Borylated arylisoquinolines: Photophysical properties and switching behavior of promising tunable fluorophores

Article abstract of DOI:10.1002/chem.201203887

A series of nine borylated arylisoquinolines has been prepared with systematic variation in their electronic properties and their photophysical properties were investigated. The color of their fluorescence can be finely tuned by changing the properties of the aryl moiety, which is involved in internal-charge-transfer processes. For example, methoxy-substituted compound 5 showed an intense green emission, whereas dimethylamino-substituted compound 6 showed an orange-red emission. These new fluorophores were tested for their potential as molecular switches with external ionic stimuli, such as protons and fluoride ions. On the one hand, protonation of the isoquinoline moiety led to fluorescence enhancement for compounds that showed weak charge transfer and fluorescence quenching for compounds that showed strong charge transfer. On the other hand, the formation of ate complexes with fluoride led to strong fluorescence quenching in all of the investigated cases. 2 in 1: Borylated arylisoquinolines are tunable and quite versatile molecular entities for the fluorescence sensing of protons and fluoride ions (see figure). A series of compounds with different electronic properties was investigated for various switching modes (quenching, enhancement, ratiometric response). Copyright

Full text of DOI:10.1002/chem.201203887

DOI: 10.1002/chem.201203887
Borylated Arylisoquinolines: Photophysical Properties and Switching
Behavior of Promising Tunable Fluorophores
[
a]
[b]
[c]
[d]
Vꢀnia F. Pais, Hamdy S. El-Sheshtawy, Rosario Fernꢁndez, Josꢂ M. Lassaletta,
[
d]
[a]
Abel Ros,* and Uwe Pischel*
Abstract: A series of nine borylated ar-
ylisoquinolines has been prepared with
systematic variation in their electronic
properties and their photophysical
properties were investigated. The color
of their fluorescence can be finely
tuned by changing the properties of the
aryl moiety, which is involved in inter-
nal-charge-transfer processes. For ex-
ample, methoxy-substituted compound
whereas
dimethylamino-substituted
the one hand, protonation of the iso-
quinoline moiety led to fluorescence
enhancement for compounds that
showed weak charge transfer and fluo-
rescence quenching for compounds
that showed strong charge transfer. On
the other hand, the formation of ate
complexes with fluoride led to strong
fluorescence quenching in all of the in-
vestigated cases.
compound 6 showed an orange-red
emission. These new fluorophores were
tested for their potential as molecular
switches with external ionic stimuli,
such as protons and fluoride ions. On
Keywords: boron · fluorescence ·
fluorides · molecular switches · sen-
sors
5
showed an intense green emission,
Introduction
ions, thus resulting in chromogenic or fluorescent sen-
[11–13,15]
sors.
The most widely employed systems are based on
Fluorophores that contain trisubstituted boron species, such
as triarylboranes or aryl boronic acids and their derivatives,
have been intensively exploited for applications as lumines-
internal-charge-transfer processes in the excited state that
involve the electron-deficient boron atom, which, upon the
coordination of F , loses its electron-accepting properties.
ꢀ
[1–6]
[7,8]
cent polymeric materials,
chromophores for the study of electron transfer,
in nonlinear optics,
as model
and as
This process is often accompanied by an “on-off” fluores-
[9,10]
[16,17]
cence switching.
Indeed, systems in which “off-on” fluo-
[11–14]
[18–20]
chemosensors.
The electron-deficient nature of a trisub-
rescence switching
or a ratiometric emission re-
[7,21–28]
stituted boron atom in chromophoric architectures provides
possibilities for the coordination of donor ligands through
sponse
are observed are of great interest for sensing
purposes. Other designs have used the classical photoin-
[29–31]
interactions with its vacant p orbital. This coordination ena-
duced electron-transfer approach,
by integrating elec-
p
bles the tuning of the photophysical properties of the system
upon the addition of donor ligands, such as fluoride (F )
tron-donating fluorophores with electron-accepting triaryl-
boranes. An additional functional aspect of fluorescent or-
ꢀ
[32]
ganoborane compounds is the implementation of switches
[33]
with multiple emissive states. Notably, another abundantly
employed design principle for fluorescent fluoride chemo-
sensors relies on anionic interactions with activated NH
[
a] V. F. Pais, Prof. Dr. U. Pischel
Centro de Investigaciꢀn en Quꢁmica Sostenible (CIQSO) and
Departamento de Ingenierꢁa Quꢁmica
[34–44]
moieties (urea, thiourea, aminonaphthalimides, etc.).
Quꢁmica Fꢁsica y Quꢁmica Orgꢂnica
Herein, we exploited a new class of fluorophores with
boronic ester groups that were structurally and electronical-
ly integrated with arylisoquinolines. These compounds were
Universidad de Huelva, Campus de El Carmen s/n
2
1071 Huelva (Spain)
Fax : (+34)959-21-99-83
E-mail: uwe.pischel@diq.uhu.es
prepared by using a recently developed borylation method
[
[
b] Dr. H. S. El-Sheshtawy
[45]
of broad synthetic scope.
The resulting fluorophores
Chemistry Department, Faculty of Science
South Valley University, 83523 Qena (Egypt)
showed electronically tunable internal-charge-transfer (ICT)
fluorescence emission in an ample spectroscopic window
and could be switched by protonation of the isoquinoline or
the formation of fluoroboronate complexes with the boronic
acid ester.
[
c] Prof. Dr. R. Fernꢂndez
Departamento de Quꢁmica Orgꢂnica, Universidad de Sevilla
C/Prof. Garcꢁa Gonzꢂlez 1, 41012 Sevilla (Spain)
d] Prof. Dr. J. M. Lassaletta, Dr. A. Ros
Instituto de Investigaciones Quꢁmicas (CSIC-US)
C/Amꢃrico Vespucio 49, 41092 Sevilla (Spain)
Fax : (+34)954-46-05-65
E-mail: abel.ros@iiq.csic.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203887.
6650
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Chem. Eur. J. 2013, 19, 6650 – 6661

FULL PAPER
Results and Discussion
Synthesis of borylated arylisoquinolines: The boron center
was integrated as a boronic acid pinacol ester. Recently, we
reported a convenient synthetic access to ortho-borylated ar-
Molecular design: A series of borylated arylisoquinolines
(
1–9, Scheme 1), the fluorescence of which can be addressed
ylisoquinolines through an iridium AHCUTNGTERNNUN(G III)-catalyzed nitrogen-
+
[45]
in two ways, that is, by the addition of protons (H ) or
directed reaction, thereby obtaining the herein-exploited
compounds 1, 4, 7, and 8 in excellent yields. The scope of
this recently introduced method was further extended
herein, thereby yielding borylated arylisoquinolines with
electron-withdrawing (2 and 3) and electron-donating sub-
stituents (5 and 6) at the 4 position of the naphthalene
moiety, as well as the borylated 1-(anthracen-1-yl)isoquino-
line (9). These reactions worked well at 558C, thus affording
the desired ortho-borylation products in good-to-excellent
yields (Scheme 3). The borylation of cyano derivative M2
Scheme 1. Structures of borylated arylisoquinolines 1–9 and their respec-
tive models (M1–M9).
ꢀ
[36,46,47]
anions (F ),
was explored. The design principle
(
Scheme 2) rested on three pillars: 1) The coordination of
Scheme 3. Directed ortho-borylations of models M2, M3, M5, M6, and
M9.
ꢀ
F ions to the boron center, thus giving rise to tetracoordi-
required elevated reaction temperatures (808C) to afford
the desired product (2). This result was attributed to the
2
ability of the cyano group to compete with the N
A
H
U
G
E
N
N
(sp ) atom
of the isoquinoline moiety for the vacant coordination site
[48]
of the catalyst.
The ortho-borylation step was performed with the corre-
Scheme 2. Molecular design of dually switchable borylated arylisoquino-
lines: a) general structure and indication of the input recognition sites
and b) schematic representation of possible ICT interactions.
sponding arylisoquinolines. Hence, the model compounds
(
M1–M9) were precursors of the final borylated products
and, thus, were directly available from this synthetic se-
quence. The synthesis of precursors M1, M4, M7, and M8
[45]
has been reported elsewhere. The newly prepared aryliso-
quinolines (M2, M3, M5, M6, and M9) were obtained from
their corresponding haloarenes by palladium-catalyzed one-
pot borylation/Suzuki-coupling, following Buchwald’s
nated fluoroboronate substituents with strong electron-do-
nating character; 2) the protonation of the isoquinoline
moiety, thereby increasing its electron-demanding character;
and 3) the possibility of observing fluorescence based on in-
ternal charge transfer (ICT).
For the purpose of studying the electronic effects on the
ICT properties of these compounds, a set of compounds
with various aryl moieties (naphthyl, acenaphthyl, pyrenyl,
anthryl) and, in the case of the naphthyl moieties, substitu-
tion with electron-withdrawing or electron-donating groups,
were prepared. The chosen synthetic approach (see below)
yielded the borylated target compounds, as well as their cor-
responding boron-free arylisoquinolines (M1–M9), which
served as photophysical models.
[49]
method (Scheme 4).
1
11
Structural characterization: The H and B NMR data for
compounds 1–9 (see the Supporting Information) provided
evidence for the absence of intramolecular BꢀN interac-
[45]
11
tions, most likely for steric reasons: The B NMR signal
appeared within the range d=30–31 ppm for all of these
2
[50]
compounds, typical for tricoordinated sp boron atoms.
On the contrary, for a BꢀN interaction, which would imply
3
sp hybridization of the boron atom, strong shielding and an
[50]
upfield shift to d=13–15 ppm should have been detected.
Another indicator of the absence of BꢀN interactions was
Chem. Eur. J. 2013, 19, 6650 – 6661
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6651

