Heterotriangulenes π-expanded at bridging positions

Article abstract of DOI:10.1021/ol501004g

A series of nitrogen-containing heterotriangulenes expanded at the bridging positions has been synthesized. Among them, a dibenzo[c,g]fluorenylidene- substituted derivative has a highly twisted conformation for the overcrowded alkene moieties, which impart a highly electron-accepting character to the electron-donating heterotriangulene skeleton and thereby an NIR absorption as well as multiredox properties with a low reduction potential.

Full text of DOI:10.1021/ol501004g

Letter
pubs.acs.org/OrgLett
Heterotriangulenes π‑Expanded at Bridging Positions
Chih-Ming Chou,† Shohei Saito,† and Shigehiro Yamaguchi*,†,‡
‡
†Department of Chemistry, Graduate School of Science, and Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya
University, Furo, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: A series of nitrogen-containing heterotriangulenes
expanded at the bridging positions has been synthesized. Among
them, a dibenzo[c,g]fluorenylidene-substituted derivative has a
highly twisted conformation for the overcrowded alkene moieties,
which impart a highly electron-accepting character to the electron-
donating heterotriangulene skeleton and thereby an NIR
absorption as well as multiredox properties with a low reduction
potential.
eterotriangulenes with a planar nitrogen atom at the center
are simple and attractive building blocks for functional
point of view.11 We now report the synthesis of a series of
heterotriangulenes 3 π-expanded at the bridging positions. In
particular, we disclose the significant impacts of different π-
expansion modes, the introduction of highly distorted CC
double bonds and the unsymmetrical annulation with more fused
benzene rings, on their structures and electronic properties.
For the synthesis of π-expanded heterotriangulenes, we first
synthesized an iodinated compound 4 as a key precursor which
was prepared according to the literature (Scheme 1).4 A C6F5-
H
organic materials.1−9 A representative example is the carbonyl-
bridged heterotriangulene 1 (Figure 1).1 Several derivatives
Scheme 1. Synthesis of π-Expanded Heterotriangulenes
Figure 1. Chemical structures of heterotriangulene derivatives.
functionalized at the phenyl rings with n-dodecyl or 4-
alkoxyphenylethynyl groups were synthesized, and their self-
assembly behavior to form nanowires or microrods was
reported.2,3 Heterotriangulene-based dendrimers with carbazole
moieties were also studied as a potential material for organic light
emitting diodes (OLEDs).4 One issue regarding their molecular
design is the poor solubility of the core triangulene skeleton in
common organic solvents, which limits the applications in
organic electronics. In this regard, the replacement of the
carbonyl bridging moieties in 1 with sp3-hybridized dimethyl-
methylene groups imparts a better material processability.
Compound 2 thus developed has been utilized as a promising
building block for the hole-transporting or -injecting materials
with a high thermal stability in OLEDs, materials with large two-
photon absorption cross section, and materials for dye-sensitized
solar cells.5−8
Heterotriangulene 2 has the intrinsic electronic nature of
triphenylamine. The expansion of the skeleton at the para-
positions of the three phenyl rings is a rational way to produce
electron-donating materials. In addition, the heterotriangulene is
a potential building block for heteroatom-embedded polycyclic
aromatic hydrocarbons, ultimately including heteroatom-doped
graphene flakes.10 The first step to this end should be expansion
of π conjugation at the bridging moieties between the phenyl
rings. However, no derivative has yet been studied from this
substituted 5 was obtained by the Pd-catalyzed Suzuki−Miyaura
coupling of 4 with pentafluorophenylboronic acid in 61% yield.12
The introduction of the C6F5 groups increased not only the
solubility but also the reactivity of the carbonyl functionality for
the next reaction. Thus, the carbonyl groups were converted to
the CC double bonds by carrying out the 3-fold Corey−Fuchs
reaction, which is useful for expanding π-systems.13
A
hexabrominated product 6 was successfully obtained in 45%
yield. This compound was further transformed by the Suzuki−
Miyaura coupling with phenylboronic acid or 2-naphthylboronic
Received: April 5, 2014
© XXXX American Chemical Society
A
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Letter
acid to the corresponding 6-fold coupling products 3a and 3b in
94 and 92% yields, respectively.
