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A route to new colorimetric pH sensors

  • Sebastian Wandtke and Dietmar Stalke EMAIL logo
Published/Copyright: February 15, 2017
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Zeitschrift für Naturforschung B
From the journal Zeitschrift für Naturforschung B Volume 72 Issue 3

Abstract

The detection of target molecules via luminescence changes of appropriate sensors is a powerful tool to determine the presence of analytes qualitatively and quantitatively. Therefore, the exploration of synthetic routes to new sensors is important to facilitate the recognition of a large range of analytes. In this report we describe such a new route to sensors and discuss the behavior of the synthesized sensors in the presence of different analytes. The prepared anthracene derivatives belong to two classes of substances. On the one hand imines were synthesized wherein the isomerization of their inherent carbon nitrogen double bond provides an effective non-radiative relaxation pathway. On the other hand amines were received by reduction of the mentioned imines. Their intramolecular charge transfer (ICT) based sensing mechanism allows the detection of protons in a reversible colorimetric fashion. Furthermore, the addition of zinc cations in the aprotic solvent DCM provides the first example showing that the detection of these colorimetric pH sensors is not limited to protons.

Keywords: anthracene; colorimetric; luminescence; pH sensor

1 Introduction

The appearance of luminescence fascinated various cultures for ages. For example, the Aztecs used a wood to tread kidney and urinary diseases that was named lignum nephriticum by the Spanish arriving in Central America in the 16th century [1], [2]. Infusions of this wood exhibited blue luminescence at their surfaces and cups were manufactured thereof and offered to monarchs in medieval times [3], [4].

A few years ago the structure of the luminescent component of lignum nephriticum was determined and the formation of this species by an oxidation reaction was proposed [5].

Aside from the beauty of luminescence phenomena a variety of applications was established to make use of light emission after excitation of chromophores (e.g. labelling [6], polymerization [7], OLED [8], microscopy [9]). One of these is the construction of luminescent sensors. After addition of specific analytes the optical output of the sensor is altered and allows a judgment on the presence or absence of this analyte.

A variety of principles is known for the detection of target molecules ranging from the frequently reviewed ones like the photo-induced electron transfer (PET) or the intramolecular charge transfer (ICT) effect to newer principles (e.g. aggregation-induced emission (AIE) or C=N isomerization based systems) [10]. A noteworthy contribution was published by Aron and coworkers in 2012 that illustrates the structural requirements for different sensing mechanisms [11]. ICT based sensors must not have a spacer between the chromophore and the receptor (responsible for the fixation of the analyte) but exhibit a conjugated π system including both, chromophore and receptor. Aron and coworkers proofed that the interruption of this conjugation by introduction of a spacer changes the sensing principle from ICT to PET [11].

The sensing mechanism determines the nature of luminescence changes upon addition of the specific analyte. Generally, there are two main different responses. The first one is the intensity variation based approach that is exhibited by sensors using the PET effect. The counterpart are colorimetric sensors varying their emission wavelength in the presence of appropriate analytes. This second response is in many cases accompanied by intensity variations, too, but in contrast to the first category the shift of emission wavelength is the determining feature. The advantage of this second category is the easy qualitative detection, in some examples simply by naked eye, [12] whereas intensity variation based sensors require in general spectrometers to compare intensities before and after addition of the analyte.

2 Results and discussion

Herein, we present a synthetic route (Scheme 1) to colorimetric luminescence sensors based on the anthracene fluorophore as well as the description of the luminescence behavior of these species.

Scheme 1: Synthetic route to the imines 2–4 via a condensation reaction and subsequent reduction to the amines 5–7.
Scheme 1:

Synthetic route to the imines 2–4 via a condensation reaction and subsequent reduction to the amines 5–7.

2.1 Synthesis and structural data

The initial point was the synthesis of 9-aminoanthracene (1), derived from the procedure of Glorius and coworkers [13]. The reaction conditions were maintained but the purification process was modified.

Starting from 1, a series of imines (2–4) was obtained by the usage of several aldehydes. Subsequent reduction of the prepared imines resulted in the amines 5–7. Noteworthy is the choice of the reducing agent. The more polar imines 2 and 4 were reduced in THF by the conventional reactants NaBH4 and LiAlH4, whereas the less polar imine 3 was treated with a 1m solution of Na[BEt3H] in toluene due to only minor conversion in the presence of the first two mentioned hydrides.

