Synthesis and characterization of new y-shaped fluorophores with an imidazole core

Article abstract of DOI:10.1007/s10895-012-1055-z

Four new y-shaped fluorophores of 4- {4,5-[2,2′-Bis(2,4,6- trimethoxyphenyl)vinyl]-1-H-imidazole-2-yl}ben-zonitrile 1a, 2-phenyl-{4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl) vinyl]-1-H-imidazole} 1b, 2- (9-anthryl)-{4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vinyl]}-

Full text of DOI:10.1007/s10895-012-1055-z

J Fluoresc (2012) 22:1159–1164
DOI 10.1007/s10895-012-1055-z
ORIGINAL PAPER
Synthesis and Characterization of New Y-shaped
Fluorophores with an Imidazole Core
Gulsiye Ozturk & Demet Karakas & Fatma Karadag &
Gulnur Ozturk & Cansu Yorgun
Received: 24 December 2011 /Accepted: 19 March 2012 /Published online: 1 April 2012
#
Springer Science+Business Media, LLC 2012
Abstract Four new y-shaped fluorophores of 4- {4,5-[2,2′-
Bis(2,4,6-trimethoxyphenyl)vinyl]-1-H-imidazole-2-yl}ben-
zonitrile 1a, 2-phenyl-{4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)
vinyl]-1-H-imidazole} 1b, 2- (9-anthryl)-{4,5-[2,2′-Bis(2,4,6-
trimethoxyphenyl)vinyl] }-1-H-imidazole 1c and 2- (4-nitro-
phenyl) – {4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vinyl] -1-H-
imidazole 1d which bear an imidazole core, were synthesized
for the first time via intermediate 1,6-Bis(2,4,6-trimethoxy-
phenyl)hexa-1,5-diene-3,4-dion with different aldehydes.
The structures of the new derivatives were confirmed by 1H
NMR, 13C NMR and FT-IR. The optical properties such as
absorption and emission maxima, Stokes’ shift and quantum
yield values were investigated in solvents of toluene, tetrahy-
drofuran and acetonitrile. The products show intense emission
maxima in the range of 440–630 nm. The imidazole deriva-
tives exhibited excellent photostabilities.
the major challenges in the area of organic NLO molecules is
to design and synthesize second-order NLO chromophores
which, simultaneously, exhibit large hyperpolarizability (β),
good optical transparency and high thermal stability. Most
attempts to design molecules with large β have relied upon
end capping an optimal π-conjugated bridge with different
donors and acceptors [16]. However, this enhancement of β is
always accompanied with a red-shift in the λmax peak, which
is so-called nonlinearity-transparency trade-off. Y-shaped
chromophores show improved trade-off compared to classical
one-dimensional dipolar chromophores, due to the contribu-
tion of the large, off-diagonal β tensorial components.
The emergence of applications of two-photon technology
to up-converted lasting, optical power limiting, photody-
namic therapy and generation of singlet oxygen, three-
dimensional micro-fabrication and data storage, and three-
dimensional fluorescence microscopy, has drawn an increas-
ing interest in the design and development of two-photon
absorbing molecules with large nonlinear absorptivity to
generate a high-energy excited state with relatively low laser
energy [1, 3, 8, 10]. The new structure–property correlation
is being sought to establish guidance for molecular design
since the ability of organic molecules to simultaneously
absorb two photons to reach the excited state depends on
molecular structures. Several strategies based on schemes
such as D-π-D, D-π-A-π-D, and A-π-D-π-A (D: electron
donating group, A: electron accepting group, and π: conju-
gated pathway) have been found to enhance the δ value, the
two-photon absorptivity [4, 5, 10]. In the comparison with
single-photon fluorescent probes, two photon fluorescent
probes excited in the near infrared (NIR) region show very
low background light, weak photo damage, highly transmis-
sion at the low incident intensity [10, 11]. These characters
can make such two-photon chromophores suitably act as
fluorescent labels in bio imagine, and the cases of two-
.
.
.
Keywords Imidazole Fluorophores Y-shaped molecules
Quantum yield
Introduction
Second-order nonlinear optical (NLO) materials have been
advancing for a few decades as a promising field with impor-
tant applications in the domain of opto-electronics and pho-
tonics [1–16]. Organic NLO molecules have proven to be of
great interest in recent years due to their extremely large
optical nonlinearities as compared to most of the inorganic
crystals that are transparent in the visible region [15]. One of
:
:
:
:
G. Ozturk (*) D. Karakas F. Karadag G. Ozturk C. Yorgun
Department of Chemistry, Faculty of Sciences,
Dokuz Eylul University,
Tinaztepe Campus,
35160 Buca, Izmir, Turkey
e-mail: gulsiyeozturk@yahoo.com

