Nature of the lowest electron transitions in the linear conjugated systems derivatives of 2,6-diphenylthiapyrylium: Cationic trimethinecyanine, neutral ethylene, its dication and cation-radical

Article abstract of DOI:10.1016/j.molstruc.2011.06.036

A series of linear conjugated systems of different types containing the 2,6-diphenylthiapyrylium residues as terminal groups have been synthesized and investigated by the combined spectral and quantum-chemical methods. The analysis of calculated by ab ini

Full text of DOI:10.1016/j.molstruc.2011.06.036

Journal of Molecular Structure 1011 (2012) 59–65
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Nature of the lowest electron transitions in the linear conjugated systems
derivatives of 2,6-diphenylthiapyrylium: Cationic trimethinecyanine, neutral
ethylene, its dication and cation-radical
M.O. Kudinova a, D.O. Melnyk b,*, O.D. Kachkovsky a, O.I. Tolmachev a
a Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska Str., 03660 Kyiv, Ukraine
b Ivano-Frankivsk National Medical University, 2 Galytska Str., 76000 Ivano-Frankivsk, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of linear conjugated systems of different types containing the 2,6-diphenylthiapyrylium residues
as terminal groups have been synthesized and investigated by the combined spectral and quantum-
chemical methods. The analysis of calculated by ab initio and semi-empirical methods of their molecular
geometry and electron structure, and nature of the lowest electron transitions has shown that four types,
cationic cyanine, neutral polyene, polyene-dication and cationic polyene-radical differ fundamentally
Received 9 January 2011
Received in revised form 25 April 2011
Accepted 24 June 2011
Available online 6 July 2011
each other by the charge distribution and alternation of bond lengths along
p-electron system, as well
Keywords:
Cyanine dyes
Polyenes
2,6-Diphenylthiapyrylium derivatives
Absorption spectra
Quantum-chemical calculation
as by nature of their lowest electron transitions and hence of shapes of the absorption bands. It was
established the cyanine-like similarity of the ‘‘middle’’ high intensive and narrow spectral band for the
cationic polyene-radical and cyanine dye, whereas the polyene with the close electron shell exhibit the
comparatively wide longwavelength bands.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
the longerwavelengths [3,6]. Also, many spectral bands of different
intensities are observed in the visible and NIR spectral region,
Cationic polymethine dyes are the most well-known among the
linear conjugated systems containing pyrylium and thiapyrylium
residues as terminal groups. They have been synthesized for more
than half century, but wide investigation of them is continued so as
their application permanently extends [1–7]. Besides traditional
using of polymethine dyes as photosensitizers in photography,
fluorescence probes, initiators for polymerization, active and pas-
sive components for tunable lasers, media exploring excited-state
absorption, non-linear optical materials, etc. [8–12]. It have invited
further wide investigation of the electron structure of different
types of the conjugated molecules, particularly of the higher ex-
cited states. Earlier we were studied in details the nature of the
electron transitions in the cationic pyrylocyanines and their hete-
roanalogues and have established that comparatively deep color,
whereas the only single typical separated highly intensive, so-
called ‘‘cyanine-like’’ spectral band appears in the spectrum of
the corresponding cation-polymethine.
On the other hand, it was established that the lowest electron
transitions in the linear conjugated systems with the compara-
tively short polymethine chain and with the complex terminal
groups containing their own extended
p-electron system, depend
on the topology of the terminal residues; moreover, S0 ? S1 and
S0 ? S2 transitions could be considered as splitting transitions
from the donor orbitals to the solitonic MO [15]. One could
suppose that the relative location of the ‘‘local’’ levels of the
complex thiapyrylium terminal groups should significantly depend
on the total charge of the conjugated system and hence should
influence principally on the nature of the first and higher electron
transitions.
