MCM-SO3H catalyzed synthesis of environment-sensitive fluorophores incorporating pyrene moiety: Optimization, fluorescence emission and theoretical studies

Article abstract of DOI:10.1016/j.jphotochem.2018.11.036

Six new highly fluorescent 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-pyrazolines were synthesized by reaction of various (E)-3-aryl-1-(pyren-1-yl)prop-2-en-1-ones with phenyl hydrazine in the presence of sulfonated mesoporous silica (MCM-SO₃H) as efficient and eco-friendly acidic catalyst. The chemical structures of all synthesized compounds were illustrated on the basis of spectral data (IR, ¹H NMR and ¹³C NMR). This was followed by photophysical properties-based absorption and emission studies of the target compounds in solutions of different solvent polarities. The microenvironment-sensitive fluorescent pyrazolines labeled with pyrene exhibited valuable fluorescence properties with emission in the range of 600–700 nm with a marked response to change in the environmental polarities. A significant and pronounced red shift was observed in the emission spectrum of 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-pyrazolines Δλ ～ 50 nm compared to the absorption spectrum Δλ ～ 10 nm upon increasing the solvent polarity. This indicated the presence of higher dipole moment in the excited state than in the ground state and the transition involved are π-π* transition through the charge transfer phenomena. The fundamental understanding of solvatochromic properties were analyzed through Lippert-Mataga and Reichardts correlations in order to estimate the change in dipole moments (Δμ) which suggested the emissive state of designed fluorescent 2-pyrazoline derivatives is of strong ICT character.

Full text of DOI:10.1016/j.jphotochem.2018.11.036

Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 306–314
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
MCM-SO H catalyzed synthesis of environment-sensitive fluorophores
3
incorporating pyrene moiety: Optimization, fluorescence emission and
theoretical studies
Nizar El Guesmia,b, Essam M. Hussein , Saleh A. Ahmed
a,c
a,c,⁎
a
Department of Chemistry, Faculty of Applied Science, Umm Al-Qura University, 21955, Makkah, Saudi Arabia
Département de chimie, Faculté des Sciences de Monastir, Avenue de l’Environnement, 5019, Monastir, Tunisia
Department of Chemistry, Faculty of Science, Assiut university, 71516, Assiut, Egypt
b
c
A R T I C L E I N F O
Keywords:
A B S T R A C T
Six new highly fluorescent 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-pyrazolines were synthesized by reaction of various
(E)-3-aryl-1-(pyren-1-yl)prop-2-en-1-ones with phenyl hydrazine in the presence of sulfonated mesoporous silica
MCM-SO
Pyrene
3
H
3
(MCM-SO H) as efficient and eco-friendly acidic catalyst. The chemical structures of all synthesized compounds
Fluorophores
Solvatochromic properties
1
13
were illustrated on the basis of spectral data (IR, H NMR and C NMR). This was followed by photophysical
properties-based absorption and emission studies of the target compounds in solutions of different solvent po-
larities. The microenvironment-sensitive fluorescent pyrazolines labeled with pyrene exhibited valuable fluor-
escence properties with emission in the range of 600–700 nm with a marked response to change in the en-
vironmental polarities. A significant and pronounced red shift was observed in the emission spectrum of 5-(aryl)-
2
-Pyrazolines
Eco-friendly
1-phenyl-3-(pyren-1yl)-2-pyrazolines Δλ ∼ 50 nm compared to the absorption spectrum Δλ ∼ 10 nm upon
increasing the solvent polarity. This indicated the presence of higher dipole moment in the excited state than in
the ground state and the transition involved are π-π* transition through the charge transfer phenomena. The
fundamental understanding of solvatochromic properties were analyzed through Lippert-Mataga and Reichardts
correlations in order to estimate the change in dipole moments (Δμ) which suggested the emissive state of
designed fluorescent 2-pyrazoline derivatives is of strong ICT character.
1. Introduction
conjugating with numerous and diverse chemical species [15]. This has
encouraged the development of new synthetic methodologies for the
Pyrene is a well-known polynuclear aromatic hydrocarbon, which
synthesis of pyrene type substances. Some of these compounds can be
used as useful key intermediates for the synthesis of precious hetero-
cyclic ring systems. Among them, 1-cinnamoylpyrenes synthesized
through the Claisen-Schmidt condensation of 1-acetylpyrene and aro-
matic aldehydes [16,17] has proved to be particularly important. 1-
Cinnamoylpyrenes have been used for the synthesis of functionalized
pyrrolidines [16], pyrrolizidines [17] and other nitrogen-based het-
erocycles. Furthermore, pyrazolines are important nitrogen-containing
five membered heterocyclic compounds. Literature survey revealed that
many pyrazoline derivatives are known to exhibit a wide range of
biological activities such as antimicrobial [18], antimalarial [19], an-
titumor [20], anti-inflammatory [20], anticonvulsant and anti-
depressants activities [21].