A. Ros, U. Pischel et al.
Scheme 4. Synthesis of precursors M2, M3, M5, M6, and M9.
1
the H signal of the pinacolate methyl groups. These protons
Figure 1. Normalized UV/Vis absorption spectra (dashed lines) and fluo-
rescence spectra (solid lines) of compounds 2, 4, and 6 in MeCN.
appeared, without exception, in the region d=0.74–
1
.01 ppm. The formation of an internal Lewis adduct should
[45]
have led to deshielding (dꢁ1.40–1.45 ppm),
which was
not observed. Further compelling evidence for the absence
of BꢀN interactions in the case of compound 1 came from a
[45]
recently reported crystal structure.
UV/Vis absorption and fluorescence properties: The most
important photophysical properties of compounds 1–9 in
MeCN are summarized in Table 1. Representative absorp-
Figure 2. Fluorescence colors of solutions (about 3.2 mm) of compounds
1
F
–9 in MeCN. The brightness of the emission depends on the product
ꢅe
Table 1. UV/Vis absorption and fluorescence properties of compounds
f
ACHTUNGTRNENUNG( labs).
1
–9 in MeCN.
F [
b]
I
fl(LW)/Ifl(SW)
t
fl
l
abs
e
l
fl
f
ꢀ
1
ꢀ1 [a]
[c]
showed appreciable brightness, which was especially evident
for compounds 5, 6, and 8.
[
nm]
A[ m cm
C
H
T
U
N
G
T
R
E
N
N
U
N
G
]
[nm]
[ns] (%)
1
2
3
4
5
6
7
8
9
284
309
286
287
309
323
291
341
371
9500
368, 492
437
0.01
0.01
0.02
0.04
0.43 123
0.36 149
3
0.32 (98.3)
0.26 (98.1)
1.00 (91.8)
2.51 (98.5)
7.81 (81.2)
6.11 (100)
7.95 (100)
3.32 (100)
3.28 (100)
[
d]
10200
8250
9700
9100
6200
8300
33000
4800
–
21
25
To gain more insight into the nature of the red-shifted
LW emission, it appeared reasonable to have a closer look
at its spectral position in relation to the aryl oxidation po-
tential. Figure 3 shows a plot of the half-wave oxidation po-
362, 494
386, 502
424, 532
420, 608
417, 527
[51,52]
tential of the aryl residues
(see Table 2) versus the ener-
0.20
41
29
6
391, 412, 528 0.27
428, 571 0.10
gies that correspond to the LW maximum. A clear trend is
evident (linear regression; r=0.963, n=9): The easier the
[
a] Value refers to labs. [b] Fluorescence quantum yield. [c] Fluorescence
lifetime of the main component (weight is given in parentheses). [d] Only
one fluorescence band was observed.
tion and fluorescence spectra are shown in Figure 1 (all
other spectra can be found in the Supporting Information).
The UV/Vis absorption spectra showed major bands in the
region 250–400 nm. In the case of compounds 8 and 9, their
spectra were complemented by the typical signatures of
pyrene and anthracene at around 340 and 370 nm, respec-
tively (see the Supporting Information). The fluorescence
spectra were generally dominated by broad and red-shifted
long-wavelength (LW) emission bands. Moreover, in gener-
al, a weak and blue-shifted short-wavelength (SW) emission
was also notable, except for compound 2, which showed
only one emission band. Figure 2 shows a photograph of the
emission colors of compounds 1–9. Whereas the emissions
of compounds 1–3 were hardly visible, compounds 4–9
Figure 3. Linear plot of the energy that corresponds to the principal fluo-
rescence emission maximum versus the half-wave oxidation potential of
the aryl moiety.
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Chem. Eur. J. 2013, 19, 6650 – 6661

Properties and Switching Behavior of Borylated Arylisoquinolines
FULL PAPER
Table 2. Redox potentials, solvatochromic effects, and fluoride-binding
constants for compounds 1–9, as well as their fluorescence modulation
upon the addition of TFA or Bu
4
NF, in MeCN.
[
a]
[b]
ꢀ1 [c]
[d]
[d]
E
ox [V]
Dl [nm]
K [m
]
Q [%]
(TFA)
Q [%]
(Bu NF)
A
H
U
G
R
N
N
A
H
U
T
E
N
N
4
[
[
[
[
[
[
[
[
[
e]
e]
e]
e]
e]
h]
e]
h]
h]
3
[f]
1
2
3
4
5
6
7
8
9
1.65
2.01
1.72
1.54
1.26
0.75
1.21
1.16
1.09
1
ꢀ2
3
A
T
N
R
N
U
G
–
–
5
59
96
99
87
91
96
70
93
85
89
96
96
78
86
98
6[g]
[f]
4
3
4
3
3
4
4
AHCTUNGTRENNUNG
5
A
H
U
T
E
U
11
26
15
19
27
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[
a] Half-wave oxidation potential of the aryl model (versus SCE in
MeCN): 1=naphthalene, 2=1-cyanonaphthalene, 3=1-fluoronaphtha-
lene, 4=1-methylnaphthalene, 5=1-methoxynaphthalene, 6=1-(N,N-di-
methylamino)naphthalene, 7=acenaphthene, 8=pyrene, 9=anthracene.
Figure 4. Solvent effects on the fluorescence spectra of compounds 6
(
red), 8 (blue), and 9 (green). Dashed lines correspond to the spectra in
toluene and the solid lines to the spectra in MeCN; all spectra are nor-
malized to 1 at their maximum intensity.
[
b] Solvatochromic shift of the LW fluorescence maximum upon switch-
ing from toluene to MeCN. [c] 1:1 Binding constant of fluoride that re-
sulted in the formation of a fluoroboronate complex, as measured by flu-
orescence titration. [d] Fluorescence quenching of the LW emission band
(
measured at the maximum wavelength) upon the addition of 10 equiva-
of the LW emission band was performed. Figure 5 shows a
lents of TFA or 60 equivalents of Bu NF. [e] See ref. [52]. [f] Fluores-
4
ꢀ1
plot of the LW emission maximum (in cm ) versus the Dim-
cence enhancement was observed by a factor of 3.6 and 4.3 for com-
pounds 1 and 2, respectively. [g] Lower limit, too large to be measured
accurately. [h] See ref. [51].
[56]
roth–Reichardt E (30) parameter for selected compounds
T
aryl group is to oxidize, the lower the energy level of the flu-
orescent state. For example, compound 6, which contained a
4
-(N,N-dimethylamino)naphthyl moiety (E (ox)=0.75 V
1/2
[51]
versus SCE in MeCN), showed the most red-shifted emis-
sion, with lmax =608 nm. On the other hand, compound 2,
which contained a hard-to-oxidize 4-cyanonaphthyl residue
[52]
(
E (ox)=2.01 V versus SCE in MeCN),
had its only
1/2
emission band at 437 nm. Based on this trend, we inferred
that ICT, through the participation of the aryl residue, was a
decisive factor in determining the photophysical behavior of
these compounds. The often-observed dual fluorescence sig-
nature of biaryl fluorophores has been interpreted along
similar lines, by invoking an equilibrium between the locally
[53]
Figure 5. Linear plot of the LW emission maximum versus the E (30) sol-
vent parameter of compounds 2 (&), 4 (&), 6 (*), and 9 (*).
T
excited (LE) and ICT states. The geometry of these latter
states, which are often discussed as “twisted” versus
“
planar” ICT states, has been the subject of some controver-
[53,54]
sy in the literature.
As can be clearly seen from the
I (LW)/I (SW) ratio, the ICT state is dominant in MeCN for
in n-hexane, toluene, CHCl , THF, MeCN, and DMF. A
fl
fl
3
the compounds investigated herein.
linear correlation (r=0.856–0.958) was observed for com-
The occurrence of ICT phenomena in the excited state
was further supported by the observation of significant sol-
vatochromic effects on the spectroscopic position of the LW
emission maximum (Table 2, Figure 4, and the Supporting
Information). In accordance with these results, the com-
pounds with the most pronounced ICT (6, 8, and 9) showed
the largest bathochromic shifts of the LW emission band
upon switching from non-polar toluene to polar MeCN:
Dl=19–27 nm. For the other compounds (1–5 and 7), the
effects were significantly smaller or even negligible (Dl=ꢀ2
to 15 nm). For representative compounds with negligible
ICT (2), moderate ICT (4), and pronounced ICT (6 and 9),
a more detailed analysis of the solvent effect on the position
pounds 4, 6, and 9: The higher the E (30), the lower the
T
emission energy. The gradient of the slope of the plots
should give some semi-quantitative insight in the degree of
ICT, which is supportive of the conclusions shown above.
Thus, for compound 4, a 2.5-times shallower slope than for
compounds 6 and 9, with pronounced ICT character, was
obtained. However, in line with the negligible ICT character
of compound 2, no influence of the solvent was observed.
[55]
[57,58]
Using the Lippert–Mataga parameter (Df)
for the analy-
sis led to similar plots (see the Supporting Information),
albeit with poorer linear-regression parameters.
Interestingly, the SW band (where visible) showed no sig-
nificant solvatochromic effect, as clearly observed for com-
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6653

