Electrochemiluminescent functionalizable cyclometalated thiophene-based iridium(III) complexes

Article abstract of DOI:10.1021/ic901660m

A family of new functional bis-cyclometalated thiophene-based cationic iridium complexes have been prepared and fully characterized. The introduction of formyl groups into the thienyl-based cyclometalating ligand (thpy-CHO) allows one to perform further functionalizability and to confer to the whole species potentially interesting perspectives as functional materials. The X-ray crystal structures of three complexes, namely, [Ir(thpy)₂bpy]PF₆, [Ir(thpy-CHO)₂bpy]PF₆, and [Ir(thpy-CHO) ₂phen]PF₆, are reported. A rich reduction voltammetric pattern of the complexes is outlined, and the effect of the substituents on the cyclic voltammetric behavior is fully elucidated. Finally, the electrochemiluminescence spectra of all of the species have been obtained in acetonitrile by annihilation of the one-electron-oxidized and-reduced forms, showing very similar features with respect to the luminescence (as both the shape and energy of the emission bands).

Full text of DOI:10.1021/ic901660m

Inorg. Chem. 2010, 49, 1439–1448 1439
DOI: 10.1021/ic901660m
Electrochemiluminescent Functionalizable Cyclometalated Thiophene-Based
Iridium(III) Complexes
Marco Bandini,* Michele Bianchi, Giovanni Valenti, Fabio Piccinelli,† Francesco Paolucci, Magda Monari,
Achille Umani-Ronchi, and Massimo Marcaccio*
ꢀ
Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Universita di Bologna, via Selmi 2, 40126
Bologna, Italy. † Current address: Laboratory of Solid state Chemistry -DB-University of Verona, Strada le
ꢁ
Grazie 15, Ca Vignal 1, 37134, Verona, Italy.
Received August 19, 2009
A family of new functional bis-cyclometalated thiophene-based cationic iridium complexes have been prepared and fully
characterized. The introduction of formyl groups into the thienyl-based cyclometalating ligand (thpy-CHO) allows one to
perform further functionalizability and to confer to the whole species potentially interesting perspectives as functional
materials. The X-ray crystal structures of three complexes, namely, [Ir(thpy)2bpy]PF6, [Ir(thpy-CHO)2bpy]PF6, and
[Ir(thpy-CHO)2phen]PF6, are reported. A rich reduction voltammetric pattern of the complexes is outlined, and the effect
ofthesubstituents onthe cyclicvoltammetric behaviorisfullyelucidated. Finally, theelectrochemiluminescence spectra of
all of the species have been obtained in acetonitrile by annihilation of the one-electron-oxidized and -reduced forms,
showing very similar features with respect to the luminescence (as both the shape and energy of the emission bands).
Introduction
efficiency (QE), and short phosphorescence lifetimes make
these species valuable candidates for numerous technological
applications. Among them, full-display technology and light-
ing are of particular relevance.4 Depending on the nature
of the organic ligands, cyclometalated iridium complexes
can be divided into two groups: neutral homo- and hetero-
leptic complexes5 of the general formulas [Ir(C∧N)3] and
[Ir(C∧N)2(X∧X)] (X = O, N) and charged analogues of
the general formulas [Ir(C∧N)2(N∧N)]þ (cationic)6 and
After the pioneering work of Forrest, Thompson, and co-
workers1 over the past decade, many efforts have been
devoted toward the design, synthesis, and photophysical
investigation of new phosphorescent cyclometalated iridium-
(III) complexes.2 Solution processability, thermal stability,
color versatility (tuning over the whole visible range has been
covered by ligand modification),3 high external quantum
*To whom correspondence should be addressed. E-mail: marco.bandini@
unibo.it (M.B.), massimo.marcaccio@unibo.it (M.M.).
(1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
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S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003,
125, 12971. (b) Okada, S.; Okinaka, K.; Iwawaki, H.; Furugori, M.; Hoshimoto,
M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Tsuboyama, A.; Takiguchi, T.; Ueno, K.
J. Chem. Soc., Dalton. Trans. 2005, 1583. (c) Dedeian, K.; Shi, J.; Sheperd, N.;
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r
2010 American Chemical Society
Published on Web 01/14/2010
pubs.acs.org/IC

1440 Inorganic Chemistry, Vol. 49, No. 4, 2010
Bandini et al.
application as second-order nonlinear optical phosphors12
and photosensitizers for hydrogen production.13
A very important and potentially appealing applicative
field of charged iridium(III) cyclometalated polypyridine
complexes is represented by their use in biological tagging
for proteins, peptides, amino acids, antibodies, oligonucleo-
tides, and so on.14 This requires adequately functionalized
cyclometalating or ancillary ligands to tether the biomole-
cules, thus obtaining luminescent labeling reagents.
Although the chemical manipulation of preformed iridium
complexes has been described,15 the use of prefunctionalized
organic counterparts in the iridium-based complexes remains
the widely employed approach.16 In this context, the use of
formyl groups was dictated by the chemical feasibility of
further chemical manipulations and tethering.
Recently, there has been a “renaissance” of electrogener-
ated chemiluminescence as a powerful biodetection tool
because it brings all of the potentialities of the luminescence
techniques with an uncommon higher sensitivity and with-
out the need for use of an exciting light. A comprehens-
ive investigation on the electrochemiluminescence (ECL) of
functionalized cyclometalated iridium(III) complexes has not
been reported so far.17
Our ongoing interest in the development of unprecedented
“outer-sphere” metal complexes based on oligo- and poly-
thienyl ligands18,19 led us to consider 2-thienylpyridine (thpy)
as a basic structural motif for the cyclometalating ligands. In
fact, the unique properties of thienyl compounds to undergo
direct and selective chemical/electrochemical polymerization
would allow the efficient incorporation of iridium phosphors
into functional organic semiconductors.
In the present study, a series of cationic iridium complexes
have been prepared by using different combinations of bis-
pyridine neutral ligands (N∧N) and thiophene-based substi-
tuted cyclometalating (C∧N) ligands (Chart 1).20 A thorough
investigation of the structural, optical, and electrochemical
properties is also reported.
Figure 1. General structures of cyclometalated iridium(III) complexes.
[Ir(C∧N)2X2]- (anionic).7 The symbols C∧N and N∧N/O∧O
represent the cyclometalating and ancillary ligands, respec-
tively, as sketched in Figure 1.
Nowadays, numerous studies account for the remarkable
potentialities and attitude of the neutral iridium(III) com-
plexes as triplet emitters for electroluminescent devices. On
the contrary, ionic transition-metal complexes (iTMCs) have
received less attention even though some peculiar physical/
chemical properties should allow one to overcome a number
of drawbacks occurring with neutral species.8
With particular concern for the cationic bis-cyclometa-
lated iridium complexes, the possibility of assembling effi-
cient single-layer nonencapsulated devices with air-stable
electrodes has been envisaged. Operational simplicity com-
bined with low turn-on voltages (usually <3 V) led to the
production of light-emitting electrochemical cells (LECs)9
and polymerically analogous LECs10 with fine tuning of the
emission color from yellow to blue.11 Moreover, in the search
for new materials, charged iridium(III) complexes have found
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Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1441
Chart 1. Neutral and Cyclometalating Ligands Used in the Present
Investigation
4H), 7.60 (dd, J1 = 1.4 Hz, J2 = 7.4 Hz, 4H), 9.00 (d, J = 5.8 Hz,
4H).