recently become one of the most widely studied organic molecules in
the field of photochemistry and photophysics. Because of its unique
properties, it has emerged as an outstanding area of area in many sci-
entific fields. The highly fluorescence properties of pyrene has rendered
it to a fluorophore of first choice in both applied and fundamental
photochemical and photophysical research [1–10]. Interestingly,
pyrene is one of few polycyclic aromatic hydrocarbons that perform
vibronic structure in its monomer fluorescence spectrum in solution
[
11]. Its derivatives have been extensively expended in many applica-
tions such as fluorescent sensors [12,13] or fluorescent probes [14]
based on virtue of their excellent fluorescence properties. Pyrene
monomer emission typically occurs between 370 and 420 nm exhibiting
a characteristic violet color and is considered as one of the most useful
frameworks for the construction of fluorogenic chemosensors by
2 -Pyrazoline derivatives are the most commonly studied pyrazo-
line-type compounds and numerous methods have been reported for
⁎
Corresponding author at: Department of Chemistry, Faculty of Applied Science, Umm Al-Qura University, 21955, Makkah, Saudi Arabia.
E-mail address: saahmed@uqu.edu.sa (S.A. Ahmed).
https://doi.org/10.1016/j.jphotochem.2018.11.036
Received 18 October 2018; Received in revised form 22 November 2018; Accepted 24 November 2018
Available online 26 November 2018
1010-6030/ © 2018 Elsevier B.V. All rights reserved.

N. El Guesmi et al.
Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 306–314
Scheme 1. Previous synthetic methodologies of 2-pyrazoline derivatives.
3 3
Scheme 2. Synthesis of 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-pyrazolines (4a–f): (a) CH Cl, AlCl , DCM, rt/ overnight; (b) substituted benzaldehyde, KOH (20%), 2-
propanol, rt/ 1.5–3 h; (c) PhNHNH , MCM-SO H (15 mg), EtOH, reflux/.1–1.5 h.
2
3
their synthesis [22]. Since the milestone work of Fischer and Knove-
nagel published in late nineteen century, the reaction of α,β-un-
saturated aldehydes and ketones with hydrazines became a commonly
used, simple and convenient procedure for the synthesis of 2-pyrazoline
derivatives [23–30]. In the last decades, some of the catalysts devel-
transformation [41–43]. In continuation with our previous work on the
use of sulfonated mesoporous silica (MCM-SO H) as a mild and high-
3
performance heterogeneous catalyst in the green synthesis of hetero-
cyclic compounds [43,44], herein, we report an effective and facile
synthesis of a series of highly fluorescent 2-pyrazoline derivatives
grafted pyrene moiety in excellent yields (88–98%) and short reaction
time. The reaction of pyrenyl chalcones and phenyl hydrazine is cata-
lyzed by employing sulfonic acid functionalized mesoporous silica
oped for this synthetic transformation includes K
radiation [31], Amberlyst 15/ toluene [32], NaOAc/ AcOH under
heating or ultrasound irradiation [33], H PW12 40 [34], triethanola-
mine [35], NaOH/ EtOH [36], NaOAc/ EtOH [37], pyridine/ EtOH
38], AcOH/ grinding [39], and CeCl / ethyl lactate [40] as shown in
2 3
CO / microwave ir-
3
O
3
(MCM-SO H), an efficient solid acid heterogeneous catalyst, with
[
3
ethanol as an environmentally benign solvent. The photophysical
properties of the synthesized 2-pyrazoline derivatives were studied. The
main objective of the present work is to perform a detailed investigation
on the photophysical properties of these organic fluorophores related to
pyrene-based fluorescent type molecules in different media. In addition,
Lippert-Mataga and Reichardts correlations were applied to calculate
the difference between the excited and ground state dipole moments
(Δμ).
Scheme 1. However, despite their potential expediency, many of these
procedures endure some disadvantages such as use of expensive cata-
lysts, hazardous, and explosive solvents, as well as, comparatively low
to moderate yield and use of expensive instruments for experimenta-
tion.
3
Recently, sulfonated mesoporous silica (MCM-SO H) has emerged
as one of the most efficient, inexpensive, eco-friendly and versatile
heterogeneous catalyst for promotion of many acid-catalyzed organic
reactions with immense potential for further exploration in organic
307

N. El Guesmi et al.
Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 306–314
Scheme 3. Model reaction for the optimization of reaction conditions.
2. Results and discussion
Table 2
3
MCM-SO H catalyzed synthesis of novel 5-aryl-1-phenyl-3-(pyren-1yl)-2-pyr-
2
.1. MCM-SO H catalyzed synthesis of 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-
3
azolines (4a–f) under optimized conditions.
pyrazolines (4a-f)
Product
Time (h)
Yield (%)
Product
Time (h)
Yield (%)
4
4
4
a
b
c
1.5
1
1
89
98
90
4d
4e
4f
1.5
1.5
1.5
88
93
91
The synthesis of a series of (E)-3-aryl-1-(pyren-1-yl)prop-2-en-1-
ones (3a-f) was carried out using the reported procedure [16,17]. The
chemical route for the synthesis of 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-
pyrazolines (4a–f) are summarized in Scheme 2, which clearly shows
that the new heterocyclic compounds incorporates both a pyrene and a
1
3
5 mg of MCM-SO H was found to be the optimum catalyst amount
required for promoting the reaction.
