Synthesis and Properties of Benzo-Fused Indeno[2,1-c]fluorenes

Article abstract of DOI:10.1002/asia.201801684

A set of fully-conjugated indenofluorenes has been synthesized and confirmed by solid-state structure analysis. The indeno[2,1-c]fluorenes and their benzo-fused analogues all contain the antiaromatic as-indacene core. The molecules possess high electron affinities and show a broad absorption that reaches into the near-IR region of the electromagnetic spectrum. All of the featured compounds reversibly accept up to two electrons as revealed by cyclic voltammetry. Analysis of molecule tropicity using NICS-XY scan calculations shows that, while the as-indacene core is less paratropic than s-indacene, benz[a]-annulation further reduces the antiaromaticity of the core. Antiaromatic strength of the as-indacene core can also be tuned by the position of fusion of additional arenes on the outer rings.

Full text of DOI:10.1002/asia.201801684

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Synthesis and Properties of Benzo-Fused Indeno[2,1-c]fluorenes
Authors: Michael Mark Haley, Tanguy Jousselin-Oba, Parker E. Deal,
Aaron G. Fix, Conerd K. Frederickson, Chris L. Vonnegut,
Abderrahim Yassar, Lev N. Zakharov, and Michel Frigoli
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To be cited as: Chem. Asian J. 10.1002/asia.201801684
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10.1002/asia.201801684
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Synthesis and Properties of Benzo-Fused Indeno[2,1-c]fluorenes
Tanguy Jousselin-Oba,[a] Parker E. Deal,[b] Aaron G. Fix,[b] Conerd K. Frederickson,[b] Chris L.
Vonnegut,[b] Abderrahim Yassar,[c] Lev N. Zakharov,[d] Michel Frigoli,*[a] and Michael M. Haley*[b]
Dedicated to Prof. François Diederich on the occasion of his retirement from ETH Zürich.
Abstract: A new set of fully-conjugated indenofluorenes has been
synthesized and confirmed by solid-state structure analysis. The
indeno[2,1-c]fluorenes and their benzo-fused analogues all contain
the antiaromatic as-indacene core. The molecules possess high
electron affinities and show a broad absorption that reaches into the
near-IR region of the electromagnetic spectrum. All of the featured
compounds reversibly accept up to two electrons as revealed by
cyclic voltammetry. Analysis of molecule tropicity using NICS-XY
scan calculations show that, while the as-indacene core is less
paratropic than s-indacene, benz[a]-annulation further reduces the
antiaromaticity of the core. Antiaromatic strength of the as-indacene
core can also be tuned by the position of fusion of additional arenes
on the outer rings.
directives of this research area has been to develop new OSCs
that are solution-processible, possess robust operational stability
to common environmental contaminants/doping, are relatively
cheap and easy to manufacture, are easily functionalized/tuned
in the final synthetic step(s), etc.[2,3] Acenes and other related
polycyclic aromatic hydrocarbons (PAHs) have been studied
intensively in this regard.[4] Of particular interest in the search for
organic electronic materials are the cyclopenta-fused PAHs[5]
(CP-PAHs, e.g., fullerenes,[6] buckybowls,[7] etc.), as the five-
membered ring(s) within these compounds impart a higher
electron affinity because of the driving force to aromatize to a
cyclopentadienyl anion by accepting an electron.[5]
Introduction
Organic semiconductors (OSCs) have been the subject of
intense academic and industrial research for several decades,
comprising one of the most significant areas of study in
materials chemistry. Such allure is to be expected as
semiconductors are fundamental to the construction of electronic
devices now ubiquitous in modern society.[1] With a drive to
make devices that we utilize every day more energy-efficient
and user friendly, there has been significant research effort
focused on developing organic electronic materials for niche
applications, such as wearable devices and flexible display
technologies that will consume a fraction of the energy of those
in use today.[2] To make this transition from the research
laboratory to commercial application feasible, one of the main
1
2
indeno[1,2-a]fluorene
indeno[1,2-b]fluorene
3
4
5
indeno[2,1-a]fluorene
indeno[2,1-b]fluorene indeno[2,1-c]fluorene
Figure 1. The five possible indenofluorene regioisomers 1–5.
