Synthesis, photochemical isomerization and photophysical properties of hydrazide-hydrazone derivatives

Article abstract of DOI:10.1039/d0nj05172a

Hydrazide-hydrazone derivatives have been utilized in molecular switches, sensing, metallo assemblies, drugs and other applications. The hydrazide-hydrazone functional group contains the -CO-NH-NCH- moiety that yields the hydrazide-hydrazone group its physical, chemical and biochemical properties; however, the application in security protection has not been explored before. Herein, the 2-(ortho-), 3-(meta-), 4-(para-) substituted pyridine conjugated to 2,6-dihydroxybenzoic acid, which containing hydrazide-hydrazone as photoswitch linker were designed and synthesized (3a-3c). Interestingly, when irradiating the ortho-substituted pyridine of molecule 3a with a light of 365 nm, obvious emission intensity enhancements were detected. However, 3a-3c before irradiation and 3b-3c after irradiation show no fluorescence. UV-vis, fluorescence, 1H NMR and Fourier transform infrared spectra and density functional theory calculations were carried out, which revealed that the hydrazide-hydrazone derivative of 3a could undergo a characteristic E to Z photoisomerization after light irradiation for 365 nm. The Z isomer of 3a may have additional intramolecular hydrogen bonds to restrict the motions of the molecule, thus increasing the fluorescence intensity of 3a. The experimental and molecular calculation results agreed and thus demonstrated the first example of invisible ink based on the hydrazide-hydrazone motif.

Full text of DOI:10.1039/d0nj05172a

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Synthesis, Photochemical Isomerization and Photophysical
Properties of Hydrazide-Hydrazone Derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Bao-Xing Wu, Hsin-Yueh Chang, Yi-Shun Liao and Mei-Yu Yeh*
DOI: 10.1039/x0xx00000x
The hydrazide-hydrazone derivatives have been utilized in molecular switches, sensing, metallo-assemblies, drug as well as
other applications. The hydrazide-hydrazone functional group contains the -CO-NH-N=CH- moiety that gives the hydrazide-
hydrazone group its physical, chemical and biochemical properties, however, the application in security protection have
not been explored before. Herein, the ortho-, meta-, para-substituted pyridine conjugated to 2,6-dihydroxybenzoic acid,
which containing hydrazide-hydrazone as photoswitch linker were designed and synthesized (3a-3c). Interestingly, when
irradiation the ortho-substituted pyridine of molecule 3a with 365 nm light, obvious emission intensity enhancements
were detected. In contrast, 3a-3c before irradiation and 3b-3c after irradiation show no fluorescence. UV-Vis, fluorescence,
1H NMR and Fourier transform infrared spectra as well as density functional theory calculations were carried out, which
revealed that the hydrazide-hydrazone derivative of 3a could undergo a characteristic E to Z photoisomerization after 365
nm light irradiation. The Z isomer of 3a may have additional intramolecular hydrogen bond to restrict the motions of
molecule, thus increasing the fluorescence intensity of 3a. The experimental and molecular calculation results agreed well
and thus demonstrated the first example of invisible ink based on the hydrazide-hydrazone motif.
magnetic field etc., have been investigated widely and also
have proven potential applications in security protection.22-28
These smart materials can alteration their output response
through external stimulation, making them possible to prevent
the information being forged. Wang and Zheng have
developed a new type of hydrochromic molecules as a double-
encryption of invisible inks.24 Kalita and coworkers reported a
rewritable fluorescent ink and confirm its practical efficacy on
various substrates by using acid/base vapors.25 Ray et al.
designed white light-emitting materials that show afterglow
1. Introduction
The hydrazide-hydrazone derivatives, which play an important
role in organic synthesis, supramolecular chemistry and
medicine chemistry, have been utilized in molecular switches,
sensing, metallo-assemblies, drug as well as other
applications.1-9 The hydrazide-hydrazone functional group
contains the -CO-NH-N=CH- moiety that has (i) the imine (C=N)
double bond to exhibit E/Z isomerism and photochromism
both in solution and in the solid state,10-12 (ii) the coexistence
of C=N and carbonyl (C=O) groups that improve the
coordination capability of metal ions,13-15 (iii) the NH and/or
C=O groups provide the hydrogen bonds to bind the
appropriate anions/cations and biomolecules,16,17 and thus
enables its use in various fields. These structural motifs give
the hydrazide-hydrazone group its physical, chemical and
biochemical properties, however, the application in invisible
inks have not been explored.
