How πextension or Structural Bending Alters the Properties of Boron-Doped Phenylene-Containing Oligoacenes

Article abstract of DOI:10.1021/acs.organomet.9b00330

Linear annulation of two 2,3-biphenylenediyl moieties to a 1,4-dibora-2,5-cyclohexadiene core via its carbon atoms leads to the boron-doped phenylene-containing oligoacene (B-POA) DBI. The optoelectronic properties of DBI are unlike those of related boron-doped polycyclic aromatic hydrocarbons (B-PAHs), such as 6,13-dihydro-6,13-diborapentacene (DBP). Herein we disclose two new B-POAs, which provide insight into fundamental structure-property relationships. The first is a π-extended DBI congener (e-DBI), in which two 2,3-benzo[b]biphenylenediyl moieties are linearly annulated to the central B₂C₄ ring and behaves partly similar to DBI: it is a strong electron acceptor and has a deep red color. In contrast to the nonluminescent DBI, it shows red fluorescence (quantum yield: 12%), which changes to blue-green upon addition of excess F^--ions. In the presence of 0.4 equiv of F^-, nearly white emission is observed (CIE1931: (0.3320|0.4056)). If biphenylenediyl moieties are angularly attached to the B₂C₄ ring via their 1,2-positions, the resulting mixture of "v"- and "z"-shaped B-POA isomers (v/z-DBI) behaves distinctly different from linear DBI: the electrode potentials required for the reversible two-electron reductions of v/z-DBI are significantly more cathodic, and v/z-DBI is an orange rather than red solid and emits green light (quantum yields 65%) upon irradiation.

Full text of DOI:10.1021/acs.organomet.9b00330

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
How π Extension or Structural Bending Alters the Properties of
Boron-Doped Phenylene-Containing Oligoacenes
Sven Kirschner, Ise Uecker, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner*
Institut fur Anorganische und Analytische Chemie, Goethe-Universitat Frankfurt (Main), Max-von-Laue-Straße 7, 60438 Frankfurt
̈
̈
(Main), Germany
S
* Supporting Information
ABSTRACT: Linear annulation of two 2,3-biphenylenediyl moieties to a 1,4-dibora-
2,5-cyclohexadiene core via its carbon atoms leads to the boron-doped phenylene-
containing oligoacene (B-POA) DBI. The optoelectronic properties of DBI are unlike
those of related boron-doped polycyclic aromatic hydrocarbons (B-PAHs), such as 6,13-
dihydro-6,13-diborapentacene (DBP). Herein we disclose two new B-POAs, which
provide insight into fundamental structure−property relationships. The first is a π-
extended DBI congener (e-DBI), in which two 2,3-benzo[b]biphenylenediyl moieties
are linearly annulated to the central B2C4 ring and behaves partly similar to DBI: it is a strong electron acceptor and has a deep
red color. In contrast to the nonluminescent DBI, it shows red fluorescence (quantum yield: 12%), which changes to blue-green
upon addition of excess F−-ions. In the presence of 0.4 equiv of F−, nearly white emission is observed (CIE1931: (0.3320|
0.4056)). If biphenylenediyl moieties are angularly attached to the B2C4 ring via their 1,2-positions, the resulting mixture of “v”-
and “z”-shaped B-POA isomers (v/z-DBI) behaves distinctly different from linear DBI: the electrode potentials required for the
reversible two-electron reductions of v/z-DBI are significantly more cathodic, and v/z-DBI is an orange rather than red solid
and emits green light (quantum yields 65%) upon irradiation.
B2C4 core, which should afford bent isomers of DBI with “v”-
and “z”-shaped scaffolds (v/z-DBI; Scheme 1).
INTRODUCTION
■
Polycyclic aromatic hydrocarbons (PAHs) are an extensively
investigated class of materials with widespread use.1 Their
properties can be enhanced further through the incorporation
of main-group elements.2 In particular, boron-doped PAHs (B-
PAHs) have found numerous applications, ranging from metal-
free catalysis3−7 to light-emitting devices5,8−11 and sensor
technology.12 Relatives of the PAHs are the phenylene-
containing oligoacenes (POAs), which contain an element of
antiaromaticity due to their incorporated biphenylene
moieties.13,14 Recently, we reported that the boron-doped
POA (B-POA) DBI shows very different properties in
comparison to its B-PAH analogues (Chart 1): in contrast to
9,10-dihydro-9,10-diboraanthracene (DBA) and 6,13-dihydro-
6,13-diborapentacene (DBP), DBI is nonemissive and a
stronger electron acceptor.15,16 The electron-withdrawing
nature of a biphenylene backbone, combined with its unique
RESULTS AND DISCUSSION
■
Synthesis of e-DBI and v/z-DBI. The synthesis of e-DBI
is based on the key intermediate 2,3-bis(trimethylsilyl)benzo-
[b]biphenylene20 (Scheme 2; see the Supporting Information
for an optimized procedure). After cyclocondensation with
BBr3 (n-hexane, 120 °C, sealed ampule) and mesitylation with
MesMgBr (toluene/THF, 0 °C), e-DBI was isolated in 27%
yield over six steps starting from commercial 2,3-dihydroxy-
naphthalene.
