Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores

The efficiency of intramolecular charge transfer (ICT)-based emission on π-aromatic-group-appended closo-ortho-carboranyl luminophores is known to be affected by structural fluctuations and molecular geometry, but investigation of this relationship has been in progress to date. In this study, four naphthyl-based bis-o-carboranyl compounds, in which hydrogen (15CH and 26CH) or trimethysilyl groups (15CS and 26CS) were appended at the o-carborane cage, were synthesized and fully characterized. All the compounds barely displayed an emissive trace in solution at 298 K; however, 15CH and 26CH distinctly exhibited a dual emissive pattern in rigid states (in solution at 77 K and in films), attributed to locally excited (LE) and ICT-based emission, while 15CS and 26CS showed strong ICT-based greenish emission. Intriguingly, the molecular structures of the four compounds, analyzed by single X-ray crystallography, showed that the C-C bond axis of the o-carborane cage in the trimethysilyl group-appended compounds 15CS and 26CS were more orthogonal to the plane of the appended naphthyl group than those in 15CH and 26CH. These features indicate that 15CS and 26CS present an efficient ICT transition based on strong exo-π-interaction, resulting in a higher quantum efficiency (Φem) for ICT-based radiative decay than those of 15CH and 26CH. Moreover, the 26CS structure revealed most orthogonal geometry, resulting in the highest Φem and lowest knr values for the ICT-based emission. Consequently, all the findings verified that efficient ICT-based radiative decay of aromatic group-appended o-carboranyl luminophores could be achieved by the formation of a specific geometry between the o-carborane cage and the aromatic plane.


Introduction
Naphthalene-based organic compounds have been widely used as components of optoelectronic materials in organic light-emitting diodes [1][2][3], photovoltaic cells [4][5][6][7][8][9][10][11], and organic thin-film transistors [12][13][14][15][16] because of their outstanding electrochemical and chemical stability and outstanding photophysical characteristics. These unique properties are mostly attributed to the electronic-abundant nature and structural rigidity originating from the dimeric fused-benzene ring formation [17]. In particular, these optical features can be manipulated by the electronic characteristics of the introduced functional groups, leading to the extensive utilization of naphthyl derivatives in various industrial fields [18][19][20][21][22][23][24]. Among the specific functional units showing consistent electronic tendency and chemical stability, the ortho-carborane of closo-type structure, a well-known icosahedral boron cluster recognized as a three-dimensional subspecies of the benzene ring [25][26][27][28], has recently become a topic of interest for inducing the revelation of intriguing photophysical characteristics [29][30][31]. The o-carborane composed of ten boron atoms intrinsically has a strong electron-withdrawing nature via C-substituents and the high polarizability of its σ-aromatic

General Considerations
All operations were performed under an inert nitrogen atmosphere using standard

Synthesis of 1,5-Diethynylnaphthalene, 15AH
THF (14 mL) and NEt 3 (10 mL) were added via cannula to a mixture of 1,5dibromonaphthalene (0.60 g, 2.1 mmol), CuI (33 mg, 0.17 mmol), and Pd(PPh 3 ) 2 Cl 2 (0.12 g, 0.17 mmol) at ambient temperature. After stirring for 10 min, ethynyltrimethylsilane (0.64 mL, 4.6 mmol) was slowly added. The reaction mixture was then heated under reflux at 80 • C for 24 h prior to cooling to ambient temperature and evaporation of the solvent to afford a dark brown residue. Filtration through celite was employed to remove the metal salts from the reaction mixtures. The products were then used in situ for the next step, without characterization. More specifically, a round-bottom flask was charged with a mixture of the crude product (1,5-bis((trimethylsilyl)ethynyl)naphthalene, 0.58 g, 1.8 mmol) and K 2 CO 3 (1.0 g, 7.2 mmol). Methanol (15 mL) was then added, and the mixture was stirred at 25 • C for 2 h. After this time, the reaction mixture was quenched by the addition of brine, and the aqueous phase was subsequently extracted with ethyl acetate (3 × 30 mL). The combined organic layer was dried over anhydrous MgSO 4 , filtered, and the volatiles were removed under vacuum to obtain a brown residue. Purification by column chromatography on silica (eluent: n-hexane) yielded 15AH. Yield = 56% (0.21 g). 1

