N-Heterocyclic Olefins as Electron Donors in Combination with Triarylborane Acceptors: Synthesis, Optical and Electronic Properties of D–π–A Compounds

Article abstract of DOI:10.1002/chem.201903118

N-heterocyclic olefins (NHOs), relatives of N-heterocyclic carbenes (NHCs), exhibit high nucleophilicity and soft Lewis basic character. To investigate their π-electron donating ability, NHOs were attached to triarylborane π-acceptors (A) giving donor (D)

Full text of DOI:10.1002/chem.201903118

DOI: 10.1002/chem.201903118
Full Paper
&
N-Heterocyclic Olefins
N-Heterocyclic Olefins as Electron Donors in Combination with
Triarylborane Acceptors: Synthesis, Optical and Electronic
Properties of D–p–A Compounds
Jiang He, Florian Rauch, Alexandra Friedrich, Daniel Sieh, Tatjana Ribbeck,
Ivo Krummenacher, Holger Braunschweig, Maik Finze,* and Todd B. Marder*
[
a]
Abstract: N-heterocyclic olefins (NHOs), relatives of N-het-
erocyclic carbenes (NHCs), exhibit high nucleophilicity and
soft Lewis basic character. To investigate their p-electron do-
nating ability, NHOs were attached to triarylborane p-accept-
ors (A) giving donor (D)–p–A compounds 1–3. In addition,
an enamine p-donor analogue (4) was synthesized for com-
parison. UV–visible absorption studies show a larger red
shift for the NHO-containing boranes than for the enamine
analogue, a relative of cyclic (alkyl)(amino) carbenes (CAACs).
Solvent-dependent emission studies indicate that 1–4 have
moderate intramolecular charge-transfer (ICT) behavior. Elec-
trochemical investigations reveal that the NHO-containing
boranes have extremely low reversible oxidation potentials
ox
+
(e.g., for 3, E
1
=ꢀ0.40 V vs. ferrocene/ferrocenium, Fc/Fc ,
=
2
in THF). Time-dependent (TD) DFT calculations show that the
HOMOs of 1–3 are much more destabilized than that of the
enamine-containing 4, which confirms the stronger donating
ability of NHOs.
Introduction
2
[1]
Three-coordinate boron is sp hybridized
and
adopts a trigonal planar geometry, which leaves an
unoccupied p-orbital. This vacant orbital can act as
an excellent p-acceptor (A) in the excited state, lead-
ing to intramolecular charge transfer (ICT) after pho-
toexcitation. As such, applications of three-coordi-
[
2]
nate boranes as selective anion sensors, nonlinear
[
3]
optical (NLO) materials, for two-photon absorption
Scheme 1. Reported strategies to narrow the energy gap of boron containing D–p–A
systems (a, b, c) and new systems reported in this study (d).
[
4]
and two-photon excited fluorescence, and live-cell
[
5]
[6]
imaging, among others have been intensively
studied.
We are interested in designing small molecules (donor (D)–
p–A type three-coordinate boranes) with narrow energy gaps.
One approach is to stabilize the lowest unoccupied molecular
orbital (LUMO) by enhancement of the electron-acceptor
strength of the boron center. Instead of using mesityls as steric
protecting groups to avoid water or other nucleophiles bind-
[7]
[8]
[9]
ing to the empty orbital of boron, Marder and others ap-
plied 2,4,6-(CF ) C H (FMes) as a new steric protecting group
3
3
6
2
[
a] J. He, F. Rauch, Dr. A. Friedrich, Dr. D. Sieh, Dr. T. Ribbeck,
Dr. I. Krummenacher, Prof. Dr. H. Braunschweig, Prof. Dr. M. Finze,
Prof. Dr. T. B. Marder
in three-coordinate boranes (Scheme 1a). A second approach
is modification of the p-linker in D–p–A systems, for example,
[10]
[3f,8e]
Institute for Inorganic Chemistry and Institute for Sustainable Chemistry &
Catalysis with Boron (ICB), Julius-Maximilians-Universitꢀt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
using pyrene or thiophenes
(Scheme 1b), potentially in-
fluencing both the highest occupied molecular orbital (HOMO)
and LUMO. The third method is to use a very electron-rich
donor group which reduces the energy gap by destabilizing
the HOMO (Scheme 1c). As an example, Marder, Braunschweig
et al. have recently reported the use of a diborene (B=B)
system as the p-donor resulting in NIR absorbing and emitting
E-mail: maik.finze@uni-wuerzburg.de
todd.marder@uni-wuerzburg.de
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201903118.
