New salicylaldehyde azine esters: Structural, aggregation induced fluorescence, electrochemical and theoretical studies

Article abstract of DOI:10.1016/j.molliq.2020.114029

The establishment of high-tech products relying on organic optoelectronics is focused towards latest strategies for improving the processability and performance. In the present study, two esters of salicylaldehyde azine SAE-F and SAE-NF were synthesized and their structural characterization were performed using FT-IR, ¹H and ¹³C NMR, UV–visible spectroscopy, mass spectrometry and single crystal-XRD technique. The bathochromic shift observed in luminescence measurements from solution to aggregate/solid state, solvent based fluorescence studies, SEM and DLS measurements confirmed the fluorescence emission from the SAE-F aggregates. The DSC data indicated the phase transitions, and the TGA measurements confirmed the thermal stability of SAE-F to withstand temperatures up to 225 °C. The morphological characterization revealed formation of homogeneous pin-hole free uniform films of SAE[sbnd]F. Cyclic voltammetry was used to estimate the HOMO and LUMO energy levels which were found to be ?5.46 eV and ?3.05 eV respectively with a band gap of 2.41 eV. The p-type semi-conductivity of SAE-F was established using hot probe technique. The dielectric measurements were carried out to measure dielectric constant and ac conductivity at different frequency ranges. The electronic data obtained based on Density Functional Theory calculations were in conformity with the experimental results. Successful fabrication of a diode and the recorded current-voltage characteristics clearly advocated SAE-F to be an excellent candidate for electro-optical devices.

Full text of DOI:10.1016/j.molliq.2020.114029

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New salicylaldehyde azine esters: Structural, aggregation induced
fluorescence, electrochemical and theoretical studies
Sneha Kagatikar, Dhanya Sunil, Dhananjaya Kekuda, Suresh D.
Kulkarni, Abdul Ajees Abdul Salam
PII:
S0167-7322(20)32993-7
DOI:
https://doi.org/10.1016/j.molliq.2020.114029
MOLLIQ 114029
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
10 May 2020
27 July 2020
8 August 2020
Please cite this article as: S. Kagatikar, D. Sunil, D. Kekuda, et al., New salicylaldehyde
azine esters: Structural, aggregation induced fluorescence, electrochemical and theoretical
studies,
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of
Molecular
Liquids
(2020),
https://doi.org/10.1016/
j.molliq.2020.114029
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New salicylaldehyde azine esters: Structural, aggregation induced
fluorescence, electrochemical and theoretical studies
Sneha Kagatikar1, Dhanya Sunil1*, Dhananjaya Kekuda2, Suresh D Kulkarni3, Abdul Ajees
Abdul Salam3
1Department of Chemistry, Manipal Institute of Technology, Manipal Academy of Higher
Education, Manipal-576104, Karnataka, India.
2Department of Physics, Manipal Institute of Technology, Manipal Academy of Higher
Education, Manipal, 576104, Karnataka, India.
3Department of Atomic and Molecular Physics, Centre for Applied Nanosciences, Manipal
Academy of Higher Education, Manipal, 576104, Karnataka, India.
* Corresponding author
Dr. Dhanya Sunil,
Associate Professor (Senior Scale),
Department of Chemistry,
Manipal Institute of Technology,
Manipal, Karnataka, India- 576104
E-mail: dhanya.s@manipal.edu

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New salicylaldehyde azine esters: Structural, aggregation induced
fluorescence, electrochemical and theoretical studies
ABSTRACT
The establishment of high-tech products relying on organic optoelectronics is focused
towards latest strategies for improving the processability and performance. In the present study,
two esters of salicylaldehyde azine SAE-F and SAE-NF were synthesized and their structural
1
13
characterization were performed using FT-IR, H and C NMR, UV-visible spectroscopy, mass
spectrometry and single crystal-XRD technique. The bathochromic shift observed in
luminescence measurements from solution to aggregate/solid state, solvent based fluorescence
studies, SEM and DLS measurements confirmed the fluorescence emission from the SAE-F
aggregates. The DSC data indicated the phase transitions, and the TGA measurements confirmed
the thermal stability of SAE-F to withstand temperatures up to 225 °C. The morphological
characterization revealed formation of homogeneous pin-hole free uniform films of SAE-F.
Cyclic voltammetry was used to estimate the HOMO and LUMO energy levels which were
found to be -5.46 eV and -3.05 eV respectively with a band gap of 2.41 eV. The p-type semi-
conductivity of SAE-F was established using hot probe technique. The dielectric measurements
were carried out to measure dielectric constant and ac conductivity at different frequency ranges.
The electronic data obtained based on Density Functional Theory calculations were in
conformity with the experimental results. Successful fabrication of a diode and the recorded
current-voltage characteristics clearly advocated SAE-F to be an excellent candidate for electro-
optical devices.
Keywords: Aggregation induced emission, Diode, Optoelectronics, Salicylaldehyde azine ester
1. Introduction
Organic fluorophores are chemical entities that display luminescence upon light
excitation, and have evolved as indispensable materials in diverse fields of science and
technology. Interestingly, many organic luminophores exhibit intense and often tunable emission
features across the whole visible spectrum, permitting extensive applications mainly in the field
of life sciences as fluorescent markers and biological probes [1, 2], and most importantly in the

