Benzodipyrrolenine-based biscyanine dyes: Synthesis, molecular structure and spectroscopic characterization

Article abstract of DOI:10.1016/j.dyepig.2009.09.007

A novel method of synthesizing long-wavelength absorbing and emitting bis-trimethine dyes that consists of condensing benzodipyrrolenine dialdehyde with quaternized heterocyclic CH-acidic compounds was used to prepare a series of biscyanines. The new meth

Full text of DOI:10.1016/j.dyepig.2009.09.007

Dyes and Pigments 85 (2010) 7–15
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Benzodipyrrolenine-based biscyanine dyes: Synthesis, molecular structure
and spectroscopic characterization
Oleksiy P. Klochko a, Iryna A. Fedyunyayeva a, Sania U. Khabuseva a, Olga M. Semenova a,
,b,
Ewald A. Terpetschnig 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:
A novel method of synthesizing long-wavelength absorbing and emitting bis-trimethine dyes that
consists of condensing benzodipyrrolenine dialdehyde with quaternized heterocyclic CH-acidic
compounds was used to prepare a series of biscyanines. The new method is more convenient than its
conventional counterepart, which relies upon condensation of a quaternized benzodipyrrolenine with
Fisher’s aldehyde, for the synthesis of a symmetrically substituted dyes that contain various heterocyclic
end-groups. Investigations of the spectral and luminescent properties of the dyes in solution revealed
that the absorption (596–717 nm) and emission (629–773 nm) maxima of the biscyanines were red-
shifted by w100 nm compared to the parent ‘‘monomeric’’ cyanine that contained only one chromo-
Received 6 May 2009
Received in revised form
17 September 2009
Accepted 24 September 2009
Available online 2 October 2009
Keywords:
Benzodipyrrolenine
Biscyanine dyes
Synthesis
Spectral properties
Fluorescence
phoric polymethine system. The prepared dyes have high extinction coefficients (ꢀ251,000 Mꢁ1 cmꢁ1
)
and quantum yield (ꢀ28%). Substitution of both terminal benzoxazole moieties with indolenine, ben-
zothiazole, 2- and 4-quinoline imparted a red-shift in the absorption and emission maxima but lowered
quantum yield.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
of the absorption bands, electron transition energy, extinction
coefficient and quantum yield. These investigations originate from
Cyanine dyes enjoy widespread use as fluorescent probes and
labels in biomedical assay [1–4], sensitive materials for optical
storage devices [5,6], photorefractive materials [7], spectral sensi-
tizers for silver halide photography and other inorganic, large band-
gap, semiconductor materials [8,9], passive photo-shutters and
laser dyes [10,11]. For most of these applications, long-wavelength
absorbing dyes having high extinction coefficients and high fluo-
rescence quantum yields are required. One of the conventional
methods used to achieve these spectral characteristics consists of
preparing the so-called biscyanine dyes, which are molecules
comprising two polymethine-type chromophoric systems.
Biscyanine molecules are of significant theoretical interest as
the model bifluorophoric system permits investigation of the
correlation between the mutual orientation of two polymethine
chromophores and their spectral properties such as the shape of
absorption and emission spectra, the number and relative intensity
the classic work of Kiprianov and co-workers [12] and other
publications [13,14]. Such publications reveal that the interaction
between two uniform, partially conjugated chromophores results
in the splitting of the long-wavelength absorption band of the
parent ‘‘monomeric’’ dye into two bands. One of these new bands is
red-shifted whilst the other is blue-shifted compared to the orig-
inal long-wavelength band of the parent dye. The magnitude of the
splitting depends on the symmetry of the interacting parent
chromophores insofar as the higher the symmetry the more
pronounced the splitting. The relative intensities of the split bands
depend on the angle between the interacting chromophores: the
larger the angle the greater is the intensity of the long-wavelength
band and the lower the intensity of the short-wavelength band.
When the angle is w180ꢂ, only the long-wavelength band is
observed. Thus, biscyanine dyes with bridged centrosymmetrical
benzodioxazole 1 [15], benzodithiazole 2 [16], benzodiimidazole 3
[17], and benzodipyrrolenine 4a, 5, 6 [18] moieties (Scheme 1) have
only one intense long-wavelength band. At the same time two
bands appear in the absorption spectra of isomeric benzodipyrro-
lenine-based biscyanines 7 and 8 and these bands have lower
intensity than those of the centrosymmetrical benzodipyrrolenine
4a [18]. Despite the favourable absorption characteristics of 4a, i.e.
* Corresponding author at: 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/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.09.007

8
O.P. Klochko et al. / Dyes and Pigments 85 (2010) 7–15
Scheme 1.
longer-wavelength absorption and higher extinction coefficient,
the fluorescent properties of this dye have not been studied.
Derivatized 4a containing other terminal heterocyclic moieties
such as benzoxazole, benzothiazole and quinoline have not been
synthesized and investigated.