2-pyrazoline moieties.
An investigation of the available literature revealed that the reac-
Based on these results, reaction of (E)-3-aryl-1-(pyren-1-yl)prop-2-
tion of chalcones and phenyl hydrazine worked better when the latter
was in excess amount. Therefore, we fixed the ratio of chalcone 3a-3f to
phenyl hydrazine to be 1:1.5. To optimize the reaction conditions, (E)-
en-1-ones (3a-f) with phenyl hydrazine in presence of MCM-SO
15 mg) in ethanol under refluxing condition furnished a simple, con-
venient and eco-friendly methodology that provided access to corre-
sponding 2-pyrazoline derivatives (4a-f) in excellent yields (Table 2).
The chemical structures of all the new synthesized compounds were
elucidated using spectroscopic techniques such as FT-IR, H NMR,
NMR, and C-DEPT-135 (see attached SI). For example, the IR spectra
showed C]N band at around 1614-1662 cm is in harmony with the
-pyrazoline skeleton. A sharp absorption band corresponding to cyano
3
H
(
3-phenyl-1-(pyren-1-yl)prop-2-en-1-one (3c) was chosen as a model
substrate and reacted with phenyl hydrazine in 1:1.5 ratio (Scheme 3).
For this cyclocondensation reaction, a series of experiments were
carried out by varying the amount of catalyst and the temperature to
reach at the optimum conditions. The results are summarized in
Table 1.
1
13
C
13
−1
2
In the preliminary study, we tried to optimize the above-mentioned
group was assigned only in the case of 5-(4-cyanophenyl)-1-phenyl-3-
model reaction in the absence and presence of MCM-SO
3
H catalyst, by
pyren-1yl)-2-pyrazoline (4e) at 2226 cm . The 1H-NMR spectra of
−1
(
mixing the reactants together in ethanol (10 ml) at room temperature.
No observable reaction was recorded in the absence of catalyst (Table 1,
entry 1). When the reaction was performed in the presence of different
compounds (4a-f) revealed that, the protons attached to the C-4 and C-5
carbon atoms of the five-membered ring (Scheme 1) showed an "ABX"
spin pattern. Chemical shifts and the coupling constant values (cf. Ex-
3
quantities of MCM-SO H, ranging from 5 to 25 mg per mmol of chal-
perimental Section) unequivocally prove the 2-pyrazoline structure.
cone, only trace amounts of the product were isolated. This could be
attributed to the fact that chalcone 3c is sparingly soluble in ethanol at
room temperature (Table 1, entries 2–6). This encouraged us to raise
the reaction temperature to reflux, which resulted in the remarkable
increase in the yield of 4c (71%) in shortened reaction time (2 h) in
13
C-NMR spectra, the chemical shift values of carbon atoms C
(45.9–46.4 ppm) and C (62.7–63.5 ppm) con-
3
(
147.5–147.8 ppm), C
4
5
1
firm the 2-pyrazoline structure deduced from the IR and H-NMR
spectroscopic data.
3
presence of 5 mg of MCM-SO H (Table 1, entry 7). Further studies
showed that on increasing the catalyst amount to 10 and 15 mg, the
reaction time was further shortened to 1 h and the yield increased to
2
.2. Solvent effect on absorption and emission spectra of 5-aryl-1-phenyl-3-
(pyren-1-yl)-pyrazolines (4a-f)
8
2% and 90% respectively. (Table 1, entries 8, 9). However, there was
The absorption spectra of the 5-aryl-1-phenyl-3-(pyren-1-yl)-pyr-
no significant change in the yield when the amount of catalyst was
azolines (4a-f) were measured in solvents with different polarity, which
showed slight variation in the absorption maxima on going from non-
polar aprotic (or slightly polar) (hexane, toluene), polar aprotic (di-
chloromethane (DCM), tetrahydrofuran (THF), acetonitrile) to polar
protic (methanol) solvents using UV–vis absorption spectrophotometer
further increased to 20 and 25 mg (Table 2, entries 10, 11). Therefore,
Table 1
3
MCM-SO H catalyzed synthesis of 5-phenyl-1-phenyl-3-(pyren-1yl)-2-pyr-
azolines 4c: Effect of the amount of the catalyst and temperature on the yield.
−
5
-3
with a concentration of 1 × 10
mol dm at ambient temperature.
ᵒ
Entry
Catalyst (mg)
Temp. ( C)
Time (h)
Yield (%)
Representation of the absorption spectra of 5-aryl-1-phenyl-3-(pyren-1-
yl)-pyrazolines (4a-f) recorded in some selective solvents are shown in
Fig. 1 and the corresponding spectral data are summarized in Table 3.