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fabrication of organic optoelectronic lighting and display devices [3–5], organic thin film
transistors [6] and fluorescent sensors [7]. Over the past few years, extensive research have been
driven in the design and synthesis of organic fluorophores that can function as desirable light
emitting components in device architectures for the next generation solid state lighting and
flexible flat panel display technology, unlike their inorganic counterparts [8–12]. In this context,
design of new organic light emitting materials is quite challenging, and study of their structure
and solid state properties have attracted much attention in improving the device efficiency [13].
Though organic materials are used either in their thin film or aggregate states in
optoelectronic devices, it is highly challenging to attain good solid-state emissive and
electrochemical properties, as most of them are strongly inclined to aggregation-caused
luminescence quenching [14]. The discovery of a novel phenomenon called aggregation-induced
emission (AIE) by Luo et al in the past decade provided a new insight for the construction of
organic solid luminophores that offer high quantum yield both in their aggregated state and
concentrated solutions [15–17]. Thus AIE features can be tapped to develop organic fluorophores
for potential optical applications in light emitting devices. Salicylaldehyde azine (SAA) and its
derivatives that comprise of N-N single bond are reported to exhibit strong AIE behavior with
their luminescence emission ranging from green to red based on the varying substituents attached
to the phenyl ring [18–20]. Moreover, SAA derivatives display excited-state intramolecular
proton transfer (ESIPT) that considerably decreases the self-quenching of luminescence,
contributing to high quantum efficiency and large Stokes shift in their aggregated
phase/concentrated solutions [21]. Moreover, the luminescence of these fluorophores can be
quenched if the hydroxyl groups placed at the ortho-position of the benzene unit that drive both
ESIPT and AIE features are blocked, but later can be recovered once they are freed [22].
Adorning SAA with varying substituents can thus be a vital approach to develop AIE-
active materials with assorted fluorescence colors that can render impending applications in
optoelectronic device fabrication. Therefore, in the present study two salicylaldehyde esters
(SAEs), viz. an unsymmetrical ester with the ortho-hydroxyl functionality of one of the phenyl
rings free (SAE-F), and a symmetrical ester with both the ortho-hydroxyl groups blocked (SAE-
NF) were prepared using simple experimental protocols. The AIE characteristics along with
structural, morphological, thermal, electrochemical and electrical properties were examined.
Based on the results assimilated, an enhanced understanding on the AIE behavior of SAE was

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acquired. DFT calculations were performed to obtain more insights into the electronic properties.
The response of SAE to an externally applied ac signal was also examined. Unoptimized bi-layer
device was fabricated with the new p-type ester SAE-F that can potentially perform as a
functional candidate for OLED applications.
2. Materials and methods
The chemicals and solvents used for the present investigation were procured from Sigma,
Finar and Spectrochem Chemicals Pvt. Ltd. The mixture of SAEs obtained after the completion
of reaction were separated using automated AKROS Flash Chromatography System. Melting
points were determined using open capillary method and are uncorrected. The IR spectra were
recorded in a Schimadzu FTIR spectrophotometer. 1H and 13C NMR spectra were obtained using
400 MHz Bruker spectrometer with CDCl3/DMSO-d6 as solvent and TMS as internal standard.
Agilent 6430 Triple Quad with electrospray ionization was used to record the mass spectrum.
The single cystal data was collected at 293 K using a Bruker Smart Apex Duo single crystal X-
ray diffractometer and the structure was solved by Olex2.solve [23] structure solution program
using Charge Flipping, and refined with the olex2.refine [24] refinement package using Gauss-
Newton minimization. Hydrogen atoms were treated as riding on their carbon parent atoms, and
all carbon bound H atoms were placed at calculated positions and were refined as riding with
their Uiso set to 1.2Ueq for CH, 1.5Ueq for CH2 and CH3 of the respective carrier atoms. The
absorbance and emission spectra were taken in 1800 shimadzu UV-visible spectrophotometer
and JASCO spectrofluorometer FP 8300 respectively. DLS (dynamic light scattering)
measurements were performed on a Malvern Zetasizer Nano ZS instrument. Spin coating was
carried out using Apex instruments Co to obtain thin films on glass slides (1.5×1.5 cm - Labtech
Pvt.Ltd) and the coating thickness was measured by means of Bruker DektakXT profilometer.
The thermal evaporation process was achieved using vacuum coating unit model no-12A4D from
Hind High Vacuum Co. (P) Ltd. The surface morphological features of the aggregates and thin
films formed were observed using Carl Zeiss EVO 18 analytical scanning microscope (SEM)
and INNOVA SPM atomic force microscope (AFM). Differential scanning calorimetry
thermogram was recorded using Mettler Toledo DSC-3 instrument at a constant heating rate of 5
°C min-1, whereas thermogravimetric analysis was carried out under nitrogen atmosphere using
Perkin Elmer TGA-4000 thermal analyser at a heating rate of 20 °C min-1. Cyclic voltammetry

