Temperature insensitive fluorescence intensity in a coumarin monomer-aggregate coupled system

Article abstract of DOI:10.1039/c4cc04245j

The emission intensities of a fluorescent monomer-aggregate coupled system, based on 7-(dimethylamino)-coumarin-3-carbaldehyde, exhibit ultra-low temperature dependence with a low temperature coefficient of only 0.05% per °C, by judicious selection of the excitation wavelength. This finding has significant implications to temperature-sensitive fluorescent applications.

Full text of DOI:10.1039/c4cc04245j

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Temperature insensitive fluorescence intensity in
a coumarin monomer–aggregate coupled system†
Xiaogang Liu,‡a Deqi Mao,‡bc Jacqueline M. Cole*ad and Zhaochao Xu*bc
Cite this: Chem. Commun., 2014,
50, 9329
Received 3rd June 2014,
Accepted 20th June 2014
DOI: 10.1039/c4cc04245j
www.rsc.org/chemcomm
The emission intensities of a fluorescent monomer–aggregate coupled This system can also serve as a reference dye (or built-in correction)
system, based on 7-(dimethylamino)-coumarin-3-carbaldehyde, exhibit for temperature sensing and temperature effect correction in
ultra-low temperature dependence with a low temperature coefficient conjunction with other temperature sensitive dyes.4 To date,
of only 0.05% per 8C, by judicious selection of the excitation wave- wholly temperature insensitive dyes are unattainable; the
length. This finding has significant implications to temperature-sensitive lowest temperature-dependence in
a fluorescence system
fluorescent applications.
requests the use of certain rigid dyes, such as rhodamine 101.
However, this dye possesses a temperature coefficient of
Fluorescent probes are essential tools in many applications, such 0.13% per 1C at 20 1C, or B6% intensity variation over a
as tracking cellular events in biological systems,1 investigating the temperature change of 50 1C.9 Developing a fluorescent
distribution and activity of catalytic zeolite particulates2 and system with an ultra-low temperature coefficient remains a
exploring the flow mixing in fluid dynamics.3 Their high spatial challenging task.
resolution and quick response characteristics also make them
Following the recent exploration of molecular aggregates and
particularly attractive in the study of micro-systems.4 However, their functionalities,10–24 we propose and demonstrate a new
the effect of temperature variation on the photophysics and photo- design concept to achieve ultra-low temperature dependence of
chemistry of fluorescent probes has often been ignored, despite emission intensity using a monomer–aggregate coupled system.
being a crucial factor for an accurate interpretation of fluorescent In the most simplified version of this design, the QE of a dye
signals.5 Fluorescence intensities or quantum efficiencies (QE) drops as temperature increases; while at the same time, the
decrease with a rise in temperature in almost all fluorescent rising temperature leads to the dissolution of non-emissive
systems, owing to the increasing frequency of dye-solvent colli- molecular aggregates, boosting the quantity of emissive mono-
sions and the internal rotations/vibrations of dye molecules; this mers. By achieving a proper balance between these two con-
results in higher nonradiative de-excitation rates. Recent studies trasting temperature dependent emission characteristics, the
have shown that the distribution of temperature is highly hetero- overall emission intensity of the fluorescent system could be
geneous and dynamic in cells6,7 and microfluidic devices.8 There- maintained at a constant level. However, the actual picture
fore, during the quantitative analysis of fluorescent signals, the could be more complicated, since different phases of aggre-
cross-sensitivity of a probe to temperature (in addition to target gates may co-exist and some molecular aggregates are highly
species/processes) must be considered or, preferably, avoided emissive, whilst most of them are non- or weakly fluorescent.
through the use of a temperature-independent fluorescent system.
Our model system is based on 7-(dimethylamino)-coumarin-
3-carbaldehyde (1) in chloroform (Scheme S1; Fig. S1–S5, ESI†).
