Toward two-photon absorbing dyes with unusually potentiated nonlinear fluorescence response

Article abstract of DOI:10.1021/jacs.0c07377

The combination of two two-photon-induced processes in a F?rster resonance energy transfer (FRET)-operated photochromic fluorene-dithienylethene dyad lays the foundation for the observation of a quartic dependence of the fluorescence signal on the excitat

Full text of DOI:10.1021/jacs.0c07377

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Communication
Towards two-photon absorbing dyes with unusually
potentiated nonlinear fluorescence response
Carlos Benitez-Martin, Shiming Li, Antonio Dominguez-Alfaro, Francisco
Najera, Ezequiel Perez-Inestrosa, Uwe Pischel, and Joakim Andréasson
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07377 • Publication Date (Web): 15 Aug 2020
Downloaded from pubs.acs.org on August 15, 2020
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Towards two-photon absorbing dyes with unusually
potentiated nonlinear fluorescence response
Carlos Benitez-Martin,†,‡,# Shiming Li,† Antonio Dominguez-Alfaro,§,¶ Francisco Najera,‡,#
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Ezequiel Pérez-Inestrosa,‡,# Uwe Pischel,§* and Joakim Andréasson†*
† Department of Chemistry and Chemical Engineering, Physical Chemistry, Chalmers University of Technology, SE-
412 96 Gothenburg, Sweden.
‡ Universidad de Málaga-IBIMA, Departamento de Química Orgánica, Campus Teatinos s/n, E-29071 Málaga,
Spain.
# Centro Andaluz de Nanomedicina y Biotecnología (BIONAND), Parque Tecnológico de Andalucía, C/ Severo
Ochoa 35, E-29590 Málaga, Spain.
§ CIQSO - Center for Research in Sustainable Chemistry and Department of Chemistry, University of Huelva,
Campus de El Carmen s/n, E-21071 Huelva, Spain.
Supporting Information Placeholder
Few reports on 3PA and 4PA dyes9-16 and their occasional use
ABSTRACT: The combination of two two-photon-induced
in bioimaging17-20 can be found in the literature. Even more
processes in
a FRET-operated photochromic fluorene-
rarely, organic dyes21 or materials22 that feature 5PA are
described. However, the mentioned drawbacks, widely limit
their generalized application from a technological point of
view, making imaging with 2PA dyes the most frequently
chosen approach.1, 3
dithienylethene dyad lays the fundament for the observation
of a quartic dependence of the fluorescence signal on the
excitation light intensity. While this photophysical behavior
is predicted for a four-photon-absorbing dye, the herein
proposed approach opens the way to use two-photon
absorbing dyes, reaching the same performance. Hence, the
spatial resolution limit, being a critical parameter for
applications in fluorescence imaging or data storage with
common two-photon-absorbing dyes, is dramatically
improved.
FRET
F
F
S
F
F
F
2 x 820 nm
F
HN
O
FL-DTEc
S
N
O
H
2 x 820 nm
or
Vis
OCH3
NO fluorescence
2 x 820 nm
UV
Multiphoton processes find extensive use in imaging
applications1-3 and data storage4 with specifically designed
organic chromophores. This preference is tightly related to
the non-linear dependence of the absorption probability on
the excitation light intensity. For two-photon absorption
(2PA) a quadratic dependence applies5-8 and for higher-order
multiphoton absorption cubic (3PA)9 or quartic (4PA)10
dependencies are expected. The practical consequence of this
photophysical phenomenon translates into a much higher
density of excited chromophores in the focal point of the
exciting laser beam as compared to areas that are out-of-
focus. If fluorescence emission is used to monitor the excited
state, this translates into superior 3D spatial resolution, for
example in fluorescence microscopy. Noteworthy, there is a
dramatic increase in the spatial resolution with an increasing
number of simultaneous excitations, i.e., 3PA and 4PA are
superior to 2PA. However, on the downside the absorption
probability of 3PA and 4PA is dramatically decreased,
requiring very high laser intensities and the excitation
wavelength would be shifted to longer than 1200 nm for
typical dyes with one-photon absorption at around 400 nm.
