Design, synthesis, and spectral luminescent properties of a novel polycarbocyanine series based on the 2,2-difluoro-1,3,2-dioxaborine nucleus

Article abstract of DOI:10.1002/ejoc.200701012

The natures of the chromophores in symmetric polymethine dyes derived from 2,2-difluoro-1,3,2-dioxaborine have been investigated. Ab initio quantum chemical calculations demonstrated that the presence of dioxaborine end residues stabilizes the frontier levels of the corresponding polymethine dye and makes electron-density distribution over the oxygen atoms in the chelate ring more even than in the analogous dye structure with boron-free acyclic end groups. A series of novel symmetric polycarbocyanines and a tricarbocyanine series with variously bridged polymethine chromophores have been synthesized from hitherto unknown pyrimidino-annelated dioxaborines. The absorption, fluorescence and 13C NMR spectroscopic data point to the polymethinic type of electron-density distribution in the 2,2-difluoro-1,3,2-dioxaborine polymethine dye molecules. The fundamental options for controlling the spectral properties of these dyes by modification of their polymethine chains have been evaluated. One of the new compounds synthesized is remarkable among the known open-chain polymethine dyes for its record high fluorescence quantum yield. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200701012

FULL PAPER
DOI: 10.1002/ejoc.200701012
Design, Synthesis, and Spectral Luminescent Properties of a Novel
Polycarbocyanine Series Based on the 2,2-Difluoro-1,3,2-dioxaborine Nucleus
Konstantin Zyabrev,*[a] Andrey Doroshenko,[b] Elena Mikitenko,[a] Yurii Slominskii,[a] and
Alexei Tolmachev[a]
Keywords: Ab initio calculations / Absorption / Dioxaborine / Fluorescence / Polymethine dye
The natures of the chromophores in symmetric polymethine
dyes derived from 2,2-difluoro-1,3,2-dioxaborine have been
investigated. Ab initio quantum chemical calculations dem-
onstrated that the presence of dioxaborine end residues sta-
bilizes the frontier levels of the corresponding polymethine
dye and makes electron-density distribution over the oxygen
atoms in the chelate ring more even than in the analogous
dye structure with boron-free acyclic end groups. A series
of novel symmetric polycarbocyanines and a tricarbocyanine
series with variously bridged polymethine chromophores
have been synthesized from hitherto unknown pyrimidino-
annelated dioxaborines. The absorption, fluorescence and
13C NMR spectroscopic data point to the polymethinic type of
electron-density distribution in the 2,2-difluoro-1,3,2-dioxa-
borine polymethine dye molecules. The fundamental options
for controlling the spectral properties of these dyes by modi-
fication of their polymethine chains have been evaluated.
One of the new compounds synthesized is remarkable
among the known open-chain polymethine dyes for its re-
cord high fluorescence quantum yield.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
well as on various physical properties.[7,8,13,15,17] The natures
of the chromophore systems and the general colour regular-
ities have so far remained unexplored.
Introduction
Since the second half of the last century, it has been
known that 2,2-difluoro-1,3,2-dioxaborines bearing
methyl or methylene group at position 4 of the heterocycle
can form deeply coloured polymethine dyes (PDs).[1–6]
a
This study, based on the nonempirical ab initio method
(UHF/6–31G**), addresses the effect of the ring-forming
BF2 moiety on the frontier level positions and the electron-
density distributions in the molecules of symmetric car-
bocyanines with variously derivatized 2,2-difluoro-1,3,2-di-
oxaborine end residues. A novel polycarbocyanine series
has been synthesized from previously unknown pyrimidino-
annelated 2,2-difluoro-1,3,2-dioxaborines, and the spectral
properties of the dyes obtained have been investigated.
A
special interest in π-conjugated systems derived from 2,2-
difluoro-1,3,2-dioxaborines has stemmed from their pecu-
liar electronic and spectral luminescent properties, such as
high hyperpolarizabilities,[7,8] wide ranges (from UV to near
IR wavelengths) and high intensities of absorption[1,2,4–12]
and fluorescence,[10–13] photosensitizing activities towards
photoconducting materials[3,4] and metallic silver,[13] and
large two-photon cross sections.[13] The wide application of
2,2-difluoro-1,3,2-dioxaborines has been reviewed.[14,15]
An effective instrument for the design of PDs with speci-
fied parameters is offered by structural variation in the po-
lymethine chains (PCs) and heterocyclic end groups.[16] This
approach calls for a deep insight into the relationships be-
tween the electronic structures and the spectral properties
of the symmetric PDs concerned. However, the literature
devoted to 2,2-difluoro-1,3,2-dioxaborine PDs is mostly fo-
cused on synthetic[1–6,8] and some colour[12,17] respects, as
Results and Discussion
The simplest symmetric 2,2-difluoro-1,3,2-dioxaborine
PD 1, with its first absorption maximum found at λmax
=
519 nm in CH3CN, was synthesized by us starting from the
boron complex of acetylacetone:[17]
[a] Institute of Organic Chemistry, National Academy of Sciences
of Ukraine,
5 Murmanskaya St., Kiev 02094, Ukraine
Fax: +38-0445732643
Dyes of this kind are regarded as oxonoles: that is, oxy-
gen analogues of cyanine dyes bearing delocalised negative
charges.[6] A classical example of an oxonol, with much the
same absorption region (λmax = 547 nm) is provided by
E-mail: zyabrev@ukr.net
[b] Department of Chemistry, Kharkov National University V. N.
