Luminescent halogen-substituted 2-(: N -arylimino)pyrrolyl boron complexes: The internal heavy-atom effect

Article abstract of DOI:10.1039/d0dt01845g

A group of new boron complexes [BPh2{κ2N,N′-NC4H3-2-C(H)N-C6H4X}] (X = 4-Cl 4c, 4-Br 4d, 4-I 4e, 3-Br 4f, 2-Br 4g, 2-I 4h) containing different halogens as substituents in the N-aryl ring have been synthesized and characterized in terms of their molecular properties. Their photophysical characteristics have been thoroughly studied in order to understand whether these complexes exhibit an internal heavy-atom effect. Phosphorescence emission was found for some of the synthesized halogen-substituted boron molecules, particularly for 4g and 4h. DFT and TDDFT calculations showed that the lower energy absorption band resulted from the HOMO to LUMO (π-π?) transition, except for 2-I 4h, where the HOMO-1 to LUMO transition was also involved. The strong participation of iodine orbitals in HOMO-1 is reflected in the calculated absorption spectra of the iodine derivatives, especially 2-I 4h, when spin-orbit coupling (SOC) was included. Organic light-emitting diodes (OLEDs) based on these complexes, in the neat form or dispersed in a matrix, were also fabricated and tested. The devices based on films prepared by thermal vacuum deposition showed the best performance. When neat complexes were used, a maximum luminance (Lmax) of 1812 cd m-2 was obtained, with a maximum external quantum efficiency (EQEmax) of 0.15%. An EQEmax of ca. 1% along with a maximum luminance of 494 cd m-2 were obtained for a device fabricated by co-deposition of the boron complex and a host compound (1,3-bis(N-carbazolyl)benzene, mCP).

Full text of DOI:10.1039/d0dt01845g

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An international journal of inorganic chemistry
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Rodrigues, K. Paramasivam, C. S. B. Gomes, N. Carmona, R. E. di Paolo, P. H. Pander, J. M. Pina, J. S.
Seixas de Melo, F. B. Dias, M. J. Calhorda, A. L. Maçanita and J. Morgado, Dalton Trans., 2020, DOI:
10.1039/D0DT01845G.
Volume 46
Number 10
14 March 2017
Pages 3073-3412
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Page 1 of 39
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DOI: 10.1039/D0DT01845G
Luminescent halogen-substituted 2-(N-arylimino)pyrrolyl
boron complexes: the internal heavy-atom effect
Ana I. Rodrigues,a Paramasivam Krishnamoorthy,a,h Clara S. B. Gomes,a,i,j Nicolas Carmona,a
Roberto E. Di Paolo,a Piotr Pander,c João Pina,d J. Sérgio Seixas de Melo,d Fernando B. Dias,c
Maria José Calhorda,e António L. Maçanita,a,b Jorge Morgadof,g and Pedro T. Gomes*,a,b
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001
Lisboa, Portugal.
b
Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049-001 Lisboa, Portugal.
c
Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom.
d
University of Coimbra, Coimbra Chemistry Centre, Department of Chemistry, Rua Larga, 3004-535 Coimbra,
Portugal.
e
BioISI -Biosystems & Integrative Sciences Institute, Departamento de Química e Bioquímica, Faculdade de
Ciências, Universidade de Lisboa, Campo Grande, Ed. C8, 1749-016 Lisboa, Portugal.
f
Instituto de Telecomunicações, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
g
Department of Bioengineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001
Lisboa, Portugal.
h
Centre for Environmental Research, Department of Chemistry, Kongu Engineering College, Perundurai, Erode
638 060, India
i
LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade NOVA de
Lisboa, 2829-516, Caparica, Portugal.
j
UCIBIO-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade NOVA
de Lisboa, 2829-516 Caparica, Portugal.
* Corresponding Author. E-mail: pedro.t.gomes@tecnico.ulisboa.pt

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Page 2 of 39
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DOI: 10.1039/D0DT01845G
Abstract
A group of new boron complexes [BPh2{2N,N’-NC4H3-2-C(H)=N-C6H4X}] (X= 4-Cl 4c, 4-
Br 4d, 4-I 4e, 3-Br 4f, 2-Br 4g, 2-I 4h) containing different halogens as substituents in the N-
aryl ring have been synthesized and characterized in terms of their molecular properties. Their
photophysical characteristics have been thoroughly studied in order to understand whether
these complexes exhibit internal heavy-atom effect. Phosphorescence emission was found for
some of the synthesized halogen-substituted boron molecules, in particular for 4g and 4h. DFT
and TDDFT calculations showed that the lower energy absorption band resulted from the HOMO
to LUMO (-*) transition, except for 2-I 4h, where the HOMO-1 to LUMO transition was also
involved. The strong participation of iodine orbitals in HOMO-1 is reflected on the calculated
absorption spectra of the iodine derivatives, especially 2-I 4h, when spin-orbit coupling (SOC) was
included. Organic light-emitting diodes (OLEDs) based on these complexes, in the neat form or
dispersed in a matrix, were also fabricated and tested. The devices based on films prepared by
thermal vacuum deposition showed the best performance. When neat complexes were used, a
maximum luminance (Lmax) of 1812 cd m-2 was obtained, with maximum external quantum
efficiency (EQEmax) of 0.15%. An EQEmax of ca. 1% along with a maximum luminance of 494
cd m-2 were obtained for a device fabricated by co-deposition of the boron complex and a host
compound (1,3-bis(N-carbazolyl)benzene, mCP).
Keywords: Boron; fluorescence; OLED; phosphorescence; photophysics; DFT calculations
2

Page 3 of 39
Dalton Transactions
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DOI: 10.1039/D0DT01845G
1. Introduction
After the report of the first multi-layered organic-light emitting diodes (OLEDs), by Tang and
Van Slyke,1 many improvements have been achieved in this type of technology to the point of
their being nowadays present in commercially available flat displays and lighting applications.
In spite of this achievement, there is a continuous search for enhancements in this technology
aiming to obtain better brightness, flexibility, stability and lower production costs.
In the production of luminescent emitters, there is already a variety of options based on
fluorescent and/or phosphorescent chromophores, or in the newest thermally activated delayed
fluorescent (TADF) molecules.2 Tetracoordinate boron complexes containing bidentate N,N-;
N,O-; N,C-; C,C-; C,O- and O,O- ligands gave rise to particularly interesting luminescent
chromophores, some of them used in OLED devices with good electroluminescent properties.3,4
Our research group has already developed a reasonable number of tetracoordinate boron
complexes bearing a 2-(N-arylformimino)pyrrolyl ligand. Their emission colour could be tuned
by varying the structural and electronic features of this scaffold. The 2-iminopyrrolyl-BPh2
complexes containing donor and acceptor groups in the N-phenyl fragment (e.g. H, 2,6-iPr2,
OMe, CN, etc.) (Chart 1, A)5 proved to be blue to bluish-green fluorescence emitters. The boron
analogues bearing fused aromatic fragments onto the C4-C5 or the C3-C4 bonds (indolyl or
phenanthropyrrolyl, respectively) (Chart 1, B) are other good examples. For instance, the
phenanthropyrrolyl derivatives exhibited fluorescence quantum yields in the range 37-61%.6
Other derivatives such as 2-(imino) and 2-(iminophenanthro)pyrrolyl-BPh2 containing N-alkyl
groups (methyl, n-octyl, i-propyl, cyclohexyl, t-butyl and adamantyl) were also reported as
violet-blue emitters (Chart 1, C).7 OLED devices based on binuclear 2-iminopyrrolyl-BPh2
derivatives (Chart 1, D) achieved luminance maxima of 4400 cd m-2.5a,8 The substitution of the
2-iminopyrrolyl ring at position 5 with aromatic substituents, such as phenyl or anthracenyl
(Anthr), originated a new family of 5-substituted 2-(N-arylformimino)pyrrolyl boron
complexes. These green and blue fluorescent emitters (Chart 1, E) were applied in the emissive
layers of OLED devices with various structures, showing external quantum efficiencies up to
2.75% along with luminances as high as 23530 cd m-2.9
3