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study was carried out using a Ivium Vertex electrochemical workstation and the measurements
were taken in a three-electrode conventional cell system using SAE-F coated ITO plate as
working electrode, calomel electrode as reference electrode and platinum wire as counter
electrode, dipped in 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as
supporting electrolyte and ferrocene/ferrocenium as internal reference at a scan rate of 100 mVs-
1. The response of SAE-F thin films to an externally applied ac signal at room temperature was
examined through dielectric spectroscopy studies using Keithley 4200 SCS semiconductor
parameter analyzer in two probe configuration. To prepare the diode, pre-cleaned indium tin
oxide (ITO) glass substrate was exposed to plasma for 20 min, and poly(3,4-
ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS) film was spin coated on to the
ITO glass and dried at 120 C for 10 min. SAE-F was spin coated and tris-(8-
hydroxyquinoline)aluminum (Alq3) was thermal evaporated on the polymer layer and the top
contact of aluminium (Al) was introduced to complete the diode. All the computational
simulations and calculations were performed using B3LYP function of Schrödinger's Materials
Science Suite with 6-31G++ basis set in vacuum [25,26].
3. Results and discussion
3.1. Synthesis
Salicylaldehyde (SA) was used as the major precursor for the synthesis of salicylaldehyde
azine (SAA), which was subsequently esterified to obtain salicylaldehyde azine esters (SAEs):
fluorescent SAE-F and non-fluorescent SAE-NF.
3.1.1. Synthesis of 2,2'-[hydrazine-1,2-diylidenedimethylylidene]diphenol (SAA)
About 0.3 mmol of hydrazine hydrate was added drop wise to 0.6 mmol of SAA in
ethanol, and was refluxed for 3h. The yellow crystals obtained were filtered, washed with
ethanol and dried.
3.1.2. Synthesis of Salicylaldehyde azine ester (SAE)

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About 1 mmol SAA in acetone was refluxed with 2 mmol ethyl chloroacetate and 4
mmol potassium carbonate (K2CO3) for 28 h at 80 °C. The solvent was evaporated, ice-cold
water was added and the precipitate formed was filtered, washed with water and dried to obtain a
mixture of two SAEs: an unsymmetrical fluorescent SAE-F and a symmetrical non-fluorescent
SAE-NF. The products were separated by flash chromatography using ethyl acetate and hexane
(2:8) as eluents to obtain SAE-F and SAE-NF. The synthetic route for SAE is presented in
Scheme 1.
Scheme 1: Synthesis of salicylaldehyde azine esters
The spectral data obtained were in agreement with the proposed structures of SAE-F and SAE-
NF.
3.1.2.1. Ethyl (2-{[(2-hydroxybenzylidene)hydrazinylidene]methyl}phenoxy)acetate (SAE-F)
White needles (35%); m.p: 148-150 C; UV-Vis (THF, nm) (Fig. S1): 298, 354; FT-IR (KBr,
cm-1) (Fig. S2): 3066 (Ar. C-H str.), 1739 (C=O str.), 1618 (C=N-N=C str.), 1205 (C-O-C str.);
1H NMR (CDCl3, 400 MHz) (Fig. S3): δ 1.286-1.321 (t, 3H, CH3, 7.2 Hz), 4.255-4.309 (q, 2H,
CH2, 7.2 Hz), 4.739 (s, 2H, OCH2), 6.828-6.848 (d, 1H) ,6.925-6.965 (m, 1H) 7.017-7.038 (d,
1H), 7.069-7.106 (t, 1H), 7.342-7.384 (m, 2H), 7.404-7.448 (m, 1H), 8.133-8.157 (dd, 1H),
8.788 (s, 1H, CH=N), 9.128 (s, 1H, CH=N); 13C NMR (DMSO-d6, 100 MHz) (Fig. S4): δ
13.958, 60.724, 65.343, 111.537, 113.453, 116.405, 118.191, 119.504, 121.613, 121.936,
123.309, 126.614, 131.036, 132.997, 133.121, 157.130, 158.616, 162.865; MS C18H18N2O4 (ESI,
m/z) (Fig. S5): 327.1 (M+1).
3.1.2.2.
Diethyl2,2'-(((hydrazine-1,2-diylidenebis(methanylylidene))bis(2,1-phenylene))bis
(oxy))diacetate (SAE-NF)
Yellow crystals (46%); m.p: 116-118 C; UV-Vis (THF, nm) (Fig. S6): 296, 334; FT-IR (KBr,
cm-1) (Fig. S7): 3074 (Ar. C-H str.), 1753 (C=O str.), 1608 (C=N-N=C str.), 1211 (C-O-C str.);
1H NMR (CDCl3, 400 MHz) (Fig. S8): δ 1.276-1.312 (t, 6H, CH3), 4.236-4.308 (m, 4H, CH2),
4.696 (s, 4H, OCH2), 6.820-6.841 (d, 2H), 7.055-7.092 (t, 2H), 7.370-7.414 (m, 2H), 8.152-
13
8.175 (dd, 2H), 9.157 (s, 2H, CH=N). C NMR (CDCl3, 100 MHz) (Fig. S9): δ 14.36, 61.69,

