Tuning the Donor-π-Acceptor Character of Arylborane-Arylamine Macrocycles

Article abstract of DOI:10.1021/acs.organomet.0c00779

Donor-π-acceptor compounds based on arylamine and arylborane moieties connected by a π-conjugated linker are attractive materials in organic electronics and imaging applications due to the strong charge transfer character that leads to low energy absorption and solvatochromic emission properties. Here we introduce a new conjugated macrocyclic system that consists of four arylborane and two arylamine units as confirmed by single-crystal X-ray structure analysis. The acceptor character of the arylboranes is enhanced by exocyclic electron-withdrawing 2,4,6-tris(trifluoromethyl)phenyl (FMes) substituent as well as the mutual interaction between adjacent boranes. The absorption, solvent-dependent emission, and electrochemical properties are studied and compared to those of other macrocyclic organoboranes. Computational studies offer additional insights into the electronic structure. Due to the enhanced acceptor character of the boranes, the LUMO orbitals are lower lying, leading to more facile reduction, red-shifted absorption and emission, and larger Stokes shifts than those found for previously studied B-N macrocycles.

Full text of DOI:10.1021/acs.organomet.0c00779

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Article
Tuning the Donor−π−Acceptor Character of Arylborane−Arylamine
Macrocycles
̈
Nurcan Baser-Kirazli, Roger A. Lalancette, and Frieder Jakle*
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ABSTRACT: Donor−π−acceptor compounds based on arylamine
and arylborane moieties connected by a π-conjugated linker are
attractive materials in organic electronics and imaging applications
due to the strong charge transfer character that leads to low energy
absorption and solvatochromic emission properties. Here we
introduce a new conjugated macrocyclic system that consists of
four arylborane and two arylamine units as confirmed by single-
crystal X-ray structure analysis. The acceptor character of the
arylboranes is enhanced by exocyclic electron-withdrawing 2,4,6-
tris(trifluoromethyl)phenyl (FMes) substituent as well as the mutual
interaction between adjacent boranes. The absorption, solvent-
dependent emission, and electrochemical properties are studied and compared to those of other macrocyclic organoboranes.
Computational studies offer additional insights into the electronic structure. Due to the enhanced acceptor character of the boranes,
the LUMO orbitals are lower lying, leading to more facile reduction, red-shifted absorption and emission, and larger Stokes shifts
than those found for previously studied B−N macrocycles.
benzothiadiazole moieties linked by acetylene have been
explored as fullerene hosts in photovoltaics,11 while the
thermally activated delayed fluorescence (TADF) of a π-
conjugated macrocycle comprised of two U-shaped dibenzo-
[a,j]phenazine acceptors and two phenylenediamine donors
has been exploited in OLED applications.12
INTRODUCTION
■
π-Conjugated donor−acceptor (D−A) molecular and poly-
meric materials are of great current interest in view of their
wide applications in optoelectronics, sensors, and bioimaging.1
Among the various classes of electron acceptors, tricoordinate
organoboranes have received much recent attention.2 Their
strong electron accepting ability stems from overlap of the
empty pz orbital on boron with the π-conjugated system. Thus,
the incorporation of tricoordinate organoborane groups into π-
conjugated systems presents an efficient way to tune the
electronic properties, providing opportunities in a wide range
of applications such as nonlinear optics and two-photon
imaging,3 electron transporters and emitters in organic light-
emitting diodes (OLEDs),4 highly selective anion sensors,5 and
stimuli-responsive materials.6 When combined with electron-
donating groups such as arylamines, intramolecular charge
transfer (ICT) results in a large electronic dipole in the excited
state.7 As a consequence, interesting emission properties arise,
such as a high fluorescence efficiency, large Stokes shifts, and
pronounced fluorescence solvatochromism.8
Among π-conjugated materials, conjugated macrocycles
stand out because of their exquisite structure and promising
functions.9 Unique properties can be achieved by incorporating
donor and acceptor moieties in these cyclic systems. In a
recent example, the Nuckolls group developed conjugated
macrocycles with bithiophene derivatives as donor and
perylene diimides as acceptor units for field effect transistor
and photodetector applications.10 Conjugated macrocycles
with alternating triphenylamine and 4,7-bisthienyl-2,1,3-
However, to date only few examples of macrocycles that
combine organoborane acceptors with electron-rich donors
have been reported.13 In 2012, our group embedded
tricoordinate organoboron into a cyclic system with arylamine
donor groups and introduced the first ambipolar B−π−N
macrocycle (A, Chart 1).14 We found that π-expanded
borazine A exhibits a small HOMO (highest occupied
molecular orbital)−LUMO (lowest unoccupied molecular
orbital) gap, resulting in strong blue fluorescence and
solvatochromic emission that is quenched by addition of
cyanide ions. Our group also introduced other B−N containing
ambipolar macrocycles, such as B, with alternate shapes and
different sizes.15 Recently, Ito and Fueno reported several
smaller ambipolar π-conjugated macrocycles (C) that are
composed of two para-phenylene and two meta-phenylene
Received: December 12, 2020
Published: January 29, 2021
https://dx.doi.org/10.1021/acs.organomet.0c00779
© 2021 American Chemical Society
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a
amine 2 was prepared from brominated precursor 1 according
to a literature procedure.14 For the second building block, 2,7-
dibromo-9,9-didodecylfluorene20 was subjected to a lithium
halogen exchange followed by treatment with Me3SiCl to give
disilylated fluorene 3 in 85% yield. Subsequently, silicon−
boron exchange with 2 equiv of BBr3 in CH2Cl2 furnished
diborylated 4 as a white powder after recrystallization from
toluene at low temperature. Having the reactive key
intermediates in hand, we performed the cyclization between
distannylated amine 2 and diborylated fluorene 4 by an
organometallic condensation under high-dilution condition.
