Triplet states and energy back transfer of carbazole derivatives

Article abstract of DOI:10.1039/c5ra09340f

Intermolecular interactions among π conjugated semiconducting molecules often give rise to totally different optical behaviours between the solid state and dilute phases. Phosphorescence spectra observed in the solid state are often lowered compared with dilute forms resulting in the red-shift of the phosphorescence spectra. Here, we demonstrate that this red-shift can be reduced by introducing side groups. We also show that such a shift is a function of interchromophoric distance with fast exponential decay. Furthermore, we show conclusively that triplet exciton transfer between the hosts and the bis[2-(4F,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(iii) can be described in terms of the Boltzmann factor using triplet energies obtained from the solid state. These results will have implications in molecular design that utilizes triplet excitons such as organic light emitting diodes and singlet fission solar cells.

Full text of DOI:10.1039/c5ra09340f

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Table of contents entry
The red-shift of triplet energies between the dilute phase and solid states is a function of
intermolecular distance.

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Triplet states and Energy Back Transfer of Carbazole
Derivatives
Kai Lin Woon1,3*, Zainal A. Hasan2,5 , Bee Kian Ong2,5, Azhar Ariffin2**, Raimonda Griniene4 ,
Saulius Grigalevicius4 , Show-An Chen3
1Low Dimensional Material Research Centre (LDMRC), Department of Physics, Faculty of
Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
2Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur,
Malaysia
3 Department of Chemical Engineering and Frontier Research Center on Fundamental and
Applied Sciences of Matters, National Tsing-Hua University, 101, Section 2, Kuang-Fu Road,
Hsinchu 30041, Taiwan, ROC
4 Department of Polymer Chemistry and Technology, Kaunas University of Technology,
Radvilenu Plentas 19, LT-50254 Kaunas, Lithuania
5 ItraMAS Corporation. Sdn. Bhd., 542A-B Mukim 1, Lorong Perusahaan Baru 2, Kawasan
Perindustrian Perai 13600, Penang, Malaysia
Correponding Authors *ph7klw76@um.edu.my , ** azhar70@um.edu.my
Abstract:
Intermolecular interactions among π conjugated semiconducting molecules often give rise to
totally different optical behaviours between the solid states and dilute phases.
Phosphorescence spectra observed in solid states are often lowered compared with dilute
forms resulting in the redꢀshift of phosphorescence spectra. Here, we demonstrate that this
redꢀshift can be reduced by introducing side groups. We also show that such a shift is a
function of interchromophoric distance with fast exponential decay. Furthermore, we show
conclusively that triplet exciton transfer between the hosts and the bis[2ꢀ(4F,6ꢀ
difluorophenyl)pyridinatoꢀC2,N](picolinato)iridium(III) can be described in terms of the
Boltzmann factor using triplet energies obtained from the solid states. These results will have
implications in molecular design that utilizes triplet excitons such as organic light emitting
diodes and singlet fission solar cells.
1

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Introduction
Triplet excitons are tightly bounded electronꢀhole pairs of total spin S = 1. These
excitons have been used to provide efficient light emission in organic light emitting diodes
(LEDs) 1ꢀ5 or to give free carriers in solar cells by dissociation of triplet excitons formed from
singlet fission 6ꢀ8. Triplet exciton confinement is a prerequisite for efficient phosphorescent
organic LEDs9ꢀ10. In contrast with phosphorescent organic LEDs, singlet fission solar cells
require a rapid triplet exciton transfer between two nearest neighbours but have a detrimental
effect for the efficiency of organic phosphorescent LEDs11. In order to obtain highly efficient
blue phosphorescent organic LEDs, hosts having high triplet energies are required. This is
usually achieved by incorporating meso, insulating or twisted configurations into the
molecular design12. However, interactions with other chromophores cannot be discounted in
solid state devices and photosynthetic reaction centres13ꢀ16. The molecular interactions can
include resonance, polarization, VanꢀderꢀWaals and exchange interactions for example17.
When the strength of the interaction is particularly strong, a stable dimer may form. For a
dimer, the mutual distance and orientation in the ground state and in the excited state is very
similar. While for the excimer, the mutual distance and orientation between the excited and
ground state equilibriums is significantly different. The strength of such interactions always
depends on the intermolecular distance. Triplet energies of organic molecules are often
determined in dilute forms in cryogenic temperatures18ꢀ20. Much consideration has been given
to reducing the effective conjugations but little effort is given to understanding how
intermolecular interactions can alter the energy levels. However, molecular interactions can
redꢀshift and even completely change the spectra observed in solid states. These can give rise
to triplet excimers such as in naphthalene21,22 and carbazole systems23,24. Since most
phosphorescent hosts are used in solid states, it is important to examine the phosphorescence
spectra of such hosts in solid and dilute forms.
Depending on the strength of the interactions, the phosphorescence spectra of
chromophores in a solid state is often redꢀshifted compared with the dilute forms. Despite the
importance of molecular interactions, there has been little work done to study how short
range interactions influence the observed phosphorescent spectra and how it differs from the
dilute forms and the monomers. This maybe the result of vanishing small phosphorescent
quantum yields from spinꢀforbidden transitions and increased nonꢀradiative loss pathways in
the solid state even at low temperatures25. Nevertheless, a greater understanding is required
on how the intermolecular interactions can alter the observed phosphorescent spectra and
their triplet energies. This is particularly relevant, for example, in determining the threshold
2

