Synthesis and characterization of new green and orange region emitting anthracene based oxazol-5-one dyes

Article abstract of DOI:10.1016/j.dyepig.2017.05.049

New anthracene oxazol-5-one dyes featuring an extended π-conjugated electron system have been successfully prepared by Erlenmeyer synthesis, structurally characterized and their spectroscopic properties were investigated by UV–vis absorption spectroscopy and fluorescence spectroscopy. The oxazol-5-ones were attached to the 9-position of the anthracene to obtain the desired structures. The dyes are having extended conjugation throughout their structure with oxazol-5-one ring as the chromophore. The spectral properties of the oxazol-5-one ring were monitored with respect to the substituents at the phenyl ring. All of the dyes synthesized show good solubility in common organic solvents. Also in order to establish whether there was any solvent affects the absorption and emission spectra of the compounds were measured in the solvents of acetonitrile, tetrahydrofuran and chloroform which have different polarity. Moreover, their absorption and emission properties were investigated in plasticized PVC film matrix. In comparison to the solution phase, the dyes displayed enhanced fluorescence emission quantum yield values when embedded in poly(vinyl chloride) polymer film. The anthracene based oxazol-5-ones were found to emit in the green portion of the spectrum for 2a and 2b, and in the orange portion of the spectrum for 2c. By attachment of nitro substituent at the para position of the phenyl ring bound to oxazol-5-one core, the fluorescence maxima could be effectively modulated from the green region of the spectra to the orange region. 4-(9-anthralydene)-2-(4-nitrophenyl)oxazol-5-one (2c) exhibits large Stokes shift up to 146 nm presumably due to intramolecular charge transfer.

Full text of DOI:10.1016/j.dyepig.2017.05.049

Accepted Manuscript
Synthesis and characterization of new green and orange region emitting anthracene
based oxazol-5-one dyes
Gulsiye Ozturk Urut, Serap Alp, Derya Topkaya
PII:
S0143-7208(17)30446-1
10.1016/j.dyepig.2017.05.049
DYPI 6011
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 3 March 2017
Revised Date: 14 April 2017
Accepted Date: 29 May 2017
Please cite this article as: Ozturk Urut G, Alp S, Topkaya D, Synthesis and characterization of new
green and orange region emitting anthracene based oxazol-5-one dyes, Dyes and Pigments (2017), doi:
10.1016/j.dyepig.2017.05.049.
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ACCEPTED MANUSCRIPT

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Synthesis and characterization of new green and orange region emitting anthracene
based oxazol-5-one dyes
Gulsiye Ozturk Urut*, Serap Alp, Derya Topkaya
Department of Chemistry, Faculty of Sciences, Dokuz Eylul University, Tinaztepe Campus,
35160, Buca, Izmir, Turkey
*Corresponding Author: Dokuz Eylül University, Department of Chemistry, Faculty of
Sciences, Tinaztepe Campus, 35160, Buca, Izmir, Turkey.
Tel: +90 232 301 9552
Fax: 0090 232 453 41 88
E-mail adresses: gulsiye.ozturk@deu.edu.tr, gulsiyeozturk@yahoo.com
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ABSTRACT
New anthracene oxazol-5-one dyes featuring an extended π-conjugated electron system have
been successfully prepared by Erlenmeyer synthesis, structurally characterized and their
spectroscopic properties were investigated by UV-vis absorption spectroscopy and
fluorescence spectroscopy. The oxazol-5-ones were attached to the 9-position of the
anthracene to obtain the desired structures. The dyes are having extended conjugation
throughout their structure with oxazol-5-one ring as the chromophore. The spectral properties
of the oxazol-5-one ring were monitored with respect to the substituents at the phenyl ring.
All of the dyes synthesized show good solubility in common organic solvents. Also in order
to establish whether there was any solvent affects the absorption and emission spectra of the
compounds were measured in the solvents of acetonitrile, tetrahydrofuran and chloroform
which have different polarity. Moreover, their absorption and emission properties were
investigated in plasticized PVC film matrix. In comparison to the solution phase, the dyes
displayed enhanced fluorescence emission quantum yield values when embedded in
poly(vinyl chloride) polymer film. The anthracene based oxazol-5-ones were found to emit in
the green portion of the spectrum for 2a and 2b, and in the orange portion of the spectrum for
2c. By attachment of nitro substituent at the para position of the phenyl ring bound to oxazol-
5-one core, the fluorescence maxima could be effectively modulated from the green region of
the spectra to the orange region. 4-(9-anthralydene)-2-(4-nitrophenyl)oxazol-5-one (2c)
exhibits large Stokes shift up to 146 nm presumably due to intramolecular charge transfer.
