π-Extended Indenofluorenes

Article abstract of DOI:10.1002/chem.201406646

A series of π-extended aromatic indenofluorene (IF) analogues with naphthalene and anthracene cores have been synthesized through acid-catalyzed intramolecular cyclization. The regioselectivity of the reaction is controlled by a combination of steric and

Full text of DOI:10.1002/chem.201406646

DOI: 10.1002/chem.201406646
Full Paper
&
Indenofluorenes
p-Extended Indenofluorenes
M. Rajeswara Rao, Antonin Desmecht, and Dmitrii F. Perepichka*[a]
Abstract: A series of p-extended aromatic indenofluorene
(IF) analogues with naphthalene and anthracene cores have
been synthesized through acid-catalyzed intramolecular cyc-
lization. The regioselectivity of the reaction is controlled by
a combination of steric and electronic factors and in some
cases several possible regioisomers have resulted from the
same precursor. The effects of ring connectivity on the opto-
electronic properties were investigated by DFT calculations,
absorption/emission spectroscopy, cyclic voltammetry, and
spectroelectrochemical studies. All regioisomers exhibited
a redshift of their absorption/emission bands relative to the
parent IF analogues, but the magnitude of this shift and
other optoelectronic properties (luminescence quantum
yield, etc.) depends on the ring connectivity in a less obvi-
ous manner.
a broad variety of applications, in blue OLEDs,[10] organic field-
effect transistors,[11] and organic solar cells.[12] Varying the con-
nectivity of the benzene rings in IF isomers a1–a5 allows for
tuning of their bandgap and triplet energy, thus affording
highly efficient electroluminescent molecular materials and
high triplet energy hosts for phosphorescent OLEDs.[13,14]
Introduction
For the past two decades the interest in structure–property re-
lationships in p-conjugated systems has been fueled by their
applications in various optoelectronic devices.[1] Fluorene,
a rigid planar aromatic hydrocarbon, and its numerous deriva-
tives[2] have gained a lot of attention in optoelectronics due to
their high stability, efficient light emission, and reasonable
charge-transport properties. Polyfluorenes are considered
bread-and-butter materials for blue organic light-emitting
diodes (OLEDs)[3] and solid-state lasers.[4] Also, fluorene-based
copolymers have found numerous applications in thin-film
transistors[5] and solar cells.[6]
A number of other structurally modified IFs have been ex-
ploited[15–17] with the goal of tailoring the electronic and optical
characteristics of thin-film devices. The p conjugation of the IF
core was further extended to the peripheral substituents
(b2[15]) and by annulation of terminal benzene rings.[18] Truxene
b3, a C3-symmetric analogue of IF, has become a highly popu-
lar core for star-shaped conjugated oligomers, with myriad ap-
plications.[19–21] Linearly extended fully fused IFs b4 and b5
have also been studied, as well-defined molecular analogues
of LPPP b1.[16] Despite the apparent variety of the IF-type struc-
tures, their properties are not dramatically different, which can
be explained by large aromatic stabilization of their benzene
ring constituents, and thus limited p-electron delocalization in
the ground state. For example, most of the emission of IFs a1–
a5 falls in the UV range of the spectrum. This “electron con-
finement” effect can be alleviated by replacing the sp3-hybrid-
ized bridge of dihydroindenofluorene with an sp2 carbon
atom, as was demonstrated for compounds b6[22] and b7.[23]
The quinoidalization of the central benzene ring and formally
antiaromatic character renders these molecules very different
electronic properties, including dramatically lowered HOMO–
LUMO gap and ambipolar charge transport. Less is known
about the structural effects of other aromatic rings in the IF
motif,[16a,18,24,25] although such extension of the p-electron
system would allow widening of the range of optoelectronic
properties while maintaining all the advantages of the rigid IF-
type structure. Of close relevance to the present paper, several
analogues of LPPP polymer with a naphthalene moiety in the
main chain have been described.[25b] Also, the diphenylanthra-
cene derivative b8 with planarizing sp3 bridges was reported.[25a]
The electronic and optical properties of fluorene principally
arise from its rigidity where an sp3 bridge carbon atom “locks”
the two benzene rings in a planar structure. However, free ro-
tation around a CÀC bond between fluorene units in corre-
sponding polymers leads to a relatively flexible structure (with
possible nonplanar conformations). To extend an oligo/poly-
phenylene p-electron structure without compromising the ri-
gidity of the formed material, sp3 bridge atoms should be in-
troduced between each neighboring aromatic ring, as exempli-
fied by indenofluorene (IF) a1 or ladder poly(p-phenylene)
(LPPP; b1).[7] An efficient p conjugation in a fully coplanar LPPP
backbone leads to a number of interesting properties includ-
ing increased photoluminescence efficiency[8] and charge mo-
bility.[9] The dihydroindeno[1,2-b]fluorene core (a1) was shown
to be a promising building block for conjugated polymers for
[a] Dr. M. R. Rao, A. Desmecht, Prof. D. F. Perepichka
Department of Chemistry and Centre for Self-Assembled
Chemical Structures
McGill University
Montreal, QC H3A 0B8 (Canada)
E-mail: dmitrii.perepichka@mcgill.ca
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406646.
