Phosphine oxide functionalized pyrenes as efficient blue light emitting multifunctional materials for organic light emitting diodes

Article abstract of DOI:10.1039/c4tc01753f

In a search for blue light emitting multifunctional materials, the electron transport enhancing diphenyl phosphine-oxide (Ph₂PO) group has been appended to blue light emitting pyrene derivatives. This design, we observe, leads to highly efficient electron transporting blue-emitters for non-doped organic light emitting devices (OLEDs) with good film formation characteristics. The superior performance is attributed to enhanced charge transport and formation of pyrene excimers assisted by thermally activated delayed fluorescence (TADF) in the device. We report the synthesis and characterization using experimental and computational methods of six such pyrene derivatives. Although three of these derivatives show quenching of luminescence in solvents at higher concentrations, in the thin film invariably all six of them exhibit typical pyrene excimer emission. X-ray crystal analysis reveals π-π stacking and the C-H...O interactions in the solid due to the PO group. The measured electron mobilities for all the compounds are higher in comparison to the standard electron transport material, Alq₃. Non-doped OLEDs with the pyrene derivatives as emitters (multi-layer configuration) as well as electron transport cum emitters (bilayer configuration) exhibit excellent efficiencies. The derivatives as emitters display a performance with current efficiencies (η_c) in the range 21.1-30.1 cd A^-1, power efficiencies (η_p) 11.0-15.76 lm W^-1, external quantum efficiencies (EQE) 7.2-9.1% and brightness 28500-42750 cd m^-2. In addition, the derivatives as electron transporting emitters demonstrate very good external quantum efficiencies in the range of 3.0-4.0%. These results demonstrate a successful strategy to obtain blue light emitting multifunctional materials for OLED applications.

Full text of DOI:10.1039/c4tc01753f

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Phosphine Oxide Functionalized Pyrenes as Efficient Blue
Light Emitting Multifunctional Materials For Organic Light
Emitting Diodes
1,2
1,2
2
Godumala Mallesham, Chidirala Swetha, Surukonti Niveditha, Maneesha
,
Esther Mohanty,1
Nanubolu Jagadeesh Babu, * Arunandan Kumar, *
3,
4
2,5,
1,5,
Kotamarthi Bhanuprakash * and Vaidya Jayathirtha Rao , *
1
Crop Protection Chemicals Division, CSIRꢀIndian Institute of Chemical Technology, Hyderabad
500007, India, Eꢀmail: jrao@iict.res.in
2
Inorganic and Physical Chemistry Division, CSIRꢀIndian Institute of Chemical Technology,
Hyderabad 500007, India, Eꢀmail: bhanu2505@yahoo.co.in
3Centre for Xꢀray Crystallography, CSIRꢀIndian Institute of Chemical Technology, Hyderabad
500007, India, Eꢀmail: jagadeesh.nanubolu@iict.res.in
4Laboratoire Interdisciplinaire Carnot de Bourgogne (ICB) UMR 6303 CNRS, Universit´e de
Bourgogne. 9, Av. Savary, BP 47870, 21078 Dijon Cedex, France.
Eꢀmail: kumar.arunandan@gmail.com
5
Network Institute for Solar Energy, New Delhi, India

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ABSTRACT
Aiming for blue light emitting multifunctional materials, electron transport enhancing
diphenylphosphineꢀoxide (Ph P=O) group is appended to blue light emitting pyrene
2
derivatives. This design, we observe, leads to highly efficient electron transporting blueꢀ
emitters for nonꢀdoped organic light emitting devices (OLEDs) with good film formation
characteristics the superior performance attributed to enhanced charge transport and
formation of pyrene excimers assisted by thermally activated delayed fluorescence
(TADF) in the device. We report the synthesis and characterization using experimental
and computational methods of six such pyrene derivatives. Although three of these
derivatives show quenching of luminescence in solvents at higher concentrations, in the
thin film invariably all six of them exhibit typical pyrene excimer emission. Xꢀray crystal
analysis reveal π−π stacking and the CꢀHꢁꢁꢁO interactions in the solid due to P=O group.
The measured electron mobilities for all the compounds are higher in comparison to
standard electron transport material Alq . Nonꢀdoped OLEDs with the pyrene
3
derivatives as emitters (multi layer configuration) as well as an electron transport cum
emitters (bilayer configuration) exhibit excellent efficiencies. The derivatives as electron
transporting emitters exhibit a performance with current efficiency (η ) in the range
c
2
1.1−30.1 cd/A, power efficiency (η ) 11.0−15.76 lm/W, external quantum efficiency
p
2
(
EQE) 3.0−4.0% and brightness 28500−42750 cd/m . In addition, the derivatives as
emitter demonstrate very good external quantum efficiency in the range of 7.2−9.1%.
These results demonstrate a successful strategy to obtain blue light emitting
multifunctional materials for OLED applications.

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KEYWORDS: synthesis of pyreneꢀphosphine oxides, thermally activated delayed
fluorescence (TADF), pyrene excimers, electron transport and emitting materials,
organic light emitting devices.
INTRODUCTION
Organic molecules with potential applications as luminescent materials have
been the focus of current research because of the attractive custom tailored design and
versatility of synthesis. An example of a practical application of such a molecule as a
material is the emissive layer in organic light emitting diodes (OLED). Since the initial
report, there has been a rapid progress of research in the field of OLED both at the
1
basic and applied level. OLEDs have potential as a technology for flat panel displays
and lighting because of their characteristic low driving voltage, high brightness, fullꢀ
colour emission, fast response time, wide viewing angle and selfꢀemitting properties
2
with an added advantage of low price. Full colour displays require the three primary
3
colours (red, green and blue) with equal stability, colour purity and improved efficiency.
Among these, green and red light emitters have been extensively studied and numerous
4
reports are available in the literature. However, blue light emitting materials are
relatively less explored due to their wide band gap and low HOMO energy level which
5
impede the charge carrier injection from the adjacent layers. Thus, the development of
blue light emitting materials with high efficiency remains as a challenge to researchers.
The basic OLED principle is that the hole (radical cation) and electron (radical
anion) should undergo recombination in emissive layer (electrochemiluminescence
6
(ECL)) to generate light.
Generally, OLEDs with fluorescent and phosphorescent

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emitters have been fabricated in multilayer configuration, possessing hole transport
layer (HTL), an emitting layer (EML) and an electron transport layer (ETL) along with
charge injection and blocking layers which are sequentially deposited in between anode
6
and cathode with each layer having their own functioning. In such configuration, holes
and electrons are injected from HTL and ETL, respectively into the emitting layer, where
the charge recombination takes place and an exciton is generated which emits light.
The major drawbacks of the multilayer configuration are the complexity and
7
manufacturing cost. To overcome these hurdles, materials with multi functional
properties, like emission and charge transport have to be developed so as to reduce the
8
number of layers in the device. Emitting materials with electron transport property are
9
relatively less explored than hole transporting emitters. Though the phosphorescent
OLEDs show high efficiency compared to fluorescent OLEDs, the colour purity, dopant
10
concentration and tripletꢀtriplet annihilations limit their applications. Recently Adachi
11ꢀ16
and coꢀworkers
have identified a new phenomenon that thermally activated
delayed fluorescence(TADF) can play a major role in enhancing the performance of
OLEDs. Overall small organic molecules have advantages like easy synthesis and
purification over polymers.
Keeping the above factors in mind and our own research interest, we aim in this
9c,17
work to develop an efficient multifunctional blue emitter.
We chose pyrene moiety for
the emitting part because of its high fluorescence quantum yield, good thermal stability
18
and more active positions to develop numerous analogues.
Though pyrene as an
emitter has been a subject of numerous investigations, most of the reported work has
19
focused on preventing its tendency to form π−π stacking and excimers.
Very few

