Reversible Photomodulation of Electronic Communication in a π-Conjugated Photoswitch-Fluorophore Molecular Dyad

Article abstract of DOI:10.1002/chem.201503419

The extent of electronic coupling between a boron dipyrromethene (BODIPY) fluorophore and a diarylethene (DAE) photoswitch has been modulated in a covalently linked molecular dyad by irradiation with either UV or visible light. In the open isomer, both moieties can be regarded as individual chromophores, while in the closed form the lowest electronic (S₀→S₁) transition of the dyad is slightly shifted, enabling photomodulation of its fluorescence. Transient spectroscopy confirms that the dyad behaves dramatically different in the two switching states: while in the open isomer it resembles an undisturbed BODIPY fluorophore, in the closed isomer no fluorescence occurs and instead a red-shifted DAE behavior prevails.

Full text of DOI:10.1002/chem.201503419

DOI: 10.1002/chem.201503419
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&
Photoswitches
Reversible Photomodulation of Electronic Communication in a
p-Conjugated Photoswitch-Fluorophore Molecular Dyad
Javier Moreno,[a] Felix Schweighçfer,[b] Josef Wachtveitl,*[b] and Stefan Hecht*[a]
Abstract: The extent of electronic coupling between
a boron dipyrromethene (BODIPY) fluorophore and a diaryl-
ethene (DAE) photoswitch has been modulated in a covalent-
ly linked molecular dyad by irradiation with either UV or visi-
ble light. In the open isomer, both moieties can be regarded
as individual chromophores, while in the closed form the
lowest electronic (S0!S1) transition of the dyad is slightly
shifted, enabling photomodulation of its fluorescence. Tran-
sient spectroscopy confirms that the dyad behaves dramati-
cally different in the two switching states: while in the open
isomer it resembles an undisturbed BODIPY fluorophore, in
the closed isomer no fluorescence occurs and instead a red-
shifted DAE behavior prevails.
Introduction
photoswitch dyads, either electron or energy transfer is re-
sponsible for the fluorescence modulation,[20–23] whereas the in-
itial open isomer is strongly emissive while the photochemical-
ly generated closed isomer exhibits a diminished fluorescence
emission. If, for example, energy transfer is responsible for fluo-
rescence quenching (demanding spectral overlap of the fluoro-
phore emission with the closed photoswitch absorption),[24,25]
the excitation energy of the fluorophore is funneled to the
closed isomer. This leads to formation of the open isomer,
causing a destructive read-out, which is a typical phenomenon
encountered in these so called turn-off fluorescence switches.
Linking the fluorophore and the DAE via a p-conjugated
bridge enables direct electronic coupling and therefore light-
induced formation of the closed isomer is efficiently trans-
duced to the fluorophore. The associated increased conjuga-
tion throughout the entire (bi)chromophore system will lead to
a bathochromic shift of the lowest-energy electronic (S0!S1)
transition (Figure 1). Importantly, if the fluorophore is chosen
in such a way that its emission does not overlap with the DAE
absorption, it can be anticipated that no cycloreversion would
occur during read-out. In fact, a molecule designed following
this working principle could be fluorescent in both the initial
and the other photoswitched state, enabling true reversible
photomodulation of its emission on demand. Considering the
stringent molecular design outlined above, a potent fluoro-
phore with an absorption band red-shifted from the closed
DAE isomer has to be employed. We therefore chose a benzo-
fused boron dipyrromethene (BODIPY) dye and attached it to
a DAE photoswitch via a modular phenylene ethynylene
spacer to be able modulate the strength of the coupling be-
tween the two components while maintaining photochrom-
ism.[26] The specific benzothiophene derived DAE was selected
