Short-step synthesis and chiroptical properties of polyaza[5]–[9]helicenes with blue to green-colour emission

Abstract

We have developed a two- or three-step synthesis of polyaza[5]–[9]helicenes by consecutive N–H/C–H coupling with a hypervalent iodine reagent.

Quantifying the Overall Efficiency of Circularly Polarized Emitters

“…Given both the large number of reported CPL‐active [ n ]helicenes and also their quite diverse structural motifs (Scheme 4), we shall distinguish and base the discussion of the surveyed compounds on five main classes: 1) purely monomeric organic [ n ]helicenes (in turn divided into 6 subclasses according to n, with n being the number of aromatic units forming the helicene scaffold); [38–65] 2) dimeric systems; [48, 50, 66–68] 3) S‐shaped systems; [69, 70] 4) systems with semimetals (B and Si) in the [ n ]helicenes core or directly linked to it; [71–76] and 5) cationic systems [77–80] (Scheme 4 and Table 4).…”

“…Comprehensively, the [6]helicenes subclass presents a median B CPL value of 8.9 m −1 cm −1 which significantly differs from its average of 29.2 m −1 cm −1 . Concerning [7]helicenes, the quantum yields are in the range of 0.23–0.66, [51, 53, 55–61] with lower values of 0.1 and 0.06 associated to the phosphahelicene Hel‐26 [52] and to the carbohelicene Hel‐28 , [54] respectively. As for the [5] and [6]helicenes, the g lum factors are in the order of 10 −4 –10 −3 .…”

“…The median B CPL value is 12.9 m −1 cm −1 and the average 17.4 m −1 cm −1 . For larger [ n ]helicenes (with n= 8, 9 and 11), the quantum yields are generally low (<0.3) and the g lum factors are again in the order of 10 −4 –10 −3 [53, 60, 62–65] . For n= 8 and n= 11, the B CPL analysis could be performed only on two cases each, so the median/average values are 4.8 m −1 cm −1 for [8]helicenes and 1.35 m −1 cm −1 for [11]helicenes.…”

Synthesis and Chiroptical Properties of Quinoxaline‐Fused Polyaza[5]–[7]helicenes with Orange‐Color CPL Emissions

“…We initially attempted to synthesize quinoxaline‐fused polyaza[5]helicene C[5] based on our second‐generation method ( Scheme 1). [21] We employed 1,4‐dibromophenazine ( 1 ) as a quinoxaline unit [25] . A double Buchwald–Hartwig ‐type cross coupling of 1 with 2‐aminopyridine ( 2 ) produced the helicene precursor N 1 , N 4 ‐di(pyridin‐2‐yl)phenazine‐1,4‐diamine ( 3 ) in 57 % yield.…”

“… [20] The helicenes A[7] emit a strong blue‐to‐yellowish green fluorescence with superior CPL activity. We also developed second‐generation polyazahelicene synthesis with a 6‐5‐6‐5‐6‐fused ring system as a core skeleton, starting from p ‐phenylenediamine, which enabled not only the two‐step synthesis of symmetric polyaza[5]‐, [7]‐, and [9]helicenes B , but also a three‐step synthesis of unsymmetric polyaza[8]helicene [21] . The [9]helicene B[9]a emits a green CPL with high g lum value of 0.027, but is hindered by a low quantum yield of 0.06.…”