This work concerns the synthesis of a series of symmetrical
biscyanine dyes based on centrosymmetrical benzodipyrrolenine
and an investigation of the molecular structure, absorption and
fluorescent properties of the dyes.
as the reference dye. The quantum yields of each sample were
independently measured 3–4 times and the average value was
calculated. The reproducibility was no worse than 5%.
FAB-MS experiments were performed using magnetic sector
mass spectrometer MI-1201E (‘‘SELMI’’ Works, Sumy, Ukraine)
equipped with commercial FAB units. Argon was used as a bom-
barding gas, the energy of the primary beam was 4.5 keV. All the
compounds except 9 were injected using glycerol as the matrix
while 9 was dissolved in 3-nitrobenzyl alcohol (NBA).
Quantum chemical semi-empirical simulations were done by
the PM3 method using standard parameterization [21,22]. Full
geometry optimization of the ground state was undertaken. PPP CI
calculations [23,24] were carried out for the PM3 optimized
geometries using the following parameters [25]: bCN ¼ ꢁ1.9 eV,
bCC ¼ ꢁ2.1 eV, EN ¼ ꢁ21.22 eV, gNN ¼ 12.98 eV.
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.
Quinaldine and lepidine for synthesis were from Aldrich. Other
chemicals, reagents and solvents were from Merck.
Absorption spectra were taken at room temperature on a Perkin-
Elmer Lambda 35 UV/Vis spectrophotometer. Absorption maxima
were obtained with an accuracy of 0.5 nm. For measurement of the
2.2. Synthesis
2.2.1. 1,3,3,5,7,7-Hexamethyl-2,6-dimethylene-1,2,3,5,6,7-
hexahydropyrrolo[2,3-f]indole (10) [26]
extinction coefficients (3), each dye (7–10 mg) was dissolved in
50 mL of methanol, the stock solution was diluted (1:2000) and the
absorbance (A) was measured in a 5-cm standard quartz cell. The dye
concentrations were in the range of 1.0–3.0 ꢃ 10ꢁ7 M. The extinction
coefficients were calculated according to the Lambert–Beer’s law.
Yield 98%; mp. 205–207 ꢂC. 1H NMR (200 MHz, DMSO-d6, ppm):
d
6.62 (s, 2H), 3.72 (d, J ¼ 3.3 Hz, 4H), 2.96 (s, 6H), 1.27 (s, 12H);
Elemental analysis: calcd (%) for C18H24N2: C, 80.55; H, 9.01; N,
10.44, found: C, 80.83; H, 9.10; N, 10.07.
The
average value was taken. The reproducibility for determining the
extinction coefficients was about 1000 Mꢁ1 cmꢁ1
3 of each dye was independently measured several times and the
2.2.2. 2-{6-(-1-Formylmethylidene)-1,3,3,5,7,7-hexamethyl-
.
1,2,3,5,6,7-hexahydropyrrolo[2,3-f]indol-2-yliden}acetaldehyde (9)
POCl3 (caution: corrosive; toxic; reacts violently with water;
incompatible with many metals, alcohols, amines, phenol, DMSO,
strong bases) (0.21 mL, 2.3 mmol) was added (with care) dropwise
to dry DMF (2 mL) at 10 ꢂC. The mixture was stirred for 1 h and
dimethylene indole 10 (286 mg, 1 mmol) in dry DMF (2 mL) was
added. The mixture was refluxed for 1 h, cooled, poured into
solution of NaOH (2.5 g) in water (15 mL) and stirred for 30 min at
room temperature. The precipitate was filtered, washed several
times with water and air-dried to give the product as a yellow solid.
Yield: 0.28 g (86%); mp. > 300 ꢂC. 1H NMR (200 MHz, DMSO-d6,
Emission spectra and quantum yields were measured in chlo-
roform and methanol at room temperature in a standard 1-cm
quartz cell on a Varian Cary Eclipse spectrofluorometer. Concen-
trations were adjusted to be 1.0 to 3.0 ꢃ 10ꢁ7 M. The spectra were
corrected. For the determination of the quantum yields, the inte-
grated relative intensities of the dyes were measured against Cy5 in
water as the reference. Optical density of the dye solutions at the
excitation wavelength (610 nm) was 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 (
determined as described in [19] using Cy5 (Scheme 1,
4
) of the dyes were
¼ 27%) [20]
4
ppm):
d
9.84 (d, J ¼ 8.8 Hz, 2H), 7.31 (s, 2H), 5.24 (d, J ¼ 8.8 Hz, 2H),

O.P. Klochko et al. / Dyes and Pigments 85 (2010) 7–15
9
3.26 (s, 6H), 1.61 (s, 12H); FAB-MS (NBA) m/z: 324 (M)þ, 325 (MH)þ;
nmax (KBr) 1604.8 cmꢁ1 (C]O); Elemental analysis: calcd (%) for
C20H24N2O2: C, 74.04; H, 7.46; N, 8.64, found: C, 73.87; H, 7.54; N,
8.50.
