Crucial Factors Regulating Intramolecular Charge-Transfer-Based Radiative Efficiency in ortho-Carboranyl Luminophores: Planarity between Substituted Biphenyl Rings

Abstract

o-Carboranyl compounds contain specific geometries, ranging from planar to orthogonally distorted biphenyl rings. Herein, 13 o-carboranyl compounds, 1HF–13PP, were synthesized and fully characterized to determine the impact of structural formation of the aromatic group appended with the o-carborane to estimate the efficiency of their radiative decay process. All the compounds exhibited significant intramolecular charge transfer (ICT)-based emission in the crystalline state at 298 K. Remarkably, increasing the distorted dihedral angles between biphenyl rings gradually decreased the emission efficiencies. Furthermore, their radiative decay constants decreased linearly with increasing dihedral angles, which demonstrated the inversely proportional relationship between these two factors. These findings distinctly suggest that the planar or distorted geometry of substituted aryl groups can strongly affect the efficiency of the ICT-based radiative process in o-carboranyl luminophores.

Introduction

Unique photophysical properties¹ of organic luminophores bearing ortho-closo-carborane (C₂B₁₀H₁₀), a well-known icosahedral boron cluster viewed as a three-dimensional variant of benzene,^2,3 have recently provided insights for applying the optoelectronic materials in the field of organic light-emitting diodes,⁴⁻⁶ organic thin-film transistors,^7,8 and photovoltaic cells.^9,10 In particular, investigations for enhancing their emissive characteristics derived from intramolecular charge transfer (ICT) transition via a strong electron-withdrawing inductive effect on C-substituents and high polarizability of their σ-aromatic framework have consistently proceeded in a decade.¹¹⁻³⁵ The crucial factors that severely affect the efficiency of the ICT-based radiative mechanism are mainly divided into two categories: (1) structural fluctuations around the o-carborane cage, such as the rotation of the cage²⁶⁻²⁸ and elongation of the C–C bond into the cage²⁹⁻³³ due to its electron-withdrawing character, and (2) geometrical characteristics of the aromatic group substituted in the cage, such as the orthogonality between the C–C bond and substituted aryl plane³⁴ and the planarity of the aryl rings.³⁵⁻³⁷ Most factors, excluding the impact of planarity, have been significantly examined by an empirical comparison between the structural isomers of various o-carboranyl compounds and their theoretical analysis based on time-dependent density functional theory (TD-DFT) calculations.³⁸ Our group has been focusing on the impact of molecular geometry, particularly the planarity of the substituted aryl rings, on ICT transition in o-carboranyl luminophores. We recently determined that this impact could be verified experimentally. A comparison of the photophysical properties of o-carboranyl biphenyl and fluorene compounds³⁵ or perfectly planar and distorted para-terphenyl compounds³⁶ revealed that radiative ICT transitions depended on the planarity of the substituted phenyl rings. Several studies have investigated the critical role of such geometrical arrangements in controlling the intrinsic electronic characteristics of o-carboranyl compounds. However, quantitative and in-depth analyses of the relationship between the photophysical properties and structural geometry of the luminescent o-carboranes bearing various aromatic groups are still rare.

Thus, 13 species of biphenyl-based o-carboranes (1HF–13PP, Chart 1), possessing different distortion angles between the biphenyl rings, were strategically designed and prepared to ascertain the relationship between the geometrical arrangement and the ICT-based radiative decay process of the o-carboranyl compounds and further establish the specific molecular geometry, which could maximize their radiative efficiency. The quantitative comparison of their ICT-based radiative decay efficiency in fixed geometry (crystalline solid structure) distinctly provided the structural key factor to enhance the luminescent behaviors. To the best of our knowledge, this is the first study where such a relationship is determined quantitatively. The detailed synthetic procedures, characterization, photophysical property analysis, and theoretical calculation results have been described further.

Chart 1. Molecular structures of biphenyl-based o-carboranyl compounds (1HF–13PP) in the present study.

Results and Discussion

Synthesis and Characterization

Detailed synthetic procedures for biphenyl-based o-carboranyl compounds, 1HF–13PP, possessing structurally different distortion angles between the biphenyl rings, are illustrated in Scheme 1 of the Supporting Information. Each acetylene precursor for o-carboranyl compounds was prepared by a palladium-catalyzed reaction, in other words, by Sonogashira coupling between phenylacetylene and either of the corresponding bromo compounds (1HFA, 2HTA, 4HNA, 5HHA, 7HDA, and 8HPA) or Suzuki–Miyaura coupling using boronic acid derivatives (6HCA and 9HMA–13PPA). The cage-forming reaction³⁹ of decaborane (B₁₀H₁₄) with each acetylene precursor afforded o-carborane compounds 1HF, 2HT, and 4HN–13PP in relatively moderate yields (37–70%, Scheme S1). In particular, subsequent treatments using 4HN with excess oxidizing agents such as 2,3-dichloro-5,6-dicyano-p-benzoquinone converted an ethyl bridge to an ethylene moiety, that is, 3HB in high yield (85%). All synthesized precursors and biphenyl-based o-carboranyl compounds were characterized by multinuclear nuclear magnetic resonance (NMR) spectroscopy (Figures S1–S40 in Supporting Information) and elemental analyses. The broad ¹H{¹¹B decoupled} peaks in the upfield region (3.5–2.5 ppm) of the o-carboranyl compounds could be assigned to total 10H atoms, and three peaks between −2 and −11 ppm in the ¹¹B{¹H} NMR spectra distinctly confirmed the presence of a closo-o-carborane cage.

The single-crystal X-ray diffraction (XRD) analysis of 1HF–13PP demonstrates their solid-state molecular structures (Figure 1). Detailed parameters, including selected bond lengths and angles, are illustrated in Tables S1 and S2 of Supporting Information. Remarkably, the results suggested that the dihedral angles (Ψ_exp = C5–C6–C9–C10, Figure 1 and Table 1) between the biphenyl rings could be altered by the intramolecular steric hindrance owing to the bulkiness of the substituted functional group in the biphenyl moiety. Consequently, distorted geometries were observed for biphenyl rings of o-carboranyl compounds, with planar (1HF: Ψ_exp = 0.7°) to nearly perpendicular structures (12MM and 13PP: Ψ_exp = 83°). These values were crucial factors in verifying the relationship between the structural geometry and the efficiency of the radiative process in the o-carboranyl compounds.

Figure 1.

Molecular structures of biphenyl-based o-carboranyl compounds (30% thermal ellipsoids, hydrogen atoms were omitted for clarity) and their dihedral angles (Ψ_exp = C5–C6–C9–C10, green-dash box) between biphenyl rings.

Table 1. Photophysical and Structural Data for Biphenyl-Based o-Carboranyl Compounds (1HF–13PP).

	λabsa/nm (ε × 10–3 M–1 cm–1)	λex/nm	λemb/nm	Φemb,c	τobsb/ns	krb,d/s–1 × 108	knrb,e/s–1 × 108	Ψexpf/deg	Ψcalcg/deg
1HF	282 (19.2), 306 (18.7)	307	477	0.341	3.25	1.05	2.03	0.67	0.12
2HT	285 (16.0), 311 (6.3)	292	542	0.317	3.05	1.04	2.24	2.4	0.74
3HB	264 (64.2), 290 (19.2)	296	483	0.303	2.91	1.04	2.40	2.8	0.29
4HN	286 (58.9), 308 (37.7)	308	461	0.281	3.01	0.934	2.39	14	20
5HH	269 (34.5)	290	453	0.229	2.97	0.771	2.60	39	37
6HC	266 (17.6)	291	479	0.227	3.76	0.604	2.06	49	48
7HD	266 (17.9)	286	469	0.251	4.17	0.602	1.80	52	49
8HP	238 (26.3), 270 (13.2)	300	480	0.241	4.50	0.536	1.69	58	51
9HM	256 (24.3)	283	493	0.182	3.79	0.480	2.16	64	54
10HV	237 (31.9), 266 (19.9)	296	451	0.115	2.53	0.455	3.50	67	57
11HI	251 (15.5)	281	452	0.074	1.75	0.423	5.29	74	59
12MM	229 (24.9), 272 (6.5)	275	469	0.112	2.88	0.389	3.08	83	89
13PP	225 (25.6), 271 (3.3)	270	492	<0.01	h			83	90

^aMeasured in THF (3.0 × 10^–5 M).

^bMeasured in a crystalline state at 298 K.

^cAbsolute PL quantum yield.

^dk_r = Φ_em/τ_obs.

^ek_nr = k_r(1/Φ_em–1).

^fExperimental dihedral angle between biphenyl rings from each X-ray crystal structure.

^gCalculated dihedral angle between the biphenyl rings from each optimized structure in the ground (S₀) state.

^hNot observed due to weak emission.

