4,5-Diaminophthalimides: highly efficient solid-state fluorophores and turn-on type fluorescent probes for hydrazine

Masaki Shimizu*, Tomokazu Tamagawa and Kenta Nishimura
Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, 1 Hashikami-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. E-mail: mshimizu@kit.ac.jp

Received 13th November 2018 , Accepted 18th December 2018

First published on 19th December 2018


We report herein that 4,5-bis(diarylamino)phthalimides exhibit efficient solid-state emission. The phthalimides were easily prepared from dimethyl 4,5-bis(diarylamino)phthalates or commercially available 4,5-dichlorophthalic acid via a few steps. The absorption spectra of the phthalimides in toluene showed strong bands at 337–374 nm and weak bands possessing two shoulders at 378–455 nm. Toluene solutions of the phthalimides fluoresced in the greenish blue to orange-yellow region with good-to-high quantum yields. In contrast, the phthalimides showed no emission in DMSO. Based on the observation, phthalimide was demonstrated to serve as a turn-on type fluorescent probe for hydrazine. The phthalimides dispersed in a thin film of poly(methyl methacrylate) and in powder form fluoresced in the blue-to-green and green-to-orange region, respectively, with high quantum yields. As the electron-donating ability of the diarylamino moieties increased, the emission spectra in solution and the solid states were red-shifted. The density functional theory calculations confirm that the photo-excitation involves an intramolecular charge transfer from diarylamino groups to imide-carbonyl moieties.


Introduction

Because of the recent advances in organic light-emitting devices (OLEDs), much attention has been paid to the design and development of π-conjugated compounds that exhibit highly efficient fluorescence in the solid state.1 The advances in organic light-emitting field-effect transistors,2 semiconducting lasers,3 and solid-state fluorescence sensing4 also rely on the development of organic solid-state emitters. Hence, studies on solid-state luminescence are pursued to develop novel luminophores that emit visible light in the solid state with high efficiency.

Phthalimide frameworks are easy to synthesize, are thermally stable, and exhibit good electron-accepting ability. Accordingly, various kinds of small molecules and polymeric compounds with phthalimide moieties are developed for application in organic transistors5 and photovoltaic cells.6 Examples of phthalimide-based luminescent materials are also available, which include N-(aminoalkyl)phthalimides,7 N-aryl-4-(dialkylamino)phthalimides,8 N-cyclohexyl-3-hydroxy- and N-cyclohexyl-3,6-dihydroxyphthalimides,9 4-aryl-N-(4-trifluorophenyl)phthalimides,10 oxydiphthalimides,11 4,5-dicarbazolyl-N-cyclohexylphthalimides,12 4-halo-N-(4-trifluorophenyl)phthalimides,13 and phthalimide-based polyimides.14 However, fluorescent properties reported for the known luminescent phthalimides were mostly those in solution. The precedents of phthalimides that exhibit fluorescence in the solid state are very limited, and the reported fluorescence quantum yields in the solid state, such as crystal, powder, and neat film, were generally lower than 0.3,15 except for 2,2′-(1S,2S)-1,2-cyclohexanediylbis[5,6-di-9H-carbazol-9-yl-1H-isoindole-1,3(2H)-dione] (Φneat[thin space (1/6-em)]film = 0.41)12 and 5-(dimethylamino)-2-(4-methylphenyl)-1H-isoindole-1,3(2H)-dione (Φcrystal = 0.58).8c Thus, the development of phthalimides that efficiently fluoresce in the solid state with high quantum yields exceeding over 0.3 remains unexplored.

During our research on the development of diaminophenylene-cored luminogens that exhibit efficient solid-state fluorescence,16 we recently demonstrated that 4,5-bis(diarylamino)terephthalates emitted blue light with good quantum yields in powder form and in a poly(methyl methacrylate) (PMMA) film.17 The electronic structure of the diaminoterephthalates can be translated as 1,2-bis(acceptor)-4,5-bis(donor)benzene. Based on this analysis, we designed 4,5-bis(diarylamino)phthalimides 1 as novel fluorophores, in which an imide functionality is employed as the equivalent of two acceptors in the 1,2-bis(acceptor)-4,5-bis(donor)benzene framework (Fig. 1). We report herein the synthesis, structures, photophysical properties, and theoretical calculations of 1, showing that 1 can serve as highly efficient solid-state emitters in the blue-to-orange spectral region with quantum yields ranging from 0.30 to 0.57.


image file: c8qm00578h-f1.tif
Fig. 1 Molecular structures of 4,5-bis(diarylamino)phthalimides 1.

