Elucidating the origin of enhanced phosphorescence emission in the solid state (EPESS) in cyclometallated iridium complexes

Article abstract of DOI:10.1002/ejic.201402495

A new mechanism for enhanced phosphorescence emission in the solid state (EPESS) in cyclometallated Ir complexes with the general formula [Ir(CN) ₂(NO)] involving distortion of the six-membered chelate ring of the ancillary ligand is proposed. Photophysical and computational studies show that neither π-stacking nor restricted rotation cause the observed EPESS in these complexes and that ligand distortions in the triplet excited state are responsible for EPESS. Copyright

Full text of DOI:10.1002/ejic.201402495

FULL PAPER
DOI:10.1002/ejic.201402495
Elucidating the Origin of Enhanced Phosphorescence
Emission in the Solid State (EPESS) in Cyclometallated
Iridium Complexes
Ashlee J. Howarth,[a] Raissa Patia,[b] David L. Davies,*[b]
Francesco Lelj,*[c] Michael O. Wolf,*[a] and Kuldip Singh[b]
Keywords: EPESS / Phosphorescence / Density functional calculations / Pi interactions / Iridium
A new mechanism for enhanced phosphorescence emission
in the solid state (EPESS) in cyclometallated Ir complexes
with the general formula [Ir(C N)2(N O)] involving distor-
tion of the six-membered chelate ring of the ancillary ligand
is proposed. Photophysical and computational studies show
that neither π-stacking nor restricted rotation cause the ob-
served EPESS in these complexes and that ligand distortions
in the triplet excited state are responsible for EPESS
∧
∧
EPESS in metal complexes has been attributed to a vari-
ety of factors including restricted intramolecular rotation
(RIR),[11] or π-stacking.[7] However, the complexity of the
excited state manifolds in these complexes makes unambig-
uous determination of the origin of EPESS difficult. Cyclo-
metallated iridium complexes are of particular interest due
to their high photoluminescence efficiencies and the ability
to colour tune their emission.[12] Two different mechanisms
have been proposed to drive EPESS in cyclometallated irid-
ium complexes. One mechanism involves restricted intramo-
Introduction
Organic light-emitting diodes (OLEDs) and light-emit-
ting electrochemical cells (LECs) require molecules that
emit intensely in the solid state.[1,2] Interactions between
molecules in the solid state are known to influence emission
behaviour. Typically, molecules that are strongly emissive in
dilute solution become less emissive in concentrated solu-
tions or in the solid state due to “aggregation caused
quenching” (ACQ).[3] In 2001, Tang and co-workers re-
ported a series of substituted 2,3,4,5-tetraphenylsiloles in
which aggregation caused an enhancement in emission in
the solid state compared to dilute solution, a phenomenon
they dubbed “aggregation induced emission” (AIE).[4,5]
Since this discovery, many organic chromophores exhibiting
AIE have been reported.[6] Recently, coordination com-
plexes of iridium,[7] platinum[8] and rhenium[9] that show
AIE have also been reported. In such complexes, emission
often arises from states with triplet character, so AIE in this
class of compounds is also called enhanced phosphores-
cence emission in the solid state (EPESS). The performance
of solid-state molecular devices depends strongly on the
molecular assembly of components. As a consequence, un-
derstanding and controlling molecular arrangements in the
solid state is pertinent to these applications.[10]
lecular rotations of substituents on the bidentate ancillary
[11]
ligand (N O or N N[13]) and the other involves π-stack-
ing of cyclometallating phenylpyridine ligands.[7,14,15]
Park et al. proposed that restricted intramolecular rota-
tion around the N–aryl bond of salicylimine ligands in the
solid state suppresses a non-radiative decay pathway giving
rise to EPESS.[11] The solid-state absorption and lumines-
cence properties of 1–4 were studied in neat films as well as
in various polymer films.[11] Due to the presence of strong
solid-state emission in the polymer films of 1–4, it was con-
cluded that the solid-state emission does not arise from an
excimeric or aggregated state.[11] Instead, a combination of
low temperature emission and TD-DFT studies were used
to conclude that rotation around the N–aryl bond in solu-
tion gives rise to a non-radiative decay pathway causing
these complexes to be non-emissive in solution.[11] It was
proposed that this pathway is slowed down or shut off in
the solid state giving rise to the observed emission.[11]
