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

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: DOI: Reference:

S0167-7322(18)34508-2 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* 1

Department of Chemistry, Manipal Institute of Technology, Manipal Academy of Higher

Organic Materials Lab, Department of Chemistry, National Institute of Technology Karnataka,

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2

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Education, Manipal-576 104, Karnataka, India.

3

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Surathkal, Mangalore-575 025, India.

Department of Atomic and Molecular Physics, Manipal Academy of Higher Education,

4

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Manipal-576 104, Karnataka, India.

Department of Physics, Manipal Institute of Technology, Manipal Academy of Higher

PURSE Lab, Mangalore University, Mangalore-574 199, India.

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5

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Education, Manipal-576 104, Karnataka, India.

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*Address for correspondence Dr. Dhanya Sunil,

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Associate Professor, Department of Chemistry,

Manipal-576104.

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Manipal Institute of Technology,

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E-mail: [email protected]

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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

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organic molecules that possess an efficient solid-state emission are excellent candidates for

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optoelectronic devices. Focusing on high demand of organic fluorophores, we herein report the

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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

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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

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the existence of various intra and inter molecular interactions and gave evidences for locked intramolecular rotations of the benzene rings in the rigid conformation of the molecule. The

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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,

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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

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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

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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

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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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ACCEPTED MANUSCRIPT 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

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easy synthesis pathways, fine tunability of optical properties, lightweight, and ease of molding.

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Consequently, there is a prodigious requirement for the design and development of many types of

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new small-molecule organic fluorophores. Evidently, construction of such molecules entails a πconjugated framework and a cogent technique for both efficient absorption of light and radiative

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transition processes [4,5].

Over the past two decades, extensive research has been carried out on the development of

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organic optoelectronic devices, which include organic light emitting field-effect transistors (OLEFETs), organic light-emitting diodes (OLEDs), organic solid-state luminescent sensors and

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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

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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

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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

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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

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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. 3

ACCEPTED MANUSCRIPT 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

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[19-23]. Nevertheless, to the best of our knowledge, very limited reports are available on the study

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of photophysical and electrochemical properties of fluorescent N-N linked diimines or azines and

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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

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established to be good templates showing aggregation induced emission (AIE) phenomenon. Ortho-vanillin (2-hydroxy-3-methoxybenzaldehyde) is a yellow crystalline solid present

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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

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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

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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

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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

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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

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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 4

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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 8methoxy-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'(F2)

[28].

Subsequently,

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[hydrazinediylidenedimethanylylidene]bis(6-methoxyphenol)

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give

the

symmetrical

fluorescent

tetrazo

dye,

bis[2-(iminomethyl)-6-methoxy-4-

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to

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alkaline solution of F2 was treated with benzene diazonium chloride solution at low temperature

(phenyldiazenyl)phenol] (F3). The synthetic pathway for the o-vanillin derivatives is depicted in

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Scheme 1.

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Scheme 1.

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2.2. Methodology

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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 13

C NMR

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eluent. FTIR spectra were recorded on Schimadzu FTIR 8400S spectrometer. 1H and

spectra were run using Bruker 400 MHz instrument and DMSO-d6 as the solvent at room

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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

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(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 5

ACCEPTED MANUSCRIPT 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.

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Single crystals of F2 were grown by slow evaporation of an ethanol:methanol (3:2) solution

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at room temperature. The tiny crystals obtained from initial crystallization setup was used as a

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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

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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 -

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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.

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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 =

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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

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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

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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

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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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ACCEPTED MANUSCRIPT The spectral data were consistent with the proposed structures of all the compounds, F1F3 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

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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

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absolute ethanol.

Yellowish white solid (92%); m.p. 108-110 C; FTIR (KBr) [cm-1]: 3051 (Ar. C-H str.), 2974

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(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)

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[ppm]:  1.300-1.335 (t, 3H, CH3),  3.925 (s, 3H, OCH3),  4.276-4.329 (q, 2H, CH2),  7.3187.358 (m, 1H, coumarin 5H),  7.412-7.466 (m, 2H, coumarin 6H and 7H),  8.729 (s, 1H,

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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

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(M+1) (Fig. S4).

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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

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of ethanol was refluxed for 2.5 h. The yellow crystalline product formed was filtered, washed using cold ethanol and dried.

