Impact of solvents on energy gap, photophysical, photometric properties for a new class of 4-HCM coumarin derivative: Nonlinear optical studies and optoelectronic applications

Article abstract of DOI:10.1016/j.molliq.2019.111383

Now a day the derivatives of biscoumarins are receiving overwhelming importance due to their application in designing organic semiconductor materials for display application. Therefore, a new photostable, light emitting and highly efficient synthesized ma

Full text of DOI:10.1016/j.molliq.2019.111383

Accepted Manuscript
Impact of solvents on energy gap, photophysical, photometric
properties for a new class of 4-HCM coumarin derivative:
Nonlinear optical studies and optoelectronic applications
A.G. Pramod, C.G. Renuka, Y.F. Nadaf, R. Rajaramakrishna
PII:
DOI:
Article Number:
Reference:
S0167-7322(19)31842-2
https://doi.org/10.1016/j.molliq.2019.111383
111383
MOLLIQ 111383
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Journal of Molecular Liquids
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Revised date:
Accepted date:
30 March 2019
12 July 2019
16 July 2019
Please cite this article as: A.G. Pramod, C.G. Renuka, Y.F. Nadaf, et al., Impact of solvents
on energy gap, photophysical, photometric properties for a new class of 4-HCM coumarin
derivative: Nonlinear optical studies and optoelectronic applications, Journal of Molecular
Liquids, https://doi.org/10.1016/j.molliq.2019.111383
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ACCEPTED MANUSCRIPT
Impact of Solvents on Energy gap, Photophysical, Photometric Properties
for a New Class of 4-HCM Coumarin Derivative: Nonlinear Optical
Studies and Optoelectronic Applications
Pramod A. G.1, Renuka C. G.1*, Nadaf Y. F.2, Rajaramakrishna R.3
1Department of Physics, Bangalore University, Bengaluru, India.
2Department of Physics and Material Research Centre, Maharani Science College for
Women Bengaluru India.
3 Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom
Rajabhat University, Nakhon Pathom, 73000, Thailand.
*Corresponding Email: renubub@gmail.com (Dr. Renuka C G)
Abstract
Now a day the derivatives of biscoumarins are receiving overwhelming importance due
to their application in designing organic semiconductor materials for display application.
Therefore, a new photostable, light emitting and highly efficient synthesized material to exhibit
display property and nonlinear optical active materials with good band gap energy, we have
synthesized 4-Hydroxy-3-[(4-hydroxy-2-oxo-2H-chromen-3-yl)(4-methoxyphenyl) methyl]-
2H-chromen-2-one (4-HCM) molecule via the simple catalytic method. The photophysical
property of studied molecule in various organic solvents having different polarity exhibited
considerably acceptable results. The results of quantum yield, lifetime and large Stoke shift
shows the compound is a good promising candidate for the laser dyes and light emitting diode
color filters. The energy band gap was determined by both experimentally and theoretically
which is found to be an average of 3.78 eV which is well matched to the semiconducting
materials band gap. The nonlinear optical absorption properties of the synthesized 4-HCM
molecule are investigated using Z-scan technique. Further, the optoelectronic properties were
investigated the CIE, color rendering index, color purity and color correlated temperature
results of the 4-HCM in all studied solvents revels that this compound exhibits blue color (CIE-
co-ordinates x = 0.14, y = 0.08) and is suitable for optoelectronic device applications.
Keywords: Coumarin; Band gap; Photostable; NLO material; Optoelectronic.
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1. Introduction:
Recent years, organic light emitting diodes (OLEDs) have fascinated significant
interest as potential low cost and environmental friendly components. Metal free organic
materials with donor-π-acceptor (D-π-A) motifs are recognized as one of the promising
strategies for improving organic light-emitting device performance. Due to this the organic
materials have received overwhelming importance in science and technology community
because of their incredible potential as bio-imaging agents, fluorescent probes, cheomosensors
and as an electroluminescent materials for organic materials, specially and particularly for
energy consumption and energy harvesting devices, such as organic light-emitting devices [1–
3], polymer light emitting diodes, thin-film transistors and photovoltaic cells. The basic
parameters that determine the performance of OLEDs in lighting systems and display
technologies are the cost of manufacturing, the power efficiency, and color chromaticity.
Thermally stable activated delayed fluorescence (TADF) attracts more attention due to its
ability to harvest triplet excitons to OLEDs. The stable blue color emission, low efficiency
fluorescent compounds, and devices are specifically attraction due to the usage of industrial
commercial products. Many efficient blue fluorescent OLEDs based on TADF materials have
been reported with high external quantum efficiencies (EQEs) over 20%.[4–6]. However, it is
under question if the potential of the blue TADF emitters have been fully materialized in terms
of efficiency. For example, 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN) which is known
as an efficient TADF material emitting sky-blue showed the EQE of 13.6% although 2CzPN
has a high photo-luminescence quantum yield (PLQY) over 85%.[4,7–10].