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20
reports actually utilize the excimer emission of pyrene in the device. As the π−π
stacking enhances the charge transport we retain it and additionally we increase the
electron affinity of the molecules by end capping electron deficient/withdrawing
diphenyl phosphine oxide group to the pyrenes. In an earlier study, end capping pyrene
derivatives with electron donating diarylamino groups to obtain bifunctional blue emitting
19l
material yielded reduced EL.
On the other hand there are reports of P=O functional
group appended to highly fluorescent molecules like anthracene and triphenylamine
21
leading to effective electron transporting blue emitters.
Using this design principle, here we have synthesized and investigated six new
diphenyl phosphineꢀoxide appended pyrene derivatives (Figure 1). In this series of
compounds, different spacers were introduced to compound 10 to improve optical and
thermal properties. The positional isomers 11 and 13 were designed to verify the
influence of molecular configuration on optoelectronic properties. The increase in
number of phenyl rings between pyrene and PO entities offers to increase the thermal
stability. Compound 15 was designed to compare the influence of non planar biphenyl
and planar flourene motif as π−bridge. Two methyl groups were appended on phenyl
ring which cause molecular non planarity through twisting of phenyl ring for compound
1
4. Photophysical, electrochemical and thermal properties evaluation of all the
compounds (10ꢀ15) have been carried out. The crystal structure of 13 was analyzed by
Xꢀray crystallography. Nonꢀdoped OLEDs were fabricated for the compounds 10-15 as
an emitter [device configuration: ITO (120 nm)/F ꢀTCNQ (2.5 nm)/αꢀNPD (45
4
nm)/emissive layer (10-15) (30 nm)/BCP (6 nm)/Alq (30 nm)/LiF (1 nm)/Al (150 nm)] as
3
well as an electron transport emitter [device configuration: ITO (120 nm)/F ꢀTCNQ (2.5
4

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nm)/αꢀNPD (45 nm)/emissive layer (10-15) (50 nm)/LiF (1 nm)/Al (150 nm)]. In addition,
5
the electron mobilities were measured for all the compounds at an electric field of 1x10
V/cm. To the best of our knowledge the present manuscript is the first report of
phosphine oxide functionalized pyrene derivatives as multifunctional blue light emitters.
Results and Discussion
Synthesis and Characterization
Scheme 1 and Scheme 2 illustrate the synthetic sequence adopted to obtain the target
30
materials 10-15 (Figure 1). Pyrene was brominated to get bromo pyrene 1 and
31
subsequently 1 was transformed to its corresponding borolane derivative 2 (Scheme
1
). In another sequence commercially available fluorene was brominated to get dibromo
32
33
derivative 3 and further it was dimethylated to obtain compound 4 (Scheme 1).
SuzukiꢀMiyaura coupling reaction was developed to make key mono bromo
intermediates 5-9 in good yields (73ꢀ85%), by reacting compound 2 with various
dibromo
derivatives
like
1,4ꢀdibromobenzene,
4,4'ꢀdibromobiphenyl,
1,3ꢀ
dibromobenzene, 1,4ꢀdibromoꢀ2,5ꢀdimethylbenzene and 2,7ꢀdibromoꢀ9,9ꢀdimethylꢀ9Hꢀ
34
fluorene (Scheme 2). The synthesized mono bromo intermediates 1 and 5-9 were
o
treated with nꢀBuLi to generate corresponding aryl lithio derivative at ꢀ78 C, quenched
with chlorodiphenylphosphine and subsequently oxidized using hydrogen peroxide to
obtain target phosphine oxide compounds (10-15) as depicted in Scheme 2. All the
1
13
intermediates and target compounds were characterized by H, C NMR, and mass
spectrometry techniques. The target compounds (10-15) were additionally characterized
31
by P NMR, EIꢀHRMS and IRꢀspectroscopic studies (see experimental section).

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Molecular Geometry, Orbital Picture and Crystal Structure
Knowledge of the geometry of the individual molecules would be of help in
understanding the properties and also the effect of substitution. Additionally comparison
of the geometries obtained by Xꢀray analysis where possible (molecule in solid state
environment), and DFT methods (isolated molecules in gas phase) would also be
helpful in understanding the intermolecular interactions in the solid. In this study only
for 13 we were able to determine the molecular structure using Xꢀray analysis shown in
Figure 2 along with the DFT optimized structure. From the Figure 2 it can be seen that
the major changes in the geometry on changing the environment from solid to gas
phase are the dihedral angles between the rings (Table S1ꢀSupporting Information).
◦
The dihedral angle between the pyrene ring and the neighbouring aryl ring is about 10
◦
smaller in the gas phase but larger changes of as much as 70 are seen in the dihedral
angles of the phenyl groups in the diphenylphosphine oxide moiety. This indicates the
22
large role of the P=O moiety in the stabilization of molecules in solid state.
Another important point is to understand the conjugation between the pyrene ring
and the phosphine oxide group in the ground state. In case of 13 in spite of the smaller
◦
dihedral angle of 55 between the pyrene ring and the phenyl group one can observe
that due to mꢀsubstitution of the diphenyl phosphine oxide group the conjugation is
decreased. But for all the other molecules the dihedral angle between the pyrene ring
and its neighbouring group dictates the conjugation. For example in 14 the optimized
◦
angle is 75 which is large and attributed to the steric effect, we can expect a decreased
conjugation. On the other hand in 11, 12 and 15 the angles vary between smaller values
◦
◦
namely, 35 ꢀ55 indicating conjugation would be effective. Thus for all the molecules this

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dihedral angle seems to play an important role in enhancing or decreasing the charge
transfer.
The frontier molecular orbitals along with the individual contributions of the
groups are shown in Figure 3. In all the molecules, the HOMO is mostly localized on the
pyrene ring. While the LUMO of 10, 13 and 14 are mainly localized on the pyrene ring,
in the case of 11, 12 and 15 the LUMO is delocalized on the non pyrene part due to
conjugation. The dipole moments estimated from DFT studies indicate that the ground
state dipole moments are small ~ 3.4−4.0 Debye (Table 1) with very little variation with
substitution.
We also obtain other near degenerate conformations with different dihedral
angles between the pyrene ring and the neighbouring aryl ring when we change our
initial geometry in the DFT methods shown as isomer 2 for 13 (Figure S1ꢀ Supporting
Information). In fact one can envisage many near degenerate local minima (rotational
conformations) due to the low rotation barrier about of this angle in these molecules in
the gas and solvent phase in the ground states. In the excited state too one can expect
say in the case of intramolecular charge transfer a change in the geometry due to this
dihedral angle. But this rotation may be restricted in the solid state due to the strong
intermolecular forces.
To get a deeper understanding of the intermolecular interactions we study the
solid state nature of 13 (Figure 4). 13 crystallizes in the triclinic P 1‾ space group with a
single molecule in the asymmetric unit (Z' = 1). The molecule is predominantly
hydrophobic due to the polar phosphine oxide. This functional group is considered to be