The solid state structures of all three imines were determined by X-ray diffraction. Two representatives are shown in Fig. 1 and the third one is displayed in S4 (cf. Supporting Information).

Fig. 1: Solid state structures of 2 (left) and 3 (right). Hydrogen atoms bound to carbon atoms are not shown. Anisotropic displacement parameters are displayed at the 50% probability level.
Fig. 1:

Solid state structures of 2 (left) and 3 (right). Hydrogen atoms bound to carbon atoms are not shown. Anisotropic displacement parameters are displayed at the 50% probability level.

An interesting structural feature is the twist between the anthracene moiety and the six membered ring of the imines 2–4, because planarity would indicate a delocalized π system through the whole molecule. 4 exhibits the most pronounced twist with an angle of 81.02(4)° due to the bulky hydroxyethoxy group. The twist in 3 is with 61.74(5)° smaller than in 2 [70.06(4)°], indicating the lowest extent of conjugation. Nevertheless, all three twists differ clearly from 0° and so a delocalized π system including the whole molecule seems unlikely despite of the formal sp2 hybridization of the carbon and nitrogen atoms connecting the two aromatic systems in each of the three molecules.

The heteroatom N2 in 3 points to the opposite direction relative to the lone pair of N1, evident from the large torsion angle N1–C11–C12–N2 of 175.9(1)°. The hydroxy group of 2, however, is oriented in the same direction [the torsion angle N1–C11–C12–C17 amounts to 3.1(2)°] as the lone pair of N1. This arrangement is fixed by an intramolecular hydrogen bond. The analogous torsion angle in 4 equals 164.3(1)°, meaning the same orientation of the hydroxyethoxy group as the nitrogen atom N2 in 3.

The intramolecular hydrogen bond in 2 consists of O1 as donor, H101 and N1 as acceptor. Often the strength of such an attractive interaction is evaluated based on geometrical criteria. Following a classification of Jeffery the observed geometry points towards a moderate strength. In this category the hydrogen–acceptor distance is found between 1.5 and 2.2Å [here 1.69(2)Å]. The angle of the three involved atoms is with 149.7(2)° larger than 130° and the distance found between the acceptor and the donor lies in the range of 2.5 to 3.2Å [here 2.590(1)Å] [14]. The geometric results of the solid state investigations are obviously at the shorter side of the categorized values. Therefore, the intramolecular hydrogen bond strength in 2 can be termed moderate, nearly strong.

Furthermore, the solid state structures of the amines 5 and 6 were identified via single crystal X-ray diffraction. Both amines show stereogenic centers at their nitrogen atoms N1. 5 crystallizes in the orthorhombic space group Fdd2 with two molecules in the asymmetric unit. The one displayed in the left part of Fig. 2 exhibits the (S) arrangement, whereas the other one is the (R) enantiomer. In the following discussion the features of the (S) enantiomer are provided. Two positional disorders are present in the solid state structure of 6 shown in the right part of Fig. 2. Both are discussed in the supporting information (S6).

Fig. 2: Solid state structures of 5 (left) and 6 (right). Hydrogen atoms bound to planar coordinated carbon atoms are not shown. Anisotropic displacement parameters are displayed at the 50% probability level.
Fig. 2:

Solid state structures of 5 (left) and 6 (right). Hydrogen atoms bound to planar coordinated carbon atoms are not shown. Anisotropic displacement parameters are displayed at the 50% probability level.

The twist between the two aromatic moieties in each of the amines is less pronounced than in the three previously discussed imines. 5 exhibits a twist of 56.96(7)°, whereas the analogous angle in 6 is 44.35(8)°. Despite of these smaller angles, a delocalized system including the whole molecule is ruled out due to the quaternary bridging atoms.

Both arrangements are fixed by intramolecular hydrogen bonds. The hydrogen–acceptor distances [5: 1.98(4)Å; 6: 2.26(3)Å] are in the range of moderate hydrogen bonds. The same is true for the donor–acceptor distances [5: 2.711(4)Å; 6: 2.765(2)Å], but only one of the angles between the three involved atoms [5: 136(3)°; 6: 114(3)°] is larger than 130°. Nevertheless, judged by the geometrical features both hydrogen bonds are moderately strong.