and invisible security ink.26 Xu et al. established
photoresponsive fluorescent ink by aggregation-induced
photodimerization of alkynylpyrene derivative.27
a
In this work, as shown in Scheme 1, the ortho-, meta-, para-
substituted pyridine conjugated to 2,6-dihydroxybenzoic acid,
which containing hydrazide-hydrazone as photoswitch linker
were designed and synthesized (3a-3c). After 365 nm light
irradiation, we found that the ortho-substituted pyridine of
molecule 3a may have additional intramolecular hydrogen
bond to stabilize its Z isomer and restrict the motions of
molecule, thus increasing the fluorescence intensity of 3a.
Herein, the structure-property-function relationships for these
new materials are analyzed by various spectroscopies, e.g. UV-
Vis, fluorescence, 1H NMR and Fourier transform infrared
spectra. Additionally, the molecular calculation studies of 3a-
3c can provide deep insights into the structure and dynamics
of photoisomerization at the atomic level, which is essential to
understand and ultimately manipulate their function. The
experimental and molecular calculation results agreed well
Invisible inks can't be seen by the naked eye under normal
conditions, until it is treated with chemicals, light, heat and
acid/base.18-21 In recent years, the materials with stimuli-
responsive characteristics, such as pH, light, temperature,
a. Department of Chemistry, Chung Yuan Christian University, 200 Chung Pei Road,
Chung-Li, Taiwan.
E-mail: myyeh@cycu.edu.tw
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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and thus demonstrated the first example of invisible ink based chromatography to yield (E)-2,6-dihydroxy-N'-(pyridin-2-
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DOI: 10.1039/D0NJ05172A
on the hydrazide-hydrazone motif.
ylmethylene)benzohydrazide (3a) in 66% yield. H NMR (400
MHz, DMSO-d6): = 6.40 (d, J= 8.1 Hz, 2H), 7.17 (t, J= 8.1 Hz,
1H), 7.41-7.44 (m, 1H), 7.87-7.91 (m, 1H), 7.96-7.98 (m, 1H),
8.39 (s, 1H), 8.62 (s, 1H), 11.73 (s, 2H), 11.90 (s, 1H). 13C NMR
(100 MHz, DMSO-d6): =104.89, 107.55, 120.68, 125.04,
133.90, 137.42, 149.34, 150.05, 153.51, 159.72. HRMS (ESI)
m/z for C13H12N3O3 [M+H]+, calcd. 258.08787, found 258.08988.
In a manner similar to that described above, a mixture of 3-
pyridinecarboxaldehyde (41.6 mg, 0.39 mmol), catalytic
amount of p-TsOH and compound 2 (100 mg, 0.39 mmol) was
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Scheme 1 Molecular design of hydrazide-hydrazone derivatives.
1
converted to 3b (60%). H NMR (400 MHz,DMSO-d6): = 6.43
(d, J= 8.2 Hz, 2H), 7.22 (t, J= 8.2 Hz, 1H), 7.49-7.53 (m, 1H),
8.14-8.17 (m, 1H), 8.48 (s, 1H), 8.63-864 (m, 1H), 8.85 (s, 1H),
11.88 (s, 1H), 12.03 (s, 2H). 13C NMR (100 MHz, DMSO-d6): =
104.02, 107.69, 124.54, 130.46, 134.13, 134.16, 147.01, 149.38,
151.40, 160.05, 166.51. HRMS (ESI) m/z for C13H12N3O3 [M+H]+,
calcd. 258.08787, found 258.08740.
2. Experimental section
2.1 Materials and Instrumentation
All chemicals and reagents were commercially available and
used as received without further purification. Dimethyl
sulphate (DMS) was purchased from SHOWA chemical. 2,6-
Dihydroxybenzoic acid, hydrazine monohydrate and p-
toluenesulfonic acid (p-TsOH) were purchased from Alfa Aesar.
