A twisted-intramolecular-charge-transfer (TICT) based ratiometric fluorescent thermometer with a mega-Stokes shift and a positive temperature coefficient

Article abstract of DOI:10.1039/c4cc08010f

The fluorescence intensity of N,N-dimethyl-4-((2-methylquinolin-6-yl)ethynyl)aniline exhibits an unusual intensification with increasing temperature, by activating more vibrational bands and leading to stronger TICT emissions upon heating in dimethyl sulfoxide. Based on the different temperature dependence at various wavelengths, as shown in the TICT fluorescence spectrum, this dye can be employed to ratiometrically detect temperature.

Full text of DOI:10.1039/c4cc08010f

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DOI: 10.1039/C4CC08010F
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A twisted-intramolecular-charge-transfer (TICT)
based ratiometric fluorescent thermometer with a
mega-Stokes shift and a positive temperature
coefficient
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
ab
bc
b
b
b
Cheng Cao, Xiaogang Liu,* Qinglong Qiao, Miao Zhao, Wenting Yin, Deqi
DOI: 10.1039/x0xx00000x
b
a
b
Mao, Hui Zhang* and Zhaochao Xu*
www.rsc.org/
The
methylquinolin-6-yl)ethynyl)aniline, exhibits an unusual on the thermally activated delayed fluorescence of fullerene C70
intensification with increasing temperature, by activating whilst increasing temperature facilitates the back-forth
more vibrational bands and leading to stronger TICT transitions between the excited states S (singlet) and T
fluorescence
intensity
of
N,N-dimethyl-4-((2- a positive temperature coefficient has also been reported based
,
1
1
2
3
emissions upon heating in dimethyl sulfoxide. Based on the (triplet). Nevertheless, such an effect is vulnerable to oxygen
different temperature dependence at various wavelengths of quenching, posing a significant limitation to its application
the TICT fluorescence spectrum, this dye can be employed to environment. Consequently, the development of new
ratiometrically detect temperature.
fluorescent thermometers, preferably with positive temperature
coefficients, ratiometric measurements and a wide working
range becomes an important research target.
In this work, we propose and demonstrate a new design
concept to achieve a positive temperature coefficient in a
Fluorescent thermometers have been actively investigated
for visualizing temperature distribution in micro-environment
with high spatial and temporal resolutions, i.e., in cells and
1
-8
micro-fluidic devices. Most of such thermometers are based
on temperature-sensitive organic dyes (such as rhodamine B) or
inorganic complexes of ruthenium or europium, whilst their
emission intensities/lifetimes decrease with rising temperature
fluorescent
intramolecular-charge-transfer (TICT) emission of
dimethyl-4-((2-methylquinolin-6-yl)ethynyl)aniline ( ; Scheme
) in dimethyl sulfoxide (DMSO). TICT emission is typically
thermometer,
employing
the
twisted-
N,N-
1
1
(
owing to the thermal activation of nonradiative deexcitation
weak, because emission from the zero vibrational level of TICT
state is forbidden; in contrast, the radiative transitions from
TICT to ground states are feasible via the assistance of higher
pathways), and this correlation affords temperature
information.
9
-13
14
Recently, CdSe quantum dots and ZnO
1
5
2
4
microcrystals have also been reported as nano-thermometers,
which, however, still possess a negative temperature coefficient.
In contrast, fluorescent probes with a positive temperature
coefficient, whose emission intensities increase with
temperature, are particularly desirable, because they can
effectively suppress background interference at high
temperature; they can also work in conjunction with one
fluorophore with a negative temperature coefficient, where
ratiometric measurements of their emission intensities provide a
built-in correction and allow quantitative detection of
and non-totally symmetrical vibrational bands of TICT state.
These vibrational bands become activated upon heating.
Consequently, the TICT emission intensity will become
intensified with increasing temperature, provided that the
increase in TICT emission rate exceeds that of non-emissive
deexcitation.