Karazin,
4 Svobody sqr., Kharkov 61077, Ukraine
1550
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Properties of a Novel Polycarbocyanine Series
structure 2.[16] It is clear that the chromophores of dyes 1 keto and hydroxy groups, the electron density is distributed
and 2 include the same number of atoms and that the main somewhat more evenly in the oxygen atom pairs 1/12 and
long-wavelength transitions are localized along the conjuga- 11/13, but this equalization is much less pronounced than
tion chains connecting oxygen atoms 1 and 11. To estimate in the dioxaborine derivative 1. The hydroxy substituents at
the effect of the dioxaborine rings on the electronic struc- positions 4 and 8 in dye 3, though stabilized by the intra-
ture of dye 1, its frontier level positions, and the S0–S1 tran- molecular hydrogen bond, merely widen the energy gap
sition energy, we performed ab initio calculations for PD 1 (from 7.66 to 8.41 eV), due to their positive inductive effect,
and oxonole 2.
in accordance with the Foerster–Dewar–Knott rule.[19]
The relevant characteristics were also calculated for the
virtual oxonole 3, representing a structure intermediate be-
tween PDs 1 and 2: like dioxaborine dye 1, it contains hy-
droxy groups at positions 4 and 8, but six-membered diox-
aborine rings are simulated with analogous chelate rings
arising from intramolecular H-bonding.
Figure 1. HOMO and LUMO shapes for PDs 1, 2 and 3 calculated
by the ab initio MO method at the UHF/6–31G** level of theory.
The previously reported computational data for the 2,2-
difluoro-1,3,2-dioxaborine nucleus concerned only 4,5,6-tri-
substituted and other functionalized derivatives.[14,18] The
calculation results listed in Table 1 suggest that the presence
of the dioxaborine rings leads to a notable lowering of the
highest occupied molecular orbital (HOMO) of PD 1 in
relation to oxonoles 2 and 3 (by 1.83 and 1.51 eV, respec-
tively).
Table 1. The HOMO and LUMO energies for PDs 1, 2 and 3 calcu-
lated by the ab initio MO method at the UHF/6–31G** level of
theory.
Figure 2. Atomic charges for PDs 1, 2 and 3 calculated by the ab
initio MO method at the UHF/6–31G** level of theory.
Dye
EHOMO [eV]
ELUMO [eV]
1
2
3
–4.27
–2.44
–2.76
4.37
5.22
5.65
As shown by us previously,[17] the boron chelate of 2-
acetyldimedone (4) reacts with electrophiles to give the
symmetric PD 5 as well as its unsymmetrical analogues.
The stabilization of the lowest unoccupied molecular or- Though the exocyclic carbonyl groups conjugated with the
bital (LUMO) of PD 1 relative to the simple oxonole 2 and dye chromophore produce no auxochromic effect (λmax
=
its substituted analogue 3 is not so significant (the corre- 519 nm in CH3CN both for 1 and for 5), they make the
sponding downward energy shifts amount to 0.85 and structures of related dyes more solvolytically stable, due to
1.28 eV). As can be seen from Figures 1 and 2, the dioxa- electron withdrawal from the dioxaborine chelate ring.
borine rings also cause an essential equalization of the
On the other hand, it has been demonstrated[12] that
HOMO coefficients and charges on oxygen atoms 1 and 12 bridge groups that make the dioxaborine ring coplanar with
(and, accordingly, 11 and 13) in comparison with those in the rest of the PD molecule can substantially increase the
the model dye 3. Contrary to this, the charge distribution molar absorption coefficients (ε) and the fluorescence quan-
and the frontier level positions for model oxonole 3, which tum yields (Φ) of such dyes. In this context, it thus appears
lacks the BF2 groups, were found to resemble those for an- even more promising, from the spectroscopic point of view,
ionic dye 2 (see Figures 1 and 2). In relation to isolated to annelate the dioxaborine chelate ring to the 1,3-dimethyl-
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K. Zyabrev et al.
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the new PDs, together with those of the previously obtained
dye 5. Though the absorption and fluorescence maxima
fl
(represented by λmax and λ , respectively) of carbocyan-
max
ine 9 are only slightly shifted to longer wavelengths in rela-
tion to PD 5, its absorption intensity (ε), fluorescence quan-
tum yield (Φ), and Stokes shift (∆ν) are significantly in-
˜
creased. To elucidate the effect of end group topology on
the π-electronic properties of 2,2-difluoro-1,3,2-dioxaborine
carbocyanines, ab initio calculations for PDs 5 and 9 were
carried out (the results are demonstrated in Figure 3).
Table 2. Spectral luminescence properties of PDs 5, 9, 12, 13 and
15.
fl
Dye λmax [nm]
λmax [nm]
λ
max
[nm] ∆ν [cm–1]
Φ
˜
(ε·10–5, –1cm–1) (ε·10–5, –1cm–1) CH2Cl2
CH3CN
CH2Cl2
5
9
519
(1.756)
525
525
(1.553)
530
(1.693)
630
(2.028)
735
(1.766)
844
(1.283)
535
546
651
760
356
553
512
448
0.11
0.31
0.82
0.43
barbituric acid (6) residue rather than to dimedone, since
the former component has a more planar molecular struc-
ture and bears an additional carbonyl group. The desired
bicyclic nucleus 7 was obtained by the known procedure,[20]
with the strong nucleophilic centre C-5 of the pyrimidine
system used for the construction of the boron chelate com-
plex.
(2.095)
12 625
(2.221)
13 725
(1.613)
15 838
(1.111)
In PD 5, the sp3-hybridized carbon atoms in the cyclo-
hexane rings cause nonplanarity of the dye structure, so
that the non-chelated carbonyl groups are turned out of the
plane of the polymethine chromophore by 18°. In spite of
the deplanarization, the atoms of the twisted carbonyl
groups have nonzero HOMO coefficients, thus implying a
Taking advantage of the highly reactive methyl group at certain degree of conjugation with the PC. PD 9 is charac-
position 8, heterocycle 7 was treated according to conven- terized by a more planar structure, as its pyrimidine moiety
tional cyanine chemistry synthetic schemes; the series of lies in the chromophore plane. The carbonyl groups at posi-
PDs 9, 12, 13 and 15 was thus produced with the goals tions 5 and 5Ј, as well as the nitrogen atoms at positions 4
of research into the dyes’ electronic structures and spectral and 4Ј, are also involved in the first electronic transition.
properties in relation to variation in PC length (Scheme 1). It is likely that the enhanced absorption and fluorescence
Table 2 presents the spectral luminescence parameters of intensity observed on going from PD 5 to 9 is partially at-
Scheme 1. Synthesis of a series of PDs: 9, 12, 13 and 15.
