Solvatochromism of symmetrical 2,6-distyrylpyridines. An experimental and theoretical study

Article abstract of DOI:10.1016/j.saa.2014.07.023

Seven symmetrical 2,6-distyrylpyridines, phenyl-substituted with hydrogen-bond donors, hydrogen-bond acceptors, halogens and hydrophobic moieties were synthesized and their spectroscopic characterization was done. Solvent effects on the absorption and fluorescence spectra were analyzed and quantified using the Kamlet-Taft and Catalán approach. The obtained results were rationalized by comparison of electrostatic potentials of the molecules in the ground and in excited state and by comparison of the frontier molecular orbitals (HOMO and LUMO), derived from quantum-mechanical calculations (HF, DFT, MP2). Analysis of the results revealed an important influence of non-specific (dispersive) interactions on the solvatochromic behavior of the compounds. 1D and 2D NMR data, in silico obtained conformational assembly of the compound, and the NMR analysis of molecular flexibility in solution (NAMFIS), were used to estimate population of conformers and to deconvolute the UV-Vis spectrum of representative derivative; inferring that the conformational assembly is more complex than was assumed in so far published literature data for this class of compounds. Along with this, the emission spectra of the representative compounds were decomposed by the Multivariate Curve Resolution analysis.

Full text of DOI:10.1016/j.saa.2014.07.023

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Solvatochromism of symmetrical 2,6-distyrylpyridines. An experimental
and theoretical study
a
a,
⇑
b
b
c
Jelena M. Markovi c´ , Nemanja P. Trišovi c´ , Dragosav Mutavd zˇ i c´ , Ksenija Radoti c´ , Ivan O. Jurani c´ ,
c
a
Branko J. Drakuli c´ , Aleksandar D. Marinkovi c´
a
Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
Institute for Multidisciplinary Research, University of Belgrade, Kneza Višeslava 1, 11000 Beograd, Serbia
Department of Chemistry, Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
b
c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Synthesis and spectroscopic
properties of symmetrical 2,6-
distyrylpyridine derivatives are
described.
ꢀ
ꢀ
ꢀ
Population of conformers in solution
was estimated by NAMFIS analysis,
from NMR data.
Solvent effects on the UV–Vis spectra
are evaluated by the Kamlet–Taft and
Catalán LSER models.
The decomposition of the emission
spectra is performed using MCR
analysis.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2014
Received in revised form 11 July 2014
Accepted 15 July 2014
Available online 24 July 2014
Seven symmetrical 2,6-distyrylpyridines, phenyl-substituted with hydrogen-bond donors, hydrogen-
bond acceptors, halogens and hydrophobic moieties were synthesized and their spectroscopic character-
ization was done. Solvent effects on the absorption and fluorescence spectra were analyzed and quantified
using the Kamlet–Taft and Catalán approach. The obtained results were rationalized by comparison of
electrostatic potentials of the molecules in the ground and in excited state and by comparison of the fron-
tier molecular orbitals (HOMO and LUMO), derived from quantum-mechanical calculations (HF, DFT,
MP2). Analysis of the results revealed an important influence of non-specific (dispersive) interactions
on the solvatochromic behavior of the compounds. 1D and 2D NMR data, in silico obtained conformational
assembly of the compound, and the NMR analysis of molecular flexibility in solution (NAMFIS), were used
to estimate population of conformers and to deconvolute the UV-Vis spectrum of representative deriva-
tive; inferring that the conformational assembly is more complex than was assumed in so far published
literature data for this class of compounds. Along with this, the emission spectra of the representative
compounds were decomposed by the Multivariate Curve Resolution analysis.
Keywords:
UV–Vis spectroscopy
Fluorescence spectroscopy
Conformational preferences
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
Molecules containing the styryl (arylvinyl) moiety represent
⇑
Corresponding author. Tel.: +381 11 3303869; fax: +381 11 3370387.
E-mail address: ntrisovic@tmf.bg.ac.rs (N.P. Trišovi c´ ).
one of the most important groups of functional dyes, with a num-
ber of favorable properties. These dyes are fluorescent, have higher
http://dx.doi.org/10.1016/j.saa.2014.07.023
386-1425/Ó 2014 Elsevier B.V. All rights reserved.
1

4
36
J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
solvatochromic comparison methods of Kamlet and Taft [16] and
Catalán [17] have been commonly applied to separate effects of
non-specific solvent–solute interactions (electrostatic effects) from
specific interactions (hydrogen bonding). Recently, it has been
demonstrated that a clear and univocal influence of solvent on
the position of absorption and fluorescence maxima cannot be
obtained using simplified solvation models. A detailed analysis of
the static and dynamic aspects of solvation should be taken into
account [18].
In this study, we report the set of seven symmetrical 2,6-
distyrylpyridines (Fig. 2) substituted with the hydrogen-bond
donors and acceptors, bulky hydrophobic moiety and bearing per-
manently charged fragment. Molecules are able to form hydrogen
bonds with the solvent, as well as intramolecular hydrogen bonds,
hydrophobic interactions and ion–dipole interactions. Compound 7
is a bent-core mesogen and its liquid crystalline behavior has
already been reported [19]. The absorption and emission spectra
of the investigated compounds in various solvents have been
investigated. Their solvatochromic properties are also described.
It is demonstrated that the interpretation of solvent effects on
the position of the absorption maxima using both Kamlet–Taft
and Catalán models should be taken into consideration cautiously.
This is further explained using electrostatic potential of molecules.
It is shown that the solvatochromic properties of the investigated
molecules originate in greater extent from non-specific solvent–
solute interactions (electrostatic effects) than from hydrogen
bonding. Furthermore, an analysis of the frontier orbitals and
charge distribution, obtained from calculations performed on
MP2, DFT and HF level of theory, is described in details.
Fig. 1. Schematic representation of preferential conformations of 2,6-distyrylpyri-
dine: s-cis/s-cis (A); s-trans/s-trans (B); s-trans/s-cis (C).
photostability, when compared to classical cyanine dyes, and they
can cover the range of the electromagnetic spectrum from the UV
to the near infrared region [1]. Styryl dyes are widely used as non-
linear optical materials (NLO), optical sensitizers, information
recording materials, laser dyes, sensors, two-photon absorption
materials, for artificial photosynthesis or photocatalysis [2–6].
Experimental part
2
,6-Distyrylpyridine and its derivatives (Fig. 1) have attracted
Synthesis
significant attention over the last decade. The physico-chemical
properties of these compounds are determined by the existence
of the dynamic equilibrium between conformational isomers,
which occurs due to the lack of the free rotation of the arylvinyl
moieties around the quasi-single bonds with the central pyridine
7]. The preferred conformation of 2,6-distyrylpyridine in solid
state is governed by the weak steric interactions between the vinyl
hydrogens and the nearby hydrogens of the phenyl or pyridyl rings
Compounds 1–7 were obtained following the synthetic protocol
shown in Scheme 1. Condensation of 2,6-lutidine with the excess
of 4-methoxybenzaldehyde in acetyc anhydride at the reflux tem-
perature afforded compound 1. Compounds 2–5 were prepared
according to the procedure described by Bergmann and Pinchas
[
[
20]. Analogously, condensation of 2,6-lutidine with the excess of
substituted 4-hydroxybenzaldehydes in acetic anhydride at the
reflux temperature gave rise to 2,6-bis[2-(substituted-4-acetoxy-
phenyl)ethenyl]pyridine. Subsequent base catalyzed hydrolysis
led to compounds 2–5. Compound 6 was synthesized in a reaction
between N-Me-2,6-lutidinium iodide and 4-hydroxybenzaldehyde
in the presence of catalytic amount of piperidine. Compound 7 was
obtained by acylation of compound 2 with the corresponding
[
8]. Ab initio calculations have revealed that among three confor-
mations, conformation A (Fig. 1) is the most stable one, in agree-
ment with the reported crystallographic data. Conformation C is
somewhat less stable and conformation B is the least stable [9].
A spectral and photochemical study of 2,6-distyrylpyridine has
shown that the relative abundance of different rotamers depends
on the solvent used [10]. Its UV–Vis spectra are complex, and
reveal the presence of two or three not well-separated electronic
transitions. The shape of spectra probably depends on the number
and the abundance of rotamers. It is assumed that formation of
intramolecular hydrogen bonds stabilizes the sterically favoured,
longer lived, rotamer [10].
4
-n-heptyloxybenzoyl chloride. A detailed description of the
experimental procedures and the characterization of reported
compounds are given in the Supplementary Material.
Pyridinium salts of 2,6-distyrylpyridines exhibit significantly
different photophysical properties in comparison with the corre-
sponding neutral forms. Wang et al. have reported that absorption
and emission maxima of 2,6-bis(4-dimethylaminostyryl)pyridine
are dramatically red shifted upon methylation [11]. They have
pointed out that alkylation of the pyridine nitrogen converts this
atom to a member of a cyanine array. Increasing the electron-
donating ability, number of electron-donating substituents, and
the coplanarity of polysubstituted pyridinium molecules is favor-
able for NLO absorption, especially for saturated absorption at
the picosecond pulse [12].
Solvatochromism has been established as an efficient tool to
study bulk and local polarity in macrosystems [13–15]. The
Fig. 2. Schematic depictions of the structure of compounds 1–7.

