Norbornadiene-bridged diarylethenes and their conversion into turn-off fluorescent photoswitches

Article abstract of DOI:10.1039/d0cc02210a

We describe the synthesis and characterization of novel diarylethene photoswitches that contain a norbornadiene bridge and operate as p-type positive photochromes. One of the double bonds of norbornadiene is furthermore utilized to attach a fluoresceine tetrazine by an iEDDA cascade reaction, thereby forming a turn-off mode fluorescent photoswitch. This journal is

Full text of DOI:10.1039/d0cc02210a

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Norbornadiene-bridged diarylethenes and their conversion into
turn-off fluorescent photoswitches
Simon M. Büllmann and Andres Jäschke
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We describe the synthesis and characterization of novel Soh et. al.8, but the reported synthesis of the DAE-fluorophore
diarylethene photoswitches that contain a norbornadiene bridge conjugates is challenging and time-demanding. Consequently,
and operate as p-type positive photochromes. One of the double there is a need for a simplified access to DAE-fluorophore
bonds of norbornadiene is furthermore utilized to attach a conjugates with strategically designed linkages. Herein we
fluoresceine tetrazine by an iEDDA cascade reaction, thereby report a simple and practical method for the preparation of
forming a turn-off mode fluorescent photoswitch.
DAE-fluorophore conjugates. Rather than using typical
cyclopentenyl bridge moieties we chose norbornadiene (NBD)
systems. NBD has two very interesting features:
Diarylethenes (DAEs) are commonly used photochromic
systems. They are widely used in materials science as high
density information storage devices1. In addition, DAEs have
found various applications in super-resolution microscopy2 and
imaging of biomolecules3, photopharmacology4, and
nanobiotechnology5. DAEs are p-type positive photochromes6.
A variant of particular interest in imaging and microscopy
applications are fluorogenic DAEs which switch between the
open and closed configuration and thereby turn on or off their
fluorescence. S-oxidized benzothiophene-DAEs are known to be
fluorescent in their closed form, while non-fluorescent in the
open form, thereby creating a turn-on mode fluorescent
photochromic system7.
However, compared to typical fluorescent dyes the
fluorescence intensity of these intrinsically fluorescent DAEs is
rather low, thereby limiting their potential applicability.
Therefore, attaching a high-performance fluorophore to a non-
fluorescent DAE may be a more viable strategy for obtaining
fluorogenic DAEs than the search for intrinsically fluorescent
ones. By selection of a suitable fluorophore, its fluorescence
should be reversibly switched between an on- and an off-mode
with the DAE acting as a conditional fluorescence quencher. In
such systems, the absorption of one isomer (open or closed
form) needs to overlap with the emission of the fluorophore for
efficient FRET quenching. For typical dithienylethenes, a well-
fitting fluorophore (in terms of the emission wavelength) is
fluorescein. Such an architecture has been demonstrated by
First, NBDs are photochromes themselves, as they isomerize
into a strained quadricyclane (QC) structure upon irradiation
with UV-light. Because of the high energy storage capacity of 96
kJ/mol, the NBD/QC couple is a promising candidate for
molecular solar thermal (MOST) devices9. The NBD-QC
interconversion can be described as t-type, negative
photochromism10: QC is metastable and thermally relaxes back
to the NBD structure (t-type), and the conjugation between the
residues observed in the NBD structure is interrupted in the QC
form (negative photochromism).
Second, NBD with its two olefinic double bonds has been
commonly used as dienophile in bioconjugation reactions, and
in particular in inverse electron-demand Diels Alder (iEDDA)
reactions where it allows the site-specific conjugation of
biomolecules to tetrazine derivatives in the presence of a
multitude of functional groups11. We therefore speculated that
the integration of an NBD bridge into a DAE photochrome
results in a structure that cannot only switch between the open
and closed DAE forms, but that also contains an olefinic double
bond that can serve as dienophile in iEDDA reactions for the
attachment of dye-tetrazine derivatives at a central position
(Fig. 1).
a.Institute of Pharmacy and Molecular Biotechnology, Heidelberg University, Im
Neuenheimer Feld 364, 69120 Heidelberg, Germany.
Fig.