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a good Hꢀbond acceptor and can result in strong hydrogen bonding usually with polar
amine or hydroxyl donor groups. However in 13, the phosphine oxide involves in
multiple CꢀHꢁꢁꢁO interactions because the molecule lacks strong hydrogen bond donors.
Three CꢀHꢁꢁꢁO interactions essentially propagate the molecular packing and stabilize the
crystal lattice ꢀ a C2ꢀH2ꢁꢁꢁO1 interaction with its inversion related molecule facilitates
dimer formation, followed by extending the dimer of molecules into a zizꢀzag tape via
C30ꢀH30ꢁꢁꢁO1 interaction, and finally by linking inversion related tapes by a third C15ꢀ
H15ꢁꢁꢁO1 interaction. The pyrene rings allow multiple π−π ring stacking interactions to be
established along the zigꢀzag tape and render additional stability to the crystal structure.
Significant nonꢀcovalent interactions in 13 along with their symmetry codes are
tabulated (Table S2ꢀ Supporting Information).
Electrochemical Properties
The redox behaviour of 10-15 was investigated by performing cyclic voltammetric (CV)
experiments with a standard threeꢀelectrode configuration electrochemical cell in dry
THF under nitrogen atmosphere, and ferrocene as the internal reference (Figure S2ꢀ
Supporting Information). The main aim is to estimate the ionization potential (IP) and the
electron affinity (EA) values of the molecules relative to the vacuum level. The IP of all
the compounds are estimated (after correcting for the ferrocene standard) from the
onset oxidation potential according to an empirical formula IP = − (̿ ** 3) . / + 4.8) eV and
EA are estimated from the onset reduction potentials according to an empirical formula
35
EA = − (̿-*
). /
ꢀ
+ 4.8) eV which are compiled in Table 1. Their potentials were
35
corrected to the saturated calomel electrode (SCE) by measuring the

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ferrocene/ferrocenium couple in this system (0.6 V versus SCE). The onset oxidation
potentials of 10-15 are estimated to be between 1.26−1.46 V corresponding to the IP
between 5.46−5.66 eV. The very little variation indicates that the oxidation occurs from
the same moiety in all the molecules, namely pyrene. The onset reduction potentials
are estimated to be between 1.6−1.85 V which corresponding to the EA of 2.35ꢀ2.60 eV.
[
9b]
For comparison, the EA of Alq , an electron transport material, is 3.0 eV.
The band
3
gaps obtained by the electrochemical methods are in the range 2.95ꢀ3.24 eV (Table 1).
IP and EA estimated by CV can be correlated to the HOMO and LUMO energy
36
values obtained from DFT calculations. Hence, to compare the DFT obtained frontier
orbital data is given in the same Table. While the HOMO values obtained from DFT are
in good agreement with the IP obtained from CV measurements, the LUMO values are
slightly underestimated by the DFT method. While it is well known that EA using DFT
methods are underestimated, additionally here this could be also due to different
conformation adapted by the molecule in the solvent (variation of LUMO is larger with
37
change in geometry in these molecules). Figure 5 depicts the typical energy level
diagram for a bilayer device with αꢀNPD as HTL and synthesized materials as nꢀtype
emitter. IP of compounds offers a minimal energy barrier for holes to be injected from αꢀ
NPD, while EA is relatively closer to the work function of cathode LiF/Al. Hence, the
compounds are suitable for hole and electron injection.

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Photophysical Properties
-5
UVꢀVis absorption studies have been carried out for 10-15 in chloroform (1.0 x 10 M)
as well as in thin film state to analyze the optical properties and to estimate their utility
for EL applications. The corresponding spectra are shown in Figure 6 and the relevant
data is summarized in Table 2. The absorption spectra of 10ꢀ15 are almost similar
having comparable high energy bands (~260–300 nm) and low energy bands (~325–
3
50 nm). There is very little variation of the λmax with substitution (346ꢀ353 nm)
indicating that the group causing this absorption in all the molecules is most probably
same. 10 and 14 exhibit fine structure (relative to 11, 12, 13 and 15) in their absorption
spectra. To understand the nature of the band solvatochromic studies in various nonꢀ
polar to polar solvents were carried out. Small or negligible solvent polarity effect was
observed in the UVꢀVis absorption spectra of these compounds (Table S3 and Figure
S3ꢀSupporting Information), indicating that there is no major change in the dipole
moment of these molecules upon excitation. In the case of the thin films, the absorption
spectra recorded are almost similar to the absorption spectra in solution as shown in the
same figure, except for a small red shift. This red shift (~10 nm) can be attributed to the
intermolecular interactions like π−π stacking or due to change in the dihedral angle in
the solid state.
22
TDꢀDFT calculations at pbe0/6ꢀ31g (d,p) level of theory have been carried out
to get deeper insight into the absorption spectra and to know the origin of electronic
transitions. The absorption maxima obtained by computational methods are in good
agreement with the experimental values (Table 2). First singlet excited state (S ) of all
1
the molecules is attributed to the HOMOꢀLUMO transition (~90%). From the orbitals

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∗
shown in Figure 3 this could be classified as π − π transition. Interestingly in the case of
1
0, 13 and 14 the orbital density is almost localized on the pyrene moiety in the HOMOꢀ
LUMO transition, while in the rest of the molecules the HOMOꢀLUMO transition
proceeds with a small charge transfer to the neighbouring aryl moiety. This can be
seen in the computed oscillator strengths of the transitions which are slightly smaller in
the case 10, 13 and 14.
Fluorescence spectra of 10ꢀ15 were recorded in chloroform solvent as well as in
the thin film. The spectra are shown in Figure 6 and the relevant data summarized in
Table 2. The emissions in chloroform are in the range of 380ꢀ420 nm which can be
attributed to substituted pyrene monomer. The fluorescence quantum yield (Φ ) of the
f
derivatives 10-15 were measured in CHCl using quinine sulphate (quantum yield:
3
23
0
.53) as standard (Table 2) and are in the range of 0.51ꢀ0.91. Optical band gaps
obtained from the intersection point of offset point of absorption and onset of emission
are given in the same Table. The band gaps lie in the region 3.10ꢀ3.38 eV
corresponding to blue emission. Interestingly the spectra of 11, 12 and 15 in chloroform
show a broad structureless peak having a small red shift compared to 10, 13 and 14.
The emission in the thin film state of all the molecules are red shifted to around 458ꢀ482
nm. This large red shift can occur due to aggregation in the solid state or due to change
in geometry of the molecules in the solid state.
Fluorescence spectra were also recorded in various polar and non polar solvents
to understand the nature of the excited state (Table 3 and Figure S4ꢀSupporting
Information). Fluorescence spectra recorded for 11, 12 and 15 in various solvents show
a red shift of almost 20ꢀ30 nm in going from hexane to a more polar solvent

