Synthetically Tailored Excited States: Phosphorescent, Cyclometalated IridiumACHTUNGTRENNUNG(III) Complexes and Their Applications

Abstract

Phosphorescent iridiumACHTUNGTRENNUNG(III) complexes are being widely explored for their utility in diverse photo- physical applications. The performance of these materi- als in such roles depends heavily on their excited-state properties, which can be tuned through ligand and sub- stituent effects. This concept article focuses on methods for synthetically tailoring the properties of bis-cyclome- talated iridiumACHTUNGTRENNUNG(III) materials, and explores the factors governing the nature of their lowest excited state.

Full text

! 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2006, 12, 7970 – 7977
7970
CONCEPTS
DOI: 10.1002/chem.200600618

Synthetically Tailored Excited States: Phosphorescent, Cyclometalated
IridiumACHTUNGTRENNUNG(III) Complexes and Their Applications
Michael S. Lowry and Stefan Bernhard*
[a]
Introduction
Luminescent transition-metal complexes are appealing for
their utility in diverse applications.
[1–5]
The effectiveness of a
complex in a specific role is determined by its excited-state
properties and can be manipulated through synthetic modifi-
cations. Early work in this field focused predominantly on
tris-diimine ruthenium(II) complexes (e.g., [RuACHTUNGTRENNUNG(bpy)
3
]
2 +
),
but these materials offered limited color tuning capability
due to thermal population of a nonemissive metal-centered
(
3
MC) state.
[6]
IridiumACHTUNGTRENNUNG(III) complexes, on the other hand,
enable broader tuning possibilities due to their increased
ligand-field stabilization energy (LFSE) and, as a conse-
quence, less thermally accessible
3
MC states. Splitting of the
d orbitals can be further enhanced with strong-field ligands
and, as a result, bis-cyclometalated iridiumACHTUNGTRENNUNG(III) complexes
have displaced ruthenium(II) compounds at the forefront of
many photochemical and photophysical investigations.
[7–12]
In particular, the reversible electrochemistry, synthetic
versatility, and robust nature of iridiumACHTUNGTRENNUNG(III) complexes
render them appealing materials for a multitude of applica-
tions. Bis-cyclometalated iridiumACHTUNGTRENNUNG(III) complexes can be pre-
pared according to the two-step synthesis presented in
Scheme 1, in which the net charge of the mononuclear com-
plex can be determined by the nature of the ancillary ligand
(Figure 1).
[11–14]
By independently modifying the cyclometa-
lating and ancillary ligands, it is possible to endow a com-
plex with specific photophysic al and electrochemical traits
and to tune its ability to perform in areas such as organic
light-emitting diodes (OLEDs),
[13–15]
luminescence-based
sensors,
[3,16,17]
and photocatalysis.
[18]
Nature of the Excited State
The two principle transitions that are observed in the long-
lived excited state of iridiumACHTUNGTRENNUNG(III) complexes are: 1) metal-
to-ligand charge transfer (MLCT) in which an electron is
promoted from a metal d orbital to a vacant p* orbital on
one of the ligands, and 2) ligand-cen tered (LC) transitions
in which an electron is promoted between p orbitals on one
of the coordinated ligands.
[6,7]
Strong spin-orbit coupling
from the iridiumACHTUNGTRENNUNG(III) center facilitates inters ystem crossing
to energetically similar triplet states and enables the forma-
tion of an emissive, mixed (triplet) excited state [T
1
; Eq (1)]
Abstract: Phosphorescent iridiumACHTUNGTRENNUNG(III) complexes are
being widely explored for their utility in diverse photo-
physical applications. The performance of these materi-
als in such roles depends heavily on their excited-state
properties, which can be tuned through ligand and sub-
stituent effects. This concept article focuses on methods
for synthetically tailoring the properties of bis-cyclome-
talated iridiumACHTUNGTRENNUNG(III) materials, and explores the factors
governing the nature of their lowest excited state.
