Efficient thermally activated delayed fluorescence of functional phenylpyridinato boron complexes and high performance organic light-emitting diodes

Article abstract of DOI:10.1039/c6tc04994j

A new series of functional phenylpyridinato boron complexes possessing thermally activated delayed fluorescence (TADF) has been strategically designed and synthesized. These boron complexes utilize phenylpyridine as the electron acceptor (A) that links to carbazole or triphenyl amine as the electron donor (D) via a core boron atom, forming four-coordinate neutral boron complexes. The selection of boron to spatially separate the donor and acceptor takes advantage of facile functionalization. The TADF properties of the resulting D-A functional materials in various solvents have been investigated via their emission spectra and associated relaxation dynamics. The results show that the operation of TADF is strongly solvent polarity dependent in fluid states, and several underlying mechanisms are discussed. OLEDs fabricated by fppyBTPA and dfppyBTPA show EL efficiencies of up to (20.2%, 63.9 cd A^?1, 66.9 lm W^?1) and (26.6%, 88.2 cd A^?1, 81.5 lm W^?1), respectively, in which a nearly 27% EQE for the dfppyBTPA device is among the most efficient TADF OLEDs so far.

Full text of DOI:10.1039/c6tc04994j

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Efficient Thermally Activated Delayed Fluorescence of Functional PhenylpyDrOidI:i1n0a.10t3o9/C6TC04994J
Boron Complexes and High Performance Organic Light-Emitting Diodes
Yi-Jiun Shiu,a,‡ Yi-Ting Chen,b,‡ Wei-Kai Lee,a Chung-Chih Wu,a,* Tzu-Chieh Lin,b Shih-
Hung Liu,b Pi-Tai Chou,b,* Chin-Wei Lu,c I-Chen Cheng,c Yi-Jyun Lien,c and Yun Chi,c,*
a
Graduate Institute of Electronics Engineering and Department of Electrical
Engineering, National Taiwan University, Taipei 10617, Taiwan; *E-mail:
wucc@ntu.edu.tw
b Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan; *E-mail:
chop@ntu.edu.tw
c Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan; *E-
mail: ychi@mx.nthu.edu.tw
‡ Authors with equal contribution.
Abstract
A new series of functional phenylpyridinato boron complexes possessing thermally
activated delay fluorescence (TADF) have been strategically designed and synthesized.
These boron complexes utilize phenylpyridine as the electron acceptor (A) that links to
carbazole or triphenyl amine as the electron donor (D) via a core boron atom, forming
four-coordinate neutral boron complexes. The selection of boron to spatially separate
donor and acceptor takes its advantage of facile functionalization. TADF properties of
the resulting D-A functional materials in various solvents have been investigated via
their emission spectra and associated relaxation dynamics. The results show that the
operation of TADF is strongly solvent polarity dependent in fluid states, and several
underlying mechanisms are discussed. OLEDs fabricated by fppyBTPA and dfppyBTPA
shows EL efficiencies of up to (20.2%, 63.9 cdA-1, 66.9 lmW-1) and (26.6%, 88.2 cdA-
1, 81.5 lmW-1), respectively, in which a nearly 27% EQE for the dfppyBTPA device is
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among the most efficient TADF OLEDs so far.
Introduction
Efficient light emitting materials are an indispensable component in the
fabrication of organic light emitting diodes (OLEDs) for flat-panel display and solid state
lighting applications. It is notable that the heavy transition-metal of phosphorescent
emitters are capable to induce fast intersystem crossing (ISC) between the singlet (S1)
and triplet (T1) excited states, giving electroluminescence with internal quantum
efficiencies (ηint) of nearly 100%.1-4 Alternatively, there is a relatively new class of
compounds, namely: thermally activated delayed fluorescence (TADF) emitters, which
can also achieve unitary ηint without employment of the third-row transition-metal
atoms.5-9 In general, TADF emitter can be enabled by reducing the spatial overlap
between the HOMO and LUMO orbitals of molecules, for which the reduced electron
exchange energy afforded the smaller energy gap between triplet (typically T1) and
singlet (S1) states, defined as ET-S (T1-S1). The smaller ET-S facilitates the fast
reverse intersystem crossing (RISC) from the non-emissive T1 to the emissive S1 excited
state upon thermal activation, rendering effective harvest of triplet excitons to the
singlet state for the generation of bright electroluminescence (fluorescence).
Bipolar molecules with suitable electron donor (D) and acceptor (A) entities are
known to foster efficient TADF by virtue of the reduced orbital overlap between these
individual constituents. Bearing this criterion in mind, many TADF molecules have been
successfully constructed by combining D and A units in a way that the mutual
orthogonality among them restricts their π-conjugation and, hence affords the smaller
ET-S. Typical donors comprise functional arylamine,10-12 carbazole,13
phenoxazine,14-16 acridine17-19 and etc., while conventional acceptors contain
carbonyl,20 phosphine oxide,21-24 sulfoxide or sulfone-containing units,25-29 triazine,30-
35
pyrimidine36, 37 and other heteroaromatic fragments for achieving the demanded
bipolar and TADF properties. More recently, efficient TADF materials bearing a wide
range of boron-containing acceptor such as phenoxaborin,38 phenazaborin,39 and
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polycyclic organoboron compounds with either 1,4-oxaborine40 or azaborine
subunit,41 were reported. Of particularly interesting are the second class of
organoboron compounds, for which the strong emissions with smaller Stokes shifts
and narrow FWHMs were attributed to their extremely rigid π-conjugated
framework.40, 41
In this work, we report a new series of TADF boron complexes, including fppyBTPA
and dfppyBTPA in which the electron-donating triphenylamine and the electron-
accepting functional phenylpyridinato fragment are linked at the spiro-bridged, four-
coordinate boron atom.42 The (phenylpyridinato-N,C2’)boron unit (ppyB) and its
analogues are known to be the excellent building blocks in assembling functional
luminescent materials.43-46 This tricyclic ppyB entity is isostructural to the PrFB
fragment, for which the hexagonal arranged phenyl cyclometalate is akin to that of the
pentagonal arranged pyrrolide unit, PrFH = 2-(3,5-bis(trifluoromethyl)-1H-pyrrol-2-
yl)pyridine.47 Moreover, the same ppyB unit is also related to the fluorene fragment,
but the inherent difference in electronegativity between boron and nitrogen atom
makes the ppyB fragment to possess greater degree of charge separation and suitable
than fluorene to serve as a core atom in construction of TADF molecules. Intrinsically,
these ppyB-containing compounds are different from that of the three-coordinate,
boron compounds such as PXZ-Mes3B and TB-3PXZ,48, 49 in which the three-coordinate
boron fragment served as the electron accepting unit to foster the charge transfer. On
the other hand, the choice of the coordinated phenylpyridinato boron complexes turns
out to be highly critical, as it can substitute the lower energy empty p-orbital of boron
fragment with the π*-orbital of similar energy on the pyridyl unit, in serving as the
electron acceptor. Equally important is the employment of more stabilized donor
orbitals, such as 4-(9H-carbazol-9-yl)phenyl (PCz) and 9-phenyl-9H-carbazol-3-yl (CzP)
fragments, for which the enlarged ET-S afforded the inefficient TADF. This has been
shown in the relevant derivatives, e.g. fppyBCzP, fppyBPCz, dfppyBCzP and dfppyBPCz,
for which results and discussion are also elaborated on the basis of both experimental
and computational approaches. Moreover, our observation is in sharp contrast to the
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closely related boron complex ppyBTPA,50 for which severe device degradation
occurred under standard OLED operating conditions and afforded extremely low
efficiency.
Results and discussion
Synthesis and characterization. As for the preparation of desired D-A complexes,
the intermediates, fppyBBr2 and dfppyBr2 were obtained from 2-(4-
fluorophenyl)pyridine (or 2-(2,4-difluorophenyl)pyridine) and BBr3 through an
electrophilic aromatic borylation catalyzed by ethyldiisopropylamine.51 The dibromide
intermediates are stable and can be handled in air without decomposition. After then,
treatment with the corresponding in-situ generated Grignard reagents afforded the
desired functional boron compounds in 47-35% yields. Their synthetic protocol is
depicted below:
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These boracyclic compounds were identified by mass spectrometry and NMR
spectroscopies, together with the X-ray structural analysis of fppyBTPA (cf. electronic
supporting information). The B-N and B-C distances of 1.624(2) and 1.610-1.623(2) Å
are comparable to the relevant bond distances observed in boracycles.52-54 All samples
exhibited adequate stability and solubility in common solvents, such as toluene,
CH2Cl2 and acetonitrile.
Solution photophysics. The UV-Vis absorption and emission spectra of fppy and
dfppy based boron complexes in different solvents at RT are shown in Figure 1 and
Figure 2, while pertinent photophysical data are listed in Table 1. As can be seen, the
lower energy absorption peaks for these complexes at 350 nm have a large molar
extinction coefficient in the order of 1.5  104 M-1cm-1 (in toluene). Thus, this
characteristic absorption can reasonably be assigned to a ligand-centered π-π*
transition, of which the spectral features in terms of peak wavelength and profile are
nearly independent of solvent polarity.
In stark contrast, all these complexes exhibit featureless emission in the visible range,
of which the peak wavelength is strongly solvent dependent, being red shifted upon
increasing the solvent polarity. For example, the emission maximum in fppyBPCz shifts
from 485 nm (in toluene), 550 nm (in CH2Cl2) to 620 nm (in CH3CN), which can be
ascribed to the photo-induced inter-ligand charge transfer (CT, vide infra), resulting in
a large change of dipole moment in the electronically excited state versus that of the
ground state, and affording a notable red-shifted emission upon increasing solvent
polarity, which is conventionally dubbed as the solvatochromism.55, 56 Similar behavior
was observed for fppyBCzP and fppyBTPA. It is worth of noting that a non-negligible
band at approx. 390 ‒ 410 nm for fppyBPCz and fppyBCzP, and became obvious for
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fppyBTPA in CH3CN due to its very weak CT emission. This short-wavelength emission
is attributed to the localized excitation (LE) emission to distinguish it from the CT
emission. This phenomenon is reminiscent of a number of compounds that exhibit
nonadiabatic or twisted intramolecular charge transfer.57, 58 Similar dual emission has
been observed recently by Adachi and co-workers on the dihydrophenazine-based
emitters exhibiting TADF.59
We also performed the time-dependent density functional (TD–DFT) calculations
to gain insight into the basic photophysical data for all titled boron complexes. As a
result, the lowest lying singlet S0 → S1 optical transition is ascribed to HOMO → LUMO
processes (see Tables S1–S6 in the Electronic Supporting Information (ESI)). Their
frontier molecular orbitals are shown in Figure 3 (in toluene) and Figure S1–S6 of ESI.
In Figure 3, the electron density distribution of LUMO is localized at fppy (or dfppy)
moiety of all complexes, while the electron density distribution of HOMO is localized
at TPA, PCz or CzP fragments of the respective compounds. It is worth of noting that
the electron density distribution of boron atom is negligible in both HOMO and LUMO
for these boron complexes, being within 0.18% to 0.59%. Therefore, the S0 → S1
transition displays a large inter-ligand charge transfer (ILCT) character, for which the
transition moment in oscillator strength is calculated to be near zero. Therefore, the
ILCT can be treated as a nonadiabatic type of electron transfer in the excited state,
giving rise to negligible ILCT absorption but prominent charge transfer emission.
We then performed the transient photoluminescence decay dynamics of these
compounds in various solutions. Figure 4 displays the PL decay profiles for fppyBTPA
and dfppyBTPA in the degassed toluene and their corresponding lifetime data are
listed in Table 1. Apparently, the emission decay for both TPA derivatives exhibited
double exponential decay kinetics, consisting of a prompt and a long decay
components, for which the time constants fitted to be 77 ns and 2.5 µs for fppyBTPA
and 75 ns and 2.7 µs for dfppyBTPA, respectively. Under the assumption of pre-
equilibrium between S1 and T1 states, the difference in energy ET-S can be derived
according to the relationship between ET-S and S1-T1 equilibrium constant Keq, which
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is expressed as ET-S = ‒RTln(Keq/3) where Keq can be deduced from the pre-
exponential factors for prompt fluorescence versus delay fluorescence components. A
factor of 3 stands for the triplet degenerate states.47 ET-S was then deduced to be ‒
1.65 and ‒1.52 kcal/mol for fppyBTPA and dfppyBTPA in toluene, respectively. As
shown in Table 1, dfppyBCzP exhibits similar double exponential decay kinetics in
toluene (31 ns, 2.6 µs), for which ET-S is calculated to be ‒2.36 kcal/mol. Interestingly,
the rest of boron complexes fppyBCzP, fppyBPCz and dfppyBCzP show solely single
exponential decay kinetics in toluene (see Table S7). For a tentative rationalization,
increasing (decreasing) the HOMO (LUMO) energy level for donor (acceptor) should
stabilize the charge transfer state, such that ET-S is expected to be smaller due to the
greater charge separation. As elaborated in the cyclic voltammetry measurement, the
oxidation potential was measured to be in the order of BTPA < BCzP < BPCz and the
reduction potential shows the order fppy > dfppy. The combination of these two
factors gives the monotonous order of the free energy for the charge transfer:
fppyBPCz > dfppyBPCz > fppyBCzP > dfppyBCzP > fppyBTPA > dfppyBTPA
As a result, charge transfer for dfppyBTPA, fppyBTPA and dfppyBCzP is more exergonic
than that for fppyBPCz, dfppyBPCz and fppyBCzP. Due to the reduced charge transfer
character, ET-S for the last three boron compounds may be too large to be
thermally accessed in toluene, resulting in insufficient TADF character.
In the more polar solvents such as CH2Cl2 and CH3CN, as shown in Table S7, even
dfppyBTPA and fppyBTPA did not show typical TADF decay behavior, i.e., the double
exponential decay kinetics. Instead, the emission decay could be well fitted by a single
exponential decay component and the resulting data are listed in Table S7. For a
tentative rationalization, recent experimental evidence in the literature has supported
the involvement of triplet local excited states (LE) from the donor or acceptor units,
other than the triplet charge-transfer (CT) in the reverse intersystem crossing (RISC).
Therefore, RISC may not occur between 1CT and 3CT but between 1CT and 3LE.60, 61 This
is attributed to the fact that the energy of the CT states is stabilized in the more polar
3
environments such that they now lie below the LE that is unaffected by solvent
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polarity. This provides an extra decay pathway, i.e., internal conversion, from 3LE to3CT.
As a result, the 3CT → 1CT RISC rather inefficient due to the large energy gap in polar
solvent, making 3CT an effective triplet reservoir for dfppyBTPA and fppyBTPA.
Photophysics in thin solid film. Photophysical measurements of these boron
complexes doped (with a doping concentration of 8 wt.%) in a large-triplet-energy
host mCPCN [9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile] or DPEPO
[bis[2-(diphenylphosphino)phenyl]ether oxide],62-66 have also been performed.
Pertinent photophysical data are listed in Table 2. Figure 5a depicts all
photoluminescence spectra (measured at RT) doped at 8 wt.% in the mCPCN host. The
featureless PL spectra in the mCPCN host are similar to their emission spectra in the
toluene solution, except for a 20 nm blue shift in the doped film for all samples due
perhaps to the restricted relaxation of the surrounding media and hence less
stabilization in the solid film.67, 68 Also, similar to that in solutions, the emission
wavelength is mainly determined by the type of the donor units (TPA, CzP, PCz)
connected to the ppyB acceptor, i.e., with PL peak wavelengths being descending in
the order of fppyBTPA/dfppyBTPA > fppyBCzP/dfppyBCzP > fppyBPCz/dfppyBPCz. It
is generally consistent with the strength of electron-donating capability of different
donor units:
TPA > CzP > PCz
and the strength of intramolecular charge-transfer characteristics, as supported by
the electrochemical results (cf. ESI). Meanwhile the number of fluoro atom
substitution on the ppyB acceptor (dfppy vs. fppy) has minor effects on emission
wavelengths.
Blue
emitting
compounds
fppyBPCz/dfppyBPCz
and
fppyBCzP/dfppyBCzP doped into DPEPO host with an exceedingly larger singlet/triplet
energies exhibit emission spectra/wavelengths similar to those in the mCPCN host
(see Table 2 and ESI). As listed in Table 2, 8 wt.% of fppyBTPA and dfppyBTPA in
mCPCN host show high PL quantum yields (PLQYs, Φ) of 72% and 95%, respectively,
while all other compounds (bearing PCz and CzP donors) exhibit much lower PLQYs of
only 6-19% in either mCPCN or DPEPO host.
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From the difference in the onset wavelengths of both fluorescence and
phosphorescence of all samples in the doped films (see Fig. S12 in ESI)
fppyBTPA/dfppyBTPA show very close onset positions in both fluorescence and
phosphorescence spectra, suggesting very small ET-S for favoring the RISC, while
fppyBPCz/dfppyBPCz and fppyBCzP/dfppyBCzP show relatively large ET-S ( 3.69 to
10.2 kcal/mol)) not so favoring for RISC in both mCPCN and DPEPO hosts. Indeed, the
room-temperature transient PL characteristics of all samples at 8 wt.% in the mCPCN
shown in Figure 5b (and those of fppyBPCz/dfppyBPCz and fppyBCzP/dfppyBCzP in
the DPEPO host, cf. ESI) indicate that fppyBTPA/dfppyBTPA in mCPCN exhibit distinct
delayed fluorescence along with the initial prompt fluorescence, while all other
compounds in either mCPCN or DPEPO host hardly show (or show very weak) delayed
fluorescence and behave more like conventional fluorescent emitters.
fppyBTPA and dfppyBTPA show very close onset positions in both fluorescence
and phosphorescence spectra in doped films (see Figure S12 in ESI). This suggests a
very small ET-S, which makes unreliable estimation of their ET-S from the spectral
onsets. Thus, for fppyBTPA and dfppyBTPA, we had instead characterized their ET-S
from the activation energies of their temperature-dependent RISC rate constants krisc.
Figures 6a and 6b depict transient PL profiles of fppyBTPA and dfppyTPA (8 wt.%
concentration) in the mCPCN host at different temperatures, respectively. Significant
dependence of the decay rates of the delayed components on the temperature can be
seen. Following the analysis method showed in literature,13, 69 the krisc at different
temperatures are extracted and are depicted as a function of the reciprocal of the
temperature in Figures 6c and 6d, from which the ET-S for fppyBTPA and dfppyBTPA
are estimated to be -0.60 kcal/mol (-26 meV) and -0.74 kcal/mol (-32 meV), and listed
in Table 2. These results confirm very small ET-S values of fppyBTPA and dfppyBTPA,
which are among the smallest ET-S values ever reported for TADF materials.
Transient PL profiles of fppyBTPA/dfppyBTPA can be fitted by two exponential
decay components as: I(t) = B1·exp(-t/p)+B2·exp(-t/d), where B1 and B2 are pre-
exponential factor, and p and d are the observed lifetimes of the prompt and delayed
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decay components. Extracted (p, d) for fppyBTPA/dfppyBTPA are (48 ns, 2.0 s) and
(46 ns, 2.4 s), respectively (Table 2). The emission quantum yields Φp and Φd for the
prompt and delayed fluorescence components, respectively, can further be derived
by Φp = Φ·B1·p/(B1·p + B2·d) and Φd = Φ·B2·d/(B1·p + B2·d).70 Determined (Φp, Φd)
for fppyBTPA/dfppyBTPA in mCPCN are (38%, 34%) and (50%, 45%), respectively
(Table 2), clearly indicating significant contribution of delayed fluorescence for these
two emitters.
OLED performance. The boron complexes fppyBTPA/dfppyBTPA with significant
delayed fluorescence and high PLQYs were applied for electroluminescent (EL) studies
using the common device architecture of: glass substrate/ITO anode/PEDOT:PSS (70
nm)/TAPC (15 nm)/mCP (5 nm)/mCPCN doped with TADF dopants (4-75 wt.%; 20
nm)/3TPYMB (50 nm)/LiF (0.5 nm)/Al (100 nm). Indium tin oxide (ITO), poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), LiF, and Al were used
as the anode, hole-injection layer, electron-injection layer and cathode,
respectively.62-65 Both di-[4-(N,N-ditolylamino)phenyl]cyclohexane (TAPC) and N,N-
dicarbazolyl-3,5-benzene (mCP) constituted the hole-transport layers (HTL), while
mCPCN was served as the host of emitting layer (EML) and tris-[3-(3-
pyridyl)mesityl]borane (3TPYMB) employed as the electron-transport layer (ETL).62-65
Details of device fabrication and material characterization (molecular structures and
energy levels of materials used in OLEDs) can be found in the supporting information.
With a same device structure, devices with different doping concentrations (from
4 to 75 wt.%) were tested (with detailed EL characteristics of these devices being
shown in supporting information). Figure 7a-7d shows EL characteristics of OLEDs with
optimized fppyBTPA/dfppyBTPA doping concentrations at 8 wt.% and 25 wt.%,
respectively, while performance parameters of both devices are summarized in Table
3. The EL spectra are similar to PL spectra in doped films, indicating pure EL from either
fppyBTPA or dfppyBTPA. These devices in general exhibit a rather low turn-on voltage
of 2.5 V and low operation voltage (e.g., 3-4 V for a brightness of 100 cdm-2). The
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optimized dfppyBTPA device based shows EL efficiencies of up to (26.6%, 88.D2OcI:d10A.10-139,/C6TC04994J
81.5 lmW-1), superior to (20.2%, 63.9 cdA-1, 66.9 lmW-1) of the fppyBTPA device, due
to superior PLQY of dfppyBTPA. Having a nearly 27% of EQE, the present dfppyBTPA
device is among the most efficient TADF OLEDs.48, 49 Such an EQE is certainly beyond
the theoretical limit of a conventional fluorescent emitter and clearly indicates
effective recycling and utilization of the triplet excitons formed during the EL process,
through the effective TADF (or reverse intersystem crossing) process. Overall, results
here clearly demonstrate that linking electron-donating and the electron-accepting
fragments through the spiro-bridged, four-coordinate boron atom provide a flexible
and powerful design for TADF emitters capable of high PLQYs and high EL efficiencies.
Preliminary stability tests had been conducted on fppyBTPA and dfppyBTPA
devices. Currently, operational lifetimes (defined as the time for the brightness to drop
to 50% of initial brightness) of these devices are not satisfactory, i.e. only minutes to
tens of minutes (under an initial brightness of 200-250 cdm-2, see Figure S20 of ESI).
Nevertheless, it already showed a significant improvement compared to the boron
complex represented by ppyBTPA,50 for which severe and rapid device degradation
occurred even under standard OLED current-voltage-luminance characterization
conditions and afforded extremely low efficiency. Hence, the electron withdrawing
fluoro substituent on ppy fragment of boron complexes appears to improve the
stability of the as-fabricated OLEDs relative to those of ppyBTPA.
Conclusion
In summary, we report the design and synthesis of a new series of four-coordinated
boron complexes anchoring with both electron donating and accepting units. Due to
the electron deficiency and saturated atomic character, the boron atom has negligible
contribution to the lower lying electronic transition and hence creates spatial
separation between donor and acceptor moieties. Among the titled boron complexes
the underlying ICT characters and TADF properties have been investigated via steady-
state and time-resolved measurements, together with computational approach.
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OLEDs based on fppyBTPA and dfppyBTPA have been fabricated successfully, which
exhibit a rather low turn-on voltage of 2.5 V and low operation voltage, e.g., 3-4 V
for a brightness of 100 cdm-2. The performances of dfppyBTPA based OLEDs (26.6%,
88.2 cdA-1, 81.5 lmW-1) are among the most efficient TADF OLEDs so far. Moreover,
our observation is in sharp contrast to the closely related boron complex ppyBTPA,50
for which severe and rapid device degradation occurred even under standard OLED
current-voltage-luminance characterization conditions and afforded extremely low
efficiency. Hence, the electron withdrawing fluoro substituent on the ppy fragment of
boron complexes has obviously improved the stability of the as-fabricated OLEDs
relative to those of ppyBTPA. Our results thus broaden the horizon for boron based D-
A complexes as emerging TADF materials that are able to harvest the triplet excitons
efficiently.
Experimental Section
General Procedures. All reactions were conducted under nitrogen atmosphere
unless otherwise noted. All solvents were dried over appropriate drying agents and
distilled prior to use. Commercially available reagents were used without further
purification. 4-Bromo-triphenylamine was obtained from monobromination of
triphenylamine using N-bromosuccinimide.71 3-Bromo-9-phenylcarbazole was
synthesized from Ullmann coupling using 3-bromocarbazole and iodobenzene in
presence of CuI,72 while 9-(4-bromophenyl)carbazole was obtained from the coupling
of carbazole and 1-bromo-4-iodobenzene.73 Reactions were monitored by pre-coated
TLC plates (0.20 mm with fluorescent indicator F254). Mass spectra were obtained on a
JEOL SX-102A instrument operating in electron impact (EI) or fast atom bombardment
1
(FAB) mode. H, 13C and 19F NMR spectra were recorded on a Varian Mercury-400 or
an INOVA‐500 instrument. 1H NMR spectra were obtained in CDCl3 and calibrated using
residual CHCl3 ( = 7.24 ppm) as an internal reference. Elemental analysis was carried
out with a Heraeus CHN-O-Rapid Elemental Analyzer. Thermal analysis was performed
using a Seiko Instrument TG/DTA320 thermogravimetric analyzer under nitrogen at a
‒ 12 ‒

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Journal of Materials Chemistry C
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heating rate of 15 °C/min. Differential scanning calorimeter (DSC) was conductDeOdI:u10s.i1n03g9/C6TC04994J
a Seiko Instrument DSC220C, with a heating and cooling rate of 10 °C /min.
Preparation of borane dibromide complexes fppyBBr2 and dfppyBBr2. To a
solution of 2-(4-fluorophenyl)pyridine (3.6 g, 20 mmol) in 50 mL of CH2Cl2, 60 mL of 1
M solution of BBr3 in CH2Cl2 (60 mmol) was slowly added at 0 C, followed by addition
of ethyldiisopropylamine (3.7 mL, 20 mmol). The mixture was warmed up to RT and
stirred for 12 hours. After then, the mixture was washed with saturated K2CO3 solution
and water (three times), and the organic layer was dried over Na2SO4, and
concentrated to dryness. The crude product was rinsed with hexane to afford air-stable,
colorless 2-(4-fluorophenyl)pyridine boron dibromide fppyBBr2 (3.7 g, 53%). The
corresponding 2-(2,4-difluorophenyl)pyridine derivative dfppyBBr2 was synthesized in
an analogous manner in 30% yield.
Characterization of fppyBBr2: 1H NMR (400 MHz, CDCl3, 298K)  (ppm) 8.91 (d, J
= 6.0 Hz, 1H), 8.15 (t, J = 8.4 Hz, 1H), 7.85 (d, J = 8.4 Hz, 1H), 7.73 (dd, J = 8.4, 6.0 Hz,
1H), 7.55-7.49 (m, 2H), 7.08 (t, J = 8.4 Hz, 1H); 19F NMR (376 MHz, CDCl3, 298K)  (ppm)
−105.18 (1F).
Characterization of dfppyBBr2: 1H NMR (400 MHz, d6-acetone, 298K)  (ppm) 9.12
(d, J = 6.0 Hz, 1H), 8.58 (t, J = 8.0 Hz, 1H), 8.28 (d, J = 8.0 Hz, 1H),7.95 (t, J = 8.0 Hz, 1H),
7.28 (d, J = 8.0 Hz, 1H), 7.15 (t, J = 8.0 Hz, 1H); 19F NMR (376 MHz, d6-acetone, 298K) 
(ppm) −104.03 (1F), −112.92 (1F).
Preparation of fppyBTPA. To a solution of fppyBBr2 (0.50 g, 1.5 mmol) in THF (10
mL) at 0 C, was slowly added a Grignard reagent, which was in-situ generated from 4-
bromotriphenylamine (1.0 g, 3.2 mmol), Mg chip (0.23 g, 9.6 mmol) and trace amount
of I2 in THF (15 mL) and stirred at RT for 12 hours. The solution was evaporated to
dryness, and the residue was dissolved into CH2Cl2, washed with H2O, dried over
MgSO4 and concentrated. The resulting product was purified by silica gel column
chromatography, eluting with a mixture of ethyl acetate and hexane (v/v = 1/5),
‒ 13 ‒

Journal of Materials Chemistry C
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followed by recrystallization from mixed CH2Cl2 and hexane to afford fppyBTDPOAI: (100..140329/C6TC04994J
g, 42%) as a light yellow solid.
Characterization of fppyBTPA: 1H NMR (400 MHz, d6-acetone, 298K)  (ppm) 8.68 (d, J
= 5.9 Hz, 1H), 8.40-8.33 (m, 2H), 8.16 (t, J = 4.2 Hz, 1H), 7.67 (t, J = 5.9 Hz, 1H), 7.35 (d,
J = 8.6 Hz, 1H), 7.23-7.18 (m, 8H), 7.13-7.07 (m, 5H), 6.98-6.92 (m, 12H), 6.88 (d, J =
8.6 Hz, 4H); 13C NMR (100 MHz, CDCl3, 298K)  (ppm) 166.4(C-F), 163.9, 157.3,
148.1(4C), 145.4 (2C), 144.2, 140.6, 133.7 (4C), 131.8, 129.0 (8C), 123.8 (8C), 123.6
(4C), 122.0 (4C), 121.5 (2C), 117.8, 117.3, 117.1, 113.8, 113.5; 19F NMR (376 MHz, d6-
acetone, 298K)  (ppm) −111.18 (1F); MS (EI) m/z: calcd for C47H35BFN3: 671.3; found:
+
671.4 [M ]; anal. calcd for C47H35BFN3: C, 84.05; H, 5.25; N, 6.26; found: C, 83.68; H,
5.69; N, 6.16.
Preparation of dfppyBTPA. Following the procedure described for fppyBTPA,
reaction of dfppyBBr2 (0.40 g, 1.1 mmol) and the in-situ generated Grignard reagent
afforded dfppyBTPA (0.36 g, 47%) as a light yellow solid.
1
Characterization of dfppyBTPA: H NMR (400 MHz, d6-acetone, 298K)  (ppm)
8.73 (d, J = 6.4 Hz, 1H), 8.42 (t, J = 7.6 Hz, 1H), 8.35 (d, J = 7.6 Hz, 1H), 7.74 (t, J = 6.4
Hz, 1H), 7.23-7.18 (m, 9H), 7.10 (d, J = 8.4 Hz, 4H), 6.98-6.92 (m, 13H), 6.88 (d, J = 8.4
Hz, 4H);13C NMR (100 MHz, CDCl3, 298K)  (ppm) 166.7(C-F), 164.2(C-F), 157.6, 144.2,
141.0 (4C), 141.0, 134.2 (8C), 131.9, 126.0 (4C), 125.7 (8C), 123.2 (2C), 120.1 (2C),
119.5 (4C), 118.2, 117.4, 117.2, 114.3, 114.0, 110.0 (4C); 19F NMR (376 MHz, d6-
acetone, 298K)  (ppm) −107.05 (1F), −114.28 (1F); MS (EI) m/z: calcd for C47H34BF2N3:
+
689.3; found: 689.4 [M ]; anal. calcd for C47H34BF2N3: C, 81.86; H, 4.97; N, 6.09; found:
C, 81.77; H, 5.04; N, 6.11.
Preparation of fppyBPCz and dfppyBPCz. Following the procedure described for
both fppyBTPA and dfppyBTPA, the reactions with the in-situ generated Grignard
reagent from 9-(4-bromophenyl) carbazole afforded fppyBPCz and dfppyBPCz in 35-
45% yields.
Characterization of fppyBPCz: 1H NMR (400 MHz, CDCl3, 298K)  (ppm) 8.67 (d, J = 5.2
‒ 14 ‒

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Hz, 1H), 8.18-8.07 (m, 6H), 7.95 (dd, J = 7.1, 3.4 Hz, 1H), 7.55-7.36 (m, 17H), 7D.2O8I:-170..120349/C6TC04994J
(m, 5H), 7.11 (t, J = 8.5 Hz, 1H); 13C NMR (100 MHz, CDCl3, 298K)  (ppm) 166.7 (C-F),
164.2, 157.7, 144.2, 141.1, 141.0 (4C), 135.6 (2C), 134.2 (4C), 131.9, 126.1 (4C), 125.7
(4C), 123.2 (4C), 121.9 (2C), 120.1 (4C), 119.5 (4C), 118.2, 117.4, 117.2, 114.3, 114.0,
110.1 (4C); 19F NMR (376 MHz, CDCl3, 298K)  (ppm) −107.94 (1F); MS (EI) m/z: calcd
+
for C47H31BFN3: 667.2; found: 667.4 [M ]; anal. calcd for C47H31BFN3: C, 84.56; H, 4.68;
N, 6.29; found: C, 84.62; H, 4.84; N, 6.30.
Characterization of dfppyBPCz: 1H NMR (400 MHz, CDCl3, 298K)  (ppm) 8.52 (d,
J = 5.76 Hz, 1H), 8.22 (d, J = 8.2 Hz, 1H), 8.08 (t, J = 7.8 Hz, 1H), 7.41 (t, J = 6.6 Hz, 1H),
7.22-7.17 (m, 8H), 7.07-7.03 (m, 10H), 6.96-6.91 (m, 7H), 6.74 (t, J = 9.9 Hz, 1H); 13C
NMR (100 MHz, CDCl3, 298K)  (ppm) 157.4 (C-F), 148.4 (C-F), 144.3, 144.2, 141.7,
141.5, 141.0 (4C), 135.8 (4C), 134.2 (2C), 126.2 (4C), 126.1 (4C), 125.7 (4C), 125.6 (4C),
123.2 (4C), 122.3, 120.3 (2C), 120.1, 119.7, 119.5, 113.1, 110.0 (4C); 19F NMR (376 MHz,
CDCl3, 298K)  (ppm) −106.37 (1F), −115.00 (1F); MS (EI) m/z: calcd for C47H30BF2N3:
+
685.3; found: 685.2 [M ]; anal. calcd for C47H30BF2N3: C, 82.34; H, 4.41; N, 6.13; found:
C, 82.59; H, 4.27; N, 6.21.
Preparation of fppyBCzP and dfppyBCzP. Following the procedure described for
both fppyBTPA and dfppyBTPA, the reactions with the in-situ generated Grignard
reagent from 3-bromo-9-phenylcarbazole afforded fppyBCzP and dfppyBCzP in
moderate yields.
Characterization of fppyBCzP: 1H NMR (400 MHz, d6-acetone, 298K)  (ppm) 8.83
(d, J = 5.6 Hz, 1H), 8.40 (d, J = 8.0 Hz, 1H), 8.29 (t, J = 8.0 Hz, 1H), 8.18-8.15 (m, 1H),
8.09 (s, 2H), 7.96 (d, J = 7.6 Hz, 2H), 7.65-7.57 (m, 9H), 7.48-7.43 (m, 3H), 7.35-7.28 (m,
6H), 7.24 (d, J = 8.0 Hz, 2H), 7.14-7.06 (m, 3H); 13C NMR (100 MHz, CDCl3, 298K)  (ppm)
166.6 (C-F), 164.0, 157.4, 144.3 (2C), 140.7 (2C), 140.6 (2C), 140.0 (2C), 138.1 (2C),
131.8, 131.6 (2C), 129.7 (4C), 126.95, 126.90 (4C), 125.2 (2C), 124.4 (2C), 123.7 (2C),
123.1 (2C), 121.6, 120.2 (2C), 119.4 (2C), 117.9, 117.3, 117.1, 113.7, 113.5, 109.4 (2C),
109.1 (2C); 19F NMR (376 MHz, d6-acetone, 298K)  (ppm) −111.27 (1F); MS (EI) m/z:
‒ 15 ‒

Journal of Materials Chemistry C
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DOI: 10.1039/C6TC04994J
+
calcd for C47H31BFN3: 667.3; found: 667.4 [M ]; anal. calcd for C47H31BFN3: C, 84.56; H,
4.68; N, 6.29; found: C, 84.55; H, 4.73; N, 6.35.
1
Characterization of dfppyBCzP: H NMR (400 MHz, d6-acetone, 298K)  (ppm)
8.91 (d, J = 5.6 Hz, 1H), 8.46-8.39 (m, 2H), 8.07 (s, 2H), 7.98 (d, J = 7.8 Hz, 2H), 7.75 (t,
J = 5.6 Hz, 1H), 7.67-7.63 (m, 4H), 7.59 (d, J = 8.0 Hz, 4H), 7.48 (t, J = 7.2 Hz, 2H), 7.36-
7.30 (m, 7H), 7.25 (d, J = 8.4 Hz, 2H), 7.14 (t, J = 7.2 Hz, 2H), 6.99 (t, J = 8.4 Hz, 1H); 13C
NMR (100 MHz, CDCl3, 298K)  (ppm) 158.4 (C-F), 144.4 (C-F), 140.7 (2C), 140.4 (2C),
139.9 (2C), 138.2 (2C), 135.9, 131.72 (2C), 131.68, 131.3, 131.0, 129.6 (4C), 126.93 (4C),
126.89 (2C), 125.9, 125.1 (2C), 124.50 (2C), 124.47, 123.8 (2C), 123.1, 121.9, 121.8,
120.2 (2C), 119.3 (2C), 118.2 (2C), 109.4 (2C), 108.9 (2C); 19F NMR (376 MHz, d6-
acetone, 298K)  (ppm) −107.22 (1F), −114.39 (1F); MS (EI) m/z: calcd for C47H30BF2N3:
+
685.3; found: 685.4 [M ]; anal. calcd for C47H30BF2N3: C, 82.34; H, 4.41; N, 6.13; found:
C, 82.46; H, 4.26; N, 6.20.
Photophysical Measurements. Steady-state absorption and emission spectra in
solution were recorded with a Hitachi (U-3310) spectrophotometer and an Edinburgh
(FS920) fluorimeter, respectively.74 Thin films for photophysical characterization were
prepared by thermal evaporation on quartz substrates at 1-2 Å/sec in a vacuum
chamber with a base pressure of <10-6 torr. Absorption spectra were characterized by
a UV-vis-NIR spectrophotometer (UV-1650 PC, Shimadzu). Photoluminescence (PL)
spectra, photoluminescence quantum yields (PLQYs), and phosphorescence spectra
were characterized by a spectrofluorimeter (FluoroMax-P, Horiba Jobin Yvon Inc.).
PLQYs of thin films or dilute solutions were determined using this spectrofluorimeter
equipped with a calibrated integrating sphere. During the PLQY measurements, the
integrating sphere was purged with pure and dry nitrogen to keep the environment
inert. The selected monochromatic excitation light was used to excite samples placed
in the calibrated integrating sphere. By comparing the spectral intensities of the
monochromatic excitation light and the PL emission, the PL quantum yields were
determined. Phosphorescence spectra of thin films or dilute solutions were
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conducted at 77 K (the liquid nitrogen temperature) by the spectrofluoDroOmI: 10e.1t0e3r9/C6TC04994J
equipped with a microsecond flash lamp as the pulsed excitation source. A 10-ms delay
time was inserted between the pulsed excitation and the collection of the emission
spectrum. Time-resolved PL (PL decay curves) was measured by monitoring the decay
of the intensity at the PL peak wavelength using the time-correlated single-photon
counting technique with a fluorescence lifetime system (FluoroCube, Horiba Jobin
Yvon Inc.) and nanosecond pulsed light excitation from a UV light-emitting diode (300
nm). The samples were placed in a vacuum cryostat chamber with the temperature
control.
Computational method. All the calculations were performed with the Gaussian
09 program package. The geometry optimization of ground states for the six boron
complexes was simulated with density functional theory (DFT) at the B3LYP/6-31g(d)
levels. The calculated optical absorptions were performed with the time-dependent
density functional theory (TD–DFT) method. The solvent effect was based on the
polarizable continuum model (PCM), which was implemented in the Gaussian 09
program. We also adopted the Mulliken population analysis (MPA) to obtain the
electron density distribution of every atom in each molecular orbital. Additionally,
upon performing the TD-DFT calculation we were able to access the assignment and
corresponding percentage of the frontier orbitals for HOMO → LUMO (or HOMO-1 →
LUMO, HOMO → LUMO+1, etc.) excitation processes.
OLED Fabrication and Characterization. OLEDs were fabricated on the ITO-coated
glass substrates with multiple organic layers sandwiched between the transparent
bottom indium-tin-oxide (ITO) anode and the top metal cathode. The PEDOT:PSS
layer was prepared by spin coating from the as-purchased aqueous dispersion of
PEDOT:PSS (Clevios, Heraeus Co.), followed by annealing on a hot plate at 130 C for
15 min. under ambient conditions. Other material layers were deposited by vacuum
evaporation in a vacuum chamber with a base pressure of <10-6 torr. All organic
materials used in experiments (except for the TADF emitters and the mCPCN host
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DOI: 10.1039/C6TC04994J
developed by ourselves) were purchased from Lumtec, Inc. All compounds were
subjected to temperature-gradient sublimation under high vacuum before use. The
deposition system permits the fabrication of the complete device structure in a single
vacuum pump-down without breaking vacuum. The deposition rate of organic layers
was kept at 0.1 - 0.2 nm/s. The doping was conducted by co-evaporation from
separate evaporation sources with different evaporation rates. The active area of the
device is 1 x 1 mm2, as defined by the shadow mask for cathode deposition. The
current-voltage-brightness (I-V-L) characterization of the light-emitting devices was
performed with a source-measurement unit (SMU) and a Si photodetector calibrated
with a spectroradiometer (DMS 201, AUTRONIC-MELCHERS GmbH). EL spectra of
devices were collected by a calibrated CCD spectrograph.
Acknowledgments. This work was supported by the Ministry of Science and
Technology of Taiwan, ROC. Single crystal X-ray structural analyses were conducted by
Prof. Gene-Hsiang Lee at the Instrumentation Center of National Taiwan University.
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DOI: 10.1039/C6TC04994J
1.0
0.5
0.0
1.0
fppyBTPA
0.5
0.0
300
400
500
600
700
1.0
0.5
0.0
1.0
0.5
0.0
fppyBPCz
300
300
400
400
500
500
600
600
700
1.0
0.5
0.0
1.0
0.5
0.0
fppyBCzP
700
 / nm
Figure 1. Absorption and emission spectra of fppyBTPA, fppyBPCz, and fppyBCzP in
solid (black), and in toluene (red), CH2Cl2 (blue), and CH3CN solution (green) at 298 K.
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DOI: 10.1039/C6TC04994J
1.0
0.5
0.0
1.0
dfppyBTPA
0.5
0.0
300
300
300
400
400
400
500
500
500
600
600
600
700
1.0
0.5
0.0
1.0
0.5
0.0
dfppyBPCz
700
1.0
0.5
0.0
1.0
0.5
0.0
dfppyBCzP
700
 / nm
Figure 2. Absorption and emission spectra of dfppyBTPA, dfppyBPCz, and dfppyBCzP
in solid (black), and in toluene (red), CH2Cl2 (blue), and CH3CN solution (green) at 298
K.
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Journal of Materials Chemistry C
(a) LUMO, B: 0.39% (b) LUMO, B: 0.43% (c) LUMO, B: 0.37% (d) LUMO, B: 0.37% (e) LUMO, B: 0.41% (f) LUMO, B: 0.35%
HOMO, B: 0.38%
HOMO, B: 0.18%
HOMO, B: 0.60%
HOMO, B: 0.38%
HOMO, B: 0.18%
HOMO, B: 0.59%
Figure 3. The frontier molecular orbitals (HOMO and LUMO) of (a) fppyBTPA, (b) fppyBPCz, (c) fppyBCzP, (d) dfppyBTPA, (e) dfppyBPCz, and (f)
dfppyBCzP in toluene. Notation “B” indicates the electron density distribution at boron atom in each orbital. The Mulliken population analysis
(MPA) in Gaussian 09 program was applied to obtain the electron density distribution in each molecular frontier orbital.
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Journal of Materials Chemistry C
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DOI: 10.1039/C6TC04994J
10000
1000
100
(a)
10
1
0
2000
4000
Time (ns)
6000
8000
(b)
10000
1000
100
10
1
0
5000
10000
Time (ns)
15000
20000
Figure 4. The fluorescence decay of (a) fppyBTPA and (b) dfppyBPTPA (black line) in
degassed toluene at 298 K. IRF: instrument response function (red line). Fitting curve
(blue line).
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DOI: 10.1039/C6TC04994J
Figure 5. (a) PL spectra and (b) transient PL profiles (measured at RT) of all doped
samples at 8 wt.% in mCPCN host.
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Journal of Materials Chemistry C
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DOI: 10.1039/C6TC04994J
Figure 6. (a), (b) Transient PL profiles of fppyBTPA and dfppyBTPA (8 wt.%
concentration) in the mCPCN host, respectively, at different temperatures. (c), (d)
Extracted krisc as a function of the reciprocal of the temperature.
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DOI: 10.1039/C6TC04994J
Figure 7 (a) EL spectra and (b) current-voltage-luminance (I-V-L) characteristics, (c)
external quantum efficiencies (EQEs), and (d) power efficiencies of fppyBTPA and
dfppyBTPA devices. The doping conc. for fppyBTPA and dfppyBTPA are at 8 wt.% and
25 wt.%, respectively. The 1931 CIE (x,y) coordinates for the fppyBTPA and dfppyBTPA
devices are (0.27,0.54) and (0.26,0.58), respectively.
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DOI: 10.1039/C6TC04994J
Table 1. Selected photophysical properties of all boron complexes in toluene at RT.
CT
ΔET-S
[kcal/mol]
11.6[a], 56.4[b] 76.8 (0.9798); 3369.6 (0.0202) 1.65
Compound
PLQY [%]
τCT [ns][b] (pre-exp factor)
[nm]
fppyBTPA
529
fppyBPCz
fppyBCzP
456
485
29.4[b]
31.9[b]
29.6 (–)
27.7 (–)
dfppyBTPA
dfppyBPCz
dfppyBCzP
535
488
486
19.9[a], 100.0[b] 75.3 (0.9752); 2697.1 (0.0248) 1.52
16.0[a], 39.2[b] 23.1 (–)
17.5[a], 44.4[b] 31.6 (0.9819); 2576.1 (0.0181) 2.36
[a] measured in aerated solution. [b] measured in degassed solution. [c] (–) indicates
that the emission can be fitted solely by a single exponential decay kinetics.
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Journal of Materials Chemistry C
Table 2. Selected photophysical properties of all boron complexes in mCPCN and DPEPO hosts (with a doping concentration of 8 wt.%, except
for an additional test on 25 wt.% dfppyBTPA).
(a)
(c)
(d)
(f)
(f)
(g)
(g)
λPL, max
Φ (b)
ES
[eV]
ET
[eV]
τp
τd
Φp
Φd
(e)
ET-S
Emitter
Host
[nm]
[%]
[kcal/mol]
0.60 e
7.61
[ns]
[μs]
2.0
---
[%]
[%]
fppyBTPA
fppyBPCz
mCPCN
mCPCN
DPEPO
mCPCN
DPEPO
mCPCN
mCPCN
mCPCN
DPEPO
mCPCN
DPEPO
494
426
420
450
456
490
508
432
427
455
456
72
48
38
---
---
34
---
---
0.8
---
45
38
---
---
---
---
2.90
2.90
15.7
6.0
3.25
3.35
3.11
3.25
2.92
2.73
3.13
3.40
3.12
3.14
2.92
2.92
2.87
2.92
2.92
2.73
2.88
2.96
2.94
2.98
10.4
16.6
11.4
15
---
9.92
fppyBCzP
15.5
16.6
95
0.29
---
14.7
---
5.53
7.61
0.74 e)
0.53 e)
5.77
dfppyBTPA (8 wt.%)
dfppyBTPA (25 wt.%)
dfppyBPCz
46
2.4
2.4
---
50
62
---
100
15.5
7.0
51
9.4
17.7
9.8
11
---
---
10.2
dfppyBCzP
15.0
19.4
---
---
4.15
---
---
3.69
a. fluorescence peak maximum; b. photoluminescence quantum yield; c. singlet energy calculated from the onset of the fluorescence; d. triplet
‒ 27 ‒

Journal of Materials Chemistry C
Page 28 of 32
energy calculated from the onset wavelength of the 77K phosphorescence; e. ET-S of fppyBTPA and dfppyBTPA were obtained from the
activation energy of temperature-dependent krisc while ET-S of other compounds were obtained from onsets of fluorescence and
phosphorescence spectra; f. lifetime of the prompt and delayed photoluminescence; g. quantum yield of the prompt and delayed fluorescence.
Table 3. EL parameters of the fppyBTPA and dfppyBTPA based OLED devices (with a doping concentration of 8 wt.% and 25 wt.%, respectively).
Current efficiency
[cd∙A-1]
Power efficiency
[lm∙W-1]
Device
Conc.
EQE [%]
fppyBTPA
8 wt.%
Peak
20.2
15.5
26.6
25.3
63.9
49.1
88.2
83.9
66.9
28.5
81.5
59.9
at 100 cd∙m-2
Peak
dfppyBTPA
25 wt.%
at 100 cd∙m-2
‒ 28 ‒

Page 29 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC04994J
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