Tunable fluorophores based on 2-(N-Arylimino)pyrrolyl chelates of diphenylboron: Synthesis, structure, photophysical characterization, and application in OLEDs

DOI: 10.1002/chem.201303607

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Fluorescence

Tunable Fluorophores Based on 2-(N-Arylimino)pyrrolyl Chelates

of Diphenylboron: Synthesis, Structure, Photophysical

Characterization, and Application in OLEDs

D. Suresh,^[a]Patrꢀcia S. Lopes,^[a]Bruno Ferreira,^[a]Clꢁudia A. Figueira,^[a]Clara S. B. Gomes,^[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]}and Maria Josꢃ Calhorda^[d]

Abstract: Reactions of 2-(N-arylimino)pyrroles (HNC₄H₃C(H)= a hydrogen atom at the 2,6-positions of the aryl groups

N-Ar) with triphenylboron (BPh₃) in boiling toluene afford

the respective highly emissive N,N’-boron chelate complexes,

[BPh₂{k²N,N’-NC₄H₃C(H)=N-Ar}] (Ar=C₆H₅(12), 2,6-Me₂-C₆H₃

(13), 2,6-iPr₂-C₆H₃(14), 4-OMe-C₆H₄(15), 3,4-Me₂-C₆H₃(16),

4-F-C₆H₄(17), 4-NO₂-C₆H₄(18), 4-CN-C₆H₄(19), 3,4,5-F₃-C₆H₂

(20), and C₆F₅(21)) in moderate to high yields. The photo-

physical properties of these new boron complexes largely

depend on the substituents present on the aryl rings of

their N-arylimino moieties. The complexes bearing electron-

withdrawing aniline substituents 17–20 show more intense

strongly restricts rotation of this moiety towards coplanarity

with the iminopyrrolyl ligand framework, inducing a shift in

the emission to the violet region (l_max=410–465 nm) and

a significant decrease in quantum yield (f =0.005, 0.023,

f

and 0.20 for Ar=2,6-Me₂-C₆H₃(13), 2,6-iPr₂-C₆H₃(14), and

C₆F₅(21), respectively, in THF), even when electron-with-

drawing groups are also present. Density functional theory

(DFT) and time-dependent DFT (TD-DFT) calculations have

indicated that the excited singlet state has a planar arylimi-

nopyrrolyl ligand, except when prevented by steric hin-

drance (ortho substituents). Calculated absorption maxima

reproduce the experimental values, but the error is higher

for the emission wavelengths. Organic light-emitting diodes

(OLEDs) have been fabricated with the new boron com-

plexes, with luminances of the order of 3000 cdm^ꢀ2being

achieved for a green-emitting device.

(e.g., f =0.71 for Ar=4-CN-C₆H₄(19) in THF), higher-energy

f

(blue) fluorescent emission compared to those bearing elec-

tron-donating substituents, for which the emission is red-

shifted at the expense of lower quantum yields (f =0.13

f

and 0.14 for Ar=4-OMe-C₆H₄(15) and 3,4-Me₂-C₆H₃(16), re-

spectively, in THF). The presence of substituents bulkier than

Introduction

in order to prepare organometallic complexes that might be

applied as pre-catalysts for various organic reactions.^[1]Biden-

tate 2-iminopyrrolyl chelating ligands, containing a pyrrolyl

anionic ring and a neutral imine as donor moieties, are one

such type of ligands,^[2]which can be considered to be structur-

ally similar to salicylaldiminate, anilidoimine, or 2-(2-pyridyl)in-

dolyl ligands.^[3]Although the first syntheses of homoleptic

metal complexes of Co^II, Ni^II, Pd^II, Cu^II, and Zn^IIcontaining li-

gands of this kind were reported back in the 1960s,^[4]there has

been a recent resurgence of interest in such systems.^[5]The

ease of steric and electronic tuning through the introduction

of aryl substituents on the imine group makes these chelates

highly flexible in terms of ligand design, which renders them

very interesting for various applications. For instance, bulky

aryl substituents on the imine group provide steric tuning and

shielding of the metal centers and hence several mono- or bis-

[2-(N-arylimino)pyrrolyl] early and late transition-metal com-

plexes have been reported in the literature as efficient a-olefin

oligo-/polymerization catalysts.^[2,6]

There has been considerable interest in the synthesis of biden-

tate chelating ligands containing an imine and a donor moiety,

[a] Dr. D. Suresh, P. S. Lopes, B. Ferreira, C. A. Figueira, Dr. C. S. B. Gomes,

Dr. P. T. Gomes, Dr. R. E. Di Paolo, Prof. A. L. MaÅanita, Dr. M. T. Duarte

Centro de Quꢀmica Estrutural

Departamento de Engenharia Quꢀmica

Instituto Superior Tꢁcnico, Universidade de Lisboa

Av. Rovisco Pais, 1049-001 Lisboa (Portugal)

E-mail: pedro.t.gomes@tecnico.ulisboa.pt

[b] Dr. A. Charas, Dr. J. Morgado

Instituto de TelecomunicaÅꢂes

Av. Rovisco Pais, 1049-001 Lisboa (Portugal)

[c] Dr. J. Morgado

Department of Bioengineering, Instituto Superior Tꢁcnico

Universidade de Lisboa

Av. Rovisco Pais, 1049-001 Lisboa (Portugal)

[d] Prof. M. J. Calhorda

Departamento de Quꢀmica e Bioquꢀmica, CQB

Faculdade de CiÞncias, Universidade de Lisboa

Campo Grande, Ed. C8, 1749-016 Lisboa (Portugal)

Besides catalytic investigation, another important focus of

interest in recent years has been the synthesis of coordination

and organometallic compounds with photoluminescent (PL)

Supporting information for this article is available on the WWW under

http://dx.doi.org/10.1002/chem.201303607.

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properties, due to their potential applications in electrolumi-

nescent (EL) displays.^[7]Luminescent chelate complexes have

proved to be particularly useful in electroluminescent displays

because of their relatively high stability and volatility.^[8]Much

effort has been devoted to the design and synthesis of new

phosphorescent materials with heavy metals such as iridium-

(III)^[9]and platinum(II).^[10]Recently, the synthesis of fluorescent

tricoordinate^[11]and tetracoordinate^[12]boron complexes has at-

tracted a great deal of attention because of their potential ap-

plications in functional materials and in sensors.^[13]Earlier re-

ports on boron-based complexes suggested that variations in

the chromophore part of the molecule would influence the

HOMO–LUMO levels and thereby the color of emission.^[12a,d]In

addition, some of these compounds exhibited good charge-

transport properties and, consequently, could be employed in

the fabrication of organic light-emitting diodes (OLEDs) and

sensors.^[14]

states of which could be assigned to mixed intraligand (p–p*)

and metal-to-ligand charge-transfer (MLCT) transitions. Their

phosphorescence quantum yields were, however, very low

(f_p<3%). Experimental and theoretical results indicated that

the photophysical properties of these complexes largely de-

pended on the N,N or N,O anionic ancillary ligands.

Very recently, in a preliminary communication, we reported

the synthesis and characterization of new fluorescent four-co-

ordinate mono- and binuclear organoboron complexes con-

taining 2-(N-arylformimino)pyrrolyl bidentate ligands (D and E,

respectively) and their successful application in single-layer,

non-doped OLEDs.^[17]In particular, the mononuclear boron

complex [BPh₂{k²N,N’-NC₄H₃C(H)=N-Ph}] D (denoted as 12 in

the present work; see Scheme 2 below) exhibited blue fluores-

cence (l^m_em^ax=479 nm) characterized by a quantum yield of

34%, this emission being assigned to a virtually pure ligand-

1

based (p–p*) transition.^[18]

Recently, we have developed new homoleptic 2-(N-arylformi-

mino)pyrrolyl zinc(II) complexes of the type [Zn{k²N,N’-2-(N-

arylformimino)pyrrolyl}₂] (A1 and A2) or [Zn{k²N,N’-2-(N-

arylformimino)phenanthro[9,10-c]pyrrolyl}₂] (A3 and A4), the

latter containing a phenanthrene fragment fused on the

These results prompted us to synthesize new boron com-

plexes containing 2-(N-arylformimino)pyrrolyl anionic ligands

and, in order to assess their emissivity and color tunability, to

study in detail their photoluminescent properties under the in-

fluence of different structural parameters of the N-aryl ring. In

the present paper, we report a series of highly emissive mono-

nuclear 2-(N-arylformimino)pyrrolyl chelates of diphenylboron,

[BPh₂{k²N,N’-NC₄H₃C(H)=N-Ar}], and their characterization by

multinuclear NMR, X-ray diffraction analysis, and cyclic voltam-

metry. Photophysical characterization of the newly synthesized

complexes was accomplished by steady-state and time-re-

solved luminescence techniques in solution. Density functional

theory (DFT) and time-dependent DFT (TD-DFT) calculations

(ADF program) were also carried out on these new boron com-

plexes, to assign their electronic transitions and to determine

their singlet excited-state properties (geometry, lifetime, emis-

sion). Furthermore, these compounds have been utilized as

emissive layers in OLED devices.

Results and Discussion

Synthesis and characterization of iminopyrrole ligand

precursors

Most of the ligand precursors were synthesized and character-

ized according to literature methods (Scheme 1).^[5d,19–24]The

new ligand precursors 6 and 10 were synthesized, and their

characterization data are presented in the Experimental Sec-

tion. In general, the 2-(N-arylformimino)pyrrole ligand precur-

C3ꢀC4 bond of the pyrrolyl ring of the ligand, and studied

their photoluminescent properties.^[15]The two phenanthrene-

fused iminopyrrolyl zinc complexes showed mainly a ligand-

1

based (p–p*) emission in the blue/green spectral region, but

their fluorescence efficiencies were low (f =3.9 and 8.8%, re-

f

spectively). Eisenberg and co-workers reported a series of neu-

tral phosphorescent copper(I) complexes of the types [Cu(N,N)-

(P,P)] and [Cu(N,O)(P,P)] containing phosphine or diphosphine

ligands and coordinated bidentate N,N ancillary ligands such

as 2-(N-phenylformimino)pyrrolyl or 2-(N-phenylformimino)in-

dolyl (B), or the corresponding parent bidentate N,O ancillary

ligands 2-formylpyrrolyl or 2-formylindolyl, respectively (C).^[16]

These complexes exhibited long-lived emissions, the excited

Scheme 1. Synthesis of 2-(N-arylformimino)pyrrole ligand precursors 2–11.

p-TSA=para-toluenesulfonic acid.

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sors (HNC₄H₃C(H)=N-Ar), with Ar=C₆H₅(2), 2,6-Me₂-C₆H₃(3),

2,6-iPr₂-C₆H₃(4), 4-OMe-C₆H₄(5), 3,4-Me₂-C₆H₃(6), 4-F-C₆H₄(7),

4-NO₂-C₆H₄(8), 4-CN-C₆H₄(9), 3,4,5-F₃-C₆H₂(10), and C₆F₅(11),

were synthesized by condensation reactions of 2-formylpyrrole

(1) with the requisite substituted aryl amines, namely aniline,

2,6-dimethylaniline, 2,6-diisopropylaniline, 4-methoxyaniline,

3,4-dimethylaniline, 4-fluoroaniline, 4-nitroaniline, 4-aminoben-

zonitrile, 3,4,5-trifluoroaniline, and 2,3,4,5,6-pentafluoroaniline,

employing standard reaction conditions (Scheme 1). The syn-

theses and purifications of these ligand precursors were

straightforward, and those with sterically bulky and electron-

donating substituents were typically obtained in good yields.

