Photophysical Properties of Oligo(phenylene ethynylene) Iridium(III) Complexes Functionalized with Metal-Anchoring Groups

Article abstract of DOI:10.1002/ejic.201501409

The electrochemical and photophysical properties of a family of conjugated ligands and their iridium(III) cyclometallated complexes are described. They consist of a series of monocationic Ir^III bis-2-phenylpyridine complexes with p-phenylethynyl-1,10-phenanthroline ligands of different length. The structure of these ligands includes terminal acetylthiol or pyridine groups, which can provide good electrical contacts between metal electrodes. Cyclic voltammetry, absorption and emission spectroscopy, laser flash photolysis and density functional theory calculations reveal that the high conjugation of the diimine ligand affords small energy gaps between the frontier orbitals. Nevertheless, the nature of the terminal substituents and the extent of the conjugation in the diimine ligand have little influence on the photophysical features at room temperature. The spectroscopic data and theoretical calculations agree that the charge-transfer nature of the emitting excited state is maintained along the series at room temperature, whereas in rigid matrices ligand-centred states also contribute to the low-temperature emission. The good conducting features of the diimine ligands, the small dependence of the HOMO-LUMO (HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) gaps of these complexes on the ligands and the charge-transfer nature of the emitting excited state make these complexes promising test beds for the study of photoconducting phenomena in molecular junctions.

Full text of DOI:10.1002/ejic.201501409

DOI: 10.1002/ejic.201501409
Full Paper
Luminescent Ir Complexes
Photophysical Properties of Oligo(phenylene ethynylene)
Iridium(III) Complexes Functionalized with Metal-Anchoring
Groups
Julia Ponce,[a] Juan Aragó,[a] Ignacio Vayá,[b] Jorge Gómez Magenti,[a] Sergio Tatay,*[a]
Enrique Ortí,[a] and Eugenio Coronado[a]
Abstract: The electrochemical and photophysical properties of ence on the photophysical features at room temperature. The
a family of conjugated ligands and their iridium(III) cyclometal- spectroscopic data and theoretical calculations agree that the
lated complexes are described. They consist of a series of mono- charge-transfer nature of the emitting excited state is main-
cationic IrIII bis-2-phenylpyridine complexes with p-phenyleth- tained along the series at room temperature, whereas in rigid
ynyl-1,10-phenanthroline ligands of different length. The struc- matrices ligand-centred states also contribute to the low-tem-
ture of these ligands includes terminal acetylthiol or pyridine perature emission. The good conducting features of the diimine
groups, which can provide good electrical contacts between ligands, the small dependence of the HOMO–LUMO (HOMO =
metal electrodes. Cyclic voltammetry, absorption and emission highest occupied molecular orbital, LUMO = lowest unoccupied
spectroscopy, laser flash photolysis and density functional the- molecular orbital) gaps of these complexes on the ligands and
ory calculations reveal that the high conjugation of the diimine the charge-transfer nature of the emitting excited state make
ligand affords small energy gaps between the frontier orbitals. these complexes promising test beds for the study of photo-
Nevertheless, the nature of the terminal substituents and the conducting phenomena in molecular junctions.
extent of the conjugation in the diimine ligand have little influ-
Introduction
and high quantum yields[23] have made IrIII complexes highly
attractive for a wide variety of applications, such as emitters in
organic light-emitting diodes (OLEDs),[24] light-emitting electro-
chemical cells (LECs),[25] and oxygen sensors.[26] However, to the
best of our knowledge, these phosphorescent complexes have
never been implemented in single-molecule junctions.
In the prototypical complex [Ir(ppy)2(N^N)]+ (Hppy = 2-phen-
ylpyridine and N^N = diimine ligand), the photoluminescent
emission occurs from a triplet metal-to-ligand charge-transfer
(3MLCT) excited state.[27,28] The nature of the substituents on
the diimine ligand has a marked influence on the luminescence
of the complexes.[28,29] For example, the contribution of triplet
ligand-centred (3LC) excited states to the photoluminescence
increases with the conjugation length.[29–33]
The field of molecular electronics has been enriched recently
by the idea of using electromagnetic radiation to modulate the
response of molecular junctions.[1–4] On this basis, the conduct-
ance of some single molecules contacted between metallic
electrodes could be optically switched through light-triggered
isomerization.[5–9] On the other hand, molecular-dipole changes
have a deep influence on the electronic structures of contacted
molecules.[10–12] According to this idea, light has also been pro-
posed as a control tool of molecular conductance in systems
that do not undergo photochemical reactions. In addition to
some experimental work,[13–15] theoretical studies in this area
support this idea.[2,16–19] For example, the effect of illumination
on junctions comprising molecules of different lengths[20,21] or
characterized by strong charge-transfer optical transitions[22]
have been studied theoretically.
In this work, we present the synthesis and characterization
of a series of monocationic photoluminescent IrIII complexes
(Ir1–Ir4, Figure 1), which incorporate conjugated ligands of dif-
ferent molecular length and are functionalized with suitable an-
choring groups for their integration into metallic molecular
junctions.[34,35] The new compounds result from the combina-
tion of iridium(III) bis-cyclometallated 2-phenylpyridine com-
plexes with four different diimine π-conjugated ligands (1–4).
Ligands 1–4 are 1,10-phenanthroline (phen) derivatives sym-
metrically functionalized at their 3- and 8-positions with 1-(S-
acetylthio)-4-phenyl and pyridine terminal groups, which are
connected to the phen core by phenylethynyl spacers of differ-
ent length. The molecular conductances of some of these li-
Cyclometallated iridium(III) compounds are the focus of in-
tense research owing to their unique and tuneable photophysi-
cal properties. Their characteristic intense photoluminescence
[a] Instituto de Ciencia Molecular (ICMol), Universidad de Valencia,
Catedrático José Beltrán 2, 46980 Paterna, Valencia, Spain
E-mail: sergio.tatay@uv.es
http://www.uv.es/taser/Index.html
[b] Departamento de Química, Universitat Politècnica de València,
Camino de Vera s/n, 46022 Valencia, Spain
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201501409.
