Fluoranthene–o-carborane dyads: exploring thermochromism and mechanochromism in crystalline systems

Abstract

Fluoranthenes, an important subclass of polycyclic aromatic hydrocarbons, have continued to attract considerable attention in the fields of synthetic organic chemistry and materials science. However, due to the strong π–π intermolecular interactions resulting from highly planar π–π conjugated structures, they usually suffer from aggregation caused quenching (ACQ) in the solid state, which limits their further applications. In this study, SOF and DOF are synthesized via a modified nickel-catalyzed Kumada coupling reaction using o-carborane with three-dimensional clusters and 3-bromofluoranthene as the starting materials. The structural characteristics, luminescence properties, and stimuli-responsive behavior of these compounds are systematically investigated through single crystal structure analysis, UV-vis absorption spectroscopy, fluorescence spectroscopy, and DFT theoretical calculations. For SOF, the formation of excimers is identified as the primary cause of thermochromism. The DOF exhibits excellent properties of aggregation-induced emission (AIE) and quantum yields as high as 18.75% in the solid state. Additionally, the transformation from crystalline to amorphous is the primary reason for the significant mechanochromism of DOF. The nearly perpendicular packing is beneficial for thermochromism, while the anti-parallel packing is likely to cause mechanochromism. These results suggest that o-carborane can play a distinctive role in precisely regulating the emission properties and stimulus response.

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1. Introduction

Organic solid-state luminescent materials have attracted considerable interest due to their potential applications as display materials, and in sensing, optoelectronic devices and biological imaging.^1–5 However, in most luminescent materials, the interaction of molecules in the solid state will lead to an increase in the probability of non-radiative transitions during the luminescence process, which results in low solid-state luminescence efficiency for these materials. Recent reports have shown that the modification of aromatic compounds by o-carborane units is one of the effective strategies to avoid ACQ.^6–10 What is more, o-carborane derivatives exhibit high-efficiency solid-state luminescence characteristics,^7,10,11 while also frequently demonstrate stimuli-responsive properties, including thermochromism, mechanochromism and other related phenomena.^12–15

Fluoranthenes (FAs), structural isomers of pyrene, are characterized by their planar and extended π-conjugated system, which results from the fusion of a naphthalene moiety with a benzene ring through an intervening cyclopentane ring. FAs are characterized by their high quantum efficiency in blue light emission, favorable electron transport properties, and commendable thermal stability. Consequently, FAs and their derivatives are predominantly utilized in the investigation of organic optoelectronic and sensing materials.^16–18 The application of FAs and their derivatives in the realm of luminescence is substantially constrained by their inherent tendency towards ACQ, which is particularly pronounced in planar molecules.¹⁹ A report has detailed that FA derivatives substituting polyphenyl rings can effectively restrain the aggregation, thereby decreasing the fluorescence quenching and the red shift in emission spectra.²⁰ However, due to the intrinsic propensity of fluorescent molecules to aggregate in the solid state, this approach to inhibiting aggregation is not entirely satisfactory and fails to fully address the demands of high-performance luminescent materials.

Herein, we introduce novel fluoranthene–o-carborane dyads that exhibit aggregation-induced emission (AIE) and mechanochromic and thermochromic properties. Other published polycyclic o-carborane compounds with a D–A structure show no stimulus-responsive behavior or only exhibit thermochromism/mechanochromism.^12,21–24 Modified fluoranthene–o-carborane dyads were designed based on the strategy of improving the solid-state emission of the reported molecules by suppressing intramolecular motions. Fluoranthene fluorophores have acquired the thermochromism of the single-substituted compound and the mechanochromism of the double-substituted counterpart through the utilization of o-carborane. From the perspective of the single-crystal structure and packing mode, the mechanism behind the compound's thermochromism and mechanochromism was explored. Correspondingly, it is revealed that the stimulus response behavior can be regulated by different stacking modes.

2. Experimental

2.1. General procedures

All reactions were carried out under a dry N₂ atmosphere. Solvents such as tetrahydrofuran (THF) and toluene were distilled from sodium/benzophenone and stored over molecular sieves. Other chemicals were purchased from Energy and Aladdin Chemical Reagent Company and used without further purification. All reactions were monitored by thin layer chromatography (TLC) and column chromatography was conducted using silica gel (200–300 mesh).

