Colour switching with photochromic vinylidene-naphthofurans

Article abstract of DOI:10.1016/j.tet.2018.10.076

A series of novel photochromic vinylidene-naphthofurans with extended conjugation, and a free hydroxyl function, were easily prepared using the Suzuki reaction. After silanization, these dyes were embedded in ormosil matrices affording solid and transpare

Full text of DOI:10.1016/j.tet.2018.10.076

Tetrahedron xxx (xxxx) xxx
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Tetrahedron
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Colour switching with photochromic vinylidene-naphthofurans
*
ꢀ
Diogo Costa, Ceu M. Sousa, Paulo J. Coelho
ꢀ
Centro de Química-Vila Real, Universidade de Tras -os-Montes e Alto Douro, 5000-801 Vila Real, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel photochromic vinylidene-naphthofurans with extended conjugation, and a free hy-
droxyl function, were easily prepared using the Suzuki reaction. After silanization, these dyes were
embedded in ormosil matrices affording solid and transparent materials that acquire different colour-
ations (violet, green, bluish), reversibly, when exposed to the UV (Sun) light, for 2 min, at room tem-
perature. The presence of an extra phenyl ring in some positions affects both the lmax of absorption of the
photochromic compounds in the uncoloured closed and open coloured form. After removal of the light
source the materials lose progressively their colouration returning to the initial uncoloured state in less
than 15 min at room temperature.
Received 4 September 2018
Received in revised form
29 October 2018
Accepted 30 October 2018
Available online xxx
Keywords:
Photochromism
Molecular switches
UV light
© 2018 Elsevier Ltd. All rights reserved.
Sol-gel
Vinylidene-naphthofurans
Thermal fading
1. Introduction
compounds that exhibit photochromic properties at room tem-
perature but, unlike common photochromic compounds, they
Photochromic colorants can change reversibly their colouration
through the action of light which promotes an isomerization re-
action leading to coloured species. For T-type photochromic com-
pounds the coloured photoisomer is thermally unstable and
returns spontaneously to the initial form when the light source is
removed [1]. These photoactive compounds have been successfully
used in the production of ophthalmic lenses that darken under
sunlight. However, although many types of photochromic com-
pounds are known, which exhibit a wide variation in behavior, only
a few have been considered by the lens industry since their re-
quirements are very specific [2]. The compounds must be uncol-
oured in the dark, be UV active, change quickly to grey after
exposure to the sunlight, develop an intense colouration, work at
room temperature, have a small dependence on temperature, re-
turn to the uncoloured state after a few minutes in the dark and
resist to photodegradation over a long period of time [3]. Nowa-
days, the industry uses mainly complex naphthopyrans that fulfill
most of the above characteristics but exhibit a rather slow fading in
the dark, which means that the compounds have a slow adaptation
to a changing UV light environment.
require an acidic environment and UV light to generate the col-
oured species [4]. The photochromic mechanism involves the UV
light promoted addition of one proton, from the environment, to
the allene group with overture of the furan ring leading to a col-
oured carbocation with two absorption bands in the visible region
(451 nm and 580 nm) (Scheme 1). In the absence of light this spe-
cies returns spontaneously to the uncoloured closed form, in few
minutes, at room temperature [5].
This system possess some characteristics that makes them
potentially useful for industrial applications especially in
ophthalmic lenses: 1) these molecules can be easily prepared from
the reaction of 2-naphthols with readily available tetraarylbut-2-
yne-1,4-diols at room temperature; 2) they are activated by the
sunlight at room temperature; 3) develop intense colours with two
absorption bands which opens the opportunity to obtain a grey
colouration; 4) bleach in few minutes, at room temperature, in the
dark [3].
The unsubstituted vinylidene-naphthofuran 1 (Scheme 1) ex-
hibits a strong absorption in the UV region (lmax at 373 nm) but a
low absorption near 400 nm. This aspect is very relevant since the
UV sunlight intensity is higher near 400 nm and the polymeric
materials used usually in the production of ophthalmic lenses, CR-
39, also absorb intensively in the UV region below 385 nm (the
transmittance drops from 90% at 400 nm to 65% at 385 nm), which
limits the absorption of light by the photoactive molecule [6]. On
Vinylidene-naphthofurans are
a new class of polycyclic
* Corresponding author.
E-mail address: pcoelho@utad.pt (P.J. Coelho).
https://doi.org/10.1016/j.tet.2018.10.076
0040-4020/© 2018 Elsevier Ltd. All rights reserved.
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BuLi followed by addition of arylketones. Following this approach
the diols 2a-c were prepared starting from p-bromobenzophenone,
benzophenone and p-methoxybenzophenone, respectively.
The reaction of 2-naphthol with diol 2a, catalyzed by p-tolue-
nesulfonic acid at room temperature, gave compound 3a in good
yield (78%) while the reaction of 6-bromo-2-naphthol with tet-
raarylbut-2-yne-1,4-diols 2b,c, afforded the bromo substituted
vinylidene-naphthofurans 3b,c (73e97%) (Scheme 3).
Scheme 1. Photochromic behavior of vinylidene-naphthofuran 1.
