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Chemical Physics 303 (2004) 317–326
www.elsevier.com/locate/chemphys
Dual emission of chalcone-analogue dyes emitting in the red region
Tarek A. Fayed *, Mohamed K. Awad
Department of Chemistry, Faculty of Science, Tanta University, 31527 Tanta, Egypt
Received 28 March 2004; accepted 9 June 2004
Available online 6 July 2004
Abstract
The photophysical properties of new synthesized chalcones namely; 1-(40-R-phenyl)-5-(40-dimethylaminophenyl)-2,4- pentadien-
1-one, [R¼H (1), Cl (2) and OCH3 (3)] were studied in different solvents by using steady-state absorption and emission spec-
troscopy. The fluorescence spectra of these chalcones exhibit dual emission in medium and polar solvents. The dual emission was
attributed to population of a polar locally excited (LE) state and a highly dipolar intramolecular charge transfer (ICT) state. The
changes in dipole moments upon excitation were calculated from the solvatochromic plots. The total fluorescence quantum yields
(/f ) were also determined, and their values are strongly dependent on the nature of substitutent and the solvent polarity. Semi-
empirical molecular orbital calculations using the atom superposition and electron delocalization molecular orbital (ASED-MO)
method were also performed to investigate the molecular and electronic structures of these chalcones in both the ground and excited
state. The change of the dipole moment upon excitation was explained on the basis of changes in the charge redistribution over the
whole skeleton of the molecules, which agree well with the experimental results. Also, the nature and energy of the electronic
transitions were elucidated and discussed in relation to the experimental data.
� 2004 Elsevier B.V. All rights reserved.
Keywords: Chalcones; Pentadien-1-one; Dual emission; Semiempirical; ASED-MO method
1. Introduction
Fluorescent dyes that emit in the red region are of
major interest in many fields of chemistry such as de-
velopment of luminescent pigments for plastics and fi-bers or fluoroionphore design for biochemical and
environmental analysis [1,2]. Among these dyes are
carbocyanines [3] and some chalcone derivatives [4].
Chalcones are well-known precursors of many naturally
occurring pigments as flavones, and are used in many
fields of applications such as UV-absorption filters in
polymers [5], in different kinds of optical materials [6], in
food industry [7] and holographic recording technolo-gies [6] as well as in medical therapy [8].
The photophysical properties of substituted chal-
cones have been studied by many researchers involving
mostly asymmetrical donor–acceptor (D/A) chalcones
* Corresponding author. Tel.: +20403120708; fax: +20403350804.
E-mail addresses: tfayed2003@yahoo.co.uk, tfayed@dec1.tanta.
edu.eg (T.A. Fayed).
0301-0104/$ - see front matter � 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2004.06.023
[4,9–14] and to a minor extent symmetrical donor–ac-
ceptor–donor chalcones [13,14]. In these systems, the
acceptor part is the carbonyl group while a 4-donor-
substituted phenyl group serves as the donor part. In
such D/A-molecules, very large changes in charge dis-tribution can be induced in the excited state upon ab-
sorption of light photons. The sudden creation of a giant
dipole (due to photoinduced intramolecular charge
transfer, ICT) can result in a strong interaction with the
surrounding medium to cause not only solvent reorga-
nization but also sometimes to structural rearrangement
in the solute itself. These changes lead, in several mol-
ecules, to distinguishable spectroscopic properties suchas a large Stokes-shifted fluorescence maximum in ad-
dition to the normal Stokes-shifted band, a phenomenon
that is called as dual luminescence. Although several
molecules, in which ICT reaction takes place, show dual
fluorescence, the origin of this process is presently under
debate [14]. The best known example that shows solvent
dependent dual fluorescence is p-dimethylaminobenzo-
nitrile [15]. Since its discovery ample experimental and
R
O
NMe2R =H (1)
Cl (2)
OCH3 (3)
Scheme 1.
318 T.A. Fayed, M.K. Awad / Chemical Physics 303 (2004) 317–326
theoretical evidences have been accumulated which
confirm that such a molecule undergoes ICT accompa-
nied by solvent assisted structural reorganization. Thedual fluorescence has been rationalized in terms of a
non-radiative conversion between a primary non-polar
excited state and a charge transfer state, which are re-
sponsible for a short-wavelength and a long-wavelength
emission, respectively. The most commonly accepted
explanation, proposed by Grabowski [16,17] and re-
viewed by Rettig [18], assumes that dielectric polariza-
tion of the solvent permits excited state rotationalisomerization, leading to a highly polar twisted ICT
state with a conformation of the Dþ and A� subunits
close to perpendicular configuration [16–19]. This leads
to a full charge separation and consequently to a large
dipole moment.
