One-pot synthesis, physicochemical and photophysical properties of deep blue light-emitting highly fluorescent pyrene-imidazole dye: A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.jphotochem.2018.06.015

A one-pot multi-component synthesis of 4,5,-diphenyl-2-(pyren-1-yl)-1H-imidazole (DPI) chromophore was performed by the reaction of benzil, pyrene-1-carboxaldehyde and ammonium acetate in acetic acid. Structure elucidation of DPI chromophore was confirmed by spectroscopic techniques (FT-IR, ¹H NMR, ¹³C NMR and mass spectra). The physicochemical and photophysical parameters of DPI chromophore such as extinction coefficient, oscillator strength, dipole moment, stokes shift and fluorescence quantum yield were calculated experimentally and theoretically on the basis of the different solvents polarity to see the effect of the solvents on DPI chromophore. DPI chromophore was also applicable for determination of critical micelle concentration (CMC) of cationic and anionic surfactant such as cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS).

Full text of DOI:10.1016/j.jphotochem.2018.06.015

Accepted Manuscript
Title: One-pot synthesis, physicochemical and photophysical
properties of deep blue light - emitting highly fluorescent
pyrene-imidazole dye: A combined experimental and
theoretical study
Authors: Salman A. Khan, Abdullah M. Asiri, Al-Anood M.
Al-Dies, Osman I. Osman, Mohammed Asad, Mohie E.M.
Zayed
PII:
S1010-6030(18)30386-1
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2018.06.015
JPC 11332
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
25-3-2018
27-5-2018
5-6-2018
Please cite this article as: Khan SA, Asiri AM, Al-Dies A-AnoodM, Osman OI, Asad
M, Zayed MEM, One-pot synthesis, physicochemical and photophysical properties
of deep blue light - emitting highly fluorescent pyrene-imidazole dye: A combined
experimental and theoretical study, Journal of Photochemistry and Photobiology, A:
Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.06.015
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One-pot synthesis, physicochemical and photophysical properties of
deep blue light - emitting highly fluorescent pyrene-imidazole dye: A
combined experimental and theoretical study
Salman A. Khan1,*, Abdullah M. Asiri1,2,**, Al-Anood M. Al-Dies, Osman I. Osman1,
Mohammed Asad1, Mohie E.M. Zayed1
1Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O. Box
80203, Jeddah 21589, Saudia Arabia
2Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz
University, P. O. Box 80203, Jeddah 21589, Saudia Arabia
*Correspondence Author e-mail adders: sahmad_phd@yahoo.co.in (S. A. Khan*),
aasiri2@kau.edu.sa (A. M. Asiri**)
Graphical Abstract:
Highlights
 One pot synthesis of (4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole (DPI)
 Spectroscopic and photophysical investigations of DPI dye
 Determine the critical micelle concentration (CMC) of CTAB and SDS
 Frontier Molecular Orbitals (FMOs) study of DPI
Abstract:
A one-pot multi-component synthesis of 4,5,-diphenyl-2-(pyren-1-yl)-1H-imidazole
(DPI) chromophore was performed by the reaction of benzil, pyrene-1-carboxaldehyde
and ammonium acetate in acetic acid. Structure elucidation of DPI chromophore was
confirmed by spectroscopic techniques (FT-IR, 1H-NMR, 13C-NMR and mass spectra).
The physicochemical and photophysical parameters of DPI chromophore such as
extinction coefficient, oscillator strength, dipole moment, stokes shift and fluorescence
quantum yield were calculated experimentally and theoretically on the basis of the
different solvents polarity to see the effect of the solvents on DPI chromophore. DPI
chromophore was also applicable for determination of critical micelle concentration
(CMC) of cationic and anionic surfactant such as cetyl trimethyl ammonium bromide
(CTAB) and sodium dodecyl sulfate (SDS).
1

Keywords: One-pot; Imidazole; Dipole moment; DFT; CMC
1. Introduction
Over the last few decades, the synthesis of heterocyclic compounds has become a
keystone of synthetic organic chemistry because of extensive multiplicity of
applications of these heterocyclic compounds in the pharmaceutical and medicinal
chemistry [1]. Five-member two nitrogen atoms including heterocyclic compound with
special reference of imidazole normally found in many biological systems such as
Olmesarten and Losartan [2]. IUPAC name of imidazole is 1,3-diaza-,2,4-
cyclopentadiene. It is a planar five-member heterocyclic compound having three carbon
atoms and two nitrogen atoms in positions 1 and 3. The imidazole moiety is an
important pharmaphore that embraces a number of pharmacological behaviors such as
antifungal, antibacterial [3], anticancer [4], anti-inflammatory and antitumor etc. [5, 6].
It is also significantly applicable in the chemical analytical field such as polymer
stabilizer [7], laser dye [8], environmental probes in bio-molecules [9] and Raman
filters [10]. The long π-bond conjugated system of the chromophores is responsible for
their colour by absorbing the UV-light at certain wavelength. The Imidazole nucleus
containing the long π-bond conjugated system are applicable in various fields of
materials science such as Organic light-emitting diodes (OLED) [11, 12], dye-
sensitized solar cells (DSSC) [13], chemosensors for the detection of metal ions [14]
and optoelectronic devices [15]. The photophysical and physicochemical parameters of
the imidazole containing chromophore such as oscillator strengths, extinction
coefficients, stokes shift, dipole moments and fluorescence quantum yields are
important parameters that prove the physical behavior of the chromophores [16]. On
the basis of literature survey, we found, lot of work have been done on imidazole
derivative with their various biological applications and also in the area of material
2

