Luminescent di- and trinuclear boron complexes based on aromatic iminopyrrolyl spacer ligands: Synthesis, characterization, and application in OLEDs

Article abstract of DOI:10.1002/chem.201500109

New bis- and tris(iminopyrrole)-functionalized linear (1,2-(HNC₄H₃-C(H)-N)₂-C₆H₄ (2), 1,3-(HNC₄H₃-C(H)-N)₂-C₆H₄ (3), 1,4-(HNC₄H₃-C(H)-N)₂-C₆H₄ (4), 4,4′-(HNC₄H₃-C(H)-N)₂-(C₆H₄-C₆H₄) (5), 1,5-(HNC₄H₃C-(H)-N)₂-C₁₀H₆ (6), 2,6-(HNC₄H₃C-(H)-N)₂-C₁₀H₆ (7), 2,6-(HNC₄H₃C-(H)-N)₂-C₁₄H₈ (8)) and star-shaped (1,3,5-(HNC₄H₃-C(H)-N-1,4-C₆H₄)₃-C₆H₃ (9)) π-conjugated molecules were synthesized by the condensation reactions of 2-formylpyrrole (1) with several aromatic di- and triamines. The corresponding linear diboron chelate complexes (Ph₂B[1,3-bis(iminopyrrolyl)-phenyl]BPh₂ (10), Ph₂B[1,4-bis(iminopyrrolyl)-phenyl]BPh₂ (11), Ph₂B[4,4′-bis(iminopyrrolyl)-biphenyl]BPh₂ (12), Ph₂B[1,5-bis(iminopyrrolyl)-naphthyl]BPh₂ (13), Ph₂B[2,6-bis(iminopyrrolyl)-naphthyl]BPh₂ (14), Ph₂B[2,6-bis(iminopyrrolyl)-anthracenyl]BPh₂ (15)) and the star-shaped triboron complex ([4′,4′′,4′′′-tris(iminopyrrolyl)-1,3,5-triphenylbenzene](BPh₂)₃ (16)) were obtained in moderate to good yields, by the treatment of 3-9 with B(C₆H₅)₃. The ligand precursors are non-emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π-conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time-dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13, in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples (11, 12, 14, and 15), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π-system, to afford green to yellow emissions. Organic light-emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m^-2, for single layer devices, increasing to 4400 cd m^-2 when a hole-transporting layer is used. Bridge the gap: Luminescent linear and star-shaped polynuclear organoboron complexes containing iminopyrrolyl ligands linked by aryl bridges were synthesized, showing good luminescent properties depending on π-conjugation lengths of the molecules (see figure). Organic light-emitting diodes (OLEDs) were successfully fabricated with the new boron complexes, achieving luminance values of up to 4400 cd m^-2.

Full text of DOI:10.1002/chem.201500109

DOI: 10.1002/chem.201500109
Full Paper
&
Luminescence
Luminescent Di- and Trinuclear Boron Complexes Based on
Aromatic Iminopyrrolyl Spacer Ligands: Synthesis,
Characterization, and Application in OLEDs**
D. Suresh,[a, e] Clara S. B. Gomes,[a] Patrꢀcia S. Lopes,[a] Clꢁudia A. Figueira,[a] Bruno Ferreira,[a]
Pedro T. Gomes,*[a] Roberto E. Di Paolo,[a] Antꢂnio L. MaÅanita,[a] M. Teresa Duarte,[a]
Ana Charas,[b] Jorge Morgado,[b, c] Diogo Vila-ViÅosa,[d] and Maria Josꢃ Calhorda[d]
Abstract: New bis- and tris(iminopyrrole)-functionalized
linear (1,2-(HNC4H3-C(H)=N)2-C6H4 (2), 1,3-(HNC4H3-C(H)=N)2-
The photophysical properties of the luminescent polyboron
compounds were measured, showing good solution fluores-
C6H4 (3), 1,4-(HNC4H3-C(H)=N)2-C6H4 (4), 4,4’-(HNC4H3-C(H)= cence quantum yields ranging from 0.15 to 0.69. DFT and
N)2-(C6H4-C6H4) (5), 1,5-(HNC4H3C-(H)=N)2-C10H6 (6), 2,6-
(HNC4H3C-(H)=N)2-C10H6 (7), 2,6-(HNC4H3C-(H)=N)2-C14H8 (8))
and star-shaped (1,3,5-(HNC4H3-C(H)=N-1,4-C6H4)3-C6H3 (9)) p-
conjugated molecules were synthesized by the condensation
reactions of 2-formylpyrrole (1) with several aromatic di- and
triamines. The corresponding linear diboron chelate com-
time-dependent DFT calculations confirmed that molecules
10 and 16 are blue emitters, because only one of the imino-
pyrrolyl groups becomes planar in the singlet excited state,
whereas the second (and third) keeps the same geometry.
Compound 13, in which planarity is not achieved in any of
the groups, is poorly emissive. In the other examples (11, 12,
14, and 15), the LUMO is stabilized, narrowing the gap be-
tween the highest occupied molecular orbital and the
lowest unoccupied molecular orbital (HOMO–LUMO), and
the two iminopyrrolyl groups become planar, extending the
size of the p-system, to afford green to yellow emissions. Or-
ganic light-emitting diodes (OLEDs) were fabricated by using
the new polyboron complexes and their luminance was
found to be in the order of 2400 cdmꢀ2, for single layer de-
vices, increasing to 4400 cdmꢀ2 when a hole-transporting
layer is used.
plexes
(Ph2B[1,3-bis(iminopyrrolyl)-phenyl]BPh2
(10),
Ph2B[1,4-bis(iminopyrrolyl)-phenyl]BPh2 (11), Ph2B[4,4’-bis-
(iminopyrrolyl)-biphenyl]BPh2 (12), Ph2B[1,5-bis(iminopyrrol-
yl)-naphthyl]BPh2 (13), Ph2B[2,6-bis(iminopyrrolyl)-naph-
thyl]BPh2 (14), Ph2B[2,6-bis(iminopyrrolyl)-anthracenyl]BPh2
(15)) and the star-shaped triboron complex ([4’,4’’,4’’’-tris(imi-
nopyrrolyl)-1,3,5-triphenylbenzene](BPh2)3 (16)) were ob-
tained in moderate to good yields, by the treatment of 3–9
with B(C6H5)3. The ligand precursors are non-emissive, where-
as most of their boron complexes are highly fluorescent;
their emission color depends on the p-conjugation length.
[a] Dr. D. Suresh, Dr. C. S. B. Gomes, P. S. Lopes, Dr. C. A. Figueira, B. Ferreira,
Prof. P. T. Gomes, Dr. R. E. Di Paolo, Prof. A. L. MaÅanita, Prof. M. T. Duarte
Centro de Quꢀmica Estrutural, Departamento de
Engenharia Quꢀmica, Instituto Superior Tꢁcnico
Universidade de Lisboa, Av. Rovisco Pais
Introduction
Research involving luminescent organic/organometallic com-
plexes has received considerable attention because of their po-
tential use in various applications such as electroluminescent
(EL) and photoluminescent (PL) devices,[1] fluorescence probes
for detection of various anions, cations, or even neutral mole-
cules.[2] Tang and Van Slyke were the first to examine electrolu-
minescent properties on AlQ3 (HQ=8-hydroxyquinoline)[3] and
then onwards various synthetic studies have been conducted
on different materials including complexes of heavy metals,
such as phosphorescent iridium(III)[4] and platinum(II),[5] and flu-
orescent complexes of boron.[6] Luminescent tri-[7] and tetra-
coordinate[8] organoboron compounds are receiving increasing
attention because of their various applications in materials and
organic devices.[9] Boron, a strong electrophile, has tendency to
fill the vacant orbital and complete the octet, four-coordinate
organoboron compounds with monoanionic ligands being in
general more stable than other organometallic derivatives.
1049-001 Lisboa (Portugal)
E-mail: pedro.t.gomes@tecnico.ulisboa.pt
[b] Dr. A. Charas, Prof. J. Morgado
Instituto de TelecomunicaÅꢂes
Av. Rovisco Pais, 1049-001 Lisboa (Portugal)
[c] Prof. J. Morgado
Department of Bioengineering
Instituto Superior Tꢁcnico, Universidade de Lisboa
Av. Rovisco Pais, 1049-001 Lisboa (Portugal)
[d] D. Vila-ViÅosa, Prof. M. J. Calhorda
Centro de Quꢀmica e Bioquꢀmica, DQB
Faculdade de CiÞncias, Universidade de Lisboa
Campo Grande, Ed. C8, 1749-016 Lisboa (Portugal)
[e] Dr. D. Suresh
School of Chemical and Biotechnology
SASTRA University, Thanjavur–613 401 (India)
[**] OLEDs=organic light-emitting diodes.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500109.
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Many examples illustrate these type of compounds, including
the mononuclear boron complexes of the type
[8a–c,e,l,n,r]
(N,N’)BR2,[8a–c,h,i,k,o,q] (N,O)BR2
and (N,C)BR2,[8a–c,i,m] in which
N,N’, N,O, and N,C are bidentate chelating heterocyclic ligands
with a neutral N and a negative Nꢀ, Oꢀ, or Cꢀ donor, respec-
tively. A limited number of oxygen-bridged[10] and ladder-type
luminescent polynuclear organoboron compounds[8a–c,f,g,j] have
also been reported.
Some of the reported four-coordinate organoboron com-
pounds can exhibit charge-transport properties and can be
employed in the fabrication of photo- and electroluminescent
devices, including organic light-emitting diodes (OLEDs).[11] The
search for new low-cost, highly efficient emitters for the manu-
facturing of OLEDs is considered of great importance because
of the high industrial demand for flat panel displays and solid-
state lighting. Intense luminescence and high carrier mobility
are the two most requested parameters for high performance
OLEDs, which are accomplished with molecules having planar
geometry along with an extended p-conjugated system.[12] The
incorporation of multiboron centers into the same p-conjugat-
ed framework intensifies the emission and enhances the
charge-transport properties,[8a–c,f,g,j,p] indicating this may be an
ideal synthetic strategy towards high-performance OLEDs. The
search for organometallic precursors for OLEDs exhibiting dif-
ferent colors of emission is another important task to achieve.