The absorption band intensities (log ε) of the compounds under in-
vestigations were found to be between 4.06 and 4.39 (Table 3) de-
pending on the substituent on both para position of the phenyl part and
solvent.
The absorption maxima of the 5-aryl-1-phenyl-3-(pyren-1-yl)-pyr-
azolines (4a-f) were detected in visible region in all solvents under
study and lie between 412 and 436 nm (Fig. 1, Table 3). A small
bathochromic shift by ∼ 16 nm in the absorption maxima was detected
1
2
3
4
5
6
7
8
9
–
5
rt
rt
rt
rt
rt
rt
reflux
reflux
reflux
reflux
reflux
overnight
overnight
overnight
overnight
overnight
overnight
2
2
1
1
1
no product
trace
trace
trace
trace
trace
71
82
90
90
88
10
15
20
25
5
10
15
20
25
10
1
1
308

N. El Guesmi et al.
Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 306–314
−
5
Fig. 1. Absorption spectra of (1 × 10
M) 5-aryl-1-phenyl-3-(pyren-1-yl)-pyrazolines (4a-f) in different solvents.
by changing the substituent from electron withdrawing group (nitro 4f,
16 nm) by electron donating group (methoxy 4b, 432 nm) in di-
unchanged on varying the solvent polarity.
4
The fluorescence emission spectra given in Fig. 2 revealed sig-
nificant solvent dependent shifts in emission maxima. For example, the
emission of 1,5-diphenyl-3-(pyren-1-yl)-pyrazoline 4c broadly shifted
bathochromically from non-polar to polar solvents, such as 647 nm in
hexane, 686 nm in methanol and 697 in acetonitrile. The substantial
red-shift of the fluorescence could be attributed to the noteworthy
difference between the excited-state charge distribution in the solute
chloromethane solution. Similar propensity was perceived for all sol-
vents (Table 3). On the other hand, for the same substituent, an overall
shift of ∼ 11 nm is observed in the absorption maxima of all com-
pounds (4a-f) by changing the solvent from methanol to toluene. Such
minor solvent effect toward the absorption spectrum for these mole-
cules designates that the ground electronic state for (4a-f) remains
309

N. El Guesmi et al.
Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 306–314
Table 3
Spectral and photophysical of 5-aryl-1-phenyl-3-(pyren-1-yl)-pyrazolines (4a-f) in different solvents.
−1
Solvent
Compound
λ
abs
λ
em
Δῡ
ε
−1
E
T
(30) kcal mol
Δf (D,n)
(
nm)
(nm)
(cm−1)
(M cm−1
)
Hexane
Toluene
THF
DCM
MeCN
MeOH
427
433
430
428
424
422
646
662
680
686
699
690
7939
7989
8545
8787
9279
9204
24,300
22,300
22,900
22,000
22,300
21,900
31.0
33.9
37.4
40.7
45.6
55.4
0.0014
0.0132
0.210
0.219
0.304
0.308
Hexane
Toluene
THF
DCM
MeCN
MeOH
430
436
433
432
428
426
647
666
682
690
697
690
7800
7921
8432
8655
9017
8981
23,400
21,400
22,100
21,600
21,800
20,900
31.0
33.9
37.4
40.7
45.6
55.4
0.0014
0.0132
0.210
0.219
0.304
0.308
Hexane
Toluene
THF
DCM
MeCN
MeOH
426
432
429
427
423
421
643
659
678
682
697
686
7922
7974
8561
8756
9293
9176
22,100
21,100
21,500
21,000
20,900
19,100
31.0
33.9
37.4
40.7
45.6
55.4
0.0014
0.0132
0.210
0.219
0.304
0.308
Hexane
Toluene
THF
DCM
MeCN
MeOH
421
428
424
423
419
417
627
645
664
666
678
670
7804
7861
8525
8626
9117
9055
11,500
11,800
12,500
12,300
12,400
12,300
31.0
33.9
37.4
40.7
45.6
55.4
0.0014
0.0132
0.210
0.219
0.304
0.308
Hexane
Toluene
THF
DCM
MeCN
MeOH
425
431
427
426
422
420
635
648
668
668
681
674
7781
7770
8449
8504
9012
8973
18,200
16,100
17,500
16,900
17,100
17,200
31.0
33.9
37.4
40.7
45.6
55.4
0.0014
0.0132
0.210
0.219
0.304
0.308
Hexane
Toluene
THF
DCM
MeCN
MeOH
416
422
419
416
415
412
537
550
567
568
580
571
5417
5515
6230
6433
6855
6759
22,900
20,700
21,200
21,500
21,200
21,000
31.0
33.9
37.4
40.7
45.6
55.4
0.0014
0.0132
0.210
0.219
0.304
0.308
and the ground-state charge distribution, resulting in stronger interac-
tions with polar solvents in the excited state.
of the excited state being more polar than the ground state for the π to
π* transition. The π to π* transition was confirmed by DFT calculations
with the corresponding HOMO and LUMO calculated orbitals (see
Supplementary Information). Alternative use of a cyano group proved
more successful and was inspired by the substantial effect of solvent on
the fluorescent properties of 5-(4-cyanophenyl)-1-phenyl-3-(pyren-1yl)-
2-pyrazoline 4e.