Besides the extended aromatic signals in the 1H NMR spectrum,
the polyaromatic compounds 3a-c exhibit two characteristic sig-
nals in the 13C NMR spectrum: a low field signal around 202 ppm,
characteristic of the allene function, and a signal at 94e95 ppm
assigned to the dihydrofuran sp [3] C-2 carbon atom. The HMBC
spectrum of compound 3c (R ¼ OMe) showed a strong correlation
between the sp [3] C-2 carbon at 95.0 ppm and the four protons H-
20 of the phenyl groups at 7.48 ppm, indicative that the p-
methoxyphenyl ring is linked to the allene group. For compound 3a
the HMBC showed a strong correlation between the sp [3] C-2
carbon at 94.3 ppm and two sets of aromatic protons (H-20 and H-
200) of the phenyl and p-bromophenyl substituents at 7.47 and
7.37 ppm, which indicates that the p-bromophenyl group in linked
to the C-2 of the dihydrofuran ring (Scheme 3).
the other hand, a significant absorption above 400 nm means that
the compound has initially an yellow colouration which is unde-
sirable. Therefore, the ideal photochromic molecule should be
uncoloured, absorb intensively in the UV region just below 400 nm
and develop a strong absorption in the visible region after UV or
sunlight irradiation [7].
In the closed form the four phenyl groups are not co-planar with
the dihydronaphthofuran ring and the compound absorbs only the
UV region, but upon ring opening there is a substantial bath-
ochromic shift indicative of the conjugation with one or more
phenyl groups. Accordingly, the introduction of substituents on the
phenyl groups will affect mainly the lmax of the open form while
the changes in the naphthalene ring may have an effect on both the
closed and open forms.
This one-pot reaction occurs via a domino reaction (Scheme 4):
under acid-catalysis the diol 2 is converted into a propargylic car-
bocation, which upon reaction with 2-naphthol provides a prop-
argylic aryl ether. Then, this intermediate performs a [3,3]-
sigmatropic Claisen rearrangement followed by enolization and
acid-catalyzed intramolecular dehydration affording the final
vinylidene-naphthofuran 3. The selectivity observed with diols 2a,c
is a consequence of the effect of the methoxy and bromine sub-
stituents on the stability of the initial propargylic carbocation. The
electron donating methoxy group will stabilize a positive charge on
carbon 1 of the diol 2c, while with the electron withdrawing
bromine atom (diol 2a) the positive charge with be formed pref-
erentially on carbon 4 (Scheme 4). Therefore, following the pro-
posed mechanism, the methoxy group with appear in the allene
phenyl rings (compound 3c), while the bromine atom with end in
the C-sp [3] phenyl rings (compound 3a).
The Suzuki reaction of compounds 3a,b with p-(hydroxymethyl)
phenylboronic acid, catalyzed by tetrakis (triphenylphosphine)
palladium afforded, respectively, the vinylidene-naphthofurans 5a
and 5b, in good yield (79e84%), while the reaction of compound 3c
with phenylboronic acid gave the phenyl substituted compound 4c
(85%) which was then deprotected with BBr3 affording the phenolic
vinylidene-naphthofuran 5c (76%) (Scheme 5).
The vinylidene-naphthofurans operate well in acid medium and
can be embedded in ormosil matrices producing uncoloured
transparent materials [8]. However, the unsubstituted vinylidene-
naphthofuran 1 shows a limited solubility in these conditions
leading to its precipitation during the condensation reactions of the
siloxane monomers and hardening of the material. To avoid this
problem it is preferable to introduce, in the molecule, a siloxane
reactive group able to establish a chemical linkage with the matrix.
This can be done by treatment of hydroxyl substituted compounds
with (triethoxysilyl)propyl isocyanate [9]. Taking into account these
considerations we designed three new vinylidene-naphthofurans
possessing an extra phenyl group, in different positions, to extend
the conjugation and induce a bathochromic shift in the wavelength
of absorption of the uncoloured and coloured forms, and a reactive
OH function necessary to introduce a reactive triethoxysilylpropyl
chain. The linkage of the photoactive molecule to the matrix was
performed from 3 different positions: from the naphthalene core
(1), the allene group (2) and from the sp [3] aromatic rings (3)
(Scheme 2). The photochromic behavior of the materials obtained
using these photoactive molecules will allow to understand the
effects of the extra conjugated aromatic ring and the point of
linkage of the molecule to the matrix.
2. Results and discussion
2.1. Synthesis
Vinylidene-naphthofurans can be easily obtained through the
acid catalyzed reaction of 2-naphthols with tetraarylbut-2-yne-1,4-
diols [4]. This cascade reaction occurs at room temperature and is
very versatile allowing the preparation of differently substituted
vinylidene-naphthofurans. The tetraarylbut-2-yne-1,4-diols can be
easily obtained through the reaction of 1,1-diarylpropynols with n-
Scheme 2. Positions of the linkage of the vinylidene-naphthofuran molecule to the
Scheme 3. Synthesis of bromo substituted vinylidene-naphthofurans 3a-c from
matrix.
naphthols.
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D. Costa et al. / Tetrahedron xxx (xxxx) xxx
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Scheme 4. Mechanism for the formation of substituted vinylidene-naphthofurans 3a,c from tetrarylbut-2-yne-1,4-diols 2a,c.
Fig. 1. UVevis absorption spectra of vinylidene-naphthofurans 3b, 5b and the parent
compound 6 substituted by a CH2OH group, in CHCl3.
Scheme 5. Synthesis of the hydroxylated vinylidene-naphthofurans 5a-c using the
Suzuki reaction and the structure of the reference compound 6 [5].
2.2. UVevis spectral analysis
Chloroform solutions of vinylidene-naphthofurans 3, 4,
5
(5.0 ꢀ 10ꢁ5 M) are uncoloured with a strong absorption in the UV
region. Figs. 1e3 show the UVevis spectra of these compounds
compared with the 7-hydroxymethyl-tetraphenylvinylidene-
naphthofuran 6.
