Benzo[1,2-d:4,5-d']bisimidazoles as a convenient platform towards dyes that are capable of excited-state intramolecular proton transfer and of two-photon absorption

Article abstract of DOI:10.1002/asia.201200539

New, strongly fluorescent benzo[1,2-d:4,5-d']bisimidazoles have been prepared by the reaction of Bandrowski's base with various aldehydes. Their structures were carefully designed to achieve efficient excited-state intramolecular proton transfer and good two-photon-absorption (2PA) cross-sections. Functional dyes that possessed both high fluorescence quantum yields and large Stokes shifts were prepared. A π-expanded D-A-D derivative that possessed φ_fl=50 % and σ₂=230 GM in the spectroscopic area of interest for biological imaging is an excellent candidate as a fluorescent probe. Thanks to the presence of two reactive amino groups, such compounds can be easily transformed into probes for bioconjugation. All of these benzo[1,2-d:4,5-d']bisimidazoles were also strongly fluorescent in the solid state. Copyright

Full text of DOI:10.1002/asia.201200539

DOI: 10.1002/asia.201200539
Benzo[1,2-d:4,5-d’]bisimidazoles as a Convenient Platform Towards Dyes
that are Capable of Excited-State Intramolecular Proton Transfer and of
Two-Photon Absorption
Mariusz Tasior,[a] Vincent Hugues,[b] Mireille Blanchard-Desce,*[b] and
Daniel T. Gryko*[a, c]
Abstract: New, strongly fluorescent
benzo[1,2-d:4,5-d’]bisimidazoles have
been prepared by the reaction of Ban-
drowski’s base with various aldehydes.
Their structures were carefully de-
signed to achieve efficient excited-state
intramolecular proton transfer and
good two-photon-absorption (2PA)
cross-sections. Functional dyes that
possessed both high fluorescence quan-
tum yields and large Stokes shifts were
prepared. A p-expanded D-A-D deriv-
ative that possessed Ffl =50% and s2 =
230 GM in the spectroscopic area of in-
terest for biological imaging is an ex-
cellent candidate as
a
fluorescent
probe. Thanks to the presence of two
reactive amino groups, such com-
pounds can be easily transformed into
probes for bioconjugation. All of these
benzo[1,2-d:4,5-d’]bisimidazoles were
also strongly fluorescent in the solid
state.
Keywords: absorption
·
dyes/
pigments · fluorescence · fluores-
cent probes · nitrogen heterocycles
Introduction
transfer of a hydroxy proton to a neighboring nitrogen- or
oxygen atom, which leads to a significant increase in the
Stokes shift.[2] This phenomenon has been utilized in the
design of fluorescent sensors,[3] laser dyes,[4] ultraviolet sta-
bilizers,[5] probes for biological environments,[6] and, recent-
ly, in organic light-emitting devices.[7] Among the many com-
pounds that display ESIPT, benzoxazoles,[8] flavones,[9] imi-
The development of new luminescent platforms for applica-
tions in bioimaging has been one of the major focal points
in organic chemistry over the last few decades. Highly selec-
tive fluorescent probes have opened new avenues in moni-
toring key functions and dynamic changes within living or-
ganisms.[1] One of the problems in fluorescence microscopy
is the relatively small Stokes shift of many commercially
available dyes. In this case, the absorption- and emission
spectra overlap, thereby leading to self-absorption and poor
distinction between excitation- and emission light in the
sample. As a consequence, typical multicolor (multiple-dye)
imaging presents a true challenge. A possible solution in-
volves modifying the core of the fluorescent probe so as to
allow excited-state intramolecular proton transfer (ESIPT)
to occur. In most classical examples, ESIPT involves the
ACTHNUTRGNEUNG
dazoles,[10] imidazo[1,2-a]pyridines,[11] and benzoquinolines[12]
have attracted the most attention. Unfortunately, the major-
ity of these compounds absorb in the UV region, which is
highly undesirable in biological applications. Ideally, a fluo-
rescent probe should absorb in the so-called “biological
window” (650–900 nm), which allows for deep tissue pene-
tration and low background fluorescence without the photo-
chemical side-effects that are often observed after ultravio-
let irradiation.[13] This combination can be accomplished
without substantial modification of the parent fluorescent
platform by exploiting the nonlinear properties of the de-
sired material, in particular, two-photon absorption. Two-
photon absorbing materials have found many applications in
various fields, such as optical limiting, multiphoton-pumped
frequency-upconversion lasing, polymerization microfabrica-
tion, 3D data storage, photodynamic therapy, and two-
photon excited fluorescence.[14] Surprisingly, there are very
few reports on molecules that simultaneously undergo both
of these aforementioned phenomena.[15] Herein, we present
our preliminary studies on two-photon absorbing benzobisi-
midazoles and their excited-state intramolecular proton-
transfer properties.