Analysis of Optical Properties Based on Theoretical Calculations

UV–vis absorption and PL spectroscopy experiments were conducted under various conditions for o-carboranyl compounds 1HF–13PP, which provided information of each photophysical characteristic (Figures 2 and S41, Tables 1, and S3). All the compounds exhibited strong absorption bands centered at λ_abs = 251–311 nm (Table 1) due to the vibronic structures undergoing spin-allowed π–π* transitions on the biphenyl group. The major bands were slightly red-shifted compared to the absorption center (Figure S42) of 1,1′-biphenyl because introducing o-carborane led to a large stabilization of the lowest unoccupied molecular orbital (LUMO) level via a strong inductive effect.⁴⁰ Specific absorption features of these o-carboranyl compounds include substantially tailing absorption bands above 300 nm, which are attributed to typical ICT transitions in the o-carborane cages, which differ from 1,1′-biphenyl. The theoretical results, which were calculated by TD-DFT based on the B3LYP/6-31G(d,p) level of theory, could demonstrate the origin of such characteristics. The ground (S₀)-state calculation for all the compounds revealed that the lowest-energy electronic transitions were mainly associated with the highest occupied molecular orbital (HOMO) → LUMO transitions (Figure S43). The HOMOs of each compound were predominantly localized on the biphenyl group (>96% in all compounds, insets of Figure S43); however, the LUMOs were substantially distributed over the o-carborane cages (21–38%). These S₀-calculation results indicate that the low-energy electronic transitions of the o-carboranyl compounds originate from substantial ICT transitions, corresponding to the o-carborane cage.

Figure 2.

PL spectra for biphenyl-based o-carboranyl compounds (1HF–13PP) in a crystalline state. The inset illustrates the emission color observed under a hand-held ultraviolet lamp (λ_ex = 265 nm).

The emissive properties of 1HF–13PP in the crystalline state (Figure 2 and Table 1) were examined by PL spectroscopy to investigate the radiative process in the specifically fixed molecular geometry. All the compounds exhibited strong (for 1HF–12MM) or moderate (for 13PP) emission in the range from λ_em = 451 nm; blue region (for 10HV) to 542 nm; yellow region (for 2HT). The emissive bands differed distinctly from those (λ_em = 308, 316 nm, Figure S42) of 1,1′-biphenyl based on the π–π* transition. Therefore, these could be conducted to reflect the ICT-based emissions involving the o-carborane cage. The results calculated for the S₁-optimized structures of the o-carboranyl compounds suggested that the LUMO–HOMO type transitions (λ_em,calc = 498–529 nm, Figure 3) primarily contributed to the emissions. Interestingly, the LUMOs and HOMOs were significantly localized on the o-carborane cage (72–75%, insets of Figure 3) and biphenyl rings (>90%), respectively. These computational results confirm that the experimentally observed emissions in the crystalline state mainly originate from a radiative decay process based on ICT transitions between o-carborane and biphenyl rings. Similarly, such ICT-based emissions could be observed for o-carboranyl compounds in the rigid state in the region of 450–550 nm (λ_em = 459–487 nm in tetrahydrofuran (THF) solution at 77 K and 470–506 nm in the film state; bluish-green emission, Figure S41 and Table S3). However, the PL spectra of o-carboranyl compounds in THF solution at 298 K did not exhibit any emissive trace in this low-energy region; furthermore, the compounds either did not emit in various organic solvents (cyclohexane, dichloromethane (DCM), ethyl acetate, and acetone) at 298 K or only demonstrated a significantly weak emissive trace. This quenched ICT-based radiative decay was ascribed to the elongation of the C–C bond of the o-carborane cage in the solution state at an ambient temperature.^{11,12,16,18,26−28,31,34,37}

Figure 3.

Frontier molecular orbitals of biphenyl-based o-carboranyl compounds 1HF–13PP at the first-excited singlet state (S₁) with their relative energies from DFT calculations (isovalue = 0.04 a.u.) and molecular orbital distributions on the o-carborane moieties of the LUMO levels (%) and on the biphenyl group of the HOMO level. The transition energy (in nm) was calculated using the TD-B3LYP method with 6-31G(d,p) basis sets.

The estimated C–C bond lengths of the o-carborane cage in all the S₁-optimized structures were approximately 2.39 Å (Table 2) and significantly exceeded those of the S₀-structures (approximately 1.77 Å) and the experimental values of 1.71–1.73 Å. Such a feature was similarly observed in calculation results for diphenyl-ortho-carborane (1,2-Ph₂-1,2-C₂B₁₀H₁₀, 1);⁴¹ C–C bond length for 1 = 1.76 Å and [1]⁻ (radical anion) = 2.39 Å. The fluctuation of the C–C bond elongation prevented the ICT-based radiative decay process, resulting in no emissive trace in PL spectra at 298 K.

Table 2. Experimental and Theoretical C–C Bond Lengths (Å) in the o-Carborane Cage for Biphenyl-Based o-Carboranyl Compounds (1HF–13PP).

		1HF	2HT	3HB	4HN	5HH
exp.a		1.73	1.73	1.72	1.72	1.72
calc.b	S0	1.77	1.76	1.76	1.77	1.76
	S1	2.39	2.40	2.40	2.39	2.39
		6HC	7HD	8HP	9HM	10HV
exp.a		1.72	1.73	1.72	1.72	1.73
calc.b	S0	1.76	1.77	1.77	1.77	1.77
	S1	2.38	2.38	2.38	2.39	2.38
		11HI	12MM	13PP
exp.a		1.72	1.72	1.71
calc.b	S0	1.77	1.76	1.76
	S1	2.39	2.39	2.40

^aExperimental values from their X-ray crystal structures (C1–C2 bond).

^bCalculated values from their ground (S₀) and the first-excited singlet state (S₁) optimized structures.

Intriguingly, the high-energy emissive trace in the 340–380 nm region could be assigned to the spin-allowed π–π*transition-based emission, that is, locally excited (LE)-based emission on the biphenyl moiety. Emissive patterns similar to those of 1,1-biphenyl (Figure S42) were observed in the PL spectra of 5HH and 10HV–13PP in all states (Figure S41 and Table S3). Moreover, the LE-based emission of 13PP was more intense than the ICT-based emission. These features confirm that the distorted geometry of the biphenyl rings plays an important role in alternating the intramolecular electronic transitions and the corresponding radiative decay mechanism.

Quantitative Analysis for the Relationship between Distorted Geometry and the ICT-Based Radiative Process in the Crystalline State

To quantitatively obtain insights into the relationship between the geometric structure of the distorted biphenyl ring and the radiative decay mechanism for ICT-based emissions, the quantum efficiencies (Φ_em) and decay lifetimes (τ_obs) of the o-carboranyl compounds were estimated in the crystalline state at 298 K (Table 1 and Figure S44). The emission decay lifetimes of the ICT-based emissions were 1.7–4.5 ns, indicating that the observed emission corresponded to fluorescence. The Φ_em values for the ICT-based emission in the crystalline state gradually decreased (Φ_em for 1HF = 34.1% decreased to 7.4% for 11HI, 11.2% for 12MM, and <1% for 13PP), upon increasing the distortional angle between the biphenyl rings (Ψ_exp for 1HF = 0.67° increased to 83° for 12MM and 13PP). Interestingly, the radiative (k_r, Table 1) and nonradiative (k_nr) decay constants calculated from Φ_em and τ_obs revealed the relationship between the efficiency of the radiative process for ICT-based emission and biphenyl geometry. The k_r values (1.05 × 10⁸ s^–1 for 1HF to 3.89 × 10⁷ s^–1 for 12MM, Table 1) for all the compounds exhibited an inversely proportional relationship with the corresponding Ψ_exp value (Figure 4, blue-dot and orange linear-fitting line), thereby estimating the accuracy of the linear-fitting equation [k_r = (−8.64 × 10⁵) × Ψ_exp + 1.06] as 99% (square R value, R² = 0.9899). However, the k_nr values were almost within the range of the 1.7–2.6 × 10⁷ s^–1 box pattern, except for those of 10HV–12MM, as they possessed significantly low Φ_em values (<12%). Additionally, this relationship was graphically illustrated using the calculated values of the dihedral angle (Ψ_calc, Table 1) between the biphenyl rings from each S₀-optimized structure (Figure S45). The k_r of 1HF–11HI was inversely related to the Ψ_calc values ([k_r = (−1.03 × 10⁶) × Ψ_calc + 1.07], R² = 0.9706). These results strongly indicate that the biphenyl geometry, particularly the distorted structure, diminishes the radiative decay process based on ICT transitions corresponding to o-carborane. Thus, the planarity of the biphenyl group appended to the o-carborane cage is a crucial factor for controlling the efficiency of the ICT-based emission.

Figure 4.

Radiative (k_r, filled blue-circle, left) and nonradiative decay constants (k_nr, hollow gray circle, right) for the biphenyl-based o-carboranyl compounds (1HF–13PP) in the crystalline state as a function of the experimental dihedral angles (Ψ_exp) from each X-ray crystal structure. The orange line is its linear fitting for the k_r values vs Ψ_exp.