Results and discussion

Synthesis

Designed phthalimides 1 were easily prepared from dimethyl 4,5-bis(diarylamino)terephthalates (aryl: C6H5, 4-CF3C6H4)17 or commercially available 4,5-dichlorophthalic acid via two or three steps (Scheme 1). Hydrolysis of the dimethyl esters with lithium hydroxide followed by condensation with aniline derivatives gave 1a–1f in good-to-high yields. Phthalimides 1g–1i were synthesized through a three-step sequence involving condensation of 4,5-dichlorophthalic acid with amine, Pd-catalyzed amination with aniline derivative,18 and Cu-catalyzed arylation. Decomposition temperatures, at which 5% mass loss occurs, of 1 ranged from 247 °C to 336 °C, indicating that 1 are thermally stable.
image file: c8qm00578h-s1.tif
Scheme 1 Synthesis of 1.

UV-Vis absorption properties in toluene

The absorption spectra of 1 in toluene are shown in Fig. 2.19 Each spectrum shows a strong absorption band at 300–370 nm and a weaker and broad band at 360–500 nm. Based on the DFT calculations, the weaker bands at longer wavelengths are ascribed to the ICT from the two diarylamino moieties to the two imide-carbonyl groups (vide infra). The spectra of 4,5-bis[(4-CF3C6H4)2N] derivatives 1f–1h are significantly blue-shifted compared with those of bis[(C6H5)2N] derivatives 1a–1e, while bis[(4-tert-butylC6H4)2N] derivative 1i shows a red-shifted spectrum with respect to that of 1a. This dependence of the absorption spectra on the electron-donating ability of an Ar2N group is consistent with the ICT nature of the optical transition of 1. Compared with the absorption spectra of the corresponding phthalates, the spectra of 1 are bathochromically shifted, indicating that the HOMO–LUMO gaps of 1 are narrower than those of the corresponding phthalates. Considering that the electron-withdrawing effect of an aminocarbonyl group, i.e. an amide moiety, is weaker than that of an alkoxycarbonyl group, the ICT attribute in 1 is expected to be weaker than those in the corresponding phthalates, which should induce a blue shift in the absorption spectra of 1 as compared with those of the corresponding phthalates. However, the results are contradictory. These findings suggest that the co-planarity of the central benzene ring and the two acceptors (aminocarbonyl groups), which contribute to enhanced π-conjugation, is more important for the HOMO–LUMO gaps than the ICT character.
image file: c8qm00578h-f2.tif
Fig. 2 Absorption spectra of 1 in toluene (10−5 M). The enlarged spectra are shown in the ESI.

Fluorescence properties in solvents

The fluorescence spectra of 1 in toluene are shown in Fig. 3, and the data are summarized in Table 1. In toluene, 1a–1e having (C6H5)2N groups as donors exhibited green fluorescence at 522–556 nm with good-to-high quantum yields (Φ = 0.33–0.68). Greenish blue emission was observed for 1f–1h having weaker electron-donating (4-CF3C6H4)2N groups compared with a (C6H5)2N group, and 1i with stronger electron-donating (4-tert-butylC6H4)2N groups showed orange-yellow emission. Moreover, when the solvent was changed from toluene to THF and chloroform, the spectra of 1a exhibited a red shift according to the solvent polarity (in THF: λem = 561 nm, Φ = 0.19; in chloroform: λem = 601 nm, Φ = 0.10), suggesting that the excited state had a charge-separated nature (Fig. S5, ESI). This behavior of the emission colors/spectra, which depend on the electron-donating ability of amino groups and solvent polarity, is consistent with the ICT mechanism.
image file: c8qm00578h-f3.tif
Fig. 3 Fluorescence spectra of 1 in toluene (10−5 M, λex = 320 nm). The enlarged spectra are shown in the ESI.
Table 1 Fluorescence data of 1
1 λem [nm] (Φ)a
In tolueneb In PMMA film In powder
a λem: wavelength of emission maximum; Φ: absolute fluorescence quantum yield determined with a calibrated integrating sphere system.b Measured at 10−5 M.
1a 529 (0.48) 524 (0.66) 577 (0.37)
1b 533 (0.51) 527 (0.57) 573 (0.54)
1c 528 (0.46) 529 (0.55) 562 (0.57)
1d 522 (0.68) 522 (0.47) 540 (0.48)
1e 556 (0.33) 538 (0.47) 605 (0.50)
1f 484 (0.03) 481 (0.39) 512 (0.37)
1g 473 (0.68) 474 (0.66) 508 (0.42)
1h 477 (0.69) 480 (0.62) 518 (0.51)
1i 575 (0.43) 551 (0.46) 588 (0.30)


