The bimolecular sensitization of nitric oxide release from weak interacting ruthenium units

Article abstract of DOI:10.1016/j.ica.2011.02.018

This work reports on the bimolecular sensitization of nitric oxide release from cis-[Ru(bpy)₂(iso)NO](PF₆)₃ (1) (iso = isoquinoline and bpy = 2,2′-bipyridine) by irradiating the MLCT transition of the chloro analog cis-[Ru(bpy)₂(iso)Cl]PF₆ (2) _. The compounds displayed peaks in the ESI-MS spectra at m/z 749.1 and m/z 578.1 ascribed, respectively, to ([1(NO⁰)-2PF ₆·CH₃OH]²⁺) and ([2-PF₆] ⁺). In the cyclic voltammograms, the nitrosyl complex presented two redox waves related to the NO ligand at 0.48 and -0.37 V (versus Ag/AgCl, NO^+/0/-1 processes), while the sensitizer showed two reversible waves at 0.79 and -1.46 V (versus Ag/AgCl, Ru^2+/3+ and bpy ^0/-1, respectively). The most important feature of this system is that the nitrosyl compound does not have significant absorption in the visible region, while the sensitizer has an intense band centered at 496 nm. The irradiation of an equimolar mixture of the two compounds in an ethanol:water solution (v:v) with light of λ > 500 nm leads to NO release, as probed by amperometric measurements. The variational method was applied, showing that the two compounds self-assembly in solution with a 1:1 stoichiometry. Fluorescence spectra acquired at 77 K provided the E_0-0 for the system and, from the thermodynamic cycle it was estimated that the photoinduced electron transfer between the species has a ΔG value of -1.59 eV.

Full text of DOI:10.1016/j.ica.2011.02.018

Inorganica Chimica Acta 370 (2011) 444–448
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
The bimolecular sensitization of nitric oxide release from weak interacting
ruthenium units
a
a
b
b
Andreia N. de Carvalho , Elisa C. Fornari , Weverson R. Gomes , Diesley M.S. Araújo ,
b
a,
⇑
Antonio E.H. Machado , Sofia Nikolaou
a
Departamento de Física e Química, Faculdade de Ciências Farmacêuticas de Ribeirão Preto, Universidade de São Paulo, Av. do Café s/n ZIP CODE 14040-903, Ribeirão Preto, Brazil
Instituto de Química, Universidade Federal de Uberlândia, P.O. Box 593, 38400-902, Uberlândia, Minas Gerais, Brazil
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 October 2010
Received in revised form 31 January 2011
Accepted 9 February 2011
Available online 21 February 2011
This work reports on the bimolecular sensitization of nitric oxide release from cis-[Ru(bpy) (iso)-
2
0
NO](PF
chloro analog cis-[Ru(bpy)
49.1 and m/z 578.1 ascribed, respectively, to ([1(NO )ꢀ2PF
6
)
3
(1) (iso = isoquinoline and bpy = 2,2 -bipyridine) by irradiating the MLCT transition of the
2
6 .
(iso)Cl]PF (2) The compounds displayed peaks in the ESI-MS spectra at m/z
0
2+
+
7
6
ꢁCH
3
OH] ) and ([2ꢀPF
6
] ). In the cyclic vol-
tammograms, the nitrosyl complex presented two redox waves related to the NO ligand at 0.48 and
+
/0/ꢀ1
ꢀ
0.37 V (versus Ag/AgCl, NO
processes), while the sensitizer showed two reversible waves at 0.79
Keywords:
Weak interactions
Ruthenium
Nitric oxide release
Visible light
Bimolecular photosensitization
2+/3+
0/ꢀ1
and ꢀ1.46 V (versus Ag/AgCl, Ru
and bpy
, respectively). The most important feature of this sys-
tem is that the nitrosyl compound does not have significant absorption in the visible region, while the
sensitizer has an intense band centered at 496 nm. The irradiation of an equimolar mixture of the two
compounds in an ethanol:water solution (v:v) with light of k > 500 nm leads to NO release, as probed
by amperometric measurements. The variational method was applied, showing that the two compounds
self-assembly in solution with a 1:1 stoichiometry. Fluorescence spectra acquired at 77 K provided the
E
0–0 for the system and, from the thermodynamic cycle it was estimated that the photoinduced electron
transfer between the species has a
D
G value of ꢀ1.59 eV.