in comparison with the corresponding neutral
a,x-disubstituted
polyenes, was connected with the appearance of the so-called sol-
itonic level (level of the positive charge or impurity level) in the
energy gap [13,14]. In the same time, it is known that polyene-
dications absorb the light at considerably longer wavelength,
400–500 nm, in contrast to cationic polyene-radical, absorbing in
The present paper concerns with the features of the generation
of the frontier and nearest molecular orbitals and with features of
the lowest electron transitions in four stable types of the symmet-
rical linear conjugated systems containing the thiapyrylium resi-
dues as symmetrical terminal groups. The main goal is to
understand the cause of the similarity of the electron structure of
the thiapyrylocarbocyanine cation with even number of the
methine groups in the polymethine chain and related polyenic cat-
⇑
Corresponding author.
E-mail address: melnyk_dm@ukr.net (D.O. Melnyk).
ion-radical with odd number of the p-centers in the chain.
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.06.036

60
M.O. Kudinova et al. / Journal of Molecular Structure 1011 (2012) 59–65
about the solitonic-like shape of the charge distribution and max-
2. Results and discussion
2.1. Conjugated systems
imum equalizing of the molecular geometry in the center of the
charge wave in the linear conjugated monoions [16]. In contrast,
the bond lengths in the neutral polyene 3 and dication 4 are seen
from Fig. 2 to differ considerably, the order of the alternation of
the lengths of the neighboring bonds being opposite in the com-
pounds 3 and 4. Meanwhile, the alternation of the bond lengths
along the chromophore of the cationic polyene-radical 5 is lower
than in other two types of the polyenes, 3 and 4; however, the
alternation degree is somewhat higher, in comparison with
polymethine dye 2. Also, the calculation gives the symmetry break-
ing in the chromophore of the cation-radical 5. This is connected
with the slight shift of the waves of the charge and spin density
from the center.
The linear conjugated systems 1 (pyrylcarbocyanine 2, neutral
polyene 3, its dication 4 and cation-radical 5) as well as initial salt
6 studied are described by following formulae:
2.4. Charge distribution
Similarly to the bond lengths in the polymethine chain, the var-
ious types of the linear conjugated systems 1 differ principally each
other by the distribution of the electron density at the carbon
atoms. It is well-known that the charges at the neighboring atoms
alternate considerably in the chromophore of the polymethine cat-
ion/anion, whereas the electron density is equalized in the neutral
polyenes [16,17].
Here An = ClO4 except 4a (An = Br) and 6a (An = BF4);
R+, R and RÅ are the terminal groups:
Here we will analyze the charge distribution calculated by ZIN-
DO/S method. Other methods have given the considerable polari-
zation of the C-H bonds, so that the calculated charge at the
hydrogen atom can reach as much as 0.146 (AM1) or even 0.167
(ab initio), while ZINDO/S gives only 0.02–0.04. The fulfilled calcu-
lations have shown that the electron distribution at the atoms in
the phenyl substituents is practically independent on the type of
the linear system; the atomic charges are practically equalized,
in contrast to the distribution of the electron density along the
main chromophore. The calculated charges at the atoms in the cat-
ionic pyrylotrimethine 2 in Fig. 3 show that the considerable alter-
nation of the electron density extends within both terminal
heterocycles. In the polyenes with closed electron shell, neutral
compound 3 and dication 4, the alternation of the charges is lower.
The minimum of the charge alternation is obtained for the cationic
polyene-radical 5. It is to be noted that the charge distribution is
unsymmetric. Also, Fig. 4 shows the symmetry breaking for the
alternation of the spin densities at atoms calculated by the follow-
ing formulas:
Here Ar = C6H5 (for compounds 2–6), Ar = C6H4–C2H5–p (for
compounds 2a–6a).
2.2. Synthesis
The synthesis of the compounds 2 and 6 is described in the pa-
pers [1,2]; the dyes 2a and 6a were obtained similarly. The poly-
enes 3 (3a) were obtained by heating of the corresponding salts
6 (6a) in pyridine [5,6]. Then, they were oxidated in the dications
4 (4a) by CuClO4 in acetonytryl or by Br2 in CH2Cl2. The solutions
of the cation-radicals 5 (5a) were obtained by mixed of the equi-
molar solutions of the neutral polyene 3 (3a) and dication 4 (4a).
