Oxidized Hemithioindigo Photoswitches—Influence of Oxidation State on (Photo)physical and Photochemical Properties

Article abstract of DOI:10.1002/chem.202002176

The photophysical and photochemical properties of sulfoxide and sulfone derivatives of hemithioindigo photoswitches are scrutinized and compared to the unoxidized parent chromophores. Oxidation results in significantly blue-shifted absorptions and mostly

Full text of DOI:10.1002/chem.202002176

Accepted Article
Accepted Article
Title: Oxidized Hemithioindigo Photoswitches - Influence of Oxidation
State on (Photo)physical and Photochemical Properties
Authors: Laura Köttner, Monika Schildhauer, Sandra Wiedbrauk, Peter
Mayer, and Henry Dube
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To be cited as: Chem. Eur. J. 10.1002/chem.202002176
Link to VoR: https://doi.org/10.1002/chem.202002176
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10.1002/chem.202002176
Chemistry - A European Journal
Oxidized Hemithioindigo Photoswitches - Influence of Oxidation
State on (Photo)physical and Photochemical Properties
Laura Köttner,†a Monika Schildhauer,†a Sandra Wiedbrauk,† Peter Mayer,† Henry Dube†§*
† Ludwig-Maximilians-Universität München, Department of Chemistry and Center for Integrated
Protein Science CIPSM, Butenandtstr. 5-13, 81377 München
§Current address: Friedrich-Alexander-Universität Erlangen-Nürnberg, Department of Chemistry and
Pharmacy, Nikolaus-Fiebiger-Str. 10, 91058 Erlangen
* E-mail: henry.dube@fau.de
a both authors contributed equally to the work
Abstract
The photophysical and photochemical properties of sulfoxide and sulfone derivatives of
hemithioindigo photoswitches are scrutinized and compared to the unoxidized parent
chromophores. Oxidation results in significantly blue-shifted absorptions and mostly reduction of
photochromism while thermal stabilities of individual isomers remain largely unaltered. Effective
photoswitching takes place at shorter wavelengths compared to parent hemithioindigos and high
isomeric yields can be obtained reversibly in the respective photostationary states. Reversible solid
state photoswitching is observed for a twisted sulfone derivative accompanied by visible color
changes. These results establish oxidized hemithioindigo photoswitches as promising and versatile
tools for robust light-control of molecular behavior for a wide range of applications.
Introduction
The hemithioindigo (HTI)[1] structural motive belongs to the class of indigoid chromophores[2] and has
been known since 1906 when it was first synthesized by Paul Friedländer.[3] It took some time since its
discovery until HTI has been recognized to undergo light-induced photoisomerization, which was first
reported in 1961 by Mostoslavski and coworkers.[4] Since then HTI has been applied as photoswitch by
a couple of groups[5] including our own[6], and has been further developed into a more mature
photoswitching system primarily by the mechanistic works of Rück-Braun, Cordes, Zinth,[5f, 5h, 7] de
Vivie-Riedle,[8] Riedle,[9] and our own group[6a, 9-10]. Other research teams also have joined these efforts
in recent years.[11] In many studies the effects of various substitution patterns at different positions of
the chromophore structure on the photo/physical properties have been explored quantitatively, which
now enables conscious design of different property profiles of this class of photoswitches. At present it
is thus well established that HTI chromophores possess many advantages permitting highly robust and
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10.1002/chem.202002176
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efficient photoswitching within the visible part of the electromagnetic spectrum. When further exploring
the structural motive of HTI, our group used oxidation of the sulfur atom to the corresponding sulfoxide
as a straight-forward method to introduce chirality into HTI chromophores with fourfold[12] substituted
central double bonds. Such structures enabled us to develop a variety of different molecular motor types
that are responsive to visible light and perform unidirectional motions upon irradiation.[9, 13] Despite this
progress, the effects of sulfur oxidation on HTI photoswitches bearing three substituents at the central
photoisomerizable double bond have to the best of our knowledge not been explored so far. Some
photophysical data of mainly sulfone HTIs are described in the literature but no photoisomerization
reactions are scrutinized.[14] In this work we set out to fill in this gap and provide a survey of the effects
of sulfur oxidation on the photoswitching properties of HTI. To this end we compare five parent HTI
structures (1-5) with the corresponding sulfoxide (HTI-SO 1-5) and sulfone (HTI-SO2 1-5) derivatives
as shown in Figure 1a. The specific substitution variations on the stilbene fragment were chosen because
electronically neutral or donating substituents are generally beneficiary for HTI photoswitching.[6a, 10a]
Likewise twisted structures such as in HTIs 4 and 5 have also been reported by our group to elicit
interesting (photo)physical properties, especially with regard to high thermal bistability and switching
performance.[10c, 10f] In this context it was deemed most effective to consistently increase the push-pull
character across the central double bond and evaluate the effects in a systematic way. For these reasons
we increased acceptor strength of the thioindigo fragment by successive increase in the oxidation state
of the sulfur atom and paired it with donor substituents on the stilbene fragment.
