Ruthenium(II) complexes containing tetrazolate group: Electrochemiluminescence in solution and solid state

Article abstract of DOI:10.1021/jp064326r

In this work, we report the results about the solution and solid-state phosphorescence emission properties of six Ru^II complexes containing various 5-substituted tetrazolate ligands. The photo- and electrochemiluminescence spectra of all compounds revealed a red shifted emission with respect to the Ru(bpy)₃²⁺. Significant changes to the light emission energy and to the efficiency and sensitivity to oxygen were also determined by varying the nature of the substituent ring of the tetrazolate ligand. Light-emitting solid devices with active layers containing solid films of the same complexes were prepared, and preliminary studies of their electroinduced emission properties were performed. The electrochemiluminescence (ECL) emission intensity of two of the six complexes was of the same order of magnitude as the reference Ru(bpy)₃ ²⁺.

Full text of DOI:10.1021/jp064326r

J. Phys. Chem. B 2006, 110, 22551-22556
22551
Ruthenium(II) Complexes Containing Tetrazolate Group: Electrochemiluminescence in
Solution and Solid State
Simone Zanarini,*,† Allen J. Bard,‡ Massimo Marcaccio,*,† Antonio Palazzi,§
Francesco Paolucci,† and Stefano Stagni§
Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna, Via Selmi 2,
I-40126, Bologna, Italy, Department of Chemistry & Biochemistry, The UniVersity of Texas at Austin,
Austin, Texas 78712, and Dipartimento di Chimica Fisica ed Inorganica, UniVersita` di Bologna,
Viale Risorgimento 4, I-40136 Bologna, Italy
ReceiVed: July 10, 2006; In Final Form: September 2, 2006
In this work, we report the results about the solution and solid-state phosphorescence emission properties of
six RuII complexes containing various 5-substituted tetrazolate ligands. The photo- and electrochemilumi-
nescence spectra of all compounds revealed a red shifted emission with respect to the Ru(bpy)32+. Significant
changes to the light emission energy and to the efficiency and sensitivity to oxygen were also determined by
varying the nature of the substituent ring of the tetrazolate ligand. Light-emitting solid devices with active
layers containing solid films of the same complexes were prepared, and preliminary studies of their
electroinduced emission properties were performed. The electrochemiluminescence (ECL) emission intensity
2+
of two of the six complexes was of the same order of magnitude as the reference Ru(bpy)3
.
Introduction
SCHEME 1: Complexes and Acronyms Used in This
Work
2+
Since Ru(bpy)3
electrochemiluminescence (ECL) was
discovered, photophysical properties of many Ru complexes
have been investigated.1,2 The intense emission of light generated
by annihilation in organic solvents or in the presence of
coreactant in water3 has been extensively studied. The potential
applications are correlated with the specific properties of the
single compound. In organic light-emitting solid devices
2+
(OLED) tests, Ru(bpy)3 showed an intense orange emission
with low applied voltages.4-7 Molecules intended for light-
emitting solid devices require in general high quantum efficien-
cies, short response times, and high durability. Ruthenium-
tris(4,7-diphenyl-1,10-phenanthroline) (RuDPP) has been used
as an oxygen sensor with fluorimetric detection in organic
solvents and in supported silicon and xerogels films.8 RuII
complexes interacting with DNA or other biomolecules often
show a change in ECL intensity proportional to the concentra-
tion, and therefore, many analytical methods based on ECL have
been developed.3,9 In this article, ECL of the RuII(bpy)(tpy)
moiety with different substituted tetrazolates (see Scheme 1)
has been tested in MeCN solution and in light-emitting solid
devices. The addition or removal of small groups around the
tetrazolate moiety allowed us to study the effect in MLCT
(metal-to-ligand charge transfer) transitions. The modification
of ECL spectra is often remarkable even in relatively similar
molecular structures. To detect biomolecules such as nucleic
acids and proteins, the use as covalent labels3 could be possible
Experimental Section
Materials and Synthesis. Unless otherwise stated, all
chemicals were obtained commercially. The organic tetrazoles
were prepared in high yields (80-95%) according to the first
established method involving 1,3 dipolar cycloaddition of the
azide anion onto the appropriate aromatic nitriles.10a The only
exception was represented by the ligand 4-TPhH, which was
synthesized by following the protocol reported by Koguro et
al.10b The precursor complexes Ru(tpy)Cl310c and [Ru(tpy)(bpy)-
Cl][PF6]10d were prepared according to literature methods. The
preparation and the structural characterization of four of the six
dyes, Ru(4-TBN)Ru, Ru(BTB)Ru, Ru(4-TBN), and Ru(4-TBN)-
because of considerable quantum efficiency and red shift with
2+
respect to Ru(bpy)3
.