So far, the color tunability of boron-based OLEDs has been
achieved by varying the substituents on the chromophore
(N,N’ or N,O) side of the molecules and/or by varying the R
substituents at the boron center.[8a–c,i,k,o] Changing the nature of
the substituents will affect the Lewis acidity of the boron
center, which in turn may affect the boron–chromophore inter-
action and the HOMO–LUMO levels, and thereby the color of
emission. An alternative method to tune the emission wave-
length and the quantum yield can be obtained by varying the
length of p-conjugation.
(N-arylformimino)pyrrolyl}2] showed negligible quantum yields
of 0.16 and 0.23%. More recently, in a preliminary communica-
tion, we reported a mononuclear iminopyrrolyl boron complex
[BPh2{k2N,N’-2-(N-phenylformimino)pyrrolyl}] (labeled 17 in the
present work; see Table 1) and two binuclear organoboron
complexes containing bridging iminopyrrolyl ligands with ex-
tended p-conjugation (labeled 11 and 12 in the present work;
see Table 1).[15] These boron complexes showed very good
photoluminescent properties, their blue or green emission
being assigned to an essentially pure ligand-based 1(p–p*)
transition.[17] The binuclear compounds exhibited good electro-
luminescent properties and were successfully used as the emit-
ting layer of non-doped OLEDs. This inspired us to synthesize
a new series of polynuclear organoboron compounds having
different spacers and thus p-conjugation lengths.
Herein, we report a new series of highly emissive linear and
star-shaped bi- and trinuclear organoboron compounds bear-
ing aromatic bis- or tris(iminopyrrolyl) spacer ligands and their
characterization by multinuclear NMR spectroscopy and cyclic
voltammetry. The photophysical characterization of the newly
synthesized complexes was performed by using steady-state
and time-resolved luminescence techniques in solution. Densi-
ty functional theory (DFT) and time-dependent DFT (TDDFT)
calculations were also carried out for these new boron com-
plexes to determine the geometry of the ground and first ex-
cited states, to assign the electronic transitions, and to try to
rationalize the luminescence behavior exhibited. Further, these
compounds were utilized in OLED devices as the emitting
layer, with some of them showing high luminance values.
2-Formiminopyrrole ligands are an important class of biden-
tate chelating ligand precursors in coordination and organo-
metallic chemistry, their complexes having a wide range of ap-
plications in various organic transformations.[13] Steric and elec-
tronic tuning through the N-aryl substituents in the imine
group is easily achieved by using a condensation reaction (see
below, I)[14] and the p-conjugation length of the molecule can
be readily altered by incorporating bridging aromatic spacers
(II),[14b,15] or by fusing aromatic groups on the edges of the pyr- Results and Discussion
role ring (III and IV; Ar=aromatic group),[14a,16] making these li-
Syntheses and characterization of bis- and tris(iminopyrrole)
ligand precursors
gands very versatile.
Earlier, our group reported the 2-formylphenanthro[9,10-
c]pyrrole ligand precursor (III), in which the p-conjugation was
extended by fusing the phenanthrene ring on the C3ꢀC4
bond, and their homoleptic zinc(II) complexes of the type
[Zn{k2N,N’-2-(N-arylformimino)phenanthro[9,10-c]pyrrolyl}2].[14a]
The linear bis(iminopyrrole) ligand precursors 2–6 have been
synthesized and characterized according to the literature
methods,[18] whereas 7–9 are herein described for the first time
(Scheme 1). The star-shaped tris(iminopyrrole) ligand precursor
9 was synthesized from its corresponding triamine.[19] In gener-
al, the linear bis(iminopyrrole) ligand precursors, 1,2-(HNC4H3-
C(H)=N)2-C6H4 (2), 1,3-(HNC4H3-C(H)=N)2-C6H4 (3), 1,4-(HNC4H3-
C(H)=N)2-C6H4 (4), 4,4’-(HNC4H3-C(H)=N)2-(C6H4-C6H4) (5), 1,5-
(HNC4H3C-(H)=N)2-C10H6 (6), 2,6-(HNC4H3C-(H)=N)2-C10H6 (7), 2,6-
(HNC4H3C-(H)=N)2-C14H8 (8), and the star-shaped 1,3,5-(HNC4H3-
1
These complexes showed mainly a ligand-based (p–p*) emis-
sion in the blue–green spectral region with low fluorescence
efficiencies (f =3.9 and 8.8%, for aryl=2,6-iPr2C6H3 and C6H5,
f
respectively). However, the extended p-conjugation played
a vital role in improving the quantum yields because the
simple iminopyrrolyl zinc analogues of the type [Zn{k2N,N’-2-
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3·MeOH, and
7
show planar
backbones for both iminopyrrol-
yl fragments, and each of their
dihedral angles with their corre-
sponding
planes are of ꢀ45.9(10) and
ꢀ17.6(12), ꢀ36.4(11), and
aromatic
bridge
ꢀ37.4(11)8 for 3 (molecules A
and B, respectively), ꢀ60.33(15)
and 25.33(17)8 for 3·MeOH, and
48.5(3)8 for 7 (half of the mole-
cule generated by symmetry).
All the remaining structural fea-
tures presented by these com-
pounds are similar to other imi-
nopyrrole molecules reported
previously.[13gꢀi,k,14b,18e,f,20]
Synthesis and characterization
of bis- and tris(iminopyrrolyl)
boron complexes
The treatment of one equivalent
of bis(iminopyrrole) 2–8 and tris-
(iminopyrrole) 9 ligand precur-
sors with, respectively, two or
three equivalents of triphenyl
boron (BPh3) in toluene heated
at reflux, overnight, under a ni-
trogen atmosphere, afforded
the corresponding linear binu-
clear organoboron compounds
Ph2B[1,3-bis(iminopyrrolyl)-phe-
nyl]BPh2 (10), Ph2B[1,4-bis(imino-
pyrrolyl)-phenyl]BPh2
(11),
Ph2B[4,4’-bis(iminopyrrolyl)-bi-
phenyl]BPh2 (12), Ph2B[1,5-bis-
Scheme 1. Synthesis of the bis- and tris(iminopyrrole) ligand precursors 2–9.
(iminopyrrolyl)-naphthyl]BPh2
C(H)=N-1,4-C6H4)3-C6H3 (9) were synthesized by the condensa-
tion reactions of 2-formylpyrrole (1) with the corresponding
aryl diamines, that is, o-phenylenediamine, m-phenylenedia-
mine, p-phenylenediamine, 4,4’-biphenylenediamine, 1,5-naph-
thalenediamine, 2,6-naphthalenediamine, 2,6-anthracenedia-
mine, and 4’,4’’,4’’’-triamino-1,3,5-triphenylbenzene employing
standard reaction conditions.[13g,h] The syntheses and purifica-
tion of these ligand precursors are in general straightforward
(except for the case of 8), being obtained mostly in good
(13), Ph2B[2,6-bis(iminopyrrolyl)-naphthyl]BPh2 (14), Ph2B[2,6-
bis(iminopyrrolyl)-anthracenyl]BPh2 (15), and the trinuclear
star-shaped compound [4’,4’’,4’’’-tris(iminopyrrolyl)-1,3,5-triphe-
nylbenzene](BPh2)3 (16) in good to average yields (Scheme 2).
However, attempts to isolate the desired Ph2B[1,2-bis(iminopyr-
rolyl)-phenyl]BPh2 from the reaction of ligand precursor 2 with
BPh3 led to uncharacterized products and continuous decom-
position in solution, probably because of the proximity, and
possible further reactivity, between the iminopyrrolyl boron
moieties. The iminopyrrolyl organoboron compounds 10–16
1
yields. Their characterization by H and 13C NMR spectroscopies
1
are consistent with the literature values.
were characterized by using H, 13C, and 11B NMR spectroscop-
Crystals suitable for single-crystal X-ray diffraction were ob-
tained for the ligand precursors 3, 3·MeOH, and 7. Perspective
views of the molecular structures of these ligand precursors
are depicted in Figures S1–S6 in the Supporting Information.
Selected bond lengths and bond angles are given as captions
in the corresponding Figures. The details of the crystal struc-
ture determinations are given in Table S1 in the Supporting In-
formation. The molecular structures of ligand precursors 3,
ies, and elemental analyses. Although the binuclear com-
pounds 11 and 12 were already reported in our preliminary
communication,[15] they are included in this full paper for com-
parison with the other derivatives.
The 11B NMR spectra of the binuclear compounds show a sin-
glet in the range of d=3.90 to 6.22 ppm (relative to F3BDOEt2),
consistent with the formation of four-coordinate boron com-
pounds (resonances for three-coordinate boron are typically
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which upon exposure led to
slow decomposition with con-
comitant regeneration of the
free ligand precursor. The syn-
thesized iminopyrrolyl organo-
boron compounds are soluble in
dichloromethane and tetrahy-
drofuran, moderately soluble in
toluene, and insoluble in diethyl
ether and n-hexane.
Compounds 10–16 are nor-
mally obtained as amorphous
powders, and many attempts to
obtain crystals suitable for
single-crystal X-ray diffraction by
using different crystallization
techniques failed. An exception
was complex 11, as an n-hexane
solvate (11·C6H14), which crystal-
lized in the monoclinic system,
C2/c space group, with half mol-
ecule of both compounds, 11
and n-hexane, generated by the
1
symmetry operations
= ꢀx,
2
1
= ꢀy, ꢀz, and 1ꢀx, y, 1.5ꢀz, re-
2
spectively (see Figure S7 in the
Supporting Information). A per-
spective view of the molecular
structure of this iminopyrrolyl
boron complex is shown in
Figure 1 (and Figure S8 in the
Supporting Information). Select-
ed bond lengths and bond
angles are given in the corre-
sponding Figure caption.
