Bis(dioxaborine) Dyes with Variable π-Bridges: Towards Two-Photon Absorbing Fluorophores with Very High Brightness

Article abstract of DOI:10.1002/chem.201704544

Bis(dioxaborine) dyes of the A-π-A format (A: acceptor, π: conjugated bridge) were prepared and photophysically characterized. The best performing dyes feature (a) visible-light absorption (>400 nm), (b) high molar absorption coefficients (up to 70000 m^?1 cm^?1), (c) Stokes shifts in the range of ca. 2500–5800 cm^?1, and (d) strong fluorescence emission with quantum yields of up to 0.74. This yields very bright-emitting dyes for one-photon excitation. However, the most intriguing feature of the dyes is their strong two-photon absorption. This was achieved by means of increased π-conjugation in the phenylene or phenylene-thiophene bridges through the variation of the conjugation length and rigidity. This provided two-photon absorption cross sections of up to 2800 GM (1 Goeppert-Mayer (GM)=10^?50 cm⁴ s photon^?1). Considering the mentioned high fluorescence quantum yields, exceptionally bright-emitting A-π-A two-photon absorbing dyes with low molecular mass are obtained. Time-dependent density-functional theory calculations corroborated the experimental results.

Full text of DOI:10.1002/chem.201704544

A Journal of
Accepted Article
Title: Bis(dioxaborine) Dyes with Variable π-Bridges: Towards Two-
Photon Absorbing Fluorophores with Very High Brightness
Authors: Álvaro Moneo Marín, João P. Telo, Daniel Collado, Francisco
Nájera, Ezequiel Pérez Inestrosa, and Uwe Pischel
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10.1002/chem.201704544
Chemistry - A European Journal
ARTICLE
Bis(dioxaborine) Dyes with Variable π-Bridges: Towards Two-
Photon Absorbing Fluorophores with Very High Brightness
Álvaro Moneo Marín,[a,b] João P. Telo,[b] Daniel Collado,[c,d] Francisco Nájera,[c,d] Ezequiel Pérez-
Inestrosa,[c,d] and Uwe Pischel*[a]
Abstract: Bis(dioxaborine) dyes of the A--A format (A: acceptor, :
conjugated bridge) were prepared and photophysically characterized. would be the capacity of dye to be stimulated in a two-photon
ideal requirements to a large extent.[1, 3, 7-19] An additional asset
The best performing dyes feature (a) visible-light absorption (>400
excitation process.[20-22] This creates the same emissive state as
conventional one-photon excitation, but makes use of near-
infrared (NIR) photons. These are of low energy and in general
spectrally well separated from the emitted light, thereby avoiding
1
1
nm), (b) high molar absorption coefficients (up to 70000 M cm ), (c)
1
Stokes shifts in the range of ca. 25005500 cm , and (d) strong
fluorescence emission with quantum yields of up to 0.74. This yields
very bright-emitting dyes for one-photon excitation. However, the
most intriguing feature of the dyes is their strong two-photon
absorption. This was achieved by means of increased -conjugation
in the phenylene or phenylene-thiophene bridges through the
variation of the conjugation length and rigidity. This provided two-
photon absorption cross sections of up to 2800 GM (1 Goeppert-
eventual
experimental
issues due to straylight
or
photodegradation. These features have been especially valued
in applications that imply imaging by fluorescence
microscopy.[23-26]
In front of this background there has been much progress
in the tailored design of molecular architectures that feature high
two-photon absorption (TPA) cross sections.[26-31] The most
popular approaches are based on -conjugated dipolar donor-
acceptor architectures (DA) or centrosymmetric quadrupolar
-conjugated dyes with terminal donor or acceptor substitution
(DD, AA, DAD, ADA). The design strategies build on
the variation of the conjugation length and the control of
intramolecular charge redistribution on excitation, either from D
to A or between the center and the A/D substituted ends of the
molecule. However, there is room for improvement because
dyes with high cross sections feature very often high molecular
weights and imply elevated synthetic labour. In addition, so far
reported AA architectures show generally inferior TPA
performance as compared to dyes that incorporate donor
moieties.
