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Article

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

by
Sanghee Yi
†,
Mingi Kim
†,
Chan Hee Ryu
,
Dong Kyun You
,
Yung Ju Seo
and
Kang Mun Lee
*
Department of Chemistry, Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon 24341, Korea
*
Author to whom correspondence should be addressed.
†
The first and second authors contributed equally to this work.
Molecules 2022, 27(19), 6565; https://doi.org/10.3390/molecules27196565
Submission received: 5 September 2022 / Revised: 26 September 2022 / Accepted: 28 September 2022 / Published: 4 October 2022
(This article belongs to the Special Issue New Boron Chemistry: Current Advances and Future Prospects)
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Abstract

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

1. Introduction

Naphthalene-based organic compounds have been widely used as components of optoelectronic materials in organic light-emitting diodes [1,2,3], photovoltaic cells [4,5,6,7,8,9,10,11], and organic thin-film transistors [12,13,14,15,16] because of their outstanding electrochemical and chemical stability and outstanding photophysical characteristics. These unique properties are mostly attributed to the electronic-abundant nature and structural rigidity originating from the dimeric fused-benzene ring formation [17]. In particular, these optical features can be manipulated by the electronic characteristics of the introduced functional groups, leading to the extensive utilization of naphthyl derivatives in various industrial fields [18,19,20,21,22,23,24]. Among the specific functional units showing consistent electronic tendency and chemical stability, the ortho-carborane of closo-type structure, a well-known icosahedral boron cluster recognized as a three-dimensional subspecies of the benzene ring [25,26,27,28], has recently become a topic of interest for inducing the revelation of intriguing photophysical characteristics [29,30,31]. The o-carborane composed of ten boron atoms intrinsically has a strong electron-withdrawing nature via C-substituents and the high polarizability of its σ-aromatic framework, resulting in intramolecular charge-transfer (ICT) transitions during photoexcitation and relaxation processes in clusters bearing electron-rich aromatic groups [30,31,32,33,34,35,36,37,38,39,40,41]. As a result, the compound in which the π-conjugated aryl group is linked to the carbon atom of the o-carborane can be called a donor (D, aromatic group)–acceptor (A, o-carborane) dyad system. The naphthyl moiety is a candidate for several π-aromatic scaffolds in the o-carboranyl dyad compounds, exhibiting interesting emissive features based on the ICT transition [40,42,43,44,45]. Chujo et al. reported emissive color-tuning properties from the blue to near-infrared energy region using bis-o-carborane-substituted naphthalene derivatives [46].
The luminescent characteristics of o-carboranyl dyad compounds can be significantly affected by the molecular geometry between the appended aromatic group and o-carborane cage and can furthermore strongly control the revelation and severance of ICT electronic transition [40,41,47,48,49]. For example, Fox et al. reported C-diazaboryl-o-carborane dyads demonstrating dual emissions from high-energy locally excited (LE) and low-energy CT states depending on the rotational motion of the diazaboryl substituents [50]. Our group also revealed that o-caboranes bearing a spirobifluorene moiety fluorescent donor could exhibit multiple photoluminescent characteristics originating from the alternation of the ICT process via inhibition of twisted fluctuations around the o-carborane cage, resulting in the dyad compounds being utilized as a visual sensory material [51]. These previous works distinctly indicate that the relationship between the geometric formation of aromatic rings and the o-carborane cage is a critical factor in adjusting the efficiency of the ICT-based radiative decay mechanism in o-carboranyl dyad luminophores. Although various aromatic group-appended D–A-type o-carboranyl luminophores revealed the changed photophysical features triggered by the twisted ICT phenomenon [52,53,54,55,56,57,58], extensive investigation of this specific molecular geometry, which can reinforce the ICT transition and enhance the ICT-based radiative decay efficiency, is still in progress.
In line with the research trends and intrinsic concept of the photophysical properties of o-carboranyl luminophores, four types of o-carboranyl D–A-type conjugated compounds based on a popular aromatic fluorophore, the naphthalene group, were investigated. We strategically designed naphthyl-bis-o-carboranyl compounds to confirm a geometric key affecting their photophysical properties and electronic transitions; o-carborane cages were introduced to the 1,5- or 2,6-position of naphthalene and hydrogen atoms (the smallest functional group, 15CH and 26CH, Figure 1) and trimethylsilyl groups (bulky group, 15CS and 26CS) were substituted with the carbon atoms on the o-carboranes. A comparison of the molecular structures and photophysical analysis of each naphthyl-based o-carborane compound enabled us to elucidate whether the specific molecular geometry is crucial to the efficiency of the ICT-based radiative decay mechanism. The detailed synthetic procedures, full characterization including molecular structure analysis, and photophysical properties (with theoretical calculations) are described below.

