Between Aromatic and Quinoid Structure: A Symmetrical UV to Vis/NIR Benzothiadiazole Redox Switch

Article abstract of DOI:10.1002/chem.202004009

Reversibly switching the light absorption of organic molecules by redox processes is of interest for applications in sensors, light harvesting, smart materials, and medical diagnostics. This work presents a symmetrical benzothiadiazole (BTD) derivative with a high fluorescence quantum yield in solution and in the crystalline state and shows by spectroelectrochemical analysis that reversible switching of UV absorption in the neutral state, to broadband Vis/NIR absorption in the 1st oxidized state, to sharp band Vis absorption in the 2nd oxidized state, is possible. For the one-electron oxidized species, formation of a delocalized radical is confirmed by electron paramagnetic resonance spectroelectrochemistry. Furthermore, our results reveal an increasing quinoidal distortion upon the 1st and 2nd oxidation, which can be used as the leitmotif for the development of BTD based redox switches.

Full text of DOI:10.1002/chem.202004009

Accepted Article
Accepted Article
Title: Between Aromatic and Quinoid Structure: A symmetric UV to Vis/
NIR Benzothiadiazole Redox-Switch
Authors: Philipp Rietsch, Sebastian Sobottka, Katrin Hoffmann, Alexey
Popov, Pascal Hildebrandt, Biprajit Sarkar, Ute Resch-
Genger, and Siegfried Eigler
This manuscript has been accepted after peer review and appears as an
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using the Digital Object Identifier (DOI) given below. The VoR will be
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202004009
Link to VoR: https://doi.org/10.1002/chem.202004009
01/2020

10.1002/chem.202004009
Chemistry - A European Journal
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Between Aromatic and Quinoid Structure: A symmetric UV to
Vis/NIR Benzothiadiazole Redox-Switch
Philipp Rietsch,[a],[†] Sebastian Sobottka,[b],[†] Katrin Hoffmann,[c] Alexey Popov,[d] Pascal Hildebrandt,[a]
Biprajit Sarkar,[b,e] Ute Resch-Genger,[c]* and Siegfried Eigler[a]*
[a]
M. Sc. Philipp Rietsch, B. Sc. Pascal Hildebrandt, Prof. Dr. Siegfried Eigler
Institute of Chemistry and Biochemistry
Freie Universität Berlin
Takustraße 3, 14195 Berlin, Germany
E-mail: siegfried.eigler@fu-berlin.de
Dr. Sebastian Sobottka, Prof. Dr. Biprajit Sarkar
Institute of Chemistry and Biochemistry
Freie Universität Berlin
Fabeckstraße 34-36, 14195 Berlin, Germany
Dr. Katrin Hoffmann, Dr. Ute Resch-Genger
Department 1, Division Biophotonics
Bundesanstalt für Materialforschung und -prüfung (BAM)
Richard Willstätter Straße 11, 12489 Berlin
E-mail: ute.resch@bam.de
[b]
[c]
[d]
[e]
Dr. Alexey Popov
Leibniz Institute for Solid State and Materials Research,
Helmholtzstraße 20, 01069 Dresden
Prof. Dr. Biprajit Sarkar
Chair of Inorganic Coordination Chemistry
Institute of Inorganic Chemistry
University of Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart, Germany
Co-First authors contributed equally to this manuscript.
