The synthesis, structure and spectral properties of new long-wavelength benzodipyrroleninium-based bis-styryl dyes

Article abstract of DOI:10.1016/j.dyepig.2010.12.004

Novel bis-styryl dyes based on a quaternized centrosymmetrical benzodipyrrolenine moiety were synthesized by reaction of a quaternized benzodipyrrolenine with two equivalents of 4-dimethylaminobenzaldehyde, 4-dimethylaminocinnamoic aldehyde or 4-(3,5-diph

Full text of DOI:10.1016/j.dyepig.2010.12.004

Dyes and Pigments 90 (2011) 201e210
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Review
The synthesis, structure and spectral properties of new long-wavelength
benzodipyrroleninium-based bis-styryl dyes
Iryna A. Fedyunyayeva a, Oleksii P. Klochko a, Plga M. Semenova a, Sania U. Khabuseva a,
Yevgen A. Povrozin a, Oksana O. Sokolyk a, Olena Yu. Stepanenko a, Ewald A. Terpetschnig b,
,b,
Leonid D. Patsenker a
*
a SSI “Institute for Single Crystals” of the National Academy of Sciences of Ukraine, 60 Lenin Ave., Kharkov 61001, Ukraine
b SETA BioMedicals, LLC, 2014 Silver Court East, Urbana, IL 61802, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel bis-styryl dyes based on a quaternized centrosymmetrical benzodipyrrolenine moiety were
synthesized by reaction of a quaternized benzodipyrrolenine with two equivalents of 4-dimethylami-
nobenzaldehyde, 4-dimethylaminocinnamoic aldehyde or 4-(3,5-diphenyl-4,5-dihydro-1H-1-pyrazolyl)
benzaldehyde. The molecular structures and spectral properties of these dyes were investigated using
PM3 and PPP CI quantum chemical simulations and compared with biscyanines and parent “monomeric”
styryl dyes. Conjugation of two uniform “monomeric” styryl chromophores to form a bis-chromophoric
system was found to cause a pronounced red-shift (w100 nm) of the absorption and emission maxima,
an increase of the extinction coefficients but also a conformational rigidization of the molecules. This
results in better vibrationally resolved absorption and emission bands and, in general, in increased
quantum yields. Bis-styryl dyes absorb and emit in the red and near-IR spectral region (643e812 nm).
They have high extinction coefficients (ꢀ133,000 Mꢁ1 cmꢁ1), large Stokes’ shifts (up to 55 nm in chlo-
roform and 90 nm in methanol), and quantum yields in chloroform ꢀ10%, whereas polar solvents
decrease quantum yields. Despite their symmetrical structures these bis-styryl dyes exhibit spectral
properties in solvents of different polarity that are more resemble those of unsymmetrical “monomeric”
styryls.
Received 19 August 2010
Received in revised form
2 December 2010
Accepted 7 December 2010
Available online 15 December 2010
Keywords:
Bis-styryl dyes
Benzodipyrrolenine
Biscyanine dyes
Synthesis
Molecular structure
Spectral properties
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
to bis-polymethine dyes [17], which until now have only been
sparsely investigated.
Red and near-infrared absorbing and emitting polymethine dyes
are currently of considerable technical interest due to their
tremendous potential for practical applications such as optical
recording techniques [1,2], non-linear optics [3,4], fluorescent
probes for biological research and biomedical assays particularly
for binding with nucleic acids [5e8] and proteins [9], tumor
imaging and photodynamic therapy [10e12]. Molecular structures
and spectral properties of the compounds containing a poly-
methine chromophore system were thoroughly investigated
[13e16]. A promising approach to design longer-wavelength dyes
with advantageous properties consists in the conjugation of two
parent “monomeric” polymethine chromophores in one molecule
Recently we reported the synthesis, molecular structures and
spectral properties of a series of bis-trimethine cyanine dyes 1ae1e
(Scheme 1) all containing a centrosymmetrical benzodipyrrolene
moiety and variable terminal heterocyclic end-groups [18]. Conju-
gation of two uniform “monomeric” trimethine chromophores to
form a bis-trimethine molecule was found to cause splitting of the
long-wavelength absorption band of the parent “monomeric” dye
into two bands, which was in good agreement with Kiprianov’s
theory [17]. One of these new bands was red-shifted while the
other was blue-shifted compared to the position of the original
long-wavelength absorption maximum in the parent dye. Also, in
the example of these dyes it was demonstrated that fusion of two
trimethine chromophores into one biscyanine molecule leads not
only to a substantial red-shift of the absorption and emission bands
but also to a noticeable increase of the extinction coefficients and
fluorescence quantum yields, which is extremely important for
many practical applications.
* Corresponding author. SSI “Institute for Single Crystals” of the National
Academy of Sciences of Ukraine, 60 Lenin Ave., Kharkov 61001, Ukraine. Tel.: þ38
057 3410272; fax: þ38 057 3409343.
E-mail address: patsenker@isc.kharkov.com (L.D. Patsenker).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.12.004

202
I.A. Fedyunyayeva et al. / Dyes and Pigments 90 (2011) 201e210
Scheme 3.
Scheme 1.
Absorption and emission spectra were measured in chloroform
and methanol at room temperature in a standard 1-cm quartz cells.
The dye concentrations for the spectral measurements were in the
range of 1.0e3.0 ꢂ 10ꢁ7 M.