The 1,2-bis(trimethylsilyl)biphenylene required for the
synthesis of v/z-DBI was synthesized from the parent
biphenylene,21−23 which was first brominated selectively at
its 2-position.24,25 2-Bromobiphenylene can subsequently be
subjected to ortho-directed lithiation at its acidic 1-position
using amide bases, such as lithium diisopropylamide (LDA)26
and lithium 2,2,6,6-tetramethylpiperidide (LiTMP; cf. the
Supporting Information) under cryogenic conditions. The fact
that Me3SiCl does not react with LiTMP in THF at −78 °C
made an in situ quenching strategy possible, which furnished 2-
bromo-1-(trimethylsilyl)biphenylene in 88% yield. Li/Br
exchange with n-BuLi and further reaction with Me3SiCl
furnished 1,2-bis(trimethylsilyl)biphenylene (90%). The cyclo-
condensation with BBr3 took place under the established
conditions (n-hexane, 120 °C, sealed ampule), and after
̈
peri distance, was also exploited by Gabbaı et al., who
constructed a series of ditopic biphenylene-1,8-diyldiboranes,
capable of trapping (ditopic) Lewis bases in a chelating
manner (Chart 1).17,18 Herein, we are exploring the field
further to gain deeper insight into the fundamental character-
istics of B-POAs. (1) The weakly luminescent DBA can be
turned into an efficient blue emitter upon benzannulation to
give DBP.15,19 We therefore also synthesized a π-extended
congener of DBI, equipped with two peripheral 2,3-
naphthylene instead of 1,2-phenylene fragments and inves-
tigated its optoelectronic properties (e-DBI; Scheme 1). (2)
We also assessed the consequences of an angular 1,2- instead
of the linear 2,3-annulation of biphenylene subunits to the
Received: May 16, 2019
© XXXX American Chemical Society
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a
Chart 1. Structures of DBA, DBP, DBI, and Selected
Biphenylene-Bridged Ditopic Boranes
Scheme 2. Syntheses of e-DBI (top) and v/z-DBI (bottom)
a
Reagents and conditions: (a) NfF (3.0 equiv), K2CO3 (5.0 equiv),
MeCN, room temperature, 1 h; (b) Me3SiCCZnCl·LiCl (2.5
equiv), Pd(PPh3)2Cl2 (2 mol %), THF, Δ, 3.5 h; (c) TBAF (2.2
equiv), DCM, 0 °C, 30 min; (d) Me3SiCCSiMe3 (excess),
CpCo(CO)2 (10 mol %), neat, Δ, hν, 10 h; (e) BBr3 (3.7 equiv),
n-hexane, 120 °C, 3 days (sealed ampule); (f) MesMgBr (2.0 equiv),
toluene/THF, 0 °C, 18 h; (g) LiTMP (1.1 equiv), Me3SiCl (6.0
equiv), THF, −78 °C, 18 h; (h) n-BuLi (1.2 equiv), Me3SiCl (2.0
equiv), THF, −78 °C, 18 h.
ful. However, reversed-phase HPLC (C18, MeOH) provided a
sample of v/z-DBI that was further enriched in v-DBI (5:1).
NMR Spectroscopy. e-DBI and v/z-DBI were charac-
terized by NMR spectroscopy in CDCl3 solutions (cf. the
Supporting Information for full details). The 1H NMR
spectrum of e-DBI contains only four biphenylene signals
and one set of mesityl resonances, which fits to the proposed
symmetric, linear structure (similar arguments hold for the
13C{1H} NMR spectrum of e-DBI). The v/z-DBI mixture
gives rise to a major and a minor set of 1H and 13C resonances.