2.2. Analytical measurements

NMR spectra were recorded on a Bruker ADVANCE III 600 MHz spectrometer (¹H NMR: 600 MHz, ¹³C NMR: 151 MHz). All chemical shifts were reported in δ units, referenced to the residual solvent resonances of the deuterated solvents for proton and carbon chemical shifts. Mass spectra were obtained using an Ultimate 3000/Q-Exactive spectrometer. Melting points were measured using a Nikon Polarizing Microscope ECLIPSE 50i POL equipped with an INTEC HCS302 heating stage. UV-vis absorption spectra were recorded on a HITACHI U-3900H. Fluorescence spectra were measured using a Horiba FluoroLog-3 spectrofluorometer (Horiba-Jobin-Yvon, Edison, NJ, USA). Absolute PL quantum yield (Φ_FL) was determined using a Horiba FL-3018 Integrating Sphere. The temperature-dependent luminescence spectra were measured using a Janis VPF-100 liquid nitrogen low temperature thermostat (Janis, USA). Powder X-ray diffraction (XRD) measurement was conducted on a Bruker D8 Advance Diffraction diffractometer in the 2θ range from 5 to 80°, with Cu K_α radiation (λ = 0.15405 nm) at 40 kV and 40 mA. The DSC experiments were performed on a German Netzsch DSC 404 F3 instrument at a scanning rate of 10 K min⁻¹. The single crystal test was carried out on a Bruker D8 VENTURE TXS PHOTON II diffractometer. The crystals of SOF and DOF were maintained at 301 K and 273 K, respectively, during data collection. Using Olex2,²⁵ the structure was solved with the SHELXT²⁶ structure solution program using intrinsic phasing and refined with the SHELXL²³ refinement package using least squares minimisation.²⁷

2.3. DFT calculations

The optimized structures and frontier orbitals (HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital) were calculated using density functional theory (DFT) and time-dependent DFT (TD-DFT) at the B3LYP/6-31G(d,p) level using the Gaussian 09W²⁸ program in both the ground and excited states. Structural optimization was performed prior to the calculation of the HOMO/LUMO energies.^29,30

2.4. Synthesis and characterization

2.4.1. Synthesis of SOF. To a flame-dried 120 mL closed Schlenk flask were added o-carborane (144.0 mg, 1.0 mmol) and dry THF (2.0 mL). The mixture was cooled to 0 °C and i-PrMgCl (0.6 mL, 1.2 mmol, 2.4 M n-hexane solution) was slowly added. The resulting solution was stirred at 0 °C for 2 h and then stirred at room temperature for 10 h under a N₂ atmosphere. After replacement of THF with toluene (4.0 mL), dehydrated nickel chloride (13 mg, 0.1 mmol) and 3-bromofluoranthene (337 mg, 1.2 mmol) were added, and the reaction mixture was heated at 120 °C under stirring for 20 h. After hydrolysis with water (10 mL) and extraction with dichloromethane (20 mL × 2), the obtained organic phases were combined and concentrated to dryness in vacuo. The residue was subjected to flash column chromatography on silica gel using petroleum ether : dichloromethane = 10 : 1 as the eluent to obtain a yellow solid. 188 mg, with a yield of 77%. M.p.: 209 °C. ¹H NMR (600 MHz, CDCl₃) δ 8.44 (d, J = 6.0 Hz, 1 H), 7. 39 (t, J = 6.0 Hz, 2 H), 7.86 (t, J = 6.0 Hz, 2 H), 7.82 (d, J = 6.0 Hz, 1 H), 7.68 (t, J = 6.0 Hz, 1 H), 7.43–7.37 (m, 2H), 4.60 (s, 1 H, cage-CH), 2.98–1.90 (B–H, bro, 10 H). ¹³C NMR (151 MHz, CDCl₃) δ 139.7, 138.2, 137.7, 133.2, 130.0, 129.2, 129.2, 128.7, 128.1, 127.4, 124.9, 121.9, 121.5, 120.4, 118.9, 61.2. HRMS: m/z calcd for C₁₈H₂₀B₁₀[M]⁺: 344.2563, found: 344.2428.