Fig. 2. UVevis absorption spectra of vinylidene-naphthofurans 3c, 4c, 5c and the
parent compound 6 substituted by a CH2OH group, in CHCl3.
(hydroxymethyl)phenyl groups in the 7 of the naphthofuran ring
(compounds 3b, 5b) has a significant effect on the lmax of the
From Fig. 1 it is clear that the introduction of bromine or p-
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Table 1
Maxima wavelength of absorption of naphthofurans 1, 3e6 (5.0 ꢀ 10ꢁ5 M, CHCl3)
before and after TFA addition.
Compound Substituents
lmax (nm) in CHCl3 lmax (nm) after TFA addition
1
6
H
373
373
373
451, 580
449, 581
442, 583
451, 582
439, 592
458, 600
455, 660
468, 663
458, 663
7-CH2OH
p-BrPh
p-PhPhCH2OH 373
7-Br
7-PhCH2OH
7-Br; PhOMe
3a
5a
3b
5b
3c
4c
5c
381
381
381
7-Ph; PhOMe 381
7-Ph; PhOH 381
coloured species with two absorption maxima at 439 and 592 nm
corresponding to a dark bluish colouration (Fig. 4).
The presence of a p-methoxyphenyl group in the allene function
and a phenyl in the position 7 of the naphthofuran (compounds 3c,
4c, 5c) induces also a noteworthy bathochromic shift, particularly
in the second absorption maxima of the cationic species (Fig. 5).
The species formed after protonation of compound 4c is green with
the maxima at 468 nm (þ17 nm) and 663 nm (þ83 nm). Upon NEt3
addition these coloured species are converted back to the initial
naphthofuran although a slightly residual absorption near 420 nm
Fig. 3. UVevis absorption spectra of vinylidene-naphthofurans 3a, 5a and the parent
compound 6 in CHCl3.
compounds promoting a bathochromic shift of 8 nm. This may be
rationalized based on the extension of the conjugation brought by
the extra phenyl ring or bromine atom. The same effect was
observed with compounds 3c, 4c, 5c: after the introduction of a
bromine or a phenyl group in the 7-position, a 8 nm bathochromic
shift of the lmax of the closed vinylidene-naphthofurans was
observed (Fig. 2).
On the contrary, the introduction of the same groups on the para
position C-sp [3] phenyl groups (compounds 3a, 5a), as well as the
deprotection of the methoxy group (compounds 4c, 5c Fig. 2) has
practically no effect on the absorption of the closed vinylidene-
naphthofurans (Fig. 3). This confirms the prediction that, in the
closed form, the four phenyl rings are not conjugated with the
dihydronaphthalene.
Hence, compounds 5b and 5c absorb at higher wavelengths in
the UV region than the parent compound 6, and they possess a free
hydroxyl function which allows the introduction of a reactive
siloxane chain necessary to link the compounds to the matrix.
2.3. Acidochromic properties
Besides photochromic properties, the vinylidene-naphthofurans
show also acidochromic properties. In the presence of strong acids
the uncoloured vinylidene-naphthofuran 1 is converted into a
stable cationic species (Scheme 1) that exhibit two absorption
maxima at 451 and 580 nm, the second one being more intense,
and bleach back to the uncoloured closed form upon neutralization
with Et3N. The parent compound 6, substituted by a CH2OH group
shows a similar behavior developing a violet colouration charac-
terized by two maxima at 449 and 581 nm. The new compounds
3e5, uncoloured in CHCl3, develop deep violet to blue colourations
after the addition of a few drops of trifluoroacetic acid (TFA)
(Table 1).
Fig. 4. Absorption spectra of naphthofuran 3b before (black), after the addition of
CF3COOH (blue) and after the subsequent addition of NEt3 (red).
The introduction of bromine or p-(hydroxymethyl)phenyl group
on the C-sp [3] phenyl group (compounds 3a, 5a) has a very limited
effect on the lmax of the open form, when compared with com-
pounds 1 and 6, which indicates that in the coloured cationic
species these substituents are not conjugated with the rest of the
molecule, adopting probably a perpendicular disposition. Therefore
the introduction of the linkage in this position does not perturb the
photochromic properties of the core molecule (Scheme 2). As ex-
pected, the introduction of a bromine or p-(hydroxymethyl)phenyl
in the position 7 of the naphthofuran (compounds 3b, 5b) induces a
significant bathochromic shift of the lmax of the cationic open form
(12e20 nm). Thus, the protonation of compound 3b generates a
Fig. 5. Absorption spectra of naphthofuran 4c before (black), after the addition of
CF3COOH (green) and after the subsequent addition of NEt3 (red).
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can be observed in almost all compounds (Figs. 4 and 5).
2.4. Photochromism
The colour change associated with the photochromic phenom-
ena is due to a significant change in the molecules structure which
10
causes an increase in the conjugation and consequently in the
l
max.
This transformation requires space to occur and therefore the
photochromic phenomena is rarely observed when the molecules
are in solid state, being more common in solution or when the
molecules are dispersed in polymeric matrices [11]. Naphthofurans
operate particularly well when adsorbed in acidic silica gel pro-
ducing a non-transparent white photochromic powder sensible to
the sunlight, at room temperature, but are not active in common
polyacrylate and polyurethane polymers used with naphthopyrans.
We have found that organically modified silica materials are
especially adequate to accumulate the naphthofurans, are
compatible with an acidic environment and generate transparent
and flexible materials whose properties can be tuned either
through the change in the fabrication conditions or in the structure
of the siloxane precursors [6,7].
