Synthesis, spectroscopic characterization, and computed optical analysis of green fluorescent cyclohexenone derivatives

Article abstract of DOI:10.1002/poc.3512

Cyclohexenone containing chalcones core is one important class of materials, which exhibit high nonlinear optical (NLO) responses and good crystallizability. The present study reports the successful development of six new fluorescent cyclohexenone derivat

Full text of DOI:10.1002/poc.3512

Research article
Received: 7 May 2015,
Revised: 26 August 2015,
Accepted: 1 October 2015,
Published online in Wiley Online Library: 6 November 2015
(wileyonlinelibrary.com) DOI: 10.1002/poc.3512
Synthesis, spectroscopic characterization, and
computed optical analysis of green fluorescent
cyclohexenone derivatives
Muhammad Faizan Nazara*, Amir Badshahb, Asif Mahmoodc,
Muhammad Naveed Zafara, Muhammad Ramzan Saeed Ashraf Janjuac,
Muhammad Asam Razaa and Riaz Hussaind
Cyclohexenone containing chalcones core is one important class of materials, which exhibit high nonlinear optical (NLO)
responses and good crystallizability. The present study reports the successful development of six new fluorescent
cyclohexenone derivatives (CDs) via conventional Robinson annulation method. The molecular structures of these newly
synthesized CDs were confirmed by using various analytical techniques such as 1H NMR, 13C NMR, FTIR, EIMS, UV–Vis spectros-
copy and single crystal X-ray diffraction. The crystallographic data revealed that the spatial structure of the representative CD
(4BE) belongs to monoclinic, P21/c space group. The results from luminescence studies show that the CDs molecules apparently
emit intense green light at room temperature in aqueous media. The relative polarity and molecular chemical stability of the
CDs molecules were predicted by measuring the molecular electrostatic potential and frontier molecular orbital energy. In
addition, the UV–Vis spectra, transition character and electronic structures of these CDs were computed by using quantum
chemical methodology. It was interesting to note that the values of computed and experimental electronic transitions (λmax
)
were in good agreement and these CDs display high hyperpolarizability (β) values. The present work will be helpful for
systematical understanding of the structures and the optical properties of CDs for studying the structure–activity relationship
that will suggest their potential application in photonic devices. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: crystallizability; cyclohexenones; fluorescent; hyperpolarizability; nonlinear optical
anticonvulsant,[24] and antitubercular activity.[25] Despite biologi-
INTRODUCTION
cal fate, extensive optical properties[26] and various chemical
functions of cyclohexenone derivatives (CDs) also make them
valuable compounds for their potential applications in organic
light-emitting devices (light emitting laser, laser pointers, light-
emitting diodes, and so on),[27] and fluorescent probing for
membranes.[28,29]
The present work demonstrates the synthesis and characterization
of six novel CDs via a convenient route of Robinson annulations. The
molecular structures of as-synthesized CDs have been confirmed by
means of micro-spectral and spectral analysis together with single
crystal X-ray diffraction. The spectral-luminescent measurements
show that the CDs molecules apparently emit intense green light
The emergence of optoelectronic technology has drawn the
world’s attention to design new organic molecules exhibiting
prodigious molecular hyperpolarizabilities with improved second
order optical nonlinearities.[1,2] In recent years, the fabrication of
efficient optoelectronic devices is a challenging endeavor for
researchers to meet the stringent requirements of high optical
performance and sustainable electro-optical responses. On the ba-
sis of theoretical and experimental investigations, several principles
including planar donor–π-conjugated bridge–acceptor (D–π–A)
type,[3] bond length alternation theory,[4] auxiliary donor and
acceptor representation of heterocycles,[5] and twisted π-electron
systems[6] have been suggested to improve the second-order
nonlinear optical (NLO) response.
Owing to the extending conjugated bridge and flexibility of
altering the molecular structure, the organic molecules are therefore
investigated as the potential candidates for optimizing the NLO
properties in optical signal processing, data storage, sensor protec-
tion, and imaging.[7–9] Among a variety of organic molecular
systems, the chalcones core in compounds offer remarkable NLO
properties owing to extensive conjugation and provision of donor
to acceptor charge transfer axis.[10–13]
* Correspondence to: Muhammad Faizan Nazar, Department of Chemistry,
University of Gujrat, Gujrat 50700, Pakistan.