Synthesis of 15CH
To a toluene solution (30 mL) of B 10 H 14 (1.4 g, 11.7 mmol) and 15AH (0.69 g, 3.9 mmol), an excess of N,N-dimethylaniline (2.5 mL, 20 mmol) was added at ambient temperature. After heating to reflux at 110 • C, the reaction mixture was stirred for 18 h. After this time, the volatiles were removed under vacuum and the resulting solid was dissolved in toluene. The solution was purified by column chromatography on basic alumina (eluent: toluene) to produce 15CH as a white solid. Yield = 53% (0.85 g). 1

Synthesis of 15CS
An n-hexane solution of n-BuLi (1.6 M, 1.6 mL, 2.5 mmol) was added dropwise to a solution of 15CH (0.41 g, 1.0 mmol) in THF (10 mL) at 0 • C. After stirring for 1 h, the reaction was slowly allowed to warm to ambient temperature, and TMSCl (0.63 mL, 5.0 mmol) was added dropwise to the mixture. The reaction mixture was then stirred for 2 h at 25 • C. After quenching with saturated aqueous NH 4 Cl (15 mL), the mixture was extracted with ethyl acetate (3 × 15 mL). The combined organic layer was then dried over anhydrous MgSO 4 , filtered, and the solvent was removed under vacuum. The product was purified by recrystallization from DCM to obtain 15CS as a white solid. Yield = 61% (0.34g). 1

UV/Vis Absorption and Photoluminescence (PL) Measurements
Solution-phase UV/Vis absorption and PL measurements for each o-carborane compound were performed in degassed THF using a 1 cm quartz cuvette (50 µM) at 298 K. PL measurements were also carried out in THF at 77 K and in the film state (5 wt% doped in PMMA on a 15 × 15 mm quartz plate (thickness = 1 mm)). The UV/vis absorption and PL spectra were recorded on Jasco V-530 (Jasco, Easton, MD, USA) and FluoroMax-4P spectrophotometers (HORIBA, Edison, NJ, USA), respectively. The absolute PL quantum yields (Φ em ) of the film samples were obtained using an absolute PL quantum yield spectrophotometer (FM-SPHERE, 3.2-inch internal integrating sphere on FluoroMax-4P, HORIBA, Edison, NJ, US) at 298 K. The fluorescence decay lifetimes of the films were measured at 298 K using a time-correlated single-photon counting (TCSPC) spectrometer (FLS920, Edinburgh Instruments, Livingston, UK) at the Central Laboratory of Kangwon National University. The TCSPC spectrometer was equipped with a pulsed semiconductor diode laser excitation source (EPL, 375 nm) and microchannel plate photomultiplier tube (MCP-PMT, 200-850 nm) detector.

X-ray Crystallography
Single-X-ray quality crystals of 15CH, 26CH, 15CS, and 26CS were grown from a DCM/n-hexane mixture. Single crystals were coated with paratone oil and mounted on a glass capillary. Crystallographic measurements were performed using a Bruker D8QUEST diffractometer (Bruker Coorperation, Billerica, MA, US), with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) and CCD area detector. The structures of 15CH, 26CH, 15CS, and 26CS were assessed using direct methods, and all nonhydrogen atoms were subjected to anisotropic refinement with a full-matrix least-squares method on F 2 using the SHELXTL/PC software package (released and presented at the Software Fayre, XXII IUCr Congress in Madrid, Spain, 2011). The X-ray crystallographic data are available in CIF format (CCDC 2184401-2184404 for 15CH, 26CH, 15CS, and 26CS), provided free of charge by the Cambridge Crystallographic Data Centre. The hydrogen atoms were placed at their geometrically calculated positions and refined using a riding model on the corresponding carbon atoms with isotropic thermal parameters. Detailed crystallographic data are provided in Tables S1 and S2 in the Supplementary Materials.

