Chiroptical dissymmetries in fluorescence excitation from single molecules of (M-2) helicene dimers.

We report on the single-molecule chiroptical properties of “right”-handed bridged triaryl amine helicene dimers, MH2. Using an experimental setup to precisely define the circular excitation polarization at the sample plane, we investigated the circular dichroic response in luminescence from individual molecules in which induced ellipticity from microscope optics is minimized. Our results comparing circular anisotropies in fluorescence excitation from MH2 and perylene diimide (PDI), an achiral, centrosymmetric chromophore, demonstrate a significant reduction in the breadth of the distribution of circular dissymmetry parameters obtained from modulation of the circularly polarized excitation source (457 nm). For PDI, we observe a symmetric distribution of circular anisotropy parameters centered about zero, with a fwhm of 0.25. For MH2, we observe an asymmetric distribution peaked at g = −0.09, with a slightly larger width as the corresponding PDI distribution. These results indicate that the large dissymmetr...