[a] Dr. M. Tasior, Prof. D. T. Gryko
Institute of Organic Chemistry
Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw (Poland)
Fax : (+48)226326681
E-mail: dtgryko@icho.edu.pl
[b] V. Hugues, Dr. M. Blanchard-Desce
Universitꢀ de Bordeaux, ISM (UMR5255 CNRS)
F 33400, Bordeaux (France)
[c] Prof. D. T. Gryko
Warsaw University of Technology
Faculty of Chemistry
Noakowskiego 3, 00-664 Warsaw (Poland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200539.
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Results and Discussion
Design and Synthesis
To the best of our knowledge, benzo[1,2-d:4,5-d’]bisimida-
zoles have not been subjected to ESIPT or 2PA studies.
Their core can be considered as relatively electron poor.
The facile attachment of electron-rich groups at the periph-
ery of this heterocycle through ethenyl or ethynyl linkers
should lead to classical quadrupolar D-p-A-p-D systems
(where D and A are electron donors and -acceptors, respec-
tively, and p denotes a p-conjugated bridge), which are
known to effectively promote simultaneous two-photon ab-
sorption.[14a–c] It has been shown that derivatives of
benzo[1,2-d:4,5-d’]bisimidazole can be utilized as sensing
molecules and that they interact with G-quadruplex
DNA.[16] Their quaternary salts were found to be strongly
fluorescent.[17] We envisioned that benzo[1,2-d:4,5-d’]bisimi-
dazoles, if suitably substituted, should be an ideal core for
the construction of two-photon absorbing and ESIPT-capa-
ble materials.
The synthesis of substituted benzo[1,2-d:4,5-d’]bisimida-
zoles has been much less explored than similar benzo[1,2-
d:4,5-d’]bisoxazole systems.[18] They have usually been syn-
thesized from 1,2,4,5-tetraaminobenzene[16a,b] or, more re-
cently, by Pd-catalyzed tandem coupling/ring closure.[19]
However, very recently, Siri and co-workers proposed
a new, elegant synthetic route to these heterocycles,[20] start-
ing from the readily available Bandrowski’s base[21] and an
appropriate aldehyde. The desired compounds were pre-
pared in just one step (as a result of nucleophilic addition
followed by prototropic rearrangement, cyclization, and oxi-
dation in air), and possessed two N-(4-aminophenyl) sub-
stituents that could be used for further functionalization.
This last feature is particularly attractive because it affords
easy access to compounds that are suitable for bioconju-
gation.
Scheme 1. Synthesis of 2-hydroxyphenyl-substituted benzobisimidazole 2.
2-allyloxybenzaldehyde with Bandrowski’s base, followed by
Claisen rearrangement, gave the expected product (2) in
moderate overall yield (Scheme 1).
Subsequently, we prepared a model compound for 2PA
studies. The condensation of Bandrowski’s base with 4-(N,N-
dimethylamino)cinnamaldehyde yielded the expected p-ex-
panded product (3, Scheme 2). Unfortunately, similar reac-
We initially focused our attention on benzobisimidazoles
for ESIPT studies. Unfortunately, the simple reaction of
Bandrowski’s base with salicylaldehyde did not lead to the
expected product. We suspected that phenol protection
would help to overcome this problem and assumed that
simple methyl ethers would not be sufficient for this purpose
because their deprotection in the presence of amino groups
might be problematic. Therefore, we decided to use an allyl
ether that can be removed thermally. Indeed, the reaction of
Abstract in Polish: W wyniku reakcji odpowiednio podsta-
wionych aldehydꢂw z zasada˛ Bandrowskiego otrzymano
nowe, silnie fluoryzuja˛ce benzobisimidazole. Ich struktura
została starannie zoptymalizowana pod ka˛tem uzyskania wy-
dajnego przeniesienia protonu w stanie wzbudzonym, naste˛-
puja˛cego w wyniku jednoczesnej absorpcji dwꢂch fotonꢂw.