Ó 2011 Elsevier B.V. All rights reserved.
1
. Introduction
Fig. 1) as an anchor between ruthenium photoactive units aiming
to promote weak interactions in solution. This ligand was chosen
because of its planar and hydrophobic structure, which might pro-
mote stacking interactions in relatively polar media [17–19].
Therefore, it is presented an example of the use of a rather simple
Since the 90s, when it was discovered that nitric oxide presents
bioregulatory characteristics with both protective and cytotoxic ef-
fects depending on its concentration [1], a huge amount of research
has focused delivering systems able to release NO in a controlled
fashion [2–5]. Of particular interest are the ruthenium complexes
that release NO under controllable stimulation, that is, in reducing
conditions or under light irradiation [6–8]. Regarding the photoin-
duced release, the major drawback of this family of ruthenium
nitrosyl complexes is that they usually have no absorption bands
in the visible region, being necessary to use some artifice to pro-
mote the photoinduced release of NO, especially within the thera-
peutic window [9]. Two of the strategies presented up to now
constitute the development of supramolecular assemblies or the
use of dyes either coordinated to the metal center or to perform
bimolecular sensitization [10–16].
inorganic complex, cis-[Ru(bpy)
bimolecular induced photorelease of NO from its analogous cis-
[Ru(bpy) (iso)NO](PF
2 6
(iso)Cl]PF , as a sensitizer to a
2
6 3
) .
2
. Experimental
2
2
.1. Synthesis
.1.1. cis-[Ru(bpy)
This procedure was adapted from the synthetic approach de-
2
(iso)NO](PF
6
)
3
ꢁHPF
6
scribed in [20] for analogous complexes. All the reagents were
commercially available and were used without further purification.
Less usual is the use of other ruthenium complex as a bimolec-
ular sensitizer. Therefore, in this work we report an approach
based on the use of the azanaphthalene ligand isoquinoline (iso,
ꢀ
4
5
00 mg of cis-[Ru(bpy)
were dissolved in 30 mL of acetone. Then, 42 mg of NaN
in 12 mL of methanol were added to the resulting solution. After
0 min, the solution was poured over an isoquinoline solution
2
(NO)
2
(NO)](PF
6
)
2
[20] (6.4 ꢂ 10 mol)
3
dissolved
1
ꢀ
3
⇑
Corresponding author. Tel.: +55 16 36024712; fax: +55 16 36331092.
E-mail address: sofian@fcfrp.usp.br (S. Nikolaou).
(826 mg, 6.4 ꢂ 10 mol, dissolved in 17 mL of acetone), and the
resulting mixture was allowed to react for 22 h, protected from
0
020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.018

A.N. de Carvalho et al. / Inorganica Chimica Acta 370 (2011) 444–448
445
ble microcomputer. A conventional three electrode cell was used,
consisting of a platinum disk and a platinum wire working as coun-
ter electrodes, respectively and an Ag/AgCl as reference electrode.
A 0.1 M solution of tetrabutylammonium hexafluorophosphate
(
6
TBAPF ) was used as supporting electrolyte and Ferrocene as
internal standard.
The photolysis of the complex, as well as of the equimolar mix-
ꢀ5
ꢀ1
ture of nitrosyl and sensitizer, were carried out from 10 mol L
water/ethanol 1:1 (v:v) deaerated mixtures. The light source was
the output from a 200 mW high pressure mercury lamp in combi-
nation with a 500 nm interference filter. This last was used to en-
able the irradiation wavelength at the desired range. A sample of
known volume was placed in a 1 cm quartz cuvette at 25 °C, being
irradiated under continuous stirring. UV–Vis spectra were recorded
after pre-defined time intervals. NO release from the complex dur-
ing visible irradiation was measured using a specific NO selective
electrode (ISO-NOP NO meter, World Precision Instruments, Sara-
sota, FL, USA). The calibration curve of the electrode was prepared
by using several dilutions of a known volume of a saturated NO
solution in 10.0 mL of deaerated acetate buffer solution (pH 7.4).
