A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films 

A. Ruseckas

I. D. W. Samuel

J. C. Ribierre

K. Knights

N. Cumpstey

P. L. Burn

S. V. Staton

V. M. Agranovich

A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films (&lt; 20 nm) where surface quenching dominates the decay.</p

Ribierre, J. C.

Ruseckas, A.

Knights, K.

Staton, S. V.

Cumpstey, N.

Burn, P. L.

Samuel, I. D. W.

University of St Andrews Research Portal

Triplet exciton diffusion and phosphorescence quenching in Iridium(III)-Centered dendrimers

University of St. Andrews - Pure

Crossref

Triplet Exciton Diffusion and Phosphorescence Quenching in Iridium(III)-Centered Dendrimers

Burn. P. L.

University of Queensland eSpace

Triplet Exciton Diffusion and Phosphorescence Quenching in Iridium(III)-Centered Dendrimers
J. C. Ribierre,1 A. Ruseckas,1 K. Knights,2 S. V. Staton,2 N. Cumpstey,2 P. L. Burn,2,3 and I. D. W. Samuel1
1Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews,
North Haugh, St. Andrews, Fife KY16 9SS, United Kingdom
2Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, OX1 3TA Oxford, United Kingdom
3Centre for Organic Photonics and Electronics, School of Molecular and Microbial Science, Chemistry Building,
University of Queensland, Queensland, 4072, Australia
(Received 30 May 2007; published 10 January 2008)
A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered
phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the
molecular structure and also by blending with a host material. It was found that triplet exciton hopping is
controlled by electron exchange interactions and can be over 600 times faster than phosphorescence
quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton
diffusion, except in very thin films (<20 nm) where surface quenching dominates the decay.
DOI: 10.1103/PhysRevLett.100.017402 PACS numbers: 78.66.Qn, 73.50.Gr, 78.40.Me, 78.55.Kz
Excitation energy transfer is an important process in
organic semiconductors and has to be taken into account
when designing new optoelectronic materials and devices.
In photovoltaic devices, the neutral excited state is gener-
ated by light absorption and must diffuse to a heterojunc-
tion with another material to be separated into charge
carriers and so provide photocurrent. In organic light emit-
ting diodes (OLEDs), exciton diffusion can lead to a de-
crease of the electroluminescence efficiency due to
quenching of the emitting state by intermolecular inter-
actions and defects [1,2], exciton interactions with charge
carriers [2,3], and exciton-exciton annihilation [4,5]. These
quenching effects are more pronounced in phosphorescent
OLEDs than in fluorescent devices because of the longer
excited state lifetime. Nevertheless, phosphorescent
OLEDs show much higher internal quantum efficiencies
due to their ability to convert both singlet and triplet
excitons into light [6–8]. A photoluminescence (PL) study
of iridium(III) complexes dispersed into a wide energy gap
host suggested that intermolecular quenching of phosphor-
escence in films is controlled by the dipole-dipole inter-
actions between emitters [9]. However, the impact of
exciton migration on phosphorescence quenching has not
been considered.
In this Letter, we compare the dynamics of triplet ex-
citon diffusion and quenching in several fac-tris(2-
phenylpyridyl)iridium(III) [Ir ppy3]-cored phosphores-
cent dendrimers. Dendrimers provide a convenient way
of changing the spacing of the core chromophores in the
solid state and hence of studying the effect of spacing on
the physics of exciton diffusion and light emission. The
triplet exciton diffusion rates are extracted from the mea-
surements of triplet-triplet annihilation and have an expo-
nential dependence on chromophore spacing. This shows
that diffusion is controlled by nearest-neighbor electron
exchange interactions [10]. Nonradiative decay in 180 nm
thick films is governed by much weaker dipole-dipole
interactions and does not depend on the triplet diffusion
rate. In much thinner films (<20 nm), phosphorescence
quenching is found to depend on exciton diffusion and can
be modeled using the diffusion equation together with
quenching at the film surface.
Five green Ir ppy3-cored dendrimers were used in
this study and their chemical structures are shown in the
insets of Fig. 1. They were studied as neat films, and
their different structures give a range of chromophore
spacings. In addition to being studied as a neat film,
the first generation IrG1 dendrimer was also blended
into a 4,4’-bis(N-carbazolyl)biphenyl (CBP) or
m-bis(N-carbazolyl)benzene (mCP) host at various con-
centrations, providing an additional way of tuning the
spacing between the dendrimer cores. Dendrimer films
were deposited by spin-coating onto precleaned quartz
substrates from chloroform solutions. Film thickness was
determined by spectroscopic ellipsometry. The film photo-
luminescence quantum yield (PLQY) was measured in an
integrating sphere [11] under a flowing nitrogen atmo-
sphere using a helium-cadmium laser with a wavelength
of 325 nm and a power of 0:2 mW. PL kinetics were
measured by the time-correlated single-photon counting
technique. For exciton-exciton annihilation measurements,
170–200 nm thick films were excited with the 3rd har-
monic of a pulsed Nd:YAG laser at a wavelength of
355 nm. The excitation light at a repetition rate of
10 kHz was focused onto a spot of 0.3 mm diameter. The
emitted light was dispersed in a monochromator and de-
