Key words
small organic molecules - temperature-dependent dual fluorescence - local minimum
conformations - ground state - excited state - thermal equilibrium
Introduction
Temperature is a crucial physical parameter in a wide range of fields such as biology,
chemistry, industry and even our daily life. Therefore, the development of thermometric
methods to detect variations of temperature is particularly important not only for
scientific research but also for technological development.[1] Among the various temperature-determining methods developed so far, the fluorescence-based
sensing methods are most promising due to their unique advantages, such as noninvasiveness,
fast response, high spatial resolution, and capability of real-time monitoring.[2] According to the output signals, the thermo-responsive fluorescent systems can be
typically classified into two categories. One relies on the absolute-intensity change
of a single emission and the other takes advantage of change in the intensity ratio
of two emission bands. The temperature sensing based on the single-emission intensity
easily suffers from errors caused by the fluorophore concentration, the power of the
excitation laser, the inhomogeneous distribution of the fluorophore, and the sensitivity
of the detector. In contrast, the ratiometric sensing mode by measuring the change
in the intensity change of two emission bands can overcome the above undesired effects.
It is thus highly desirable to achieve temperature-dependent dual fluorescence.
To achieve the fluorescence ratiometric sensing of temperature, two approaches are
generally adopted. One is to refer the signal of indicator to that of a temperature-insensitive
fluorophore. The other is to utilize the dual emission of a single component. From
the practical point of view, the latter method is more attractive because it can avoid
the extra calibration due to the different physiochemical properties between two different
components. The hereto reported fluorophores for the ratiometric fluorescence sensing
of temperature mainly focus on hybrid nanoparticles,[3] polymers[4] and rare-earth complexes,[5] which either require complicated fabrication processes or are unstable in polar
solvents and easily quenched by oxygen. In this context, the small organic molecule
(SOM) fluorophores of a single component can provide good solutions to these problems.[6] However, the design of such fluorophores is a very challenging issue. According
to the Kashaʼs rule, which states that “regardless of which electronic state of a
given multiplicity is excited, the emitting electronic level of a given multiplicity
is the lowest excited level of that multiplicity,”[7] the single-component SOM-based fluorophores generally display only one fluorescence
band from the lowest singlet excited state (S1). As a consequence, dual fluorescence is an abnormal photophysical phenomenon, let
alone the dual fluorescence that is sensitive to temperature.
Theoretically, the prerequisite for the temperature-dependent dual fluorescence from
a single-component SOM is the presence of two emissive species, which may be chemically
isomeric or conformationally isomeric. Here we are mainly interested in the latter
situation. In addition, the population of two emissive species is sensitive to temperature.
The careful examination on the reported examples of temperature-dependent dual fluorescence
of SOMs found that the presence of two emissive species may be caused by the presence
of two local minimum conformations in the ground state (S0) that interchange with each other,[8]–[10] the presence of two local minimum conformations in the S1 state originating from significant conformational relaxation upon excitation,[11]–[14] and the presence of thermal equilibrium between S1 and the second excited state (S2)[15]–[18] ([Figure 1]). Considering the great potential utilization of SOM fluorophores showing temperature-dependent
dual fluorescence for the ratiometric fluorescence sensing of temperature and the
significant difficulty to obtain such fluorophores, we would like to present a short
review on the existing samples. Especially, the analysis of the intrinsic mechanism
behind experimental phenomena is significantly important. Herein, we mainly focus
on the illustration of the three mechanisms through introduction of representative
examples. Hopefully, the discussions in this short review will provide some important
guidelines for the new design of such fluorophores.
Figure 1 Schematic presentations for the mechanisms of temperature-dependent dual fluorescence,
which arise from the presence of (a) two local minimum conformations in the S0 state, (b) two conformers in the S1 state originating from significant conformational relaxation upon excitation, and
(c) the thermal equilibrium between S1 and S2 states.
Presence of Two Local Minimum Conformations in the S0 State
Presence of Two Local Minimum Conformations in the S0 State
For most fluorescent SOMs, there exists only one local minimum conformation in the
S0 state, which thus corresponds to the global minimum conformation in the S0 state. Upon excitation, this conformation relaxes to the global minimum conformation
of the S1 state and emits fluorescence. In some circumstances, one local minimum conformation
may surpass a certain energy barrier and transform to another one in the S0 state. Namely, there exist two local minimum conformations that are thermally equilibrated
in the S0 state ([Figure 1a]). In addition, the two stable conformers may display different electronic structures
and thus different absorption and fluorescence properties due to their different stereostructures.