Keywords: iridium · luminescence · OLEDs · photo-
chemistry · structure–property relationships
[a] M. S. Lowry, Prof. S. Bernhard
Department of Chemistry, Princeton University
Princeton, New Jersey 08544 (USA)
Fax: (+ 1) 609-258-7657
E-mail: bern@princeton.edu
Scheme 1. Synthesis of a heteroleptic, bis-cyclometalated iridiumACHTUNGTRENNUNG(III)
complex. A dichloro-bridged diiridium dimer involving a cyclometalating
ligand (e.g., 2-phenylpyridine, ppy, blue) is isolated and subsequently
cleaved using an ancillary ligand (e.g., 2,2’-bipyridine, bpy, red) to yield
mononuclear iridiumACHTUNGTRENNUNG(III) complexes (e.g., [IrACHTUNGTRENNUNG(ppy)
2
ACHTUNGTRENNUNG(bpy)]
+
).
Chem. Eur. J. 2006, 12, 7970 – 7977 ! 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org
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CONCEPTS

in which the coefficients a and b refer to the contributions
of the
3
MLCT and
3
LC states, respectively.
[6,7,19]
An excited-
state manifold (a ! b) is presented in Figure 2.
Y
T
1
¼ aY
MLCT
þ bY
LC
ð1Þ
Tremendous efforts have been made to understand the
nature of the excited state as well as the structural factors
by which it is controlled. A combination of excited-state
lifetime measurements,
[9]
time-resolved luminescence spec-
troscopy,
[8,20,21]
and computational investigations
[22,23]
have
provided significant insight into the nature of the excited
state in bis-cyclometalated iridiumACHTUNGTRENNUNG(III) complexes. For ex-
ample, Hay accurately described the HOMO and
HOMO&1 as metal and cyclometalating ligand (p) orbitals
and the LUMO and LUMO+1 as vacant (p*) ligand orbi-
tals in [IrACHTUNGTRENNUNG(ppy)
2
ACHTUNGTRENNUNG(bza)] (in which ppy= 2-phenylpyridine,
bza = 1-phenyl-1,3-butanedione) by using time-dependent
density-functional theory (TD-DFT).
[23]
Similar results have
been attained with other neutral and ionic iridiumACHTUNGTRENNUNG(III) com-
plexes.
[11,14,22,23]
Thus, it appears feasible for the excited state
of bis-cyclometalated iridiumACHTUNGTRENNUNG(III) complexes to contain
mixed MLCT–LC character (Figure 3). It is worth noting
that these calculations correspond to the absolute energy of
the associated orbitals (i.e.,
singlet transition energies for
which the
1
LC state is signifi-
cantly larger than the
1
MLCT
state) and that singlet–triplet
stabilization energies must be
considered before further pre-
dictions can be made about the
energy of the emissive
state.
[7,11]
Tuning Strategies
Excited-state mixing occurs
when sufficient overlap is pres-
ent between the
3
LC and
3
MLCT states.
[24]
Thus, it is possible to control the energy of
the lowest excited state by deliberately adjusting the energy
of metal and ligand orbitals, which can be achieved through
substituent effects
[25,26]
or by changing the ligand parent
structure entirely (e.g., ppy vs. 1-phenylpyrazole, ppz).
[14, 27]
Structural control is facilitated by the multistep synthesis
Figure 1. Bis-cyclometalated (shown in blue) iridiumACHTUNGTRENNUNG(III) luminophores. The net charge of the complex is de-
termined by the ancillary ligand(s) (shown in red): a) cationic [IrACHTUNGTRENNUNG(dFCF
3
ppy)
2
ACHTUNGTRENNUNG(dtbbpy)]
+
PF
6
&
(dFCF
3
ppy = 5-
trifluoromethyl-2-(2’,4’-difluorophenyl)-pyridine; dtbbpy = 4,4’-di-tert-butyl-2,2’-bipyridine), b) neutral [Ir-
ACHTUNGTRENNUNG(dFpz)
2
ACHTUNGTRENNUNG(CF
3
ppz)] (dFpz = 1-(2’,4’-difluorophenyl)-pyrazole; CF
3
ppz = 2-(5-trifluoromethylpyrazol-3-yl)-pyri-
dine), and c) anionic [TBA]
+
[IrACHTUNGTRENNUNG(ppy)
2
(CN)
2
]
&
(TBA
+
= tetrabutylammonium, CN
&
= cyanide).
