Photochemical switching of luminescence and singlet oxygen generation by chemical signal communication

Article abstract of DOI:10.1039/b900712a

Photoluminescence in the far red spectral region and photosensitised generation of singlet oxygen, with associated near-IR emission, are reversibly controlled by near-UV or violet light in a communicating ensemble of molecular switches.

Full text of DOI:10.1039/b900712a

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Photochemical switching of luminescence and singlet oxygen generation
by chemical signal communicationw
Serena Silvi,a Edwin C. Constable,b Catherine E. Housecroft,b Jonathon E. Beves,b
Emma L. Dunphy,b Massimiliano Tomasulo,c Francisco M. Raymo*c and Alberto Credi*a
¸
Received (in Cambridge, UK) 13th January 2009, Accepted 10th February 2009
First published as an Advance Article on the web 20th February 2009
DOI: 10.1039/b900712a
Photoluminescence in the far red spectral region and photosensitised
generation of singlet oxygen, with associated near-IR emission, are
reversibly controlled by near-UV or violet light in a communicating
ensemble of molecular switches.
state, which in turn exhibits phosphorescence in the near
infrared (NIR).
The ensemble is composed of a reversible merocyanine-type
photoacid and an osmium polypyridine complex that functions
as a two-state luminescent switch (Scheme 1). On irradiation
with near-UV or violet light (e.g., 400 nm), ME-H1 is converted
into the spiropyran SP and releases a proton into the solution.15
The Os21 species is an [Os(pytpy)2]21 complex (pytpy ¼
40-(pyridin-4-yl)-2,20:60,200-terpyridine).16 Upon the addition of
Photochemical switches are molecular or supramolecular
species in which properties or functions can be switched on
and off by light.1,2 The design and preparation of molecule-based
systems in which optical outputs can be modulated reversibly in
response to optical inputs is a scientifically stimulating objective,3
with potential outcomes in fields such as materials science and
information and communication technology.
As shown by biological structures,4 the type and utility of
the functions that can be obtained with photochemical
switches depend on the degree of complexity and organisation
of the chemical species that receive and process the photons. In
the frame of artificial systems, one approach is that of linking
together, either by weak interactions or covalent bonds, a
discrete number of molecular components.5 Along this line, a
large number of multicomponent (supramolecular) systems
exhibiting photoinduced functions such as light harvesting,6
charge separation,6 catalysis,7 control of acid–base properties,8
drug delivery,9 logic operations,10 and molecular mechanical
motions,5,11 have been investigated.
two equivalents of acid, Os21 is converted into the Os-H2
41
species, protonated on the pendant pyridyl nitrogen atoms,
which possesses different absorption and luminescence
properties.16 Specifically, on protonation both the intensity
and lifetime of the luminescence from the triplet metal-to-ligand
charge-transfer (MLCT) state are strongly quenched, and the
band maximum is red shifted (Os21: lmax ¼ 755 nm, Irel ¼ 100,
t ¼ 115 ns; Os-H241: lmax ¼ 843 nm, Irel ¼ 3, t ¼ 20 ns).w We
envisaged that the protonation state of the osmium complex
could be controlled by light using ME-H1 as a photoacid,
A different modular approach for constructing photo-
chemical molecular devices with predetermined functionalities
relies on the combined operation of structurally unlinked
but functionally connected molecular switches. Recently, we
applied this strategy to develop prototypes of molecule-based
memories,12 simple mechanical machines,13 and logic gates.14
Here we describe the design and operation of a chemical
ensemble in acetonitrile solution which employs a near-UV or
violet light input to control a photoluminescence output in the
far-red spectral region. This system can also be used to switch
on or off the generation of oxygen in the lowest singlet excited
a
`
Dipartimento di Chimica ‘‘G. Ciamician’’, Universita di Bologna,
via Selmi 2, 40126 Bologna, Italy. E-mail: alberto.credi@unibo.it;
Fax: +39 051 2099456
b Department of Chemistry and Swiss Nanoscience Institute,
University of Basel, Spitalstrasse 51, CH 4056 Basel, Switzerland.
E-mail: edwin.constable@unibas.ch
c Center for Supramolecular Science, Department of Chemistry,
University of Miami, 1301 Memorial Drive, Coral Gables,
FL 33146-0431, USA. E-mail: fraymo@miami.edu;
Fax: +1 305 2844571
w Electronic supplementary information (ESI) available: Materials
and methods, absorption spectra of the Os complexes and
irradiation–equilibration cycles. See DOI: 10.1039/b900712a
Scheme 1 Structures and coupled operation of the ME-H1/SP
photoacid system and the Os21/Os-H2 switch in MeCN solution
41
by means of photoinduced proton exchange.