2.4.2. Synthesis of DOF. To a flame-dried 120 mL closed Schlenk flask, o-carborane (144.0 mg, 1.0 mmol) was added and dissolved in THF (2.0 mL). The mixture was cooled to 0 °C and i-PrMgCl (1.3 mL, 2.5 mmol, 2.4 M n-hexane solution) was slowly added. The resulting solution was stirred at 0 °C for 2 h and then stirred at room temperature for 10 h under a N₂ atmosphere. After replacement of THF with toluene (4.0 mL), dehydrated nickel chloride (26 mg, 0.2 mmol) and 3-bromofluoranthene (675 mg, 2.4 mmol) were added. The reaction mixture was heated at 120 °C under stirring for 20 h. After hydrolysis with water (10 mL) and extraction with dichloromethane (20 mL × 2), the obtained organic phases were combined and concentrated to dryness in vacuo. The residue was subjected to flash column chromatography on silica gel using petroleum ether : dichloromethane = 10 : 1 as the eluent to obtain a yellow solid. 155 mg, yield 28%. M.p.: 290 °C. ¹H NMR (600 MHz, CDCl₃) ¹H NMR (600 MHz, CDCl₃) δ 8.80 (d, J = 6.0 Hz, 2 H), 7.88 (d, J = 6.0 Hz, 2 H), 7.81 (d, J = 6.0 Hz, 2 H), 7.74 (d, J = 6.0 Hz, 2 H), 7.68–7.65 (m, 2 H), 7.59 (d, J = 6.0 Hz, 2 H), 7.39 (d, J = 12.0 Hz, 2 H), 7.28 (t, J = 6.0 Hz, 2 H), 7.21 (t, J = 6.0 Hz, 2 H), 2.99–2.23 (B–H, bro, 10 H). ¹³C NMR (151 MHz, CDCl₃) δ 140.3, 139.9, 137.8, 137.5, 134.6, 133.1, 129.5, 128.9, 128.6, 127.8, 126.1, 125.8, 121.9, 121.2, 120.1, 118.6, 90.8. HRMS: m/z calcd for C₃₄H₂₈B₁₀ [M]⁺: 544.3303, found: 544.3166.

3. Results and discussion

3.1. Design and synthesis of SOF and DOF

The synthetic routes toward fluoranthene-based compounds SOF and DOF are presented in Scheme 1. First, o-carborane was converted into o-carborane magnesium salt under the action of a nucleophile (i-PrMgCl), and then it was subjected to a nickel-catalyzed Kumada cross-coupling reaction with 3-bromofluoranthene to finally produce SOF and DOF.³¹ Due to the small steric hindrance around the spherical o-carborane in SOF, free rotation may occur in both the ground state and the excited state. Therefore, it is expected to exhibit dual emission properties with a low quantum yield.^32,33 The distorted conformation of DOF was formed due to structural restriction caused by the steric hindrance of another fluoranthene. Simultaneously, rotation could be inhibited by the inductive effect of the adjacent fluoranthene, leading to unusual optical properties and pressure sensitivity.^13,14 All compounds were characterized by ¹H NMR and ¹³C NMR spectroscopy, as well as HRMS measurements (Fig. S16–S21, ESI†). SOF and DOF exhibit good stability and solubility in common organic solvents.

Scheme 1 Synthetic routes to SOF and DOF. (a) THF, 1.2 eq. i-PrMgCl, 0 °C for 2 h and then room temperature, 10 h; (b) toluene, 1.2 eq. 3-bromofluoranthene, NiCl₂, 120 °C, 10 h; (c) THF, 2.5 eq. i-PrMgCl, 0 °C for 2 h and then room temperature, 10 h; and (d) toluene, 2.5 eq. 3-bromofluoranthene, NiCl₂, 120 °C, 20 h.

3.2. X-ray crystallography

For gaining insights into the relationship between the crystal structure and photophysical properties, the crystal structure was thoroughly studied. The single-crystals of SOF (CCDC 2340882†) and DOF (CCDC 2323066†) were obtained using CH₂Cl₂ and n-hexane solutions via slow evaporation using a solvent layering method. All hydrogen atoms were geometrically fixed using the riding model. Crystal data and structure refinement are given in Table S1 (ESI†). SOF was in a monoclinic crystal system and crystallized in the space group of Cc with four SOF molecules in one unit cell and unit cell parameters of a = 10.47 Å, b = 24.56 Å, c = 7.24 Å, α = 90.00°, β = 92.49°, and γ = 90.00°, and DOF was in a monoclinic crystal system and crystallized in the space group of P2₁/n with four DOF molecules in one unit cell and unit cell parameters of a = 19.32 Å, b = 6.63 Å, c = 26.86 Å, α = 90.00°, β = 106.80°, and γ = 90.00°.