ꢂ 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of Creative Commons Attri-
bution NonCommercial License, which permits use, distribution and repro-
duction in any medium, provided the original work is properly cited and is
not used for commercial purposes.
[11]
quadrupolar systems, but these compounds are rather unsta-
ble in air.
Amines are among the most efficient and well-studied p-
[12]
electron donors in organic materials, and triarylamines con-
Chem. Eur. J. 2019, 25, 1 – 9
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nected to triarylboranes were found to be suitable materials
introduction of the NHO or enamine moiety. Thus, the corre-
sponding silyl-protected brominated alcohols were treated
[
13]
for OLEDs. However, using two nitrogen atoms linked 1,1 to
a C=C double bond, known as an N-heterocyclic olefin (NHO),
as a donor (Scheme 1d), to the best of our knowledge, has
never been combined with a three-coordinate borane accept-
with nBuLi at ꢀ788C, followed by the addition of FBMes ,
2
giving the intermediate boranes 6a, 6b, 6c in high yields. Sub-
sequent consecutive treatment with PBr and tetrabutylammo-
3
[
14]
or. NHOs are widely used as catalysts in transesterification,
nium bromide (TBAB) in CH Cl at 08C, deprotection and bro-
2
2
[
15]
to turn CO2 into valuable chemicals,
promote polymeri-
mination in one step, gave precursors in good (7a and 7b) to
acceptable (7c) yields. TBAB is necessary in these reactions,
[
16]
zation, etc.
Due to the donating effect of two nitrogen
[18]
atoms, as well as the 6p-electrons of the imidazole ring, the
exocyclic C=C bond becomes highly polarized and electron
otherwise the products are formed in much lower yields. All
compounds were fully characterized by NMR, high-resolution
mass spectrometry (HRMS), as well as elemental analysis.
Single crystals of 7a and 7b suitable for X-ray diffraction analy-
sis were grown from dichloromethane/hexane at ꢀ308C and
of 7c by evaporation of a hexane solution at room tempera-
ture. These precursors were then treated with 2 equiv of 1,3-
bis-(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr, for 7a, 7b
and 7c) or 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrroli-
dine-2-ylidene (CAAC, for 7a), respectively, at room tempera-
ture. The solutions turned reddish-orange (for reactions with
IPr) or yellow (for reaction with CAAC), immediately. After
workup and crystallization, 1–4 were isolated in 90, 87, 85, and
[
17]
rich, thus making NHOs potential strong donors.
With this in mind, we designed and synthesized four differ-
ent boranes, 1–4 (Scheme 2), with 1–3 having an NHO as the
donating group, whereas in 4, an enamine is the donating
group. The two extra methyl groups in 2 versus 1 should im-
prove the stability by protecting the exocyclic C=C bond. For a
more efficient and p-electron-rich linker, borane 3 was de-
signed. Herein, we report about the synthesis and properties
of these new donor–p–acceptor three-coordinate boranes.
45% yields, respectively.
The identity and purity of 1–4 was confirmed by NMR spec-
troscopy, HRMS, and elemental analysis. The chemical shifts in
11
1
the B{ H} NMR spectra are in the typical range of three-coor-
11
dinate boranes (d( B)=69.6, 71.9, 57.5, and 73.3 ppm for 1, 2,
11
3
, and 4, respectively). Compound 3 has the highest field B
chemical shift which is attributed to the more efficient conju-
1
gation and donor ability of the thienyl group. In the H NMR
spectra, the signals of the protons of the exocyclic double
bond appear at 4.00 (1), 3.69 (2), 4.38 (3), and 4.50 ppm (4).
1
Another interesting observation is that the H NMR spectra of
Scheme 2. Three-coordinate boranes 1–4 developed in this study.
