Studies of 2-azaazulenium derivatives - 3: The nature of electron transitions and spectral properties of styryl dyes containing terminal groups of different types

Article abstract of DOI:10.1016/j.molstruc.2012.08.034

This paper presents the results of a quantum-chemical study of the molecular geometry and electron structure as well as spectral measurements of absorption and ¹³C NMR spectra of dimethylaminostyryls, methoxystyryls, and methylstyryls bearing 2

Full text of DOI:10.1016/j.molstruc.2012.08.034

Journal of Molecular Structure 1033 (2013) 215–226
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Studies of 2-azaazulenium derivatives – 3: The nature of electron transitions
and spectral properties of styryl dyes containing terminal groups of different types
c
b
a
Julia L. Bricks a, Alona V. Stanova b, , Aleksey B. Ryabitsky , Valeriy M. Yashchuk , Aleksey D. Kachkovsky
⇑
a Department of Colour and Structure of Organic Compounds, Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska str., 02660 Kyiv, Ukraine
b Physics Department, Taras Shevchenko National University of Kiev, 4 Glushkova Prosp., 03187 Kyiv, Ukraine
c Spoluka Chemical Company, 5 Murmanska str., 02660 Kyiv, Ukraine1
h i g h l i g h t s
"
"
"
Cyanine dyes explored by spectroscopic methods and quantum chemical calculations.
Classification of cyanine dyes is proposed basing on the research.
Features in the absorption spectra of AA dyes caused by the local molecular orbital.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2012
Received in revised form 19 August 2012
Accepted 20 August 2012
Available online 30 August 2012
This paper presents the results of a quantum-chemical study of the molecular geometry and electron
structure as well as spectral measurements of absorption and 13C NMR spectra of dimethylaminostyryls,
methoxystyryls, and methylstyryls bearing 2-azaazlenium (AA) moiety in comparison with analogous
dyes containing 2-benzimidazolim and 4-pyrylium residues. Based on the analysis of both calculations
and experimental data, it was concluded that these types of extremely unsymmetrical cyanines differ
between each other only slightly in the ground state with respect to the charge distribution and molec-
ular geometry while their spectral properties reveal a considerable difference between dimethylaminos-
tyryls and methoxystyryls due to the lowest disposition of the lone electron pair level of the oxygen atom
and hence, limiting basicity of the p-methoxyphenylene residue. It was shown that appearance of a high-
positioned local level generated by AA terminal group caused the inversion of the delocalized and quasi-
local electron transitions in AA-methoxystyryl and hence drastic changes in its absorption spectrum.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Absorption
Azaazulenes
Dyes/pigments
Quantum-chemical calculations
NMR spectroscopy
1. Introduction
monomethine and trimethine cyanine derivatives of 2-azaazulene
in our preceding papers [15,16]. There we have shown that 2-
Styryl dyes have numerous successful applications [1]. For
example, they can be used as fluorescence probes, laser active
and passive components, as optical sensitisers, as recording media
(optical disks) and potential objects for molecular electronics.
One of the most significant problems of the color theory of
polymethine dyes is investigation of the dependency of the elec-
azaazulenium (AA) residue can be treated as a low basic terminal
group. Its donor properties are provided by the participation not
of the highest occupied molecular orbital (HOMO) but of the next
HOMO-1, in contrast to the typical Brooker’s terminal residues
with their donor HOMOs. It enables us to propose a new classifica-
tion of terminal groups of cyanine dyes and hence a classification
of symmetrical and unsymmetrical cyanines. It was shown that
the nature of the highest electron transitions (delocalized or local)
in cyanine dyes depends on the relative dispositions of local and
delocalized molecular orbitals (MOs). In unsymmetrical trimethine
cyanines containing terminal groups of different types, unusual
spectral properties could occur, for example, the negative
deviation.
tronic transitions and
p-electron density distribution upon the
electron asymmetry degree of dye molecules [2–9]. It was found
both experimentally [2,5,6,10] and theoretically [3,4,11–14] that
going from symmetrical cyanine dyes to chemically unsymmetrical
cyanines affects considerably their spectral properties. We have re-
ported a study of properties of symmetrical and unsymmetrical
In our previous paper devoted to the properties of styryls and
their heteroanalogues [17], we have shown that p-methoxyphenyl-
ene residue in methoxystyryls should be considered as a special
type of the terminal group with an extremely low electron donor
⇑
Corresponding author. Tel.: +380 671949208.
E-mail addresses: jbricks@ioch.kiev.ua (J.L. Bricks), diakova_a@ukr.net
(A.V. Stanova).
1
www.lifechemicals.com.
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.08.034

