The role of alkyl substituents in deazaadenine-based diarylethene photoswitches

Article abstract of DOI:10.3762/bjoc.12.106

Diarylethenes are an important class of reversible photoswitches and often claimed to require two alkyl substituents at the carbon atoms between which the bond is formed or broken in the electrocyclic rearrangement. Here we probe this claim by the synthesis and characterization of four pairs of deazaadenine-based diarylethene photoswitches with either one or two methyl groups at these positions. Depending on the substitution pattern, diarylethenes with one alkyl group can exhibit significant photochromism, but they generally show poor stability towards extended UV irradiation, low thermal stability, and decreased fatigue resistance. The results obtained provide an important direction for the design of new efficient DNA photoswitches for the application in bionanotechnology and synthetic biology.

Full text of DOI:10.3762/bjoc.12.106

The role of alkyl substituents in deazaadenine-based
diarylethene photoswitches
Christopher Sarter, Michael Heimes and Andres Jäschke*§
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1103–1110.
Institut für Pharmazie und Molekulare Biotechnologie, Universität
Heidelberg, 69120 Heidelberg, Germany
doi:10.3762/bjoc.12.106
Received: 23 March 2016
Accepted: 13 May 2016
Published: 01 June 2016
Email:
Andres Jäschke* - jaeschke@uni-hd.de
* Corresponding author
Associate Editor: C. Stephenson
§ Homepage: http://www.jaeschke.uni-hd.de
© 2016 Sarter et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
diarylethene; nucleoside; electrocyclic rearrangement;
photochromism; photoswitch
Abstract
Diarylethenes are an important class of reversible photoswitches and often claimed to require two alkyl substituents at the carbon
atoms between which the bond is formed or broken in the electrocyclic rearrangement. Here we probe this claim by the synthesis
and characterization of four pairs of deazaadenine-based diarylethene photoswitches with either one or two methyl groups at these
positions. Depending on the substitution pattern, diarylethenes with one alkyl group can exhibit significant photochromism, but
they generally show poor stability towards extended UV irradiation, low thermal stability, and decreased fatigue resistance. The
results obtained provide an important direction for the design of new efficient DNA photoswitches for the application in bionano-
technology and synthetic biology.
Introduction
Most biomacromolecules are per se not responsive to light. Most reported applications use azobenzenes, while diaryl-
Their conversion into photoresponsive molecules opens up new ethenes, spiropyrans, fulgides and hemithioindigos are applied
and innovative applications in materials sciences, bionanotech- less frequently [18-30].
nology, synthetic biology, and biomedical diagnostics, and re-
versible photoswitching is a particularly attractive goal [1-12]. Diarylethenes are an important class of photoswitches. Origi-
Light is a powerful and convenient trigger for manipulating nally developed for optical information storage, they found
structure and function, as it is non-invasive, provides high many applications in materials sciences, nanotechnology, and
spatio-temporal resolution, and offers the option of orthogo- biomimetic chemistry [31-42]. Their most typical architecture is
nality [13,14]. One of the common approaches for introducing shown in Figure 1A: two heteroaromatic, most often five-mem-
photoresponsiveness into biomolecules is their covalent functio- bered heterocyclic rings are connected via a cyclopentenyl
nalization with small-molecule photoswitches [1,2,13,15-21]. bridge, which may also be perfluorinated. Substituents on the
1103

Beilstein J. Org. Chem. 2016, 12, 1103–1110.
