Simple azo dyes provide access to versatile chiroptical switches

Article abstract of DOI:10.1002/ejoc.201500485

Azo dyes have played an important role in the development of the chemical industry for 150 years. The azo-core of these dyes can undergo trans to cis photoisomerization, which allows azobenzene derivatives to act as light triggered molecular switches. Here, we showed that simple derivatization of Sudan I provides access to chiroptical molecular switches, and that the properties of these switches can be readily tuned by modification of the molecular structure. The synthesis, characterization, photoisomerization, thermal stability, chiral HPLC resolution, determination of absolute configuration and chiroptical properties of chiroptical switches based on Sudan I are reported. ortho-difluorinated Sudan I derivatives have improved thermal stabilities and switching properties compared to switches based on Sudan I itself. Transfer of stereochemical information from a non-racemic chiral unit to the π-conjugated system of the dye and exciton-coupled circular dichroism are both observed. The chiral unit influences the geometry and hence the spectra of cis and trans-isomers differently, which is the mechanistic basis of chiroptical switching. The azo-core of dyes can undergo trans to cis photoisomerization, which allows azobenzene derivatives to act as light triggered molecular switches. Simple derivatization of Sudan I using a chiral auxiliary provides access to chiroptical molecular switches. The properties of these switches can be readily tuned by modification of the molecular structure.

Full text of DOI:10.1002/ejoc.201500485

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DOI: 10.1002/ejoc.201500485
Simple Azo Dyes Provide Access to Versatile Chiroptical Switches
Emmanuel Anger[a] and Stephen P. Fletcher*[a]
Keywords: Dyes / Molecular switch / Electronic circular dichroism / Chirality
Azo dyes have played an important role in the development
of the chemical industry for 150 years. The azo-core of these
dyes can undergo trans to cis photoisomerization, which al-
lows azobenzene derivatives to act as light triggered molec-
ular switches. Here, we showed that simple derivatization of
Sudan I provides access to chiroptical molecular switches,
and that the properties of these switches can be readily tuned
by modification of the molecular structure. The synthesis,
characterization, photoisomerization, thermal stability, chiral
HPLC resolution, determination of absolute configuration
and chiroptical properties of chiroptical switches based on
Sudan I are reported. ortho-difluorinated Sudan I derivatives
have improved thermal stabilities and switching properties
compared to switches based on Sudan I itself. Transfer of
stereochemical information from a non-racemic chiral unit to
the π-conjugated system of the dye and exciton-coupled
circular dichroism are both observed. The chiral unit
influences the geometry and hence the spectra of cis and
trans-isomers differently, which is the mechanistic basis of
chiroptical switching.
Humans appear to have been purposefully coloring tex- azo dye produces a light-triggered chiroptical switch where
tiles for at least 30,000 years,[1] and Perkin’s discovery of differences in optical rotation and circular dichroism (CD)
mauveine, the first synthetic dye, may have been the spark spectra between the cis and trans isomers are observed (Fig-
that ignited the modern chemical industry.[2] Azobenzene ure 1, b). These switches are easy to prepare and we use
was first described in 1834[3] and synthesized by Noble in simple synthetic modifications to tune the thermal stability
1856.[4] An azobenzene unit is the core structural feature of and circular dichroism spectra of the dyes. Switches con-
azo dyes which have tremendous contemporary and histori- taining fluoro-substituents have improved thermal stability
cal importance to chemistry.[5]
and exhibit dramatic differences in optical rotation (inver-
In 1937, Hartley characterized the cis isomer of azobenz- sion of sign) and CD spectra (in the 250 to 500 nm range)
ene;[6] and observation that azobenzenes undergo trans to between the trans and cis isomers.
cis photoisomerization allowed the design of useful molec-
ular switches.[7,8] Besides in the dye industry,[5] aryl-substi-
tuted azo-compounds have found applications as diverse as
pH indicators[9] and in optical data storage,[10] and are
widely used across materials and biological science.[11,12] Be-
cause of their extensive applications and relative ease of
synthesis, there are many commercially available azo dyes[5a]
and these molecules represent a valuable bank of easily ac-
Figure 1. Concept of converting an industrial dye to a light-trig-
cessible and modifiable molecules for the development of
gered chiroptical switch.
new applications.
Light-triggered chiroptical molecular switches based on
overcrowded alkenes,[13] diarylethenes[14] fulgides[15] azo-
benzenes[16] and ensembles of molecules (gels, liquid crys-
tals, etc.) have been extensively studied[17] with the aim of
developing materials capable of non-destructive readout be-
tween switchable optical states. Here, we demonstrate that
simply adding a chiral auxiliary to a commercially available
The sheer number of commercially available or known
azo dyes with different spectral characteristic, solubilities,
structural features and preexisting functional groups for
derivatization suggests that this approach could be used to
access a vast number of inexpensive tailor-made chiroptical
switches with highly tunable properties.
