Giant Amplification of Photoswitching by a Few Photons in Fluorescent Photochromic Organic Nanoparticles

Article abstract of DOI:10.1002/anie.201510600

Controlling or switching the optical signal from a large collection of molecules with the minimum of photons represents an extremely attractive concept. Promising fundamental and practical applications may be derived from such a photon-saving principle. W

Full text of DOI:10.1002/anie.201510600

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201510600
German Edition: DOI: 10.1002/ange.201510600
Photoswitching
Hot Paper
Giant Amplification of Photoswitching by a Few Photons in
Fluorescent Photochromic Organic Nanoparticles
Jia Su, Tuyoshi Fukaminato,* Jean-Pierre Placial, Tsunenobu Onodera, Ryuju Suzuki,
Hidetoshi Oikawa, Arnaud Brosseau, FranÅois Brisset, Robert Pansu, Keitaro Nakatani, and
RØmi MØtivier*
Abstract: Controlling or switching the optical signal from
a large collection of molecules with the minimum of photons
represents an extremely attractive concept. Promising funda-
mental and practical applications may be derived from such
a photon-saving principle. With this aim in mind, we have
prepared fluorescent photochromic organic nanoparticles
(NPs), showing bright red emission, complete ON–OFF
contrast with full reversibility, and excellent fatigue resistance.
Most interestingly, upon successive UV and visible light
irradiation, the NPs exhibit a complete fluorescence quenching
and recovery at very low photochromic conversion levels
(< 5%), leading to the fluorescence photoswitching of 420 Æ 20
molecules for only one converted photochromic molecule. This
“giant amplification of fluorescence photoswitching” origi-
nates from efficient intermolecular energy-transfer processes
within the NPs.
fied quenching” effect has been often observed in condensed
solid states or well-ordered supramolecular assemblies.[3]
However, most of these systems suffer from a lack of
reversibility. Only very few examples exploit this effect in
photochromic fluorescent materials in order to achieve
“amplified photoswitching”.[4]
Hybridization of a photochromic acceptor unit and
a fluorescent donor unit in materials, such as nanoparticles
(NPs), quantum dots, polymer dots, or up-converting par-
ticles, is a typical approach to prepare fluorescent photo-
switchable systems,[4a,c,d,5] because the hybridization is a con-
venient way to prevent formation of non-fluorescent aggre-
gates. However, in these systems, it is often difficult to achieve
efficient photoswitching and good fatigue resistance because
of the heterogeneous distribution of molecular arrangements
or photodegradation issues. Otherwise, the preparation of
photoswitchable fluorescent NPs from a single fluorescent
photochromic molecular component appears to be a partic-
ularly promising approach to reach excellent fluorescence
photoswitching properties.[4a] In such NPs, a quite high density
of chromophores can be obtained, providing a way to enhance
the brightness and maximize the intermolecular FRET
efficiency, leading to a groundbreaking amplification of
fluorescence photoswitching. In this approach, the aggrega-
tion induced emission (AIE)[6] is often utilized to overcome
concentration (aggregation) fluorescence quenching. How-
ever, the fluorescence quantum yields of photoswitchable
fluorescent NPs based on AIE systems are unfortunately not
sufficient (a few percent) compared to the original AIE unit.
Indeed, the photoswitching unit disrupts the ideal packing of
the AIE moiety and the fluorescence process competes with
the photoreaction process.[4a,6d]
T
riggering property changes of a large number of molecules
by only a few photons is a challenge, one which when solved
would represent a significant energy- and time-saving added
value in the field of molecular fluorescent photoswitches,[1]
which are applied in ultrahigh density all-optical data storage,
tunable biological sensing, or super-resolution fluorescence
imaging.[2] With this aim in mind, fluorescence-quenching
based on the long-range intermolecular Fçrster Resonance
Energy Transfer (FRET), which enables a single molecular
acceptor to quench the fluorescence of surrounding multiple
donor fluorophores is of particular interest. Such an “ampli-
[*] Dr. J. Su, Dr. J.-P. Placial, A. Brosseau, Dr. R. Pansu, Prof. K. Nakatani,
Dr. R. MØtivier
PPSM, ENS Cachan, CNRS
Herein we developed photoswitchable fluorescent NPs
based on a diarylethene (DAE) benzothiadiazole (BTD)
dyad PF (Figure 1a) with excellent fluorescence and photo-
switching properties. The DAE unit undergoes reversible
cyclization and cycloreversion reactions between its open-
form (OF) and closed-form (CF) under UV and visible
irradiation even in the solid state.[7] The BTD fluorophore has
been selected for its strong fluorescence properties in both
solution and solid states, its large Stokes shift, and high
photostability.[8] BTD derivatives maintain their bright red
fluorescence emission even when additional functional units
are connected to the BTD backbone. Therefore, excellent
photochromic and fluorescence properties are anticipated by
combining covalently DAE and BTD moieties into PF dyads
and then gathering the PF molecules into NPs, providing
UniversitØ Paris-Saclay
94235 Cachan (France)
E-mail: metivier@ppsm.ens-cachan.fr
Prof. T. Fukaminato
Dpt Appl. Chem. & Biochem.