Owing to the thermal equilibrium, the population of each local minimum conformation
is sensitive to temperature. The increase of temperature is expected to increase the
population of the local minimum conformation with relatively higher energy. Therefore,
the individual excitation of both conformers will give rise to temperature-dependent
dual fluorescence. The change trend of fluorescence as temperature varies is expected
to be closely related to the relative stability and the emission properties of each
conformer. Due to different absorption and emission properties of two conformational
structures, both fluorescence and absorption change with the change in temperature.
So far, it has been very difficult to predict which conformer is more stable. In addition,
the more stable conformer in the S0 state does not necessarily show a higher stability in the S1 state, which makes it very hard to compare two emission wavelengths and the change
trend of fluorescence.
Triarylboranes represent good examples to illustrate this mechanism. Owing to the
Lewis acidity of the trivalent boron center, it can accept electrons from Lewis basic
donors, such as N, P, or O, to form reversible B–X bond coordination bonds. By finely
tuning the intramolecular relative positions and steric hindrance between boron and
Lewis basic donors, it becomes possible to modulate the B–X bond strength to achieve
the reversible transformation between three-coordinated and four-coordinated species,[8]–[10],[19] which may be stimulated by temperature. One pioneering system possessing switchable
B–X bonds is the bithiophene Lewis pair 1 ([Figure 2]),[8] in which diphenylphosphine oxide P(O)Ph2 and dimesitylboryl (BMes2) were introduced at 3,3′-poistions of 2,2′-bithiophene. One important structural
feature of 1 is the flexibility of the bithiophene backbone, which allows for interannular C–C
bond rotation, leading to the flexible switching between the closed form as a Lewis
adduct and the open form as an unbound Lewis pair. In addition, the flexible switching
of 1 between two different forms is greatly influenced by the H-bonding capability of
the solvent, which was strongly supported by the IR and NMR spectra. In weak or non-H-bonding
donor solvents, such as hexane, the closed forms are present. However, the open form
is favored in a strong H-bonding solvent, such as MeOH, and two structures coexist
in weak H-bond-donating solvents like CH2Cl2. It is well known that tri-coordinated boron exhibits strong electron-accepting ability.
The formation of the B–O bond in 1 between the B center and the O atom of P=O resulted in the great difference in the
electronic structures and thus different absorption and emission properties between
the open and closed forms. Theoretical calculations demonstrate the S0→S1 transition of the open form is characterized by a charge-transfer (CT) process from
a mesityl-localized π-orbital to boron-centered LUMO due to the electron-accepting
ability of BMes2.[20] In contrast, the S0→S1 excitation of the closed form features a bithiophene π–π* transition along with a
CT from a mesityl π-orbital to the bithiophene-localized LUMO. Consistent with the
solvent dependence of flexible switching between the open and closed forms of 1, both the absorption and emission are sensitive to the solvent. In very weak or non-H-bonding
donor solvents, such as hexane, the spectra show a dominant band at ≈ 370 nm. However,
this band becomes weaker and the bands between 260 and 350 nm become stronger with
increasing H-bonding ability. Regarding the emission, this compound shows bright greenish
yellow fluorescence (Φ
F = 0.60) at 540 nm in methanol and emits a bright blue emission at 440 nm in hexane,
which correspond to the open and closed forms, respectively ([Figure 2c]). Moreover, the emission of 1 in alcohol is highly temperature-dependent. At room temperature, where the open form
is the major species, the emission is dominated by the fluorescence at ≈ 540 nm. As
the temperature decreases, the initially dominant band at 540 nm decreases with a
concomitant increase in the emission band at 440 nm, suggesting lowering the temperature
is favorable for the formation of the closed form. The temperature dependence of the
equilibrium between the two forms of 1 is reasonably explained by their entropy difference. Comparatively, the closed form
has a lower entropy due to the increased rigidity. It was noted that the structure
of phosphine oxide plays an important role in the flexible switching between Lewis
adducts and unbound Lewis pairs. The analogous compound containing di(i-propyl) phosphine oxide behaves similarly to 1 and the analogue with dimesityl phosphine oxide always exists as the unbound Lewis
pairs of the open form. This may be due to that the B–O bond strength is greatly affected
by electronic and steric effects of the substituent on the phosphor atom.
