# Structural insight into the photoinduced E→Z isomerisation of cinnamate embedded in ZnAl and MgAl layered double hydroxides

Zita Timár, Gabor Varga, Márton Szabados, Krisztián Csankó, Tünde Alapi, Claude Forano, Vanessa Prevot, Pál Sipos, István Pálinkó

<figure><img src="image_1.png" /></figure>

## To cite this version:

Zita Timár, Gabor Varga, Márton Szabados, Krisztián Csankó, Tünde Alapi, et al.. Structural insight into the photoinduced E→Z isomerisation of cinnamate embedded in ZnAl and MgAl layered double hydroxides. Journal of Molecular Structure, 2020, 1219, pp.128561. 10.1016/j.molstruc.2020.128561. hal-02901669

HAL Id: hal-02901669

https://hal.science/hal-02901669

Submitted on 27 Nov 2020

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Structural insight of photoinduced E→Z isomerisation of cinnamate embedded in ZnAl- and MgAl layered double hydroxides

Zita Timára<sup>a,b</sup>, Gábor Varga<sup>a,b</sup>, Márton Szabados<sup>a,b</sup>, Krisztián Csankó<sup>c</sup>, Tünde Alapi<sup>d</sup>, Claude Forano<sup>e</sup>, Vanessa Prevot<sup>e</sup>, Pál Sipos<sup>b,d</sup>, István Pálinkó<sup>a,b*</sup>

## ABSTRACT

Hybrid E-cinnamate- and Z-cinnamate-intercalated layered double hydroxides (Mg<sub>2</sub>Al- E- or Z-Cin LDH and Zn<sub>2</sub>Al- E- or Z-Cin LDH) were prepared by the co-precipitation method, and structurally characterised by powder X-ray diffractometry, UV-Vis, FT-IR and <sup>13</sup>C CP MAS solid-state NMR spectroscopies to gain further insights on the arrangement of the organic anions in the interlamellar domain. UV light irradiation induced E-Z isomerisation reaction was subsequently attempted in the solid state and in methanolic suspension. Although reaction was observed in the solid state; however, E-Z isomerisation mainly occurred in the slurry phase. The fact that there was no isomerisation when E-Cin was solely adsorbed on the surfaces of pristine LDHs highlights that the reaction took place in the interlayer region. Similar behaviour was observed for the two LDH compositions proving that the LDH structures acted as nanoreactors confining the photoinduced isomerisation.

**Keywords:** Layered Double Hydroxides; Cinnamate; E-Z isomerisation, photoinduced reaction, nanoreactors.

## 1. Introduction

Having well-defined spaces at the nanoscale, inorganic solid-phase nanoreactors, such as (nano)materials with porous and low-dimensional structures (0-D, 1-D, 2-D), such as mesoporous materials or certain zeolites, can not only control selectivities and rates of confined chemical reactions, but the conformation, the stereochemistry and the size of the products as well [1e5]. An additional advantage if structural modifications are possible, is to tailor the nanoreactor properties to needs [1-10].

Layered double hydroxides (LDH) ([M<sup>II</sup><sub>1-x</sub>M<sup>III</sup><sub>x</sub>(OH)<sub>2</sub><sup>q+</sup>][X<sup>q-</sup><sub>x/q</sub>.nH<sub>2</sub>O]), which are well-known lamellar inorganic materials with positively charged layers ([MII1-xMIIIx(OH)<sub>2</sub><sup>q+</sup>]) and negatively charged interlayer gallery ([X<sup>q-</sup><sub>x/q</sub>.nH<sub>2</sub>O]) and expanding structure under intercalation, meet the above requirements. Therefore, they have good potential of acting as nanoreactors, especially when their action partner(s) is/are intercalated into the interlayer domain[8,11e17]. Indeed, the high density of hydrophilic OH groups (~12OH/nm<sup>2</sup>) covering the

<footnote><sup>a</sup> Department of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged, H-6720, Hungary</footnote>

<footnote><sup>b</sup> Materials and Solution Structure Research Group, Institute of Chemistry, University of Szeged, Aradi Vértanúk tere 1, Szeged, H-6720, Hungary</footnote>

<footnote><sup>c</sup> Biological Research Centre, Temesvári krt. 62, Szeged, H-6726, Hungary</footnote>

<footnote><sup>d</sup> Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, Szeged, H-6720, Hungary</footnote>

<footnote><sup>e</sup> Université Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, F-63000 Clermont-Ferrand, France</footnote>