An existence of the radical particles was confirmed by the EPR
spectrum presented in the Fig. 1.
l
D
q
¼ ðꢀ1Þ ðql
ꢀ
qlþ1Þ;
ð1Þ
2.3. Optimized molecular geometry
where ql = ql(a) ꢀ q (b) is a difference of the densities of
a
- and b-
l
The optimization of the molecular geometry performed by the
ab initio method (STO 6-31G⁄⁄) has shown that both the neutral
molecule 2 and the ions 3–6 are practically planar, as it is typical
electrons at the
lth atom. The Fig. 4 demonstrates that phases of
the wave of the electron densities of the opposite spin are also
opposite, both waves being intersected at one of the central atom,
but not at the middle of the central bond; hence this leads to the
symmetry breaking in the electron density distribution and equilib-
rium molecular geometry.
for the
p-electron systems. Only the phenyl substituents in the
thiapyrylium residue are twisted slightly, at 15–17° relatively to
the molecular plane, what is in a good accordance with the exper-
imental data obtained by NMR spectroscopy for the salt 6 [2]. Here,
we will suppose that the main chromophore includes the internal
polymethine chain and both heterocycles, whereas the four phenyl
substituents disturb relatively small the charge distribution at
atoms and molecular geometry of the main chromophore system.
The calculated optimized lengths of carbon–carbon bonds in the
main chromophore (terminated by two sulfur atoms) for the vari-
ous types of the linear conjugated systems derivatives of the thia-
pyrylium are presented in Fig. 2. It should be noted that the
carbon–carbon bond lengths in all phenyl residues in all types of
the linear systems 2–5 and salt 6 are practically equalized and
are approximately 1.39–1.40 Å. One can see from Fig. 2 that the
all carbon–carbon bonds are maximum equalized in the monocat-
ionic polymethine dye 2. It accords with the theoretical conception
2.5. Positions of electron levels and shapes of molecular orbitals
Before starting the analysis of the nature of electron transitions
related to the spectral bands to understand the reasons of the
significant difference let us examine the position of the frontier
and nearest electron levels and shape of the corresponding MOs
in the conjugated systems of the neutral molecule 3 and ions 2,
4–6. Calculated data are presented in Figs. 2 and 3. Firstly, one
can see from Fig. 5a that the lowest unoccupied MO (LUMO) is
totally and uniformly delocalized along the whole molecule,
whereas the highest occupied MO (HOMO) and next MO, HOMO-
1, extends practically only over the open polymethine chain and

M.O. Kudinova et al. / Journal of Molecular Structure 1011 (2012) 59–65
61
both heterocyclic rings, excepting all four phenyl substituents. It
should be to noted the next two vacant orbitals, LUMO + 1 and
LUMO + 2 are local MOs and do not extend over the polymethine
chain. Both orbitals have their molecular node at the atom con-
nected with the chain, similar to the HOMO in the initial salt 6
(compare with Fig. 6a); hence, they do not conjugate with the rest
of the p-electron system and remain degenerated MOs.
Removing of one electron from the collective system of the neu-
tral polyene 3, i.e. going to the cationic polyene-radical 5 is accom-
panied by considerable shifting down of the energy gap, as it is
seen from Fig. 5b. Similarly to the neutral molecule 3, the LUMO
in the radical 5 for the both
delocalized along the whole conjugated system, while two next
-spin-orbitals, LUMO + 1 and LUMO + 2, are local. In contrast,
a-electron is totally and uniformly
H, G
3290
3295
3300
3305
3310
a
Fig. 1. EPR spectrum of the cation-radical 5.
both lowest vacant b-MOs are totally delocalized, whereas both
highest occupied b-MOs are seen from Fig. 5b to be local, although
they are not degenerated. Also, going to the cation-polyene causes
extending of the delocalization of the highest occupied a-MOs over
the phenyl substituents. In the same time, the calculation without
taking into consideration the spin of the electron (RHF version)
gives delocalization of the single occupied MO (SOMO) to be only
over the chain and both heterocycles (Fig. 6c), similarly to the
HOMO in both the neutral molecules (Fig. 5a) and the cationic
polymethine dye 2 (Fig. 6b). One can see in Fig. 6c that the highest
double occupied MO and LUMO are delocalized, including the phe-
nyl substituents, whereas the HOMO-1 is local.