Figure 2 (a) Chemical structure of 1 and its intramolecular thermochromic B – O switching. (b) LUMO diagrams of 1. (c) Normalized emission and excitation spectra of 1closed
in hexane (black trance) and 1open
in methanol (red trace). (d) Emission spectra of 1 in MeOH/EtOH (v/v = 1 : 4) at variable temperatures. Adapted with permission from
Ref. 8a. Copyright 2015 American Chemical Society.
Another more recent example of triarylborane with switchable B–O bond and temperature-dependent
dual fluorescence is an ortho-BMes2-substituted benzaldehyde derivative 2,[9] in which the B–O bond is formed between the B center and the O atom of C=O instead
of P=O. It was well established by NMR experiments that this compound exists as the
open form in various solvents. Upon addition of water to the THF solution of 2, the B–O bond formation is favored to form the Lewis adduct, probably due to the
possible interaction of water with the amino group, which would reduce the electron-donating
ability of the amino group to the boron center and thus enhance the electrophilicity
of the boron center to facilitate the B–O bond formation. Again, this molecule emits
distinct fluorescence in its two different forms. In THF, where 2 exists solely as the open form; a bluish green fluorescence at 490 nm (Φ = 0.61) was observed. This fluorescence is typical of a CT transition. After the
water content reaches 70%, where the closed form is dominant, the solution ultimately
gives orange-red fluorescence at a much longer wavelength (λ
em = 605 nm; Φ = 0.02). Moreover, the switching between the open and closed forms of 2 is also responsive to temperature, which was only observed below the melting points
of the solvent (m. p. = 205 K for 2-methoxyethyl ether [MOE] and 176 K for CH2Cl2). From the temperature of m. p. to 77 K, a decrease in temperature facilitates the
switching from the open form to the closed form, causing the decrease of shorter wavelength
fluorescence band and a simultaneous increase of longer wavelength fluorescence band.
This change trend of fluorescence with varying temperature is similar to 1. Above the m. p., the fluorescence is gradually blue-shifted with increased intensity
as temperature increases, which is ascribed to the CT character for the emission in
the open form. If the aldehyde group is replaced by a methyl ketone group, the corresponding
analogue exists only in its closed form due to the stronger nucleophilicity of ketone
as the result of the electron-donating ability of methyl. In contrast to the full
characterization of fluorescence at various temperatures, the influence of temperature
on the absorption was not examined for 1 and 2. However, structural analysis through the NMR technique undoubtedly proved the reversible
conversion between the open and closed forms in the S0 state for these two compounds.
In addition to the B–O bond, the switchable B–N bond has also been utilized to design
stimuli-responsive triarylboranes. Similar to the triarylboranes with switchable B–O
bond, an increase in temperature usually leads to the cleavage of B–N bond, giving
rise to the temperature dependence of fluorescence. However, examples of such triarylboranes
showing dual fluorescence are very limited because one of two species is probably
nonemissive. One example of triarylborane with switchable B–N bond and temperature-dependent
dual fluorescence is the diborylated bis(thienyl)benzothiadiazole derivative 3 ([Figure 3]).[10] Although the intramolecular B–N bond of this compound is absent in the X-ray crystal,
the mono-closed form exists to some extent in solution. According to the density functional
theory (DFT) calculations, the Gibbs free energy between the open and mono-closed
forms is small (ΔG = 1.02 kcal · mol−1), which supports the possible formation of both isomers and an easy transformation
between them. Moreover, the ratio of the mono-closed form increases with decreasing
temperature. This was supported by the temperature-dependent absorption spectra, in
which the bands at 575 and 422 nm were assigned to the absorption of the open and
closed forms, respectively. Based on the analysis of the absorbance by using the oscillator
strengths obtained from time-dependent DFT (TD-DFT) calculations, the abundance ratio
of the open and mono-closed forms in toluene was estimated to be 85 : 15 at 100 °C,
47 : 53 at 20 °C, and 9 : 91 at −40 °C. Along with facile transformation between the
open and mono-closed forms, the fluorescence is sensitive to temperature ([Figure 4]).[10] In toluene solution, the two fluorescence bands corresponding to the open and closed
forms are located at 568 nm (Φ
F = 0.12, λ
ex = 470 nm) and 718 nm (Φ
F = 0.02, λ
ex = 590 nm), respectively. Upon cooling, the orange-yellow band at ≈ 560 nm gradually
decreases and almost disappears at −40 °C. On the contrary, the near-infrared fluorescence
band at ≈ 720 nm is increased. It was noted that the two different forms of 3 have to be excited with different wavelengths due to the significant difference of
their absorption. This still provides a good example to achieve temperature-dependent
dual fluorescence by utilizing the switchable B–N bond. Clearly, utilization of the
switchable B–X (B–O/B–N) bond is an efficient strategy for the design of SOMs showing
temperature-dependent dual fluorescence. It seems the response temperature range is
highly influenced by the B–X bond strength. It is still a very tricky problem to control
the B–X bond strength and to tune the response temperature range.