In a manner similar to that described above, a mixture of 4-
pyridinecarboxaldehyde (41.6 mg, 0.39 mmol), catalytic
amount of p-TsOH and compound 2 (100 mg, 0.39 mmol) was
1
converted to 3c (79%). H NMR (400 MHz, DMSO-d6): = 6.41
(d, J= 8.2 Hz, 2H), 7.19 (t, J= 8.2 Hz, 1H), 7.66 (d, J= 6.1 Hz, 2H),
8.40 (s, 1H), 8.66 (d, J=6.1 Hz, 2H), 11.85 (s, 2H), 11.92 (s, 1H).
13C NMR (100 MHz, DMSO-d6): = 104.42, 107.66, 121.59,
134.16, 141.76, 147.10, 150.76, 159.92, 166.57. HRMS (ESI)
m/z for C13H12N3O3 [M+H]+, calcd. 258.08787, found 258.08774.
2-Pyridinecarboxaldehyde,
3-pyridinecarboxaldehyde,
4-
pyridinecarboxaldehyde and methanol were obtained from
1
Acros Organics. H and 13C NMR spectra were measured on a
Bruker AVANCEII 400 NMR spectrometer using DMSO-d6 as
solvent. UV-Vis spectra were recorded on a Shimadzu UV-2550
spectrometer, fluorescence emission spectra were measured
on a Shimadzu RF-1501 fluorescence spectrophotometer and
Fourier transform infrared (FT-IR) spectra were obtained on
Jasco 4600 FT/IR infrared spectrophotometer.
2.3 Computational Methodology
Geometry optimization and the theoretical electrostatic
potential maps were performed through density functional
theory (DFT) using B3LYP methods (Becke’s three-parameter
hybrid functional using the correlation functional of Lee, Yang,
and Parr, which includes both local and non-local terms
correlation functionals) at the 6-31G(d) level.29, 30 The highest
occupied molecular orbital- lowest unoccupied molecular
orbital (HOMO-LUMO) analysis has been performed to explain
the charge transfer place within the molecule.30 Potential
energy curves (PEC) were constructed starting from the E
optimized geometry by constraining the N–N=C–C dihedral
angle while optimizing all the other structural parameters. The
constrained angle was incremented in 20◦ steps, between the
E and Z minima, and the energy of the optimized structure
obtained at each point.
2.2 Synthesis
Synthesis of 2,6-dihydroxy benzohydrazide (2): Compound 2
was synthesized and purified according to literature
procedure.9 To the stirred solution of 2,6-dihydroxybenzoic
acid (0.5 g, 3.25 mmol), DMS (0.28 mL, 2.9 mmol), potassium
carbonate (0.45 g, 3.3 mmol) in 10 mL of acetone. After
reaction at 55◦C for 24 hrs, the mixture was filtered off and the
acetone was evaporated by rotary evaporator. The residue
was extracted with ethyl acetate (3 x 25 mL), and the
combined organic phase was dried with anhydrous magnesium
sulfate and concentrated to give crude compound 1.
Subsequently, compound 1 was dissolved in 10 mL methanol
(MeOH) and hydrazine monohydrate (1.5 mL, 29.8 mmol) was
added dropwise in the above solution for 2 hrs. After the
reaction, MeOH was evaporated and the crude product was
purified by silica column chromatography to obtain compound
2 in 55% overall yield.
3. Results and discussion
3.1 Synthesis of hydrazide-hydrazone molecules
The ortho-, meta-, para-substituted pyridine conjugated to 2,6-
dihydroxybenzoic acid, which containing hydrazide-hydrazone
as photoswitch linker were designed. The three new
compounds of hydrazide-hydrazone derivatives (3a-3c) of this
study were synthesized according to modified literature
procedures.7-9, 31, 32 Scheme 2 displayed the synthetic routes
for these hydrazide-hydrazone derivatives of 3a-3c.
