Enantioselectivity of a tartaric acid amide linked zinc bisporphyrinate towards amino acid esters

Dyes and Pigments 176 (2020) 108223

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Enantioselectivity of a tartaric acid amide linked zinc bisporphyrinate towards amino acid esters Jiao Wang , Zhihao Zhang , Chuanjiang Hu *, Yong Wang ** College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, Jiangsu, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Porphyrin Enantioselectivity Chiral recognition Circular dichroism

We have designed and synthesized a new chiral zinc bisporphyrinate ([Zn2(D-BTABis)]) and investigated its enantioselectivity towards amino acid esters. Our studies reveal the binding of D-type of amino acid esters to this chiral zinc bisporphyrinate is more favorable than L-type of amino acid esters, the corresponding enantiose­ lectivity (α) is 8.4 for the phenylalanine ethyl esters. NMR studies suggest these guests are monodentate ligands in the host-guest complexes. Further DFT calculations rationalize the CD results and enantioselectivity. When the chiral host interacts with the D-enantiomers of these guests, the steric repulsion interactions between the linker and the guest are weaker, and the tartaryl group adopts the conformation A with a larger positive torsion angle. These cause the complexes with D-type of amino acid esters are more energetically favorable than those with Ltype of amino acid esters. Our studies provide one good bisporphyrin system for enantiodiscrimination of monodentate guests.

1. Introduction In recent decades, bisporphyrins have been intensively investigated in chirality related research fields since they have distinct properties, such as an intense UV–visible absorption and strong coupling in­ teractions between porphyrin chromophores [1–6]. It is also known that chiral porphyrin systems have been widely used in molecular recogni­ tion [7–9], asymmetric synthesis [10,11], resolution of racemic mix­ tures [12–14] enantioselective extraction [15] and circularly polarized luminescence materials [16] etc. Among them, chiral bisporphyrins are of particular interest due to their potential applications in enantiose­ lective recognition, enantioselective separation etc. But so far, there are limited reports on chiral bisporphyrins [17–28]. In most of these reports, guests are bidentate ligands. For example, Hayashi et al. synthesized a chiral bisporphyrin linked by the binaphthyl, the host shows good enantioselectivity towards lysine [17]. Crossley et al. synthesized bismetalloporphyrin analogues of Troger’s base, which showed enantioselective binding of histidine esters and lysine benzyl ester [18]. Ema and Sakai used a chiral bisporphyrin with an isophthalic linker, which functioned as a chiral shift reagent for bidentate guests, such as chiral diamine derivatives [19]. Jiang and Bian have recently developed 1,10 -binaphthalene-bridged bisporphyrins [26,

27], which show enantioselectivity towards diamines. But so far, no monodentate ligands have been reported as guests in the chiral bisporphyrin systems. It still remains a challenging task to design highly efficient chiral discrimination bisporphyrin systems for more general guests, such as monodentate chiral guests. We have been recently studied on bisporphyrins [29–35], and we are interested in chiral bisporphyrins as well. Currently, we have designed a chiral bisporphyrin as shown in Fig. 1, which has the following feature: i) There are coordination sites (zinc atoms) and hydrogen bonding sites (amide groups) in the host; ii) More importantly, since the amide is at the orth-position on meso-phenyl groups, this special arrangement forces hydrogen bonding and coordination interactions to occur on the same guest. It could also cause other interactions, such as repulsion or π-π interactions between the guests and linker, then increase the chiral recognition ability for the host; iii) Since the linker is a tartaryl group containing two chiral centers, the interactions between the chiral guests and linker could lead to enantioselectivity. Herein, we report the enantioselectivity of this new chiral bisporphyrin towards amino acid esters. In order to better understand the new system, we chose the following amino acid esters with simple substituents R on chiral carbon as guests. The results show the new host has great enantioselectivity towards amino acid esters (especially PheOEt) and amino acid esters

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Hu), [email protected] (Y. Wang). https://doi.org/10.1016/j.dyepig.2020.108223 Received 18 December 2019; Received in revised form 14 January 2020; Accepted 17 January 2020 Available online 18 January 2020 0143-7208/© 2020 Elsevier Ltd. All rights reserved.