[Ir(thpy-CHO)2Cl]2 (1b). Yield: 90% (137 mg). 1H NMR
(CD2Cl2, 300 MHz): δ 6.57 (s, 4H), 6.92 (t, J = 5.7 Hz, 4H),
7.80-7.92 (m, 8H), 9.15 (d, J = 6.0 Hz, 4H), 9.65 (s, 4H).
[Ir(thpy-Me)2Cl]2 (1c). Yield: 69% (120 mg). 1H NMR
(CDCl3, 300 MHz): δ 2.28 (s, 12H), 5.64 (br s, 4H), 6.55 (t,
J = 9.3 Hz, 4H), 7.37 (d, J = 12.0 Hz, 4H), 7.53-7.61 (m, 4H),
8.90 (d, J = 7.8 Hz, 4H).
To a solution of 1 (1 equiv) in 20 mL of a degassed CH2Cl2/
MeOH (1:1) mixture was added all at once 2 equiv of a bis-imine
ligand (N∧N), and the mixture was stirred in the dark for 4 h at
reflux. The red solution was then cooled to room temperature,
anhydrous NH4PF6 was added (4 equiv), and the mixture was
stirred overnight in the dark at room temperature. After eva-
poration of the solvent under reduced pressure, 25 mL of H2O
was added with the aim of removing excess NH4PF6. The solid
obtained was washed with diethyl ether (20 mL) and dried under
vacuum.
[Ir(thpy)2bpy]PF6, starting from 0.067 mmol of 1a. Yield:
77% (82 mg). MS-ESI: m/z 669 ([Ir(thpy)2bpy]þ). 1H NMR
(CD2Cl2, 200 MHz): δ6.37 (d, J= 4.4 Hz, 2H), 6.83 (t, J=6.4Hz,
2H), 7.40 (d, J = 5.8 Hz, 2H), 7.46 (d, J = 4.8 Hz, 2H), 7.53 (d,
J = 5.4 Hz, 2H), 7.59 (d, J = 6.2 Hz, 2H), 7.72-7.77 (m, 2H),
7.98 (d, J = 4.4 Hz, 2H), 8.14 (t, J = 7.6 Hz, 2H), 8.49 (d, J =
8.4 Hz, 2H). Elem anal. Calcd for C28H20F6IrN4PS2: C, 41.32; H,
2.48; N, 6.88. Found: C, 41.40; H, 2.44; N, 6.95.
[Ir(thpy-CHO)2bpy]PF6, starting from 0.074 mmol of 1b.
Yield: 81% (104 mg). MS-ESI: m/z 725 ([Ir(thpy-CHO)2bpy]þ).
1H NMR (CDCl3, 300 MHz): δ 6.87 (s, 2H), 7.04 (dt, J1 =
2.1 Hz, J2 = 6.6 Hz, 2H), 7.50-7.58 (m, 4H), 7.77-7.83 (m,
4H), 8.04 (d, J = 5.4 Hz, 2H), 8.21 (t, J = 8.1 Hz, 2H), 8.55 (d,
J = 8.4 Hz, 2H), 9.84 (s, 2H). Elem anal. Calcd for C30H20F6Ir-
N4O2PS2: C, 41.42; H, 2.32; N, 6.44. Found: C, 41.51; H, 2.22;
N, 6.55.
[Ir(thpy-Me)2bpy]PF6, starting from 0.064 mmol of 1c. Yield:
50%. MS-ESI: m/z 637 ([Ir(thpy-Me)2bpy]þ). 1H NMR (CDCl3,
300 MHz): δ 2.49 (s, 6H), 5.96 (s, 2H), 6.73-6.77 (m, 2H), 7.30
(d, J = 4.2 Hz, 2H), 7.39 (d, J = 8.1 Hz, 2H) 7.49-7.51 (m, 2H),
7.54-7.61 (m, 2H), 7.95-7.97 (m, 2H), 8.15 (t, J = 8.1 Hz, 2H),
8.64 (d, J = 8.1 Hz, 2H). Elem anal. Calcd for C30H24F6Ir-
N4PS2: C, 40.82; H, 2.87; N, 6.66. Found: C, 40.75; H, 2.78;
N, 6.70.
the solvent resonance as the internal standard (deutero-
chloroform: δ 7.27 ppm). Data are reported as follows: chemical
shift, multiplicity (s = singlet, d = doublet, pd = pseudodoub-
let, t = triplet, q = quartet, br = broad, br s = broad singlet, m =
multiplet), coupling constants (Hz). Gas chromatography-
mass spectrometry (GC-MS) spectra were taken by electron
impact ionization at 70 eV on a Hewlett-Packard 5971 with GC
injection. They are reported as m/z (relative intensity). Liquid
chromatography-electrospray ionization mass spectrometry
(MS-ESI) spectra were obtained with an Agilent Technologies
MSD1100 single-quadrupole mass spectrometer. Chromato-
graphic purification was done with 240-400 mesh silica gel.
IR analysis were performed with a FT-IR Nicolet 205 spectro-
photometer, and the spectra are expressed by wavenumber
(cm-1). Elemental analyses were carried out by using a EACE
1110 CHNOS analyzer. Melting points were determined with a
€
Buchi 150 melting point unit and are not corrected.
Materials. All anhydrous solvents were supplied by Fluka
in Sureseal bottles and used as received. 2-Ethoxyethanol was
distilled from MgSO4. IrCl3 3H2O was supplied by Strem. 5-(2-
3
Pyridyl)thiophene-2-carbaldehyde (thpy-CHO) was synthesized
by following a known protocol.21 5-(2-Pyridyl)-2-methylthio-
phene (thpy-Me) was synthesized via a variant of ref 21: thpy
(1 equiv), n-BuLi (1.1 equiv), MeI (1.1 equiv). Yield: 30% (not
1
optimized). H NMR (CDCl3, 300 MHz): δ 2.55 (s, 3H), 6.79
(s, 1H), 7.10-7.13 (m, 1H), 7.28 (d, J = 3.3 Hz, 1H), 7.38-7.42
(m, 1H), 7.60-7.67 (m, 1H), 8.55 (s, 1H). GC-MS: m/z 51 (10),
78 (9), 97 (12), 130 (19), 141 (17), 175 (100, M). dmbpy = 5,50-
dimethyl-2,20-dipyridyl, thpy = 2-thienylpyridine, phen =
1,10-phenanthroline, and 2,20-bipyridine (bpy) were procured
from commercial sources and used without further purification.
Synthesis of the Cationic Cyclometalated Iridium(III) Com-
plexes.22 To a degassed ethoxyethanol/water mixture (3:1, 4 mL)
[Ir(thpy-CHO)2dmbpy]PF6, starting from 0.020 mmol of 1b.
Yield: 67% (24 mg). MS-ESI: m/z753 ([Ir(thpy-CHO)2-
1
dmbpy]þ). H NMR (CDCl3, 300 MHz): δ 2.32 (s, 6H), 6.85
(s, 2H), 7.17 (br s, 2H), 7.57 (br s, 2H), 7.65 (s, 2H), 7.79-7.84
(m, 4H), 7.95-7.99 (m, 2H), 8.50-8.53 (m, 2H), 9.83 (s, 2H).