However, the ligand precursors bearing electron-withdrawing

groups were always obtained with unreacted starting amines

and thus had to be purified by vacuum sublimation, leading to

low to moderate yields. All compounds were characterized by

¹H, ¹³C, and, in the case of fluorinated derivatives, ¹⁹F NMR

spectroscopies, their spectra being consistent with those re-

ported in the literature.

and C₆F₅(21), were characterized by H, ¹³C, ¹¹B, and, in the

case of fluorinated compounds, ¹⁹F NMR spectroscopies, as

well as by elemental analysis. Where possible, single-crystal

X-ray diffraction analysis was also performed. Although the or-

ganoboron compound [BPh₂{k²N,N’-NC₄H₃C(H)=N-C₆H₅}] (12)

has already been reported in our earlier communication,^[17]it is

also included in this full paper. The absence of NH proton sig-

nals in the ¹H NMR spectra of the complexes confirmed the for-

mation of the neutral tetracoordinate boron mononuclear

complexes. The ¹¹B NMR chemical shifts of these compounds

are in the range d=4.88–5.81 ppm, consistent with those of

other four-coordinate boron derivatives^[12](resonances for

three-coordinate boron are typically downfield shifted, appear-

ing at dꢁ +25 ppm and above^[11]). In general, these iminopyr-

rolyl organoboron compounds could be handled in air for peri-

ods of at least an hour, but, upon prolonged exposure, under-

went a relatively slow decomposition process leading to regen-

eration of the corresponding free ligand precursor. However,

compound 21 proved to be extremely sensitive to air and

moisture (in the solid state or in solution) as well as very unsta-

ble in solution even under nitrogen, undergoing very fast de-

composition with concomitant regeneration of the ligand pre-

cursor. This instability, most likely due to the high electronega-

tivity of the C₆F₅group, could be monitored by UV/Vis absorp-

tion spectroscopy and affected the electrochemical measure-

ments. Compounds 13–21 were found to be soluble in most

of the common organic solvents, such as dichloromethane,

tetrahydrofuran, and toluene, and moderately soluble in

n-hexane.

Crystals suitable for single-crystal X-ray diffraction analysis

were obtained for the ligand precursors 3–6 and 8. The struc-

tural features of these compounds are similar to those of other

previously reported iminopyrrole molecules.^{[5d,e,h,15,19]}In fact,

the crystal structure of compound 4 has been reported pre-

viously.^[5h]However, a new polymorphic form was obtained in

the present work when the compound was crystallized from

toluene double-layered with n-hexane. Perspective views of

the molecular structures of the ligand precursors 3–6 and 8

are given in Figures S1–S5 in the Supporting Information. Se-

lected bond lengths and angles are given in the captions to

these figures. Details of the crystal structure determinations

are given in Table S1 in the Supporting Information.

Crystals suitable for X-ray diffraction analysis were obtained

for compounds 13–17, 19, and 21, enabling the determination

of their crystal structures. Perspective views of the molecular

structures of these iminopyrrolyl boron complexes are shown

in Figure 1 (and Figure S6 in the Supporting Information) and

selected bond distances and angles are given in Table 1.

In general, the boron center in each of the complexes dis-

plays a typical distorted tetrahedral geometry with C1 symme-

try. The iminopyrrolyl ligand is chelated to the boron atom

through the two nitrogen atoms to form a five-membered

ring. The respective bite angles (N1-B1-N2) range from

93.81(17)8 to 95.30(8)8. Throughout the series [BPh₂{k²N,N’-

NC₄H₃C(H)=N-Ar}] (13–17, 19, and 21), the BꢀC and BꢀN bond

distances are rather invariant, with average B1ꢀC_ipsoand

B1ꢀN1(pyrrolyl) distances of 1.610 and 1.561 ꢅ, respectively,

the latter being shorter by 0.073 ꢅ compared to the average

B1ꢀN2(imine) distance (1.634 ꢅ), as previously observed for

compound 12.^[17]

Synthesis and characterization of iminopyrrolyl boron

complexes

The organoboron compounds 13–21 (Scheme 2) were ob-

tained in good to moderate yields by reactions of the corre-

sponding iminopyrrole ligand precursors with triphenylboron.

Furthermore, the iminopyrrolyl fragments coordinated to

boron are virtually planar in all of the compounds, but the re-

spective aryl groups bonded to the iminic nitrogen are not co-

planar. The dihedral angle (defined as C6-N2-C7-C12) depends

on the bulkiness of the substituents at 2,6-positions of the N-

aryl groups. For instance, within the group of complexes bear-

ing 2,6-substituted aryl groups (13, 14, and 21, with Me, iPr

and F substituents, respectively), the N-aryl plane is nearly per-

pendicular to that of the iminopyrrolyl fragment in 13 and 14,

as reflected in very high dihedral angles (81.6(3)8 for 13,

Scheme 2. Synthesis of iminopyrrolyl-BPh₂complexes 12–21.

The mixtures were heated overnight in refluxing toluene under

nitrogen atmosphere. The iminopyrrolyl organoboron com-

pounds [BPh₂{k²N,N’-NC₄H₃C(H)=N-Ar}], with Ar=2,6-Me₂-C₆H₃

(13), 2,6-iPr₂-C₆H₃(14), 4-OMe-C₆H₄(15), 3,4-Me₂-C₆H₃(16), 4-F-

C₆H₄(17), 4-NO₂-C₆H₄(18), 4-CN-C₆H₄(19), 3,4,5-F₃-C₆H₂(20),

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Figure 1. Perspective views of the molecular structures of iminopyrrolyl boron complexes 13–17, 19, and 21. Ellipsoids are drawn at the 30% probability

level. All calculated hydrogen atoms have been omitted for clarity.

Photoluminescence properties

75.3(6)8 for 14). A dihedral angle of 70.3(3)8 is observed for 21,

owing to the lower steric volume of the F atom compared to

the Me and iPr groups. In the remaining complexes 15–17 and

19, in which H atoms occupy the 2,6-positions of the N-aryl

ring, the dihedral angles are considerably smaller (41.82(19)8

for 15, 39.84(16)8 for 16, 45.7(2)8 for 17, and 27.4(2)8 for 19).

The other structural features of these boron complexes resem-

ble those of similar analogues^{[12a,d,17,19]}and will be further dis-

cussed below, when comparing them with those of excited

states.

The UV/Vis absorption and fluorescence spectra of complexes

12–21 in THF are shown in Figure 2 (and Figure S8 in the Sup-

porting Information) and relevant photophysical data, listed in

order of increasing absorption wavelength maxima, are sum-

marized in Table 3. In contrast to their non-emissive ligand pre-

cursors 2–11, most of the boron complexes are highly emissive

in solution.

The absorption spectra show wavelength maxima (corre-

!

sponding to the S_1,1S_0,0vibronic transition) within the range

353–421 nm in THF, which depend on the steric and electronic

properties of the N-aryl substituents. The absorption spectra of

the 2,6-substituted compounds (13, 14, and 21) are considera-

bly blue-shifted with respect to that of the unsubstituted

parent compound (12). This feature is due to the large torsion

angle between the N-aryl ring and the iminopyrrolyl fragment

(>708, see the last row of Table 1), which prevents p-conjuga-

tion (resonance) between the two fragments. On the other

hand, substitution at the 3-, 4-, or 5-positions of the N-aryl ring

leads to red-shifts of the absorption maxima, the magnitudes

of which also depend on the electron-donating or electron-

withdrawing properties of these substituents. The correlation

of the absorption energy maxima with IPꢀEA (Table 2 and Fig-

ure S9a in the Supporting Information), which assumes propor-

tionality between the S₀-S₁and HOMO–LUMO energy gaps, is

surprisingly good (except for compound 21), given the ab-

sence of CIS (configuration interaction of states),^[27]which is re-

quired for conversion of orbital energies into state energies.

The fluorescence emission maxima (412–505 nm) correspond

to the S_1,0!S_0,1vibronic transition, except in the case of the

Electrochemical properties

Cyclic voltammetry measurements on compounds 12–21 were

performed in electrolyte solutions in dichloromethane. The re-

spective onset oxidation and reduction potentials were used,

after conversion to the absolute scale taking the Fc/Fc⁺(ferro-

cene/ferrocenium ion redox couple) as a reference, to estimate

their ionization potentials (IP) and electron affinities (EA).^[25]

The values obtained are summarized in Table 2.

With the exception of compounds 13, 14, and 21, bearing

particularly twisted aryl groups owing to restricted rotation

about the N-aryl bond, the ionization potentials are clearly cor-

related with the electron-donating or electron-withdrawing

properties of the substituents, as represented by the sum of

their classical Hammett s constants^[26](Table 2 and Figure S7a

in the Supporting Information). The electron affinities also

show a correlation, although that of the strongly conjugated

nitro substituent (compound 18) falls outside the relevant

trend line (Figure S7b in the Supporting Information).

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is, a rather planar ground-state

geometry, in agreement with the

lowest computationally calculat-

ed dihedral angle between the

iminopyrrolyl and aryl moieties

in this complex (25.078, see

Figure 3). Accordingly, com-

pound 19, which has the second

lowest dihedral angle (25.88),

shows the second best vibration-

ally resolved emission (Figure 2).

As observed for their absorp-

tions, the fluorescence of the

Table 1. Selected bond distances [ꢅ] and angles [8] for complexes 13–17, 19, and 21.

13^[a]

14^[a]

15

16

17

19

21^[a]

Distances

N1ꢀC2

N1ꢀC5

N2ꢀC6

N2ꢀC7

N1ꢀB1

1.376(3)

1.347(3)

1.297(3)

1.439(3)

1.561(4)

1.645(4)

1.594(4)

1.615(4)

1.364(6)

1.356(6)

1.299(6)

1.452(6)

1.541(7)

1.625(7)

1.605(7)

1.591(8)

1.375(2)

1.347(2)

1.304(2)

1.424(2)

1.566(2)

1.634(2)

1.618(2)

1.604(2)

1.371(2)

1.407(2)

1.3749(15)

1.3465(15)

1.3083(14)

1.4301(14)

1.5660(15)

1.6306(16)

1.6206(18)

1.6189(16)

1.5061(17)

1.3794(19)

1.3467(19)

1.3172(18)

1.4285(18)

1.565(2)

1.633(2)

1.621(2)

1.608(2)

1.3593(17)

1.381(2)

1.337(2)

1.319(2)

1.426(2)

1.569(3)

1.630(2)

1.603(3)

1.613(3)

1.439(3)

1.145(2)

1.381(3)

1.343(3)

1.313(3)

1.418(3)

1.561(3)

1.644(3)

1.603(4)

1.611(4)

1.343(3)

N2ꢀB1

C_aryl1ꢀB1

C_aryl2ꢀB1

[b]

XꢀC_N-aryl-p

[b,c]

YꢀXꢀC_N-aryl-p

[d]

XꢀC_N-aryl-m

1.5047(17)

1.342(3)

1.339(3)

1.332(3)

1.347(3)

violet-emitting

2,6-substituted

[e]

XꢀC_N-aryl-o

1.501(4)

1.496(4)

1.508(7)

N-aryl sterically hindered com-

pounds 13, 14, and 21 is sub-

stantially shifted to shorter

wavelengths with respect to that

of 12, while that of the remain-

ing blue to blue-greenish-

emitting compounds is shifted

to longer wavelengths. Again,

some correlation with the

LUMO-HOMO energy gap is ob-

served (Table 2 and Figure S9b

in the Supporting Information),

except in the cases of com-

pounds 18 and 21.

