Controlled conversion of isomers in a hybrid biphotochromic system

Article abstract of DOI:10.1021/ol062055x

(Chemical Equation Presented) The photochromic performance of a hybrid system connecting naphthopyran and dithienylethene was investigated, and the photochemistry of eight different isomers was explored by choosing an appropriate wavelength of light.

Full text of DOI:10.1021/ol062055x

ORGANIC
LETTERS
2006
Vol. 8, No. 21
4931-4934
Controlled Conversion of Isomers in a
Hybrid Biphotochromic System
Ste´phanie Delbaere,* Gaston Vermeersch, Michel Frigoli,†,‡ and Georg H. Mehl†
CNRS UMR 8009, Laboratoire de Physique, Faculte´ de Pharmacie, UniVersite´ de Lille
2, F-59006 Lille Cedex, France
stephanie.delbaere@uniV-lille2.fr
Received August 21, 2006
ABSTRACT
The photochromic performance of a hybrid system connecting naphthopyran and dithienylethene was investigated, and the photochemistry
of eight different isomers was explored by choosing an appropriate wavelength of light.
Reversible light-induced transformation between several
states allows the exploration of fundamental relationships
of electromagnetic radiation with organic matter and the
elucidation of correlations between chemical structure,
electronic properties, reactivity and the controlled intercon-
version of species.1 Naphthopyrans2 whose photochromism
is based on the ring opening of a pyran group have been
intensively investigated as a result of their high colorability,
rapid thermal relaxation and fatigue resistance, properties
relevant to applications in the optical glass industry3 and
optoelectronics.4 More recently, the thermally irreversible
photocyclization reactions of 1,2-dithienylethenes, materials
that can be tuned to very high photoconversion quantum
yields and whose absorption spectra can be modulated
rationally by suitable chemical construction, are becoming
increasingly significant.5 Multiphotochromic systems incor-
porating a dithienylethene moiety are in the focus of research,
as the controlled sequential interaction of photons promises
synergies in the design of such materials and their electronic
properties.6 Furthermore, ultrahigh information data storage
systems might be obtainable as a result of the information
density inherent in such materials. Information could be
encoded according to the multispectral properties of such
compounds either in their bistable mode or for time-resolved
read-out.7 It is crucial, however, that the interaction of the
“read-out beam” with the information carrying species can
be controlled, to deal with the issue of data erasure during
read-out.
Recently, some of us reported the synthesis and photo-
chromic properties of a hybrid system, OD-CN: 3-(1,2-bis-
(2,5-dimethyl-3-thienyl)perfluorocyclopentene) 3-phenyl-
† Department of Chemistry, University of Hull, Hull HU6 7RX, UK.
‡ Current address: University of Versailles, Lavoisier Institute, France.
(1) (a) Du¨rr, H.; Bouas-Laurent, H. Photochromism: Molecules and
Systems; Elsevier: Amsterdam, 1990. (b) Crano, J. C.; Guglielmetti, R. J.
Organic Photochromic and Thermochromic Compounds; Plenum Press:
New York, 1999. (d) Feringa, B. L. Molecular Switches; Wiley-VCH: New
York, 2001. (e) Irie, M. Photo-ReactiVe Materials for Ultrahigh-Density
Optical Memory; Elsevier: Amsterdam, 1994.
(5) (a) Irie, M. Chem. ReV. 2000, 100, 1685-1716. (b) Norsten, T. B.;
Peters, A.; McDonald, R.; Wang, M.; Branda, N. R. J. Am. Chem. Soc.
2001, 123, 7447-7448. (c) Higashiguchi, K.; Matsuda, K.; Tanifugi, N.;
Irie, M. J. Am. Chem. Soc. 2005, 127, 8922-8923. (d) Kobatake, S.;
Yamada, T.; Uchida, K.; Kato, N.; Irie, M. J. Am. Chem. Soc. 1999, 121,
2380-2386.
(2) Becker, R. S.; Michl, J. J. Am. Chem. Soc. 1966, 88, 5931-5933.
(3) Crano, J. C.; Flood, T.; Knowles, D.; Kumar, A.; Van Gemert, B.
Pure Appl. Chem. 1996, 68, 1395-1398.
(4) (a) Sakai, H.; Ueno, A.; Anzai, J.; Osa, T. Bull. Chem. Soc. Jpn.
1986, 59, 1953-1956. (b) Xuzhi, Q. Vision Ease Lens Inc., WO03044022,
2003. (c) Nelson, C. M.; Chopra, A.; Petrovskaia, O.; Knowles, D. B.; Van
Gemert, B.; Kumar, A. Transitions Optical Inc., EP1214311, 2002.
(6) Mrozek, T.; Go¨rner, H.; Daub, J. Chem.-Eur. J. 2001, 7, 1028-
1040. Jung, I.; Choi, H.; Kimb, E.; Lee, C.-H.; Kang, S. O.; Ko, J.
Tetrahedron 2002, 61, 12556-12263. Wigglesworth, T.; Myles, A.; Branda,
N. Eur. J. Org. Chem. 2005, 7, 1233-1238.
10.1021/ol062055x CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/21/2006

