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Article

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

by
Hyomin Jin
1,†,
Seonah Kim
1,†,
Hye Jin Bae
2,
Ji Hye Lee
1,
Hyonseok Hwang
1,
Myung Hwan Park
3,* and
Kang Mun Lee
1,*
1
Department of Chemistry, Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon 24341, Korea
2
Department of Chemistry, KAIST, Daejeon 34142, Korea
3
Department of Chemistry Education, Chungbuk National University, Cheongju 28644, Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2019, 24(1), 201; https://doi.org/10.3390/molecules24010201
Submission received: 5 December 2018 / Revised: 31 December 2018 / Accepted: 3 January 2019 / Published: 7 January 2019
(This article belongs to the Special Issue Advances in Materials Derived from Polyhedral Boron Clusters)
Browse Figures
Versions Notes

Abstract

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

1. Introduction

Icosahedral carboranes (C2B10H12) 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-C2B10H12) have been widely investigated as a novel family of optoelectronic materials for various functional and photonic applications [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23] 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) transition between the two components [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51]. Thus, the unprecedented luminescent properties of numerous o-carboranyl compounds emerge from the ICT-based emissive features [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57]. For those reasons, o-carborane-containing organic and organometallic luminophores have been extensively spotlighted in optoelectronic research fields.
Recently, it has been reported that the intrinsic nature derived from the ICT transition in o-carboranyl compounds can be controlled by molecular geometry and structural fluctuation. Fox and co-workers [32] have reported a series of C-diazaboryl-o-carboranes that exhibit switching of emissive states between a locally excited high-energy state and low-energy ICT state. This phenomenon depends on the dihedral angle between the diazaboryl moiety and C–C bond of o-carborane. In addition, various fluorophores possessing o-carboranyl groups have exhibited multiple photoluminescence (PL) originating from the alternation of the twisted ICT state [43,44,45,46,47,48,49,50,51]. These results clearly indicate that structural features can play an important role in controlling the intrinsic photophysical and electronic characteristics of o-carboranyl compounds.
Thus far, there have been several studies on the unique emission behavior of fluorescent aromatic derivatives conjugated with the C atoms of o-carborane, which result from structural variations such as the rotation of appended aromatic groups. However, a detailed investigation of the correlation between such structural variations and the photophysical properties of o-carboranyl compounds has been rarely performed. Our group has recently presented for the first time the dramatic change of the emissive ICT transition caused by distortion of biphenyl rings through comparison of the photophysical properties of biphenyl- and fluorene-based o-carboranyl compounds [58].
Therefore, to intimately investigate the efect of structural variation in the aromatic group appended to o-carborane, we examined the photophysical properties of a series of o-carboranyl compounds, namely 2-phenylpyridine (ppy) and 2-(benzo[b]thiophen-2-yl)pyridine (btp) with an o-carborane substituent at the C4 position of the pyridine ring (CB1 and CB2, respectively, Figure 1), and their corresponding Me2B-C∧N-chelated compounds CB1B and CB2B. The UV-vis and PL experiments were performed to examine how the efficiency of the emissive ICT transition is affected by structural variation, especially the planarity of the aryl groups linked to the o-carborane cage. Moreover, the experimental and theoretical structural features and electronic transition states of CB1, CB2, CB1B, and CB2B are presented herein in detail.