sciences, but no one reported synthesis of imidazole as a donor-acceptor chromophore,
our aim to design highly fluorescence and highly photostable long pi bond conjugated
system containing with donor and acceptor chromophore can be use as organic light
emitting diode. In this manuscript we are reporting synthesis of 4,5-diphenyl-2-(pyren-
1-yl)-1H-imidazole (DPI) chromophore by the one-pot reaction of benzil, pyrene-1-
carboxaldehyde and ammonium acetate. The physicochemical and photophysical
properties such as extinction coefficients, stokes shifts, oscillator strengths, dipole
moments and fluorescence quantum yields of the chromophore in different solvents on
the basis of the polarity were determined experimentally. Density Functional Theory
(DFT) was also used to accompaniment the experimentally physical behavior of the
newly synthesized molecule.
2. Experimental
2.1. Chemicals and reagents
All the chemical and reagents for this experiment like pyrene-1-carboxaldehyde, benzil,
ammonium acetate and acetic acid were obtained from Acros Organic. All solvents
were used HPLC grade.
2.2. Apparatus
The reaction was examined by 0.2 mm silica gel (60F-254) coated thin layer
chromatography. Melting point of DPI dye was determined by using a Stuart Scientific
Co. Ltd apparatus. The IR spectra of the compound (DPI) was measured by a Perkin
Elmer FT-IR spectrometer. NMR spectra (1H & 13C) of the compound was measured
by a 850MHZ Bruker AVANCE™ III HD. Mass spectra were measured by using Direct
Analysis in Real Time (DART) on a JOEL AccuToF LC-plus JMS–T100LP, time-of-
flight mass spectrometer (DART-ToF MS). Absorption and emission spectra DPI dye
3

was recorded in different solvent by Shimadzu UV-1650 PC spectrophotometer and
Shimadzu RF 5301 PC spectrofluorophotometer using a 1 cm rectangular quart cell.
2.3.Synthesis of 4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole (DPI)
DPI chromophore was prepared by one-pot reaction of pyrene-1-carboxaldehyde (4.14
g, 0.018 mol), benzil (3.0 g, 0.018 mol) and ammonium acetate (4.15 g, 0.018 mol) in
acetic acid (20 ml) in a 50 ml round-bottom flask. Mixture was refluxed for 2 h at 150
oC in an oil bath. Progress of the reaction was monitored by the TLC. After complete
the reaction, the reaction mixture was cooled to room temperature and precipitate was
removed by filtration and 200 ml of distilled water was added to the filtrate heavy
precipitate obtained filtered off, washed with water several times, dry and recrystallized
from methanol and chloroform (5:5) (Scheme 1) [17].
Color: orange, shape: powder, Yield: 97%; m.p.: 299 oC; IR vmax/cm-1: 3435 (NH stretch),
1579 (NH bend), 3036, 2953, 2923 (CHaromatic stretch), 1728 (C=N), 1603, 1443(C=C),
1
1355 (C-N); H NMR (850 MHz, DMSO-d6, 훿ppm): 13.00 (s, 1H, NH), 9.56 (d, 4H,
CHAromatic, J = 8.9 Hz), 8.54 (d, 1H, CHAromatic, J = 7.4 Hz), 8.42 (d, 1H, CHAromatic, J
= 8.6 Hz), 8.36 (t, 4H, CHAromatic, J = 7.6 Hz), 8.32 (d, 2H, CHAromatic, J = 9.6 Hz), 8.25
(dd, 1H, CHAromatic, J = 12.0 and 8.7 Hz), 8.12 (t, 2H, CHAromatic, J = 7.5 Hz), 7.69 (d,
1H, CHAromatic, J = 7.1 Hz), 7.63 (d, 1H, CHAromatic, J = 7.1 Hz), 7.48 (t, 1H, CHAromatic
,
13
J = 7.05 Hz), 7.42-7.37 (m, 1H, CHAromatic); C NMR (213 MHz, DMSO-d6, 훿ppm ):
146.43 (C=N), 138.06, 135.73, 131.46, 131.43, 131.28, 130.87, 129.17, 129.01, 128.91,
128.81, 128.48, 128.41, 128.39, 128.30, 127.80, 127.75, 127.15, 127.02, 126.40,
126.05, 125.72, 125.25, 125.12, 124.89, 124.31 (C-aromatic); MS (DART-To FMS)
m/z: exact mass 420.16 found 421.57 with a base peak at 231.08.
3. Results and discussion
4

