Boradipyrromethenecyanines derived from conformationally restricted nuclei

Article abstract of DOI:10.1016/j.dyepig.2010.01.017

A series of meso-polymethine-substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes were synthesized based on the reaction of meso-methyl borondipyrromethene with a number of hemicyanine derivatives. The dyes obtained exhibited a weak short-wavelength absorption, a strong long-wavelength absorption and weak fluorescence. Upon protonation, the long-wavelength band disappeared while the intensity of?the short-wavelength band increased markedly. The properties of the dyes were closely related to those merocyanine dyes rather than borondipyrromethenes.

Full text of DOI:10.1016/j.dyepig.2010.01.017

Dyes and Pigments 87 (2010) 17e21
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Dyes and Pigments
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Boradipyrromethenecyanines derived from conformationally restricted nuclei
*
Viktor P. Yakubovskyi, Mykola P. Shandura, Yuriy P. Kovtun
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska Street, 02660 Kyiv, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of meso-polymethine-substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes were synthesized
based on the reaction of meso-methyl borondipyrromethene with a number of hemicyanine derivatives.
The dyes obtained exhibited a weak short-wavelength absorption, a strong long-wavelength absorption
and weak fluorescence. Upon protonation, the long-wavelength band disappeared while the intensity
of the short-wavelength band increased markedly. The properties of the dyes were closely related to
those merocyanine dyes rather than borondipyrromethenes.
Received 13 October 2009
Received in revised form
19 January 2010
Accepted 20 January 2010
Available online 2 February 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
BODIPY
Boradiaza-s-indacene
Polymethine dye
Merocyanine
Long-wavelength dye
1. Introduction
are the very positions which can be protonated, unlike terminal
groups [18,19]. Therefore, in case of B dialkylamino groups cannot
The dyes derived from borondipyrromethene (BODIPY) have
attracted considerable attention over the past two decades on account
of their excellent thermal, chemical and photochemical stability,
high molar absorption coefficient, high fluorescence quantum yield,
general insensitivity to solvent polarity and pH, large two-photon
cross-section for multiphoton excitation, lack of ionic charge and good
solubility [1e4]. However, most BODIPY compounds have absorption
maxima below 600 nm. As long-wavelength dyes are important [5,6]
there are many synthetic approaches for modifying the BODIPY
system so as to red shift their absorption maxima.
be referred to as true “end groups” as they are considered in the
ideology of polymethine dyes. Thus, these NR2 groups in structures
B provide considerable spectral effects but do not change the
nature of colour of the BODIPY core. Derivatives C meso-function-
alized with p-dimethylaminophenyl groups [20e25] show much
the same absorption as the unsubstituted BODIPY nucleus and, like
B compounds, drastically reduced fluorescence.
We have recently established that meso-substitution of BODIPY
with more complex heterocycle-terminated polymethine chains
results in the formation of merocyanine dyes (see compounds 5ae7a
in Scheme 3) and thus qualitatively change the very nature of the
BODIPY absorption [26]. These dyes are mainly characterized by
two electronic transitions, one long-wavelength polymethinic and
the other short-wavelength localized in the borondipyrromethene
moiety. In terms of conventional cyanine nomenclature, we have
introduced the term boradipyrromethenecyanines to describe these
new substituted BODIPY systems.
One of the most promising approaches to the modification of
BODIPY is its peripheral functionalization with conjugated chro-
mophores. A wealth of studies have been devoted to the synthesis
of
dimethylamino auxochromes [12e17] (see Scheme 1). The styryl
moieties at the -positions cause a noticeable bathochromic shift in
a-substituted BODIPYs A [7e11] including derivatives B with
a
lambda max of the dye. Introduction of additional 4-dialkylamino
substituents into A (structure B) results in even more pronounced
spectral changes, with bathochromic shifts around 80 nm. Under
acidic conditions only the diethylamino groups are protonated,
leading to the optical properties that are very similar to those of A,
where Ar ¼ Ph. As known, in cyanine dyes the chain carbon atoms
The present work is directed towards the synthesis of dyes
structurally similar to 5ae7a and containing the bridged bor-
ondipyrromethene residue 4 as one end group. It is known that the
bridging of a-aryl substituents causes significant bathochromic shifts
in the absorption of BODIPYs as well as an increase in their molar
absorption coefficients and fluorescence quantum yields [27,28].
Thus, our concern is with the effects caused by these structural
changes on the spectra of the corresponding boradipyrromethene
merocyanines.
* Corresponding author.
