High-contrast and reversible mechanochromic luminescence of a D–π–A compound with a twisted molecular conformation

Abstract

By integrating a triphenylamino donor (D) and a (2,2-dicyanovinyl)phenyl acceptor (A) with an anthryl π-spacer, a mechanochromic luminescent (ML) D–π–A compound, TPAANDCV, has been constructed. The single crystals of TPAANDCV display efficient green emission with a good fluorescence quantum yield (Φ_F) of 0.35. Upon grinding the crystals, a dramatic emission-colour variation from green to orange-red has been observed, and such a variation can be erased through annealing or solvent-vapour fuming. Moreover, the emission of the ground sample is also bright with a Φ_F value of 0.11. The powder XRD measurements indicate that the emission-colour change upon grinding is caused by a partial conversion of the ordered solid-state structures to an amorphous state. The high-contrast and reversible fluorescent response of TPAANDCV towards mechanical stress, as well as the bright emission of both the unground and ground states, enable TPAANDCV to be a good ML material.

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Introduction

Mechanochromic luminescent (ML) materials with switched solid-state luminescent properties in response to mechanical stress have attracted much attention because of their potential applications in sensors, information storage, security papers and optoelectronic devices.¹ So far, a number of ML materials based on metal complexes^2,3 and organic compounds^4,5 have been reported. Their luminescence shows either a colour change⁵ or on/off switching^3b,4k under mechanical stress, and the response behaviour is normally related to the variation of molecular packing modes and/or molecular conformations, two key factors determining the luminescent properties of a solid material. For practical applications, such as sensing, a large variation of emission colours and the efficient emission of both the pressed and unpressed states facilitate naked-eye detection; a reversible colour change is beneficial to the reuse of the material. Therefore, the development of materials possessing all these favourable properties is attractive.

Luminescent bipolar compounds with donor–(π-spacer)–acceptor (D–π–A) structures are a class of fascinating materials widely used in various applications, such as nonlinear optics (NLO),⁶ organic light-emitting diodes (OLEDs)⁷ and fluorescent probes.⁸ Recently, a few D–π–A compounds have also been found to exhibit mechanochromic luminescence.⁹ Besides the intramolecular charge-transfer (ICT) emission, a relatively flexible and non-planar molecular structure with multiple intramolecular rotation sites is another general feature of this type of system.¹⁰ Compounds with such a structural feature can allow the variations of not only the molecular packing modes but also the molecular conformations^4e,f,n,o when mechanical stress is applied on their solid samples, which is unlike the case of structurally rigid compounds/chromophores whose conformation is hardly changed. Therefore, this structural feature could be viewed as an advantage for enhancing florescence sensitivity towards mechanical stress. In addition, the luminescent properties of D–π–A compounds can be readily tuned by facile modifications of the D, A or π-spacer moiety,¹¹ which facilitates the development of ML materials with diverse response behaviours. These attractive properties of D–π–A compounds provide a broad prospect for achieving good ML materials based on this type of system.

In line with our continuous interest in ML materials, we have prepared a new D–π–A ML compound, TPAANDCV, by adopting a triphenylamino (TPA) donor, an anthryl (AN) π-spacer and a (2,2-dicyanovinyl)phenyl (DCV) acceptor (Scheme 1). It exhibits reversible mechanochromic fluorescence, showing variable emission with high-contrast colours and high luminance upon mechanical grinding. Herein, the detailed studies are reported.

Scheme 1 Syntheses of TPAANDCV. Reagents and conditions: (i) Pd(PPh₃)₄, K₂CO₃, toluene, EtOH, H₂O, reflux; (ii) CHCl₃, Et₃N, r.t.

Results and discussion

Syntheses

As can be seen in Scheme 1, TPAANDCV has been readily synthesized through the Knoevenagel condensation reaction of malononitrile and a formyl-substituted precursor (1) generated from the Suzuki cross coupling reaction of 4-formylphenylboronic acid and 4-(10-bromoanthracene-9-yl)triphenylamine. Both the precursor (88%) and TPAANDCV (76%) were obtained in high yields, and their structures were confirmed by NMR spectroscopy, elemental analysis and mass spectra.

