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

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.


Introduction
Icosahedral carboranes (C 2 B 10 H 12 ) are well-known boron-rich clusters that can be often regarded as 3D analogues of organic aryl derivatives [1]. Among them, organic and organometallic complexes that contain closo-o-carborane (closo-1,2-C 2 B 10 H 12 ) have been widely investigated as a novel family of optoelectronic materials for various functional and photonic applications  because they definitely possess excellent thermal and electrochemical stabilities. They also realize unique photophysical properties induced by the electron-withdrawing character of the o-carborane unit [24][25][26][27][28]. Introduction of this unit to an aryl group leads to the formation of a donor-acceptor conjugated system owing to the strongly electron-withdrawing C atoms and highly delocalized 3D aromaticity of the o-carborane cage. Such a good conjugated system results in intramolecular charge transfer (ICT) of Kangwon National University, Gangwon-do, Korea). The UV-vis absorption and PL spectra were recorded on the Jasco V-530 (JASCO International Co., Ltd., Tokyo, Japan) and Horiba FluoroMax-4P spectrophotometers (Horiba FluoroMax ® , Salem, NH, USA), respectively. Fluorescence decay lifetimes (τ) were measured at 298 K using a time-correlated single-photon counting spectrometer (FLS920, Edinburgh Instruments, at the Central Laboratory of Kangwon National University) equipped with an EPL-375 ps pulsed semiconductor diode laser as the excitation source and microchannel plate photomultiplier tube (200-850 nm) as the detector. Absolute PL quantum yields (Φ em ) were obtained at 298 K using an absolute PL quantum yield spectrophotometer (FM-SPHERE, 3.2-inch internal integrating sphere in FluoroMax-4P).

Synthesis of 1a
To a mixture of 3 (0.77 g, 4 mmol) and Pd(PPh 3 ) 4 (0.60 g, 0.52 mmol) in THF (30 mL) was successively added phenylboronic acid (0.58 g, 4.8 mmol) and Na 2 CO 3 (1.27 g, 12.0 mmol) in H 2 O (10 mL). The mixture was stirred and refluxed at 80 • C for 24 h. After cooling it to room temperature, 30 mL of water was added. The organic portions were dried over MgSO 4 and filtered. Following evaporation of the solvent under reduced pressure, the yellow residue was purified by column chromatography (eluent: dichloromethane/n-hexane = 1:3, v/v) to yield 1a as a pale yellow oil (0.74 g, Yield = 79%). 1

Synthesis of CB1
To a toluene solution (100 mL) of B 10 H 14 (0.46 g, 3.77 mmol) and 1a (0.68 g, 2.90 mmol) was slowly added an excess amount of Et 2 S (2.5 equiv. for B 10 H 14 ) at room temperature. The reaction mixture was further stirred at 110 • C for 3 d. After cooling it to room temperature, the solvent was removed under vacuum, and then methanol (50 mL) was added. The precipitated yellow solid was filtered and re-dissolved in toluene. The solution was purified by passing through an alumina column, and the solvent was removed in vacuo to afford CB1 as a white solid. Recrystallization from an acetone/methanol mixture gave 0.51 g of CB1 (Yield = 50%). 1

Synthesis of CB1B
To a stirred solution of CB1 (0.41 g, 1.17 mmol) and diisopropylamine (0.20 mL, 1.17 mmol) in dichloromethane (2.0 mL) at 0 • C was added BBr 3 (1.0 M in dichloromethane, 3.5 mL, 3.5 mmol). After stirring the reaction mixture at room temperature for 48 h, saturated K 2 CO 3 aqueous solution was added. The organic layer was separated, and the aqueous layer was extracted with dichloromethane (3 × 10 mL). The combined organic portions were dried over MgSO 4 and filtered. Evaporation of the solvent under reduced pressure afforded a crude solid residue. Recrystallization from a dichloromethane/n-hexane mixture afforded the BBr 2 -C ∧ N-chelated compound of CB1 as a white solid (0.50 g, Yield = 81%). This compound was characterized by 1 H NMR spectra only and then used in the subsequent methylation reaction in situ. 1  To a stirred solution of the BBr 2 -C ∧ N-chelated compound of CB1 (0.30 g, 0.57 mmol) in toluene (5.0 mL) at room temperature was added AlMe 3 (2.0 M in toluene, 0.63 mL, 1.26 mmol). After stirring the mixture at room temperature for 30 min, the reaction was quenched by adding distilled water (7 mL). The organic layer was separated, and the aqueous layer was extracted with ethyl acetate (3 × 10 mL). The combined organic portions were dried over MgSO 4 and filtered. Evaporation of the solvent under reduced pressure afforded CB1B. Recrystallization from a dichloromethane/n-hexane mixture afforded CB1B as a white solid (0.14 g, Yield = 61%). 1 13

