Organic fluorescent thermometers based on borylated arylisoquinoline dyes

Article abstract of DOI:10.1002/chem.201402027

Borylated arylisoquinolines with redshifted internal charge-transfer (ICT) emission were prepared and characterized. Upon heating, significant fluorescence quenching was observed, which forms the basis for a molecular thermometer. In the investigated temp

Full text of DOI:10.1002/chem.201402027

DOI: 10.1002/chem.201402027
Full Paper
&
Boron Compounds
Organic Fluorescent Thermometers Based on Borylated
Arylisoquinoline Dyes
Vꢀnia F. Pais,[a] Josꢁ M. Lassaletta,[b] Rosario Fernꢂndez,[c] Hamdy S. El-Sheshtawy,[d]
Abel Ros,*[b] and Uwe Pischel*[a]
Abstract: Borylated arylisoquinolines with redshifted internal
charge-transfer (ICT) emission were prepared and character-
ized. Upon heating, significant fluorescence quenching was
observed, which forms the basis for a molecular thermome-
ter. In the investigated temperature range (283–323 K) an
average sensitivity of ꢀ1.2 to ꢀ1.8%Kꢀ1 was found for the
variations in fluorescence quantum yield and lifetime. In the
physiological temperature window (298–318 K) the average
sensitivity even reaches values of up to ꢀ2.4%Kꢀ1. The ther-
mometer function is interpreted as the interplay between
excited ICT states of different geometry. In addition, the for-
mation of an intramolecular Lewis pair can be followed by
11B NMR spectroscopy. This provides a handle to monitor
temperature-dependent ground-state geometry changes of
the dyes. The role of steric hindrance is addressed by the in-
clusion of a derivative that lacks the Lewis pair formation.
Introduction
Previous studies of organic compounds in which charge
transfer was exploited for the photophysical design of molecu-
lar thermometers include rhodamine dyes (rhodamine B and
rhodamine 3B),[3,6] 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) deriv-
atives,[7,8] 6-dodecanoyl-2-dimethylaminonaphthalene (Laur-
Temperature is a fundamental physical parameter that needs
to be known and controlled in the most diverse applications,
from engineering to life sciences. In recent years the exploita-
tion of the temperature dependence of photophysical phe-
nomena has given rise to the development of molecular ther-
mometers that deliver an optical output response.[1,2] The key
principle for such devices lies in the temperature-controlled
equilibration or competition between two states that have dif-
ferentiated optical signatures; these are most often fluores-
cence or phosphorescence. Unlike the majority of conventional
thermometers, chemical systems provide the appealing poten-
tial for small-scale integration that has found applications in,
for example, microfluidic systems[3] or for temperature map-
ping in cells.[4,5]
dan),[8] N,N-dimethylaminobenzonitrile,[9] and
a dipyren-1-
yl(2,4,6-triisopropylphenyl)borane.[10] However, on other occa-
sions, the temperature-controlled formation of exciplexes,[11,12]
excimers in bis-pyrenes[13] or bis-napthalenes,[14] and the phe-
nomenon of delayed fluorescence, observed for C70 fuller-
ene[15,16] or the acridine yellow dye,[17] was implemented with
equal success for the purpose of molecular thermometry. Fluo-
rophore-modified organic polymers in which temperature-de-
pendent conformational changes and connected variations of
the microenvironment of the fluorophore constitute the basic
design principle were also used abundantly.[4,18–24] Inorganic lu-
minescent compounds and materials such as nanoparti-
cles,[25–29] metal–organic frameworks,[30] metal clusters,[31] and
organometallic complexes[32–38] have received increased atten-
tion as well.