2. Materials and Methods

2.1. General Considerations

All operations were carried out under an inert N2 atmosphere using standard Schlenk and glove box techniques. Anhydrous-grade toluene and tetrahydrofuran (THF) (Sigma–Aldrich, St. Louis, MO, USA) were dried by passing through an activated alumina column and storing over activated molecular sieves (5 Å). Spectrophotometric-grade toluene was used as received from Alfa Aesar. Na2CO3, copper(I) iodide (CuI), diethyl sulfide (Et2S), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4), triethylamine, bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2), boron tribromide (BBr3, 1.0 M in dichloromethane), trimethylaluminum (AlMe3, 2.0 M in toluene), and poly(methylmethacrylate) (PMMA) were purchased from Sigma–Aldrich; n-hexane, dichloromethane, methanol, acetone, and ethyl acetate, from Alfa Aesar; benzo[b]thien-2-yl-boronic acid, phenylboronic acid, 2-chloro-4-iodopyridine, 1-hexyne, trimethylamine, and N,N-diisopropylamine, from TCI Chemicals; and decaborane (B10H14), from KatChem. These commercially available reagents and solvents were used without further purification. Deuterated solvents were purchased from Cambridge Isotope Laboratories and used after drying over activated molecular sieves (5 Å). The NMR spectra of all compounds were recorded at ambient temperature on the Bruker Avance 400 spectrometer (400.13, 100.62, and 128.38 MHz for 1H, 13C, and 11B, respectively). Chemical shifts are given in ppm and referenced against either external Me4Si (1H and 13C) or BF3·Et2O (11B). Elemental analyses were performed on the EA3000 elemental analyzer (Eurovector, at the Central Laboratory 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 (Φem) were obtained at 298 K using an absolute PL quantum yield spectrophotometer (FM-SPHERE, 3.2-inch internal integrating sphere in FluoroMax-4P).

2.2. Synthesis of 2-Chloro-4-(hex-1-yn-1-yl)pyridine (3)

Toluene (7 mL) and triethylamine (1:9, v/v) were added using a cannula to a mixture of 2-chloro-4-iodopyridine (1.80 g, 7.5 mmol), CuI (100 mg), and Pd(PPh3)2Cl2 (289 mg) at room temperature. After stirring the resulting dark brown slurry for 15 min, 1-hexyne (1.29 mL, 11.3 mmol) was added. The reaction mixture was then refluxed at 80 °C for 24 h. The volatiles were removed using a rotary evaporator to afford a dark-gray residue. The crude product was purified by silica column chromatography (eluent:dichloromethane/n-hexane = 1:2) to yield 3 as an ivory solid (1.43 g, Yield = 98%). 1H NMR (CDCl3): δ 8.28 (d, J = 5.1 Hz, 1H), 7.28 (s, 1H), 7.15 (dd, J = 5.1, 1.3 Hz, 1H), 2.43 (t, J = 7.0 Hz, 2H), 1.59 (m, 2H), 1.47 (m, 2H), 0.95 (t, J = 7.3 Hz, 3H). 13C NMR (CDCl3): δ 151.60, 149.38, 135.28, 126.24, 124.46, 97.75 (acetylene-C), 77.44 (acetylene-C), 30.33, 22.02, 19.18, 13.56. Anal. Calcd for C11H12ClN: C, 68.22; H, 6.25; N, 7.23. Found: C, 68.01; H, 6.11; N, 7.14.

2.3. Synthesis of 1a

To a mixture of 3 (0.77 g, 4 mmol) and Pd(PPh3)4 (0.60 g, 0.52 mmol) in THF (30 mL) was successively added phenylboronic acid (0.58 g, 4.8 mmol) and Na2CO3 (1.27 g, 12.0 mmol) in H2O (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 MgSO4 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%). 1H NMR (CDCl3): δ 8.58 (d, J = 5.0 Hz, 1H), 7.96 (d, J = 8.0 Hz, 2H), 7.69 (s, 1H), 7.42 (m, 3H), 7.17 (d, J = 5.0 Hz, 1H), 2.44 (t, J = 7.1 Hz, 2H), 1.61 (m, 2H), 1.49 (m, 2H), 0.95 (t, J = 7.3 Hz, 3H). 13C NMR (CDCl3): δ 157.44, 149.49, 138.96, 133.03, 129.09, 128.72, 126.90, 124.07, 122.75, 95.65 (acetylene-C), 78.74 (acetylene-C), 30.50, 22.04, 19.19, 13.60. Anal. Calcd for C17H17N: C, 86.77; H, 7.28; N, 5.95. Found: C, 86.44; H, 7.07; N, 5.60.