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J Fluoresc (2012) 22:1159–1164
photon excited fluorescence sensors for ions have been
reported [8, 10].
Synthesis
In order to be compatible with existing semiconductor
processing technology, organic NLO chrompohores for de-
vice application are required to be highly thermally stable for
sustaining the processing and operating temperatures [7].
Heterocyclic imidazoles based on NLO chromophores have
received increasing interest due to their excellent thermal
stability, improved molecular nonlinearity and high solubility
[1,6,8-11].
We report here the synthesis of four new Y-shaped deriva-
tives of 4- {4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vinyl]-1-H-
imidazole-2-yl}benzonitrile 1a, 2-phenyl-{4,5-[2,2′-Bis(2,4,6-
trimethoxyphenyl)vinyl]-1-H-imidazole} 1b, 2- (9-anthryl)-
{4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vinyl] }-1-H-imidazole
1c and 2- (4-nitrophenyl) – {4,5-[2,2′-Bis(2,4,6-trimethoxy-
phenyl)vinyl] -1-H-imidazole 1d, having an imidazole core.
Their optical properties such as absorption maxima, fluores-
cence emission maxima, Stokes’ shifts, singlet energies, fluo-
rescence quantum yield values and photostabilities in
tetrahydrofuran, toluene and acetonitrile were also investigated.
1,6-Bis(2,4,6-trimethoxyphenyl)hexa-1,5-diene-3,4-dione
(2). 2,3-butanedione (0.36 ml , 5 mmol) was added to the
methanol (20 ml) solution of 2,4,6- trimethoxybenzaldehyde
(2 g , 10 mmol). Then, piperidine (0.56 ml, 5.56 mmol) and
glacial acetic acid (0.32 ml, 5.55 mmol) were added as catalyst
to the stirred mixture. The mixture was refluxed for 6 h under
nitrogen atmosphere, during which orange crystals started to
form. After cooling to room temperature, the crystals were
filtered and washed with methanol for several times to obtain
the title compound with a yield of 30 %; mp 223–224 °C;
FT-IR (KBr): νmax 1650, 1551, 1207 cm-1; 1H NMR
(500 MHz, DMSO-d6): δ 3.88 (s, 6H), 3.90 (s, 12 H), 6.33
(s, 4H), 7.38 (d, J08.3 Hz, 2H), 8.02 (d, J08.3 Hz, 2H),; 13
C
NMR (500 MHz, DMSO-d6): δ 53.7, 56.4, 91.4, 105.1, 119.6,
138.3, 161.9, 164.2, 193.2; HRMS (ESI-TOF): m/z Anal.
Calcd. for C24H27O8 (M+ + H)0443.1706; Found: 443.1712.
General methods for the synthesis of 1a-d Glacial acetic
acid (1.91 ml) and 2 (0.1 g , 0.23 mmol) were added to a
50 mL round bottom flask. The mixture was allowed to stir
at room temperature until the entire solid was dissolved.
Aldehyde (0.23 mmol) was then added, followed by the
addition of ammonium acetate (0.28 g, 3.6 mmol). The
reaction mixture was magnetically stirred at 118 °C under
nitrogen atmosphere for 6 h and then cooled to room tem-
perature. The mixture was poured into 19,1 ml of ice water
and neutralized by %10 of sodium carbonate solution to a
pH 6.5-7.0. The formed precipitate was filtered and washed
with water for several times. Then redissolved in ethyl
acetate and purified by column chromatography using ethyl
acetate / hexane as eluent.
4- {4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vinyl]-1-H-
imidazole-2-yl}benzonitrile (1a) Yield 49 %; mp 174 °C;
FT-IR (KBr): νmax 3204, 2223, 1630, 1454, 1328, 1204, cm-1;
1H NMR (500 MHz, DMSO-d6): 3.84 (s, 6H), 3.84 (s,
12H), 6.56 (s, 4H), 7.33 (d, J08.3 Hz, 2H), 7.48 (d, J0
3.4 Hz, 2H), 7.51 (d, J03.4 Hz, 2H), 7.58 (d, J08.3 Hz, 2H),
12.65 (s, 1H); 13C NMR (500 MHz, DMSO-d6): δ 55.2, 55.8,
91.2, 105.2, 118.1, 119.1, 123.6, 125.7, 130.8, 132.4, 132.8,
144.4, 149.7, 159.1, 161.9; HRMS (ESI-TOF): m/z Anal.
Calcd. for C32H32N3O6 (M+ + H)0554.2291; Found:
554.2354.
Experimental
General
All solvents were of analytical grade and purchased from
Merck (Darmstadt, Germany), Fluka (Buchs, Switzerland),
and Riedel (Seelze, Germany). All melting points were mea-
sured in sealed tubes using an electrothermal digital melting
points apparatus (Southend, UK) and are uncorrected. Infrared
spectra were recorded on a Perkin Emler (Massachusetts,
1
USA) FTIR infrared spectrometer (spectrum BX-II). H-
NMR and 13C-NMR spectra were recorded on Bruker
AC500 (500 MHz) (Coventry, UK). Ultra performance liquid
chromatography coupled to mass spectrometry detection
(UPLC-MS) was performed with a Waters Alliance systems
(gradient mixtures of acetonitrile/water) equipped with Acq-
uity UPLC columns. The Waters systems consisted of a
Waters Separations Module 2695, a Waters Diode Array de-
tector 996, a LCT Premier XE mass spectrometer, and a
Waters Mass Detector ZQ 2000. Analytical and preparative
thin layer chromatography (TLC) were carried out using silica
gel 60F254 (Merck). Column chromatography was carried out
by using 70–230 mesh silica gel (0.063-0.2 mm, Merck). UV/
visible absorption spectra were recorded with Schimadzu UV-
1601 spectrophotometer (Tokyo, Japan). All fluorescence
measurements were undertaken by using Varian-Carry Eclipse
spectrofluorimeter (Mulgrave, Australia). 1-hydroxypyrene-
3,6,8-trisulfonate trisodium salt (HPTS) purchased from Fluka
was used as reference standard for fluorescence quantum
yield calculations of 1a-d.
2-phenyl-{4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vinyl]-1-
H-imidazole} (1b) Yield 78 %; mp 135 °C; FT-IR (KBr):
1
νmax 3200, 1602, 1454, 1327, 1203; H NMR (500 MHz,
DMSO-d6): 3.83 (s, 6H), 3.91 (s, 12H), 6.32 (s, 4H), 7.27
(d, J08.2 Hz, 2H), 7.41-7.38 (m, 5H), 7.51 (d, J04.4 Hz,
2H), 12.36 (s, 1H); 13C NMR (500 MHz, DMSO-d6): δ
55.2, 55.9, 91.2, 117.4, 125.5, 126.4, 127.6, 128.2, 128.5,
130.4, 136.1, 146.4, 158.1, 163.3; HRMS (ESI-TOF): m/z
Anal. Calcd. for C31H33N2O6 (M+ + H)0529.2339; Found:
529.2189.

