Synthesis of 2-methyl-5-(5-phenyl substituted-1,3,4 oxadiazole-2-yl) quinazolin-4-one fluorescent brightening agent: Computational and experimental comparison of photophysical structure

Article abstract of DOI:10.1016/j.molstruc.2019.01.039

Report is about the synthesized new range of oxadiazole substituted quinazoline and studied its electronic distribution to attribute fluorescent properties. B3LYP Density Functional Theory (DFT) computational optimization was studied to observe the effect

Full text of DOI:10.1016/j.molstruc.2019.01.039

Accepted Manuscript
Synthesis of 2-methyl-5-(5-phenyl substituted-1,3,4 oxadiazole-2-yl) quinazolin-4-
one fluorescent brightening agent: Computational and experimental comparison of
photophysical structure
Vikas Patil, Vikas S. Padalkar, Nagaiyan Sekar, Satish V. Patil, Jamatsing Rajput
PII:
S0022-2860(19)30049-3
DOI:
https://doi.org/10.1016/j.molstruc.2019.01.039
MOLSTR 26090
Reference:
To appear in: Journal of Molecular Structure
Received Date: 22 September 2018
Revised Date: 9 January 2019
Accepted Date: 10 January 2019
Please cite this article as: V. Patil, V.S. Padalkar, N. Sekar, S.V. Patil, J. Rajput, Synthesis of 2-
methyl-5-(5-phenyl substituted-1,3,4 oxadiazole-2-yl) quinazolin-4-one fluorescent brightening agent:
Computational and experimental comparison of photophysical structure, Journal of Molecular Structure
(2019), doi: https://doi.org/10.1016/j.molstruc.2019.01.039.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis of 2-Methyl-5-(5-phenyl substituted-1,3,4 oxadiazole-2-yl) quinazolin-4-one Fluorescent
Brightening Agent: Computational and Experimental Comparison of Photophysical Structure
Vikas Patilab*, Vikas S. Padalkarb, Nagaiyan Sekarb Satish V. Patilc, Jamatsing Rajputc
,
a) University Institute of Chemical Technology, North Maharashtra University, Jalgaon
425001(India) Email: ce12vs.patil@pg.ictmumbai.edu.in
b) Department of Intermediate and Dyestuff technology, Institute of Chemical Technology,
,Mumbai - 400 019. India.
c) School of Life Sciences, North Maharashtra University, Jalgaon 425001(India)
Abstract: Report is about the synthesized new range of oxadiazole substituted quinazoline and
studied its electronic distribution to attribute fluorescent properties. B3LYP Density Functional
Theory (DFT) computational optimization was studied to observe the effect of electron donor and
acceptor substituent’s on photophysical properties, electronic state and energy level. DFT
computational optimization was performed by Polarizable Continuum Model (PCM) of solvation
strictly in the gas phase and DMF maintaining C1 symmetry in ground state geometry structure.
UV-vis and fluorescence spectroscopic methods help in understanding the relationship between the
electron donor and acceptor functional groups on the photophysical properties. Eventually
comparing experimental spectral emission and DFT computations were envisage understanding the
changes of the electronic transition, energy levels, and electronic orbital distribution in the
substituted quinazoline structure. These compounds have good fluorescent brightening properties
hence studied and applied as fluorescent brightening agent on polyester fiber.
Keywords: quinazoline, oxadiazole, TD-DFT, fluorescent, vertical excitation
Introduction
In dyestuff industry quinazoline plays an important role to carry chromatic di-azo and water
solubilizing sulphonic acid group [1-3]. Quinazoline core structure were widely utilized as the
fluorescent brightening agent. While substituted quinazolines being fluorescent in nature and was
used in many functional applications [3]. Naphthotriazole and annelated quinazoline were reported
as fluorescent whitening agents for synthetic fibers well before in 1970 [4]. Similarly, more
advanced methods are also reported for the synthesis of fluorescent compound by fused azoles and
respective components which are useful as functional application [5, 6]. Quinazoline as well as
other organic fluorescent dyes plays the role of heat-resistant fluorescent whitening agents which
has ᴫ bond conjugation [2, 7]. As an application point of view quinazoline was synthesized and
used as a luminescent paper ink containing unique sulphonamide (λmax 523 nm) and carboxamido
1

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phenyl substituent’s [1, 2, 4]. Fluorescent compounds gain the attention of organic chemist due to
its very high sensible level thereby acts as ON-OFF agents for decent applications [8]. Where the
size of synthetic dyes plays an important role, which may be reduced by physical method to the
desire extent for high sensitive and functional applications [8, 9].
Fluorescent properties of organic compounds are governed by the electronic distribution of the core
motif thereby active substituent plays a vital role in the final fluorescent properties of the molecule
[10-12]. The single molecular fluorescence aspects are also important while screening against the
protein and large biomolecules [12-16]. Hence; it becomes phenomenal to study and understand the
structural relationship between electron donating-withdrawing groups on the fluorescent properties.