J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
437
The UV–Vis spectra were recorded using spectroscopy grade
ꢂ5
ꢂ3
solvents (Fluka) at 1 ꢁ 10 mol dm
concentration, with a
Shimadzu 1700 spectrophotometer. Fluorescence spectra of the
compounds 1–7 were measured using a Fluorolog-3 spectrofluo-
rimeter (Jobin Yvon Horiba, France) equipped with a 450 W Xe
lamp and a photomultiplier tube. The slits on the excitation and
emission beams were fixed at 4 nm and 2 nm, respectively. The
integration time was 0.1 s. The spectra were corrected for dark
counts. Each reported value was average of three scans. All mea-
surements were performed at controlled temperature of 25.0 °C.
For the compounds 1–7 emission spectra were measured upon
excitation at the wavelength of the absorption maximum in the
corresponding solvent. Additionally, for the compounds 2 and 7
series of emission spectra were collected, by excitation at different
wavelengths, with a 5 nm-step. The excitation/emission range was
2
3
80–450 nm/360–700 nm for the compound 2, and 270–340 nm/
50–530 nm for the compound 7. Decomposition of the series of
emission spectra was performed using Multivariate Curve Resolu-
tion (MCR) [21], in the Unscrambler 9.7 software.
Computational details
In order to estimate abundance of conformers of compound 1
in CHCl solution, we generated 200 conformers by molecular
3
mechanics conformational search in AMMP program [22]. Boltzmann
Jump algorithm was used, with SP4 force field. VegaZZ was used as
a GUI [23]. Conformers differing by 10° in any torsion were
considered as non-redundant and were included in conformational
assembly. Each conformer included in conformational assembly
was minimized by SP4 force field, with 20 steps and gradient
threshold value of 0.01. Assembly of conformers and NMR data
(
integrals of NOESY signals and coupling constants, using weight
ratio of 1:0.1) were imported in Janocchio program [24], where
the input for NAMFIS analysis was prepared. NAMFIS analysis
was performed in program DISCO [25]. The initial geometries of
all compounds were obtained in MOPAC2012 [26] with PM6 semi-
empirical molecular-orbital method [27]. The geometries of three
‘
ideal’ conformations of derivative 1 were fully optimized by MP2
method with the 6-311G basis set in vacuum in Gaussian03
program [28]. Geometries of all studied molecules were (also) opti-
mized on DFT level of theory, and 6-311G basis set for compounds
1
–5 and 7. For compound 6, the DGDZVP basis set was used,
because of presence of the iodine in the molecule. Energies of
frontier orbitals are derived by self-consistent field analysis of
optimized geometries (keyword SCF = tight), for the each case
reported. Electrostatic potential of molecules were derived from
total SCF density for the ground states, or from total CI density
for excited states of molecules, both obtained on HF level of theory
with 6-311G basis set. Configuration interactions (CI) calculations
were performed considering 30 singlet states. Molecular orbitals
are depicted in Jmol [29]. UV–Vis spectra were calculated by semi-
empirical ZINDO/S method [30] or by TD-DFT calculations, using
Scheme 1. Synthetic route to compounds 1–7.
Spectral measurements
3
implicit solvation in CHCl (IEFPCM – integral equation formalism
Structures of the synthesized compounds were confirmed by
instrumental methods. 1H and C nuclear magnetic resonance
variant of the polarizable continuum model) in Gaussian09 pro-
gram [28]. We used equilibrium solvation model for both the
ground and the excited states and the default cavitation model
(UFF radii scaled by 1.1). Both DFT and TD-DFT calculations were
performed with the B3LYP functional. Molecular interaction fields
(MIF) around studied molecules were computed using O1 (HBD)
and OC2 (HBA) probes of programme GRID [31–33]. Grid resolu-
tion was set to 0.5 Å, and rotation of OH groups around CAO axis
in response to probes was allowed only (directive MOVE = 0).
Atomic charges of molecules were calculated by GRID built-in
semiempirical method (GRIN directive IHAC = 1), in order to retain
consistency of GRID methodology.
13
(
NMR) spectra were recorded on a Varian Gemini 2000, or Bruker
AVANCE 500 instruments, on 200/50, or 500/125 MHz. NMR spec-
tra were recorded in CDCl , or DMSO-d , using TMS as the internal
3
6
standard. Chemical shifts (d) are reported in ppm. Multiplicities of
signals are given as follows: s – singlet, d – doublet, t – triplet,
qu – quintet, m – multiplet. NOESY spectrum of compound 1 in
ꢂ2
CDCl
3
(7 mg/0.5 mL; 4.9 ꢁ 10 M) was recorded on Bruker Avance
instrument on 500 MHz, with mixing time of 1 s. Fourier-
transform infrared spectra (FT-IR) were recorded on a Bomem
ꢂ1
MB 100 spectrophotometer (
m
max are given in cm ) on KBr pellets.