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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) 7

ACCEPTED MANUSCRIPT 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

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further stirred at 0 °C for 1 h and allowed to slowly attain room temperature under stirring. The

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dried and finally recrystallized from absolute ethanol.

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product formed was filtered, washed with 100 mL of 10 % brine solution, subsequently with water, Yellow solid (55%); m.p. 178-180 C; FTIR (KBr) [cm-1]: 3466 (O-H str.), 2937 (methyl C-H

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asym. str.), 2839 (methyl C-H sym. str.), 1622 (C=N str.), 1577 (N=N str.) (Fig. S9); 1H NMR (400 MHz, DMSO-d6) [ppm]:  3.835 (s, 6H, OCH3),  6.897-8.996 (m, 14 Ar.H),  10.890 (s, 13

C NMR (100 MHz, DMSO-d6) δ = 56.37, 114.31, 115.76, 116.06,

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2H, CH=N) (Fig. S10);

118.83, 119.77, 122.49, 129.26, 148.45, 148.96, 163.24 (Fig. S11); MS C28H24N6O4: 301.11 (M-

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3.2. Photophysical studies

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208) (Fig. S12).

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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

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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*

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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 8

ACCEPTED MANUSCRIPT 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 opt

( E g ) of the compounds was calculated [37], which varies from 3.27 eV to 3.43 eV as given in Table 1. Evidently, the observed shifts suggested the presence of aggregation in solid state of F2

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and F3. In fact, these results are in good agreement with findings of previous studies reported for

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4-hydroxy-3-methoxybenzaldehyde azine, wherein fascinating photochromic properties of it was

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ascribed to both non-solvent aggregate and concentration induced aggregates [15].

Table 1

λEm

Colour

Stokes

λcut

E ga

(nm)

(nm)

CH3OH

shift

off

(eV)

Comp.

in

CH3OH

(thin

No.

CH3OH

(thin

film)

-1

(cm )

(nm)

11069

376

314

426

Violet

(625)

(Orange)

394

Violet

(629)

(red)

8474

HOMOc (eV)

LUMOc (eV)

Egc (eV)

3.30

-1.30

-6.70

-3.40

n

-6.75

-2.57

4.18

362

3.43

-1.62

-6.51

-3.08

p

-5.88

-2.05

3.83

3.27

-1.71

-6.26

-2.99

p

-5.82

-2.51

3.31

6466

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F3

blue (blue)

Semi conductor type

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313

F2

486 (482)

LUMOb (eV)

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316 F1

HOMOb (eV)

(V)a

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film)

red E onset

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λAbs

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Photophysical, electrochemical and computational data of fluorophores F1-F3

 1

379

1 

a

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7 -1 Stokes shift, λ Abs  λ Em    λ  λ  x 10 cm Em   Abs

Eg calculated from UV-vis spectra [37-39]; Eg (eV)=1240/ [(λcut off) (nm)] red

b

HOMO and LUMO are energy levels calculated [39,40]as ELUMO = - E onset - 4.70(eV), Eg = ELUMO- EHOMO

c

Quantum mechanical calculations B3LYP/def TZVPP level

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 9

ACCEPTED MANUSCRIPT 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

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addition of water to 99%, lowered the emission intensity in F2, which could be due to its reduced

the water volume as evident from Fig. 1 (C and D).

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Fig. 1.

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solubility. In contrast, there was no fluorescence gradation pattern perceived in F3 on increasing

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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

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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%

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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

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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

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leveled-off tails that appeared in the visible region, can be attributed to the light-scattering effects

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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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ACCEPTED MANUSCRIPT 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

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different concentrations (1×10-4 - 1×10-2 M). The variation in fluorescence intensity, with

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concentration is presented in Fig. 3 (A). Besides, the spectra exhibited a weak new peak

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(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

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enhancement of these J-bands, which is being bathochromically shifted when compared to the non-

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aggregated form, has confirmed the formation of J-aggregates by F2 in concentrated solutions.

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Fig. 3.

3.2.3. Mechanism of AIE

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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

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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

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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.