3.1.Characterization of DPI
4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole (DPI) chromophore was prepared by the
one-pot reaction of benzil, pyrene-1-carboxaldehyde and ammonium acetate in acetic
acid. The structure of novel DPI chromophore was confirmed by the FT-IR, 1H-NMR
and 13C-NMR and mass spectroscopic techniques. FT-IR spectra of DPI chromophore
show bands at 3435 and 1728 cm-1 which are characteristic to the N-H and C=N groups
vibrations, respectively, with no any band 1658 cm-1 which is conformed that carbonyl
groups of benzil and pyrene-1-carboxaldehyde are utilized for the formation of
imidazole ring. The structure of DPI chromophore was further conformed by 1H-NMR
1
spectra. The H-NMR spectra of DPI chromophore was measured in DMSO-d6, its
shows a singlet peak at δ 13.00 due to presence of N-H group in imidazole ring. The
1H-NMR spectra also showed doublet (d), doublet of doublets (dd), triplet (t) and
13
multiplet (m) due to present of 19 aromatic protons (Fig. S1). C-NMR spectra also
confirmed the formation of DPI chromophore, its shows signal at δ 146.43 due to
prescence of azo-carbon of the imidazole ring and signal in range of δ124.31-138.06,
13
due to aromatic carbon. The detailed C-NMR spectrum of the DPI chromophore is
given in the experimental section. Finally, structure of DPI chromophore was
confirmed by the mass spectra, its shows a molecular ion peak m/z at 421.57 with a
base peak at 231.08 (Fig. S2).
3.2. Spectra behavior of DPI chromophore in different solvents
The absorbance spectra of the 4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole (DPI)
chromophore (1 x 10-5 M) were obtained in nine different polarity solvents. As shown
in the Fig. 1, the polarities of the solvents have considerably affected the absorption
maxima with a red shift 16 nm from n-Hexane (354 nm) to DMSO (370 nm), indicating
the polar character of DPI chromophore in the ground state. As expected, red shift is
5

because of strong intramolecular charge transfer (ICT) from push to pull group (CNHC
group to CNC moiety) as found in other related compounds [18].
As Fig. 2 shows, on excitation with 365 nm, the fluorescence spectrum of DPI
chromophore displays a strong bathochromic shift on increasing the solvents polarity.
The emission profile undergoes a red-shift of 30 nm in emission maxima from n-
Hexane (440 nm) to DMSO (470 nm), indicating photoinduced intramolecular singlet
excited state charge transfer (ICT); which occurs from the electron push group (CNHC
group) to the electron pull group (CNC moiety). This indicates that the polarity of DPI
chromophore enhances on excitation [19]. Hence, the fluorescence profile of DPI
chromophore is more effective to the polarity of the solvents than in the absorption
profile; signifying that the charge transfer is much significant in the excited state in
comparison to the ground state; which means that the polar solvents stabilize highly
dipolar excited states.
Emission spectra of different concentrations of DPI chromophore in CHCl3 were also
measured exhibited almost same fluorescence maxima, only emission intensity changed
with the concentration (Fig. 3).
The absorption and emission energy (Ea and Ef) of DPI chromophore in various solvents
are associated with the empirical Dimroth polarity parameter ET(30) of the solvents
(Fig. 4) [20]. Energy of the absorption and emission of DPI chromophore versus
polarity of the various solvents was find linear correlation (Eqs. 1 and 2). Polar aprotic
solvents such as DMSO and DMF shows away from the averaged line indicating that
the nature of the emitting state is different for this class of solvents. A linear association
between the energy of the absorption and emission versus polarity of various solvent
was obtained (Eqs. 1 and 2).
Ea = 84.05 – 0.038 x ET(30)
(1)
6

Ef = 66.82 – 0.083 x ET(30)
(2)
The solvatochromic behavior of the DPI chromophore based on the linear correlation
between absorption and emission maxima (λab and λem) and polarity solvent functions
is well-known for determining excited and ground state dipole moments. Lippert-
Mataga Eqs. (3) and (4), were used to determine the energy difference between the
ground and excited states i.e. Stokes shift and change in dipole moment between the
excited singlet and ground state [21].
2(e  g )2
 st

f Const.
(3)
(4)
hca3
 1
2 1
n2 1
2n2 1
f


where (vst) is the stokes shift, which increases with increasing the polarity of the
solvents, h is Planck’s constant, c is the speed of light, a is the Onsager cavity radius, n
is the refractive index of the solvent and ε is the dielectric constant. The Onsager cavity
radius was taken as 5.6 Ao. Fig 5 shows the plot of the orientation polarizability of the
solvents versus Stokes shift of DPI chromophore in different solvent (f). Change in
dipole moments () were calculated between the singlet excited and ground states
from the slop plot of orientation of polarizability of the different solvent and Stokes
shift () of the DPI chromophore in different solvents and found 8.27 Debye, Positive
value indicating that the excited sate is more polar than the ground state.
The transition dipole moment (μ12) and oscillator strength (f) of electronic transition for
DPI chromophore from ground to excited singlet state (So → S1) in different solvent
were calculated by using the following equations (5, 6) [22].
 4.32109   ( ) d
(5)
f
7