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acetonitrile/methanol. This behaviour is absent in 10, 13 and 14 which exhibit almost no
red shift. The possible reason for this is that the former molecules are excited to a local
excited state (HOMOꢀLUMO) which then relaxes to an intramolecular charge transfer
state (ICT) with change in the geometry. On the other hand in 10, 13 and 14 the excited
state do not show any charge transfer with the charge being localized almost on the
pyrene moiety even after excitation.
Changes in the emission spectra with increasing concentration of the compound
were also studied in chloroform solvent (Figure S5ꢀSupporting Information). With
increasing concentration the intensity of the emission spectra of 13 at 390 nm
decreases, and a new peak at 430 nm is observed which increases in intensity with
increasing concentration. This effect is also seen in 10 and 14 where the red shifted
peaks are at 430 and 420 nm, respectively. The emission spectra of 10, 13 and 14 at
low energy wavelength in thin film are similar in high concentration solution spectra and
different in low concentration solution, which can be attributed to emission from pyrene
excimer. In fact the thin film emission has a broad emission, typical of excimer emission.
On the other hand in 11, 12 and 15 with increasing concentration in the solvent,
self quenching is observed. The intensity of the peak decreases with no formation of a
new red shifted peak. In the corresponding thin film spectra, we find that there are broad
peaks around 450ꢀ470 nm like in the case of 10, 13 and 14. Based on the red shift and
the previous data of 10, 13 and 14 we believe this emission to be also due to the
formation of excimers. The quenching effect in the solvent could be caused by change
in geometry (ICT) in the excimer on excitation (rotation of the pyrene ring) leading to
non radiative decay of the excited state. On the other hand, the strong intermolecular

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interactions restrict the rotation in the solid which leads to a radiative decay. Thus, we
attribute that the emissions 450ꢀ482 nm in the solid in all the molecules 10-15 arising
due to the excimer emission. Importantly, all the compounds exhibit their emission in
the blue region (380ꢀ480 nm) in solution as well as in thin film state.
To understand the mechanism of luminiscence and also to identify the species
responsible for emission, luminiscence lifetime measurements were carried out for
compound 13. Femto laser set up was used to determine the short lived fluorescence
ꢀ5
lifetimes for 13. In lower concentration (10 M) where the steady state data shows no
excimer formation, the short lifetime component (5ns) may be attributed to the lifetime of
the locally excited singlet state and the longer lifetime (12 ns) to charge separation
between pyrene and diphenyl phospheneoxide. Fluorolog3 spectrometer equipped with
flash lamp was used for determining delayed emission for 13. The fluorescence lifetime
ꢀ5
ꢀ2
decay of 13 at 10 and 10 M concentrations was measured at emission wavelength
90 and 460 nm (TableꢀS4, Figure S6ꢀsupporting information). Delayed emission of 13
was checked with 0.1 ms delay after flash, the spectra shows delayed emission with a
3
ꢀ5
ꢀ2
lifetime of 9 ꢂs at lower concentration (10 M), at higher concentration (10 M) 7 ꢂs and
7 ꢂs (Figure 7) involving excimer formation. The delayed emission with 9 ꢂs life time is
the delayed fluorescence of 13, since it has overlap with the fluorescence spectrum of
9
1
3. The origin of this delayed fluorescence is interesting that it arises from a possible
11ꢀ16
close lying triplet.
Both fluorescence and delayed fluorescence spectra are nearly
coincident hence the delayed component maybe attributed to thermally activated
38,39
delayed fluorescence.
The proposal of close lying triplet comes from the
40
observations recorded on phosphine oxide containing small molecules having triplet

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energy, E ~3.1 eV. The singlet formed upon absorption of light undergoes inter system
T
crossing to give triplet state, which undergoes reverse intersystem crossing (RISC) to
populate delayed singlet leading to delayed fluorescence (Figure 7) or it can interact
with another molecule (at higher concentrations) leading to excimer fluorescence. The
reverse intersystem crossing associated with TADF arises due to close lying singlet and
triplet states.
Thermal Properties
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)
measurements were carried for 10-15 to obtain thermal decomposition (T ) and glass
d
transition temperatures (T ), respectively (Figure S7 –Supporting Information). The
g
obtained T and T data are presented in Table 4. The compounds 10-15 exhibit good
d
g
o
thermal stability with T (corresponding to 5% weight loss) in the range of 341ꢀ444 C. In
d
a repeated DSC scans, relatively high T values were observed in second heating scan
g
o
and are in between 73 −125 C. Among the six compounds, high T values were noticed
g
o
o
o
for 12 (104 C), 14 (112 C) and 15 (125 C) than other compounds (10, 11 and 13),
which could be attributed to the increased molecular size and molecular weight.
Crystallization temperature as exothermic peak in DSC scans was not observed for all
the compounds indicating their amorphous nature, which is required for better
performance of OLEDs. Melting points exhibited in DSC scans as sharp endothermic
o
peaks were found to be in between 188 − 273 C.
Electroluminescent (EL) Properties

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EL studies were performed for 10ꢀ15 using them as electron transport emitters with the
following configuration (i): ITO (120 nm) F ꢀTCNQ (2.5 nm)/αꢀNPD (45 nm)/emissive
4
layer 10ꢀ15 (50 nm)/LiF (1 nm)/Al (150 nm). In this device, ITO (indium tin oxide) and
LiF/Al are the anode and the cathode, respectively; αꢀNPD (4,4’ꢀbis[Nꢀ(1ꢀnaphthyl)ꢀNꢀ
phenyl amino] biphenyl) is the hole transporting layer (HTL). The F ꢀTCNQ (2,3,5,6ꢀ
4
tetrafluoroꢀ7,7’,8,8’ꢀtetracyanoquinodimethane) is utilized for efficient hole injection from
41
ITO to αꢀNPD and its thickness is used as optimized by Priyanka Tyagi et al for
enhanced efficiency and life time of OLEDs. Then the EL spectrum, Current density –
Voltage – Luminescence (JꢀVꢀL) characteristics and CIE coꢀordinates are characterized
by EL devices. Figure 8 depicts the EL spectrum for all the molecules and their
emission was found to be in 465ꢀ491 nm wavelength range. Peak wavelength and CIE
coꢀordinates for each device are listed in Table 5. JꢀVꢀL characteristics of the bilayer
devices using these molecules as emitter are presented in Figure 9. The EL devices
constructed using 10ꢀ15 as emitters show good EL performance over a wide range of
current density. The current and power efficiency were calculated for each device and
are tabulated in Table 5. The onset voltage and maximum brightness data have also
been included in the same Table. All the devices were found to possess very low onset
voltage, 3.5 V. The key point to be noted about the device is that these devices are not
having any doped transport layer. Therefore, the onset voltage of 3.5 V indicates very
high electron mobilities for these molecules. This also indicates charge carrier balance
inside the emissive layer. Bilayer devices are found to possess very high brightness
2
around 30,000 cd/m . The efficiency of these devices are found to be very high with
current efficiency (η ) ~16.0 cd/A and power efficiency (η ) ~8.0 lm/W. The high
c
c

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efficiency data indicate that these molecules have very high electron mobilities and
emission efficiencies. Further, we have also measured the external quantum efficiencies
for these devices at 6.0 V. The quantum efficiencies are found to be in the range of 3.0–
4
.0% and these quantum efficiencies are very high for bilayer blue fluorescent OLEDs.
4c,d
If a 1:3 ratio of singlet to triplet excitons
and 20% out coupling efficiency for OLED
41b,c
structure is considered
, the internal quantum efficiency of these OLEDs comes in
the range of 60–80%, which is very close to the measured fluorescence efficiencies for
these molecules. Therefore, these bilayer devices have charge carrier balance factor
very close to unity, which indicates the electron mobility of these molecules is very close
41d
to the hole mobility in αꢀNPD.
Furthermore, to have a complete analysis of the EL properties of these
molecules, we also tested them as an emitter with a multilayer device structure as (ii):
ITO (120 nm)/F ꢀTCNQ (2.5 nm)/αꢀNPD (45 nm)/Emissive layer 10ꢀ15 (30 nm)/BCP (6
4
nm)/Alq (30 nm)/LiF (1 nm)/Al (150 nm), which has an additional BCP as hole blocking
3
41eꢀg
layer and Alq electron transport layer.
Figure 10 depicts the JꢀL characteristics for
3
these devices. The efficiency data are listed in the Table 6. Multilayer OLEDs show very
high efficiencies as close to 16 lm/W power efficiency, 45 Cd/A current efficiency and
9
.1% external quantum efficiency. From this data, it can be seen that the results of
these compounds as electron transporting emitters are very close to results as emitters.
The difference in the efficiency data is nearly two times for multilayer device in
comparison to bilayer as compared to the previous reports on emitting organic
9b,42
molecules it is nearly 5ꢀ10 times.
For reference, the OLED with Alq as emissive
3
layer with the same structure has power efficiency close to 2 lm/W and pyrene