Elem anal. Calcd for C32H24F6IrN4O2PS2: C, 41.81; H, 2.69; N,
6.24. Found: C, 41.91; H, 2.56; N, 6.33.
[Ir(thpy-CHO)2phen]PF6, starting from 0.076 mmol of 1b.
Yield: 67%. MS-ESI: m/z 748 ([Ir(thpy-CHO)2phen]þ), 145
([PF6]-). 1H NMR (CD3CN, 200 MHz): δ 6.87 (dt, J1 = 1.4 Hz,
J2 = 7.6 Hz, 2H), 7.10 (s, 2H), 7.42 (d, J = 5.8 Hz, 2H),
7.65-7.92 (m, 6H), 8.25 (s, 2H), 8.35 (dd, J1 = 1.2 Hz, J2 =
5.0 Hz, 2H), 8.70 (dd, J1 = 1.2 Hz, J2 = 8.6 Hz, 2H), 9.85 (s,
2H). Elem anal. Calcd for C32H20F6IrN4O2PS2: C, 43.00; H,
2.26; N, 6.27. Found: C, 42.90; H, 2.31; N, 6.35.
were added 100 mg (0.28 mmol) of IrCl3 3H2O and 0.70 mmol
3
(2.5 equiv) of the desired carbometalable ligand (thpy, thpy-CHO,
or thpy-Me). The mixture was refluxed overnight (∼16 h) and then
cooled down. The volatiles were removed under reduced pressure,
and the deep-red solid was recovered by centrifugation. The
resulting solid was washed with water (20 mL) and diethyl ether
(20 mL) and recovered by centrifugation. The resulting deep-red
solid was dissolved in CH2Cl2. Traces of insoluble material were
removed by filtration. Evaporation of the solvent led to a red-
orange solid, which was used without any further purification.
[Ir(thpy)2Cl]2 (1a).23 Yield: 68% (115 mg). 1H NMR (CDCl3,
200 MHz): δ 5.92 (d, J = 5.2 Hz, 4H), 6.62 (dt, J1 = 1.4 Hz,
J2 = 7.4 Hz, 4H), 7.06 (d, J = 5.2 Hz, 4H), 7.50 (d, J = 7.0 Hz,
Crystal Data. The diffraction experiments were carried out at
room temperature on a Bruker SMART Apex II CCD-based
diffractometer using graphite-monochromated Mo KR radia-
˚
tion (λ = 0.710 73 A). Intensity data were measured over the full
diffraction sphere using 0.3° wide ω scans. The software
SMART24 was used for collecting frames of data, indexing
reflections, and determining the lattice parameters. The collected
(21) Nerenz, H.; Meier, M.; Grahn, W.; Reisner, A.; Schmaelzlin, E.;
€
Stadler, S.; Meerholz, K.; Brauchle, C.; Jones, P. G. J. Chem. Soc., Perkin
Trans. 2 1998, 437–448.
(24) SMART & SAINT Software Reference Manuals (Windows NT
Version), version 5.051; Bruker Analytical X-ray Instruments Inc.: Madison,
WI, 1998.
(22) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767.
€
(23) (a) Colombo, M. G.; Gudel, H. U. Inorg. Chem. 1993, 32, 3081.
(b) McGee, K. A.; Mann, K. R. Inorg. Chem. 2007, 46, 7800.

1442 Inorganic Chemistry, Vol. 49, No. 4, 2010
Bandini et al.
frames were then processed for integration by software
SAINT,24 and an empirical absorption correction was applied
with SADABS.25 The structures were solved by direct methods
(SIR97)26 and subsequent Fourier syntheses and refined by full-
matrix least-squares calculations on F2 (SHELXTL),27 attribut-
ing anisotropic thermal parameters to all non-hydrogen atoms.
The aldehydic and aromatic hydrogen atoms that were located
on the Fourier difference map were placed in calculated posi-
tions and refined with idealized geometry with Uiso(H) =
1.2Ueq(C). The complex [Ir(thpy)2bpy]PF6 crystallized with
one-third of an ordered water molecule and a lattice solvent
(presumably MeOH) could not be modeled reasonably, so the
SQUEEZE routine found in PLATON was used. [Ir(thpy-
CHO)2bpy]PF6 and [Ir(thpy-CHO)2phen]PF6 crystallized with
solvent molecules (dichloromethane, DCM). Crystal data and
experimental details are reported in Table S1 (Supporting
Information).
solvent was typically (1-2) ꢀ 10-5 mbar. Voltammograms were
recorded with an AMEL model 552 potentiostat or a custom-
made fast potentiostat30 controlled by an AMEL model 568
programmable function generator. The potentiostat was inter-
faced to a Nicolet model 3091 digital oscilloscope, and the data
were transferred to a personal computer by the program Anti-
gona.31 Minimization of the uncompensated resistance effect in
the voltammetric measurements was achieved by the positive-
feedback circuit of the potentiostat. Digital simulations of the
CV curves were carried out by either Antigona or DigiSim 3.0.
Determination of the potentials, for the irreversible processes,
was obtained by digital simulation of the CV curves,32 utilizing a
best-fitting procedure of the experimental curves recorded at
different scan rates spanning over, at least, 2 orders of magni-
tude.
Absorption and Emission Spectroscopy and ECL. UV-visible
absorption spectra were recorded on a UV-vis-near-IR spec-
trophotometer (Varian CARY5). Photoluminescence (PL) mea-
surements were performed with a Varian (Cary Eclipse)
spectrofluorimeter. The absorbance and luminescence spectra
of the complexes were obtained in ACN solutions. The relative
luminescence efficiency was measured using an optically diluted
ACN solution (OD < 0.1 at the excitation wavelength) and
CCDC-737663-CCDC-737665 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Electrochemical Measurements. Tetrabutylammonium hexa-
fluorophosphate (TBAH; electrochemical grade from Fluka)
was used as received as a supporting electrolyte. Acetonitrile
(ACN), DCM, and tetrahydrofuran (THF) were purified and
dried as previously reported,28 stored in a specially designed
Schlenk flask, and protected from light.
2þ
calibrated with Ru(bpy)3 as a standard according to the
procedure reported elsewhere.33
The ECL measurements were carried out in an ACN solution
with TBAH as the supporting electrolyte, under the same strictly
aprotic conditions as those described above, in the Electro-
chemical Measurements section. A one-compartment three-
electrode airtight cell, with high-vacuum O-rings and glass
stopcocks, was used for ECL measurements.34 The working
electrode consisted of a platinum side-oriented, 2-mm-diameter
disk sealed in glass while the counter electrode was a platinum
spiral and the reference electrode was a quasi-reference silver
wire.
All of the solvents were distilled via a closed system into an
electrochemical cell, containing the supporting electrolyte and
the species under examination, immediately before the experi-
ment was performed. Electrochemical experiments were carried
out in an airtight single-compartment cell described elsewhere,28
using platinum as the working and counter electrodes and a
silver spiral as a quasi-reference electrode. The drift of the quasi-
reference electrode was negligible during the time required for
an experiment. All of the E1/2 potentials have been directly
obtained from cyclic voltammetric (CV) curves as averages of
the cathodic and anodic peak potentials and by digital simula-
tion for those processes, which are not Nernstian, or for
processes closely spaced in multielectron voltammetric peaks.