<footnote>* To whom correspondence should be addressed.</footnote>

<footnote>E-mail: palinko@chem.u-szeged.hu (I. Pálinkó)</footnote>
surface of the interlayer gallery added to the presence
of positive (intralayer metal cations) and negative
(interlayer anions) sites may favour selective binding
interactions for organic substrates in a well-defined
environment, may act as H-bond catalyst [18]. This
was shown by Yu and He [19] for the L-proline
confined in LDH or by Dutta and Tummanapelli
using computational investigation to describe
interaction between the anionic layered host and a
neutral guest molecule (Chloranyl) [20]. Hybrid
LDHs may also have important role in prebiotic
chemistry carrying chiral information due to the
intercalated/adsorbed optically active amino acids in
anionic forms [21e23]. The high number of papers on
hybrid LDH materials demonstrate the high capacity
of these 2-D materials to intercalate a large variety of
organic molecules and macromolecules offering an
open field for confined catalytic reactions
[24]. However, there are not many examples of LDH
acting as a nanoreactor for in situ catalytic
transformations [25-27]. Alkene E,Z isomers are
involved in the synthesis of a large variety of highly
valuable organic molecules, therefore, there is a need
to find better strategies the stereoselective synthesis
of E and Z isomers. Direct pathways to the formation
of Z olefins is still rare. Therefore, E/Z isomerisation
appears as a green strategy to reach this challenge
instead of more atom costly Z-selective catalytic
olefin cross-metathesis [28]. Recently Li et al. [29]
reported the efficient photocatalytic E to Z
isomerisation of cinnamyl derivatives.
Photoisomerisation of indoline spirobenzopyran was
achieved in the interlayer of MgAl LDHs [30,31];
however, the role of the LDH support was not
discussed. J. Valim et al. [32] achieved the inter-
calation of p-chlorocinnamate in MgAl LDH
(dbs¼1.98 nm); however, under UV-irradiation
dimerization competed with E to Z isomerisation.
These preliminary results were not investigated much
further. The recent examples reporting light-assisted
cis-trans isomerisation in LDH mainly concern
azobenzene molecules [33,34]. In one of our earlier
works [14], the E isomers of various acrylate ions
were intercalated into CaFe-LDH, and the samples
were irradiated with a non-heating xenon light source
working in the 220e400 nm wavelength range.
Topotactic [2+2] cyclo-dimerization was achieved
solely. To promote the cycle-dimerization, domains
were needed in the host layered structure where the
intercalated anions were in close proximity and in
suitable arrangement. Surprisingly, E-Z
isomerisation was not observed. Acrylic acid
derivatives on UV irradiation preferentially
underwent cyclodimerization in solution too, E-Z

isomerisation was rare, occurring only when
methanol was used as the solvent [35]. The E-Z
isomerisation of the E-cinnamic acid can be followed
by UV-Vis spectroscopy: the conveniently detectable
absorption maxima for the E and the Z compounds
are at 268 nm and 258 nm, respectively [36,37]. The
positions of the absorption maxima did not change
even when the anionic forms of these compounds
were intercalated into LDH matrices [38]. In the
current experimental work leading to this
contribution, the possibilities for the photo induced
E-Z isomerisation to occur in the presence of Zn₂Al
or Mg₂Al LDHs were investigated. To high-light the
role of the organic anion confinement into the layered
domain, the E-cinnamate (Cin) was either admixed to
a suspension of LDH (Zn₂Al or Mg₂Al) or
intercalated into the LDH structure. The aim was to
explore if any of the LDH forms can be identified as
efficient nanoreactor(s) and promote the formation of
the Z-Cin. Techniques such as powder X-ray
diffractometry (PXRD), Fourier-transformed infrared
(FTIR), ultraviolet-visible (UV-Vis) and solid-state 13C Cross Polarization Magic Angle Spinning Nuclear Magnetic Resonance (13C CP MAS NMR)
spectroscopies were used to comprehensively
characterise the materials and to follow in situ the
isomerisation reaction.

**2. Experimental**

E-Cinnamic acid (E-Cin), Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, Mg(NO₃)₂·6H₂O, methanol, ethanol and NaOH were purchased from Sigma Aldrich and used as received. Z-cinnamic acid (Z-Cin) was prepared in our laboratory starting from the E isomer through the sequence of (2R,3S)-dibromo-3-phenylpropanoic acid3-phenylpropiolic acid Z-cinnamic acid. The last step was a Z-selective partial hydrogenation over Lindlar's palladium catalyst. Details are to be found in Ref. [39].