Li et al. have suggested that π-stacking of phenylpyridine
∧
∧
[a] Department of Chemistry, University of British Columbia
Vancouver, BC V6T 1Z1, Canada
E-mail: mwolf@chem.ubc.ca
http://groups.chem.ubc.ca/wolf/
3
[b] Department of Chemistry, University of Leicester
Leicester, LE1 7RH, UK
ligands lowers the energy of an emissive MLLCT state be-
low that of a non-emissive triplet ligand (3L) state, resulting
in an increase in emission in the solid state compared to
solution.[7] This explanation was first proposed from studies
of 5–7, where 5 does not show EPESS in contrast to 6 and
7.[7] The triplet energies of the ancillary ligands (3L) of 5–7
http://www2.le.ac.uk/departments/chemistry/people/academic-
staff/david_l_davies
[c] Dipartimento di Scienze, Università della Basilicata
85100 Potenza, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402495.
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cillary ligand to the metal atom. We show that this also
∧
∧
applies to [Ir(C N)2(O O)] complexes and hence is consis-
tent with all the experimental observations made previously
by Park[11] and Li[7,15] and may apply more widely to other
transition metal complexes.
Results and Discussion
Synthesis and Photophysical Properties
were measured and 6 and 7 were determined to have low-
lying triplet energies.[7] From these results, it was proposed
that π-stacking lowers an emissive 3MLLCT state below the
Complexes 3, 4, 7 and 11–17 were studied to test the
effect of restricted N–aryl and N–alkyl rotations as well as
π-stacking on EPESS. Increasing steric bulk of the R group
from phenyl to fluoranthyl to 2,6-diisopropylphenyl was
used to test the effects of restricted N–R rotation. IrIII com-
plexes with phenylpyrazole cyclometallating ligands were
used as a starting point to test the effects of π-stacking on
EPESS, since phenylpyrazole ligands have been shown to
be less likely than phenylpyridine ligands to π-stack in the
solid state.[17] Complexes 3, 4, 7 and 11–17 were prepared
by treating the appropriate IrIII dimer with the correspond-
ing salicylimine proligand under microwave irradiation at
100 °C for 30 min. All complexes were characterized by
high resolution mass spectrometry and 1H NMR spec-
troscopy (COSY, NOESY, TOCSY and HSQC) allowing
full assignment of all resonances. X-ray crystal structures
were also obtained for 15–17.
3
low-lying, non-emissive L state (in 6 and 7) giving rise to
emission in the solid state.[7] Follow-up studies on 8–10
were used to dispute the theory of restricted rotation pro-
posed by Park et al. since 8 and 9 have ancillary ligand
substituents, which are not free to rotate yet still show
EPESS.[15] Li et al. used X-ray crystallography[7,15] as well
as emission spectroscopy on micro-aggregates formed in
acetonitrile/water mixtures,[7] to suggest that the phenylpyr-
idine ligands π-stack and give rise to the solid-state emis-
sion observed.
There are some contradictory results in the data reported
by Park and Li respectively which suggest that neither of
the interpretations proposed by these groups is correct.
Park et al. have shown that strong solid-state emission is
observed in polymer films[11] (when π-stacking interactions
are diminished[16]). This observation is inconsistent with the
interpretation put forward by Li et al. as the cause of
EPESS. The studies by Li et al. in which strong solid-state
emission is observed in complexes with ancillary ligand sub-
stituents unable to rotate[15] do not support Park’s hypothe-
Complexes 3, 4, 7 and 11–17 show either very weak or no
sis of how EPESS is generated. Herein, we propose a new phosphorescence at room temperature in dichloromethane
mechanism to explain EPESS in cyclometallated Ir com- solution (Figure S1, Supporting Information). In stark con-
∧
∧
plexes with the general formula [Ir(C N)2(N O)] (3, 4, 7 trast to this observation, all of the complexes display en-
and 11–17). Using a combination of photophysical and hanced phosphorescence emission in the solid state in both
computational studies we show that neither π-stacking nor powder and crystalline forms. A thin film of each complex
restricted rotation of ancillary ligand substituents cause the was prepared by drop casting of dichloromethane solutions
observed EPESS in these complexes. Rather, we propose of 3, 4, 7 and 11–17 onto a glass slide and emission was
that the cause of EPESS in these complexes is a distortion observed with λmax between 580 and 635 nm, measured at
of the bonding of the six-membered chelate ring of the an- 298 K (Figure 1). Although the solid-state quantum yields
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are not particularly high, these complexes still display restricted rotation around the N–aryl bond is not the major
EPESS and can be used to study the mechanism that drives cause of EPESS in these complexes.
this phenomenon.