Our work within this class of molecules has focused mainly
on the fully conjugated indenofluorenes (IFs),[8] a family of
cyclopenta-fused hydrocarbons comprised of five structural
isomers (1–5, Figure 1).[9] Although known since 1957,[10] these
π-electron-rich compounds were essentially ignored over the
subsequent 50+ years.[11] In 2011 we reported the synthesis and
structural characterization of stable ethynylated derivatives of
[1,2-b]IF (2)[12] and showed that such molecules could be used
as OSCs in ambipolar OFETs.[13] This area has since seen
tremendous growth with 150+ published studies on
indenofluorenes and related diindenoacene/diarenoindacene
topologies. All five regioisomers are now known, culminating in
2017 with the identification of the [1,2-a]IF scaffold (1).[14] Like
the [2,1-b]IF (4) derivative prepared by Tobe in 2013,[15] IF 1
exhibits a high degree of diradical character, making it (and 4)
rather unstable. On the other hand, IF regioisomers 2, 3, and 5
possess considerably reduced diradical character; thus, stable
derivatives can be prepared by inclusion of bulky and/or
electron-withdrawing groups on the apical carbons of the five-
membered rings, i.e., the sites of greatest spin density.[8]
[a]
[b]
T. Jousselin-Oba, Dr. M. Frigoli
UMR CNRS 8180, Institut Lavoisier de Versailles
UVSQ, Université Paris-Saclay
45 Avenue des Etats-Unis, 78035 Versailles Cedex, France
E-mail: michel.frigoli@uvsq.fr
P. E. Deal, Dr. A. G. Fix, Dr. C. K. Frederickson, Dr. C. L. Vonnegut,
Prof. M. M. Haley
Department of Chemistry & Biochemistry and Materials Science
Institute
1253 University of Oregon
Eugene, Oregon 97403-1253, United States
E-mail: haley@uoregon.edu
[c]
[d]
Dr. A. Yassar
UMR CNRS 7647, LPICM-École Polytechnique
91128 Palaiseau Cedex, France
Dr. L. N. Zakharov
CAMCOR – Center for Advanced Materials Characterization in
Oregon
University of Oregon
Eugene, Oregon 97403-1433, United States
Supporting information for this article is given via a link at the end of
the document.
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CO2Me
Bpin
We have been pursuing two complementary strategies for
tailoring the electronic properties of our CP-PAHs, namely (1)
elongation of the quinoidal central core while maintaining the
same fused outer ring[16] and (2) variation of the fused outer ring
while retaining the same core motif.[17] This latter strategy is the
easier of the two as synthesis of the requisite dione precursors
is in general relatively straightforward. While a majority of the
reported studies have focused on [1,2-b]IF 2 and other closely
related D2h-symmetrical diindenoacenes, we disclosed the initial
foray into the C2v-symmetric [2,1-c]IF scaffold in 2013.[18] Given
our recent study on benzo-fused [1,2-b]IFs,[17c] we decided to
reinvestigate this particular regioisomer. Herein we report the
synthesis and characterization of the new benzo-fused
congeners 6-8 and compare their optical and electronic
properties with previously reported derivatives 5a-5c.
12
Pd2(dba)3
S-Phos
MeO2C
Br
TfOH
K3PO4, PhMe
H2O, 100 °C
ClCH2CH2Cl
80 °C
OTf
MeO2C
11
13 (76%)
5a, R = CCSii-Pr3 (46%)
5b, R = 3,5-(CF3)2C6H3 (39%)
5c, R = 2,4,6-Me3C6H2 (49%)
1. RLi, THF
2. H2O
O
O
3. SnCl2, PhMe
(TFA)
6a, R = CCSii-Pr3 (60%)
6b, R = 3,5-(CF3)2C6H3 (22%)
r.t.–80 °C
R
R
9 (no ring fusion)
10 (78%, benzo-fused)
i-Pr3Si
Sii-Pr3
Scheme 1. Synthesis of Indeno[2,1-c]fluorenes 5a-c and 6a-b.
5a, R = CCSii-Pr3
5b, R = 3,5-(CF3)2C6H3
5c, R = 2,4,6-Me3C6H2
The synthesis of unknown diones 14 and 15, the precursors
of IFs 7 and 8, respectively, also followed the “outside-in”
Friedel-Crafts pathway. Starting again from 11, Suzuki cross-
coupling with ester 16[24] afforded diester 17, which underwent
Friedel-Crafts acylation using TfOH to furnish 14 (Scheme 2).
