Synthesis and thermal modulation of the fluorescence of a pyrene-arylmaleimide dyad and its Diels-Alder adduct

Article abstract of DOI:10.1016/j.tetlet.2005.04.135

The fluorescence of a pyrene-arylmaleimide dyad and its Diels-Alder adduct can be thermally modulated through reversible Diels-Alder reaction of arylmaleimide unit with furan. Moreover, the fluorescence intensities of the Diels-Alder adduct of dyad 1 with furan vary linearly with the temperatures of the corresponding solutions, demonstrating that these molecules are interesting compounds for studies of fluorescent molecular thermometers.

Full text of DOI:10.1016/j.tetlet.2005.04.135

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 4609–4612
Synthesis and thermal modulation of the fluorescence of
a pyrene-arylmaleimide dyad and its Diels–Alder adduct
Zhuo Wang,a,b Deqing Zhanga,* and Daoben Zhua,*
aThe Key Laboratory of Organic Solids, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, China
bGraduate School, Chinese Academy of Sciences, Beijing 100080, China
Received 25 March 2005; revised 29 April 2005; accepted 30 April 2005
Available online 23May 2005
Abstract—The fluorescence of a pyrene-arylmaleimide dyad and its Diels–Alder adduct can be thermally modulated through rever-
sible Diels–Alder reaction of arylmaleimide unit with furan. Moreover, the fluorescence intensities of the Diels–Alder adduct of
dyad 1 with furan vary linearly with the temperatures of the corresponding solutions, demonstrating that these molecules are inter-
esting compounds for studies of fluorescent molecular thermometers.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Temperature is an important property of environment,
thus monitoring temperature is essential in many fields.
However, in some cases, like in micro-sized domains
or harsh environments, a conventional thermometer is
impractical.1 In this respect, the development of fluores-
cent molecular thermometers is highly desirable2a,b and
has the edge over physical thermal probes of tempera-
ture for fluid systems and systems under electromagnetic
irradiation in that flow patterns and field lines are not
disturbed by the presence of these molecular probes.2c–f
Several fluorescent molecular thermometers have been
described. They are based on exciplexe,3 excimers,4
cyclodextrin complexes of phosphors and intramolecu-
lar charge-transfer fluorophores,5 twisted TICT fluoro-
phores,1a polymers showing temperature-induced phase
transition,6 spin crossover complexes,7 vibrational
energy changes in the excited state,8 quasi-reversible
photodissociation,9 and lanthanide complexes.10 Herein,
we report the synthesis and thermal modulation of the
pyrene-1,4-disubstituted arylmaleimide dyad 1 (abbrev.
as arylmaleimide, Scheme 1) and its Diels–Alder (D–
A) adduct 2. The results indicate that these molecules
are interesting compounds for studies of fluorescent
molecular thermometers.
The design rationale is based on the tuning of the
photoinduced electron transfer (PET) process within
dyad 1 through the reversible D–A reaction between
arylmaleimide unit and furan. In general, maleimide
is both a good electron acceptor and a reactive dieno-
phile. For instance, maleimide can react with furan
leading to the corresponding D–A adduct, and at ele-
vated temperature the retro D–A reaction occurs
(Scheme 2).11
O
O
CH O(CH ) O
2 5
N
O
2
CH O(CH ) O
N
O
2
2 5
O
+
O
2
dyad 1
Scheme 1.
*
Corresponding author. Tel.: +86 10 6263 9355; fax: +86 10 6255 9373; e-mail: dqzhang@infoc3.icas.ac.cn
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.04.135