Figure 3 (a) Chemical structure of 2 and its intramolecular thermochromic B – O switching. (b) Emission spectra of 2 in MOE at various temperatures. (c) Emission spectra of 2 in CH2Cl2 at various temperatures. Adapted with permission from Ref. 9. Copyright 2018 Royal
Society of Chemistry.
Figure 4 (a) Chemical structure of 3 and its intramolecular thermochromic B – N switching. (b) Thermochromism of 3 in toluene. The wavelength of the longest absorption bands and photographs of solutions
are included. Temperature-dependent fluorescence spectra of 3 in toluene when excited at (c) 370 nm and (d) 540 nm. Adapted with permission from
Ref. 10. Copyright 2017 Wiley-VCH.
Presence of Two Local Minimum Conformations in the S1 State
Presence of Two Local Minimum Conformations in the S1 State
Generally, the fluorescence arises from the initial minimum conformation in the S1 state, which is also the global minimum and is reached through structural relaxation
upon excitation from the S0 state. From the initial minimum (S1
1), if the fluorophore overcomes a certain energy barrier and can reach another more
stabilized local minimum (S1
2) through much more significant structural relaxation, the dual fluorescence from
both local minima may be observed ([Figure 1b]). In this situation, the initial minimum (S1
1) usually emits at the shorter wavelength. The increase in temperature would facilitate
the transformation from S1
1 to S1
2, leading to the increasing fluorescence intensity ratio of the longer wavelength
band. Since both fluorescence bands originate from the S1 state, the Kashaʼs rule still works. In addition, the absorption is independent of
the temperature since two emission bands arise from the S1 state.
One typical example of SOMs that show temperature-dependent dual fluorescence following
this mechanism is a class of N,N’-disubstituted-dihydrophenazine derivatives,[11] which were reported by Tian and coworkers. Among them, the N,N’-diphenyl (DPAC) and N-phenyl-N’-fluorenyl (FIPAC) dihydrophenazines are two representative compounds ([Figure 5]). In the S0 state, they adopted a nonplanar distorted structure for the dihydrophenazine framework
as a result of steric hindrance between the N,N’-disubstituents and phenanthrene ring. Upon excitation, the initial CT S1 state with saddle-shaped non-planar structure for the dihydrophenazine moiety emits
blue fluorescence due to the limited overall conjugation. In the electronically excited
S1 state, the molecule easily undergoes bent-to-planar vibration and finally reaches
a planarized S1 state, which emits red fluorescence as a result of an elongated conjugation. This
phenomenon was coined as vibration-induced emission (VIE).[12] Since the intramolecular vibration is greatly dependent on rigidity of the surrounding
environment, which is sensitive to temperature and viscosity, the emission can be
tuned by modulation of the intramolecular vibrations through changing the temperature.