1
and 2 show broad signals in C D , but in CD Cl , all signals
6 6 2 2
are sharp with well-resolved HꢀH couplings. In contrast, the
1
H NMR signals of 3 and 4 are well-resolved in both solvents,
Results and Discussion
C D and CD Cl .
6
6
2
2
Stability tests indicate that 1–3 decompose slowly in solu-
Synthesis
tion in air, so they have to be handled under an inert atmos-
phere. In stark contrast, compound 4 is a bench-stable yellow
solid and shows no decomposition even in common solvents
for several months, as evident from NMR spectroscopy.
The synthesis of the four boranes is shown in Scheme 3. Our
strategy was to employ bromomethyl triaryl boranes 7a, 7b,
7
c as precursors for the final step of the synthesis, namely the
Scheme 3. Synthesis of three-coordinate boranes 1–4.
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Figure 1. Molecular structures of 1–4 from single-crystal X-ray diffraction data at 100 K. Atomic displacement ellipsoids are drawn at the 50% probability
level, and hydrogen atoms are omitted for clarity. For 4, only one of two symmetrically independent molecules is shown. With regard to the aryl rings
bonded to boron atoms, the central ring is labelled P1 and the terminal rings are labelled P3 and P4. The 5-membered nitrogen-containing ring is labelled
P2. The pyrrolidine moiety of one of the two molecules of 4 is disordered, and only the part with 87% occupancy is shown.
Crystal structures
from a saturated acetonitrile solution and of 4 by crystalliza-
Reddish-orange single crystals suitable for X-ray diffraction
analysis of 1 and 3 were grown by evaporation of a hexane so-
lution at room temperature. Single crystals of 2 were obtained
tion from a dichloromethane/acetonitrile solution at ꢀ308C.
The molecular structures in the solid state are shown in
Figure 1 and selected bond lengths (ꢁ) and interplanar angles
o
Table 1. Selected bond lengths [ꢁ] and angles [ ] of 1–4.
Compound
1
2
3
4
4
[
a]
[a]
(
molecule 1)
(molecule 2)
B1ꢀC (P1)
B1ꢀC (P3)
B1ꢀC (P4)
1.547(2)
1.581(2)
1.583(2)
17.69(7)
56.72(6)
60.31(6)
359.9(1)
1.379(2)
1.388(2)
1.387(2)
37.76(7)
1.552(3)
1.588(3)
1.585(3)
17.86(11)
56.45(5)
66.42(6)
360.0(2)
1.361(3)
1.395(2)
1.407(2)
57.74(7)
1.512(3)
1.586(3)
1.583(3)
11.72(10)
58.10(8)
61.13(8)
360.0(2)
1.382(3)
1.376(2)
1.381(2)
24.65(7)
1.564(3)
1.572(3)
1.581(3)
27.13(10)
58.76(9)
50.36(7)
360.0(2)
1.337(3)
1.389(3)
–
1.558(3)
1.572(3)
1.584(3)
29.13(10)
58.07(9)
50.56(7)
360.0(2)
1.334(3)
–
]
]
]
B1C
B1C
B1C
3
3
3
-P1
-P3
-P4
Sum ]CB1C
h (C=C)
CꢀN1
CꢀN2
1.392(3)
79.89(8)
]P1-P2
83.89(9)
With regard to the aryl rings bonded to boron atoms, the central ring is labelled P1 and the terminal rings are labelled P3 and P4. The 5-membered nitro-
gen containing ring is labelled P2. [a] In borane 4, the boron and nitrogen atoms are labeled B1 and N1 in molecule 1 and B2 and N2 in molecule 2.
