4-(4,5-Diphenyl-1H-imidazole-2-yl)phenol: Synthesis and Estimation of Nonlinear Optical Properties using Z-Scan Technique and Quantum Mechanical Calculations

Abstract

In this study, 4-(4,5-Diphenyl-1H-imidazole-2-yl) phenol is successfully synthesized, and its nonlinear optical properties (NLO) are investigated both experimentally and theoretically. Theoretical investigations have been done by using TD-DFT and B3LYP functional with usual 6-31+G(d,p) basis set. The results of HOMO-LUMO and NBO analysis show the low energy gap, high total dipole moment, and hyperpolarizabilities (β, γ) as well as the presence of dipolar excited states with relatively significant dipole-moment changes which are linked to the nonlinearity. The z-scan technique confirmed the NLO properties of title compound. The nonlinear absorption coefficient, refractive index, and third-order susceptibility were found to be 4.044 × 10 −1 cmW −1 , 2.89 × 10 −6 cm 2 W −1 and 2.2627 × 10 −6 esu, respectively. The negative sign of n 2 indicated the occurrence of self-defocusing nonlinearity. The results show that the title compound can been used as potential NLO material.

Full text

170 Acta Chim. Slov. 2021, 68, 170–177
Mostaghni: 4-(4,5-Diphenyl-1H-imidazole-2-yl)phenol: ...
DOI: 10.17344/acsi.2020.6299
Scientic paper
4-(4,5-Diphenyl-1H-imidazole-2-yl)phenol:
Synthesis and Estimation of Nonlinear Optical
Properties using Z-Scan Technique and Quantum
Mechanical Calculations
Fatemeh Mostaghni
Department of Chemistry , Payam Noor University, P.O. BOX 19395-4697 Tehran, Iran
* Corresponding author: E-mail: mostaghnif@yahoo.com
Received: 10-26-2020
Abstract
In this study, 4-(4,5-Diphenyl-1H-imidazole-2-yl) phenol is successfully synthesized, and its nonlinear optical properties
(NLO) are investigated both experimentally and theoretically. eoretical investigations have been done by using TD-
DFT and B3LYP functional with usual 6-31+G(d,p) basis set. e results of HOMO-LUMO and NBO analysis show the
low energy gap, high total dipole moment, and hyperpolarizabilities (β, γ) as well as the presence of dipolar excited states
with relatively signicant dipole-moment changes which are linked to the nonlinearity. e z-scan technique conrmed
the NLO properties of title compound. e nonlinear absorption coecient, refractive index, and third-order suscepti-
bility were found to be 4.044 × 10−1 cmW−1, 2.89 × 10−6 cm2W−1 and 2.2627 × 10−6 esu, respectively. e negative sign
of n2 indicated the occurrence of self-defocusing nonlinearity. e results show that the title compound can been used
as potential NLO material.
Keywords: Nonlinearity; z-scan technique; hyperpolarizabilities; triaryl imidazoles.
1. Introduction
e development of photonic technology, nonlinear
optical materials have received widespread attention both
from the research as well as industrial point of view.1–3 In
recent years, many studies have been reported by research-
ers to nd new compounds with high nonlinear optical
properties. e essential requirements of suitable photonic
materials are their high nonlinearity, fast response time,
chemical stability, and ease of molecular design.4–8
In this context, π-conjugated organic materials have
received more attention due to their high nonlinearity and
fast response times resulting from the ease of polarizability
of the extended mobile π-electron clouds across the mole-
cule.9–13
Unlike inorganic materials in which band structure
phenomena cause nonlinear phenomena, in organic ma-
terials and polymers, these phenomena arise from the
transition of an electron from the highest occupied mo-
lecular orbital (HOMO) to the lowest unoccupied mo-
lecular orbital (LUMO) that caused a transition of the
dipole moment from the ground state to the excited
state.14–18
Dierent types of organic compounds with extensive
conjugated π system are expected to exhibit nonlinear op-
tical properties because of π–π interactions that allow an
intramolecular charge transfer (ICT).19–29 Moreover, ICT
is responsible for the broadening of the absorption spec-
trum, and the reduction of the optical bandgap.30–32
Over the past two decades, small organic molecules
have been a subject of increasing research interest for their
potential applications in organic electronics.33 Further-
more, it was shown that the π-conjugated bridges based on
heterocyclic rings had improved stability relative to other
polyenes.34–37
Among these, various types of imidazole derivatives
have received widespread attention due to their piezoelec-
tric, photochromic, and thermochromic properties. ey
are widely used in optoelectronics, superconductors, molec-
ular photonics, sensors, and optical data storage devices.37–39
In this study, 4-(4,5-Diphenyl-1H-imidazole-2-yl)
phenol was synthesized by one-pot three-component syn-

