"This	is	the	peer	reviewed	version	of	the	following	article:	Angew. Chem. 2017, 129 (17),
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Photochromism

and

Dual-Color

Fluorescence

in

a

Polyoxometalate-benzospiropyran Molecular Switch

[a]

[b]

[c]

[d]

Dr. A. Parrot, A. Bernard, Dr. A. Jacquart, Dr. E. Derat, Prof. A.
Proust, Dr. G. Izzet
Institut Parisien de Chimie Moléculaire UMR CNRS 8232
Sorbonne Universités, UPMC-Paris06,
4 Place Jussieu, F-75005 Paris, France
E-mail: guillaume.izzet@upmc.fr
Dr. S. A. Serapian, Prof. C. Bo
Catalan Institute of Chemical Research (ICIQ)
The Barcelona Institute of Science and Technology
Avinguda dels Països Catalans 16, 43007 Tarragona, Spain
Dr. O. Oms, Dr. A. Dolbecq, Prof. P. Mialane,
Institut Lavoisier de Versailles, UMR CNRS 8180
Université de Versailles Saint-Quentin en Yvelines,
Université Paris Saclay,
45 Avenue des Etats-Unis, F-78035 Versailles cedex, France
E-mail: pierre.mialane@uvsq.fr
Dr. R. Métivier
PPSM, UMR CNRS 8531
ENS Cachan
61 av. Président Wilson, 94235 Cachan cedex France

Arnaud Parrot,[a] Aurélie Bernard,[a] Aurélie
Jacquart,[a] Stefano Artin Serapian,[b] Carles
Bo,[b] Etienne Derat,[a] Olivier Oms,[c] Anne
Dolbecq,[c] Anna Proust,[a] Rémi Métivier,[d]
Pierre Mialane,*[c] and Guillaume Izzet*[c]

Abstract: The photophysical properties of a Keggin-type polyoxometalate (POM) covalently bounded to a benzospiropyran (BSPR) unit have
been investigated. These studies reveal that (i) both closed and open forms are emissive with distinct spectral features (λem, (closed form) =
530 nm, λem, (open form) = 670 nm); (ii) the fluorescence of the BSPR unit of the POM-based hybrid is considerably enhanced compared to
BSPR reference compounds. While the fluorescence excitation energy of the BSPR reference compounds (370 nm) is close to the intense
absorption responsible of the photochromic character (350 nm), the fluorescence excitation of the hybrid is shifted to lower energy (400 nm),
facilitating the population of the emissive state. Combined NOESY NMR and theoretical calculations of the closed form of hybrid give an
intimate understanding of the structural conformation adopted by the hybrid and show that the nitro-aryl moieties of the BSPR is folded toward
the POM, which should affect the electronic properties of the BSPR.

Photo-responsive molecules and materials are currently receiving considerable attention regarding their potentiality for the
development of advanced photonic devices in various applications such as information storage, optical switches, protection, smart
labelling and super-resolution microscopy.[1] Particularly, the elaboration of molecular systems combining photo-switching and
fluorescence properties constitute a promising research field because of the high sensitivity, resolution, contrast and fast response
times of the fluorescence. Furthermore, the emergence of single-molecule fluorescence spectroscopy opens the route to high
memory density nanodevices, in which a single molecule would work as one bit of memory,[2] and thus strongly supports the
molecular approach. Until now, the development of molecular systems displaying photo-modulated emissive properties has mostly
relied on the association of luminescent and photochromic components in a molecular assembly.[3] In these systems, the
transformation of the photochromic component is exploited to modulate the emission intensity of the luminescent component on the
basis of electron and/or energy transfer.[4] However, molecular systems displaying tuneable fluorescence features, i.e. having
different emission energies, according to the form of the photochromic unit, have been scarcely reported in the literature.[5] The
coexistence of photochromic and fluorescence properties, in a single photoactive unit, is rarely achieved since in case of fast internal
conversion only one photophysical property (photochromic vs fluorescence) is observed. Furthermore the ns timescale of
fluorescence[6] is considerably slower than the fs to ps timescale of the ring opening/closure dynamics occurring in the photochromic
events.[7]
POMs are nanosized molecular oxo cluster that are currently receiving considerable attention because of their wide range of
properties and their potential applications in various fields such as analytical chemistry, medicine and catalysis, but also optics.[8],[9]
Noticeably, efficient modulation of the emission intensity of a Eu3+ center embedded in a purely inorganic POM compound has been