Addition of lithium (triisopropylsilyl)acetylide and subsequent
reductive dearomatization with SnCl2 gave the fully conjugated
[2,1-c]IF 7 in good overall yield.
7
i-Pr3Si
Sii-Pr3
R
R
6a, R = CCSii-Pr3
6b, R = 3,5-(CF3)2C6H3
8
CO2Me
Figure 2. Indeno[2,1-c]fluorene derivatives described in this study.
MeO2C
Bpin
16
Br
Pd2(dba)3, S-Phos
Results and Discussion
K3PO4, PhMe
H2O, 100 °C
OTf
Synthesis. Diones 9[19] and 10,[20] the precursors of 5 and 6,
respectively, can be prepared via the previously reported
pathway of Diels-Alder cycloaddition of the requisite
cyclopentadienone followed by cheletropic elemination of CO
and then “inside-out” Friedel-Crafts acylation onto the outer
benzenes. Alternatively, we explored the “outside-in” Friedel-
Crafts route as well to avoid the sometimes-difficult task of
cyclopentadienone formation. As shown in Scheme 1, Suzuki
cross-coupling of naphthalene 11[21] to ester 12 furnished diester
13, which could be cyclized with TfOH[22] to give dione 10.
Addition of the lithiated nucleophile to either 9 or 10 followed by
SnCl2-mediated reductive dearomatization[23] of the central
arene afford the desired [2,1-c]IF derivatives as green-black
solids. Whereas the SnCl2 reaction proceeded smoothly at room
temperature for 5a and 6a, the addition of catalytic TFA with
moderate heating (50-80 °C) was necessary for the successful
generation of the arylated systems. It should be noted that we
did try to prepare 6c in analogy to 5c; however, we were unable
11
MeO2C
17 (85%)
O
1. LiCCSii-Pr3
THF
2. H2O
TfOH
7 (73%)
ClCH2CH2Cl
80 °C
3. SnCl2
PhMe
r.t.
O
14 (82%)
Scheme 2. Synthesis of Indeno[2,1-c]fluorene 7 via Dione 14.
To our surprise, attempts to perform the analogous double
Suzuki cross-coupling on 11 using isomeric ester 18[25] were not
successful; thus, a revised synthesis is shown in Scheme 3
where each indene unit is introduced in a stepwise manner.
to perform
a successful reductive dearomatization on the
intermediate diol.
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CO2Me
Bpin
O
18
MeO2C
Br
1.5 equiv.TfOH
Py•HCl
Tf2O, Py
Pd2(dba)3, S-Phos
K3PO4, PhMe
H2O, 100 °C
ClCH2CH2Cl
r.t.
µW, 225 °C
CH2Cl2
r.t
OMe
19
OMe
20 (54%)
OMe
21 (95%)
O
1. LiCCSii-Pr3
THF
O
18
O
2. H2O
Pd2(dba)3, S-Phos
10 equiv.TfOH
8 (62%)
K3PO4, PhMe
H2O, 100 °C
3. SnCl2
PhMe
r.t.
ClCH2CH2Cl
80 °C
OTf
MeO2C
O
22 (88%)
Scheme 3. Synthesis of Indeno[2,1-c]fluorene 8 via Dione 15.
23 (80%)
15 (89%)
Cross-coupling 18 to 2-bromo-3-methoxynaphthalene (19) gave
20, which could be cyclized with TfOH to furnish ketone 21.
Demethylation with Py•HCl and then triflate formation provided
22. A second Suzuki cross-coupling with 18 and again Friedel-
Crafts acylation of 23 using TfOH gratifyingly furnished dione 15.