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because SP and ME-H1 exhibit smaller and larger pKa values
than that of the pyridinium ion, respectively.12,13
(Fig. 1, curves c), indicating that thermal equilibration with
regeneration of Os21 and ME-H1 has occurred (Scheme 1).
41
The UV–visible absorption spectrum (Fig. 1, curve a) of a
MeCN solution containing 8.4 mM Os21 and 18 mM ME-H1 is
essentially identical to the sum of the spectra of separated
components. The sharp and intense bands at l o 350 nm are
due to ligand-centered (LC) transitions of the metal complex,
the band with lmax ¼ 401 nm is ascribed to the absorption of
ME-H1, and the bands with lmax ¼ 487 and 670 nm are,
respectively the 1MLCT and 3MLCT absorption bands typical
Interestingly, the switching between Os21 and Os-H2 is
accompanied by a significant change in the emission intensity
(Fig. 1, inset),16 indicating that in this system a luminescence
signal in the far red spectral region can be switched off by a
light input at the opposite extremity of the visible spectrum,
and switched on again by heating. As noted above, the
optical output reading can be performed with excitation at
l 4 500 nm (in our experiments, lexc ¼ 607 nm) and thus it
does not interfere with the switching process. In fact, cycling
experiments performed on the same solution indicate that the
luminescence switching is reversible.w
of Os21 16
. The luminescence spectrum (inset of Fig. 1, curve a)
shows a band with lmax ¼ 755 nm, assigned to emission from
3
the MLCT state of Os21
.
From the absorption spectrum reported in Fig. 1, curve a, it
can be noticed that (i) ME-H1 can be photoexcited with good
selectivity in the spectral region around 400 nm, and (ii)
exclusive excitation of the metal complex is possible at
l 4 500 nm, regardless of its protonation state. Irradiation
of the solution at 400 nm causes profound changes in the
absorption and luminescence spectra of the solution (Fig. 1).
At the photostationary state (Fig. 1, curves b), obtained after
60 min of irradiation in our conditions,w the ME-H1 absorp-
tion band at 401 nm has disappeared completely, an absorp-
tion shoulder at ca. 350 nm has grown up, and the MLCT
absorption bands of the metal complex have become more
intense and bathochromically shifted. The MLCT lumines-
cence intensity and lifetime are quenched by factors of B30
Another interesting feature of this system arises from the
fact that the lifetime of the 3MLCT excited state is
41 16
.
substantially shortened on going from Os21 to Os-H2
Therefore, the chance that such an excited state becomes
involved in bimolecular events is greatly diminished upon
protonation of the metal complex. It follows that the photo-
induced proton transfer between ME-H1 and Os21 could be
used to implement light control on the efficiency of bimolecular
photosensitisation processes that originate from the osmium
species.
To demonstrate the feasibility of this idea, we relied on the
3
fact that the MLCT excited state of oligopyridine complexes
of Ru(II) and Os(II)17 is able to generate singlet oxygen (1D)
with good efficiency by energy transfer to the triplet ground
state (3S) of O2.18,19 Hence, we investigated the formation of
singlet oxygen (monitored by its characteristic phosphores-
cence at 1270 nm) in air equilibrated solutions upon excitation
of the osmium species in the 3MLCT absorption band
(l ¼ 600–780 nm).z The experiment can be described in
terms of the following sequence of reactions (S0 denotes the
electronic ground state of the osmium complex):
and 6, respectively. A careful comparison with the spectra of
41
separated SP and Os-H2
suggests that (i) the ME-H1
species has been photoconverted to SP, and (ii) the protonated
complex Os-H241 has been obtained (Scheme 1). Upon leaving
the solution for five days in the dark at room temperature the
original absorption and luminescence spectra are restored
Os (S0) þ hn (600–780 nm) - Os (3MLCT)
Os (3MLCT) þ O2 (3S) - Os (S0) þ O2 (1D)
O2 (1D) - O2 (3S) þ hn0 (1270 nm)
(1)
(2)
(3)
In fact, we found that in air equilibrated MeCN solution Os21
photosensitises the formation of O2 (1D) with a quantum yield
FD ¼ 0.41 upon excitation at 607 nm. Conversely, no lumines-
cence at around 1270 nm, indicative of the presence of singlet
41
oxygen, was observed in the case of Os-H2
.