In addition to the improvement of the thermally activated delay fluorescence materials
with high photoluminescence quantum yield, good stability and good colour purity, a selection
of the proper host and charge transferring materials along with the device structures
optimization are essential to completely profit from the potential of the materials by
maximizing out coupling efficiency and achieving good electrical balance [11–13]. Bipolar
hosts, double emitting layers, exciplex forming and mixed hosts instead of hole or electron
transporting single hosts have been designed to get good energy balance without any electrical
reduction in conventional phosphorescent and fluorescent OLEDs [14–17]. Nowadays Organic
semiconductors have been also widely used in medicine and energy conversion [18]. The effect
of solvents should be understood in detail intending to understand and estimate the
performances of electronic devices in different media. Hence, the optical electronic behaviour
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of organic semiconductors can be fundamentally understood by two properties i.e., refractive
index and energy band gap Eg for designing optoelectronic devices [19].
Apart from the applications of biscoumarin compounds, they attaining great importance
in the field of fluorescent laser grade dyes. generally organic dyes possess large molecular
hyperpolarizability because of electron delocalization with the conjugated system and hence
having significant nonlinearities. Furthermore, over the past decades, great importance has
been paid for organic dyes towards the development of nonlinear optical (NLO) devices
particularly for optical limiting devices which effectively safeguard the photosensitive
elements from intense radiation (optical) and while reside idle for low-intensity levels [20].
Attention is also due to their promising technological applications in fields like optical
computing, telecommunications, optical information processing, optical computing, etc. [20–
24]. Nevertheless, the rapid development in the field of telecommunication requires novel NLO
materials with improved nonlinearity, low optical limiting threshold, high-density data storage
and also high-speed electro-optical information processing is very much essential. In present
years, NLO and fluorescence research fields purely focusing on inorganic semiconductors,
conjugated polymer and organic molecules [25]. In view of this, organic compounds consisting
of (D-π-A) donor π acceptor functionality are of great for enhanced NLO property and high
emission yield and of course several properties of the dye in solution mainly depend on the
nature of their corresponding solvents environment [26]. The increase of optical response in
organic molecules is not only due to the high level of π conjugated charge transfer but it is also
of an increase in the conjugation length which plays an essential determining role in the
delocalization of π electrons under light excitation. Also, only a few researchers are reported
that [27], the biscoumarin dyes showed excellent optical properties. There is an endless demand
for novel organic materials, for the high-technology industry and nonlinear devices and also
for the biological sciences.
In view of this, coumarin dyes have received special substantial attention from the
research community because of their key role in pharmaceutical agents, like anticoagulants
HIV inhibitors, anti-inflammatory, and antioxidant agents. Due to the long spectral range, give
higher fluorescence quantum yields, photostability, and good solubility in the safest solvents
[28,29]. Further, among coumarin derivatives, dimeric coumarins biscoumarins are the ones
which occupying an interesting position and these exist naturally in microorganisms in a large
number of plants [29]. Nonetheless, biscoumarins have drawn intense interest in recent years
due to their versatile medical and biological application viz., anticancer, antibacterial, anti-
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inflammatory and antioxidant agents and many more. In addition, these dyes can strongly
exhibit tubulin aggregation also played an important role against cancer [30–35].
Despite the numerous reports on the fluorescent performance of 4-HCM biscoumarin
only few reports appeared concerning, and the attention was dedicated to the OLEDs,
biological applications or their preparation as intermediates in the synthesis of more
complex molecules. To the best of our knowledge, none examples were reported on the study
of the photophysical properties, NLO studies of biscoumarins family. Hence, the present work
involves the systematic investigation of optical (absorption and emission), NLO studies (using
Z-scan technique at 532 nm in CW mode), photometric (CIE, CCT, CRI and color purity) and
photophysical (Lifetime and quantum yield, etc.) properties of synthesised 4-Hydroxy-3-[(4-
hydroxy-2-oxo-2H-chromen-3-yl)(4-methoxyphenyl)methyl]-2H-chromen-2-one (4-HCM)
compound dissolved in suitable solvents.
2. Synthesis of 4-HCM Coumarin and Methods
2.1. Chemicals used
The spectroscopic grade solvents viz. cyclohexane, 1,4-dioxane, chloroform, carbon
tetrachloride, dichloromethane, butanol, methanol, and acetonitrile were procured from SD
Fine chemicals and used without further purification (99.9% purity for all solvents). The
samples were prepared using these solvents and the concentration of the solution was
maintained for about 10-5 to 10-6 M because of self-aggregation and inner filter effects.
2.2. Synthesis Route
We synthesized biscoumarin by catalyst method, choosing the acetic acid as a catalyst.
The mixture of aryl aldehydes (1mmol), 4-hydroxycoumarin (3 mmol) and Acetic acid (12 mL)
was taken in a 100mL beaker and refluxed for 1-1.2h. By using thin layer chromatography
(TLC) (Eluent: n-hexane and Ethyl acetate) the progress of the reaction was monitored. The
vanishing of a solvent in vacuum afforded the crude product which was recrystallized from
ethanol to get 4-Hydroxy-3-[(4-hydroxy-2-oxo-2H-chromen-3-yl) (4-methoxyphenyl)
methyl]-2H-chromen-2-one (4‒HCM) bis coumarins of 92% yield. Fig. 1(a and b) shows the
synthesis scheme and free radicle mechanism of biscoumarin.