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Properties of a Novel Polycarbocyanine Series
the heterocyclic nucleus (as in PD 9) results in increasing
stabilization of the dye frontier levels: when passing from 1
to 5 and then to 9, the initial HOMO energy is lowered by
0.25 eV in each modification, while the corresponding val-
ues for the LUMOs amount to 0.29 and 0.21 eV.
The electronic structures of carbocyanines 5 and 9 ap-
proach the polymethinic type, as evidenced by the vinylene
shifts of close to 100 nm (the typical value for symmetric
PDs), as well as by the alternation of positive and negative
atomic charges in the chromophore, supported both by the
quantum chemical computational results for the two dyes
(see Figure 3) and by the 13C NMR spectroscopic data for
PD 9 (see Table 4).
Table 4.[a] The chemical shifts of the 13C NMR signals and the
corresponding ab initio calculated charges on the carbon atoms in
the chromophore of PD 9.
Carbon atom number
δ [ppm]
q
9
10
8
11
12
173.3
90.0
165.3
108.2
152.9
0.880
–0.428
0.551
–0.334
0.024
[a] The data for half of the symmetric main chromophore are listed.
For the δ values of the other carbon atoms see Exp. Sect.
It can be seen from Table 4 that the calculated charges
correlate qualitatively with the chemical shifts of the 13C
NMR signals from the carbon atoms in the PC. A positive
charge on a C atom in the dye chromophore corresponds
to its chemical shift in the 13C NMR spectrum being be-
tween 153 and 173 ppm, while a negative charge of a chro-
mophore C atom corresponds to its chemical shift in the
13C NMR spectrum being close to 100 ppm. A discrepancy
for the atom C-12 may arise from the overestimated C–H
bond polarization typical of the computational method
used.
Analysis of absorption band shapes and, in particular,
halfwidths (S) for PDs 9, 12, 13 and 15 (see Figure 4 and
Table 5) reveals a gradual weakening of the main spectral
maxima for tricarbo- and tetracarbocyanines, together with
a band broadening on going from dichloromethane to the
more polar acetonitrile. The absorption band of the tetra-
carbocyanine 15 demonstrates an extended short-wave-
length tail, which can hardly be associated with vibrational
transitions. Nor does it result from higher excitations (e.g.,
the S0ǞS2 transition), as evidenced by previous stud-
ies[21a,21b] involving fluorescence anisotropy measurements
of the fluorescence excitation spectra and two-photon ab-
sorption measurements for some cationic PDs.[21a,21b] This
feature is due, rather, to the increased contribution of the
electronically unsymmetrical resonance structure of the dye,
which is favoured by the absorption shift to the near IR
region and by the increasing solvent polarity.[21] Import-
antly, the electron symmetry break for PD 15 occurs at
shorter wavelengths than for cationic and anionic PDs,[21]
Figure 3. HOMO and LUMO shapes, atomic charges and molecu-
lar projections (side views) for dioxaborine PDs 5 and 9 calculated
by the ab initio MO method at the UHF/6–31G** level of theory.
tributable to the better conjugation between the heterocy-
clic end residues and the PC in the latter molecule, whereas
the larger Stokes shift of PD 9 can be attributed to the
electron effect of the nitrogen atoms in the pyrimidine moi-
ety. The effect of the end group structure on the dye frontier
MO energies is illustrated in Table 3.
Table 3. HOMO and LUMO energies for PDs 5 and 9 calculated
by the ab initio MO method at the UHF/6–31G** level of theory.
Dye
EHOMO [eV]
ELUMO [eV]
5
9
–4.52
–4.77
4.08
3.87
Comparison of the data listed in Tables 1 and 3 suggests and also for the previously described symmetric tetracar-
that the annelation of the simple dioxaborine residues (in bocyanine with the 2,2-difluoro-1,3,2-dioxaborine end
PD 1) with the alicyclic moieties (as in PD 5) and then with groups.[8]
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K. Zyabrev et al.
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λmax, by a value of about 100 nm, while the Stokes shift
reduces on passing from shorter to longer vinylogues. The
luminescence efficiencies of cyanines 9, 12 and 13 also be-
have like those of cationic dyes. On going from the carbo-
to the dicarbocyanine, the Φ value increases to a record
high value among the known open-chain PDs (0.82) and
then reduces for tricarbocyanine. The latter effect is proba-
bly due to the enhancement of internal conversion, which
becomes a significant deexcitation channel for most PDs
with the absorption wavelengths over 700 nm.[23]
Introduction of bridge groups into the various positions
of the polymethine chromophore is known to be an advan-
tageous approach to PD design, enabling fine and efficient
tuning of absorption and luminescence in a specified spec-
tral region.[24] As has been shown previously,[12] cyclizations
of different portions of the conjugated chain with the aid
of bridge groups makes it possible to vary the positions and
intensities of the absorption and fluorescence maxima of
dioxaborine carbocyanines. It is noteworthy that the copla-
narization of the dioxaborine nucleus and the trimethine
chain can cause not only spectral band shifts but also dis-
tortions of the chromophore system arising from bridge-
induced steric hindrance. As a result, the fluorescence quan-
tum yields of bridged dioxaborine carbocyanines can de-
crease drastically. Thus, in order to study a possibly “pure”
effect of various bridge groups on the colour of dioxaborine
PDs, with the minimized bridge-induced geometry pertur-
bations, we synthesized tricarbocyanines 21–25 (Scheme 2)
in which the end heterocyclic nuclei are sufficiently distant
from the substituents in the PC.
Figure 4. Absorption spectra for PDs 9, 12, 13 and 15 in CH2Cl2
(solid lines) and CH3CN (dashed lines).
Table 5. Absorption band halfwidths for PDs 9, 12, 13 and 15.