The data of selected bond lengths and dihedral angles for SOF and DOF in the crystal state are shown in Table 1. The dihedral angle of SOF is nearly zero, which is not conducive to ICT, thereby presenting LE luminescence.³² Conversely, the dihedral angle of DOF is approximately perpendicular, which is conducive to ICT and shows luminescence in the ICT state. The C_cage–C_cage bond length in DOF (1.777 Å) is elongated compared with that in o-carborane (1.631 Å), which is consistent with the theoretical calculation results. The results show that the connection of two fluorophores on the two carbons of o-carborane extends the C_cage–C_cage bond length but simultaneously inhibits the vibration of the C_cage–C_cage and enhances the luminous efficiency.

Table 1 Dihedral angles (Ψ°) between the C–C bonds of the o-carborane and the fluoranthene planes, and C–C bond lengths (Å) in the o-carborane of SOF and DOF

Sample	Ψ/°			C–C bond/Å
	Exp.^a	Calc.^b		Exp.^a	Calc.^b
		S0	S1		S0	S1
a Experimental values from X-ray crystal structures. b Calculated values from the ground (S0) and first excited singlet state (S1) optimised structures.
SOF	0.10	0.04	0.02	1.68	1.68	1.69
DOF	78.24	76.82	76.81	1.78	1.87	1.87

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As shown in Fig. S1 (ESI†) and Fig. 1a, the angle between the planes where FA is located in two adjacent SOF molecules is 7.1°, and the maximum distance between the planes is 3.577 Å. However, the FA planes in the two DOF molecules are parallel to each other, with the distances of 7.448 Å and 12.105 Å as shown in Fig. 1b. There is evidence for a π–π interaction between adjacent SOF molecules. However, the distance between adjacent DOF molecules is too long for a π–π interaction to occur. In addition, DOF and SOF exhibit different packing modes as shown in Fig. S2 and S3 (ESI†). The SOF molecules in each unit cell adopt nearly perpendicular packing, whereas DOF exhibits anti-parallel packing.

Fig. 1 Molecular structures and the surface-to-surface distance measurement of the fluoranthene group plane of (a) SOF and (b) DOF (hydrogen atoms are omitted for clarity, and thermal ellipsoids are displayed at 30% probability).

3.3. UV-vis absorption and emission spectra

The UV-vis absorption spectra of SOF and DOF in THF are shown in Fig. 2, and the photophysical properties are summarized in Table 2. For SOF and DOF, two absorption peaks can be attributed to the π–π transition of the FA part and have similar shapes. The molar absorption coefficient of DOF is twice that of SOF, which may be related to the fact that DOF has two FA substituents while SOF has one.²⁸ It is further confirmed that the absorption peak is attributed to the π–π transition of the FA unit, and there is no significant electronic coupling between the FAs and the o-carborane moiety in the ground state.³⁴ Compared with the absorption spectrum of SOF, the spectrum of DOF exhibits a red shift, indicating that the molecular structure has become more conjugated.³⁵ At the same time, the UV-vis absorption spectra of SOF and DOF in different polar solvents were recorded (Fig. S4, ESI†). The absorption intensity and wavelength did not change significantly, indicating that the polarity of the solvent has no effect on its absorption.³⁴ From the UV-vis absorption spectra of SOF and DOF in the solid state (Fig. S5, ESI†), the absorption peaks of SOF and DOF around 400 nm can be attributed to the π–π transition of the FA part and have similar shapes. Compared with the absorption spectrum of SOF, the absorption spectrum of DOF exhibits a red shift. This is mainly due to the larger conjugation degree of DOF than that of SOF.

Fig. 2 UV-vis absorption spectra of SOF and DOF in THF (c = 1.0 × 10⁻⁵ M, 25 °C).