Figure 1. General photochromic reaction of OD-CN (OD, open dithienylethene; CD, cyclized dithienylethene; ON, open naphthopyrans;
CN, closed naphthopyrans; TTC, trans-transoid-cis; CTC, cis-transoid-cis; TTT, trans-transoid-trans; CTT, cis-transoid-trans); numbers
1
refer to H chemical shifts (ppm) of selected protons.
naphthopyran.8 Four different states were detected by UV-
vis spectroscopy. However, as the individual photochromes
form isomers, such a system consisting of a dithienylethene
group (termed OD/CD, as open/cyclized dithienylethene) and
a naphthopyran group (here named CN/ON for closed/open
naphthopyran) consists of at least 10 colored states (Figure
1). Provided that these isomers are identifiable and addres-
sable, an isomer density of approximately 10 states per nm3
can be estimated, leading in turn to a system with potentially
1019 states (theoretically addressable isomers) per cm3 in the
solid state.9 For the investigation of this switching behavior
the initial challenge is the identification of the species formed,
the selective and controlled addressing of individual photo-
chromic units, and the conversion of the system in the photon
mode. To achieve this, NMR spectroscopy of samples
irradiated under controlled conditions is uniquely suitable,
as this technique measures directly and quantitatively the
concentration of different species (isomers).10
In initial experiments, the structures of the species formed
on irradiation were identified; in a second series of experi-
ments, the interconversion of species was monitored. Ir-
radiation of a solution of OD-CN at 313 nm at 293 K allows
the monitoring of the cyclization of the dithienylethene group
(a colorless solution turns pink). At this temperature, the open
forms of naphthopyran are not accumulated sufficiently to
be detected by NMR because of their low thermal stability.
1
In H NMR spectra, peaks appear at 5.82 (24%) and 5.87
3
(33%) ppm (each with doublet multiplicity, J ) 9 Hz), at
5
5.76 (57%) ppm (multiplet with J ) 1.2 Hz), and at 6.56
(24%) and 6.66 (33%) ppm (singlet resonances). On the basis
(7) (a) Tsivgoulis, G. M.; Lehn, J.-M. AdV. Mater. 1997, 9, 627-630.
(b) Fernandez-Acebes, A.; Lehn, J.-M. AdV. Mater. 1999, 11, 910-913.
(c) Morimoto, M.; Kobatake S.; Irie, M. AdV. Mater. 2002, 14, 1027-
1029. (d) Morimoto, M.; Kobatake S.; Irie, M. J. Am. Chem. Soc. 2003,
125, 11080-11087. (e) Higashiguchi, K.; Matsuda, K.; Tanifuji, N.; Irie,
M. J. Am. Chem. Soc. 2005, 127, 8922-8923. (f) Wigglesworth, T.; Branda,
N. Chem. Mater. 2005, 17, 5473-5480.
(9) Vm ) Mw/FNA; F ) 103 kg m-3; Mw ) 628, 92; Vm ) 1.04 nm3. As
the molecular volume does not change, this results in 10 theoretical states
per nm. It is noted that this is only a theoretical number, unlikely to be
achieved in practice.
(10) Delbaere, S.; Micheau, J.-C.; Teral Y.; Bochu, C.; Campredon M.;
Vermeersch, G. Photochem. Photobiol. 2001, 74, 694-699. Delbaere, S.;
Micheau, J.-C.; Vermeersch, G. J. Org. Chem. 2003, 68, 8968-8973.
Delbaere, S.; Micheau, J.-C.; Frigoli, M.; Vermeersch, G. J. Org. Chem.
2005, 70, 5302-5304.
(8) Frigoli M.; Mehl, G. H. Angew. Chem., Int. Ed. 2005, 44, 5048-
5052.
4932
Org. Lett., Vol. 8, No. 21, 2006