[a] Dr. V. F. Pais, Dr. U. Pischel
CIQSO—Center for Research in Sustainable Chemistry
and Department of Chemical Engineering, Physical Chemistry
and Organic Chemistry, University of Huelva
Campus El Carmen s/n, 21071 Huelva (Spain)
E-mail: uwe.pischel@diq.uhu.es
Recently, we reported on a new class of internal charge-
transfer (ICT) fluorophores that are based on borylated aryliso-
quinoline (BAI) dyes.[39,40] These compounds show redshifted
fluorescence that can be fine-tuned by electron-donor substi-
tution. The structural integration of a boronic ester moiety
confers photophysical properties to these dyes that are not
observed for the nonborylated analogues. The ICT character of
the dyes was established by correlations of fluorescence with
redox properties, solvatochromic effects, and density functional
theory (DFT) calculations.[39]
[b] Prof. Dr. J. M. Lassaletta, Dr. A. Ros
Institute for Chemical Research (CSIC-US)
c/ Amꢀrico Vespucio 49, 41092 Seville (Spain)
E-mail: abel.ros@iiq.csic.es
[c] Prof. Dr. R. Fernꢁndez
Department of Organic Chemistry, Universidad of Seville
C/Prof. Garcꢂa Gonzꢁlez 1, 41012 Seville (Spain)
[d] Dr. H. S. El-Sheshtawy
Akin to the observations made for the archetypal NBD fluo-
rophore,[7] it was anticipated that the temperature-dependent
interconversion between fluorescent ICT states and nonfluores-
cent “twisted” intramolecular charge-transfer (TICT) states
Chemistry Department, Faculty of Science
South Valley University, 83523 Qena (Egypt)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402027.
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could be at the origin of a similar effect for the BAI dyes 1–3
investigated herein. Furthermore, we expected that the BꢀN
interaction (see below), which can be followed by the charac-
BꢀN interactions for these dyes in solution. On the other hand,
compound 3 contains a methyl group in the ortho-aryl posi-
tion, and consequently the formation of an intramolecular
Lewis pair is sterically hindered. In agreement with this conno-
tation, the 11B NMR spectroscopic signal for dye 3 is downfield-
shifted to d=30.7 ppm (which corresponded to trigonal planar
B), and the 1H NMR spectroscopic signal of the pinacolate
methyl groups was found upfield-shifted at d=0.79 ppm.
Optical properties
The UV/Vis absorption and fluorescence spectra of the investi-
gated dyes at room temperature are shown in Figure 1 (for nu-
merical data see Table 1). Dyes 1 and 2 show significantly red-
shifted and broad absorption bands with maxima at 377 and
420 nm, respectively. In accordance with a recent report,[40]
these are assigned to charge-transfer (CT) bands. However, the
absorption maxima vary dramatically with the possibility to
engage in BꢀN interactions, and such a ground-state CT band
was not seen for the sterically hindered 3 (see above). Hence,
the UV/Vis absorption spectrum of 3 features a band at
283 nm that corresponds to localized p–p* transitions of the
aryl residue and a sharp peak at 321 nm that is typical of iso-
quinolines. Furthermore, dyes 1–3 correspond well with the
generally established excited-state ICT character of BAI dyes.[39]
They show broad emissions in the turquoise to green-yellow
teristic 11B NMR spectroscopic signal,[41,42] would provide a
convenient handle to allow the direct monitoring of the tem-
perature-dependent ground-state structural changes of the
fluorophores.
Results and Discussion
Synthesis and structural characterization of BAI dyes 1–3
Dyes 1–3 were prepared by borylation of their respective pre-
cursors A1–A3 by using a recently reported synthetic approach
that is based on the IrIII-catalyzed nitrogen-directed ortho-CꢀH
borylation of arylisoquinolines.[43]
The borylation reaction was carried out at 808C and afforded
the dyes in excellent yields for
products 1 and 2 (89 and 94%)
and in a moderate yield for 3
(43%). Precursor A1 was pre-
pared by means of a Pd0-cata-
lyzed Suzuki coupling of 1-chlo-
roisoquinoline with 4-methoxy-
phenylboronic acid.[40] For the
preparation of A2, the methoxy
group of A1 was treated with
lithium 4-methylpiperazin-1-ide
in a nucleophilic aromatic substi-
tution.[40] Finally, A3 was ob-
tained in a Suzuki coupling of 1-
chloroisoquinoline
with
2-
methyl-4-methoxyphenylboronic
acid. The described synthetic
procedures are summarized in
Scheme 1 (see the Experimental
Section also).