Angles

C2-N1-C5

C2-N1-B1

C5-N1-B1

C6-N2-C7

C6-N2-B1

C7-N2-B1

N1-B1-N2

107.2(2)

112.4(2)

139.2(2)

120.6(2)

110.7(2)

128.6(2)

94.69(19)

116.0(2)

113.8(2)

109.0(2)

111.3(2)

110.0(2)

81.6(3)

107.1(5)

112.7(4)

139.1(5)

119.7(4)

110.9(4)

128.4(4)

94.9(4)

114.7(5)

108.5(4)

115.8(5)

111.6(4)

109.7(4)

75.3(6)

107.88(13)

112.14(12)

139.68(14)

121.64(13)

111.03(12)

126.86(12)

94.90(11)

117.82(13)

112.91(12)

110.28(12)

107.25(12)

111.26(11)

41.82(19)

107.46(9)

112.00(9)

140.45(10)

121.03(9)

110.52(9)

128.28(9)

95.30(8)

112.91(9)

111.61(9)

110.72(9)

110.07(9)

114.99(9)

39.84(16)

107.78(12)

112.51(12)

139.50(13)

121.43(12)

110.87(12)

127.51(11)

94.90(11)

115.76(12)

112.07(12)

109.96(12)

109.59(12)

112.70(12)

45.7(2)

107.70(15)

112.20(14)

140.05(16)

122.30(15)

110.76(14)

126.89(14)

95.35(13)

118.88(16)

110.45(15)

108.82(15)

111.02(14)

110.02(14)

27.4(2)

107.7(2)

113.00(18)

139.3(2)

122.52(19)

111.30(18)

124.78(18)

93.81(17)

119.2(2)

111.77(19)

112.2(2)

110.81(17)

105.93(18)

70.3(3)

C_aryl1-B1-C_aryl2

N1-B1-C_aryl1

N1-B1-C_aryl2

N2-B1-C_aryl1

N2-B1-C_aryl2

C6-N2-C7-C12^[f]

[a] Due to the similarity of the parameters for the molecules in the asymmetric unit, only bond distances and

angles for molecule A are presented. [b] X=O (15), C (16, 19), F (17, 21). [c] Y=C (15), F (17). [d] X=C (16), F

(17). [e] X=C (13, 14), F (17). [f] For molecule B, the dihedral angle C6-N2-C7-C12 is 80.9(3)8 in 13, 79.4(6)8 in

14, and 75.1(3)8 in 21.

The fluorescence quantum

yields and lifetimes are dictated

by the non-radiative rate con-

stants (k_nr) (the variation of the

radiative rate constant k_famong

all of the compounds is relatively

small). The 2,6-substituted compounds 13 and 14 show the

Table 2. Ionization potentials (IP) and electron affinities (EA) of com-

plexes 12–21, estimated from cyclic voltammetry measurements, derived

largest values of k_nr(ca. 10^{10 ꢀ1}) and, consequently, very low

s

IPꢀEA values, energies of the absorption maximum (E_a^m_b^a_s^x) and of the first

fluorescence quantum yields (0.005 and 0.023). Conversely, the

highest quantum yields are observed for 18, 19, and 20, which

show the lowest k_nrvalues (ca. 10⁸s^ꢀ1). In order to clarify the

origin of these differences in the k_nrvalues, triplet formation

quantum yields f_Twere measured for 13, 12, and 19, as com-

pounds representative of high, medium, and low values of k_nr,

respectively (Table 4).

0ꢀ0

vibronic transition (E ), and the sum of Hammett s constants of the cor-

em

responding N-aryl substituents.

max

abs

0ꢀ0

[a]

Cmpd N-aryl ring

IP

EA

IPꢀEA

E

s_o+s_m+s_p

em

substituents [eV] [eV] [eV]

[eV] [eV]

12

13

14

15

16

17

18

19

20

21^[b]

none

5.64 2.82 2.82

5.82 2.56 3.26

5.83 2.56 3.27

5.45 2.74 2.71

5.52 2.74 2.78

5.66 2.86 2.80

5.83 3.43 2.40

5.78 3.06 2.72

5.79 2.99 2.80

5.58 3.01 2.57

3.24 2.75

0

2,6-Me₂

2,6-iPr₂

4-OMe

3,4-Me₂

4-F

4-NO₂

4-CN

3,4,5-F₃

2,3,4,5,6-F₅

3.51 3.20 ꢀ0.34

3.51 3.21 ꢀ0.46

3.21 2.65 ꢀ0.29

3.23 2.72 ꢀ0.02

For the 2,6-dimethyl-substituted compound 13, no triplet

formation was found, indicating that exclusively internal con-

version is responsible for the non-radiative decay of the singlet

excited state S₁of this compound. The exceptionally high

value of k_icis also responsible for the abnormally low fluores-

cence quantum yield and lifetime (36 ps) of 13 within this

series of compounds. For the parent (unsubstituted) com-

pound 12, f_Tis also low (ca. 1%) and internal conversion is

similarly the dominant mechanism of non-radiative decay.

Finally, the cyano-substituted compound 19 has a tenfold

lower k_icrelative to 12, most likely due to the extended p-con-

jugation of the cyanophenyl and iminopyrrolyl moieties, which

may increase the rigidity of the complex in the S₁excited

state.

3.25 2.77

2.95 2.67

3.07 2.74

3.18 2.79

3.38 2.87

0.05

0.78

0.66

0.73

1.23

[a] Sum of the Hammett constants (ref. [26]). [b] Compound unstable in

solution.

nitro-substituted compound 18, for which the maximum corre-

sponds to the S_1,0!S_0,0transition. The higher intensity of the

S_1,0!S_0,0transition of 18 indicates a smaller change in the

equilibrium geometries of its ground and excited states, that

Chem. Eur. J. 2014, 20, 1 – 16

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of the boron compounds 12–21. The geometries of the

ground state and the first excited singlet state (obtained by

promoting one electron from the HOMO to the LUMO) were

fully optimized. The BP86 functional and a triple-z basis set, as

used for geometry optimization of complex 12 (only hydrogen

atoms on the arene ring) and others in a previous study,^[18]

were also employed here in order to compare the present re-

sults with previously obtained data relating to other boron

compounds. Time-dependent DFT (TD-DFT)^[30]was used to

obtain the absorption spectra and the lifetimes of the first ex-

cited singlet states (same functional and basis set). Some calcu-

lations were performed in the gas phase and others also in

THF, as specified in the text below.

Molecular geometries

The optimized geometries of the ground states and the first

excited singlet states are shown in Figure 3 (and Figure S12 in

the Supporting Information), emphasizing the dihedral angle

between the iminopyrrolyl and aryl moieties. The agreement

between the calculated and experimental dihedral angles of

the ground-state compounds is generally very good, consider-

ing that rotation about the NꢀC bond is essentially unhindered

(Figure 3). The best agreements are observed for 19 (CN), with

values of 27.48 and 25.88, and 14, with values of 75.38 and

74.28. In 15, 16, and 17, the deviations range from about 10 to

158. No experimental structures could be obtained for 18 and

20. The largest deviations are observed in 13 (Me₂) and 21 (F₅),

and can probably be attributed to dimeric association of the

molecules through weak CꢀH···p hydrogen bonds in the corre-

sponding crystal structures. The aryl ring is distorted in order

to maximize hydrogen bonding. This interaction energy was

not calculated, since it is outside the scope of this work. For

the compound with the bulkiest ortho substituents, 14 (iPr₂),

a perfect agreement between the calculated and experimental

dihedral angles was observed, despite also existing as a dimer.

Based on the good agreement between the experimental and

calculated structural parameters, we trust this methodology to

provide reliable estimates of the geometrical features of com-

pounds 18 and 20, for which no X-ray structures are available.

Some trends are evident in these structures, namely that the

dihedral angle is always higher than 258 in the ground-state

structures and drops to close to zero (planar ligand) in the sin-

glet excited states of all of the compounds, except those with

two ortho substituents (13 (Me₂), 14 (iPr₂), 21 (F₅)). As men-

tioned above, the reason for this is the steric hindrance of the

substituents, which prevents the attainment of planarity in this

state, and is reflected in the higher angles of 41.678 in 14

(iPr₂), 35.788 in 13 (Me₂), and 22.048 in 21 (F₅).

Figure 2. Normalized a) absorption and b) emission spectra of complexes

12–21 in THF.

It is interesting to note that, except for the compounds

bearing aryl groups with substituents at the 2,6-positions, 13,

14, and 21, general trends are observed for the emissions of

complexes 12–21: the use of increasingly electron-withdrawing

N-aryl substituents shifts the emission wavelength maxima to

higher energies and increases the fluorescence quantum yields

(see Figures S10a and S10b in the Supporting Information, re-

spectively). These electronic effects, along with the steric effect

of 2,6-substitution, enable color tuning of this molecular

system in the range from violet to blue to bluish-green (see

Figure S11 in the Supporting Information).

Interestingly, these geometrical features seem to correlate

well with the low experimental fluorescence quantum yields f

f

(Table 3). In particular, the nonplanarity of the ligand in the sin-

glet excited state of the hindered 2,6-substituted N-aryl deriva-

tives may be associated with very low f values of 0.005 for 13

(Me₂) and 0.023 for 14 (iPr₂). Compounds 15–20 (and also

12^[18]), with virtually planar ligands in their singlet excited

f

Density functional calculations

DFT calculations^[28](ADF^[29]program) were performed in order

to understand the influence of substituents on the properties

states, display much higher values of f. The ligand in the sin-

f

&

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glet state of compound 21 is far from planar (22.048),

although less so than in 13 and 14 (35.88 and 41.78,

respectively), but its fluorescence quantum yield is

higher than those of other compounds, such as 15

and 16. In this compound, the steric effect is accom-

panied by the electronic effect of the five electroneg-

ative fluorine atoms, and it cannot be analyzed sepa-

rately.

Table 3. Wavelength maximum (l_a^m_b^a_s^x) and molar extinction coefficient (e_max) of the

first absorption band, wavelength maximum (l^m_em^ax) and wavelength of the first vibron-

ic transition (l⁰_em^ꢀ0) of the emission band, fluorescence quantum yield (f), fluorescence

f

lifetime (t_f), radiative rate constant (k_f), and sum of non-radiative rate constants (k_nr) of

boron complexes 12–21 in THF at 293 K.

max

abs

max

em

0ꢀ0[b]

[e]

Cmpd N-aryl ring Ss_i^[a]

l

e_max

l

f

t_f^[c]

k_f^[d]

k_nr

em

substituents

[nm] [10⁴Lmol^ꢀ1[nm] [nm]

[ns] [ns^ꢀ1] [ns^ꢀ1

]

cm^ꢀ1

]

The energies and three-dimensional representa-

tions (Molekel^[31]) of the HOMOs and LUMOs of all of

the compounds (determined in THF) are shown in

Figure 4 (and Figure S13 in the Supporting Informa-

tion). Both the HOMO and the LUMO of all of the

compounds are essentially located on the iminopyr-

rolyl ligand, the contribution of the aryl moiety being

negligible in 14 (iPr₂) and 13 (Me₂), as well as in the

LUMO of 21 (F₅).

