Sulfonium polyoxometalates: A new class of solid-state photochromic hybrid organic-inorganic materials

Article abstract of DOI:10.1021/ic302477p

For the very first time, sulfonium polyoxometalate (POM) assemblies are shown to develop efficient solid-state photochromism in ambient conditions. The optical properties of the already known Rb_0.75(NH₄) _5.25[(Mo₃O₈)₂O(O ₃PC(CH₂S(CH₃)₂)OPO₃) ₂]·8H₂O (1) and a new material (Me ₃S)₄[Mo₈O₂₆] (2) under UV excitation are investigated by diffuse reflectance spectroscopy, revealing that the color change effect is highly tunable playing with the nature of the POM. A mechanism involving the photoreduction of Mo⁶⁺ cations associated with electron transfers from the sulfonium cations toward the POMs is proposed.

Full text of DOI:10.1021/ic302477p

Communication
pubs.acs.org/IC
Sulfonium Polyoxometalates: A New Class of Solid-State
Photochromic Hybrid Organic−Inorganic Materials
Khadija Hakouk,† Olivier Oms,‡ Anne Dolbecq,‡ Hani El Moll,‡ Jerome Marrot,‡ Michel Evain,†
́
̂
Florian Molton,§ Carole Duboc,§ Philippe Deniard,† Stephane Jobic,† Pierre Mialane,*,‡
́
and Remi Dessapt*,†
́
†Institut des Mater
́
iaux Jean Rouxel, Universite
́
de Nantes, CNRS, 2 rue de la Houssinier
̀
e, BP 32229, 44322 Nantes cedex, France
‡Institut Lavoisier de Versailles, UMR 8180, Universite
Versailles cedex, France
́
de Versailles Saint-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035
§Universite
́ ́ ́ ́
Joseph Fourier Grenoble 1/CNRS, Departement de Chimie Moleculaire, UMR 5250, Institut de Chimie Moleculaire de
Grenoble, FR-CNRS-2607, BP 53, 38041 Grenoble cedex 9, France
S
* Supporting Information
functionalized bisphosphonate (BP) ligands are directly grafted
on dodeca- (Mo12(BP)4)7 and hexamolybdate (Mo6(BP)2)8
cores.
ABSTRACT: For the very first time, sulfonium poly-
oxometalate (POM) assemblies are shown to develop
efficient solid-state photochromism in ambient conditions.
The optical properties of the already known
Rb0.75(NH4)5.25[(Mo3O8)2O(O3PC(CH2S(CH3)2)-
OPO3)2]·8H2O (1) and a new material (Me3S)4[Mo8O26]
(2) under UV excitation are investigated by diffuse
reflectance spectroscopy, revealing that the color change
effect is highly tunable playing with the nature of the
POM. A mechanism involving the photoreduction of Mo6+
cations associated with electron transfers from the
sulfonium cations toward the POMs is proposed.
A new approach in rupture with those developed above could
consist in developing hybrid POMs for which the photo-
chromism mechanism involves an electron transfer from the
organic cation toward the POM by freeing itself the
dependence of the hydrogen-bonding network. With this aim,
sulfonium cations R1R2R3S+ (Ri are alkyl chains) clearly appear
as promising candidates to generate such assemblies, because
the S atom possesses both the positive charge susceptible to
directly interact with the nucleophilic oxo ligands of the POM,
and a nonbonding electron pair, which could be transferred
toward the POM. Nevertheless, to the best of our knowledge,
only one sulfonium/POM system, Rb0.75(NH4)5.25
-
[(Mo3O8)2O(O3PC(CH2S(CH3)2)OPO3)2]·8H2O9 (1), has
been successfully synthesized, and its potential photochromic
properties have not been explored. We thus present here the
investigation of its optical properties. In addition, we report a
new supramolecular assembly (Me3S)4[Mo8O26] (2) as a good
model material to point out the active role of the sulfonium
cations in the photoactivity of sulfonium POMs. The
photochromic properties of 2 have been studied and compared
with those of (HDEA)2(NH4)2[Mo8O26] (3) (HDEA+ is the
n recent years, considerable efforts have been made to
1
Ielaborate efficient solid-state photochromic materials
(organic, inorganic, or hybrid), because their ability to tune
their optical properties with light is of great interest in a wide
range of technological and marketable applications, such as
smart windows, cosmetics, multicolor smart painting, UV
sensors, optical switches, and 3D high-density optical data
storage. One of the promising systems in this field are hybrid
organic−inorganic materials based on polyoxometalates2
(POMs) and organoammonium cations (OACs).3 Notably,
they offer a wide range of photoinduced coloration, with
remarkable coloration contrast and highly tunable coloration
and fading rates.3b,4 The photochromism mechanism of the
OAC/POM systems has been investigated in detail3a,4 and is
described in terms of a UV-induced photoreduction of the
POM concomitant with an electron transfer assisted by H atom
displacement from the OAC toward the POM (see Figure S1 in
the Supporting Information, SI, for the detailed mechanism).