Dyes
1 and 2 show their
11B NMR spectroscopic signals (in
CD3CN at 293 K) at around d=
14–15 ppm, typical of tetracoor-
dinated boron.[41,42] Furthermore,
in the 1H NMR spectra of these
compounds,
the
pinacolate
methyl groups were observed as
a single signal at dꢁ1.30 ppm.
Both observations corroborate
Scheme 1. Reaction conditions: a) [Pd(PPh3)4] (3 mol%), Na2CO3 (2 equiv), THF/MeOH/H2O (see composition of sol-
vent mixture in the Experimental Section), 1108C, overnight; b) 2-pyridinecarboxaldehyde N,N-dibenzyl hydrazine
(1 mol%), [{Ir(m-OMe)cod}2] (cod=cyclooctadiene; 0.5 mol%), 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBPin;
unambiguously the existence of 5 mol%), THF, 808C; and c) THF, argon, reflux, overnight.
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Figure 1. Long-wavelength UV/Vis absorption bands (full lines) and corre-
sponding fluorescence spectra (dashed lines) of the investigated compounds
(1: blue; 2: red; 3: green) at 298 K. The spectra are normalized to 1 at their
respective maxima.
spectral range that corresponds to fluorescence maxima be-
tween 470 and 560 nm (see Figure 1) and typically large
Stokes shifts (91 nm for 1, 139 nm for 2, and 213 nm for 3). In
comparison to the methoxy group in 1 and the methyl and
methoxy groups in 3, the stronger electron-donating pipera-
zinyl substituent in 2 lowers the energy of the emitting ICT
state, thereby resulting in a significantly more redshifted emis-
sion. The involvement of CT in the lowest-energy transitions
was corroborated by density functional theory (DFT) calcula-
tions. The results confirm that the HOMO is mainly located on
the electron-donating aryl residue, whereas the LUMO has its
major contribution from the isoquinolyl moiety that plays the
role of the electron acceptor (see contour plots of frontier orbi-
tals in Figure 2). The HOMO–LUMO energy gap DE qualitatively
follows the trend of emission energies (see fluorescence
maxima in Table 1) of the excited ICT states: DE(2)<DE(3)<
DE(1).
Figure 2. Contour plots of HOMO and LUMO orbitals of the dyes 1–3 and
their energies [eV].
interaction and consequently the planarization of the biaryl
system on the one hand and the fluorescence quantum yield
on the other hand. However, although dye 2 clearly features
a BꢀN interaction, its relatively low-lying emissive charge-trans-
fer state (most redshifted emission maximum) preferably deac-
tivates through nonradiative pathways. Thereby the emission
quantum yield is diminished. In accordance with the discussion
of related BAI dyes, this finds its explanation in the energy-gap
law.[39] These few notions demonstrate that structure–property
relationships are certainly more complex than it appears at
first sight.
Temperature-dependent geometrical changes
The fluorescence quantum yields of the dyes vary between
0.05 and 0.16 (see Table 1). Interestingly, dye 1 has an emission
quantum yield that is three times higher than the sterically
hindered dye 3; electronically they differ by only one methyl
substituent. This observation proposes a relation between BꢀN
To obtain insight into the ground-state structural changes of
the compounds upon variation of the temperature, the
11B NMR spectroscopic signal was followed. In Figure 3 the ex-
ample of dye 2 is shown. Upon changing gradually from 283
to 323 K, an accentuated shift
change of the signal toward
Table 1. Photophysical properties of dyes 1–3 in acetonitrile.