14

2,6-iPr₂

2,6-Me₂

2,3,4,5,6-F₅

4-F

ꢀ0.46 353 1.7

412 386 0.023 0.13 0.18 7.52

413 388 0.005 0.036 0.13 27.6

462 432 0.20 1.04 0.19 0.77

478 448 0.25 1.53 0.16 0.49

479 451 0.34 1.90 0.18 0.35

492 456 0.14 1.22 0.11 0.70

505 468 0.13 1.07 0.12 0.81

474 445 0.47 2.72 0.17 0.19

482 453 0.71 3.28 0.22 0.09

466 464 0.50 1.83 0.27 0.27

13

ꢀ0.34 353 1.7

1.22 367 2.1

0.06 381 1.9

21^[f]

17

12^[g]none

0

383 1.7

16

15

20

19

18

3,4-Me₂

ꢀ0.24 384 2.0

ꢀ0.27 386 2.1

0.74 390 2.0

0.67 404 2.2

0.78 421 2.0

4-OMe

3,4,5-F₃

4-CN

4-NO₂

[a] Ss_i=s_o+s_m+s_p(ref. [26]). [b] From spectral decomposition with sums of four

Gaussians. [c] From single exponential decays. [d] k_f=f/t_f. [e] k_nr=(1ꢀf)/t_f. [f] Com-

f

pound unstable in solution. [g] Ref. [17].

Electronic state transitions

The absorption maxima were calculated using the

TD-DFT approach described above, both in the gas

Figure 3. Optimized geometries of the iminopyrrolyl boron complexes 13–21 in the ground state and in the first excited singlet state, showing the dihedral

angles (8).

Chem. Eur. J. 2014, 20, 1 – 16

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duce very well the experimental values for most of the com-

pounds, with deviations between 0 and 0.11 eV, exceptions

being 14 (0.25 eV), 15 (0.25 eV), and 18 (0.35 eV). The SO

values show better agreement for compound 15 (0.04 eV), but

greatly overestimate the energy of the absorption maxima by

a mean deviation of 0.29 eV for all the other compounds. The

introduction of the solvent (THF) effect in the calculation of ab-

sorption energies also led to large deviations from the experi-

mental values, depending on the nature of the substituents.

With electronegative substituents, the calculated energies

were lower than the experimental values, the largest devia-

tions being observed for compounds 18 (NO₂) and 15 (OMe)

(0.76 and 0.44 eV, respectively), followed by the fluorinated de-

rivatives (17, 20, 21) with smaller deviations (0.23, 0.16,

0.20 eV). For the other species, the shifts were in the opposite

direction and smaller (0.1–0.2 eV). In summary, the gas-phase

values were, in general, closer to the experimental ones.

The low-energy transition was 90% HOMO (H) to LUMO (L)

for compound 12 (H), both orbitals being delocalized over the

aryliminopyrrolyl moiety.^[18]Two other compounds, 13 and 19,

were studied in more detail.

Table 4. Fluorescence quantum yield (f) and lifetime (t_f), triplet forma-

f

tion quantum yield (f_T) and lifetime (t_T), and radiative (k_f), internal con-

version (k_ic), and intersystem crossing (k_isc) rate constants of boron com-

plexes 13, 12, and 19 in THF at 293 K.

[a]

[b]

Cmpd N-aryl ring

f

t_f

f_Tt_T

k_f

k_nr

k_isc

k_ic

substituents

[ns]

[ms] [ns^ꢀ1] [ns^ꢀ1] [ns^ꢀ1] [ns^ꢀ1

]

13

12

19

2,6-Me₂

none

4-CN

0.005 0.036 0

0.34 1.9 0.01 1.87 0.18

0.71 3.28 0.2 89.5 0.22

–

0.13 27.6 0.00 27.60

0.35 0.01

0.09 0.06

0.34

0.03

[a] k_isc=f_T/t_f. [b] k_ic=k_nrꢀk_isc

.

In 13 (2,6-Me₂), the low-energy band has contributions from

three transitions: H-5!L (98%); H-7!L (49%)+H!L (40%);

H-6!L (71%)+H!L (19%). The HOMO and LUMO are local-

ized on the iminopyrrolyl part of the ligand, while H-5 and H-7

are essentially on the aryl ring and H-6 has some contribution

from boron (see Figure 5, as well as Figure S14 in the Support-

ing Information). Thus, the band is characterized by significant

intraligand charge transfer from the aryl to the iminopyrrolyl

moiety, which may contribute to the measured low fluores-

cence yield (Table 3).

Figure 4. The energies (eV) and three-dimensional representations of the

HOMOs and LUMOs (in THF) of the iminopyrrolyl boron complexes 13–21.

phase (GP) and in THF, since the solvent often influences the

absorption features. This methodology, however, does not

allow the calculation of the radiative lifetime of the singlet ex-

cited state (1/k_f), which requires the introduction of spin-orbit

coupling. It can be obtained from a scalar relativistic TD-DFT

approach, which uses a perturbative method including the in-

fluence of spin-orbit coupling, SOPERT (SO),^[32]and has been

shown to yield better results when using a hybrid PBE0 func-

tional in conjunction with an all-electron basis set (TZ2P was

still used). Having performed this calculation, we used other

available information on absorption spectra for comparison

with our standard approach.

The calculated absorption energy maxima are given along

with the experimental values in Table 5. The GP values repro-

Table 5. Calculated (GP, THF, and SO) and experimental energy maxima

[eV] of the lowest energy band.

Figure 5. The nature of the transitions leading to the lower-energy absorp-

tion bands for 13 (2,6-Me₂) (left) and 19 (CN) (right).

max

abs

max

abs

max

abs

Cmpd

N-aryl ring

E_a^m_b^a_s^x(exp)

E

(GP)

E

(THF)

E

(SO)

substituents

[eV]

In 19 (CN), the low-energy band has contributions from two

transitions: H!L (84%)+H-5!L (5%); H-5!L (88%)+H!L

(6%). The HOMO and LUMO are localized on the cyanoarylimi-

nopyrrolyl ligand, while H-5 is almost completely on the pyrrol-

yl moiety, with a small contribution from boron (Figure 5). In

this compound, the intraligand contribution of the pyrrolyl

group is dominant, even compared to the unsubstituted com-

pound 12, which may be responsible for the increased fluores-

cence yield (Tables 3 and 4).

14

13

21

17

12

16

15

20

19

18

2,6-iPr₂

2,6-Me₂

2,3,4,5,6-F₅

4-F

3.51

3.38

3.25

3.24

3.23

3.21

3.18

3.07

2.95

3.76

3.62

3.38

3.25

3.29

3.16

2.96

3.23

3.14

2.60

3.79

3.75

3.18

3.02

3.34

3.17

2.77

3.02

3.16

2.19

3.92

3.86

3.70

3.47

3.48

3.41

3.25

3.47

3.40

3.24

none

3,4-Me₂

4-OMe

3,4,5-F₃

4-CN

4-NO₂

&

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The emission wavelength was calculated from the energy

difference between the excited singlet state and the ground

state with the same geometry, both in the gas phase (GP) and

in THF. The geometry was optimized using the standard ap-

proach in ADF (BP86 functional) as described above. Repro-

ducing emission energies is difficult, as has been reported by

other authors. Since new and (in principle) better methodolo-

gies have become available for optimizing the excited states

with TD-DFT, combining several excitations and affording the

emission energy directly, we thought that it would be interest-

ing to compare them with the established approach involving

only the HOMO to LUMO excitation. The radiative lifetimes

(1/k_f) of the singlet excited states were obtained from SO cal-

culations, as mentioned above, allowing calculation of the k_f

values. All of the results are collected in Table 6 and compared

with the experimental values (emission maxima).

agreement is perfect for compounds 12 and 13. For the com-

pounds with p-donating substituents, such as F (17, 20) or

OMe (15), the calculated fluorescence rate constants are very

close to the experimental values, whereas for those bearing p-

accepting groups such as cyanide (19) or nitro (18), the agree-

ment is poor. Compound 21, with five electronegative F sub-

stituents, two of them in ortho positions, represents the largest

challenge for the calculation method, and the calculated excit-

ed singlet-state lifetime was the furthest away from the experi-

mental value.

Electroluminescence properties

Light-emitting diodes, LEDs, with the simple ITO/PEDOT:PSS/

complex (12–21)/Ca/Al structure, were firstly prepared, with

films of the emissive complexes being deposited by spin-coat-

ing inside a glove box. The relative positions of the

frontier energy levels of the components used in the

max

Table 6. Calculated (GP, THF, and TD-DFT) and experimental energy maxima (E ) of

the emission bands and calculated (SO) and experimental fluorescence rate constants

(k_f) of boron complexes 12–21.

various LED structures are shown in Figure S17 in the

Supporting Information. The emissions obtained from

LEDs based on the complexes with the lowest solu-

tion PL quantum yields, 13 and 14 (containing 2,6-

substituted N-aryl rings), were very dim. We could

not record their electroluminescence (EL) spectra. In

addition, LEDs based on the complexes with strongly

electron-accepting groups (18, 20, and 21) also

showed very low emission intensities, in spite of their

em

max

em

max

em

max

em

max

em

Cmpd N-aryl ring

E

(exp)

E

(GP)

E

(THF)

E

(TD-DFT) k_f(exp) k_f(SO)

substituents [eV]

[eV]

[ns^ꢀ1

]

[ns^ꢀ1

]

14

13

21

17

12

16

15

20

19

18

2,6-iPr₂

2,6-Me₂

2,3,4,5,6-F₅

4-F

3.01

3.00

2.68

2.59

2.52

2.46

2.62

2.57

2.66

2.12

1.97

2.05

1.86

1.89

1.83

1.76

1.88

1.87

2.02

2.17

2.02

2.09

1.92

1.96

1.88

1.79

1.95

1.93

nc^[a]

3.02

nc^[a]

3.00

2.94

3.01

3.25

3.23

2.73

nc^[a]

0.18

0.13

0.19

0.16

0.18

0.11

0.12

0.17

0.22

0.27

0.01

0.13

0.004

0.22

0.18

0.25

0.19

0.14

0.08

0.11

none

moderate solution fluorescence quantum yields (f).

3,4-Me₂

4-OMe

3,4,5-F₃

4-CN

f

Among these, it is worth mentioning the results ob-

tained for 18, which showed a solution quantum

yield of 50%, yet the emission intensity of its LED

was the lowest among the series. It is also interesting

to note that for these complexes there is no elec-

tron-injection barrier with respect to the calcium

cathode, pointing to an “excess” electron injection

4-NO₂

[a] Not converged.

The GP energy values are systematically much higher for all

of the compounds, which is not surprising since they rely on

a single HOMO–LUMO excitation. The solvent correction (THF

values) shifts all calculated values closer to the experimental

ones, but only by a small amount. For instance, the calculated

emission of compound 13 (Me₂) is increased from 1.97 to

2.02 eV, but is still too far from the experimental value of

3.00 eV. On the other hand, the TD-DFT values reproduce the

observed energies more closely, almost perfectly for 13, but

with deviations of 0.16 eV for 19 (CN) and 0.35–0.79 eV for the

other compounds. Although these calculations require much

longer times, the inclusion of several excitations leads to

a good agreement between calculated and experimental emis-

sions.

with respect to the hole injection from PEDOT:PSS. It is also

possible that there may be significant PL quenching in the

solid state compared to solution, contributing to the very poor

device performance. However, LEDs based on complexes 15,

16, 17, and 19 showed significant emission intensities, with 16

exhibiting a maximum luminance of 614 cdm^ꢀ2with a peak EL

efficiency of 0.2 cdA^ꢀ1(see Table 7). Although 16 showed only

a modest solution photoluminescence (PL) quantum yield (f)

f

of 14% (see Table 3), it was the complex that performed best

in this simple LED structure. From Figure S17 (Supporting Infor-

mation), we conclude that, among these four complexes, 15

and 16 have similar electron- and hole-injection barriers. The

difference in LED emission efficiencies may possibly result from

differences in charge transport, assuming that these complexes

The calculation of emission energies is still a challenge and

therefore several computational approaches must be tested.