Next going to the polyenic dication 4 leads to the further shift-
ing of the electron levels down, so that the lowest vacant level is
positioned below the highest occupied level in the neutral polyene
3 (compare Fig. 5c and a). Two lowest vacant orbitals, LUMO and
LUMO + 1, are totally delocalized, whereas the two next MO are lo-
cal. In contrast to vacant MOs, both carbon atoms of such short
chain in the dication of the ethylene 4 do no take part in the occu-
Fig. 2. Carbon–carbon bond lengths in main chromophore of the compounds 2–5;
for sake convenience, bond in the cycles are considered as equivalent; two central
bonds in polymethine dye 2 are presented by one point.
pied MOs, because of the absence of the own
p-electrons. This is
responsible for the specific nature of the lowest electron transi-
tions in the polyene dications, as it was established by us earlier
for the ethylene-dication containing flavilium residues [13,14].
At last, we consider the frontier and the nearest MOs in the
polymethine dye 2. One can see from comparing Fig. 6b with the
Fig. 5b that the energy gap in both cations, 2 and 5, is situated
approximately close. Both HOMO-1 and HOMO-2 are degenerated
local orbitals; their shapes are practically the same as the HOMO in
the initial cationic salt 6 (Fig. 6a). Because the MO nodes at the
atom connecting with the polymethine chain, this MO does not
conjugate with the rest of the
p-system. The LUMO as well
LUMO + 1 are seen from Fig. 6b to be delocalized over the whole
molecule.
Fig. 3. Alternation of charges at carbon atoms in main chromophore of the
compounds 2–5; for sake convenience, value
polymethine 2 are presented by one point.
D
q
for two pair of central atoms in
l
In the simplest scheme, in the RHF approximation, the position
of the single occupied MO in the radical could be treated as pro-
ducing of the additional level in the energy gap of the virtual sys-
tem with closed electron shell, i.e. with double occupied orbitals
(Fig. 6c). Such model enables to treat more simple and illustrative
the electron transitions.
Δρ, e.u.
0.8
2.6. Electron transitions and absorption spectra
0.4
0
The measured absorption spectra of the compounds 2–6 solu-
tions in the same region (UV, visible and near IR) are presented
in Fig. 7 and in Table 1. The simplest spectrum is observed for
the cationic polymethine dye 2; the high-intensive and compara-
tively narrow spectral band in the longwavelength part and more
intensive and wider band in the UV region. Comparing the spectra
of the dye 2 and salt 6, one can see that the position of the second
spectral band of the trimethincyanine coincides approximately
with the position of the corresponding initial salt. It is seen from
Table 2 that the first band is connected practically only with the
# atom
1
2
3
4
5
6
7
8
-0.4
-0.8
Fig. 4. Alternation of spin densities at carbon atoms in main chromophore of
cation-radical 5.

62
M.O. Kudinova et al. / Journal of Molecular Structure 1011 (2012) 59–65
E, eV
0
LUMO
HOMO
α
-MOs
β
-MOs
-3
-6
LUMO
LUMO
LUMO
-9
HOMO
HOMO
-12
HOMO
c
a
b
-15
Fig. 5. Positions of the levels and shape of the MOs in neutral polyene 3 (a) [AM1 approximation, RHF version], cationic radical-polyene 5 (b) [UHF], polyene dication 4 (c)
[RHF].