50
1
Mayer (GM) = 10 cm4 s photon ). Considering the mentioned high
fluorescence quantum yields, exceptionally bright emitting AA
two-photon absorbing dyes with low molecular mass are obtained.
Time-dependent density-functional theory calculations corroborated
the experimental results.
Introduction
In recent years boron-containing molecular organic architectures
have attracted elevated interest because of their photophysical
properties, such as strong light absorption and emission.[1-3] This
is reflected in high brightness, being the product of the molar
absorption coefficient and the emission quantum yield. If these
properties are combined with light absorption at wavelengths
longer than 400 nm and emission in the spectral region >500
nm, as well as significant Stokes shifts, then dyes with potential
in bioimaging or optoelectronic applications are identified.[2, 4-6]
There are several examples of organoboron dyes that fulfil these
[a]
[b]
A. Moneo Marin, Dr. U. Pischel
CIQSO - Center for Research in Sustainable Chemistry and
Department of Chemistry
University of Huelva
Campus de El Carmen s/n, E-21071 Huelva (Spain)
E-mail: uwe.pischel@diq.uhu.es
A. Moneo Marin, Dr. J. P. Telo
Centro de Química Estructural, Instituto Superior Técnico
Universidade de Lisboa
Av. Rovisco Pais, 1049-001 Lisboa (Portugal)
Dr. D. Collado, Dr. F. Nájera, Prof. Dr. E. Pérez-Inestrosa
Department of Organic Chemistry, IBIMA, University of Málaga
Campus Teatinos s/n, E-29071 Málaga (Spain)
Dr. D. Collado, Dr. F. Nájera, Prof. Dr. E. Pérez-Inestrosa
Andalusian Center for Nanomedicine and Biotechnology–BIONAND
Parque Tecnológico de Andalucía, E-29590 Málaga (Spain)
[c]
[d]
Scheme 1. Strategies for improving the TPA properties of bis(dioxaborine)
dyes and structures of the herein investigated dyes 17.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201704544
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ARTICLE
Recently, we have reported tetracoordinate borylated
arylisoquinolines and boronic-acid derived dyes that show
quantum yields of 0.60.7. Consequently, the brightness ( ×

fluo) of some of these dyes is very high,[19] reaching values of
significant TPA cross sections (up to 300 GM, 1 GM = 1050 cm4
about 4200046000 M1cm1 for the bis(dioxaborines) 24.
These interesting and highly advantageous optical
s photon1).[14,
In the present work we have focused on
32]
bis(dioxaborines), another class of tetracoordinate organoboron
dyes. Previous studies have demonstrated that they are
characteristics are complemented by Stokes shifts of ca.
25005600 cm1 and fluorescence lifetimes in the range of 14
ns (except for dye 1). The herein observed Stokes shifts are
comparable to those obtained for other boron(III) dye
architectures with heterocylic ligands.[3] The absolute values are
indicative for significant electronic rearrangements on excitation.
interesting candidates for organic electronics,[33-36] non-linear
38]
optical materials,[37,
as TPA dyes,[39-43] and as fluorescent
probes.[43-45] The dioxaborine moiety is a strong electron-
acceptor unit and so far reported dyes are of the
abovementioned AA or ADA format. It was shown that the
AA bis(dioxaborines) dyes show significant TPA cross
sections of up to 500 GM for excitation with visible light around
600 nm.[39-41] For bis(dioxaborines) with a carbazole as electron-
donor bridge high TPA cross sections (530 GM) were measured
for excitation at 700 nm.[39, 41] Bis(dioxaborines) with biphenylene
bridges (either flexible or rigidified) show also some TPA activity
for excitation longer than 700 nm, but unfortunately with rather
modest cross sections (ca. 1020 GM).[40, 41] With the idea to
improve the TPA features in the NIR region we oriented towards
the further extension of the conjugation length, combined with
increased planarization or the electronic integration of
thiophenes (see structures in Scheme 1). Indeed, we were able
to show a dramatic increase of the TPA cross sections to values
of about 20003000 GM for excitation in the 700800 nm NIR
spectral region. This observation and the generally high
emission quantum yields propose some of the herein
investigated compounds as very bright-emitting TPA dyes.
