Highly fluorescent [7]carbohelicene fused by asymmetric 1,2-dialkyl-substituted quinoxaline for circularly polarized luminescence and electroluminescence

Hayato Sakai, Sho Shinto, Jatish Kumar, Yasuyuki Araki, Tomo Sakanoue, Taishi Takenobu, Takehiko Wada, Tsuyoshi Kawai, Taku Hasobe

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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 (Φ<inf>FL</inf>) of [7]Hl-NAIQx was 0.25, which is more than 10 times larger than that of the reference [7]Hl (Φ<inf>FL</inf> = 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<inf>lum</inf> (normalized difference in emission of right-handed and left-handed circularly polarized light) was estimated to be 4.0 × 10<sup>-3</sup>. The electroluminescence of an organic light-emitting diode utilizing [7]Hl-NAIQx was successfully observed.

Original languageEnglish
Pages (from-to)13937-13947
Number of pages11
JournalJournal of Physical Chemistry C
Volume119
Issue number24
DOIs
Publication statusPublished - 2015 Jun 18

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Electronic, Optical and Magnetic Materials
  • Surfaces, Coatings and Films
  • Energy(all)

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