4610
Z. Wang et al. / Tetrahedron Letters 46 (2005) 4609–4612
K2CO3 led to the mixture of endo and exo isomers of
O
O
O
N
O
R1
R1
R1
R1
2.15 The separation of exo and endo 2 was achieved with
column chromatography. Structural assignment of the
endo and exo isomers was made based on the 1H
NMR spectra (see Supplementary data) according to
the previous results:16 the one with the chemical shift
of a-H at 3.67 ppm was endo 2 and that with the chemi-
cal shift of a-H at 2.98 ppm was exo 2. The retro D–A
reaction of the endo and exo isomers of 2 led to dyad
1.17 The retro D–A reaction of the endo isomer occurred
when the solution was heated to 60 ꢁC in benzene. The
endo 2 was completely transferred to dyad 1 when the
solution was kept at 60 ꢁC for 3h. But, the correspond-
ing retro D–A reaction of exo 2 could not take place
until the solution was heated to 100 ꢁC in toluene.
R1
R1
R
+
O
N
+
O
N
R
R
O
O
O
furan
Scheme 2.
maleimide
exo
endo
Compared with arylmaleimide itself, its D–A adduct
with furan shows weak electron accepting ability.12
Thus, it is possible to tune the electron accepting ability
of arylmaleimide through the reversible D–A reaction
with furan. Accordingly, in a dyad formed by linking
arylmaleimide to a fluorescent donor unit such as pyr-
ene, it would be possible to tune the intramolecular
PET process within this dyad, which in turn modulates
the fluorescence of pyrene unit. On one hand, it can be
expected that dyad 1 is weakly fluorescent owing to
the intramolecular PET process, and the fluorescence
of its D–A adduct 2 with furan (Scheme 1) would be en-
hanced, due to the fact that the electron acceptor unit in
the D–A adduct 2 becomes weaker and thus the corre-
sponding PET process would be prohibited. The D–A
adduct 2 can be converted to dyad 1 by heating. There-
fore, a thermal induced fluorescence switch can be real-
ized based on dyad 1 through reversible D–A reaction
with furan. It is worthwhile to mention that Li and co-
workers13 have reported very recently the reversible
fluorescence and aggregation behavior of multi-malei-
mides bearing electron-donating chromophores. On
the other hand, the reaction rates of both the D–A reac-
tion of arylmaleimide with furan and the corresponding
retro D–A reaction should be dependent on the temper-
ature of the solution. As a result, the fluorescence inten-
sity of the solution would become dependent on the
temperature of the solution. Thus, dyad 1 and its D–A
adduct 2 might be interesting compounds for studies
of fluorescent molecular thermometers.
As expected, dyad 1 showed rather weak fluorescence
(Fig. 1) with low quantum yield18 at 20 ꢁC (UF = 0.022
in DMF) due to the intramolecular PET reaction be-
tween the excited pyrene unit and arylmaleimide unit.
On the contrary, both endo and exo 2 were strongly fluore-
scent. The quantum yields of endo and exo 2 were deter-
mined to be 0.148 and 0.159, respectively, at 20 ꢁC in
DMF. The strong fluorescence of endo and exo 2 was
due to the fact that the D–A adduct of arylmaleimide
and furan showed weak electron accepting ability com-
pared with arylmaleimide and as a result, the PET pro-
cess would be prohibited. These results indicated that
the fluorescence of dyad 1 can be reversibly modulated
in the presence of furan through the reversible D–A
reaction, and accordingly a thermally induced fluores-
cence switch can be established based on dyad 1.
The fluorescence spectra of dyad 1 in the presence of fur-
an or furfuryl alcohol were measured in the temperature
range of 25–60 ꢁC. The fluorescence intensity of the
solution was enhanced by increasing the temperature
(see Supplementary data). Such fluorescence enhance-
ment was obviously due to the transformation of dyad
1
1 to 2. But, on the basis of H NMR data (see Supple-
mentary), the molar ratio of dyad 1 that was trans-
formed to endo and exo 2 was rather small.
The synthesis of the D–A adduct 2 is shown in Scheme
3. Reaction of 3, which was prepared by the reaction of
1-pyrenemethanol pyrene and 1,5-dibromopentane with the
usual procedure, and 414 in the presence of anhydrous
The fluorescence spectra of endo 2 were measured at
different temperatures in DMF (Fig. 2A). Before each
measurement the solution of endo 2 was kept for
O
N
CH O(CH ) Br
2
2 5
HO
O
+
6000
O
O
dyad 1
5000
α
endo 2
4
3
O
α
4000
3000
2000
1000
0
CH2O(CH2)5O
N
O
K CO
2
3
+
DMF
O
endo 2
CH2O(CH2)5O
O
α
N
O
α
350
400
450
500
Wavelength (nm)
exo 2
Figure 1. Fluorescence spectra (kex = 340 nm) of dyad 1 (1.0 · 10À5 M)
and endo 2 (1.0 · 10À5 M) in DMF.
Scheme 3.