The E1/2 values, referenced to the saturated calomel electrode
(SCE), have been determined by adding ferrocene at the end of
each experiment as an internal standard and measuring them
with respect to the ferrocinium/ferrocene couple standard
potential. The temperature-dependent ferrocinium/ferrocene
couple standard potential was measured with respect to SCE
by a nonisothermal arrangement according to the method out-
lined by Weaver et al.29 The potentials thus obtained were not
corrected for the unknown liquid junction potential between the
organic phase and the aqueous SCE solution.
For a given solution, two or three records were made to check
the temporal stability of the system investigated.
The annihilation reaction was obtained by pulsing the work-
ing electrode between the first oxidation and first reduction peak
potentials of the complex with a pulse width of 0.1 s. The ECL
signal generated by the potential step program was measured
with a photomultiplier tube (PMT; Hamamatsu R4220p) placed
a few millimeters from the cell, and in front of the working
electrode, inside a darkbox. A voltage in the range of 750-1000 V
was supplied to the PMT. The light/current/voltage curves were
recorded by collection of the preamplified PMT output signal
(using an ultralow-noise Acton Research model 181), with the
second input channel of the ADC module of the Autolab
instrument, without any correction for the PMT response at
different wavelengths. ECL spectra have been recorded by
insertion of the same PMT in a dual-exit monocromator
(Acton Research model Spectra Pro2300i) and collection of
the signal as described above. The photocurrent detected at the
PMT was accumulated for 1-3 s, depending on the emission
intensity, for each monocromator wavelength step (usually
1 nm). Entrance and exit slits were fixed to the maximum value
of 3 mm.
The cell containing the supporting electrolyte and the electro-
active compound was dried under a vacuum at 100-120 °C for
at least 60 h before each experiment. The pressure measured in
the electrochemical cell prior to trap-to-trap distillation of the
(25) Sheldrick, G. M. SADABS, program for empirical absorption correction;
€
€
University of Gottingen: Gottingen, Germany, 1996.
(26) Altomare, A.; Burla, M. C.; Cavalli, M.; Cascarano, G. L.; Giacovazzo,
C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.
J. Appl. Crystallogr. 1999, 32, 115.
(30) Amatore, C.; Lefrou, C. J. Electroanal. Chem. 1992, 324, 33.
(31) Mottier, L. Antigona; University of Bologna: Bologna, Italy, 1999.
(32) Spieser, B. In Electroanalytical Chemistry. A Series of Advances;
Bard, A. J., Rubistein, I., Eds.; Marcel Dekker, Inc.: New York, 1996; Vol. 19, p 1
and references cited therein.
(33) Stagni, S.; Colella, S.; Palazzi, A.; Valenti, G.; Zacchini, S.; Paolucci,
F.; Marcaccio, M.; Albuquerque, R. Q.; De Cola, L. Inorg. Chem. 2008, 47,
10509.
(34) (a) Zanarini, S.; Della Ciana, L.; Marcaccio, M.; Marzocchi, E.;
Paolucci, F.; Prodi, L. J. Phys. Chem. B 2008, 112, 10188. (b) Stagni, S.;
Palazzi, A.; Zacchini, S.; Ballarin, B.; Bruno, C.; Marcaccio, M.; Paolucci, F.;
Monari, M.; Carano, M.; Bard, A. J. Inorg. Chem. 2006, 45, 695.
(27) Sheldrick, G. M. SHELXTLplus (Windows NT version) Structure
Determination Package, version 5.1; Bruker Analytical X-ray Instruments Inc.:
Madison, WI, 1998.
(28) (a) Bruno, C.; Benassi, R.; Passalacqua, A.; Paolucci, F.; Fontanesi,
C.; Marcaccio, M.; Jackson, E. A.; Scott, L. T. J. Phys. Chem. B 2009, 113,
ꢁ
1954. (b) La Pensee, A. A.; Bickley, J.; Higgins, S. J.; Marcaccio, M.; Paolucci, F.;
Roffia, S.; Charnock, J. M. J. Chem. Soc., Dalton Trans. 2002, 4095. (c) Guldi,
D. M.; Maggini, M.; Menna, E.; Scorrano, G.; Ceroni, P.; Marcaccio, M.;
Paolucci, F.; Roffia, S. Chem.;Eur. J. 2001, 7, 1597.
(29) Yee, E. L.; Cave, R. J.; Guyer, K. L.; Tyma, P. D.; Weaver, M. J.
J. Am. Chem. Soc. 1979, 101, 1131.

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1443
ECL yields were determined against the annihilation reaction
Scheme 1. Synthesis of the μ-Chloro-Bridged Dimers 1a-1c
of a standard ECL system [i.e., Ru(bpy)32þ] in an ACN solution
containing TBAH. The ECL yields were obtained by a chron-
oamperometric experiment using the following expression:35
ΦECL ¼ ΦECL°ðIQ°=I°QÞ
where ΦECL° is the ECL efficiency of the standard under the
same experimental conditions (ΦECL° = 0.05),36 I and I° are the
integrated ECL intensities of the compound and standard
systems, and Q and Q° are the charges (in Coulombs) passed
for the complex and standard species, respectively. It has been
estimated that the ECL efficiency can be confidently given with
an error of 15%.
Scheme 2. Synthesis of the Cationic Cyclometalated Iridium(III)
Complexes
Results and Discussion
The development of newfunctionalized bis-cyclometalated
iridium(III) complexes is currently of considerable interest.
Many advances in this endeavor have focused on the intro-
duction of binding sites on the ancillary neutral ligands.6a,37
However, the search for unprecedented tailor-designed func-
tionalization of cyclometalating ligands that allow increas-
ingly rapid access to structural complexity remains a
preeminent goal.16 Finally, although the emission color of
such a class of iridium complexes is generally governed by the
cyclometalating ligand, also neutral bis-pyridine (ancillary)
ligands have been demonstrated to actually affect both the
quantum yield and emission color tunability of iTMCs.38
Therefore, for a systematic investigation, several bis-imine
type ligands (e.g. bpy, dmbpy, and phen) were taken into
account. Electron-withdrawing formyl groups are known to
strongly affect all of the electronic properties of cationic
mixed-ligand iridium complexes [(C∧N)2Ir(N∧N)]þ, and to
adequately assess such a role, unsubstituted (thpy) and
complementary (thpy-Me) ligands were also considered.
The target iridium μ-chloro-bridged dimers of the general
formula (C∧N)2Ir(μ-Cl)2Ir(C∧N)2 were obtained in good-to-
excellent yields (68-90%) via a known protocol that involves
temperature. In all three structures, iridium(III) adopts a
distorted octahedral geometry, with the C and N atoms
belonging to cyclometalated ligands in cis and trans config-
urations, respectively (Figure 2).
The nitrogen atoms of the bis-imine ligands are in trans to
the carbon ones, and the Ir-N(bis-imine) bond lengths fall in
˚
the range of 2.114-2.141 A (Table 1). These distances are
significantly longer than the Ir-N(thpy) bond lengths (range
˚
2.031-2.072 A) in mutual trans positions, in agreement with
what is usually observed for related compounds.39 This
behavior is ascribable to the stronger trans influence of the
sp2 σ-donor carbon with respect to the sp2 nitrogen of
unsaturated R,R0-bis-imine (bpy and phen).