The molecular structure of
compound 11·C6H14 is very simi-
lar to that reported previously[15]
for compound 11 as a dichloro-
methane solvate. The boron
center in each moiety displays
a distorted tetrahedral geome-
try. Each iminopyrrolyl fragment
is chelating one boron center
through two nitrogen atoms,
with the consequent formation
of a five-membered ring with
a
bite angle (N1-B1-N2) of
97.4(6)8. The average B1ꢀCPh
distance is 1.647(11) ꢅ, whereas
Scheme 2. Synthesis of bi- and trinuclear organoboron complexes 10–16.
those of B1ꢀN1(pyrrolyl) and
downfield-shifted, appearing above dꢁ +25 ppm). The ab-
B1ꢀN2(imine) are 1.550(12) and 1.608(11) ꢅ, respectively. The
iminopyrrolyl fragments coordinated to the boron center are
nearly planar, exhibiting a dihedral angle with the aromatic
bridge (defined as C6-N2-C7-C9) of 46.1(11)8, which is similar
to the corresponding angles observed in the 11·CH2Cl2 struc-
ture reported previously (ꢀ47.21(17) and 47.2(3)8).[15]
1
sence of an NH proton broad resonance in the H NMR spectra
confirms the formation of the complexes. The CH=N protons
appear in the 1H NMR spectra as singlet resonances in the
range of d=8.20–8.67 ppm. These binuclear organoboron
compounds are slightly air and moisture-sensitive in solution,
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functional, see the Computational details below), as was dis-
cussed in previous works in which several crystal structures
were available.[14b,15,17] As mentioned above in the description
of the structure of 11·C6H14, the bond lengths around boron
do not vary significantly, the dihedral angle over the aromatic
bridge of the iminopyrrolyl fragment being the relevant struc-
tural parameter. To enable a comparison of the results, the
same methodology as before was used. The geometries of
both the ground state and first excited singlet state (obtained
by promoting one electron from the HOMO to the LUMO)
were fully optimized without any symmetry constraints.
The ground state geometries were modelled after the avail-
able X-ray structures[15,17] and were used as an estimate to opti-
mize the excited state geometries. The optimized structures
are depicted in Figure 2, emphasizing the Cimino-Nimino-Carylꢀipso
-
Carylꢀortho dihedral angle that characterizes the planarity of the
iminopyrrolyl ligand. The calculated angle is very close to 25–
278 for all complexes except 13, in which it reached ꢁ528 in
both sides of the molecule, and 10, which was 25.04 and
ꢀ32.568 in each side. The value of 26.078 calculated for 11 is
smaller than the experimentally determined 46.1(11)8 for
11·C6H14 and ꢀ47.21(17) and 47.2(3)8 for the previously report-
ed 11·CH2Cl2,[15] respectively, probably because of the intermo-
lecular interactions between adjacent diboron molecules and
the co-solvate (see Figure S3 in Supporting Information of
Ref. [15]), as observed previously.[15] This complex is perfectly
symmetric, although no symmetry was considered in the calcu-
lations, and the bond lengths are all within 0.05 ꢅ of the ex-
Figure 1. Perspective view of the molecular structure of 11. The ellipsoids
were drawn at 50% probability level. All the hydrogen atoms and the n-
hexane solvent molecule have been omitted for clarity. Selected bond
lengths [ꢅ]: N1ꢀC2 1.383(9), N1ꢀC5 1.331(9), N2ꢀC6 1.279(9), N2ꢀC7
1.416(9), N1ꢀB1 1.550(12), N2ꢀB1 1.608(11), C10ꢀB1 1.645(12), C16ꢀB1
1.649(11); selected bond angles [8]: N2-B1-C10 110.0(7), N2-B1-C16 111.9(6),
N1-B1-N2 97.4(6), N1-B1-C10 111.8(7), N1-B1-C16 111.6(7), C10-B1-C16
113.1(7), C6-N2-C7-C9 46.1(11).
Molecular geometries and electronic structures
The molecular geometries of the boron complexes may be
well reproduced by DFT[21] calculations (ADF program,[22] BP86
Figure 2. Optimized geometries of the iminopyrrolyl boron complexes 10–16 in the ground state and in the first excited singlet state, showing the dihedral
angles [8].
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perimental ones. In other com-
plexes there is a small asymme-
try between the two sides of
the molecule, which becomes
ꢁ7.58 in 10. There are two
kinds of spacers and the C-C-C-C
dihedral angle in the flexible
ones (12 and 16) adjusts to
a value between 31 and 358.
Therefore, the behavior of 13 is
unusual and steric constraints
seem to be the cause.
The geometries of the first ex-
cited singlet states are related
to the photoluminescence prop-
erties of these species (see the
next section). They were also
calculated, as described above,
by promoting one electron from
the HOMO to the LUMO. The
general trend is a narrowing of
Figure 3. Energies [eV] and three-dimensional representations of the HOMOs and LUMOs of the complexes 10–17
(gas phase).
the C-N-C-C dihedral angle, reaching values lower than 98 (an-
other methodology for determining this structure with
TDDFT[23] leads to values lower than 18; see below) in both
sides of the molecule. However, compound 10 again behaves
differently, with 5.09/ꢀ26.808 (or ꢀ1.25/32.62 in TDDFT), lead-
ing to a very asymmetric excited state. Similarly, the star-
shaped 16 also displays one almost planar moiety (3.528),
whereas the other two keep the geometry observed in the
ground state. As expected, compound 13 remains different. Al-
though the two dihedral angles drop to about half, they are
still very high (26.18/ꢀ26.40 and 37.28/ꢀ30.738 in TDDFT). As
observed before,[14b,15,17] rotation of the iminopyrrolyl boron di-
phenyl fragments (about the Nimino-Carylꢀipso bond) in relation to
the aryl bridging groups, toward coplanarity, is a key feature of
these molecules, which helps them to reach maximum p-de-
localization in the excited state.
The HOMOs and LUMOs of the compounds 10–17 are
shown, together with their relative energies, in Figure 3. The
nature of the frontier orbitals is responsible for the geometry
trends discussed above. The two complexes 10 and 16, with
asymmetric excited states, have the HOMO localized in one
part of the molecule and the LUMO in the other one, whereas
the other species which exhibit symmetric singlet excited state
structures also display symmetric orbitals. Compound 13 has
a HOMO with a strong localization in the bridge, whereas the
LUMO has no contribution from it. The same tendency, though
with a small contribution in the LUMO, is observed in 12 and
14.
Figure 4. a) Absorption and b) fluorescence emission spectra of the boron
complexes 10–17, in THF, at 208C. The fluorescence spectra show very dif-
ferent band structures from unstructured (13) to very well vibronically re-
solved (11).
below).[14b,15] Relevant experimental absorption data, listed by
increasing order of absorption wavelength maxima, are sum-
marized in columns 3 and 4 of Table 1.
Photoluminescence properties
UV/Vis Absorption
The higher torsion angles of compound 13, because of the
considerable steric hindrance exerted by the hydrogen atoms
at positions 4 and 8 of the 1,5-naphthalene-di-yl bridge (adja-
cent to the imininopyrrolyl diphenylboron 1,5-substituents),
induce a blueshift in relation to the mononuclear compound
The UV/Vis solution absorption spectra of the polynuclear
boron complexes 10–16 are shown in Figure 4a along with
that of the previously reported parent mononuclear boron
complex [BPh2(k2N,N’-2-(N-phenylformimino)pyrrolyl)] 17 (see
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light elements, so the different results arise from the
all-electron basis set and the hybrid PBE0 functional.
As a consequence it is computationally much more
demanding.
Table 1. Experimental and calculated absorption data of complexes 10–17.[a]
max
abs
[b]
max
abs
max
abs
max
abs
max
abs
Complex Bridge
l
emax
E
(exp)
E
(GP)
E
(THF)
E
(SO)
[nm]
[eV]
[eV]
[eV]
[eV]
The calculated absorption maxima of the boron
complexes 10–17 are listed in columns 6, 7, and 8 of
Table 1. With two exceptions, the mononuclear 17
and the meta-phenylene bridge 10, the SO method
leads to the best reproduction of the absorption en-
ergies, with a maximum shift of 0.1 eV. The experi-
mental absorption maxima for 10 and 17 are well
described by the BP86 approach in gas phase. The
solvent effect is sometimes almost negligible (11 and
13), either changing the absorption energy in the
wrong direction (14–17), or correcting the gas phase
value (11 and 12).
The absorption maximum of the reference mono-
nuclear complex 17 is assigned essentially to an
intraligand HOMO to LUMO excitation, which is
a p!p* transition within the iminopyrrolyl group,
including the N-phenyl substituent.[17] The nature of
the frontier orbitals (and thus the transition) was
shown to depend on the substituents.[14b] The com-
plex with a 1,5-naphthalene-di-yl bridge (13) is the
13
372
4.0
3.33
3.10
3.11
3.24
17[c]
10
no bridge
383
400
1.7
5.1
3.24
3.10
3.29
3.20
3.34
2.96
3.48
3.40
16
403
7.0
3.08
2.84
2.74
3.18
12[c]
11[c]
419
428
4.7
3.0
2.96
2.90
2.34
2.47
2.41
2.52
2.98
2.93
14
15
430
447
3.4
3.3
2.88
2.77
2.36
2.11
2.28
2.04
2.88
2.64
[a] Experimental wavelength maximum (lmabasx), molar extinction coefficient (emax) and
energy (Emabasx) of the first absorption band, and calculated energy of the absorption
maximum using the BP86/GP, BP86/THF, and PBE0/SO methods (E
and E (SO), respectively). [b] 104 Lmolꢀ1 cmꢀ1. [c] Refs. [14b] and [15].
max
abs
max
abs
(GP), E
(THF),
max
abs
17.[15] Compounds 11, 12, 14, and 15 show the largest red-
shifts; the smaller redshift of 12 with respect to 11, 14, and 15
results from the larger values (and
only one with the low-energy band exhibiting a blueshift rela-
tive to the absorption of 17. In this species, the excitation lead-
ing to this low-energy absorption is 96% HOMO!LUMO+
1 (Figure 5 and Figure S9 in the Supporting Information), and
number) of dihedral angles be-
tween the aromatic moieties,
which counteract the redshift
effect of increasing the p-conjuga-
tion length from 11 to 12 or 14.
