Controlling fluorescence resonance energy transfer of donor-acceptor dyes by Diels-Alder dynamic covalent bonds

Article abstract of DOI:10.1039/d1cc00165e

Two new dyes consisting of an aromatic amine donor and dansyl acceptor were synthesized, where the intramolecular donor and acceptor are connected by Diels-Alder dynamic covalent bonds. These new dyes display a switchable fluorescence resonance energy transfer through reversible formation and cleavage of Diels-Alder bonds. Single crystal X-ray diffraction revealed that Diels-Alder bonds are longer and weaker than normal single bonds. Dynamic covalent properties enable the mutual conversion of the two dyes by maleimide exchanges, where a new higher energy transfer efficiency system can be constructed.

Full text of DOI:10.1039/d1cc00165e

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Controlling fluorescence resonance energy
transfer of donor–acceptor dyes by Diels–Alder
dynamic covalent bonds†
Cite this: Chem. Commun., 2021,
7, 3275
5
Received 10th January 2021,
Accepted 23rd February 2021
Yanhui Cui, Fen Li and Xin Zhang
*
DOI: 10.1039/d1cc00165e
rsc.li/chemcomm
9
,10
Two new dyes consisting of an aromatic amine donor and dansyl Diels–Alder reactions have been reported,
the relationship
acceptor were synthesized, where the intramolecular donor and between dynamic covalent bonds and dye optical behaviours is
1
0d
acceptor are connected by Diels–Alder dynamic covalent bonds. rarely studied.
The use of dynamic covalent bonds to mod-
These new dyes display a switchable fluorescence resonance ulate energy transfer has never been reported before. In this
energy transfer through reversible formation and cleavage of work, we designed and synthesized two new dyes (Fig. 1), where
Diels–Alder bonds. Single crystal X-ray diffraction revealed that an energy donor and acceptor are connected by dynamic
Diels–Alder bonds are longer and weaker than normal single bonds. covalent bonds within a molecule. Their optical behaviours were
Dynamic covalent properties enable the mutual conversion of the investigated. In particular, we demonstrate the modulation of FRET
two dyes by maleimide exchanges, where a new higher energy by reversible, dynamic Diels–Alder covalent bonds.
transfer efficiency system can be constructed.
Dyes 1 and 2 were synthesized by a four-step reaction
procedure (Fig. 2a). We first synthesized a 1,4-substiuted
Fluorescence resonance energy transfer (FRET) has been furan-derivative; however, it cannot perform the Diels–Alder
recently extensively studied because of widespread applications reaction with maleimides due to the steric hindrance (Fig. 2b).
1
2
such as in biosensors, photoluminescent nanomaterials,
Thus, we designed and synthesized 1,3-substiuted furan deri-
3
4
fluorescence probes, chemosensor, and white-light-emitting vatives from dansyl chloride through a Sonogashira reaction by
optical devices and materials. The energy transfer efficiency is using and CuI as catalysts. Finally, dyes 1 and 2 were obtained
5
inversely proportional to the sixth power of the donor-to- by an efficient and clean Diels–Alder reaction of 3 with the
acceptor distance; thus, energy transfer is highly sensitive to corresponding maleimides at room temperature. For Diels–
any small changes occurring in donor-to-acceptor distance. The Alder adduct 1 or 2, two isomers were formed firstly, i.e., exo
extreme distance dependence feature of energy transfer has and endo (Fig. S5, ESI†). The endo product is kinetically
been generally used as a ‘‘spectroscopic ruler’’ in the fields of favoured and unstable, and the exo product is thermodynami-
6
10b,14
photonics, biology and chemistry. Recently, Ghosh et al. cally stable.
reported a FRET method to study the dynamic behaviours of endo product decreased, and the exo product increased. In this
As the Diels–Alder reaction proceeded, the
1a,7
amphiphilic polymer aggregates and supramolecular polymers.