The best results have been obtained starting from mixtures of
tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), phenyl-
triethoxysilane (PTES), 1,2-ethanediol, chloroacetic acid, water, THF
and the photochromic compound. The sol obtained after stirring
these precursors at room temperature, is cured, at 50 ꢂC, for several
days affording an uncoloured transparent photochromic solid ma-
terial. During the hydrolysis and condensation reactions of the
precursors a three dimensional siloxane porous network is formed
in which the chloroacetic acid and the photochromic molecule are
retained, while the solvents are expelled [10].
However, we have noticed that during the curing process the
naphthofurans tend to precipitate as the space within the porous
structure decreases. To avoid this it is preferable to introduce a
reactive siloxane chain in the compounds which allows them to
become part of the siloxane network (Scheme 6) [7].
Thus, before adding the photochromic dyes 5a-c, 6 to the sol,
these compounds, possessing reactive hydroxyl groups, were
treated with 3-(triethoxysilyl)propyl isocyanate at 60 ꢂC for 7 days
affording the respective silanized compounds, which, due to their
high reactivity under acidic conditions, were not purified and used
as such in the preparation of the doped ormosil materials (Scheme
7).
Scheme 7. Silanization of the vinylidene-naphthofuran 5b.
To prepare the photochromic materials a solution of TEOS/
MTES/PTES/1,2-ethanediol/water/chloroacetic acid (1/1/1/3.4/4.2/
0.48 M ratio) was stirred at 25 ꢂC for 7 h and then a solution of the
silanized photochromic compounds 5a-c, 6 in THF was added
(0.025 M ratio). The resulting sol was transferred into a petri dish
and cured at 50 ꢂC for 7 days affording transparent solid disks with
1.0 mm thickness. Once the materials were removed from the oven
some cracks appeared.
The transparent uncoloured siloxane matrix has no relevant
absorption in the UV region between 300 and 400 nm. The mate-
rials doped with the silanized compounds 5a-c, 6 were all uncol-
oured in the dark. The UVevis spectra of the materials doped with
the silanized naphthofurans 5a-c, 6 (Fig. 6) show that, in solution,
the compounds substituted by an aromatic group in the 7 position
(5b,c) absorb at higher wavelengths than the reference compound
6, while, in the matrix, the opposite situation occurs: the
substituted naphthofurans 5b,c, absorb at lower wavelengths
which indicates that probably, in the thick matrix, the extra aro-
matic ring in the naphthalene group adopts preferentially a
perpendicular configuration preventing conjugation.
The uncoloured materials doped with the silanized naph-
thofurans 5a-c, 6 were exposed to a UV lamp (370 nm, 36 W) for
2 min. All materials developed a light colouration with
tween 0.16 and 0.33 (Table 2). The UVevis spectra of the materials
doped with the silanized reference compound 6 show, after UV
exposure, the development of two bands near 450 and 576 nm in
accordance with the behavior observed in solution after addition of
a strong acid. The material doped with compound 5a, exhibit also
the same expected behavior (maxima at 450 and 578 nm) with a
violet colouration.
DAbs be-
The introduction of a phenyl group in the 7 position (compound
5b) generates a bathochromic shift (þ20 nm) of the second band:
the material doped with this compound develops
a bluish
Fig. 6. UVe vis spectra of the materials doped with the silanized compounds 5a-c and
6.
Scheme 6. Model of the siloxane material doped with the photochromic dyes.
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D. Costa et al. / Tetrahedron xxx (xxxx) xxx
Table 2
Photochromic properties of the materials doped with compounds 5a-c, 6 after UV exposure (2 min): Absorption maxima, colour, absorbance variation, bleaching rate con-
stants, half-life times (t1/2) and t3/4
.
Compound
lmax (nm) under UV
Colour
Abs
k1 (minꢁ1);
k2 (minꢁ1
t1/2 (min)
t3/4 (min)
)
6
450, 576
450, 578
457, 595
458, 606
violet
violet
blue
0.33
0.16
0.21
0.17
0.38; 0.066
0.34; 0.064
0.39; 0.064
0.37; 0.054
3.11
3.11
3.18
3.35
8.89
8.74
9.75
11.09
5a
5b
5c
green
colouration characterized by two bands at 457 and 595 nm, which
are comparable to those observed by acidochromism (Fig. 7,
Table 1).
However, while the addition of acid to a CHCl3 solution of
compound 4c, substituted with a phenyl group in the 7 position
and a methoxyphenyl group in the allene, leads to a large bath-
ochromic shift of the maxima wavelength (maxima at 468 and
663 nm), the material doped with the parent silanized compound
5c develop, after UV exposure, two bands at lower wavelengths
(458 and 606 nm) indicating a less efficient conjugation in the
matrix. Therefore the behavior observed in solution is not
completely transmitted to the matrix (Fig. 8).
When the light source was removed the materials lose pro-
gressively their colouration following a bi-exponential decay with
two rate constants of similar amplitude, the second being 6e7
times slower than the first (Fig. 9, Table 2). This means that there is
initially a fast colour decay followed by a slower colour fading.
According, the materials exhibit an half-life time of 3 min and an
higher t3/4 (9e11 min). However, after 15 min the materials have
lost approximatively 80% of their colouration and the residual
colour is undetectable by the human eye. This process can be
repeated several times in the same day without significant changes
in the behavior although after a few days the photochromic prop-
erties tend to be attenuated. The materials show a comparable
Fig. 9. Normalized fading kinetics of the material doped with silanized naphthofuran
5b measured at 595 nm. Inset graphic (ln Abs vs time) shows the deviation from a
monoexponential behavior.
behavior under sunlight, although the colour intensities were
slightly lower.