F
F
S
F
F
F
F
HN
O
FL-DTEo
S
N
O
H
OCH3
fluorescence
F
F
F
S
F
F
F
HN
O
N
S
O
OCH3
H
FL-m
DTE-m
Figure 1. Top: working principle of a photochromic dyad
having a DTE appended to a 2PA dye. Bottom: Structures of
model compounds.
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In this work we introduce a molecular design that builds on
essentially coincides with the charge-transfer (one-photon)
absorption band of the chromophore, when the wavelength
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8
2PA, but presenting the above described advantages of a
nonlinear dependence beyond a 2PA system. The basic
approach relies on the structural and photophysical
integration23, 24 of a 2PA dye (a fluorene derivative, FL) with a
scale of the former is divided by 2, i.e., λ2PA  2λ1PA
.
Importantly, the two-photon-excited fluorescence spectrum
is the same as observed by one-photon excitation into the
absorption band of the chromophore (λabs,max = 378 nm),
confirming that the same excited state is involved in either
process.
dithienylethene photoswitch (DTE) in a dyad; see Figure 1.25-
27
Opposed to multiphotonically addressable DTE switches that
are electronically integrated in π-extended architectures,28-31
the two chromophores are electronically decoupled by
implication of an insulating spacer. However, in such
architectures photophysical communication via electron- or
energy-transfer is possible.32-35 In our specific case Förster
resonance energy transfer (FRET), implying the colored
closed form DTEc but not the colorless open form DTEo, was
envisioned.36-38 As illustrated in Figure 1, on two-photon
excitation the emission of the FL chromophore is quenched
When integrated into the FL-DTE dyad (see Supporting
Information for details on the synthesis of the ring-open
form; the ring-closed form is generated in situ by UV-light
irradiation) the fluorene shows its fluorescence only when
the DTE is encountered in its open form; see Figure 2 and
Table 1. However, for the closed form a significant spectral
overlap (J = 1.54  1015 nm4 M-1 cm-1) between the FL emission
and the DTEc absorption (see data in Table 1) yields a
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practically quantitative FRET process (ΦFRET
 1); see
by
a FRET process, which ultimately sensitizes the
Supporting Information for details. The critical FRET radius
R0 was determined as 53 Å (see Supporting Information), far
larger than the modeled distance between the centers of the
two chromophores (R = 22 Å). For the colorless FL-DTEo the
experimental fluorescence quantum yield and lifetime are
identical to that of the model FL-m (Table 1), being reasoned
with the zero spectral overlap integral for this form. This
confirms the absence of electronic communication between
the FL excited state and the DTEo ground state. After
formation of FL-DTEc on irradiation with UV light (302 nm;
Φo-c = 0.50), a residual fluorescence (ca. 6% of that of FL-
DTEo) is observed, which, however, is attributed to the
minor content of the open form in the photostationary state
isomerization of DTEc to its colorless form. In the latter the
unquenched fluorescence of the FL chromophore is observed
on two-photon excitation.
As follows from the nonlinear (quadratic) dependence of the
2PA probability on the excitation light intensity, the
combination39 of two two-photon-induced processes, i.e., a)
FRET sensitized DTEcDTEo isomerization and thereby the
generation of the emissive form of the dyad (FL-DTEo) and
b) two-photon excited fluorescence, should lead to a quartic
dependence for the first instances of the photochromic
conversion. On the other extreme, once all DTEc is
converted into DTEo, the “normal” quadratic dependence,
arising from two-photon excited fluorescence, would apply.
(DTEc-m/DTEo-m
=
94/6; determined by 1H-NMR
spectroscopy for the model DTE). Hence, in practical terms
quantitative FRET is operative for FL-DTEc, while no such
pathway is observed for FL-DTEo. Although photoinduced
electron transfer from FL to DTEc cannot be excluded, it is
very unlikely that that this process contributes significantly
to the quenching (see Supporting Information). The
Table 1. Photophysical key data of FL-DTE, DTE-m, and
Fl-m in methanol.