216
J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
level, which remains under the highest occupied level of the chain.
It leads to a considerable hypsochromic shift of the first absorption
maxima of methoxystyryls relatively to styryls. The quantum-
chemical calculations have shown that the nature of the second
transition S0 ? S2 in methoxystyryls have been changed, in com-
parison with corresponding dimethylamino styryls.
At the same time, earlier it was reported that in symmetrical
and unsymmetrical polymethine dyes with AA terminal groups
specific electron transitions from local MOs take place [15,16].
The direct experimental observation of the local transitions in
absorption spectra was not often possible because they could be
hidden under the high-intensive band connected with the first
delocalized transition. For example, it is in contrast to the spectra
of pyrylocyanines and their heteroanalogues, when the local tran-
sitions are reveal themselves as separate low-intensive spectral
bands. It could be expected that the combination of AA terminal
group, with its local electron level, and a specific p-methoxypheny-
len group in methoxystyryl will result in appearance of local
transitions.
31G(d,p)/B3LYP method with (TD DFT in text) and semi-empirical
AM1 method.
2.2. Synthesis
A series of styryl dyes reported here was synthesized by con-
ventional cyanine chemistry approach [6,24]. This approach in-
volves heating a mixture of p-R-substituted benzaldehydes and
the corresponding quaternary salts of nitrogen containing hetero-
cycles or 4-methyl-2,6-di(tert-Bu/or Ph)-pyrylium salts in an
appropriate solvent. Structures of all synthesized dyes see in
Table 1.
The purity of the dyes have been checked by TLC and their
structures were confirmed by NMR (1H NMR and 13C NMR) and
elemental analysis.
Starting materials and solvents for the synthesis and spectro-
scopic measurements were purchased from Sigma–Aldrich.
AA-Styryls (General procedure for AA dyes): A solution of 1 mmol
of 2-butyl-1,3,6-trimethyl-cyclohepta[c]pyrrolium tetrafluorobo-
rate and 1.5 mmol of p-dimethylamino benzaldehyde, or p-anisal-
dehyde, or p-methylbenzaldehyde in dimethylsulfoxide (2 mL)
was heated at 50 °C for 15–20 min. The solvent was evaporated
in vacuum. The residue was triturated with diethyl ether
(5 ꢀ 5 mL) and dissolved in ethanol (2 mL). After cooling to room
temperature the precipitated dye was filtered off, washed with
diethyl ether (2 ꢀ 10 mL) and purified by recrystallization from
ethanol.
This paper presents the results of a quantum-chemical study of
the electron structure and electron transitions features as well as
spectral measurements of absorption and NMR spectra of dimeth-
ylaminostyryls (DMA-styryls), methoxystyryls (OMe-styryls), and
methylstyryls (Me-styryls) bearing 2-azaazlenium moiety in com-
parison with analogous dyes containing 2-benzimidazolim and 4-
pyrylium residues.
2-Butyl-6-{2-[4-(dimethylamino)phenyl]vinyl}-1,3-dimethyl cy-
clohepta[c]pyrrolium tetrafluoroborate AA-DMA was obtained
from 2-butyl-1,3-trimethylcyclohepta[c] pyrrolium tetrafluoro-
borate and p-dimethylaminobenzaldehyde [25].
2. Experimental section
2.1. Measurements and quantum-chemical calculations
2-Butyl-6-[2-(4-methoxyphenyl)vinyl]-1,3-dimethylcyclohepta [c]
pyrrolium tetrafluoroborate AA-OMe was obtained from 2-butyl-
1,3-trimethylcyclohepta[c] pyrrolium tetrafluoroborate and p-
anisaldehyde [25].
UV/Vis absorption spectra were recorded on Shimadzu UV-3100
spectrophotometer.
All NMR measurements were carried out on Varian GEMINI 2000
spectrometer with 1H and 13C frequencies of 400.07 and
100.61 MHz, respectively at 293 K. Tetramethylsilane was used as
a standard for d scale calibrating. 1H NMR spectra were recorded
with spectral width 8000 Hz and numbers of points 32000; 13C
NMR spectra were recorded with spectral width 30,000 Hz and
numbers of points 128,000. 1HA1H COSY [18] spectra were ac-
quired into 2048 (F2) and 512 (F1) time-domain data matrix and
2048 (F2) ꢀ 2048 (F1) frequency-domain matrix after zero-filling.
NOESY [19] spectra were acquired, if necessary, with parameters
similar to COSY spectra. Mixing times were determined prelimin-
ary from T1-measurement experiment for each sample by conven-
tional inversion-recovery method. Heteronuclear chemical shift
correlation (HETCOR) [20] was used to determine 1HA13C attach-
ment with 2048 (F2) ꢀ 256 (F1) time-domain matrix and 2048
(F2) ꢀ 1024 (F1) frequency-domain matrix after zero-filling. The
average value of one bond constant JCH was set to 140 Hz. HETCOR
for determination long range correlation had very similar parame-
ters and average value of multibond CAH coupling constant was
set to 8 Hz.
The quantum-chemical calculations were carried out to study
the dependence of the electron structure and electron transitions
on molecular constitution. The equilibrium geometry of dye mole-
cules in the ground state was optimized using GAUSSIAN03 [21]
program set with DFT approximation. A functional B3LYP [22]
was chosen, in combination with the standard 6-31G(d,p) basis
set. In case of need for some structures the conformational analysis
was carried out using scanning of the potential energy surface to
search the optimal conformation with the energy minimum.
Charge distribution in atoms was calculated using the theory of
Natural Bond Orbital analysis (NBO) [23]. The electron transition
characteristics were calculated using two methods: TD DFT/6-
2-Butyl-6-[2-(4-methylphenyl)vinyl]-1,3-dimethylcyclohepta
[c]pyrrolium tetrafluoroborate AA-Me was obtained from 2-bu-
tyl-1,3-trimethylcyclohepta[c] pyrrolium tetrafluoroborate
and p-methylbenzaldehyde was obtained from 2-butyl-
1,3-trimethylcyclohepta[c] pyrrolium tetrafluoroborate and
p-methylbenzaldehyde. Yield 34%, m.p. 159–162 °C (decom-
position); kmax = 464 nm,
e
= 17100 Mꢁ1 cmꢁ1 (in acetonitrile).
1H NMR (CDCl3, 400 MHz) d: 8.55 (d, J = 10.3 Hz, 2H, AAz), 7.65
(d, J = 16.5 Hz, 1H. ACH@CHA), 7.59 (d, J = 10.3 Hz, 2H, AAz),
7.51 (d, J = 8.0 Hz, 1H, C6H4), 7.22 (d, J = 8.0 Hz, C6H4), 7.07
(d, J = 16.5 Hz, ACH@CHA), 4.25–4.18 (m, 2H, NCH2), 2.75 (s,
6H, 2 ꢀ CH3 AAz), 2.40 (s, 3H, C6H4CH3), 1.73–1.63 (m, 2H,
NCH2CH2), 1.49–1.36 (m, 2H, CH3CH2), 0.89 (t, J = 7.3 Hz, 3H,
CH3CH2). Elemental analysis calcd. (%) for C24H28BF4N
(417.29): C 69.08, H 6.76, N 3.36%; found: C 69.32, H 6.44, N
3.58%.
Preparation of Py-tBu-styryl dyes. A solution of 2,6-di-tert-butyl-
4-methylpyrylium perchlorate (1 mmol) and p-dimethylamino-
benzaldehyde, or p-anisaldehyde, or p-methylbenzaldehyde
(1.1 mmol) in acetanhydride (2 mL) was heated for 1 h at 130 °C.
After cooling, the reaction mixture was triturated with diethyl
ether (50 mL) and the obtained solid was filtered and crystallized
from the mixture of ethanol–diethyl ether (1:2).
2,6-di-tert-Butyl-4-{-2-[4-(dimethylamino)phenyl]vinyl}pyryli-
um perchlorate, Py-tBu-DMA was described in [25].
2,6-di-tert-Butyl-4-[2-(4-methoxyphenyl)vinyl]pyrylium per-
chlorate Py-tBu-OMe was described in [25].
2,6-di-tert-Butyl-4-[2-(4-methylphenyl)vinyl]pyrylium
rate Py-tBu-Me. Yield 45%, m.p. 198–200 °C (decomposition);
kmax = 411 nm,
= 32300 Mꢁ1 cmꢁ1 (in acetonitrile). 1H NMR
(CDCl3, 400 MHz) d: 1H NMR (CDCl3, 400 MHz) d: 8.31 (d,
perchlo-
e

