Highly fluorescent materials derived from ortho-vanillin: Structural, photophysical electrochemical and theoretical studies

Article abstract of DOI:10.1016/j.molliq.2018.11.067

Small-molecule organic fluorophores are highly in demand attributed to their extensive prospective in material and biomedical applications. Particularly, luminescent π-conjugated organic molecules that possess an efficient solid-state emission are excellent candidates for optoelectronic devices. Focusing on high demand of organic fluorophores, we herein report the synthesis of three organic fluorescent materials derived from o?vanillin, viz. an ester (F1), an azine (F2) and an azo dye (F3). Interestingly, F2 exhibited very intense luminescence in its aggregate phase due to the restriction in intra-molecular rotation (RIR), as demonstrated by solution thickening studies. Further, its Single Crystal X-ray Crystallography (SCXRD) study suggested the existence of various intra and inter molecular interactions and gave evidences for locked intra-molecular rotations of the benzene rings in the rigid conformation of the molecule. The bathochromic shift in fluorescence from solution to solid phase was confirmed by its thin-film emission spectrum, which evidences the formation of J-aggregates. The observed RIR, development of J-aggregates and high conjugation in F2 impart an excellent fluorescence in its aggregated state. Thin films of both F2 and F3 on ITO plates exhibited a bathochromic shift with a deep orange to red photoluminescence on UV excitation. Furthermore, the morphological characterization revealed the presence of clear dense grains in case of F2 and F3, while the DSC analysis indicated phase transitions of all the derivatives. As seen from dielectric measurement studies, the azo dye F3 exhibited the highest dielectric constant among the three derivatives. The electronic and photophysical data based on Density Functional Theory (DFT) and Time Dependent-DFT (TD-DFT) calculations are in agreement with the experimental results. All the above data clearly advocate that, the synthesized fluorophoric o?vanillin derivatives are excellent candidates for electro-optical devices.

Full text of DOI:10.1016/j.molliq.2018.11.067

Accepted Manuscript
Highly fluorescent materials derived from ortho-vanillin:
Structural, photophysical electrochemical and theoretical studies
Sachin Poojary, Madhukara Acharya, Abdul Ajees Abdul Salam,
Dhananjaya Kekuda, Upendra Nayek, S. Madan Kumar, Airody
Vasudeva Adhikari, Dhanya Sunil
PII:
S0167-7322(18)34508-2
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2018.11.067
MOLLIQ 9977
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
1 September 2018
7 November 2018
14 November 2018
Please cite this article as: Sachin Poojary, Madhukara Acharya, Abdul Ajees Abdul Salam,
Dhananjaya Kekuda, Upendra Nayek, S. Madan Kumar, Airody Vasudeva Adhikari,
Dhanya Sunil , Highly fluorescent materials derived from ortho-vanillin: Structural,
photophysical electrochemical and theoretical studies. Molliq (2018), https://doi.org/
10.1016/j.molliq.2018.11.067
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ACCEPTED MANUSCRIPT
Highly Fluorescent Materials Derived from Ortho-Vanillin: Structural, Photophysical
Electrochemical and Theoretical Studies
Sachin Poojary1, Madhukara Acharya2, Abdul Ajees Abdul Salam3, Dhananjaya Kekuda4,
Upendra Nayek3, Madan Kumar S5, Airody Vasudeva Adhikari2, Dhanya Sunil1*
1Department of Chemistry, Manipal Institute of Technology, Manipal Academy of Higher
Education, Manipal-576 104, Karnataka, India.
2Organic Materials Lab, Department of Chemistry, National Institute of Technology Karnataka,
Surathkal, Mangalore-575 025, India.
3Department of Atomic and Molecular Physics, Manipal Academy of Higher Education,
Manipal-576 104, Karnataka, India.
4Department of Physics, Manipal Institute of Technology, Manipal Academy of Higher
Education, Manipal-576 104, Karnataka, India.
5PURSE Lab, Mangalore University, Mangalore-574 199, India.
*Address for correspondence
Dr. Dhanya Sunil,
Associate Professor,
Department of Chemistry,
Manipal Institute of Technology,
Manipal-576104.
E-mail: dhanya.s@manipal.edu
1

ACCEPTED MANUSCRIPT
Highly Fluorescent Materials Derived from Ortho-Vanillin: Structural, Photophysical
Electrochemical and Theoretical Studies
ABSTRACT
Small-molecule organic fluorophores are highly in demand attributed to their extensive
prospective in material and biomedical applications. Particularly, luminescent π-conjugated
organic molecules that possess an efficient solid-state emission are excellent candidates for
optoelectronic devices. Focusing on high demand of organic fluorophores, we herein report the
synthesis of three organic fluorescent materials derived from o-vanillin, viz. an ester (F1), an azine
(F2) and an azo dye (F3). Interestingly, F2 exhibited very intense luminescence in its aggregate
phase due to the restriction in intra-molecular rotation (RIR), as demonstrated by solution
thickening studies. Further, its Single Crystal X-ray Crystallography (SCXRD) study suggested
the existence of various intra and inter molecular interactions and gave evidences for locked intra-
molecular rotations of the benzene rings in the rigid conformation of the molecule. The
bathochromic shift in fluorescence from solution to solid phase was confirmed by its thin-film
emission spectrum, which evidences the formation of J-aggregates. The observed RIR,
development of J-aggregates and high conjugation in F2 impart an excellent fluorescence in its
aggregated state. Thin films of both F2 and F3 on ITO plates exhibited a bathochromic shift with
a deep orange to red photoluminescence on UV excitation. Furthermore, the morphological
characterization revealed the presence of clear dense grains in case of F2 and F3, while the DSC
analysis indicated phase transitions of all the derivatives. As seen from dielectric measurement
studies, the azo dye F3 exhibited the highest dielectric constant among the three derivatives. The
electronic and photophysical data based on Density Functional Theory (DFT) and Time
Dependent-DFT (TD-DFT) calculations are in agreement with the experimental results. All the
above data clearly advocate that, the synthesized fluorophoric o-vanillin derivatives are excellent
candidates for electro-optical devices.
Keywords: Aggregation, Azine, Dielectric, DFT, Electrochemical, Fluorophore, X-ray
Crystallography
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1. Introduction
Fluorescent compounds have turned out to be indispensable materials in the broad arena of
science and technology. Their wide applications include usage in probes, paints, indicators and
light-emitting devices, which play a central role in our daily life and hence in scientific research
[1-3]. Organic fluorescent compounds are superior to other materials due to several factors like
easy synthesis pathways, fine tunability of optical properties, lightweight, and ease of molding.