reached by photo-reduction of the polyoxotungstate framework.[10] On another hand, spiropyrans constitute an important class of
photochromic organic compounds.[11] Recently, a quenching of the luminescence of the BODIPY fragment in a photochromic
spiropyran-polyoxomolybdate-BODIPY complex has been evidenced.[12] But systems – incorporating POM or not – with different
emission energies remain largely to be developed.
closed form

open form

NO 2
NO

N
O

NO 2

I

Sn

4 TBA

Sn

4 TBA

Sn

4 TBA

N
O

N
O

hν
ΔT

TBA.KSn[BSPR]

NO 2
TBA.KSn[MC]

TBA.KSn[I]

BSPR

NO 2
BSPR-tolyl

Figure 1. Photoisomerization process occurring in the organo-tin POM hybrid KSn[BSPR]. Framed: Molecular representation of the reference compounds KSn[I],
BSPR and BSPR-tolyl described in this study

We previously described the synthesis and photochromic properties (both in solution and in the solid state) of a new family of
polyoxometalate (POM)-based organic/inorganic hybrids, in which polyoxotungstates are covalently linked to a spiropyran or a
benzospiropyran unit.[13] The KSn[BSPR][14] compound (Figure 1) is illustrative of the materials reported in the study mentioned above,
and can be described as a tin-monosubstituted Keggin [PW11O39Sn]4- anion covalently connected to a benzospiropyran entity. We
herein report that this complex displays significant fluorescence properties under near visible irradiation while the starting
benzospiropyran is scarcely luminescent. More importantly, in this hybrid, it is demonstrated that the organic photochromic unit is
emissive in both isomeric forms, with distinct emissive features, while retaining its photochromic properties. The enhanced
fluorescence of the closed form arising from the covalent combination of organic and inorganic components is tentatively rationalized
thanks to a computational study.

Figure 2. (a) UV-vis absorption spectra recorded for KSn[BSPR] (black), the starting POM platform KSn[I] (green) and the benzospiropyran references BSPR
-5
(blue) and BSPR-tolyl (purple) in 3.10 M acetonitrile solution. Insert: enhancement of the absorption spectra in the 380 – 650 nm range. (b) Emission (red, λexc =
400 nm) and excitation spectra (blue, λem = 530 nm) of KSn[BSPR] in acetonitrile solution.

The synthesis and electrochemical properties of KSn[BSPR] have been previously reported.[13] For the present study we also
designed a new benzospiropyran reference compound named BSPR-tolyl in order to have the most accurate benzospiropyran
reference to the POM-based hybrid KSn[BSPR]. The synthesis of BSPR-tolyl involves a Sonogashira cross-coupling reaction
between the alkyne-terminated BSPR compound with 4-iodotoluene.
Between 250 and 350 nm, the absorption spectrum of KSn[BSPR] displays intense absorption bands attributed to both oxygen-tometal charge transfer bands localized on the POM (ca. 270 nm) and π → π* transitions localized on the organic unit (Figure 2a).
Above 350 nm, the absorption spectrum of KSn[BSPR] is mostly dominated by the benzospiropyran unit, since the POM itself does
not contribute to the spectral profile in the visible. Finally, around 400 nm, KSn[BSPR] has a weak absorption that is even higher than
those of the reference compounds BSPR and BSPR-tolyl (Figure 2a (insert)). In acetonitrile (MeCN) solution, KSn[BSPR] exhibits a
prominent emission band at λem = 530 nm (φ = 4.0 %, Figure 2b) when excited in the UV-visible spectrum down to at least 350 nm.
The excitation spectrum of KSn[BSPR] at λem = 530 nm in MeCN exhibits a broad absorption between 350 nm and 450 nm with a
maximum at 400 nm (Figure 2b) matching with the previously mentioned slight absorption at ca. 400 nm. As regards the reference
compounds, BSPR and BSPR-tolyl display a weaker emission with a slight hypsochromic shift (φ(BSPR) = 0.6 %, φ (BSPR-tolyl) =
1.5 %). Furthermore, their excitation spectra exhibit a maximum at higher energy (i.e. 370 nm, Figure S4). Finally the fluorescence
lifetime of the POM-based hybrid KSn[BSPR] and the reference compound BSPR-tolyl are similar with τ ≈ 8-9 ns, suggesting that