f
2
12

(6)
4.72 107 Emax
where v is the wavenumber measured in cm-1, ε is the numerical value for molar
extinction coefficient is measured in dm-3 mol-1 cm-1 and Emax is the energy the
maximum absorption band in cm-1. Calculated value of transition dipole moment and
oscillator strength are listed in the table 1, which designate that the transition So → S1
is strongly allowed.
The empirical Dimroth polarity parameters, ET (30) and ETN of DPI chromophore were
calculated according to the following equations [23].
ET (solvent) 30.7
ETN 
(7)
32.4
28591
ET (solvent) 
(8)
λmax
where (λmax) corresponds to the peak wavelength (nm) in the red region of the of the
intramolecular charge transfer absorption of DPI chromophore. The red
(bathochromic) shift from n-Hexane to DMSO designates that the photoinduced
intramolecular charge transfer (ICT) arises in the singlet state, and the polarity of DPI,
chromophore therefore, increases on excitation.
3.3.Fluorescence quantum yield
The fluorescence quantum yield (Φf) of the DPI chromophore was calculated in
different solvents with the reference of fluoresce dye solution (Φf 0.95 in 0.1 NaOH) as
stranded dye. The following equation (Eq. 9) has been applied for the calculated of
fluorescence quantum yield (Φf).
2
I x A x n
r
Ir x Axn

 
(9)
r
f
2
r
8

Where Φf is the fluorescence quantum yield of DPI chromophore, Φr is the fluorescence
quantum yield of the reference dye (fluoresce), A is the absorbance at the excitation
wavelength, I is the integrated fluorescence intensity and n is the refractive index of the
solvent. The subscript r refers to the reference fluorescence of a known quantum yield
[24].
The fluorescence quantum yields (Φf) of DPI chromophore have been calculated in
different solvents and the values are mentioned in the table 1. The polarity of the
solvents strongly affected on the value of the fluorescence quantum yields of DPI
chromophore. Additionally, the fluorescence quantum yield of DPI chromophore in
different solvents and their relationships with the ET(30) of the various solvents are
shown in Fig 6, where ET(30) is the solvent polarity factor recognized by Reichardt. As
presented in the table 1, the fluorescence quantum yield of DPI chromophore increases
with increasing polarity of the solvent; which increases from 0.40 in n-Hexane to 0.68
in DMSO (a non-polar solvent to highly polar solvent). This result is associated to the
prevalence of negative solvatokinetic effect (n-Hexane to DMSO) [25]. One main
reason for the negative solvatokinetic (the increase of Φf is accompanied by a
appropriate enhancement of intramolecular charge transfer) could be due to the
biradicaloid charge transfer involving the un-bridged double bonds and the other cause
could be related to the proximity effect for a chromophore with n-π* and π-π* electron
configuration. In other words, in non-polar solvents, these effects will result in effective
non-radiative decay of the excited states.
3.4.Effect of surfactant on emission spectrum of DPI
9

Due to pyrene carboxaldehyde well-known probes for the deamination of the CMC of
CTAB, pyrene containing imidazole (DPI) can be applicable to determine the CMC of
the surfactants. Two surfactants namely sodium dodecyl sulphate (SDS) and cetyl
trimethyl ammonium bromide (CTAB) surfactants as an anionic and cationic surfactant
were chosen for estimated the emission behavior of the DPI chromophore. The two
specified surfactants were selected because the ionic charges possessed by DPI
chromophore can be influenced by the negatively charged SDS and the positively
charged CTAB. Thus, the charge attraction accounts for the DPI chromophore
fluorescence behavior. The fluorescence emission spectra of DPI chromophore in the
absence and presence of SDS and CTAB were measured. The fluorescence intensity of
DPI chromophore decreases with an increase of the SDS concentration (2 x 10-3 up to
1.8 x 10-2 M). Moreover, reductions that are more significant, noticed in fluorescence
intensities of DPI with SDS. The quenching of emission intensity of DPI chromophore
upon increasing SDS concentration can likely be recognized to the association of DPI
chromophore with SDS. It can be observed that there was a successive decrease in the
relative fluorescence intensity of DPI chromophore with an increase in the SDS
concentration, strongly providing that there was an interaction between DPI and SDS
Fig. 7. The emission intensity of DPI chromophore increases with increasing the
concentration of CTAB from 2 x 10-4 up to 1.8 x 10-3 M. Such enhancement in the
emission intensity of 1 x 10-5 M DPI chromophore at fixed concentrations with an
increase in the CTAB concentration may likely be ascribed to the association
mechanism of DPI chromophore with CTAB Fig. 8. It seems that the DPI dye molecule
are located in the hydrocarbon core of CTAB aggregates, while in SDS they are located
at the micelle-water interface, with quenching role of water [26]. The emission intensity
of DPI chromophore decreases with increasing concentration of surfactant SDS and
10