2
, 16-19
temperature with a high sensitivity.
To this end, a number
of polymer-based fluorescent thermometers, which consist of a
thermo-responsive polymer and a polarity-sensitive fluorophore
fragment (such as benzofurazan), have been reported with
2
0-22
positive temperature coefficients,
because polymers offer a
decreasing
micro-environmental polarity around the Scheme 1 The Synthesis Route of
1.
fluorophore with a rise in temperature. Unfortunately, these
polymer-based thermometers require complicated synthesis
Compound
1 was synthesized in good yield as shown in
procedures; their temperature sensing functionality only Scheme 1 (Supporting Information; Fig. S1—S3). The UV—
operates around the polymer phase transition point (where vis absorption solvatochromism of is very weak (Fig. 1a). For
significant micro-polarity changes occur) and the corresponding example, the peak UV—vis absorption wavelength (λabs) of
1
1
working range is thus limited (i.e., < 10 ºC). A fluorophore with changes from 332 nm in n-hexane to 338 nm in DMSO, by only
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6
nm. In contrast, the peak emission wavelength (λem) of
solution varies considerably from non-polar to polar solvents
Fig. 1b). For instance, peak λem is registered at 380 nm in
1 in
ꢁ
ꢁ
− 1 ꢂꢃ − 1
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DOI: 10.1039/C4CC08010F
(eq. 2)
Δꢀ′ =
−
(
n
-
ꢃ
+ 2 2ꢂ + 4
hexane and ~550 nm in DMSO, with a change of ~170 nm. The
substantial bathochromic shift in peak λem results in a mega-
Stokes shift of ~210 nm in DMSO. A mega-Stokes shift is
beneficial for reducing the overlap between the UV—vis
where
ε and n refer to the relative dielectric constant and refractive index
of a solvent, respectively.
The formation of TICT state in
1 can be further rationalised
absorption and emission spectra of
a fluorophore and
via both viscosity-dependent emission measurements and
density functional theory (DFT) based calculations. In the
mixtures of ethanol and glycerol at various ratios, the UV—vis
absorption spectra of
contrast, as more glycerol is added into the mixture, affording a
higher viscosity, the corresponding emission intensity of is
minimizing the re-absorption of the emitted photons (via the so
called “inner-filter” effect), and thus improves the signal-to-
2
5
noise ratio of fluorescent imaging.
1 remain little changed (Fig. 2a). In
1
significantly enhanced, i.e., by approximately three times from
pure ethanol to pure glycerol (Fig. 2b). Such viscosity-
dependent fluorescence intensification is a typical characteristic
2
4, 27
of TICT emission.
Furthermore, DFT calculations show
from the S to S state mainly
that the photo-excitation of
1
0
1
involves electron transitions from the highest occupied
molecular orbital (HOMO) to both the lowest occupied
molecular orbital (LUMO) and LUMO+1 (Fig. 2c). Such
transitions result in substantial intramolecular charge transfer
(ICT) from the phenyl ring to the quinolone ring of 1. The large
extent of ICT is accompanied by significant molecular
geometry relaxation and drives the formation of TICT state at
the excited state of
group. The substantial geometry relaxation in conjunction with
the greatly increased polarity of in the TICT state is
responsible for the mega-Stokes shift of and its large
1, via the rotation of the dimethyl-amino
1
1
2
5, 28
fluorescence solvatochromism.
Fig. 1 (a) The UV—vis absorption and (b) fluorescence spectra of
solvents. (c) The Lippert-Mataga plot for the fluorescence transition energies of
] = 10 μM; the fluorescence spectra were obtained by exciting the samples at
1 in various
1
.
[1
their respective peak UV—vis absorption wavelengths. Note that the scales in (b)
is arbitrary, owing to varied slit sizes in use; please refer to Table S1 for exact
fluorescence quantum yields in various solvents.