While fluorescent properties are truly controlled by the rate of photoinduced electron transfer from
the electron donor to the singlet excited state of the electron acceptor fluorophore itself in the
excited state [17-19]. The fluorescent molecular properties are more prominent towards the protein
and biomolecules where the effect of solvation on the electronic properties plays important role in
probing the molecular behaviour [20, 21]. On the contrary oxidation and reduction potential of the
core structure of fluorophore has also plays vital role in the fluorescence properties. The oxidation
potential and reduction potential of any fluorophore more likely dependable on electron density on
central core structure. Which often controlled the fluorescence properties of substituted fluorophore
[22, 23]. It is noted that electron donor substitution at 2-position and an acceptor at 6-position gives
red-shifted absorption and complimentary red-shifted emission 1 [4-9, 24].
N
D
N
A
R
O
1
In the present study, we have reported combined experimental and theoretical photo-
physical properties of novel quinazolinone derivatives. The effects of various substituent groups
significantly influence absorption and emission properties of structure in DMF solvent. The
experimental photophysical properties are compared with the computed properties obtained from
DFT and TD-DFT computations.
Experimental
Materials and Methods:
All the commercial reagents and solvents were purchased from S. D. Fine Chemicals Pvt. Ltd. and
they were used without purification and all the solvents were of the spectroscopic grade. The
absorption spectra of the dyes were recorded on a Spectronic Genesys 2 UV-Visible
spectrophotometer with the measurement range of 250-600 nm in DMF at 1ꢀ10-6 mol L-1
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concentration and emission spectra were recorded on Varian Cary Eclipse fluorescence
spectrophotometer with the range of 350-600 nm using freshly prepared solutions in DMF at the
concentration of 1ꢀ10-6 mol L-1. The excitation wavelength used for fluorescence measurements
was absorption maxima of the compounds in DMF. The FT-IR spectra were recorded on a Perkin-
Elmer Spectrum 100 FT-IR Spectrometer with resolution of 0.5 cm-1, range of 4000 to 500 cm-1, 4
1
(four) number of scan and with cooling MCT (Mercury Cadmium Telluride) detector. H NMR
spectra were recorded on VXR 300 MHz in deuterated DMSO [(CD3)2SO] instrument using TMS
as an internal standard. Mass spectra were recorded on Finnigan mass spectrometer by Electro
Ionisation (EI) technique due to low molecular weight compound. All the final compounds were
gravity column purified before analysis and use as a fluorescent brightening agent.
Relative Fluorescence Quantum Yield Calculations:
The fluorescence quantum yields of compounds in DMF were estimated for which anthracene was
used as the standard [12, 25]. Fluorescence quantum yields were calculated using the comparative
method [12, 25]. Absorption and emission characteristics of the standards and compounds 6a-6g
were measured in DMF at different concentrations (1, 2, 3, 4, and 5 ppm). Further a linear plots of
emission intensity against the absorbance intensity were obtained and a gradient were calculated for
the standard and compound 6a-6g. All the measurements were done by keeping the parameters such
as solvent and slit width constant. Emission and excitation slit width was 2.5 unit maintained in all
measurements. Equation (1) was used to calculate relative fluorescence quantum yields of
synthesized compounds 6a-6g [12, 25].
2
Gradx ηx
φx = Φst ×
×
------------------Equation (1)
2
Gradst
ηst
Where:
Φx
= Fluorescence quantum yield of compound
Φst
= Fluorescence quantum yield of standard sample
Gradx = Gradient from the plot of integrated fluorescence intensity against
absorbance of the studied compound
Gradst = Gradient of standard sample
ηx
ηst
= Refractive index of solvent used for synthesized compound
= Refractive index of solvent used for standard sample
3

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Experimental:
Synthesis of Ethyl 2-methyl-4-oxo-3, 4-dihydroquinoline-5-carboxylate 4
The esterification of 5 g of 3-nitrophtallic acid was performed at reflux in 15 mL of ethanol. The
dehydration catalyst p-toluene sulphonic acid (0.1 g) was used in catalytic amount. 3-Nitrobenzene-
1,2-dicarboxylate ester was separated after 2 hours of reflux the completion of reaction was
checked by TLC. Extraction was performed in ethyl acetate to separate un-esterified 3-nitrophtallic
acid which was separated as neutralization salt with the treatment of Na2CO3. 3-Nitrobenzene-1,2-
dicarboxylate was reduced in presents of 5% Pd supported on carbon in hydrogen gas pressure
reactor at 5 Kg/cm2. Methanol was used as solvent for reduction of diethyl 3-nitrobenzene-1,2-
dicarboxylate. The crude diethyl 3-aminobenzene-1,2-dicarboxylate was used for further reaction
after vacuum distillation. Acetonitrile (100 mL) and 3-aminobenzene-1,2-dicarboxylate (2.5 g) was
reacted by continuously aerated dry hydrochloric acid gas. HCl gas catalyzed cyclisation reaction
yielded intermediate ethyl 2-methyl-4-oxo-3,4-dihydroquinoline -5-carboxylate (4) separated in
cold water under continuous stirring Scheme 1.
