Syntheses and photophysical properties of new iminopyrrolyl boron complexes and their application in efficient single-layer non-doped OLEDs prepared by spin coating

Article abstract of DOI:10.1039/c2dt30487b

Efficient non-doped OLEDs have been achieved using new binuclear tetracoordinate organoboron complexes containing 2-(N-aryl)formiminopyrrolyl ligands. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2dt30487b

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 8502
www.rsc.org/dalton
COMMUNICATION
Syntheses and photophysical properties of new iminopyrrolyl boron
complexes and their application in efficient single-layer non-doped OLEDs
prepared by spin coating†
D. Suresh,a Clara S. B. Gomes,a Pedro T. Gomes,*a Roberto E. Di Paolo,a António L. Maçanita,a
Maria José Calhorda,b Ana Charas,c Jorge Morgadoc,d and M. Teresa Duartea
Received 16th February 2012, Accepted 16th April 2012
DOI: 10.1039/c2dt30487b
geometry along with extended π-conjugated systems.4 Recent
Efficient non-doped OLEDs have been achieved using new
reports suggested that compounds containing π-conjugated
ladder-type skeletons coordinated to multi Lewis acidic boryl
groups constrain the π-conjugated framework to intensify the
emission, and enhance the electron-transport properties.5 This
indicates that the incorporation of multiboron centres into rigid
conjugated π-systems may be an ideal synthetic strategy to
achieve high performance OLEDs. Only few rigid multiboron-
containing π-systems have been obtained to date owing to the
lack of efficient synthons, whose preparation involves several
reaction stages.
The 2-(N-aryl)formiminopyrrole ligand precursors are an
example of such synthons where the extended π-conjugation
moiety can be readily attained by incorporating various aromatic
spacers via condensation reactions.6 The π-conjugation can also
be extended through fusing aromatic groups on the edges of
pyrrole ring, as reported by our group with blue/green light emit-
ting 2-(N-aryl)formiminophenanthro[9,10-c]pyrrolyl zinc com-
plexes.7 However, their potential application as OLEDs was not
successful. This prompted us to synthesise and characterise new
boron complexes of 2-(N-aryl)formiminopyrrolyl ligands and
study their remarkable photoluminescent and electroluminescent
properties.
The 2-(N-aryl)formiminopyrrolyl ligand precursors 1–3 herein
reported were prepared and characterised according to the
reported literature methods.6,7 Refluxing the 2-(N-aryl)formimi-
nopyrrole ligand precursors 1–3 and triphenylborane in toluene,
followed by crystallisation, afforded the target compounds 4–6
in good yields (Scheme 1). These complexes were completely
characterised by multinuclear NMR spectroscopy, elemental ana-
lyses and/or single crystal X-ray diffraction.8 All the complexes
are bright yellow solids, which show fluorescence in solution
and in solid state, and are sensitive to air and moisture.
binuclear tetracoordinate organoboron complexes containing
2-(N-aryl)formiminopyrrolyl ligands.