Keywords: Anthracene, Oxazol-5-one, Fluorescent, Erlenmeyer reaction, PVC
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1. Introduction
Organic compounds, which have been designed based on different heterocycles, have
been extensively studied for their application in many fields of chemistry, biochemistry,
biology and industry [1-19]. Most of the molecules are derived from five- and six-membered
rings containing one or more nitrogen and/or oxygen heteroatom. For the synthesis of organic
compounds for variety of applications, oxazol-5-one is a valuable chemical intermediate [1-
22]. Unsaturated oxazol-5-one bearing heterocyclic derivatives can be prepared by reacting of
aromatic aldehydes with hippuric acid in dry acetic anhydride which is the most convenient
methods [1-6,8,9,11,13-16,18,20-22].
Due to the dependence of the photochemical and photophysical properties of the
oxazol-5-ones on their structures, they found broad application areas such as medicine,
biosensors, probes, fluorophores, chromophores, non-linear optical materials, photonics,
electronics, photoswitches, organic light-emitting diodes and electrophotographic
photoreceptors [1-9,15-18]. Their large optical properties in solid state as well as in solutions
originate from extensive delocalized π-electron distribution [4,6,13,15].
Anthracenes are a well-known polycyclic aromatic hydrocarbon compounds which
find variety of applications in diverse areas such as optoelectronics, high energy physics,
bioimaging, sensing and medicine [23-42]. Owing to the extended π-electron system and
excellent optical properties such as well-resolved absorption and emission bands, high
fluorescence quantum yield values, nanosecond lifetime, high stability and superior
electrochemical properties, anthracene derivatives find applications in organic optoelectronic
devices including thin film transistors, solar cells, organic semiconductors, supramolecular
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materials, lasers, photovoltaic devices, molecular transistors, molecular switches,
fluorophores, fluoroionophores, luminescentsensors, polymers and light emitting diodes [23-
42]. Although different emission colours have been obtained for anthracene-based small
molecules, most of the literature reports describe molecules with emission in blue region
[23,24,26,28,31-33,39,40]. Anthracene derivatives are particularly valued for easy
modification, deep-blue emission and high photostability, which makes them attractive for
manufacture of blue emitters [23-26,28,31-33,39,40].
Despite the advantages mentioned above, owing to their unsuitable photophysical
properties, unsubstituted anthracene and its derivatives are hard to be directly used for special
applications such as bio-imaging due to a short emission wavelength, poor solubility and lack
of functional groups [23-25,32]. Moreover, representative anthracene units can undergo
intermolecular interaction like excimer or exciplex formation resulting in the decrease of their
electroluminescent performances, therefore adequate molecular design is necessary for
advanced anthracene derivatives [31,33]. For decades, great efforts have been devoted to
develop new fluorescent derivatives with good solubility, long absorption and emission
wavelength and large Stokes shift values [23-25,30,31]. Among the fluorophores, anthracene
has been widely applied in the field of fluorescent molecules because it is easy to modify the
structure [23,27,30-32,35,36]. It has been reported in the literature that structural
modifications especially at 9 and 10-positions of the anthracene molecule can greatly improve
anthracenes emission properties, thus many anthracene based structures have been
synthesized in this manner [23-25,31,32,34-37,39-41].
Inspired by the unique properties of anthracene mentioned above and our promising
results obtained with our previously synthesized oxazol-5-ones [20-22,43,44] and literature
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reveal, herein, we designed and synthesized anthracene oxazol-5-one dyes of 4-(9-
anthralydene)-2-phenyloxazol-5-one (2a), 4-(9-anthralydene)-2-(4-tolyl)oxazol-5-one (2b) 4-
(9-anthralydene)-2-(4-nitrophenyl)oxazol-5-one (2c) by introducing the oxazol-5-one moiety
to the 9- position of the anthracene molecule. The effect of substitution was monitored by the
change of substituents located at the phenyl ring. The UV-vis absorption and emission
properties of these fluorophores were also investigated in the solvents of acetonitrile (ACN),
chloroform (CHCl3), tetrahydrofuran (THF) and in immobilized poly(vinyl chloride) (PVC)
phase.