* Authors to whom correspondence should be addressed. E-mail:
simone.zanarini@unibo.it (S.Z.).
† Dipartimento di Chimica “G. Ciamician”, Universita` di Bologna.
‡ The University of Texas at Austin.
§ Dipartimento di Chimica Fisica ed Inorganica, Universita` di Bologna.
10.1021/jp064326r CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/25/2006

22552 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Zanarini et al.
Me, used in this work have been described elsewhere.2 The
synthesis of the new cationic complexes Ru(TPh) and Ru(4-
Tpy) has been performed by adopting the same two-step
procedure. This latter involved the preliminary chloride abstrac-
tion from the precursor species [Ru(tpy)(bpy)Cl][PF6] followed
by reaction with the desired tetrazolate ligand. Typically, a
0.500-g aliquot of [Ru(tpy)(bpy)Cl][PF6] (0.74 mmol) was
placed into a light-protected 100-mL round-bottomed flask and
was dissolved in 15 mL of deaerated acetone. A slight excess
(1.1 equiv) of AgPF6 was added, and the resulting mixture was
stirred with reflux for 2 h. Then, the precipitated AgCl was
removed by filtration through a Celite pad and the filtrate was
added slowly to an acetone (10 mL) solution of the desired
tetrazolate ligand (0.85 mmol). Once the addition was complete,
the deep red reaction mixture was heated at the reflux temper-
ature for 8-10 h. The solution was cooled to room temperature,
was concentrated to ca. half of the initial volume, was added to
10 mL of an aqueous solution containing 0.5 g of NH4PF6, and
was extracted with dichloromethane until the aqueous phase
became colorless. The organic layers were combined and dried
over MgSO4, and the solvent was removed in vacuo. The
resulting crude products were purified by alumina-filled column
chromatography with acetone/toluene mixtures as the eluent.
The target complexes [Ru(TPh)] and [Ru(4-TPy)] were both
eluted as the second red-brown fraction (acetone/toluene: 1.5/1
v/v) after a first purple band identified as the starting [Ru(tpy)-
(bpy)Cl][PF6]. X-ray structural determinations of both com-
plexes are in progress and will be reported elsewhere. [Ru(TPh)]:
0.300 g, 52% (calculated with respect to the initial [Ru(tpy)(bpy)-
Cl][PF6]). ESI-MS(Waters ZQ-4000 instrument; acetonitrile as
the solvent): m/z 636, [M - PF6-]+. Anal. Calcd. for C32H24N9-
RuPF6:C, 49.17; H, 3.07; N, 16.13. Found (ThermoQuest Flash
1112 Series EA instrument): C, 49.22; H, 3.08, N, 14.25. [Ru-
(4-TPy)]: 0.360 g, 62%. ESI-MS: m/z 637, [M - PF6-]+. Anal.
Calcd. For C31H23N10RuPF6:C, 47.57; H, 2.94; N, 17.90.
Found: C, 47.24; H, 2.97, N, 17.94.
focused on the output of a grating spectrometer (Holographics,
Inc.). The CCD camera general setup was described before.11
All ECL spectra have been recorded with a pulse width of 0.1
s and a 4-min exposure time.