Compound 10 (meta-phenylene
bridged) shows a redshift in rela-
tion to the mononuclear 17, but
its absorption is blueshifted rela-
Figure 5. The frontier orbitals of Ph2B[1,5-bis(iminopyrrolyl)-naphthyl]BPh2
(13) involved in the low-energy HOMO!LUMO+1 transition.
tive to the para-phenylene bridged analogue 11, because of
a higher steric hindrance between the two iminopyrrolyl di-
phenyl boron moieties, leading to lower p-conjugation length.
The spectrum of the star-shaped trinuclear compound 16 is
only slightly shifted to the red in relation to its binuclear ana-
logue 10, again resulting from the existence of dihedral angles
between aromatic rings, which decrease the overall p-conjuga-
tion length of the molecule.
is also an intraligand charge transfer (ILCT) band, involving the
two iminopyrrolyl groups and the bridge (p!p*). The energy
is thus higher than in a HOMO!LUMO transition. Usually,
these intraligand transitions do not give rise to strong
emissions.
The molar extinction coefficients (column 4 of Table 1) in-
crease with the number of N-iminopyrrolyl units roughly dou-
bling or tripling with respect to 17 for the bi- and trinuclear
complexes, respectively, a trend that would be expected if the
chromophores were mutually independent (with no p-conju-
gation).
The binuclear species with boron atoms bridged by the 4,4’-
biphenylene (12) or the para-phenylene (11) were analyzed
before.[17] Their lowest-energy transitions are mainly HOMO!
LUMO and indeed they are observed at higher wavelengths
than in 17 as the HOMO–LUMO gap decreases (see Figure 3),
because of the stabilization of the LUMO. Complex 14 has
a bridge similar to 13, but binds the rest of the molecule by
different carbon atoms (2,6 instead of 1,5). The low-energy
band can be assigned to a HOMO!LUMO transition (97%)
(Figure 6 and Figure S10 in the Supporting Information). The
HOMO is delocalized over the iminopyrrolyl and the bridge
fragments, but the bridge practically does not contribute to
Absorption spectra were calculated for all the compounds
using a TDDFT[23] approach with the BP86 functional, as in ge-
ometry optimization, in both the gas phase (GP) and in THF, as
well as with the PBE0 functional within the SOPERT (SO) meth-
odology[24] (see the Computational details). This method, which
considers the spin-orbit coupling, is required to calculate the
excited state lifetimes. Spin-orbit coupling is not relevant in
Chem. Eur. J. 2015, 21, 1 – 18
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analogy occurs between 11 and 10, respectively, 1,4- and 1,3-
phenylene. This complex displays a wide band, calculated at
3.40 eV (SO), consisting of three intense excitations, starting
always in HOMO and HOMOꢀ1 and ending at LUMO and
LUMO+1 (Figure 7 and Figure S11 in the Supporting Informa-
tion). In contrast to the orbitals shown in Figures 5 and 6 (also
in those of 11, 12, and 17), which extend over the full biden-
tate ligand (two iminopyrrolyl and one bridge), the orbitals of
10 occupy only approximately one half of the ligand, so that
two pairs of occupied or empty orbitals are needed to describe
the transition. Note that this is one of the complexes with an
asymmetric singlet excited state. The different nature of the
low-energy absorption band may be responsible for the differ-
ent photophysical properties.
Figure 6. The frontier orbitals of Ph2B[2,6-bis(iminopyrrolyl)-naphthyl]BPh2
(14) involved in the low-energy HOMO!LUMO transition.
the LUMO. This is not a pure IL transition as described for 13
(Figure 4) and suggests that emission will be more likely.
These results show how the nature of the bridge influences
the nature of the low-energy transition leading to the very dif-
ferent colors and properties of isomers of 13 and 14. A similar
Fluorescence emission
The fluorescence spectra of all polynuclear boron complexes
(Figure 4b), including 13, are redshifted up to 68 nm with re-
spect to 17 (columns 3 and 4 of Table 2).
The difference between the wavelengths of the 0!0 vibron-
ic transition (le0mꢀ0) and absorption maximum (lmabasx) (calculated
from Tables 1 and 2) is substantially larger for 13 (124 nm)
than for the remaining compounds (68–84 nm; Figure 4), indi-
cating a large conformational change occurring between
ground and first excited states, because of a significant de-
crease in the N-iminopyrrolyl-naphthyl-N-iminopyrrolyl torsion-
al angles. Indeed, as referred, the two C-N-C-C dihedral angles
drop from 52.31/ꢀ51.92 to 26.18/ꢀ26.408. The observed loss
of vibrational structure in the emission spectrum of 13 is con-
sistent with the frustrated planarization attempt of the
molecule.
Figure 7. The frontier orbitals of Ph2B[1,3-bis(iminopyrrolyl)-phenyl]BPh2 (10)
involved in the low-energy transition (HOMO/HOMOꢀ1!LUMO/LUMO+1).
Table 2. Experimental and calculated fluorescence data of complexes 10–17.[a]
0-0
max
em
[c]
[d]
[e]
max
em
Complex
Bridge
l
l
f
f
tf
kf (exp)[b]
fT
knr
kisc
kic
Ememax(exp)
[eV]
E
(TDDFT)
kf (SO)
em
[nm]
[nm]
[ns]
[nsꢀ1
]
[nsꢀ1
]
[nsꢀ1
]
[nsꢀ1
]
[eV]
[nsꢀ1
]
13
496
531
0.16
1.4[f]
0.12
0.025
0.01
0.65
0.02
0.63
2.33
2.94
0.21
17[g]
10
no bridge
451
468
479
468
0.34
0.43
1.9
0.18
0.16
0.35
0.21
0.01
–
0.34
–
2.59
2.65
2.94
2.82
0.22
0.11
2.7(5)
16
477
510
0.46
2.1(4)
0.21
0.25
–
–
2.43
3.28
nc[h]
12
11
496
512
531
512
0.64
0.69
1.9(4)
2.2(2)
0.33
0.31
0.19
0.14
–
–
2.33
2.42
2.97
3.02
0.55
0.40
ꢁ0
ꢁ0
0.14
14
15
507
519
541
554
0.63
0.32
1.8
0.35
0.17
0.20
0.36
–
–
2.29
2.24
2.98
2.78
nc[h]
nc[h]
1.8(6)
0.03
0.02
0.34
[a] Experimental wavelengths of the fluorescence maximum (lemmax) and first vibronic S1!S0 transition (l0-0); fluorescence quantum yield (f), lifetime (tf) and
f
em
triplet formation quantum yield (fT); rate constants of: fluorescence (kf (exp)), intersystem crossing (kisc) and internal conversion (kic); sum of non-radiative
max
em
rate constants (knr); experimental energy maxima (Eemmax (exp)), in THF, at 293 K; calculated (TDDFT) energy maxima (E
(TDDFT)) and calculated fluores-
cence rate constants (kf (SO)). [b] kf =f/tf; [c] knr =(1ꢀf)/tf; [d] kisc =fT/tf; [e] kic =knrꢀkisc. [f] From double exponential decays with t1 =1.4 ns (0.9) and
f
f
t2 =0.35 ns (0.1). [g] Refs. [14b] and [15]. [h] Not calculated.
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The binuclear compound 10 (meta-phenylene-bridged) is
a blue-emitter, whereas 11 (para-phenylene bridged) emits in
the green region, this shift being assigned to an approximately
planar excited-state conformation of the latter complex.[17] The
excited state of 10 displays an asymmetric geometry, in which
one of the aryl-bridge-iminopyrrolyl groups becomes planar
and the second keeps the dihedral angle at ꢀ26.808. Thus, the
p-extension of 10 in the excited-state is similar to that of the
mononuclear boron complex 17, by this reason exhibiting
a hypsochromic shift in relation to 11. The trinuclear com-
pound 16 also emits in the blue-green region and also displays
an asymmetric excited state geometry, in which only one of
the three aryl-bridge-iminopyrrolyl groups became planar.
Therefore, compound 16 also behaves as a boron mononu-
clear compound having a 1,1’-biphenyl imino substituent.
Complexes 10 and 16 are the only ones in which just a single
aryl-bridge-iminopyrrolyl boron moiety achieves planarity in
the singlet excited state.
The calculated radiative rate constant kf values (column 14
of Table 2) were obtained from the excited state lifetimes
given by the SO approach and in general give a relative good
agreement with the experimental ones.
In summary, the use of different aromatic spacers with in-
creasing p-conjugation length between two or more iminopyr-
rolyl diphenylboron moieties shifts the emission maxima to
lower energies and increases the fluorescence quantum yield.
This enables the color tuning of this polynuclear molecular
system in the range of blue to yellow (see Figure S12 in the
Supporting Information).
Electroluminescent properties and light-emitting devices
Cyclic voltammetry measurements were used to determine the
ionization potentials (IP) and electron affinities (EA) of com-
pounds 10–17 to establish the electron- and hole-injection
barriers from the corresponding electrodes in the LED struc-
tures. These studies were carried out in dichloromethane solu-
tions with tetrabutylammonium tetrafluoroborate or tetrabuty-
lammonium perchlorate as electrolyte salt, at room tempera-
ture and under inert atmosphere (N2). IP and EA were obtained
from the oxidation and reduction onset potentials, after being
converted to the absolute scale using the Fc/Fc+ (ferrocene/
ferrocenium ion redox couple) as external reference.[15] The
values obtained are summarized in Table 3, along with the en-
ergies of the HOMOs and LUMOs of the complexes 10–17 cal-
culated by using DFT. As expected, the values of IP correlate
well with those of the energies of the HOMOs (except for com-
pound 13, which shows the peculiar structural features de-
scribed above), with the IP values differing between 0.2 and
0.4 eV from the calculated ones (see Figure S13a in Supporting
Although the structural features of the ground and singlet
excited states correlate well with the shifts in emission relative
to the reference mononuclear compound 17, the calculation of
emission energies remains difficult (Table 2 and Table S2 in
Supporting Information). There is a large difference between
the energy of the excited state and the energy of the corre-
sponding ground state with the same geometry (column 4 of
Table S2), and the emission energy is underestimated for all
the complexes. The consideration of the solvent effect (THF,
column 5 of Table S2) corrects the values in the right direction,
but in very small amounts, the estimation remaining poor. A
TDDFT methodology (column 13 of Table 2 or column 6 of
Table S2) for optimizing the first singlet excited state and ob-
taining directly the emission energy overestimates it, but gives
a good approximation for compound 10 (2.82 eV
compared with the experimental 2.65 eV).