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66.29, 112.65, 122.23, 123.81, 127.98, 132.50, 157.44, 157.75, 168.81. MS C22H24N2O6 (ESI,
m/z) (Fig. S10): 413.1 (M+1).
3.2. Single crystal-XRD
Single crystals of SAE-NF were grown using 1:1 mixture of methanol and water.
Supplementary Table 1 (ST1) summarizes the crystal data and structure refinement details of
SAE-NF. The molecule crystallized in a monoclinic system with space group C2/c, and the
asymmetric unit was composed of a half molecule. The entire molecule was generated through
an inversion centre by the symmetry equivalent 1/2-x,1/2-y,-z through the midpoint of the N-N
bond (Fig. 1A). The complete molecule adopted a co-planar conformation creating an extended
conjugated system. The selected geometric parameters and hydrogen-bond geometries of SAE-
NF are presented in ST2–6. The PLATON [27] calculations showed that there is no deviation in
bond lengths and bond angles within 5.0  level, except the final R-factors, which are slightly
high (ST1) due to incomplete data. We are in the process of improving both the crystal quality as
well as data collection methodology. Though the structure quality needs some improvement, the
possible hydrogen bonding and packing may provide some insight into understanding the three-
dimensional arrangements of the molecule. Overall, a four possible hydrogen-bonding pattern
was observed with strong intermolecular C6-H6…O3 (donor-hydrogen distance: 0.930(6)Å,
hydrogen-acceptor distance: 2.385(5)Å, donor-acceptor distance: 3.301(5)Å, donor-acceptor
angle: 168.2(4)°, and symmetry: x,1-y,1/2+z), intramolecular C1-H1…O1 (donor-hydrogen
distance: 0.931(5)Å, hydrogen-acceptor distance: 2.364(5)Å, donor-acceptor distance:
2.704(5)Å,
donor-acceptor angle: 101.2(4)°). In addition to C-H...O interactions, a …
interaction between the phenyl rings (symmetry: 1/2-x,1/2-y,1-z) with a distance of 3.742(3)Å
existed. Besides, a weak C-H..N intermolecular interaction with C4-H4…N1 (donor-hydrogen
distance: 0.929Å, hydrogen-acceptor distance: 2.703Å, donor-acceptor distance: 3.602Å
donor-acceptor angle: 163.25°) was also observed.
The angle between the two phenyl rings was zero degree, which suggested that the
molecule was planar, and the C9=O3 of SAE-NF was slightly puckered away from the plane
with an angle of 11.44°. Though C-H…N intramolecular hydrogen bond was absent, SAE-NF
displayed weak C-H…O intramolecular hydrogen bonds instead of C-H…N. Fig. 1B shows the

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molecular packing of SAE-NF, which in turn is related to the association of each individual
molecule with another. A single-molecule of SAE-NF was connected through hydrogen bond
network with four adjacent molecules as shown in Fig. 1C. The extended ester arms of the
phenyl rings played a significant role interconnecting the molecules via C-H…O and C-H..N
hydrogen bonds (Fig. 1C). Each SAE-NF molecule was found to orient at a slope of 31.62° and
formed three layers with four another molecules of the same kind. The two symmetry molecules
were situated in the center, and other two symmetry molecules were positioned at both the ends
with an equidistance of 3.497Å (Fig. 1D).
Fig. 1. (A) The molecular structure of SAE-NF with atom numbering shown in thermal ellipsoid
with a 50% probability level, (B) molecular packing diagrams of SAE-NF and (C) close view of
hydrogen bond network of SAE-NF. The molecules are depicted in ball and stick model, and the
carbon atoms of the parent model are displayed in green. The adjacent four molecules involved
in the hydrogen bonding network are marked as Mol1, Mol2, Mol3, and Mol4 respectively. (D)
Arrangement of symmetry molecules (red) in a plane concerning the parent atom (blue) is shown
for SAE-NF. The distance between the planes and the angle are marked.
Further, the intermolecular interactions of SAE-NF were studied using the Hirshfeld
surface (HS) analysis, together with three-dimensional (3D) HS maps, two-dimensional (2D)
fingerprint plots (FP), and electrostatic energy frameworks. The HSs mapped with dnorm and the
respective shape index associated 2D-FPs were generated using the CrystalExplorer software
(Version 3.1), and the CIF of SAE-NF structure was used as the input file. The C–H⋯O
hydrogen bonds appeared as large red areas and C-H…N as small red areas (Fig. 2A). The
contacts highlighted in red in the HS maps denote the intermolecular associations with distances
shorter than the sum of the van der Waals radii [28]. Fig. 2B-F illustrates the FPs for SAE-NF
for All, O-H (18.8%), C-H (16.6%), N-H (5.4%) and H-H (52.5%) contributions. The three
major spikes belonged to O-H, and H-H contributions, whereas four minor spikes represented C-
H and N-H (Fig. 2B). The H⋯H contacts presented a relatively large contribution of 52.5 %
(Fig. 2F). The Fig. S11 and ST7 summarizes the other interactions (C-All, H-All, O-All and N-
All) with their percentage contributions.