Very few were reported about enones containing
butadiene bridge [20]. Therefore, we report on a detailed
study of ICT process of some D/A-chalcones, namely,1,5-diphenyl-2,4-pentadien-1-one (Scheme 1) by using
steady state absorption and emission measurements.
Quantum chemical studies using semiempirical molecu-
lar orbital calculations were also performed in order to
elucidate theoretically the effect of charge redistribution
in the excited state on the photophysical properties of
these chalcones. The molecular and electronic structures
were investigated using the same method. Also, thestudy aims at exploring the mechanism of dual fluores-
cence exhibited by these D/A-molecules. The investi-
gated dyes were chosen depending on their valuable
spectroscopic properties such as intense absorption in
the visible region and strongly red-shifted red emission
in highly polar solvents. Moreover, the emission prop-
erties of these chalcones can be tuned by changing the
acceptor moiety.
2. Experimental
2.1. Samples and solvents
The investigated chalcones 1, 2 and 3 were synthe-
sized by condensation of equivalent amounts from 4-(dimethylamino)-cinnamaldehyde and 40-substitutedacetophenone in a water–ethanol mixture (30%, v/v)
using NaOH as a base [21]. After crystalization from
hexane, the products were characterized by TLC, ele-
mental analysis, UV–Vis. and fluorescence measure-
ments. All the employed solvents were of spectroscopic
grade from Aldrich and used as received. The following
abbreviations are used throughout the text: methanol,MeOH; ethanol, EtOH; n-propanol, PrOH; n-butanol,
BuOH; acetonitrile, ACN; dimethylformamide, DMF;
dimethylsulfoxide, DMSO; acetone, AC; dichlorome-
thane, DCM; ethylacetate, EA; diethylether, DEE; tol-
uene, Tol; n-hexane, Hex. The used water was double
distilled.
2.2. Spectral measurements
Steady state absorption and emission measurements
were carried out using a Shimadzu UV-3101PC scan-
ning spectrophotometer and a Perkin–Elmer LS 50B
spectrofluorometer, respectively. For the measurement
of fluorescence quantum yields (/f ) only diluted solu-
tions were used (optical densities at the excitation
wavelength is less than 0.2). The samples were excitedaround their absorption maximum. Fluorescien in 0.1 M
NaOH (/f ¼ 0:93) was used as a standard for /f de-
termination [22]. All measurements were carried out
under red light at 25 �C� 1 using fresh solutions (10�5
M).
2.3. Theoretical method
The atom superposition and electron delocalization
molecular orbital (ASED-MO) theory [23,24] was used.
This is a semiempirical theoretical approach based on
partitioning the electronic charge density into free atom
parts and a component due to electron delocalization
bond formation. Using the electrostatic theorem, the
forces on the nuclei are integrated as the atoms are
brought into a molecular configuration to yield a re-pulsive energy due to rigid atom densities and an at-
tractive energy due to electron delocalization. The sum
is the exact molecular binding energy. In the ASED-MO
method the atom superposition energy is calculated
from the actual atomic densities and the electron delo-
calization is approximated as the change in the total
one-electron valence orbital energy obtained by using a
modified extended H€uckel Hamiltonian. This techniquehas already been applied for predicting molecular
structures, reaction mechanisms, and electronic and vi-
brational properties (see recent Refs. [25–27]). Atomic
parameters used in the calculations are given in Table 1.
Because of ionicity, the ionization potentials for N 2s
and 2p are decreased by 2.5 eV from the atomic values
[28]. The N 2s and 2p Slater exponents are decreased by
0.30 au from the atomic values of Clementi and Rai-mondi [29] to get a reasonable bond length. Unshifted
parameters are used for C, O, Cl and H, Table 1. In all
reported results for equilibrium structures, the bond
Table 1
Atomic parameters used in the calculations: principle quantum numbers, n; ionization potentials, IP (eV); orbital exponents, f (a.u.)