The most interesting characteristic of dynamic covalent
chemistry is the reversible formation and cleavage of covalent
bonds. Therefore, the dynamic feature allows the exchanges of
a variety of different moieties in thermodynamic equilibrium
8
systems. Diels–Alder reactions are most interesting dynamic
covalent reactions, which can occur at mild room temperature,
and retro-Diels–Alder reaction occurs at an elevated tempera-
ture without adding any catalyst. Although many interesting
School of Chemical Engineering and Technology, Collaborative Innovation Center of
Chemistry Science and Engineering, Tianjin University, Tianjin, 300072, China.
E-mail: xzhangchem@tju.edu.cn
†
Electronic supplementary information (ESI) available. CCDC 2063061. For ESI
Fig. 1 Chemical structures of donor–acceptors 1 and 2, dansyl-based
energy acceptor 3, and carbazole and triphenylamine-based energy
donors 4 and 5.
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1cc00165e
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Fig. 2 (a) Synthetic route towards donor–acceptors 1 and 2. Reagents
and conditions: (i) CH
2 h; (iii) C OH, CH
b) No reaction occurring due to steric hindrance.
2
Cl
2
, r.t., 5 min; (ii) Et
3
N, THF, Pd(PPh
3
)
4
, CuI, 70 1C,
, r.t., 70 h.
1
2
H
5
3
COOH, Pd/C, NaBH
4
, r.t., 48 h; (iv) CHCl
3
(
Fig. 4 (a) Spectral overlap of acceptor absorption and donor emission:
work, both 1 and 2 are pure exo isomers according to NMR UV-vis absorption spectrum of dansyl acceptor 3, and fluorescence
ꢀ
5
emission spectra of donors 4 and 5. [3] = [4] = [5] = 5.0 ꢁ 10 M,
spectroscopy (Fig. S3 and S8, ESI†).
l
ex = 295 nm and 310 nm for 4 and 5, respectively. Spectral overlaps are
shown by crosslines. Excitation-dependent 3D fluorescence spectra of
Donors 4 and 5 were synthesized according to our previous
1
1b
procedure.
The absorption spectrum of 1 (or 2) is equal to
ꢀ5
D–A 1 (b) and 2 (c) in dichloromethane. c = 5.0 ꢁ 10 M. Insets in (b) and
the absorption spectral sum of acceptor 3 and donor 4 (or 5)
(
c): CIE 1931 chromaticity diagrams. The points indicated by circles are the
(Fig. 3a and b). This suggests that the covalent bond connection fluorescence colour coordinates (0.24, 0.47) for 1, and (0.23, 0.44) for 2.
between the donor and acceptor has no influence on their
ground state absorption. Acceptor 3 shows characteristic green
emission from the dansyl chromophore at 508 nm (Fig. 3c
and d). Donors 4 and 5 show the characteristic deep blue
emission from carbazole and triphenylamine chromophores
at 346 nm and 371 nm, respectively (Fig. 3c and d). Donor–
acceptor dyes 1 and 2 only show green emission at 508 nm from
the dansyl acceptor with the fluorescence quantum yields of
carbazole or triphenylamine donors almost disappear in 1
and 2 (Fig. 3c and d). These results point to a prominent FRET
occurring from the aromatic amine donor to the dansyl accep-
tor in 1 and 2.
To confirm the energy transfer mechanism, excitation-
dependent 3D fluorescence spectroscopy of 1 and 2 was mea-
sured (Fig. 4b and c). When the carbazole donor of 1 was
excited at 293 nm, we observed the dansyl acceptor emission of
1
1a
0.627 and 0.264, respectively. The fluorescence colour coordi-
nates are (0.24, 0.47) and (0.23, 0.44) for 1 and 2, which falls
into the visible green light region in the CIE 1931 chromaticity
diagram (Fig. 4b and c). The characteristic emissions of
1
at 508 nm. In other words, when the carbazole donor and the
dansyl acceptor of 1 were selectively excited at 293 nm and
50 nm, respectively, we observed the same green emission
3
from the dansyl acceptor at 508 nm (Fig. 4b). The same
phenomenon was also observed for 2 (Fig. 4c). We further
measured the time-resolved fluorescence spectroscopy of 1
and 2. When the donor of 1 or 2 was excited at 345 nm or
370 nm, dansyl acceptor fluorescence decay at 508 nm shows a
rising time of 0.5–0.6 ns, which is approximately equal to the
rapid decay of the donor of 1 or 2 (0.3–0.5 ns) (Fig. S20, ESI†).