A. Ros, U. Pischel et al.
pound 9 (Figure 4). Hence, the SW bands were assigned to a
locally excited (LE) state. Furthermore, the relative intensi-
ty of the SW band increased with decreasing solvent polarity
(
e.g., for compound 9, I (LW)/I (SW)ꢁ6 versus 2 in MeCN
fl
fl
and toluene, respectively). This result was in line with the
assignment of an ICT state to the LW band, because a lower
solvent polarity would destabilize the ICT state and, thus,
emission from the LE state gained more weight.
In the overall series, the fluorescence quantum yield (Ff;
Table 1) was the highest for compound 5 (F =0.43). For the
f
naphthalene derivatives with more-positive aryl oxidation
potentials (1–4), much-lower emission quantum yields were
noted (F <0.05). This result provides further indication of
f
the importance of ICT phenomena, especially for com-
pounds with easily oxidizable aryl groups. Compound 6,
which had the energetically lowest-lying emissive ICT state,
showed a small drop in quantum yield (F =0.36) compared
f
to compound 5. This behavior can be understood by the
energy-gap law: The lower lying the emissive state, the more
competitive the non-radiative deactivation processes. Similar
trends for the emission quantum yields were noted for deriv-
atives with aryl residues with different extents of conjuga-
tion (1, 7, 8, and 9). Anthracene derivative 9 (with the
lowest-lying ICT state in this set) showed a significantly
lower emission quantum yield compared to acenaphthene
derivative 7 or pyrenyl-substituted derivative 8 (Table 1).
The lifetimes of the ICT emission were measured as com-
plementary data and they followed a similar trend to the
quantum yields (Table 1; for detailed analysis, see the Sup-
porting Information). The emission of the compounds with
the weakest ICT was very short lived (t<0.4 ns for com-
pounds 1 and 2) and it reached a maximum for compounds
with significant ICT character (tꢁ8 ns for compounds 5 and
Figure 6. HOMO and LUMO contour plots of the optimized molecular
geometries (at the B3LYP/6-31+G
, and 6.
ACHTUNGTRENUNNG( d,p) level of theory) of compounds 2,
4
The energies of the frontier orbitals in this series corre-
sponded very well with the expected values, based on the
substituent effects (Figure 7). For example, cyano-substitut-
ed compound 2 had the lowest-lying HOMO because it was
the hardest to oxidize. On the other hand, the N,N-dimethyl-
AHCTUNGTRENNUNGa mino substituent, as the strongest electron-donating group,
gave rise to the highest-lying HOMO (easiest to oxidize) for
compound 6. Moreover, the trends in the LUMO energies
of the compounds in the naphthalene subseries (1–6) also
corresponded to their expected electronic properties. Com-
pound 2, as the easiest reducible compound, had the lowest-
lying LUMO (Figure 7), with the highest contribution from
the Bpinꢀcyanonaphthalene residue. The major contribution
7
) with some decrease for compounds with energetically
low-lying ICT states (6: tꢁ6 ns; 8 and 9: tꢁ3 ns).
Density functional theory (DFT) calculations of compounds
1
–6: The direct participation of the aryl residue in the ob-
served fluorescence behavior was further corroborated by
the results of DFT calculations. For consistency and to facili-
tate the direct comparison of these results, calculations were
performed on the naphthalene-derived subseries of com-
pounds 1–6. Contour plots of the frontier orbitals of the op-
in the other compounds (1 and 3–6) came from the isoqui-
noline moiety and, thus, they showed comparable and con-
sistently higher LUMO energies. This very satisfactory cor-
relation between the electronic structure and the calculated
timized structures (at the B3LYP/6-31+G ACHUTNGRENNUG( d,p) level of
theory) of three selected examples (which were differentiat-
ed by the electron-donating propensity of their naphthyl res-
idue) are shown in Figure 6. For compounds 4 and 6, the
HOMO was localized over the borylated aryl residue,
whereas the isoquinolinyl group made the largest contribu-
tion to the LUMO. The same picture was obtained for com-
pounds 1, 3, and 5 (see the Supporting Information), which
strongly suggested that the ICT involved an aryl moiety as a
donor and the isoquinolinyl group as an acceptor. Com-
pound 2 was an exception, in which both the HOMO and
LUMO were mainly located on the borylated aryl residue,
in agreement with the assignment of a LE state as the
lowest-energy transition.
Figure 7. DFT-calculated HOMO and LUMO energies of naphthalene-
derived compounds 1–6.
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Chem. Eur. J. 2013, 19, 6650 – 6661

Properties and Switching Behavior of Borylated Arylisoquinolines
FULL PAPER
energies gave us confidence for also comparing their
HOMO–LUMO energy differences (DE). Thus, we found
that the relative trend in the energetic position of the LW
emission band was correctly expressed in qualitative terms
by the calculations (Figure 7). Again, compound 2 was the
only exception, which could be reasoned by the different lo-
calization of the LUMO (Figure 6).
observed very weak SW emission for compound 6 (Table 3
and Supporting Information). In general, the emission quan-
tum yields were much lower than those observed for the
borylated analogues, often lower than 0.01. The same trend
also applied to the emission lifetimes, except for compounds
with weak ICT character in their borylated derivatives.
From these observations, we concluded that boron substitu-
tion indeed played an important role in establishing the
emissive properties of the borylated arylisoquinolines. How-
ever, upon protonation (the addition of 10 equivalents of tri-
fluoroacetic acid, TFA), the isoquinoline moiety turned into
a cationic isoquinolinium group, a much stronger electron
acceptor, and, again, broad red-shifted LW emission bands
became visible at similar positions as observed for the bory-
Comparison with non-borylated analogues: For the sake of
comparison and for confirming the role of the boronic ester
substituent, non-borylated models M1–M9 were also includ-
ed in this study. Their photophysical data in MeCN are sum-
marized in Table 3. The absorption spectra did not change
+
lated analogues (Figure 8 for M4H as an example). This
Table 3. UV/Vis absorption and fluorescence properties of arylisoquino-
behavior was assigned as the formation of an ICT state,
which was also supported by the appearance of new red-
shifted absorption features for the protonated model com-
pounds at l=330–450 nm (see the Supporting Information).
Compound M6 was an exception, which only showed fluo-
rescence quenching of the band at l=553 nm upon protona-
tion. The new emissive ICT state was presumably so low-
lying that its quantum yield was very small because of a
strongly competing non-radiative decay process (energy-gap
law). A similar reasoning also applied to anthryl derivative
M9, for which a very small new emission at l=632 nm was
observed upon protonation.
lines M1–M9 in MeCN.
ꢀ1
ꢀ1 [a]
[b]
[c]
[d]
Model
l
abs [nm] e [m cm
]
l
fl [nm]
l
fl [nm]
F
f
t
f
[ns]
M1
M2
M3
M4
M5
M6
M7
M8
M9
282
312
285
287
310
323
297
341
362
10200
9800
10150
10800
9800
6600
9700
33100
7700
370
431
376
388
425
553
418
439
433
501
465
493
515
580
–
560
599
632
0.01 4.96
<0.01 1.93
<0.01 3.75
<0.01 1.70
<0.01 0.64
0.06 3.92
<0.01 0.53
0.02 0.89
0.09 1.00
[
[
e]
f]
[
a] Value refers to labs. [b] Fluorescence maximum of the red-shifted LW
band on the addition of 10 equivalents of TFA. [c] Fluorescence quantum
yield of the unprotonated compound. [d] Multiexponential decays (see
the Supporting Information); lifetimes are stated as average fluorescence
lifetimes. [e] Only quenching was observed, with no new LW bands.
Fluorescence modulation by the addition of an acid: Based
on the photophysical effects upon protonation of the non-
borylated model compounds (see above), it was of interest
to examine the fluorescence response of borylated ana-
logues 1–9 on the addition of acid (10 equiv TFA). Their re-
sponse was clearly dependent on the electronic conditions
of the system. Compounds 1–3, which contained aryl resi-
dues that were either weak electron donors/acceptors (1, 3)
or strong acceptors (2), yielded strong-to-moderate fluores-
cence enhancements (for compound 1, see Figure 9 and
Table 2) or only showed very minor quenching effects (com-
pound 3). All of the other borylated derivatives (4–9) gave
moderate-to-almost-quantitative fluorescence quenching of
the LW band, that is 59% quenching for compound 4 and
[
f] Maximum of a very weak new fluorescence band.
dramatically in the absence of the Bpin substituent (see the
Supporting Information). However, obviously, the fluores-
cence spectra only showed one (generally weak) emission
band, close to the SW band of the corresponding borylated
derivative (where observable). This result is shown in
Figure 8 for methyl-substituted naphthalene derivative M4
as an example. Compound M6 was an exception, which
showed a band that was spectroscopically unrelated with the
8
7–99% quenching for compounds 5–9 (for compound 6,
see Figure 9, Table 2, and the Supporting Information).
These observations were interpreted as follows: Deriva-
tives 1 and 2 only showed minor inherent excited-state ICT
character, owing to the protonation of the isoquinoline
group, which transformed this moiety into a strongly elec-
tron-accepting isoquinolinium cation. The result was an in-
creased degree of ICT, which was accompanied by fluores-
cence enhancement. In agreement with a slightly narrowed
energy gap, red-shifts of the emission band of compound 1
by about 7 nm and of compound 2 by about 20 nm were
noted.
In principle, one could anticipate this behavior for the
complete series of compounds 1–9. However, for the deriva-
tives in which a pronounced ICT was already noted for the
Figure 8. Comparison of the normalized fluorescence spectra of com-
pounds M4, M4H (with 10 equivalents of TFA), and 4 in MeCN.
+
Chem. Eur. J. 2013, 19, 6650 – 6661
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6655