Photoluminescence and Lifetime. Photoluminescence (PL)
measurements were performed with a Varian Cary Eclipse
fluorimeter in acetonitrile (Fisher Scientific HPLC grade) with
concentrations between 10-3 and 10-5 M. PL experiments were
performed after 20 min degassing with Ar in a specially
homemade O-ring sealed cell. Samples were then progressively
exposed to air to study oxygen sensitivity. Absorbance spectra
were recorded by a Varian Cary 5 UV-vis-NIR spectropho-
tometer using the same sealed cell used for photoluminescence
measurements in MeCN solutions. Lifetimes were measured in
an air-equilibrated sample using an IBH TCSPC model 5000 F
(time-correlated single photon counting) with a PMT detector
(Applied Voltage: 2150 V). The emission wavelength was
selected by a computer-controlled monochromator. Two cutoffs
(590 and 610 nm) were used during measurements. Lifetimes
were then calculated by fitting the data with a single-exponential
decay function.
Solid Device Preparation. Light-emitting solid devices were
prepared as reported4,5 previously with Ru(bpy)32+ by Bard and
co-workers. The inorganic thin layer was prepared on ITO glass
(Delta Technologies, Ltd., 100 Ω, square) by spin coating from
a MeCN solution (1-4% w/v of complex). A small drop of
In/Ga eutectic (from Aldrich) over the active layer was used as
counter electrode and was connected to the reference electrode
lead. Positive charge was injected through the underlying ITO
glass used as working electrode (Figure S2, Supporting Infor-
mation). For Ru(4-TBN)Ru and Ru(4-TBN)Me complexes, both
spin coating and an alternative film deposition method were
used: a few drops of an acetonitrile solution (0.1 up to 2%
w/v) were dried in air on ITO surface. In this way, a thicker
film could be obtained. Light intensity was measured using a
photodiode positioned under the emitting thin layer as described
earlier.4,5
Electrochemistry and Electrochemiluminescence. Cyclic
voltammetry was carried out with a model 660 electrochemical
workstation (CH Instruments, Austin, TX). Electrodes were
polished with 0.05-µm alumina and then were ultrasonicated
and thoroughly rinsed with Milli-Q water and acetone before
each run. The quasi-reference electrode was a coiled silver wire.
A platinum disk (approximate diameter 2 mm) was used as
working electrode and a coiled Pt wire was used as an auxiliary
electrode. The E1/2 values for first reduction and oxidation are
referred to SCE (saturated calomel electrode), and they have
been calculated adding ferrocene as an internal standard. In ECL
and electrochemical experiments, acetonitrile (Fisher scientific
HPLC grade) was used as received and 1 mM solution of the
Results and Discussion
Electrochemiluminescence and Electrochemistry. The ECL
spectra of a 1 mM Ar degassed acetonitrile solution of Ru-
(bpy)3(PF6)2 obtained by direct annihilation between cation and
anion has been taken as reference. The sample was run several
times, and the highest emission intensity was of 300 000
Arbitrary Units (A.U.) with maxima at 610 nm. Detailed
voltammetric studies under ultradry conditions (MeCN/TBAPF6)
have been reported by our group for Ru(4-TBN)Ru, Ru(BTB)-
Ru, and Ru(4-TBN).2 For the remaining three complexes and
the reference Ru(bpy)32+, the cyclic voltammetric curves in Ar
degassed MeCN are showed in Figure 1.