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Gradual enhancement in the emission intensity of F2 with increasing amounts of glycerol (1080%) 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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ACCEPTED MANUSCRIPT 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

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were unique. The structural features and relevant refinement details regarding the crystal structure

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analysis of F2 are shown in Table 2. The atomic coordinates of all the non-hydrogen atoms with

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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-

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hydrogen atoms. The positional and isotropic displacements of the hydrogen atoms are given ESITable S3. The thermal ellipsoid diagram of F2 is depicted in Fig. 4. The SCXRD studies revealed

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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

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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

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shift. Besides, close face to face packing in solid state can form excimers also, which produce large

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bathochromic shifts in solid phase.

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Table 2 Crystal data and structure refinement for F2 C16 H16N2O4

Formula weight

300.31

Temperature

293(2) K

Wavelength

0.71073 Å

Crystal system

Monoclinic

Space group

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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Molecular formula

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ACCEPTED MANUSCRIPT γ = 90° 3

Volume

735.29(10) Å

Z

2

Density (calculated)

1.356 mg/m

Absorption coefficient

0.099 mm

F(000)

316

Crystal size

0.28 × 0.22 × 0.18 mm

Theta range for data collection

2.184 to 31.075°

Index ranges

-8<=h<=8, -25<=k<=26, -9<=l<=7

Reflections collected

10032

-1

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T

3

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Independent reflections

3

2184 [R(int) = 0.0375] 99.4 %

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Completeness to theta = 25.242

°

Semi-empirical from equivalents

Max. and min. transmission

0.984 and 0.970

Refinement method

Full-matrix least-squares on F2

Goodness-of-fit on F2

2184 / 0 / 105

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Data / restraints / parameters

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Absorption correction

1.039

R1 = 0.0495, wR2 = 0.1181

R indices (all data)

R1 = 0.0724, wR2 = 0.1319

Largest diff. peak and hole

0.192 and -0.129 e.Å

CCDC deposition number

1849903

-3

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ED

Final R indices [I>2sigma(I)]

Fig. 4.

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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

13

ACCEPTED MANUSCRIPT [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].

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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 0.91(2) 1.83(2) 2.6415(13) 148.0(18)

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O1-H1...N1#1 0.91(2) 1.83(2) 2.6415(13) 148.0(18) C8-H8B…Cg(1) #2 0.96 2.84 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

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Fig. 5.

search

of

the

web-based

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A

Cambridge

Structural

Database

(CSD;

https://www.ccdc.cam.ac.uk) gave 21 entries (ESI-Table S5) with structural similarity to F2.

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Among them, two structures (CSD ID: SALIAZ05 and ANISAZ02) do not contain threedimensional coordinates, and the remaining 19 structures were taken for comparative studies with

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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

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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).

14

ACCEPTED MANUSCRIPT 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

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both crystallize in P21/n space group with half molecule in the asymmetric unit. Whereas,

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YORROM01 crystallizes in P21/c with whole molecule in the asymmetric unit with cell volume 3

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(1487.7Å ), 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

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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

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(Fig. S18).

Further, among the three entries of (E,E)-4-hydroxy-3-methoxybenzaldehyde azine which

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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

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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

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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.

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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

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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(2,3dimethoxybenzylidene)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 15

ACCEPTED MANUSCRIPT 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

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of F2 was restricted by various intra and inter molecular interactions in the solid state. The strong

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intra-molecular hydrogen bonds between the hydrogens attached to the phenolic oxygens and the

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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

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rings of the neighboring azine entities and the molecules are stabilized through C8-H8B… (C1C6) interactions with H…Cg – 2.84 Å in addition to van der Waals forces. Thus, absence of the

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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

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aggregation has been formed by arrangement of one azine above the other due to their strong intermolecular association as evident from the zig-zag packing pattern. Nevertheless, these

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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

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thus confirmed that azine F2 is AIE active.

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3.4. Analysis of self-assembly behaviour of F2

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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 16

ACCEPTED MANUSCRIPT 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.

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Fig. 6.

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AFM permits the morphological analysis of thin films made from semiconducting

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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

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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

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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

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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.

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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

Fig. 7.

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structure induced enhanced emission in F2.

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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 F1F3 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 17

ACCEPTED MANUSCRIPT 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

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requirements to select the organic materials for optoelectronic devices [37]. The phase transition

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behavior of the materials, F1-F3 were examined by DSC, as these processes play a significant role

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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

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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

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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

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found to exhibit a lower melting temperature than that of azine F2.