Figure 1: Diarylethene photoswitches. A: classical design and photoisomerization reaction [27]. B: purine-based photoswitches (1st generation)
[43,44], C: pyrimidine-based photoswitches (2nd generation) [45].
aryl rings tune the photophysical and chemical properties, such Therefore, we set out to clarify this matter. If pyrimidine-based
as absorption and isomerization wavelengths, thermal stability, diarylethenes with one methyl group are good photoswitches,
and fatigue resistance.
purine derivatives might work as well with just one methyl
group. If this hypothesis was true, the synthesis of such photo-
In 2010 our group reported the first diarylethene-based photo- switches would be much simplified. Therefore, we report in the
switchable nucleosides in which a purine nucleobase repre- present communication the synthesis of four pairs of 7-deaza-
sented one of the aryl rings of the photoswitch (Figure 1B). Im- adenosine-based photoswitches with one or two methyl groups
portantly and different from most other approaches towards present, and their thorough characterization.
photoswitchable nucleic acids, the nucleobase constituted an
Results and Discussion
active part of the photoswitch (rather than an appendage),
changing its bonding and hybridization upon switching [43,44]. Design and synthesis of the photoswitches
This new class of diarylethenes showed promising photophysi- The design of the target compounds 1a–d and 2a–d is shown in
cal and photochemical properties, such as the location of the Scheme 1. One aryl system is a 7-deaza-2’-deoxyadenosine,
photostationary states, reversibility of switching, thermal while the other one is a 2-methylthiophene derivatized with four
stability, and fatigue resistance.
different substituents in the 5-position. The only distinction be-
tween the two series of compounds is the absence (1a–d) or
In 2013 we developed a 2nd generation of diarylethene-based presence (2a–d) of a methyl group in position 8 of 7-deazaade-
photoswitchable nucleosides. The design of these molecules nine. For the synthesis of these 8 compounds we used a conver-
challenged two of the established design rules for diarylethene- gent approach based on a central Suzuki cross-coupling reac-
based photoswitches (Figure 1C) [45]: (1) They incorporated a tion (Scheme 1).
six-membered pyrimidine ring, and (2) – more importantly –
only one methyl group, rather than two, was present at the posi- For the compounds with one methyl group (1a–d), 7-deaza-7-
tions where the bond is formed in the electrocyclic rearrange- iodo-2’-deoxyadenosine was synthesized as described [48],
ment. Still, these new diarylethenes had very favorable switch- while 7-deaza-7-iodo-8-methyl-2’-deoxyadenosine (9), re-
ing properties [45]. These results were surprising, considering quired for the photoswitches with two methyl groups (2a–d),
that two alkyl groups at these positions are considered as essen- was synthesized using an optimized route based on our original
tial for reversible switching throughout the published literature work (Scheme 2) [43].
[27,39]. Experimental evidence for this requirement is, howev-
er, scarce. Few studies compare symmetrical switches with zero This route started from commercial 6-chloro-7-deazapurine and
and two alkyl groups [39,46,47], but we were not able to locate involved a transient protection of ring nitrogen N9 with
a single publication where otherwise identical switches with one benzenesulfonyl chloride, resulting in a four-fold increased
and two alkyl groups are compared.
yield of 6 compared to the original route.
1104

Beilstein J. Org. Chem. 2016, 12, 1103–1110.
Scheme 1: Synthesis of 7-deaza-2’-deoxyadenosine photoswitches with one and two methyl groups via Suzuki cross-coupling [45].
Scheme 2: Optimized route for synthesis of 7-deaza-7-iodo-8-methyl-2’deoxyadenosine (9).
The other part of the diarylethene, namely the cyclopentenyl bility, were investigated. The main text shows only the analyti-
bridge with the attached substituted thiophene ring was cal data for new pyridyl switch 1b in comparison to the previ-
furnished as shown in Scheme 3 and activated as boronic acid ously published and well-studied switch 2b [43]; all other spec-
pinacolate ester. Suzuki coupling yielded the eight target com- tra are presented in Supporting Information File 1 Figures
pounds in yields between 36% and 97%. Two of these com- S1–S5. Figure 2 and Figure S1 (Supporting Information File 1)
pounds (2b,c) are identical to the ones reported previously [43], show the changes in the UV–vis spectra upon irradiation with
while the other 6 represent new compounds.
UV light (366 nm, hand lamp) in acetonitrile.