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gands have been investigated already in mechanically con-
trolled break junctions.[36] Therefore, complexes Ir1–Ir4 com-
bine the low resistance of molecular wires 1–4 with the rich
photophysics of IrIII complexes.
[Ir(ppy)2(N^N)]+ complexes functionalized with phenyl-
ethynyl groups have been reported before.[37,38] However, the
interplay between the conjugation and different anchoring
groups in Ir1–Ir4 makes it difficult to anticipate the nature of
the emitting excited state and, thus, their behaviour at illumi-
nated molecular junctions. For the first time, this work brings
together electro- and spectrochemical techniques along with
theoretical calculations in the study of iridium(III) bis-cyclome-
tallated 2-phenylpyridine complexes with phenylethynyl-substi-
tuted diimine ligands.
Results and Discussion
Chemical Synthesis
The synthesis of ligands 1–4 was accomplished by the se-
quence of Sonogashira-type cross-coupling reactions between
3,8-dibromo-1,10-phenanthrolines and substituted phenylacet-
ylenes displayed in Scheme 1. Ethynyl derivatives 1 and 3 have
been reported previously,[39,40] whereas 2 and 4 have been syn-
thesized for the first time. The synthesis of the short molecular
Figure 1. Chemical structures of iridium complexes Ir1–Ir4.
Scheme 1. (a) nBuLi, chlorotrimethylsilane (TMSCl), THF (68 %); (b) Pd(PPh3)Cl2, CuI, THF, iPr2NH (74 % for 7, 50 % for 8); (c) K2CO3, MeOH (97 % for 9, 91 %
for 10); (d) 11 (2 equiv.), Pd(dba)2, PPh3, CuI, THF, DIEA (40 % for 1, 63 % for 2); (e) 4-iodopyridine (2 equiv.), Pd(PPh3)4, CuI, THF, DIEA (84 % for 3); (f) 4-
iodopyridine (2 equiv.), Pd(dba)2, PPh3, CuI, THF, DIEA (96 % for 4). DIEA = N,N-diisopropylethylamine, dba = dibenzylideneacetone.
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wires 1 and 3 was accomplished by coupling 3,8-dibromo-1,10- versible for the acetylthiol-terminated compounds Ir1 and Ir2
phenanthroline (5)[41] with commercial ethynyltrimethylsilane than for the pyridine-terminated derivatives Ir3 and Ir4, this
to afford 7 in good yield. For the longer ligands 2 and 4, 5 may only reflect the different tendency of the acetylthiol deriva-
was coupled with [(4-ethynylphenyl)ethynyl]trimethylsilane (6), tives to become adsorbed on the electrodes following the
which had been previously prepared by a reported proce- redox process.[36]
dure,[42] that is, the reaction of 1,4-diethynylbenzene with nBuLi
in tetrahydrofuran (THF) and the subsequent addition of chloro-
trimethylsilane. The desilylation of 7 and 8 under basic condi-
tions (K2CO3 in methanol) afforded diethynyl derivatives 9 and
10, respectively, in high yield.[43] These diethynyl-terminated
phenanthrolines were then coupled under Sonogashira condi-
tions with 2 equiv. of 1-(S-acetylthio)-4-iodobenzene (11) to af-
ford 1 and 2 or with commercially available 4-iodopyridine to
afford 3 and 4. Derivative 11 was prepared in high yield by the
reduction of 4-iodobenzenesulfonyl chloride.[44] Alternatively, 2
was synthesized in 24 % yield by following the procedure de-
scribed in the Supporting Information and shown in Scheme S1.
Complexes Ir1–Ir4 were prepared from the dichlorido-
bridged IrIII dimer [Ir(ppy)2Cl]2.[45] The treatment of this salt with
1 equiv. of ligands 1–4 in dichloromethane/methanol mixtures
under reflux under inert conditions and the subsequent treat-
ment with an excess of KPF6 afforded monocationic
[Ir(ppy)2(L)][PF6] (L = 1–4) complexes in reasonable yields
Table 1. Oxidation and reduction potentials versus SCE for Ir1–Ir4.
Ecred [V]
Eaox [V]
Electrochemical gap[a] [V]
Ir1
Ir2
Ir3
Ir4
–0.96
–1.03
–0.94
+1.39
+1.39
+1.42
2.35
2.42
2.36
2.35
–0.95
+1.40
fac-Ir(ppy)3
–2.26[b]
+0.75[b]
[a] The electrochemical gaps were obtained as the differences between the
cathodic peak of the reduction process (Ecred) and the anodic peak of the
red
ox
oxidation process (Eaox). [b] The E1/2 and E1/2 values of fac-Ir(ppy)3 from
ref.[50] have been added for comparison. The electrochemical measurements
were performed in anhydrous dimethylformamide.
At negative potentials, a single reduction process is visible
between –1.03 and –0.96 V, and the ratio between the cathodic
and anodic peaks is again far from one for Ir1 and Ir2. The
potential of the reduction process, particularly for the pyridine-
terminated derivatives, is far higher than that described for the
parent complex [Ir(ppy)2(phen)][PF6] (E1/2 = –1.28 V vs. SCE).[28]
This suggests that the reduction processes for Ir1–Ir4 are cen-
tred on the N^N ligand, as mentioned in previous reports[28–31]
and discussed below.
1
(Scheme 2). In the H NMR spectra of all of the complexes (see
Supporting Information), the ppy ligands present one set of
eight signals, each one of which integrates to two protons. This
equivalency is consistent with the selective formation of the
isomer in which the Ir–Cppy bonds are trans to the Ir–Nphen
bonds according to the so-called “trans effect”.[23]
Photophysical Properties
The absorption spectra of ligands 1–4 and their iridium com-
plexes Ir1–Ir4 in dichloromethane are shown in Figure 2, and
the corresponding absorption maxima and molar extinction co-
efficients are listed in Table 2. The short ligands 1 and 3 exhib-
ited two strong absorption bands, the first at λ = 289 and
283 nm (ε ≈ 55000
340 nm (ε ≈ 60000
M
–1 cm–1) and the second at λ = 351 and
Scheme 2. Synthesis of Ir1–Ir4: (a) CH2Cl2/MeOH (2:1), reflux; (b) excess KPF6
(54 % for Ir1, 35 % for Ir2, 40 % for Ir3, 41 % for Ir4).