1
A white solid; H NMR (CDCl3 400 MHz): δ 3.80 (s, 3H, OCH3), 6.04 (s, 1H, CH),
13
6.88-7.69 (m, 12H, Ar-H), 8.00 (s, 1H, OH), 8.07 (s, 1H, OH) ppm. C NMR (CDCl3 100
MHz): δ 35.7, 55.8, 114.01, 116.63, 124.39, 124.88, 126.95, 127.65, 132.74, 146.92, 147.67,
152.24, 158.45, 163.92, 164.25 ppm. HRMS: [M+H] = m/z 443.1649; Anal. Calculated for
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C26 H18O7: C, 70.59; H, 4.11; found: C, 70.56; H, 4.09 %. The NMR and HRMS spectra were
shown in Fig. S1, S2, and S3 (Supporting information).
3. Experimental Techniques
The linear absorption spectra were recorded using Perkin Elmer Lambda–35 UV–Vis
spectrometer operating in the range of 200–1000 nm equipped with deuterium and halogen
lamps with a resolution of ±1 nm at room temperature (RT). The fluorescence spectra were
collected at RT with the help of Hitachi F-2000 fluorescence spectrophotometer operating in
the range of 220 ‒ 800 nm with a resolution of ±5 nm having a light source of 150 W Xenon
lamp. The absorption and fluorescence measurements were repeated for several time, and the
spectra were consistent in all the times. Also, the instruments were well calibrated before the
1
13
scan. The H NMR and C NMR spectrum were recorded for the studied compound at 400
MHz using a JEOL ECS-400 spectrometer in CDCl3 solvent to verify the presence of hydrogen
and carbon environment.
The fluorescence lifetime was recorded using Time-Correlated Single Photon Counting
technique (TCSPC) (Edinburgh Instruments, Model: FSP920) along with a Hamamatsu PMT
detector. The excitation source of 408 nm wavelength diode laser (<100ps, 1MHZ) was used
and the device response function was ~260 ps at FWHM. Fluorescence decays measured with
an excitation beam of vertically polarized and fluorescence was collected at the magic angle
(54.730). For all fluorescence detection, the spectral bandwidth is kept at ~7.0 nm. Fluorescence
decay data analysis in the present work was done by iterative reconvolution and nonlinear least
square method. The fluorescence decay curve analysis was carried out by using the software
IBH (DAS-6) based on reconvolution technique.
3.1. NLO
The nonlinear absorption of synthesized biscoumarin dye was analyzed by Z‒scan
technique [36]. Z‒scan setup is a single-beam technique which offers simplicity, high
sensitivity for simultaneous measuring of both magnitude and sign of non-linear absorption
and non-linear refraction. And in which the samples were moved along the propagation
direction of the beam i.e., along the +Z and –Z axis positions. The intensity of the transmitted
beam was recorded using a detector placed after the sample and the Z‒scans for solutions were
carried out by utilizing a standard 1mm glass cuvette. In Z‒scan measurements diode-pumped
Nd: YAG laser (HOLMARC Model: HO-ED-LOE-03) of continuous wave with 5x103 W/cm2
input intensity and 50 mW output power operating at 532 nm wavelength has been used and
the laser beam was focused by 5 cm focal length lens, leads to Rayleigh length of Zo = 1.51
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mm and a radius of the beam waist of 16 μm. Throughout the experiment, the concentration of
the solution was kept 10-5 to 10-6 M. The schematic Z‒scan experimental setup is presented in
Fig. 9.
3.2. Computational Method
4-Hydroxy-3-[(4-hydroxy-2-oxo-2H-chromen-3-yl)(4-methoxyphenyl) methyl]-2H-
chromen-2-one is optimized in ground state geometry in vacuum phase with Density
Functional Theory-Becke’s 3 Lee-Yang-Parr/6-311(d) (DFT-B3LYP/6-311(d)) basic set using
Gaussian 09W software [37]. As B3LYP functional has been reported to give reasonably good
results for most of the organic molecules[38] The addition of these polarization function was
very important for improving the representation of the molecular electron density. During
geometry optimizations, every bond length, bond angle and dihedral angle were allowed to
relax free of constraints. Many properties can be calculated using the DFT theory such as
optimization energy, geometrical parameters, molecular electrostatic potential maps (MEP),
reactivity parameters and the Highest Occupied Molecular Orbital (HOMO), Lowest
Unoccupied Molecular Orbital (LUMO) plots, energy levels and energy gaps (Eg) have been
estimated in different solvents using DFT-B3LYP/6-311(d) basic set. The total electron
density, Electrostatic potential, and Molecular electrostatic potential maps are analyzed with
the same basic set in a vacuum and in one of the studied solvent that is in octanol for the studied
molecule. GaussView 5.0.8 programs have been used to extract the calculation results, and
visualize the optimized structures, the frontier molecular orbitals (FMOs) and molecular
electrostatic potential (MEP) maps. The color chromaticity, CIE color coordinate were
determined by employing OSRAM software.
3.3. Methods
-1
The evaluation of mass extinction coefficient mass (Lg cm-1) at a given wavelength
the light absorbed per unit mass density of the compound which is a useful parameter in the
optoelectronic application and is given by equation.
