Investigations on the photochromic properties of 2,6-bis(5-bromo-2-hydroxybenzylidene)cyclohexanone

Article abstract of DOI:10.1039/c6pp00466k

The network of chemical reactions of 2,6-bis(5-bromo-2-hydroxybenzylidene)cyclohexanone (BHBC) when subjected to light and different pH values has been investigated. The pH dependent species involved in the chemical network have been identified and charac

Full text of DOI:10.1039/c6pp00466k

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Investigations on photochromic properties of 2,6-bis(5-bromo-2-
hydroxybenzylidene)cyclohexanone
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
a
a
a*
L. N. Corîci , S. Shova , V. Badea , D. Aparaschivei , O. Costisor , L. Cseh
DOI: 10.1039/x0xx00000x
The network of chemical reactions of 2,6-bis(5-bromo-2-hydroxybenzylidene)cyclohexanone (BHBC) when submitted to
light and different pH values has been investigated. The pH dependent species involved in the chemical network have
been identified and characterized by NMR and UV-VIS spectroscopy. Direct pH jumps were carried out by adding strong
acid to equilibrated solutions of trans-chalcone (Ct) forming the flavylium cation which was stable only at extremely acidic
conditions (pH < 0.5). The single crystal X-ray study and NMR analysis has confirmed the structure of new flavylium cation.
www.rsc.org/
2
-
In the case of reverse pH jump, the Ct species interconverted instantaneously into deprotonated trans-chalcone (Ct )
around pH 12. A new colorless compound 3,11-dibromo-7,8-dihydro-6H-chromeno[3,2-d]xanthenes (B-B) isolated from
the equilibrated solution of trans-chalcone species in methanol after long periods of time (100 h) in dark conditions have
been isolated and fully characterized by NMR and X-ray diffraction. The rate of the reaction increased when the solution of
trans-chalcone was exposed to light and total conversion of Ct into spiropyran-like compound (B-B) was achieved in about
3
0 minutes. The B-B form was stable at in neutral and basic conditions, while at low pH values it converts into cationic
+
form AH .
1
5
+
changes in pH. The flavylium cation (FAH ) is only stable at
pH < 1. When the pH changes from 1 to 7 the cationic species
is immediately transformed into the quinoidal base (FA)
through a fast proton transfer reaction. In the same time,
Introduction
The development of novel photochromic materials has gained
an increased interest in the last decades due to their industrial
applications in optical memory devices, sensors, switches,
+
competitive hydration of FAH can also occurs to give the
1
-6
hemiketal (FB) which, after a ring-opening tautomerization,
leads to cis-2,4’-dihydroxychalcone (FCc). By irradiation at 313
nm isomerization of FCc into trans-2,4’-dihydroxychalcone
intelligent windows and displays.
Flavylium derivatives
constitute an attractive family of organic compounds
comprises anthocyanins, the natural pigments responsible for
the red and blue colors prevailing in flowers, fruits and
(
FCt) takes place. A direct pH jump from 1 to 12 or higher
+
determines the deprotonation of FAH leading to the
7
processed beverages. This class of colorants, including
2
-
16
formation of thermodynamically stable FCt species. It was
also reported that the rates of hydration, tautomerization and
isomerization strongly depend on the solvent, and type and
positions of the functional groups attached to the flavylium
synthetic or naturally molecules, are of practical interest with
the possibility of achieving reversible changes in color when
submitted to different external stimuli, such as light,
8
-12
temperature and pH.
Moreover, this family of compounds
1
1
7,18,19
3
core.
Moreover, it was demonstrated that the change in
can even mimic the properties of neurons. Generally, all
these compounds hold the same 2-arylbenzopyrilium
+
colour of FAH solutions depends on the amount of water and
2
0
flavylium cation concentration in solution.