Figure 2. The energetic closeness and degree of overlap between
3
MLCT
and
3
LC states results in the formation of a mixed lowest excited state
(T
1
). The excited molecule relaxes to the ground state through radiative
(k
r
) and nonradiative (k
nr
) pathways.
Figure 3. Molecular orbital diagram for [IrACHTUNGTRENNUNG(dFCF
3
ppy)
2
ACHTUNGTRENNUNG(dtbbpy)]
+
(Fig-
ure 1a) obtained from DFT calculations. The HOMO is composed pri-
marily of metal d orbitals with some contributions from ligand (ppy-
based, phenyl) orbitals, while the LUMO and LUMO+1 are localized on
separate ligands (bpy- and ppy-based, respectively). Ligand-field splitting
is eviden t between the HOMO (“t
2g
”) and LUMO+12 (“e
g
*”). The cal-
culated orbital energies are corroborated by experimental evidence
(cyclic voltammetry).
www.chemeurj.org ! 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2006, 12, 7970 – 7977
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S. Bernhard and M. S. Lowry

shown in Scheme 1 in which different ligands are independ-
ently added to the coordination sphere.
When both classes of ligands provide orbitals that partici-
pate in the excited-state transitions, the cyclometalating
ligand tends to be associated with the
3
LC transition and the
ancillary ligand with the
3
MLCT transition.
[23,24]
In such
cases, the excited state can be tuned directly through ligand
modifications, because each ligand is linked to a different
transition. By monitoring the effect of various ligand permu-
tations within the coordination sphere, it is possible to gain
insight into the factors that govern the photophysical and
electrochemical behavior of heteroleptic iridiumACHTUNGTRENNUNG(III) com-
plexes, and to tailor materials with specific excited-state
properties.
[11, 28]
In some excited complexes, interligand electron transfer
(ILET) from the cyclometalating ligand to the ancillary
ligand is possible. The net properties of these materials can
be controlled almost exclusively by the ancillary ligand. For
instance, Park"s group observed emission across the visible
spectrum by changing the structure of the ancillary ligand in
a series of complexes that underwent ILET prior to emis-
sion.
[29]
Campagna"s group further demonstrated that if the
structure of the ancillary ligand is modified such that some
of its p density overlaps the plane of chelation for the cyclo-
metalating ligand, another charge transfer transition (sigma-
bond-to-ligand charge transfer, SBLCT
[30]
) is feasible.
[31]
As
a result, electron density can be transferred from a carbon–
metal s bond to the ancillary ligand, and site-specific tuning
can be achieved.
When only the cyclometalating ligand contributes p* orbi-
tals at an appropriate energy for the excited-state transi-
tions, both transitions tend to be associated with the cyclo-
metalating ligand. Meanwhile the ancillary ligand (e.g., 2,4-
pentanedione, acac) plays a more passive role in determin-
ing the nature of the excited state; influencing the energy of
the metal orbitals by inductive communication through s
bonds.
[32]
Thus, even a “nonparticipating” ancillary ligand
can be used to adjust the energy of the MLCT state as well
as the degree of mixing between the states.
One of the most effective methods for tuning the energy
of the lowest excited state involves changing the degree of
conjugation in the structure of the participating (cyclometa-
lating and/or ancillary) ligands.
[11,27,28]
As the coordination
sphere becomes more diffuse, the corresponding orbitals are
stabilized. It follows, then, that the excited-state transitions
can be tuned by altering the size of the ligands and also by
localizing electron density in discrete regions of the mole-
cule or by partia lly destroying ligand aromaticity. For exam-
ple, bulky pendant groups can be used to distort a ligand
from planarity, which will destabilize its p orbitals, and, as a
result, increase the size of the associated
3
MLCT or
3
LC
transition. A critical drawback of this approach is that en-
hanced internal strain may promote nonradiative decay,
whereby higher excited-state energy is attained at the ex-
pense of other important properties (e.g., luminous intensity
and excited-state lifetime).