The current value, in nA, was recorded for each added volume. NO
concentration was calculated according to the NO molar fraction
Fig. 1. Structural representation of the complexes cis-[Ru(bpy)
2 6 3
(iso)NO](PF ) and
cis-[Ru(bpy) (iso)Cl]PF
2
6
.
light. Acetone was removed from the reaction medium by evapora-
tion under reduced pressure, and 10 mL of a saturated NH PF
aqueous solution were added to the remaining residue. After
hour under refrigeration, an orange precipitate was formed. It
was isolated by filtration, washed with diethyl ether, and dried un-
der vacuum. Then, 690 mg of the crude material (cis-[Ru(bpy)
iso)(NO )]PF ) were dissolved in 25 mL of methanol. Next, 5 mL
4
6
ꢀ3
ꢀ1
solubility of NO (2.1 ꢂ 10 mol L at 25 °C). The sensitivity of this
1
apparatus ranged from 1 nM to 20 M, with a 2-mm sensor, which
l
directly detected NO concentration by an amperometric technique.
The luminescence spectra for both compounds (Ru–NO and
Ru–Cl) was done at 77 K in a glass matrix of anhydrous ethanol,
using a Hitachi F-4000 spectrofluorimeter equipped with accesso-
ries for low temperature measurements. For both species the
excitation wavelength was set at 496 nm. The excitation and emis-
sion slits were fixed at an aperture of 2.5 mm. The voltage applied
to the photomultiplier was 700 V. The concentration of Ru–NO and
2
(
2
6
of HPF6(conc) were added to this solution and the reaction mixture
was allowed to react for 1 h. The resulting yellowish solid was
collected by filtration, washed with methanol and diethyl ether,
and dried under vacuum. Yield 230 mg (23.7%). Anal. Calc. C,
3
0.2; H, 2.1; N, 7.3. Found: C, 30.8; H, 2.1; N, 7.5%; FT-IR (KBr):
ꢀ1
ꢀ5
m
(NO) = 1953 cm
.
Ru–Cl in these measurements were respectively 1.1 ꢂ 10 and
ꢀ
6
9
.8 ꢂ 10 mol/L. From these spectra it was possible to estimate,
for both complexes, the energy of the excited states involved in
NO photolabilization.
2 6
2.1.2. cis-[Ru(bpy) (iso)Cl]PF
Two hundred milligram of [Ru(bpy)
2
Cl
2
]ꢁ2H
2
O [21] (0.38 mmol)
pH values were recorded on a Labmeter pHmeter model PH2. In
was dissolved in 20 mL of ethanol. After adding 0.1 mL of isoquin-
oline (0.78 mmol) dissolved in 0.9 mL of ethanol, the resulting
solution was allowed to react under reflux for 90 min, in the pres-
ence of a nitrogen atmosphere and light protected. An excess of
+
order to evaluate the pH for the conversion of coordinated NO into
ꢀ
NO2 , absorption spectra of the nitrosyl complex solutions under
different pH values were registered. Variations in the absorbance
values of the bands, recorded in the visible region, were plotted
against the pH. The curves were differentiated for determination
4 6
NH PF was added and the reaction medium was kept overnight
under refrigeration. The resulting dark orange solid was collected
on a filter and the crude material was dissolved in a minimum vol-
of the pK
a
values. Mcllvaine buffer solutions were used for the pH
range 2.5–7.5, while the Kolthoff buffer was used for pH 10 [22].
ume of CH
alumina column. The main fraction was eluted with a mixture of
0% CH CN:CH Cl , and evaporated to dryness. After dissolving in
a minimum volume of CH Cl , the solution was filtered onto
0 mL of diethyl ether, yielding a precipitated which was collected
2 2
Cl and purified by chromatography, using a neutral
ꢀ
1
For the pH range 0–2.5, trifluoroacetic acid and NaOH 0.1 mol L
solutions were employed. Other pH values were obtained by com-
binations of NaOH and HCl solutions. Aiming to maintain an
approximately constant ionic strength during the experiments, all
2
3
2
2
2
2
5
ꢀ
1
solutions were prepared from KCl 0.1 mol L solutions.
by filtration, washed with diethyl ether, and dried under vacuum.
Yield 188 mg (66%). Anal. Clac. C, 48.1; H, 3.2; N, 9.6. Found: C,
The continuous variation method was applied to analyze the
behavior of nitrosyl complex and sensitizer mixtures in acetonitrile
solutions, probing the variation of the corrected absorbance at
4
8.0; H, 3.4; N, 9.4%.
3
30 nm with the molar fraction of the nitrosyl at a total concentra-
ꢀ5
ꢀ1
2.2. Measurements
tion 6.48 ꢂ 10 mol L [21].