M
–1 cm–1), respectively. These maxima are
significantly shifted to lower energies as the conjugation length
increases (ligands 2 and 4), and the bathochromic effect is
higher for the pyridine-terminated molecular backbones. As for
related ligands, we assign the lower- and higher-energy bands
Electrochemical Characterization
The cyclic voltammograms of Ir1–Ir4 are displayed in Figure S1, to long- and short-axis polarized π→π* transitions, respec-
and relevant electrochemical data are collected in Table 1. The tively.[37,38,51] The absorption bands of Ir1–Ir4 are broader than
electrochemical measurements revealed oxidation and reduc- those of the bare ligands 1–4 (Figure 2, bottom). The spectra
tion half-wave potentials more than 1.8 V apart and the high show an intense band (ε > 5 × 104
M
–1 cm–1) below λ = 350 nm
–1 cm–1) in the λ = 350–
redox stability of the complexes. All of the complexes display and a less-intense band (ε > 4 × 104
M
an oxidation process centred at 1.40 V. This oxidation process 450 nm range. On the basis of previous reports,[28–31,33,37] we
has been observed previously for other bis-cyclometallated IrIII assign the first band to π→π* transitions centred on the ppy
complexes[28–31,46,47] such as [Ir(ppy)2(phen)]+ [1.36 V vs. satu- and phen ligands. The second broad band is the result of the
rated calomel electrode (SCE)][28] and has been associated with overlap of π→π* LC transitions with spin-allowed 1MLCT transi-
the bis-cyclometallated Ir(III/IV) oxidation. In agreement with tions. For Ir3, the latter transitions are distinguishable as a sepa-
this assignment, the oxidation potential presents minor varia- rate band at λ ≈ 400 nm. The redshift (up to 30 nm) of the
tions along the Ir1–Ir4 series. The irreversibility of this process bands into the λ = 360–390 nm range compared with those
denotes the participation of the ppy moiety in the redox proc- observed for 1–4 is in part attributed to the stabilization of the
ess, as supported by previous reports and our theoretical calcu- π-electron system of the phen chromophore as a result of the
lations (see below).[48,49] Although the oxidation is more irre- coordination to the cationic IrIII centre (see below).[51,52]
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Table 2. Photophysical properties of ligands 1–4 and complexes Ir1–Ir4 in dichloromethane and ethanol solutions.[a]
τ[e]
CH2Cl2
τ
[f] EtOH
kr[g] CH2Cl2
knr
[b]
[b,c]
[g]
λ
max CH2Cl2 [nm] λem[b] CH2Cl2
λem
EtOH 278 EtOH 77 K
λem
ET[b,d] CH2Cl2
278 K [eV]
φair
CH2Cl2
278 K
φAr
CH2Cl2
278 K 278 K [ns]
(ε, 103
M
–1 cm–1
)
278 K [nm]
77 K [μs]
278 K
CH2Cl2 278
K [nm]
[nm]
[106 s–1
]
K [10 s–1]
1
2
3
4
Ir1
Ir2
Ir3
Ir4
289 (55), 351 (62)
305 (45), 367 (74)
283 (57), 340 (60)
300 (39), 367 (64)
272 (69), 380 (46)
314 (66), 395 (51)
290 (56), 362 (38)
301 (53), 389 (51)
244 (45), 283 (45)
384
403
384
394
638
638
648
640
561
568
511
2.48
2.37
2.56
2.35
2.22
2.21
2.13
2.19
0.76
0.83
0.12
0.79
0.84
0.15
<1
<1
<1
564
0.90
0.93
<1
644
644
658
562 (552)
563 (563)
588 (512)
566 (566)
510[h]
0.097
0.113
0.057
0.049
0.151
0.179
0.073
0.073
0.4
502
564
202
460
1900
5.7, 18.8
4.5, 15.1
6.7, 18.2
6.8, 20.6
3.6
0.30
0.32
0.36
0.16
0.21
1.7
1.5
4.6
2.0
0.32
646
492[h]
[h]
fac-Ir(ppy)3
[a] The concentrations were 3 × 10–6
fluorescence quantum yield, τ = luminescence lifetime, kr = radiative emission constant, and knr = nonradiative emission constant. [b] λexc (1–4) = 300 nm,
exc (Ir1–Ir4) = 375 nm. [c] The emission maxima were registered with delays of 50 μs for 1–4 and 5 (50) μs for Ir1–Ir4. [d] Determined in deaerated
M. λmax = absorption maximum, ε = molar attenuation coefficient, λem = emission maximum, ET = triplet energy, φ =
λ
dichloromethane from the 10 % increase phosphorescence spectra. [e] Measurements in deaerated dichloromethane solution at the emission maximum
(λexc = 375 nm); the lifetimes for 1–4 are shorter than 1 ns and could not be resolved by our system. [f] Measurements in deaerated ethanol solid matrix at
560 nm (λexc = 355 nm), the lifetimes correspond to biexponential fits. [g] kr and knr in dichloromethane solution were calculated according to the equations:
kr = φAr/τ and knr = (1 – φAr)/τ. [h] The data for fac-Ir(ppy)3 from ref.[50] have been added for comparison. In this case, the emission and lifetime measurements
were performed in 2-methyltetrahydrofuran.
Figure 3. Normalized photoluminescence spectra of ligands 1–4 (λexc
=
300 nm) and complexes Ir1–Ir4 (λexc = 375 nm) in dichloromethane at 298 K.