Dalton Transactions
Page 4 of 39
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DOI: 10.1039/D0DT01845G
Chart 1. 2-(N-arylformimino)pyrrolyl-BPh2 complexes previously reported by our research
group.
Interestingly, in all the previous studies, the observation of triplet formation via intersystem
crossing was only identified in a single case (for a molecule of the type A with R= 4-CN, see
Chart 1).5 We describe in the present work a set of new halogen-substituted 2-(N-
arylformimino)pyrrolyl-BPh2 complexes, which were designed to investigate the possible
existence of internal heavy-atom effect,10 promoting the triplet state formation via a spin-orbit
coupling mechanism, namely for the heavier substituents. The photophysical properties of the
new boron derivatives were thoroughly studied in order to conclude about the origin of light
emission in these compounds. Additionally, the new halogen-substituted 2-(N-
arylimino)pyrrolyl boron complexes were characterized by NMR spectroscopy, elemental
analysis and single crystal X-ray diffraction. DFT and TDDFT studies complemented the work
by providing the geometry of the ground state, the singlet and triplet first excited states, as well
as the nature of the absorption bands and an understanding of the importance of spin-orbit
coupling in the emission of the halogenated complexes. At a later stage, OLEDs based on these
complexes as emissive materials were fabricated and their performance assessed.
4

Page 5 of 39
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DOI: 10.1039/D0DT01845G
2. Results and Discussion
2.1 Synthesis and characterization of halogen-substituted 2-iminopyrrole ligand
precursors and halogen-substituted 2-iminopyrrolyl boron complexes
The halogen-substituted 2-formiminopyrrole ligand precursors 3c-3h were synthesized via a
condensation reaction of 2-formylpyrrole 111 with different anilines 2c-2h containing halogen
substituents. The mixtures were refluxed at ca. 130 ºC, in toluene, with removal of water
(Scheme 1), except in the case of 3c that was refluxed in (a mixture of) xylenes.
Scheme 1. Synthesis of halogen-substituted 2-(N-arylformimino)pyrrole ligand precursors 3c-
3h. The synthesis of compound 3c was carried out in xylenes.
The resulting ligand precursors 3c-3h were obtained in moderate to high yields. Their molecular
13
characterization was performed by NMR spectroscopy (1H and C) and elemental analysis.
The 2-(N-arylformimino)pyrrole ligands 3a and 3b had already been reported by us5a,c and other
research groups.12 The syntheses of the p-chlorine, p-bromine, p-iodine,
m-bromine and o-bromine derivatives 3c-3g, although already reported in the literature,12a,b,d,e,13
were carried out following the typical condensation procedure used in our group.5–9,12f The
reaction of halogen-substituted 2-(N-arylformimino)pyrrole ligand precursors 3c-h with
triphenylboron under reflux, in toluene and under nitrogen, afforded the respective
iminopyrrolyl boron complexes 4c-4h in moderate to high yields (Scheme 2).
The final 2-(N-arylformimino)pyrrolyl-BPh2 complexes 4c-4h, containing halogen
substituents at the N-phenyl ring, were molecularly characterized by different techniques, such
as NMR spectroscopy (1H, 13C and 11B), elemental analysis and single-crystal X-ray diffraction
(complexes 4c, 4d and 4g).
5

Dalton Transactions
Page 6 of 39
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DOI: 10.1039/D0DT01845G
Scheme 2. Synthesis of the 2-(N-arylformimino)pyrrolyl-BPh2 complexes 4a-4h.
The formation of the four-coordinate boron complexes 4c-4h is suggested by the absence of the
NH proton resonance in the 1H NMR and the appearance of a 11B singlet in the range of  4.73-
5.34 (three-coordinate 11B resonances occur at higher fields in the region ca. 25 ppm).
The iminopyrrolyl boron complexes 4a and 4b, previously reported in the literature,5 were
included in this work as reference complexes, for comparison with complexes 4c-4h.
2.2 X-Ray diffraction studies
Single crystal X-ray diffraction structures were obtained for halogen-substituted
2-iminopyrrolyl-BPh2 complexes 4c, 4d and 4g. Figure 1 presents the perspective views of their
molecular structures. Crystallographic data for these complexes and the most significant bond
distances and angles are listed in Table S1 and Figures S1-S3 of the Electronic Supporting
Information (ESI), respectively.
Molecules 4c and 4d, containing the substituent at the p-position of the aryl ring, far away
from the BPh2 and 2-iminopyrrolyl fragments, exhibit dihedral angles between the N-phenyl
ring and the 2-iminopyrrolyl fragment of 45.19(16)º and -46.4(7)º, respectively, similar to that
observed in 4a.5a,c However, in these perspective views, it is clear that the
N-phenyl core of the o-bromine complex 4g appears to be approaching orthogonality relative
to the 2-iminopyrrolyl fragment, with a significantly higher dihedral angle of -69.4(3)º,
59.8(3)º, 72.0(3)º, and -70.4(3)º, for molecules A, B, C, and D, respectively (defined as C6-N2-
C7-C8), owing to the high atomic radius of the bromine atom, exerting its bulkiness over the
BPh2 fragment.
6

Page 7 of 39
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The bite angles, corresponding to N1-B1-N2, have typical values of 94.91(12)º (D4OcI): ,109.1403.89/(D40)DºT01845G
(4d) and 94.86(18)º, 94.88(18)º, 94.87(18)º, and 94.60(18)º (4g, molecules A, B, C, and D,
respectively), consistent with modestly distorted tetrahedral geometries.
C3
C3
C6
C2
C6
C2
C9
C8
C4
C9
C4
C8
C10
C7
C7
N2
Br1
N2
C10
N1
C19
C5
Cl1
C11
C12
N1
C5
B1
C12
C11
B1
C20
C14
C13
C13
C19
C20
C14
(a)
(b)
C3
C6
C2
Br1
C10
C4
C8
C7
C9
N2
C11
C12
N1
C20
C5
B1
C13
C19
C14
(c)
Figure 1. Perspective view of 2-iminopyrrolyl-BPh2 complex (a) 4c, (b) 4d and (c) 4g
(molecule A). The calculated hydrogen atoms were omitted for clarity and the ellipsoids were
drawn at the 50% probability level.
2.3 Photophysical studies
The photophysical properties of complexes 4c-4h were studied in solution and in solid state and
compared to complexes 4a and 4b. For solution studies, diluted THF solutions (c < 2.7×10-5 M)
were prepared. In the case of solid state measurements, complexes 4c-4h were mixed with
ZEONEX and drop-casted on quartz or sapphire disks. The normalized absorption and emission
spectra of the halogen-substituted iminopyrrolyl-BPh2 complexes 4c-4h, along with their emission
colours, are presented in Fig. 2. No emission was found in the films of 4h dispersed in ZEONEX.
7

Dalton Transactions
Page 8 of 39
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DOI: 10.1039/D0DT01845G
4c
4d
4e
4f
4g
4h
Figure 2. Normalized absorption and emission spectra of complexes 4c-4h in solution (THF)
and ZEONEX film, at 293 K. Colours of the respective complexes in THF solution, under
UV-irradiation at 365 nm, are also presented (bottom).
8

Page 9 of 39
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The absorption wavelength maxima of complexes 4c-4h in THF solution are witDhOinI: 10th.10e393/6D07D-T01845G
395 nm range (Table 1). Films of these complexes dispersed in ZEONEX show similar maxima
(365-395 nm range), despite the small but significant decrease of the dielectric constant ( =
7.6 in THF and 2.5 in ZEONEX). The molar extinction coefficients at the maximum wavelength
(max) in THF (Table 1) display values characteristic of 2-iminopyrrolyl-BPh2 complexes,
within the 1.5-2.8×104 L mol-1cm-1 range, comparable to those of compounds 4a-4b (1.7 and
1.9×104 L mol-1cm-1, respectively)5, and of the similar complexes already published.5–9
The fluorescence quantum yield (f) values of complexes 4a-4g in THF are within the 0.18-
0.48 range, but for complex 4h (the N-2-iodophenyl derivative) it is more than one order of
magnitude lower (f = 0.01) and non-measurable in ZEONEX films. However, the moderately
high molar singlet extinction coefficient of 4h (max =1.5×104 L mol-1cm-1, Table 1) points to an
allowed π-π* character of the lowest lying singlet excited state, similar to the other complexes.
The increase of the f for the p-substituted N-phenyl complexes (0.25 up to 0.48, for p-F 4b to
p-I 4e, respectively) indicates the absence of spin-orbit coupling when the heavy-atom is
attached at the para position, because the increase of the halogen atom mass (FClBrI),
should increase the intersystem crossing rate constant (kisc), thus decreasing the fluorescence
quantum yield, f = kf/(kic+kisc).
The fluorescence decays of complexes (4a to 4g) in THF were single exponentials with
fluorescence lifetimes (f) around 20.5 ns, from which the values of the fluorescence (kf) and
sum of the non-radiative rate (knr= kic+kisc) constants were obtained (Table 1). The exception
was the fluorescence decay of complex 4h that required a sum of three exponential terms with
decay times (i) equal to 0.027, 0.16 and 2.27 ns and pre-exponential coefficients (Ai) equal to
0.62, 0.30 and 0.08, respectively, to be properly fitted (Figure S4, in ESI). This complexity
indicates the presence of a fast additional excited-state process (a photoreaction), responsible
for the extremely small fluorescence quantum yield of 4h (f = 0.01). Briefly, the two shortest
decay times are assigned to the quenched fluorescence decay of 4h and the longest time to the
emission of a photoproduct (see detailed discussion in ESI). In order to evaluate approximate
values for the fluorescence (kf) and the sum of the non-radiative (knr) constants, the quenched
lifetime of 4h was assumed equal to the average time of the two shortest decay times
(average=Aii2/AiI = 0.13 ns).
9