Consequently, there is a prodigious requirement for the design and development of many types of
new small-molecule organic fluorophores. Evidently, construction of such molecules entails a π-
conjugated framework and a cogent technique for both efficient absorption of light and radiative
transition processes [4,5].
Over the past two decades, extensive research has been carried out on the development of
organic optoelectronic devices, which include organic light emitting field-effect transistors
(OLEFETs), organic light-emitting diodes (OLEDs), organic solid-state luminescent sensors and
organic solid-state lasers [6-8]. Though in solution state, organic fluorophores have been
extensively tested in biological and analytical fields, they turned out to be non-luminescent in the
solid phase owing to the aggregation-caused quenching (ACQ) [9]. As solid materials are
employed directly in these optoelectronic devices, the prominence of organic chromophores that
unveil strong emission of visible light in their solid phase is impressively enhancing in the area of
organic electronics. Yet, it is very challenging to attain highly effective emission from organic
solids, as most of these organic molecules are strongly inclined to quenching of the luminescence
in their condensed state. Numerous fascinating approaches, like introduction of bulky substituents
to avert close molecular packing in the solid phase, exploitation of a propeller/spiro-shaped core
molecule, aggregation-induced fluorescence, dendritic substituent protection, cross-dipole
stacking, enhanced Intramolecular Charge-transfer Transitions (ICT) and formation of J-aggregate
[10-16] were successfully explored to circumvent the ACQ effect and to accomplish materials with
intense solid-state luminescence. However, developing stable, efficient small organic
chromophores that are pure color-emitting in the solid state still remains as a demanding task.
Therefore, the molecular design and development of novel molecules that emit intense
luminescence with high efficiency in the solid phase are strongly preferred for the modification,
and applications in high-performance optoelectronic devices.
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It has been well-established that good solid-state emissive and electrochemical properties
are critical for organic fluorophores in order to improve their optoelectronic device performance.
A review on literature reveals that interesting chemical systems like azines and azodyes have been
widely used to modify the properties of liquid crystals, polymers and as structural controllers of
biomolecules, like proteins, DNA and RNA [17,18]. Moreover, they find applications in non-linear
optics, molecular switch systems and to fabricate molecular-level memory devices and machines
[19-23]. Nevertheless, to the best of our knowledge, very limited reports are available on the study
of photophysical and electrochemical properties of fluorescent N-N linked diimines or azines and
azodyes as well as their use in optical devices [24-26]. Certain azine derivatives were shown to
display non-solvent aggregate as well as concentration induced aggregates and hence they were
established to be good templates showing aggregation induced emission (AIE) phenomenon.
Ortho-vanillin (2-hydroxy-3-methoxybenzaldehyde) is a yellow crystalline solid present
in the extracts and essential oils of many plants and, with distinctly different properties from its
more ubiquitous isomer vanillin. In the present study, we synthesized three fluorophoric
compounds, viz. ester (F1), azine (F2) and azo dye (F3) from o-vanillin using simple experimental
protocols. These azine molecules were expected to display AIE properties with high luminescence
and have a pronounced potential for the development of discrete optical applications. The
structural characterization of the F1-F3 was performed by FTIR, NMR and mass spectral
techniques. The electrochemical and photophysical behavior of these molecules in solution as well
as in solid state were examined to obtain the necessary data. AIE and aggregation characteristics
in azine F2 were examined using UV-visible and fluorescence spectroscopy. The response of these
molecules to an externally applied ac signal was monitored through dielectric spectroscopy. DFT
and TD-DFT calculations were carried out to get insights into their electronic, and photophysical
properties. Based on the results acquired, we have shown that the molecular structure and packing
arrangement determined by SCXRD play significant roles in the photophysical characteristics of
azine F2, that deliver an expedient approach to acquire an improved understanding on the AIE
behavior.
2. Experimental
2.1. Chemicals and materials
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The chemicals were procured from Sigma-Aldrich and Spectrochem Ltd. and were used
without any further purification. o-Vanillin and malonic ester were subjected to undergo reaction
under sonication in presence of catalytic amount of piperidine to yield fluorescent ethyl 8-
methoxy-2-oxo-2H-1-benzopyran-3-carboxylate (F1) [27] The ethanolic solution of F1 was
refluxed with excess of 99% hydrazine hydrate to generate a symmetrical fluorophoric azine, 2,2'-
[hydrazinediylidenedimethanylylidene]bis(6-methoxyphenol) (F2) [28]. Subsequently, an
alkaline solution of F2 was treated with benzene diazonium chloride solution at low temperature
to give the symmetrical fluorescent tetrazo dye, bis[2-(iminomethyl)-6-methoxy-4-
(phenyldiazenyl)phenol] (F3). The synthetic pathway for the o-vanillin derivatives is depicted in
Scheme 1.
Scheme 1.
2.2. Methodology
The reaction progress was monitored by thin layer chromatography, using aluminium
sheets pre-coated with aluchrosep silica gel 60/UV254 and cyclohexane-ethyl acetate mixture as
1
13
eluent. FTIR spectra were recorded on Schimadzu FTIR 8400S spectrometer. H and C NMR
spectra were run using Bruker 400 MHz instrument and DMSO-d6 as the solvent at room
temperature. Mass spectra were taken in Agilent 6510 series mass spectrometer. Melting points
were determined by open capillary method and are uncorrected. Differential Scanning Calorimetry
(DSC) measurements were carried out in a Shimadzu DSC-60 instrument. The UV-Visible
absorption spectra and the fluorescence emission spectra were obtained at room temperature using
Analytik Jena SPECORD S 600 spectrophotometer and Jasco FP 6200 spectrophotometer
respectively. The surface morphology of thin solid films was examined by means of Carl Zeiss
EVO 18 analytical scanning microscope (SEM) and INNOVA SPM atomic force microscope
(AFM). The cyclic voltammetry study of the compounds F1-F3 was carried out by using Ivium
Vertex electrochemical workstation. A thin film of the molecule using chloroform as a solvent was
prepared on a glassy carbon electrode by drop-cast method that functions as a working electrode.