The photoluminescence spectra of Ir1–Ir4 at room tempera-
ture are shown in Figure 3 (right), and their photophysical prop-
erties are summarized in Table 2. The emission quantum yields
of the complexes are much lower than those observed for the
bare ligands and are significantly affected by the presence of
oxygen. The emission lifetimes of Ir1–Ir4 at room temperature
range in the sub-microsecond time scale and are more than
two orders of magnitude longer than those found for 1–4
(τ < 1 ns). All of these features are consistent with the triplet
nature of the emissive excited state for all of the Ir com-
plexes.[53]
Figure 2. Normalized absorption spectra of ligands 1–4 (top) and complexes
Ir1–Ir4 (bottom) in dichloromethane.
The photoluminescence spectra of ligands 1–4 in dichloro-
methane at 298 K (λexc = 300 nm) are shown in Figure 3. The
emissions are centred in the λ = 380–400 nm range and show
relatively high quantum yields (φ), which are essentially unaf-
fected by the presence of oxygen and evidence the fluores-
cence character of the emission (Table 2). The enhancement of
the π conjugation results in higher φ values and the stabiliza-
In contrast to those observed for 1–4, the emission shapes
for Ir1–Ir4 in dichloromethane solution at 298 K are broad and
unstructured (Figure 3, right), as is typical for emissions from
MLCT excited states,[54] and the emission shifts by 6–10 nm
towards longer wavelengths in more polar ethanol (Table 2).
The emissions of Ir1 and Ir3 appear redshifted compared
tion of the lower-energy transitions. The fluorescence lifetimes with that of [Ir(ppy)2(phen)]+ (575 nm).[28] Therefore, the emis-
(τ) of 1–4 were less than 1 ns and could not be resolved by our sion is most likely localized in the N^N unit. However, in contrast
experimental setup.
to the observations for ligands 1–4 and previous re-
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ports,[37,38,51,55] a further increase in the conjugation of the di- states, which, according to their lifetime, energy and shape are
3
3
imine ligand does not lead to an energy decrease of the emis- assigned to MLCT and LC states.
sion (compare Ir2 and Ir4 with Ir1 and Ir3, respectively, in
Table 2).
Transient Absorption
The photoluminescence decay traces of Ir1–Ir4 in dichloro-
methane, measured at the emission maxima, can be accurately
fitted to single exponential functions. The lifetimes from these
fits range from 0.20 to 0.56 μs and are longer for the acetylthiol
compounds than for the pyridine-terminated derivatives
(Table 2). The increase of the conjugation length of the diimine
ligand also leads to an increase of the lifetime. The quantum
yields are higher for the acetylthiol-terminated derivatives than
for the pyridine-terminated derivatives, and all of them are in
the range reported for other [Ir(ppy)2(N^N)]+ complexes with
MLCT emissive excited states.[30,33,46]
Laser flash photolysis (LFP) measurements were performed at
room temperature with samples in deaerated dichloromethane
at λexc = 355 nm. The LFP of 1–4 affords long-lived absorption
species that are highly quenched by oxygen and low-energy
triplet acceptors such as ꢀ-carotene (ET = 1.00 eV). Therefore,
they are assigned to triplet 3π→π* excited states. The transient
absorption spectra of 1–4 are shown in Figure S3 and are char-
acterized by a broad absorption band throughout the visible
region, which extends to the near-infrared, with triplet lifetimes
of 9.3–16.5 μs (see Table S1).
Interestingly, compared with those observed at room tem-
perature, the low-temperature phosphorescence decay curves
measured at λ = 560 nm for Ir1–Ir4 show biexponential decays
with lifetimes for the two component differing by ca. 10 μs
(Table 2). The emission spectra of Ir3 recorded in ethanol glass
at different delay times (5 and 50 μs) are shown in Figure 4. In
this case, the two components of the emission are well-sepa-
rated in energy and could be clearly resolved. The short-lived
(6.7 μs) emission that peaks at λ ≈ 600 nm is unstructured and
appears blueshifted by 1800 cm–1 compared with the room-
temperature emission. This behaviour is typical of MLCT emit-
ters and originates from the rigidochromic effect.[54] The long-
lived emission (18.2 μs) is structured and peaks at the same
energy (512 nm) as that measured for 3 under equivalent ex-
perimental conditions (Table 2). Similarly, the emission spectra
of Ir1, Ir2 and Ir4 shows structured bands with maxima that are
close to those found for the low-temperature emissions of the
corresponding ligands (Figure S2). However, in this case, both
components peak at similar wavelengths, and the different con-
tribution to the emission could not be time-resolved. At low
temperature, 3MLCT states increase in energy, and 3LC states
can increase their contribution to the emission.[55] Thereby, at
77 K, Ir1–Ir4 show dual emission from two different triplet
The transient absorption spectra of Ir1–Ir4 (Figure 5) contrast
with those obtained for ligands 1–4. The negative signal at λ ≈
400 nm is assigned to ground-state bleaching, according to the
absorption spectra (Table 2). The absorption species with λmax
≈ 510 nm are largely quenched by oxygen with rate constants
of ca. 6 × 108 –1 s–1 (see Figure 5, b and d) and by low-energy
M
triplet acceptors such as ꢀ-carotene; as for related IrIII com-
3
plexes, they are assigned to MLCT excited states.[37,38,55] Addi-
tionally, the decay traces of Ir1–Ir4 at λ = 510 nm can be accu-
rately fitted with a first-order exponential function. The lifetimes
range from 0.24 to 0.54 μs and are very close to the photolumi-
nescence decay components detected by emission spectro-
scopy (Table 2). Finally, the bleaching observed at λ ≈ 640 nm
is assigned to the photoluminescence detected in the emission
spectra measured in deaerated dichloromethane solution at
room temperature (see Table 2).
Figure 5. Transient absorption spectra of (a) Ir2 and (c) Ir4 in deaerated di-
chloromethane solution recorded 0.01 (circles), 0.1 (squares), 0.3 (triangles),
and 1 μs (inverted triangles) after the laser pulse. The arrows show the evolu-
tion of the absorption band with time. Decay traces of (b) Ir2 and (d) Ir4
monitored at λ = 510 nm in N2 (dot), air (solid), and O2 (dash). The insets
show the Stern–Volmer plots for quenching by oxygen. All measurements
were performed at 298 K, λexc = 355 nm.
