Reversible light-driven redox switching of multifunctional dipolar ruthenium(III/II) pentaammine(4,4′-bipyridinium) complexes

Article abstract of DOI:10.1021/ja034712b

We report the first example of a molecular switch of multifunctional dipolar ruthenium(III/II) pentaammine-N-methyl-(4,4-bipyridinium) complexes, exclusively driven by light. This is achieved by using a two-phase (water/benzene) system in which RuIII/II complexes are soluble only in the water phase. The reversible redox switching is triggered by the selective irradiation of the water and the benzene compartments with 254 and 528 nm light, respectively. Copyright

Full text of DOI:10.1021/ja034712b

Published on Web 04/17/2003
Reversible Light-Driven Redox Switching of Multifunctional Dipolar
Ruthenium(III/II) Pentaammine(4,4′-bipyridinium) Complexes
Salvatore Sortino,* Salvatore Petralia, and Santo Di Bella*
Dipartimento di Scienze Chimiche, UniVersit a` di Catania, Viale Andrea Doria 8, I-95125 Catania, Italy
Received February 17, 2003; E-mail: ssortino@mbox.unict.it; sdibella@mbox.unict.it
The achievement of multifunctional molecular switches is of
paramount interest in the growing field of molecular electronics.1
Dipolar ruthenium(II) ammine complexes of 4,4′-bipyridinium (bpy)
ligands possess unique characteristics as multifunctional redox
switches.3 Actually, the reversible chemical oxidation of these
complexes is accompanied by a dramatic change of their linear
,2
II
and nonlinear optical properties. While Ru complexes possess
distinct linear (absorption maxima in the 580-640 nm region) and
nonlinear optical features associated with the their intense, low-
energy metal-to-ligand charge-transfer (MLCT) electronic transi-
III
tions, Ru analogues are transparent in the whole visible region
Figure 1. Absorption spectra of 1 in the water phase (s) before and (- - -)
4
and possess negligible second-order optical nonlinearity. In addi-
after 5 min of 254 nm irradiation. The inset shows the redox-switching of
tion, we have recently demonstrated that self-assembled monolayers
the absorbance maximum (monitored at 580 nm) upon consecutive cycles
of 254 and 528 nm irradiation.
III/II
of these Ru complexes on an optically transparent metal electrode
5
can be reversibly switched chemically.
Scheme 1
Light is a very appealing trigger to switch molecular properties.
Its easy manipulation makes light-controlled molecular switches
2
ideally suited to molecular electronics. Indeed, a large variety of
1
examples is reported in the literature. The goal of the present study
is the light-controlled, reversible redox switching of multifunctional
III/II
III/II
dipolar [Ru (NH
3
)
5
-N-methyl-4,4′-bipyridinium](PF
6
)
n
(1
)
6
complexes. This represents a difficult task, because photoinduced
processes involving Ru
precluded due to their photolability. In fact, Ru
III/II
ammine complexes are, in principle,
III/II
ammine
Scheme 2
complexes of pyridine and related ligands give photosubstitution
7
in both aqueous solution and nonaqueous solvents. Analogously,
II
the direct or sensitized photoexcitation of 1 leads to photodegra-
dation.8 To circumvent this problem, we have thus explored
alternative strategies.
We report here on the reversible redox photoswitching of 1III/II
complexes, which represents the first example of a multifunctional
molecular switch involving dipolar metal complexes, having both
linear and nonlinear optical properties, exclusively controlled by
9
light. The approach is sketched in Scheme 1. It implies a two-
phase (water/benzene) system in which 1II/III salts are soluble only
in the water phase. Noteworthy features include (i) photooxidation
II
of 1 by a phenoxy radical generated upon 254 nm irradiation of
III
phenol (PhOH) in the water phase; (ii) photoreduction of 1 at
water/benzene interface by irradiation at 528 nm of zinc tetraphenyl
porphyrin (ZnTPP) in the benzene phase. The above features, within
absorption band is observed after 5 min of steady-state irradiation,
II
III
proper experimental conditions (vide infra), avoid either the direct
according to the oxidation of 1 to the 1 counterpart. The
photooxidation route can be explained through the mechanism
illustrated in Scheme 2. It is well established that UV irradiation
of aqueous solutions of PhOH leads to the formation of hydrated
III/II
or sensitized photoexcitation of 1
complexes and, hence, any
photodegradation. The processes involved in our proposed scheme
are described as follows.
Photooxidation Route. This step was photoinitiated by PhOH
-
•
10
•
electrons (e ) and phenoxy radicals (PhO ). PhO offers ap-
aq
(
5 × 10 M), which is in large excess with respect to 1II (10
-
3
-5
propriate thermodynamic and kinetic prerequisites as oxidizing
II
•
-
M). Under these experimental conditions the 254 nm irradiation of
the water compartment leads to the selective excitation of PhOH,
which absorbs almost all incident photons (g95%), thus preventing
species of 1 . Actually, its reduction potential (E°PhO /PhO ) +
0.72 V vs SCE),11 higher than that of 1 (E° 1III/II ) 0.46 vs SCE),
and its decay on a sufficiently long time scale (hundreds of µs)
make feasible the quenching of PhO by 1 through an electron-
II
4
1
2
II
•
II
any significant direct excitation of 1 .
As displayed in Figure 1, the disappearance of the MLCT optical
transfer pathway. Moreover, the presence of an efficient scavenger
5610
9
J. AM. CHEM. SOC. 2003, 125, 5610-5611
10.1021/ja034712b CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
ZnTPP•+ and TEOA not only prevents the potential back electron-
transfer processes but also restores the starting ZnTPP, thus
minimizing undesired side reactions.
