Design and synthesis of non-symmetric phenylpyridine type ligands. Experimental and theoretical studies of their corresponding iridium complexes

Article abstract of DOI:10.1016/j.poly.2016.07.029

In this work three non-symmetric phenylpyridine type ligands, L1, L2 and L3, were designed, and their corresponding Iridium complexes, C1, C2 and C3, synthetized, in order to understand the effect of ligand asymmetry on the properties of the complexes, and to explore their potentiality in devices. The complexes were structurally characterized by NMR experiments and by X-ray Diffraction, and physicochemically by technics as UV/Vis and cyclic voltammetry. Theoretical DFT calculations of the energy and electronic density of the frontier orbitals of the complexes under study were also performed. The energy of the HOMO and LUMO correlated well with the experimental electrochemical data, and supported the understanding of the processes observed.

Full text of DOI:10.1016/j.poly.2016.07.029

Polyhedron 118 (2016) 159–170
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Design and synthesis of non-symmetric phenylpyridine type ligands.
Experimental and theoretical studies of their corresponding iridium
complexes
a
a,
⇑
a
b
a,
⇑
C. Iturbe , B. Loeb , M. Barrera , I. Brito , A. Cañete
a
Facultad de Química, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago, Chile
Departamento de Química, Universidad de Antofagasta, Avda. Angamos 601, Antofagasta, Chile
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work three non-symmetric phenylpyridine type ligands, L1, L2 and L3, were designed, and their
corresponding Iridium complexes, C1, C2 and C3, synthetized, in order to understand the effect of ligand
asymmetry on the properties of the complexes, and to explore their potentiality in devices. The com-
plexes were structurally characterized by NMR experiments and by X-ray Diffraction, and physicochem-
ically by technics as UV/Vis and cyclic voltammetry. Theoretical DFT calculations of the energy and
electronic density of the frontier orbitals of the complexes under study were also performed. The energy
of the HOMO and LUMO correlated well with the experimental electrochemical data, and supported the
understanding of the processes observed.
Received 12 June 2016
Accepted 22 July 2016
Available online 5 August 2016
Keywords:
Non-symmetric phenylpyridine ligands
Iridium complexes
Molecular calculations
NMR spectroscopy
Ó 2016 Elsevier Ltd. All rights reserved.
Electrochemical studies
1
. Introduction
maxima to the red. Additionally, in complexes with ppy type
ligands the HOMO electronic density is localized both over the
Transition Metal Chromophores have attracted great attention
for their capacity to respond in different applications. In this ambit,
phenyl ring and the metal center, while the LUMO level is centered
in the pyridine ring [8]. In literature, the reported synthetic proce-
dures on substituted bpy or ppy type ligands has been mainly been
focused on symmetrically substituted ligands.
The present study is oriented to the understanding of the effect
of the symmetry of the ligand, and its substituents, on some of the
properties of the corresponding complexes. The ligands used are
non-symmetric derivatives of ppy, and are tested on Iridium com-
2
+
polypiridyl compounds of transition metals, as Ru(bpy)
cyclometallated complexes, as Ir(ppy) (acac), are good triplet emit-
ters and have suitable spectral properties for different applications,
e.g., in OLEDs devices, or as dyes fixed to TiO in solar cells among
3
, or
2
2
,
others [1,2]. For this kind of complexes, the ligand plays an impor-
tant role to induce different photophysical properties or electro-
chemical behaviors [3]. Accordingly, in the literature there are
2
plexes of type Ir(R-ppy) (acac). The ligands and complexes were
many spectral studies with ligands based on 2-phenylpyridine
spectroscopically and crystallographically characterized. A theo-
retical study was also performed, in order to understand the ten-
dencies observed in the experimental results for the series of
Iridium complexes with non-symmetric ppy ligands studied.
0
(
ppy) or 2,2 -bipyridine (bpy), coordinated to different transition
metals [4]. Complexes with red-orange to deep red phosphores-
cence and high emission quantum yields have been obtained. This
kind of properties are related to the gap between the lowest unoc-
cupied molecular orbital (LUMO) to the highest occupied molecu-
lar orbital (HOMO). In special, it is possible to adjust the HOMO–
LUMO gap simply by changing the ligand framework; this change
affects directly the emission color [5–7]. For example, given its
donor properties, in complexes with ligands of 2-phenylpyridine
2. Experimental
2.1. General methods
(
ppy) type, the HOMO energy level is destabilized compared to
All starting materials were commercially available and used as
received without further purification. Thin Layer Chromatography
(TLC) was performed on silica gel 60 F254, using aluminum plates
and visualized with vanillin stain and UV lamp. Flash chromatogra-
phy was carried out on hand packed columns of silica gel 60 (230–
the equivalent complexes with bpy type ligands. This reduces the
HOMO–LUMO energy gap, and shifts the absorption and emission
⇑
Corresponding authors.
4
00 mesh). All NMR experiments were recorded on a Bruker
http://dx.doi.org/10.1016/j.poly.2016.07.029
277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
0

1
60
C. Iturbe et al. / Polyhedron 118 (2016) 159–170
Avance of 400 MHz, 1H NMR spectra were recorded at 400 MHz
C-Br) cm . 1H NMR (400 MHz, CDCl
ꢀ1
, ppm) d 8.36 (d, J = 5.1 Hz,
3
using CDCl
3
1
as the solvent and TMS as internal standard
1H), 7.85 (s, 1H), 7.65 (d, J = 5.0 Hz, 1H), 3.82 (s, 3H).
3
(
0.00 ppm). C NMR spectra were recorded in CDCl
3
at 101 MHz.
The electrochemical characterization of the Iridium complex was
performed by means of cyclic voltammetry (CV) in ACN as a sol-
vent, using a three-electrode cell consisting of a platinum working
electrode, a platinum wire auxiliary electrode and standard
Ag/AgCl electrode as the reference electrode with a scan rate of
3.1.1.3. Methyl 2-(m-tolyl)isonicotinate (L1). In a micro reactor ves-
sel, bromopyridne (3) (3.1 eq), 30 mL of DME, and Pd(Ph )
3 4
(0.015 eq) were added. The mixture was stirred for 10 min under
inert atmosphere. When the time was over, boronic acid (4,
1.2 eq) and K
at its boiling point for 2 days. Finally the reaction was filtered
under vacuum and the filtrate was extracted with CHCl . The crude
2 3
CO (2.4 eq) were added. The mixture was heated
ꢀ1
ꢀ1
ꢀ1
1
00 mV * s
.