E-mail address: kovtun@ioch.kiev.ua (Y.P. Kovtun).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.01.017

18
V.P. Yakubovskyi et al. / Dyes and Pigments 87 (2010) 17e21
Ar
B
effect in absorption is accompanied by a significant increase in the
N
fluorescence quantum yield (see Scheme 2). At the same time, the
spectral properties of dyes 5e8 are much like those of analogous
compounds 5ae7a (see Fig. 1 and Table 1). They also exhibit two
absorption bands: long-wavelength having the polymethinic origin
and short-wavelength assigned to the electronic transition in the
borondipyrromethene moiety. The latter is red-shifted due to
bridging, so that the two bands are closer to each other than for
dyes 5ae7a. In DMF solutions, the absorption of merocyanines 5e7
is shifted only 20e33 nm to longer wavelengths and a little more
intense relative to respective dyes 5ae7a (see Table 1). This unex-
pectedly slight difference in spectral behaviour between bridged
and unbridged dyes is attributable to the competition between the
batho- and hyper-chromic effect of bridging, on the one hand, and
the opposite trend caused by the less polymethinic character of the
bridged dyes, on the other hand. Indeed, bridging makes the bor-
ondipyrromethene nucleus, as well as its boron-free precursor, less
electron-withdrawing; at the same time, it is known that any
decrease in the donor-acceptor strength of end groups brings the
corresponding neutral merocyanines further from the so-called
cyanine limit [30] and hence blue-shifts their absorption.
The borondipyrromethene local absorption peak which appears
for indolenine dye 5 as a shoulder of the polymethinic band
becomes increasingly separate for dyes 6e8 as the basicity of the
second end group rises. The band separation is caused by an
increasing red shift of the long-wavelength polymethinic absorp-
tion as a result of the approach to the cyanine limit with rising
donoreacceptor difference of dye end groups. Accordingly, the
long-wavelength and short-wavelength bands of dye 8 (much
weaker than those of the other dyes in the series) are practically
resolved from each other.
Ar
B
N
N
F
F
N
F
N
R
R
R
R
F
Ar
Ar
N
F
N
B
F
N
N
B
C
A
Scheme 1.
2. Results and discussion
Compound 4 was synthesized similarly to its analogue 4a [26].
Pyrrole 1 [29] was condensed with triethyl orthoacetate in the
presence of p-toluenesulfonic acid to give the tosylate salt of
dipyrromethene 2 (see Scheme 2).
Unlike the previously described unbridged analogue [26], this
salt appears to be quite stable in solutions; the 1H NMR spectrum in
DMSO-d6 exhibits no signs of dissociation. One can therefore assume
that the ethylene bridging of the dipyrromethene moiety makes this
unit more basic. The absorption spectrum of 2 in dichloromethane
likewise suggests its stability, whereas a significantly reduced
molar absorption coefficient in more polar DMF points to the partial
dissociation of the salt (see Table 1). When treated with triethyl-
amine in ethanol, salt 2 is converted to compound 3 characterized by
the 1H NMR-detectable prototropic tautomerism of ethylene 3a and
meso-methyldipyrromethene form 3b. The former is observed in
highly polar DMSO-d6, and the latter in weakly polar CDCl3. Salt 2
was boiled in chloroform with boron trifluoride etherate in the
presence of Hünig's base to produce 4 in high yield (90%); 4 is also
obtainable from 3a under the same conditions.
The meso-methyl group of compound 4 appears to be quite
reactive in condensations with (2-acetanilidovinyl) heterocyclic
derivatives (see Scheme 3). A series of dyes 5e8 containing, respec-
tively, 2-indolenyl, 2-benzothiazolyl, 2-quinolyl, and 4-quinolyl as the
second heterocyclic end residue (in the order of increasing basicity)
were obtained. The corresponding cyanine condensations proceeded
quite smoothly to furnish the dyes desired in good yields (45e65%). It
should be noted that even dye 8 with the very basic 4-quinolyl end
nucleus was prepared successfully, whereas its unbridged analogue
proved inaccessible via the conventional procedure [26].
For all the dyes obtained, just as for their unbridged analogues
5ae7a, protonation with strong acid results in a complete disap-
pearance of the long-wavelength band and an intensity increase of
the short-wavelength band. This effect is exemplified by the
absorption of protonated dye 6 in Fig. 1.