4
38
J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
Results and discussion
The experimentally obtained fluorescence and absorption max-
ima and the corresponding Stokes shifts ( ) of compounds (2–6)
Dk
s
Solvent effects on the absorption and emission spectra
measured in six solvents are shown in Table 2. The fluorescence
spectra of all compounds in methanol and the fluorescence spectra
of compound 2 in selected solvents are shown in Fig. 4a and b,
respectively. With increasing solvent polarity, both absorption
and emission maxima undergo a bathochromic shift, the latter
being more pronounced than the former, except for the compound
4. Furthermore, the emission spectra in polar solvents (methanol,
formamide) are broad and structureless, when compared with
spectra recorded in less polar solvents (e.g. ethyl acetate). This
indicates the formation of an ICT state rapidly after the excitation
[36]. The methylated compound 6 has emission maxima at longer
wavelengths than the neutral compounds (Fig. 4a), but the small-
est shift of the absorption and the fluorescence maxima was
observed in the solvents used, compared to neutral compounds
(Table 2). Thus, the Stokes shift values for compounds 2–6 increase
with the solvent polarity, implying that their dipole moment is lar-
A preliminary investigation of solvent effects on the absorption
maxima shift was performed in solvents with different polarity and
hydrogen-bonding ability. A significant shift of the absorption
maxima of compound 1 was not observed, while compound 7
appeared insoluble in majority of the solvents used. The solvato-
chromic behavior of compounds 2–6 was further investigated by
recording their absorption spectra in 24 solvents in the spectral
range from 200 to 600 nm. The effect of different concentration
ꢂ4
ꢂ5
ꢂ3
of the compounds, in the range from 10 to 10 mol dm , on
the position of absorption maxima (kmax) was also studied. It was
observed that variation in concentrations did not influence the
position or the shape of absorption bands, confirming that there
is no aggregation of the studied compounds in the chosen solvents.
The most intensive absorption maxima are shown in Table 1, while
representative spectra are shown in Fig. 3a (for compounds 2–6 in
methanol) and Fig. 3b (for compound 2 in several typical solvents).
As can be seen in Table 1, an increase in solvent polarity shifted the
absorption maxima of compounds 2–5 to the longer wavelengths
e g
ger in the excited state than in the ground state (l > l ).
Conformational assembly of compound 1
(
bathochromic effect). The absorption maxima of the neutral com-
pounds in aprotic solvents are shifted to the shorter wavelengths,
in comparison with protic solvents (hypsochromic effect). The
reverse situation was observed for compound 6. Solvents with
the highest hypsochromic and bathochromic shifts are marked in
The shape of the UV–Vis spectra of 2,6-distyrylpiridines and
structurally similar compounds are commonly ascribed to simulta-
neous existences of a few rotamers in solution. This phenomenon
has been examined using different spectroscopic techniques [37],
mostly fluorescent. Literature reports on NMR investigation of
rotamerism of 2,6-distyrylpiridines and similar compounds are
not frequent, most probably because inherent time-scale of nuclear
transition, as well as relatively low sensitivity of the method. On
the other hand, conformational preferences of flexible molecules
in solution have been frequently examined by NMR [38,39]. NMR
spectra of flexible molecules represent an average conformation
of the compound, due to the fast conformational exchange during
time scale of spectrum recording. Such average conformation, most
often, does not correspond to the conformation of any real con-
former that exists in solution. NAMFIS analysis [24] was tailored
to resolve structures and the relative populations of several con-
formers from the NMR data and the conformational assemblies
of studied molecules.
Table 1. An unusually large shift of
–5 in acetic acid is caused by the protonation of the pyridine
nitrogen. Because compound 6 is N-methylated, a moderate nega-
tive solvatochromic shift of k = 35 nm was observed. Such behav-
Dk = 93–110 nm for compounds
2
D
ior is characteristic of pyridinium salts [34,35]. Nevertheless, the
absorption maxima of the methylated compound 6 undergo a sig-
nificant bathochromic shift, when compared to the neutral forms.
Table 1
Position of the absorption maxima of compounds 2–6 in selected solvents.
Solvent
Compound
2
3
4
5
6
k (nm)
We aimed to estimate population of rotamers of compound 1 in
chloroform using H NMR and NOESY spectra and NAMFIS analysis.
Compound 1 was chosen because of well-resolved NOESY signals
in its spectrum recorded in chloroform. Afterwards, the UV–Vis
spectra of the most abundant rotamers, as found by NAMFIS anal-
ysis, were calculated and compared with the experimentally
1
439c
1
,4-Dioxane
307
298
329
324
329
326
326
326
332
331
334
326
348
353
329
329
348
350
349
330
350
335
303
299
306
300
302
298
307
305
306
301
300
360
382
302
300
301
300
387
300
310
326
302
325
309
308
309
328
327
329
309
328
352
400
323
348
349
350
390
329
332
b
b
b
b
Acetonitrile
Anisole
404
a
306
302
299
305
309
308
324
304
321
321
320
300
321
319
322
321
322
324
408c
321
306
307
110
–
–
a
Diethyl ether
Dichloromethane
Diisopropyl ether
Dimethylacetamide
Dimethylformamide
Dimethyl sulfoxide
Ethyl acetate
Ethanol
429
b
a
–
3
obtained UV–Vis spectrum of compound 1 in CHCl . The section
418
416
416
434
417
410
415
1
of the H NMR spectrum of compound 1 in chloroform that corre-
sponds to the aromatic and vinylic protons, is shown in Fig. 5.
All NMR signals are well-resolved, and coupling constants of
1
6.02 Hz unambiguously show the trans-configuration of the
Ethylene glycol
Formamide
Chloroform
vinylic bonds (7.07 and 7.65 ppm). Only pattern of the 4-MeOAPh
AB protons (6.92 and 7.55 ppm) point to few rotamers around
PhACH@CHA bonds. NOESY signal between 4-MeOA hydrogens
and Ph AB protons, unambiguously show that the doublet on
.92 ppm belongs to protons on Ph ring in ortho-position to MeOA
group (Fig. 6a). From the section of NOESY spectrum from 6.80 to
.30 ppm, the spatial vicinity of the proton of ACH@CHA moiety
at 7.65 ppm and both the meta-pyridine hydrogens and the phenyl
hydrogens ortho- to vinyl bond was obvious (Fig. 6b). On the other
hand, the protons of the ACH@CHA moiety at 7.08 ppm are close
to the phenyl hydrogens ortho- to the vinyl bond. So, average con-
formation of molecule appeared very different from three confor-
mations shown in Fig. 1, which have been found as the local
minima obtained by geometry optimization on high level of theory
a
–
2
-Propanol
Methanol
-Butanol
N-Methylformamide
-Propanol
427
412
428
417
425
432
406
434
438
1
6
1
Pyridine
7
Acetic acid
tert-Butanol
Tetrahydrofuran
Toluene
420c
343
331
328
96
391c
300
302
306
93
410c
345
327
324
108
a
–
D
k (nm)
35
a
b
c
Insoluble in given solvent.
Solvent with the highest hypsochromic shift.
Solvent with the highest bathochromic shift.

