Helicene Monomers and Dimers: Chiral Chromophores Featuring Strong Circularly Polarized Luminescence

Abstract

The synthesis and chiroptical properties of a series of enantiomerically pure, C2‐symmetrical carbo[6]helicene dimers are reported. Two helicene cores are connected through a buta‐1,3‐diyne‐1,4‐diyl linker or a heteroaromatic bridge and bear arylethynyl substituents at their 15‐positions. This ensures the possibility of extended electronic communication throughout the whole molecule. The new chromophores exhibit intense ECD spectra with strong bands in the UV/Vis region well above 400 nm. The anisotropy factor gabs (defined as Δϵ/ϵ) reaches values up to 0.047, which are unusually large for single organic molecules. They also display blue fluorescence, with good quantum yields (Φf∼0.25). The emitted light is circularly polarized to an outstanding extent: in some cases, the luminescence dissymmetry factor glum=2(IL−IR)/(IL+IR) attains values of |0.025|. To the best of our knowledge, such values are among the highest ever reported for non‐aggregated organic fluorophores.

Quantifying the Overall Efficiency of Circularly Polarized Emitters

“…The g lum values are in the order of 10 −4 –10 −3 , thus translating into low B CPL values with a median value of 5 m −1 cm −1 and an average value of 4.9 m −1 cm −1 . [6]Helicenes [44–50] are a less homogeneous subclass: compounds with an extended π‐conjugation ( Hel‐18 , Hel‐20 , Hel‐22 , Hel‐23 and Hel‐24 ) present high quantum yields (0.41–0.75), [46, 47, 50] whereas Hel‐17 , Hel‐19 and Hel‐21 have lower quantum yields(<0.1) [44, 45, 47–49] . In most cases, the g lum factors are similar to those recorded for [5]helicenes, that is, in the order of 10 −4 –10 −3 , with only Hel‐21 [48, 49] being endowed with a higher g lum value (2.5×10 −2 ).…”

“…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).…”

“…Dimeric systems ( Hel‐47 – Hel‐55 ) show extinction coefficients in the range 42 000–95 000 m −1 cm −1 , with emission maxima falling in the blue‐red region (420–650 nm) [48, 50, 66–68] . In most case, the quantum yields are between 0.22 and 0.55.…”

Simple Perylene Diimide Cyclohexane Derivative With Combined CPL and TPA Properties

“…This property is characterized by the preferential emission of left (I L ) or right handed (I R ) circularly polarized light with respect to the entire emission (I L +I R )/2, being usually described by the dimensionless Kuhn factor of the emission, g lum = 2(I L -I R )/(I L +I R ). For simple organic molecules (SOMs), these values usually range from 0.01 to 0.0001, being scarce values higher than 0.01 (Sato et al, 2017;Ito et al, 2018;Takaishi et al, 2018Takaishi et al, , 2019Takaishi et al, , 2020Han et al, 2019;Reine et al, 2019;Schaack et al, 2019;Zheng et al, 2019).…”

Helicene-derived aggregation-induced emission conjugates with highly tunable circularly polarized luminescence

“…2a left, see Supplementary methods for synthetic details) by linking helicene and TPE through a conjugated alkynyl bridge. 38,39 In the absorption spectra ( Fig. 2b left), both 2-TPEH and 4-TPEH showed a bathochromic shift for the major bands with the absorption extending to > 400 nm, clearly reflecting a strong π-conjugation between helicene and TPE through the alkynyl bridge.…”