Diversely halogenated spiropyrans - Useful synthetic building blocks for a versatile class of molecular switches

Article abstract of DOI:10.1016/j.dyepig.2016.08.039

Spiropyrans are dyes that can be reversibly switched to a highly colored merocyanine form by a number of stimuli such as light, mechanical force or temperature. To make use of these molecules, there is a requirement to functionalize them appropriately. Herein we report a library of spiropyrans bearing two (pseudo) halide functional groups on either half of the molecule. Such halide substituents are valuable, because they themselves may be used as reactive sites in cross-coupling reactions, for example. Different combinations of halides, for which different reactivities in cross-coupling reactions may be expected, will facilitate selective consecutive cross-coupling reactions and condensations. Data concerning the UV/vis characteristics, the photostationary equilibria of the materials as well as the half-life of the merocyanine forms in solution are presented.

Full text of DOI:10.1016/j.dyepig.2016.08.039

Dyes and Pigments 136 (2017) 292e301
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Diversely halogenated spiropyrans - Useful synthetic building blocks
for a versatile class of molecular switches
a
b
a
a, c, d, *
€
Mathias Schulz-Senft , Paul J. Gates , Frank D. Sonnichsen , Anne Staubitz
a Otto-Diels-Institute for Organic Chemistry, University of Kiel, Otto-Hahn-Platz 4, 24098, Kiel, Germany
b School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK
c Institute for Organic and Analytical Chemistry, University of Bremen, Leobener Str. NW2 C, 28359, Bremen, Germany
d MAPEX Center for Materials and Processes, University of Bremen, Bibliothekstraße 1, 28359, Bremen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Spiropyrans are dyes that can be reversibly switched to a highly colored merocyanine form by a number
of stimuli such as light, mechanical force or temperature. To make use of these molecules, there is a
requirement to functionalize them appropriately. Herein we report a library of spiropyrans bearing two
(pseudo) halide functional groups on either half of the molecule. Such halide substituents are valuable,
because they themselves may be used as reactive sites in cross-coupling reactions, for example. Different
combinations of halides, for which different reactivities in cross-coupling reactions may be expected, will
facilitate selective consecutive cross-coupling reactions and condensations. Data concerning the UV/vis
characteristics, the photostationary equilibria of the materials as well as the half-life of the merocyanine
forms in solution are presented.
Received 18 July 2016
Received in revised form
14 August 2016
Accepted 16 August 2016
Available online 21 August 2016
Keywords:
Molecular switch
Spiropyran
Organic synthesis
3H-indole
© 2016 Elsevier Ltd. All rights reserved.
Photochromism
1. Introduction
to be present to detect switching events visually (Fig. 1).
The photochemical properties of the open merocyanine form,
Molecular switches are molecules that can exist in two or more
metastable states. They can be transformed from one state into the
other by applying external stimuli such as changes in pH, irradia-
tion with light or by mechanical force [1e3]. Molecular switches are
therefore instrumental for the development of new “intelligent”
materials, [4] or molecular machines [5]. Spiropyrans belong to the
most important molecular switches, because of the exceptionally
large variety of diverse stimuli that can be used for switching: Their
isomerization between a closed spiropyran form and an open
merocyanine form can be induced by light, [6] pH, [7e10] the
presence of ions, [11e13] pressure, [14] mechanical force
[1,2,14e21] as well as electric fields [19,22]. Of those stimuli, the
light induced switching provides to be easily accessible and
nondestructive. In addition, the absorption coefficient for the
merocyanine forms is extremely high (ca. 45,000 L molꢀ1 cmꢀ1 at
550 nm) [23] so that only very low concentrations of switches have
with its closed ring systems connected by an sp3 carbon centre, are
in striking contrast to the properties of the spiropyran.
Whereas spiropyrans consist of two separated
tems and therefore absorb only in the UV region of the spectrum,
merocyanines possess one planar -system. In this -system, the
p-electron sys-
p
p
electrons are delocalized across the entire molecule, which causes a
broad absorption maximum at ca. 550 nm. Irradiation with light of
wavelengths that correspond to the absorption maxima leads to a
decrease in the concentration of the irradiated species. Therefore,
UV irradiation induces the ring opening, whereas visible light fa-
cilitates the ring closure (Fig. 1).
Since 2007, spiropyrans have been used as mechanophores [6].
Therefore, they represent a relatively rare class of molecules in
which a mechanical force induces a chemical transformation.
Because spiropyrans are also chromophores, it is possible to visu-
alize the switching event. The mechanical rupture of the bond
between the spiro carbon atom and the oxygen atom can be
induced by grinding of the solid spiropyran [2]. The use of spi-
ropyrans as mechano active sensors in materials (such as polymers)
requires a covalent connection of the mechanophore to polymeric
chains [1]. This construction principle enables the material to
* Corresponding author. Institute for Organic and Analytical Chemistry, Univer-
sity of Bremen, Leobener Str. NW2 C, 28359, Bremen, Germany.
E-mail
addresses:
astaubitz@oc.uni-kiel.de,
staubitz@uni-bremen.de
(A. Staubitz).
http://dx.doi.org/10.1016/j.dyepig.2016.08.039
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
293
Fig. 1. top: Switching states of spiropyrans. The closed spiropyran form (SP) undergoes a ring opening upon irradiation with UV light (365 nm, purple flash) to the open mer-
ocyanine form (MC). Irradiation with visible light (565 nm, green flash) induces a ring closure and drives the equilibrium towards the SP side. Bottom: Absorption spectra and
photographs of the two states. The absorption spectrum of the SP is plotted in black, the spectrum of the MC in red. Flashes and vertical lines represent the wavelengths used for
switching. Whereas the colorless SP form does not absorb at wavelengths higher than 400 nm, the solution of MC shows a dark purple color which results from a broad absorption
with a maximum wavelength of approx. 550 nm. The solution in acetonitrile had a concentration of 65 mmol/L and irradiation times of 30 s were used. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
respond to stretching or compression by a color change [6].
halves of the molecule (indoline and chromene) with groups of
different reactivity promises a broader variety of options for further
functionalizations and thus a wider applicability of spiropyrans.