lower field (Dd= +2.8 ppm) was
observed in CD3CN. Similar
labs [nm][a,b] lf [nm][a,c] Ff[a,d] tf [ns] ([%])[a,e] ln(A [sꢀ1])[f] Ea [kJmolꢀ1
]
Q [%][h]
SA [%Kꢀ1
Ff tf
]
[g]
[i]
changes were monitored for dye
1 (see the Supporting Informa-
tion). However, the sterically hin-
dered compound 3 showed no
temperature effect on the
11B NMR spectroscopic signal
(see the Supporting Informa-
tion). This does not mean that
the torsional angle between the
aryl planes in 3 is not altered
upon heating, but this process is
not seen in the 11B NMR spectra
1
2
3
377
(9650)
420
468
559
496
0.16
0.07
0.05
0.97 (37)
2.56 (63)
0.59 (88)
5.39 (12)
1.65 (92)
4.60 (8)
29.5
26.7
28.5
23.8
13.6
20.7
73
52
72
ꢀ1.8 ꢀ1.5
ꢀ1.2 ꢀ1.2
ꢀ1.8 ꢀ1.6
(12150)
283
(6500)
[a] Measured at 298 K. [b] UV/Vis absorption maximum; the corresponding molar absorption coefficient
e [mꢀ1 cmꢀ1] is given in parentheses. [c] Fluorescence maximum of corrected spectra. [d] Fluorescence quantum
yield measured against quinine sulfate as standard. [e] Fluorescence lifetimes measured by single-photon
counting. [f] Preexponential factor according to the Arrhenius equation [Eq. (1)]. [g] Activation energy for non-
radiative decay; r2 ꢂ0.996. [h] Fluorescence quenching upon heating from 283 to 323 K. [i] Average sensitivity
(in the temperature range of 283–323 K) based on the temperature-dependent quantum yield or lifetime data.
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Figure 3. Temperature dependence of the 11B NMR spectra of dye 2 in
[D3]acetonitrile. The dashed red line is meant to guide the eye (see also
Figure 4). The spectra were recorded between 283 and 323 K in steps of 5 K.
Figure 4. Temperature dependence of the UV/Vis absorption (left) and fluo-
rescence spectra (right) of dye 2 in acetonitrile upon heating from 283 to
323 K.
owing to the absence of BꢀN interactions. As can be deduced
from the fact that even at higher temperature (343 K) the
11B NMR spectroscopic resonance signals of dyes 1 and 2
remain at lower positions (d<20 ppm) than that of compound
3 (dꢁ31 ppm), the temperature increase leads to an equilibri-
um between the more planar low-temperature form and the
twisted high-temperature form (see Scheme 2 for the exam-
Scheme 2. Schematic representation of the temperature-dependent ground-
state equilibrium between the planar (left) and twisted forms (right) of bor-
ylated arylisoquinolines (R=methoxy (1) or N-methylpiperazinyl (2)).
Figure 5. Temperature dependence of a) the fluorescence lifetime, b) emis-
sion quantum yield, c) and 11B NMR spectroscopic signal of dye 2 in acetoni-
trile. The lines are meant to guide the eye.
ples of dyes 1 and 2). The partial thermally induced conversion
between the planar form (BꢀN) interaction and a nonplanar
form (with no BꢀN interaction) is also expressed by a reduced
intensity of the CT bands (see above) of dyes 1 and 2 in the
UV/Vis absorption spectrum (see Figure 4 for dye 2, and the
Supporting Information).