Addition of lithium (triisopropylsilyl)acetylide and subsequent
reductive dearomatization with SnCl2 gave the fully conjugated
[2,1-c]IF 8 in good overall yield. Unfortunately, attempts to
prepare the third naphtho-fused regioisomer were unsuccessful,
likely because of severe steric crowding. Similarly, attempts to
prepare the analogue of 14 minus the benz[a]-fusion afforded a
mixed 5-6-7-membered ring product and not the desired dione
(Scheme S1 in the Supporting Information).
which lengthens by ~0.06 Å from 1.355-1.359 Å in 5c to 1.418 Å
in 6a, the latter value typical of the fused arene bond in adjoining
rings, cf. bond g. Bond b lengthens a more modest ~0.015-0.025
Å because of its (now) benzylic nature, whereas nearly all of the
remaining bond lengths are essentially the same between the
two structures (i.e., a benzo-fused biindenylidene). To minimize
steric interactions with the bulky TIPS groups, the triple bonds
bend away from the ‘bay’ region C–H units ca. 1.4-6.3° from the
preferred linearity of sp-hybridized carbon atoms.
X-ray analysis. Single crystals of 6a suitable for X-ray
diffraction were obtained via slow crystallization from toluene.
The resultant crystal structure (Figure 3) shows that the
molecule possesses a slight helicene-like axial chirality, with
equal occupancy between the M- and P-isomers.[26] The dihedral
angles between the average planes of the central six-membered
ring, the bridging five-membered ring, and the terminal six-
membered ring of 6a are 5.5/7.0° and 6.2/7.6°, respectively,
values that are slightly larger than those found in the X-ray
structure of 5c (P-isomer: 2.3/4.3° and 2.0/2.4°; M-isomer:
4.8/6.6° and 4.7/3.7°).[18] Nonetheless, the barrier to inversion is
extremely small as no appreciable line broadening is observed
in the proton NMR spectrum upon cooling. The molecules pack
as essentially isolated molecules (i.e., no cofacial stacking) with
only a single close intermolecular C–C contact of 3.42 Å.
Comparison of the bond lengths in 6a with those in the two
independent molecules of 5a clearly shows the effect of benz[a]-
fusion to the [2,1-c]IF core (Table 1). Most affected is bond a,
Figure 3. Side and top view of the X-ray structure of 6a. Ellipsoids drawn at
the 30% probability level and hydrogens omitted for clarity.
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Figure 4. Electronic absorption spectra of (left) 5a-c and 6a-b in CH2Cl2 and (right) 6a, 7, and 8 in CHCl3.
weak low-energy bands in the [2,1-c]IF derivatives are similarly
observed in the spectrum of 11,12-dimesityl[2,1-a]IF (based on
3),[27] which also possesses the as-indacene core. This is in
contrast to the spectra of [1,2-b]IFs, which are based on the s-
indacene skeleton, as the λmax of the lowest energy peak of the
derivatives of 2 to date has been less than 615 nm.[12,13]
Calculations revealed that the source of the low energy bands in
the C2v-symmetric [2,1-c]IF core is an allowed HOMO"LUMO
(π"π*) transition, whereas this same transition is forbidden in
the D2h-symmetric [1,2-b]IF skeleton.[28]
Table 1. Select Bond Lengths (Å) of [2,1-c]IFs 5c and 6a.
a
b
b'
h'
R
R
e
e'
c
c'
f
f'
h
d
g
g'
bond
a
5c-P[a]
5c-M[a]
6a
Comparison of the data for 5 and 6 readily show the effect of
benz[a]-annulation, in that the absorption bands blue-shift by
roughly 15-30 nm (Figure 4, Table 2) as do the absorption cut-
offs up to 50 nm, indicating decreased conjugation within the as-
indacene core. On the other hand, analogous benzannulation of
the [2,1-a]IF scaffold produces a 250 nm red-shift between the
spectra (800"1050 nm); however, unlike 6 where the x-ray data
show the extra ring to be a fused benzene (vide supra), the x-ray
data of the benz[c]indeno[2,1-a]fluorene derivative indicate the