Computational Calculation Studies
The optimized geometries for the ground (S 0 ) and first excited (S 1 ) states of all ocarboranyl compounds in THF were obtained at the B3LYP/6-31G(d,p) [59] level of theory. The vertical excitation energies at the optimized S 0 geometries and the optimized geometries of the S 1 states were calculated using time-dependent density functional theory (TD-DFT) [60] at the same level of theory. Solvent effects were evaluated using the conductor-like polarizable continuum model (CPCM) based on the integral equation formalism of the polarizable continuum model (IEFPCM), with THF as the solvent [61]. All geometry optimizations were performed using the Gaussian 16 program [62]. The percent contribution of a group in a molecule to each molecular orbital was calculated using the GaussSum 3.0 program [63]. The most stable geometries were determined by constructing one-dimensional potential energy surfaces as a function of each dihedral angle (Ψ, Table S11) by rotating the carboranes of 15CH and 26CH between approximately 0 • and 180 • at intervals of 15 • to yield 13 initial conformations for each compound. Conformations that exhibited physically impossible atomic overlaps were excluded from further geometric optimization. The dihedral angle was fixed, whereas other geometric variables were fully relaxed for geometry optimization and energy calculation of the resulting initial conformations using the Gaussian 16 software program [62]. The calculation method in this study does not provide accurate information for electronic transition of the o-carboranyl compounds and only gives an indirect hint to elucidate their transitions corresponding to specific photophysical characteristics.
The solid-state molecular structures of the four o-carboranyl compounds were also determined by X-ray single-crystallography (inset figures in Figure 1; detailed parameters, including selected bond lengths and angles, are provided in Tables S1 and S2 in the Supplementary Materials). The crystal structures of the four compounds revealed planar naphthyl groups and icosahedral formation of o-carborane cages. Interestingly, bulky trimethylsilyl group-substituted compounds (15CS and 26CS) showed a significantly perpendicular orientation between the naphthalene plane and the bonding axis of the C-C bond in the o-carborane cage (Ψ = 78 • for 15CS and 82 • for 26CS, Table 1), whereas the C-C bond axes of 15CH and 26CH were almost parallel to the naphthalene plane, as evidenced by the Ψ values (5.3 • for 15CH and 1.8 • for 26CH). Such a different geometry between silyl-and H-substituted compounds is a decisive factor in controlling their photophysical characteristics (vide infra).