2.2.5.1. Dye 4a. Yield 40 mg, 82%; mp. > 300 ꢂC (dec). 1H NMR
(200 MHz, DMSO-d6, ppm):
d
8.31 (t, J ¼ 13.4 Hz, 2H), 7.85 (s, 2H),
7.65 (d, 2H), 7.53–7.39 (m, 8H), 7.38–7.25 (m, 2H), 7.11 (d, J ¼ 7.8 Hz,
4H), 6.45 (d, J ¼ 13.4 Hz, 4H), 3.69 (s, 6H), 3.65 (s, 6H), 2.28 (s, 6H),
1.73 (s, 12H) 1.7 (s, 12H); FAB-MS (Gl) m/z 621 (Cat-CH3)þ, 635 (Cat-
H)þ, 636 (Catþþþeꢁ)þ, 807 (Cat þ An)þ; Elemental analysis: calcd
(%) for C58H66N4O6S2: C, 71.13; H, 6.79; N, 5.72; S, 6.55; found: C,
71.33; H, 6.73; N, 5.69; S, 6.40.
2.2.3. General procedure to synthesize quaternary salts (12a–12e)
Corresponding nitrogen containing heterocyclic compound
(1 equivalent) was melted with methyl 4-methyl-1-benzenesulfo-
nate (1.3 equivalents) at 130 ꢂC for 3 h. The mixture was cooled to
room temperature, treated with isopropanol, filtered and dried.
2.2.5.2. Dye 4b. Yield 23 mg, 50%; mp. > 300 ꢂC (dec). 1H NMR
(200 MHz, DMSO-d6, ppm):
d
8.29 (t, J ¼ 13.5 Hz, 2H), 7.86 (d,
2.2.3.1. 1,2,3,3-Tetramethyl-3H-indolium 4-methyl-1-benzenesulfo-
J ¼ 7.0 Hz, 2H), 7.77 (d, J ¼ 7.0 Hz, 2H), 7.70 (s, 2H), 7.60–7.49 (m,
4H), 7.46 (d, J ¼ 7.6 Hz, 4H), 7.10 (d, J ¼ 7.6 Hz, 4H), 6.31 (d,
J ¼ 13.5 Hz, 2H), 6.20 (d, J ¼ 13.5 Hz, 2H), 3.82 (s, 6H), 3.60 (s, 6H),
2.28 (s, 6H), 1.69 (s, 12H); FAB-MS (Gl) m/z 569 (Cat-CH3)þ, 583
(Cat-H)þ, 584 (Catþþþeꢁ)þ, 755 (Cat þ An)þ; Elemental analysis:
calcd (%) for C52H54N4O8S2: C, 67.36; H, 5.87; N, 6.04; S, 6.92; found:
C, 67.32; H, 5.94; N, 6.09, S 6.80.
nate (12a). Yield 87%; 1H NMR (200 MHz, DMSO-d6, ppm):
d 7.94–
7.86 (m,1H), 7.84–7.78 (m,1H), 7.66–7.57 (m, 2H), 7.46 (d, J ¼ 7.6 Hz,
2H), 7.09 (d, J ¼ 7.6 Hz, 2H), 3.96 (s, 3H), 2.75 (s, 3H), 2.28 (s, 3H), 1.5
(s, 6H); Elemental analysis: calcd (%) for C19H23NO3S: C, 66.06; H,
6.71; N, 4.05; S, 9.28; found: C, 66.84; H, 6.64; N, 4.10; S, 8.81.
2.2.3.2. 2,3-Dimethyl-1,3-benzoxazol-3-ium 4-methyl-1-benzenesul-
fonate (12b). Yield 75%; 1H NMR (200 MHz, DMSO-d6, ppm):
d
7.48
2.2.5.3. Dye 4c. Yield 32 mg, 68%; mp. > 300 ꢂC (dec). 1H NMR
(d, J ¼ 7.6 Hz, 2H), 7.24–7.15 (m, 2H), 7.12 (d, J ¼ 7.6 Hz, 2H), 6.95 (d,
J ¼ 8.4 Hz, 1H), 6.84 (t, J ¼ 7.4 Hz, 1H), 3.01 (s, 3H), 2.28 (s, 3H), 1.68
(s, 3H); Elemental analysis: calcd (%) for C16H17NO4S: C, 60.17; H,
5.37; N, 4.39; S, 10.04; found: C, 60.36; H, 5.34; N, 4.27; S, 9.92.
(200 MHz, DMSO-d6, ppm):
d
8.08 (d, J ¼ 8.2 Hz, 2H), 7.98 (t,
J ¼ 13.5 Hz, 2H), 7.87 (d, J ¼ 8.2, 2H), 7.73 (s, 2H), 7.65 (t, J ¼ 8.2, 2H),
7.51 (t, J ¼ 8.2 Hz, 2H), 7.46 (d, J ¼ 7.6 Hz, 4H), 7.09 (d, J ¼ 7.6 Hz, 4H),
6.76 (d, J ¼ 13.5 Hz, 2H), 6.25 (d, J ¼ 13.5 Hz, 2H), 3.93 (s, 6H), 3.61 (s,
6H), 2.28 (s, 6H), 1.69 (s, 12H); FAB-MS (Gl) m/z 601 (Cat-CH3)þ, 615
(Cat-H)þ; Elemental analysis: calcd (%) for C52H54N4O6S4: C, 65.11;
H, 5.67; N, 5.84; S, 13.37; found: C, 65.31; H, 5.71; N, 5.89; S, 13.04.
