Stimuli responsive reversible high contrast off–on fluorescence switching of simple aryl-ether amine based aggregation-induced enhanced emission materials

Abstract

Aryl-ether amine based simple Schiff base molecules (1–5) showed aggregation induced enhanced emission (AIEE) in the solid state and rare stimuli responsive fluorescence off–on switching. The hard grounding of 1 resulted in irreversible fluorescence tuning from greenish-yellow to green. Heating or solvent exposure did not result in any fluorescence reversibility. Interestingly, the hard grounding of 2–5 led to the quenching of the solid state fluorescence and heating/solvent exposure produced clear bright fluorescence. Importantly, 2–5 exhibited reversible off–on fluorescence switching for several cycles without significant change in fluorescence intensity. It is noted that the turn-on fluorescence of 2–5 was slightly blue shifted compared to the initial solids. PXRD studies suggest that the switching of dark to bright fluorescence and vice versa of 2–5 is due to the reversible change of crystalline to amorphous phase and more planarization of the twisted structure. Single crystal analysis of 1 and 5 confirmed the twisted molecular conformation and strong intermolecular interactions in the crystal lattice, which led to AIEE by restricting free rotation and rigidifying the fluorophores in the solid state. The high contrast dark and bright fluorescence switching of 2–5 could be of potential interest in optoelectronic applications.

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Introduction

The development of stimuli responsive organic fluorescent materials, particularly with repeated fluorescence switching, has received considerable interest because of their potential applications in sensors, displays, switches, security inks, optoelectronic devices, and data storage.^1,2 In general, the switching of organic solid state fluorescence has often been achieved either by altering fluorophore packing and conformation or controlling the intermolecular interactions in solid state structures.^3–5 External stimuli, such as mechanical force, heat and solvent exposure, often change the molecular conformation or phase of materials (crystalline to amorphous) and lead to the switching of solid state fluorescence. Organic molecules that exhibit temperature dependent molecular packing in crystals show highly reversible fluorescence switching.⁶ The inclusion and removal of guest molecules in the crystal lattices of supramolecular organic fluorescent host materials lead to fluorescence switching.⁵ In recent years, the aggregation induced enhanced emission (AIEE) phenomenon, in which fluorophores are weak or non-emissive in solution but have strong emission in the aggregated state, has been successfully used for the development of efficient solid state fluorescent materials.⁷ The rigidification of fluorophores in the aggregated state restricts intramolecular rotation and activates radiative decay. Some of the strongly fluorescent AIEE materials exhibit external stimuli responsive fluorescence switching due to the change of fluorophore conformation or phase of the materials.^8,9 For example, conformationally twisted tetraphenylpyrene, pyrene- and anthracene-based liquid crystals, cyanostilbene and triphenylamine derivatives exhibit reversible fluorescence switching with external force.¹⁰ Most of these materials show only color transformation with external force rather than a change in fluorescence efficiency, which hampers the materials used for high-contrast fluorescence recording.

Park et al. reported the unique example of high contrast reversible fluorescence on–off by external force using a dicyanodistyrylbenzene based donor–acceptor–donor triad.¹¹ Solvent inclusion/removal in coordination complexes show dramatic fluorescence changes in intensity.¹² However, the examples of fluorescent materials that exhibit external stimuli, such as heat, pressure and vapor, responsive fluorescence on–off switching with high contrast are rarely reported compared to those with a change of color.^6a,13,14 The integration of the anthracene unit with the AIEE active diphenylquinoxaline core led to heat responsive fluorescence on–off in the solid state.¹⁵ Zhang et al. recently reported heating and mechanical force induced fluorescence on–off in a triphenylamine based fluorophore.¹⁶ In our continuing effort to develop new organic solid state fluorescence switching materials,¹⁷ we report herein the synthesis of aryl-ether amine based Schiff base molecules, which exhibit AIEE in the solid state and stimuli responsive reversible fluorescence off–on switching (Scheme 1). Schiff bases were chosen considering their facile synthesis and good photophysical properties, which have found applications in many research fields such as chemical molecular optoelectronics, photochromism and medicine.¹⁸ The greenish-yellow fluorescence of 1 in the solid state was tuned to green by hard grounding. 1 did not show any fluorescence switching. However, 2–5 exhibited clear off–on fluorescence switching and showed reversible dark and bright fluorescence with external force. The greenish-yellow or yellow fluorescence of 2–5 was quenched by hard grounding and heating or solvent exposure turned-on the fluorescence. The turn-on fluorescence of 2–5, after exposure to external stimuli (grounding/heat/solvent vapor), was slightly blue shifted compared to the initial solids. Importantly, the fluorescence off–on switching of 2–5 could be reproduced for several cycles by repeated grounding and heating. PXRD studies suggest that fluorescence on–off switching could be attributed to the reversible change of the crystalline to amorphous phase. Single crystal analysis of 1 and 5 showed a twisted molecular conformation and strong intermolecular interactions in the crystal lattice, which rigidify the fluorophores and induced AIEE. Thus, simple Schiff base organic molecules show rare high contrast stimuli responsive fluorescence off–on switching in the solid state, which could be of interest in optoelectronic device studies.