due to its relatively blue-shifted absorption band in the closed
isomer as well as outstanding fatigue resistance.[7]
Molecular switches, which upon irradiation with light efficiently
and reversibly interconvert between two stable isomers, have
been widely studied in the past and continuously improved.[1–5]
Nowadays their accurate and on-demand implementation in
complex molecular systems is possible with applications for
such nanoengineered constructs ranging from optoelectronic
devices to stimuli-responsive materials.[6,7] Diarylethenes
(DAEs)[8] experience a dramatic increase in conjugation upon
photoinduced isomerization with UV light, and some of their
derivatives possess a high fatigue resistance.[5,8] These attri-
butes can be exploited to modulate or even unlock secondary
functions at the molecular or supramolecular level by using
light as a remote control with high spatial and temporal reso-
lution.[9] Among the many chemical and physical properties of
molecular switches, fluorescence emission is of particular im-
portance since fluorescent molecules can be used as probes
for intracellular processes[10–12] as well as for optical recording
materials with fluorescence readout.[13–16] Furthermore, stable
photoswitchable fluorophores are specifically required in novel
super-resolution techniques capable of overcoming the diffrac-
tion limit of light.[17–19]
Several approaches have been followed to modulate the
emission of fluorophores in compounds bearing an attached
photochromic DAE derivative. In most of these fluorophore–
[a] Dr. J. Moreno, Prof. Dr. S. Hecht
Department of Chemistry, Humboldt-Universität zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: sh@chemie.hu-berlin.de
[b] F. Schweighçfer, Prof. Dr. J. Wachtveitl
Institute of Physical and Theoretical, Goethe-Universität Frankfurt
Max-von-Laue Strasse 7, 60438 Frankfurt/M. (Germany)
E-mail: wveitl@theochem.uni-frankfurt.de
Herein, we report the novel BODIPY-DAE dyad 1, including
its synthesis and photophysical and photochemical properties,
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503419.
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tion reactions. On the one hand, the non-symmetrical DAE 4
was obtained by a sequential nucleophilic substitution of octa-
fluorocyclopentene with two different lithiated hetaryl, that is,
thiophene and benzothiophene, residues. The preparation of
building block 2 from 2,4-dibromo-5-methylthiophene via
Suzuki cross-coupling with a suitable phenylethynylene was
straightforward. The synthesis of benzothiophene 3 was vastly
optimized when compared to previous reports where at least
three equivalents of the rather expensive and volatile octa-
fluorocyclopentane had to be employed.[28] In our own initial
work related to the synthesis of symmetrical DAE precursors,
we observed that employing Et2O instead of THF as the sol-
vent had a dramatic effect on the reaction, leading to isolation
of exclusively the mono-coupled product despite the use of
a two-fold excess of lithiated benzothiophene. This important
observation was adapted in our new method, which now uses
only an equimolar amount of octafluorocyclopentene and lithi-
tated methylbenzothiophene. Indeed, lithiation of 3-bromo-
methyl-benzo[b]thiophene in anhydrous Et2O at À788C with n-
BuLi, and subsequent addition of one equivalent of octafluoro-
cyclopentene led to quantitative formation of 3 and an excel-
lent 92% yield after purification by column chromatography.
Our optimized reaction conditions are generally applicable for
the cost-efficient preparation of new non-symmetrical DAEs
with exceptional fatigue resistance. On the other hand, synthe-
sis of the precursor BODIPY fragment 6 was accomplished by
an initial reduction of phthalimide with AlCl3 under super-
acidic conditions and high pressure in cyclohexane to yield iso-
indoline,[29,30] which was subsequently chloroformylated under
modified Vilsmeier–Haack conditions,[31] yielding compound 5.