As already mentioned, dipyrromethene
2 and hence its
boron compound 4 are weaker electron-acceptors than their
unbridged analogues. Accordingly, the 1H NMR signal of the
methyl group in compound 4 is shifted upfield from that in 4a
(cf. the respective chemical shifts of 2.64 and 2.76 ppm). Like-
wise, dyes 5e7 exhibit upfield shifts (of about 0.2 ppm) for the
proton resonances of the polymethine chain in comparison to
respective analogues 5ae7a.
As expected, compound 4 is more deeply coloured than its
unbridged counterpart 4a; a noticeable batho- and hyper-chromic
Solvatochromism of the merocyanines obtained is illustrated
by the absorption of dye 5 which demonstrates batho- and
i
ii
NH
N
NH
HN
NH
N
N
H
TosOH
3a
1
3b
2
iii
iii
N
N
N
N
B
B
F
F
F
F
4a
4
λabs= 621 nm (CH2Cl2)
λabs= 544 nm (CH2Cl2)
ε = 57000, φ = 0.64
ε = 134000, φ = 0.86
Scheme 2. Reagents: (i) CH3(OEt)3, TosOH$H2O; (ii) Et3N, EtOH; (iii) BF3$Et2O, Et(i-Pr)2N.

V.P. Yakubovskyi et al. / Dyes and Pigments 87 (2010) 17e21
19
Ph
Ac
N
N
N
X
X
N+
H
N+
N
Ph
-
Y
Tos
N
N
B
N
N
F
B
F
F
N
N
F
B
F
F
4
X = C(CH3)2 (5), X = S (6),
X = CH=CH (7)
8
N
X
N
N
F
B
F
X = C(CH3)2 (5a), X = S (6a),
X = CH=CH (7a)
Scheme 3.
hyper-chromic effects with increasing solvent polarity (see Fig. 2).
Solvent effects decrease as the basicity of the second heterocyclic
nucleus rises: dyes 5e8 show the respective red shifts of 38, 21, 10,
and 9 nm on going from dichloromethane to DMF. This tendency is
consistent with the fact that the bridged dyes with the second end
group of high basicity are closer to the so-called cyanine limit and
hence less solvent-sensitive.
The comparative analysis of the two series shows that for
the bridged dyes with weakly and moderately basic end resi-
dues 5 and 6 solvatochromic red shifts are larger than those
for the unbridged 5a and 6a (Table 1). This suggests that
dyes 5e6 are further from the cyanine limit and hence more
solvatochromic than 5ae6a due to the decreased electron-
withdrawing ability of the bridged borondipyrromethene end
nucleus. As the basicity of the second heterocyclic nucleus rises
further, the solvatochromic effects for bridged and unbridged
dyes are equalized (7 and 7a).
3. Experimental
Electronic absorption spectra were recorded on a Shimadzu UV-
3100 spectrophotometer. 1H (300 MHz, 25 ꢀC, Si(CH3)4 as internal
standard) and 19F NMR (188 MHz, CFCl3 as internal standard)
spectra were obtained with a Varian VXR-300 instrument. LC/MS
spectra were recorded using a liquid chromatography/mass spec-
trometric system consisting of an Agilent 1100 Series high-perfor-
mance liquid chromatograph equipped with a diode-matrix and an
Agilent LC/MSD SL mass-selective detector. Fluorescence spectra
were recorded on Solar CM 2203 fluorescence spectrophotometer.
Relative fluorescence quantum yields (4) were determined relative
to indodicarbocyanine [31] (for compound 4) and pentamethyne
dioxaborinate [32] (for compounds 5e8).
3.1. Compound 2
Compound 4, being a typical BODIPY, exhibits a narrow fluo-
A mixture of pyrrole 1 (1.69 g, 10 mmol), p-toluenesulfonic acid
(1 g, 5.25 mmol), and triethyl orthoacetate (5.5 mL, 30 mmol) was
stirred at room temperature for 20 min. After diluting with ethyl
acetate (30 mL) the reaction mixture was allowed to stand for
20 min and the product was filtered off. Yield 1.78 g, 66%. M.p.
rescence band with
a quantum yield of 0.86. Polymethine-
substituted dyes 5-e8 as well as their analogues 5ae7a are weakly
fluorescent, particularly in polar solvents; the protonated dyes are
likewise non-fluorescent.
Table 1
Optical properties of prepared compounds.