The solutions were 1 ꢁ 10 mol/L
in the
corresponding complex, and contained tetrabutylamonium hex-
3
ꢀ1
ꢀ1
afluorophosphate 1 ꢁ 10 mol/L as supporting electrolyte. The
emission spectra were recorder on a PerkinElmer LS 55 Fluores-
cence spectrometer with slit of 2.5 cm. The absorption spectra
were recorder on a Shimadzu UV-3101PC UV–VIS-NIR Spectropho-
tometer. The X-rays diffraction were obtained on a STOE IPDS II
two-circle. The structure was solved by direct methods using
product was purified by silica gel chromatography using petroleum
benzine/ethyl acetate (1:1) as mobile phase. The contents of the
organic phase were removed in vacuo to obtain an orange oil cor-
responding to methyl 2-(m-tolyl)isonicotinate, L1, with 80% yield.
1
3
H NMR (400 MHz, CDCl , ppm) d, 8.81 (d, J = 5.0 Hz, 1H), 8.28
(s, 1H), 7.88 (s, 1H), 7.82 (d, J = 7.8 Hz, 1H), 7.75 (dd, J = 5.0,
1
3
1
1
.3 Hz, 1H), 7.37 (t, J = 7.7 Hz, 1H), 7.27 – 7.23 (m, 1H), 3.97 (s,
SHELXS; and Figures were created using the PLATON software.
Finally, for the quantum yields for all complexes, using [Ru
1
3
H), 2.44 (s, 3H).
3
C NMR (101 MHz, CDCl , ppm) 165.79,
58.62, 150.31, 138.57, 138.37, 138.14, 130.36, 128.79, 127.78,
27.64, 124.19, 121.05, 119.96, 119.77, 77.16, 52.75, 21.52.
(
3 6
bpy) ](PF )2, as reference compound was calculated using stan-
dard reported methods [9].
3.1.1.4. Methyl 2-phenylisonicotinate (L2). This ligand was obtained
3
. Computational details
with 86% yield using a similar procedure than the one described for
L1.
1H NMR (400 MHz, CDCl3, ppm) d, 8.75 (d, J = 4.8 Hz, 1H), 8.21
(s, 1H), 7.97 (d, J = 7.2 Hz, 2H), 7.68 (d, J = 4.8 Hz, 1H), 7.39 (dt,
All calculations were performed on ADF framework [10] and the
geometries were optimized with the OPBE [11] exchange correla-
tion functional, including the scalar relativistic correction [12] in
conjunction with ZORA-TZP and DZP basis set for Iridium and
atoms from the first row. Hirschfeld fragment analysis [13] was
employed to study the composition of the molecular orbitals in
sets of four fragments, including the metal core(M), the ancillary
acac ligand(L), and the phenylpyridine ligand, which in turn is
decomposed in the two fragments: phenyl(F) and piridyne(P).
The TDDFT calculations were performed in presence of acetonitrile
as solvent and employing a Klamt surface [14]. The first 40 allowed
and not allowed excitations were calculated with ZORA-SZ basis
set for C,H,N,O and the SOAP exchange correlation potential [15].
1
3
J = 21.7, 7.0 Hz, 3H), 3.90 (s, 3H). C NMR (101 MHz, CDCl3,
ppm), 165.88, 158.57, 150.55, 138.61, 138.2, 129.58, 128.97,
127.10, 121.23, 119.81, 52.83.
3.1.1.5. Methyl 2-(m-tolyl)pyridine (L3). This ligand was obtained
with 86% yield using a similar procedure than the one described
for L1.
1
H NMR (400 MHz, CDCl , ppm) d 8.53 (d, J = 4.9 Hz, 1H), 7.97
3
(d, J = 7.2 Hz, 2H), 7.52 (s, 1H), 7.45 (t, J = 7.3 Hz, 2H), 7.39
(d, J = 7.2 Hz, 1H), 7.02 (d, J = 4.4 Hz, 1H), 2.38 (s, 3H). 13C NMR
(101 MHz, CDCl , ppm) d 157.45, 149.50, 147.79, 139.62, 128.81,
3
1
28.70, 127.01, 123.20, 121.60, 21.28.
3.1. Procedure for ligand synthesis
2
3.2. General procedure to obtain [Ir(C^N)2(l-Cl)] dimers [17]
3
3
.1.1. Preparation of methyl 2-(m-tolyl)isonicotinate (L1) [16]
.1.1.1. 2-Bromoisonicotinic acid (2). A mixture of 2-bromo-4-
All dimers were synthetized by the following procedure:
To a 100 mL round bottom flask 1 eq of IrCl in methylethylene
3
methylpiridine, 1, (9.27 g, 54 mmol) in 490 mL of water, and of
KMnO (16 g, 11 mmol) in 250 mL of water were stirred for 5 h
at 110 °C. The resulting mixture was filtered and the filtrate was
reduced to 1/3 and acidified with HCl until pH 3. The white precip-
itated obtained was filtered and dried, Yield: 36% of 2-bromoison-
icotinic acid.
glycol (5 mL for each 150 mg) were added. The flask was attached
to a condenser and the system set under nitrogen atmosphere, and
then the corresponding L(i) ligand (2.5 eq) was added. The reaction
was carried out at reflux in a silicone bath for a period of 12 h.
Then, the heating was stopped and the reaction allowed to cool.
The contents of the flask were removed in vacuo to afford the crude
compound was purified with a minimal amount of hexane/ethyl
acetate (1:1) and stirred a few seconds. Finally, a few drops of hex-
ane were added to precipitate the complex that was separated
from the solution by filtering, yield 98%.
4
FT-IR (KBr)
m, 3100–2350 (stretching O–H), 1710 (str. C@O),
1
cm
0
597 (str. C@C), 1546 (str. C@N), 1285 (str. C–O), 667 (str. C-Br)
ꢀ
1
1
.
6
H NMR (400 MHz, DMSO-d , ppm) d, 8.58 (dd, J = 5.0,
.8 Hz), 7.95 (dd, J = 1.4, 0.8 Hz), 7.84 (dd, J = 5.0, 1.4 Hz).
3
.1.1.2. Methyl 2-bromoisonicotinate (3). A mixture of 2-bromoison-
3.2.1. Bis[
l
-chloro di-(2-m-tolylisonicotinate)iridium III] (5),
H NMR (400 MHz, CDCl , ppm) d 9.31 (d, J = 6.0 Hz, 3H), 8.42 (s,
3H), 7.44 (s, 3H), 7.39–7.08 (m, 5H), 6.45 (d, J = 7.5 Hz, 3H), 5.77 (d,
1
icotinic acid, 2, (0.200 g, 0.99 mmol) in acetone and K
1
time was over CH
ture refluxed for 2 h. Finally the mixture was cooled and left over
ice, and the product extracted with CHCl . The organic phase was
dried with Na SO , and purified by silica gel chromatography with
CHCl as mobile phase. The contents of the organic phase were
removed in vacuo to obtain methyl 2-bromoisonicotinate with
3
PO
4
(0.290 g,
3
.09 mmol) was stirred for 30 min at room temperature. When the
1
3
3
I (0,155 g, 1.09 mmol) was added and the mix-
J = 7.9 Hz, 3H), 4.12 (s, 9H), 2.15 (s, 10H). C NMR (101 MHz,
CDCl , ppm) d 169.30, 164.80, 151.54, 142.26, 140.85, 137.12,
130.87 (d), 129.99, 125.00, 120.80, 117.69, 52.89, 20.65.
