Thermally Triggered Isomerization in a Naphthalene-Based Acylhydrazone with Solid-State Optical Nonlinearity Response

Article abstract of DOI:10.1002/ejic.202000767

Under the assistance of the state-of-the-art meta-dynamics simulations, in this contribution, we report a push-pull-type acylhydrazone 4(Z) ([Z)-2-(naphthalen-2-yloxy)-N'-(pyridine-2-ylmethylene)acetohydrazide] connecting an electron-donating naphthalene ether group with an accepting pyridyl unit, which could be thermally converted to its configurational isomer 4(E) ([E)-2-(naphthalen-2-yloxy)-N'-(pyridine-2-ylmethylene)acetohydrazide]. The thermally triggered Z-to-E configurational isomerization not only involves up to five chemical bonds undergoing 180° rotation in the backbone of acylhydrazone, but changes the crystal packing symmetry from a centrosymmetric space group to a non-centrosymmetric one. Most impressively, the acentric crystal arrangement of 4(E) exhibits a second-harmonic generation (SHG) active response, about 2.5 times than that of KH₂PO₄ (KDP) standard.

Full text of DOI:10.1002/ejic.202000767

Communication
doi.org/10.1002/ejic.202000767
EurJIC
European Journal of Inorganic Chemistry
Nonlinear Optical Acylhydrazone
Thermally Triggered Isomerization in a Naphthalene-Based
Acylhydrazone with Solid-State Optical Nonlinearity Response
[
a,d][‡]
[a][‡]
[a]
[b]
[c]
Miaoling Huang,
Rongxing Qiu,
Zhonghua Pan, Dan Tian, Yunwen Tao,*
[
a]
[a,e]
Jinqing Lin,* and Geng-Geng Luo*
Abstract: Under the assistance of the state-of-the-art meta- figurational isomerization not only involves up to five chemical
dynamics simulations, in this contribution, we report a push- bonds undergoing 180° rotation in the backbone of acyl-
pull-type acylhydrazone 4(Z) ([Z)-2-(naphthalen-2-yloxy)-N′- hydrazone, but changes the crystal packing symmetry from a
(
pyridine-2-ylmethylene)acetohydrazide] connecting an elec- centrosymmetric space group to a non-centrosymmetric one.
tron-donating naphthalene ether group with an accepting pyr- Most impressively, the acentric crystal arrangement of 4(E) ex-
idyl unit, which could be thermally converted to its configura- hibits a second-harmonic generation (SHG) active response,
tional isomer 4(E) ([E)-2-(naphthalen-2-yloxy)-N′-(pyridine-2-yl- about 2.5 times than that of KH PO (KDP) standard.
2
4
methylene)acetohydrazide]. The thermally triggered Z-to-E con-
Nonlinear optical (NLO) materials play important roles in constituent functional molecules from a centrosymmetric pack-
SHG, optical telecommunication and optical signal process- ing to an acentric one with apparent NLO response change. In
[
1,2]
ing.
Generally, NLO crystalline materials include inorganic, this regard, inorganic and organic-inorganic hybrid NLO crystal-
organic and organic-inorganic hybrid solids. Over the last dec- line materials have been reported to activate SHG active re-
ades, intensive attentions have been paid to searching for new sponse change by thermal energy.[5–8] By comparison, organic
crystalline NLO materials that exhibit efficient second-order NLO crystals with concomitant solid-state second-order NLO
[
3,4]
NLO activities.
Recently, controllable NLO crystals with ap- properties change by thermal stimuli are rarely reported possi-
parent SHG response change between different states by exter- bly due to their low melting point, low thermal stability and
nal stimuli (heat, pressure, pH etc.), have become another excit- low laser-induced damage threshold.
[
5–8]
ing new branch of NLO crystalline materials.