-
complex (as PF6 salt) was prepared with 0.1 M TBAPF6
(Aldrich) as supporting electrolyte. Similar results were obtained
with TBABF4 (Aldrich). Before each ECL experiment, the
sample was deaerated with Ar for 20 min in a specially made
cell (Figure S1, Supporting Information). The exact concentra-
tion of oxygen in solution following deaeration was not
measured. Preliminary potential cycling was performed sys-
tematically until a reproducible voltammogram between the first
reduction and the first oxidation was obtained. The ECL signal
during cyclic voltammetry was measured with a photomultiplier
tube (PMT, Hamamatsu R4220p) placed on the side of the
electrochemical cell. A voltage of 750 V was supplied to the
PMT. A charge-coupled device (CCD) camera (Photometrics
CH260) cooled below -135 °C interfaced to a personal
computer was used to obtain ECL spectra. The camera was
In the cyclic voltammetry experiments preliminary to ECL
ones, the complexes showed in general chemical reversibility
in first oxidation and reduction wave (ia/ic very close to 1). If
the scan was repeated for 100 cycles including only the first
reduction and oxidation (scan rate 10 V/s), no change in
reversibility was observed. For all the complexes, including Ru-
(bpy)32+, the presence of oxygen seriously affects the quality
of reduction wave which becomes irreversible when the cell is
air equilibrated (see Figure 1b). Ru(4-TBN)Ru shows two
reversible oxidation processes and two very close reduction
waves assigned to weakly interacting Ru centers bridged by
the asymmetric ligand.2 Fluorescence, ECL, and electrochemical
data for all complexes and the reference one are summarized
in Table 1. All molecules evidenced a smaller energy gap [EI,ox
-

Ruthenium(II) Complexes Containing Tetrazolate
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22553
Figure 1. Cyclic voltammetric curves of 1 mM complexes in 0.1 M TBAPF6/MeCN solution recorded with a 2-mm diameter Pt disk working
electrode; T ) 25 °C; scan rate 0.2 V/s. (a) Ru(bpy)32+; (b) Ru(4-TBN)Me, air equilibrated (dashed line) and degassed (solid line) solution; (c)
Ru(TPh). The small voltammetric wave at about 0.8 V is attributed to the oxidation process of the precursor species [Ru(tpy)(bpy)Cl]+, which is
not luminescent;2 (d) Ru(TPy). In all voltammograms, potential is referred to SCE.
2+
TABLE 1: PL, ECL, and Electrochemical Data of the Ru(bpy)3 (Reference Compound) and the Six Complexes Investigateda
compound
λ
max,abs/nm (MLCT) λmax,PL/nm φrel (%) E (I Ox.)/V (vs SCE) E (I Red.)/V (vs SCE) λmax,ECL/nm most stable ion in ECL
2+
Ru(bpy)3
455
487
463
487
487
487
460, 490
630
690
660
670, 700
690
690
670, 700
100
7.4
11.6
5.7
7.2
9.0
+1.20
+1.12
+1.25
+1.01
+1.03
+1.01
+1.02
-1.48
-1.31
-1.34
-1.42
-1.39
-1.39
-1.35
610
690
720
730
740
700
690
anion
anion
cation
anion
anion
cation
anion
Ru(4-TPZ)
Ru(4-TBN)Me
Ru(4-TBN)
RuTPh
Ru(BTB)Ru
Ru(4TBN)Ru
15.3
a Energy gap can be estimated by the electrochemical data. The most stable ion was determined during ECL experiment comparing the relative
light intensity during positive and negative potential pulse in double potential step chronoamperometry. φrel (%) is calculated taking Ru(bpy)32+ as
reference (100% efficiency).
E
I,red], expressed in eV with respect to the reference, and the
not symmetric; a tail at longer wavelength is clearly visible.
The spectral resolution in the ECL experimental setup does not
allow one to clearly separate the lower energy shoulder as in
PL experiments.
ECL maximum wavelength is always red shifted with respect
to Ru(bpy)32+. Upon changing the substituent, the emission
shifts from 690 to 740 nm (see Table 1, Figure 2 and Figure
S3). In general, the ECL maxima are also red shifted from the
corresponding PL by about 10-50 nm.