Final treatment with POCl3 under basic conditions accompa-
nied by addition of kryptopyrrole and after complexation with
BF3·OEt2 yielded BODIPY 6. Model compound Ph-bdp was ob-
Figure 1. Concept of photomodulation of fluorescence in p-conjugated
DAE-BODIPY dyad 1: Chemical structures and photochromism and associat-
ed representation of the envisioned bathochromically shifted S0!S1 transi-
tion of the BODIPY fluorophore, in principle enabling dual fluorescence and
non-destructive readout.
including both stationary and transient spectroscopic charac-
terization. Our results are relevant for the design of molecular
systems that enable photocontrol of fluorescence without in-
volving energy transfer while at the same time providing
a deeper understanding of electronic coupling in photochro-
mic molecular dyads.[20,27]
Results and Discussion
Synthesis
The synthesis of target DAE-BODIPY dyad 1 (Scheme 1) was ac-
complished in a modular convergent fashion, coupling BODIPY
6 to DAE 4 in the final step following a sequence of desilyla-
tion, lithiation, stannylation, and Pd-catalyzed Stille alkynyla-
Scheme 1. Synthesis of DAE-BODIPY dyad 1 and reference compound Ph-bdp.
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tained by Stille alkynylation of 6 with tributyl(phenylethynyl)-
stannane. Further details of the synthesis of all compounds,
their purification, and all characterization data are collected in
the Supporting Information.
mined cycloreversion yield of Fc!o =0.02 is in the common
range for phenyl substituted DAEs.[7]
Inspection of the absorption spectrum of the DAE-BODIPY
dyad 1-o (Figure 2, top left) shows a broad intense band in the
UV region, which is ascribed to the p!p* singlet–singlet tran-
sition of the DAE moiety. Note that the shape is significantly
broadened in comparison to 4-o since in the same spectral
region the S0!S2 transition of the BODIPY fluorophore is locat-
ed. Furthermore, a comparison of the spectral characteristics of
1-o and 1-c in the UV region (Figure 2, bottom left) leads to
the conclusion that the band originally corresponding to the
p!p* transition in the parent DAE 4-o is bathochromically
shifted and centered at 378 nm. The same effect is observed
for the previously mentioned S0!S2 transition assigned to the
BODIPY, which is red-shifted from 322 nm in model compound
Ph-bdp (Supporting Information, Figure S1) to 333 nm in 1-o.
The same red-shift is observed for the main S0!S1 transition
of the BODIPY band centered at 624 nm in Ph-bdp and at
636 nm in 1-o. These spectral shifts are attributed to the elec-
tronic ground state coupling associated with an elongated p-
conjugated system in the dyad, established between the
BODIPY system and the DAE photoswitch in its open isomer.
Upon irradiation with 313 nm (Figure 2, bottom left), the in-
tense S0!S1 transition centered at 636 nm associated with the
BODIPY fluorophore markedly decreases accompanied by for-
mation of a strong red-shifted shoulder owing to the build-up
of an intense new band assigned to the BODIPY unit in the
ring-closed dyad 1-c. Furthermore, ring-closure is associated
with the growth of a less intense band in the 400–600 nm
range as well as the decrease of the p!p* transition at
378 nm assigned to the DAE unit.
UV/Vis spectroscopy
After their successful preparation, the photochromic behavior
of DAE-BODIPY dyad 1 and also the DAE moiety 4 was investi-
gated. The absorption spectrum of the open DAE isomer 4-o
in CH2Cl2 (Figure 2, top right) exhibits a broad intense band in
The extent of photoconversion was measured by ultrahigh-
performance liquid chromatography coupled to mass spec-
trometry detection (UPLC-MS) and provided the isomer ratios
1-o:1-c in the PSS as 74:26 in acetonitrile and 56:44 in CH2Cl2,
respectively. The quantum yield for the cyclization induced at
313 nm irradiation was determined as Fo!c =0.004, which is
common for fluorophore–photoswitch molecular dyads, since
a large spectral overlap in the UV region imposes a low effi-
ciency to address the electronic transition responsible for pho-
tochemical ring-closure.[24] Upon visible-light irradiation at
546 nm, the cycloreversion was initiated with the concomitant
raise of the 378 nm band and decrease of the broad visible
Figure 2. UV/Vis absorption spectra and photoinduced ring-closure of DAE-
BODIPY dyad 1 (left) and DAE unit 4 (right). Top: absorption spectra of the
open isomer (c), the PSS (a), and the calculated pure closed isomer
(g) in CH2Cl2 at 258C are shown. Bottom: evolution of the absorption
spectra during the course of ring-closure induced by irradiation with UV
light (lirr =313 nm) in CH2Cl2 at 258C.