3
3
2
4
3
3.2.2. Bis[l-chloro di(2-phenylisonicotinate)iridium III] (6)
1
Yield 99%. H NMR (400 MHz, CDCl
3
, ppm) d 9.32 (s, 1H), 8.45 (s,
8
2% yield.
FT-IR (KBr)
1H), 7.63 (d, J = 7.6 Hz, 1H), 7.20 (d, J = 5.5 Hz, 1H), 6.81 (t,
J = 7.3 Hz, 1H), 6.61 (t, J = 7.4 Hz, 1H), 5.90 (d, J = 7.7 Hz, 1H), 4.12
3
m = 3089 (str. C–H sp ), 2955 (str. C–H sp2), 1722
1
3
(
str. C@O), 1588 (str. C@C aromatic), 1546 (str. C@N), 670 (str.
(s, 3H).
3
C NMR (101 MHz, CDCl , ppm) d 169.64, 164.96,

C. Iturbe et al. / Polyhedron 118 (2016) 159–170
161
1
1
51.89, 145.32, 142.80, 137.73, 130.63, 130.02, 124.66, 122.14,
21.35, 118.15, 53.23.
2-phenylisonicotinate, (L2), m-tolyl)pyridine, (L3), and used as
starting materials to synthetize Iridium (III) cyclometalated com-
plexes, in order to evaluate the effect of ligand asymmetry on the
complex properties, and eventually to predict their potentiality
in solar cells and OLED devices. Ligands were synthetized using
2-bromo-4-methylpyridine 1 as starting material; the methyl
group of 1 was oxidized by conventional methodology using
3.2.3. Bis[
l-chloro di-2-(m-tolyl)pyridinyl)iridium III] (7)
1
Yield > 99%. H NMR (400 MHz, CDCl
3
, ppm) d 9.24 (d, J = 4.5 Hz,
1
6
2
1
7
H), 7.82 (d, J = 7.4 Hz, 1H), 7.66 (t, J = 6.9 Hz, 1H), 7.29 (s, 1H),
.76–6.66 (m, 1H), 6.40 (d, J = 7.2 Hz, 1H), 5.80 (d, J = 7.6 Hz, 1H),
1
3
.10 (s, 3H). C NMR (101 MHz, CDCl
3
, ppm) d 152.14, 143.95,
4
KMnO to prepare the respective acid 2. Subsequently, this acid
36.26, 130.74, 130.43, 124.77, 122.19, 118.58, 77.70, 77.38,
7.06, 21.21.
was sterificated by methyl iodide in basic media to obtain 3. By a
Suzuki–Miyaura reaction between 3 and 4 it was possible to obtain
the L1 ligand (Scheme 1).
By the same strategy, the ester 3, or 2-bromopyridine, and the
respective boronic acid under Suzuki–Miyaura conditions allowed
to obtain the C^N ligands L2 and L3 (Scheme 2).
3
2
.3. General procedure to obtain [Ir(C^N) (acac)] [17]
To a 100 mL round bottom flask 1 eq of [Ir(C^N)
eq of acac in presence of TBAOH were added, using CH
2
(
l
-Cl)]
2
and
To obtain the Ir(C^N)
prepare the corresponding dimer intermediates [Ir(C^N)
in this case, these intermediates were synthesized by the reaction
between IrCl and the respective C^N ligand to obtain the follow-
ing complexes: [Ir(L1) -Cl)] , (5), [Ir(L2) -Cl)] , (6) and [Ir
2
(acac) complexes it is necessary to first
2
2
Cl as sol-
2
(l-Cl)] ;
2
vent. This reaction was refluxed for a period of 12 h. Then, the heat-
ing was stopped and the reaction allowed to cool. The contents of
the flask were removed in vacuo to afford the crude compound,
which was purified by successive washes with hexane. The precip-
itated complex was separated from the solution by filtration.
2
3
2
(l
2
2
(l
2
(
L3)
Subsequently, the Ir(C^N)
L2) (acac) C2 and Ir(L3) (acac) C3, were synthetized by the inter-
-Cl)] by acetylacetone
2 2
(l-Cl)] , (7) [18–20].
2
2
(acac) compounds, Ir(L1) (acac) C1, Ir
(
2
2
3.3.1. Acetylacetonate-di-2-(m-tolyl)isonicotinate iridium III, C1
1
change of chlorine atoms in [Ir(C^N)
acac) [17]:
These new complexes were characterized by different technics
2
(l
2
3
H NMR (400 MHz, CDCl , ppm) d 8.63 (d, J = 5.9 Hz, 2H), 8.38 (s,
(
2
2
1
1
1
H), 7.63 (dd, J = 5.9, 1.6 Hz, 2H), 7.48 (s, 2H), 6.57 (d, J = 8.6 Hz,
H), 6.10 (d, J = 7.7 Hz, 2H), 5.20 (s, 1H), 4.04 (s, 5H), 2.20 (s, 6H),
1
3
in order to obtain as much information as possible about the prop-
erties of the complexes. In addition to consider aspects as the influ-
ence of the acac ligand, or of metalation, on the properties of the
complexes, the effect of loss of symmetry on the 2-phenylpyridine
ligands, and the electronic effect of donors or acceptor substituents
on it, has to be analyzed. As a whole, this information should help
to predict if C1, C2 or C3 would be suitable for solar cells or OLED
applications (see Schemes 3 and 4).
3
.78 (s, 6H). C NMR (101 MHz, CDCl , ppm) d 184.77, 169.40,
65.01, 148.28, 144.24, 143.70, 143.26 (d), 137.50, 132.41,
30.95, 129.99, 125.12, 120.63, 117.66, 100.34, 52.75, 28.55, 20.85.
3.3.2. Acetylacetonate-di-2-phenylisonicotinate iridium III, C2
1
3
H NMR (400 MHz, CDCl , ppm) d 8.65 (d, J = 5.9 Hz, 2H), 8.42
(
s, 2H), 7.66 (d, J = 6.5 Hz, 4H), 6.85 (t, J = 7.4 Hz, 2H), 6.71 (t,
J = 7.3 Hz, 2H), 6.22 (d, J = 7.5 Hz, 2H), 5.22 (s, 1H), 4.04 (s, 6H),
1
3
1
1
1
3
.79 (s, 6H). C NMR (101 MHz, CDCl , ppm) d 185.23, 169.77,
4.1.1. NMR experiments
65.24, 148.66, 147.81, 143.86, 138.07, 133.09, 129.87, 124.69,
21.22, 120.07, 118.13, 100.68, 53.07, 28.83.