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of triplet ionization in singlet fission solar cells26, suppression of triplet excimers and
effectiveness of triplet exciton confinement in phosphorescent LEDs. This study is carried out
using a number of carefully selected organic semiconducting materials and their components,
mainly carbazole based derivatives. The phosphorescent spectra obtained in solid states can
be considered as a combination of interchromophore interaction and its contribution to the
perturbation of the zeroꢀorder (“dilute”) spectra. We report the difference between the spectra
obtained in dilute forms and solid states. We show that such a red shift follows a fast
exponential decay with distance. We go further to show how triplet energy transfer between
bis[2ꢀ(4,6ꢀdifluorophenyl)pyridinatoꢀC2,N](picolinato)iridium(III) (Firpic) and the host is
affected.
Experimental Methods
Materials Synthesis
The synthesis of compounds 1 – 11 is shown in Scheme 1. The structures of intermediates
and final products were confirmed by proton (1H) and carbon (13C) nuclear magnetic
resonance (NMR) spectroscopy, infrared (IR) spectroscopy and mass spectrometry (MS). All
commercially available starting materials, reagents and solvents were used as supplied, unless
otherwise stated. All these materials were obtained from Aldrich or Merck. All reactions were
carried out without exclusion of air unless otherwise stated and the temperatures were
measured externally. Microwave irradiated reactions were performed in a CEM Microwave
MARS 6, Xpress Plus vessel. For compounds with RMM < 800 g/mol mass spectra were
recorded using a Schimadzu QP5050A gas chromatography/mass spectrometer with electron
impact (EI) at a source temperature of 200 °C. For compounds with RMM > 800 g mol, mass
spectra were analyzed using a Bruker reflex IV matrixꢀassisted laser desorption/ionization
(MALDI) timeꢀofꢀflight (TOF) MS. A 384ꢀwell microliter plate format was used with a scout
target. Samples were dissolved in DCM with HABA (2ꢀ(4hydroxyphenylazo)benzoic acid)
matrix (1:10 DCM:HABA). IR spectra were recorded using a PerkinꢀElmer Spectrum 400
1
Fourier transformꢀinfrared (FTꢀIR) Spectrometer, H NMR and 13C spectra were recorded
using either a JEOL Lambda FTꢀ400 spectrometer, Bruker Avance III FTꢀ400 spectrometer or
Bruker Avance III FTꢀ600 MHz and an internal standard of tetramethylsilane (TMS).
Aluminumꢀbacked TLC plates coated with silica gel (60 F, Merck) were used to monitor the
progress of reactions. Purification of intermediates and final products were performed by
flash column chromatography, using silica gel 60 (0.063ꢀ0.200 mm) obtained from Merck.
The experimental details are found in the Supporting Information. Poly{3,6ꢀdi(tꢀbutyl)ꢀ9ꢀ[4ꢀ
(3ꢀmethyloxetanꢀ3ꢀylmethoxy)phenyl]carbazole} (PtPC) and poly{9ꢀ[4ꢀ(3ꢀmethyloxetanꢀ3ꢀ
3