tected with a cooled Hamamatsu microchannel plate pho-
tomultiplier tube RU-3809U-50. Measurements were
performed at the wavelength corresponding to the maxi-
mum of the PL spectrum of our samples and in a vacuum of
<8 104 mbar. PL kinetics at low excitation density
were identical to those measured after excitation at
395 nm with a Picoquant pulsed laser diode, which was
used for thickness-dependent PL studies.
PRL 100, 017402 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending11 JANUARY 2008
0031-9007=08=100(1)=017402(4) 017402-1 © 2008 The American Physical Society
Figure 1 shows the PL kinetics measured at different
initial excitation densities N0, which were calculated from
the absorption and the incident laser energy density. PL
decays are independent of excitation density for N0 < 2
1017 cm3. At higher excitation densities, phosphores-
cence decays faster because of triplet-triplet annihilation
[4,5,12,13]. The density of triplet excitons N, which is
directly proportional to the PL intensity, can be described
by the rate equation: dN=dt  N= N2, where  is
the exciton lifetime at low excitation density and  is the
annihilation rate, which represents the encounter rate of
excitons. In the case of time independent , the solution is
 Nt  N0 expt=
1 N01 expt=	 : (1)
We analyzed the decays for the time interval of t < 0:5 s
where decays are exponential at low excitation density. All
the PL decays fit well with Eq. (1) and the  values are
plotted in Fig. 2(a) as a function of the center-to-center
distance between phosphorescent molecules R. This inter-
molecular spacing was calculated by considering the mole-
cules as hard spheres and using a film density of
1:1 g=cm3. Neutron reflectivity measurements gave a
film density for Ir ppy3 cored dendrimers ranging from
1.06 to 1:14 g=cm3 with about 10% uncertainties. The 
values strongly decrease with increasing R and the depen-
dence on interchromophore distance is exactly the same in
neat films and in the IrG1 blends. This indicates that both
the dilution of the phosphorescent emitters into a suitable
host and the dendritic structure offer the possibility to
finely control the exciton annihilation rate and thus the
exciton diffusion processes.
In the case of a diffusion controlled annihilation in 3D,
the annihilation rate  is related to the exciton diffusion
constant D by:   4DRa, where Ra is the reaction
radius at which annihilation is faster than hopping and
can be taken as R. Because of fast exciton dephasing,
exciton diffusion occurs by incoherent hopping and in a
3D system D is related to the nearest-neighbor hopping
rate, kh, by: D  R2kh=3 [14]. In Fig. 2(b), kh is found to
decrease exponentially with increasing R in the range of
1.8 to 3.2 nm, indicating that diffusion is by an electron
exchange interaction. Our experimental data were fitted by:
 lnkh  lnk0h  R Rv; (2)
where  and k0h are an attenuation factor and the hopping
rate at the van der Waal’s distance Rv. The best fit gives
  0:33 A1 and kh  4 109 s1 at Rv  1 nm.
The PLQY values and the PL lifetime , obtained from
the low excitation density kinetics by fitting them to
single exponential decay, increase with R [Fig. 2(c)].
We calculated the radiative and nonradiative decay rates
kR and kNR using PLQY  kR=kR  kNR and 1= 
kR  kNR. For IrG1 blends, kR is independent of the spac-
ing and equal to 7 105 s1 which is within 10% of the
reported values [9,15]. In dendrimers with carbazole den-
drons (Ircarb) kR  5:6 105 s1 is independent of gen-
eration, whereas the highly branched G1 dendrimer shows
kR  3:7 105 s1. Lower values of kR can be explained
by a larger amount of ligand character in the predominantly
metal-to-ligand charge transfer transition.
0.0 0.5 1.01015
1017
1019
1021
N
N
Ir
3
R2
R2
N
 (c
m
-
3 )
0.0 0.5 1.0
1E15
1E16
1E17
1E18
1E19
1E20
1E21
N
N
Ir
3
N
N
R2
R2
R2
R2
Ircarb- G2     R=24.6 Å
0.0 0.5 1.0
1E15
1E16
1E17
1E18
1E19
1E20
1E21
N
N
Ir
3
N
N
N
N
R2
R2
R2
R2 N
R2
R2
N
R2
R2
Ircarb- G3           R=30.8 Å
0.0 0.5 1.01015
1017
1019
1021
N
 (c
m
-
3 )
0. 0.5 1.01E5
1E6
1E7
1E8
1E9
1E20
1E2
OR1
R1O
N
Ir
3
N N
mCP
IrG1:mCP (20:80 wt.%)
          R=31.3 Å  
Ircarb- G1     R=19.7 Å
0. 0.5 1.01E5
1E6
1E7
1E8
1E9
1E20
1E21
N N
N
I r
3
R 3
R 3
highly branched G1   R=20.2 ÅIrG1   R=18.3 Å
0.0 0.5 1.010
15
1017
1019
1021 IrG1:CBP (80:20 wt.% )
        R=19.6 Å
N
 (c
m
-
3 )
Time (µs)
0.0 0.5 1.0
1E5
1E6
1E7
1E8
1E9
1E20
1E21
IrG1:CBP (50:50 wt.% )
R=23 Å
CBP
Time (µs)
0.0 0.5 1.0
1E5
1E6
1E7
1E8
1E9
1E20
1E21
IrG1:CBP (20:80 wt.% )
R=31.3 Å
Time (µs)
FIG. 1. Intensity dependent PL kinet-
ics of several phosphorescent den-
drimers. Solid lines are the fits calculated
using Eq. (1). The chemical structure of
the dendrimers used in this work and
their estimated diameter R are given in
the insets. R1  2-ethylhexyl, R2 
(9,9-di-n-propylfluoren-2-yl), and R3 
4-(9,9-di-n-propylfluoren-2-yl)phenyl.
PRL 100, 017402 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending11 JANUARY 2008
017402-2
The nonradiative decay rate kNR, shown in Fig. 2(b), can
be expressed as the sum of the nonradiative deactivation
rate of isolated molecules, kNRi, and the quenching rate
due to intermolecular interactions, kq, so that kNR 
kNRi  kq. Highly diluted dendrimers give kNRi 
1:8 105 s1 for IrG1 blends and Ircarb dendrimers,
which indicates similar deactivation rates of isolated den-
drimers. As shown in Fig. 2(b), kNR can be described by:
 kNR  kNRi  1