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Page 10 of 39
max
max
), fluorescence
em
Table 1. Wavelength maximum (
) and molar extinction coefficient (max) of the absorption band, wavelength maximum (
abs
quantum yields (f and PL), fluorescence lifetimes (f and PL), radiative rate constants (kf and kPL) and sum of non-radiative rate constants (knr)
in solution (THF) and in solid state (ZEONEX 480 film), respectively, of boron complexes 4a-4h, at 293K.
Solution (THF)
Solid state (ZEONEX 480 film)
c
d
e
f
a
b
max
abs
max
em
max
abs
max
em
Complex
X
kf
knr
kPL
knr
max
f
f
PL
PL




(ns-1)
(ns-1)
(ns-1)
(ns-1)
(ns)
1.9
1.5
2.1
2.4
2.5
2.6
2.0
(ns)
(nm)
383
381
391
393
395
389
367
369
(nm)
479
478
487
487
492
478
464
470
(nm) (nm)
4a
4b
4c
4d
4e
4f
None g
4-F g
4-Cl
4-Br
4-I
1.7
1.9
2.2
1.7
2.8
1.8
1.7
1.5
0.34
0.25
0.30
0.38
0.48
0.37
0.18
0.01
0.18
0.16
0.14
0.16
0.19
0.14
0.09
0.35
0.49
0.33
0.26
0.21
0.24
0.42
7.66
h
h
h
h
h
h
h
h
h
h
h
h
391
390
395
387
367
365
490 0.66
491 0.62
494 0.53
470 0.59
445 0.31
4.0
3.9
3.5
3.8
0.17
0.16
0.15
0.16
0.09
0.10
0.13
0.11
3-Br
2-Br
2-I
4g
4h
2.7
0.11
0.26
0.13 i 0.08
j
j
j
j
j
a 104 L mol-1 cm-1; b From single exponential decays; c kf = f /f; d knr = (1-f)/f; e kPL = PL /PL; f knr = (1-PL)/PL; g Ref. 5; h not available; i average decay time from
two exponential terms; j non-emissive.
10

Page 11 of 39
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The kf values of complexes 4a-4f in THF are similar (within the 0.14-0.19DnOsI:-110.1r0a3n9/gDe0)D,T01845G
decreasing to 0.09 and 0.08 ns-1 for the ortho-substituted complexes 4g and 4h. This diminution
is confirmed by TDDFT calculations (see Table 3 and the following discussion in section 2.4).
The non-radiative rate constant (knr) values are within the 0.35  0.14 ns-1 range except for
complex 4h whose knr value (7.66 ns-1) is 20-fold larger than those of the other complexes,
consistent with the presence of the additional non-radiative process indicated by the multi-
exponential fluorescence decay.
View Article Online
In ZEONEX films, the photoluminescence rate constants (kPL) are similar to those in THF,
but the non-radiative rate constants are lower, explaining the higher values of the
photoluminescence quantum yields (PL) in ZEONEX.
In order to split the non-radiative rate constants (knr) of the new complexes (4c to 4h) into
their internal conversion (kic) and intersystem crossing (kisc) rate constants, the triplet formation
quantum yields (T) and the triplet lifetimes (T) were also measured, using nanosecond-laser
flash photolysis. The transient absorption spectra of complexes 4c-4g decayed single-
exponentially with lifetimes (T) within the 35-51 s range (Table 2). Complex 4h was again
the exception, with decays at abs< 440 nm generally requiring sums of two exponential terms
plus a constant to be properly fitted, which indicates the presence of other absorbing species
besides the triplet. For longer wavelengths (abs = 450 nm), where only T1→Tn transitions are
expected, acceptable fits with single exponentials plus a constant (
Aabs = a × exp ( - t/τT) +
) were obtained with values of  = 28 s, a = 0.67 and b = 0.33 (Figure S5, ESI). The triplet
b
T
lifetime T = 28 s (Table 2), is slightly shorter than that of 4g (35 s), as expected from the
larger spin-orbit coupling effect on the T1S0 transition. The significant value of b (0.33) is
consistent with the absorption of ground-state species resulting from photoreaction of 4h. The
results (T, kisc = T /f and kic = knr - kisc) are shown in Table 2.
The kisc values are small (0 to 0.03 ns-1) for the para-substituted 4c-4e series, slightly
increase for the m-substituted 4f (0.05 ns-1) and clearly increase for the ortho-substituted
compounds 4g and 4h (0.11 and 0.39 ns-1), indicating that spin-orbit coupling is most efficient
when the heavy-atom is bonded at the ortho position. Actually, 4h is the only compound where
the introduction of the spin-orbit coupling in the calculations induces significant changes in the
calculated absorption spectrum (see below Section 2.4).
The kic values of compounds 4c-4g are within the 0.250.06 ns-1 range, the kic value of 4h
(7.27 ns-1) being much larger than those of the other compounds, as expected. This means that,
in the case of 4h, knr =kisc+kic+kreaction
.
11