Scheme 1 Molecular structures of 1–5.

Materials and methods

2-Fluoro-nitrobenzene, 4-fluoro-nitrobenzene, phenol, 2-cyanophenol, NaBH₄, Pd/C, 2-hydroxy naphthaldehyde and 2-hydroxy-4-methoxy benzaldehyde were obtained from Sigma-Aldrich and were used as received. Solvents were obtained from Merck India. All chemicals were used as received. Aryl-ether amine precursors and Schiff base molecules (1–5) were synthesized following our recently reported procedure (Scheme S1†).¹⁹ The melting points of the compounds were obtained from DSC spectra (Fig. S1†).

Synthesis of 1–3

1 was synthesized by adding 2-hydroxy-4-methoxy benzaldehyde (1.1 mmol) dissolved in ethanol (20 mL) to a stirring solution of 2-aminophenoxybenzene (1 mmol) in ethanol (5 mL) dropwise under ambient conditions. The resultant reaction mixture was refluxed for 4 h. The product was precipitated upon cooling the reaction mixture to room temperature. The precipitate was filtered, washed with ethanol and dried under vacuum. 2 was synthesized using the same procedure, except 2-hydroxy naphthaldehyde (1.1 mmol) in ethanol was used, whereas 3 was prepared using 4-aminophenoxybenzene (1 mmol) and 2-hydroxy naphthaldehyde (1.1 mmol) in ethanol.

1. Yield = 85%. Mp 117.2 °C. ¹H NMR (400 MHz, chloroform-d) δ 13.54 (s, 1H), 8.57 (s, 1H), 7.32–7.27 (m, 3H), 7.23–7.19 (m, 3H), 7.07–7.05 (m, 2H), 6.98–6.95 (m, 2H), 6.45–6.42 (dd, J = 1.2, 2.0 Hz, 2H), 3.80 (s, 3H). ¹³C NMR (100 MHz, chloroform-d) δ 164.5, 164.2, 162.0, 157.8, 149.3, 140.4, 133.6, 129.8, 127.4, 124.9, 122.9, 121.3, 120.4, 117.6, 113.3, 107.2, 101.2, 55.5. m/z calcd for C₂₀H₁₇NO₃ (M + H): 319.12, found: 320.1.

2. Yield = 90%. Mp 163.2 °C. ¹H NMR (400 MHz, chloroform-d) δ 15.40 (s, 1H), 9.32 (s, 1H), 7.97 (d, J = 8.4 Hz, 1H), 7.70 (d, J = 9.2 Hz, 1H), 7.63 (d, J = 8 Hz, 1H), 7.49–7.45 (m, 2H), 7.36–7.25 (m, 3H), 7.23–7.19 (m, 2H), 7.12–7.02 (m, 4H), 6.96 (d, J = 9.2 Hz, 1H). ¹³C NMR (100 MHz, chloroform-d) δ 173.1, 157.0, 153.1, 148.8, 137.4, 135.8, 133.5, 129.9, 129.5, 128.2, 127.2, 127.0, 124.5, 123.6, 123.6, 123.4, 120.1, 119.0, 118.8, 118.6, 108.9. m/z calcd for C₂₃H₁₇NO₂ (M + H): 339.13, found: 340.1.