λabs,max
ε
λf,max
Φfd
τfe
(nm)a
(M-1cm-1)b
(nm)c
(ns)
switching is reversed by shining red light on the dyad (Φc-o
=
FL-DTEo
FL-DTEc
DTEo-m
DTEc-m
336
600
302
592
378
34900
10900
42600
18700
20400
555
0.36 2.4
0.008) and 100% of the open form is observed in the
corresponding photostationary state.
f
f
f
-
-
-
-
-
-
-
g
g
g
g
g
 (nm)
-
600 700 800 900 1000 1100 1200 1300 1400
g
-
160
1.0
FL-m
555
0.36 2.4
140
120
a
b
Longest-wavelength absorption maximum.
Molar
c
absorption coefficient at λabs,max
maximum.
.
Fluorescence emission
Fluorescence quantum yield; 15% error.
d
e

100
Fluorescence lifetime, measured by time-correlated single-
f
80
photon counting; 5% error. FL-DTEc is non-fluorescent.
0.5
Some residual fluorescence is observed due to the presence
of 6% FL-DTEo in the photostationary state. g Not measured;
essentially not fluorescent.
60
40
20
The choice of the FL chromophore was motivated by its
favorable photophysical properties (see Table 1 and spectra in
the Supporting Information).40 In methanol the separate FL
model chromopore (FL-m, see structure in Figure 1) shows a
broad intramolecular charge-transfer fluorescence with a
maximum at 555 nm (Φf = 0.36, τf = 2.4 ns). The 2PA
spectrum features a maximum at 770 nm with a significant
2PA cross section (σ2PA) of 156 GM. The 2PA spectrum
0.0
300
0
700
400
500
600
 (nm)
Figure 2. One-photon absorption (left) and fluorescence
(right) spectra of FL-DTEo (black solid lines); 10 M in
methanol. Note that the absorption spectrum of FL-DTEo
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shows the FL band as a shoulder at ca. 384 nm and the
Figure 3. a) Kinetics of fluorescence buildup on irradiation
of FL-DTEc (10 M in methanol) with 820-nm laser light at
full intensity (FI, blue points) and at half intensity (HI, red
points). See Supporting Information for the associated rate
constants. Inset: Zoom in on the first 20 seconds. b) Ratio of
time-dependent fluorescence intensities shown in a). The
fluorescence excitation wavelength was 820 nm.
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accordingly adapted 2PA spectrum (red dashed line and red
points) coincides spectrally with that feature. The two-
photon-excited fluorescence spectrum (black dashed line
and black points) resembles the conventional fluorescence
spectrum (black solid line).
Having established the FRET behavior of the dyad we
proceeded to obtain experimental proof for the two-photon-
excited processes in the FL-DTE dyad; i.e., FRET-induced
ring-opening of FL-DTEc and fluorescence from FL-DTEo.
The 2PA spectrum of FL-DTEo (Figure 2) coincides in shape
and cross-section (λ2PA,max = 770 nm, σ2PA = 150 GM) with the
one of FL-m (see Supporting Information). The operation in
the two-photon regime is confirmed by the double
logarithmic plot of the fluorescence versus the laser
intensity, yielding the expected slope of 2 (see Supporting
Information). As for FL-m, the two-photon-excited
fluorescence of FL-DTEo spectrally coincides with the
emission resulting from conventional one-photon excitation
(Figure 2). Importantly, starting with the dyad in its closed
non-fluorescent form (FL-DTEc) the irradiation with 820-nm
laser light yields the fast increase of the FL fluorescence
emission (see below). Noteworthy, at half of this wavelength,
i.e., 410 nm, the DTE photochrome shows negligible
absorption (zero for DTEo and < 1000 M-1cm-1 for DTEc).
Hence, it can be safely assumed that 2PA of the DTE at the
chosen excitation wavelength is at most minor, while the FL
features a significant cross section (86 GM at 820 nm). Thus,
the formation of the fluorescent FL-DTEo form by direct
two-photon-induced ring opening is unlikely and instead the
above proposed FRET-sensitized ring-opening isomerization
is operative.