2. Materials and Methods

2.1. General Considerations

All operations were performed under an inert nitrogen atmosphere using standard Schlenk and glove box techniques. Anhydrous solvents (tetrahydrofuran (THF), triethylamine (NEt3), and toluene; Sigma-Aldrich, St. Louis, MO, USA) were dried by passing through an activated alumina column and were then stored over activated molecular sieves (5 Å). Spectrophotometric-grade solvents (THF, methanol (MeOH), ethyl acetate, n-hexane, and dichloromethane (DCM)) were used as received from Alfa Aesar (Haverhill, MA, USA). All commercial reagents were used without any further purification after purchase from Sigma-Aldrich (1,5-dibromonaphthalene, 2,6-dibromonaphthalene, ethynyltrimethylsilane, bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2), copper(I) iodide (CuI), 1.6 M n-butyllithium in hexane (n-BuLi), ammonium chloride (NH4Cl), magnesium sulfate (MgSO4), potassium carbonate (K2CO3), N,N-dimethylaniline, trimethylsilyl chloride (TMSCl), and poly(methyl methacrylate) (PMMA)), and Alfa Aesar (decaborane (B10H14), and basic alumina). Deuterated solvents (chloroform (CDCl3), dichloromethane (CD2Cl2), and THF-d8) were purchased from Cambridge Isotope Laboratories (Tewksbury, MA, USA) and dried over activated molecular sieves (5 Å). All nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance 400 spectrometer (400.13 MHz for 1H and 1H{11B}, 100.62 MHz for 13C, and 128.38 MHz for 11B{1H}) at ambient temperature (Bruker Corporation, Billerica, MA, USA). Chemical shifts are given in ppm and are referenced to external tetramethylsilane (Me4Si) (1H, {11B}1H, and 13C) or BF3·Et2O ({1H}11B).

2.2. Synthesis of 1,5-Diethynylnaphthalene, 15AH

THF (14 mL) and NEt3 (10 mL) were added via cannula to a mixture of 1,5-dibromonaphthalene (0.60 g, 2.1 mmol), CuI (33 mg, 0.17 mmol), and Pd(PPh3)2Cl2 (0.12 g, 0.17 mmol) at ambient temperature. After stirring for 10 min, ethynyltrimethylsilane (0.64 mL, 4.6 mmol) was slowly added. The reaction mixture was then heated under reflux at 80 °C for 24 h prior to cooling to ambient temperature and evaporation of the solvent to afford a dark brown residue. Filtration through celite was employed to remove the metal salts from the reaction mixtures. The products were then used in situ for the next step, without characterization. More specifically, a round-bottom flask was charged with a mixture of the crude product (1,5-bis((trimethylsilyl)ethynyl)naphthalene, 0.58 g, 1.8 mmol) and K2CO3 (1.0 g, 7.2 mmol). Methanol (15 mL) was then added, and the mixture was stirred at 25 °C for 2 h. After this time, the reaction mixture was quenched by the addition of brine, and the aqueous phase was subsequently extracted with ethyl acetate (3 × 30 mL). The combined organic layer was dried over anhydrous MgSO4, filtered, and the volatiles were removed under vacuum to obtain a brown residue. Purification by column chromatography on silica (eluent: n-hexane) yielded 15AH. Yield = 56% (0.21 g). 1H NMR (CDCl3): δ 8.40 (d, J = 8.0 Hz, 2H), 7.78 (d, J = 7.2, 2H), 7.53 (t, J = 8.4 Hz, 2H), 3.49 (s, 2H, acetylene-H). 13C NMR (CDCl3): δ 133.26, 131.83, 127.38, 126.14, 120.28, 82.44 (acetylene-C), 81.46 (acetylene-C). Anal. Calcd for C14H8: C, 95.42; H, 4.58. Found: C, 94.89; H, 4.23.

2.3. Synthesis of 2,6-Diethynylnaphthalene, 26AH

A procedure analogous to that for 15AH was employed utilizing 2,6-dibromonaphthalene (1.4 g, 5.0 mmol), CuI (76 mg, 0.40 mmol), Pd(PPh3)2Cl2 (0.28 g, 0.40 mmol), and ethynyltrimethylsilane (1.5 mL, 11 mmol). Subsequently, the reaction between the crude 2,6-bis((trimethylsilyl)ethynyl)naphthalene (1.4 g, 4.2 mmol) and K2CO3 (2.3 g, 17 mmol) afforded the crude 26AH, which was purified by silica gel column chromatography (eluent: n-hexane). Yield = 42% (0.37 g). 1H NMR (CDCl3): δ 7.99 (s, 2H), 7.75 (d, J = 8.4 Hz, 2H), 7.54 (d, J = 8.4 Hz, 2H), 3.18 (s, 2H, acetylene-H). 13C NMR (CDCl3): δ 132.41, 132.06, 129.36, 127.89, 120.49, 83.68(acetylene-C), 78.23(acetylene-C). Anal. Calcd for C14H8: C, 95.42; H, 4.58. Found: C, 94.95; H, 4.25.

2.4. Synthesis of 15CH

To a toluene solution (30 mL) of B10H14 (1.4 g, 11.7 mmol) and 15AH (0.69 g, 3.9 mmol), an excess of N,N-dimethylaniline (2.5 mL, 20 mmol) was added at ambient temperature. After heating to reflux at 110 °C, the reaction mixture was stirred for 18 h. After this time, the volatiles were removed under vacuum and the resulting solid was dissolved in toluene. The solution was purified by column chromatography on basic alumina (eluent: toluene) to produce 15CH as a white solid. Yield = 53% (0.85 g). 1H{11B} NMR (THF-d8): δ 9.08 (d, J = 8.9 Hz, 2H), 7.95 (d, J = 7.7 Hz, 2H), 7.59 (t, J = 8.3 Hz, 2H), 5.65 (s, 2H, CB-H), 2.94 (br s, 4H, CB-BH), 2.53 (br s, 6H, CB-BH), 2.34 (br s, 5H, CB-BH), 2.27 (br s, 5H, CB-BH). 13C NMR (THF-d8): δ 132.12, 130.98, 128.55, 128.49, 125.74, 78.43(CB-C), 64.04(CB-C). 11B{1H} NMR (THF-d8): δ −3.61 (br s, 6B, CB-BH), −9.62 (br s, 6B, CB-BH),−10.22 (br s, 4B, CB-BH), −14.09 (br s, 4B, CB-BH). Anal. Calcd for C14H28B20: C, 40.76; H, 6.84. Found: C, 40.55; H, 6.65.