Z. Wang et al. / Tetrahedron Letters 46 (2005) 4609–4612
4611
6000
4000
2000
of dyad 1 formed in the solution increased linearly with
the temperature. As mentioned above, endo 2 showed
strong fluorescence while dyad 1 displayed weak fluores-
cence. Thus, it was understandable that the fluorescence
intensity of the solution of endo 2 decreased linearly with
6000
5000
4000
3000
2000
1000
0
o
60 C
60
1.0
70
80
90
1
o
the temperature. The results of H NMR and fluores-
cence spectra were consistent.
Temperature (ºC)
90 C
0.8
0.6
0.4
Similarly, the fluorescence spectra of exo 2 were also mea-
sured after the solution was heated. As mentioned above,
the retro D–A reaction of the exo isomer of 2 occurred at
elevated temperature. As shown in Figure 2B, the fluores-
cence intensity of the exo isomer of 2 did not decrease
until the temperature reached 100 ꢁC. Further fluores-
cence reduction was observed by increasing the tempera-
ture of the solution, but the fluorescence intensity reached
the minimum after the solution was heated at 130 ꢁC for
10 min. As for the endo isomer of 2, there was a good lin-
ear plot of the fluorescence intensity at 394 or 376 nm ver-
sus the temperature of the solution as shown in the inset of
Figure 2B. Similarly, the relative amount of exo 2 (in
molar) that was converted into dyad 1 increased linearly
with the temperature (see inset of Fig. 2B).20
70 75 80 85 90
Temperature (ºC)
350
400
450
500
Wavelength (nm)
A
6000
4000
2000
7000
6000
5000
4000
3000
2000
1000
0
100 ºC
130 ºC
100
110
120
130
Temperature (ºC)
0.8
0.6
0.4
As control experiments, the fluorescence spectra of 1-
pyrenemethanol were measured in the mixed solvent of
THF/furan and DMF under the same conditions em-
ployed for dyad 1 and the endo and exo isomers of 2.
The results indicated that variation of the fluorescence
spectra of 1-pyrenemethanol under these conditions
was rather minor (see Supplementary data). Thus, it
could be ruled out that the change of the fluorescence
intensity of dyad 1 and endo and exo 2 was due to the
physical thermal effect.
110 115 120 125 130
Temperature (ºC)
350
400
450
500
Wavelength (nm)
B
Figure 2. (A) Fluorescence spectra (kex = 340 nm) of the DMF
solution of endo 2 (1.0 · 10À5 M) recorded over the temperature range
60–90 ꢁC; Inset shows the IF at 394 nm versus temperature profile, and
the profile for the relative amount (in molar, x) of endo 2 that was
transformed to dyad 1 versus the temperature; (B) fluorescence spectra
(kex = 340 nm) of the DMF solution of exo 2 (1.0 · 10À5 M) recorded
over the temperature range 100–130 ꢁC; Inset shows the IF at 394 nm
versus temperature profile, and the profile for the relative amount (in
molar, x) of exo 2 that was transformed to dyad 1 versus the
temperature.
In summary, the pyrene-arylmaleimide dyad 1 and its
D–A adduct 2 were synthesized and characterized. Dyad
1 was weakly fluorescent, while its D–A adduct, endo
and exo 2 showed strong fluorescence. This was because
arylmaleimide and its D–A adduct showed different elec-
tron accepting abilities, and as a result the PET pro-
cesses within dyad 1 and endo and exo isomers of 2
could be tuned through such D–A reaction. Accord-
ingly, a thermally induced fluorescence switch can be
established based on dyad 1. The fluorescence spectra
of dyad 1 containing furan (and furfuryl alcohol) and
those of endo and exo 2 were measured after being
heated at different temperatures. For the solutions of
endo and exo 2, the fluorescence intensities show good
linear relations with the temperatures of the correspond-
ing solutions in the ranges of 60–90 and 100–130 ꢁC,
respectively. These unique properties indicate that endo
and exo 2 are interesting compounds for studies of fluo-
rescent molecular thermometers. Further investigations
including structural optimization (e.g., spacer and chro-
mophore unit) and dynamics of the corresponding D–A
reaction are underway.
10 min at the temperature.19 As shown in Figure 2A, the
fluorescence intensity of the solution started to decrease
when it was heated to 60 ꢁC, and further decrease was
observed at temperature higher than 60 ꢁC. Such fluores-
cence reduction was due to the transformation of the
endo isomer of 2 into dyad 1 through the retro D–A reac-
tion. But, the decrease of fluorescence intensity stopped
when the temperature reached 90 ꢁC. This was probably
due to the fact that the major fraction (ꢀ90%) of endo 2
in the solution was transformed to dyad 1 after the solu-
tion was heated at 90 ꢁC for 10 min. The profile of the
fluorescence intensity of the solution, measured either
at 394 or 376 nm, versus temperature gave good linear
relationship (see the inset of Fig. 2A).
1H NMR spectra of the solution of endo 2 were recorded
after it was heated at 70, 80, and 90 ꢁC, respectively, for
10 min (see Supplementary data). Based on these 1H
NMR data, the relative amount (in molar) of endo 2 that
was transformed to dyad 1 was estimated at each tem-
perature,20 and it increased linearly with the tempera-
Acknowledgments
1
ture as illustrated in the inset of Figure 2A. The H
NMR data also indicated that the amount (in molar)
The present research was financially supported by
NSFC, Chinese Academy of Sciences and State Key