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3.7. Electrochemical properties

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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,

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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

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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 18

ACCEPTED MANUSCRIPT 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

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their charge injection capabilities and HOMO and LUMO energy levels. The normalized

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voltammograms are shown in Fig. S23. The cyclic voltammetric traces of F1-F3 encompass a

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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

US

outlined in Table 1. The solution electrochemistry of F1-F3 thus confirmed that all three molecules can behave as wide-band gap semiconductors.

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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

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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

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3.8. Dielectric studies

PT

semiconductors (Table 1).

ED

readout that was obtained, while applying the hot and cold probes, indicated F2 and F3 as p-type

The studies on dielectric properties of the compounds F1-F3 derived from o-vanillin were

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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 (  r ) of F1-F3 were calculated by using the formula:

εr 

Cd εoA

19

ACCEPTED MANUSCRIPT 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

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the dielectric constant which is due to the inability of the interfacial polarization to respond to the

CR

𝜎𝑎𝑐 = ωεo ε′ tanδ

IP

high frequency. Further, the ac conductivity (𝜎𝑎𝑐 ) was calculated by the given formula: where 𝜔 is the angular frequency, 𝜀 ′ is the real part of the dielectric constant and 𝑡𝑎𝑛𝛿 is the loss

US

factor. The values of the ac conductivity calculated for the F1-F3 samples were 5×10-6, 7.2×10-6

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and 9.1×10-6 ohm-cm, respectively at a frequency of 50 kHz.

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Fig. 8.

π  σ ac  A n (T)sin(n  1) ωs 2 

ED

The frequency dependence of the ac conductivity ( σ ac ) is specified by the power law as [60]:

PT

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

CE

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

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𝜎𝑎𝑐 values 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 𝜎𝑎𝑐 values 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 20

ACCEPTED MANUSCRIPT 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-

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LUMO energy levels as attained from T-mole 3D visualizer are demonstrated in Fig. 9. The

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calculated HOMO and LUMO energies and energy gaps (Eg) are listed in Table 1. As seen from

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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

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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

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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,

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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

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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

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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.

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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

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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 21

ACCEPTED MANUSCRIPT 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

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peak at around 3700 cm-1 corresponding to OH functionality, as seen from the simulated spectra

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of F2 and F3. Thus, the calculated IR absorption frequencies soundly match with the

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experimentally obtained FTIR data. Convincingly, the theoretically obtained UV-Visible and

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vibration spectral data are consistent with the experimentally acquired spectral data.

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Fig. 9.

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Fig. 10.

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4. Conclusion

Three materials, F1-F3 were prepared from o-vanillin by easy synthetic methods. Their

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chemical structures as well as photophysical, electrochemical, thermal, and dielectric properties were investigated. Compounds F2 and F3 exhibited weak fluorescence in dilute solutions and

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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

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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. 22

ACCEPTED MANUSCRIPT

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

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Manipal FAIMER Institute (M-FIILIPE).

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References

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[1] S. Fluor, E. Kim, Y. Lee, S. Lee, S.B. Park, Discovery, understanding, and bioapplication of organic fluorophore: A case study with an indolizine-based novel fluorophore, Acc. Chem.

US

Res. 48 (2015) 538-547.

[2] L. Yuan, W. Lin, K. Zheng, L. Hea, W. Huang, Far-red to near infrared analyte-responsive

AN

fluorescent probes based on organic fluorophore platforms for fluorescence imaging, Chem. Soc. Rev. 42 (2013) 622-661.

M

[3] K.P. Carter, A.M. Young, A.E. Palmer, Fluorescent sensors for measuring metal ions in living systems, Chem. Rev. 114 (2014) 4564-4601.

ED

[4] R. Jin, X. Zhang, W. Xiao, D. Luo, Theoretical investigations of the photophysical properties of star-shaped π-conjugated molecules with triarylboron unit for organic light-emitting diodes

PT

applications, Int. J. Mol. Sci. 18 (2017) 2178-2188. [5] S. Toyota, M. Kurokawa, M. Araki, K. Nakamura, T. Iwanaga, New π-conjugated system with

CE

a rigid framework of 1,8-anthrylene-ethynylene cyclic dimer and its monoanthraquinone analogue, Org. Lett. 9 (2007), 3655-3658.