Dye
CH2Cl2
DMF
labs, nm
fwhma, cmꢁ1
lem, nm (4f)
labs, nm
$10ꢁ3, Mꢁ1 cmꢁ1
)
fwhma, cmꢁ1
lem, nm (4f)
(
3
$10ꢁ3, Mꢁ1 cmꢁ1
)
(
3
2
4
5
6
7
8
606 (108)
621 (134)
664 (103)
699 (119)
741 (145)
797 (49)
661 (108)
679 (110)
710 (125)
875
625
603 (47)
967
685
631 (0.86)
723 (0.011)
741 (0.005)
n.d.b
617 (113)
702 (107)
724 (123)
751 (140)
806 (56)
682 (116)
693 (105)
718 (128)
2334
2569
1170
1163
1631
2289
1432
2762
1627
1026
1056
1634
1558
1334
n.d.b
n.d.b
n.d.b
n.d.b
736 (<0.001)
729 (<0.001)
754 (<0.001)
n.d.b
5ac
6ac
7ac
712 (0.006)
726 (0.005)
745 (0.004)
a
Full width at half-maximum height.
Too weak to be reliably detectable.
The data taken from Ref. [26].
b
c

20
V.P. Yakubovskyi et al. / Dyes and Pigments 87 (2010) 17e21
3.3. Compound 4
1,5
1,0
0,5
0,0
To a stirred solution of salt 2 (1.5 g, 2.8 mmol) in dry chloroform
7
(50 mL), first boron trifluoride etherate (5 mL, 40 mmol) and then N,
N-diisopropylethylamine (2 mL, 12 mmol) were added. After the
reaction mixture was refluxed with stirring for 1 h, it was cooled and
washed with water (3 ꢂ 50 mL), and the organic layer was dried over
Na2SO4. Chloroform was evaporated and the residue was chroma-
tographed on silica gel (using a 3:1 mixture of hexane and ethyl
acetate as eluent). Yield 1.03 g, 89%. M.p. >250 ꢀC. 1H NMR (DMSO,
6
5
4
+
6 + H
8
300 MHz):
d
¼ 2.64 (s, 3H, CH3), 2.73 (q, J ¼ 6.3 Hz, 4H, CH2), 2.90
(q, J ¼ 6.3 Hz, 4H, CH2), 7.38e7.43 (m, 8H), 8.53 (d, J ¼ 7 Hz, ArH). 19
F
NMR (DMSO, 188 MHz):
d
¼ ꢁ136.43 (q). LC-MS: m/z 392 ([M ꢁ 19
(F)]þ). Anal. calcd. for C26H21BF2N2: C, 76.1; H, 5.12; N, 6.83. Found: C,
76.25; H, 5.32; N, 6.9.
3.4. General procedure for the synthesis of dyes 5e7
300
400
500
600
700
800
900
Wavelength
To a mixture of compound 4 (0.1 g, 0.25 mmol) and the corre-
sponding hemicyanine (0.3 mmol) in acetic anhydride (1 mL),
triethylamine (0.04 g, 0.4 mmol) was added. The reaction mixture
was boiled for 2e3 min and allowed to stand at room temperature
overnight. The precipitated product was filtered off.
Fig. 1. Absorption spectra of dyes 4 and 5e8 in DMF (C ¼ 1 ꢂ10ꢁ5 mol/l).
174e176 ꢀC. 1H NMR (DMSO, 300 MHz):
d
¼ 2.30 (s, 1H, CH3)
2.88e2.94 (m, 7H,), 3.02 (q, 4H, CH2), 7.12e7.19 (m, 3H), 7.46e7.57
(m, 9H), 8.19 (d, J ¼ 3.6 Hz, 2H, ArH), 12.68 (s, 1H, NH). LC-MS: m/z
536 ([M þ H]þ). Anal. calcd. for C33H30N2SO3: C, 74.16; H, 5.62; N,
5.24. Found: C, 74.37; H, 5.4; N, 7.51.
3.5. Dye 5
Yield 0.08 g, 52%. M.p. > 250 ꢀC. 1H NMR (DMSO, 300 MHz):
d
¼ 1.63 (s, 6H, CH3), 2.72 (q, J ¼ 7 Hz, 4H, CH2), 2.86 (q, J ¼ 7 Hz, 4H,
CH2), 3.48 (s, 3H, NCH3), 6.24 (d, J ¼ 12.6 Hz, 1H, CH), 7.07e7.36 (m,
3.2. Compound 3
12H), 7.48 (d, J ¼ 8.1 Hz, 1H, HetH), 8.17 (t, J ¼ 12,9 Hz, 1H, CH), 8.46
(d, J ¼ 7.2 Hz, 2H, ArH). 19F NMR (DMSO,188 MHz):
¼ ꢁ134.44 (q).
d
To a suspension of salt 2 (1.07 g, 2 mmol) in ethanol (10 mL),
triethylamine (0.4 g, 4 mmol) was added, followed by stirring the
reaction mixture at room temperature for 30 min. The resulting
precipitate was filtered off and washed with ethanol. Yield 0.54 g,
LC-MS: m/z 594 ([M þ H]þ). Anal. calcd. for C39H34BF2N3: C, 78.92;
H, 5.73; N, 7.08. Found: C, 79.07; H, 5.4; N, 7.1.