E-mail: faizan_qau@yahoo.com
a M. F. Nazar, M. N. Zafar, M. A. Raza
Department of Chemistry, University of Gujrat, Gujrat 50700, Pakistan
b A. Badshah
National Center for Nanoscience and Technology, Beiyitiao No. 11,
Zhongguancun, Beijing 100190, China
Cyclohexenones are cyclic chalcones derive from the Michael
additions of ethyl acetoacetate to chalcones, followed by internal
aldol condensation.[14–18] Literature revealed that the cyclic
chalcones possess numerous pharmacological properties inclu-
ding anti-cancer,[19] anti-HIV,[20] anti-fungal,[21] anti-tumor,[22,23]
c A. Mahmood, M. R. S. A. Janjua
Department of Chemistry, University of Sargodha, Sargodha 40100, Pakistan
d R. Hussain
Physics Division (PD), PINSTECH, P.O. Nilore, 45650, Islamabad, Pakistan
J. Phys. Org. Chem. 2016, 29 152–160
Copyright © 2015 John Wiley & Sons, Ltd.

OPTICAL ANALYSIS OF NOVEL FLUORESCENT CYCLOHEXENONE DERIVATIVES
at room temperature in aqueous media. To explore the structure–
activity relationship, relative polarity and molecular chemical stability
of the CDs molecules were predicted by measuring the molecular
electrostatic potential (MEP) and frontier molecular orbital energy.
In order to fully explore the properties of these newly
synthesized CDs, NLO and electronic transitions of these com-
pounds were computed using the time dependent density
functional theory (TD-DFT). Our results revealed that the investi-
gated CDs are likely to have exciting NLO properties with high
hyperpolarizability (β) values and computed, and experimental
electronic transitions (λmax) were in good agreement. The
generation of these synthesized CDs having high responsive
optical moieties may encourage the reader for their possible
selection as potential NLO molecules that will possibly lead to
the molecular and material engineering pathways in the future.
synthesized compounds was achieved mostly through recrystalliza-
tion, the use of solvent extraction, or by making preparative thin layer
chromatography or column chromatography whenever required.
Steady-state fluorescence measurements
Steady-state fluorescence was performed on a Perkin Elmer LS 55
Luminescence Spectrometer (Waltham, MA, USA) with PC controlled
software FinWinLab. The Photo Multiplier tube voltage was kept at
665 V. Good resolution of the bands was obtained at the slit width
(ex. 6.0 nm, em. 6.0 nm) with 3.0 × 10ꢀ5 mol dmꢀ3 concentration
of the CDs. The excitation wavelength was taken as 420 nm. The
scan range used was from 450–700 nm. Polarizers were kept clear,
and no cutoff was operating during the scan. The temperature of
the cell was fixed with the help of an external circulator water bath.
Spectrophotometric (UV–visible) measurements
MATERIALS AND METHODS
Spectrophotometric measurements were performed on a Perkin-
Elmer Lambda 20 ultraviolet-visible spectrophotometer with 1.0 cm
quartz cells at a temperature of 25.0 0.1 °C. The scan range used
was from 200–500 nm. The concentration of CDs was fixed to
3.0 × 10ꢀ5 mol dmꢀ3 to obtain the absorbance value in the range
of Beer Lambert law.
Reagents
The reagents; ethyl acetoacetate, 2-chlorobenzaldehyde 4-chloro-
benzaldehyde, 2-methoxybenzaldehyde, 3-methoxybenzaldehyde,
4-methoxybenzaldehyde, 4-ethoxybezaldehyde, and 4-bromoace-
tophenone were purchased from Fluka (Germany). The liquid
reagents were distilled at their boiling points, and the solid
reagents were characterized by recording their melting points. No
further purification was required. Sulphuric acid and hydrochloric
acid (37%) were obtained from Stedee Ltd. The solvents chloro-
form, ethyl acetate, absolute ethanol, and pet ether were purchased
from Sigma Aldrich (Germany). All the solvents were used after
necessary purification and drying according to the standard proce-
dures. The dried solvents were stored over molecular sieves (4 Å).
General synthetic methods
The synthetic strategy for the titled compounds is outlined in the
following Scheme 1. The chalcone analog 2 (3 mmol) and ethyl
acetoacetate (0.39 g, 0.40 mL,and 3 mmol) were refluxed for 2 h
in 10–15 ml ethanol in the presence of 0.5 ml 10% NaOH. The
reaction mixture was then poured with intensively stirring into
200 mL ice-cold water and kept at room temperature until the
reaction product separated as a solid, which was filtered off
and recrystallized from ethanol.