2.2.3.3. 2,3-Dimethyl-1,3-benzothiazol-3-ium 4-methyl-1-benzene-
sulfonate (12c). Yield 90%; 1H NMR (200 MHz, DMSO-d6, ppm):
d
8.42 (d, J ¼ 8.2 Hz, 1H), 8.28 (d, J ¼ 8.2 Hz, 1H), 7.95–7.74 (m, 2H),
7.46 (d, J ¼ 7.6 Hz, 2H), 7.09 (d, J ¼ 7.6 Hz, 2H), 4.19 (s, 3H), 3.16 (s,
3H), 2.28 (s, 3H); Elemental analysis: calcd (%) for C16H17NO3S2: C,
57.29; H, 5.11; N, 4.18; S, 19.12; found: C, 57.34; H, 5.22; N, 4.32; S
18.56.
2.2.5.4. Dye 4d. Yield 31 mg, 66.5%; mp. > 300 ꢂC (dec). 1H NMR
(200 MHz, DMSO-d6, ppm):
d
8.41–8.2 (m, 6H), 8.1 (d, J ¼ 8.7 Hz,
2H), 8.03 (d, J ¼ 8.7 Hz, 2H), 7.88 (t, J ¼ 7.8 Hz, 2H), 7.67–7.55 (m,
4H), 7.48 (d, J ¼ 7.8 Hz, 4H), 7.1 (d, J ¼ 7.8 Hz, 2H), 6.72 (d,
J ¼ 13.0 Hz, 2H), 6.28 (d, J ¼ 13.0 Hz, 2H), 4.11 (s, 6H), 3.58 (s, 6H),
2.28 (s, 6H), 1.73 (s, 12H); FAB-MS (Gl) m/z 589 (Cat-CH3)þ, 603
(Cat-H)þ; Elemental analysis: calcd (%) for C56H58N4O6S2: C, 71.01;
H, 6.17; N, 5.91; S, 6.77; found: C, 71.21; H, 6.23; N, 5.86; S, 6.53.
2.2.3.4. 1,2-Dimethylquinolinium 4-methyl-1-benzenesulfonate (12d).
Yield 84%; 1H NMR (200 MHz, DMSO-d6, ppm):
d
9.07 (d, J ¼ 8.4 Hz,
1H), 8.56 (d, J ¼ 9.0 Hz, 1H), 8.38 (d, 8.0 Hz, 1H), 8.21 (t,
J ¼ 8.0 Hz,1H), 8.01 (d, J ¼ 8.7 Hz,1H), 7.97 (t, J ¼ 8.0 Hz,1H), 7.46 (d,
J ¼ 7.6 Hz, 2H), 7.09 (d, J ¼ 7.6 Hz, 2H), 4.43 (s, 3H), 3.06 (s, 3H) 2.28
(s, 3H). Elemental analysis: calcd (%) for C18H19NO3S: C, 65.63; H,
5.81; N, 4.25; S, 9.73; found: C, 65.54; H, 5.62, N 4.22; S, 10.12.
2.2.5.5. Dye 4e. After cooling, product was precipitated with ether
and recrystallized from 2-propanol to give the title compound 4e
(yield 16 mg, 35%); mp. > 300 ꢂC (dec). 1H NMR (200 MHz, DMSO-
d6, ppm):
d
8.58 (d, J ¼ 8.5 Hz, 2H), 8.52 (d, J ¼ 8.5 Hz, 2H), 8.31 (t,
2.2.3.5. 1,4-Dimethylquinolinium 4-methyl-1-benzenesulfonate (12e).
J ¼ 13.4 Hz, 2H), 8.13–7.96 (m, 6H), 7.78 (t, J ¼ 7.4 Hz, 2H), 7.55 (s,
2H), 7.48 (d, J ¼ 7.8 Hz, 4H), 7.25 (d, J ¼ 13.4 Hz, 2H), 7.10 (d,
J ¼ 7.8 Hz, 4H), 6.24 (d, J ¼ 13.4 Hz, 2H), 4.24 (s, 6H), 3.54 (s, 6H),
2.27 (s, 6H),1.72 (s,12H); FAB-MS (Gl) m/z 589 (Cat-CH3)þ, 603 (Cat-
H)þ, 604 (Catþþþeꢁ)þ; Elemental analysis: calcd (%) for
C56H58N4O6S2: C, 71.01; H, 6.17; N, 5.91; S, 6.77; found: C, 71.10; H,
6.21; N, 6.08; S, 6.46.