new ring to be quinoidal and thus the core is highly
conjugated.[28] Within the ethynylated benz[a]-series of 6a, 7 and
8, linear fusion in 7 with two additional benzene rings further
blue-shifts the bands, with the absorption cut-off almost 100 nm
shorter, whereas angular fusion in 8 affords a 250 nm red-shift in
its spectrum (800"1050 nm). These latter results are in contrast
to the same experiments in the naphtho-fused s-indacenes,
where the additional two benzene rings always resulted in a red-
shift of the absorption bands regardless of the mode of ring
fusion.[17c] Nonetheless, as noted above, the low energy bands
observed for the [2,1-c]IF core are forbidden for the [1,2-b]IFs;
therefore, direct comparison of the UV-vis results between the
various IF skeletons may not entirely be valid. Varying solvent
polarity in the UV-Vis experiments showed no appreciable
changes, thus indicating negligible intramolecular charge
transfer character. Similar to other fully conjugated IFs,
compounds 5-8 are non-emissive due to rapid non-radiative
decay via a conical intersection.[29]
1.359(2)
1.428(2)
1.434(2)
1.472(2)
1.471(2)
1.367(2)
1.371(2)
1.371(2)
1.468(2)
1.464(2)
1.414(2)
1.415(2)
1.482(2)
1.483(2)
1.355(2)
1.439(2)
1.433(2)
1.468(2)
1.469(2)
1.364(2)
1.370(2)
1.377(2)
1.464(2)
1.461(2)
1.413(2)
1.419(2)
1.474(2)
1.475(2)
1.418(6)
1.452(6)
1.460(5)
1.462(5)
1.462(5)
1.364(6)
1.384(5)
1.376(5)
1.471(2)
1.471(2)
1.413(2)
1.413(2)
1.469(3)
1.469(3)
b
b'
c
c'
d
e
e'
f
f'
g
g'
h
h'
[a] Reference [18].
Electronic absorption data. The UV-vis spectra of 5a-c, 6a-b,
7 and 8 (Figure 4) display strong absorbances in the higher
energy range and a broad low-energy absorbance stretching out
to the near-IR region (550-1050 nm). It is noteworthy that the
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6a
7a
8
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential (V)
Figure 5. Cyclic voltammograms of (left) 5a-c and 6a-b and (right) 6a, 7, and 8 in CH2Cl2.
Table 2. Summary of optical and electrochemical data of compounds 5–8
electrochemical
optical
Cmpd
5a[a]
5b[a]
5c[a]
6a[a]
6b[a]
Ered1 (V)
–0.66
–0.71
–1.05
–0.63
–0.82
Ered2 (V)
–1.11
–1.02
–1.51
–1.06
–1.08
Eox (V)
1.07
ELUMO (eV)
–3.98
EHOMO (eV)
–5.71
—–
Egap (eV)
λmax (nm)
Egap (eV)[c]
1.49
1.73
—–
360, 380, 451, 480, 647
348, 365, 441, 465, 627
345, 363, 421, 447, 603
347, 364, 438, 465, 615
328, 419, 440, 571
—- [d]
1.02
–3.93
1.60
–3.59
–5.66
–5.89
—–
2.08
1.87
—–
1.54
1.25
–4.01
1.57
—- [d]
–3.82
1.68
6a[b]
7[b]
–1.16
–1.27
–0.96
–1.58
–1.64
–1.35
0.73
0.56
0.59
–3.64
–3.53
–3.84
–5.53
–5.36
–5.39
1.89
1.83
1.55
347, 364, 438, 465, 615
1.57
1.78
1.24
331, 387, 411, 437, 606, 654
320, 348, 468, 498, 764
8[b]
[a] CV recorded in Oregon using 1-5 mM of analyte in 0.1 M Bu4NOTf/CH2Cl2 using a scan rate of 50 mV s–1. The working electrode was a glassy carbon
electrode with a Pt coil counter electrode and Ag wire pseudoreference. Values reported as the half-wave potential (vs SCE) using the Fc/Fc+ couple (0.46 V)
as an internal standard; see reference [30]. Energy levels were approximated by ELUMO = –(4.68 + Ered1) and EHOMO = –(4.68 + Eox1); see reference [31]. [b] CV
recorded in France using 1-2 mM of analyte in 0.1 M Bu4NPF6/CH2Cl2 using a scan rate of 100 mV s–1. The working electrode was a glassy carbon electrode
with a Pt coil counter electrode and Ag/AgCl (EtOH) as pseudo reference electrode. Values reported as the half-wave potential (vs Fc/Fc+) using the Fc/Fc+
couple (0.51 V) as an internal standard. Energy levels determined by ELUMO = –(4.8 + Ered1) and EHOMO = –(4.8 + EOx1). [c] The optical HOMO-LUMO energy gap
was determined as the intersection of the x-axis and a tangent line that passes through the inflection point of the lowest energy absorption. [d] Reversible
oxidation was not achieved.