J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
217
Table 1
Spectral properties of styryl dyes.
Het+ (Xꢁ)
R
Dye abbreviation
Absorption spectra, kmax (nm)
(
e
ꢀ 10ꢁ4, (Mꢁ1 cmꢁ1)) in CH3CN
N(CH3)2
OCH3
CH3
AA-DMA
AA-OMe
AA-Me
681 (4.82), 334 (1.56)
656 (0.27), 497 (2.39), 304 (1.68)
660 (0.26), 464 (1.71), 338 (1.19), 295 (1.71)
n-Bu
N
(BF4)
N(CH3)2
OCH3
CH3
Py-tBu-DMA
Py-tBu-OMe
Py-tBu-Me
577 (7.87), 299 (1.26)
446 (3.78), 307 (0.90)
411 (3.23), 309 (1.11)
O
N(CH3)2
OCH3
Py-Ph-DMA
Py-Ph-OMe
628 (9.46), 386 (2.18)
481 (5.9), 415 (3.08), 271 (2.93)
(BF4)
Ph
O
Ph
N(CH3)2
OCH3
CH3
BIm-DMA
BIm-OMe
BIm-Me
397 (1.69), 281 (1.45)
343 (2.74)
329 (3.23)
N
N
(I)
J = 14.8 Hz, 1H, ACH@CHA), 7.90 (s, 2H, Py), 7.78 (d, J = 8.0 Hz, 2H,
C6H4), 7.44 (d, J = 14.8 Hz, 2H, ACH@CHA), 7.20 (d, J = 8.0 Hz, 2H,
C6H4), 2.37 (s, 3H, CH3C6H4), 1.49 (s, 18H, 2 ꢀ t-Bu). Elemental
analysis calcd. (%) for C22H29BF4O (396.27): C 66.68, H 7.38%;
found: C 66.42, H 7.30%.
4-{2-[4-(dimethylamino)phenyl]vinyl]-2,6-diphenylpyrylium
perchlorate Py-Ph-DMA was described in [26].
4-[2-(4-methoxyphenyl)vinyl]-2,6-diphenylpryrlium perchlo-
rate Py-Ph-OMe was described in [27].
2 -{2-[4-(dimethylamino)phenyl]vinyl}-1,3-dimethyl-1H-3,1-ben-
zimidazol-3-ium iodide BIm-DMA. A mixture of 1,2,3-trimethyl-1
H-3,1-benzimidazol-3-ium iodide (0.58 g, 2 mmol), p-dimethyl-
aminobenzaldehyde (0.4 g, 2.68 mmol), and DBU (five drops) was
heated in pyridine (7 mL) at 110 °C for 3 h. After cooling the orange
precipitate was filtered off, washed with pyridine (2 mL), hot water
(10 mL), methanol (5 mL), diethyl ether (5 mL), dried and purified
by recrystallization from DMSO. Yield 0.52 g (62%); m.p. 297–
300 °C (decomposition); 1H NMR (400 MHz, [D6]DMSO, 25 °C,
TMS): d = 8.00–7.94 (m, 2H, BIm), 7.78 (d, J = 8.9 Hz, 2H, C6H4),
7.74 (d, J = 16.5 Hz, 1H, ACH@CHA), 7.66–7.60 (m, 2H, BIm), 7.19
(d, J = 16.5 Hz, 1H, ACH@CHA), 6.78 (d, J = 8.9 Hz, 2H, C6H4), 4.11
(s, 6H, 2 ꢀ CH3 BIm), 3.05 (s, 6H, N(CH3)2). Elemental analysis
calcd. (%) for C19H22IN3 (419.3): I 30.27, N 10.02; found: I 30.05,
N 10.24.
ACH@CHA), 7.70–7.65 (m, 2H, BIm), 7.41 (d, J = 16.7 Hz, 1H,
ACH@CHA), 7.11 (d, J = 8.8 Hz, 2H, C6H4), 4.14 (s, 6H, 2 ꢀ CH3
BIm), 3.85 (s, 3H, OCH3). Elemental analysis calcd. (%) for C17H16IN2-
O: I 32.44, N 7.16; found: I 32.27, N 7.12.
2 -[2-(4-methylphenyl)vinyl]-1,3-dimethyl-1H-3,1-benzimidazol-
3-ium iodide BIm-Me was obtained in a similar way from 1,2,3-tri-
methyl-1H-3,1-benzimidazol-3-ium iodide (0.58 g, 2 mmol) and
p-methylbenzaldehyde (0.6 g, 5 mmol). Purification was by recrystal-
lization from DMSO (twice). Yield 0.370 g (47%); m.p. 298–299.5 °C
(decomposition); 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS):
d = 8.03–7.98 (m, 2H, BIm), 7.74–7.67 (m, 3H, BIm ACH@CHA),
7.17 (d, J = 8.2 Hz, 2H, C6H4), 6.77 (d, J = 12.7 Hz, 1H, ACH@CHA),
3.73 (s, 6H, 2 ꢀ CH3 BIm), 2.29 (s, 3H, CH3). Elemental analysis calcd.
(%) for C18H19IN2: I 32.52, N 7.18; found: I 32.32, N 7.25.
3. Results and discussion
3.1. Objects and their spectral properties
The spectral properties of studied dyes are presented in Table 1.
Scheme 1 below demonstrates a general structure of investigated
dyes.
3.2. Optimized molecular geometry in the ground state
2-[2-(4-methoxyphenyl)vinyl]-1,3-dimethyl-1H-3,1-benzimidazol
-3-ium iodide BIm-OMe. A mixture of 1,2,3-trimethyl-1H-3,1-ben-
zimidazol-3-ium iodide (0.58 g, 2 mmol), p-anisaldehyde (0.544 g,
4 mmol) and piperidine (0.1 g) was heated in pyridine (7 mL) at
130 °C for 8 h. Then p-anisaldehyde (0.27 g, 2 mmol) was added,
and the heating was continued for 6 h. After cooling the precipitate
was filtered off, washed with hot water (10 mL), ethanol (5 mL),
diethyl ether (2 ꢀ 5 mL), dried and purified by recrystallization from
DMSO (twice). Yield 0.450 g (55%); m.p. 272–275 °C (decomposi-
tion); 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): d = 8.07–8.01
(m, 2H, BIm), 7.93 (d, J = 8.8 Hz, 2H, C6H4), 7.81 (d, J = 16.7 Hz, 1H,
The quantum-chemical calculations have shown that almost all
molecules are practically planar, which is typical for conjugated
Scheme 1.