Dye
n
S [cm–1]
(CH2Cl2)
S [cm–1]
(CH3CN)
9
1
2
3
4
818
812
849
810
764
836
922
12
13
15
1262
As shown by the spectroscopic data in Table 6, the effects
To minimize solvation effects, the luminescence proper- of cyclic bridge groups on the absorption band positions
ties of the PDs obtained were studied in dichloromethane, are much the same for dioxaborine PDs as for cationic
a weakly polar solvent. From analysis of the fluorescent dyes.[24] The dimethylene and trimethylene bridge groups,
measurements for the PD series 9, 12 and 13 (see Table 2) when introduced into the central positions of the PCs (as
it follows that, as in the case of cationic PDs,[22] the PC in PDs 21 and 22, 23 and 24) give rise to regular batho-
lengthening causes a regular bathochromic shift of the fluo- chromic shifts of spectral maxima, the former residue causing
fl
rescent maxima λ , as well as the absorption maxima more pronounced changes. The presence of vinylene groups
max
Scheme 2. Synthesis of series of PDs 21–25.
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Properties of a Novel Polycarbocyanine Series
Table 6. Spectral luminescence properties of PDs 21–25. The differences in absorption and fluorescent maximum positions, ∆λmax and
fl
∆λ , are calculated with reference to open-chain tricarbocyanine 13.
max
fl
fl
Dye
λmax [nm]
λmax [nm]
∆λmax [nm]
CH3CN/CH2Cl2
λ
[nm]
∆λ
[nm]
∆ν [cm–1]
Φ
˜
max
max
[ε 10–5 –1·cm–1]
CH3CN
[ε 10–5 –1·cm–1]
CH2Cl2
CH2Cl2
21[a]
22
773
738
(1.896)
778
(2.001)
755
(2.355)
667
(2.077)
783
746
(1.551)
786
(1.682)
759
(1.656)
677
(1.619)
48/48
13/11
800
768
40
8
271
384
0.20
0.07
0.09
0.15
23
24
25
53/51
812
784
698
52
24
407
420
444
30/24
–58/–58
–62
[a] Because of the high lability of dye solutions, spectra were measured only qualitatively.
bound to the same positions in the chromophore (PD 25) due hinder the nucleophilic solvation of the chromophore,
causes absorption and fluorescence maxima to shift hyp- which is typical of PD solutions in acetonitrile and nor-
sochromically. It should be noted that the observed band mally leads to broadened absorption bands (resulting from
shifts are almost insensitive to the nature of the solvent and the superposition of bands for variously solvated dye
have closely similar magnitudes in absorption and fluores- forms).
cence spectra. Contrastingly, the band intensities and
shapes prove to depend substantially on the solvent param-
eters. Dye 13 dissolved in dichloromethane exhibits the Dye
most intense absorption among the tricarbocyanines under
Table 7. Absorption band halfwidths for tricarbocyanines 22–25.
S [cm–1]
S [cm–1]
(CH3CN)
(CH2Cl2)
study (see Table 2). In this solvent, the bridge groups and
the meso-phenyl substituent in the chromophore slightly re-
duce the molar extinction coefficients of PDs 22, 23, 24
and 25 (by 12.2, 4.8, 6.2, and 8.3%, respectively) and the
absorption band halfwidths also change negligibly (Fig-
ure 5, Table 5 and Table 7). Contrary to this, acetonitrile
solutions of the dyes (Figure 6) show increases in absorp-
tion intensities on passing from unsubstituted PD 13 to
bridged PDs 22, 23, 24 and 25 (by 17.5, 24.0, 46.0, and
28.8%, respectively – cf. Table 2 and Table 6). In addition,
the dye absorption bands become much narrower on chro-
mophore bridging (cf. Tables 5 and 7). Such a trend may be
due to the fact that the bridge groups and the phenyl resi-
22
23
24
25
866
858
893
877
817
798
764
906
Figure 6. Absorption spectra of tricarbocyanines 13 and 22–25 in
CH3CN.
The meso-phenyl substituents in tricarbocyanines 23 and
24 act as slight electron acceptors and hence should cause
bathochromic shifts, according to the Foerster–Dewar–
Knott rule.[19] This effect is only slight in PD 23, containing
the strongly electron-donating dimethylene bridge: the red
shifts amount to 5/3 nm (CH3CN/CH2Cl2) for absorption
and to 12 nm (CH2Cl2) for fluorescence. For comparison,
PD 24, containing the slightly electron-donating trimethyl-
Figure 5. Absorption spectra of tricarbocyanines 13 and 22–25 in
CH2Cl2.
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K. Zyabrev et al.
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ene bridge, exhibits much stronger bathochromic shifts of performed for anionic PDs 1–3 enabled the elucidation of
absorption and fluorescence, so that the respective values the effects of BF2 groups on the frontier level positions and
are 17/13 nm (CH3CN/CH2Cl2) and 16 nm (CH2Cl2).
electron-density distributions in dioxaborine dye molecules.
Unlike dye absorption intensity in dichloromethane, In relation to the analogous dye structures with boron-free
which is almost insensitive to bridging and substitution ef- acyclic end groups, dioxaborine PDs have somewhat in-
fects, fluorescence of bridged and phenyl-substituted PDs creased first transition energies and notably stabilized
22, 23, 24 and 25 in the same solvent is characterized by HOMOs and LUMOs. At the same time, the involvement
decreased quantum yields in relation to the open-chain un- of the BF2 group in the chelate dioxaborine ring causes
substituted PD 13 (cf. Table 2 and Table 6). It can thus be charge equalization on the adjacent oxygen atoms but ex-
assumed that the luminescence efficiency falls off because erts practically no effect on the electron-density distribution
of the electronic factors rather than steric perturbations of in the PC.
the PCs by substituents. To verify this conjecture, we ana-
To study the design potential for dioxaborine PDs, we
lysed the AM1-calculated bond orders in the PCs of com- synthesized the annelated polycarbocyanine series 9, 12, 13
pounds 13, 22, 23, 24 and 25 (see Table 8). As can be seen and 15, as well as the tricarbocyanine series 21–25 with the
from the computational results, introduction of a dimethyl- variously bridged chromophores. As shown by enhanced
ene and a trimethylene bridge and a phenyl ring (PD 22, 23 absorption and fluorescence, and by the lower-lying frontier
and 24) into the chromophore leads to bond equalization, energy levels of PD 9 in relation to PD 5, the novel hetero-
with their order tending to 1.5. Therefore, photoisomeriza- cyclic nucleus 7 shows much promise in the construction of
tions become less probable in the excited state.
various π-conjugated systems with favourable hole-blocking
and electron-injection behaviour.[18]
Table 8. AM1-calculated bond orders in the PCs of tricarbocyan-
ines 13, 22, 23, 24 and 25.