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the optoelectronic properties of
these polycyclic aromatic hydro-
carbons.
Results and Discussion
Synthesis
We identified naphthalene/an-
thracene triflates 1a–d as the
key intermediates that provide
access to the entire series of de-
signed isomers (Scheme 1). The
former were obtained in good
yields (65–80%) by reaction of
the corresponding diols with
triflic anhydride, in the presence
of triethylamine. The precursor
naphthalenediols were commer-
cially available whereas anthra-
Herein, we report a series of new molecular IF analogues
based on naphthalene (BINs) and anthracene (BIAs) cores, with
various connectivities. Different regioisomers were achieved
through an expedient synthesis utilizing double electrophilic
cyclization of the substituted acene precursors. The optical,
electrochemical, and structural properties of the BIN/BIA com-
pounds were explored and compared with those of IF homo-
logues to understand the effect of p-electron topology on
cenediols were synthesized from the corresponding dihydrox-
yanthraquinones (see the Supporting Information). Suzuki–
Miyaura coupling of the triflates 1a–d with 2-(ethoxycarbonyl)-
phenylboronic acid gave intermediates 2a–d in 45–65% yields.
The diesters 2a–d were subjected to nucleophilic addition of
p-tolyllithium or p-trifluoromethylphenyllithium, thereby afford-
ing the tertiary alcohols 3a–d. Subsequent acid-catalyzed intra-
molecular cyclization of 3a–d in neat trifluoroacetic acid (TFA)
Scheme 1. Synthesis of 2,6-/1,5-naphthalene and anthracene indenofluorenes (BINs and BIAs). OTf=triflate.
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at room temperature afforded the target BINs and BIAs.
Screening of reaction temperatures, catalyst (HCl, TFA, and
BF3·Et2O), and concentration did not show a significant effect
either on the yield or the isomeric ratios. Statistically, the cycli-
zation on each side might occur through one of the two
carbon atoms of the acene unit neighboring the carbinol sub-
stituent, to yield three possible regioisomers. In contrast to the
previously reported synthesis of isomeric (not p-extended)
IFs,[15–17] intramolecular cyclization of 2,6-disubstituted diols 3a
and 3b led to almost exclusive formation of 1,5-cyclized sym-
X-ray crystal structure analysis
Single crystals of 1,5-N(6,6) and 1,5-A(5,5) were grown by
physical vapor transport at approximately 1508C and ambient
pressure, in a flow of argon, whereas slow evaporation of
chloroform solution was employed for 2,6-A-CH3. In the latter
case, the prolonged exposure to air and light (over 3 months)
led to the unexpected formation of the endoperoxide of 2,6-
A-CH3, which nevertheless proved the regiochemistry of this
isomer (Figure 1e,f). All three crystals are triclinic with the
1
metric isomers (2,6-N and 2,6-A), confirmed by H NMR spectra
of reaction mixtures. This could be attributed to the known
higher nucleophilicity of peri positions of naphthalene/anthra-
cene rings.[26] In contrast, the intramolecular cyclization of 1,5-
diols 3c and 3d led to the formation of several regioisomers.