1 Schematic illustration of the photochromism of a norbornadiene-bridged
†
Electronic supplementary information (ESI) available: Details of sample
diarylethene (NBD-DAE). The double bond for tetrazine conjugation is highlighted in
red.
preparation, determination of photophysical properties, analytical data and
calculation of the second order iEDDA rate constants. See DOI
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Table 1. Photophysical properties of NBD-DAEs (1a-c) and the rearranged photoswitch 11
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In addition to the potential iEDDA reactivity, such a design raises
the question which of the two photochromes will dominate the
photophysical properties of the NBD-DAE system, since they
possess markedly different types of photochromism (t-type vs.
p-type, negative vs. positive). To the best of our knowledge, the
incorporation of NBD as a bridging unit in a DAE system has not
been reported before and represents an interesting hybrid of
different photoactive units. Not only can important
photophysical insight be gained; such a system could be of
practical use for imaging applications with fluorescence
readout.
in acetronitrile. Quantum yields were determined with an updated quantum yield
DOI: 10.1039/D0CC02210A
determination setup by Megerle et. al. 14
To answer these questions, we developed a synthetic approach
to NBD-DAE photochromes (Fig. 2A). The synthesis of
symmetric and asymmetric photoswitches was initiated from
2,3-dibromonorbornadiene, which was synthesized from
commercial bicyclo[2.2.1]hepta-2,5-diene (2,5-norbornadiene)
according to a literature procedure12. Symmetric NBD-DAEs
1a,b were obtained in a single step via Suzuki-Miyaura reaction
with the corresponding 3-borylated thiophenes 8 a,b at room
temperature (Fig. 2B). The synthesis of asymmetric NBD-DAEs
required the substitution of a bromine by a chlorine atom at the
norbornadiene structure in order to achieve regioselectivity at
the 2 and 3 position in the subsequent Suzuki-Miyaura
Isomerization of the NBD-structure to the QC-structure, as
reported in the literature10, was not detected during the
photophysical experiments.
Instead, we identified a new absorption band in the visible
wavelength range, indicating the cyclization of the DAE-moiety
(Fig. 1, Fig. S1-S4).
To exclude the possibility that the norbornadiene isomerizes to
its valence isomer QC, we analysed the reaction mixture before
and after UV irradiation by 1H-NMR spectroscopy. If the
cyclization of the NBD-DAE system was the preferred reaction
pathway, we would expect the signal for the vinylic protons to
remain in the olefinic region (6-7 ppm) upon irradiation with
UV-light. This was indeed the case, and no additional signals in
the aliphatic region (3.8 ppm) were detected (Fig. S5).
Furthermore, the splitting of the NMR signal for the vinylic
proton into two distinct signals (Fig S5B) upon irradiation with
UV-light indicated the formation of two diastereomers. Thus,
irradiation resulted exclusively in the cyclization of the DAE
core, and no significant QC formation occurred. DAE cyclization
quantum yields were high, ranging from 35%-54%, while the
cycloreversion quantum yields were more than one order of
magnitude smaller, which is common for cyclopentene-bridged
DAEs9,15 (Table 1). The ratio of ring-opening and ring-closing
quantum yields at the irradiated absorption wavelength (300
nm) strongly favours ring-closure. Thus, in the photostationary
state (PSS) high yields of the closed-ring isomer (91%-98%) were
determined by HPLC (Fig. S6-S11). Upon irradiation with vis-
light, complete cycloreversion back to the initial isomer was
observed, since the open isomer does not absorb in the visible
wavelength range. However, good reversibility of the NBD-DAE
system was only measured for compound 1a (Fig. S12A).
Compounds 1b,c possess a reduced reversibility (Fig S12B-S14),
likely due to the electron-donating nature of the methoxy group
at the thiophene moiety, which is in agreement with
literature16. Electron-rich DAEs have been reported to undergo
a side reaction to yield an annulated isomer of the closed form
upon extensive irradiation with UV-light17. The closed-ring
isomer of all NBD-DAE systems was found to be thermally stable
at 20°C and 50°C for 1h (Fig. S15-S17). Thus, the linkage of two
thienyl moieties via NBD results in a fully functional p-type
positive photochrome that has all characteristics of a DAE. Next
we analysed whether the NBD bridge can indeed be used as
reactions. At room temperature, cross coupling of compound
3
with the 3-borylated thiophene only occurred at the more
reactive bromine position, while by increasing the temperature
to 70°C, the chlorine position was reactive, too. Using this
synthetic strategy, the selective addition of two different
thiophene moieties was achieved.
Fig. 2 (A) Synthesis of symmetric and asymmetric NBD-DAEs. (I) 1. tBuLi, 2. TsCl, THF; (II)
Pd2(dba)3, P(tBu)3, CsF, rt., THF; (III) Pd2(dba)3, P(tBu)3, CsF, 70°C, THF; (IV) Pd2(dba)3,
P(tBu)3, (8a,b), CsF, rt., THF. (B) Synthesis of the 3-borylated thiophenes. 1,3-dibromo-
4-methylthiophne was converted to compounds 8a,b in a series of lithiation, borylation
and Suzuki-Miyaura cross coupling reactions. (V) 1. n-BuLi, 2. B(OBu)3, Et2O; (VI)
Pd(dppf)Cl2, Na2CO3, Ar-I, H2O, THF; (VII) 1. n-BuLi, 2.B(OBu)3, Et2O.