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Table 2. Fluorescence lifetime (f), radiative rate constant (kf), sum of non-radiative rDaOteI: 1c0o.1n03s9ta/Dn0tDsT01845G
(knr), triplet formation quantum yield (T), triplet lifetime (T), intersystem crossing rate constant
(kisc) and internal conversion rate constant (kic), of boron complexes 4c-4h, in THF, at 293 K.
b
c
d
e
a
kf
knr
kisc
kic
f
T
Complex
X
T
(ns-1) (ns-1)
(ns-1)
(ns-1)
(ns)
2.1
(µs)
f
f
f
f
4c
4d
4e
4f
4-Cl
4-Br
4-I
0.14
0.16
0.19
0.14
0.09
0.08
0.33
0.26
0.21
0.24
0.42
7.66
2.4
0.04
0.08
0.14
0.21
0.05
51
37
54
35
28
0.02
0.03
0.05
0.11
0.39
0.25
0.18
0.19
0.31
7.27 h
2.5
3-Br
2-Br
2-I
2.6
4g
4h
2.0
0.13g
a From single exponential decays. b kf = f /f; c knr = (1-f)/f; d kisc = T /f; e kic = knr - kisc. f No
signal was observed that could be attributed to the triplet state absorption; g average decay time
from two exponential terms; h value of kic+kreaction
.
The photoreactivity of 4h may be due to the contribution of the p(I)*(iminopyrrolyl) transition
for the S1 state leading to some charge transfer from de iodine p-orbitals (HOMO-1) to the LUMO
orbital, essentially located on the iminopyrrolyl moiety (see Fig. 5 and following discussion in
Computational studies). Actually, homolytic photodissociation of iodine has been observed for
iodine-substituted aromatic rings or other chromophores.14 In the case of 4h, the reaction is
described by two fast decay times, which is consistent with reversible homolytic cleavage of the
iodine-C bond followed by separation of the resulting radicals (see ESI). Complete elucidation
would require further photochemical studies that are beyond the purpose of this work.
Figure 3 compares the emission spectra of diluted solutions of complexes 4e-4h in 2-MeTHF
glasses, at 77 K, measured at two different delay times after excitation: t=0 ms, to collect all
luminescence (fluorescence and phosphorescence), and t=0.05 ms, with most of the
fluorescence being gated-out and exclusively collecting phosphorescence.
Phosphorescence emission at t=0.05 ms was not detected for complex 4c, being very week
for complexes 4d, 4e and 4f (close to the background noise). For complexes 4g and 4h, the ortho-
substituted bromine and iodine compounds, phosphorescence was clearly observed indicating
that the internal heavy-atom effect is highest in these two compounds, as mentioned before. The
smaller phosphorescence emission intensity of 4h relative to 4g results from the concurrent
photoreaction of 4h, leading to a four-fold smaller T (Table 2). It is worth noting that at t=0
ms the phosphorescence intensity of complex 4h competes with that of fluorescence and complex
4g shows residual phosphorescence peaking at 518 nm and 561 nm.
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Figure 3. Fluorescence and phosphorescence emission of complexes 4e-4h, in 2-MeTHF, at
77 K, measured with a 10 ms sample window, at 0 (left) and 0.05 ms (right) delay times after
excitation.
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DOI: 10.1039/D0DT01845G
2.4 Computational studies
The ground state geometry of all the complexes was obtained from DFT calculations (ADF
program) using the PBE0 functional, with a TZP basis set for all atoms, considering spin-orbit
coupling (SOPERT) and the effect of the THF solvent (method A). Calculations were also
performed with the B3LYP functional (method B) and, in both cases, using a D3 Grimme
correction (methods A/D3 and B/D3, more in Computational details). TDDFT calculations
were used to determine the geometry of the first singlet and triplet excited states. The
geometries obtained for complexes 4a-4h are shown in Figure 4. Although geometries for 4a
and 4b have been published,5 they had not been calculated in these conditions for 4b (method
A)5c and had not been reported in detail for 4a.9
In the ground state, the relevant dihedral angle C6-N2-C7-C8 in the 2-iminopyrrolyl ligand
varies between 35º and 39º (or -36º and -38º) for the complexes without substituents (4a)
and with substituents in positions 3 and 4 (4c-f). The two complexes 4g and 4h, with ortho
substituents display a much higher angle (58 and 59º, respectively), owing to steric repulsion
with the pyrrolyl group. These results agree in general with the values obtained for this angle
by X-ray crystallography, namely 45.19(16)º for 4c, -46.4(7)º for 4d, and -69.4(3)º, 59.8(3)º,
72.0(3)º, and -70.4(3)º, for the four independent molecules of 4g. The higher values in the solid
can be assigned to the repulsion between adjacent molecules.
In both singlet and triplet excited states, the C6-N2-C7-C8 dihedral angle approaches zero
for complexes 4a-f. The other two complexes behave differently. While in the 2-Br derivative
4g, the dihedral angle drops significantly from -59º to -29º in the singlet and -32º in the triplet,
in the 2-I derivative 4h the dihedral angle increases from -58º to -66 º in the singlet, but drops
to -32º in the triplet. These values are very similar to those obtained by the other approaches
referred above9 and the one we used in earlier publications,5 as shown in Table S2.
TDDFT calculations were used to calculate the absorption spectrum of all the complexes
(method A with a TZ2P basis set; results from other approaches are given in ESI). The
absorption maxima for all the complexes are given in Table 3.
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Figure 4. Calculated geometries of the ground state (DFT) and of the first singlet and triplet
excited states (TDDFT) of the complexes 4a-4h; the numbers represent the C6-N2-C7-C8
dihedral angle (º) for each species.
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Table 3. Calculated (method A) and experimental transition energy maxima (eV) of the lowest
energy absorption band ( max), of the emission band ( max), and calculated and experimental
Eabs
Eem
fluorescence rate constants (kf) of boron complexes 4a-4h in solution.
max
max (eV)
Calc.
kf (ns-1)
Calc. a
(
eV)
Exp.
E
Eem
abs
Complex
X
Calc.
Exp.
2.59 a
2.59 a
2.55
2.55
2.52
2.59
2.67
2.64
Exp.
0.18 b
0.16 b
0.14
0.16
0.19
0.14
0.09
0.08
4a
4b
4c
4d
4e
4f
None
4-F
3.45
3.62
3.53
3.53
3.50
3.61
3.82
3.75
3.24a
3.25 a
3.17
3.16
3.14
3.19
3.38
3.36
2.73
2.67
2.66
2.65
2.61
2.76
2.83
2.54
0.54
0.56
0.60
0.62
0.62
0.58
0.49
0.36
4-Cl
4-Br
4-I
3-Br
2-Br
2-I
4g
4h
a kf(calc)=1/calc Ref. 5 b Values from Ref. 5.
The calculated values are higher than the experimental ones, but they reflect the trends, namely
the significantly higher absorption energies for 4g and 4h. The lowest energy absorption band
results for all complexes, except for 4h, from a HOMO to LUMO transition in the following
percentage (oscillator strength): 4a 96.2 (1.05), 4b 95.9 (0.98), 4c 96.3 (1.11), 4d 96.5 (1.14),
4e 96.7 (1.19), 4f 95.8 (1.03), 4g 92.0 % (0.78). The same band for complex 4h arises from a
HOMO to LUMO (87%) and a HOMO-1 to LUMO (8%) transition, with an oscillator strength
of 0.65.
The HOMOs and LUMOs of 4a-4h and HOMO-1 of 4h are shown in Figure 5, with their
energies (see also Table S3). It is very clear that the energies are almost exactly the same for
the 4-substituted complexes (4b-4e), the LUMOs being slightly destabilized and the HOMOs
stabilized for the 2-substitued ones (4g-4h), while 3-Br (4f) only exhibits the stabilization of
the HOMO. The HOMOs and LUMOs are localized in the iminopyrrolyl for 4a-4g, with a very
small participation of the halogen in both HOMO and LUMO (4-F), or in the HOMO (4-Cl, Br,
I), the contribution of the halogen increasing from F to I. The transition can be assigned as an
intraligand (IL) * (iminopyrrolyl) with a small (4b-4e) or no (4g) participation of the halogen.
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Figure 5. Energies of the HOMOs and LUMOs of complexes 4a-4h and HOMO-1 of 4h with
their 3D representation (arrows indicate double occupation of the HOMOs).
The orbitals of 4h are different, since the contribution of the iminopyrrolyl ligand practically
does not include the N-phenyl substituent, both in the HOMO and LUMO. The HOMO-1,
however, is localized in two phenyls (on the iminopyrrolyl and on the boron), as well as in
iodine, being C-I -antibonding. In this complex the transition is essentially *
(iminopyrrolyl) with some p(I) * (iminopyrrolyl) charge transfer character.
The calculated fluorescence rate constants (kf) are higher than the experimental ones, but
they follow the main trend, with the values for 4g and 4h being the smallest. Since the
calculations included spin orbit coupling (SOC), it was possible to analyse its role. It can be
seen by the percentage of triplet states contributing to the absorption bands and the position of
the maxima. In almost all complexes, the first excited state is ~100% singlet (for instance the
number for 4-Br (4d) is 99.5%) and there is no shift in the absorption maximum when the