Theoretical Modeling for the Effect of Distorted Geometry on Orbital Localization

The impact of the structural geometry of the biphenyl rings on the ICT-based radiative decay process was further verified by calculating the low-energy transitions assignable to ICT within o-carborane (LUMO → HOMO in THF) for 5HH in the S₁ state as Ψ varied from 0 to 90° (Figure 5). The results revealed that the oscillator strengths (f_calc) of the transition dramatically decreased as the biphenyl rings became orthogonally distorted (f_calc = 0.5439 at Ψ = 0° to 0.2181 at 90°, Figure 5a and Table S4). Moreover, the computed emission wavelengths (λ_calc) gradually changed to the values attributed to the LE-based high-energy emission upon increasing Ψ (λ_calc = 536.95 nm at Ψ = 0° to 430.11 nm at 90°). In addition, the molecular orbital contribution of the o-carborane cage (zone 1, Figure 5a) and terminal phenyl ring (zone 3) to the LUMO and HOMO, respectively, was maximized (71 → 74% and 46 → 51%, respectively, Figure 5b). However, the molecular orbital contribution of the bridged phenyl ring (zone 2) to the HOMO decreased (44 → 36%) as the distortion of the biphenyl ring became perpendicular (Ψ = 0 to 90°). These observations imply that the distortion of biphenyl rings leads to the isolation (inset in Figure 5b when Ψ = 90°) of the orbital contribution for LUMO and HOMO, resulting in the prevention of the ICT transition between o-carborane and the appended phenyl group. All the calculation results strongly suggest that the distorted geometry of the biphenyl rings prohibits the efficient ICT transition corresponding to the o-carborane cage. Consequently, the combined experimental and theoretical findings verify that the molecular geometry, particularly the planarity of aromatic rings linked to the o-carborane cage, is a critical factor in regulating the efficiency of the ICT-based radiative decay mechanism in o-carborane luminophores.

Figure 5.

(a) Computed emission wavelengths (λ_calc) and oscillator strengths (f_calc) as a function of the dihedral angle (Ψ) in each first-excited (S₁) state. (b) Effects of Ψ in S₁-optimized structures on the contributions of the o-carborane cage to the LUMO (zone 1; green-line), bridged phenyl ring to the HOMO (zone 2; purple-line), and terminal phenyl ring to the HOMO (zone 3; dark yellow-line). Insets are frontier orbitals at Ψ = 0 and 90°.

Conclusions

In conclusion, the biphenyl-based o-carborane luminophores of the 13 species of 1HF–13PP, with various distortion geometries on the biphenyl group, were prepared and structurally characterized to establish the relationship between the geometric conformations and photophysical features. Intriguingly, their radiative decay efficiencies for ICT-based emissions in the crystalline state linearly decreased in accordance with the increasing dihedral angles between the biphenyl rings in the crystal structures. Consequently, the results confirm that the planarity of the aromatic groups linked to o-carborane is crucial for controlling the ICT-based radiative decay of o-carborane. Thus, the results of this study make a significant contribution to developing o-carborane-based organic luminophores, exhibiting specific photophysical characteristics.

Experimental Section

General Considerations

All the operations were performed in an inert nitrogen atmosphere using standard Schlenk and glovebox techniques. Anhydrous solvents (acetonitrile (ACN), THF, toluene, trimethylamine (NEt₃), dichloroethane (DCE), and DCM), purchased from Sigma-Aldrich, were dried by passing through an activated alumina column and stored over activated molecular sieves (5 Å). Spectrophotometric-grade solvents (cyclohexane, n-hexane, methanol (MeOH), ethyl acetate, toluene, THF, DCM, and acetone) were used as received from Alfa Aesar. Commercial reagents purchased from Sigma-Aldrich (N-bromosuccinimide (NBS), p-toluenesulfonic acid monohydrate (TsOH), 2-isopropoxy-4,4,5,5-tetramethyl[1,3,2]-dioxaborolane, 1,4-dibromobenzene, 2-iodobiphenyl, phenylacetylene, 2-bromo-9H-fluorene, 2-bromotriphenylene, 4-bromobiphenyl, 4-bromophenylboronic acid, 2-bromotoluene, 2-bromostyrene, 2-bromocumene, 2,6-dimethylbenzeneboronic acid, 2,6-diisopropylbenzeneboronic acid, 1.6 M n-butyllithium in hexane (n-BuLi), copper(I) iodide (CuI), potassium carbonate (K₂CO₃), sodium thiosulfate (Na₂S₂O₃), magnesium sulfate (MgSO₄), diethyl sulfide (Et₂S), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), saturated HCl aqueous solution, and poly(methyl methacrylate) (PMMA), 1,1′-biphenyl), decaborane (B₁₀H₁₄) (Alfa Aesar), TCI (9,10-dihydrophenanthrene, tetrakis(triphenylphosphine)palladium(0) (Pd(PPh₃)₄)), and bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh₃)₂Cl₂) were used without any further purification. 2-bromo-2′-methyl-1,1′-biphenyl (precursor for 6HCA),⁴² 2′-bromo-2,6-dimethyl-1,1′-biphenyl (precursor for 7Br),⁴³ (4-(phenylethynyl)phenyl)boronic acid (precursor for 6HCA, 9HMA, 10HVA, and 11HIA),³⁷ and 1-bromo-4-(phenylethynyl)benzene (precursor for 12MMA and 13PPA)³⁷ were prepared according to previously reported procedures. Deuterated solvents, chloroform-d₁ (CDCl₃), and dichloromethane-d₂ (CD₂Cl₂) were purchased from Cambridge Isotope Laboratories Inc., and dried over activated molecular sieves (5 Å). All NMR spectra were recorded on a Bruker Avance 400 MHz NMR spectrometer (Bruker; 400.13 MHz for ¹H and ¹H{¹¹B}, 100.62 MHz for ¹³C, and 128.38 MHz for ¹¹B{¹H}) at an ambient temperature. Chemical shifts are expressed in ppm and referenced against external tetramethylsilane (Me₄Si) (¹H, ¹H{¹¹B}, and ¹³C) or BF₃·Et₂O (¹¹B{¹H}). Elemental analyses were performed on an EA3000 instrument (EuroVector) at the Central Laboratory of Kangwon National University.

Synthesis of 2-Bromo-9,10-dihydrophenanthrene, 4Br

ACN (25 mL) was added to a mixture of 9,10-dihydrophenanthrene (0.84 mL, 5.0 mmol), NBS (0.98 g, 5.5 mmol), and TsOH, (29 mg, 0.15 mmol) at 50 °C, and the reaction mixture was subsequently refluxed for 12 h. After quenching with excess saturated aqueous Na₂S₂O₃, the mixture was extracted with ethyl acetate (50 mL × 3). The organic layer was dried with anhydrous MgSO₄, and the volatiles were removed under vacuum. Purification via recrystallization from MeOH gave 4Br (0.67 g) as a white solid (yield = 52%). Except for elemental analyses, other characterization techniques were not performed due to its poor solubility in any organic solvents. Anal. calcd for C₁₄H₁₁Br: C, 64.89; H, 4.28. Found: C, 64.55; H, 4.01.

Synthesis of 4″-Bromo-2,6-dimethyl-1,1′:2′,1″-terphenyl, 7Br

Boronification: n-BuLi (1.6 M in n-hexane, 1.4 mL, 2.2 mmol) was added to a solution of 2′-bromo-2,6-dimethyl-1,1′-biphenyl (0.49 g, 1.9 mmol) in THF (20 mL) at −78 °C. After stirring for 3 h, 2-isopropoxy-4,4,5,5-tetramethyl[1,3,2]-dioxaborolane (0.42 g, 2.2 mmol) in THF (10 mL) was added dropwise. The reaction mixture was gradually heated to 25 °C and further stirred for 12 h. After quenching with concentrated HCl aqueous solution (20 mL), the mixture was extracted with ethyl acetate (50 mL × 3). The organic layer was dried over anhydrous MgSO₄, and the solvents were removed through rotary evaporation. The boronificated crude product was used in situ for the next step without any purification. Suzuki–Miyaura coupling reaction: a cannula was utilized to add toluene (8.0 mL) and distilled water (1.6 mL) to a mixture of the boronificated product (0.49g, 1.6 mmol), 1,4-dibromobenzene (0.75 g, 3.2 mmol), Pd(PPh₃)₄ (92 mg, 80 μmol), and K₂CO₃ (0.44 g, 3.2 mmol). The reaction mixture was refluxed at 120 °C for 4 h. After extracting with ethyl acetate (10 mL × 3), the organic layer was dried over anhydrous MgSO₄, and the volatiles were removed through rotary evaporation to give a dark brown residue, which was purified by column chromatography on silica gel (eluent: n-hexane only) to afford 7Br (0.17 g) as a white solid. Yield = 32%. ¹H NMR (CD₂Cl₂): δ 7.43 (d, J = 8.0 Hz, 2H), 7.42 (m, 1H), 7.29 (d, J = 8.0 Hz, 2H), 7.17 (m, 1H), 7.08 (t, J = 8.2 Hz, 1H), 6.98 (d, J = 8.2 Hz, 4H), 1.92 (s, 6H, −CH₃). ¹³C NMR (CD₂Cl₂): δ 140.84, 140.80, 139.87, 139.26, 136.36, 131.09, 130.91, 130.88, 130.30, 128.17, 127.96, 127.57, 127.46, 121.11, 20.88 (−CH₃). Anal. calcd for C₂₆H₁₇Br: C, 71.23; H, 5.08. Found: C, 71.55; H, 5.12.