Solution photophysics

In nonpolar cyclohexane, TPAANDCV exhibits three absorption bands (Fig. 1): a weak and broad lowest-energy shoulder band at around 450 nm; a higher-energy band with fine structures in the range of 350–400 nm; and a highest-energy band with the maximum located at 304 nm. According to TD-DFT (M06-2X/6-31G(d,p)) calculations, the lowest-energy shoulder can be assigned to the S1 ← S0 transition (oscillator strength (f): 0.25) which is dominated by LUMO ← HOMO−1 and contributed slightly from LUMO ← HOMO (Table S1†). As can be seen in Fig. 2, HOMO−1, HOMO and LUMO are located at the AN, TPA and DCV moieties, respectively. Therefore, the shoulder band is originated from ICT transitions. The neighbouring higher-energy and structured absorption band can be assigned to the S2 ← S0 transition (Table S1†) which is dominated by the π–π* transition (LUMO+1 ← HOMO−1) of AN unit with a small contribution of the electronic transition from TPA to AN moieties (LUMO+1 ← HOMO). The highest-energy band is likely originated from a number of ICT and TPA-centred transitions (Table S1 and Fig. S1†). From TPAANDCV to precursor 1, the replacement of DCV unit with 4-formylphenyl group leads to the disappearance of the shoulder and hypsochromically shifts the absorption edge from ca. 500 nm to ca. 435 nm, while showing tiny influence (<4 nm) on the peak wavelengths of the other two bands (Fig. 1). TD-DFT calculations indicate that the S1 ← S0 transition (f = 0.38) of 1 also possesses certain ICT characters (Table S2 and Fig. S2†). The peak wavelengths of the two strong higher-energy absorption bands of TPAANDCV are almost independent on solvent polarity, showing a small variation within 3 nm from cyclohexane to THF. Similarly, only slight influence of solvent polarity on the absorption spectra of precursor 1 can be observed (Fig. S3†). Interestingly, the weak shoulder band of TPAANDCV becomes inconspicuous in toluene and almost invisible in more polar Et₂O and THF (Fig. 1), while the absorption edge at ca. 500 nm is not obviously changed. The reason for the gradual disappearance of the shoulder is not fully understood yet.

Fig. 1 UV-vis absorption spectra of TPAANDCV and 1.

Fig. 2 Molecular orbitals of TPAANDCV calculated at the M06-2X/6-31G(d,p) level of theory using the optimized geometry.

In solution, the emission properties of TPAANDCV strongly depend on solvent polarity. As can be seen in Fig. 3, the emission maximum (λ_em) of TPAANDCV, located at 521, 575 and 602 nm in cyclohexane, toluene and diethyl ether, respectively, exhibits significant red shift with increased solvent polarity. Correspondingly, the emission colour varies from green to orange-red. In more polar solvents, like THF and CH₂Cl₂, the compound becomes non-emissive. The continuously red-shifted emission colours and emission spectra with gradually increased solvent polarity, which clearly shows the solvatochromic tendency of TPAANDCV, can also be observed in a series of solutions with cyclohexane/dichloromethane mixed solvents (Fig. S4†). The observed positive solvatochromism indicates the formation of a highly polarized ICT excited state.¹² Compared with compound 1 which shows the λ_em values of 452, 468 and 472 nm in cyclohexane, toluene and diethyl ether (Fig. S5†), respectively, TPAANDCV with stronger DCV acceptor¹³ exhibits significantly red-shifted emission in corresponding solvent. In addition, unlike TPAANDCV, 1 is emissive in THF and even highly polar DMF (Fig. S5†). The fluorescence quantum yields (Φ_F) of TPAANDCV are measured to be 0.61 in cyclohexane, 0.56 in toluene, and 0.30 in Et₂O, showing a decrease with increased solvent polarity. The non-emissive nature of TPAANDCV in more polar CH₂Cl₂/THF is likely attributed to the formation of a highly twisted ICT (TICT) state which is usually generated by the push–pull systems in polar solvents and featured by weak/no emission due to the electronically decoupled molecular orbitals involved in the emission process.^11,14 As the TICT state is formed after the geometric and energetic relaxation of the Franck–Conden state, a rigid environment that prevents the relaxation is able to restrict the formation of TICT state and thus leads to the intensified and higher-energy emission.¹¹ When the non-emissive room-temperature solution of TPAANDCV in polar 2-MeTHF (10⁻⁶ M) is cooled to 80 K, an intense green emission (λ_em = 544 nm) of the formed solid solution can be observed, consistent with the formation of a TICT state in polar solvents (Fig. S6†).

Fig. 3 Emission spectra of TPAANDCV (10⁻⁵ M) in different solvents. Inset: photographs of TPAANDCV in cyclohexane (left), toluene (middle) and diethyl ether (right) taken under UV light.