AC

[6] H. Ulla, M.R Kiran, B. Garudachari, M.N. Satyanarayan, G. Umesh, A.M. Isloor, Blue emitting halogen–phenoxy substituted 1,8-naphthalimides for potential organic light emitting diode applications, Opt. Mater. 37 (2014) 311-321. [7] L. Song, Y. Hu, N. Zhang, Y. Li, J. Lin, X. Liu, Improved performance of organic lightemitting field-effect transistors by interfacial modification of hole-transport layer/emission layer: incorporating organic heterojunctions, ACS Appl. Mater. Interfaces 8 (2016), 1406314070. [8] Y. Hu, J. Lin, L. Song, Q. Lu, W. Zhu, X. Liu, Vertical microcavity organic light emitting field-effect transistors, Sci. Rep. 6 (2016) 23210. 23

ACCEPTED MANUSCRIPT [9] S.W. Thomas III, G.D. Joly, T.M. Swager, Chem. Rev. 107 (2007) 1339-1386. [10]

S.P. Anthony, Organic solid-state fluorescence: strategies for generating switchable and

tunable fluorescent materials. ChemPlusChem 77 (2012) 518-553. [11]

J. Wang, Y. Zhao, C. Dou, H. Sun, P. Xu, K. Ye, J. Zhang, S. Jiang, F. Li, Y. Wang, Alkyl

and dendron substituted quinacridones: synthesis, structures, and luminescent properties, J. Phys. Chem. B 111 (2007) 5082-5089. Y. Li, F. Li, H. Zhang, Z. Xie, W. Xie, H. Xu, B. Li, F. Shen, L. Ye, M. Hanif, D. Ma, Y.

T

[12]

IP

Ma, Tight intermolecular packing through supramolecular interactions in crystals of cyano

CR

substituted oligo(para-phenylene vinylene): a key factor for aggregation-induced emission, Chem. Commun. (2007) 231-233.

Z. Xie, B. Yang, F. Li, G. Cheng, L. Liu, G. Yang, H. Xu, L. Ye, M. Hanif, S. Liu, D. Ma,

US

[13]

Y. Ma, Cross dipole stacking in the crystal of distyrylbenzene derivative:  the approach toward

[14]

AN

high solid-state luminescence efficiency, J. Am. Chem. Soc. 127 (2005) 14152-14153. T. E. Kaiser, H. Wang, V. Stepanenko, F. wrthner, Supramolecular construction of

M

fluorescent J‐ Aggregates based on hydrogen‐ bonded perylene dyes, Angew. Chem. 119 (2007) 5637-5640.

S. Dineshkumar, A. Muthusamy, Investigation of aggregation induced emission in 4-

ED

[15]

hydroxy-3-methoxybenzaldehyde azine and polyazine towards application in (opto)

PT

electronics: synthesis, characterization, photophysical and electrical properties, Des. Monomers Polym. 20 (2017) 234-249. A. Wakamiya, K. Mori, S. Yamaguchi, 3‐ Boryl‐ 2,2′‐ bithiophene as a versatile core

CE

[16]

skeleton for full‐ color highly emissive organic solids, Angew. Chem. 119 (2007) 4351-4354. Y. Lu, M. Nakano, T. Ikeda, Photomechanics: Directed bending of a polymer film by light,

AC

[17]

Nature 425 (2003) 145. [18]

A.J. Hammood, A.F.H. Kased, A.J.M. Al-Karawi, S.R. Raseen, I.H.R. Tomi, A.B.O. Ali

Efficient and practical method for the synthesis of hydrophobic azines as liquid crystalline materials, Mol. Cryst. Liq. Cryst. 648 (2017) 114-129. [19]

T. Gao, Y. Xue, Z. Zhang, W. Que, Multi-wavelength optical data processing and recording

based on azo-dyes doped organic-inorganic hybrid film, Opt. Express 26 (2018) 4309-4317.