2.2 Luminescence properties

The route presented in Scheme 1 provides access to two different classes of substances. On the one hand imines with their carbon nitrogen double bond were synthesized. This type of molecules is known to be nearly non-luminescent because the isomerization of the double bond opens an effective non-radiative relaxation pathway. In some previously reported cases the coordination of an analyte hindered the isomerization and an increase in intensity was monitored [15]. This can be used to detect coordinated analytes and therefore some derivatives containing a carbon nitrogen double bond were used as sensors for metal cations (e.g. for Mg2+ [16], Al3+ [17], [18], [19] Fe3+ [20], Hg2+ [21]). The three imines 2–4 are also nearly non-luminescent but exhibit only small variations of their optical properties upon addition of different metal cations. Another disadvantage of the imines is their low stability towards acids indicated by their back reaction to 1 and the corresponding aldehyde.

The amines 5–7 on the other hand show an interesting behavior at different concentrations of acid. Their basic absorption and emission properties are displayed in the supporting information (S7–S12).

The blue line in Fig. 3 represents the emission of the first investigated amine 5 and exhibits a broad band at around 530nm. After addition of hydrochloric acid up to a concentration of 10−3m (approximately 100eq.), enormous luminescence changes are obvious. The intensity is strongly increased and the emission wavelength is shifted to a maximum at 416nm. The curve shape is also altered to a vibrational structured one. A larger excess of acid results in a moderate decrease of intensity but even in a 1m solution of hydrochloric acid (approximately 100 000 eq.) the emission of the protonated species is stronger than the initial luminescence of 5.

Fig. 3: Emission spectra of 5 (10−5m in methanol) at different concentrations of aqueous hydrochloric acid (λex=396nm).
Fig. 3:

Emission spectra of 5 (10−5m in methanol) at different concentrations of aqueous hydrochloric acid (λex=396nm).

This colorimetric response of 5 towards protons can be explained in analogy to the changes observed from Aron and coworkers investigating a series of imidazolium salts [11]. In the non-protonated state an intramolecular charge transfer (ICT) from the amine to the anthracene moiety occurs. Protonation disables this charge transfer and the emission changes dramatically.

A disadvantage of 5 was its observed decomposition to a species showing a stronger emission in a solution of methanol over a few days (displayed in S13 supporting information). This low stability is a drawback with regard to application.

The investigation of 6 showed over days a constant emission of its solution in methanol but the addition of hydrochloric acid led to decomposition upon protonation (Fig. 4).

Fig. 4: Emission spectra of 6 (10−5m in methanol) at different concentrations of aqueous hydrochloric acid (λex=396nm).
Fig. 4:

Emission spectra of 6 (10−5m in methanol) at different concentrations of aqueous hydrochloric acid (λex=396nm).

The blue curve of Fig. 4 resembles the one in Fig. 3, indicating similar emission of both amines. But in contrast to 5, addition of acid to 6 decreases the detected luminescence. At a concentration of 10−3m hydrochloric acid the hypsochromic shift is also observable but higher concentrations of acid resulted in a disappearance of this signal.

So 5 exhibits a colorimetric response towards protons but is not sufficiently stable in methanol. 6, however, is stable in methanol over days but decomposes after addition of HCl to a non-luminescent species.

The third investigated amine 7 is also stable in methanol (S14). Its response to hydrochloric acid is displayed in Fig. 5.

Fig. 5: Emission spectra of 7 (10−5m in methanol) at different concentrations of aqueous hydrochloric acid (λex=396nm).
Fig. 5:

Emission spectra of 7 (10−5m in methanol) at different concentrations of aqueous hydrochloric acid (λex=396nm).

The emission of 7, represented by the blue line, is similar to those of 5 and 6. After raising the concentration of hydrochloric acid to 10−3m, a huge increase of intensity as well as a shift of the emission wavelength of more than 100nm were observed. This colorimetric behavior is reminiscent of that of 5 towards protons but even more pronounced in terms of intensity variation. The combination of 7’s stability in methanol and the enormous changes of its luminescence properties in the presence of protons make 7 a promising colorimetric pH sensor.