2.5. Synthesis of 26CH

A procedure analogous to that for 15CH was employed utilizing B10H14 (0.46 g, 3.8 mmol) and 26AH (0.26 g, 1.5 mmol), and N,N-dimethylaniline (0.95 mL, 7.5 mmol). Purification by column chromatography on basic alumina (eluent: toluene) afforded 26CH as a white solid. Yield = 73% (0.45 g). 1H{11B} NMR (CD2Cl2): δ 8.00 (s, 2H), 7.86 (d, J = 8.7 Hz, 2H), 7.63 (d, J = 8.8 Hz, 2H), 4.19 (s, 2H, CB-H), 2.66 (br s, 4H, CB-BH), 2.57 (br s, 4H, CB-BH), 2.42 (br s, 2H, CB-BH), 2.31 (br s, 10H, CB-BH). 13C NMR (CD2Cl2): δ 132.89, 132.85, 129.67, 127.58, 126.21, 76.71 (CB-C), 60.94 (CB-C).11B{1H} NMR (CD2Cl2): δ −3.14 (br s, 3B, CB-BH), −5.18 (br s, 3B, CB-BH), −9.88 (br s, 4B, CB-BH), −11.73 (br s, 6B, CB-BH), −13.65 (br s, 4B, CB-BH). Anal. Calcd for C14H28B20: C, 40.76; H, 6.84. Found: C, 40.60; H, 6.71.

2.6. Synthesis of 15CS

An n-hexane solution of n-BuLi (1.6 M, 1.6 mL, 2.5 mmol) was added dropwise to a solution of 15CH (0.41 g, 1.0 mmol) in THF (10 mL) at 0 °C. After stirring for 1 h, the reaction was slowly allowed to warm to ambient temperature, and TMSCl (0.63 mL, 5.0 mmol) was added dropwise to the mixture. The reaction mixture was then stirred for 2 h at 25 °C. After quenching with saturated aqueous NH4Cl (15 mL), the mixture was extracted with ethyl acetate (3 × 15 mL). The combined organic layer was then dried over anhydrous MgSO4, filtered, and the solvent was removed under vacuum. The product was purified by recrystallization from DCM to obtain 15CS as a white solid. Yield = 61% (0.34g). 1H NMR (CD2Cl2): δ 9.25 (d, J = 9.0 Hz, 2H), 8.32 (d, J = 7.7 Hz, 2H), 7.57 (t, J = 9.0 Hz, 2H), 3.30 (br s, 4H, CB-BH), 2.67 (br s, 6H, CB-BH), 2.48 (br s, 8H, CB-BH), 2.21 (br s, 2H, CB-BH), −0.31 (s, 18H, –Si(CH3)3). 13C NMR (CD2Cl2): δ 135.85, 133.30, 129.42, 129.22, 125.54, 85.99 (CB-C), 81.55 (CB-C), −0.29 (–Si(CH3)3). 11B{1H} NMR (CD2Cl2): δ −0.41 (br s, 3B, CB-BH), −2.06 (br s, 3B, CB-BH), −8.48 (br s, 6B, CB-BH), −11.00 (br s, 8B, CB-BH). Anal. Calcd for C20H44B20Si2: C, 43.13; H, 7.96. Found: C, 42.98; H, 7.80.

2.7. Synthesis of 26CS

A procedure analogous to that for 15CS was employed utilizing n-BuLi (1.6 M, 0.78 mL, 1.3 mmol), 26CH (0.20 g, 0.50 mmol), and TMSCl (0.32 mL, 2.5 mmol). The product was purified by recrystallization from n-hexane to obtain 26CS as a white solid. Yield = 44% (0.12 g). 1H{11B} NMR (CD2Cl2): δ 8.18 (s, 2H), 7.88 (d, J = 8.4 Hz, 2H), 7.80 (d, J = 7.4 Hz, 2H), 3.04 (br s, 4H, CB-BH), 2.38 (br s, 12H, CB-BH), 2.26 (br s, 4H, CB-BH), −0.13 (s, 18H, –Si(CH3)3). 13C NMR (CD2Cl2): δ 133.02, 132.39, 131.53, 129.54, 129.52, 83.24 (CB-C), 77.63 (CB-C), −0.27 (–Si(CH3)3). 11B{1H} NMR (CD2Cl2): δ −0.23 (br s, 4B, CB-BH), −3.33 (br s, 4B, CB-BH), −8.89 (br s, 6B, CB-BH), −10.84 (br s, 3B, CB-BH), −12.53 (br s, 3B, CB-BH). Anal. Calcd for C20H44B20Si2: C, 43.13; H, 7.96. Found: C, 42.89; H, 7.77.