3. Conclusion
A set of new vinylidene-naphthopyrans possessing an extra ar-
omatic ring and a reactive hydroxyl function were easily prepared
in 3 steps. These compounds were then silanized and embedded in
a siloxane matrix producing transparent uncoloured photochromic
materials that developed violet, green or bluish colourations when
exposed to the UV or sunlight. The introduction of these sub-
stituents affects both the lmax of absorption of the photochromic
compounds in the closed (before UV) and open form (after UV)
although the results observed in solution are not the same as those
observed in the matrix. The introduction of an aromatic ring in the
7 position, compound 5b, leads to a bathochromic shift in the lmax
of absorption of the closed form (381 nm) approaching the 400 nm
limit, and also in the lmax of absorption of the open form generating
a bluish coloured species with two maxima at 458 and 600 nm. UV
exposure of the material doped with this compound, for 2 min,
generates a blue colouration that fades to the initial uncoloured
state within 15 min.
Fig. 7. UVe Vis spectra of the material doped with the silanized naphthofuran 5b
before and after UV exposure.
4. Experimental section
4.1. General remarks
All reaction were monitored by thin-layer chromatography on
aluminum plates coated with Merck silica gel 60 F254 (0.25 mm). IR
spectra were obtained in a Shimadzu IRAffinity spectrometer using
Fig. 8. Photos of the material doped with the silanized compound 5c after total or
partial exposure to the sunlight.
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D. Costa et al. / Tetrahedron xxx (xxxx) xxx
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an ATR mode. Melting point were determined using a microscope
ethanol.
URAC with a heating plate and are uncorrected. The 1H and 13C
NMR spectra were recorded at 298 K on a Bruker ARX 400 spec-
trometer (at 400.13 and 100.62 MHz). High resolution electrospray
ionization time-of-flight (ESI-TOF) mass spectra and electron-
impact ionization time-of-flight mass spectra were determined
with a VG AutoSpec M spectrometer. Diol 2b was prepared ac-
cording to reference [4].
4.3.1. 2-(4-Bromophenyl)-1-(2,2-diphenylvinylidene)-2-phenyl-
1,2-dihydronaphtho[2,1-b]furan (3a)
Slightly yellow solid obtained from 2-naphthol and diol 2a
(0.96 g, 78%). Mp 183.5e188.0 ꢂC. IR (cmꢁ1): 3054, 3027, 1624, 1591,
1518,1490,1251, 960, 809, 750, 690. 1H NMR (400 MHz, CDCl3): 8.27
(d, J ¼ 5.5 Hz, 1H), 7.90e7.80 (m, 2H), 7.49 (d, J ¼ 5.0 Hz, 2H), 7.47 (d,
J ¼ 5.0 Hz, 2H), 7.40e7.25 (m, 15H), 7.21 (m, 1H), 7.15 (m, 1H), 6.96
(d, J ¼ 5.4 Hz, 1H). 13C NMR (100 MHz, CDCl3): 202.7, 157.6, 142.5,
142.1, 136.2, 131.9, 131.5, 131.2, 130.2, 130.1, 129.7, 129.1, 129.0, 128.7,
128.5, 128.4, 128.0, 127.9, 127.8, 127.4, 127.2, 127.1, 123.8, 122.2,
122.0, 117.6, 114.6, 113.3, 112.3, 94.3. EI-MS (TOF) m/z (%): 578
([Mþ2]þ, 100), 576 (Mþ, 98), 501 (44), 499 (45), 420 (84), 391 (29),
331 (33), 313 (85), 302 (41), 165 (41). HRMS calcd for C38H25OBr:
576.1089, found: 576.1080.
4.2. General procedure for the synthesis of diols 2a-c
n-Butyllithium (2 eq, 3.85 mL, 2.5 M in hexane) was added
dropwise to
a solution of 1,1-diarylprop-2-yn-1-ol (1.00 g;
4.8 mmol) in dry THF (25 mL) at 0 ꢂC and the cold mixture was
maintained under constant stirring for 30 min. The adequate
benzophenone (1.1 eq) was added, at once, to the solution and the
resulting mixture stirred at room temperature for 22 h. The solvent
was removed, HCl (5%, 30 mL) was added and the aqueous phase
was extracted with ethyl acetate (3 ꢀ 30 mL). The combined
organic phases were dried (Na2SO4) and the solvent removed under
reduced pressure to give an oil which was purified by recrystalli-
zation from CH2Cl2/petroleum ether.