temperature effect is caused by the change in solvent proper-
ties that would ultimately lead to variations in the polarity
function and solvatochromic effects.[39] The fluorescence modu-
lation is reversible and upon cooling back to 283 K the initial
emission signal level was restored in all cases. The cycling can
be repeated at least ten times without significant differences
being observed (see Figure 6 for the example of dye 2). Akin
to the interpretations of related probes that are based on
charge-transfer phenomena, we propose the conversion of
a fluorescent ICT state into a nonfluorescent (or less fluores-
cent) TICT state in the rationalization of our observations.[7] The
temperature-dependent changes of the ground-state geome-
try (see above) give a hint of the conformational flexibility of
the dyes and increased torsional angles upon heating. Similar
effects are expected for the excited state in which the popula-
tion of a less fluorescent TICT state is promoted with increasing
temperature. Experimentally this assumption is indirectly evi-
Molecular fluorescent thermometer
In a further step we were interested in harnessing the changes
of the fluorescence of the dyes as a convenient and readily de-
tectable output signal. Upon gradual heating from 283 to
323 K, a pronounced quenching of the ICT emission intensity
that varied between 52 and 73% was observed (see Figures 4
and 5 for the example of BAI dye 2 and the Supporting Infor-
mation for the other dyes). The fluorescence maxima are not
shifted (dyes 1 and 3) or suffer only a minor blueshift by ap-
proximately 8 nm for dye 2. This observation excludes that the
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ꢀ
ꢁ
Ea
RT
kðTÞ ¼ Aexp ꢀ
ð2Þ
ð3Þ
1 DX
SA ¼
X
ref DT
On the one hand, the plot of ln(knr) versus 1/T according to the
Arrhenius equation [Eq. (2) with the rate constant k(T)=knr(T)
at temperature T, A as pre-exponential factor, R as gas con-
stant, and Ea as activation energy] yields Arrhenius-type behav-
ior for all investigated dyes (Figure 7, filled circles) and the re-
sulting activation energies (Ea) vary between 13.6 (dye 2) and
23.8 kJmolꢀ1 (dye 1) (see Table 1). Hence, the notion of the in-
volvement of a thermally induced nonradiative decay pathway
(such as the invoked TICT) is supported by the analysis of the
temperature-dependent excited-state kinetic data. The order of
magnitude of the activation energies is comparable with those
for related ICT chromophores.[7] However, as often observed for
organic fluorophores, the radiative rate constant kf is practically
independent of the temperature [see Figure 7, empty circles;
with k(T)=kf(T) in Eq. (2)].
Figure 6. Cycling of dye 2 for ten consecutive cycles of heating (to 323 K)
and cooling (to 283 K).
denced by the aforementioned fluorescence quenching for
dyes with larger torsional angles (compare dyes 1 and 3;
Table 1). Thus, the TICT state provides an efficient channel of
nonradiative excited-state deactivation and thereby leads to
the observed reduction of the fluorescence quantum yield.
In the same way as observed for the quantum yield (normal-
ly monitored through fluorescence intensity), the concentra-
tion-independent excited-state lifetime tf can also be used for
temperature monitoring. This bears advantages such as the in-
dependency of the signal reading on the fluorophore distribu-
tion in heterogeneous media, which for fluorescence intensity
measurements can be only overcome with ratiometrically re-
sponding systems. Akin to the observations made for the
quantum yield, the fluorescence lifetime is reduced with in-
creasing temperature (Figure 5 for dye 2). With the quantum
yield and lifetime data at hand, the corresponding kinetic con-
stants for the radiative (kf) and nonradiative (knr) excited-state
deactivation were calculated for each temperature [see Eq. (1)].
The data for dye 2 are plotted in Figure 7 (for the other dyes,
see the Supporting Information) [Eqs. (2) and (3)]:
The applicability of molecular thermometers is commonly
rated by the average sensitivity SA of the fluorescence quan-
tum yield (intensity) or lifetime toward the temperature
change [Eq. (3) in which X: Ff or tf].[36] The calculated values
for this parameter are listed in Table 1 and are in the order of
ꢀ1.2 to ꢀ1.8%Kꢀ1 for the full investigated temperature range
of 283–323 K. In the physiological temperature window (298–
318 K) even higher values apply: up to ꢀ2.4%Kꢀ1 (dye 1) for
the fluorescence intensity/quantum yield measurements and
up to ꢀ2.0%Kꢀ1 (dye 3) for the lifetime-based thermometer.
These data compare nicely with some of the best-performing
molecular optical thermometers based on lanthanide com-
plexes or those based on quantum dot luminescence.[2] On the
basis of a 1% precision of the measurement of the fluores-
cence intensity, a temperature resolution of approximately
ꢄ0.4–0.68C can be estimated for the most sensitive dyes,
1 and 3.