Kumamoto University
2-39-1 Kurokami, Chuo-ku, Kumamoto 860–8555 (Japan)
E-mail: tuyoshi@kumamoto-u.ac.jp
Dr. T. Onodera, R. Suzuki, Prof. H. Oikawa
Inst. of Multidisciplinary Res. for Adv. Materials, Tohoku University
Katahira 2-1-1, Aoba-ku, Sendai (Japan)
Dr. F. Brisset
ICMMO, Paris-Sud University, CNRS UniversitØ Paris-Saclay
91405 Orsay (France)
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201510600.
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suspension of PF(OF) NPs, as shown in Figure 1b, exhibits
a main band in the UV range with a maximum at 308 nm,
which corresponds to the overlay of two bands originating
from both fluorescent and photochromic dyes, whereas the
second absorption band at 447 nm matches the fluorescent
moiety only (see Supporting Information, Section 2.1). PF-
(OF) NPs are emissive in the red region (lmax = 615 nm), with
a high fluorescence quantum yield FF = 0.65 and a lifetime of
9.6 ns (see Supporting Information, Section 2.9), that are
comparable to the corresponding values in THF solution
(FF = 0.67 and a lifetime of 7.8 ns). Upon irradiation in the
UV (up to 380 nm), the typical absorption band of the
PF(CF) appears in the 500–700 nm range of the spectrum, as
shown in Figure 1b. When the photostationary state (PSS) is
reached, the conversion yield was estimated by high-perfor-
mance liquid chromatography (HPLC) to be 0.73 for 313 nm
illumination, which is lower than PF in THF solution (0.93).
Under alternate UV-visible irradiation cycles, the fluores-
cence of the NPs suspension can be completely switched OFF
and ON again, many times, without any alteration of the
fluorescence levels of both states (Figure 1c and SI Sec-
tion 2.7). Additionally, the contrast between the ON and OFF
states reaches an extremely high value of 10000:1, and the
fluorescence levels of the system can be probed continuously
over 1 h without any change of the signal (see SI Section 2.8)
under excitation at 447 nm corresponding to the absorption
band of BTD unit, in which the open-form (respectively,
closed-form) of the photochromic unit has no absorbance
(respectively, very weak absorbance). Furthermore, the
25 nm-NPs contain about 7000 PF molecules, yielding
a brightness of n ” e ” FF ꢀ 7 ” 107 LmolÀ1 cmÀ1 at 450 nm,
that is more than 100 times brighter than most common
quantum dots.[10]
Most interestingly, the fluorescence switching behavior of
the PF molecular system appears rather different in solution
and in NPs suspension, in terms of both efficiency and rate of
switching. In THF solution, the normalized fluorescence
intensity versus conversion yield (IF vs. CY, see SI Sec-
tion 2.10) correlation plot displayed in Figure 2a shows
a linear behavior for the forward reaction (under UV light,
blue markers) and the backward reaction (under visible light,
red markers) between the PF(OF) state (high fluorescence
level) and the PSS (low fluorescence level). This linear
response is compatible with a simple intramolecular FRET
process.[11] It is clearly visualized on a series of cuvette
pictures containing the PF molecule dissolved in THF,
illuminated at increasing UV irradiation times, together
with the corresponding conversion yield determined by
absorption control (Figure 2b). The minimum level of
fluorescence depends on the PSS composition and cannot
reach zero (Figure 2a). In contrast, the IF versus CY
correlation plot presented in Figure 2c, corresponding to
the NPs suspension of PF, shows a different behavior. The
initial fluorescence of the PF(OF) NPs decreases dramatically
at very low conversion yield. More than 90% of the whole
fluorescence is quenched for only 1% of PF(CF), and the
fluorescence can be considered to reach almost zero (more
than 99% fluorescence quenching) for only 5% of PF(CF).