J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
439
Fig. 3. Representative UV–Vis spectra. Normalized absorption spectra of: (a) compounds 2–6 in methanol, and (b) compound 2 in selected solvents.
Table 2
Absorption and fluorescence maxima, and the Stokes shift (
s
Dk , given in bold) of compounds 2–6 in selected solvents.
Compound
Solvent
Dk (nm)
a
b
c
d
ACN
EtOAc
Chloroform
DMA
MeOH
Formamide
2
3
k
k
Abs: (nm)
298
408
110
304
397
93
300
407
107
309
411
102
319
435
116
320
443
123
22
46
max
Fluor:
(nm)
max
D
k
s
(nm)
Abs:
324
425
101
326
404
78
329
412
83
332
434
102
350
446
96
329
444
115
26
42
k
k
(nm)
max
Fluor:
max
(nm)
s
Dk (nm)
4
5
k
k
Abs: (nm)
299
407
301
399
302
405
307
478
301
401
382
445
83
79
max
Fluor:
(nm)
max
D
k
s
(nm)
108
302
98
309
103
323
171
328
100
349
63
400
Abs: (nm)
98
k
max
Fluor:
max
421
399
410
414
422
516
117
k
(nm)
D
k
s
(nm)
119
90
87
86
73
116
e
6
k
k
Abs: (nm)
404
514
110
434
516
82
–
418
509
91
412
517
105
415
479
64
30
38
max
Fluor:
e
(nm)
–
max
D
k
s
(nm)
–
a
b
c
Acetonitrile.
Ethyl acetate.
Dimethylacetamide.
Methanol.
d
e
Insoluble in given solvent.
Fig. 4. Representative fluorescence spectra. Normalized fluorescence spectra of: (a) compounds 2–6 in methanol, and (b) compound 2 in selected solvents.

4
40
J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
Fig. 5. The section of the 1H NMR spectrum of compound 1 in chloroform that include signals of the aromatic and vinylic protons.
Fig. 6. The sections of the 1NOESY spectrum of compound 1: (a) signal between 4-MeOA protons and protons of the phenyl ring in ortho-position to MeOA group; (b) signals
of aromatic and vinylic protons.
(
HF, DFT and MP2) [9], for structurally similar 2,6-distyrylpyridine
derivative.
NAMFIS analysis recognized three conformers as the most
abundant ones (Fig. 7, I–III). Those three conformers accounts for
1% of all conformers in solution. We normalized abundance of
during data preparation for NAMFIS analysis, i.e. coupling con-
stants and NOESY signals were assigned to each PhACH@CHA
moiety, and only the protons of the MeO-methyl group were con-
sidered as equivalent, more accurate structure–spectra assignment
will be assessed for unsymmetrically substituted derivatives. For
example, see reference [40] where NMR spectra of unsymmetri-
cally substituted derivatives was described.
The positions of absorption maxima at ꢃ320 and 340 nm in the
calculated and experimentally obtained UV–Vis spectra are in fair
agreement (Fig. 8). The most red-shifted peak in calculated UV–
9
those three conformers to 100%. An UV–Vis spectrum of each con-
former is calculated by the ZINDO/S method [30], and the intensi-
ties of obtained spectra were scaled according to abundance of
conformers found by NAMFIS analysis. The overlapped, calculated,
UV–Vis spectra are shown in Fig. 8.
The calculated UV–Vis spectra of the most abundant conformers
derived from NOESY data and NAMFIS analysis are in obvious dis-
agreement when compared to the experimentally obtained UV–Vis
spectrum of derivative 1 in CHCl . One of the possible reasons for
3
this is difference in concentrations of the NMR and the UV–Vis
Vis spectrum (conformer I, vmax = 340 nm calculated; observed
348 nm for the most red-shifted peak in the experimentally
obtained spectrum) are significantly broader, comparing to the
most red-shifted peak in the experimentally obtained UV–Vis spec-
trum. In this conformation (I), PhA to ACH@CHA torsion of ꢃ85° in
one of the styryl fragments was found, while the same torsions is
significantly lower in the other styryl part. This conformation in
ACH@CHAPyACH@CHA part roughly resembles conformation A
depicted in Fig. 1 (s-cis/s-cis). The peak in the calculated spectra
of conformation III is blue-shifted with respect to the real spectrum
samples. NMR spectra of derivative 1 are recorded in concentration
ꢂ2
of 4.9 ꢁ 10 M, while the UV–Vis spectrum is recorded in concen-
ꢂ5
1
tration of 1 ꢁ 10 M. Next, we used symmetrically substituted
compound, so in H NMR spectrum only one pair of doublets for
both vinylic bonds and for aromatic protons of the Ph moieties
appeared. Although we treated derivative 1 as unsymmetrical
(vmax = 284 nm calculated, observed 308 nm for the most

J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
441
3
Fig. 7. (a–c) The most abundant conformers of compound 1 in CHCl solution, as found by NOESY data and NAMFIS analysis. (d–f) Local minima of compound 1 with different
geometries of PyACH@CH bonds, obtained by MP2 calculations. Dipoles are also shown. For clarity, the dipole of s-trans/s-trans conformer (f) is scaled with 0.85.
Fig. 8. (a) Experimentally obtained UV–Vis spectrum of compound 1 in chloroform; (b) calculated UV–Vis spectra of conformers I–III shown in Fig. 7a–c.
blue-shifted peak in the experimentally obtained spectrum) and
this peak is (again) significantly broader than in the experimentally
obtained UV–Vis spectrum. In this conformation PyA to ACH@CHA
torsion of ꢃ83°, and the PhA to ACH@CHA torsion of ꢃ48° in one
styryl fragment were found, while in other PhA to ACH@CHA
torsion is of ꢃ41°. This conformation in ACH@CHAPyACH@CHA
part roughly resembles conformation B depicted in Fig. 1 (s-trans/
s-trans). The only conformation with the fairly predicted UV–Vis
spectrum is conformation II, with the calculated absorption
maximum at 316 nm (the experimentally obtained absorption
maximum of the ‘middle’ peak was at 321 nm). Although in this
conformation both PyA to ACH@CHA and PhA to ACH@CHA
torsions appeared significant (different from almost coplanar ‘ideal’
conformations schematically depicted in Fig. 1), such conformation
is less symmetrical in ACH@CHAPyACH@CHA part, comparing to
conformations I and III. So, the calculated UV–Vis spectrum of
conformation II, which most resemble conformation having s-cis
configuration in the one styryl part and the s-trans configuration
in the other styryl part, appeared closest to the middle part of
experimentally obtained spectrum of compound 1 in CHCl , both
by the position of absorption maximum and by the width of the
absorption peak. Most probably, other two conformations that fit
to absorption maxima (and peak widths) at 308 and 348 nm in
3
3
experimentally obtained spectrum of compound 1 in CHCl can
be found among similar conformations having s-cis configuration
in the one styryl part and the s-trans configuration in the other.
HOMO ? LUMO and HOMO ? LUMO+1 transitions contribute
mainly to absorption peaks at 340 and 316 nm of conformations I
and II, respectively; while HOMO ? LUMO and HOMOꢂ1 ?
LUMO+2 transitions contribute to the absorption peak on 284 nm
of conformation III. Those molecular orbitals are shown in Fig. S1
in Supplementary Material.
In order to further examine hypothesis of preferred conformers
3
of compound 1 in CHCl , we optimized geometry of compound 1 in
three ‘ideal’ conformations, schematically depicted in Fig. 1, to sta-
tionary points (local minima) by MP2 calculations and 6-311G