Scheme 2. Synthesis of p-expanded benzobisimidazole 3.
tions with phenylpropargyl aldehyde derivatives only led to
tar-like products. Thus, we decided to prepare compound 4
from trimethylsilylpropynal (Scheme 3). Subsequent alkyne
deprotection yielded highly insoluble compound 5, which
was completely unreactive to palladium-catalyzed coupling
´
Dzie˛ki obecnosci dwꢂch reaktywnych grup aminowych zsyn-
´
tetyzowane zwia˛zki daja˛ moz˙liwosci dalszych modyfikacji
i uz˙ycia w dwufotonowej mikroskopii fluorescencyjnej.
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Benzo[1,2-d:4,5-d’]bisimidazoles as a Platform for Dyes
Figure 1. Absorption (solid line), emission (dashed line), and 2PA spectra
(dashed–dotted line) of compound 1.
Scheme 3. Attempted approach to another p-expanded benzobisimida-
zole.
with 4-bromo-N,N-dimethylaniline. For this reason, we at-
tempted a “sila”-Sonogashira coupling reaction with highly
soluble compound 4, but all attempts were met with failure.
It is worth pointing out that the prepared compounds can
very often be purified by simple crystallization from the re-
action mixture.
Figure 2. Absorption (solid line), emission (dashed line), and 2PA spectra
(dashed–dotted line) of compound 2 in CH2Cl2.
this model compound, compound 2 exhibits both shifted ab-
sorption (380 nm versus 330 nm) and emission spectra
(506 nm versus 430 nm). The enol syn form exhibits ESIPT
owing to favorable electronic and structural properties,
whereas ESIPT is prohibited in both the anti and open
forms owing to unfavorable thermodynamic parameters.
Although the formation of
Linear Photophysical Properties
The spectroscopic properties of the prepared benzo[1,2-
d:4,5-d’]bisimidazoles are summarized in Table 1 and Fig-
ures 1–3. 2-(2’-Hydroxyphenyl)benzimidazole is a prototype
of molecules that exhibit ESIPT.[22] When compared with
the enol anti conformer in com-
pound 2 is blocked, the overall
Table 1. Spectroscopic data for compounds 1, 2, and 3 in CH2Cl2.
max
max
[a]
Compound
labs [nm]
emax [mÀ1 cmÀ1
]
lem [nm]
Stokes shift [cmÀ1
]
F [%]
19.7[b]
16.5[b]
14[b,d]
7[b,e]
1.5[b,f]
50[c]
number of possible conformers
is four. The distribution of the
conformers is governed by the
substitution pattern and by the
polarity of the solvent, with
polar and/or protic solvents fa-
voring the enol open conformer
(stabilized by intermolecular
hydrogen bonds with the sol-
1
2
313
3.53ꢃ104
6.36ꢃ104
6.28ꢃ104
2.03ꢃ104
407
500[d]/506
501[e]/501[f]
7380
6480
383[d]/381/378[e]/376[f]
365[d]/363/360[e]/360[f]
307
3
437
301
3.26ꢃ104
1.34ꢃ104
488
2390
[a] Stokes shift=1/labsÀ1/lem; [b] standard: quinine bisulfate; [c] standard: fluoresceine; [d] measured in tolu-
ene; [e] measured in MeCN; [f] measured in MeOH.
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Mireille Blanchard-Desce, Daniel T. Gryko et al.
FULL PAPER
ingly high in non-polar solvents, even though ESIPT very
often causes a significant decrease in emission strength
owing to competitive internal conversion followed by vibra-
tional relaxation. The fluorescence quantum yield decreased
with an increase in the polarity of the solvent (Table 1).
Park and co-workers showed that tetrasubstituted imidazole
derivatives (and compound 2 can be considered as such)
that contain a 2-hydroxyphenyl substituent at position 2 un-
dergo ESIPT and display unusually high fluorescence quan-
tum yields (0.18).[7] Interestingly, compound 1, in which
ESIPT is blocked, possesses a relatively large Stokes shift,
thus suggesting a highly distorted excited state.
Compared to compounds 1 and 2, the absorption of com-
pound 3 in CH2Cl2 is significantly red-shifted as a result of
extended p-conjugation, although, surprisingly, emax is not
larger (Figure 3). The strongest absorption band of com-
pound 3 has several shoulders, which may either be due to
aggregation or to the existence of different conformers. Ag-
gregation typically leads to emission-quenching, so the fact
that compound 3 is strongly fluorescent in solution (with the
quantum yield reaching 50%) favors the second possibility.