Meanwhile, no fluorescence was observed when 1a was dissolved in DMSO. The fluorescence quenching in DMSO is presumably ascribed to the intermolecular electronic interaction between the charge-separated excited state and surrounding DMSO molecules. Phthalimides are known to react with hydrazine to produce primary amine and 2,3-dihydro-1,4-phthalazinedione. Based on the reaction, several phthalimides, which are non-fluorescent in solution, are developed as turn-on type fluorescent probes for hydrazine.20 Then, we checked the hydrazine detection ability of phthalimides of type 1 in DMSO using 1i. Hydrazine monohydrate and various amines including amino acids, such as L-histidine methyl ester dihydrochloride, D-glucosamine hydrochloride, L-cysteine, DL-homocysteine, glutathione, n-octylamine, aniline, and dibenzylamine, were dissolved in DMSO (2 mL), respectively, and a DMSO solution of 1i (10−5 M, 0.4 mL) was added to the colorless solutions. As shown in Fig. 4, only the hydrazine-dissolved solution immediately became fluorescent just after the addition of 1i, indicating that 1 has a potential of fluorescent probes for hydrazine. The hydrazine detection mechanism for the known phthalimide-based probes involves the release of fluorescent primary amines through the reaction of the probes with hydrazine.21 On the other hand, in the case of 1i, the released amine was aniline, which was non-fluorescent in the visible region. Hence, the detection mechanism of the present system is different from those of the known probes and involves the production of fluorescent 2,3-dihydro-1,4-phthalazinedione derivative.22


image file: c8qm00578h-f4.tif
Fig. 4 (a) Optical images of amine-dissolved DMSO solution (10−4 M) and (b) fluorescence images of 1i-added DMSO solution: (1) none (control), (2) hydrazine monohydrate, (3) L-histidine methyl ester dihydrochloride, (4) D-glucosamine hydrochloride, (5) L-cysteine, (6) DL-homocysteine, (7) glutathione, (8) n-octylamine, (9) aniline, and (10) dibenzylamine.

Fluorescence properties in the solid states

The fluorescent behavior of 1 dispersed in the PMMA film (1 wt%) was investigated.23 The emission maxima (λem) of the 1-doped films were almost similar to those in toluene, except for 1e and 1i, whose emission maximum was blue-shifted by 18 and 24 nm, respectively (Table 1 and Fig. 5). The similarity of the luminescence spectra to those in toluene suggests that there are no electronic intermolecular interactions between 1 dispersed in the PMMA film and that the molecular conformations of 1 in the polymer matrix resemble those in solution. The blue-shifted spectra of 1e and 1i in PMMA film may imply that the molecular conformations are more twisted in polymer film than in solution. The quantum yields of 1-doped PMMA films were good to high (Φ = 0.39–0.66), indicating that 1 is a good candidate for dopants in light-emitting diodes and luminescent sensing films.
image file: c8qm00578h-f5.tif
Fig. 5 Fluorescence spectra of 1 in PMMA film (λex = 320 nm). The enlarged spectra are shown in the ESI.