The fluorescence quantum yields (column 5 of
Table 3. Ionization potentials (IP), electron affinities (EA), and IPꢀEA values of com-
plexes 10–17, estimated from cyclic voltammetry measurements, and corresponding
energies of HOMOs and LUMOs, determined by DFT (THF).
Table 2) increase more or less parallel to the absorp-
tion wavelength and the extent of delocalization,
except for 15, reaching a respectable value of 0.69
for 11. Inspection of columns 7 and 9 of Table 2
shows that such increase results from both the in-
crease in the radiative rate constant kf and the de-
crease of the sum of non-radiative rate constants knr
(sum of internal conversion, kic, and intersystem
crossing, kisc), from top to bottom. Compound 15
shows smaller kf and larger knr values than the best
emitters. The lower value of kf likely results from sub-
stitution at positions 2 and 6 of the anthracene
bridge instead of the 9 and 10 positions that are
Cyclic voltammetry
DFT
Complexes
Bridge
IP
[eV]
EA
[eV]
IPꢀEA
HOMO
[eV]
LUMO
[eV]
[eV]
13
4.95
2.73
2.22
ꢀ5.32
ꢀ2.87
17[a]
10
no bridge
5.64
5.64
2.82
3.12
2.82
2.52
ꢀ5.41
ꢀ5.43
ꢀ2.88
ꢀ3.05
16
5.58
3.21
2.37
ꢀ5.33
ꢀ3.00
1
aligned with the lowest 1Bu! Ag transition dipole
moment of anthracene.[25]
12[a]
11[a]
5.53
5.50
3.23
3.44
2.30
2.06
ꢀ5.22
ꢀ5.20
ꢀ3.07
ꢀ3.12
To clarify the variation of knr values, triplet forma-
tion quantum yields were evaluated for representa-
tive compounds 13, 11, and 15 (column 8 of Table 2).
As found with other related boron complexes,[14b] the
results show that intersystem crossing is unimportant
(kisc !kic), that is, knr is approximately equal to the
rate constant of internal conversion, kic.
14
5.49
5.39
3.27
2.93
2.22
2.46
ꢀ5.11
ꢀ5.02
ꢀ3.12
ꢀ3.20
15
[a] Ref. [15].
Chem. Eur. J. 2015, 21, 1 – 18
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Information). Instead, the EA values show a much larger differ-
ence from the calculated LUMO energies (see Figure S13b in
the Supporting Information), which can be explained by the
fact that the calculated LUMO refers to the neutral species,
whereas EA refers to the stability of the anionic species.
(having a higher work function of ꢀ2.7 eV) should not change
the device performance significantly. We believe that the effect
is likely due to differences in film properties, though we do
not have a clear explanation for it. Sublimation of the com-
pounds (except for 16 (decomposes) and 17 (not tested)) led
to devices with better performance than those prepared by
spin coating. Some of the compounds led to films with poor
quality, sometimes evidencing the presence of small aggre-
gates. These are expected to have a detrimental effect on EL
properties, because exciton quenching is favored when com-
paring with amorphous films. We therefore conclude that, al-
though films with good to reasonable quality were obtained
by spin coating, the sublimation leads to better performing de-
vices. LEDs based on sublimed 12 leads to a maximum lumi-
nance of 2439 cdmꢀ2 with an efficiency of 0.38 cdAꢀ1. Surpris-
ingly, whereas the luminance is higher than that of the LED
prepared by spin coating, the EL efficiency is lower. Complex
15 is the compound that leads to a more significant improve-
ment of performance when changing from spin coating to
sublimation. This may be related to the presence of the anthra-
cenyl moiety, a large p system that can induce significant
stacking.
Light-emitting diodes were fabricated by using films pre-
pared either by spin coating, with structure ITO/PEDOT:PSS/
boron-complex/Ba or Ca/Al, or by sublimation, ITO/PEDOT:PSS/
boron-complex/Ba/Al. It was found that compound 13 is not
soluble enough to prepare films with adequate thickness for
use in LEDs and 16 could not be sublimed. Devices were also
prepared with a hole-transporting/electron-blocking layer of
TPD deposited by sublimation, prior to the sublimation of the
boron complexes (ITO/PEDOT:PSS/TPD/boron-complex/Ba/Al).
The results obtained for the various device structures are
shown in Table 4. This Table includes the previously reported
results for 11,[15] 12,[15] and 17,[14b,15] and additional data for de-
vices based on 11 and 12.
For the single-layer devices with Ca cathodes, the maximum
luminance (958 cdmꢀ2) and external quantum efficiency
(EQEmax =0.084%) was achieved with 11, whereas the mini-
mum (0.35 cdmꢀ2) was achieved with 17, for which an EQEmax
of 1.5ꢆ10ꢀ4 % was obtained. The use of Ba, instead of Ca, only
in two cases (10 and 12) led to significant increase of lumi-
nance and efficiency, reaching a maximum luminance of
2000 cdmꢀ2 and EQEmax =0.145% for 12, this being the best
performing device in the series, prepared by spin coating.
Since Ca (work function of ꢀ2.9 eV) already ensures an ohmic
contact concerning electron injection, its substitution by Ba
The insertion of TPD creates a small hole-injection barrier at
its interface with PEDOT:PSS of about 0.2 eV, and, by having
a LUMO at about ꢀ2.3 eV, acts as a barrier against electron
leakage, thereby improving the charge balance within the
active layer and the probability for electron-hole recombina-
tion.
Table 4. Electroluminescent properties (maximum values of luminance, Lmax [cdmꢀ2], luminous efficiency, fEL,max [cdAꢀ1], and quantum efficiency, EQEmax
[%]). Thickness of the active layer: 60–80 nm. TPD thickness is 20 nm and that of PEDOT:PSS is 40 nm.
Complex
Bridge
Spin coating (Ca)
Spin coating (Ba)
Sublimed (Ba)
TPD/Subl/Ba
L
f
max =109
EL,max =0.14
EQEmax =0.047
L
f
max =83
EL,max =0.65
EQEmax =0.23
13
17
no bridge
[a]Lmax =0.35
[a]fEL,max =3.8ꢆ10ꢀ4
[a]EQEmax =1.5ꢆ10ꢀ4
L
f
max =6.7
EL,max =0.0055
EQEmax =0.002
L
f
max =14
EL,max =0.0146
EQEmax =0.005
L
f
max =737
EL,max =0.261
EQEmax =0.10
L
f
max =67
EL,max =0.0264
EQEmax =0.010
10
16
L
f
max =64
EL,max =0.077
L
f
max =53
EL,max =0.015
EQEmax =0.051
EQEmax =0.052
[a]Lmax =844
L
f
max =2000
EL,max =0.53
EQEmax =0.145
L
f
max =2439
EL,max =0.38
EQEmax =0.10
max =233
EL,max =0.12
EQEmax =0.030
max =658
EL,max =0.11
EQEmax =0.0286
max =410
EL,max =0.086
EQEmax =0.034
L
f
max =3916
EL,max =0.70
EQEmax =0.19
max =1212
EL,max =0.36
EQEmax =0.095
max =4370
EL,max =1.36
EQEmax =0.36
max =221
EL,max =0.039
EQEmax =0.016
12
11
14
15
[a]fEL,max =0.19
[a]EQEmax =0.052
[a]Lmax =958
L
f
L
f
[a]fEL,max =0.30
[a]EQEmax =0.084
L
max =69
EL,max =0.0094
EQEmax =0.0025
max =13
EL,max =0.0031
EQEmax =0.0023%
L
max =57
EL,max =0.0030
EQEmax =0.0008
max =14
EL,max =0.0046
EQEmax =0.0034
L
f
L
f
f
f
L
f
L
f
L
f
L
f
[a] Previously reported.[15]
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Figure 8. a) Hole- (Dh) and electron- (De) injection barriers for the various
boron complexes, defined as the energy offset at PEDOT:PSS/IP and EA/Ca
interfaces, respectively; b) Relation between EL quantum efficiency of the
various single layer devices and solution PL quantum efficiencies (f).
f
Figure 8a (and Figure S14a in the Supporting Information)
shows the electron- and hole-injection barriers for the single-
layer devices, considering the work function of PEDOT:PSS as
ꢀ5.2 eV, that of Ca as ꢀ2.9 eV, and the IP and EA data shown
in Table 3 (though this data was obtained for solutions, it is as-
sumed that the values should be similar to solid state). The
data for 13 is not shown, because, as mentioned above, this
compound is not soluble enough to prepare films by spin
coating and was not used in devices with Ca cathodes. For the
other compounds, there is a small hole-injection barrier and
the electron-injection barrier is null. For the Ba-based devices,
no electron-injection barrier should exist for any of the com-
pounds. In general, with the exception of 13, the current flow-
ing through all single-layer devices is expected to be electron-
dominated.
Figure 9. Characteristics of the devices based on 14. a) Current (I) and lumi-
nance (L); and b) luminous efficiency as a function of the applied voltage.
Devices are identified according to the preparation method, spin coating
(“soln”) or sublimation (“sub”), the cathode material (Ca and Ba) and when
the hole-transporting layer of TPD is present; c) comparison between the
emission spectra of the devices, prepared by spin coating (noisy spectrum)
and by sublimation, with solution and film photoluminescence spectra.
tures based on 14 and their emission spectra. We observed
that, in general, the fluorescence spectra for films overlap or
are redshifted with respect to those recorded for the solutions
(shown in Figure 4b). We attribute this redshift to intermolecu-
lar interactions in the solid state that tend to stabilize excited
states. EL spectra are either overlapping the solid state PL or
appear slightly blueshifted. This is indicative that the excited
state responsible for the EL emission is the same as that re-
sponsible for the PL. Interference effects within the device
(which behaves as an optical cavity) are likely the cause of the
slight differences between EL and solid-state PL spectra.