Yield 81%; 1H NMR (200 MHz, DMSO-d6, ppm):
d
9.34 (d, J ¼ 6 Hz,
1H), 8.51 (t, J ¼ 9.6 Hz, 2H), 8.27 (t, J ¼ 9.6 Hz, 1H), 8.10–8.00 (m,
2H), 7.46 (d, J ¼ 7.6 Hz, 2H), 7.09 (d, J ¼ 7.6 Hz, 2H), 4.57 (s, 3H), 3.00
(s, 3H), 2.28 (s, 3H); Elemental analysis: calcd (%) for C18H19NO3S: C,
65.63; H, 5.81; N, 4.25; S, 9.73; found: C, 65.50; H, 5.73; N, 4.20; S,
10.05.
2.2.4. Dye 4a (Method A)
2-(1,1,3-trimethyl-2,3-dihydro-1H-2-indenyliden) acetaldehyde
[27] (150 mg, 0.75 mmol) was dissolved in 5 mL acetic anhydride
and then benzodipyrrolenine 11 (182 mg, 0.3 mmol) was added.
The mixture was refluxed for 1 h, cooled and the product was
precipitated with ether. The raw product was dissolved in
a minimal amount of nitromethane and precipitated with ether to
give the title compound 4a (250 mg, yield of 85%).
3. Results and discussion
3.1. Synthesis
The key intermediate for the synthesis of biscyanines 4a–4e is
the quaternary salt 11 [26] that can be obtained by quaternization
of 2,3,3,6,7,7-hexamethyl-3H,7H-benzo[1,2-b:3,4-b’]dipyrrole (ben-
zodipyrrolenine) with dimethylsulfate or methyltosylate. Biscya-
nine 4a was reported to be synthesized with 42% yield by heating to
reflux of the quaternized benzodipyrrolenine 11 with Fisher’s
aldehyde in acetic anhydride followed by removing of the solvent
under the reduced pressure (Scheme 2, Method A) [18]. According
to this procedure we synthesized compound 4 with a 62% yield.
Furthermore instead of removing the solvent we precipitated the
product with ether and after purification obtained 4a in 85% yield.
2.2.5. General procedure to synthesize dyes 4a–4e (Method B)
2-[1,3,3,5,7,7-hexamethyl-6-(2-oxoethylidene)-5,7-dihydropyrrolo
[2,3-f]indol-2(1H,3H)-ylidene]acetaldehyde 9 (0.05 mmol) was dis-
solved in acetic anhydride (2 mL) and the corresponding nitrogen
containing heterocyclic compound 12a–12e (0.12 mmol) was added.
The mixture was refluxed for 40 min and processed as described above
to give the title compound 4a–4e.

10
O.P. Klochko et al. / Dyes and Pigments 85 (2010) 7–15
Scheme 4.
Scheme 2.
3.2. Molecular structure
Nevertheless the above Method A seems not rational for
The molecular structures of biscyanines 4a–4e were studied
using the PM3 semi-empirical quantum-chemical method. The
simulations show that the dye molecules are flat, i.e. both terminal
and central heterocycles are in plane. Quaternized nitrogen atoms
of the central and terminal heterocycles (or quinoline benzene ring
in case of 4e) can be directed in one side (syn-form) or opposite
sides (anti-form). Conformational analysis evidences that the syn-,
syn-conformer (Fig. 1) is more preferable compared to both anti-,
anti- and syn-,anti-forms by 4.2–11.3 kcal/mol and 1.7–7.4 kcal/mol,
respectively (Table 1). Change of other torsion angles in the poly-
methine chain causes even more pronounced energy increase and
therefore formation of the corresponding conformers is not very
probable. According to the Boltzmann distribution the biscyanine
molecules exist at room temperature mostly in the syn, syn-form. In
this form the angle between two trimethine chromophore systems
is very close to 180ꢂ.
synthesizing of biscyanines with variable terminal heterocyclic
moieties because it requires prior formylation of each heterocyclic
compound used as the terminal end-group (Scheme 3).
Therefore we explored another approach via the previously
unknown benzodipyrrolenine diformyl derivative 9 (Scheme 4,
Method B) [28]. Benzodipyrrolenine quaternary salt 11 cannot be
formylated and therefore this salt was initially transformed into
dimethylene benzodiindole 10 by treatment with aqueous
ammonia. The Vilsmeier formylation of 10 with POCl3 in an excess
of DMF yielded the dialdehyde 9 in 86% yield.
Bis-trimethine dyes 4a–4e were obtained with 35–82% yield by
condensation of dialdehyde 9 with quaternized 2,3,3-trimethy-
lindolenine 12a, 2-methylbenzoxazole 12b, 2-methylbenzothiazole
12c, 2- and 4-methylquinolines (12d and 12e), respectively
(Scheme 4). Reactions were accomplished by refluxing in acetic
anhydride. Structures of the synthesized dyes were confirmed with
1H NMR (DMSO-d6) and FAB-MS. Four doublet and two triplet
signals of methine hydrogen atoms in the 1H NMR spectra are
located between 5.6 and 8.8 ppm.