A. Ros, U. Pischel et al.
pounds 6 and 9 in a 1:1 mixture of MeCN/water. As expect-
ed, upon the addition of 10 equivalents of TFA, strong fluo-
rescence quenching (about 90%) was observed.
Fluoride-ion-binding properties and photophysical effects:
The use of boron-substituted fluorophores in chemosensing
is often connected to their interactions with fluoride ions
ꢀ
(
F ) in a Lewis acid/Lewis base reaction. The binding con-
stants for the formation of 1:1 adducts upon the addition of
tetra-n-butylammonium fluoride are listed in Table 2 (for an
example titration curve of compound 9, see Figure 10). For
Figure 9. Effect of the addition of 10 equivalents of TFA to solutions of
compounds 1 and 6 in MeCN. The spectra were normalized to 1 at their
maximum intensity. The solid lines correspond to the spectra of the un-
protonated compounds and the dashed lines correspond to the spectra of
the protonated compounds.
unprotonated compounds (especially for compounds 5–9),
the additional energetic stabilization of the ICT state by iso-
quinolinium formation favored their preferential deactiva-
tion through non-radiative processes (energy-gap law).
Hence, the emission quantum yield of the new ICT state
(
upon protonation of the isoquinoline) was drastically de-
creased, thus resulting in the observed fluorescence quench-
ing. The observation of fluorescence enhancement or
quenching in the investigated series is a fine balance be-
tween the energetic positioning of the ICT states of the pro-
Figure 10. Fluorescence titration curve of compound 9 in MeCN with
Bu NF. The intensity of the LW emission band was monitored. The solid
4
line corresponds to the fitting according to 1:1 binding.
[59]
tonated compounds.
The fluorescence response to a large excess of selected
3
most compounds, the values varied between 3ꢅ10 and 2ꢅ
+
+
2+
2+
2+
4
ꢀ1
metal cations (60 equiv Na , Ag , Mg , Pb , and Zn )
10 m . However, derivatives 2 and 3 showed larger binding
6
ꢀ1
was also investigated for some representative compounds (2,
constants, exceeding 10 m in the case of cyano-substituted
derivative (2). This result was attributed to the electron-
withdrawing mesomeric effect of the CN substituent and the
electron-withdrawing inductive effect of the F substituent,
both of which increased the Lewis acidity of the boron
atom. Similar trends have been reported for other boronic
4
, 6, and 9; for detailed data, see the Supporting Informa-
+
tion). With Na ions, no fluorescence modulation was ob-
served. However, divalent metal cations (such as Pb or
Zn ) led, in some cases (compounds 6 and 9), to very
strong fluorescence quenching (81–100%). Such effects are
often interpreted as being the result of heavy-atom-induced
2+
2
+
[60,61]
acid derivatives.
Globally, the observed binding affini-
2+
intersystem crossing (Pb ) or redox quenching (d-metal cat-
ties for fluoride are comparable with that of other boronic
2+
[13]
ions, like Zn ). However, because many of these cations
form Brønsted acidic aquo complexes with trace water or
hydrate water, the observed fluorescence quenching may ac-
tually be the result of isoquinoline protonation. The ob-
served fluorescence enhancement of compound 2 (by up to
acid derivatives in organic solvents. For example, for com-
pound 9, the notion of the formation of a stable 1:1 adduct
ꢀ
with F was further supported by its unambiguous detection
by electrospray ionization mass spectrometry, which showed
a corresponding peak at m/z=450 (see the Supporting Infor-
mation). Similar verifications by mass spectrometry were
2
+
2+
4
00%) on the addition of Pb or Zn is analogous to the
ꢀ
[62]
observations made on the addition of TFA and contrary to
the expected nature of the quenching by these metal cations.
Hence, these combined observations provide strong indica-
tion that protonation is indeed the decisive factor in the flu-
orescence modulation. Thus, the effects of the metal ions
themselves are postulated to be of minor importance.
made for a F adduct of ferrocenyl boronic acid ester.
ꢀ
The formation of the 9·F adduct was also followed by
B NMR spectroscopy. The addition of 0.5 equivalents of
Bu NF to a solution of compound 9 resulted in the immedi-
1
1
4
1
1
ate appearance of a B signal at about d=7.0 ppm
(Figure 11). When 1.0 equivalent of Bu NF was added,
4
The fluorescence experiments in this study were per-
formed in MeCN as the solvent. To show that the fluores-
cence response upon protonation should also be observable
in aqueous media, the experiments were repeated for com-
almost all of the signal at d=31.1 ppm, which corresponded
to tricoordinated boron in the boronic acid ester, had disap-
peared and only the resonance signal at d=7.0 ppm, which
was assigned to the tetracoordinated boron in the fluorobor-
6656
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Chem. Eur. J. 2013, 19, 6650 – 6661

Properties and Switching Behavior of Borylated Arylisoquinolines
FULL PAPER
Figure 12. Fluorescence titration of compound 9 in MeCN with Bu
The LW band at lmax =571 nm diminished and a new SW band at lmax
20 nm developed. At l=494 nm, an isoemissive point was maintained
throughout the titration. Inset shows the change in emission color upon
the addition of fluoride.
4
NF.
=
1
1
Figure 11. B NMR spectra of compound 9 in [D
cessive addition of Bu NF (0.5–2.0 equiv). The spectra were recorded in
boron-free NMR tubes.
3
]MeCN upon the suc-
4
4
onate complex, was observable. In a recent report, similar
1
1
ꢀ
ate complexes showed B NMR signals in the range d=7–
center. Again, as for compound 2, the binding of F
[63]
8
ppm. The observed upfield shift reflected the change in
quenched all of the transitions in which the LUMO was in-
volved. The fine-structure and spectral position of the evolv-
ing band at about 420 nm pointed to emission from the LE
p,p* state of the anthracene chromophore itself.
the coordination number, as well as the geometry at the
boron center.
The binding event was accompanied by strong-to-almost-
quantitative fluorescence quenching (Table 2). The forma-
For some representative cases (compounds 2, 4, 6, and 9)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3
tion of the F adduct turned the boronic acid ester into an
their interactions with other anions (Cl , Br , CN , NO ,
ꢀ
ꢀ
electron-donating fluoroboronate substituent. The related
CH COO , and H PO ) were also investigated. However,
3 2 4
with the exception of cyano-substituted compound 2, which
showed fluorescence quenching upon the addition of
60 equivalents of CN (27% quenching) or H PO4 (60%
ꢀ
fluorodihydroxyboronate substituent (B(OH) F ) is one of
2
[13]
the strongest known inductively donating groups. Hence,
ꢀ
ꢀ
ꢀ
it can be assumed that the F coordination results in in-
2
creased electron-donating propensity of the aromatic moiet-
ies, which leads to an energetically lower-lying ICT state
and, hence, to fluorescence quenching, according to the
energy-gap law. Alternatively, excited-state electron transfer
from the fluoroboronateꢀaryl residue to the isoquinoline
quenching), no alteration of the fluorescence properties was
observed. Hence, these compounds showed a very selective
response to fluoride ions.
As described above for the addition of TFA, it was of in-
terest to verify the possibility of observing fluorescence
modulation by the addition of fluoride (60 equiv) to repre-
sentative compounds (6 and 9) in a water-containing
medium (MeCN/water, 1:1). Although the degree of fluores-
cence quenching was lower, it was possible to observe a sig-
nificant effect, that is, 14 and 22% quenching for com-
pounds 6 and 9, respectively. The Lewis basicity of fluoride
is counterbalanced by the formation of hydrogen-bonding
interactions with surrounding water molecules and, thus,
within this scenario, our observations are gratifying. Howev-
er, for a clear on-off fluorescence response, it is desirable to
keep the presence of water to a minimum.
[14]
part could be responsible for the quenching mechanism.
Electron- or charge-transfer-induced fluorescence quenching
upon the formation of a fluoroboronate has precedence in
[64]
the literature. By taking a closer look at compound 2, it
was obvious that the LUMO did not involve the isoquino-
line group (as for the other naphthalene-derived com-
pounds), but instead involved the vacant boron p orbital.
p
ꢀ
As is often observed for triarylboranes, F binding quenched
[21,23]
the transition that involved the LUMO.
Interestingly, compound 9 showed exceptional response
that was well-differentiated from the generally observed on-
off fluorescence modulation (Figure 12). The quenching of
the LW emission band at 571 nm was accompanied by the
growth of a blue-shifted SW band at about 420 nm, thus
giving access to ratiometric sensing. Similar observations
have occasionally been discussed for other boron-containing
Conclusion
A series of borylated arylisoquinolines was prepared and
their photophysical properties were investigated. The emis-
sion properties of these compounds were fine-tuned by var-
iation of the aryl moiety and its electron-withdrawing or
electron-donating substitution pattern. On the one hand,
compounds with aryl moieties that were harder to oxidize
[7,21–28]
fluorophores.
DFT calculations (see the Supporting In-
formation) of compound 9 showed that the HOMO was lo-
cated on the anthryl moiety and that the LUMO involved
the anthrylꢀBpin part. This result matched with an ICT
from the aromatic p system to the electron-deficient boron
Chem. Eur. J. 2013, 19, 6650 – 6661
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6657