Synthesis of hydrazide-hydrazone molecules (3a-3c): 2-
Pyridinecarboxaldehyde (41.6 mg, 0.39 mmol) and catalytic
amount of p-TsOH were added to a solution of compound 2
(100 mg, 0.39 mmol) in 1.5 mL absolute ethanol (EtOH) with
stirring at room temperature for overnight. The reaction
mixture was concentrated and purified by silica column
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Compound 2 were synthesized from the reaction of 2,6-
dihydroxybenzoic acid with DMS in the presence of potassium
carbonate in acetone to obtain crude compound 1. Without
further purification, we then proceeded directly to the
reaction of compound 1 with hydrazine monohydrate in MeOH
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DOI: 10.1039/D0NJ05172A
to afford compound
Pyridinecarboxaldehyde,
2
in 55% two steps yield. 2-
3-pyridinecarboxaldehyde, 4-
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pyridinecarboxaldehyde were reaction with compound 2 in the
presence of p-TsOH in EtOH, respectively, to yield the
hydrazide-hydrazone derivatives of 3a-3c (60-79%).
OH
O
OH
O
NH2NH2 ,H2O
DMS , K2CO3
acetone
O
OH
OH
MeOH,
RT overnight
55% (2 steps)
OH
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OH
EtOH, TsOH
66%
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OH
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OH
O
N
N
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O
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NH2
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H
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H
EtOH, TsOH
60%
OH
O
OH
3b
2
Fig. 1 UV-Vis absorption spectra of (a) 3a (black), 3b (green) and 3c (blue) before
irradiation and (b) 3a before irradiation (black) and after irradiation (red) with
365 nm light in DMSO (Molecule concentration: 50 M).
N
OH
N
N
H
EtOH, TsOH
79%
OH
3c
Scheme 2 Synthetic routes for hydrazide-hydrazone derivatives of 3a-3c.
3.2 Photophysical Analysis and Characterization
UV-Vis absorption and fluorescence emission spectra of
hydrazide-hydrazone derivatives of 3a-3c at the concentration
of 50 M in DMSO are presented in Fig. 1, Fig. S1 and Fig. 2. As
shown in Fig. 1 and Fig. S1, the absorption maximum for 3a, 3b
and 3c are ca. 322, 321 and 330 nm, respectively. We observed
that when irradiation with 365 nm light for 5 hrs, the shapes of
the absorption spectra changed in 3a, while 3b and 3c almost
retained their absorption features. The blue shift of the
absorption spectrum of 3a was obtained after irradiation,
probably due to the photoisomerization of the E form to the Z
form.33 Fig. 2 displayed the fluorescence spectra of 3a-3c with
the excitation wavelength of their corresponding absorption
maximum. Remarkably, among them, only 3a revealed
dramatically enhanced emission intensity after irradiation with
365 nm light. Furthermore, we compared the optical images of
3a-3c under UV illumination, a bright blue light was detected
in 3a after 365 nm light irradiation, which is consistent with
the observation of the fluorescence profile (Fig. 2 inset).
Fig. 2 Fluorescence spectra of 3a, 3b and 3c before irradiation (black for 3a,
green for 3b, blue for 3c) and after irradiation (red for 3a, orange for 3b, purple
for 3c) with 365 nm light in DMSO (Molecule concentration: 50 M). The inset
showed the optical images of 3a-3c taken under UV illumination (left: before
irradiation; right: after irradiation).