UV-vis Absorption and PL Measurements
Solution UV-vis absorption and PL measurements for the o-carboranyl compounds were performed in degassed toluene (5.0 × 10 −5 M) at 298 K using a 1-cm quartz cuvette. The PL measurements were also carried out in toluene solution at 77 K and film (5 wt % doped on PMMA) on 1.5 × 1.5 cm quartz plates (thickness = 1 mm) at 298 K. The absolute PL quantum yields (Φ em ) at the solution and film states were obtained at 298 K using an absolute PL quantum yield spectrophotometer (FM-SPHERE, 3.2-inch internal integrating sphere on FluoroMax-4P).

X-ray Crystallography
Single crystals of CB2 and CB2B suitable for X-ray diffraction were grown from a dichloromethane/n-hexane mixture. The single crystals were coated with Paratone oil and mounted onto glass capillaries. Crystallographic measurements were performed on the Bruker D8 QUEST CCD area detector diffractometer with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and all non-hydrogen atoms were subjected to anisotropic refinement using the full-matrix least-squares method on F 2 on the SHELXTL/PC package to obtain the X-ray crystallographic data in CIF format (CCDC 1878407 and 1878406 for CB2 and CB2B, respectively). Hydrogen atoms on the carbon and boron atoms were placed at their geometrically calculated positions and refined as riding on the corresponding carbon atoms with isotropic thermal parameters. Detailed crystallographic data are given in Tables S1 and S2 in the Supplementary Material.

Theoretical Calculations
The ground (S 0 ) and first excited state (S 1 ) structures of the o-carboranyl compounds, CB1, CB1B, CB2, and CB2B were optimized using density functional theory (DFT) with the B3LYP functional. The 6-31G(d) basis set was used for all atoms [59]. The electronic transition energies were calculated using time-dependent DFT (TD-DFT) [60] based on the hybrid B3LYP functional (TD-B3LYP), which also accounts for electron correlation. All calculations were performed using the GAUSSIAN 09 program [61]. The percent contribution of a functional group to each molecular orbital was calculated using the GaussSum 3.0 program [62].
Molecules 2018, 23, x FOR PEER REVIEW 6 of 13 All o-carboranyl compounds were fully confirmed by multinuclear ( 1 H, 13 C, and 11 B) NMR spectroscopy (Figures S1-S7) and elemental analysis. The 1 H and 13 C NMR spectra of CB1B and CB2B show the expected resonances corresponding to (C ∧ N)BMe2. In particular, a characteristic singlet signal assignable to B−CH3 was detected at 0.05 and 0.15 ppm for CB1B and CB2B, respectively. Additionally, in the 11 B NMR spectra of CB1B and CB2B, the shoulder signals in the region around δ 0 ppm and three broad peaks from δ −3 to −10 ppm clearly reveal the presence of tetracoordinated boron atoms and closo-o-carborane cage, respectively. The X-ray diffraction study revealed the molecular structures of CB2 and CB2B in the solid state ( Figure 2, detailed parameters and selected bond lengths and angles are listed in Tables S1 and S2). These results distinctly demonstrate that the o-carborane cage is appended at the C4 position of the pyridine ring of the btp moiety. In particular, the CB2B structure clearly exhibit a tetracoordinated structure with bidentate chelation by the boron atom. .