휀
훼푚푎푠푠
=
(1)
(2)
⁄
푀푤
퐴
휀 =
푏∗푐
Here, ‘ε’ (Lmol-1 cm-1) is the molar extinction coefficient, ‘A’ is the absorption of light
by the compound for a particular temperature, ‘b’ is the distance of light which travelled
through the sample, ‘c’ is the concentration and ‘Mw’ molecular weight of the compound. The
optical band gap energy or the forbidden band gap of optical transition Eg depends on
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absorption coefficient ‘’ (cm-1) and photon energy ‘h’ (eV) which is given is given by below
equation [39].
∗
1/2
(
)
 h = 푍 (h − 퐸푔)
(3)
Here, Z* is constant.
The quantum yield (Φ) provides significant information about the excited state,
radiationless transitions, and coupling of electronic to vibrational states [40–42]. However, it
is also used to determine the appropriateness of laser media, sample purity, and chemical
structure, the relative ‘Φ’ can be calculated by using Eqn. 4 [43,44].
2
(
)
퐹
푢
푂퐷 푛
푠
푢
Φ = Φ푠
(4)
2
(
)
푛
푠
퐹 푂퐷
푠
푢
Where Φs is the quantum yield of the standard reference, Fu and Fs are the integrated areas
under the curve of emission spectra of unknown and standard sample, ODu and ODs is the
optical density of the unknown and standard sample, 푛푢 and 푛푠 are the refractive index of both
unknown and standard sample.
The decay rate constants such as radiative (kr) and non-radiative (knr) process were
determined by using the below equations, where 휏0 is the average lifetime of the sample
[45,46].
Φ
푘푟 =
(5)
(6)
휏
0
1
푘푛푟
=
− 푘푟
휏
0
4. Results and Discussion
4.1. Optical properties
4.1.1. UV-Visible absorption studies
The absorption spectra of the biscoumarin derivative studied in this article is presented
in Fig. 2 for the solvents used in this experiment and all measurements were done at RT. The
4-HCM in different polarities of organic solvents had intense S0 state to S1 state absorption
band in the wavelength range of 263‒313 nm. Table 1 listed by changing the solvent polarity
as no or weak influence on the absorption peak is observed. From the Fig. 2 it is clearly noticed
that the absorption peak in chloroform showed very strong absorption maxima at 305 nm, the
absorption decreased and ended in cyclohexane at 311 nm (in UV region) and is due to
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intermolecular charge transfer (ICT) transitions, and this range also attributed to the π to π*
transition [47,48]. 4-HCM showed the absorption maxima have hypsochromic shift by 15 nm,
little influences on shape and position of the absorption spectra in all studied solvents.
However, the blue shift in excitation spectra is attributed to the π to π* transition and also
evident in both polar and non-polar solvents, because of a large degree of stabilization of the
ground state in same environments [49–51]. The formation of hydrogen-bonded solvated
clusters in between polar, non-polar solvents and probe molecule the blue-shifted spectra is
often reported in charge transferring systems [48]. The molar extinction and mass extinction
coefficient considerably enhanced in all studied solvents for 4-HCM compound (Table 1).
4.1.2. Theoretical and experimental linear optical energy band gap (Eg)
The linear coefficient of absorption as a function of photon energy is used to study the transition
of electrons and band structure. By using the below equation [52], the () linear absorption
coefficients of the compound in studied solvents were measured.
퐴
( )
  = 2.303 (푑)
(7)
here, ‘A’ is the absorbance and ‘d’ is the thickness of the cuvette used in the experiment (since
the samples were in the solution phase). Davis and Mott [52] have interpreted earlier in their
article the relationship between photon energy of the incident photon ‘h’and ‘’ can be written
in the general equation as presented in Eqn. 3.
The typical plots of h2 versus ‘h’ (eV) (solution phase) of 1-HCM in solvents are
presented in Fig. 3 shows the linearity in the curves by the arrows which confirm that the
electronic transitions are direct allowed transitions. The optical band gap (Eg) is estimated by
means of extrapolating the linear region to X-axis (Fig. 3). Hence, the calculated band gap Eg
values are tabulated in Table 2. As we can conclude by observing Table 2 the experimental
energy band gap values of the studied molecule is found in the range between 3.52 to 3.78 eV.
The Eg showed a minimum in carbon tetrachloride and chloroform solvents, this is because of
deprotonation (removal of hydrogen). From the results, the variation in band gap energy values
is noticed. Thus, the redshift in band gap energy was observed by rising different solvents
polarity. The degree of structural order and disorder was led to changes in Eg values and also
which will change the distribution of energy levels within the band gap [19]. Here, Eg values
were shifted (Table 2) and are observed that the values are increase or decrease (with respect
to dielectric constant, refractive index and viscosity) when the sample was exposed in the
different solvent environment.
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By the method of DFT-B3LYP/6-31G (d) basic set (ground state) the HOMO, LUMO
energies and theoretical band gap energy were presented in Table 2. The molecular orbitals of
4-HCM in all studied solvents are shown in Fig. S4 (Supporting information). The orbital
diagram was plotted with the counter value of 0.02 a.u., and yellow color (Fig. S4) indicates
positive and blue color indicates negative value of the density distribution plots of HOMO and
LUMO shows the typical  molecular orbital characteristics, from this analysis we infer that
the lowest lying singlet to singlet absorption and electronic transition of * type. From Fig.