Fig. 2 (a) The UV—vis absorption and (b) fluorescence spectra of
1 in ethanol-
glycerol mixtures. [ ] = 5 μM; the fluorescence spectra were obtained by exciting
1
It is worth noting that the emission spectrum of
1 in n-
the samples at 380 nm. (c) The frontier molecular orbitals of
obtained via DFT calculations.
1 in DMSO,
hexane exhibits a clear shoulder; however, the fluorescence
spectra become featureless with large Stokes shifts, as solvent
polarity increases (Fig. 1b). Moreover, the transition energies of
The emission intensities of
1 between ~450 and ~600 nm
the fluorescence (
vem) and Δf, the orientation polarisability (eq.
exhibit an unusual positive temperature coefficient, with a
temperature coefficient of 0.5% per ºC, while that in the long
wavelength region (> 600 nm) decreases slightly with
increasing temperature in DMSO (Fig. 3a). Accordingly, the
1
), demonstrate an approximate linear relationship in most
solvents, except that in
correlation coefficient,
n
r
-hexane in a Lippert-Mataga plot
= 0.9287; Fig. 1c; Table S1).
(
Interestingly, by re-plotting vem against Δf’ (eq. 2; which is
fluorescence quantum yield of
1 increases as temperature rises
applicable when TICT state is formed upon the photoexcitation
of a dye), an even higher correlation coefficient of 0.9507 is
in DMSO (Table S2). This extraordinary effect can be
rationalised considering the nature of TICT emissions. As
temperature rises, more vibrational bands at higher energy
levels in TICT state become active, which boosts the
probability of radiative electron transitions from TICT to
ground states. Not surprisingly, the associated peak emission
2
6
obtained (excluding data in
n-hexane; Fig. S4). These unusual
spectral characteristics indicate the formation of TICT state in
the excited state of
1
.
ꢁ
− 1
ꢂꢃ − 1
ꢃ
wavelength of
1 also displays a blue-shift from 551 nm at 25 ºC
Δꢀ =
−
(eq. 1)
2
ꢁ + 1 2ꢂ + 1
to 538 nm at 65 ºC, because TICT emissions associated with
2
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higher vibrational bands contribute relatively more as spectrum of rhodamine 6G (represented by the grey shadowed
2
4
temperature increases (Fig. 3a).
area in Fig. 3c) matches the emission spectrum of 1 very well.
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It should be pointed out the overall temperature dependence As a result, an emission peak at 567 nm, owing to rhodamine
DOI: 10.1039/C4CC08010F
of fluorescence intensity depends on two competing factors, the 6G, became apparent even when the mixture was excited at 330
rates of radiative TICT emission and non-emissive deexcitation. nm.
Both rates increase with rising temperature. While the former is
relatively more significant, such as in DMSO, demonstrates a limitations of existing fluorescence temperature sensors.
positive temperature coefficient (Fig. 3a); in contrast, an Fluorescence lifetime based thermometers require relatively
The TICT based thermometer of 1 circumvents several
1
1
1
inverse trend is found in other solvents, such as ethyl acetate long measurement time and sophisticated equipment.
EA; Fig. 3b), owing to more substantial non-emissive Fluorescence intensity based measurements are more
(
deexcitation at high temperature. Consequently, the emission straightforward. However, the emission intensity also depends
quantum yield of 1 drops with increasing temperature (Table on dye concentration and illumination intensity. Careful initial
S2). In fact, the fluorescence intensities of most TICT dyes calibration is thus unavoidable when a single type of dyes is
1
1
decrease at high temperature.