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ylmethoxy)phenyl]carbazole} (PPC) are newly synthesized polymers. The chemical
structures of the materials are given in Figure 1. The experimental details of the synthesis are
provided in the Supporting Information. Poly(9ꢀvinylcarbazole (PVK), 2,7ꢀ
Bis(diphenylphosphoryl)ꢀ9,9'ꢀspirobifluorene
(SPPO13),
3,5ꢀdi(9Hꢀcarbazolꢀ9ꢀyl)
tetraphenylsilane (SIMCP2) and 4,4′,4″ꢀtrisꢀ(Nꢀcarbazolyl)ꢀtriphenlyamine (TcTa) are
purchased from Lumtec.
Low Temperature Phosphorescent Spectra Measurement
Thin film samples were dropꢀcasted onto a glass substrate heated at 60oC from chloroform
solution containing hosts at a concentration of 40 mg/mL. For a dilute system, the hosts are
prepared at 0.1mg/ml dissolved in dichlorobenzene. The samples were attached to the sample
holder in a nitrogen cryostat (Janis). The samples were excited at 350 nm by a 150 fs pulsed
Ti:sapphire laser (SpectraꢀPhysics Hurricane) at 1kHz repetition rate in conjunction with an
ultrafast optical parametric amplifier (Quantronix TOPAS). The luminescence from the film
was allowed to pass through a long pass filter (395nm) and a monochromator (Princeton
Instruments Acton SP 2150) and then to a gated intensified CCD camera (Andor ICCD
334T). The intensified CCD was operated synchronously with gate delay of 100 ꢀs for all
samples with a detection window width of 850µs. To increase the signal to noise ratio, each
final spectrum was obtained by averaging the accumulated spectra for at least 600,000 laser
pulses with a gain of at least 100. The laser intensity was about 7 µJ/pulse. Measurements
were carried out in a cryostat under vacuum of 10 ꢀ7 Torr at 77 K for all samples. Origin 8.5 is
used for the fitting. The spectra were fitted to Gaussian functions using least square
regression. Good fitting is obtained when R2 > 0.99 with the least number of Gaussian peaks.
The locations of the first peaks were then extracted from the least square fitting.
Time Resolved Lifetime Measurement
Thin film samples (~200nm) were spinꢀcoated from the chlorobenzene solutions containing
the hosts and Firpic at a concentration of 1% by weight. The samples were excited at 400nm
by a Ti:Sapphire laser using a similar setꢀup used for the determination of triplet energies. A
synchronous delay of 20ns is used with a detection window of 20ns. Measurements were
carried out in a cryostat (Janis) under vacuum of 10 ꢀ7 Torr at 77 K for all samples. The timeꢀ
resolved phosphorescence is measured up to 7ꢀs.
Molecular Modelling and Molecular Dynamic Simulation
Gaussian 09 is used to model the torsional angle and potential barrier using the PM6 method
at the ground state. For molecular dynamic simulations, the ground state molecular
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configurations are optimized using the PM6 method to obtain the lowest energy conformer.
For polymers, we duplicated the monomer and connected it back onto the original strand and
repeat the ground state configurations optimization until there are 32 units of chromophores.
We duplicate the whole polymer chain with close contact between the chains in Chemdraw
3D. The steric energy between the polymeric chains is minimized using the MM2 Level. We
repeat the same procedure until we obtained 8 polymeric chains giving us 256 units of
chromophores in cubic arrangement. The same procedure is also applied for the small
molecules. The steric energies of an ensemble of molecules/polymers are minimized and
terminated when the minimum RMS Gradient is less than 0.100. The ensemble of molecules
is then brought up to a target temperature of 300K with a heating rate of 1Kcal/atom/s with a
2.0fs step interval. A program is written in Labview to find the minimum contact distance
between two nitrogen atoms in two different molecules. A histogram of minimum contact
distance between each of the chromophores (interchromophoric distance) can be plotted.
Result and discussion
Phosphorescence spectra
The chemical structures of materials used in this study are given in Figure 1. In order to
understand how each substituent affects the overall molecular triplet energies, we
investigated the triplet energies of the substituents. All the compounds studied here, except
carbazole monomers, are amorphous. The lack of ability to form crystal is due to the presence
of bulky side groups and/or the highly twisted structure.27,28 Figure 2(a) shows the normalized
phosphorescent spectra of carbazole in a dilute form and dropꢀcasted onto thin film. The
spectra between the two states are very different. It is important to note that fluorescence and
phosphorescent spectra of the solid states are completely different in all cases investigated.
There is no delay fluorescence observed. In the dilute phase, the 0ꢀ0 vibronic peak is located
at 2.98 eV, consistent with published data22. However, the spectrum of the dropꢀcasted