2.1. *Preparation of cinnamate-intercalated Zn₂Al LDH and Mg₂Al LDH*

E-Cin-intercalated Zn₂Al LDH (Zn₂Al-E-Cin LDH) and Mg₂Al LDH (Mg₂Al-E-Cin LDH) were prepared by the co-precipitation method using (deionised) water-ethanol (1:1) solvent mixture. A mixture containing 0.30 M M(II)-nitrate and 0.15 M Al(III)-nitrate (M(II)/Al(III) molar ratio=2.0) in 25 cm³ solvent mixture was added simultaneously with an appropriate amount of sodium hydroxide solution (0.3 M) to maintain the pH constant (pH= 8.0 for ZnAl LDH and pH=10.5 for MgAl LDH). 0.065 mol of E-cinnamic acid (0.065 mol) (E-Cin/Al(III) molar
ratio=17.33) was initially placed into the reactor. After addition, the suspension was stirred over-night at room temperature under N₂ atmosphere. The obtained slurry was filtered, washed with deionised water and ethanol several times, and dried at 60° C overnight. Z-Cin-intercalated substances (M(II)₂Al-Z-Cin LDH) were synthesized in the same way.

## 2.2. Interlayer photoinduced isomerisation of E-Cin.
The reaction was attempted by irradiating the samples with a mercury or monochromatic UV lamps, working at 365 nm and 254 nm, respectively, at 298 K. Photoreactions were performed on (i) 0.4 g E-Cin-intercalated LDH solid samples, (ii) a slurry (0.4 g of the composite was sus-pended in 250 cm³ of methanol) and (iii) a solution of pure E-Cin (0.1 g dissolved in 250 cm³ methanol), admixed with 0.4 g of pristine MgAl LDH or ZnAl LDH. After conducting the reactions for a given time duration (from 1 to 24 h), the solid components were filtered, washed with methanol several times and dried at room temperature in desiccator filled with P₂O₅.

## 2.3. Instrumental methods of sample characterisation and detecting E-Z isomerisation

The X-ray diffractograms (XRD) of the solid samples were recorded on a Rigaku Miniflex II (Japan) X-ray diffractometer using CuKα radiation (k¼1.5418 Å), with 40 kV accelerating voltage at 30 mA. The FT infrared spectra were registered in diffuse reflection mode (DRS) on a BIO-RAD Digilab Division FTS-65A/896 FT-IR spectrophotometer with 4 cm⁻¹ resolution. The 1850-600 cm⁻¹ wavenumber range was investigated. The isomerisation reaction was followed in the liquid as well as the solid phase by UV-Vis spectroscopy using a Shimadzu UV-1650 spectrophotometer equipped with diffusion reflectance accessory. Solid-state NMR spectra were collected on a Bruker 17.6 T superconducting magnet operating at 13C Larmor frequencies of 188.6 MHz. The experiments were performed at a spinning frequency of 12 kHz with a Bruker double-resonance 4 mm probe head. Cross-polarization (CP) contact times were set to 0.25 ms to favour short CeH distances, with 1024 scans for signal accumulation.

# 3. Results and discussion

## 3.1. Synthesis of cinnamate-intercalated ZnAl LDH or MgAl LDH

The XRD patterns of M(II)₂Al(OH)₆NO₃·nH₂O pristine LDH and their E-Cin- and Z-Cin-intercalated derivatives with M(II)=Mg (A) and Zn (B) are given

in Fig. 1. They orderly displayed the (00l) diffraction line series characteristic of layered structure beside the typical (012) and (110) reflections that account for the structure of the layer. They are all consistent with a rhombohedral unit cell, analogous to that of nitrate-containing hydrotalcite (JCPDS#89e5434). Unit cell parameters and basal spacings are reported in Table S1.

As expected, the intercalation of the E-cinnamate led to an increase of the interlayer distance for the cinnamate-containing derivatives with a similar dᵢnter of 17.7 Å for both Mg₂Al-E-Cin and Zn₂Al-E-Cin, in good agreement with the literature [40,41]. Performing the intercalation with the home-made Z-Cin resulted in materials with larger basal spacings of 21.8 Å and 20.8 Å for the Mg₂Al-Z-Cin LDH and the Zn₂Al-Z-Cin LDH, respectively. Since the dimensions of the Z-Cin ion are 4.98 Å×2.26 Å×7.71 Å by PM3 semi empirical calculations (Fig. S1) and the layer thickness for both LDHs is 4.8 Å, a bilayer arrangement of Z-Cin among the layers seems quite feasible. Similar conclusion can be drawn for E-Cin intercalated in both LDH, with a lower expansion of the interlayer distance due to the structural difference between the two isomers (Fig. S1). It is to be noted that the Z-Cin intercalated LDH structures are novel, since the Z-isomer is not accessible easily. To the best of our knowledge, Z-isomer has not been intercalated into LDH of any kind as yet. The FT-IR spectra of Mg₂Al-E, Z-Cin LDH samples and that of the pristine Mg₂AlNO₃ LDH are displayed in Fig. 2. Based on the assignments reported by Kalinowska et al. for cinnamate alkali salts [42] all vibration bands were clearly identified (Table S2). The usual characteristic bands of LDH structures (d-OH vibration at 1643 cm⁻¹ or Al-O-Al skeletal stretching vibrations at 704 cm⁻¹ [43]) were present. Free and intercalated E- and Z-cinnamate ions displayed a series of vibration bands characteristic of the different groups, i.e. the aryl group with β(CH)ar and v(CH)ar vibrations often combined with other bands, the alkene or cinnamic group with typical β(CH)cin at 1244 cm⁻¹ and the carboxylate function with the characteristic vas(COO⁻) at 1550±12 cm⁻¹ and vs(COO⁻) occurring at 1413 cm⁻¹ and 1394 cm⁻¹ for E-Cin and Mg₂Al-E-Cin, respectively, and 1357 cm⁻¹ and 1363 cm⁻¹ for Z-Cin and Mg₂Al-Z-Cin, respectively. Note that the differences between of asymmetric and symmetric carboxylate vibrations Δ(vas(COO⁻)-vs(COO⁻)) (Table S1, last column) for the free and intercalated cinnamate isomers are noticeably different being 133/143 cm⁻¹ and 205/187 cm⁻¹ for the E and Z
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** X-ray diffractograms of the a) $M(II)_2Al-NO_3LDH$, b) $M(II)_2Al-E-Cin LDH$, c) $M(II)_2Al-Z-Cin LDH$ for A: $M(II)=Mg$ and B: $M(II)=Zn$.</figcaption></figure>