Compared to polymers of similar benzobisoxazoles, both the
absorption- and emission maxima of compound 3 are blue-
shifted by more than 70 nm.[18d] Such behavior may be ex-
plained by the stronger electron-withdrawing character of
the benzobisoxazole, which leads to a more polarized struc-
ture. Similarly, compound 2 has a rich, red-shifted absorp-
tion spectrum in CH2Cl2 with three major bands and one
shoulder. This behavior is commonly interpreted as being
a result of the presence of different conformers (Scheme 4).
Figure 3. Absorption (solid line), emission (dashed line), and 2PA spectra
(dashed–dotted line) of compound 3.
Non-Linear Photophysical Properties
Two-photon absorption measurements of compounds 1–3
were performed by using the TPEF method (Table 2). As
Table 2. Two-photon data for 1, 2 and 3 measured in CH2Cl2.
max
max
Compound 2lOPA [nm] lTPAmax [nm] s2 [GM][a] s2maxF [GM]
1
2
626
762
726
614
874
602
ꢀ710
ꢁ7.4
>1.5
2
730
14
Scheme 4. Three possible 2-(2’-hydroxyphenyl)benzimidazole conformers.
3
740
228
114
vent) and non-polar solvents favoring the enol syn- and anti
conformers (Scheme 4). The absorption spectrum of com-
pound 2 was measured in four different solvents and its
maximum was around 380 nm (p–p* transition). The fluo-
rescence spectra of compound 2 in CH2Cl2 and in toluene
only showed a single ESIPT band (at 506 and 501 nm, re-
spectively, Figure 2). These bands were shifted to longer
wavelengths by almost 100 nm compared to compound
1 and had a Stokes shift of 6480 cmÀ1.
Additional bands appeared in MeCN (403 nm, 17% of
the overall fluorescence intensity) and MeOH (405 nm, 2%
of the overall fluorescence intensity), thereby indicating
conformational equilibrium in solution in the ground state.
The fluorescence quantum yield of compound 2 was surpris-
[a] GM=Goeppert-Mayer unit; 1 GM=10À50 cm4 sphotonÀ1
.
expected, compound 3 gave the highest 2PA cross-section
(228 GM) owing to extended p-overlap and pronounced di-
polar character. In combination with high fluorescence
quantum yield, this property leads to relatively good two-
photon brightness. On the other hand, the results obtained
for compound 2 suggest that the 2-hydroxyphenyl substitu-
ent in the enol syn conformer is not an efficient electron
donor in the D-A-D system. Apparently, the decreased ri-
gidity, together with moderate conjugation in compound 2,
causes a significant drop in 2PA cross-section.
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Benzo[1,2-d:4,5-d’]bisimidazoles as a Platform for Dyes
zation of the product. The precipitate was filtered off and then recrystal-
lized from CHCl3 and DMSO. The resulting orange crystals were washed
with EtOH and dried under high vacuum to give 300 mg (16%) of com-
pound 3. M.p.: >3008C; 1H NMR (500 MHz, CF3COOD): d=3.42 (s,
12H; NCH3), 6.98 (d, J=16.5 Hz; CH=CH), 7.67, 7.79 (AA’BB’, J=
7.8 Hz, 8H; C6H4), 7.93 (s, 2H; benzobisimidazole), 7.96, 8.08 (AA’BB’,
J=7.8 Hz, 8H; C6H4), 8.17 ppm (d, J=16.5 Hz; CH=CH), NH2 signal
was not detected; 13C NMR (125 MHz, CF3COOD): d=49.3, 100.7,
110.8, 123.3, 129.1, 131.9, 133.3, 134.9, 135.9, 138.1, 146.4, 149.1,
Conclusions
In summary, three new benzo[1,2-d:4,5-d’]bisimidazoles
were synthesized and their optical properties were studied.
Compound 2, which contained two 2-hydroxyphenyl units
and emitted yellow–green light, underwent efficient ESIPT,
although the Ffl value remained remarkably high (16.5%).
Ground-state conformational equilibrium in solvents of dif-
ferent polarity and proticity was observed, although, after
excitation, the dominant radiative decay pathway was
ESIPT. A benzo[1,2-d:4,5-d’]bisimidazole-based D-p-A-p-D
system was also synthesized for the first time. The introduc-
tion of this rarely seen building block as an electron accept-
or inside the two-photon absorbing scaffold was successfully
investigated, thereby resulting in the formation of a new dye
that displayed good two-photon brightness.