Photophysical and photochemical
characterization
Three differences are immediately apparent between the two
classes of compounds: (1) At any given time point, the absorp-
For diarylethenes 1a–d and 2a–d, the photophysical and photo- tion cross-section is much higher with two methyl groups. To
chemical properties, in particular their photochromicity, switch- exclude solubility effects, we normalized the absorbance at the
ing kinetics and reversibility, fatigue resistance, and thermosta- respective maximum of the closed-ring isomer to the 7-deaza-
1105

Beilstein J. Org. Chem. 2016, 12, 1103–1110.
Figure 2: Spectral changes of the pyridyl switch with one (1b) and two (2b) methyl groups upon irradiation with UV light. A 60 µM solution of the com-
pound in acetonitrile was irradiated for the indicated time intervals (see inset) with UV light (366 nm) and after each interval a UV–vis absorption spec-
trum was recorded.
Scheme 3: Synthesis of the boronic acid pinacolate esters.
adenosine absorbance at 280 nm of the open-ring form, yielding tion, the one-methyl compounds are not (see insets in the
values between 0.08 and 0.35 for one (1a–d) and 0.25 to 0.67 Figures). Absorbance decreases after reaching a maximum,
for two (2a–d) methyl groups (Table 1). Among the com- implying that the closed-ring form undergoes a further light-de-
pounds with one methyl group, only naphthyl derivative 1d pendent irreversible reaction. This decay is strongest for phenyl
showed appreciable photochromicity (ratio 0.35). The normal- derivative 1c which loses ~50% of its absorbance upon an irra-
ized absorption cross-section is 1.75–6.73-fold higher for the diation of 1 hour. The same three tendencies can also be ob-
two-methyl compounds. The strongest methyl group effect is served when the UV spectra are measured in ethanol and metha-
observed for the pyridyl switches (Figure 2). (2) For all com- nol (Supporting Information File 1, Figures S2 and S3), indicat-
pounds, the absorption maximum of the closed-ring form is ing that the differences between the one- and two-methyl
blue-shifted for the one-methyl compounds, in comparison to switches are not due to solvent effects.
the two-methyl derivatives. This shift is between 14 (naphthyl,
1d vs 2d) and 50 nm (phenyl, 1c vs 2c, Table 1). (3) While all This latter observation already indicated poor reversibility and
two-methyl compounds are stable towards further UV irradia- fatigue resistance for the one-methyl switches. We therefore
1106

Beilstein J. Org. Chem. 2016, 12, 1103–1110.
Table 1: Absorption characteristics of diarylethenes 1a–d and 2a–d.
R
λmax,vis (1a–d)
Aλmax,vis /A280 nm (1a–d)
λmax,vis (2a–d)
Aλmax,vis /A280 nm (2a–d)
Ratioa
Δλ21b
a
b
c
d
432 nm
489 nm
435 nm
482 nm
0.08
0.10
0.14
0.35
447 nm
505 nm
485 nm
496 nm
0.32
0.67
0.25
0.66
3.96
6.73
1.75
1.86
15
16
50
14
a(Aλmax,vis/A280nm (2a–d))/(Aλmax,vis/A280 nm (1a–d)). bλmax,vis(2a–d) − λmax,vis(1a–d).
undertook absorption measurements over multiple cycles of visible range, which is indicative of the closed-ring form. The
ring closure by UV and ring opening by vis irradiation only for other two peaks (at 21.2 and 22.8 min) are not colored and are
pyridyl switch 1b, which had shown the highest photostability much larger (in terms of integrated A254). These findings indi-
(Figure 2). These measurements revealed almost complete loss cate that a large proportion of compound 1d has undergone an
of photoswitchability over 7 cycles, whereas the compound 2b irreversible side reaction and the colored closed-ring form is
showed only slight deterioration (Figure 3). The more electron- only a minor component after extended irradiation. The iden-
rich two-methyl switches 2a,c and d showed reduced reversibil- tical m/z values rule out additions or eliminations and instead
ity, as also known for electron-rich classical diarylethenes (Sup- suggest a rearrangement, such as the 1,2-dyotopic rearrange-
porting Information File 1, Figure S4) [49]. Interestingly, re- ment described for diarylethenes [49-51]. This hypothesis is
versibility and fatigue resistance of the two-methyl switches are further supported by the appearance of two new signals in the
slightly improved in more polar solvents methanol and ethanol NMR spectrum of an extensively irradiated solution of 1d at
(Supporting Information File 1, Figure S5).