218
J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
systems. Only both phenyl substituents in the Py-Ph-dyes lie out of
the main plane of the pyrylium heterocycle through approximately
25° (ab initio or Hartree–Fock method (HF)), whereas the DFT
method gives essentially minor value of the torsion angle 10.5°.
BIm residues in the corresponding dyes also turned out the main
chromophore because of considerable sterical hindrances: through
40° in the case of BIm-DMA dye and through 44° in the case of dye
BIm-OMe.
In contrast to symmetrical AA-cyanines with their practically
equalized lengths of carbonAcarbon bonds in the open chain
[4,5,11] a regular alternation of lengths of the neighboring bonds
in the chromophore of unsymmetrical AA-DMA and AA-OMe is
observed. As an illustration, the bond lengths values in AA dye
molecules are presented in Fig. 1. Bond lengths numeration see
in Scheme 2. One can see that not only the bond lengths in the
open chain alternated, but the bond lengths in the benzene ring
of both dyes also demonstrate the appreciable alternation.
To analyze the dependence of the bond length alternation (BLA)
on the topology of the terminal groups, we will use an amplitude
Dlv calculated by the following equation [28]:
Scheme 2.
3.3. Charge distribution and 13C NMR shifts
In our previous papers we have established that it is observed a
considerable cyanine-like alternation of the charges at the neigh-
boring carbon atoms along the chromophore for symmetrical and
unsymmetrical derivatives of AA cyanines, what was confirmed
experimentally by means of NMR spectroscopy. So, 13C chemical
shifts (d ) for the neighboring carbon atoms are dislocated in the
opposite fields, what agrees with the calculated electron density
l
values (q ) at the atoms.
l
The same alternations of both atomic charges (q ) and the cor-
l
responding NMR 13C signals (d ) are obtained for styryl dyes AA-,
l
Py-tBu-, BIm-. The measured chemical shifts of investigated dyes
D
l
¼ ðl ꢁ l þ1Þ
ð1Þ
v
v
v
are collected in Table 2, while the alternation of the values
Dq
l
and calculated by formulae (2) [29], and (3) for the open chain
and benzene ring (as a branched chromophore) are pictured in
Figs. 3 and 4.
D
d
l
where lv is the length of the vth bond.
The plot of values |Dlv| versus the numbers of the bond pair
along the chromophore for synthesized dye series are pictured in
Fig. 2. First of all, one can see that the amplitude of the BLA (Dlv)
in the open chain of methoxystyryls-OMe exceeds this parameter
l
D
q
¼ ðꢁ1Þ ðq ꢁ q þ1Þ
ð2Þ
l
l
l
in the branched part (benzene ring); whereas the values Dlv for
l
D
@
¼ ðꢁ1Þ ð@ ꢁ @ þ1Þ
ð3Þ
l
l
l
the benzene ring and the open chain of styryls-DMA are compara-
ble, especially in the dyes with the low basic variable terminal
group R. It has to be noted that the method DFT taking into consid-
eration the electron correlation gives the longer double (formally)
bond and hence the less degree of the BLA (Fig. 2).
A noticeable diminishing of 13C shifts alternation for atoms C1
and C2 in AA-dyes is observed due to a considerable remoteness
of these atoms from the heteroatom of the AA heterocycle, unlike
this one in the case of Py-tBu- and BIm-dyes.
The analysis of the results of quantum-chemical calculations of
all dyes has shown that parameter Dlv for the bond lengths in the
open chain is more sensitive (in comparison with the bonds in the
benzene ring) to the donor strength of the variable residue R. For
The calculations gave practically the same magnitudes of
parameter
Dq , for the pairs of carbon atoms in the benzene ring
l
for the series of the studied dyes. Only the degree of the electron
densities alternation for the first pair (DFT, point 1 in Fig. 3a and
c) of the atoms (starting from the nearest one to the heteroatom)
is higher for the OMe-styryls than for the DMA-styryls.
example, the DFT method gives the following order of value D v
l
for the bond pair 2 (Fig. 2a,
styryls:
c = 2) in the open chain of the DMA-
The maximum differences in the alternation degree
Dq for the
l
same pair of atoms in both series of the dyes (DMA- and OMe-styr-
yls) are obtained for the open chain, especially nearly to the vari-
able terminal residues Het (C1). One can see from Fig. 3 that the
increasing of the donor properties (or basicity) of the terminal
group in the series Py-t-Bu < AA < BIm (independently on the nat-
ure of the second end group: N(CH3)2 in DMA-styryls or OCH3 in
OMe-styryl), is accompanied by a regular decreasing of values
BIm > AA > Py-t-Bu ꢂ Py-Ph
It agrees with the conclusion about the low basicity of the AA
terminal group drawn in our previous papers [15,16]. It has to be
noted that similar regularities for OMe-styryls and Me-styryls are
not clearly pronounced. The data obtained by ab initio (HF) method
have also proved the fact of lower basicity of the AA, in comparison
with the high basicity of the BIm residue.
D
q , as it is evident from the comparison of the first and second
l
Fig. 1. Bond length values for the dye AA-molecules, calculated using (a) Density-Functional Theory (DFT) method and (b) Hartree–Fock (HF) method;
v is the number of the
bond.

J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
219
Fig. 2. Alternation of the calculated bond length for studied dyes, (a–e) – DFT method; (b–f) – HF method;
c is the number of the bond pair (Scheme 1 above shows carbon
atom numeration for the part of the molecule that used here for calculations).
Table 2
Experimental chemical shifts 13C d of investigated styryl dyes recorded in [D6]DMSO.
l
Carbon atom numbera
AA-DMA
AA-OMe
AA-Me
Py-tBu-DMA
Py-tBu-OMe
Py-tBu-Me
BIm-DMA
BIm-OMe
BIm-Me
1
2
3
4
5
6
7
162.76
129.19
143.38
128.30
131.05
114.76
161.97
162.80
125.72
146.57
123.56
132.59
112.40
153.26
162.01
129.45
142.86
132.06
128.57
129.48
141.73
160.75
116.24
152.90
123.14
134.71
112.32
154.48
164.48
120.35
151.95
127.18
133.09
114.62
163.66
165.16
122.01
151.73
131.62
130.52
129.81
143.94
152.17
100.49
147.12
121.61
130.49
111.52
149.00
148.49
105.02
146.17
126.96
130.46
114.46
161.64
148.61
108.16
145.88
131.29
128.21
129.78
140.22
a
The numbering of carbon atoms see Scheme 1.
points in Fig. 3. On the contrary, the alternation of the bond lengths
increases for the same points in the dye series considered
localized practically in the open PC. Then, cyanines with two donor
terminal groups can be presented as the following system:
Dlv
above (Fig. 2).
D1A½
p
ꢃþAD2 or D1A½AꢃþAD2
It is reasonable that a similar trend should be observed for the
alternation of the chemical shifts,
Dd . Indeed, the alternation of
l
where the chain could be treated as an acceptor A. Scheme 3 makes
clear this generalization on example of concrete dye – Bim-Me.
It is to be noted that the donor levels, generated predominantly
by the lone electron pairs (LEPs) of nitrogen atoms, are positioned
higher than the highest delocalized orbital of the main chromo-
phore in cationic cyanine dyes with short PC. Then, we have
proposed to consider two highest occupied MOs as donor orbitals
the chemical shifts is seen from Fig. 4 to be appreciable for the
two first pairs of atoms in the chromophore, while differences in
the magnitudes
Dd for the rest atom pairs of the conjugated p-
l
system are less (compare Figs. 3 and 4).
Thus, the increasing of the asymmetry degree upon going to
dyes with more basic variable terminal group leads to increasing
of BLA degree and to decreasing of the charge alternation, mainly,
in the open chain. On the other hand, the difference in the electron
densities and molecular geometry between DMA-styryls and corre-
sponding OMe-styryls is far lower.
(which, of course, are conjugated with delocalized
p-electron
system of the PC [30]. The donor MOs, /(D1) and /(D2), interact be-
tween each other to give two new splitting MOs: symmetrical (4)
and unsymmetrical (5) linear combinations:
u1 ¼ ð2Þꢁ1=2f/ðD1Þ þ /ðD2Þg
ð4Þ
3.4. Relative positions of donor and delocalized levels
Earlier it was established [30] that terminal groups can make
the main contributions to the highest occupied MOs of cyanine
dyes with a comparatively short polymethine chain (PC). It was
shown that the positive charge in the cationic polymethine dye is
u2 ¼ ð2Þꢁ1=2f/ðD1Þ ꢁ /ðD2Þg
Fig. 5 shows schematically the splitting of the donor levels in
symmetrical (Sym) dyes (D1@D2„D).
ð5Þ