The suitability of the proposed scheme to achieve the reversible
II/III
molecular switching of 1
complexes was tested by consecutive
phototriggered redox cycles. As displayed in the inset of Figure 1,
the disappearance/restoration of the MLCT absorption band provides
direct evidence that a reversible redox switching occurs.
In summary, we have shown the first example of a molecular
III/II
3 5
switch of multifunctional dipolar (NH ) Ru bpy complexes,
exclusively driven by light. The adopted strategy allowed the
photocontrolled redox switching of such photolabile systems,
otherwise not feasible by their direct or sensitized photoexcitation.
Such a strategy might represent a general method to accomplish
the redox switching of photolabile species. Moreover, the two-phase
approach is also of relevance in the perspective of photocontrolled,
•
Figure 2. Kinetic traces for the decay of PhO observed upon 266 nm
-3
laser excitation of PhOH (1 × 10 M in aqueous solution) (a) in the absence
II
and (b) in the presence of 1 (3 µM). The faster components of the decay
3
traces are due to *PhOH, whose absorption is superimposed to that of
•
15
•
II
PhO . The inset shows the plot for the quenching of PhO by 1 .
III/II
21
Ru -based self-assembled monolayer molecular switches.
of hydrated electrons, such as 2-chloroethanol (2ClEtOH),13
Acknowledgment. This research was supported by the MIUR.
prevents the back reduction of 1III during the steady-state illumina-
II
•
tion. Noteworthy, the electron transfer between 1 and PhO not
Supporting Information Available: Details of experimental
procedures and laser flash photolysis experiments (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
II
only switches off the MLCT optical absorption of 1 but also offers
the advantage of restoring the starting PhOH (see Scheme 2).14
The proposed mechanism is substantiated by complementary
•
References
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•
12
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•
As shown in Figure 2, PhO is efficiently quenched by addition of
II
1
, with a diffusion-controlled bimolecular rate constant (k
q
) 8.5
9
-1 -1
×
10 M
s ) (see inset Figure 2).
(
(
(
(
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II
15
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3
16
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II
II
-5
3
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-
5
5
28 nm irradiation of ZnTPP (10 M), which is soluble only in
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(
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9
b
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II
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(
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route makes use of the well-known properties of porphyrins to
undergo interfacial electron-transfer reactions with coreactants
(
(
(
17
dissolved in a different phase. In our case, ZnTPP presents suitable
18
prerequisites for a cross-phase photoinduced electron transfer to
•
(14) Under our working conditions, the concentration of PhO generated is
III
1
. In fact, the high quantum yield (ca 0.9) of the ZnTPP excited
II
always lower than that of 1 (see SI). This criterion ensures that the
quenching process predominates over PhO dimerization.
3
•
triplet state ( *ZnTPP) photogeneration, its reduction potential
E°ZnTPP•+/3* ZnTPP ) -0.55 vs SCE), much lower than that of 1
E°1III/II ) 0.46 vs SCE), and its sufficiently long lifetime (ca 1
III
(15) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1.
(
(
(
15
16) This value is shorter than that reported in the literature, likely due to
4
self-quenching processes occurring at the high PhOH concentration used.
3
III
(17) See, for example: Eugster, N.; Fermin, D. J.; Girault, H. H. J. Phys. Chem.
B 2002, 106, 3428 and references therein.
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ms) make feasible the quenching of *ZnTPP by 1 . According
to this view, the complete restoration of the MLCT absorption of
II
1
is observed after ca. 60 min irradiation of the benzene phase.
(
19) (a) The values of the quenching constants generally reported for interfacial
3
Although we detected the typical decay signal of *ZnTPP at 470
nm by laser flash photolysis measurements, it was not possible to
get any reliable evidence for the above quenching process under
our experimental conditions.19 However, the mechanism illustrated
in Scheme 2 is supported by the effect of triethanolamine (TEOA)
photoinduced electron-transfer involving zinc porphyrin triplets and
6
7
-1 -1 19b
suitable electron acceptors, are ca. 10 -10 M
s
.
According to these
3
III
data, the *ZnTPP lifetime in the presence of 1 is expected to be
shortened not more than 10%. Such variations are within the experimental
error of our laser flash photolysis measurements. (b) Dung Duong, H.;
Brevet, P. F.; Girault, H. H. J. Photochem. Photobiol., A 1998, 117, 27.
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H. J. Org. Chem. 1988, 53, 1703.
(
(
(
1 mM) added as a sacrificial electron donor. In fact, no significant
21) (a) Actually, the interface between a self-assembled monolayer and a liquid
degradation of ZnTPP is observed in the presence of TEOA during
the photoreduction route. As is well documented,20 reactions
between TEOA and oxidized zinc porphyrins take place very rapidly
due to the favorable energetics. In our case, the reaction between
2
1b
can be comparable to our reactant species at liquid/liquid interface. (b)
See, for example: Squitieri, E.; Benjamin, I. J. Phys. Chem. B 2001, 105,
6412.
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J. AM. CHEM. SOC.
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