It was reported that other three families of structurally related
2
compounds such as styrylflavylium, naphthoflavylium and
(flavylium) core and follow the same pH-dependent network of
reversible chemical reactions. An illustrative example of
chemical reaction network of flavylium-type compound is
1
22
2
3-25
1
4
xantylium
follow an identical network of chemical
illustrated in Scheme 1. The aqueous 4’-hydroxyflavylium can
be interconverted in different forms using light excitation or
reactions as flavylium derivatives.
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2
4,26
(Scheme 3).
In order to identify the stable species, the
NMR analyses were performed in different pH media: neutral
13
1
pH 6), acidic (pH < 0.5) and basic (pH > 12). The H and
(
C
NMR spectra of neutral species are reported in ESI (Fig. S10,
Fig. S11). The NMR spectra corresponding to the basic
conditions, recorded immediately after the addition of
deuterated sodium hydroxide, revealed the total conversion of
2
-
1
Ct into deprotonated base Ct (see ESI, Fig. S7, Fig. S9). The H-
NMR spectrum recorded after 7 days confirmed the stability of
the species in basic conditions (ESI, Fig. S8). The flavylium
+
cation, AH , was detected only in very acidic medium at pH <
0.5. The NMR spectra of this species were recorded after 4
days upon the addition of HCIO4 (70% w/w) when the
thermodynamic equilibrium was established and the total
+
conversion of Ct into AH was reached. After a period of two
months, in dark conditions, red crystals suitable for X-ray
crystal analysis have been developed from the solution. Full
1
4
Scheme 1 Structural transformations of the 4’-hydroxyflavylium ion.
1
characterization and assignments of H and C signals for AH
13
+
The aim of the present work was to discover new photochromic
systems based on xanthylium derivatives and to study the influence
of substituents on the network of chemical reaction. The designed
was achieved by COSY, HSQC, HMBC and NOESY spectra (see
ESI, Fig. S1-S6).
According
to
single-crystal
X-ray
diffraction
of
molecule
was
2,6-bis(5-bromo-2-hydroxy-benzylidene)
AHClO ·0.375H O compound crystalizes in P2 space group of
4
2
1
+
cyclohexanone (BHBC) (Scheme 2) which can be further monoclinic system. The crystal is built up from AH cations,
functionalized by attaching certain substituents on 5, 5’ positions. ClO4 anions and co-crystalized water molecules in 1:1:0.375
-
Our previous studies on functionalization of opened form (Ct) of ratio. The symmetric part of the unit cell contains four cations,
BHBC led to many products difficult to isolate. We have chemically identical but crystallographic independent (denoted
demonstrated that B-B form of the compound without bromine, as A, B, C and D) with very close geometric parameters. As an
example, Fig. 1 shows the structure of cation AH , component
+
2
,6-bis(2-hydroxy-benzylidene)cyclohexanone, is a stable species in
2
4
A.
basic condition. Therefore, in the present study, we focused the
attention on the influence of external stimuli on the stability of
The crystal structure of AHClO ·0.375H O is described as a packing
4
2
of 2D supramolecular layers which grow parallel to ac the
crystallographic plane through π-π stacking interactions (Fig. 2). The
values of centroid-to-centroid distances are in the range of
spiropyran-like
species
of
2,6-bis(5-bromo-2-hydroxy-
benzylidene)cyclohexanone. The stability of the species involved in
the network of chemical reactions of BHBC has been investigated
and characterized.
3
.489÷3.704 Å. Besides this, the intermolecular O-H···O and C-H···O
-
hydrogen bonding involving ClO4 anions and coordinated water
molecules provides additional strengthening of supramolecular
motif.
1
13
2-
The assignments of the H and C NMR peaks of Ct, Ct and
+
AH species are reported in Table 1.
Scheme 2 Chemical structure of 2,6-bis(5-bromo-2-hydroxybenzylidene)
The color shift of the species as a function of pH has been also
studied by UV-VIS spectroscopy. The solutions at acid pH
values (pH < 0.5) are reddish and exhibit a broad band with a
maximum absorption at 498 nm, corresponding to the
cyclohexanone (BHBC).