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Figure 1
a) Molecular structures of parent HTIs 1-5 and oxidized derivatives HTI-SO 1-5 and HTI-SO2
1-5. b) Structures of HTI 4 and selected oxidized HTIs in their Z isomeric form in the crystalline
state. HTI-SO 1-4 crystallized as racemic mixtures, only the (S)-entiomeric forms are depicted
for clarity.
Results and Discussion
Synthesis
Synthesis of oxidized HTI derivatives 1-5 followed either of two different approaches, oxidation of
established HTI photoswitches using H2O2/AcOH or NaBO3·4 H2O/AcOH, or condensation of oxidized
benzothiophenones (sulfoxide 6 or sulfone 7) with the corresponding aldehydes 8 or 9 (Scheme 1).
Synthetic procedures are detailed in the Supporting Information together with comprehensive
characterization (see also Figures S14 – S25 for NMR spectra). Mixtures of the corresponding HTI-SO
and HTI-SO2 products were usually obtained under the first conditions, which could easily be separated
by conventional column chromatography i.e. in the synthesis of HTI-SO and HTI-SO2. Typically the
isolated yields of HTI-SOs were much lower than of the corresponding HTI-SO2s because of prolonged
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reaction times and an excess of H2O2 added (HTI-SO : HTI-SO2 = 5% : 58% for 2, 26% : 35% for 4).
However in case of 5 the opposite behavior was observed (HTI-SO : HTI-SO2 = 95% : 5%). HTI-SO 1
.
was obtained after oxidation of the corresponding HTI 1 with NaBO3 4 H2O in 20% yield. HTI-SO 3
was synthesized in 40% yield by condensation of 9-formyljulolidine (8) with known benzothiophenone-
sulfoxide (6).[15] HTI-SO2 derivatives 1 and 3 were obtained by condensation of the respective aldehydes
9 and 8 with commercially available benzothiophenone-sulfone (7) in 50% and 67% yield, respectively.
Crystals suitable for structural analysis were obtained for the Z isomers of HTI 4, HTI-SO 1 to 4, and
HTI-SO2 derivatives 1-5 as shown in Figure 1b (see also Tables S4 – S8 in the Supporting Information).
In the crystalline state it can be observed that the molecular structures of oxidized HTIs 1-3 are still
almost completely planar structures facilitating efficient conjugation between the central
photoisomerizable double bond and the stilbene fragments. This finding shows that oxidation at the
sulfur atom does not induce substantial sterical hindrance at this position to lead to clashes with adjacent
molecular fragments. However, the molecular structures of derivatives 4 and 5 are substantially distorted
primarily because of the increased sterical hindrance induced by their methoxy and methyl substituents.
The sterical effect is much more pronounced in the methyl-substituted derivatives of HTI 5.
.
NaBO3 4H2O,
AcOH,
23-40 °C
O
O
O
R
S
O
S
O
piperidine,
toluene,
100 °C
O
R
HTI-SO
H2O2 30%,
AcOH,
23 °C
S
+
R
O
O
O
R
S
piperidine,
toluene,
100 °C
S
O
O
O
O
R
HTI-SO2
Scheme 1
Different synthetic approaches to obtain oxidized HTI photoswitches 1-5.
Thermal Isomerizations
For many applications of photoswitches thermally stable switching states are required to ensure full
light-addressability of the system. Since metastable states are usually generated by at least one
photoreaction the kinetics of their spontaneous thermal decays are important parameters for
photoswitches. For HTIs the E isomers are typically metastable and slowly decay thermally back to the
Z isomers. We have measured the thermal decay of the E isomers for oxidized HTIs in toluene solution
and analyzed them with the Eyring equation to derive the corresponding free activation enthalpies DG‡
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(for details see Figures S1 – S3 and Tables S1 and S2 in the Supporting Information). All DG‡ values
are reported in Table 1. The free activation enthalpies for thermal double bond isomerizations range
from 21.4 kcal/mol (HTI 3) to >33.0 kcal/mol (HTI-SO and HTI-SO2 5), which is quite high compared
to many conventional photoswitching systems. A possible reason for the significantly higher barriers of
thermal isomerization of oxidized derivatives of HTI 5 may be rooted in a positive interaction between
the oxygen atoms on the sulfur atom and the hydrogen atoms of the neighboring ortho-methyl groups.