Emission behaviour of the aggregates formed in THF/H₂O mixtures

Though TPAANDCV is non-emissive in THF, it forms luminescent aggregates when a certain amount of water, a non-solvent for TPAANDCV, is added to its THF solution (10⁻⁵ M). As can be seen in Fig. 4, when the water fraction (f_w, vol%) is less than 40%, the solute does not tend to aggregate and no obvious emission can be detected. After the f_w is increased to 50%, the emission becomes detectable but still extremely weak. Significant enhancement of the emission is achieved when the f_w is higher than 60%, due to the aggregation of a large amount of the solute. At a f_w of 90%, the fluorescence intensity is further enhanced by ca. 60 times compared with that at a f_w of 50%. The above-described florescence enhancement phenomenon of TPAANDCV from THF solution to aggregates is similar to those observed for other donor–acceptor compounds and termed aggregation-induced emission (AIE).¹⁵ It has been confirmed that the AIE behaviours of compounds with push–pull structures can be a result of the restricted formation of a TICT state,^15a,b which may also play a key role for the florescence enhancement of TPAANDCV. The aggregates show orange-red emission with the emission maximum at ca. 620 nm, which does not show obvious shift towards different f_w values. The emission of the aggregates is dramatically red shifted compared with that of the solid 2-MeTHF solution (λ_em = 544 nm) formed at 80 K. This is probably attributed to the strong intermolecular interactions in the aggregates.

Fig. 4 (a) Emission spectra of TPAANDCV in THF/H₂O mixtures (10⁻⁵ M) with different water fractions; (b) plot of emission intensity vs. water fractions. The inset shows the emission enhancement at a water fraction of 90%.

Luminescence mechanochromism

The single crystals of TPAANDCV, obtained by slow evaporation of its solution in the CH₂Cl₂/CH₃OH mixed solvent, exhibit strong green emission with the λ_em located at 532 nm (Fig. 5) and a Φ_F value of 0.35. Through grinding the crystals with a pestle in a mortar, a large variation of the emission colour from green to orange-red, as well as a huge red shift (>80 nm) of the λ_em from 532 to 615 nm, can be observed, suggesting the high-contrast mechanochromic fluorescence of TPAANDCV (Fig. 5). Notably, both the pristine crystals and the ground powder display bright emission, with the Φ_F values of 0.35 and 0.11, respectively. When the ground powder is annealed at 150 °C for 1 min or fumed by the dichloromethane vapour for 30 s, bright green emission can be recovered, indicating the reversibility of the ML behaviour. The high-contrast and reversible variation of the luminescent colours and the bright emission of both the ground and unground states indicate that TPAANDCV is a good ML material. We note that the ground powder and the TPAANDCV aggregates formed in the THF/H₂O mixtures have close λ_em values. However, the separated aggregates do not show obvious emission-colour change upon dichloromethane-vapour fuming for 1 min, indicating that the aggregates are not identical to the ground powder.

Fig. 5 Fluorescence spectra of the crystals, ground powder and annealed and vapour-fumed ground powders of TPAANDCV. Inset shows the photographs of TPAANDCV before and after grinding.

In order to investigate the underlying origin of the mechanochromism of TPAANDCV, the powder X-ray diffraction (PXRD) measurements were carried out (Fig. 6). The PXRD pattern of the pristine crystals shows a number of intense and sharp peaks which are consistent with those simulated from the single-crystal structure (Fig. S7†), suggesting the unique crystalline phase of the collected sample. Upon grinding, these peaks become much weaker and show a superposition over a newly generated weak and broad diffuse halo (Fig. 6 and S8†), indicating that most of the pristine crystals with ordered molecular packing structures convert to an amorphous phase.¹⁶ The diffraction pattern of the ground powder is hardly changed by further grinding, resulting in relatively stable emission spectra and Φ_F values for the ground powder. Similar phenomena of incomplete conversion have been observed in other ML systems.¹⁷ Annealing or vapour fuming of the ground power results in the disappearance of the diffuse halo and the intensification of diffraction peaks, indicating the recovery of original crystalline state. From the PXRD results, it can be concluded that the conversion between crystalline and amorphous states is responsible for the reversible mechanochromism of TPAANDCV.

Fig. 6 PXRD patterns of the crystals, ground powder and the annealed and vapour-fumed ground powders of TPAANDCV.