Considering the major isomer first, we note that its
biphenylene halves are related by symmetry and give rise to
six proton resonances. One of these signals appears upfield-
shifted at 4.71 ppm, indicating that the corresponding proton
experiences a shielding magnetic anisotropy effect from the
nearby mesityl π system.27 All of these features are in
agreement with a kinked B-POA. The fact that its two mesityl
substituents are magnetically inequivalent unequivocally
identifies the dominant product as v-DBI. A full character-
ization of the minor product z-DBI was precluded by the small
portion of this substance in the mixture and pronounced
overlap of the v- and z-DBI proton resonances.
Scheme 1. Structures of the B-POA DBI, Its π-Extended
Derivative e-DBI, and the Bent Isomers v/z-DBI
A comparison of the 13C{1H} NMR spectra of e-DBI and v-
DBI with those of their constituent hydrocarbons (e-DBI,
benzo[b]biphenylene; v-DBI, biphenylene) revealed that only
those 13C nuclei belonging to the rings directly attached to the
B atoms are significantly deshielded, which is in accord with
previous observations.15,28 As is typical for triarylboranes,29 no
resonances were detectable in the 11B{1H} NMR spectra of e-
DBI and v/z-DBI.
Crystal Structure of e-DBI. Crystals of e-DBI suitable for
X-ray analysis were obtained by recrystallization from toluene.
The molecule possesses Ci symmetry in the solid state (Figure
1). In contrast to the planar DBI,15 but similar to the higher
all-carbon [N]phenylenes,30 e-DBI shows a sigmoidal
curvature when it is viewed along the imaginary B···B axis.
This distortion is mainly due to the fact that the planar 2,3-
naphthylene moieties include dihedral angles of 13.5(1)° with
protection of the boron atoms with mesityl substituents, a
mixture of v/z-DBI was isolated by column chromatography
(26%; Scheme 2). The stoichiometric ratio of v- vs z-DBI in
this mixture amounted to 4:1. All attempts to separate the
isomers (crystallization, normal-phase HPLC) were unsuccess-
B
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Table 1. Electrochemical and Spectroscopic Data for DBI, e-
DBI, and v/z-DBI
a
bc
,
b c
,
bd
,
E
1/2/V
λabs/nm
λex/nm
λem/nm
ΦPL/%
e
f
DBI
−1.52
−2.30
570
399
330
317
f
e-DBI
−1.51
−2.27
−2.70
−3.41
570
364
646
672
12
Figure 1. Structure of e-DBI in the solid state (C atoms, black; B
atoms, green; H atoms omitted for clarity, only Mes-Cipso is shown).
Selected bond lengths (Å) and angles (deg): B−Cendo = 1.564(3)/
1.570(3), B−Cexo= 1.583(3), C1−C6 = 1.373(3), C5−C6 =
1.418(3), C4−C5 = 1.376(3), C5−C9 = 1.504(3), C6−C8 =
1.509(3), C7−C8 = 1.357(3), C8−C9 = 1.451(3), C9−C10 =
364
346
331
315
250
495
464
425
400
288
256
g
g
1.347(3); Cendo−B−Cendo = 119.79(17), Cendo−B−Cexo
=
v/z-DBI
−1.82
−2.51
256
506
540
585
65
120.27(17)/119.91(17). Symmetry operator for generating equivalent
atoms: (i) 2 − x, 1 − y, 1 − z.
the essentially planar DBA core. The π-electron distribution
within each naphthylene fragment can be approximated by
drawing a Clar’s sextet in either the inner or the outer
benzenoid ring. An inspection of the C−C bond length
distribution agrees better with the latter description: with an
average value of 1.352 Å, C7−C8 and C9−C10 are the
shortest C−C bonds in the entire molecule while the
connecting C8−C9 bond is rather long (1.451(3) Å). The
C7C8−C9C10 fragment thus possesses characteristic
features of a butadiene moiety (cf., s-trans-1,3-butadiene:
CC, 1.348(1) Å (exp),31 1.338 Å (calcd);32 C−C, 1.468(2)
Å (exp), 1.454 Å (calcd)). In contrast, the C−C bond lengths
in the outer benzenoid ring are more evenly distributed (range:
1.372(3)−1.427(3) Å) and thus more typical of delocalized π
bonds (cf. C6H6: average 1.378 Å).33
The pronounced tilt at the four-membered rings of the
benzo[b]biphenylene substructures has no significant influence
on the C5−C9/C6−C8 bond lengths (average 1.507 Å), as
they are still similar to the corresponding values in pristine
biphenylene (1.511 Å)34 and DBI (1.516 Å);15 a comparison
with pristine benzo[b]biphenylene is not possible due to
disorder problems.35 C5−C9/C6−C8 are best regarded as
elongated σ bonds with only minimal delocalization of π
electrons across them (length of a typical C(sp2)−C(sp2)
single bond: 1.47 Å36). When all relevant structural parameters
are taken together, the four-membered-ring fragments of e-
DBI are more reminiscent of [4]radialenes than cyclo-
butadienes. Significantly longer exocyclic (1.583(3) Å) than
endocyclic (average 1.567 Å) B−C distances reflect the steric
repulsion of the bulky mesityl substituents as well as a partial
BC double-bond character within the DBA substructure.
