Experimental and Theoretical Studies of the Spectroscopic Properties of Chalcone Derivatives

Article abstract of DOI:10.1007/s10895-016-1981-2

Newly synthesized, differently substituted chalcones (1,3-diaryl-2-propen-1-ones) have been studied using steady-state and time-resolved techniques combined with quantum-chemical modelling. To explore spectroscopic structure - property relationships the substituent (acceptor moiety) was chosen according to systematic variation in the Hammett parameter. It was shown that photophysical properties of the studied donor-acceptor (D-A) molecules can be predicted in terms of a simple model from the properties of individual chromophores (composite-model of decoupled moieties: donor (D) and acceptor (A)). The results of spectroscopic measurements also indicate that for investigated D-A fluorophores in medium-polar solvent, the initially populated, locally excited (LE) state (where the fundamental role plays donor moiety (D*-A)) reacts further to produce intramolecular charge transfer (ICT) state. Additionally, the experimental absorption (M_ge) and fluorescence (M_eg) transition dipole moments were calculated on the basis of spectroscopic data and compared with results of our quantum-chemical calculations. The absorption transition dipole moment was found to vary linearly with the Hammett substituent coefficient (σ).

Full text of DOI:10.1007/s10895-016-1981-2

J Fluoresc
DOI 10.1007/s10895-016-1981-2
ORIGINAL ARTICLE
Experimental and Theoretical Studies of the Spectroscopic
Properties of Chalcone Derivatives
Marek Pietrzak1 & Marek Józefowicz2 & Agnieszka Bajorek1 & Janina R. Heldt2
Received: 2 August 2016 /Accepted: 3 November 2016
#
Springer Science+Business Media New York 2016
Abstract Newly synthesized, differently substituted chalcones
(1,3-diaryl-2-propen-1-ones) have been studied using steady-
state and time-resolved techniques combined with quantum-
chemical modelling. To explore spectroscopic structure - prop-
erty relationships the substituent (acceptor moiety) was chosen
according to systematic variation in the Hammett parameter. It
was shown that photophysical properties of the studied donor-
acceptor (D-A) molecules can be predicted in terms of a simple
model from the properties of individual chromophores (compos-
ite-model of decoupled moieties: donor (D) and acceptor (A)).
The results of spectroscopic measurements also indicate that for
investigated D-A fluorophores in medium-polar solvent, the ini-
tially populated, locally excited (LE) state (where the fundamen-
tal role plays donor moiety (D*-A)) reacts further to produce
intramolecular charge transfer (ICT) state. Additionally, the ex-
perimental absorption (Mge) and fluorescence (Meg) transition
dipole moments were calculated on the basis of spectroscopic
data and compared with results of our quantum-chemical calcu-
lations. The absorption transition dipole moment was found to
vary linearly with the Hammett substituent coefficient (σ).
Introduction
The changes in the distribution of the electron density upon
photoexcitation of the luminescent molecule very often cause
that spectroscopic, photophysical and photochemical proper-
ties of the molecule in the excited states differ diametrically
from the properties in its ground state. This statement is par-
ticularly richly supported by the example of the molecules
possessing an electron donor (D) and an electron acceptor
(A) groups, in case of which the photoinduced intramolecular
electron transfer process has been observed [1, 2]. The mech-
anism of this phenomenon has become the subject of a widely
spread discussion among many scientific groups, right after
the Lippert’s discovery of an anomalous fluorescence of
DMABN molecule in a polar environment [1–3].
The large class of donor-acceptor (D-A) molecules is inter-
esting for two reasons: theoretical - because they are an excel-
lent candidate to studies the general theory of photoinduced
intramolecular charge transfer (ICT) process and practical –
because: (1) such molecules are often very sensitive to the
microenvironment and therefore they have been examined
for application as optical probes, (2) they have been also ex-
tensively studied due to their potential applications in various
optical devices (in nonlinear optics (NLO)) [1–12]. In light of
the presented information, one can completely understand the
fact that the development of the research into the photochem-
istry and photophysics of various organic D-A compounds has
been very intensive for the last several years [1–17].
.
Keywords Donor–acceptor system Intramolecular charge
.
transfer Chalcones, Azachalcones
This article is dedicated to the memory of Professor Józef Heldt
(1934–2015).
* Marek Józefowicz
fizmj@univ.gda.pl
Chalcones are aromatic ketones occur in nature; consist of
an electron donor and electron acceptor groups connected by a
π-conjugated spacer. The importance of such chalcones lies in
their medical applications. Research over the last decade has
shown that chalcones exhibit antimalarial [18], anti-infective
[19], anticancer [20], anti-inflammatory [21], antifungal [22],
antioxidant [23], and antituberculosis [24] properties.
1
Faculty of Chemical Technology and Engineering, UTP University
of Science and Technology, ul. Seminaryjna 3,
85-326 Bydgoszcz, Poland
2
Institute of Experimental Physics, University of Gdańsk, ul. Wita
Stwosza 57, 80-952 Gdańsk, Poland