4
42
J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
basis set in vacuum (Fig. 7d–f). Vibration analysis showed that no
imaginary frequencies exist, confirming that local minima were
obtained. Predicted thermal energy and zero-point vibrational
energies of conformers, energies of frontier orbitals and numerical
values of dipoles are shown in Table 3. Dipoles are also depicted in
Fig. 7. It should be noted that for the 4-MeO-substituted derivative
As a consequence of N-methylation of the central pyridine ring,
and the presence of the iodine counter-ion, the s-cis/s-trans confor-
mation of compound 6 was found as the most stable. The HOMO
orbital is delocalized over the whole system, while LUMO is pri-
marily located on the styrylpyridinium core with minimal density
on the iodide counter anion. Because basis set used for compound
6 is different from the basis set used for geometry optimization of
the neutral compounds, energies of the frontier orbitals and their
difference for this compound cannot be compared with the rest
in the set.
(1) significantly lower differences in energies of three conformers
were obtained, comparing to data reported in Ref. [9], obtained
for the unsubstituted compound. This can be a consequence of
somewhat different structures of compounds, as well as the differ-
ences in basis sets used.
The UV–Vis spectra of all conformations obtained by MP2
method were calculated by semiempirical ZINDO/S method, and
by TD-DFT calculations with 6-311G basis set. The calculated
UV–Vis spectra are shown in Supplementary Material (Fig. S2).
Obviously, the predicted UV–Vis spectra obtained by ZINDO/S
method are (at least by the position of the absorption maxima)
something more like to the experimentally obtained spectrum of
compound 1, but are still in significant extent far from the real
spectrum, considering both the width of the peaks and the posi-
tions of absorption maxima. A more detailed examination of this
type will be performed on unsymmetrically substituted derivatives
in future.
Linear solvation energy correlation analyses
The Kamlet–Taft [16] and Catalán [17] linear solvation energy
relationship (LSER) models were used in order to investigate
effects of solvent–solute interactions on the absorption maxima
shifts (solvatochromic properties). The three-parameter Kamlet–
Taft model (Eq. (1)) describes hydrogen bond donating (
a) and
hydrogen bond accepting (b) capacities, along with dipolarity/
*
polarizability (p ) of solvents.
m
~
¼
~
mmax;0 þ a ꢄ
a
þ b ꢄ b þ s ꢄ pꢅ
ð1Þ
max
The significance of each of those effects on the solvatochromic
properties is expressed by the weights of terms a, b and s. Although
this is the most frequently used LSER model, it does not separate
dipolarity and polarizability effects of solvents. The newer, Catalán
LSER model (Eq. (2)), provides different parameters, SP and SdP, for
those two effects, respectively, along with parameters which
describe hydrogen bond donating (SA) and hydrogen bond accept-
ing (SB) abilities of solvents.
Nature of the frontier molecular orbitals
The optimized molecular geometries, their highest occupied
molecular orbitals (HOMOs) and lowest unoccupied molecular
orbitals (LUMOs) are shown in Fig. 9.
The molecular geometries of neutral compounds 1–5 and 7, are
optimized in their s-cis/s-cis geometries to stationary point, on DFT
level of theory (B3LYP/6-311G). Energies of frontier orbitals are
shown in Table 4. Qualitatively, HOMO of the neutral compounds
is delocalized over the entire molecule, while, with the exception
of compounds 4 and 7, LUMO is shifted towards the central pyri-
dine ring. The introduction of the weakly electron-donating ethoxy
groups in compounds 3 and 5 does not produce any appreciable
change in the position of HOMO and LUMO orbitals with respect
to compound 2. The energy gap of these compounds resembles
that of compound 2, although the involvement of the oxygen
atoms in HOMO slightly lowers its energy. On the other hand,
the introduction of the strong electron-withdrawing nitro group
in compound 4 causes a shift of the electron density of LUMO
towards the outer phenyl rings. LUMO and LUMO+1 orbitals are
almost degenerated for compound 4 (Fig. 9 and Table 4). In addi-
tion, the introduction of the nitro group leads to a stronger stabil-
ization of both HOMO and LUMO orbitals, and the energy gap for
this compound is the lowest, when compared with other neutral
compounds. It should be noted that the magnitude of dipole
moment of the neutral compounds increases with increasing num-
ber of substituents in the outer rings, and the electron-withdraw-
ing ability of substituents (Table 4). In compound 7, which is
symmetrical as the other compounds, both LUMO and LUMO+1
orbitals are delocalized over its bent core in something unsymmet-
rical fashion, and energies of both HOMO and LUMO are lowered
with respect to compound 2.
m
~
¼
~
mmax;0 þ a ꢄ SA þ b ꢄ SB þ c ꢄ SP þ d ꢄ SdP
ð2Þ
max
The solvent parameters of both models are given in Supplementary
Material (Table S1). The results of the multiple linear regression
analysis obtained by the Kamlet–Taft and Catalán models are
shown in Tables 5 and 6, respectively. All correlation coefficients
(R) calculated at the 95% confidence level are higher than 0.91, for
all regressions. Therefore, those equations are suitable for the anal-
ysis of solvatochromic behavior. The success of the quantification
and interpretation of solvent effects on the position of the most
intense absorption band is illustrated in Fig. S3 (Supplementary
Material) by plots of
mmax measured (
mexp) versus mmax calculated
(mcalc).
In both models, coefficient a provides information about inter-
actions between hydrogen-bond donating (HBD) moiety of sol-
vents and the appropriate hydrogen-bond accepting (HBA)
fragments of compounds, while coefficient b describes the specific
solvation of compounds by hydrogen bonding to the HBA moieties
of solvents.
One can expect that HBD solvents interact with the pyridine
nitrogen, or with the oxygen atoms of OH, EtO or NO groups of
2
molecules, but the strength of such interactions are different from
compound to compound. The accessibility and the strength of
interactions of the hydrogen-bonding moieties of the molecules,
in their ground state, with HBA and HBD were estimated using
GRID [31–33] probes that represent the hydroxyl group bound to
Table 3
Predicted stabilities, energies of frontier orbitals and dipoles of three conformers of 4-MeO derivative (1), Fig. 7d–f, as obtained by MP2 calculations with 6-311G basis set on
T = 298.16 K and P = 1 atm, without applied solvent model.
Conformer
Thermal E (kcal/mol)
Zero-point vibrational E (kcal/mol)
E HOMO (Hartree)
E LUMO (Hartree)
Dipole (Debye)
s-trans/s-trans
s-cis/s-trans
s-cis/s-cis
250.597
249.642
250.233
235.272
234.739
234.783
ꢂ0.27507
ꢂ0.27290
ꢂ0.27184
0.07087
0.07100
0.06877
4.528
2.674
2.289