R0
R

6
; (3)
where R0 is the Fo¨rster radius [16]. The best fit gives R0 
1:5 nm for the IrG1 blends and 2 nm for the neat films of
Ircarb dendrimers, which is very similar to the Fo¨rster
radius calculated from the spectral overlap of phosphor-
escence and absorption spectra of the Ir ppy3 cores [9,15].
The results show that kq follows a dependence of R6.
There is a recent report of this dependence in blends of
Ir ppy3 and it was ascribed to ‘‘dampening’’ of excitation
energy by Fo¨rster type dipole-dipole interactions [9]. In the
present work we show that the results can be explained by
excitation transfer to a quencher.
Figure 2(d) shows that the triplet exciton hopping can be
over 600 times faster than the phosphorescence quenching.
We propose that transfer to the quencher is slow, limiting
the quenching rate, and making the phosphorescence de-
cays insensitive to hopping rate. The decay would then be
exponential with a rate of kR  kNRi  kq, where kq is the
transfer rate to the quencher [17]. Our results indicate that
Dexter transfer to the quenchers is inefficient, which can be
explained by a type II energy level offset between the
emitter and quencher. The energy level offset would need
to be sufficiently smaller than the exciton binding energy
(1 eV) to prevent a quenching of the triplet excitons by
charge transfer [18,19] but large enough to make Dexter
transfer to the quencher inefficient. Another possible ex-
planation is that the highest occupied molecular orbital
(HOMO) of the quencher is localized on the metal ion in
contrast to the HOMO of the normal core, which is delo-
calized between the metal ion and ligands, so that the
distance becomes too large for hole tunneling to the
quencher. A possible quencher of this type is the mer
isomer [20] which shows a higher HOMO energy with a
more pronounced metal character than the fac isomer.
An average number of hops by an exciton during its
lifetime is given by kh=k where k  1= is the decay rate.
The probability to visit the same site twice on a random
walk in a 3D system is negligible. Therefore, the ratio kh=k
also represents the number of molecules sampled by triplet
excitons. Figure 2(d) shows that this ratio varies from 220
in IrG1 neat film to 5 in Ircarb-G3. Hence in all cases the
migration of triplet excitons in Ir ppy3-cored dendrimers
involves successive short range hops of the excitons be-
tween the cores until excitations decay radiatively or
nonradiatively.
The results presented so far have all been for films with
thicknesses in the range 170–200 nm. To gain a further
insight into the phosphorescence quenching and exciton
diffusion processes, we also studied the influence of the
film thickness on the PL kinetics in IrG1 and Ircarb-G1
neat films and in the blend containing 20 wt% of IrG1 in
CBP (Fig. 3). In both neat films, the PL decays were not
significantly modified when the film thickness varied from
180 to 60 nm but a strong increase of the phosphorescence
quenching when reducing the film thickness below 20 nm
was observed. The dynamics of the exciton density in the
case of quenching by an interface can be described by the
diffusion equation:
 