˚
The Ir-C bond distances (1.995-2.018 A) are within the
interval observed for the corresponding distance in iridium-
(III) C∧N chelates with similar geometry and ligand environ-
ments.40 The bite angles of the thpy ligands (79.7-80.2°) in
[Ir(thpy)2bpy]PF6, [Ir(thpy-CHO)2bpy]PF6, and [Ir(thpy-
CHO)2phen]PF6 lie in the same range as that observed in
the literature for similar complexes. The bite angles of the bis-
imine ligands (range 76.9-78.3°) are in the same range as that
reported earlier for IrIIIbpy fragments.41
the treatment of IrCl3 3H2O with an excess of the desired
3
cyclometalating ligand (thpy, thpy-CHO, and thpy-Me) in a
solvent mixture of ethoxyethanol/water (Scheme 1).22
Finally, the cationic bis-cyclometalated complexes were
obtained by reaction of the dimers (1) with the bis-imine
ligands (N∧N) in a CH2Cl2/MeOH mixture, followed by a
metathesis reaction with NH4PF6 (Scheme 2). The deep-red
air-stable complexes were isolated in moderate-to-good
yields (50-81%) and were fully characterized by HPLC-MS
and 1H NMR analysis. High-purity samples of complexes for
electrochemical, spectroscopic, and luminescence investiga-
tions were obtained by crystallization.
It is worth mentioning that incorporation of the aldehyde
function in the 50 position of the thpy ligand does not change
significantly the bond lengths involving the metal center. On
the contrary, coordination of the phen ring, with respect to
the bpy one, leads to a slight shortening of the Ir-N(thpy)
˚
bond [2.031(16) A]. In addition, in the crystal structures of
Crystals suitable for X-ray diffraction experiments have
been obtained for [Ir(thpy)2bpy]PF6, [Ir(thpy-CHO)2b-
py]PF6, and [Ir(thpy-CHO)2phen]PF6 by the slow diffusion
of hexane into CH2Cl2 solutions of the complexes at room
[Ir(thpy-CHO)2bpy]PF6 and [Ir(thpy-CHO)2phen]PF6, the
CHO functions are coplanar with the thpy-conjugated system
with a cis arrangement of O1 with respect to S1.
€
(39) (a) van Diemen, J. H.; Haasnoot, J. G.; Hage, R.; Muller, E.; Reedijk,
J. Inorg. Chim. Acta 1991, 181, 245. (b) Garces, F. O.; Dedeian, K.; Keder, N. L.;
Watts, R. J. Acta Crystallogr. 1993, C49, 1117. (c) Nord, G.; Hazell, A. C.;
Hazell, R. G.; Farver, O. Inorg. Chem. 1983, 22, 3429. (d) Urban, R.; Kramer, R.;
Shahram, M.; Polborn, K.; Wagner, B.; Beck, W. J. Organomet. Chem. 1996,
517, 191.
(35) Electrogenerated Chemiluminescence; Bard, A. J., Ed.; Marcel Dekker:
New York, 2004.
(36) Rubinstein, L.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 6641.
(37) (a) Neve, F.; Crispini, A.; Campagna, S.; Serroni, S. Inorg. Chem.
1999, 38, 2250. (b) Neve, F.; Crispini, A. Eur. J. Inorg. Chem. 2000, 1039.
(c) Plummer, E. A.; Hofstraat, J. W.; De Cola, L. J. Chem. Soc., Dalton Trans.
2003, 2080. (d) Neve, F.; La Deda, M.; Crispini, A.; Bellusci, A.; Puntoriero, F.;
Campagna, S. Organometallics 2004, 23, 5856.
(38) Dragonetti, C.; Falciola, L.; Mussini, P.; Righetto, S.; Roberto, D.;
Ugo, R.; Valore, A.; De Angelis, F.; Fantacci, S.; Sgamellotti, A.; Ramon,
M.; Muccini, M. Inorg. Chem. 2007, 46, 8533.
€
(40) Colombo, M.-G.; Brunold, T.-C.; Riedner, T.; Gudel, H. U.;
€
€
Fortsch, M.; Burgi, H.-B. Inorg. Chem. 1994, 33, 545.
(41) (a) Hazell, A. C.; Hazell, R. G. Acta Crystallogr. 1984, C40, 806.
(b) Alexander, B. D.; Johnson, B. J.; Johnson, S. M.; Boyle, P. D.; Kann, N. C.;
Mueting, A. M.; Pignolet, L. H. Inorg. Chem. 1987, 26, 3506. (c) Albano, V. G.;
Bellon, P. L.; Sansoni, M. Inorg. Chem. 1969, 8, 298.

1444 Inorganic Chemistry, Vol. 49, No. 4, 2010
Bandini et al.
Figure 2. Crystal structures of the cations of [Ir(thpy)2bpy]PF6, [Ir(thpy-CHO)2bpy]PF6, and [Ir(thpy-CHO)2phen]PF6. The PF6 anion and the
crystallization solvents are omitted for clarity. The primed atoms in [Ir(thpy-CHO)2phen]PF6 are generated by a symmetry operation (symmetry
1
3
code: I, -x þ /2, y, -z þ /2).
a
˚
Table 1. Selected Bond Lengths (A) and Angles (deg) for [Ir(thpy)2bpy]PF6, [Ir(thpy-CHO)2bpy]PF6, and [Ir(thpy-CHO)2phen]PF6
[Ir(thpy)2bpy]PF6
[Ir(thpy-CHO)2bpy]PF6
[Ir(thpy-CHO)2phen]PF6
Ir1-N1
Ir1-C2
Ir1-N2
Ir1-C11
Ir1-N3
Ir1-N4
2.072(4)
1.998(5)
2.063(4)
2.006(5)
2.124(4)
2.114(4)
2.066(2)
1.995(3)
2.064(2)
2.002(3)
2.128(3)
2.116(2)
Ir1-N1
Ir1-C2
Ir1-N3
2.031(16)
2.018(19)
2.141(14)
N1-Ir1-C2
N2-Ir1-C11
N3-Ir1-N4
N1-Ir1-N2
N3-Ir1-C2
N4-Ir1-C11
80.2(2)
79.9(2)
77.2(2)
170.6(2)
171.4(2)
173.0(2)
80.00(11)
80.19(11)
76.85(9)
169.69(9)
173.44(10)
173.99(11)
N1-Ir1-C2a
N3-Ir1-N3a
N1-Ir1-N1a
N3-Ir1-C2a
79.7(6)
78.3(9)
172.9(8)
173.0(6)
1
3
a Symmetry transformations used to generate equivalent atoms: -x þ /2, y, z þ /2.
Electronic Absorption Spectroscopy and Luminescence
Properties. The absorption spectra of all complexes were
recorded at room temperature in an ACN solution and
are reported in Figure 3. The UV region displays the
species, the vibronic structure, although still present, is
less evident. According to the published works concern-
ing analogous iridium(III) cyclometalates,43,44 the occur-
rence of a similar emission shape is attributed to the
presence of an excited state resulting from the mixing of
1
ligand-centered (LC) π f π* transitions as the most
3
3
intense absorption bands, for each species (Table 2). The
shoulders appearing in the range of 350-450 nm are likely
due to charge-transfer (CT) transitions, with their nature
being both spin-allowed (singlet-to-singlet metal-to-
ligand charge transfer, 1MLCT) and spin-forbidden
(singlet-to-triplet metal-to-ligand charge transfer, 3MLCT).