Electrochemistry. Cyclic voltammograms (CV) of molecules
5-8 are shown in Figure 5 and the results compiled in Table 2.
The CV data indicate that like the [1,2-b]IFs (2) and [2,1-a]IFs
(3), the family of [2,1-c]IFs is electron deficient. The compounds
can reversibly accept up to two electrons and possess LUMO
energies estimated at –3.5 to –4.0 eV, analogous to those found
in derivatives of 2;[12b,13a] however, the HOMO energies of 5 are
roughly 0.15 eV higher, thus leading to reduced gap energies
compared to their identically substituted analogues of 2 (e.g.,
1.89 eV for 2a vs 1.73 eV for 5a).[12b] Benz[a]-fusion appears to
lower the HOMO energy level back close to 2 such that energy
gaps are nearly identical (e.g., 1.89 eV for 2a vs. 1.87 eV for 6a).
Unlike the absorption data, inclusion of the two additional
benzene rings in 7 and 8 lowers the energy gap to 1.83 and 1.55
eV, respectively, regardless of the mode of ring fusion and thus
the CV results closely parallel those obtained for the naphtho-
fused s-indacene congeners (energy gap of 1.75 and 1.57 eV,
respectively).[17c]
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uncontaminated by proximity to other parts of the molecule, and
mirror the results. The resultant plots (Figure 7) resemble those
of the [1,2-b]IF derivatives.[17c] The NICSπZZ values for each ring
are summarized in Table 3.
C
A
B
A
B
A
B
as-indacene
s-indacene
2'
Table 3. Summary of NICS1.7πZZ values
B
B
E
A
C
A
Ring[a]
3'
B
cmpd
as-ind
s-ind
2’
A
14
23
5
B
C
D
E
F
C
A
23
26
7
D
C
7'
5'
–9
E
A
E
A
3’
6
10
9
–8
B
5’
3
–10
–11
–12
–12
–12
–13
B
C
D
6’
1
7
–13
–13
–11
–14
–11
C
8'
6'
7’
0
4
–16
–15
E
A
8’
4
11
6
F
E
A
24’
25’
1
–16
B
4
12
–16
C
B
[a] Ring assignments shown in Figure 6.
D
C
As shown in Figure 7a, the NICS-XY scans reveal that as-
indacene (teal) is less antiaromatic than s-indacene (fuchsia),
especially in the central 6-membered ring (14 vs. 23 ppm, ring A
in Table 3). Nonetheless, fusion of the outer benzenes (ring C)
to create the indenofluorene skeleton essentially negates this
difference, as the chemical shift values for the ghost atoms now
vary less than 2.5 ppm, i.e., all three IF isomers are comparable
in their moderate-to-weak paratropicity and moderately strong
diatropicity in the indacene core and outer benzenes,
respectively. Fusion of a third arene in the benz[a]-position of 5’
(ring E) further reduces indacene paratropicity and slightly
increases benzene ring C diatropicity (Figure 7b) in 6’ and 24’
by ca. 2-3 ppm. Performing the NICS-XY scan along the axis of
benz[a]-fusion (Figure 7c) illustrates that there is little difference
in the central 6-membered ring A of the indacene as one
changes from benzo- (6’) to naphtho-fusion (24’). 1,2-Naphtho-
fusion (e.g., 8’, 25’) does increase indacene paratropicity by ca.
3 ppm; nonetheless, rings C and D become progressively more
aromatic, which is reflected in the chemical shift values in 7’, 8’
and 25’ (Figure 7d) from –11/–13 ppm to –14/–16 ppm.
Analogous to the [1,2-b]IF congeners,[17c] these naphtho-fused
derivatives show the same trend within the indacene core:
derivatives having a lower bond order of ring fusion (e.g., 7’,
bond order of 1.33) display reduced internal paratropicity
strength, whereas derivatives having a higher bond order of ring
fusion (e.g., 8’ and 25’, bond order of 1.66) exhibit increased
internal paratropicity strength, when both are compared to 6
(bond order of 1.5). As might be expected based on our DNI
studies (vide supra),[17c] compound 8 is easier to reduce than 7
because of its larger paratropicity.
