4,4′-Bithiazole-based tetraarylenes: New photochromes with unique photoreactive patterns

Article abstract of DOI:10.1039/c2cc35587f

Five 4,4′-bithiazole-based tetraarylenes were prepared and their photochromic behavior investigated. With their 1,3,5,7-octatetraene photoreactive backbone, they offer not only more available sites for further functionalizations, but also a novel design principle for the development of a new class of biphotochromes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc35587f

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4,40-Bithiazole-based tetraarylenes: new photochromes with unique
photoreactive patternsw
a
a
a
a
b
´ ´ ´ ´
Gildas Gavrel, Pei Yu,* Anne Leaustic, Regis Guillot, Remi Metivier and
Keitaro Nakatanib
Received 1st August 2012, Accepted 23rd August 2012
DOI: 10.1039/c2cc35587f
Five 4,40-bithiazole-based tetraarylenes were prepared and their
photochromic behavior investigated. With their 1,3,5,7-octatetraene
photoreactive backbone, they offer not only more available sites for
further functionalizations, but also a novel design principle for
the development of a new class of biphotochromes.
Two 6p-electrocyclizations are now possible within such an
octatetraene structure, as shown in Scheme 1b. The central
diene moiety (C3–C6) is involved in both electrocyclizations. In
the case of symmetrically substituted octatetraene, the two
photochromic reactions would result in an identical photo-
product, while two different photoproducts could be expected
for dissymmetric octatetraenes. If verified, the latter would
constitute a new class of biphotochromes with two mutually
exclusive photochromic reactions at the molecular scale, which
is different from the conventional biphotochromes where
the two photochromic hexatriene backbones are structurally
independent from each other.12 Molecules derived from such
a photochromic backbone are interesting not only because
they provide more available sites that can be further function-
alized to meet specific demand, but they also offer a unique
opportunity to investigate parameters controlling the two
different and competing electrocyclization pathways in the
case of dissymmetric octatetraenes. To illustrate the potential
of such photochromic compounds, we have focused our
attention on a few 4,40-di(2-phenyl) thiazole-based symmetric
and dissymmetric tetraarylenes.
Since the first report in the late 1980s, photochromic diarylethenes
have attracted widespread attention as versatile light-driven
switching elements for the design of various photoresponsive
systems.1 This popularity is mainly due to their generally good
photochromic behavior (thermal irreversibility and high
photo-resistance) and also to their synthetic flexibility. Based
on the pericyclic reaction between 1,3,5-hexatriene open form
and 1,3-cyclohexadiene closed form (Scheme 1a), the photo-
chromism of diarylethenes proved to be quite tolerant toward
various structural patterns.2 Indeed, although most of them
have a basic structure with two side aryl groups and a cyclic
ethene moiety, such as perfluorocyclopentene, cyclopentene,
maleic anhydride or maleimide, recent works have also shown
that one of the aryl groups can be replaced by substituted
olefin groups.3 More interesting is the introduction of variously
functionalized ethene moieties, leading to multifunctional diaryl-
ethene-based molecular switches.4–11 In this work, we report on a
family of easily accessible photochromes, termed tetraarylenes,
that possess a 1,3,5,7-octatetraene core structure with unique
photochromic reaction patterns (Scheme 1).
The choice of 4,40-di(2-phenyl)dithiazole as the central diene
moiety is explained both by the ease of its synthetic access and
by the excellent photochromic properties of thiazole-containing
diarylethenes either as a side aryl group1a,13–15 or as a central
ethene moiety.16 Besides thiazole groups, thiophene and oxazole
groups are also used as the two side aryl groups in order to assess
their impact on the photochromic properties.
As outlined in Scheme 2, 1o–5o can be easily prepared in three
steps. A double Hantzsch condensation reaction of thiobenzamide
with 1,4-dibromo-2,3-butanedione, both commercially available,
Scheme 1
a
´
´
Institut de Chimie Moleculaire et des Materiaux d’Orsay, UMR
´
ˆ
8182, Universite Paris-Sud 11, Bat. 420, 91405 Orsay, France.