The experiments were conducted by using Pt electrode as a counter electrode and Ag/AgCl as a
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reference electrode, immersed in an electrolyte [0.1M tetrabutylammonium tetrafluoroborate in
acetonitrile] at a scan rate of 100 mV/s. All the computational, i.e. DFT and TD-DFT calculations
were carried out using B3LYP function with def TZVP basis set in vacuum. The Turbomole V7.1
software package was used to perform all the simulations. The dielectric studies of the films were
performed at room temperature using Keithley 4200 SCS semiconductor parameter analyzer in
two probe configuration.
Single crystals of F2 were grown by slow evaporation of an ethanol:methanol (3:2) solution
at room temperature. The tiny crystals obtained from initial crystallization setup was used as a
seed to grow the appropriate crystals for X-ray diffraction studies. The preliminary cell
determination, as well as the three-dimensional data collection for F2, was carried out in a Rigaku
Saturn 72+ single crystal X-ray diffractometer using graphite monochromatized MoK radiation
(=0.71075). The cell parameters were refined by the least-squares method in the  range of 2 -
31°. A complete data set was processed using CrystalClear software [29]. Structure solution by
direct methods using the SHELXS97 [30] exposed the positions of all non-hydrogen atoms of F2.
The trial structures were refined by the least-squares method until convergence, the final R-factor
was found to be 0.0495 for F2. Carbon-bound H atoms were placed geometrically, with C-H =
0.93 Å, and 0.96 Å (methyl) forced to ride on their parent atoms with Uiso(H) = 1.2Ueq(C) or Uiso(H)
= 1.5Ueq(Cmethyl). The phenolic H atom was positioned in a difference Fourier map and was also
permitted to ride with Uiso(H) = 1.2Ueq(O). The final refinement converged to an R-value of 0.0495
with Δρmax, Δρmin (e Å−3) being 0.192 and -0.129, respectively. These calculations were carried out
using the package SHELXL [31] and WinGX [32]. The thermal ellipsoid diagrams of F2 were
prepared by ORTEP-3 [33]. Tabulation of atomic and thermal parameters was done using the
software CIFTAB [30]. The characterization of rings was done from the values of puckering
parameters [34] and symmetry parameters were obtained using PARST97 [35]. Molecular packing
diagrams were drawn using Mercury [36]. The relevant crystallographic data for F2 have been
deposited at Cambridge Crystallographic Data Centre (CCDC) with CCDC Number 1849903.
3. Results and discussion
3.1. Chemistry
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The spectral data were consistent with the proposed structures of all the compounds, F1-
F3 obtained from o-vanillin.
3.1.1. Ethyl 8-methoxy-2-oxo-2H-1-benzopyran-3-carboxylate (F1)
Equimolar mixtures of o-vanillin and diethyl malonate in 10 mL of ethanol was
homogenized and a catalytic quantity of piperidine (2 drops) was added. Further, the mixture was
sonicated for 1.5 h at 60 C. The solution was then poured into finely crushed ice; the solid product
formed was filtered, washed with diethyl ether and dried. Finally, it was recrystallized from
absolute ethanol.
Yellowish white solid (92%); m.p. 108-110 C; FTIR (KBr) [cm-1]: 3051 (Ar. C-H str.), 2974
(methyl C-H asym. str.), 2839 (methyl C-H sym. str.), 1763 (lactone C=O str.), 1693 (C=O str.),
1612 and 1570 (Ar. C=C str.), 1244 (Ar. C-O-C str.) (Fig. S1); 1H NMR (400 MHz, DMSO-d6)
[ppm]:  1.300-1.335 (t, 3H, CH3),  3.925 (s, 3H, OCH3),  4.276-4.329 (q, 2H, CH2),  7.318-
7.358 (m, 1H, coumarin 5H),  7.412-7.466 (m, 2H, coumarin 6H and 7H),  8.729 (s, 1H,
coumarin 4H) (Fig. S2); 13C NMR (100 MHz, DMSO-d6) δ = 14.54, 54.65, 61.73, 116.91, 118.32,
118.80, 121.65, 125.25, 144.36, 146.71, 149.39, 156.19, 163.07 (Fig. S3); MS C13H12O5: 249.07
(M+1) (Fig. S4).
3.1.2. 2,2'-[Hydrazinediylidenedimethanylylidene]bis(6-methoxyphenol) (F2)
A mixture of 10 mmol of compound F1 and 25 mmol of 99% hydrazine hydrate in 50 mL
of ethanol was refluxed for 2.5 h. The yellow crystalline product formed was filtered, washed using
cold ethanol and dried.
Yellow solid (45%); m.p. 186-188 C; FTIR (KBr) [cm-1]: 3298 (O-H str.), 3045 (Ar. C-H str.),
2929 (methyl C-H asym. str.), 2850 (methyl C-H sym. str.), 1624 (C=N str.), 1573 (N=N str.) (Fig.
S5); 1H NMR (400 MHz, DMSO-d6) [ppm]:  3.847 (s, 6H, OCH3),  6.907-6.947 (t, 2H, phenyl
4H and 4'H ),  7.132-7.151 (d, 2H, phenyl 3H and 3'H),  7.292-7.311 (d, 2H, phenyl 5H and
5'H),  9.004 (s, 2H, CH=N),  10.898 (s, 2H, OH) (Fig. S6); 13C NMR (100 MHz, DMSO-d6) δ
= 54.36, 115.75, 118.82, 119.76, 122.50, 148.14, 148.96, 163.25 (Fig. S7); MS C16H16N2O4:
301.11 (M+1) (Fig. S8).
3.1.3. Bis[2-(iminomethyl)-6-methoxy-4-(phenyldiazenyl)phenol] (F3)
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In a separate flask, 2 mmol of freshly distilled aniline was dissolved in minimum quantity
of concentrated HCl and it was added in small proportion under continuous stirring to a solution
of 2 mmol of NaNO2 in 2 mL of water, maintained at 5 °C to prepare the diazonium chloride
solution. About 1 mmol of F2 dissolved in 2 mL of water containing of 1 mmol of NaOH and 4
mmol of Na2CO3 maintained at 0 °C was added in small quantity with stirring to the prepared
diazonium chloride solution keeping the temperature between 0-5 °C. The reaction mixture was
further stirred at 0 °C for 1 h and allowed to slowly attain room temperature under stirring. The
product formed was filtered, washed with 100 mL of 10 % brine solution, subsequently with water,
dried and finally recrystallized from absolute ethanol.