1
ð1Þ
Ff ¼ kf ꢃ tf with tf ¼
kf þ knr
Conclusion
In conclusion, borylated arylisoquinolines show a pronounced
dependence of their ICT fluorescence on the temperature. The
sensitive (SA between ꢀ1.2 to ꢀ1.8%Kꢀ1) molecular thermom-
eters were tested in the range of 283–323 K, including the
physiologically relevant temperature window. Some of the
thermometers can be operated with visible-light excitation and
provide spectrally well-separated emission outputs. The inter-
play between planar and twisted ground-state geometries was
studied by taking advantage of the 11B NMR spectroscopic
signal as an indicator for the formation of an intramolecular
Lewis pair. The temperature-dependent excited-state behavior,
as monitored by fluorescence, is rationalized by the interplay
between ICT and TICT states, which has precedence in the liter-
ature.[7] The analysis of the temperature-dependent kinetic
constants provides evidence for the thermal activation of
a nonradiative excited-state decay channel. The implication of
Figure 7. Arrhenius plot for the radiative (kf, empty circles) and nonradiative
rate constants (knr, filled circles) of excited-state deactivation of dye 2 in ace-
tonitrile. The major lifetime component was analyzed.
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calculations were performed to check for the stationary points on
the potential-energy surface. The frontier orbital energies were cal-
culated by the time-dependent density-functional theory (TD-DFT)
at the B3LYP/6-31+G(d,p) level of theory.
planar versus nonplanar geometries is further corroborated by
observations of a reduced fluorescence quantum yield of a ster-
ically hindered derivative. Potentially, the boronic ester opens
possibilities to functionalize biologically relevant carbohydrates
with the thermometer function. By building on the photophys-
ical foundation that was laid in this work, such studies are cur-
rently underway in our laboratories.
Synthetic procedures
Compounds A1, A2, and 2 were prepared following described
methodologies.[40]
Experimental Section
1-(4-Methoxy-2-methylphenyl)isoquinoline (A3): A Schlenk tube
was charged with a solution of the corresponding 1-chloroisoqui-
noline (197 mg, 1.2 mmol), 2-methyl-4-methoxyphenylboronic acid
(239 mg, 1.44 mmol), Na2CO3 (254 mg, 2.4 mmol), and [Pd(PPh3)4]
(41 mg, 3 mol%). After three cycles of vacuum–argon, toluene
(1 mL), MeOH (0.2 mL), and H2O (0.25 mL) were added sequentially.
The reaction mixture was stirred at 1108C overnight, cooled to
room temperature, quenched with H2O (10 mL), and extracted with
CH2Cl2 (3ꢄ10 mL). The organic layer was dried over MgSO4, filtered,
concentrated, and the residue was purified by flash chromatogra-
phy on silica gel (EtOAc/n-hexane 1:3!1:1) to give A3 (220 mg,
74%) as a light yellow viscous oil. 1H NMR (400 MHz, CDCl3): d=
8.61 (d, J=6.0 Hz, 1H; ArꢀH), 7.88 (d, J=8.4 Hz, 1H; ArꢀH), 7.71–
7.65 (m, 3H; 3ꢄArꢀH), 7.49 (t, J=7.2 Hz, 1H; ArꢀH), 7.27 (d, J=
3.2 Hz, 1H; ArꢀH), 6.90–6.86 (m; 2ꢄArꢀH), 3.89 (s, 3H; OCH3),
2.08 ppm (s, 3H; CH3); 13C NMR (100 MHz, CDCl3): d=161.3 (C),
159.6 (C), 142.2 (CH), 138.1 (C), 136.4 (C), 131.7 (C), 130.8 (CH),
130.0 (CH), 127.8 (C), 127.6 (CH), 127.1 (C), 126.8 (CH), 119.7 (CH),
115.7 (CH), 110.9 (CH), 55.0 (OCH3), 20.1 ppm (CH3); HRMS (EI): m/z
calcd for C17H14NO: 248.1075 [M+ꢀ1]; found: 248.1069.