The calculated electron-density distribution for PD 9 and
its 13C NMR signals, as well as the electronic spectral char-
acteristics of the vinylogous series 9, 12, 13 and 15, point
to polymethinic natures of the chromophores in the diox-
aborine dyes. Dye 12 exhibits a record high fluorescence
quantum yield (0.82) among the known open-chain poly-
methine dyes.
Modification of the PC in tricarbocyanine 13 by intro-
duction of various substituents leads to the PD series 21–
25, with their optical behaviour resembling that of structur-
ally similar cationic PDs.[24] However, bridging of the PC
with a vinylene group, as in PD 21, does not cause fluores-
cence enhancement, contrary to what has been observed for
analogous indotricarbocyanines.[24]
Bond of the
dye molecule
13
22
23
24
25
a
b
c
1.322
1.373
1.363
1.323
1.377
1.344
1.334
1.361
1.370
1.325
1.373
1.347
1.276
1.437
1.273
This study demonstrates well that structural variation
both of end nuclei and of the PCs of dioxaborine dyes offers
the advantage of efficient, wide-ranging control of their op-
tical properties, thus showing much practical promise.
At the same time, the electron-donating effects of the di-
methylene and trimethylene groups and the acceptor effect
of the phenyl substitute cause the fluorescence maximum to
shift notably to longer wavelengths (about 800 nm). Thus,
it is likely that the reduced quantum yields of PDs 22–24
are mainly attributable to internal conversion, which is con-
sistent with the fact that the Φ values decrease as the ba-
thochromic effect of the substituent in the PC rises. Intro-
duction of a vinylene bridge results in hypsochromically
shifted spectral maxima and hence in a decreased prob-
ability of internal conversion. On the other hand, dye 25
exhibits a more pronounced bond alternation in the PC and
thus affords more possibilities for photoisomerizations than
its unsubstituted counterpart 13. It is evident that deeper
insight into the fluorescence quenching mechanisms for
PDs 22–25 will require a special study.
Experimental Section
General: Charge distributions and frontier molecular orbital ener-
gies for dyes 1, 2, 3, 5 and 9 were determined for fully optimized
ground-state geometries by standard ab initio calculations with the
aid of the HyperChem. Package at the Unrestricted Hartree–Fock
(UHF) level of theory and with 6–31G** as a basis set.
1H NMR spectra were obtained with Varian VXR 300 and Bruker
Avance DRX 500 instruments at 300 and 500 MHz, respectively
(25 °C); the 13C NMR spectrum was obtained with a Varian Gem-
ini 2000 instrument at 100 MHz (25 °C). Electronic absorption
spectra were recorded on a Shimadzu UV-3100 spectrophotometer.
Fluorescence spectra were taken on Cary Eclipse and Hitachi
F4010 fluorescence spectrophotometers and were fully corrected.
The fluorescence quantum yields (Φ) of the dyes synthesized were
determined by the known method[25] relative to Rhodamine 6G (Φ
= 0.95 in EtOH)[26] for compounds 5 and 9, Nile Blue (Φ = 0.27
in MeOH)[27] for compound 12, indotricarbocyanine (Φ = 0.28 in
EtOH)[23,28] for compounds 13, 22, 23 and 24 and Rhodamine 800
Conclusions
In summary, we have investigated the electronic struc-
tures and spectral properties of symmetric PDs derived
from 2,2-difluoro-1,3,2-dioxaborine. Ab initio calculations (Φ = 0.086 in MeOH)[29] for compound 25.
1556
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 1550–1558

Properties of a Novel Polycarbocyanine Series
Boron Complex 7: A mixture of 1,3-dimethylbarbituric acid (6,
4.68 g, 30 mmol) and glacial acetic acid (4.5 mL) was heated at
reflux until a clear solution was obtained, followed by cooling to
room temperature. Boron trifluoride etherate (4.6 mL, 36 mmol)
was added, and the reaction mixture was heated at reflux for 1 min.
After the system had cooled to room temperature, acetic anhydride
(8.5 mL, 90 mmol) was added. The obtained mixture was heated at
95 °C for 4 h, allowed to cool to room temperature and allowed to
stand for 2 h. The resulting solid was suspended in diethyl ether
(30 mL). The precipitate was filtered off, washed with diethyl ether
(3 ϫ 15 mL), and used in the next steps without further purifica-
tion. Yield 6.7 g (91%); m.p. 172–173 °C. 1H NMR (300 MHz,
[D6]acetone, TMS): δ = 3.29 (s, 3 H, NCH3), 3.22 (s, 3 H, NCH3),
2.66 (s, 3 H, CH3) ppm. C8H9BF2N2O4 (245.98): calcd. C 39.06, H
3.69, N 11.39; found C 39.55, H 3.62, N 11.48.
[D3]acetonitrile, HMDS): δ = 7.76 [t, 3J(H,H) = 13.2 Hz, 2 H,
2ϫCH], 7.26–7.35 (m, 3 H, 2ϫCH + CH), 6.36 [t, 3J(H,H) =
13.2 Hz, 2 H, 2ϫCH], 3.28 (s, 6 H, 2ϫNCH3), 3.17 (s, 6 H,
2ϫNCH3), 3.07 [q, 3J(H,H) = 7.2 Hz, 6 H, 3ϫCH3CH2N+H],
1.17 [t, 3J(H,H)
= 7.2 Hz, 9
H, 3ϫCH3CH2N+H] ppm.
C27H35B2F4N5O8 (655.21): calcd. C 49.49, H 5.38, N 10.69; found
C 48.99, H 5.35, N 11.00.