The initially formed bromide-substituted macrocycle was
directly treated with 4 equiv of the organolithium reagent
Chart 1. B−π−N Donor−Acceptor Macrocycles
F
FMesLi in toluene to introduce the electron-deficient Mes
moieties that also sterically protect the boron centers. The
crude product was subjected to column chromatography on
silica gel using hexanes with 10% (v/v) toluene as the eluent.
The new macrocycle, B4N2-FMes, was isolated as a yellow
powder in an overall yield of 33% over two steps by keeping a
solution of the purified product in pentane at −24 °C.
a
Tip = 2,4,6-tri-iso-propylphenyl, Ar1 = 4-t-butylphenyl, Ar2 = 4-
The structure of B4N2-FMes was confirmed by gel
permeation chromatography (GPC), MALDI-TOF MS, and
multinuclear NMR analyses. The GPC data for the purified
macrocycle indicate a monodisperse product with a molecular
weight of Mn = 2450 Da (Mw/Mn = 1.004), which is close to
the theoretical value of 2768.2606 Da (Figure 1a). Additional
methoxyphenyl.
bridges.16 They found significantly different electronic and
optical properties depending on the specific placement of
boron and nitrogen in the cyclic systems.
In this study, we pursued a new ambipolar π-conjugated
macrocycle, B4N2-FMes, consisting of triarylamine donors
paired with highly electron-deficient bis(boryl)fluorene units
(Scheme 1). The electron-acceptor character of the boryl
Scheme 1. Synthesis of Macrocycle B4N2-FMes
groups is enhanced by (i) the pendent 2,4,6-tris-
(trifluoromethyl)phenyl (FMes) groups8c,17 that also endow
the compounds with enhanced stability due to steric shielding
and (ii) the mutual electronic interactions18 between the
boron atoms across the π-conjugated fluorene bridge. We
anticipated that the enhancement of the acceptor character
Figure 1. (a) Gel permeation chromatography (GPC) trace (THF, 1
mL/min), (b) MALDI-TOF mass spectrum (positive ion mode), and
1
(c) aromatic region of the H NMR spectrum of B4N2-FMes.
F
using Mes groups could have a strong impact on the ICT
characteristics.8c,19 The photophysical and electronic proper-
ties of B4N2-FMes are evaluated in comparison to previously
reported ambipolar macrocycles, while computational studies
offer important insights into the electronic structure.
evidence for the cyclic structure is provided by a positive-ion
MALDI-TOF mass spectrum that shows a dominant ion peak
at 2768.2026 Da for the molecular ion and matches well the
calculated isotope pattern (Figure 1b). The inversion
symmetric structure of B4N2-FMes was further verified by
1
RESULTS AND DISCUSSION
The synthesis of the key building blocks for the targeted
macrocycle is presented in Scheme 1. Distannylated triphenyl-
multinuclear NMR analyses in CDCl3. As expected, the H
■
NMR spectrum for B4N2-FMes shows two singlets for the
F
fluorene and Mes groups and six doublets in the aromatic
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region can be assigned to the fluorene and triarylamine groups
(Figure 1c).
Single crystals for X-ray structure analysis were obtained by
slow vapor diffusion of acetonitrile into a solution of
B4N2-FMes in dichloroethane. The macrocycle crystallized
The electronic structure of B4N2-FMes was examined by
cyclic voltammetry. Two consecutive reduction waves were
observed at −2.02 and −2.18 V vs Fc0/+ redox couple (Figure
3). The splitting is attributed to moderate electronic
̅
in the centrosymmetric space group P1 with a single main
molecule and two dichloroethane solvent molecules in the unit
cell (Figure 2). The alkylated fluorene C9-positions are
Figure 3. Cyclic voltammetry data for B4N2-FMes in THF
(reduction, left) and DCM (oxidation, right) containing 0.1 M
[Bu4N][PF6] (1 × 10−3 mol L−1, v = 200 mV s−1; vs Fc0/+).
communication between the boron atoms across the arylamine
linkers based on a comparison with results for macrocycles A
and B (Table 1). Interactions between boron atoms across the
fluorene bridges are expected to result in two additional waves
at more negative potentials. These could not be observed, even
in scans with a more extended voltage range, possibly due to
the instability of the corresponding polyanionic species in the
presence of the fluorinated mesityl groups. A single reversible
oxidation wave at 0.74 V is attributed to generation of
arylamine radical cations, which suggests that mutual
interaction between the arylamine groups on opposite sides
of the cycle are negligible. Compared with macrocycles A (Ered
= −2.53 V) and B (Ered = −2.30 V), the first reduction of
B4N2-FMes (Ered = −2.02 V) occurs at significantly less
negative potential. Considering also computational data for the
nonfluorinated direct analog B4N2-Mes′ (vide infra), we
conclude that the FMes groups play an important role in
lowering the LUMO energy.