Table 1. Photophysical data of the bis(dioxaborine) dyes 17 in aerated
acetonitrile.[a]
Stokes
shift
(cm1)[f]

abs,max (nm)
fluo,max
(nm)[c]
fluo
(ns)[e]
[d]
fluo
 (GM)[g]
[ (M cm1)][b]
1
[h]
1
363 [37000]
418
413
0.18
0.61
< 0.2
0.92
3678
2864
20 [700]
12 [770]
2[i]
370 [71000]
377 [57000]
440 [68000]
442 [55000]
428 [48000]
440 [47000]
744 [700]
109 [750]
3
4
5
6
7
479
506
496
535
506
0.74
0.67
0.45
0.67
0.35
1.60
2.52
1.59
2.03
4.13
5645
2951
2462
4706
3922
2800 [730]
337 [890]
1674 [710]
45 [880]
2002 [730]
200 [890]
Results and Discussion
1684 [770]
106 [880]
Synthesis of bis(dioxaborines)
The bis(dioxaborine) dyes 17 were synthesized from acetylated
aromatic compounds by treatment with BF3CH3COOH complex
according to reported protocols for related dyes.[46] The isolated
yields were 8696%, except for dye 1 (71% yield). Non-
commercial acetylated aromatic compounds were prepared by
direct double Friedel-Crafts acetylations of the parent
hydrocarbon, or by standard Suzuki or Ullmann coupling
reaction of acetylated precursors. The detailed experimental
procedures and analytical data can be found in the Supporting
Information.
[a] Compound
3 required the addition of 0.3 vol% DMF. The TPA
measurements were done with DMF as additive (0.2 vol%, except for dye 3
where 2.4 vol% were used. [b] Longest-wavelength absorption maximum, in
square brackets the corresponding molar absorption coefficient is given. [c]
Fluorescence emission maximum. [d] Fluorescence emission quantum yield;
15% error. [e] Fluorescence lifetime measured by time-correlated single-
photon-counting; 5% error. For the compounds 47 multiexponential decays
were registered and the lifetime of the most dominant contribution is given. [f]
The corresponding energies in eV are: 0.46 (1), 0.36 (2), 0.70 (3), 0.37 (4),
0.31 (5), 0.58 (6), 0.49 (7). [g] TPA cross sections were measured in the range
of 7001000 nm; in square brackets the corresponding TPA wavelength is
provided. The highest cross section and the one corresponding to the longest
wavelength maximum are given. [h] No significant TPA properties were
measured. [i] The UV/vis-absorption spectrum and emission quantum yield of
an analogous compound, containing n-propyl instead of methyl, were reported
for dichloromethane as solvent: max,abs = 373 nm, fluo = 0.68; ref. [39]. These
data compare nicely with the herein obtained values for 2 in acetonitrile.
