Triarylborane-Based Helical Donor–Acceptor Compounds: Synthesis,Photophysical, and Electronic Properties

The synthesis and characterization of 10-(dimesitylboryl)-N,N-di-p-tolylbenzoc]phenanthren-4-amine (3-B(Mes)₂-4]helix-9-N(p-Tol)₂ 1 ) and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzoc,g]phenanthren-8-amine (3-B(Mes)₂-5]helix-12-N(p-Tol)₂ 2 ) are reported herein. Their electrochemical and photophysical properties have been studied experimentally and theoretically. The donor and acceptor-substituted helicene derivatives exhibit moderate fluorescence quantum yields in THF (Φ_f=0.48 and 0.61 for 1 and 2 , respectively), which are higher than unsubstituted ones (Φ_f=0.18 for 4]helicene; Φ_f<0.05 for n]helicenes (n≥5)). In the solid state, the Φ_f values are higher (Φ_f=1.00 and 0.55 for 1 and 2 , respectively) than those in solution, most likely due to the restrictions of molecular motions. The S₁←S₀ transitions of 1 and 2 are predominately HOMO→LUMO transitions. Upon excitation with UV light, the interplanar angle between the two terminal aryl rings of the 5]helix core of 2 decreases (S₁ state compared with S₀ state), which is similar to placing a spring under an external force.