Analysis of Photophysical Properties with Theoretical Calculation
The absorption and emissive properties of the naphthyl-bis-o-carboranyl compounds 15CH, 26CH, 15CS, and 26CS were investigated using UV/Vis absorption and PL spectroscopy, respectively ( Figure 2 and Table 2). All compounds in THF exhibited major absorption bands centered at λ abs = 278-301 nm, which were attributed to the vibronic structures undergoing spin-allowed π-π* transitions of the naphthalene moiety. These bands were also observed in the absorption spectrum of the mother-scaffold compound (naphthalene, λ abs = 276 and 285 nm, Figure S11). However, the lowest absorption bands for the o-carboranyl compounds were relatively red-shifted (λ abs = 322-330 nm) compared to the absorption band (λ abs = 313 nm) of naphthalene because of the lowest unoccupied molecular orbital (LUMO) stabilization effect of o-carborane [67]. Furthermore, the lowenergy absorption of these compounds includes tailing of the absorption bands at 350 nm. These features indicate that the bands can be attributed to the typical ICT transitions in the o-carborane cages. Time-dependent density functional theory (TD-DFT) calculations for the S 0 state of the o-carboranyl compounds clearly confirmed these characteristics (vide infra). The origin of the electronic transitions of the four naphthyl-o-carboranyl compounds was determined using TD-DFT calculations [60] (Figure 3). Each calculated structure was based on the X-ray crystal structure (Figure 1). The calculation results for the S0 state demonstrated that the major lowest-energy transitions were dominantly assignable to the highest occupied molecular orbital (HOMO) → LUMO (Figure 3). Although both the HOMO and LUMO of each naphthyl o-carborane compound were mainly localized on the naphthyl group (HOMO > 90% and LUMO > 84%, Tables S4, S6, S8, and S10), the LUMO   1 50 µM in THF. 2 50 µM, observed at 298 K. 3 Measured in the film state (5 wt% doped in PMMA). 4 50 µM in THF/water (1/9, v/v). 5 Absolute PL quantum yield. 6 k r = Φ em /τ obs . 7 k nr = k r (1/ Φ em −1). 8 Not observed due to weak emission.
The origin of the electronic transitions of the four naphthyl-o-carboranyl compounds was determined using TD-DFT calculations [60] (Figure 3). Each calculated structure was based on the X-ray crystal structure ( Figure 1). The calculation results for the S 0 state demonstrated that the major lowest-energy transitions were dominantly assignable to the highest occupied molecular orbital (HOMO) → LUMO (Figure 3). Although both the HOMO and LUMO of each naphthyl o-carborane compound were mainly localized on the naphthyl group (HOMO > 90% and LUMO > 84%, Tables S4, S6, S8, and S10), the LUMO levels were further distributed over the o-carborane cages (>12%). These results suggest that the lowest-energy electronic transition for the four o-carboranyl compounds originates from the naphthyl-centered π-π* LE transition with substantial ICT transitions between the naphthyl moiety and o-carboranyl cage.  The emissive properties of the four o-carboranyl compounds were examined using PL measurements under various conditions ( Figure 2 and Table 2). Interestingly, the PL spectra of all the compounds in the THF solution at 298 K exhibited non-emissive characteristics. On the other hand, these compounds showed intense emissive patterns in rigid states (THF at 77 K and film), and the hydrogen-substituted o-carboranyl compounds 15CH and 26CH exhibited specific dual-emission patterns centered at λem = ~345 and ~480 nm, and trimethylsilyl-compounds 15CS and 26CS exhibited strong emission only in the region of λem = 470-500 nm. As a result, greenish emission was observed in the film states of all compounds under a hand-held UV lamp (Figure 2, inset figures). In particular, the emission of naphthalene was centered at λem = 322 and 336 nm ( Figure S11), which proves that the high-energy emissions centered at λem = ~345 nm in the rigid states for 15CH and 26CH can be attributed to the LE transition of the naphthalene moiety. Moreover, the low-  orbitals for 15CH, 26CH, 15CS, and 26CS at their ground state (S 0 ) and the first excited singlet state (S 1 ) with their relative energies from DFT calculation (isovalue 0.04). The transition energy (in nm) was calculated using the TD-B3LYP method with 6-31G(d,p) basis sets.