Unfortunately, as the establishment of direct H-bonds between
POMs and OACs is not systematic,5 certain materials do not
develop any photochromic activity. Hence, new strategies have
been recently elaborated to better control the OAC/POM
interface. For example, the hydrogen-bonding network can be
+
diethylammonium cation (CH3CH2)2NH2 ), which is a new
OAC/POM material and contains the same octamolybdate
unit. Finally, a mechanism is proposed to describe the UV-
induced color change effects in 1 and 2.
1 contains the [(Mo3O8)2(O)(O3PC(CH2S(CH3)2)(O)-
PO3)2]6− polyanion9 (Mo6(Sul)2), which is built upon two
sulfonium bisphosphonate ligands covalently grafted on a
hexanuclear {Mo6} core (Figure 1a). The {Mo6} core is
composed of two trimeric units which are corner-shared
connected via a common O atom. Single crystals of 2 and 3
have been obtained at room temperature (SI). Single X-ray
diffraction (Table S1 in the SI) and FT-IR spectroscopy
(Figure S2 in the SI) analyses reveal that both materials contain
the discrete β-[Mo8O26]4− block (Mo8) (Figure 1b), which is
anticipated in materials based on layered 2/∞[MonO3n+1 2− (n =
]
5, 7, 9) units.6 Alternatively, a new family of intrinsic
photochromic POMs has been elaborated, in which OAC-
Received: November 13, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic302477p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Figure 1. Mixed ball and stick and polyhedral representations of (a)
the Mo6(Sul)2 anions in 1 (yellow tetrahedra = PO3C, blue octahedra
= MoO6), (b) the Mo8 anion surrounded by the Me3S+ cations in 2
(blue octahedra = MoO6). In 1 and 2, the sulfonium groups develop
electrostatic and van der Waals interactions with the POM units (the
S···O interactions are represented as dotted lines).
Figure 2. (a) Evolution of the photogenerated absorption in 1 after 0,
1, 2, 4, 6, 8, 10, 12.5, 20, 30, 40, 50, 90, and 120 min of UV irradiation
(365 nm). Insert: Photographs of powder of 1 at different UV
irradiation time (in min). (b) Evolution of the photogenerated
absorption in 2 after 0, 5, 10, 20, 30, 40, 50, 60, 70, 90, 100, and 120
min of UV irradiation (365 nm). Insert: Photographs of powder of 2 at
different 365 nm UV irradiation time (in min).
built upon eight distorted edge-sharing {MoO6} octahedra.10 In
1 and 2, both the sulfonium cations and the POM units
strongly interact via electrostatic and van der Waals
interactions. The sulfonium group adopts a trigonal pyramidal
geometry with all C−S−C angles far smaller than 109°, due to
the space at the S atom lone electron pair. Notably, the S···O
distances are 3.22 and 3.33 Å in 1 (within inter and
intramolecular interactions respectively), and in the range
2.77−3.30 Å in 2, most of them being shorter than 3.32 Å that
is the sum of the O and S atom van der Waals radii.11 Such
interactions between the positively charged sulfonium S and
electron rich O atoms are commonly observed in protein
structures.12 The supramolecular 3D-network of 3 is built upon
The color changes in 1 and 2 are due to the photoreduction
of the POMs. The formation of reduced metallic centers can be
well evidenced by EPR spectroscopy. The spectra of 1 and 2
recorded after UV irradiation are characteristic of the signatures
of materials incorporating 4d1 96Mo5+ ions (S = 1/2; I = 0),13
with g-values of g1 = 1.924, g2 = 1.916, and g3 = 1.887 for 1 and
g1 = 1.937, g2 = 1.922, and g3 = 1.880 for 2 (Figure S6 in the
SI). The coloration kinetics of 1, 2 and 3 have been quantified
by analyzing their reflectance values versus the irradiation time
(R(t)). Details of the coloration kinetic parameters are given in
Table S2 in the SI. In OAC/POM systems, the decrease of
Mo8 blocks, NH4 and HDEA+ cations which connect the
+
POMs via strong N−H···O interactions (Figure S3 in the SI).