J Fluoresc
Additionally, chalcones have been also extensively studied
due to their potential applications in various optical devices
[25] and due to their unique microenvironment-dependent op-
tical properties [26].
the studied D-π-A dyes could be explained in terms of a
composite-molecule model based on the electronic states of
the donor and acceptor subunits [4, 26, 27].
The most essential quantity which determines the transition
probabilities of the absorption and emission processes is tran-
sition dipole moment (Mge and Meg ) between ground (g) and
excited (e) states . It is well known that, the knowledge of the
absorption and fluorescence transition dipole moments
(values and orientation) is a helpful to get information
about the character of the excited state and the relaxa-
tion mechanism (physico-chemical processes in bulk
solution) [15, 16]. The results of the steady-state and
time-resolved spectroscopic measurements of the mole-
cules under study in methylcyclohexane (MCH) and
ethyl acetate (EA) were used to calculate the electric
transition dipole moments of the absorption and emission pro-
cesses. Obtained experimental data were compared with the-
oretical results of quantum-chemical calculations. It is impor-
tant to note that good correlation was found between the ab-
sorption transition dipole moment and the Hammett substitu-
ent coefficient (σ).
Despite the widespread use of chalcones in medicine, ma-
terials science (nonlinear optical (NLO) materials) and pho-
tonic technologies, no systematic and profound studies have
been carried out to explain spectroscopic and photophysical
properties of the investigated, newly synthesized dyes. In a
continuation of our recent works on the spectroscopic proper-
ties of D-A molecular systems [13–17], in this paper we report
the synthesis of new differently substituted chalcones (1,3-
diaryl-2-propen-1-ones) (see Scheme 1). In order to obtain
more insight into the photophysics of newly synthesized
chalcones, it is necessary to studies their fundamental spectro-
scopic properties. Because one of our aims is to design D-π-A
fluorescence probes, which exhibit strong solvatochromism,
the purpose of the present paper is to present comprehensive
studies of the substitution effect on the spectral properties of
the investigated dyes.
We have recently shown, that the photophysical properties
of the studied some D-A biphenyl derivatives can be predicted
in terms of a simple model from the properties of individual
chromophores (composite-model of decoupled moieties: do-
nor (D) and acceptor (A)) [14, 16]. Taking into account above,
we want to predict whether or not the electronic structure of
Theoretical Background
As it was mentioned earlier, the most essential quantity which
determines the transition probabilities of the absorption and
emission processes are absorption and fluorescence transition
dipole moments (Mge and Meg), defined by the integral
[28, 29]:
O
O
O
N
N
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
*
+
K00
A02
A02
A02
K00A02
K01A02
K02A02
X
Mge ¼ e Ψg
z r Ψ and
ð1Þ
ꢀ
i
i
e
ꢀ
i
O
O
O
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
*
+
N
N
X
N
ꢀ
ꢀ
ꢀ
Meg ¼ e Ψe
z r Ψ
;
ꢀ
(1)
i
i
g
N
ꢀ
i
K01
where Ψg, Ψe are wavefunctions of the ground and excited
states, respectively; ri is the position vector of the i-th particle
(electron or nucleus) of charge zie in the molecule.
According to Strickler and Berg [30], the radiative transi-
tion probability can be calculated from the steady-state spec-
troscopic measurements using the expression:
O
O
O
N
N
N
N
K02
O
N
ꢁ
ꢂ
Z
−1
ꢃ ꢄ
8π⋅2303cn3F
O
−3
ν~F
O
k F
¼
ε ν~ dlnν~;
ð2Þ
N
NnA
N
N
A02
K03
K03A02
where the integration is extended over the whole electronic
absorption band, N is Avogadro’s number, n is the refractive
Scheme 1 Chemical structure of the investigated molecules

J Fluoresc
ꢅ
ꢆ
−1
index of the medium, c is the velocity of light in vacuo. ν~−F3
can be calculated as:
Quantum yields of the molecules under study in EA were
measured by a relative method using Coumarin I in ethanol as
standard [31]. The fluorescence quantum yield was calculated
from the relationship:
Z
ꢃ ꢄ
F ν~ dν
ꢃ ꢄ
ꢁ
ꢂ
−1
−3
ν~F
Z
¼
:
ð3Þ
Z
IðνÞdν
1−10−D n2
F ν~
S
Z
dν
ΦF ¼ ΦS
;
ð7Þ
3
1−10−D
nS2
ν~
ISðνÞdν
Knowing the experimental values of the fluorescence quan-
where ΦF is the fluorescence quantum yield of studied
fluorophore, the integral is the area under the corrected fluo-
rescence spectrum, D is the optical density of the studied mol-
ecule, n is the refractive index of the solvent , and the subscript
S refers to the standard fluorophore in all cases.
tum yield (ΦF) and fluorescence decay time (τF), and measur-
ing the absorption band (εðν~Þ ) of the studied molecule, we are
able to calculate radiative transition probability (kF), absorp-
tion transition dipole moment (Mge) and fluorescence transi-
tion dipole moment (Meg), using Eqs. (4–6) [28, 29]:
ΦF
k F
¼
;
ð4Þ
ð5Þ
ð6Þ
Computational Details
τ F
Z
ꢃ ꢄ
ꢀ
ꢀ
ꢀ
3hc⋅2303
2
ꢀ
Mge
¼
¼
ε ν~ dlnν~;
All calculations concerning the spectroscopic properties
of molecules under study have been performed using
the CAChe WS 5.04 program. The geometrical struc-
tures of the investigated molecules has been determined
using the PM3 semiempirical molecular orbital method
at the Restricted Hartree Fock (RHF) level including
single excitation configuration interaction (CIS). The
PM3 method based on neglect of diatomic differential
overlap (NDDO) approximation takes into account va-
lence electrons only and for simplicity some of the two
electron integrals are omitted. The ZINDO semiempiri-
cal method has been used in calculations of electronic
singlet Franck– Condon (FC) state energies for UV-
visible spectra. The calculations were conducted for
the neutral, free molecule (closed-shell) in the vacuum.
Additionally to those data, the oscillator strengths of the
transition between the corresponding states S0–Si and
electric transition dipole moment were determined. In
the calculations performed the wave functions used in-
cluded all electronic states, which are generated by sin-
gle excitation of electrons in the highest occupied
(HOMO) and lowest unoccupied (LUMO) molecular
orbitals.
8π3Nn
ꢀ
ꢀ
ꢀ
ꢀ
ΦF
3h
2
Meg
⋅
:
τ F
64π4n3 < ν~F>3
Experimental Details
Measurements
All starting reagents and solvents were purchased from
Aldrich Chemical Co. The solvents used (methylcyclohexane
(MCH) and ethyl acetate (EA)) for spectroscopic studies were
of the highest grade commercially available and were used
without further purification.
Melting points were determined on the Buchi melting point
1
apparatus MP-1. The H (400 MHz) and 13C (100 MHz)
NMR spectra were recorded with the use of a Bruker
Ascent™ 400 NMR spectrometer. Dimethylsulfoxide
(DMSO-d6) was used as a solvent and tetramethylsilane as
an internal standard. The IR spectra were recorded using a
Bruker spectrophotometer Vector 22 by the KBr pellet
technique.
Absorption and fluorescence spectra were recorded using a
Shimadzu UV Multispec-1501 spectrophotometer and a
Hitachi F-4500 spectrofluorometers. The fluorescence life-
times were measured using an Edinburgh Instruments,
single-photon counting system (FLS920P spectrometer). The
apparatus utilizes for the excitation a picosecond diode laser
generating pulses of about 58 ps at 373 nm. Short laser pulses
in combination with a fast microchannel plate photodetector
and ultrafast electronics make a successful analysis of fluores-
cence decay signals with a resolution of few picoseconds pos-
sible. The fluorescence light was monitored at the magic angle
in respect to the polarized exciting beam.
Synthesis
Benzylideneacetone (A02) was synthesized according to the
literature [32], from acetophenone and acetone. Melting point
135–136 °C, 1H NMR (DMSO-d6) δ (ppm): 7.53 (d,
J = 8.8 Hz, 2H), 7.51 (d, J = 16.2 Hz, 1H, −CH=), 6.72 (d,
J = 8.9 Hz, 2H), 6.54 (d, J = 16.1 Hz, 1H, −CH=), 2.98 (s, 6H,
N(CH3)2), 2.27 (s, 3H, C(O)CH3); 13C NMR (DMSO-d6) δ
(ppm): 197.9 (CO), 152.3 (C), 144.6 (CH), 130.5 (CH), 122.5
(CH), 121.9 (C), 112.3 (CH), 40.1 N(CH3)2, 27.5 (C(O)CH3).

