Improvement of Solid‐State Excimer Emission of the Aryl–Ethynyl‐o‐Carborane Skeleton by Acridine Introduction

Abstract

We report highly‐efficient and solid‐state excimer emission based on the acridine–o‐carborane dyad possessing the ethynyl spacer. The previous pyrene‐modified o‐carborane showed excimer emission only at 77 K in the crystalline state, meanwhile, the current acridine‐modified molecule presented excimer emission with high efficiency (ΦPL = 0.23) in the crystalline state at room temperature. From single‐crystal X‐ray crystallography, it was indicated that two acridine moieties were stacked and the third acridine molecule was out of alignment. Such a packing mode could restrict exciton splitting over the columnar packing structure. Moreover, molecular interactions through the nitrogen atom in the acridine moiety and the hydrogen atom in the o‐carborane unit contribute to suppression of molecular motions. As a consequence, an improved emission efficiency was obtained. This study demonstrates that the ethynyl–o‐carborane skeleton should work as the excimer‐inducible component in the solid state.

Recent Progress in the Development of Solid‐State Luminescent o‐Carboranes with Stimuli Responsivity

“…It is proposed that electronic conjugation is drastically changed by the formation of a dative bond between fluoride and tricoordinate boron at the substituent. Modified o ‐carboranes having stimuli‐responsive luminochromism in the solid state have been reported ( 76 – 85 ) . The details will be discussed in Section 5.…”

“…Other types of AIE‐active o ‐carborane derivatives are listed in Table and Scheme . Influences on AIE properties were investigated by changing the substituents on the o ‐carborane unit ( 44 – 75 ) . For these compounds, solid‐state excimer emission, single‐molecule white emission, aggregation‐induced electrochemiluminescence, and thermally activated delayed fluorescence were observed.…”

“…Next, to obtain solid‐state excimer emission at ambient temperature, the heterocycle– o ‐carborane dyad 83 was designed (Figure ) . As mentioned above, o ‐carborane has electron‐accepting character at the carbon atoms and electron‐donating character at the boron atom most distant from carbon.…”

Aggregation-induced emission characteristics of o-carborane-functionalized fluorene and its heteroanalogs: the influence of heteroatoms on photoluminescence

“…All compounds exhibited dual emissions consisting of the locally excited (LE) and ICT emission bands in the solution state; the former emission band was attributed to the LE (localized at the aryl backbone) state emission, and the latter belonged to the ICT state emission because the intensity of the ICT emission is sensitive to solvent polarity. 36 Moreover, we measured the emission spectra at the low temperature of 77 K in a 2-methyltetrahydrofuran (2-MeTHF) solution, where the molecular motions are completely frozen (Fig. S3, ESI †).…”

“…Significantly, in contrast to these features from the LE and ICT emissions at room temperature, all compounds showed strong broad emissions in the longer-wavelength region but the sharp bands with vibrational structures were similar to those at room temperature, indicating that the emission bands should be from the LE and ICT states in solutions. 36 Owing to the introduction of fluorene and its heteroanalogs into the two carbon atoms of optically functional ''elementblock'' o-carborane, [37][38][39] CDC, CDN, CDO and CDS showed a typical AIE characteristic: the weak emission in a dilute THF solution was enhanced upon aggregate formation in solution and also in the solid state. The AIE characteristics of CDC, CDN, CDO and CDS were further proved by studying their PL behaviors in THF and THF/water mixtures with different water fractions (f w ) (Fig.…”