Esterification of Sudan I with (S)-Naproxen chloride
gave the racemic ester trans-1a in 65% yield (Scheme 1, a)
due to the formation of an achiral ketene intermediate dur-
ing the reaction.[18] Isomerization of 1a using a light-emit-
ting diode (LED) with λmax = 570 nm gave a 1:1 mixture of
trans-1a and cis-1b (Scheme 1, b). The cis and trans isomers
can be separated by flash column chromatography and the
[a] Department of Chemistry, University of Oxford, Chemistry
Research Laboratory,
12 Mansfield Road, Oxford, OX1 3TA, U.K.
E-mail: stephen.fletcher@chem.ox.ac.uk
research.chem.ox.ac.uk/stephen-fletcher.aspx
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500485.
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NMR spectra (see Supporting Information) of cis-1b indi- the ester group of (+)-1a and (–)-1a and comparing the op-
cates restricted rotation about the azo group. Compound tical rotation of the resulting acids to previously reported
1b has a half-life of 3 d at 25 °C in CDCl3.
specific rotations.[19]
Chiroptical properties of enantiopure (R) and (S)-1 were
studied using pure cis-1b and trans-1a isomers. The asym-
metry in molecules 1 is derived from (S)-Naproxen chloride
and we empirically observe that the switches have higher
specific rotations [trans-(S)-1a = +100; cis-(S)-1b = +280]
than the starting acid chloride. ECD spectra of (S)-Nap-
roxen chloride in acetonitrile display two active CD bands
at 260 nm (Δε
= =
+4 m–1 cm–1) and 230 nm (Δε
–6 m–1 cm–1). Compound trans-(S)-1a displays four active
CD bands between 200 and 550 nm (see Scheme 1, c). The
two first ECD bands at 205 nm (Δε = –12.1 m–1 cm–1) and
235 nm (Δε = +11.1 m–1 cm–1) correspond to a positive exci-
ton-coupled circular dichroism (ECCD) phenomenon[20]
between the naphthyl and azo dye moieties of (S)-1a. The
two active weak broad bands in the near UV/Vis region at
300 nm (Δε = –0.9 m–1 cm–1) and 460 nm (Δε = 0.8 m–1 cm–1)
are attributed to induction of chirality from the stereogenic
center to the extended π-conjugated system of Sudan I.
Compound cis-(S)-1b displays four ECD active bands be-
tween 200 and 500 nm. Two first narrow bands, a negative
band at 210 nm (Δε = –19.9 m–1 cm–1) and a positive band
at 235 nm (Δε = +29.9 m–1 cm–1), are stronger than the cor-
responding bands found in trans-(S)-1a indicating that the
ECCD phenomenon is more prominent in the cis-isomer.
The last two bands, a negative broad band between 250 and
320 nm (Δε = –2.9 m–1 cm–1) and a positive band at 430 nm
(Δε = +1.6 m–1 cm–1), corresponding to the chiral induction,
are slightly stronger than those found in the ECD spectrum
of (S)-1a (see Scheme 1, c).
Fluorination of symmetrical ortho-azobenzenes has been
reported to modify the relative energies of the n-π* absorp-
tion in azo-isomers, this causes band separation between
cis- and trans-isomers so that they can be selectively in-
terconverted using different wavelengths of light, addition-
ally these cis-isomers of ortho-tetrafluoroazobenzene also
display longer thermal isomerization half-lives, which was
attributed to the negative inductive effect of the fluorines
that decreases the repulsive interaction of the nitrogen lone
pairs.[21]
In an attempt to improve the trans to cis isomerization
behavior of our switch described above, we prepared gram
quantities of ortho-difluorinated Sudan I by allowing the di-
azonium salt of the 2,6-difluoroaniline and 2-naphthol to
react in presence of KOH below 5 °C. Esterification then
Scheme 1. a) Synthesis, resolution and determination of absolute
configuration of 1a; b) photoisomerization of (S)-1a to (S)-1b; c)
CD spectra of (S)- and (R)-1a–b in solution in acetonitrile (c =
6ϫ10–6 m).