carbazole thin film is drastically redꢀshifted by 0.68 eV with the first peak located at 2.30 eV.
This is far lower in energy than most triplet excimer emissions obtained from carbazole
derivatives.22,23,29 It could be the result of very strong interactions between carbazole
molecules since the dropꢀcasted films are expected to produce polycrystalline carbazole
microcrystals. We also observed a second peak located at 1.80 eV of unknown origin.
Figure 2(b) shows the normalized phosphorescent spectra of carbazole functionalized with
tertꢀpentyl at the 3 and 6 positions of carbazole in dilute form and dropꢀcasted thin film. The
whole spectrum of 3,6ꢀdiꢀtertꢀpentylꢀ9Hꢀcarbazole is redꢀshifted by 0.05 eV compared to the
monomer in dilute form. However, in the dropꢀcasted thin film, the first peak is increased to
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2.58 eV compared to 2.30 eV for carbazole monomers. This indicates that the tertꢀpentyl acts
as an bulky substituent, effectively reducing the intermolecular interactions. However the
peak at 1.80 eV can be observed with a reduced intensity. By functionalizing carbazole with
3,6ꢀdiꢀtertꢀpentylꢀ9Hꢀcarbazole at the 3 and 6 positions to give 3,6ꢀbis(3,6ꢀditertꢀpentylꢀ
carbazolꢀ9ꢀyl)carbazole (TPC), the triplet energy in the dilute form is further reduced from
2.93 eV to 2.81 eV as shown in Figure 2(c). This indicates that the triplet exciton
wavefunction might not be fully confined into the monomeric carbazole. Nevertheless, the
first peak of the dropꢀcasted thin film has increased to 2.68 eV for TPC. Unlike carbazole
and 3,6ꢀdiꢀtertꢀpentylꢀ9Hꢀcarbazole, where the phosphorescence is featureless,
phosphorescence from TPC contains vibronic features. The difference between the first peak
of dropꢀcasted thin film and dilute form (∆ESD) for TPC is reduced to 0.13 eV compared with
0.68 eV and 0.35 eV for carbazole and 3,6ꢀdiꢀtertꢀpentylꢀ9Hꢀcarbazole respectively. When
two TPC compounds are coupled at the 1 and 3 position of the benzene ring to give 1,4ꢀ
bis(3,6ꢀbis(3,6ꢀditertꢀpentylꢀcarbazolꢀ9ꢀyl)carbazolꢀ9ꢀyl)benzene (TPC-CCP), the triplet
energy in the dilute form is further reduced to 2.78 eV. However, the ∆ESD is reduced to 0.05
eV resulting in the triplet energy in the thin film to be 2.73 eV. The decrease of the triplet
energy from 3,6ꢀdiꢀtertꢀpentylꢀ9Hꢀcarbazole to TPC can be explained by the increase of the
‘effective’ conjugation as evident from the photoluminescence spectra in the dilute solutions
as indicated in Figure 3. Even though there is a ‘break’ of conjugation as a result of the
torsional angle between two carbazole substituents for TPC, the ‘break’ is not complete.
Depending on the torsional potential, torsional angle and temperature, Boltzmann statistics
dictates that there will be a range of possible torsional angles30. The torsional angle between
two carbazoles is modelled using a Gaussian PM6 resulting in a potential barrier of ~0.2 eV
with a torsional angle of 47.4o for TPC as shown in Figure 4. Compared with 4,4′ꢀbisBis(9ꢀ
carbazoleyl)ꢀ2,2′ꢀdimethylꢀbiphenyl (CBDP), it has a potential barrier of ~0.8 eV with a
torsional angle of 82o. The number of near planar conformations between two carbazoles for
TPC-CCP can be calculated using Boltzmann distribution30 (See Supplementary information
Fig1). Ideally the torsional potential barrier is significantly higher than the thermal energy
and Gaussian disorder. In order to further confirm whether steric hindrance can reduce the
∆ESD, we measure a larger range of compounds. For 3,6ꢀbis(diphenylphosphoryl)ꢀ9Hꢀ
carbazole (POCARB) and 2,7ꢀBis(diphenylphosphoryl)ꢀ9,9'ꢀspirobifluorene (SPPO13), the
role of phosphine oxides is to increase solubility in polar solvents such as alcohol. The
diphenylphosphinic moiety/group also increases the intermolecular distance between two
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chromosphores. As expected, the ∆ESD is 0.07 eV and 0.06 eV for POCARB and SPPO13
respectively as shown in Figures 5(a) and 5(b). For PVK, the monomers being separated by
vinyl (two carbons), ∆ESD is 0.29 eV. Poly{3,6ꢀdi(tꢀbutyl)ꢀ9ꢀ[4ꢀ(3ꢀmethyloxetanꢀ3ꢀ
ylmethoxy)phenyl]carbazole}
(PtPC)
and
poly{9ꢀ[4ꢀ(3ꢀmethyloxetanꢀ3ꢀ
ylmethoxy)phenyl]carbazole} (PPC) are newly synthesized polymers. Both are separated by
three carbons and one oxygen atom in the main chain. In PtPC, the carbazole is
functionalized with tertꢀbutyl at the 3 and 6 positions. This results in the reduction of ∆ESD
from 0.24eV (PPC) to 0.14eV (PtPC) as illustrated in Figure 5(c) and 5(d) respectively. We
also note that the functionalisation of tertꢀbutyl or tertꢀpentyl results in a slight reduction of
triplet energy in the dilute form as evident in 3,6ꢀdiꢀtertꢀpentylꢀ9Hꢀcarbazole and PtPC. This
can be explained by the effect of hyperconjugation. In terms of the molecular orbital shape of
nonsubstituted carbazole, electron distribution is rich at the 3ꢀand 6ꢀpositions of the HOMO