24

ACCEPTED MANUSCRIPT [20]

R. Martínez, I. Ratera, A. Tárraga, P. Molina, J. Veciana, A simple and robust reversible

redox–fluorescence molecular switch based on a 1,4-disubstituted azine with ferrocene and pyrene units, Chem. Commun. (2006) 3809-3811. [21]

E. Anger, S.P. Fletcher, Simple azo dyes provide access to versatile chiroptical switches,

Eur. J. Org. Chem. 17 (2015) 3651-3655. [22]

M. Sliwa, S. Létard, I. Malfant, M. Nierlich, P.G. Lacroix, T. Asahi, H. Masuhara, P. Yu,

T

K. Nakatani, Design, Synthesis, structural and nonlinear optical properties of photochromic M. Ziółek, K. Filipczak, A. Maciejewski, Spectroscopic and photophysical properties of

CR

[23]

IP

crystals:  toward reversible molecular switches, Chem. Mater. 17 (2005) 4727-4735.

salicylaldehyde azine (SAA) as a photochromic Schiff base suitable for heterogeneous studies,

[24]

US

Chem. Phys. Lett. 464 (2008) 181-186.

H. Joo, B. Le, Y. Seo, Azo-pyrene–based fluorescent sensor of reductive cleavage of

[25]

AN

isomeric azo functional group, Tetrahedron Lett. 58 (2017) 679-681. A.B. Tathe, N. Sekar, Red emitting coumarin-azo dyes: synthesis, characterization, linear

M

and non-linear optical properties-experimental and computational approach. J. Fluoresc. 26 (2016) 1279-1293.

F. Cisnetti, R. Ballardini, A. Credi, M.T. Gandolfi, S. Masiero, F. Negri, S. Pieraccini, G.P.

ED

[26]

Spada, Photochemical and electronic properties of conjugated bis(azo) compounds: an

[27]

PT

experimental and computational study, Chem. Eur. J. 10 (2004) 2011-2021. H. Takahashi, H. Takechi, K. Kubo, T. Matsumoto, Ethyl 8-meth-oxy-2-oxo-2H-1-

[28]

CE

benzopyran-3-carboxyl-ate, Acta Cryst. E62 (2006) o2553-o2555. A.M. Islam, A.H. Bedair, F.M. Aly, A.M.S. El-Sharief, F.M. El-Masry, Synthesis and

AC

reactions of coumarin-3-N-bromo-arylcarboxamides, Indian J. Chem. 19B (1980) 224-227. [29]

Expert, C.-S. Tokyo, Japan 2011.

[30]

G.M. Sheldrick, SHELXS97 and SHELXL97. Program for Crystal Structure Solution and

Refinement. University of Göttingen, Göttingen, (1997). [31]

G.M. Sheldrick, A short history of SHELX, Acta Cryst. Sect. A: Found. Crystallogr. 64

(2008) 112-122. [32]

L.J. Farrugia, WinGX and ORTEP for Windows: an update, Appl. Crystallogr. 45 (2012)

849-854.

25

ACCEPTED MANUSCRIPT [33]

L. J. Farrugia, ORTEP-3 for Windows - a version of ORTEP-III with a Graphical User

Interface (GUI), J. Appl. Cryst. 30 (1997) 565-566. [34]

D. Cremer, J.A. Pople, General definition of ring puckering coordinates, J. Am. Chem.

Soc. 97 (1975) 1354-1358. [35]

M.J. Nardelli, PARST95 - an update to PARST: a system of Fortran routines for calculating

molecular structure parameters from the results of crystal structure analyses, Appl. Crystallogr.

C.F. Macrae, I.J. Bruno, J.A. Chisholm, P.R. Edgington, P. McCabe, E. Pidcock, L.

IP

[36]

T

28 (1995) 659.

CR

Rodriguez-Monge, R. Taylor, J. Streek, P.A. Wood, Mercury CSD 2.0 – new features for the visualization and investigation of crystal structures, J. Appl. Cryst. 41 (2008) 466-470. H. Ulla, B. Garudachari, M.N. Satyanarayan, G. Umesh, A.M. Isloor, Blue organic light

US

[37]

emitting materials: synthesis and characterization of novel 1,8-naphthalimide derivatives Opt.

[38]

AN

Mater. 36 (2014) 704-711.

Y. Kim, H.M. Hwang, L. Wang, I. Kim, Y. Yoon, H. Lee, Solar-light photocatalytic

M

disinfection using crystalline/amorphous low energy bandgap reduced TiO2, Sci. Rep. 6 (2016) 25212.