Despite the large variety of biscyanine dyes that were synthe-
sized and investigated [17,19e23], there is a pronounced lack of
data for bis-styryl dyes. Similar to cyanines, styryls belong to
a subclass of polymethine dyes bearing a positive charge delo-
calized within the polymethine chain; however, contrary to
cyanines which contain an odd number of methine groups, styryl
dyes have an even number of these groups in the polymethine
chain. These classes of dyes are very different in the regards to their
molecular and electronic structures and spectral properties
[24e26]. While a number of styryl dyes with a single polymethine
chromophore and various heterocyclic and aromatic end-groups
are described in the literature [16 and refs. cited therein], the only
centrosymmetrical bis-styryl dye that was previously reported is
the benzodioxazole-based dye 2 (Scheme 2) [21]. The absorption
spectrum of this dye shows only one intense long-wavelength
band, which was red-shifted by 86 nm as compared to the parent
“monomeric” dye. The molecular structure and fluorescence
properties of dye 2 were not investigated. Bis-styryl dyes with
a centrosymmetrical benzodipyrroleninium moiety instead of
a benzodioxazole central bridge have not been described, although
just indolenine based polymethines are of main practical value due
to their advantageous photophysical properties such as higher
photo stability [26e28]. Thus, the majority of commercially avail-
able long-wavelength polymethine dyes such as Cy series (GE
Healthcare), Alexa Fluor dyes (Life Technologies), DyLight dyes
(Dyomics), IRDyes (Li-Cor Biosciences), Seta and Square dyes (SETA
Biomedicals) are all based on indolenine or benzoindolenine
heterocyclic end-groups [28].
Absorption spectra were recorded on a PerkinElmer Lambda 35
UV/Vis spectrophotometer. Absorption maxima were obtained with
an accuracy of 0.5 nm. For measurement of the extinction coeffi-
cients (3), each dye (7e10 mg) was dissolved in 50 mL methanol, the
stock solution was diluted (1:2000) and the absorbance (A) was
measured in a 5-cm standard quartz cell. The extinction coefficients
were calculated according to the LamberteBeer’s law. The 3 of each
dye was independently measured several times and the average
value was taken. The reproducibility for determining the extinction
coefficients was about 1000 Mꢁ1 cmꢁ1
.
Emission spectra and quantum yields (FF) were taken on a Varian
Cary Eclipse spectrofluorometer. The spectra were corrected. Emis-
sion maxima were obtained with an accuracy of 0.5 nm. For the
determination of the quantum yields of dyes 1a and 3ae3c, the
integrated relative intensities of the dyes were measured against Cy5
(GE Healthcare) in water as the reference. To avoid aggregation the
absorbances of the bis-styryl dyes solutions at the excitation wave-
length (610 nm) were between 0.12 and 0.18 measured in a 5-cm cell.
The emission spectra of the solutions were recorded and the absolute
quantum yields (FF) were determined as described in [29] using Cy5
(
FF ¼ 27% [30]) as the reference dye for bis-styryl dyes. Rhodamine B
(FF ¼ 97% in ethanol [31]) was used as the reference dye to determine
the quantum yields of trimethines 4ae4c. The quantum yield of each
dye was independently measured 3e4 times and the average value
was calculated. The reproducibility was no worse than 5%.
This paper is aimed at describing the synthesis of a series of bis-
styryl dyes 3ae3c (Scheme 3) containing a benzodipyrroleninium
moiety as the central bridging group and further to investigate the
molecular structures and spectral properties of these dyes and to
compare them to the “monomeric” styryls 4ae4c and bis-trime-
thines 1ae1e (Scheme 1).
Quantum chemical semi-empirical simulations were carried out
by the PM3 method using standard parameterization [32,33]. Full
geometry optimization of the ground state was undertaken. PPP CI
calculations [34,35] were done for the PM3 optimized geometries
using the following parameters: bCN ¼ ꢁ1.9 eV, bCC ¼ ꢁ2.1 eV,
EN ¼ ꢁ21.22 eV, gNN ¼ 12.98 eV [36].
2-(1,1,3-trimethyl-2,3-dihydro-1H-2-indenyliden) acetaldehyde
(Fisher’s aldehyde) and 4-dimethylaminobenzaldehyde were purchased
from SigmaeAldrich, 3-(4-dimethylaminophenyl) acrylaldehyde
was from Merck. 4-(3,5-diphenyl-4,5-dihydro-1H-1-pyrazolyl)
benzaldehyde was synthesized according to [37]. 2-(3-1,3,3,5,7,7-
hexamethyl-6-[3-(1,3,3-trimethyl-3H-2-indoliumyl)-2-propenylid-
ene]-1,2,3,5,6,7-hexahydropyrrolo[2,3-f]indol-2-yliden-1-propenyl)-1,
3,3-trimethyl-3H-indolium (1a) was synthesized according to [18].
Other chemicals, reagents and solvents were from Merck.
2. Experimental
2.1. General
Chemical reactions were monitored by TLC (Silica Gel 60 F254
TLC Plates, Merck). 1H NMR spectra were measured on a Varian
Mercury-VX-200 (200 MHz) spectrometer in DMSO-d6 using TMS as
an internal standard.
2.2. Synthesis
2.2.1. 1,2,3,3,5,6,7,7-Octamethyl-3,7-dihydropyrrolo[2,3-f]
indolediium diiodide (5) [38]
0.45 g of benzodipyrrolenine [38] was refluxed in large excess of
methyl iodide for 7 h. The reaction mixture was diluted with ether;
Scheme 2.

