Synthesis, Resolution, Configurational Stability, and Properties of Cationic Functionalized [5]Helicenes

Abstract

A straightforward approach to the synthesis of two different series of cationic [5]helicenes has been achieved including, in dioxa series, the possibility to introduce aromatic functional groups at the periphery of the helical structure. While photophysical study highlights that the introduction of aryl substituents at position 23 of the helical moieties has a negligible impact on the optical properties, styryl substituents allow a welcoming extension of the conjugation pathways. Finally, a redshift of the optical properties was evidenced upon introduction of nitrogen atoms in the helicene scaffold, leading to particularly good fluorescence efficiencies in the red domain for a helicenic dye. Detailed information on racemization kinetics was collected for the most stable species upon direct HPLC resolution or, when configurational lability was too high, through VT-HPLC analysis on chiral stationary phase (ΔG ‡ values ranging from 85.0 to 137.1 kJ.mol -1 and above). 8AMix c 12 4.8 10 -2 86.7 9.7 10 -2 d 85.0 8B EtOH 70 4.7 10 -3 111.5 9.4 10 -3 d 109.5 10A Mix c 40 5.1 10 -2 95.3 1.0 10 -1 d 93.4 10A Mix c 46 8.9 10 -2

Synthesis of Helical and Planar Extended‐Phenanthridinium Salts

“…It was earlier shown that monoaza[5]helicenes rapidly racemize at room temperature and that racemization barriers are lower than for the corresponding carbon analogues [13] . Furthermore, the position of the nitrogen significantly impacts the configurational stability [14] and relatively low barriers were observed for the racemization of cationic diaza[5]helicenes [15] . We thus examined the racemization of the synthesized compound 1 after separation by HPLC on a chiral stationary phase.…”

“…[13] Furthermore, the position of the nitrogen significantly impacts the configurational stability [14] and relatively low barriers were observed for the racemization of cationic diaza [5] helicenes. [15] We thus examined the racemization of the synthesized compound 1 after separation by HPLC on a chiral stationary phase. A sample of enantioenriched 1 in acetonitrile was heated to 40 °C and the racemization was monitored by HPLC (Figure 1).…”

Synthesis of Functionalized Six-Membered-Ring Azahelicenes

“…The brominated product 33b can be further reacted under Suzuki-Miyaura and Sonogashira conditions to yield the diphenyl (36) and diphenylethynyl (37) derivatives (Scheme 29). A very recent paper [57] shows the preparation of a cationic Br-substituted diaza [5] Intermediate 38 can be prepared on a multigram scale and can also be used for the preparation of structurally similar dioxa-and azaoxa-helicenes. It is also possible to prepare the related compounds 39 and 40 (Figure 6) through the transformation of dioxaderivatives.…”

Synthesis of Methyl-Substituted Aza[7]helicene with Enhanced Configurational Stability and Chiroptical Performance

“…However, the relatively low racemization energy barrier of A7H makes it difficult to maintain its chiral configuration at high temperatures during the chemical derivatization or the thermal processing for device fabrication, thus limiting the applications of A7H-based chiral materials. Herein, we report the synthesis and characterizations of a new tetramethyl-substituted aza [7]helicene (TMA7H) by introducing methyl groups at the terminal of the A7H backbone to increase the racemization energy barrier [49][50][51][52][53][54] (Fig. 1b).…”