isomers, respectively, due to the different structures of the isomers and thus different interactions with the LDH hydroxylated layers. Therefore, these specific data can be used as probe values for the stereoisomers. These values are characteristic of a symmetric interaction between two adjacent OH layers and the carboxylate groups. The IR spectra of Zn-LDH and its intercalated derivatives displayed very similar vibrations (Fig. S2) due to analogous structures.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** IR spectra of the A: pristine $Mg_2Al-NO_3LDH$, B: sodium E-Cin, C: sodium Z-Cin, D: $Mg_2Al-E-Cin LDH$, E: $Mg_2Al-Z-Cin LDH.</figcaption></figure>

$^{13}$C CP MAS solid-state NMR spectroscopy is an efficient tool to learn about the structure of the

cinnamate salts in the solid state and those intercalated in the LDHs.

The experimental $^{13}$C CP-MAS NMR spectra of sodium salts and LDH-intercalated E,Z-cinnamate are displayed in Fig. 3 with spectral deconvolutions. NMR results confirmed the presence of E-Cin and Z-Cin isomers embedded in both $Zn_2Al$ and $Mg_2Al$ LDH structures as already shown by XRD and FTIR characterizations. Moreover, NMR analysis allowed a clear differentiation of E and Z isomers, particularly in the 130-150 ppm chemical shift range, the $^{13}$C spectra being better resolved for the E isomer in this region. Attempts to resolve the spectra were performed using Lorentzian deconvolutions. Chemical shifts of the different carbon atoms are given in Table 1 and structurally assigned according to Januar et al. [44] and Hanai et al. [45]. From Table 1, main differences between chemical shifts of intercalated and free E- and Z-cinnamates appeared for the carbon atoms of the aromatic cycle. Intercalation of cinnamate anions in the LDH galleries probably forced there orientation of the aryl groups in order to minimize the repulsion energies and optimise the cinnamate packing. This is enhanced particularly for the Z-Cin isomer; when intercalated in LDH, a new peak appeared at 104.1 ppm. One must also notice that the $Zn_2Al-E,Z-Cin$ samples displayed much broader NMR bands than $Mg_2Al-E,Z-Cin$.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Experimental and deconvoluted ¹³C SS-CP MAS NMR spectra of a) E-Cin, b) Zn₂Al-E-Cin, c) Mg₂Al-E-Cin, d) Z-Cin, e) Zn₂Al-Z-Cin, f) Mg₂Al-Z-Cin. (*) traces of cinnamic acid.</figcaption></figure>

## 3.2. *Photoinduced interlayer isomerisation of E-Cin*

To detect and possibly to follow the EeZ isomerisation reaction, there are four markers in our hands: (i) changes in the basal spacing detected by XRD, (ii) the distances between the asymmetric and symmetric infrared vibrations of the carboxylate ions, (iii) the positions of the UV absorbance maxima and (iv) the MAS¹³CP NMR spectra are different for the

**Table 1**

¹³C δ chemical shift extracted from Lorentzian deconvolution of NMR spectra for sodium salts and intercalated E or Z-cinnamate.

