A triarylboron-based fluorescent thermometer: Sensitive over a wide temperature range

Article abstract of DOI:10.1002/anie.201102390

Feeling blue: The luminescence of a triarylboron compound has a high quantum yield (at least 0.64) over a wide temperature range (-50 to +100 °C) and changes from green to blue as the temperature is increased (see picture). The luminescence color was dete

Full text of DOI:10.1002/anie.201102390

Communications
DOI: 10.1002/anie.201102390
Fluorescence
A Triarylboron-Based Fluorescent Thermometer: Sensitive Over a
Wide Temperature Range**
Jiao Feng, Kaijun Tian, Dehui Hu, Shuangqing Wang, Shayu Li,* Yi Zeng, Yi Li,* and
Guoqiang Yang*
Temperature is one of the most frequently measured variables
as it is a principal thermodynamic property.[1] Many types of
thermometers, which utilize various kinds of temperature-
dependant physical properties, such as volume, electric
potential, and electric conductance, have been developed
for quantitative temperature measurement.[2] Precise temper-
ature measurements in various environments require higher
thermometer performance. For instance, in situ large-area or
gradient temperature measurements with high spatial reso-
lution, which are often required in marine research, under-
ground geochemistry, wind tunnels, and automobile and
aircraft industries, present a serious challenge to traditional
thermometers.[3] A sensor with size from tens of micrometers
to several millimeters is required for temperature detection in
most thermometers, such as liquid-in-glass thermometers,
thermocouples, and thermistors. An array of these sensors is
used to achieve this goal, but has disadvantages because of
complications, high cost, and low spatial resolution.
The intrinsic limitation of mechanical or electrical ther-
mometers encourages the development of optical thermom-
eters that operate for large-area or fluidic samples.[4] Among
the available optical methods, infrared thermometers that use
the principle of blackbody radiation are flexible and easy to
use, but can only measure the temperature of surfaces, thus
limiting their applications.[5] Luminescence-based temper-
ature sensors have received more attention because of their
fast response, high spatial resolution, and safety of remote
handling.[6] To date, several luminescent materials based on
phosphors, dyes, or metal–ligand complexes have been
reported for temperature detection,[4g,7] which operates by
using the temperature-dependant luminescence intensity and/
or decay time of these compounds.[8] As measuring the
luminescence decay requires a relatively long time, the
intensity-based approach using a fast camera is more appli-
cable for large-area or gradient temperature measurements.[9]
However, the luminescence intensity is also affected by the
quantity of the luminophore, excitation power, and the
sample morphology. These drawbacks reduce the accuracy
and thus restrict the general utility of these techniques.[10] To
improve the performance of the luminescent thermometer,
some systems based on the intensity ratio of two emissive
compounds were developed.[7j,11] In these cases, the differ-
ences of physicochemical features between two compounds
might require extra calibration before data collection.[12]
Meanwhile, as a consequence of increased thermal activation
of radiationless processes with increasing temperature, a
significant decrease of the luminescence quantum yield is an
inevitable problem, except for some materials.[8b] Therefore, it
is important to develop novel single-luminophore thermom-
eters with high and stable luminescence quantum yield over a
wide temperature range. Additionally, when aiming to
facilitate fast and direct observation for temperature distri-
bution, significant thermosensitive hue transformation is also
required for these thermometers.[13]
In general, the luminescence quantum yield of organic
compounds decreases with increasing temperature. Some
compounds, such as twisted intramolecular charge transfer
(TICT) compounds, exhibit a total luminescence intensity
maintenance or even an enhancement from lower temper-
ature to room temperature.[14] This process is concomitant
with a luminescence colorimetric change that results from the
shift of the thermal equilibrium between local excited state
emission (LE) and the TICT excited state emission.[15]
However, it is difficult to apply the reported TICT com-
pounds in thermometers. Almost all TICT compounds have
only moderate luminescent quantum yields and the lumines-
cence decreases to very low levels at higher temperatures.[16]
Recently, several arylboron compounds were found to have
high luminescence quantum yields, even above room temper-
ature.[17] Following our interest in the design of novel highly
luminescent systems,[18] we designed a thermosensitive mol-
ecule that combined the advantages of TICT compounds with
two reverse luminescence intensity changes, and arylboron
compounds with high luminescence. This molecule can be
used as a sensitive luminescent colorimetric thermometer for
in situ large-area or gradient temperature measurements over
a wide temperature range and with high spatial resolution.
[*] J. Feng, K. J. Tian, D. H. Hu, Dr. S. Q. Wang, Dr. S. Y. Li,
Prof. Dr. G. Q. Yang
Beijing National Laboratory for Molecular Sciences
Key laboratory of Photochemistry, Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 (China)
E-mail: shayuli@iccas.ac.cn
gqyang@iccas.ac.cn
Dr. Y. Zeng, Prof. Dr. Y. Li
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing 100190 (China)
E-mail: yili@mail.ipc.ac.cn
[**] We acknowledge the National Natural Science Foundation of China
(grant nos. 20703049, 20733007, 20873165, 50973118), the
National Basic Research Program (2007CB808004, 2009CB930802),
and Chinese Academy of Sciences.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102390.