Yield: 58 %, Melting point = 212 oC (Lit 212 oC) [10, 11, 26].
FT-IR (KBr): 2978 (CH3 Out of plane), 2910 (-CH2 methylene), 1739 (-C=O ester), 1674 (-C=O
Amide), 1613 (C=N), 1503 (C=C Aromatic), 1468 (C-C-O), 1322 (C-H ion phase), 1273 (-CH2 ),
1180, 1024, 873, 779, 678 (C-O-C sting) cm-1.
1H-NMR (CD3)2SO (300 MHz): δ = 1.41 (t, 3H, -CH3), 2.55 (s, 3H, -CH3), 4.40 (q, 2H, -CH2),
7.42 (d, 1H, J = 8.4 Hz, Ar-H), 7.7 (d, 2H, J = 9.9 Hz, Ar-H), 11.88 (s, 1H, NH).
Mass: m/z 235 (M+H)+.
Synthesis of 2-Methyl-4-oxo-3,4-dihydroquinazoline-5-carbohydrazide 5
2-methyl-4-oxo-3,4-dihydroquinazoline-5-carbohydrazide 5 was prepared by reaction of Ethyl-2-
methyl-4-oxo-3,4-dihydroquinazoline-5-carboxylate (5g) with 80 % hydrazine hydrate (15 mL)
under reflux condition for two hours.
Yield: 89 %, Melting point = > 300 oC.
FT-IR (KBr): 3165 (-NH2), 2981 (-CH3 Out of plane), 2917 (-CH2), 1665 (Amid –C=O), 1607
(Hydrazide –C=O), 1562 (-C=N), 1478 (C-C-O), 1383 (-CH3 primary band), 1261 (-CH2-), 1193
(C-C bond), 1041 (C-O lattise mode), 843 (NH ring mode), 721, 663 (C-O-C ring mode) cm-1.
1H-NMR (CD3)2SO (300 MHz): δ 2.20 (s, 3H, -CH3), 5.26 (s, 2H, Amine –NH2), 7.72 (d, 1H, J =
22.00 Hz, Ar-H), 7.79 (d, 1H, J = 6.7 Hz, Ar-H), 7.82 (dd, 1H, J = 22.00, 6.7 Hz, Ar-H).
Mass: m/z 219 (M+H)+.
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COOH
COOH
COOEt
COOEt
COOEt
EtOH
H2
Dry HCl
CH3CN
Pd/C
PTSA
COOEt
NO2
NO2
NH2
3
2
1
N
N
CONHNH2
Ar
COOEt
O
O
O
O
ArCOOH
POCl3
80% NH2NH2
N
NH
CH3
N
NH
N
NH
CH3
CH3
6a-6g
5
4
HO
Ar =
OH
Cl
Cl
OH
Cl
OH
6a
NO2
6f
OH
6g
NO2
6c
6b
6e
6d
Scheme 1: Synthesis of ethyl 2-methyl-4-oxo-3,4-dihydroquinazoline-5-carboxylate 4 intermediate
and 2-methyl-5-(5-phenyl substituted-1,3,4 oxadiazole-2-Yl) quinazolin-4(3H)-one 6a-6g.
General experimental procedure for synthesis 6a-6g
2-Methyl-4-oxo-3,4-dihydroquinazoline-5-carbohydrazide 5 (0.0045 moles) and an equimolar
quantity of different substituted carboxylic acid were refluxed in phosphorus oxychloride for
eighteen hours. Carboxylic acid and substituted hydrazine undergoes the cyclo-condensation
reaction. After completion of reaction confirmed by TLC the reaction mixture cooled to room
temperature and completely vacuum distilled POCl3 at low temperature. The compete degassing
performed for one hour under vacuum. The dry reaction mass diluted with ice cold water in the
reaction flask and neutralized with 10% Na2CO3 and filter the product. Wash the product with the
copious amount of water. Finally dried the product in an oven at 50o C gave different oxadiazol-2-
yl-2-ethylquinazolin-4(3H)-one 6a-6g Scheme 1.
5-[5-(2,4-Dihydroxyphenyl)-1, 3, 4-oxadiazol-2-yl]-2-methylquinazolin-4(3H)-one 6a
Yield: 69 %, Melting point = 228 oC
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FT-IR (KBr, cm-1): 3289 (Phenolic –OH), 2987 (-CH3 Out of plane), 1789 (C=N oxadiazole),
1616 (C=O amide), 1589 (C=C aromatic), 1567 (C=C aromatic), 1455 (N-H amide), 1289 (C-O
aromatic), 1189 (N-N oxadiazole), 988 (-CH3 primary).