One-photon optical properties
The bis(dioxaborines) 17 were characterized with respect to
their one-photon UV/vis-absorption (OPA) and fluorescence
properties in acetonitrile solution. The corresponding data are
summarized in Table 1 and the spectra are shown in Figure 1
and 2. Generally it can be affirmed that the investigated dyes are
strong absorbers with long-wavelength maxima in the spectral
range between ca. 370 and 440 nm, featuring molar absorption
coefficients () as high as 70000 M1cm1. The fluorescence of
the dyes was observed in the blue to green-yellow spectral
range (maxima between 413 and 535 nm) and shows maximum
The investigated dyes can be divided into two sub-series
regarding the variations of the -bridge that connects the two
dioxaborine moieties. Sub-series I contains the dyes 14 with
para-phenylene linkers. These dyes show a systematic variation
of their UV/vis-absorption and fluorescence maxima with
increasing length of the bridge (13) and consequently
increasing -conjugation (Figure 1). Each addition of a para-
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phenylene yielded a bathochromic shift of the UV/vis-absorption
maximum by 7 nm. Also for the fluorescence maxima a
bathochromic shift was observed for the stepwise extension of
the -bridge, amounting to 82 nm when comparing the dyes 1
and 3, with one and three phenylene units, respectively. The
case of dye 4 is significant because the increased planarization
of the bridge causes even higher -conjugation. This leads to a
further shift of the absorption and fluorescence maxima by 63
nm and 27 nm, respectively, compared to the terphenylene-
bridged bis(dioxaborine) 3 with the same conjugation length;
Figure 1.
Sub-series II is constituted by the compounds 57 with
thiophene-containing bridges. These dyes feature absorption
maxima at wavelengths longer than 400 nm, i.e., 430440 nm,
and emission maxima close to 500 nm and in some case even
at ca. 540 nm (dye 6); Figure 2. Noteworthy, the substitution of
one or two para-phenylene units by thiophene (dyes 2, 3 versus
57) yields significant bathochromic shifts of the UV/vis-
absorption and emission bands. These observations are
reasoned with the higher -conjugation in thiophene bridges as
48]
compared to phenylene bridges.[47,
Interestingly, the
connectivity of the thiophene in the bridge plays an important
role. While the direct neighborhood of thiophenes to the
dioxaborine moieties yields a red-shifted UV/vis-absorption
maximum, the inclusion of thiophene as central part of the
bridge red-shifts the emission maximum (compare dyes 6 and 7;
Table 1). The emission quantum yields are moderate to high,
being somewhat lower for the dyes that incorporate two
thiophene units, i.e., 5 and 7.
The global photophysical trends are well reproduced by
time-dependent density-functional-theory (TD-DFT) calculations
[B3LYP/6-31+G(d)],[49, 50] accounting for acetonitrile as solvent
by including the polarization-continuum model (PCM); see Table
2. The S1←S0 excitation is dominated by the LUMO←HOMO
transition. The same frontier orbitals are involved in the S1→S0
emission. Consequently, the experimental UV/vis-absorption
and emission maxima follow the trend that is dictated by the
HOMOLUMO energy bandgap, being largest for dye 1 and
smallest for the dyes with higher -conjugation, i.e., 47. Once
more, also the calculated bandgaps confirm nicely that the
increased rigidity (dye 4) or the integration of the comparably
stronger electron-donating thiophene as bridge element (dyes
57) increases the level of -conjugation.
Figure 1. UV/vis-absorption (black) and fluorescence spectra (red) of the dyes
14 in acetonitrile. The fluorescence spectra were obtained by excitation at the
wavelength of maximum absorbance.
Two-photon absorption (TPA) properties
The most intriguing property of the herein investigated dyes are
their TPA cross sections. In the present work the TPA
characteristics between 700 nm and 1000 nm were studied by
the method of two-photon-induced fluorescence (TPIF).[51] In the
context of TPA-active compounds with DD, AA, DAD
or ADA architecture, centrosymmetry is generally identified as
a structural key element, while -conjugation constitutes the
electronic ingredient.[29] Centrosymmetry is fulfilled for the
conformations of the dyes 1, 3, 4, and 5 that are shown in
Scheme 1. Note for example that the lowest-energy ground-
state conformations of these dyes, having the two dioxaborine
units in anti-orientation, feature a dipole moment of practically
zero Debye (see Table 2). More calculated structural data for the
dyes 17 can be found in the Supporting Information.