This trend is highlighted for a series of fluorescence cuvette
Figure 1. a) Chemical structure and photochromism of the PF molec-
ular dyad (DAE unit gray/blue, BTD unit red). b) Absorption (empty
curves) and emission spectra (shaded curves) of a suspension of
PF NPs (10À5 m in H2O/THF 80:20) in its open-form (OF, red curves),
closed-form (CF, blue curves), and at the photostationary state (PSS,
dotted blue curves). c) Fluorescence intensity of a suspension of PF
NPs (10À5 m in H2O/THF 80:20) for several UV (313 nm,
0.28 mWcmÀ2)-visible (575 nm, 3.25 mWcmÀ2) irradiation cycles.
a promising design strategy to maximize the performance of
photoswitchable fluorescent NPs.
PF was synthesized from an appropriate hydroxy-substi-
tuted photochromic fragment and a benzyl bromide substi-
tuted fluorescent unit, in one step with almost quantitative
yield, as shown in Scheme S1 in Supporting Information
Section 1. Typical absorption and fluorescence characteriza-
tion (spectra, conversion yields, photochromic and fluores-
cence quantum yields, lifetimes) of PF(OF) and PF(CF) in
THF solution are given in the Supporting Information
Sections 2.2, 2.4, and 2.9. NPs of PF were prepared by the
reprecipitation method in a H2O/THF 80:20 mixture.[9]
Atomic force microscopy (AFM) and electron microscopy
on NPs deposited on glass or silicon substrates from the
suspension provided an average diameter of 25 Æ 10 nm (see
Supporting Information, Section 2.5). Powder X-ray diffrac-
tion (PXRD) on the NPs provided hallow patterns as shown
in the Supporting Information Section 2.5, indicating that NPs
are amorphous. The absorption spectrum of the initial
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packing of PF molecules being quite dense in the solid matrix
of the NPs, a PF(CF) molecule can actually play the role of
energy acceptor for many neighboring PF(OF) molecules
located in its Fçrster volume of interaction (the detailed
discussion is described in SI Sections 2.3 and 2.11).[12] As
a first qualitative picture, the number of PF molecules
included in the Fçrster sphere of a single PF(CF) acceptor
(with diameter 2 ” R0 = 9.8 nm) reaches 420, over 7000 PF
molecules are estimated to be contained in a NP of 25 nm
diameter. The validity of a FRET-induced giant amplification
of fluorescence photoswitching observed in PF NPs suspen-
sions was further examined by means of a complete numerical
simulation based on a Fçrster energy-transfer model. The IF
versus CY profile in NPs was calculated using straightforward
assumptions, considering spherical NPs composed of evenly
space-distributed and randomly oriented PF molecules. Our
simplified model assumes that a given PF(OF) molecule is not
quenched when the neighboring PF molecules, located at
a distance shorter than the Fçrster radius R0, are all in the
PF(OF) state, and fully quenched in all other cases (see SI
Section 2.11). The calculated number of emitting molecules
versus the number of converted molecules is in agreement
with the experimental data, as shown in Figure 3a. Conse-
quently, the principle of FRET-induced giant amplification of
fluorescence photoswitching, based on intermolecular dipole–
dipole interactions at long distances between PF(OF) and
PF(CF), is identified to be the main photophysical process at
the origin of the strong nonlinear behavior of the fluorescence
quenching in PF NPs. Note that, in the present case, exciton
diffusion process[3e, 4d] plays a minor role in the photophysics
of such NPs, since the spectral overlap between emission and
absorption of the BTD fluorophores is negligible (Figure 1b
and SI Section 2.1–2.3). The general Scheme of the FRET-
induced giant amplification of fluorescence photoswitching is
illustrated in Figure 3b. The number of quenched PF
molecules per converted PF(CF) unit can be deduced from
the initial slope of the curves plotted in Figure 3a, providing
a huge amplification factor: around 420 Æ 20 PF molecules
are estimated experimentally (310 from the numerical
simulation, see SI) to be quenched by intermolecular energy
transfer to a single PF(CF) acceptor molecule. Another way
to figure out this amplification is to express it in terms of
photons needed to switch OFF the whole fluorescence of the
NPs. From this point of view, around 40 PF molecules are
switched OFF per photon absorbed by the photochromic unit
(see detailed calculation in SI Section 2.11), leading to
a groundbreaking improvement of practical efficiency in
photoswitchable materials.