To comprehend the polarity effect of 5-aryl-1-phenyl-3-(pyren-1-yl)-
pyrazolines in various solvents, solvent dependent spectral shifts was
investigated. The Lippert–Mataga’ Eqs. (1) and (2) [49–52] shows that
the solvent dependence of the Stokes shift for a compound depends on
the change in the dipole moment of the fluorescence moiety upon ex-
citation, the refractive index, and the dielectric constant of the solvents
being used [49–55]:
There are some remarkable features: firstly, the presence of a nitro
group in the phenyl ring of 5-(4-nitrophenyl)-1-phenyl-3-(pyren-1-yl)-
pyrazoline 4f diminishes its fluorescence quantum yield and fluores-
cence brightness drastically when compared with (4a-e). Secondly, the
compounds (4a-e) showed very similar photophysical properties in
various solvents. These results indicate that the 5-aryl-1-phenyl-3-
(
pyren-1-yl)-pyrazolines (4a-f) exhibit highly solvent-dependent pho-
tophysical properties despite their structural similarities.
Furthermore, a comparative study on the impact between cyano and
nitro functionality toward a solvatochromism for many systems was
known in the literature studies where the cyano group offered strong
emission and solvent-dependence in comparison to nitro. [45–48] The
shift of the fluorescence spectrum as a function of time reflects the
reorganization of solvent molecules around the excited 5-aryl-1-phenyl-
2
2
(μ − μ )
2
e
g
ε − 1
n − 1
¯
⎛
⎝
⎞
⎠
Δϑst =
−
+ constant
3
2
3-(pyren-1-yl)-pyrazoline molecules 4a-f. Therefore, following excita-
hca
2ε + 1
2n + 1
(1)
tion, the solvent cage undergoes a relaxation, that is, a reorganization,
leading to a relaxed state of minimum free energy. The higher the po-
larity of the solvent, the lower the energy of the relaxed state and the
larger the redshift of the emission spectrum. The fluorescence of 4e and
Where Δ ϑ¯ is the difference between the absorption and emission
st
−1
maxima expressed in wave numbers (cm ), h is Planck’s constant, c is
the velocity of light in vacuum, a is the radius of the Onsager cavity
around the fluorophore, ε and n are the dielectric constant and the
index of refraction of the solvents, respectively, μe and μg are the dipole
moments in the excited and ground states, respectively. The Lippert–-
Mataga plot can be obtained by plotting the Stokes shift vs. the term in
the brackets in the above equation, referred to as the orientation po-
larizability (Δf) of the solvent, which measures both electron mobility
and dipole moment of the solvent molecule.
4f was significantly and differently shifted in various solvents. This is
evident in Fig. 2, where the emission maximum of 4e is shifted upwards
of 250 nm, while 4f is shifted by 150 nm in similar solvents. To some
extent, the observed shifts are not surprising given the excited state is
known to be more perturbed for 4e than 4f by solvent effects than the
ground state. This may perhaps be due to a non-radiative relaxation
pathway available to the nitro group. On the other hand, this is a result
310

N. El Guesmi et al.
Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 306–314
−
5
Fig. 2. Normalized fluorescence emission spectra of (1 × 10
M) 5-aryl-1-phenyl-3-(pyren-1-yl)-pyrazolines (4a-f) in different solvents.
correlation of Lippert–Mataga plot supports the occurrence of photo
2
ε − 1
n − 1
Δf =
−
2
2
ε + 1
2n + 1
(2)
induced intramolecular charge transfer in all compounds (4a–f).
The data of quantum chemical calculations of compounds (4a–f)
obtained by Gaussian program [56] (DFT/B3LYP functional using
Fig. 3 represents the Lippert–Mataga plot of 5-aryl-1-phenyl-3-
pyren-1-yl)-pyrazolines in different solvents. A plot of Δῡ versus Δf
(
6–31 G-(d) orbital base) were used to determine the radius of the On-
gives a straight line for solvent in the range 0.0014 ≤ Δf ≤ 0.308, with
sager cavity. The values of a for investigated fluorophores pyrazolo-
2
a correlation coefficient of 0.977 ≤ R ≤ 0.992 (Fig. 3). The linear
311

N. El Guesmi et al.
Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 306–314
hexane, toluene, tetrahydrofuran, dichloromethane and acetonitrile
and the other for protic solvents like methanol suggesting a specific
interaction of 5-aryl-1-phenyl-3-(pyren-1-yl)-pyrazolines with me-
thanol. 5-Aryl-1-phenyl-3-(pyren-1-yl)-pyrazolines in aprotic solvents
showed general solvent effect whereas in methanol it deviated from the
general solvent effect due to specific interactions such as the hydrogen
bonding ability with 5-aryl-1-phenyl-3-(pyren-1-yl)-pyrazolines (4a-f).