All the UV–Vis spectra were recorded on a Hitachi U-3501 spec-
trophotometer in different solvents, at solution concentrations of
3. Results and discussion
ꢀ5
ꢀ1
approximately 10 mol L . The infrared spectra were registered
on an FTIR/Nicolet spectrometer, model Protegé 460, from KBr pel-
lets. The mass spectrometer employed in the analyses was the ul-
trO-TOFQ-ESI-TOF Mass Spectrometer (Bruker Daltonics, Billerica,
EUA), operating in the positive mode, using methanol:water
3.1. ESI-MS and ESI-MS/MS structural characterization
Electrospray ionization mass spectrometry (ESI-MS) as well as
its tandem version (ESI-MS/MS) has been increasingly used for
structural characterization of organometallics [23,24], inorganic
molecules [25], supermolecules, and their ionic forms [26]. ESI is
particularly suited for such species due to its ability to transfer,
via a unique and gentle ion evaporation process, pre-formed ions
(
20:80, v/v) as sample solvent.
The electrochemical experiments were carried out in approxi-
mately 1 ꢂ 10 mol L acetonitrile solutions, using an AUTOLABÒ
ꢀ
3
ꢀ1
potentiostat/galvanostat model PGSTAT 30, coupled to a compati-

4
46
A.N. de Carvalho et al. / Inorganica Chimica Acta 370 (2011) 444–448
directly from solutions to the gas phase environment of mass spec-
trometers [27].
In the ESI-MS spectrum of the nitrosyl cis-[Ru(bpy)
2
(iso)-
NO](PF , the molecular ion corresponding to the neutral
6
) ꢁHPF
3
6
complex is not detected. Instead, it is observed a series of isotopo-
logue ions centered at m/z 749.1 and m/z 719.1 (separated by one
m/z unit) and m/z 271 and m/z 207 (separated by 1/2 m/z unit),
characteristic of ruthenium complexes, which is a multiple isotope
1
04
102
101
100
99
element ( Ru, 18.7%;
1
Ru, 31.6%;
Ru, 17%;
Ru, 12.6%; Ru,
9
8
96
2.7%; Ru, 1.88% and Ru, 5.52%) [25]. The former ions corre-
spond to singly-charged ones and the last ones correspond to dou-
ble charged ions.
The ion centered at m/z 749.1 was ascribed to the fragment
2
+
0
1+
{
[Ru (bpy)
2
isoNO ]PF
6
ꢁCH
3
OH} , a gaseous ion originated from
ꢀ
the loss of two PF6 counter-ions and the adduct formation with
one methanol molecule. The loss of the neutral NO ligand gener-
ates the fragment {[Ru (bpy)
The existence of the {[Ru (bpy)
shows that, at least in gaseous phase, the formally charged unit
2+
1+
2
iso]PF
6
ꢁCH
0
3
OH} , at m/z 719.1.
2
+
1+
2
isoNO ](PF
6
)
2
ꢁCH
3
OH} fragment
0
‘
‘Ru(II)-NO ’’ is stable.
The doubly charged fragments observed at m/z 271 and m/z 207
+
+
were assigned respectively to the loss of NO and of NO plus the
isoquinoline ligands from the original compound, leading to the
Fig. 2. Electronic spectra of the complexes in acetonitrile solutions (___) cis-
2
+
ꢀ5
ꢀ1
[
Ru(bpy)
2
]
fragment. Within the studied m/z window, in the dis-
[Ru(bpy)
2
(iso)NO](PF
.
)
6 3
, 1.48 ꢂ 10 mol L and (———) cis-[Ru(bpy)
2 6
(iso)Cl]PF ,
ꢀ
5
ꢀ1
10
mol L
sociation ESI MS/MS spectra of the four ions depicted above, it is
observed a peak at m/z 157, which corresponds to a protonated
bipyridine molecule, suggesting that the complex further dissoci-
⁄
(
p
?
p
dp p⁄
intraligand and Ru ? NO ) centered at 300 nm
4
ꢀ1
ꢀ1
(e
= 4.3 ꢂ 10 cm mol L) and a shoulder around 350 nm. This
ates to collapse the structure.