The relatively high intensity of these latter bands can be
rationalized by the strong spin-orbit coupling effect,
arising from the presence of a third-row transition metal
in a high oxidation state such as iridium(III).42
All of the complexes are luminescent at room tempera-
ture in an ACN solution, with emission maxima of
around 600 nm. In particular, among the series represented
by the three aldehyde-containing compounds, e.g., [Ir-
(thpy-CHO)2bpy]þ, [Ir(thpy-CHO)2dmbpy]þ, and [Ir-
(thpy-CHO)2phen]þ, the PL spectra display vibronic-
structured emission bands, whereas for the remaining two
comparable percentages of LC and MLCT states. Si-
milar results were very recently obtained for a series of
iridium(III) tetrazolate species,33 for which enhancement
of the 3LC-3MLCT energy gap, due to replacement of a
more electron-withdrawing ligand, makes the emission a
structureless band, consistent with the prevalent 3MLCT
character of the excited state.
In a degassed ACN solution, the complexes reported
here show emission quantum yields (Φ) ranging from
1.8%, as in the case of [Ir(thpy-CHO)2bpy]þ, to 0.5% for
the thpy complexes. The appreciable decrease of the
quantum yield values, which is observed in air-equili-
brated solutions, provides confirmation of the triplet
character of the emissions. The introduction of the formyl
group onto the thpy ligands stabilizes the occupied and
virtual molecular orbitals with a consequent red shift of
the emission band. Interestingly, the quantum yield of PL
is much lower because the emission occurs at lower wave-
length (i.e., [Ir(thpy)2bpy]þ and [Ir(thpy-Me)2bpy]þ),
according to the energy gap law.43
€
(42) Colombo, M. G.; Hauser, A.; Gudel, H. U. Top. Curr. Chem. 1994,
171, 143.
(43) (a) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti,
F. Top. Curr. Chem. 2007, 281, 143 and references cited therein. See also:
(b) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni, L.; Encinas, S.;
Barigelletti, F. Chem. Soc. Rev. 2000, 29, 385.
Electrochemical Behavior. The electrochemical proper-
ties of the cyclometalated complexes were investigated in
ACN, DCM, and THF solutions by cyclic voltammetry,
at room temperature. As previously reported33,38,45 and
(44) (a) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.;
Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada,
S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (b) Orselli, E.;
€
Kottas, G. S.; Konradsson, A. E.; Coppo, P.; Frohlich, R.; De Cola, L.; van Dijken,
(45) (a) Nazeeruddin, Md. K.; Wegh, R. T.; Zhou, Z.; Klein, C.; Wang,
€
€
Q.; De Angelis, F.; Fantacci, S.; Gratzel, M. Inorg. Chem. 2006, 45, 9245.
A.; B€uchel, M.; Borner, H. Inorg. Chem. 2007, 46, 11082.

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1445
Figure 4. CVcurvesofa1mMsolutionofcomplexes(a) [Ir(thpy)2bpy]þ
and (b) [Ir(thpy-Me)2bpy]þ in a 0.07 M TBAH/THF electrolyte
(working electrode, platinum disk, 125 μm diameter; T = 25 °C; scan
rate = 1 V/s).
Figure 3. Absorption and emission spectra of the complexes in an ACN
solution (0.2 mM) at 25 °C. (a) Absorption spectra: 1 (black line),
[Ir(thpy-CHO)2bpy]þ; 2 (red line), [Ir(thpy)2bpy]þ; 3 (blue line), [Ir-
observed. While the first three peaks correspond to
reversible one-electron reductions, the fourth is irrever-
sible likely because of a follow-up reaction coupled to the
reduction process. The introduction of the methyl group
on the cyclometalating ligand (e.g., 2-(5-methylthiophen-
2yl)pyridine) ꢁ thpy-Me; see the species [Ir(thpy-Me)2-
bpy]þ) changed the CV pattern of the corresponding
iridium species only slightly (Figure 4b). In particular,
while the first reduction peak remained virtually unaf-
fected by the substitution, a negative shift (by 20 and
60 mV, respectively) of the second and third reductions
was observed (Table 3). This would suggest that the first
reduction is localized into the ancillary ligand (bpy) and
the second and third ones into the cyclometalating ligand.
The fourth cathodic peak would then correspond to the
second reduction of the bpy ligand, a process that is
known to often trigger follow-up reactions that would
explain the irreversible character of this peak.46
(thpy-Me)2bpy]þ. (b) Emission spectra:
1 (black line), [Ir(thpy-
CHO)2bpy]þ; 2 (green line), [Ir(thpy-CHO)2phen]þ; 3 (orange line),
[Ir(thpy-CHO)2dmbpy]þ; 4 (red line), [Ir(thpy)2bpy]þ; 5 (blue line), [Ir-
(thpy-Me)2bpy]þ.
Table 2. Adsorption and Emission Data for the Iridium Complexes Investigated
in an ACN Solution at 25 °C
absorption
abs (nm)
emission
λem (nm)
ΦPh
species
λ
(%)a
[Ir(thpy-CHO)2bpy]PF6
[Ir(thpy)2bpy]PF6
[Ir(thpy-
282, 309, 433
275, 309, 408
276, 312, 438
633
612
636
1.8
0.9
1.1
CHO)2dmbpy]PF6
[Ir(thpy-CHO)2phen]PF6
[Ir(thpy-Me)2bpy]PF6
273, 309, 436
280, 308, 410
633
623
1.8
0.5
a QE of degassed solutions.
also shown by the density functional theory (DFT)
calculations (vide infra), the electronic structures of these
iridium species are characterized by a delocalization of
the frontier molecular orbitals over both the metal and
ligands. Accordingly, the oxidation of this class of
iridium(III) species can be mainly attributed to the metal
center, with a substantial contribution from the ligands,
while, conversely, the reduction processes are mainly
localized on the ligands, with only a partial contribution
from the metal center. The potentials of the various
processes are collected in Table 3. In the region of positive
potential, explored up to about 1.5 V, all of the complexes
exhibited a single one-electron reversible oxidation pro-
cess. Conversely, in the negative potential region, all of
the complexes display a rich voltammetric pattern largely
associated with the reduction of both the cyclometalating
ligand and the ancillary ligands. As an example, the
CV curve of the complex [Ir(thpy)2bpy]þ is shown in
Figure 4a, where four subsequent reduction peaks are
The above attribution of reduction processes was con-
firmed by replacement of the (weak) electron-donating
methyl group by a strong electron acceptor, namely,
aldehyde. Figure 5a shows the CV curve of complex
[Ir(thpy-CHO)2bpy]þ. The first reduction is still occur-
ring at about the same potential, although slightly less
negative (-1.1 V), as a consequence of the second-order
effect of the -CHO substituents on the thpy ligands.
Then, two one-electron closely spaced processes follow at
about -1.5 V, and further three CV peaks can be ob-
served in the last part of the CV curve, before solvent/
electrolyte breakthrough. The two overlapping reduction
peaks (i.e., the second and third processes at about -1.5 V)
can be confidently attributed as the first process onto the
ꢂ
ꢂ
(46) (a) Krejcik, M.; Vlcek, A. A. J. Electroanal. Chem. 1991, 313, 243.
(b) Roffia, S.; Marcaccio, M.; Paradisi, C.; Paolucci, F.; Balzani, V.; Denti, G.;
Serroni, S.; Campagna, S. Inorg. Chem. 1993, 32, 3003. (c) Marcaccio, M.;
Paolucci, F.; Fontanesi, C.; Fioravanti, G.; Zanarini, S. Inorg. Chim. Acta 2007,
360, 1154.