Atom s p
n IP f n IP f
C 2 16.590 1.6580 2 11.260 1.6180
N 2 17.830 1.6237 2 12.040 1.6170
O 2 28.480 2.2459 2 13.620 2.2266
Cl 3 24.540 2.3562 3 13.010 2.0388
H 1 13.600 1.2000
T.A. Fayed, M.K. Awad / Chemical Physics 303 (2004) 317–326 319
lengths are variationally optimized to the nearest degree.
In all calculations the C–H bond lengths are kept con-
stant at 1.10 �A.
3. Results and discussion
3.1. Absorption and fluorescence spectra
The normalized absorption and emission spectra of 1
in solvents of different polarity are shown in Fig. 1 and
the corresponding spectroscopic data of 1–3 are col-lected in Table 2. The spectra reveal an intense struc-
300 400 500 6000.0
0.5
1.0EtOH
Et.acetate
ACN
n-hexaneAbs
orba
nce
Wavelength (nm)
450 500 550 600 650 7000.0
0.4
0.8
1.2 tolueneEtOH
ACNacetone
Et.acetaten-hexane
Em
issi
on in
tens
ity (
a.u.
)
Wavelength (nm)
(a)
(b)
Fig. 1. Normalized absorption (a) and fluorescence (b) spectra of
compound 1 in the mentioned solvents.
tureless absorption band in the visible region (emax
ranges from 32 to 42� 103 lmol�1 cm�1). This band
suffers a bathochromic shift as the solvent polarity is
increased indicating an increase in the dipole moment ofthe Franck–Condon (FC) excited state. Similar obser-
vations were found in the case of 2 and 3. Therefore, this
absorption band is attributed to the charge transfer
from the dimethylanilino group to the keto–phenyl
fragment via the butadiene bridge. Also, Increasing the
acceptor strength from OCH3– via 1 to chloro-substi-
tuted keto–phenyl group leads to a pronounced batho-
chromic shift, which supports the charge transfer natureof this band. This is exemplified by a good correlation
between the band maximum and the rp – Hammett
constant [30] in a polar solvent such as ACN
[ka ¼ 432þ 15:8rp r ¼ 0:999]. Further confirmation for
the charge transfer direction comes from the observed
blue shift of the absorption maximum in strongly hy-
drogen bonding solvents like water and MeOH, Table 2.
Such behaviour is consistent with the restriction ofcharge transfer from the –NMe2 group due to hydrogen
bonding interactions between solvent molecules and the
electron lone pair on the donor group. In fact, molecular
association is ruled out since the Beer’s law is verified
within the concentration range from 5� 10�4 to
5� 10�6 M.
The oscillator strength (f ) was also calculated from
the relation [31]
f ¼ 4:32� 10�9emaxDm1=2; ð1Þwhere Dm1=2 is the half width at absorption maximum in
cm�1. The obtained values range from 0.67 to 0.86 (no
general trend was found with changing the solvent po-larity). These large values indicate that the involved
electronic transition is analogous to the strongly allowed
p–p� transition associated with a planar configuration
[32].
Unlike the moderate effect of solvent polarity on the
absorption spectra of 1–3, their fluorescence spectra are
strongly affected by changing the solvent. In non-polar
solvents such as Hex, the emission spectra are structuredwhile in weakly polar Tol, the spectra are structureless
with a maximum around 512–517 nm and a tail ex-
tending to over 600 nm. However, a slight increase in
the solvent polarity, from Tol to DEE (e ¼ 2:4 and 4.3,
Table 2
Absorption (ka) and emission (kf ) maxima of 1–3 measured in solvents having different polarities
Solvent 1 2 3
ka (nm) kf (nm) ka (nm) kf (nm) ka (nm) kf (nm)
Hex 407 479 505sh 416 488 525sh 408 474 503sh
Tol 427 512 435 517 – 429 512 –
DEE 419 524 554sh 425 530 560sh 421 524 558sh
EA 422 534 557sh 431 546sh 568 425 533 572sh
DCM 432 540sh 568 442 543sh 577 438 545 564sh
DMSO 444 545sh 595 452 – 605 445 542sh 589
AC 429 537sh 570 435 543sh 584 429 544sh 566
DMF 435 540sh 586 441 550sh 596 435 542sh 577
ACN 432 542sh 587 436 552sh 596 428 548sh 577
BuOH 440 538sh 600 448 538sh 608 442 540sh 596
PrOH 440 538sh 605 448 538sh 611 443 541sh 600
EtOH 442 542sh 610 446 540sh 616 443 541sh 606
MeOH 440 540sh 612 440 540sh 618 439 541sh 610
H2O 423 – – 430 – – 366 – –
Sh, shoulder (lower intensity).