increases with increasing concentration of surfactant CTAB, with an abrupt change in
emission intensity occurring at surfactant concentration of 6.79 x 10-3 and 9.51 x 10-4
mo dm-3 which are very close to the critical micelle concentration (CMC) of SDS and
CTAB (Fig. 9 and 10) [27]. Thus, DPI chromophore can be employed as a probe and
quencher to determine the CMC of the surfactants.
3.5. Fluorescence quenching of DPI with ethylene glycol
The fluorescence quenching of DPI chromophore in dioxan was studied by using polar
protic solvent ethylene glycol as quenchers Fig. 11. As follows from these figure, the
fluorescence spectra undergo very complex changes on adding different concentration
of ethylene glycol, i.e., the emission intensity of DPI chromophore decreases with
increasing the concentration of ethylene glycol and also shifted to longer wavelength,
possess changed half widths and band profiles of the emission spectrum. This behavior
indicates, that in such a solution an extra factor contributes to the well-known dipole-
dipole interaction, i.e., hydrogen-bonding interactions between the ethylene glycol
molecule and imidazole group of DPI chromophore, not only in the molecule and
ethylene glycol, not only in the S0 state, but also in the excited, S1, state [28]. The Stern–
Volmer constants (KSV) was calculated from the Stern– Volmer plots shown in Fig 12.
The KSV constant was determined as 0.40 M-1 in ethylene glycol. The dependence of
fluorescence characteristics on ethylene glycol properties suggest a potential
application of DPI chromophore to probe of the polarity and hydrogen bonding
properties of its local microenvironment.
I0 / I f 1 Ksv [Q]
(10)
11

where Io and If are the relative integrated fluorescence intensities without and with the
quencher concentration [Q] and Ksv (Stern-Volmer constant).
3.6.DFT Calculations
The molecular structure of DPI chromophore was optimized using the long-range
corrected (LC) column-attenuating (CAM) method of the hybrid Becke's three
parameter Lee-Young-Parr correlation functional (CAM-B3LYP) [29] of the density
functional theory (DFT) with double-zeta and polarization functions on heavy atoms
basis set [6-31+G*]. A global minimum on the potential energy profile of DPI was
located and evidenced by the absence of any imaginary vibrational wavenumber. Time-
Dependent density functional theory (TD-DTF) [30] and the polarizable continuum
model (PCM) method [31] with the 6-31+G* basis set were applied to simulate the
Ultra-Violet spectra of DPI in n-Hexane (C6H12), chloroform (CHCl3), tetrahydrofuran
(THF), ethanol (C2H5OH), methanol (CH3OH), acetonitrile (CH3CN) and dimethyl
sulphoxide (DMSO). All these calculations were performed using the Gaussian09
Suites of program [32] together GaussView [33] for monitoring the geometry of DPI
chromophore. Version 3.1 of natural bond orbital (NBO) program [34] was applied to
compute the electric charges and hyperconjugative energies of DPI chromophore.
3.7.Geometry
Table 2 lists a few bond lengths and angles of the optimized structure of the gas-phase
molecule DPI chromophore that is depicted by Fig.13. They are computed by using
CAM-B3LYP/6-31+G* level of theory. It is worth extracting some geometrical
remarks from Table 2 in conjunction with Fig.13: (1) the pyrene and imidazole rings
are out-of-plane by ca. 37O as shown by the dihedral angle N5-C1-C7-C8, (2) the
dihedral angles N5-C4-C32-C33 and C4-C3-C43-C45 indicate that the two benzene
rings are off-plane with the imidazole ring by ca. 30O and 44O, respectively, (3) All
12

carbon-carbon bonds are doubly bonded or partially multiply bonded. This feature does
not exclude even C1-C7, C3-C4, C3-C43 and C4-C32 bond lengths being shorter than
expected by 0.064, 0.065, 0.065 and 0.061Å, respectively, compared to the typical C-
C single bond of ethane [35], (4) the computed bond angles of 105O-129O show clearly
the sp2 hybridization scheme over the entire macromolecule.
3.8.UV-Visible Spectra
The π→π* and n→π* electronic transitions from the higher occupied molecular orbitals
(HOMO) to the lower unoccupied molecular orbitals (LUMO), collectively called
frontier molecular orbitals (FMOs) lead to UV-Vis. absorption spectra of π-conjugated
organic molecules [36]. The macromolecule DPI has many double bonds and lone pairs
on the nitrogen atoms. Table 3 lists the experimental UV-Vis. absorption and emission
electronic transitions of DPI chromophore in different solvents as mention in table 3
together with their theoretical peers computed by using PCM/TD-CAM-B3LYP/6-
31+G* level of theory. Overall, excellent agreement between the experimental and
simulated absorption and emission wavelength maxima is obtained. On the one hand,
the absorption maximum wavelengths are theoretically underestimated. The
discrepancy between the experimental and estimated maximum wavelengths increases
from ca. 7nm in n-Hexane to ca. 24nm in DMSO. On the other hand, the emission
wavelength maxima are overvalued compared to their measured analogues. The
variance between them does not exceed ca. 25nm. It is worth mentioning that the
estimated maximum absorption wavelengths are gradually blue-shifted while the
emission wavelength maxima are progressively red-shifted with the increase of solvent
polarity. The non-linear dependence of solvatochromic shifts of the absorption maxima
and their oscillator strengths on the dielectric constant of the solvents could be
explained in terms of the intermolecular hydrogen bonds between solute and solvent
13