Tetracoordinate mononuclear boron compounds containing che-
lating N,O-, N,N- and N,C-chromophores are a family of
efficient emitters, and several complexes have been designed and
synthesised.1 Variations on the chromophore part of the molecule
influence the HOMO (highest occupied molecular orbital)–
LUMO (lowest unoccupied molecular orbital) energies and
thereby the colour of emission.1a,c In addition, some of these
compounds exhibit electron-transport properties and, conse-
quently, can be employed in making photoluminescent (PL) and
electroluminescent (EL) devices, including organic light-emit-
ting diodes (OLEDs) and sensors.1a–c,2 Nevertheless, their per-
formances have been under par. Recently, a rapid progress has
been observed in the production of low-cost, high efficiency
materials for making OLEDs, mainly for application in flat-panel
displays.3 For high performance OLEDs, the intense lumines-
cence and high carrier mobility are the two most important par-
ameters, which are accomplished with molecules having planar
aCentro de Química Estrutural, Departamento de Engenharia Química,
Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco
Pais, 1049-001 Lisboa, Portugal. E-mail: pedro.t.gomes@ist.utl.pt;
Tel: +351 218419612
bDepartamento de Química e Bioquímica, CQB, Faculdade de Ciências,
Universidade de Lisboa, Campo Grande, Ed. C8, 1749-016 Lisboa,
Portugal
cInstituto de Telecomunicações, Av. Rovisco Pais, 1049-001 Lisboa,
Portugal
dDepartamento de Bioengenharia, Instituto Superior Técnico,
Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa,
Portugal
†Electronic supplementary information (ESI) available: Experimental
details: syntheses and characterisation of compounds 4–6; X-ray crystal
data and structure refinements for complexes 4 and 5 (Table S1), mol-
ecular structure of 4 (Fig. S1) and crystal packing of 4 and 5 (Fig. S2
and S3); spectroscopic measurements and fluorescence decays (Fig. S4–
S6); computational studies including calculated absorption spectra of
and molecular orbitals of 4 (Fig. S7 and S8); voltammograms of com-
plexes 4–6 (Fig. S9); OLED fabrication and characterisation (Fig. S10
and S11); energy level diagram of the components involved in the
“single-layer” devices based on complexes 4–6 (Fig. S12). CCDC
867381 and 867382. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2dt30487b
The molecular structure of 5 is depicted in Fig. 1 (4 is
depicted in ESI†), along with the selected metric parameters. It
is notable that, in both compounds, the boron atom has a charac-
teristic pseudo-tetrahedral geometry, in which the chelating 2-(N-
aryl)formiminopyrrolyl ligands show bite angles (N–B–N) of
94.95(10) and 94.87(15)°, and C–B–C angles of 115.56(11)
(C13–B1–C19) and 115.79(17)° (C10–B–C16), for 4 and 5,
respectively. The average B–C bond distances are 1.612 Å
for both complexes. The two B–N distances (1.6327(19) and
8502 | Dalton Trans., 2012, 41, 8502–8505
This journal is © The Royal Society of Chemistry 2012

View Online
1.5687(18) Å for 4; 1.632(3) and 1.562(3) Å for 5) are different,
owing to the different nature of N1 and N2 atoms.7,9 The dihe-
dral angles between the 2-formiminopyrrolyl and the aromatic
imine substituent planes (C6–N2–C7–C12 in 4, and C6–N2–
C7–C9 in 5) are −47.21(17) and 47.2(3)°, respectively,
suggesting no severe restricted rotation around the C–Nimine
bond.10 In addition, there is no interfacial π–π interaction in the
supramolecular arrangement of 4 and 5 probably because of the
large steric hindrance of the tetrahedral BPh2 moieties. This
structural feature plays an important role in reducing molecular
aggregation in the solid state.
The UV-Vis absorption and fluorescence spectra in THF are
shown in Fig. 2 and relevant data are summarised in Table 1.
The 2-(N-aryl)formiminopyrrole ligand precursors 1–3 are
non-emissive while their boron complexes are highly emissive in
both solution and thin films. The mononuclear boron complex 4
0–0
em
absorbs at λabs = 383 nm and emits blue light at λ = 451 nm
(first vibronic transition). The binuclear compounds 5 and 6
show substantial bathochromic shifts both in absorption (45 nm
and 36 nm, respectively) and emission (61 nm and 46, respect-
ively) when compared to the mononuclear compound 4 and
show intense green fluorescence. Preliminary results of
TD-DFT11 calculations (ADF programme12) are in agreement
with these trends of absorption and emission, namely the absorp-
tion of compound 4 at 377 nm (HOMO to LUMO ILCT).8 The
smaller red-shifts of 6 with respect to 5 result from the larger
values (and number) of dihedral angles between the aromatic
moieties, which counteract the red-shift effect of increasing the
π-conjugation length from 5 to 6.
Scheme 1 Synthesis of 2-(N-aryl)formiminopyrrolyl boron complexes
4–6.