the UV region, which is attributed to the p!p* singlet–singlet
transition. Upon irradiation with 313 nm UV light, the growth
of an intense band within the 460-650 nm range and another
yet less intense broad band between 370–460 nm was ob-
served (Figure 2, bottom right). The buildup of the long wave-
length band indicating formation of the closed isomer 4-c was
accompanied by a simultaneous decrease of the intense-band
UV absorption centered at 318 nm. The ratio of the open and
closed isomers in the photostationary state (PSS) 4-o:4-c of
DAE 4 was estimated as 15:85 based on the analysis of the
1H NMR spectrum of the PSS mixture. The isomerization quan-
tum yield for the cyclization at 313 nm was determined as Fo!
c =0.63. This value is relatively high and is most likely due to
a preferential population of the photoactive anti-parallel con-
formation in solution. Upon irradiation of the closed isomer
(PSS mixture) with visible light at 546 nm, cycloreversion was
initiated with the concomitant rise of the 318 nm absorption
maximum and decrease of the broad visible bands. The deter-
bands. The determined cycloreversion quantum yield Fc!o
=
0.004 is relatively high for a DAE having such an elongated p-
system. Although the PSS attained by irradiation of 1-o with
313 nm is rather low, an emerging new band can be distin-
guished, which is significantly red-shifted to the fluorophore
absorption maximum (636 nm). Performing a singular value de-
composition (SVD) analysis, it was possible to obtain the simu-
lated absorption spectrum of pure 1-c, which is in good agree-
ment with the spectra obtained from the UPLC-traces and the
ones calculated by differential subtraction (Supporting Infor-
mation, Figure S3). Importantly, the absorption spectra of nei-
ther the open nor the closed isomer of the dyad are simply
the sum of the spectra of reference BODIPY Ph-bdp and DAE
4, indicating an electronic coupling between both individual
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components. Importantly, the appearance of a new red-shifted
absorption band upon photoisomerization validates the effect
of increased electronic conjugation in the dyad upon cycliza-
tion.
ing to the actual amount of closed isomer (1-c) in the PSS mix-
ture (Supporting Information, Figure S2). Importantly, upon cy-
cloreversion with visible light the fluorescence emission could
be completely restored, enabling several switching cycles to
be performed without noticeable fatigue (Figure 3, right).
Fluorescence spectroscopy
Transient absorption spectroscopy
To investigate the change of electronic communication
beyond the ground state, stationary fluorescence spectra of
the dyad (Figure 3) and also the reference BODIPY Ph-bdp
(Supporting Information, Figure S1) were also obtained and
the key emission parameters determined (Table 1). As observed
To elucidate the origin of the fluorescence quenching in the
closed form, ultrafast transient absorption (TA) spectroscopy
was performed. Excitation of the PSS mixture of the reference
DAE (4-PSS) at 600 nm (black circles in Figure 4, bottom)
shows a fast relaxation (ca. 4 ps) to the ground state as known
for other DAE derivatives[34,35] (Supporting Information, Fig-
ure S6). In the case of the dyad 1-o (gray squares in Figure 4,
bottom) the BODIPY moiety is excited at 600 nm resulting in
a very long-lived excited state with a life time of several nano-
seconds (Figure 4, top). The signal consists of an excited state
absorption (ESA) between 450 and 600 nm and a combination
of ground state bleach (GSB) and stimulated emission (SE) at
Figure 3. Left: Corrected normalized fluorescence emission spectra of 1-o
(c) and in the PSS (a) obtained in CH2Cl2 (lexc =600 nm, cꢀ10À6 m).
Right: Relative fluorescence emission intensity upon alternating 313 nm
(gray bar) and 690 nm irradiations (white bar) to carry out successive switch-
ing cycles of 1.
