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

DOI: 10.1002/chem.201500109

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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 cdm^ꢀ2, for single layer de-

vices, increasing to 4400 cdm^ꢀ2when 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

htt p ://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 N^ꢀ, O^ꢀ, or C^ꢀ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-iPr₂C₆H₃and 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]BPh₂from the reaction of ligand precursor 2 with

BPh₃led 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·C₆H₁₄is 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 B1ꢀC_Ph

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·CH₂Cl₂struc-

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 C_imino-N_imino-C_arylꢀipso

-

C_arylꢀorthodihedral 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·C₆H₁₄and ꢀ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 N_imino-C_arylꢀipsobond) 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

e_max

E

(exp)

E

(GP)

E

(THF)

E

(SO)

[nm]

[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 (l^m_ab^a_s^x), molar extinction coefficient (e_max) and

energy (E^m_ab^a_s^x) 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] 10⁴Lmol^ꢀ1cm^ꢀ1. [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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7

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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 (l_e⁰_m^ꢀ0) and absorption maximum (l^m_ab^a_s^x) (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

f

t_f

k_f(exp)^[b]

f_T

k_nr

k_isc

k_ic

E^m_em^ax(exp)

[eV]

E

(TDDFT)

k_f(SO)

em

[nm]

[ns]

[ns^ꢀ1

]

[ns^ꢀ1

]

[ns^ꢀ1

]

[ns^ꢀ1

]

[eV]

[ns^ꢀ1

]

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

0.14

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]

1.8(6)

0.03

0.02

0.34

[a] Experimental wavelengths of the fluorescence maximum (l_e^m_m^ax) and first vibronic S₁!S₀transition (l^0-0); fluorescence quantum yield (f), lifetime (t_f) and

f

em

triplet formation quantum yield (f_T); rate constants of: fluorescence (k_f(exp)), intersystem crossing (k_isc) and internal conversion (k_ic); sum of non-radiative

max

em

rate constants (k_nr); experimental energy maxima (E_e^m_m^ax(exp)), in THF, at 293 K; calculated (TDDFT) energy maxima (E

(TDDFT)) and calculated fluores-

cence rate constants (k_f(SO)). [b] k_f=f/t_f; [c] k_nr=(1ꢀf)/t_f; [d] k_isc=f_T/t_f; [e] k_ic=k_nrꢀk_isc. [f] From double exponential decays with t₁=1.4 ns (0.9) and

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 k_fvalues (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 k_fand the de-

crease of the sum of non-radiative rate constants k_nr

(sum of internal conversion, k_ic, and intersystem

crossing, k_isc), from top to bottom. Compound 15

shows smaller k_fand larger k_nrvalues than the best

emitters. The lower value of k_flikely 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]

13

4.95

2.73

2.22

ꢀ5.32

ꢀ2.87

17^[a]

10

no bridge

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 ¹B_u! A_gtransition 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 k_nrvalues, 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

(k_isc!k_ic), that is, k_nris approximately equal to the

rate constant of internal conversion, k_ic.

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].

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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 cdm^ꢀ2with an efficiency of 0.38 cdA^ꢀ1. 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 cdm^ꢀ2) and external quantum efficiency

(EQE_max=0.084%) was achieved with 11, whereas the mini-

mum (0.35 cdm^ꢀ2) was achieved with 17, for which an EQE_max

of 1.5ꢆ10^ꢀ4% 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 cdm^ꢀ2and EQE_max=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, L_max[cdm^ꢀ2], luminous efficiency, f_EL,max[cdA^ꢀ1], and quantum efficiency, EQE_max