The reversibility of the analyte binding was investigated and the results are shown in Fig. 6.

Fig. 6: Emission spectra of 7 (10−5m in methanol) before and after alternately added 100eq. aqueous hydrochloric acid and 100eq. aqueous sodium hydroxide (λex=396nm).
Fig. 6:

Emission spectra of 7 (10−5m in methanol) before and after alternately added 100eq. aqueous hydrochloric acid and 100eq. aqueous sodium hydroxide (λex=396nm).

The addition of 100eq. hydrochloric acid resulted in the same striking changes of wavelength and intensity discussed for Fig. 5. To verify the reversible analyte binding, the same amount of base (100eq. NaOH) was injected. The resulting black line of Fig. 6 is almost identical to the blue line (for 7 in MeOH), proving the possibility to reverse the changes. Two further cycles of protonation and subsequent deprotonation were carried out and confirmed the multiple addressability of both states despite of the loss of intensity after each switch.

For a prove of principle, the solvent was changed from methanol to dichloromethane and zinc bromide was used as analyte to investigate if this type of molecules are also capable to detect metal cations.

The addition of one equivalent of zinc bromide did not change the emission of 7 significantly, recognizable in the course of the red line being directly on top of the blue one (Fig. 7). However, further four equivalents changed the emission dramatically. Similar to protonation, the addition of zinc bromide results in a hypsochromic shift of more than 100nm and an enormous increase of intensity. This increase gets even more pronounced the higher the concentration of zinc cations becomes. Therefore 7 can not only work as a colorimetric pH sensor but can also detect zinc cations in a colorimetric fashion.

Fig. 7: Emission spectra of 7 (10−5m in DCM) before and after addition of zinc bromide in DCM (λex=396nm).
Fig. 7:

Emission spectra of 7 (10−5m in DCM) before and after addition of zinc bromide in DCM (λex=396nm).

So far this metal cation recognition is limited to the solvent dichloromethane. In the polar protic solvent methanol changes of comparable extent were not observed.

3 Conclusions

With the synthetic approach presented here we established a new reliable route to novel colorimetric sensors that can most likely be extended to further derivatives. The so far prepared representatives show promising optical properties especially for proton sensing. This kind of anthracene derivatives provides reversible colorimetric pH sensors but is not limited to protons as analyte. The last example (addition of zinc bromide) proves the possibility to expand the range of analytes to metal cations.

4 Experimental section

The used starting materials and solvents were purchased from ABCR, Sigma-Aldrich and Deutero. If necessary, the solvents were dried due to standard laboratory techniques. All reactions were carried out under Schlenk conditions in dried nitrogen or argon atmosphere and resulting sensitive substances were stored in an argon glovebox.

NMR spectra were recorded at a Bruker Avance III 300 instrument. The measurements were carried out in 1–10% solutions of deuterated solvents and the residual proton signals of the solvents were chosen as internal standards. The labelling scheme used for the assignment of the atoms in the molecules is equal to that of the solid state structures given in the supporting information. For EI mass spectra the instrument MAT 95 was used.

UV/Vis and fluorescence measurements: All experiments were carried out in 10−5m solutions of the respective anthracene derivative in degassed solvents. The UV/Vis spectra were recorded at a Jasco V-650 double beam spectrometer and a FluoroMax-4 from HORIBA Jobin Yvon was the used spectrofluorometer.

4.1 Crystal structure determinations

4.1.1 Choice and application of crystals

Air sensitive crystals were extracted from Schlenk flasks under a counterflow of argon and placed in perfluorinated polyether oil on a microscope slide [22], [23]. A suitable single crystal was chosen with the help of a polarizing microscope and mounted, inside a droplet of oil, at the top of a MiTeGen loop. The loop was moved immediately to the diffractometer, where the droplet was frozen due to the nitrogen flow with a temperature of 100K surrounding the crystal. The oil solidified in a glass‐like manner and fixed the crystal during the data collection.