two isomers. Irradiation was first performed directly on E-Cin intercalated inMg₂Al and Zn₂Al LDH in the solid state in order to evaluate the ability of the LDH interlayer confined spaces to absorb light and favour an *in situ* reactivity toward isomerisation. No structural change was by powder XRD nor using FT-IR for the intercalated ZnAl LDH sample. XRD patterns for the irradiated solid samples were
<table><thead><tr><th>C atom</th><th>Na-E-Cin</th><th>Zn<sub>2</sub>Al-E-Cin</th><th>Mg<sub>2</sub>Al-E-Cin</th><th>Na-Z-Cin</th><th>Zn<sub>2</sub>Al-Z-Cin</th><th>Mg<sub>2</sub>Al-Z-Cin</th></tr></thead><tbody><tr><td>1</td><td>174.0</td><td>175.2</td><td>175.2</td><td>175.9</td><td>175.2</td><td>175.5</td></tr><tr><td>2</td><td>127.3</td><td>—</td><td>127.4</td><td>126.3</td><td>124.0</td><td>123.4</td></tr><tr><td>3</td><td>140.9</td><td>—</td><td>—</td><td>—</td><td>141.2</td><td>141.4</td></tr><tr><td>1'</td><td>138.6</td><td>138.1</td><td>139.0</td><td>136.9</td><td>133.6</td><td>135.7</td></tr><tr><td>2'/6'</td><td>134.8</td><td>—</td><td>—</td><td>129.2</td><td>129.0</td><td>128.2</td></tr><tr><td>3'/5'</td><td>129.7</td><td>128.2</td><td>129.7</td><td>130.7</td><td>128.0</td><td>128.2</td></tr><tr><td>4'</td><td>136.3</td><td>135.7</td><td>135.9</td><td>132.3</td><td>130.4</td><td>133.7</td></tr></tbody></table>

identical to those of the precursors (Fig. 4Ac and S3Ac). Although the v(v<sub>as</sub>(COO<sup>-</sup>) - v<sub>s</sub>(COO<sup>-1</sup>)) infrared probe remained unchanged for both the free and the intercalated cinnamate (Figs. 4B and S3B), the FT-IR spectrum of the intercalated MgAl LDH sample irradiated in the solid state (Fig. 4Bc) indicates that isomerisation proceeded to some extent (see, the similar profiles for the set of bands in the 1450-1300 cm<sup>-1</sup> range for spectra c and d; in spectrum d a band at 1363 cm<sup>-1</sup> is the sure sign of the appearance of the Z isomer in the irradiated slurry). Combining the results of these two measurement types, it is safe to state that no double bond isomerisation took place in the solid state for the intercalated ZnAl LDH sample; however, it proceeded in the intercalated MgAl LDH sample. The photoreaction was attempted on cinnamate-intercalated samples suspended in methanol and characterization measurements were performed on solid samples extracted from the slurry. As far as the host structure is concerned, no change could be detected in the basal spacing by PXRD, and the powder patterns remained very similar before and after irradiation (compare patents a) and d) in Fig. 4 and Fig. S3), which is not in favour of a structural modification of the intercalated species. However, in the FTIR spectrum recorded after irradiating the E-Cin-intercalated samples in the slurry, the difference

Δ(v<sub>as</sub>(COO<sup>-</sup>) - v<sub>s</sub>(COO<sup>-1</sup>)) became 160 cm<sup>-1</sup> (Table S1, last column), which is a strong indication that E-Z isomerisation did proceed in the slurry of both composites. This indication is further strengthened by the results of UV-Vis measurements of the supernatants. As a reference, UV-Visible spectra of E-Cin and Z-Cin sodium salts in water are used (Fig. 1 in Ref. [36]). The UV-Vis spectra of the supernatants of the mixtures of E-cinnamate and Mg<sub>2</sub>Al LDH or Zn<sub>2</sub>Al LDH confirmed that they only contained one isomer even after lengthy irradiation of the mixtures indicated by the absorption maximum at 269 nm, typical of the E-Cin ion. There was no observable band at 258 nm, which would reveal the presence of the Z isomer (Figs. S4 and S5). However, the characteristic absorption maximum gradually shifted towards lower wavelength on increasing the irradiation time of the slurries containing the intercalated substances (Fig. S6). As is seen in Fig. 5 (and Fig. S6 for Zn<sub>2</sub>Al), the 2 h irradiation treatment was sufficient to achieve appreciable extent of isomerisation indicated by the intense band at 258 nm. The above-described results mean that the observed isomerisation in the slurry of the hybrid Mg<sub>2</sub>Al-E-Cin LDH was not due to Cin adsorbed on the outer surface of the LDH, but it took place in the interlayer region.

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** (A) X-ray diffractograms and (B) FTIR spectra of a) Mg<sub>2</sub>Al-E-Cin, b) Mg<sub>2</sub>Al-Z-Cin, c) Mg<sub>2</sub>Al-E-Cin (solid state) irradiated at 254 nm for 2 h, d) Mg<sub>2</sub>Al-E-Cin (slurrye suspended in methanol) irradiated at 254 nm for 2 h.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** The deconvoluted UV-Vis absorbance spectra of the supernatant of the Mg₂Al-E-Cin LDH slurry, A : before the irradiation (there is one component only), B: after irradiation for 2 h (could be decomposed to two components).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** ¹³C CP MAS NMR spectra of the a) Mg₂Al-E-Cin (in slurry -suspended in methanol) irradiated on 254 nm for 2 h, b) Mg₂Al-Z-Cin and c) Mg₂Al-E-Cin.</figcaption></figure>

¹³C CP MAS solid-state NMR can also be used as local probe for detecting and, at least semi-quantitatively, following the E-Z isomerisation reaction.