Tetracarbocyanine 15: Triethylamine (0.3 mL) was added to a
stirred suspension of compound 14[24] (0.114 g, 0.3 mmol) in acetic
anhydride (1.5 mL). The mixture was stirred until the blue precipi-
tate was dissolved (for about 10 min), and boron complex 7
(0.149 g, 0.6 mmol) was then added. After the system had been
stirred for another 30 min, ether (30 mL) was added and the re-
sulting mixture was allowed to stand for 1 h. The precipitate was
filtered off, washed with diethyl ether (3ϫ15 mL) and water
(2ϫ10 mL) and dried. The resulting solid was suspended in ethyl
acetate (2 mL); the precipitate was filtered off and washed with
ethyl acetate (2ϫ0.75 mL). Yield 0.035 g (16%); m.p. Ͼ310 °C. 1H
Hemicyanine 8: A mixture of boron complex 7 (0.49 g, 2 mmol) and
N-ethoxymethylidenaniline (ethyl isoformanilide, 0.33 g, 2.2 mmol)
was heated at 60 °C for 1 h and then allowed to cool to room tem-
perature. The resulting solid was suspended in diethyl ether
(25 mL), and the precipitate was filtered off, washed with diethyl
ether (2ϫ10 mL), and recrystallized from acetic anhydride. Yield
0.56 g (81%); m.p. 227–229 °C (decomp). 1H NMR (300 MHz,
[D6]DMSO, TMS): δ = 11.93 [d, 3J(H,H) = 13.5 Hz, 1 H, NH],
3
NMR (300 MHz, [D3]acetonitrile, HMDS): δ = 7.90 [t, J(H,H) =
3
13.0 Hz, 2 H, 2ϫCH], 7.30 [d, J(H,H) = 13.0 Hz, 2 H, 2ϫCH],
6.25–6.41 (m, 3 H, 2ϫCH + CH), 3.29 (s, 6 H, 2ϫNCH3), 3.19
(s,
3ϫCH3CH2N+H], 2.43 (s, 4 H, 2ϫCH2) 1.18 [t, 3J(H,H) = 7.2 Hz,
H, 3ϫCH3CH2N+H], 0.97 (s,
H, 2ϫCH3) ppm.
6 = 7.2 Hz, 6 H,
H, 2ϫNCH3), 3.08 [q, 3J(H,H)
8.75 [t, 3J(H,H) = 13.5 Hz, 1 H, CH], 7.20–7.52 (m, 5ϫHAr
H), 3.31 (s, H, NCH3), 3.21 (s, H, NCH3) ppm.
+
9
6
1
3
3
C34H45B2F4N5O8 (749.38): calcd. C 54.50, H 6.05, N 9.35; found
C 54.12, H 6.12, N 9.63.
C15H14BF2N3O4 (349.10): calcd. C 51.61, H 4.04, N 12.04; found
C 52.00, H 3.99, N 12.10.
3-Anilinomethylene-2-phenylcyclopenta-1,4-diene-1-carbaldehyde
(20): Compound 18[24] (1.16 g, 3 mmol) was dissolved in CHCl3
(120 mL) at reflux, a solution of 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ, 1.36 g, 6 mmol) in CHCl3 (100 mL) was
added, and the mixture was allowed to stand for 2 h. The precipi-
tate was filtered off and washed with CHCl3 (3ϫ30 mL). The crys-
talline product was mixed with nitromethane (40 mL) and heated
at reflux for 1 h. After cooling to room temperature, it was allowed
to stand for 4 h. The solid was filtered off, washed with nitrometh-
ane (2ϫ10 mL), and suspended in CHCl3 (15 mL) without traces
of EtOH. On addition of triethylamine (1 mL), the mixture was
stirred for 10 min and purified by column chromatography on alu-
minium oxide 90 (standardized, Merck) (CHCl3/MeOH 100:0.5).
Yield 0.12 g (15%); m.p. 109–110 °C. 1H NMR (500 MHz, [D6]-
DMSO, TMS): δ = 11.24 [d, 3J(H,H) = 11.0 Hz, 1 H, NH], 9.49
(s, 1 H, CHO), 7.68 [d, 3J(H,H) = 11.0 Hz, 1 H, CH], 7.36–7.51
Carbo-, Dicarbo- and Tricarbocyanines 9, 12, and 13 (General Pro-
cedure): Triethylamine (0.2 mL) was added to a thoroughly tritu-
rated mixture of boron complex 7 (0.25 g, 1 mmol), hemicyanine
8 (0.35 g, 1 mmol) or malonaldehyde dianil hydrochloride (0.13 g,
0.5 mmol), or glutaconaldehyde dianil hydrochloride (0.14 g,
0.5 mmol) in acetic anhydride (1.5 mL). The mixture was heated at
75 °C for 30 min, and was then allowed to cool to room tempera-
ture and to stand for 2 h. The resulting solid was suspended in
diethyl ether (30 mL). The precipitate was filtered off, washed with
diethyl ether (2ϫ15 mL) and (for compounds 12, 13) with water
(2ϫ10 mL), dried, and purified by a specific method.
Carbocyanine 9: Purification by recrystallization from CH3CN.
1
Yield 0.5 g (84%); m.p. 253–254 °C. H NMR (300 MHz, [D6]ace-
tone, TMS): δ = 8.98 [t, 3J(H,H) = 13.5 Hz, 1 H, CHβ], 7.57 [d,
3
3J(H,H) = 13.5 Hz, 2 H, CHα + CHγ], 3.48 [q, J(H,H) = 7.0 Hz,
3
3
6 H, 3ϫCH3CH2N+H], 3.40 (s, 6 H, 2ϫNCH3), 3.28 (s, 6 H,
2ϫNCH3), 1.43 [t, 3J(H,H) = 7.0 Hz, 9 H, 3ϫCH3CH2N+H] ppm.
13C NMR ([D3]acetonitrile, HMDS): δ = 173.3 (C-9), 165.3
(C-8), 161.1 (C-7), 152.9 (C-12), 151.0 (C-5), 108.2 (C-11), 90.0 (C-
10), 48.6 (3ϫCH3CH2N+H), 29.5 (NCH3), 28.6 (NCH3),
9.5 (3ϫCH3CH2N+H) ppm. C23H31B2F4N5O8 (603.14): calcd. C
45.80, H 5.18, N 11.61; found C 45.23, H 5.23, N 11.72.