Effects of π-Stacking
To test the possible effects of π-stacking[7,15] or aggrega-
tion in complexes 3, 4, 7 and 11–17, 2 wt.-% of each com-
plex was dispersed into a PMMA matrix, allowing isolation
of the molecules from one another in the solid state.[16] In
all ten complexes, the solid-state emission maxima in
PMMA are blueshifted by 21–47 nm (Figure 2) compared
to the corresponding complexes in the undiluted solid state
Figure 1. Solid-state emission spectra of 3, 4, 7 and 11–17, λex
=
(Table 1). Redshifts in solid-state absorption and emission
spectra are typically observed due to intermolecular inter-
actions,[18] and the blueshifts observed here confirm that
intermolecular solid-state interactions are less prevalent
when the complexes are placed in the polymer matrix (i.e.
the molecules are more isolated). In addition, in each case,
the emission quantum yield of the complex in PMMA is
higher than that of the solid (Table 1), demonstrating that
the solid-state emission is partially quenched in the aggre-
gated form. These results are consistent with the more com-
mon aggregation caused quenching (ACQ) effect[3] and
demonstrate that π-stacking or aggregation of these com-
plexes in the solid state is not giving rise to EPESS. In fact,
π-stacking or aggregation is actually detrimental to the en-
hancement of the emission observed. In addition, the π-
stacking that has been previously reported[7,15] always in-
volved phenylpyridine cyclometallating ligands. We also ob-
serve EPESS behaviour in complexes bearing phenylpyr-
azole ligands which typically do not show long-range π-
stacking in the solid state.[17]
400 nm.
Effects of Restricted Rotation
Previously, restricted intramolecular rotation around the
N–aryl bond of the salicylimine ligand in the solid state was
proposed to suppress a non-radiative decay pathway giving
rise to EPESS.[11] Contrary to this proposal, our results
show that there is no correlation between the size of the
substituent on the salicylimine ligand and the presence or
absence of emission in solution. In addition, there is no
correlation between size and the emission quantum yield of
the complex in the solid state (Table 1). If rotation around
the N–aryl (or N–alkyl) bond was giving rise to a non-radi-
ative pathway, then complexes in which rotation is hindered
or fixed would be expected to be emissive in solution and
hence not show significant EPESS. The 1H NMR spectrum
of 15 shows four inequivalent doublets (δ = 1.18, 1.03, 0.87
and 0.62 ppm) corresponding to the methyl groups of the
isopropyl substituents as well as two septets (δ = 3.48 and
2.68 ppm) corresponding to the CH groups of the isopropyl
substituents; complex 17 shows similar features. The obser-
vation of inequivalent diisopropyl substituents on the di-
isopropylphenyl moiety confirms that there is no rotation
of the N–aryl substituent on the NMR timescale, yet com-
plexes 15 and 17 still demonstrate EPESS. These results, as
well as others reported in the literature showing complexes
with fixed substituents that display EPESS,[15] confirm that
Figure 2. Solid-state emission spectra of 3, 4, 7 and 11–17 in a
PMMA matrix.
Table 1. Solid-state emission wavelengths and quantum yields of
neat solids and solids in a PMMA matrix. Measurements taken in
air.
Complex Solution Solution Solid Solid PMMA PMMA
X-ray Crystallography
λem
f
λem
f
λem
f
The X-ray structures of 15–17 have been determined, and
they all show the expected atom connectivities. The struc-
ture of 15 is shown in Figure 3 and those for 17 and 16 are
in Figures S2 and S3, respectively (Supporting Infor-
mation). Diagrams showing a lack of evidence for π-stack-
ing in all three structures are provided in Figure S4 (Sup-
porting Information). The Ir–N(salicylimine) bond, where
the N atom is trans to the C atom of the cyclometallating
ligand, is much longer than the Ir–N(ppy/ppz) bonds. In
complex 15 the phenylpyrazole of one molecule is coplanar
3
–
–
–
–
–
–
–
–
–
–
609 0.058
633 0.012
600 0.045
602 0.080
598 0.044
632 0.014
580 0.028
570
604
576
578
574
604
559
569
534
570
0.134
0.097
0.096
0.086
0.139
0.086
0.047
0.099
0.117
0.109
4
7
11
12
13
14
15
16
17
–
–
589
593
584
–
0.013
ϽϽ 0.001[a] 592 0.049
ϽϽ 0.001[a] 581 0.036
–
607 0.028
[a] Quantum yield was too low to be accurately measured.