3. Yield = 82%. Mp 132.6 °C. ¹H NMR (400 MHz, chloroform-d) δ 15.53 (s, 1H), 9.33 (s, 1H), 8.09 (dd, J = 4 Hz, 1H), 7.78 (dd, J = 4.4 Hz, 1H), 7.71 (d, J = 7.6 Hz, 1H), 7.51 (t, J = 7.6 Hz, 1H), 7.39–7.32 (m, 5H), 7.16–7.04 (m, 6H). ¹³C NMR (100 MHz, chloroform-d) δ 168.8, 157.0, 156.0, 154.8, 141.1, 136.2, 133.1, 129.9, 129.3, 128.0, 127.4, 123.5, 123.5, 121.8, 119.8, 118.9, 108.9. m/z calcd for C₂₃H₁₇NO₂ (M + H): 339.13, found: 340.1.

Synthesis of 4 and 5

An ethanol solution (30 mL) of aldehyde (1.1 mmol, 2-hydroxy-4-methoxy benzaldehyde (4), and 2-hydroxy naphthaldehyde (5)) was added dropwise into 2-(2′-aminophenoxy)benzene carboxylic acid (1 mmol) dissolved in ethanol (10 mL) under stirring at room temperature. After the addition, the reaction mixture was refluxed for 10 h. After the cooling of reaction mixture, it produced a precipitate, which was filtered and washed with ethanol and dried under vacuum.

4. Yield = 85%. Mp 158.8 °C. ¹H NMR (300 MHz, d₆-DMSO) δ 13.50 (s, 1H), 12.84 (s, 1H), 8.91 (s, 1H), 7.83 (dd, J = 1.8 Hz, 1H), 7.57–7.54 (m, 2H), 7.46 (d, J = 8.7 Hz, 1H), 7.28–7.25 (m, 3H), 6.97–6.94 (m, 1H), 6.81 (d, J = 8.1 Hz, 1H), 6.49 (dd, J = 2.7, 2.4 Hz, 1H), 6.37 (d, J = 2.4 Hz, 1H), 3.77 (s, 3H). ¹³C NMR (75 MHz, d₆-DMSO) δ 166.2, 163.6, 163.4, 162.6, 155.8, 149.0, 138.9, 134.0, 133.4, 131.5, 124.7, 122.9, 120.0, 119.7, 118.3, 112.8, 106.6, 100.6, 55.3. m/z calcd for C₂₁H₁₇NO₅ (M + H): 363.11, found: 364.1.

5. Yield = 86%. Mp 224 °C. ¹H NMR (300 MHz, d₆-DMSO) δ 15.79 (s, 1H), 12.98 (s, 1H), 9.68 (s, 1H), 8.45 (d, J = 8.4 Hz, 1H), 8.08–8.05 (dd, J = 3.1 Hz, 1H), 7.92 (d, J = 7.8 Hz, 1H), 7.84 (d, J = 9.3 Hz, 1H), 7.72 (d, J = 7.5 Hz, 1H), 7.61–7.53 (m, 2H), 7.34–7.22 (m, 4H), 7.08 (d, J = 8.1 Hz, 1H), 6.86–6.78 (m, 2H). ¹³C NMR (300 MHz, d₆-DMSO) δ 173.4, 166.0, 154.7, 153.1, 148.9, 137.4, 133.7, 133.3, 131.7, 128.9, 128.0, 127.0, 126.2, 124.2, 123.8, 123.7, 123.3, 123.2, 120.9, 120.0, 119.0, 117.3, 108.3. m/z calcd for C₂₄H₁₇NO₄ (M + H): 383.12, found: 384.1.

Spectroscopy and structural characterization

Absorption and fluorescence spectra were obtained using PerkinElmer Lambda 1050 and Jasco fluorescence spectrometer-FP-8200 instruments. The fluorescence quantum yields (Φ_f) of solid samples were measured using a Horiba Jobin Yvon model FL3-22 Fluorolog spectrofluorometer with an integrating sphere. Powder X-ray diffraction (PXRD) patterns were obtained using an XRD-Bruker D8 Advance XRD with Cu Kα radiation (λ = 1.54050 Å) operated in the 2θ range from 10° to 50°. Single crystals of 1 and 5 were coated with paratone-N oil and diffraction data was measured at 100 K with synchrotron radiation (λ = 0.62998 Å) on an ADSC Quantum-210 detector at 2D SMC with a silicon (111) double crystal monochromator (DCM) at the Pohang Accelerator Laboratory, Korea. CCDC-1418264 (1), 1417738 (5) contains the supplementary crystallographic data for this study.†