J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
443
Fig. 9. HOMO and LUMO orbitals of compounds 1–7, as obtained by DFT calculations. LUMO and LUMO+1 orbitals of compounds 4 and 7 are almost symmetrical in respect to
plane defined by the N and the C4 atom of the pyridine ring. Both LUMO and LUMO+1 are depicted for those compounds.
Table 4
Energies of frontier molecular orbitals and their difference, E gap, dipoles and Mülliken charges at the pyridine N of compounds 1–5 and 7.
Compound
E HOMO (Hartree)
E LUMO (Hartree)
E gap (Hartree)a
Dipole (Debye)
Mülliken charge at pyridine N (a.u.b)
1
2
3
4
5
7
ꢂ0.19881
ꢂ0.20250
ꢂ0.20281
ꢂ0.22270
ꢂ0.20622
ꢂ0.20923
ꢂ0.06032
0.13849
0.13962
0.14016
0.09555
0.13635
0.13677
1.823
2.004
4.152
8.176
10.257
2.166
ꢂ0.4187
ꢂ0.4188
ꢂ0.4198
ꢂ0.4192
ꢂ0.4177
ꢂ0.4161
ꢂ0.06288
ꢂ0.06265
c
c
ꢂ0.12715 (ꢂ0.12320)
ꢂ0.06987
ꢂ0.07246 (ꢂ0.06945)
a
b
c
E gap is difference in energies of HOMO and LUMO orbitals.
Atomic units.
LUMO+1. For compounds 4 and 7, we found very similar energies of LUMO and LUMO+1 orbitals, which are almost degenerated. In compound 4, LUMO orbital covers one
half of the molecule, while LUMO+1 orbital cover the other half. In compound 7, both LUMO and LUMO+1 orbitals in something unsymmetrical fashion cover
AC(O)OAPhACH@CHAPyACH@CHAPhAO(O)CA, but are mutually symmetrical. Because of this, both LUMO and LUMO+1 orbitals of compounds 4 and 7 are shown in Fig. 9.
Table 5
Regression data (±sd) for the Kamlet–Taft model.
ꢂ1
Ra
sdb (cmꢂ1
)
Fc
sign. Fc
nd
Compound
m
0
(cm
)
a
b
s
ꢂ6
ꢂ8
ꢂ4
ꢂ4
ꢂ4
e
f
2
3
4
5
6
34,374 (±344)
31,337 (±237)
32,065 (±962)
33,453 (±594)
21,999 (±484)
ꢂ1114 (±255)
ꢂ1741 (±179)
ꢂ4259 (±634)
ꢂ2481 (±450)
1238 (±249)
ꢂ2137 (±338)
ꢂ1092 (±225)
7132 (±1216)
ꢂ2152 (±576)
ꢂ1094 (±432)
ꢂ1035 (±403)
ꢂ586 (±299)
ꢂ5014 (±856)
ꢂ1760 (±718)
2932 (±534)
0.924
0.958
0.945
0.910
0.886
396
274
811
646
326
32.9
62.5
30.4
20.9
13.4
<1 ꢁ 10
<1 ꢁ 10
<1 ꢁ 10
<1 ꢁ 10
<5 ꢁ 10
21
21
15
17
g
h
15i
a
b
c
d
e
f
Correlation coefficient.
Standard deviation.
Fisher test of significance.
Number of solvents.
Excluded solvents: acetic acid, pyridine, toluene.
Excluded solvents: acetic acid, N-methylformamide, formamide.
Excluded solvents: acetic acid, dichloromethane, methanol, chloroform, N-methylformamide, acetonitrile, anisole, toluene, ethanol.
Excluded solvents: acetic acid, toluene, ethanol, 1-propanol, acetonitrile, formamide, N-methylformamide.
g
h
i
Excluded solvents: dichloromethane, formamide, ethylene glycol, pyridine. Insoluble in: anisole, diethyl ether, diisopropyl ether, chloroform, toluene.