Theoretical Calculations
Figure 4. Normalized luminescence spectra at λexc = 375 nm of ligand 3 and
complex Ir3 in dichloromethane (300 K) and in ethanol glass (77 K) measured
at different delays.
Density functional theory (DFT) calculations were performed at
the B3LYP/(6-31G**+LANL2DZ) level for cationic complexes Ir1–
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Ir4 in the presence of dichloromethane (see Computational De-
tails). The isolated phenanthroline-based ligands 1–4 were also
computed at the B3LYP/6-31G** level for comparison. The cal-
culations of the electronic ground states (S0) correctly predict
near-octahedral coordination geometries for the Ir centres and
provide geometrical parameters in good agreement with the
experimental data for similar Ir-based complexes. The optimized
values calculated for the bond lengths and bond angles that
define the coordination spheres of the iridium centres of all of
the complexes are collected in Table S2.
The energies calculated for the highest occupied molecular
orbitals (HOMOs) and lowest unoccupied molecular orbitals
(LUMOs) of the phenanthroline-based ligands 1–4 and com-
plexes Ir1–Ir4 are shown in Figure 6, and the isovalue contours
computed for the frontier molecular orbitals of Ir1 and Ir3 are
shown in Figure 7 as representative examples. The topologies
of the molecular orbitals of Ir2 and Ir4 (Figure S4) are identical
to those of Ir1. Similarly to those of related ppy-based cyclo-
metallated Ir complexes,[31–33] the HOMO is composed of a mix-
ture of IrIII dπ orbitals (t2g) and phenyl π orbitals with little
contribution from the pyridine rings of the cyclometallated li-
gands. As the family of complexes Ir1–Ir4 only differs in the
diimine ligand, the energy of the HOMO remains almost con-
stant along the series and is slightly more stable for Ir3 and Ir4
(Figure 6, b). Therefore, the theoretical calculations support the
participation of the ppy ligands in the first oxidation process
and fully justify the small variation of the oxidation potentials
observed from Ir1 (+1.39 V) and Ir2 (+1.39 V) to Ir3 (+1.42 V)
and Ir4 (+1.40 V). In contrast to the HOMO, the LUMO is com-
pletely localized over the diimine ligands and is mainly centred
on the phenanthroline core. Compared with the LUMO of the
parent [Ir(ppy)2(phen)]+ complex with an energy of –2.35 eV,[28]
the LUMO is stabilized by the attachment of phenylethynylene
Figure 7. Schematic representation showing the isovalue contours ( 0.03 a.u.)
and the energy values [eV] calculated for the HOMOs and LUMOs of Ir1 (left)
and Ir3 (right).
transitions from HOMO→LUMO excitation have an MLCT nature
mixed with some ligand-to-ligand charge-transfer (LLCT) char-
acter for all of the complexes. Additionally, for Ir1, Ir2 and Ir4,
3
the HOMO–1 is located on the diimine ligands; thus, a LC ex-
cited state described by the HOMO–1→LUMO one-electron
3
promotion can appear close in energy to the MLCT state. This
effect is more likely for the more conjugated Ir2 and Ir4 (Fig-
ure S4), for which both the HOMO–1 and HOMO–2 spread over
the diimine ligand and are closer in energy to the HOMO. In
contrast, the HOMO–1 of Ir3 is localized on the Ir-ppy environ-
ment; and the HOMO–2 and HOMO–3, which show some con-
tribution from the diimine skeleton, are significantly stabilized
(Figure 7).
Thus, 3LC excited states for Ir3 are expected to appear at
groups at the 3- and 8-positions of the phen ligand, and the higher energies than those of Ir1, Ir2 and Ir4. This analysis of
HOMO–LUMO gap decreases from 3.18 eV in [Ir(ppy)2(phen)]+ the frontier molecular orbitals supports the well-separated
3
3
to ca. 2.75 eV in Ir1 and Ir3. However, the attachment of an emission from the LC and MLCT states observed experimen-
additional phenylethynylene unit in Ir2 and Ir4 does not lead tally for Ir3 at low temperature.
to a further stabilization of the LUMO; thus, the HOMO–LUMO
gap is similar for all the complexes, in agreement with the elec-
trochemical data.
To characterize the nature of the emitting excited states in
more detail, the molecular structures of the lowest triplet ex-
cited states (T1) were optimized by using the spin-unrestricted
A wider inspection of the energy positions along with the UB3LYP approach. After full-geometry relaxation, the T1 state is
atomic orbital compositions at the optimized ground-state ge- computed to lie in the range 1.89–2.05 eV above S0 (adiabatic
ometry of the frontier molecular orbitals (Figures 7 and S4) can energy differences, ΔE in Figure 8), in reasonable agreement
provide qualitative information about the nature of the low- with the triplet energies experimentally registered from the on-
lying triplet excited states. The lowest-energy triplet electronic set of the phosphorescence (Table 2). As illustrated in Figure 8
Figure 6. Energy diagram for the HOMOs and LUMOs of (a) ligands 1–4 and (b) complexes Ir1–Ir4.
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(b and c), the unpaired-electron spin-density distributions cal- plexes. The spectroscopic measurements and DFT calculations
culated for Ir1 and Ir3 (Ir: 0.48 e, ppy: 0.50 e, 2: 1.02 e) perfectly suggest that the room-temperature emission of the iridium
match the topology of the HOMO→LUMO excitation (Figure 7). complexes can be unambiguously assigned to 3MLCT excited
3
Similar spin-density distributions are found for Ir2 and Ir4. states in all cases. Nevertheless, the contributions of LC states
Therefore, the calculations clearly indicate that the emitting T1 to the emission increase in rigid matrices at low temperature.
state at room temperature mainly results from the HO- This could be clearly observed in the low-temperature emission
MO→LUMO monoexcitation and implies an electron transfer of Ir3, for which the long-lived component of emission perfectly
from the Ir–ppy environment to the conjugated diimine ligand matches the emission of ligand 3 at room temperature. Accord-
3
for Ir1–Ir4. Therefore, T1 has MLCT character with some LLCT ing to DFT calculations, this behaviour arises from the different
contribution, in good concordance with the spectroscopic and distribution of the HOMO of Ir3 compared with those of Ir1, Ir2
electrochemical data at room temperature.
and Ir4.