Compound 3a was subjected to typical Scholl reaction
conditions, namely the treatment with an excess amount (12
equiv) of FeCl3 in CH2Cl2/CH3NO2 degassed with argon at 0 °C
for 30 min (Scheme 2).14 A 10-ring-fused compound 7 was
bond length of 1.36 Å.16 The bowl depth, the distance between
the nitrogen atom and the plane defined by three para-carbon
atoms of the benzene rings, is 1.1 Å. This result demonstrates
that this planarized skeleton still has tolerance toward the
structural deformation. A similar bowl-shaped structure is also
reported for analogous bridged triphenylborane derivatives,
although they take the bowl-shaped structure only in the excited
state or in the one electron-reduced state.17 Second, there is
severe steric congestion between the diarylvinylidene moieties
and the C6F5 groups. As a consequence, the three vinylidene
moieties in 3a significantly deviate in the same direction, while
the two vinylidene moieties in 7 are distorted in the opposite
direction (Figure 2a and Figure S3, Supporting Information).
This steric repulsion is likely responsible for the bowl-shaped
structure in 3a. Third, in the structure of 9, the three
dibenzo[c,g]fluorenylidene groups all adapt twisted overcrowded
alkene conformations (Figure 2b and Figure S4, Supporting
Information). The unit cell consists of five crystallographically
independent molecules, all of which take the twisted alkene
conformations similar to each other. This structure is worth
noting since common overcrowded alkenes, like 9,9′-bi-
(fluorenylidene), tend to have a folded structure as the more
stable conformer compared to the twisted conformer.18 The
DFT (B3LYP/6-311G**) structure optimization indicated that
the structure with the twisted overcrowded alkene geometries is
more stable than the folded congener by 4.9 kcal mol−1 in zero-
point corrected energies. This result suggests that the twist of the
alkene units is not due to a packing force. The variable
temperature 1H NMR spectra in the range of 25−100 °C did not
show a significant change (Figure S16, Supporting Information),
suggesting that a high energy barrier for the structural change
between the twisted and folded conformers suppresses the
formation of the folded one or that the amount of folded species
is negligible in the thermal equilibrium. The steric repulsion
between the planar dibenzo[c,g]fluorenyl groups and the C6F5
groups is again likely responsible for this structure, while the
central triphenylamine core has a relatively planar geometry. This
highly distorted structure gives rise to intriguing photophysical
properties (vide infra).
The expansion of the π skeleton at the bridging moieties
significantly alters the photophysical properties. The absorption
and fluorescence data in CH2Cl2 for 3 and 7−9 are summarized
in Table 1, together with that of 26c for comparison. The 3-fold
vinylidene-expanded 3a and 3b have the respective longest
absorption maximum wavelengths (λabs) at 377 and 397 nm,
which are red-shifted by 76 and 96 nm from 2 (λabs = 301 nm),
respectively. Compound 7 with a dibenzochrysene substructure
shows a more red-shifted absorption band with the λabs of 471
nm. More notably, the dibenzo[c,g]fluorenylidene-expanded
derivative 9 has a broad absorption band that reaches the near-
infrared (NIR) region with the λabs of 797 nm (Figure 3).
To rationalize the significant impact of the π expansion on the
absorption properties, we conducted DFT structural optimiza-
tion and TD-DFT calculations at the B3LYP/6-31G* level of
theory.19 Figure 4 shows a comparison of the electronic
structures among 2, 3b, 9, and 8 (Figures S10−15, Supporting
Information). For the TD-DFT calculation of 9, the optimized
geometry with the twisted conformation for the three
fluorenylidene moieties was employed.