[37]. The interaction between the solvent and solute could explain the slight fluctuations
in the computed absorption wavelength maxima [38].
In Table 4 are registered the property parameters [39] of the solvents used for the
solvation of DPI chromophore together with the calculated absorption maxima in these
solvents. It is clear that the calculated absorption wavelengths do not correlate with the
solvent polarity scale [ET(30)] as the later does not include the hydrogen-bond acceptor
(HBA) beside both the hydrogen–bond donor (HBD) and solvent dielectric constants
[40]. Likewise the solvent polarity parameter (Δf) alone is not adequate for pinpointing
the solvatochromic behaviour of DPI in the elected solvents. These two observations
indicate that specific solute-solvent interactions prevail in the solvation of DPI
chromophore [41]; which necessitate the use of a multiparametric solvatochromic scale
of Kamlet and Taft [42,43].
As shown in Table 4, the absorption wavelength is blue-shifted of 3.2nm in going from
a nonpolar n-hexane (α=0.00, β=0.00, π*=-0.4) to the acetonitrile (α=0.19, β=0.40,
π*=0.75) as the latter being highly polar and a good HBA, on the one hand. On the
other hand, a blue shift of 3.4nm occurs in going from n-hexane to methanol (α=0.98,
β=0.66, π*=0.60) as a result of a large HBD ability and moderate HBA and polar
abilities. The anomalous solvatochromic behaviour of DPI chromophore in DMSO
(α=0.00, β=0.76, π*=1.00) is attributed to its high ability as a HBA together with its
relatively high polarity.
The mapped electrostatic potential for excited and ground states of the DPI
chromophore applying HF/321G level of theory is depicted in Fig.14. The figure shows
clearly that around the CNC moiety (acceptor) an intensive negative charge is built up;
while around the CNHC group (donor) a denser positive charge is intensified in both
the ground and excited states. These theoretical judgments support our experimental
14

findings that DPI chromophore could be used for the determination of the critical
micelle concentrations of both SDS and CTAB surfactants.
3.9.Frontier Molecular Orbitals (FMOs)
Fig.15 depicts the gas-phase DPI frontier molecular orbitals (FMOs). CAM-B3LYP/6-
31+G* level of theory was applied to compute them. On the one hand, the HOMO is
localized mainly over the imidazole ring and the two adjacent phenyl groups as π-
bonding orbitals (donor orbital). On the other hand, the LUMO extends entirely over
the pyrene ring as π*-antibonding orbital (acceptor orbital). This situation facilitates the
intramolecular charge transfer (ICT) within the macromolecule. A detailed description
of the ICT will be presented in a separate section devoted to natural bond orbital (NBO)
analysis.
Table 5 lists the HOMO and LUMO energies of the solvated DPIs, together with their
energy gaps (E.G.). The later parameters could be used as measures for the
intermolecular solute-solvent charge transfer. As Table 4 shows energy gaps of 5.648-
5.681eV which could easily ease the π→π* and n→π* charge transfer and, therefore,
could evidently generate UV-Vis. spectra. It is noteworthy that the energy gaps of the
solvated DPI chromophore a directly related to their nature (protic or aprotic) and/or
polarities, i.e. their dielectric constants. These effects steadily stabilize both the
HOMOs and LUMOs by varying degrees and, therefore, result in incremental small
increases of the energy gaps.
The electronic chemical potential (µ) that indicates the motion of electrons in a
chemical species [44], the chemical hardness (η) that examines the stability and
reactivity of a chemical species [45] and the global electrophilicity index (ω) that
estimates the stabilizing energy when a chemical species accepts additional electronic
charge from the medium [46] are listed in Table 5. As Table 5 shows the magnitudes
15