J Fluoresc (2012) 22:1159–1164
1161
Scheme 1 Synthesis of
1a-d. i) butane-2,3-dione,
piperidine; ii) ArCHO,
NH4Ac, glacial HAc
1a
1b
1c
1d
Ar
2- (9-anthryl)-{4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vi-
nyl] }-1-H-imidazole (1c) Yield 62 %; mp 159 °C; FT-IR
(KBr): νmax 3250, 1602, 1454, 1329, 1203; 1H NMR
(500 MHz, DMSO-d6): 3.82 (s, 6H), 3.87 (s, 12H), 6.56
(s, 4H),7.15 (d, J08.8 Hz, 2H), 7.64-7.50 (m, 9H), 7.76 (d,
J04.01 Hz, 2H), 12.75 (s, 1H); 13C NMR (500 MHz, DMSO-
d6): δ 55.3, 55.8, 91.1, 117.1, 125.6, 126.0, 126.6, 126.7, 126.8,
128.0, 128.4, 129.1, 130.7, 130.9, 139.3, 144.5, 159.0, 159.8;
HRMS (ESI-TOF): m/z Anal. Calcd. for C39H37N2O6 (M+ +
H)0629.2652; Found: 629.2784.
2- (4-nitrophenyl) – {4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)
vinyl] -1-H-imidazole (1d) mp 145 °C; FT-IR (KBr): νmax
3200, 1690, 1513, 1454, 1326, 1203, cm-1; 1H NMR
(500 MHz, DMSO-d6): δ 3.841(s, 6H), 3.92 (s, 12 H),
6.56 (s, 4H), 7.37 (d, J08.3 Hz, 2H), 7.50 (d, J03.5 Hz,
2H), 7.53 (d, J03.6 Hz, 2H), 7.60 (d, J08.2 Hz, 2H), 12.78
(s, 1H); 13C NMR (500 MHz, DMSO-d6): δ 55.4, 55.9,
91.2, 107.3, 124.2, 126.0, 131.3, 135.9, 140.7 144.1,
149.7, 159.0, 159.8, 160.5; HRMS (ESI-TOF): m/z Anal.
Calcd. for C31H32N3O8 (M+ + H)0574.2189; Found:
574.2299.
Results and Discussion
Scheme 1 describes the synthesis of 4- {4,5-[2,2′-Bis(2,4,6-
trimethoxyphenyl)vinyl]-1-H-imidazole-2-yl}benzonitrile
1a, 2-phenyl-{4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vinyl]-
1-H-imidazole} 1b, 2- (9-anthryl)-{4,5-[2,2′-Bis(2,4,6-tri-
methoxyphenyl)vinyl] }-1-H-imidazole 1c and 2- (4-nitro-
phenyl) – {4,5-[2,2′-Bis(2,4,6-trimethoxyphenyl)vinyl] -1-H-
imidazole 1d, with a yield from 49 to 86 %, via intermediate
1,6-Bis(2,4,6-trimethoxyphenyl)hexa-1,5-diene-3,4-dione 2,
which was obtained by the reaction of 2,3-butanedione with
2,4,6- trimethoxybenzaldehyde. The structures of 2 and 1a-d
were confirmed by 1H and 13C NMR, FT-IR and UPLC.
Next, the optical properties of 1a-d were investigated
(Table 1, Figs. 1, 2 and 3). The absorption maxima of 1a-
Table 1 Absorption and fluo-
abs
em
Compound
1a
Solvent
λmax
εmax
λmax
Δλ
ES
Φ
rescence emission data for
compounds 1a-d maximum
absorption wavelengths
(λmaxabs, nm), maximum
emission wavelengths
Toluene
336
383
382
340
375
356
385
383
382
391
388
386
9600
486
519
547
440
466
462
579
589
452
630
445
445
150
136
165
100
91
58.7
55.0
52.2
64.8
61.2
61.8
49.3
48.4
63.1
45.3
64.1
64.1
0.1295
0.3888
0.2577
0.3169
0.5420
0.5731
0.1281
0.0301
0.0115
0.0123
0.0073
0.0061
Tetrahydrofuran
Acetonitrile
Toluene
20300
18200
13400
18800
20000
18300
31800
23000
8800
(λmaxem, nm), molar extinction
coefficients (ε, l mol-1 cm-1),
Stokes’ shifts, Δλ (nm), singlet
energies, Es (kcal mol-1),
and fluorescence quantum
yields, Φ in toluene,
1b
1c
1d
Tetrahydrofuran
Acetonitrile
Toluene
106
194
206
70
Tetrahydrofuran
Acetonitrile
Toluene
tetrahydrofuran,
and acetonitrile
239
57
Tetrahydrofuran
Acetonitrile
32600
29700
59