(m, 7HAr), 7.28 [d, J(H,H) = 7.3 Hz, 2ϫHAr], 7.17 [t, J(H,H) =
7.3 Hz, 1ϫHAr], 7.02 [d, 3J(H,H) = 5.0 Hz, 1 H, CH], 6.70 [d,
3J(H,H) = 5.0 Hz, 1 H, CH] ppm. C19H15NO (273.33): calcd. C
83.49, H 5.53, N 5.12; found C 83.64, H 5.42, N 5.13.
Tricarbocyanines 21–25 (General Procedure): Triethylamine
(0.15 mL) was added to a mixture of boron complex 7 (0.185 g,
0.75 mmol) and compound 16[24] (0.12 g, 0.38 mmol) or compound
17[24] (0.12 g, 0.38 mmol), or compound 18 (0.15 g, 0.38 mmol) or
compound 19[24] (0.15 g, 0.38 mmol), or aldehyde 20 (0.1 g,
0.38 mmol) in acetic anhydride (1.5 mL). The mixture was heated
at 75 °C for 15 min and cooled to room temperature. After addition
of ether (30 mL), the mixture was allowed to stand for 1.5 h. The
precipitate was filtered off, washed with diethyl ether (3ϫ20 mL)
and (for compounds 21–24) with water (2ϫ10 mL), dried, and
purified by a specific method.
Dicarbocyanine 12: The crude dye was dissolved in CH3CN (7 mL)
and the solid was filtered off. Ether (30 mL) was added to the fil-
trate and the mixture was allowed to stand for 1 h. The precipitate
was filtered off and washed with diethyl ether (2ϫ15 mL). Yield
1
0.27 g (87%); m.p. 249–250 °C. H NMR (300 MHz, [D3]acetoni-
3
trile, HMDS): δ = 7.98 [t, J(H,H) = 13.2 Hz, 2 H, 2ϫCH], 7.34
3
3
[d, J(H,H) = 13.2 Hz, 2 H, 2ϫCH], 6.45 [t, J(H,H) = 13.2 Hz, 1
H, CH], 3.29 (s, 6 H, 2ϫNCH3), 3.18 (s, 6 H, 2ϫNCH3), 3.07 [q,
3J(H,H) = 7.2 Hz, 6 H, 3ϫCH3CH2N+H], 1.17 [t, 3J(H,H) =
7.2 Hz, 9 H, 3ϫCH3CH2N+H] ppm. C25H33B2F4N5O8 (629.18):
calcd. C 47.72, H 5.29, N 11.13; found C 47.48, H 5.33, N 11.20.
Tricarbocyanine 21: The dye was dissolved in CH3CN (30 mL) and
the solid was filtered off. Ether (100 mL) was added to the filtrate
and the mixture was allowed to stand for 2 h. The precipitate was
filtered off and washed with diethyl ether (3ϫ30 mL). Yield 0.10 g
(30%); m.p. Ͼ310 °C. 1H NMR (300 MHz, [D3]acetonitrile,
Tricarbocyanine 13: The dye was purified similarly to dicarbocyan-
1
ine 12. Yield 0.25 g (76%); m.p. 168–169 °C. H NMR (300 MHz,
Eur. J. Org. Chem. 2008, 1550–1558
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1557

K. Zyabrev et al.
FULL PAPER
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HMDS): δ = 7.68 [d, J(H,H) = 13.5 Hz, 2 H, 2ϫCH], 7.22 [d,
3J(H,H) = 13.5 Hz, 2 H, 2ϫCH], 7.00 (s, 1 H, CH), 3.29 (s, 6 H,
3
2ϫNCH3), 3.19 (s, 6 H, 2ϫNCH3), 3.05 [q, J(H,H) = 7.3 Hz, 6
H, 3ϫCH3CH2N+H], 2.71 (s, 4 H, 2ϫCH2), 1.17 [t, 3J(H,H) =
7.3 Hz, 9 H, 3ϫCH3CH2N+H] ppm. C29H37B2F4N5O8 (681.25):
calcd. C 51.13, H 5.48, N 10.28; found C 51.18, H 5.44, N 10.11.
Tricarbocyanine 22: The dye was dissolved in CH3CN (30 mL) and
the solid was filtered off. Ether (100 mL) was added to the filtrate
and the mixture was allowed to stand for 2 h. The precipitate was
filtered off and washed with diethyl ether (2ϫ25 mL). Yield 0.11 g
(32%); m.p. 293 °C. 1H NMR (300 MHz, [D3]acetonitrile, HMDS):
δ = 9.31 (s, 1 H, NH), 7.58 [d, J(H,H) = 13.2 Hz, 2 H, 2ϫCH],
3
7.38 [d, J(H,H) = 13.2 Hz, 2 H, 2ϫCH], 7.14 (s, 1 H, CH), 3.28
(s, 6 H, 2ϫNCH3), 3.18 (s, 6 H, 2ϫNCH3), 3.07 [q, 3J(H,H) =
7.2 Hz, 6 H, 3ϫCH3CH2N+H], 2.36 [t, 3J(H,H) = 6.0 Hz, 4 H,
2ϫCH2], 1.76 [q, 3J(H,H) = 6.0 Hz, 2 H, CH2], 1.17 [t, 3J(H,H)
= 7.2 Hz, 9 H, 3ϫCH3CH2N+H] ppm. C30H39B2F4N5O8 (695.27):
calcd. C 51.82, H 5.65, N 10.07; found C 51.68, H 5.59, N 10.08.