Photophysical Properties
The photophysical properties of o-carboranyl compounds, CB1, CB1B, CB2, and CB2B, were investigated by UV-vis absorption and PL experiments ( Figure 3). The non-chelated CB1 and CB2 display major absorption bands at λabs = 286 and 328 nm, respectively ( Table 1). The broadness of these absorption bands strongly indicate that they can be assigned to not only a spin-allowed π-π * transition in the aryl groups (ppy or btp), but also ICT transition between the o-carborane and aryl groups. The absorption bands of the chelated CB1B and CB2B, which are assignable to a π-π * transition in the aryl groups, are also similarly exhibited in the high-energy region at λabs = 283 and 314 nm, respectively. Interestingly, CB1B and CB2B also show a dominant low-energy absorption band at λabs = 343 and 391 nm, respectively, which correlate with ICT from o-carborane to either the ppy or btp moiety (see calculated data below).
The fluorescent properties of the o-carboranyl compounds were further examined through PL All o-carboranyl compounds were fully confirmed by multinuclear ( 1 H, 13 C, and 11 B) NMR spectroscopy (Figures S1-S7) and elemental analysis. The 1 H and 13 C NMR spectra of CB1B and CB2B show the expected resonances corresponding to (C ∧ N)BMe 2 . In particular, a characteristic singlet signal assignable to B−CH 3 was detected at 0.05 and 0.15 ppm for CB1B and CB2B, respectively. Additionally, in the 11 B NMR spectra of CB1B and CB2B, the shoulder signals in the region around δ 0 ppm and three broad peaks from δ −3 to −10 ppm clearly reveal the presence of tetracoordinated boron atoms and closo-o-carborane cage, respectively. The X-ray diffraction study revealed the molecular structures of CB2 and CB2B in the solid state (Figure 2, detailed parameters and selected bond lengths and angles are listed in Tables S1 and S2). These results distinctly demonstrate that the o-carborane cage is appended at the C4 position of the pyridine ring of the btp moiety. In particular, the CB2B structure clearly exhibit a tetracoordinated structure with bidentate chelation by the boron atom.
Molecules 2018, 23, x FOR PEER REVIEW 6 of 13 All o-carboranyl compounds were fully confirmed by multinuclear ( 1 H, 13 C, and 11 B) NMR spectroscopy (Figures S1-S7) and elemental analysis. The 1 H and 13 C NMR spectra of CB1B and CB2B show the expected resonances corresponding to (C ∧ N)BMe2. In particular, a characteristic singlet signal assignable to B−CH3 was detected at 0.05 and 0.15 ppm for CB1B and CB2B, respectively. Additionally, in the 11 B NMR spectra of CB1B and CB2B, the shoulder signals in the region around δ 0 ppm and three broad peaks from δ −3 to −10 ppm clearly reveal the presence of tetracoordinated boron atoms and closo-o-carborane cage, respectively. The X-ray diffraction study revealed the molecular structures of CB2 and CB2B in the solid state (Figure 2, detailed parameters and selected bond lengths and angles are listed in Tables S1 and S2). These results distinctly demonstrate that the o-carborane cage is appended at the C4 position of the pyridine ring of the btp moiety. In particular, the CB2B structure clearly exhibit a tetracoordinated structure with bidentate chelation by the boron atom. .

Photophysical Properties
The photophysical properties of o-carboranyl compounds, CB1, CB1B, CB2, and CB2B, were investigated by UV-vis absorption and PL experiments (Figure 3). The non-chelated CB1 and CB2 display major absorption bands at λabs = 286 and 328 nm, respectively ( Table 1). The broadness of these absorption bands strongly indicate that they can be assigned to not only a spin-allowed π-π * transition in the aryl groups (ppy or btp), but also ICT transition between the o-carborane and aryl groups. The absorption bands of the chelated CB1B and CB2B, which are assignable to a π-π * transition in the aryl groups, are also similarly exhibited in the high-energy region at λabs = 283 and 314 nm, respectively. Interestingly, CB1B and CB2B also show a dominant low-energy absorption band at λabs = 343 and 391 nm, respectively, which correlate with ICT from o-carborane to either the ppy or btp moiety (see calculated data below).
The fluorescent properties of the o-carboranyl compounds were further examined through PL