A. Ros, U. Pischel et al.
showed emission maxima in the blue-shifted spectral region
for example cyano-substituted compound at lmax
37 nm), whereas, on the other hand, compounds 6 and 9,
were purchased from Aldrich in the highest available quality. The metal
cations were used in the form of their perchlorates (highest available
(
4
2
=
quality from Aldrich; NaClO
(ClO ·6H O).
4 4 4 2 4 2 2
, AgClO , Mg ACTHNUGRTENNU(G ClO ) , Pb ACTHNGURTENNUNG( ClO ) ·3H O, Zn-
A
H
U
G
R
N
U
G
4
)
2
2
which contained the easiest-to-oxidize aryl residues, showed
LW emission maxima at lmax =608 and 571 nm, respectively.
These ICT phenomena, which were implicated in the excit-
ed-state behavior, were further corroborated by the observa-
tion of significant solvatochromic effects and have been
characterized by DFT calculations. An analysis of the fron-
tier orbitals of the optimized molecular geometries (at the
All of the reactions were performed in oven-dried Schlenk tubes under
an argon atmosphere by employing standard techniques. Anhydrous
THF was obtained by using Grubbs-type anhydrous solvent drying col-
umns. Anhydrous 1,4-dioxane was obtained by distillation from sodium
by using benzophenone as an indicator. EtOAc, MeOH, n-hexane, and
toluene were purchased from Carlo Erba. Bis(pinacolato)diboron
(
B
2
pin
and Aldrich, respectively, and used without further purification. [{Ir
OMe)(cod)} (cod=1,5-cyclooctadiene), Sphos ligand, [Pd (dba)
dba=dibenzylideneacetone), and [Pd(PPh were purchased from
2
) and pinacolborane (HBpin) were purchased from Alfa Aesar
ACHTUNGTRENNUNG( m-
B3LYP/6-31+G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(d,p) level of theory) of the naphthalene-de-
A
H
U
G
R
N
U
G
2
]
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
]
(
A
H
U
G
R
N
U
G
3 4
) ]
rived compounds showed that ICT proceeded from the aryl
residue to the isoquinoline part of the molecule (except for
compound 2). The general importance of boron substitution
for the described photophysical phenomena became obvious
by including the non-borylated arylisoquinolines for the
sake of comparison. In general, their maximum emission
wavelengths were strongly blue-shifted and the quantum
yields were drastically decreased.
The molecular design of the title compounds enabled the
dual switching of their emission properties by the addition
of an acid (protonation at the isoquinoline N atom) or fluo-
ride ions (fluoroboronate formation with the Bpin moiety).
As a general trend, protonation caused significant fluores-
cence enhancement for compounds with weak ICT character
Strem and used without further purification. 1-Chloroisoquinoline was
purchased from Aldrich. Products 1, 4, 7, and 8 and their corresponding
non-borylated models (M1, M4, M7, M8) were prepared according to lit-
[
45]
erature procedures.
All photophysical measurements were performed on air-equilibrated sol-
utions in MeCN at RT (typically adjusting an optical density of 0.1–0.2 at
the excitation wavelength), by using quartz cuvettes with a 1 cm optical
path length. UV/Vis absorption spectra were recorded on a UV-1603
spectrophotometer (Shimadzu). Fluorescence spectra were recorded on a
Cary Eclipse fluorimeter (Varian) and corrected for the spectroscopic re-
sponse of the setup. To record the fluorescence spectra of the compounds
in the absence of additives, the samples were excited at the UV/Vis ab-
sorption maximum. The fluorescence quantum yields (error ꢂ25%) were
[
65]
f
determined by using quinine sulfate (F =0.55 in 0.05m sulfuric acid)
as a reference and corrected for the difference between the refractive in-
[30]
dices of water and MeCN. The excitation wavelengths for the fluores-
(
compounds 1 and 2) and strong quenching for compounds
4
cence measurements on the addition of TFA or Bu NF were chosen to
coincide with the isosbestic points of the UV/Vis absorption titration
spectra. Lifetime measurements were performed by means of time-corre-
lated single-photon-counting (Edinburgh instruments FLS 920) with pico-
second pulsed EPLEDs (EPLED 280, EPLED 330, EPLED 340, or
EPLED 360) for excitation close to the respective absorption maximum.
Deconvolution analysis of the decay curves yielded the corresponding
fluorescence lifetimes. For this purpose, the instrument-response function
was recorded by using a light-scattering Ludox solution.
with pronounced ICT character (compounds 5–9). On the
addition of fluoride, almost quantitative emission quenching
was observed, which, in the case of compound 9, was accom-
panied by the build-up of a blue-shifted emission, thus re-
sulting in a ratiometric response. Hence, these borylated ar-
ylisoquinolines not only constitute an interesting new class
of fluorophores, but they also possess potential as ion-trig-
gered molecular switches. Their potential applications may
include fluoride sensing in non-aqueous media or the use as
fluorescent tags with tunable emission color. In particular,
compound 6 and its derivatives are of interest because of its
strongly red-shifted emission signature and significant quan-
tum yields.
DFT calculations: All calculations were performed by using the Gaussi-
[
66]
an03 program. The ground-state geometries were calculated by apply-
ing the Kohn–Sham density functional theory (DFT) with the Becke3
Lee–Yang–Parr hybrid functional (B3LYP) method by using the 6-31+
G ACHUTNGRENNUG( d,p) basis set for the full structural optimization and frequency calcula-
tions. Analytical frequency calculations were performed to verify the sta-
tionary points on the potential-energy surface. To calculate the frontier
orbital energies, time-dependent density functional theory (TD-DFT)
was applied at the B3LYP/6-31+G
ACHTUNGTRENNUNG( d,p) level of theory.
General procedure for the synthesis of models M2, M3, M5, M6, and
[
49]
Experimental Section
M9: According to a literature procedure, a pre-dried Schlenk tube was
charged with [Pd (dba) (11 mg, 0.012 mmol), SPhos (39 mg,
0.096 mmol), pin (305 mg, 1.2 mmol), the haloarene (254 mg,
2
A
H
U
G
R
N
U
G
3
]
1
B
2
2
Materials and methods: H NMR spectra were recorded at 300, 400, or
1
3
1.2 mmol), and potassium acetate (196 mg, 2.00 mmol). After three cycles
of vacuum/argon-flushing, anhydrous 1,4-dioxane (4.0 mL) was added
through the septum with a syringe. The reaction mixture was heated at
5
00 MHz and C NMR spectra were recorded at 75, 100, or 125 MHz,
with the solvent peak used as an internal reference (CDCl : d=7.26 and
7.0 ppm for H and C, respectively); B NMR spectra were recorded
with complete proton decoupling at 160 MHz by using BF ·Et O as a
3
1
13
11
7
1
108C for 5 h. At this point,
(1.0 mmol, 163 mg) in 1,4-dioxane (1.0 mL) and an aqueous solution of
PO (5m, 1.0 mL) were added by syringe into the reaction flask. Then,
a solution of 1-chloroisoquinoline
3
2
1
1
19
standard (d=0.00 ppm for B NMR). F NMR spectra were recorded
K
3
4
with complete proton decoupling at 377 MHz by using trichlorofluorome-
1
9
the reaction mixture was heated at 1108C for a further 4 h, before being
cooled to RT and diluted with water (10 mL) and EtOAc (10 mL). The
organic phase was separated and the aqueous phase was extracted with
EtOAc (3ꢅ10 mL). The combined organic phases were dried over anhy-
drous MgSO , filtered, and concentrated to dryness. The products were
4
purified by flash chromatography on silica gel (n-hexane/EtOAc).
thane as a standard (d=0.00 ppm for F NMR). Column chromatogra-
phy was performed on silica gel (Merck Kieselgel 60). Analytical thin
layer chromatography was performed on aluminum-backed plates (1.5ꢅ
5
6
.0 cm) that were pre-coated (0.25 mm) with silica gel (Merck, Silica Gel
0 F254). The compounds were visualized by exposure to UV light or by
dipping the plates in a solution of 5% (NH
4 2 7 2
) Mo O24·4H O in EtOH
(
95%, w/v), followed by heating. MeCN (Scharlab) was of spectroscopic
4-(Isoquinolin-1-yl)-1-naphthonitrile (M2): According to the general pro-
[
67]
grade. Trifluoroacetic acid (TFA) was purchased from Aldrich. The dif-
cedure, starting from 4-bromo-1-naphthonitrile
(290 mg, 1.25 mmol);
ferent anions were applied as their tetra-n-butylammonium salts
flash chromatography on silica gel (n-hexane/EtOAc, 4:1) gave com-
pound M2 (134 mg, 48% yield) as a brown solid. M.p. 115–1178C;
(
4 3 3 2 4 4 2
Bu NX; X=Cl, Br, CH COO, NO , H PO , CN; Bu NF·3H O), which
6658
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Chem. Eur. J. 2013, 19, 6650 – 6661