This phenomenon has also been observed for other environment-sen-
sitive fluorophores and has been explained as the result of emission
from two different excited states: one with higher dipole moment that is
stabilized in polar environments and another less polar excited state
responsible for the emission in less polar media [58–60,64,65].
3. Conclusion
In conclusion, we have synthesized new 5-(aryl)-1-phenyl-3-(pyren-
1yl)-2-pyrazolines in excellent yield by the reaction of (E)-3-aryl-1-
(
pyren-1-yl)prop-2-en-1-ones and phenyl hydrazine under simple and
convenient reaction conditions using sulfonated mesoporous silica
MCM-SO H) as efficient, eco-friendly and versatile heterogeneous
(
3
Fig. 3. Correlation of the Stokes shift with the solvent polarity function (Δf) for
acidic catalyst in ethanol as a reaction medium. The fluorescence
emission properties exhibited that all compounds 5-(aryl)-1-phenyl-3-
5
-aryl-1-phenyl-3-(pyren-1-yl)-pyrazolines.
(
pyren-1yl)-2-pyrazolines (4a-f) under investigation are intramolecular
pyrene consisted of 7.40, 7.33, 7.11, 6.84, 7.15 and 7.29 Å for 4a–f,
respectively, was taken as half of the average distance, which corre-
sponds to the longest distance across the molecule where charge se-
paration can take place [57]. The changes of dipole moment upon the
transition into excited state were determined using the slope of the lines
charge transfer fluorophores and exhibit significant solvatochromism.
Uv–vis and fluorescence emission of the investigated compounds (4a-f)
indicated a highly solvent polarity dependency. Lippert-Mataga and
T
E (30) values plots for (4a-f) in different solvents showed sharp in-
crease of Δf value with increase in the polarity of the aprotic solvents
and decrease in alcoholic solvents due to hydrogen bonding between
solute and solvent. Dipole moment calculations suggested that the ex-
cited state of pyrazolines (4a-f) is more polar than the ground state.
in the Fig. 3. Calculated values (??
e g
-?? ) consisted of 12.90, 12.10,
12.20, 11.42, 11.98 and 13.08 D for 4a–f respectively. These values are
located at the range of 5–20 D, values reported for other environment-
sensitive fluorophores, such as the values of 5.5 D obtained for 6-N,N-
dimethylamino-2,3-naphthalimide [58], 7.09 and 8.07 D for chalcone
derivative 3-(1-methyl-1H-pyrrol-2-yl)-1-naphthalen-2-yl propenone
4
. Material and methods
[
59], 3-(1-Benzyl-1H-3-indol-3-yl)-1-naphthalen-2-yl propenone [60],
respectively, 20.78 D for 2-Butyl-6-(4-methoxystyryl)-1H-benzo[d,e]
isoquinoline-1,3(2H)-dione [61].
All solvents used purchased from Sigma-Aldrich are spectroscopic
grade and used without further purifications. Melting points were de-
termined on a Stuart SMP3 melting point apparatus and are un-
corrected. IR spectra were recorded on a Shimadzu IR-3600 FT-IR
spectrometer in KBr pellets. NMR spectra were acquired on a Bruker
In the case of charge transfer in the molecule, the gross solvent
polarity indicator scale such as E
of Stokes shift vs. the E (30) values of the solvents is given in Fig. 4. The
plot illustrates two sets of linearity: one for aprotic solvents such as
T
(30) is more applicable [62,63]. Plot
T
1
13
Avance 500 instrument (at 500 MHz for H, 125 MHz for C) in CDCl
solutions, using residual solvent signals as internal standards. Electronic
absorption spectra were recorded using UV/VIS/NIR spectro-
mol/L in six aprotic and
3
a
−
5
photometer in concentration of 1 × 10
protic solvents with different polarity. The measurements were ac-
quired in room temperature in the spectral range of 190–1000 nm by
using a computer aided double-beam Shimadzu 3600 UV–VIS–NIR
spectrophotometer with a resolution of 0.1 nm. Fluorescence spectra
were performed using Horiba Spectrofluorometer (model FluoroMax-
4).
4
2
.1. General procedure for the synthesis of 5-(aryl)-1-phenyl-3-(pyren-1yl)-
-pyrazolines (4a–f)
A mixture of the appropriate (E)-3-aryl-1-(pyren-1-yl)prop-2-en-1-
one (3a-f, 1.0 mmol), phenyl hydrazine (162 mg, 1.5 mmol) and MCM-
SO H (15 mg) in ethanol (10 ml) was under reflux with stirring for
–1.5 h. After completion of reaction (monitored by TLC using CH Cl
2
3
1
2
as eluent), the mixture was cooled to room temperature. The reaction
mixture was concentrated under reduced pressure then chloroform
(
10 ml) was added. The catalyst was filtered off, the filtrate was then
evaporated under reduced pressure and the title compounds (4a–f)
were obtained, washed with little amount of ethanol, filtered off, and
dried under vacuum. The product was obtained essentially pure in all
cases.