Results and discussion

The synthesis of the amine precursors and corresponding Schiff base molecules (1–5) are shown in Scheme S1.† The reaction between phenol and 2-fluoro-nitrobenzene or 4-fluoro-nitrobenzene in the presence of K₂CO₃ in DMSO at 110 °C yielded the corresponding aryl-ether with a nitro group at the ortho or para position. The subsequent nitro group reduction produced the amine precursors for 1–3. The reaction between 2-cyanophenol and 2-fluoro-nitrobenzene in the presence of K₂CO₃ in DMSO at 110 °C resulted in the formation of 2-(2-nitrophenoxy)benzonitrile. The nitro group reduction followed by the oxidation of CN to COOH produced the amine precursor for 4 and 5. The Schiff base molecules (1–5) were synthesized using a simple condensation reaction between the amine and aromatic aldehydes in ethanol. The synthesized Schiff base compounds (1–5) showed greenish-yellow to yellow fluorescence in the solid state. However, 1–5 did not show any fluorescence in solution state, which could be due to the free rotation of the single bond and isomerism of the imine (C=N).²⁰ The observation of fluorescence only in the solid state suggests the AIEE phenomena in 1–5.

1 showed greenish-yellow fluorescence with an emission peak at 520 nm (Fig. 1, Φ_f = 40%). The strong grounding of 1 blue shifted this emission to 508 nm and showed green fluorescence (Φ_f = 43%). Heating or solvent vapor exposure did not result in any fluorescence switching. The methoxy-salicylaldehyde group was replaced with naphthaldehyde in 2. The isomeric compound of 2 and 3 also showed greenish-yellow solid state fluorescence (Fig. 2). The fluorescence spectra of 2 and 3 showed emission at 532 and 527 nm, respectively (Φ_f = 46% (2), 53% (3)). In contrast to 1, the strong grounding of 2 and 3 exhibited the quenching of fluorescence (Φ_f = 5% (2), 7% (3)). The grounded powder showed only very weak fluorescence. Interestingly, the quenched fluorescence of the 2 and 3 grounded powder was switched to bright green fluorescence upon heating or solvent vapor exposure. Digital images and fluorescence spectra confirmed the bright green fluorescence with enhanced intensity. It is noted that the grounding and heating of 2 and 3 blue shifted their fluorescence. 2 showed fluorescence at 525 nm along with a small hump at 500 nm after grounding and heating, whereas 3 exhibited two clear fluorescence peaks at 503 and 522 nm after grounding and heating. The excitation spectra of 2 and 3 showed a small change after grounding and heating (Fig. S2 and S3†). Importantly, 2 and 3 showed reversible fluorescence off–on switching (dark to bright fluorescence) without significant loss of intensity for more than five cycles by repeated grounding and heating (Fig. 3).

Fig. 1 Fluorescence tuning of 1 (digital images and fluorescence spectra) by hard grounding. λ_exc = 365 nm.

Fig. 2 Digital images and fluorescence spectra of off–on fluorescence switching of (a) 2 and (b) 3. λ_exc = 365 nm.

Fig. 3 External stimuli induced reversible off–on fluorescence switching between the dark and bright state of (a) 2 and (b) 3.