Results and discussion
+
formation of AH cation. The solutions are stable for long
In the present study we have investigated the behavior of 2,6-
periods at room temperature. As the pH values increase (1 <
pH < 7), the solutions become yellowish, corresponding to the
neutral form of the compound. The absorption band of 498 nm
is not detectable for the neutral species, and it practically
disappears as the pH value increases (Fig. 3). At pH values in
the range of 8.2 and 12.5, the 360 nm band of the protonated
neutral form (Ct) was gradually shifted to a lower energy 460
nm band at basic pH, due to the formation of the
bis(5-bromo-2-hydroxybenzylidene)cyclohexanone
(BHBC)
when submitted to external stimuli. The attention has focused
on the isolation, identification and characterization of the
species involved in the network of chemical reactions, which
are similar to structurally related flavylium-type compounds.
The synthesis of BHBC was performed in basic conditions
according to the procedure described previously for the
2
3
2
-
synthesis of 2,6-bis(2-hydroxybenzylidene)cyclohexanone.
deprotonated base Ct , where the saturation was observed at
pH 12.5 (Fig. 4a). Therefore, the successive deprotonation
gives rise to an orange shift of the absorption bands. As shown
The halochromic behavior of the compound considering the
changes in color due to the pH shift has been studied. This
ability can be attributed to the various species involved in the
interconversion of the compound in acidic or basic conditions
2
-
in Scheme 3, the dissociation of Ct to generate Ct occurs via
-
the formation of Ct as reported previously for the non-
2
| J. Name., 2012, 00, 1-3
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24
brominated compound. The superimposed UV-VIS spectra
disclose the presence of a slightly distorted isosbestic point at
~
402 nm which can be due to the similarity of the molar
-
2-
absorption coefficient of the three compounds (Ct, Ct and Ct )
Fig. 4a). The compound can be stored at high pH values due to
(
the stability of the solutions at room temperature. The 460 nm
absorption at different pH showed an ideal titration profile
with a transition midpoint pKa at ~ 9.5 (Fig. 4b).
+
Fig. 1 Crystal structure of AH cation (component A) with thermal ellipsoids
at 50% probability level.
Scheme 3 Chemical transformation of BHBC depending on pH conditions.
+
Fig. 2 Partial view of the crystal structure of AH along b axis. Centroid-to-
centroid distances are shown in dashed orange line.
1
13
2-
+
Table 1 H-NMR and C-NMR data for Ct, Ct and AH species in neutral, basic and acidic conditions.
Position of
H/C in the
molecular
structure
1
13
1
13
1
13
H –NMR
C –NMR
H –NMR
C –NMR
H –NMR
C –NMR
δ (ppm)
δ (ppm)
23.0
δ (ppm)
δ (ppm)
23.4
δ (ppm)
δ (ppm)
19.7
1
2
3
4
5
6
7
8
9
1.79, m, 2H
1.80, m, 2H
1.67, s, 2H
2.85, t, 2H
28.2
2.89, (t, J = 5.3 Hz, 2H)
28.5
2.81, s, 2H
27.2
2.85, t, 2H
28.2
2.89, (t, J = 5.3 Hz, 2H)
28.5
2.64, s, 2H
26.2
-
132.4
132.4
190.4
136.6
110.2
155.9
-
132.7
132.7
191.2
135.8
103.1
168.4
-
133.0
128.6
172.3
151.9
124.7
131.0
-
-
-
-
-
-
7.86, s, 1H
8.16, s, 1H
8.46, s, 1H
-
-
-
-
-
7.92 (d, J = 2.2 Hz, 1H)
7
.07 (dd, J = 8.8, 2.7 Hz,
H)
1
1
0
1
6.80, d, 1H
116.9
132.1
121.8
127.1
-
122.5
140.5
1
7
