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 (phi(f)=0.48 and 0.61 for 1 and 2, respectively), which are higher than unsubstituted ones (phi(f)=0.18 for [4]helicene; phi(f)n]helicenes (n >= 5)). In the solid state, the phi(f) values are higher (phi(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.