalyzed azide-alkyne cycloaddition (CuAAc) reaction in the final step, which simplifies access to novel derivatives. We illustrate the benefits of this approach using stereopure and (E) and (Z)-aggregates, powders and crystals of the molecule and show that emission properties are considerably dependent on their stereoconfiguration.

Fig. 1. Different strategies to obtain (E/Z) isomers of TPE derivative.

Fig. 2. Synthesis of DTS-1 following two different synthetic paths.

Fig. 3. Crystal structure of (E)-DTS-1 (top) and (E)-DTS-2 (bottom). Central double bonds are highlighted with a rectangle to show specific packing: herringbone (orange, (E)-DTS-1) and β-motif (blue, (E)-DTS-2). Zooms show specific intermolecular interactions: T-contacts in DTS-1 and triazole π-stacking in (E)-DTS-2.

Fig. 4. Structure of the synthesized diphenylamino-ditriazolostilbene derivatives.

Fig. 5. (a) Structure of the isolated diastereoisomers of DTS-2 (b) partial ¹H NMR spectra of (E)-DTS-2 (blue), of irradiated (E)-DTS-2 (black) and (Z)-DTS-2 (cyan). (c) Isomerization yield of the initially pure (E) (blue) and (Z) (cyan) in CD₂Cl₂ (10⁻³ M) upon photoirradiation at 365 nm (4.7 mW cm⁻²) determined by NMR.

Fig. 6. Absorption (dashed lines) and normalized emission (solid lines) for (E)-DTS-1 (yellow curve), (E)-DTS-2 (blue curve), (Z)-DTS-2 (cyan curve) in THF, 10⁻⁵ M. Note that no fluorescence is observed for (E)-DTS-1.

Fig. 7. Emission spectra of (E)-DTS-2 (a) and (Z)-DTS-2 (c) with different DMSO/water fractions (f_w) at 10⁻⁵ M. Plots of I/I₀ of (E)-DTS-2 (b) and (Z)-DTS-2 (d), where I is the intensity at a given f_w and I₀ is the initial intensity in DMSO.

Fig. 8. (a) Photoluminescence spectra of crystalline powder of (E)-DTS-1 (yellow), (E)-DTS-2 in α powder (pink), β powder (blue) and (Z)-DTS-2 (cyan). (b) XRD powder diffraction patterns for (E)-DTS-1 (yellow), (E)-DTS-2 in α (blue) and β (pink) and (Z)-DTS-2 (cyan). (c) Optical microscope images of (E)-DTS-1 (yellow), (E)-DTS-2 in α (blue) and β (pink) powder and (Z)-DTS-2 (cyan) under white light (left) and UV-light (right).

Fig. 9. PES scans along the central double bond dihedral angle. At the ground state (black), a large energy barrier prevents thermal isomerization. At the first excited state (grey), two minima allowing for deexcitation via fluorescence (green pathway) coexist with a conical intersection (CI, orange) which results in (E/Z) photoisomerization (EZI, red pathway).

Fig. 10. (top) Superposition of optimized structures of (E)-DTS-2 at the ground state (grey) and the first excited state (blue): (a) optimized in solution, (b) central phenyl and azide groups with hydrogen atoms omitted of the same structures, (c) optimized in crystal (d) central phenyl and azide groups with hydrogen atoms omitted of the same structures. (bottom) Table of computed ethylene dihedral angle values (°) of (E)-DTS-2 found at the minima of energy of the PES in solution and in crystal.