2.8. UV/Vis Absorption and Photoluminescence (PL) Measurements

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

2.9. X-ray Crystallography

Single-X-ray quality crystals of 15CH, 26CH, 15CS, and 26CS were grown from a DCM/n-hexane mixture. Single crystals were coated with paratone oil and mounted on a glass capillary. Crystallographic measurements were performed using a Bruker D8QUEST diffractometer (Bruker Coorperation, Billerica, MA, US), with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) and CCD area detector. The structures of 15CH, 26CH, 15CS, and 26CS were assessed using direct methods, and all nonhydrogen atoms were subjected to anisotropic refinement with a full-matrix least-squares method on F2 using the SHELXTL/PC software package (released and presented at the Software Fayre, XXII IUCr Congress in Madrid, Spain, 2011). The X-ray crystallographic data are available in CIF format (CCDC 2184401–2184404 for 15CH, 26CH, 15CS, and 26CS), provided free of charge by the Cambridge Crystallographic Data Centre. The hydrogen atoms were placed at their geometrically calculated positions and refined using a riding model on the corresponding carbon atoms with isotropic thermal parameters. Detailed crystallographic data are provided in Tables S1 and S2 in the Supplementary Materials.

2.10. Computational Calculation Studies

The optimized geometries for the ground (S0) and first excited (S1) states of all o-carboranyl compounds in THF were obtained at the B3LYP/6-31G(d,p) [59] level of theory. The vertical excitation energies at the optimized S0 geometries and the optimized geometries of the S1 states were calculated using time-dependent density functional theory (TD-DFT) [60] at the same level of theory. Solvent effects were evaluated using the conductor-like polarizable continuum model (CPCM) based on the integral equation formalism of the polarizable continuum model (IEFPCM), with THF as the solvent [61]. All geometry optimizations were performed using the Gaussian 16 program [62]. The percent contribution of a group in a molecule to each molecular orbital was calculated using the GaussSum 3.0 program [63]. The most stable geometries were determined by constructing one-dimensional potential energy surfaces as a function of each dihedral angle (Ψ, Table S11) by rotating the carboranes of 15CH and 26CH between approximately 0° and 180° at intervals of 15° to yield 13 initial conformations for each compound. Conformations that exhibited physically impossible atomic overlaps were excluded from further geometric optimization. The dihedral angle was fixed, whereas other geometric variables were fully relaxed for geometry optimization and energy calculation of the resulting initial conformations using the Gaussian 16 software program [62]. The calculation method in this study does not provide accurate information for electronic transition of the o-carboranyl compounds and only gives an indirect hint to elucidate their transitions corresponding to specific photophysical characteristics.

3. Results and Discussion

3.1. Synthesis and Characterization

Naphthyl-bis-o-carboranyl compounds 15CH, 26CH, 15CS, and 26CS, in which the o-carborane cages are appended at the C1 and C5-positions of the C2 and C6-positions of the naphthyl group, were synthesized as shown in Figure 1. Sonogashira coupling reactions between ethynyltrimethylsilane and dibromonaphthanlene precursors (1,5-dibromonaphthalene for 15AH and 2,6-dibromonaphthalene for 26AH) and consecutive reactions with a weak base (K2CO3) for the deprotection of trimethylsilyl groups produced bis-ethynylnaphthalene compounds (15AH and 26AH) in moderate yields (56% and 42%, respectively). The 1,5-or 2,6-bis [1-o-carboran-1-yl] naphthalene compounds (15CH and 26CH) were prepared via cage-forming reactions with B10H14 using 15AH or 26AH in the presence of N,N-dimethylaniline (yields of 53% and 73%, respectively) [64,65,66]. Furthermore, lithiation of 15CH and 26CH with n-butyllithium followed by reaction with trimethylsilyl chloride afforded 1,5 or 2,6-bis [2-(trimethylsilyl)-1-o-carboran-1-yl]naphthanlene compounds (15CS and 26CS) in 61% and 44% yields, respectively. The produced o-carboranyl compounds were stable in air and light and decomposed at detectable levels for more than six months under ambient conditions. All the precursors and prepared o-carboranyl naphthalene compounds (15CH, 26CH, 15CS, and 26CS) were fully characterized using multinuclear (1H, 1H{11B}, 13C, and 11B{1H}) NMR spectroscopy (Figures S1–S10 in the Supplementary Materials) and elemental analysis. Each of the 1H{11B} NMR spectrum of four o-carboranyl compounds exhibited broad singlet peaks in the region of 3.3–2.2 ppm (totally integrated to 20 H atoms), confirming the existence of –BH units in the closo-o-carborane cages. In particular, a sharp signal around 0.0 ppm were observed in the 1H{11B} NMR spectra for 15CS and 26CS, which was attributed to the trimethylsilyl groups appended to o-carboranes. In addition, several broad singlet peaks were observed between −0 and −15 ppm in the 11B{1H} NMR spectra of all naphthyl-o-carborane compounds, which clearly confirmed the presence of the o-carborane cage.
The solid-state molecular structures of the four o-carboranyl compounds were also determined by X-ray single-crystallography (inset figures in Figure 1; detailed parameters, including selected bond lengths and angles, are provided in Tables S1 and S2 in the Supplementary Materials). The crystal structures of the four compounds revealed planar naphthyl groups and icosahedral formation of o-carborane cages. Interestingly, bulky trimethylsilyl group-substituted compounds (15CS and 26CS) showed a significantly perpendicular orientation between the naphthalene plane and the bonding axis of the C-C bond in the o-carborane cage (Ψ = 78° for 15CS and 82° for 26CS, Table 1), whereas the C-C bond axes of 15CH and 26CH were almost parallel to the naphthalene plane, as evidenced by the Ψ values (5.3° for 15CH and 1.8° for 26CH). Such a different geometry between silyl- and H-substituted compounds is a decisive factor in controlling their photophysical characteristics (vide infra).