320 T.A. Fayed, M.K. Awad / Chemical Physics 303 (2004) 317–326
respectively), leads to development of a shoulder at the
longer wavelength side of the main band. Further in-crease of the solvent polarity enhances the emission in-
tensity of the longer wavelength band relative to that of
the shorter wavelength one. These changes are accom-
panied by continuous bathochromic shift in the emission
maximum (more than 125 nm on going from Hex to
EtOH), thus, supporting the charge transfer nature and
the increased dipole moment of the excited singlet state.
Comparing the fluorescence spectra of compounds 1, 2and 3 in the same solvent (especially moderately polar
solvents like EA and DCM) helps to determine the role
of the acceptor group in appearance of the dual emis-
sion. Fig. 2 reveals significant differences in the fluo-
rescence spectra of the three D/A-chalcones recorded in
DCM. As can be seen, the intensities of the merged long
and short wavelength emission bands of 3 are compa-
rable. On the other hand, the intensity of the lower en-ergy fluorescence band of 1 is enhanced relative to that
of the higher energy one, whereas for 2 the higher energy
450 500 550 600 650 7000.0
0.5
1.0
321
Flu
ores
cenc
e in
tens
ity (
a.u.
)
Wavelength (nm)
Fig. 2. Fluorescence spectra of compounds 1–3 in dichloromethane.
emission band is hardly detected. These changes were
attributed to the increased acceptor strength of theketo–phenyl group on going from methoxy to chloro-
substituted chalcones, which enhances and stabilizes the
separated charges.
The fluorescence excitation spectra of the respective
emission bands of 1–3 measured in ACN solvent, as an
example, are nearly matched and almost resemble the
lower energy absorption band. Fig. 3 depicts the exci-
tation spectra of 1 as an illustrative example. In con-trast, the emission spectra (not shown here) measured at
different excitation wavelengths in ACN do not display
any changes in their shapes (except for changes of in-
tensity). Therefore, the dual emission exhibited by the
present compounds is due to the presence of one species
in the ground state.
The appearance of dual fluorescence is also docu-
mented for 1 in Hex–EtOH binary mixtures, Fig. 4. Asthe EtOH concentration is increased, the long wave-
300 350 400 450 500 5500
100
200
300
400
Inte
nsity
(a.
u.)
Wavelength (nm)
Fig. 3. Fluorescence excitation spectra of 1 in ACN monitored at
kem ¼ 592 nm (–––) and 543 nm (- - -).
450 500 550 600 650 7000
40
80
120
10
50.0
50
25
100
75
15
Wavelength (nm)
Flu
ores
cenc
e in
tens
ity (
a.u.
)
Fig. 4. Fluorescence spectra of 1 in hexane–EtOH mixed solvents. The
EtOH ratios (v/v%) are shown in the figure.
0.00 0.05 0.10 0.15 0.20 0.25 0.30
3600
4200
4800
5400
6000
∆νst,(
cm-1)
∆f
Fig. 5. Plots of the Stokes shifts of compound 1 (�), 2 (s) and 3 (j)
vs. the solvent polarity function Df , Eq. (2).
S
S
o
1
ICT
FC
emission at lowerenergy
emission at higherenergy
excitation
LE
Scheme 2.
T.A. Fayed, M.K. Awad / Chemical Physics 303 (2004) 317–326 321
length fluorescence band suffers continuous red shift and
grows at the expense of the short wavelength band.
Similar behaviour was also found for 2 and 3.