.31 (dd, J = 8.7, 2.4
7.87 (dd, J = 9.1, 2.2 Hz,
6.57 (d, J = 8.8 Hz, 1H)
Hz, 1H)
-
1H)
1
1
1
1
1
1
2
3
4
5
6
7
125.0
131.4
136.6
110.2
131.4
125.0
-
132.7
131.6
135.8
103.1
131.6
132.7
7.62 (d, J = 9.1 Hz, 1H)
119.5
153.7
140.3
111.6
132.2
123.0
7.41 (d, J = 2.4 Hz, 1H)
7.29 (d, J = 2.7 Hz, 1H)
-
7.86, s, 1H
8.16, s, 1H
8.22, s, 1H
-
-
-
7.41 (d, J = 2.4 Hz, 1H)
7.29 (d, J = 2.7 Hz, 1H)
7.19 (d, J = 2.0 Hz, 1H)
-
-
-
7.00 (dd, J = 8.8, 2.2 Hz,
1H)
7
.31 (dd, J = 8.7, 2.4
1
8
132.1
6.57 (d, J = 8.8 Hz, 1H)
127.1
135.7
Hz, 1H)
7
.07 (dd, J = 8.8, 2.7 Hz,
1
2
9
0
6.80, d, 1H
-
116.9
155.9
121.8
168.4
6.48 (d, J = 8.8 Hz, 1H)
117.4
155.5
1
H)
-
-
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In comparison, the solutions of Ct in methanol in dark within only 30 minutes (Fig. 5c). In conclusion, we can affirm
conditions are not stable and in a slow process an interesting that photo-irradiation and high temperatures can have a
colorless species is formed according to the spectral significant influence on the formation of B-B by increasing the
modifications shown in Fig. 5a. The spectral changes indicate rate of the process.
that the Ct species leads to the formation of a product lacking In order to investigate the stability of this compound, the
of absorption in the visible, showing a disruption of π – π* equilibrated solutions of B-B at neutral pH were submitted to
conjugation of the aromatic ring. During a period of 100 h, the pH jumps to the basic and acidic regions. In basic medium, B-B
absorption band at 360 nm is blue shifted due to the presence was stable, while in very acidic conditions (pH < 0.5) it was
of Ct, Cc, B and B-B species. In dark conditions, the solution quickly transformed into flavylium cation. The absorption
develops from yellow to colorless, forming transparent crystals spectra of B-B from neutral pH to acidic pH values are
+
suitable for single X-ray crystal analysis. The crystals have been represented in Fig. 8. The AH species started to form
washed with methanol, dried at room temperature for 24 h immediately (10 seconds) after the addition of HClO
4
(70% wt).
OD in order to yield the spectrum The kinetics of the transformation of B-B into AH was studied
in time (~ 8 h) until the system reached the equilibrium. It was
+
and solubilized in CD
showed in Fig. 6.
3
1
13
Full characterization and assignments of H and C signals for observed that at pH values higher than 1, the B-B is very
the formed colorless product was achieved by COSY, HSQC, stable, while increasing the pH does not give any Ct back.
HMBC and NOESY spectra (see ESI, Fig. S12-S17) and allowed
us to identify a spiropyran-like structure linked by a propano-
bridge,
3,11-dibromo-7,8-dihydro-6H-chromeno[3,2-d]
xanthene (Fig. 7). In the IR spectrum (see ESI, Fig. S18) the
-1
absorption band at 951 cm could be assigned to the presence
of stretching vibrations νO-C-O corresponding to spiro form
2
7-29
according to the literature.
henceforth named B-B.
The compound has been
The X-ray crystallographic analysis confirms the compound B-B
able to crystallize in P-1 space group of triclinic system and its
molecular crystal structure consists of the isolated neutral
molecules, as shown in Fig. 7. No co-crystallized solvent has
been found in the crystal.