S4 the HOMO of 4-HCM, the electron density is localized on 4-hydroxy coumarin moiety in
that double bond C19-C20 and C10-C11 has maximum electron density. The LUMO is `mainly
localized on the 4-methoxybenzaldehyde moiety in that singlet oxygen O28 and C48-O29
double bond has the maximum distribution of electron density. All the molecular electron
densities of HOMO and LUMO shows in studying solvents have a similar observation. The
calculated HOMO to LUMO energy gap values varies from 3.26 to 3.27 eV in all solvents,
which corresponds to the blue region of the visible spectrum (Fig. 4). Since the average
discrepancies in between the observed and determined band gap Eg were found to 0.37 eV [53–
55]. Nevertheless, it is very much important to point out that the obtained difference between
theoretical and experimental results can be ascribed due to the fact that the experimental results
belonged to solution phase in which there are several inter and intramolecular interactions
within the solvent and solution can be seen. Whereas, in the case of theoretical calculations
(using the software) it may be due to a solvent phase of an isolated single molecule. Thus, this
band gap (wide) and the optical transparency of the compound with the lower Eg can be
specially preferred for light emitting diode and laser diodes in the field of optoelectronics. Fig.
5 shows the typical theoretical energy band gap of 4-HCM in 1, 4-dioxane solvent [56].
4.2 Photoluminescence Studies
Photoluminescence spectra of 4-HCM (Fig. 6) is investigated at RT in all studied
solvent with varying polarity index to examine the solvent effect on the excited state. Emission
maxima (em), fluorescence quantum yield (Φ), fluorescence lifetime (τ), radiative decay (kr)
and non-radiative decay (knr) rates [57] for 4-HCM in all studied solvents are tabulated in Table
3. At 314 nm excitation in all solvents leads to an intense emission three bands (1max = 349,

2max = 412 and 3max = 436 nm) with the same shape emission spectrum as in Fig.6, clearly
confirms with the three intense bands of the emission due to anti-conformer present in the
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solution in smaller concentration [53,57]. The most common photophysical phenomenon arose
from intermolecular charge transfer which is featured with emission redshift and intensity
decreases in different solvent polarities. The Stoke shift between the excitation and emission
maxima is relatively large (Table 3) as has also been observed for in all solvents. The quantum
yield can be calculated by using the above-mentioned equation (Eqn. 4) using decanol as a
reference sample and the obtained results are tabulated in Table 3. The 4-HCM in all solvents
energy yield of fluorescence is less than unity because of Stoke’s losses. This is further
supported by more knr and fewer kr values. The decay rate constants radiative (kr) and non-
radiative (knr) (Table 3) were calculated using Eqn. (5 and 6), the emissive rate constants for
4-HCM in all studied solvents having different polarities clearly suggests more effective
transient displacement of charges between the ground and excited state. Obtained results are
strongly suggesting as the solvent polarity varies, the relative quantum yield also varies from
72 to 91%. This result confirms that the fluorescence emission of 4-HCM in excited energy
states strongly depends on the solvent environment. Fig. 7 shows the fluorescence lifetime
decay curves of 4-HCM molecule in all studied solvents at RT, are recorded experimental setup
with magic angle confirmation keeping the emission polariser at 54.730, the decay profiles
shows single exponential which confirms the dominant singlet nature of the radiative exciton.
The fluorescence lifetime of 4-HCM varies from 4.8 to 6 ns in polar and non-polar solvents
confirms compound is fluorescent material. Because of the high values of Φ and short lifetime
of 1-HCM, probably confirms the absence of twisted intermolecular charge transfer (TICT)
character in the excited state. Therefore, the fluorescence is from intramolecular charge transfer
(ICT). Obtained results of quantum yield, lifetime and Stoke’s shift shows the compound is a
good promising candidate for the laser dyes and LED color filters [58,59].
4.3 Photostability
To study the photostability of the 4-HCM, we inspected the photostabilities of our
synthesized compound by exposing in all solvents used in this experiment. The solution phase
of 4-HCM under the 365 nm UV light irradiation in air. As shown in Fig. 8, the
photoluminescence major emission peak of the compound with the maximal peak at around
349 nm was decreased as the exposure time of compound to the UV light was increased. After
6 hours of irradiation by UV light (with a UV power intensity of 5W), the 4-HCM maintained
95% of its initial light emission at the maximum photoluminescence emission peaks. Hence,
the above results demonstrated that our compound showed good photostability. From the
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literature report, derivatives of biscoumarin generally showed good photostability which is
rather unusual among fluorescent dyes [60].