We then performed the photosensitisation experiments with
the osmium complex in the presence of the ME-H1 photoacid.
The results are summarised in Fig. 2. Upon 607-nm excitation
of an air equilibrated MeCN solution containing 8.4 mM Os21
and 18 mM ME-H1, we observed the singlet oxygen phosphores-
cence band shown in Fig. 2, curve a. Irradiation of the solution
Fig. 1 Absorption and (inset) luminescence spectra (air equilibrated
MeCN, r.t.) of a solution containing 8.4 mM Os21 and 18 mM ME-H1
(a). The dashed lines show the absorption and (inset) luminescence
changes observed on irradiation of the solution at 400 nm until a
41
at 400 nm for 60 min generates the Os-H2 and SP species;
for this solution we detected no emission peak in the
NIR range, as shown in Fig. 2, curve b. Successive thermal
re-equilibration of the solution regenerates Os21 and ME-H1,
and the singlet oxygen phosphorescence can again be seen
upon excitation of the metal complex (Fig. 2, curve c).
As noted above, the output reading in this system can be
performed non-destructively on excitation with l 4 500 nm
41
photostationary state containing Os-H2 and SP is reached (b). In
particular, spectra a and b in the inset are ascribed to emission from
the Os21 and Os-H2 species, respectively. Thermal re-equilibration
41
(five days in the dark at r.t.) regenerates the original spectra (c). For
the luminescence spectra, excitation was performed at an isosbestic
point at 607 nm.
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reset is thermally driven and thus it does not imply the
addition of chemicals and accumulation of byproducts.
The main limitation of this system in view of real applications
is the very slow thermal reset, which however could be
exploited to implement memory effects.12
This work was supported by the US National Science
Foundation
(CAREER
Award
CHE-0237578
and
CHE-0749840), the Swiss National Science Foundation, and,
in Italy, MIUR (PRIN 2006034123) and Universita di Bologna.
Notes and references
z The excitation spectra show that, as expected, the lowest excited
state (3MLCT) of these Os complexes is obtained with unit efficiency
regardless of the wavelength of the absorbed light.
1 Molecular Switches, ed. B. L. Feringa, Wiley-VCH, Weinheim,
2001.
Fig. 2 Near-infrared phosphorescence spectra (lexc ¼ 607 nm) of an
air equilibrated MeCN solution containing 8.4 mM Os21 and 18 mM
ME-H1 (a), and of the same solution after exhaustive irradiation at
400 nm (b) and successive re-equilibration for five days at r.t. (c). The
inset shows the O2 (1D) phosphorescence intensity at 1270 nm for a
solution containing Os21 (12 mM) and ME-H1 (24 mM) as a function
of the dose of light absorbed by the osmium complex at 450 nm. Note
that in this experiment the same wavelength is employed for both
irradiating the photoacid and exciting the Os species.
2 Photochromism: Molecules and Systems, ed. H. Durr and
H. Bouas-Laurent, Elsevier, Amsterdam, 2003.
¨
3 (a) F. M. Raymo and S. Giordani, Proc. Natl. Acad. Sci. U. S. A.,
2002, 99, 4941; (b) F. M. Raymo and M. Tomasulo, Chem.–Eur. J.,
2006, 12, 3186; (c) D. Gust, T. A. Moore and A. L. Moore, Chem.
´
Commun., 2006, 1169; (d) J. Andreasson, S. D. Straight,
T. A. Moore, A. L. Moore and D. Gust, J. Am. Chem. Soc.,
2008, 130, 11122.
4 See, e.g. A. Amunts, O. Drory and N. Nelson, Nature, 2007,
447, 58.
because light in this spectral range is not absorbed by
the photoacid. However, an interesting behaviour arises
if the solution is irradiated at a wavelength absorbed by both
the ME-H1 species and the osmium complexes. We chose
5 (a) V. Balzani, A. Credi and M. Venturi, Molecular Devices and
Machines—Concepts and Perspectives for the Nanoworld,
Wiley-VCH, Weinheim, 2nd edn, 2008; (b) R. Ballardini,
A. Credi, M. T. Gandolfi, F. Marchioni, S. Silvi and M. Venturi,
Photochem. Photobiol. Sci., 2007, 6, 345; (c) S. Sortino, Photochem.
Photobiol. Sci., 2008, 7, 919.