Table 1. Molar absorptivities, absorption maxima, emission maxima,
Stokes shifts, and relative fluorescence quantum yields of 1-o and Ph-
bdp in CH2Cl2.
Compound
e
lmax
lem
Stokes shift
[cmÀ1
Ff
[104 LmolÀ1 cmÀ1
]
[nm]
[nm]
]
1-o
Ph-bdp
10.8
9.5
636
624
644
631
195.3
177.8
0.63
0.66
for the absorption spectra, the maximum of the emission of
1-o is bathochromically shifted by 13 nm when compared to
Ph-bdp. This red-shift can be explained with the increased
length of the p-system in the dyad as compared to the parent
BODIPY, in agreement with an electronic coupling between
both moieties in the open form of the dyad. The relative fluo-
rescence quantum yield determined for Ph-bdp was Ff =0.66,
which is in line with related BODIPY fluorophores.[32] Interest-
ingly, the relative fluorescence quantum yield of 1-o was
slightly lower (Ff =0.63), which may be attributed to a fast in-
tramolecular charge transfer process (ICT) from the electron-
rich thiophene to the BODIPY acceptor similar to those previ-
ously described previously.[33] On the contrary, the emission in-
tensity of the PSS mixture (1-PSS), obtained after irradiation of
1-o with UV light, was significantly reduced (Figure 3, left). The
associated degree of fluorescence quenching was determined
to be of 44% in CH2Cl2 and of 26% in acetonitrile, correspond-
Figure 4. TA data of 1-o (top) and single transient traces (bottom) of GSB
and ESA of 1-o (lprobe =642 nm, 503 nm, respectively) and 4-PSS
(lprobe =572 nm (with background subtracted), 465 nm, respectively) excited
&
*
at 600 nm (gray , black , respectively), and 1-PSS (lprobe =673 nm,
~
489 nm, respectively) excited at 685 nm (red ).
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l>600 nm. In the PSS mixture of the dyad (1-PSS), also the
closed isomer (1-c) is excited at this wavelength owing to the
overlap of the absorption bands. Therefore, this initially record-
ed data set contained the dynamics of both open and closed
isomers. For this reason, measurements were performed using
lexc =685 nm to allow for rather selective excitation of the red-
shifted absorption band of the closed isomer (1-c, red triangles
in Figure 4, bottom).
of excited open isomer. Therefore, the presence of excited 1-o
in the PSS mixture upon excitation at lexc =685 nm is attribut-
ed to ring-opening of excited 1-c rather than direct excitation
of 1-o.
In a global lifetime analysis,[36] four time constants were nec-
essary to describe the obtained data satisfactorily. The first
decay with t1 =180 fs was required to describe a fast process
immediately following photoexcitation. This process lies within
the time resolution of the experiment (ca. 100 fs) and essen-
tially describes an ultrafast relaxation in the excited state. The
time constant t4, which is larger than the time range of the ex-
periment, was fixed to a value of 2.6 ns. This value was ob-
tained from the measurements of the open isomer excited at
600 nm and describes the decay of the small amount of open
isomer within the PSS. Spectra at fixed delay times and the
DAS of these two time constants look very similar for both
data sets (Figure 5, bottom; Supporting Information, Fig-
ure S4), an observation that consolidates this assignment. The
decay of the main signal is associated with t2 =2.6 ps. It con-
tributes with large negative amplitudes in the range of the
broad GSB signal. The spectral characteristics of this DAS
(Figure 5, bottom) fit well with the (negative) steady state ab-
sorption spectrum of the pure closed isomer (Figure 2, top
left). Therefore, this time constant is assigned to the repopula-
tion of the ground state of the closed dyad (1-c). Finally, the
DAS at t3 =19 ps with its derivative-like shape can be attribut-
ed to a cooling process of the closed isomer after relaxation to
the electronic ground state. This behavior is already known for
other dithienylethene-BODIPY conjugates[34] as well as for indi-
vidual DAEs.[35]
In these TA measurements on 1-PSS, a much faster decay of
the excited state is observed (Figure 5, top). The ESA (lprobe
=
450–570 nm) is narrower than in the open isomer and the GSB
signal is broader, now ranging from 580 nm to 700 nm. The
main decay associated with 1-c in the PSS mixture occurs
within the first 10 ps (see transient traces in Figure 5, bottom).