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thesis using CoFe2O4. en for the rst time, the nonlinear
optical properties of this molecule are measured by the
Z-scan technique and quantum mechanical methods. e
results showed that the title compound could be a good
candidate with the potential application in optoelectronic
devices.
2. Experimental
All chemicals were analytical grade and purchased
from Sigma-Aldrich. Deionized water served as reacting
medium. e melting point was measured by an Electro-
thermal-9200 melting point apparatus. IR spectra were
measured on the FTIR-6300 spectrometer (KBr). 1HNMR
spectra were recorded on Bruker ADVANC DRX 400
spectrometer, using DMSO as solvent.
2. 1. Synthesis of 4-(4,5-Diphenyl-1H-
imidazole-2-yl) Phenol
To the solution of benzoin (1 mM), 2-hydroxy benz-
aldehyde (1 mM), ammonium acetate (4mM) in ethanol
(10 mL) was added 5 mol% CoFe2O4 nanocatalyst. e
mixture was reux in 50 °C, and the progress of the reac-
tion was controlled by TLC using the mixture eluent
(n-Hexane: Ethyl acetate 4:1). Aer completion of the re-
action (30 minutes), the catalyst was separated by an exter-
nal magnet, and the reaction mixture was allowed to cool.
en the precipitate was collected by ltration, washed
with water, and recrystallized using ethanol.
e product (gure 1) was characterized by FTIR
and 1H NMR and Mass spectroscopy. Yield 87%; purity >
96%; mp: 278 °C (275–276 °C).40
sample was dissolved in DMF and measured in quartz cell
in the range of 320 to 1000 nm (Figure 2). e spectrum
showed absorption peaks at 340 nm, and 406 nm.
2. 3. Z-Scan Measurement
e determining of the third-order nonlinear optical
properties of the sample was carried out using the z-scan
technique as a standard method. In this method, a very
thin sample of matter is exposed to a laser beam while the
sample was translated across the focal zone along the z-ax-
is. e light passing through the sample is recorded as a
function of the sample position relative to the beam focal
point. e Z-Scan method is used to determine the non-
linear refractive index and absorption coecient in close
and open aperture congurations, respectively.
2. 4. Computational Method
I performed density functional theory (DFT) and
time-dependent DFT (TD-DFT) calculations on the title
compound using the Gaussian 09 suite of soware.41 e
calculations were carried out by using TD-DFT and B3LYP
functional.42,43 e usual 6-31G+(d,p) basis set was em-
ployed in the calculations. Numerous quantities including
molecular structures, ionization potentials, electronega-
tivity, HOMO-LUMO energies, and the HOMO-LUMO
energy gap, dipole moments, polarizability, and hyperpo-
larizability Have been measured and discussed.
3. Results and Discussion
3. 1. UV-Vis Absorption Spectrum
As can be seen in Figure 2, this sample has shown the
absorption bands in the UV-Vis region between 340–470
nm. e absorption observed at 406 nm due to the π→π*
transition. e wide transparency of the sample in the vis-
Figure 1. Molecular structure of 4-(4,5-Diphenyl-1H- imidazo-
le-2-yl) phenol
IR (KBr, cm−1): ν/cm–1 : 3598, 3445, 2996, 2468,
1643, 1613, 1546, 1506, 1490, 1240, 764, 698, 1H NMR:
δH(ppm) (300 MHz, CDCl3): 8.03 (s, 1H, NH), 7.63–7.66
(m, 2H), 7.43–7.45 (m, 2H), 7.22–7.26 (m, 4H), 7.07–7.09
(d, 2H), 6.77–6.86 (m, 4H), GC-MS (IE, 70 eV) m/z (%)
312 (M+, 100.0), 208, 165, 89, 77, 51, 41.
2. 2. UV-Vis Absorption Spectrum
e UV-Vis absorption spectrum of the sample was
recorded using a spectrometer (Jenway model 6310). e
Figure 2. UV-Vis absorption spectra of 4-(4,5-Diphenyl-1H- imi-
dazole-2-yl) phenol