Therefore, to make spiropyrans amenable as electrophiles in
cross coupling reactions, a library of spiropyrans was synthesized
that contained several substitution patterns of bromide, iodide and
trifluoromethanesulfonyl as leaving groups.
Thus, mechanical forces applied to the periphery of the mole-
cule are being transferred to the central bond. DFT calculations
suggest a functionalization of the spiropyran at both, the chromene
and the indole side, which was confirmed experimentally [14].
However, the possibilities to access these two positions are very
limited. To date, a covalent incorporation of spiropyrans to the
backbones of polymers was achieved by six methods: electro-
polymerization, [24] introduction of an ATRP initiator by ester
condensation to a phenolic spiropyran, followed by radical poly-
merization, [18] polyurethane (PU) formation, [17] hydrosilation,
[19] ring-opening polymerization (ROP) with ε-caprolactone, [15]
ring-opening metathesis polymerization (ROMP), [21] and poly-
condensation by Suzuki coupling [25e27]. The incorporation into
polysiloxanes by hydrosilation as well as the usage of ATRP, ROP or
ROMP methods or polycondensations to form PU use hydroxyl
groups at the spiropyran. In contrast, due to a limited availability of
spiropyrans with halide functions, very few examples of function-
alization of spiropyrans by cross coupling have been reported
[25e27]. Especially the differentiating functionalization of the two
2. Experimental part1
Spiropyrans 1 are typically synthesized by a condensation re-
action of a Fischer's base and a salicylaldehyde 2 (Scheme 1, section
2.2.1) [28]. The Fischer's base is released in situ from an indolium
salt 3 under basic conditions. To establish a general procedure (9
examples) for the versatile functionalization of spiropyrans, it was
1
Further information concerning the used chemicals (supplier, purity, and pu-
rification procedures), equipment and experimental data (detailed procedures,
purification, characterization, NMR spectra) are provided in the supporting
material.

294
M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
Scheme 1. Synthetic overview of the syntheses of spiropyrans with X1 and X2 ¼ Br, I, OH. a) ethanol, piperidine (2 eq.), reflux, yields varying from 50 to 98%.
phenol 4 (Scheme 3). By a magnesium (II) directed ortho-for-
mylation with paraformaldehyde, the aldehyde functions were
selectively introduced in moderate to good yields of 73% (X2 ¼ Br)
or 49% (X2 ¼ I, Lit. 84% [29]) to obtain the salicylaldehydes 5a and
5b. The procedure for the synthesis of the 2-hydroxy-3-
iodobenzaldehyde was reported before [29] and could be adapted
for the synthesis of the bromo functionalized species. A meta-se-
lective nitration was performed to prepare the nitro-
salicylaldehydes 2a and 2b in moderate yields of 59% (X2 ¼ Br) or
39% (X2 ¼ I). An equimolar amount of fuming nitric acid in acetic
acid was used as nitrating agent.
The synthesis of 3-hydroxysalicylaldehyde (2c) started from
ortho-vanillin (6) [14]. First, the nitro group was introduced using
fuming nitric acid in acetic acid as nitrating agent in a yield of 56%
(Lit. 50% [14]). Afterwards, the methoxy group was transferred into
a hydroxy group using hydrobromic acid in an excellent yield of 95%
(Lit. 86% [14]).
Scheme 2. Introduction of trifluoromethylsulfonates. a) Tf2O, pyridine, DCM, 0e40 ꢁC.
crucial to synthesize these precursors 2 (section 2.1.1) and 3 (sec-
tion 2.1.2) bearing the desired functional groups, halides and
hydroxides.
The hydroxy groups were to be converted into tri-
fluoromethylsulfonates after the spiropyran formation (Scheme 2).
Therefore, we established a versatile synthesis (5 examples) using
trifluoromethylsulfonic anhydride as electrophile in DCM/pyridine
as solvent/base system under a nitrogen atmosphere (section
2.2.2).
2.1. Syntheses of the precursor molecules
2.1.2. Indolium salts
2.1.1. Salicylaldehyde derivatives
The 3-halide-5-nitrosalicylaldehydes (2a and 2b) were synthe-
sized in a two-step reaction sequence, starting from 2-halide
The indole species were prepared as indolium iodide salts 3aec
in a linear synthetic route, starting from 1,4-substituted phenyl
hydrazines 7 (Scheme 4). It was possible to prepare the trimethyl-
Scheme 3. Synthetic routes to obtain the salicylaldehyde precursors. a) paraformaldehyde, magnesium chloride, trimethylamine, THF, reflux. b) nitric acid/acetic acid, 15e40 ꢁC. c)
nitric acid in acetic acid, 15 ꢁC. d) hydrobromic acid, reflux. Overall yields: X2 ¼ Br: 43%, X2 ¼ I: 19%, X2 ¼ OH: 53%.

M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
295
Scheme 4. Synthetic route to obtain indolium salts. a) 3-methyl-2-butanone; for X1 ¼ OMe: ethanol, reflux; for X2 ¼ Br, I: acetic acid, reflux. b) for X1 ¼ OMe: hydrobromic acid,
reflux. c) iodomethane, reflux. Overall yields: X1 ¼ Br: 73%, X2 ¼ I: 73%, X1 ¼ OH: 56%.