Owing to their limitations, it is not yet possible to give

a simple picture of emission that has a reliable predictive capa-

bility.

have similar f values in the solid state and in solution, with 16

exhibiting more equilibrated hole and electron mobilities.

f

Complexes 15–20, those in this new series that showed the

highest solution f values, were further tested in LEDs, for

f

which they were deposited by sublimation on top of either PE-

DOT:PSS or N,N’-bis(3-methylphenyl)-N,N’-diphenylbenzidine

(TPD)-coated PEDOT:PSS. Barium was used as cathode material,

protected with an overlayer of Al. Among the ITO/PEDOT:PSS/

The excited singlet-state lifetimes are given as fluorescence

rate constants (k_f). The values are generally close to the experi-

mental ones, with small deviations in both directions. The

Chem. Eur. J. 2014, 20, 1 – 16

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tron-hole recombination. In fact,

we observed a significant im-

provement in performance of

the LEDs based on these six

complexes (15–20), although the

devices based on 18 and 20 still

showed very poor performances.

Among the other LEDs, those

based on 16 showed the best

performance, as observed for

the other device types, with

a maximum luminance of nearly

3000 cdm^ꢀ2and a maximum EL

efficiency of 1.4 cdA^ꢀ1. In fact,

the devices based on complexes

15 and 16, with electron-donat-

ing groups on the N-aryl moiety,

displayed the highest luminan-

ces, in spite of the modest solu-

Table 7. Maximum luminance (L_max) in [cdm^ꢀ2], luminous EL efficiency (f_EL,max) in [cdA^ꢀ1], and external quan-

tum efficiency (EQE_max) in [%] of LEDs based on complexes 12–21. Films were prepared by spin-coating (in the

case of the Ca-based LEDs) or by sublimation.

Complex N-aryl ring

ITO/PEDOT:PSS/complex/ ITO/PEDOT:PSS/com-

ITO/PEDOT:PSS/TPD/com-

plex/Ba (sublimed)

substituents

Ca (spin-coating)

plex/Ba (sublimed)

12

13

14

15

16

17

18

19

20

21

none

L_max

0.35

–

f_EL,max3.8ꢆ10^ꢀ4

EQE_max1.5ꢆ10^ꢀ4

2,6-Me₂

2,6-iPr₂

4-OMe

3,4-Me₂

4-F

L_max

0.096

f_EL,max5.6ꢆ10^ꢀ6

EQE_max6.6ꢆ10^ꢀ6

L_max

0.085

f_EL,max3.56ꢆ10^ꢀ6

EQE_max1.92ꢆ10^ꢀ6

–

22

–

500

L_max

f_EL,max0.036

EQE_max0.013

L_max

f_EL,max0.2

EQE_max0.079

L_max

f_EL,max0.005

31

0.006

0.002

755

0.31

0.12

53

0.03

0.01

0.28

4.8ꢆ10^ꢀ4

1.6ꢆ10^ꢀ4

80

0.072

0.027

0.26

2.09ꢆ10^ꢀ5

8.2ꢆ10^ꢀ6

–

0.16

0.058

2965

1.4

0.48

384

0.22

0.087

2.1

2.85ꢆ10^ꢀ3

9.9ꢆ10^ꢀ4

246

0.089

0.036

1.4

8.5ꢆ10^ꢀ5

3.4ꢆ10^ꢀ5

–

614

43

tion f values (13% and 14%, re-

f

EQE_max0.002

spectively) of these complexes.

4-NO₂

4-CN

L_max

0.066

f_EL,max2.36ꢆ10^ꢀ5

LEDs based on 19, the complex

EQE_max8.2ꢆ10^ꢀ6

showing the highest solution f

f

L_max

–

(71%), were not the best per-

forming. Figure 6 (and Fig-

ure S15 in the Supporting Infor-

mation) compares the perform-

ances of LEDs based on 16 and

19, having barium cathodes, and

with or without an HT/EBL made

of TPD. In both cases, the inser-

tion of TPD led to an increase in

f_EL,max

EQE_max

L_max

f_EL,max0.002

EQE_max0.001

3,4,5-F₃

0.012

2,3,4,5,6-F₅L_max

0.28

f_EL,max6.4ꢆ10^ꢀ5

–

EQE_max3.3ꢆ10^ꢀ5

sublimed complex/Ba devices, those based on 16 showed the

best performance, as found in the Ca-based LEDs, with a maxi-

mum luminance of 755 cdm^ꢀ2and a maximum EL efficiency of

0.31 cdA^ꢀ1. As observed for the first series of LEDs, those

based on complexes 18 and 20 showed the poorest perform-

ances. The use of Ba in place of Ca should improve the elec-

tron-injection efficiency in the case of complexes 15, 16, and

possibly also 17 (albeit to a lesser extent), maintaining the

ohmic electron injection for the remaining complexes of the

studied series. We attribute the observed differences in per-

formance for the devices based on complexes 18–20 to the

different film preparation processes (sublimation vs solution

spin-coating). We consider that the variation in the performan-

ces of the devices based on complexes 15–17 is inconsistent

with reduction of the electron-injection barrier upon the use of

Ba (which should intrinsically improve the EL efficiency, but

was not realized for 15). Again, we believe that the change in

the film preparation process is the main contributing factor.

The insertion of a TPD hole-transporting/electron-blocking

layer (HT/EBL), with a HOMO lying slightly lower in energy

than the PEDOT:PSS workfunction, should improve hole injec-

tion for all of the complexes by creating an intermediate level

between the PEDOT:PSS workfunction and their HOMO levels.

In addition, its high-lying LUMO prevents electrons from escap-

ing to the PEDOT:PSS electrode, thereby improving the elec-

the maximum luminance and also enhanced the EL efficiency

(in the case of 16, it led to an increase in the maximum EL effi-

ciency from 0.31 to 1.4 cdA^ꢀ1, whereas in the case of 19 it led

to a much more modest increase from 0.07 to 0.09 cdA^ꢀ1). It is

also interesting to note that while the solution PL efficiency of

Figure 6. Current (I, filled symbols) and luminance (L, open symbols) for

LEDs based on 16 (film thickness 80 nm) and 19 (film thickness 60 nm). Re-

sults are shown for ITO/PEDOT:PSS/complex/Ba/Al and ITO/PEDOT:PSS/TPD/

complex/Ba/Al device structures.

&

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19 was higher than that of 16 (71% versus 14%), the reverse

was true for the EL efficiencies. This behavior could be due to

differences in PL efficiency in the solid state (with detrimental

stronger dipole interactions in the case of 19), higher hole-in-

jection barrier, due to a higher ionization potential of 19 with

respect to that of PEDOT:PSS (5.2 eV), and possibly also differ-

ences in hole and/or electron mobilities.

LED characteristics of which are shown in Figure 6) and 18,

which bears a strongly electron-withdrawing group (NO₂) and,

in spite of its reasonable solution PL efficiency of 50%, yielded

poorly performing LEDs. For complex 16, the solution and

thin-film PL spectra are similar (with a red-shift of the emission

maximum from 490 to 505 nm on going from solution to

a thin film), with the EL spectrum having a maximum at an in-

termediate wavelength (Figure 7A). The solid-state PL spec-

trum of complex 19 shows a significantly red-shifted maximum

(516 nm) with respect to the solution PL (maximum at

482 nm), and a stronger tail at longer wavelengths (Figure 7B).

Surprisingly, the EL spectrum is not as shifted toward lower en-

ergies as the PL spectrum. This is somewhat unexpected, as

we would suppose the accessible excited states to be the

same under both photo and electrical excitation. We do not

yet have a definitive explanation for this observation as, in

view of the spectral shapes, interference effects are unlikely to

be the cause. For complex 18, the red-shift of the thin-film PL

spectrum with respect to that obtained from the solution is

even more pronounced, with the maximum increasing from

about 466 nm to 524 nm (Figure 7C). This effect, combined

with the loss of vibronic resolution, indicates that a different

lowest-energy state appears in the solid state, which is most

likely of intermolecular origin. The emission of the 18-based

LEDs was too weak to allow us to record an EL spectrum,

which indicates that, in spite of the high solution PL efficiency

of 50%, the lowest-energy state formed in the solid state has

insufficient emission efficiency.

From the results shown in Table 7, it can be concluded that

the presence of electron-withdrawing substituents on the N-

aryl fragment is generally detrimental to device performance.

Complexes 17 and 19, with fluoro and cyano substituents in

the para position, respectively, show intermediate performan-

ces. With an increase in the number of groups and/or their

electron-withdrawing strengths, the performance decreases

even further.

Figure 7 (and also Figure S16 in the Supporting Information)

compares the PL spectra of three complexes, 16 and 19 (the

Conclusion

Several tetracoordinate mononuclear organoboron complexes

containing 2-(N-arylformimino)pyrrolyl ligands with N-aryl ring

substituents of different electronic and steric natures have

been synthesized and characterized by multiple methods. X-

ray crystallography studies have confirmed chelation of these

ligands to the Lewis acidic diphenylboron moiety. The resulting

complexes have a pseudo-tetrahedral geometry about the

boron atoms, and the dihedral angle between the N-aryl plane

and the iminopyrrolyl fragment depends upon the bulkiness of

the substituents in the 2,6-positions of the aryl groups. It was

found that the electronic properties of these fluorescent com-

pounds could be varied in a predictable manner by changing

the electron-donating or electron-withdrawing character of the

aniline substituents. For instance, the color of emission could

be tuned from blue to bluish-green by increasing the electron-

donating strength of the substituents. This change was at the

expense of the fluorescence quantum yield. DFT calculations

have shown the geometry of the iminopyrrolyl ligand in the

excited singlet state to be almost planar, except when ortho

substituents are present, in contrast to the nonplanar ground-

state structure. The low-energy band leading to emission is es-

sentially a HOMO to LUMO transition. The more delocalized

frontier orbitals seem to be associated with higher fluores-

cence quantum yield, whereas the introduction of some

charge transfer between the ligands has the opposite effect.

DFT methods lead to very good estimates of the absorption

Figure 7. Comparison of emission spectra (PL recorded in solution and from

thin films, and EL of the thin films) for complexes (A) 16, (B) 19, and (C) 18.