4.4 Non-Linear Optical (NLO) properties
The Z‒scan is one of the best tools because it is a highly sensitive and powerful technique for
testing the NLO properties of the materials, which revealed the induced absorption in this
sample, making them as potential candidates for optical limiting applications. The strength of
the non-linear properties was compared with other organic systems under the same
experimental measurements. In the present investigation on the NLO absorption measurements
of synthesized 4-HCM were done using the Z-scan technique at 532 nm laser in open aperture
(OA) mode. Z-scan experiment performed in all nine selected solvents at RT and having the
same concentration and Fig. 10 shows spatial profiles of 4-HCM in solvents. During the
measurements, the cuvette having a thickness of 1mm was loaded with the solution moved
along the propagation (Z‒axis), the beam was spatially filtered using a lens of focal length 7.5
cm. Based on the position the sample experiences a different output intensity of the laser beam
means that a far field the output intensity is almost constant, however, it is varying when the
samples reached near to the focus [61]. The experimental measurements are repeated several
times to confirm the signature and for good consistency in the results also all the evaluated
values are within 10% error. From Fig. 10, it is clear that the 4-HCM dissolved in the above-
mentioned solvents depicted a dip at the focal point. This valley or dip at the focal point of the
Z‒scan is the signature of reverse saturable absorption (RSA) of kind of nonlinear behavior
present in the compounds.
In order to calculate the nonlinear absorption coefficient (NLO) (βeff), we considered
the intensity dependent nonlinear absorption relation, [62, and 63]
α (I) = α + βeff I
(8)
here ‘I’ is the laser beam intensity and ‘α’ is the linear absorption coefficient.
In OA Z‒scan mode the normalized transmission is given by [18, 19]
푚
{
(
)}
⁄
푞 − 푧,0
표
∞
(
)
∑
푇 = 푧, 푆 = 1 =
(9)
3
푚=0
2
(푚+1)
Where qo is the free factor given by
퐼 퐿
푞표 푧 = [훽
]
(10)
푒푓푓
0
푒푓푓
2
( )
푍
1+(
)
푍
0
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here, Leff is the effective thickness of the compound and Io is the maximum laser beam intensity
at the focus. A fit to Eqn. 10 of the open aperture data gives the nonlinear absorption coefficient
βeff for the 4-HCM.
The reverse saturable absorption (positive nonlinear absorption) has been also found to
occur in other organic materials [61–63]. And positive signature was observed in a 4-HCM
compound in the different polarity of solvents. It has been reported that, the RSA kind of
nonlinear property can be attributed to any of the NLO mechanisms, such as two-photon
absorptions (2PA) (ground state to the excited state), excited state absorption (ESA) (first
triplet state to higher triplet state), nonlinear scattering, free carrier absorption (FCA), or even
the combination of all these processes [64]. ESA process is ascribed from nonlinear absorption
in organic molecules, the FCA process is attributed in metals and metal nanoparticles [64].
Further, it has been explained that, if the nonlinearity observed is only due to 2PA, then the
nonlinear absorption coefficient βeff should be independent of input intensity I0 i.e. βeff should
be constant with respect to variation in input intensity [64,65]. Nonetheless, the variation of
βeff values with I0 shown Table 4 clearly revealed that the βeff value is almost constant with I0,
hence we observed nonlinear absorption in our 4-HCM is due to the 2PA assisted RSA process
[21,66–68]. One of the best exploits of RSA kind of nonlinear process in materials or dyes can
be useful as an optical limiter. In practical, an optical limiter is opaque to high input intensities
while transmitting low light intensities [69,70]. In the present investigation, we have noticed
RSA kind of nonlinearity and also the evaluated βeff values are comparable with that those of
other derivatives reported [71] under CW regime, hence the investigated 4-HCM are most
beneficial for optical limiting application in CW regime at the visible region.
4.5. Photometric parameters.
The organic materials have received overwhelming importance in organic light-
emitting devices (OLEDs), polymer light-emitting diodes (PLEDs) and display device
applications it is important to determine the characteristics of color emission. The first
mathematical representation of colors generally accepted is the Commission Internationale de
I’Eclairage 1931 (CIE-1931) color system [72]. Hence, the color coordinate values have been
determined by using Osram Sylvania CIE co-ordinates software. This system consists of three
primary color stimuli symbolized by X, Y and Z tristimulus integrals
12

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푋
푥 =
푦 =
(11)
(12)
푋+푌+푍
푌
푋+푌+푍
Where,
750
( ) ( )
푋 = ∑ 푆푃퐷 휆 푥 휆 푑휆
̅
190
750
( ) ( )
푌 = ∑ 푆푃퐷 휆 푦̅ 휆 푑휆
190
750
( ) ( )
푍 = ∑ 푆푃퐷 휆 푧 휆 푑휆
̅
190
From Fig. 11 it is clear that the CIE coordinates lie in deep blue region for chloroform,
cyclohexane, 2-methoxyethanol, butanol, 1,4-dioxane, carbon tetrachloride, methanol,
dichloromethane, acetonitrile, and the corresponding x and y values are tabulated in Table 5. it
is observed that this blue light emission approaches to the blue light as even change in polarities
of the solvents. It is found that 4-HCM showed color coordinate (x = 0.14  0.15 and y =
0.078  0.086) almost near to ideal blue light which can be used for optoelectronic
applications. Because of an electron withdrawing strength of 4-methoxybenzaldehyde emission
tuning is more highlighted. x and y chromacity values of 4-HCM in all solvents are well
matched to National television standard committee system (NTSC) for the ideal blue
chromacity coordinate (0.14, 0.08) [73]. Color correlated temperature (CCT) U' V' and DUV
these are an important aspect of color appearance how cool (bluish) or how warm (yellowish)
light appearance. DUV with the positive sign for above and negative sign for below the Planckian locus
is defined as the distance from the test light source to the nearest point on the Planckian locus of the
CIE coordinates [72]. The CCT was determined by transforming the x, y coordinates of the light
source to U'V', of the Planckian locus by using the following equations.