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behave as monodentate guests. To the best of our knowledge, it is the first chiral bisporphyrin system that has great enantioselectivity towards monodentate chiral guests. This system has been investigated by UV–Vis, CD spectroscopy, NMR and DFT calculations.

¼ 6.3 Hz, 2H), 8.33 (d, J ¼ 4.1 Hz, 2H), 8.23–8.08 (m, 8H), 7.97 (dd, J ¼ 20.8, 9.8 Hz, 6H), 7.88 (dd, J ¼ 14.6, 7.3 Hz, 4H), 7.80–7.64 (m, 14H), 7.59 (s, 2H), 7.51 (d, J ¼ 7.3 Hz, 2H), 7.42 (t, J ¼ 7.7 Hz, 4H), 7.20 (t, J ¼ 7.4 Hz, 2H), 5.38 (d, J ¼ 19.1 Hz, 2H), 4.52 (d, J ¼ 7.4 Hz, 4H), 4.17 (t, J ¼ 7.1 Hz, 2H), 3.69 (t, J ¼ 7.3 Hz, 4H), 3.47 (s, 4H). UV–vis (CH2Cl2): λmax (logε) 417 (5.97), 515 (4.55), 550 (4.32), 591 (4.26), 646 (4.56). Anal. Calcd for C106H72N10O6: C, 80.49; H, 4.59; N, 8.86. Found: C, 80.45; H, 4.62; N, 8.77.

2. Experimental section 2.1. General Triethylamine was treated with potassium hydroxide. CH2Cl2 was distilled over with CaH2. The fresh distilled amino acid esters and zinc 5(2-aminophenyl)-10,15,20-triphenylporphyrinate were prepared ac­ cording to the literature [36,37]. 1 H NMR spectra were recorded on a Bruker AVANCE 400 MHz spectrometer in CDCl3 using tetramethylsilane (TMS) as the internal standard. 13C NMR spectra were recorded with CDCl3 (77.16 ppm) as internal reference. 1H–1H COY spectrum was recorded on an Agilent 600 MHz NMR instrument. Circular Dichroism spectra were recorded on an AVIV Model 410 spectropolarimeter. UV–visible spectra were recorded on a Shimadzu UV-3150 spectrometer. Elemental analyses (C, H and N) were taken on an Elementar Vario EL III analytical instrument. UV–vis and CD titration experiments were carried out as follows. Different amounts of optically active amino acid esters solution was added to the [Zn2(D-BTABis)] solution in CH2Cl2 at 298 K. Then UV–vis or CD spectra were recorded. CD spectra were recorded in millidegrees, and normalized according to the concentrations of [Zn2(D-BTABis)]. 1 H NMR titration measurements were performed by adding different amounts of LeuOEt solution to zinc bisporphyrinate solution in CDCl3.

2.3. Synthesis of zinc bisporphyrinate [Zn2(D-BTABis)] To the solution of D-BTABis (0.35 g, 0.22 mmol) in the mixed solvent of CHCl3 and CH3OH (120 mL, 2:1), Zn(CH3COO)2 (0.20 g, 1.2 mmol) was added. The mixture was washed with water after 2 h’ refluxing. Then the organic layer was collected. A purple solid was obtained after rotary evaporation. Then it was purified by silica gel chromatography (CH2Cl2/petroleum ether ¼ 1:1) (0.34 g, yield 90%). 1H NMR (400 MHz, CDCl3) δ 9.04 (d, J ¼ 4.6 Hz, 2H), 8.98–8.88 (m, 6H), 8.71 (d, J ¼ 4.7 Hz, 2H), 8.41 (t, J ¼ 5.8 Hz, 4H), 8.29 (d, J ¼ 7.2 Hz, 2H), 8.21 (dd, J ¼ 16.6, 6.8 Hz, 6H), 8.13–8.03 (m, 6H), 7.95–7.50 (m, 22H), 7.45 (dd, J ¼ 14.8, 6.1 Hz, 4H), 7.22 (t, J ¼ 7.0 Hz, 2H), 5.37 (s, 2H), 4.44 (d, J ¼ 7.1 Hz, 4H), 3.80 (t, J ¼ 7.2 Hz, 2H), 3.58 (t, J ¼ 7.6 Hz, 4H). UV–vis (CH2Cl2): λmax (logε) 419 (5.96), 553 (4.67), 596 (4.12). 13C NMR (101 MHz, CDCl3) δ:163.49, 160.61, 150.29, 150.18, 149.96, 149.78, 149.71, 149.61, 149.23, 142.74, 142.48, 142,29, 136.75, 134.84, 134.68, 134.54, 134.45, 134.35, 133.92, 133.51, 132.24, 132.21, 132.18, 132.13, 131.98, 130.89, 130.38, 130.01, 129.66, 129.04, 127.79, 127.61, 127.54, 126.96, 126.79, 126.71, 126.65, 126.49, 126.44, 125.45, 124.11, 123.88, 123.23, 121.90, 121.68, 120.68, 120.18, 112.54, 71.61. Anal. Calcd for C106H68N10O6Zn2: C, 74.52; H, 4.01; N, 8.20. Found: C, 75.66; H, 4.09; N, 8.26.