Interestingly, naphthalene and anthracene diols showed a dif-
ferent regiocontrol. Although symmetric products have pre-
vailed in both reactions, naphthalene derivative 3c preferred
to form six-membered cycles, thus yielding two isomers 1,5-
N(6,6) and 1,5-N(5,6) in 60:40 ratio. In contrast, its anthracene
congener 3d produced 1,5-A(5,5), 1,5-A(6,6), and 1,5-A(5,6) in
95:3:2 ratio.[27] Symmetric (5,5)/(6,6) isomers are less soluble
than asymmetric (5,6) isomers, which allowed pure isomers to
be obtained by recrystallization. The combined yields of both
isolated isomers are in each case above 50%. Their structures
were ascertained by a combination of 1H NMR spectroscopy
and X-ray crystallography (see below).
The different isomeric ratios observed for naphthalene and
anthracene can be rationalized by two factors: 1) higher nucle-
ophilicity of the aromatic carbon atoms in the peri position
(which favors formation of 6,6-isomers) and 2) relative steric
strain of the isomers (which favors formation of less hindered
5,5-isomers). Density functional theory (DFT) calculations
showed that 1,5-N(6,6) is destabilized with respect to 1,5-
N(5,6) and 1,5-N(5,5) isomers by 0.8 and 1.6 kcalmolÀ1, respec-
tively, as a result of mild steric strain. A competition between
this steric strain (of the corresponding transition state), which
should be smaller for cyclization through five-membered rings,
and higher nucleophilicity of the peri positions in the naphtha-
lene (that favors cyclization through six-membered rings) ex-
plains why both (5,6) and (6,6) isomers are formed in similar
quantities (40:60). In the anthracene series, the steric strain is
much more pronounced: 1,5-A(6,6) is thermodynamically less
stable than 1,5-A(5,6) and 1,5-A(5,5) isomers by 11.3 and
21.6 kcalmolÀ1, respectively (see the Supporting Information).
Such large destabilization of the (6,6) and (5,6) isomers, which
should also be reflected in the corresponding transition states,
overshadows the difference in nucleophilicity and explains the
almost exclusive formation of the (5,5) isomer. A relevant inter-
play of electronic and steric factors on the isomeric ratio of IFs
a1/a2 through a similar electrophilic cyclization reaction has
been recently investigated in detail.[28]
Figure 1. Crystal structures of 1,5-N(6,6) (a,b), 1,5-A(5,5) (c,d), and 2,6-A-CH3
endoperoxide (e,f); (a), (c), and (e): top view and (b), (d), and (f): side view.
¯
space group P1. In 1,5-A(5,5) the p-conjugated core is nearly
planar (dihedral angle between the anthracene and terminal
benzene rings of 4.48; Figure 1c,d) whereas a substantial
“saddle-shaped” twist was noticed in 1,5-N(6,6) (19.88; Fig-
ure 1a,b). This can be attributed to steric clashes between the
tolyl substituents and peri hydrogen atoms of the naphthalene
core, and the relative flexibility of six-membered rings, which
allow minimization of the steric repulsions by distortion.
In the crystal of 1,5-A(5,5), the molecules are arranged in
one-dimensional slipped stacks with a very limited p overlap
(of three carbon atoms) of the terminal benzene rings and in-
terplanar distance of 3.74 ꢁ. In addition, there are a few weak
CÀH···p contacts between tolyl substituents of adjacent 1,5-
A(5,5) molecules with an interaction distance of approximately
3.2 ꢁ. Similarly, 1,5-N(6,6) also showed no p stacks in the crys-
tal packing with only weak p–p interactions taking place
through CÀH···p contacts (ꢀ3.2 ꢁ).
Electronic structure calculations
All synthesized compounds are readily soluble in common
organic solvents (CHCl3, THF, ether, etc.) except 1,5-A(5,5),
which was only soluble in hot chlorinated solvents (e.g., tetra-
chloroethane or trichlorobenzene).
The calculated (B3LYP/6-31G(d)) gas-phase structures resemble
closely those found in the solid state by X-ray analysis. The
conjugated backbone in all (5,5) isomers is almost planar (ꢀ2–
58) whereas (5,6) and (6,6) isomers possess significant twist dis-
tortion (12–228 and 19–308, respectively). The calculated bond
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Figure 2. DFT (B3LYP/6-31G(d)) calculated frontier molecular orbital energies and surface topologies of BINs and BIAs.
lengths in the central IF core
match the experimentally ob-
served values within 0.01 ꢁ, thus
reaffirming the accuracy of the
chosen theoretical level for
these molecular systems.