Scheme 2. Dehydrogenative Coupling of 3
isolated as the main product in 45% yield. Notably, when the
reaction was conducted in the presence of air, a dicarbonyl-
substituted 10-ring-fused system 8 was obtained in 39% yield
instead of 7. This compound is likely produced by a [2 + 2]
cycloaddition of singlet oxygen15 to the olefin moieties in 7,
which is followed by C−C bond cleavage. Although we also
attempted the dehydrogenative annulation of 3a and the partially
cyclized 7 using DDQ/Lewis acids or photocyclization methods,
the reactions only produced unidentified complex mixtures. In
contrast, when 3b was employed as a substrate for the reaction
with FeCl3 at 0 °C, the dehydrogenative coupling smoothly took
place at the naphthyl groups to give a dibenzo[c,g]-
fluorenylidene-substituted heterotriangulene 9 as the main
product in 40% yield. The low aromaticity of the naphthyl
groups may facilitate the reaction. All the π-expanded
heterotriangulenes 3a, 3b, 7, 8, and 9 have good solubilities in
the common organic solvents, such as toluene, Et2O, THF, and
CH2Cl2, even at room temperature. For example, the solubilities
of 3b and 9 in CH2Cl2 are 25.3 and 23.0 mg/mL, respectively.
Single crystals of 3a, 7, and 9 suitable for the X-ray diffraction
analysis were obtained by a vapor diffusion method in different
solvent systems (Figure 2 and Supporting Information). There
Figure 2. (a) Crystal structure of 7 and (b) DFT-optimized structure of
9 calculated at the B3LYP/6-31G* level of theory.
are several notable features in their crystal structures. First,
whereas the 3-fold bridged triphenylamine core generally takes a
planar structure,1,2 that in 3a has a bowl-shaped structure (Figure
S1, Supporting Information). The sum of the C−N−C angles is
351.8°, and the N−C bond distances are 1.417(3)−1.425(2) Å,
which are significantly elongated from the ordinary Csp2−N
The HOMO of 3b is delocalized in the central triphenylamine
core with the contribution of the three vinylidene bridging
moieties. Despite the expansion of the π conjugation, the
HOMO level of 3b is 0.38 eV lower than that of 2, indicative of
B
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localized on the vinylidene moieties. The highly distorted
geometries decreases the π* orbital of the double bond moieties,
resulting in the significant decrease in the LUMO level from 2 to
3b by 1.34 eV. As a result, 3b has the HOMO−LUMO transition
band with the lower energy compared to that for 2. In addition, it
is also noteworthy that the intramolecular charge transfer
character in the HOMO−LUMO transition for 3b results in the
significant increase in the oscillator strength (f = 0.215)
compared to 2 (f = 0.0111), which is also an important outcome
for the expansion of π skeleton with the three vinylidene
moieties.
The ring closure from 3b to 9 further alters the electronic
structure to a considerable extent. The formation of the
fluorenylidene bridges increases the contribution of the vinyl-
idene moieties to the HOMO delocalization, resulting in a slight
increase in the HOMO level by 0.14 eV. A more dramatic change
is suggested in the LUMO level from 3b to 9 by 1.61 eV due to
the more significant distortion of the olefin moieties.
Table 1. Photophysical and Electrochemical Properties of
Heterotriangulene Derivatives
photophyscial properties in CH2Cl2
redox potential
a
b
c
e
e
λabs
log ε
λem
E1/2
E1/2
red
(V)
d
compd
(nm)
(M−1 cm−1
)
(nm)
Φf
(Vo)x
f
2
301
377
397
471
530
797
416
449
490
517
569
i
h
h
3a
3b
7
4.36
4.43
4.50
4.26
4.57
g
g
g
0.40
0.39
0.20
1.02
0.30
−2.49
−2.15
−2.29
−1.49
−1.13
8
0.20
9
g
a
b
Only the longest absorption maxima are shown. Molar extinction
coefficient at the longest absorption maximum. Emission maxima.
Absolute fluorescence quantum yield determined by a calibrated
c
d
e
integrating sphere system within errors of 3%. Determined by cyclic
voltammetry with Bu4NPF6 (0.1 M) at a scan rate of 0.1 V s−1.