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with that of another (Figure S4a, Supporting Information) emission from 13 and 4 is redshifted relative to the other
although the two planes are considerably offset with only complexes whereas 14 and 16 are blueshifted. This is due
minor overlapping of the two pyrazole rings (centroid– to a shift in the LUMO from being localized on the salicyl-
∧
∧
centroid distance of 3.42 Å). The plane of the N O ligand imine portion of the N O ancillary ligand (blueshift) to
∧
(N,C,C,C,O) is considerably distorted from the IrC2 plane being delocalized onto the N–aryl moiety of the N O ancil-
forming an interplanar angle of 27.6° (Figure 3). The corre- lary ligand (redshift).
sponding phenylpyridine complex 17 shows some π-stack-
ing of the phenylpyridine ligands, which are at an angle of
7.1° to each other although again the rings are offset from
one another (centroid–centroid distance of pyridine in one
molecule and phenyl in another is 4.16 Å). The angle be-
∧
tween the plane of the N O ligand and IrC2 plane is 24.5°
(Figure S2, Supporting Information). In complex 16, the
phenylpyridine ligands in adjacent molecules are almost co-
planar but are hardly overlapped (centroid–centroid dis-
tance of pyridine in one molecule and pyridine in another
∧
is 3.80 Å). The angle between the plane of the N O ligand
and IrC2 plane is 22.2° (Figure S3, Supporting Infor-
mation). Overall, there is very little evidence in any of these
Figure 4. HSOMO-1 (α) and HSOMO (α) images as well as
ground-state frontier molecular orbital images from unrestricted
DFT (UDFT) and TD-DFT calculations respectively for 11 and 3.
HSOMO: Highest Singly Occupied Molecular Orbital.
structures for a significant degree of π-stacking. A notable
∧
feature in all three cases is the bending of the N O plane
away from the IrC2 plane. The reasons for this are not obvi-
ous. However, there are a number of weak interactions be-
tween molecules in the solid state. For example, in 15 there
are close contacts between the salicylimine phenol ring and
DFT and UDFT calculations were performed to help
the C–H bond of a pyrazole, and of an isopropyl group, understand the origins of EPESS. For complex 11, the
from two different adjacent molecules. In all cases, these ground-state calculation shows that in the most stable struc-
short contacts suggest restricted motion of the molecules in ture the plane of the phenol imine ligand and the equato-
the solid state.
rial[20] plane are almost coplanar (angle between planes =
5.94°). However, in the triplet excited state the phenol imine
tilts with respect to the equatorial plane leading to two
stable conformations of this state [tilted up (43.54°) or
down (–25.65)]. The cause of this distortion can be traced
to the fact that the LUMO has antibonding character be-
tween Ir and the phenol imine ligand. Thus, the plane of
the phenol imine tilts with respect to the equatorial plane
to reduce antibonding interactions and to allow population
of this orbital in the triplet excited state (T1). Hence, the
conformers do not arise from a distortion of the phenol
imine ancillary ligand. Rather, the distortion is in how this
ligand bonds to the metal atom. This ligand distortion does
not require a rotatable imine substituent, therefore EPESS
in complexes that do not have rotatable groups can also be
explained by this mechanism. The largest barrier separating
the two conformers of 11 is only 15.7 kJ/mol (Figure 5),
Figure 3. ORTEP and wireframe representation of 15 showing the
angle between the IrC2 and N O plane.
∧
DFT Calculations
To better understand the emission observed in complexes and, as a result, they can rapidly interconvert in solution at
such as 3, 4, 7 and 11–17 detailed DFT calculations were room temperature. This geometric change in the excited
carried out. TD-DFT calculations show that the lowest en- state gives rise to a non-radiative decay pathway which de-
3
ergy triplet consists mainly of a LUMO–HOMO transition activates the MLLCT state in solution at room tempera-
justifying the use of an unrestricted DFT (UDFT) ap- ture. Similar distortions were observed in complexes 3, 4, 7
proach to study the first triplet state. UDFT calculations of and 12–17 (Figure S7, Supporting Information), suggesting
3, 4, 7 and 11–17 reveal that the HSOMO and HSOMO- that the non-radiative decay pathway has the same origin
1[19] contain both Ir metal character as well as contributions in all cases. Ligand distortions in solution have been re-
from each of the three ligands (Figure 4 and S5 in Support- ported as the cause of non-radiative decay in a series of
ing Information), consistent with emission from a 3MLLCT EPESS-active Pt complexes containing similar N O chelat-
∧
state. This suggests that the nature of both the cyclometall- ing ligands.[21,22] Calculations that simulate a solid-state
∧
ating ligands as well as the ancillary N O ligand is impor- matrix (see Supporting Information), show that the T1 state
tant for the observed emission. DFT results show that the is much less distorted than in solution. This leads to an
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enhancement in the contribution of the Franck–Condon
factor for the 3MLLCT emission and hence makes this
transition more likely in the solid state.