D.D. Babu, H. Cheema, D. Elsherbiny, A. El-Shafeib, A.V. Adhikari, Molecular

ED

[39]

engineering and theoretical investigation of novel metal-free organic chromophores for dye-

[40]

PT

sensitized solar cells, Electrochim. Acta 176 (2015) 868-879. P. Naik, R. Su, M.R. Elmorsy, D.D. Babu, A. El-Shafeib, A.V. Adhikari, Molecular design

CE

and theoretical investigation of new metal-free heteroaromatic dyes with D-p-A architecture as photosensitizers for DSSC application, J. Photochem. Photobiol. A: Chem. 345 (2017) 63-

[41]

AC

73

F. Würthner, T.E. Kaiser, C.R. Saha-Möller, J-Aggregates: from serendipitous discovery

to supramolecular engineering of functional dye materials, Angew. Chem. Int. Ed. 50 (2011) 3376-3410. [42]

F.H. Allen, O. Johnson, G.P. Shields, B.R. Smith, M. Towler, CIF applications. XV.

enCIFer: a program for viewing, editing and visualizing CIFs, J. Appl. Crystallogr. 37 (2004) 335-338. [43]

G. Arcovito, M. Bonamico, A. Domenicano, A.Vaciago, Crystal and molecular structure

of salicylaldehyde azine, J. Chem. Soc. B (1969) 733-741. 26

ACCEPTED MANUSCRIPT [44]

Xue-Xiang Xu, Xiao-Zeng You, Zhen-Fan Sun, Xin Wang, Hai-Xin Liu, 2,2'-

Azinodimethyldiphenol, Acta Crystallogr. C: Cryst. Str. Commun. 50 (1994) 1169-1171. [45]

S. Liu, Y. Chen, J. Dai, H. Liu, Synthesis and crystal structure of salicylaldehyde azine,

Hecheng Huaxue 12 (2004) 219-221. [46]

Puerta, M. Carmen, Valerga, Pedro, CCDC 1407807: Experimental crystal structure

determination, CSD Communication (2015). S.G. Teoh, S.B. Teo, G.Y. Yeap, H.K. Fun, A novel formation of salicylaldehyde azine

T

[47]

IP

and related azines from methylhydrazones induced by tin (IV) chloride and crystal structure of

[48]

CR

3-methoxysalicylaldehyde azine. Main Group Met. Chem. 17 (1994) 595-602. R. Lu, W. Wang, X. Lü, S. Zhao, 2-{[2-(2-Hydroxy-3-methoxybenzylidene) hydrazin-1-

[49]

US

ylidenemethyl}-6-methoxyphenol. Acta Crystallogr. E 67 (2011) o2702-o2702. Y. Qu, X.M. Sun, (E,E)-4-Hydroxy-3-methoxybenzaldehyde azine. Acta Crystallogr. E

[50]

AN

61(2005), o3828-o3830.

B. Schmitt, T.I. Gerber, E.C. Hosten, R. Betz, Crystal structure of (E,E)-4-hydroxy-3-

M

methoxybenzaldehyde azine–a second polymorph, C16H16N2O4, Z. Kristallogr. - New Cryst. Struct. 229 (2014) 470-472.

J.Y. Jin, K. Zhang, L.X. Zhang, A.J. Zhang, Y.Z. Liu, Crystal structure of N, N'-bis (4-

ED

[51]

methoxy-benzylidene)hydrazine, C16H16N2O2. Z. Kristallogr. - New Cryst. Struct.

PT

222(2007) 391-392.

Z. Fu, 2-Methoxybenzaldehyde azine, Acta Crystallogr. E 63 (2007) o2993.

[53]

Q. Ali, I. Anis, M.R. Shah, S.W. Ng, 2,3-Dimeth-oxy-benzaldehyde azine, Acta

CE

[52]

Crystallogr. E 67 (2011) o531. S.V. Sereda, S.V. Shelyazhenko, Y.A. Fialkoy, N.S. Pivovarova, V.A. Mithailik,

AC

[54]

Kristallografia, 37 (1992) 894-896. [55]

J. Qiu, B. Yin, 4-(Di-ethyl-amino)-salicyl-aldehyde azine, Acta Crystallogr. E 67 (2011)

o1092. [56]

K.O. Ukoba, A.C. Eloka-Eboka, F.L. Inambao, Review of nanostructured NiO thin film

deposition using the spray pyrolysis technique, Renew. Sust. Energ. Rev. 82 (2018) 29002915.