E-mail: pei.yu@u-psud.fr; Fax: +33 1 69 15 47 54;
Tel: +33 1 69 15 61 83
b
´
PPSM, ENS Cachan, UMR 8531, 61 av. President Wilson, 94235
Cachan cedex, France
w Electronic supplementary information (ESI) available: Experimental
details and spectroscopic data. CCDC 889067, 889068 and 889294.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c2cc35587f
Scheme 2 Synthesis of 1o–5o: (a) (COCH2Br)2/MeOH/reflux;
(b) Br2/MeCN–CHCl3/reflux or NBS–DMF, 70 1C; (c) ArB(OR)2,
Pd(PPh3)4, CsF/dioxane/reflux.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 10111–10113 10111

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furnished in high yield 4,40-di(2-phenyl)thiazole, which was then
brominated to give 5,50-dibromo-4,40-di(2-phenyl)thiazole. For
symmetric 1o–4o, the final step was a double Suzuki–Miyaura
coupling reaction with the corresponding heteroaryl boronic
acid (ester). For the preparation of dissymmetric 5o, stepwise
Suzuki–Miyaura coupling reactions proved to be troublesome
because of purification and partial debromination problems.
Nevertheless, it turned out that simultaneous running of the
two Suzuki–Miyaura coupling reactions provided 5o as the
major product, which could be readily isolated by chromato-
graphy from the two expected symmetric tetraarylenes 2o
interconversion between 1o and 1c. The coloration–bleaching
cycle can be repeated many times without detectable fatigue,
as shown in the inset of Fig. 1. It is interesting to compare
these data to those reported for the corresponding triangle
terarylene with three phenylthiazolyl groups.16a Not surprisingly,
the spectral changes are similar, given the almost identical
hexatriene structures involved in the two photochromic
reactions. However, the thermal stability of 1c, estimated by
monitoring the decay of the visible absorption band at various
temperatures, is considerably decreased, with a half-life of
24 days at room temperature for 1c as compared to 3.3 years
for the terarylene counterpart.15a One possible explanation
to such a difference could be that the bulky substituent
(phenylthiazole) in 1c would induce more strain in its mole-
cular structure, thus decreasing its stability as compared to its
terarylene counterpart with a methyl group as a substituent.15a
A similar effect has been reported and successfully exploited to
prepare conventional diarylethenes17 as well as triangle terarylenes18
with tunable thermal fading rate of their closed forms.
As can be seen in Table 1, both the absorption spectra of
these tetraarylenes and the thermal stability of their closed
forms can be significantly tuned by varying the nature of the
two side aryl groups. Moreover, they are all characterized by
pretty high cyclization quantum yields while those of their
cycloreversions are in the usual range for diarylethenes. It is
reasonable to think that such tetraarylenes in their open forms are
prone to adopt, in order to minimize steric hindrance, the photo-
active anti-parallel conformations rather than the photoinactive
parallel ones. The fact that 1o, 2o and 5o all possess antiparallel
conformations in their molecular structures, as revealed by their
single crystal data (Fig. S7–S9, ESIw), is also in favor of similar
conformations in solution for such tetraarylenes.
1
and 3o. 1o–5o are fully characterized by H NMR, 13C NMR,
HRMS, elemental analyses and single crystal structure determina-
tions for 1o, 2o and 5o (ESIw).
1o–4o display good and qualitatively similar photochromic
properties in solution and they all can be switched between
their colorless open forms (1o–4o) and colored closed forms
(1c–4c) by alternate UV and visible light irradiation, as
monitored by using electronic absorption and 1H NMR
spectroscopy (for 2o–4o, Fig. S2–S4, ESIw). Their main photo-
chromic data are gathered in Table 1, and exemplified by the
UV-Vis spectral changes of 1o upon light irradiation (Fig. 1)
and discussed below.
The initial solution of 1o is colorless and characterized by an
intense absorption at 320 nm. Upon UV irradiation at 320 nm,
the solution rapidly turns blue with the appearance of a broad
band in the visible region as a result of the ring-closing reaction
to produce 1c. The presence of isosbestic points indicates clean
Table 1 Main photochromic data of symmetric tetraarylenes in MeCN
lmax emax
(nm) (Mꢀ1 cmꢀ1
Fo-c
Fc-o
Conversion
(%) (320 nm)
)
(lirr/nm) (lirr/nm) t1/2
The solution photochromic behavior of dissymmetric 5o
is qualitatively similar to those of symmetric ones (Fig. S5,
ESIw). However, two photoproducts can be expected as shown
in Scheme 3.