Differential scanning calorimetry (DSC) measurements were also carried out to understand the mechanochromism of TPAANDCV. Upon heating, the pristine crystals of TPAANDCV only exhibit one endothermic peak at 272 °C corresponding to the melting of the sample, while the ground powder experiences two exothermic recrystallization processes at around 105 and 140 °C and then melts at the melting point of the pristine crystals (Fig. S9†). These observations indicate that heating the ground sample at 150 °C is able to realize the recovery of the original crystalline state, consistent with the PXRD results and the observed recovery of the green emission through annealing.

As for compound 1, grinding its blue-green emissive single-crystals (λ_em = 481 nm, Φ_F = 0.35) obtained from vacuum sublimation leads to an emission red shift of only ca. 13 nm (Fig. 7). The original emission colour can also be recovered by annealing (160 °C, 2 min) or dichloromethane-vapour fuming (1 min) of the ground powder. PXRD studies suggest that the interconversion between crystalline and amorphous states is responsible for the reversible fluorescence mechanochromism (Fig. S10†), similar as the case of TPAANDCV. The much stronger mechanochromism of TPAANDCV, compared with 1, implies that the 2,2-dicyanovinyl group plays an important role in realizing the strong emission-colour response towards grinding. The large grinding-induced emission red shift of TPAANDCV is accompanied by a large red shift (ca. 100 nm) of the fluorescence excitation spectra, while the weak mechanochromism of 1 is consistent with a slight shift (<10 nm) of the excitation spectra (Fig. 8).

Fig. 7 Fluorescence spectra of the crystals, ground powder and annealed and vapour-fumed ground powders of 1. Inset shows the photographs of 1 before and after grinding.

Fig. 8 Fluorescence excitation spectra of the crystals (solid line) and ground powders (dash line) of TPAANDCV (black) and compound 1 (red). The emission maxima of corresponding samples were adopted as the monitored wavelength, i.e. 532 and 481 nm for the crystals of TPAANDCV and 1, respectively, and 615 and 494 nm for the ground powders of TPAANDCV and 1, respectively.

Single-crystal structures

To further understand the mechanism of the ML behaviour of TPAANDCV at a molecular level, single-crystal structures of TPAANDCV and compound 1 were resolved. In both compounds, the anthracene ring exhibits large dihedral angles with the 1,4-substituted phenyl rings in the donor (76.3° for TPAANDCV and 62.8° for 1) and acceptor (86.3° for TPAANDCV and 87.1° for 1) moieties, resulting in a highly twisted molecular conformation (Fig. 9a and S11†). The large dihedral angles are attributed to the repulsion between the hydrogen atoms at 1,4,5,8-positions of the anthracene ring and their adjacent hydrogen atoms on 1,4-substituted phenyl groups.¹⁸ Notably, the DCV unit in TPAANDCV is twisted, showing a dihedral angle of 38.7° between the 2,2-dicyanovinyl group and C₆H₄ ring. This is different from the case in an analogous compound, 4-(2,2-dicyanovinyl)triphenylamine,^9f where the corresponding dihedral angle is only ca. 6° and the DCV unit is almost planar. In contrast to the DCV unit, the 4-formylphenyl moiety in 1 is almost planar (Fig. S11†). In the DFT-optimized ground-state structure of TPAANDCV, the large dihedral angles between the 1,4-substituted phenyl rings and the anthracene ring are well reproduced, while a fully planar DCV unit is observed (Fig. S12†). Thus, the twisted DCV moiety in the crystal structure is likely a result of specific molecular packing arrangements. In the crystal of TPAANDCV, molecules are packed together through rich intermolecular C–H⋯π hydrogen bonds, as well as C–H⋯N hydrogen bonds between the DCV units (Fig. 9). The congested environments around the DCV unit caused by the formation of hydrogen bonds may be a reason for its twisted geometry. Due to the highly twisted molecular conformation, there are no intermolecular π⋯π interactions in the crystals of both TPAANDCV and 1, which contributes to their good Φ_F values in the crystalline state.¹⁹

Fig. 9 (a) Molecular structure of TPAANDCV from single-crystal X-ray diffraction. Hydrogen atoms are omitted for clarity. Atomic displacement ellipsoids are drawn at 50% probability; (b) C–H⋯N hydrogen bonds in the crystal; (c) C–H⋯π hydrogen bonds in the crystal.