Synthesis of 4-Bromo-1,1′:2′,1″-terphenyl, 8Br

THF (60 mL) and distilled water (15 mL) were injected via a cannula to a mixture of 2-iodobiphenyl (1.7 mL, 10 mmol), 4-bromophenylboronic acid (2.0 g, 10 mmol), Pd(PPh₃)₄ (1.1 g, 1.0 mmol), and K₂CO₃ (4.1 g, 30 mmol) at 90 °C and then stirred for 12 h at the same temperature. The reaction mixture was quenched with distilled water (50 mL) and extracted with ethyl acetate (50 mL × 3). After drying over anhydrous MgSO₄, the organic volatiles were removed under vacuum to afford a dark brown oil. The residue was purified by column chromatography on silica gel (eluent: n-hexane only) to yield 8Br (0.62 g) as a white solid. Yield = 29%. ¹H NMR (CD₂Cl₂): δ 7.42 (t, J = 4.8 Hz, 3H), 7.40 (m, 1H), 7.35 (d, J = 7.8 Hz, 2H), 7.24 (m, 3H), 7.14 (m, 2H), 7.03 (d, J = 8.0 Hz, 2H). ¹³C NMR (CD₂Cl₂): δ 141.65, 141.06, 140.95, 139.66, 131.99, 131.34, 131.08, 130.73, 130.24, 128.39, 128.23, 127.99, 127.02, 120.99. Anal. calcd for C₁₈H₁₃Br: C, 69.92; H, 4.24. Found: C, 69.89; H, 4.10.

General Synthetic Routes for Acetylene Derivatives: 1HFA, 2HTA, 4HTA, 5HHA, 7HDA, and 8HPA

The acetylene derivatives (1HFA, 2HTA, 4HTA, 5HHA, 7HDA, and 8HPA) were synthesized by following the scheme with an adequate amount of starting materials. Toluene (14 mL) and NEt₃ (7.0 mL) were added via a cannula to the mixture of each bromo precursor (Scheme S1), CuI, and Pd(PPh₃)₂Cl₂ at 110 °C. After stirring for 15 min, injection of phenylacetylene of three equivalents per bromo precursor made the resulting dark brown slurry. Subsequently, the reaction mixture was refluxed for 24 h. The volatiles were removed with a rotary evaporator, affording a dark gray residue. After washing with MeOH, followed by n-hexane, column chromatography on silica gel (eluent: n-hexane only) was performed to purify acetylene derivatives from the crude products.

Data for 2-(Phenylethynyl)-9H-fluorene, 1HFA

2-Bromo-9H-fluorene (0.74 g, 3.0 mmol), CuI (48 mg, 0.24 mmol), Pd(PPh₃)₂Cl₂ (190 mg, 0.26 mmol), and phenylacetylene (1.0 mL, 9.0 mmol) provided 1HFA (0.63 g) as an ivory solid. Yield = 79%. ¹H NMR (CDCl₃): δ 7.80 (d, J = 6.4 Hz, 1H), 7.77 (d, J = 8.0 Hz, 1H), 7.73 (s, 1H), 7.57 (dd, J = 7.1, 3.6 Hz, 4H), 7.40 (t, J = 8.2 Hz, 1H), 7.36 (d, J = 7.8 Hz, 3H), 7.32 (d, J = 8.0 Hz, 1H), 3.92 (s, 2H, −CH₂−). ¹³C NMR (CDCl₃): δ 143.72, 143.35, 142.03, 141.25, 131.72, 130.66, 128.50, 128.32, 128.29, 127.31, 127.07, 125.24, 123.61, 121.43, 120.34, 119.94, 90.31 (acetylene–C), 89.53 (acetylene–C), 36.90 (−CH₂−). Anal. calcd for C₂₁H₁₄: C, 94.70; H, 5.30. Found: C, 94.70; H, 5.28.

Data for 2-(Phenylethynyl)triphenylene, 2HTA

2-Bromotriphenylene (0.31 g, 1.0 mmol), CuI (19 mg, 0.10 mmol), Pd(PPh₃)₂Cl₂ (70 mg, 0.10 mmol), and phenylacetylene (0.35 mL, 3.0 mmol) provided 2HTA (0.25 g) as a white solid. Yield = 76%. ¹H NMR (CDCl₃): δ 8.85 (s, 1H), 8.65 (m, 5H), 7.80 (dd, J = 8.5, 1.5 Hz, 1H), 7.68 (m, 4H), 7.65 (dd, J = 8.0, 2.4 Hz, 2H), 7.41 (m, 3H). ¹³C NMR (CDCl₃): δ 131.85, 130.19, 130.11, 130.08, 129.87, 129.69, 129.46, 129.25, 128.59, 128.56, 127.76, 127.74, 127.54, 127.52, 126.99, 123.66, 123.60, 123.57, 123.51, 123.46, 123.38, 122.05, 90.34 (acetylene–C), 89.95 (acetylene–C). Anal. calcd for C₂₆H₁₆: C, 95.09; H, 4.91. Found: C, 94.94; H, 4.90.

Data for 2-(Phenylethynyl)-9,10-dihydrophenanthrene, 4HNA

4Br (0.47 g, 1.8 mmol), CuI (12 mg, 0.14 mmol), Pd(PPh₃)₂Cl₂ (110 mg, 0.16 mmol), and phenylacetylene (0.59 mL, 5.4 mmol) provided 4HNA (0.25 g) as a white solid. Yield = 49%. ¹H NMR (CD₂Cl₂): δ 7.76 (d, J = 7.8 Hz, 1H), 7.75 (d, J = 7.8 Hz, 1H), 7.56 (t, J = 6.4 Hz, 1H), 7.55 (d, J = 4.8 Hz, 1H), 7.48 (dd, J = 8.0, 1.7 Hz, 1H), 7.43 (s, 1H), 7.40 (d, J = 8.5 Hz, 1H), 7.37 (m, 2H), 7.32 (m, 1H), 7.26 (d, J = 4.2 Hz, 2H), 2.89 (s, 4H, −CH₂CH₂−). ¹³C NMR (CD₂Cl₂): δ 138.04, 135.06, 134.18, 131.91, 131.51, 130.54, 128.88, 128.83, 128.69, 128.62, 128.26, 127.43, 124.18, 124.08, 123.71, 122.27, 89.99 (acetylene–C), 89.91 (acetylene–C), 29.23 (−CH₂−), 29.20 (−CH₂−). Anal. calcd for C₂₀H₁₄: C, 94.45; H, 5.45. Found: C, 94.28; H, 5.33.

Data for 4-(Phenylethynyl)-1,1′-biphenyl, 5HHA

4-Bromobiphenyl (0.70 g, 3.0 mmol), CuI (48 mg, 0.24 mmol), Pd(PPh₃)₂Cl₂ (190 mg, 0.26 mmol), and phenylacetylene (1.0 mL, 9.0 mmol) provided 5HHA (0.67 g) as a white solid. Yield = 87%. ¹H NMR (CDCl₃): δ 7.62 (d, J = 7.8 Hz, 2H), 7.61 (s, 4H), 7.56 (t, J = 2.4 Hz, 1H), 7.55 (d, J = 1.8 Hz, 1H), 7.46 (t, J = 7.6 Hz, 2H), 7.38 (t, J = 2.4 Hz, 1H), 7.35 (m, 3H). ¹³C NMR (CDCl₃): δ 141.10, 140.49, 132.17, 131.76, 129.01, 128.52, 128.43, 127.78, 127.17, 123.42, 122.30, 90.19 (acetylene–C), 89.43 (acetylene–C). Anal. calcd for C₂₀H₁₄: C, 94.45; H, 5.55. Found: C, 94.12; H, 5.48.

Data for 2,6-Dimethyl-4″-(phenylethynyl)-1,1′:2′,1″-terphenyl, 7HDA

7Br (0.54 g, 1.6 mmol), CuI (24 mg, 0.12 mmol), Pd(PPh₃)₂Cl₂ (100 mg, 0.14 mmol), and phenylacetylene (0.53 mL, 4.8 mmol) provided 7HDA (0.34 g) as a white solid. Yield = 59%. ¹H NMR (CDCl₃): δ 7.48 (t, J = 8.2 Hz, 3H), 7.42 (m, 2H), 7.33 (dd, J = 5.3, 2.0 Hz, 5H), 7.19 (d, J = 7.3 Hz, 1H), 7.08 (t, J = 8.2 Hz, 3H), 6.98 (d, J = 7.5 Hz, 2H), 1.94 (s, 6H, −CH₃). ¹³C NMR (CDCl₃): δ 141.49, 140.63, 140.17, 139.10, 136.17, 131.68, 131.12, 130.63, 130.09, 128.93, 128.48, 128.35, 127.82, 127.68, 127.40, 127.22, 123.40, 121.44, 89.81 (acetylene–C), 89.49 (acetylene–C), 20.95 (−CH₃). Anal. calcd. for C₂₈H₂₂: C, 93.81; H, 6.19. Found: C, 93.72; H, 6.08.