J Fluoresc
4-Dimethylamino-chalcone (K00A02) was synthesized
following a reported standard procedure [33], from
acetophenone and 4-(dimethylamino)benzaldehyde. Yield:
Result and Discussion
Steady-State Absorption Spectra
1
72 %. Melting point 110–111 °C; H NMR (DMSO-d6) δ
(ppm): 8.10 (d, J = 8.0 Hz, 2H), 7.73–7.50 (m, 7H), 6.74 (d,
J = 9.0 Hz, 2H), 3.01 (s, 6H, N(CH3)2); 13C NMR (DMSO-d6)
δ (ppm): 188.5 (CO), 151.8 (C), 145.2 (CH), 138.3 (C), 132.4
(CH), 130.7 (CH), 128.5 (CH), 128.1 (CH), 121.9 (C), 115.9
(CH), 111.6 (CH), 39.6 (CH3); IR (KBr) (cm−1): 1648, 1598,
1565, 1532, 1439, 1380, 1346, 1178, 1019, 816.
As can be seen in Scheme 1, the investigated chalcone deriva-
tives consist of an electron donor and electron acceptor parts
connected by a π-conjugated spacer. In order to confidently
treat the electronic structure of a molecule consisting of two
different chromophoric parts (D-A) in terms of a simple model
from the properties of individual chromophores (composite-
model of decoupled moieties: donor (D) and acceptor (A)), it
is necessary to have information about the absorption spectra of
the acceptor and donor subunits. Figs. 1 and 2 show the steady-
state absorption spectra of the donor (D), acceptor (A) units and
studied D–A systems in non-polar MCH at room temperature.
For the all investigated D-A molecules in MCH, the absorp-
tion spectrum presents two intensive bands (short-wavelength
(SW) in region 220–275 nm and long-wavelength (LW) in re-
gion 340–425 nm) and one weak, board absorption band ob-
served at 275–340 nm. The folded structure of the weak, board
absorption band located at 275–340 nm, as well as LW absorp-
tion band in MCH suggests that both analyzed bands consist of
two electronic transitions. Taking into account above, we used a
linear combination of Gaussian functions to decompose the
discussed absorption bands into two Gaussian profiles. Figs. 1
and 2 show the example of decomposition of LW absorption
band indicating the presence of two separated bands that corre-
spond to the S0 → S1 and S0 → S2 transitions. The Gaussian
intensity maxima positions correspond to the S0 → Sn transi-
tions for n = 1,2,3,4 of the studied compounds are collected in
Table 1. As can be seen in Figs. 1 and 2, and Table 1, the
position of the two long-wavelength absorption bands (S0 →
S1 and S0 → S2) undergo only a slight shift, whereas S0 → S3
and S0 → S4 transitions undergo a pronounced shift on changing
the acceptor subunit. Figure 3 shows a plot of the experimental
absorption energies (S0 → Sn; n = 1,2,3,4) of all investigated D-
A dyes as a function of Hammett constant characterizing accep-
tor moiety. It is clear from Fig. 3, that the analyzed data follow a
roughly linear dependence on Hammett constant. It is also im-
portant to note that the value of the linear slope for S0 → S3 and
S0 → S4 transitions was higher than that of S0 → S1 and S0 →
S2, which signals that the long-wavelength transitions (S0 → S1
and S0 → S2) possess a less pronounced charge transfer charac-
ter in comparison to the S0 → S3 and S0 → S4 transitions.
To further characterize the electronic structure of the donor-
acceptor dyes in terms of a simple model from the properties
of individual chromophores, we tested also the absorption
spectra of the acceptor and donor subunits in non-polar
methylcyclohexane (see Figs. 1 and 2). In the case of all stud-
ied acceptor subunits (K00, K01, K02 and K03) in MCH, two
broad absorption bands are observed in region 200–300 nm.
The absorption spectrum of donor moiety (A02) possesses
also two bands in region 200–400 nm: the broad and
Tested azachalcones were prepared adopting the procedure
described in literature [34].
A general method for synthesis of azachalcones.
To a solution of an apropriate acetylpyridine (10 mmol) and
4-(dimethylamino)benzaldehyde (10 mmol) in pyridyne
(6 mL) was added dimethylamine (12 mmol). The flask was
sealed and stored at 20 °C for 7 days. Then poured onto
250 mL of ice water, stirred for 0.5 h and filtered. The residue
was recrystallized twice from water-ethanol (1:1).
4-Dimethylamino-4′-azachalcone (K01A02) was synthe-
s i z e d f r o m 4 - a c e t y l p y r i d i n e a n d
4-(dimethylamino)benzaldehyde. Yield: 83 %. Melting point
129–130 °C; 1H NMR (DMSO-d6) δ (ppm): 8.80 (d,
J = 6.0 Hz, 2H), 7.94 (d, J = 6.0 Hz, 2H), 7.74 (d,
J = 14.8 Hz, 1H, −CH=), 7.73 (d, J = 9.4 Hz, 2H), 7.56 (d,
J = 15.4 Hz, 1H, −CH=), 6.75 (d, J = 8.8 Hz, 2H), 3.02 (s, 6H,
−N(CH3)2); 13C NMR (DMSO-d6) δ (ppm): 188.1 (CO),
152.2 (C), 150.5 (CH), 146.9 (CH), 144.5 (C), 131.2 (CH),
121.5 (C), 121.4 (CH), 115.3 (CH), 111.7 (CH), 39.7 (CH3);
IR (KBr) (cm−1): 1647 (C = O), 1575, 1546, 1526, 1346,
1190, 1047, 814, 651.
4-Dimethylamino-2′-azachalcone (K02A02) was synthe-
s i z e d f r o m 2 - a c e t y l p y r i d i n e a n d
4-(dimethylamino)benzaldehyde. Yield: 78 %. Melting point
137–138 °C; 1H NMR (DMSO-d6) δ (ppm): 8.77 (d,
J = 4.6 Hz, 1H), 8.06–7.98 (m, 3H), 7.78 (d, J = 15.8 Hz,
1H, −CH=), 7.69–7.63 (m, 3H), 6.76 (d, J = 8.8 Hz, 2H), 3.02
(s, 6H, N(CH3)2); 13C NMR (DMSO-d6) δ (ppm): 188.5
(CO), 154.6 (C), 152.6 (C), 149.5 (CH), 145.7 (CH), 138.0
(CH), 131.2 (CH), 127.6 (CH), 122.7 (CH), 122.4 (C), 115.2
(CH), 112.3 (CH), 40.1 (CH3); IR (KBr) (cm−1): 1652, 1583,
1562, 1528, 1350, 1182, 1033, 799.
4-Dimethylamino-3′-azachalcone (K03A02) was synthe-
s i z e d f r o m 3 - a c e t y l p y r i d i n e a n d
4-(dimethylamino)benzaldehyde. Yield: 71 %. Melting point
1
127–129 °C; H NMR (DMSO-d6) δ (ppm): 9.28 (s, 1H),
8.79 (d, J = 4.8 Hz, 1H), 8.42 (d, J = 7.6 Hz, 1H), 7.78–7.54
(m, 5H), 6.75 (d, J = 9.0 Hz, 2H), 3.02 (s, 6H, NCH3); 13C NMR
(DMSO-d6) δ (ppm): 187.7 (CO), 152.8 (CH), 152.2 (C), 149.4
(CH), 146.0 (CH), 135.6 (CH), 133.5 (C), 131.1 (CH), 123.8
(CH), 121.8 (C), 115.7 (CH), 111.7 (CH), 39.7 (CH3);
IR (KBr) (cm−1): 1644, 1585, 1574, 1549, 1526, 1437, 1371,
1189, 1054, 803.