Yellow solid (55%); m.p. 178-180 C; FTIR (KBr) [cm-1]: 3466 (O-H str.), 2937 (methyl C-H
1
asym. str.), 2839 (methyl C-H sym. str.), 1622 (C=N str.), 1577 (N=N str.) (Fig. S9); H NMR
(400 MHz, DMSO-d6) [ppm]:  3.835 (s, 6H, OCH3),  6.897-8.996 (m, 14 Ar.H),  10.890 (s,
2H, CH=N) (Fig. S10); 13C NMR (100 MHz, DMSO-d6) δ = 56.37, 114.31, 115.76, 116.06,
118.83, 119.77, 122.49, 129.26, 148.45, 148.96, 163.24 (Fig. S11); MS C28H24N6O4: 301.11 (M-
208) (Fig. S12).
3.2. Photophysical studies
3.2.1. Electronic absorption and emission studies in solution and thin film state
The maximum λAbs of F1-F3 were obtained from their respective UV-Visible spectra
recorded in methanol at 10-5 M concentration (Fig. S13). The electronic absorption spectra
displayed bands located at about 250 and 320 nm that could be assigned to the π→π* and n→π*
transitions, respectively. All the three derivatives F1, F2 and F3, when viewed under UV lamp of
wavelength 340 nm showed bright blue, orange and red fluorescence, respectively. The
corresponding photographs obtained are shown in Fig. S14. The photoluminescence (PL) spectra
of F1-F3 recorded in methanol are depicted in Fig. S15. As seen from the spectra, the compounds
displayed weak blue/violet PL emission in methanol. In order to study PL spectra in thin film state,
transparent thin films of all the three fluorophores were prepared by spin coating technique on
indium tin oxide (ITO) coated glass substrates. Fig. S15 shows their PL emission spectra in thin
film state, which are not analogous to those recorded in methanol. The λEm bathochromically
shifted to 625 nm and 629 nm in case of F2 and F3, respectively. Stoke’s shift, the difference
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between the maximum excitation and maximum emission wavelengths in cm-1, was calculated.
The observed high Stoke’s shift values (Δνst) in the range 6466-11069 cm-1 in solution state
ensured that, there is no re-absorption of the emitted radiation [37]. Further, the optical band gap
Eopt
(
) of the compounds was calculated [37], which varies from 3.27 eV to 3.43 eV as given in
g
Table 1. Evidently, the observed shifts suggested the presence of aggregation in solid state of F2
and F3. In fact, these results are in good agreement with findings of previous studies reported for
4-hydroxy-3-methoxybenzaldehyde azine, wherein fascinating photochromic properties of it was
ascribed to both non-solvent aggregate and concentration induced aggregates [15].
Table 1
Photophysical, electrochemical and computational data of fluorophores F1-F3
a
c
λAbs
(nm)
in
λEm
(nm)
Colour
CH3OH
(thin
Stokes
shift
(cm-1)
λcut
Eg
HOMOb
(eV)
LUMOb
(eV)
Semi
conductor
type
HOMOc
(eV)
LUMOc
(eV)
Eg
red
onset
(V)a
E
(eV)
(eV)
off
Comp.
No.
CH3OH
(thin
(nm)
CH3OH
film)
film)
316
313
314
486
blue
11069
8474
6466
376
362
379
3.30
3.43
3.27
-1.30
-1.62
-1.71
-6.70
-6.51
-6.26
-3.40
-3.08
-2.99
n
p
p
-6.75
-5.88
-5.82
-2.57
-2.05
-2.51
4.18
3.83
3.31
F1
F2
(482)
(blue)
426
Violet
(625)
(Orange)
F3
394
Violet
(red)
(629)


1
1
7
Stokes shift,
cm-1




λAbs  λEm


x10
λAbs λEm


aEg calculated from UV-vis spectra [37-39]; Eg (eV)=1240/ [(λcut off) (nm)]
red
onset
bHOMO and LUMO are energy levels calculated [39,40]as ELUMO = -
c Quantum mechanical calculations B3LYP/def TZVPP level
- 4.70(eV), Eg = ELUMO- EHOMO
E
3.2.2. Aggregation-induced emission properties of F2
To investigate the aggregation induced emission process, absorption and fluorescence
spectra of F2 and F3 in dilute (1×10-5 mol/L) THF solution with increasing water volume was
recorded. The fluorescence spectra of F2 presented no noticeable variations as the water volume
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fraction (fw) was increased from 10% to 60% as depicted in Fig. 1 (A and B), but the intensification
in fluorescence emission was perceived at 70%. Interestingly, a dramatic fluorescence
enhancement with a maximum emission intensity was observed, when fw was 80%. The
fluorescence of F2 solution was almost retained at the maximum value at fw = 90%. This could be
ascribed to the RIR phenomenon in the aggregate state, wherein the molecular conformation is
fixed, the non-radiative path is blocked, and the luminescence is enhanced. However, subsequent
addition of water to 99%, lowered the emission intensity in F2, which could be due to its reduced
solubility. In contrast, there was no fluorescence gradation pattern perceived in F3 on increasing
the water volume as evident from Fig. 1 (C and D).
Fig. 1.
The formation of aggregates has been further confirmed by the alterations in the UV-visible
spectra of F2 in THF/water mixtures with changing water volume fractions (Fig. 2 (A)). THF
solution of F2 with fw = 10% displayed two absorption peaks at 251 and 315 nm. These two
absorption peaks essentially continued to remain at the same positions, with a rise in fw from 10%
to 60% and the absorption peak at 315 nm abruptly increased when fw was further enhanced to
70% and gradually increased at fw = 80%, as presented in Fig. 2 (B). However, a gradual fall in
intensity of absorption peak at 315 nm was observed when water volume was increased to 90%
and a sudden reduction in the absorption intensity was detected at 99% water volume. Besides, the
leveled-off tails that appeared in the visible region, can be attributed to the light-scattering effects
and suggest aggregate formation.
Fig. 2.
The UV-visible absorption of F2 in THF/water mixtures revealed immense variations at fw
≥ 70% which were in agreement with the fluorescence emission spectra. Hence, the intense
luminescence ought to be due to the molecular aggregate formation. Further, illumination of F2 in
solid state (Fig. S14) and in solid thin film (Fig. S16) with a 365-nm UV lamp, produced bright
orange fluorescence. Thus, faint fluorescence was observed in dilute F2 solutions, whereas strong
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emission was perceived in concentrated solutions, as well as in the solid phase, which is in
alignment with the feature defined by the AIE phenomenon.