The TPA spectra of the dyes 27 are shown in Figure 3. In
line with the above discussed rationale, dye 3 has a significantly
higher  value than dye 2 (see Table 1). However, it was the
locking of the bridge conformation in 4 that turned out to push
the TPA cross section of bis(dioxaborines) beyond currently
known literature values for AA dyes with similar conjugation
length, reaching 2800 GM at 730 nm. This observation
combined with an emission quantum yield of 0.67 leads to the
Figure 2. UV/vis-absorption (black) and fluorescence spectra (red) of the dyes
57 in acetonitrile. The fluorescence spectra were obtained by excitation at the
wavelength of maximum absorbance.
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conclusion that dye 4 belongs to the TPA AA dyes with
highest brightness ( × fluo; 1876 GM); see for comparison
other dyes in the Supporting Information. Even for the longest-
wavelength TPA maximum at 890 nm an appreciable cross
section of 337 GM was measured for dye 4, being comparable
to the performance of other boron-containing fluorescent
architectures.[14, 19-22]
Table 2. Calculated electronic and photophysical data for the bis(dioxaborine) dyes 17.[a]
Absorption
Emission
f[b]
Dominant component
(%)[c]
Emax,calc
(eV)[d]
Emax,exp
(eV)[d]
f[b]
Dominant component
(%)[c]
Emax,calc
(eV)[d]
Emax,exp
(eV)[d]
ΔEHOMO-LUMO
S0)
S1)
(eV)[f]
(D)[e]
(D)[e]
1
2
3
4
5
6
7
1.050
1.605
1.704
1.807
1.437
1.701
1.734
LUMO←HOMO (99)
LUMO←HOMO (99)
LUMO←HOMO (98)
LUMO←HOMO (99)
LUMO←HOMO (100)
LUMO←HOMO (99)
LUMO←HOMO (99)
3.47
3.25
3.04
2.80
2.72
2.65
2.70
3.42
3.35
3.29
2.82
2.81
2.90
2.82
1.323
2.106
2.511
2.383
1.723
2.316
2.684
LUMO→HOMO (100)
LUMO→HOMO (99)
LUMO→HOMO (99)
LUMO→HOMO (99)
LUMO→HOMO (100)
LUMO→HOMO (99)
LUMO→HOMO (99)
2.82
2.55
2.35
2.33
2.10
2.03
2.01
2.97
3.00
2.59
2.45
2.50
2.32
2.45
0.01
5.55
0.00
0.00
0.22
5.98
3.09
0.01
1.15
0.01
0.01
2.46
7.92
2.10
3.89
3.69
3.47
3.17
3.08
3.04
3.10
[a] Calculated by TD-DFT (see text), corresponding to the S1←S0 transition (absorption) and S1→S0 transition (emission). [b] Oscillator strength. [c] Percentage
contribution approximated by 2 × (ci)2 × 100%; ci is the coefficient for the particular "orbital rotation". [d] Calculated (calc) and experimental (exp) energies. [e]
Dipole moment in the ground state (S0) and first singlet excited state (S1). [f] HOMO-LUMO energy bandgap.
yield (see discussion above). However, dye 6 maintains a high
emission quantum yield (fluo = 0.67), giving rise to a brightness
of 1341 GM (at 730 nm).
The dyes 4, 6, and 7 with the highest TPA cross sections
in this work stand out among other DD and AA
architectures with similar spectral positions of the TPA bands.[28,
29]
As an orientation we normalized the TPA cross sections for
the number of -electrons (Ne) in the conjugated bridge (/Ne).
Our best performing dyes feature values of ca. 70110 GM per
-electron, while typical values for other AA structures do not
exceed 1020 GM per -electron. Even DD dyes, that are
known to have generally higher cross sections,[29] do reach at
best 5075 GM per bridge -electron; see comparison with
literature-known systems in the Supporting Information.