At a molecular level, the varied solid-state emission of a D–π–A compound can be caused by the change of molecular packing features and the variation of molecular conformations that affect the ICT characters. However, these two factors are normally interrelated, leading to the difficulties in analysing and separating their respective influences on emission colours. As suggested in other studies on donor–acceptor type of ML compounds with twisted structures, the planarization of the molecule may occur upon grinding and contributes to the red-shifted emission.^{4o,9e,10a,16b,17} When the crystals of TPAANDCV are ground, the highly-ordered molecular arrangements are destroyed, which may diminish of steric congestion around the DCV unit and result in its planarization. Such a planarization may be partly responsible for the red shift of the emission spectra as well as the excitation spectra. Further planarization of the whole molecule may also occur. However, the extent of the planarization might be small, as deduced from the fact that compound 1 with a similar twisted skeleton only exhibits weak mechanochromism. Considering that the DCV group is a stronger and more extended acceptor than the 4-formylphenyl group, stronger and richer dipole–dipole interactions may exist in the ground powder of TPAANDCV and serve as another responsible factor for the hugely red-shifted emission and excitation spectra of the ground powder.²⁰

Conclusions

In summary, a new D–π–A compound, TPAANDCV, has been synthesized through linking a triphenylamino donor and a (2,2-dicyanovinyl)phenyl acceptor with an anthryl π-bridge. Forced by the hydrogen–hydrogen repulsion, the compound exhibits a highly twisted conformation, which prevents the π–π stacking in the crystalline state and contributes to the bright green emission of the single crystals. The D–π–A structure endows the compound with an ICT state that results in solvatochromic emission with the emission colours ranging from green to orange-red. TPAANDCV displays AIE behaviours in THF/H₂O mixtures, forming orange-red emissive aggregates. Most importantly, it exhibits excellent mechanochromic luminescent properties with a high-contrast and reversible colour change between green and orange-red, as well as bright emission for both the ground and unground states. The PXRD results indicate that the colour change upon grinding is caused by the partial conversion of the ordered molecular packing structures to an amorphous state. Our results will facilitate the further development of D–π–A type of mechanochromic luminescent materials.

Experimental

General information

4-(10-Bromoanthracene-9-yl)triphenylamine was synthesized according to literature procedures.²¹ All other starting materials were purchased from commercial sources and used without further purification. Solvents for synthetic reactions were analytical grade and used as received, while the solvents for photophysical measurements were spectroscopic grade. ¹H and ¹³C{¹H} NMR spectra were recorded on Varian Mercury 300 MHz and Bruker Mercury 500 MHz spectrometers, respectively, with tetramethylsilane as the internal standard. Mass spectra were recorded on a Thermo Fisher ITQ 1100 mass spectrometer. Element analyses were performed on a Flash EA 1112 elemental analyser. PXRD data were collected at 298 K on a Bruker SMART-CCD diffractometer. DSC measurements were performed on a Netzsch DSC 204 F1 instrument at a heating rate of 10 °C min⁻¹ under nitrogen. UV-vis absorption spectra were recorded by a Specord 210 Plus spectrophotometer (Analytik Jena AG, Germany). The emission spectra of solutions and solids were recorded using a Shimadzu RF-5301 PC spectrometer and a Maya2000 Pro CCD spectrometer, respectively. The excitation spectra of solids were measured using a Shimadzu RF-5301 PC spectrometer. The fluorescence quantum yields of solutions and solids were measured at an excitation wavelength of 365 nm by using a calibrated integrating sphere combined with an Edinburgh FLS920 spectrometer. The absolute error of the given quantum yields is within ±0.03 as estimated by multiple measurements.

Single-crystal X-ray diffraction

Single crystal X-ray diffraction data were collected on a Rigaku RAXIS-PRID diffractometer using the ω-scan mode with graphite-monochromator Mo Kα radiation. The structures were solved with direct methods using the SHELXTL programs and refined with full-matrix least-squares on F². Non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated and refined isotropically.

AIE experiments

A THF solution of TPAANDCV with a concentration of 1 × 10⁻⁴ M was prepared. 0.4 mL of the solution was transferred to a quartz cuvette. According to the desired final concentration of 1 × 10⁻⁵ M and the required water fractions (0% to 90%), an appropriate amount of THF was added at first, and then water was added dropwise under stirring (Table S3†). The emission spectrum was checked immediately after the solution was prepared.