The solid-state photochromic properties of the aforemen-
tioned compounds have been investigated in ambient
conditions by diffuse reflectance spectroscopy. White micro-
crystalline powders of 1, 2, and 3 exhibit optical gaps of 356 nm
(3.48 eV), 333 nm (3.72 eV) and 344 nm (3.60 eV),
respectively (Figure S4 in the SI). Under UV excitation at 365
nm (3.4 eV), the color of 1 gradually shifts to purplish-red with
a high coloration contrast (Figure 2a), in agreement with the
growth of a broad absorption band in the visible (λmax = 508
nm). This latter is quite comparable with those of other
photochromic Mo6(BP)2 once irradiated,8b confirming that the
nature of the POM core dictates the photogenerated hue.3b
Under similar UV exposure, 2 develops a light grayish-brown
color (Figure 2b) (two broad absorption bands at λmax = 454
and 559 nm). The color change of 2 is clearly noticeable even if
it is less pronounced than in 1. By comparison, 3 develops a
R
λmax(t) (i.e., the reflectance at the wavelength of the maximum
absorption in the visible) with the irradiation time is correlated
to the decrease of the concentration of reducible Mo6+ cations,
according to a pseudo second order kinetic law.4a It is worth
noting the similar kinetic law for 1 and 2 (see SI for the detailed
coloration kinetic model). The R508(t) vs t curve relative to 1 is
perfectly fitted as [R508(t) − R508(∞)]−1 = At + B (Figure S7 in
the SI), and the coloration rate constant is directly proportional
to the slope of this linear curve. 1 shows a fast photoresponse
(t1/2 = 4.84 min), and it competes with the faster photochromic
Mo6(BP)2 materials8b (the coloration rate constant ratio k1/ki
is in the range 6.3−0.6) (Table S3 in the SI). 2 shows a
coloration rates slower than 1 (t1/2 = 30 min), however, its
coloration change is much faster than that of 3 (t1/2 = 125 min;
and k2/k3 = 4) (Figure S8 in the SI), proving that the
photoreduction of the Mo8 unit is more efficient in 2 than in 3.
The slow photochromic effects in 2 and 3 can be attributed to
the high difficulty to reduce the Mo8 anion.2a
very poor photoresponse. Its color shifts to pale gray (λmax
=
454 nm) (Figure S5 in the SI) with a low coloration contrast
(the color change is weakly detectable with naked eyes only
after 90 min).
1 and 2 show reversible color change effects. The bleaching
of both materials once irradiated occurs in air and at room
B
dx.doi.org/10.1021/ic302477p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
temperature (Figure S9 in the SI), and the fading process is
slower than the coloration one (typically, for UV irradiation
duration of 2 h which corresponds to a photoreduction degree
Y(t) of about 96 and 80% in 1 and 2, respectively (Figure S10
in the SI), the color of the samples totally disappears after 3 h).
Similarly as observed for OAC/POM systems,3a,4b the
bleaching of 1 and 2 is correlated to the back oxidation of
the Mo5+ cations by O2 in ambient conditions (no color change
occurs when 1 and 2 are kept in O2 free atmosphere). The
fading process is strongly accelerated by keeping the samples
under moderate heating at 50 °C (the purple coloration of 1
disappears after a few minutes). To date, for 1 and 2, about 15
coloration/fading cycles at room temperature as well as at 50
°C can be performed without detecting any fatigue resistance
with naked eyes.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data of 2 and 3 in CIF format,
experimental procedures, single-crystal X-ray diffraction data
of 2 and 3, FT-IR, EPR, and UV−vis spectra, and
complementary detailed coloration kinetic models. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: (+33)240-373-995 (R.D.); (+33)139-254-381 (P.M.). E-
mail: remi.dessapt@cnrs-imn.fr (R.D.); mialane@chimie.uvsq.fr
(P.M.).
■
Notes
Considering all these results, the photochromic effect in 1
and 2 could arise according to the mechanism proposed in
Scheme 1. The UV excitation induces an electron transfer into
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the CNRS, the Minister
■
̀
e de
́
Scheme 1. Proposed Mechanism of Photocoloration in 1 and
2 Involving an Electron Transfer from the Sulfonium Cation
l’Enseignement Super
National de RPE Interdisciplinaire”, Grant FR-CNRS 3443, and
́
ieur et de la Recherche, the “TGE Reseau
a
Towards the Adjacent POM
the ANR-11-BS07-011-01 BIOOPOM.
REFERENCES
■
(1) (a) Irie, M. Chem. Rev. 2000, 100, 1685−1716. (b) Ohkoshi, S.-I.;
Tsunobuchi, Y.; Matsuda, T.; Hashimoto, K.; Namai, A.; Hakoe, K.;
Tokoro, H. Nature Chem 2010, 2, 539−545. (c) Pardo, R.; Zayat, M.;
Levy, D. Chem. Soc. Rev. 2011, 40, 672−687. (d) Wang, M.-S.; Xu, G.;
Zhang, Z.-J.; Guo, G.-C. Chem. Commun. 2010, 46, 361−376.
(2) (a) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-
Verlag: New York, 1983. (b) Long, D.-L.; Burkholder, E.; Cronin, L.
