Bridging the visible: The modulation of the absorption by more than 450 nm

Article abstract of DOI:10.1021/ol101672v

The coexistence of 18 isomers, identified by NMR spectroscopy and gathered into six states with very different absorption properties, is observed upon irradiation with selective wavelengths of a triphotochromic system joining two naphthopyran entities thr

Full text of DOI:10.1021/ol101672v

ORGANIC
LETTERS
2010
Vol. 12, No. 18
4090-4093
Bridging the Visible: The Modulation of
the Absorption by More than 450 nm
Ste´phanie Delbaere,*,† Gaston Vermeersch,† Michel Frigoli,‡,§ and
Georg H. Mehl*,‡
UniVersite´ Lille Nord de France, CNRS UMR 8516, UDSL, Lille, F-59006 Lille Cedex,
France, and UniVersity of Hull, Department of Chemistry, Hull HU6 7RX, UK
stephanie.delbaere@uniV-lille2.fr; g.h.mehl@hull.ac.uk
Received July 20, 2010
ABSTRACT
The coexistence of 18 isomers, identified by NMR spectroscopy and gathered into six states with very different absorption properties, is
observed upon irradiation with selective wavelengths of a triphotochromic system joining two naphthopyran entities through a dithienylethene
bridge. The sequential electronic coupling of photochromic groups resulted in an absorption behavior reaching far into NIR.
The efficient modulation of the absorption spectra of organic
materials is a challenge involving a wide spectrum of
interests, ranging form the design and application of dyes to
the use of panchromatic materials as sensitivity enhancers
for solar cells and to fundamental studies focused on
understanding the electronic parameters responsible for the
extension of the absorption.1 For photochromic materials
where irradiation, typically in the UV range, leads to
reversible transformations, resulting in colored forms with
absorption maxima in the visible region,2 the extension of
the absorption far into the NIR has not yet been much in the
focus of research. However, the increase in the length of
the π-conjugation is expected to generate systems absorbing
in the red-end of the visible spectrum or even with infrared-
absorbing properties, paving the way to develop applications
as materials for write-once and erasable memories.3 To
achieve this goal, several strategies of connecting two or
more photochromic groups to prepare multiphotochromic
switches have been reported.4
Usually, UV/vis spectroscopy, as a relatively fast tech-
nique, is employed to record changes. However, no detailed
information can be gained concerning the structures of the
species formed. This makes NMR investigations invaluable,
even though this technique is slower and requires a higher
concentration of samples.5 Moreover, when multiple species
are formed through irradiation and a number of potential
reaction pathways become possible, only an unambiguous
identification of species can clarify the processes actually
† Universite´ Lille Nord de France.
‡ University of Hull.
§ Current address: Institut Lavoisier de Versailles, UMR CNRS 8180,
UVSQ, Versailles, France.
(3) (a) Emmelius, M.; Pawlowski, G.; Vollmann, H. W. Angew. Chem.,
Int Ed. 1989, 28, 1445–1471. (b) Gilat, S.; Kawai, S.; Lehn, J.-M.
Chem.sEur. J. 1995, 1, 275–284.
(1) (a) Sreejith, S.; Carol, P.; Chithra, P.; Ajayaghosh, A. J. Mater. Chem.
2008, 18, 264–274. (b) Son, H.-J.; Han, W.-S.; Yoo, D.-H.; Min, K.-T.;
Kwon, S.-N.; Ko, J.; Ook Kang, S. J. Org. Chem. 2009, 74, 3175–3178.
(2) (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. (c) Feringa, B. L. Molecular Switches; Wiley-VCH: New
York, 2001. (d) Irie, M. Photo-ReactiVe Materials for Ultrahigh-Density
Optical Memory; Elsevier: Amsterdam, 1994.
(4) (a) Samat, A.; Lokshin, V.; Chamontin, K.; Levi, D.; Pepe, G.;
Guglielmetti, R. Tetrahedron 2001, 57, 7349–7359. (b) Mrozek, T.; Go¨rner,
H.; Daub, J. Chem.sEur. J. 2001, 7, 1028–1040. (c) Myles, A. J.;
Wigglesworth, T. J.; Branda, N. AdV. Mater. 2003, 15, 745–748. (d)
Higashiguchi, K.; Matsuda, K.; Tanifuji, N.; Irie, M. J. Am. Chem. Soc.
2005, 127, 8922–8923.
(5) Delbaere, S.; Vermeersch, G. J. Photochem. Photobiol. C, Photo-
chem. ReV. 2008, 9, 61–80.
10.1021/ol101672v  2010 American Chemical Society
Published on Web 08/23/2010