Table 2 The photophysical properties of SOF and DOF

Sample	λ /nm	ε	λ em /nm			Φ (%)
			Sol^d	Agg^e	Solid^g	Sol.^d	Agg.^e	Solid^g
a Absorption wavelength (c = 1.0 × 10^−5 M, 20 °C). b Molar extinction coefficient: ε: 10^4 M^−1 cm^−1. c Excitation wavelengths of SOF and DOF were 369 nm and 371 nm, respectively (c = 1.0 × 10^−5 M, 20 °C). d In THF. e f w = 99%. f Fluorescence quantum yields using a calibrated integrating sphere. g Solid state.
SOF	369	1.1	476	438	492, 608	7.5	4.7	0.7
DOF	371	2.3	458	577	533	5.6	6.5	18.8

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The fluorescence emission spectra of SOF and DOF in different solvents were recorded (Fig. 3). For SOF, the emission peak intensity around 475 nm decreases with the increase of solvent polarity. Interestingly, DOF has the dual-emission characteristic properties. The peak intensity of DOF at 450 nm is almost unchanged, but the intensity of the peak at the longer wavelength decreases, showing a remarkable solvatochromic shift as the solvent polarity increases. To determine the origin of the fluorescence emission peak, a Lippert–Mataga plot was constructed. As depicted in Fig. S6 (ESI†), the slopes of the emission bands around 450 and 550 nm are 5168 cm⁻¹ and 9089 cm⁻¹ for DOF, respectively. On the other hand, the slope of emission band around 470 nm for SOF is 5874 cm⁻¹. These data indicate that the luminescent bands around 450 nm for SOF and 470 nm for DOF should be assigned to the LE state.³³ Moreover, the luminescent bands around 550 nm for DOF can be assigned to the ICT emission.

Fig. 3 Fluorescence emission spectra of (a) SOF (λ_ex = 369 nm) and (b) DOF (λ_ex = 371 nm) in different solvents (c = 1.0 × 10⁻⁵ M, 25 °C).

The fluorescence spectra of SOF and DOF in THF/water mixtures with different water fractions (f_w) are depicted in Fig. 4. As we all know, the planar and twisted conformations in o-carborane-containing compounds are favorable for generating LE and ICT or twisted intramolecular charge transfer (TICT) emission, respectively.³² For SOF, the emission peak intensity initially decreases and then increases with the increase in f_w. When the f_w increases from 70% to 90%, the wavelength red-shifted from 460 to 482 nm (a shift of 22 nm). At the beginning, the concentration of SOF was high, which was conducive to the π–π interaction. With the increase of f_w, the solubility of SOF decreased, leading to a decrease in the emission peak intensity of SOF. As the f_w further increases, the effect of o-carborane inhibiting the π–π interaction gradually emerges, and the emission peak intensity gradually increases. As evidenced in Table 1, SOF demonstrates a dihedral angle of almost zero between the C–C bonds of the o-carborane and the fluoranthene, which is the LE trend of the SOF. The LE trend does not completely overcome the effect of the solubility and the steric hindrance of o-carborane, so the ICT emission peak does not appear. Therefore, the emission color of SOF remains blue with increasing f_w. For DOF, the emission band around 450 nm in THF solution was attributed to LE state emission. With the increase in the f_w, an ICT emission band around 580 nm emerged, while the intensity of the LE state emission weakened (Fig. 4b). These results suggest that DOF has no intermolecular interactions in the aggregated state, and the emission originates solely from the ICT state.³⁶ Inset of Fig. 4b illustrates that DOF emits blue fluorescence in THF and yellow fluorescence in the mixed solvent of THF/H₂O (f_w = 99%) under 365 nm UV irradiation. These results show that the intermolecular interaction becomes weak or disappears in the aggregated state, and DOF exhibits the properties of AIE.²³

Fig. 4 Fluorescence spectra of (a) SOF (λ_ex = 369 nm) and (b) DOF (λ_ex = 371 nm) in THF/H₂O mixtures with different water volume fractions (c = 1.0 × 10⁻⁵ M, 25 °C), Inset: emission images of (a) SOF and (b) DOF in THF (left) and the mixed solvent of THF/H₂O (1/99) (right) under 365 nm UV light illumination.