Fig. 6 shows the fluorescence spectra of 1 in powder, which are significantly red-shifted compared with those in toluene and PMMA film. The fluorescence images of 1a, 1c, 1e, and 1g in PMMA film and powder are shown in Fig. 7 as examples for a marked color difference between the doped polymer film and the powder. This is probably because molecular proximity in the condensed state, i.e., in the powder form, can give rise to intermolecular electronic interactions that cannot occur in toluene or in PMMA film and thus lead to an increase in the effective conjugation length of each luminophore. In the series of bis[(C6H5)2N] derivatives 1a–1e, the emission color was drastically altered from green to orange upon changing the aromatic substituent R of an imide moiety. The fluorescence quantum yields of 1 in powder (Φ = 0.30–0.57) are noteworthy, all of which are higher than the reported values so far for known phthalimides in the solid state except for two cases.15


image file: c8qm00578h-f6.tif
Fig. 6 Fluorescence spectra of 1 in powder (λex = 320 nm). The enlarged spectra are shown in the ESI.

image file: c8qm00578h-f7.tif
Fig. 7 (a) Fluorescence images of 1a, 1c, 1e, and 1g dispersed in the PMMA film; (b) fluorescence images of 1a, 1c, 1e, and 1g in powder, recorded under irradiation using a UV lamp (λex = 365 nm).

Molecular and crystal structures

Single crystals suitable for X-ray diffraction analysis were obtained from the recrystallization of 1d from CH2Cl2/EtOH solution.24 The molecular and crystal structures are shown in Fig. 8. Compound 1d crystallizes in the monoclinic space group P21/c. As shown in Fig. 8a, each phenyl group of Ph2N moieties is directed upside down with respect to the central benzene ring. As the bond angles of carbon–nitrogen–carbon skeletons in Ph2N moieties are 117.88°, 118.90°, and 121.53°, and 117.33°, 117.44°, and 121.72°, respectively, the two nitrogen atoms are found to be sp2 hybridized; hence, the lone pair of electrons is not fully conjugated with the π-orbitals of the central benzene ring. The 2,4,6-Me3C6H2 group is oriented perfectly perpendicular (90.6°) to the benzene plane. Presumably due to the twisted molecular conformation, there is no π–π stacking in the crystal lattice (Fig. 8b), which aids in reducing the excited state energy loss through the Dexter mechanism. Instead, hydrogen bonds between the carbonyl groups and the benzene hydrogen of the central benzene ring are observed, which partially restrict the molecular motion leading to the loss of the excited state energy (Fig. 8c). Hence, one of the possible reasons for the efficient emission of powder 1d is such a packing motif.
image file: c8qm00578h-f8.tif
Fig. 8 (a) Molecular and (b and c) crystal structures of 1d.

Molecular orbital calculations

To gain insight into the electronic structure of 1, we carried out DFT calculations on 1a, 1f, 1f′ (Ar = 4-CF3C6H4, R = C6H5), and 1i at the B3LYP/cc-pVDZ level of theory25 using the Gaussian 09 package (revision D.01).26 The orbital drawings of the HOMOs and LUMOs are shown in Fig. 9, and the energies are listed in Table 2. In each 1, the HOMOs are primarily developed over the Ph2N moieties, the central benzene ring, and an imide nitrogen. In the case of 1f, the benzene ring of the imide moiety, i.e. 4-tert-butylphenyl group, also participates in the HOMO. As the HOMO of 1f′ is also extended over the phenyl group of an imide moiety, the spread of the HOMOs on aryl groups in the imide moiety of 1f and 1f′ is ascribed to the presence of the CF3 groups, but not to the tert-butyl group. Meanwhile, the LUMOs are localized on the central benzene and two carbonyl groups, and no lobes are spread over the benzene rings of the imide moiety in all cases. Time-dependent DFT (TD-DFT) calculations confirmed that the optical transition of 1 was the HOMO to LUMO transition. Thus, the photo-excitation of 1 involved ICT from two Ar2N groups to two carbonyl groups of the imide moiety. As shown in Fig. 2, the absorption spectra exhibited a red shift in the order of 1f, 1a, and 1i. The calculated energy gaps of the HOMOs and LUMOs (ΔEHOMO–LUMO) become smaller in the order of 1f, 1a, and 1i. The consistency between the trend of HOMO–LUMO energy gaps and the bathochromic shift of the absorption spectra demonstrates the validity of the DFT calculations as a method for analyzing the electronic structures of 1.
image file: c8qm00578h-f9.tif
Fig. 9 HOMO and LUMO drawings of 1a, 1f, 1f′, and 1i.
Table 2 HOMO and LUMO energies, transition configuration, and oscillator strength of 1a, 1f, 1f′, and 1ia
  1a 1f 1f′ 1i
a Calculated at the B3LYP/cc-pVDZ level using the Gaussian 09 (revision D01).b ΔEexp: energy gap between HOMO and LUMO, estimated from the wavelength of the absorption edge.
LUMO (eV) –2.23 –2.77 –2.81 –2.13
HOMO (eV) –5.40 –6.14 –6.17 –5.21
ΔELUMO–HOMO (eV) 3.17 3.37 3.36 3.08
ΔEexp (eV)b 3.49 3.65 Not available 3.32
Transition configuration (coefficient) HOMO → LUMO (0.70151) HOMO → LUMO (0.69862) HOMO → LUMO (0.69924) HOMO → LUMO (0.70223)
Oscillator strength 0.0780 0.0616 0.0728 0.0910