Induced dipolar moments are a pursued feature in molecular
electronics. The charge-transfer nature of the emitting excited
states of Ir1–Ir4, together with the small dependence of the
HOMO–LUMO gap on the molecular length and the substitu-
ents (anchoring groups) of the diimine 1–4 ligands and the
good conducting features reported for ligands 1 and 3 makes
the complexes reported herein promising test beds for the
study of photoconducting phenomena in molecular junctions.
This work represents a first stage of the construction of nano-
scale optoelectronic devices with metallic complexes.
Experimental Section
Chemical Synthesis: All of the chemicals and solvents used were
purchased from commercial sources and used without further puri-
fication, unless mentioned. The starting chlorido-bridged iridium
dimer [Ir(ppy)2Cl]2 was prepared according to the literature proce-
dure.[45]
Figure 8. (a) Schematic energy diagram showing the adiabatic energy differ-
ence (ΔE) between the S0 and T1 states and the emission energy (Eem) from
T1 calculated for Ir1–Ir4. Unpaired-electron spin-density contours (0.003 a.u.)
calculated for the fully relaxed T1 state of (b) Ir1 and (c) Ir3.
Instrumentation: The 1H NMR spectra were acquired with a Bruker
AVANCE DRX 300 spectrometer. The spectra were referenced to the
residual proton resonances of the solvent. Electrospray (ES) mass
spectra were obtained with a Waters Micromass ZQ spectrometer
in the positive-ion mode. The absorption spectra of samples in
To estimate the phosphorescence emission energies, the ver-
tical energy differences between T1 and S0 were computed by
performing single-point calculations of S0 at the optimized min-
imum-energy geometries of T1 (Eem in Figure 8). The theoretical
emission energies follow the same trend as the maximum verti-
cal emission of the emission bands observed at room tempera-
ture; Ir1, Ir2 and Ir4 emit at almost the same energy, and Ir3
emits at slightly lower energies (Table 2).
3 × 10–6
M dichloromethane solutions were recorded with a Shi-
madzu UV-2501PC spectrophotometer with 1 cm path-length
quartz cuvettes. The electrochemical measurements were per-
formed under nitrogen in a glovebox with an Autolab PGSTAT 128N
potentiostat and a three-electrode electrochemical cell consisting
of a glassy carbon working electrode, a platinum wire counter elec-
trode and a silver wire pseudoreference electrode.
Conclusions
Detailed synthetic procedures can be found in the Supporting Infor-
mation.
The syntheses of a family of highly conjugated, phenanthroline-
based diimine ligands, functionalized with suitable terminal
groups to anchor them to metal electrodes, and their corre-
Laser Flash Photolysis Measurements: A pulsed Nd:YAG SL404G-
10 laser (Spectron Laser Systems) was used at the excitation wave-
sponding cyclometallated monocationic IrIII complexes have length of 355 nm. The single pulses were ca. 10 ns in duration, and
the energy was less than 15 mJ/pulse. The detecting light source
was a pulsed Lo255 Oriel xenon lamp. The laser flash photolysis
system consisted of a pulsed laser, a Xe lamp, a 77200 Oriel mono-
chromator, an Oriel photomultiplier tube (PMT) system made up of
a 77348 side-on PMT tube, a 70680 PMT housing, and a 70705 PMT
power supply. A Tektronix TDS-640A oscilloscope was used. The
output signal from the oscilloscope was transferred to a personal
computer. All transient measurements were recorded in dichloro-
methane or ethanol by employing 10 × 10 mm2 4 mL quartz cells,
which were purged with nitrogen or oxygen for at least 10 min
been described. In these complexes, the energy of the LUMO,
centred on the diimine ligand, is greatly stabilized by the in-
creased conjugation of ligands 1–4 with respect to that of the
bare phen ligand. Accordingly, the electrochemical and photo-
physical features of Ir1–Ir4 show lower HOMO–LUMO gaps than
that of the reference complex [Ir(ppy)2(phen)][PF6]. However,
the HOMO–LUMO gap does not follow the expected trend with
the extension of the conjugation of the diimine ligand ob-
served in other monocationic IrIII complexes and remains al-
most unchanged from Ir1 and Ir3 to the more conjugated Ir2 before acquisition. All of the experiments were performed at room
temperature.
and Ir4. As a result, the emission wavelength remains almost
invariable along the series, except for Ir3, the emission of which
Phosphorescence Measurements: The phosphorescence spectra
is slightly redshifted with respect to those of the other com- were obtained with a Photon Technology International TimeMaster
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1857 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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[10] M. Elbing, R. Ochs, M. Koentopp, M. Fischer, C. von Hänisch, F. Weigend,
F. Evers, H. B. Weber, M. Mayor, Proc. Natl. Acad. Sci. USA 2005, 102, 8815–
8820.
[11] Y. Lee, S. Yuan, L. Yu, Sci. China Chem. 2011, 54, 410–414.
[12] P. Jiang, G. M. Morales, W. You, L. Yu, Angew. Chem. 2004, 116, 4571;
Angew. Chem. Int. Ed. 2004, 43, 4471–4475.
TM-2/2003 spectrofluorometer equipped with a pulsed Xe lamp.
The instrument was operated in time-resolved mode with a variable
delay time. The compounds were dissolved in ethanol in a quartz
tube (5 mm of diameter) and cooled to 77 K. The absorbance of
the samples was 0.3 at the excitation wavelength (355 nm).
Fluorescence Measurements: The fluorescence decay traces were
recorded with an EasyLife X system from OBB with a PTI lifetime
detector. The solutions were purged with N2 for at least 10 min. The
experiments were performed at room temperature (λexc = 375 nm).
[13] S. Battacharyya, A. Kibel, G. Kodis, P. A. Liddell, M. Gervaldo, D. Gust, S.
Lindsay, Nano Lett. 2011, 11, 2709–2714.