Next, we studied the photophysical properties of
B4N2-FMes in solvents of varying polarity to gain insights
into the ICT characteristics. As a solid, B4N2-FMes is light
yellow and shows a strong blue-green emission (ϕF = 15.8, λem
= 477 nm) when exposed to UV light (Figure 4a), whereas A
and B form colorless crystals, the former giving rise to only a
weak blue emission in the solid state. In DCM solution, the
absorption spectrum of B4N2-FMes shows three distinct bands
with maxima at 407 (ε = 1.06 × 105 M−1 cm−1), 382, and 347
nm (Figure 4b). The lowest energy absorption maxima of
B4N2-FMes are found at wavelengths much longer than those
of B (385 (sh), 357 nm) but slightly shorter than those of A
(420, 390 nm). The emission of B4N2-FMes in DCM at 520
nm (ϕF = 0.90; τF = 7.1 ns) is very strong and largely
redshifted compared to both A (460 nm) and B (436 nm).
This effect is attributed to more dominant intramolecular
charge transfer (ICT) as a result of the electron-withdrawing
FMes groups on the boron atoms. The absorption data proved
to be essentially independent of solvent polarity, but the
emission maxima experience a strong bathochromic shift with
increasing solvent polarity (Figure 4c,d).
Figure 2. X-ray structure plot of B4N2-FMes (50% thermal
ellipsoids). Hydrogen atoms and two cocrystallized dichloroethane
solvent molecules are omitted for clarity. Selected bond distances (Å)
and angles (deg): B1−C1 1.521(10), B1−C32 1.567(10), B1−C23
1.596(10), N1−C4 1.411(8), N1−C7 1.415(8), N1−C13 1.440(8),
C10A-B2 1.545(11), C39−B2 1.563(11), C46−B2 1.603(11), C1−
B1−C32 123.6(6), C1−B1−C23 118.5(6), C32−B1−C23 117.9(6),
C10A−B2−C39 125.4(7), C10A−B2−C46 116.6(6), C39−B2−C46
118.0(6), C4−N1−C7 123.1(5), C4−N1−C13 118.4(5), C7−N1−
C13 118.4(5).
pointing inward and both the triarylamines and triarylboranes
adopt an almost perfect trigonal planar geometry as is evident
from the sum of the angles at N and B (360°). When
comparing the endo- and exocyclic angles at N for B4N2-FMes
(123.1(5) vs 118.4(5)°) a slight widening of the endocyclic
angles is apparent that suggests a modest amount of ring strain.
A similar widening of the endocyclic C−B−C angles to
123.6(6)/125.4(7)° is observed. Furthermore, the exocyclic
C−B and C−N bonds in B4N2-FMes are relatively longer than
the endocyclic C−B and C−N bonds, suggesting more
favorable π-orbital overlap within the macrocycle.
Another interesting aspect is the observation of a short B···F
contact for each of the CF3 groups. The interatomic B−F
distances range from 2.54 to 2.67 Å, resulting in a distorted
trigonal bipyramidal geometry. The B−F distances are within
the range of other previously reported ortho-CF3 substituted
arylboranes.8c,17b,20 The computed distances of 2.64−2.67 Å
are also in a similar range and such short contacts have
previously been associated with weak bonding interactions in
the order of a few kcal/mol based on computational
studies.17b,20a These interactions may somewhat diminish the
F
overall electron-withdrawing effect of the Mes substituents.
However, it is important to note that the 11B NMR shift of
65.6 ppm above is well within the range of typical tricoordinate
boranes, suggesting that the interactions and resulting
electronic effects should be relatively weak.
The pronounced positive solvatochromic effect in the
emission, but not the absorption spectra, is attributed to
ICT from triarylamine donor to triarylborane acceptor sites, a
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Table 1. Comparison of Electrochemical Data for Boron-Based Donor−π−Acceptor Macrocycles
a
a
a
a
b
b
b
compound
E1 (V)
E2 (V)
E1 (V)
E2 (V)
EHOMO (eV)
ELUMO (eV)
ΔEgap (eV)
ox
ox
red
red
A14
B15
B4N2-FMes
0.46
0.95
0.74
0.65
d
−2.53
−2.30
−2.02
−2.72
−5.26
−5.75
−5.54
−2.27
−2.50
−2.78
2.99
3.25
2.76
c
c
c
−2.42
d
c
−2.18
a
Reduction and oxidation potentials from square wave or cyclic voltammetry measurements (0.1 M Bu4NPF6 in THF/DCM; 200 mV/s; vs Fc/
b
c
Fc+). Estimated ELUMO = −(4.8 + E1red) eV and EHOMO = −(4.8 + E1ox) eV. Additional waves observed for A at E3red = −2.84 V and E3ox= 0.94 V;
d
for B at E3 = −2.68 V and E4 = −2.79 V; no additional waves observed for B4N2-FMes at more negative potentials. Not observed.
red
red
ability of polar solvent molecules to approach and stabilize the
ICT excited state, and possibly the effects of the weak
interactions between boron and the CF3 groups discussed
above.