Developing a
In the quest for new and efficient organic NLO crystals with
promising approach for solid-state NLO transformation strongly SHG response change, we focus our attention on the hydraz-
depends on the relative arrangement and orientation of the ones, which have had an important place in chemistry for a
[
9]
long time. Hydrazone-based compounds can undergo light
or thermal E/Z isomerization around the imine C=N bond. As a
subclass of the hydrazone, acylhydrazone compounds exhibit
many advantages including easy crystallization, simple synthe-
sis and high modifiability. Most studies on the isomerization of
acylhydrazones are focused on the solution, but rarely per-
formed in the solid state. We anticipated that the E/Z isomer-
ization of acylhydrazones in the crystalline state by thermal
stimuli can form different intra- and intermolecular interactions,
thus enabling a favorable acentric arrangement in the molec-
ular crystal and large difference in the SHG efficiency.
[
a] M. Huang, R. Qiu, Z. Pan, Prof. Dr. J. Lin, Prof. Dr. G.-G. Luo
Key Laboratory of Environmental Friendly Function Materials,
Ministry of Education, College of Materials Science and Engineering,
Huaqiao University,
Xiamen 361021, P.R. China
E-mail: linlab@hqu.edu.cn
ggluo@hqu.edu.cn
b] Prof. Dr. D. Tian
[
10]
[
College of Materials Science and Engineering, Co-Innovation Center of
Efficient Processing and Utilization of Forest Resources, Nanjing Forestry
University,
Nanjing 210037, P.R. China
[
[
c] Dr. Y. Tao
On the other hand, meta-dynamics simulations based on the
recently developed tight-binding quantum chemical calcula-
tions represents a powerful approach to comprehensively ex-
plore molecular conformational flexibility, which can comple-
Department of Chemistry, Southern Methodist University,
3
215 Daniel Avenue, Dallas, Texas 75275-0314, United States
E-mail: ywtao.smu@gmail.com
d] M. Huang
College of Chemical Engineering and Materials, Quanzhou Normal
University,
[
11,12]
ment experimental studies.
However, to the best of our
knowledge, no attempts using these inspiring simulations have
been made to accelerate discovery of acylhydrazone isomers.
The synthesis of the naphthalene-based acylhydrazone 4(Z)
was accomplished in a straightforward manner, which involves
the condensation coupling reaction between hydrazide 3 (Fig-
ure S1) and 2-pyridylcarbaldehyde. Compound 4(Z) was isolated
Quanzhou 362000, P.R. China
e] Prof. Dr. G.-G. Luo
[
[
Instrumental Analysis Center, Huaoqiao University,
Xiamen 361021, P.R. China
‡] These authors contributed equally to this work.
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000767.
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as needle-like white crystals, which were fully characterized by ric alignment is not a prerequisite for second-order NLO effects.
NMR spectroscopy, mass spectrometry and single-crystal X-ray Given that non-rod-shaped molecular crystals have been shown
[
16]
crystallography. The FT-IR spectrum of 4(Z) shows stretching to have high tendency to pack non-centrosymmetrically,
–1
modes at 1643–1597 and 1221–1184 cm that are characteris- Therefore, it is reasonable for us to explore other configura-
tics of νC=N moiety. The H NMR spectrum of 4(Z) in CDCl on tional isomers of 4(Z), aiming to obtain non-centrosymmetric
1
3
a Bruker Advance III 500 MHz spectrometer reveals one set of molecular alignment in their crystalline states. Careful analysis
signals, having a characteristic low-field shifted hydrazone N–H of the 4(Z)'s structure reveals that it has a series of rotatable
proton resonance at 15.4 ppm. The signal (δ = 15.4) is assigned covalent bonds in its backbone, including C–O, C–C, C–N, N–N,
to the strong intramolecular hydrogen bond between the C=N and C–C bonds from naphthalene moiety to pyridyl unit
hydrazone proton and the pyridinyl nitrogen and naphthalene consecutively (Scheme 1 and Figure S4). It is known that the
ether oxygen, implying that 4(Z) adopts the Z configuration in configurational changes involving more rotatable chemical
[
10,14]
solution.