4612
Z. Wang et al. / Tetrahedron Letters 46 (2005) 4609–4612
9. de Silva, A. P.; Gunaratne, H. Q. N.; Jayasekera, K. R.;
OÕCallaghan, S.; Sandanayake, K. R. A. S. Chem. Lett.
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Basic Research Program (G2000077505). D.-Q.Z.
thanks National Science Fund for Distinguished Young
Scholars.
Supplementary data
11. Reversible D–A reactions have been utilized for the studies
of macromolecular architectures, thermally cleavable/reas-
sembling dendrons and dendrimers, and NLO functional
materials: (a) Chen, X.; Dam, M.; Ono, K.; Mal, A.; Shen,
H.; Nutt, S. R.; Sheran, K.; Wudl, F. Science 2002, 295,
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12. Reduction potential of 4-methoxyarylmaleimide (5, see
Supplementary data) was measured with the differential
pulse voltammetry to be À1.03V in DMF (potential are
reported by reference to SCE), no reduction wave was not
detected for the D–A adduct of 5 with furan (compound 6,
see Supporting data) under identical conditions.
Synthesis and characterization of dyad 1, the endo and
exo 2 and the reference compounds 5 and 6; the fluores-
cence spectra of dyad 1 in DMF/furfuryl alcohol; the
profiles for the relative fluorescence intensity versus
the reaction time for endo and exo 2; the plots of relative
intensity (I/I0) versus temperature for the solutions of
dyad 1, endo and exo 2 and 1-pyrenemethanol in differ-
ent temperature range; 1H NMR spectra of dyad 1, endo
and exo 2 after being heated; the differential pulse vol-
tammogram of compound 5 in DMF. Supplementary
data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2005.04.135.
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2 in DMF was prepared and carefully sealed. The solution
was kept in a constant temperature trough for 10 min (the
temperature was controlled with a thermocouple). After
that, the fluorescence spectrum of endo 2 was measured
using a Hitachi-4500 spectrometer in a quartz cell. The
fluorescence spectra of exo 2 were measured similarly.
20. The yields of dyad 1 after heating endo 2 at 70, 80, and
90 ꢁC for 10 min were estimated to be 0.31, 0.58, and 0.90,
respectively, based on the 1H NMR data; the yields of
dyad 1 after heating exo 2 at 110, 120, and 130 ꢁC for
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1
on H NMR data.