of 1D and 2D NMR experiments, the two photoproducts were
assigned to the possible two diastereomers (formed as
racemates) for a cyclized dithienylethene and closed naph-
thopyran, CD-CN1 and CD-CN2. Indeed, the photogenerated
closed-ring form (CD) has either SS or RR asymmetric
structure,5a and the stereochemistry at the sp3 carbon in CN
is either R or S. Consequently, the two couples of enantio-
mers (RR-R)SS-S and RR-S)SS-R) can be distinguished by
NMR.
Irradiation of a solution (1.8 mM of OD-CN) at 365 nm
at 227 K (the colorless solution turns red via orange) revealed
the partial conversion of OD-CN into three photoproducts,
which displayed resonances at δ 9.35, 9.01 and 7.99 ppm
(each with doublet multiplicity, 3J ≈ 12 Hz). Thermal
evolution in the dark indicated conversion toward the initial
form OD-CN with different rates (227k∆ (δ 9.35 ppm) ) 9 ×
10-6 s-1; 227k∆ (δ 9.01 ppm) ) 1.5 × 10-4 s-1; 227k∆ (δ 7.99
ppm) ) very slow). Such rate constants and 2D NMR
experiments allowed unambiguous attribution of these three
forms to open diarylethene with open naphthopyran groups
(OD-TTC, OD-CTC, and OD-TTT). Irradiation of OD-CN
Figure 3. Time-evolution concentrations at 227 K under 313 nm
light of OD-CN (black), OD-TTC (pink), OD-CTC (purple), OD-
TTT (orange), CD-CN1 + CD-CN2 (red), CD-TTC (blue), CD-
CTC (green). (a) [OD-CN]0 ) 100%, (b) [OD-CN]0/[CD-CN]0 )
54%/46%.
At the end of cumulative irradiation (2400 s), about 30% of
structures with open dithienylethene and open naphthopyran
groups were obtained (17%, 9%, and 4% of OD-TTC, OD-
CTC, and OD-TTT, respectively), and compounds with
cyclized dithienylethene represented 47% of the solution
(17%, 20%, and 10% of CD-CN, CD-TTC, and CD-CTC,
respectively).
Figure 2. Typical 1H NMR spectra of ODCN (a) and mixture after
irradiation at 313 nm (b).
at 227 K at 313 nm (the solution turns brown via orange/
red) allowed the observation of the five previously assigned
photoproducts: CD-CN1, CD-CN2, OD-TTC, OD-CTC, and
OD-TTT. In addition, new intense doublet signals (3J ) 12
Hz) at 9.12 and 8.82 ppm, correlated in concentration with
singlet resonances at 5.75 and 5.76 ppm were detected. Both
were thermally unstable (227k∆ (δ 9.12 ppm) ) 5 × 10-6 s-1
and 227k∆ (δ 8.82 ppm) ) 2.1 × 10-5 s-1). Chemical shifts,
multiplicity, and thermal rate constants were used to assign
both structures to CD-TTC and CD-CTC, respectively.
To estimate the pathways for photocyclization of dithien-
ylethene moiety and photo-opening of the naphthopyran part
in more detail, a sample of OD-CN was subjected to 20
periods of 120 s of irradiation with 313 nm light and the
spectra were recorded at 227 K (Figure 3). After irradiation
of OD-CN at 313 nm for 120s, 13% of the material converted
into a ∼1:1:1 mixture of OD-TTC, OD-CTC, and CD-CN
() CD-CN1 + CD-CN2). After 240 s of photolysis, the ratio
changed to favor OD-CTC and CD-CN (1:2:2) and signals
for OD-TTT, CD-TTC and CD-CTC were also observed.
A similar experiment was repeated but with preliminary
irradiation of the sample with 313 nm light at 293 K to
generate a mixture of OD-CN and CD-CN (54%:46%).
Irradiation (120s) generated OD-TTC and CD-TTC (ratio
∼1:1) and OD-CTC and CD-CTC (ratio ∼1:1), but concen-
trations of CTC were twice as high as those of TTC. At the
end of cumulative photolysis, the conversion to OD-ON was
similar to the previous experiment (17% corresponding to
∼30% of initial OD-CN), while higher for the formation of
CD-ON structures (∼40%), indicating that they were formed
mainly from CD-CN.
To confirm the prevalent process at low temperatures,
irradiation of a solution of OD-CN at 227 K at 313 nm for
1 h was performed, and this resulted in a mixture containing
∼17% of OD-CN, ∼52% of OD-ON isomers, ∼7% of CD-
CN isomers, and 24% of CD-ON isomers. The accumulation
Org. Lett., Vol. 8, No. 21, 2006
4933

of photoproducts “OD-ON” identifies the naphthopyran
group as the most active group under these conditions.
The effect of photobleaching in the visible was investigated
for a sample preirradiated at 313 nm in irradiation intervals
(434 nm) of 15 min. All of the photoproducts decreased along
monoexponential-type curves from which pseudo rates have
been extracted. The faster rate corresponded to the decrease
in CD-CN (5.5 10-4 s-1), while rates for OD-CTC and CD-
TTC were about three times lower and those for OD-TTT
and OD-TTC were five times lower.
In summary, the photochromic performance of a linked
naphthopyran-dithienylethene system was investigated, and
the photochemistry of eight different isomers was explored.
Irradiation with 365 nm light gives rise to the thermally
reversible opening of naphthopyran ring. Irradiation at 313
nm leads to the closure of the dithienylethene moiety and
the opening of the naphthopyran group, generating seven
different structures in different concentrations. The enrich-
ment of the closed dithienylethene is achieved more opti-
mally at higher temperatures. The structures CD-ON are
thermally reversible but with rate constants for bleaching
considerably lower than those of their corresponding OD-
ON forms.
Acknowledgment. The 500 MHz NMR facilities were
funded by the Re´gion Nord-Pas de Calais (France), the
Ministe`re de la Jeunesse. de l’Education Nationale et de la
Recherche (MJENR), and the Fonds Europe´ens de De´vel-
oppement Re´gional (FEDER). M.F. acknowledges support
by the EPSRC (UK).
Supporting Information Available: Details of irradiation
techniques and NMR experiments. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL062055X
4934
Org. Lett., Vol. 8, No. 21, 2006