Fig. 4. Correlation of the Stokes shift with E
-yl)-pyrazolines.
T
(30) for 5-aryl-1-phenyl-3-(pyren-
1
312

N. El Guesmi et al.
Journal of Photochemistry & Photobiology A: Chemistry 371 (2019) 306–314
2226 (CN), 1662 (C = N) cm . H NMR (500 MHz, CDCl ): δ 9.74 (d,
3
−1 1
4
(
.1.1. 5-(4-Dimethylaminophenyl)-1-phenyl-3-(pyren-1yl)-2-pyrazoline
4a)
Orange crystals; mp 187–189 °C. IR (KBr): νmax = 3038 (CH
J = 8.0 Hz, 1H, CH), 8.99 (d, J = 7.5 Hz, 1H, CH), 8.37 (d, J = 7.5 Hz,
1H, CH), 8.30 (d, J = 8.0 Hz, 1H, CH), 8.21-8.17 (m, 1H, CH), 8.10-8.0
(m, 2H, 2CH), 7.97 (d, J = 5.5 Hz, 1H, CH), 7.77 (d, J = 5.5 Hz, 1H,
CH), 7.62-7.60 (m, 3H, 3CH), 7.47-7.43 (m, 2H, 2CH), 7.14 (d,
−
1
1
arom.), 2885 (CH aliph.), 1614 (C = N) cm
CDCl ): δ 9.77 (d, J = 9.0 Hz, 1H, CH), 8.26 (d, J = 9.0 Hz, 1H, CH),
.20 (d, J = 6.5 Hz, 1H, CH), 8.16 (d, J = 6.5 Hz, 1H, CH), 8.06-7.96
m, 5H, 5CH), 7.88 (d, J = 7.5 Hz, 1H, CH), 7.25-7.24 (m, 5H, 5CH),
.83 (broad d, 1H, CH), 6.89 (d, J = 7.5 Hz, 2H, 2CH), 5.24 (dd,
. H NMR (500 MHz,
3
3
8
(
6
J = 6.5 Hz, 2H, 2CH), 7.03-6.95 (m, 2H, 2CH), 5.19 (dd, J = 2.5 Hz,
3
), 4.00 (dd, J = 14.0 Hz, 2J = 14.5 Hz, 1H, H
3
J = 14.5 Hz, 1H, H
c
b
),
3
2
) ppm. 13C NMR (125 MHz,
3.31 (dd, J = 2.5 Hz, J = 14.5 Hz, 1H, H
a
3
3
3
J = 5.5 Hz,
J = 14.0 Hz, 1H,
H
c
), 4.07 (dd,
J = 14.0 Hz,
), 3.46 (dd, J = 5.5 Hz, J = 16.2 Hz, 1H, H ),
): δ 149.9 (C), 147.6
3
C ), 144.9 (C), 131.4 (C), 131.2 (C), 130.9 (C), 130.1 (C), 129.4 (C),
CDCl
3
3
): δ 152.7 (C), 147.8 (C ), 147.6 (C), 144.3 (C), 141.8 (C), 139.6
2
3
2
J = 16.2 Hz, 1H, H
.89 (s, 6H, 2CH ) ppm. C NMR (125 MHz, CDCl
3
b
a
(C), 134.8 (C), 133.1 (CH), 132.3 (CH), 131.5 (CH), 131.4 (CH), 130.8
(CH), 129.3 (CH), 129.1 (CH), 128.9 (CH), 127.2 (CH), 126.8 (CH),
126.3 (CH), 126.0 (CH), 125.7 (CH), 125.5 (CH), 125.3 (CH), 124.5
1
3
2
(
3
1
1
28.9 (C), 128.6 (C), 128.5 (C), 127.8 (CH), 127.3 (CH), 126.8 (CH),
26.6 (CH), 126.5 (CH), 126.0 (CH), 125.9 (CH), 125.4 (CH), 125.2
(CH), 119.8 (CH), 113.5 (CH), 111.6 (CN), 110.2 (CH), 62.9 (C
(C ) ppm.
5
), 45.9
4
(
CH), 125.1 (CH), 124.6 (CH), 124.5 (CH), 119.1 (CH), 113.5 (CH),
1
13.0 (CH), 63.2 (C
5
), 46.4 (C
4
), 40.5 (CH
3
) ppm.