2.4. Synthesis of 2a

A procedure analogous to that for 1a using 3 (0.58 g, 3.0 mmol), benzo[b]thiophene-2-ylboronic acid (0.64 g, 3.6 mmol), Pd(PPh3)4 (0.35 g, 0.30 mmol), and Na2CO3 (0.95 g, 9.0 mmol) was employed to afford 2a as a white solid (0.77 g, Yield = 88%). 1H NMR (CDCl3): δ 8.52 (d, J = 5.1 Hz, 1H), 7.85 (t, J = 4.6 Hz, 1H), 7.81 (s, 1H), 7.78 (t, J = 4.6 Hz, 1H), 7.75 (s, 1H), 7.34 (m, 2H), 7.14 (dd, J = 5.1, 1.4 Hz, 1H), 2.45 (t, J = 7.1 Hz, 2H), 1.62 (m, 2H), 1.49 (m, 2H), 0.96 (t, J = 7.3 Hz, 3H). 13C NMR (DMSO): δ 152.56, 149.46, 144.28, 140.67, 140.36, 132.97, 125.06, 124.48, 124.11, 122.54, 121.66, 121.30, 96.21 (acetylene-C), 78.38 (acetylene-C), 30.43, 22.02, 19.18, 13.58. Anal. Calcd for C19H7NS: C, 78.31; H, 5.88; N, 4.81. Found: C, 78.12; H, 5.70; N, 4.65.

2.5. Synthesis of CB1

To a toluene solution (100 mL) of B10H14 (0.46 g, 3.77 mmol) and 1a (0.68 g, 2.90 mmol) was slowly added an excess amount of Et2S (2.5 equiv. for B10H14) 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%). 1H NMR (CDCl3): δ 8.72 (d, J = 5.3 Hz, 1H), 7.98 (d, J = 6.8 Hz, 2H), 7.91 (d, J = 0.8 Hz, 1H), 7.49 (m, 3H), 7.43 (dd, J = 5.3, 1.8 Hz, 1H), 3.70‒1.61 (br, 10H, CB-BH), 1.79 (t, J = 8.6 Hz, 2H), 1.38 (m, 2H), 1.09 (m, 2H), 0.73 (t, J = 7.3 Hz, 3H). 13C NMR (CDCl3): δ 158.69, 150.45, 140.06, 138.09, 129.90, 129.04, 127.09, 123.28, 122.08, 82.35 (CB-C), 80.75 (CB-C), 35.08, 31.63, 22.10, 13.52. 11B NMR (CDCl3): δ −3.73 (br s, 1B), −4.57 (br s, 1B), −10.83 (br s, 8B). Anal. Calcd for C17H27B10N: C, 57.76; H, 7.70; N, 3.96. Found: C, 57.70; H, 7.55; N, 3.72.

2.6. Synthesis of CB2

A procedure analogous to that for CB1 using B10H14 (0.42 g, 3.45 mmol), 2a (0.77 g, 2.65 mmol), and Et2S (2.5 equiv. for B10H14) was employed. Recrystallization from an acetone/methanol mixture afforded CB2 as a yellow solid (0.45 g, Yield = 41%). 1H NMR (CDCl3): δ 8.65 (d, J = 5.3 Hz, 1H), 7.97 (s, 1H), 7.90 (s, 1H), 7.86 (m, 2H), 7.38 (m, 3H), 3.56‒1.68 (br, 10H, CB-BH), 1.81 (t, J = 8.0 Hz, 2H), 1.40 (m, 2H), 1.10 (m, 2H), 0.74 (t, J = 7.3 Hz, 3H). 13C NMR (CDCl3): 153.81, 150.50, 143.28, 140.93, 140.21, 140.05, 125.69, 124.85, 124.45, 123.56, 122.65, 122.36, 120.94, 82.40 (CB-C), 80.40 (CB-C), 35.10, 31.64, 22.10, 13.51. 11B NMR (CDCl3): δ −3.65 (br s, 1B), −4.47 (br s, 1B), −10.82 (br s, 8B). Anal. Calcd for C19H27B10NS: C, 55.72; H, 6.64; N, 3.42. Found: C, 55.50; H, 6.44; N, 3.22.