The ring-closure reaction of Ct to form B-B species has been
accelerated by incubating the solution of BHBC in methanol at
5
5 °C. The total conversion of Ct into B-B was reached after 7
-
4
-1
hours as shown in Fig. 5b.
Fig. 3 UV-VIS spectra of the species in neutral (1.2 x 10 mol L BHBC in
-4 -1
2 2 4
MeOH/H O = 1:1), acidic (1.4 x 10 mol L BHBC in MeOH/H O/HClO 70% =
-4 -1
Surprisingly, by exposing a fresh solution of BHBC at neutral pH
to light irradiation at 365 nm, the product exhibits the same
absorption spectrum of the cyclic ketal B-B. In these
conditions, total conversion of BHBC into B-B was achieved
6
:1:1) and basic pH (0.84 x 10 mol L BHBC in MeOH/NaOH 1M = 1:1).
b
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a
b
-4
-1
Fig. 4 (a) Absorption spectra of BHBC in various pH buffer solutions (2.1 x 10 mol L BHBC in buffer/MeOH 1:1). The increase and decrease in intensities
with increase in pH are indicated in arrows; (b) Absorption at 460 nm at different pH;
a
b
c
-4
-1
Fig. 5 (a) Formation of B-B from equilibrated solution of BHBC in dark conditions (1.7 x 10 mol L BHBC in MeOH); (b) Formation of B-B from equilibrated
-
4
-1
-4
solution of BHBC at 55 °C (1.3 x 10 mol L BHBC in MeOH); (c) Formation of B-B from equilibrated solution of BHBC upon irradiation at 356 nm (1.7 x 10
-
1
mol L BHBC in MeOH).
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Fig. 7 X-ray molecular structure of B-B. Thermal ellipsoids are drawn at 50%
1
Fig. 6 H NMR spectrum of B-B crystals in CDCl
3
.
probability level.
ppm, coupling constants are reported in Hz and following
abbreviations are used for splitting pattern: s (singlet), d
(
doublet), dd (doublet of doublets), t (triplet), and m
multiplet). NMR assignments have been carried out on the
(
1
13
basis of H, C, DEPT 135 and 2D NMR spectra: DQF-COSY,
standard multiplicity-edited HSQC (HSQCEDETGPSISP) and
standard HMBC (HMCETGPL3ND). The NMR measurements: (i)
basic pH of the BHBC solution in deuterated methanol (0.7 ml)
was reached by adding 40 µl of 40% wt NaOD solution in D
and (ii) the acidic pH of BHBC in deuterated methanol (0.2 ml)
was reached by adding 0.5 ml of 40% wt DClO solution in D O.
The NMR spectra of B-B species was recorded in CDCl . The
2
O;
4
2
3
FTIR spectra were recorded on a Cary 630 FTIR from Agilent
Technologies, recorded as KBr pellets. The UV-VIS absorption
spectra were recorded on an Agilent Cary 60
spectrophotometer. The pH of the solutions was measured
using an Oakton pH/°C meter pH 10 series. The BHBC solutions
were prepared using distilled water and methanol.
Experiments involving pH jumps were performed by adding 1
mL of a solution of BHBC in methanol (0.43 mM) to 1 mL of
glycine 0.1 M/NaOH 0.1 M buffer solution (pH previously
adjusted) in order to obtain the final desired pH value. The
irradiation of BHBC was performed at 365 nm using a TLS - 260
from Newport equipped with a 350 W Mercury arc-lamp.
Fig. 8 Spectral variation upon pH jumps from equilibrated solutions of B-B in
methanol to lower pH values. The spectra were collected by adding 2.5 ml
4
HClO (70% wt) to 1 ml solution of B-B in MeOH (0.43 mM).
Experimental
Materials and methods
5-bromo-2-hydroxybenzaldehyde and hydrochloric acid were
purchased from Merck & Co. Sodium hydroxide was from
Lach-Ner (CZ) and cyclohexanone was purchased from
Chimopar (RO). All the other reagents and solvents were of
analytical grade.