C1: The complexation of the monoanionic ligand acetylaceto-
nate in 5 is detectable by the appearance of two signals in the 1
H
NMR spectrum due to the presence of acac (Fig. S1), corresponding
to an allyl proton (5.2 ppm) and the methyl group of acac
3
.3.3. Acetylacetonate-di-2-(m-tolyl)pyridine iridium III, C3
1
3
H NMR (400 MHz, CDCl , ppm) d 8.50 (d, J = 5.4 Hz, 2H), 7.82
(
2.20 ppm). Additional signals at 1.78 ppm and 4.0 ppm are
(
(
d, J = 7.9 Hz, 2H), 7.69 (t, J = 7.7 Hz, 2H), 7.37 (s, 2H), 7.10
t, J = 6.1 Hz, 2H), 6.54 (d, J = 7.1 Hz, 2H), 6.15 (d, J = 7.7 Hz, 2H),
assigned to the corresponding methyl protons and to the methyl
ester groups present in the phenyl ring and pyridine ring, respec-
tively. Table 1 shows the complete chemical shift assignment for
.20 (s, 1H), 2.18 (s, 3H), 1.77 (s, 3H). 1 C NMR (101 MHz, CDCl
3
5
3
,
ppm) d 184.62, 168.79, 148.31, 144.77, 143.35, 136.73, 132.87,
1
1
13
the H and C spectra of C1 obtained by means of the concerted
30.62, 129.61, 124.76, 121.37, 118.36, 100.44, 28.88, 21.21.
4
. Results and discussion
MeO C
CH3
2
4.1. Synthesis and characterization
N
N
L3
L2
In the present work three non-symmetric phenylpyridine-based
ligands were synthetized:
2-(m-tolyl)isonicotinate, (L1),
Scheme 2. Structure of 2-phenylpyridine ligands L2 and L3.
CH3
CO H
CO Me
2
2
1
) K PO , acetone
KMnO4
reflux
3
4
1
)
2
) CH3I
N
Br
N
Br
N
Br
3
1
2
CO Me
2
MeO C
CH3
2
B(OH)2
Pd(PPh3)4
2
)
+
K CO , DME
2
3
N
N
Br
4
3
CH3
L1
Scheme 1. Synthetic route to obtain 2-phenylpyridine, L1.

1
62
C. Iturbe et al. / Polyhedron 118 (2016) 159–170
C
N
methylethylene glycol
N
C
Cl
Cl
C
N
2
IrCl xH O
+
4(C^N)
3
2
Ir
Ir
reflux
N
C
COOCH
3
COOCH
3
COOCH
3
COOCH
3
H C
H3C
3
CH3
CH3
CH3
CH3
N
N
N
N
N
N
Cl
Cl
Cl
Cl
Cl
Cl
Ir
Ir
Ir
Ir
Ir
Ir
N
N
N
N
N
N
H3C
H3C
H3COOC
COOCH3
H COOC
COOCH3
3
5
6
7
Scheme 3. Preparation of [Ir(C^N)
2
(l
-Cl)]
2
; 5, 6, and 7.
C
N
C
Ir
N
N
C
Cl
Cl
C
N
O
O
N
C
O
O
TB AOH
Ir
Ir
+
2
,
2 2
CH Cl
reflux
N
C
COOCH
3
COOCH3
CH
3
CH3
N
Ir
N
N
Ir
N
N
O
O
O
Ir
N
O
O
O
CH3
CH
3
COOCH
3
COOCH
3
C1
C2
C3
2 2
Scheme 4. Preparation of [Ir(C^N) (acac)] type complexes; C1, C2, and C3.
application of a variety of one- and two-dimensional techniques:
COSY, detecting one-bond (C–H), heteronuclear multiple quantum
coherence (HMQC), and long-range two- and three-bond C–H
the quaternary
143.39 ppm which correlates at long-range with H
examining the structure of C1, H correlates long-range with the
quaternary carbons C12 and C . The resonance at 117.66 ppm is
assigned to C , because it is long- range connected with H . The
C
7
corresponds to the signal resonating at
e
and H . By
f
b
heteronuclear multiple bond connectivity (HMBC).
1
1
2
D
H homonuclear COSY spectrum (Fig. 1) showed the
4
b
expected two-spin system as two doublets resonating at 7.63 and
.63 ppm. The more deshielded doublet at 8.63 ppm corresponds
to H (proton numbering given in the Figure of Table 1) and that
at 7.63 corresponds to H ,. The spectrum also showed other 2
two spin systems, attributed to the vicinal coupling between H
and H . However so far it is not possible to indicate which is which.
assignment of the other quaternary carbon resonance was straight-
forward. The quaternary carbon resonating at 184.77 ppm is
assigned as C16.
8
a
b
d
4
.1.2. X-ray crystal structures
Crystal Structures for complexes Ir(L1)
(C2), and Ir(L3) (acac) (C3) were determined. Displacement ellip-
e
1
13
2 2
(acac) (C1), Ir(L2) (acac)
The complete H and C NMR chemical shift assignments for
1
2
C1 was deduced from the concerted application of H-detected
soids drawings of the compounds C1, C2, and C3 are displayed in
Figs. 2–4, respectively, and the crystal data and coordination geo-
metrical parameters are listed in Tables 2 and 3. The crystallo-
graphic data for all the structures reported have been deposited
‘
‘one-bond” and long-range (C,H) correlation experiments. The
one-bond proton-carbon chemical shift correlation was estab-
lished using the HMQC sequence (Fig. S2); all the ‘‘one-bond”
(
C,H) connectivities found between the pairs of mutually coupled
1
in the Cambridge Database . All the crystals were obtained from
atoms are given in Table 1.
The HMBC spectrum shows long-range correlations between Ha
and three carbon resonances (Fig. S3). Examining the structure of
1
Crystallographic data (excluding structure factors) for the structures reported in
this paper have been deposited in the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 997795–997797. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: (044) 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk).
C1, the resonance corresponding to C
and C can be identified. C quaternary carbons show long range
correlation with H and H , while C correlates with H and H . Also,
2
, and two quaternary carbons
C
3
5
5
c
f
6
c
d

C. Iturbe et al. / Polyhedron 118 (2016) 159–170
163
Table 1
Assignment of 1H and 13C NMR of [Ir(L1)
2
(acac)] (C1).