Compound characterization techniques
The molar ratio, physical, FTIR, 1H NMR, 13C NMR, and EIMS
data for these compounds are given below:
Rf values were calculated by using precoated silica gel aluminum
backed plates Kiesel gel 60 F254 Merck (Germany) using ethylacetate :
pet-ether (1 : 4) as developing solvents. Melting points of the
compounds were determined in open capillaries using Gallenkamp
melting point apparatus and are uncorrected. The fourier transform
infrared spectroscopy (FTIR) spectral data were recorded on Bio-
Ethyl 4-(4-bromophenyl)-6-(4-ethoxyphenyl)-2-oxocyclohex-3-
enecarboxylate (4BE)
Yellow solid, Yield 3.5g (65.0%); m.p.: 126–127 °C. 1H NMR (CDCl3,
300 MHz): δ 1.07 (t, J = 7.2 Hz, 3H), 4.08 (q, J = 7.2 Hz, 2H), 3.03 (dd,
J1 = 2.2Hz, J2 = 18.0, 1H), 2.71–2.81 (m, 1H), 3.73 (dd J1 = 2.9 Hz,
J2 = 6.9, 2H), 6.53 (s, 1H), 7.44 (dd J1 = 2.8Hz, J2 = 6.85, 2H), 7.02
(dd J1 = 2.5Hz, J2 = 6.9, 2H), 6.85 (dd J1 = 2.1Hz, J2 = 6.6, 2H), 7.23
(dd J1 = 3.0Hz, J2 = 6.6, 2H), 1.43 (t, J = 7.2 Hz, 3H), 4.09 (q,
J = 7.1Hz, 2H). 13C NMR (CDCl3, 300 MHz): δ 193.6, 169.3, 160.3,
Rad Merlin Spectrophotometer using KBr discs. H NMR and 13C
1
NMR spectra were recorded on Bruker (300 MHz) AM-250 spectrom-
eter in CDCl3 solution using Tetramethylsilane (TMS) as internal
standard. Electron ionization mass spectrometry (EIMS) was recorded
on Agilent mass spectrometer. Purity of each compound was
ascertained by thin layer chromatography. The purification of
Scheme 1. General methodology for the synthesis of cyclohexenone derivatives
J. Phys. Org. Chem. 2016, 29 152–160
Copyright © 2015 John Wiley & Sons, Ltd.
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M. F. NAZAR ET AL.
157.4, 141.9, 133.6, 133.6, 129.3, 129.3, 128.1, 128.1, 126.9, 126.9,
123.5, 116.3, 116.3, 63.4, 61.0, 59.7, 43.5, 36.0, 14.8, 14.0. IR (KBr,
cmꢀ1): 681, 1510, 1605, 1663, 1733, 2981. MS (EI): m/z (%) = 372
(30), 222(100), 194(18), 51(3), 39(2), 344(6), 299(2), 119(9), 91 (20).
Ethyl 4-(4-bromophenyl)-6-(4-methoxyphenyl)-2-oxocyclohex-
3-enecarboxylate (4BM4)
Yellow solid, Yield 3.0 g (62.0%); m.p.: 119–121 °C. 1H NMR (CDCl3,
300 MHz): δ 1.08 (t, J = 7.1Hz, 3H), 4.06 (q, J = 7.1Hz, 2H), 3.04 (dd,
J1 = 2.3Hz, J2 = 17.0, 1H), 2.79–2.87 (m, 1H), 3.70 (dd J1 = 3.2 Hz,
J2 = 6.95, 2H), 6.54 (s, 1H), 7.45 (dd J1 = 2.91 Hz, J2 = 7.0, 2H), 7.04
(dd J1 = 2.6 Hz, J2 = 6.9, 2H), 7.24 (dd J1 = 2.1Hz, J2 = 6.6, 2H), 6.90
(dd J1 = 2.1Hz, J2 = 6.6, 2H), 3.81 (s, 3H). 13C NMR (CDCl3,
300 MHz): δ 193.5, 169.5, 157.4, 157.4, 141.9, 133.8, 133.7, 129.3,
129.3, 128.1, 128.1, 126.9, 126.9, 123.4, 119.3, 119.3, 61.0, 59.6,
55.2, 43.3, 35.9, 14.0. IR (KBr, cm-1): 640, 1060, 1499, 1608, 1662,
1736, 2975. MS (EI): m/z (%) = 358(19), 222(100), 194(18), 51(5), 39
(2), 328(8), 297(3), 119(8), 91 (20).
Ethyl 4-(4-bromophenyl)-6-(2-chlorophenyl)-2-oxocyclohex-3-
enecarboxylate (4BC2)
1
Fluffy white solid, Yield 2.9 g (61.0%); m.p.: 109–110 °C. H NMR
(CDCl3, 300 MHz): δ 1.07 (t, J = 7.1 Hz, 3H), 4.06 (q, J = 7.2 Hz, 2H),
3.04 (dd, J1 = 2.3 Hz, J2 = 17.0, 1H), 2.75–2.85 (m, 1H), 3.68 (dd
J1 = 3.0 Hz, J2 = 6.9, 2H), 6.53 (s, 1H), 7.42 (dd J1 = 3.0 Hz, J2 = 7.0,
2H), 7.04 (dd J1 = 2.5 Hz, J2 = 6.9, 2H), 7.27 (m, 4H). 13C NMR (CDCl3,
300 MHz): δ 193.5, 169.5, 157.3, 142.1, 133.6, 133.6, 133.1, 129.2,
129.2, 128.9, 128.1, 128.1, 127.6, 127.1, 126.5, 123.4, 61.0, 59.5,
43.3, 35.9, 14.0. IR (KBr, cmꢀ1): 677, 756, 1483, 1605, 1665, 1738,
2976. MS (EI): m/z (%) = 362(35), 222(100), 194(25), 51(5), 39(4),
334(9), 299(5).
