Chemiluminescence Resonance Energy Transfer Efficiency and Donor–Acceptor Distance: from Qualitative to Quantitative

Article abstract of DOI:10.1002/anie.202102999

Since its birth in 1967, the utilization of chemiluminescence resonance energy transfer (CRET) has made substantial progress in a variety of fields for its unique features. However, the quantitative relationship between CRET efficiency and donor–acceptor

Full text of DOI:10.1002/anie.202102999

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How to cite:
International Edition: doi.org/10.1002/anie.202102999
German Edition: doi.org/10.1002/ange.202102999
Chemiluminescence
Chemiluminescence Resonance Energy Transfer Efficiency and
Donor–Acceptor Distance: from Qualitative to Quantitative
Jinhui Lou+, Xiaofang Tang+, Haoke Zhang, Weijiang Guan,* and Chao Lu*
Abstract: Since its birth in 1967, the utilization of chemilumi-
nescence resonance energy transfer (CRET) has made sub-
stantial progress in a variety of fields for its unique features.
However, the quantitative relationship between CRET effi-
ciency and donor–acceptor distance has not yet been deter-
mined owing to the difficulty in designing the variable lengths
between chemiluminescent donors and acceptors. Herein, we
synthesized three kinds of tetraphenylethene (TPE)-anchored
cationic surfactants with aggregation-induced emission (AIE)
characteristics. For the first time, it is quantitatively demon-
strated that the CRETefficiency is inversely proportional to the
sixth power of distance between luminol donors and TPE
acceptors. The details disclosed in this contribute have
provided the solid evidence that CRET follows Fçrster
resonance theory. Our strategy would build a promising
platform to control donor–acceptor distance, allowing to the
interdisciplinary applications of CRET.
evidence to support the statement that CRET efficiency is
proportional to the inverse sixth power of the donor–acceptor
distance. Accordingly, CRET is limited to be qualitative,
rather than quantitative. Such a gap would greatly limit the in-
depth understanding and multidisciplinary applications of
CRET. Therefore, it is essential to pursue a feasible strategy
to measure the quantitative relationship between CRET
efficiency and donor–acceptor distance.
Currently, luminol donors have been attempted to cova-
lently link to suitable fluorescent acceptors to construct
intramolecular CRET, where the donors and acceptors were
separated by a rigid spacer with a fixed length.[7,8] However,
the variable lengths have not yet been achieved due to two
major difficulties: 1) as the spacer length increases, the poor
water solubility of luminol molecule would make its lumines-
cence efficiency decrease;[7] 2) intermolecular CRET inevi-
tably occurs owing to Brownian motion in solution, affecting
the efficiency of intramolecular CRET.[17] To solve these
obstacles, it is highly required to maintain the water solubility
of luminol as well as to confine the luminol donors and the
acceptors in separated regions.
Cationic micelles in aqueous solution can provide the
ideal separated regions, where anions and organic fluorescent
molecules are confined at the micelle interface and inside the
micelle, respectively.[18] In principle, when the luminol anions
in a basic solution are electrostatically attracted to the micelle
interface, the distances between the luminol donors and the
fluorescent acceptors are able to be controlled by tuning the
position of the fluorescent acceptors inside the micelle.
Accordingly, the corresponding CRET efficiencies are quan-
titatively measured. However, the conventional acceptors
inside the micelle often suffer from aggregation-caused
quenching (ACQ) problem, leading to a dramatic drop in
their luminescence performances.[19–21] Supposing that if
aggregation-induced emission (AIE)-active acceptors can be
fixed at different positions inside the micelles, their intense
luminescence properties in the aggregated state could be
exhibited without ACQ problem.