δ = 2.53 and 2.71 (Supporting Information File 1, Figure S7), in
agreement with values reported for isolated products of this re-
arrangement reaction [49]. Methyl switch 1a, pyridyl switch 1b
and phenyl switch 1c, on the other hand, decomposed into a
multitude (>15) of different products, according to the HPLC
chromatograms, indicating multiple degradation pathways. Due
to this effect, the ratio between open- and closed-ring isomer in
the pseudo-photostationary state cannot be determined.
To understand why the one-methyl compounds are so rapidly
degraded upon continued UV irradiation, we carried out HPLC
and LC–MS analysis of solutions of 1b–d after different times
of UV irradiation and compared them with 2b–d (Figure 4).
While no additional peaks appear in the chromatograms of the
two-methyl compounds after reaching the photostationary state
(where the closed-ring isomer dominates with more than 95% in Finally, we analyzed the thermostability of the closed-ring form
all cases according to the integrated peak areas, Figure 4), the of the photoswitches. In all cases, the two-methyl compounds
1-methyl switches behave differently: For naphthyl switch 1d 2a–d had higher thermostability. For the pyridyl switch
the single peak corresponding to the open-ring form at 20 min (Figure 5), the two-methyl variant 2b is absolutely stable at
(m/z of 539.21, strong absorption at 254 nm, no absorption at 25 °C and loses about 5% within 60 min at 50 or 70 °C, where-
520 nm) largely disappeared, whereas after extended UV irradi- as the one-methyl version 1b is already unstable at room tem-
ation three new peaks are detected by absorption at 254 nm, all perature and rapidly degrades (40% in 4 min) at 70 °C. With the
with the same m/z value of 539.21 (Supporting Information other three substituents, the same tendency can be observed
File 1, Figure S6). One of them (at 22.2 min) also absorbs in the (Supporting Information File 1, Figure S8).
Figure 3: Reversibility measurements of deazaadenosine photoswitches with one and two methyl groups. A 60 µM solution of the compound in aceto-
nitrile was irradiated with UV light until the (pseudo-)photostationary state was reached. After recording the UV–vis spectrum, the solution was irradi-
ated with vis light for the same time. This procedure was repeated 7 times.
1107

Beilstein J. Org. Chem. 2016, 12, 1103–1110.
Figure 4: Stability analysis of compounds 1b–d and 2b–d upon different times of UV irradiation, monitored by HPLC with detection at 254 nm and at
the absorption maximum of the closed-ring form. ORI: open-ring isomer, CRI: closed-ring isomer, BP: byproduct.
Conclusion
pursued two aims: i) we wanted to probe the common claim that
In the present study, we synthesized four pairs of substituted two alkyl groups are required for photochromic diarylethenes
7-deaza-2’-deoxyadenosine photoswitches bearing either one or by direct comparison, and ii) we wanted to test whether our
two methyl groups at the position where the new bond is prior successful violation of this “rule” with photochromic
formed in the electrocyclic rearrangement. This investigation pyrimidine nucleosides could be expanded to purine nucleo-
1108

Beilstein J. Org. Chem. 2016, 12, 1103–1110.
Figure 5: Thermostability measurements of the 7-deazaadenosine nucleosides. A 60 µM solution of the compound in acetonitrile was irradiated with
UV light until the (pseudo)-photostationary state was reached. Then UV irradiation was switched off and the absorption at the visible light maximum of
the closed ring form was measured over 1 h.