The time-resolved results directly reveal energy transfer pro-
cesses in dyes 1 and 2.
The energy transfer of these dyes was further quantified by
1
2
using eqn (1)–(3).
Ð
1
4
FDðlÞeAðlÞl dl
0
JðlÞ ¼
Ð
(1)
1
FDðlÞdl
0
1
ꢀ
ꢁ
R0 ¼ 0:211 k Zꢀ4FDJðlÞ 6
2
(2)
(3)
R06
Fig. 3 (a and b) UV-vis absorption spectra of donor–acceptors 1 and 2,
E ¼
6
6
R0 þ rDA
ꢀ5
acceptor 3, and donors 4 and 5 in dichloromethane, c = 5.0 ꢁ 10 M.
c) Fluorescence spectra of 1, 3, and 4 in dichloromethane, lex = 295 nm,
(
where FD is the fluorescence quantum yield of the donor. The
donor-to-acceptor distances (rDA) are 17.3 Å and 18.8 Å for 1 and
2, respectively (semiempirical AM1 calculations).
ꢀ5
c = 5.0 ꢁ 10 M. Inset in (c): Photograph of dichloromethane solutions of
ꢀ
5
1
and 4 under 365 nm UV lamp, [1] = 5.0 ꢁ 10 M. (d) Fluorescence
5
spectra of 2, 3, and 5 in dichloromethane, lex = 310 nm, c = 5.0 ꢁ 10ꢀ M.
Inset in (d): Photograph of dichloromethane solutions of 2 and 5 under
According to eqn (1), the overlapping integrals (J(l))
65 nm UV lamp, [2] = 5.0 ꢁ 10ꢀ5 M.
14
ꢀ1
ꢀ1
ꢀ4
3
were calculated to be 1.21 ꢁ 10 cm
M
nm
and
3
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1
3
ꢀ1
ꢀ1
ꢀ4
9
.64 ꢁ 10 cm
M
nm for 1 and 2, respectively, from the
spectral overlap of donor emission and dansyl acceptor absorp-
tion (Fig. 4a). According to eqn (2), the F o¨ rster critical distances
(R0) at which the 50% donor energy transfers to the acceptor
were then calculated to be 28.2 Å and 17.4 Å for 1 and 2,
respectively. According to eqn (3), we finally obtained a high
energy transfer efficiency (E) of up to 94.9% for 1, and 38.6%
for 2. The energy transfer rates (k
T
) of 1 and 2 were calculated to
ꢀ
1
ꢀ1
be 2.37 ns and 0.90 ns respectively, according to equation
6
12
T 0 D D
k = (R /rDA) (1/t ), where t is fluorescence lifetime of the
donor. Owing to intramolecular energy transfer, the Stokes
shifts of 1 and 2 are very large (4180 nm, Table S2, ESI†),
which enables them to become excellent candidates for fluor-
escent probes with high microenvironment sensitivity proper-
ties (Fig. S22, ESI†).