Moreover, we measured the low temperature emission spectra at 77 K in a 2-methyltetrahydrofuran (2-MeTHF) solution (Fig. S8, ESI†). For SOF, it is noteworthy that the LE emission was observed at both 293 K and 77 K, indicating that the molecular structure retains its initial configuration. This observation implies that intramolecular rotation within the o-carborane moiety persists even at cryogenic temperatures (77 K), which can be attributed to the distinctive spherical architecture of o-carborane.³² For DOF, it is observed that an emission peak occurs at 450 nm, and notably, a new emission peak emerges at 550 nm under cryogenic conditions (77 K). This phenomenon strongly supports that the dual-emission attributes of DOF should originate from the LE and ICT states.³⁷

As shown in Fig. 5, the emission properties of SOF and DOF in the solid state were investigated. Interestingly, we found that SOF has two emission peaks. While there is an emission peak at 470 nm, a new emission peak also appears at 608 nm. The newly observed emission peak at 608 nm is attributed to the π–π interactions between SOF molecules. These π–π interactions lead to the formation of excimers, which result in the additional emission peak.²¹ For DOF, only one ICT emission band is observed. The molecular packing of DOF provides sufficient space for the o-carborane unit to rotate. This rotational freedom allows for the effective ICT process to occur. In the absence of fluorophore substituents, the rotation of the o-carborane unit is not restricted, leading to the dominant ICT emission. As shown in Fig. 5b, two compounds appear as yellow powder under daylight. Under 365 nm UV light irradiation, yellow emission was observed from SOF, and strong yellow emissions were observed from DOF. The absolute emission quantum yields in the solid state were also evaluated (Table 2). The Φ_F values of SOF and DOF were 0.65% and 18.75%, respectively. These strong ICT state emissions with high Φ_F were attributed to the restriction of the rotation at the o-carborane unit by the inductive effect of the fluorophore at the adjacent carbon atom in o-carborane.^12,38–40

Fig. 5 (a) Normalized fluorescence spectra of SOF (λ_ex = 369 nm) and DOF (λ_ex = 371 nm) in the solid state, (b) photographs of compounds as solid powders under day light (left), and fluorescence images under UV illumination (365 nm) (right).

In order to further explore the effect of temperature on the luminescence of SOF and DOF, we measured the fluorescence spectra of solid-state SOF and DOF at different temperatures (Fig. S7, ESI†). The peak intensity of SOF gradually decreases with the increasing temperature, while that of DOF shows no significant changes. From the normalized spectra shown in Fig. 6a and b, it can be seen that the minor emission peak around 450 nm for SOF and DOF at low temperatures is attributed to LE emission. At low temperatures, the reduced thermal motion of molecules stabilizes the LE state and enhances its emission intensity. As the temperature increases, the thermal energy promotes non-radiative transitions from the LE state to the ground state, leading to a gradual decrease in the LE emission intensity.²² What is more, the CIE coordinates of SOF change linearly with the increasing temperature, while there is almost no change in those of DOF from the CIE diagram (Fig. 6c and d) of the emission of SOF and DOF. The photographs of SOF and DOF as solid powders in liquid nitrogen and at room temperature, along with videos illustrating the temperature change process, are provided (Fig. 6c and d and Videos S1 and S2, ESI†). The above results show that the emission wavelength intensity of SOF at 608 nm is affected by temperature, resulting in SOF having thermochromic properties, while DOF does not have thermochromic properties.

Fig. 6 Normalized fluorescence spectra of (a) SOF and (c) DOF in the solid state with different temperatures. The CIE diagram of the emission of (b) SOF and (d) DOF at different temperatures, inset: photographs of solid-state compounds at 77 K (left) and room temperature (right) under ultraviolet light.

In SOF, there are π–π interactions between adjacent molecules. These π–π interactions lead to the formation of excimers. The formation and stability of excimers are highly sensitive to temperature changes.⁴¹ At different temperatures, the balance between the monomer emission and excimer emission shifts, resulting in temperature-dependent emission spectra. When the temperature increases, the thermal motion of molecules intensifies, which can enhance the π–π interactions, promoting the formation of excimers and increasing the intensity of the excimer emission peak.⁴² In contrast, DOF does not exhibit temperature-dependent emission spectra. This is mainly attributed to the larger distance between the planes of fluoranthene in DOF molecules, which prevents effective π–π interactions and makes it difficult to form excimers. Additionally, due to the rigidity of the molecules, the structural alterability is relatively poor.^12,29 As a result, the emission spectrum of DOF is primarily dominated by monomer emission, and its emission properties are less affected by temperature changes.⁴³