O
O
Ar
Cl
i
HN
H2N
Ar
+
OH
OH
O
O
1
ii
+
1
O
O
N
O
Ar
2
Scheme 1. The synthesis of 2 dyes (Ar: 2a: phenyl; 2b: p-tolyl; 2c: p-nitrophenyl) (i: dil.
NaOH sol.; ii: (CH3CO)2O, CH3COONa).
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2. Experimental
2.1. Materials and Instrumentation
All the solvents (spectroscopic grade and for synthesis), starting materials for synthesis and
the PVC membrane components were purchased either from Merck, Fluka or Sigma Aldrich.
The polyester support (Mylar type) was provided by Du Pont. The reaction completion was
followed by TLC (thin layer chromatography) using silica gel plates (SiO2, Merck 60 F254)
and UV light at 254 nm and 325 nm for visualization.
The compounds were characterized by FT-IR (spectra were recorded as KBr pellets by using
a Perkin Elmer Spectrophotometer), NMR (Varian Mercury AS 400 NMR spectrometer at
1
13
400 MHz for H NMR and 100 MHz for C NMR) and by measuring the melting points in
sealed tubes using an electrothermal digital melting points apparatus (Southend, UK) and are
uncorrected. Elemental analysis (CHN) of the dyes was carried out in a FLASH 2000
analyzer (Thermo Scientific, Billerica, MA, USA). The samples (2 −3 mg) were catalytically
combusted at 1200 °C. The steady-state absorption and emission spectra were recorded at
room temperature using a Shimadzu 164 UV-1601 spectrophotometer and a Varian-Cary
Eclipse Fluorescence spectrofluorimeter, respectively.
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2.2. Synthesis
2.2.1. Synthesis of 1 derivatives
The hippuric acid derivatives (1), precursors of oxazol-5-ones were synthesized by the
literature method [45,46].
2.2.2. Synthesis of 2 derivatives
Aldehyde (1 eq.), hippuric acid (1 eq.), sodium acetate (1 eq.) and acetic anhydride (3 eq.)
were placed in a flask and stirred at room temperature for 10 minutes and then under 100-
120˚C during 2 hours. The mixture changed colour upon fusion of all the reagents. After
cooling down to room temperature, some ethanol was added and the product precipitated. The
solid obtained was filtered and washed first with cold ethanol and then with hot water. If
further purification was needed some of the compounds were recrystallized in a suitable
solvent. Details of the amounts used for the synthesis and the characterization of the oxazol-5-
1
13
one derivatives by melting point determination, FT-IR, H NMR and C NMR are given in
the following sub-sections.
4-(9-anthralydene)-2-phenyloxazol-5-one (2a)
1.0 g (0.0048 mol) 9-antraldehyde, 0.86 g (0.0048 mol) hippuric acid, 0.66 g (0.0048 mol)
sodium acetate. Orange solid; yield 14 %; mp: 242˚C. FT-IR (KBr): υ (cm-1) 3048 (=C−H),
1
1796 (-O-C=O), 1658 (-C=N-), 1322 (-C-N), 1185 (O-C=O). H NMR (CDCl3, 400 MHz, δ
(ppm)): 8.55 (s, 1H), 8.33 (s, 1 H, Anthracene-CH=), 8.12-8.10 (m, 2H), 8.05-8.03 (m, 2H,),
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7.98-7.96 (m, 2H), 7.54-7.48 (m, 5H), 7.42-7.38 (2, 2H). 13C NMR (CDCl3, 100 MHz, δ
(ppm)): 166.6, 164.1, 131.3, 130.3, 130.2, 130.1, 128.9, 128.8, 128.5, 126.5, 125.9, 125.5,
125.4. Anal. Calcd. for C24H15NO2: C 82.50, H 4.33, N 4.01, O 9.16; found: C 82.43, H 4.41,
N 3.95, O 9.21.