It was found that the insertion of the TPD layer improves
the maximum luminance and efficiency of the devices based
on complexes 11–14 and has a detrimental effect on the devi-
ces based on 10 and 15. In cases in which the TPD insertion
leads to a current reduction, the performance of the device im-
proved, evidencing a better electron-hole balance. The best
performing device was that based on 14, with a maximum lu-
minance of 4370 cdmꢀ2 and
a maximum efficiency of
1.36 cdAꢀ1. Figure 9 (and Figure S15 in the Supporting Infor-
mation) compares the performance of the various device struc-
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gas was supplied in cylinders by specialized companies (Air Liq-
uide) and purified by passage through 4 ꢅ molecular sieves. Unless
otherwise stated, all reagents were purchased from commercial
suppliers (e.g., Acros, Aldrich, Fluka) and used without further pu-
rification. All solvents to be used under an inert atmosphere were
thoroughly deoxygenated and dehydrated before use; they were
dried and purified by heating at reflux over a suitable drying agent
followed by distillation under nitrogen. The following drying
agents were used: Sodium (for toluene, diethyl ether, and tetrahy-
drofuran (THF)), and calcium hydride (for n-hexane and dichloro-
methane). Solvents and solutions were transferred by using a posi-
tive pressure of nitrogen through stainless steel cannula and mix-
tures were filtered in a similar way by using a modified cannula
that could be fitted with glass fiber filter disks.
NMR spectra were recorded on a Bruker Avance III 300 (1H, 13C and
11B) spectrometer. Deuterated solvents were dried by storage over
4 ꢅ molecular sieves and degassed by the freeze–pump–thaw
method. Spectra were referenced internally to residual protio sol-
vent (1H) or solvent (13C) resonances and are reported relative to
tetramethylsilane (d=0 ppm). 11B NMR spectra were referenced to
Et2O·BF3.[26] All chemical shifts are quoted in d (ppm) and coupling
constants given in Hz. Multiplicities were abbreviated as follows:
broad (br), singlet (s), doublet (d), triplet (t), quartet (q), heptet (h),
and multiplet (m). For air- and/or moisture-sensitive materials, sam-
ples were prepared in J. Young NMR tubes in a glovebox. Elemen-
tal analyses were obtained from the IST elemental analysis services.
Conclusion
Several polynuclear organoboron complexes containing 2-for-
miminopyrrolyl ligands with different p-conjugation lengths
were synthesized and characterized by multiple methods, ena-
bling the color tuning of this polynuclear molecular system in
the range of blue to yellow. By incorporating more than one 2-
formiminopyrrolyl diphenylboron moiety into a linear or star-
shaped central core, the photophysical properties of the mole-
cule can be greatly enhanced in relation to the parent mono-
nuclear one (compound 17). The 1,4-phenylene bridged binu-
clear complex 11, having longer p-conjugation length, shows
emission in the green region with an excellent fluorescence
quantum yield of 0.69, whereas the 1,3-phenylene bridged bi-
nuclear complex 10, being stereochemically more hindered
and thus possessing a shorter p-conjugation length, emits in
the blue region with a lower quantum yield of 0.43. DFT and
TDDFT calculations allowed the determination of the ground
and first excited singlet state geometries, and the classification
of the boron compounds in three groups. The excited state of
10 and 16 is characterized by its asymmetry, namely only one
iminopyrrolyl boron group becomes coplanar with the aryl
bridge, whereas the other retains the high dihedral angle, so
that these two compounds behave like the mononuclear refer-
ence complex, with a blue emission. Complex 13 exhibits large
dihedral angles on both sides of the molecule (ꢁ528 in the
ground state), which drop significantly in the excited state, but
stay far from 08 (ꢁ268), and emits with poor quantum yields.
The nature of the frontier orbitals and the low energy transi-
tions differ between these two types of compounds (10, 16,
and 13) and the third one (11, 12, 14, and 15). The first excited
singlet state geometries of the third type of boron complexes
show coplanarity between iminopyrrolyl groups and the aryl
bridge throughout the molecule, with an extended p-system,
with variable size. The LUMO is therefore stabilized leading to
a smaller HOMO–LUMO separation, which parallels the absorp-
tion and emission properties. The absorption maxima can be
well reproduced for all the compounds, with one of the meth-
odologies tested, but emission energies are more difficult to
reproduce and require more computationally demanding
approaches.
Syntheses
General procedure for bis(iminopyrrole) ligand precursors 2–6:
The bis(iminopyrrole) ligand precursors 1,2-(HNC4H3-C(H)=N)2-C6H4
(2),[18a] 1,3-(HNC4H3-C(H)=N)2-C6H4 (3),[18b] 1,4-(HNC4H3-C(H)=N)2-C6H4
(4),[18c] 4,4’-(HNC4H3-C(H)=N)2-C6H4-C6H4 (5),[18c] 1,5-(HNC4H3-C(H)=
N)2-C10H6 (6),[18d] were synthesized according to literature proce-
dures with a slight modification. In a round-bottom flask fitted
with a condenser and a CaCl2 guard tube, two equivalents of 2-for-
mylpyrrole, one equivalent of the corresponding aromatic diamine,
and a catalytic amount of p-toluenesulfonic acid were suspended
in absolute ethanol (20 mL). The reaction mixture was heated to
reflux overnight, turning yellow-orange, and allowed to cool to
room temperature. All the volatiles were removed and washed
with small amounts of CH2Cl2, and dried to obtain the correspond-
ing bis(iminopyrrole) ligand precursors in moderate to good yields
(50–78%).
2,6-(HNC4H3-C(H)=N)2ꢀC10H6 (7): In a round-bottom flask fitted
with a condenser and a CaCl2 guard tube, two equivalents of 2-for-
mylpyrrole, one equivalent of the naphthalene-2,6-diamine,[27] and
a catalytic amount of p-toluenesulfonic acid were suspended in ab-
solute ethanol (20 mL). The reaction mixture was heated to reflux
for 48 h, in the presence of molecular sieves, turning orange-
brown with concomitant precipitation of a dark-brown solid. The
mixture was allowed to cool to room temperature, filtered, and the
isolated bis(iminopyrrole) 7 was dried under vacuum. Yield 0.61 g
Non-doped EL devices were fabricated by using these com-
pounds as both emitter and ambipolar charge-transporting
materials, which exhibit high brightness and efficiency, reach-
ing luminances up to 2400 cdmꢀ2 and efficiencies of
0.38 cdAꢀ1 in single layer structures of 12. The insertion of
a TPD hole-transporting/electron-blocking layer between PE-
DOT:PSS and the active layer leads to luminances in the order
of 4400 cdmꢀ2 and efficiencies of 1.36 cdAꢀ1 (for 14).
1
(65%). H NMR (300 MHz, [D6]DMSO): d=11.82 (br, 2H), 8.48 (s, 2H,
CH=N), 7.92 (d, 3JHH =9 Hz, 2H, naphthyl), 7.63 (s, 2H, naphthyl),
3
7.46 (d, JHH =9.0 Hz, 2H, naphthyl), 7.07 (s, 2H, pyrr), 6.76 (s, 2H,
pyrr), 6.25 ppm (s, 2H, pyrr); 13C{1H} NMR (300 MHz, [D6]DMSO): d=
150.4, 149.0, 131.9, 130.8, 128.6, 124.0, 121.9, 117.0, 116.6,
109.8 ppm; elemental analysis calcd (%) for C20H16N4: C 76.90, H
5.16, N 17.94; found C 76.55, H 5.53, N 18.24.
Experimental Section
General procedures
All experiments dealing with air- and/or moisture-sensitive materi-
als were performed in an inert atmosphere by using a dual
vacuum/nitrogen line and standard Schlenk techniques. Nitrogen
2,6-(HNC4H3-C(H)=N)2ꢀC14H8 (8): In a round-bottom flask fitted
with a condenser and a CaCl2 guard tube, two equivalents of 2-for-
mylpyrrole, one equivalent of the anthracene-2,6-diamine,[28] and
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a catalytic amount of p-toluenesulfonic acid were suspended in ab-
solute ethanol (20 mL). The reaction mixture was heated to reflux
for 48 h, in the presence of molecular sieves, turning reddish-
brown with concomitant precipitation of a brown solid. The mix-
ture was allowed to cool to room temperature and filtered, the re-
sulting brown powder being dried under vacuum. After several
washings with n-hexane, diethyl ether, and toluene, the product
still contained a substantial amount of impurities. Due to its insolu-
bility in current organic solvents, recrystallizations were not carried
out. Therefore, it was impossible to determine the yield and fully
characterize this bis(iminopyrrole) ligand precursor 8, although the
(300 MHz, CD2Cl2): d=8.42 (s, 2H, CH=N), 7.17 (s, 22H, BPh2 +pyrr),
3
7.15 (s, 4H, phenyl), 7.02 (d, JHH =4.2 Hz, 2H, pyrr), 6.53 ppm (dd,
4
3JHH =4.2 Hz, JHH =2.1 Hz, 2H, pyrr); 13C{1H} NMR (75 MHz, CD2Cl2):
d=150.0, 145.8, 140.8, 134.5, 133.2, 131.9, 127.7, 126.8, 122.7,
117.8, 115.7 ppm; 11B NMR (96.29 MHz, CD2Cl2): d=5.48 ppm; ele-
mental analysis calcd (%) for C40H32B2N4: C 81.38, H 5.46, N 9.49;
found: C 81.28, H 5.11, N 8.95.