The photophysical properties of the NBD-DAE systems (1a-c
)
are summarized in table 1. Absorption spectra of all
photochromic compounds upon irradiation with UV-light can be
found in the supporting information (Fig. S1-S4). These spectra
reveal that the synthesized NBD-DAEs possess classic properties
of p-type positive photochromes, i.e., the DAE dominates the
properties (Table 1).
2 | J. Name., 2012, 00, 1-3
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dienophile in one-step biorthogonal click reactions with almost 40 nm, relative to 1a (Table 1, Fig. S2B). A high switching
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fluorescein-labelled tetrazines. Since compound 1a had the yield (Fig. S9) was observed due to the hDigOhI:r1a0t.1io03o9/fDc0yCcCli0za22ti1o0nA
highest reversibility, it was selected for further investigations quantum yield and cycloreversion quantum yield determined
(Fig. 3). For those initial experiments, 1,4-di-(2-pyridyl)- upon UV irradiation (Tab. 1). After ten switching cycles, 39% of
tetrazine was used as diene, because of its fast reaction kinetics functional photochromic compound remained (Fig. S14A), the
in iEDDA reactions17.
Compound 1a has two alkene functionalities in the NBD moiety. cyclopentadiene bridge to rearrangement16 or oxidation15.
One of them is part of the DAE core and thereby sterically less Interestingly, the two click products (13 14) have markedly
decline likely being due to the susceptibility of the electron rich
,
accessible, while the other is unsubstituted and freely different photochromic properties. Product 14 that still has the
accessible for iEDDA. In fact, the click reaction was only bicyclic structure showed a high switching yield of 98% as well
detected at the less substituted alkene, which yields the as high reversibility of 81% after ten cycles (Fig. S10, S14B). Ring-
dihydropyridazine
9. However, HPLC, mass and NMR opening and ring-closing quantum yields of 14 at the irradiated
spectroscopy of the reaction products (Fig. S18-S25) indicated absorption wavelength (300 nm) strongly favour ring-closure
(ɸOC=0.46, ɸCO=0.06), while the cycloreversion quantum yield
(ɸCO=0.008 at 505 nm) is more than one order magnitude
smaller, which correlates with cyclopentene-bridged DAEs9,15
.
Product 13 that is formed the absence of PIDA still undergoes
cyclization, but significantly increased irradiation times are
required to reach the PSS. Furthermore, iEDDA reaction rates of
the NBD-DAE system (1a) and the CPD-DAE (11) were
determined according to a procedure described by Knall et al.17
(Fig. S26-S27). As anticipated, due to its higher ring strain, the
NBD-DAE reacted slightly faster than the CPD-DAE (Tab. 1).
Nevertheless, the CPD-DAE still exhibited an adequate reactivity
to undergo the second iEDDA reaction to form compound 13
.
Using the information about the reactivity of NBD-DEA 1a in
iEDDA reactions with tetrazines, we envisioned that the
reaction cascade in the absence of PIDA can be easily utilized to
fluorescently tag NBD-DAE 1a with a fluorescein labelled
tetrazine (Figure 4A).
Fig. 3 (A) Reaction scheme of the strain promoted iEDDA of 1a with 1,4-dimethyl-
tetrazine in methanol. (B) Reaction scheme of the strain promoted iEDDA with the
addition of PIDA ((diacetoxyiodo)benzene) in methanol.
The direct reaction pathway in the presence of PIDA (Fig. 3B)
was not suitable for this approach, due to the instability of
fluorescein-linked tetrazines in the presence of an oxidant.
that dihydropyridazine
9 never accumulates during the
reaction. Instead, a rearrangement takes place in which 3,6-
dimethylpyridazine is eliminated. In this rearrangement the
NBD converts to a cyclopentadiene (CPD) which further
rearranges to its most stable isomer 11 (Fig. 3A, Fig. S18). If
equimolar starting concentrations of diene and dienophile were
used, the CPD switch 11 could be isolated and characterized by
NMR spectroscopy (Fig. S24-S25). If an excess of diene was
used, compound 11 reacted as dienophile and underwent a
second iEDDA to form 12, followed by an oxidation (Fig 3A, Fig.