In this work, three kinds of AIE-active cationic surfac-
tants are designed and synthesized by covalently incorporat-
ing tetraphenylethylene (TPE) into the 4th, 8th, and 12th
methyl groups adjacent to the cationic headgroup of dodecyl
trimethylammonium bromide (C12TAB), namely C8-TPE-
C4TAB, C4-TPE-C8TAB, and TPE-C12TAB (Scheme 1). The
as-prepared three kinds of cationic micelles in aqueous
solution own almost the same critical micelle concentration
(CMC) and micelle size. Luminol anions are electrostatically
anchored at the micelle interface. As the distance between
luminol donors and TPE acceptors increases from 11.9 to
22.4 ꢀ, the CRET efficiency decreases from 99.14% to
Introduction
In 1967, chemiluminescence resonance energy transfer
(CRET) was first proposed by simulating fluorescent donors
in fluorescence resonance energy transfer (FRET) with
chemiluminescence (CL).[1,2] Since then, CRET is subjectively
assumed to follow Fçrster resonance theory, namely, the
energy transfer efficiency is inversely proportional to the sixth
power of the donor–acceptor distance.[3–5] In comparison to
FRET, CRET offers several inherent advantages, including
elimination of external photoexcitation, absence of autofluor-
escence and photobleaching, and thus it could provide
excellent performances in immunoassays, macromolecular
analysis, small molecule detection, bioimaging, and therapeu-
tic applications.[6–16] Unfortunately, there is no experimental
[*] J. Lou,[+] X. Tang,[+] Dr. W. Guan, Prof. Dr. C. Lu
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
Beijing 100029 (China)
E-mail: wjguan@mail.buct.edu.cn
luchao@mail.buct.edu.cn
Dr. H. Zhang
MOE Key Laboratory of Macromolecules Synthesis of Functionali-
zation, Department of Polymer Science and Engineering
Zhejiang University, Hangzhou 310027 (China)
Prof. Dr. C. Lu
Green Catalysis Center, and College of Chemistry
Zhengzhou University, Zhengzhou, 450001 (China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202102999.
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Similarly, para-hydroxy-TPE (TPE-OH) reacted with 1,12-
dibromododecane to form TPE-C12Br.[22] In the presence of
TiCl4 and zinc dust, McMurry coupling of BP-C4Br and BP-
C8, and BP-C8Br and BP-C4 could produce C8-TPE-C4Br and
C4-TPE-C8Br.[23] Their molecular structures were verified
using 1H nuclear magnetic resonance (1H NMR) spectra
(Supporting Information, Figures S2–S4). Thereafter, three
bromine-functionalized intermediates (C8-TPE-C4Br, C4-
TPE-C8Br, and TPE-C12Br) were reacted with excess trime-
thylamine to afford three target products: C8-TPE-C4TAB,
C4-TPE-C8TAB, and TPE-C12TAB.[22] The H NMR spectra
1
Scheme 1. Preparation of three kinds of AIE-active cationic micelles for
the quantitative relationship between CRET efficiency and donor–
acceptor distance.
(Figure 1) indicated that all hydrogen atoms were in the
correct positions and the corresponding peak integrations
were consistent with their structures. Moreover, the 13C NMR
spectra (Supporting Information, Figures S5–S7) and posi-
tive-ion mass spectra (Supporting Information, Figures S8–
S10) gave further validation of the successful synthesis of C8-
TPE-C4TAB, C4-TPE-C8TAB, and TPE-C12TAB.
72.39%. The measured CRET efficiency is inversely propor-
tional to the sixth power of distance. These data are in
excellent agreement with Fçrster resonance theory. Our
strategy provides the first experimental evidence for deter-
mining the quantitative relationship between CRET efficien-
cy and donor–acceptor distance.
The as-prepared C8-TPE-C4TAB, C4-TPE-C8TAB, and
TPE-C12TAB were typical amphiphilic molecules consisting
of the same quaternary ammonium headgroup and hydro-
phobic tail (TPE and an alkyl chain with 12 carbon atoms).
Their logP (octane/water partition coefficient) values were
calculated to be 5.96, 5.96, and 6.57, indicating their similar
amphiphilicity.[24] Moreover, the amphiphilic properties were
also compared by experimentally determining the CMC.[24,25]
The conductivities (k) of the aqueous solutions of C8-TPE-
C4TAB, C4-TPE-C8TAB, and TPE-C12TAB at different con-
centrations were plotted and shown in the Supporting
Information, Figure S11. Their conductivities increased line-
arly with increasing their concentrations, with breaks at about
46, 44, and 45 mM, respectively, above which the linear slope
became smaller. The break indicated the CMC, which was due
to the binding of counterions to the formed ionic micelles.[26]
Obviously, C8-TPE-C4TAB, C4-TPE-C8TAB, and TPE-
C12TAB had a similar CMC, allowing them to form micelles
and perform CRET measurements at the same concentra-
tions above CMC.
Results and Discussion
The chemical structures and synthetic routes of C8-TPE-
C4TAB, C4-TPE-C8TAB, and TPE-C12TAB were shown in
Figure 1 and the Supporting Information, Figure S1, respec-
tively. In detail, under basic conditions, 4-hydroxybenzophe-
none (BP-OH) was converted into BP-C4Br, BP-C8, BP-C8Br,
and BP-C4 by reacting with 1,4-dibromobutane, 1-bromooc-
tane, 1,8-dibromooctane, and 1-bromobutane, respectively.