[†]
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: Reversibly switching the light absorption of organic
and most of the known optical redox switches consist of complex
molecules by redox processes is of interest for applications in sensors, molecular structures, which employ tetrathiafulvalenes or quinone
light harvesting, smart materials, and medical diagnostics. Here, we
present a symmetric benzothiadiazole (BTD) derivative with a high
fluorescence quantum yield in solution and in the crystalline state and
show by spectroelectrochemical analysis that reversible switching of
UV-absorption in the neutral state, to broad-band Vis/NIR absorption
in the 1st oxidized state, to sharp band Vis absorption in the 2nd
oxidized state, is possible. For the one-electron oxidized species,
motifs as redox systems.[3] Bard et al. investigated the redox
behaviour of symmetric dithienylbenzothiadiazoles (BTD) and
focused on the electrochemiluminescence (ECL) in solution and
solid films.[4] Recently, Liang et al. presented a symmetric
triphenylamine substituted BTD, with one reversible oxidation and
reduction process, strong ECL, and used it as a sensor for
dopamine.[5] However, BTDs have not been considered for
absorption or colour switching through redox processes. However,
as we recently demonstrated switching the absorption between
UV and Vis/NIR by redox processes seems to be feasible with
small molecule BTD derivatives.[6] These derivatives (Fig. 1A)
however, lacked the desired reversibility of the redox processes.
Herein, we investigate the fundamental properties of a symmetric
benzothiadiazole derivative by a combination of spectroscopy,
electrochemistry, spectroelectrochemistry and theoretical
calculations. Spectroelectrochemical measurements (CV-
UV/Vis/NIR) of this highly emissive molecule, that has a
fluorescence quantum yield of up to 93%, show that its absorption
can be switched between the UV (neutral form), to multi band
Vis/NIR absorption (1st Ox.), to a sharp single band Vis absorption
(2nd Ox.) and back to UV. Electron paramagnetic resonance
spectroelectrochemical measurements (EPR-SEC) reveal the
formation of a delocalized radical after the first oxidation and DFT
calculations indicate an increasing quinoidal distortion upon the
first and second oxidation, which was not considered before.
formation of
a delocalized radical is confirmed by electron
paramagnetic resonance spectroelectrochemistry. Furthermore, our
results reveal an increasing quinoidal distortion upon the 1st and 2nd
oxidation, which can be used as the leitmotif for the development of
BTD based redox switches.
Changing the color (or opacity) of a material by the application of
voltage is known as electrochromism. This effect is commonly
used to tint rear-view mirrors in cars or to control heat transfer
through surfaces (smart windows). To date, most of the employed
electrochomic materials are metal oxides, e.g. WO3, with only
some organic exceptions, mainly viologens.[1]
Reversibly switching the absorbance and/or the fluorescence by
redox processes bears a high potential for future applications in
light harvesting, sensors, and medical diagnostics. Small
molecular compounds have been in the focus of research as
functional dyes, probes and sensors for neutral and ionic analytes
and as active components in organic light emitting diodes
(OLEDs), optical devices, and lasers.[2] However, only few of
these investigations employ spectroelectrochemical methods,
1
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spectra in the solid state (Fig. 2B, orange curve) resembles the
emission spectrum in n-hexane.
The Φfl of 1 is maximal in DCM and DMSO with values of 93%,
and minimal in EtOH with a value of 64%. We proved that the
compound follows the Lambert-Beer law, even in EtOH, where it
is least soluble by two dilution series, one in DCM and one in
EtOH (Figure S20). This reduction in Φfl is therefore ascribed to
hydrogen bonding interactions with the protic solvent and the BTD
core and the ICT character of the dye´s emission in this solvent,
as it was shown before.[6, 9b, 10] The fluorescence lifetimes (τ) of
about 9 ns in n-hexane and up to 12 ns in DCM are relatively long
compared to other BTD, which possess lifetimes in the range of
2-10 ns.[6]
Figure 1: Spectroelectrochemical investigations of A) recent work: Only partly
reversible, reductive redox switching between UV and Vis/NIR absorption
realized with benzothiadiazoles featuring an alkyne moiety. B) This work: shows
reversible redox switching between UV, Vis/NIR and Vis absorption with two
fully reversible oxidative processes.
Results and Discussion
We started the synthesis of the symmetrical di-methoxyphenyl
BTD 1 from o-phenylenediamine following a published procedure
(Fig. 2A).[7] The Suzuki reaction of 4,7-dibromo-2,1,3-
benzothiadiazole with 2.2 eq. of (4-methoxyphenyl)boronic acid
and 0.1 eq. of Pd(PPh3)4 afforded 1 in a yield of 91%.