4
44
J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
Table 6
Regression data (±sd) for the Catalán model.
ꢂ1
Ra
sdb (cmꢂ1
)
Fc
sign. Fc
nd
Compound
m
0
(cm
)
a
b
c
d
ꢂ6
ꢂ8
ꢂ4
ꢂ5
ꢂ4
e
f
2
3
4
5
6
35,603 (±1032)
32,582 (±628)
43,326 (±2467)
41,979 (±2227)
24,077 (±760)
ꢂ2796 (±378)
ꢂ3971 (±359)
ꢂ5726 (±1004)
ꢂ8407 (±1182)
771 (±273)
ꢂ2480 (±410)
ꢂ1597 (±247)
5876 (±1740)
ꢂ3199 (±981)
ꢂ1682 (±405)
ꢂ2871 (±1297)
ꢂ2326 (±812)
614 (±374)
443 (±252)
4440 (±1491)
1340 (±967)
2263 (±384)
0.950
0.975
0.935
0.914
0.931
322
211
892
843
265
34.4
66.2
19.1
18.9
19.4
<1 ꢁ 10
<1 ꢁ 10
<1 ꢁ 10
<1 ꢁ 10
<1 ꢁ 10
20
19
16
20
g
h
ꢂ24,002 (±3750)
ꢂ13,525 (±2904)
ꢂ1880 (±952)
17i
a
b
c
d
e
f
Correlation coefficient.
Standard deviation.
Fisher test of significance.
Number of solvents.
Excluded solvents: N-methylformamide, acetic acid, pyridine, dimethyl sulfoxide.
Excluded solvents: N-methylformamide, acetic acid, formamide, ethylene glycol, diisopropyl ether.
Excluded solvents: N-methylformamide, chloroform, anisole, toluene, dichloromethane, 1,4-dioxane, acetic acid, pyridine.
Excluded solvents: N-methylformamide, acetic acid, ethylene glycol, 1-propanol.
g
h
i
Excluded solvents: N-methylformamide, dichloromethane. Insoluble in: anisole, diethyl ether, diisopropyl ether, chloroform, toluene.
an aliphatic alkyl chain (HBD probe) and the aliphatic ether oxygen
compound 1, which did not exert observable solvatochromism.
For compound 4, the largest difference in negative lobes of electro-
static potential in the ground and excited state can be observed on
(
HBA probe). The set of the solvents used for the experimental
evaluation of solvatochromic properties of the studied molecules
comprises a number of aliphatic alcohols and ethers. MIFs depicted
on isocontour levels of ꢂ4.5 kcal/mol for HBD probe, and ꢂ2.0 kcal/
mol for HBA probe are shown in Fig. S4 in Supplementary Material.
The steric hindrance of the pyridinium nitrogen with the
ACH@CHA moieties for interaction with HBD probe is obvious.
Interactions of the ‘lone’ OH groups on the outer phenyl moieties
in compound 2 with HBD probe is of significantly lower strength
2
the one of NO groups. All described is consistent with the regres-
sion coefficients derived from the Kamlet–Taft and Catalán models.
Both terms associated with the HBD and HBA influence of solvent
(a and b) are of the same sign (except term b for compound 4, as
explained above) and have relative low variation from compound
to compound. Both Kamlet–Taft coefficient s and Catalán coeffi-
cient c showed largest variability. So, the solvatochromic behavior
of compounds 2–6 can be in a considerable extent ascribed to non-
specific electrostatic interactions with the solvent, rather than to
specific hydrogen bonding. Most probably that such behavior
stems from the high symmetry of the investigated compounds.
Considering the Catalán model, it can be seen that the magnitude
of the polarizability (coefficient c) is higher than that of the dipo-
larity (coefficient d). Most probably because of the presence of
2
than the interactions of proximal EtO and OH; or EtO, NO and
OH groups in compounds 3–5. For compound 2, interactions with
HBD probe is depicted on isocontour level of ꢂ3.8 kcal/mol, in
order that MIF be visible. It should be noted that HBD probe exerts
significantly weaker interactions with the NO
. MIF of HBD probe associated with the NO
2
group of compound
2
group is visible on
4
isocontour level of ꢃ ꢂ3.5 kcal/mol (data not shown). The ester
carbonyl O exerts stronger interactions with HBD probe than ester
AC(O)OA, in chosen conformation, most probably due to the steric
hindrance of this oxygen by the vicinal phenyl rings.
2
electron-withdrawing substituents (NO and Br), the highest val-
ues of coefficient d was obtained for compounds 4 and 5, which
also have the largest dipole moments in the ground state (Table 4).
A hypsochromic shift with increasing solvent dipolarity/polariz-
ability (positive s coefficient) is observed for compound 6.
The charge distribution in the ground state of the molecules
significantly changes upon introduction of different substituents
in the outer phenyl rings. This is illustrated by the graphical
depiction of the negative lobe of the electrostatic potential (ESP)
of compounds 1–5, 7 in the ground state, obtained from total
self-consistent field (SCF) density (Fig. 10). Surfaces are depicted
on arbitrary chosen isocontour level, which provide good distinc-
tion among the compounds studied; and negative lobe of ESP
was chosen, because it covers moieties of all heteroatoms in mol-
ecules, which participate in (intermolecular) hydrogen bonding.
Such differences cannot be observed by considering the calculated
atomic charges on, for example, the pyridine nitrogen (see Table 4)
or on any other heteroatom in the molecules. Along with this, the
frontier molecular orbitals (HOMO and LUMO), depicted in Fig. 9,
provide a different kind of information. In this place, it should be
noted that electrostatic potential of a molecule is a physical obser-
vable, while atomic charges and molecular orbitals are not. In the
case of compound 4, most probably, the formation of a strong
intramolecular hydrogen bond between the nitro and hydroxyl
groups efficiently hinders HBD ability of the OH group to solvent,
resulting in a positive sign of coefficient b. In the case of compound
Multivariate Curve Resolution (MCR) analysis
The emission spectra of compounds 2 and 7 were compared in
order to analyze effect of esterification of the hydroxyl group of
compound 2 on the position of emission maxima and the complex-
ity of the spectra.
The series of emission spectra of the compounds 2 and 7 in
chloroform, as well as the results of corresponding MCR analysis
of the emission profiles are shown in Fig. 11. The emission spectra
of compound 2 contain five components with the emission max-
ima positioned at 392, 415, 480, 518 and 553 nm. The spectrum
of compound 7 contains four components with the maxima at
380, 394, 404 and 457 nm. The spectral components correspond
to the discrete emitting structures in the molecules. The higher
number of spectral components in the spectrum of compound 2
in comparison with the compound 7 indicates higher number of
discrete emitting structures in compound 2 and consequently
more complex spectrum. The components of the spectrum of
compound 2 are red shifted, comparing with the components of
the spectrum of compound 7. This might be attributed to the
electron-accepting competition between the pyridine ring and
ester groups in compound 7. It also should be noted that the alkyl
tails in compound 7 can promote self-association of molecules,
especially in polar solvents.
5, halogen bonding of bromine with HBA of solvents should also be
considered [41].
Difference in charge distribution of compounds in the ground
and in excited state can be clearly seen in Fig. 10. For all
compounds, negative lobe of ESP covers a greater part of the pyri-
dine moiety in the excited state, when compared to the ground
state of the molecule. The lowest difference can be observed for

J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
445
Fig. 10. Negative lobe of the electrostatic potential of compounds 1–5, 7 in ground and in excited states, obtained from total self-consistent field (SCF) density, or total CI
ꢂ2
density of geometries optimized on DFT level of theory, depicted on isocontour level of 0.02, or 0.03 (for compound 4) e Å
.
Fig. 11. Fluorescence landscapes (left) and corresponding spectral components (right) obtained by the MCR analysis, for the compound 2 (upper panel) and compound 7
lower panel) in chloroform.
(