Upon increasing the delay times, the decay associated spectra
(DAS) shift to shorter wavelengths and show the same spectral
characteristics as the excited open isomer in the lexc =600 nm
measurement (Supporting Information, Figure S4, right). Note
that TA measurements with lexc =685 nm on pure 1-o (Sup-
porting Information, Figure S7) show only a very small amount
All of these observations lead to the conclusion that in the
closed dyad (1-c) the BODIPY and the DAE are electronically
coupled. Owing to this coupling, they behave as one chromo-
phore with relaxation dynamics comparable to the ones of the
pure DAE (see Figure 4, bottom). The fast non-radiative relaxa-
tion proceeds through at least one conical intersection as
widely discussed for various DAE derivatives.[37] Importantly, an
energy transfer from BODIPY to DAE as another alternative re-
laxation pathway can be ruled out owing to the absence of
any signals of excited pure DAE. Upon irradiation at lexc
=
685 nm, predominantly 1-c could be excited. Once 1-c is in its
excited state, the excitation energy is employed either in the
non-radiative deactivation to the ground state or in the photo-
isomerization to 1-o. This process takes place without any fluo-
rescence emission. Indeed, steady-state experiments using irra-
diation of the PSS mixture at 690 nm demonstrated the gener-
ation of the open similarly to direct excitation of the DAE-band
centered at 546 nm. The associated quantum yield for ring-
opening following 690 nm irradiation (Fc!o =0.004) equals
that when performing the irradiation at 546 nm (or 565 nm) as
expected by Kasha’s rule, which predicts ring-opening to occur
from the lowest excited state.[38] This finding seemingly contra-
dicts a recent report of a wavelength dependency for the cy-
cloreversion quantum yield of two other DAE derivatives,
which was explained by a thermal barrier in the photoreactive
excited state.[39] However, in our case, merging of the DAE and
the BODIPY fragments creates one new chromophore with an
Figure 5. TA data of 1-PSS (top) excited at lexc =685 nm and associated DAS
of 1-PSS (bottom).
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Conclusion
In summary, we have prepared a new covalent bichromophoric
construct, dyad 1, composed of a far-red emitting BODIPY fluo-
rophore directly p-conjugated to a DAE photoswitch. In the
open isomer (1-o) both individual subunits retain their intrinsic
functions, that is, fluorescence emission and photoisomeriza-
tion, as shown by comparison with related reference com-
pounds. However, in the closed form (1-c) strong electronic
coupling is established and results in one new chromophore
system, which combines the features of both individual sub-
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spectral data show that upon excitation of the closed isomer
(1-c) in the red (lexc =685 nm) the typical BODIPY fluorescence
is lost and instead DAE cycloreversion is taking place. Hence,
our system allows on the one hand for modulation of BODIPY
fluorescence and on the other hand represents a DAE, in
which ring opening can be triggered by visible light in the far-
red with reasonable efficiency. Current efforts in our laborato-
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allow for quantitative switching and enhanced fluorescence
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Acknowledgements
We thank Dr. Lars Dworak and Dr. Markus Braun for insightful
discussions. Generous support by the German Research Foun-
dation (DFG via SFB 658, project B8 as well as grants WA 1850/
4-1 and WA 1850/6-1) and the European Research Council (ERC
via ERC-2012-STG 308117 “Light4Function”) is gratefully ac-
knowledged. BASF AG, Bayer Industry Services, and Sasol Ger-
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Published online on December 15, 2015
Chem. Eur. J. 2016, 22, 1070 – 1075
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