Luminescent coatings: White-color luminescence from a simple and single chromophore with high anticorrosion efficiency

Article abstract of DOI:10.1016/j.dyepig.2019.108146

Luminescent coatings have potential commercial applications. Here, the photoluminescence, thermal, and corrosion inhibition properties of reported Schiff bases have been studied. Moreover, quantum chemical calculations have been done to realize the working mechanism of their properties. Interestingly, we succeeded to get a single-chromophore white-color emission from a small Schiff base molecule through a simple technique, where the relative emission intensities at the long and short wavelengths of the visible range were controlled by introducing electron-donating or withdrawing groups. Furthermore, the compounds were found thermally stable and can emit efficiently until 124 °C. Also, the electrochemical impedance spectroscopy, electrochemical frequency modulation, Tafel polarization, and surface characterizations have been investigated. The studied compounds showed high inhibition efficiencies (ex. 93%) and played a great role in retarding the stainless-steel corrosion in 2 M H₂SO₄. These measurements confirmed that the existence of these Schiff bases can reduce the double-layer capacities, corrosion current densities, and corrosion rate simultaneously with increasing the charge transfer resistance values. The studied materials as luminescent coatings may be employed for mixed-type inhibitors and can provide a strategy for the development of new organic materials capable of producing white-color emission from a simple and single molecule.

Full text of DOI:10.1016/j.dyepig.2019.108146

Dyes and Pigments 175 (2020) 108146
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Luminescent coatings: White-color luminescence from a simple and single
chromophore with high anticorrosion efficiency
,
,
Osama Younisa b, Emad E. El-Katoria, Reda Hassaniena *, Ashraf S. Abousalemc,
Osamu Tsutsumib
a Chemistry Department, Faculty of Science, The New Valley University, El-Kharja, 72511, Egypt
b Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, Kusatsu, 525-8577, Japan
c Quality Control Laboratory, Operations Department, JOTUN, 11835, Egypt
A R T I C L E I N F O
A B S T R A C T
Keywords:
Luminescent coatings have potential commercial applications. Here, the photoluminescence, thermal, and
corrosion inhibition properties of reported Schiff bases have been studied. Moreover, quantum chemical cal-
culations have been done to realize the working mechanism of their properties. Interestingly, we succeeded to get
a single-chromophore white-color emission from a small Schiff base molecule through a simple technique, where
the relative emission intensities at the long and short wavelengths of the visible range were controlled by
introducing electron-donating or withdrawing groups. Furthermore, the compounds were found thermally stable
and can emit efficiently until 124 �C. Also, the electrochemical impedance spectroscopy, electrochemical fre-
quency modulation, Tafel polarization, and surface characterizations have been investigated. The studied
compounds showed high inhibition efficiencies (ex. 93%) and played a great role in retarding the stainless-steel
corrosion in 2 M H2SO4. These measurements confirmed that the existence of these Schiff bases can reduce the
double-layer capacities, corrosion current densities, and corrosion rate simultaneously with increasing the charge
transfer resistance values. The studied materials as luminescent coatings may be employed for mixed-type in-
hibitors and can provide a strategy for the development of new organic materials capable of producing white-
color emission from a simple and single molecule.
Luminescent coatings
White luminescence
Single chromophore
Corrosion
DFT calculations
1. Introduction
cost on the fabrication process [12]. In other words, organic molecules
with an emission spectrum covering the entire visible-light range of
White-light luminescent organic materials have increased attention
due to their possible applications in light sources and the next genera-
tion of displays [1–3]. The production of white-light organic solids
depended mainly on the combination of emitters with three primary
colors as red, green, and blue [3,4] or two complementary colors like
yellow and blue [5] to result in an emission spectrum that covers the full
visible-light region [6]. Additionally, other approaches have been pro-
posed such as multilayer architectures, a single layer of blended emitting
polymers, and doping small fluorescent or phosphorescent molecules
into polymer hosts [7–10]. Consequently, most of the organic
white-light devices were frequently manufactured based on complex
systems. Moreover, the long-term device function of these systems may
lead to the problem of the components phase separation [11]. Therefore,
single-component white-light-emitting materials should have various
benefits like phase stability, facile fabrication, reproducibility, and low
wavelengths with proper relative intensities are highly desirable. For
that purpose, several concepts have been considered such as the
excited-state intramolecular proton transfer [13], frustrated energy
transfer [14], monomer and excimer fluorescence [15], fluorescence
and phosphorescence [16], and prompt and delayed fluorescence [17].
However, felicitous examples designed by these concepts with high ef-
ficiency and appropriate Commission Internationale de l’Eclairage (CIE)
are still very rare [18]. Furthermore, most of the designed
single-component white-light-emitting organic materials reported so far
have complicated optimizations and complex molecular structures.
Thus, new approaches to realize the full-color emission from a single and
simple molecule in the solid state are in great demand for the fabrication
of organic light-emitting devices [19]. Recently, we have discovered
novel liquid crystal polymers that can emit the white color from a single
chromophore after heating to the liquid crystal phase as a result of the
* Corresponding author.
E-mail address: reda.h@scinv.au.edu.eg (R. Hassanien).
https://doi.org/10.1016/j.dyepig.2019.108146
Received 17 October 2019; Received in revised form 13 December 2019; Accepted 15 December 2019
Available online 19 December 2019
0143-7208/© 2019 Elsevier Ltd. All rights reserved.