employed in a thermometer. While a ratiometric thermometer
using two types of dyes with distinct temperature coefficients
provides a built-in correction, these two types of dyes may
photo-bleach at different rates, thus compromising the
4
reliability of the thermometer. Moreover, the inhomogeneous
distribution of dyes (or concentration ratios) in a complex
system is a major concern for accurate interpretations of
emission intensity signals for both the one-dye and two-dye
thermometers. In contrast, the thermometer based on
1 offers a
ratiometric temperature sensing mechanism, while only one
type of dyes is employed. This self-calibration effect makes the
thermometer insensitive to dye concentration variations and
photo-bleaching effects, thus greatly improving measurement
accuracy. In addition, although the emission brightness of TICT
dyes is normally low, the quantum yields of
1 are good,
measured to be 55.2% and 18.9% at 25 °C in EA and DMSO,
respectively (Table S2). Since the temperature calibration curve
of
1 is solvent-dependent and viscosity-dependent, this
thermometer is most suitable for a system with minimal solvent
and viscosity variations. It can also serve as a calibration
Fig. 3 Temperature dependence of the fluorescence spectra of (a)
excited at 360 nm ([ ] = 5 µM); (b) in EA excited at 360 nm ([
mixture of and rhodamine 6G in DMSO excited at 330 and 540 nm, respectively;
] = 5 µM; [rhodamine 6G] = 1 µM. Temperature was varied from 25 to 65 C at
a step change of 10 C. The corresponding temperature dependence of emission
intensity ratios and the associated best-fit equations of (d) in DMSO at 500 and
00 nm; (e) in EA at 440 and 520 nm; (f) and rhodamine 6G mixture in DMSO
at 500 and 563 nm.
1
in DMSO reference for existing one-dye and two-dye thermometers to
1
1
1] = 1 µM); (c) the
further gauge their accuracies, i.e., in cells where the
environment mostly consists of water.
1
[
1
º
º
In conclusion, we have demonstrated a TICT induced
emission system with a positive temperature coefficient and a
1
6
1
1
mega-Stokes shift based on
1. This unusual positive
temperature coefficient is realised by significantly activating
vibrational bands of TICT state, which greatly facilitates TICT
emissions with respect to non-emissive deexcitation with a rise
in temperature. Owing to different temperature dependence of
TICT fluorescence at various wavelengths, the intensity ratios
of these emissions can be used to ratiometrically detect
temperature. It is expected that this new sensor design strategy
will have a significant impact on the development of
The different temperature dependence of TICT emissions at
varied wavelengths can be employed for ratiometric
temperature measurements (Fig. 3). The ratios of emission
intensities at 500 and 600 nm in DMSO ([
nm) affords an excellent linear correlation with temperature
changes, which demonstrates the potential of using as a
1] = 5 µM; λex = 360
1
thermometer (Fig. 3d). A similar temperature calibration curve
ratiometric fluorescent thermometers. However,
1 has poor
has also been constructed in other solvents, i.e., EA, although
solubility in aqueous solution and relatively short absorption
wavelengths. Developing water-soluble and bathochromically
shifted TICT dyes is the subject of our future work, for
measuring temperature in biological systems. The molecular
origins of the unusually high quantum yields and positive
the overall emission intensity of
1 drops as temperature
increases in this case (Fig. 3e). The reversibility of this
fluorescence system was also investigated by heating and
cooling it between 25 °C and 65 °C at a step change of 10 °C
for 10 cycles in DMSO. The fluorescence intensities at different
temperatures remain consistent among all cycles, demonstrating
an excellent reversibility.
temperature coefficients associated with the TICT emission of
are also currently under investigation.
1
Compound
1 can also work in conjunction with a
Z.X. thank financial supports from the National Natural
Science Foundation of China (21276251; 21422606), Ministry
of Human Resources and Social Security of PRC, the 100
talents program funded by Chinese Academy of Sciences. X.L.
temperature sensitive fluorophore, i.e., rhodamine 6G with a
negative temperature coefficient, to ratiometrically detect
temperature in DMSO (Fig. 3c, f). It is interesting to note that
Förster resonance energy transfer (FRET) from
1 to rhodamine
6
G occurs in this mixture, because the UV—vis absorption
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