2.7. 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 BBr3 (1.0 M in dichloromethane, 3.5 mL, 3.5 mmol). After stirring the reaction mixture at room temperature for 48 h, saturated K2CO3 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 MgSO4 and filtered. Evaporation of the solvent under reduced pressure afforded a crude solid residue. Recrystallization from a dichloromethane/n-hexane mixture afforded the BBr2-C∧N-chelated compound of CB1 as a white solid (0.50 g, Yield = 81%). This compound was characterized by 1H NMR spectra only and then used in the subsequent methylation reaction in situ. 1H NMR (CDCl3): δ 8.93 (d, J = 6.3 Hz, 1H), 8.05 (d, J = 1.5 Hz, 1H), 7.87 (d, J = 7.4 Hz, 1H), 7.82 (d, J = 7.7 Hz, 1H), 7.72 (dd, J = 6.3, 1.9 Hz, 1H), 7.61 (t, J = 7.4 Hz, 1H), 7.46 (t, J = 7.6 Hz, 1H), 3.44‒1.75 (br, 10H, CB-BH), 1.86 (t, J = 8.5 Hz, 2H), 1.45 (m, 2H), 1.17 (m, 2H), 0.78 (t, J = 7.3 Hz, 3H).
To a stirred solution of the BBr2-C∧N-chelated compound of CB1 (0.30 g, 0.57 mmol) in toluene (5.0 mL) at room temperature was added AlMe3 (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 MgSO4 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%). 1H NMR (CDCl3): δ 8.45 (d, J = 6.1 Hz, 1H), 8.11 (s, 1H), 7.89 (d, J = 7.7 Hz, 1H), 7.65 (d, J = 7.2 Hz, 1H), 7.53 (dd, J = 6.0, 1.7 Hz, 1H), 7.47 (t, J = 7.2 Hz, 1H), 7.33 (t, J = 7.5 Hz, 1H), 3.46‒1.66 (br, 10H, CB-BH), 1.84 (t, J = 8.5 Hz, 2H), 1.42 (m, 2H), 1.13 (m, 2H), 0.74 (t, J = 6.3 Hz, 3H), 0.05 (s, 6H, B(CH3)2). 13C NMR (CDCl3): δ 157.84, 142.82, 142.70, 134.03, 131.30, 129.47, 125.56, 122.69, 121.93, 119.65, 82.80 (CB-C), 79.52 (CB-C), 35.23, 31.70, 22.06, 13.50, 8.83 (B(CH3)2). 11B NMR (CDCl3): δ 0.53 (B(CH3)2, 1B), −3.19 (br s, 1B), −4.48 (br s, 1B), −10.71 (br s, 8B). Anal. Calcd for C19H32B11N: C, 58.01; H, 8.20; N, 3.56. Found: C, 57.92; H, 8.10; N, 3.34.