Electrochemistry. Both e-DBI and v/z-DBI show two
redox waves with features of electrochemical reversibility in the
cathodic regions of their cyclic voltammograms (THF, 0.1 M
[n-Bu4N][PF6]; vs FcH/FcH+). According to the first
reduction event, e-DBI (E1/2 = −1.51 V) possesses the same
electron affinity as DBI (−1.52 V) but a higher affinity than v/
z-DBI (−1.82 V). This trend persists for the second reduction
event, which again proves e-DBI (−2.27 V) to be more readily
reducible than v/z-DBI (−2.51 V). Two further redox
transitions of e-DBI are irreversible (Epc= −2.70, − 3.41 V)
and likely originate from one-electron uptake at each of the
2,3-naphthylene moieties. Neither e-DBI nor v/z-DBI showed
redox events in the anodic regions of their cyclic voltammo-
grams. Electrochemical and spectroscopic data for DBI, e-DBI,
and v/z-DBI are given in Table 1.
a
b
Conditions: THF, 0.1 M [n-Bu4N][PF6], vs FcH/FcH+. In
c
cyclohexane solution. Values assignable to vibrational bands are
italicized. Quantum yields were determined from degassed samples
d
e
using a calibrated integrating sphere. Values taken from ref 15.
f
g
Onset value of a flat plateau. Irreversible redox event.
UV/Vis Spectroscopy. All electronic spectra were
recorded on cyclohexane solutions (Figure 2a). e-DBI is a
deep red compound, whereas v/z-DBI possesses an orange
color. As in the case of DBI,15 the red color of e-DBI
originates from a very broad absorption plateau with an onset
wavelength of approximately 570 nm (Figure S29 in the
Supporting Information). The UV/vis spectrum of the 5:1
mixture of bent isomers v/z-DBI shows a nicely resolved
longest-wavelength absorption band at λmax = 495 nm and a
distinct vibrational fine structure. In this respect, v/z-DBI
resemble a known derivative of DBA carrying the 1,2-
biphenylenediyl fragment at only one side.27 The linear DBI
is nonemissive, likely due to a low-lying doubly excited state.15
Twofold benzannulation in e-DBI turns on a red fluorescence,
associated with a very broad, poorly resolved emission band at
λem = 646 nm (Figure 2b, right; ΦPL = 12%). In addition to π
extension, also the simple change in the annulation mode from
2,3 (DBI)- to 1,2-annulation (v/z-DBI) leads to appreciable
fluorescence (λem = 506 nm, ΦPL = 65%).
In the case of e-DBI, we also observed two very small
emission bands at wavelengths of λ = 438 and 465 nm (cf. the
Supporting Information), which became more pronounced
with increasing water content of the solvent and are thus likely
attributable to adducts with H2O or OH−. For a further
investigation of the phenomenon, we resorted to a titration of
e-DBI in CHCl3 using [n-Bu4N]F·3H2O (TBAF) as a more
quantifiable ligand source (Figure 2b).
As the amount of added TBAF was increased, the original
main absorption band of e-DBI at 364 nm gradually vanished
and two new bands emerged instead (λabs = 328 and 465 nm).
Only after 2 equiv of TBAF had been added did the absorption
spectrum of the mixture not change further with increasing F−
concentration. Accordingly, the emission spectrum of e-DBI in
the presence of 1 equiv of TBAF still showed the emission
band of native e-DBI at 646 nm, together with a second,
hypsochromically shifted band at 477 nm. After the addition of
2 equiv of F− ions, the band at 646 nm was completely
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Figure 2. (a) UV/vis absorption (left) and emission spectra (right) of DBI, e-DBI, and v/z-DBI in cyclohexane. DBI is nonfluorescent. (b) UV/vis
absorption and emission spectra of e-DBI in the presence of varying amounts of [n-Bu4N]F·3H2O (TBAF) in dry and alcohol-free CHCl3.
