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

The synthesis and characterization of 10‐(dimesitylboryl)‐ N , N ‐di‐ p ‐tolylbenzo[ c ]phenanthren‐4‐amine (3‐B(Mes) 2 ‐[4]helix‐9‐N( p ‐Tol) 2 1 ) and 13‐(dimesitylboryl)‐ N , N ‐di‐ p ‐tolyldibenzo[ c , 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 S 1 ←S 0 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 (S 1 state compared with S 0 state), which is similar to placing a spring under an external force.