The ability of B4N2-FMes to bind fluoride anions as a model
Lewis base was also investigated. Upon addition of 15 equiv of
(n-Bu4N)F·3H2O (TBAF) to macrocycle B4N2-FMes in THF
the color of the solution immediately changed from bright
yellow to orange and within seconds to colorless with a light
brownish tint. 11B NMR analysis confirmed the formation of
tetracoordinate boron sites and 19F NMR data were consistent
with fluoride coordination (Figure S17). However, the latter
also suggested a significant extent of protiodeborylation with
formation of the free tris(trifluoromethyl)benzene, which is
attributed to the presence of traces of acid as a common
impurity of commercial TBAF samples. Negative ion-mode
electrospray-ionization mass spectrometry (ESI-MS) data for
the anion complexes were also acquired in THF (Figure S18).
Upon addition of 15 equiv of (n-Bu4N)F to macrocycle
B4N2-FMes and stirring for 1 h, we identified a series of
species that contain multiple fluorides bound to boron; these
include [B4N2-FMes + 2F−]2− (m/z = 1403.1689),
[B4N2-FMes + 3F− + TBA]2− (m/z = 1533.8184), and
[B4N2-FMes + 4F− + 2TBA]2− (m/z = 1664.4724). The
NMR and ESI-MS data indicate that B4N2-FMes can be
readily converted to polyanionic macrocycles with multiple
fluoride ions bound to the boron Lewis acid sites.
Figure 4. (a) Photographs of solid B4N2-FMes taken in daylight
(top) and upon irradiation at 365 nm (bottom). (b) UV−vis
absorption and emission spectra in DCM (1.90 × 10−5 M) and solid
state emission for B4N2-FMes. (c) Photographs of solutions in
solvents of varying polarity upon irritation at 365 nm (left to right:
hexane, toluene, dioxane, CHCl3, DCM, and THF). (d) Emission
spectra in solvents of varying polarity. (e) Lippert−Mataga plot for
the solvatochromic emission of B4N2-FMes in comparison to
macrocycle B.
Macrocycle B4N2-FMes was further studied by computa-
tional methods. The structure of the macrocycle in the gas
phase with methyl in place of dodecyl groups in the 9,9-
positions of the fluorene π-bridges (B4N2-FMes′) was
optimized by DFT methods at the B3LYP/6-31G(d) level of
theory. Similar computations were also performed on the
direct nonfluorinated analog B4N2-Mes′ with mesityl in place
of fluoromesityl groups on boron. The lowest energy
conformers have the fluorene C9 methylene bridge pointing
toward the inside of the macrocycle as also seen in the X-ray
crystal structure (Figure S21). Overall, the geometric
parameters of B4N2-FMes′ are remarkably similar to those
found for the solid-state structure of B4N2-FMes (Table S2),
suggesting that the structure is well-reproduced. The computed
frontier orbitals for B4N2-FMes′ are illustrated in Figure 5.
The degenerate HOMO and HOMO−1 of B4N2-FMes′ are
localized on the triphenylamine units, whereas the degenerate
LUMO and LUMO+1 are delocalized over the conjugated π-
systems of the fluorene units and the boron centers with only
small contributions from the triphenylamine units. Neither the
HOMO nor the LUMO show any contribution from the
exocyclic FMes groups. When compared to the respective
nonfluorinated macrocycle, B4N2-Mes′, both the HOMO and
LUMO levels of B4N2-FMes′ are significantly lower in energy
by 0.20 and 0.24 eV, respectively. Thus, the attachment of the
electron-deficient pendent FMes groups affects the LUMO
phenomenon that is typical of Ar2B−π−NAr2 systems. The
results of the solvent-dependent emission studies were further
analyzed using a Lippert−Mataga plot shown in Figure 4e, in
which the Stokes shift (Δυ = υA − υF) is plotted versus the
̅
solvent polarity, expressed in terms of f = f(εS) − f(n2) with εS
as the solvent permittivity and n the refractive index of the
solvent. The slope in the Lippert−Mataga plot is correlated
with the change in dipole moment between the ground and
excited states and thus provides insights into the charge
transfer character and the stabilization of the excited state of
B4N2-FMes in dependence on the solvent polarity. Although
the Stokes shifts in all solvents are much larger for B4N2-FMes
relatively to nonfluorinated carbazole-derived macrocycle B,
the slope derived from the Lippert−Mataga analysis is similar
for both compounds, indicating that the change in dipole
moment is comparable for the different macrocycles. This
unexpected result could be due to the presence of the nonpolar
dodecyl group in compound B4N2-FMes, which may affect the
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comparison to that for previously reported macrocycle B, while
the HOMO is similar in energy. Thus, the ambipolar character
of B4N2-FMes is more dominant and the Stokes shifts for the
emission are much larger. A comparison of computational data
for B4N2-FMes and nonfluorinated analog B4N2-Mes
suggests that this is a result of both the electron-withdrawing
FMes groups and the replacement of carbazole by triphenyl-
amine donor moieties. Macrocycle B4N2-FMes is highly
luminescent both in solution and the solid state, giving rise
to an intense blue to green emission, depending on the polarity
of the environment. Surprisingly, the slope in the Lippert−
Mataga plot is relatively modest, similar to macrocycle B. This
unexpected result is attributed to the presence of nonpolar
dodecyl groups that limit the effective stabilization of the
excited state by polar solvent molecules. The Lewis acidic
boron centers in B4N2-FMes also lend themselves to
complexation with fluoride anions suggesting potential utility
in anion detection.
