Effect of Planarity of Aromatic Rings Appended to o-Carborane on Photophysical Properties: A Series of o-Carboranyl Compounds Based on 2-Phenylpyridine- and 2-(Benzo[b]thiophen-2-yl)pyridine

Abstract

Herein, we investigated the effect of ring planarity by fully characterizing four pyridine-based o-carboranyl compounds. o-Carborane was introduced to the C4 position of the pyridine rings of 2-phenylpyridine and 2-(benzo[b]thiophen-2-yl)pyridine (CB1 and CB2, respectively), and the compounds were subsequently borylated to obtain the corresponding C∧N-chelated compounds CB1B and CB2B. Single-crystal X-ray diffraction analysis of the molecular structures of CB2 and CB2B confirmed that o-carborane is appended to the aryl moiety. In photoluminescence experiments, CB2, but not CB1, showed an intense emission, assignable to intramolecular charge transfer (ICT) transition between the aryl and o-carborane moieties, in both solution and film states. On the other hand, in both solution and film states, CB1B and CB2B demonstrated a strong emission, originating from π-π * transition in the aryl groups, that tailed off to 650 nm owing to the ICT transition. All intramolecular electronic transitions in these o-carboranyl compounds were verified by theoretical calculations. These results distinctly suggest that the planarity of the aryl groups have a decisive effect on the efficiency of the radiative decay due to the ICT transition.

Photophysical Properties of Spirobifluorene-Based o-Carboranyl Compounds Altered by Structurally Rotating the Carborane Cages

“…The Sonogashira-coupling reaction between ethynyltrimethylsilane and 2- or 4-bromo-9,9′-spirobifluorene produced (9,9′-spirobi[fluoren]-2-ylethynyl)trimethylsilane ( S1 ) and (9,9′-spirobi[fluoren]-4-ylethynyl)trimethylsilane ( S2 ) in high yields (75% for S1 and 80% for S2 ). The protonated compounds E1 and E2 were obtained by the treatment of S1 and S2 with a weak base (K 2 CO 3 ), after which the 2- and 4- o -carborane-substituted spirobifluorenes C1 and C2 were respectively prepared from E1 and E2 while using decaborane-promoted (B 10 H 14 -promoted) cage-forming reactions in the presence of Et 2 S (Figure 1) [61,62,63]. All of the compounds were fully characterized by multinuclear ( 1 H, 13 C, and 11 B{ 1 H}) NMR spectroscopy (Figures S1–S6) and elemental analysis.…”

“…The ICT character of the low-energy emission from C2 was confirmed by the solvatochromism that was observed in various solvents of different polarity (Table 1 and Figure S8b). These interesting features suggest that the ICT-based radiative decay processes in C1 and C2 are significantly enhanced in rigid molecular states that effectively prevent structural fluctuations, such as the C−C bond variations that occur in o -carborane cages [8,35,55,61,62]. Indeed, the DFT-optimized structures of the ground (S 0 ) and first-excited singlet (S 1 ) states of both C1 and C2 confirm such structural fluctuations.…”

“…In addition, various o -carborane-containing donor-acceptor systems exhibit intriguing photophysical properties through twisted intramolecular charge-transfer (TICT) processes [43,44,45,46,47,48,49,50,51,52,53]. Along with these previous studies, our group reported significant changes in radiative ICT mechanisms by modulating the planarities of several phenylene groups that were substituted on o -carborane cages [60,61,62]. Accordingly, we clearly demonstrated that the appropriate combination of molecular geometry and donor plays a key role in implementing promising optoelectronic features.…”

“…Indeed, these desirable electronic properties are due to the unique characteristics of the o -carborane unit, such as its strongly electron-deficient nature, as well as its highly polarizable σ-aromaticity. In addition, these properties facilitate the formation of various donor‒acceptor systems, leading to intriguing intramolecular charge-transfer (ICT) transitions between numerous π -conjugated aromatic groups and o -carborane cages [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62]. Hence, these features have become ultimate sources for generating specific luminescence behavior in a variety of o -carborane-based compounds.…”

Deboronation-Induced Ratiometric Emission Variations of Terphenyl-Based Closo-o-Carboranyl Compounds: Applications to Fluoride-Sensing

“…Such ICT characteristics can induce unique luminescence behavior in various o-carborane-based organic luminophores . Interestingly, such an intramolecular radiative mechanism activated by the ICT transitions in the o-carboranyl luminophores has been found amenable to modifications via variations to the structure of the o-carborane cages or appended aryl groups [32,[43][44][45][46][47][48][49][50][51][52][53]70] and their molecular geometries [66][67][68][69]. Furthermore, the direct control of the ICT-based emission in the closo-o-carboranyl compounds involves the conversion of closo-o-carboranes to nido-o-species (o-1,2-C 2 B 9 H 12 − , one boron atom removed analog of the closo-o-carborane cage) by reaction with nucleophilic anions.…”