1
3
12
14
10
H
3
CO
2
C
Hc Hf
CH
9
3
g
4
h
3
11
5
Hb
He
2
6
N
8
1
7
Ha
Ir
Hd
2
O
O
1
7
17
16
16
H
3
C
CH
3
15
j
j
Hi
1H*
d (ppm)
13C*
d (ppm)
13C**
d (ppm)
Fig. 2. Displacement ellipsoids drawing of C1 showing the labeling scheme of
atoms at 30% probability level. The acetonitrile solvent molecule was omitted for
clarity.
H
H
H
H
H
H
H
H
H
H
a
b
c
d
e
f
8,63
7,63
8,38
6,1
6,57
7,48
4,04
2,2
C
C
1
2
148,28
120,63
117.66
132,41
130,95
125,12
165,01
20,85
C
C
C
C
C
C
C
3
137,5
169,4
143,14
143,39
129,99
52,75
28,55
5
C4
6
C
C
C
C
C
C
C
8
7
between 175.31(15) and 176.7(2)°. The shortest coordination dis-
tances are associated with the Ir–C bonds while the longest are
related to Ir–O bonds. The average Ir–O bond length observed in
the complexes [2.147(4) Å] is considerably longer than the mean
Ir–O value of 2.088 Å reported in the Cambridge Crystallographic
Database and reflects the pronounced trans influence of the phenyl
rings.
9
10
13
17
11
12
14
15
16
g
h
i
5,2
1,78
100,34
184,77
j
*
*
Shows the direct bond between H-C.
Quaternary carbon.
*
A comparison of the complexes indicates that complex C2 pos-
sesses the shorter Ir–O distances [2.142(3) vs 2.149(4) Å] indica-
tive of a comparatively weaker transophobic behavior [21].
In the three complexes, the C–Ir–C and N–Ir–N bond angles
assume the order C2 < C3 < C1; C3 < C2 < C1 respectively, while
the O–Ir–O angles assume the order C3 < C1 < C2. Geometrical
parameters such as bond lengths and angles around the Ir atom
^
are in a similar range than reported for other (C N)
2
Ir(acac) com-
plexes [22–24].
In the complex C1, C2 and C3 the dihedral angles between pyr-
idine and phenyl rings of the ppy ligands are 8.4(4)° and 5.5(4)°;
5
.1(2)° and 3.7(14)° respectively. The relatively small dihedral
angles between the aromatic rings are consistent with a large
amount of conjugation across the ppy ligand. The electronic prop-
erties of the aromatic system are not affected by this effect [25].
4.2. Spectrophotometric study
UV–Vis absorption spectra for complexes C1, C2 and C3 mea-
sured in acetonitrile solution at room temperature are shown in
Fig. 6, and the corresponding data in Table 4. The spectral data
for the corresponding dimer precursors is shown in Table S4. As
reported in literature [22] for compounds of type C1, C2 and C3,
Fig. 1. COSY spectrum of C1.
mixed solution of dichloromethane and n-hexane. Complex C1 crys-
tallizes with one acetonitrile molecule which lies on a twofold axis.
In the complexes C2 and C3, the complete molecule is generated by
crystallographic twofold symmetry, with the Ir and C2 atoms lying
on the rotation axis. For complexes C1 and C2, the crystal structure
is stabilized only by van der Waals interactions. However for
complex C3, the crystal structure is stabilized by van der Waals
interactions and by further stacking interaction between phenyl
and pyridine moieties, in which the stacking distance is 3.6876
strong bands with several maximum up to 300 nm appear and
⁄
are attributed to intraligand (
p–p
) transitions. At lower-energy,
absorption bands corresponding to metal-to-ligand charge transfer
(MLCT) transitions are also present.
Regarding these last bands, it can be observed that in the case of
C3, the presence of a donor methyl group in the L3 ligand raises the
energy of the LUMO orbital, enhancing therefore the HOMO–LUMO
gap and explaining in this way the hypsochromic shift of the
absorption pattern of this complex in regard to the other
complexes of the series. Additionally, the presence of an electro
withdrawing group in L1 and L2, by stabilizing the LUMO orbital,
also explains the tendency observed.
(
17) Å and a = 3.70(14)° (a = dihedral angle between centroids
defined by N11/C12-C16 and C21/C26 planes), Fig 5. As depicted in
Figs. 2–4, the three complexes adopt distorted-octahedral coordina-
tion geometry with a cis-O,O, cis-C,C, and trans-N,N chelate disposi-
tion, and with angles of the trans ligands at the metal center range
The emission spectrum of all complexes was registrated in ace-
tonitrile solution, and is shown in the Supplementary Material. The

1
64
C. Iturbe et al. / Polyhedron 118 (2016) 159–170
Fig. 3. Displacement ellipsoids drawing of C2 showing the labeling scheme of atoms at 30% probability level.
3
3
e.g. a triplet intraligand IL and a triplet charge transfer CT excited
state [26].
4.3. Electrochemical studies
The complexes reported in this work possess different func-
tional groups in their structure, in order to enable electronic trans-
fer (ET) modulation. Despite that all complex show both the
oxidation of the metal and the reduction of the ligand, the present
study was mainly focused on the metal oxidation process.
In Fig. 7 the cyclic voltammetry of C1, C2 and C3 is displayed.
For all complexes a quasi-reversible signal between 0.70 and
0
.95 V was observed. These results correlate well with the struc-
ture of the complexes because the complex that is easier to oxidize
ox
(E1/2 = 0.733V) is C3, due to the presence of a methyl group that con-
fers electron donating capacity to the ligand, facilitating the abstrac-
tion of one electron from the metal. On the other hand, in C2 the
presence of a withdrawing group generates a reduction of electronic
density on the metallic center, increasing the energy needed to
remove an electron. For C1 the presence of both types of substituents
balances the described behaviors, reflected in an oxidation potential
between both values. Specifically, a difference of 116 mV between
C1 and C3 and of 205 mV between C2 and C3 is observed, while a dif-
ference of 89 mV between C1 and C2 is observed, (see Table 6 and
Fig. 7).
Fig. 4. Displacement ellipsoids drawing of C3 showing the labeling scheme of
atoms at 30% probability level.