Fluorescence quantum yields of 4–6 in THF were measured
relative to those of tetrathiophene (for 4) and pentathiophene
(for 5 and 6).13 Notably, the quantum yields for the binuclear
compounds 5 (0.69) and 6 (0.64) almost double that of the
Fig. 1 Perspective view of molecular structure of 5. The ellipsoids
were drawn at 30% probability level. All the hydrogen atoms were
omitted for clarity. Selected bond lengths (Å): N1–C5, 1.345(3); N1–
C2, 1.378(3); N2–C6, 1.307(2); N2–C7, 1.427(3); N1–B1, 1.562(3);
N2–B1, 1.632(3); C10–B1, 1.616(3); C16–B1, 1.614(3). Selected bond
angles (°): N2–B1–C10, 108.42(16); N2–B1–C16, 111.02(16); N1–B1–
N2, 94.87(15); N1–B1–C10, 111.40(16); N1–B1–C16, 113.32(17);
C10–B1–C16, 115.79(17).
Fig. 2 Absorption (left) and fluorescence emission (right) spectra of
complexes 4–6 in THF.
Table 1 Wavelength maximum of the first absorption band (λmabasx) and respective molar extinction coefficient (εmax), wavelength of the first vibronic
emission transition (λe0m–0), fluorescence quantum yield (ϕf), lifetime (τf) and rate constant (kf) and sum of non-radiative rate constants (knr) of boron
complexes 4–6, in THF at 293 K. Ionization potential (IP) and electron affinity (EA) values, as determined by cyclic voltammetry, and DFT calculated
HOMO and LUMO energies, are also shown
λmabasx (nm)
εmax
λ0em–0 (nm)
ϕf
τf (ns)
kfb (ns−1
)
knrc (ns−1
)
IP/EA (eV)
HOMO/LUMO (eV)
a
Comp
4
5
6
383
428
419
1.3
2.8
1.8
451
512
497
0.34
0.69
0.64
1.90
2.22
1.94d
0.18
0.31
0.33
0.35
0.14
0.19
5.64/2.82
5.01/3.44
4.98/3.23
−5.94/−2.94
−5.22/−3.21
−5.22/−3.14
a 104 L mol−1 cm−1 b kf = ϕf/τf. c knr = (1 − ϕf)/τf. d Major decay time.
.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8502–8505 | 8503

View Online
mononuclear compound 4 (0.34) in THF, being similar to those
reported for the above mentioned ladder-type binuclear boron
species5 despite their less constrained framework. Fluorescence
decays of 4 and 5, measured at three emission wavelengths
(onset, maximum, and tail of the fluorescence spectra), were
globally well fitted with single exponential functions, while
those of complex 6 required a sum of two exponential terms for
global fitting (available in ESI†). For 6, besides the main decay
1.94 ns component (the complex lifetime), a shorter decay time
(13 ps), appearing as a decay at the onset of the emission spec-
trum and as a rise-time at longer wavelengths, was observed.
This fast component is assigned to torsional relaxation of the
four aromatic moieties of 6, from twisted to more planar confor-
mations in the excited state, as found with different conjugated
organic polymers and oligomers in solution.14,15 This trend is
also reproduced in the optimised geometries of the ground and
first excited singlet states. Moreover, if the conformational relax-
ation rate constant (kCR) is equated to the reciprocal of the
shorter decay time,15a its value in THF at 293 K is kCR = 7.7 ×
1010 s−1, a value slightly higher than those reported for pheny-
lene–vinylene trimers in other solvents of similar viscosity.15a,16
The fluorescence quantum yields of complexes 5 and 6 are ca.
twice that of 4, due to the changes of both the radiative (kf) and
radiationless (knr) rate constants. The larger kf values of 5 and 6
result from the increase in the transition dipole moment (DFT
calculated transition dipole moments are 2.34, 3.44, and 3.28
a.u., respectively, for 4, 5 and 6), and in the molar extinction
coefficient (εmax) values. The twisting of the biphenyl group
reduces the effective π-conjugation length, being likely the cause
of the smaller transition dipole moment of 6 with respect to 5.