Theoretical studies

All calculations (DFT and TD-DFT) were carried out with the program package Gaussian 09 (Rev. D.01).²² The ground-state geometry was optimized by DFT using the B3LYP functional²³ in combination with the 6-31G(d,p) basis set. The molecular structure as determined by X-ray crystallography was used as the input for optimizing the ground-state geometry. The optimized geometries were confirmed to be local minima by performing frequency calculations and obtaining only positive (real) frequencies. Based on these optimized structures, the gas-phase lowest-energy vertical transitions were calculated (singlets, ten states) by TD-DFT using the M06-2X functional²⁴ in combination with the 6-31G(d,p) basis set.

Syntheses

9-(4-Formylphenyl)-10-(4-(diphenylamino)phenyl)anthracene (1). Aqueous K₂CO₃ (2.0 M, 4 mL) and ethanol (2 mL) were added to a mixture of 4-(10-bromoanthracene-9-yl)triphenylamine (1.0 g, 2 mmol), 4-formylphenylboronic acid (0.6 g, 4 mmol) and toluene (45 mL). After the obtained mixture was degassed, Pd(PPh₃)₄ (87 mg, 0.075 mmol) was added under a nitrogen atmosphere. The reaction mixture was heated at reflux for 12 h (under N₂) and then cooled to r.t. The solvents were evaporated under vacuum and the resulting residue was extracted with dichloromethane and water. The organic phase was washed with brine and then evaporated to dryness. Purification by column chromatography (silica gel, 1.5 : 1 dichloromethane/petroleum ether) produced the desired compound as a yellow-green solid (926 mg, 88.2%). The single crystal of the compound was obtained by vacuum sublimation. ¹H NMR (300 MHz, DMSO, ppm): δ 10.22 (s, 1H), 8.20 (d, J = 9 Hz, 2H), 7.80–7.69 (m, 4H), 7.56–7.42 (m, 7H), 7.42–7.34 (m, 5H), 7.26–7.19 (m, 6H), 7.13 (t, J = 7.3 Hz, 2H). ¹³C NMR (500 MHz, CDCl₃, ppm): δ 192.03, 147.80, 147.35, 146.21, 137.93, 135.72, 135.17, 132.26, 132.07, 130.08, 129.87, 129.56, 129.45, 127.33, 126.34, 125.57, 125.14, 124.80, 123.22, 123.03. MS (m/z): 525.5 [M]⁺. Anal. calcd (%) for C₃₉H₂₇NO: C, 89.11; H, 5.18; N, 2.66. Found: C, 89.40; H, 5.12; N, 2.65.

9-(4-(2,2-Dicyanovinyl)phenyl)-10-(4-(diphenylamino)phenyl)-anthracene (TPAANDCV). Compound 1 (787 mg, 1.5 mmol) and malononitrile (297 mg, 4.5 mmol) were dissolved in CHCl₃ (40 mL), and then one drop of triethylamine was added. The mixture was stirred at r.t. for 12 h under a nitrogen atmosphere. The volatiles were removed under vacuum, and then the residue was purified by column chromatography (silica gel, 1 : 1 dichloromethane/petroleum ether) to produce a crude product. The recrystallization of the crude product through slow evaporation of its solution in CH₂Cl₂/CH₃OH mixed solvent produced the pure TPAANDCV as light-yellow crystals (0.65 g, 75.7%). ¹H NMR (300 MHz, DMSO, ppm): δ 8.76 (s, 1H), 8.24 (d, J = 8.2 Hz, 2H), 7.77 (d, J = 8.2 Hz, 4H), 7.50 (d, J = 7.8 Hz, 6H), 7.45–7.31 (m, 6H), 7.28–7.17 (m, 6H), 7.13 (t, J = 7.3 Hz, 2H). ¹³C NMR (500 MHz, CDCl₃, ppm): δ 159.51, 147.76, 147.40, 146.80, 138.36, 134.31, 132.91, 132.02, 130.91, 130.22, 130.07, 129.47, 127.46, 126.05, 125.84, 125.24, 124.83, 123.27, 122.96, 113.89, 112.79, 82.89. MS (m/z): 573.5 [M]⁺. Anal. calcd (%) for C₄₂H₂₇N₃: C, 87.93; H, 4.74; N, 7.32. Found: C, 87.98; H, 4.67; N, 7.31.

Acknowledgements

This work was supported by the National Basic Research Program of China (2015CB655000), the National Natural Science Foundation of China (51173067 and 91333201) and Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT101713018).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra and additional data for single-crystal structures (CCDC 1056935 for TPAANDCV and 1410073 for 1), TD-DFT calculations and PXRD, DSC and luminescence measurements. CCDC 1056935 and 1410073. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra12050k

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