@Nx; t
@t
 Nx; t

D@
2Nx; t
@x2
: (4)
The boundary condition at the quenching interface is
D@Nx; t=@x  vNx; t, where v is the surface quench-
ing velocity. The PL intensity is proportional to the exciton
density integrated over the film thickness and, in the case
of fast exciton diffusion (D
 vd where d is the film
thickness), is given by:
 
Z d
0
Nx; tdx  N0d exp



1

 v
d

t

; (5)
where N0 is the initial exciton density. As shown in
Fig. 3(a) and 3(b), Eq. (5) describes our data well with
γ
µ
FIG. 2. Panel (a) shows the triplet-triplet annihilation rate as a
function of the distance between Ir ppy3 cores. Panel (b) shows
the distance dependence of the hopping kh and the nonradiative
decay kNR rates. The dotted line is the fit obtained with Eq. (2)
and the dashed lines are fits by Eq. (3). Panel (c) displays PLQY
and the PL lifetimes in the absence of annihilation with solid
lines as guides for the eye. In panel (d) the ratios kh=kq (open
symbols) and kh=k (solid symbols) are plotted as a function of
the distance between Ir ppy3 cores. In these graphs, the upward
pointing triangles are the data obtained from the neat film and
the blends of IrG1 in a CBP host, the crosses from the blend
containing 20 wt% of IrG1 in mCP, the circles from the neat
films of the dendrimers Ircarb G1, G2, and G3, and the down-
ward pointing triangles are from the highly branched G1 den-
drimer.
PRL 100, 017402 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending11 JANUARY 2008
017402-3
v  0:86 cm s1 for Ircarb-G1 and v  0:35 cm s1 for
IrG1, which shows that the phosphorescence quenching at
the interface is more than twice as fast in Ircarb-G1. It is
worth noting that this model does not fit the decay curves
for films thicker than 20 nm which is to be expected as
vdD. These results suggest that there is quenching
associated with one or both interfaces. The strength of
the surface quenching was also found to be strongly de-
pendent on the dielectric substrate used (data not shown)
which suggests that the substrate-film interface is more
likely to be responsible for this process. Additional experi-
ments would be needed to clarify the mechanism of this
surface quenching but we can already exclude the possi-
bility that faster decays in thinner films could be due to an
enhancement of dipole-dipole interactions because the
overlap between absorption and emission spectra and
hence Fo¨rster radius were independent of film thickness.
Finally, we found that the PL decays in the IrG1 blend
[Fig. 3(c)] with an average chromophore spacing of 3.1 nm
(assuming a homogenous blend) are independent of the
film thickness. This result shows that the phosphorescence
quenching at the surface can be totally suppressed by
increasing the intermolecular distance between chromo-
phores and thus reducing the exciton diffusion.
In summary, we have examined the PL properties of
phosphorescent Ir ppy3-cored dendrimers with different
molecular sizes in neat films and in blends. Triplet exciton
diffusion rates obtained from triplet-triplet annihilation
studies show exponential dependence on the spacing dis-
tance between phosphorescent cores in the range of 1.8 to
3.2 nm and an attenuation factor of 0:33 A1. This behav-
ior confirms that triplet exciton diffusion is ruled by a
Dexter mechanism. In contrast, the phosphorescence
quenching rate in the volume of the films is governed by
Fo¨rster type dipole-dipole interactions and is up to 615
times slower than the hopping rate. The direct comparison
of these rates indicates that concentration quenching and
triplet migration in Ir ppy3-cored dendrimers are different
processes. The slower quenching than diffusion can be
explained by a type II energy level offset between the
emitters and quenchers. A different regime in which dif-
fusion becomes important exists in very thin films
(<20 nm) where additional quenching of triplet excitons
is observed at the film surface and can be effectively
overcome by increasing the intermolecular spacing be-
tween chromophores.
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FIG. 3. PL kinetics of IrG1 neat film (a), Ircarb-G1 neat film
(b), and the IrG1 : CBP (20:80 wt%) blend (c) for different film
thicknesses at low excitation intensity. The decay curves in (c)
are almost identical and have therefore been shifted vertically for
clarity. Solid lines are the fits obtained using Eq. (5).
PRL 100, 017402 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending11 JANUARY 2008
017402-4


https://doi.org/10.1103/PhysRevLett.100.017402

Triplet exciton diffusion and phosphorescence quenching in Iridium(III)-Centered dendrimers

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