Table 1
Synthesized spiropyrans. The starting materials 2aec and 3aec were used as equimolar solution in EtOH (17 mol/L), 2 eq. of piperidine were added and the solution was heated
to reflux for 3e5 h.
Entry
Compound
X1
X2
Yielda
1
2
3
4
5
6
7
8
9
9a
9b
9c
9d
9e
9f
9g
9h
9i
Br
Br
Br
I
I
I
OH
OH
OH
Br
I
OH
Br
I
OH
Br
I
78%
50%
88%
79%
62%
75%
90%
83%
98%b
OH
a
The spiropyrans were obtained as solids and are likely mixtures of open and closed form.
Reported yield: 99% [14].
b
3H-indoles bearing bromide (8a), iodide (8b) and methoxy (8c)
74% [32]). The 5-hydroxy indolium salt 3c was synthesized using a
method previously reported [14,18].
Further details (exact procedures, yields and analytical data) are
provided in the supplementary material.
substituents at the 5-position via a Fischer indole synthesis in good
yields ranging from 76% to 84%. The methoxy group in the 5-
position was cleaved with a yield of 79% using hydrobromic acid
to give the 5-hydroxy indole 8d. Three trimethyl-3H-indoles (8a, b,
d) were successfully transferred into tetramethyl-3H-indolium io-
dide salts (3aec) in excellent yields >86% using iodomethane as
both, solvent and reagent.
A method which had already been reported for the synthesis of
5-bromo indole was used to synthesize both, the 5-bromo (8a, 84%,
Lit. 97% [30]) and 5-iodo (8b) precursor. The synthesis of the 5-iodo
indolium salt 3b has been reported earlier using different condi-
tions which gave lower yields compared to our route (70% [31] and
2.2. Syntheses of spiropyrans
2.2.1. Spiropyran condensations
Spiropyrans were synthesized from indolium salts 3aec and
salicylaldehydes 2aec using a versatile piperidine promoted pro-
cedure in ethanol as solvent (Table 1). The base was required to
induce the in situ formation of 2-methyleneindolines (Fischer's
bases) as reactive species from indolium salts 3. For the synthesis of

296
M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
Table 2
Introducing trifluoromethanesulfonyl groups into spiropyrans. Trifluoromethanesulfonyl anhydride (1.2 eq. per OH group) was used as electrophile, pyridine (4 eq. per OH
group) as base and DCM (0.25 mol/L) as solvent.
0
0
Entry
Compound
X1
X2
Yielda
1
2
3
4
5
9j
9k
9l
9m
9n
Br
I
OTf
OTf
OTf
OTf
OTf
Br
74%
56%
43%
43%
89%
I
OTf
a
The spiropyrans were obtained as solids and are likely to predominantly consist of the open merocyanine form.
spiropyrans, the use of indolium salts is advantageous compared to
directly using the corresponding Fischer's bases as starting mate-
rials, because indolium salts are stable against air and moisture and
as solids easy to handle. All spiropyran products were obtained
after crystallization from the reaction mixture. If necessary, they
were further purified by recrystallization. The spiropyrans bearing
either bromide, iodide or hydroxy functions showed a negative
photochromism on silica gel. This means that they have ring
opened to give the zwitterionic merocyanine isomer whose hy-
droxy groups can interact with the silica surface by hydrogen
bonding leading to severe yield losses for the purification via col-
umn chromatography [11].
groups using column chromatography or filtration through silica gel.
Nevertheless, in the case of 9k e m (Table 2, Entry 2 to 4), the
binding of the merocyanine isomer to silica gel was noticeable
resulting in somewhat lower yields of 43 to 56%. Especially the
trifluoromethanesulfonyl groups at the indolene side of the mole-
cule (9l and 9m, entries 3 and 4) showed significant binding to silica
gel, so that 9l could not be purified by chromatography. Therefore,
several washing and precipitation steps were needed to remove
pyridine and trifluoromethanesulfonic anhydride from the product.
This also resulted in loss of yield.
2.3. Sample preparation for UV/vis and PSS characterizations
The colors of the reaction mixtures and the corresponding iso-
lated products were mostly dark brown, violet or blue. This sug-
gested the presence of merocyanine species in the solid. We
characterized the products using solution based techniques like
NMR- or UV/vis-spectroscopy and therefore could not determine
the exact composition of the solids as synthesized. The spiropyrans'
solvatochromism [6,33] was used to optimize the solvents for the
NMR-characterization in order to obtain pure spectra of the closed
or the corresponding open species, respectively.
A stock solution of each new spiropyran (100 mL, ca. 0.1 mmol/L)
in acetonitrile was prepared. A defined volume (1e5 mL) was taken
out of this solution and diluted to 10 mL to obtain the various
desired sample concentrations (ca. 10e50 mmol/L).
The exact concentrations of the stock solutions are provided in
the supplementary material.
For the determination of the ratio between spiropyran and
merocyanine species in dark environment by 1H NMR spectroscopy,
500 mL of the most concentrated solution (ca. 50 mmol/L) and 50 mL
of deuterated acetonitrile were combined.