2.2. Synthesis of (þ)-2,3-dibenzoyl-D-tartaric acid amide linked bisporphyrin (D-BTABis)

2.4. Computation methods

The reaction was performed under anaerobic condition. To a 100 mL of Schlenk glass containing (þ)-2,3-dibenzoyl-D-tartaric acid (0.10 g, 0.28 mmol), SOCl2 (10 mL) was added. After 10 h’ refluxing under N2, the excess SOCl2 was removed under vacuum. The resulted white solid was dissolved in anhydrous CH2Cl2 (30 mL), then 0.50 g of 5-(2-ami­ nophenyl)-10,15,20-triphenylporphyrinate (0.78 mmol) and 150 μL of Et3N (1.07 mmol) was added. After for 15 min’ stirring in an ice bath, the mixture was warmed up to RT. The reaction was monitored by TLC and it was complete after 8 h. The solvent was removed by rotary evaporator, a purple solid was obtained. It was further purified by silica gel chromatography (CH2Cl2). 0.39 g of pure product was obtained. (yield 89%). 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.94 (d, J ¼ 4.1 Hz, 2H), 8.81 (dd, J ¼ 14.7, 4.5 Hz, 6H), 8.59 (d, J ¼ 3.8 Hz, 2H), 8.42 (d, J

Theoretical studies on [Zn2(D-BTABis)], the 1:1 and 1:2 host–guest complexes formed between [Zn2(D-BTABis)] and D-/L-LeuOEt were performed by density functional theory calculation at WB97XD/6-31G* level using Gaussian 09 suite of program [38]. No symmetry constraints was employed to investigate the optimized geometries. 3. Results and discussion 3.1. UV–visible spectral studies In order to investigate the chiral discrimination abilities of [Zn2(D-

Fig. 1. Structural formulas for the host and guests. 2

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Dyes and Pigments 176 (2020) 108223

BTABis)] towards the amino acid esters, UV–Visible titration experi­ ments have been carried out by adding amino acid ethyl esters into the zinc bisporphyrinate solution. For the host [Zn2(D-BTABis)], the spec­ trum shows one Soret band at 419 nm. For D-LeuOEt, the titration spectral changes are shown in Fig. 2. During the titration, a new band is formed at 428 nm. The band intensity at 419 nm decreases and the band intensity at 428 nm increases when the ligand concentrations increase. Such red shift is most likely caused by the coordination of a N-containing guest to zinc porphyrinate. But there could be also contributions from the conformational changes in a bisporphyrin induced by external ligation. Job’s plots (Fig. S6 in the supplementary data) show a peak at around 0.33, which indicates that the 1:2 complexes are formed between [Zn2(D-BTABis)] and D-LeuOEt. Since zinc is generally five-coordinate in these zinc porphyrinate systems, there are most likely the following two equilibria in solution. ½Zn2 ðD