Table 1. Experimental redox and DFT calculated properties of BINs and BIAs.
Sample
E01ox [V][a] E02ox [V][a] HOMO[b] [eV] LUMO[c] [eV] DEop[d] [eV] Theoretical [eV]
DETh [eV]
HOMO
LUMO
2,6-N
2,6-A
1,5-N(5,6)
1,5-N(6,6)
1,5-A(5,5)[e] 0.62
0.83
0.64
0.72
0.81
1.28
1.19
1.21
1.24
–
À5.6
À5.4
À5.5
À5.6
À5.4
À5.3
À2.3
À2.6
À2.3
À2.5
À2.6
À2.7
3.27
2.81
3.15
3.09
2.76
2.61
À5.14
À4.89
À5.17
À5.15
À4.92
À4.85
À1.28
À1.72
À1.43
À1.45
À1.79
À1.82
3.86
3.17
3.74
3.70
3.13
3.03
The calculated frontier orbital
energies of all the BINs and BIAs
are summarized in Figure 2 and
Table 1. As expected, all the BIAs
showed higher HOMO and lower
LUMO energies relative to BINs
and IF, which led to a decreased
bandgap (DETh) in line with the
1,5-A(6,6)
0.49
1.07
[a] Redox potentials versus ferrocene (Fc), in 0.2m Bu4NPF6 in CH2Cl2. [b] Calculated from the half-wave oxida-
tion potentials using the equation HOMO=À(4.8 + E0 vs. Fc/Fc+). [c] Calculated as LUMO=[HOMO+DEop].
1ox
[d] Calculated from the red edge of the absorption spectra. [e] Recorded in 1,2-dicholoroethane.
Optical properties
extended p conjugation. Both HOMO and LUMO are spread
over the entire BIN/BIA backbone (Figure 2). Their topologies
are qualitatively similar for both series, regardless of the cycli-
zation direction. A noticeable contribution from the sp3 carbon
of six-membered ring containing (5,6) and (6,6) isomers to the
HOMO and LUMO can be attributed to a hyperconjugation
effect.
The absorption and emission spectra of BIN/BIA derivatives in
dilute CHCl3 solutions are shown in Figure 3 and the derived
photophysical data are summarized in Table 2, along with the
literature data for other IF derivatives. The absorption spectra
of these chromophores are substantially (11–110 nm) redshift-
ed relative to IF a1 (lmax =347 nm),[16e] which can be attributed
Table 2. Photophysical data of BINs and BIAs in CHCl3 in comparison with the literature data for IFs.
Sample
labs [nm]
lem [nm]
Stokes shift [cmÀ1
]
Ff[a]
tf [ns]
kr[b] [108 sÀ1
]
knr[c] [108 sÀ1
]
Radical cation labs [nm]
IF a1[16e][d]
IF a2[14a][e]
IF a3[14d][e]
IF a4[14d][e]
IF a5[14e][e]
BIA b8[25a][f]
2,6-N-CH3
2,6-A-CH3
1,5-N(5,6)
1,5-N(6,6)
1,5-A(5,5)[h]
1,5-A(6,6)
347
322
333
342
338
488
358, 372 sh
432
381
353
326
338
344
343
518
382
440
399
404
447
471
490
381
444
170
38
61
23
54
–
2.0
1.2
2.7
3.9
7.0
–
2.1
3.6
1.6
1.8
3.5
3.2
1.9
4.9
0.3
1.4
–
3.1
3.1
1.0
1.2
–
–
–
–
–
–
–
431
1186
700[g]
420
1184
1021
564
–
–
–
30
22
70
72
41
32
1.4
0.6
4.4
4.0
1.1
1.0
3.3
2.2
1.9
1.6
1.7
2.1
456, 494, 523, 803, 1093, 1284
575, 624, 683, 1056, 1234
434, 560, 593, 836, 1055, 1233
429, 532, 571, 857, 1084, 1258
poor solubility, not measured
631, 692, 766, 1092, 1268
388
436
460
508
[a] The relative quantum yields (F) were measured using standard procedures with reference to 9,10-diphenylanthracene in cyclohexane (F=0.9).[30]
[b] kr =Ff/tf. [c] knr =1/tÀkr’. [d] R=tolyl, measured in CH2Cl2. [e] R=spirofluorene, measured in THF. [f] R=Me and p-decylphenyl. [g] Assuming the
shoulder is a (0,0) transition, a nominal Stokes shift between the absorption/emission peak maxima is 1755 cmÀ1. [h] Measured in 1,1,2,2-tetrachloroethane.