S20-S23). A similar reaction pathway for NBD in iEDDA reactions
with tetrazines was already described by Warrener et. al18.
Additionally, we found that the rearrangement that degrades
the NBD structure can be prevented by the addition of PIDA
((diacetoxyiodo)benzene)19. However, PIDA was found
unsuitable for iEDDA reactions with 1,4-(2-dipyridyl)-tetrazine,
as various side products were formed, probably due to the high
reactivity of this tetrazine derivative. Hence, we used the more
stable 1,4-dimethyltetrazine as diene in the direct iEDDA
reaction (Fi. 3B). With 1,4-dimethyltetrazine significantly less
side products were formed, and compound 14 could be
isolated. In the presence of PIDA, dihydropyridazine
9 was
Fig. 4 (A) Reaction scheme of the iEDDA-cascade reaction with an excess of the
fluorescein-modified tetrazine. (B) Illustration of the cyclization and cycloreversion
reaction of the fluorescently tagged DAE. (C) Left: Absorption spectrum of compound 15
upon irradiation with 310 nm LED at the defined time points. Right: Fluorescence
emission spectra of compound ref upon irradiation with 310 nm LED at the defined time
points.
oxidized before the rearrangement could take place, and was
converted to 14 (Fig. 3B). The CPD-DAE 11, as well as the two
click products 13 and 14 were isolated and characterized. Upon
UV-induced photocyclization, the absorption maximum in the
visible range of CPD-DAE 11 shows a bathochromic shift of
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Therefore, the indirect route (via the CPD intermediate) was Conflicts of interest
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DOI: 10.1039/D0CC02210A
There are no conflicts to declare.
chosen and NBD-DAE 1a was treated with an excess (3 eq.) of
the fluorescein-linked tetrazine. After 10 h reaction at rt, 1a was
fully consumed and the DAE-fluorophore conjugate 15 was
formed with > 95% conversion (Fig. 4B, Fig. S28). Figure 4C
illustrates the absorbance spectrum of 15 upon irradiation with
UV-light at various time points. A new absorption band was
formed which is slightly red-shifted relative to the fluorescein
absorption band at 460 nm, indicating the conversion of the
DAE core structure to its closed isomer. Next, the fluorescence
Acknowledgements
The authors thank Dr. M. Baalmann for advice on tetrazine synthesis,
H. Rudy for mass spectrometry, and Dr. K. Höfer (all Heidelberg
University) for comments on the manuscript. Furthermore, the
authors thank Henrieta Derondeau (LMU Munich) for her help in
calculating the quantum yields.
properties of 15 were investigated. As anticipated, high
Notes and references
fluorescence intensity was observed in the open form. As
illustrated in Fig. 4C, the fluorescence intensity decreased
gradually after the irradiation with the 310 nm UV-light, due to
an energy transfer of the fluorophore to the closed form of the
DAE core (Fig 4B). Furthermore, the initial fluorescence
intensity can be restored by irradiation the sample with visible
light. HPLC analysis of the fluorescently tagged DAE after
irradiation with UV-light revealed a switching yield of 60% to the
closed isomer (Fig. S11), in agreement with the observed
fluorescence quenching efficiency of 59% (Fig. 4C). Thus, the
quenching ratio of the fluorescence directly correlates with the
amount of closed isomer in the PSS, as previously shown for a
structurally different DAE by Soh et. al.8
In summary, we describe in this communication the synthesis
and application of photochromic compounds that contain a
NBD structural motif and act as p-type positive photochromes.
A set of four novel NBD-DAE systems has been synthesized and
studied, containing symmetric as well as asymmetric aromatic
residues at the periphery. All of them were identified as p-type
positive photochromes undergoing a reversible cyclization
reaction typical for DAE’s upon irradiation with UV-light or
visible-light respectively. Their photophysical properties,
including quantum yields, composition of the photostationary
state, reversibility and thermostability, were found to correlate
well with literature-known DAE’s. An isomerization reaction to
the QC valenz isomer, which is typical for photochromic
molecules consisting of a NBD structure, does not take place to
any significant extent, as confirmed by NMR spectroscopy.
Introducing electron rich residues at the periphery of the NBD-
DAE significantly decreased their reversibility, while other
photophysical properties (quantum yield, PSS and
thermostability) remained essentially unaltered. Furthermore,
we demonstrated the utilization of the alkene functionality of
the NBD structure in iEDDA reactions and fluorescently tagged
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DOI: 10.1039/D0CC02210A
A new class of reversible fluorogenic diarylethenes,
assembled by iEDDA chemistry, with high switching
efficiencies and quantum yields is described.