To enable that effective CRET could occur between
luminol donors and TPE acceptors in the micelle solution, we
studied ultraviolet-visible (UV/Vis) absorption and fluores-
cence properties of the aqueous solutions of C8-TPE-C4TAB,
C4-TPE-C8TAB, and TPE-C12TAB at different concentra-
tions, as well as CL spectrum of luminol in water. As shown in
Figure 2a–c, UV-vis absorption wavelengths of C8-TPE-
C4TAB, C4-TPE-C8TAB, and TPE-C12TAB were about 250
and 317 nm, corresponding to the absorption of the benzene
and TPE groups, respectively.[22] Their absorbances at 317 nm
increased linearly with increasing their concentrations from
10 to 100 mM (Supporting Information, Figure S12), indicat-
ing no change in the conjugation degree of the TPE groups
before and after micelle formation.[22,24] On the other hand,
their fluorescence emission wavelengths were around 485 nm
under the same conditions (Figure 2a–c). Interestingly, the
relationship between fluorescence intensity and concentra-
tion was consistent with the relationship between conductivity
and concentration, showing two different linear relationships
below and above CMC due to changes in the aggregation
state of TPE groups (Supporting Information, Fig-
Figure 1. Chemical structures and the corresponding 1H NMR spectra
of a) C8-TPE-C4TAB, b) C4-TPE-C8TAB, and c) TPE-C12TAB. Asterisks
indicate solvent peaks, namely, water, methanol, and DMSO. TMS=te-
tramethylsilane.
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indicating that the TPE groups could intensely emit in the
micelles. In addition, the spectral overlap between donor
emission and acceptor absorption was essential for RET to
occur.[27–29] As shown in Figure 2d, the CL spectrum of
luminol was recorded in the presence of 5 mM of H2O2 and
10 mM of Co2+, which could overlap with the absorption
spectra of the micelle solutions of C8-TPE-C4TAB, C4-TPE-
C8TAB, and TPE-C12TAB.[2,16] It was worth noting that high
CRET efficiency can still be obtained by shortening the
donor–acceptor distance in the presence of the small overlap
between the emission of donor and the absorption of accept-
or.[27–29] Therefore, C8-TPE-C4TAB, C4-TPE-C8TAB, and
TPE-C12TAB could act as the high-performance CRET
acceptors in the micelles.
To ensure that the TPE acceptors were anchored at
different positions of the micelles, transmission electron
microscope (TEM), dynamic light scattering (DLS) assays
and theoretical calculations were carried out for the three
micelle solutions.[30–35] As shown in Figure 3a–c, the TEM
images showed that the three kinds of micelles had a spherical
morphology with the diameters ranging from 5.0 to 5.5 nm.
The DLS data (Figure 3d–f) indicated that the hydrodynamic
diameters of C8-TPE-C4TAB, C4-TPE-C8TAB, and TPE-
C12TAB micelles were about 6.19, 6.25, and 5.85 nm, respec-
tively. Their hydrodynamic radii were further estimated by
Figure 2. Absorption (black lines) and fluorescence emission (red
lines) spectra of the aqueous solutions of a) C8-TPE-C4TAB, b) C4-TPE-
C8TAB, and c) TPE-C12TAB at concentrations of 10–100 mM. d) Normal-
ized absorption spectra of C8-TPE-C4TAB, C4-TPE-C8TAB, and TPE-
C12TAB (black dotted lines), and normalized CL spectrum of luminol
(red solid line).
ure S13).[22,24] The fluorescence quantum yields of the three
micelle solutions (80 mM) were 8.45%, 15.19%, and 9.10%,
Figure 3. TEM images and DLS data of micelle solutions, and energy-minimized molecular structures of a),d),g) C8-TPE-C4TAB, b),e),h) C4-TPE-
C8TAB, and c),f),i) TPE-C12TAB.