BTABis�

½Zn2 ðD

BTABisÞL�

þ þ

K1

L

⇌ L

K2

⇌

½Zn2 ðD ½Zn2 ðD

BTABisÞL� BTABisÞðLÞ2 �

Table 1 Formation Constants (Kf), Enantioselectivities (α), and Chiral Recognition En­ ergies (ΔΔG� , kJ⋅mol 1) of [Zn2(D-BTABis)] towards amino acid esters by UV–Vis titration data. K1, K2 D-LeuOEt L-LeuOEt D-ValOEt L-ValOEt D-AlaOEt L-AlaOEt D-PheOEt L-PheOEt D-PhgOEt L-PhgOEt

2.9 1.1 1.6 1.1 5.6 4.3 6.8 1.3 1.6 1.3

Kf 4

� 10 , 4.1 � 104, 2.2 � 104, 3.6 � 104, 2.6 � 103, 6.6 � 103, 8.2 � 104, 3.0 � 104, 1.9 � 104, 1.9 � 104, 1.9

3

� 10 � 103 � 103 � 103 � 102 � 102 � 103 � 103 � 103 � 103

1.2 2.4 5.8 2.9 3.7 3.5 2.1 2.5 3.0 2.5

α 8

� 10 � 107 � 107 � 107 � 106 � 106 � 108 � 107 � 107 � 107

△△G

5.0

4.0

2.0

1.8

1.1

0.24

8.4

5.3

1.2

0.45

K1 and K2 are the equilibrium constants for the equilibria 1 and 2;Kf is the overall formation constant for the 1:2 host-guest complexes;α ¼ Kf(D)/Kf(L);△△G ¼ -RTLnα.

(1)

disappeared, and a new negative couplet was formed at longer wave­ length. In comparison, the addition of excess of D-LeuOEt to [Zn2(DBTABis)] leads a new positive CD couplet. CD titration spectra are given in the supplementary data. It is obvious that those spectra for the hostguest complexes with D- or L-amino acid esters are not mirror-symmetric to each other. That is reasonable because that these host-guest com­ plexes with enantiomers of amino acid esters are diastereomers when the host is chiral. The binding constants were also calculated with the program SQUAD. Table S1 in the supplementary data lists the corresponding fitting results. All these Kf values are consistent with those from UV–vis spectra. These results further confirmed the enantioselectivity of the new chiral zinc bisporphyrinate towards amino acid esters.

(2)

For the above-mentioned two equilibria, their binding constants were calculated with the SQUAD program [39]. For D-PheOEt, the equilibrium constants K1 and K2 are 6.8 � 104 and 3.0 � 103, respec­ tively, which are similar to those observed in other amide-linked bis­ porphyrin systems [34]. The overall formation constant Kf is found to be 2.1 � 108. For L-PheOEt, Kf is found to be 2.5 � 107. It is obvious that the formation constant for the complex between the host and each enan­ tiomer of amino acid esters is different, which indicates the host has enantiodiscrimination ability towards amino acid esters. For all com­ plexes, the formation constants Kf were listed in Table 1. The enantio­ selectivities (α) of [Zn2(D-BTABis)] towards amino acid esters vary from 1.1 to 8.4. The difference in α for those amino acid esters could be caused by the different stereochemical features as shown in the computational studies. The enantioselectivity is 8.4 for the host with D-/L-PheOEt, the corresponding chiral recognition energy (ΔΔG� ) is 5.3 kJ mol 1.

3.3.

1

H NMR studies

The coordination behaviors were also investigated by NMR spectra. In order to obtain distinguishable NMR signals, we chose leucine ethyl ester as the guest since it has an alkyl rather than an aryl as the sub­ stituent. 1H NMR titration experiments have been carried out by adding L-LeuOEt into [Zn2(D-BTABis)] solution. The assignments of the signals of L-LeuOEt were based on the titration spectra (Fig. 4) and 1H–1H COSY spectrum (Fig. S23). For Hd, He, Hf and Hg, the titration spectra can be used to track their original locations. For Ha, Hb and Hc, the 1H–1H COSY spectrum clearly shows the correlations among them, which lead to the corresponding assignments. For NH, it is assigned to the signal at 3.7 ppm since such signal disappears upon addition of D2O (Fig. S22). Upon addition of L-LeuOEt, the resonances of the ligand protons display remarkable upfield shifts. A maximum complexation-induced chemical shift (CIS) value is found for NH (Δδ ¼ 5.35 ppm) due to its direct coordination to zinc. For the rest, the proton of chiral carbon, Ha, shows the second largest upfield shift (Δδ ¼ 4.65 ppm).