The yield of dye 4a synthesized according to Method B (Scheme 4)
is 82%, which is substantially higher than that reported by Mihailenko
et al. [18] (42%), and very close to the value of 85% that we obtained
following the above improved Method A (Scheme 2). Thus the
proposed approach to synthesize bis-trimethine dyes via diformyl
derivative 9 seems to be good alternative for the previously described
condensation of the Fisher’s aldehyde with benzodipyrrolenine. The
new approach is more convenient for the synthesis of a series of
symmetrically substituted dyes with variable terminal end hetero-
cyclic moieties.
The chromophore part of biscyanine molecules is presented
as a di-cation with a delocalized positive charge. According to
PM3 (Table 1) and PPP simulations the positive charge (q)
predominates on the nitrogen atoms of the terminal heterocy-
cles (qTerm) but also (to a lesser degree) on nitrogen atoms of the
central benzodipyrrolenine moiety (qCentr). Thus the electron
Scheme 3.
Fig. 1. Representative conformations of biscyanines 4a–4e.

O.P. Klochko et al. / Dyes and Pigments 85 (2010) 7–15
11
Table 1
The formation enthalpy (DH) of different conformers of biscyanines, the electron charges on nitrogen atoms of the central (qCentr) and terminal (qTerm) heterocycles of the
syn,syn-forms, and the chemical shifts of methyl hydrogen atoms of the central (dCentr) and terminal (dTerm) heterocycles.
Molecule
DHsyn,syn [kcal/mol]
DHsyn,anti [kcal/mol]
DHanti,anti [kcal/mol]
qTerm
qCentr
dTerm [ppm]
dCentr [ppm]
Dd [ppm]
4a
4b
4c
4d
4e
414.7
388.3
469.3
452.5
453.2
418.8
390.8
471.7
456.4
456.8
422.9
392.5
474.5
463.8
460.7
þ0.392
þ0.303
þ0.380
þ0.414
þ0.407
þ0.226
þ0.223
þ0.228
þ0.191
þ0.178
3.69
3.82
3.93
4.11
4.24
3.65
3.60
3.61
3.58
3.54
0.04
0.22
0.32
0.53
0.70
density on the polymethine chains is shifted from the terminal
heterocycles towards the central heterocyclic unit. The positive
charge qTerm subsequently increases in the order of 2-
benzoxazolium < 2-benzothiazolium < 2-indoleninium < 4-
quinolinium < 2-quinolinium, while the positive charge qCentr
concentration in the range of 1.0–3.0 ꢃ 10ꢁ7 M, which is evidence
that the shorter wavelength component is not due to aggregation.
The vibrational structure of the absorption band is more apparent
in case of dye 4a with the most symmetrical chromophore systems.
An increase of the chromophores asymmetry, estimated by Dd
values (Table 1), and an increase of the solvent polarity cause
a smoothing of the absorption and emission bands (Fig. 3). This
effect originates from an increase of the dye molecule polarity and
increase of interaction between the dye and solvent molecules
[35,36]. At the same time no correlation between the Dd values and
the absorption and emission band half-widths (Dn1/2) measured in
chloroform and methanol (Table 2) was found.
Absorption and emission maxima of biscyanines are red-
shifted by about 90–120 nm compared to those for the corre-
spondent parent trimethines 13–21 (Table 2). To analyze
correlation between spectral properties and molecular structure of
both biscyanines and parent trimethines we simulated the spatial
and electronic structure of these molecules in the ground and first
excited states an calculated the electron transitions under excita-
tion. The simulations were performed using semi-empirical
quantum-chemical PM3 CI and PPP CI methods. An example of the
calculated singlet electron transitions is shown on Fig. 2. These
simulations evidence that the long-wavelength band of biscyanines
is connected with S0 / S1* transitions mostly formed by super-
position of two configurations: HOMO / LUMO (higher contri-
bution) and (HOMO ꢁ 1) / (LUMO þ 1) (lower contribution).
Calculated by PM3 CI method structures of the molecular orbitals
participating in excitation S0 / S1* (Fig. 4) and the localization
numbers obtained by the PPP CI method (Fig. 5) indicate that the
decreases
in
the
order:
2-benzothiazolium
>
2-
indoleninium > 2-benzoxazolium > 2-quinolinium > 4-quino-
linium, which does not correlate with the previous sequence.
Because of the non-correlation between qTerm and qCentr these
electron charges cannot be used to make an unambiguous
conclusion about the relative donating and withdrawing ability
of the heterocyclic end-groups. It is worth mentioning that
Brooker in his papers [29–31] investigated spectral shifts of the
long-wavelength absorption band of heterocyclic compounds
and concluded that the electron donor ability increases in the
order of 2-indoleninium
<
2-benzoxazolium
<
2-
benzothiazolium < 2-quinolinium < 4-quinolinium, which is
different from our findings on the strength of the calculated
electron charge distribution.