of η and µ indicate that DPI chromophore in DMSO is the hardest, most stable and least
reactive amongst the solvated chromophore, while being the softer, least stable and
most reactive in n-Hexane. In addition, the ω values show that the solvation of DPI by
DMSO fabricates the strongest electrophile amongst the solvated DPIs, compared to
being the weakest electrophile (or a nucleophile) when solvated by n-Hexane [47].
3.10. Natural Bond Orbital Analysis
The Natural Bond Orbital (NBO) analysis [48] has gained wide endorsement in dealing
with hyperconjugative interactions [49]. This is accomplished through analyzing
second order perturbation energies (E(2)) given by the relation:
E(2) = ∆Eij = qi (Fij)2/∆ε
(11)
where Fij is the NBO Kohn-Sham off-diagonal matrix elements, qi is the occupancy of
the donor orbital(i), and ∆ε is difference between the energies of the donor orbital (i)
and an acceptor orbital (j). Table 6 registers the second order perturbation (E(2))
hyperconjugative energies of DPI chromophore in the gas-phase that trace the charge
transfer from the donor imidazole ring to the pyrene ring acceptor. They were computed
by employing HF/321G/CAM-B3LYP/6-31+G* level of theory. They are classified as
π→π*, σ→σ* and n→π* electronic charge transfer interactions. The π→π* and n→π*
hyperconjugative interactions are most effective and availed totals of 174.24 and 148.61
kcal/mol, respectively; whereas the σ→σ* transitions are extremely weak and
contributed only 18.04 kcal/mol to the stabilization of the DPI chromophore. The DPI
chromophore substrate intramolecular weak charge transfers from the imidazole moiety
toward the phenyl groups are shown by the σC4-N5→σ*C32-C34, σC4-N5→σ*C3-C43, πC3-
C4→π*C32-C34 and πC3-C4→π*C43-C44 interactions which stabilized DPI chromophore by
1.44, 4.66, 11.72 and 9.26 kcal/mol, respectively. Comparatively, large charge is also
transferred from the imidazole ring at the pyrene moiety. These are symbolized by the
16

πC1-N5 →π*C7-C9, πC7-C9 →π*C8-C10, πC8-C10 →π*C14-C19 and πC14-C19 →π*C20-C24 transitions
which availed 8.64, 36.46, 31.62 and 37.99 kcal/mol to the stabilization of the studied
macromolecule. These results are in excellent agreement with our observed UV-Vis.
spectra and are properly correlated with our theoretical predictions mentioned earlier.
4. Conclusion
Novel 4, 5-diphenyl-2-(pyren-1-yl)-1H-imidazole (DPI) chromophore was prepared by
one-pot equimolar reaction of benzil, pyrene-1-carboxaldehyde and ammonium acetate
in acetic acid. Structure of the synthesized DPI chromophore was confirmed by
spectroscopic techniques and elemental analysis together with DFT study,
Experimental and theoretical absorbance and emission spectra of DPI chromophore
showed bathochromic shifts with increasing polarities of the solvents (n-Hexane to
DMSO) to gather with the polar protic and aprotic solvents. Photophysical parameters
such as extinction coefficient, dipole moment, stokes shift, oscillator strength and
fluorescence quantum yield of the DPI chromophore was estimated. Its affected by the
polarities and types of the solvents. The DPI chromophore can be use as probe or
quencher to determine the critical micelle concentration of SDS and CTAB surfactants.
Acknowledgments
The authors are thankful to the King Abdulaziz City for Science and Technology
(KACST) for providing the financial support for the work under Grant no 1-17-01-009-
0046.
17

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22

Scheme 1: Synthesis of 4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole (DPI)
23

Table 1. Spectral data and fluorescence quantum yield (f) of dye (DPI) in different
solvents
ETN
Solvent
ET (30)
Kcal
ε
f
μ 12
Debye
Φf
 st
(cm-1)
f
ab(nm) em(nm)
M -1cm-1
mol-1
DMSO 0.263
0.441
45.1
370
365
470
27290 0.63
7.02
5751 0.68
DMF 0.274
EtOH 0.288
MeOH 0.308
0.404
0.654
0.762
0.259
43.8 368
51.9
55.4 366
39.1
45.6
36
37.4
31.1
31.1
466
450
453
453
461
448
222
439
27430 0.62
24000 0.49
22500 0.47
24100 0.54 6.43
22600 0.53
23250 0.51
40000 0.44 5.76
19400 0.42
19080 0.41
6.95
6.15
6.03 5421
5714 0.56
5715 0.61
0.68
5625 0.53
5555 0.54
5511 0.48
5551 0.43
5469 0.40
5497 0.38
CHCl3
0.148
361
367
358
356
354
CH3CN 0.304 0.472
Dioxan 0.021 0.164
THF 0.10
6.40
6.21
0.210
0.006
0.006
n-Hexane 0.0014
n-Heptane 0.0004
5.61
5.47
353
438
24

Table 2: Selected bond lengths (Å), bond angles and dihedral angles (degrees) of DPI
which have been estimated by applying CAM-B3LYP/6-31+G* level of theory.
Parameter
C1-C7
Bond length
1.471
Parameter
N5-C1-C7
N6-C1-C7
N5-C1-N6
Bond Angle
128.5
C1-N5
C1-N6
1.315
1.367
121.4
110.1
N6-C3
C3-C4
C4-N5
1.379
1.382
1.376
1.470
1.474
1.399
1.400
C1-N5-C4
C1-N6-C3
N5-C4-C3
107.0
108.5
109.8
104.7
-36.8
30.1
C3-C43
C4-C32
C32-C34
C7-C9
N6-C3-C4
N5-C1-C7-C8
N5-C4-C32-C33
C4-C3-C43-C45
43.7
25