4.3.2. 7-Bromo-1-(2,2-diphenylvinylidene)-2,2-diphenyl-1,2-
dihydronaphtho[2,1-b]furan (3b)
Slightly yellow solid obtained from 6-bromonaphth-2-ol and
diol 2b (0.86 g, 97%). Mp 251.2e253.5 ꢂC. IR (cmꢁ1): 3083, 3058,
3026, 1568, 1507, 1270, 952, 760, 688. 1H NMR (400 MHz, CDCl3):
8.10 (d, J ¼ 9.0 Hz,1H), 7.96 (d, J ¼ 1.8 Hz,1H), 7.68 (d, J ¼ 8.9 Hz,1H),
7.52 (m, 1H), 7.42 (m, 4H), 7.30e7.15 (m, 13H), 7.11 (dd, J ¼ 7.2 Hz,
J ¼ 1.5 Hz, 4H). 13C NMR (100 MHz, CDCl3): 202.6, 157.93, 142.7,
136.2, 131.2, 131.0, 130.9, 130.7, 128.7, 128.4, 128.21, 128.15, 128.0,
127.8, 127.2, 123.8, 117.6, 117.2, 114.4, 113.8, 113.5, 95.0. EI-MS (TOF)
m/z (%): 578 ([Mþ2]þ, 71), 576 (Mþ, 76), 501 (40), 499 (45), 420
(100), 409 (26), 313 (57), 300 (26), 165 (44). HRMS calcd for
4.2.1. 1-(p-Bromophenyl)-1,4,4-triphenylbut-2-yne-1,4-diol (2a)
Obtained from p-bromobenzophenone (1.87 g, 83%). Mp
117.3e120.1 ꢂC. IR (KBr, cmꢁ1): 3408, 1596, 1492, 1442, 1395, 1208,
1127, 1073, 1013, 986, 892, 818, 745, 691, 611. 1H NMR (400 MHz,
CDCl3): 7.60e7.50 (m, 6H), 7.50e7.40 (m, 4H), 7.35e7.20 (m, 9H),
3.11 (s, 1H), 3.05 (s, 1H). 13C NMR (100 MHz, CDCl3): 144.5, 144.2,
143.8, 131.4, 128.4, 128.3, 128.0, 127.9, 127.8, 125.9, 125.8, 121.8, 90.3,
89.4, 74.5, 74.1. EI-MS (TOF) m/z (%): 450 (Mþ-18, 8), 436 (31), 434
(35), 347 (8), 276 (15), 267 (10), 265 (35), 263 (14), 207 (22), 189
(15), 184 (18), 182 (18), 178 (28), 165 (19), 105 (100). HRMS calcd for
C38H25OBr: 576.1089, found: 576.1081.
4.3.3. 7-Bromo-1-(2-(4-methoxyphenyl)-2-phenylvinylidene)-2,2-
diphenyl-1,2-dihydronaphtho[2,1-b]furan (3c)
C
28H19OBr (M-H2O): 450.0619, found: 450.0619.
Slightly yellow solid obtained from 6-bromonaphth-2-ol and
diol 1c (1.06 g, 73%). Mp 222.2e224.4 ꢂC. IR (cmꢁ1): 3068, 3025,
2956, 2834, 1572, 1504, 1443, 1242, 1161, 1037, 961, 880, 809, 697. 1H
NMR (400 MHz, CDCl3): 8.10 (d, J ¼ 8.9 Hz, 1H), 7.95 (d, J ¼ 2.0 Hz,
1H), 7.67 (d, J ¼ 8.9 Hz, 1H), 7.52 (dd, J ¼ 8.9, J ¼ 1.5 Hz, 1H),
7.44e7.39 (m, 4H), 7.30e7.20 (m, 11H), 7.11 (m, 2H), 7.02 (d,
J ¼ 8.8 Hz, 2H), 6.76 (d, J ¼ 8.8 Hz, 2H), 3.79 (s, 3H). 13C NMR
(100 MHz, CDCl3): 202.3, 159.40, 157.9, 142.9, 142.8, 136.5, 131.2,
130.9, 130.7, 129.8, 128.7, 128.5, 128.4, 128.3, 128.2, 128.1, 127.9,
127.8, 126.8, 123.8, 117.2, 114.2, 114.0, 113.9, 113.5, 95.0, 55.3. EI-MS
(TOF) m/z (%): 608 ([Mþ2]þ, 99), 606 (Mþ, 100), 531 (52), 529 (56),
450 (56), 313 (24), 300 (38), 289 (33), 197 (70). HRMS calcd for
4.2.2. 1,1,4,4-Tetraphenylbut-2-yne-1,4 diol (2b)
Obtained from benzophenone (1.51 g, 81%). Mp: 201.4e203.0 ꢂC.
IR (KBr, cmꢁ1): 3532, 3456, 3061, 3019, 2918, 2843, 2364, 2339,
1600, 1499, 1448, 1331, 1213, 1138, 1070, 1028, 995, 911, 886, 776,
743, 693, 642, 609. 1H NMR (400 MHz, CDCl3): 7.59 (d, J ¼ 7.9 Hz,
8H), 7.39e7.23 (m, 12H), 2.86 (s, 2H, OH). EI-MS (TOF) m/z (%): 372
(Mþ-18, 32), 371 (11), 370 (14), 356 (28), 344 (22), 267 (22), 265
(27), 252 (11), 208 (10), 207 (14), 182 (64), 181 (10), 179 (13), 178
(17), 165 (19), 05 (100), 77 (50). HRMS calcd for C28H20O (M - H2O):
372.1514, found: 372.1503.
C
39H27O2Br: 606.1194, found: 606.1196.
4.4. General procedure for the synthesis of naphthofurans 5a-b, 4c
mixture of naphthofuran (1,15 mmol), boronic acid
4.2.3. 1-(p-Methoxyphenyl)-1,4,4-triphenylbut-2-yne-1,4-diol (2c)
Obtained from p-methoxybenzophenone (1.43 g, 71%). Mp
106.4e108.8 ꢂC. IR (KBr, cmꢁ1): 3375, 1590, 1509, 1446, 1362, 1241,
1174, 996, 892, 839, 778, 691. 1H NMR (400 MHz, CDCl3): 7.59e7.50
(m, 6H), 7.35 (d, J ¼ 7.1 Hz, 2H), 7.32e7.26 (m, 9H), 6.85 (d,
J ¼ 8.6 Hz, 2H), 3.79 (s, 3H, OCH3), 3.03e2.90 (m, 2H). 13C NMR
(100 MHz, CDCl3): 159.1, 144.8, 144.7, 137.0, 128.3, 128.7, 127.8, 127.7,
127.4, 126.0, 125.9, 113.6, 90.2, 89.8, 74.5, 74.2, 55.3. EI-MS (TOF) m/z
(%): 420 (Mþ, 18), 225 (31), 386 (100), 374 (28), 143 (47), 265 (31),
252 (26), 237 (26), 207 (14), 182 (14), 165 (38). HRMS calcd for
A
3
(1,76 mmol), Pd(PPh3)4 (40 mg; 0.0346 mmol) in toluene (3 mL),
ethanol (3 mL) and K2CO3 (aqueous 2 M solution, 3 mL) was heated
under reflux for 4e5 h. The black solution was filtered, water
(10 mL) was added, and the aqueous phase extracted with ethyl
acetate (3 ꢀ 10 mL). The combined organic phases were dried
(Na2SO4), filtered, and the solvent removed under reduced pressure
to provide an oil which was purified by recrystallization from
CH2Cl2/ethanol.