Generally in J-aggregate, the supramolecular self-organization of molecules occurs under
the influence of a solvent, additive or concentration. This results in the bathochromic shift of the
emission band with increasing sharpness as well as higher absorption coefficient [41,42]. To
establish the formation of J-aggregates, the emission spectra of F2 in THF were recorded at three
different concentrations (1×10-4 - 1×10-2 M). The variation in fluorescence intensity, with
concentration is presented in Fig. 3 (A). Besides, the spectra exhibited a weak new peak
(aggregation band) at a higher wavelength of 589 nm, in addition to the usual emission bands in
case of dilute THF solution of F2. The observed concentration dependent fluorescent intensity
enhancement of these J-bands, which is being bathochromically shifted when compared to the non-
aggregated form, has confirmed the formation of J-aggregates by F2 in concentrated solutions.
Fig. 3.
3.2.3. Mechanism of AIE
Subsequent to the studies on the optical properties of F2 in THF/water mixtures, the
mechanism of AIE was further investigated. It is an already established fact that, if an RIR process
is existent, then the molecules that exhibit AIE should display more fluorescence in increasingly
viscous solution, since the thickening process hinders intra-molecular rotations. Glycerol being a
highly viscous liquid which is completely miscible with methanol, the emission characteristics of
F2 in glycerol/methanol mixtures (1×10−5 mol/L with different fractions of glycerol) was checked.
Gradual enhancement in the emission intensity of F2 with increasing amounts of glycerol (10-
80%) was observed (Fig. 3 (B)). In addition, the emission wavelengths at 590-602 nm revealed
little shift with enhancing glycerol fraction, which suggested that the augmentation in emission
intensity is primarily owing to the viscosity effect, but not to molecular aggregation. These results
indicated that emission of F2 can efficiently increase with rise in the viscosity of the solvent
mixture, confirming that the AIE has been caused by the RIR process.
3.3. Structure of F2 and its effect on AIE
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In order to well comprehend the association between molecular packing and AIE
properties, single crystals of F2 suitable for X-ray diffraction studies were grown and good quality
crystals were used to determine the three dimensional structure of F2 using SCXRD studies. A
yellow colored single crystal of F2 with 0.28 × 0.22 × 0.18 mm3 dimensions was used for intensity
data collection at room temperature (293 K). Azine F2 crystallized in monoclinic system with the
space group P21/n. A total of 10032 reflections were collected for F2, of which 2184 reflections
were unique. The structural features and relevant refinement details regarding the crystal structure
analysis of F2 are shown in Table 2. The atomic coordinates of all the non-hydrogen atoms with
their equivalent isotropic displacements factors are given in Electronic Supplementary Information
(ESI)-Table S1. ESI-Table S2 summarizes the anisotropic displacements parameters of the non-
hydrogen atoms. The positional and isotropic displacements of the hydrogen atoms are given ESI-
Table S3. The thermal ellipsoid diagram of F2 is depicted in Fig. 4. The SCXRD studies revealed
that F2 molecule is planar. Generally, molecules having planar structure, or which can attain planar
structure in solid state give maximum Stoke’s shift due to greater intermolecular interactions
compared to non-planar molecules. Further, molecules having rotatable groups tend to attain
planarity in solid state, which brings about increase in their conjugation, and hence causes red
shift. Besides, close face to face packing in solid state can form excimers also, which produce large
bathochromic shifts in solid phase.
Table 2
Crystal data and structure refinement for F2
Molecular formula
Formula weight
Temperature
C16 H16N2O4
300.31
293(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Monoclinic
P 21/n
Unit cell dimensions
a = 5.9882(4) Å
b = 18.6467(11) Å
c = 6.8646(6) Å
= 90°
β= 106.408(8)°
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γ = 90°
Volume
Z
735.29(10) Å3
2
Density (calculated)
Absorption coefficient
F(000)
1.356 mg/m3
0.099 mm-1
316
Crystal size
0.28 × 0.22 × 0.18 mm3
2.184 to 31.075°
-8<=h<=8, -25<=k<=26, -9<=l<=7
10032
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
2184 [R(int) = 0.0375]
Completeness to theta = 25.242° 99.4 %
Absorption correction
Semi-empirical from equivalents
Max. and min. transmission
Refinement method
0.984 and 0.970
Full-matrix least-squares on F2
2184 / 0 / 105
Data / restraints / parameters
Goodness-of-fit on F2
1.039
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0495, wR2 = 0.1181
R1 = 0.0724, wR2 = 0.1319
0.192 and -0.129 e.Å-3
1849903
Largest diff. peak and hole
CCDC deposition number
Fig. 4.
The bond length and bond angles are within the expected ranges and are shown in Fig. 4
(B). The torsion angles involving all the atoms for F2 are given in ESI-Table S4 and hydrogen
bonds are listed in Table 3, respectively. Intra-molecular O1-H1…N1 and symmetry equivalent
O1-H1…N1 hydrogen bonds play a key role in stabilizing the overall shape of the molecule and
Fig. 5 (A) shows the packing of molecule viewed down the a-axis. In the crystal, the molecules
are interlinked and stabilized through C8-H8B… (C1-C6) interactions with H…Cg – 2.84 Å
[symmetry code ½+x, ½-y, ½+z] in addition to van der Waals forces. The asymmetric unit of the
F2 is composed of one half molecule and the whole molecule is generated by two fold screw axis
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[symmetry equivalent ½-x, ½+y, ½-z] through the mid-point of the N-N bond. The entire molecule
has adopted co-planar conformation forming an extended conjugated system [34].
Table 3
Hydrogen bonds for F2
____________________________________________________________________________
D-H...A
d(D-H)Å
d(H...A)Å
d(D...A)Å
<(DHA)°
____________________________________________________________________________
O1-H1...N1
O1-H1...N1#1
C8-H8B…Cg(1) #2 0.96
0.91(2)
0.91(2)
2.84
1.83(2)
1.83(2)
2.6415(13)
2.6415(13)
148.0(18)
148.0(18)
3.5174(15) 128.0
____________________________________________________________________________
Symmetry transformations used to generate equivalent atoms:
#1 -x+1,-y,-z
#2 -1/2+x,1/2-y,1/2+z
Fig. 5.
A
search of the web-based Cambridge Structural Database (CSD;
https://www.ccdc.cam.ac.uk) gave 21 entries (ESI-Table S5) with structural similarity to F2.