J Fluoresc
λ (nm)
λ (nm)
Fig. 1 Absorption spectra of
K00A02 (a), K01A02 (a’), A02
(b, b’) and K00 (c), K01 (c’) in
MCH. The bars under the
200
1,0
250
300
350
400
200
250
300
350
400
450
1,0
O
O
A
B
C
A'
B'
C'
absorption curves give the
theoretical electronic state energy
values and corresponding
oscillator strengths of the studied
molecules in vapour-phase
N
N
0,8
0,6
0,4
0,2
0,8
0,6
0,4
0,2
N
0,0
1,0
0,0
1,0
O
O
N
N
0,8
0,6
0,4
0,2
0,8
0,6
0,4
0,2
0,0
1,0
0,0
1,0
O
O
0,8
0,6
0,4
0,2
0,0
0,8
0,6
0,4
0,2
0,0
N
200
250
300
350
400
200
250
300
350
400
450
λ (nm)
λ (nm)
structureless long-wavelength band in the region of 275–
400 nm and short-wavelength band in the 200–275 nm region.
As a further step, to test whether some bands in the absorp-
tion spectra of the investigated D-A dyes originate from the
donor and/or acceptor parts, a series of quantum-chemical cal-
culations were carried out. Table 1 assembles theoretical elec-
tronic state energy values and corresponding oscillator strengths
for the donor, acceptor units and investigated D–A molecules.
Analyzing the data assembled in Table 1 in connection with the
graphical presentation in Figs. 1 and 2 (the bars under the ab-
sorption curve give the theoretical electronic state energy and
corresponding oscillator strengths), one can state that:
&
&
two discussed transitions participate in the LW absorption
spectrum of D-A dyes in the ratio which slightly depends
on the chemical structure of the studied molecules,
the two long-wavelength absorption bands (corresponding
to S0 → S1 and S0 → S2 transitions) of D-A and D mol-
ecules participate in the absorption spectrum in the ratio
which agree well with the percentage value of participa-
tion of oscillator strengths (f S ; f S ) in the long-
1
2
wavelength absorption spectrum.
&
the calculated S0 → S1 and S0 → S2 transitions in the
absorption spectrum of D-A molecules correspond mainly
to the S0 → S1 and S0 → S2 transitions of the donor (A02)
molecule, whereas for all investigated D-A systems new
transitions (S0 → S3 and S0 → S4) at about 320 nm and
300 nm are present (these transitions are absent in the
spectrum of free donor molecule).
&
&
the long-wavelength absorption band of all studied different-
ly substituted chalcones and donor moiety consists of two
overlapping components (S0 → S1 and S0 → S2 transitions),
S0 → S1 and S0 → S2 transitions of the investigated D-A
molecules are shifted to longer wavelength by about
40 nm compared to the S0 → S1 and S0 → S2 transition
of A02 (donor moiety),
These behaviors are understandable in terms of a simple
model from the properties of individual chromophores (com-
posite-model of decoupled moieties: donor (D) and acceptor