4푥
푈′ =
푉′ =
(13)
(14)
−2푥+12푦+3
9푦
−2푥+12푦+3
The CCT values for 4-HCM in tabulated solvents are found to be 5969 to 8479 K (Table 5)
and shown in Fig. 12.
Also, from McCamy’s equation [74] the CCT value can be calculated as follows,
CCT = 449 n3 + 3525 n2 + 6823.3 n + 5520.33
(15)
13

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(푥−0.3320)
(0.1858−푦)
where 푛 =
and x, y chromacity coordinate, for 4-HCM compound in all studied
solvents. From this method the average CCT was found to 6756 K. Therefore, the 4-HCM was
highly useful for the production of artificial blue light to be similar to those natural blue light
simulates the natural color of daylight applications. The Du'v' output indicates error between
the test lamp CCT and 4-HCM CCT as computed by software. As with MacAdam ellipses,
however, the noticeable difference is dependent on many factors [75], so our results are
considered as acceptable Du'v' errors.
Color rendering index (CRI) is the quantitative measurement of the capability of a light
source to reproduce the colors of different objects truly in comparison with natural light or an
ideal source [75,76]. The general color rendering index (CRI) is given by
1
8
8
∑
퐶푅퐼 =
푖=1 푅푖
(16)
The detailed calculation of Ri (special color rendering index) was given by Aritra Ghosh et. al.
[75]. The performance of 4-HCM in all solvents of different polarities have high CRI values
which are in average are greater or equal to 90% (Table 5). This result can be concluded that
higher achievable CRI values also offers higher CCT values for interior light applications
[77,78]. The color purity is an important property to estimate the possibility of the newly
synthesized organic derivative for potential applications [73]. The color purity of the 4-HCM
is estimated by using the below equation [79,80]
2
2
√(x−x) + (y−푦 )
i
Color purity =
x 100%
(17)
2
2
√(푥 −x) + (y−푦 )
d
i
Where X, Y are CIE chromaticity coordinate values of the studied compound, Xi, Yi is the color
coordinate of white illumination and (xd, yd) is the CIE coordinate of the dominant emission wavelength.
In this report the (xi, yi) = (0.310, 0.316), (xd, xd) = (0.142, 0.084). However, these results are also
tabulated in Table 5 revealed that the observed the color coordinates of 4-HCM slightly vary
with different solvents due to decrease in intensity as well as the area of the photoluminescence
emission. Thus, the 4-HCM possesses high color purity and good CIE chromaticity coordinate
also they would have the potential application of organic light emitting devices and display
applications [81].
14

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5. Conclusions
In this present work, we have determined experimental and theoretical determination
energy band gap of the newly synthesized derivative 4-Hydroxy-3-((4-hydroxy-2-oxo-2H-
chromen-3-yl) (4-methoxyphenyl) methyl)-2H-chromen-2-one (bis coumarin) these results
were well matched with a semiconducting materials band gap. Further, NLO absorption
measurements have been carried out by the method of Z‒scan technique using Nd: YAG laser
operating at 532 nm in the CW regime. The OA Z‒scan plots clearly signify the RSA kind of
nonlinearity due to 2PA and hence the studied 4-HCM can be used as an optical limiter for the
protection of human eyes and other sensitive devices from high intensity laser radiation. The
photostability was studied for the 4-HCM under UV-light irradiation for 6 hours which
exhibited the compound is stable in all studied solvent. The present paper also reports on CIE
(Commission Internationale de I’Eclairage) color purity, Color Rendering Index (CRI) and
Color correlated temperature (CCT) of previously mentioned compound dissolved in different
solvents to understand the photometric properties. To this end, the 4-HCM in all studied
solvents showed that this compound exhibits blue color (CIE-coordinates x = 0.14, y = 0.08)
and is a promising material for optoelectronic device applications.
15

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Figure 1. (a). Synthesis procedure of 4-Hydroxy-3-((4-hydroxy-2-oxo-2H-chromen-3-yl)(4-
methoxyphenyl) methyl)-2H-chromen-2-one (4-HCM). (b) Free radical mechanism of 4-HCM
molecule.
Figure 2. UV-Visible absorption spectrum of 4-HCM in all studied solvents.
Figure 3. Energy band gap plots of all studied solvents in 4-HCM
Figure 4. Plot of energy gap values of 4-HCM in all studied solvents calculated using DFT-
B3LYP/6-31G (d) basic set in ground state.
Figure 5. HOMO, LUMO and theoretical energy band gap of 4-HCM in 1, 4-Dioxane
solvent.
Figure 6. Photoluminescence Spectra of 4-HCM at room temperature.
Figure 7. Fluorescence lifetime decay curves of 4-HCM in all studied solvents.
Figure 8. Photoluminescence spectra intensity of 4-HCM in all solvents under the 365 nm
UV light illumination in air.
Figure 9. Schematic experimental open aperture Zscan setup.