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A luminescent pyrene-containing triarylboron molecule,
dipyren-1-yl(2,4,6-triisopropylphenyl)borane (DPTB;
Scheme 1) has been designed and synthesized based on the
following considerations: 1) an electron-deficient boron atom
with an empty p orbital is well known as a highly effective
Scheme 1. Mechanism of emission changes with temperature.
electron acceptor, and the pyrene (Py) group is known as an
electron donor, so an intramolecular charge-transfer excited
state should be present;[19] 2) the contribution of these two Py
groups to the luminescent excited state is possibly not
equivalent because of the large steric hindrance in the
compact Py–B–Py structure; 3) any substituents that may
enhance radiationless decay are avoided and the rigid Py
group is chosen for facilitating the high luminescence
quantum yield;[20] 4) the sterically bulky substituent 2,4,6-
triisopropylphenyl (tipp) group has been proved to be an
effective stabilizer for the boron compound (details of the
synthesis are provided in the Supporting Information).[21]
DPTB shows temperature-dependant green to blue
luminescence with a very high quantum yield (Table 1) over
a wide temperature range (À50 to + 1008C). This intense
thermosensitive emission over a wide range conquers the
Figure 1. a) Absorption and b) corrected emission spectra of DPTB
recorded between À50 and 1008C (excitation wavelength 410 nm).
Table 1: Quantum yields F of DPTB in MOE at different temperatures.
lifetime measurements (see the Supporting Information). The
components with shorter- and longer-wavelength lumines-
cence bands are assigned to the emissions from the LE and
TICT states of DPTB, respectively (see the Supporting
Information). Similar to the case of other dual-luminescent
TICT compounds,[15,16] the temperature strongly influenced
the dynamic equilibrium between the LE and TICT excited
states of DPTB. The luminescence color was determined by
the population of the two distinct excited-state conforma-
tions. The lower-energy TICT excited state is preferentially
occupied with decreasing temperature, and, as expected a
bathochromic shift of the luminescence is observed. Upon
heating the system, the molecular motion that crosses the
thermal barrier between the two excited states increases the
population of the LE state,[22] thus resulting in a hypsochromic
luminescence. A feasible luminescence sensing mechanism
for temperature detection is shown in Scheme 1.
T [8C] À50 À30 À10 10
30
50
70
90
100
F
0.64 0.68 0.73 0.78 0.80 0.83 0.81 0.74 0.67
limitation that high temperatures induce a low signal/noise
ratio, thus suggesting that DPTB is an excellent candidate for
a reliable and absolute luminescent temperature sensor.
Figure 1 shows the absorption and corrected emission
spectra of DPTB in 2-methoxyethyl ether (MOE) at various
temperatures. MOE, which is a chemically stable liquid with a
relatively low melting point and high boiling point, was
chosen as the medium for DPTB in order to extend the testing
temperature range. An absorption band that extends from
350 nm to 460 nm, with a peak around 420 nm, is observed at
room temperature. The absorption band is not temperature-
sensitive and only shows very few changes over the whole
temperature range. In contrast, the emission profile of the
compound shows completely different features. At room
temperature, DPTB exhibits an abnormally wide lumines-
cence band that is identified as dual luminescence by its decay
Accuracy is another important factor for evaluating the
luminescent thermometer systems. As shown in Figure 1b,
the luminescence spectrum gradually shifts to higher energy
with increasing temperature. There is a good linear relation-
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ship between the maximum emission wavelength and temper-
temperature range of À50 to + 1008C according to the
resolution of current commercial colorimeters.
ature (Figure 2a). This linear relationship can be fitted as a
function of T= 1673.3À3.34lmax with correlation coefficient
0.9988, where lmax is the maximum emission wavelength (nm),
For further evaluation of the reversibility of the DPTB-
MOE system, 30 cycles of the temperature-dependent
emission were conducted between À508C and 1008C with
cycling rates of 308CsÀ1 heating and 108CsÀ1 cooling. The
spectra at different temperatures are exactly same for each
cycle, even at 1008C (Figure S5a), and are thus indicative of
the excellent reversibility of this system. Furthermore, the
isothermal test of stirring and heating at 1008C for 3 h was
carried out to examine the stability of the degassed system.