Photophysical Properties
The photophysical properties of o-carboranyl compounds, CB1, CB1B, CB2, and CB2B, were investigated by UV-vis absorption and PL experiments (Figure 3). The non-chelated CB1 and CB2 display major absorption bands at λ abs = 286 and 328 nm, respectively ( Table 1). The broadness of these absorption bands strongly indicate that they can be assigned to not only a spin-allowed π-π * transition in the aryl groups (ppy or btp), but also ICT transition between the o-carborane and aryl groups. The absorption bands of the chelated CB1B and CB2B, which are assignable to a π-π * transition in the aryl groups, are also similarly exhibited in the high-energy region at λ abs = 283 and 314 nm, respectively. Interestingly, CB1B and CB2B also show a dominant low-energy absorption band at λ abs = 343 and 391 nm, respectively, which correlate with ICT from o-carborane to either the ppy or btp moiety (see calculated data below).
The fluorescent properties of the o-carboranyl compounds were further examined through PL measurements at various conditions (Figure 3b−d and Table 1). The emission spectra of CB2 and CB2B in toluene (at 298 K) exhibit an intense emission at λ em = 552 and 505 nm, respectively, that tail off to 650 nm. The emission spectrum of CB1 does not show any peak, while that of CB1B show a faint emissive trace from 380 to 550 nm. On the other hand, the emission spectra of all compounds at 77 K are enhanced relative to those at 298 K. In particular, the emission spectra of CB1B shows a dual emissive pattern that can be divided to high-(above 450 nm) and low-energy (below 450 nm) regions, that of CB2B shows an intense emission from 430 to 600 nm, while those of CB1 and CB2 exhibit a single, broad emission band in the low-energy region. According to theoretical data, which will be discussed below, the low-energy emission of all compounds is closely associated with the ICT transition between the o-carborane and aryl groups, while the high-energy emission at 384 and 456 nm for CB1B and CB2B, respectively, closely involves π-π * transition in the BMe 2 -chelated aryl group. Thus, these features strongly indicate that radiative decay due to π-π * transition in the aryl groups and ICT transition associated with the o-carborane unit can be amplified in the rigid molecular state. These results are attributed to the inhibition of structural fluctuation, such as variation in the C−C bond in o-carborane and free rotation of the o-carborane cage [10,32,58]. Indeed, the calculated optimized structures of all compounds at the S 0 and S 1 states distinctly present evidence supporting structural fluctuation. Specifically, the C−C bond length (2.38-2.42 Å) in o-carborane at the S 1 state (the structure that the one side of icosahedron is elongated) becomes significantly longer than that at the S 0 state (≈ 1.73 Å) ( Table 2), consistent with previous studies [10,39,58].
The most interesting feature is that CB2 shows significant emission in solution at both 298 and 77 K, while CB1 shows no emission in solution at 298 K and a weak emissive trace at 77 K. From these emissive characteristics, the Φ em values of CB1 and CB2 in solution at 298 K are estimated to be <1% and 13%, respectively. The difference between the emissive properties of these non-chelated o-carboranyl compounds seem to be strongly correlated with their structure. Our group has already reported that radiative decay resulting from ICT transition in o-carboranes can be efficiently generated by maintaining the planarity of the aryl rings [58]. The optimized S 0 and S 1 structures of CB2 specifically exhibit considerably similar dihedral angle between the pyridine and benzothiophene rings (Ψ calc = 0.8 • for S 0 and 1.2 • for S 1 ), whereas those of CB1 show significantly different Ψ calc (22.2 • for S 0 and 0.3 • for S 1 ) ( Table 2). The experimental dihedral angle for CB2 (Ψ exp = 2.9 • ), determined from the molecular structure at the solid state, is also similar to Ψ calc . These structural features clearly indicate that the planarity of the aryl groups of CB2 can be sufficiently maintained in spite of repeated conversion between the S 0 and excited states by an external energy. Because structural stability efficiently evokes radiative decay due to ICT transition involving o-carborane, CB2 shows a high Φ em , moderate radiative decay constant (k r = 2.2 × 10 7 s −1 ), and low non-radiative decay constant (k nr = 1.5 × 10 8 s −1 ) in solution at 298 K (Table 1) [58]. The measured Φ em of CB1B and CB2B, resulting from radiative decay due to π-π * transition in the aryl groups, are 3% and 6%, respectively, at 298 K.
The PL spectra for the film state (PMMA film doped with 5 wt% o-carboranyl compound) at 298 K exhibit emissive patterns similar to those in solution at 77 K (Figure 3d). The high-energy emissions of CB1B and CB2B corresponding to π-π * transition in the chelated aryl groups are still distinctively observed in the region centered at 424 and 473 nm, respectively, and extend to the low-energy emission region (ca. 600 nm) assigned to the ICT transition involving o-carborane. The estimated Φ em of CB1B and CB2B are 8% and 9%, respectively, which are significantly enhanced compared with those in solution. These results arise from the efficient radiative mechanism induced by inhibition of structural fluctuation, such as elongation of the C−C bond and free rotation of the o-carborane cage, in the rigid solid state. Indeed, the k nr values of both CB1B and CB2B in the film state increase by more than twice (1.6 × 10 7 and 1.1 × 10 8 s −1 , respectively) as those in solution at 298 K.
Interestingly, the PL spectrum of CB2 in the film state (Figure 3d inset) shows a Φ em nearly twice enhanced (25%) as that in solution at 298 K; however, the Φ em values of CB1 in the solution and film states are similar. The emission band mainly involves ICT transition between o-carborane and aryl groups (see the calculated data below). The rigidity of both compounds can inhibit structural fluctuation, although the considerable difference between their emissive properties strongly reveal that maintaining the planarity of the aryl rings promotes radiative decay [58]. Consequently, the k r of CB2 in the film state (1.04 × 10 9 s −1 ) is 50 times higher than that in solution (2.20 × 10 7 s −1 ) at 298 K ( Table 1).      S0 S1 S0 S1 S0 S1 S0 S1 Dihedral angle determined from the X-ray structure ( Figure 2).