Among them, two structures (CSD ID: SALIAZ05 and ANISAZ02) do not contain three-
dimensional coordinates, and the remaining 19 structures were taken for comparative studies with
F2 (Fig. S17). The basic moiety of 2,2'-azinodi-2-hydroxytoluene has four entries in the CSD:
SALIAZ [43], SALIAZ01 [44], SALIAZ05 [45], SALIAZ10 [46]. As three dimensional
coordinates are not available for SALIAZ05, it is not used for further comparison studies. The
remaining three structures are crystallized in monoclinic space group P21/c (SALIAZ) and P21/n
(SALIAZ01, SALIAZ10) with half molecules in the asymmetric unit. All the structures are
perfectly planar. However, in SALIAZ01 the hydrogen atoms of the hydroxyl group are facing
opposite each other, which prevent the intra-molecular hydrogen bond formation. Superimposition
of these molecules with all common 18 non-hydrogen atoms yielded the Root Mean Square
Deviation (RMSD) of 0.037Å, suggesting similar conformations for the molecules (Fig. S17 and
ESI-Table S5).
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The
crystal
structure
of
2-([2-(2-hydroxy-3-methoxybenzylidene)hydrazin-1-
ylidene]methyl)-6-methoxyphenol (F2) was first reported by Teoh et al. (CSD ID: YORROM)
[47], and Lu et al. (CSD ID: YORROM01) [48]. However, the F2 structure reported in this study
has comparatively higher resolution and all the crystallographic parameters are better than the
other two structures - YORROM and YORROM01. The structures of F2 and YORROM share
high resemblance with slight deviation with cell volume and interchange of cell parameters, and
both crystallize in P21/n space group with half molecule in the asymmetric unit. Whereas,
YORROM01 crystallizes in P21/c with whole molecule in the asymmetric unit with cell volume
(1487.7Å3), which is almost double compared to that of F2 and YORROM. The superposition of
the structures of YORROM and YORROM01 with F2 yield the RMSD of 0.097 Å and 0.054 Å
for all 22 atoms and the terminal CH3 moiety has slight deviation of 0.21 Å. Interestingly, packing
pattern of the YORROM01 (γ type) and F2 (herringbone) molecules are different from each other
(Fig. S18).
Further, among the three entries of (E,E)-4-hydroxy-3-methoxybenzaldehyde azine which
are structural isomers of F2, GAWFOA [49] and GAWFOA01 contain two molecules in the
asymmetric unit, while GAWFOA02 [50] contains two half molecules and one complete molecule
in the asymmetric unit. Hence, GAWFOA and GAWFOA01 exhibit the same type of crystal
packing (Fig. S19A-C), whereas GAWFOA02 adopts a different pattern of crystal packing as
depicted in Fig. S19D-F. Moreover, it is interesting to note that these molecules are not planar and
the two phenyl rings form an angle of 43.21º.
In addition, yet another structure 4,4'-di(N)-methoxybenzalazine has eight entries in the
CSD (CSD ID: ANISAZ – ANISAZ07). Among them, seven structures are crystallized in the Cc
space group with similar cell parameters and have one complete molecule in the asymmetric unit
(Fig. S20A-C). The eighth structure (CSD ID: ANISAZ05) [51] has two half molecules in the
asymmetric unit and is crystallized in the triclinic centrosymmetric space group Pī, and adopts a
different type of crystal packing (Fig. S20D-F), when compared to the other seven structures.
Besides, four more azines: 2-methoxybenzaldehyde azine (CSD ID: EDEPAF [52]), bis(2,3-
dimethoxybenzylidene)hydrazine (CSD ID: IKAXAU [53]), 4-trifluoromethoxybenzaldehyde
azine (CSD ID: LACGUR [54]) and 2,2'-(hydrazine-1,2-diylidenedimethylylidene)bis(5-
(diethylamino)phenol) (CSD ID: URAQEK [55]) are also available in the CSD. EDEPAF (RMSD
0.04 Å) and LACGUR (0.08Å) adopt similar conformation as that of F2, whereas IKAXAU varies
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slightly (RMSD 0.20Å) as presented in Fig. S17. Similarly, the superposition of F2 SCXRD
structure with the theoretical model calculated from DFT (RMSD: 0.07 Å for 16 atoms) suggests
that both the structures adopt similar conformation, except slight variation in the orientation of
CH3 and OH moieties.
Thus, the nature of the substituents on the phenyl rings and the presence of OH group
adjacent to the diimine linkage play a pivotal role in the conformation of the azines. The flexibility
of F2 was restricted by various intra and inter molecular interactions in the solid state. The strong
intra-molecular hydrogen bonds between the hydrogens attached to the phenolic oxygens and the
azine nitrogens (O−H···N) lock the intra-molecular rotation enabling a planar structure with
effective electronic delocalization within F2. In addition, there is no overlap between the benzene
rings of the neighboring azine entities and the molecules are stabilized through C8-H8B… (C1-
C6) interactions with H…Cg – 2.84 Å in addition to van der Waals forces. Thus, absence of the
overlap between the phenyl units of adjacent azine units, with no evident face-to-face π−π stacking
contacts causes waning of π–π interactions and hence the emission cannot be quenched. The
aggregation has been formed by arrangement of one azine above the other due to their strong inter-
molecular association as evident from the zig-zag packing pattern. Nevertheless, these
conformations assist in avoiding the π-π stacking contacts in the aggregate state, thereby
preventing fluorescence quenching, and hence boosting the emission. The crystallographic results
thus confirmed that azine F2 is AIE active.
3.4. Analysis of self-assembly behaviour of F2
The surface morphology of the thin films prepared by thermal vapor deposition of THF/water
mixture with fw = 80% and two different concentrations (0.0001 and 0.1 molar) of F2 in THF
solutions on ITO coated glass substrates were obtained by SEM. No ordered microstructures were
observed at fw = 80% as displayed in Fig. 6 (A), due to fast aggregation of molecules forming
smaller aggregates. Interestingly, withered tree-shaped microparticles were observed (Fig. 6 (B))
with 0.0001 molar F2 solution as the molecular aggregation process was slow. When the
concentration was increased to 0.1 molar, concomitant increase in aggregation was noticed as
presented in Fig. 6 (C). The variations in the surface morphology suggested that the shape and size
of the microstructures are linked to the THF/water ratio as well as to the concentration of the F2
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solution. The thin films of F2 obtained from the THF/water mixture (fw = 80%) and single crystal
of F2 were examined under a fluorescence microscope (200X magnification). The fluorescent
images of F2 are presented in Fig. S21. The F2 aggregates emitted orange fluorescence, while F2
single crystal emitted intense fluorescence.