I.A. Fedyunyayeva et al. / Dyes and Pigments 90 (2011) 201e210
203
the obtained precipitate was filtered off, washed with ether and
dissolved in chloroform. The salt 5 was filtered off and washed with
chloroform. Yield: 0.45 g (47%). 1H NMR (200 MHz, DMSO-d6,
J ¼ 9 Hz, arpm H), 7.83e7.62 (4H, m, indolenine), 7.25 (1H, d,
J ¼ 15.7 Hz, ethylene), 6.87 (2H, d, J ¼ 8.5 Hz, arpm H), 3.96 (3H, s,
NþCH3), 3.19 (6H, s, N(CH3)2), 1.74 (6H, s, C(CH3)2). Anal. Calcd. for
C21H25N2I: S, 58.29; H, 5.78; N, 6.48%. Found: S, 58.17; H, 5.62; N,
6.53%.
ppm):
d 8.53 (2H, s, arom H), 4.01 (6H, s, NeCH3), 2.82 (6H, s, 2,2-
CH3), 1.59 (12H, s, C(CH3)2). Anal. Calcd. for C18H26N2I2: S, 41.24; H,
4.95; N, 5.34%. Found: S, 41.64; H, 5.07; N, 5.74%.
2.2.5.2. 2-(E)-2-[4-(3,5-diphenyl-4,5-dihydro-1H-1-pyrazolyl)
2.2.2. 2,6-Di[2-(4-dimethylaminophenyl)-1-ethenyl]-1,3,3,5,7,7-
hexamethyl-3,7-dihydropyrrolo[2,3-f]indolediium diiodide (3a)
phenyl]-1-ethenyl-1,3,3-trimethyl-3H-indolium iodide (4b). Yield
80%; mp. 245 ꢃC 1H NMR (200 MHz, DMSO-d6, ppm):
d 8.29 (12H, d,
A
mixture of 100 mg (0.2 mmol) of benzodipyrrolenine
J ¼ 15.4 Hz, ethylene), 8.04 (2H, d, J ¼ 8.1 Hz, arpm H), 7.9e7.2 (15H,
m, arom. H and 1H ethylene), 7.17 (2H, d, J ¼ 8.4 Hz, arpm H),
5.92e5.80 (2H, m, pyrazoline ring), 4.03(1H, d, pyrazoline ring),
3.96(3H, s, NþCH3), 1.75(6H, s, C(CH3)2). Anal. Calcd. for C34H32N3I:
S, 66.94; H, 5.25; N, 6.89%. Found: S, 66.90; H, 5.34; N, 6.96%.
quaternary salt 5 and 60 mg (0.42 mmol) of 4-dimethylamino-
benzaldehyde in 5 mL of acetic anhydride was refluxed for 40 min.
After cooling the reaction mixture was diluted with ether, the
precipitate was filtered, washed with ether and column purified
(Silica gel 60, 20% methanolechloroform) to yield 70 mg (47%) of
3a; mp. 263e265 ꢃC 1H NMR (200 MHz, DMSO-d6, ppm):
d
8.34
2.2.5.3. 2-[(1E,3E)-4-(4-dimethylaminophenyl)-1,3-butadienyl]-
(2H, d, J ¼ 15.5 Hz, ethylene), 8.21 (2H, s, dipyrrolenine arpm H),
8.09 (4H, d, J ¼ 8.9 Hz, arpm H), 7.25 (2H, d, J ¼ 15.4 Hz, ethylene),
6.91 (4H, d, J ¼ 8.9 Hz, arpm H), 4.01 (6H, s, NþCH3), 3.19 (12H, s, N
(CH3)2), 1.81 (12H, s, C(CH3)2). Anal. Calcd. for C36H44N4I2: S, 54.92;
H, 5.59; N, 7.12%. Found: S, 54.85; H, 5.70; N, 6.91%.
1,3,3-trimethyl-3H-indolium iodide (4c). Yield 75%; mp. 240 ꢃC 1H
NMR (200 MHz, DMSO-d6, ppm): d 8.31 (1H, t,15 Hz, CH), 7.85e7.45
(7H, m, indolenine, arom., ethylene), 7.29 (1H, t, J ¼ 14.7 Hz, CH),
6.95 (1H, d, J ¼ 14.7 Hz, CH), 3.89 (3H, s, NþCH3), 3.08 (6H, s, N
(CH3)2), 1.71 (6H, s, C(CH3)2). Anal. Calcd. for C23H27N2I: S, 60.21; H,
5.89; N, 6.11%. Found: S, 60.35; H, 5.71; N, 6.19%.
2.2.3. 2,6-Di-2-[4-(3,5-diphenyl-4,5-dihydro-1H-1-pyrazolyl)
phenyl]-1-ethenyl-1,3,3,5,7,7-hexamethyl-3,7-dihydropyrrolo[2,3-f]
indolediium diiodide (3b)
3. Results and discussion
A
mixture of 100 mg (0.2 mmol) of benzodipyrrolenine
3.1. Synthesis
quaternary salt 5 and 140 mg (0.4 mmol) of 4-(3,5-diphenyl-4,5-
dihydro-1H-1-pyrazolyl)benzaldehyde [37] in 5 mL of acetic anhy-
dride was refluxed for 30 min. After cooling the reaction mixture
was diluted with ether, filtered, washed with ether and column
purified (Silica gel 60, 10% methanoldchloroform) to give 70 mg
(45%) of 3b; mp. 297e299 ꢃC 1H NMR (200 MHz, DMSO-d6, ppm):
Bis-styryl dyes 3ae3c were synthesized in good yields (45e60%)
via condensation of benzodipyrrolenine di-quaternary salt 5 [38]
with two equivalents of 4-dimethylaminobenzaldehyde, 4-(3,5-
diphenyl-4,5-dihydro-1H-1-pyrazolyl)benzaldehyde [37] and 4-dime-
thylaminocinnamoic aldehyde, respectively, in acetic anhydride
under reflux (Scheme 4). The formation of undesirable mono-
condensation product was avoided by addition of an excess of
aldehyde. The “monomeric” styryl dyes 4ae4c were obtained
under the same reaction conditions with quaternized indolenine 6
as the starting material.
d
8.33 (2H, d, J ¼ 15 Hz, ethylene), 8.29 (2H, s, dipyrrolenine arpm
H), 8.09 (4H, d, J ¼ 8.1 Hz, arpm H), 8.2e7.3 (20H, m, arom. H and 2H
ethylene), 7.19 (4H, d, J ¼ 8.3 Hz, arpm H), 5.93e5.85(4H, m, pyr-
azoline ring), 4.03(6H, s, NþCH3 and 2H, pyrazoline ring), 1.80(12H,
s, C(CH3)2). Anal. Calcd. for C62H58N6I2: S, 65.21; H, 5.08; N, 7.36%.