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theoretical calculations. The energy-minimized molecular
whole spectrum of the luminol and TPE.[8] It was found that
structures (Figure 3g–i) were optimized using the density
functional theory (DFT) method B3LYP with the base 6-
31G(d,p).[33,34] The distances between the C atom of the end
group and the N atom were calculated to be 27.6, 27.9, and
27.1 ꢀ, respectively, while the distance between the C atom of
the headgroup and the N atom was calculated to be 1.5 ꢀ. The
thickness of solvation layer was about 2 ꢀ.[31] By summing the
molecular length, the headgroup radius, and the thickness of
the solvation layer,[30–32] the estimated hydrodynamic radii of
the three micelles were 3.11, 3.14, and 3.06 nm, which
approximately equaled those obtained by DLS. It was worth
noting that the neighboring TPE molecules were packed with
overlapping benzyl fragments (ca. 4.3 ꢀ) in the crystal
lattice.[35] The hydrodynamic diameter of TPE-C12TAB ob-
tained by DLS (5.85 nm) fell within the estimated range
(5.69–6.12 nm). Moreover, the effects of the presence of
Co(NO3)2 on the micelle aggregates and photophysical
properties were investigated. As shown in the Supporting
Information, Figure S14, the hydrodynamic diameters of the
three micelles remained unchanged after the addition of
Co(NO3)2 because low concentrations of neutral electrolytes
caused no change in the micelle size.[36] The fluorescence
spectra of the micelle solutions of C8-TPE-C4TAB, C4-TPE-
C8TAB, and TPE-C12TAB in the absence and presence of
Co(NO3)2 are given in the Supporting Information, Fig-
ure S15. There was a slight increase in the fluorescence
intensity with the addition of 10 mM of Co(NO3)2. The
fluorescence enhancement is because the shielding effect of
neutral electrolytes can reduce the electrical repulsion
between the charged headgroups, allowing the tighter packing
to further restrict the intramolecular motions of TPE
groups.[36] Therefore, the anchored positions of the TPE
acceptors in the three micelles were at 11.9, 17.3, and 22.4 ꢀ
from the cationic headgroups, respectively.
Luminol has a pKa value of ca. 6.7, and it easily loses
a proton in basic solution to form the luminol monoan-
ion.[37–39] By physically mixing an alkaline solution of luminol
with an aqueous solution of cationic micelles, the luminol
monoanions could be electrostatically adsorbed onto the
cationic headgroups of micelles.[40] The strong electrostatic
attraction between luminol monoanions and headgroups of
cationic micelles ensures that the luminol monoanions could
be anchored at the micelle interface, rather than diffuse inside
the micelles.[41–43] Accordingly, the distance between the
luminol anions and the TPE acceptors was equal to the
distance between the cationic headgroups and the center of
the TPE acceptors in the C8-TPE-C4TAB, C4-TPE-C8TAB,
and TPE-C12TAB micelles (i.e., 11.9 ꢀ, 17.3 ꢀ, and 22.4 ꢀ).
Figure 4 and the Supporting Information, Figures S16–S18
show the CL spectra of different concentrations of luminol
ranging from 40 to 110 mM in the presence of 80 mM of C8-
TPE-C4TAB, C4-TPE-C8TAB, and TPE-C12TAB, respectively.
Luminol could deprotonate to form luminol anion as the pH
values of luminol solution and the final solutions containing
micelles, luminol, and cobalt ions (Supporting Information,
Table S1) were higher than the pKa of luminol. The CRET
efficiency was determined through dividing the integral area
of the TPE emission spectrum by the integral area of the
the CRET efficiency was almost constant with increasing
luminol concentrations until 60 mM, above which the CRET
efficiency decreased sharply. These results indicated that the
adsorption of the micelle interface was saturated when the
luminol concentration reached 60 mM. Further increases in
the luminol concentrations could make them dissolve in the
bulk solution rather than localize on the micelle interface,
resulting in the longer donor–acceptor distances. To maintain
the luminol donor-TPE acceptor distance constant, the
luminol concentrations in this work were employed within
60 mM. In addition, the electrostatic interaction between the
luminol anions and the three cationic micelles was examined
by determining the zeta potentials of the three micelle
solutions in the absence and presence of 60 mM of luminol.
Figures 4c, f, and i show that the zeta potentials of the C8-
TPE-C4TAB, C4-TPE-C8TAB, and TPE-C12TAB micelles
were 53.7, 53.4, and 55.5 mV. The addition of luminol could
reduce their values to 40.3, 40.1, and 39.0 mV. The partial
neutralization of zeta potentials clearly indicated that the
luminol anions were adsorbed at the micelle interface through
electrostatic interaction. Accordingly, 11.9 ꢀ, 17.3 ꢀ, and
22.4 ꢀ can be used as the CRET donor–acceptor distances
when the luminol concentrations were controlled within
60 mM.