3.2. Analysis of Photophysical Properties with Theoretical Calculation

The absorption and emissive properties of the naphthyl-bis-o-carboranyl compounds 15CH, 26CH, 15CS, and 26CS were investigated using UV/Vis absorption and PL spectroscopy, respectively (Figure 2 and Table 2). All compounds in THF exhibited major absorption bands centered at λabs = 278–301 nm, which were attributed to the vibronic structures undergoing spin-allowed π–π* transitions of the naphthalene moiety. These bands were also observed in the absorption spectrum of the mother-scaffold compound (naphthalene, λabs = 276 and 285 nm, Figure S11). However, the lowest absorption bands for the o-carboranyl compounds were relatively red-shifted (λabs = 322–330 nm) compared to the absorption band (λabs = 313 nm) of naphthalene because of the lowest unoccupied molecular orbital (LUMO) stabilization effect of o-carborane [67]. Furthermore, the low-energy absorption of these compounds includes tailing of the absorption bands at 350 nm. These features indicate that the bands can be attributed to the typical ICT transitions in the o-carborane cages. Time-dependent density functional theory (TD-DFT) calculations for the S0 state of the o-carboranyl compounds clearly confirmed these characteristics (vide infra).
The origin of the electronic transitions of the four naphthyl-o-carboranyl compounds was determined using TD-DFT calculations [60] (Figure 3). Each calculated structure was based on the X-ray crystal structure (Figure 1). The calculation results for the S0 state demonstrated that the major lowest-energy transitions were dominantly assignable to the highest occupied molecular orbital (HOMO) → LUMO (Figure 3). Although both the HOMO and LUMO of each naphthyl o-carborane compound were mainly localized on the naphthyl group (HOMO > 90% and LUMO > 84%, Tables S4, S6, S8, and S10), the LUMO levels were further distributed over the o-carborane cages (>12%). These results suggest that the lowest-energy electronic transition for the four o-carboranyl compounds originates from the naphthyl-centered π–π* LE transition with substantial ICT transitions between the naphthyl moiety and o-carboranyl cage.
The emissive properties of the four o-carboranyl compounds were examined using PL measurements under various conditions (Figure 2 and Table 2). Interestingly, the PL spectra of all the compounds in the THF solution at 298 K exhibited non-emissive characteristics. On the other hand, these compounds showed intense emissive patterns in rigid states (THF at 77 K and film), and the hydrogen-substituted o-carboranyl compounds 15CH and 26CH exhibited specific dual-emission patterns centered at λem = ~345 and ~480 nm, and trimethylsilyl-compounds 15CS and 26CS exhibited strong emission only in the region of λem = 470–500 nm. As a result, greenish emission was observed in the film states of all compounds under a hand-held UV lamp (Figure 2, inset figures). In particular, the emission of naphthalene was centered at λem = 322 and 336 nm (Figure S11), which proves that the high-energy emissions centered at λem = ~345 nm in the rigid states for 15CH and 26CH can be attributed to the LE transition of the naphthalene moiety. Moreover, the low-energy emissions of all the o-carboranyl compounds in the region over 450 nm were severely red-shifted compared with that of naphthalene, indicating that these corresponded to ICT involving o-carborane (vide infra). Such differences in the emission features between THF at 298 K and the rigid states typically originate from restricted structural fluctuations in the rigid state, and the elongation of the C-C bond in the o-carborane cage during the excitation process is known to diminish the efficiency of the ICT-based radiative decay mechanism [31,52,53,68,69,70]. Indeed, the calculated lengths of the C-C bonds for the o-carborane cages of all the compounds in the S1 state were significantly longer (ca. 2.40 Å, Table 1) than those in the S0 state (1.6–1.7 Å) and the experimentally measured values based on the solid-state crystal structures (ca. 1.7 Å).
The calculation results of the four o-carboranyl compounds in the S1-optimized structures revealed that the major transitions associated with low-energy emission involve a LUMO → HOMO transition (Figure 3). The LUMO and HOMO of the compounds were predominantly localized on the o-carborane cage (>76%, Tables S4, S6, S8, and S10) and the naphthyl group (>81%). These results strongly suggest that the low-energy emission around 450–550 nm observed in the rigid states is primarily manifested from a radiative decay process based on ICT between the o-carborane and naphthyl moieties.
To further investigate the origin of the greenish emission in the solid (film) state for these compounds, PL measurements were performed in a THF/distilled-water mixture (50 μM) (Figure 4 and Table 2). Intriguingly, the intensity of the low-energy emission for trimethylsilyl-substituted compounds 15CS and 26CS was dramatically enhanced in the region from 450 to 550 nm upon increasing the water fraction (fw). Consequently, the most aggregated state in THF/water (fw = 90%) exhibited intense greenish emission (λem = 501 nm for 15CS and 490 nm for 26CS), which was significantly similar to that observed in the film state. These features are characteristic of aggregation-induced emission (AIE) phenomena. The structural rigidity could originate from the aggregation effect, resulting in an increase in the efficiency of the ICT-based radiative decay. Unfortunately, such an AIE phenomenon was not observed in the PL spectra of 15CH and 26CH THF/water mixtures.