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
formation of different aggregated species [20,21]. Moreover, other
several materials have been reported in which the luminescence
behavior is sensitive to the molecular aggregation without changing the
molecular structure of the luminescent core [22–27].
chromatographic plates were visualized under UV at 254 nm. Elemental
Analysis system GmbH-VarioEL V.3 micro-analyzer was used to perform
the elemental analyses and the obtained results were found in a good
agreement with their calculated values. Mass spectra were observed
using a JEOL JMS-600 apparatus. IR spectra were recorded on a KBr disc
method (FT/IR-610, JASCO). 1H nuclear magnetic resonance (NMR)
spectra were measured using a JEOL ECS-400 spectrometer at 400 MHz,
and the residual solvent proton was used as an internal reference.
Typically, five reported [41,43–47] N-benzylidenepyridine-2-amine
compounds (Fig. 1) were prepared by the reaction of the stoichiometric
ratio of 2-aminopyridine and benzaldehyde or one of its substituents
(2-hydroxy, 4-methyl, 4-methoxy, and 3-nitro). Each reactant was dis-
solved in a minimum amount of methanol and mixed with the other
reactant, followed by adding a few drops of piperidine. The solution was
Recently, photoluminescence was considered as a suitable charac-
terization method to investigate the mechanism of the coating corrosion,
where some luminescent coatings have a response to external stimuli
such as organic vapors and temperature change [28]. Also, optical
sensors are low-cost systems for the early indications of the corrosion in
the constructions to reduce the cost of corrosion problems like mainte-
nance [29]. Moreover, several potential applications in the safety field,
outdoor furniture, components of the architecture, and interior designs
are expected from luminescent coatings. However, only a few lumines-
cent coatings were reported [30]. Therefore, we focused in this work on
materials with both corrosion-inhibition activity and luminescence
emission.
�
refluxed for 2 h at 65 C then concentrated by rotary evaporator. The
Schiff base compounds were filtered and washed with ethanol then
recrystallized from hot methanol to give yellow precipitates with a yield
of 74–86%.
Due to the high strength, high corrosion resistance, and workability
of stainless steel (SS), it is used in numerous applications like beverage
and food industry [31], pharmaceutical and chemical industry [32],
water and oil pipelines [33], petrochemical industry [34], architectural
applications, water supply or desalination plants [35], and ship or naval
structures [36]. However, the presence of aggressive ions decreases the
resistance of SS towards the localized corrosion. Hydrochloric and sul-
furic acids are used for industrial acid cleaning, acid pickling, oil well
acidizing, and acid descaling, so the use of acid corrosion inhibitors is
useful in the industry to diminish or avoid the material loss while it is
exposed to the acid. It was found that introducing multiple bonds or
heteroatoms that have high charge density like oxygen, sulfur, and ni-
trogen into the organic compounds improves the inhibition efficiency
against the acid corrosion [37,38]. Many organic compounds were used
as corrosion inhibitors to protect SS in acid medium [39]. The existence
of atoms with unshared electrons among the structure of these com-
pounds lies behind their inhibition action [40].
2.2.1. Synthesis of NPA-H
According to the general procedures, reacting 2-aminopyridine
(2.82 gm, 0.03 mol) and freshly distilled benzaldehyde (3.05 ml, 0.03
mol) gave 4.5 gm (82% yield) of the title compound after recrystalli-
zation from hot methanol, m.p. 110 �C. Anal. Calcd. for C12H10N2
(182.23): C, 79.10; H, 5.53; N, 15.317%. Found: C, 79.50; H, 5.19; N,
–
15.61%. IR:
ν
(cmÀ 1) 1595 (C N). H NMR (400 MHz, CH OH-d , δ
–
3
4
–
–
–
–
(ppm)): 8.42 (s, 1H, N CH aliphatic), 8.02 (d, J ¼ 5.46 Hz, 1H, N CH
C5H4N), 7.87 (d, J ¼ 4.55 Hz, 1H, C5H4N), 7.57–7.48 (m, 1H C6H5 and
1H C5H4N), 7.45 (t, J ¼ 7.92 Hz, 1H, C6H5), 7.34 (m, 1H, C6H5), 7.02 (m,
1H, C6H5), 6.67 (t, J ¼ 6.78 Hz, 1H, C5H4N), 6.58 (t, J ¼ 6.96 Hz, 1H,
C6H5).
2.2.2. Synthesis of NPA-2OH
Schiff bases gain popularity due to their ease preparation from
readily available starting materials [41]. The great attention in Schiff
bases is due to their industrial applications such as dyes and pigments as
well as corrosion inhibitors for steel, copper, and aluminum [42].
Although there are wide applications of Schiff bases in catalysis,
analytical and biological systems, photoluminescence, thermal, and
corrosion-inhibition studies, pyridine-based Schiff bases are compara-
tively less. Here, luminescent coatings from simple molecules of re-
ported Schiff bases were synthesized and evaluated for
photoluminescence, thermal studies, and corrosion inhibition potential.
These molecules gave a single-chromophore white-color emission
through a simple technique where the relative emission intensities at the
long and short wavelengths of the visible range were controlled by
introducing electron-donating or withdrawing groups. The studied
compounds were found also as suitable inhibitors to mitigate the SS
corrosion in 2 M sulfuric acid solution.
As described in the general procedures, the reaction of 2-aminopyr-
idine (2.82 gm, 0.03 mol) with freshly distilled 2-hydroxybenzaldehyde
(3.14 ml, 0.03 mol) produced 5.1 gm (86% yield) of the title compound
�
after recrystallization from hot methanol, m.p. 68 C. Anal. Calcd. for
C12H10N2O (198.08): C, 72.71; H, 5.09; N, 14.13; O, 8.07%. Found: C,
72.56; H, 5.12; N, 14.20%. IR:
ν
(cmÀ 1) 1606 (C N), mass spectrum m/
–
–
1
–
–
z ¼ 79.03. H NMR (400 MHz, CH OH-d , δ (ppm)): 8.49 (s, 1H, N CH
3
4
–
–
aliphatic), 8.39 (d, J ¼ 7.91 Hz, 1H, N CH C H N), 8.25 (d, J ¼ 7.82 Hz,
4
1H, C5H4N), 7.98–7.84 (m, 1H C6H4 and 51H C5H4N), 7.44 (m, 1H,
C6H4), 7.35 (m, 1H, C6H4), 6.65–5.45 (m, 1H C6H4 and 1H C5H4N), 4.91
(s, 1H, OH).
2.2.3. Synthesis of NPA-4OMe
This compound was obtained, according to the general procedures,
by reacting 2-aminopyridine (2.82 gm, 0.03 mol) with freshly distilled
4-methoxybenzaldehyde (3.65 ml, 0.03 mol) to produce 4.7 gm (74%
yield) of NPA-4OMe which was recrystallized from hot methanol, m.p.
104 �C. Anal. Calcd. for C13H12N2O (182.23): C, 73.56; H, 5.70; N,
2. Experimental
13.20; O, 7.54%. Found: C, 73.50; H, 5.29; N, 13.61%. IR:
ν
(cmÀ 1) 1602
1
–
2.1. Materials
(C
–
N), mass spectrum m/z ¼ 79.03. H NMR (400 MHz, CH3OH-d4, δ
–
–
–
(ppm)): 8.45 (s, 1H, N CH aliphatic), 8.00 (m, 1H, N CH C H N), 7.86
–
5
4
2-Aminopyridine, benzaldehyde, 2-hydroxybenzaldehyde, 3-nitro-
benzaldehyde, 4-methylbenzaldehyde, and 4-methoxybenzaldehyde
used for this research are of synthesis grade chemicals purchased from
Sigma-Aldrich. Solvents were used without further purification as they
are with analytical grade.
(m, 1H, C5H4N), 7.57–7.27 (m, 3H C6H4 and 1H C5H4N), 6.70–6.53 (m,
1H C6H4 and 1H C5H4N), 3.32 (s, 3H, OCH3).
2.2.4. Synthesis of NPA-4Me
As explained in the general procedures, the reaction of 2-aminopyr-
idine (2.82 gm, 0.03 mol) and freshly distilled 4-methyl-benzaldehyde
(3.53 ml, 0.03 mol) resulted in 4.8 gm (82% yield) of the title com-
pound, m.p. 75 �C. Anal. Calcd. for C13H12N2 (196.25): C, 79.56; H,
2.2. Synthesis
6.16; N, 14.27%. Found: C, 79.58; H, 6.11; N, 14.61%. IR:
ν
(cmÀ 1) 1602
The melting points were determined on a Gallen-Kamp melting point
apparatus and uncorrected. The purity of the synthesized Schiff bases
was verified by thin-layer chromatography (TLC) using a Merck pre-
coated silica gel plate (MERCK, 60F) and the developed
–
(C N). NMR was not measured because of the poor solubility of NPA-
–
4Me.
2