4.1.2 Data collection, structure solution and refinement

The majority of data sets were recorded at a Bruker APEX II Quazar bearing an Incoatec Mo IμS or an APEX II Ultra equipped with a Bruker TXS Mo source. Both diffractometers use monochromatic MoKα radiation (λ=0.71073Å). An exception was the measurement of 5, where CuKα radiation (λ=1.54178Å) was applied to determine its solid state structure. The associated diffractometer was equipped with a SMART6000 detector and a Bruker TXS source. The reflections were detected by combined ω and π scans with a step width of 0.3° or 0.5° in the respective directions. The data were integrated with Saint, [24] and a multi-scan absorption correction (Sadabs [25]) and a 3λ correction were applied [26]. All structures were solved using the program Shelxt [27] and refined with Shelxl [28] in the graphical user interface shelxle [29]. The positions of hydrogen atoms were refined using riding models [30] and their displacement parameters were constrained to their pivot atoms. Hydrogen atoms attached to heteroatoms (oxygen or nitrogen) have been found in the Fourier‐density‐difference map and were refined with DFIX command to set the distance to tabulated values.

CCDC 1403684 (2), 1403685 (3), 1403686 (4), 1403687 (5), 1403688 (6) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

See below for further crystallographic data contained in the Supporting Information.

4.2 Synthesis of 9-anthracenesalicylimine (2)

To a suspension of 1 (1.00eq., 1.50g, 7.76mmol) in 30mL ethanol salicylaldehyde (1.50eq., 11.6mmol, 1.24mL) was added. The reaction mixture was stirred firstly at 78°C for 6h and then overnight at room temperature. The precipitated solid was filtered and washed with dried degassed ethanol (3 times 4mL) to yield 2.14g (7.20mmol, 93%) of 2 as a yellow solid. – 1H NMR (300MHz, [D6]DMSO): δ/ ppm=7.04–7.13 (m, 2H, H14,16), 7.48–7.60 (m, 5H, H2,3,6,7,15), 7.84 (dd, 3JHH=7.7Hz, 4JHH=1.7Hz, 1H, H13), 7.98 (d, 3JHH=7.6Hz, 2H, H1,8), 8.13 (d, 3JHH=7.6Hz, 2H, H4,5), 8.48 (s, 1H, H10), 8.95 (s, 1H, H11), 12.47 (s, 1H, OH). – 13C{1H} NMR (75MHz, [D6]DMSO): δ/ ppm=116.81 (s, 1C, C16), 119.47 (s, 2C, C12,14), 122.01 (s, 2C, C8a,9a), 123.09 (s, 3C, C1,8,10), 125.75 (s, 2C, C3,6), 125.92 (s, 2C, C2,7), 128.30 (s, 2C, C4,5), 131.28 (s, 2C, C4a,10a), 132.08 (s, 1C, C13), 133.95 (s, 1C, C15), 142.95 (s, 1C, C9), 160.16 (s, 1C, C17), 168.83 (s, 1C, C11). – MS (EI, 70eV): m/z (%)=297 (100) [M]+, 204(9) [M‐C6H4OH]+, 176(10) [C14H8]+. – Anal. calcd. for C21H15NO (297.35gmol−1): C 84.82, H 5.08, N 4.71; found: C 84.65, H 5.05, N 4.73.

4.3 Synthesis of 9-anthracenepicolylimine (3)

To a suspension of 1 (1.00eq., 0.99g, 5.12mmol) in 20mL ethanol 2‐pyridinecarboxaldehyde (1.50eq., 0.73ml, 7.68mmol) was added. After stirring the reaction mixture at 78°C for 2.5h, the volatile components were removed under reduced pressure. The crude reaction product was purified via recrystallization from hexane (15mL) to yield 1.30g (4.60mmol, 90%) 3 as an orange solid. – 1H NMR (300MHz, [D6]DMSO): δ/ ppm=7.43–7.57 (m, 4H, H2,3,6,7), 7.66 (dd, 3JHH=7.5Hz, 3JHH=4.8Hz, 1H, H15), 7.97 (d, 3JHH=8.9Hz, 2H, H1,8), 8.07–8.14 (m, 3H, H4,5,14), 8.41 (s, 1H, H10), 8.50 (d, 3JHH=7.9Hz, 1H, H12), 8.65 (s, 1H, H11), 8.81 (d, 3JHH=4.8Hz, 1H, H16). – 13C{1H} NMR (75MHz, [D6]DMSO): δ/ ppm=121.04 (s, 2C, C8a,9a), 121.59 (s, 1C, C13), 122.41 (s, 1C, C10), 123.30 (s, 2C, C1,8), 125.47 (s, 2C, C2,7), 125.70 (s, 2C, C3,6), 126.27 (s, 1C, C15), 128.16 (s, 2C, C4,5), 131.31 (s, 2C, C4a,10a), 137.35 (s, 1C, C14), 144.49 (s, 1C, C9), 149.88 (s, 1C, C16), 153.49 (s, 1C, C12), 166.39 (s, 1C, C11). – MS (EI, 70eV): m/z (%)=282(100) [M]+, 204(73) [M‐C5H4N]+, 176(18) [C14H8]+. – Anal. calcd. for C20H14N2 (282.34gmol−1): C 85.08, H 5.00, N 9.92; found: C 84.91, H 5.06, N 10.01.