3.3. Quantitative Comparison for ICT-Based Radiative Decay Efficiency

The absolute quantum efficiency (Φem) and decay lifetime (τobs) of each o-carboranyl compound in the film state were investigated to compare the efficiency of the radiative decay process quantitatively. Intriguingly, the Φem values of 15CS and 26CS were moderately estimated at 10.5% and 18.4%, respectively (Table 2), while those for 15CH and 26CH could not be measured owing to their inefficient radiative decay processes (<1%). Such differences in radiative efficiencies between the two series of naphthyl o-carboranyl compounds correlate with their structural geometry; in particular, the formation of the o-carborane cage appended to the plane of the aromatic group [40,41,47,48,49]. In general, the ICT transition on o-carboranyl luminophores can be intensified by increased delocalization through exo-π-interactions between the tangential p-orbital (σ*-character) on the substituted carbon atom of the o-carborane cage and the π-delocalized system of the appended aromatic group [40,41,71]. Consequently, the C-C bond axis of o-carborane becomes perpendicular to the aromatic plane to maintain the exo-π-interaction. Indeed, the calculated dihedral angles (Ψ) between the C-C bond axis and naphthyl plane in the optimized structures of the four compounds were orthogonally arranged according to the excited states (ΨS0 = 30–85° → ΨS1 = 82–90°, Table 1) for the σ*–π* conjugation between the carboranyl cages and naphthyl groups. However, the experimental Ψ values of 15CH (5°, Table 1) and 26CH (2°) from solid-state crystal structures indicated that the geometric relationship between the C-C bond axis and naphthyl planes was planar, whereas those of 15CS (78°) and 26CS (82°) were orthogonally maintained; the bulkiness of trimethylsilyl groups substituted to the o-carborane cages led to orthogonality. These structural features were the main reason why the radiative efficiencies of 15CH and 26CH in the solid state were lower than those of 15CS and 26CS. Furthermore, the restriction of exo-π-interaction for 15CH and 26CH generates the isolation of excitons on the naphthyl group [40], resulting in the hindrance of the ICT transition and the natural concentration of excitons to favor the LE transition from the naphthyl group. Consequently, 15CH and 26CH demonstrated strong LE-based emissions in the high-energy region (Figure 2 and Table 2), differing from those of 15CS and 26CS.
The τobs values for 15CS and 26CS in the film state were estimated to 3.7 and 6.4 ns, respectively (Table 2 and Figure S12), indicating distinctly fluorescent characteristics. A comparison of the radiative (kr; Table 2) and nonradiative (knr) decay constants of both compounds, which were calculated by the Φem and τobs values, revealed a slight difference in the efficiency of the ICT-based radiative process between the two o-carboranyl compounds. The kr values of both compounds were the same (2.9 × 107 s−1, Table 2), whereas the knr value (2.4 × 108 s−1) of 15CS was approximately two times higher than that (1.3 × 108 s−1) of 26CS. These findings imply that the relatively low delocalization between the o-carborane cage and naphthyl group of 15CS in the solid state leads to inefficiency of the ICT-based radiative decay process compared with 26CS. Indeed, the experimental Ψ value (82°, Table 1) of 26CS verified that this geometry was relatively closer to the orthogonal structure than that (78°) of 15CS. Consequently, all the observed results verified that geometric formation around the o-carborane cage plays a significant role in controlling the efficiency of the ICT-based radiative decay mechanism on the aromatic group-appended o-carboranyl luminophores.

3.4. Theoretical Calculations of the Effect of Geometry around the O-carborane Cages

To elucidate the effect of the structural geometry around the o-carborane cages on the electronic transitions, the alternation of low-energy transitions assignable to ICT corresponding to the o-carborane (LUMO to HOMO in the S1-states, Figure 3) was calculated (B3LYP/6-31G(d,p) basis set) for 15CH and 26CH as the change in Ψ (the dihedral angle between the C-C bond axis of the o-carborane and naphthyl plane) from 0° (=360°) to 180° (Figure 5 and Table S11). The orbital occupation of the LUMOs on the o-carborane cage for both 15CH and 26CH was maximized (> 95%) when the C-C axis was planar to the naphthyl plane (Ψ = 0° and 180°), whereas the localization of the HOMOs on the naphthyl groups did not change significantly (85–89% for 15CH and 81–83% for 26CH). In addition, the occupation of the LUMOs for both compounds gradually decreased as Ψ values had orthogonality (90°). These results indicate that the π-conjugated delocalization between the o-carborane and naphthyl group became blocked in accordance with being close to Ψ = 0° and 180°, and that the orbital distribution of each LUMO and HOMO was thoroughly isolated in the planar geometry, resulting in the inhibition of the ICT transition between the o-carborane and naphthyl groups. The oscillator strengths (fcalc) computed for each transition of both compounds were maximized at Ψ = 90° and equaled zero at = 0° and 180°. These features strongly suggest that the orthogonality of the C–C bond axis to the plane of the appended naphthyl group significantly affects the ICT transitions of the o-carborane cage. The experimental and theoretical findings demonstrate that the structural geometry around the o-carborane cage plays a decisive role in enhancing the efficiency of the ICT-based radiative decay mechanism in the D–A-type o-carboranyl luminophores.