Figure 6. Excited triplet state geometry of 18 and 5. The green
plane is the equatorial plane, and the red plane is the acac ligand
plane.
In the complexes demonstrating EPESS (3, 4, 6, 7 and
11–19), the ground-state HOMO and/or LUMO must con-
∧
∧
tain significant contributions from the N O or O O ancil-
lary ligand (Figures 4, S5 and S9). In complex 5, which does
not show EPESS, the HOMO and LUMO do not show any
contribution from the acac ancillary ligand. Although the
origins of the excited state ligand distortions are very com-
plex, these results suggest that involvement of the ancillary
ligand is a prerequisite for distortion to occur.
Figure 5. Computed energy profile for the excited triplet state ge-
ometry of 11 relative to the half chair inversion of the six-mem-
bered ring. The green plane is the equatorial plane, and the red
plane is the phenol imine ligand plane. Blue arrows show the transi-
tion vectors related to the interconversion between minima and are
associated with the imaginary frequency of 23.72i cm–1.
Conclusions
In addition to IrIII complexes with N O ligands, com-
∧
Most examples of EPESS-active iridium complexes in
the literature contain ancillary ligands bound through a
flexible six-membered chelate ring.[7,14,15] This suggests that
distortions of the ancillary ligand relative to the equatorial
plane may more broadly serve as the origin of EPESS in
such complexes. The use of six-membered rings may there-
fore be a useful design criterion for the synthesis of EPESS-
active complexes. The few EPESS-active complexes that
contain a five-membered chelate,[13,24] have very bulky an-
cillary ligand substituents, and, in these cases, EPESS is al-
most certainly due to restricted rotation. The relative ener-
gies of the cyclometallating and ancillary ligands are likely
∧
plexes with O O ancillary ligands have also been shown to
demonstrate EPESS and to be of interest in OLED applica-
tions.[7,12,17c,23] To further test our theory, calculations were
performed on a series of known complexes (5, 6, 18, and
∧
19) containing bidentate O O ancillary ligands. These com-
plexes also contain ancillary ligands with six-membered
chelate rings and 6, 18 and 19 have been shown to demon-
strate EPESS.[7,12,17c,23]
3
also important since the emission results from a MLLCT
state involving orbitals on all three ligands. Additionally, it
is important to ensure that the transition involves orbitals
that have a significant contribution from the atoms of the
six-membered chelate ring and not orbitals exclusively on
Complex 18 demonstrates EPESS,[17c,23] whereas com-
plex 5 is strongly emissive in both solution and the solid
state[7,12] and therefore fails to display EPESS. Calculations
of 18 show that ligand distortions occur in the excited trip-
let state, which is in agreement with our interpretation (Fig-
ure 6). Calculations of 5, however, show that ligand distor-
tions do not occur in the excited triplet state, and instead
the acac ancillary ligand remains planar with respect to the
equatorial plane (Figure 6). This is convincing evidence that
excited state ligand distortions give rise to EPESS. Com-
plexes 6 and 19 both demonstrate EPESS[7,12,23] and consis-
tent with our interpretation, calculations show that both
complexes undergo excited state ligand distortions (Fig-
ure S8, Supporting Information).
∧
the C N ligands and/or the periphery of the complex.
Experimental Section
General: All experiments were performed under nitrogen, using
standard Schlenk-line techniques. Deuterated solvents were pur-
chased from Cambridge Isotope Laboratories Inc. Chloro(1-phen-
ylpyrazole)iridium(III) dimer [IrCl(ppz)2]2,[25] chloro(2-phenylpyr-
idine)iridium(III) dimer [IrCl(ppy)2]2,[25] 2-[(phenylimino)methyl]-
phenol (NOPh),[11] 2-[(1-naphthalenylimino)methyl]phenol (NON-
apht),[26] 2-[(3-fluoranthenylimino)methyl]phenol (NOFluor),[11]
and 2-({[2,6-bis(1-methylethyl)phenyl]imino}methyl)phenol (NO-
iProp2Ph)[27] were prepared according to literature procedures. 2-
{[(1-Methylethyl)imino]methyl}phenol (NOiProp) was prepared by
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a typical Schiff base condensation reaction in ethanol. All other
solvents and reagents were obtained from commercial sources and
product as a dark orange precipitate. The product was purified by
column chromatography using CH2Cl2 to elute any unreacted NO-
Fluor and then EtOAc/CH2Cl2 (1:1) to elute complex 4 (0.040 g,
yield 38%).