When the luminol concentrations were 40, 50, and 60 mM,
the average values of the corresponding CRET efficiencies
were 99.14 Æ 0.07%, 93.41 Æ 0.10%, and 72.39 Æ 0.22%, re-
spectively. To ensure that micelle formation was critical to the
calculations, the energy transfer behavior was further per-
formed at lower concentration than CMC. The CL spectra of
luminol at 60 mM in the presence of C8-TPE-C4TAB, C4-TPE-
C8TAB, and TPE-C12TAB at 20 mM are shown in the
Supporting Information, Figure S19. The corresponding
CRET efficiencies were calculated to be 64.06%, 81.85%,
and 60.73%, respectively. These results indicated that luminol
anions could bind to positively charged C8-TPE-C4TAB, C4-
TPE-C8TAB, and TPE-C12TAB molecules to shorten their
distance for CRET to occur. However, in the absence of
micelle, the donor–acceptor distance was unknown. There-
fore, the quantitative relationship between CRET efficiency
and donor–acceptor distance must be performed in the
presence of micelle.
With knowing the CRET efficiency and the distance
between luminol donors and TPE acceptors, their Fçrster
distances were calculated to be 26.3 Æ 0.3, 26.9 Æ 0.1, and
26.3 Æ 0.1 ꢀ, respectively (E = R0 / (R06 + r6), where E was the
6
CRET efficiency, and R0 and r corresponded to the Fçrster
distance and the donor–acceptor distance, respectively).[3–5]
Apparently, they had nearly the same Fçrster distance.
Moreover, the efficiency of resonance energy transfer as
a function of distance was given by E = (R0/r)j/[(R0/r)j + 1],
where R0 was the distance corresponding to 50% transfer and
j was the exponent of the distance dependence.[3–5] In the
current micelle systems of C8-TPE-C4TAB, C4-TPE-C8TAB,
and TPE-C12TAB, the calculated j was 6.0 Æ 0.1.[3–5] Therefore,
it was concluded that the CRETefficiency was proportional to
the inverse sixth power of the donor–acceptor distance, which
agreed well with Fçrster resonance theory.
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Figure 4. CL spectra of luminol (at 40, 50, and 60 mM) in the presence of a)–c) C8-TPE-C4TAB, d)–f) C4-TPE-C8TAB, and g)–i) TPE-C12TAB micelles
at 80 mM. Insets: corresponding zeta potentials of C8-TPE-C4TAB, C4-TPE-C8TAB, and TPE-C12TAB micelles in the absence (red, M) and presence
(blue, M+L) of 60 mM of luminol. Measurement time: 1.25 s, H2O2 concentration: 5 mM, catalyst: 10 mM of Co2+
.
Acknowledgements
Conclusion
This work was supported by National Natural Science
Foundation of China (21974008, 21804006, and 21521005),
Beijing Natural Science Foundation (2212013), and the
Fundamental Research Funds for the Central Universities
(buctrc201820).
We have synthesized the three kinds of AIE-active
cationic surfactants. TPE in AIE-active cationic surfactant
could act as a CRETacceptor by attaching to the 4th, 8th, and
12th methyl groups adjacent to the cationic headgroup. The
three donor–acceptor distances were obtained by electro-
statically attracting the luminol anions onto the cationic
headgroups of the micelle interface. The CRETefficiency was
calculated to be inversely proportional to the sixth power of
distance, which agreed well with Fçrster resonance theory.
This work not only establishes a feasible method for verifying
the occurrence of the Fçrster resonance between chemilumi-
nescent donors and acceptors, but also better understands the
quantitative relationship between CRET efficiency and
CRET donor–acceptor distance. The facile method devel-
oped in this study has the potential to greatly inspire the
current research of CRET in multiple disciplines.
Conflict of interest
The authors declare no conflict of interest.
Keywords: aggregation-induced emission ·
chemiluminescence · energy transfer · Fçrster resonance ·
micelles
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Manuscript received: March 1, 2021
Revised manuscript received: March 31, 2021
Accepted manuscript online: April 5, 2021
Version of record online: && &&
,
&&&&
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Chemiluminescence
More than 50 years after the birth of
chemiluminescence resonance energy
transfer (CRET), the first experimental
evidence of the quantitative relationship
between CRET efficiency and donor–
acceptor distance was achieved using
aggregation-induced emission (AIE)-
active micelles.
J. Lou, X. Tang, H. Zhang, W. Guan,*
C. Lu*
&&&& — &&&&
Chemiluminescence Resonance Energy
Transfer Efficiency and Donor–Acceptor
Distance: from Qualitative to
Quantitative
&&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!