Figure 2: A) Synthetic route to symmetrical benzothiadiazole 1 starting from
ortho-phenylenediamine. i) SOCl2, DCM, 5h, reflux, yield = 93%. ii) HBr, Br2,
yield = 76%. iii) THF/H2O, 2.2 eq. of (4-methoxyphenyl)boronic acid, K2CO3,
Pd(PPh3)4, yield = 91%. The fluorescence quantum yield (Φfl) and fluorescence
lifetime (τ) in DCM is given. B) Absorption spectra (solid) and normalized
fluorescence spectra in solution (dotted, exc. at 400 nm, n-hexane = black, DCM
= green, DMSO = blue, EtOH = red) and the solid state (orange, exc. at 355 nm)
of 1. The inset shows the crystalline sample under irradiation with light of 366
nm. The dye concentration was 10-5M.
Subsequently, we performed a photophysical characterization of
BTD 1 in four solvents of varying polarity and proticity, namely n-
hexane (apolar and aprotic), dichloromethane (DCM; medium
polarity, aprotic), dimethylsulfoxide (DMSO; polar and aprotic),
and ethanol (EtOH; polar and protic) as well as in the solid state.
The absorption and normalized fluorescence spectra of 1 are
shown in Fig. 2B. In DCM, the molar extinction coefficient (ε) of
the π-π* transition at 280 nm is 20,161 M-1 cm-1 and about 8,611
M-1 cm-1 for the HOMO-LUMO transition, very likely the S0-S1, at
The push-pull BTDs, which we investigated in another study[6]
showed an 50% increase of Φfl from polar, protic EtOH to apolar,
protic 1-decanol (1-dec). In contrast, the Φfl of the symmetric
BTD 1 increases by only about 9% from EtOH to 1-dec (Table S4).
As demonstrated by our measurements in different mixtures of
EtOH and polyethylene glycol (PEG), which have a constant
polarity and an increasing viscosity, the increase of Φfl in primary
alcohols can be assigned to a change in viscosity (1-dec is about
12 times more viscous than EtOH (Table S2)). This difference
between symmetric and unsymmetric push-pull BTDs goes hand
400 nm.[8] The fluorescence spectra show
a
positive
solvatochromism, indicated by the arrows in Fig. 2B, of about 82
nm (n-hexane: 473 nm vs. DMSO: 555 nm). That is characteristic
for BTDs as well as compounds with intramolecular charge-
transfer (ICT) character, such as push-pull coumarins, and
indicates an increasing dipole upon excitation.[9] The fluorescence
2
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in hand with differences in their ground state and excited state
dipole moments, and the resulting charge separation and charge
distribution in the respective intramolecular charge-transfer (ICT)
and the locally excited (LE) states. The population of the ICT state
is preferred in polar solvents for structures, which enable stronger
charge separation, e.g. push-pull systems.[9b, 10-11] Those rational
is in agreement with our finding, that the change of dipole moment
upon excitation, derived from the Lippert-Mataga plot, of 1 is
12.4 Debye and thus smaller compared to our push-pull
derivatives with values up to 18.0 Debye (Fig. S6).