41
450 nm because at this wavelength Os21 and Os-H2 have
the same absorption coefficient.w In fact, starting from the
ME-H1/Os21 state, the NIR phosphorescence band of singlet
oxygen is initially observed, but it progressively fades out on
increasing the irradiation time (i.e., the dose of light absorbed
by the Os complex) because of the transformation of ME-H1
6 V. Balzani, A. Credi and M. Venturi, ChemSusChem, 2008, 1, 26,
and references therein.
7 D. Sud, T. B. Norsten and N. R. Branda, Angew. Chem., Int. Ed.,
2005, 44, 2019.
8 (a) V. Lemieux, M. D. Spantulescu, K. K. Baldridge and
N. R. Branda, Angew. Chem., Int. Ed., 2008, 47, 5034;
into SP and concomitant conversion of Os21 into the shorter-
(b) M. V. Peters, R. S. Stoll, A. Kuhn and S. Hecht, Angew.
Chem., Int. Ed., 2008, 47, 5968.
¨
41
lived Os-H2
species (Scheme 1). Therefore, the photo-
9 E. B. Caruso, E. Cicciarella and S. Sortino, Chem. Commun., 2007,
5028.
10 A. P. de Silva and S. Uchiyama, Nat. Nanotech., 2007, 2, 399.
11 E. R. Kay, D. A. Leigh and F. Zerbetto, Angew. Chem., Int. Ed.,
2007, 46, 72.
sensitised formation of O2 (1D) takes place at low doses of
light, whereas it becomes inefficient at high doses (Fig. 2,
inset). Such a behaviour could be useful for the design of
self-regulating systems20 for applications, e.g., in photodynamic
therapy.
12 F. M. Raymo, R. J. Alvarado, S. Giordani and M. A. Cejas, J. Am.
Chem. Soc., 2003, 125, 2361.
13 S. Silvi, A. Arduini, A. Pochini, A. Secchi, M. Tomasulo,
F. M. Raymo, M. Baroncini and A. Credi, J. Am. Chem. Soc.,
2007, 129, 13378.
14 S. Silvi, E. C. Constable, C. E. Housecroft, J. E. Beves,
E. L. Dunphy, M. Tomasulo, F. M. Raymo and A. Credi,
Chem.–Eur. J., 2009, 15, 178.
15 F. M. Raymo, S. Giordani, A. J. P. White and D. J. Williams,
J. Org. Chem., 2003, 68, 4158.
16 E. C. Constable, C. E. Housecroft, A. Cargill Thompson,
P. Passaniti, S. Silvi, M. Maestri and A. Credi, Inorg. Chim. Acta,
2007, 360, 1102.
17 P. P. Laine, S. Campagna and F. Loiseau, Coord. Chem. Rev.,
2008, 252, 2552.
18 F. Wilkinson, W. P. Helman and A. B. Ross, J. Phys. Chem. Ref.
Data, 1993, 22, 113.
19 (a) Q. G. Mulazzani, H. Sun, M. Z. Hoffman, W. E. Ford and M.
A. J. Rodgers, J. Phys. Chem., 1994, 98, 1145; (b) C. Tanelian,
C. Wolff and M. Esch, J. Phys. Chem., 1996, 100, 6555;
(c) A. A. Abdel-Shafi, D. R. Worrall and A. Y. Ershov, Dalton
Trans., 2004, 30.
20 S. D. Straight, G. Kodis, Y. Terazono, M. Hambourger,
T. A. Moore, A. L. Moore and D. Gust, Nat. Nanotech., 2008,
3, 280.
In summary, the coupled operation of the acid–base
switchable complex Os21 and the photoacid system ME-H1/SP
has enabled us to devise a chemical ensemble in which a violet
light input controls (a) a photoluminescence output in the far
red spectral region, and (b) the photosensitised generation of
singlet oxygen—and associated NIR phosphorescence—with
self-regulating behaviour.
This molecular switching ensemble exhibits a number of
interesting features, namely: (i) it can process input and output
optical signals in the visible region; (ii) at the same time, a
near-UV light input can control a light output in the near IR,
thus bypassing the whole visible range; (iii) the outputs
correspond to wavelengths in a spectral region (far red/near
infrared) that is interesting, for instance, in communication
technology and diagnostics; (iv) the photoluminescence output
reading can be performed in a non-destructive manner; (v)
owing to its reversibility and stability, the system can be cycled
for several times without appreciable loss of signal, and (vi) the
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1486 | Chem. Commun., 2009, 1484–1486