Properties and Switching Behavior of Borylated Arylisoquinolines
FULL PAPER
1
13
H NMR (500 MHz, CDCl
3
): d=8.71 (d, J=5.5 Hz, 1H; Ar-H), 8.37 (d,
Ar-H), 7.34 ppm (t, J=8.5 Hz, 1H; Ar-H); C NMR (125 MHz, CDCl
3
):
J=8.5 Hz, 1H; Ar-H), 8.07 (d, J=7.5 Hz, 1H; Ar-H), 7.98 (d, J=8.5 Hz,
d=160.5 (C), 142.4 (CH), 137.0 (C), 136.5 (C), 131.8 (C), 131.8 (C), 131.5
(C), 130.7 (C), 130.2 (CH), 129.1 (CH), 128.5 (CH), 128.3 (C), 127.8
(CH), 127.7 (CH), 127.2 (CH), 127.2 (CH), 126.8 (CH), 126.6 (CH),
1
7
7
1
H; Ar-H), 7.83 (d, J=5.5 Hz, 1H; Ar-H), 7.75–7.70 (m, 2H; Ar-H),
.65 (d, J=7.5 Hz, 1H; Ar-H), 7.51 (d, J=8.5 Hz, 1H; Ar-H), 7.48–
1
3
.44 ppm (m, 3H; Ar-H); C NMR (125 MHz, CDCl
3
): d=162.4 (C),
125.5 (CH), 125.3 (CH), 124.9 (CH), 124.6 (CH), 120.3 ppm (CH);
+
58.4 (C), 142.2 (CH), 136.4 (C), 132.6 (C), 132.0 (C), 131.8 (CH), 130.5
HRMS (EI): m/z calcd for C23
H
15N: 305.1204 [M] ; found: 305.1200; ele-
(
(
1
CH), 128.6 (CH), 127.9 (CH), 127.7 (C), 127.6 (CH), 127.0 (CH), 126.9
mental analysis calcd (%) for C23
C 90.52, H 5.08, N 4.44.
H15N: C 90.46, H 4.95, N 4.59; found:
CH), 126.8 (CH), 126.6 (CH), 125.5 CH), 120.9 (CH), 117.7 (C),
+
10.8 ppm (CN); HRMS (EI): m/z calcd for C20
H
11
N
2
: 279.0922 [Mꢀ1] ;
General procedure for the Ir-catalyzed borylation reactions: Following a
found: 279.0913; elemental analysis calcd (%) for C20
4
H
12
N
2
: C 85.69, H,
[45]
recently reported method, a pre-dried Schlenk tube was charged with
.31, N, 9.99; found: C 85.74, H, 4.28, N, 9.80.
2 2
the substrate (M2, M3, M5, M6, or M9) and B Pin (1 equiv). After three
[
72]
1
-(4-Fluoronaphthalen-1-yl)isoquinoline (M3): According to the general
cycles of vacuum/argon-flushing, the catalyst stock solution (1 mL per
0.5 mmol substrate) and HBPin (5% mol) were added and the reaction
mixture was stirred at 55–808C until the starting material had been com-
pletely consumed (by TLC). The mixture was cooled to RT, concentrated
to dryness, and purified by column chromatography on silica gel (n-
hexane/EtOAc or toluene/EtOAc).
[
68]
procedure, starting from 1-bromo-4-fluoronaphthalene
(515 mg,
2.3 mmol); flash chromatography on silica gel (n-hexane/EtOAc, 6:1)
gave compound M3 (350 mg, 67% yield) as a colorless solid. M.p. 108–
1
1
8
108C; H NMR (400 MHz, CDCl
3
): d=8.73 (d, J=5.6 Hz, 1H; Ar-H),
.25 (d, J=8.3 Hz, 1H; Ar-H), 7.97 (d, J=8.1 Hz, 1H; Ar-H), 7.79 (d,
J=5.6 Hz, 1H; Ar-H), 7.72 (t, J=7.4 Hz, 1H; Ar-H), 7.64 (d, J=8.3 Hz,
H; Ar-H), 7.60–7.52 (m, 2H; Ar-H), 7.47–7.38 (m, 3H; Ar-H), 7.32 ppm
4
-(Isoquinolin-1-yl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1-
1
naphthonitrile (2): According to the general procedure, starting from
compound M2 (90 mg, 0.32 mmol); flash chromatography on silica gel
1
3
(
dd,
J
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(H,F)=9.8 Hz, J=8.1 Hz, 1H; Ar-H);
C NMR (100 MHz,
(C,F)=251 Hz; C), 142.4 (CH), 136.5
(C,F)=4 Hz; C), 133.1 (d, J(C,F)=4 Hz; C), 130.3 (CH),
28.4 (C), 127.7 (CH), 127.6 (CH), 127.3 (CH), 127.0 (CH), 126.3 (CH),
1
3
CDCl ): d=159.7 (C), 159.1 (d, JACHTGNRUETNNUG
(
toluene/EtOAc, 7:1) gave compound 2 (82 mg, 63% yield) as a light-
(
C), 133.7 (d, J
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
H
U
G
R
N
U
G
1
3
brown solid. M.p. 149–1518C; H NMR (500 MHz, CDCl ): d=8.67 (d,
1
1
1
J=6.0 Hz, 1H; Ar-H), 8.42 (s, 1H; Ar-H), 8.34 (d, J=8.0 Hz, 1H; Ar-
H), 7.93 (d, J=8.0 Hz, 1H; Ar-H), 7.79 (d, J=6.0 Hz, 1H; Ar-H), 7.71 (t,
J=8.0 Hz, 1H; Ar-H), 7.67 (t, J=8.0 Hz, 1H; Ar-H), 7.45–7.36 (m, 3H;
2
3
26.0 (CH), 123.9 (d,
20.4 (CH), 108.9 ppm (d,
J ACHTUNGTRENNUNG( C,F)=16 Hz; C), 120.8 (d, J ACHTUNGTREUNNNG( C,F)=5 Hz; CH),
2
19
J ACHUTNGRENNUG( C,F)=20 Hz; CH); F NMR (377 MHz,
CDCl
3
): d=ꢀ122.0 ppm; HRMS (EI): m/z calcd for C19
H
11FN: 272.0876
12FN:
Ar-H), 7.32 (d, J=7.5 Hz, 1H; Ar-H), 0.95 (s, 6H; 2ꢅCH
3
), 0.76 ppm (s,
): d=159.6 (C), 141.6 (C), 137.5
CH), 136.0 (C), 133.5 (C), 131.9 (C), 130.1 (C), 129.3 (CH), 129.0 (C),
127.7 (CH), 127.6 (CH), 127.1 (CH), 126.8 (CH), 126.6 (CH), 125.2
+
[
Mꢀ1] ; found: 272.0884; elemental analysis calcd (%) for C19
H
13
6
(
3 3
H; 2ꢅCH ); C NMR (75 MHz, CDCl
C 83.50, H 4.43, N 5.12; found: C 83.70, H 4.55, N 5.22.
1
-(8-Methoxynaphthalen-1-yl)isoquinoline (M5): According to the general
[
69]
procedure, starting from 1-bromo-4-methoxynaphthalene
(564 mg,
(CH), 120.2 (CH), 117.8 (CH), 110.0 (CN), 83.7 (2ꢅC), 24.3 (2ꢅCH
24.0 ppm (2ꢅCH
d=31.0 ppm (br s); HRMS (EI): m/z calcd for C26
2 2
[M] ; found: 406.1840; elemental analysis calcd (%) for C26H23BN O :
3
),
):
1
1
2
2
1
.4 mmol); flash chromatography on silica gel (n-hexane/EtOAc, 4:1!
3
), (C-B not observed); B NMR (160 MHz, CDCl
3
:1) gave compound M5 (228 mg, 40% yield) as a light-yellow solid. M.p.
