Control of Chiroptical Properties of Axially Chiral Naphthalene Oligomers

We developed several naphthalene oligomers with contiguous chiral axes as chiroptical dyes. RS-alternating oligomers with the 2, 2'-methylenedioxy bridges take a V-shaped conformation with a notably extensive conjugation in the rod direction, leading to high fluorescence in solution and in the solid state (Phi(FL, solution) = 0.64 and Phi(FLsolid) = 0.13 for an octamer). Theoretical studies indicated that the dihedral angles of naphthalene rings were held steady 48-49 degrees, and both the HOMO and LUMO were spread throughout the molecule. It should be noted that the bridged R,R,R,R,R,R,R)-naphthalene octamer showed more intense fluorescence (FL) and circularly polarized luminescence (CPL) (Phi(FL, solution) = 0.90, g(lum, solution) = + 2.2 x 10-3,Phi(FL, solid) = 0.22, and g(lum, solid) = + 7.0 x 10-3). The high Phi(FL) and glum values were due to the rigidity. Moreover, the octamer molecules were incapable of forming efficiently stacked structures due to the non-planar conformation, which led to a high Phi(FL) solid value in the solid state. A(R,R, R)-naphthalene tetramer possessing eight pyrenes displayed intense excimer-FL and CPL (Phi(FLsolution) = 0.25, g(lum, solution) = + 0.034,Phi(FL, solid) = 0.28, and g(lum, solid) = +5.3 x 10-3). Pyrenes densely connected to a naphthalene tetramer via ester linkers were conformationally rigid with right-handed chirality and were important for obtaining strong CPL.