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ible region enables it for the second harmonic generation
that required for all NLO materials.
e band gap energy was determined using the
Tauc relation.44 e band gap value obtained using the
direct transition. Extrapolating the linear part of the
curves to the X axis yield the bandgap equal to 2.54 eV
(Figure 3).
coecient resulting from the two-photon absorptions
(TPA) whereby a molecule simultaneously absorbs two
photons that are inherently weak at low intensities of
light.
e nonlinear absorption coecient β was calculat-
ed by the following equations:45
(1)
(2)
(3)
(4)
Where I0 is peak on-axis irradiation at the focal
point, Z is the sample position at the minimum transmit-
tance, Z0 is diraction length, T is the total transmittance,
Le is the eective thickness of the sample and β is NLA
coecient. e nonlinear absorption coecient is tabulat-
ed in table 1.
3. 3. Nonlinear Refractive Index
e sign and magnitude of the nonlinear refractive
index, and real part of χ(3) were determined from the
closed aperture z-scan data. e normalized transmittance
of the sample as a function of distance from the focus point
is plotted in Figure 5.
As gure 5 shows a peak-valley conguration, the
nonlinear refractive index is negative and indicates self de-
focusing nonlinearity.
Figure 3. Plot (αhv)2 vs. photon energy
3. 2. Nonlinear Absorption Coecient
e nonlinear absorption coecient, the nonlinear
susceptibility, and imaginary part of χ(3) were determined
from open aperture z-scan data. For this purpose, the nor-
malized transmittance of open aperture Z-scan at wave-
lengths 532 nm was plotted as a function of sample posi-
tion (Figure 4).
Figure 4. Open aperture Z-Scan data for 4-(4,5-Diphenyl- 1H-imi-
dazole-2-yl) phenol solution.
As can be seen from Figure 4, the curve shows a
valley shape, indicating a positive nonlinear absorption
Figure 5. Closed aperture Z-Scan data for 4-(4,5-Diphenyl- 1H-im-
idazole-2-yl) phenol solution