2.2.2. Transformation of hydroxy groups into
trifluoromethanesulfonyl groups
The functionalization of the hydroxy groups to give the corre-
sponding trifluoromethanesulfonyl groups was accomplished using
trifluorosulfonyl anhydride as trifluorosulfonylating agent and pyr-
idine as base in DCM as a solvent (Table 2). Under strict exclusion of
oxygen and water, it was possible to obtain the five desired spi-
ropyrans in good yields varying from 43% to 89%. The transformation
of hydroxy groups into trifluoromethane-sulfonyl groups sup-
pressed the inverse photochromism on silica gel. Therefore, it was
possible to purify the spiropyrans with trifluoromethanesulfonyl
2.4. Photochromic properties
All the synthesized spiropyrans were examined with respect to
their photochromic properties. UV/vis spectra of all spiropyrans
were measured while ensuring that measurements were taken in a
concentration range in which the Beer-Lambert law is valid: Solu-
tions of each spiropyran in acetonitrile were prepared in five
different concentrations to obtain the absorption coefficients. This
was done for the photostationary states after irradiation with

M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
297
Table 3
Absorption maxima and half-life times from the photostationary states. In some cases, a half-life time could not be determined, as explained in the footnotes.
Entry
Compound
X1
X2
Absorption maximuma
* transition of chromene
Half-life time t1/2 (min:sec)a
pꢀ
p
Merocyanine transition
After PSS565
After PSS365
1
2
3
4
5
6
7
8
9a
9b
9c
9d
9e
9f
9g
9hd
9j
9k
9l
9m
9n
Br
Br
Br
I
I
I
OH
OH
Br
I
OTf
OTf
OTf
Br
I
OH
Br
I
OH
Br
I
OTf
OTf
Br
I
315 nm
306 nm
355 nm
308 nm
314 nm
355 nm
320 nm
320 nm
308 nm
311 nm
331 nm
335 nm
304 nm
556 nm
559 nm
568 nm
559 nm
561 nm
570 nm
547 nm
549 nm
536 nm
538 nm
561 nm
557 nm
537 nm
23:07
22:16
n.n.b
25:07
08:51
n.n.b
15:21
11:28
00:55
14:23
13:21
01:12
n.n.c
285:32
271:45
253:17
208:11
n.n.b
n.n.c
9
n.n.c
10
11
12
13
n.n.c
02:43
02:41
56:30
n.n.b
OTf
121:20
a
In acetonitrile, concentrations ranged between 10 and 50
The absorption spectra of the PSS (565 nm) and of the equilibrium in the dark showed no differences.
The concentration of merocyanine at the PSS (365 nm) was lower than the concentration of merocyanine in dark environment.
Because the UV/vis spectrum of the highest concentration showed a strong deviation from the expected linear increase, only the four other concentrations were used for
m
mol/L.
b
c
d
the calculation of molar absorbances.
Fig. 2. 1H NMR spectroscopic determination of the ratio between 9eMC and 9eSP form in dark equilibrium. a) Structures of the species present in solution. The investigated protons
are highlighted. b) Excerpt of the aromatic region of the 1H NMR spectrum of the solution in acetonitrile after relaxation in dark environment for 24 h (sample concentration:
34 m
mol/L). The specific protons are highlighted in orange (9eSP) and green (9eMC). Integration gives a ratio of 96/4. Grey boxes are used to mark residual impurities of deuterated
acetonitrile. The third integral is used to determine the error of the integration. In this example, the integral of the baseline is 0.01. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)

298
M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
visible and UV light (565 nm and 365 nm) as well as for the equi-
libria in the dark. The exact ratio (and thus, with the information of
the total sample amount the concentration) of open and closed
form in the dark was determined by 1H NMR-measurements and
used to calculate both, the molar absorption coefficient of the
merocyanine form and its amount in the PSS365
.
The spectra and calculations for the diiodo functionalized dye 9e
will be described in greater detail as an example. All other spectra
are provided in the supplementary material. Detailed information
on the used UV-light and visible light irradiation sources are also
provided there.
2.4.1. Photostationary state: visible light (
l
¼ 565 nm)
Before the sample solutions were measured, the photochemical
equilibrium between closed spiropyran and open merocyanine
form was shifted towards the closed form by irradiation with visible
light (
l
¼ 565 nm) until the UV/vis spectra did not change anymore,
which typically took about 30 s. For the determination of the ab-
sorption coefficient, solutions with five different concentrations in
the range of 5e50
The spectra showed two major absorption bands, representing
* transitions in the indolene (~250e290 nm) and the chro-
mmol/L of spiropyrans in acetonitrile were used.
pꢀp
mene moiety (~300e350 nm) [33,34]. In the photostationary
equilibrium under these conditions no merocyanine form was
present2 (which had a typical broad absorption band at ca.
500e600 nm) [6]. Since only the chromene moiety contributes to a
light yellow color of the solution, this absorption wavelength (in the
case of 9e: 314 nm, see Table 3 for all maxima values) could be used
for the calculation of the molar extinction coefficient. Because these
absorption spectra do not contain any significant absorption at
wavelengths higher than 500 nm, no merocyanine form is present.
An overview of the determined properties of all spiropyran dyes
9aen is provided in Table 3. A detailed description of the used
analysis procedure and the plotted UV/vis spectra of the PSS565 and
their concentration dependence are provided in the supplementary
material.