3.2. CD spectral studies The chiral discrimination abilities of [Zn2(D-BTABis)] towards amino acid esters were also confirmed by electronic CD spectroscopy. CD spectra were measured on the mixture of [Zn2(D-BTABis)] and enantiopure amino acid ethyl esters. The results are provided in Fig. 3 and Table 2. In Fig. 3, the chiral zinc bisporphyrinate itself shows a negative CD couplet at 425 and 416 nm. For all these host-guest com­ plexes, the spectra show CD couplets in the Soret band. Upon titration of L-LeuOEt to [Zn2(D-BTABis)], the original negative CD couplet

Table 2 CD values of complexes of [Zn2(D-BTABis)] with various of amino acid esters. Amino acid esters

Complexes Δε,λnm

L-LeuOEt L-ValOEt L-AlaOEt L-PheOEt L-PhgOEt

Fig. 2. UV–Visible spectral changes of [Zn2(D-BTABis)] (2.05 � 10 6 M) in CH2Cl2 upon addition of D-LeuOEt at 298 K, the host to guest molar ratio: 1:0 to 1:1400. Arrows show absorbance changes during the titration.

101,435 99,426 88,436 77,428 93,435 91,426 140,434 108,426 91,435 68,426

Aaobs ¼ Δε1-Δε2,cm 1M 1. 3

Amino acid esters

Complexes Δε,λnm

Aobs

200

D-LeuOEt

208

165

D-ValOEt

184

D-AlaOEt

248

D-PheOEt

159

D-PhgOEt

140,432 68,425 78,432 25,425 52,430 2424 101,433 36,426 79,432 21,425

Aaobs

103 54 137 100

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Fig. 3. CD spectra of [Zn2(D-BTABis)] (2.05 � 10 6 M) before (black) and after the addition of excess amino acid esters (red, D-enantiomer; blue, L-enantiomer). (a) LeuOEt, (b) ValOEt, (c) AlaOEt, (d) PheOEt, (e) PhgOEt. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

As we also notice, the proton resonances of L-LeuOEt showed stepby-step downfield shifts upon titration. It indicates there are several products involved, which are likely 1:1 and 1:2 complexes. The 1H NMR signals are the average signals from all ligand containing species, such as the ligand, 1:1 and 1:2 complexes. When the concentrations of the ligand increase, the ratios among these species change and the percentage of ligand increases, so the proton resonances shift downfield. On the other hand, the CIS values for the complex of [Zn2(D-BTABis)] with LeuOEt are very similar to those for the monozinc bisporphyrinate system [35], in which amino acid esters unambiguously function as monodentate guests. So it further confirms the monodentate nature of the amino acid esters in this chiral bisporphyrin system.

3.4. Computational studies In order to understand CD spectra and the enantioselectivity of this chiral bisporphyrin towards amino acid esters, DFT calculations were performed and the optimized structures were obtained. As shown in Fig. 5, there are two types of hydrogen bonds in the host-guest com­ plexes. One is formed between NH2 in the guest and the carbonyl oxygen in the linker, another is between the amide NH in the linker and the carbonyl oxygen in the guest. Both types of hydrogen bonds have been reported in the literatures [34,40]. It is known that the CD signs depend on the torsion angle between electric transition dipole moments (ETDMs) [41]. In the optimized structures, the torsion angle ϕ of 4