2.8. Synthesis of CB2B

A procedure analogous to that for the BBr2-C∧N-chelated compound of CB1 using CB2 (0.45 g, 1.09 mmol), diisopropylamine (0.20 mL, 1.39 mmol), and BBr3 (1.0 M in dichloromethane, 3.27 mL, 3.27 mmol) was employed. Recrystallization from a dichloromethane/n-hexane mixture afforded the BBr2-C∧N-chelated compound of CB2 as a yellow solid (0.57 g, Yield = 90%). This compound was characterized by 1H NMR spectra only and then used in the subsequent methylation reaction in situ. 1H NMR (CDCl3): δ 8.86 (d, J = 6.3 Hz, 1H), 8.21 (dd, J = 6.8, 1.8 Hz, 1H), 7.92 (dd, J = 6.9, 1.5 Hz, 1H), 7.68 (d, J = 1.5 Hz, 1H), 7.59 (dd, J = 6.3, 1.8 Hz, 1H), 7.50 (m, 1H), 3.47‒1.68 (br, 10H, CB-BH), 1.89 (t, J = 8.52 Hz, 2H), 1.45 (m, 2H), 1.19 (m, 2H), 0.79 (t, J = 7.3 Hz, 3H).
A procedure analogous to that for CB1B using the BBr2-C∧N-chelated compound of CB2 (0.30 g, 0.52 mmol) and AlMe3 (2.0 M in toluene, 0.55 mL, 1.1 mmol) was employed. Recrystallization from a dichloromethane/n-hexane mixture afforded CB2B as a pale-yellow solid (0.10 g, Yield = 43%). 1H NMR (CDCl3): δ 8.39 (d, J = 6.1 Hz, 1H), 8.00 (m, 1H), 7.92 (dd, J = 6.3, 2.8 Hz, 1H), 7.71 (d, J = 1.5 Hz, 1H), 7.41 (m, 3H), 3.57‒1.74 (br, 10H, CB-BH), 1.86 (t, J = 8.54 Hz, 2H), 1.43 (m, 2H), 1.15 (m, 2H), 0.75 (t, J = 7.3 Hz, 3H), 0.15 (s, 6H, B(CH3)2). 13C NMR (CDCl3): δ 154.49, 146.38, 143.03, 142.96, 140.26, 131.99, 126.68, 126.46, 124.71, 123.47, 120.60, 119.44, 82.82 (CB-C), 79.40 (CB-C), 35.21, 31.72, 22.07, 13.52, 8.13 (B(CH3)2). 11B NMR (CDCl3): δ 0.14 (B(CH3)2, 1B), −3.19 (br s, 1B), −4.47 (br s, 1B), −10.56 (br s, 8B). Anal. Calcd for C19H32B11N: C, 55.72; H, 6.64; N, 3.42. Found: C, 55.62; H, 6.42; N, 3.25.

2.9. 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 (Φem) 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).

2.10. 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 F2 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.

2.11. Theoretical Calculations

The ground (S0) and first excited state (S1) 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].

3. Results and Discussion

3.1. Synthesis and Characterization

The synthetic pathways for all compounds with an o-carboranyl unit appended at the C4 position of the pyridine ring (CB1, CB1B, CB2, and CB2B) are illustrated in Figure 1. The Sonogashira reaction between 1-hexyne and 2-chloro-4-iodopyridine produces 3 in high yield (98%). After the Suzuki–Miyaura coupling reaction between 3 and phenylboronic acid (for 1a) or benzo[b]thien-2-yl-boronic acid (for 2a), CB1 and CB2 are synthesized via cage formation with decaborane (B10H14) in the presence of Et2S (Scheme 1) [58,63,64,65]. The C∧N-chelated compounds CB1B and CB2B were finally obtained in moderate yield (61% and 42%, respectively) via simple borylation of CB1 and CB2, respectively, with BBr3, followed by treatment with AlMe3.
All o-carboranyl compounds were fully confirmed by multinuclear (1H, 13C, and 11B) NMR spectroscopy (Figures S1–S7) and elemental analysis. The 1H and 13C 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 11B 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.

3.2. 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 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 λem = 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 BMe2-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 S0 and S1 states distinctly present evidence supporting structural fluctuation. Specifically, the C−C bond length (2.38–2.42 Å) in o-carborane at the S1 state (the structure that the one side of icosahedron is elongated) becomes significantly longer than that at the S0 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 Φem 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 S0 and S1 structures of CB2 specifically exhibit considerably similar dihedral angle between the pyridine and benzothiophene rings (Ψcalc = 0.8° for S0 and 1.2° for S1), whereas those of CB1 show significantly different Ψcalc (22.2° for S0 and 0.3° for S1) (Table 2). The experimental dihedral angle for CB2 (Ψexp = 2.9°), determined from the molecular structure at the solid state, is also similar to Ψcalc. 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 S0 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 Φem, moderate radiative decay constant (kr = 2.2 × 107 s−1), and low non-radiative decay constant (knr = 1.5 × 108 s−1) in solution at 298 K (Table 1) [58]. The measured Φem 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 Φem 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 knr values of both CB1B and CB2B in the film state increase by more than twice (1.6 × 107 and 1.1 × 108 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 Φem nearly twice enhanced (25%) as that in solution at 298 K; however, the Φem 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 kr of CB2 in the film state (1.04 × 109 s−1) is 50 times higher than that in solution (2.20 × 107 s−1) at 298 K (Table 1).