Considering the four different mononuclear complexes, the
higher efficiency was for the ligands containing tetrazolate
bound with pyridine and benzene. The introduction of the
cyanide group in RuTPh (i.e., the species Ru(4-TBN)) causes
a decrease of ECL intensity of about 75%. When a methyl group
is then inserted directly on the tetrazolate ring (Ru(4-TBN)-
Me), the intensity went back to higher values. Another interest-
ing comparison is between Ru(4-TBN) and Ru(4-TBN)Ru. The
addition of a second Ru(bpy)(tpy) center causes an increase of
ECL efficiency of 6 times. As stated above, the two Ru centers
in Ru(4-TBN)Ru behave independently,2,12 so the inclusion of
the second oxidation and reduction localized on the second Ru
ECL intensities for mononuclear compounds (see Figure 2)
2+
are lower than for Ru(bpy)3 but are of the same order of
magnitude. ECL spectra of the dinuclear complexes Ru(4-TBN)-
Ru and Ru(BTB)Ru with respect to Ru(bpy)32+ under the same
experimental conditions described above have been recently
reported (see Figure S3).2 The relative ECL intensity for the
six complexes are shown in Table 2. One of the two dinuclear
Ru complexes, Ru(4-TBN)Ru, shows an emission about 3 times
more intense than the reference one, which can be observed by
the naked eye in a dark room.2 The emission intensity can be
tuned by simply including or not including the second reduction
and oxidation wave. The emission peak recorded by CCD is
center causes the ECL intensity to double. Considering the
2+
tetrazolate complex Ru(4-TBN)Ru with respect to Ru(bpy)3
,

22554 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Zanarini et al.
Figure 2. ECL emission spectra of the four mononuclear tetrazolate-
based complexes, obtained by annihilation of one-electron oxidized and
reduced forms. All ECL spectra were collected for 1 mM compound
in 0.1 M TBAPF6/MeCN solution, T ) 25 °C, and accumulation time
4 min.
Figure 3. Absorbance spectra of 0.1 mM solutions of Ru(4-TBN)Ru
(in acetonitrile and dichloromethane) and Ru(4-TBN) (acetonitrile only).
TABLE 2: Estimated Relative ECL Emission Efficiency of 1
2+
mM Complex in 0.1 M TBAPF6/MeCN Solution (Ru(bpy)3
Used as Reference) Cycling the Potential between the I
Oxidation and I Reductiona
compound
max ECL intensity (A.U)
rECL-Int. (%)
Ru(4-TPZ)
Ru(4-TBN)Me
Ru(4-TBN)
RuTPh
227 869
135 405
45 625
187 232
75
45
15
62
45
120
290
100
Ru(BTB)Ru
134 928
Ru(4TBN)Rub
372 410 (I Ox., I Red.)
864 597 (II Ox., II Red)
303 181
Ru(bpy)32+ (ref)
a The relative ECL intensities (rECL-Int.) have been calculated by
b The potential has
2+
rECL-Int.(%) ) 100*I
,
ECL MAX/IECL,MAX,Ru(bpy)
3
also been cycled between II Ox. and II Red.
the relative PL quantum efficiency is 15.3% (φrel in Table 1)
and the relative ECL intensity is 120% (rECL-Int. in Table
2).The mismatching of photo- and electrochemiluminescence
is necessarily correlated with processes prior to generation of
the excited emitting state. The moderate presence of oxygen
Figure 4. Concentration effect on the normalized intensity of the 700-
nm emission of Ru(4-TBN)Ru in MeCN.
2+
has been found to affect ECL efficiency of Ru(bpy)3 much
more than the tetrazolate complexes suggesting the possibility
to perform measurements under high vacuum conditions.
Photoluminescence and Broadened Emission of Ru(4-
TBN)Ru. Absorbance Spectra and Air SensitiVity. The absor-
bance and maximum emission wavelength of all complexes are
shown in Table 1. All complexes (excluding Ru(4-TBN)Ru and
Ru(4-TBN)) show, as expected, a single emission in MLCT
region between 630 and 700 nm and a single absorbance
maximum between 450 and 490 nm. The sensitivity to oxygen
concentration is in general very low. Only Ru(4-TBN)Ru and
Ru(4-TBN)Me have shown a decrease of 20-30% in maximum
emission intensity when exposed to air, which is still very
different from the sensitivity of the Ru(DPP)3 tested under
similar conditions.