5. Jäschke, A. FEBS Lett. 2012, 586, 2106–2111.
sides. Our analysis reveals that – depending on the substituents
doi:10.1016/j.febslet.2012.05.040
– the one-methyl 7-deaza-2’-deoxyadenosine compounds can
6. Rothemund, P. W. K. Nature 2006, 440, 297–302.
indeed show photochromicity; in the case of a naphthyl substit-
doi:10.1038/nature04586
uent even to a significant extent. In all cases, however, the
7. Zhang, D. Y.; Turberfield, A. J.; Yurke, B.; Winfree, E. Science 2007,
absorption cross-section was much smaller than for the
2-methyl compounds. More importantly, the one-methyl deriva-
tives showed lower photostability, thermal stability, reversibili-
ty and fatigue resistance. Significant UV-irradiation-dependent
side reactions were observed. From a practical point of view,
one can therefore conclude that for these (i.e., deazaadenosine-
based) photoswitches two methyl groups are more suitable than
one. Overall, the results obtained provide an important direc-
tion for the design of new efficient photoswitches for applica-
tion in bionanotechnology and synthetic biology.
318, 1121–1125. doi:10.1126/science.1148532
8. Babii, O.; Afonin, S.; Berditsch, M.; Reißer, S.; Mykhailiuk, P. K.;
Kubyshkin, V. S.; Steinbrecher, T.; Ulrich, A. S.; Komarov, I. V.
Angew. Chem., Int. Ed. 2014, 53, 3392–3395.
doi:10.1002/anie.201310019
9. Fujimoto, K.; Kajino, M.; Sakaguchi, I.; Inouye, M. Chem. – Eur. J.
2012, 18, 9834–9840. doi:10.1002/chem.201201431
10.Fujimoto, K.; Maruyama, T.; Okada, Y.; Itou, T.; Inouye, M.
Tetrahedron 2013, 69, 6170–6175. doi:10.1016/j.tet.2013.05.044
11.Reisinger, B.; Kuzmanovic, N.; Löffler, P.; Merkl, R.; König, B.;
Sterner, R. Angew. Chem., Int. Ed. 2014, 53, 595–598.
doi:10.1002/anie.201307207
12.Vomasta, D.; Högner, C.; Branda, N. R.; König, B.
Angew. Chem., Int. Ed. 2008, 47, 7644–7647.
Supporting Information
doi:10.1002/anie.200802242
13.Feringa, B. L.; Browne, W. R., Eds. Molecular Switches; Wiley-VCH:
Weinheim, Germany, 2011.
Supporting Information File 1
Experimental part and additional spectra of investigated
compounds.
14.Maurice Goeldner, R. G.; Givens, R., Eds. Dynamic Studies in Biology:
Phototriggers, Photoswitches and Caged Biomolecules; Wiley-VCH:
Weinheim, Germany, 2005.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-106-S1.pdf]
15.Ferreira, R.; Nilsson, J. R.; Solano, C.; Andréasson, J.; Grøtli, M.
Sci. Rep. 2015, 5, No. 9769. doi:10.1038/srep09769
16.Goldau, T.; Murayama, K.; Brieke, C.; Steinwand, S.; Mondal, P.;
Biswas, M.; Burghardt, I.; Wachtveitl, J.; Asanuma, H.; Heckel, A.
Chem. – Eur. J. 2015, 21, 2845–2854. doi:10.1002/chem.201405840
17.Horstmann, B.; Korbus, M.; Friedmann, T.; Wolff, C.; Thiele, C. M.;
Meyer-Almes, F.-J. Bioorg. Chem. 2014, 57, 155–161.
doi:10.1016/j.bioorg.2014.10.004
Acknowledgements
This work was supported by the Helmholtz Initiative on Synthe-
tic Biology (SO 078) and BMBF grant 031A170C. The authors
thank Sandra Suhm, Heiko Rudy and Tobias Timmermann for
technical assistance.