Figure 10. Open aperture Zscan traces of 4-HCM. (Bubbles in red color indicates
experimental and solid dashed line indicates theoretical fit)
Figure 11. Plot of color coordinate values for 4-HCM in all studied solvents. (Inset zoom
scale of CIE coordinates)
Figure 12. CCT plot of 4-HCM in in all studied solvents.
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Table 1. Solvent polarity parameters, absorption maxima, Mass and Molar extinction
coefficient of 4-HCM
Mass extinction Molar extinction
Solvents
Dielectric
Constanta
()
Refractive
Indexb
(n)
abs
(nm)
coefficient αmass
Coefficient
ε x 104
(Lg-1 cm-1)
(L mol-1 cm-1)
Cyclohexane
1,4 -Dioxane
Carbon tetrachloride
Chloroform
2.01
2.21
2.24
4.8
1.424
1.42
311.66
311.83
312.72
305.01
275.99
312.33
273.88
275.78
278.69
4.53
8.18
19.9
26.3
24.4
18.6
13.4
7.02
9.03
2.1
3.6
1.459
1.444
1.437
1.421
1.359
1.326
1.324
8.8
11.6
10.8
8.2
2-Methoxyethanol
Dichloromethane
Butanol
16.9
9.1
22.4
32.6
37.5
5.9
Methanol
3.1
Acetonitrile
3.9
a and b parameters are taken from [48]
Table 2. Experimental and Theoretical energy band gap of 4-HCM.
Solvents
HOMO
(eV)
LUMO
(eV)
Band gap (eV)
Eg(Theo) Eg(Exp)
Cyclohexane
-3.6585
-3.6599
-3.6598
-3.6683
-3.6712
-3.6719
-3.6746
-3.6748
-3.6749
-0.4024
-0.4025
-0.4026
-0.4029
-0.4030
-0.4031
-0.4033
-0.4035
-0.4038
3.256
3.572
3.257
3.265
3.268
3.269
3.271
3.271
3.271
3.78
3.64
3.52
3.59
3.75
3.64
3.71
3.72
3.74
1,4 -Dioxane
Carbon tetrachloride
Chloroform
2-Methoxyethanol
Dichloromethane
Butanol
Methanol
Acetonitrile
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Table 3. Photophysical properties of 4-HCM
Solvents
Stokes shift
∆=(fl -
abs) (nm)
88.39
Quantum
Yield
Life
kr
knr
fl
(nm)
Time (ns) (10-7 s-1) (10-7 s-1)
Cyclohexane
1,4 -Dioxane
Carbon tetrachloride
Chloroform
400.05
428.26
405.95
431.02
451.52
428.78
425.03
337.98
400.94
0.821(±0.001)
0.783(±0.001)
0.915(±0.001)
0.811(±0.001)
0.935(±0.001)
0.853(±0.001)
0.802(±0.001)
0.726(±0.001)
0.729(±0.001)
4.80.1)
5.10.1)
5.50.2)
6.10.4)
5.20.1)
5.70.3)
5.60.1)
5.40.2)
5.10.1)
17.1
15.3
1.66
13.5
1.79
14.9
14.3
13.4
14.2
3.7
4.2
1.5
3.1
1.2
2.5
3.5
5.0
5.3
116.43
93.23
126.01
175.53
116.37
152.03
62.2
2-Methoxyethanol
Dichloromethane
Butanol
Methanol
Acetonitrile
122.25
Table 4. Non-linear absorption coefficient of 4-HCM.
Solvents
eff (Non-linear
absorption coefficient)
10-3 (cm/W)
5.24
Cyclohexane
1,4 -Dioxane
Carbon tetrachloride
Chloroform
4.71
5.04
4.87
3.13
2-Methoxyethanol
Dichloromethane
Butanol
4.52
4.87
4.06
Methanol
3.87
Acetonitrile
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Table 5. Photometric parameters of 4-HCM.
Solvents
CIE Coordinate
U'
V'
CCT
(K)
DU'V'
CRI Color Purity
in %
X
Y
Cyclohexane
1,4 -Dioxane
Carbon tetrachloride
Chloroform
0.1673
0.0879
0.1798 0.2126 6244 -0.0102
89
93
93
98
98
94
96
92
90
86.27
91.06
93.21
93.36
93.41
88.86
95.79
93.38
94.51
0.1589
0.1549
0.1555
0.1522
0.1585
0.1527
0.1525
0.1518
0.0827
0.0809
0.0789
0.0857
0.0945
0.0789
0.0852
0.0812
0.1729 0.2025 6214
0.1692 0.1988 6297
0.1710 0.1953 5969
-0.009
-0.012
-0.008
2-Methoxyethanol
Dichloromethane
Butanol
0.1634 0.2071 7341 -0.0088
0.1660 0.2228 8479 -0.0105
0.1645 0.1947 6427
0.1640 0.2062 7219 -0.0085
0.1654 0.1991 6617 -0.013
-0.011
Methanol
Acetonitrile
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Highlights
 Synthesis of coumarin molecule
 Exhibits good energy band gap for optoelectronic device application.
 Zscan profiles clearly demonstrates the presence of reverse saturable absorption.
 A promising nonlinear optical material for optical limiting application in CW regime.
 x=0.14, y=0.08 coordinates shows material is suitable for blue color OLED.
26

Figure 1

Figure 2

Figure 3