On the basis of aforementioned results the structures of CDs are
given in Table 1.
Computational procedure
All computations were performed using Gaussian 09 program
package (Gaussian Inc., Wallingford, CT),[30] and all systems were
optimized using DFT calculations. No symmetry constraint was
imposed during geometry optimization. For geometry optimization,
B3LYP functional and 6-31 + G* basis set were used; which have
been known to be quite reliable in having compromise between ac-
curacy of the results and computational time. Coulomb-attenuating
method CAM-B3LYP functional, a long-range corrected form of
B3LYP, was used to calculate absorption spectra of CDs. Ten (10)
lowest singlet–singlet excitation energies were computed. The
absorption spectra of the CDs were simulated by TD-DFT, a most
popular methods for calculating electron absorption spectra in
quantum chemistry[31,32] because of the accuracy and reliability.
Moreover, we used polarized continuum model to calculate the
absorption spectra.[33,34] When a molecule is placed in a uniform
static electric field, its electronic energy can be written as a series
involving coefficients identified as permanent multipole moments
and polarizabilities.[5] The first hyperpolarizabilities are evaluated
by finite field approach with the DFT level of theory. In finite field
method when a molecule is subjected to the static electric field,
the energy of the molecule is expressed by Eqn (1).
Ethyl 4-(4-bromophenyl)-6-(4-chlorophenyl)-2-oxocyclohex-3-
enecarboxylate (4BC4)
Yellow solid, Yield 2.8 g (60.0%); m.p.: 111–112 °C. 1H NMR (CDCl3,
300 MHz): δ 1.08 (t, J = 7.1 Hz, 3H), 4.07 (q, J = 7.1 Hz, 2H), 3.05 (dd,
J1 = 2.4 Hz, J2 = 16.5, 1H), 2.79–2.88 (m, 1H), 3.72 (dd J1 = 2.9 Hz,
J2 = 7.0, 2H), 6.54 (s, 1H), 7.41 (dd J1 = 2.9 Hz, J2 = 6.9, 2H), 7.03
(dd J1 = 2.6 Hz, J2 = 6.9, 2H), 7.28 (dd J1 = 1.2 Hz, J2 = 4.5, 2H), 7.30
(dd J1 = 2.1 Hz, J2 = 6.6, 2H). 13C NMR (CDCl3, 300 MHz): δ 193.7,
169.5, 157.5, 141.1, 133.7, 133.6, 133.5, 129.2, 129.2, 128.6, 128.6,
128.1, 128.1, 127.6, 127.6, 123.5, 61.0, 59.5, 43.4, 36.0, 14.0. IR
(KBr, cmꢀ1): 665, 760, 1502, 1600, 1670, 1736, 2977. MS (EI): m/z
(%) = 362(35), 222(100), 194(20), 51(5), 39(3), 334(8), 299(2).
Ethyl 4-(4-bromophenyl)-6-(2-methoxyphenyl)-2-oxocyclohex-
3-enecarboxylate (4BM2)
1
Fluffy white solid, Yield 2.9 g (61.0%); m.p.: 118–120 °C. H NMR
(CDCl3, 300 MHz): δ 1.08 (t, J = 7.1 Hz, 3H), 4.07 (q, J = 7.1 Hz, 2H),
3.03 (dd, J1 = 2.3 Hz, J2 = 17.0, 1H), 2.80–2.87 (m, 1H), 3.69 (dd
J1 = 3.0 Hz, J2 = 7.1, 2H), 6.54 (s, 1H), 7.54 (dd J1 = 2.9 Hz, J2 = 7.0,
2H), 7.01 (dd J1 = 2.6 Hz, J2 = 7.0, 2H), 7.10 (m, 4H), 3.81 (s, 3H).
13C NMR (CDCl3, 300 MHz): δ 193.5, 169.4, 157.3, 155.8, 141.9,
133.7, 133.5, 129.3, 129.3, 128.0, 128.0, 127.3, 127.0, 123.4,
120.8, 114.0, 55.2, 61.0, 59.5, 43.5, 36.0, 14.0. IR (KBr, cmꢀ1): 630,
1045, 1495, 1607, 1662, 1735, 2975. MS (EI): m/z (%) = 358(30),
222(100), 194(20), 51(5), 39(2), 328(7), 297(2), 119(10), 91 (20).