Theoretical Calculations and Orbital Analysis
To elucidate the nature of the electronic transitions in the o-carboranyl compounds, TD-DFT optimization of the S0 and S1 structures of CB1, CB2, CB1B, and CB2B were carried out using the B3LYP functional (Figures 4 and S11, Tables 3 and S3). The calculated geometries were optimized from the X-ray crystal structures of CB2 and CB2B. To include the effects of the toluene solvent [60,61], a conductor-like polarizable continuum model was also used. The computational data for the S0 state show that the lowest-energy electronic transition for the non-chelated compounds (CB1 and CB2) is the highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) transition ( Figure S11 and Table S3). The HOMOs of both compounds are entirely localized on the aryl moiety (>99%, Tables S5 and S9), whereas the orbital contribution of the o-carborane unit to the LUMOs is slightly higher at >17%. These results indicate that the lowest energy absorption of CB1 and CB2 can be mainly attributed to π-π * transition in the aryl moieties, with a minor contribution from ICT transition between the o-carborane and aryl groups. On the other hand, the calculated data for the optimized S0 geometries of the chelated compounds (CB1B and CB2B) show that the lowest energy absorption mainly involves the two major transitions (fcalc > 0.12, Figure S11 and Table S3) from the HOMO to LUMO and LUMO + 1. The HOMO and LUMO + 1 levels of both compounds are predominantly localized over the BMe2-chelated aryl rings (>96%, Tables S7 and S11 in Supplementary Materials), whereas the LUMO is distributed over not only the aryl rings (∼84%), but also the o-carborane moiety (∼16%). These results suggest that the absorption spectra of CB1B and CB2B can be largely attributed to π-π * transition on the chelated aryl ring, with substantial contribution from the ICT transition associated with the o-carborane moiety, as in the non-chelated compounds. In addition, all calculated data for the optimized S0 structures match the experimental Dihedral angle determined from the X-ray structure ( Figure 2).