the presence of the tolyl group or the POM do not affect the radiative/nonradiative rate constants of the emitting state. These findings
indicate that the nature of the emissive state of all compounds is identical and thus centred on the BSPR unit. All photophysical data
are reported in Table 1.
Table 1. Photophysical data of the compounds in MeCN solutions
[a]

Head 1

λem,max, nm

λexc,max, nm

τ, ns

φ

TBA.KSn[BSPR]

528

400

8.9

4±1%

BSPR-tolyl

512

370

8.4

1.5±0.4%

BSPR

517

370

n.c.

0.6±0.2%

[a] Quantum yields were recorded with an excitation wavelength corresponding to the maximum of the excitation spectrum.

The photochromic properties of KSn[BSPR] have been studied in solution. In its closed form, the colourless benzospiropyran
molecule contains a Cspiro-O bond, which can be cleaved under near-UV irradiation, leading to a zwitterionic merocyanin (MC) isomer
(Figure 1). As the MC form is unstable in acetonitrile at room temperature, the photogeneration of the open form was performed at 30°C where both isomeric forms of the benzospiropyran units are kinetically stable. Under UV irradiation (335 nm, 12 mW.cm-2), the
colourless solution of KSn[BSPR] in MeCN quickly shifts to purple with two absorption bands in the visible region, at 400 nm and 585
nm, attributed to the appearance of the MC form, and reaches maximum coloration after 40 min of irradiation (Figure 3a). By
comparison, these bands arise at similar wavelengths for an irradiated BSPR solution.[13] When the irradiation is performed at higher
wavelength (365 nm), similar absorption appears, albeit less intense (Figure S5), indicating that in the photostationary state, the open
form vs closed form ratio is lower using lower energy wavelength. The purple sample is stable at temperature below -10°C but quickly
decolours when warming to room temperature. The intensity of the fluorescence at 530 nm decreases upon photo-irradiation while a
new emission band at 670 nm, attributed to the emission of the MC form as reported in the literature,[3a, 15] concomitantly appears
(Figure 3b).

-5

Figure 3. UV-vis absorption (a) and emission (b) spectral changes of KSn[BSPR] in acetonitrile solution (4.10 M, T = -30°C) during photoirradiation (335 nm).
Black curves correspond to the spectra of KSn[BSPR] before irradiation; purple curves correspond to the spectra of the sample in the photolysis steady-state.

When warming to room temperature, the fluorescence of the closed form at 530 nm is almost retrieved with a ca. 8% decrease
indicating that a slight decomposition may occur during the photolysis (Figure S6).
In order to understand the role of the POM in the enhanced fluorescence of KSn[BSPR] compared to the reference BSPR
compounds, we performed a series of theoretical calculations using density functional theory (DFT). As a wide range of conformers
resulting from the rotation of the benzospiropyran unit around the methylene linker are potentially accessible, we undertook a 2D
1
H/1H NOESY NMR study of both BSPR-tolyl and KSn[BSPR] in CD3CN to establish their spatial organization. The 1H NMR spectra
(600 MHz, 300 K) of KSn[BSPR] and BSPR-tolyl display a unique set of signals with sharp peaks. The NOESY NMR spectra of
BSPR-tolyl (Figure S2) and KSn[BSPR] (Figure S3) displays same NOE correlations involving H3/H9, H3’/H9, H3/H10, H3’/H10, H3/H11
and H2/H14 proton pairs (Figure 4). This indicates that the benzospiropyran unit of BSPR-tolyl and KSn[BSPR] adopts a similar
conformation in which the nitroaryl moiety is folded and engages C-H/π interactions with the tolyl/aryltin unit. Furthermore, it also
implies that, in KSn[BSPR], the POM has little effect on the spatial organization of the benzospiropyran unit and that the
enhancement of the fluorescence emission in the hybrid most likely does not arise from a conformational change of the
benzospiropyran unit. Quantitative analysis of 2D NOE cross-peaks of the above protons allows to derive the distances separating
them (Table S1). We observe that the nitroaryl moieties is slightly more folded in KSn[BSPR] than in BSPR-tolyl. While BSPR display
two chirality centres (Cspiro and N), BSPR-tolyl and KSn[BSPR] exists as two potential sets of diastereoisomers. Nonetheless, circular
dichroism experiments of BSPR-tolyl and KSn[BSPR] showed no signals, pointing to equal stereoisomer population in solution.