2.4.2. Photostationary state: UV light (
l
¼ 365 nm)
The solutions used in section 2.4.1 were irradiated with UV light
(
l
¼ 365 nm) until an equilibrium was reached, which typically
Fig. 3. Relaxation of the 9e system to an equilibrium (Sample concentration: 38 mmol
took approx. 30 s. Plots of the obtained absorption spectra contain
an absorption maximum at ca. 550 nm (For 9e: 561 nm). Since the
spiropyran form 9aenSP does not absorb at wavelengths higher
than 500 nm, the observed absorption maximum depends
completely on the amount of merocyanine 9aenMC present in the
solution. A detailed description of the used analysis procedure and
the plotted UV/vis spectra of the PSS365 and their concentration
dependence are provided in the supplementary material.
in acetonitrile). Top: After stopping irradiation with green light (565 nm, until the
photostationary state was reached), the merocyanine absorption at 561 nm increased
with a half-life time of 8 min 51 s bottom: Irradiation of the sample with UV light
(365 nm, until the photostationary state was reached) enriched the merocyanine
species. After removal of the illumination source, this was found to decay with a half-
life time of 13 min 21 s. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
open form which formed under dark conditions by both, UV/vis and
1H NMR spectroscopy. The UV/vis solution or the NMR sample,4
respectively, were kept dark for at least 24 h prior to the mea-
surement. Following the earlier described procedure, it was
possible to calculate the concentration dependence of the absorp-
tion maximum corresponding to the merocyanine transition
(typically circa 550 nm). In the case of 9e, it is 828 L (mol cm)ꢀ1. By
1H NMR spectroscopy, the ratio 9eMC/9eSP was determined to be 4/
96. For a comparison of the two species, the signals corresponding
to protons of the ethylene bridge (H-3 and H-4) in the chromene
moiety were used (Fig. 2). These protons form doublets, with
coupling constants of circa 3JZ ¼ 10.5 Hz for 9eSP and 3JE ¼ 16.5 Hz
2.4.3. Equilibrium in a dark environment
For all synthesized dyes 9aen, the PSS365 decayed with a half-
life time for the decay of less than 30 min3. Therefore, it was not
possible to directly determine the amount of merocyanine in the
PSS365 via 1H NMR spectroscopy. Due to the known formation of J-
and H-aggregates by merocyanines, [6] increasing the concentra-
tion for faster NMR measurements was not an option.
It was possible to measure the equilibrium between closed and
2
In the case of 9c and 9f, after irradiation with visible light, the solution con-
tained a significant amount of the merocyanine species in the PSS.
3
For the dyes 9gek, a decay measurement was not performed, because the
concentration of merocyanine was lower at the PSS365 compared to dark conditions.
This is because the merocyanine form also absorbs at this wavelength, leading to
the formation of the spiropyran form, an effect that is not present in the dark
equilibrium.
4
A detailed procedure of sample preparation is provided in the supplementary
material.

M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
299
for 9eMC
Thereby, the molar extinction coefficient of 9eMC was calculated
as follows:
.
observed maximum wavelengths between 536 nm (9j; entry 9) and
570 nm (9f; entry 6). Again, a change in the substitution of the
chromene side results in large changes of the absorption maxima's
wavelengths. Trifluoromethylsulfonyl groups induce
a hyp-
828
4% mol cm
L
L
εMC;561
¼
¼ 20700
sochromic shift of about 20 nm compared to bromide and iodide
functionalities, whereas hydroxy groups induce a bathochromic
shift of about 10 nm compared to the halide functionalities. Varying
the substitution of the indole side (X1) does not result in a larger
change of the absorption wavelength. For example, compounds
9a,d,g,l (entries 1,4,7,11) all bear a bromide as functional group at
the chromene side, while the functional groups at the indole side
vary: bromide (9a; entry 1), iodide (9d; entry 4), hydroxy (9g; entry
7), Trifluoromethylsulfonyl (9l; entry 11). The observed absorption
wavelengths vary only between 547 (9g; entry 7) and 561 nm (9l;
entry 7). For this absorption, hydroxy groups induce a hyp-
sochromic shift, compared to the dyes with bromide or iodide at
this position. Trifluoromethylsulfonyl groups at the indole side do
not significantly influence the absorption maximum of the mer-
ocyanine transition significantly.
mol cm
2.4.4. Relaxation of the photostationary states
Eventually, we also examined the relaxation of the photosta-
tionary states (Fig. 3). To analyze the relaxation from the spiropyran
form, the sample was irradiated at 565 nm to reach the photosta-
tionary state with enhancement of the spiropyran; then the light
was switched off and the UV/vis spectra measured in dependence
of the time. The absorbance at 561 nm, which corresponds to the
absorption maximum increased with a half-life time of 8 min 51 s.
The same procedure was used for analyzing the relaxation from the
merocyanine form: irradiation at 365 nm to enhance the mer-
ocyanine concentration to equilibrium, switching off the light and
recording the absorbance at 561 nm. The half-life time in this case
was 13 min 21 s.
Relaxation of the systems after the PSS565 takes place with
longer half-life times (08:51e285:32 min) than the relaxation after
PSS365 (0:55e56:30 min). For 3 compounds (9c,f,m; entries 3,6,12),
it was not possible to determine the half-life time, because the
absorption spectra at the PSS565 and the equilibrium in dark are the
same. The shortest half-life time was observed for compound 9e
(entry 5), which bears two iodide functionalities. Compounds with
either hydroxy groups at the indole side (9g,h; entries 7,8) or tri-
fluoromethylsulfonyl groups at the chromene side (9j,k; entries
9,10) show the slowest relaxation with half-life times ranging from
208 to 285 min. Two compounds with trifluoromethylsulfonyl
groups at the chromene side (9j,k; entries 9,10) and two com-
pounds with hydroxy groups at the indole side (9g,h; entries 8,9)
could not be analyzed concerning their decay after PSS365, because
the amount of merocyanine was higher in the dark equilibrium. The
other compounds with trifluoromethylsulfonyl or hydroxy groups
show the fastest decay, with half-life times ranging from 55 s (9c;
entry 3) to 2:41 min (9m, entry 12). Most other relaxations show
half-life times in the range of approx. 10e15 min, only the dye with
two trifluoromethylsulfonyl groups (9n, entry 13) shows a signifi-
cantly slower decay (t1/2 ¼ 56:30 min).