Conclusions

We have developed 4,5-bis(diarylamino)phthalimides as novel luminophores that emit fluorescence with high efficiency in the solid state such as in powder form and in doped PMMA films. The solid-state emission color can be altered from blue to orange by changing the electron-donating properties of diarylamino moieties and a substituent on an imide nitrogen. The molecular design of phthalimides 1 is based on the concept that the construction of the 1,2-bis(acceptor)-4,5-bis(donor)benzene structure is effective for inducing twisted molecular conformation and ICT transition resulting in a large Stokes shift, both of which are essential for achieving highly efficient solid-state luminescence. The luminescent phthalimides are readily prepared and their molecular modification is easy, which are attractive features for future applications in solid-state light-emitting devices and chemical/biological sensing.

Experimental

Typical procedure for the synthesis of 1a–1f

A 200 mL round-bottomed flask was charged with a magnetic stir bar, dimethyl 4,5-bis(diphenylamino)phthalate (6.71 g, 12.7 mmol), and THF (18 mL), and the solution was stirred at room temperature for 5 min. To the flask was added aq. LiOH solution (0.95 M, 40.1 mL, 38.1 mmol). The resulting mixture was stirred at 80 °C for 17 h. After cooling at room temperature, the reaction mixture was acidified with aq. HCl (6.0 M) to give precipitate. The precipitate was collected by suction filtration and dried at 80 °C under vacuum for 12 h, giving crude 4,5-bis(diphenylamino)phthalic acid (6.26 g, 12.5 mmol, 95%) as pale yellow solid which was used for the next step without purification. A 15 mL vial was charged with a magnetic stir bar, 4,5-bis(diphenylamino)phthalic acid (0.25 g, 0.50 mmol), acetic acid (4 mL), and 4-tert-butylaniline (88 μL, 0.55 mmol). The resulting mixture was stirred at 100 °C for 4 h. After cooling at room temperature, distilled water was added to the reaction mixture, giving a precipitate. The precipitate was collected by suction filtration and purified by recrystallization from dichloromethane/hexane solution to give 1b (0.24 g, 0.39 mmol, 77%) as yellow solid. The solid was further recrystallized from acetone and from a mixture of chloroform/ethanol for the measurement of photophysical properties. The characterization data of 1a–1f are described in the ESI.