Broadened Emission and RelatiVe Lifetime. When the absor-
bance spectra of Ru(4-TBN)Ru and mononuclear Ru(4-TBN)
in acetonitrile are compared (Figure 3), an additional absorbance
peak appears at 460 nm in the MLCT region in addition to the
490-nm peak present in both compounds. The two peaks at 460
and 490 nm are more intense in dichloromethane (DCM) than
in MeCN (Figure 3). Both Ru(4-TBN)Ru and Ru(4-TBN)
showed a broadened emission with respect to the other four
compounds with a short wavelength difference between the
maxima at 670 and 700 nm (Figure 4). The maximum intensity
of both emissions of Ru(4-TBN)Ru is exactly twice the
corresponding emissions of the mononuclear complex Ru(4-
TBN). This can be explained in terms of the number of Ru
centers that can emit from the single lower energy center. There
is a clear increase of the intensity of the 700-nm emission with
the concentration in Ru1 and Ru2 complexes (Figure 4).
Temperature also affects the relative intensity of the second peak
(Figure 5). In a 1 mM acetonitrile solution of Ru(4-TBN)Ru as
the temperature is increased from 20 to 50 °C, the emission
8

Ruthenium(II) Complexes Containing Tetrazolate
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22555
Figure 6. Light/current/potential curves of a solid device prepared by
drying a drop of 4% w/v acetonitrile solution of Ru(4-TBN)Ru.
Figure 5. Effect of temperature on the emission band of 1 mM solution
of Ru(4-TBN)Ru in MeCN.
acetone) or drying a deposited drop from a lower concentration
solution did not improve the emission intensity.
band at 670 nm decreases more than that at 700 nm. There is
no selectivity with the excitation wavelength on the 670- and
700-nm emission. At room temperature, the two peaks seem to
be thermally equilibrated and the emissions take place at a
similar rate. According to our results, low temperature stabilizes
the 670-nm emission and moderately high temperature stabilizes
the 700-nm emission (Figure 5).
Ru(4-TBN)Ru. The solubility was 4% w/v in acetonitrile.
Using spin coating, precipitation and crystallization on ITO
surface resulted in an inhomogeneous film with pinholes. No
steady-state emission was reached, and a maximum light
intensity of about 0.6 µW/mm2 was measured (applied potential
4 V; red-orange light was observed by the naked eye).
Reproducibility was not good because of pinhole distribution
in the sample.
The lifetimes of the 670- and 700-nm emissions for both
mono- and dinuclear species are identical within the experi-
mental error (15.7 ns and 16.0 ns, respectively),13 which
excludes the possibility of artifacts and distortions (low
concentration conditions). Lifetimes measured on integrated
spectra do not change significantly suggesting the similar nature
of the two emissions. Electrochemical data assigned the first
two reductions to the tpy (lower energy) and bpy ligand,
respectively.2 The fact that both emissions are present even in
the mononuclear compounds suggests that they are localized
on the same Ru center. In general, Kasha’s rule14 states that no
multiple MLCT emissions are possible from the same Ru center,
but in the case of mixed ligand complexes, one can sometimes
expect the presence of a few MLCT states with similar energies.
The more reasonable explanation for the dual emission seems
to be the presence of a vibrational structure in the emission
from the tpy-Ru MLCT or, alternatively, from bpy-Ru MLCT
transition. Further and more detailed studies on the photophysics
are necessary to thorougly understand this broadened emission.