Therefore, these two compounds show temperature-dependent fluorescence. For the solution
of FIPAC in n-butanol, the temperature dependence of fluorescence was mainly observed within the
temperature range of 133 – 233 K (−140 to −40 °C). At 133 K, this compound only shows
the blue emission at 425 nm, which corresponds to the deactivation of the intrinsically
saddle-shaped structure of the S1 state. As the temperature increases, this band decreases gradually with concomitant
emergence of a red fluorescence band, which becomes dominant and is maximized at 610 nm
at 233 K. Such a change in fluorescence was ascribed to the more facile skeletal motion
toward the planarization with increasing temperature and thus minimizing rigidity
of the surrounding environment. Similar temperature-dependent fluorescence was also
observed for DPAC within the temperature range of 133 – 193 K (−140 to −80 °C) and it emits blue fluorescence
and orange-red fluorescence at 133 and 193 K, respectively. For the solution of the
above two compounds, the fluorescence response to temperature is only limited to a
relatively narrow temperature range. In addition, a very low temperature is required
to trigger the fluorescence response. This may be caused by the small energy barrier
for the geometry transformation of dihydrophenazine. In contrast, the required temperature
to trigger the fluorescence response of a solid sample is too high due to a very large
energy barrier. The relatively narrow temperature scope and the too high/too low temperature
for the fluorescence response restrict their practical applications. In order to modify
the temperature sensing range to a suitable scope, Mei and coworkers designed three
fluidic dihydrophenazine dyes, DPC, DPSi-1, and DPSi-2, by attaching flexible side alkyl/siloxane chains onto the dihydrophenazine core
([Figure 6]).[13] Compared with the solid sample, the reduced intermolecular interactions of the pure
fluidic substance will lower the energy barrier of conformation transformation in
the electronically excited S1 state and thus tune the dynamic range of fluorescence temperature sensing to a suitable
range. It was well demonstrated that all these three dihydrophenazine derivatives
can serve as excellent ratiometric fluorescence thermometers with high sensitivity,
a broad temperature range (5 – 135 °C) and fairly good reversibility. For example,
DPSi-1 exhibits linear response within the temperature range of 50 – 135 °C with sensitivity
up to 3.40% per °C at 135 °C. Moreover, the fluorescence color changes from blue to
greenish-blue and near-white and finally to orange-red with increasing temperature,
which is easily detected by the naked eyes.
Figure 5 (a) Chemical structures of N,N’-disubstituted-dihydrophenazine DPAC and FIPAC. (b) Emission mechanism for the multifluorescence of FIPAC. (c) Temperature-dependent fluorescence spectra of FIPAC in n-butanol from 233 to 133 K with various viscosities. The arrows indicate the direction
of fluorescence response upon reducing the temperature. Inset: photographs under 365 nm
UV light. Adapted with permission from Ref. 11. Copyright 2015 American Chemical Society.
Figure 6 (a) Schematic illustration of the working principle of the ratiometric thermometers
constructed based on the VIE mechanism. (b) Chemical structure of AIE-active N,N’-diphenyl dihydrophenazines, DPC, DPSi-1 and DPSi-2. (c) Emission spectra of DPSi-1 noumenon recorded when the temperature was increased from 5 to 135 °C. Inset: The
fitted plot of ln(I
580/I
490) versus T, photograph of a melting-point tube filled with DPSi-1 under illumination of UV light at 365 nm when it was subjected to site-reliable thermal
treatment, and fluorescence photographs of DPSi-1 scribbled on a ceramic heater at different temperatures. Adapted with permission
from Ref. 13. Copyright 2018 Wiley-VCH.
In addition to N,N’-disubstituted-dihydrophenazine derivatives, another representative example of SOMs
showing temperature-dependent dual fluorescence as a result of the dynamic conformational
change in the excited state is the cyclooctatetraene (COT) derivative 4 ([Figure 7]),[14] which was reported by Yamaguchi and coworkers. This compound consists of a flexible
COT core unit and two emissive rigid rings of anthraceneimide. At room temperature,
the CH2Cl2 solution of 4 displays green fluorescence with the maximum wavelength located at 520 nm together
with vibronic peaks at 561 and 611 nm as well as a weak shoulder band in a higher
energy region around 460 nm. The fluorescence spectrum in 2-methyltetrahydrofuran
(MTHF) solution is very close to that in CH2Cl2. Importantly, this compound shows a dramatic emission change between a green fluorescence
and a blue fluorescence with decreasing temperature and thus with increasing viscosity
of MTHF. From 296 to 163 K, the green fluorescence of 4 is only gradually increased in intensity without any significant shifts in energy,
which results from the retarded nonradiative decay process at lower temperature. However,
the further decrease of temperature led to the decrease of intensity for the green
fluorescence covering 500 – 650 nm and a concomitant increase of the blue fluorescence
shoulder band in the shorter wavelength region from 420 to 500 nm. At 133 K, the intensities
of the two bands are comparable to each other. At 77 K, compound 4 finally shows the sole blue fluorescence at 433 nm with vibronic peaks at 460 and
491 nm. In contrast to the remarkable temperature dependence of fluorescence, no temperature
dependence was observed for the excitation spectra from 296 to 77 K, regardless of
the emission wavelengths. Theoretical calculations for the model compound 4′ demonstrated that the most stable geometry (the global minimum) in the S1 state is a planar conformation, for which the bent angle (θ) of COT is 0°, while the V-shaped structure (θ = 40.6°) is most stable in the S0 state. In addition, 4′ also has a local minimum at θ = 22.8°. Herein, the temperature-dependent dual fluorescence for 4 was well explained by the conformational transformation of the COT core unit from
the V-shaped structure to the planar structure in S1, which gave the blue and green fluorescence, respectively. At lower temperatures,
the conformational change in the S1 state is more restricted, leading to a decreased intensity of the green fluorescence
and an increased intensity of the blue fluorescence. Probably due to the very small
energy barrier for the conformational change, the temperature-dependent dual fluorescence
was only observed below 163 K. When the temperature is higher than 163 K, the temperature
change only leads to a change in the fluorescence intensity. The observation of blue
fluorescence in solution at room temperature was explained by the small energy difference
between the V-shaped and planar conformers in the S1 state, as a result of which the V-shaped conformer exists with a certain percentage
(7.69% at 296 K). Although the authors did not explore the utility of compound 4 for ratiometric temperature fluorescence sensing, it definitely provides a good example
for the temperature-dependent dual fluorescence and suggests that the combination
of an emissive rigid skeleton with a COT core is an effective strategy to achieve
this unusual property.