Typical procedure for the synthesis of 1g–1i

A 70 mL screw vial was charged with a magnetic stir bar, 4,5-dichlorophthalic acid (3.01 g, 12.8 mmol), acetic acid (10 mL), and isopropylamine (1.65 mL, 19.2 mmol). The vial was heated at 100 °C for 48 h and then cooled at room temperature. Addition of water to the vial gave precipitate, which was collected by suction filtration and dried at 40 °C under vacuum for 12 h. The crude 4,5-dichlorophthalimide (3.26 g, 12.6 mmol, 98%) was used for the next step without purification. An 80 mL Schlenk flask was charged with a magnetic stir bar, 4,5-dichlorophthalimide (2.99 g, 11.6 mmol), K2CO3 (6.4 g, 46 mmol), 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos, 0.66 g, 1.39 mmol), and Pd2(dba)3 (0.32 g, 0.35 mmol) and capped with a rubber septum. The flask was evacuated and filled with argon. This evacuation–argon introduction was repeated twice. To the flask were added toluene (30 mL) and 4-trifluoromethylaniline (3.65 mL, 29 mmol). The resulting solution was stirred at room temperature for 5 min and then at 100 °C for 16 h. To the reaction mixture were added ethyl acetate (40 mL) and water (50 mL). The aqueous layer was extracted with ethyl acetate (50 mL × 3), and the combined organic layer was dried over anhydrous MgSO4 and concentrated in vacuo. The crude product was purified by gel permeation chromatography to give the corresponding 4,5-bis(arylamino)phthalimide (5.0 g, 9.9 mmol, 85%) as yellow solid, which was used for the next step for further purification. An 80 mL Schlenk flask was charged with a magnetic stir bar, 4,5-bis(arylamino)phthalimide (2.5 g, 4.9 mmol), Cu powder (94 mg, 1.45 mmol), and K2CO3 (2.7 g, 19.7 mmol) and capped with a rubber septum. The flask was evacuated and filled with argon. This evacuation–argon introduction was repeated twice. To the flask was added 4-trifluoromethyliodobenzene (20 mL). The resulting mixture was stirred at 200 °C for 49 h. To the reaction mixture were added dichloromethane (30 mL) and water (50 mL). The aqueous layer was extracted with dichloromethane (30 mL × 3), and the combined organic layer was dried over anhydrous MgSO4 and concentrated in vacuo. The crude product was purified by silica gel column chromatography (hexane only to hexane/EtOAc 3[thin space (1/6-em)]:[thin space (1/6-em)]1) to give 1g (3.2 g, 4.0 mmol, 82%) as yellow-green solid. The solid was further recrystallized from chloroform/ethanol and from a mixture of chloroform/hexane for the measurement of photophysical properties. The characterization data of 1g–1i are described in the ESI.

Preparation of doped PMMA films

The sample (1 mg) was placed in a glass tube and dissolved in a toluene solution (1 mL) of PMMA (99 mg). The resulting solution was dropped onto a quartz plate (10 mm × 10 mm) and spin-coated at 300 rpm for 40 s; then the spinning speed was increased to 1000 rpm over a period of 60 s. The deposited film was dried for 12 h in air and under reduced pressure (6.7 × 10−2 Pa) for 4 h.

Measurement of UV-visible absorption and fluorescence spectra

UV-visible absorption spectra were measured with a Shimadzu UV-2550 spectrometer. Fluorescence spectra and absolute quantum yields were recorded using a calibrated integrating sphere with a Hamamatsu Photonics C9920-02 Absolute PL Quantum Yield Measurement System.

Theoretical calculations

Both density functional theory (DFT) and time-dependent DFT calculations were carried out at the B3LYP/cc-pVDZ level using the Gaussian 09 package (revision D.01). The initial geometries for structural optimizations were based on the molecular structure of 1d determined by X-ray diffraction analysis of a single crystal.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan (KAKENHI: 15H03795, 15H00740, 15H00996, and 15K13671). We would like to thank Prof. Takashi Yumura (Kyoto Institute of Technology) for valuable discussion on theoretical results.

Notes and references

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  22. A plausible mechanism for hydrazine sensing is shown in the ESI.
  23. When, for example, the content of 1i in the PMMA film decreased from 1 wt% to 0.1 wt%, or increased to 5 and 10 wt%, the fluorescence quantum yields decreased from 0.46 to 0.40, 0.34, and 0.32.
  24. Data were measured on a Bruker SMART APEX diffractometer (MoKα radiation, λ = 0.71073 Å) at 300 K. The structures were solved by direct methods using SHELXTL program and refined with full-matrix least-squares on F2. CCDC 1553093 (1d).
  25. DFT calculations at the cam-B3LYP/cc-pVDZ level were also carried out and the results are summarized in Table S1 (ESI). The HOMO–LUMO energy gaps determined by the calculations at the B3LYP/cc-pVDZ level were closer to the experimentally determined gaps than those at the cam-B3LYP/cc-pVDZ level.
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Footnote

Electronic supplementary information (ESI) available: Absorption and fluorescence spectra of 1a in solvents, crystal data of 1d, and 1H and 13C NMR charts. CCDC 1553093. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8qm00578h

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