The UV/Vis absorption spectroscopy (Figure S2) bonds will be more difficult than the changes with fewer rotata-
of 4(Z) in CHCl shows a series of absorption bands centered at ble bonds. We wonder whether it is possible to achieve the
3
λmax = 262, 285, 305 and 320 nm (shoulder). A simulated elec- configurational changes with all rotatable bonds simultane-
tronic absorption spectrum (Figure S2) of 4(Z) based on TD- ously for 4(Z) under thermal stimuli, namely all colored chemi-
DFT calculations displays general agreement with experimental cal bonds in 4(Z) (Scheme 1 and Figure S4) are expected to
data, further confirming the Z configuration of 4(Z).
undergo substantial 180° rotations, leading to the non-rod-
The Z configuration of 4(Z) is unambiguously evidenced by shaped molecule of 4(E).
single-crystal X-ray diffraction (SCXRD), which reveals that 4(Z)
crystallizes in the Pbca centrosymmetric group with one acyl-
hydrazone molecule co-crystallized with one water in its asym-
metric structure unit. As displayed in Figure 1, structural analy-
sis shows the 4(Z) has an almost planar geometry, as displayed
by the dihedral angle of 0.76(4)° between the acylhydrazone
plane [–C(=O)–NH–N=] and the pyridine plane and that be-
tween the –C(=O)–NH–N= plane and the naphthalene plane.
Alternatively, 4(Z) can be viewed as a tridentate NNO ligand
that accommodates a hydrogen atom. There is an intramolec-
ular hydrogen bond [N–H···N, 2.682(2) Å, 134.5(1)°] between the
pyridyl nitrogen (N3) and the acylhydrazone NH proton, leading
to the formation of a six-member ring. The naphthalene ether
oxygen atom O1 also forms the other five-membered intra-
molecular hydrogen-bonding interaction [N–H···O, 2.585(2) Å,
Scheme 1. Thermal-induced Z → E configurational isomerization in a naphth-
alene-based acylhydrazone of 4(Z) involved up to five chemical bonds under-
going 180 ° rotation.
1
07.8(1)°] with the acylhydrazone proton. Obviously, the pres-
Therefore, we firstly utilize the root-mean-square-deviation
RMSD)-based meta-dynamics simulations as a valuable tool in
ence of the two strong intramolecular hydrogen bonds would
be helpful to lock and stabilize 4(Z) in the Z configuration.
Neighboring 4(Z) molecules are linked by water through abun-
dant intermolecular O–H···O, O–H···N and C–H···O hydrogen
bonds into a supramolecular structure (Figure S3).
(
[
15]
accelerating conformers discovery. As portrayed in Figure 2, the
[
10,11]
newly developed meta-dynamics algorithm
which is simi-
lar to previously known simulated annealing, proves to be suc-
cessful in conformational searching of acylhydrazone com-
pounds: for an efficient conformational sampling using 4(Z) as
Figure 1. ORTEP drawings of 4(Z) with 50 % probability ellipsoids. H atoms
are placed in calculated positions, except for that of the acylhydrazone N1
atom, which were refined. The water molecule of 4(Z) is not shown for clarity.
The original purpose of designing the push-pull type N-acyl-
hydrazone connecting an electron-donating naphthalene ether
group with an accepting pyridyl moiety is to search for possible
NLO molecular crystals. Unfortunately, the 4(Z)'s centrosymmet-
Figure 2. Profile of RMSD (in units Å) for meta-dynamics simulations at
GFN2-xTB level for the push-pull type hydrazone isomer. The 4(Z) isomer was
taken as the starting structure (marked in red) while the 4(E) isomer was
th
found with lowest RMSD at 5759 step (marked with blue dot).
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molecular starting structure, 6000 steps of molecular dynamics
were performed in the xtb 6.2 program, and E configuration
4
(E) (with respect to the C=N double bond) with the lowest
th
RMSD value was generated at 5759 step during molecular dy-
namics simulations. As a result, we found that under thermal
stimuli, C–C, C–N, N–N, C=N and C–C bonds from naphthalene
moiety to pyridyl unit consecutively in 4(Z) were rotated by
180° while the C–O bond was rotated by 90° (Figure S4), form-
ing the isomer 4(E).