To better understand the photophysical behavior of hydrazide-
hydrazone derivatives of 3a-3c (Fig. 1, Fig. S1 and Fig. 2), the
photoisomerization was confirmed by 1H NMR and FT-IR
1
spectroscopies. Fig. 3 showed the H NMR spectra of 3a-3c
before and after irradiation with 365 nm light. In the case of
3a, we observed the hydrazone NH and HC=N protons at 11.90
and 8.39 ppm, respectively. Upon irradiation with 365 nm
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light, the downfield shift of NH proton (12.50 ppm) and an Since 3a presented interesting photophysical properties, we
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upfield shift of HC=N signal (7.76 ppm) were detected, which further investigation the concentration DeOffIe: 1c0t.1o03n9/tDh0eNJU0V51-7V2iAs
can be attributed to the E to Z photoisomerization (Fig. 3a and absorption and fluorescence emission spectra of 3a after 365
3b).34 FT-IR spectra of 3a were depicted in Fig. 4 and the nm light irradiation. As shown in Fig. S3, we measured the
characteristic peaks at ca. 1550, 1650, 3305 cm-1 were absorption spectra of 3a with different concentrations (50 M,
assigned to C=N, C=O, and NH groups, respectively.35 After 500 M and 5,000 M). It was found that the absorption band
irradiation, the NH signal was shifted to lowerwavenumber, at 375 nm increasingly strengthened with a gradual increase in
indicating the formation of intramolecular hydrogen bonds in the concentration of 3a, suggesting that the ground state
the Z isomer of 3a.36 This intramolecular hydrogen bond may aggregates can be formed. Furthermore, the concentration
restricted the rotation of the molecule, thus increasing the dependence of the fluorescence spectra of 3a were carried out
fluorescence intensity of 3a.9 In contrast, 3b and 3c didn't with excitation wavelength at 315 nm (Fig. 5). It was observed
exhibit obvious changes in the 1H NMR and FT-IR spectra that with increasing the concentration of 3a from 50 M to
before and after irradiation treatment compared to that of 3a 5,000 M, a red-shift emission peak from 356 to 379 nm and a
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(Fig. 3c-3f and Fig. S2).
new emission band around 450 nm appeared, which
accompany the color changes in this optical images taken
under UV illumination (Fig. 5 inset). For comparison, the
optical images of 3a before irradiation were also taken,
showing that there is no fluorescence in the solution and solid
states (Fig. S4 and S5). In addition, we have recorded the
colors of 3a at different concentrations without and with 365
nm light treatment, the yellow solution was observed at high
concentration after 365 nm light irradiation taken under
ambient room lighting (Fig. S6).
1
Fig. 3 Comparison of H NMR spectra of (a, b) 3a, (c, d) 3b and (e, f) 3c before
irradiation (black) and after irradiation (red) with 365 nm light in DMSO-d6.
Fig. 5 Concentration dependence of fluorescence spectra of 3a after irradiation
with 365 nm light in DMSO (black for 50 M, red for 500 M and green for 5,000
M). The inset showed the optical images of 3a at high concentration taken
under UV illumination.
Based on the above property of the compound 3a, we attempt
to use it on filter paper. We have manually written the word
“CYCU” by using a DMSO solution of 3a, as shown in Fig. 6a,
after drying the filter paper, the word “CYCU” can't be found
under ambient room lighting. Upon 365 nm light irradiation for
a while, the word “CYCU” is visible by naked eye easily under
both a visible light and a hand-held UV lamp (Fig. 6b and 6c).
These experimental results are consistent with the observed
photophysical properties of 3a.
Fig. 4 Comparison of FT-IR spectra of 3a before irradiation (black) and after
irradiation (red) with 365 nm light in DMSO (Molecule concentration: 5,000 M).
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configuration. Fig. 8b displayed the optimized structures of E
and Z isomers as well as transition state (TS) of 3a-3c, we
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found that an intramolecular hydrogen bond can be formed in
Z form of 3a, suggesting the Z isomer has lower total energy
than that of E isomer (Fig. S8, the distances of N-H···O and N-H··
·N are 1.94 Å and 2.04 Å, respectively). The visualization of the
charge distributions and the electrostatic potential of 3a-3c
were shown Fig. 9. The calculation results indicated that
negative electrostatic potentials were localized around the
oxygen and nitrogen atoms of dihydroxybenzoic and pyridine
moieties, resulting in higher electrostatic repulsion of 3a and
thus causing the rotation of the N-N bond between the two
residues. These results imply that 3a easily isomerizes to the Z
isomer by irradiation of the E isomer. Additionally, we also
calculated the absorption spectra of 3a-E and 3a-Z (Fig. S9),
these results are comparable with the experimental data
obtained in Fig. 1b. Overall, we systematically studied the
effect of ortho-, meta-, para-substituted pyridine conjugated
to 2,6-dihydroxybenzoic acid, which containing hydrazide-
hydrazone as photoswitch linker, and investigated structure-
property-function relationships. We concluded that when
irradiation the ortho-substituted pyridine of molecule 3a with
365 nm light may result additional intramolecular hydrogen
bond to stabilize its Z isomer and restrict the motions of
molecule, causing the strong emission peaks observed in 3a-Z.