[33–35]
Alternatively, recent investi-
gations of iridiumACHTUNGTRENNUNG(III) complexes involving terdentate cyclo-
metalating ligands (e.g., 2,6-diphenyl-pyridine
[36,37]
and 1,3-
bis-(1-methyl-benzimidazol-2-yl)benzene
[38]
) have shown
that low-energy emission can be readily achieved in com-
plexes containing ligands with extended p systems.
[36–38]
Another promising method for tuning (and fine-tuning)
the excited-state properties of iridiumACHTUNGTRENNUNG(III) complexes in-
volves deliberate functionalization of the ligands through
the use of substituent groups. B y modifying the symmetry
and inductive influence of a ligand with different substitu-
ents, it is possible to control metal–ligand bonding as well as
ligand orbital energies and, thus, to control the nature of the
lowest excited state. Tremendous color versatility has been
achieved with iridiumACHTUNGTRENNUNG(III) luminophores in this manner
(Figure 4), and a broad range of excited-state lifetimes
(from nanoseconds to several microseconds) as well as phos-
phorescent yields (approaching 100 %) have been report-
ed.
[11, 14]
In particular, electronic effects have been considered due
to their profound influence on orbital energies as well as the
relative ease with which electron-withdrawing (e.g., -F,
-CF
3
) and electron-donating (e.g., -CACHTUNGTRENNUNG(CH
3
)
3
, -OCH
3
) groups
can be incorporated into the ligand structure. Electron-with-
drawing substituents tend to stabilize the HOMO by remov-
ing electron density from the metal, whereas donating
groups have an inverse effect.
[11,14,39,40]
This relationship is
convoluted by the fact that withdrawing groups may also
lower the energy of the LUMO (i.e., increasing the electron
affinity of the parent ligand).
[14]
Fortunately, the cyclometa-
lating and ancillary ligands can be separately substituted
with electron-withdrawing and ACHTUNGTRENNUNG-donating groups in hetero-
leptic complexes, which enables deliberate control over the
excited state.
The position of these substituents with respect to the co-
ordinating carbon of a cyclometalating ligand will strongly
influence the LFSE of the resulting complex.
[41]
For exam-
ple, electron-withdrawing groups meta to the site of coordi-
nation (and donating groups ortho or para to this position)
increase the field strength of the ligand and concomitantly
enhance d-orbital splitting. Interestingly, opposing spe ctro-
scopic trends have been observed for fluoro (-F) and tri-
fluoromethyl (-CF
3
) substituents at the same position of a
cyclometalating ring despite their similar electronic ef-
fects.
[40]
This difference has been attributed to the mesomer-
ic and inductive ability of the fluorine atom as opposed to
the purely inductive ability of the trifluoromethyl group.
Thus, it is important to consider the total impact of a sub-
stituent group when designing ligand systems for excited-
state tuning.
Due to the high number of ligand and substituent varia-
bles that influence the nature of the excited state in hetero-
leptic iridiumACHTUNGTRENNUNG(III) complexes, their electrochemical and pho-
tophysical properties are not always easy to predict a priori.
To this end, our group has developed a set of combinatorial
procedures—involving parallel synthesis and high-through-
put screening—to examine the effect of multiple structural
(and site-dependent) variables in tandem.
[11]
As our under-
standing of structure–property relationships improves, we
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CONCEPTS
Luminescence

have found that DFT calculations provide an excellent
method for modeling orbital configurations and extrapolat-
ing the energy of the lowest excited state.
[11,14]
These tech-
ACHTUNGTRENNUNGniques in conjunction with the other strategies presented
herein have facilitated and expedited the design of useful
materials for various optoelectronic,
[4,39]
analytical,
[3,16]
and
photocatalytic applications.
[18]
Four areas in which the highly
tunable nature of iridiumACHTUNGTRENNUNG(III) complexes have improved the
performance of inorganic materials are discussed below.