1
2
1
6
1
24
E ¼ Eð0Þ ꢀ μiFi ꢀ αijFiFj ꢀ βijkFiFjFk ꢀ
γijklFiFjFkFl ꢀ … (1)
where E(0) is the energy of molecule in the absence of an electronic
field, μ is the component of the dipole moment vector, α is the linear
polarizability tensor, β and γ are the first and second hyper-
polarizability tensors whereas i, j and k label the x, y, and z compo-
nents, respectively. Average polarizability (αav) is determined by
considering only the diagonal elements.[35]
Ethyl 4-(4-bromophenyl)-6-(3-methoxyphenyl)-2-oxocyclohex-
3-enecarboxylate (4BM3)
Fluffy white solid, Yield 2.8 g (60.0%); m.p.: 121–123 °C. 1H NMR
(CDCl3, 300 MHz): δ 1.08 (t, J = 7.2 Hz, 3H), 4.08 (q, J = 7.2 Hz, 2H),
3.02 (dd, J1 = 2.2 Hz, J2 = 17.5, 1H), 2.81–2.89 (m, 1H), 3.71 (dd
J1 = 3.1 Hz, J2 = 7.0, 2H), 6.53 (s, 1H), 7.44 (dd J1 = 2.85 Hz,
J2 = 6.9, 2H), 7.03 (dd J1 = 2.5 Hz, J2 = 7.0, 2H), 7.20 (m, 4H),
3.81 (s, 3H). 13C NMR (CDCl3, 300 MHz): δ 193.6, 169.4, 160.4,
157.4, 142.1, 133.8, 133.7, 129.5, 129.2, 129.2, 128.1, 128.1,
126.9, 123.4, 111.5, 110.4, 55.3, 61.0, 59.5, 43.4, 35.9, 14.0. IR
(KBr, cmꢀ1): 628, 1050, 1488, 1609, 1660, 1738, 2970. MS (EI):
m/z (%) = 358(20), 222(100), 194(18), 51(4), 39(3), 328(7), 297
(2), 119(9), 91 (19).
ꢀ
ꢁ
αav ¼ 1=3 αxx þ αyy þ αzz
(2)
Ten components (βxxx, βxxy, βxyy, βyyy, βxxz, βxyz, βyyz, βxzz, βyzz
,
and βzzz) produced by Gaussian 09 are used to calculate total
hyperpolarizability (βtot):[35]
hꢂ
ꢃ
ꢂ
ꢃ
βtot
¼
βxxx þ βxyy þ βxzz 2 þ βyyy þ βxxy þ βyzz 2 (3)
ꢂ
ꢃ i
2
þ
βzzz þ βxxz þ βyyz
1=2
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OPTICAL ANALYSIS OF NOVEL FLUORESCENT CYCLOHEXENONE DERIVATIVES
Table 1. Structures and Rf values of cyclohexenone derivatives
Compound
Structure
Molecular formula
Molecular weight (g/mol)
443.32
aRf values × 100
4BE
C
23
H23BrO
4
61
4BC
4BC
2
C
21
H
18BrClO
18BrClO
3
433.72
433.72
429.30
429.30
58
59
55
56
4
C
21
H
3
4BM
2
C
22
H
21BrO
4
4BM
3
C
22
H
21BrO
4
4BM
4
C
22
H21BrO
4
429.30
58
aSolvent for Rf values = Pet-ether : ethyl acetate (4 : 1)
Michael addition followed by internal Claisen condensation to
produce cyclohexenone analogs, as outlined in Scheme 1.
The organic compounds were separated out by pouring the
reaction mixture into water. All compounds precipitated after being
kept for several days. The solid products were separated by
filtration, dried, and recrystallized from ethanol. The yields of the
cyclocondensation were good, varying from 59.0 to 65.0%. Struc-
tural analysis of the newly synthesized cyclohexenone was carried
out by analytical and spectral data. The IR spectra of these
RESULTS AND DISCUSSION
Synthesis of cyclohexenone
The reaction of 4-Bromoacetophenone and benzaldehyde
analogs in the presence of high basic condition endured Claison
Schmidt condensation to produce heterocyclic analogs of
chalcone. The reaction of heterocyclic chalcone analogs with
ethyl acetoacetate in the presence of basic condition underwent
J. Phys. Org. Chem. 2016, 29 152–160
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M. F. NAZAR ET AL.
compounds revealed a sharp strong absorption band around
1733–1738 cmꢀ1 that can be correlated with the presence of the
ester moiety in the structure of cyclohexenone. Furthermore,
another sharp strong absorption band at ca. 1660–1670 cmꢀ1 can
be assigned to the conjugated ketonic group. The bands around
2970–2981 cmꢀ1 accounted for the methylene (CH2), while the
bands around 1483–1510 cmꢀ1 and 1600–1609 cmꢀ1 can be
assigned to C = C(Ar) and C = C functions, respectively.
compound (4BE) is determined by single-crystal X-ray diffrac-
tion.[36] Figure 1 represents the molecular structure and packing
diagram of 4BE with structural features.