3.2. Dipole moment change
The three D/A-chalcones are characterized by an in-
crease in the Stokes shift (D�mst) with increasing the sol-
vent polarity. This points to stronger stabilization of the
excited state in polar solvents. The solvatochromic shifts
were processed by applying the simplified Lippert–
Mataga relation [33,34]
D�tst ¼ðle � lgÞ
2
hca3Df þ Const:; ð2Þ
Df ¼ e� 1
2eþ 1� n2 � 1
2n2 þ 1: ð3Þ
In this relation Dl ð¼ le � lgÞ is the change in dipole
moment upon excitation, h is Planck’s constant, c is thespeed of light and a is the Onsager radius. e and n are thedielectric constant and refractive index of the solvent,
respectively. The data in hydroxylic solvents were ex-cluded to avoid specific solute–solvent interactions. The
Lippert–Mataga plots (Fig. 5) clearly demonstrate a
change in the slope at an intermediate polarity region
(Df > 0:2). This behaviour indicates that the polar
character of the emitting state changes on going from
less to highly dipolar solvents. The changes of dipole
moment upon excitation were calculated from the slopes
Table 3
Slopes (m) of the solvatochromic plots according to Eq. (1), molecular radiu
Compound M1ðrÞ Dl1 (D) m
1 6118 (0.99) 15.4
2 6646 (0.99) 16.74 1
3 5398 (0.99) 16.56 1
The correlation coefficients are given in parenthesis.
of the plots and collected in Table 3. The Onsager radiuswas estimated from the optimized distance between the
two farthest atoms in the direction of charge separation
within the molecules (a ¼ 40% of this distance [35]). As
can be seen, the dipole moment changes are much larger
in highly polar solvents than those in less polar solvents.
Nevertheless, both values indicate highly polar excited
states compared to the corresponding ground state.
Such a property is a prerequisite to obtain relevant hy-perpolarizability [36], and suggests that the present D/A-
chalcones can serve as good candidate components of
non-linear optical materials.
To explain these results as well as the observed dual
emission in moderately and highly polar solvents, the
following scheme has to be considered (cf. Scheme 2).
According to this scheme, the dual emission arises
from two states termed as locally excited (LE) state andan ICT state. It is necessary to mention that the
s (a) and the changes of dipole moments (Dl) of 1–3
2ðrÞ Dl2 (D) a (�A)
9856 (0.96) 19.58 7.3
5569 (0.97) 25.63 7.5
3676 (0.98) 26.36 8.0
30 32 34 36 38 40 42 44 460.00
0.02
0.04
φ f
ET(30), (kcal/mole)
Fig. 6. Plots of /f vs. the solvent polarity parameter ETð30Þ for the
investigated chalcones; 1 (j), 2 (�) and 3 (s).
Table 4
Fluorescence quantum yields (/f ) of 1–3 in different solvents measured
at 25 �C
Solvent 1 2 3
Hex 0.005 0.0074 0.0083
Tol 0.012 0.013 0.013
DEE 0.013 0.0244 0.013
EA 0.023 0.037 0.018
DCM 0.028 0.039 0.020
DMSO 0.023 0.030 0.011
AC 0.031 0.042 0.022
DMF 0.030 0.031 0.026
ACN 0.027 0.026 0.019
BuOH 0.014 0.013 0.014
PrOH 0.009 0.009 0.012
EtOH 0.0044 0.004 0.0078
MeOH 0.0014 0.0014 0.0024
322 T.A. Fayed, M.K. Awad / Chemical Physics 303 (2004) 317–326
red-shifted absorption spectra in polar solvents, indi-
cates that the initial FC excited state is already polar.
So, we can assume that the LE state, which predomi-
nates in less polar solvents and emits at higher energy, is
also polar. The LE state is obtained directly after re-laxation of the initially formed FC excited state via re-
arrangement of the solvent molecules into the most
favorable alignment with the excited state dipole. The
polar nature and increased dipole moment of the LE
state is supported by the pronounced red-shift of the
short wavelength fluorescence band maximum on going
from Hex to EA. However, the position of this band is
less sensitive to the polarity change in highly polar sol-vents, Table 2. This is due to the less polar nature of the
LE state when compared to the ICT state.
In solvents with polarity larger than EA, another
highly dipolar excited state becomes the lowest state
which is also emissive. This state emits at lower energy
and exhibits large red-shift as the solvent polarity is
increased from DCM to MeOH. This behaviour is
consistent with the larger Dl values calculated in highlypolar solvents, and indicates an ICT state. The forego-
ing results show also that the equilibrium between both
the LE and ICT states (their relative contribution to the
emission band) depends strongly on both the solvent
polarity and the acceptor strength of the keto–phenyl
group, see Figs. 1 and 2.