E, eV
0
-3
D4 D5
D1
S1
S2 S3
LUMO
SOMO
LUMO
S1
LUMO
-6
-9
D
2 D3
HOMO
HDOMO
-12
HOMO
-15
a
b
c
Fig. 6. Scheme of electron transitions in cationic systems: salt 6 (a), polymethine dye 2 (b) and radical-polyene 5 (c); AM1 approximation, RHF version.
electron transition between the antisymmetrical and symmetrical
the frontier MOs. Because of fact that the frontier MOs taking part
into this transition are of opposite symmetry, the first transition is
antisymmetrical, A1 ? B1, and is polarized along the polymethine
chain; hence its transition dipole momentum is comparative large:
f1 = 1.876.
The second and the third electron transitions are seen from
Fig. 6b to involve practically degenerated local MOs as well as
the LUMO. The calculation shows that both transitions are allowed
and are polarized perpendicular to the polymethine chromophore.
Data in Table 2 show that the splitting of these local transitions is
negligible.

M.O. Kudinova et al. / Journal of Molecular Structure 1011 (2012) 59–65
63
(1 + R ), where a is constant and R is the distance between the
2
1
0
lm
lm
3
4
6
l-th and
m-th atoms overestimates the electron interaction if the
atoms are removed to a great distance from each other [18]. Then,
using the same OWF for both totally delocalized and local MOs
leads to the too low energies of the higher electron transitions. In
this paper, we have used different methods for the comparative
estimation of the energies of the first and higher electron transi-
tions, optimistically supposing that divergence between the exper-
imental and experimental transition energies is no so considerable
to establish correctly the order of the transitions of the different
types: delocalized or local. The central and largest part of our work
was to find the nature of the first and higher transitions in the dyes
1–6, and hence to explain correctly the main reason for the consid-
erable difference between their spectra. Similar methodology was
proposed by one of us in the previous work [19].
Practically, the generation of the calculated transitions
|S0 ? S2 > and |S0 ? S3 > indicates that both transitions correspond
to the same observed spectral band with the maximum 400 nm. It
was shown above, that the local HOMO-1 and HOMO-2 with their
nodes at the carbon atom connected with the chain in the dye 2 are
practically the same as the highest occupied orbital of the salt 6. At
the same time, the LUMO in both compounds, 2 and 6, is the soli-
tonic or charge level [20]. Taking into consideration that the salt is
1
0
300 400 500 600
2
5
300
400
500
600
700
800
900 1000 1100 1200
Fig. 7. Absorption spectra of the compounds 2–6.
Table 1
Spectral properties of compounds 2 (2a)–6 (6a) in acetonitryl.
Compound kmax, nm (
e
ꢁ 10ꢀ4
)
an electron-balanced
p
-system (n
p
-electrons are delocalized at n
-system of the
-centers), then going from the salt
2
2a
3
3a
4
4a
5
752 (19.5); 394 (1.8)
760 (20.72); 413 (2.11)
p-centers), in contrast to the electron – excessive
p
dye 3 (n + 1
p-electrons and n
p
500 (7.45)
504 (–); 513a (4.66)
to the corresponding trimethinecyanine should be slightly accom-
panied by shifting of the energy gap up, what is in a good agree-
ment with the direct calculation (compare Fig. 6a and b). As a
result, the distance between local levels and solitonic level should
be close in the both dye 2 and corresponding salt 5; the observed
spectra confirm this conclusion. The similar nature of the short-
wavelength spectral band in the spectra of the thiapyrylocyanines
(as well as of their heteroanalogues) and of the first band in the
spectra of the corresponding salts was additionally confirmed by
the spectral and quantum-chemical investigations of the viny-
logues series of these dyes: the lengthening of the polymethine
chain leads to regular batochromic shift of the longwavelength
band at 120–130 nm, whereas the position of the shortwavelength
band remains practically the same and coincides with the position
of the first spectral band of the corresponding salt [7,21].