4-(9-anthralydene)-2-(4-tolyl)oxazol-5-one (2b)
1.0 g (0.0048 mol) 9-antraldehyde, 0.93 g (0.0048 mol) p-tolylglycine, 0.66 g (0.0048 mol)
sodium acetate. Orange solid; yield 11 %; mp: 251˚C. FT-IR (KBr): υ (cm-1) 3043 (=C−H),
1
1794 (-O-C=O), 1657 (-C=N-), 1326 (-C-N), 1171 (O-C=O). H NMR (CDCl3, 400 MHz, δ
(ppm)): 8.54 (s, 1H), 8.28 (s, 1H, Anthracene-CH=), 8.12-8.10 (m, 2H), 8.05-8.03 (m, 2H),
7.87-7.85 (m, 2H), 7.51-7.49 (m, 4H); 7.21-7.19 (d, 2H), 2.38 (s, 3H, -CH3). 13C NMR
(CDCl3, 100 MHz, δ (ppm)): 166.8, 164.3, 144.6, 137.1, 133.5, 131.3, 130.1, 130.0, 128.9,
128.5, 127.7, 126.5, 126.0, 125.5, 122.8, 122.6, 22.5. Anal. Calcd. for C25H17NO2: C 82.63, H
4.72, N 3.85, O 8.80; found: C 82.55, H 4.79, N 3.92, O 8.74.
4-(9-anthralydene)-2-(4-nitrophenyl)oxazol-5-one (2c)
1.0 g (0.0048 mol) 9-antraldehyde, 1.1 g (0.0048 mol) p-nitrobenzoylglycine, 0.66 g (0.0048
mol) sodium acetate. Dark red solid; yield 9 %; mp: 294˚C. FT-IR (KBr): υ (cm-1) 3052
(=C−H), 1800 (-O-C=O), 1649 (-C=N-), 1521 - 1350 (−NO2), 1316 (-C-N), 1160 (O-C=O).
1H NMR (CDCl3, 400 MHz, δ (ppm)): 8.60 (s, 1H), 8.57 (s, 1H, Anthracene-CH=), 8.26-8.24
(d, 2H), 8.15-8.05 (m, 6H), 7.54-7.51 (m, 4H). 13C NMR (CDCl3, 100 MHz, δ (ppm)): 166.9,
164.4, 144.8, 131.6, 130.3, 130.2, 129.8, 129.6, 129.1, 128.7, 126.8, 126.7, 126.2, 125.7,
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122.8. Anal. Calcd. for C24H15NO2: C 73.09, H 3.58, N 7.10, O 16.23; found: C 72.99, H
3.57, N 7.24, O 16.20.
2.3.Fluorescence quantum yield studies
The fluorescence quantum yield values of the dyes were determined by using Fluorescein in
0.1 M NaOH (Φstd = 0.95) as reference and were calculated by using the following equation
(1) [47]:
2
ΦF = Φstd x (F Astdη2) / (FstdA ηstd
)
(1)
Where: F and Fstd are the areas under the fluorescence emission curves of the samples and the
standard, respectively. A and Astd are the respective absorbance values of the samples and
standard at the excitation wavelength, respectively, and η and ηstd the refractive indexes of the
medium used for the samples and standard, respectively.
2.4. Polymer film preparation
The polymer films were prepared from a mixture of 120 mg of PVC (high molecular weight),
240 mg of plasticizer (bis-(2-ethylhexyl)phthalate (DOP) or bis-(2-ethylhexyl)adipate)
(DOA)), equimolar lipophilic anionic additive potassium tetrakis-(4-chlorophenyl)borate
(PTCPB) to the dyes and 1.5 mL of tetrahydrofuran (dry). The concentration of the
derivatives were between 10-3 - 10-4 M (about 20 mmol dye kg-1 polymer). The prepared
mixtures contained 33 % PVC and 66 % plasticizer by weight. The resulting cocktails were
spread onto a 125 µm polyester support (Mylar type) having a width of 120 mm by knife
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coating. Polyester provides a mechanical support for thin films of PVC which are impossible
to be handled without it. The polymer support is optically fully transparent and exhibits good
adhesion to PVC. In order to avoid the damage from the ambient air of laboratory and keep
the photostability of the membranes, the obtained PVC films were stored in a THF containing
desiccator and in the dark. The resulting dye doped PVC films were placed diagonally into the
measuring cuvette in order to achieve better reproducibility of the measurements.