3. Results and discussion
The absorption spectra and photochromic properties of 13
formerly unknown spiropyrans were investigated. Table 3 contains
the wavelengths of the absorption maxima of the
of the chromene and merocyanine moieties as well as the observed
half-life times of the systems' relaxation from the photostationary
states towards the equilibrium in dark. The
p p* transitions
ꢀ
p p* transitions of the
ꢀ
chromene moieties in the closed forms 9aenSP vary between
304 nm (9nSP; Table 3, entry 13) and 355 nm (9c,f; entries 3,6). As
expected, a change of the substitution at this side of the molecule
results in a large change of these absorption maxima. While bro-
mide (9a,d,g,l; entries 1,4,7,11), iodide (9b,e,h,m; entries 2,5,8,12)
and trifluoromethylsulfonyl groups (9j,k,n; entries 9,10,13) at the
chromene side show similar wavelengths between 304 and
320 nm, hydroxyl functions (9c,f; entries 3,6) result in a bath-
ochromic shift with absorption maxima at wavelengths of 355 nm.
A similar trend can be found for the merocyanine transition with
Table 4
Determination of the molar extinction coefficient of merocyanine and the merocyanine: spiropyran ratio at the PSS (365 nm). A table showing errors of the 1H NMR integration
and resulting errors of the other properties is provided in the supplementary material.
Entry
Compound
Equilibrium in dark environment
PSS (365 nm)
Molar absorption (L (mol cm)ꢀ1
)
Ratio [MC]:[SP]
ε
MC (L (mol cm)ꢀ1
)
Molar absorption (L (mol cm)ꢀ1
)
Ratio [MC]:[SP]
1
2
3
4
5
6
7
8
9a
9b
9c
9d
9e
9f
9g
9h
9j
9k
9lc
9m
9n
1858
1082
2424
1576
828
2209
31,621
15,851
21,876
23,936
336
08:92
12:88
07:93
07:93
04:96
06:94
58:42
56:44
59:41
52:48
02:98
16:84
27:73
23,225
9327
30,591
11,500
27,520
19,770
8320
24,215
1747
1972
6885
5274
24,402
6292
11,532
100:0a
100:0a,b
79:31
88:12
40:60
66:34
03:97
07:93
19:81
11:89
100:0d
100:00d
45:55
34,628
22,514
20,700
36,817
54,519
28,305
37,078
43,031
16,800
1806
9
10
11
12
13
289
6879
25,477
a
The calculated εMC led to an apparent amount of more than one equivalent of merocyanine in the PSS365, which is due to an integration error in this 1H NMR spectroscopic
measurement of 0.01. However, the observed extinction coefficient in the PSS is included in the error ranges. Plots of all NMR spectra including integrals and a detailed table
with errors are provided in the supplementary material.
b
A different proton signal was used for integration, due to an overlap of the ethylene protons of the merocyanine species.
c
Due to very low total absorptions of the merocyanine transition, it was necessary to use concentrations from 60 to 100 m
mol/L for UV/vis and 1H NMR spectroscopy.
d
The calculated εMC led to an apparent amount of more than one equivalent of merocyanine in the PSS365. The measured absorptions in the dark were close to the device's
detection limits.

300
M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
For 13 spiropyran dyes 9aen we were able to determine the
Lindhorst for the donation of a 6 month Ph. D. position for M. S.-S..
This research has been supported by the Institutional Strategy of
the University of Bremen, funded by the German Excellence
Initiative.
molar extinction coefficients of the merocyanine transitions (see
Table 4). Therefore, the absorption at the equilibrium state in dark
was plotted against the overall concentration. The obtained values
range from 289 L (mol cm)ꢀ1 (9m; entry 12) to 31,621 L (mol cm)ꢀ1
(9g; entry 7). While compounds with only halide functionalities
(9a,b,d,e; entries 1,2,4,5) or trifluoromethylsulfonyl functional
groups at the indole side (9l,m; entries 11,13) show low absorptions
coefficients at approx. 1000 L (mol cm)ꢀ1, hydroxy groups at the
chromene side rise the absorption coefficients to approx.
2500 L (mol cm)ꢀ1 (9c,f; entries 3,6). The introduction of hydroxy
groups at the indole side increases the measured absorptions
further to values of 15 or 30 kL (mol cm)ꢀ1 (9g,h; entries 7,8).
Transformation of hydroxy to trifluoromethylsulfonyl groups at the
chromene side increases the absorption to approx.
20 kL (mol cm)ꢀ1 (9j,k; entries 9,10). These absorptions originate
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.08.039.
References
[1] Caruso MM, Davis DA, Shen Q, Odom SA, Sottos NR, White SR, et al. Me-
chanically-induced chemical changes in polymeric materials. Chem Rev
2009;109:5755e98. http://dx.doi.org/10.1021/cr9001353.
[2] Beyer MK, Clausen-Schaumann H. Mechanochemistry: the mechanical acti-
vation of covalent bonds. Chem Rev 2005;105:2921e48. http://dx.doi.org/
10.1021/cr030697h.
[3] Schulz-Senft M, Lipfert M, Staubitz A. Mechanopolymerchemie: Molekulare
Wirkung durch Kraft. Chem Unserer Zeit 2014;48:200e14. http://dx.doi.org/
10.1002/ciuz.201400640.