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The optical gaps of BINs and BIAs deduced from the red
edge of absorption spectra are in the range of approximately
3.2–3.5 and 2.7–2.9 eV, respectively. The optical gap contrac-
tion by about 0.5 and 0.8 eV observed in BIAs relative to BINs
and IF, respectively, is brought about by a raised HOMO and
lowered LUMO (see above), a result of extended p conjuga-
tion.
All compounds exhibited intense blue to cyan fluorescence
with vibronically split bands at approximately 380–430 and
440–500 nm for BINs and BIAs, respectively. The Stokes shifts
are small, consistent with the planar and rigid structure of the
molecules. However, the observed trend of the Stokes shift:
BINs>BIAs~IF (490 cmÀ1) (Table 2) does not follow the exten-
sion of p conjugation. The most likely explanation lies in the
change of the exciton delocalization: a full delocalization be-
tween three benzene rings in IF, to partial localization of the
central naphthalene or anthracene core in BINs and BIAs, re-
spectively. The emission quantum yield (f) of BIN isomers (30–
f
72%) is consistently higher than that of BIAs (22–41%) and in
both series, the 1,5-substituted derivatives showed high quan-
tum yields (32–72%) relative to 2,6-congeners (22–30%). Anal-
ysis of radiation lifetimes suggests that both trends could be
attributed to faster radiative decay of the naphthalene versus
anthracene derivatives, and of 1,5- versus 2,6-isomers, although
the structural reason behind this is not immediately clear. The
nonradiative decay rates are similar for BIN/BIA derivatives, as
can be expected based on their highly rigid structure.
Figure 3. Absorption and emission spectra of a) BINs and b) BIAs.
Electrochemical properties
to the extension of p conjugation. Both BINs and BIAs display
characteristic vibronic splitting (ꢀ1400 cmÀ1), indicative of the
rigidity of their structure, in accordance with the previously re-
ported IFs.[15]
The electrochemical properties of all BINs and BIAs were mea-
sured in CH2Cl2 solution by cyclic voltammetry and the corre-
sponding data are summarized in Table 1. Figure 4 shows the
representative cyclic voltammograms of 1,5-N(5,6) and 1,5-
N(6,6). All BIN and BIA derivatives feature one single-electron
reversible oxidation wave, followed by a second, irreversible
multielectron oxidation. The first oxidation potential decreases
in the range IF (1.44 V vs. Ag/AgCl for a1;[16a] ca. 1.0 V vs. Fc/
Fc+) to BIN (ꢀ0.8 V vs. Fc/Fc+) to BIA (ꢀ0.6 V vs. Fc/Fc+), con-
sistent with the extension of p conjugation. From the half-
wave oxidation potentials, HOMO levels were determined. The
The connectivity of the terminal benzene rings with the
acene core has a significant effect on optical properties, in line
with the previous reports for IFs a1–a5. The absorption and
emission bands of 1,5-BINs/BIAs are redshifted relative to their
2,6-isomers by 4–30 nm, which suggests a more effective elec-
tronic communication through the acene moiety via 1,5-posi-
tions compared to 2,6-postions. Also, comparison with the
known analogue b8 suggests a further enhancement of conju-
gation through 9,10-positions.[25a] A similar effect was observed
by us recently for structurally related distyrylanthracene iso-
mers,[29] and was rationalized by smaller bond-length alterna-
tion along the 9,10- vs 1,5- vs 2,6-pathway. Appreciable elec-
tronic effects were also noticed in regioisomers of 1,5-BIN and
1,5-BIA with identical p-electron structure but different fusion
(five- versus six-membered rings). The absorption spectrum of
1,5-N(6,6) was redshifted relative to 1,5-N(5,6) by 10 nm, and
1,5-A(6,6) exhibited a greater redshift of 27 nm relative to 1,5-
A(5,5). This observation could most likely be attributed to
combined consequences of distortion of the acene ring and
a hyperconjugation effect (see above) in (5,6) and (6,6) isomers.