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J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
Fig. 3. Alternation of atomic charges
D
q
for investigated dyes; (a–e) – calculated by DFT method; (b–f) – calculated by HF method. l is the number of the atoms pair (carbon
l
atom numeration see Scheme 1).
Fig. 4. Alternation of experimental 13C NMR shifts
Dd (DMSO-d6) for styryl dyes AA-, Py-tBu- and BIm-: (a) DMA-styryls; (b) OMe-styryls; (c) Me-styryls (carbon atom
l
numeration see Scheme 1).
(2) The original donor levels are positioned under the highest
occupied level of the PC, nevertheless one of the splitting
donor levels proves to be the highest level of the dye, simi-
larly to Brooker’s dyes. This dye type can be named as pseudo-
Brooker dyes (see Fig. 5b).
(3) The original donor levels are positioned too low, so the
donor splitting level cannot exceed the highest occupied
level of the PC; consequently, the HOMO of this dye type is
the delocalized MO of the PC, similarly to the unsubstituted
polymethine cation. We propose to name such dyes as non-
Brooker’s dyes (see Fig. 5c).
Scheme 3.
There are three main types of relative positions of splitting do-
nor levels and the higher occupied level of the PC:
It has to be noted additionally that the terminal groups in
pseudo-Brooker’s and non-Brooker’s dyes can contain local MOs
positioned higher than the donor orbital of the terminal residues.
The first type (Fig. 5a) can be presented by a typical Sym cyanine
where D = BIm. Sym azaazulenecyanine (D = AA) can be considered
(1) The original donor levels are positioned higher than the level
of the PC. The higher occupied level of the dye is the splitting
donor levels,
u1. We propose to name the symmetrical dyes
with such a disposition of the splitting donor levels as Brook-
er’s dyes (see Fig. 5a).

J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
221
Fig. 5. Splitting of donor levels in symmetrical cationic polymethine dyes: (a) Brooker’s type; (b) pseudo-Brooker type; (c) non-Brooker-type.
as an example of the second type of cationic polymethine dyes
(Fig. 5b). The detailed investigation presented in our previous pa-
pers has shown that the HOMO of the terminal AA residue is a local
orbital and therefore does not take part in the conjugation with the
PC. Instead of it, HOMO-1 with non-zero coefficient at the carbon
atom bonded with the chain is a donor orbital. The interaction of
such MOs of both terminal groups in Sym dye (D = AA) results in
HOMO-1 in the Py-tBu-OMe with the low basic pyrylium residue
is the local MO, as one can see from Fig. 7d, whereas the
HOMO-1 in the dye Py-tBu-DMA (Fig. 7c) could be treated (in the
framework of our approach) as the second splitting donor orbital,
similarly to BIm-DMA.
The scheme of MOs (and hence electron transitions) arrange-
ment in unsymmetrical dyes, especially, in OMe-styryls, becomes
considerably more complicated when the variable terminal group
R has the local level. So, introducing of phenyl substituents instead
of tert-butyl-groups in pyrylium heterocycle (upon going from
dyes Py-tBu-DMA and Py-tBu-OMe to the corresponding Py-Ph-
DMA and Py-Ph-OMe) is accompanied by an appearance of a spe-
cific MO located only at the atoms of the terminal group, as one
can see comparing Fig. 7c and d with Fig. 7e and f.
the HOMO, which is one of the splitted donor MOs u1
.
Finally, the third type of the relative disposition of donor and
delocalized MOs is realized in Sym dye (D = OCH3) with the lone
electron pair (LEP) of oxygen atoms providing the donor MOs: both
splitting donor levels remain lower than the highest occupied level
of the chain.
In unsymmetrical cyanines (D1 – D2), the energies of both do-
nor levels are different. Taking into consideration that the interac-
tion of levels is in inverse proportion to the distance between
them, the splitting energy should decrease in unsymmetrical
polymethine dyes. Schematically, the dependence of splitting ef-
fect on the different distances in two types of unsymmetrical
polymethine dyes, DMA-styryl and OMe-styryl, is illustrated in
Fig. 6. In the DMA-styryl model, both terminal residues contain
nitrogen atoms with their high donor levels hence the distance be-
tween both original donor levels is not too large, as one can see
from Fig. 6a. Then we would expect that an interaction between
them is comparatively high, similarly to symmetrical cyanines.
The situation reverses principally in the OMe-styryls model; the
corresponding quasi-donor level (generated by the LEP of the oxy-
gen atom) should be disposed rather lower. Fig. 6b demonstrates
that the energy of the splitting in such unsymmetrical dye should
be practically negligible, in comparison with symmetrical cyanines
or even with unsymmetrical dyes, containing both nitrogenous ter-
minal residues (Fig. 6a). Thus, the nature of the HOMO-1 (and
hence all electron transition involving this orbital) should be
essentially different in DMA-styryls and related OMe-styryls.
Certainly, mentioned above scheme is strongly simplified. The
initial donor MOs also interacts with the delocalized MOs of the
main chromophore.
The HOMOs in DMA-styryls, with both nitrogen containing res-
idues, are shifted upwards in comparison with the corresponding
orbital in OMe-styryls, containing only one heterocyclic nitroge-
nous terminal group (Fig. 7). At the same time, the position of
the lowest vacant level is seen to be less sensitive to the replace-
ment of the dimethylamino group by a methoxy group. Conse-
quently, we can conclude that the methoxyphenylene residue
does not produce the high positioned highest occupied level, in
contrast to the corresponding nitrogen-containing a DMA terminal
group in DMA-styryls. Additionally, it should be noted that the
This local orbital is the HOMO-1 in both DMA-styryl and OMe-
styryl. The next HOMO-2 in DMA-styryl is totally delocalized along
the whole chromophore, similarly to the LUMO. The vacant local
orbitals obtained from calculation are situated higher and do not
take part in the lowest electron transitions.
Similarly, the calculations give the local HOMO-1 in the dyes
bearing azaazulenium residue AA-DMA and AA-OMe, as it is dem-
onstrated in Fig. 7g and h.
3.5. A general scheme of electron transitions in styryls of different
types: the principal distinctions
The absorption bands observed in visible and near IR spectra of
the typical Brooker’s symmetrical and unsymmetrical cyanine dyes
are known to correspond to the electron transitions involved some
highest occupied MOs and some vacant orbitals [5,14–16]. The
lowest electron transition is connected practically only with the
highest occupied MO and the lowest unoccupied orbital, so that
|S1> ꢂ |HOMO ? LUMO>. Regarding to the next state, |S2>, its nat-
ure depends most commonly on the topology of the terminal
groups.
It was shown above (see Fig. 5) that donor terminal groups with
their donor MOs generate two highest occupied orbitals. As a rule,
in the cationic polymethine dyes, the distance between two lowest
vacant levels exceeds considerably the distance between splitted
donor levels or even the distance between the highest occupied le-
vel and the fifth level below [30]. Then, the first two the lowest
transitions involved both |HOMO> and |HOMO-1> in Brooker’s
type dyes (Fig. 5a) could be treated as coupling transitions. It is
obvious that the magnitude of donor levels splitting should be
maximal in the symmetrical dyes, because of the degeneration of
donor levels. Increasing of the distance between the donor levels
in unsymmetrical dyes should be accompanied by decreasing of