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C5–C15–C50 -C150 is used to present the torsion angles between ETDMs. In the optimized structure as shown in Fig. 5a, the host [Zn2(D-BTABis)] adopts an anticlockwise twist with the torsion angle ϕ is 76� . Based on the exciton chirality method [41], the exciton bisignate CD reflects the “exciton chirality”: (i) If the exciton CD shows negative first and positive second Cotton effects (CEs), the two ETDMs constitute a counterclockwise twist (positive exciton chirality). (ii) If the exciton CD shows positive first and negative second CEs, the two ETDMs constitute a clockwise twist (negative exciton chirality). So the negative torsion angle corresponds to negative exciton chirality. For D-LeuOEt, all the 1:1 and 1:2 complexes, [Zn2(D-BTABis)(D-LeuOEt)] and [Zn2(D-BTABis) (D-LeuOEt)2], adopt clockwise twists with the torsion angle ϕ of þ62� and þ109� , respectively, which correspond to positive exciton chirality; For L-LeuOEt, both 1:1 and 1:2 complexes adopt anticlockwise twists with the torsion angle ϕ of 42 and 64� , respectively, which corre­ spond to negative exciton chirality. So our DFT calculations suggest that the torsion angles ϕ in [Zn2(D-BTABis)(D-LeuOEt)] and [Zn2(D-BTABis) (L-LeuOEt)] have opposite signs. According to exciton chirality method, it indicates the reverse twists of ETDMs in two complexes. All the pre­ dicted signs of CD signals are consistent with the experimental results. For both 1:1 and 1:2 complexes, the computational results also suggest that the complexes with D-LeuOEt as the guest are more ener­ getically favorable than those with L-LeuOEt (the difference in energy is

Fig. 4. 1H NMR in the selected region for the titration of [Zn2(D-BTABis)] (5.0 � 10 3 M) with L-LeuOEt in CDCl3 at 298 K. The amount of L-LeuOEt are (a) 0.4 equiv., (b) 2.0 eq., (c) 4.0 eq., (d) 8.0 eq., (e) 10.0 eq., and (f) pure L-LeuOEt for comparison.*Impurity from H2O. **Impurity from grease.

Fig. 5. DFT optimized structures. a) The host [Zn2(DBTABis)], which forms the clockwise twist. b) 1:1 Host-guest complex [Zn2(D-BTABis)(D-LeuOEt)], which forms the clockwise twist. c) [Zn2(D-BTABis) (L-LeuOEt)], which forms the anticlockwise twist. d) 1:2 Host-guest complex [Zn2(D-BTABis)(D-LeuOEt)2], which forms the clockwise twist. e) [Zn2(D-BTABis) (L-LeuOEt)2], which forms the anticlockwise twist. Hydrogen and isopropyl groups of the chiral carbon atoms are marked by blue circles, the tartary groups are marked by red circles. Hydrogen bonds are labeled as red dash lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

5

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shown in Figs. S24 and S25), which is also consistent with the enan­ tioselectivity determined by spectroscopic results. What is the structural feature leading to the enantioselectivity? We are more concerned about the conformations of those chiral groups: the chiral guest and the chiral tartaryl group in the host. For leucine ethyl ester, four different groups are covalently bonded to the chiral carbon: NH2, ester, hydrogen and R group, where R is the different substituent for various different amino acids. Because of the coordination in­ teractions and hydrogen bonding interactions for the porphyrin subunit as shown in Fig. 5, the NH2 and the ester group in these host-guest complexes are constrained in similar positions, but the positions of hydrogen and R group could be different. On the other hand, the tartaryl group, as the linker in this chiral bisporphyrin, is relative flexible, which could lead to different confor­ mations with either positive or negative twist of the two porphyrin chromophores, and the corresponding CD signals will be also different. As shown in Fig. 6, in the optimized structure of [Zn2(D-BTABis)(DLeuOEt)], the hydrogen atom is facing the linker, which could lower the steric repulsion between the linker and the guest. Correspondingly, the tartaryl group adopts the conformation A. In this conformation, two largest groups (L) are far away from each other with a torsion angle of þ67� . The positive sign is also consistent with the torsion angle ϕ of ETDMs, which corresponds to the positive CD signals as the experi­ mental results. In the case of [Zn2(D-BTABis)(L-LeuOEt)], the R group (isopropyl) is facing the linker, which could cause greater repulsion between the linker and the guest. Correspondingly, the tartaryl group adopts the confor­ mation B. As shown in Fig. 6, two largest groups form a torsion angle of 40� . The negative value is also consistent with the torsion angle ϕ,