´ˇ
Cigan et al. showed that this dye formed highly emissive
a Cavendish Laboratory, Department of Physics, University of Cambridge,
J. J. Thomson Avenue, Cambridge CB3 0HE, UK. E-mail: jmc61@cam.ac.uk
b Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road,
Dalian 116023, China. E-mail: zcxu@dicp.ac.cn; Fax: +86-411-84379648
c State Key Laboratory of Fine Chemicals, Dalian University of Technology,
2 Linggong Road, Dalian 116012, China
d Argonne National Laboratory, 9700 S Cass Avenue, Argonne, IL 60439, USA
† Electronic supplementary information (ESI) available: Synthesis, characteriza-
tion, experimental and computational details. See DOI: 10.1039/c4cc04245j
‡ These authors contributed equally.
H-aggregates in methanol and ethanol, but they did not observe
molecular aggregates in several other solvents, such as CHCl3 and
ethyl acetate (EA).25 However, we find that emissive H-aggregates
exist in CHCl3, EA, and several other solvents tested in-house,
including cyclohexane, toluene, tetrahydrofuran (THF), and
dimethyl sulfoxide (DMSO). The molecular aggregation in
CHCl3 is very subtle, as reflected by the almost perfect match in
the normalized UV-Vis absorption spectra of 1 at different con-
centrations (Fig. 1a). In addition, in the 2D excitation-emission
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Fig. 1 (a) Normalized UV-Vis absorption spectra of 1 in CHCl3; (b) 2D
excitation-emission map of 1 in CHCl3 ([1] = 2 mM; T = 25 1C); (c)
excitation-dependent emission profiles of 1 in CHCl3 ([1] = 2 mM; T =
25 1C); the emission spectra have been scaled arbitrarily for clarity.
Fig. 2 Temperature dependence of fluorescence intensities of 1 ([1] =
4 mM; slit size = 2.5 : 2.5 nm) excited at: (a) lex = 395 nm; (b) lex = 410 nm;
(c) lex = 415 nm and (d) lex = 450 nm in CHCl3. Temperature dependence
(from 5 to 50 1C) of the ratios of fluorescence intensities and the
associated best-fit equations (e) at 475 nm, excited at 450 and 410 nm,
respectively; (f) at 460 nm, excited at 450 and 410 nm, respectively; (g) at
475 and 460 nm, excited at 410 nm; (h) at 475 and 460 nm, excited at
450 nm. Two vertical lines at 460 and 475 nm are drawn in (b) and (d),
respectively, to illustrate the different temperature dependence of the
emission intensities at these two wavelengths.
Fluorescence excitation spectra of 1 in CHCl3 ([1] = 4 mM) at: (d) lem
406 nm (slit size = 5 : 10 nm); (e) lem = 436 nm (slit size = 5 : 5 nm) and (f)
em = 470 nm (slit size = 2.5 : 2.5 nm). (g) Proposed aggregation model and
=
l
the calculated structures of the monomer and dimer of 1 in CHCl3.
map of 1, only one significant peak is observed, indicating the
presence of just one species in solution (Fig. 1b).
Nevertheless, a closer look, via the excitation-dependent
emission profiles of 1, does suggest the presence of molecular concentration and temperature prevent us from accurately deter-
aggregates (Fig. 1c). When excited at 350 nm, 1 exhibits two mining the aggregate formation constants.
secondary peaks at B406 and B440 nm, in addition to the main
This coumarin monomer–aggregate coupled system possesses a
peak at 470 nm. As the excitation shifts to longer wavelengths, the unique temperature dependence (Fig. 2a–d). While excited at
main peak becomes increasingly dominant, rendering the secondary 395 nm, where strong absorption is observed owing to dimer
peaks undetectable. The fluorescence excitation spectra were also formation, the fluorescence intensity unusually exhibits an increase
probed, at the emission wavelengths of 406, 436 and 470 nm. These with rising temperature (Fig. 2a). We attribute this effect to a
spectra demonstrate three distinct peaks at B370, B395 and relatively large increase in dimer quantity (by dissolving high-order
B445 nm, in turn, and suggest the co-existence of three species in aggregates) with respect to the drop in dimer QE. The considerable
the solution of 1 in CHCl3 (Fig. 1d–f and Fig. S6–S9, ESI†).