energy level31. The electrons in the sigma bond interact with the πꢀorbital to give a more
extended molecular orbital. The photoluminescence is redꢀshifted indicating a reduced bandꢀ
gap and hence reduced triplet energy.
It is clear that the difference in triplet energies measured in the solid states and dilute
phases (whether it is triplet excimers from dimers or monomers) must be a function of the
distance between two chromophores. (ie, each carbazole in the polymeric chain of PPC is
distinct from each other as the phosphorescent spectra of PPC is very similar to carbazole
itself while for example in TPC, the carbazoles within the same molecule are not
spectroscopically distinct with each other). Here, the minimum distance between two
spectroscopically distinct chromophores is referred to as the interchromophoric distance as
measured from the nitrogen atom of the chromophore.
Correlation of ∆ESD with interchromophoric distance
Because organic semicoductors are generally amorphous, it is not a trival task to determine
how each of the chromophores are separated from each other in an solid state by
experimental methods. However, it does not deter some researchers trying to obtain the
interchromophoric distance, r between two carbazoles using highꢀlevel ab initio
calculations32. The contact distance between two carbazoles in poly(9ꢀvinylcarbazole) (PVK)
is simulated with more than 200 carbazole units in a given simulation to obtain a statistically
meaningful result. An MM2 force field is used. Phosphine oxide is not parameterized within
the MM2 and hence not included the calculations. The obtained mean r of PVK is 0.39 nm
with a standard deviation of 0.05nm. Most r have a normal (Gaussian) or logꢀnormal
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probability density function (see supplementary information Figure 2). This behaviour has
been observed in amorphous solids exhibiting a random network structure33. Amorphous
organic semiconductors and polymers are disordered and have been shown to exhibit logꢀ
normal distributions for interchromophoric distance34,35. It is interesting to speculate that such
positional disorder distributions might contribute to the Gaussian distribution of the density
of states. Figure 6 shows the correlation of ∆ESD with mean r for a range of compounds along
with their standard deviations. tpc-CCP has the largest r (1.79 nm) and the largest standard
deviation with just slightly less than 10% having r less than 1 nm. It is noted here that the r
for tpc-CCP is twice as large as the distance from the nitrogen atom at carbazole to the
furthest hydrogen atom at the tertꢀpentyl substituent (0.85nm). This type of characteristic has
been observed in nematic semiconducting liquid crystalline phases exhibiting bulky side
groups using Xꢀray Diffraction36. For TPC, the r is almost half of tpc-CCP which is 0.82
nm. It is not surprise since the bulky side groups are on only one side of the carbazole. In the
solid states, emission often occurs at lower energies. Hence the redꢀshifted spectra is
observed. Such red shift of energy should depend on the interchromophic distance. Hence, we
fit the data with equation below.
ꢀE = Joe−2γr (1)
where r is the interchromophric distance, ꢁ is the electronic wavefunction localization and Jo
is the constant of dimension of energy. Fitting of equation (1) on Figure 5 yields Jo = 1.08 eV,
ꢁ = 0.32nmꢀ1 with an error of 0.08 eV. These parameters could be used to predict the triplet
energies in the solid state for carbazole based materials once the r is known. Triplet energies
computated in a vacuum using timeꢀdependent density functional theory can be used to
predict the triplet states in solid states using this correction38.
Energy Back transfer
In order to study the effect of energy back transfer between the Firpic and solution
processable host, we carried out time resolved phosphorescence measurements. 3,5ꢀdi(9Hꢀ
carbazolꢀ9ꢀyl) tetraphenylsilane (SIMCP2), 4,4′,4″ꢀtrisꢀ(Nꢀcarbazolyl)ꢀtriphenlyamine
(TcTa) and SPPO13 are solution processable hosts which have been used to produce blue
phosphorescent organic LEDs 39ꢀ41. The lifetimes of the guest molecules are always used as
an indication of the effectiveness of triplet confinement42,43. It should exhibit clear
monoexponential decay as an indication of good triplet exciton confinement. Time resolved
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phosphorescence is carried out on Firpic doped hosts at 10% by weight. The Firpic doped
hosts are excited at 400nm. Since the absorption band of the hosts are all less than 370nm, the
excitons created by pulsed excitation are Firpic excitons. There is a clear expotential decay of
radiative lifetime reducing with increasing triplet energy as measured in solid states as shown
in Figure 7. Only TcTa shows expontential decay with two lifetimes. Here, we took the