Materials and methods
1H and 13C NMR spectra were recorded at 400 and 100 MHz, re-
spectively, using the solvent peak for CDCl3 as the internal refer-
ence (d=7.26 and 77.0 ppm for H and 13C, respectively). 11B NMR
spectra were recorded with complete proton decoupling at
160 MHz by using BF3·Et2O (d=0.00 ppm) as internal standard.
Electrospray ionization (EI) mass spectra and high-resolution mass
spectra were recorded using a QTRAP mass spectrometer (hybrid
triple quadrupole/linear ion trap mass spectrometer). Flash column
chromatography was carried out on silica gel (35–70 or 70–
200 mm).
1
All reactions were carried out in oven-dried Schlenk tubes under
an argon atmosphere. Anhydrous 1,4-dioxane was obtained by dis-
tillation from sodium using benzophenone as indicator. Anhydrous
tetrahydrofuran (THF) was obtained by using Grubbs-type solvent
drying columns. Ethyl acetate (EtOAc), methanol (MeOH), n-hexane,
dichloromethane (CH2Cl2), and toluene were purchased from com-
mercial suppliers and used without further purification. Reagents,
metallic precursors, and ligands for the syntheses were purchased
from commercial suppliers and used as received.
General procedure for the Ir-catalyzed borylation: Following a re-
cently described method,[43] a dried Schlenk tube was charged with
the substrate and B2Pin2 (1 equiv). After three cycles of vacuum–
argon flushing, catalyst stock solution (1 mL)[47] per 0.5 mmol sub-
strate and HBPin (5% mol) were added. The reaction mixture was
stirred at 808C until quantitative consumption of the starting ma-
terial was achieved. The mixture was cooled to room temperature,
concentrated to dryness, and purified by column chromatography
or by precipitation.
The photophysical measurements were performed on air-equili-
brated acetonitrile solutions, typically adjusting an optical density
of approximately 0.1 at the excitation wavelength and using
quartz cuvettes with 1 cm optical path length. Room-temperature
(298 K) UV/Vis absorption spectra were recorded using a UV-1603
spectrophotometer from Shimadzu, and the fluorescence spectra
were measured using a Cary Eclipse fluorimeter from Varian.
Steady-state temperature-dependent photophysical measurements
were carried out using a Perkin–Elmer Lambda 750 UV/Vis spectro-
photometer or a fluorimeter from Edinburgh instruments with
a Peltier system for temperature control (10–508C). For the temper-
ature-dependent fluorescence measurements the samples were ex-
cited at wavelengths of minimal absorbance variation throughout
the experiment. The lifetime measurements were accomplished
using time-correlated single-photon-counting (Edinburgh instru-
ments FLS 920). As excitation source a picosecond-pulsed UV-LED
(EPLED 280, l=283.2 nm, pulse width fwhm 706.5 ps) or picosec-
ond-pulsed diode lasers (EPLED 330, l=334.6 nm, pulse width
fwhm 758.7 ps; EPLED 360, l=367.4 nm, pulse width fwhm
745.5 ps) were used. Deconvolution analysis of the decay kinetics
yielded the fluorescence lifetimes. The instrument response func-
tion was obtained with a light-scattering Ludox solution. The fluo-
rescence quantum yields (error ꢄ10%) were determined with
quinine sulfate (Ff =0.55 in 0.05m sulfuric acid)[44,45] as reference
and corrected for refractive index differences between water and
acetonitrile.