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
Fig. 1. Molecular structure of the studied materials.
2.2.5. Synthesis of NPA-3NO2
lowest energy level, then the surface was enlarged to a supercell with
dimensions of (10 � 10 cm) and a vacuum slab up to 30 Å capping the
metal surface. This provides enough free surface area for placing the
inhibitors along with water molecules on the metal surface for modeling
the interaction. The dimensions of the simulation box are (24.82 �
24.82 � 41.03 Å) with only one inhibitor molecule in each vacuum and
water simulation conditions. The COMPASS force field was selected
because of its recognition as a high-quality forcefield to consolidate
parameters of inorganic and organic compounds. The energy conver-
gence tolerance was set at 10À 4 kcal/mol at maximum force of 0.005
kcal/mol/Å and displacement of 5 � 10À 5 Å. Fine quality was opted for
all simulation courses [51].
When 2-aminopyridine (2.82 gm, 0.03 mol) and 3-nitrobenzalde-
hyde (4.53 gm, 0.03 mol) reacted as in the general procedures, 5.8 gm
(85% yield) of the title compound were formed, m.p. 146 �C. Anal.
Calcd. for C12H9N3O2 (227.22): C, C, 63.43; H, 3.99; N, 18.49; O,
(cmÀ 1) 1602 (C N).
–
14.08%. Found: C, 63.50; H, 4.11; N, 18.60%. IR:
ν
–
1
–
–
H NMR (400 MHz, CH OH-d , δ (ppm)): 8.49 (s, 1H, N CH aliphatic),
3
4
–
–
8.40 (m, 1H, N CH C H N), 8.07–7.76 (m, 2H C H ), 6.66–7.31 (m, 1H
C6H4 and 1H C5H4N),56.472–6.53 (m, 1H C6H4 and 42H C5H4N).
6
2.3. Thermal properties
The thermal stability was studied by thermogravimetric analysis-
differential thermal gravimetry (TGA-DTG) using a Shimadzu DTG-
60AH instrument. While the thermodynamic properties and the lack
of liquid crystallinity were studied by the polarizing optical microscopy
(POM, Olympus BX51 microscope) equipped with a hot stage (Instec
HC302 hot stage and mk1000 controller) and the differential scanning
calorimetry (DSC; SII X-DSC7000). Both DSC and POM were done in air
2.5. Optical properties
UV–visible (UV–Vis) absorption spectra were measured using the
JASCO V-550 absorption spectrometer. The steady-state luminescence
spectra were recorded by a Hitachi F-7000 fluorescence spectrometer
using the R928 photomultiplier (Hamamatsu) detector. Quantum yields
of the luminescence were measured with a calibrated integrating sphere
(Hitachi). The sample was placed inside a 1 mm quartz cell and fixed on
a homemade heating stage to measure the photoluminescence spectra at
high temperatures.
at a heating rate of 5 �C minÀ 1
.
2.4. Quantum-chemical calculations
Computational calculations at the density functional theory (DFT)
level were performed on the lowest energy conformers of the studied
compounds using ORCA 4.1 with the B3LYP functional and a def2-SVP
basis set [48]. For the sake of having comparable outputs from
another method, the electronic properties of tested compounds were
investigated using the DMOL3 module implemented in Materials Studio
7.0 (Accelrys Inc., San Diego, CA, USA). The gradient-corrected func-
tional method (GGA) was adopted with a double numeric plus polari-
zation (DNP) basis set and BLYP functional [49]. The calculated
quantum parameters like the energies of the lowest unoccupied molec-
ular orbital (LUMO) and highest occupied molecular orbital (HOMO),
HOMO-LUMO energy gap, dipole moment, binding energy, electron
affinity, ionization potential, global hardness, the fraction of electrons
transferred, and electronegativity were calculated from the compounds
optimized geometries at the ground state [50]. Monte Carlo simulation
was applied to the compounds in a simulation box with periodic
boundary conditions to explore their adsorption behavior on the Fe
surface using Materials Studio 7.0 (Accelrys Inc., San Diego, CA, USA).
As SS was the metal of concern in our study and since the major element
of the SS alloys is Fe, we explored the interaction of the compounds with
Fe surface. In this simulation method, Fe (1 1 0) plane surface was
cleaved from Fe unit cell. The surface of the metal was first relaxed to the
2.6. Metal sample
The composition (wt%) of the used 316L SS specimens was as fol-
lows; C: 0.03%, Mn: 2.0%, Si: 0.75%, P: 0.045%, S: 0.03%, Cr: 18–20%,
Ni: 12%, Mo: 2.0%, N: 0.1%, and Fe: 64.05%.
2.7. Solutions
The applicable solution used in this study was a 2 M H2SO4 solution
prepared using double distilled water and calibrated by a sodium car-
bonate standard solution. Dilution of 98.3% H2SO4 with demineralized
water was used to prepare 2 M Sulfuric acid solution. The used doses of
the inhibitors are 1, 4, 7, 10, 13 and 16 ( � 10À 6) M.
2.8. Potentiodynamic polarization (PP)
3-electrode cell with a Pt electrode (HANNA Instruments Co.)
counter was used to perform PP tests using the reference saturated
calomel electrode (SCE) (HANNA Instruments Co.). The working elec-
trode of 316L SS was cut to sheets and fixed in epoxy, the surface flat
area was 1.0 cm2. The working electrode was polished with papers grit
3