Figure 5. Selected computed frontier molecular orbitals for
B4N2-FMes′ (RB3LYP/6-31G(d)); hydrogen atoms are omitted for
clarity, isovalue 0.02).
EXPERIMENTAL SECTION
■
Materials and General Methods. n-BuLi (1.6 M in hexanes),
BBr3, (n-Bu4N)F (TBAF), and N,N,N′,N′-tetramethyl-
ethylenediamine (TMEDA) were purchased from Aldrich. Other
chemicals were purchased as follows: MesBr from Acros, Me3SiCl
from Alfa Aesar, and Me3SnCl from Strem chemicals. Me3SiCl and
TMEDA were distilled from CaH2, and all other commercially
available chemicals were used as received without further purification.
Distannylated triphenylamine 214 and 2,7-dibromo-9,9-didodecyl-
fluorene21 were prepared according to literature procedures.
Tetrahydrofuran (THF) and diethyl ether were distilled from Na/
benzophenone prior to use. Hexanes and toluene were purified using
a solvent purification system (Innovative Technologies; alumina/
copper columns for hydrocarbon solvents). Dichloromethane (DCM)
and CDCl3 were distilled from CaH2 and degassed via several freeze−
pump−thaw cycles for use with air-sensitive compounds. All reactions
and manipulations involving reactive borane or organolithium species
were carried out under an atmosphere of prepurified nitrogen using
either Schlenk techniques or an inert-atmosphere glovebox.
level, and B4N2-FMes′ is expected to be more readily reduced
than nonfluorinated B4N2-Mes′. However, according to our
calculations, the HOMO−LUMO gap in the ground state does
not change significantly for B4N2-FMes′ (3.04 eV) relative to
that of the respective mesityl derivative B4N2-Mes′ (3.08 eV).
This also suggests that the larger gap of previously studied
compound B15 of 3.54 eV is associated primarily with the
placement of a carbazole rather than a triphenylamine donor
unit.
Time-dependent DFT calculations were performed both at
the RB3LYP/6-31G(d) and rcam-B3LYP/6-31G(d) levels of
theory. Relative to the experimental data, with the RB3LYP
functional the transition energies were somewhat under-
estimated, but with the rcam-B3LYP functional they were
greatly overestimated. The results are otherwise qualitatively
similar, hence for simplicity we discuss only the results from
the RB3LYP calculations (data for the rcam-B3LYP calcu-
lations are included in the Supporting Information). As
typically observed for symmetric macrocycles, the lowest
energy transition for B4N2-FMes′/B4N2-Mes is symmetry-
forbidden (HOMO → LUMO, 478 nm, f = 0.005/HOMO →
LUMO, 474 nm, f = 0.000). Therefore, the experimentally
observed longest wavelength absorptions are assigned to the
much more intense higher energy S0 → S2 (445 nm, f = 0.749/
438 nm, f = 0.775) and S0 → S3 (424 nm, f = 0.703/419 nm, f
= 0.883) transitions, which by and large correspond to
HOMO−1 to LUMO and HOMO to LUMO+1 excitations
with strong charge transfer character.
NMR data were acquired at ambient temperature unless noted
otherwise, using the following apparatuses: 500.2 MHz 1H, 470.6
MHz 19F, and 125.8 MHz 13C NMR data were recorded on a 500
MHz Bruker Auto AVANCE spectrometer; 499.9 MHz 1H and 160.4
MHz 11B NMR data were recorded on another 500 MHz Bruker
AVANCE spectrometer that was equipped with a 5 mm PH SEX
500S1 11B−H/F-D probe; 599.7 MHz H, 150.8 MHz 13C, 564.2
1
MHz 19F, and 192.4 MHz 11B NMR data were recorded on a Varian
INOVA 600 spectrometer. 11B NMR spectra were acquired with
boron-free quartz NMR tubes either on the Varian INOVA 600 with a
boron-free 5 mm dual broadband gradient probe (Nalorac, Varian
Inc., Martinez, CA) or the 500 MHz Bruker Auto AVANCE with a 5
mm PH SEX 500S1 11B−H/F−D probe. H and 13C NMR spectra
1
were referenced internally to solvent signals (CDCl3: 7.27 ppm for 1H
NMR, 77.16 ppm for 13C NMR) and all other NMR spectra were
referenced externally to SiMe4 (0 ppm). The following abbreviations
are used for signal assignments: s = singlet, d = doublet, t = triplet, q =
quartet, m = multiplet, br = broad.
CONCLUSIONS
■
A new B−N containing ambipolar macrocycle, B4N2-FMes, is
MALDI-MS and ESI-MS measurements were carried out on an
Apex-ultra 7T Hybrid FT-MS (Bruker Daltonics). MALDI-TOF MS
measurements were carried out on a Bruker Ultraflextreme in
reflectron mode with delayed extraction. Anthracene (10 mg/mL)
was used as the matrix for the MALDI-TOF MS analyses, mixed with
the samples (10 mg/mL in chloroform) in a 1:1 ratio, and then
spotted on the wells of a target plate. Red phosphorus was used for
calibration.