4.4 Synthesis of 9-anthracene(o-(β-hydroxyethoxy)benzyl)imine (4)

To a suspension of 1 (1.00eq., 5.28mmol, 1.02g) in 5mL ethanol a solution of o-(β-hydroxyethoxy)benzaldehyde (2.46eq., 13.0mmol, 2.16g) in 5mL ethanol was added. The reaction mixture was stirred while heated to 78°C over 19h. Afterwards, the reaction flask was cooled at −33°C for 3d and the crystallized solid was filtered under inert atmosphere, washed with cooled ethanol (three times 4mL) and dried under reduced pressure. The crystalline solid was dissolved in 12mL ethyl acetate and the volatile components were removed under reduced pressure. The crude product was recrystallized from hexane (20mL) and ethyl acetate (1mL) to obtain the yellow solid 7 in a yield of 1.11g (3.25mmol, 61%). – 1H NMR (300MHz, [D6]DMSO): δ/ ppm=3.63 (q, 3JHH=5.2Hz, 2H, H19), 4.09 (t, 3JHH=5.2Hz, 2H, H18), 4.80 (t, 3JHH=5.2Hz, 1H, OH), 7.17–7.26 (m, 2H, H14,16), 7.42–7.56 (m, 4H, H2,3,6,7), 7.62(t, 3JHH=7.9Hz, 1H, H15), 7.93 (d, 3JHH=8.6Hz, 2H, H1,8), 8.09 (d, 3JHH=8.3Hz, 2H, H4,5), 8.35–8.40 (m, 2H, H10,13), 9.01 (s, 1H, H11). – 13C{1H} NMR (75MHz, [D6]DMSO): δ/ ppm=59.39 (s, 1C, C19), 70.42 (s, 1C, C18), 113.24 (s, 1C, C16), 120.92 (s, 1C, C14), 121.42 (s, 1C, C10), 121.57 (s, 2C, C8a,9a), 123.59 (s, 2C, C1,8), 123.74 (s, 1C, C12), 125.19 (s, 2C, C2,7), 125.69 (s, 2C, C3,6), 126.98 (s, 1C, C13), 128.13 (s, 2C, C4,5), 131.40 (s, 2C, C4a,10a), 133.83 (s, 1C, C15), 146.30 (s, 1C, C9), 159.15 (s, 1C, C17), 161.50 (s, 1C, C11). – MS (EI, 70eV): m/z (%)=341 (98) [M]+, 296(29) [M–C2H5OH]+, 193(100) [C14H11N]+, 119(44) [C7H5NO]+. – Anal. calcd. for C23H19NO2 (341.40gmol−1): C 80.92, H 5.61, N 4.10; found: C 79.75, H 5.30, N 3.81.