H
2 2
23BN O : 406.1853
1
+
52–1548C; H NMR (500 MHz, CDCl
3
): d=8.70 (d, J=5.7 Hz, 1H; Ar-
H), 8.39 (d, J=8.4 Hz, 1H; Ar-H), 7.91 (d, J=8.4 Hz, 1H; Ar-H), 7.73
d, J=5.7 Hz, 1H; Ar-H), 7.69–7.66 (m, 2H; Ar-H), 7.51 (d, J=7.8 Hz,
H; Ar-H), 7.48 (td, J=7.6, 1.1 Hz, 1H; Ar-H), 7.42–7.39 (m, 2H; Ar-
H), 7.34 (td, J=7.8, 1.1 Hz, 1H; Ar-H), 6.96 (d, J=7.8 Hz, 1H; Ar-H),
C 76.86, H 5.71, N 6.90; found: C 77.00, H 5.65, N 6.78.
(
1
1
-(4-Fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)naphthalen-1-
yl)isoquinoline (3): According to the general procedure, starting from
compound M3 (137 mg, 0.5 mmol); flash chromatography on silica gel
1
3
4
.10 ppm (s, 3H; OCH
3 3
); C NMR (125 MHz, CDCl ): d=160.6 (C),
(
toluene/EtOAc, 3:1) gave compound 3 (134 mg, 67% yield) as a viscous
1
55.9 (C), 142.4 (CH), 136.5 (C), 133.2 (C), 130.1 (CH), 129.4 (C), 128.5
1
light-yellow oil. H NMR (400 MHz, CDCl
3
): d=8.65 (d, J=5.8 Hz, 1H;
(
(
C), 128.0 (CH), 127.9 (CH), 127.0 (CH), 126.8 (CH), 126.7 (CH), 125.7
CH), 125.6 (C), 125.2 (CH), 122.1 (CH), 119.9 (CH), 103.2 (CH),
Ar-H), 8.20 (d, J=8.4 Hz, 1H; Ar-H), 7.89 (d, J=8.2 Hz, 1H; Ar-H),
7
7
0
1
.73 (d, J=5.8 Hz, 1H; Ar-H), 7.65–7.61 (m, 2H; Ar-H), 7.56 (t, J=
.6 Hz, 1H; Ar-H), 7.42–7.31 (m, 4H; Ar-H), 0.94 (s, 6H; 2ꢅCH ),
): d=160.9 (C),
(C,F)=3 Hz; C),
(C,F)=4 Hz; C), 129.7 (CH), 127.3 (CH), 127.0
5
[
5.6 ppm (OCH
3
); HRMS (EI): m/z calcd for
Mꢀ1] ; found: 284.1079; elemental analysis calcd (%) for C20
C 84.19, H 5.30, N 4.91; found: C 84.15, H 5.52, N 4.95.
-(Isoquinolin-1-yl)-N,N-dimethylnaphthalen-1-amine (M6): According to
the general procedure, starting from 4-bromo-N,N-dimethylnaphthalen-1-
C
20
H
14NO: 284.1075
3
+
H
15NO:
13
3 3
.74 ppm (s, 6H; 2ꢅCH ); C NMR (100 MHz, CDCl
1
4
58.2 (d,
J AHCTUNGTRENNUGN( C,F)=251 Hz, C), 141.9 (CH), 140.2 (d, J ACHTUNGTRENNUNG
3
4
136.0 (C), 133.9 (d,
(CH), 127.0 (CH), 126.8 (CH), 126.5 (CH), 125.0 (d, J CAHUTNGTRNEUNGN( C,F)=17 Hz; C),
120.4 (d,
JACHTUNGTRENNUNG
2
[
70]
3
2
amine
(300 mg, 1.2 mmol); flash chromatography on silica gel (n-
J
A
H
U
G
E
N
N
J AHCTUNGTERNNUN(G C,F)=18 Hz;
hexane/EtOAc, 5:1) gave compound M6 (163 mg, 55% yield) as a vis-
CH), 83.4 (2ꢅC), 24.4 (2ꢅCH
3
3
1
11
19
cous orange/yellow oil. H NMR (300 MHz, CDCl
3
): d=8.68 (d, J=
B NMR (128 MHz, CDCl
CDCl
3
5
1
7
1
1
.7 Hz, 1H; Ar-H), 8.33 (d, J=8.6 Hz, 1H; Ar-H), 7.91 (d, J=8.3 Hz,
3
C
25
H
2
:
+
H; Ar-H), 7.73 (d, J=5.7 Hz, 1H; Ar-H), 7.72–7.64 (m, 2H; Ar-H),
.50–7.38 (m, 4H; Ar-H), 7.32–7.27 (m, 1H; Ar-H), 7.19 (d, J=7.7 Hz,
C
25
H
2
1
3
H; Ar-H), 2.98 ppm (s, 6H; N
3 2 3
ACHTUNGTERUNN(NG CH ) ); C NMR (75 MHz, CDCl ): d=
1
60.8 (C), 151.6 (C), 142.2 (CH), 136.5 (C), 133.6 (C), 131.4 (C), 130.2
yl)isoquinoline (5): According to the general procedure, starting from
compound M5 (143 mg, 0.5 mmol); flash chromatography on silica gel (n-
(
(
CH), 128.8 (C), 128.4 (C), 128.0 (CH), 127.8 (CH), 127.1 (CH), 126.8
CH), 126.4 (CH), 126.1 (CH), 125.1 (CH), 124.4 (CH), 120.0 (CH),
hexane/EtOAc, 2:1) gave compound 5 (175 mg, 85% yield) as a yellow
1
2
13.2 (CH), 45.2 ppm (N
A
H
U
G
R
N
U
G
3
)
2
); HRMS (EI): m/z calcd for C21
H
17
N
2
:
1
solid. M.p. 154–1568C; H NMR (500 MHz, CDCl
3
): d=8.64 (d, J=
+
5
1
.8 Hz, 1H; Ar-H), 8.34 (d, J=8.5 Hz, 1H; Ar-H), 7.85 (d, J=8.2 Hz,
H; Ar-H), 7.68 (d, J=5.8 Hz, 1H; Ar-H), 7.59 (t, J=7.5 Hz, 1H; Ar-
21 18 2
C H N
1
H), 7.46 (t, J=8.5 Hz, 1H; Ar-H), 7.44 (d, J=8.2 Hz, 1H; Ar-H), 7.31–
7.24 (m, 4H; Ar-H), 4.11 (s, 3H; OCH ), 0.92 (s, 6H; 2ꢅCH ), 0.74 ppm
); C NMR (125 MHz, CDCl ): d=161.9 (C), 154.6 (C),
141.9 (CH), 136.9 (C), 135.9 (C), 133.3 (C), 129.9 (C), 129.5 (CH), 127.6
(CH), 126.8 (C), 126.5 (CH), 126.5 (CH), 126.4 (CH), 126.3 (CH), 125.9
[
71]
dure, starting from 1-chloroanthracene (254 mg, 1.2 mmol); flash chro-
matography on silica gel (n-hexane/EtOAc 4:1) gave compound M9
3
3
1
3
(s, 6H; 2ꢅCH
3
3
1
(
(
223 mg, 73% yield) as
500 MHz, CDCl
a
yellow solid. M.p. 120–1228C; H NMR
): d=8.76 (d, J=5.5 Hz, 1H; Ar-H), 8.54 (s, 1H; Ar-
3
H), 8.17 (d, J=8.5 Hz, 1H; Ar-H), 8.01 (d, J=8.5 Hz, 1H; Ar-H), 7.97 (s,
H; Ar-H), 7.96 (d, J=7.0 Hz, 1H; Ar-H), 7.81 (d, J=6.0 Hz, 1H; Ar-
H), 7.70–7.66 (m, 3H; Ar-H), 7.61 (t, J=8.5 Hz, 1H; Ar-H), 7.58 (d, J=
.5 Hz, 1H; Ar-H), 7.43 (t, J=7.5 Hz, 1H; Ar-H), 7.38 (t, J=8.5 Hz, 1H;
(CH), 121.8 (CH), 119.3 (CH), 107.6 (CH), 83.1 (2ꢅC), 55.6 (OCH
3
),
), (C-B not observed); B NMR
): d=31.2 ppm (br s); HRMS (EI): m/z calcd for
1
1
1
24.3 (2ꢅCH
(160 MHz, CDCl
26 3
C H26BNO : 411.2006 [M] ; found: 411.1998; elemental analysis calcd
3 3
), 24.1 ppm (2ꢅCH
3
+
8
Chem. Eur. J. 2013, 19, 6650 – 6661
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6659