1H NMR (CD3)2SO, 300 MHz): δ 2.49 (s, 3H, -CH3), 5.38 (s, 1H), 6.10 (s, 1H, Ar-H), 6.33 (d, 1H,
J=9.8, Ar-H), 6.64 (d, 1H, J=8.6, Ar-H), 7.45 (dd, 1H, J=9.8, 8.6, Ar-H), 7.59 (s, 1H, Ar-H), 7.76
(d, 1H, 11.8, Ar-H), 7.97 (s, 1H, Ar-H)
Mass: m/z 337 (M+H)+
5-[5-(2-Hydroxyphenyl)-1, 3, 4-oxadiazol-2-yl]-2-methylquinazolin-4(3H)-one 6b
Yield: 71 %, Melting point = 295-305 oC
FT-IR (KBr, cm-1): 3345 (Phenolic –OH), 2890 (-CH3 Out of plane), 1689 (C=O amide), 1545
(C=C aromatic), 1435 (N-H amide), 1287 (C-O aromatic), 1150 (N-N oxadiazole), 978 (-CH3
primary).
1H NMR (CD3)2SO, 300 MHz): δ 2.42(s, 3H, -CH3), 7.07(s, 1H, Ar-H), 7.24(d, 1H, J=8.86, Ar-
H), 7.07(dd, 1H, J=8.86, 11.65, Ar-H), 7.62(d, 1H, J=11.65, Ar-H), 7.59(s, 1H, Ar-H), 7.76(1d, 1H,
J=11.8, Ar-H), 7.97(s, 1H, Ar-H), 8.21(s, 1H, Ar-H).
Mass: m/z 320 (M+H)+
5-[5-(2-Hydroxynaphthalen-1-yl)-1, 3, 4-oxadiazol-2-yl]-2-methylquinazolin-4(3H)-one 6c
Yield: 68 %, Melting point = 265 oC
FT-IR (KBr, cm-1): 3401 (Phenolic –OH), 2978 (-CH3 Out of plane), 1667, 1639 (C=O amide),
1556 (C=C aromatic), 1446 (N-H amide), 1267 (C-O aromatic), 1151 (N-N oxadiazole), 965 (-CH3
primary).
1H NMR (CD3)2SO, 300 MHz): δ 2.61(s, 3H, -CH3), 6.09(s, 1H), 7.04(s, 1H, Ar-H), 7.79(s, 1H,
Ar-H), 8.04(d, 1H, J=9.1, Ar-H), 7.44(dd, 1H, J=9.1, 8.79, Ar-H), 7.53(dd, 1H, J=8.79, 7.4, Ar-H),
8.41(d, 1H, J=7.4, Ar-H), 7.59(s, 1H, Ar-H), 7.76(dd, 1H, J=11.1, 7.65, Ar-H), 7.97(s, 1H, Ar-H),
9.07(s, 1H, Ar-H)
Mass: m/z 372 (M+H)+
5-[5-(2-Chlorophenyl)-1, 3, 4-oxadiazol-2-yl]-2-methylquinazolin-4(3H)-one 6d
Yield: 73 %, Melting point = 220 oC
FT-IR (KBr, cm-1): 3389 (N-H amide), 3018 (-CH3 Out of plane), 1767 (C=N oxadiazole), 1670,
1569 (C=C aromatic), 1440 (N-H amide), 1397, 1251 (C-O aromatic), 1176 (N-N oxadiazole), 978
(-CH3 primary).
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1H NMR (CD3)2SO, 300 MHz): 2.42(s, 3H, -CH3), 7.56(d, 1H, J=8.19, Ar-H), 7.35(dd, 1H,
J=8.19, 7.6, Ar-H), 7.39(dd, 1H, J=7.6, 11.1, Ar-H), 7.73(d, 1H, J=11.4, Ar-H), 7.59(s, 1H, Ar-H),
7.76(d, 1H, J=8.3, Ar-H), 7.97(s, 1H, Ar-H), 9.1(s, 1H, Ar-H).
Mass: m/z 339 (M+H)+
5-[5-(2, 6-Dichloro-4-nitrophenyl)-1, 3, 4-oxadiazol-2-yl]-2-methylquinazolin-4(3H)-one 6e
Yield: 78 %, Melting point = 241 oC
FT-IR (KBr, cm-1): 3401 (N-H amide), 3031 (-CH3 Out of plane), 1775 (C=N oxadiazole), 1666
(C=O amide), 1581 (C=C aromatic), 1441 (N-H amide), 1254 (C-O aromatic), 1176 (N-N
oxadiazole), 978 (-CH3 primary).