Data for 4-(Phenylethynyl)-1,1′:2′,1″-terphenyl, 8HPA

8Br (0.93 g, 3.0 mmol), CuI (57 mg, 0.30 mmol), Pd(PPh₃)₂Cl₂ (250 mg, 0.36 mmol), and phenylacetylene (1.1 mL, 9.0 mmol) provided 8HPA (0.20 g) as a white solid. Yield = 20%. ¹H NMR (CD₂Cl₂): δ 7.53 (d, J = 4.2 Hz, 1H), 7.51 (d, J = 2.4 Hz, 1H), 7.44 (s, 4H), 7.39 (d, J = 7.8 Hz, 2H), 7.35 (m, 3H), 7.23 (t, J = 7.6 Hz, 3H), 7.14 (t, J = 8.8 Hz, 4H). ¹³C NMR (CD₂Cl₂): δ 141.74, 141.37, 140.70, 139.96, 131.71, 131.32, 130.86, 130.57, 130.01, 130.01, 128.49, 128.37, 128.16, 127.93, 127.72, 126.76, 123.41, 121.40, 89.78 (acetylene–C), 89.52 (acetylene–C). Anal. calcd. for C₂₆H₁₈: C, 94.51; H, 5.49. Found: C, 94.88; H, 5.12.

General Synthetic Routes for Acetylene Derivatives, 6HCA, and 9HMA–13PPA

The acetylene derivatives (6HCA and 9HMA–13PPA) were synthesized by following the scheme with an adequate amount of starting materials. THF (10 mL) and distilled water (5.0 mL) were added via a cannula to a mixture of each bromo and boronic acid precursor (Scheme S1), Pd(PPh₃)₄, and K₂CO₃ at 90 °C. In addition, the reaction mixture was stirred for 12 h at the same temperature, quenched by distilled water (20 mL), and then extracted with ethyl acetate (50 mL × 3). The organic layer was dried over anhydrous MgSO₄, and the organic solvents were removed under vacuum to give a dark brown oil. The crude product was purified by column chromatography on silica gel (eluent: n-hexane only) to afford acetylene derivatives.

Data for 2-Methyl-4″-(phenylethynyl)-1,1′:2′,1″-terphenyl, 6HCA

2-Bromo-2′-methyl-1,1′-biphenyl (1.3 g, 5.4 mmol), (4-(phenylethynyl)phenyl)boronic acid (0.53 g, 2.4 mmol), Pd(PPh₃)₄ (0.30 mg, 0.43 mmol), and K₂CO₃ (2.2 g, 16 mmol) provided 6HCA (0.87 g) as a white solid. Yield = 47%. ¹H NMR (CDCl₃): δ 7.48 (dd, J = 8.9 Hz, 4H), 7.42 (t, J = 8.4 Hz, 1H), 7.33 (m, 6H), 7.17 (m, 1H), 7.15 (t, J = 7.6 Hz, 2H), 7.09 (t, J = 8.0 Hz, 3H), 1.91 (s, 3H, −CH₃). ¹³C NMR (CDCl₃): δ 141.64, 141.15, 140.44, 140.41, 135.88, 131.69, 131.16, 130.96, 130.69, 130.03, 129.91, 129.51, 128.47, 128.34, 127.75, 127.55, 127.31, 125.56, 123.43, 121.37, 89.80 (acetylene–C), 89.53 (acetylene–C), 20.18 (−CH₃). Anal. calcd for C₂₇H₂₀: C, 94.15; H, 5.85. Found: C, 93.85; H, 6.10.

Data for 2-Methyl-4′-(phenylethynyl)-1,1′-biphenyl, 9HMA

2-Bromotoluene (0.34 g, 2.0 mmol), (4-(phenylethynyl)phenyl)boronic acid (0.53 g, 2.4 mmol), Pd(PPh₃)₄ (230 mg, 0.20 mmol), and K₂CO₃ (0.83 g, 6.0 mmol) provided 9HMA (0.30 g) as a white solid. Yield = 57%. ¹H NMR (CD₂Cl₂): δ 7.61 (s, 1H), 7.58 (m, 2H), 7.56 (d, J = 2.4 Hz, 1H), 7.40 (m, 1H), 7.38 (d, J = 4.8 Hz, 2H), 7.35 (s, 1H), 7.33 (s, 1H), 7.28 (m, 2H), 7.24 (m, 2H), 2.29 (s, 3H, −CH₃). ¹³C NMR (CD₂Cl₂): δ 142.50, 141.56, 135.71, 131.92, 131.65, 130.80, 129.92, 129.73, 128.82, 128.73, 127.91, 126.22, 123.64, 122.01, 89.84 (acetylene–C), 89.58 (acetylene–C), 20.57 (−CH₃). Anal. calcd for C₂₁H₁₆: C, 93.99; H, 6.01. Found: C, 94.94; H, 5.96.

Data for 4′-(Phenylethynyl)-2-vinyl-1,1′-biphenyl, 10HVA

2-Bromostyrene (0.18 g, 1.0 mmol), (4-(phenylethynyl)phenyl)boronic acid (0.27 g, 1.2 mmol), Pd(PPh₃)₄ (120 mg, 0.10 mmol), and K₂CO₃ (0.41 g, 3.0 mmol) provided 10HVA (0.23 g) as a white solid. Yield = 83%. ¹H NMR (CDCl₃): δ 7.67 (dd, J = 8.4, 2.4 Hz, 1H), 7.61 (s, 1H), 7.59 (m, 2H), 7.57 (d, J = 4.8 Hz, 1H), 7.40 (dd, J = 8.0, 2.4 Hz, 1H), 7.38 (m, 4H), 7.35 (t, J = 3.8 Hz, 2H), 7.32 (td, J = 8.4, 2.4 Hz, 1H), 6.73 (dd, J = 17.5, 11.0 Hz, 1H, −CHCH₂), 5.73 (dd, J = 17.5, 1.3 Hz, 1H, −CHCH₂), 5.23 (dd, J = 11.0, 1.3 Hz, 1H, −CHCH₂). ¹³C NMR (CDCl₃): δ 140.95, 140.20, 135.88, 135.85, 131.76, 131.41, 130.04, 129.97, 128.50, 128.41, 127.89, 127.88, 126.04, 123.42, 122.12, 115.19, 91.56 (acetylene–C), 91.56 (acetylene–C). Anal. calcd for C₂₂H₁₆: C, 94.25; H, 5.75. Found: C, 94.11; H, 5.69.

Data for 2-Isopropyl-4′-(phenylethynyl)-1,1′-biphenyl, 11HIA

2-Bromocumene (0.31 mL, 2.0 mmol), (4-(phenylethynyl)phenyl)boronic acid (0.53 g, 2.4 mmol), Pd(PPh₃)₄ (230 mg, 0.20 mmol), and K₂CO₃ (0.83 g, 6 mmol) provided 11HIA (0.30 g) as a white solid. Yield = 50%. ¹H NMR (CD₂Cl₂): δ 7.58 (t, J = 7.8 Hz, 4H), 7.39 (m, 5H), 7.31 (d, J = 8.0 Hz, 2H), 7.22 (t, J = 7.8 Hz, 1H), 7.18 (t, J = 7.8 Hz, 1H), 3.06 (m, 1H, −CH(CH₃)₂), 1.17 (d, J = 6.9, 6H, −CH(CH₃)₂). ¹³C NMR (CD₂Cl₂): δ 146.78, 142.68, 140.81, 131.94, 131.57, 130.07, 129.89, 128.84, 128.74, 128.33, 126.02, 125.77, 123.65, 121.98, 89.84 (acetylene–C), 89.56 (acetylene–C), 29.81 (−CH(CH₃)₂), 24.37 (−CH(CH₃)₂). Anal. calcd for C₂₃H₂₀: C, 93.20; H, 6.80. Found: C, 93.15; H, 6.75.

Data for 2,6-Dimethyl-4′-(phenylethynyl)-1,1′-biphenyl, 12MMA

1-Bromo-4-(phenylethynyl)benzene (0.77 g, 3.0 mmol), 2,6-dimethylbenzeneboronic acid (0.54 g, 3.6 mmol), Pd(PPh₃)₄ (350 mg, 0.30 mmol), and K₂CO₃ (1.2 g, 9.0 mmol) provided 12MMA (0.23 g) as a white solid. Yield = 27%. ¹H NMR (CD₂Cl₂): δ 7.62 (d, J = 8.0 Hz, 2H), 7.57 (dd, J = 8.4, 2.8 Hz, 2H), 7.39 (m, 3H), 7.16 (d, J = 8.4, Hz, 3H), 7.11 (d, J = 6.4 Hz, 2H), 2.03 (s, 6H, −CH₃). ¹³C NMR (CD₂Cl₂): δ 141.86, 141.58, 136.23, 132.04, 131.94, 129.76, 128.83, 128.72, 127.70, 127.63, 123.68, 121.89, 89.67 (acetylene–C), 89.57 (acetylene–C), 20.90 (−CH₃). Anal. calcd for C₂₂H₁₈: C, 93.57; H, 6.43. Found: C, 93.45; H, 6.35.