Found: S, 65.32; H, 5.24; N, 6.96%.
3.2. Molecular structures
2.2.4. 2,6-Di[4-(4-dimethylaminophenyl)-1,3-butadienyl]-
1,3,3,5,7,7-hexamethyl-3,7-dihydropyrrolo[2,3-f]indolediium
diiodide (3c)
96 mg (0.18 mmol) of benzodipyrrolenine quaternary salt 5 and
95 mg (0.54 mmol) of 3-(4-dimethylaminophenyl) acrylaldehyde
were dissolved in 5 mL of acetic anhydride and the mixture was
refluxed for 1 h. After cooling, the reaction mixture was diluted
with benzene, filtered, washed with benzene and dried. Yield
90 mg (60%); mp. 320e323 ꢃC 1H NMR (200 MHz, DMSO-d6, ppm):
The structures of bis-styryls 3ae3c and parent styryls 4ae4c
were confirmed by 1H NMR data. Similarly to bis-trimethine dyes
1ae1e [18], the spinespin coupling constant (J) of the methine
hydrogens are in the range of 13.5e15.0 Hz indicating that the
polymethine chain in both series 3ae3c and 4ae4c is in all-trans
conformation. This conformation seems not to depend on the
d
8.34 (2H, t, J ¼ 14.7 Hz, CH), 8.25 (2H, s, dipyrrolenine arom H),
7.78 (2H, d, J ¼ 14.7 Hz, CH), 7.61 (4H, d, J ¼ 8.8 Hz, arom H), 7.33
(2H, t, J ¼ 14.7 Hz, CH), 6.94 (2H, t, J ¼ 15 Hz, CH), 6.85 (4H, d,
J ¼ 8.8 Hz, arom H), 3.91 (6H, s, NþCH3), 3.11 (12H, s, N(CH3)2), 1.78
(12H, s, indolenine C(CH3)2). Anal. Calcd. for C40H48N4I2: S, 57.24; H,
5.72; N, 6.68%. Found: S, 57.38; H, 5.59; N, 6.75%.
2.2.5. General procedure for synthesis of styryl dyes 4ae4c
A mixture of 0.30 mmol of 1,2,3,3-tetramethylindolenium iodide
6 and 0.32 mmol of corresponding aldehyde was refluxed in 10 mL
of acetic anhydride for 1 h. The reaction mixture was poured into
water and the solvent was removed under reduced pressure by
a rotary evaporator. The residue was re-crystallized from ethanol.
2.2.5.1. 2-(4-Dimethylaminophenyl)-1-ethenyl]-1,3,3-trimethyl-3H-
indolium iodide (4a). Yield 82%; mp.175e177 ꢃC 1H NMR (200 MHz,
DMSO-d6, ppm):
d
8.31 (1H, d, J ¼ 16 Hz, ethylene), 8.07 (2H, d,
Scheme 4.

204
I.A. Fedyunyayeva et al. / Dyes and Pigments 90 (2011) 201e210
Table 1
terminal end-groups (1a, 3a, 3b, 4a, 4b) or the chain length
(dimethines 3a and 4a or tetramethines 3c and 4c). The confor-
mational analysis using the quantum chemical semi-empirical
method PM3 [32,33] indicates a global minimum of the potential
energy just for this all-trans form. In this conformation the angle
between principle axes of the two uniform parent chromophores in
bis-styryl molecules is close to 180ꢃ. The conformation of the
terminal dimethylamino groups in dyes 3a, 3c, 4a and 4c ensures
Rotational energy barriers (D
Hs) in polymethines (4ae4c, 7) and bis-polymethines
(1a, 3ae3c).
Dye
D
Hs
(
f1)
D
Hs
(f2
)
D
Hs
(
f3)
D
Hs
(f4
)
D
Hs
(
f5)
D (f6)
Hs
[kcal/mol] [kcal/mol] [kcal/mol] [kcal/mol] [kcal/mol] [kcal/mol]
1a
3a
3b
3c
4a
4b
4c
7
18.33
13.04
11.50
10.85
5.57
7.45
7.32
13.31
7.82
>35
>35
7.87
>35
>35
15.60
13.55
e
19.23
e
9.00
13.92
11.12
8.31
7.14
7.15
e
e
9.37
9.01
6.49
5.62
5.54
4.28
e
e
e
10.25
e
9.11
e
their strong conjugation with the main chromophore
peelectron
e
e
system. The 1,3-diphenylpyrazolinyl moiety in dyes 3b and 4b has
a planar form and is also well-conjugated with the polymethine
chromophore. The 5-phenyl ring is out of plane with the pyrazoline
ring and therefore not in conjugation with the main chromophore
system.
6.73
e
15.55
13.55
5.30
13.31
Importantly, quantum chemical simulations show that the
minimal energy barrier of the internal rotation in bis-styryl mole-
cules 3ae3c is higher compared to that of the parent styryl dyes
4ae4c by a factor of about 1.5e1.7 (Fig. 1, Table 1). Thus the minimal
rotational barrier for the “monomeric” styryl dye 4a corresponding
to the rotation around the single bond connected to indolenine
moiety (f1) is 5.57 kcal/mol while for the bis-styryl 3a this barrier is
9.37 kcal/mol and corresponds to the rotation around the single
bonds connected to the dimethylamino group (f6). The rotational
less than for the parent trimethine dye 7 (13.31 kcal/mol and
13.55 kcal/mol) (Table 1). Contrary to styryl dyes, the “dimeriza-
tion” of trimethine 7 to 1a decreases the
DH
s of f2 and f5 by 5.73
and 4.31 kcal/mol but increases the
D
H
s of f1 and f4 by 5.02 and
5.68 kcal/mol, respectively, which does not cause a pronounced
change of the overall conformational flexibility of the molecule.