As a general observation for the investigated compounds,
the most intense TPA bands (at ca. 720770 nm) are not related
to the transition that corresponds to the longest-wavelength OPA
bands (see Figure 4). This suggests that the strongest TPA
transitions implicate higher excited states, namely the S2 state.
Figure 3. TPA spectra of the dyes 2 (black), 3 (red), 4 (dark blue), 5 (blue), 6
(green), and 7 (magenta) in acetonitrile. The spectra of dyes that belong to the
sub-series I (24) are symbolized by filled circles and for sub-series II (57)
In agreement the calculated energy of the S2←S0 transition
divided by two compares quite well with the energy
corresponding to the TPA wavelength maximum of the strongest
band. At the same time the less strong TPA feature at
wavelengths longer than 850 nm correlates well with the lowest
energy transition (S1←S0); see Figure 4 for the examples of 4
and 6.
Similar conclusions can be drawn from symmetry
considerations. For the centrosymmetrical molecules 3, 4, and 5
the symmetry of the S0, S1, and S2 states is Ag, Au, and Ag,
respectively. Thus, in accordance with the selection rules the
oscillator strength of the S2←S0 transition is zero in the OPA
process. This is reversed for the TPA process in
centrosymmetric chromophores,[29] and hence, the S2←S0 (Ag to
Ag) transition is TPA-active for these dyes.
empty circles are used.
The strategy of fine-tuning the -conjugation of the bridge
was also followed in sub-series II, consisting of the thiophene-
containing dyes 57. A first hint on the improved conjugation of
the bridge can be obtained by comparing the cross sections of
the dyes 2 and 5. Both have the same conjugation length, but
the thiophene variant features a much higher TPA cross section
(see Table 1 and Figure 3). The same observation applied for
the dyes 6 and 7, where in comparison to dye 3 one or two
thiophenes substitute phenylene(s). This electronic variation
yielded  values of 2002 GM (at 730 nm) and 1684 (at 770 nm)
for
6 and 7, respectively. The associated brightness is
somewhat affected for dye 7, due to its lower emission quantum
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symmetry considerations (see above). The increased
quadrupolar polarization applies as well to the non-
centrosymmetrical dyes 6 and 7, giving reason to their high TPA
cross sections (see NTO analysis in the Supporting Information).
Conclusions
Bis(dioxaborine) dyes with varying degree of -conjugation
length and rigidity of the bridge element were prepared and
photophysically characterized. As bridges phenylene, thiophene,
or mixed phenylene-thiophene linkers were tested. In general
the dyes show high fluorescence quantum yields, reaching
0.60.7, and are strong light absorbers. This gives rise to very
high brightness (up to 46000 M1cm1) for one-photon excitation
of some of the investigated dyes.
In cases where the bridge of the AA architectures
supports an extended -conjugation (determined by rigidity,
number of -electrons, HOMO-LUMO gap) exceptionally high
TPA cross sections  were measured (up to 2800 GM), being
among the highest that were so far observed for quadrupolar
DD and AA architectures with similar conjugation and low
molecular weight. The high emission quantum yields provide
outstanding TPA-related brightness of up to 1900 GM to some of
the investigated dyes. These dyes can be foreseen to find
applications especially in optoelectronic materials.
Figure 4. Comparison of OPA spectra (bottom) and TPA spectra (top) spectra
of 4 (right) and 6 (left). The x-axis of the two-photon spectra was divided by 2,
to allow a comparison.
Experimental Section
General synthetic procedures
General procedure for double acetylation. A solution of the aromatic
compound (1 equiv) in dichloromethane was added dropwise during 20
min to a flask charged with a stirred mixture of anhydrous aluminium
chloride (2.3 equiv) and acetyl chloride (2.1 equiv) in dichloromethane at
0 ºC. The mixture was kept at 0 ºC for 3 hours and then added to ice
water, containing HCl. The precipitate was isolated by filtration and
washed with copious amounts of water.
Figure 5. NTO orbitals for the S1←S0 transition (bottom) and for the S2←S0
transition (top) for dye 4.