74%. 1H NMR (CDCl3, 300 MHz):
d
¼ 2.54 (s, CH3), 2.87 (q, J ¼ 7 Hz,
3.6. Dye 6
4H, CH2), 3.01 (q, J ¼ 7 Hz, 4H, CH2), 6.89 (s, 2H, pyrrole H),
Yield 0.09 g, 63%. M.p. >250 ꢀC. 1H NMR (DMSO, 300 MHz):
7.32e7.42 (m, 6H), 7.86 (d, J ¼ 7.5 Hz, 2H, ArH) 4b; 1H NMR (DMSO,
d
¼ 2.7 (q, J ¼ 7.2 Hz, 4H, CH2), 2.84 (q, J ¼ 7 Hz, 4H, CH2), 3.79
300 MHz):
d
¼ 2.64 (q, J ¼ 7 Hz, 4H, CH2), 2.85 (q, J ¼ 7 Hz, 4H, CH2),
(s, 3H, NCH3), 6.67 (d, J ¼ 12.6 Hz, 1H CH), 7.06 (s, 2H, pyrrole H),
7.17e7.35 (m, 8H), 7.52 (t, J ¼ 8.1 Hz, 1H, HetH), 7.66 (d, J ¼ 8.4 Hz,
1H, HetH), 7.86e7.96 (m, 2H), 8.41 (d, J ¼ 8.4 Hz, 2H, ArH). 19F NMR
5.34 (s, 2H, C]6;CH2), 6.15 (s, 2H, pyrrole H), 7.09e7.19 (m, 6H), 7.69
(d, J ¼ 7 Hz, 2H, ArH) 4a. LC-MS: m/z 364 ([M þ H]þ). Anal. calcd. for
C26H22N2: C, 86.19; H, 6.08; N, 7.73. Found: C, 86.27; H, 5.92; N, 7.5.
(DMSO, 188 MHz):
d
¼ ꢁ133.94e134.4 (m). LC-MS: m/z 584
([M þ H]þ). Anal. calcd. for C36H28BF2N3S: C, 74.1; H, 4.8; N, 7.2.
Found: C, 74.27; H, 5.05; N, 7.1.
3
3.7. Dye 7
2
1,0
4
Yield 0.09 g, 64%. M.p. >250 ꢀC. 1H NMR (DMSO, 300 MHz):
1
d
¼ 2.69 (q, J ¼ 7.2 Hz, 4H, CH2), 2.82 (q, J ¼ 7.2 Hz, 4H, CH2), 3.96
(s, 3H, NCH3), 6.74 (d, J ¼ 12.3 Hz, 1H, CH), 7.08 (s, 2H, pyrrole H),
7.13e7.19 (m, 3H), 7.26e7.33 (m, 4H), 7.46 (t, J ¼ 7.2 Hz, 1H, HetH),
7.73e8.05 (m, 5H), 8.36e8.43 (m, 3H). 19F NMR (DMSO, 188 MHz):
d
¼ ꢁ133.77e134.05 (m). LC-MS: m/z 580 ([M þ H]þ). Anal. calcd. for
0,5
C38H31BF2N3: C, 78.9; H, 5.4; N, 7.26. Found: C, 79.05; H, 5.24; N, 7.11.
3.8. Dye 8
To a mixture of compound 4 (0.1 g, 0.25 mmol) and 4-(2-ace-
tanilidovinyl)-1-methylquinolinium tosylate (0.12 g 0.3 mmol) in
pyridine (1 mL), triethylamine (0.04 g, 0.4 mmol) was added. The
reaction mixture was heated to boiling and boiled for 2e3 min,
followed by cooling to room temperature. After evaporation of
pyridine in vacuo, the residue was chromatographed on silica gel
(using dichloromethane as eluent). Yield 0.06 g, 43%. M.p. >250 ꢀC.