HPLC on a chiral non-racemic stationary phase was gave trans-compounds 2a (70% yield) (Scheme 2, a). Com-
used to separate the enantiomers of trans-1a (see Support- pounds cis-2b were obtained by irradiating 2a at the tail of
ing Information). Using repeated 100 μL [10 mg/mL in a the n–π* absorption band (see Supporting Information for
solvent mixture of hexane/chloroform (8:2)] injections and UV/Vis spectra) using a LED (λmax = 570 nm). After
a Chiralpak® IC analytical column with an eluent mixture 30 min of irradiation the photo-stationary state (PSS) was
of hexane/chloroform/2-propanol (79:20:1) and collecting reached giving a mixture of isomers, of which 92% were cis-
enantiomerically pure material with an automated semi- isomers 2b. cis-2b was purified using flash column
1
preparative fraction collector allowed about 30 mg of chromatography over silica gel. H and 13C NMR spectra
enantiopure (R) and (S)-1a to be separated using less than of 2b show some broad signals, suggesting that free rotation
a liter of solvent within a working day. The absolute config- of the azo-moiety is restricted. Compound 2b has a thermal
uration of the enantiomers was determined by hydrolysis of half-life of about 15 d in solution at room temperature.
2
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Simple Azo Dyes Provide Access to Versatile Chiroptical Switches
tion of Naproxen chloride (Table 1). Interestingly, the spe-
cific rotation of cis-(S)-2b is almost the same magnitude,
but opposite in sign, of the specific rotation of trans-(S)-2a.
trans-(S)-2a displays four active CD bands between 200 and
550 nm (see Scheme 2, c). The ECD bands at 210 nm (Δε =
–14.2 m–1 cm–1) and 235 nm (Δε = +14.5 m–1 cm–1) are due
to ECCD phenomenon between the two naphthyl moieties.
The two broad bands in the near UV/Vis region between
250 and 450 nm (Δε = –2.5 m–1 cm–1) and at 490 nm (Δε =
+1.5 m–1 cm–1) are attributed to the stereogenic center influ-
encing the geometry of the extended π-conjugated system of
ortho-difluoro-Sudan I unit. Compound cis-(S)-2b displays
four ECD active bands between 200 and 500 nm. The first
two narrow bands, a negative band at 215 nm (Δε =
–14.2 m–1 cm–1) and a positive band at 240 nm (Δε =
+19.2 m–1 cm–1), remain almost unchanged from those
found in trans-(S)-2a, however the last two bands, a positive
broad band between 250 and 320 nm (Δε = +4.3 m–1 cm–1)
and a negative band at 430 nm (Δε = –10.4 m–1 cm–1), corre-
sponding to asymmetry about the cis-azo unit, are com-
pletely inverted and stronger in intensity than those found
in the ECD spectrum of trans-(S)-2a (see Scheme 2, c).
Table 1. Specific rotation of trans (a) and cis (b) isomers of com-
pounds (S)-1–3.
OA (°·mL·g–1·dm–1)
Starting acyl chloride
(S)-1
(S)-2
(S)-3
+66[a]
+66[a]
+19[a]
a (trans)
b (cis)
+100[b]
+420[b]
+180[b]
+280[b]
–380[b]
–175[b]
[a] From literature in dichloromethane. [b] Measured in dichloro-
methane at 25 °C with the D line (c = 0.1).
To study the origins of the observed chiroptical proper-
ties, the CD active Naproxen-based chiral auxiliary in 1 and
2 was replaced by a saturated chiral auxiliary to eliminate
Scheme 2. a) Synthesis and resolution of 2a; b) photoisomerization
of (S)-2a to (S)-2b; c) CD spectra of (S)- and (R)-2a–b in solution exciton-coupling. Esterification of difluoro-Sudan I with
in acetonitrile (c = 8.5ϫ10–6 m).
(S)-2-methylbutyryl chloride gave racemic 3a in 65% yield
(Scheme 3, a). Compound 3b was obtained by irradiating
Selective cis to trans isomerization was achieved by irra- 3a in the tail of the n–π* absorption band using a LED
diation with a 435 nm LED, giving a photostationary state (λmax = 570 nm). After 30 min of irradiation the photo-sta-
of 89:11 for trans-2a/cis-2b (Scheme 2, b). Thermal conver- tionary state was reached (10:90, trans/cis) (Scheme 3, b),
sion of cis-2b to trans-2a can also be accomplished by heat- and 3b has a thermal half-life of 16 d at room temperature.
ing at 80 °C for 1 h in acetonitrile. These light-triggered chi- After separating the enantiomers of 3 by HPLC the abso-
roptical switches are robust as determined by 1H NMR lute configurations were determined by base mediated
spectroscopy, no detectable degradation was observed after cleavage of the esters in (+)-3a and (–)-3a and examining
ten trans/cis and cis/trans isomerization cycles.