1H NMR (CD3)2SO, 300 MHz): δ 2.4(s, 3H, -CH3), 7.91(s, 1H, Ar-H), 7.67(d, 1H, J=12.5, Ar-H),
7.08(s, 1H, Ar-H), 8.31(s, 2H, Ar-H), 9.21(s, 1H)
Mass: m/z 419 (M+H)+
2-Methyl-5-[5-(4-nitrophenyl)-1, 3, 4-oxadiazol-2-yl] quinazolin-4(3H)-one 6f
Yield: 78 %, Melting point = 251 oC
FT-IR (KBr, cm-1): 3352 (N-H amide), 2988 (-CH3 Out of plane), 1775 (C=N oxadiazole), 1656,
1622 (C=O amide), 1531 (C=C aromatic), 1443 (N-H amide), 1251 (C-O aromatic), 1176 (N-N
oxadiazole), 978 (-CH3 primary).
1H NMR (CD3)2SO, 300 MHz): δ 2.61(s, 3H, -CH3), 7.21(s, 1H, Ar-H), 7.46(d, 1H, J=13.9, Ar-
H), 7.68(s, 1H, Ar-H), 8.19(d, 2H, J=11.5, Ar-H), 7.95(d, 2H, J=9.2, Ar-H), 9.04(s, 1H)
Mass: m/z 350 (M+H)+
5-[5-(4-Hydroxyphenyl)-1, 3, 4-oxadiazol-2-yl]-2-methylquinazolin-4(3H)-one 6g
Yield: 69 %, Melting point = 190-198 oC
FT-IR (KBr, cm-1): 3451 (Phenolic –OH), 2985 (-CH3 Out of plane), 1769 (C=N oxadiazole),
1591 (C=C aromatic), 1529 (C=C aromatic), 1439 (N-H amide), 1396, 1255 (C-O aromatic), 1171
(N-N oxadiazole), 968 (-CH3 primary).
1H NMR (CD3)2SO, 300 MHz): δ 2.49(s, 3H, -CH3), 7.13(d, 2H, 14.9, Ar-H), 7.96(d, 2H, J=14.9,
Ar-H), 7.49(s, 1H, Ar-H), 7.55(d, 1H, J=11.8, Ar-H), 7.79(s, 1H, Ar-H), 9.04(s, 1H), 11.1(s, 1H)
Mass: m/z 320 (M+H)+
Computational Details:
Effect of electron donor and acceptor on photophysical properties, electronic state and energy level
was investigated of oxadiazole substituted quinazoline. The ground state geometry of the
compounds in their C1 symmetry was optimized using the fixed parameter in the gas phase and in
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DMF solvent using density functional theory [27] and Polarizable Continuum Model (PCM)
solvation model. The functional used in this study was B3LYP. The B3LYP method is a
complementary combination of Becke’s three parameter exchange functional (B3) [28, 29] with the
nonlocal correlation functional by Lee, Yang, and Parr (LYP) [30]. The basic set used for all the
atoms was 6-31G(d) [31-33]. The vibrational frequencies of the optimized structures were
computed using the same method to verify that the optimized structures correspond to local minima
on the potential energy surface. TD-DFT computations were carried out on the optimized
structures. Since the fluorophores synthesized here are the largely donor-acceptor type and there are
no fused rings in the system. It was decided to do the excited computations at the same level of
theory. The excited state computations in terms of optimization in the framework of PCM model
has its own implications [34-37] however, considering the fact that we wanted to assess the general
trend in the experimentally observed vertical excitations and emissions (in the same series of
generic structures) a simple and traditional computational strategy has been chosen. The vertical
excitation energies at the ground-state equilibrium geometries were calculated with TD-DFT [38].
The low-lying first singlet excited state (S1) of each tautomer was relaxed using the TD-DFT to
obtain its minimum energy geometry. The difference between the energies of the optimized
geometries in the first singlet excited state and the ground state was used in computing the
emissions [39]. All electronic structure computations were carried out using the Gaussian 09
program [40, 41]. As depicted in Figure 1 a computational optimized geometry shows quinazoline
core in one plane and substituent at 5th position are at out of the plane from quinazoline minimizing
stereochemical interactions. Hence the mesomeric effect is very negligible at 5th position and
dominating inductive effect governs the absorption and emission properties of the quinazoline core.
Figure 1: Molecular structure of substituted quinazoline 6a extracted from DFT optimization.
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General procedure of brightning polyester fabric by fluorescent brightning agent:
Dyeing of polyester fabric was carried out using the high-temperature high-pressure method in
Rossari Labtech Flexi Dyer dyeing machine at a material to liquor ratio of 1:20. 2% fluorescent
compounds were used for dying (calculated on the weight of the fabric). All the synthesized
fluorescent compounds are having less solubility in water. Initially the compounds were dissolved
in 5 mL N,N-dimethylformamide and diluted with 15 mL buffered solution of pH 5 made by using
sodium acetate and acetic acid in water. The mixture was ultra-sonicated for 15 min to obtain a fine
dispersion. Metamol was used as a dispersant. The polyester fabric was dyed using the above
solution and metamol as the dispersing agent. The dye bath temperature was raised at a rate of 3°C
°
min-1 to 130 C, maintained at this temperature for 60 minutes, and rapidly cooled to room
temperature. The dyed fabrics were rinsed with cold water and allowed to dry in the open air [5,
10].