which corresponds to the negative CD signals as the experimental results. Obviously, the torsion angle between two L groups in [Zn2(D-BTA­ Bis)(D-LeuOEt)] is much larger than that in [Zn2(D-BTABis)(L-LeuOEt)]. The overall interactions cause the [Zn2(D-BTABis)(D-LeuOEt)] is more energetically favorable than [Zn2(D-BTABis)(L-LeuOEt)], which is consistent with the enantioselectivity obtained from the spectroscopic results. So the binding affinities of this chiral bisporphyrin towards Dtype of amino acid esters are greater than towards L-type of amino acid esters. We also notice the enantioselectivity values are different for different amino acid esters, α increases in the following order:AlaOEt < PhgOEt < ValOEt < LeuOEt < PheOEt. Such order is roughly consistent with the bulkiness order of R groups of these amino acids: methyl (in AlaOEt) < iso-propyl (in ValOEt) < iso-butyl (in LeuOEt) < benzyl (in PheOEt). The only exception is PhgOEt. If we consider the rigidity of the phenyl group in PhgOEt, that could lead to greater repulsion interactions when the phenyl is facing the porphyrin plane in [Zn2(D-BTABis)(D-PhgOEt)], then raises its energy. That will decrease the energetic difference be­ tween [Zn2(D-BTABis)(D-PhgOEt)] and [Zn2(D-BTABis)(L-PhgOEt)], and lower the corresponding enantioselectivity value. Compared with the phenyl group, the benzyl group in PheOEt is also bulky. But the benzyl group is more flexible since it has one sp3 hybridized carbon, which allows the phenyl in the benzyl group to adopt orientations away from the porphyrin plane. That avoids the great steric repulsion as for the phenyl group. In a word, for D-type of amino acid esters, the host-guest interactions lead to the conformation A of the tartaryl group and the clockwise twist of two porphyrin chromophores, which results in the positive exciton chirality; for L-type of amino acid esters, the host-guest interactions lead to the conformation B of the tartaryl group and the anticlockwise twist of two porphyrin chromophores, which results in the negative exciton chirality. 4. Conclusion We have designed and synthesized a new tartaric acid amide-linked zinc bisporphyrinate. UV–vis and CD spectra reveals it has great enan­ tioselectivity towards amino acid esters. Our measurements demon­ strate the enantiodiscrimination abilities of this chiral zinc bisporphyrinate towards amino acid esters. NMR studies also suggest amino acid esters behave as monodentate ligands. DFT calculations further rationalize the CD results and enantioselectivity. In general, when [Zn2(D-BTABis)] interacts with the D-type of amino acid esters, the tartaryl group adopts the conformation A with a larger L-L torsion angle, leading to more energetically favorable complexes with the Dtype of amino acid esters than those with L-type of amino acid esters. Our studies provide one good bisporphyrin system for enantiodiscrimi­ nation of monodentate guests. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 6. In the optimized structures of the 1:1 complexes, the orientation of the amino acid ester leads to different steric repulsions with the linker. (top) When the hydrogen atom of the chiral carbon is facing the linker, the overall in­ teractions lead to the conformation A of the tartaryl group and the clockwise twist. (bottom) When the isopropyl group is facing the linker, the overall in­ teractions lead to the conformation B of the tartaryl group and the anticlock­ wise twist. The tartary groups are marked by red circles. Hydrogen bonds are labeled as red dash lines. L represents the largest group in size, (it is associated with porphyrin subunits), M represents a middle size group (a benzoate group), S represents the smallest group (a hydrogen atom). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

CRediT authorship contribution statement Jiao Wang: Investigation, Data curation, Writing - original draft. Zhihao Zhang: Validation, Visualization. Chuanjiang Hu: Project administration, Supervision, Resources, Conceptualization, Methodol­ ogy, Writing - review & editing. Yong Wang: Formal analysis, Software. Acknowledgments We thank the National Nature Science Foundation of China for 6

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Dyes and Pigments 176 (2020) 108223

financial support (No. 21271133), Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, the Natural Science Founda­ tion of Jiangsu Province (BK20161275), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201905), Soochow University Analysis and Testing Center and the Political Science-preponderant Discipline of Jiangsu Province.

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