Next, the luminescence lifetimes of SOF and DOF in different aggregation states were tested, as shown in Fig. 7 and Fig. S9 (ESI†). The radiative transition rate constants and non-radiative transition rate constants of SOF and DOF were quantitatively calculated based on their fluorescence lifetimes and quantum yields (Table S3, ESI†). In each state, the non-radiative transition rate constants of the two compounds are larger than the radiative transition rate constants. For SOF in the solid state, the radiative transition rate constant was mere 1/15th of that in the aggregated state, while the non-radiative transition rate constant was halved. Conversely, DOF in the solid state exhibited a radiative transition rate constant triple that of the aggregated state, alongside a halved non-radiative transition rate constant. These results strongly imply that the restricted vibration of C_cage–C_cage bonds and rotational motion of the o-carborane moiety can lead to emissions of the ICT state with high luminesce efficiencies. As illustrated in Fig. 7 and Table S3 (ESI†), when SOF is monitored at a 608 nm light source, the fluorescence lifetime exhibits an increase of one order of magnitude. This significant enhancement is a characteristic feature of excimer formation.³⁹

Fig. 7 Decay profiles of fluorescence lifetime measurement of (a) SOF, (b) SOF and (c) DOF monitored at (a) 492 nm, (b) 608 nm, and (c) 533 nm (λ_ex = 371 nm, 25 °C).

3.4. Mechanochromism behavior

As shown in Fig. 8a and Fig. S10 (ESI†), the luminescence color of the FA and SOF remains unchanged after grinding. Interestingly, we unexpectedly found that the DOF compound exhibits exciting mechanochromic properties. The luminescence color of DOF after grinding and fumigation changes. “XJU” we wrote was initially orange and then recovered to green after solvent fumigation, but traces of “XJU” can still be seen. We speculate that the grinding process alters the packing mode of the compounds, leading to partial overlap of the pressure-induced FA, which in turn emit orange fluorescence. FA, due to its rigid planar structure, is more likely to form tight π–π interaction. Although there is overlap in the FA part of SOF molecules, the perpendicular packing and the angle between the planes of FA (Fig. S1 and S2, ESI†) mean that grinding has little effect on their packing mode.

Fig. 8 (a) Photographs of DOF after grinding and fuming under ultraviolet light; (b) fluorescence spectra, (c) PXRD patterns and (d) DSC curves of pristine (single crystal state), ground and fumed powders of DOF.

There is a peak at 525 nm before grinding and after fumigation recovery, and a red shift of about 50 nm occurs after grinding (Fig. 8b). The compound exhibits significant luminescence stability, and the luminescent color after grinding does not change after a long period of placement. In order to test the stability of the mechanochromic properties of DOF, the emission intensity and wavelength of DOF were monitored, as shown in Fig. S11 (ESI†). After four cycles, there was almost no change in the luminescence wavelength and intensity of the compound. The disordered powder after grinding can be completely restored to the original state after exposure to dichloromethane vapor, indicating that the mechanochromism of the DOF compound has excellent reversibility.

Powder XRD was used to characterize the changes in the powder crystallites of pristine, ground and fumed powders of DOF, and the results are shown in Fig. 8c. The diffraction peak intensity of the compound in its initial state is high and sharp, indicating that the compound possesses an ordered crystal structure in this state. In contrast, when the compound is ground, the peak shape of some diffraction peaks of the compound DOF becomes wider, and the intensity decreases or even disappears, indicating that the ordered microcrystalline structure in the compound DOF is destroyed after pressure grinding. It shows that the luminescence change of the compound is due to its transition from the crystalline state to the disordered state.^13,40,41

The thermodynamic change process of the powder before and after the pressure treatment was studied by DSC, and the results are shown in Fig. 8d. It can be seen from the DSC curves of the compounds in each state that an endothermic peak appears at 153.2 °C, which does not belong to the endothermic peak of the melting point (around 290 °C) of the compound. The disappearance of this endothermic peak after grinding indicates that during the heating process, the metastable microcrystalline phase in the compound transforms into a stable amorphous state through an exothermic recrystallization process, which is consistent with the XRD results. After the grinding treatment, the endothermic peak reappeared following fumigation with the organic solvent dichloromethane, indicating that pressure can induce the transition of molecules from a metastable crystalline state to a disordered state.²⁴

