Search for: All records

The rationalization of the molecular parameters that influence the intensity and sign of circularly polarized luminescence (CPL) for chiral emitters is a challenging task and remains of high interest for future chiral optoelectronic applications. In this report, we explore the design of novel chiral donor–acceptor structures based on C 2 -symmetric bicarbazole systems and compare the influence of the type of chirality, namely axial versus helical, and the electron withdrawing strength of the acceptor units on the resulting photophysical and CPL properties. By using carbonyl-based acceptors with both axial and helical electron donors, CP-Thermally Activated Delayed Fluoresence (TADF) can be obtained, whose efficiency depends on the dihedral angle between the carbazole moieties, related to the axial and helical chirality of the compounds. The latter also impacts the intensity of the CPL, which shows an opposite trend as a function of the polarity of the solvent, with a notably strong increase of the luminescence dissymmetry factor, g lum , for the helical donor–acceptor compounds related to a subtle reoarganization of the intramolecular charge-transfer process. 

While the development of chiral molecules displaying circularly polarized luminescence (CPL) has received considerable attention, the corresponding CPL intensity, g lum, hardly exceeds 10 −2 at the molecular level owing to the difficulty in optimizing the key parameters governing such a luminescence process. To address this challenge, we report here the synthesis and chiroptical properties of a new family of π-helical push–pull systems based on carbo[6]helicene, where the latter acts as either a chiral electron acceptor or a donor unit. This comprehensive experimental and theoretical investigation shows that the magnitude and relative orientation of the electric ( μe ) and magnetic (μ m ) dipole transition moments can be tuned efficiently with regard to the molecular chiroptical properties, which results in high g lum values, i.e. up to 3–4 × 10 −2 . Our investigations revealed that the optimized mutual orientation of the electric and magnetic dipoles in the excited state is a crucial parameter to achieve intense helicene-mediated exciton coupling, which is a major contributor to the obtained strong CPL. Finally, top-emission CP-OLEDs were fabricated through vapor deposition, which afforded a promising g El of around 8 × 10 −3 . These results bring about further molecular design guidelines to reach high CPL intensity and offer new insights into the development of innovative CP-OLED architectures. 

The photophysical and chiroptical properties of a novel, chiral helicene‐NHC−Re(I) complex bearing anN‐(aza[6]helicenyl)‐benzimidazolylidene ligand are described, showing its ability to emit yellow circularly polarized luminescence. A comparative analysis of this new system with other helicene‐Re(I) complexes reported to date illustrates the impact of structural modifications on the emissive and absorptive properties.

 

π-Helical push–pull dyes were prepared and their (chir)optical properties were investigated both experimentally and computationally. Specific fluorescent behaviour of bis-substituted system was observed with unprecedented solvent effect on the intensity of circularly polarized luminescence (CPL, dissymmetry factor decreasing from 10 −2 to 10 −3 with an increase in solvent polarity) that was linked to a change in symmetry of chiral excited state and suppression of interbranched exciton coupling. The results highlight the potential of CPL spectroscopy to study and provide a deeper understanding of electronic photophysical processes in chiral π-conjugated molecules. 

The first enantiopure chiral‐at‐rhenium complexes of the formfac‐ReX(CO)₃(:C^N) have been prepared, where :C^N is a helicene‐N‐heterocyclic carbene (NHC) ligand and X=Cl or I. These have complexes show strong changes in the emission characteristics, notably strongly enhanced phosphorescence lifetimes (reaching 0.7 ms) and increased circularly polarized emission (CPL) activity, as compared to their parent chiral models lacking the helicene unit. The halogen along with its position within the dissymmetric stereochemical environment strongly affect the photophysics of the complexes, particularly the phosphorescence quantum yield and lifetime. These results give fresh insight into fine tuning of photophysical and chiroptical properties of Re‐NHC systems.

 

The first enantiopure chiral‐at‐rhenium complexes of the formfac‐ReX(CO)₃(:C^N) have been prepared, where :C^N is a helicene‐N‐heterocyclic carbene (NHC) ligand and X=Cl or I. These have complexes show strong changes in the emission characteristics, notably strongly enhanced phosphorescence lifetimes (reaching 0.7 ms) and increased circularly polarized emission (CPL) activity, as compared to their parent chiral models lacking the helicene unit. The halogen along with its position within the dissymmetric stereochemical environment strongly affect the photophysics of the complexes, particularly the phosphorescence quantum yield and lifetime. These results give fresh insight into fine tuning of photophysical and chiroptical properties of Re‐NHC systems.