¹³C CP MAS solid-state NMR measurements clearly attested the E-Z isomerisation did partially occur in the slurry (Fig. 6), since after the irradiation, the peaks characteristic of Z-Cin appeared for the irradiated sample at 128.1, 133.5 and 140.0 ppm.

Although the partial photoinduced isomerisation of MII₂Al-E-Cin into MII₂Al-Z-Cin was accompanied by basal spacing expansions; however, they did not reach the basal spacings of the Z-Cin intercalated materials. We suggest that in the slurry, the interlayer water content is gradually replaced by methanol, *i.e.* the interlayer environment is changing. Because of this and the partial and gradual formation of the intercalated Z isomer in presence of the unreacted

intercalated E isomer, a different arrangement was adopted compared to that of the pure MII₂Al-Z-Cin. Similar behaviour toward Cin photoisomerisation was observed for both LDH hybrid matrices, which seems to indicate that the chemical composition of the LDH layer did not play key role in the photo-induced process.

**4. Conclusions**

Hybrid Mg₂Al and Zn₂Al intercalated by E and Z cinnamate (Cin) were obtained by direct co-precipitation. Besides the already described LDH samples intercalated by E-Cin, two novel pure Z-Cin-intercalated LDH derivatives were prepared, and structurally characterized by a range of techniques by various spectroscopic methods and powder X-ray diffractometry. Two distinct interlayer distances were obtained in this work by modifying the nature of the Cin isomer (E or Z) involved in the process. LDH intercalated by Z-Cin led to a larger interlamellar distance compared to intercalated E-Cin in good agreement with a bilayer arrangement of the organic anions in between the Mg₂Al or Zn₂Al layers. The characterization of the hybrid phases by FTIR, UV-Vis and solid-state NMR spectroscopies allowed to highlight the main differences between the two intercalated isomers, while the nature of the LDH layer did not influence the orientation of the intercalated Cin anions. Starting from LDH intercalated with the E-Cin isomer, partial isomerisation of the organic species by UV light irradiation was evidenced. There action took place in the interlayer region, but mainly when a slurry phase was used, in which there was possibility of interlayer solvent exchange in favour of molecular rearrangement.

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2020.128561.

**References**

[1] K. Renggli, P. Baumann, K. Langowska, O. Onaca, N. Bruns, W. Meier, Selective and responsive nanoreactors, Adv. Funct. Mater. 21 (2011) 1241-1259.

[2] J. Liu, S.Z. Qiao, J.S. Chen, X.W. Lou, X.R. Xing, G.Q. Lu, Yolk/shell nanoparticle s:new platforms for nanoreactors, drug delivery and lithium-ion batteries, Chem. Commun. 47 (2011) 12578-12591.

[3] A.N. Khlobystov, Carbon nanotubes: from nano test tube to nano-reactor, ACS Nano 5 (2011) 9306*9312.
[4] T.H. Tran-Thi, R. Dagnelie, S. Crunaire, L. Nicole,[15] M. Wei, J. Guo, Z. Shi, Q. Yuan, M. Pu, G. Rao, Optical chemical sensors based on hybrid organic-inorganic solegel nanoreactors, Chem. Soc. Rev. 40(2011) 621-639.

[5] I. Palinko, Z. Konya, Kukovecz A, I. Kiricsi,[16] M. Wei, X. Tian, J. He, M. Pu, G. Rao, H. Yang, Zeolites, in: R. Vajtai (Ed.), Springer Handbook of Nanomaterials, Springer-Verlag, Berlin, Heidelberg, 2013, pp. 819-857. Ch. 22.

[6] L. Song, W. Shi, C. Lu, Confinement effect in layered double hydroxide nano-reactor: improved optical sensing selectivity, Anal. Chem. 88 (2016)8188-8193.

[7] M.Q. Zhao, Q. Zhang, J.Q. Huang, F. Wei, Hierarchical nanocomposites derived from nanocarbons and layered double hydroxides properties, synthesis, and applications, Adv. Funct. Mater. 22 (2012) 675-694.

[8] M. Wei, Z.Y. Shi, D.G. Evans, X. Duan, Study on the intercalation and interlayer oxidation transformation of L-cysteine in a confined region of layered double hydroxides, J. Mater. Chem. 16 (2006) 2102-2109.