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derivative in same configuration, the efficiency goes as high as to 10 lm/W and gives a
43
difference in efficiency by a factor of 5. This indicates that our molecules possess very
high electron mobility in comparison to Alq . Therefore, in configuration (i), the injected
3
electrons into the emitting molecules from cathode traversed very quickly to the
HTL/EML interface, where the recombination zone is located and high fluorescence
efficiencies of these molecules give rise to very high luminous efficiencies of the
devices. Hence, these molecules serve the purpose of both electron transport as well as
emission. The selected thickness was 50 nm which leads to a recombination zone
located near the HTL/EML interface to avoid cathode induced quenching, as obtained
44
by other researchers.
Recently reported efficiencies (external quantum) for
45a
synthesized blue emitters are 2.4% based on carbozoleꢀdimesitylborane by Lin et al,
45b
45c
5
.0% on triphenylamineꢀimidazole by Li et al, 3.1% and 2.0 % by Zhang et al and
45d
45e,f
Yu et al . The other reports also range from 3.0ꢀ6.0%
. The higher reported
efficiencies in our case clearly signify the improvement made in our synthesized
compounds.
We have measured the electron mobilities of these molecules using time of flight
46
ꢀ4
ꢀ4
2
method. The electron mobilities are found to be in the range of 1x10 – 5x10 cm /Vs
5
measured at an electric field of 1x10 V/cm (Table 5). To understand the reason for the
high values, we carry out the calculation of the transfer integrals, using computational
chemistry, between the molecules in the crystal of 13 and for comparison in an
47
unsubstituted pyrene crystal obtained from literature (Figure 11). The values of the
transfer integral for both electron and hole transport in the unsubstituted pyrene is large
but only in one direction, while from the figure it is clear that the charge transport in 13

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has more possible channels with slightly smaller transfer integrals. This increase in
channels we believe leads to larger electron mobilities. These results justify our
arguments for the high efficiencies as electron transport emitters. The measured
mobilities in our molecules were found to be an order of magnitude higher than the
41h, 46
standard electron transport material Alq3.
Also from the electron mobility data, the
low efficiencies in case of 14 can be explained. This molecule was found to have a
three times lower mobility in comparison to other materials and therefore, give rise to
low efficiencies due to a charge carrier balance factor less than unity. These EL studies
infer that the molecules synthesized in these studies possess excellent nꢀtype emitting
properties.
EXPERIMENTAL SECTION
Synthesis and Characterization:
Various mono bromo intermediates (5ꢀ9) were synthesized using SuzukiꢀMiyaura
coupling reaction and were further used to attain target phosphine oxide derivatives (10ꢀ
1
5).
Synthesis of 1-(4-bromophenyl)pyrene (5)
1
,4ꢀDibromobenzene (3.0 g, 12.8 mmol), compound 2 (3.8 g, 11.5 mmol), and
Pd(PPh ) catalyst (0.74 g, 0.6 mmol) were mixed in toluene (80 mL). Aqueous K CO
3
3
4
2
(5.2 g, 38.5 mmol) was slowly added to the reaction mixture and nitrogen gas was
purged for 30 min. The reaction mixture was heated to reflux for 12 h under nitrogen

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atmosphere (reaction progress was monitored by TLC). After completion of the reaction,
it was diluted with ethyl acetate (150 mL). Organic layer was washed with brine solution,
water, dried over anhydrous Na SO , filtered and the solvent was removed in vacuo.
2
4
The residue was purified by column chromatography using nꢀhexane as eluent to afford
o
1
compound 5 as colourless solid. Yield: 83% (3.4 g); mp 142ꢀ144 C; H NMR (300 MHz,
CDCl , δ): 8.18 (d, J = 7.9 Hz, 2H), 8.16ꢀ8.09 (m, 2H), 8.07 (s, 2H), 8.03ꢀ7.96 (m, 2H),
3
13
7
.90 (d, J = 7.9 Hz, 1H), 7.67 (d, J = 8.3 Hz, 2H), 7.47 (d, J = 8.3 Hz, 2H); C NMR (75
MHz, CDCl , δ): 144.87, 136.05, 133.09, 132.13, 131.99, 131.92, 131.26, 130.88,
3
1
1
30.71, 130.53, 128.58, 128.44, 128.21, 127.80, 127.68, 127.25, 127.18, 126.04,
+
25.26, 124.97, 124.77, 124.57; EIꢀMS: m/z 356 [M] .
Synthesis of 1-(4'-bromobiphenyl-4-yl)pyrene (6)
Compound 6 was synthesized according the procedure similar to that of compound 5,
using 4,4'ꢀdibromobiphenyl (3.0 g, 9.7 mmol) and compound 2 (2.8 g, 8.7 mmol) as
synthetic precursors, Pd(PPh ) (0.56 g, 0.5 mmol) as catalyst and K CO (3.9 g, 29.0
3
4
2
3
mmol) as base. The obtained residue was purified by column chromatography using
o
1
chloroform: hexane (1:20) as eluent. Yield 80% (3.0 g); mp 217ꢀ220 C; H NMR (300
MHz, CDCl , δ): 8.28ꢀ8.16 (m, 4H), 8.13ꢀ8.10 (m, 2H), 8.08ꢀ8.00 (m, 3H), 7.79ꢀ7.69 (m,
3
+
4
H), 7.66ꢀ7.57 (m, 4H); EIꢀMS: m/z 434 [M] .
Synthesis of 1-(3-bromophenyl)pyrene (7)
Compound 7 was synthesized according the procedure similar to that of compound 5,
using 1,3ꢀdibromobenzene (3.0 g, 12.8 mmol) and compound 2 (3.8 g, 11.5 mmol) as
synthetic precursors, Pd(PPh ) (0.74 g, 0.6 mmol) as catalyst and K CO (5.2 g, 38.5
3
4
2
3