C
29H24O3: 420.1725, found: 420.1724.
4.3. General procedure for the synthesis of vinylidene-
naphthofurans 3a-c
4.4.1. 1-(2,2-Diphenylvinylidene)-2-(p-hydroxymethylbiphenyl)-2-
phenyl-1,2-dihydronaphtho[2,1-b]furan (5a)
A solution of naphthol (2.38 mmol), tetraarylbut-2-yn-1,4-diol
2a-c (1 eq), p-TSA (catalytic) in CHCl3 (20 mL) was stirred 5 h at
room temperature. The solvent was removed under reduced pres-
sure and the residue was purified by recrystallization from CH2Cl2/
Obtained from naphthofuran 3a and phenylboronic acid (0.49 g,
79%). Orange crystals. Mp 80.3e84.7 ꢂC. IR (cmꢁ1): 3310, 3054,
3022, 2928, 1623, 1491, 1269, 1247, 958, 807. 1H NMR (400 MHz,
CDCl3): 8.25 (d, J ¼ 8.6 Hz, 1H), 7.82 (t, J ¼ 8.0 Hz, 2H), 7.65 (m, 1H),
Please cite this article as: D. Costa et al., Colour switching with photochromic vinylidene-naphthofurans, Tetrahedron, https://doi.org/10.1016/
j.tet.2018.10.076

8
D. Costa et al. / Tetrahedron xxx (xxxx) xxx
7.61e7.4 (m, 6H), 7.4e7.1 (m, 17H), 4.73 (s, 2H), 1.6 (s, 1H). 13C NMR
(100 MHz, CDCl3): 202.8, 157.8, 142.8, 142.2, 140.5, 140.1, 140.0,
136.4, 132.1, 132.0, 131.8, 130.0, 129.1, 129.0, 128.8, 128.5, 128.4,
128.3, 128.2, 128.1, 127.8, 127.7, 127.41, 127.38, 127.3, 126.9, 123.6,
122.2, 117.4, 114.8, 113.4, 112.4, 94.9, 65.0. EI-MS (TOF) m/z (%): 604
(Mþ, 19), 602 (31), 588 (10), 525 (27), 498 (18), 421 (31), 313 (28),
277 (100), 201 (25). HRMS calcd for C45H32O2: 604.2402, found:
604.2410.
112.8, 94.9. EI-MS (TOF) m/z (%): 590 (Mþ, 62), 512 (100), 496 (17),
407 (16), 183 (11). HRMS calcd for C44H30O2: 590.2246, found:
590.2238.
4.5. Preparation of the photochromic materials
A solution of naphthofuran 5a-c, 6 (0.160 g), 3-(triethoxysilyl)
propyl isocyanate (1eq) in dry THF was stirred at 50e60 ꢂC for 7
days. The solvent was removed at reduced pressure and residue
dried under high vacuum affording the corresponding silanized
naphthofurans that were not purified. A mixture of triethox-
4.4.2. 7-Hydroxymethyl-1-(2,2-diphenylvinylidene)-2,2-diphenyl-
1,2-dihydronaphtho[2,1-b]furan 5b
Obtained from naphthofuran 3b and p-(hydroxymethyl)phe-
nylboronic acid. Yellowish crystals (88 mg, 84%). Mp
229.3e231.2 ꢂC. IR (cmꢁ1): 3564, 3053, 3026, 1596, 1508, 1253, 958,
805. 1H NMR (400 MHz, CDCl3): 8.33 (d, J ¼ 8.6 Hz, 1H), 8.05 (d,
J ¼ 1.4 Hz, 1H), 7.87 (d, J ¼ 8.8 Hz, 1H), 7.76 (dd, J ¼ 8.7 Hz, J ¼ 1.5 Hz,
1H), 7.69 (d, J ¼ 8.1 Hz, 2H), 7.6e7.4 (m, 6H), 7.3e7.1 (m, 17H), 4.77
(s, 2H), 1.6 (s, 1H). 13C NMR (100 MHz, CDCl3): 202.8, 157.9, 142.9,
140.3, 139.7, 136.4, 135.9, 132.0, 130.3, 128.9, 128.7, 128.4, 128.1,
127.9, 127.7, 127.5, 127.32, 127.27, 126.8, 122.7, 117.4, 114.7, 113.5,
112.9, 94.9, 65.0. EI-MS (TOF) m/z (%): 604 (Mþ, 6), 602 (36), 588
(16), 525 (31), 510 (14), 435 (12), 395 (17), 368 (41), 353 (30), 255
ymethlysilane (1117
5.5 mmol), phenyltriethoxysilane (1355
glycol (1047 L,18.72 mmol), chloroacetic acid (250 mg, 2.65 mmol)
and water (420
L, 23 mmol) was stirred for 7 h at 25 ꢂC and then a
solution of the above silanized naphthofurans (20 mg, 0.025 mmol)
in THF (730 L) was added and this solution and maintained in
vigorous stirring for 30 min. The solution was then transferred to a
petri dish and kept covered, in an oven at 50 ꢂC. After 4 days, the
petri dish was uncovered and maintained at 50 ꢂC for more 3 days
providing a transparent material.
m
L, 5.5 mmol), tetraethoxysilane (1248
mL,
mL, 5.5 mmol), ethylene
m
m
m
(27), 213 (42). HRMS calcd for
604.2390.