calculation is performed with SOC, and the same happens for the F, Cl, and Br derivatives. The
situation changes for the iodine containing molecules. In 4e (4-I), the first excited state is 99.3%
singlet, and small amounts (< 1%) of several triplet states, while the maximum shifts from 355
to 356 nm with SOC. The effect is more pronounced for 4h (2-I), where the first excited state
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Figure 6. Calculated absorption spectrum of complex 4h: with SOC (red) and without SOC
(blue).
is 92.7% S1, 3.6% T1. The 2 nm shift of the absorption maximum is visible in Figure 6 for 4h
(a similar picture is shown in Figure S6, ESI, for 4e).
These features are in relatively good agreement with the experimental results described in
the previous section, since the larger effect of spin-orbit coupling is observed in 4h being much
smaller in the second iodine derivative 4e. The effect of bromine is not detected in the
calculations, but it is also smaller in the emission features. We calculated the energy of the
phosphorescence by optimizing with TDDFT the lowest triplet excited state. Table S4, ESI,
shows the calculated energies of singlet and triplet states of the complexes 4a to 4h using
E00
various methods. For 4g, the experimental triplet energy,
, is 2.39 eV (518 nm) (Figure 3),
T
while the calculated value is 1.89 eV (method A, Table S4 in ESI). This is much lower than the
experimental energy. The method had not been tested for the calculation of the energy of
triplets, since this effect was not observed previously in this family.
A similar role played by iodine has been described for gallium and aluminium corrole
complexes and their iodinated derivatives (three and four iodine atoms).15 TDDFT calculations
with and without spin-orbit coupling showed the participation of iodine in occupied orbitals,
and its increase with the number of I atoms in the molecule, and reflected the strength of spin-
orbit coupling. Despite the different nature of the absorption spectrum of these complexes, the
charge transfer states from iodine to corrole parallel the charge transfer from iodine to
iminopyrrolyl and the analogous role played by iodine.
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2.5 Electrochemical properties
The new halogen-substituted complexes 4c-4h and parent complexes 4a-4b were characterized
by cyclic voltammetry (CV) (Figures S7-S12, ESI). The measurements were carried out in
tetrabutylammonium perchlorate/dichloromethane electrolyte solutions, at room temperature
and under inert (N2) atmosphere. The oxidation and reduction onset potentials were used to
calculate the electron affinities (EA) and ionization potentials (IP), after conversion to the
absolute scale with the Fc/Fc+ (ferrocene/ferrocenium ion redox couple) as external reference.5a
As the energy level of Fc/Fc+ (ferrocene/ferrocenium ion redox couple) is at 4.80 eV below the
vacuum level, we calculated IP (eV) = Eonset,ox (V) + (4.80 - EFc/Fc+) and EA (eV) = Eonset,red (V)
+ (4.80 ‒ EFc/Fc+), where EFc/Fc+ represents the measured half-wave potential of Fc/Fc+. The
values obtained are summarized in Table 4 along with the energies of the HOMOs and LUMOs
of the corresponding complexes calculated by DFT with solvent correction (CH2Cl2).
Table 4. Ionization potentials (IP), electron affinities (EA) and IP-EA values of complexes
4a-4h, estimated from cyclic voltammetry measurements, and corresponding energies of
HOMOs and LUMOs determined by DFT (CH2Cl2). All values in eV.
Cyclic Voltammetry
DFT (CH2Cl2)
EHOMO ELUMO
Complex
X
IP
EA
IP-EA
4a
4b
4c
4d
4e
4f
None
4-F
5.64 a
5.66 a
5.70
5.67
5.68
5.68
5.78
5.86
2.82 a 2.82
2.86 a 2.80
-5.51
-5.49
-5.53
-5.52
-5.49
-5.60
-5.62
-5.60
-2.98
-2.98
-2.99
-2.99
-2.98
-3.01
-2.91
-2.91
4-Cl
4-Br
4-I
2.81
2.85
2.88
2.81
2.73
2.70
2.89
2.82
2.80
2.87
3.05
3.16
3-Br
2-Br
2-I
4g
4h
a Values from Ref. 5.
The values of ‒IP show a correlation with the energies of the HOMOs, although with IP values
differing between 0.08 and 0.26 eV from the calculated ones (see Fig. S13, ESI). The
‒EA values also correlate with the calculated LUMO energies, with differences varying
between 0.10 and 0.21 eV (see Fig. S13, ESI).
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2.6 Electroluminescence studies
The materials presented above (4b-4h) were tested as emissive materials in organic light-
emitting diodes (OLEDs), with OLEDs based on complex 4h failing to show any measurable
emission. Thin films of the complexes were prepared by either spin-coating or vacuum thermal
deposition. The performance of the full set of devices tested can be found in the ESI.
The best performing OLEDs were obtained when the boron complexes thin films were
prepared by vacuum thermal deposition.
Table 5 shows the performance parameters of a group of devices based on films of
complexes 4c, 4d and 4g prepared by thermal vacuum deposition, with the structure: ITO (100
nm)/PEDOT:PSS (40 nm)/TPD (20 nm)/Complex (ca. 80 nm)/Bphen (11 nm)/LiF (1.5 nm)/Al
(80 nm).
Table 5. Characteristics of OLED devices based on vacuum thermal deposited complexes 4c,
4d and 4g including maximum luminance (Lmax, cd m-2), external quantum efficiency
(EQEmax, %), current efficiency (L, cd A-1), and Commission Internationale de l’Éclairage
(CIE) colour coordinates.
Complex
X
Lmax
EQEmax
CIE
Lmax
ITO/PEDOT:PSS/TPD/Complex/Bphen/LiF/Al
4c
4d
4g
4-Cl
4-Br
2-Br
1812
488
4
0.15
0.03
0.006
4.9×10-1
1.2×10-1
1.2×10-2
0.30, 0.51
0.33, 0.53
0.29, 0.38
In this series, 4c-based OLED showed the best performance, with a maximum luminance of
1812 cd m-2, with an EQE of 0.15%. It is worth pointing out that for OLEDs based on all three
complexes, 4c, 4d and 4g, the maximum emission is red shifted with respect to PL emission of
the corresponding sublimed films (Figure S15 in the ESI).
A multilayer OLED based on 4e with structure ITO/HAT-CN (10 nm)/TAPC (25 nm)/mCP
co 10% 4e (25 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (100 nm) (Figure 7(a)), was also prepared
by vacuum thermal deposition.
Commercially available hexaazatriphenylenehexacarbonitrile (HAT-CN) was used as a hole
injection layer and N,N-bis(4-methylphenyl)-benzenamine (TAPC) was used as hole-
transporting layer. 2,2’,2’-(1,3,5-benzinetriyl)-tris(1-phenyl-1-benzimidazole) (TPBi) was
selected as electron-transporting material, lithium fluoride (LiF) as electron injection layer with
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aluminium (Al) as the cathode. Complex 4e was dispersed in mCP (1,3-Bis(N-
DOI: 10.1039/D0DT01845G
carbazolyl)benzene) (10%), which was selected due to its energy levels: HOMO at -5.9 eV,
below that of 4e (considering EHOMO= -IP=-5.68 eV) and LUMO at -2.4 eV, above that of 4e
(ELUMO= -EA=-2.88 eV).
Figure 7 presents the performance data of this device, which showed a turn on voltage of 5.1
V at 5 cd m-2 reaching a maximum luminance of 494 cd m-2 with a maximum EQE of 0.90%
and a maximum current efficiency of 2.07 cd A-1. The obtained EL spectrum (CIE coordinates
at 100 cd m-2 of 0.28, 0.43) is slightly red shifted with respect to the PL spectra, which could
be due to interference effects.
(a)
(b)
(c)
(d)
Figure 7. Characteristics of OLED devices fabricated using complex 4e as the emitter and
mCP as host: (a) Schematic device energy level structure; (b) Electroluminescence spectra
(EL) compared with the film photoluminescence (PL) and solution fluorescence spectra in
THF (Sol); (c) External Quantum Efficiency (EQE) vs. Luminance; and (d) Current Density
(filled symbols)/Luminance (open symbols) vs. Voltage.
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The OLEDs prepared with molecules 4e-4h did not clearly show the phosDpOhI:o1r0e.1s0c39e/nD0cDeT01845G
contribution to the electroluminescence spectrum. In the case of 4g, there is some indication
that it may be present, but a definite conclusion cannot be drawn (see Figure S18 in the ESI).
The absence of a clear phosphorescence contribution to the OLEDs emission is attributed to a
significant charge-induced triplet quenching, promoted by the triplets long lifetime, which
makes the observation of this emission very difficult.16
3. Conclusions
A set of halogen-substituted 2-iminopyrrolyl-BPh2 complexes was synthesized and
characterized in terms of their molecular and photophysical properties. The internal heavy-atom
effect was strongly controlled by the position of the halogen atom in the N-aryl ring of the
ligand moiety, being negligible for para-I substitution (kisc 0.03 ns-1) and most effective for
ortho-I substitution (kisc 0.39 ns-1). Accordingly, complex 4h exhibited the greatest
phosphorescence emission at 77 K. DFT and TDDFT calculations reproduced well the
absorption and emission energies and provided geometries of the first singlet and triplet excited