1-[4-Methoxy-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phe-
nyl]isoquinoline (1): Following a general procedure for Ir-catalyzed
borylation, starting from A1 (59 mg, 0.25 mmol) and carrying out
the reaction at 808C, precipitation from the reaction crude with
pentane, trituration, and filtration afforded 1 (85 mg, 94%) as
a yellow-green solid. 1H NMR (400 MHz, CDCl3): d=8.83 (d, J=
8.0 Hz, 1H; ArꢀH), 8.48 (d, J=6.4 Hz, 1H; ArꢀH), 8.18 (d, J=8.4 Hz,
1H; ArꢀH), 7.89 (d, J=8.0 Hz, 1H; ArꢀH), 7.81 (dd, J=8.0, 7.2 Hz,
1H; ArꢀH), 7.71 (dd, J=8.0, 7.2 Hz, 1H; ArꢀH), 7.60 (d, J=6.4 Hz,
1H; ArꢀH), 7.34 (d, J=2.4 Hz, 1H; ArꢀH), 6.87 (dd, J=8.4, 2.4 Hz,
1H; ArꢀH), 3.92 (s, 3H; OCH3), 1.41 ppm (s, 12H; 4ꢄCH3); 13C NMR
(100 MHz, CDCl3): d=162.2 (C), 157.4 (C), 139.1 (C), 134.5 (CH),
132.4 (CH), 131.9 (C), 128.6 (CH), 127.7 (2 ꢄ CH), 127.3 (CH), 125.1
(C), 120.0 (CH), 116.6 (CH), 113.5 (CH), 80.3 (2ꢄC), 55.3 (OCH3),
27.2 ppm (4ꢄCH3) (CꢀB not observed); 11B NMR (160 MHz, CDCl3):
d=13.1 ppm (brs); HRMS (EI): m/z calcd for C22H24BNO3: 360.1768
[M+ꢀ1]; found: 360.1771.
The calculations were performed with the Gaussian 03 program.[46]
The ground-state geometries were calculated by applying the
Kohn–Sham density-functional theory (DFT) with the Becke3-Lee–
Yang–Parr hybrid functional (B3LYP) method using the 6-31+G(d,p)
basis set for the full structural optimization. Analytical frequency
1-[4-Methoxy-2-methyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)phenyl]isoquinoline (3): Following a general procedure
for Ir-catalyzed borylation, starting from A3 (62 mg, 0.25 mmol)
and carrying out the reaction at 808C, flash chromatography on
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Acknowledgements
Financial support from the Spanish MINECO (CTQ2011-28390
for U.P., CTQ2010-15297 and CTQ2010-14974 for A.R.), the Eu-
ropean Union (FEDER), and the Junta de Andalucꢅa (2008/FQM-
3685 for U.P., 2008/FQM-3833 and 2009/FQM-4537 for A.R.,
PhD fellowship for V.F.P.) is gratefully acknowledged. A.R. was
supported by a Marie Curie Reintegration Grant (FP7-PEOPLE-
2009-RG-256461) and by the Consejo Superior de Investiga-
ciones Cientꢅficas with a JAE-Doc Fellowship. H.S.E.-S. thanks
the Computational Laboratory for Analysis, Modeling, and Visu-
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Chem. Eur. J. 2014, 20, 1 – 9
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C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
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ployed to facilitate dissolution). The resulting red-brown solution was
kept under argon.
[47] The catalyst stock solution was prepared by dissolving 2-pyridinecar-
boxaldehyde N,N-dibenzyl hydrazone (37.6 mg, 0.125 mmol) and [{Ir(m-
OMe)(cod)}2] (41 mg, 0.063 mmol) in dry tetrahydrofuran to give a solu-
tion with a total volume of 25 mL (sonication for one hour was em-
Received: February 4, 2014
Published online on && &&, 0000
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FULL PAPER
Getting warmer: A series of molecular
fluorescent thermometers with good to
very good sensitivity is reported. Bory-
lated arylisoquinolines show pro-
&
Boron Compounds
V. F. Pais, J. M. Lassaletta, R. Fernꢁndez,
H. S. El-Sheshtawy, A. Ros,* U. Pischel*
nounced fluorescence quenching upon
heating (see figure), which can be re-
verted by cooling. The analysis of kinet-
ic parameters revealed the involvement
of a thermally activated nonradiative ex-
cited-state decay channel.
&& – &&
Organic Fluorescent Thermometers
Based on Borylated Arylisoquinoline
Dyes
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These are not the final page numbers! ÞÞ