Figure 3. Emission spectra of e-DBI, [e-DBI·F]−, 2-BMes2-benzo[b]biphenylene, and benzo[b]biphenylene in DCM.
suppressed and exclusively the band at 477 nm remained. This
latter emission appears at wavelengths similar to those of the
previously mentioned small features, which supports our
assumption that they originate from e-DBI·(H2O/OH−)
complexes.
mediating a π interaction between the two 2,3-benzo[b]-
biphenylenediyl units, which should therefore be largely
isolated from each other. As a result, we would expect a blue
emission, reminiscent of that of pristine benzo[b]biphenylene
(Figure 3; cf. the Supporting Information for the syntheses of
the two model compounds). In summary, the 2 equiv of TBAF
necessary for the complete quenching of the e-DBI emission is
just required to push the association/dissociation equilibrium
toward the formation of [e-DBI·F]−.
The following facts strongly suggest that the association/
dissociation equilibrium between e-DBI and F− ions does not
go beyond the stage of the monoadduct [e-DBI·F]−. (1) A Job
plot of the e-DBI/TBAF system using the most intense of the
emerging bands (λ = 453 nm) has its maximum at χTBAF = 0.5
(CHCl3, Figure S31 in the Supporting Information). (2) Even
the stronger Lewis acid DBI forms only monoadducts [DBI·
F]−.15 (3) Two nearly isosbestic points are conserved during
the TBAF titration (Figure 2b, left). (4) The green
fluorescence of the monoadduct [e-DBI·F]− is very similar
to the emission color of the tricoordinate monoborane 2-
BMes2-benzo[b]biphenylene. In the putative diadduct [e-DBI·
(F)2]2−, neither of the two boron atoms would be capable of
The [e-DBI·F]− emission band (ΦPL = 42%) and e-DBI
absorption band do not overlap significantly, and the
photoluminescence colors of both compounds are comple-
mentary to each other. This offers a possibility to create white
light through proper adjustment of the amount of added F−
ions. Indeed, irradiation of a mixture of e-DBI and 0.4 equiv of
TBAF in DCM at 325 nm produced nearly white light, thereby
rendering e-DBI a boron-based stimuli-responsive material
(CIE1931: (0.3320|0.4056); Figure 4).37,38
D
DOI: 10.1021/acs.organomet.9b00330
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ORCID
Sven Kirschner: 0000-0003-4843-4695
Michael Bolte: 0000-0001-5296-6251
Hans-Wolfram Lerner: 0000-0003-1803-7947
Matthias Wagner: 0000-0001-5806-8276
Notes
The authors declare no competing financial interest.
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CONCLUSION
■
The compound DBI, which consists of two 2,3-biphenylene-
diyl moieties linked through two B−Mes bridges, represents an
archetypal boron-doped phenylene-containing oligoacene (B-
POA). DBI is distinguished from related boron-doped
polycyclic aromatic hydrocarbons (B-PAHs) by its higher
electron affinity and lack of photoluminescence. A change in
the connection mode from 2,3- to 1,2-biphenylenediyl shifts
the first reduction potential of the kinked v/z-DBI isomers by
0.31 V to more cathodic values and turns on an appreciable
turquoise fluorescence (λem = 506 nm, ΦPL = 65%).
Benzannulation on both sides of DBI leads to the extended
congener e-DBI, which is a comparably strong electron
acceptor and a red emitter (λem = 646 nm, ΦPL = 12%). Our
results thus demonstrate that seemingly subtle structural
modifications can have a dramatic effect on the optoelectronic
properties of B-POAs. Moreover, e-DBI forms the green-
emitting monoadduct [e-DBI·F]− and thus behaves as a
stimuli-responsive material, capable of producing white light
upon addition of 0.4 equiv of [n-Bu4N]F.
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Fluoride Ion Complexation and Sensing Using Organoboron
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00330.
■
S
́
(13) Miljanic, O. Ţ.; Vollhardt, K. P. C. [N]Phenylenes: A Novel
Experimental details and characterization data (PDF)
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Properties of Phenylene-Containing Oligoacenes. J. Am. Chem. Soc.
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Accession Codes
CCDC 1915623 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for M.W.: matthias.wagner@chemie.uni-frankfurt.de.
■
(16) Kirschner, S.; Bao, S.-S.; Fengel, M. K.; Bolte, M.; Lerner, H.-
W.; Wagner, M. Aryl-Aryl Coupling in a Polycyclic Aromatic
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