Dual emission may also arise from population of a
twisted ICT state in addition to the normal ICT state. Infact, semi-empirical calculations reveal highly planar
structures for the studied D/A-chalcones in both the
ground and excited state. Also, the strong electronic
interaction between the donor and acceptor subunits
could result in flattening of the excited molecule.
Therefore, the long wavelength emission band exhibited
by the present molecules, in polar solvents, could not be
attributed to a twisted ICT state. For these reasons, theobserved dual emission is explained in terms of a po-
larity-induced change in the energetic ordering of two
polar emissive states having largely different charge
transfer characters.
A complete localization of electronic positive and
negative charges on the donor and acceptor groups
separated by 12.5 �A leads to an excited state dipole
moment of 60 D [37]. This value seems to be greaterthan those could be calculated from the Dl values ob-
tained here. Thus electronic excitation of the present
chalcones leads to a p–p� state with a partial charge
transfer character.
3.3. Fluorescence quantum yields
Although the compounds under investigation are nothighly fluorescent, the total fluorescence quantum yield
(/f ) depends strongly on the solvent polarity and the
acceptor strength of the keto–phenyl group, Table 4.
Plots of /f vs. ETð30Þ, Reichardt’s empirical solvent
polarity scale [38], in non-hydroxylic solvents, are shown
in Fig. 6. These plots indicate that the /f value increases
with increasing the solvent polarity (a negative solvat-
okinetic effect), except for the region of highly polar
solvents as in ACN. Also, the relative /f values decreasein the order 2 > 1 > 3.
For interpretation of the emission behavior of the
present D/A-chalcones, the possible deactivation routes
of the excited singlet state have to be considered. These
routes include rotation around certain C–C single bonds
(conformational changes), coupling of the excited singlet
and triplet states through an enhanced intersystem
crossing, and twisting around the C@C bond (trans/cisphotoisomerization). Based on the results of semi-em-
pirical calculations (see later), conformational changes
due to rotation around C7–C8 single bonds are excluded
due to unstability of the obtained conformer (its energy
is ca. 15 eV higher than that of the conformer shown in
Fig. 7). Also, it was reported that intersystem crossing
Fig. 7. The calculated molecular structure of the investigated compounds.
T.A. Fayed, M.K. Awad / Chemical Physics 303 (2004) 317–326 323
and population of the triplet state do not contribute
effectively to deactivation of the excited D/A-chalcones
containing an ethylenic bond [10]. Accordingly, we can
assume that the singlet–triplet intersystem crossing does
not play an effective role in fluorescence quenching forthe present compounds. In analogy to D/A-stilbenes [18]
the non-emissive twisted C@C species, with biradicaloid
properties and reduced dipole moment, enhances the
non-radiative decay of the excited singlet state. A fur-
ther possibility for strong fluorescence quenching is
connected with the proximity of n; p� and p; p� states
[10]. Therefore, the negative solvatokinetic effect, ob-
served here, can be explained considering these two ef-fects. Steady state irradiation of compound 1 (as an
example) in Hex and ACN solutions, using 365 nm light,
results in a significant occurrence of trans–cis photo-
isomerization. This was concluded from the changes in
the absorption spectra. However, the changes are com-
parable in both solvents (spectra not shown). Hence, in
non-polar solvents like Hex, formation of the weakly
polar biradicaloid state alone cannot explain the lower/f values. Most probably in these solvents, the emitting
state (with polar characters) is significantly destabilized
to experience perturbation by the proximity effect and
vibronic coupling to an energetically close lying n� p�
state [9]. This opens an effective non-radiative deacti-
vation pathway for the excited singlet state, which ex-
plains the observed negative solvatokinetic effect.
As in the case of solvent sensitive fluorescent com-
pounds, the /f decreases in highly polar proton donor
solvents like alcohols, Table 4. This effect is due to ef-ficient hydrogen bond formation between the solvent
molecules and the carbonyl group of the fluorophore.
According to charge density calculations, the carbonyl
group of the present chalcones becomes more negatively
charged in the excited state (see later). So, we expect that
hydrogen-bonding interaction is playing a key role in
enhancing the radiationless deactivation of the excited
singlet state.