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J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
Fig. 6. Schematic disposition of delocalized and donor (D1 and D2) levels as well as two first electron transitions in DMA-styryl (a) and OMe-styryl (b).
Fig. 7. Shapes of the frontier and nearest MOs and the two lowest electron transitions in dyes BIm-DMA (a), BIm-OMe (b), Py-tBu-DMA (c), Py-tBu-OMe (d), Py-Ph-DMA (e),
Py-Ph-OMe (f), AA-DMA (g), and AA-OMe (h). AM1 method.
the splitting energy. Then, one could expect two main features that
distinguish DMA-styryls from OMe-styryls:
(2) The strong interaction between two donor levels and those
owing to their splitting in the DMA-styryls results in the
appreciable decreasing of the first electron transition energy
and hence in the comparatively long wave (deep) color of
these dyes. The situation is different for methoxy substituted
analogous, where the first electron transition depends
practically on the one donor quasi-local level position with-
out essential additional decreasing of the transition energy
by the interaction with another donor level generated by
the LEP of the oxygen atom (Fig. 6a and b).
(1) The two lowest electron transitions S0 ? S1 and S0 ? S2 in
DMA-styryls as dyes with highly positioned donor levels of
both terminal groups, are coupled transitions involved split-
ted donor levels and the same lowest vacant level (Fig. 5a),
similarly to the symmetrical cyanine. Whereas S0 ? S2 tran-
sition in OMe-styryls (Fig. 5b) should be connected with the
delocalized MO of the chain.

J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
223
Fig. 8. Normalized absorption spectra of investigated dyes and symmetrical dyes Sym (D1@D2„D) recorded in acetonitrile: (a) AA-derivatives; (c) BIm derivatives; (d) Py-tBu
derivatives; (e) Py-Ph derivatives. (b) Shows AA-OMe dye in different solvent, acetonitrile (CH3CN) and methylene chloride (CH2Cl2).
(Fig. 8e) causes not only the bathochromic shift (82 nm) of the first
spectral band, but also the appearance of the comparatively low
intensive absorption band with the maximum at 386 nm, which
corresponds to the practically degenerated electron transitions,
S0 ? S2 and S0 ? S3 (Table 3), involving the local MOs HOMO-1
(their shapes are the same in DMA-styryls and OMe-styryl Py-
Ph-DMA, Py-Ph-OMe, see Fig. 7e and f). Similarly, it was estab-
lished earlier, [15] that the short wavelength absorption in the
spectra of dye Sym, D = AA is connected with the local transitions.
Aiming to compare DMA-styryls and OMe-styryls we discuss
their spectra and two lowest electron transitions, taking into con-
sideration the nature of the variable terminal groups.
Of course, this scheme is considerably simplified, however, such
theoretical assumption helps to interpret the spectral data
correctly. On the other hand, the above mentioned scheme
(Fig. 5) becomes complicated, when the terminal groups have a
high positioned local MO. As a result, the nature of the lowest tran-
sitions and hence the shape of the spectral bands could undergo
the considerable transformations.
3.6. Absorption spectra and a nature of electron transitions
One can see from Fig. 8 that the typical high intensive and high
selective long wavelength absorption bands are observed in the
spectra of all symmetrical dyes Sym (D1@D2„D). The band maxi-
mum is regularly red shifted in series Sym: D = AA (Fig. 8a),
D = BIm (Fig. 8c), D = Py-tBu (Fig. 8d), D = Py-Ph (Fig. 8e), in respect
to increasing of the effective length of the terminal residue. Intro-
ducing two phenyl substituents in the pyrylium heterocycle
3.6.1. Dyes derivatives of cyclohepta[c]pyrrolium (2-azaazulenium
dyes)
The particular emphasis of our investigation of the influence of
the local level on the absorption spectra was devoted to the pair of