18.Özçoban, G.; Halbritter, T.; Steinwand, S.; Herzig, L.-M.;
Kohl-Landgraf, J.; Askari, N.; Groher, F.; Fürtig, B.; Richter, C.;
Schwalbe, H.; Suess, B.; Wachtveitl, J.; Heckel, A. Org. Lett. 2015, 17,
1517–1520. doi:10.1021/acs.orglett.5b00397
References
1. Gautier, A.; Gauron, C.; Volovitch, M.; Bensimon, D.; Jullien, L.; Vriz, S.
Nat. Chem. Biol. 2014, 10, 533–541. doi:10.1038/nchembio.1534
2. Kamiya, Y.; Takagi, T.; Ooi, H.; Ito, H.; Liang, X.; Asanuma, H.
ACS Synth. Biol. 2015, 4, 365–370. doi:10.1021/sb5001092
3. Bath, J.; Turberfield, A. J. Nat. Nanotechnol. 2007, 2, 275–284.
doi:10.1038/nnano.2007.104
19.Brieke, C.; Rohrbach, F.; Gottschalk, A.; Mayer, G.; Heckel, A.
Angew. Chem., Int. Ed. 2012, 51, 8446–8476.
doi:10.1002/anie.201202134
20.Szymański, W.; Beierle, J. M.; Kistemaker, H. A. V.; Velema, W. A.;
Feringa, B. L. Chem. Rev. 2013, 113, 6114–6178.
doi:10.1021/cr300179f
4. Douglas, S. M.; Dietz, H.; Liedl, T.; Högberg, B.; Graf, F.; Shih, W. M.
Nature 2009, 459, 1154. doi:10.1038/nature08165
1109

Beilstein J. Org. Chem. 2016, 12, 1103–1110.
21.Barrois, S.; Wagenknecht, H.-A. Beilstein J. Org. Chem. 2012, 8,
905–914. doi:10.3762/bjoc.8.103
46.Kellogg, R. M.; Groen, M. B.; Wynberg, H. J. Org. Chem. 1967, 32,
3093. doi:10.1021/jo01285a035
22.Barrois, S.; Beyer, C.; Wagenknecht, H.-A. Synlett 2012, 711–716.
doi:10.1055/s-0031-1290599
47.Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 6136–6138.
doi:10.1021/jo00261a035
23.Broichhagen, J.; Frank, J. A.; Johnston, N. R.; Mitchell, R. K.; Šmid, K.;
Marchetti, P.; Bugliani, M.; Rutter, G. A.; Trauner, D.; Hodson, D. J.
Chem. Commun. 2015, 51, 6018–6021. doi:10.1039/c5cc01224d
24.Broichhagen, J.; Schönberger, M.; Cork, S. C.; Frank, J. A.;
Marchetti, P.; Bugliani, M.; Shapiro, A. M. J.; Trapp, S.; Rutter, G. A.;
Hodson, D. J.; Trauner, D. Nat. Commun. 2014, 5, No. 5116.
doi:10.1038/ncomms6116
48.Seela, F.; Peng, X. Synthesis 2004, 1203–1210.
doi:10.1055/s-2004-822382
49.Herder, M.; Schmidt, B. M.; Grubert, L.; Pätzel, M.; Schwarz, J.;
Hecht, S. J. Am. Chem. Soc. 2015, 137, 2738–2747.
doi:10.1021/ja513027s
50.Higashiguchi, K.; Matsuda, K.; Kobatake, S.; Yamada, T.; Kawai, T.;
Irie, M. Bull. Chem. Soc. Jpn. 2000, 73, 2389–2394.
doi:10.1246/bcsj.73.2389
25.Fihey, A.; Jacquemin, D. Chem. Sci. 2015, 6, 3495–3504.
doi:10.1039/c5sc00856e
51.Ikeda, H.; Kawabe, A.; Sakai, A.; Namai, H.; Mizuno, K.