C45H32O2: 604.2402, found:
4.6. Measurement of the photochromic properties
The UVeVis spectra of the materials, before and after UV irra-
diation was recorded using a UVeVis spectrophotometer (CARY 50
Varian). To activate the molecules an UV lamp (370 nm, 36 W) was
positioned over the samples and the UV light was switch on for
2 min producing a violet/green colouration. Then the light source
was removed and the absorbance at lmax recorded over time to
measure the fading kinetics of the samples. All measurements were
made at 20 1 ꢂC. The rate constant (k) was calculated by the fitting
of the bleaching curves to a bi-exponential decay equation:
A ¼ A0þA1.exp (-k1t)þ A2.exp (-k2t)þ where A0 is the initial
absorbance of the sample before UV irradiation and k1 and k2 two
rate constants.
4.4.3. 1-(2-(4-Methoxyphenyl)-2-phenylvinylidene)-2,2,7-
triphenyl-1,2-dihydronaphtho[2,1-b]furan (4c)
Obtained from naphthofuran 3c and phenylboronic acid. Grey
crystals (0.59 g, 85%). Mp 179.9e183.6 ꢂC. IR (cmꢁ1): 3058, 3001,
2957, 2832, 1592, 1508, 1240, 1171, 944, 828, 771, 692. 1H NMR
(400 MHz, CDCl3): 8.35 (d, J ¼ 8.6 Hz, 1H), 8.06 (s, 1H), 7.88 (d,
J ¼ 8.8 Hz, 1H), 7.77 (d, J ¼ 8.7 Hz, 1H), 7.70 (d, J ¼ 7.7 Hz, 2H),
7.54e7.40 (m, 6H), 7.40e7.20 (m, 13H), 7.10 (d, J ¼ 8.7 Hz, 2H), 6.82
(d, J ¼ 8.7 Hz, 2H), 3.83 (s, 3H). 13C NMR (100 MHz, CDCl3): 202.5,
159.3, 157.9, 143.1, 143.0, 140.9, 136.7, 136.3, 131.9, 130.3, 129.9,
128.9, 128.8, 128.7, 128.6, 128.4, 128.1 (2C), 127.9, 127.7, 127.4, 127.2,
127.1, 126.9, 122.7, 117.0, 114.5, 113.81, 113.7, 112.8, 94.9, 55.3. EI-MS
(TOF) m/z (%): 604 (Mþ, 82), 573 (12), 527 (100), 511 (9), 496 (19),
407 (12), 376 (17), 363 (14), 300 (11), 289 (12). HRMS calcd for
Acknowledgements
C
45H32O2: 604.2402, found: 604.2428.
The authors acknowledge FCT (Portugal's Foundation for Sci-
ence and Technology) and FEDER for financial support through the
research project POCI-01-0145-FEDER-016726 and PTDC/QEQQOR/
0615/2014.
4.4.4. 1-(2-(4-Hydroxyphenyl)-2-phenylvinylidene)-2,2,7-
triphenyl-1,2-dihydronaphtho[2,1-b]furan (5c)
BBr3 (1.0 M; 0.33 mL) was added to a solution of naphthofuran
4c (0.20 g; 0.33 mmol) in dry CH2Cl2 (15 mL) at room temperature.
After stirring for 5 h water (15 mL) was added to the green dark
solution and the aqueous phase extracted with CH2Cl2 (3 ꢀ 15 mL).
The combined organic phases were dried (Na2SO4), filtered and the
solvent removed under reduced pressure to provide an oil which
was purified by column chromatography using 15% EtOAc/petro-
leum ether. The compound was further purified by recrystallization
from CH2Cl2/petroleum ether to afford white crystals (0.15 g, 76%).
Mp 120.3e124.7 ꢂC. IR (cmꢁ1): 3347, 3060, 3023, 1597, 1495, 1448,
1264, 1167, 1024, 952, 755. 1H NMR (400 MHz, CDCl3): 8.29 (d,
J ¼ 8.6 Hz, 1H), 8.01 (s, 1H), 7.83 (d, J ¼ 8.8 Hz,1H), 7.73 (d, J ¼ 8.5 Hz,
1H), 7.65 (d, J ¼ 7.9 Hz, 2H), 7.5e7.35 (m, 6H), 7.35e7.10 (m, 17H),
7.01 (d, J ¼ 8.3 Hz, 2H), 6.70 (d, J ¼ 8.5 Hz, 2H), 4.8 (s, 1H). 13C NMR
(100 MHz, CDCl3): 202.4, 157.9, 155.3, 143.1, 142.9, 140.9, 136.6,
136.3, 131.9, 130.3, 130.1, 128.9, 128.8, 128.7, 128.4, 128.12, 128.09,
127.9, 127.7, 127.4, 127.2, 127.1, 126.9, 122.6, 116.9, 115.3, 114.5, 113.6,
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ꢀ
Please cite this article as: D. Costa et al., Colour switching with photochromic vinylidene-naphthofurans, Tetrahedron, https://doi.org/10.1016/
j.tet.2018.10.076