DFT energy-optimized structures of BSPR-tolyl and KSn[BSPR] were then obtained starting from input structures in which the
BSPR unit display a folded geometry towards the tolyl/aryltin unit. Whereas we only considered one type of chirality at the Cspiro
centre, we considered both possible inversions of the N centre, thus generating the diastereoisomer pairs KSn[BSPR].1 /
KSn[BSPR].2, and BSPR-tolyl.1 / BSPR-tolyl.2 (Table S1 and on-line).***iochemBD reference***
At the level of theory used in our calculations, in both pairs of diastereoisomers the .2 partner is always found to exhibit greater
energetic stability: KSn[BSPR].2 is more stable than KSn[BSPR].1 by 2.78 kcal mol-1; whereas BSPR-tolyl.2 is more stable than
BSPR-tolyl.1 by 2.28 kcal mol-1.
We observe that for all structures the H-H distances are in good agreement with the experimental distances obtained from the 2D
H/1H NOESY NMR spectra (except H3/H10 for which a ca. 0.6 Å discrepancy is calculated). Consequently no proton-proton distance
constraints were further input into these structures.
1

7

6

8

5
4

9

N
H 3'
N

10

H3

O

11

O

12

14
2

13

NO 2

NO 2

1

Sn

4-

Figure 4. Representation of energy-optimized structures of BSPR and KSn[BSPR].

Subsequently, time-dependent DFT (TD-DFT) calculations were performed on energy-optimized structures of KSn[BSPR].1 /
KSn[BSPR].2 and the benzospiropyran references BSPR-tolyl.1 / BSPR-tolyl.2 and BSPR.1 / BSPR.2. The calculated UV-visible
absorption spectra of all compounds are dominated by intense bands around 350 nm, in agreement with the experimental spectra.
However despite a minor transition at ca. 450 nm corresponding to a BSPR (HOMO) ! BSPR (LUMO) transition, we did not observe
additional bands in the 370-400 nm region that could be attributed to a transition populating the BSPR emitting state. Owing to the
molecular complexity of the hybrid, it appears thus complicated to accurately model the effect of the POM on the electronic properties
of the BSPR in KSn[BSPR]. Yet both the fluorescence maximum energy and the fluorescence excitation maximum energy of
KSn[BSPR] are slightly red-shifted compared to BSPR reference compounds. This suggests that the electronic properties of the
BSPR moiety are affected by the POM, which is located in its vicinity because of the folding of the nitroaryl ring. This minor energy
shift has significant consequences, since the absorption responsible for the fluorescence is less hindered by the intense absorption at
ca. 350 nm that is responsible of the photochromic behaviour of the BSPR. The fluorescence lifetime of all BSPR compounds being
similar, the difference in the emission intensity between the hybrid and the reference compounds mostly arises from a greater
population of the emitting state in KSn[BSPR]. In both photochromic forms of the BSPR, the excitation energy leading to the emitting
state is lower than that of the excitation leading to photochromism reactions. This favours the coexistence of photochromic and
fluorescence properties since the dynamics of the photochromic events are known to be much faster that the fluorescence decay. In
this respect, further computational studies should be undertaken on KSn[BSPR] and the two reference systems, with particular focus
on the energetics of the open MC forms, and those of the first singlet excited state previously reported by Celani et al.[16]
In conclusion we have described an unprecedented example of a molecular photoswitch based of a covalent POM-BSPR
conjugate that displays specific fluorescence features in each of its two-photochromic forms. Interestingly, the fluorescence of the
closed form of the hybrid is significantly higher than that of reference BSPR compounds. This is attributed to a more easily accessible
emitting state in the hybrid, likely due to a folding of the BSPR towards the POM. This system opens new insights for the
development of molecular optical switches.