+
is a typical spectrum profile for this kind of mononuclear ruthe-
nium–nitrosyl complex [20], in which the MLCT bands are dramat-
ically shifted to lower wavelengths due to the strong backdonation
The molecular ion corresponding to cis-[Ru(bpy)
2
(iso)Cl] was
observed at m/z 578.1. This gaseous phase ion loses its ligands
2
+
+
leading to the fragments [Ru(bpy)
at m/z 449 and [Ru(bpy)(iso)Cl. H
2
(iso)] at m/z 271, [Ru(bpy)
2
Cl]
+
+
between the NO ligand and the d ruthenium levels.
p
2
O] at m/z 440. These peaks pres-
Regarding the voltammetric behavior of the investigated
compounds, both follow the same trends already reported for their
ent the isotopic profile characteristic of the ruthenium element and
their separations are m/z = ½ for the doubly charged fragment at
m/z 271 and m/z = 1 for the singly charged ions at m/z 449 and
m/z 440.
analogues [20]. Complex cis-[Ru(bpy)
reversible waves (
= 60 mV) at E1/2 = ꢀ1.46 and 0.79 V (versus
Ag/AgCl) ascribed to the redox couples bpy
whereas cis-[Ru(bpy) (iso)NO](PF displays a quasi-reversible
wave (
2 6
(iso)Cl]PF features two
DE
p
ꢀ
1/0
2+/+3
and Ru
,
2
6 3
)
3.2. Absorption spectra and electrochemical measurements
D
E
p
ꢃ80 mV) at 0.48 V (versus Ag/AgCl), assigned to the
0/+1
NO
ꢀ
oxidation process, and one irreversible wave at Epc =
0.37 V (versus Ag/AgCl) assigned to the reduction NO
In order to probe the possibility to assemble an aggregate in
solution between cis-[Ru(bpy) (iso)NO](PF and cis-[Ru(bpy)
iso)Cl]PF in a relatively polar medium such as acetonitrile, and
Table 1 summarizes the spectral and voltammetric data for the
studied compounds. Fig. 2 presents the spectra.
0/ꢀ1
.
2 6
Concerning the absorption spectra of cis-[Ru(bpy) (iso)Cl]PF ,
2
6
)
3
2
-
the main features observed may be easily assigned by comparing
to reported data for analogous [Ru(bpy) (L)Cl]PF complexes where
(
6
2
6
to address the aggregate stoichiometry, the continuous variation
method was applied [31]. In this method the total concentration
of the species is kept constant, and the ratio between ruthenium
nitrosyl and sensitizer (the ruthenium chloro complex) varied from
L is a pyridinic ligand (Fig. 2) [28–30]. The intense band in the ultra-
⁄
violet region can be assigned to the isoquinoline ligand
p
?
p
tran-
sitions and in the visible region one can observe the MLCT bands of
the [Ru(bpy) (iso)] moiety, at 360 and 496 nm. In its turn, the
absorption spectrum of the cis-[Ru(bpy) (iso)NO](PF complex
2
0
to 1. Fig. 3 presents the dependence of the absorbance at 330 nm
2
6 3
)
with the molar fraction of the nitrosyl complex under investigation
for five different mixtures of both compounds in acetonitrile.
The maximum of this curve should correspond to the ratio of
the compounds in which there is the maximum interaction
between then and, consequently, it is the aggregate stoichiometry.
In the case under investigation in this work, the plot has a typical
profile of a 1:1 stoichiometry aggregate, displaying a well defined
maximum for a 0.5 M ratio. This qualitative result suggests that
in acetonitrile displays an absorption band in the UV region
Table 1
Absorption and voltammetric data for the compounds cis-[Ru(bpy)
2 6 3
(iso)NO](PF )
and cis-[Ru(bpy)
2
(iso)Cl]PF
6
collected form acetonitrile solutions.
Compound
Electronic spectra data
4
ꢀ1
ꢀ1
k
max (nm)/
e
ꢂ 10 (mol L cm
)
⁄
p
?
p
MLCT
MLCT
the nitrosyl cis-[Ru(bpy)
Ru(bpy) (iso)Cl]PF interact in solution, possibly by
between the coordinated polyaromatic isoquinoline molecules
17,18].