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
1200 in size. Before tests and at a natural potential, the electrode was
immersed in H2SO4 for half an hour until it reached the steady-state. The
used potential range was from À 500 to þ500 mV vs. Open circuit po-
tential OCP (Eocp). The Equipment of electrochemical measurements
were calibrated using a dummy cell and all tests were done in freshly
prepared solutions and the outcome data were always recorded at least 3
times to check the results reproducibility. The inhibition efficiency (IE
%) and the surface coverage (θ) can be calculated as in Equation (1):
methylbenzaldehyde, 4-methoxybenzaldehyde, or 3-nitrobenzaldehyde,
respectively. The purity of the synthesized compounds was proven from
their sharp melting points and through the TLC. The elemental and
spectral analyses confirmed the molecular structure of the prepared
compounds as detailed in the experimental section. NMR and IR spectra
of the studied Schiff bases are shown in Figures S1–S2 in the Electronic
Supporting Information (ESI).
3.2. Thermal properties
IE% ¼ 100 � θ ¼ 100 � [1-(Icorr(inh)/ Icorr(free))] … … … .
(1)
TGA-DTG was used to check the thermal stability of the studied
where, Icorr(inh) and Icorr(free) are the corrosion current densities in the
Schiff bases, Fig. S3 (in the ESI). NPA-H, NPA-2OH, NPA-4OMe, NPA-
presence and absence of the investigated inhibitors, respectively.
4Me, and NPA-3NO2 gave thermal decomposition temperatures (Tdec
,
temperature of 5% weight loss) of 139, 141, 124, 164, and 150 �C,
respectively. Thus, these materials have the required thermal stability
for applications at high temperatures. Using POM and DSC, none of
these Schiff bases showed liquid crystallinity because of not having
flexible chains within their molecular structures.
2.9. Electrochemical impedance spectroscopy (EIS)
All EIS tests were accomplished at Eocp of 30 � 1 �C through a large
frequency range of (1 � 105 Hz to 0.1 Hz). The semi-circle width de-
scribes the charge transfer resistance (Rct) corresponding to the polari-
zation resistance (Rp) and inversely proportional to the corrosion
current density (Icorr) value. The increase in Rct reflects the growth in the
double layer thickness that was adsorbed by the applied inhibitors.
Furthermore, the double-layer capacitance (Cdl) data decrease by
introducing the investigated inhibitors into the corrosive medium. Cdl is
calculated from the next Equation:
3.3. Quantum-chemical calculations
As a part of the investigation, we attempted to explore the electronic
and molecular properties of the compounds with the application of DFT
computations and find an established correlation between the quantum-
chemical parameters and the practically-obtained IE% of the compounds
[52]. By analyzing the electron density of Frontiers molecular orbital
(FMOs), the chemical interactions and compounds performance as
corrosion inhibitors can be anticipated with better insights [51,53]. It
can be observed from Fig. 2 that the electron density surface for HOMO
was spread over the entire structure of the compounds. Likewise, the
LUMO was distributed over the entire structure except for compound
NPA-3NO2 where the presence of the nitro group decreases the electron
density on the benzene ring. Table S1 (in the ESI) lists the
quantum-chemical parameters calculated for the studied compounds at
the ground state. It can be noted that the introduction of the
electron-donating group is associated with an increase in HOMO energy
(EHOMO) and a decrease in the energy gap (Egap) ¼ ELUMO – EHOMO. The
higher the EHOMO, the lower the ionization potential (IP ¼ - EHOMO), and
the higher the ability of the compound to donate electrons to suitable
electrophiles in chemical interactions. Whereas, The ELUMO is associated
with the electron affinity of the compounds, which describes the com-
pound ability to accept electrons in a chemical interaction. The lower
the ELUMO, the higher the electron affinity (EA ¼ - ELUMO) of the com-
pound and the stronger its ability to accept free electrons from electron
donors. Egap is an important stability index of compounds. The lower
value of Egap is consistent with a higher chemical reactivity, increasing
in the softness, and more inhibition efficiency. Egap decreases in the
order NPA-H > NPA-4Me > NPA-4OMe > NPA-3NO2. While the EHOMO
decreases in the order NPA-4OMe > NPA-4Me > NPA-H > NPA-3NO2.
The decreasing order of EHOMO coincides with the increasing efficiency
of the compounds to inhibit stainless steel corrosion in acid solutions.
This can be interpreted by the presence of an electron pushing group
which increases the electron-donating ability as the compounds are rich
in electrons that can transfer from HOMO (non-bonding & anti-bonding)
orbitals to the vacant d-orbitals of Fe atoms.
Cdl ¼ 1/(2
π
fm ax.Rct) … … … .
(2)
where fmax is the frequency maximum and Rct is the charge transfer
resistance.
IE% and θ given from the EIS analyses were obtained from Equation
(3).
IE% ¼ 100 � θ ¼ 100 � [1-(R�ct/Rct)] … … …
(3)
where Roct and Rct are the charge transfer resistance in the absence and
presence of the investigated inhibitors, respectively.
2.10. Electrochemical frequency modulation (EFM)
The two frequencies 2 and 5 Hz were used to do EFM tests. The base
frequency of 1 Hz with 32 cycles and a perturbation signal with an
amplitude of 10 mV were used, so the waveform repeats after 1s. The
causality factors (CF2 and CF3), corrosion current density (Icorr), and
anodic and cathodic Tafel slopes (β and β ) were measured utilizing the
a
c
maximum peaks. The inhibition efficiencies (IEEFM%) was calculated
according to Equation (1). All the electrochemical tests were done using
Gamry apparatus PCI300/4 Potentiostat/Zra/Galvanostat; EIS300 was
utilized for the software of EIS, and 5.5 Echem Analyst was used for
plotting the outcomes, value fitting, and the measurements.
2.11. Surface analysis
To examine the surface morphology, the used specimens were
immersed in 2 M H2SO4 in the presence or absence of 16 � 10À 6 M of the
investigated inhibitors for 12 h at 30 �C. After gentle washing with
distilled water and careful drying, the specimens were examined. The
corroded 316L SS was inspected by model Wet–SPM (Scanning probe
microscopy) Shimadzu, Japan and was utilized for the Atomic force
microscopy (AFM) tests.
3.4. Optical properties
The optical behavior was studied for the powders at the room and
higher temperatures as well as for the solutions, the results were shown
in Table 1, Figs. 3–6 and Figs. S4–S12 (in the ESI). The solid-state NPA-H
emitted a broad emission spectrum that covered the full-visible region
with maximum emission intensities at 343, 464, and 546 nm to produce
a white-color emission with CIE coordinates of (0.29, 0.31), Fig. 3a and
b and Table 1. Sometimes, the multi-color emission from a single emitter
can be explained by the formation of different aggregated species and
3. Results and discussion
3.1. Synthesis
The Schiff bases NPA-H, NPA-2OH, NPA-4OMe, NPA-4Me, and
NPA-3NO2 (Fig. 1) were synthesized in a one-pot reaction of 2-amino-
pyridine
and
benzaldehyde,
2-hydroxybenzaldehyde,
4-
4