Table 3: The experimental UV-Vis. maximum absorption (λabs/nm) and emission
(λem/nm) wavelengths for DPI in different solvents compared with the simulated ones
and oscillator strengths applying TD-CAM-B3LYP/6-31+G* level of theory. Dielectric
constants of solvents were also included as estimates of the polarity of solvents.
Solvent
Absorption Wavelengths
Emission Wavelengths Dielectric
Constant
λexpt.
354
362
356
365
366
367
370
λtheor.
346.5
345.8
344.8
343.6
343.1
343.3
344.4
f
1.040
1.052
1.033
1.011
0.998
1.003
1.032
λexpt.
440
451
444
450
453
461
470
λtheor.
f
n-Hexane
CHCl3
440.7 1.329
460.7 1.499
466.6 1.544
474.7 1.601
475.6 1.607
475.8 1.609
476.4 1.613
1.9
4.8
7.6
24.5
32.7
37.5
46.7
THF
C2H5OH
CH3OH
CH3CN
DMSO
26

Table 4: The property parametersa of the solvents used to solvate DPI showing the
polarity ET(30), the solvent polarity function (Δf), the hydrogen-bond donor (HBD)
ability (α), the hydrogen-bond acceptor (HBA) ability (β), and polarity/polarizability
(π*) that affect the absorption wavelengths (λtheor.) calculated by using TD-CAM-
B3LYP/6-31+G* level of theory.
Solvent
ET (30)
Kcal mol-1
31.1
Δf
α
β
π*
λtheor.
n-Hexane
CHCl3
0.0014
0.148
0.210
0.288
0.308
0.304
0.263
0.00
0.20
0.00
0.86
0.98
0.19
0.00
0.00 -0.40
346.5
345.8
344.8
343.6
343.1
343.3
344.4
31.7
37.4
51.9
55.4
45.6
45.1
0.10
0.55
0.75
0.66
0.40
0.76
0.58
0.58
0.54
0.66
0.75
1.00
THF
C2H5OH
CH3OH
CH3CN
DMSO
a: Marcus, Y. The properties of organic liquids that are relevant to their use as solvating
solvents. Chem. Soc. Rev. 1993, 22, 409–416
27

Table 5: The HOMO (eV), LUMO (eV) the energy gap (E.G./eV), the electronic
chemical potential (µ/eV), the chemical hardness (η/eV), the global electrophilicity
index (ω/eV), and the Dipole Moments (D.M./Debye) for the ground state DPI in
different solvents. They have been estimated applying CAM-B3LYP/6-31+G* level of
theory.
Parameter Gas Phase
n-Hexane
-6.585
-0.937
5.648
-3.761
2.824
CHCl3
-6.645
-0.975
5.670
-3.810
2.835
THF
-6.661 -6.683
-0.987 -1.002
5.674
-3.824 -3.843
2.837
2.577
C2H5OH CH3OH CH3CN DMSO
HOMO
LUMO
E.G.
-6.513
-0.901
5.612
-3.707
2.806
-6.685
-1.004
5.681
-3.845
2.841
-6.686
-1.005
5.681
-3.846
2.841
-6.687
-1.006
5.681
-3.847
2.841
5.681
µ
2.841
2.599
η
2.449
2.504
2.560
2.602
2.603
2.605
ω
D.M.(g)
D.M.(ex)
Diff.
3.205
---
---
3.837
3.418
0.419
4.446
4.045
0.401
4.624
4.233
0.391
4.863
4.483
0.380
4.889
4.510
0.379
4.896
4.518
0.378
4.914
4.537
0.377
28

Table 6: The second order perturbation (E(2)) estimation of the hyperconjugative
energies (kcal/mol) of DPI that follow that charge transfer from the donor imidazole
ring to the pyrene and phenyl rings acceptor. They were calculated using CAM-
HF/321G/B3LYP/6-31+G* level of theory.
Interaction
Energy
11.43
27.12
8.64
36.46
31.62
37.99
11.72
9.26
Interaction
Energy
3.13
4.87
3.94
1.44
πC43-C44 →π*C3-C4
πC3-C4 →π*C1-N5
πC1-N5 →π*C7-C9
πC7-C9 →π*C8-C10
πC8-C10 →π*C14-C19
πC14-C19 →π*C20-C24
πC3-C4 →π*C32-C34
πC3-C4 →π*C43-C44
σC1-N5 →σ*C1-C7
σC1-C7 →σ*C7-C8
σC1-C7 →σ*C7-C9
σC4-N5 →σ*C32-34
σC4-N5 →σ*C3-C43
n1N5→σ*C1-N6
n1N6→π*C1-N5
n1N6→π*C3-C4
18.04
174.24
4.66
10.72
81.64
56.25
148.61
29