3. Results and discussion
3.1. Synthesis and structure
Scheme 1 depicts the synthetic routes and molecular structures of compounds 2(a-c)
(Fig.1). Firstly, we synthesized the key precursors, hippuric acid derivatives (1), by the
literature methods [45,46]. Then using Erlenmeyer reaction 2 dyes were easily prepared by
condensation reaction of 9-anthraldehyde with the prepared precursors, hippuric acid for 2a,
p-tolylglycine for 2b and p-nitrophenylglycine for 2c in the presence of sodium acetate and
acetic anhydride. The synthons for these dyes, all hippuric acid precursors are colourless
crystalline compounds. However, after the condensation of the molecules acquires orange
colour for 2a and 2b, and dark red for 2c. This is probably because during the condensation,
the bond formation between the molecular pairs is brought about when filled π-orbitals of
donor overlap with the depleted orbitals of the acceptor as indicated in the literature [4]. The
two new molecular orbitals are formed, the lower energy molecular orbitals for the
compounds is occupied in the ground state. The transition from this molecular orbital to the
new upper molecular orbital is responsible for the new lower energy absorption band
observed in Figure 2.
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O
O
N
O
N
O
N
R
O
O
2a (R-H) and 2b (R-CH3)
2c
Fig. 1. The structure and conjugation pathway for 2 dyes.
The frequencies of the stretching vibrations of the C=N group was represent at 1658,
1657 and 1649 cm-1 for 2a, 2b and 2c, respectively, in line with the literature [2,13,15,16,18].
The strong electron withdrawing nitro group for 2c decreases the vibration band of C=N
group to 1649 cm-1, which also indicates the charge transfer in this dye is from electron
donating 9-anthralydene moiety to the electron withdrawing nitro group (Figure 1) and is in
agreement with the absorption and emission maxima. The characteristic absorption band of
the carbonyl group was found at 1796, 1794 and 1800 cm-1 for 2a, 2b and 2c, respectively.
Due to the conjugation pathway from 9-anthryl group to carbonyl group for 2a and 2b dyes,
the stretching vibration frequency is lower than that of the one for 2c, which is also in
agreement with the absorption and emission properties.
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3.2. Absorption and emission properties in solution phase
The synthesized anthryl-oxazol-5-one dyes UV/Vis and emission spectra were
measured in diverse solvents with different polarity, acetonitrile (polarity 0.460), chloroform
(polarity 0.259) and tetrahydrofuran (polarity 0.207), to establish whether there was any
solvent effect [9]. The compounds show absorption maxima between 433-495 nm and
emission maxima between 515-620 nm (Figure 2, Figure 3 and Table 1), which are different
from the anthracenes characteristic structural absorption approximately between 300-390 nm
and emission between 360-460 nm [23], indicating formation of a new chromophore systems
for 2 dyes as shown in Figure 1. Obviously seen in Figure 2 and Table 1, all the synthesized
molecules absorption properties exhibit similar sensitiveness towards solvent polarity, i.e
blue-shifts in more polar solvent of acetonitrile. The emission maxima of 2c presents
somewhat different sensitivity towards polarity as it is can be seen form Figure 3 and Table 1.
In comparison to 2a and 2b, which exhibits red-shift in emission maxima in more polar
solvent of acetonitrile, the 2c dye characterizes a red-shift of the fluorescence band in less
polar solvents. Similar evidence has been reported in the literature for branched oxazol-5-one
dyes studied by Jedrzewska and co-workers [1]. The emission wavelength of the dyes of 2a
and 2b is red-shifted only slightly in acetonitrile than that in the solvents with mediate
polarity and the emission of 2c is blue-shifted in polar solvent of acetonitrile. This might be
because the photo-induced charge transfer is not significant, in other words the dipole
moment of the excited state is not much larger than the ground state. It would arise also from
lowered difference of equilibrium geometries between ground and excited states.
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2a
2b
2c
2c
Fig. 2. The UV-vis absorption spectra of 2 in acetonitrile (ACN), chloroform (CHCl3) and
tetrahydrofuran (THF).
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2a
2b
2c
Fig. 3. The fluorescence spectra of 2 in acetonitrile (ACN), chloroform (CHCl3) and
tetrahydrofuran (THF).