[%]). 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

EQE_max=0.047

L

f

_max=83

_EL,max=0.65

EQE_max=0.23

13

17

no bridge

^[a]L_max=0.35

^[a]f_EL,max=3.8ꢆ10^ꢀ4

^[a]EQE_max=1.5ꢆ10^ꢀ4

L

f

_max=6.7

_EL,max=0.0055

EQE_max=0.002

L

f

_max=14

_EL,max=0.0146

EQE_max=0.005

L

f

_max=737

_EL,max=0.261

EQE_max=0.10

L

f

_max=67

_EL,max=0.0264

EQE_max=0.010

10

16

L

f

_max=64

_EL,max=0.077

L

f

_max=53

_EL,max=0.015

EQE_max=0.051

EQE_max=0.052

^[a]L_max=844

L

f

_max=2000

_EL,max=0.53

EQE_max=0.145

L

f

_max=2439

_EL,max=0.38

EQE_max=0.10

_max=233

_EL,max=0.12

EQE_max=0.030

_max=658

_EL,max=0.11

EQE_max=0.0286

_max=410

_EL,max=0.086

EQE_max=0.034

L

f

_max=3916

_EL,max=0.70

EQE_max=0.19

_max=1212

_EL,max=0.36

EQE_max=0.095

_max=4370

_EL,max=1.36

EQE_max=0.36

_max=221

_EL,max=0.039

EQE_max=0.016

12

11

14

15

^[a]f_EL,max=0.19

^[a]EQE_max=0.052

^[a]L_max=958

L

f

L

f

^[a]f_EL,max=0.30

^[a]EQE_max=0.084

L

_max=69

_EL,max=0.0094

EQE_max=0.0025

_max=13

_EL,max=0.0031

EQE_max=0.0023%

L

_max=57

_EL,max=0.0030

EQE_max=0.0008

_max=14

_EL,max=0.0046

EQE_max=0.0034

L

f

L

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 cdm^ꢀ2and

a maximum efficiency of

1.36 cdA^ꢀ1. 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 CaCl₂guard 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 CaCl₂guard 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 cdm^ꢀ2and efficiencies of

0.38 cdA^ꢀ1in 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 cdm^ꢀ2and efficiencies of 1.36 cdA^ꢀ1(for 14).

1

(65%). H NMR (300 MHz, [D₆]DMSO): d=11.82 (br, 2H), 8.48 (s, 2H,

CH=N), 7.92 (d, ³J_HH=9 Hz, 2H, naphthyl), 7.63 (s, 2H, naphthyl),

3

7.46 (d, J_HH=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 CaCl₂guard 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, J_HH=4.2 Hz, 2H, pyrr), 6.53 ppm (dd,

4

³J_HH=4.2 Hz, J_HH=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, ³J_HH=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, J_HH=3.9, J_HH=0.9 Hz, 2H, pyrr), 6.60 ppm

3

4

(dd, J_HH=3.9, J_HH=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

(d, J_HH=7.6 Hz, 2H, anthracenyl), 7.21 (s, 2H, pyrr), 7.11 (d, J_HH

=

3

7.6 Hz, 2H, anthracenyl), 7.00 (d, 2H, J_HH=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 CaCl₂guard 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, J_HH=3.9, J_HH=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, ³J_HH=9.0 Hz, naphthyl), 7.41 (d, J_HH

=

9.0 Hz, 2H, naphthyl), 7.28–7.18 (m, 22H, BPh₂+pyrr), 7.09 (d, ³J_HH

3.9 Hz, 2H, pyrr), 6.58 ppm (dd, ³J_HH=3.9, ⁴J_HH=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, ³J_HH=9.0 Hz, 2H, anthracenyl), 7.81 (s, 2H, anthracenyl),

7.43 (d, ³J_HH=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

4

pyrr), 7.10 (d, J_HH=3.6 Hz, 2H, pyrr), 6.60 ppm (dd, J_HH=3.9, J_HH

=

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

³J_HH=3.9, ⁴J_HH=0.6 Hz, 3H, pyrr), 6.60 ppm (dd, ³J_HH=3.9, J_HH

=

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;

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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 Mo_Karadiation

(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 R_intand relatively low ratio of observed/unique re-

flections. Crystal 11·C₆H₁₄showed 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 U_iso(H)=1.2U_eq(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 (f_T) 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 (e_T) were obtained by the singlet depletion

technique (e_T=e_SꢆDOD_T/DOD_S, in which the DOD_Sand DOD_Tare

the singlet and triplet differential optical densities, respectively,

and e_Sis the ground state molar extinction coefficient).^[44]

The f_Tvalues 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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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 FundaÅ¼o 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

Published online on && &&, 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 cdm^ꢀ2

.

&

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18

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ÝÝ These are not the final page numbers!

Article Doi

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

DOI: 10.1002/chem.201500109

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Article abstract of DOI:10.1002/chem.201500109

Full text of DOI:10.1002/chem.201500109

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