Fig. 6.
AFM permits the morphological analysis of thin films made from semiconducting
materials and provides good topographic contrast and lateral resolution when imaging surfaces.
To afford a large surface examination of the micro-structural arrays, the morphology and
roughness of the thin films were inspected using AFM. The images of the film qualities, porosities
and topographical structures are portrayed in Fig. 7. Dense grain morphology was observed for
THF/water mixture with fw = 80% and 0.1 molar F2 solutions prepared by spin coating with rms
roughness of 1.22 and 5.59 nm, respectively (Fig. 7 (A and C)). Whereas 0.0001 molar F2 solution
presented a rms roughness of 0.685 nm as presented in Fig. 7 (B). Maximum aggregation was thus
found in F2 solution of higher concentration and THF /water mixture with higher water volume.
Thus, from the photophysical, structural and surface morphological studies it has been concluded
that restricted inhibition to rotation, formation of J-aggregates and higher conjugation due to planar
structure induced enhanced emission in F2.
Fig. 7.
3.5. Analysis of surface morphology of F1-F3
The development of uniform amorphous thin films with favorable morphology is vital for
device applications [56]. The morphology of the films needs to be prudently monitored, since it is
related to the transport features of the devices. Easy solution processability of the fluorophores F1-
F3 permitted the use of inexpensive solution based spin coating techniques and fabrication
methods for device development [57]. The surface morphologies of the thin films prepared on ITO
coated glass substrates were examined by SEM. The images presented in Fig. 6 (D-G) afforded
evidences validating the ability of the three fluorophores in forming homogeneous pin-hole free
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uniform films. The role of the morphology on the dielectric properties is explained in the
succeeding sections.
3.6. Thermal studies of F1-F3
High thermal stability and capability to endure high temperature ranges are key
requirements to select the organic materials for optoelectronic devices [37]. The phase transition
behavior of the materials, F1-F3 were examined by DSC, as these processes play a significant role
on the stability and hence lifetime of any optoelectronic device. DSC scans of the molecules did
not exhibit any crystallization events or glass transitions. The DSC plots recorded at a constant
heating rate of 10 °C min-1 are presented in Fig. S22. The traces showed that all the compounds
unveil clear sharp endothermic melting peaks, and are consistent with the melting points measured
by capillary method. The melting points of F1-F3 are in the range of 96-198 oC. The thermograms
of the derivatives also indicated that they are stable at high temperatures. The azo dye F3 was
found to exhibit a lower melting temperature than that of azine F2.
3.7. Electrochemical properties
Generally, selection of suitable materials that can act as the active semiconducting layer is
the first and crucial step in device fabrication as the charge generation, separation and transfer,
occur at the semiconductor material on illumination with light. Obviously, an efficient exciton
dissociation takes place at the donor/acceptor interface for a bilayer organic solar cell configuration
and OLED. To accomplish the required energy demands in this process and hence to produce the
photo-current, there should be transfer of electron from the donor to the acceptor. In order to permit
this electron flow, the lowest unoccupied molecular orbital (LUMO) level of the donor should be
above that of the acceptor. Meanwhile, the highest occupied molecular orbital (HOMO) level of
the donor should be above that of the acceptor to allow hole transfer from the acceptor to donor.
In fact the band gap, which is the energy needed to promote a valence electron is related to the
HOMO/LUMO gap analogous to the valence and conduction band gaps in semiconducting
materials [58]. Therefore, it is imperative to determine HOMO and LUMO energy levels and the
band gap between them, while choosing a new material for such applications. For any organic
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semiconductor, an oxidation process is defined by HOMO level, which is the energy necessary to
extract an electron from a molecule, and a reduction process is implied by LUMO level, which
represents the energy required to inject an electron to a molecule. These redox features can be
characterized using cyclic voltammetry method and the energy band diagram can be estimated by
measuring the redox potentials Ered and Eox.
The cyclic voltammetry behavior of the synthesized fluorophores was carried out to inspect
their charge injection capabilities and HOMO and LUMO energy levels. The normalized
voltammograms are shown in Fig. S23. The cyclic voltammetric traces of F1-F3 encompass a
single cathodic reduction wave that is completely reversible, exemplifying the reduction of these
compounds. The reduction potentials of the azines and their HOMO-LUMO energy levels are
outlined in Table 1. The solution electrochemistry of F1-F3 thus confirmed that all three
molecules can behave as wide-band gap semiconductors.
Further, we were prompted to ascertain the hole-transport (p-type) and electron- (n-type)
behavior of the three semiconducting molecules F1-F3. So, the ‘hot-probe’ experimentation that
allows a very modest method to distinguish among p-type and n-type semiconductors was
performed to perceive the transport characteristic of the compounds [59]. The negative voltage
readout that was obtained, while applying the hot and cold probes, indicated F2 and F3 as p-type
semiconductors (Table 1).
3.8. Dielectric studies
The studies on dielectric properties of the compounds F1-F3 derived from o-vanillin were
performed in the frequency range from 1 Hz to 100 MHz. The thin films of ~600 nm were prepared
on pre-patterned ITO coated glass substrates, which served as one of the electrodes for the
dielectric measurements. Aluminium top contacts of thickness 700 nm were prepared by thermal
evaporation technique. The constructed device acts as a parallel plate capacitor with an active area
of 0.01 cm2. The deviation of dielectric constant with log frequency is displayed in Fig. 8. The
dielectric constant ( ) of F1-F3 were calculated by using the formula:
r
Cd
εr 
εoA
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where C is the capacitance of the crystal, d is the thickness of the fluorophores F1-F3, ɛo is the free
space permittivity and A is the active area of the device. The frequency response of the dielectric
constant is evident in the graph and it was observed that F3 has shown a highest dielectric constant
among the three series, while F1 exhibiting the lowest value. Dense grain morphology of the F3
samples resulted in a dielectric constant of ~6.3. On the other hand, the porous F1 has resulted in
a dielectric constant of ~4.8. The increase in the applied frequency caused a marginal decrease in
the dielectric constant which is due to the inability of the interfacial polarization to respond to the
high frequency. Further, the ac conductivity (휎푎푐) was calculated by the given formula:
휎푎푐 = ωεoε′tanδ
where 휔 is the angular frequency, 휀′ is the real part of the dielectric constant and 푡푎푛훿 is the loss
factor. The values of the ac conductivity calculated for the F1-F3 samples were 5×10-6, 7.2×10-6
and 9.1×10-6 ohm-cm, respectively at a frequency of 50 kHz.