states, which showed how steric hindrance of 2-substituents prevented the iminopyrrolyl
ligands from achieving planarity in both situations. Evidence of spin-orbit coupling in the
absorption spectra was only found for the molecules containing iodine and was more visible
for the ortho-I derivative. OLEDs were fabricated based on solution processed and vacuum
thermal evaporated films of complexes 4b-4h, the best one giving an external quantum
efficiency (EQEmax) of 0.15% along with a luminance maximum (Lmax) of 1812 cd m-2 for 4c.
An optimized structure was later prepared with 2-iminopyrrolyl boron complex 4e as emissive
layer mixed with mCP as host material. The latter structure gave rise to improved EQE of nearly
1%, along with a maximum luminance of 494 cd m-2.
4. Experimental Section
4.1 General
All experiments dealing with air- and/or moisture-sensitive materials were carried out under
inert atmosphere using a dual vacuum/nitrogen line and standard Schlenk techniques. Nitrogen
gas was supplied by Air Liquide and purified by passage through 4 Å molecular sieves. Unless
otherwise stated, all reagents were purchased from commercial suppliers (e.g. Acrös, Aldrich,
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Fluka, Alfa Aesar) and used without further purification. All solvents to be used under inert
DOI: 10.1039/D0DT01845G
atmosphere were thoroughly deoxygenated and dehydrated 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 and diethyl ether), calcium hydride (for
n-hexane and dichloromethane). Solvents and solutions were transferred using a positive
pressure of nitrogen through stainless steel cannulae and mixtures were filtered in a similar way
using modified cannulae that could be fitted with glass fibre filter disks.
Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III 300
spectrometer at 299.995 MHz (1H), 75.4296 MHz (13C) and 96.2712 MHz (11B). Deuterated
solvents were dried by storage over 4 Ǻ molecular sieves and degassed by the freeze-pump-
thaw method. Spectra were referenced internally using the residual protio solvent resonance
(1H) and the solvent carbon (13C) resonances relative to tetramethylsilane (δ=0) and referenced
11
externally using 15% BF3.OEt2 (δ=0) for B. All chemical shifts are quoted in δ (ppm) and
coupling constants given in hertz. Multiplicities were abbreviated as follows: broad (br), singlet
(s), doublet (d), doublet of doublets (dd), triplet (t) and multiplet (m). For air- and/or moisture
sensitive materials, samples were prepared in J. Young NMR tubes in a glovebox. Elemental
analyses were obtained from the IST elemental analysis services.
The reagent 2-formylpyrrole (1)11 was prepared according to the literature.11 Materials used
for the device fabrication were purchased from Sigma Aldrich (LiF(99.995%), TPD, TAPC),
Alfa Aesar (Al wire (99.9995%)), and LUMTEC (TPBi, mCP, HAT-CN).
4.2 Syntheses
Synthesis of 2-(4-chlorophenylformimino)pyrrole (3c): 4-Chloroaniline 2c (0.38 g, 3.0
mmol), 2-formylpyrrole 1 (0.28 g, 3.0 mmol) and a catalytic amount of p-toluenesulphonic acid
were suspended in xylene (35 mL) in a 50 mL round-bottom flask fitted with a Soxhlet, a
condenser and a CaCl2 guard tube. The orange mixture was refluxed at ≈130 ºC, for 4 days.
After reaching room temperature, the reaction mixture was filtered, and all volatiles removed.
The brown oil obtained was extracted with hot n-hexane, giving rise to an orange solution that
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was concentrated and kept at -20 ºC. After some hours, a brown solid correspoDnOdIi: n10g.10t3o9/Dth0DeT01845G
1
desired product 3c was filtered and dried under vacuum. Yield, 0.15 g (25%). H NMR (300
MHz, CD2Cl2):  9.47 (br, 1H, NH), 8.23 (s, 1H, CH=N), 7.33 (d, 3JHH= 9 Hz, 2H, N-Ph-H3 +
N-Ph-H5), 7.11 (d, 3JHH= 9 Hz, 2H, N-Ph-H2 + N-Ph-H6), 6.98 (s, 1H, H5-pyrr), 6.70 (d, 3JHH=
3 Hz, 1H, H3-pyrr), 6.32 (t, 3JHH= 3 Hz, 1H, H4-pyrr).
Synthesis of 2-(4-bromophenylformimino)pyrrole (3d): 4-Bromoaniline 2d (0.51 g, 3.0
mmol), 2-formylpyrrole 1 (0.28 g, 3.0 mmol), a catalytic amount of p-toluenesulphonic acid
and MgSO4 (to remove any water from the reaction mixture) were suspended in toluene (30
mL) in a 50 mL round-bottom flask fitted with a condenser and a CaCl2 guard tube. The brown
mixture was refluxed overnight, at 100 ºC. After reaching room temperature, the reaction
mixture was filtered, and all volatiles removed. The brown solid obtained was extracted with
hot n-hexane, giving rise to a yellow solution that was concentrated and kept at -20 ºC. After
some hours, a beige solid corresponding to the desired product 3d was filtered and dried under
vacuum. Yield, 0.47 g (63%). 1H NMR (300 MHz, CD2Cl2):  9.99 (br, 1H, NH), 8.24 (s, 1H,
CH=N), 7.49 (d, 3JHH= 9 Hz, 2H, N-Ph-H3 + N-Ph-H5), 7.06 (d, 3JHH= 9 Hz, 2H, N-Ph-H2 + N-
Ph-H6), 6.87 (s, 1H, H5-pyrr), 6.71 (d, 3JHH= 3 Hz, 1H, H3-pyrr), 6.30 (t, 3JHH= 3 Hz, 1H, H4-
pyrr).
Synthesis of 2-(4-iodophenylformimino)pyrrole (3e): 4-Iodoaniline 2e (0.66 g, 3.0 mmol),
2-formylpyrrole 1 (0.28 g, 3.0 mmol), a catalytic amount of p-toluenesulphonic acid and
MgSO4 (to remove any water from the reaction mixture) were suspended in toluene (30 mL) in
a 50 mL round-bottom flask fitted with a condenser and a CaCl2 guard tube. The brown mixture
was refluxed at 100 ºC, overnight. After reaching room temperature, the reaction mixture was
filtered, all volatiles removed, and a small quantity of toluene added. The resulting brown
solution was stored at -20 ºC, giving a brown solid corresponding to the desired product 3e.
Yield, 0.36 g (41%). 1H NMR (300 MHz, CD2Cl2):  9.81 (br, 1H, NH), 8.22 (s, 1H, CH=N),
7.68 (d, 3JHH= 9 Hz, 2H, N-Ph-H3 + N-Ph-H5), 6.96 (s, 2H, N-Ph-H2 + N-Ph-H6), 6.93 (s, 1H,
H5-pyrr), 6.73 (d, 3JHH= 3 Hz, 1H, H3-pyrr), 6.32 (t, 3JHH= 3 Hz, 1H, H4-pyrr).
Synthesis of 2-(3-bromophenylformimino)pyrrole (3f): 3-Bromoaniline 2f (1.72 g, 10.0
mmol), 2-formylpyrrole 1 (0.96 g, 10.1 mmol) and a catalytic amount of p-toluenesulphonic
acid were suspended in toluene (30 mL) in a 50 mL round-bottom flask fitted with a Soxhlet
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apparatus, a condenser and a CaCl2 guard tube. The orange mixture was refluxed at 130 ºC, for
DOI: 10.1039/D0DT01845G
3 days. After reaching room temperature, the reaction mixture was filtered, and all volatiles
removed. The brown oil obtained was extracted with hot n-hexane, giving rise to a red solution
that was concentrated and kept at -20 ºC. After some hours, a brown solid corresponding to the
1
desired product 3f was filtered and dried under vacuum. Yield, 1.80 g (72%). H NMR (300
MHz, CD2Cl2):  10.11 (br, 1H, NH), 8.26 (s, 1H, CH=N), 7.37 (s, 1H, N-Ph-H2), 7.35 (s, 1H,
N-Ph-H4), 7.26 (t, 3JHH= 9 Hz, 1H, N-Ph-H5), 7.15 (d, 3JHH= 3 Hz, 1H, N-Ph-H6), 6.89 (s, 1H,
H5-pyrr), 6.76 (d, 3JHH= 3 Hz, 1H, H3-pyrr), 6.33 (t, 3JHH= 3 Hz, 1H, H4-pyrr). 13C{1H} NMR
(75 MHz, CDCl3):  153.3 (N-Ph-Cipso), 150.9 (CH=N), 130.6 (N-Ph-C5), 130.5 (C2-pyrr),
128.4 (N-Ph-C4), 124.1 (N-Ph-C2), 123.9 (N-Ph-C6), 122.9 (N-Ph-C3), 120.2 (C5-pyrr), 117.9
(C3-pyrr), 110.8 (C4-pyrr). Anal. Calcd (%) for C11H9BrN2: C, 53.04; H, 3.64; N, 11.25. Found:
C, 53.06; H, 3.55; N, 11.14.
Synthesis of 2-(2-bromophenylformimino)pyrrole (3g): 2-Bromoaniline 2g (1.73 g, 10.0
mmol), 2-formylpyrrole 1 (0.95 g, 10.0 mmol) and a catalytic amount of p-toluenesulphonic
acid were suspended in toluene (30 mL) in a 50 mL round-bottom flask fitted with a Soxhlet
apparatus, a condenser and a CaCl2 guard tube. The orange mixture was refluxed at 130 ºC, for
2 days. After reaching room temperature, the reaction mixture was filtered, and all volatiles
removed. The brown oil obtained was extracted with hot n-hexane, giving rise to an orange
solution that was concentrated and kept at -20 ºC. After some hours, a beige solid corresponding
to the desired product 3g was filtered and dried under vacuum. Yield, 1.36 g (54%). 1H NMR
(300 MHz, CD2Cl2):  9.77 (br, 1H, NH), 8.11 (s, 1H, CH=N), 7.60 (d, 3JHH= 6 Hz, 1H, N-Ph-
H3), 7.29 (t, 3JHH= 6 Hz, 1H, N-Ph-H5), 7-08-6.88 (m, 3H, N-Ph-H4 + N-Ph-H6 + H5-pyrr), 6.70
3
3
(d, JHH= 3 Hz, 1H, H3-pyrr), 6.30 (t, JHH= 3 Hz, 1H, H4-pyrr). 13C{1H} NMR (75 MHz,
CDCl3):  150.9 (N-Ph-Cipso), 150.6 (CH=N), 133.1 (N-Ph-C3), 130.7 (C2-pyrr), 128.6 (N-Ph-
C5), 126.4 (N-Ph-C4), 123.7 (N-Ph-C6), 120.1 (C5-pyrr), 118.7 (N-Ph-C2), 117.4 (C3-pyrr),
110.8 (C4-pyrr). Anal. Calcd (%) for C11H9BrN2: C, 53.04; H, 3.64; N, 11.25. Found: C, 53.07;
H, 3.57; N, 11.25.
Synthesis of 2-(2-iodophenylformimino)pyrrole (3h): 2-Iodoaniline 2h (0.50 g, 2.3 mmol),
2-formylpyrrole 1 (0.24 g, 2.5 mmol) and a catalytic amount of p-toluenesulphonic acid (p-
TSA) were suspended in toluene (30 mL) in a 50 mL round-bottom flask fitted with a Soxhlet
apparatus, a condenser and a CaCl2 guard tube. The salmon-coloured mixture was refluxed
25