The smaller knr values of complexes 5 and 6 (due to less
efficient internal conversion and/or intersystem crossing) com-
bined with the larger kf values lead to the significant improve-
ment in their quantum efficiencies.
which we attribute to intermolecular interactions in solid state,
stabilising the excited state (Fig. S11, ESI†). It should be men-
tioned that no reports are found on LEDs based on low molecu-
lar weight boron complexes prepared by solution methods,
though organoborane polymers have been used.17 There are
several reports addressing the study of LEDs with boron com-
plexes, prepared by vacuum sublimation and combining various
layers to improve the charge balance and/or to block the excitons
away from the electrodes.5a,c Very high efficiencies and lumi-
nances are reported for such multilayer structures, where the use
of at least a hole-conduction layer is mandatory to achieve such
high performance, to compensate for the higher electron-trans-
port ability of the boron complexes. This unbalanced charge
transport in boron-based complexes is consistent with the poorer
performance of the single-layer LEDs based on compound 4,
which has a much higher ionization potential. In this work, we
explored the combined ambipolarity and light emission of these
new boron complexes, evidencing very promising results.
In summary, new four-coordinate mono- and binuclear orga-
noboron complexes containing 2-(N-aryl)formiminopyrrolyl
moieties were synthesised and characterised, showing intense
fluorescence properties. Non-doped EL devices were fabricated
using these compounds as both emitter and ambipolar charge-
transporting materials, the two binuclear ones exhibiting high
brightness and efficiency. We consider that this comprehensive
work opens a new avenue towards boron complexes-based
LEDs, with potential applications in displays.
We thank the Fundação para a Ciência e Tecnologia, Portugal,
for financial support (Projects PTDC/QUI/65474/2006, PEst-
OE/QUI/UI0100/2011-2012, PEst-OE/QUI/UI0612/2011 and
PEst-OE/EEI/LA0008/2011) and for fellowships to D.S. and
C.S.B.G. (SFRH/BPD/47853/2008 and SFRH/BPD/64423/2009,
respectively), and the Portuguese NMR Network (IST-UTL
Centre) for providing access to the NMR facility.
The ionization potential (IP) and the electron affinity (EA) of
complexes 4–6 were determined by cyclic voltammetry, as
detailed in the ESI.† The obtained values are compared with the
DFT calculated HOMO and LUMO energies in Table 1. The cal-
culated values reflect the trends observed experimentally, both
results affording similar variation along the series, with com-
pound 4 showing a stabilisation of the HOMO and destabiliza-
tion of the LUMO with respect to complexes 5 and 6.
Complexes 4–6 were tested as emissive layers in non-doped
single-layer light-emitting diodes, with indium–tin oxide anodes,
covered with a hole-injection layer of PEDOT:PSS (polystyrene
sulphonic acid-doped poly(3,4-ethylene dioxythiophene)), and
calcium cathodes. Details of device preparation and characteris-
ation can be found in the ESI.† The emissive layer was prepared
by spin coating from THF solutions of the pure materials. LEDs
based on 4 show a poor performance, which can be attributed to
its lower fluorescence efficiency and in particular to the much
lower HOMO energy, which leads to a poorer electron/hole
balance within the emissive layer (Fig. S12, ESI†). Instead, the
performance of the LEDs based on neat diboron complexes 5
and 6 is remarkable, considering that these are single-layer
devices. Maximum luminances in the order of 103 cd m−2 and
maximum EL efficiencies of ca. 0.3 cd A−1 are obtained
(Fig. S10, ESI†). The electroluminescence spectra are in general
red-shifted by ca. 20 nm with respect to the solution PL spectra,
Notes and references
1 (a) Q.-D. Liu, M. S. Mudadu, R. Thummel, Y. Tao and S. Wang, Adv.
Funct. Mater., 2005, 15, 143; (b) H. Amarne, C. Baik, S. K. Murphy and
S. Wang, Chem.–Eur. J., 2010, 16, 4750; (c) S. Kappaun,
S. Rentenberger, A. Pogantsch, E. Zojer, K. Mereiter, G. Trimmel, R. Saf,
K. C. Mőller, F. Stelzer and C. Slugovc, Chem. Mater., 2006, 18, 3539;
(d) B. J. Liddle, R. M. Silva, T. J. Morin, F. P. Macedo, R. Shukla, S.
V. Lindeman and J. R. Gardinier, J. Org. Chem., 2007, 72, 5637.
2 (a) S. L. Hellstrom, J. Ugolotti, G. J. P. Britovsek, T. S. Jones and
A. J. P. White, New J. Chem., 2008, 32, 1379; (b) T.-R. Chen, R.-
H. Chien, M.-S. Jan, A. Yeh and J.-D. Chen, J. Organomet. Chem., 2006,
691, 799; (c) K. Rurack, M. Kollmannsberger and J. Daub, Angew.