The Diels–Alder reaction between furan and maleimide
groups is dynamically reversible. To obtain precision bond
information on Diels–Alder dynamic covalent bonds, we
synthesized model compound Diels–Alder adduct M (Fig. 5a),
and obtained high quality single crystals. Its single crystal X-ray
diffraction structure shows that the Diels–Alder bond length is
1
3
1
.57 Å (Fig. 5b), which is longer than that of the normal single
bond (1.54 Å). Thus, the Diels–Alder bond strength is much
lower than that of a normal covalent single bond, which leads
to the easy occurrence of retro Diels–Alder reaction at an
Fig. 6 (a) Cleavage of dynamic covalent bonds (shown in red) through the
1
4
1
increased temperature. For D–A 1, the retro Diels–Alder addi- retro Diels–Alder reaction of 1 in CD
3
CN at 75 1C. (b) H NMR spectral
changes before and after the retro Diels–Alder reaction from 0 h (top) to
tion (Fig. 6a) can occur at 75 1C in CD CN according to NMR
measurements. NMR peaks of protons of 1 at d 6.07–6.03 ppm
3
1
2 h (bottom). (c) Fluorescence spectral changes before and after the retro
ꢀ5
Diels–Alder reaction from 0 h to 12 h. lex = 294 nm, [1] = 5.0 ꢁ 10 M in
CH Cl . (d) Reformation of dynamic covalent bonds (shown in red)
through the Diels–Alder reaction of 3 and carbazole-based maleimide in
(Hl), d 5.07–5.02 ppm (Hm), d 4.73–4.65 ppm (Hh), d 4.16–4.09
2
2
ppm (Hn), and d 3.16–3.05 ppm (Hp and Hq), gradually
1
decrease after the retro Diels–Alder addition occurs (Fig. 6b). CD
3
CN at room temperature. (e) H NMR spectral changes before
0
0
and after the Diels–Alder reaction from 0 h (top) to 72 h (bottom).
(f) Fluorescence spectral changes before and after the Diels–Alder reac-
tion from 0 h to 72 h. lex = 294 nm.
NMR peaks at d 6.96–6.93 ppm (Hp and Hq ), d 6.93–6.90 ppm
0
0
0
(
Hl ), d 5.96–5.93 ppm (Hm ), d 4.60–4.54 ppm (Hh ), and d
.53–4.47 ppm (Hn ) appear, which are assigned to protons of
0
4
acceptor 3 and carbazole-based maleimide. The newly forming
carbazole maleimide is not fluorescent due to the quenching bonds, which leads to the separation of the energy donor and
10a,b,11
effect of a maleimide group.
Fluorescence intensity drama- acceptor, and prevents energy transfer from the donor to the
tically decreases after the retro Diels–Alder addition occurring acceptor. When the donor was photoexcited, almost no acceptor
(
Fig. 6c). This is due to the cleavage of the dynamic Diels–Alder emission can be observed (Fig. 6c).
After the above reaction system was cooled to room tempera-
ture, the fluorescence of the system was resorted (Fig. 6f). NMR
spectroscopic measurements reveal that Diels–Alder addition
occurs (Fig. 6d), which indicates that NMR peaks of protons of
1
appear again (Fig. 6e). Thus, the dynamic Diels–Alder bonds of
D–A 1 are reformed, which leads to the reconstruction of the
energy transfer system of 1. Thus, the intensity of fluorescence
emission increases significantly (Fig. 6f).
We further explored the dynamic nature of the Diels–Alder
bond of 1 and 2. In our previous work, we reported the
1
0b
exchange of furan groups in a dynamic reaction system.
Here, we investigated the exchange of maleimide groups in
the dynamic Diels–Alder addition process. An excess of
Fig. 5 (a) Chemical structures of model compound M. Dynamic covalent
bonds are indicated in red. (b) Molecular structure (ORTEP) of model
compound M determined by single-crystal X-ray diffraction, top view (left)
and side view (right). Thermal ellipsoids are drawn at a 50% probability
level. All hydrogen atoms are omitted for clarity.
carbazole-based maleimide was added into D–A 2 in CD CN
3
1
and the mixture was heated at 75 1C. We used H NMR
spectroscopy to monitor the dynamic exchange progress
(Fig. 7 and Fig. S23. ESI†). The NMR peaks of the protons of
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3
CD CN at 75 1C, where 2 is converted into 1, and a new higher fluores-
cence resonance energy transfer (FRET) efficiency (E = 94.9%) system is
1
constructed. H NMR spectral changes during the conversion of 2 into 1 in
9
(a) D. Loco, R. Spezia, F. Cartier, I. Chataigner and J. P. Piquemal,
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the dynamic Diels–Alder addition system. [2] = 0.025 M, [carbazole-based
maleimide] = 0.075 M. Reaction time = 0, 3, 6, 9, 12, 24, 36, 48, 72 h from
top to bottom.