1
used as received. H NMR and 13C NMR spectra were obtained
using a Bruker AV-400 spectrometer and referenced to the residual
protonated solvent peak. NMR spectra were assigned using a com-
bination of COSY, NOESY, TOCSY and HSQC experiments. Elec-
trospray ionization mass spectrometry data were obtained with a
Bruker Esquire LC ion trap mass spectrometer. Microwave reac-
tions were performed with a Biotage Initiator 2.5 microwave syn-
thesizer. Fluorescence spectroscopy data were collected using a
Photon Technology International QuantaMaster fluorimeter. So-
lid-state emission spectra were obtained at 298 K by drop-casting
a dichloromethane solution of each complex onto a glass slide. Ab-
solute quantum yields were determined with an integrating sphere
coupled to the PTI fluorimeter. Neat solid quantum yields were
obtained from drop-casting of a dichloromethane/hexane (1:1)
solution of each complex. PMMA matrix quantum yields were
drop-cast from dichloromethane.
Synthesis of Ir(ppy)2(NONapht) (7): This complex has been re-
ported previously[7] using different synthetic procedures. [IrCl-
(ppy)2]2 (0.070 g, 0.065 mmol), NONapht (0.035, 0.14 mmol) and
sodium carbonate (0.015 g, 0.14 mmol) were placed in a microwave
vial with ethanol (3.5 mL). The suspension was degassed with ni-
trogen for 4 min. The vial was placed in the microwave reactor and
heated under microwave irradiation at 100 °C (18 bar, 155 W) for
30 min. The solvent was then removed in vacuo and the solid resi-
due dissolved in CH2Cl2 (10 mL). The resulting yellow-orange solu-
tion was passed through a Celite pad and the filtrate reduced to
approximately 2 mL in volume. Layering with hexanes yielded the
desired product as a yellow precipitate. The product was purified
by column chromatography using CH2Cl2 to elute any unreacted
NONapht and then EtOAc/CH2Cl2 (1:1) to elute complex 7
(0.036 g, yield 37%).
X-ray Diffraction: Data were collected with a Bruker Apex 2000
CCD diffractometer using graphite monochromated Mo-Kα radia-
tion (λ = 0.7107 Å). The data were corrected for Lorentz and pola-
risation effects, and empirical absorption corrections were applied.
The structure was solved by direct methods and with structure re-
finement on F2 employing SHELXTL version 6.10.[28] Hydrogen
atoms were included in calculated positions (C–H 0.95–1.00 Å, O–
H 0.84 Å) riding on the bonded atom with isotropic displacement
parameters set to 1.5Ueq(O) for hydroxy H atoms, 1.5Ueq(C) for
methyl hydrogen atoms and 1.2Ueq(C) for all other H atoms. All
non-hydrogen atoms were refined with anisotropic displacement
parameters without positional restraints. Disordered solvent in 16
was removed using the Squeeze option in PLATON.[29] Figures
were drawn using the program ORTEP.[30] CCDC-987738 (15),
CCDC-987739 (16), CCDC-987737 (17), contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of Ir(ppz)2(NOPh) (11): [IrCl(ppz)2]2 (0.070 g,
0.068 mmol), NOPh (0.030, 0.15 mmol) and sodium carbonate
(0.016 g, 0.15 mmol) were placed in a microwave vial with ethanol
(3.5 mL). The suspension was degassed with nitrogen for 4 min.
The vial was placed in the microwave reactor and heated under
microwave irradiation at 100 °C (18 bar, 155 W) for 30 min. The
solvent was then removed in vacuo and the solid residue dissolved
in CH2Cl2 (10 mL). The resulting yellow solution was passed
through a Celite pad and the filtrate reduced to approximately
2 mL in volume. Layering with hexanes yielded the desired product
as a bright orange precipitate. The product was purified by column
chromatography using CH2Cl2 to elute any unreacted NOPh and
then EtOAc/CH2Cl2 (1:1) to elute complex 11 (0.031 g, yield 34%).
Synthesis of Ir(ppz)2(NONapht) (12): [IrCl(ppz)2]2 (0.070 g,
0.068 mmol), NONapht (0.037, 0.15 mmol) and sodium carbonate
(0.016 g, 0.15 mmol) were placed in a microwave vial with ethanol
(3.5 mL). The suspension was degassed with nitrogen for 4 min.
The vial was placed in the microwave reactor and heated under
microwave irradiation at 100 °C (18 bar, 155 W) for 30 min. The
solvent was then removed in vacuo and the solid residue dissolved
in CH2Cl2 (10 mL). The resulting yellow-orange solution was
passed through a Celite pad and the filtrate reduced to approxi-
mately 2 mL in volume. Layering with hexanes yielded the desired
product as a bright yellow precipitate. The product was purified
by column chromatography using CH2Cl2 to elute any unreacted
NONapht and then EtOAc/CH2Cl2 (1:1) to elute complex 12
(0.029 g, yield 29%).