409 (3.31)
474 (2.62); 411 (3.53)
1031 (4.19); 904 (2.6); 813 (1.79); 603 (9.2); 546 (6.32); 347
(2.28);
5a
6
6a
1040 (–); 908 (–); 817 (–); 605 (–); 556 (–); 364 (–);
390 (1.695)
415 (2.32)
a
In toluene.
Here we should note that the calculated energies of the first
and, especially, higher transitions for the cationic linear conjugated
systems disagree with the spectral data. It is well-known that there
is a serious problem concerning the correlation between the calcu-
lated and experimental energies simultaneously for both the first
and the higher electron transitions. The parameter OWF (Overlap
Weight Factor) in the semi-empirical method ZINDO/S is con-
nected indirectly with the of the electron-electron repulsion clm
Going from the polymethine dye 2 to the related polyenic con-
jugated systems 3 and 4 with the closed electron shell is seen from
Fig. 7 to be accompanied by the significantly hypsochromic shift of
the absorption band. This spectral phenomenon, i.e. the consider-
able distinction in the energy gaps upon comparing two types of
in the integrals Jij and Kij in the transition energy:
ꢀ ei + 2Kij ꢀ Jij. The standard Mataga–Nishimoto’s formulas, clm = a/
D
E(S0 ? Sp) = ej
Table 2
Calculated data of compounds 2–6 (AM1 approximation).
Dye
Transition
k (nm)
f
Symmetry
Main configuration, Tp,i?j
6 (salt)
S
S
S
S
S
S
S
S
S
S
S
S
0 ? S1
0 ? S2
0 ? S1
0 ? S2
0 ? S3
0 ? S1
0 ? S2
0 ? S3
0 ? S4
0 ? S1
0 ? S2
0 ? S3
398
338
509
404
402
422
415
352
0.722
0.145
1.899
0.017
0.069
0.005
1.082
0.049
0.005
B1
A1
Bu
|S1> = 0.96 |H ? L>
|S2> = 0.89 |H-1 ? L> + 0.45 |H ? L + 1>
|S1> = 0.99 |H ? L>
|S2> = 0.86 |H ? L + 1>
|S3> = 0.81 |H ? L + 2>
|S1> = 0.55 |H ? L + 1 > -0.74 |H-1 ? L>
|S2> = 0.74 |H ? L > -0.55 |H -1 ? L + 1>
|S3> = 0.81 |H-2 ? L> + 0.45 |H-3 ? L + 1>
|S4> = 0.81 |H-3 ? L> + 0.45 |H -2 ? L + 1>
|S1> = 0.99 |H ? L>
3 (Pe-Tprl)
Ag (Loc+)
Bu (Locꢀ)
Ag (Loc+)
Bu (Locꢀ)
Ag (Loc+)
Bu (Locꢀ)
Bu
Ag (Loc+)
Bu (Locꢀ)
Bu
4 (Pe-DC-Prl)
351
2 (Pm-Prl)
566 (747)
339 (561)
337 (554)
1235 (1308)
541 (787)
428 (600)
408 (580)
1.876 (1.218)
0.149 (0.381)
0.049 (0.387)
0.282 (0.358)
0.008 (0.010)
1.670 (0.387)
0.001 (0.006)
|S2> = 0.83 |H-2 ? L>
|S3> = 0.81 |H-1 ? L>
5 (Pe-R-Prl)
D
D
D
D
0 ? D1
0 ? D2
0 ? D3
0 ? D4
|D1> 2 |SOMO ? LUMO>
Ag
Bu
Ag
|D2> 2 |HDOMO ? SOMO>
|D3> 2 |HDOMO ? LUMO>
|D4> 2 |HDOMO-1 ? LUMO>
Comments. Data in brackets are obtained by ZINDO/S methods (OWF = 0.3).