The emissive properties of the four o-carboranyl compounds were examined using PL measurements under various conditions ( Figure 2 and Table 2). Interestingly, the PL spectra of all the compounds in the THF solution at 298 K exhibited non-emissive characteristics. On the other hand, these compounds showed intense emissive patterns in rigid states (THF at 77 K and film), and the hydrogen-substituted o-carboranyl compounds 15CH and 26CH exhibited specific dual-emission patterns centered at λ em =~345 and~480 nm, and trimethylsilyl-compounds 15CS and 26CS exhibited strong emission only in the region of λ em = 470-500 nm. As a result, greenish emission was observed in the film states of all compounds under a hand-held UV lamp (Figure 2, inset figures). In particular, the emission of naphthalene was centered at λ em = 322 and 336 nm ( Figure S11), which proves that the high-energy emissions centered at λ em =~345 nm in the rigid states for 15CH and 26CH can be attributed to the LE transition of the naphthalene moiety. Moreover, the low-energy emissions of all the o-carboranyl compounds in the region over 450 nm were severely red-shifted compared with that of naphthalene, indicating that these corresponded to ICT involving o-carborane (vide infra). Such differences in the emission features between THF at 298 K and the rigid states typically originate from restricted structural fluctuations in the rigid state, and the elongation of the C-C bond in the o-carborane cage during the excitation process is known to diminish the efficiency of the ICT-based radiative decay mechanism [31,52,53,[68][69][70]. Indeed, the calculated lengths of the C-C bonds for the ocarborane cages of all the compounds in the S 1 state were significantly longer (ca. 2.40 Å, Table 1) than those in the S 0 state (1.6-1.7 Å) and the experimentally measured values based on the solid-state crystal structures (ca. 1.7 Å).
The calculation results of the four o-carboranyl compounds in the S 1 -optimized structures revealed that the major transitions associated with low-energy emission involve a LUMO → HOMO transition (Figure 3). The LUMO and HOMO of the compounds were predominantly localized on the o-carborane cage (>76%, Tables S4, S6, S8, and S10) and the naphthyl group (>81%). These results strongly suggest that the low-energy emission around 450-550 nm observed in the rigid states is primarily manifested from a radiative decay process based on ICT between the o-carborane and naphthyl moieties.
To further investigate the origin of the greenish emission in the solid (film) state for these compounds, PL measurements were performed in a THF/distilled-water mixture (50 µM) ( Figure 4 and Table 2). Intriguingly, the intensity of the low-energy emission for trimethylsilyl-substituted compounds 15CS and 26CS was dramatically enhanced in the region from 450 to 550 nm upon increasing the water fraction (f w ). Consequently, the most aggregated state in THF/water (f w = 90%) exhibited intense greenish emission (λ em = 501 nm for 15CS and 490 nm for 26CS), which was significantly similar to that observed in the film state. These features are characteristic of aggregation-induced emission (AIE) phenomena. The structural rigidity could originate from the aggregation effect, resulting in an increase in the efficiency of the ICT-based radiative decay. Unfortunately, such an AIE phenomenon was not observed in the PL spectra of 15CH and 26CH THF/water mixtures.
Molecules 2022, 26, x FOR PEER REVIEW 10 of AIE phenomenon was not observed in the PL spectra of 15CH and 26CH THF/water mi tures.