Chem., Int. Ed., 2001, 40, 385.
3 (a) T. K. S. Wong, in Handbook of Organic Electronics and Photonics,
ed. H. S. Nalwa, American Scientific Publishers, Stevenson Ranch, CA,
2008, vol. 2, pp. 413–472; (b) S. Wang, Coord. Chem. Rev., 2001, 215,
79.
4 (a) C. Du, S. Ye, Y. Liu, Y. Guo, T. Wu, H. Liu, J. Zheng, C. Cheng,
M. Zhuab and G. Yua, Chem. Commun., 2010, 46, 8573; (b) A. Kraft, A.
C. Grimsdale and A. B. Holmes, Angew. Chem., Int. Ed., 1998, 37, 402.
5 (a) Z. Zhang, H. Bi, Y. Zhang, D. Yao, H. Gao, Y. Fan, H. Zhang,
Y. Wang, Y. Wang, Z. Chen and D. Ma, Inorg. Chem., 2009, 48, 7230;
(b) D. Li, Z. Zhang, S. Zhao, Y. Wang and H. Zhang, Dalton Trans.,
2011, 40, 1279; (c) D. Li, Y. Yuan, H. Bi, D. Yao, X. Zhao, W. Tian,
Y. Wang and H. Zhang, Inorg. Chem., 2011, 50, 4825.
6 C. Pozo-Gonzalo, J. A. Pomposo, J. Rodríguez, E. Y. Schmidt, A.
M. Vasil’tsov, N. V. Zorina, A. V. Ivanov, B. A. Trofimov, A.
I. Mikhaleva and A. B. Zaitsev, Synth. Met., 2007, 157, 60.
8504 | Dalton Trans., 2012, 41, 8502–8505
This journal is © The Royal Society of Chemistry 2012

View Online
7 C. S. B. Gomes, P. T. Gomes, M. T. Duarte, R. E. Di Paolo, A.
L. Maçanita and M. J. Calhorda, Inorg. Chem., 2009, 48, 11176.
8 Details in (ESI†).
9 S. A. Carabineiro, L. C. Silva, P. T. Gomes, L. C. J. Pereira, L. F. Veiros,
S. I. Pascu, M. T. Duarte, S. Namorado and R. T. Henriques, Inorg.
Chem., 2007, 46, 6880.
10 C. S. B. Gomes, D. Suresh, P. T. Gomes, L. F. Veiros, M. T. Duarte, T.
G. Nunes and M. C. Oliveira, Dalton Trans., 2010, 39, 736.
11 S. J. A. van Gisbergen, J. G. Snijders and E. J. Baerends, Comput. Phys.
Commun., 1999, 118, 119.
12 ADF2009, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam,
The Netherlands, http://www.scm.com
13 R. S. Becker, J. S. de Melo, A. L. Maçanita and F. Elisei, Pure Appl.
Chem., 1995, 67, 9.
14 F. Momicchioli, M. C. Bruni and I. Baraldi, J. Phys. Chem., 1972, 76, 3983.
15 (a) R. E. Di Paolo, J. S. de Melo, J. Pina, H. D. Burrows, J. Morgado and
A. L. Maçanita, ChemPhysChem, 2007, 8, 2657; (b) R. E. Di Paolo, H.
D. Burrows, J. Morgado and A. L. Maçanita, ChemPhysChem, 2009, 10,
448; (c) F. B. Dias, J. Morgado, A. L. Maçanita, F. P. da Costa, H.
D. Burrows and A. P. Monkman, Macromolecules, 2006, 39, 5854.
16 R. E. Di Paolo, B. Gigante, M. A. Esteves, N. Pires, C. Santos, M.
H. Lameiro, J. S. de Melo, H. D. Burrows and A. L. Maçanita, Chem-
PhysChem, 2008, 9, 2214.
17 F. Jäkle, Chem. Rev., 2010, 110, 3985.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8502–8505 | 8505