4.4. Theoretical calculations
presence of the electron withdrawing ester substituent in C1 and
C2 generates dual emission and the expected displacement of the
emission maxima to lower energy. Additionally, a close to one
order of magnitude higher quantum yield is observed for C3, as
can be seen in Table 5. In general, the three complexes studied in
this work show a considerable weaker emission than the parent
C0 compound. This behavior is probably due to a combination of
the shift to lower energies of the emission, generated specially
by the presence of the esther substituent in C1 and C2, and the
introduction of substituents that increases the number of vibra-
tional modes available for non-radiative deactivation. Indeed, C3
is a better emitter than C1 and C2. Although dual emission is not
uncommon for Iridium(III)–polypyridine complexes in glass at
low temperature, it is more rare in fluid solutions at room temper-
ature, while some recent examples can be found in literature
4.4.1. Geometry optimization
In order to obtain the closest approximation to the real geome-
try of the molecules, obtained by X-ray crystallography, several
exchange–correlation functional were tested on C3. The mean
absolute deviation (MAD) was calculated and the xc correlation
functional that minimizes these values, was chosen. In this sense,
the OPBE functional appeared to be the best performing when
employed in conjunction with a scalar relativistic correction. The
values found are displayed in Table 6.
Comparing these values with those from crystallographic data
of Table 3, a mean deviation of 0.019, 0.015 and 0.011 Å is found
respect to the interatomic experimental distance, and a net differ-
ence of ±1° for bond angles was observed. When analyzing the
above values, it can be seen that, as expected, the negative charge
on the carbon atoms of the phenyl fragment enlarges the metal-
acac oxygen atomic distance, i.e. the octahedral symmetry appears
[
26,27]. This behavior has been explained on the basis of an
interruption of communication between the two emitting states,

C. Iturbe et al. / Polyhedron 118 (2016) 159–170
165
Table 2
Crystal data for complexes C1, C2 and C3.
Properties
Complex C1
2(C29 28Ir N
669.82
293(2)
Complex C2
31IrN
743.80
173(2)
0.71073
Complex C3
2 6
27IrN O
715.74
173(2)
0.71073
0.140 ꢁ 0.110 ꢁ 0.020
monoclinic
C2/c
12.8967(6)
13.7225(5)
15.1286(7)
90
92.677(4)
90
2674.5(2)
4
1.778
5.041
Emprical formula
Fw (Dalton)
T (K)
Radiated used (k(Å))
Crystal size (mm )
Crystal system
Space group
a (Å)
b (Å)
c (Å)
H
2
O
2
)ꢂ(CH
3
CN)
C
33
H
2
O
6
C
31
H
0.71073
3
0.30 ꢁ 0.26 ꢁ 0.24
monoclinic
C2/c
26.144(5)
9.4460(19)
22.784(5)
90
106.63(3)
90
5391.2(19)
4
0.320 ꢁ 0.280 ꢁ 0.130
orthorhombic
Pccn
7.4478(5)
23.430(2)
17.4357(15)
90
90
90
3042.6(4)
4
a
(°)
b (°)
(°)
c
3
V (Å )
Z
q
l
calc (g cmꢀ3)
1.600
4.983
1.624
4.435
ꢀ
1
(mm
)
F(000)
2560
1472
1408
Scan range h (°)
Number of total reflections
1.63–26.14
10608
2.870–25.007
9121
2.514–25.341
15,171
Number of unique reflections (Rint
)
5245 (0.0280)
5245/0/327
1.080
2668 (0.0426)
2668/0/194
0.969
2455 (0.0691)
2455/0/184
1.087
Number of data/restraints/parameters
2
Goodness-of-fit (GOF) on F
R
R
1
, wR
, wR
2
[I>
(all data)
r
2(I)]
0.0437 and 0.1224
0.0506 and 0.1225
0.0437 and 0.1168
R1 = 0.0466 and 0.1167
0.0202 and 0.0482
0.0214 and 0.0487
1
2
Table 3
Selected bond distances (Å) and bond angles for complexes 1, 2 and 3.
Complex
Ir–C (Å)
1.993(6)
Ir–O (Å)
Ir–N (Å)
N–Ir–N(°)
C–Ir–C(°)
O–Ir–O(°)
C–Ir–N(°)
N–Ir–O(°)
C–Ir–O (°)
C1
2.143(5)
2.149(4)
2.049(5)
2.054(5)
175.3(2)
94.2(2)
88.03(19)
80.5(2)
95.7(2)
87.0(2)
96.5(2)
96.9(2)
86.4(2)
88.0(2)
2.003(6)
176.7(2)
175.3(2)
89.9(2)
9
8
6.4(3)
0.9(3)
C2
C3
1.985(5)
1.994(3)
2.142(3)
2.155(2)
2.031(4)
2.029(3)
174.3(2)
89.1(3)
88.45(18)
87.80(12)
80.70(18)
5.20(17)
80.73(11)
94.21(9)
94.61(14)
89.49(14)
91.16(9)
90.61(11)
175.31(15)
91.43(18)
174.67(9)
9
172.54(13)
91.42(16)
94.03(11)
Fig. 5. A partial view of the crystal packing of the complex 3, showing the weak p–p
stacking interactions.
Fig. 6. Absorption bands of C1, C2 and C3 complexes in acetonitrile.
with a minor distortion, in a similar way than reported for analo-
gous complexes [22].
played on Fig. 8 show the same orbital ordering tendency for the
three complexes under study in regard to the unsubstituted Ir
(
2
ppy) (acac) complex, C0, used as reference. The differences
4
.4.2. Molecular orbital structure
Molecular orbital calculations were performed with scalar rela-
between them can be understood in terms of Hirschfeld fragment
analysis of the molecular orbitals for the four complexes. In this
sense, the phenylpyridine ligand is decomposed in two
tivistic correction in presence of acetonitrile as solvent. Results dis-

1
66
C. Iturbe et al. / Polyhedron 118 (2016) 159–170
Table 4
the 4-position of the pyridine ring in C1 is also reflected in the cal-
Absorption maxima for complexes C1, C2 and C3 in acetonitrile.
culated results, particularly in a raise of 0.16 eV in the HOMO, with
the consequent reduction of the band gap from 2.03 to 1.98 eV.
Experimentally, this is observed by a further displacement of the
emission to lower energy. These results are in agreement with sim-
ilar substituent electronic effects reported by Baranoff [28].
Fig. 8(b) displays the composition of each molecular orbital in
terms of the molecular fragments (P,F,M,L). As a general survey it
can be mentioned that the HOMO is composed mostly by M
Maximum absorption (nm)
C1
C2
C3
k
k
k
k
k
1
2
3
4
5
265
320
374
424
531
k
k
k
k
k
1
2
3
4
5
253
262
340
380
500
k
k
k
k
1
2
3
4
261
319
369
400
(
+
50%) and F (40%) components which result from a mixture of d
⁄
p
atomic orbitals. The HOMO-1 also contains a combination of
Table 5
metal and ligand orbitals but in this case the ligand is acac (60%).
At deeper energies the HOMO-2 is mainly metallic (76%) with
small contribution of orbitals from all the ligands.
Emission maximums and emission quantum yields of C1, C2 and C3.