153.9 ppm; UV/Vis (CH2Cl2):
l (e)=437 (3.26), 301 nm (1.34ꢃ
10À4 dm3 molÀ1 cmÀ1); emission max: 488 nm; IR (KBr): n˜ =811, 1184,
1363, 1433, 1518, 1606 cmÀ1; HRMS (ESI): m/z calcd for C40H39N8:
631.3292 [M+H+]; found: 631.3262.
Synthesis of Compound 4
A mixture of Bandrowski’s base (1.27 g, 4 mmol) and 3-(tris-isopropylsil-
yl)-2-propynal (3.36 g, 16 mmol) in absolute EtOH (60 mL) was heated
at reflux for 24 h under an Ar atmosphere. The reaction mixture was
cooled to RT and left undisturbed for 24–48 h to allow for slow crystalli-
zation of the product. The precipitate was filtered off to give 881 mg of
compound 4 as off-white crystals. The supernatant was evaporated with
silica and purified by column chromatography on silica gel (CH2Cl2 then
CH2Cl2/MeOH, 98:2) and crystallized from EtOH to give an additional
477 mg of the product. Overall yield: 1.36 g, 48%; M.p.: >3008C;
Experimental Section
1H NMR (500 MHz, CDCl3): d=1.00–1.02 (m, 42H; CH
ACTHUNGTRNEUNG(CH3)2), 3.89 (s,
Synthesis of Compound 1
4H; NH2), 6.80, 7.28 (AA’BB’, J=8.6 Hz, 8H; C6H4), 7.52 ppm (s, 2H;
benzobisimidazole); 13C NMR (125 MHz, CDCl3): d=11.1, 18.4, 96.1,
99.0, 99.3, 115.3, 126.4, 127.9, 134.2, 138.3, 141.4, 146.9 ppm; IR (KBr):
n˜ =740, 1361, 1428, 1518, 1627, 2161, 2864, 2942, 3215, 3339 cmÀ1; HRMS
(ESI): m/z calcd for C42H56N6NaSi2: 723.3997 [M+Na+]; found: 723.4027.
A mixture of Bandrowski’s base (1.27 g, 4 mmol) and 2-allyloxybenzalde-
hyde (1.43 g, 8.8 mmol) in absolute EtOH (100 mL) was heated at reflux
for 24 h under an Ar atmosphere. The reaction mixture was cooled to RT
and left undisturbed for a further 24–48 h to allow slow crystallization of
the product. The precipitate was filtered off and washed with EtOH to
give 1.55 g (64%) of compound 1 as off-white crystals. M.p.: 2558C (dec./
Claisen rearrangement); 1H NMR (500 MHz, CF3COOD): d=4.53 (d,
J=5.5 Hz, 4H; CH2CH=CH2), 5.22 (d, J=16.5 Hz, 2H; CH2CH=CH2),
5.27 (d, J=9.5 Hz, 2H; CH2CH=CH2), 5.81–5.85 (m, 2H; CH2CH=CH2),
7.05–7.11 (m, 4H; C6H4), 7.41 (d, J=1.5 Hz, 2H; C6H4), 7.63–7.67 (m,
2H; C6H4), 7.75, 7.87 (AA’BB’, J=8.5 Hz, 8H; C6H4NH2), 8.07 ppm (s,
2H; benzobisimidazole), NH2 signal was not detected; 13C NMR
(125 MHz, CF3COOD): d=72.4, 101.1, 110.6, 115.7, 121.3, 124.0, 128.4,
131.3, 131.5, 132.8, 133.4, 133.9, 135.2, 136.8, 139.4, 155.2, 159.6 ppm; UV/
Vis (CH2Cl2): l (e)=313 nm (3.53ꢃ10À4 dm3 molÀ1 cmÀ1); emission max:
407 nm; IR (KBr): n˜ =757, 1360, 1431, 1518, 1633, 3462 cmÀ1; HRMS
(ESI): m/z calcd for C38H33N6O2: 605.2659 [M+H+]; found: 605.2676.