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mmol) as base. The obtained residue was purified by column chromatography using
o
1
hexane as eluent. Yield 85% (3.5 g); mp 125ꢀ127 C; H NMR (300 MHz, CDCl , δ):
3
8
7
7
1
1
.21 (d, J = 7.8 Hz, 2H), 8.17 (d, J = 7.8 Hz, 1H), 8.13ꢀ8.07 (m, 3H), 8.06ꢀ8.00 (m, 2H),
.93 (d, J = 7.9 Hz, 1H), 7.79 (t, J = 1.5 Hz, 1H), 7.60 (d, J = 8.1 Hz, 1H), 7.55 (d, J =
13
.6 Hz, 1H), 7.42 (t, J = 7.8 Hz, 1H); C NMR (75 MHz, CDCl , δ): 143.26, 135.85,
3
33.33, 131.35, 130.85, 130.81, 130.23, 129.80, 129.21, 128.33, 127.80, 127.64,
27.38, 127.27, 126.05, 125.28, 124.99, 124.82, 124.72, 124.67, 124.57, 122.44; EIꢀ
+
MS: m/z 356 [M] .
Synthesis of 1-(4-bromo-2,5-dimethylphenyl)pyrene (8)
Compound 8 was synthesized according the procedure similar to that of compound 5,
using 1,4ꢀdibromoꢀ2,5ꢀdimethylbenzene (3.0 g, 11.4 mmol) and compound 2 (3.4 g,
1
0.3 mmol) as synthetic precursors, Pd(PPh ) (0.66 g, 0.6 mmol) as catalyst and
3 4
K CO (5.2 g, 38.5 mmol) as base. The obtained residue was purified by column
2
3
o
1
chromatography using hexane as eluent. Yield 73% (2.9 g); mp 119ꢀ121 C; H NMR
(
300 MHz, CDCl , δ): 8.17 (d, J = 7.9 Hz, 2H), 8.16ꢀ8.11 (m, 1H), 8.07 (s, 2H), 8.01ꢀ7.93
3
(m, 2H), 7.80 (d, J = 7.7 Hz, 1H), 7.68 (d, J = 9.1 Hz, 1H), 7.56 (s, 1H), 7.21 (s, 1H),
1
3
2
1
1
.43 (s, 3H), 1.96 (s, 3H); C NMR (75 MHz, CDCl , δ): 139.86, 136.36, 136.04,
3
34.88, 133.38, 132.87, 131.33, 130.89, 130.62, 128.77, 127.60, 127.38, 127.35,
27.09, 126.02, 125.17, 125.08, 124.99, 124.73, 124.66, 124.50, 123.91, 22.38, 19.47;
+
EIꢀMS: m/z 384 [M] .
Synthesis of 1-(7-bromo-9,9-dimethyl-9H-fluoren-2-yl)pyrene (9)

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Compound 9 was synthesized according the procedure similar to that of compound 5,
using compound 4 (3.0 g, 8.6 mmol) and compound 2 (2.5 g, 7.7 mmol) as synthetic
precursors, Pd(PPh ) (0.49 g, 0.4 mmol) as catalyst and K CO (3.5 g, 25.7 mmol) as
3
4
2
3
base. The obtained residue was purified by column chromatography using hexane as
o
1
eluent. Yield 85% (3.1 g); mp 186ꢀ188 C; H NMR (300 MHz, CDCl , δ): 8.25 (d, J = 2.0
3
Hz, 1H), 8.24 (d, J = 3.4 Hz, 1H), 8.21 (dd, J = 7.6, 1.1 Hz, 1H), 8.17 (d, J = 7.6 Hz, 1H),
8
7
.11 (d, J = 0.6 Hz, 2H), 8.06ꢀ8.01 (m, 3H), 7.86 (d, J = 7.8 Hz, 1H), 7.70ꢀ7.66 (m, 2H),
13
.64ꢀ7.61 (m, 2H), 7.52 (dd, J = 8.1, 1.8 Hz, 1H), 1.57 (s, 6H); C NMR (75 MHz,
CDCl , δ): 155.89, 153.43, 140.63, 137.90, 137.82, 137.20, 131.43, 130.89, 130.54,
3
1
1
30.19, 129.74, 128.46, 127.60, 127.51, 127.38, 126.19, 125.99, 125.16, 125.12,
+
24.88, 124.79, 124.63, 121.47, 121.13, 119.88, 47.23, 27.04; EIꢀMS: m/z 474 [M] .
Synthesis of diphenyl(pyren-1-yl)phosphine oxide (10)
To a suspension of 1 (3.0 g, 10.7 mmol) in dry THF (120 mL) was added nꢀBuLi (6.4
o
mL, 12.8 mmol, 2.0 M in cyclo hexane) drop wise at ꢀ78 C, under nitrogen atmosphere,
which turns to a greenish suspension. Stirring was continued for another 1 h, and then
chlorodiphenylphosphine (3.0 mL, 16.1 mmol) was added drop wise while maintaining
the same temperature, the reaction mixture turned to clear pale yellow solution. The
resulting mixture was gradually warmed to ambient temperature, stirred for 2 h. After
completion of the reaction (monitored by TLC), dry methanol (10 mL) was added. The
organic compound was extracted with dichloromethane (3 x 80 mL), and the combined
organic layer was dried over sodium sulphate, filtered and evaporated under reduced
pressure to get white solid. The residue obtained was dissolved in dichloromethane (50
mL) and H O (10 mL, 30%) was added dropwise. After stirring for 4 h at room
2
2

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temperature, water (60 mL) was added and the organic phase was extracted with
dichloromethane (3 x 40 mL), dried over anhydrous Na SO , filtered and the solvent
2
4
was removed in vacuo. The crude reaction mixture was purified by column
chromatography using hexane: ethyl acetate (1:1) as eluent to afford compound 10
which further purified by precipitating in chloroform and diethyl ether (1:3) mixture. Yield
o
1
6
8
4
1
1
3
4
8% (3.2 g); mp 243 C; H NMR (300 MHz, CDCl , δ): 8.93 (d, J = 9.1 Hz, 1H), 8.27ꢀ
3
.15 (m, 3H), 8.11ꢀ8.00 (m, 4H), 7.80ꢀ7.67 (m, 5H), 7.60ꢀ7.52 (m, 2H), 7.51ꢀ7.42 (m,
13
H); C NMR (75 MHz, CDCl , δ): 134.16, 133.89, 132.50, 132.22, 132.09, 131.85,
3
31.22, 131.06, 130.94, 130.34, 129.82, 128.87, 128.66, 128.50, 127.06, 126.44,
31
ꢀ1
26.28, 126.15, 123.59, 123.41; P NMR (500 MHz, CDCl , δ): 32.886; IR (KBr, cm ):
3
+
049, 1585, 1187, 847, 695; EIꢀMS: m/z 402 [M] ; EIꢀHRMS: m/z Calcd for C H OP:
28
19
+
ꢀ1
ꢀ1
02.11735 [M] ; found: 402.11720; UVꢀVis (CHCl , nm) (ε, M cm ): 271 (36177), 282
3
(57035), 325 (18305), 339 (37890), 353 (52457), 379 (8534).
Synthesis of diphenyl(4-(pyren-1-yl)phenyl)phosphine oxide (11)
Compound 11 was prepared according the procedure similar to that of 10, using 5 (2.5
g, 7.0 mmol), nꢀBuLi (4.2 mL, 8.4 mmol, 2.0 M in cyclohexane) and
chlorodiphenylphosphine (1.9 mL, 10.5 mmol) as synthetic precursors. The obtained
residue was purified by column chromatography (hexane/ethyl acetate 1:1) followed by
o
1
precipitation in chloroform: diethyl ether (1:3). Yield 72% (2.4 g); mp 208 C; H NMR
(
300 MHz, CDCl , δ): 8.19 (t, J = 7.8 Hz, 2H), 8.15 (d, J = 7.5 Hz, 1H), 8.11 (d, J = 9.2
3
Hz, 1H), 8.07 (d, J = 1.4 Hz, 2H), 8.03ꢀ7.98 (m, 2H), 7.93 (d, J = 7.8 Hz, 1H), 7.88ꢀ7.78
13
(
m, 6H), 7.72 (dd, J = 8.1, 2.4 Hz, 2H), 7.60ꢀ7.50 (m, 6H); C NMR (75 MHz, CDCl , δ):
3