[14] D. Gerster, J. Reichert, H. Bi, J. V. Barth, S. M. Kaniber, A. W. Holleitner, I.
Visoly-Fisher, S. Sergani, I. Carmeli, Nat. Nanotechnol. 2012, 7, 673–676.
[15] M. Vadai, N. Nachman, M. Ben-Zion, M. Bürkle, F. Pauly, J. C. Cuevas, Y.
Selzer, J. Phys. Chem. Lett. 2013, 4, 2811–2816.
[16] S. M. Parker, M. Smeu, I. Franco, M. A. Ratner, T. Seideman, Nano Lett.
2014, 14, 4587–4591.
[17] H. Löfas, B. O. Jahn, J. Wärna, R. Emanuelsson, R. Ahuja, A. Grigoriev, H.
Ottosson, Faraday Discuss. 2014, 174, 105–124.
[18] L. Y. Hsu, D. Xie, H. Rabitz, J. Chem. Phys. 2014, 141, 124703.
[19] H. Cao, M. Zhang, T. Tao, M. Song, C. Zhang, J. Chem. Phys. 2015, 142,
084705.
[20] J. K. Viljas, F. Pauly, J. C. Cuevas, Phys. Rev. B 2007, 76, 033403.
[21] J. K. Viljas, F. Pauly, J. C. Cuevas, Phys. Rev. B 2008, 77, 155119.
[22] M. Galperin, A. Nitzan, J. Chem. Phys. 2006, 124, 234709.
[23] L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura, F. Barigelletti, Top. Curr.
Chem. 2007, 281, 143–203.
[24] R. C. Evans, P. Douglas, C. J. Winscom, Coord. Chem. Rev. 2006, 250, 2093–
2126.
[25] R. D. Costa, E. Ortí, H. J. Bolink, F. Monti, G. Accorsi, N. Armaroli, Angew.
Chem. 2012, 124, 8300; Angew. Chem. Int. Ed. 2012, 51, 8178–8211.
[26] A. Ruggi, F. W. B. van Leeuwen, A. H. Velders, Coord. Chem. Rev. 2011,
255, 2542–2554.
Computational Details: DFT calculations were performed with the
D.01 revision of the Gaussian 09 program package[56] with the
Becke three-parameter B3LYP exchange-correlation functional[57]
together with the 6-31G** basis set for C, H, N, O, and S[58] and the
“double-ꢁ” quality LANL2DZ basis set for Ir.[59] The geometries of
the singlet ground state (S0) and the lowest-energy triplet state (T1)
were optimized within the C2 symmetry group. The geometry of
the first triplet state was calculated at the spin-unrestricted UB3LYP
level with a spin multiplicity of three. All calculations were per-
formed in the presence of the solvent (dichloromethane). Solvent
effects were considered within the self-consistent reaction field
(SCRF) theory by using the SMD keyword that performs a polarized
continuum model (PCM)[60–62] calculation by using the solvation
model of Truhlar et al.[63] The SMD solvation model is based on the
polarized continuous quantum chemical charge density of the so-
lute (the “D” in the name stands for “density”).
[27] Y. Ohsawa, S. Sprouse, K. A. King, M. K. DeArmond, K. W. Hanck, R. J.
Watts, J. Phys. Chem. 1987, 91, 1047–1054.
[28] C. Dragonetti, L. Falciola, P. Mussini, S. Righetto, D. Roberto, R. Ugo, A.
Valore, F. De Angelis, S. Fantacci, A. Sgamellotti, M. Ramon, M. Muccini,
Inorg. Chem. 2007, 46, 8533–8547.
[29] K. K. W. Lo, C. K. Chung, T. K. M. Lee, L. H. Lui, K. H. K. Tsang, N. Zhu,
Inorg. Chem. 2003, 42, 6886–6897.
[30] F. Neve, M. La Deda, A. Crispini, A. Bellusci, F. Puntoriero, S. Campagna,
Organometallics 2004, 23, 5856–5863.
[31] Q. Zhao, S. Liu, M. Shi, C. Wang, M. Yu, L. Li, F. Li, T. Yi, C. Huang, Inorg.
Chem. 2006, 45, 6152–6160.
[32] X. Zeng, M. Tavasli, I. F. Perepichka, A. S. Batsanov, M. R. Bryce, C. J.
Chiang, C. Rothe, A. P. Monkman, Chem. Eur. J. 2008, 14, 933–943.
[33] R. V. Kiran, C. F. Hogan, B. D. James, D. J. D. Wilson, Eur. J. Inorg. Chem.
2011, 4816–4825.
[34] R. Huber, M. T. González, S. Wu, M. Langer, S. Grunder, V. Horhoiu, M.
Mayor, M. R. Bryce, C. Wang, R. Jitchati, C. Schönenberger, M. Calame, J.
Am. Chem. Soc. 2008, 130, 1080–1084.
Acknowledgments
The authors would like to especially acknowledge Prof. Hans U.
Güdel for his valuable inspiration and friendship. The present
work has been funded by the European Union (EU), (ERC Ad-
vanced Grant SPINMOL), the Spanish Ministerio de Economía y
Competitividad (MINECO) (projects MAT2014-56243, CTQ2015-
71154, and Unidad de Excelencia María de Maeztu MDM-2015-
0552), and the Generalitat Valenciana (Prometeo/2012/053,
PrometeoII/2013/006 and ISIC-Nano). J. P., I. V. and S. T. thank
the Ministerio de Ciencia e Innovación (MICINN) for their pre-
doctoral fellowships and JdlC contracts. J. A., I. V., and S. T. also
thank the European Union (EU) for their FP7-PEOPLE-2012-IEF-
329513, PCIG12GA-2012-334257 and MSCA-IF-657465, and FP7-
PEOPLE-2012-CIG-321739 grants, respectively.
[35] W. Hong, D. Z. Manrique, P. Moreno-García, M. Gulcur, A. Mishchenko,
C. J. Lambert, M. R. Bryce, T. Wandlowski, J. Am. Chem. Soc. 2012, 134,
2292–2304.