We found a clear correlation (r ¼ 0.95) between the chemical
shifts of the methyl hydrogens located on the nitrogen atoms of the
terminal (dTerm) and central (dCentr) heterocycles in the 1H NMR
spectra (DMSO-d6) of biscyanines (Table 1). These shifts indicate
that the electron donor ability of the terminal heterocycles
increases in the order of 2-indoleninium < 2-benzoxazolium < 2-
benzothiazolium < 2-quinolinium < 4-quinolinium. This sequence
completely coincides with the Brooker’s series obtained from the
shifts of the absorption bands. Increasing difference between the
chemical shifts Dd
asymmetry of polymethine chains increases in the order:
4a < 4b < 4c < 4d < 4e.
¼
dTerm
ꢁ
dCentr indicates that the electron
S0 / S1* transition affects mostly two polymethine chains
including nitrogen atoms of both central and terminal heterocyclic
moieties. The angle between these two trimethine chromophore
systems is very close to 180ꢂ and the electron transition is polarized
along the principle axis of these chromophores.
The discussed spatial and electron structure of biscyanines have
strong influence on their spectral properties.
3.3. Spectral properties
The simulations indicate that the conjugation of two parent
trimethines to form a biscyanine dye does not cause a noticeable
change in the spatial and conformational structure of the parent
chromophores. Localizations and polarizations of the S0 / S1*
transitions are also almost constant (Fig. 5). The nature of the
heterocyclic end-groups does not noticeably impact the confor-
mational structure of the molecules, the orientation of the two
conjugated chromophores or the localization of the electron exci-
tation but can substantially affect the transition energy and elec-
tron distribution in both ground and first excited states. The
alteration of electron distribution upon the nature of the end
heterocycles might lead to various positive and negative solvation
effects for the investigated biscyanine dyes.
Indeed the location of the absorption and emission bands of bis-
cyanines was experimentally found to be strongly dependent upon
the structure of terminal heterocycles (Table 2). Thus a substitution of
indolenine moiety with 2-quinoline results in the long-wavelength
shift of the absorption (by 37 nm in chloroform and 22 nm in
methanol) and emission maxima (63 and 61 nm). Introduction of 4-
quinoline moiety causes even higher red-shifts. Substitution of an
Absorption and emission spectra of biscyanines 4a–4e were
measured in chloroform and methanol. The spectral characteristics
such as the absorption and emission maxima (lmax), extinction
coefficients (3), Stokes’ shifts (Dnst), quantum yields (Q.Y.), absorp-
tion and emission band half-widths (Dn1/2) are given in Table 2. For
a comparison the same table also includes the spectral character-
istics of parent trimethine dyes 13–21 taken from [32,33]. Repre-
sentative absorption and emission spectra of biscyanines in the
example of 4a are shown on Fig. 2.
Shape of Spectra. As predicted by Kiprianov’s theory [12–14],
due to the linear orientation of the two chromophores, all biscya-
nine dyes 4a–4e show only one intensive long-wavelength
absorption band in visible spectral range (Fig. 2). This band consists
of two components: a longer-wavelength component with higher
intensity and a shorter wavelength component with lower inten-
sity. Similar to other cyanine dyes these components can be
attributed to the vibrational transitions [34]. The shape of the
absorption and emission spectra is found to be not dependent upon

12
O.P. Klochko et al. / Dyes and Pigments 85 (2010) 7–15
Table 2
Spectral characteristics of biscyanines and parent trimethine dyes.
[Mꢁ1 cmꢁ1
]
lmax (Em) [nm] Dnst [cmꢁ1
]
Q.Y. [%] Dn1/2 (Abs)b [cmꢁ1
]
Dn1/2 (Em)c [cmꢁ1
]
a
Dye
Solvent lmax (Abs) [nm]
CHCl3 658
MeOH 644
3
4a
677
664
430
470
18.8
10.4
1908
1875
1835
1930
N
251,000
N
N
N
2 TsO
4b
4c
CHCl3
MeOH 596
616
640
629
610
880
28.3
12.6
2173
2308
1812
2180
O
N
N
O
N
154,000
205,200
N
N
2TsO
CHCl3
MeOH 640
664
688
675
525
810
23.0
6.0
1985
2165
1000
2200
S
N
S
N
N
2 TsO
4d
4e
CHCl3
MeOH 666
695
740
725
875
1220
4.0
0.5
2322
2230
2400
2800
N
N
176,000
133,700
N
N
N
2 TsO
CHCl3
MeOH 717
707
755
773
900
1010
3.8
0.03
2099
2209
1420
1930
N
N
N
2TsO
13
CH2Cl2 550
135,000
134,000
570
568
638
743
4
3
N/A
N/A
EtOH
545
N
N
I
I
14
15
DMSO 490
EtOH 482
N/A
156,000
515
500
991
747
9.3
4
N/A
N/A
N/A
N/A
O
N
O
N
CH2Cl2 560
EtOH 558
151,000
138,000
580
577
616
590
6
5
S
N
S
N
I
16
17
DMSO 615
EtOH 605
N/A
178,500
645
756
0.4
N/A
N/A
N/A
N/A
N
N
I
CH2Cl2 713
DMSO 710
N/A
N/A
732
734
364
461
2
3.6
N
N
I
18
19
20
n-PrCN 512
N/A
112,700
534
n/d
805
n/d
4.6
n/d
N/A
N/A
N/A
N/A
O
N
EtOH
510
N
N
N
I
CH2Cl2 550
EtOH 546
142,000
126,000
572
573
699
863
5
3
S
N
I
I
CH2Cl2 571
EtOH 561
N/A
106,900
607
622
1039
1748
0.7
0.3
N/A
N/A
N/A
N/A
N
21
CH2Cl2 618
EtOH 601
N/A
79,800
644
653
653
1324
1.6
0.7
N
N
TsO
a
Dnst d Stokes’ shift.