Synthesis of Compound 5
Alkyne 4 (881 mg, 1.25 mmol) was dissolved in THF (30 mL) and TBAF
(1m solution in THF, 3 mL, 3 mmol) was added whilst stirring. A heavy,
amorphous precipitate was immediately formed. After 1 h, the solvent
was removed under reduced pressure and the residue was triturated with
MeOH, thereby yielding fine crystals that were filtered off and washed
with plenty of MeOH. This product was not further purified owing to its
low solubility in common organic solvents and was subjected in this form
to Pd-catalyzed coupling reactions. 1H NMR (500 MHz, CF3COOD): d=
4.34 (s; CCH), 7.98, 8.04 (AA’BB’, J=8.7 Hz, 8H; C6H4), 8.15 ppm (s,
2H; benzobisimidazole), NH2 signal was not detected; 13C NMR
(125 MHz, CF3COOD): d=66.1, 101.4, 102.0, 128.7, 131.1, 132.1, 134.8,
135.0, 135.1, 138.4 ppm; HRMS (ESI): m/z calcd for C24H17N6: 389.1509
[M+H+]; found: 389.1518.
Synthesis of Compound 2
Ether 1 (420 mg, 0.5 mmol) was added in one portion to boiling Ph2O
(20 mL). The reaction mixture was heated to reflux under an Ar atmos-
phere for 10 min. After that time, the mixture was cooled and hexanes
(80 mL) was slowly added. The resulting pale-yellow precipitate was fil-
tered off and washed with plenty of hexanes to give 376 mg (89%) of
compound 2. M.p.: >3008C; 1H NMR (500 MHz, CF3COOD): d=3.76
(d, J=6.5 Hz, 4H; CH2CH=CH2), 5.43–5.48 (m, 2H; CH2CH=CH2), 5.56
(dd, 2J=1 Hz, 3J=10.2 Hz, 2H; CH2CH=CH2), 6.21–6.29 (m, 2H;
CH2CH=CH2), 7.26 (t, J=7.8 Hz, 2H; C6H3), 7.44 (dd, 3J=1.3 Hz, 4J=
8.1 Hz, 2H; C6H3), 7.78 (dd, 3J=1 Hz, 4J=7.5 Hz, 2H; C6H3), 8.11, 8.22
(AA’BB’, J=8.7 Hz, 8H; C6H4), 8.32 ppm (s, 2H; benzobisimidazole),
NH2 and OH signals were not detected; 13C NMR (125 MHz,
CF3COOD): d=36.7, 100.8, 109.6, 120.1, 124.2, 128.6, 130.0, 131.2,
131.38, 131.41, 134.0, 135.3, 136.0, 136.9, 140.0, 154.4, 156.6 ppm; UV/Vis
Acknowledgements
Financial support from the Foundation for Polish Science (Grant no.
TEAM/2009-4/3) is gratefully acknowledged.
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(CH2Cl2):
l
(e)=381
(6.36),
363
(6.28),
307 nm
(2.03ꢃ
10À4 dm3 molÀ1 cmÀ1); emission max: 506 nm; IR (KBr): n˜ =750, 1170,
1248, 1367, 1422, 1460, 1518, 1618, 3371, 3470 cmÀ1; HRMS (ESI): m/z
calcd for C38H33N6O2: 605.2659 [M+H+]; found: 605.2679.
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Synthesis of Compound 3
A mixture of Bandrowski’s base (954 mg, 3 mmol) and 4-dimethylamino-
cinnamaldehyde (2.1 g, 12 mmol) in absolute EtOH (60 mL) was heated
at reflux for 24 h under an Ar atmosphere. The reaction mixture was
cooled to RT and left undisturbed for 24–48 h to allow for slow crystalli-
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Received: June 18, 2012
Published online: && &&, 0000
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
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FULL PAPER
Or dye trying: Strongly fluorescent
benzobisimidazoles were prepared by
the reactions of Bandrowski’s base
with various aldehydes. Their proper-
ties were carefully tuned to achieve
efficient excited-state intramolecular
proton transfer and strong two-photon
absorption. Their superb optical prop-
erties, in addition to the presence of
two reactive amino groups, make them
good candidates to be transformed
into probes for bioconjugation.
Functional Dyes
Mariusz Tasior, Vincent Hugues,
Mireille Blanchard-Desce,*
Daniel T. Gryko*
&&&& — &&&&
Benzo[1,2-d:4,5-d’]bisimidazoles as
a Convenient Platform Towards Dyes
that are Capable of Excited-State
Intramolecular Proton Transfer and of
Two-Photon Absorption
Chem. Asian J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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