4.5 Synthesis of 9-anthracenesalicylamine (5)

To a suspension of NaBH4 (5.00eq., 6.73mmol, 0.25g) in 20mL THF a yellow solution of 2 (1.00eq., 1.35mmol, 0.40g) in 20mL THF was added. The reaction mixture was stirred for 18h at room temperature. The volatile components were removed under reduced pressure and the residue was dissolved in chloroform (25mL) and water (20mL). The aqueous layer was once again extracted with chloroform (20mL) and the combined organic phases were washed with water (30mL). After drying the organic layer with magnesium sulfate and filtration the volatile components were removed under reduced pressure. The crude product was recrystallized from hexane (10mL) and chloroform (3.5mL) to isolate 5 as a yellow solid in a yield of 0.30g (1.00mmol, 74%). – 1H NMR (300MHz, [D6]DMSO): δ/ ppm=4.42 (d, 3JHH=7.3Hz, 2H, H11), 5.58 (t, 3JHH=7.3Hz, 1H, NH), 6.76 (td, 3JHH=7.7Hz, 4JHH=1.1Hz, 1H, H14), 6.85 (dd, 3JHH=7.7Hz, 4JHH=1.1Hz, 1H, H16), 7.09 (td, 3JHH=7.7Hz, 4JHH=1.6Hz, 1H, H15), 7.35–7.49 (m, 5H, H2,3,6,7,13), 8.00 (d, 3JHH=8.0Hz, 2H, H4,5), 8.15 (s, 1H, H10), 8.39 (d, 3JHH=8.3Hz, 2H, H1,8), 9.69 (s, 1H, OH). – 13C{1H} NMR (75MHz, [D6]DMSO): δ/ ppm=50,70 (s, 1C, C11), 114.87 (s, 1C, C16), 118.84 (s, 1C, C14), 119.97 (s, 1C, C10), 123.66 (s, 2C, C1,8), 124.24 (s, 2C, C2,7), 124.75 (s, 2C, C8a,9a), 125.26 (s, 2C, C3,6), 126.48 (s, 1C, C12), 127.98 (s, 1C, C15), 128.51 (s, 2C, C4,5), 129.15 (s, 1C, C13), 131.92 (s, 2C, C4a,10a), 142.63 (s, 1C, C9), 155.14 (s, 1C, C17). – MS (EI, 70eV): m/z (%)=299(17) [M]+, 193(100) [M–C7H5OH]+, 176(18) [C14H8]+. – Anal. calcd. for C21H17NO (299.37gmol−1): C 84.25, H 5.72, N 4.68; found: C 82.96, H 5.39, N 4.55.

4.6 Synthesis of 9-anthracenepicolylamine (6)

To a solution of 3 (1.00eq., 0.78g, 2.76mmol) in 15mL toluene a 1 m solution of Na[BEt3H] (2.50eq., 6.91mmol, 6.91mL) in toluene was added. The reaction mixture was stirred under inert atmosphere for 20h at room temperature. The volatile components were removed under reduced pressure and the residue was dissolved in ethyl acetate (20mL) and water (50mL). The aqueous layer was twice extracted with ethyl acetate (20mL each) and the combined organic layers were dried with magnesium sulfate. After filtration the volatile components were removed under reduced pressure and the crude product was purified via column chromatography (pentane and ethyl acetate 20:1 → pentane and ethyl acetate 2:1) and recrystallization from hexane (10mL) and ethyl acetate (2mL). 6 was isolated in a yield of 0.18g (0.63mmol, 23%) as an orange solid. – 1H NMR (300MHz, [D6]DMSO): δ/ ppm=4.61 (d, 3JHH=6.8Hz, 2H, H11), 6.11 (t, 3JHH=6.8Hz, 1H, NH), 7.29 (dd, 3JHH=7.5Hz, 3JHH=5.2Hz, 1H, H15), 7.38–7.49 (m, 4H, H2,3,6,7), 7.57 (d, 3JHH=7.5Hz, 1H, H13), 7.77 (td, 3JHH=7.5Hz, 4JHH=1.8Hz, 1H, H14), 8.00 (d, 3JHH=7.5Hz, 2H, H4,5), 8.15 (s, 1H, H10), 8.39 (d, 3JHH=7.8Hz, 2H, H1,8), 8.57 (d, 3JHH=5.2Hz, 1H, H16). – 13C{1H} NMR (75MHz, [D6]DMSO): δ/ ppm=56.20 (s, 1C, C11), 119.92 (s, 1C, C10), 122.00 (s, 1C, C13), 122.22 (s, 1C, C15), 123.65 (s, 2C, C1,8), 124.27 (s, 2C, C2,7), 124.35 (s, 2C, C8a,9a), 125.27 (s, 2C, C3,6), 128.53 (s, 2C, C4,5), 131.92 (s, 2C, C4a,10a), 136.66 (s, 1C, C14), 142.35 (s, 1C, C9), 148.83 (s, 1C, C16), 159.42 (s, 1C, C12). – MS (EI, 70eV): m/z (%)=284(87) [M]+, 192(100) [M–C6H6N]+, 165(31) [C13H9]+. – Anal. calcd. for C20H16N2 (284.35gmol−1): C 84.48, H 5.67, N 9.85; found: C 84.00, H 5.39, N 9.87.