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J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
Table 3
Wavelengths (k) and oscillator strengths (f) of the lowest electron transitions of studied dyes calculated by TD DFT and AM1 methods.
Dye
Transition
TD DFT
AM1
k (nm)
f
Main configuration
k (nm)
f
Main configuration
BIm-DMA
S0 ? S1
S0 ? S2
467
355
1.160
0.060
0.79 |H ? L>
0.81|H-1 ? L>
434
268
1.328
0.028
0.91 |H ? L>
0.88 |H-1 ? L>
BIm-OMe
S0 ? S1
S0 ? S2
431
359
0.990
0.016
0.81 |H ? L>
0.95 |H-1 ? L>
355
268
0.870
0.102
0.95 |H ? L>
0.91 |H-1 ? L>
Py-t-Bu-DMA
Py-t-Bu-OMe
Py-Ph-DMA
S0 ? S1
S0 ? S2
489
432
1.160
0.003
0.92 |H ? L>
0.95 |H-1 ? L>
494
317
1.578
0.074
0.92 |H ? L>
0.70 |H-2 ? L>
S0 ? S1
S0 ? S2
464
416
0.990
0.021
0.76 |H ? L>
0.94 |H-1 ? L>
455
309
1.315
0.074
0.92 |H ? L>
0.65 |H-1 ? L>
S0 ? S1
S0 ? S2
S0 ? S3
556
485
458
1.200
0.010
0.210
0.76 |H ? L>
0.89 |H ? L + 1>
0.82 |H-1 ? L>
548
381
300
1.557
0.586
0.015
0.87 |H ? L>
0.88 |H-1 ? L>
0.78 |H ? L + 1>
Py-Ph-OMe
AA-DMA
AA-OMe
S0 ? S1
S0 ? S2
S0 ? S3
525
476
459
1.050
0.210
0.005
0.80 |H ? L>
0.89 |H-1 ? L>
0.82 |H-3 ? L>
478
398
336
1.309
0.520
0.002
0.86 |H ? L>
0.89 |H-1 ? L>
0.68 |H-2 ? L>
S0 ? S1
S0 ? S2
S0 ? S3
627
552
519
0.011
1.310
0.001
0.93 |H-1 ? L>
0.74 |H ? L>
0.96 |H-2 ? L>
555
534
366
1.487
0.083
0.176
0.85 |H ? L>
0.91 |H-1 ? L>
0.79 |H ? L + 1>
S0 ? S1
S0 ? S2
S0 ? S3
636
524
432
0.012
1.150
0.013
0.93 |H-1 ? L>
0.74 |H ? L>
0.89 |H-2 ? L>
557
496
369
0.059
1.282
0.292
0.93 |H-1 ? L>
0.88 |H ? L>
0.82 |H-2 ? L>
the related dyes AA-DMA and AA-OMe (Fig. 8a). Taking into con-
sideration that electron transition, which involves occupied local
orbital corresponds to the lowest transition in the origin salt of
the variable heterocycle, and remembering that the azaazulenium
salt absorbs rather deeply (634 nm [15]), we have expected the
most unusual phenomena in the spectrum of DMA-styryl and,
especially, methoxystyryl dye, which are derivatives of the non-
alternant heterocycle ? azaazulene. Although the local level is
seen from Fig. 7g and h to be not the highest occupied level, nev-
ertheless the energy of the local transition can be lower than the
energy of the transition between the delocalized frontier levels,
as it is obtained by the calculations (Table 3). This is connected
with the different contribution of the integrals of the electron
interaction, Kij and Jij, in the formulas for the electron transition be-
tween any i- and j-MOs (6) for the delocalized and local transitions.
(1) the delocalized transition manifesting itself as an intensive
cyanine-like spectral band;
(2) the local transition, which exhibit the comparatively low
and wide band.
To confirm our theoretical assumption, we have synthesized not
only AA-OMe, but also the corresponding dye AA-Me without any
donor group with the LEP. The measured absorption spectra of
both dyes are seen from Fig. 8a to support our hypothesis. One
can clearly see that going from AA-DMA to AA-OMe is accompa-
nied by the drastic transformation of the absorption spectrum:
the short wavelength band becomes the typical cyanine-like band
with its maximum at 497 nm, whereas the long wavelength band
is wide and low intense, its shape being similar to the first band
in the spectrum of azaazulenium salt. Also, in order to show that
the shape of the absorption spectrum of the AA-OMe dye is the
consequence of the peculiarities of electronic structure but not
the influence of the solvent [31], we used two different solvent ace-
tonitrile (CH3CN) and methylene chloride (CH2Cl2). As it seen from
Fig. 8b, absorption spectra forms are similar and show the same
tendency, the first electron transition is not intensive, only some
shift of the maximum of the next intensive band is present.
Thus, basing on the general assumption about the relative dis-
positions of the electron transition of different nature in dyes with
the high positioned local level as well as on the calculated data, ob-
tained in both quantum-chemical methods, we conclude that the
long wavelength spectral band with its maximum approximately
at 680 nm corresponds to electron transition involving the local
orbital (HOMO-1), while the second narrow and high intense band
with its maximum at 497 nm is generated by the transition
between the totally delocalized frontier MOs. As an additional con-
firmation of this conclusion about the nature of two spectral bands,
the spectrum of dye AA-Me could be considered: the spectra of
both dye AA-OMe and its analogue AA-Me are practically the same,
only the short wavelength cyanine-like band is shifted slightly
hypsochromically and the local transition manifests itself more
clearly – as a separated wide long wavelength band with its
maximum at 660 nm. Consequently, the performed investigations
confirm undoubtedly our statement about the inversion of cya-
nine-like and local electron transitions which observed as a princi-
pal change of the shape of spectral bands in the absorption spectra.
D
Ei!j
¼
ej
ꢁ
ei þ 2Kij ꢁ Jij
ð6Þ
here ej and ei are energies of the orbital involving in the transition.
One could see from Table 3 that both methods predict a signif-
icant decreasing of the oscillator strengths (f) for the transition
from the local HOMO-1 in respect to the transition that involves
HOMO. In styryl AA-DMA, the distance between the first and the
second transitions is minimal. The AM1 predicts that the local
transitions should be the second transition shifted hypsochromi-
cally at 21 nm, in respect to the first transition, whereas the TD
DFT method gives the opposite disposition of the transitions
|HOMO> ? |LUMO> and |HOMO-1> ? |LUMO> with the larger
distance between them: 75 nm. Both methods predict negligible
oscillator strength for the local transition. Thus, we cannot estab-
lish the position of the virtual spectral band corresponding to the
local transition. We could only suppose that some broadening of
the long wavelength band with the maximum at 681 nm is likely
to be connected with nearby two electron transitions.
For methoxystyryl AA-OMe, both methods have predicted
unambiguously the lower energy of the local transition with low
oscillator strength so, that corresponding low intensive spectral
band should be shifted bathochromically: at 112 nm (TD DFT) or
61 nm (AM1). I.e. the inversion of two lowest electron transitions
of different nature should be observed:

J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
225
Besides, the compounds AA-OMe and AA-Me are the first dyes
derivatives of AA, in which the local transition manifest itself as
an individual spectral band.
dye BIm-Me is shifted hypsochromically in comparison with dye
BIm-OMe only by 14 nm and reproduces its band shape. Then we
can argue that the methoxy group in dye BIm-OMe practically does
not affect the electron transition, in contrast to dye BIm-DMA, in
which the presence of two nitrogenous terminal groups with their
high positioned donor levels provides a considerable splitting of
the donor energy levels and hence appreciable raising of the high-
est occupied level of the dye.
3.6.2. Dyes derivatives of benzimidazolium
As it was shown above, the styryl BIm-DMA is a typical Brook-
er’s cyanine containing both nitrogenous terminal groups: a high
basic BIm residue and a low basic p-dimethylaminophenylen resi-
due. Both HOMO and HOMO-1 involved in S0 ? S1 and S0 ? S2
transitions are totally delocalized MOs, the splitting between them
is comparatively not large. Then, the two lowest electron transi-
tions are described correctly by the scheme presenting in Fig. 5a,
similarly to other Brooker’s unsymmetrical polymethine dyes.
We can estimate quantitatively the degree of asymmetry of dye
BIm-DMA using the deviation D calculated by Brooker’s formulae:
3.6.3. Dyes derivatives of 2,6-di-tert-Bu-pyrylium
Similarly to the large bathochromic shift of the long wavelength
band maximum observed in the absorption spectra of symmetrical
dyes Sym upon replacing of benzimidazolium terminal groups by
the 2,6-di-tBu-pyrylium residues, a considerable spectral effect is
seen from Fig. 8c and d to be observed upon going to: BIm-
DMA ? Py-tBu-DMA and BIm-OMe ? Py-tBu-OMe. Nevertheless,
the analysis of the measured spectral data and corresponding
calculated results shows that the nature of the first electron tran-
sitions in unsymmetrical dyes Py-tBu-DMA and Py-tBu-OMe is
the same as in corresponding dyes BIm-DMA and BIm-OMe. I.e.,
2,6-di-tBu-pyrylium terminal group does not produce any specific
additional MOs which could principally change the type of the first
and the second electron transitions. The measured spectrum of sty-
ryl Py-tBu-DMA and the spectrum of the corresponding parent
symmetric cyanine dyes exhibit also the reasonable deviation:
D = 21.5 nm, however, the value D is less than this parameter for
the styryl BIm-DMA with high basic variable terminal groups.
Consequently, decreasing of the deviation points to decreasing of
the basicity of 2,6-di-tBu-pyrylium terminal group, that agrees
with the conclusion about the comparatively low basic properties
of the pyrylium heterocycle, obtained by the analysis of the charge
distribution.
Spectral effect upon going from styryl Py-tBu-DMA to dye Py-
tBu-OMe, 131 nm (5090 cmꢁ1), is also large, but it is smaller than
in the pair of dyes BIm-DMA ? BIm-OMe. At least, the replacement
of AOCH3 group by the methyl substituent in dye Py-tBu-Me
causes only the relative insignificant hypsochromic shift of the
long wavelength band maximum without appreciable change of
its shape, in comparison with dye Py-tBu-OMe, supporting the
statement about the same nature of the first electron transition
in these dyes.
D ¼ ðk1 þ k2Þ=2 ꢁ kas
ð7Þ
where kas is the absorption maximum of the unsymmetrical dye
while k1 and k2 are the maxima of the corresponding symmetrical
parent molecules.
The measured spectra of the corresponding dyes give:
D = 150 nm. Such large value points to a considerable difference
in basicities of both terminal groups in BIm-DMA styryl.
The calculation results obtained by the means of time depended
(TD) DFT method are seen from Table 3 and give the second tran-
sition S0 ? S2 that involves the |HOMO-1>. Then, this transition
could be treated as the second splitting transition. It follows from
Table 3 that it should be shifted hypsochromically upon 112 nm,
but the calculated oscillator strength f2 is too small this transition
to be observed in the absorption spectra. It should be noticed that
the semi-empirical AM1 method gives the second transition to in-
volve the |HOMO-1> too, nevertheless the distance between the
first and second transition is also rather large: 166 nm.
One can see from Fig. 8c that going from BIm-DMA dye to its
methoxy analogue BIm-OMe is accompanied by a considerable
hypsochromic effect. As far as the interaction of the donor levels
in methoxystyryls should be significantly smaller, in comparison
with corresponding DMA-styryls (compare Fig. 6a and b), the en-
ergy of the first electron transition should be considerably higher,
and hence the absorption band maximum is necessarily shifted in
the short wavelength spectral region. The observed hypsochromic
effect is 54 nm (3953 cmꢁ1) (see Fig. 8c).
It has been observed that the calculated spectral effect is lower
for TD DFT – 36 nm and higher for AM1 method – 79 nm, that is
connected, first of all, with a considerable discordance between
the calculated wavelength of the first electron transition kcalc and
the observed spectral band maximum kmax for the styryl with both
nitrogenous terminal groups, while the corresponding values, kcalc
and kmax for OMe-styryl are seen from Table 3 and Fig. 8c to be
comparatively close.
The calculation gives the second transition S0 ? S2 in OMe-sty-
ryl BIm-OMe that involves the local orbital |HOMO-1>, as one can
see from Fig. 7b. The corresponding spectral band should appear at
359 nm (TD DFT) but none appreciable absorption is observed in
the spectrum up to 300 nm, that could be connected with too
low oscillator strength (f2 = 0.016).
Such considerable hypsochromic shift of the long wavelength
absorption band in spectrum of BIm-OMe, in comparison with
the spectrum of its nitrogen-containing analogue BIm-DMA, con-
firms the hypothesis about the change of the nature of the two
lowest electron transitions in related dyes BIm-OMe and BIm-
DMA. We could even assume that the first electron transition in
the OMe-styryl is similar to that in the corresponding BIm salt,
but with exo-cyclic conjugated substituent. To confirm this idea,
the corresponding dye BIm-Me without any second donor residue
was synthesized. One could see from Fig. 8c that spectral band for
3.6.4. Dyes derivatives of 2,6-di-Ph-pyrylium
The regularities concerning the long wavelength absorption
band observed for the pair of dyes DMA-styryl and OMe-styryl
discussed above remain the same also for Py-Ph-DMA and related
Py-Ph-OMe dye, only the absorption maximum is blue shifted
(from 628 nm to 481 nm). The calculated wavelength of the first
electron transition (see Table 3) differs somewhat from the exper-
imental result. Nevertheless, both methods help to interpret the
nature of transitions correctly, as well as to give the considerable
distance between the first and the next transition.
The relatively large deviation that appears in the spectrum of
styryl: D = 12.5 nm, testifies about the difference in the basicities
of terminal groups. Naturally, going from DMA-styryl to OMe-
styryl is accompanied by a considerable hypsochromic shift of
the band maximum that points at the difference of the nature of
the first electron transitions, similarly to the dyes mentioned above
(Py-tBu-and BIm-).
However, it was shown above (see Fig. 7c) that introducing of
two phenyl substituents instead of tert-Bu groups in the pyrylium
heterocycle leads not only to exceeding of the total length of the
conjugated system, but to the appearance of the high positioned
level, which corresponds to the HOMO-1, located entirely on 2,6-
di-Ph-pyrylium residue in both styryl Py-Ph-DMA and related
dye Py-Ph-OMe. Then, the electron transition involving the local

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J.L. Bricks et al. / Journal of Molecular Structure 1033 (2013) 215–226
MO should appear in the spectra. One can see from Fig. 8e that
separated appreciable spectral band with maximum at 386 nm is
observed in the short wavelength region, similar to the spectrum
of symmetrical pyrylotrimethine cyanine Sym (D = Py-Ph). The cal-
culation by AM1 method of dye Py-Ph-DMA is seen from Table 3
and gives the wavelength of the second transition: 381 nm that
is close to the experimental maximum. This transition is described
by only one configuration: local HOMO-1 ? LUMO. Also, AM1
method predicts a comparatively high oscillator strength that
agrees with sufficient intensity of this transition to manifest itself
as a separated band in the spectrum. The distance between two
first maxima, corresponding to the transitions from the highest
splitting donor orbital (HOMO) and from local orbital (HOMO-1),
is 242 nm, whereas the calculations give the smaller value:
167 nm, that is connected with the divergence between the exper-
imental and calculated wavelengths of the first transition:
ꢂ628 nm–548 nm = 80 nm.
in AA-methoxystyryl and hence drastic changes in its absorption
spectrum.
Acknowledgement
Financial support by Science and Technology Centre in Ukraine
(STCU Project 4270) is gratefully acknowledged.
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.molstruc.2012.
08.034.
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