Potentials are given against the ferrocene/ferrocenium couple. The
first oxidation and reduction potentials in CH2Cl2 are shown, except
f
As a result, the dibenzo[c,g]fluorenylidene moieties act as the
strong electron-accepting moieties, resulting in the significantly
lower transition energy of 1.45 eV, consistent with the
experimentally observed NIR absorption band.
for the reduction potentials of 3a, 3b, and 7 in THF. Reference 6c.
Below 0.01. Irreversible redox waves were observed. Peak potentials
g
h
i
are shown. Not observed.
In the fluorescence spectra, while the diarylvinylidene-
substituted heterotriangulenes 3a, 3b, and 7 only show a faint
fluorescence, the dibenzo[c,g]fluorenylidene-substituted 9 does
not show any fluorescence. The weak or nonfluorescence
characteristics are likely due to the highly distorted structure of
the olefin moieties, which may accelerate the nonradiative decay
process from the lowest excited singlet state. In contrast, the 10-
ring-fused compound 8 with a rigid skeleton shows a relatively
intense yellow emission with the maximum wavelength of 569
nm in CH2Cl2. The fluorescence quantum yield (Φf) is 0.20. Its
fluorescence property depends on the solvents. While the
compound shows a very small Stokes shift of 246 cm−1 in
cyclohexane, it increases to 1692 cm−1 in DMF (Figure S6,
Supporting Information). The Lippert−Mataga plot of the
Stokes shift as a function of the solvent orientation polarizability
shows a good linear relationship. The dipole moment change
(Δμe−g) value between the ground state and the excited state is
13.6 D, while the calculated dipole moment in the ground state in
the gas phase is 5.76 D. The DFT calculation of 8 revealed that
while the HOMO is mainly delocalized on the dibenzochrysene
moiety, the LUMO has a major contribution from the
heterotriangulene with the dicarbonyl-substituted structure.
The intramolecular charge transfer character gives rise to the
Figure 3. UV−vis absorption spectra of 3a, 3b, 7, and 9 measured in
CH2Cl2. Inset: photographs of their CH2Cl2 solutions.
large dipole moment change Δμe−g
.
The other notable feature of the expanded heterotriangulene is
its multiredox properties. Their electrochemical properties were
investigated by cyclic voltammetry (Table 1). As the
representative compounds, the cyclic voltammograms of 3b, 8,
and 9 are shown in Figure 5, among which the latter two
compounds show reversible multistep redox waves for both the
oxidation and reduction. In comparison with the dinaphthylvi-
nylidene-substituted 3b, the ring closure to the dibenzofluor-
enylidene substructure in 9 results in the increased reversibility of
the oxidation process and, more importantly, the significant
positive shift of the reduction potentials. The half redox potential
for the first reduction is −1.13 V (vs Fc/Fc+), which is
comparable to that of C60 (−1.0 V vs Fc/Fc+),20 a representative
electron-accepting compound. The differential pulse voltamme-
try study demonstrated that the first reduction process involves
the two-electron reduction in one step. This compound can be
reduced up to a tetraanion with a high reversibility,
Figure 4. Energy levels of the Kohn−Sham HOMOs and LUMOs for 2,
3b, 9, and 8 with the transition energies and oscillator strengths
calculated at the B3LYP/6-31G* level.
the electron-withdrawing effect of the bridging moieties. In
contrast, the characteristics of the LUMOs for 3b and 2 are
totally different from each other, the former of which is mainly
C
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Letter
ACKNOWLEDGMENTS
■
This work was supported by CREST, JST. C.-M.C. appreciates
support of postdoctoral fellowships from the Japan Society for
the Promotion of Science (JSPS).
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ASSOCIATED CONTENT
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Experimental procedures, characterization data, and X-ray crystal
structures (CIF) of 3a (CCDC-990662), 7 (CCDC-990663),
and 9 (CCDC-990918); photophysical properties, cyclic
voltammograms, and theoretical calculations for all new
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yamaguchi@chem.nagoya-u.ac.jp.
Notes
The authors declare no competing financial interest.
D
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