We note, however, that a blueshift was observed earlier for the
sterically congested IFs a2 and a3 compared with unhindered
a1 and a4 with the same p-electron structure (Table 2).[14a]
Figure 4. Cyclic voltammograms of 1,5-N(5,6) and 1,5-N(6,6) in 0.2m
Bu4NPF6 in CH2Cl2.
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estimated HOMO level is progressively raised from IF (ꢀ
À5.9 eV) to the BINs (À5.5–À5.6 eV) and BIAs (ꢀÀ5.3–
À5.4 eV), which might be useful for reducing the hole injection
barrier in organic electronic devices.
Due to the large HOMO–LUMO gap of these compounds, no
reduction was observed within the electrochemical window of
the solvent (À2.3–+1.7 V vs. Fc/Fc+). The LUMO energies
were thus approximated by adding the optical gap to the elec-
trochemically determined HOMO level. Comparing these exper-
imental HOMO and LUMO values with the calculated ones sug-
gests that DFT overestimates the energy of frontier orbitals by
approximately 0.4 eV for the HOMO and approximately 0.9 eV
for the LUMO and, as a result, slightly overestimates the gap.
However, all the discussed trends in electronic energies are re-
produced well by the calculations.
Spectroelectrochemistry
Open-shell radical cations, formed in the above-discussed one-
electron oxidation, act as charge carriers in organic electronic
devices in which their chemical and thermodynamic stability
plays a crucial role. Therefore, studies of the electronic struc-
ture and stability of these species are essential for designing
efficient optoelectronic materials. The UV/Vis–near-infrared
(NIR) spectra of radical cations of BIN and AIN formed by in
situ oxidation in a spectroelectrochemical cell are shown in
Figure 5. Upon applying the progressively increasing anodic
potential, the intense absorption band at 380/400 nm of the
neutral molecules attenuates while the new absorption bands
concomitantly grow in the Vis and NIR range of the spectrum
(Table 2). Clear isosbestic points (e.g., 333 and 362 nm for 2,6-
N-CH3; 328, 428, and 438 nm for 2,6A-CH3) indicate that only
two interconverting species (neutral and radical cation) are
present over the course of the oxidation.
The newly appearing absorption in the visible range (450–
650 nm) can be assigned to the SOMO-to-LUMO transitions,
whereas the NIR bands (>700 nm) are due to HOMO-to-SOMO
transitions.[31] Such attribution is in line with the asymmetric
position of the SOMO in the HOMO–LUMO gap of radical cat-
ions, for example, 2,6A-CH3: HOMO (À8.4 eV), SOMO (À8.0 eV),
LUMO (À5.0 eV) (gas-phase calculations; the absolute energies
in solution would be significantly different). Surprisingly, the
first NIR absorption band of BIA radical cations shows no pro-
nounced redshift versus that of BIN radical cations (slight red-
shift for 1,5-isomers; slight blueshift for 2,6-isomers), although
the expected redshift is observed for the new absorption band
in the visible range. This could likely be attributed to a more
pronounced solvent/electrolyte stabilization of the smaller (less
delocalized) radical cation of BIN relative to BIA.[32]
Figure 5. Spectroelectrochemical spectra of 1,5-N(5,6) (a) and overlaid ab-
sorption spectra of four cation radicals (b). c) Photograph of 1,5-BIN solu-
tions in the neutral (left) and radical cation (right) state.
radical state. Such a property, along with the high stability of
the oxidized form, makes these materials of potential interest
for electrochromic devices.[33]
Conclusion
We have synthesized p-extended IF analogues (BINs/BIAs)
through a high-yield acid-catalyzed cyclization of naphthalene
and anthracene precursors. The effect of regioisomerism on
the structural, optical, and electrochemical properties of these
compounds has been studied experimentally and computa-
tionally. Crystallographic analysis and DFT calculations reveal
that the conjugated backbone of (5,5) isomers is nearly planar,
whereas a considerable distortion is present in the (5,6) and
(6,6) isomers. Compared with the parent IF, the extended
Application of a mild reductive potential of À100 mV quanti-
tatively transforms the radical cations back to the neutral state,
as manifested by the complete disappearance of the long-
wavelength absorption and full recovery of the original spec-
trum.