Figure 7 (a) Chemical structure of cyclooctatetraene derivatives. (b) Temperature-dependent
fluorescence spectra of 4 in Me-THF from 163 K (solution) to 77 K (glass). λ
ex = 350 nm. (c) Calculated potential energy diagram for the S0 and S1 states of 4′ with fixed bent angle θ. The constrained geometry optimization was performed in the S1 state at the PBE0/des-SV(P) level. Adapted with permission from Ref. 14a. Copyright
2013 American Chemical Society.
Presence of Thermal Equilibrium between S1 and S2 States
Presence of Thermal Equilibrium between S1 and S2 States
Although most fluorescent SOMs obey Kashaʼs rule and only show one fluorescence band
from the S1 state, the anti-Kasha fluorescence from the upper excited states, generally the S2 state, is possible under some circumstances. Two prototype mechanisms are usually
responsible for the anti-Kasha fluorescence from the S2 state.[21] One is that the S0→S2 transitionʼs possibility is large and the S1–S2 energy gap is large, which retards the internal conversion from S2 to S1. The other is that the S1–S2 energy gap is small, which enables the thermal population of S2 from S1. For the first case, the relative intensity of two fluorescence bands mainly depends
on the rate of radiative and non-radiative decay processes from each excited state
and thus basically is not affected by temperature. On the contrary, the high temperature
would populate the S2 state and thus increase the intensity ratio of shorter wavelength to longer wavelength
when the thermal equilibrium between S1 and S2 exists due to the small energy gap ([Figure 1c]).
The SOMs with electron donor (D) and electron acceptor (A) units connected through
π-linkers usually exhibit an intramolecular CT excited state, which is formed via
the geometric rearrangement (planarization or twisting at the bridge between donor
and acceptor units) following excitation to the locally excited (LE) state or by direct
excitation from the S1 state.[22] As the CT emission is highly sensitive to the surrounding environments, such as
the solvent polarity, viscosity and temperature of the medium, such fluorophores are
promising for the design of fluorescent sensors.[23] In addition to the CT emission, the emission from the LE state can also be detected
sometimes. The thermal population of the LE state from the CT state might enable the
ratiometric temperature sensing. One example of such fluorophores is the T-shaped
pyridoquinoxaline derivative PQCz-T ([Figure 8a]),[15] in which two electron-donating carbazole groups were introduced at 2,7-positions.