OLED applications
Transition-metal-based, organic light-emitting diodes
(OLEDs) are considered viable candidates for flat-panel dis-
plays due to their color versatility, low operation voltages,
and phosphorescent efficiency.
[42,43]
The first complex-based
OLED was assembled by Tang and VanSlyke in 1987 con-
taining an Alq
3
chromophore (q = 8-hydroxyquinoline) that
emits green light (550 nm) from a singlet excited state.
[44]
Considerable attention has since been placed on altering
device characteristics through strategic modifications to the
chromophore. In particular, complexes containing a heavy
transition metal center, such as iridiumACHTUNGTRENNUNG(III),
[45]
rutheni-
ACHTUNGTRENNUNGum(II),
[46]
or osmium(II)
[47]
are targeted for electrolumines-
cence studies because of the increased theoretical limit for
emission efficiency from triplet emitting complexes (nearly
100 %) over singlet emitters (% 25 %).
[48]
OLEDs that are constructed with neutral complexes typi-
cally consist of the luminescent chromophore embedded in
an organic matrix (e.g., 4,4’-N,N’-dicarbazolylbiphenyl,
CBP), sandwiched between multiple layers of charge trans-
port materials (e.g., 2,9-dimethyl-4,7-diphenyl-1,10-phenan-
throline, BCP and 4,4’-bis-[N-(naphthyl-N-phenylamino)bi-
phenyl, a-NPD, respectively), and capped with a low work-
function cathode (e.g., LiF/Al) and a transparent anode
(e.g., indium–tin oxide, ITO; Figure 5a).
[49]
By systematically
adjusting the nature of the excited state in the chromophore,
Thompson and Forrest have observed emission across the
visible spectrum.
[15,28]
Similarly, Holmes and Friend fine-
tuned electroluminescence in the green-to-blue regime
through synthetic modifications to the chromophore and,
more importantly, improved the operational lifetime of the
devices through the use of more stable (i.e., less labile) an-
cillary ligands.
[50]
Neutral iridiumACHTUNGTRENNUNG(III) complexes have spur-
red tremendous interest (beyond the scope of this manu-
Figure 4. Color versatility expressed by a series of six cationic iridiumACHTUNGTRENNUNG(III) luminophores. Their structures are listed in order of increasing emission wave-
length (blue to red). The evolution of
3
LC (vibrationally structured, high-energy bands) and
3
MLCT character (structureless, low-energy bands) in the lu-
minescence spectra are indicative of a mixed excited state.
Figure 5. a) A multilayer device ensemble involving an organic material
(e.g., CBP) doped with a neutral or anionic iridiumACHTUNGTRENNUNG(III) chromophore.
This matrix is sandwiched between hole- (e.g., a-NPD) and electron-in-
jecting (e.g., Alq
3
, q = 8-hydroxyquinoline) layers as well as a hole block-
ing layer (e.g., BCP) and capped with a low work-function cathode (e.g.,
LiF/Al) and a transparent anode (e.g., ITO). b) Single-layer device ge-
ometry in which a cationic iridiumACHTUNGTRENNUNG(III) chromophore is spin-coated on an
ITO substrate and capped with a gold electrode. c) Electroluminescence
from [IrACHTUNGTRENNUNG(ppy)
2
ACHTUNGTRENNUNG(dtbbpy)]ACHTUNGTRENNUNG(PF
6
) in a single-layer, air-stable device (560 nm).
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7974
S. Bernhard and M. S. Lowry

script) over the course past ten years and a concise review
of the advances in this field is overdue.
Recently, cationic iridiumACHTUNGTRENNUNG(III) complexes have emerged in
OLED applications due to their ability to electrolumines-
cence from a single-layer of neat complex sandwiched di-
rectly between two air-stable electrodes, such as ITO and
gold (Figure 5b, c).
[45]
This simple device geometry is made
feasible by the presence of mobile counterions that redis-
tribute in the presence of an applied potential and facilitate
charge injection at the electrodes. Additionally, charge-
transport occurs through a hopping mechanism, which ena-
bles low turn-on voltages (< 3 V) and device operation from
an alternating current (AC) power source.
