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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