longer lifetime as an indication of energy back transfer. The phosphorescence spectra of
SIMCP2 and TcTa are given in the supplemtary information Figure S3.
The radiative lifetime of Firpic which is 1.2ꢁs 44 is far longer than the minimum hopping time
of a triplet exciton between two molecules which can be as low as a few picoseconds45.
Assuming that thermal equilibrium is reached between the excited Firpic molecules and the
next nearest hosts in the system, the population density between triplet excitons in the Firpic
and the host molecules can be calculated using the rate equations:
dnIr
= −kIrnIr + kHEnho − kEH nIr − γ
IRnI2r (2)
dt
dnhost
dt
= −khonho − kHEnho + kEH nIr − γ
honh2o (3)
where nIr is the triplet exciton population density of Firpic, kIr is the population decay rate for
Firpic, kHE is the transfer rate from the host (H) to the emitter (E), kEH is the transfer rate from
the emitter (E) to the host (H), nho is the triplet exciton population density of the host and kho
is the population decay rate for the host. kHE and kEH are the transfer rates usually given by the
Marcus hopping rate for high temperatures or MillerꢀAbraham for a low temperature
regime46,47. Since Firpic has a radiative quantum efficency of near 99% ignoring nonꢀ
radiative decay, kIr becomes the Einsten A coeffient. kho is the sum of the radiative and nonꢀ
radiative decay rates for the triplet excitons of the host molecules located nearest to the Firpic
molecules. γho and γIr are the tripletꢀtriplet annihiation quenching rates for the host and the
emitter respectively. The total number of triplet excitons, N, in that system is simply the sum
of nIr and nho. MaxwellꢀBoltzmann distribution between energy levels in Firpic and the host
molecules can be given as:
Eho
−
EIr
nIr
KT
= e
(4)
nho
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where Eho and EIr are the triplet energy levels of the host and Firpic determined in the solid
states. Eho and EIr in equation (4) are the mean values of Gaussian broadened density of states
triplet energy levels assuming both have the same variant σ. K is the Boltzmann constant and
T is the triplet exciton temperature. Note that the equation 4 does not depend on whether the
triplet states originate from the triplet exciton, from a monomer, dimer or excimer, as it is the
energy levels which are important as long as the lifetime of the triplet excimers are
significantly longer than the Firpic. This is true since phosphorescence from the host can be
observed even 800 ꢁs after laser pulse excitation for both doped and undoped host.
Summing equations (2) and (3) gives:
dN
= −kIrnIr − khonho −γ γ
IRnI2r − honh2o
(5)
dt
which is independent of the hopping rate given by Marcus and the MillerꢀAbraham transfer
rate. Equation (6) can be further simplified by considering several facts. Firstly, the decay of
host triplets is significantly longer than the radiative lifetime of Firpic. Hence khonho <<
kIrnIr . Secondly, the fluorination on the ppy ligand of Firpic hinders selfꢀquenching
interactions. The photoluminescence quantum yield of Firpic in an inert matrix at
concentrations of ≤ 10% has been shown to approach 99%48. It is believed that the
fluorination on the ppy ligand hinders selfꢀquenching interactions. Hence, the selfꢀquenching
of emitterꢀemitter is too small to have any significant effects. The laser intensity was spread
out on a spot of 0.2cm2. The initial excitation densities can be calculated from the absorption
at 400nm to give an initial exciton density of less than 1017 cmꢀ3. However, this exciton
density is too small to cause an appreciable hostꢀhost quenching.49 Furthermore, under typical
experimental conditions, the low temperature tripletꢀtriplet annihilation rate, γ, is of the order
10ꢀ14 cm3/s.50 This is usually explained by blocked triplet diffusion at extremely low
2
temperatures.51 Since the experiment is carried out at 77K, this means that khoNho2 >> γho Nho
by three order of magnitude. Hence the hostꢀhost quenching will not play any significant role.
Equation (5) tends to simplify into:
dN
= −kIr nIr (6)
dt
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Using equation (4) and keep in mind that N = nIr + nho equation (6) becomes:
dN
dt
N
= −kIr
(7)
EIr −Eho
KT
(1+ e
)
Integrating (7) becomes:
EIr
−Eho
KT
τ =τo + (τ
oeKT )e
(8)
where ꢂo is the radiative lifetime of Firpic embedded in the host. A monoꢀexponential fit
performed for Figure 7 results in ꢂo =1.32 ꢀs, an effective temperature of 664 K and a Firpic
triplet energy of 2.55 eV. This is higher than the bulk/lattice temperature. The excitons are
always coupled with phonons within the molecule. Optical relaxation and the reorganization
of the molecular structure from hoping from one molecule to another generate phonons. The
high effective temperature might correspond to the vibronic deformation of the ring modes of