64
M.O. Kudinova et al. / Journal of Molecular Structure 1011 (2012) 59–65
the linear conjugated system with the same terminal groups, poly-
methinic and polyenic, is explained in the framework of the Dae-
hne’s triad theory by the considerable alternation of the lengths
of the carbon–carbon bonds along the chromophore [17], what
agrees with the results of our calculations discussed above. The
calculated characteristics for the neutral molecule 3 the polyene-
dication 4 are presented in Table 2.
the SOMO (HDOMO ? SOMO and SOMO ? LUMO) are generated,
as it is shown in the Fig. 7. These configurations with the same
MO should interact substantially, so that two lowest doublet states
are described by the following mixture:
jD0 ! D1 >ꢂ jHDOMO ! SOMO > þjSOMO ! LUMO >
jD0 ! D2 >ꢂ jHDOMO ! SOMO > ꢀjSOMO ! LUMO >
One can see that three first electron transitions are described
practically by single configuration with the jump of electron from
the HOMO to the LUMO, LUMO + 1 and LUMO + 2, correspondingly.
Because of the local type of the LUMO + 1 and LUMO + 2, the sec-
ond and third transitions prove to be degenerated; their oscillator
strengths are too low these transitions to be observed in the
absorption spectra. Then, the comparatively wide spectral band
with maximum at 500 nm corresponds only to the first electron
transition. However, the intensity of this band is appreciably lower,
in comparison with the longwavelength absorption band of the
cationic polymethine dye; so, the extinctions are as follows:
We suppose that the |D0 ? D1 > and |D0 ? D1 > are just these
electron transitions which correspond to the two additional spec-
tral bands in the spectra of the polyene-radical positioned batho-
chromically, in comparison with the main intensive cyanine-like
spectral band, what is observed experimentally (Fig. 7). However,
one can see from the Table 2 that the calculations by the AM1 or
ZINDO/S methods using the MOs obtained in RHF approximation
as basis set for the building the configurations give too low ener-
gies of both transitions.
In contrast, the energy of the third transition with the compar-
atively large oscillator strength, |D0 ? D3>, is too high in the AM1
approximation. The ZINDO/S with the same parameter OWF using
for the polymethine dye 2 results in the close wavelength of this
transition. Consequently, we can propose that the intensive cya-
nine-like spectral band at 603 nm with the largest extinction,
e
= 7.45 ꢁ 10ꢀ4 (polyene 3),
e
= 19.50 ꢁ 10ꢀ4 (polymethine 2),
In contrast to the neutral disubstituted-ethylene 3 with its delo-
calized HOMO and HOMO-1 (Fig. 5a), the two highest occupied
orbitals in the polyene-dication 4 are seen from Fig. 6c to be degen-
erated and are local. On the other hand, both lowest vacant orbitals
are solitonic MOs and hence should be considered as coupled orbi-
tals. Consequently, the two lowest transitions prove to be the
mixed local transitions, as it is seen from calculated data collected
in the Table 2. The first transition is forbidden, while the second
transition is allowed. The performed calculation gives the similar
mixing of the next configurations involving the pair degenerate
orbitas: HOMO-2 and HOMO-3 (Table 2). It should be noted that
increasing of the number of the single excited configurations upon
calculations by the configurational interaction methods leads to
the regular increasing of oscillator strengths of the allowed third
transition, f3, whereas the value f2 decreases. Then, we can sup-
pose, that band maxima at 410 nm in the absorption spectra of
the dication 4 correspond to the S0 ? S3 transition, while the
shoulder in the right side of the main spectral band (what is shifted
approximately at 50–70 nm) is connected with the S0 ? S2 transi-
tion. The calculation gives the distance between these transitions,
which close to experimental data.
At last, let us consider the absorption spectrum and, corre-
spondingly, the nature of the electron transitions in the conjugated
system 5 with the open electron shell. Here, we are restricted only
by the qualitative analysis of the relationships between the ob-
served maxima in the visible and near IR regions, as well as by
comparing of the absorption spectra of the two cationic systems:
the radical 5 with the open electron and polymethine dye 2 with
the closed shell.