Chem. Eur. J. 2014, 20, 1 – 16

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whereupon it turned yellow-orange. It was then allowed to cool,

CH₂Cl₂was added, and the suspension was filtered through Celite

and washed through with more CH₂Cl₂. After removal of all vola-

tiles from the filtrate in vacuo, the product was dissolved in reflux-

ing n-hexane. The dark-brown oil was separated from the superna-

tant orange solution, and the latter was stored at ꢀ208C to yield

orange-yellow crystals of the iminopyrrole ligand precursor. For fur-

ther details of the experimental procedure and product characteri-

zation, see the Supporting Information.

maxima, but emission is more difficult to predict, even by

using the more recent methodologies. Single-layer devices,

with a PEDOT:PSS hole-injection layer, have been fabricated

with the prepared complexes as emissive layers, films of which

were prepared either by spin-coating from solution or by subli-

mation. As expected, the complexes that showed the lowest

solution photoluminescence (PL) efficiencies (12, 13, 14, and

21) yielded very poorly emissive devices, and LEDs based on

18, which has a solution PL efficiency of 50%, also showed

very poor performance. The best performing devices were ob-

tained with 16, even though it has only a modest solution PL

efficiency of 14%. Upon insertion of a hole-transporting/elec-

tron-blocking layer of TPD, the performance was greatly im-

proved, with the devices based on 16 showing a maximum lu-

minance of 2965 cdm^ꢀ2with a maximum EL efficiency of

General procedure for the syntheses of diphenylboron

derivatives, [BPh₂{k²N,N’-NC₄H₃C(H)=N-Ar}] (13–21)

In a typical experiment, an equimolar mixture of triphenylboron

and the requisite iminopyrrolyl ligand precursor in toluene (15–

25 mL) was stirred and heated at reflux, under nitrogen atmos-

phere, overnight. The reaction mixture was allowed to cool to

room temperature, then concentrated under vacuum to a volume

of 5 mL and double-layered with hexane. The resulting solution

was kept at ꢀ208C to afford the corresponding crystalline boron

complex. For further details of the experimental procedure and

product characterization, see the Supporting Information.

1.4 cdA^ꢀ1

.

Experimental Section

General procedures

X-ray data collection

All experiments involving air- and/or moisture-sensitive materials

were carried out under inert atmosphere using a dual vacuum/ni-

trogen line and standard Schlenk techniques. Nitrogen gas was

supplied in cylinders (Air Liquide) 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 purification. All solvents to be used

under inert atmosphere were thoroughly deoxygenated and dehy-

drated before use. They were dried and purified by refluxing over

a suitable drying agent followed by distillation under nitrogen. The

following drying agents were used: sodium (for toluene, diethyl

ether, and tetrahydrofuran (THF)) and calcium hydride (for

n-hexane and dichloromethane). Solvents and solutions were trans-

ferred using a positive pressure of nitrogen through stainless steel

cannulae and mixtures were filtered in a similar way using modi-

fied cannulae that could be fitted with glass fiber filter disks.

Crystallographic and experimental details of the crystal structure

determinations are listed in Tables S1 and S2 in the Supporting In-

formation. The crystals were selected under an inert atmosphere,

covered with polyfluoroether oil, and mounted on a nylon loop.

Crystallographic data for compounds 3–6, 8, 13–17, 19, and 21

were collected at 150 K using graphite-monochromated Mo_Karadi-

ation (l=0.71073 ꢅ) on a Bruker AXS-Kappa APEX II diffractometer

equipped with an Oxford Cryosystems open-flow nitrogen cryostat.

Cell parameters were retrieved using Bruker SMART^[33]software

and refined using Bruker SAINT^[34]on all observed reflections. Ab-

sorption corrections were applied using SADABS.^[35]Structure solu-

tion and refinement were performed by direct methods with the

programs SIR2004^[36]and SHELXL^[37]included in the package of pro-

grams WINGX-Version 1.80.05.^[38]Except for the NH hydrogen

atoms in the compounds, all hydrogen atoms were inserted in ide-

alized positions and allowed to refine riding on the parent carbon

atom. Graphic representations were prepared with ORTEP-III.^[39]

Data have been deposited with the CCDC under deposition num-

bers CCDC-957737 for 3, 957738 for 4, 957739 for 5, 957740 for 6,

957741 for 8, 957742 for 13, 957743 for 14, 957744 for 15, 957745

for 16, 957746 for 17, 957747 for 19, and 957748 for 21. CCDC-

957737–957748 contain the supplementary crystallographic data

for this paper. These data can be obtained free of charge from The

Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/

data_request/cif.

Nuclear magnetic resonance (NMR) spectra were recorded on

a Bruker Avance III 300 (¹H, ¹³C, ¹¹B, and ¹⁹F) spectrometer. Deuter-

ated solvents were dried by storage over 4 ꢅ molecular sieves and

degassed by the freeze-pump-thaw method. Spectra were refer-

enced internally to residual protio-solvent (¹H) or solvent (¹³C) reso-

nances and are reported relative to tetramethylsilane (d=0). ¹¹B

and ¹⁹F NMR spectra were referenced to Et₂O·BF₃and CFCl₃, respec-

tively. All chemical shifts are quoted in d (ppm) and coupling con-

stants are given in Hz. Multiplicities are 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 service.

Cyclic voltammetry studies

Cyclic voltammetry measurements were carried out with a Solartron

potentiostat using a standard three-electrode cell, with a saturated

calomel reference electrode, a platinum wire as counter electrode,

and a platinum disk as working electrode. The compounds were

dissolved in freshly distilled dichloromethane containing 0.2m elec-

trolyte salt (tetrabutylammonium tetrafluoroborate or tetrabutyl-

ammonium perchlorate).

General method for the synthesis of iminopyrrole ligand

precursors

This synthetic procedure was based on other similar methods de-

scribed in the literature.^[5d,19–24]In a typical experiment, an equimo-

lar ratio of 2-formylpyrrole and a substituted aniline, a catalytic

amount of p-toluenesulfonic acid, and MgSO₄(to remove any

water from the reaction mixture) were suspended in absolute etha-

nol (20 mL) in a round-bottomed flask fitted with a condenser and

a CaCl₂guard tube. The mixture was heated to reflux overnight,

Ionization potential (IP) and electron affinity (EA) values were esti-

mated from the onset of oxidation and reduction potentials, re-

spectively. To convert the values on the electrochemical scale to an

absolute scale, referred to the vacuum, we used ferrocene as a ref-

&

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benzophenone

TT

sample

DOD

max

erence and considered the energy level of ferrocene/ferrocenium

(Fc/Fc⁺) to be 4.80 eV below the vacuum level, as detailed in

ref. [17].

e

ꢀ^s_T^ample

¼

_benzophenoneꢀ_T^benzophenone

ð2Þ

sample

DOD_max

e

TT

benzophenone

TT

with e

¼ 7200 m^ꢀ1cm^ꢀ1and ꢀ_T^benzophenone¼ 1:

Spectroscopic measurements

Computational studies

Absorption and fluorescence spectra were measured on 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.

Density functional theory calculations^[28]were performed using the

Amsterdam density functional program package (ADF).^[29]Gradient-

corrected geometry optimizations,^[42]without symmetry con-

straints, were performed using the local density approximation of

the correlation energy (Vosko–Wilk–Nusair),^[43]and the generalized

gradient approximation (Becke’s^[44]exchange and Perdew’s^[45]corre-

lation functionals). Relativistic effects were treated with the ZORA

approximation.^[46]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.^[30]The sol-

vent effect was included with the COSMO approach in ADF in

single-point calculations on the optimized geometries. The geome-

try of the excited 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 (TD-DFT) formal-

ism, with the influence of spin-orbit coupling effect (SOPERT),^[32]

was used to calculate the excited-state lifetimes. In these calcula-

tions, complete basis sets were used for all elements (as above,

but without any frozen cores) with the hybrid PBE0 functional.^[47]

We checked that the absorption spectra calculated by this ap-

proach were the same as those obtained under the same condi-

tions but without including spin-orbit coupling since all the atoms

are light. TD-DFT optimizations of the first singlet excited state

were also performed, using the Gaussian 09 software,^[48]for techni-

cal reasons, with the PBE0 functional^[47]and a 6–31G** basis set for

all atoms.^[49]

Fluorescence decays were measured 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 Millennia Xs solid-state laser (Spectra

Physics). Decays longer than 2 ns were remeasured at reduced rep-

etition rate (4 MHz) using a pulse picker (Spectra Physics, Model

3980). 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 an SPC-630 acquisition board (Becker

and Hickl, GmbH). The instrumental response function (IRF) was

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 5ꢆ10³counts at the maximum were reached. Fluorescence

decays were deconvoluted from the IRF using the modulation

functions method (Sand program).^[41]

Flash photolysis

Triplet-state absorption spectra and quantum yields were mea-

sured with a laser flash photolysis apparatus (Applied Photophy-

sics) pumped by an Nd:YAG laser (Spectra Physics). Transient spec-

tra were obtained by monitoring the optical density (OD) change

at intervals of 10 nm over the range 300–700 nm and averaging at

least ten decays at each wavelength. Second-order kinetics was ob-

served for the decay of the lowest triplet state. Excitation was at

355 nm with an unfocused beam. Special care was taken in deter-

mining triplet yields to have optically matched dilute solutions

(absꢁ0.2–0.3 in a 10 mm square cell) and low laser energy (2 mJ)

to avoid multiphoton and triplet-triplet (T-T) annihilation effects.

The triplet molar absorption coefficients were determined by the

singlet depletion technique, according to the well-known relation-

ship:

The structures were modeled on those of compounds 13–17, 19,

and 21 described above. Three-dimensional representations of the

orbitals were obtained with Molekel^[31]and electronic spectra with

Chemcraft.^[50]

Light-emitting diode studies

Light-emitting diodes were fabricated on glass/ITO substrates

(ITO=indium tin oxide), which were cleaned with detergent, dis-

tilled water, acetone, and isopropanol. They were treated with

oxygen plasma, prior to the deposition of PEDOT:PSS (poly(3,4-eth-

ylenedioxythiophene) doped with polystyrene sulfonic acid,

CLEVIOS P VP.AI 4083 from Heraeus Clevios GmbH) by spin-coat-

ing. The PEDOT:PSS films (40 nm thick, as measured with a DEKTAK

profilometer) were annealed in air for 2 min at 1208C, and then

transferred to a nitrogen-filled glove box. Some devices were pre-

pared with an approximately 20 nm thick, hole-transporting/elec-

tron-blocking layer of TPD (N,N’-bis(3-methylphenyl)-N,N’-diphenyl-

benzidine, from Aldrich), which was thermally deposited on top of

the PEDOT:PSS.

ðe_SÞðDOD_TÞ

ð1Þ

e_T¼

ðDOD_SÞ

where both DOD_Sand DOD_Twere obtained from the triplet transi-

ent absorption spectra. Triplet formation quantum yields were de-

rived from these and actinometry with benzophenone. The inter-

system-crossing yields (f_T) for the compounds were obtained by

comparing the DOD at 530 nm of solutions of benzophenone in

benzene (standard) optically matched (at the laser excitation wave-

length) with that of the compound using the following equation:

Films of the complexes 12–21 were deposited on top of the PE-

DOT:PSS by spin-coating from solutions in THF, inside the glove

box. The thicknesses of the films of the complexes were in the

range 60–100 nm. The substrates were then placed inside an evap-

Chem. Eur. J. 2014, 20, 1 – 16

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Y. Wang, Chem. Commun. 2000 , 155 1 –1 55 2; d) S. Lamansky, P. Djuro-

vich, D. Murphy, F. Abdel-Razzaq, H. E. Lee, C. Adachi, P. E. Burrows, S. R.

Forrest, M. E. Thompson, J. Am. Chem. Soc. 2001 , 123 , 430 4 –4 312.

[9] a) J. M. Fernꢁndez-Hernꢁndez, C.-H. Yang, J. I. Beltran, V. Lemaur, F. Polo,

R. Frohlich, J. Cornil, L. De Cola, J. Am. Chem. Soc. 2011, 133, 10543–

10548; b) C.-H. Yang, Y.-M. Cheng, Y. Chi, C.-J. Hsu, F.-C. Fang, K.-T.