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overnight, at 130 ºC. After reaching room temperature, the reaction mixture wasDfOilIt:e10r.e10d3,9/aDn0DdT01845G
all volatiles were removed. The brown oil obtained was extracted with hot n-hexane, giving
rise to a yellow solution that was concentrated and kept at -20 ºC. After some hours, a brown
solid corresponding to the desired product 3h was filtered and dried under vacuum. Yield, 0.48
g (72%). 1H NMR (300 MHz, CD2Cl2):  9.56 (br, 1H, NH), 8.10 (s, 1H, CH=N), 7.88 (d, 3JHH=
6 Hz, 1H, N-Ph-H3), 7.35 (t, 3JHH= 6 Hz, 1H, N-Ph-H5), 7.06-6.94 (m, 2H, N-Ph-H6 + H5-pyrr),
6.89 (t, 3JHH= 6 Hz, 1H, N-Ph-H4), 6.72 (s, 1H, H3-pyrr), 6.33 (s, 1H, H4-pyrr). 13C{1H} NMR
(75 MHz, CDCl3):  152.7 (N-Ph-Cipso), 150.0 (CH=N), 139.2 (N-Ph-C3), 130.5 (C2-pyrr),
129.5 (N-Ph-C5), 126.7 (N-Ph-C4), 123.4 (N-Ph-C6), 118.5 (C5-pyrr), 116.9 (C3-pyrr), 110.9
(C4-pyrr), 95.6 (N-Ph-C2). Anal. Calcd (%) for C11H9IN2: C, 44.62; H, 3.06; N, 9.46. Found:
C, 44.70; H, 3.07; N, 9.41.
Synthesis of [BPh2{k2N,N’-NC4H3C(H)=N-4-Cl-C6H4}] (4c): A solution of 2-(4-
chlorophenylformimino)pyrrole 3c (0.20 g, 1.0 mmol), in toluene, was added to triphenylboron
(0.24 g, 1.0 mmol) (also in toluene). The green solution was refluxed under nitrogen overnight.
The resulting dark green solution was cooled to room temperature, filtered and all volatiles
removed under vacuum. Crystals of the desired boron complex 4c suitable for single crystal X-
ray diffraction studies were obtained by double layering with toluene and n-hexane. The
solution was filtered, and the crystals dried under vacuum. Yield, 0.17 g (47%). 1H NMR (300
MHz, CD2Cl2):  8.48 (s, 1H, CH=N), 7.31-7.16 (m, 15H, N-Ph-H2 + N-Ph-H3 + N-Ph-H5 +
N-Ph-H6 + B-Ph-Hortho + B-Ph-Hmeta + B-Ph-Hpara + H5-pyrr), 7.08 (d, 3JHH= 3Hz, 1H, H3-pyrr),
3
4
13
6.58 (dd, JHH= 6 Hz, JHH= 3 Hz, 1H, H4-pyrr). C{1H} NMR (75 MHz, CDCl3):  150.8
(CH=N), 141.2 (N-Ph-Cipso), 134.7 (C2-pyrr), 133.5 (B-Ph-Cortho), 133.1 (N-Ph-C4), 132.3 (C5-
pyrr), 129.8 (N-Ph-C3 + N-Ph-C5), 127.9 (B-Ph-Cmeta), 127.1 (B-Ph-Cpara), 123.8 (N-Ph-C2 +
N-Ph-C6), 118.0 (C4-pyrr), 116.0 (C3-pyrr), B-Ph-Cipso resonance absent. 11B NMR (96.29
MHz, CD2Cl2):  4.73. Anal. Calcd (%) for C23H18BClN2: C, 74.93; H, 4.92; N, 7.60. Found:
C, 75.09; H, 4.94; N, 7.67.
Synthesis of [BPh2{k2N,N’-NC4H3C(H)=N-4-Br-C6H4}] (4d): A solution of 2-(4-
bromophenylformimino)pyrrole 3d (0.45 g, 1.8 mmol), in toluene, was added to triphenylboron
(0.44 g, 1.8 mmol) (also in toluene). The yellow solution was refluxed under nitrogen overnight.
The resulting dark green solution was cooled to room temperature, the solvent concentrated
under vacuum and stored at -20 ºC. In the following day, crystals of the desired boron complex
26