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Fig. 2: (A) Hirshfeld surfaces (HS) for SAE-NF (HSs are mapped with dnorm). The red spots
indicate the possible hydrogen bond, which are marked appropriately. 2D fingerprint plots of (B)
SAE-NF full, (C) O-H, (D) C-H/H-C, (E) N-H/H-N and (F) H-H are presented. The minor
contributions of C-C (3.0%), C-N or N-C (0.8%), C-O or O-C (0.5%), N-N (0%), N-O or O-N
(0.6%), and O-O (1.9%) are not shown.
In order to gain comprehensive information on the crystal packing and better visualize
the interaction topologies of SAE-NF, the energy framework analysis was performed using
CrystalExplorer software (Version 3.1). The electrostatic, dispersion, polarization and exchange-
repulsion energies of interaction between molecules were calculated for SAE-NF (ST8) using
B3LYP/6-31G(d.p) electron density functions. Energies between molecular pairs are presented
as cylinders (radius 180) by joining the centroid pairs of molecules. The energy frameworks for
Eele (Fig. 3A), Edisp (Fig. 3B), and Etot (Fig. 3C) are shown in red, green, and blue,
respectively. The largest pairwise energy calculated was -53.9 kJ/mol (ST8), and the dispersion
energy dominated the electrostatic energy. The dominating interactions were found between N-N
bonds and the energy frameworks formation was like crossed molecular sheets.
Fig. 3: Energy frameworks for SAE-NF. View of molecular arrangement of SAE-NF down
along the c axis are shown in ball and stick models. A) Represents the electrostatic term
(highlighted in red), B) dispersion term (green), and C) total interaction energy (blue). The
hydrogen atoms are omitted for clarity.
In spite of several trials, the single crystals obtained for SAE-F were too small to be used for SC-
XRD analysis.
3.3. Photo-physical measurements
In order to gain an in-depth understanding of light-emitting mechanisms of SAEs, their
solid-state emission characteristics were observed. Though the fluorescence wavelengths were
almost same for both the esters (SAE-F: 546 nm and SAE-NF: 548 nm), SAE-F emitted with a
five-fold fluorescence intensity compared to SAE-NF (Fig. 4A). Due to high greenish-yellow

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emissive feature and a quantum yield of 0.28, SAE-F was further investigated as an OLED
material.
3.3.1. Solution phase photo physical characteristics:
The absorption and emission spectra of SAE-F in various solvents (1×10-4 M) are
presented in Fig. S12. No appreciable shift in the absorption and emission wavelength occurred
with varying solvent polarity. Interestingly, the fluorescence spectra of all the studied SAE-F
solutions showed an emission maxima in the range of 428-446 nm in comparison with a red
shifted fluorescence maxima observed at 546 nm in solid state. As maximum fluorescence
intensity was observed for SAE-F dissolved in THF among the studied solvents, all further
solution phase photophysical measurements were performed using THF as solvent. The
absorption maxima of THF solutions of SAE-F observed at 298 and 353 nm corresponded to
ππ* and nπ* transitions, respectively. The same solution exhibited maximum fluorescence
at 435 nm, whereas a significant bathochromic shift of 113 nm was observed in its solid state
emission. The appearance of a new peak in the solid state was interesting and could be due to the
formation of J-aggregates leading to aggregation induced emission (AIE).
3.3.2. Investigation of AIE feature of SAE-F
Classically, AIE phenomena arise predominantly due to restriction of intramolecular
motions (RIM), which is further accompanied by aggregation. The self-alignment of molecules
to form supramolecular aggregates generally occurs under the influence of either concentration,
solvent or an additive.
Concentration based studies: To explore the development of J-aggregates, the absorbance and
the emission spectra of SAE-F in THF at various concentrations were recorded (Fig. 4B and C).
Interestingly, a concentration dependent decrease in the emission intensity at 435 nm was
observed. Besides, the spectra exhibited a weak new emission at 545 nm, which could be due to
aggregation of SAE-F molecules with rise in concentration. The observed concentration
dependent reduction in fluorescence intensity at 435 nm owing to non-aggregated form, and the
appearance of new red shifted emission at 545 nm with increasing intensities with rise in SAE-F
concentration established the formation of J-aggregates in their concentrated solutions. We