[36] J. Ponce, C. R. Arroyo, S. Tatay, R. Frisenda, P. Gaviña, D. Aravena, E. Ruiz,
H. S. J. Van Der Zant, E. Coronado, J. Am. Chem. Soc. 2014, 136, 8314–
8322.
Keywords: Iridium · N ligands · Photophysical properties ·
Density functional calculations · Anchoring groups · Molecular
junctions
[37] K. D. Glusac, S. Jiang, K. S. Schanze, Chem. Commun. 2002, 8, 2504–2505.
[38] K. Y. Kim, R. T. Farley, K. S. Schanze, J. Phys. Chem. B 2006, 110, 17302–
17304.
[39] M. J. E. Resendiz, J. C. Noveron, H. Disteldorf, S. Fischer, P. J. Stang, Org.
Lett. 2004, 6, 651–653.
[40] J. W. Ciszek, J. M. Tour, Tetrahedron Lett. 2004, 45, 2801–2803.
[41] Y. Saitoh, T. Koizumi, K. Osakada, T. Yamamoto, Can. J. Chem. 1997, 75,
1336–1339.
[42] S. Huang, J. M. Tour, J. Org. Chem. 1999, 64, 8898–8906.
[43] R. Ziessel, J. Suffert, M. Youinou, J. Org. Chem. 1996, 61, 6535–6546.
[44] D. T. Gryko, C. Clausen, K. M. Roth, N. Dontha, D. F. Bocian, W. G. Kuhr,
J. S. Lindsey, J. Org. Chem. 2000, 65, 7345–7355.
[1] T. Shamai, Y. Selzer, Chem. Soc. Rev. 2011, 40, 2293–2305.
[2] M. Galperin, A. Nitzan, Phys. Chem. Chem. Phys. 2012, 14, 9421–9438.
[3] S. V. Aradhya, L. Venkataraman, Nat. Nanotechnol. 2013, 8, 399–410.
[4] S. Rigaut, Dalton Trans. 2013, 42, 15859–15863.
[5] D. Dulić, S. J. van Der Molen, T. Kudernac, H. T. Jonkman, J. J. D. De Jong,
T. N. Bowden, J. van Esch, B. L. Feringa, B. J. van Wees, Phys. Rev. Lett.
2003, 91, 207402.
[6] A. C. Whalley, M. L. Steigerwald, X. Guo, C. Nuckolls, J. Am. Chem. Soc.
2007, 129, 12590–12591.
[7] E. S. Tam, J. J. Parks, W. W. Shum, Y. W. Zhong, M. B. Santiago-Berríos, X.
Zheng, W. Yang, G. K. L. Chan, H. D. Abruña, D. C. Ralph, ACS Nano 2011,
5, 5115–5123.
[45] S. Sprouse, K. A. King, P. J. Spellane, R. J. Watts, J. Am. Chem. Soc. 1984,
106, 6647–6653.
[8] S. Lara-Avila, A. V. Danilov, S. E. Kubatkin, S. L. Broman, C. R. Parker, M. B.
Nielsen, J. Phys. Chem. C 2011, 115, 18372–18377.
[9] Y. Kim, T. J. Hellmuth, D. Sysoiev, F. Pauly, T. Pietsch, J. Wolf, A. Erbe, T.
Huhn, U. Groth, U. E. Steiner, E. Scheer, Nano Lett. 2012, 12, 3736–3742.
[46] M. Lepeltier, T. K. M. Lee, K. K. W. Lo, L. Toupet, H. Le Bozec, V. Guerchais,
Eur. J. Inorg. Chem. 2005, 110–117.
Eur. J. Inorg. Chem. 2016, 1851–1859
www.eurjic.org
1858
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[47]
[48]
[49]
[50]
[51]
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Mont-
gomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, revision D.01,
Gaussian, Inc., Wallingford CT, 2009.
L. Zhao, K. Ghosh, Y. Zheng, M. M. Lyndon, T. I. Williams, P. J. Stang, Inorg.
Chem. 2009, 48, 5590.
F. Neve, A. Crisping, S. Campagna, S. Serroni, Inorg. Chem. 1999, 38,
2250–2258.
P. Didier, I. Ortmans, A. K. Mesmaeker, R. J. Watts, Inorg. Chem. 1993, 32,
5239–5245.
A. B. Tamayo, B. D. Alleyne, P. I. Djurovich, S. Lamansky, I. Tsyba, N. N. Ho,
R. Bau, M. E. Thompson, J. Am. Chem. Soc. 2003, 125, 7377–7387.
K. A. Walters, K. D. Ley, C. S. P. Cavalaheiro, S. E. Miller, D. Gosztola, M. R.
Wasielewski, A. P. Bussandri, H. van Willigen, K. S. Schanze, J. Am. Chem.
Soc. 2001, 123, 8329–8342.
[57] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789.
[58] M. M. Francl, W. J. Pietro, W. J. Hehre, J. S. Binkley, M. S. Gordon, D. J.
DeFrees, J. A. Pople, J. Chem. Phys. 1982, 77, 3654–3665.
[59] P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 299–310.
[60] J. Tomasi, M. Persico, Chem. Rev. 1994, 94, 2027–2094.
[61] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 2005, 105, 2999–3093.
[62] C. S. Cramer, D. G. Truhlar, in: Solvent Effects and Chemical Reactivity (Eds.:
O. Tapia, J. Bertrán), Kluwer, Dordrecht, The Netherlands, 1996, p. 1–80.
[63] A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B 2009, 113,
6378–6396.
[52] E. S. Manas, L. X. Chen, Chem. Phys. Lett. 2000, 331, 299–307.
[53] F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry: A Comprehen-
sive Text, John Wiley, New York, 1980.
[54] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Zelewsky,
Coord. Chem. Rev. 1988, 84, 85–277.
[55] F. Lafolet, S. Welter, Z. Popovic, L. De Cola, J. Mater. Chem. 2005, 15,
2820–2828.
[56] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nak-
atsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
Received: December 3, 2015
Published Online: March 21, 2016
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