4.1.6. 5-(4-Nitrophenyl)-1-phenyl-3-(pyren-1yl)-2-pyrazoline (4f)
Red crystals; mp 203–205 °C. IR (KBr): νmax = 3030 (CH arom.),
−
1 1
4.1.2. 5-(4-Methoxyphenyl)-1-phenyl-3-(pyren-1yl)-2-pyrazoline (4b)
3
1635 (C = N) cm . H NMR (500 MHz, CDCl ): δ 9.69 (d, J = 8.5 Hz,
Orange crystals; mp 159–161 °C. IR (KBr): νmax = 3034 (CH arom.),
1H, CH), 8.26 (d, J = 8.5 Hz, 1H, CH), 8.21 (d, J = 6.5 Hz, 1H, CH),
8.17 (d, J = 6.5 Hz, 3H, 3CH), 8.06-8.02 (m, 3H, 3CH), 7.94 (d,
J = 8.0 Hz, 1H, CH), 7.79 (d, J = 6.5 Hz, 1H, CH), 7.49 (d, J = 6.5 Hz,
−
1 1
2890 (CH aliph.), 1625 (C = N) cm
3
. H NMR (500 MHz, CDCl ): δ
9.75 (d, J = 8.5 Hz, 1H, CH), 8.25 (d, J = 8.5 Hz, 1H, CH), 8.20 (d,
J = 6.0 Hz, 1H, CH), 8.15 (d, J = 6.0 Hz, 1H, CH), 8.03-7.94 (m, 4H,
CH), 7.85 (d, J = 6.5 Hz, 1H, CH), 7.29 (d, J = 7.0 Hz, 2H, 2CH), 7.24
d, J = 6.0 Hz, 2H, 2CH), 7.20 (d, J = 7.0 Hz, 2H, 2CH), 6.86-6.85 (m,
2H, 2CH), 7.27-7.23 (m, 2H, 2CH), 7.10 (d, J = 6.5 Hz, 2H, 2CH), 6.88
(broad d, 1H, CH), 5.27 (dd, J = 5.0 Hz, 3J = 13.5 Hz, 1H, H
3
), 4.07
), 3.37 (dd, J = 5.0 Hz,
) ppm. C NMR (125 MHz, DMSO-d ): δ 149.7 (C),
), 147.4 (C), 144.2 (C), 131.6 (C), 131.3 (C), 130.7 (C), 129.2
4
(
c
3
2
3
(dd, J = 13.5 Hz, J = 16.2 Hz, 1H, H
b
3
3
2
13
3
H, 3CH), 5.23 (dd, J = 5.0 Hz, J = 14.0 Hz, 1H, H
c
), 4.06 (dd,
), 3.43 (dd,
) ppm. C NMR (125 MHz, CDCl ): δ
3
), 144.7 (C), 134.5 (C), 131.4 (C), 131.3 (C), 130.8
J = 16.2 Hz, 1H, H
147.6 (C
a
6
3
2
J = 14.0 Hz,2 J = 16.5 Hz, 1H, H
b
), 3.75 (s, 3H, CH
3
3
3
13
J = 5.0 Hz, J = 16.5 Hz, 1H, H
58.9 (C), 147.6 (C
a
3
(C), 128.9 (CH), 128.6 (CH), 128.2 (CH), 127.2 (CH), 126.9 (CH), 126.2
(CH), 125.9 (CH), 125.7 (CH), 125.4 (CH), 125.3 (CH), 124.6 (CH),
1
(
(
C), 129.0 (C), 128.6 (C), 127.9 (C), 127.2 (CH), 127.1 (CH), 126.4
CH), 126.2 (CH), 126.1 (CH), 125.9 (CH), 125.4 (CH), 125.3 (CH),
124.5 (CH), 124.4 (CH), 119.9 (CH), 113.5 (CH), 62.7 (C
ppm.
5 4
), 45.9 (C )
125.2 (CH), 124.6 (CH), 124.4 (CH), 119.2 (CH), 114.5 (CH), 113.5
(
CH), 63.0 (C ), 55.2 (CH ), 46.4 (C ) ppm.
5
3
4
Appendix A. Supplementary data
4
.1.3. 1,5-Diphenyl-3-(pyren-1yl)-2-pyrazoline (4c)
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.11.
036.
Orange crystals; mp 195–197 °C. IR (KBr): νmax = 3035 (CH arom.),
−
1 1
1
1
8
3
655 (C = N) cm . H NMR (500 MHz, CDCl ): δ 9.75 (d, J = 9.0 Hz,
H, CH), 8.25 (d, J = 9.0 Hz, 1H, CH), 8.20 (d, J = 6.0 Hz, 1H, CH),
.15 (d, J = 6.0 Hz, 1H, CH), 8.03-7.99 (m, 3H, 3CH), 7.96 (d,
References
J = 7.5 Hz, 1H, CH), 7.84 (d, J = 7.5 Hz, 1H, CH), 7.39-7.37 (m, 2H,
CH), 7.35-7.33 (m, 2H, 2CH), 7.26-7.23 (m, 3H, 3CH), 7.19 (d,
2
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3
2
J = 13.5 Hz, 1H, H
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8
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3
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