4. Conclusions

Herein, we prepared hydrogen (15CH and 26CH) or trimethyl silyl groups (15CS and 26CS) substituted with four naphthyl-based bis-o-carboranyl compounds and investigated the relationship between their molecular geometries and photophysical characteristics. Although these o-carboranyl compounds barely exhibited emissive patterns in solution at 298 K, intense greenish emission assignable to each LE- and ICT-based emission (for 15CH and 26CH) or ICT-based emission only (for 15CS and 26CS) were observed in rigid states (in solution at 77 K and in the film state). Intriguingly, the solid-state molecular structures of the four compounds revealed that the C-C bond axis of the o-carborane cage in the trimethysilyl group-substituted o-carboranyl compounds 15CS and 26CS was more orthogonal to the plane of the appended naphthyl group than those in the hydrogen-substituted o-carboranyl compounds 15CH and 26CH. These findings verify the efficient σ*–π* conjugation between the o-carborane and naphthyl groups in 15CS and 26CS via strong exo-π interaction. Such features result in a higher Φem for the emission of solid 15CS and 26CS than those of 15CH and 26CH. Furthermore, the most orthogonal geometry of 26CS exhibits the highest Φem and lowest knr values for the ICT-based radiative decay process. Consequently, the results indicate that geometric formation around the o-carborane cage plays a decisive role in the efficiency of the ICT-based radiative mechanism in o-carboranyl organic luminophores.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/molecules27196565/s1: 1H, 1H{11B}, 13C, and 11B{1H} NMR spectra (Figures S1–S10), crystallographic data and parameters (Tables S1 and S2), UV-vis absorption and PL spectra for naphthlene (Figure S11), emission decay curves (Figure S12), and computational calculation details (Figures S13–S16 and Tables S3–S19) (PDF).

Author Contributions

S.Y., C.H.R., D.K.Y. and Y.J.S. synthesized the compounds, analyzed the data, and wrote the paper. M.K. conducted the computational study and analyzed the data. K.M.L. analyzed all the data and wrote the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant (NRF-2020R1A2C1006400 and NRF-2021M3H4A1A02055684 for K. M. Lee) funded by the Ministry of Science and ICT and the Ministry of Education.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article or Supplementary Material.