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4d suitable for single crystal X-ray diffraction studies were obtained. The solutionDwOaI:s10f.1i0lt3e9/rDe0dD,T01845G
and the crystals dried under vacuum. Yield, 0.34 g (45%). 1H NMR (300 MHz, CD2Cl2):  8.49
(s, 1H, CH=N), 7.41 (d, 3JHH= 9 Hz, 2H, N-Ph-H3 + N-Ph-H5), 7.31-7.18 (m, 11H, H5-pyrr +
3
B-Ph-Cortho + B-Ph-Cmeta + B-Ph-Cpara), 7.15 (d, JHH= 9 Hz, 2H, N-Ph-H2 + N-Ph-H6), 7.09
3
4
3
4
(dd, JHH= 6 Hz, JHH= 3 Hz, 1H, H3-pyrr), 6.58 (dd, JHH= 6 Hz, JHH= 3 Hz, 1H, H4-pyrr).
13C{1H} NMR (75 MHz, CDCl3):  150.7 (CH=N), 141.6 (N-Ph-Cipso), 134.8 (C2-pyrr), 133.5
(B-Ph-Cortho), 132.8 (N-Ph-C3 + N-Ph-C5), 132.3 (C5-pyrr), 128.0 (B-Ph-Cmeta), 127.1 (B-Ph-
Cpara), 124.0 (N-Ph-C2 + N-Ph-C6), 121.0 (N-Ph-C4), 118.1 (C4-pyrr), 116.1 (C3-pyrr), B-Ph-
Cipso resonance absent. 11B NMR (96.29 MHz, CD2Cl2):  4.77. Anal. Calcd (%) for
C23H18BBrN2: C, 66.87; H, 4.39; N, 6.78. Found: C, 66.72; H, 4.36; N, 6.83.
Synthesis of [BPh2{k2N,N’-NC4H3C(H)=N-4-I-C6H4}] (4e): A solution of 2-(4-
iodophenylformimino)pyrrole 3e (0.36 g, 1.2 mmol), in toluene, was added to triphenylboron
(0.29 g, 1.2 mmol) (also in toluene). The greenish yellow solution was refluxed overnight under
nitrogen. The resulting solution was cooled to room temperature and all volatiles removed
under vacuum. A green powder of the desired boron complex 4e was obtained by double
layering a toluene solution with n-hexane. The solution was filtered, and the powder dried under
vacuum. Yield, 0.25 g (45%). 1H NMR (300 MHz, CD2Cl2):  8.50 (s, 1H, CH=N), 7.60 (d,
3JHH= 9 Hz, 2H, N-Ph-H3 + N-Ph-H5), 7.33-7.14 (m, 11H, H5-pyrr + B-Ph-Hortho + B-Ph-Hmeta
3
3
+ B-Ph-Hpara), 7.08 (d, JHH= 6 Hz, 1H, H3-pyrr), 7.03 (d, JHH= 9 Hz, 2H, N-Ph-H2 + N-Ph-
H6), 6.58 (dd, 3JHH= 6 Hz, 4JHH= 3 Hz, 1H, H4-pyrr). 13C{1H} NMR (75 MHz, CDCl3):  150.5
(CH=N), 142.2 (N-Ph-Cipso), 138.8 (N-Ph-C3 + N-Ph-C5), 134.8 (C2-pyrr), 133.5 (B-Ph-Cortho),
132.4 (C5-pyrr), 128.0 (B-Ph-Cmeta), 127.1 (B-Ph-Cpara), 124.2 (N-Ph-C2 + N-Ph-H6), 118.1
(C4-pyrr), 116.1 (C3-pyrr), 92.2 (N-Ph-C4), B-Ph-Cipso resonance absent. 11B NMR (96.29
MHz, CD2Cl2):  5.05. Anal. Calcd (%) for C23H18BIN2: C, 60.04; H, 3.94; N, 6.09. Found: C,
60.07; H, 3.93; N, 6.08.
Synthesis of [BPh2{k2N,N’-NC4H3C(H)=N-3-Br-C6H4}] (4f): A solution of 2-(3-
bromophenylformimino)pyrrole 3f (0.57 g, 2.3 mmol), in toluene, was added to triphenylboron
(0.55 g, 2.3 mmol) (also in toluene). The green solution was refluxed overnight under nitrogen.
The resulting dark green solution was cooled to room temperature and filtered. The solvent was
concentrated under vacuum and the final solution stored at -20 ºC, giving rise to a yellow
powder corresponding to the desired boron complex 4f, which was dried under vacuum. Yield,
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DOI: 10.1039/D0DT01845G
1
0.25 g (45%). H NMR (300 MHz, CD2Cl2):  8.48 (s, 1H, CH=N), 7.40 (s, 1H, N-Ph-H2),
7.37 (d, 3JHH= 9 Hz, 1H, N-Ph-H4), 7.33-7.18 (m, 12H, N-Ph-H6 + B-Ph-Hortho + B-Ph-Hmeta
B-Ph-Hpara + H5-pyrr), 7.16 (d, 3JHH= 9 Hz, 1H, N-Ph-H5), 7.10 (d, 3JHH= 3 Hz, 1H, H3-pyrr),
+
3
4
13
6.59 (dd, JHH= 6 Hz, JHH= 3 Hz, 1H, H4-pyrr). C{1H} NMR (75 MHz, CDCl3):  150.9
(CH=N), 143.8 (N-Ph-Cipso), 134.7 (C2-pyrr), 133.5 (B-Ph-Cortho), 132.6 (C5-pyrr), 130.9 (N-
Ph-C5), 130.5 (N-Ph-C4), 128.0 (B-Ph-Cmeta), 127.2 (B-Ph-Cpara), 125.2 (N-Ph-C2), 123.1 (N-
11
Ph-C3), 121.4 (N-Ph-C6), 118.3 (C4-pyrr), 116.4 (C3-pyrr), B-Ph-Cipso resonance absent. B
NMR (96.29 MHz, CD2Cl2):  5.09. Anal. Calcd (%) for C23H18BBrN2: C, 66.87; H, 4.39; N,
6.78. Found: C, 66.81; H, 4.23; N, 6.73.
Synthesis of [BPh2{k2N,N’-NC4H3C(H)=N-2-Br-C6H4}] (4g): A solution of 2-(2-
bromophenylformimino)pyrrole 3g (0.46 g, 1.8 mmol), in toluene, was added to triphenylboron
(0.47 g, 1.9 mmol) (also in toluene). The greenish blue solution was refluxed under nitrogen
overnight. The resulting dark blue solution was cooled to room temperature, the solvent
concentrated under vacuum and stored at -20 ºC. In the following day, crystals of the desired
boron complex 4g suitable for single crystal X-ray diffraction studies were obtained. The
solution was filtered, and the crystals dried under vacuum. Yield, 0.39 g (53%). 1H NMR (300
MHz, CD2Cl2):  8.31 (s, 1H, CH=N), 7.58 (dd, 3JHH= 6 Hz, 4JHH= 3 Hz, 1H, N-Ph-H3), 7.29
(s, 1H, N-Ph-H5), 7.25-7.03 (m, 13H, N-Ph-H4 + H3-pyrr + H5-pyrr + B-Ph-Hortho + B-Ph-Hmeta
+ B-Ph-Hpara), 6.95 (dd, 3JHH= 6 Hz, 4JHH= 3 Hz, 1H, N-Ph-H6), 6.66 (dd, 3JHH= 6 Hz, 4JHH= 3
13
Hz, 1H, N-Ph-H4). C{1H} NMR (75 MHz, CDCl3):  156.4 (CH=N), 141.5 (N-Ph-Cipso),
135.1 (C2-pyrr), 134.1 (N-Ph-C3), 133.4 (B-Ph-Cortho), 132.4 (N-Ph-C5), 129.4 (C5-pyrr), 128.2
(N-Ph-C4), 127.8 (B-Ph-Cmeta), 127.4 (N-Ph-C6), 126.9 (B-Ph-Cpara), 119.9 (N-Ph-C2), 117.6
11
(C4-pyrr), 116.0 (C3-pyrr), B-Ph-Cipso resonance absent. B NMR (96.29 MHz, CD2Cl2): 
5.34. Anal. Calcd (%) for C23H18BBrN2: C, 66.87; H, 4.39; N, 6.78. Found: C, 66.96; H, 4.26;
N, 6.76.
Synthesis of [BPh2{k2N,N’-NC4H3C(H)=N-2-I-C6H4}] (4h): A solution of 2-(2-
iodophenylformimino)pyrrole 3h (0.31 g, 1.0 mmol), in toluene, was added to triphenylboron
(0.25 g, 1.0 mmol) (also in toluene). The orange solution was refluxed overnight under nitrogen.
The resulting dark brown suspension was cooled to room temperature and all volatiles removed
under vacuum. The brown powder of the desired boron complex 4h was obtained by double
layering a toluene solution with n-hexane. The solution was filtered, and the powder dried under
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vacuum. Yield, 0.33 g (69%). 1H NMR (300 MHz, CD2Cl2):  8.26 (s, 1H, CH=N), 7.83 (d,
DOI: 10.1039/D0DT01845G
3
3JHH= 9 Hz, 1H, N-Ph-H3), 7.29 (t, JHH= 9 Hz, 1H, N-Ph-H4), 7.26-7.04 (m, 12H, H3-pyrr +
H5-pyrr + B-Ph-Hortho + B-Ph-Hmeta + B-Ph-Hpara), 7.00 (t, 3JHH= 9 Hz, 1H, N-Ph-H4), 6.90 (d,
13
3JHH= 9 Hz, 1H, N-Ph-H6), 6.67 (s, 1H, H4-pyrr). C{1H} NMR (75 MHz, CDCl3):  156.3
(CH=N), 144.7 (N-Ph-Cipso), 140.6 (N-Ph-C3), 134.2 (C2-pyrr), 133.5 (B-Ph-Cortho), 132.4 (N-
Ph-C5), 129.6 (N-Ph-C4), 129.0 (C5-pyrr), 127.7 (B-Ph-Cmeta), 126.9 (B-Ph-Cpara), 126.7 (N-
Ph-C6), 117.5 (C4-pyrr), 115.8 (C3-pyrr), 96.1 (N-Ph-C2), B-Ph-Cipso resonance absent. 11B
NMR (96.29 MHz, CD2Cl2):  5.31. Anal. Calcd (%) for C23H18BIN2: C, 60.04; H, 3.94; N,
6.09. Found: C, 60.36; H, 3.94; N, 6.04.
4.3 X-ray data collection
The crystallographic data for complexes 4c, 4d and 4g were collected using graphite
monochromated Mo-K radiation ( = 0.71073 Å) on a Bruker AXS-KAPPA APEX II
diffractometer equipped with an Oxford Cryosystem open-flow nitrogen cryostat, at 150 K, and
the crystals were selected under inert atmosphere, stored in polyfluoroether oil and mounted on
a nylon loop. Cell parameters were retrieved using Bruker SMART software and refined using
Bruker SAINT on all observed reflections. Absorption corrections were applied using
SADABS.17 Structure solution and refinement were performed using direct methods with the
programs SIR2004,18 SIR2014,19 SIR201819 and SHELXL,20 included in the package of
programs WINGX-Version2014.1.21 All hydrogen atoms were inserted in idealized positions
and allowed to refine riding on the parent carbon atom, with C-H distances of 0.95 Å for
aromatic H atoms and with Uiso(H) = 1.2Ueq(C). Graphic presentations were prepared with
ORTEP-III.21 Data was deposited in CCDC under the deposit numbers 2004594 for 4c,
2004595 for 4d, and 2004596 for 4g.
4.4 Cyclic voltammetry measurements
Cyclic voltammetry (CV) measurements were performed on a Solartron potentiostat in a three-
electrode cell with a 0.1 M tetrabutylammonium perchlorate (TBAClO4)/CH2Cl2 supporting
electrolyte, at a scan rate of 50 mV/s, at room temperature and under inert atmosphere (N2).
The reference electrode, counter electrode and working electrode used were a saturated calomel
electrode (SCE), a platinum wire and a platinum disk, respectively.
29