Results and Discussion:
The intermediate ethyl 2-methyl-4-oxo-3,4-dihydroquinazoline-5-carboxylate 4 was prepared from
3-nitro-phthalic acid by esterification, reduction and cyclisation reaction Scheme 1 [17, 18].
Further the intermediate 5 (2-methyl-4-oxo-3,4-dihydroquinazoline-5-carbohydrazide) was
synthesized from 4 reacting with hydrazine hydrate. 2-Methyl-5-(5-phenyl substituted-1,3,4
oxadiazole-2-yl) quinazolin-4(3H)-one (6a-6g) was synthesized by reacting the intermediate 5 with
different carboxylic acids in a single step. Synthesis of compounds 6a-6g involves a cyclisation
reaction in phosphorous oxychloride at reflux temperature for 18 hrs. On completion of the reaction
phosphorous oxychloride was distilled out completely from the reaction mixture under vacuum.
The thick reaction mixture was poured into ice cold water and neutralized with 10% Na2CO3. The
product was filtered, washed with cold water, and dried. The yield of this reaction ranges from 68-
78%. The synthesis details are summarized in Scheme 1.
Computational Study:
In case of oxadiazole series, the difference between experimental absorption, emission and vertical
excited computed by DFT and theoretical emission computed by TD-DFT are in good agreement
except for the compound 6e for absorption and vertical excitation. Observed absorption, emission,
computed vertical excitations and emission are summarized in Table 1. %D character is expressed
as deviation obtained from experimental results. As depicted Table 1 maximum absorption
deviation occur 43 % for 6e. For emission maxima high deviation % obtained from experimental
results in case of 6g molecule while 6e has the absorption maxima shifts from 276 to 297 nm with
43 %D. All the other values are in well agreement with experimentally obtained values. This is clue
9

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for synthesis of molecules before its experimental setup to find out the preliminary absorption and
emission properties.
aStoke
TD-DFT
Vertical
Excitation
%
%
D
Orbital
Contribution
s Shift
ꢀλ
f
bΦ
D
(nm)
(nm)
(cm-1)
nm
eV
342
282
300
314
276
312
376
335
320
343
306
397
378
327
3.698
3.873
3.613
4.048
3.118
3.272
3.791
0.259
0.342
0.121
0.387
0.005
0.109
0.280
2
427
402
485
430
428
431
462
396
382
409
369
473
436
388
7.0
5820
0.002
0.017
0.021
0.102
0.253
0.203
0.10
6a
6b
6c
6d
6e
6f
H → L (98 %)
H → L (96 %)
H → L (98 %)
H → L (93 %)
H → L (99 %)
H → L (99 %)
H → L (97 %)
13
14
02
43
21
13
4.8 10585
15 12714
14
8591
10 12867
1.3
15
8849
4850
6g
a Stokes shift, b Quantum yield, f Oscillator strength, Analyses were carried out at room temperature
(25°C); λexp experimentally observed absorption and emission, (% D) % Deviation between vertical
excitation and experimental absorption and experimental emission and computed (TD-DFT)
emission.
Table 1: Observed UV-visible absorption, computed absorption, observed emission and computed
emission of compounds 6a-6g in DMF.
The HOMO-LUMO energies of the compounds 6a-6g were obtained computationally and
summarized in Figure 2. The HOMO-LUMO energy difference for all the compounds is almost
similar with slight change. In case of the compounds 6b, 6e and 6f the difference between HOMO-
LUMO is large as compared to the compounds 6a, 6c, 6d and 6g accounting for a blue shift in the
compounds 6b, 6e and 6f. This is in agreement with the experimental absorption data except for the
compound 6g (Figure 2).
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Figure 2. Graph of HOMO-LUMO energies of compounds 6a-6g in DMF.