increase in dimer quantity with temperature is also reflected by a
These three species are assigned as high-order H-aggregates, small but consistent blue shift of 3 nm in the UV-Vis absorption
low-order H-aggregates (mainly dimers) and monomer units, spectra of 1 from 5 to 50 1C, towards the dimer absorption peak
respectively (Fig. 1g). The absorption band peak at B445 nm (Fig. S12–S14, ESI†). In contrast, the under-compensation of mono-
and the associated emission peak at B470 nm exhibit broad mers leads to a decrease in emission intensity as temperature
profiles, in contrast to the narrow bands indicative of J-aggregates.26 increases (Fig. 2d). By choosing an intermediate or ‘‘optimal’’ excita-
TD-DFT calculations suggest that these peaks are related to the tion wavelength, i.e., 415 nm, the decrease in monomer emission
monomers of 1 (ESI†). The remaining two species in CHCl3 exhibit can be balanced by the intensification of dimer emission. Conse-
blue-shifted peaks, larger Stokes shifts than the monomers of 1 and quently, the overall emission intensities of this system remain little
broad absorption/emission profiles, which are typical characteristics changed, with a small variation of only B3% over a temperature
of H-aggregates.20 Indeed, TD-DFT calculations show that the change of 45 1C, or a very low effective temperature coefficient of
H-dimer exhibits a hypsochromic shift with respect to the monomer B0.06% per 1C (Fig. 2c). As the contribution of dimer emission
of 1 (ESI†). The presence of H-aggregates in CHCl3 is not surprising, increases at high temperatures, the peak fluorescent wavelength (lf)
considering that similar aggregates have been reported in methanol of 1 also experiences a small blue shift of 5 nm from 5 to 50 1C.
and ethanol.25 Moreover, slight upfield shifts in the NMR spectra of
The balance mechanism in the fluorescence intensity of 1
1 were also observed as its concentration increased in CHCl3 and is also revealed via fluorescent lifetime measurements. When
methanol, corroborating our conclusion that molecular aggregates excited at 405 nm, the fluorescence decay dynamics of emissions
are present (Fig. S10 and S11, ESI†). However, the subtle changes between 415 and 475 nm of 1 follows a double-exponential decay
in the molar absorptivity of 1 in CHCl3 with respect to dye with two distinct time-constants of 1.1 and 2.9 ns (Table S1, ESI†).
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Table 1 The ‘‘optimal’’ excitation wavelengths and associated fluores-
cence intensity/peak emission wavelength variations of 1 in CHCl3 over
different concentrationsa
for the monomer–aggregate equilibrium. In fact, we have also
found that coumarin 545 possesses similar temperature insen-
sitivity in THF, methanol and DMSO. In particular, its tem-
perature coefficient in methanol is as low as 0.025% per 1C. To
our best knowledge, this result represents the lowest tempera-
ture coefficient among all fluorescence systems reported to
date. More details about this compound will be reported in a
forthcoming publication.
These systems stand to be useful for temperature-sensitive
applications. In particular, they could be employed to monitor
solvent temperatures and the associated dynamic processes
(such as heat transfer27,28) with high spatial and temporal
resolution in a contact-less way.29 In conventional two-color laser-
induced fluorescent thermometry, for example, the emission inten-
sity ratio of a temperature-sensitive dye (usually rhodamine B) and
a temperature-insensitive reference dye (typically rhodamine 101) is
‘‘Optimal’’
Concentration excitation
Intensity variation lf shift from
(mM)
wavelength (nm) from 5 to 50 1C (%) 5 to 50 1C (nm)
2
4
8
20
40
415
415
410
410
400
2.4
3.5
3.5
2.6
2.3
5
6
4
4
3
a
The ‘‘optimal’’ excitation wavelength refers to the excitation wave-
length affording the minimal fluorescence intensity variations over
temperature changes. During the determination of this ‘‘optimal’’
wavelength, the excitation wavelength is varied at a step size of 5 nm
across the entire first absorption band of 1.