Encouraged by the above promising simulated results, we
set out to reproduce a process of making 4(E). Experimentally,
a heating of 4(Z) crystals up to 170 °C and subsequent cooling
to room temperature (r.t.) lead to a Z-to-E isomerization to 4(E),
as evidenced by X-ray diffraction (XRD) measurements (Fig-
ure 3). Before heating, the XRD pattern of the sample 4(Z) (Fig-
ure 3b) was consistent with that simulated one from the result
of SCXRD analysis (Figure 3a). After heating at 170 °C and cool-
ing to r.t., the XRD pattern was changed and a new set of peaks
were observed (Figure 3c) which were well consistent with
[
17]
those for 4(E) (Figure 3d), indicating the formation of 4(E).
Obtained 4(E) crystal has block morphology compared to 4(Z)
needle-like shape). SCXRD analysis reveals that 4(E) crystal ex-
(
hibits a non-centrosymmetric packing with a monoclinic space
Figure 4. (a) ORTEP drawings of 4(E) with 50 % probability ellipsoids. H atoms
are placed in calculated positions, except for that of the hydrazone N1 atom,
which were refined. (b) Crystal packing for 4(E) showing Hirshfeld surfaces of
the central molecules mapped with dnorm. The most significant intermolecular
interactions are N–H···N hydrogen bonds (①). (c) 4(E) molecules build a
one-dimensional lambda-shaped arrangement along b-axis through strong
N–H···N hydrogen bond interactions.
group symmetry P2 . The structure of 4(E) resembles the pre-
1
dicted one by meta-dynamics/GFN2-xTB simulations. In the
structure of 4(E), the acylhydrazone plane and the electron-
accepting pyridyl group show a good co-planarity (Figure 4a),
while the electron-donating naphthalene ether unit is nearly
orthogonal to the hydrazone plane with a dihedral angle of
7
9.64(9)°. From a comparison with 4(Z), it is clear that no intra-
white-blue color mapping of the surface (red, shorter contacts;
white, contacts around the van der Waals separation; blue,
longer contacts). As illustrated in the dnorm-mapped Hirshfeld
surfaces (Figure 4b), the molecular surfaces suggest two signifi-
cant N–H···N intermolecular hydrogen bonds exist in the 4(E)
crystal. The H-bonding sites include one site corresponding to
the hydrogen atom on the NH group in hydrazone that acts as
the hydrogen-bond donor, and the other site nitrogen atom
corresponding to the nitrogen atom (N3) on the electron ac-
ceptor group that serves as the hydrogen-bond acceptor. Im-
pressively, with the help of the two strong N–H···N hydrogen
bonds, 4(E) molecules build a 1D lambda-shaped arrangement
along the principal crystallographic polar b-axis (Figure 4c). In
addition, these lambda-shaped molecular assemblies stack up
through C–H/π interactions (centroid-naphthalene hydrogen
distance of 3.143 Å) to form a 2D layer (Figure S5).
molecular H-bonds formed in 4(E). Most importantly, five chem-
ical bonds including C–C, C–N, N–N, C=N and C–C bond in the
configuration of 4(E) undergo 180° rotation (Scheme 1). By
comparison, photoisomerization of hydrazone in solid state
usually rotates only one to two chemical bonds by 180° rota-
[
13]
tion.
The thermal stabilities of the 4(Z) and 4(E) crystals were in-
vestigated by using thermogravimetric analysis combined with
differential scanning calorimetry (TGA-DSC) on a Mettler Toledo
–
1
Figure 3. XRD patterns of (a) single-crystal simulated 4(Z), (b) measured 4(Z)
crystalline powder, (c) measured 4(E) crystalline powder which is prepared
by heat treatment of 4(Z) at 170 °C and cooling to r.t., (d) single-crystal simu-
lated 4(E), and (e) measured 4(E) crystalline powder after exposure to water
vapor.
