Chiroptical Dissymmetries in Fluorescence Excitation from Single Molecules of (M-2) Helicene Dimers

Abstract

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 dissymmetry parameters (|g| > 0.5) in fluorescence excitation described in our original report (Hassey, R.; et al. Chirality 2008, 20, 1039-1046 and Hassey, R.; et al. Science 2006, 314, 1437-1439) were indeed affected by (at the time, unknown) linear polarization artifacts. However, the present results on MH2 provide compelling evidence for single-molecule circular dissymmetries much larger than solution or thin-film ensemble values, defined primarily by the enhanced rotatory strength (relative to the monomer), and restricted orientation at the sample surface.

Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

“…Note that bridged triarylamines were used as models for several calculations of ECD, VCD and CPL. 211,207,212,213,214…”

Giant Circular Dichroism in Individual Carbon Nanotubes Induced by Extrinsic Chirality

“…Despite its common use on macroscopic samples, detection of CD from single molecules is nontrivial as the difference in absorption is typically less than a part in a thousand. The use of superchiral optical fields can enhance enantioselectivity [3,4], but CD spectroscopy at a single molecule level still remains a challenge [5][6][7][8][9]. In contrast to the smallness of the effect in molecules, giant CD at optical frequencies has been demonstrated in metamaterials [10][11][12][13].…”

Synthesis of [7]Helicene Enantiomers and Exploratory Study of Their Conversion into Helically Chiral Iodoarenes and Iodanes

“…Helicenes are polycyclic molecules composed of ortho-fused aromatic rings that confer an inherently chiral helical shape to their structures. [1][2][3][4] Owing to their uniques tructural, electronic and chiroptical properties, these compounds are the focus of considerable attention in variousf ields of research, [3][4][5] notably for their potential utility in asymmetricc atalysis, [6][7][8][9][10] molecular recognition, [11][12][13][14][15] supramolecular chemistry, [16][17][18] molecular machines design, [19][20][21][22][23] liquid crystal technology, [24][25][26][27] organic (opto)electronics, [28][29][30][31][32] as well as new materials. [33][34][35][36] Several of these applications necessitate access to optically pure helicenes.…”