Anodic Deposition of Enantiopure Hexahelicene Layers

Abstract

Helicenes are polyaromatic compounds with chiral properties useful for many applications in optoelectronics, separation processes, chiral recognition and catalysis. Here we focused on the electrochemistry of carbo[n]helicenes (n=5,6,7). The cyclic voltammograms of racemic mixtures of target compounds in acetonitrile/0.1 M tetrabutylammonium perchlorate at a glassy carbon electrode reveal the diffusion‐controlled reactions in both anodic and cathodic potential regions. Electrochemical behaviors are different for individual helicenes, [7]helicene undergoes redox transformation easily in comparison to the other investigated compounds, which is in agreement with DFT (density functional theory) calculations. Generally, the multi‐component anodic process of helicenes is observable at potentials from +1.5 to +2.5 V, leading to the formation of deposited structures (layers) on the electrode surface. The helicenes were electrodeposited onto transparent indium tin oxide (ITO) electrodes and characterized by atomic force microscopy, UV/Vis, Raman spectroscopy and ellipsometry. Finally, the anodic deposition of P and M enantiomers of [6]helicene was performed using ITO substrates, resulting in the formation of enantiopure layers of nanometer thicknesses, as confirmed by circular dichroism spectroscopy. The discovered electrosynthetic procedure opens up a new possibility for the immobilization of chiral helicene layers onto solid supports.

Single Molecule Conductance of Electroactive Helquats: Solvent Effect

“…There are only two reports on the single molecule conductance properties of uncharged diazahelicenes, where Nheteroatoms served as the anchoring groups. [30,31] In the first work the theoretical description of mechanical tuning of the single molecule conductance was emphasized, whereas in the second one the single molecule conductance of oxa [9]helicene terminated by pyridine units was reported to be 8.8 × 10 À 4 G/G 0 (log(G/G 0 ) = À 3.06), which is in reasonable agreement with our measurements assuming that this molecule has n equal to 9 and contains anchoring groups.…”

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

“…It thus appeared that ( M )‐ 17 a is prone to oxygenation on sites other that its iodine atoms, notably on positions 9 and 10 of the helicene framework (see Scheme and the Supporting Information). This unfortunate sensitivity of ( M )‐ 17 a toward oxygenation is corroborated by the electrochemical studies of Storch and his colleagues, who recently reported that [7]carbohelicenes are more sensitive to oxidation than their smaller [5] and [6]helicene counterparts . Nevertheless, the conversion of carvacrol ( 18 ) into biscarvacrol ( 19 ) clearly indicated that the bis‐iodinated helicene ( M )‐ 17 a could be in part transformed into an iodosyl (ArIO) or an iodyl (ArIO 2 ) species, which was capable of transferring an oxygen atom onto a 2‐methylphenol substrate with a moderate but real level of enantiocontrol (see Scheme ).…”

“…This unfortunate sensitivity of (M)-17 a toward oxygenationi sc orroboratedb yt he electrochemical studies of Storch and his colleagues, who recently reported that [7]carbohelicenes are more sensitive to oxidation than their smaller [5] and [6]helicene counterparts. [34] Nevertheless, the conversionofcarvacrol(18)into biscarvacrol (19)clearly indicatedt hat the bis-iodinated helicene (M)-17 a could be in part transformed into an iodosyl (ArIO) or an iodyl (ArIO 2 )s pecies, which was capable of transferringa no xygen atom onto a 2-methylphenol substrate with am oderate but real level of enantiocontrol (see Scheme 7). However,t he low yield of the reaction, the use of the iodane reagent in stoichiometric amountsa nd the difficulties of producing it in pure form were arguments against the continuation of this work.…”

“…[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. [2][3][4] Although af ew asymmetric synthesis approaches have relied on the use of chiral reagents or catalysts, [10,[37][38][39][40]…”

“…[51][52][53] Thus, the separation of helicene racemates by chiral chromatography remains today the most commonly used access to separated pairs of helicene enantiomers. [15,34,[60][61][62][63] Despite the wide range of applications of helicenes, their helical chirality has yet never been evaluated as inductor of asymmetry in hypervalent iodine-mediated transformations. [64] Our long-standing interest in this field thus prompted us to examine the stereodiscriminating potential of enantiopure iodinated helicenes.…”