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1
1
1
44.98, 144.95, 136.13, 133.20, 132.17, 132.04, 131.98, 131.82, 131.34, 130.94,
30.78, 130.73, 130.57, 128.62, 128.46, 128.28, 127.83, 127.72, 127.28, 127.24,
31
26.07, 125.32, 125.00, 124.85, 124.70, 124.62; P NMR (500 MHz, CDCl , δ): 29.141;
3
ꢀ1
+
IR (KBr, cm ): 3041, 1589, 1197, 831, 692; EIꢀMS: m/z 478 [M] ; EIꢀHRMS: m/z Calcd
+
ꢀ1
ꢀ1
for C H OP: 478.14865 [M] ; found: 478.14809; UVꢀVis (CHCl , nm) (ε, M cm ): 270
34
23
3
(33937), 281 (49877), 330 (28075), 346 (44332).
Synthesis of diphenyl(4'-(pyren-1-yl)-[1,1'-biphenyl]-4-yl)phosphine oxide (12)
Compound 12 was prepared according the procedure similar to that of 10, using 6 (2.5
g, 5.8 mmol), nꢀBuLi (3.5 mL, 6.9 mmol, 2.0 M in cyclohexane) and
chlorodiphenylphosphine (1.6 mL, 8.7 mmol) as synthetic precursors. The obtained
residue was purified by column chromatography (hexane/ethyl acetate 1:1) followed by
o
1
precipitation in chloroform: diethyl ether (1:3). Yield 75% (2.4 g); mp 244 C; H NMR
(
300 MHz, CDCl , δ): 8.24ꢀ8.15 (m, 4H), 8.09 (s, 2H), 8.05ꢀ7.98 (m, 3H), 7.81ꢀ7.70 (m,
3
1
3
1
1
1
1
1
5
2H), 7.60ꢀ7.47 (m, 6H); C NMR (75 MHz, CDCl , δ): 144.24, 141.11, 138.64, 136.85,
3
33.21, 132.74, 132.60, 132.15, 132.01, 131.41, 131.15, 130.88, 130.69, 130.50,
28.61, 128.45, 127.59, 127.51, 127.47, 127.34, 127.19, 127.03, 126.03, 125.18,
3
1
ꢀ1
25.03, 124.87, 124.66; P NMR (500 MHz, CDCl , δ): 28.996; IR (KBr, cm ): 3047,
3
+
594, 1180, 849, 696; EIꢀMS: m/z 554 [M] ; EIꢀHRMS: m/z Calcd for C H OP:
40
27
+
ꢀ1
ꢀ1
54.17995 [M] ; found: 554.17986; UVꢀVis (CHCl , nm) (ε, M cm ): 269 (45661), 281
3
(66729), 330 (32635), 347 (53724).
Synthesis of diphenyl(3-(pyren-1-yl)phenyl)phosphine oxide (13)

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Compound 13 was prepared according the procedure similar to that of 10, using 7 (2.5
g, 7.0 mmol), nꢀBuLi (4.2 mL, 8.4 mmol, 2.0 M in cyclohexane) and
chlorodiphenylphosphine (1.9 mL, 10.5 mmol) as synthetic precursors. The obtained
residue was purified by column chromatography (hexane/ethylacetate 1:1) followed by
o
1
precipitation in chloroform: diethyl ether (1:3). Yield 66% (2.2 g); mp 196 C; H NMR
(
300 MHz, CDCl , δ): 8.17 (d, J = 7.8 Hz, 2H), 8.14 (d, J = 7.8 Hz, 1H), 8.05 (d, J = 2.9
3
Hz, 2H), 8.03ꢀ7.93 (m, 3H), 7.91 (d, J = 7.8 Hz, 2H), 7.86ꢀ7.75 (m, 6H), 7.66 (td, J = 7.6,
1
3
2
1
1
1
1
8
.9 Hz, 1H), 7.56ꢀ7.45 (m, 6H); C NMR (75 MHz, CDCl , δ): 141.45, 141.36, 136.08,
3
34.07, 134.05, 133.92, 133.84, 133.37, 132.80, 132.55, 132.09, 132.01, 131.97,
31.95, 131.31, 130.93, 130.85, 130.80, 130.73, 128.61, 128.56, 128.52, 128.46,
28.25, 127.73, 127.62, 127.47, 127.23, 126.02, 125.25, 124.91, 124.79, 124.67,
31
ꢀ1
24.59, 124.51; P NMR (500 MHz, CDCl , δ): 29.01; IR (KBr, cm ): 3039, 1582, 1187,
3
+
+
51, 697; EIꢀMS: m/z 478 [M] ; EIꢀHRMS; m/z Calcd for C H OP: 478.14865 [M] ;
34
23
ꢀ1
ꢀ1
found: 478.14852; UVꢀVis (CHCl , nm) (ε, M cm ): 272 (38723), 281 (57657), 330
3
(33286), 348 (45803).
Synthesis of (2,5-dimethyl-4-(pyren-1-yl)phenyl)diphenylphosphine oxide (14)
Compound 14 was prepared according the procedure similar to that of 10, using 8 (2.5
g, 6.5 mmol), nꢀBuLi (3.9 mL, 7.8 mmol, 2.0 M in cyclohexane) and
chlorodiphenylphosphine (1.8 mL, 9.8 mmol) as synthetic precursors. The obtained
residue was purified by column chromatography (hexane/ethyl acetate 1:1) and was
further purified by precipitating in chloroform: diethyl ether (1:3). Yield 58% (1.9 g); mp
o
1
1
88 C; H NMR (300 MHz, CDCl , δ): 8.21 (t, J = 7.9 Hz, 2H), 8.17 (d, J = 7.2 Hz, 1H),
3

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8
9
.10 (s, 2H), 8.03ꢀ7.99 (m, 2H), 7.84 (d, J = 7.9 Hz, 1H), 7.83ꢀ7.77 (m, 4H), 7.67 (d, J =
.1 Hz, 1H), 7.61ꢀ7.50 (m, 6H), 7.31 (d, J = 4.3 Hz, 1H), 7.09 (d, J = 14.3 Hz, 1H), 2.47
13
(
s, 3H), 1.90 (s, 3H); C NMR (75 MHz, CDCl , δ): 144.58, 140.10, 139.99, 135.91,
3
1
1
1
1
35.07, 134.92, 134.28, 134.14, 133.96, 133.79, 133.56, 132.19, 132.04, 131.93,
31.78, 131.26, 130.82, 130.68, 128.64, 128.47, 127.68, 127.45, 127.27, 126.69,
ꢀ1
26.02, 125.21, 125.03, 124.92, 124.45, 21.23, 19.75; IR (KBr, cm ): 3048, 2917,1598,
+
186, 848, 717; EIꢀMS: m/z 506 [M ]; EIꢀHRMS: m/z Calcd for C H OP: 506.17995
36
27
+
ꢀ1
ꢀ1
[
M] ; found: 506.17775; UVꢀVis (CHCl , nm) (ε, M cm ): 269 (42209), 279 (67110), 315
3
(18706), 330 (41780), 345 (59413).
Synthesis of (9,9-dimethyl-7-(pyren-1-yl)-9H-fluoren-2-yl)diphenylphosphine oxide
15)
(
Compound 15 was prepared according the procedure similar to that of 10, using 9 (2.5
g, 5.3 mmol), nꢀBuLi (3.2 mL, 6.3 mmol, 2.0 M in cyclohexane) and
chlorodiphenylphosphine (1.5 mL, 7.9 mmol) as synthetic precursors. The obtained
residue was purified by column chromatography (hexane/ethyl acetate 1:1) followed by
o
1
precipitation in chloroform: diethyl ether (1:3). Yield 79% (2.5 g); mp 273 C; H NMR
(
300 MHz, CDCl , δ): 8.22 (d, J = 9.1 Hz, 2H), 8.18 (d, J = 7.6 Hz, 1H), 8.15 (d, J = 7.6
3
Hz, 1H), 8.08 (s, 2H), 8.05ꢀ7.97 (m, 4H), 7.90 (d, J = 7.6 Hz, 1H), 7.82 (dd, J = 7.8, 2.3
13
Hz, 1H), 7.78ꢀ7.70 (m, 5H), 7.62 (d, J = 7.8 Hz, 1H), 7.58ꢀ7.46 (m, 7H), 1.56 (s, 6H); C
NMR (75 MHz, CDCl , δ): 154.52, 154.10, 153.94, 142.69, 141.48, 137.59, 136.87,
3
1
1
33.51, 132.10, 131.97, 131.84, 131.35, 131.27, 131.19, 130.81, 130.54, 129.88,
29.74, 128.50, 128.35, 127.52, 127.39, 127.30, 126.46, 126.34, 125.95, 125.09,