The same synthetic procedure was used for the synthesis of
other members of the series.
Absorption and emission spectra of cyclohexenone
derivatives
Figures S1–S6, Supporting Information shows 1H-NMR and
13C-NMR spectra and a summary of integrals and peak positions
of some selected cyclohexenone derivatives. One of the repre-
sentative compounds of cyclohexenone series, ethyl 4-(4-bromo-
phenyl)-6-(4-ethoxyphenyl)-2-oxocyclohex-3-enecarboxylate
(4BE), has been described here in details.
The UV–Visible absorption spectra and fluorescence emission
spectra of CDs are shown in Fig. 2.
The electronic absorptions of CDs were located around
425 nm. Their excitation wavelengths were all fixed at 420 nm.
It is found that their intensity of fluorescence differs from each
other and their emission maximum (λem) occurred at 513 nm,
display green luminescence in aqueous medium.
In the 1H-NMR spectrum of the compound (4BE), the ethyl
protons resonated as a triplet and a quartet at 1.07 and 4.08 ppm
integrating for three and two protons, respectively. The characteris-
tic signal in the 1H NMR spectrum of compound (4BE) is however the
singlet of the vinylic proton in the cyclohexenone rings, which
occurs at approximately 6.53 ppm integrating for one proton, and
confirms that the intramolecular cyclocondensation subsequent to
the Michael addition actually took place. The signal due to methy-
lene protons appeared as two doublets at 3.03 and at 3.73 ppm,
respectively, thereby indicating that they are diastereotropic
protons. The signal due to methane appeared as a multiplet in
the range of 2.71–2.81 ppm integrating for one proton. As for the
protons in the aromatic region are concerned, the protons in the
aromatic ring are at 6.85 ppm, 7.02 ppm, 7.23 ppm, and 7.44 ppm.
The 1H-NMR spectra substantiated the results of the IR analysis.
The 13C NMR spectrum of 4BE shows a characteristic peak at
169.30 ppm, which confirms carbonyl carbon of the ester group. A
peak at 193.65 ppm confirms the presence of the carbonyl carbon
of ketone. A peak at 133.64 shows the presence of vinylic carbon.
The mass spectrum of 4BE is also in accordance with the
proposed structure showed characteristic peaks; however, the
molecular ion peak is absent. The fragment peak at m/z 372 is due
to the loss of ester moiety. The base peak in the mass spectrum
appeared at m/z 222 as a result of retro-Diels–Alder fission of cyclo-
hexene ring. The peaks appeared at m/z 344 and 299 are due to the
loss of carbon monoxide molecule and side chain, respectively.
To understand the disposition of the constituent parts in the
topical compounds, the single crystal of one of the analogous
Electronic structure and molecular reactivity descriptors of CDs
Frontier molecular orbitals, highest occupied molecular orbital-
lowest unoccupied molecular orbital (HOMO-LUMO), significantly
affect the electronic spectra and reactions between molecules.[37,38]
The HOMO–LUMO distribution patterns of as-synthesized CDs are
shown in Fig. 3.
600
4BE
4BC2
1.2
500
4BC4
Absorption
4BM2
0.9
400
4BM3
4BM4
300
0.6
200
0.3
0.0
Emission
100
0
360
420
480
540
600
660
Wavelength (nm)
Figure 2. The normalized optical absorption and fluorescence emission
spectra of aqueous solution (3% ethanol) of cyclohexenone derivatives
with the concentration of 3.0 × 10ꢀ5 mol dmꢀ3
Figure 1. X-ray molecular structure of (4BE). The displacement ellipsoids are drawn at the 30% probability level, and the H atoms are shown as small
spheres of arbitrary radii, (b) The packing diagram of the (4BE), viewed along the c-axis showing the weak C—H---O and C—H---π electron interactions
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OPTICAL ANALYSIS OF NOVEL FLUORESCENT CYCLOHEXENONE DERIVATIVES
Figure 3. Surfaces of the frontier molecular orbitals of synthesized cyclohexenone derivatives
The stability and reactivity of a molecule can be determined
by HOMO–LUMO energy gap (Egap). Because the kinetic stability,
chemical reactivity, chemical hardness–softness, and chemical
reactivity changes with change in energy gap, determining the
HOMO–LUMO energy gap is a way for qualitative estimation of
linear and NLO behavior of a molecule.[39,40] The global chemical
reactivity parameters of molecules have been defined in terms of
hardness (η), softness (S), chemical potential (μ), electronegati-
vity (χ), and electrophilicity index (ω). Using Koopman’s theorem
for closed-shell molecules,[41,42] the global reactivity descriptors
are related as follows:
measures the tendency or capability of a species to accept elec-
trons and is calculated using the electronic chemical potential
and chemical hardness as shown in Eqn (3).[43,44] It is a measure
of the stabilization in energy after a system accepts additional
amount of electronic charge from the environment. Among the
typical CDs, 4BM2 is the strongest nucleophile, while 4BC4 is
the strongest electrophile (Table 2).