Figure 2. a),c) Fluorescence intensity versus conversion yield (CY)
correlation plots and b),d) photographs of sample cuvettes for PF.
a),b) in solution (2”10À6 m in THF) and c),d) in NPs suspension
(10À5 m in H2O/THF 80:20), under increasing UV (blue dots) and
visible (red dots) exposure times. Irradiation conditions: for UV,
313 nm light (a: 45 mWcmÀ2, b,d: 50 mWcmÀ2, c: 41 mWcmÀ2), and for
visible, 575 nm light (a: 9.45 mWcmÀ2, c: 3.25 mWcmÀ2) were
employed.
images shown in Figure 2d, where the fluorescence is entirely
switched OFF in a few seconds of 313 nm irradiation
(50 mWcmÀ2) (see SI Movie S1). This non-linear fluorescence
photoswitching behavior can be compared to other photo-
switchable fluorescent multichromophoric systems men-
tioned above,[4a–c] but in the present case, the quenching
efficiency has been greatly enhanced by several orders of
magnitude (see below).
Such a “giant amplification of fluorescence photoswitch-
ing” can actually be ascribed by a very efficient intermolec-
ular FRET process between the fluorescent units and the
photochromic moieties in their closed form within the NPs
prepared exclusively from the PF molecular dyads. Indeed,
the Fçrster radius R0 was calculated to be 4.9 nm, with k2 =
0.496 for fixed molecules with random orientations. The
Fluorescence images of individual PF NPs deposited on
a glass cover slide, recorded under continuous excitation at
488 nm and displayed in Figures 4a–c, show that individual
PF NPs can be efficiently and rapidly switched OFF and ON
under UV light (365 nm, 80 mWcmÀ2) and visible light
(488 nm, 150 mWcmÀ2) excitation: the ON!OFF (resp.
OFF!ON) transition requires 150–200 ms (resp. 1.0–
1.5 sec) illumination time. The fluorescence signal of an
individual NP under UV–visible irradiation sequences, shown
in Figure 4d, clearly demonstrates the perfect reversibility
and excellent fatigue resistance at the single NP level. A
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decrease of only 10% of the individual NP
fluorescence was observed after 100 cycles,
and a very high ON/OFF contrast up to
600:1 was estimated in the first 20 cycles.
In conclusion, the PF NPs show out-
standing state-of-the-art properties in the
field of photochromic-fluorescent nanoma-
terials: easy preparation method, small size
(25 Æ 10 nm), bright red emission, high pho-
tostability, and excellent performances in
terms of photoswitching, with very high
contrast (10000:1) and fatigue resistance.
These outstanding properties are based on
a “giant amplification of fluorescence pho-
toswitching”, which is due to a very efficient
intermolecular Fçrster energy transfer at
the origin of the collective quenching of
more than 400 molecules per switching
event. Such strong nonlinear photoswitch-
ability is highly beneficial to many applica-
tions which require sensitive fluorescent
photoswitches working with the minimum
number of photons, such as optical record-
ing or subdiffractive fluorescence microsco-
py.
Figure 3. a) Percentage of emitting PF molecules versus percentage of converted PF(CF)
molecules in NPs, plotted from experimental (blue dots) and simulated data (black curve).
The slopes at the origin are shown, and the intercept with the x-axis quantifies the amplified
switching effect reached in this system. b) Illustration of the FRET-induced “giant
amplification of fluorescence photoswitching” in NPs. Left: every NP is initially composed
of emissive PF(OF) molecules densely packed together (depicted in gray/bright red
molecular dyads) subjected to a limited number of excitation hopping steps by energy
migration. Right: when a few molecules are promoted to the non-fluorescent PF(CF) (blue/
dark red dyads) at very low UV irradiation conditions, a large number of PF(OF) are
quenched (gray/dark red dyads) by long-range intermolecular FRET process PF(OF)!
PF(CF) within each Fçrster sphere.
Acknowledgements
We thank Agence Nationale de la Recher-
che (France) for funding through Nano-
PhotoSwitch and AZUR projects (K.N. and
R.M.). This work was supported by JSPS
KAKENHI, the Ministry of Education,
Culture, Sports, Science and Technology
(MEXT) Grant-in-Aid for Young Scientist
(A) (No. 24685021) and a Grant-in-Aid for
Scientific Research on Innovative Areas
“Photosynergetics” (no. 26107008). PHEN-
ICS network (CNRS GDRI), CNRS–JSPS
program, SAKURA program (JSPS-
Campus France), and ENS Cachan (invited
professorship to T.F.) are acknowledged for
supporting the collaboration.
Keywords: amplification · FRET ·
nanoparticles · photochromism ·
photoswitching
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Received: November 16, 2015
Published online: January 28, 2016
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