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o
(
) are listed in Table 1. The three aryl groups attached to
[
a]
Table 2. Cyclic voltammetry data for boranes 1–4.
boron adopt propeller-like configurations in all four com-
ox
1
red
1
[c]
[c]
pounds. The BC moieties are planar with the sum of the C-B-C
E
[V]
E
[V]
HOMO [eV]
LUMO [eV]
3
=2
=2
bond angles equal to 3608. The interplanar angles between
1
2
3
4
ꢀ0.36
ꢀ0.36
ꢀ0.40
0.27
ꢀ2.86
ꢀ2.82
ꢀ2.86
ꢀ2.66
ꢀ4.37
ꢀ4.38
ꢀ4.33
ꢀ5.00
ꢀ2.00
ꢀ2.05
ꢀ2.00
ꢀ2.20
the BC plane and the aryl groups bonded to boron depend
3
on the steric demand of the aryl moieties. The terminal mesityl
rings P3 and P4 are strongly twisted with respect to the BC3
plane (50–668, Table 1), a behavior that is generally observed
[
b]
4 6
[a] Measured in THF in the presence of 0.1m nBu NPF , potential sweep
ꢀ
1
+
[
3g–i,4e,8c,e,19]
rates of 250 mVs , half-wave potentials are given against the Fc/Fc
couple. [b] Partially reversible half-wave oxidation potential. [c] Calculated
from the onset potentials of the first oxidation and reduction waves, re-
spectively, assuming that the HOMO of Fc lies 4.8 eV below the vacuum
level.
in triarylboranes.
The bridging aryl rings P1, which
are sterically less demanding, are only slightly twisted with re-
spect to the BC plane (12–298, Table 1). The BꢀC bond lengths
3
lie in the expected range. They are longer to the mesityl
groups (1.572(3)–1.588(3) ꢁ), whereas they are significantly
shorter to the bridging rings, that is, 1.547(2)–1.564(3) ꢁ for
the phenyl and xylyl rings (1, 2, and 4) and shortest at
1.512(3) ꢁ for the thiophene group in 3 (Table 1). In 1–3 the
exocyclic C=C double bond (h, Figure 1) length is significantly
longer (1.361(3)–1.382(3) ꢁ, Table 1) than a normal C=C double
[
20]
bond or the exocyclic C=C bond of 1,3-bis(2,6-diisopropyl-
phenyl)-2-methylene-2,3- dihydro-1H-imidazole (IPr=CH2,
.332(4) ꢁ). This suggests some degree of charge transfer in
[
17]
1
the ground state and a polarized ground state in 1–3. This is
not the case for 4, in which h=1.334(3) ꢁ, close to the expect-
ed C=C double bond length. A pronounced bond-length alter-
nation (0.036(3) ꢁ, Table S3, Supporting Information) is ob-
served for the phenyl and xylyl units of 1 and 2 consistent
with a partially quinoidal structure. This indicates strong conju-
gation between the boron centers and the bridging units,
which also suggests ground-state ICT. The bond-length alterna-
tion is less pronounced in 4 (0.018(3) ꢁ). The interplanar angle
between the bridging ring P1 and the N-heterocyclic carbene
ring P2, which are connected through the exocyclic C=C
double (h) and a CꢀC single bond (g), varies strongly among
the compounds. Although these are smallest for 1 (388) and 3
Figure 2. Cyclic voltammograms of 1(black, solid), 2 (red, solid), 3(blue,
dash), and 4 (pink, dash).
2. The reversible reduction potentials of the NHO donor com-
pounds are about 0.2 V more negative than that of the enam-
ine donor compound. These reduction potentials are all com-
parable to those of other structurally related D–p–A bo-
(258), the angle is larger for 2 (588), which has a sterically more
demanding bridging ring, and largest for 4 (808 and 848).
Borane 4 has the largest dihedral angle between rings P1 and
P2, probably due to less effective conjugation between the
boron center and the bridging unit, which also is reflected by
less bond length alternation of the bridge-phenyl group.
[21]
ranes. Obviously, the donor ability of the NHO or enamine
unit does not have a large influence on the electron-accepting
ability of the three-coordinate boron center in our compounds.
In sharp contrast to their very similar reduction potentials,
large differences were found for their oxidation potentials de-
pending on the donor moiety. Compounds 1 and 2 have the
Electrochemical properties
ox
same half-wave oxidation potential (ꢀ0.36 V) and E
1
of 3
=
2
To investigate their electrochemical properties, 1–4 were also
studied by cyclic voltammetry (Table 2). Boranes 1–3 show
both a reversible reduction wave and a reversible oxidation
wave, whereas 4 reveals only a reversible reduction wave and
a partially reversible oxidation wave (Figure 2). The reversible
(ꢀ0.40 V) is shifted to a more negative value by 40 mV, only.