J Fluoresc
λ (nm)
λ (nm)
Fig. 2 Absorption spectra of
K02A02 (a), K03A02 (a’), A02
(b, b’) and K02 (c), K03 (c’) in
MCH. The bars under the
absorption curves give the
theoretical electronic state energy
values and corresponding
200
1,0
250
300
350
400
200
250
300
350
400
450
1,0
O
N
O
A
B
C
A'
N
N
0,8
0,6
0,4
0,2
0,8
0,6
0,4
0,2
N
oscillator strengths of the studied
molecules in vapour-phase
0,0
1,0
0,0
1,0
O
O
B'
C'
N
N
0,8
0,6
0,4
0,2
0,8
0,6
0,4
0,2
0,0
1,0
0,0
1,0
O
O
0,8
0,6
0,4
0,2
0,0
0,8
0,6
0,4
0,2
0,0
N
N
200
250
300
350
400
450
200
250
300
350
400
λ (nm)
λ (nm)
(A)) [4, 26, 27]. Taking into account above, it can be conclud-
ed that, for the investigated D-A molecules, the long-
wavelength absorption band may originate mainly from the
absorption of the substituted donor moiety.
S(nπ)) predicted by numerical calculations is not observed
in solution because the weak absorption band (f = 0.001) of
nπ character is overlapped by the second transition peak. On
the basis of the one electron LUMO and HOMO characteris-
tics, it is evident that the first electronic transition which can
be observed in solution (see S0 → S1 transition in Tab. 1) for
all investigated D-A molecules and for donor part corresponds
to the promotion of an electron from HOMO to LUMO.
Moreover, we found that, the calculated S0 → S1 transition
of A02 is described by the same linear combination of HOMO
to LUMO one electron π-orbital transitions as the long-
wavelength S0 → S1 transition of the studied D-A dyes (see
Fig. 4 and Table 1).
As it was shown in Table 1, the S0 → S2 transition of A02
comes from the HOMO-LUMO(+1) (69 %) and HOMO (−1)-
LUMO (19 %) orbital transitions, whereas the S0 → S2 tran-
sition of D-A molecules comes from HOMO-LUMO (+2)
(55–64 %) and HOMO (−3)-LUMO (17–26 %) transitions.
It is important to note that, molecular orbitals (MO) associated
with the S0 → S2 transition are qualitatively identical for all D-
Molecular Orbital Calculations
We carried out theoretical molecular orbital calculations to get
a better understanding of the electronic structure of the mole-
cules presented in this study in terms of a simple composite-
model of decoupled moieties. Figure 4 presents the three low-
est unoccupied and five highest occupied molecular orbitals
involved in electronic transitions calculated for free acceptor
subunit (K00), free donor part (A02), and K00A02 molecule.
Additionally, Table 1 gives the percentage value of participa-
tion of HOMO (m) and LUMO (n) orbitals in electronic
transitions.
As can be seen in Fig. 4, for all studied systems (D-A, A,
D), the lowest-energy transition can be mainly represented by
transition of pure nπ character. The first transition (S0 →

J Fluoresc
exp
max
Table 1 Theoretical and experimental (obtained using a linear combination of Gaussian profiles) energy values of the singlet excited states (νtmheaox ,ν
(in nm and cm−1)), their oscillator strengths ( f ) and transition compositions (nχm[%]) of the tested dyes
Compound
Transient state
Area (%)
f
nχm[%]a
ðcm−1Þ =λmexapx ðnmÞ
theo
exp
max
theo
max
ν
ðcm−1Þ =λmax ðnmÞ
ν
K00
S (nπ)
S1
32,010 / 312.4
36,284 / 275.6
42,283 / 236.5
49,776 / 200.9
25,176 / 397.2
28,129 / 355.5
29,878 / 334.7
40,833 / 244.9
42,827 / 233.5
47,939 / 208.6
49,776 / 200.9
22,361 / 447.2
26,532 / 376.9
28,264 / 353.8
31,817 / 314.3
33,910 / 294.9
38,139 / 262.2
40,453 / 247.2
48,216 / 207.4
31,427 / 318.2
34,530 / 289.6
39,124 / 255.6
44,563 / 224.4
51,440 / 194.4
22,492 / 444.6
25,394 / 393.8
26,860 / 372.3
30,647 / 326.3
32,584 / 306.9
38,139 / 262.2
41,459 / 241.2
49,358 / 202.6
31,526 / 317.2
34,650 / 288.6
38,986 / 256.5
45,269 / 220.9
51,546 / 194.0
22,316 / 448.1
25,588 / 390.8
27,078 / 369.3
30,931 / 323.3
32,906 / 303.9
38,580 / 259.2
41,982 / 238.2
50,100 / 199.6
31,726/ 315.2
34,176 / 292.6
0.001
0.015
0.458
0.351
0.001
1.000
0.290
0.320
0.129
0.480
0.272
0.001
1.000
0.260
0.173
0.163
0.290
0.099
0.656
0.001
0.051
0.183
0.392
0.510
0.001
1.000
0.240
0.173
0.163
0.290
0.099
0.656
0.001
0.049
0.236
0.435
0.489
0.001
1.000
0.280
0.173
0.103
0.293
0.094
0.656
0.001
0.053
0χ2[71] , 2χ2[27]
0χ1[49] , 1χ0[30]
0χ0[81]
0χ1[36] , 1χ0[32]
0χ3[58] , 2χ3[30]
0χ0[94]
1χ0[69] , 0χ1[19]
0χ1[70] , 1χ0[23]
0χ2[75] , 2χ0[10]
1χ1[70] , 3χ0[15]
2χ1[53] , 1χ2[25]
0χ4[72]
35,971 / 278.0
42,535 / 235.1
S2
S3
A02
S (nπ)
S1
27,624 / 362.0
30,084 / 332.4
48
52
S2
S3
S4
S5
S6
K00A02
S (nπ)
S1
25,400 / 393.7
27,541 / 363.1
31,625 / 316.2
33,875 / 295.2
46
54
0χ0[89]
S2
2χ0[64] , 0χ3[17]
0χ1[40] , 1χ0[38]
0χ1[45] , 1χ0[29]
0χ3[63] , 2χ0[24]
0χ2[55] , 4χ0[16]
2χ3[69]
0χ2[69] , 2χ2[27]
0χ1[57] , 1χ0[27]
0χ0[78]
0χ1[39] , 1χ0[32]
0χ1[82]
0χ4[58] , 0χ3[25]
0χ0[78]
2χ0[56] , 0χ3[18]
0χ1[46] , 1χ0[37]
0χ1[53] , 1χ0[28]
0χ3[66] , 2χ0[28]
0χ2[57] , 4χ0[22]
2χ3[78]
0χ2[66] , 2χ2[29]
0χ1[59] , 1χ0[29]
0χ0[73]
0χ1[36] , 1χ0[35]
0χ1[88]
0χ4[62] , 0χ3[22]
0χ0[74]
S3
S4
S5
S6
S7
K01
S (nπ)
S1
35,063 / 285.2
37,736 / 265.1
S2
S3
S4
K01A02
S (nπ)
S1
24,444 / 409.1
26,288 / 380.4
29,674 / 337.0
31,696 / 315.5
38
62
S2
S3
S4
S5
S6
S7
K02
S (nπ)
S1
35,002 / 285.7
37,679 / 265.4
S2
S3
S4
K02A02
S (nπ)
S1
24,594 / 406.6
26,344 / 379.6
30,534 / 327.5
32,658 / 306.2
42
58
S2
2χ0[59] , 0χ3[21]
0χ1[49] , 1χ0[33]
0χ1[57] , 1χ0[28]
0χ3[64] , 2χ0[25]
0χ2[53] , 4χ0[23]
2χ3[81]
S3
S4
S5
S6
S7
K03
S (nπ)
S1
0χ2[62] , 2χ2[32]
0χ1[56] , 1χ0[27]
34,530 / 289.6