F
introduced that contains Mes pendent groups on boron not
only to provide improved stability but also to enhance the
electron-deficient character. A rare X-ray structure analysis of
this large macrocycle is presented, revealing a modest amount
of ring strain as evidenced by widened endocyclic C−B−C and
C−N−C angles. π-Conjugation within the conjugated cyclic
system is reflected in relatively shorter endocyclic C−B and
C−N bonds. Electrochemical studies and DFT calculations
reveal a significantly lower LUMO level for B4N2-FMes in
GPC analyses were carried out in THF (1 mL/min) using a Waters
Empower system equipped with a 717 plus autosampler, a 1525
binary HPLC pump, a 2998 photodiode array detector, and a 2414
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refractive index detector. For separation, the samples were passed
through a series of styragel columns (Polymer Laboratories, two
columns 5 μm/500 Å), which were kept in a column heater at 35 °C.
The columns were calibrated with narrow polystyrene standards (580,
935, 1860, 2980, 4900, 6940, 13270, 18340 Da, Polymer
Laboratories).
kept stirring overnight. The reaction solution was diluted with diethyl
ether (50 mL) and quenched with water (150 mL). The aqueous
layer was washed with diethyl ether (3 × 100 mL). The combined
organic layers were washed with water (200 mL) and dried over
Na2SO4. All volatile components were removed under high vacuum to
1
give 3 as an orange oil (4.15 g, 85% yield). H NMR (600 MHz,
UV−visible (UV−vis) absorption data were acquired on a Varian
Cary 5000 UV−vis/NIR spectrophotometer. The fluorescence data
and lifetimes were measured using a Horiba Fluorolog-3 spectro-
fluorometer equipped with a 350 nm nanoLED and a FluoroHub R-
928 detector. Absolute quantum yields (ΦF) were determined on the
HORIBA Fluorolog-3 using a precalibrated Quanta-φ integrating
sphere. Light from the sample compartment is directed into the
sphere via a fiber-optic cable and an F-3000 Fiber-Optic Adapter and
then returned to the sample compartment (and to the emission
monochromator) via a second fiber-optic cable and an F-3000 Fiber-
Optic Adapter.
Cyclic voltammetry CV and square wave voltammetry experiments
were carried out on a CV-50W analyzer from BASi. The three-
electrode system consisted of a Au disk as working electrode, a Pt wire
as counter electrode, and a Ag wire as the reference electrode. The
voltammograms were recorded with ca. 10−3−10−4 M solutions in
THF (reduction scans) or DCM (oxidation scans) containing
Bu4N[PF6] (0.1 M) as the supporting electrolyte. The scans were
referenced after the addition of a small amount of ferrocene as
internal standard. The potentials are reported relative to the
ferrocene/ferrocenium couple (vs Fc/Fc+).
CDCl3) δ (ppm) 7.69 (d, J = 7.2 Hz, 2H), 7.48 (d, J = 7.2 Hz, 2H),
7.47 (s, 2H), 1.96 (m, 4H), 1.3−1.0 (m, 36H), 0.88 (t, J = 7.2 Hz,
6H), 0.68 (m, 4H), 0.32 (s, 18H). 13C NMR (150.8 MHz, CDCl3) δ
(ppm) 150.2, 141.9, 139.2, 131.9, 127.8, 119.2, 55.0, 40.1, 32.1, 30.1,
29.8−29.3 (multiple signals), 23.8, 22.9, 14.3, −0.7. 29Si NMR (119.2
MHz, CDCl3) δ (ppm) −4.0. MALDI-TOF (+ mode): m/z =
646.5316 ([M+], calcd for C43H74Si2 646.5324).
Synthesis of 2,7-Bis(dibromoboryl)-9,9-didodecylfluorene
(4). In a glovebox, a solution of 2,7-bis(trimethylsilyl)-9,9-
didodecylfluorene (3) (1.62 g, 2.50 mmol) in 15 mL of CH2Cl2
was transferred into a 100 mL Schlenk flask. A solution of BBr3 (1.50
g, 5.99 mmol) in 15 mL of CH2Cl2 was added dropwise to the
reaction flask at 0 °C. The reaction mixture was stirred overnight at
room temperature. All volatile components were removed under high
vacuum. The residue was recrystallized from toluene at −34 °C to
1
give 4 as a white solid (1.55 g, 74% yield). H NMR (500 MHz,
CDCl3) δ (ppm) 8.30 (d, J = 7.5 Hz, 2H), 8.23 (s, 2H), 7.89 (dd, J =
7.5, 0.5 Hz, 2H), 2.10 (m, 4H), 1.3−1.0 (m, 36H), 0.87 (t, J = 6.0 Hz,
6H), 0.60 (m, 4H). 13C NMR (150.8 MHz, CDCl3) δ (ppm) 152.1,
146.6, 138 (br, B−C), 137.6, 132.2, 120.8, 55.7, 39.9, 32.1, 29.9−29.2
(multiple signals 23.9, 22.8, 14.3. 11B NMR (160.3 MHz, CDCl3) δ
(ppm) 57.2 (w1/2 = 5100 Hz).