0,0
300
400
500
600
700
800
Wavelength
Fig. 2. Solvatochromism of dye 5 (C ¼ 1 ꢂ10 ꢁ5 mol/l). 1, hexane
(
(
lmax ¼ 626 nm,
lmax ¼ 664 nm,
3
3
¼ 92,000); 2, toluene
(
lmax ¼ 650 nm,
3
¼ 98,000); 3, CH2Cl2
¼ 103,000); 4, DMF (lmax ¼ 702 nm,
3
¼ 107,000).
1H NMR (DMSO, 300 MHz):
d
¼ 2.69 (q, J ¼ 6.9 Hz, 4H, CH2),

V.P. Yakubovskyi et al. / Dyes and Pigments 87 (2010) 17e21
21
[13] Yu YH, Descalzo AB, Shen Z, Rohr H, Li Q, Wang YW, et al. Mono- and Di(dime-
thylamino)styryl-substituted borondipyrromethene and borondiindomethene
dyes with intense near-infrared fluorescence. Chemistry - An Asian Journal
2006;1-2:176e87.
[14] Saki N, Dinc T, Akkaya EU. Excimer emission and energy transfer in cofacial
boradiazaindacene (BODIPY) dimers built on a xanthene scaffold. Tetrahedron
2006;62:2721e5.
[15] Peng X, Du J, Fan J, Wang J, Wu Y, Zhao J, et al. A selective fluorescent sensor for
imaging Cd2þ in living cells. Journal of the American Chemical Society
2007;129:1500e1.
[16] Deniz E, Isbasar GC, Bozdemir OA, Yildirim LT, Siemiarczuk A, Akkaya EU.
Bidirectional switching of near IR emitting boradiazaindacene fluorophores.
Organic Letters; 2008:3401e3.
2.82 (q, J ¼ 6.9 Hz, 4H, CH2), 4.07 (s, 3H, NCH3), 6.7 (s, 2H, pyrrole H),
7.11e7.38 (m, 8H), 7.61 (d, J ¼ 7.2 Hz, 1H, HetH), 7.68e7.73 (m, 1H),
7.93 (d, J ¼ 3.3 Hz, 2H, HetH), 8.25 (d, J ¼ 6.9 Hz, 1H, HetH),
8.37e8.52 (m, 4H). 19F NMR (DMSO,188 MHz):
d
¼ ꢁ133.61e133.91
(m). LC-MS: m/z 580 ([M þ H]þ). Anal. calcd. for C38H31BF2N3: C,
78.9; H, 5.4; N, 7.26. Found: C, 79.1; H, 5.3; N, 7.15.
4. Conclusions
Merocyanines 5e8 derived from a conformationally restricted
(bridged) borondipyrromethene nucleus represent long-wave-
length polymethine dyes and much resemble, by spectral behav-
iour, their analogues 5ae7a with flexible conformations of phenyl
substituents. Absorption spectra of dyes 5e8 also contain two
bands, one assigned to the polymethinic transition and the other to
the local transition in the borondipyrromethene moiety. It has
been shown that bridging mainly affects the location of the short-
wavelength band and slightly affects the long-wavelength band.
Like conformationally free dyes 5ae7a, merocyanines 5e7 exhibit
weak fluorescence (only in nonpolar media).
[17] Baruah M, Qin W, Flors C, Hofkens J, Vallee RAL, Beljonne D, et al. Solvent and
pH dependent fluorescent properties of
a dimethylaminostyryl bor-
ondipyrromethene dye in solution. Journal of Physical Chemistry Part A 2006;
110:5998e6009.
[18] James TH. The theory of the photographic process. New York: Macmillan; 1973.
[19] Tolmachev AI, MYu Kornilov, Karaban EF. Direction of protonation in poly-
methine dyes. Theoretical and Experimental Chemistry 1976;12:644.
[20] Werner T, Huber C, Heinl S, Kollmannsberger M, Daub J, Wolfbeis O. Novel
optical pH-sensor based on a boradiaza-indacene derivative. Fresenius' Jour-
nal of Analytical Chemistry 1997;359:150.
[21] Kollmannsberger M, Gareis T, Heils S, Brue J, Daub J. Electrogenerated chem-
iluminescence and proton-dependent switching of fluorescence: functional-
ized difluoroboradiaza- s-indacene. Angewandte Chemie International Edition
1997;36:1333e5.
[22] Kollmannsberger M, Rurack K, Resch-Genger U, Daub J. Ultrafast charge
transfer in amino-substituted boron dipyrromethene dyes and its inhibition
by cation complexation: a new design concept for highly sensitive fluorescent
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