Page 27 of 58
Journal of Materials Chemistry C
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DOI: 10.1039/C4TC01753F
31
1
25.01, 124.97, 124.77, 124.58, 120.54, 119.86, 119.68, 47.28, 26.90; P NMR (500
ꢀ1
MHz, CDCl , δ): 29.932; IR (KBr, cm ): 3038, 2961, 1598, 1434, 1192, 844, 696; EIꢀMS:
3
+
+
m/z 594 [M] ; EIꢀHRMS: m/z Calcd for C H OP: 594.21125 [M] ; found: 594.21218;
43
31
ꢀ1
ꢀ1
UVꢀVis (CHCl , nm) (ε, M cm ): 268 (27423), 280 (38296), 312 (29979), 351 (45056).
3
Measurements
1
All chemicals are reagent/analytical grade and used without further purification. H
13
NMR and C NMR were recorded on a Bruker Avance (75 MHz) spectrometer in CDCl
3
31
with TMS as the internal standard. P NMR spectra were recorded on Avance IIIꢀ500
MHz spectrometer in CDCl with reference to phosphoric acid. Mass spectra obtained
3
by using electron ionization (EI) ion trap mass spectrometry (Thermofinnigan, Sanzox,
CA, USA) were recorded on a VGꢀAUTOSPEC spectrometer. Elemental analyses were
performed using a VarioꢀEL elemental analyzer. UVꢀVis absorption spectra were
measured using a Jasco Vꢀ550 spectrophotometer. Steady state fluorescence spectra
were recorded using a Spex Fluorologꢀ3 spectrofluorometer. The fluorescence quantum
yields (Φ ) were estimated by integrating the fluorescence bands and by using quinine
f
17
sulphate as the standard. The glassꢀtransition temperature (T ) of the compounds
g
were measured using differential scanning calorimetry (DSC) under a nitrogen
ꢀ1
atmosphere and at a heating rate of 10 °C min using a DSC Q200 (TA instruments).
The T was determined from the second heating scan. Thermogravimetric analysis
g
(TGA) was performed using a TGA/SDTA 851e (Mettler Toledo) in the temperature
ꢀ1
range of 30–550 °C under a nitrogen atmosphere at a heating rate of 10 °C min . Cyclic
voltammetric measurements were performed on a PCꢀcontrolled CHI 620C
electrochemical analyzer (CH instruments). Cyclic voltammetric experiments were

Journal of Materials Chemistry C
Page 28 of 58
View Article Online
DOI: 10.1039/C4TC01753F
ꢀ1
performed in 1 mM solution of degassed dry THF at a scan rate of 100 mV s using 0.1
M tetrabutylammoniumperchlorate (TBAP) as the supporting electrolyte. The glassy
carbon was used as the working electrode, Ag/AgCl as the reference electrode and
platinum wire as the counter electrode. The working electrode surface was first polished
with 1 mm alumina slurry, followed by 0.3 mm alumina slurry on a micro cloth. It was
then rinsed with Millipore water and also sonicated in water for 5 min. The polishing and
sonication steps were repeated twice.
Fluorescence lifetime measurements
Steady state and time resolved phosphorescence measurements were carried out in
Fluorologꢀ3 spectrofluorometer using flash lamp with 0.1 ms delay after flash at RT.
Time correlated single photon counting was used to measure the fluorescence lifetime
in the nanosecond time scale. The femtosecond pulses at 8 MHz repetition rate were
obtained from fractional part of MaiTai output passing through femtosecond Pulse
Selector (3980ꢀ5S, Spectra Physics). The excitation pulses at 370 nm were generated
by frequency doubling the 740nm Tiꢀsapphire output with 0.5 mm BBO crystal. This
excitation pulses are focused to the sample using Fluorescence Upꢀconversion set up
(FOG100 system CDP System). The time distribution data of fluorescence intensity
were recorded on a SPCꢀ130 TCSPC module (Becker & Hickl).The instrument response
function was 250 ps.
Crystal Structure Determination

Page 29 of 58
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC01753F
The pyrene derivative 13 was crystallized from CH OH–CHCl (1: 4) and the single
3
3
crystal structure was determined. It crystallizes into the monoclinic P21/c space group.
All H atoms were positioned geometrically and treated as riding on their parent C atoms,
with CꢀH distances of 0.93ꢀ0.97 Å, and with U (H) = 1.2U (C) or 1.5U for methyl
iso
eq
eq
atoms. Crystal data for 13: C H OP, M = 478.49, colourless needle, 0.42 x 0.12 x 0.10
34
23
3
mm , triclinic, space group Pꢀ1 (No. 2), a = 9.0088(9), b = 9.6445(9), c = 14.0042(13) Å,
3
α
= 96.309(2),
β
= 98.1750(10), γ = 92.359(2)°, V = 1195.1(2) Å , Z = 2, D = 1.330
c
3
g/cm , F
= 500, CCD area detector, MoKα radiation, λ = 0.71073 Å, T = 293(2)K,
000
2
1
θmax = 50.0º, 11563 reflections collected, 4195 unique (R = 0.0174). Final GooF =
int
.062, R1 = 0.0350, wR2 = 0.1011, R indices based on 3860 reflections with I >2σ(I)
2
ꢀ1
(
refinement on F ), 325 parameters, ꢀ = 0.142 mm . CCDC 979234 contains the
18
supplementary crystallographic data for this paper.
Computational Methodology
To analyse the absorption spectra and molecular orbitals, quantum chemical
calculations have been carried out. Optimized geometries of the molecules were
25
obtained using density functional theory (DFT), at B3LYP/6ꢀ311g(d,p) level of theory
26
as implemented in the Gaussian09 software package. Frequency calculations were
carried out to ensure that the minima obtained is a stationary point on the potential
energy surface. Molecular orbital surfaces are visualized using Gaussview. Solvent
27
effects are simulated using polarizable continuum model (PCM). Lowest singlet
excitation energies are calculated using time dependent density functional theory