X-ray crystal for compound B-B
X-ray diffraction data were collected with an Oxford-
Diffraction XCALIBUR E CCD diffractometer equipped with
Elemental analysis was performed on an elemental analysis
system VarioMICRO cube from Elementar Analyses. The NMR
graphite-monochromated MoKα radiation. Single crystals was
positioned at 40 mm from the detector, and 592 and 376
spectra were recorded on
a Bruker Advance III 500
13
frames were measured each of 5 and 100 s over 1° scan for
+
AH and B-B, respectively. The unit cell determination and data
1
spectrometer (500.13 MHz for H, 125.77 MHz for C) at 298
K. The center of the solvent signal was used as an internal
integration were carried out using the CrysAlis package of
1
standard, which was related to TMS with δ 3.31 ppm for H
3
Oxford Diffraction. The structures were solved by direct
0
1
and δ 49.15 ppm for C. Chemical shifts δ are reported in
3
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3
methods using Olex2 software with the SHELXS structure B-B was confirmed by X-ray study on single crystal and it can
1
solution program and refined by full-matrix least-squares on F² be transformed in other species only in very acidic media.
3
with SHELXL-97. Hydrogen atoms attached to carbon were The presence of bromine atoms on the molecule has as a
2
+
placed in fixed, idealized positions and refined as rigidly consequence the shift to lower pH values of AH form
bonded to the corresponding non-hydrogen atoms. Positional compared to the 2,6-bis(2-hydroybenzylidene)cyclohexanone.
parameters of the H attached to O atoms were obtained from
difference Fourier syntheses and verified by the geometric
Acknowledgements
parameters of the corresponding hydrogen bonds. Two of four
-
asymmetric ClO
4
anions were found to be disordered and their
This work was supported by funds from Romanian Academy,
Program 4, Project 4.1.
positional parameters were refined in combination with PART
and SADI restraints and the displacement parameters for
paired components were constrained to be equivalent.
Notes and References
Synthesis of BHBC
‡
C
1
Crystal data for AHClO
ClO (Mr = 533.35 g
3.2991(6) Å, b = 20.2762(12) Å, c = 15.0236(7),
4 2
·0.375H O:
4
.0 g 5-bromo-2-hydroxybenzaldehyde (20 mmol) was
20
H15.75Br
2
4
⋅
mol-1), monoclinic, a =
= 91.184(5)
, Z = 8, 37533 coll.
dissolved in 40 mL ethanol, while stirring at room
temperature. 1.03 mL of cyclohexanone (0.981 g, 10 mmol)
were then added and the mixture was stirred vigorously for 1
h. 24 g of a NaOH 20 % (w/w) solution were added dropwise,
slowly, by maintaining the temperature below 40 °C and the
reaction was carried out at room temperature for 24 h. Then,
β
°,
3
V = 4050.3(3) Å , T = 200 K, space group P2
refl., 11582 indep. (Rint = 0.0745), Gof = 1.014, R
2
wR(F ) = 0.2093. CCDC - 1522904.
1
1
= 0.0758,
‡
C
=
Crystal data for B-B:
(Mr = 446.13 g
11.1199(15) Å, c = 11.382(2),
= 83.992(12) , V = 849.3(2) Å , T = 293 K, space
orange precipitate was formed which was filtered dried and group P-1, Z = 2, 6057 coll. refl., 3003 indep. (R = 0.0620), Gof
20
H14Br
2
O
2
⋅
mol-1), triclinic, a = 6.9626(10) Å, b
= 75.755(14)
100 mL of distilled water was added and the mixture was
α
°, β =
3
neutralized to pH 5.5 – 6.0 by using HCl 6 M solution. An 89.506(14)
°
,
γ
°
int
2
1
recrystallized from methanol/water while adjusting the pH of = 0.986, R = 0.0406, wR(F ) = 0.0785. CCDC - 1509599.
the solution to 1.5 – 2.0 using HCl 6 M solution. A yellowish
1
2
3
4
M. Irie, T. Fukaminato, K. Matsuda and S. Kobatake,
Photochromism of diarylethene molecules and crystals:
powder was obtained, ƞ = 65.4 %. Elemental analysis calcd. for
: C 51.75, H 3.47, found: C 51.82, H 3.44 %.