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assume that the intermolecular forces such as dipole-dipole, van der walls may be responsible for
this kind of aggregation.
Fig. 4. A) Solid state emission spectra of SAE-F and SAE-NF, B) absorption spectra of SAE-F
in THF and C) emission spectra of SAE-F in THF showing concentration dependent aggregate
formation.
Solvent-based studies: The organic AIEgens are generally insoluble in water. Consequently,
addition of large amounts of water to an AIEgen dissolved in a suitable solvent can destabilize
the solution to trigger their aggregation [29]. Hence the ability of SAE-F to exhibit AIE was
further investigated by recording the emission spectra of its THF solution on addition of different
water fractions to induce aggregate formation (Fig. 5A). About 1×10-3 M SAE-F stock solution
was prepared and appropriate amounts of THF/THF and water were successively added and
mixed well to furnish 1×10-4 M solutions of various water fractions (0–99 vol %). The addition
of increasing amount of water led to the reduction in fluorescence emission observed at 435nm.
However, the solution phase fluorescence intensity at 545 nm plotted against the water fractions
(Fig. 5B) displayed a drastic increase in the intensity when the water fraction rose to 90%. This
enhancement in fluorescence intensity could be attributed to the phenomenon of restricted
inhibition to intramolecular rotation in the aggregated form of SAE-F, wherein the conformation
of the molecule is fixed and the non-radiative pathway is blocked, which is explained in the
subsequent section. However, the emission intensity was lowered on further rise in water volume
to 99%, which could be ascribed to the reduced solubility of SAE-F at this higher water content
in the mixture [30].
The formation of aggregated SAE-F nanoparticles was evident from the particle size
analyses using DLS. The particle size of SAE-F in THF (0% water) could not be detected by the
analyzer, thereby confirming the complete dissolution of SAE-F as isolated species. However, an
increase in the average particle size to 179 nm was observed in THF-water mixture containing
90% water, proving the larger size due to aggregate generation (Fig. S13). Besides, the
aggregate formation was further established by analyzing the SEM images of SAE-F in THF
alone and in THF-water mixture with 90% water. The presence of aggregated SAE-F
morphology was clearly evident from Fig. S14.

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3.3.3. Mechanism of AIE
One of the major contributing factors for AIE is restriction of intramolecular rotation
(RIR) [17]. Thus in order to explore whether the AIE property of SAE-F was due to hindrance to
intra-molecular rotations, solution thickening studies were adopted by addition of increasing
concentration of a viscous liquid like glycerol. The emission characteristics of methanolic
solution (1×10-4 M) of SAE-F with different fractions of glycerol (completely miscible with
methanol) were recorded and is presented in Fig. 5C. As expected, a gradual increase in the
fluorescence at 545 nm, which peaked to a maximum for 90% glycerol solution, and was four
times more intense than in pure methanol was observed. The emission wavelength did not show
any shift with different glycerol fractions, which advocated that the steady escalation in emission
intensity was mainly due to the viscosity effect, confirming the RIR mechanism for AIE of SAE-
F [31].
Fig. 5. A) Emission spectra of SAE-F in THF with increasing water fractions, B) bar diagram of
% water fractions vs. intensity at 545 nm and C) emission spectra of SAE-F in methanol with
increasing glycerol fractions.
The chemical structure of SAE-F is comprised of salicylaldimine and benzaldimine units
which are connected via a rotatable N−N, rather than a C−C bond. The ortho-hydroxyl group in
the salicylaldimine unit can engage in intramolecular hydrogen bonds, thereby restricting the N-
N bond rotations in the aggregated state (Fig. 6). This can rigidify the SAE-F molecular structure
and activate the RIR process, thus endowing efficient solid/aggregate phase emission [20,32].
Fig. 6. Mechanism of AIE and ESIPT phenomena involved in SAE-F.
3.4. Thermal Characterization
It is important to ensure the capability of SAE-F to withstand high temperature ranges for
potential use in optoelectronic devices. DSC and TGA analysis that play a vital role in
understanding phase transitions were carried out to gain insights on the thermal stability of the