Synthesis of Ir(ppy)2(NOPh) (3): This complex has been reported
previously[11] using different synthetic procedures. [IrCl(ppy)2]2
(0.070 g, 0.065 mmol), NOPh (0.028, 0.14 mmol) and sodium carb-
onate (0.015 g, 0.14 mmol) were placed in a microwave vial with
ethanol (3.5 mL). The suspension was degassed with nitrogen for
4 min. The vial was placed in the microwave reactor and heated
under microwave irradiation at 100 °C (18 bar, 155 W) for 30 min.
The solvent was then removed in vacuo and the solid residue dis-
solved in CH2Cl2 (10 mL). The resulting yellow solution was passed
through a Celite pad and the filtrate reduced to approximately
2 mL in volume. Layering with hexanes yielded the desired product
as an orange-yellow precipitate. The product was purified by col-
umn chromatography using CH2Cl2 to elute any unreacted NOPh
and then EtOAc/CH2Cl2 (1:1) to elute complex 3 (0.026 g, yield
29%).
Synthesis of Ir(ppz)2(NOFluor) (13): [IrCl(ppz)2]2 (0.070 g,
0.068 mmol), NOFluor (0.048, 0.15 mmol) and sodium carbonate
(0.016 g, 0.15 mmol) were placed in a microwave vial with ethanol
(3.5 mL). The suspension was degassed with nitrogen for 4 min.
The vial was placed in the microwave reactor and heated under
microwave irradiation at 100 °C (18 bar, 155 W) for 30 min. The
solvent was then removed in vacuo and the solid residue dissolved
in of CH2Cl2 (10 mL). The resulting yellow-brown solution was
passed through a Celite pad and the filtrate reduced to approxi-
mately 2 mL in volume. Layering with hexanes yielded the desired
product as a dark orange precipitate. The product was purified by
column chromatography using CH2Cl2 to elute any unreacted NO-
Fluor and then EtOAc/CH2Cl2 (1:1) to elute complex 13 (0.046 g,
yield 43%).
Synthesis of Ir(ppy)2(NOFluor) (4): This complex has been reported
previously[11] using different synthetic procedures. [IrCl(ppy)2]2
(0.070 g, 0.065 mmol), NOFluor (0.046, 0.14 mmol) and sodium
carbonate (0.015 g, 0.14 mmol) were placed in a microwave vial
with ethanol (3.5 mL). The suspension was degassed with nitrogen
for 4 min. The vial was placed in the microwave reactor and heated
under microwave irradiation at 100 °C (18 bar, 155 W) for 30 min.
The solvent was then removed in vacuo and the solid residue dis-
solved in CH2Cl2 (10 mL). The resulting yellow-brown solution was
passed through a Celite pad and the filtrate reduced to approxi-
mately 2 mL in volume. Layering with hexanes yielded the desired
Synthesis of Ir(ppz)2(NOiProp) (14): [IrCl(ppz)2]2 (0.070 g,
0.068 mmol), NOiProp (0.024, 0.15 mmol) and sodium carbonate
Eur. J. Inorg. Chem. 2014, 3657–3664
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
(0.016 g, 0.15 mmol) were placed in a microwave vial with ethanol
(3.5 mL). The suspension was degassed with nitrogen for 4 min.
The vial was placed in the microwave reactor and heated under
microwave irradiation at 100 °C (18 bar, 155 W) for 30 min. The
solvent was then removed in vacuo and the solid residue dissolved
in CH2Cl2 (10 mL). The resulting yellow-brown solution was
passed through a Celite pad and the filtrate reduced to approxi-
mately 2 mL in volume. Layering with hexanes yielded the desired
product as a yellow precipitate. The product was purified by col-
umn chromatography using CH2Cl2 to elute any unreacted NOi-
Prop and then EtOAc/CH2Cl2 (1:1) to elute complex 14 (0.040 g,
yield 46%).