The rotational barriers decrease in the order of 3a > 3b > 3c,
which results in increase of the conformational flexibility of the
molecules.
The PM3-calculated energy barriers of the internal rotation and
the electron density distributions evidence that both bis-styryl and
parent styryl dyes exist mostly in the canonical (contributing) form
A with a positive charge on the indolenine or benzodipyrroleni-
nium moieties (Fig. 1). Because of this the delocalized positive
charge in bis-styryl molecules 3ae3c is shifted from the terminal
nitrogens (qTerm) to the central benzodipyrrolenine nitrogens
energy for the benzodipyrroneninium (
minophenyl ( f5)) moieties in the bis-styryl 3a are also about
Hs
twice as high as compared to f5) and f6) of the parent
Hs Hs
styryl 4a. Thus, the “dimerization” of styryl 4a to 3a causes
a pronounced increase of the three rotational barriers f1),
Hs
Hs Hs
f5 and f6 by 7.47 kcal/mol, 6.78 kcal/mol and
3.75 kcal/mol, respectively. The rotation around f2 in both 3a and
4a is not probable due to the high
Hs
A similar increase in
s is also observed for dyes 3b, 4b and 3c,
D (f1)) and dimethyla-
Hs
D
(
D
(
D
(
D
(
D
(
)
D
(
)
D
.
(qCentr), which results in a positive
the same time opposite effect (negative
D
q ¼ qCentr ꢁ qTerm (Table 2). At
D
H
D
q) was observed for bis-
4c (Table 1). The minimal rotational barrier in these molecules
corresponds to the rotation around pyrazoline moiety and dime-
thylamino group, respectively.
trimethine dyes such as 1a, where a positive charge predominated
on the terminal heterocyclic end-groups [18].
Remarkably, bis-styryl molecules have lower polarity compared
Based on this result one can conclude that the conjugation of
two parent styryl chromophores to a bis-styryl system results in an
increase of the conformational rigidity: the molecule containing
two conjugated chromophores becomes less flexible and statisti-
cally more planar.
Importantly such a correlation is not observed in the series of
bis-trimethine dyes 1ae1e. The minimal rotational energy barriers
in the bis-trimethine 1a (7.82 kcal/mol and 9.00 kcal/mol) are even
to parent styryls: the
lower than those for 4ae4c. This should cause less pronounced
solvation effects in bis-styryls compared to parent styryls.
The quantum chemical simulations of the electron density
distribution are in good agreement with 1H NMR data: similar to
bis-trimethine dyes [18] we found clear correlations between the
Dq values in 3ae3c are about 4e5.5 times
hydrogen chemical shifts (
calculated charges (q) on nitrogens of central and terminal moieties
d) for the bis-styryl molecules and the
Fig. 1. Resonance forms and internal rotation angles in polymethine and bis-polymethine molecules.

I.A. Fedyunyayeva et al. / Dyes and Pigments 90 (2011) 201e210
205
Table 2
The PM3-calculated electron charges on nitrogen atoms of the central (qCentr) and terminal (qTerm) moieties of dyes 1a, 3ae3c, 4ae4c, 7 and the experimental chemical shifts of
methyl hydrogen atoms at nitrogens of the central (dCentr) and terminal (dTerm) moieties.
Molecule
Dye
qTerm
qCentr
D
q
dTerm [ppm]
dCentr [ppm]
Dd [ppm]
ꢁ0.04
1a
þ0.392
þ0.226
ꢁ0.166
þ0.076
þ0.050
3.69
3.65
N
N
N
N
N
2 TsO
2I
3a
3b
þ0.187
þ0.198
þ0.263
þ0.248
3.19
4.01
4.03
þ0.82
N
N
N
e
e
Ph
Ph
N
N
N
N
N
N
N
N
Ph
N
Ph
N
2I
3c
7
þ0.173
þ0.303
þ0.090
þ0.235
þ0.303
þ0.390
þ0.062
0.0
3.10
3.90
þ0.80
2I
N
N
N
I
4a
þ0.300
3.19
3.96
þ0.77
N
I
4b
4c
þ0.104
þ0.065
þ0.377
þ0.380
þ0.273
þ0.315
e
3.96
3.89
e
Ph
N
N
N
Ph
N
I
3.08
þ0.81
N
I
as well as between Dd and
D
q values. The proton signals of the
methanol are given in Table 3 while the representative absorption
and emission spectra are shown in Figs. 2 and 3. For an analysis of
the correlations between the spectral properties and the molecular
structures we simulated the spatial and electronic structures of
these molecules and calculated the electron transitions under
excitation. The simulations were carried out using semi-empirical
quantum chemical methods PM3 CI [32,33] and PPP CI [34e36].