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fluorophore. The DSC thermogram of SAE-F presented in Fig. 7A indicated a sharp endothermic
o
peak at 153 C due to its melting, which was in accordance with that determined by open
capillary method. Besides, the thermogram did not reveal any other heat changes suggesting its
o
stability to endure temperature up to 250 C. The TGA curves of SAE-F shown in Fig. 7B
o
indicated towards a one-step weight loss. The compound was highly stable up to 225 C,
followed by a gradual decrease in the mass till ~340 oC. Initial weight loss of ~71 % may be due
to the breakdown of side chains and gradual degradation of backbone structure [33].
Fig. 7. A) DSC and B) TGA thermograms of SAE-F under nitrogen atmosphere
3.5. Measurement of optical parameters of thin films
Photophysical measurements in solution gave a clear idea about the RIR mechanism for
AIE phenomenon with J-aggregate formation, but it was important to know the optical property
of the thin films formed by SAE-F for the OLED fabrication. THF solutions of different weight
percentages (0.5%, 1%, 1.5% and 2%) of SAE-F were spin coated on clean glass slides by
varying the rotations per minute (1000, 1500 and 2000 rpm) to prepare thin films of varying
thickness. The results of the optical measurements performed are presented in Fig. S15 and data
are presented in ST9. It was observed that 150-200 nm thick spin coated film obtained by spin
coating 1 weight% SAE-F solution at 1500 rpm displayed maximum fluorescent intensity.
The difference in the structure and properties between the ground and excited states of
luminophores is indicated by Stokes shift [20,33-35]. The appreciably high Stokes shift of 7000
cm-1 calculated for SAE-F suggested that the emitted radiation is not re-absorbed. The optical
band gap (Egopt) which involves the excitation of electrons from the valance band to conduction
band using photons of selected frequency was found to be wide for SAE-F with 3.72 eV in its
thin film state. The effective number of the electrons involved in the transition between the
ground and excited state is denoted by the oscillator strength, which is proportional to the area
under the absorption band. The strength of transition or oscillator strength for SAE-F was
determined as 0.00047 (value between 0 to 1 is ideal) [36,37].
The material performance for electroluminescence was evaluated by the comparison of
fluorescence emission color with the color coordinates. With the help of emission spectra of thin

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films at 397 nm, CIE chromaticity coordinates of SAE-F were found to be x = 0.45 and y = 0.53
and is presented in Fig. 8. With these coordinates it can be noted that SAE-F can be used as
potential candidate for the fabrication of yellowish-green OLED.
Fig. 8. Chromaticity diagram of SAE-F
3.6. Morphological analysis of thin films
The formation of amorphous and uniform thin films is crucial for any device application
[34] as their transport characteristics are strongly related to film morphology. The surface of thin
films obtained by spin coating (1 weight % at 1500 rpm; 150-200 nm) was carefully monitored.
The SEM images presented in Fig. 9A validated the ability of SAE-F to form homogeneous and
uniform pin-hole free films. Examination of intrinsic geometry and micro structural dimensions
of thin films through AFM revealed the roughness vales such as Rmax, Ra and Rq to be 149, 23.2
and 26.7 nm respectively. Topographical structures, porosities and film qualities of the spin
coated films are portrayed in AFM photographs shown in Fig. 9B.
Fig. 9. A) SEM and B) AFM and of SAE-F spin coated on glass substrate.
3.7. Electrochemical measurements
The two important parameters for any electroluminescent material are highest occupied
and lowest unoccupied molecular orbitals (HOMO & LUMO), which are related to its redox
potentials and decides the hole-/electron-injecting ability in organic light emitting diodes. In
order to measure the HOMO/LUMO energy levels and the charge injection capability of SAE-F,
its thin film was subjected to cyclic voltammetry (CV) analysis [15, 35]. The cyclic
voltammogram depicted in Fig. S16 exhibited the reduction onset and oxidation onset potentials
at -1.25 V and 0.16 V respectively. In comparison with the onset potentials of ferrocene (4.8 eV
versus vacuum), the HOMO and LUMO energy levels of SAE-F were estimated to be -3.05 eV
and -5.46 eV respectively with an electrochemical band gap of 2.14 eV. The electrochemical