Single crystals of B4N2-FMes for X-ray analysis were obtained in
the form of yellow needles by slow vapor diffusion of acetonitrile into
a solution of B4N2-FMes in dichloroethane with a small amount of
dichloromethane for increased solubility. X-ray diffraction intensities
were collected on a Bruker SMART APEX II CCD Diffractometer
using Cu Kα (1.54178 Å) radiation at 100(2) K. The structure was
refined by full-matrix least-squares based on F2 with all reflections.22
Non-hydrogen atoms were refined with anisotropic displacement
coefficients, and hydrogen atoms were treated as idealized
contribution. SADABS absorption correction was applied.23
DFT calculations were performed with the Gaussian09 or
Gaussian16 suite of programs. The input files were generated in
Chem3D and preoptimized in Spartan ‘08 V 1.2.0. Geometries were
then optimized in Gaussian09 or Gaussian16 using the hybrid density
functional RB3LYP with a 6-31G(d) basis set.24 Frequency
calculations were performed to confirm the presence of local minima
(only positive frequencies). Vertical excitations were calculated by
TD-DFT methods at the RB3LYP/6-31G(d) and rcam-B3LYP/6-
31G(d) level in Gaussian09 or Gaussian16.
Synthesis of B4N2-FMes. In a glovebox, a solution of 2 (68.9 mg,
0.11 mmol) in 100 mL of toluene and a solution of 4 (92.6 mg, 0.11
mmol) in 100 mL of toluene were simultaneously added through two
different addition funnels to a three-necked round-bottomed flask
containing 300 mL of toluene over a period of 8 h with stirring. The
reaction mixture was kept stirring for 3 days at room temperature.
Separately, 1,3,5-tris(trifluoromethyl)benzene (0.31 g, 1.10 mmol)
was charged into a Schlenk flask under nitrogen protection and
dissolved in 100 mL of anhydrous diethyl ether. A solution of n-BuLi
(1.0 mL, 1.6 M in hexanes, 1.6 mmol) was added dropwise via syringe
at −78 °C. The reaction mixture was stirred for another 0.5 h at −78
°C, then allowed to warm to room temperature and stirred for 4.5 h.
F
The solvent was removed under high vacuum to obtain MesLi as a
light yellow solid. The lithium salt was redissolved in 50 mL of dry
toluene, and to this solution was added the previously prepared
borane macrocycle solution at −78 °C. The reaction mixture was
allowed to warm up to room temperature and kept stirring for 3 days.
A precipitate was removed by filtration through a fritted glass disk.
The crude product was purified by column chromatography on silica
gel using hexanes/toluene mixture (10% v/v) as the eluent. A solution
of the purified product in pentane was kept at −24 °C to give
1
The identity and purity of all new compounds was verified by H,
13C, 11B, 19F, and 29Si NMR spectroscopy according to elemental
composition. High-resolution MALDI-TOF mass spectra were
acquired except for highly air-sensitive species 4. Macrocycle B4N2-
FMes was additionally characterized by HSQC and HMBC 2D NMR
studies, GPC, single-crystal X-ray diffraction analysis, and elemental
analysis.
1
B4N2-FMes as a light green solid (50.0 mg, 33% yield). H NMR
(500 MHz, CDCl3) δ (ppm) 8.14 (s, 8H), 8.05 (s, 4H), 7.74 (d, J =
8.0 Hz, 4H), 7.58 (d, J = 8.0 Hz, 8H), 7.34 (d, J = 8.5 Hz, 4H), 7.31
(d, J = 8.0 Hz, 8H), 7.25 (d, J = 8.0 Hz, 4H), 7.10 (d, J = 8.5 Hz, 4H),
2.24 (m, 8H), 1.31 (s, 18H), 1.25−1.07 (m, 72H), 0.81 (t, J = 7.3 Hz,
12H), 0.71 (m, 8H). 13C NMR (150.8 MHz, CDCl3) δ (ppm) 150.9,
150.7, 149.2, 147.8, 144.4, 143.1, 139.9 (CH), 139.6, 136.7 (CH),
133.8 (q, J (13C, 19F) = 32 Hz, C−CF3), 133.2, 132.4 (CH), 131.4 (q,
J (13C, 19F) = 35 Hz, C−CF3), 127.3 (CH), 126.9 (CH), 126.0 (CH),
123.8 (q, J (13C, 19F) = 274 Hz, CF3), 123.0 (q, J (13C, 19F) = 273 Hz,
CF3), 121.4 (CH), 120.2 (CH), 55.8, 40.9, 34.7, 32.0, 31.5, 30.5,
30.0−29.4 (multiple signals), 24.2, 22.8, 14.2 (B−C signals not
detected). 11B NMR (160.3 MHz, CDCl3) δ (ppm) 65.6 (w1/2 = 5200
Hz). 19F NMR (470.4 MHz, CDCl3) δ (ppm) −56.3 (s), −63.0 (s).