Theoretical Calculations and Orbital Analysis
To elucidate the nature of the electronic transitions in the o-carboranyl compounds, TD-DFT optimization of the S 0 and S 1 structures of CB1, CB2, CB1B, and CB2B were carried out using the B3LYP functional ( Figure 4 and Figure S11, Table 3 and Table S3). The calculated geometries were optimized from the X-ray crystal structures of CB2 and CB2B. To include the effects of the toluene solvent [60,61], a conductor-like polarizable continuum model was also used. The computational data for the S 0 state show that the lowest-energy electronic transition for the non-chelated compounds (CB1 and CB2) is the highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) transition ( Figure S11 and Table S3). The HOMOs of both compounds are entirely localized on the aryl moiety (>99% , Tables S5 and S9), whereas the orbital contribution of the o-carborane unit to the LUMOs is slightly higher at >17%. These results indicate that the lowest energy absorption of CB1 and CB2 can be mainly attributed to π-π * transition in the aryl moieties, with a minor contribution from ICT transition between the o-carborane and aryl groups. On the other hand, the calculated data for the optimized S 0 geometries of the chelated compounds (CB1B and CB2B) show that the lowest energy absorption mainly involves the two major transitions (f calc > 0.12, Figure S11 and Table S3) from the HOMO to LUMO and LUMO + 1. The HOMO and LUMO + 1 levels of both compounds are predominantly localized over the BMe 2 -chelated aryl rings (>96%, Tables S7 and S11 in Supplementary Materials), whereas the LUMO is distributed over not only the aryl rings (∼84%), but also the o-carborane moiety (∼16%). These results suggest that the absorption spectra of CB1B and CB2B can be largely attributed to π-π * transition on the chelated aryl ring, with substantial contribution from the ICT transition associated with the o-carborane moiety, as in the non-chelated compounds. In addition, all calculated data for the optimized S 0 structures match the experimental UV-vis absorption spectra well.
Based on the computational data for the S 1 states of CB1 and CB2, the major transition for the lowest-energy emission is the HOMO → LUMO transition (Figure 4 and Table 3). While the LUMOs of both compounds are significantly localized on the entire o-carborane moiety (∼85% , Tables S5 and  S9), the HOMOs dominantly occupy the aryl groups (>99%). These results strongly suggest that the experimentally observed emission in the rigid states, namely the solution at 77 K and film (solid) state, dominantly originates from ICT between the o-carborane and aryl moieties. On the other hand, the major low-energy emissions of CB1B and CB2B are bipartitely assigned to the HOMO → LUMO and HOMO → LUMO+1 transitions ( Figure 3 and Table 3). Although both the HOMO and LUMO+1 are mostly focused on the chelated aryl moieties (>90%, Tables S7 and S11), the LUMO has a significant contribution of around 77% from the o-carborane moiety. These results strongly suggest that the intense emissions in the high-energy region centered at ca. 380 nm for CB1B and ca. 450 nm for CB2B originate from π-π * transitions in the chelated aryl groups. Additionally, the tailed emission traces in the low-energy region below 500 nm are clearly attributed to the ICT transition from the o-carborane unit to the chelated aryl group. Consequently, all electronic transitions occurring in each o-carboranyl compound were precisely analyzed through theoretical calculation.    1 Singlet energies for the vertical transition calculated using the optimized S1 geometries.

Conclusions
The ppy-and btp-based o-carboranyl compounds (CB1 and CB2) and their BMe 2 -C ∧ N chelated compounds (CB1B and CB2B) were synthesized and fully characterized. The solid-state structures of CB2 and CB2B, analyzed by single-crystal X-ray diffraction, clearly exhibited the o-carborane cage substituent at the C4 position for the pyridine ring and tetracoordinated dimethylboryl center of CB2B. CB1B and CB2B in the solution and film states demonstrated strong emission centered at ca. 450 and 500 nm, respectively, originating from π-π * transition in the aryl group; furthermore, the tailing off to 650 nm is attributed to ICT transition between the o-carborane and aryl groups. While CB1 exhibited faint emissions in toluene solution at 298 K and the film state, CB2 showed intense emissions in both states, which are assignable to radiative decay due to the ICT transition. On the other hand, the dihedral angle between the aromatic rings of CB1 and CB2 in the optimized S 0 and S 1 structures clearly revealed that the planarity of the btp groups of CB2 could be maintained, while the ppy groups of CB1 freely rotated from the ground to the excited states. These results distinctly suggest that the planarity of aryl groups appended to o-carborane have a decisive effect on the efficiency of the radiative decay due to the ICT transition.

Conflicts of Interest:
The authors declare no conflict of interest.