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Table 1
Absorption and fluorescence emission maxima (λ, nm), Stokes shifts (∆λ, nm), molar
extinction coefficients (ε, 103 L.mol-1.cm-1) and fluorescence quantum yields (ΦF) for
compounds 2a–c in solutions of acetonitrile (ACN), chloroform (CHCl3), tetrahydrofuran
(THF) and PVC polymer film.
max
max
Compound Solvent Plasticiser λabs
λemis
ꢀλ
ε
ΦF
ACN
CHCl3
THF
436
456
540
104 13.8 0.0003
62 17.0 0.0006
75 16.2 0.0008
72 452.2 0.0469
77 452.2 0.0337
107 11.8 0.0006
68 14.8 0.0007
75 14.4 0.0014
75 447.1 0.0712
79 447.1 0.0503
518
517
524
524
540
521
515
525
524
547
614
620
603
604
442
452
447
433
453
440
450
445
467
495
474
486
483
2a
2b
2c
DOP
DOA
PVC
ACN
CHCl3
THF
DOP
DOA
PVC
ACN
CHCl3
THF
80
8.2 0.0007
119 9.3 0.0012
146 8.6 0.0050
117 437.8 0.0315
121 437.8 0.0348
DOP
DOA
PVC
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The synthesized compounds have linear π-conjugated framework end-capped by
electron-donating methyl (2b) and electron-accepting nitro group (2c) (Figure 1). The
anthracene moiety was chosen as electron donor group. In order to compare the effect of the
electron accepting and electron donating groups on the absorption and emission properties of
the synthesized fluorophores, we attached p-tolyl as electron donor (D) group (2b), p-
nitrophenyl group as electron acceptor (A) group (2c) and as a reference phenyl group (2a) to
the 4th position of the oxazol-5-one ring. The dye 2b is substituted in a D-π-D arrangement,
while 2c is substituted in a D-π-A arrangement. Compound 2a was prepared for comparison.
We anticipate that depending on donor/acceptor strength of the substituents, the
oxazol-5-one heterocycle can act as an electron acceptor by accommodating an excess charge
on the carbonyl group or on the nitro group (Figure 1). 2a and 2b dyes possess charge-transfer
between the electron-donating 9-antralidene moiety and the electron-withdrawing carbonyl
group of the oxazol-5-one ring (Figure 1). The pair of electrons in 9-anthralydene moiety
exerts positive mesomeric effect. Thus positive charge is acquired by 9-anthrylidene group
and the negative charge by the oxygen of carbonyl group for 2a and 2b, and of nitro group for
2c. For 2c dye the charge-transfer pathway is extended and different from the one for 2a and
2b due to strong electron accepting nitro group, thus it is again from 9-antralydene electron
donating group, but this time to strong electron accepting nitro group (Figure 1). This increase
of the dimensionality of donor-π-acceptor 2c dye causes a red-shift of both absorption and
emission maxima of 2c in comparison to 2a and 2b dyes (Table 1, Figure 2, Figure 3). The
absorption maxima are shifted up to 495 nm and emission maxima are shifted up to 620 nm
for 2c. The little shift in emission in ACN when comparing 2a and 2b with 2c can be
attributed to the difference in their attitude toward solvent polarity which is in the opposite
direction. For 2a and 2b red-shift is observed in the polar solvent of ACN, while for 2c blue-
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shift is observed in the same solvent. Opposite to THF and CHCl3 where there is a large red-
shift in emission of 2c in comparison to 2a and 2b, the slight red shift in ACN is due to the
fact that 2c derivative’s dipole dipole interaction with polar nitro group is predominant, thus
its electron withdrawing ability and hence conjugation degree decreases. As a result while
there is a red-shift of approximately 100 nm in emission maxima of 2c in THF and CHCl3 in
comparison to 2a and 2b, there is only 20 nm red-shift in ACN.
It is a desired property to have a dipolar structure bearing electron donating substituent
linked to the oxazol-5-one skeleton (electron acceptor) by a π-conjugated bridge [1]. Such
structure is conductive in intramolecular charge transfer. 4-(9-anthralydene)-2-(4-
nitrophenyl)oxazol-5-one (2c) exhibits large Stokes shift up to 146 nm presumably due to
intramolecular charge transfer (Table 1).