Chem. Soc. Rev. 2007, 36, 105−121. (c) Dolbecq, A.; Dumas, E.;
Mayer, C. R.; Mialane, P. Chem. Rev. 2010, 110, 6009−6048.
a
The mechanism depicted here implies a MoO terminal O atom,
but it can be quite transposable to the bridging O atoms.
(d) Kortz, U.; Muller, A.; Salgeren, J. V.; Schnack, J.; Dalal, N. S.;
̈
Dressel, M. Coord. Chem. Rev. 2009, 253, 2315−2327. (e) Gouzerh, P.;
Proust, A. Chem. Rev. 1998, 98, 77−111.
the POM from an oxo ligand to an adjacent Mo6+ site. Then,
the excited electron should be trapped onto the Mo atom, while
concomitantly, the S atom lone electron pair of a sulfonium
group should be strongly polarized toward the POM to stabilize
the electron-deficient O atom. Consequently, the coloration
should be due to the photoreduction of Mo6+ cations into Mo5+
ones and the photogenerated hole should partially move onto
the S atom. The spatially segregated charge carrier allows
maintaining the coloration after switching off the UV
irradiation.
In conclusion, we have successfully highlighted that
sulfonium polyoxometalates develop efficient solid-state photo-
chromism in ambient conditions. Their UV-induced coloration
can be modulated by playing with the nature of the POM unit.
The photoreduction of the POM has been evidenced and the
sulfonium cations (covalently grafted on the POM or not) act
as electron donors, which stabilize the reduced Mo5+ centers.
Importantly, compared to their OAC counterparts, the
sulfonium/POM systems are more promising photoactive
materials, because the electron transfers at the organic−
inorganic interface do not require direct H-bonding interactions
between the POM and the organic moiety, and the S···O
contacts seem more easily predictable. We are now embarked
on a prospective research of new sulfonium/POM assemblies
to consolidate the proposed coloration mechanism and to
investigate the fading one. Notably, the influence of the nature
of the sulfonium cations (size, alkyl side-chain length) on the
coloration and fading kinetics is under study.
́
(3) (a) Yamase, T. Chem. Rev. 1998, 98, 307−325. (b) Coue, V.;
Dessapt, R.; Bujoli-Doeuff, M.; Evain, M.; Jobic, S. Inorg. Chem. 2007,
46, 2824−2835. (c) He, T.; Yao, J. Prog. Mater. Sci. 2006, 51, 810−
879.
́
(4) (a) Dessapt, R.; Collet, M.; Coue, V.; Bujoli-Doeuff, M.; Jobic, S.;
Lee, C.; Whangbo, M.-H. Inorg. Chem. 2009, 48, 574−580.
(b) Dessapt, R.; Gabard, M.; Bujoli-Doeuff, M.; Deniard, P.; Jobic,
S. Inorg. Chem. 2011, 50, 8790−8796.
(5) Fang, R.-Q.; Zhang, X.-M.; Wu, H.-S.; Ng, S. W. Acta Crystallogr.
2004, E60, m359−m361.
(6) Bujoli-Doeuff, M.; Dessapt, R.; Deniard, P.; Jobic, S. Inorg. Chem.
2012, 51, 142−149.
(7) Compain, J.-D.; Deniard, P.; Dessapt, R.; Dolbecq, A.; Oms, O.;
́
Secheresse, F.; Marrot, J.; Mialane, P. Chem. Commun. 2010, 46,
7733−7735.
(8) (a) Sergienko, V. S. Russ. J. Coord. Chem. 2001, 27, 723−753.
(b) El Moll, H.; Dolbecq, A.; Mbomekalle, I. M.; Marrot, J.; Deniard,
P.; Dessapt, R.; Mialane, P. Inorg. Chem. 2012, 51, 2291−2302.
(9) El Moll, H.; Zhu, W.; Oldfield, E.; Marleny Rodriguez Albelo, L.;
Mialane, P.; Marrot, J.; Vila, N.; Mbomekalle,
C.; Dolbecq, A. Inorg. Chem. 2012, 51, 7921−7931.
(10) Hubbard, D. J.; Johnston, A. R.; Sanchez Casalongue, H.;
Narducci Sarjeant, A. N.; Norquist, A. J. Inorg. Chem. 2008, 47, 8518−
8525.
́ ̀
I. M.; Riviere, E.; Duboc,
(11) Bondi, A. J. Phys. Chem. 1964, 68, 441−451.
(12) Markham, G. D.; Norrby, P.-O.; Bock, C. W. Biochemistry 2002,
41, 7636−7646.
(13) Baffert, C.; Boas, J. F.; Bond, A. M.; Kogerler, P.; Long, D.-L.;
̈
Pilbrow, J. R.; Cronin, L. Chem.Eur. J. 2006, 12, 8472−8483.
C
dx.doi.org/10.1021/ic302477p | Inorg. Chem. XXXX, XXX, XXX−XXX