MALDI-TOF (+ mode): m/z = 2768.2026 (100%) ([M+], calcd for
C154H162B4F36N2 2768.2606), 2755.3673 (30%) ([M+ − CH3], calcd
for C153H159B4F36N2 2755.2727). Elemental analysis for
C154H162B4F36N2: calcd C 66.82, H 5.90, N 1.01. Found C 65.60,
H 6.14, N 0.92% (the slightly low carbon content is likely due to
incomplete combustion as a result of carbide formation).
Caution: BBr3 is highly reactive and corrosive and must be handled
with care. All reactions involving BBr3 were performed using fluorinated
grease. n-Butyl lithium is highly reactive and must be handled accordingly.
The reaction byproduct Me3SnBr is highly toxic, and appropriate
precautions must be taken during synthesis and product isolation.
Synthesis of 2,7-Bis(trimethylsilyl)-9,9-didodecylfluorene
(3). Compound 3 was prepared by an adaptation of the literature
procedure for 2,7-bis(trimethylsilyl)-9,9-dihexylfluorene.25 2,7-Dibro-
mo-9,9-didodecylfluorene (5.00 g, 7.57 mmol) was dissolved in 200
mL of anhydrous diethyl ether, and distilled TMEDA (2.80 mL, 18.7
mmol) was added. A solution of n-BuLi (11.4 mL, 1.6 M in hexanes,
18.2 mmol) was added dropwise with stirring through an addition
funnel at −78 °C over a period of 1 h. The reaction solution was
stirred for an addition 1 h at −78 °C, followed by stirring at room
temperature for 2 h. The reaction flask was cooled down to −78 °C,
and neat Me3SiCl (2.14 g, 19.7 mmol) was added dropwise. The
reaction mixture was allowed to warm up to room temperature and
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Organometallics 2021, 40, 520−528

Organometallics
pubs.acs.org/Organometallics
Article
P.; Ye, M.-Y.; Bru
̈
ckner, A.; Schmidt, J.; Thomas, A. Donor−acceptor
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00779.
■
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Accession Codes
CCDC 2049896 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.
AUTHOR INFORMATION
Corresponding Author
■
̈
Frieder Jakle − Department of Chemistry, Rutgers University-
Newark, Newark, New Jersey 07102, United States;
orcid.org/0000-0001-8031-9254; Email: fjaekle@
rutgers.edu
Authors
Nurcan Baser-Kirazli − Department of Chemistry, Rutgers
University-Newark, Newark, New Jersey 07102, United
States
̀
5−12. (b) Charlot, M.; Porres, L.; Entwistle, C. D.; Beeby, A.;
Marder, T. B.; Blanchard-Desce, M. Investigation of two-photon
absorption behavior in symmetrical acceptor−π−acceptor derivatives
with dimesitylboryl end-groups. Evidence of new engineering routes
for TPA/transparency trade-off optimization. Phys. Chem. Chem. Phys.
2005, 7, 600−606. (c) Ji, L.; Fang, Q.; Yuan, M.-S.; Liu, Z.-Q.; Shen,
Y.-X.; Chen, H.-F. Switching High Two-Photon Efficiency: From
3,8,13-Substituted Triindole Derivatives to Their 2,7,12-Isomers. Org.
Lett. 2010, 12, 5192−5195. (d) Griesbeck, S.; Michail, E.; Rauch, F.;
Ogasawara, H.; Wang, C.; Sato, Y.; Edkins, R. M.; Zhang, Z.; Taki,
M.; Lambert, C.; Yamaguchi, S.; Marder, T. B. The Effect of
Branching on the One- and Two-Photon Absorption, Cell Viability,
and Localization of Cationic Triarylborane Chromophores with
Dipolar versus Octupolar Charge Distributions for Cellular Imaging.
Chem. - Eur. J. 2019, 25, 13164−13175. (e) Griesbeck, S.; Michail, E.;
Wang, C.; Ogasawara, H.; Lorenzen, S.; Gerstner, L.; Zang, T.;
Nitsch, J.; Sato, Y.; Bertermann, R.; Taki, M.; Lambert, C.;
Yamaguchi, S.; Marder, T. B. Tuning the π-bridge of quadrupolar
triarylborane chromophores for one- and two-photon excited
fluorescence imaging of lysosomes in live cells. Chem. Sci. 2019, 10,
5405−5422.
Roger A. Lalancette − Department of Chemistry, Rutgers
University-Newark, Newark, New Jersey 07102, United
States; orcid.org/0000-0002-3470-532X
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00779
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was financially supported by the National
Science Foundation (NSF) under grants CHE-1664975 and
CHE-1954122. A 500 MHz NMR spectrometer (MRI
1229030) and an X-ray diffractometer (CRIF 0443538) were
purchased with support from the NSF and Rutgers University.
We thank Dr. Yohei Adachi for performing some of the
computational studies.
(4) (a) Brend’Amour, S.; Gilmer, J.; Bolte, M.; Lerner, H.-W.;
Wagner, M. C-Halogenated 9,10-Diboraanthracenes: How the
Halogen Load and Distribution Influences Key Optoelectronic
Properties. Chem. - Eur. J. 2018, 24, 16910−16918. (b) Matsui, K.;
Oda, S.; Yoshiura, K.; Nakajima, K.; Yasuda, N.; Hatakeyama, T. One-
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