3.3. 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 (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 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 UV-vis absorption spectra well.
Based on the computational data for the S1 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.

4. Conclusions

The ppy- and btp-based o-carboranyl compounds (CB1 and CB2) and their BMe2-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 S0 and S1 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.

Supplementary Materials

The following are available online at https://www.mdpi.com/1420-3049/24/1/201/s1, multinuclear NMR spectra (1H, 13C, and 11B) of o-carboranyl compounds (Figures S1–S7), X-ray crystallographic data in CIF format (Tables S1 and S2), and computational data (Figures S11–S15 and Tables S3–S19).

Author Contributions

H.J., S.K., and H.J.B. performed the experiments for synthesis of compounds and analyzed the data; M.H.P. and K.M.L. analyzed the data and wrote the paper; J.H.L. and H.H. performed the experiments for theoretical calculation and analyzed these data and wrote the paper.

Funding

This work was supported by the Basic Science Research Program (2016R1C1B1008452 for M. H. Park and 2018R1D1A1B07040387 for K. M. Lee) and the Basic Research Laboratory (2017R1A4A1015405 for K. M. Lee) funded by the Ministry of Science, ICT through the National Research Foundation of Korea (NRF).

Conflicts of Interest

The authors declare no conflict of interest.

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Sample Availability: Samples of the o-carboranyl compounds (CB1, CB2, CB1B, and CB2B) are available from the authors.
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Figure 1. Synthetic procedure for o-carboranyl compounds (CB1, CB2, CB1B, and CB2B). Reaction conditions: (i) 1-hexyne, CuI, PdCl2(PPh3)2, NEt3/toluene, r.t., 24 h. (ii) Pd(PPh3)4, Na2CO3, THF/H2O (4:1, v/v), 80 °C, 24 h. (iii) B10H14, Et2S, toluene, 110 °C, 72 h. (iv) BBr3, (i-Pr)2NEt, DCM, r.t., 48 h. (v) AlMe3, toluene, r.t., 0.5 h.
Figure 1. Synthetic procedure for o-carboranyl compounds (CB1, CB2, CB1B, and CB2B). Reaction conditions: (i) 1-hexyne, CuI, PdCl2(PPh3)2, NEt3/toluene, r.t., 24 h. (ii) Pd(PPh3)4, Na2CO3, THF/H2O (4:1, v/v), 80 °C, 24 h. (iii) B10H14, Et2S, toluene, 110 °C, 72 h. (iv) BBr3, (i-Pr)2NEt, DCM, r.t., 48 h. (v) AlMe3, toluene, r.t., 0.5 h.
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Figure 2. X-ray crystal structures of CB2 (left) and CB2B (right) (50% thermal ellipsoids). H atoms are omitted for clarity.
Figure 2. X-ray crystal structures of CB2 (left) and CB2B (right) (50% thermal ellipsoids). H atoms are omitted for clarity.
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Figure 3. (a) UV-vis absorption spectra in toluene (5.0 × 10−5 M). Photoluminescence (PL) spectra in toluene (5.0 × 10−5 M) at (b) 298 and (c) 77 K. (d) PL spectra in film state (poly(methylmethacrylate) (PMMA) film doped with 5 wt % o-carbonyl compound). Inset figures show the color of the emission of each state under UV irradiation (λex = 354 nm).
Figure 3. (a) UV-vis absorption spectra in toluene (5.0 × 10−5 M). Photoluminescence (PL) spectra in toluene (5.0 × 10−5 M) at (b) 298 and (c) 77 K. (d) PL spectra in film state (poly(methylmethacrylate) (PMMA) film doped with 5 wt % o-carbonyl compound). Inset figures show the color of the emission of each state under UV irradiation (λex = 354 nm).