[4]
The discovery
of novel cationic luminophores has been catalyzed by the
development of an exhaustive library of prospective materi-
als using the combinatorial procedures that were developed
in our group.
[11]
Recently, we reported yellow (560 nm),
[45]
green (531 nm)
[51,52]
and blue-green (497 nm)
[14]
electrolumi-
nescence from complexes containing a ppy–bpy backbone.
By adjusting the size (and, thus, energy) of the aromatic li-
gands, Thompson"s group achieved red electroluminescence
(635 nm) and also observed a slight hypsochromic shift
(492 nm) over these pioneering complexes.
[27]
Nevertheless,
efforts to optimize cationic systems in OLED applications
and to achieve pure blue electroluminescence (450 nm) are
ongoing.
OLED fabrication has also been successful with anionic
complexes of the form [IrACHTUNGTRENNUNG(ppy)
2
X
2
]
&1
(X = CN
&
, NCO
&
), in
which device assemblies resemble the multilayer ensembles
utilized for neutral chromophores.
[13]
In such cases, the
pseudo-halogen (X) influences the energy of the metal d or-
bitals by adjusting the LFSE and, thus, provides a viable
platform for color tuning. As such, yellow (556 nm; X=
CN
&
) and blue-green (500 nm; X= NCO
&
) emission have
been observed.
[13]
Notably, device stability and efficiency
also improved in systems with large LFSE (in which the
3
MC state is no longer accessible), but pursuit of pure blue
electroluminescence persists.
Oxygen Sensor Applications
The long-lived triplet excited state of luminescent iridium-
ACHTUNGTRENNUNG(III) complexes enables efficient energy transfer with the
triplet ground state of molecular oxygen, resulting in lumi-
nescence quenching and the formation of singlet oxygen.
[53]
Consequently, iridiumACHTUNGTRENNUNG(III) luminophores can be utilized as
oxygen probes in various medicinal, chemical, and environ-
mental sensors.
[16,54]
Oxygen concentration is quantified ac-
cording to sudden changes in the luminescent properties of
the complex and, thus, materials with high quantum yield
and long excited-state lifetime (i.e., several microseconds)
are highly desirable for facile, sensitive detection. Mixed-
ligand iridiumACHTUNGTRENNUNG(III) complexes are particularly intriguing due
to their broadly tunable excited-state properties, durability,
and high chem ical stability in the presence of singlet
oxygen.
[16]
A significant setback to the implementation of
solid-state iridiumACHTUNGTRENNUNG(III)-based sensors has been the occur-
rence of self-quenching between neighboring molecules.
One potential solutio n involves separating luminescent dyes
by embedding them in oxygen permeable polymer matrices,
but the sensitivity of such systems is limited by low loading
concentrations in order to prevent dye aggregation.
[16]
Ef-
forts to improve the loading concentration have included co-
valently binding the luminophores to the polymeric host
[55]
as well as adjusting the charge and size of the dye mole-
cules,
[56]
but investigations in this field are ongoing.
Bioanalytical Applications
The rich electrochemical and photophysical characteristics
of iridiumACHTUNGTRENNUNG(III) complexes also render them strong candi-
dates for bioanalytical applications. The intense emission
and long excited-state lifetimes of the iridiumACHTUNGTRENNUNG(III) com-
plexes enable sensitive, time-resolved detection and their
large Stokes shift helps minimize self-quenching between
dispersed materials.
[57]
More importantly, luminescent
iridiumACHTUNGTRENNUNG(III) complexes can be used to label biomaterials
due their ability to either covalently (e.g., aldehyde–amine
cross-linking)
[58]
or noncovalently (e.g., DNA intercala-
tion)
[59]
bind biological substrates. Bound complexes typical-
ly express different luminescent properties than their free
analogs due to changes in the rigidity and hydrophobicity of
the surrounding environment.
[59]
Thus, luminescent labels
provide a facile method for monitoring bioconjugation reac-
tions and quantifying the binding affinity between different
substrates.
[3,60]
A comprehensive review of the role of
iridiumACHTUNGTRENNUNG(III) and other transition-metal complexes in bioana-
lytical applications as well as methods for improving their
luminescent properties was reported by Lo and co-workers
in 2005.