1446 Inorganic Chemistry, Vol. 49, No. 4, 2010
Bandini et al.
Table 3. Half-Wave (E1/2) Redox Potentials (vs SCE) of Uncoordinated Ligands and Iridium Complexes at 25 °C
E1/2(red) (V)b
species
E1/2(ox) (V)a
I
II
III
IV
V
VI
thpy
thpy-CHO
-2.27
-1.48
-1.14
-1.19
-1.25
-1.17
-1.19
-2.86c
-2.01
-1.40
-2.08
-1.40
-1.41
-2.10
[Ir(thpy-CHO)2bpy]þ
[Ir(thpy)2bpy]þ
1.42
1.20
1.40
1.41
1.07
-1.49
-2.35
-1.60
-1.53
-2.41
-1.99
-2.53c
-2.05
-2.02
-2.64c
-2.21
-2.57
[Ir(thpy-CHO)2dmbpy]þ
[Ir(thpy-CHO)2phen]þ
[Ir(thpy-Me)2bpy]þ
-2.30
-2.26
-2.68
-2.51
a Oxidation potentials measured in a DCM solution. b Reduction processes measured in a THF solution. c Irreversible process whose E1/2 potential has
been determined by digital simulation of the CV curve.
first reduction moves toward more negative potentials and
the following two closely spaced processes are split into two
well-defined CV peaks. Again, a different electronic mod-
ulation of the ancillary ligand by replacement of bpy with
another one, in this case the ligand phen, reproduces a CV
pattern very similar to that of the parent species.
The attribution, discussed above, of the processes as
mainly centered on the different ligands for the various
complexes is sketched in the diagram of Figure S1
(Supporting Information), where the reduction processes
of the various complexes are related to each other as well
as to those of the uncoordinated ligands.
In order to support the nature of the various electro-
chemical processes, quantum chemical calculations were
undertaken at the DFT level for two of the most repre-
sentative iridium complexes of the whole family, namely,
[Ir(thpy)2(bpy)]þ and [Ir(thpy-CHO)2(bpy)]þ. All of the
calculations were carried out using the Gaussian 03
program.47 A preliminary screening of the most appro-
priate level of theory indicated that the best results were
obtained at B3LYP/LanL2DZ,48 which use a double-ζ
qualitybasisset D9549a forthecarbon, hydrogen, nitrogen,
(47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M. Toyota, K. Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J.; Ayala, P. Y.; Morokuma, K.; Voth, G.
A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.;
Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
phen]þ, in a 0.08 M TBAH/THF electrolyte (working electrode, platinum
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
Figure 5. CV curves of a 1 mM solution of complexes (a) [Ir(thpy-
CHO)2bpy]þ, (b) [Ir(thpy-CHO)2dmbpy]þ, and (c) [Ir(thpy-CHO)2-
B. G.; Chen, W.; Wong, M. W.; Gonzalez C.; Pople, J. A. Gaussian 03,
revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(48) A combination of different functionals (BP86, B3LYP, and B3P86)
and basis sets [i.e., (i) 6-31G* on carbon, hydrogen, nitrogen, and oxygen
atoms and LanL2DZ ECP on sulfur and iridium atoms, (ii) 6-31G* on
disk, 125 μm diameter; T = 25 °C; scan rate = 1 V/s).
two thpy-CHO ligands, as is further explained below and
also on the basis of the quantum mechanical calculations
(vide infra). The redox behavior of the uncoordinated
ligands allowed one to attribute the successive two reduc-
tions of the CV curve as the electron pairing onto the thpy
ligands while the last process is the second reduction of
the bpy ligand. The last process is affected by a follow-up
reaction, and it becomes more reversible as the scan rate is
increased.
In Figure 5b,c, the CV curves of the two homologous
iridium species, where only the ancillary ligand bpy has been
changed, are shown. When bpy is replaced with dmbpy, the
carbon, hydrogen, nitrogen, and oxygen atoms and SDD ECP on sulfur and
iridium atoms, and (iii) LanL2DZ, i.e., D95, on carbon, hydrogen, nitrogen,
and oxygen atoms and Los Alamos ECP plus DZ on sulfur and iridium
atoms] has been used in the screening calculations. Geometry optimization
using the combinations B3LYP/(6-31G*, LanL2DZ) and B3LYP/LanL2DZ
gave very similar results and the lowest deviations of the bond lengths and
bond angles from the X-ray values.
(49) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;
Schaefer, H. F., III, Ed.;Plenum: New York, 1976; Vol. 3, p 1.(b) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299. (c) The ECP on the iridium replaced the
inner-core electrons, leaving the outer-core [(5s)2(5p)6] electrons and the (5d)6
valence electrons of iridium(III).

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1447
Table 4. ECL Data for the Iridium Complexes Investigated in an ACN Solution
at 25 °C
species
λECL (nm)
Φ
ECL (%)a
[Ir(thpy-CHO)2bpy]þ
[Ir(thpy)2bpy]þ
642
608
635
640
630
0.5
0.4
0.3
0.3
0.1
[Ir(thpy-CHO)2dmbpy]þ
[Ir(thpy-CHO)2phen]þ
[Ir(thpy-Me)2bpy]þ
a The ECL efficiency was calculated as reported in the Experimental
Section.
ligands (see Table 3), and, in turn, it also determines the
occurrence of an electrochemically accessible second re-
duction on thpy-CHO when chelated to the metal center.
Moreover, as far as the first three reductions are con-
cerned, comparisons of the corresponding gas-phase
theoretical results, for the two iridium species, are quali-
tatively in good agreement with the electrochemical find-
ings (see Figure S2 in the Supporting Information).
ECL. The PL efficiency of the iridium complexes
investigated in this work prompted us to consider these
molecules as relatively good candidates for ECL studies.17g
The ECL spectra of all of the species have been obtained
in ACN by annihilation of the one-electron-oxidized
and -reduced forms, and the data are collected in Table 4.
The emission, although not very intense, shows a stable
and reproducible behavior over a wide number of experi-
ments performed with the same solution; thus, it is
promising for a possible use in devices.
Figure 6. Plot of the frontier molecular orbital surfaces for the species
[Ir(thpy-CHO)2(bpy)]þ showing the main localization of the orbitals
involved in the redox processes: (a) HOMO (first oxidation) mainly
centered on the iridium with a contribution from the thienyl moieties;
(b) LUMO (first reduction) centered on bpy; (c and d) second and third
unoccupied molecular orbitals (i.e., second and third reduction processes)
centered on the two thpy ligands. Optimized geometries were at the
B3LYP/LanL2DZ level of theory.
Concerning the energetics of the ECL process, on the
basis of the emission wavelengths of the various com-
pounds, the energy needed to generate the excited states,
from which the emission takes place, ranges from 1.95 eV
for [Ir(thpy-CHO)2dmbpy]þ to 2.03 eV for [Ir(thpy)2-
bpy]þ. As in a typical annihilation ECL experiment, the
energy to generate the emitting excited state is provided
by the homogeneous electrochemical reaction between
the electrochemically generated oxidized and reduced
species; the energy requirements above must be fulfilled
by the electrochemical data. Thus, the annihilation reac-
tion enthalpy, ΔH°ann = ΔG°ann þ TΔS°, must be high
enough to generate the emitting state. ΔG°ann is obtained
from the half-wave potential difference between the first
oxidation and first reduction waves in the cyclic voltam-
mogram [ΔE1/2(ox/red)], which is comprised of, for the
compounds investigated here, between 2.3 and 2.6 V. If
we consider an entropy contribution ≈0.1 eV,35,36,17b we
obtain a minimum value of 2.2 eV for ΔH°ann, which is,
for all of the species, higher than the minimum energy
required for the population of the excited state (estimated
from the emission spectra, vide supra).