J Fluoresc
Table 1 (continued)
Compound
Transient state
Area (%)
f
nχm[%]a
ðcm−1Þ =λmexapx ðnmÞ
theo
exp
max
theo
max
ν
ðcm−1Þ =λmax ðnmÞ
ν
S2
38,685 / 258.5
46,318 / 215.9
51,440 / 194.4
22,391 / 446.6
25,826 / 387.2
27,450 / 364.3
31,104 / 321.5
33,256 / 300.7
38,491 / 259.8
42,017 / 238.0
49,850 / 200.6
37,850 / 264.2
0.256
0.493
0.509
0.001
1.000
0.224
0.157
0.099
0.325
0.080
0.602
0χ0[77]
0χ1[38] , 1χ0[32]
0χ1[81]
0χ4[66] , 0χ3[21]
0χ0[72]
2χ0[55] , 0χ3[26]
0χ1[46] , 1χ0[36]
0χ1[55] , 1χ0[28]
0χ3[66] , 2χ0[27]
0χ2[57] , 4χ0[25]
2χ3[79]
S3
S4
K03A02
S (nπ)
S1
24,685 / 405.1
27,086 / 369.2
30,694 / 325.8
32,415 / 308.5
41
59
S2
S3
S4
S5
S6
S7
a The percentage of a transition from the (HOMO-m)th orbital to the (LUMO + n)th orbital
A molecules, as illustrated with the MO plots of selected ex-
amples (A, D, D-A) in Fig. 4. Furthermore, the calculated
S0 → S2 transition of A02 is described by the same linear
combination of HOMO to LUMO one electron π-orbital tran-
sitions as the S0 → S2 band of the studied D-A dyes.
of the HOMO and LUMO transitions, we can also state that
the calculated S0 → S6 transition of titled molecules is mainly
located on the acceptor unit and it is described by the same
linear combination of HOMO to LUMO one electron π-
orbital transition like S0 → S3 transition of the free acceptor
molecule (see Fig. 4 and Table 1).
In conclusion, it is evident that for all studied D-A systems,
some analyzed MOs correspond to the molecular orbitals of
the free donor and free acceptor parts (see Fig. 4). Thus, after
Maus and Rettig [4, 26, 27] and our previous studies [16], it is
possible to suggests that the electronic structure of the studied
dyes can be approximately described within composite-model
of decoupled moieties: donor and acceptor.
In contrast to the behavior in S0 → S1 and S0 → S2 transi-
tions, the S0 → S3 and the S0 → S4 transitions of the D-A
0
molecules comprises mainly χ1 HOMO-LUMO transition.
Furthermore, it is evident that the S0 → S3 and the S0 → S4
transitions possess strongly intramolecular charge transfer
(ICT) character (see Fig. 4). Tab. 1 shows that the electron
transfer character of the HOMO (−1)-LUMO transition in-
creases from 40 % for K00A02 to 49 % for K02A02 (S0 →
S3 transition) and from 45 % for K00A02 to 57 % for K02A02
(S0 → S4 transition). It is also clearly seen in Table 1 and
Fig. 4 that the analyzed transition does not occur in A02 mol-
ecule. On examining the electron density π-orbital characters
Transition Dipole Moments
As it was mentioned, the knowledge of the absorption and
fluorescence transition dipole moments (values and orienta-
tion) is a helpful to get information about the relaxation mech-
anism of the investigated dyes [15, 16]. Taking into account
above, the absorption (Mge) and fluorescence (Meg) transition
dipole moments were calculated, on the basis of experimental
data (see Theoretical background), and compared with results
of our quantum-chemical calculations.
420
K02A02
K00A02
400
K01A02
K03A02
380
360
S0 - S1
S0 - S2
Absorption Transition Dipole Moments
340
S0 - S3
S0 - S4
320
Table 2 assembles the theoretical values of absorption transi-
tion dipole moments of the S0 → S1 and S0 → S2 transitions
for D-A, D, and A molecules. In order to explore the role of
the solute-solvent interactions, the theoretical values are
complemented by experimental values of absorption transi-
tion dipole moments in non-polar methylcyclohexane
(MCH) and medium-polar ethyl acetate (EA). Analyzing the
data obtained from the steady-state spectroscopic
300
0,0
0,1
0,2
0,3
0,4
Hammett constant
Fig. 3 The dependence of the experimental absorption energies (S0 →
Sn; n = 1,2,3,4) vs. Hammett constant of the studied D-A molecules

J Fluoresc
Fig. 4 Molecular orbitals involved in the main electronic transitions of K00 (column 1), A02 (column 2) and K00A02 (column 3)