The absorption spectral shapes of bis-styryls were found to be in
good agreement with Kiprianov’s theory [17]: because of the linear
combination of two parent chromophores only one intense, long-
wavelength absorption band, red-shifted by 90e110 nm compared
to that of parent “monomeric” styryl dyes is observed. Quantum
chemical simulations evidence that, similarly to the bis-trimethines
1ae1e [18] this absorption is based on an S0 / S1* electron tran-
sition corresponding to the superposition of two configurations:
a larger HOMO / LUMO contribution and a smaller (HOMO ꢁ 1)
/ (LUMO þ 1) contribution (Fig. 4). The transition S0 / S1* is
localized mostly within the two polymethine chains, the nitrogen
atoms of dimethylamino groups (dyes 3a, 3c) or pyrazoline nitro-
gens (dye 3b) and the central benzodipyrroleninium nitrogens. The
structures of boundary orbitals, localization and polarization of the
methyl groups at the central benzodipyrrolene nitrogens (dCentr) of
dye 1a are observed at higher magnetic fields as compared to those
of the methyl hydrogens at the terminal indolenine nitrogens
(dTerm), showing a negative Dd while positive Dd’s were found for
bis-styryl dyes 3a and 3c (Table 2). The aromatic hydrogen signals
of the benzodipyrroleninium (3ae3c) can be found at lower
magnetic fields (8.21, 8.29 and 8.24 ppm, respectively) showing
higher positive charge on the central moiety as compared to the
biscyanine 1a (d 7.85 ppm). The different pattern of the charge
distribution in the bis-styryl molecules 3ae3c compared to bis-
cyanine 1a is probably due to the different charge alteration on the
polymethine chains: bis-styryls have the even number of methine
groups while biscyanines have an odd number. The nature of the
different charge distribution in conventional (“monomeric”)
cyanine and styryl dyes was discussed elsewhere [24e26].
3.3. Spectral properties
The spectral characteristics of the investigated bis-styryl dyes
3ae3c and parent styryls 4ae4c measured in chloroform and

206
I.A. Fedyunyayeva et al. / Dyes and Pigments 90 (2011) 201e210
Table 3
Spectral characteristics of bis-polymethine and parent polymethine dyes.
Dye
Solvent
lmax (Abs) [nm]
3
[Mꢁ1 cmꢁ1
]
lmax (Em) [nm]
FF [%]
Dnst[cmꢁ1
]
1a
3a
3b
CHCl3
MeOH
658
644
677
664
18.8
10.4
430
460
N
N
250,000
133,000
80,000
N
N
N
2 TsO
2I
CHCl3
MeOH
655
643
683
680
9.2
1.2
630
830
N
N
N
CHCl3
MeOH
697
675
736
737
10.0
1.7
760
1250
Ph
Ph
N
N
N
N
N
N
N
N
Ph
N
Ph
N
2I
3c
7
CHCl3
MeOH
757
720
812
810
2.4
1.0
900
1540
100,000
134,000
2I
CHCl3
MeOH
550
545
570
568
4.0
3.0
640
740
N
N
N
I
4a
CHCl3
MeOH
563
545
592
590
1.7
0.9
870
1400
83,200
N
I
4b
4c
CHCl3
MeOH
600
568
655
659
7.4
3.0
1400
2530
Ph
N
N
73,000
70,700
N
Ph
N
I
CHCl3
MeOH
645
605
697
697
5.5
2.5
1160
2130
N
I
Fig. 2. Normalized absorption and emission spectra of bis-styryl dyes 3a (solid line),
3b (dash line) and 3c (short dash line) in chloroform.
Fig. 3. Absorption and emission spectra of parent styryl dyes 4a (solid line), 4b (dash
line) and 4c (short dash line) in chloroform.

I.A. Fedyunyayeva et al. / Dyes and Pigments 90 (2011) 201e210
207
electron transition S0 / S1* in the biscyanines 3ae3c are also very
similar to those in bis-trimethines 1ae1e and the parent styryl
molecules 4ae4c (Fig. 5).
2550 cmꢁ1 and 2600 cmꢁ1, respectively, for the emission, which
can be attributed to the above discussed, more rigid structure (
Hs
values in Table 1) and lower polarity ( q values in Table 2) of bis-
styryl molecules compared to “monomeric” styryls.
The extinction coefficients ( ) of bis-styryl dyes are always higher
compared to parent styryls (Table 2), however, these values are
lower than those for bis-trimethine cyanines such as 7 described in
[18]. The parent styryls also have lower extinction coefficients
compared to the “monomeric” trimethines. An extension of the
conjugated chain in both bis-styryl 3a and “monomeric” styryl 4a
by a vinylene group to form vinylogs 3c and 4c decreases the
extinction coefficients. The substitution of the dimethylamino
group in 3a and 3c with a pyrazoline moiety (dyes 3b and 4b) also
D
D
The long-wavelength absorption and emission maxima of bis-
polymethine dyes 1a and 3ae3c are between 643e757 nm and
664e812 nm, respectively. The terminal end-groups strongly
influence the S0 / S1* transition energy and the electron distri-
bution in both the ground and the first excited states. In general,
these maxima show a pronounced red-shift in the order of
1a z 3a < 3b < 3c, which is in good agreement with the quantum
chemical simulations showing a decrease of the S0 / S1* energy in
the same order. The only exception was found for the absorption
maxima of 1a and 3a that were almost identical. An extension of
the polymethine chains in bis-styryl dye 3a by one vinylene group
to form dye 3c causes a significant red-shift of the absorption
3
reduces the
3 values. Such effect of initially increasing and later
decreasing extinction coefficients upon extension of the conjugated
chain in polymethine dyes was already discussed in the example of
monomeric polymethines elsewhere [13]. It was found to be con-
nected with an increase of the conformational flexibility of their
maxima in chloroform and methanol by 1970 cmꢁ1 and 1780 cmꢁ1
,
respectively, and an even more pronounced red-shift of the emis-
sion maxima (2220 cmꢁ1 and 2300 cmꢁ1). Remarkably, in the
parent styryls 4a and 4c these shifts are even higher: 2260 cmꢁ1 in
chloroform and 1820 cmꢁ1 in methanol for the absorption and
molecules, which is also in good agreement with our data (
values in Table 1).