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Figure 4. Relative energies of frontier molecular orbitals of o-carboranyl compounds at the first excited singlet state (S1) calculated by density functional theory (DFT) (isovalue = 0.04). The transition energy (in nm) was calculated at the TD-B3LYP/6-31G(d) level.
Figure 4. Relative energies of frontier molecular orbitals of o-carboranyl compounds at the first excited singlet state (S1) calculated by density functional theory (DFT) (isovalue = 0.04). The transition energy (in nm) was calculated at the TD-B3LYP/6-31G(d) level.
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Table
Table 1. Photophysical data of o-carboranyl compounds.
Table 1. Photophysical data of o-carboranyl compounds.
Compoundλabs1/nm(ε × 10−3 M−1 cm−1)λex/nmλem/nmΦem3
298 K 177 K 1Film 2298 K 1Film 2
CB1286 (8.8)286− 4445− 4<0.01<0.01
CB1B283 (12.4), 343 (8.8)337451384, 4544240.030.08
CB2328 (19.6)3285524875100.130.25
CB2B314 (21.4), 391 (7.0)3915054564730.060.09
Compoundτ/nskr5/× 108s−1knr6/× 108s−1
298 K 1Film 2298 K 1Film 2298 K 1Film 2
CB1− 4− 4----
CB1B5.55.00.050.161.761.84
CB25.90.240.2210.41.4731.3
CB2B1.20.830.501.17.811.0
1c = 5.0 × 10−5 M in toluene. 2 Measured in film state (5 wt % doped on PMMA) at 298 K. 3 Absolute PL quantum yield. 4 Not observed due to weak emission. 5 kr = Φem /τ. 6 knr = kr(1/Φem−1).
Table
Table 2. C−C bond length (Å) and N−C−C−C dihedral angle (Ψ, °) between aromatic rings (curved dotted lines in figure) in the optimized ground (S0) and first excited singlet state (S1) structures of o-carboranyl compounds
Table 2. C−C bond length (Å) and N−C−C−C dihedral angle (Ψ, °) between aromatic rings (curved dotted lines in figure) in the optimized ground (S0) and first excited singlet state (S1) structures of o-carboranyl compounds
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CB1CB1BCB2CB2B
S0S1S0S1S0S1S0S1
C−C1.722.391.732.421.722.381.742.40
Ψcalc22.20.3--0.81.2--
Ψexp1--2.9-
1 Dihedral angle determined from the X-ray structure (Figure 2).
Table
Table 3. Major low-energy electronic transitions in o-carborane compounds at the first excited singlet state (S1) calculated at the TD-B3LYP/6-31G(d) level 1.
Table 3. Major low-energy electronic transitions in o-carborane compounds at the first excited singlet state (S1) calculated at the TD-B3LYP/6-31G(d) level 1.
λcalc/nmfcalcAssignment
CB1468.980.0282HOMO → LUMO (99.6%)
CB1B466.86
360.99		0.2415
0.2817		HOMO → LUMO (99.6%)
HOMO → LUMO+1 (74.8%)
CB2515.130.0137HOMO → LUMO (99.7%)
CB2B522.90
433.83		0.1849
0.1971		HOMO → LUMO (99.8%)
HOMO → LUMO+1 (70.2%)
1 Singlet energies for the vertical transition calculated using the optimized S1 geometries.

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Jin, H.; Kim, S.; Bae, H.J.; Lee, J.H.; Hwang, H.; Park, M.H.; Lee, K.M. 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. Molecules 2019, 24, 201. https://doi.org/10.3390/molecules24010201

AMA Style

Jin H, Kim S, Bae HJ, Lee JH, Hwang H, Park MH, Lee KM. 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. Molecules. 2019; 24(1):201. https://doi.org/10.3390/molecules24010201

Chicago/Turabian Style

Jin, Hyomin, Seonah Kim, Hye Jin Bae, Ji Hye Lee, Hyonseok Hwang, Myung Hwan Park, and Kang Mun Lee. 2019. "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" Molecules 24, no. 1: 201. https://doi.org/10.3390/molecules24010201

APA Style

Jin, H., Kim, S., Bae, H. J., Lee, J. H., Hwang, H., Park, M. H., & Lee, K. M. (2019). 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. Molecules, 24(1), 201. https://doi.org/10.3390/molecules24010201

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