Fig. 8.
σ
The frequency dependence of the ac conductivity ( ac ) is specified by the power law as [60]:
π


σac  An (T) sin(n 1) ωs


2


where the parameter ‘s’ is the frequency dependent parameter and symbolizes many body
interactions of electrons, charges and impurities and An(T) is a constant for a particular
temperature. The power law nature of the graph is represented in Fig. 8 (C), which depicts the
short range hopping of the charge carriers through the trapped sites. A notable difference in the
휎푎푐values was observed at lower frequencies with the ac conductivity showing an increasing trend
with increasing frequency. This could be ascribed to the dipole-dipole interaction between the
ionic charges. At higher frequencies the 휎푎푐values begin to converge indicating that a single
conduction mechanism prevails at high frequencies.
3.9. Theoretical calculations
The adiabatic quantum (DFT and TD-DFT) calculations are reflected to be one of the most
consistent and efficient computational approaches and is mostly employed in the field of organic
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electronics to obtain a profound insight into the electronic distribution and molecular structures of
semiconducting molecules, in addition to their optical feature in the excited state. In the present
study, the DFT simulations were performed to gather a better understanding of the geometric and
electronic structures of the three fluorophoric molecules. In the initial step, the stable ground state
of the new molecules were optimized at B3LYP/def TZVPP level [61-63]. The optimized
molecular geometries of F1-F3 and their corresponding electron distribution of their HOMO-
LUMO energy levels as attained from T-mole 3D visualizer are demonstrated in Fig. 9. The
calculated HOMO and LUMO energies and energy gaps (Eg) are listed in Table 1. As seen from
the results, the HOMO energy levels of F1-F3 are -6.75, -5.88 and -5.82 eV respectively, while
LUMO energy levels are -2.57, -2.05 and -2.51 eV, respectively. Subsequently, their band gaps
were calculated to be 4.18, 3.83 and 3.31 eV, respectively. 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. According to Fig. 9,
in F1 the HOMO was distributed on the phenyl ring at ground state, whereas in the excited state,
LUMO was concentrated on the pyran as well as on the ester side chain. In F2, the ground state
HOMO was averagely spread over both the aromatic rings and in the excited state, LUMO was
intense over the central CH=N-N=CH moiety. Whereas in case of F3, the ground state HOMO
was concentrated on phenol and partly on azo group and in the excited state, LUMO was spread
over mainly on the azo group. Here, in F1 the electron cloud has moved from the electron donor
phenyl group to the electron accepting pyran ring indicating donor-acceptor phenomenon in it.
Similarly, in case of F2 the wide-spread electron cloud has shifted from two donor groups towards
electron accepting CH=N-N=CH group, showing some sort of donor-acceptor interactions from
HOMO to LUMO. Further, they seem to have π and π* character and hence S0-S1 electronic
transition is definitely π - π* in nature. Also, F2, with the HOMO, distributed on the aromatic rings
and in its neighborhood (-CH=N-), showcases that the oxidation should occur predominantly in
this region, indicating that this area has the highest density of negative charge [61-63]. Further, the
distribution of HOMO and LUMO electrons in F3 signifies the movement of charge from electron
donor phenol group to electron withdrawing diazo functionality to some extent through the
extended conjugation.
Further, TD-DFT simulations were performed to the optimized geometry of molecules that
resulted from DFT simulations obtained at ground state, in order to compute vertical excitation
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and their response in the excited molecules, using B3LYP basis set. The simulated absorption
spectrum of F1 displays two separate peaks that corresponds to π-π* transitions and ICT process,
which is nearly same as the experimentally gained UV-Visible absorption spectra, as evidenced
from Fig. 10 (A). These results confirmed that the basis set and (XC) functional chosen are
rationally suitable for the above mentioned calculations. Fig. 10 (B) depicts the simulated vibration
spectrum of F1-F3 as estimated using TD-DFT calculations [64]. The molecules exhibit a broad
peak at around 3700 cm-1 corresponding to OH functionality, as seen from the simulated spectra
of F2 and F3. Thus, the calculated IR absorption frequencies soundly match with the
experimentally obtained FTIR data. Convincingly, the theoretically obtained UV-Visible and
vibration spectral data are consistent with the experimentally acquired spectral data.
Fig. 9.
Fig. 10.
4. Conclusion
Three materials, F1-F3 were prepared from o-vanillin by easy synthetic methods. Their
chemical structures as well as photophysical, electrochemical, thermal, and dielectric properties
were investigated. Compounds F2 and F3 exhibited weak fluorescence in dilute solutions and
displayed a red shift with increased fluorescence intensities in their solid state. AIE-based azine
derivative F2 displayed a strongly enhanced emission in the aggregated state because of inhibition
to intra-molecular rotation, which was confirmed by solution thickening studies. Further, its planar
structure with extended π-conjugation and a herringbone packing pattern, as confirmed by SCXRD
analysis and theoretical studies, facilitate intense solid state luminescence in the aggregated state.
The surface morphology investigations together with dielectric studies suggested that pin hole free,
dense grain morphology is desired for obtaining a high dielectric constant. Their HOMO levels
are in between -6.26 and -6.70 eV, LUMO levels are in the order of -2.99 to -3.40 eV and the band
gap values were found to be in the range of 3.27-3.43 eV. The theoretical results obtained by DFT
and TD-DFT simulations give support to the experimental findings. Based on all the above said
facts, these compounds can be viewed as potential candidates for optoelectronic applications.
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Acknowledgement
AA acknowledges the research grant provided by the Manipal Academy of Higher
Education (MAHE) (MAHE/REG/TD/DAMP-(T)) under the fellowship program offered by the
Manipal FAIMER Institute (M-FIILIPE).
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Legends for schemes and figures
Scheme 1. Synthesis of F1-F3 from o-vanillin
Fig. 1. (A) Fluorescence spectra of F2, (B) Variation of emission intensity with water fractions for
F2, (C) Fluorescence spectra of F3 and (D) Variation of emission intensity with water fractions
for F3
Fig. 2. (A) UV-visible spectra of F2 and (B) Variation in absorption intensity of F2 with water
fractions
Fig. 3. Fluorescence spectra of F2 (A) at three different concentrations in THF and (B) in
methanol-glycerol mixtures with different glycerol fractions
29