*
C0
[
Ir(ppy)
2
](acac)
C1
C2
C3
Looking the unoccupied molecular orbitals, it follows that the
LUMO and LUMO + 1 are very close in energy, and mainly com-
posed by the pyridyl (P1, P2) fragments of each ppy ligands. In a
truly octahedral symmetry complete degeneration would be
observed, but calculations and crystallographic data show that
the octahedra are distorted, and that the two ppy ligands are asym-
metric. This asymmetry is the cause of the lost of degeneracy of the
LUMO, that is split in two sub-levels each one composed by only
one ligand. The energy gap created between the LUMO and
LUMO + 1 is of 30 meV for C0, and diminishes to 24 meV for C3,
with the methyl substituted ppy ligand, while it increases to 40
and 45 meV when the methyl ester group is present like in C1
and C2. At this point it is interesting to highlight that for all the
complexes the HOMO, HOMO ꢀ 1,LUMO and LUMO + 1 contains
the same fragment composition, however they differ in their% of
participation. Nevertheless, at the LUMO + 2 a noticeable dissimi-
larity appears that differences C0 and C1 from C2 and C3. In the
case of the first two, this molecular orbital is centered on the ancil-
lary (L) ligand while for the second pair it is centered on the ppy
ligand.
k
max (nm)
516
622
597
611
9.01 ꢁ 10
520
6
57
ꢀ
3
ꢀ3
1.40 ꢁ 10ꢀ2
ø
emis
0.34
2.24 ꢁ 10
*
Ref. [22], in 2-methyltetrahydrofuran solutions.
0.000006
-
--- C2
--- C1
--- C3
-
-
0.000004
0.000002
0.000000
-0.000002
-0.000004
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
4.5. Absorption and emission
+
E/V vs. Ag /AgCl
Calculated TDDFT values for the complexes studied in presence
Fig. 7. Cyclic Voltammetry for C1, C2 and C3 in ACN, HFPBA 0.1 mol/L, scan rate of
00 mV/s.
of acetonitrile as solvent are displayed in Table 7. The forthcoming
analysis starts with complex C3, which has a methyl substituent on
para position on the phenyl fragment of the cyclometalated ligand.
The absorption properties of the C3 complex should be quite sim-
1
Table 6
2
ilar to those reported for the parent complex (ppy) Ir(acac), C0. In
Calculated interatomic distances (Å) and bond angles.
general this type of complexes displays a strong absorption in the
UV region (300–400 nm) and two less intense bands in the visible
region, above 400 nm. For C3 a low intensity band at 443 nm is
found, with oscillator strength of 0.026. This band is composed
by an HOMO-1 to LUMO electronic transition with two competing
electron transfer reaction involved: a MLCT from the metal to the
pyridine ring (M ? P), and an electronic (L ? P) transition from
the acac ligand to the P fragment, which can be described as LLCT.
Moving to lower energy values, an absorption can be found at
488 nm, with oscillator strength of f = 0.009, associated to the
HOMO–LUMO transition and composed by two excitations: one
with MLCT character and another Ligand Centered excitation
(LC), originated on a charge transfer from the Phenyl (F) to the Pyr-
idyl (P) fragment. The same assignment is proposed for the bands
of complex C0 .
Complex
Ir–C
Ir–O
Ir–N
N–Ir–N
C–Ir–C
O–Ir–O
C1
C2
C3
1.972
1.967
1.968
2.163
2.171
2.166
2.007
2.003
2.015
174
174
176
92
91
90
88
85
88
components: the phenyl (F) and pyridyl (P) fragments, while the
other fragments are labeled as: (M) for the metal core and (L) for
the ancillary ligand (acac). Additionally, the phenylpyridyne
ligands are differentiated by labels 1 and 2. Results are depicted
in Fig. 8(a).
Among these three complexes C3 possesses the higher energy
gap (2.42 eV). The methyl substituent on the cyclometallated ring
causes a destabilization of the HOMO in 0.16 eV and 0.11 eV in
the LUMO, with respect to the unsubstituted complex, C0. On the
3
When the electron withdrawing COOCH group is introduced
other hand, the electron withdrawing substituent (–COOCH
complex C2, located on the pyridine ring, causes the stabilization
of the HOMO (0.18 eV) and more markedly, of the LUMO
3
), on
on the para position of the pyridine ring as in complex C2, a
remarkably change in the absorption spectra is observed: a med-
ium intensity band appears in the visible region at 403 nm with
oscillator strength of 0.100 followed by a lower energy excitation
at 536 nm with oscillator strength of f = 0.052 and displaying MLCT
and LLCT character.
(
0.66 eV). The net result is the reduction of the band gap energy
of this complex which is reflected in a bathochromic shift of the
emission maxima (Table 5). The presence of a methyl group in

C. Iturbe et al. / Polyhedron 118 (2016) 159–170
167
Fig. 8. (a) Molecular Orbital Diagram, (b) Composition of molecular orbital and its energies in term of fragment components (blue: phenyl (F), grey: pyridyl (P), pink: metal
M) and yellow: acac (L). Phenylpyridine ligands are differentiated by labels 1 and 2. (Color online.)
(
In principle the COOCH
character causes a stabilization of the LUMO from ꢀ3.72 eV to
4.44 eV which cause a bathochromic shift of the absorption band,
originated on the HOMO-1 to LUMO transition, which for C3 occurs
at 443 nm and in C2, moves to 536 nm. The influence of the ester
substituent is not only limited to a change in the energy at which
this electronic transition occurs. It is also accompanied with an
increase in the oscillator strength from 0.26 to 0.52.
3
group with its electron withdrawing
Analyzing the C1 complex, which is equivalent to C2, calcula-
tions reveals that this substituent do not alter significantly the
position of the kmax of both transition (403 and 532 nm). However,
an increase on the oscillator strength of the higher energy transi-
tion is observed.
On the other hand, and looking to the composition of the HOMO
and HOMO-1 for C1, C2, and C3 it can be seen that the presence of a
methyl ester group does not alter meaningfully the composition of
ꢀ

1
68
C. Iturbe et al. / Polyhedron 118 (2016) 159–170
Table 7
Calculated values from TDDFT.