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Fig. 1 Absorption spectra of 1a-d in acetonitrile
d were observed in the 336–391 nm range. Figure 1 shows
the absorption spectra of 1a-d in acetonitrile. In the absorp-
tion maxima of 1a and 1b bathochromic shifts were ob-
served as the solvent polarity increases, while in the
absorption maxima of 1c and 1d there are slight hypsochro-
mic shifts indicating that the absorption maxima were not
affected so much by solvent polarity for 1c and 1d (Fig. 1).
While 1a and 1b show the longest wavelength absorption
maxima in tetrahydrofuran, 1c and 1d show the longest
wavelength absorption maxima in toluene (Table 1). Among
all the derivatives 1, 1d has the longest absorption maxima at
391 nm (in toluene). This can be attributed to the enhanced
conjugation of 1d due to the strong electron accepting nitro
group.
The maximum emission wavelengths of 1a-d are in the
range of 440–630 nm as summarized in Table 1. Figure 2
depicts the fluorescence spectra of 1a-d in toluene. In fluo-
rescence maxima of 1a and 1b bathocromic shifts were
observed as the solvent polarity is increased, while for 1c
and 1d dramatical hypsochromic shifts were observed as the
solvent polarity is increased. Among all the derivatives 1,
1d has the longest absorption maxima at 630 nm (in toluene)
Fig. 3 The photostability test results of 1a-d in tetrahydrofuran after
Fig. 2 Emission spectra of 1a-d in toluene
1 h of monitoring

