Synthesis and Chiroptical Properties of Hexa‐, Octa‐, and Deca‐azaborahelicenes: Influence of Helicene Size and of the Number of Boron Atoms

Abstract

Four members of a new class of cycloborylated hexa- to deca-helicenes (1a–d) have been prepared in enantiopure forms, along with cycloplatinated deca-helicenes (1d′, 1d1) further extending a family of cycloplatinated hexa- to octa-helicenes reported previously. The azabora[n]helicenes display intense electronic circular dichroism and strong optical rotations, whose dependence on the size of the helix (n = 6, 8, or 10) and number of boron atoms (1 or 2) has been examined in detail both experimentally and theoretically. The photophysical properties (unpolarized and circularly polarized luminescence) of these new fluorescent organic helicenes have been measured and compared with the corresponding organometallic phosphorescent cycloplatinated derivatives (1a1–d1).

Phosphahelicenes: From Chiroptical and Photophysical Properties to OLED Applications

“…Note that no noticeable involvement of P(O)R group was revealed in the low‐energy excitations of 1′ ‐ 4′ implying that its main role is to rigidify the molecule, increase the size of the helical structure and extend its π‐electron conjugation up to the terminal phosphole‐phenyl rings. Its effect is thus very similar to that of BR 2 moiety in our recently reported azaborahelicene derivatives …”

“…The highest quantum yield is obtained for the phosphafluorene‐type phospha[6]helicene oxide 2 ( Φ f =32 %). Furthermore, at 77 K, the emission spectra of the rigid phosphafluorene‐based compounds 2 and 4 demonstrate fluorescence emission accompanied by phosphorescence which is typical for helicenic structures . For compounds 1 , 3 and 5 , which display pendant phenyl rings at position 5 of the phosphole ring, only fluorescent emission is observed indicating that such freely rotating group promotes non‐radiative deactivation processes.…”

“…An appealing strategy to modify emission properties of helicenes consists in introducing heteroatom(s) or heterocycle(s) in the helical structure. For example, significant improvements of helicenes’ fluorescence properties have been recently achieved via introduction of such heterocycles as carbazole, maleimide, siloles, oxaborine, or azaborole . Among other possible heterocycles, the phosphole ring is an interesting subunit for tuning emission properties of helicenes since phosphole derivatives are known to be efficient fluorescent emitters for organic light‐emitting diodes (OLEDs) .…”

Azabora[5]helicene Charge‐Transfer Dyes Show Efficient and Spectrally Variable Circularly Polarized Luminescence

“…However, despite the fact that several BODIPY‐based chiral architectures were found to show significant CPL effects, azabora[ n ]helicenes, that incorporate boron directly in the helical architecture, were reported only in a single work so far . These systems build on borylated arylpyridine N,C‐chelates.…”

Exploring potentialities and limitations of stapled o‐oligo(phenyleneethynylene)s (o‐OPEs) as efficient circularly polarized luminescence emitters

“…Therefore, the search for new and efficient CPL‐emitting compounds is now a growing research field. Among all the potential emitting structures, the development of single organic monomolecular (SOM) emitters, with reasonable dissymmetry factors, is very appealing. Thus, for example, they have been suggested as crucial materials for the development of circularly polarized laser emission .…”