[9] W.Y. Shi, M. Wei, D.G. Evans, X. Duan, Tunable photoluminescence properties of fluorescein in a layered double hydroxide matrix and its application in sensors, J. Mater. Chem. 20 (2010) 3901e3909.

[10] Y.B. Song, S.J. Zhu, S.Y. Xiang, X.H. Zhao, J.H. Zhang, H. Zhang, Y. Fu, B. Yang, Tunable photoluminescence properties of fluorescein in a layered double hydroxide matrix and its application in sensors, Nanoscale 6 (2014)4676-4682.

[11] J. Sun, H. Liu, X. Chen, D.G. Evans, W. Yang, X. Duan, Synthesis of graphene nanosheets with good control over the number of layers within the two-dimensional galleries of layered double hydroxides, Chem. Commun. 48(2012) 8126-8128.

[12] A.L. García-Ponce, V. Prevot, B. Casal, E. Ruiz-Hitzky, Intracrystalline reactivity of layered double hydroxides: carboxylate alkylations in dry media, New J. Chem. 24 (2000) 119e121.

[13] V. Prevot, B. Casal, E. Ruiz-Hitzky, Intracrystalline alkylation of benzoate ions into layered double hydroxides, J. Mater. Chem. 11 (2001) 554-560.

[14] D.F. Sranko, S. Canton, A. Enghdahl, Sz Murath, A. Kukovecz, Z. Knya, M. Sipiczki, P. Sipos, I. Palinko, Radiation induced topotactic [2+2] dimerisation of acrylate derivatives among the layers of a CaFe layered double hydroxide followed by IR spectroscopy, J. Mol. Struct. 1044 (2013) 279-285.

[15] M. Wei, X. Tian, J. He, M. Pu, G. Rao, H. Yang, L. Yang, T. Liu, E.G. Evans, X. Duan, Study of the in situ post intercalative polymerization of metanilic anions intercalated in NiAl-layered double hydroxides under a nitrogen atmosphere, Eur. J. Inorg. Chem. (2006) 3442-3450.

[17] F. Leroux, C. Taviot-Gueho, Fine tuning between organic and inorganic hoststructure: new trends in layered double hydroxide hybrid assemblies, J. Mater. Chem. 15 (2005) 3628e3642.

[18] G. Hincapie, D. Lopez, A. Moreno, Infrared analysis of methanol adsorption on mixed oxides derived from Mg/Al hydrotalcite catalysts for transesterification reactions, Catal. Today 302 (2018) 277e285.

[19] C. Yu, J. He, Synergic catalytic effects in confined spaces, Chem. Commun. 48(2012) 4933-4940.

[20] D. Dutta, A.K. Tummanapelli, Spectroscopic and computational investigations on the origin of charge transfer between included neutral guest molecules and a functionalized anionic layered host, Phys. Chem. Chem. Phys. 18 (2016)22379-22389.

[21] H.C. Greenwell, P.V. Coveney, Layered double hydroxide minerals as possible prebiotic information storage and transfer compounds Orig, Life Evol. Biosph. 36 (2006) 13-37.

[22] V. Erastova, M.T. Degiacomi, D.G. Fraser, H.C. Greenwell, Mineral surfacechemistry control for origin of prebiotic peptides, Nat. Commun. 8 (2017)1-9, 2033.

[23] P.V. Coveney, J.B. Swadling, J.A.D. Wattis, H.C. Greenwell, Theory, modelling and simulation in origins of life studies, Chem. Soc. Rev. 41 (2012)5430-5446.

[24] C. Taviot-Gueho, C. Forano, C. Mousty, V. Prevot, G. Renaudin, F. Leroux, Tailoring materials properties of hybrid LDH for the development of innovative applications, Adv. Funct. Mater. 28 (2018) 1-33, 1703868.

[25] M. Wei, X. Zhang, D.G. Evans, X. Duan, X. Li, H. Chen, Rh-TPPTS intercalated layered double hydroxides as hydroformylation catalyst, AIChE J. 53 (2007)2916e2924.

[26] S. Nakagaki, K.A.D.F. Castro, G.M.U. Halma, V. Prevot, C. Forano, F. Wypych, Anionic iron(III) porphyrin immobilized on/into exfoliated macroporous layered double hydroxides as
catalyst for oxidation reactions, J. Braz. Chem.Soc. 25 (2014) 2329-2338.

[27] G. Varga, V. Kozma, V.J. Kolcsar, A. Kukovecz, Z. Konya, P. Sipos, I. Palinko, Gy Szollosi, β-Isocupreidinate-CaAl-layered double hydroxide composites-heterogenized catalysts for asymmetric Michael addition, Mol. Catal. 482(2020) 1-7, 110675.