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
Fig. 2. HOMO and LUMO electron density maps obtained at DFT level for the investigated compounds (ORCA 4.1).
various excited states as a result of the different intra- or intermolecular
interactions (e.g. hydrogen bonding, C–H … , conformational changes,
and stacking) [54–56]. However, this mechanism cannot be taken as
NPA-4Me was shifted from the white color to produce light red-color
emission with CIE coordinates of (0.40, 0.39) and (0.40, 0.37), respec-
tively, as in Fig. 3a and b and Table 1. On the other hand, introducing the
electron-withdrawing group (m-nitro) in NPA-3NO2, clearly diminished
the emission intensity at the longer wavelength region to give a spectral
shape with a stronger emission at the short wavelengths �500 nm.
NPA-3NO2 exhibited cyan-color emission with CIE coordinates of (0.27,
0.27). Powders of NPA-H, NPA-4OMe, NPA-4Me, and NPA-3NO2 gave
emission quantum yields (Φem) of 3.6, 1.9, 0.7, and 1.0%, respectively.
The emission, absorption, and excitation spectra of solutions of the
Schiff bases were measured and presented in Fig. 4 to study the effect the
molecular aggregation. Compounds NPA-4OMe and NPA-3NO2 were
dissolved in DMF and their photophysical properties were investigated
for diluted (2 � 10À 6 M) and concentrated (2 � 10À 3 M) solutions. Since
NPA-H is not soluble in the common organic solvents at room temper-
ature, KBr was finely ground then mixed well with different wt% of
ground NPA-H to prepare high (40 wt%) and low (4 � 10À 3 wt%)
concentration solid-state solutions [57]. NPA-4OMe and NPA-3NO2
gave an absorption band at 300 nm in the UV region, but both were
transparent in the visible-light range that is a significant property for the
organic-luminescent materials. The resemblance of the excitation spec-
trum of each compound and its absorption spectrum verifies the simi-
larity between its excited and ground states. Also, it can be observed that
the excitation spectra of NPA-4OMe and NPA-3NO2 are mostly
matching to indicate that the substitution on the aromatic ring does not
have a significant effect on the excited state of their solutions. Solutions
with low concentrations (black lines in Fig. 4, 4 � 10À 3 wt% of KBr
solution of NPA-H and 2 � 10À 6 M of DMF solutions of NPA-4OMe and
NPA-3NO2) gave a strong emission at the short wavelengths of the
visible range (363–378 nm) to give CIE coordinates of (0.28, 0.26),
(0.19, 0.11), and (0.19, 0.11), respectively as shown in Table 1 and
Fig. 4 inset. On the other hand, solutions with high concentrations (blue
lines in Figs. 4, 40 wt% of KBr solution of NPA-H and 2 � 10À 3 M of DMF
solutions of NPA-4OMe and NPA-3NO2) exhibited obvious improve-
ment of the emission intensity at the longer wavelengths (>400 nm) and
had CIE coordinates of (x, y) ¼ (0.26, 0.26), (0.24, 0.19), and (0.25,
0.26), respectively to produce a spectral shape similar to that of the
solid-state material. Using different solution concentrations of NPA-H
and NPA-3NO2 changed significantly the spectral shape and the relative
emission maximum intensities, while this change was slight with
NPA-4OMe. It can be speculated that the molecular aggregation of
π
π-π
the reason for the broad emission from NPA-H where its emission or
excitation spectra did not show a significant dependence on the exci-
tation or emission energies, as shown in Fig. S4 (in the ESI). To get more
insight into the mechanism of this interesting broad emission, the
luminescence behavior of the solid-state derivatives NPA-4OMe,
NPA-4Me, and NPA-3NO2 was studied and compared with NPA-H.
Similar to the parent compound NPA-H, the emission or excitation
spectra of the other three derivatives do not depend on the excitation or
emission wavelengths, Fig. 3c–d and Figs. S5–S6 (in the ESI). These
results confirm that the broad spectrum emitted from any of these ma-
terials is not attributed to the emission from different excited states. All
the studied four materials exhibited broad spectral bands that covered
the entire visible range but with different relative intensities through the
short (400–550 nm) and long (550–700 nm) wavelengths, Fig. 3a. The
relative emission intensity of NPA-H at the range of 400–500 nm was
slightly higher than that at 500–700 nm. By introducing
electron-donating groups (p-methoxy or p-methyl) in NPA-4OMe and
NPA-4Me, the maximum emission intensity at the longer wavelength
region was enhanced significantly relative to that at the shorter wave-
lengths. Because of this distortion in the relative intensities along the
visible range wavelengths, the emission color of NPA-4OMe and
Table 1
Experimental photophysical data of the studied materials.
a
e
λabs
(nm)
CIEb
(x,y)
CIEc
(x,y)
CIEd
(x,y)
Φem
(%)
NPA-H
–
0.28, 0.26
0.19, 0.11
–
0.26, 0.26
0.24, 0.19
–
0.29, 0.31
0.40, 0.39
0.40, 0.37
0.27, 0.27
3.6
1.9
0.7
1.0
NPA-4OMe
NPA-4Me
NPA-3NO2
300
–
300
0.19, 0.11
0.25, 0.26
a
λabs: maximum absorption wavelength measured in DMF solutions (2 � 10À 5
M).
b
CIE of diluted solutions (4 � 10À 3 wt% KBr solution of NPA-H and 2 � 10À 6
M DMF solution of NPA-4OMe and NPA-3NO2).
c
CIE of concentrated solutions (40 wt% KBr solution of NPA-H and 2 � 10À 3
M DMF solution of NPA-4OMe and NPA-3NO2).
d
e
CIE of solids.
Φem: emission quantum yield measured for solids.
5