Firpic52. The consequence of equation (4) is that in order to reduce the energy back transfer
from the emitter to the host to less than 1%, the hosts must have a triplet energy of at least
0.26 eV higher than the emitters. The equation does not capture the diffusion of the triplet
excitons far away from the Firpic–host interface which can result in failed back transfer and
hence quenching, as evident from the observed phosphorescence detected with a gate delay of
650ꢀs (see supplementary information Figure 4). It is surprising to see that phosphorescence
from Firpic doped hosts are observed except above 2.80eV. The triplet exciton generated by
the Firpic molecule can be transferred to the neighbouring host molecule as shown in Figure
8(a). The triplet energy can also be backꢀtransferred to the Firpic molecule. Triplet excitons
generated at the host molecules will then undergo randomꢀwalks with diffusion taking place
in an energetic downward direction and, by thermal excitation, also in an energetic upward
direction. Over time, the exciton tends to occupy the tail of the Gaussian density of states
resulting in the loss of the triplet exciton as shown in Figure 8(b). Or it might find its way
back to the Firpic molecules as shown in Figure 8(c).
Conclusions
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The effect of side groups on the energy shift of phosphorescent spectra in the solid state has
been quantified. The reduction of energy shifts in the solid state can be related to the increase
in interchromophoric distance. By using triplet energies determined in the solid state, we can
show clearly that energy transfer between Firpic and the host is governed by the Boltzmann
factor. This implies that the host should have a triplet energy greater than 2.80 eV in order to
confine the triplet exciton generated in Firpic. This new found understanding will have
implications in molecular design for triplet based optoelectronic devices.
Acknowledgements
This research is funded by Chancellery High Impact Research Grant (UM.C/625/1/HIR/195)
and (UM.C/625/1/HIR/208) and by Grant No. MIPꢀ024/2013 from the Research Council of
Lithuania. S.A. Chen wishes to thank the Ministry of Education and Ministry of Science and
Technology of the Republic of China for the financial aid through project NSCꢀ101ꢀ2120ꢀMꢀ
007ꢀ004, NSCꢀ102ꢀ2633ꢀMꢀ007ꢀ002 and NSC 102ꢀ2221ꢀEꢀ007ꢀ131
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Figure Legends
Figure 1: Molecular structure and the names of materials used in determining the triplet
energies.
o
Scheme 1: Reagents: (a) KI,KIO3, AcOH,80oC,24 h; or NBS, toluene, DMF, 0 C; (b) 2ꢀ
Chloroꢀ2ꢀmethylbutane, AlCl3, Dichloromethane(dry),R.T, 24 h; (c)AC2O,reflux, 4h; (d)
Cu2O DMAc,165oC, 48 h (e) KOH,DMSO,THF,H2O,reflux, 4h ; (f)Cu, K2CO3,18ꢀCrownꢀ
6,O
ꢀDichlorobenzene, 180oC, 24 h or 48 h; (g) H2O, CH3CN, ꢁw, 90 oC, 1h; (h) NiCl2.6H2O,
Zn, 2,2ꢀbipyridine, 99.5% ethanol, 70 oC, 5d.
Figure 2: Phosphorescence spectra in dilute phase or in the form of molecular aggregate
formed from dropꢀcasting. ∆ESD is the difference in triplet energies between the two states.
All triplet energies are determined from the 0ꢀ0 vibronic peak using multiꢀGaussian fitting.
(a) Carbazole (b) 3,6ꢀdiꢀtertꢀpentylꢀ9Hꢀcarbazole (c) TPC (d) tpcꢀCCP.
Figure 3: The normalized photoluminescence emission. The carbazole monomer has a peak
at 3.50eV. Substitution of the weak electron donating group decreases the peak to 3.40eV for
3,6ꢀdiꢀtertꢀpentylꢀ9Hꢀcarbazole. The effective conjugation for TPC and TPCꢀCCP is larger
than carbazole resulting in the redꢀshift of photoluminescence emission.
Figure 4: The torsional angles for CDBP, TPC and TPCꢀCCP. The red lines along the
molecules in the figure are the measured dihedral angle.
Figure 5: Phosphorescence spectra in dilute form or in the form of molecular aggregate from
dropꢀcasting (a) POCARB (b) SPPO13 (c)PPC and (d) PtPC.
Figure 6: Correlation of ∆ESD with mean interchromophoric distance for a range of
compounds and the error bars are their respective standard deviation. ∆ESD follows a exp(ꢀ
2R) decay. ꢃ is the error of fitting. The interchromophoric distance for disordered carbazole is
obtained from Ref. 37.
Figure 7: Correlation of the radiative lifetime of Firpic in various hosts. The higher the triplet
energy, the shorter the radiative lifetime.
Figure 8: The three main pathways for energy transfer of a triplet exciton generated at the
Firpic molecule. (a) Energy transfer between the nearest neighbouring host molecule and
the Firpic molecules governed by the Boltzmann factor (b) Down-hill transfer via random
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walk away from the Firpic before becoming lost. (c) Multiphonon absorption. The triplet
exciton hopped among the host molecules and later finds its way back to the Firpic
molecule.
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Scheme 1
179x249mm (300 x 300 DPI)