1
0
0
2
at
d
5.98–6.02 ppm (Hl ),
d
4.95–4.98 ppm (Hm ),
Chem. – Eur. J., 2020, 26, 11503–11510; (d) S. L. Xiang, Q. X. Hua,
W. L. Gong, N. H. Xie, P. J. Zhao, G. J. Cheng, C. Li and M. Q. Zhu,
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0
0
0
d 4.02–4.09 ppm (Hn ), and d 2.94–3.06 ppm (Hp and Hq )
gradually decrease. New NMR peaks appear and gradually
increase at d 6.04–6.07 ppm (Hl), d 5.03–5.06 ppm (Hm),
d 4.09–4.16 ppm (Hn), and d 3.06–3.16 ppm (Hp and Hq),
which are assigned to the protons of 1 (Fig. 7). These NMR
1
1
1 (a) X. H. Li, Y. Wang, F. Li and X. Zhang, Dyes Pigm., 2020,
182, 108474; (b) Y. Wang, Q. Zhang, J. B. Gong and X. Zhang, Dyes
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2 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, New
York, 2006. F
the wavelength range l to l + Dl; e
the acceptor; k is the orientation factor, and usually takes the value
D
(l) is area under fluorescence curve of the donor in
experiment results reveal that
2 is converted into 1,
A
is the extinction coefficient of
where triphenylamine- and carbazole-based maleimides are
exchanged. In the dynamic covalent system, of particular
interest is the construction of a new energy transfer system
involving conversion of 2 into 1, where the energy transfer
efficiency increases from 38.9% to 94.9%.
2
of 2/3; Z is the refractive index of the medium.
1
3 Crystallographic data for the model compound: 0.20 ꢁ 0.18 ꢁ
3
0
.12 mm , triclinic, a = 5.3407(11), b = 12.745(3) Å, c = 8.872(4) Å,
3
a = 90.001, b = 92.51(3)1, g = 90.001 and V = 1283.3(4) Å , space group
21, Z = 2, rcalcd = 1.135 g cm , l(MoKa) = 0.71073 Å, T = 113 K,
ꢀ3
P
13 184 reflections, 3160 unique (3160 observed, R(int) = 0.0374),
In summary, we synthesized two new donor–acceptor dyes,
which display efficient intramolecular FRET from carbazole or
triphenylamine donors to dansyl acceptors up to 94.9%. Their
energy transfer can be modulated by reversible formation and
cleavage of dynamic covalent bonds. The two dyes can be
converted each other by maleimide exchanges in the dynamic
covalent system. We demonstrate an unprecedented method to
control energy transfer by using dynamic covalent bonds. The
National Natural Science Foundation of China (grant number
1 2
R = 0.0709, wR = 0.1968, for 276 parameters and 1 restraints. The
single-crystal X-ray diffraction data of the model compound was
deposited in the Cambridge Crystallographic Data Centre (CCDC
2063061†)
21975177 and 21674079) is acknowledged for financial support.
.
Conflicts of interest
1
4 (a) V. Froidevaux, M. Borne, E. Laborbe, R. Auvergne, A. Gandini and
B. Boutevin, RSC Adv., 2015, 5, 37742–37754; (b) C. J. Kloxin and
C. N. Bowman, Chem. Soc. Rev., 2013, 42, 7161–7173.
The authors declare no conflict of interest.
3
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