4.7 Synthesis of 9-anthracene(o-(β-hydroxyethoxy)benzyl)amine (7)

To a suspension of LiAlH4 (5.00eq., 8.05 mmol, 306 mg) in THF (26 mL) a solution of 4 (1.00eq., 1.61mmol, 0.55g) in THF (26 mL) was added dropwise. The reaction mixture was stirred for 18h at room temperature. The volatile components were removed under reduced pressure and ethyl acetate (15mL) and water (15 mL) were added. The insoluble constituents were filtered off and washed with ethyl acetate and water followed by phase separation. After extraction of the aqueous phase with ethyl acetate the combined organic layers were dried with magnesium sulfate and filtered, and the volatile components were removed under reduced pressure. The crude product was recrystallized from hexane (10mL) and ethyl acetate (10mL) to isolate the orange solid 7 in a yield of 0.19g (0.55mmol, 34%). – 1H NMR (300MHz, [D6]DMSO): δ/ ppm=3.75 (q, 3JHH=5.2Hz, 2H, H19), 4.05 (t, 3JHH=5.2Hz, 2H, H18), 4.44 (d, 3JHH=7.4Hz, 2H, H11), 4.92 (t, 3JHH=5.2Hz, 1H, OH), 5.60 (t, 3JHH=7.4Hz, 1H, NH), 6.87 (td, 3JHH=7.8Hz, 4JHH=0.9Hz, 1H, H14), 7.00 (d, 3JHH=7.8Hz, 1H, H16), 7.23 (td, 3JHH=7.8Hz, 4JHH=1.5Hz, 1H, H15), 7.37 (d, 3JHH=7.8Hz, 1H, H13), 7.38–7.49 (m, 4H, H2,3,6,7), 8.00 (d, 3JHH=7.1Hz, 2H, H4,5), 8.16 (s, 1H, H10), 8.38 (d, 3JHH=8.2Hz, 2H, H1,8). – 13C{1H} NMR (75MHz, [D6]DMSO): δ/ ppm=50.76 (s, 1C, C11), 59.63 (s, 1C, C19), 69.72 (s, 1C, C18), 111.46 (s, 1C, C16), 120.12 (s, 1C, C10), 120.18 (s, 1C, C14), 123.73 (s, 2C, C1,8), 124.27 (s, 2C, C2,7), 124.97 (s, 2C, C8a,9a), 125.24 (s, 2C, C3,6), 128.32 (s, 2C, C12,15), 128.46 (s, 2C, C4,5), 129.11 (s, 1C, C13), 131.87 (s, 2C, C4a,10a), 142.47 (s, 1C, C9), 156.56 (s, 1C, C17). – MS (EI, 70eV): m/z (%)=343(100) [M]+, 192(58) [M–CH2C6H4OC2H4OH]+, 165(24) [C13H9]+, 151(22) [CH2C6H4OC2H4OH]+, 133(43) [CH2C6H4OC2H3]+, 107(27) [CH2C6H4OH]+. – Anal. calcd. for C23H21NO2 (343.42gmol−1): C 80.44, H 6.16, N 4.08; found: C 78.83, H 6.14, N 3.88.

4.8 Supporting information

Crystallographic and other supporting data associated with this article can be found in the online version (DOI: 10.1515/znb-2016-0229).

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Supplemental Material:

The online version of this article (DOI: 10.1515/znb-2016-0229) offers supplementary material, available to authorized users.

Received: 2016-10-31
Accepted: 2016-12-16
Published Online: 2017-2-15
Published in Print: 2017-3-1

©2017 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.1515/znb-2016-0229
Keywords for this article
anthracene; colorimetric; luminescence; pH sensor

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
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  4. A new furan carboxamide and two potential precursors from a terrestrial streptomycete
  5. A Co(II) complex based on a mixed N- and O-donor: synthesis, structural characterization, and properties
  6. Synthesis, structural characterization, and fluorescence of a Cd(II) coordination polymer with N-donor ligands
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