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
Fig. 3. (a) Normalized emission spectra of powders
at λex ¼ 360 nm (red: NPA-H, green: NPA-4OMe,
blue: NPA-4Me, black: NPA-3NO2). (b) CIE chro-
maticity diagram of powders emission colors at λex
¼
360 nm (red: NPA-H, green: NPA-4OMe, blue: NPA-
4Me, black: NPA-3NO2). (c) Normalized excitation
spectra of NPA-4OMe powder at different emission
wavelengths (λem ¼ 545 nm (red), 570 nm (green),
614 nm (blue)). (d) Normalized emission spectra of
NPA-4OMe powder at different excitation wave-
lengths (λex ¼ 340 nm (red), 360 nm (green), 377 nm
(blue), 467 nm (black)). (For interpretation of the
references to color in this figure legend, the reader is
referred to the Web version of this article.)
Fig. 4. Absorbance (green, 2 � 10À 5 M DMF solu-
tion), excitation (red, 2 � 10À 5 M DMF solution, λem
¼ 380 nm), and emission (blue “concentrated” and
black “diluted”, λex ¼ 340 nm) of NPA-H (dotted
lines), NPA-4OMe (solid lines), and NPA-3NO2
(dashed lines), where dotted blue: 40 wt% KBr solu-
tion, solid and dashed blue: 2 � 10À 3 M DMF solu-
tion, dotted black: 4 � 10À 3 wt% KBr solution, solid
and dashed black: 2 � 10À 6 M DMF solution. Inset:
CIE chromaticity diagram of NPA-3NO2 emission
colors at λex ¼ 340 nm (violet: 2 � 10À 6 M, blue: 2 �
10À 3 M, black: powder). (For interpretation of the
references to color in this figure legend, the reader is
referred to the Web version of this article.)
NPA-H and NPA-3NO2 is affected by the solution concentration easier
than NPA-4OMe. This decrease in the responsiveness of NPA-4OMe
aggregation to the solution concentration may attribute to the
p-methoxy group that can enhance the compound solubility. The
computed absorption wavelengths (λabs) was studied in DMF using
TD-DFT (B3LYP) method. The calculated λabs of NPA-H, NPA-4Me, and
NPA-4OMe were 328, 347, 319 nm and the major participating FMOs to
give major excitations were found from (HOMO→LUMO), (HOMO-1-
→LUMO), and (HOMO→LUMO), respectively.
were compared, a major change will be observed. The powders and
concentrated solutions have similar spectra and red-shifted emission
maxima compared to their diluted solutions that are common for most
organic-luminescent materials because the formed aggregates can
enhance the π-π interactions [58–60]. From the above results, it can be
understood that the luminescence at wavelengths >450 nm came from
the aggregation of the molecules where it was seen only in the
concentrated solutions and powders. While the emission bands at
363–378 nm observed with the diluted solutions can be assigned as the
monomer emission.ggregation.
Powders of NPA-4OMe and NPA-3NO2 (Fig. 3c and Fig. S6a in the
ESI) have different excitation spectra than their solutions (Fig. 4), as
evidence that the molecular aggregation has a substantial effect on their
excited states. If the emission spectra of the powders (Fig. 3) of the
studied materials and their diluted and concentrated solutions (Fig. 4)
It has been reported that introducing electron-donating groups into
Schiff bases can reduce the energy gap between HOMO and LUMO to
result in a red shift, and the opposite effect was obtained with electron-
withdrawing groups [61]. The emission spectra obtained in the current
6

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
Fig. 5. Emission spectra of NPA-H (λex ¼ 360 nm) at
different temperatures: (a) first heating, (b) first
cooling, (c) and (d) are normalization of (a) and (b),
respectively. (e) Maximum emission intensity at 433
nm as a function of the temperature (heating: red,
cooling: blue). (f) CIE chromaticity diagram of the
emission colors before any thermal treatment (black)
and at 100 �C during the first cooling (green). (For
interpretation of the references to color in this figure
legend, the reader is referred to the Web version of
this article.)
study are in good accordance with this explanation where the emission
intensity at longer/shorter wavelengths was enhanced by inserting
electron-donating/withdrawing groups, respectively. As known, Egap in
the gas state may differ than that in the solid state due to the molecular
aggregation and the intermolecular interactions that affect the elec-
tronic distribution around the molecule and consequently affect Egap, so
the calculated Egap values in Fig. 2 (gas phase) do not agree with the
expected one for the solid state. Currently, we are working to improve
both the purity and the luminescence efficiency (higher Φem) of the
white-color emission from these simple Schiff bases through several
strategies. One suggested strategy is controlling Egap between HOMO
and LUMO to get similar relative emission intensities through the whole
visible range. This suggestion can be achieved by introducing other
electron-donating and/or withdrawing groups into NPA-H. Bulky
groups can also be inserted to generate a steric hindrance and restrict the
rotations of the phenyl rotors in addition to forming effective inter/in-
tramolecular interactions to suppress the non-radiative relaxations and
enhance Φem [60]. Another way is by inserting some side chains to the
molecular structure that will not affect the electronic structure of the
luminescent core but can modify the structure of the molecular aggre-
gates. The next study includes NPA-2OH and other derivatives, and the
time-dependent DFT calculations and single-crystal X-ray structural
analyses will be performed and discussed in detail.
heating and cooling to check the stability of their emission behavior
under severe conditions like high temperatures (Figs. 5–6 and
Figs. S7–S12 in the ESI). The studied materials still emit efficiently at
high temperatures. Decreasing the emission intensity on heating
because of the effect of the thermally-activated molecular motion,
however, the emission intensity increased again on cooling as a result of
decreasing the activated molecular motion. In spite of this, the studied
materials are still emitting strongly at high temperatures as the calcu-
lation of the luminescence curve integration area indicated. Besides
their emission at high temperatures, these Schiff bases are thermally
stable as discussed before. subsequently, these compounds are suitable
for use at high temperatures. The emissions after heating are similar to
that before heating with some change in the spectral shape which
directed the emission color for more white, especially with NPA-4OMe
and NPA-4Me, as can be seen from Fig. 6. 1H NMR was measured after
the heating and cooling processes, and no change was detected. Exci-
tation spectra in Fig. S12 (in the ESI) showed a small modification after
heating to indicate that the excited state was affected slightly by heating
that may due to some changes in the aggregate structure. Then, if the
aggregate structure of these Schiff bases can be controlled by heating,
pure white-color emission will be obtained.
To study the materials photostability, the luminescence behavior of
NPA-H as a representative example was observed under continuous
irradiation for 8 h (λex ¼ 360 nm, 6.5 mW cmÀ 2) at room temperature in
air without any thermal treatment, Fig. S13 (in the ESI). Φem values at
Additionally, the photoluminescence behavior of powders of the
Schiff bases was investigated at high temperatures through two cycles of
7