J Fluoresc (2012) 22:1159–1164
1163
(Table 1, Fig. 2). This can be attributed to the enhanced
conjugation of 1d due to the strong electron accepting nitro
group. Thus, also the largest Stokes’ shift value is observed
for 1d as 239 nm (in toluene) among the derivatives 1
(Table 1). The Stokes’ shift values for 1a and 1b increase
slightly as the solvent polarity is increased according to
the results obtained for toluene and acetonitrile, while
the Stokes’ shift values for 1c and 1d decrease dramat-
ically as the solvent polarity increases according to the
results obtained for toluene and acetonitrile. The high
Stokes’ shift values observed for 1a-d, confers the ad-
vantage of better spectral resolution in emission based
studies and is a desired property in commercially im-
portant fluorophores.
1b exhibited promising fluorescent efficiency, while 1c and
1d which contain anthryl and nitro moieties exhibited low
fluorescent ability. All derivatives displayed large Stokes’
shift values and excellent photostabilities. The remarkable
photophysical properties of the derivatives 1a-b, would
make them candidates for the development of fluorescent
sensors for different applications.
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Φ ¼ Φstd ꢀ F Astd η2 = Fstd A ηs2td
where F and Fstd are the areas under the fluorescence emis-
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and Astd are the respective absorbance of the samples and
standard at the excitation wavelength, respectively, and η
and ηstd the refractive indices of solvents used for the sam-
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quantum yield values up to 0.5731 and the fluorescent
efficiencies are enhanced in polar solvents (Table 1). In
addition, the highest quantum yield values were observed
for 1b indicating that the introduction of a phenyl group
drastically increase the quantum yield value. For 1c and 1d,
quantum yield values decreased, indicating that the intro-
duction of an anthryl and nitro moieties onto Y-shaped
molecules skeleton decreased the quantum yield values.
For 1c and 1d quantum yield values decreased as the solvent
polarity increased. This can be attributed to the fact that
highly polar solvents provide 1c and 1d freedom to rotate
and vibrate which can be possible sources of the non-
radiative transitions.
The photostabilities of 1a-d were recorded with a steady-
state spectrofluorimeter in toluene, thetrahydrofuran and ace-
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Conclusion
In summary, we achieved four new Y-shaped fluorophores
with emission maxima in the range of 440 – 630 nm. 1a and

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