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Figure 1
97x63mm (300 x 300 DPI)

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Figure 2
499x383mm (96 x 96 DPI)

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Figure 3
61x47mm (300 x 300 DPI)

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Figure 4
61x47mm (300 x 300 DPI)

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Figure 5
61x47mm (300 x 300 DPI)

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Figure 6
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Figure 7
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Figure 8
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Supporting Information
Triplet Energy Confinement and Transfer in Organic Semiconducting
Molecular Assemblies
K. L. Woon1,3, Z. A. Hasan2,5 , B.K.Ong2,5, A. Ariffin2, R. Griniene4 ,S. Grigalevicius4 , Show-An Chen2
1Low Dimensional Material Research Centre (LDMRC), Department of Physics, Faculty of Science,
University of Malaya, 50603 Kuala Lumpur, Malaysia
2Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
3Department of Chemical Engineering and Frontier Research Center on Fundamental and Applied
Sciences of Matters, National Tsing-Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 30041,
Taiwan, ROC
4 Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu
Plentas 19, LT-50254 Kaunas, Lithuania
5 ItraMAS Corporation. Sdn. Bhd., 542A-B Mukim 1, Lorong Perusahaan Baru 2, Kawasan
Perindustrian Perai 13600, Penang, Malaysia
1. Materials Synthesis.
3,6-Diiodo-9H-carbazole (2b)1
Carbazole (5.0 g, 29.94 mmol) was dissolved in acetic acid (50 ml) and then warmed to 80oC.
Potassium iodide (6.51 g, 39.22 mmol) and potassium iodate (4.99 g, 23.35 mmol) was
added to this solution and the mixture was refluxed for 24 hours. The crude product was
diluted with water and filtered under vacuum. The brown precipitates were stirred in
solution of sodium sulfite for 1 hour and filtered with vacuum. The product was
recrystallized from dichloromethane to give brownish solid.(9.08 g, 72 %). Mp 145-150°C; IR
1
(υmax, cm-1): 3412, 1378; H NMR (ppm, 400 Mhz, CDCl3) δH : 8.32 (2H, s), 8.10 (1H, s, N-H),
7.69 (2H, dd, J = 1.08, 7.40 Hz), 7.22 (2H, d, J = 8.48 Hz ); 13C NMR (ppm, 100 Mhz, CDCl3) δc :
138.51, 134.82, 129.39, 124.58, 112.69, 82.45 ; MS (GC) [m/z]: calcd for C12H7I2N, 418.87;
found, 419.00.

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3, 6-Di-tert-pentyl-9H-carbazole (3)
The mixture of carbazole (10.0 g, 59.88 mmol), anhydrous AlCl3 (7.98g, 58.88mmol) and
CH2Cl2 (200 ml) in a three-necked flask cooled to 0oC was added dropwise a solution of 2-
chloro-2-methylbutane (14.75 ml, 119.76 mmol) in CH2Cl2 (40 ml). After addition, the
mixture was stirred for 10 min at the same temperature. Then the ice bath was removed
and the reaction was stirred for 24 hours. The mixture poured into ice-water (500 ml) and
extracted with dichloromethane. The combined organic phase was dried over MgSO4. After
filtered, the filtration was evaporated to give grey crude which was recrystallized from
ethanol to afford white powder (9.33 g, 51%). Mp 215-220 °C; IR (υmax, cm-1): 3417; 2962; 1H
NMR (ppm, 400 Mhz, CDCl3) δH : 8.01 (2H, s), 7.83 (1H, s, N-H), 7.39 (2H, d, J = 8.48 Hz), 7.33
(2H, d, J = 8.52 Hz), 1.76 (4H, q, J = 7.40, 7.44 Hz), 1.41(12H, s), 0.71 (6H, t, J = 7.40 Hz); 13C
NMR (ppm, 100 Mhz, CDCl3) δc :140.41, 137.96, 124.03, 123.31, 117.06, 109.96, 37.90,
37.42, 29.15, 9.31; MS (GC) [m/z]: calcd for C22H29N, 307.23; found, 307.00.
N-(3, 6-diiodo-9H-carbazol-9-yl)ethanone (4)
3, 6-Diiodo-9H-carbazole (10 g, 23.87 mmol) was dissolved in acetic anhydride (50 ml). The
mixture was refluxed for about 4 hours. The precipitates were filtered under vacuum and
wash with water several times to afford white powder (8.76 g, 79 %). Mp 215-220°C; IR
(υmax, cm-1):3062, 1703; 1H NMR (ppm, 400 Mhz, CDCl3) δH : 8.25 (2H, s), 7.96 (2H, d, J = 8.80