TGA/DSC3+ instrument at a heating rate of 10 °C min (Figure
S6a–b). The thermo-diagram in TGA-DSC shows that 4(Z) is sta-
ble up to 90 °C (Figure S6a), beyond which it loses a water
molecule with the low melting temperature T = 90 °C. By com-
m
parison, 4(E) exhibits relatively moderate melting point Tm
=
Hirshfeld surface calculations[18] provide additional insights 170 °C (Figure S6b) beyond which it starts melting with no de-
into the intermolecular interactions in the crystal packing of composition. The initial weight loss temperature T of 4(E) ap-
i
4
(E). For the visualization we use a mapping of the normalized pears at 280 °C. Thus, 4(E) crystals clearly exhibits high thermal
contact distance, dnorm. The parameter dnorm defines a red- stability.
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Naturally, acentric orientation of 4(E) in the crystalline lattice tude greater than that of KDP standard in SHG signal, and high
pushes us to study its solid-state SHG property by using the thermal stability. To the best of our knowledge, this is a rare
Kurtz–Perry powder technique. Approximate estimations are example of thermally induced configurational isomerization
carried out on a pulse Q-switched Nd:YAG laser (1.064 μm). The and crystal transition with a concomitant SHG property change
well-known optically nonlinear KDP crystal is used as standard. (off → on) observed in N-acylhydrazone-based molecular crys-
As observed in Figure 5, SHG measurements indicate that the tals.
efficiency of 4(E) is roughly 2.5 times that of KH PO (KDP) refer-
ence in the 100–200 μm particle size range. The SHG efficiency
2
4
Deposition Numbers 2002884–2002885 contain the supplementary
crystallographic data for this paper. These data are provided free of
of 4(E) can be compared with the previously reported values of charge by the joint Cambridge Crystallographic Data Centre and
[
3]
some known organic NLO crystals. As expected, centrosym- Fachinformationszentrum Karlsruhe Access Structures service
metric alignment of 4(Z) molecules within a crystal causes the www.ccdc.cam.ac.uk/structures.
second order NLO response to cancel to zero. Furthermore, no Supporting Information (see footnote on the first page of this
damage was observed during the entire SHG experimental article): Based on the above considerations, in this work, we design
process, indicating enough damage threshold of the organic a push-pull type acylhydrazone 4(Z) [(Z)-2-(naphthalen-2-yloxy)-N′-
(
pyridine-2-ylmethylene) acetohydrazide] consisting of a N-acylhy-
4
(E) crystal.
drazone bridge linked between a naphthalene ether electron donor
and an ortho-pyridine acceptor. Under the guidance of meta-
dynamics simulations at GFN2-xTB level, the molecular crystal of
4(Z) could be converted to its configuration isomer, 4(E) [(E)-2-
(naphthalen-2-yloxy)-N′-(pyridine-2-ylmethylene) acetohydrazide]
by heat stimuli. Compared with its isomer 4(Z), 4(E) with up to five
chemical bonds undergoing 180° rotation is observed (Scheme 1).
Usually, isomerization of acylhydrazone in the solid state by light
activation proceeds via one to two chemical bonds by 180° rota-
[
13]
tion.
Additionally, the Z-to-E isomerization spontaneously
changes the crystal packing symmetry from a centrosymmetric
space group Pbca in 4(Z) to an acentric P21 in 4(E). An apparent
alternation of SHG response was also observed in the acentric 4(E).
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (21641011 and 21971113). The au-
thors thank Instrumental Analysis Center of Huaqiao University
for the support of test analysis.
Figure 5. Comparison on SHG intensities for 4(E), KDP and 4(E) after exposure
to water vapor.
It has been reported that the state-of-the-art organic NLO Keywords: Acyhydrazone · Configurational isomerization ·
crystals such as N,N-dimethylamino-N′-methylstilbazolium p-tol- Optical nonlinearity response · meta-Dynamics simulations
uenesulfonate and its derivatives easily absorb moisture to form
centrosymmetric hydrates, which leads to second-order NLO in-
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