Tricarbocyanine 23: The dye was dissolved in CH2Cl2 (20 mL) and
the solid was filtered off. Ether (70 mL) was added to the filtrate
and the mixture was allowed to stand for 2 h. The precipitate was
filtered off and washed with diethyl ether (2ϫ25 mL). Yield 0.18 g
(66%); m.p. Ͼ310 °C. 1H NMR (300 MHz, [D3]acetonitrile,
HMDS): δ = 7.45–7.49 (m, 3 H), 7.27–7.31 (m, 4 H), 7.18–7.22 (m,
2 H), 3.24 (s, 6 H, 2ϫNCH3), 3.18 (s, 6 H, 2ϫNCH3), 3.05 [q,
3J(H,H) = 7.0 Hz, 6 H, 3ϫCH3CH2N+H], 2.82 (s, 4 H, 2ϫCH2),
[14] J. Fabian, H. Hartmann, J. Phys. Org. Chem. 2004, 17, 359–
369.
[15] B. Domercq, C. Grasso, J.-L. Maldonado, M. Halik, S. Barlow,
S. R. Marder, B. Kippelen, J. Phys. Chem. B 2004, 108, 8647–
8651.
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Polymethine Dyes. Structure and Properties, St. Kliment Ohrid-
ski Univ. Press, Sofia, 1991, pp. 67–68.
[17] K. V. Zyabrev, A. Ya. Il’chenko, Yu. L. Slominskii, A. I. Tol-
machev, Ukr. Khim. Zh. 2006, 72, 56–63 (in Russian).
[18] C. Risko, E. Zojer, P. Brocorens, S. R. Marder, J. L. Brédas,
Chem. Phys. 2005, 313, 151–157.
1.16 [t, 3J(H,H)
= 7.0 Hz, 9
H, 3ϫCH3CH2N+H] ppm.
C35H41B2F4N5O8 (757.36): calcd. C 55.51, H 5.46, N 9.25; found
C 55.43, H 5.48, N 9.31.
[19] a) Th. Forster, Z. Phys. Chem. 1940, 48, 12–31; b) M. J. S. De-
war, J. Chem. Soc. 1950, 2329–2334; c) E. B. Knott, J. Chem.
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1999, 341, 167–172.
[21] a) R. S. Lepkowicz, O. V. Przhonska, J. M. Hales, J. Fu, E. W.
van Stryland, M. V. Bondar, Yu. L. Slominskii, A. D. Kach-
kovskii, Chem. Phys. 2004, 305, 259–270; b) J. Fu, L. A. Pad-
ilha, E. W. van Stryland, O. V. Przhonska, M. V. Bondar,
Yu. L. Slominskii, A. D. Kachkovskii, J. Opt. Soc. Am. B 2007,
24, 56–66; c) J. Fabian, J. Mol. Struct.: THEOCHEM. 2006,
766, 49–60; d) A. B. Ryabitsky, A. D. Kachkovskii, O. V.
Przhonska, J. Mol. Struct: THEOCHEM. 2007, 802, 75–83.
[22] A. A. Ishchenko, N. A. Derevyanko, V. A. Svidro, Opt. Spek-
trosk. 1992, 72, 110–114 (in Russian).
[23] A. A. Ishchenko, Structure and Spectral Luminescent Properties
of Polymethine Dyes, Naukova Dumka, Kiev, 1994, p. 46 (in
Russian).
[24] A. I. Tolmachev, Yu. L. Slominskii, A. A. Ishchenko in Near-
Infrared Dyes for High Technology Applications, NATO ASI
Series, 3. High Technology, vol. 52 (Eds.: S. Daehne, U. Resch-
Genger, O. S. Wolfbeis), Kluwer Academic Publishers, Dord-
recht, Boston, London, 1998, pp. 385–415.
Tricarbocyanine 24: The dye was purified similarly to tricarbocyan-
1
ine 23.Yield 0.15 g (52%); m.p. 204–206 °C. H NMR (300 MHz,
3
[D3]acetonitrile, HMDS): δ = 7.36–7.48 (m, 5 H), 7.20 [d, J(H,H)
= 13.2 Hz, 2 H, 2ϫCH], 7.08–7.16 (m, 4 H), 3.26 (s, 6 H,
3
2ϫNCH3), 3.17 (s, 6 H, 2ϫNCH3), 3.04 [q, J(H,H) = 7.0 Hz, 6
H, 3ϫCH3CH2N+H], 2.51 (s, 4 H, 2ϫCH2), 1.90 (s, 2 H, CH2),
1.18 [t, 3J(H,H)
= 7.0 Hz, 9
H, 3ϫCH3CH2N+H] ppm.
C36H43B2F4N5O8 (771.37): calcd. C 56.05, H 5.62, N 9.08; found
C 55.93, H 5.70, N 9.12.
Tricarbocyanine 25: The crude dye was dissolved in CH3CN
(15 mL) at 50 °C and the solid was filtered off. Ether (50 mL) was
added to the filtrate and the mixture was allowed to stand for 2 h.
The precipitate was filtered off and washed with diethyl ether
(2ϫ15 mL). Yield 0.18 g (63%); m.p. Ͼ310 °C. 1H NMR
(300 MHz, [D3]acetonitrile, HMDS): δ = 7.86–7.91 (m, 3 H), 7.40–
7.52 (m, 4 H), 7.20–7.26 (m, 2 H), 6.79 (s, 2 H, 2ϫCH), 3.28 (s, 6
H, 2ϫNCH3), 3.21 (s, 6 H, 2ϫNCH3), 3.01 [q, 3J(H,H) = 7.2 Hz,
6
H, 3ϫCH3CH2N+H], 1.14 [t, 3J(H,H)
= 7.2 Hz, 9 H,
3ϫCH3CH2N+H] ppm. C35H41B2F4N5O8 (757.36): calcd. C 55.51,
H 5.46, N 9.25; found C 55.43, H 5.48, N 9.31.
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Acknowledgments
The authors are very grateful to Prof. A. A. Ishchenko, Prof. A. D.
Kachkovskii and Dr. M. L. Dekhtyar for helpful discussions.
[1] J. A. VanAllen, G. A. Reynolds, J. Heterocycl. Chem. 1969, 6,
29–35.
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Foutter, Appl. Opt. 1992, 31, 7034–7038.
Received: October 25, 2007
Published Online: February 5, 2008
1558
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 1550–1558