D
Hs
The Stokes’ shifts (DnSt) noticeably increase in the same order as
the red-shifts of the absorption and emission bands
(1a < 3a < 3b < 3c) and this increase becomes even more
pronounced in polar solvents (e.g. methanol compared to chloro-
form). Typically the Stokes’ shifts in methanol are higher compared
to those in chloroform due to a substantial blue-shift of the
absorption maxima. The increase of the Stokes’ shift was found to
correlate well with the absolute difference between the calculated
electron charges on the nitrogens of the central and terminal
moieties (D
q) but also with the 1H NMR Dd values in Table 2. Such
a correlation evidences that the Stokes’ energy losses originate not
only from intramolecular conformational motion but are also due
to intermolecular rearrangements between the solvated dye and
solvent molecules in the excited state.
In general, an increase of the Stokes’ shift is observed when the
dye molecule polarity and/or solvent polarity increase. Thus, the
centrosymmetrical trimethine 7 with zero dipole moment and
D
q ¼ 0 exhibits a minimal DnSt (Table 3), but the DnSt increases
simultaneously with the dye polarity for styryls 4ae4c. Remark-
ably, bis-polymethines 1a and 3ae3c, which are centrosymmetrical
and virtually have zero dipole moments also demonstrate a strong
correlation of the Stokes’ shifts with the
Dq and Dd values. This
correlation can be explained by the fact that the bis-styryl mole-
cules consist of two uniformly unsymmetrical chromophores that
behave as relatively independent polar moieties (Fig. 5) and
therefore the Stokes’ energy losses correlate with the Dq of each of
these chomophore systems. Nevertheless, the Stokes’ shifts of the
bis-polymethines are always smaller compared to the parent
“monomeric” dyes, which is due to their more rigid structure
Fig. 4. Structure of the boundary orbitals in molecules 3a, 3b and 3c obtained by PM3
CI method.
Fig. 5. Localization and polarization of electron transition S0 / S1* in bis-styryl 3a and
parent styryl 4a (PPP CI method).

208
I.A. Fedyunyayeva et al. / Dyes and Pigments 90 (2011) 201e210
Table 4
The smoothened alteration of the electron density (electronic
charges and bond orders) in the excited state compared to the
ground state causes a negative solvatochromic but negligible sol-
vatofluorochromic effects. The spectral maxima of bis-styryl dyes
3ae3c measured in methanol are blue-shifted as compared to
chloroform by 11e26 nm in the absorption but this shift does not
exceed 3 nm in the emission (Table 3).
Similar correlations between the electron density distribution in
the ground and excited states and the solvation effects in the
absorption and emission spectra were also observed for bis-tri-
methines 1ae1e [18].
The vibrational structures of the absorption and emission bands
of cyanine dyes is known to smoothen with increased conjugated
chain length (trimethines < pentamethines < heptamethines) [13]
while the spectra of styryl dyes are always smoother compared to
cyanines, which is mostly due to low energy barriers for the
internal rotations of the polymethine chain and/or the terminal
end-groups [14e16]. Surprisingly, the smoother vibrational struc-
tures in the spectra of parent styryl dyes 4ae4c (Fig. 3) is easier to
identify upon an increase of the conjugation chain length in bis-
styryl dyes 3ae3c (Fig. 2). This effect can be attributed to the above-
mentioned increase of the conformational rigidity (Table 1) and
decrease of polarity (Table 2) of bis-styryl molecules 3ae3c as
compared to “monomeric” styryls 4ae4c. It is worth mentioning
that this effect is not observed for the bis-trimethine 1a and the
parent trimethine 7: both dyes exhibit well-resolved vibrational
structures, which can be explained by the higher rotational ener-
gies of styryls 1a and 7 compared to bis-styryls 3ae3c.
The electron charges on the nitrogen atoms of the central (qCentr) and terminal
(qTerm) moieties of dyes 1a and 3ae3c in the ground and first excited states (PPP CI
method).
Molecule
Ground state
First excited state
qCentr
qTerm
D
q
q*Centr
q*Term
Dq*
1a
3a
3b
3c
þ0.180
þ0.466
þ0.482
þ0.451
þ0.514
þ0.348
þ0.180
þ0.322
þ0.334
ꢁ0.118
ꢁ0.302
ꢁ0.129
þ0.205
þ0.488
þ0.480
þ0.477
þ0.500
þ0.414
þ0.232
þ0.372
þ0.295
ꢁ0.074
ꢁ0.248
ꢁ0.105
(higher
between DnSt
1ae1e [18].
DH
s) and lower polarity (lower
D
q). A similar correlation
, Dq and Dd was also reported for bis-trimethines
Solvation effects affecting the absorption and emission maxima,
Stokes’ shifts, vibrational structures of the spectral bands, and the
quantum yields can be understood from an analysis of the electron
density distribution in the ground and first excited states. The PM3-
and PPP-calculated positive charge (q) in the ground state of bis-
styryl dyes 3ae3c is localized mostly on the nitrogen atoms of the
central benzodipyrroleninium moiety (Tables 2 and 4), which is in
good agreement with the 1H NMR data in Table 2.
As can be seen from Table 4 under excitation the q values in
3ae3c increase (negative electronic charges decrease) at both the
central and terminal nitrogen atoms, however, the increase is more
pronounced for the terminal nitrogens (q*Term ꢁ qTerm) than for the
central nitrogens (q*Centr ꢁ qCentr). As a result, the difference
between the charges on the terminal and central nitrogens in the
The vibrational structures of the absorption and emission bands
of bis-styryl dyes become more pronounced in the same order as
the increase of the rotational barriers
vibrational structures, which are very well identified in chloroform,
becomes less pronounced but still well-defined in higher polar
solvents such as methanol (Fig. 6).