2
(iso)NO](PF
6
)
3
and the sensitizer cis-
[
[
Ru(bpy)
Ru(bpy)
2
(iso)Cl](PF
6
)
360/1.2
350 (shoulder)
496/0.98
[
2
6
p
-interaction
2
(iso)NO](PF
6
)
3
300/4.3
Electrochemical data
[
E1/2 (V vs. Ag/AgCl)
ꢀ
1/0
Ru2+/3+
NO1+/0
0/1ꢀa
bpy
NO
3
.3. Photophysical assays
[
[
Ru(bpy)
Ru(bpy)
2
(iso)Cl](PF
6
)
ꢀ1.46
0.79
2
(iso)NO](PF
6
)
3
0.48
ꢀ0.37
+
It is well established in literature that the NO ligand must be
a
Epc value.
0
reduced to NO , being thus released from the ruthenium–polypyr-

A.N. de Carvalho et al. / Inorganica Chimica Acta 370 (2011) 444–448
447
idine compounds [20]. One way to do this is to irradiate the com-
dp p⁄
pound in a CT transition, usually the Ru ? NO , a high energy
photoredox level that leads to NO0 release in the excited state.
However, for therapeutical application of this property, the irradi-
ation should be done in higher wavelengths. Since the compound
2 6 3
cis-[Ru(bpy) (iso)NO](PF ) does not display absorption bands in
the visible region, it is worth to probe the possibility of sensitizing
the NO release with the irradiation of a second weak interacting
2 6
unit such as the cis-[Ru(bpy) (iso)Cl]PF complex. Our hypothesis
is that, since isoquinoline ligand acts as a site for the stacking of
these compounds in solution, this assembly would promote a more
efficient transfer of the excitation energy from the sensitizer to the
nitrosyl compound than a simple collisional mechanism.
It is worth mentioning that at physiological pH the actual spe-
cies present in aqueous solution is the nitro instead of nitrosyl,
+
ꢀ
since conversion of the coordinated NO to coordinated NO in
2
2 6 3
the compound cis-[Ru(bpy) (iso)NO](PF ) was observed at pH
5
.67 (Supplementary material). However, it has already been dem-
onstrated that nitro bipyridine ruthenium compounds are also able
to release nitric oxide, due to a photoinduced homolytic dissocia-
ꢀ
2+
Fig. 4. Chronoamperogram of an equimolar mixture of cis-[Ru(bpy)
2
(iso)NO](PF
6 3
)
tion of NO from a coordinate NO2 , yielding a Ru –H
2
O complex
ꢀ5
ꢀ1
and cis-[Ru(bpy)
1 v:1v).
2 6
(iso)Cl]PF in 10 mol L solutions in a water: ethanol mixture
and NO as photoproducts [32,33]. Besides, both compounds have
poor solubility under physiological conditions. Therefore, at this
stage of the investigation, the photolysis experiments were carried
out in a 1:1 (v:v) mixture of water:ethanol. Fig. 4 displays the qual-
(
the nitrosyl, respectively), when excited in the visible region
35]. In the particular case of the nitrosyl complex, there is only
itative results of the cis-[Ru(bpy)
2
6 3
(iso)NO](PF ) photolysis in the
[
absence and in the presence of cis-[Ru(bpy)
2
(iso)Cl]PF .
6
a residual absorbance at kexi = 496 nm, which likely does not have
Actually, the irradiation of the chloro complex MLCT band leads
to the photoinduced release of NO from the nitrosyl unit cis-[Ru(b-
py) (iso)NO](PF ) , showing that the transfer of excitation energy
2 6 3
⁄
a significant contribution of the Ru
d
p
?
p
NO charge transfer since
this last transition typically occurs in the UV region [20]. From the
low temperature spectra, the value of E0–0 were calculated to be
is efficient enough to promote the sensitization. Since the energy
of the singlet charge transfer states of the chloro compound is low-
er than that of the nitrosyl (as seen in their electronic spectra), the
energy transfer mechanism might be ruled out as a pathway for the
sensitization [34]. Therefore, we consider a photoinduced oxidative
electron transfer as the most probable mechanism for the excita-
tion energy transfer.
E
0–0 = 1.90 eV for the chloro species and E0–0 = 2.04 eV for the
nitrosyl species. Considering an oxidative transfer in the excited
state, the following thermodynamic cycle was elaborated: (Scheme
1
equation below, taking the values of E1/2 from the cyclic voltammo-
grams (Table 1):
0
oxi
) and the
DG
for the electron transfer was calculated from the
0
oxi
0
oxi
0
3þ=2þ
Þ þ E ðNOþ=0Þꢄ; e:g:
0
In order to determine the driving force for such a reaction, low
temperature (77 K) luminescence spectra of these compounds
were recorded, being shown in Fig. 5.
