Electronically tunable luminescent bis(biaryl-substituted triarylamine) ((bTAA)₂) electrophores prepared via...

Bis(biaryl-substituted triarylamines) ((bTAA)₂) are readily synthesized by oxidative homocoupling from modularly accessible biaryl-substituted triarylamine (bTAA) monomers. The reactivity of the intermediary formed radical cations and the regioselectivity of the dimerization can be controlled by varying the electronic nature of the employed functional groups and the biaryl substitution. The preferred coupling positions are predominantly determined by the positioning of the biaryl substituent, yielding a single product for para- and ortho-substituted dimers. However, for the meta-substituted dimers three possible regioisomeric products can be formed and in one case identified by NMR spectroscopy, but isolated as inseparable mixtures. Electron-donating groups facilitate a broader delocalization of electron density, while electron-withdrawing substituents enhance the localization of spin density on the para-position of unsubstituted phenyl rings in the radical cations, supported by DFT-calculated spin density distributions as well as cyclic voltammograms of bTAA monomers. The cyclic voltammograms of the dimers exhibit two chemical reversible oxidation processes, i.e. typical for multistage Wurster-type redox systems with oxidation potentials dependent on the substitution pattern. Expansion onto strong acceptor moieties additionally induces reversible reduction processes. The photonic properties of the diverse library of (bTAA)₂ dimers are assessed by experimental (absorption and emission spectroscopy) and theoretical (TD-DFT calculations) investigations. Upon photonic excitation, the dimers exhibit positive emission solvatochromicity ranging from blue to orange emission colors with quantum yields of up to 59% in solution and 24% in the solid state. Highly sterically twisted donor-acceptor-conjugates were additionally investigated in a rigid polymethylmethacrylate (PMMA) matrix resulting in quantum yields of up to 38%.