Indication for this additional stabilization are derived from the relatively short distance between the
oxygen atom and the corresponding methyl-carbon atom in the crystal structures. When comparing the
results with the unoxidized parent HTIs it can be seen that oxidation of the sulfur atom usually does not
lead to significant effects with respect to the thermal isomerization reactions and no linear trends are
observed for the changes seen. Free activation enthalpy differences DG‡ in the range of 1.4 to 4.5
kcal/mol are observed upon oxidation of HTIs. These effects are not very strong for thermal double
bond rotations rendering oxidized HTIs very bistable photoswitches as well. The only notable exception
to this behavior is HTI system 4 for which oxidation leads to a significant reduction in thermal stability
of up to 8.2 kcal/mol for HTI-SO 4. The resulting barrier of 23.3 kcal/mol is nevertheless still high
enough to warrant bistable photoswitching for most applications.
Photophysical and Photochemical Properties
For all chromophores reported here the molar absorptions were recorded in CH2Cl2 and/or toluene
solution for both the Z and respective E isomers (see Figures S4 – S6 in the Supporting Information).
As it can clearly be seen in Figure 2 oxidation of the sulfur atom leads to a noticeable hypsochromic
shift of the absorption. Interestingly the first oxidation to the sulfoxide HTI-SO derivatives induces the
strongest hypsochromic shift of the absorption. Further oxidation to the sulfones HTI-SO2 does not result
in further hypsochromic shift. Instead the absorption remains essentially the same as compared to the
corresponding sulfoxides. Apparently the electronic effects of sulfoxide or sulfone-oxidation states are
very similar in these chromophores. Hypsochromic shifting occurs for both isomers, but stronger so for
the E isomers in most cases. The result is an overall reduction in the photochromism of these compounds.
This observation can at least partly be explained by the more similar electron withdrawing character of
the carbonyl and oxidized sulfur functions connected to the central double bond. Because of this
similarity the electronic character of the molecule does not change very much when switching from the
Z to the E isomer. The notable exception to this trend is the series of HTIs 4 where photochromism is
actually improved slightly as shown in Figure 2d. Upon oxidation of the sulfur atom molar absorption
increases in comparison to the unoxidized parent HTIs in the series 1, 2, and 4 but decreases slightly for
the series 3. For series 5 the effects are more complicated with notable decreases of the Z isomer
absorptions while the E isomers absorptions actually increase significantly upon oxidation. In all cases
the typical two-band structure of the absorption observed for unoxidized HTIs develops into a mainly
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single-band structure for both HTI-SO and HTI-SO2 derivatives. With regard to solvent changes the
absorptions of oxidized HTIs are not particularly solvatochromic except for derivatives bearing strong
electron-donor groups, i.e. 3 (see Figure S7 and Table S3 in the Supporting Information). Absorption
maxima and molar absorptions are summarized in Table 1 for all compounds.
Figure 2
Comparison of molar absorptivities e for the Z and E isomers of HTIs and the corresponding
oxidized derivatives. Molar absorptivities were measured in CH2Cl2 solution for the series 2, 4, and 5 and in
toluene solution for 1 and 3. Spectra of HTIs are shown in grey, of sulfoxides HTI-SOs in red, and of sulfones
HTI-SO2s in blue, of Z isomers as solid and of E isomers as broken lines (see legend in sub-Figure a). a) Molar
absorptivities of HTI series 1. b) Molar absorptivities of HTI series 2. c) Molar absorptivities of HTI series 3. Note
that molar absorptivities of the E isomer of 3-SO2 could not be obtained due to inefficient photoisomerization and
strong signal overlapping of Z and E isomer in 1H NMR spectra. d) Molar absorptivities of HTI series 4. e) Molar
absorptivities of HTI series 5. f) Enlarged section of the molar absorptivities of HTI series 5.