[4] Natali M, Giordani S. Molecular switches as photocontrollable “smart” re-
ceptors. Chem Soc Rev 2012;41:4010e29. http://dx.doi.org/10.1039/
C2CS35015G.
[5] Wiley: Molecular Switches, 2nd, Completely Revised and Enlarged Edition, 2
Volume Set - Ben L. Feringa, Wesley R. Browne n.d, http://eu.wiley.com/
WileyCDA/WileyTitle/productCd-3527313656.html (accessed May 13, 2015).
[6] Klajn R. Spiropyran-based dynamic materials. Chem Soc Rev 2014;43:148e84.
http://dx.doi.org/10.1039/c3cs60181a.
[7] Florea L, Diamond D, Benito-Lopez F. Photo-responsive polymeric structures
based on spiropyran. Macromol Mater Eng 2012;297:1148e59. http://
dx.doi.org/10.1002/mame.201200306.
solely from absorptions of the merocyanine species 9aenMC
.
Thereby, the ratios 9MC:9SP were determined by 1H NMR spec-
troscopy under exclusion of light and could be used to calculate the
molar extinction coefficients εMC of the merocyanine forms. These
values range from 9327
L
(mol cm)ꢀ1 (9b; entry 2) to
54,519 L (mol cm)ꢀ1 (9g; entry 7).
The measured absorptions at the PSS365 were divided by the
calculated molar extinction coefficients to obtain the ratio
[MC]:[SP]. Three compounds (9a,b,m; entries 1,2,12) show a com-
plete switch. For the other compounds, the amount of 9MC is be-
tween 3 and 88%. The four compounds which had the highest molar
absorption in the dark show significantly lower absorptions at the
PSS365 and thereby a smaller proportion of 9MC is present (3e19%;
9fek; entries 6e10).
[8] Raymo FM, Giordani S. Signal processing at the molecular level. J Am Chem
Soc 2001;123:4651e2. http://dx.doi.org/10.1021/ja005699n.
[9] Wojtyk JTC, Wasey A, Xiao N-N, Kazmaier PM, Hoz S, Yu C, et al. Elucidating
the mechanisms of acidochromic spiropyran-merocyanine interconversion.
J Phys Chem A 2007;111:2511e6. http://dx.doi.org/10.1021/jp068575r.
[10] Keum S-R, Lee K-B, Kazmaier PM, Buncel E. A novel method for measurement
of the merocyanine-spiropyran interconversion in non-activated 1, 3, 3-
trimethylspiro- (2H-1-benzopyran-2, 20-indoline) derivatives. Tetrahedron
Lett 1994;35:1015e8. http://dx.doi.org/10.1016/S0040-4039(00)79953-9.
[11] Kinashi K, Nakamura S, Imamura M, Ishida K, Ueda Y. The mechanism for
negative photochromism of spiropyran in silica: negative photochromism of
4. Conclusion
Thirteen novel spiropyran dyes were synthesized and charac-
terized in terms of their UV/vis spectra and their switching prop-
erties. Eight of these dyes were synthesized in good to excellent
yields (50e90%) by condensation reactions. In addition, a versatile
method (five examples) was established that gives access to tri-
fluoromethane sulfonyl groups as functionalization of spiropyrans
in good yields (43e89%). Importantly, the functional groups that
were introduced (I, Br, OTf) are all highly suitable for cross-coupling
reactions, enabling access to spiropyrans functionalized with un-
saturated groups which was not possible before. Owing to the
combination of halide with/and hydroxyl functionalities on the
molecules, condensation reactions with the hydroxyl groups can be
performed on one end of the molecule, whereas the halide gives
access to cross-coupling based extensions of the spiropyran scaf-
fold. The decay times of the photostationary states at the absorp-
tion maxima merocyanine (365 and 565 nm) were determined and
vary between 55 s and 56 min for 365 nm and between 9 min and
285 min for 565 nm. It was possible to determine the amount of
merocyanine in the photostationary states (365 nm) (3e100%) by
applying a combined UV/vis and 1H NMR spectroscopic analysis.
This method also allowed us to calculate the molar extinction co-
efficients of the merocyanine species (9.3e25 k (L (mol cm)ꢀ1)).
Work is ongoing to establish selective cross-coupling procedures
for this new library of spiropyrans.
spiropyran.
poc.1926.
J Phys Org Chem 2012;25:462e6. http://dx.doi.org/10.1002/
[12] Kinashi K, Harada Y, Ueda Y. Thermal stability of merocyanine form in spi-
ropyran/silica composite film. Thin Solid Films 2008;516:2532e6. http://
dx.doi.org/10.1016/j.tsf.2007.04.120.
[13] Wojtyk JTC, Kazmaier PM, Buncel E. Modulation of the spiropyr-
anemerocyanine reversion via metal-ion selective complexation: trapping of
the “transient” cis-merocyanine. Chem Mater 2001;13:2547e51. http://
dx.doi.org/10.1021/cm010038q.
[14] Davis DA, Hamilton A, Yang J, Cremar LD, Van Gough D, Potisek SL, et al. Force-
induced activation of covalent bonds in mechanoresponsive polymeric ma-
terials. Nature 2009;459:68e72. http://dx.doi.org/10.1038/nature07970.
[15] O'Bryan G, Wong BM, McElhanon JR. Stress sensing in polycaprolactone films
via an embedded photochromic compound. ACS Appl Mater Interfaces
2010;2:1594e600. http://dx.doi.org/10.1021/am100050v.
[16] Kingsbury CM, May PA, Davis DA, White SR, Moore JS, Sottos NR. Shear
activation of mechanophore-crosslinked polymers. J Mater Chem 2011;21:
8381. http://dx.doi.org/10.1039/c0jm04015k.