7
8
9
The beforehand made observations lay the fundament for the
proof-of-principle experiment, which we expected to confirm
our initial assumption that the combination of two-photon-
induced processes yields a dependence of the fluorescence
intensity versus laser intensity that goes beyond the “normal”
quadratic relationship and which in theory should
approximate a quartic function. In Figure 3 the kinetics of
the buildup of the FL fluorescence on 820-nm light
irradiation of FL-DTEc at full and half laser intensity are
shown. The time-dependent ratio of the kinetics shows a
quotient of 10 in the first instances of the irradiation (ca. 1.5 s
under the chosen irradiation conditions). For a “pure” quartic
dependence a value of 16 (24) would be expected. The
deviation from this number can be modeled and led to the
presence of already 6% FL-DTEo in the initial solution,
corresponding to the experimental value observed for the
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photostationary
state
of
the
open-to-closed
photoisomerization (see Supporting Information). At longer
irradiation times, and with the build-up of successively
higher concentrations of the open fluorescent form FL-
DTEo, this quotient decays and approaches a value of 4.
Expectedly, this corresponds to the sole two-photon-excited
fluorescence of FL-DTEo. Noteworthy, the fitting of the
build-up kinetics (at full and half laser intensity) yielded two
rate components: a fast one, assigned to the FRET-induced
isomerization in the focal volume, and a second slower one
ascribed to diffusion of molecules out of the focal volume of
the excitation light beam. Interestingly, the ratio of the rate
constants of the fast components at different laser intensities
shows a value of 4.2, clearly corroborating the two-photon
nature of the isomerization (see Supporting Information).
a)
In summary, we propose an innovative approach towards dye
systems that enables the harnessing of a dramatically
improved dependence of the fluorescence signal on the
excitation light intensity. Conventional two-photon-
absorbing fluorophores, in concordance with photophysical
theory, yield a quadratic dependence. By combining a 2PA
dye with a photochromic system that can be switched by
two-photon-initiated FRET and the resulting form of the
dyad showing two-photon-excited fluorescence, two 2PA
processes are entangled. This would lead effectively to a
b)
quartic dependence for
a photochromic system with
quantitative (100%) conversion between the isomeric forms.
We have been able to approach this theoretical limit,
reaching a factor of 10 for comparing excitation with full and
half laser intensity (as opposed to solely 4 for exciting the
2PA dye alone). Noteworthy, in practical terms this still
outperforms a 3PA dye, for which a factor of 8 (23) is
expected. Hence, the presented approach enables to
overcome the photophysically imposed limitations of 2PA
dyes, while maintaining the technological benefits of
working in the two-photon excitation regime. Our
structurally
modular
approach
(non-conjugative
combination of the two functional units) allows matching
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Corresponding Author
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a-son@chalmers.se (J.A.); uwe.pischel@diq.uhu.es (U.P.)
Present Addresses
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¶ CIC biomaGUNE, Paseo de Miramón 182, 20014 Donostia-
San Sebastián, Spain and Polymat, University of the Basque
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photon excitation of amplified spontaneous emission in a nonlinear
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W. Efficient Three-Photon Excited Deep Blue Photoluminescence
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The authors declare no competing financial interests.
ACKNOWLEDGMENT
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Lin, T.-C.; Chien, W.; Mazur, L. M.; Liu, Y.-Y.; Jakubowski,
K.; Matczyszyn, K.; Samoc, M.; Amini, R. W. Two- and three-photon
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We acknowledge financial support by the Swedish Research
Council (VR, grant 2016-0360 for J.A.), the Spanish Ministry
for Science, Innovation, and Universities (CTQ2017-89832-P
for U.P. and CTQ2016-75870-P, RD16/0006/0012 and
PID2019-104293GB-I00 for E.P.-I.), Universidad de Málaga-
Junta de Andalucía (UMA18-FEDERJA-007 for E.P-I.), and
FEDER. C.B-M. is grateful for an FPU fellowship
(FPU16/02516). Computer resources were provided by the
SCBI (Supercomputing and Bioinformatics Center) of the
University of Málaga. The Centre for Cellular Imaging at the
University of Gothenburg and the National Microscopy
Infrastructure (NMI, funded under VR-RFI 2016-00968)
provided technical assistance in the microscopy-related
experiments.
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