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Because of the less pronounced photochromism of oxidized HTI derivatives their photoisomerization
reactions could be rendered less efficient in generating excess of a particular isomer. To test the
photoswitching behavior solutions with the respective oxidized HTI in either CH2Cl2 or toluene were
irradiated to the photostationary state (pss) at different wavelengths and the maximum attainable isomer
1
enrichment was quantified using H NMR spectroscopy (see Figures S8 – S13 in the Supporting
Information). The results are summarized in Table 1. As it turns out oxidized HTIs in general undergo
quite efficient photoisomerizations with E isomer enrichment reaching up to 77% and Z isomer
enrichment reaching >90% for the best performing derivatives as (shown exemplarily in Figure 3 for
HTI-SO 3 and 4 and HTI-SO2 2 and 4). For all oxidized derivatives - except in the series of 3 - light
with <400nm is most efficient for the Z to E photoisomerization direction but also blue light of 405 nm
is still effective in most cases. In general it can be seen that there is some decline in the achievable
isomer enrichment in the photoisomerization reactions if the sulfur atom is oxidized. HTI-SO derivatives
are most of the time the least efficient photoswitches within a particular series and thus again no linear
trend with increasing oxidation state is observed. In series 5 HTI-SO2 is more efficient than HTI-SO in
the Z to E photoisomerization most likely because of the significantly higher absorption of the E isomer
across the spectrum. The best performing sulfoxide photoswitches are HTI-SO 3 and 4 reaching 73%
and 75% E isomer enrichment as well as 100% and 76% Z isomer enrichment in the pss, respectively.
The best performing sulfone photoswitches are HTI-SO2 2 and 4 reaching 70% and 77% E isomer
enrichment as well as 92% and 83% Z isomer enrichment in the pss, respectively. Oxidized HTI
derivatives in the series 3 possessing strong electron donating groups lose photoswitching capability in
more polar solvents, which is indicative of a possible competing deexcitation pathway that involves
significant charge transfer in the excited state.[10b, 10c] For this reason photoisomerization reactions of
derivatives 3 were measured in the less polar toluene solvent exclusively.
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1
Figure 3
Photoswitching cycles of oxidized HTI derivatives in solution as followed by H NMR
spectroscopy starting from pure Z isomers. Signals of photogenerated E isomers are highlighted in red for HTI-
SOs and in blue for HTI-SO2s. a) Photoswitching of HTI-SO 3 with blue (470 nm) and green (530 nm) light in
toluene-d8 solution. b) Photoswitching of HTI-SO 4 with UV (365 nm) and blue (435 nm) light in CD2Cl2 solution.
c) Photoswitching of HTI-SO2 2 with UV (365 nm) and blue (435 nm) light in CD2Cl2 solution. d) Photoswitching
of HTI-SO2 4 with UV (365 nm) and blue (435 nm) light in CD2Cl2 solution.
It should be noted at this time that strong isomer enrichment is desirable for most photoswitching
applications but is not necessary for applications in molecular machine building, especially molecular
motors, where efficient photoswitching of both isomers is desirable at actually the same wavelength of
irradiation. Therefore it can be stated that oxidized HTIs possess high potential to serve as valuable
molecular motives for both (visible)-light induced photoswitching and molecular machine building.
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Table 1
Quantitative comparison of the physical and photophysical properties of the structurally related
HTIs, HTI-SOs, and HTI-SO2s 1-6. Data were measured in CH2Cl2 or CD2Cl2 solutions. Data
+
marked with * were measured in toluene or toluene-d8 solutions and half-lives marked with
represent linearly extrapolated approximations from high temperature measurements without
taking into account temperature effects on DG‡. The abbreviation “a” is used as SI-unit for years.
Half-life
l
max; e of Z isomers
l
max; e of E isomers
DG‡ (therm.
E/Z)*
/kcal mol-1
Isomer yield in the pss
/% (nominal LED nm)
of pure E
isomer at
25 °C
HTI
most redshifted abs. most redshifted abs.