Quantitative Comparison for ICT-Based Radiative Decay Efficiency
The absolute quantum efficiency (Φem) and decay lifetime (τobs) of each o-carboran compound in the film state were investigated to compare the efficiency of the radiativ decay process quantitatively. Intriguingly, the Φem values of 15CS and 26CS were mode ately estimated at 10.5% and 18.4%, respectively (Table 2), while those for 15CH and 26C could not be measured owing to their inefficient radiative decay processes (<1%). Suc differences in radiative efficiencies between the two series of naphthyl o-carboranyl com pounds correlate with their structural geometry; in particular, the formation of the o-ca borane cage appended to the plane of the aromatic group [40,41,[47][48][49]. In general, th ICT transition on o-carboranyl luminophores can be intensified by increased delocaliz

Quantitative Comparison for ICT-Based Radiative Decay Efficiency
The absolute quantum efficiency (Φ em ) and decay lifetime (τ obs ) of each o-carboranyl compound in the film state were investigated to compare the efficiency of the radiative decay process quantitatively. Intriguingly, the Φ em values of 15CS and 26CS were moderately estimated at 10.5% and 18.4%, respectively (Table 2), while those for 15CH and 26CH could not be measured owing to their inefficient radiative decay processes (<1%). Such differences in radiative efficiencies between the two series of naphthyl o-carboranyl compounds correlate with their structural geometry; in particular, the formation of the o-carborane cage appended to the plane of the aromatic group [40,41,[47][48][49]. In general, the ICT transition on o-carboranyl luminophores can be intensified by increased delocalization through exo-π-interactions between the tangential p-orbital (σ*-character) on the substituted carbon atom of the o-carborane cage and the π-delocalized system of the appended aromatic group [40,41,71]. Consequently, the C-C bond axis of o-carborane becomes perpendicular to the aromatic plane to maintain the exo-π-interaction. Indeed, the calculated dihedral angles (Ψ) between the C-C bond axis and naphthyl plane in the optimized structures of the four compounds were orthogonally arranged according to the excited states (Ψ S0 = 30-85 • → Ψ S1 = 82-90 • , Table 1) for the σ*-π* conjugation between the carboranyl cages and naphthyl groups. However, the experimental Ψ values of 15CH (5 • , Table 1) and 26CH (2 • ) from solid-state crystal structures indicated that the geometric relationship between the C-C bond axis and naphthyl planes was planar, whereas those of 15CS (78 • ) and 26CS (82 • ) were orthogonally maintained; the bulkiness of trimethylsilyl groups substituted to the o-carborane cages led to orthogonality. These structural features were the main reason why the radiative efficiencies of 15CH and 26CH in the solid state were lower than those of 15CS and 26CS. Furthermore, the restriction of exo-π-interaction for 15CH and 26CH generates the isolation of excitons on the naphthyl group [40], resulting in the hindrance of the ICT transition and the natural concentration of excitons to favor the LE transition from the naphthyl group. Consequently, 15CH and 26CH demonstrated strong LE-based emissions in the high-energy region ( Figure 2 and Table 2), differing from those of 15CS and 26CS.
The τ obs values for 15CS and 26CS in the film state were estimated to 3.7 and 6.4 ns, respectively (Table 2 and Figure S12), indicating distinctly fluorescent characteristics. A comparison of the radiative (k r ; Table 2) and nonradiative (k nr ) decay constants of both compounds, which were calculated by the Φ em and τ obs values, revealed a slight difference in the efficiency of the ICT-based radiative process between the two o-carboranyl compounds. The k r values of both compounds were the same (2.9 × 10 7 s −1 , Table 2), whereas the k nr value (2.4 × 10 8 s −1 ) of 15CS was approximately two times higher than that (1.3 × 10 8 s −1 ) of 26CS. These findings imply that the relatively low delocalization between the o-carborane cage and naphthyl group of 15CS in the solid state leads to inefficiency of the ICT-based radiative decay process compared with 26CS. Indeed, the experimental Ψ value (82 • , Table 1) of 26CS verified that this geometry was relatively closer to the orthogonal structure than that (78 • ) of 15CS. Consequently, all the observed results verified that geometric formation around the o-carborane cage plays a significant role in controlling the efficiency of the ICT-based radiative decay mechanism on the aromatic group-appended o-carboranyl luminophores.