Data for 2,6-Diisopropyl-4′-(phenylethynyl)-1,1′-biphenyl, 13PPA

1-Bromo-4-(phenylethynyl)benzene (0.54 g, 2.1 mmol), 2,6-diisopropylbenzeneboronic acid (0.51 g, 2.5 mmol), Pd(PPh₃)₄ (240 mg, 0.21 mmol), and K₂CO₃ (0.87 g, 6.3 mmol) provided 13PPA (71 mg) as a white solid. Yield = 10%. ¹H NMR (CDCl₃): 7.59 (d, J = 8.0 Hz, 2H), 7.56 (dd, J = 8.4, 2.4 Hz, 2H), 7.37 (m, 4H), 7.21 (d, J = 7.8 Hz, 2H), 7.17 (d, J = 8.0 Hz, 2H), 2.60 (m, 2H, −CH(CH₃)₂), 1.08 (d, J = 6.9 Hz, 12H, −CH(CH₃)₂). ¹³C NMR (CDCl₃): δ 146.84, 141.12, 139.00, 131.76, 131.46, 129.80, 128.53, 128.42, 128.18, 123.46, 122.71, 121.63, 89.63 (acetylene–C), 89.41 (acetylene–C), 30.41 (−CH(CH₃)₂), 24.25 (−CH(CH₃)₂). Anal. calcd for C₂₆H₂₆: C, 92.26; H, 7.74. Found: C, 92.21; H, 7.69.

General Synthetic Routes for the Biphenyl-Based o-carborane Compounds: 1HF, 2HT, and 4HN–13PP

An excess Et₂S (3.0 equivalent of an acetylene derivative) was added to a toluene solution (30 mL) of decaborane (B₁₀H₁₄, 1.5 equivalent per acetylene derivative) and each acetylene derivative (Scheme 1). After stirring at 120 °C, the reaction mixture was further refluxed for 24 h. The solvent was removed with a rotary evaporate. Purification through column chromatography on basic aluminum oxide (eluent: toluene only) and recrystallization was performed to afford biphenyl-based o-carborane compounds as a white solid.

Scheme 1. Synthetic Routes for Biphenyl-Based o-Carboranyl Compounds (1HF–13PP).

Reaction conditions: (i) phenylacetylene, Pd(PPh₃)₂Cl₂, CuI, NEt₃/toluene (1/2, v/v), 110 °C, 24 h; (ii) B₁₀H₁₄, Et₂S, toluene, 120 °C, 24 h; (iii) NBS, TsOH, ACN, 50 °C, 12 h; (iv) TsOH, DDQ, DCE, 90 °C, 12 h; (v) Pd(PPh₃)₄, K₂CO₃, THF/H₂O (2/1, v/v), 90 °C, 12 h; (vi) 2-isopropoxy-4,4,5,5-tetramethyl[1,3,2]-dioxaborolane, THF, 25 °C, 12 h; and (vii) Pd(PPh₃)₄, K₂CO₃, toluene/H₂O (5/1, v/v), 120 °C, 4 h.

Data for 1HF

Decaborane (0.42 g, 3.5 mmol), Et₂S (0.74 mL, 6.9 mmol), and 1HFA (0.61 g, 2.3 mmol) in toluene afforded 1HF as a white solid. Recrystallization from a mixture of DCM/n-hexane provided 0.45 g (1.2 mmol) of crystalline 1HF. Yield = 51%. ¹H{¹¹B} NMR (CDCl₃): δ 7.69 (d, J = 7.2 Hz, 1H), 7.59 (s, 1H), 7.51 (d, J = 7.7 Hz, 2H), 7.46 (d, J = 6.8 Hz, 3H), 7.33 (m, 2H), 7.19 (t, J = 7.1 Hz, 1H), 7.11 (t, J = 7.3 Hz, 2H), 3.76 (s, 2H, −CH₂−), 3.33 (br s, 2H, CB–BH), 2.56 (br s, 5H, CB–BH), 2.36 (br s, 3H, CB–BH). ¹³C NMR (CDCl₃): δ 143.92, 143.68, 143.19, 140.28, 130.86, 130.77, 130.27, 129.78, 129.05, 128.39, 127.83, 127.42, 127.13, 125.28, 120.62, 199.47, 86.16 (CB–C), 85.63 (CB–C), 36.87 (−CH₂−). ¹¹B{¹H} NMR (CDCl₃): δ −3.48 (br s, 2B), −10.10 (br s, 4B), −11.50 (br s, 4B). Anal. calcd for C₂₁H₂₄B₁₀: C, 65.60; H, 6.29. Found: C, 65.56; H, 6.21.

Data for 2HT

Decaborane (0.55 g, 4.5 mmol), Et₂S (0.97 mL, 9.0 mmol), and 2HTA (0.98 g, 3.0 mmol) in toluene afforded 2HT as a white solid. Recrystallization from a mixture of DCM/n-hexane provided 0.87 g (2.0 mmol) of crystalline 2HT. Yield = 65%. ¹H{¹¹B} NMR (CDCl₃): δ 8.70 (s, 1H), 8.58 (t, J = 7.4 Hz, 2H), 8.45 (t, J = 8.1 Hz, 2H), 8.35 (d, J = 8.8 Hz, 1H), 7.65 (dt, J = 17.7, 8.3 Hz, 5H), 7.53 (d, J = 7.2 Hz, 2H), 7.09 (m, 3H), 3.49 (br s, 1H, CB–BH), 2.73 (br s, 2H, CB–BH), 2.63 (br s, 4H, CB–BH), 2.46 (br s, 3H, CB–BH). ¹³C NMR (CDCl₃): δ 130.93, 130.80, 130.76, 160.41, 130.36, 130.15, 129.30, 129.09, 128.64, 128.55, 128.50, 128.27, 127.99, 127.66, 127.59, 126.30, 123.70, 123.54, 123.50, 123.46, 123.18, 85.67 (CB–C), 85.53 (CB–C). ¹¹B{¹H} NMR (CDCl₃): δ −2.31 (br s, 2B), −8.92 (br s, 4B), −10.23 (br s, 4B). Anal. calcd for C₂₆H₂₆B₁₀: C, 69.93; H, 5.87. Found: C, 69.81; H, 5.87.

Data for 4HN

Decaborane (0.16 g, 1.3 mmol), Et₂S (0.28 mL, 2.6 mmol), and 4HNA (0.24 g, 0.86 mmol) in toluene afforded 4HN as a white solid. Recrystallization from n-hexane provided 0.13 g (0.32 mmol) of crystalline 4HN. Yield = 37%. ¹H{¹¹B} NMR (CD₂Cl₂): δ 7.62 (d, J = 7.2 Hz, 1H), 7.50 (d, J = 8.0 Hz, 3H), 7.36 (d, J = 8.4 Hz, 1H), 7.31 (s, 1H), 7.24 (m, 3H), 7.17 (t, J = 8.4 Hz, 3H), 3.33 (br s, 2H, CB–BH), 2.74 (tt, J = 7.8, 3.9 Hz, 4H, −CH₂CH₂−), 2.53 (br s, 5H, CB–BH), 2.33 (br s, 3H, CB–BH). ¹³C NMR (CD₂Cl₂): δ 138.08, 137.80, 136.75, 133.27, 131.14, 131.02, 130.90, 130.65, 129.62, 129.57, 128.74, 128.69, 128.62, 127.41, 124.27, 123.71, 86.04 (CB–C), 29.14 (−CH₂CH₂−), 28.95 (−CH₂CH₂−). ¹¹B{¹H} NMR (CD₂Cl₂): δ −2.70 (br s, 2B), −9.14 (br s, 4B), −10.57 (br s, 4B). Anal. calcd for C₂₂H₂₆B₁₀: C, 66.30; H, 6.58. Found: C, 65.98; H, 6.32.

Data for 5HH

Decaborane (0.48 g, 3.9 mmol), Et₂S (0.84 mL, 7.8 mmol), and 5HHA (0.67 g, 2.6 mmol) in toluene afforded 5HH as a white solid. Recrystallization from n-hexane provided 0.47 g (1.2 mmol) of crystalline 5HH. Yield = 48%. ¹H{¹¹B} NMR (CD₂Cl₂): δ 7.51 (t, J = 8.0 Hz, 6H), 7.41 (m, 4H), 7.34 (t, J = 7.2 Hz, 1H), 7.27 (t, J = 7.4 Hz, 1H), 7.18 (t, J = 7.6 Hz, 2H), 3.34 (br s, 1H, CB–BH), 2.53 (br s, 6H, CB–BH), 2.33 (br s, 3H, CB–BH). ¹³C NMR (CD₂Cl₂): δ 143.18, 139.47, 131.53, 131.12, 130.95, 130.71, 129.92, 129.27, 128.76, 128.50, 127.29, 127.09, 86.00 (CB–C), 85.73 (CB–C). ¹¹B{¹H} NMR (CD₂Cl₂): δ −2.56 (br s, 2B), −9.06 (br s, 4B), −10.47 (br s, 4B). Anal. calcd for C₂₀H₂₄B₁₀: C, 64.49; H, 6.49. Found: C, 64.21; H, 6.28.