Wong, P.-T. Chou, C.-H. Chang, M.-H. Tsai, C.-C. Wu, Angew. Chem. 2007 ,

119 , 247 0 –2 473; Angew. Chem. Int. Ed. 2007 , 46 , 241 8 –2 421; c) L. He, L.

Duan, J. Qiao, D. Zhang, L. Wang, Y. Qiu, Org. Electron. 2010 , 11 , 118 5 –

1191.

oration chamber, and the top metal cathode (calcium), about

40 nm thick, was deposited at a base pressure of 2ꢆ10^ꢀ6mbar

through a shadow mask, defining pixel areas of 4 mm². A protect-

ing, approximately 60 nm thick, aluminum layer was deposited on

top. Alternatively, films of complexes 15–20 (with thicknesses rang-

ing from 50 to 115 nm) were prepared by thermal deposition on

top of either PEDOT:PSS or TPD-coated PEDOT:PSS. Device struc-

tures were completed by the deposition of barium (ca. 40 nm

thick) protected by an overlayer of aluminum (ca. 60 nm thick).

[10] a) J. Kavitha, S. Y. Chang, Y. Chi, J. K. Yu, Y. H. Hu, P. T. Chou, S. M. Peng,

A. J. Carty, Adv. Funct. Mater. 2005 , 15 , 22 3 –2 29; b) X. Yang, Z. Wang, S.

Madakuni, J. Li, G. E. Jabbour, Adv. Mater. 2008 , 20 , 240 5 –2 40 9; c) M. A.

Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R.

Forrest, Nature 1998, 395, 151–154.

Devices were tested under vacuum using a K2400 Source Meter

and a calibrated silicon photodiode, as described previously.^[51]

Electroluminescence (EL) spectra were obtained with a CCD spec-

trograph (from Ocean Optics or from ScanSci). External quantum

efficiencies were estimated as detailed in ref. [51].

[11] a) Z. M. Hudson, S. Wang, Acc. Chem. Res. 2009 , 42 , 158 4 –1 596; b) P.

Chen, R. A. Lalancette, F. Jꢇkle, Angew. Chem. 2012 , 124 , 811 8 –8 122;

Angew. Chem. Int. Ed. 2012 , 51 , 799 4 –7 998; c) M. Halik, W. Wenseleers,

C. Grasso, F. Stellacci, E. Zojer, S. Barlow, J. L. Bredas, J. W. Perry, S. R.

Marder, Chem. Commun. 2003 , 149 0 –1 491; d) C. D. Entwistle, T. B.

Marder, Chem. Mater. 2004 , 16 , 457 4 –4 585; e) C. D. Entwistle, T. B.

Marder, Angew. Chem. 2002 , 114 , 305 1 –3 056; Angew. Chem. Int. Ed.

2002 , 41 , 292 7 –2 931; f) R. Bernard, D. Cornu, J. P. Scharff, R. Chiriac, P.

Miele, Inorg. Chem. 2006 , 45 , 874 3 –8 748; g) R. Bernard, D. Cornu, P. L.

Baldeck, J. Caslavsky, J. M. Letoffe, J. P. Scharff, P. Miele, Dalton Trans.

2005 , 306 5 –3 071; h) F. Jꢇkle, Chem. Rev. 2010, 110, 3985–4022.

[12] a) Q.-D. Liu, M. S. Mudadu, R. Thummel, Y. Tao, S. Wang, Adv. Funct.

Mater. 2005 , 15 , 14 3 –1 54; b) H. Amarne, C. Baik, S. K. Murphy, S. Wang,

Chem. Eur. J. 2010 , 16 , 475 0 –4 761; c) S. Kappaun, S. Rentenberger, A.

Pogantsch, E. Zojer, K. Mereiter, G. Trimmel, R. Saf, K. C. Mçller, F. Stelzer,

C. Slugovc, Chem. Mater. 2006 , 18 , 353 9 –3 547; d) B. J. Liddle, R. M.

Silva, T. J. Morin, F. P. Macedo, R. Shukla, S. V. Lindeman, J. R. Gardinier, J.

Org. Chem. 2007 , 72 , 563 7 –5 646; e) Y. Liu, J. Guo, H. Zhang, Y. Wang,

Angew. Chem. 2002 , 114 , 19 0 –1 92; Angew. Chem. Int. Ed. 2002 , 41 ,

18 2 –1 84; f) Z. Zhang, H. Bi, Y. Zhang, D. Yao, H. Gao, Y. Fan, H. Zhang, Y.

Wang, Z. Chen, D. Ma, Inorg. Chem. 2009 , 48 , 723 0 –7 23 6; g) D. Li, Z.

Zhang, S. Zhao, Y. Wang, H. Zhang, Dalton Trans. 2011 , 40 , 127 9 –1 285;

h) D. Li, Y. Yuan, H. Bi, D. Yao, X. Zhao, W. Tian, Y. Wang, H. Zhang, Inorg.

Chem. 2011 , 50 , 482 5 –4 831; i) C.-C. Cheng, W.-S. Yu, P.-T. Chou, S.-M.

Peng, G.-H. Lee, P.-C. Wu, Y.-H. Song, Y. Chi, Chem. Commun. 2003 ,

262 8 –2 629; j) J. Ugolotti, S. Hellstrom, G. J. P. Britovsek, T. S. Jones, P.

Hunt, A. J. P. White, Dalton Trans. 2007 , 142 5 –1 432; k) J. Yoshino, A.

Furuta, T. Kambe, H. Itoi, N. Kano, T. Kawashima, Y. Ito, M. Asashima,

Chem. Eur. J. 2010 , 16 , 502 6 –5 03 5; l) N. Kano, A. Furuta, T. Kambe, J.

Yoshino, Y. Shibata, T. Kawashima, N. Mizorogi, S. Nagase, Eur. J. Inorg.

Chem. 2012 , 158 4 –1 587; m) J. Yoshino, N. Kano, T. Kawashima, Chem.

Commun. 2007 , 55 9 –5 61; n) P. T. Gomes, D. Suresh, C. S. B. Gomes, P. S.

Lopes, C. A. Figueira (IST), World Patent WO2013039413, 2013; o) D.

Suresh, P. T. Gomes in Advances in Organometallic Chemistry and Cataly-

sis: The Silver/Gold Jubilee International Conference on Organometallic

Chemistry Celebratory Book, 1st ed. (Ed.: A. J. L. Pombeiro), John Wiley &

Sons, Hoboken, 2014, Chap. 36, pp. 485-492; p) D. Li, H. Zhang, Y.

Wang, Chem. Soc. Rev. 2013 , 42 , 841 6 –8 433.

Acknowledgements

We thank the FundaÅ¼o para a CiÞncia e Tecnologia, Portugal,

for financial support (Projects PTDC/QUI/65474/2006, PEst-OE/

QUI/UI0100/2013, PEst-OE/QUI/UI0612/2013, and PEst-OE/EEI/

LA0008/2013) and for fellowships to D.S., P.S.L., B.F., C.A.F., and

C.S.B.G. (SFRH/BPD/47853/2008, SFRH/BD/88639/2012, SFRH/

BD/80122/2011, SFRH/BD/47730/2008, and SFRH/BPD/64423/

2009, respectively).

Keywords: boranes · fluorescence · N-iminopyrrolyl ligands ·

OLEDs · density functional calculations

[1] a) G. Parshall, S. Ittel, Homogeneous Catalysis: The Applications of

Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.,

Wiley, New York, 1992; b) B. Cornils, W. A. Herrmann, Applied Homoge-

neous Catalysis with Organometallic Compounds, 2nd ed., Wiley-VCH,

Weinheim, 2002.

[2] K. Mashima, H. Tsuguri, J. Organomet. Chem. 2005 , 690 , 441 4 –4 423, and

references therein.

[3] T. Li, J. Jenter, P. W. Roesky, Z. Anorg. Allg. Chem. 2010 , 636 , 214 8 –2 155.

[4] R. H. Holm, A. Chakravorty, L. J. Theriot, Inorg. Chem. 1966 , 5 , 62 5 –6 35.

[5] See, for instance: a) T. K. Panda, K. Yamamoto, K. Yamamoto, H. Kaneko,

Y. Yang, H. Tsurugi, K. Mashima, Organometallics 2012 , 31 , 226 8 –2 274;

b) J.-S. Mu, Y.-X. Wang, B.-X. Li, Y.-S. Li, Dalton Trans. 2011 , 40 , 349 0 –

3497; c) H. Tsurugi, Y. Matsuo, K. Mashima, J. Mol. Catal. A 2006 , 254 ,

13 1 –1 37; d) R. M. Bellabarba, P. T. Gomes, S. I. Pascu, Dalton Trans.

2003 , 443 1 –4 43 6; e) S. A. Carabineiro, L. C. Silva, P. T. Gomes, L. C. J.

Pereira, L. F. Veiros, S. I. Pascu, M. T. Duarte, S. Namorado, R. T. Henri-

ques, Inorg. Chem. 2007 , 46 , 688 0 –6 890; f) S. A. Carabineiro, P. T.

Gomes, L. F. Veiros, C. Freire, L. C. J. Pereira, R. T. Henriques, J. E. Warren,

S. I. Pascu, Dalton Trans. 2007 , 546 0 –5 47 0; g) S. A. Carabineiro, R. M.

Bellabarba, P. T. Gomes, S. I. Pascu, L. F. Veiros, C. Freire, L. C. J. Pereira,

R. T. Henriques, M. C. Oliveira, J. E. Warren, Inorg. Chem. 2008 , 47 , 889 6 –

8911; h) C. S. B. Gomes, D. Suresh, P. T. Gomes, L. F. Veiros, M. T. Duarte,

T. G. Nunes, M. C. Oliveira, Dalton Trans. 2010 , 39 , 73 6 –7 48; i) C. S. B.

Gomes, S. A. Carabineiro, P. T. Gomes, M. T. Duarte, Inorg. Chim. Acta

2011 , 367 , 15 1 –1 57.

[13] a) S. Sole, F. P. Gabbai, Chem. Commun. 2004, 1284–1285; b) W. Yang, H.

He, D. G. Drueckhammer, Angew. Chem. 2001 , 113 , 176 4 –1 768; Angew.

Chem. Int. Ed. 2001 , 40 , 171 4 –1 718; c) J. Killoran, L. Allen, J. F. Gallagher,

W. M. Gallagher, D. F. O’Shea, Chem. Commun. 2002 , 186 2 –1 863;

d) Z. Q. Liu, Q. Fang, D. Wang, G. Xue, W. T. Yu, Z. S. Shao, H. M. Jiang,

Chem. Commun. 2002 , 290 0 –2 901; e) S. Yamaguchi, T. Shirasaka, S.

Akiyama, K. Tamao, J. Am. Chem. Soc. 2002 , 124 , 881 6 –8 817.

[14] a) S. L. Hellstrom, J. Ugolotti, G. J. P. Britovsek, T. S. Jones, A. J. P. White,

New J. Chem. 2008 , 32 , 137 9 –1 387; b) T.-R. Chen, R.-H. Chien, M.-S. Jan,

A. Yeh, J.-D. Chen, J. Organomet. Chem. 2006 , 691 , 79 9 –8 04; c) K.

Rurack, M. Kollmannsberger, J. Daub, Angew. Chem. 2001 , 113 , 39 6 –

399; Angew. Chem. Int. Ed. 2001 , 40 , 38 5 –3 87.