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However, ∆Tp-v is related to the nonlinear refractive
index n2 by the following equations:46
(5)
(6)
(7)
In the above equation, ∆Tp-v is the distance between
the peak and valley transmittance which is obtained from
gure 5, I0 is the intensity in the focused sample, ra is the
radius of aperture, ωa is the radius of the beam at the aper-
ture, S is the linear transition, and n2 is the nonlinear re-
fractive index.
Besides, changes in the induction reectance index
were determined by the following equation.
(8)
3. 4. ird Order Susceptibility χ(3)
e real and imaginary parts of the third-order sus-
ceptibility are related to the nonlinear refractive index n2
and the nonlinear absorption coecient β respectively.
Where the imaginary part of the third-order suscep-
tibility is calculated from the nonlinear absorption coe-
cient using the following equation.
(9)
Moreover, the real part of the third order susceptibil-
ity can be obtained from the nonlinear refractive index
through the relationship:
(10)
In the above equations, β is the nonlinear absorption
coecient, n2 is the nonlinear refractive index, n0 is the
linear refractive index, c is the speed of light in the vacuum
and ε0 is the permeability coecient in the vacuum.
However, the absolute value of nonlinear third-order
susceptibility can be obtained by the following equation.
(11)
e values of the third-order nonlinear susceptibil-
itiies obtained for 4-(4,5-Diphenyl-1H-imidazole-2-yl)
phenol are listed in table 1.
e high values of negative nonlinear refractive in-
dex and the third-order susceptibility of the sample, which
are associated with a 2PA resonance enhancement, indi-
cate that it can be potentially used as an optical limiter to
protect tools and human eyes.
3. 5. Electronic Structure and One-Photon
Absorption
Electronic properties such as ionization potential
(IP), hardness (η), soness (S), and electron anity (EA),
can be evaluated from HOMO and LUMO energies.
HOMO and LUMO energies, Band gap energy, hardness,
soness ionization potential (IP), and electron anity
(EA) are summarized in Table 2.
Quantum mechanical calculations play an important
role in the understanding of the relationship between the
molecular structure and the nonlinear optical properties
of the compounds. ere are many factors contribute to
enhancing the properties of NLO compounds such as: low
bandgap energy, high dipole moment, reversing of ground-
state charge distribution and the π-electronic cloud redis-
tribution via the π-conjugated system.47 In conjugated or-
ganic materials, electrons in π bond are delocalized and
have more motions rather than other electrons. In this case
the π-bond electrons can easily move in the whole mole-
cule space. Increasing the electron charge distribution will
result in a larger hyperpolarizability, which is linked to the
nonlinearity.
e orbitals involved in the main transitions are
shown in gure 6. As can be seen, the HOMO orbital
is delocalized over the whole molecule. By contrast,
the LUMO is mostly located over two phenyl rings 1
and 2.
Table 1. Calculated third order nonlinear optical parameters of the
title compound
n2 × 10–6 β × 10–1 Re (χ(3)) × 10–3 Im (χ(3)) × 10–5 χ(3) × 10–6
cm2/W cm/W esu esu esu
–2.89 4.044 –1.5016 8.89 2.2627
Table 2. eoretically computed HOMO and LUMO energies, Bond gap energy (Egap),
Hardness (η), soness (S), Ionization potential (IP), Electron anity (EA)
EHOMO ELUMO η S IP EA Egap
eV eV eV eV KJ/mol KJ/mol eV
–4.9980 –1.8098 1.594 0.314 482.225 174.622 3.18

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Moreover, LUMO+1 is based more on the phenyl
ring 2 and phenolic ring while, LUMO+2 is mostly delo-
calized on the two phenyl rings. Consequently, electron
transition from ground to excited states facilitate an elec-
tron density transfer.
e short-circuit current density is an important
component of the photoelectric conversion eciency
(PCE) is determined by the light-harvesting eciency
(LHE).
e light-harvesting eciency (LHE) was approxi-
mately calculated from oscillator strength (f) using the fol-
lowing equation:48
LHE = 1−10−f (12)
As can be seen from table 3, both H→L and H→L+1
transitions dominantly dictate the electronic absorption
prole of the studied molecule with suciently high f-val-
ues. However, the value of excitation energy related to
H→L transition is comparable to the band gap value ob-
tained from the Tauc equation. By contrast, the oscillator
strength value of H→L+2 transition is too low to contribute
to the absorption spectra. As expected, the low energy
electronic excitations have substantial ICT character.
Both H→L and H→L+1 transitions with the high
LHE will have a high short-circuit current density and so
the high photoelectric conversion eciency.
e partially density of states (PDOS) spectra of the
title compound was also obtained using quantum mechan-
ical calculations. According to PDOS spectra diagram
(Figure 7), band gap was low which is in good agreement
with other results.
Figure 6. Frontier orbitals of 4-(4,5-Diphenyl-1H-imidazole-2-yl) phenol
Table 3. Excitation energies (Eex), oscillator strengths (f), light harvesting eciencies (LHE), and electronic
transitions congurations of the title compound at TD-DFT-B3LYP/6-31+G(d,p) level in DMSO
Excited State Eex f LHE Transition assignment
82 –> 83 (S) 2.9654 eV 418.10 nm 0.5678 0.7295 H→L (99.18%)
82 –> 84 (S) 3.3587 eV 369.14 nm 0.2601 0.4506 H→L+1 (96.24%)
82 –> 85 (S) 3.5015 eV 354.09 nm 0.0031 0.0071 H→L+2 (96.52%)
Figure 7. Calculated DOS spectra of 4-(4,5-Diphenyl-1H-imida-
zole-2-yl) phenol