Acknowledgments

We thank Dongwook Kim (Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS)) for assistance with X-ray diffraction experiments.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 27 06565 g001 550
Figure 1. Synthetic procedures for the naphthyl-bis-o-carboranyl compounds. Reagents and conditions: (i) CuI, Pd(PPh3)2Cl2, ethynyltrimethylsilane, THF/NEt3, 80 °C, 24 h. (ii) K2CO3, methanol, 25 °C, 2 h. (iii) B10H14, N,N-dimethylaniline, toluene, 110 °C, 18 h. (iv) n-BuLi, TMSCl, THF, 25 °C, 2 h. Insets (dash-line box) show the X-ray crystal structures of 15CH, 26CH, 15CS, and 26CS (40% thermal ellipsoids), with H atoms on aromatic and aliphatic carbons omitted for clarity.
Figure 1. Synthetic procedures for the naphthyl-bis-o-carboranyl compounds. Reagents and conditions: (i) CuI, Pd(PPh3)2Cl2, ethynyltrimethylsilane, THF/NEt3, 80 °C, 24 h. (ii) K2CO3, methanol, 25 °C, 2 h. (iii) B10H14, N,N-dimethylaniline, toluene, 110 °C, 18 h. (iv) n-BuLi, TMSCl, THF, 25 °C, 2 h. Insets (dash-line box) show the X-ray crystal structures of 15CH, 26CH, 15CS, and 26CS (40% thermal ellipsoids), with H atoms on aromatic and aliphatic carbons omitted for clarity.
Molecules 27 06565 g001
Molecules 27 06565 g002 550
Figure 2. UV-vis absorption (left side) and PL spectra (right side) for (a) 15CH (λex = 305 nm), (b) 26CH (λex = 285 nm), (c) 15CS (λex = 329 nm), and (d) 26CS (λex = 333 nm). Black line: absorption spectra in THF (50 μM), blue line: PL spectra in THF (50 μM) at 298 K, green line: PL spectra in THF (50 μM) at 77 K, and orange line: PL spectra in film (5 wt% doped with PMMA) at 298 K. Inset figures show the emission color in the film state under irradiation by a hand-held UV lamp (λex = 365 nm).
Figure 2. UV-vis absorption (left side) and PL spectra (right side) for (a) 15CH (λex = 305 nm), (b) 26CH (λex = 285 nm), (c) 15CS (λex = 329 nm), and (d) 26CS (λex = 333 nm). Black line: absorption spectra in THF (50 μM), blue line: PL spectra in THF (50 μM) at 298 K, green line: PL spectra in THF (50 μM) at 77 K, and orange line: PL spectra in film (5 wt% doped with PMMA) at 298 K. Inset figures show the emission color in the film state under irradiation by a hand-held UV lamp (λex = 365 nm).
Molecules 27 06565 g002
Molecules 27 06565 g003 550
Figure 3. Frontier molecular orbitals for 15CH, 26CH, 15CS, and 26CS at their ground state (S0) and the first excited singlet state (S1) with their relative energies from DFT calculation (isovalue 0.04). The transition energy (in nm) was calculated using the TD-B3LYP method with 6-31G(d,p) basis sets.
Figure 3. Frontier molecular orbitals for 15CH, 26CH, 15CS, and 26CS at their ground state (S0) and the first excited singlet state (S1) with their relative energies from DFT calculation (isovalue 0.04). The transition energy (in nm) was calculated using the TD-B3LYP method with 6-31G(d,p) basis sets.
Molecules 27 06565 g003
Molecules 27 06565 g004 550
Figure 4. PL spectra of (a) 15CS (λex = 329 nm) and (b) 26CS (λex = 333 nm) in THF/distilled water mixtures (50 μM). Inset figures show the emission color in each state under irradiation by a hand-held UV lamp (λex = 365 nm).
Figure 4. PL spectra of (a) 15CS (λex = 329 nm) and (b) 26CS (λex = 333 nm) in THF/distilled water mixtures (50 μM). Inset figures show the emission color in each state under irradiation by a hand-held UV lamp (λex = 365 nm).
Molecules 27 06565 g004
Molecules 27 06565 g005 550
Figure 5. (a) Orbital contributions of the o-carborane cage (CB) and the naphthyl moiety (Nap) to the LUMO and HOMO, respectively, as a function of Ψ in S1-optimized structures for 15CH and 26CH. (b) Computed emission wavelengths (λcalc in nm, pink lines) and oscillator strengths (fcalc, green lines) for each low-energy transition (LUMO to HOMO).
Figure 5. (a) Orbital contributions of the o-carborane cage (CB) and the naphthyl moiety (Nap) to the LUMO and HOMO, respectively, as a function of Ψ in S1-optimized structures for 15CH and 26CH. (b) Computed emission wavelengths (λcalc in nm, pink lines) and oscillator strengths (fcalc, green lines) for each low-energy transition (LUMO to HOMO).
Molecules 27 06565 g005
Table
Table 1. Dihedral angles between the naphthalene plane and the bonding axis of the C-C bond in the o-carborane cage (Ψ = C16–C15–C1(C2)–C2(C1) and bond lengths (C16–C15) of the C-C bond for 15CH, 26CH, 15CS, and 26CS.
Table 1. Dihedral angles between the naphthalene plane and the bonding axis of the C-C bond in the o-carborane cage (Ψ = C16–C15–C1(C2)–C2(C1) and bond lengths (C16–C15) of the C-C bond for 15CH, 26CH, 15CS, and 26CS.
15CH26CH15CS26CS
Exp.1Calc.2Exp.1Calc.2Exp.1Calc.2Exp.1Calc.2
S0S1S0S1S0S1S0S1
Ψ/°5.343.489.31.830.989.278.074.982.582.085.189.7
C-C/Å1.671.692.391.571.642.411.741.762.421.701.722.40
1 Experimental values from their X-ray crystal structures. 2 Calculated values from their ground (S0) and the first excited singlet state (S1) optimized structures.
Table
Table 2. Photophysical data for naphthyl-bis-o-carboranyl compounds 15CH, 26CH, 15CS, and 26CS.
Table 2. Photophysical data for naphthyl-bis-o-carboranyl compounds 15CH, 26CH, 15CS, and 26CS.
Compoundλabs 1/nm
(ε × 10−3 M−1 cm−1)		λex
/nm		λem/nm
THF 277 K 1Film 3fw = 90% 4
15CH301 (7.7), 328 (1.4)305-8346, 495349, 499-8
26CH279 (6.3), 326 (0.7)285-8344, 460345, 489-8
15CS278 (5.0), 322 (0.6)329-8495499501
26CS282 (5.9), 330 (0.8)333-8471485490
CompoundΦem 3,5τobs3/nskr3,6/× 107 s−1knr3,7/× 108 s−1
15CH<0.01-7--
26CH<0.01-7--
15CS0.1053.672.862.44
26CS0.1846.402.881.28
1 50 μM in THF. 2 50 μM, observed at 298 K. 3 Measured in the film state (5 wt% doped in PMMA). 4 50 μM in THF/water (1/9, v/v). 5 Absolute PL quantum yield. 6 kr = Φem /τobs. 7 knr = kr(1/ Φem−1). 8 Not observed due to weak emission.
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Yi, S.; Kim, M.; Ryu, C.H.; You, D.K.; Seo, Y.J.; Lee, K.M. Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores. Molecules 2022, 27, 6565. https://doi.org/10.3390/molecules27196565

AMA Style

Yi S, Kim M, Ryu CH, You DK, Seo YJ, Lee KM. Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores. Molecules. 2022; 27(19):6565. https://doi.org/10.3390/molecules27196565

Chicago/Turabian Style

Yi, Sanghee, Mingi Kim, Chan Hee Ryu, Dong Kyun You, Yung Ju Seo, and Kang Mun Lee. 2022. "Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores" Molecules 27, no. 19: 6565. https://doi.org/10.3390/molecules27196565

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