Compound
Electronic transition
1a
k (nm)
Oscillator strength
Main excitation
Nature
Description
3
C1
532
405
0.051
0.114
88%H1 ? L0
48%H0 ? L2
M + L ? P
M ? P
MLCT/ LLCT
1b
MLCT
MLCT/ LC
MLCT/ LLCT
MLCT/ LC
3
28%H2 ? L1
M + F ? P + F
M + L ? P
M + F ? P
M + F ? P
M ? P
3
C2
C3
2a
536
403
0.052
0.100
85%H1 ? L0
57%H0 ? L2
3
2b
3
5
2
7%H0 ? L2
2%H2 ? L1
MLCT/ LC
MLCT
MLCT/ LLCT
3
3a
443
0.026
90%H1 ? L0
M + L ? P
Table 8
Composition and calculated values for the two lowest triplets.
a
State
k
EM (nm)
Mono excitation
Description
Experimental
622-657b
Complex C1
c
c
c
T
T
1
622
637
MLCT/ LC
c
2
MLCT/ LC
Complex C2
c
c
c
T
T
1
609
622
MLCT/ LC
c
597-611b
2
MLCT/ LC
Complex C3
c
c
c
c
T
T
1
516
521
MLCT/ LC/ LLCT
c
c
520b
516c
2
MLCT/ LC/ LLCT
Complex C0
c
c
c
c
T
T
1
503
508
MLCT/ LC/ LLCT
c
c
2
MLCT/ LC/ LLCT
a
b
c
Calculated.
This work.
Ref. [22].
these orbitals. This is not the case for the LUMO and LUMO + 1,
where the composition of the molecular fragments is quite differ-
ent for C1 and C0 respect to C2 and C3, where for the later the Phe-
nyl fragment does not contribute. Apparently the absence of this
fragment could result in an increase of the oscillator strength on
the transition involving these orbitals, allowing some electron
transfer process which in the presence of the F2 fragment is
forbidden.
Moreover, the gap between the two triplets remains constant
(5 nm) and is not influenced by this substituent. A different picture
is found when an ester group is present on the pyridine ring (C2),
which results in a bathocromic shift of the calculated emission
mean value, (kEM(T1)+
kEM(T2))/2, from 518 nm to 616 nm.
Another point to mention is the increase of gap between the two
triplets (13 nm) giving rise to a broadening of the emission band.
Interestingly, adding a methyl group to the phenyl ring as in com-
plex C1, results in new red shift displacement of the calculated
emission mean value to 630 nm. The larger bathocromic shift
occurring from 506 nm in C0 to 630 nm on C1 can be understood
in terms of the collaborative effect occurring between the electron
withdrawing substituent on the pyridine ring, which stabilizes the
LUMO, and the electron donating substituent on the phenyl ring
which destabilize the HOMO. This theoretical considerations corre-
late well with the experimental emission spectra (Fig. S16, Suple-
mentary Material) in the sense that C2 shows a broadening of
the emission band, while in C1, where the gap between T1 and
T2 is enhanced, dual emission is clearly observed.
Table 8 displays values for the two triplet non allowed lowest
excitations, T1 and T2. Because of their energetic proximity and
similar nature, these excitations need to be examined together;
they involve HOMO–LUMO and HOMO–LUMO + 1 excitations,
while the LUMO and LUMO + 1 are quasi degenerate levels. A third
triplet can also be mentioned, but it occurs at higher energies and
its role is not relevant for the present discussion. In general for the
3
complexes under study two type of transitions are found: MLCT
3
and LC. For the case of complexes with no substituent on the pyr-
idyl ring (C0, C3), a third process is observed that correspond to a
Ligand to Ligand Charge Transfer, arising between two similar frag-
ments, but in different ligands. This new transition results from the
increased participation of a Phenyl fragment (10–13%) in the LUMO
and LUMO + 1 of the above mentioned complexes which is not
found on the other complexes.
Regarding emission, considering in first place the unsubstituted
complex C0, an emission arising from T1 and T2 is predicted at 503
and 508 nm, above the experimental value of 516 nm. C3, with a
methyl substituent on the phenyl ring, shows a bathocromic shift
on both triplets which is also observed on the experimental value.
It can be mentioned that a preliminar analysis of the applicabil-
ity of the compounds in devices was tested. In this line, IPCE anal-
ysis was performed for [Ir(L1)
due that both complexes have an ester group to act as an anchor
to TiO when incorporated in a photoelectrochemical Grätzel type
2 2
(acac)], C1, and [Ir(L2) (acac)], C2,
2
solar cell prototype. C1 showed a higher efficiency than C2, while
both complexes have quite low yield efficiency compared to
N3 (cis-Bis(isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato
6
ruthenium(II)) and [Ir(dtp)(tpy-COOH)]PF , an Iridum complex

C. Iturbe et al. / Polyhedron 118 (2016) 159–170
169
observed. In this sense, the withdrawing electronic effect of the
ester group seems to predominate. By theoretical and electrochem-
ical studies it was established that for C3, the methyl electron
donor group destabilizes the HOMO and LUMO levels, generating
a an energy gap of 2.17 eV, greater than for C1 and C2, causing evi-
dently two spectroscopic consequences, the first is a bathochromic
shift of the emission maxima, and the second concerns the dual
emission band produced by S1 and Triplet close levels (1.72–
1
.78 eV) allowing to observe a joined fluorescence and phospho-
rescence bands. Finally, it can be mentioned that, for the reasons
given above, although not behaving in the range of the expected
values for a good response, C3 would be the best compound to
be tested in light emitting devices, while the di-substituted com-
pound C1 would be the best to be tested as dye in a solar cell.
Acknowledgements
This work was supported by the project Fondecyt 1110991 and
the Project ‘‘Puente VRI” of the Pontificia Universidad Católica de
Chile.
Fig. 9. Action spectra for complexes C1 and C2.
with an efficiency of 63% [29], under the same experimental condi-
tions. Fig. 9 shows the action spectra for complexes C1 and C2.
The difference observed between C1 and C2 can be explained by
the enhanced panchromaticity of C1 in the red part of the visible
region of the spectra. The injection capacity of C1 could also be
enhanced because of the presence of an electronic donating sub-
Appendix A. Supplementary data
CCDC contains the supplementary crystallographic data for
9
97795–997797. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
3
stituent as CH that induces an increment of charge density avail-
able to be injected [30]. It has been postulated in literature that a
raise in the degree of asymmetry – in this case due to different sub-
stituent groups on the phenyl and pyridine rings can cause an
1
016/j.poly.2016.07.029.
increase in the value of the molar extinction coefficient e.
Regarding the potentiality of these complexes in emitting
devices, can be seen in Table 7, compound C3 is the best emitter
of the series. This is not surprising, in the sense that according to
Fig. 8 this compound shows a greater energy gap than C1 and C2,
that diminishes the non radiative decay paths, according to the
energy gap law. Nevertheless, Evans [31] establishes some require-
ments to be fulfilled by complexes of this type to be efficient in
OLED devices. Among the variables analyzed are the emission
wavelength, lifetime and phosphorescence quantum yield. Consid-
ering the quantum yield, the requirement in this case is that it
should have a value of at least 0.25 at 298 K [32]. As C3 displays
a quantum yield of 0.014, according to this criteria, its expected
performance in an OLED device would be low [33].
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