Molecular electrostatic potential map
Another handy tool to evaluate the structure–reactivity relation-
ship of the molecules is MEP. It is usually related to electronic
density and relative polarity of the molecules and very efficient
to predict the active sites for electrophilic and nucleophilic
attack.[45,46] Figure 4 shows the MEP surface map of the synthe-
sized CDs carried out by B3LYP using 6-31 + G* basis set. The
electrophilic attack is illustrated by red (negative) regions,
whereas nucleophilic reactivity is shown by the blue (positive)
regions and the green region indicates the parts of the molecule,
where electrostatic potentials are close to zero.
I ꢀ A
ꢀðI þ AÞ
I þ A
1
η
μ2
2η
η ¼
;
μ ¼
;
χ ¼
;
S ¼
;
ω ¼
(4)
2
2
2
where, I = ꢀ EHOMO and A = ꢀ ELUMO are the ionization potential
and electron affinity of the compounds. HOMO and LUMO
energies and related global chemical reactivity descriptors of
the synthesized compounds are listed in Table 2.
The larger the HOMO–LUMO energy gap, the harder and more
stable/less reactive the molecule. The results indicate that 4BC4 is
harder and less reactive, while 4BC2 is softer and most reactive
among the synthesized CDs. The trend in electronic chemical po-
tential for the CDs is 4BC2 > 4BE > 4BM2 > 4BM4 > 4BM3 > 4BC4.
The greater the electronic chemical potential, the less stable or
more reactive is the molecule. Therefore, 4BC2 is the most reac-
tive, and 4BC4 is the least reactive of all newly synthesized CDs.
Global electrophilicity index, the most important descriptor,
The region for electrophilic attack (red) is localized on the oxygen
atoms having double bond with carbon (C = O group), whereas, the
nucleophilic attack is localized on major part of molecule.
Nonlinear optical properties
In the designing of optical materials, especially used in signal
processing, communication technology, optical memory devices,
Table 2. Calculated HOMO and LUMO energies and global reactivity parameters of cyclohexenone derivatives in eV
Compound
EHOMO
ELUMO
Egap
I
A
η
μ
χ
S
ω
4BE
ꢀ5.91
ꢀ3.46
ꢀ6.34
ꢀ6.00
ꢀ6.05
ꢀ5.94
ꢀ1.95
ꢀ1.70
ꢀ2.15
ꢀ1.87
ꢀ1.95
ꢀ1.95
3.97
1.76
4.19
4.12
4.09
3.99
5.91
3.46
6.34
6.00
6.05
5.94
1.95
1.70
2.15
1.87
1.95
1.95
1.98
0.88
2.10
2.07
2.05
2.00
ꢀ3.93
ꢀ2.58
ꢀ4.25
ꢀ3.94
ꢀ4.00
ꢀ3.95
3.93
2.58
4.25
3.94
4.00
3.95
0.51
1.14
0.48
0.48
0.49
0.50
3.90
3.78
4.30
3.75
3.90
3.90
4BC2
4BC4
4BM2
4BM3
4BM4
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M. F. NAZAR ET AL.
Figure 4. Molecular electrostatic potential surface map of cyclohexenone derivatives
and optical switches, the investigations of NLO properties are
incredibly essential. Hence, the values of average polarizability
(α) and static hyperpolarizability (β) associated with the intramo-
lecular charge transfer of CDs are measured and tabulated in Table 3.
Order of average polarizability (αav) of compounds was found
to be 4BC2 > 4BE > 4BM4 > 4BM3 > 4BM2 > 4BC4. Moreover, the
polarizability of a molecule showed dependence on the energy
gap between HOMO and LUMO. For a molecule to be
polarisable, a relatively small energy gap is usually required.
Additionally, literature suggests that a relatively large linear
polarizability is required to obtain large hyperpolarizabilities.[47]
The static hyperpolarizabilities were calculated by using CAM-
B3LYP method, and the computed βtot values of CDs are outlined
in Table 3. These values are reported as 10ꢀ33 esu. The order βtot
of six systems was 4BC2 > 4BE > 4BM4 > 4BM3 > 4BM2 > 4BC4.
Hyperpolarizabilities of all systems were found higher than the
reported values for p-nitroaniline.[13]
With the aim to fully explore the origin of the second-order
NLO properties of the synthesized compounds, a better eluci-
dation of a structure–property relationship was required.