Data for 6HC

Decaborane (0.46 g, 3.8 mmol), Et₂S (0.81 mL, 7.5 mmol), and 6HCA (0.86 g, 2.5 mmol) in toluene afforded 6HC as a white solid. Recrystallization from a mixture of DCM/n-hexane provided 0.52 g (1.1 mmol) of crystalline 6HC. Yield = 45%. ¹H{¹¹B} NMR (CD₂Cl₂): δ 7.39 (d, J = 8.0 Hz, 4H), 7.33 (t, J = 7.2 Hz, 2H), 7.23 (d, J = 8.3 Hz, 3H), 7.16 (t, J = 7.2 Hz, 3H), 7.06 (t, J = 7.4 Hz, 1H), 7.00 (d, J = 7.4 Hz, 1H), 6.94 (d, J = 7.4 Hz, 1H), 6.88 (d, J = 8.3 Hz, 2H), 3.23 (br s, 2H, CB–BH), 2.47 (br s, 6H, CB–BH), 2.28 (br s, 2H, CB–BH), 1.56 (s, 3H, −CH₃). ¹³C NMR (CD₂Cl₂): δ 143.97, 140.98, 140.63, 139.59, 135.83, 131.23, 130.98, 130.82, 130.73, 130.62, 130.45, 130.16, 129.93, 129.57, 129.00, 128.65, 128.13, 127.99, 127.46, 125.75, 85.90 (CB–C), 85.83 (CB–C), 19.82 (−CH₃). ¹¹B{¹H} NMR (CD₂Cl₂): δ −2.63 (br s, 2B), −9.21 (br s, 4B), −10.61 (br s, 4B). Anal. calcd for C₂₇H₃₀B₁₀: C, 70.10; H, 6.54. Found: C, 70.05; H, 6.25.

Data for 7HD

Decaborane (0.17 g, 1.4 mmol), Et₂S (0.29 mL, 2.7 mmol), and 7HDA (0.32 g, 0.90 mmol) in toluene afforded 7HD as a white solid. Recrystallization from DCM provided 0.30 g (0.63 mmol) of crystalline 7HD. Yield = 70%. ¹H{¹¹B} NMR (CD₂Cl₂): δ 7.37 (d, J = 8.0 Hz, 3H), 7.32 (d, J = 8.0 Hz, 3H), 7.21 (t, J = 7.4 Hz, 2H), 7.12 (m, 3H), 7.05 (d, J = 7.8 Hz, 1H), 6.87 (m, 4H), 3.20 (br s, 1H, CB–BH), 2.46 (br s, 6H, CB–BH), 2.27 (br s, 3H, CB–BH), 1.72 (s, 6H, −CH₃). ¹³C NMR (CD₂Cl₂): δ 143.77, 140.45, 139.38, 139.27, 136.13, 131.01, 130.89, 130.73, 130.61, 130.39, 130.14, 129.04, 129.02, 128.61, 128.37, 127.94, 127.59, 127.34, 85.85 (CB–C), 85.79 (CB–C), 20.71 (−CH₃). ¹¹B{¹H} NMR (CD₂Cl₂): δ −2.64 (br s, 2B), −9.22 (br s, 4B), −10.62 (br s, 4B). Anal. calcd for C₂₈H₃₂B₁₀: C, 70.55; H, 6.77. Found: C, 70.35; H, 6.51.

Data for 8HP

Decaborane (0.55 g, 4.5 mmol), Et₂S (0.97 mL, 9.0 mmol), and 8HPA (0.99 g, 3.0 mmol) in toluene afforded 8HP as a white solid. Recrystallization from DCM provided 0.81 g (1.8 mmol) of crystalline 8HP. Yield = 60%. ¹H{¹¹B} NMR (CD₂Cl₂): δ 7.45 (d, J = 8.0 Hz, 2H), 7.39 (m, 3H), 7.34 (d, J = 6.8 Hz, 1H), 7.28 (t, J = 7.4 Hz, 3H), 7.21 (t, J = 8.0 Hz, 2H), 7.18 (d, J = 7.8 Hz, 1H), 7.10 (t, J = 7.5 Hz, 2H), 6.92 (d, J = 7.9 Hz, 4H), 3.25 (br s, 1H, CB–BH), 2.49 (br s, 6H, CB–BH), 2.30 (br s, 3H, CB–BH). ¹³C NMR (CD₂Cl₂): δ 144.14, 141.24, 140.84, 139.16, 131.10, 131.03, 130.92, 130.67, 130.67, 130.63, 130.10, 130.03, 129.11, 128.70, 128.49, 128.34, 127.95, 126.91, 85.92 (CB–C), 85.83 (CB–C). ¹¹B{¹H} NMR (CD₂Cl₂): δ −2.66 (br s, 2B), −9.15 (br s, 4B), −10.57 (br s, 4B). Anal. calcd for C₂₆H₂₈B₁₀: C, 69.61; H, 6.29. Found: C, 69.55; H, 6.11.

Data for 9HM

Decaborane (0.18 g, 1.4 mmol), Et₂S (0.31 mL, 2.9 mmol), and 9HMA (0.26 g, 0.96 mmol) in toluene afforded 9HM as a white solid. Recrystallization from a mixture of acetone/MeOH provided 0.25 g (0.64 mmol) of crystalline 9HM. Yield = 67%. ¹H{¹¹B} NMR (CD₂Cl₂): δ 7.45 (d, J = 8.0 Hz, 4H), 7.24 (t, J = 8.2 Hz, 1H), 7.19 (d, J = 7.4 Hz, 2H), 7.14 (t, J = 7.8 Hz, 3H), 7.07 (d, J = 8.0 Hz, 2H), 7.03 (d, J = 8.0 Hz, 1H), 3.30 (br s, 2H, CB–BH), 2.53 (br s, 5H, CB–BH), 2.31 (br s, 3H, CB–BH), 2.04 (s, 3H, −CH₃). ¹³C NMR (CD₂Cl₂): δ 144.39, 140.58, 135.57, 131.10, 130.92, 130.86, 130.76, 130.62, 129.63, 129.43, 129.40, 128.64, 128.16, 126.19, 85.91 (CB–C), 85.79 (CB–C), 20.31 (−CH₃). ¹¹B{¹H} NMR (CD₂Cl₂): δ −2.57 (br s, 2B), −9.14 (br s, 4B), −10.52 (br s, 4B). Anal. calcd for C₂₁H₂₆B₁₀: C, 65.25; H, 6.78. Found: C, 65.15; H, 6.62.

Data for 10HV

Decaborane (0.15 g, 1.3 mmol), Et₂S (0.27 mL, 2.5 mmol), and 10HVA (0.23 g, 0.83 mmol) in toluene afforded 10HV as a white solid. Recrystallization from n-hexane provided 0.19 g (0.48 mmol) of crystalline 10HV. Yield = 58%. ¹H{¹¹B} NMR (CD₂Cl₂): δ 7.59 (d, J = 7.6 Hz, 1H), 7.48 (d, J = 8.0 Hz, 4H), 7.33 (t, J = 8.0 Hz, 4H), 7.28 (d, J = 7.8 Hz, 4H), 7.18 (t, J = 7.7 Hz, 2H), 7.13 (t, J = 6.6 Hz, 3H), 6.35 (dd, J = 11.0, 8.4 Hz, 1H), 5.62 (d, J = 8.8 Hz, 1H), 5.13 (d, J = 8.8 Hz, 1H), 3.33 (br s, 2H, CB–BH), 2.56 (br s, 6H, CB–BH), 2.34 (br s, 2H, CB–BH). ¹³C NMR (CD₂Cl₂): δ 143.28, 139.41, 136.02, 135.61, 131.12, 130.90, 130.80, 130.62, 129.99, 129.97, 129.65, 128.67, 128.44, 128.11, 126.24, 115.54, 85.92 (CB–C), 85.68 (CB–C). ¹¹B{¹H} NMR (CD₂Cl₂): δ −2.61 (br s, 2B), −9.19 (br s, 4B), −10.54 (br s, 4B). Anal. calcd for C₂₂H₂₆B₁₀: C, 66.30; H, 6.58. Found: C, 66.15; H, 6.52.

Data for 11HI

Decaborane (0.18 g, 1.5 mmol), Et₂S (0.32 mL, 3.0 mmol), and 11HIA (0.30 g, 1.0 mmol) in toluene afforded 11HI as a white solid. Recrystallization from n-hexane provided 0.18 g (0.44 mmol) of crystalline 11HI. Yield = 44%. ¹H{¹¹B} NMR (CDCl₃): δ 7.43 (dd, J = 7.6, 3.5 Hz, 4H), 7.32 (d, J = 3.6 Hz, 2H), 7.23 (d, J = 7.3 Hz, 1H), 7.14 (t, J = 7.7 Hz, 3H), 7.04 (d, J = 7.8 Hz, 2H), 7.00 (d, J = 7.8 Hz, 1H), 3.29 (br s, 2H, CB–BH), 2.65 (m, 1H, −CH(CH₃)₂), 2.57 (br s, 5H, CB–BH), 2.37 (br s, 3H, CB–BH), 1.04 (d, J = 6.8 Hz, 6H, −CH(CH₃)₂). ¹³C NMR (CDCl₃): δ 146.21, 144.16, 139.51, 130.81, 130.78, 130.43, 130.13, 129.48, 129.20, 128.30, 128.28, 125.70, 125.51, 85.32 (CB–C), 85.23 (CB–C), 29.32 (−CH(CH₃)₂), 24.16 (−CH(CH₃)₂). ¹¹B{¹H} NMR (CDCl₃): δ −3.33 (br s, 2B), −10.15 (br s, 4B), −11.40 (br s, 4B). Anal. calcd for C₂₃H₃₀B₁₀: C, 66.63; H, 7.29. Found: C, 66.58; H, 7.21.