Different attempts were made: spin coating at lower speeds
(400, 300, 200, 100 rpm), for shorter times (1 min, 30 s, 15 s)
and using lower concentrations (4, 3, 2, 1% w/v). However,
the film properties did not change. The steady-state emission
was reached when a 4% acetonitrile solution was dried on an
ITO substrate (Figure 6). The film thickness increased from the
previously estimated values of 100-200 nm to 500-1000 nm
showing iridescence. The durability was improved but the
response time (30 min reaching the maximum intensity) and
the maximum intensity (about 0.05 µW/mm2 at 4 V) strongly
decreased as compared with spin-coated film. The behavior was
2+
similar to that observed for Ru(bpy)3 when the device was
2+
prepared and operated in a drybox.7 The Ru(bpy)3 gave a
crystalline insulating and iridescent film when a drop from a
similar solution was dried. An explanation of this behavior is
the increased time necessary to cross the diffusion layer and
the higher amount of solvent molecules retained by tetrazolate-
based complexes as compared to Ru(bpy)32+. This could also
explain the reversible memory effect of potential conditioning
on response times (Figure 7). By waiting for some minutes after
the switching off of the activation potential of the device, the
response time increased again because of the thermal disorder
effect on the double layer generated by the voltage application
in a previous operation. In this thick, but still conductive, film,
the kinetics are slowed and a continuous decrease of open-circuit
current is observed after operation.
Ru(4-TBNMe). Solid devices have been prepared, and an
intensity of 0.1 µW/mm2 was obtained. With an applied potential
of 5 V, the emission spectrum was recorded using the previously
described CCD camera setup.4 The maximum was in the NIR
region at 780 nm. The solubility in acetonitrile is much lower
than that of Ru(4-TBN)Ru, and the active film was prepared
by drying a drop of a 0.4% w/v solution. The film is still
crystalline and is affected by pinholes. The response time was
Preliminary Solid Device Tests. Solid devices based on the
reference compound, Ru(bpy)32+, were first prepared using the
same method as reported earlier.4 If observed under an optical
microscope, the inorganic thin layer on ITO surface appears
yellow and homogeneous. The steady-state current was small
and stable up to 2.25 V suggesting the absence of pinholes and
the good quality of the coverage. The maximum light intensity
was estimated to be 1.75 µW/mm2 and was easily visible in
the daylight. The Ru(bpy)3(ClO4)2 solubility in acetonitrile is
about 40 mg/mL. For four out of the six complexes (excluding
Ru(4-TBN)Ru and Ru(4-TBN)Me), pinholes and short-circuit
effects could not be avoided and the emission was very weak
(2 orders of magnitude less intense) compared to Ru(bpy)2+
.
This is due to the low solubility in acetonitrile and to the
successive crystallization on the ITO surface that prevent the
formation of a homogeneous film. Changing the solvent (DCM,

22556 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Zanarini et al.
support from MIUR (PRIN 2004035330 & 2004030719),
University of Bologna, and the National Science Foundation
(CHE 0304925 & 0202136) are gratefully acknowledged.
Supporting Information Available: Experimental setup
used for the ECL experiments in solution, scheme for the
measurements performed on solid-state devices, and ECL spectra
of complexes Ru(4-TBN)Ru, Ru(BTB)Ru, and the reference
2+
species Ru(bpy)3 recorded under the same experimental
conditions of the mononuclear compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
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Figure 7. Light/current/time curves obtained applying 5 V switching
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about 10 times lower than for Ru(bpy)3 but faster than that
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for Ru(4-TBN)Ru. The film structure withstood, and the
emission continued to be observed, up to 10 V; the response
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A reversible memory effect in response times was again
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Concluding Remarks. This work shows the possibility of
altering the ECL emission wavelength by changing the groups
around the tetrazolate ligand. In general, for this class of
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shifted with respect to Ru(bpy)32+. In degassed acetonitrile
solutions, Ru(4-TBN)Ru showed a surprisingly intense ECL
emission, suggesting the use of Ru(4-TBN)Ru for the prepara-
tion of light-emitting solid devices, which show emission
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2+
intensity comparable to that of Ru(bpy)3 based devices. In
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phore for the design of NIR (780 nm) light-emitting solid
devices.
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excitation laser wavelength. The cell was air equilibrated.
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Acknowledgment. The authors would like to thank Francesco
Barigelletti for help in lifetime measurements and for useful
suggestions. The authors would also like to thank Jong Hyeok
Park and Cedric Hurth for useful discussions. The financial