4,4’-[Ph2B(k2N,N’-NC4H3-C(H)=N)]2-(C6H4-C6H4) (12): In the same
manner as described above, a mixture of 5 (0.200 g, 0.59 mmol)
and BPh3 (0.286 g, 1.18 mmol) afforded 12 as a yellow solid. Yield:
0.346 g (86%). 1H NMR (300 MHz, CD2Cl2): d=8.60 (s, 2H, CH=N),
7.50 (d, 3JHH =9.0 Hz, 4H, phenyl), 7.38–7.20 (m, 26H, BPh2 +
1
expected resonances were present in the H NMR spectrum along
3
4
with other impurities. This led us to use the crude product without
further purification in the subsequent reaction with BPh3 (see
below). 1H NMR (400 MHz, [D6]DMSO): d=11.96 (br, 2H), 8.30 (s,
2H, anthracenyl), 7.98 (s, 2H, CH=N), 7.75 (s, 2H, anthracenyl), 7.47
phenyl+pyrr), 7.09 (dd, JHH =3.9, JHH =0.9 Hz, 2H, pyrr), 6.60 ppm
3
4
(dd, JHH =3.9, JHH =1.8 Hz, 2H, pyrr); 13C{1H} NMR (75 MHz, CD2Cl2):
d=149.9, 141.4, 138.4, 134.4, 133.1, 131.5, 127.9, 127.5, 127.4,
126.6, 122.3, 117.4, 115.3 ppm; 11B NMR (96.29 MHz, CD2Cl2): d=
3.90 ppm; elemental analysis calcd (%) for C46H36B2N4: C 82.90, H
5.44, N 8.41; found: C 82.45, H 5.30, N 8.16.
3
3
(d, JHH =7.6 Hz, 2H, anthracenyl), 7.21 (s, 2H, pyrr), 7.11 (d, JHH
=
3
7.6 Hz, 2H, anthracenyl), 7.00 (d, 2H, JHH =6.4 Hz, pyrr), 6.27 ppm
(s, 2H, pyrr).
1,5-[Ph2B(k2N,N’-NC4H3-C(H)=N)]2-C10H6 (13): In the same manner
as described above, a mixture of 6 (0.154 g, 0.49 mmol) and BPh3
(0.240 g, 0.99 mmol) afforded 13 as a yellow solid. Yield: 0.244 g
(77%). 1H NMR (300 MHz, CD2Cl2): d=8.24 (s, 2H, CH=N), 7.38–
7.30 (m, 2H, naphthyl), 7.28–7.04 (m, 24H, BPh2 +naphthyl), 6.97–
1,3,5-(HNC4H3ꢀC(H)=N-1,4-C6H4)3ꢀC6H3 (9): Three equivalents of
2-formylpyrrole, one equivalent of 4’,4’’,4’’’-triamino-1,3,5-triphenyl-
benzene,[19] and a catalytic amount of p-toluenesulfonic acid were
suspended in absolute ethanol (20 mL) in a round-bottom flask,
fitted with a condenser and a CaCl2 guard tube. The mixture was
heated to reflux overnight turning to a yellow solution. The mix-
ture was allowed to cool and all volatiles were removed. The prod-
uct was recrystallized from hot ethanol and stored at ꢀ208C to
3
4
6.95 (m, 4H, pyrr), 6.73 ppm (dd, JHH =3.9, JHH =1.8 Hz, 2H, pyrr);
13C{1H} NMR (75 MHz, CD2Cl2): d=155.7, 137.9, 134.1, 132.9, 132.0,
131.5, 130.1, 127.3, 126.6, 125.2, 122.8, 122.5, 117.0, 115.1 ppm;
11B NMR (96.29 MHz, CD2Cl2): d=6.22 ppm; elemental analysis
calcd (%) for C44H34B2N4: C 82.52, H 5.50, N 8.75; found: C 82.76, H
5.47, N 8.56.
1
give 9 as a yellow powder. Yield 1.47 g (72%). H NMR (300 MHz,
CDCl3): d=8.34 (s, 3H, CH=N), 7.03 (br, 3H, pyrr), 6.74 (brs, 3H,
pyrr), 6.34 ppm (br, 3H, pyrr); elemental analysis calcd (%) for
C39H30N6: C 80.39, H 5.19, N 14.42; found: C 80.32, H 5.34, N 14.34.
2,6-[Ph2B(k2N,N’-NC4H3-C(H)=N)]2-C10H6 (14): In the same manner
as described above, a mixture of 7 (0.317 g, 1.02 mmol) and BPh3
(0.484 g, 2.0 mmol) afforded 14 as a yellow solid. Yield: 0.470 g
General procedure for bi- and trinuclear organoboron com-
plexes (10–16): In a typical experiment, two or three equivalents
of triphenyl boron and one equivalent of desired bis- or tris(imino-
pyrrolyl) ligand precursors in toluene (15–25 mL) were heated to
reflux under nitrogen atmosphere overnight. The reaction mixture
was brought to room temperature and then concentrated under
vacuum to 5 mL and double layered with n-hexane. The resulting
solution was kept at ꢀ208C to afford the corresponding boron
complexes. Except in the case of 11, these compounds were ob-
tained as amorphous powders. Further crystallizations were at-
tempted, including double layering with CH2Cl2/n-hexane.
1
(72%). H NMR (300 MHz, CD2Cl2): d=8.62 (s, 2H, CH=N), 7.63 (s,
3
2H, naphthyl), 7.56 (d, 2H, 3JHH =9.0 Hz, naphthyl), 7.41 (d, JHH
=
=
9.0 Hz, 2H, naphthyl), 7.28–7.18 (m, 22H, BPh2 +pyrr), 7.09 (d, 3JHH
3.9 Hz, 2H, pyrr), 6.58 ppm (dd, 3JHH =3.9, 4JHH =1.8 Hz, 2H, pyrr);
13C{1H} NMR (75 MHz, CD2Cl2): d=150.6, 140.3, 134.9, 133.5, 132.2,
132.1, 129.6, 127.9, 127.0, 121.9, 120.0, 118.0, 115.9 ppm; 11B NMR
(96.29 MHz, CD2Cl2): d=5.45 ppm; elemental analysis calcd (%) for
C44H34B2N4: C 82.52, H 5.35, N 8.75; found: C 82.21, H 4.99, N 8.62.
2,6-[Ph2B(k2N,N’-NC4H3-C(H)=N)]2-C14H8 (15): In the same manner
as described above, a mixture of impure 8 (see above, 0.276 g,
0.76 mmol) and BPh3 (0.365 g, 1.51 mmol) afforded 15 as a yellow
Attempt to synthesize 1,2-[Ph2B(k2N,N’-NC4H3-C(H)=N)]2-C6H4: In
the same manner as described above, a mixture of 2 (0.120 g,
0.50 mmol) and BPh3 (0.242 g, 1.00 mmol) afforded a yellow solid
that could not be characterized and would decompose in solution.
1
solid. Yield (after several recrystallizations): 0.224 g (42%). H NMR
(300 MHz, CD2Cl2): d=8.67 (s, 2H, CH=N), 8.11 (s, 2H, anthracenyl),
7.83 (d, 3JHH =9.0 Hz, 2H, anthracenyl), 7.81 (s, 2H, anthracenyl),
7.43 (d, 3JHH =9.0 Hz, 2H, anthracenyl), 7.34–7.20 (m, 22H, BPh2 +
1,3-[Ph2B(k2N,N’-NC4H3-C(H)=N)]2-C6H4 (10): In the same manner
as described above, a mixture of 3 (0.120 g, 0.50 mmol) and BPh3
(0.242 g, 1.00 mmol) afforded 10 as yellow solid. Yield: 0.165 g
(73%). 1H NMR (300 MHz, CD2Cl2): d=8.20 (s, 2H, CH=N), 7.27–
7.15 (m, 22H, BPh2 +phenyl), 7.13–7.08 (m, 3H, phenyl+pyrr), 7.07–
7.04 (m, 3H, phenyl+pyrr), 6.61–6.59 ppm (m, 2H, pyrr);
13C{1H} NMR (75 MHz, CD2Cl2): d=150.1, 142.9, 134.3, 133.1, 131.9,
130.1, 127.6, 126.7, 120.7, 117.7, 115.8, 115.3 ppm; 11B NMR
(96.29 MHz, CD2Cl2): d=5.05 ppm; elemental analysis calcd (%) for
C40H32B2N4·0.5CH2Cl2: C 76.87, H 5.25, N 8.85; found: C 77.35, H
5.26, N 9.09.
1,4-[Ph2B(k2N,N’-NC4H3-C(H)=N)]2-C6H4 (11): In the same manner
as described above, a mixture of 4 (0.120 g, 0.50 mmol) and BPh3
(0.242 g, 1.00 mmol) afforded 11 as a yellow solid. Yield: 0.238 g
(81%). The crystals suitable for single-crystal X-ray diffraction stud-
ies were obtained by keeping the dichloromethane solution of the
compound double layered with n-hexane, at ꢀ208C. 1H NMR
3
3
4
pyrr), 7.10 (d, JHH =3.6 Hz, 2H, pyrr), 6.60 ppm (dd, JHH =3.9, JHH
=
1.5 Hz, 2H, pyrr); 13C{1H} NMR (75 MHz, CD2Cl2): d=150.4, 139.5,
135.0, 133.6, 132.1, 131.6, 130.9, 130.0, 128.0, 127.1, 127.0, 121.3,
120.3, 118.0, 115.8 ppm; 11B NMR (96.29 MHz, CD2Cl2): d=
5.71 ppm; elemental analysis calcd (%) for C44H34B2N4: C 83.50, H
5.26, N 8.11; found: C 83.43, H 5.34, N 7.94.
