Photoswitch based on remarkably simple naphthopyrans

Article abstract of DOI:10.1016/j.tetlet.2005.02.091

Photochromic switches built around 5-carbonyl-3,3-diphenyl-3H-naphtho[2,1- b]pyran unit have been synthesized. Their open forms are constituted of a 1:1 ratio of TC and TT-isomers according to ¹⁹F and ¹H NMR investigations. Electrocyclization of TT isomers back to the closed form is efficiently achieved upon visible irradiation whereas it has been found to be a particularly unfavoured thermal process. These simple 2H-chromenes could act as molecular photoswitch at ambient temperature.

Full text of DOI:10.1016/j.tetlet.2005.02.091

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3257–3259
Photoswitch based on remarkably simple naphthopyrans
a
Xavier Sallenave, Stephanie Delbaere, Gaston Vermeersch, Ahmed Saleh and
b
b
c
´
Jean-Luc Pozzoa,*
a
´
Photochemical Nanoscience, CNRS UMR 5802, 351 crs Liberation, 33405 Talence Cedex, France
´
b
´
NMR and Photochemistry, CNRS UMR 8009, Faculte de Pharmacie, Universite Lille 2, 59006 Lille, France
cChemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
Received 6 January 2005; revised 15 February 2005; accepted 16 February 2005
Available online 26 March 2005
Abstract—Photochromic switches built around 5-carbonyl-3,3-diphenyl-3H-naphtho[2,1-b]pyran unit have been synthesized. Their
open forms are constituted of a 1:1 ratio of TC and TT-isomers according to 19F and 1H NMR investigations. Electrocyclization of
TT isomers back to the closed form is efficiently achieved upon visible irradiation whereas it has been found to be a particularly
unfavoured thermal process. These simple 2H-chromenes could act as molecular photoswitch at ambient temperature.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Photochromism is usually defined as the reversible pho-
toinduced transformation between two molecular states
whose absorption spectra are significantly different.1 Be-
sides this colour change, the light-induced transforma-
tion causes reversibly important structural and
physico-chemical changes, organic photochromes could
also offer the unique possibility to modify the self-assem-
bly process of the individual molecules and also the
resulting supramolecular network by means of light.2
The current interest in miniaturizing the components
of electronics down to the molecular level is a major
driving force in the research of molecular systems with
switchable properties.3 In the course of development of
novel applications, synthesis of readily available com-
pounds, whose photochromic properties can be used
to create photoreversible systems is highly desired.
Among the numerous organic photochromes already re-
ported, few of them possess two thermally stable states.4
Obviously diarylethenes constitute the most promising
molecules among the rare bistable photochromes and
have been extensively developed since the pioneering
work of Irie.5 The 2H-chromenes and more precisely
the 3,3-diphenyl-3H-naphtho[2,1-b]pyrans have been
studied during the past decades due to their easy access
and pronounced resistance to fatigue, which have pro-
moted their use in ophthalmic applications.6 Photochro-
mism in 2H-chromenes involves photocleavage upon
UV irradiation of the C–O bond, leading to the forma-
tion of open and coloured isomers being assigned as the
Transoid-Cis (TC) and the Transoid-Trans (TT) photo-
merocyanines,7 which reversibly revert back by electro-
cyclization to the colourless closed form.
TC has been found to be the predominant structure
formed exhibiting a fast thermal decay at ambient
temperature, which is suitable for variable optical trans-
mission materials, TT open forms representing usually
0–15%.8
Here we report the synthesis and the photochromic
properties of a new series based on the 3,3-diphenyl-
3H-naphtho[2,1-b]pyrans incorporating a carbonylated
substituent on position 5. Such naphthopyrans bearing
a methoxycarbonyl group 1a,b are easily prepared in
two steps starting from 3-hydroxy-2-naphthoic acid,
which is first converted to its methyl ester and then
reacts with conveniently substituted 1,1-diphenylprop-
2-yn-1-ol in the presence of p-toluenesulfonic acid as
catalyst to afford the photochromic compound.9 UV
irradiation of toluene solution of 1b (5 · 10À4 M)
reveals
a broad absorption in the visible-region
centred at 449 nm. When irradiation ceases partial
fading is observed following a monoexponential decay
(kD(1) = 0.66 sÀ1) until an equilibrium is reached. The
remaining thermostable absorbance has been found to
correspond to Amax/2. When this solution is kept in
the dark a significant decrease can only be detected after
48 h indicating that the conversion of the remaining
Keywords: Photoswitch; Naphthopyrans; 19F NMR; Photochromism.
*
Corresponding author. Tel.: +33 540 000 2752; fax: +33 540 000
6158; e-mail: jl.pozzo@lcoo.u-bordeaux1.fr
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.02.091