3.5. DFT calculations

In order to further understand the electronic structure of the compound and estimate the geometric structure of SOF and DOF, density functional theory (DFT) was used to calculate the molecular orbital at the B3LYP/6-31G(d,p) theoretical level using the Gaussian 09W program. On the basis of the optimized ground state geometry, the singlet excited state was calculated using time-dependent density functional theory (TD-DFT). The optimized ground state geometries are shown in Fig. S12 and S13 (ESI†). The results show that the electron cloud of the HOMO energy level of compound SOF is mostly distributed on the FA skeleton, with a small portion distributed on the o-carborane cage. Compared with the HOMOs of SOF and DOF, the o-carborane cage is involved in the composition of more LUMOs to a certain extent, indicating that the o-carborane cage may participate in the excited state.

As shown in Fig. S14 and S15 (ESI†), TD-DFT calculations were carried out based on the optimized molecular ground state configuration. The calculated results are in good agreement with the experimental UV-vis absorption spectra. The lowest energy absorption bands of compounds SOF and DOF are 353 nm and 350 nm, which are attributed to the transitions from HOMO → LUMO and HOMO → LUMO+1, respectively. The HOMO molecular orbital electron cloud of compound SOF is mainly distributed over the two benzene rings of FA, and the LUMO molecular orbital electron cloud is distributed across the entire FA molecule. The electron cloud of the compound DOF extends from the loosely arranged HOMO in the FA part to the tightly arranged LUMO+1.

In the excited state (Fig. 9), for SOF, the electron clouds of the HOMO and HOMO−1 are distributed on the carbon–carbon single bond of FA and o-carborane, while the electron clouds of the LUMO and LUMO+1 have a tendency to shift to o-carborane due to the electron-withdrawing effect of o-carborane. This means that there will be peaks of the LE state and excimer, and the LE state will dominate, which is confirmed by the solid-state emission spectrum of SOF. In the solid-state emission spectrum, there are LE state emission peaks and excimer emission peaks, and the LE state peak is higher than the latter. The electron cloud of the HOMO energy level of the compound DOF is distributed only on one FA group, while the electron cloud of the LUMO energy level of DOF is distributed across the entire molecular skeleton. This indicates that DOF exhibits the trend of LE and ICT upon excitation.

Fig. 9 Energy diagrams and the frontier orbital contribution of (a) SOF and (b) DOF, and their lowest and second-lowest-energy transitions estimated by TD-DFT calculations at the B3LYP/6-31G(d,p) level.

4. Conclusion

In summary, multifunctional luminescent molecules, SOF and DOF, were successfully synthesized in high yield via a modified nickel-catalyzed Kumada cross-coupling reaction incorporating FA and o-carborane to enhance electronic interactions. The SOF shows dual emissions and thermochromism as well as AIE properties. Moreover, the DOF not only has the AIE properties but also exhibits mechanochromism. The nearly perpendicular packing is conducive to thermochromism, while the anti-parallel packing is conducive to mechanochromism. Furthermore, the crystal transformation serves as a critical factor contributing to mechanochromism. Our findings may be useful not only for the future molecular structural design of new multifunctional luminescent materials but also for developing stimuli-sensitive materials.

Author contributions

N. Li was responsible for experimental investigations and formal analysis, data presentation, and computational investigation. J. Chi and Y. Lv were responsible for experimental investigations, computational investigation, and visualization. X. Wu and J. Guo were in charge of methodology and conceptualization, computational investigations, writing the original draft, review and editing of the manuscript, and funding acquisition.

Data availability

Data are provided within the manuscript or ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (22105163), the Open Project of Key Laboratory in Xinjiang Uygur Autonomous Region of China (2023D04032), and the Science and technology innovation leader of Xinjiang Uygur Autonomous Region of China (2022TSYCLJ0043).

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Footnote

† Electronic supplementary information (ESI) available: UV-Vis absorption and fluorescence spectra, and DFT calculations. CCDC 2340882 and 2323066. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5nj01482d

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