H NMR (500 MHz, CD OD), δ (ppm): 8.16 (s, 1H), 7.29 (d, J =
20 2 3
C H16Br O
memories, switches, and actuators, Chem. Rev., 2014, 114
2174–12277;
,
1
3
1
2
.7 Hz, 2H), 7.07 (dd, J = 8.8, 2.7 Hz, 2H), 6.57 (d, J = 8.8 Hz,
H), 2.89, (t, J = 5.3 Hz, 4H), 1.80 (m, 2H).
X. Guo, J. Zhou, M. A. Siegler, A. E. Bragg and H. E. Katz,
Visible-light-triggered molecular photoswitch based on
2
1
3
reversible E/Z isomerization of
a
1,2-dicyanoethene
C NMR (500 MHz, CD
3
OD), δ (ppm): 191.2, 168.4, 135.8,
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1
32.7, 131.6, 127.1, 121.8, 103.1, 28.5, 23.4.
-1
FTIR (cm ): 3368, 3938, 2865, 2359, 2328, 1655, 1604, 1560,
1
489, 1474, 1412, 1286, 1259, 1218, 1168, 1144, 1113, 981,
26, 877, 815, 755, 626, 569.
9
Conclusions
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6
K. Morganstern, Isomerization reactions on single adsorbed
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In spite of the fact that only a few studies of the pH network of
chemical reaction involving xanthylium compounds have been
reported, we proved that the new compound 2,6-bis(5-bromo-
2-hydroxybenzylidene)cyclohexanone follows the same
network of chemical reaction as flavylium derivatives. This new
photochromic system can be selectively “lock” in three
7
8
V. Cheynier, Polyphenols in foods are more complex than
often thought, Am. J. Clin. Nutr. 2005, 81, 223S-229S;
A. M. Diniz, C. Pinheiro, V. Petrov, A. J. Parola and F. Pina,
+
different species: xanthylium cation (AH ), chromeno-xanthene
2
-
Synthesis and characterization of a symmetric bis-7-
(
B-B) and deprotonated trans-chalcone (Ct ).
The trans-chalcone form is transformed into xanthylium cation
red color) at very low values of pH, but reverts back to the
hydroxyflavylium containing a methyl viologen bridge, Chem.
Eur. J., 2011, 17, 6359-6368;
H. Horiuchi, H. Shirase, T. Okutsu, R. Matsushima and H.
(
9
1
Hiratsuka,
Photochromism
of
2-hydroxy-4'-
yellowish trans-chalcone at pH values higher than 0.5. The
+
structure of AH was confirmed by single crystal X-ray study. At
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0 K. Tokumura, N. Taniguchi, T. Kimura and R. Matsushima,
Primary processes in photochromic reaction of 4-
basic pH values, trans-chalcone is spontaneously transformed
into anionic form, displaying an orange color. An interesting
feature of this system is related to the formation of xanthylium
cation via a stable chromeno-xanthene intermediate, which
was isolated and fully characterized by NMR. The structure of
diethylamino-4′-dimethylamino-2-hydroxychalcone
toluene, Chem. Lett., 2001, 30, 126-127;
in
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C6PP00466K
ARTICLE
Journal Name
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| J. Name., 2012, 00, 1-3
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Table of contents
The new photochromic system, 2,6-bis(5-bromo-2-hydroxybenzylidene)cyclohexanone, can be selectively “lock” in
three different species when submitted to various stimuli (pH, light, temperature).