This small difference is caused by the more electron-rich thien-
yl bridge. Compounds 1–3 are easily oxidized and show far
ox
=
2
more negative oxidation potentials than 4 (E
1
=0.27 V). This
larger difference indicates that the NHO is far more electron
rich than the enamine, and also suggests a much smaller
HOMO–LUMO gap in NHO-containing 1–3 compared to en-
amine-containing 4. The comparably low reversible oxidation
potentials of 1–3 are possibly the reason for their air-sensitivity
(see above).
reduction waves are attributed to the BMes moieties and the
2
oxidation processes are related to the NHOs or the enamine
moiety. The half-wave reduction potentials of 1 and 3 at
ꢀ
2.86 V are the most negative potentials among the four com-
red
pounds. As expected, 4 (E
1
=ꢀ2.66 V) has the most positive
=
2
half-wave reduction potential. The half-wave reduction poten-
red
tial of 2 (E =2
1
=ꢀ2.82 V) is 40 mV more positive than that for 1,
which may be due to the larger dihedral angle between rings
P1 and P2, as discussed above and the more rigid structure of
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Table 3. Photophysical data for 1–4 at room temperature.
[
a]
[b]
F
ꢀ1
Solvent
l
abs [nm]
l
em [nm]
F
t
F
[ns]
Stokes shift [cm ]
4
ꢀ1 ꢀ1
(
e [10 cm m ])
[
c]
[c]
Hexane
Toluene
THF
485 (5.8)
495 (8.5)
504 (7.4)
–
499
528
572
576
578
1263
2359
–
0.15
0.15
0.11
2.1
<1.0
1.9
1
2
3
4
Solid
Hexane
Toluene
THF
484 (3.0)
494 (3.4)
501 (3.2)
–
510
555
649
569
0.012
0.29
0.033
0.074
<1.0
7.3
1053
2225
4552
–
1.6
[
d]
Solid
[
e]
[e]
[e]
[e]
[e]
Hexane
Toluene
THF
508 ( )
525
547
575
591
(N.D.)
(N.D.)
(N.D.)
638
1098
1766
–
[
e]
[e]
[e]
516 ( )
[
e]
522 ( )
–
[
d]
Solid
0.037
[
c]
Hexane
Toluene
THF
Acetonitrile
Solid
411 (2.2)
421 (3.1)
421 (2.9)
426 (3.6)
–
432
454
475
501
467
<1.0
<1.0
<1.0
<1.0
<1.0
1183
1727
2700
3514
–
0.005
0.012
0.031
0.003
[a] Lowest-energy absorption maximum. [b] Absolute fluorescence quantum yields measured using an integrating sphere. [c] Not determined due to very
weak emission. [d] 2: t <1.0 (87.6%), t =1.1 (11.7%), t =3.8 ns (0.7%); Borane 3: t <1.0 (87.7%), t =1.9 (12.2%), t =11.2 ns (0.1%). [e] Not determined
1
2
3
1
2
3
due to slow decomposition in highly dilute solution.
ꢀ
1
Photophysical properties
has the largest Stokes shift (4552 cm ) among all four boranes
in different solvents. In the solid state, 1 exhibits the highest
quantum yield (0.11) and 3 has the lowest-energy emission
(591 nm) but with a much lower quantum yield (0.037). It was
not possible to determine the molar extinction coefficient,
quantum yield and life-time of 3 in solution accurately due to
slow decomposition in highly dilute solution.
UV–visible absorption spectra of 1–4 were measured in differ-
ent solvents and the data are listed in Table 3. Compounds 1–4
have a strong, structureless lowest-energy absorption band in
toluene with maxima at 495, 494, 516, and 421 nm, respective-
ly (Figure 3), which can be attributed to ICT. The lowest-energy
absorption of 4 in solution is similar to that of related D–p–
[
3i,4d]
BMes compounds.