[15] C. S. B. Gomes, P. T. Gomes, R. E. Di Paolo, A. L. MaÅanita, M. T. Duarte,

M. J. Calhorda, Inorg. Chem. 2009 , 48 , 1117 6 –1 1186.

[16] M. G. Crestani, G. F. Manbeck, W. W. Brennessel, T. M. McCormick, R. Ei-

senberg, Inorg. Chem. 2011 , 50 , 717 2 –7 188.

[6] a) G. J. P. Britovsek, V. C. Gibson, D. F. Wass, Angew. Chem. 1999 , 111 ,

44 8 –4 68; Angew. Chem. Int. Ed. 1999 , 38 , 42 8 –4 47 and references

therein; b) V. C. Gibson, S. K. Spitzmesser, Chem. Rev. 2003 , 103 , 28 3 –

313 and references therein.

[7] a) T. K. S. Wong, Handbook of Organic Electronics and Photonics, Vol. 2

(Ed.: H. S. Nalwa), American Scientific Publishers, Stevenson Ranch, CA,

2008, pp. 413–472; b) C. H. Chen, J. Shi, Coord. Chem. Rev. 1998 , 171 ,

16 1 –1 74; c) S. Wang, Coord. Chem. Rev. 2001 , 215 , 7 9 –9 8.

[8] a) Z. Shen, P. E. Burrows, V. Bulovic, S. R. Borrest, M. E. Thompson, Sci-

ence 1997 , 276 , 200 9 –2 011; b) H. Aziz, Z. D. Popovic, N.-X. Hu, A.-M.

Hor, G. Xu, Science 1999 , 283 , 190 0 –1 90 2; c) Y. Li, Y. Liu, W. Bu, J. Guo,

[17] a) D. Suresh, C. S. B. Gomes, P. T. Gomes, R. E. Di Paolo, A. L. MaÅanita,

M. J. Calhorda, A. Charas, J. Morgado, M. T. Duarte, Dalton Trans. 2012 ,

&

Chem. Eur. J. 2014, 20, 1 – 16

www.chemeurj.org

14

ꢄ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Full Paper

41 , 850 2 –8 50 5; and the following two errata: b) D. Suresh, C. S. B.

Gomes, P. T. Gomes, R. E. Di Paolo, A. L. MaÅanita, M. J. Calhorda, A.

Charas, J. Morgado, M. T. Duarte, Dalton Trans. 2012, 41, 14713; c) D.

Suresh, C. S. B. Gomes, P. T. Gomes, R. E. Di Paolo, A. L. MaÅanita, M. J.

Calhorda, A. Charas, J. Morgado, M. T. Duarte, Dalton Trans. 2013, 42,

16969.

[36] M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G. L. Cascarano, L.

De Caro, C. Giacovazzo, G. Polidori, R. Spagna, J. Appl. Crystallogr. 2005 ,

38 , 38 1 –3 88.

[37] G. M. Sheldrick, SHELXL-97, A Computer Program for Refinement of

Crystal Structure, University of Gçttingen, Gçttingen, 1997.

[38] L. J. Farrugia, J. Appl. Crystallogr. 1999 , 32 , 83 7 –8 38.

[39] M. N. Burnett, C. K. Johnson, ORTEP-III: Oak Ridge Thermal Ellipsoid Plot

Program for Crystal Structure Illustration, Oak Ridge National Laboratory

Report ORNL-6895, 1996.

[18] M. J. Calhorda, D. Suresh, P. T. Gomes, R. E. Di Paolo, A. L. MaÅanita,

Dalton Trans. 2012 , 41 , 1321 0 –1 3217.

[19] C. S. B. Gomes, C. A. Figueira, P. S. Lopes, D. Suresh, P. T. Gomes, M. T.

Duarte, Acta Crystallogr. Sect. C 2011 , 67 , o31 5 –o 318.

[20] T. Yasumoto, T. Yamagata, K. Mashima, Organometallics 2005 , 24 , 337 5 –

3377.

[40] R. F. Rodrigues, P. F. da Silva, K. Shimizu, A. A. Freitas, S. A. Kovalenko,

N. P. Ernsting, F. H. Quina, A. MaÅanita, Chem. Eur. J. 2009 , 15 , 139 7 –

1402.

[21] P. J. Figiel, A. Sibaouih, J. U. Ahmad, M. Nieger, M. T. Raeisaenen, M. Le-

skelae, T. Repo, Adv. Synth. Catal. 2009 , 351 , 262 5 –2 632.

[22] A. Ali-Bey, H. F. Loukil, G. Gosselin, A. Mathieu, A. Mesli, J. Heterocycl.

Chem. 1982 , 19 , 61 7 –6 24.

[41] G. Striker, V. Subramaniam, C. A. M. Seidel, A. Volkmer, J. Phys. Chem. B

1999 , 103 , 861 2 –8 617.

[42] a) L. Versluis, T. Ziegler, J. Chem. Phys. 1988 , 88 , 32 2 –3 28; b) L. Fan, T.

Ziegler, J. Chem. Phys. 1991 , 95 , 740 1 –7 408.

[23] M. Asadi, M. H. Ghatee, S. Torabi, K. Mohammadi, F. Moosavi, J. Chem.

Sci. 2010 , 122 , 53 9 –5 48.

[43] S. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 1980 , 58 , 120 0 –1 211.

[44] A. D. Becke, J. Chem. Phys. 1988 , 88 , 105 3 –1 062.

[24] B.-C. Xu, T. Hu, J.-Q. Wu, N.-H. Hu, Y.-S. Li, Dalton Trans. 2009 , 885 4 –

8863.

[45] a) J. P. Perdew, Phys. Rev. B 1986 , 33 , 882 2 –8 824; b) J. P. Perdew, Phys.

Rev. B 1986 , 34 , 740 6 –7 406.

[25] J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Bꢇssler, M. Porsh,

J. Daub, Adv. Mater. 1995 , 7 , 55 1 –5 54.

[46] E. van Lenthe, A. Ehlers, E. J. Baerends, J. Chem. Phys. 1999 , 110 , 894 3 –

8953.

[26] a) C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991 , 91 , 16 5 –1 95; b) M.

Segala, Y. Takahata, D. P. Chong, J. Mol. Struct. THEOCHEM 2006 , 758 ,

6 1 –6 9.

[47] a) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996 , 77 , 386 5 –

3868; b) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1997 , 78 ,

139 6 –1 396.

[27] See, for example: a) B. Foresman, M. Head-Gordon, J. A. Pople, M. J.

Frisch, J. Phys. Chem. 1992 , 96 , 13 5 –1 49; b) M. Head-Gordon, R. J. Rico,

M. Oumi, T. J. Lee, Chem. Phys. Lett. 1994 , 219 , 2 1 –2 9; c) M. Head-

Gordon, D. Maurice, M. Oumi, Chem. Phys. Lett. 1995 , 246 , 11 4 –1 21.

[28] R. G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules,

Oxford University Press, New York, 1989.

[29] a) SCM, ADF2009, Theoretical Chemistry, Vrije Universiteit, Amsterdam,

The Netherlands, http://www.scm.com; b) G. te Velde, F. M. Bickelhaupt,

E. J. Baerends, C. F. Guerra, S. J. A. van Gisbergen, J. G. Snijders, T. Zieg-

ler, J. Comput. Chem. 2001 , 22 , 93 1 –9 67; c) C. F. Guerra, J. G. Snijders,

G. te Velde, E. J. Baerends, Theor. Chem. Acc. 1998, 99, 391–403.

[30] a) S. J. A. van Gisbergen, J. A. Groeneveld, A. Rosa, J. G. Snijders, E. J.

Baerends, J. Phys. Chem. A 1999 , 103 , 683 5 –6 84 4; b) A. E. Rosa, J. Baer-

ends, S. J. A. van Gisbergen, E. van Lenthe, J. A. Groeneveld, J. G. Snijd-

ers, J. Am. Chem. Soc. 1999 , 121 , 1035 6 –1 0365; c) S. J. A. van Gisbergen,

A. Rosa, G. Ricciardi, E. J. Baerends, J. Chem. Phys. 1999 , 111 , 249 9 –

2507; d) S. J. A. van Gisbergen, J. G. Snijders, E. J. Baerends, Comput.

Phys. Commun. 1999 , 118 , 11 9 –1 38.

[48] Gaussian 09, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.

Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Pe-

tersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J.

Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R.

Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.

Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bear-

park, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.

Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J.

Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V.

Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,

A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Moro-

kuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, K. J. Dannenberg, S. Dap-

prich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski,

D. J. Fox, Gaussian, Inc., Wallingford, CT, 2009.

[49] a) R. Ditchfield, W. J. Hehre, J. A. Pople, J. Chem. Phys. 1971 , 54 , 72 4 –

728; b) W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972 , 56 ,

225 7 –2 261; c) P. C. Hariharan, J. A. Pople, Mol. Phys. 1974 , 27 , 20 9 –2 14;

d) M. S. Gordon, Chem. Phys. Lett. 1980 , 76 , 16 3 –1 68; e) P. C. Hariharan,

J. A. Pople, Theor. Chim. Acta 1973 , 28 , 21 3 –2 22.

[50] http://www.chemcraftprog.com/index.html.

[51] J. Morgado, A. Charas, J. A. Fernandes, I. S. GonÅalves, L. D. Carlos, L.

Alcꢁcer, J. Phys. D: Appl. Phys. 2006 , 39 , 358 2 –3 587.

[31] S. Portmann, H. P. Lꢈthi, Chimia 2000, 54, 766–770.

[32] F. Wang, T. Ziegler, J. Chem. Phys. 2005 , 123 , 15410 2 –1 54113.

[33] SMART software for the CCD detector system, Version 5.625, Bruker AXS

Inc., Madison, WI, USA, 2001.

[34] SAINT software for the CCD detector system, Version 7.03, Bruker AXS

Inc., Madison, WI, USA, 2004.

[35] G. M. Sheldrick, SADABS, Program for Empirical Absorption Correction,

University of Gçttingen, Gçttingen, 1996.

Received: September 11, 2013

Published online on && &&, 0000

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FULL PAPER

Fluorescent N,N’-boron chelate com-

plexes: Mononuclear boron complexes

of 2-(N-arylimino)pyrrolyl emit violet to

bluish-green colors in solution (see

figure, ITO=indium tin oxide, PE-

DOT:PSS=poly(3,4-ethylenedioxythio-

phene):poly(styrene sulfonic acid)), de-

pending on the substituents on the N-

aryl group. Organic light-emitting

diodes have been successfully fabricat-

ed with the new boron complexes, ach-

ieving luminances of the order of 3000

cdm^ꢀ2.

&

Fluorescence

D. Suresh, P. S. Lopes, B. Ferreira,

C. A. Figueira, C. S. B. Gomes,

P. T. Gomes,* R. E. Di Paolo,

A. L. MaÅanita, M. T. Duarte, A. Charas,

J. Morgado, M. J. Calhorda

&& – &&

Tunable Fluorophores Based on 2-(N-

Arylimino)pyrrolyl Chelates of

Diphenylboron: Synthesis, Structure,

Photophysical Characterization, and

Application in OLEDs

&

Chem. Eur. J. 2014, 20, 1 – 16

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16

ꢄ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Article Doi

Tunable fluorophores based on 2-(N-Arylimino)pyrrolyl chelates of diphenylboron: Synthesis, structure, photophysical characterization, and application in OLEDs

DOI: 10.1002/chem.201303607

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Article abstract of DOI:10.1002/chem.201303607

Full text of DOI:10.1002/chem.201303607

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