occurring. This is essential for the design of materials with
improved properties.
mental setup led to the observation of four PS which
correspond to single and/or combined absorption behaviors
of six different states.
Recently, we investigated the behavior of a biphotochromic
system, consisting of connected naphthopyran and dithie-
nylethene entities. Under UV irradiation, four different states
were identified by UV-vis spectroscopy, characterized by
very different absorption properties.6 These were unambigu-
ously identified by NMR spectroscopy as the coexistence of
seven different isomers.7,8 Notable was the extension of the
absorption in excess of 700 nm, giving rise to the question
of whether a triphotochromic system would extend the
absorption further in the NIR and how many species occur
and which reaction pathways are possible. Therefore the
triphotochromic hybrid CN-OD-CN, shown in Scheme 1,
was synthesized9 and investigated.
Irradiation with light at wavelengths above 350 nm triggers
selectively the reactions outlined in the left pathway of
Scheme 1. Monitoring the spectral modifications10 shows a
change from colorless to orange (λmax ) 456 nm (2.96 ×
10-5 mol·L-1) states 2 and 3, Figure 1), which is anticipated
Scheme 1
.
Principal Mode of Action of the Multifunctional
Photochromic System CN-OD-CN
Figure 1
.
Absorption spectra of toluene solution 1 (2.96 × 10-5
mol·.L-1) before irradiation (black), upon irradiation with 365 nm
(orange), upon irradiation with 313 nm light (green), and after
irradiation with 313 nm and thermal bleaching (pink).
for the opening of naphthopyrans. A return to the colorless
form 1 in the dark typical for naphthopyrans occurs.
Detailed NMR experiments at 227 K of more concentrated
solutions (10-3 M) to take into account the different
sensitivities of NMR and UV/vis spectrometers were carried
1
out. In the H NMR spectra recorded before irradiation
(Figure 2a), all signals of CN-OD-CN are split, and peak-
intensities measurements made it possible to correlate them
with the two parallel/antiparallel11 conformers (55-45%)
expected for dithienyl-based photochromes. Upon irradiation
at 365 nm, partial conversion of 1 into six different
thermoreversible forms occurs (Figure 2b). Not only species
where one of the naphthopyran groups is opened (state 2),
such as CTC-OD-CN, TTC-OD-CN, or TTT-OD-CN, but
also forms where both groups are activated could be detected,
such as CTC-OD-CTC, TTC-OD-TTC, and TTC-OD-
CTC (state 3). These photoproducts have been characterized
on the basis of specific chemical shifts,12 peak-intensities
measurements and constants of thermal relaxation. The
associated thermally reverse pathways are at 227 K: 1) CTC-
OD-CTC (7 × 10-4 s-1) f CTC-OD-CN (8 × 10-5 s-1)
f CN-OD-CN, 2) TTC-OD-CTC (3 × 10-4 s-1) and TTC-
OD-TTC (3 × 10-5 s-1) and TTT-OD-CN (very slow) f
TTC-OD-CN (1 × 10-5 s-1) f CN-OD-CN.
The principal and expected mode of action of the multi-
functional photochromic system CN-OD-CN (state 1) is
illustrated in Scheme 1. To check this hypothesis, the
switching properties were investigated by a series of steady-
state experiments in which light was switched on or off and
spectra were recorded until a photostationary state (PS) was
obtained. The sequential approach along with the experi-
(6) Frigoli, M.; Mehl, G. H. Angew. Chem., Int. Ed. 2005, 44, 5048–
5052.
(7) Delbaere, S.; Vermeersch, G.; Frigoli, M.; Mehl, G. H. Org. Lett.
(10) UV-vis spectra are displayed in the Supporting Information.
(11) Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990,
63, 1311–1315.
2006, 8, 4931–4934
.
(8) Delbaere, S.; Micheau, J. C.; Frigoli, M.; Mehl, G. H.; Vermeersch,
G. J. Phys. Org. Chem. 2007, 20, 929–935
.
(12) Delbaere, S.; Micheau, J.-C.; Teral, Y.; Bochu, C.; Campredon,
M.; Vermeersch, G. Photochem. Photobiol. 2001, 74, 694–699.
(9) The synthesis is reported in the Supporting Information.
Org. Lett., Vol. 12, No. 18, 2010
4091