[28] S.J. Meek, R.V. O'Brien, J. Llaveria, R.R. Schrock, A.H. Hoveyda, Catalytic Z-se-lective olefin cross-metathesis for natural product synthesis, Nature 471(2011) 461-466.

[29] H. Li, H. Chen, Y. Zhou, J. Huang, J. Yi, H. Zhao, W. Wang, L. Jing, Selectivesynthesis of Z-cinnamyl ethers and cinnamyl alcohols via visible light promoted photocatalytic E to Z isomerization, Chem.-An Asian J. 15 (2020) 555-559.

[30] H. Tagaya, T. Kuwahara, S. Sato, J. Kadokawa, M. Karasu, K. Chiba, Photoisomerization of indoline spirobenzopyran in layered double hydroxides, J. Mater. Chem. 3 (1993) 317-318.

[31] H. Tagaya, S. Sato, T. Kuwahara, J. Kadokawa, K. Masa, K. Chiba, Photo-isomerization of indoline spirobenzopyran in anionic clay matrixes of layered double hydroxides, J. Mater. Chem. 4 (1994) 1907-1912.

[32] J. Valim, B.M. Kariuki, J. King, W. Jones, Photoactivity of cinnamate-intercalates of layered double hydroxides, Mol. Cryst. Liq. Cryst. Sci. Tech. A211 (1992) 271-281.

[33] G. Abellan, E. Coronado, C. Marti-Gastaldo, A. Ribera, J.L. Jorda, H. Garcia, Photo-switching in a hybrid material made of magnetic layered double hydroxides intercalated with azobenzene molecules, Adv. Mater. 26 (2014) 4156-4162.

[34] J. Han, D. Yan, W. Shi, J. Ma, H. Yan, M. Wei, D.G. Evans, X. Duan, Layer-by-layer ultrathinfilms of azobenzene-containing polymer /layered double hydroxides with reversible photoresponsive behavior, J. Phys. Chem. B 114(2010) 5678-5685.

[35] M. D'Auria, G. Piancatelli, A. Vantaggi, Photochemical dimerization of methyl 2-furyl- and 2-thienylacrylate and related compounds in solution, J. Chem.Soc. Perkin 1 (1990) 2999-3002.

[36] J. Li, M. Zhao, H. Zhou, H. Gao, L. Zheng, Photo-induced transformation of wormlike

micelles to spherical micelles in aqueous solution, Soft Matter 8(2012) 7858-7864.

[37] M.L. Salum, P.A. Manez, F.J. Luque, R. Erra-Balsells, Combined experimental and computational investigation of the absorption spectra of E- and Z-cinnamic acids in solution: the peculiarity of Z-cinnamics, J. Photochem. Photo-biol., B 148 (2015) 128-135.

[38] Y. Li, L. Tang, X. Ma, X. Wang, W. Zhou, D. Bai, Synthesis and characterization of Zn-Ti layered double hydroxide intercalated with cinnamic acid forcosmetic application, J. Phys. Chem. Solid. 107 (2017) 62-67.

[39] K. Csanko, PhD Dissertation University of Szeged, Ch. 4.5 General Methods for the Stereoselective Synthesis of Z-Cinnamic Acids, 2015, pp. 36e38 (down-loadable from, http://doktori.bibl.u-szeged.hu/2538/1/Csanko_Krisztian_PhD_Dissertation.pdf).

[40] Y. Kameshima, A. Nakada, T. Isobe, A. Nakajima, K. Okada, The effect of UVradiation on cinnamate/layered double hydroxide (LDH) composites, J. Ceram.Soc. Jpn. 121 (2013) 303-307.

[41] W. Sun, Q. He, L. Lu, H. Liu, Synthesis and properties of layered double hydroxides intercalated with cinnamic acid series organic UV ray absorbents, Mater. Chem. Phys. 107 (2008) 261-265.

[42] M. Kalinowska, R. Swisłocka, W. Lewandowski, The spectroscopic (FT-IR, FT-Raman and $^{1}$H, $^{13}$C NMR) and theoretical studies of cinnamic acid and alkali metal cinnamates, J. Mol. Struct. 834 (2007) 572-580.

[43] V. Rives, S. Kannan, Layered double hydroxides with the hydrotalcite-type structure containing $Cu^{2+}$, $Ni^{2+}$ and $Al^{3+}$, J. Mater. Chem. 10 (2000) 489e495.

[44] S.E. Januar, P. Sugita, B. Arifin, Identification of trans-cinnamic acid in Sinyo Nakal (Duranta repens) fruits'methanol extract, Int. Res. J. Pure Appl. Chem. 8(2015) 73-80.

[45] K. Hanai, A. Kuwae, T. Takai, H. Senda, K.-K. Kunimoto, A comparative vibrational and NMR study of cis-cinnamic acid polymorphs and trans-cinnamic acid, Spectrochim. Acta, A 57 (2001) 513-519.