A comparison of the lowest-energy ab-
2
Theoretical studies
sorption bands shows a large red shift (e.g., in toluene,
ꢀ
1
ꢀ1
approx. 3500 cm for 1 and 2, 4300 cm for 3) between
NHO-containing 1–3 and enamine-containing 4, indicating the
stronger p-electron donating ability of the NHO than the en-
DFT calculations were carried out to gain deeper insight into
the electronic and photophysical properties of 1–4. Optimized
ground-state structures were obtained at the B3LYP/6–
31+G(d) level of theory using the crystal structures as the
starting geometries. The solid-state structures were nicely re-
produced. Similar to the crystal structures, the mesityl CꢀB
bonds are about 0.04 (1), 0.03 (2), 0.06 (3), and 0.03 ꢁ (4)
longer than the p-bridge BꢀC bonds. In comparison with ex-
ꢀ
1
amine. The redshift of about 20 nm (860 cm ) observed for 3
compared to 1 confirms the more efficient conjugation of a
thienyl compared with a phenyl group, which was discussed
for the electrochemical data (see above). Another interesting
finding is that 1 has a very large molar extinction coefficient
4
ꢀ1 ꢀ1
4
ꢀ1 ꢀ1
(
8.5ꢂ10 cm
m
in toluene and 7.4ꢂ10 cm
m
in THF) for
perimental values, the angles between the BC plane and the
3
the lowest-energy absorption. With increasing solvent polarity,
positive absorption solvatochromism (approx. 15 nm from
hexane to THF) was observed in all four boranes, which sug-
gests polarized ground states and moderate ground-state
p-bridge are slightly larger for 1–3 (D=3.908, 0.718 and 2.328
respectively) and smaller for 4 (D=4.128 and 6.138). The opti-
mized structures also exhibit a quinoidal distortion of the p-
bridge. The mean quinoidal distortions of the phenylene-
bridged compounds 1 and 4 are 0.03 and 0.026 ꢁ, respectively.
In the xylene-bridged derivative 2, it is not sensible to use the
mean distortion because the methyl groups on the donor side
also influence the aromatic bond lengths. In this case, the
bonds a/b are 0.02 ꢁ longer than c/f, whereas e/d are 0.03 ꢁ
longer (Figure 1). In the thiophene-bridged 3, all bond lengths
are similar to those in the crystal structure.
[
3i,22]
dipole moments,
caused by ground-state ICT.
All of the boranes show weak to moderate emission in solu-
tion and in the solid state. Solvent-dependent emission studies
indicate that all of the boranes show a moderate redshift with
increasing solvent polarity. This is a typical phenomenon ob-
served in D–p–A compounds due to ICT, resulting in a more
polarized excited state, which is stabilized by a higher-polarity
solvent relative to the ground state. The Stokes shifts of the
boranes also increase with increasing solvent polarity. In THF, 2
The optimized structures also reproduce the shortened CꢀC
single bond (g) and the elongated exocyclic C=C double bond
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HOMOs of 1–3 are very similar in energy (DE<0.05 eV), as are
the LUMOs, which indicates only a small influence of the differ-
ent bridges on the frontier orbitals. Borane 4, however, exhibits
a lower HOMO as well as LUMO energy (DE=0.5 and 0.2 eV,
respectively, in comparison to 1), which nicely fits with the
electrochemical study. This is due to the significantly lower
donor strength of the enamine than the NHO.
Table 4. Lowest-energy transitions calculated at the B3LYP/6–31+G(d)
level of theory. (H=HOMO; L=LUMO).
Compound Transition E
l
f
Major
L
Dipole
[
eV] [nm]
contributions
moment [D]
!
1
2
3
4
S
1
S
1
S
1
S
1
S
0
2.63 471 0.59 H!L (98%) 0.54 6.27
2.56 484 0.36 H!L (99%) 0.46 5.53
2.65 467 0.62 H!L (98%) 0.64 7.03
2.99 415 0.75 H!L (98%) 0.56 6.26
!
S
0
!
S
0
!
S
0
Subsequently, TD-DFT calculations were carried out at the
!