Figure 2.
1H NMR aromatic part of toluene solution of CN-OD-CN (10-3 mol·L-1) at 227 K (a) before, (b) after irradiation with 365 nm,
and (c) after irradiation with 313 nm.
These measurements confirm that both naphthopyran
groups react, an experimental observation which is not
possible directly from steady-state UV/vis measurements. It
is also noted that although π-conjugation of the open
diarylethene group has been extended through conjugation
with the opened naphthopyran groups so that the molecules
do absorb at 365 nm, no electrocyclization of the diarylethene
moiety occurs.
Here NMR investigations are crucial. Irradiation of 1 at
293 K with 313-nm light reveals the partial conversion of
state 1 into three photoproducts resulting from cyclization
in dithienylethene part (Figure 3). As the naphthopyrans are
Irradiation of 1 at 313 nm has the potential to transform
all photochromic groups, schematically shown in the middle
and right paths of Scheme 1. In a steady-state UV/vis
experiment with time, an absorption reaching from the UV
through the visible into the NIR can be detected, as shown
in Figure 1. This is associated with the initial colorless
solution of 1 turning into a very dark solution on irradiation.
This very dark solution turns red in the dark. Irradiation with
visible light turns this red solution colorless, irradiation in
the UV (313 nm) returns the previously observed dark color
whose absorption reaches up to ∼850 nm. The kinetics of
fading at selected wavelengths (658 nm: 9.57 × 10-4 s-1,
700 nm: 11.10 × 10-4 s-1 and 840 nm: 32.89 × 10-4 s-1)
allow fitting to a monoexponential function. On the other
hand, for 750 nm (9.75 × 10-4 s-1 (47%), 30.67 × 10-4 s-1
(53%)] and for 800 nm (10.24 × 10-4 s-1 (19%), 34.48 ×
10-4 s-1 (81%)), the best fading fits are obtained with a
biexponential function. This suggests that the fully conju-
gated systems involving the electronic participation of all
three groups are the only absorbing species above 820 nm.
However, the fading characteristics of the species present at
658 nm species can be explained by two groups electronically
connected of the type CD-ON.
Figure 3.
1H NMR spectrum of toluene solution of CN-OD-CN
(10-3 mol·L-1) after irradiation with 313 nm light at 293 K.
either l or d and the conrotatory photocyclization of dithie-
nylethene produces two enantiomeric closed-ring isomers R,R
and S,S originating from asymmetric carbon atoms, these
three are identified to be the three possible diastereomers,
being arbitrary designed as d-R,R-d () l-S,S-l) and d-S,S-d
() l-R,R-l) for signals 2 and signals 3, respectively, and
d-R,R-l () l-S,S-d) for lines 1 and 4.
Repeating the NMR experiments at 227 K with 313 nm
light (Figure 1c) allows the identification of the structures
4092
Org. Lett., Vol. 12, No. 18, 2010

CN-CD-CN (state 4), CTC-CD-CN, TTC-CD-CN (state
5), where both the diarylethene group (closed) and one
naphthopyran group are involved. As previously noticed for
state 4, CTC-CD-CN and TTC-CD-CN both exist under
two couples of diastereomeric forms being arbitrary designed
as R,R-d () S,S-l) and S,S-d () R,R-l).
Scheme 2. Summary of Reactions of the Six Photochromic
States Corresponding to the 18 Identified Structures
Additionally, the structures CTC-CD-CTC, TTC-CD-
TTC, and CTC-CD-TTC (state 6) were detected. Monitor-
ing of the fading kinetics indicates unambiguously a process
where first one of the two open naphthopyrans is closed and
then the other one. Although the accurate measurement of
peak intensities is difficult because of superimposed lines
and low concentrations of species, the thermal relaxation has
been monitored at different temperatures to estimate the
stability. At 227 K, CTC-CD-CTC bleaches (9.0 × 10-5
s-1) into CTC-CD-CN (0.85-1.4 × 10-5 s-1). Both exhibit,
however, a higher stability than their homologous structures
without cyclization (CTC-OD-CTC and CTC-OD-CN).
More remarkable is the thermal stability of structures TTC-
CD-TTC and TTC-CD-CN at 227 K. A significant value
of the rate constant of bleaching has been estimated at 254
K for TTC-OD-TTC (3 × 10-5 s-1) f TTC-OD-CN. This
latter evolves toward CN-OD-CN at 273K (9.4 × 10-5 s-1).
Finally, a fresh sample was preliminarily irradiated with
365 nm light to produce states 2 and 3 and then irradiated
with 313 nm. No cyclization of the molecules identified by
the states 2 and 3 into states 5 and 6 was observed. This
clearly indicates that no photocyclization occurs when the
naphthopyran is in an open configuration, or in other words,
it means no pathways 2 f 5 or 3 f 6 are open to the system.
In the same way, no reverse thermal opening of diarylethene
was detected.
the results show that switching of the naphthopyran group
is possible, independent of the state of the diarylethene group.
However, the diarylethene group cannot be closed if a
naphthopyran group is open. It was found that the open
naphthopyran groups are thermally more stable if the
diarylethene is closed.
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).
To conclude, the present work reports on the coexistence
of 18 isomers gathered into six different states (Scheme 2).
This system provides useful lessons for the design of future
systems. The sequential electronic coupling of photochromic
groups allows for the design of materials whose absorption
reaches far into NIR, beyond the absorption of the individual
photochromes. The relatively large bathochromic shift change
in the absorption maxima, moving from two (OD-CN) to
three (CN-OD-CN) active groups of about 100 nm, suggests
that this approach is well below some limit. Additionally,
Supporting Information Available: Details of irradiation
techniques, UV-vis experiments, and NMR experiments.
X-ray crystal structure of 1 (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
OL101672V
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4093