27

ACCEPTED MANUSCRIPT [57]

J.Y. Na, B. Kang, D.H. Sin, K. Cho, Y.D Park, Understanding solidification of

polythiophene thin films during spin-coating: effects of spin-coating time and processing additives, Sci. Rep. 5 (2015) 13288. [58]

G. Li, Y. Zhao, J. Li, J. Cao, J. Zhu, X. Sun, Q. Zhang, Synthesis, characterization, physical

properties, andoled application of single bn-fused perylene diimide. J. Org. Chem. 80 (2015) 196-203. G. Golan, A. Axelevitch, B. Gorenstein, V. Manevych, Hot-Probe method for evaluation

T

[59]

S.S.N. Bharadwaja, S.B. Krupanidhi, Alternating current electrical properties of

CR

[60]

IP

of impurities concentration in semiconductors. Microelectron. J. 37 (2006) 910-915.

antiferroelectric lead zirconate thin films by pulsed excimer laser ablation, J. Appl. Phys. 88

[61]

US

(2000) 2072.

P. Naik, R. Su, M.R. Elmorsy, A. El-Shafei, A.V. Adhikari. New di-anchoring A–π-D–π-

AN

A configured organic chromophores for DSSC application: sensitization and co-sensitization studies. Photochem. Photobiol. Sci. 17 (2018) 302-314. A.D. Becke, A new mixing of Hartree–Fock and local density‐ functional theories, J.

Chem.Phys. 98 (1993) 1372-1377.

C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy formula

ED

[63]

M

[62]

into a functional of the electron density, Phys. Rev. B 37 (1988) 785-789. P. Naik, M.R. Elmorsy, R. Su, D.D. Babu, A. El-Shafei, A.V. Adhikari, New carbazole

PT

[64]

based metal-free organic dyes with D-π-A-π-A architecture for DSSCs: Synthesis, theoretical

AC

CE

and cell performance studies, Sol. Energy 153 (2017) 600-610.

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Legends for schemes and figures

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Scheme 1. Synthesis of F1-F3 from o-vanillin

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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

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Fig. 4. (A) The molecular structure of F2 showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 50% probability level. Atoms labeled with the suffix ‘I’ are generated using the symmetry operator (½-x, ½+y, ½-z); (B) The bond lengths and bong angles of all the non-hydrogen atoms (including bond length of O1-H1) of F2 are represented with atom-labeling.

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lengths are depicted in the left half and bond angles on the right half.

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Atoms labeled with the suffix ‘‫ ’׳‬are generated using the symmetry operator (½-x, ½+y,½-z). Bond

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Fig. 5. (A) The intermolecular interactions of F2 are shown along a direction. The intra-molecular O-H…N hydrogen bonds as well as intermolecular C-H…π interactions are presented. For

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additional clarity, the hydrogen atoms which are not involved in the interactions are not displayed; (B) The crystal packing of F2 is shown along c direction in a herringbone stacking pattern showing

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C-H...π interactions stabilizing the molecules.

Fig. 6. SEM images of (A) F2 in THF/water mixture with fw = 80%, (B) 0.0001 molar solution of

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magnification and (G) tetrazo dye F3

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F2, (C) 0.1 molar solution of F2, (D) ester F1, (E) F2 with 10 μm magnification (F) F2 with 3 μm

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Fig. 7. AFM images of (A) F2 in THF/water mixture with fw = 80%, (B) 0.0001 molar solution of

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F2 and (C) 0.1 molar solution of F2

Fig. 8. (A) Dielectric constant (B) dielectric loss factor and (C) ac conductivity of compounds F1-

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F3 as a function of frequency

Fig. 9. Optimized geometry and FMO levels of o-vanillin fluorophores F1-F3

Fig. 10. (A) Simulated absorption spectrum of F1-F3, (B) Simulated IR spectrum of F1-F3

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RIR and J-aggregate formation impart good fluorescence in conjugated F2

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Electronic and photophysical results are in agreement with theoretical data

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These fluorophores are excellent candidates for electro-optical devices

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