Highly Fluorescent [7]Carbohelicene Fused by Asymmetric 1,2-Dialkyl-Substituted Quinoxaline for Circularly Polarized Luminescence and Electroluminescence

Abstract

A new 1,2-dialkylquinoxaline-fused [7]­carbohelicene ([7]­Hl-NAIQx) was designed and synthesized by asymmetrically introducing two alkyl chains onto the quinoxaline unit. Direct alkylation of the quinoxaline ring of quinoxaline-fused helicene leads to discontinuity in the conjugated structure. In the single-crystal analysis, the parent quinoxaline-fused [7]­carbohelicene ([7]­Hl-Qx) was found to have a helical structure formed by two phenanthrene units and a nonplanar twisted angle between the phenanthrene and quinoxaline units. In contrast, [7]­Hl-NAIQx possesses an almost planar aromatic structure between the alkyl-quinoxaline and phenanthrene units (torsion angle: 179°), in addition to the similar helical structure between the two phenanthrene units. The steady-state absorption, fluorescence, and circular dichroism (CD) spectra of [7]­Hl-NAIQx were significantly red-shifted compared to those of [7]­Hl-Qx and [7]­carbohelicene ([7]­Hl). These spectral changes were mainly explained by electrochemical measurements and density functional theory calculations. Moreover, the absolute fluorescence quantum yield (ΦFL) of [7]­Hl-NAIQx was 0.25, which is more than 10 times larger than that of the reference [7]­Hl (ΦFL = 0.02). Such a large enhancement of the fluorescence of [7]­Hl-NAIQx has provided excellent circularly polarized luminescence (CPL). The value of the anisotropy factor g lum (normalized difference in emission of right-handed and left-handed circularly polarized light) was estimated to be 4.0 × 10–3. The electroluminescence of an organic light-emitting diode utilizing [7]­Hl-NAIQx was successfully observed.

Synthetic Control of Photophysical Process and Circularly Polarized Luminescence of [5]Carbohelicene Derivatives Substituted by Maleimide Units

“…This is the first example of CPL signals of [5]helicene derivatives. As compared to the reported CPL results of carbohelicenes and their derivatives containing more than six benzene rings (g lum : ~10 -2 -10 -3 ), 56,68,71 HeliIm and MeO-HeliIm demonstrated the similar g lum values. These results indicate that [5]carbohelicene derivative are good candidates as chiroptical molecules.…”

“…[43][44][45][46][47][48] In particular, chiral luminescence materials and compounds indicate a possibility for circularly polarized luminescence (CPL) as new chiroptical properties, which is the differential emission of right-and left-circularly polarized light. [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] However, the fluorescence quantum yields (Φ FL ) of helicene derivatives are generally extremely low because the intersystem 4 crossing (ISC) pathways are highly accelerated (Φ ISC : ~0.9). [65][66][67] Therefore, synthetic strategies for improvements of Φ FL values are definitely required.…”

“…[65][66][67] Therefore, synthetic strategies for improvements of Φ FL values are definitely required. 56,[68][69][70][71] Additionally, [5]carbohelicenes consisted by five benzene rings are generally unable to show the chirality because of the only overlapped hydrogen atoms in the terminal benzene rings as compared with carbohelicenes fused by more than six benzene rings. [72][73] The CPL of carbohelicenes containing more than six benzene rings are already examined, 74 whereas the CPL of [5]carbohelicene derivatives have yet to be reported, so far.…”

Stable Enantiomers Displaying Thermally Activated Delayed Fluorescence: Efficient OLEDs with Circularly Polarized Electroluminescence

“…emitters to fabricate CP-OLEDs with intense circularly polarized electroluminescence (CPEL), and they mainly include emissive polymers decorated with chiral sidechains, [9] achiral conjugated polymers with chiral dopants, [10] as well as chiral emissive molecules [11] and complexes. [12] Although several CP-OLEDs have intense CPEL, with the dissymmetry factors (g EL )even as high as 1.13,[10e],they are mainly based on the chiral transition-metal complexes and polymers,and the efficiencies of the devices are also generally low.…”

“…[12] Although several CP-OLEDs have intense CPEL, with the dissymmetry factors (g EL )even as high as 1.13,[10e],they are mainly based on the chiral transition-metal complexes and polymers,and the efficiencies of the devices are also generally low. [9][10][11][12] Therefore,the development of CP-OLEDs with both high efficiencies and intense CPEL, especially those based on small organic emitters,w ould be practical for future display and photonic technologies.…”