Theoretical calculations suggest that the S0→S1 excitation of PQCz-T corresponds to the transition from the HOMO mainly located on the electron-donating
carbazole units to the LUMO predominantly located on the π-electron-deficient central
framework. The obvious HOMO–LUMO charge separation implies the facile D–A rotation
and the twisted intramolecular CT (TICT) characteristics in the S1 state, which was proved by the substantial fluorescence solvatochromism from 564 nm
in toluene and 631 nm in dichloromethane. The HOMO and LUMO distributions for the
optimized S1 geometry, which are very similar to those of the optimized S0 geometry, confirmed the CT character of the S1 state. Notably, the fluorescence of PQCz-T in THF is very sensitive to temperature. As the temperature drops from room temperature
(298 K) to 263 K, the yellow fluorescence at 580 nm is gradually red-shifted to 610 nm
with an increased intensity. On the contrary, the emission is blue-shifted with the
gradual increase in temperature. Especially, a new peak around 415 nm was observed
at 338 K. The good linear relationship between the ratio of fluorescence intensities
(415 and 610 nm) and temperature illustrated the promising utilization of this compound
for the ratiometric fluorescence temperature sensing. The appearance of the blue-emitting
fluorescence at 415 nm was explained by authors through the dynamic equilibrium between
the LE and TICT states. Upon heating, the molecule with the intensified molecular
motion is able to cross the thermal barrier between TICT and LE states and the preferential
population of the LE state causes the blue shift of the fluorescence. However, the
inherent feature of the LE state was not further investigated. Judging from the intramolecular
charge transfer character of S1, the LE state of PQCz-T is probably assigned to the S2 state. A careful examination of the excitation spectra of this band and the theoretical
composition of S0→S2 excitation using TD-DFT calculations based on the optimized S0 geometry probably will provide a deeper insight into this point. The temperature-responsive
emission property is very unique to PQCz-T. The temperature influence on the fluorescence is much less significant for its regioisomer
PQCz-V, in which two carbazole groups were introduced at 3,6-positions. The HOMO of PQCz-V is delocalized over the whole molecule due to the more efficient conjugation from
the donor to the acceptor, which would probably make the molecule more rigid and rules
out the emergence of the temperature-dependent fluorescence behavior.
Figure 8 (a) Chemical structures of PQCz-T and PQCz-V. (b) The obtained ground state HOMO and LUMO distributions and the respective energy
values of PQCz-T and PQCz-V obtained via DFT calculations at the B3LYP/6 – 31 G(d,p) level. (c) Fluorescence
spectra of PQCz-T in THF from 263 to 338 K (λ
ex = 340 nm). (d) The plot of the ratio of fluorescence intensity I
415/(I
415 + I
610) of PQCz-T versus T. Adapted with permission from Ref. 15. Copyright 2018 Royal Society of Chemistry.
Through thermal population of the LE state from the TICT state, Yang and co-workers
have developed a series of fluorescence temperature sensors based on triarylboranes
([Figure 9]),[16],[24] which exhibit temperature-dependent dual fluorescence. The prototype compound is
DPTB, in which the electron-deficient boron facilitates CT transitions in the excited
state. In addition, the contributions of the two pyrene units are not equivalent to
each other in the excited state due to their different orientations around the boron
atom. Although this compound displays only one fluorescence band at room temperature,
the fluorescence decay dynamics suggested that this band is dual fluorescence. The
shorter- and longer-wavelength bands are assigned to the emissions from the LE and
TICT states, respectively. Owing to the merging of two emission bands, an increase
in temperature from −50 to 100 °C (in MOE) is observed, causing consecutive hypsochromic
shifts of fluorescence, and limiting its application as a ratiometric fluorescence
thermometer. Following this design, Qian and co-workers designed tripyrenephosphine
oxide C3 to realize the ratiometric fluorescence measurement of temperature ([Figure 9]).[17] In polar solvents, this compound can show well-separated LE and CT emissions, which
originate from the intra-pyrene π–π* transition and inter-pyrene CT. The fluorescence
spectra of C3 in MOE at various temperatures clearly demonstrate its utility as a ratiometric fluorescence
thermometer. With the increase of temperature from −50 to 100 °C, the CT emission
gradually decreases while the LE emission gradually increases. The plot of fluorescence
intensity ratio between two emission bands versus temperature can be well fitted with
a fifth-order polynomial with a correlation coefficient of 0.998. The temperature
sensitivities (dI/dT) range from 0.012 °C−1 (−50 to 30 °C) to 0.024 °C−1 (30 – 100 °C). There is no doubt that the CT state of DPTB and C3 corresponds to the S1 state. However, whether the LE state corresponds to the S2 state or another conformation of the S1 state has not been well clarified.
Figure 9 (a) Chemical structure of DPTB and the mechanism of its emission changes with temperature. (b) Corrected emission
spectra of DPTB in MOE recorded between −50 and 100 °C (λ
ex = 410 nm). (c) Chemical structure and emission spectra of C3 in MOE recorded from −50 to 100 °C (λ
ex = 360 nm). (d) Temperature dependence of the ratio of fluorescence intensity (I
380/I
480) of C3. Adapted with permission from Refs. 16 and 17. Copyright 2011 Wiley-VCH and 2017
Royal Society of Chemistry, respectively.