Dn1/2 (Abs) d Absorption band half-width.
Dn1/2(Em) d Emission band half-width.
b
c

O.P. Klochko et al. / Dyes and Pigments 85 (2010) 7–15
13
Fig. 2. Absorption and emission spectra of biscyanine 4a in methanol and electron
transitions calculated by PPP CI method.
indolenine with a 2-benzothiazole does not result in a sensible
change of absorption wavelength (red-shift by 6 nm in chloroform
and blue-shift by 4 nm in methanol) but the fluorescence maximum
is red-shifted by 11 nm in both solvents. The absorption and emission
maxima of the benzoxazole derivative 4b are blue-shifted as
compared to the indolenine-based dye 4a. We found clear correla-
tions between the absorption and emission maxima measured in
both chloroform and methanol (Table 2) and Dd values (Table 1). The
correlation coefficients (r) are in the range of 0.77–0.88 depending on
the solvent system. Only indolenine-based dye 4a, which has longer
absorption and emission maxima as compared to benzoxazole
derivative 4b, demonstrates a pronounced deviation from the
regression. Elimination of 4b from the regressions results in notice-
able increase of the correlation coefficients (r ¼ 0.94–0.99). Similar
correlations were found for parent trimethines 13–21, where ben-
zoxazole derivatives were also dropped out of the regressions.
The spectral characteristics of biscyanines are noticeably
dependent upon the solvent system (Table 2). All dyes except 4e
exhibit a hypsochromic blue-shift of the absorption maxima when
chloroform is replaced with methanol. The most pronounced blue-
shift of 29 nm is found for 4d. The emission maxima in methanol
are also blue-shifted compared to those in chloroform but this
effect is less pronounced for the emission maxima as compared to
the absorption maxima, which probably is the result of the lower
polarity these dye molecules have in the excited state. Thus the
Fig. 4. Structure of the boundary orbitals in molecule 4b obtained by PM3 CI method.
calculated by the PM3 method dipole moment of 4a is 0.23 D in the
ground state and 0.17 D in the first excited state. Dye 4e exhibits
red-shifted absorption (10 nm) and emission (18 nm) maxima
when chloroform is substituted with methanol. An increase in
solvent polarity causes an increase of the Stoke’s shifts and
decrease of the quantum yields, which is most likely the result of an
increase in internal conversion caused by interaction between the
solvent of higher polarity and the polar dye molecules in the excited
state.
Similar to the parent trimethine dyes 13–21 [37,38] the above
spectral shifts caused by solvent polarity and the heterocyclic end-
groups in biscyanine dyes 4a–4e can be understood by considering
the donor-acceptor abilities of these heterocycles. The heterocyclic
end-groups in trimethine dyes 13–21 and biscyanine dyes 4a–4e
substantially differ in their electron donating and electron with-
drawing properties. The different electronic effects caused by the
heterocyclic end-groups in the parent trimethine dyes 13–21 are
known to cause an imbalance in the molecules electronic
symmetry in the ground state but in the excited state the electron
Fig. 3. Influence of terminal heterocycles on the long-wavelength absorption bands of
biscyanine dyes 4a–4e in methanol.
Fig. 5. Localization and polarization of electron transition S0 / S1* in biscyanine 4a
and parent trimethine 13 molecules (PPP CI method).

14
O.P. Klochko et al. / Dyes and Pigments 85 (2010) 7–15
density evens out substantially: the more the heterocycles end-
groups differ from each other the stronger the electron charge
redistribution under excitation [36]. The charge redistribution in
the excited state initiates intramolecular (conformational) and
intermolecular (between dye and solvent molecules) relaxation
which results in the Stokes’ shift increase due to the increase of
both the molecular asymmetry and the solvent polarity [37]. In
addition because the unsymmetrical trimethine dyes 18–21 have
substantially higher polarity in the ground state compared to the
excited state, their absorption spectra are more sensitive towards
the solvent polarity than the emission spectra.
Similar effects are observed for biscyanine dyes 4a–4e. Despite
the biscyanine molecules are symmetric they consist of two
uniformly unsymmetrical trimethine chromophores. In order to
quantify this asymmetry we used the chemical shifts (Dd) observed
for the methyl hydrogens that are located on the nitrogen atoms of
terminal and central heterocycles (Table 1). Dd shows a minimal
value in 4a indicating the highest chromophore symmetry as
Acknowledgement
This work was supported by the National Academy of Sciences
of Ukraine, project No. 0107U000487.
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