3258
X. Sallenave et al. / Tetrahedron Letters 46 (2005) 3257–3259
1
TC
R
1
1
R
R
∆
O
2
1
R
UV
O
O
R
O
O
+
2
R
2
R
O
Vis.
1a R1= Ph, R2= OMe
1
1
R
R
1b R1= 4'F-Ph, R2= OMe
1c R1= Ph, R2= Me
TT
1d R1= 4'F-Ph, R2= Me
Figure 1. Photochromic equilibrium for 3,3-diphenyl-3H-naphthopy-
rans 1a–d.
isomer towards the closed form is particularly an unfa-
voured thermal process. (kD(2) ꢀ 10À6 sÀ1). In contrast
complete bleaching is simply achieved by irradiation
with visible light (k = 450 nm) (Fig. 1). As depicted in
Figure 2, this has been repeatedly cycled without any
noticeable photodegradation. This remarkable feature
prompts us to prepare some additional examples of
naphthopyrans bearing carbonylated groups. This has
been achieved by transforming the previously prepared
esters into methylketones.9 These derivatives 1c–d exhi-
bit similar photochromic response. This attractive
behaviour has also been investigated using NMR
spectroscopy. At low temperature, a solution (10À2 M
in toluene) has been irradiated with UV light
1
(259 < k < 388 nm). 19F and H NMR spectra recorded
Figure 3. 19F NMR spectra of 1b in toluene at 243 K. (a) Before
irradiation, (b) after 10 min of UV irradiation (time t = 0 of thermal
fading), (c) t = 1 h 30 min, (d) t = 2 h 30 min, (e) t = 6 h and (f) t = 12 h
30 min after end of irradiation.
before and after irradiation indicate the conversion of 1b
into three new structures. Indeed, two pairs of 19F reso-
nances between À109 and À111 ppm are detected and
are straightforwardly assigned to the fluorine atoms
borne by phenyl groups in both transoid isomers of
photomerocyanines, TC and TT (Fig. 3).8 The other
new signal at À113.6 ppm is attributed to an allenyl–
naphthol A, which is produced from TC-isomer.10 The
time-evolution of irradiated solution has been followed
at 243 K by recording NMR spectra at regular time
intervals. Only isomer TC decreased rapidly
(kD = 3.310À4 sÀ1), while TT and A present no signifi-
cant time-evolution (Fig. 3c–f). Consequently, we ob-
serve in spite of rather different experimental
conditions of temperature and concentrations, similar
results when compared with previous spectrokinetic
measurements. The same 1:1 ratio of isomers TC and
TT is obtained and only TC is thermoreversible.
pared. Fifty percent of the open forms are constituted
by the TT isomer for all the investigated derivatives.
These latter species have been shown to convert back
to the closed forms extremely slowly through a thermal
process and very efficiently using visible irradiation. All
these properties, which were determined at ambient tem-
perature, enable the use of these chromenes as P-type
photochromes, that is, photoswitches.
Acknowledgements
The 300 MHz NMR facilities were funded by the Re-
`
gion Nord-Pas-de-Calais, the Ministere de la Jeunesse,
de lÕEducation Nationale et de la Recherche, and FED-
ER. This collaborative work was realized within the
framework CNRS GDR 2466.
The 3,3-diphenyl-3H-naphtho[2,1-b]pyrans substituted
by a carbonyl group in the 5-position are easily pre-
Supplementary data
OD
UV
Vis.
Supplementary data associated with this article can be
found, in the online version, at 10.1016/j.tetlet.
2005.02.091.
∆
0.8
0.6
0.4
0.2
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t (min)
Figure 2. Absorbance evolution at 460 nm (kmax) for 1b according UV
or Vis irradiation or thermal decay.

X. Sallenave et al. / Tetrahedron Letters 46 (2005) 3257–3259
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