Hence, by using the sum-overstates approach, a two-state
model linked between β and a low-lying charge-transfer (CT)
transition has been recognized and reported earlier.[49] In the
static case, the following model expression was employed to
estimate βCT
:
Δμgmfgm
E3gm
βCT
∝
(5)
In Eqn (3), Δμgm is the change of dipole moment between the
ground and mth excited state, fgm is the oscillator strength of the
transition from the ground state (g) to the mth excited state (m),
and Egm is transition energy. In the framework of two-state model
expression, the hyperpolarizability is proportional to the product
of the transition dipole moments and oscillating strength and is
inversely proportional to the cube of the transition energy. There-
fore, well-performing NLO material must possess a low-energy CT
Theoretical optical absorption studies
In order to investigate the electronic transitions in all synthesized
CDs, the TD-DFT calculations were successfully carried out. In
TD-DFT calculations, the first 10 spin-allowed singlet–singlet
excitations were calculated. The percentage contributions of
molecular orbitals for bands formation were obtained by using
GausSum 2.2.4 program.[48] Transition energy (Egm), maximum
wavelengths, oscillator strength (fgm), main configurations, and
transition character are presented in Table 4.
excited state with large oscillator strength.[50] These factors (Δμgm
,
fgm, and Egm) are closely related with each other and are governed
by choice/strength of the donor/acceptor along with the conju-
gated bridge. The most favorable combination of these factors
can provide a larger β value.
All compounds showed absorbance in the visible region. It is
suggested that the maximum absorption wavelength is an
Table 3. Computed polarizability (α) and hyperpolarizability (β) values of cyclohexenone derivatives
Compound
Polarizability (1 × 10ꢀ27 esu)
Hyperpolarizability (1 × 10ꢀ33 esu)
αXX
αYY
αZZ
αav
βXXX
βXXY
βXYY
βYYY
βtot
4BE
242.48
224.97
200.94
217.96
249.14
281.34
253.05
281.31
182.60
165.50
204.32
189.45
159.53
189.18
126.90
142.86
111.53
165.56
218.35
231.82
170.15
175.44
188.33
212.12
944.06
884.94
628.14
ꢀ412.92
505.45
807.34
387.30
1050.73
981.96
931.95
ꢀ938.20
420.86
308.16
924.72
709.12
386.28
1014.62
156.23
867.67
1099.34
634.45
656.34
734.12
807.89
4BC2
4BC4
4BM2
4BM3
4BM4
ꢀ1018.83
ꢀ635.58
902.20
1016.3
708.34
506.23
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J. Phys. Org. Chem. 2016, 29 152–160

OPTICAL ANALYSIS OF NOVEL FLUORESCENT CYCLOHEXENONE DERIVATIVES
Table 4. Time dependent density functional theory calculation data of synthesized cyclohexenone derivatives
a
b
Compound
λmax (nm)
λmax (nm)
Egm (eV)
ƒgm
MO transition
4BE
431
419
420
429
428
427
424
413
423
422
412
423
2.98
3.03
2.86
2.89
2.91
2.94
0.075
0.089
0.045
0.071
0.070
0.072
H → L (73%)
H-2 → L (69%)
H-2 → L (83%)
H-1 → L (80%)
H → L (77%)
H → L (75%)
4BC2
4BC4
4BM2
4BM3
4BM4
Absorption maximum:
aExperimental value,
bComputed value
approximate measure of the achievable transparency.[51] In the
case of 4BE and 4BM3 systems, the HOMO to LUMO transition
is deemed most probable and of low energy. For all other
systems, significantly low transition energy was required
(3.03–2.86 eV). Henceforth, to excite the as-synthesized CDs,
the violet wavelength (λ = 420 nm) is required as mentioned
in emission spectra of these molecules.
Literature revealed that low transition energy results the larger β
value.[52] Our results show that the order of transition energy was
opposite to that of the order of hyperpolarizability. Moreover, the
transition energy showed a decreasing order; 4BC2 < 4BE < 4-
BM4 < 4BM3 < 4BM2 < 4BC4, whereas the NLO response showed
an increasing trend; 4BC2 > 4BE > 4BM4 > 4BM3 > 4BM2 > 4BC4.
Literature suggests that the oscillating strength (fgm) is directly
related to the length of conjugation.[53] Hence, the oscillating
strength of all newly synthesized compounds was recorded in
the following order: 4BC2 > 4BE > 4BM4 > 4BM2 > 4BM3 > 4BC4.
From Tables 3 and 4, it can be clearly observed that the strong
oscillating strength and small transition energy were responsible
for large βtot values. Here, the oscillator strength can be directly
obtained from the TD-DFT calculations.
Acknowledgements
Authors express gratitude to the Department of Chemistry,
Quaid-i-Azam University Islamabad Pakistan for provision of lab
facility and gratefully acknowledge the Higher Education Com-
mission of Pakistan for sponsorship.
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Additional supporting information can be found in the online
version of this article at the publisher’s website.
1
Supplementary data associated with this article shows H-NMR,
13C-NMR spectra and a summary of integrals and peak positions
of 4BE, 4BC4, and 4BM4.
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