D
D
G
G
¼ ꢀ
¼ ꢀ
D
D
E ¼ ꢀ½ðE0—0 ꢀ E ðRu
E ¼ ꢀ½ð1:90 ꢀ 0:79 þ 0:48Þꢄ ¼ ꢀ1:59 eV:
3
As it can be seen, both complexes exhibit the typical MLCT
Therefore, the reasonable large negative value obtained for
0
oxi
emission of ruthenium–polypyridine compounds, between 550
D
G
strongly suggests that the actual mechanism of sensitization
and 700 nm (kemi = 651 nm and kemi = 607 nm for the chloro and
for this sort of system is a photoinduced electron transfer.
Fig. 3. Continuous variation plot for different mixtures of sensitizer and nitrosyl
Fig. 5. Emission spectra recorded from ethanol glasses for (a) cis-[Ru(bpy)
2
(iso)-
ꢀ
6
ꢀ1
ꢀ5
ꢀ1
compounds, showing the dependence of the absorbance at 330 nm with the molar
NO](PF
6
)
3
, 9.8 ꢂ 10 mol L and (b) cis-[Ru(bpy)
2
6
(iso)Cl]PF , 1.1 ꢂ 10 mol L
,
ꢀ
5
ꢀ1
fraction of the nitrosyl (10 mol L solutions in acetonitrile).
at 77 K (liquid nitrogen), kexc = 496 nm, slits f
1
2
= f = 2.5 nm.

4
48
A.N. de Carvalho et al. / Inorganica Chimica Acta 370 (2011) 444–448
Scheme 1
[
[
[
3] A.A. Eroy-Reveles, P.K. Mascharak, Fut. Med. Chem. 1 (2009) 1497.
4] T.S. Kurtikyan, P.C. Ford, Coord. Chem. Rev. 252 (2008) 1486.
5] P.C. Ford, S. Wecksler, Coord. Chem. Rev. 249 (2005) 1382.
4
. Conclusions
This work reported the synthesis and characterization of two
[6] E. Tfouni, F.G. Doro, A.J. Gomes, R.S. da Silva, G. Metzker, P.G.Z. Benini, D.W.
Franco, Coord. Chem. Rev. 254 (2010) 355.
[
[
[
[10] D. Maranho, R.G. de Lima, F.L. Primo, R.S. da Silva, A.C. Tedesco, Photochem.
Photobiol. 85 (2009) 705.
11] Z.N. da Rocha, R.G. de Lima, F.G. Doro, E. Tfouni, R.S. da Silva, Inorg. Chem.
Commun. 11 (2008) 737.
[12] P.C. Ford, Acc. Chem. Res. 41 (2008) 190.
13] M.J. Rose, N.L. Fry, R. Marlow, L. Hinck, P.K. Mascharak, J. Am. Chem. Soc. 130
2008) 8834.
[14] M.J. Rose, M.M. Olmstead, P.K. Mascharak, J. Am. Chem. Soc. 129 (2007) 5342.
[15] R.S. Silva, M.S.P. Marchesi, C. Khin, C.N. Lunardi, L.M. Bendhack, P.C. Ford, J.
Phys. Chem. B 111 (2007) 6962.
16] M.G. Sauaia, R.G. Lima, A.C. Tedesco, R.S. da Silva, Inorg. Chem. 44 (2005) 9946.
17] E.C. Fornari, M.S.P. Marchesi, A.E.H. Machado, S. Nikolaou, Polyhedron 28
(2009) 1121.
18] E. Buisine, K. Villiers, T.J. Egan, C. Biot, J. Am. Chem. Soc. 128 (2006) 12122.
19] R. Sarma, J.B. Baruah, J. Mol. Struct. 920 (2009) 350.
20] M.G. Sauaia, R.S. da Silva, Trans. Met. Chem. 28 (2003) 254.
21] B.P. Sullivan, D.J. Salmon, T.J. Meyer, Inorg. Chem. 17 (1978) 3334.
22] T. Morita, R.M.V. Assumpção, Manual de soluções, reagentes e solventes, Ed.
Edgard & Blücher Ltda, Brazil, 1968.
ruthenium–polypyridines complexes, the nitrosyl cis-[Ru(bpy)
iso)NO](PF and the sensitizer cis-[Ru(bpy) (iso)Cl]PF . The irra-
2
7] C.N. Lunardi, R.S. da Silva, L. Bendhack, J. Braz. Chem. Soc. 42 (2009) 87.