In 2019, our group disclosed a class of o,o’-substituted binaphthyl derivatives displaying temperature-dependent dual fluorescence
([Figure 10]),[18] which is well explained by the thermal equilibrium between S1 and S2. In the o,o’-substituted binaphthyl BNMe2-BNaph, the electron-accepting dimesitylboryl (BMes2) and the electron-donating N,N-dimethyl amino (NMe2) were introduced to each naphthyl unit. Unlike the normal donor–π–acceptor (D–π–A),
this compound essentially consists of two independent D–π–A subunits.[18a] It was found that this compound displays two well-separated fluorescence bands in
polar solvents such as THF, MOE, and MeCN. In addition, the dual fluorescence of BNMe2-BNaph is highly sensitive to temperature. In MOE, both emission bands (~478 nm and ~565 nm
as shoulder peaks) become stronger with the shorter wavelength increasing more rapidly,
which causes a significant increase of the fluorescence intensity ratio I
478/I
565 from 0.29 to 4.95 and a remarkable fluorescence color change from yellow to blue.
In addition, there is a good linear relationship between the fluorescence intensity
ratio and temperature with the best sensitivity up to 4.7%/°C observed over a wide
range of temperature from −20 to 80 °C, which indicates the great potential application
of BNMe2-BNaph as a ratiometric fluorescence thermometer. In conjunction with the experimental study
and theoretical calculations, the dual fluorescence of BNMe2-BNaph was assigned to the deactivation of S1 and S2, which have the inter-subunit CT character and the mixed intra-subunit CT and LE
characters, respectively. The very similar calculated excitation energies of S1 and S2 (ΔE = 0.07 eV) suggest a thermal equilibrium, which explains the temperature-dependent
dual fluorescence of BNMe2-BNaph. The (NPh2)-substituted analogue compound, BNPh2-BNaph, provided essentially identical calculations. However, this compound shows only one
fluorescence band in various solvents and the dual emission was observed only in the
circularly polarized luminescence spectra, which may be explained by the close energies
of the S1→S0 and S2→S0 deactivations and thus the merging of two emission bands. Moreover, the other o,o’-substituted binaphthyls, which also consist of two D–π–A subunits with BMes2 replaced by CHO (CHONMe2-BNaph) and CN (CNNMe2-BNaph), respectively, also show temperature-dependent dual fluorescence with fluorescence
changes in a similar manner to BNMe2-BNaph,[18b] indicating the general utility of this molecular design for temperature-dependent
dual fluorescence properties.
Figure 10 (a) Schematic presentation and structures of chemical structure of o,o’-substituted binaphthyls. (b) Fluorescence spectra and (c) fluorescence intensity
ratios of the bands at 478 and 565 nm of BNMe2-BNaph at various temperatures. (d) The Kohn–Sham energy levels, pictorial drawing of frontier
orbitals, and transitions of BNMe2-BNaph in the ground and excited states, calculated at TD/PBE0/6 – 31 G(d) levels.
Conclusions and Outlook
In summary, we have briefly summarized the examples of SOMs that show temperature-dependent
dual fluorescence, which is a very unusual property but promising for the ratiometric
fluorescence sensing of temperature. The examples provided here were included according
to the inherent mechanism for the temperature-dependent dual fluorescence behavior,
which may take place because of the presence of two local minimum conformations that
are thermally equilibrated in the S0 state, the presence of two local minimum conformations in the S1 state as a result of significant structural relaxation upon excitation, and the presence
of thermal equilibrium between S1 and S2 states. Ideally, the two fluorescence bands from a single SOM are intensive and well
separated. As the temperature changes, one emission becomes stronger while the other
becomes weaker, leading to a remarkable change in the fluorescence intensity ratio
between the two emission bands. In addition, the temperature response of dual fluorescence
occurs in the appropriate temperature range according to various environments. Despite
some reported examples of SOMs showing temperature-dependent dual fluorescence, it
is still very hard for the rational molecular design to achieve the above performances.
Hopefully, the discussion of the present examples, especially the inherent mechanism
behind the experimental phenomena, will provide important basis for the design of
new fluorophores that display temperature-dependent dual fluorescence with improved
performances and their practical applications.
Funding Information
This work was supported by the National Natural Science Foundation of China (21 971 150).