The luminescence spectra at 1008C show perfect matching
before and after durable heating (Figure S5b). Additional
thermogravimetric analysis reveals that DPTB does not
dissociate upon heating to 3008C. In comparison with the
structure of other boron compounds,[23] we can conclude that
the bulky substituent 2,4,6-triisopropylphenyl group separates
the central boron atom and surrounding molecules to prevent
the possible Lewis acidic/basic reaction, and contribute to
better stability of DPTB. The good reversibility and stability
make the DPTB–MOE system a good candidate for high-
performance thermometers.
To demonstrate the usefulness of the DPTB–MOE
system, we have applied it to the temperature gradient in a
fluid, as this situation is often encountered in industrial,
atmospheric, and marine research. A quartz tube filled with
DPTB–MOE solution was heated from the top and cooled
from the bottom (Figure 3a). A color-change pattern of the
temperature gradient is observed in the nonconvected vertical
fluid. The bottom is green and the top is blue, while the color
of the middle part continuously changes from green through
Figure 2. a) Temperature dependence of maximum emission wave-
length of DPTB. b) CIE chromaticity diagram showing the temperature
dependence of the (x,y) color coordinates of DPTB.
T is the temperature of the DPTB–MOE system (8C).
Calculations show that the corresponding wavelength shift
per degree centigrade is as large as 0.30 nm, which can be
readily measured by using a modern UV/Vis fluorescence
spectrometer, and is thus indicative of an accuracy better than
18C.
Although the thermosensitive wavelength shift method is
sufficiently sensitive for practical temperature detection, the
broad emission band introduces some difficulties to accu-
rately identify the maximum. To facilitate the use of this
system, the temperature-dependant spectra are transformed
to the Commission Internationale de LꢀEclairage (CIE) 1931
coordinates. Figure 2b shows the color change of the lumi-
nescence in the CIE (x,y) chromaticity diagram at different
temperatures. The system color shifts between green and blue,
which can be easily observed by the naked eye and captured
by a single CCD camera. In this case, the accuracy of this
approach is highly dependent on the CCD camera and the
software, and is calculated to be approximately 28C within the
Figure 3. a) Photographs of fluorescence at different temperatures (8C)
and the gradient fluorescence of DPTB solution in a quartz tube
(central). b) The characters A and C were written at low temperature
with DPTB solution in a polyvinyl chloride (PVC)/polypropylene (PP)
sandwich structure.
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cyan to blue as the temperature of the fluid increases.
According to the direct color observation with the naked eye
or a camera, the environmental temperature can be readily
estimated or measured by comparison with the temperature-
dependent CIE chromaticity diagram. This visualization
technique provides a useful tool for the detection of temper-
ature distribution.
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detecting the temperature distribution in a certain area, as is
often required in the automobile and aircraft industries, was
also examined. The DPTB–MOE solution was sealed in a
sandwich structure PVC–porous PP–PVC film (5 ꢁ 5 cm2, can
be enlarged to any size). The thickness of the film is 60 mm,
which is flexible and easy to adhere to the surface to map the
temperature distribution. As an example, Figure 3b shows the
characters A and C that were cooled from the bottom of the
film. The image of the characters is sharp and clear.
Considering the excited-state conformation changes in a
time range of pico- or nanoseconds,[24] the edge roughness of
the pattern is only determined by the thermal diffusion
coefficient of the conductive media. In the DPTB–MOE
system, the thermal diffusion coefficient of MOE media is
about 10À12 m2 sÀ1 KÀ1.[25] The edge roughness of the DPTB–
MOE system is estimated to be 30–40 mm when the image
capture time is set to 10 ms, indicative of a high spatial
resolution. Therefore, the thin-film system containing DPTB–
MOE can be used as a thermometer for in situ large-area
temperature measurement.
In some cases, such as industrial painting applications,
fluorescent dyes need to be placed in a polymer rather than in
a sandwich structure. High viscosity and low polarity hinder
the formation of the TICT state. No notable fluorescence
color changes are observed when DPTB is placed in low-
polarity polymers such as poly(decyl methacrylate) (Fig-
ure S6). It has been found that DPTB shows an apparent
temperature-dependant wavelength shift in polyurethane
with a low glass transition temperature. Further experiments
are needed to clarify this phenomenon, and the results will be
reported elsewhere.
In summary, a novel luminescent thermometer has been
developed by using a triarylboron compound, which has a
high luminescence quantum yield over a wide temperature
range and exhibits temperature-dependant luminescence.
This thermometer can be applied over a temperature range
of À50 to + 1008C with high stability and reversibility. By
using this thermometer, the luminescence spectra or the
luminescence color can be correlated to the temperature
values. The accuracy of former is better than 18C, and the
latter can be observed directly by naked eye or camera, thus
facilitating in situ large-area or gradient temperature meas-
urements with the accuracy of 28C. The liquid thermometer
can be fabricated in various forms and can thus be adapted for
use in different research areas.
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Received: April 6, 2010
Published online: July 7, 2011
Keywords: fluorescence · photochemistry · thermochromism ·
triarylboron compounds
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