The frontier molecular orbitals of the compounds (HOMO and LUMO) 6a-6g in DMF solvent are
summarized in Table 2. The compounds contain two heterocyclic systems - quinazoline and
substituted oxadiazole. In excited state compounds 6a-6g are more planar as compared to ground
state Table S1 (supporting material), this clearly indicates that intramolecular charge transfer
occurs in the excited state. In the case of the compound 6a HOMO, the electron density is spread
across the oxadiazole and phenyl ring substituted at the 5th position of oxazole unit, while in the
LUMO, the electron density is localized along the quinazolinone and oxadiazole unit. Such an
electron density distribution indicates that quinazoline acts as donor and phenyl ring acts as
acceptor via oxadiazole π system, a similar trend is observed for the compound 6b. In the case of
the compound 6c HOMO electron density is spread over oxadiazole and 5-substituted hydroxy
naphthalene unit, but in the case of LUMO electrons are migrated from oxadiazole to 5-substituted
hydroxy naphthalene unit which indicates that oxadiazole unit acts as donor and hydroxyl
substituent at the 5th position of naphthalene as an acceptor. In case of compound 6d HOMO
electron density is distributed over oxadiazole and 5-substituted hydroxyl naphthalene unit, while
in LUMO the electrons are distributed over the entire molecule. In the case of the compounds 6e
and 6f HOMO, the electron density is distributed over 5-substituted groups of oxadiazole units,
while in LUMO the electron density is distributed over quinazolinone and oxadiazole units for
compound 6e. The 5-substituent group acts as an electron donor and quinazoline and oxadiazole as
an acceptor for the compound 6e. In LUMO of 6f the electron density is distributed over the entire
molecule except for heterocyclic unit of quinazoline. This act indicates that oxadiazole and
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quinazoline units behave as an acceptor. The electron distribution pattern of the compound 6g is
similar to the compound 6d, in 6g oxadiazole and 5-substituted unit act as a donor while
quinazoline unit acts as an acceptor.
HOMO
LUMO
HOMO
LUMO
6a
6c
6e
6b
6d
6f
6g
Table 2. HOMO-LUMO diagrams of compound 6a-6g in DMF.
Photophysical Properties of Compounds 6a-6g:
The absorption and emission properties of all the compounds 6a-6g were studied in the solvent
dimethylformamide. All the absorption-emission studies were performed at room temperature using
solutions of concentration 1x10-6 M. The synthesized compounds are fluorescent in solution under
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irradiation of UV light. The effect of electron donor and electron acceptor groups on the absorption
and the emission properties of the compounds were studied. The absorption and emission spectra of
all the compounds are shown in Figure 3.
Figure 3: Emission spectra of compounds 6a-6g and in DMF
The compounds 6b and 6e showed blue shifted absorption, while compound 6g showed red-shifted
absorption. All the compounds absorb in UV and near visible region. The compounds 6g showed
red-shifted absorption as compared to the other compounds. The compounds 6a, 6c and 6d show
dual intense absorption in UV region. The absorption intensity of the compound 6g is
comparatively high as compared to the other studied compounds. The compounds 6a-6g are
structurally similar, only aromatic system attached to oxadiazole core are different. The compound
6g shows red-shifted absorption as well as emission; this is due to the electron donor –OH group on
the aromatic system para to the oxadiazole core. The compounds 6d, 6e and 6f show blue-shifted
emission due to the electron acceptor groups attached to the oxadiazole core. In the case of the
compounds 6a and 6b the electron donor – OH group is attached to the aromatic system at the ortho
position of the oxadiazole unit and they showed blue-shifted emission. Ortho-substituted
carboxylic acids on quinazolinone show blue shift while p-substituted carboxylic acids show a red
shift in absorption. The fluorescence emission of the compounds ranges from 402-485 nm, all the
compounds emit in the near-visible region. The compounds which emit in the near-visible region
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are generally used as fluorescent brightening agents. The compounds 6c and 6g showed red-shifted
emission as compared to 6a, 6b, 6d, 6e and 6f. The compound 6d showed high fluorescence
intensity as compared to the compounds 6b-6g. Fluorescence quantum yield of the compounds was
calculated in DMF by the relative method. The Fluorescence quantum yield value 6d, 6e, 6f and 6g
is higher than the compounds 6a, 6b and 6c. The fluorescence quantum yield is tenfold higher for
the compounds 6d, 6e, 6f and 6g than for the compounds 6b and 6c and hundredfold higher for the
compound 6a. Electron withdrawing groups present on carboxylic acid moiety enhance the
fluorescence quantum yield as compared to the electron donating groups presents on the carboxylic
acid unit. The photophysical properties of the compounds are summarised in Table 1.
Thermo gravimetric analysis: To more deep insight the molecules the thermos gravimetric
analysis shows that the molecules are thermally stable above the 300o C. Amongst these molecules
6a having the sharp peak of weight loss rest of the molecules have the continuous weight loss
profile. The thermo gravimetric analysis envisaged to apply these molecules as fluorescent
brightening agents. Where the process of fluorescent brightening agent application is at temperature
130o C with pressure and these molecules have thermal stability more than 130o C. (Figure 4).
Figure 4: Thermo gravimetric analysis of compounds 6a-6g.
Fluorescent Brightening agent:
The colorimetric parameters of the whitened polyester fabrics using synthesized fluorescent
molecules 6a-6g were recorded on a reflectance spectrophotometer CE-7000A Gretag-Macbeth [5,
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10]. CIE 1976 Color Space method was used to evaluate the color values of synthesized compounds
6a-6g on polyester fabrics in terms of L*, a* and b* (Tables 3). All the compounds have a good
affinity towards the polyester fabrics at high temperature and gave whitening with a blue tinge on
polyester fabrics. The whiteness index values of the fabrics dyed with compounds 6a-6g are
summarized in Table 3. The colorfastness properties of these dyes were correlated with
commercially used whitening agents.