Theoretical Calculations of the Effect of Geometry around the O-carborane Cages
To elucidate the effect of the structural geometry around the o-carborane cages on the electronic transitions, the alternation of low-energy transitions assignable to ICT corresponding to the o-carborane (LUMO to HOMO in the S 1 -states, Figure 3) was calculated (B3LYP/6-31G(d,p) basis set) for 15CH and 26CH as the change in Ψ (the dihedral angle between the C-C bond axis of the o-carborane and naphthyl plane) from 0 • (=360 • ) to 180 • ( Figure 5 and Table S11). The orbital occupation of the LUMOs on the o-carborane cage for both 15CH and 26CH was maximized (> 95%) when the C-C axis was planar to the naphthyl plane (Ψ = 0 • and 180 • ), whereas the localization of the HOMOs on the naphthyl groups did not change significantly (85-89% for 15CH and 81-83% for 26CH). In addition, the occupation of the LUMOs for both compounds gradually decreased as Ψ values had orthogonality (90 • ). These results indicate that the π-conjugated delocalization between the o-carborane and naphthyl group became blocked in accordance with being close to Ψ = 0 • and 180 • , and that the orbital distribution of each LUMO and HOMO was thoroughly isolated in the planar geometry, resulting in the inhibition of the ICT transition between the o-carborane and naphthyl groups. The oscillator strengths (f calc ) computed for each transition of both compounds were maximized at Ψ = 90 • and equaled zero at = 0 • and 180 • . These features strongly suggest that the orthogonality of the C-C bond axis to the plane of the appended naphthyl group significantly affects the ICT transitions of the o-carborane cage. The experimental and theoretical findings demonstrate that the structural geometry around the o-carborane cage plays a decisive role in enhancing the efficiency of the ICT-based radiative decay mechanism in the D-A-type o-carboranyl luminophores.
( Figure 5 and Table S11). The orbital occupation of the LUMOs on the o-carborane cage for both 15CH and 26CH was maximized (> 95%) when the C-C axis was planar to the naphthyl plane (Ψ = 0° and 180°), whereas the localization of the HOMOs on the naphthyl groups did not change significantly (85-89% for 15CH and 81-83% for 26CH). In addition, the occupation of the LUMOs for both compounds gradually decreased as Ψ values had orthogonality (90°). These results indicate that the π-conjugated delocalization between the o-carborane and naphthyl group became blocked in accordance with being close to Ψ = 0° and 180°, and that the orbital distribution of each LUMO and HOMO was thoroughly isolated in the planar geometry, resulting in the inhibition of the ICT transition between the o-carborane and naphthyl groups. The oscillator strengths (fcalc) computed for each transition of both compounds were maximized at Ψ = 90° and equaled zero at = 0° and 180°. These features strongly suggest that the orthogonality of the C-C bond axis to the plane of the appended naphthyl group significantly affects the ICT transitions of the ocarborane cage. The experimental and theoretical findings demonstrate that the structural geometry around the o-carborane cage plays a decisive role in enhancing the efficiency of the ICT-based radiative decay mechanism in the D-A-type o-carboranyl luminophores.

Conclusions
Herein, we prepared hydrogen (15CH and 26CH) or trimethyl silyl groups (15CS and 26CS) substituted with four naphthyl-based bis-o-carboranyl compounds and investigated the relationship between their molecular geometries and photophysical characteristics. Although these o-carboranyl compounds barely exhibited emissive patterns in solution at 298 K, intense greenish emission assignable to each LE-and ICT-based emission (for 15CH and 26CH) or ICT-based emission only (for 15CS and 26CS) were observed in rigid states (in solution at 77 K and in the film state). Intriguingly, the solid-state molecular structures of the four compounds revealed that the C-C bond axis of the o-carborane cage in the trimethysilyl group-substituted o-carboranyl compounds 15CS and 26CS was more orthogonal to the plane of the appended naphthyl group than those in the hydrogen-substituted o-carboranyl compounds 15CH and 26CH. These findings verify the efficient σ*-π* conjugation between the o-carborane and naphthyl groups in 15CS and 26CS via strong exo-π interaction. Such features result in a higher Φ em for the emission of solid 15CS and 26CS than those of 15CH and 26CH. Furthermore, the most orthogonal geometry of 26CS exhibits the highest Φ em and lowest k nr values for the ICT-based radiative decay process. Consequently, the results indicate that geometric formation around the o-carborane cage plays a decisive role in the efficiency of the ICT-based radiative mechanism in o-carboranyl organic luminophores.
Author Contributions: S.Y., C.H.R., D.K.Y. and Y.J.S. synthesized the compounds, analyzed the data, and wrote the paper. M.K. conducted the computational study and analyzed the data. K.M.L. analyzed all the data and wrote the paper. All authors have read and agreed to the published version of the manuscript.