1o 321
45 000
23 000
41 000
14 000
43 000
16 000
48 000
23 000
0.55 (334)
—
0.67 (334)
—
0.59 (313)
—
0.41 (334)
—
—
0.03 (657) 24 d
—
0.04 (657) 16.5 d
—
0.03 (550) 91 d
—
0.03 (550) 5.4 h
82
86
90
62
1c 612
2o 329
2c 632
3o 279
3c 574
4o 299
4c 634
From its absorption spectral changes alone, it is difficult to
conclude with the presence of only one closed form (5c1 or 5c2)
or both (Scheme 3): despite their structural differences, their
absorption bands are expected to overlap in the visible region.
However, 1H NMR monitoring of a UV irradiated solution of
5o elucidates this, by clearly showing the formation of two new
photoproducts along with the presence of 5o (Fig. S6, ESIw).
By comparing the 1H NMR spectra with those of two symmetric
tetraarylenes 2o and 3o upon UV irradiation, the two new
photoproducts are identified unambiguously as the two expected
cyclized products 5c1 and 5c2 (Fig. S6, ESIw). The proportion of
5c1 and 5c2 varies with irradiation time and reaches 35/65 in the
photostationary state (320 nm) with 84% of 5o being converted.
The two photochromic reactions are fully reversible as both
the initial UV-Vis and 1H NMR spectra are completely
Fig. 1 Absorption spectra of 1o (red) and its photostationary state
(blue) (320 nm) in acetonitrile (2.2 ꢁ 10ꢀ5 M) at room temperature.
Scheme 3
c
10112 Chem. Commun., 2012, 48, 10111–10113
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restored upon irradiation at 600 nm. As it is difficult to isolate
pure 5c1 and 5c2 in sufficient amount, the thermal stability of
each closed form cannot be easily and precisely evaluated.
Nevertheless, monitoring the decay of the broad absorption
band in the same way as for 1c–4c allows us to get a rough
estimate of their overall thermal stability, with a half-life of
88 days at room temperature for a mixture of 5c1 and 5c2.
The quantum yield determinations of both cyclization and
cycloreversion are also made more complex by the formation
of the two closed forms (5c1 and 5c2) and their largely over-
lapped absorption spectra. To overcome these difficulties,
analytical amounts of 5c1 and 5c2 were separated by HPLC
and (Fig. S1, ESIw) their absorption spectra and molar absorp-
tion coefficients measured (Fig. S5, ESIw). Then, UV-visible
absorption spectra of a 5o sample as a function of light irradia-
tion time on a high-rate CCD spectrograph were recorded. Every
absorption spectrum of the photochromic mixture at any time
was carefully deconvoluted into the sum of its components
(5o, 5c1, 5c2) by an automatized procedure, yielding their corres-
ponding time-evolution concentration profiles. The numerical
fitting of such concentration profiles as a function of time for
several UV and visible wavelengths of irradiation, using a
3-components kinetic photochromic scheme, provides the
following individual photochromic quantum yields: F (5o -
5c1) = 0.23 ꢂ 0.02; F (5o - 5c2) = 0.20 ꢂ 0.01; F (5c1 - 5o) =
0.02 ꢂ 0.01; F (5c2 - 5o) = 0.02 ꢂ 0.01. No significant
dependence of quantum yields on excitation wavelength (334
and 365 nm for the cyclization, 547 and 657 nm for the
cycloreversion) is observed. In contrast, the concentration
profile of the three components varies with the light excitation
wavelength (Fig. S7, ESIw).
Finally, 1o and 5o are also found to display crystalline state
photochromism (data not shown), and this is in accordance
with their molecular structures since, in addition to their
antiparallel conformations, the distance between the two
photoreactive carbon atoms involved in each of the two
cyclization pathways is much shorter (3.441 and 3.467 A for
1o, 3.423 and 3.445 A for 5o) than 4.2 A, the upper limit for
the electrocyclization to take place in the crystalline state.19
In conclusion, we have described a new family of easily accessible
4,40-dithiazole-based tetraarylenes with unique photoreactive
patterns, good and tunable photochromic properties. With their
octatetraene backbone they offer not only more available sites for
further functionalisations but also provide a novel design principle
for elaboration of a new class of biphotochromes, of which 5o
represents the first example. Synthesis of other dithiazolyl-based
tetraarylenes as well as extension of such octatetraene structure to
other aryl groups are underway and will be reported in due time.
The authors gratefully acknowledge ANR for its financial
support (ANR-07-NANO-025, Nanophotoswitch) and Emilie
Kolodziej of ICMMO for HPLC analyses of a UV irradiated
solution of 5o.
Notes and references
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Chem. Commun., 2012, 48, 10111–10113 10113