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
Fig. 6. Emission spectra at λex ¼ 360 nm of (a) NPA-4OMe before any thermal treatment (blue) and at 70 �C during the second heating process (red) and (b) NPA-
4Me before any thermal treatment (blue) and at 120 �C during the second heating process (red). (c) and (d) CIE chromaticity diagrams of the emission colors of (a)
and (b), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
different periods were calculated from the integration area under the
3.5. Electrochemical measurements
luminescence curve. After irradiation for 8 h, Φ showed negligible
em
enhancement from 3.6% to 3.7% and the emission spectral shape was
slightly changed that may due to some trivial changes in the molecular
arrangement or packing. Thus, NPA-H presented high photostability
that is important for the practical application. Since the corrosion-
related results were obtained from coatings of the Schiff base mole-
cules, the emission spectrum of NPA-H thin film deposited on the steel
substrate (Fig. S14 in the ESI) was also measured and compared to the
powder emission spectrum. The compound thin film on the steel sub-
strate is still emitting with some change in relative emission intensities
along the different wavelengths of the visible range compared to the
powder emission spectrum. This change may attribute to the effect of the
small thickness of the film on the molecular aggregation.
3.5.1. Potentiodynamic polarization measurements (pp)
Fig. 7 represents the effect of the investigated inhibitors on the
anodic and cathodic polarization curves of 316L SS in 2 M H2SO4 so-
lution. Both the anodic and cathodic reactions were suppressed after
adding the investigated compounds, this indicates that the anodic
dissolution and the hydrogen evolution reaction were weakened by
these compounds. Parameters of the electrochemical corrosion kinetic
such as corrosion potential (Ecorr), β , β , and Icorr obtained from the
a
c
polarization curves extrapolation were summarized in Table S2 in the
ESI. From the parallel cathodic Tafel curves (Fig. 7), it can be speculated
that the hydrogen evolution is activation-controlled, and the presence of
the inhibitors does not affect the reduction mechanism. Adding the in-
hibitor to the acid solution broadens the region between the linear part
of anodic and cathodic branches of the polarization curves, similar re-
sults have been reported [62]. Increasing the inhibitor concentration
Fig. 7. Potentiodynamic polarization curves for the dissolution of 316L SS in 2 M H2SO4 in the absence and presence of the investigated inhibitors at 30 �C, (a) NPA-
2OH and (b) NPA-4OMe.
8

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
changed slightly the values of βa and βc to indicate the effect of these
Schiff bases on the metal dissolution and hydrogen evolution kinetics.
The studied inhibitors decreased both the cathodic and anodic currents
with a small shift in Ecorr (47 mV), Table S2 in the ESI.
constant, as evidence of the successful adsorption of the inhibitors on the
surface at both the cathodic and anodic sites. Deviation from the perfect
circuit shape may due to the frequency dispersion of interfacial
impedance. This phenomenon is commonly attributed to the metal
surface inhomogeneity that arises from the surface roughness or inter-
facial phenomena [63]. The impedance data obtained from other tech-
niques affirm the inhibition behavior of the studied Schiff bases as
inhibitors. From the data of Table S3, the presence of these additives
greatly improved IE% and significantly decreased Icorr values. The
reduction order in Icorr closely correlates with that found from the
studies of potentiodynamic polarization. Furthermore, the reduction in
Icorr values follows the same order found in Cdl values, so IE% found from
the electrochemical impedance spectroscopy and polarization curves are
in good agreement [64].
3.5.2. Electrochemical impedance spectroscopy (EIS)
EIS measurements were performed in acid solution (2 M H2SO4) at
30 �C in the presence or absence of the investigated inhibitors, Fig. 8.
The difference in impedance at higher and lower frequencies gives the
charge transfer resistance (Rct). Equation (2) provides the calculation of
Cdl and the frequency at which the impedance imaginary component is
maximal (-Zmax). Fig. S15 (in the ESI) shows the equivalent circuit that
illustrates the metal/electrolyte interface, where Rs, CPE, and Rct are the
solution resistance, constant phase element, and charge transfer resis-
tance, respectively. IE% and EIS parameters were calculated and pre-
sented in Table S3 (in the ESI). The impedance diagrams consist of one
big capacitive loop, as seen from Fig. S15 (in the ESI). The presence of
inhibitors in the acidic solutions enhanced the value of Rct to confirm
that the charge-transfer process controls the SS corrosion. The increase
in Rct values, and thus IE%, can be attributed to the formation of an
adherent film on the metal surface as a result of the gradual substitution
of water molecules by the molecules of the adsorbed inhibitor. There-
fore, we may conclude that the double layer thickness decreases by the
formation of this film that covers the metal surface. The values of Cdl
decrease to the maximum extent in the presence of inhibitors. Increasing
the inhibitor concentration decreased the values of Cdl at the metal/
solution interface as a result of the decrease in the local dielectric
3.5.3. Electrochemical frequency modulation (EFM)
For online corrosion monitoring, EFM is considered a new electro-
chemical technique [65]. EFM is a rapid and nondestructive technique
for the corrosion rate measurements, where EFM can give the corrosion
current values without any need to study the Tafel constants. The
non-linear nature of the corrosion process causes the potential distortion
by one or more sine waves to create responses at more frequencies than
the applied signal frequencies. However, with EFM this non-linear
response has information about the corroding system enough to calcu-
late the corrosion current directly. Causality factors are the reason
behind the great strength of the EFM, these causality factors work as an
internal check on the validity of the experimental EFM [66]. Fig. 9
Fig. 8. (a, b) Nyquist and (c, d) Bode diagrams for the dissolution of 316L SS in 2 M H2SO4 in the absence and presence of the investigated inhibitors at 30 �C, (a, c)
NPA-2OH and (b, d) NPA-4OMe.
9