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band gap clearly indicated the wide band gap semiconducting behavior of SAE-F and also
enabled to choose the materials for various layers to fabricate the diode [38–40].
Further, in order to perceive the hole-transport (p-type) and electron-transport (n-type)
behavior of SAE-F, ‘hot-probe’ experiment was employed [41]. The application of hot and cold
probes resulted in a negative readout, which confirmed SAE-F to be a p-type semiconducting
material.
3.8. Diode Fabrication and measurements
A diode was constructed using SAE-F with the configuration ITO/PEDOT:PSS/SAE-
F/Alq3/Al. ITO was used as transparent anode on which PEDOT:PSS was spin coated to form
the hole transport layer. SAE-F was spin coated (1500 rpm) on PEDOT:PSS layer after
dissolving in THF. Alq3 as electron transport layer was thermal evaporated on the SAE-F layer
under vacuum (7×10-6 mbar) and a layer of Aluminum was also vacuum evaporated on Alq3
which completed the diode structure.
An efficient diode was prepared using SAE-F and plots of current vs voltage and log I vs
voltage are shown in the Fig.10A and B respectively. The forward biased I-V curve recorded for
the voltage range from -2 to 4 V revealed that the leakage current is very low and was in the
order of 10-9 A at -2 V. Ideality factor of the diode, which is a measure of how closely the
constructed diode obeys ideal diodic equation is generally one for an ideal diode, but can be
more than one for p-n diodes. The ideality factor of the fabricated diode was calculated to be
55.28 [42,43]. RR factor, which is the ratio of the forward current to reverse current at a
particular applied voltage was found to be 125.86. High values of RR factor is an indication of
high charge transfer in the device [44]. These results justified that SAE-F can be used as p-type
material to fabricate a diode.
Fig. 10: Plot of A) current vs voltage and B) ln(I) vs voltage. C) Dielectric constant and D) ac
conductivity of SAE-F as a function of frequency
3.9. Dielectric studies

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Dielectric constant is an important parameter which is the ratio of permittivity of
substance to the permittivity of free space. ITO coated plate was used as one of the electrodes
above which ~225 nm of thin films were spin coated and thermally evaporated, accordingly. The
prepared device worked well as a parallel plate capacitor with an active area of 0.02 cm2.
Dielectric property of the SAE-F coated device was studied in the frequency range of 1 kHz to
50 kHz and frequency dependent dielectric constant was calculated to be ~17.3 for SAE-F as
shown in Fig. 10C. With increase in the frequency, there was marginal decrease in the dielectric
constant which was due to the inability of the interfacial polarization to respond to the high
frequency. Further, the ac conductivity (??????) of the device was found to be 6.16×10-7 ohm-cm
at 20 kHz. Fig. 10D denotes the power law which relates to the short range hopping of the charge
carriers through the trapped sites and the plot showed a linear increase in the ac conductivity
with increasing frequency which could be due to the dipole-dipole interaction between the ionic
charges [45].
3.10. Theoretical studies
The DFT simulations were performed with Schrodinger material suite software package
to generate the optimized molecular geometry of SAE-F and then to comprehend the electron
density distributions in the molecular orbital energy levels [46]. In the initial step, the stable
ground state of the new molecules were optimized at B3LYP. Optimized geometry of the
molecule and electronic distribution in HOMO and LUMO levels were designed using 3D model
builder and is presented in Fig. 11. The HOMO and LUMO energy levels of SAE-F were -5.44
and -1.60 eV with the energy band gap (Eg) of 3.84 eV, which was comparable to 3.72 eV from
optical measurements. These theoretically calculated values vary slightly from the experimental
values and these deviations may be attributed to the various effects of conformations of these
molecules in vacuum, solutions and solid.
Fig. 11: a) Image of optimized structure b) HOMO and c) LUMO levels of SAE-F.
4. Conclusion

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Based on the highly emitting salicylaldehyde azine fluorophores, a salicylaldehyde azine
ester, SAE-F was prepared and its structural, optical, electrochemical and electrical properties
were investigated. The ester was found to be a promising AIEgen with enhanced fluorescence
properties in its aggregate and solid state. The thermal analysis data suggested the stable nature
of the ester upto 200 C. Surface morphology studies indicated that spin coated samples had
smoother and even surface. Based on the electrochemical band gap, a bilayered device was
fabricated and the electrical measurements indicated good diode behavior using SAE-F as a p-
type semiconducting material.
Acknowledgement
Dr. Abdul Ajeees Abdul Salam acknowledges the intramural research grant provided by MAHE
(MAHE/DREG/PhD/IMF/2019).
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CRediT author statement
New salicylaldehyde azine esters: Structural, aggregation induced fluorescence,
electrochemical and theoretical studies
Sneha Kagatikar: Performed experiments, collected data and prepared draft manuscript.
Dhanya Sunil: Conceptualization, Methodology, chemical characterization studies and
manuscript reviewing.
Dhananjaya Kekuda: Electrical measurements and device fabrication.
Suresh D Kulkarni: Fluorescence studies.
Abdul Ajees Abdul Salam: Single crystal data analysis

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Declaration of interests
☒ 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.
Ms. Sneha Kagatikar
Dr. Dhanya Sunil
Dr. Dhananjaya Kekuda
Dr. Suresh D Kulkarni
Dr. Abdul Ajees Abdul Salam

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Graphical abstract
Highlights
 Synthesis and structural characterization of two new salicylaldehyde azine esters
 Evaluation of aggregation induced emission property which is favorable for solid state
devices
 Fabrication of a diode based on electrochemical and electrical measurements.

Figure 1

Figure 2

Figure 3

Figure 4