J Fluoresc
Table 2 Experimental and
theoretical values of transition
dipole moments (in D) of the
molecules under study
Compound
K00
State
Mx
My
Mz
Mge(theo.)
Mge(exp.)
Meg(exp.)
S1
S2
0.022
–4.50
–0.182
1.66
0.011
0.005
0.184
4.80
0.27
3.12
A02
S1
S2
–0.54
–0.36
0.24
0.03
–0.58
–0.01
0.83
0.36
0.72
0.53
K00A02
K01
S1
S2
7.92
4.94
–0.96
0.84
2.29
0.19
8.30
5.01
7.11 / 8.49*
7.45*
7.21*
7.89*
7.11*
6.13 / 5.11*
S1
S2
0.032
4.26
–0.244
0.70
0.009
0.08
0.246
4.32
0.32
3.14
K01A02
K02
S1
S2
7.77
5.04
–1.01
0.79
–1.99
0.12
8.08
5.10
6.69 / 8.89*
2.94 / 4.46*
S1
S2
0.026
4.44
–0.217
0.39
0.016
0.09
0.219
4.46
0.29
3.49
K02A02
K03
S1
S2
7.83
4.88
–0.87
0.92
2.42
0.17
8.24
4.97
6.52 / 8.52*
5.29 / 4.63*
S1
S2
0.028
4.31
–0.222
0.66
0.017
0.06
0.224
4.36
0.34
3.30
K03A02
S1
S2
7.81
4.92
–0.87
0.95
2.12
0.13
8.14
5.01
6.92 / 8.95*
4.18 / 4.44*
*values obtained in EA
measurements (experimental values of transition dipole mo-
ments) and quantum-chemical calculation (theoretical values
of transition dipole moments), one can state that:
8,4
8,2
8,0
A
K02A02
K00A02
K01A02
K03A02
&
quantitative analysis of the absorption transition dipole
moments for all D-A fluorophores clearly demonstrates a
dependence on the acceptor substituent. Figure 5 shows a
plot of the theoretically calculated absorption transition
dipole moments between S0 - S1 and S0 – S2 states versus
Hammett parameter. It is clearly seen that the calculated
0,25
0,20
0,15
K03
K01
K02
Mge values for the studied D-A molecules follow a rough-
ly linear dependence on Hammett constant (correlation
coefficient larger than 0.85). Moreover, similar depen-
dences are found for free acceptor moieties (see Fig. 5).
absorption transition dipole moment of the S0 - S1 elec-
tronic transition obtained using quantum-chemical calcu-
lations is similar for all studied D-A dyes and changes
from 8.08 D (K01A02) to 8.30 D (K00A02).
the experimentally established absorption transition dipole
moments depend on the solvent polarity. On examining
the Mge values obtained for each of the fluorophores in
MCH and EA (see Table 2), it is evident that the Mge
values determined for the non-polar system differ from
those calculated for the medium-polar EA by about
20 %. It indicates that the radiative transition probability
is strongly affected by the environment. On the other
hand, the average values of Mge in MCH and EA (calcu-
lated using Eq. (5)) show good agreement with the theo-
retical one.
K00
0,0
0,1
0,2
0,3
0,4
Hammett constant
&
&
5,2
5,0
4,8
4,6
4,4
4,2
B
K03A02
K02A02
K00A02
K01A02
K00
K02
K01
K03
0,0
0,1
0,2
0,3
0,4
&
for acceptor molecules i.e., K00, K01, K02 and K03,
experimentally obtained Mge values for S0-S1 transition
in MCH are higher in comparison to theoretically one,
Hammett constant
Fig. 5 Theoretically calculated absorption transition dipole moments
between S0-S1 (a) and S0- S2 (b) states versus Hammett constants

J Fluoresc
title compounds in non-polar MCH is very low and fluores-
cence lifetimes are very short, we decided to calculate Meg for
all title compounds only in medium-polar ethyl acetate.
Figure 6 shows the absorption and fluorescence spectra of
D-A molecules in EA. For all studied D-A dyes, the fluores-
cence intensity distribution in EA is described by a classical
Gaussian shape. Such shape is characteristic for fluorescence
spectra when the motions in the fluorophore environment oc-
cur simultaneously or faster than the emission. For these cases
a very large number of different solute-environment space
configurations and excited molecules in different space con-
formations are possible [1, 2, 13–17].
K00A02
K01A02
K02A02
K03A02
1,0
0,8
0,6
0,4
0,2
0,0
A
200
250
300
350
400
450
500
In order to analyze quantitatively the excited-state behavior
of four D-A fluorophores, fluorescence kinetics in EA were
analyzed. Fluorescence decays were deconvoluted to the best
fit of a sum of exponentials. They can be reasonably well fitted
by biexponential decays with the shorter component (with a
small contribution) and the longer one as a major decay com-
ponent. On the other hand, the fluorescence decay of A02
molecule in EA is fitted by a mono-exponential function
(see Table 3). On examining the fluorescence decay times
and preexponential factors (A1 and A2 - describing the contri-
bution of the short and long fluorescence decay component to
the total emission), obtained for each of the fluorophores in
medium-polar EA, it is evident that two fluorescence emitting
centers exist and the participation of the time decay compo-
nents in the total emission distinctly depend on the
substituents.
λ (nm)
K00A02
1,0
0,8
0,6
0,4
0,2
0,0
B
K01A02
K02A02
K03A03
400
450
500
550
600
650
700
750
λ (nm)
The short fluorescence decay time (τ1) determined for title
compounds changes from 0.31 ns (K00A02) to 0.59 ns
(K02A02) and is about two times longer in comparison to that
obtained for free donor moiety (A02) (0.2 ns). The
preexponential factor A1 (describing the contribution of fast
decay component) changes from 7 % for K01A02 to 36 % for
K00A02. In the meantime, the decay time of the slow com-
ponent (τ2) changes insignificantly (for all molecules except
K00A01) from 1.52 ns (K03A02) to 2.04 ns (K02A02).
Taking into account the results of our previous publications
concerning different D-A systems [13–17], as well as present
steady-state and time-resolved spectroscopic measurements
and quantum-chemical calculations, one can state that two
Fig. 6 Absorption (a) and fluorescence (b) spectra of investigated dyes
in ethyl acetate
whereas opposite dependence is observed for the S0-S2
transition.
Fluorescence Transition Dipole Moments
The theories reviewed above (see Section 2) allow also eval-
uation of the fluorescence transition dipole moment, Meg,
characterizing an electron transition between excited (e) and
ground (g) states. Because fluorescence quantum yield of the
Table 3 Fluorescence decay times, fluorescence quantum yield and radiative rate constants of the investigated molecules in EA
Compound
λobs (nm)
τ1(ns)
A1 (%)
τ2(ns)
A2 (%)
<τ>*(ns)
Φ
kF ⋅ 109 (s−1
)
K00A02
K01A02
K02A02
K03A02
A02
493
533
519
508
439
0.31
0.49
0.59
0.55
0.20
36.4
6.7
0.68
1.57
2.04
1.52
63.6
93.3
90.3
81.6
0.60
1.55
2.00
1.45
0.12
0.34
0.41
0.36
0.20
0.22
0.21
0.25
9.7
18.4
100
Ã<*τ >¼ ∑iAiτ2i =∑iAiτi