a set of d polarization functions [D95(d)] was used. Hydrogen
atoms not involved in any hydrogen bonds were described by the
same Dunning basis set that does not include p polarization func-
tions. For Ir, the new double-ζ Stuttgart[36] basis set including f-
polarization functions and relativistic effects by a fully relativistic
small core pseudopotential[33] (SDD09) were used and not the de-
fault SDD as included in Gaussian 09, Revision C.01. The ultrafine
option with 99590 grid points was used thoroughly for the integral
calculations for all atoms except Ir, where a total of 1566228 grid
points were used. The first triplet-state structure was computed
using the unrestricted approach. All energy-minimized structures
were characterized by the calculation of the Hessian matrix in order
to check that they were minima and not simple stationary points
on the molecular Born–Oppenheimer energy surface. Wave-
functions were checked against possible internal instability.[37,38]
TD-DFT calculations were performed by increasing the initial con-
figuration space for the Davidson[39] diagonalization, unlike the de-
fault option in the program. A dichloromethane solvent environ-
ment was simulated by the self-consistent reaction field.[40] The vi-
brational band structure was evaluated by computing the Frank–
Condon contribution[41,42] between the ground-state S0 structure
and T1 structures and their harmonic vibrational properties. Fur-
Synthesis of Ir(ppz)2(NOiProp2Ph) (15): [IrCl(ppz)2]2 (0.070 g,
0.068 mmol), NOiProp2Ph (0.042, 0.15 mmol) and sodium carb-
onate (0.016 g, 0.15 mmol) were placed in a microwave vial with
ethanol (3.5 mL). The suspension was degassed with nitrogen for
4 min. The vial was placed in the microwave reactor and heated
under microwave irradiation at 100 °C (18 bar, 155 W) for 30 min.
The solvent was then removed in vacuo and the solid residue dis-
solved in CH2Cl2 (10 mL). The resulting orange-brown solution
was passed through a Celite pad and the filtrate reduced to approx-
imately 2 mL in volume. Layering with hexanes yielded the desired
product as an orange-yellow precipitate. The product was purified
by column chromatography using CH2Cl2 to elute any unreacted
NOiProp2Ph and then EtOAc/CH2Cl2 (1:1) to elute complex 15
(0.027 g, yield 26%).
∧
ther T1 structures were computed by constraining the O N-Ph li-
gand conformation of the N–Ir–O–C and C–N–Ir–O dihedral as
in the ground state, while optimizing all the remaining geometrical
parameters to simulate the effect of the solid state constraining
those internal degrees of freedom. All calculations were performed
using Gaussian 09, Revision C.01.[43]
Synthesis of Ir(ppy)2(NOiProp) (16): [IrCl(ppy)2]2 (0.070 g,
0.065 mmol), NOiProp (0.023, 0.14 mmol) and sodium carbonate
(0.015 g, 0.14 mmol) were placed in a microwave vial with ethanol
(3.5 mL). The suspension was degassed with nitrogen for 4 min.
The vial was placed in the microwave reactor and heated under
microwave irradiation at 100 °C (18 bar, 155 W) for 30 min. The
solvent was then removed in vacuo and the solid residue dissolved
in CH2Cl2 (10 mL). The resulting yellow-brown solution was
passed through a Celite pad and the filtrate reduced to approxi-
mately 2 mL in volume. Layering with hexanes yielded the desired
product as a yellow precipitate. The product was purified by col-
umn chromatography using CH2Cl2 to elute any unreacted NO-
iProp and then EtOAc/CH2Cl2 (1:1) to elute complex 16 (0.037 g,
yield 43%).
Supporting Information (see footnote on the first page of this arti-
cle): NMR assignments, additional photophysical and crystallo-
graphic data and DFT calculations.
Acknowledgments
We thank the Natural Sciences and Engineering Research Council
(NSERC) of Canada for funding this research. D. L. D. thanks the
Leverhulme Trust for a Study Abroad Fellowship. F. L. thanks the
National Interuniversity Consortium of Materials Science and
Technology (INSTM) for financial support for a visiting leave from
Italy.
Synthesis of Ir(ppy)2(NOiProp2Ph) (17): [IrCl(ppy)2]2 (0.070 g,
0.065 mmol), NOiProp2Ph (0.040 g, 0.14 mmol) and sodium carb-
onate (0.015 g, 0.14 mmol) were placed in a microwave vial with
ethanol (3.5 mL). The suspension was degassed with nitrogen for
4 min. The vial was placed in the microwave reactor and heated
under microwave irradiation at 100 °C (18 bar, 155 W) for 30 min.
The solvent was then removed in vacuo and the solid residue dis-
solved in CH2Cl2 (10 mL). The resulting yellow-orange solution
was passed through a Celite pad and the filtrate reduced to approx-
imately 2 mL in volume. Layering with hexanes yielded the desired
product as an orange precipitate. The product was purified by col-
umn chromatography using CH2Cl2 to elute any unreacted NO-
iProp2Ph and then EtOAc/CH2Cl2 (1:1) to elute complex 17
(0.037 g, yield 37%).
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Received: June 2, 2014
Published Online: July 15, 2014
Eur. J. Inorg. Chem. 2014, 3657–3664
3664
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim