Polymorphism, fluorescence, and optoelectronic properties of a borazine derivative

Article abstract of DOI:10.1002/chem.201204598

We have prepared a new borazine derivative that bears mesityl substituents at the boron centers and displays exceptional chemical stability. Detailed crystallographic and solid-state fluorescence characterizations revealed the existence of several polymorphs, each of which showed different emission profiles. In particular, a bathochromic shift is observed when going from the lower- to the higher-density crystal. Computational investigations of the conformational dynamics of borazine 1 in both the gas phase and in the solid state using molecular dynamics (MD) simulations showed that the conformation of the peripheral aryl groups significantly varies when going from an isolated molecule (in which the rings are able to flip over the 90° barrier at RT) to the crystals (in which the rotation is locked by packing effects), thus generating specific nonsymmetric intermolecular interactions in the different polymorphs. To investigate the optoelectronic properties of these materials by fabrication and characterization of light-emitting diodes (LEDs) and light-emitting electrochemical cells (LECs), borazine 1 was incorporated as the active material in the emissive layer. The current and radiance versus voltage characteristics, as well as the electroluminescence spectra reported here for the first time are encouraging prospects for the engineering of future borazine-based devices. Copyright

Full text of DOI:10.1002/chem.201204598

FULL PAPER
DOI: 10.1002/chem.201204598
Polymorphism, Fluorescence, and Optoelectronic Properties of a Borazine
Derivative
Simon Kervyn,[a] Oliver Fenwick,[b] Francesco D. Stasio,[b] Yong Sig Shin,[b]
Johan Wouters,[a] Gianluca Accorsi,*[c] Silvio Osella,[d] David Beljonne,*[d]
Franco Cacialli,*[b] and Davide Bonifazi*[a, e]
Abstract: We have prepared a new bor-
azine derivative that bears mesityl sub-
stituents at the boron centers and dis-
plays exceptional chemical stability.
Detailed crystallographic and solid-
state fluorescence characterizations re-
vealed the existence of several poly-
morphs, each of which showed differ-
ent emission profiles. In particular, a
bathochromic shift is observed when
going from the lower- to the higher-
density crystal. Computational investi-
gations of the conformational dynamics
of borazine 1 in both the gas phase and
in the solid state using molecular dy-
namics (MD) simulations showed that
the conformation of the peripheral aryl
groups significantly varies when going
from an isolated molecule (in which
the rings are able to flip over the 908
barrier at RT) to the crystals (in which
the rotation is locked by packing ef-
fects), thus generating specific nonsym-
metric intermolecular interactions in
the different polymorphs. To investi-
gate the optoelectronic properties of
these materials by fabrication and char-
acterization of light-emitting diodes
(LEDs) and light-emitting electro-
chemical cells (LECs), borazine 1 was
incorporated as the active material in
the emissive layer. The current and ra-
diance versus voltage characteristics, as
well as the electroluminescence spectra
reported here for the first time are en-
couraging prospects for the engineering
of future borazine-based devices.
Keywords: boron · doping · molec-
ular modeling · nitrogen · polymor-
phism
Introduction
ous types of fascinating bulky or discrete (nano)structures,
each of which display unique structural, chemical, and physi-
cal properties.[1] Among others, polycyclic aromatic hydro-
carbons (PAHs) possess many attractive electronic proper-
ties in terms of conductivity, adsorption, and emission prop-
erties.[1b,2] Among the different functionalization strategies
with hydrophobic, hydrophilic, electron-donating, or elec-
tron-withdrawing groups to tune their self-assembly, solubili-
ty, absorption, and emission behavior, the replacement of
the carbon atoms with electron-deficient or/and electron-
rich heteroatoms—that is, atom doping—in the all-carbon
networks strongly affects the frontier orbitals, thus dramati-
cally altering their electronic properties.[3] In particular, the
replacement of two carbon atoms by one boron and one ni-
trogen atom in aromatic structures leads to isoelectronic
structural mimics[3a,b,4] that possess local dipole moments as
the bonding now contains a significant polarity contribu-
tion.[3b–d,4b] Such polarity is known to considerably alter the
character of the frontier molecular orbitals, thus affecting
the optical properties of the molecular system, and generally
giving rise to larger energy gaps[5] than those of their all-
carbon analogues. In this respect, the inorganic mimic of
benzene, borazine, first reported by Stock et al. in 1926,[6]
possesses a wide energy gap of approximately 6 eV,[7] al-
though it displays similarities in both geometry and in
formal topology of the p-molecular orbitals.[8] The UV-emis-
sive properties of borazine and of its potential oligomeric
derivatives[5,9] make such molecular substrates attractive
candidates to be implemented as the active layer in UV-
The abundance and availability of carbon as a natural ele-
ment together with its ability to form different types of ho-
moatomic covalent bonds allows the development of numer-
[a] Dr. S. Kervyn, Prof. J. Wouters, Prof. D. Bonifazi
Department of Chemistry and Namur Research College (NARC)
University of Namur (UNamur), Rue de Bruxelles 61
Namur, 5000 (Belgium)
E-mail: davide.bonifazi@fundp.ac.be
[b] Dr. O. Fenwick, Dr. F. D. Stasio, Y. S. Shin, Prof. F. Cacialli
Department of Physics and Astronomy (CMMP Group) and
London Centre for Nanotechnology
University College London, Gower Street
London, WC1E 6BT (UK)
E-mail: f.cacialli@ucl.ac.uk
[c] Dr. G. Accorsi
Molecular Photoscience Group
Istituto per la Sintesi Organica e la Fotoreattivitꢀ (ISOF)
Consiglio Nazionale delle Ricerche (CNR)
Via P. Gobetti 101, 40129, Bologna (Italy)
E-mail: gianluca.accorsi@isof.cnr.it
[d] Dr. S. Osella, Dr. D. Beljonne
Laboratory for Chemistry of Novel Materials
University of Mons; Place du Parc, 20, 7000 Mons (Belgium)
E-mail: david.beljonne@umons.ac.be
[e] Prof. D. Bonifazi
Department of Chemical and Pharmaceutical Sciences and
INSTM UdR Trieste, University of Trieste,
Trieste, Piazzale Europa 1, Trieste (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204598.
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light-emitting diodes (LEDs), and consequently for
a
number of potentially high-impact applications.[10] However,
the susceptibility of the BN bonds to undergo hydrolysis in
the presence of moisture (e.g., borazine rapidly decomposes
to boric acid, ammonia, and H2 in the presence of ambient
humidity) has been a major deterrent to the widespread syn-
thetic development of BN-doped aromatic hydrocarbons
and their use in optoelectronic devices; to date, only one
single example describing their role as charge transporter
has been reported.[11] Attempts to significantly prevent the
hydrolytic decomposition of borazines by introduction of
various substituents were not successful until bulky substitu-
ents were placed in the ortho positions of B-aryl groups. In
particular, following the works of Brown et al.[12] and Haw-
kins et al.[13] that described B-mesityl derivatives, in 1965
Nagasawa[14] reported an apparently moisture-stable B-tri-
phenyl-N-trimesitylborazine that bore two methyl groups at
the ortho positions of the aryl substituents. Recently, Yama-
guchi and co-workers also reported stable borazine deriva-
tives in which visible-emitting anthryl substituents were in-
serted at the B position.[15] Inspired by these approaches, in
this work we have prepared a new borazine derivative that
displays exceptional chemical stability, in which the B-atom
centers bear mesityl substituents. Detailed crystallographic
and fluorescence characterizations showed the existence of
several polymorphs,[16] each of which showed a different
emission profile in the solid state, thus suggesting a unique
interplay between intermolecular forces and conformations.
In particular, a redshifted emission emerges when passing
from the lower- to the higher-density crystal. Molecular
modeling simulations were performed to assess the electron-
ic structure of the borazine derivatives and analyze their pe-
culiar luminescence characteristics in the solid state. To in-
vestigate the emissive properties under an externally applied
bias, light-emitting electrochemical cells (LECs) were fabri-
cated by incorporating borazine 1 as the active material.
These devices were then characterized by measurement of
the current and radiance versus voltage characteristics, as
well as of their electroluminescence spectra so as to enable
the evaluation of the optoelectronic properties of the bora-
zine-based active layer.
Scheme 1. Synthesis of borazines 1 and 2.
nylborazine 2. When subjected to hydrolysis in water-con-
taining mixtures, compound 1 exhibited extraordinary stabil-
ity, which suggested that the ortho-methyl substituents exert
a steric hindrance, thus chemically shielding the B-atom cen-
ters. As expected, borazine 2 completely decomposes under
the same conditions.
When compound 1 was crystallized from cyclohexane,
CH2Cl2, or pentane, several polymorphs that belong to the
¯
R3c, R32, and P21/n space groups were obtained, respective-
ly (for the crystallographic details, see the Supporting Infor-
mation). In all polymorphs, the X-ray molecular structure
reveals a deviation from orthogonal arrangement of the aryl
substituents and the borazine ring with an interplanar angle
between 63.14 to 83.688 depending on the polymorph and
on the substituent (see below). As previously observed in
other literature reports,[18] the intra-annular distance values
between B and N atoms are between 1.40 and 1.46 ꢂ,
whereas the internal angle of the borazine cycle is between
114 and 1248. The central BN core is nearly planar with a
torsion angle of approximately 1.58. Similar structural char-
acteristics have been also observed for molecule 2.[18b]
The quasi-orthogonal arrangement of the aryl substituents
forces the methyl substituents to nest atop the B atoms
À
(C1 B1=3.0 ꢂ), thus shielding the electrophilic atoms from
nucleophilic addition reactions (Figure 1). Interestingly, me-
chanically grinding any of the formed crystals yielded the
same crystalline powder (see XRD pattern in the Support-
ing Information) of an unknown space group, here labeled
“powder X”. As clearly seen from the crystal packing dis-
played in Figure 2, solid-state arrangements in the space
¯
Results and Discussion
groups R32 and R3c feature tubular voids with average di-
ameters of 6.0 and 6.8 ꢂ, respectively, whereas P21/n crystals
were revealed to be compact.
Synthesis and spectroscopic characterization: By following
the facile synthetic procedure of Brown et al. and Groszos
et al.,[17] trimesitylborazine derivative was synthesized by
using a one-pot procedure that involved BCl3, as shown in
Scheme 1. The reaction of BCl3 with aniline produced the
corresponding B-trichloro-N-triphenylborazine, which was
subsequently treated with the organometallic mesityl deriva-
tive (both Li-derived and Grignard reagents gave similar re-
sults) to afford B-trimesityl-N-triphenylborazine in a moder-
ate yield (see the detailed experimental procedure and full
characterization in the Supporting Information). The same
procedure was also used for preparing reference hexaphe-
The electronic absorption spectra of borazine 1, obtained
in three different solvents (i.e., CH2Cl2, THF, and CH3CN),
cover the UV spectral window in the approximately 200–
300 nm range, as depicted in Figure S17a in the Supporting
Information. As expected, the excitation spectra fully match
those of the corresponding absorption profiles, indicating
the ligand-centered (LC) nature of the emitting states. The
photoluminescence (PL) spectra in solution of the borazines
along with those of molecular reference hexaphenylbenzene
(HPB) are depicted in Figure 3. Remarkably, radiative emis-
sion is observed over spectral ranges that extend down to
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Properties of a Borazine Derivative
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Figure 1. Space-filling (above) and ORTEP (below) representation of the X-ray crystal structure of borazines a) 1 (space group: P21/n) and b) 2 (space
group: Pna21). B: pink, N: blue, C: gray. Hydrogen atoms are omitted for clarity.
less than 300 nm, which is in line with the expectation of sig-
nificant UV-emitting species. No appreciable effect could be
detected on the spectral profiles by changing the solvent po-
larity (e=1.55, 1.75, and 3.45 for CH2Cl2, THF, and CH3CN,
respectively; Figure S17b in the Supporting Information), as
further confirmed by the photoluminescence quantum yields
(Fem) and relative lifetimes (t), which are substantially un-
changed (Fem =7.7%, t=7.1 ns; Fem =6.6%, t=7.2 ns; and
Fem =7.2%, t=7.6 ns in CH2Cl2, THF, and CH3CN, respec-
tively). In contrast, the solid-state emission features for the
different borazine polymorphs of molecule 1 dramatically
differ (Figure 3, bottom). To investigate the influence of the
packing effects on the fluorescence properties, steady-state
emission studies were performed. Depending on the differ-
ent crystal packing, various emissive profiles that extended
over a remarkably large spectral range (from <300 to
>500 nm) were observed. Two main features are observed
around 310 and 360 nm, with relative intensities sensitive to
the density of the solid: the PL of the more compact poly-
morph, P21/n, displays the most intense peak at approxi-
mately 360 nm, whereas the least compact polymorphs (R32
¯
and R3c) emit mostly at lower wavelength, very close to the
solution case. In contrast, the total lifetimes (t) are substan-
tially similar (tꢀ11 ns) for all polymorphs, thus indicating
similar deactivation processes.
Modeling of the electronic properties: To shed further light
on the electronic structure and optical properties of the bor-
azine derivatives both in solution and in the solid state, we
resorted to ab initio time-dependent density functional
theory (TD-DFT) and semiempirical (AM1/SCI[19] and
INDO/SCI[20]) quantum-chemical calculations. Starting with
the analysis of the one-electron energy diagram computed
for borazines 1 and 2 and that of the reference HPB mole-
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G. Accorsi, D. Beljonne, F. Cacialli, D. Bonifazi et al.
(labeled iv in Table 1) that
peaks at approximately 6 eV,
the calculations also indicate a
second less-intense band (la-
beled iii) together with a set of
weakly optically allowed (la-
beled ii) and almost dark (la-
beled i) electronic transitions at
lower energies.
For the sake of illustration,
the INDO/SCI-simulated opti-
cal absorption spectrum of mol-
ecule 1 is displayed in Figure 5
together with the corresponding
spectrum as measured in
CH3CN. A reasonable agree-
ment is found between the ex-
perimental and theoretical re-
sults (although all predicted
transitions are slightly redshift-
ed relative to the experiment;
Figure 5), with band iii appear-
ing as a shoulder on the red
side of the dominant band iv
and a much weaker feature ii in
¯
Figure 2. View of the crystal packing along the c axis of borazine 1 in space groups a) R3c, b) R32, and
the near-UV region. The lowest
singlet electronic excitations
computed at the INDO/SCI
and AM1/SCI level are located
c) P21/n. The yellow surfaces surround the crystal voids. d) TGA profile (black curve) and its derivative (red)
of borazine 1 as obtained under an inert atmosphere.
cule, the DFT/HSE[21] results displayed in Figure 4
(on the basis of geometric structures fully optimized
at the HSE level using the polarized 6-31G* Gaus-
sian basis set)[22] show a simultaneous (de)stabiliza-
tion of the HOMO (LUMO) molecular orbital,
Table 1. Optical transitions calculated for a single molecule of borazine 1 at different
levels of theory. The intensity of the transitions based on their relative oscillator
strengths is indicated between the parentheses: very weak (vw), weak (w), medium
(m), and strong (s).
which results in an increased energy band
G
CAM-B3LYP
[eV]
LC-wPBE
[eV]
INDO/SCI
[eV]
AM1/SCI
[eV]
label
going from HPB to hexaphenylborazine 2. Notably,
whereas the shape of the LUMO remains essential-
ly unchanged, the HOMO shows smaller contribu-
tions on the central BN core in borazine 2 than that
which derives from the central hexa-substituted
phenyl ring of HPB (see Figure S21 in the Support-
ing Information). These changes reflect the partial
5.1–5.3 (vw)
5.4–5.6 (m)
5.7–5.8 (m)
5.8–6.2 (s)
5.2–5.4 (vw)
5.5–5.6 (w)
5.9–6 (m)
6–6.2 (s)
4.29–4.38 (vw)
4.43–4.53 (w)
5–5.5 (m)
3.82–3.91 (vw)
3.98–4.18 (w)
5.3–5.5 (m)
5.5–6.1 (s)
5.6–6.25 (s)
ionic character of the BN bonds, as de
scribed by Molteni
at around 4.3 and 3.8 eV, respectively, yet as these carry a
vanishingly small oscillator strength they cannot be unam-
biguously identified on either the simulated or the measured
spectrum. In a simplified picture, the spatial extent of the
electronic excitations changes (the localization of the elec-
tronic transition dipole can be locally mapped by the transi-
tion density distributions as shown in Figures S8–S11 in the
Supporting Information) from being delocalized over the
whole (including the central borazine core) molecule for
transition iv to being localized on the aryl side groups in iii
and only on a couple of phenyl or mesityl rings, respectively,
for excitations ii and i. The increasing localization of the
electronic transition dipole as a function of the decreasing
transition energy is accompanied by reduced absorption
cross sections. Thus, on the basis of these calculations, the
and co-workers.[10] As expected, the presence of weakly elec-
tron-donating methyl moieties on the mesityl groups in mol-
ecule 1 causes a minor high-energy shift of the frontier MOs
relative to that of the naked hexaphenylborazine 2.
To describe the nature of the lowest electronic excited
states in borazine 1, TD-DFT analysis using long-range cor-
rected functionals (namely, CAM-B3LYP[23] and LC-
wPBE[24]) and semiempirical simulations were also per-
formed. A consistent qualitative picture for the optical ab-
sorption spectrum of molecule 1 emerges from these calcu-
lations (although there are significant quantitative differen-
ces). These give rise to multiple electronic excitations that
(based on their nature) can be collected in four bands, as de-
picted in Table 1. In addition to the very strong absorption
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Properties of a Borazine Derivative
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Figure 5. INDO/SCI-simulated absorption spectrum for borazine
1
(dashed trace) together with the corresponding spectrum measured in
CH3CN (solid trace). The bands are labeled according to Table 1.
emission bands observed in solution at approximately
310 nm (4 eV) can be assigned to the electronic excited
states that belong to the calculated band i. This interpreta-
tion is reasonable because the integrated INDO/SCI oscilla-
tor strength over that band of 1.3ꢃ10À2 translates to a radia-
tive (gas-phase) lifetime value of 94 ns, which is in excellent
agreement with the value of 92 ns as experimentally estimat-
ed from the measured total lifetime and quantum efficiency
values in CH2Cl2 (see above). Notably, the low-emission ra-
diative decay rate is also in line with the limited LED quan-
tum efficiency (see below). One can thus speculate that the
presence of a dual-emission signal in the solid (i.e., the two
emissive features at approximately 310 and approximately
350 nm with polymorph-dependent relative intensities)
arises from a removal of the degeneracy of the electronic
excitations associated with band i. This effect is likely in-
duced by conformation constraints imposed on the peripher-
al aryl groups due to a packing effect, which is expected to
differently affect the interplanar angle of the phenyl and
mesityl units, respectively. These solid-state effects can po-
tentially slow down nonradiative decay pathways that in-
volve conformational degrees of freedom, a phenomenon
known as aggregation-induced emission.[25] It is therefore
not surprising that, compared to the emission profiles as ob-
tained from solution studies, the most drastic changes in PL
spectra are observed for the highest-density P21/n poly-
morph. We note that INDO/SCI calculations performed on
Figure 3. Top: Normalized emission spectra (lexc =268 nm) of borazines 1
and 2 and of reference molecules mesitylene and HPB in CH3CN.
Bottom: Normalized emission spectra (lexc =264 nm) of the different bor-
azine polymorphs in the solid state: polymorph space group R32 (solid
line), R3c (dashed line), P21/n (dotted line), and ground powder (full
circle).
¯
¯
clusters of molecule 1 as arranged in the R3c, R32, and
P21/n space groups did not show any significant difference
in the emission profile (namely, the presence of the low-
energy PL emissive band) with respect to that obtained for
the single molecule modeled in the vacuum, thus ruling out
any effects that are derived from the excitonic delocalization
or the formation of J-type aggregates (which is not surpris-
ing as dipole interactions are vanishingly small for weakly
allowed excitations). According to AM1/SCI calculations,
the lowest triplet excited states of 1 in the P21/n space group
Figure 4. DFT/HSE one-electron energy diagram of borazines 1 and 2 in
comparison to reference all-carbon HPB analogue.
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G. Accorsi, D. Beljonne, F. Cacialli, D. Bonifazi et al.
are located at around 2.4 eV (ꢀ517 nm) above the singlet
ground state, and thus too far in energy to contribute to the
photoluminescence signal at approximately 350 nm (al-
though they might be partly responsible for the broad elec-
troluminescence band that extends far into the visible).
However, preliminary investigations of the conformational
dynamics of borazine 1 in both the gas phase and in the
solid state by using molecular dynamics (MD) simulations
(Universal force field, 500 ps simulations in the NVT ensem-
ble at 300 K) showed that the conformation of the peripher-
al aryl groups significantly varies when going from an isolat-
ed molecule (in which the rings are able to flip over the 908
barrier at RT) to the crystals (in which the rotation is
locked by packing effects). In addition, one can observe that
the three mesityl substituents of 1 experience different
chemical environments owing to the specific intermolecular
interactions in the P21/n polymorph, unlike the R32 poly-
morph in which the torsion histograms are essentially super-
imposable (Figure S26 in the Supporting Information). In
principle, this asymmetry is expected to remove the degen-
eracy associated with the lowest electronic excitations and
reshuffle the distribution of the oscillator strengths. INDO/
SCI calculations performed on 200 snapshots extracted from
the MD simulations indicate an energy splitting of approxi-
mately 0.1 eV, which is in line (albeit smaller) with the ex-
perimental energy difference of approximately 0.56 eV ob-
served between the two emission peaks for polymorph P21/
n.
for the LECs, or of molecule 1 for the LEDs. For LECs, the
solution was spin-cast from a mix of toluene and THF (1:1),
with a concentration of 2 wt% of solids, of which 50 wt%
was borazine 1, and 50 wt% was PEO/LiOTf with a 20:1
molar ratio of the CH2CH2O moieties in PEO to LiOTf. We
instead used toluene as a solvent for the LEDs with a
2 wt% concentration of borazine emitter 1. A low work
function anode of LiF (6 nm)/Ca (30 nm)/Al (150 nm) was
evaporated to facilitate electron injection, which as men-
tioned above is notoriously difficult for high-gap materials.
The current density and radiance versus voltage characteris-
tics of a typical LEC that incorporates molecule 1 and those
of an LED are depicted in Figure 6a,c. Charge injection
into the LEC is clearly observed, with a strongly nonlinear
dependence of the current density J on the applied voltage
V, as is typical for organic and polymeric LEDs. We ach-
ieved current densities of >100 mAcmÀ2 at somewhat
higher voltages (ꢀ15 V) than we would expect for most mo-
lecular diodes and LECs. We consider this an indication of
the intrinsically low charge mobility in molecule 1, and of
the existence of further margins for optimization of charge
transport and possibly of injection. The current turn-on volt-
age (arbitrarily defined at 10À3 mAcmÀ2) is observed at ap-
proximately 3 V, whereas a clearly observable change of
slope in the JV characteristics suggests that bipolar injection
is achieved at approximately 5 V, which is only marginally
higher than the energy gap of the “bluest polymorphs” as
inferred from the PL spectra, in line with the results for “or-
ganic LECs” in which emission is observed at voltages simi-
lar to the modulus of the energy gap (in eV).
Engineering of LED and LEC devices: We also prepared
LEDs and LECs that incorporated borazine 1 in the active
layer to investigate the emission properties under electrical
injection conditions. As with all materials that emit towards
the blue/UV end of the available spectrum,[26] charge injec-
tion tends to be more difficult than with lower-gap materials
as a result of the increased difficulty of matching the work
function of the electrodes with the higher-lying lowest-unoc-
cupied molecular orbitals (LUMOs) and the lower-lying
highest-occupied molecular orbitals (HOMOs). In particu-
lar, the relatively low electron affinity (À1.0 to À1.3 eV) of
molecule 1 makes electron injection a challenge compared
to hole injection. The best results were expected with an
LEC architecture, in which the active material is blended
with an ion-transporting polymer such as polyethylene oxide
(PEO) and a salt[27] to exploit the buildup of mobile ions at
the electrode interfaces to reduce charge-injection barriers.
However, we also fabricated and characterized LEDs that
incorporated an active layer made of the neat derivative 1
and obtained nearly comparable results. The device struc-
ture is shown as an inset in Figure 6 for the LECs and
LEDs, and is built on an indium–tin oxide (ITO) anode,
with an 80 nm hole-injection layer of poly(3,4-ethylenedi-
Concomitant luminescence was observed with a threshold
of approximately 9.5 V (Figure 6a), in which this is defined
as the voltage at which the radiance reaches 0.2 mWmÀ2
(owing to the presence of emission in the UV range, at
wavelengths <400 nm, we reported the optical output in ra-
diometric units, that is, radiance in mWmÀ2, rather than
photometric units, or luminance in cdmÀ2), which clearly ex-
ceeded our noise level (ꢀ0.1 mWmÀ2). The slight discrepan-
cy in the radiative emission turn-on voltage with respect to
the occurrence of bipolar injection is a result of the higher
noise in our detection electronics relative to the radiative
emission, owing to extrinsic factors (different noise levels in
the measurement of the current and radiative emission).
Similar results were found for the representative LED, the
characteristics of which are reported in Figure 6c, although
they show slightly higher noise, and the external quantum
efficiency is approximately an order of magnitude higher
than that for the LECs. The spectral distribution of the
emission, as reported in Figure 6d, clearly demonstrates
emission at wavelengths below 400 nm, albeit with a broad
spectrum that spans the whole of the visible range as well.
Similar results were found for the LEC, the spectrum of
which is displayed in Figure 6b. In addition, of potential in-
terest for white-emitting LEDs and applications in the illu-
mination sector, or for backlights, we note that a very large
spectral width is in fact to be expected on the basis of the
various polymorphs identified, and the possible existence of
ACHTUNGTRENNUNGoxyACHTUNGTRENNUNGthiophene) (PEDOT)/polysodium styrene sulfonate
(PSS) spin-cast from a 2.8% solution in water (Sigma Al-
drich 560596). The active layer consisted either of a blend of
emitter 1, polyethylene oxide as the ion transporter, and
lithium triflate (LiOTf) as the salt that provided mobile ions
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Properties of a Borazine Derivative
FULL PAPER
Figure 6. a) Current and radiance versus light characteristics of an LEC incorporating an active layer of emitter 1, blended with PEO as the ion trans-
porter and LiOTf as the salt that provides mobile ions. The 3.5 mm2 device was fabricated with the vertical structure ITO/PEDOT:PSS (80 nm)/active
layer/LiF (6 nm)/Ca (30 nm)/Al (150 nm), which is indicated schematically as an inset. b) The electroluminescence spectrum obtained at a bias of 17 V of
such a device, showing broad emission from the near-UV through the visible spectrum. c) Current/voltage/light characteristics of an LED incorporating
an active layer of emitter 1. The 3.5 mm2 device was fabricated with the vertical structure ITO/PEDOT:PSS (80 nm)/active layer/LiF (6 nm)/Ca (30 nm)/
Al (150 nm). d) The EL spectrum of such a device obtained at a bias of 13.8 V, showing broad emission from the near-UV through the visible spectrum.
other polymorphs not yet identified in our fundamental
characterization of the materials. Furthermore, application
of electric fields, charge transport, and potential mild heat-
ing during device operation might indeed favor the forma-
tion of a mixture of polymorphs. A discrepancy between PL
and electroluminescent (EL) emission has been observed in
the past for some early blue-emitting materials, which was
mainly caused by the formation of aggregates, excimers, or
exciplexes.[26a] Formation of aggregated species could also be
responsible for relatively low EL quantum efficiency[28] as
reported here (ꢀ10À4 %).
spectroscopic response. In particular, by using molecular dy-
namics (MD) simulations, computational investigations of
the conformational dynamics of borazine 1 in both the gas
phase and in the solid state showed that the conformation of
the peripheral aryl groups significantly varies when going
from an isolated molecule (in which the rings are able to
flip over the 908 barrier at RT) to the crystals (in which the
rotation is locked by packing effects), thus generating differ-
ent chemical environments in the different polymorphs
owing to the specific intermolecular interactions. This asym-
metry is expected to remove the degeneracy associated with
the lowest electronic excitations and to reshuffle the distri-
bution of the oscillator strengths, thereby generating an
energy splitting that is responsible for the two-peak emissive
behavior. In parallel, we have been able to exploit their
electroluminescent properties in LEDs and LECs that incor-
porate borazine 1 as the active material in the emissive
layer. Despite a relatively low EL quantum efficiency
Conclusion
In conclusion, we have prepared a new UV-emitting bora-
zine derivative that bears mesityl substituents and displays
exceptional chemical stability toward hydrolysis. Detailed
crystallographic and fluorescence characterizations showed
the existence of several polymorphs, each of which showed
a different emission profile in the solid state, thus suggesting
a unique interplay between intermolecular forces and the
A
of UV electroluminescence from borazine-based materials,
and we thus consider these data to be particularly encourag-
ing for the prospects of borazine-based devices. The use of
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Acknowledgements
D.B. gratefully acknowledges the EU through the ERC Starting Grant
“COLORLANDS” project, the FRS-FNRS (FRFC contract nos.
2.4.550.09 and MIS no. F.4.505.10.F), by the Belgian Government (IUAP
no. P06-27 “Functional Supramolecular Systems”), the “Loterie Natio-
nale”, the “TINTIN” ARC project (09/14-023) and the University of
Namur (internal funding). S.K. thanks the FRIA and the University of
Namur for his doctoral fellowships. G.A. thanks the Italian National Re-
search Council (CNR, MACOL, PM.P04.010). This work is supported by
the European Commission Seventh Framework Program (FP7/2007-
2013) under grant agreement number 238177 (Project ITN-SUPERIOR)
and by the Belgian National Fund for Scientific Research (FNRS). D.B.
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Properties of a Borazine Derivative
FULL PAPER
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Received: December 26, 2012
Published online: && &&, 0000
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G. Accorsi, D. Beljonne, F. Cacialli, D. Bonifazi et al.
Polymorphism power rangers: A new
borazine derivative that bears mesityl
substituents at the boron centers and
displays exceptional chemical stability
has been prepared (see figure).
Detailed crystallographic and solid-
state fluorescence characterizations
revealed the existence of several poly-
morphs, each of which shows different
emission profiles.
Borazines
S. Kervyn, O. Fenwick, F. D. Stasio,
Y. S. Shin, J. Wouters, G. Accorsi,*
S. Osella, D. Beljonne,* F. Cacialli,*
D. Bonifazi* .................. &&&& — &&&&
Polymorphism, Fluorescence, and
Optoelectronic Properties of a Bora-
zine Derivative
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These are not the final page numbers!