Table 1: Molar extinction coefficients (ε) of the two absorption maxima (see
Figure 2B), fluorescence quantum yields, determined absolutely with an
integrating sphere setup (see SI), and fluorescence lifetime of compound 1 in
solvents of different polarity and proticity. The values for the normalized
Dimroth-Reichardt Parameter ETN were taken from Ref. [9a]
N
ET
Solvent
ε ( M-1 cm-1)
Փ [%]
τ [ns]
0.006
0.309
0.444
0.654
n-hexane
DCM
5,521 / 2,166
20,161 / 8,611
3,090 / 3,511
6,669 / 2,557
--
83
93
93
64
66
8.92
11.97
11.74
10.06
--
DMSO
EtOH
Solid
Electrochemical characterization
We performed cyclic voltammetry (CV) as well as
spectroelectrochemical (UV/Vis/NIR and electron paramagnetic
resonance (EPR)) measurements of 1 in a 0.1 M Bu4NPF6
solution of DCM. In the cyclic voltammogram, the compound
shows one reversible reduction process at -1.96 V and two
electrochemically reversible one-electron oxidation processes at
+0.87 V and +1.09 V (Fig. 3A and Table S5), in agreement with
earlier investigations of a symmetric dithienyl BTD derivative.[4]
Figure 3: A) Cyclic voltammogram of 1 measured at 100 mV/s in 0.1 M Bu4NPF6
solution of DCM in oxidative direction first (purple) and reductive direction first
(black). B) Proposed semiquinone and quinone structures of 1+ and 12+, which
are supported by DFT calculations at D3-BP86/def2-TZVP level of theory.
The push-pull BTDs recently investigated by us bear all an alkyne
moiety and undergo irreversible redox processes.[6] However,
BTD with push-pull systems have been published, which show
reversible redox processes.[12] We therefore conclude, that the
alkyne moiety induces a follow-up reaction leadings to this
irreversibility, which is not favourable for the design of optical
redox switches. The reversibility of compound 1 supports this
assumption. Furthermore, and in contrast to the push-pull BTDs,
the redox responses of 1 are independent of the scan direction,
and the reduction and 2nd oxidation are incoherent (Fig. S8A).
This is in accordance with other electrochemical investigations of
BTDs.[13] We employed UV/Vis/NIR-spectroelectrochemistry
(SEC), coupling cyclic voltammetry with UV/Vis/NIR spectroscopy,
to probe the chemical reversibility of the redox processes and the
follow-up reactions of 1, 1+, and 12+. This was done using OTTLE
(optically transparent thin layer electrochemical) cells filled with a
defined starting concentration of 1 of about 1x10-4M in DCM.[14]
We furthermore undertook TD-DFT calculations at the
B3LYP/def2-TZVP level to rationalize the electronic structure of
the redox states of 1 (Fig. S13-S17 and Table S8-S15).
3
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moieties and to a charge transfer from the BTD π-system to the
adjacent phenyl rings (Fig. 5A), which are electron-deficient after
double oxidation.
In recent studies, electrochemiluminescence (ECL) was observed
for symmetric BTDs in thin films.[4b, 5] Therefore, we measured the
fluorescence intensity during the oxidation. The strong emission
of the neutral compound (excitation at 405 nm) is reduced upon
oxidation and restored upon re-reduction (Fig. S18). However,
excitation at the absorption maximum of 12+ (638 nm), yields no
fluorescence signal at all (Fig. S19). We therefore conclude, that
fluorescence ON/OFF redox switching at an excitation
wavelength of 405 nm is possible.
Figure 4: A) Absorption spectra of 1 before (red) and after the first oxidation
(blue). The proposed structure of NIR absorbing 1+ species is shown in the inset
in blue. B) EPR spectra of 1 measured after oxidation (black) and simulated
(red). Inset: Spin density of 1+ calculated at the PBE0/IGLO-III level.
The initial visible band of 1 at 408 nm is attributed to a π-π*
transition from the π-systems of the electron-rich methoxy-
phenlyene moieties to the central electron-poor BTD π-system.
The spectrum is well reproduced, however slightly red-shifted as
expected for this level of theory (Fig. S13). The absorption
spectrum of 1+ shows four distinct bands in the visible and NIR-
region, at 531, 666, 832 and 1124 nm (Fig. 4A) with molar
extinction coefficients of 10,000- 20,000 M-1 cm-1. Similar spectra
with comparable bandwiths are observed for related semiquinone
anion radicals, which are isolobal to diamino radical cations and
the here investigated BTD derivative, and where interpreted as
the vibrational fine structure of the radical compound.[15] In fact,
the results of the geometrical optimization (D3-BP86/def2-TZVP)
for 1, 1+ and 12+ predict an increasing quinoidal distortion of the
aromatic system upon oxidation, in analogy to its isolobal
congeners (Fig. 3B). Whereas the absorption spectra of 12+
resembles the calculated one (Fig. S15), the calculated spectrum
of 1+ differs from the measured one (Fig S14 and Table S11).