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
displays the current response containing the input frequencies along
with the frequency components which are the multiples, difference, or
sum of the two input frequencies. Icorr, βa, βc, CF2, and CF3 were calcu-
lated from the larger peaks. These kinetic parameters of the electro-
chemical corrosion were determined at 30 �C using different
concentrations of the examined inhibitors in 2 M H2SO4 solution and
listed in Table S4 in the ESI. IE%, calculated from Equation (4),
increased with increasing the inhibitor concentrations. CF2 and CF3 in
Table S4 are similar to their theoretical values of 2.0 and 3.0,
respectively, demonstrating the good quality of the measured data. Also,
the calculated IE% from EIS measurements and Tafel polarization are in
good agreement with the values obtained from EFM measurements.
3.5.4. AFM study
Extra morphological estimation of the surface of 316L SS was ach-
ieved by AFM in the presence or absence of the investigated inhibitors at
30 �C with a concentration of (16 � 10À 6 M) after inundation for 12 h.
The AFM micrographs are presented in Fig. 10a–d and the average
Fig. 9. EFM spectra for the dissolution of 316L SS in 2 M H2SO4 in the absence and presence of the investigated inhibitors at 30 �C, (a) NPA-2OH and (b) NPA-4OMe.
10

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
roughness parameters were listed in Table S5 in the ESI. The micro-
graphs show that the 316L SS sample has a coarse surface caused by the
effect of the acid corrosion [67]. On the other hand, the 316L SS spec-
imens were smoothened in the presence of 16 � 10À 6 M of the investi-
gated inhibitors in 2 M H2SO4 solution which inhibited the surface
corrosion, Fig. 10c and d. It can be speculated that the reduction in the
316L SS specimens’ roughness (Table S5) is due to the creation of a thin
protective film of inhibitors adsorbed on the 316L SS surface.
combination of some or all the three mentioned types [68]. Regarding
inhibitors, the IE% depends on numerous factors such as molecular size,
number and charge density of adsorption sites, mode of interaction with
the metal surface, the formation of metallic complexes, and heat of
hydrogenation [69]. Most organic inhibitors have one or more polar
groups including an atom of sulfur, oxygen, or nitrogen, each atom
represents as a chemisorption center. The inhibition effect of these
compounds depends mainly on the electron density around the chemi-
sorption center, where higher electron density improves the efficiency of
the inhibitor. NPA-4OMe showed higher IE% than NPA-2OH. The co-
ordination surface is formed through the nitrogen and oxygen atoms of
the hetero ring, so these Schiff bases can be adsorbed in a flat orientation
via a bidentate form. It has been reported that the adsorption mode
3.5.5. Chemical structure of the studied inhibitors and the corrosion
inhibition
The first step in the mechanism action of the applied inhibitors is
their adsorption at the metal-solution interface. There are possible four
kinds of adsorptions of organic molecules at the metal-solution inter-
face: 1) interaction of the molecule unshared pairs of electrons with the
depends on the metal affinity towards the
π-electron clouds of the
aromatic-ring system [70]. This behavior of organic compounds can be
explained based on the relationship between the structure and corrosion
inhibition. IE% of organic compounds was correlated by linear free
metal, 2) interaction of the molecule
π electrons with the metal, 3)
electrostatic attraction of the charged molecules and metal, and 4)
Fig. 10. 3D AFM images of (a) 316L SS free in polished state, (b) exposed to 2 M H2SO4 solution, (c) exposed to 2 M H2SO4 solution containing 16 � 10À 6 M of NPA-
2OH, and (d) exposed to 2 M H2SO4 solution containing 16 � 10À 6 M NPA-4OMe after 12 h at 30 �C.
11

O. Younis et al.
Dyes and Pigments 175 (2020) 108146
energy relationships (LFER) with their Hammett constituent constants
as shown in Equation (4) [71].
withdrawing groups enabled controlling the energy gap between HOMO
and LUMO. Via this technique, the relative emission intensities along the
shorter and longer wavelengths of the visible range could be controlled.
The present strategy might allow the development of new organic ma-
terials capable of producing white-color emission from a simple and
single molecule.
log R ¼ - ρσ … … … .
(4)
where R is the corrosion rate,
ρ
is the reaction constant, while
σ
is the
Hammett constant and considered as a measure of the electron density
around the reaction center.
ρ
values are positive or negative for con-
is the slope of the plot of vs.
Declaration of competing interest
stituents that attract or donate electrons.
ρ
σ
log IE% or R, and its sign elucidates if the process was inhibited by the
electron density increase or decrease at the reaction center. The sensi-
tivity of the inhibition process to the electronic effects is indicated by the
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.
magnitude of ρ. If the adsorption centers are not conjugated to the ring,
this may weaken the dependence of the adsorption character on the
electron density of the ring. Also, the electron-releasing substituents
increase the inhibition. NPA-4OMe has the highest inhibition efficiency
CRediT authorship contribution statement
Osama Younis: Investigation, Formal analysis, Writing - original
draft, Writing - review & editing. Emad E. El-Katori: Investigation,
Formal analysis, Writing - original draft. Reda Hassanien: Supervision,
Investigation, Formal analysis, Writing - original draft. Ashraf S.
Abousalem: Investigation, Formal analysis, Writing - original draft.
Osamu Tsutsumi: Supervision.
(IE% ¼ 93%) as a result of the high electron-donating p-OCH3 group (
σ
¼ – 0.27) which enhances the delocalized π-electrons and its oxygen
atom may add an active center to the molecule. NPA-2OH has IE% ¼ 89
that is lower than that of NPA-4OMe because the former one has an o-
OH group in the benzylideneamino moiety which has a lower sharing
electron density to the molecule as a result of the steric hindrance effect
due to its presence in the o-position.
Acknowledgments
3.5.6. Monte Carlo simulations
A part of the corrosion system was modeled and studied using the
Monte Carlo search code by setting one inhibitor molecule for free
interaction with Fe (110) surface [72,73]. The simulation studies were
performed on the lowest energy form of the compounds. In Monte Carlo
simulation, the system reaches an equilibrated state when the variation
in the temperature and energy become in a balanced state [74]. The
output energy parameters such as adsorption energy, deformation en-
ergies, total adsorption, and rigid absorption obtained from Monte Carlo
simulations were listed in Table S6 (in the ESI) and taken when the
system reaches an equilibrium state. Fig. S16 (in the ESI) presents the
equilibrium configuration of the inhibitors tested in the presence and
absence of aqueous solution. It was found that all inhibitors take a
near-flat position on the Fe surface, indicating higher surface coverage
of compounds on the corroding metal substrate. This, in turn, gives
OT and OY acknowledge financial supports from Japan-Egypt
Research Cooperative Program (JSPS and MOSR-STDF), JSPS KAKEN-
SHI (18K05265, and 18H03764), and Cooperative Research Program of
the Network Joint Research Center for Materials and Devices (Tokyo
Institute of Technology).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2019.108146.
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