Firstly, some spectral maxima are observed in the spectra of the
radical 5 positioned more bathochrhromically, in comparison with
the longwavelength spectral band of the dye 2 with the same total
charge.
e
= 9.21 ꢁ 10ꢀ4, is evidently connected with the |D0 ? D3 > transi-
tion. Similarly to the first electron transition in the cationic
polymethine dye 2, this transition in the cationic polyene-radical
involves one of the splitting donor MO as a starting orbital and sol-
itonic orbital (orbital of the hole or positive charge) as a final MO.
As a result, the longwavelength spectral band in the spectrum of
the polymethine 2 and high intensive and selective band in the
spectrum of the radical 5 are of the same cyanine-like nature.
Of course, the better correlation between calculated and spec-
tral data can be reached by the methods using the spin-orbital
(UHR approximation) what involves considerable difficulties upon
the constructing the configurational interaction. Here, we have at-
tempted only to connect the observed bands with different inten-
sities in the spectrum cationic polyene-radical 5 with the possible
electron transition, as well as to explain the cyanine-like similarity
of the intensive spectral band at 603 nm with longwavelength
band in the spectra of the cationic compounds of the another type
of the linear conjugated system with the same terminal groups.
3. Experimental section
UV/Vis absorption spectra of acetonytryl solutions were re-
corded on a Shimadzu UV-3100 spectrophotometer. All NMR mea-
surements were carried out on Varian GEMINI 2000 spectrometer
with 1H and 13C frequencies of 400.07 and 100.61 MHz, respec-
tively at 293 K. Tetramethylsilane was used as standard for d scale
calibrating. 1H NMR spectra were recorded with spectral width
8000 Hz and numbers of points 32000.
Secondly, the comparative narrow and more intensive band
with the typical cyanine-like vibronical maximum at the short-
wavelength shoulder is observed in the spectrum of the radical 5,
which is positioned approximatively in the same spectral region,
as the first band in the spectrum of the dye 2 (see the correspond-
ing curves in Fig. 7).
To interpret the observed spectrum, we use the simplified
scheme of the electron levels transitions obtained by calculation
in the restricted HF approximation, presented in Fig. 6c. In such
model, it is convenient to treat the change in the electronic struc-
4,40-(Ethandilyden)-bis[2,6-diphenyl)-4H-thiapyran] 3 and its
ethyl-substituted analogue 3a was obtained similarly to [3–5]. 1H
NMR (in CDCl2): 3a 1.245 (t, 6H, 2ꢁ CH3, J 7.5 Hz); 1.252(n, 6H,
2ꢁ CH3, J 7.5 Hz); 2.688 (c-wide., 8H, 4ꢁ CH2); 6.757 (c, 2H, H-
chain); 7.040–7.499 (v, 16H, H-aromatic., 4H, b0, b-H).
4,40-{(1,2-vinylen)bis[2,6-di(p-ethylphenyl)thiapyrylium]}
dibrom 4a 1H NMR (in CDCl2): 1.337 (n, 12H, 4ꢁ CH3, J 7.8 Hz);
2.83 (q., 8H, 4ꢁ CH2, J 7.8 Hz); 7.563 (d, 8H, H-aromatic., J
7.8 Hz); 8.0185 (d, 8H, H-aromatic., J 8.1); 8.851(c, 2H, H-chain);
9.207 (c, 4H, 2 b-H). C44H42Br2S2. Calcd.% Br 20.11. Found% Br
20.35;
ture upon going from the
p-system of the polymethine-cation 2 to
the -system of the polyene-cation 5 as an appearance of the addi-
p
2,6-(p-ethyl)phenyl-4-methylthiapyrylium tetraftorborat 6a
tional single occupied level (SOMO) in the energy gap. The two
C22H23BF4S. Calcld.% C 65.12; H 5.67; F 18.64. Found% C 65.20; H
new electron configurations with lower energies, connected with
5.72; F 18.64.

M.O. Kudinova et al. / Journal of Molecular Structure 1011 (2012) 59–65
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65
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