Tuning of electronic properties via labile N→B-coordination in conjugated organoboranes

Abstract

In this report we demonstrate that labile intramolecular N→B-Lewis pair formation can serve to tailor the properties of π-conjugated electronic materials. A series of boranes has been prepared by copper-catalyzed 1,3-dipolar azide–alkyne ‘click’-cycloaddition with a bifunctional borane (B₂-H), which allowed the introduction of electronically varied aryl-substituents on the triazole ring (p-MeOPh, p-MePh, p-F₃CPh), and also the preparation of a polymer (PTrz) with number-average molecular weight M_n = 7.2 kDa (PDI = 2.1, DP_n ≈ 8.2). The optical and electrochemical properties of the boranes have been investigated in detail, and complemented by DFT calculations and anion binding studies. ¹¹B NMR and dynamic ¹H NMR show that the N→B-coordination persists in solution but is comparatively weak, and the boranes undergo rapid thermal exchange between closed, N→B-coordinated and open, non-coordinated conformers. UV-vis-absorption spectra exhibit only a minor impact of the derivatization, while fluorescence measurements show a direct correlation between the peripheral substituents and the observed Stokes shifts (3200 to 10 000 cm⁻¹), fluorescence lifetimes (τ = 23–11 ns) and quantum yields (φ = 17.3–6.4%). Titration with cyanide shows that the strength of the N→B-bond can be incrementally tuned by variation of the peripheral substituents, to allow for direct binding of cyanide to boron.