They are assigned to dimer and monomer emissions, respectively. used to derive fluid temperature.9,30,31 Since these two dyes have
According to their varied contributions at different emission different photo-bleaching rates, the reliability of this thermometry
wavelengths and the overall emission spectrum, the monomer can be compromised.4 In contrast, only one type of dye would
and dimer emission spectra have been extracted (Fig. S15, ESI†). be sufficient when employing this monomer–aggregate coupled
Similar effects of temperature insensitive fluorescence intensities system. By exciting this system at different wavelengths, distinct
are also observed in 1 at higher concentrations (Table 1 and temperature dependence at the same emission wavelength, varying
Tables S2 and S3, ESI†). In these samples, the relative weight of from negative to positive temperature coefficients, can be realized,
monomer emission increases, probably because the low-order aggre- owing to altered contributions from the monomer and dimer
gates start to grow and their QE decreases accordingly. Therefore, the emissions (Fig. 2b and d; Tables S1–S5, ESI†); the resulting
optimal excitation wavelengths shift towards the dimer absorption fluorescent intensity ratios provide valuable information about
peak. However, these shifts are, in general, small, indicating that a the solvent temperature (Fig. 2e and f). Similarly, by exciting this
fixed excitation wavelength (i.e., 415 nm) can be employed over a monomer–aggregate coupled system at the same wavelength, the
wide concentration range to achieve ultra-low temperature depen- ratios of fluorescence intensities at different emission wavelengths
dence in this monomer–aggregate coupled fluorescent system. can also serve as a temperature indicator (Fig. 2g and h). Interest-
At 415 nm, the absorptivity of this coumarin monomer–aggregate ingly, this temperature sensing functionality is not limited in
coupled system amounts to B1.95 Â 104 L molÀ1 cmÀ1 and its QE CHCl3, but also works in other solvents, such as THF, EA, ethanol
is B40%, suggesting good fluorescence brightness.
and DMSO (Fig. S18–S23, ESI†). The only requirement for such a
This temperature dependence was investigated in other temperature sensor is the involvement of two emission species with
solvents. In solvents with similar polarities as compared to distinct temperature coefficients.
CHCl3, such as EA and THF, it was found that the increase of
In conclusion, a new design concept to achieve ultra-low
dimer emission counterbalances the decrease in monomer temperature dependence in a fluorescent system has been
fluorescence, resulting in very low temperature coefficients at proposed and demonstrated using an equilibrated monomer–
carefully selected excitation wavelengths (0.05 and 0.07% per 1C in aggregate coupled system based on 1. By balancing the QE
EA and THF, respectively; Fig. S16 and S17; Tables S4 and S5, variation and the relative quantities of monomers and dimers
ESI†). This suggests that the low temperature dependence is not (which vary due to aggregate formation/dissolution) over different
specific to CHCl3, but can be realized in other solvents as well. temperatures, the temperature dependence of this system can be
However, in lower polarity solvent (such as cyclohexane and adjusted by simply varying excitation wavelengths. While the
toluene) and higher polarity solvents (such as methanol, ethanol demonstration of ultra-low temperature dependence has been
and DMSO), the emission intensities of both monomers and herein confined to 1 in CHCl3, EA and THF, it is expected that this
aggregates decrease as temperature increases. Hence, the tem- design concept can also be extended to other dyes, which probably
perature dependence is relatively high, regardless of the excitation function in a different set of solvents, such as aqueous solution,
wavelength. These facts are attributed to significant decreases thus enabling a new and diverse range of temperature-sensitive
in QE, which cannot be offset by high-order aggregate dissolution. fluorescent applications. In addition, it has been demonstrated
The ultra-low temperature dependence demonstrated in this that the monomer–aggregate coupled system can be used for
monomer–aggregate coupled system of 1 could also be realized temperature sensing applications.
with other dyes that are prone to molecular aggregation, and
The authors wish to thank the EPSRC UK National Service for
probably in different solvents, via controlling the molecular Computational Chemistry Software (NSCCS), based at Imperial
structures of dyes (and their associated steric hindrance effects College London, for supporting this work. X.L. is indebted to the
and electrostatic interactions) and the solvent composition; Singapore Economic Development Board for a Clean Energy Scholar-
this essentially determines the thermodynamic parameters ship. J.M.C. thanks the Fulbright Commission for a UK–US Fulbright
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