4
46
J.M. Markovi ´c et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 435–446
[
[
5] M. Matsui, K. Ooiwa, A. Okada, Y. Kubota, K. Funabiki, H. Sato, Dyes Pigm. 99
2013) 916–923.
6] E. Lukovskaya, Y. Glazova, Y. Fedorov, A. Bobylyova, A. Mizerev, A. Moiseeva, A.
Anisimov, A. Peregudov, O. Fedorova, Dyes Pigm. 104 (2014) 151–159.
Conclusion
(
We reported synthesis and spectral properties of symmetrical
,6-distyrylpyridine derivatives. To the best of our knowledge, for
2
[7] G. Bartocci, A. Spalletti, U. Mazzucato, in: J. Waluk (Ed.), Conform. Anal. Mol.
Excit. States, Wiley, New York, 2000 (Chapter 5).
the first time in literature the NAMFIS analysis, from NMR data
and in silico obtained conformational assembly, was used to esti-
mate population of conformers in solution of the representative
compound from this class (compound 1). Results of such analysis
were used in attempt to explain the shape of the UV–Vis spectrum
of compound, which comprise few overlapped absorption maxima.
The goal is achieved with a limited success, but reveals that confor-
mation assembly of compound is more complex than was assumed
in so far published literature data, which relay on three ‘ideal’ con-
formers mutually different in the configuration (s-cis or s-trans) of
PyACH@CHA bonds.
Spectral and photophysical properties of the investigated com-
pounds were studied in solvents of the different polarity and the
different hydrogen bonding ability. It was found that the neutral
compounds show positive solvatochromic properties, while the
compound with the N-methylated pyridinium nitrogen exhibits a
negative solvatochromism. Solvent effects on the shifts of the fluo-
rescence maxima are more pronounced when compared with the
absorption spectra. The obtained Stokes shift values (>100 nm in
some cases) can be related to the strong ICT character of the inves-
tigated compounds. ICT transitions were activated to a different
extent depending on the substitution pattern of the outer phenyl
rings, or N-methylation of the central pyridine ring.
[
8] V.M. Chapela, M.J. Percino, C. Rodríguez-Barbarín, J. Chem. Crystallogr. 33
2003) 77–83.
[9] F.J. Melendez, O. Urzúa, M.J. Percino, V.M. Chapela, Int. J. Quantum Chem. 110
2010) 838–849.
10] L. Giglio, U. Mazzucato, G. Musumarra, A. Spalletti, Phys. Chem. Chem. Phys. 2
2000) 4005–4012.
[11] H. Wang, R. Helgeson, B. Ma, F. Wudl, J. Org. Chem. 65 (2000) 5862–5867.
(
(
[
(
[
12] X. Xu, W. Qiu, Q. Zhou, J. Tang, F. Yang, Z. Sun, P. Audebert, J. Phys. Chem. B 112
2008) 4913–4917.
13] P.P. Madeira, C.A. Reis, A.E. Rodrigues, L.M. Mikheeva, B.Y. Zaslavsky, J. Phys.
Chem. B 114 (2010) 457–462.
14] I. Shehatta, Helv. Chim. Acta 85 (2002) 2125–2137.
15] P. Vallat, W. Fan, N. El Tayar, P.-A. Carrupt, B. Testa, J. Liq. Chromatogr. 15
1992) 2133–2151.
[16] M.J. Kamlet, J.-L.M. Abboud, M.H. Abraham, R.W. Taft, J. Org. Chem. 48 (1983)
877–2887.
17] J. Catalán, J. Phys. Chem. B 113 (2009) 5951–5960.
18] A. Marini, A. Muñoz-Losa, A. Biancardi, B. Mennucci, J. Phys. Chem. B 114
(2010) 17128–17135.
(
[
[
[
(
2
[
[
[19] J. Markovic
´
, N. Trišovi c´ , T. Tóth-Katona, M. Mil cˇ i c´ , A. Marinkovi c´ , C. Zhang, A.J.
Jákli, K. Fodor-Csorba, New J. Chem. 38 (2014) 1751–1760.
20] E.D. Bergmann, S. Pinchas, J. Org. Chem. 15 (1950) 1184–1190.
21] J. Mendieta, M.S. Diaz-Cruz, M. Esteban, R. Tauler, Biophys. J. 74 (1998) 2876–
2888.
22] R.W. Harrison, J. Comput. Chem. 14 (1993) 1112–1122.
23] A. Pedretti, L. Villa, G. Vistoli, J. Comput. Aided Mol. Des. 18 (2004) 167–173.
http://www.ddl.unimi.it>.
[24] D.A. Evans, M.J. Bodkin, S.R. Baker, G.J. Sharman, Magn. Reson. Chem. 45 (2007)
95–600.
25] O. Atasoylu, G. Furst, C. Risatti, A.B. Smith III, Org. Lett. 12 (2010) 1788–1791.
26] J.J.P. Stewart, MOPAC2012, Stewart Computational Chemistry, Colorado
Springs, CO, USA. <http://openmopac.net>.
[
[
[
[
<
5
[
[
Acknowledgements
[
[
27] J.J.P. Stewart, J. Mol. Model. 13 (2007) 1173–1213.
28] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian03/09, Revision C.02 2003/2009.
The Ministry of Education, Science and Technological Develop-
ment of the Republic of Serbia supported this work; Grants No.
1
72013, 172035 and 173017. The work reported makes use of
results produced by the High-Performance Computing Infrastruc-
ture for South East Europe’s Research Communities (HP-SEE), a
project cofunded by the European Commission (under Contract
Number 261499) through the Seventh Framework Programme
HP-SEE (http://www.hpsee.eu/). Authors gratefully acknowledge
computational time provided on PARADOX cluster at the Scientific
Computing Laboratory of the Institute of Physics Belgrade
(
SCL-IPB), supported in part by the Serbian Ministry of Education,
[
29] Jmol: an open-source Java viewer for chemical structures in 3D. <http://
www.jmol.org/>.
30] J.E. Ridley, M.C. Zerner, Theor. Chem. Acc. 32 (1973) 111–134.
31] G. Cruciani (Ed.), Molecular Interaction Fields. Applications in Drug Discovery
and ADME Prediction, Wiley-VCH, Zürich, Switzerland, 2006.
Science and Technological Development under Projects No.
ON171017 and III43007, and by the European Commission under
FP7 Projects PRACE-3IP and EGI-InSPIRE.
[
[
[
[
[
32] GRID Version 22c, Molecular Discovery Ltd. <http://www.moldiscovery.com>.
33] P.J. Goodford, J. Med. Chem. 28 (1985) 849–857.
34] P. Jacques, B. Graff, V. Diemer, E. Ay, H. Chaumeil, C. Carré, J.-P. Malval, Chem.
Phys. Lett. 531 (2012) 242–246.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.07.023.
[35] H. Xiao, C. Mei, N. Ding, T. Wei, Y. Zhang, B. Li, J. Photochem. Photobiol., A:
Chem. 273 (2014) 29–33.
[
36] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, Singapore,
006.
[37] U. Mazzucato, F. Momicchioli, Chem. Rev. 91 (1991) 1679–1719.
2
References
[
[
38] D.O. Cicero, G. Barbato, R. Bazzo, J. Am. Chem. Soc. 117 (1995) 1027–1033.
39] C.D. Blundell, M.J. Packer, A. Almond, Bioorg. Med. Chem. 21 (2013) 4976–
[
1] T. Deligeorgiev, A. Vasilev, S. Kaloyanova, J.J. Vaquero, Color. Technol. 126
2010) 55–80.
4987.
(
[
[
40] I. Kikaš, B. Carlotti, I. Škori c´ , M. Šindler-Kulyk, U. Mazzucato, A. Spalletti, J.
Photochem. Photobiol., A: Chem. 244 (2012) 38–46.
41] T. Clark, M. Hennemann, J.S. Murray, P. Politzer, J. Mol. Model. 13 (2007) 291–
[
[
[
2] C.-C. Lee, A.T. Hu, Dyes Pigm. 59 (2003) 63–69.
3] C. Qin, M. Zhou, W. Zhang, X. Wang, G. Chen, Dyes Pigm. 77 (2008) 678–685.
4] S. Shettigar, G. Umesh, P. Poornesh, K.B. Manjunatha, A.M. Asiri, Dyes Pigm. 83
296.
(
2009) 207–210.