Data for 12MM

Decaborane (0.15 g, 1.2 mmol), Et₂S (0.26 mL, 2.4 mmol), and 12MMA (0.23 g, 0.81 mmol) in toluene afforded 12MM as a white solid. Recrystallization from DCM provided 0.15 g (0.38 mmol) of crystalline 12MM. Yield = 47%. ¹H{¹¹B} NMR (CDCl₃): δ 7.44 (dd, J = 8.8, 4.0 Hz, 4H), 7.22 (t, J = 7.3 Hz, 1H), 7.13 (q, J = 7.3 Hz, 3H), 7.05 (d, J = 7.4 Hz, 2H), 6.90 (d, J = 8.0 Hz, 2H), 3.29 (br s, 2H, CB–BH), 2.58 (br s, 5H, CB–BH), 2.37 (br s, 3H, CB–BH), 1.80 (s, 6H, −CH₃). ¹³C NMR (CDCl₃): δ 143.32, 140.28, 135.66, 131.01, 130.76, 130.10, 129.12, 129.03, 128.23, 127.58, 127.46, 85.26 (CB–C), 85.18 (CB–C), 20.64 (−CH₃). ¹¹B{¹H} NMR (CDCl₃): δ −3.37 (br s, 2B), −10.36 (br s, 4B), −11.46 (br s, 4B). Anal. calcd for C₂₂H₂₈B₁₀: C, 65.97; H, 7.05. Found: C, 65.91; H, 7.04.

Data for 13PP

Decaborane (51 mg, 0.42 mmol), Et₂S (90 μL, 0.84 mmol), and 13PPA (95 mg, 0.28 mmol) in toluene afforded 13PP as a white solid. Recrystallization from a mixture of DCM/n-hexane provided 70 mg (0.15 mmol) of crystalline 13PP. Yield = 55%. ¹H{¹¹B} NMR (CD₂Cl₂): δ 7.45 (t, J = 6.8 Hz, 4H), 7.27 (q, J = 7.6 Hz, 2H), 7.14 (t, J = 8.8 Hz, 4H), 6.92 (d, J = 8.2 Hz, 2H), 3.33 (br s, 2H, CB–BH), 2.56 (br s, 6H, CB–BH), 2.34 (br s, 2H, CB–BH), 2.16 (m, 2H, −CH(CH₃)₂), 0.95 (d, J = 6.8 Hz, 12H, −CH(CH₃)₂). ¹³C NMR (CD₂Cl₂): δ 146.83, 143.48, 138.30, 131.11, 130.89, 130.81, 130.42, 129.84, 129.15, 128.55, 128.47, 122.85, 85.94 (CB–C), 85.90 (CB–C), 30.45 (−CH(CH₃)₂), 24.12 (−CH(CH₃)₂). ¹¹B{¹H} NMR (CD₂Cl₂): δ −2.46 (br s, 2B), −9.29 (br s, 4B), −10.61 (br s, 4B). Anal. calcd for C₂₆H₃₆B₁₀: C, 68.38; H, 7.95. Found: C, 68.31; H, 7.90.

Synthesis of 3HB

DCE (5.0 mL) was added via a cannula to a mixture of 4HN (0.20 g, 0.50 mmol), TsOH (95 mg, 0.50 mmol), and DDQ (0.34 g, 1.5 mmol). The reaction mixture was then refluxed at 90 °C for 12 h. The volatiles were removed under vacuum to afford a dark green residue. Purification through column chromatography on silica gel (eluent: ethyl acetate/n-hexane = 1/20, v/v) gave 3HB (0.17 g) as a white solid. Yield = 85%. ¹H{¹¹B} NMR (CDCl₃): δ 8.54 (d, J = 7.7 Hz, 1H), 8.41 (d, J = 8.8 Hz, 1H), 7.96 (s, 1H), 7.85 (d, J = 8.0 Hz, 1H), 7.72 (d, J = 8.9 Hz, 1H), 7.62 (m, 4H), 7.48 (d, J = 7.8 Hz, 2H), 7.11 (dt, J = 8.4, 4.6 Hz, 3H), 3.43 (br s, 2H, CB–BH), 2.66 (br s, 3H, CB–BH), 2.59 (br s, 3H, CB–BH), 2.42 (br s, 2H, CB–BH). ¹³C NMR (CDCl₃): δ 132.63, 131.47, 131.34, 131.17, 130.77, 130.74, 130.32, 129.44, 128.80, 128.78, 128.43, 128.18, 127.90, 127.67, 127.17, 126.80, 123.05, 122.97, 85.65 (CB–C), 85.30 (CB–C). ¹¹B{¹H} NMR (CDCl₃): δ −2.26 (br s, 2B), −8.94 (br s, 4B), −10.28 (br s, 4B). Anal. calcd for C₂₂H₂₄B₁₀: C, 66.64; H, 6.10. Found: C, 66.48; H, 6.19.

Ultraviolet–Visible Absorption and Photoluminescence Measurements

Ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectra of all biphenyl-based o-carboranyl compounds 1HF–13PP were acquired using V530 (Jasco) and FluoroMax-4P (HORIBA) spectrophotometers, respectively. The UV–vis absorption and PL measurements for the solution were performed at 298 K in oxygen-free and anhydrous THF with a 1 cm quartz cuvette (3.0 × 10^–5 M). Furthermore, the PL spectra were measured in THF solution (3.0 × 10^–5 M) at 77 K, in THF/distilled water mixtures (3.0 × 10^–5 M) at 298 K, in the crystalline state at 298 K, and in the film state at 298 K. Thin films of o-carborane compounds in PMMA were obtained by spin coating THF solution (1 mL) of PMMA (50 mg) containing each o-carborane compound (5 wt % vs PMMA) on a quartz plate of dimensions 10 mm × 10 mm (thickness = 1 mm). The absolute PL quantum yields (PLQYs) in the crystalline state, THF solution (3.0 × 10^–5 M), and film state were obtained at 298 K on a Fluoromax-4P spectrophotometer equipped with a 3.2 inch integrating sphere (FM-sphere, HORIBA). Fluorescence decay lifetimes of o-carborane compounds in the crystalline and film states were measured using a time-correlated single-photon counting spectrometer (FLS920-Edinburgh instrument at the Central Laboratory of Kangwon National University) equipped with an EPL-375ps pulsed semiconductor diode laser excitation source and a microchannel plate photomultiplier tube (MCP-PMT, 200–850 nm) detector.

X-ray Crystallography

Single X-ray quality 1HF–13PP crystals were grown from each mixture of organic solvents for X-ray diffractometry. Each single crystal was coated with Paratone oil and mounted on a glass capillary. Crystallographic measurements were performed using a Bruker D8 Quest CCD area detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structures of o-carboranyl compounds were assessed using direct methods, and all non-hydrogen atoms were subjected to anisotropic refinement with a full-matrix least-squares method on F² using a SHELXTL/PC software package. X-ray crystallographic data are available in the CIF format (CCDC 2142550–2142562 for 1HF–13PP) and are provided free of charge by the Cambridge Crystallographic Data Centre. 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 and selected bond lengths and angles are provided in Tables S1 and S2.

Computational Calculation Studies

The geometries of all biphenyl-based o-carboranyl compounds 1HF–13PP in their ground (S₀) and first-excited (S₁) states in THF were optimized at the B3LYP/6-31G(d,p)⁴⁴ level of theory. Vertical excitation energies in the optimized S₀ and S₁ state geometries were calculated using the TD-DFT method³⁸ at the same level of theory. The solvent effects were evaluated using a self-consistent reaction field method based on the integral equation formalism of the polarizable continuum model with THF as the solvent.⁴⁵ Most stable geometries were determined by constructing one-dimensional potential energy surfaces as a function of each dihedral angle (Ψ, Figure 5 and Table S4) by rotating a phenyl ring of each biphenyl group between approximately 0 and 90° at intervals of 5° to yield 19 initial conformations for each compound. Conformations that exhibited physically impossible atomic overlaps were excluded from further geometric optimizations. The dihedral angle was fixed, while other geometric variables were fully relaxed for geometry optimization and energy calculation of the resulting initial conformations using the Gaussian 16 software program.⁴⁶ The contribution (%) of a group in a molecule for each molecular orbital was calculated using the GaussSum 3.0 software program.⁴⁷

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c03344.

• ¹H, ¹H{¹¹B}, ¹³C, and ¹¹B{¹H} NMR spectra for the o-carboranyl compounds and their precursors; crystallographic data and parameters; UV–vis absorption and PL spectra for the o-carboranyl compounds and 1,1′-biphenyl; emission decay curves; and computational calculation details (PDF)

• Crystallographic data for 13PP (CIF)

• Crystallographic data for 12MM (CIF)

• Crystallographic data for 11HI (CIF)

• Crystallographic data for 10HV (CIF)

• Crystallographic data for 9HM (CIF)

• Crystallographic data for 3HB(CIF)

• Crystallographic data for 8HP(CIF)

• Crystallographic data for 7HD(CIF)

• Crystallographic data for 6HC(CIF)

• Crystallographic data for 4HN(CIF)

• Crystallographic data for 2HT(CIF)

• Crystallographic data for 1HF (CIF)

• Crystallographic data for 5HH (CIF)

Author Contributions

All authors have given approval to the final version of the article.

The authors declare no competing financial interest.