25'
24'
Figure 6. Structures for comparison in the NICS-XY scan computations;
lettering corresponds to ring assignments in Figure 7 and Table 3.
NICS-XY Scan Calculations. To compare the effects of
benzo-fusion on the aromaticity/antiaromaticity of the
indeno[2,1-c]fluorene skeleton, as well as to compare the three
closed-shell IF regioisomers, we performed nucleus independent
chemical shift (NICS) computations on the structures shown in
Figure 6 using the NICS-XY scan and π-only methods.[32] Parent
versions of each compound without the mesityl or alkynyl groups
on the five-membered rings were used to simplify comparisons
as well as make the most efficient use of computational
resources; prior results have also shown that inclusion of these
groups in the computed structures has minimal effect on the
outcome of the NICS-XY scans.[16c,17c] Our initial assessment led
to some surprising results, in that the NICS-XY scans were
lopsided. Although not too evident in the scans for 5’, 6’ and 24’,
the results for 7’, 8’ and especially 25’ were noticeably
misshapen (see Supporting Information, Figure S1). As noted
earlier, the [2,1-c]IF geometry is helical, and this helicity is
exacerbated in 25’, where the ghost atom at the end of the
NICS-XY scan ends up essentially underneath the top ring
(Figure S2), thus making it difficult to parse out the actual
NICSπZZ value for that ring. Despite their helicity, these
molecules still are symmetric; therefore, we can create a good
approximation of the full graph by taking the first half of the scan
where the points are on top of the molecule, i.e.,
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Figure 7. NICSπZZ-XY scans of a) as- and s-indacene and IFs 2’, 3’ and 5’, b) IFs 5’, 6’ and 24’ along the pentacyclic core, c) IFs 6’-8’ and 24’-25’ along the axis
of benz[a]-fusion, and d) IFs 7’, 8’ and 25’ along the heptacyclic core. The vertical lines correspond to the ring centers labeled in Figure 6.
Conclusions
Acknowledgements
Benzo-fused analogues of the indeno[2,1-c]fluorene skeleton
containing the as-indacene core have been synthesized. The
addition of the extra benzene ring enables the variation of the
annelation mode and has been achieved by two different routes.
The benzo[a]-fusion reduces slightly the energy band gap
without altering the high electron affinity of the π-system
(comparison 5a/6a). In contrast to the indeno[1,2-b]fluorene
family, the absorption behavior of the [2,1-c]IFs displays allowed
low-energy transitions in the near-IR region and the electron
acceptability increases with increasing paratropicity of the
system. Remarkably, dinaphtho-as-indacene 8 has the highest
electron affinity, an absorption maximum at 762 nm with a
lambda edge up to 1050 nm, and could behave as an ambipolar
semiconductor. Among all diacenoindacenes with a closed-shell
configuration in the ground state synthesized to date,[8]
compound 8 is unique to surpass 1000 nm of the near-IR region.
Future studies will examine other modes of annelation upon the
indeno[2,1-c]fluorene scaffold.
We thank the National Science Foundation (CHE-1565780) to
M.M.H. and Agence nationale de la recherché (ANR-16-CE07-
0024 GATE) to M.F. for support of this research. Mass
spectrometry capabilities in the CAMCOR facility are supported
by the NSF (CHE-1625529).
Keywords: indenofluorenes • diacenoindacenes • Friedel-Crafts
acylation • antiaromaticity • NICS calculations
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Entry for the Table of Contents
FULL PAPER
Containing an antiaromatic as-
indacene as the core motif, a family
of fully-conjugated indeno[2,1-
c]fluorenes and related benzo-fused
analogues has been prepared and
fully characterized. The molecules
possess high electron affinity and
show a broad absorption that
reaches into the near-IR region. The
tropicity of the molecules has been
probed by NICS-XY scan
Tanguy Jousselin-Oba, Parker E. Deal,
Aaron G. Fix, Conerd K. Frederickson,
Chris L. Vonnegut, Abderrahim Yassar,
Lev N. Zakharov, Michel Frigoli,* and
Michael M. Haley*
Increasing paratropicity and electron affinity
R
R
R
R
R
R
R = CCSii-Pr3
Page No. – Page No.
Synthesis and Properties of Benzo-
Fused Indeno[2,1-c]fluorenes
calculations.
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