Calculations which consider also the solvent molecules and
hence the solvation shell of 1+ showed no improvement (Fig
S16A). However, if the multiple transitions of 1+ are indeed
interpreted as the vibrational fine structure, then the calculated
transitions lie in between the two measured couples (531+666
and 832+1124 nm) respectively. The UV/Vis spectrum of 12+ is
dominated by an intense transition at 654 nm, which is mostly
assigned to π-π* transitions of the oxidized methoxyphenyl
Figure 5: A) Absorption spectra of 1+ (blue) and 12+ (green). The proposed
structures in a rectangle of the respective color are in the inset. B) Absorption
spectra of 1 in the neutral state (red), after 1st oxidation (blue), after the 2nd
oxidation (green) and after re-reduction (orange).
Interestingly, the reduction of 1 appears to be electrochemically
reversible, however the initial absorption spectrum is not obtained
after a complete reductive cycle in UV/Vis/NIR-SEC (Fig. S10).
Upon reduction, very broad Vis and NIR bands emerge without
distinguishable peaks (Fig. S10A), which do not change, even
after re-oxidation. This may indicate electro-polymerization
occurring on the electrode. Since the SEC experiments take place
on a considerably longer time scale (1-3 minutes) than the CV
experiments to ensure full conversion, the electro-generated
species have more time to participate in subsequent chemical
reactions.
The chemical stability and reversibility of the redox couples 1/1+
and 1+/12+ were determined to be 95% and verified by spectral
recovery in an anodic/cathodic scan reversal (Fig. 5B).
The radical cation 1+ was further investigated by EPR
spectroscopy. For this purpose, the radical was generated in situ
via electrolysis (Fig. 4B black curve). The radical shows an
isotropic signal at room temperature with a g-value of 2.0028 and
4
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a rich hyperfine structure. This suggests a strong delocalization of
the unpaired electron over the whole π-system, which is
reproduced by our DFT calculations (PBE0/IGLO-III) suggesting
a delocalized spin population (see Fig. 4B red curve). It was
possible to simulate the EPR spectrum by considering hyperfine
coupling to two 14N nuclei (I = 1) and to 10 1H nuclei (I = ½) with
different coupling constants (see SI for details).
Conclusion
We developed a reversible organic redox switch, which enables
absorption or color switching between the UV, Vis/NIR, and Vis,
as well as fluorescence ON/OFF switching. The EPR spectrum of
the 1st oxidized state unravelled a strong delocalization of the
unpaired electron over the whole π-system, which is reproduced
by our DFT calculations and is accompanied by increasing
quinoidal distortion, which can be used as the leitmotif for the
development of BTD-based redox switches. This symmetric
benzothiadiazole is easily accessible in three synthetic steps and
the compound shows a high fluorescence quantum yield of more
than 90% in solution and 66% in the solid state.
Acknowledgements
The High ‐ Performance Computing resources of the
Zentraleinrichtung für Datenverarbeitung (ZEDAT) of Freie
Universität Berlin are kindly acknowledged.
Keywords: spectroelectrochemistry • fluorescence • molecular
switch • benzothiadiazole • quinone
5
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Entry for the Table of Contents
We use the quinoidial distortion induced in benzothidiazoles upon oxidation as the leitmotif to design an absorption redox switch. By
spectroelectrochemical analysis, absorption, fluorescence and the localization of the radical are investigated during the redox
processes. The symmetric benzothiadiazole derivative can thus be used as a UV to Vis/NIR absorption switch or as a fluorescence
ON/OFF switch. With the deeper understanding of the quinodial distortion, it can furthermore be used as basis for the development of
other small organic redox switches.
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