The ECL experiments were carried out by pulsing the
potential between the first oxidation and first reduction
processes by both cyclic voltammetry and double-step
potential techniques. The ECL spectra recorded in dry
ACN for the species [Ir(thpy-CHO)2bpy]þ, [Ir(thpy-CHO)2-
phen]þ, and [Ir(thpy)2bpy]þ are shown in Figure 7a-c,
respectively, and no differences in the ECL signals were
observed when the potential window was extended in the
reduction to include the second process. Concerning the
mechanism of the excited-state generation, further studies
are currently underway, and we can only speculate that, most
and oxygen atoms of the ligands and a relativistic effective
core potential (ECP) for the iridium and sulfur atoms.49b,c
Geometry optimization of the iridium complexes was
carried out without any symmetry constraint, and the
resulting calculated structures are in reasonably good
agreement with the crystallographic data. The compar-
ison of the calculated bond lengths for the optimized
geometries with the X-ray data (vide supra) shows that
˚
the differences are, on average, within 0.03 A.
For all of the species investigated, the plot of the highest
occupied molecular orbital (HOMO) is centered on the
metal with a substantial contribution from the thpy
ligands, as is also reported for similar iridium cyclometa-
lated complexes.33 Conversely, the lowest unoccupied
molecular orbital (LUMO) is mainly localized on the
dipyridyl ligand. The two successive unoccupied molecu-
lar orbitals calculated for the pristine cationic complex
[Ir(thpy)2(bpy)]þ (e.g., LUMOþ1 and LUMOþ2) have
close energy values and are mainly localized on the two
thpy ligands. Modeling of the reduced species [Ir(thpy)2-
(bpy)]- and [Ir(thpy)2(bpy)]2- indicates that the most
stable electronic states are the triplet and quartet, respec-
tively, which means that the first three electrons intro-
duced are unpaired and mainly localized on the three
ligands. Concerning the species [Ir(thpy-CHO)2(bpy)]þ,
the quantum chemical calculations, at the same level of
theory, show a very similar molecular orbital sketch
(Figure 6), with the difference that the unoccupied mole-
cular orbitals localized on the thpy ligands have a lower
energy because of the presence of the aldehydic groups.
This makes such ligands easier to reduce, as is also
shown by the electrochemical results of the uncoordinated

1448 Inorganic Chemistry, Vol. 49, No. 4, 2010
Bandini et al.
annihilation or (ii) the formation of excited-state aggre-
gates, at the concentration used to obtain the spectrum.
Because no change in the absorption and PL spectra was
observed before and after extensive potential cycling, this
suggests that in the case of [Ir(thpy-CHO)2bpy]þ the
second hypothesis holds (e.g., the formation of excited-
state aggregates must be invoked).
ECL yields were determined using [Ru(bpy)3]2þ as a
standard by comparison of the measured integrated
photon intensities, taking into account the differences in
the electric charges passed through the solution studied.
The ECL efficiency (in photons emitted/electron transferred
between reduced and oxidized forms of the parent
molecule) is related directly to the yield of the excited
state upon annihilation (Φes) and to the emission quantum
yield (Φ0) of a given emitter (ΦECL = ΦesΦ0). The results
obtained imply that the yield for the formation of the
excited state is about 30-40%, which would make these
systems promising for consideration in future development.
Conclusions
In this paper, a new class of air-stable cationic-functiona-
lized bipyridyl-cyclometalated iridium(III) complexes, with
thienyl-based ligands, have been synthesized and thoroughly
characterized by studying their crystallographic, spectro-
scopic, luminescence, and electrochemical properties. The
aldehydic functionality on the cyclometalated ligands makes
these iridium complexes particularly interesting for further
functionalization of reductive amination reactions. The elec-
trochemical investigation showed a rich pattern of reduction
processes that have been successfully attributed through a
combined comparison of the CV behaviors of the iridium
species and the free ligands. The various complexes are
luminescent, with efficiency lower than that of similar cyclo-
metalated phenylpyridine-based iridium(III) species but
within the same range as that of previously reported thie-
nyl-containing iridium(III) compounds. Interestingly, the
complexes with the formyl-functionalized thienyl ligands
have the highest luminescence efficiency of the series. More-
over, the ECL results show a trend very similar to that of the
luminescence ones, featuring, in some cases, a relatively good
ECL efficiency, which makes such systems potentially inter-
esting in the field of new materials for electrochemiluminescent
biodetection or polymeric light-emitting devices.
Figure 7. ECL spectra of the (a) [Ir(thpy-CHO)2bpy]þ, (b) [Ir(thpy-
CHO)2phen]þ, and (c) [Ir(thpy)2bpy]þ obtained by annihilation of one-
electron-oxidized and -reduced forms. All ECL spectra were collected for
a 1 mM compound in a 0.07 M TBAH/ACN electrolyte solution, at25 °C.
likely, the main annihilation path does involve the direct
formation of the triplet state.
The ECL spectra of the iridium compounds investi-
gated are characterized by broad emission bands com-
prised of between 520 and 750 nm. The maxima were
between 600 and 650 nm. When compared to the lumi-
nescence, the ECL spectra show some differences (see
Figure S3 in the Supporting Information), being slightly
broader in shape, with λmax at about the same value or
eventually shifted to lower energy (by e10 nm for
[Ir(thpy-CHO)2bpy]þ). This difference is probably due
to the lower resolution of the ECL instrument where the
spectra were collected. This is also the reason why for
some iridium complexes the vibronic structure is less
evidenced in the ECL spectra than in the PL one. Any-
way, information relative to the vibronic structure can
still be extracted from the experimental spectra, as shown
in Figures S4 and S5 in the Supporting Information.
It is sometimes reported that the observed ECL band is
significantly shifted at a longer wavelength region as
compared to the PL spectrum, and this might be due to
(i) the formation of side products generated during the
Acknowledgment. Acknowledgment is made to the
ꢀ
Ministero dell’Istruzione, Universita e Ricerca, MIUR,
FIRB Project (Progettazione, preparazione e valutazione
biologica e farmacologica di nuove molecole organiche
quali potenziali farmaci innovativi), University of Bolog-
na, and LATEMAR (centre of excellence financed by
MIUR). INSTM and CINECA are also acknowledged
for a computing time grant for use of the IBM SP5
mainframe. This paper is dedicated to Prof. Saverio
Florio in occasion of his 70th birthday.
Supporting Information Available: Crystallographic data
table of [Ir(thpy)2bpy]PF6, [Ir(thpy-CHO)2bpy]PF6, and [Ir-
(thpy-CHO)2phen]PF6, DFT-calculated energies and molecular
orbital compositions, diagram of the reduction processes of the
iridium complexes and the uncoordinated ligands, and Gaussian
deconvolution of the PL and ECL spectra of the species [Ir(thpy-
CHO)2phen]þ and [Ir(thpy)2bpy]þ in ACN. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.