B3LYP/6–31+G(d) level of theory. In the gas phase, the S1
S
0
transitions of all four derivatives are almost exclusively HOMO-
to-LUMO transitions (Table 4). For ICT-based transitions it is rec-
ommended to use the Coulomb attenuated functional CAM-
[23]
B3LYP. We have carried out these calculations as well (see
the Supporting Information). However, the calculations per-
formed using B3LYP more accurately reproduced the energies
found experimentally. Given that this was unexpected, we fur-
[24]
ther determined the overlap coefficients (L) (Table 4). Com-
pounds 1–4 exhibit L coefficients around 0.5 for their lowest-
energy transitions, which indicates only moderate ICT character
in the excited state. For this reason, the B3LYP functional was
used for the TD-DFT calculations as well.
This moderate ICT character of the lowest-energy absorp-
tions could be due to an already partially polarized ground
Figure 3. UV–visible absorption (top) and emission spectra (bottom) of bo-
ranes 1–4 in toluene.
(h) that connects the donor to the bridge for 1–3. In
4
, this convergence is more pronounced than in the
crystal structure. This might be due to a significantly
smaller angle between the P1 and P2 rings (33.558
vs. 80/848) in 1–3. This would lead to an increased
interaction between the donor and acceptor which
is most likely responsible for the changes in these
bond lengths.
For 1–3, the HOMO is mainly delocalized over the
p-bridge, the exocyclic C=C double bond, and the
imidazole ring (Figure 4). For 4, the HOMO is mainly
delocalized over the p-bridge, the exocyclic C=C
double bond, and the nitrogen of the pyrrolidine
ring. Interestingly, the boron also contributes to the
HOMO in all compounds confirming the ground-
state ICT. In all compounds, the LUMO is mainly lo-
calized on boron and the p-bridge, with the two me-
sityl groups and the exocyclic C=C double bond also
contributing to some extent, along with a small con-
tribution from the imidazole or pyrrolidine ring. The
Figure 4. HOMO and LUMO of 1–4, calculated at the B3LYP/6–31+G(d) level of theory
and corresponding energies.
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Full Paper
state, which would decrease the change in dipole moment
and necessity for solvent rearrangement upon excitation. All
calculated structures exhibit moderate dipole moments in the
ground state (Table 4), which also supports that assumption.
The moderate positive absorption solvatochromism of all com-
pounds with increasing solvent polarity also confirms the mod-
erate ground-state dipole moments.
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Four different D–p–A boranes were synthesized in three steps
each, providing an efficient synthetic strategy for introducing
N-heterocyclic olefins (NHO) into boron-containing D–p–A sys-
tems. Photophysical studies show that NHO is a much stronger
electron donor than an enamine (or amine). The electrochemi-
cal investigations reveal extremely low reversible oxidation po-
tentials for the three NHO-containing boranes 1–3 compared
with those of enamine- (4) or amine-containing boranes. DFT
calculations indicate a much higher HOMO for 1–3 in agree-
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moiety. Our studies confirmed the electron-rich property of
NHOs, suggesting their potential for use as donors in other
push–pull systems. The reversible oxidation potentials of 1–3
suggest that radical cations may be isolable, which is currently
under investigation.
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Keywords: donor–acceptor systems · electrochemistry · N-
heterocyclic olefins · photophysical properties · triarylboranes
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Accepted manuscript online: August 31, 2019
Version of record online: && &&, 0000
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FULL PAPER
&
N-Heterocyclic Olefins
J. He, F. Rauch, A. Friedrich, D. Sieh,
T. Ribbeck, I. Krummenacher,
H. Braunschweig, M. Finze,* T. B. Marder*
Donor–acceptor N-heterocyclic olefin
tremely low reversible oxidation poten-
tials for the NHO–boranes, indicating
the electron rich properties of the NHO.
(NHO)–triarylboranes show solvato-
&& – &&
chromism in both absorption and emis-
sion. Electrochemical studies reveal ex-
N-Heterocyclic Olefins as Electron
Donors in Combination with
Triarylborane Acceptors: Synthesis,
Optical and Electronic Properties of
D–p–A Compounds
Chem. Eur. J. 2019, 25, 1 – 9
www.chemeurj.org
9
ꢀ 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