The carboxylic acid group was introduced at the ortho position of 1 and 2 to further rigidify the fluorophore in the solid state via intermolecular H-bonding interactions (4 and 5). 4 exhibited strong greenish-yellow fluorescence at 517 nm (Fig. 4a, Φ_f = 58%). However, strong grounding of 4 completely quenched the fluorescence (Φ_f = 10%). Heating/solvent exposure switched the fluorescence from dark to bright greenish yellow. The fluorescence spectra showed enhanced fluorescence at 504 nm. The excitation spectra of 4 showed clear peaks at 370 and 492 nm (Fig. S4†). Strong grounding resulted in the disappearance of peak at 370 nm and the blue shift of 480 nm peak to 465 nm. Although heating did not result in a clear peak at 370 nm, the intensity was clearly enhanced with slight red shift of the 465 nm peak to 478 nm. This result supports the fluorescence change of 4 by hard grounding and heating. The fluorescence of 4 can also be repeatedly switched off–on for several cycles by hard grounding and heating (Fig. 4b). The as-synthesized powder of 5 showed strong yellow fluorescence at 536 nm (Fig. 5a, Φ_f = 48%). Surprisingly, the crystals of 5 showed only very weak fluorescence (Φ_f = 6%). However, slight breaking of the crystals lead to a strong enhancement in fluorescence intensity (Φ_f = 34%). Similar to 2–4, hard grounding quenched the fluorescence intensity of 5 (Φ_f = 5%) and heating/solvent exposure produced bright greenish-yellow fluorescence. The strongly ground powder showed very weak fluorescence and heating resulted in the strong enhancement of intensity. The fluorescence λ_max was slightly blue shifted from 536 to 532 nm after grounding and heating. The excitation spectra of the crystals and strongly grounded powder did not show any clear peak; however, the slightly broken crystals and heated samples exhibited strong perfectly matching peaks (Fig. S5†). The solid state fluorescence of 5 also exhibited repeated fluorescence off–on switching from the dark to bright state for several cycles without significant loss of intensity by grounding and heating (Fig. 5b). Thus, the simple aryl-ether based Schiff base compounds (2–5) exhibit rare stimuli responsive fluorescence off–on switching and 1 showed fluorescence tuning. Importantly, 2–5 showed reversible dark and bright fluorescence switching with good contrast for several cycles.

Fig. 4 (a) Digital images and fluorescence spectra of 4 and (b) external stimuli induced reversible fluorescence off–on switching. λ_exc = 365 nm.

Fig. 5 (a) Digital images and fluorescence spectra of 5 and (b) external stimuli induced reversible fluorescence off–on switching. λ_exc = 365 nm.

In an effort to understand the molecular arrangement and AIEE phenomena of 1–5 in the solid state, we attempted to grow single crystals from different solvents, which include methanol, ethanol, ethyl acetate, chloroform, dichloromethane, acetonitrile and acetone. 2 and 3 gave only thin plates from all the solvents, which did not show any X-ray diffraction. 4 formed an amorphous powder with less crystalline phases. However, single crystals of 1 and 5 were obtained from methanol and dimethylsulfoxide, respectively (Table S1 and S2†). 1 and 5 showed a twisted molecular conformation in their crystal lattice (Fig. 6). The phenyl group of the aryl ether adopted a perpendicular orientation with respect to the imine attached phenyl group in both 1 and 5. It is noted that the conformationally flexible aryl-ether is known to adopt different conformations in crystals including four different conformations in a single supramolecular structure.²¹ The imine and methoxy-salicylaldehyde are in the same plane in 1, whereas the naphthaldehyde in 5 is slighted twisted (Fig. 6b and d). The hydroxy group of 1 and 5 formed strong intramolecular H-bonding interactions with the imine nitrogen in the crystal lattice (Fig. 7). Furthermore, the strong intermolecular π–π interactions between the imine phenyl group and methoxy phenyl group produced a stair-like network structure in the crystal lattice (Fig. 7a). In 5, intermolecular H-bonding between the hydroxyl groups led to the formation of 1-D chains (Fig. 7b). Strong C–H⋯O (imine hydrogen and acid carbonyl oxygen) and π–π intermolecular interactions between the imine naphthyl groups led to the interdigitation of the chains with the face-to-face molecular arrangement of imine naphthyl groups (Fig. 7c). The face-to-face arrangement of the imine naphthyl group might be the reason for fluorescence quenching in the crystals of 5 (Fig. 5a). The strong face to face π–π interactions are known to quench the solid state fluorescence of π-conjugated aromatic molecules.²² The slight breaking might have disturbed the face–face arrangement, which turned on the fluorescence. Although 1 also showed π–π interactions in the crystal lattice, it showed a slip-stacking molecular arrangement between the two different aromatic molecules. The structural studies of 1 and 5 clearly indicate the strong intra and intermolecular interactions in their crystal lattices. Thus, it is expected that 2–4 also could have strong intermolecular interactions in the solid state due to their structural similarity. These strong intermolecular interactions could restrict free rotation and imine isomerism, which induces AIEE in the solid state. The restriction of free rotation in Schiff base molecules by molecular design showed AIEE.²³

Fig. 6 Molecular conformation with intramolecular H-bonding interactions in the crystals lattice of (a and b) 1 and (c and d) 5. N (blue), O (red), H (white); H-bonds (broken line). H-bond distances are marked in Å.