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Fig. 6 Invisible ink experiment of 3a (a) before irradiation and (b) after irradiation
taken under ambient room lighting and (c) after irradiation taken under UV
illumination.
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3.3 Molecular Calculation
Molecular orbitals (HOMO-LUMO) and their properties such as
electron distributions and energy are very useful for
researchers to predict photochemical processes, chemical
stability, photophysical properties and so on.37-40 The energy
diagram of frontier molecule orbital (eV) as well as the HOMO
and LUMO energy of 3a, 3b and 3c calculated by DFT/B3LYP/6-
31G(d) method is shown in Fig. 7. The calculated energy band
gaps are 4.19 eV (3a), 4.23 eV (3b) and 4.10 eV (3c), which are
in excellent agreement with the experimental photophysical
results. Moreover, we compared the difference between the
electron distributions in HOMO and LUMO orbitals (3a-3c) and
only significant electron density shift from the
dihydroxybenzoic group to pyridine moiety in 3c, indicating
that 3c possessed good intramolecular charge transfer
characteristics.
Fig. 7 Molecular orbital amplitude plots of HOMO and LUMO energy levels of 3a,
3b and 3c, calculated by using DFT at the B3LYP/6-31G(d) level.
In order to provide more insight into the photochemical
isomerization processes of hydrazide-hydrazone derivatives of
3a-3c, the potential energy curves (PEC) and theoretical
electrostatic potential maps were carried out.41, 42 As can be
seen from Fig. 8a and Fig. S7, the PEC of 3a-3c were
constructed by constraining the N-N=C-C dihedral angle to
adopt fixed angles, from 180◦ to 0◦ degrees in 20◦ increments.
The calculated relative energy barriers of 3a-3c were 48.7, 49.8
and 50.1 kcal/mol, respectively. In addition, the calculated
relative stabilities of the Z isomers were -4.5, 4.1, and 5.3
kcal/mol, respectively, indicating that 3a has the most stable Z
Fig. 8 (a) The computed potential energy curves for E to Z isomerization of 3a
(black), 3b (green) and 3c (blue) along a reaction coordinate corresponding to
the N–N=C–C dihedral angle. (b) The optimized structures of E and Z isomers as
well as TS of 3a, 3b and 3c, calculated by using DFT at the B3LYP/6-31G(d) level.
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Acknowledgements
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This work was supported by the Ministry of Science and
Technology of the Republic of China, Taiwan (MOST 109–2113-
M-033-006- )
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Fig. 9 3D representation of the electrostatic potentials around the molecules of
(a) 3a, (b) 3b and (c) 3c, determined using DFT at the B3LYP/6-31G(d) level.
4. Conclusions
In conclusion, the ortho-, meta-, para-substituted pyridine
conjugated to 2,6-dihydroxybenzoic acid, which containing
hydrazide-hydrazone as photoswitch linker were designed and
synthesized (3a-3c). We found that only newly discovered
hydrazide-hydrazone derivative of 3a displayed strong
fluorescence emission upon 365 nm light irradiation, while
other didn't have any fluorescence. We studied the structure-
property-function relationships for 3a-3c through UV-Vis,
fluorescence, 1H NMR, FT-IR spectra as well as DFT calculations.
The experimental and molecular calculation results indicated
that the hydrazide-hydrazone derivative of 3a could undergo a
characteristic E to Z photoisomerization after 365 nm light
irradiation. The
Z isomer of 3a may have additional
intramolecular hydrogen bond to restrict the motions of
molecule, thus illustrated the increasing the fluorescence
intensity of 3a and proven its potential application in security
protection.
Conflicts of interest
There are no conflicts to declare.
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The structure-property relationships for the hydrazide-hydrazone derivativDeOsI: w10.e10r3e9/D0NJ05172A
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investigated to provide new insights into the design of photo-responsive materials
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