J Fluoresc
emitting states exist in investigated D-A fluorophores in
medium-polar EA. The comparable values of the τ1 for the
studied D-A dyes and τ in the case of A02, suggest that the τ1
decay component originates mainly from the emission of the
locally excited donor moiety (D*-A). It is also evident that in
medium-polar EA, the initially populated locally excited (LE)
state (where the fundamental role plays donor moiety (D*-A))
reacts further to produce emissive, intramolecular charge
transfer (ICT) state (τ2 decay component).
moments is present as a result of a photoinduced rearrange-
ment of nuclear coordinates (e.g., intramolecular motions such
as rotation of chromophores), therefore experimental and the-
oretical studies of transition dipole moment can be used as an
indicator of configuration changes of the molecule in the ex-
cited state.
Acknowledgments This work was supported by the National Science
Center of Poland (Decision No. DEC-2012/07/B/ST4/01417).
To gain further insight into the spectroscopic properties of
the investigated fluorophores, the fluorescence quantum yield,
radiative rate constants and fluorescence transition dipole mo-
ment were determined. Quantum yields of the molecules un-
der study were measured by a relative method using Coumarin
I in ethanol as standard, radiative rate constants were calculat-
ed, for all studied dyes in EA, using Eq.(4) and fluorescence
transition dipole moment was calculated, on the basis of ex-
perimental data, using Eq. (6). It is noteworthy that an average
value of the fluorescence decay time has been calculated using
the formula: < τ >¼ ∑iAiτ2i =∑iAiτi. Analyzing the data ob-
tained from the steady-state and time-resolved spectroscopic
measurements (see Tables 2 and 3), one can state that:
References
1. Waluk J (ed) (2000) Conformational analysis of molecules in excit-
ed states. Wiley VCH, New York
2. Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103 (2003)
3899 and references there in.
3. Lippert E, Luder W, Luder H (eds) (1962) Advances in molecular
spectroscopy. Pergamon, New York
4. Maus M, Rettig W, Jonusauskas G, Lapouyade R, Rulliere C
(1998) J Phys Chem A 102:7393
5. Zachariasse KA, Druzhinin SI, Mayer P, Kovalenko SA,
Senyushkina T (2009) Chem Phys Lett 484:224
6. Schmidhammer U, Megerle U, Lochbrunner S, Riedle E, Karpiuk J
(2008) J Phys Chem A 112:8487
7. Mohammed OF, Vauthey E (2008) J Phys Chem A 112:3823
8. Megerle U, Selmaier F, Lambert C, Riedle E, Lochbrunner S (2008)
Phys Chem Chem Phys 10:6245
9. Mohammed OF, Vauthey E (2010) Chem Phys Lett 487:246
10. Krawczyk P, Pietrzak M, Janek T, Jędrzejewska B, Cysewski P
(2016) J Mol Model 22:125
11. Grimsdale AC, Chan KL, Martin RE, Jokisz PG, Holmes AB
(2009) Chem Rev 109:897
12. Arias AC, MacKenzie JD, McCulloch I, Rivnay J, Salleo A (2010)
Chem Rev 110:3
13. Józefowicz M, Heldt JR, Bajorek A, Paczkowski J (2009) Chem
Phys 363:88
14. Józefowicz M, Fita P, Kasprzycki P, Heldt JR (2013) J Phys Chem
A 117:4136
15. Józefowicz M, Heldt JR, Heldt J (2012) J Lumin 132:755
16. Józefowicz M, Bajorek A, Pietrzak M, Heldt JR (2013)
Spectrochim Acta A 113:137
17. Józefowicz M, Bajorek A, Pietrzak M, Heldt JR, Heldt J (2014) J
Lumin 153:152
&
for three studied D-A dyes (K01A02, K02A02, K03A02),
the fluorescence quantum yield value is essentially the
same (0.35) and it is about three times larger than for
K00A02. The similar behavior is noted for <τ> value.
the radiative rate constant (kF) is practically of the acceptor
substituent independent (indicating that the radiative tran-
sition probability is not affected by the acceptor moiety).
for investigated molecules, the experimentally calculated
&
&
&
Meg value only slightly depends on the acceptor substitu-
ent and equals about (7.5 0.5) D.
for all D-A dyes in medium-polar solvent, the ratio Mge/
Meg is not close to unity. Noted differences between the
absorption and emission transition dipole moments (i.e.,
Mge ≠ Meg) confirm that the change of the electronic and
molecular structure take place in the excited state.
18. Pingaew R, Saekee A, Mandi P, Nantasenamat C, Prachayasittikul
S, Ruchirawat S, Prachayasittikul V (2014) Eur J Med Chem 85:65
19. Mahapatra DK, Bharti SK, Asati V (2015) Eur J Med Chem 101:
496
20. Zhao L, Mao L, Hong G, Yang X, Liu T (2015) Bioorg Med Chem
Lett 25:2540
Conclusions
21. Zhang XW, Zhao DH, Quan YC, Sun LP, Yin XM, Guan LP (2010)
Med Chem Res 19:403
22. Tomar V, Bhattacharjee G, Kamaluddin A, Kumar (2007) Bioorg
Med Chem Lett 17:5321
23. Kim HJ, Kim SH, Lee YH, Kim DW, Mo K, Kim HJ, Park SH, Jin
C, Kim N-J, Lee YS (2015) Bull Kor Chem Soc 36:1335
24. Lin Y-M, Zhou Y, Flavin MT, Zhou L-M, Niea W, Chen F-C (2002)
Bioorg Med Chem 10:2795
25. R.F. Bhajantri, V. Ravindrachary, A. Harisha, Ismayil, C.
Ranganathaiah, Phys. Status Solidi C 6 (2009) 2429.
26. Maus M, Rettig W, Bonafoux D, Lapouyade R (1999) J Phys Chem
103:3388
Newly synthesized chalcones have been studied using steady-
state and time-resolved techniques combined with quantum-
chemical modelling. We have shown that the electronic struc-
ture of the studied D-A dyes can be approximately described
within composite-model of decoupled moieties: donor and ac-
ceptor. We believe that the results presented in this work and the
proposed model is relevant for a wide class of chalcones
possessing strong electron donor and acceptor groups.
The presented results also clearly show that the difference
between absorption and fluorescence transition dipole
27. Maus M, Rettig W (1997) Chem Phys 218:151

J Fluoresc
28. Mataga N, Kubota T (1970) Molecular interactions and electronic
spectra. Marcel Dekker, New York
29. Birks JB (1975) Organic molecular photophysics, vol 2. Willey,
London
32. Wolter RJ, Caurran C (1960) J Am Chem Soc 82:4153
33. Li R, Kenyon GL, Cohen FE, Chen X, Gong B, Dominguez JN,
Davidson E, Kurzban G, Miller RE, Nuzum EO, Rosenthal PJ,
McKerrow JH (1995) J Med Chem 38:5031
30. Strickler SJ, Berg RA (1962) J Chem Phys 37:814
31. Olmsted J III (1979) J Phys Chem 83:2582
34. Szucs L, Durinda J, Krasnec L, Heger J (1966) Chem Zvesti 20:817