The quantum yield (FF) in both low-polar (chloroform) and polar
(methanol) solvents increases in the order: 3c < 3a < 3b < 1a.
excited state (Dq*) is less compared to that in the ground state (Dq),
and is responsible for the less pronounced alteration of the elec-
tronic charges in the excited state. The same pattern of the charge
redistribution under excitation was observed for bis-trimethines
1ae1e with the exception that positive charge predominates on the
DH
s: 3c < 3b < 3a. The
terminal nitrogens [18]. The
D
P values (
D
P ¼ P* ꢁ P0) in Table 5
evidence that the bond orders (P) in bis-styryls 3ae3c and bis-
trimethines 1ae1e also become smoother in the excited state (P*):
the bond orders of double bonds reduce under excitation while the
single bonds orders remain almost unchanged. The electron density
redistribution upon excitation in bis-styryls 3ae3c increases the
contribution of the resonance form B in the excited state (Fig. 1).
Typically the
methines. Due to the higher excited state polarity of bis-styryls as
compared to bis-trimethines ( q* in Table 2) the difference in FF is
even more pronounced in a polar solvent.
FF’s of bis-styryl dyes are lower compared to bis-tri-
D
Table 5
The bond orders in molecules 3ae3c in the ground (P0) and first excited (P*) states (PPP CI method).
Bond
1
1
6
7
1
8
9
Ph
11
7
13
6
N
N
10
3
6
3
4
5
2
4
5
7
2
8
N
3
4
5
2
8
N
N
N
11
10
9
11
12
N
9
10
Ph
3a
3c
3b
P0
P*
DP
P0
P*
DP
P0
P*
DP
1e2
2e3
3e4
4e5
5e6
6e7
7e8
8e9
9e10
10e5
8e11
10e11
11e12
12e7
10e13
0.602
0.587
0.684
0.545
0.526
0.754
0.529
0.530
0.751
0.528
0.577
e
0.544
0.582
0.661
0.533
0.514
0.757
0.513
0.515
0.753
0.517
0.593
e
ꢁ0.058
ꢁ0.005
ꢁ0.023
ꢁ0.012
ꢁ0.012
þ0.003
ꢁ0.016
ꢁ0.015
þ0.002
ꢁ0.011
þ0.016
e
0.620
0.564
0.712
0.509
0.552
0.722
0.594
0.595
0.720
0.552
0.351
e
0.560
0.578
0.679
0.510
0.539
0.729
0.575
0.577
0.726
0.540
0.371
e
ꢁ0.060
þ0.014
ꢁ0.033
þ0.001
ꢁ0.013
þ0.007
ꢁ0.019
ꢁ0.018
þ0.006
ꢁ0.012
þ0.020
e
0.582
0.615
0.645
0.612
0.683
0.533
0.533
0.747
0.540
e
0.543
0.608
0.631
0.602
0.664
0.531
0.521
0.752
0.527
e
ꢁ0.039
ꢁ0.006
ꢁ0.015
ꢁ0.010
ꢁ0.019
ꢁ0.002
ꢁ0.012
þ0.005
ꢁ0.013
e
e
e
e
0.542
0.744
0.536
0.554
0.528
0.748
0.525
0.568
ꢁ0.013
þ0.005
ꢁ0.011
þ0.014
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e

I.A. Fedyunyayeva et al. / Dyes and Pigments 90 (2011) 201e210
209
The synthesized bis-styryl dyes absorb at 643e757 nm with
high extinction coefficients (up to 133,000 Mꢁ1 cmꢁ1), have rela-
tively large Stokes’ shifts (up to 55 nm in chloroform and 90 nm in
methanol), and emit at 680e812 nm. Despite their symmetrical
structures, bis-styryls behave similarly to unsymmetrical “mono-
meric” styryl dyes but their absorption and emission maxima are
red-shifted by about 100 nm, which is important for many practical
applications. Comparable to biscyanine dyes they are more sensi-
tive to the solvent polarity showing negative solvatochromism and
reduced fluorescence in polar solvents. These characteristics of bis-
styryl dyes demonstrate their potential as biomedical probes for
proteins, lipids and cells.
Acknowledgement
This work was supported by the National Academy of Sciences
of Ukraine, project No. 0110U000488. The authors would like to
thank Dr. Vladimir Musatov (SSI “Institute for Single Crystals” of the
National Academy of Sciences of Ukraine) for the 1H NMR
measurements.
Fig. 6. Absorption and emission spectra of bis-styryl dyes 3a in chloroform (solid line)
and methanol (dash line).
The quantum yield of bis-styryl dye 3a is higher compared to
parent styryl 4a (Table 3) by factor of 5.4 in chloroform and 1.3 in
methanol. The more pronounced increase of FF in chloroform
compared to methanol upon dimerization of 4a to 3a evidences
that the increase is mostly due to an increase of conformational
rigidity but not a decrease of molecular polarity. A similar increase
of FF (by factor of w5) is observed for the bis-trimethine 1a
compared to parent trimethine 7. This increase is not so much
attributed to molecular rigidity but a decrease in cis-trans-photo-
isomerization tendency, which was already described for the ben-
zodithiazole-based bis-trimethines [39,40]. The FF of the bis-styryl
3b in chloroform is also higher than for the parent dye 4b but it
substantially decreases in methanol.
An extension of the polymethine chain in monomeric styryl 4a
by a vinylene group to form 4c causes a substantial increase in FF
(Table 3). At the same time the longest-wavelength bis-styryl 3c
exhibits a smaller quantum yields compared to both the parent
styryl 4c and the bis-styryl 3a. More likely this is due to an
increased radiationless transition caused by a vibrationally induced
internal conversion, which is known to substantially increase in the
spectral range above 700 nm [13].
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