[3]
Photocatalytic Water Splitting
The challenge of converting solar radiation to conveniently
usable forms of energy has been co nsidered by many re-
searchers. One promising solution involves harnessing solar
energy through a photocatalytic cycle to split water into hy-
drogen and oxygen.
[5,61,62]
This process can be conceptual-
ized as two half-reactions in which water is oxidized to mo-
lecular oxygen and reduced to molecular hydrogen. In par-
ticular, the high gravimetric energy density and clean com-
bustion products of hydrogen render it a highly viable
source of fuel and, as such, significant attention has been
placed on hydrogen production. Typical photoreduction
schemes employ a transition-metal photosensitizer in con-
junction with an electron relay to collect and store radiant
energy and convert protons into molecular hydrogen.
[5,61]
Heteroleptic iridiumACHTUNGTRENNUNG(III) complexes are appealing photo-
sensitizers due to their highly tunable properties, and, in
fact, we recently reported an iridiumACHTUNGTRENNUNG(III) photosensitizer
that exhibits tremendous improvements in hydrogen produc-
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7975
CONCEPTS
Luminescence

tion over the previous state-of-the-art literature standard,
[RuACHTUNGTRENNUNG(dmphen)
3
]
2+
(dmphen = 4,7-dimethyl-1,10-phenanthro-
line).
[14]
This increase in hydrogen production has been
traced to the improved reducing strength (i.e., ability to
pass an electron to the electron relay) of iridiumACHTUNGTRENNUNG(III) com-
plexes over ruthenium(II) complexes. An overview of the
ground and excited-state redox properties of these materials
were reported in a recent publication.
[14]
Other highly promising iridiumACHTUNGTRENNUNG(III) complexes have also
been targeted by using a high-throughput technology that
was developed in our laboratory, whereby multiple photo-
sensitizers were simultaneously screened under variable re-
action conditions. A custom-built parallel photoreactor—
consisting of a series of ultra-bright light emitting diodes
(500 mW ' 10% at 465 nm) connected to a multiwell
sample holder mounted on an orbital shaker—was em-
ployed along with a Ni/Pd thin film commercial hydrogen
sensor (Figure 6).
[18]
This methodology has helped us pin-
point structural and photophysical traits that are conducive
to hydrogen productio n and also provided insight into the
interplay between the photosensitizer and electron relay in
the catalytic loop, yet considerable strides still need to be
made before a sustainable “hydrogen economy” can be de-
veloped.
Conclusions and Prospects
The nature and energy of the excited state in mixed-ligand
iridiumACHTUNGTRENNUNG(III) complexes can be manipulated by deliberate
chemical synthesis. Synthetic control is facilitated by the de-
pendence of the excited state on the orbital configurations
within a molecule as well as the relative ease by which dif-
ferent parent ligand structures can be coordinated to the
metal center. Consequently, heteroleptic iridiumACHTUNGTRENNUNG(III) com-
plexes can be tailored to express specific luminescent prop-
erties or to serve a specific functional role (e.g., cross-link-
ing) and iridiumACHTUNGTRENNUNG(III)-based materials are being explored for
a plethora of applications. The improved performance of
iridiumACHTUNGTRENNUNG(III) complexes over alternate transition metal cen-
ters as well as the straightforward manner in which their
properties can be tuned has positioned heteroleptic iridium-
ACHTUNGTRENNUNG(III) complexes at the forefront of modern photochemistry
and we can expect that they will remain there throughout
the foreseeable future.
Acknowledgements
This work was supported by the National Science Foundation (Career
Award No. CHE-0449755), the Princeton Center for Complex Materials,
which is a Materials Research Science and Engineering Center of the Na-
tional Science Foundation (DMR-9632275), and a Camille and Henry
Dreyfus Foundation New Faculty Award. We would also like to thank
George G. Malliaras and Andr eas Schrag for helpful conversations and
the contr ibution of photographs.
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Published online: August 25, 2006
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CONCEPTS
Luminescence