[Ph2B(k2N,N’-NC4H3-C(H)=N-1,4-C6H4)]3-1,3,5-C6H3 (16): In the
same manner as described above, a mixture of
9 (0.160 g,
0.27 mmol) and BPh3 (0.200 g, 0.82 mmol) afforded 16 as a yellow
solid. Yield: 0.249 g (84%). 1H NMR (300 MHz, CD2Cl2): d=8.61 (s,
3H, CH=N), 7.70 (s, 3H, phenyl) 7.68–7.57 (m, 6H, phenyl), 7.41–
7.38 (m, 6H, phenyl), 7.35–7.21 (m, 33H, phenyl+pyrr), 7.10 (dd,
4
3JHH =3.9, 4JHH =0.6 Hz, 3H, pyrr), 6.60 ppm (dd, 3JHH =3.9, JHH
=
1.8 Hz, 3H, pyrr); 13C{1H} NMR (75 MHz, CD2Cl2): d 150.0, 141.5,
141.0, 139.4, 134.4, 133.1, 131.5, 129.0, 127.9, 127.5, 126.6, 124.7,
122.2, 117.5, 115.3 ppm; 11B NMR (96.29 MHz, CD2Cl2): d=4.11 ppm;
Chem. Eur. J. 2015, 21, 1 – 18
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elemental analysis calcd (%) for C75H57B3N6·0.75CH2Cl2: C 79.91, H
5.18, N 7.38; found: C 79.89, H 4.90, N 6.53.
measured using a LUDOX scattering solution in water with trans-
mittance at the excitation wavelength matched to that of the
sample (FWHM=19 ps). The IRF and sample signals were collected
until 5 K counts at the maximum were reached. Fluorescence
decays were deconvoluted from the IRS using the modulation
functions method (Sand program).[41]
X-ray data collection
Crystallographic and experimental details of crystal structure deter-
minations are listed in Table S1 of the Supporting Information. The
crystals were selected under an inert atmosphere, covered with
polyfluoroether oil, and mounted on a nylon loop. Crystallographic
data for ligand precursors 3, 3·MeOH, 7, and for complex 11·C6H14
were collected by using graphite monochromated MoKa radiation
(l=0.71073 ꢅ) on a Bruker AXS-KAPPA APEX II diffractometer
equipped with an Oxford Cryosystem open-flow nitrogen cryostat,
at 150 K. Cell parameters were retrieved using Bruker SMART[29]
software and refined using Bruker SAINT[30] on all observed reflec-
tions. Absorption corrections were applied using SADABS.[31] Struc-
ture solution and refinement were performed using direct methods
with the programs SIR97,[32] SIR2004,[33] and SHELXL[34] included in
the package of programs WINGX-Version 1.80.05.[35] Non-hydrogen
atoms were refined with anisotropic thermal parameters. All the
structures refined to a perfect convergence, even though some of
the crystals were of poorer quality, such as 3 and 11·C6H14, which
presented high Rint and relatively low ratio of observed/unique re-
flections. Crystal 11·C6H14 showed the presence of a disordered n-
hexane molecule, for which some restraints were applied and, al-
though all the carbon atoms were refined with anisotropic thermal
parameters, it was impossible to insert the corresponding hydro-
gen atoms. Moreover, due the presence of other disordered sol-
vent molecules in the same crystal and since it was impossible to
attain a good disorder model, the PLATON/SQUEEZE[36] routine was
applied. Except for the NH hydrogen atoms in compounds, all hy-
drogen atoms were inserted in idealized positions and allowed to
refine riding on the parent carbon atom, with CꢀH distances of
0.95, 0.98, and 0.84 ꢅ for aromatic, methyl, and hydroxyl H atoms,
respectively, and with Uiso(H)=1.2Ueq(C). Graphic presentations
were prepared with ORTEP-III[37] and Mercury.[38] CCDC-1022744 (3),
CCDC-1022745 (3·MeOH), CCDC-1022746 (7), and CCDC-1022747
(11·C6H14) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Triplet–triplet absorption spectra and singlet-to-triplet intersystem
crossing yields (fT) were measured using a previously described
laser-flash photolysis apparatus (Applied Photophysics) pumped by
a Nd:YAG laser (Spectra Physics).[42,43] Triplet yields, were measured
with optically matched dilute solutions (ODꢁ0.2 in a 10 mm
square cell) and low laser energy (ꢂ2 mJ). The triplet molar ab-
sorption coefficients (eT) were obtained by the singlet depletion
technique (eT =eS ꢆDODT/DODS, in which the DODS and DODT are
the singlet and triplet differential optical densities, respectively,
and eS is the ground state molar extinction coefficient).[44]
The fT values were obtained by comparing the triplet DOD at
525 nm of a benzene solution of benzophenone (the standard)
with that of the compounds (optically matched at the laser wave-
length) as described elsewhere.[42,43] First-order kinetics was ob-
served for the lowest triplet state decays of all the compounds.
Computational studies
DFT calculations[21] were performed by using the Amsterdam Densi-
ty Functional program package (ADF).[22] Geometry optimizations
were carried out with gradient correction,[45] without symmetry
constraints, and using the Local Density Approximation of the cor-
relation energy (Vosko–Wilk–Nusair),[46] and the Generalized Gradi-
ent Approximation (Becke’s[47] exchange and Perdew’s[48] correla-
tion functionals). The ZORA approximation was considered for rela-
tivistic effects.[49] Unrestricted calculations were performed for ex-
cited singlet states. The core orbitals were frozen for B, C, and N
(1s). Triple z Slater-type orbitals (STO) were used to describe the
valence shells of B, C, and N (2s and 2p). A set of two polarization
functions was added to B, C, and N (single z 3d, 4f). Triple z Slater-
type orbitals (STO) were used to describe the valence shells of H
(1s) augmented with two polarization functions (single z 2s, 2p).
Time-dependent DFT calculations in the ADF implementation were
performed to determine the excitation energies.[23] The solvent
effect was included with the COSMO approach in ADF in single-
point calculations of absorption spectra. The geometry of the excit-
ed state was calculated by promoting one electron from the
HOMO to the LUMO with S=0. The perturbative method in the
time-dependent density-functional theory (TDDFT) formalism with
the influence of spin-orbit coupling effect (SOPERT)[24] was used to
calculate the excited singlet-states lifetimes. In these calculations,
complete basis sets were used for all elements (same as above,
without any frozen core) with the hybrid PBE0 functional.[50] The
absorption spectra calculated with this approach were the same
that were obtained in the same conditions without including spin-
orbit coupling since all the atoms are light. TDDFT optimizations of
the first singlet excited state were also performed, using the Gaus-
sian 09 software,[51] for technical reasons, with the PBE0 function-
al[50] and a 6–31G** basis set for all atoms.[52]
Spectroscopic measurements
Absorption and fluorescence spectra were measured in a Beckman
DU-70 spectrophotometer and a SPEX Fluorolog 212I, respectively.
The fluorescence spectra were collected with right angle geometry,
in the S/R mode, and corrected for instrumental wavelength de-
pendence. Fluorescence quantum yields in THF at 293 K were mea-
sured by using tetra-, penta-, and hexathiophene in dioxane as
standards (f =0.18, 0.33, and 0.41, respectively).[39]
f
Fluorescence decays were measured by using the time-correlated
single-photon counting (TCSPC) technique as previously de-
scribed.[40] The pulsed (82 MHz) excitation source was a Ti:Sapphire
Tsunami laser pumped with a solid-state laser Millennia Xs (Spectra
Physics). The Tsunami output (720–900 nm) was frequency doubled
and vertically polarized. The sample emission was passed through
a polarizer set at the magic angle and a Jobin–Yvon H10 mono-
chromator, and finally detected with a microchannel plate photo-
multiplier (Hamamatsu, R3809u-50 MCP-PT). A fraction of the Tsu-
nami output was detected with a PHD-400-N photodiode (Becker
and Hickl, GmbH) for generation of the start signal. Start and stop
signals were processed with a SPC-630 acquisition board (Becker
and Hickl, GmbH). The instrumental response function (IRF) was
The structures were modeled after that of compound 11 described
above. Three-dimensional representations of the orbitals were ob-
tained with Molekel[53] and electronic spectra with Chemcraft.[54]
Cyclic voltammetry studies
Cyclic voltammetry measurements were carried out with a Solartron
potentiostat using a standard three-electrode cell, with a saturated
&
&
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ferrocenium (Fc/Fc+) to be 4.80 eV below the vacuum level, as de-
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Devices were prepared and characterized as described earlier.[15]
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with oxygen plasma, prior to the deposition of PEDOT:PSS
(poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic
acid, CLEVIOS P VP.AI 4083 from Heraeus Clevios GmbH) by spin
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and then transferred into a nitrogen filled glovebox. Some devices
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and a calibrated silicon photodiode, as described previously.[55] The
electroluminescence (EL) spectra were obtained with a CCD spec-
trograph (from Ocean Optics or from ScanSci). External quantum
efficiency values were estimated as detailed in Ref. [55].
Acknowledgements
We thank the Fundażo para a CiÞncia e Tecnologia, Portugal,
for financial support (Projects PTDC/QUI/65474/2006, UID/QUI/
00100/2013, UID/MULTI/00612/2013, UID/EEA/50008/2013 and
RECI/QEQ-QIN70189/2012) and for fellowships to D.S., B.F.,
P.S.L., C.A.F., C.S.B.G., and D.V.V. (SFRH/BPD/47853/2008, SFRH/
BD/80122/2011, SFRH/BD/88639/2012, SFRH/BD/47730/2008,
SFRH/BPD/64423/2009, and SFRH/BD/81017/2011, respectively).
We also thank Dr. J. S. Seixas de Melo and Dr. J. Pina (CCC-DQ,
University of Coimbra) for their support in the flash-photolysis
experiments.
Keywords: boron
·
density functional calculations
·
fluorescence · ligands · luminescence
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Received: January 11, 2015
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FULL PAPER
Bridge the gap: Luminescent linear and
star-shaped polynuclear organoboron
complexes containing iminopyrrolyl li-
gands linked by aryl bridges were syn-
thesized, showing good luminescent
properties depending on p-conjugation
lengths of the molecules (see figure).
Organic light-emitting diodes (OLEDs)
were successfully fabricated with the
new boron complexes, achieving lumi-
&
Luminescence
D. Suresh, C. S. B. Gomes, P. S. Lopes,
C. A. Figueira, B. Ferreira, P. T. Gomes,*
R. E. Di Paolo, A. L. MaÅanita,
M. T. Duarte, A. Charas, J. Morgado,
D. Vila-ViÅosa, M. J. Calhorda
&& – &&
Luminescent Di- and Trinuclear Boron
Complexes Based on Aromatic
Iminopyrrolyl Spacer Ligands:
Synthesis, Characterization, and
Application in OLEDs
nance values of up to 4400 cdmꢀ2
.
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Chem. Eur. J. 2015, 21, 1 – 18
www.chemeurj.org
18
ꢄ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!