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3. 6. Second-Order Nonlinear Optical (NLO)
Response
e linear isotropic polarizability indicates the ca-
pacity of changing the charge density in the system under
the inuence of an external eld. However, the magnitude
of the isotropic polarizability α and anisotropy of polariz-
ability (Δα) are calculated using the polarization compo-
nents as follows.49,50
(13)
(14)
e rst hyperpolarizability (β) , which is studied us-
ing second harmonic generation (SHG) is:
(15)
Also, the direction of charge transfer in the title com-
pound was determined by the ratio of βvec and βtotal using
the following equations:
(16)
where βvec is the vector component of rst hyperpo-
larizability.51
(17)
e second hyperpolarizability (γ), which is studied
using third-harmonic generation (THG) is:
(18)
e isotropic polarizability (α), the anisotropy of the
polarizability (∆α), the vector component of the rst hy-
perpolarizability and hyperpolarizabilities (β, γ) of the title
compound are listed in the table 4.
unidirectional charge transfer in the title compound.
erefore it is a good candidate for future studies of non-
linear optical properties.
4. Conclusion
I have synthesized 4-(4,5-Diphenyl-1H-imidaz-
ole-2-yl)phenol as an attractive material for potential ap-
plication in nonlinear optics. e nonlinear optical prop-
erties of the title compound are investigated using the
z-scan technique and quantum mechanical calculations.
Both theoretical and experimental results reveal that the
title compound exhibits large optical nonlinearity. e cal-
culation of the HOMO-LUMO energy gap showed that the
eventual charge transfer interactions occure within the
molecule. Furthermore, the high value of total static dipole
moment and hyperpolarizabilities (β, γ) were found for
the title compound, which was attributed to the positive
contribution of their conjugation. e calculated transi-
tion dipole moments for ground and excited states indicat-
ed an electron density transfer. Besides, the ratio of βvec/
βtotal indicated the unidirectional charge transfer in the ti-
tle compound. In summary, from all theoretical studies, it
was concluded that the title compound can use as potential
NLO molecule.
e theoretical results are conrmed by the nonlin-
ear refractive index, and the nonlinear absorption coe-
cient were determined by z-scan techniques. e magni-
tude and sign of the nonlinear refractive index (n2)
determined using close aperture z-scan. n2 was in the
range of 10–6 cm2/W. e negative sign of n2 indicated the
occurrence of self-defocusing phenomena due to the local
variation of the refractive index with temperature. e
measured nonlinear absorption coecient (β) by open ap-
erture z-scan was in the range of 10–1 cm/W associated
with the two-photon absorption (TPA) eect. Finally, the
physicochemical studies on the title compound revealed
the essential property of the title compound for applica-
tion in the eld of nonlinear optic.
Acknowledgments
e author are grateful to the Payame Noor Univer-
sity for encouragements.
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Povzetek
V prispevku opisujemo sintezo spojine 4-(4,5-difenil-1H-imidazol-2-il)fenol in eksperimentalne ter teoretične raziskave
njenih nelinearnih optičnih lastnosti. Teoretične raziskave so bile izvedene z uporabo funkcij TD-DFT in B3LYP z običa-
jno nastavitvijo 6-31 ++G(d,p). Rezultati analiz HOMO-LUMO in NBO kažejo nizko vrednost prepovedanega pasu,
visok dipolni moment in hiperpolarizabilnost (β, γ) kot tudi prisotnost dipolarnih vzbujenih stanj z razmeroma visokimi
spremembami dipolnega momenta, povezanimi z nelinearnostjo. Metoda z-skeniranja je potrdila NLO lastnosti spojine.
Nelinearni absorpcijski koecient, refrakcijski indeks in susceptibilnost tretjega reda znašajo 4.044 × 10−1 cmW−1, 2.89 ×
10−6 cm2W−1 in 2.2627 × 10−6 esu. Negativna vrednost n2 kaže na samo-defokusiranje in nelinearnost. Rezultati kažejo,
da bi spojino lahko uporabljali kot potencialni NLO material.
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