3.4. Quantum chemical calculations
The ASED-MO theory is applied to investigate the
molecular geometric and electronic structures of com-
pounds 1–3 in the ground and excited states. The opti-
mized molecular structures obtained by optimizing their
bond lengths, bond angles and dihedral angles, areshown in Fig. 7. The calculations show stable planar
structures with a minimum energy when the dihedral
angles are equal to zero. The calculated bond lengths for
C4–C7, C7–C8, C9–C10 and C11–C12 are somewhat
longer, with single bond characters, than those of
324 T.A. Fayed, M.K. Awad / Chemical Physics 303 (2004) 317–326
C8–C9, and C10–C11, with double bond characters, Ta-
ble 5. These values are in agreement with the expected
ones, and are consistent with the calculated bond orders.
However, these bond lengths show slight changes upon
excitation. Contrarily, the calculations show an inver-sion of bond order values upon excitation where the
bonds characterized by higher bond orders in the
ground state exhibit lower bond orders in the excited
state and vice versa. Meanwhile, the C–O bond length is
the shortest one and is lengthened from 1.22 �A in the
ground state to 1.27 �A in the excited state. This is ac-
companied by a decrease of the bond order, as con-
firmed by the calculations of M€uliken overlappopulation, see Table 5. Also, the calculations show that
the molecular structures of the ground and excited states
are slightly affected by the chloro- and methoxy-sub-
stituents.
Our calculations suggest an increasing of the dipole
moments of the investigated compounds upon excita-
tion, which agree with the experimental results. This
comes from studying the distribution of the positive andnegative charges over the whole skeleton of the mole-
cules in the ground and excited states. In the excited
state, a polar structure is shown for compound 1 with a
Table 6
The calculated net charges on selected atoms of 1–3 obtained from ASED-M
Atom 1 2
O18 )0.8523 ()1.0195) )0.8644 ()1.023O22 – –
Cl – )0.1293 ()0.154N19 0.1869 (0.5524) 0.1944 (0.5446
C7 0.7023 (0.5660) 0.6965 (0.5661
C8 )0.1299 ()0.1213) )0.1404 ()0.130C9 0.1230 ()0.0450) 0.1189 ()0.052C10 )0.0644 (0.332) )0.0551 (0.0309
C11 0.0683 ()0.1064) 0.0688 ()0.118C12 )0.5343 (0.0651) )0.0593 (0.0638
C15 0.1421 (0.1206) 0.1421 (0.1181
Table 5
The bond lengths (�A) and (bond orders) of selected bonds obtained from A
Bond 1 2
Ground Excited Ground
C4–C7 1.64 (0.7927) 1.60 (0.8314) 1.61 (0.8046)
C7–C8 1.59 (0.8582) 1.57 (0.9118) 1.60 (0.8315)
C8–C9 1.52 (1.0476) 1.54 (0.9688) 1.51 (1.0553)
C9–C10 1.57 (0.8916) 1.56 (0.9219) 1.57 (0.8864)
C10–C11 1.53 (1.0467) 1.53 (1.0062) 1.51 (1.0536)
C11–C12 1.58 (0.8831) 1.58 (0.8765) 1.58 (0.8772)
C15–N19 1.58 (1.0280) 1.56 (1.0007) 1.57 (0.9812)
N19–C20 1.74 (0.6454) 1.73 (0.6584) 1.74 (0.6497)
C7–O18 1.22 (1.0241) 1.27 (0.9120) 1.22 (1.0469)
C1–Cl – – 1.76 (0.6960)
C1–O22 – – –
O22–C23 – – –
positive charge of 0.8293 e on the dimethylaniline moi-
ety and )0.5169 e on the keto–phenyl group. Also, upon
excitation the charge density on the oxygen atom in-
creases from )0.8523 to )1.0195 e. Moreover, the higher
positive charge on the aniline moiety comes from in-creasing of the positive charge localized on N19 from
0.1869 to 0.6624 e. Meanwhile, in the ground state,
smaller positive and negative charges are calculated,
with values of 0.2378 and )0.1380 e, respectively, which
indicates a lower apparent molecular dipole moment.