[17] Lee CK, Davis DA, White SR, Moore JS, Sottos NR, Braun PV. Force-induced
redistribution of a chemical equilibrium. J Am Chem Soc 2010;132:16107e11.
http://dx.doi.org/10.1021/ja106332g.
[18] Potisek SL, Davis DA, Sottos NR, White SR, Moore JS. Mechanophore-linked
addition polymers.
10.1021/ja076189x.
J Am Chem Soc 2007;129:13808e9. http://dx.doi.org/
[19] Wang Q, Gossweiler GR, Craig SL, Zhao X. Cephalopod-inspired design of
electro-mechano-chemically responsive elastomers for on-demand fluores-
cent patterning. Nat Commun 2014;5:4899. http://dx.doi.org/10.1038/
ncomms5899.
[20] Gossweiler GR, Brown CL, Hewage GB, Sapiro-Gheiler E, Trautman WJ,
Welshofer GW, et al. Mechanochemically active soft robots. ACS Appl Mater
Interfaces 2015;7:22431e5. http://dx.doi.org/10.1021/acsami.5b06440.
[21] Gossweiler GR, Kouznetsova TB, Craig SL. Force-rate characterization of two
spiropyran-based molecular force probes. J Am Chem Soc 2015;137:6148e51.
http://dx.doi.org/10.1021/jacs.5b02492.
Acknowledgements
This project was supported by the Special Research Area 677
“Function by Switching” of the Deutsche Forschungsgemeinschaft
(DFG). Initial funding was provided by the DECHEMA (Gesellschaft
für Chemische Technik und Biotechnologie) by a Max-Buchner-
fellowship for A. S.. We are grateful to Prof. Dr. Thisbe K.
[22] Wagner K, Byrne R, Zanoni M, Gambhir S, Dennany L, Breukers R, et al.
A
multiswitchable poly(terthiophene) bearing a spiropyran functionality:
understanding photo- and electrochemical control. J Am Chem Soc 2011;133:
5453e62. http://dx.doi.org/10.1021/ja1114634.

M. Schulz-Senft et al. / Dyes and Pigments 136 (2017) 292e301
301
[23] Buback J, Kullmann M, Langhojer F, Nuernberger P, Schmidt R, Würthner F,
et al. Ultrafast bidirectional photoswitching of a spiropyran. J Am Chem Soc
2010;132:16510e9. http://dx.doi.org/10.1021/ja1062746.
[24] Wagner K, Zanoni M, Elliott ABS, Wagner P, Byrne R, Florea LE, et al.
A merocyanine-based conductive polymer. J Mater Chem C 2013;1:3913.
http://dx.doi.org/10.1039/c3tc30479e.
[29] Brady RM, Hatzis E, Connor T, Street IP, Baell JB, Lessene G. Synthesis of
conformationally constrained benzoylureas as BH3-mimetics. Org Biomol
Chem 2012;10:5230e7. http://dx.doi.org/10.1039/c2ob25618e.
€
[30] Volker SF, Renz M, Kaupp M, Lambert C. Squaraine dyes as efficient coupling
bridges between triarylamine redox centres. Chem Eur J 2011;17:14147e63.
http://dx.doi.org/10.1002/chem.201102227.
[25] Sommer M, Komber H. Spiropyran main-chain conjugated polymers. Macro-
[31] Fegan A, Shirude PS, Balasubramanian S. Rigid cyanine dyenucleic acid labels.
Chem Commun 2008:2004e6. http://dx.doi.org/10.1039/B801629A.
[32] Gerowska M, Hall L, Richardson J, Shelbourne M, Brown T. Efficient reverse
click labeling of azide oligonucleotides with multiple alkynyl Cy-Dyes applied
to the synthesis of HyBeacon probes for genetic analysis. Tetrahedron
2012;68:857e64. http://dx.doi.org/10.1016/j.tet.2011.11.041.
[33] Keum S-R, Roh S-J, Ahn S-M, Lim S-S, Kim S-H, Koh K. Solvatochromic
behavior of non-activated indolinobenzospiropyran 6-carboxylates in
aqueous binary solvent mixtures. Part II1. Dyes Pigments 2007;74:343e7.
http://dx.doi.org/10.1016/j.dyepig.2006.02.013.
mol
Rapid
Commun
2013;34:57e62.
http://dx.doi.org/10.1002/
marc.201200688.
[26] Komber H, Müllers S, Lombeck F, Held A, Walter M, Sommer M. Soluble and
stable alternating main-chain merocyanine copolymers through quantitative
spiropyranemerocyanine conversion. Polym Chem 2014;5:443. http://
dx.doi.org/10.1039/c3py00853c.
[27] Metzler L, Reichenbach T, Brügner O, Komber H, Lombeck F, Müllers S, et al.
High molecular weight mechanochromic spiropyran main chain copolymers
via reproducible microwave-assisted Suzuki polycondensation. Polym Chem
2015;6:3694e707. http://dx.doi.org/10.1039/C5PY00141B.
[34] Tyer NW, Becker RS. Photochromic spiropyrans. I. Absorption spectra and
evaluation of the .pi.-electron orthogonality of the constituent halves. J Am
Chem Soc 1970;92:1289e94. http://dx.doi.org/10.1021/ja00708a031.
[28] Minkin VI. Photo-, Thermo-, Solvato-, and Electrochromic Spiroheterocyclic
Compounds. Chem Rev 2004;104:2751e76. http://dx.doi.org/10.1021/
cr020088u.