/nm; L mol-1 cm-1
435; 15,600
347; 21,200
353; 29,700
442; 17,900
376; 29,400
383; 39,100
500; 46,000
492; 42,700
501; 39,200
442; 12,300
366; 15,400
368; 18,300
420; 5,900
/nm; L mol-1 cm-1
461; 9,900
81%/88%* E (420 nm)
100%/95%* Z (490 nm)
164 a+
22 a+
1
30.9
29.7
28.2
26.4
30.9
29.4
21.4
22.8
-
56%/52%* E (305 nm)
73%/73%* Z (385 nm)
SO-1
SO2-1
2
350; 20,000
361; 24,000
467; 13,700
385; 24,800
401; 31,700
532; 40,000
496; 38,600
502; 36,600
436; 6,200
62%/64%* E (305 nm)
90%/79%* Z (405 nm)
1.7 a+
3 d+
83% E (420 nm)
100% Z (530 nm)
56% E (365 nm)
93% Z (450 nm)
164 a+
13 a+
SO-2
SO2-2
3
70% E (365 nm)
92% Z (435 nm)
82%* E (490 nm)
100%* Z (617 nm)
9 min
1.6 h
73%* E (470 nm)
100%* Z (530 nm)
SO-3
SO2-3
4
-
-
-
93%/87%* E (365/420* nm)
17%/50%* Z (505/530* nm)
31.7
23.3
24.7
33.0
>33.0
>33.0
633 a+
3.8 h
75% E (365 nm)
76% Z (435 nm)
SO-4
SO2-4
5
356; 10,100
365; 11,200
425; 3,600
77% E (365 nm)
83% Z (435 nm)
1.7 d
85% E (405 nm)
94% Z (505 nm)
5,700 a+
>5,700 a+
>5,700 a+
64% E (365 nm)
95% Z (435 nm)
SO-5
SO2-5
352; 5,600
352; 6,400
54% E (365 nm)
95% Z (435 nm)
347; 7,000
399; 3,400
345; 3,000
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As it was mentioned already above photochromism is reduced when oxidizing the sulfur atom and as a
result changes in isomer composition are oftentimes not readily visible by the naked eye. The latter is
also true for many unoxidized HTI photoswitches. This situation is different for the oxidized HTI series
1 where only the absorptions of the E isomers are crossing the 400 nm threshold. As a result colorless
solutions of pure Z isomers turn yellow upon photoswitching for HTI-SO 1 and HTI-SO2 1 (see Figure
4 also for a comparison of the photochromism within the full series 1). A similar behavior is also
observed for HTI-SO2 5. However, for HTI-SO2 5 this easily visible color change is not confined to
solution but also occurs in the solid and crystalline state evidencing effective photoswitching of this
derivative in much more confined environments. To illustrate this, we have irradiated amorphic films
of the pure compound with alternating 365 nm and 420 nm light to reversibly affect color changes in
the solid state (Figure 4d). Such behavior is of great interest for optical materials applications e.g. for
information storage.
Figure 4
Color changes upon photoswitching of oxidized HTIs to the pss (Z isomers left, E isomer
enriched right). a) Photoswitching of unoxidized HTI 1 in CH2Cl2 solution. b) Photoswitching of HTI-SO 1 in
CH2Cl2 solution. c) Photoswitching of HTI-SO2 1 in CH2Cl2 solution. d) Photoswitching of HTI-SO2 5 with
365 nm and 420 nm light in the solid state.
In summary we have compared the performance of a series of novel oxidized HTI photoswitches, both
sulfoxide and sulfone derivatives, with their parent unoxidized HTI chromophores. We have shown that
oxidized HTIs represent valuable novel photoswitches providing efficient photoisomerization reactions
and high thermal stabilities of their metastable states. Particularly interesting are twisted derivatives that
allow to observe the photoisomerization process with the naked eye both in solution and the solid state.
Scrutiny of the chiroptical properties of chiral sulfoxide HTI-SOs is currently under way in our
laboratories and will be reported in due course. With these findings we hope to bring novel
photoswitching motives to the attention of molecular scientists seeking reliable light responsive tools
that elicit predictable structural changes upon irradiation.
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Acknowledgements
H. Dube thanks the “Fonds der Chemischen Industrie” for a Liebig fellowship (Li 188/05) and the
Deutsche Forschungsgemeinschaft (DFG) for an Emmy Noether fellowship (DU 1414/1-1). We further
thank the Deutsche Forschungsgemeinschaft (SFB 749, A12) and the Cluster of Excellence ’Center for
Integrated Protein Science Munich’ (CIPSM) for financial support.
Keywords: photoswitches • photochemistry • isomerization • hemithioindigo • physical chemistry
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10.1002/chem.202002176
Chemistry - A European Journal
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10.1002/chem.202002176
Chemistry - A European Journal
TOC Figure
A series of sulfur-oxidized hemithioindigo chromophores are introduced as versatile photoswitch
motives for light-controlled molecular behavior.
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