Res. Chem. Intermed. 2009, 35, 893–908.
doi:10.1007/s11164-009-0072-2
26.Frank, J. A.; Moroni, M.; Moshourab, R.; Sumser, M.; Lewin, G. R.;
Trauner, D. Nat. Commun. 2015, 6, 7118. doi:10.1038/ncomms8118
27.Irie, M. Chem. Rev. 2000, 100, 1685–1716. doi:10.1021/cr980069d
28.Lohse, M.; Nowosinski, K.; Traulsen, N. L.; Achazi, A. J.;
von Krbek, L. K. S.; Paulus, B.; Schalley, C. A.; Hecht, S.
Chem. Commun. 2015, 51, 9777–9780. doi:10.1039/c5cc02811f
29.Trauner, D. Beilstein J. Org. Chem. 2012, 8, 870–871.
doi:10.3762/bjoc.8.97
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
30.Zhang, X.; Heng, S.; Abell, A. D. Chem. – Eur. J. 2015, 21,
10703–10713. doi:10.1002/chem.201501488
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
31.Okuda, J.-y.; Tanaka, Y.; Kodama, R.; Sumaru, K.; Morishita, K.;
Kanamori, T.; Yamazoe, S.; Hyodo, K.; Yamazaki, S.; Miyatake, T.;
Yokojima, S.; Nakamura, S.; Uchida, K. Chem. Commun. 2015, 51,
10957–10960. doi:10.1039/c5cc02200b
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
32.Hammarson, M.; Nilsson, J. R.; Li, S. M.; Lincoln, P.; Andréasson, J.
Chem. – Eur. J. 2014, 20, 15855–15862. doi:10.1002/chem.201405113
33.Velema, W. A.; Szymanski, W.; Feringa, B. L. J. Am. Chem. Soc. 2014,
136, 2178–2191. doi:10.1021/ja413063e
(http://www.beilstein-journals.org/bjoc)
34.Chen, X.; Wehle, S.; Kuzmanovic, N.; Merget, B.; Holzgrabe, U.;
König, B.; Sotriffer, C. A.; Decker, M. ACS Chem. Neurosci. 2014, 5,
377–389. doi:10.1021/cn500016p
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.12.106
35.Bonacchi, S.; El Garah, M.; Ciesielski, A.; Herder, M.; Conti, S.;
Cecchini, M.; Hecht, S.; Samorì, P. Angew. Chem., Int. Ed. 2015, 54,
4865–4869. doi:10.1002/anie.201412215
36.Baroncini, M.; d'Agostino, S.; Bergamini, G.; Ceroni, P.; Comotti, A.;
Sozzani, P.; Bassanetti, I.; Grepioni, F.; Hernandez, T. M.; Silvi, S.;
Venturi, M.; Credi, A. Nat. Chem. 2015, 7, 634–640.
doi:10.1038/nchem.2304
37.Irie, M. Pure Appl. Chem. 2015, 87, 617–626.
doi:10.1515/pac-2015-0208
38.Irie, M. Photochem. Photobiol. Sci. 2010, 9, 1535–1542.
doi:10.1039/c0pp00251h
39.Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803–808.
doi:10.1021/jo00239a022
40.Irie, M.; Fulcaminato, T.; Matsuda, K.; Kobatake, S. Chem. Rev. 2014,
114, 12174–12277. doi:10.1021/cr500249p
41.Gostl, R.; Hecht, S. Chem. – Eur. J. 2015, 21, 4422–4427.
doi:10.1002/chem.201405767
42.Luo, Q.; Liu, M.; Cheng, R.; Liu, Y. Dyes Pigm. 2015, 113, 602–608.
doi:10.1016/j.dyepig.2014.09.026
43.Singer, M.; Jäschke, A. J. Am. Chem. Soc. 2010, 132, 8372–8377.
doi:10.1021/ja1024782
44.Singer, M.; Nierth, A.; Jäschke, A. Eur. J. Org. Chem. 2013,
2766–2769. doi:10.1002/ejoc.201300261
45.Cahová, H.; Jäschke, A. Angew. Chem., Int. Ed. 2013, 52, 3186–3190.
doi:10.1002/anie.201209943
1110