A. Ros, U. Pischel et al.
(
%) for C26
N 3.53.
-(Isoquinolin-1-yl)-N,N-dimethyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
H
26BNO
3
: C 75.92, H 6.37, N 3.41; found: C 76.10, H 6.32,
[8] J. C. Collings, S.-Y. Poon, C. Le Droumaguet, M. Charlot, C. Katan,
L.-O. Pꢇlsson, A. Beeby, J. A. Mosely, H. M. Kaiser, D. Kaufmann,
W.-Y. Wong, M. Blanchard-Desce, T. B. Marder, Chem. Eur. J. 2009,
4
1
5, 198.
lan-2-yl)naphthalen-1-amine (6): According to the general procedure,
starting from compound M6 (149 mg, 0.5 mmol); flash chromatography
on silica gel (n-hexanes/EtOAc, 2:1) gave compound 6 (182 mg, 86%
yield) as an orange solid. M.p. 166–1688C; H NMR (500 MHz, CDCl
d=8.64 (d, J=5.5 Hz, 1H; Ar-H), 8.33 (d, J=8.5 Hz, 1H; Ar-H), 7.87 (d,
J=8.0 Hz, 1H; Ar-H), 7.70 (d, J=5.5 Hz, 1H; Ar-H), 7.61 (t, J=8.0 Hz,
[
9] R. Stahl, C. Lambert, C. Kaiser, R. Wortmann, R. Jakober, Chem.
Eur. J. 2006, 12, 2358.
1
[10] H. C. Schmidt, L. G. Reuter, J. Hamacek, O. S. Wenger, J. Org.
Chem. 2011, 76, 9081.
11] T. W. Hudnall, C.-W. Chiu, F. P. Gabbaꢈ, Acc. Chem. Res. 2009, 42,
3
):
[
1
8
(
H; Ar-H), 7.52 (s, 1H; Ar-H), 7.50–7.48 (m, 2H; Ar-H), 7.33 (t, J=
.0 Hz, 1H; Ar-H), 7.30–7.25 (m, 2H; Ar-H), 2.99 (s, 6H; N(CH ), 0.93
); C NMR (125 MHz, CDCl ):
d=162.2 (C), 150.2 (C), 142.0 (CH), 139.2 (C), 136.0 (C), 133.6 (C), 130.2
388.
A
H
U
G
R
N
U
G
3
)
2
[12] Z. M. Hudson, S. Wang, Acc. Chem. Res. 2009, 42, 1584.
[13] C. R. Wade, A. E. J. Broomsgrove, S. Aldridge, F. P. Gabbaꢈ, Chem.
Rev. 2010, 110, 3958.
1
3
s, 6H; 2ꢅCH
3
), 0.75 ppm (s, 6H; 2ꢅCH
3
3
(
(
(
C), 129.9 (C), 129.7 (CH), 127.8 (CH), 127.3 (CH), 126.6 (CH), 126.4
CH), 126.0 (CH), 125.8 (CH), 124.1 (CH), 119.3 (CH), 118.1 (CH), 83.1
[14] Z. Guo, I. Shin, J. Yoon, Chem. Commun. 2012, 48, 5956.
[15] R. Martꢁnez-MꢂÇez, F. Sancenꢀn, Chem. Rev. 2003, 103, 4419.
[16] T. W. Hudnall, F. P. Gabbaꢈ, J. Am. Chem. Soc. 2007, 129, 11978.
[17] M. H. Lee, T. Agou, J. Kobayashi, T. Kawashima, F. P. Gabbaꢈ,
Chem. Commun. 2007, 1133.
2ꢅC), 45.3 (N
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(CH
3
)
2
), 24.4 (2ꢅCH
3
), 24.2 ppm (2ꢅCH
3
), (C-B not ob-
1
1
served); B NMR (160 MHz, CDCl
m/z calcd for C27 : 424.2322 [M] ; found: 424.2315; elemental
: C 76.42, H 6.89, N 6.60; found:
3
): d=31.4 ppm (br s); HRMS (EI):
+
H
2 2
29BN O
analysis calcd (%) for C27
C 76.52, H 7.00, N 6.68.
H29BN
2
O
2
[18] Y. Kubo, A. Kobayashi, T. Ishida, Y. Misawa, T. D. James, Chem.
Commun. 2005, 2846.
[
19] K. M. K. Swamy, Y. J. Lee, H. N. Lee, J. Chun, Y. Kim, S.-J. Kim, J.
Yoon, J. Org. Chem. 2006, 71, 8626.
1
-(2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)anthracen-1-yl)isoquino-
line (9): According to the general procedure, starting from compound
M9 (153 mg, 0.5 mmol); flash chromatography on silica gel (n-hexane/
EtOAc, 2:1) gave compound 9 (205 mg, 95% yield) as a light-brown
[
20] G. Zhou, M. Baumgarten, K. Mꢉllen, J. Am. Chem. Soc. 2008, 130,
1
2477.
1
[21] S. Yamaguchi, T. Shirasaka, S. Akiyama, K. Tamao, J. Am. Chem.
Soc. 2002, 124, 8816.
[
solid. M.p. 170–1728C; H NMR (500 MHz, CDCl
3
): d=8.73 (d, J=
5
7
5
1
.7 Hz, 1H; Ar-H), 8.50 (s, 1H; Ar-H), 8.13 (d, J=8.5 Hz, 1H; Ar-H),
22] Y. Kubo, M. Yamamoto, M. Ikeda, M. Takeuchi, S. Shinkai, S. Ya-
maguchi, K. Tamao, Angew. Chem. 2003, 115, 2082; Angew. Chem.
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23] X. Y. Liu, D. R. Bai, S. Wang, Angew. Chem. 2006, 118, 5601;
Angew. Chem. Int. Ed. 2006, 45, 5475.
.99 (d, J=8.6 Hz, 1H; Ar-H), 7.95–7.88 (m, 3H; Ar-H), 7.79 (d, J=
.8 Hz, 1H; Ar-H), 7.68 (d, J=8.6 Hz, 1H; Ar-H), 7.63 (t, J=8.2 Hz,
H; Ar-H), 7.46 (d, J=8.5 Hz, 1H; Ar-H), 7.42 (t, J=7.7 Hz, 1H; Ar-
[
H), 7.32 (d, J=7.5 Hz, 1H; Ar-H), 7.29 (d, J=7.5 Hz, 1H; Ar-H), 0.96 (s,
6
1
3
H; 2ꢅCH
3 3 3
), 0.76 ppm (s, 6H; 2ꢅCH ); C NMR (125 MHz, CDCl ):
[
[
[
[
24] D.-R. Bai, X.-Y. Liu, S. Wang, Chem. Eur. J. 2007, 13, 5713.
25] M.-S. Yuan, Z.-Q. Liu, Q. Fang, J. Org. Chem. 2007, 72, 7915.
26] Y. Sun, S. Wang, Inorg. Chem. 2009, 48, 3755.
27] Z. C. Xu, S. K. Kim, S. J. Han, C. Lee, G. Kociok-Kohn, T. D. James,
J. Yoon, Eur. J. Org. Chem. 2009, 3058.
d=161.7 (C), 145.0 (C), 142.0 (CH), 136.1 (C), 132.7 (C), 132.1 (C), 131.7
(
(
C), 130.8 (C), 129.8 (C), 129.7 (CH), 129.2 (CH), 128.8 (CH), 127.8
CH), 127.7 (CH), 127.5 (CH), 126.7 (CH), 126.5 (CH), 126.3 (CH),
1
CH
26.2 (CH), 125.9 (CH), 125.1 (CH), 119.7 (CH), 83.2 (2ꢅC), 24.4 (2ꢅ
1
1
3
), 24.1 ppm (2ꢅCH
3
), (C-B not observed); B NMR (160 MHz,
CDCl
3
): d=31.2 ppm (br s); HRMS (EI): m/z calcd for C29
H
26BNO
2
:
[28] H. Pan, G.-L. Fu, Y.-H. Zhao, C.-H. Zhao, Org. Lett. 2011, 13, 4830.
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Chem. 2008, 6, 2468.
+
4
31.2057 [M] ; found: 431.2060; elemental analysis calcd (%) for
C
29
H
26BNO
2
: C 80.75, H 6.08, N 3.25; found: C 80.83, H 6.12, N 3.18.
[
30] B. Valeur, Molecular Fluorescence: Principles and Applications, 1st
ed., Wiley-VCH, Weinheim, 2001.
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Acknowledgements
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32] Y. Kim, H.-S. Huh, M. H. Lee, I. L. Lenov, H. Zhao, F. P. Gabbaꢈ,
Chem. Eur. J. 2011, 17, 2057.
Financial support by MINECO, Madrid (CTQ2011-28390 to U.P. and
CTQ2010-15297 and CTQ2010-14974 to A.R.), the European FEDER
funds, and the Junta de Andalucꢁa (2008/FQM-3685 to U.P. and 2008/
FQM-3833 and 2009/FQM-4537 to A.R.) is gratefully acknowledged.
A.R. thanks the European Union for a Marie Curie Reintegration Grant
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(
FP7-PEOPLE-2009-RG-256461) and the Consejo Superior de Investiga-
ciones Cientꢁficas (CSIC, Madrid) for a JAE-Doc Fellowship. H.S.E.-S.
thanks the Computational Laboratory for Analysis, Modeling, and Visu-
alization at the Jacobs University Bremen (Germany) for computing
time.
[
[
39] D. Curiel, A. Cowley, P. D. Beer, Chem. Commun. 2005, 236.
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6660
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Chem. Eur. J. 2013, 19, 6650 – 6661

Properties and Switching Behavior of Borylated Arylisoquinolines
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Received: October 30, 2012
9
69.
Published online: March 28, 2013
Chem. Eur. J. 2013, 19, 6650 – 6661
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6661