8] M.J. Rose, P.K. Mascharak, Coord. Chem. Rev. 252 (2008) 2093.
9] A.E.H. Machado, Quim. Nova 23 (2000) 237.
(
6
)
3
2
6
diation of an equimolar mixture of the two compounds in an eth-
anol:water solution (1 v:1 v) with radiation of k > 500 nm (the
sensitizer MLCT band) leads to NO release, as probed by ampero-
metric measurements. The application of the variational method
to the system suggested that the two compounds self-assembly
[
[
in solution with a 1:1 stoichiometry. Also, based on the value of
D
(
0
oxi
G
(calculated from Section 2), the sensitization mechanism
could be assigned to a photoinduced electron transfer.
The data reported here proved that the strategy of using a pair
of two analogues coordination complexes is a promising route to
develop NO releasing system. In the next steps of this work, the is-
sue of using this sort of species in physiological medium will be
addressed.
[
[
[
[
[
[
[
Supplementary material
[
23] (a) C. Chevrin, J. Le Bras, A. Roglans, D. Harakat, J. Muzart, New J. Chem. 31
(2007) 121;
The ESI MS/MS mass spectra, infrared spectrum and the pKa
plots are available as Supplementary Material at www.elsevier.
com.
(
8
b) P. Wang, G.R. Newcome, C. Wesdemiotis, Int. J. Mass Spectrom. 255 (2006)
6.
[
[
24] O. Bortolini, V. Conte, Mass Spectrom. Rev. 25 (2006) 724.
25] M.N. Eberlin, D.M. Tomazela, K. Araki, A.D.P. Alexiou, A.L.B. Formiga, H.E. Toma,
S. Nikolaou, Organometallics 25 (2006) 3245.
26] (a) F.C. Gozzo, L.S. Santos, R. Augusti, C.S. Consorti, J. Dupont, M.N. Eberlin,
Chem. Eur. J. 10 (2004) 6187;
Acknowledgment
[
The authors would like to thank the Brazilian agencies FAPESP,
FAPEMIG, CAPES and CNPq for financial support. Prof. Roberto
Santana da Silva for essential discussions and cession of some
equipment, and Prof. Norberto Peporine Lopes and João Carlos
Tomaz for the mass spectra acquisition.
(b) M. Vazquez, M.R. Bermejo, M. Licchelli, A.M. Gonzalez-Nova, R.M. Pedrido,
C. Sangregorio, L. Sorace, A.M. Garcia-Deibe, J. Satunartin, Eur. J. Inorg. Chem.
(
2005) 3479.
[
[
27] (a) A.E.M. Crotti, R.L. Vessecchi, J.L.C. Lopes, N.P. Lopes, Quim. Nova 29 (2006)
287;
(b) R.B. Cole, Electrospray Mass Spectrometry, John Wiley, New York, 1997;
(
c) R. Colton, A. D’Agostinho, J.C. Traeger, Mass Spectrom. Rev. 14 (1995) 79.
28] R.W. Callahan, G.M. Brown, T.J. Meyer, Inorg. Chem. 14 (1975) 1443.
Appendix A. Supplementary material
[29] S. Nikolaou, H.E. Toma, Polyhedron 20 (2001) 253.
[
[
[
30] S. Nikolaou, H.E. Toma, Trans. Met. Chem. 33 (2008) 1059.
31] P. Job, Ann. Chim. 9 (1928) 113.
32] D.C.A.S. de Santanaa, T.T. Pupo, M.G. Sauaia, R.S. da Silva, R.F.V. Lopez, Int. J.
Pharmac. 391 (2010) 21.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.02.018.
[
33] R.G. Lima, M.G. Sauaia, D. Bonaventura, A.C. Tedesco, R.F.V. Lopez, L.M.
Bendhack, R.S. da Silva, Nitric Oxide 358 (2004) 2643.
34] Th. Foster, Discuss. Faraday Soc. 27 (1959) 7.
35] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. Von Zelewsky,
Coord. Chem. Rev. 84 (1988) 85.
References
[
[
[
[
1] L.J. Ignarro, Nitric Oxide Biology and Pathobiology, Academic Press, San Diego,
000.
2] S. Sortino, Chem. Soc. Rev. 39 (2010) 2903.
2