the optical rotations of recovered 2-methylbutyric acid.[23]
HPLC with a chiral non-racemic stationary phase using The specific rotation of trans-(S)-3a is more than nine times
an analytical column was used to separate the enantiomers higher than the specific rotation of (S)-2-methylbutyryl
of 2 in a manner similar to that used above to obtain 1 chloride. As observed with 2, the specific rotation of cis-
(see Supporting Information for HPLC traces and detailed (S)-3b is almost the opposite of trans-(S)-3a. trans-(S)-3a
conditions). The absolute configurations were determined displays only two weak and broad active CD bands from
by ester hydrolysis of (+)-2a and (–)-2a and comparing the 320–550 nm. The first is between 320 and 420 nm (Δε =
optical rotation of the resulting acid products with known –0.1 m–1 cm–1) and the other is centered at 490 nm (Δε =
values.[22]
+0.3 m–1 cm–1) (Scheme 3, c). cis-(S)-3b displays two active
The chiroptical properties of (R)- and (S)-2 were studied bands, a first positive broad active band between 260 and
using chemically and enantiomerically pure trans (2a) and 320 nm (Δε = +0.5 m–1 cm–1) and a second negative active
cis (2b) isomers (Scheme 2, c). The specific rotation of trans- band at 430 nm (Δε = –1.0 m–1 cm–1). As observed in (S)-2
(S)-2a is more than six times higher than the specific rota- ECD study, the CD-active bands of the cis-(S)-3b are
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stronger in intensity and with the opposite sign of those in
trans-(S)-3a.
Figure 2. X-ray crystallographic structures of cis-rac-2b (top) and
cis-(R)-3b (bottom). Hydrogen atoms have been omitted for clarity.
significantly different optical rotations and CD spectra of
the cis- and trans-isomers The transfer of stereochemical
information from the chiral ester to the π-conjugated dye
system and exciton coupling significantly differ in photo-
switchable isomers which can be read out without dis-
turbing the isomeric ratio. Difluoro-substitution, as in 2
and 3, gave improved control over cis-trans isomerization,
more dramatic changes in chiroptical properties, and im-
proved thermal stability of the cis-isomers. Study of ortho-
difluorinated Sudan I bearing a saturated chiral ester shows
that CD bands in 270–550 nm range is due to through space
transmission of chiral information from the stereogenic cen-
ter to the azo dye unit.
Scheme 3. a) Synthesis and resolution of 3a; b) photoisomerization
of (S)-3a to (S)-3b; c) CD spectra of (S)- and (R)-trans-3a and (S)-
and (R)-cis-3b in acetonitrile (c = 6ϫ10–5 m).
Since the chiral auxiliary in 3 is CD silent, we conclude
that the CD spectra of these compounds 3 are caused by
chiral induction and that the CD-active bands between 200
and 250 nm in compounds 1 and 2 can only be attributed
to an ECCD phenomenon between the naphthalene units.
Specific rotation values and ECD spectroscopy studies
demonstrate that the chiroptical properties of dyes 1–3 are
strongly dependent on (i) the trans or cis configuration of
the dye, (ii) substitution (i.e. H vs. F atoms) of the π-conju-
gated system, (iii) the grafted stereogenic center.
Single crystals of rac-2b and (R)-3b (Figure 2) were ob-
tained by slow crystallization (diffusing pentane vapor over [2] S. Garfield, in: Mauve: How One Man Invented a Colour that
dichloromethane solutions) and X-ray crystallographic
studies of rac-2b (P21/n group) and (R)-3b (P21 group)
clearly show the difluorinated-phenyl ring occupying a side
of the central naphthyl moiety where it points away from
the ester group.
In summary, simply adding a chiral auxiliary to a com-
mon industrially used azo dye gives robust and tunable chir-
optical switches. Esterification of Sudan I and ortho-di-
Acknowledgments
The authors thank the Engineering and Physical Sciences Research
Council (EPSRC) for support of this research though a Career Ac-
celeration Fellowship (EP/H003711/1) to S. P. F. and standard
grants EP/K006630/1, EP/M002144/1. The authors thank R. Tir-
foin (Univ. of Oxford) for assistance with the X-ray studies.
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Received: April 16, 2015
Published Online: ᭿
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E. Anger, S. P. Fletcher
SHORT COMMUNICATION
Chiral Industrial Azo Dye
E. Anger, S. P. Fletcher* ................... 1–6
Simple Azo Dyes Provide Access to Versa-
tile Chiroptical Switches
Keywords: Dyes / Molecular switch / Elec-
tronic circular dichroism / Chirality
The azo-core of dyes can undergo trans to
cis photoisomerization, which allows azo-
benzene derivatives to act as light triggered
molecular switches. Simple derivatization
of Sudan I using a chiral auxiliary provides
access to chiroptical molecular switches.
The properties of these switches can be re-
adily tuned by modification of the molec-
ular structure.
6
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