From Table 1, it can be clearly seen that the fluorescence quantum yield values of the
dyes changes regularly in solution media. In the polar solvent of ACN, the fluorescence
intensities of 2 derivatives are quenched. The decrease in fluorescence quantum yield upon
increasing solvent polarity could be related to some interactions between the excited solute
molecules and solvent molecules, which change the excited-state configuration and thus the
fluorescence intensity [1]. The highest quantum yield values for solution phase for all the dyes
studied were obtained in THF. The rank for the quantum yield values is as 2c > 2b > 2a in
solution phase.
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3.3. Absorption and fluorescence emission properties in PVC film matrix
Absorption and fluorescence emission data of all three oxazol-5-one molecules in
PVC are summarized in Table 1. The absorption maxima of 2 derivatives were in the rage of
524-603 nm (Fig 4). The PVC films which were plasticized with DOP, were slightly red-
shifted in the range of 3-5 nm with respect to DOA plasticized polymer films. Compared to
the absorption maxima in ACN and THF red-shift and in CHCl3 blue-shift were observed in
PVC matrix for all the molecules (Table 1, Fig. 4). The emission maxima in PVC film were
observed between 524-604 nm. The emission maxima of 2a and 2b were red-shifted in PVC
in comparison to ACN and THF, but blue-shifted in comparison to ACN. However 2c
emission maxima were blue-shifted in ACN in comparison to PVC, but red-shifted in
comparison to CHCl3 and THF. However there were no significant differences between
plasticizers used in PVC. Similar to the results obtained in solution phase also in PVC film
both absorption and emission maxima of 2c are red-shifted in comparison to 2a and 2b, which
can also be attributed to the longer conjugation pathway of 2c as shown in Fig. 1.
Fluorescence quantum yield values of the dyes in polymer film matrix has enhanced about 30-
to 50 fold, with respect to the quantum yield values observed in solution phase (Table 1).
Induced steric effect in immobilized PVC matrix, with respect to solvated derivatives in
solution phase, may support the increase in fluorescence emission. Moreover the molar
absorption coefficients are about 30- to 50 times higher in polymer matrix in comparison to
solution phase. The data obtained reveals that the dye molecules absorb and fluorescence
better in immobilized PVC film.
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Fig. 4. The absorption spectra of 2 in plasticized PVC film.
At the end, it should be highlighted that the synthesized oxazol-5-one molecules
present absorption in the visible region (Fig. 2, Fig. 5), which depending on application is a
relevant feature in an efficient photoswitch; because in this case, low-energy light could be
used; this is a required condition especially in biological media in which more energetic light
could damage the system [9]. Moreover, most of the anthracene incorporating molecules in
the literature are defined as blue emitters [23-26,28,31-33,39,40]. We propose novel
anthracene based fluorophores which have emission in the different regions of the spectra, i.e
2a and 2b have emission in blue range of the spectrum and 2c in the orange region.
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4. Conclusion
In the present study, we have synthesized new substituted oxazol-5-ones incorporating
1
9-anthracene moiety and characterized structurally by FT-IR, H-NMR and 13C-NMR
techniques. The frequencies of the stretching vibrations of the C=N and C=O group were
found to be significantly influenced by substituent effects. In order to demonstrate and
understand the substituent effect on spectral properties, we also investigated their absorption
and emission properties in the solvents of acetonitrile, tetrahydrofuran and chloroform and
solid matrix of PVC. The oxazol-5-one molecules exhibited much higher fluorescence
emission in immobilized polymer film matrices with respect to solution phase. The newly
synthesized oxazol-5-one dyes have absorption in the visible region, which is a desired
property for biological applications and are fluorescent molecules with emission in the green
portion of the spectra for 2a and 2b, and in the orange portion of the spectra for 2c. Also
absorption and emission properties were in agreement with the structural features indicating
that the charge transfer direction for 2a and 2b is from 9-antralidene moiety to carbonyl
group, while for 2c it changes to nitro group. These results show the control of spectral
properties by the electron donating/withdrawing effect of the substituent. The asymmetric
polarization induced by the presence of electron donor and electron acceptor groups in these
π-electron conjugated molecules entails an extensive delocalization of the π-electron
distribution, which is expected to favour their nonlinear interaction with light.
Acknowledgements
The research was supported by Scientific Research Funds of Dokuz Eylul University
with Grant Number of 2005.KB.FEN.015.
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