Fig. 7 (a) Stair-like network structure formation via π–π interactions in 1 and (b and c) linear chain and interdigitation chain via H-bonding and π–π interactions in 5. N (blue), O (red), H (white); H-bonds (broken line). H-bond and π–π interaction distances are marked in Å.

The external stimuli induced switching or tuning of organic solid state fluorescence is generally due to the change of molecular conformation or phase (crystalline to amorphous).^3,4 To gain insight on the switching and tuning of 1–5 solid state fluorescence, PXRD measurements were performed before and after applying an external force. The perfect matching of the simulated and experimental diffraction pattern of 1 confirms the phase purity of the sample (Fig. S6†). The strong grounding of 1 modified the strong diffraction to weak diffraction peaks and this suggests the formation of a partial amorphous phase. The change of crystalline to amorphous phase of 1 blue shifted its fluorescence λ_max from 520 to 508 nm. 2 and 3, which formed thin needles, showed clear diffraction peaks (Fig. 8a and S7†). Strong grounding lead to the formation of an amorphous phase; however, heating converted the amorphous to a crystalline phase. PXRD studies of 4 showed only weak diffraction peaks, which indicate the formation of more amorphous phases (Fig. S8†). The weak diffraction peaks also disappear or their intensity is reduced upon strong grounding and heating showed slight recovery of the peaks. Similar to 1, the perfect matching of the simulated and experimental patterns confirms the phase purity of 5 (Fig. S9†). The slight breaking of 5 produced more peaks, and strong grounding lead to the formation of an amorphous phase (Fig. 8b). Heating or solvent exposure converted the amorphous phase to a crystalline phase. Thus, PXRD studies indicate that the switching off–on fluorescence of 2–5 and fluorescence tuning in 1 is due to the conversion of the crystalline to amorphous phase with external stimuli. However, the reason for the fluorescence tuning in 1 and switching off–on in 2–5 by external stimuli is not clear. Single crystal studies of 5 showed face to face molecular organization in the solid state, which quenches the fluorescence. Therefore, it is speculated that strong grounding promotes face to face assembly in naphthaldehyde substituted compounds (2, 3 and 5) and this might quench the fluorescence. More structural studies are required to clearly understand the off–on fluorescence switching of Schiff base compounds, which are currently underway in our lab.

Fig. 8 PXRD pattern of (a) 2 and (b) 5.

Conclusion

In conclusion, we have demonstrated external stimuli dependent rare high contrast fluorescence off–on switching using simple aryl-ether amine based Schiff base molecules (1–5), which showed aggregation induced enhanced emission (AIEE) in the solid state. 1 showed irreversible fluorescence tuning from greenish-yellow to green upon the hard grounding of its solid. Heating or solvent exposure did not switch the fluorescence of 1. In contrast, strong grounding of the 2–5 solids resulted in the quenching of solid state fluorescence. Interestingly, heating/solvent exposure of the strongly grounded powder (2–5) induced clear bright fluorescence. Importantly, reversible fluorescence off–on switching in 2–5 was demonstrated for more than five cycles by repeated grounding and heating. PXRD studies suggest that the switching dark and bright fluorescence of 2–5 is due to the reversible change of crystalline to amorphous phase. The single crystal analysis of 1 and 5 confirmed the twisted molecular conformation and strong intermolecular interactions in the crystal lattice, which restrict free rotation and rigidifies the fluorophores. Thus, rare high contrast dark and bright fluorescence switching have been realized using simple Schiff base molecules, which could be of interest in optoelectronic device applications.

Acknowledgements

Financial supports from DST, New Delhi, India (DST Fast Track Scheme No. SR/FT/CS-03/2011 (G) and SR/FST/ETI-284/2011(c)) and CRF facility, SASTRA University are acknowledged with gratitude. AK and PSH thanks SASTRA University for research fellowship. “X-ray crystallography at the PLS-II 2D-SMC beamline” was supported in part by MSIP and POSTECH. We thank Dr C. R. Ramanathan for helping in NMR spectra.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Absorption, fluorescence spectra, crystallographic table (1 (CCDC No. 1418264) and 5 (1417738)). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra17570d

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