General borylation procedure. A flask was charged with a mixture of
the acetylated aromatic compound (1 equiv) and acetic anhydride (3
equiv) and then heated to 45 ºC under nitrogen atmosphere. Then a
solution of BF3CH3OOH complex (2 equiv) was added. The mixture was
kept at 45 ºC for additional 6 hours. Then, the mixture was filtered and
the solid was washed with acetic acid, ethyl acetate, and diethyl ether.
Natural transition orbitals (NTOs) are very appropriate to
describe the electronic re-distributions in excitation events.[29]
The NTOs were calculated for the two lowest-energy transitions,
i.e., S1←S0 and S2←S0, providing a visualization of the electron
and hole distributions. Figure 5 shows the NTOs of the best
performing dye 4 (information on the other dyes can be found in
the Supporting Information). On the one hand, it can be clearly
seen that the hole is evenly localized on the bridge for both
transitions. On the other hand, the electron is spread over the
whole molecule for the S1←S0 transition. This situation is quite
different for the S2←S0 transition, where the electron is
concentrated at the acceptor ends of the dyes, thereby further
increasing the quadropolar polarization. This picture explains
well the variation of the TPA cross sections involving either S1 or
S2 (see experimental data and discussion above) and
complements the conclusions that were drawn from the
Detailed information on the preparation of each dye and the analytical
characterization can be found in the Supporting Information.
Photophysical measurements
Photophysical experiments were carried out at room temperature with
aerated and optically diluted (absorbance at excitation wavelength ca.
0.10.2) acetonitrile solutions. The UV/vis-absorption spectra were
recorded with
a
UV-1603 spectrophotometer (Shimadzu). The
fluorescence spectra were obtained with a Cary Eclipse fluorimeter
(Varian). Fluorescence excitation spectra, monitoring the emission
maximum, were measured to confirm the identity of the observed
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10.1002/chem.201704544
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ARTICLE
emissions. The fluorescence quantum yields were determined with 4-
(SFRH/BD/87439/2012). We gratefully acknowledge the
computer resources, technical expertise, and assistance
provided by the SCBI (Supercomputing and Bioinformatics
Center) of the University of Málaga.
amino-1,8-naphthalimide (fluo = 0.48 in acetonitrile)[19] or quinine sulfate
53]
(fluo = 0.55 in 0.05 M H2SO4)[52,
as reference and corrected for
refractive index differences of the solvents for the sample and the
reference. The lifetime measurements were performed by means of time-
correlated single-photon-counting (Edinburgh instruments FLS 920) with
a picosecond pulsed UV-LED (EPLED 360, = 367 nm, pulse width
fwhm 745 ps) or a picosecond pulsed laser diode (EPL 445,  = 442 nm,
pulse width fwhm 78 ps) as excitation source. The lamp profile was
obtained with a light-scattering colloidal solution (Ludox).
Keywords: bis(dioxaborines) • fluorescence • two-photon
absorption • dyes • photophysics
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Ciência e Tecnologia (grant UID/QUI/00100/2013) and by the
Spanish Ministerio de Economía, Industria y Competitividad
(grant CTQ2014-54729-C2-1-P for U.P.; CTQ2013-41339-P and
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Entry for the Table of Contents
ARTICLE
Shine bright: Near-infrared two-
photon excitation of bis(dioxaborines)
with exceptional cross sections yields
strong fluorescence. The rationally
designed dyes with variable -
conjugation belong to the brightest
emitting two-photon absorbers with
acceptor--acceptor architecture
known to date.
Alvaro Moneo Marín, João P. Telo,
Daniel Collado, Francisco Nájera,
Ezequiel Pérez-Inestrosa, and Uwe
Pischel*
Page No. – Page No.
Bis(dioxaborine) Dyes with Variable
π-Bridges: Towards Two-Photon
Absorbing Fluorophores with Very
High Brightness
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