Experimental Section

Experimental Details. Acetonitrile was obtained from commercial sources and used as received. KSn[I], KSn[BSPR] and BSPR were synthesized
according to the literature procedure.[13, 17] Photolysis reactions were performed with a Hg/Xe lamp (Hamamatsu, LC6 Lightningcure, 150 W) equipped
with filters of appropriate wavelengths. UV-visible spectra were recorded on a Agilent Cary5000 spectrophotometer connected to a Hellma 661.202-UV
all-quartz immersion UV-visible probe (10 mm optical path length) via a Hellma 662.000-UV/NIR Fibre-Optic Cable Interface and 2m long 041.102-UV
quartz optic fibres. Fluorescence spectra of solutions were obtained on a Horiba Jobin Yvon FluoroLog-3 spectrofluorometer. Low temperature
fluorescence measurements were made with an Oxford Instruments Optistat DN cryostat. All NMR experiments were performed on a Bruker Avance III
600 MHz equipped with a 5 mm z-gradient broad-band fluorine observation (BBFO) probe in CD3CN.The intensity of the crosspeaks in the 1H/1H
NOESY experiments were calibrated using the crosspeak for H11/H12 correlation as an internal standard, as this internuclear distance is known. The
distances were then calculated using a 1/r6 dependence.
Theoretical calculations. All calculations discussed in this work were carried out using the Gaussian09 package.[18] Closed-shell DFT optimizations of
all structures (constrained and unconstrained) were carried out using the PBE0 density functional;[19] in conjunction with Grimme’s D3 dispersion
corrections[20] and Becke-Johnson damping[21] (-D3(BJ)) to more accurately model dispersion effects (particularly in aromatic and alkynylic components
of the BSPR moieties). At this stage, electrons on oxygen, carbon, hydrogen, phosphorus, and nitrogen atoms were modeled using the D95 basis
set[22] with an extra d polarization function (as implemented by Gaussian09); on the other hand, tin and tungsten electrons were modeled, respectively,
with the MWB46[23] and MWB60[24] effective core potentials (ECPs) and basis sets. Single-point TD-DFT calculations on resulting structures were
carried out using the methodology previously described by Deblonde et al. for deriving spectra of POMs:[25] the PBE0-D3(BJ) functional was retained,
but this time all electrons were modelled using the def2TZVP ECP and basis set.[26] In all cases, solvent effects (acetonitrile) were implicitly taken into
account using the polarizable continuum model.[27] All calculations are available on-line.***iochemBD reference***

Keywords: molecular photoswitch • polyoxometalates • photochromism • emerging property • density functional calculations.

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COMMUNICATION
Small cause, big effect. A covalent
polyoxometalate(POM)–
benzospiropyran(BSPR) dyad
represents an unprecedented
example of a molecular switch that
displays specific fluorescence feature
in each photochromic form. The
fluorescence of the BSPR unit of the
POM-based hybrid is considerably
enhanced compared to BSPR
reference compounds attributed to a
more easily accessible emitting state
in the hybrid.

Arnaud Parrot, Aurélie Bernard, Aurélie
Jacquart, Stefano Artin Serapian, Carles
Bo, Etienne Derat, Olivier Oms, Anne
Dolbecq, Anna Proust, Rémi Métivier,
Pierre Mialane,* and Guillaume Izzet*
Page No. – Page No.
Photochromic and Dual-Color
Fluorescence in a Polyoxometalatebenzospiropyran Molecular Switch