Leuco
phore white White
BSB
74.6
Sera
Sera
Hostalu
x
ESR50
75.4
Blank
Polyester
6a
6b
6c
6d
6e
6f
6g
PB
73.9
PN
71.2
48.0
50.8
54.1
76.5
-0.3
0.4
46.3
49.2
50.6
75.6
-0.9
2.2
44.6 45.8
47.4 48.3
45.5 49.6
74.4 75.0
44.6
47.7
45.4
74.6
-1.5
6.0
45.9
48.6
49.1
75.2
-0.6
3.1
44.3
47.5
43.3
74.5
-2.0
8.2
44.5
47.6
43.8
74.6
-1.8
7.8
X
Y
80.1
83.1
91.7
-28.2
2.1
77.7
105.7
90.6
1.1
75.0
96.8
89.4
0.6
79.9
105.4
91.6
Z
L*
a*
b*
C*
ho
-0.9
5.6
5.7
-0.1
2.3
2.4
-0.1
-16.1
16.1
274.0
0.26
-12.2
12.2
273.2
0.07
-14.0
14.0
0.5
2.4
6.2
3.2
8.5
8.0
3.5
133.6
0.25
112.8
0.36
99.6 93.4
0.53 0.36
104.6
0.58
100.7
0.40
104.1
0.66
103.2
0.61
142.5
0.04
269.4
0.09
K/S
Stensbay
Whitenes
74.1
64.0
66.0
45.0
54.5 67.4
53.0 44.7
51.7
54.3
63.8
47.3
43.4
60.1
45.5
59.0
0.5
142.4
189.7
128.0
162.3
134.0
181.8
Taube
Whitenes
92.1
Table 3. Color properties of compounds 6a-6g and commercially used optical whiteners.
Conclusion:
Synthesis and application of novel oxadiazole substituted quinazoline compounds have been
1
performed and characterized by H-NMR, Mass and IR spectroscopy. The novel oxadiazol
substituted quinazoline and electronic distribution over the structure concluded the fluorescent
properties of compounds. Which states that fluorescent properties are independent of substituents
attached to the 5th position. Photophysical properties, electronic state and energy level by B3LYP
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Density Functional Theory (DFT) computational optimization confirm the dependency of
fluorescence on electron donor and acceptor substituents. Eventually quinazolines show the basic
requirements for the fluorescent brightening agent’s an absorption in ultra-violet region and
emission at visible region (around 400 nm). Compounds gave whitening with a blue tinge on
polyester fabrics has potential as emerging new fluorescent brightening agents. The significant a*,
b* and K/S values are more for electron withdrawing -NO2 group substituent 6d and 6f. The K/S
value for 6d and 6f are 0.58 and 0.66 which are more than commercial Leucophore BSB (0.04),
Sera White PB (0.26), Sera white PN (0.07) and Hostalux ESR 50 (0.09) while the conditions for
application of brightening agents was similar. The compared experimental photophysical properties
with theoretical data obtained by DFT computation results are in close agreement with theoretical
properties. This is a step towards the minimization of laboratorial work before the synthesis of
complex structure desirable to find out the essential properties by computational optimization.
Acknowledgments: Vikas Patil and Vikas Padalkar are thankful to the Institute of Chemical
Technology, Mumbai. Vikas Patil is thankful to University Grant Commission (UGC) New Delhi
for UGC-FRP and startup grant. Science and Engineering Research Board (SERB), Department of
Science and Technology (DST) New Delhi for Fastrack scheme of a young scientist (SB/FT/CS-
106/2014).
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Table and Figure captions
Table 1: Observed UV-visible absorption, computed absorption,
observed emission and computed emission of compounds 6a-6g in DMF.
Table 2: HOMO-LUMO diagrams of compound 6a-6g in DMF.
Table 3: Colour properties of compounds 6a-6g and commercially used optical whiteners.
Figure 1: Molecular structure of substituted quinazoline 6a extracted from DFT optimization.
Figure 2: Molecular structure of substituted quinazoline 8a extracted from DFT optimization.
Figure 3: Graph of HOMO-LUMO energies of compounds 6a-6g in DMF.
Figure 4: Emission spectra of compounds 6a-6g and 8a-8g in DMF
Scheme1. Synthesis of 2-methyl-5-(5-phenyl substituted-1,3,4 oxadiazole-2-Yl) quinazolin-
4(3H)-one 6a-6g.
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ꢀ New range oxadiazole-quinazoline based of fluorescent brightening agent.
ꢀ C1 symmetry PCM model DFT-TD-DFT computational of molecular structure.
ꢀ Oscillator strength and % deviation by vertical excitation and λabs/λem.
ꢀ Studied electronic transition, energy levels, electronic orbital distribution.