The charge distribution over the whole skeleton of
molecules 2 and 3 is also calculated, Table 6, to study
the effect of substituents on the effective dipole momentof the ground and excited states. The calculations show
that the ground state dipole moment of compound 2 is
smaller than that of compound 1. It has a positive
charge equals to 0.2273 e on the aniline moiety and a
negative charge of )0.1196 e on the chloro-phenyl
group. For compound 3, a higher ground state dipole
moment, with a positive and a negative charge equals to
0.2728 e and )0.1413 e, respectively, is suggested. Incontrast, the polarity of both compounds increases upon
excitation. In the case of compound 2, the positive
charge on the positive center of the molecule increases to
O calculations in ground and (excited) states
3
3) )0.8944 ( )0.9673))0.3573 ()0.2813)
7) –
) 0.2607 (0.5395)
) 0.6820 (0.5591)
7) )0.1466 ()0.1590)8) 0.1176 ()0.0751)) )0.0701 (0.0152)
3) 0.0649 ()0.1303)) )0.0787 (0.0556)
) 0.1424 (0.1113)
SED-MO calculations in ground and excited states
3
Excited Ground Excited
1.60 (0.8328) 1.63 (0.7927) 1.61 (0.8252)
1.57 (0.9130) 1.58 (0.8582) 1.57 (0.8768)
1.54 (0.9647) 1.51 (1.0476) 1.50 (1.0066)
1.56 (0.9248) 1.57 (0.8916) 1.58 (0.8966)
1.53 (1.0021) 1.51 (1.0467) 1.50 (1.0308)
1.58 (0.8789) 1.58 (0.8831) 1.58 (0.8758)
1.56 (1.0017) 1.54 (1.0280) 1.54 (1.0255)
1.73 (0.6596) 1.74 (0.6454) 1.72 (0.6631)
1.27 (0.9114) 1.23 (1.0241) 1.25 (0.9321)
1.78 (0.6231) – –
– 1.24 (0.9369) 1.22 (0.9957)
– 1.41 (0.6036) 1.42 (0.5960)
Fig. 8. Correlation diagram of the calculated electronic structure for the investigated compounds.
Table 7
Types of possible transitions and their excitation energies, DE (eV), obtained from the ASED-MO method
Transition 1 2 3
n ! p� 1.090 1.141 1.158
p ! p� 2.024 2.097 1.793
n ! p� 2.450 2.306 2.378
p ! p� 3.028 (3.044) 3.028 (2.98) 3.034 (3.077)
The DE values calculated from the absorption maximum in Hex are given in parentheses.
T.A. Fayed, M.K. Awad / Chemical Physics 303 (2004) 317–326 325
0.8321 e while the negative charge becomes )0.4660 e.
Similarly, the calculated positive and negative charges
for compound 3 upon excitation increase reaching
0.8083 and )0.3608 e, respectively.
We used ASED-MO calculations to investigate the
nature of electronic transitions and calculate their en-
ergies. The calculated orbital energies and electronic
structures of the investigated compounds are shown inFig. 8. The figure shows that the highest occupied mo-
lecular orbital, HOMO, of compound 3 is more desta-
bilized than that of compound 2 (0.043 and 0.014 eV,
respectively, relative to that of 1). The lowest unoccu-
pied molecular orbitals, LUMO, are also destabilized by
0.111 and 0.065 eV, respectively. This leads to a de-
creasing of the energy gap between the HOMO and
LUMO levels in the case of compound 2 more than inthe case of compound 3. The sequence of some elec-
tronic transitions obtained from ASED-MO calcula-
tions are summarized in Table 7. Their energy is slightly
dependent on the substituent. In all the studied com-
pounds, the lowest transitions are assigned to n–p� ex-
citation. This corresponds to an electronic transition
from the HOMO level, at )11.262 eV and significantly
localized on the nitrogen lone-pair, to the LUMO level,at )10.172 eV with complete p� characters and consid-
erable contribution from C8 and C9 atoms. In view of
the position of the energy levels involved in these tran-
sitions and the lower excitation energy (�1.1 eV), these
states cannot be ignored while interpretating the fluo-
rescence of the present D/A-chalcones. Also, there is
another possible n ! p� transition with excitation en-
ergy ranging from 2.3 to 2.45 eV, which is due to a
transition from the oxygen lone-pair orbital, shown inFig. 8 with a large amplitude on the oxygen atom, to the
LUMO level with complete p� characters. The calcula-
tions indicate a further excitation with an energy around
3.03 eV, corresponding to a p ! p� transition. This is
due to a transition from the molecular orbital at )13.200eV, significantly localized on C8, C9, C12 and C13 atoms,
to the LUMO level with complete p� characters. The
energy of this transition is in a good agreement with theexperimental results, Table 7.
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