This redox interconversion is also manifested in a characteris-
tic color change, from the colorless neutral state to crimson
red for 1,5-N(5,6) and deep purple for 1,5-N(6,6) in the cation
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1361 cmÀ1; HRMS: m/z calcd for C28H24O4: 447.1566 [M+Na];
found: 447.1548.
acene core endows redshifted absorption and emission spectra
and lowered oxidation potentials. An electrochemical oxidation
of all regioisomers led to reversible formation of the corre-
sponding radical cations. An associated color change and new
characteristic absorption bands in the visible and NIR region of
the spectrum make these materials of potential interest for
electrochromic devices. Future work will focus on application
of BIN/BIA derivatives as monomers for emissive conjugated
polymers and their use in optoelectronic devices.
1
2b: Light yellow powder (67%, 0.75 g); H NMR (400 MHz, CDCl3):
d=8.44 (s, 2H), 8.02 (d, J=8.6 Hz, 2H), 7.95 (s, 2H), 7.91 (d, J=
7.6 Hz, 2H), 7.61–7.58 (m, 2H), 7.55–7.53 (m, 2H), 7.55–7.53 (m,
2H), 7.49–7.43 (m, 4H), 4.08 (q, J=7.2 Hz, 4H), 0.89 ppm (t, J=
7.2 Hz, 6H); 13C NMR (125 MHz, CDCl3): d=168.8, 142.4, 138.7,
131.6, 131.4, 131.3, 131.0, 130.9, 130.0, 127.6, 127.4, 127.3, 126.8,
126.3, 61.0, 13.7 ppm; IR: n˜ =2978 (CÀH), 1712 (C=O), 1594,
1480 cmÀ1; HRMS: m/z calcd for C32H26O4: 475.1909 [M+H]; found:
475.1905.
2c: White powder (66%, 0.67 g); 1H NMR (400 MHz, CDCl3; ob-
served two rotamers): d=8.08 (m, 2H), 7.66 (m, 4H), 7.58 (m, 4H),
7.44 (m, 4H), 7.30 (m, 2H), 7.85 (m, 4H), 7.30 (d, J=6.4 Hz, 2H),
4.09–3.81 (m, 4H), 0.86 (t, J=7.2 Hz, 2H), 0.62 ppm (t, J=7.2 Hz,
4H); 13C NMR (125 MHz, CDCl3): d=167.7, 167.5, 141.4, 141.3,
140.0, 139.9, 132.1, 132.0, 131.98, 131.92, 131.7, 131.4, 131.3, 130.1,
130.0, 127.6, 127.5, 125.9, 125.7, 125.2, 125.1, 60.6, 60.5, 13.6,
Experimental Section
Materials and methods
All chemicals were used as received unless otherwise noted. UV/
Vis/NIR absorption spectra were obtained using a JASCO V-670
spectrophotometer and fluorescence spectra were acquired using
a Cary Eclipse spectrophotometer. Lifetime measurements were
done with an Edinburgh Instruments Mini Tau lifetime fluorimeter
with an EPL 405 laser (excited at 405 nm). All electrochemical ex-
periments were performed at room temperature with a CHI-770
electrochemical workstation. Spectroelectrochemical experiments
were carried out in a thin-layer quartz cuvette containing a Pt
mesh working electrode, a platinum wire counter electrode, and
a Ag/AgNO3 reference electrode. Cyclic voltammetry measure-
ments were performed in 0.2m tetrabutylammonium hexafluoro-
phosphate solution in CH2Cl2 (or 1,2-dichloroethane) with a Pt disk
as a working electrode, Ag/AgNO3 as a reference electrode, and
a Pt wire as a counter electrode. Scan rates were 100 mVsÀ1 and
ferrocene was added at the end of experiments as an internal stan-
dard. DFT calculations of the optimized molecular structures (in the
gas phase) and the molecular orbital energies were carried out at
the B3LYP/6-31G(d) level as implemented in Gaussian 09.[34] Unre-
stricted UB3LYP/6-31G(d) was used for cation radicals. Frequency
calculations were performed for all isomers with distorted aromatic
core and showed no negative frequencies. The molecular orbital
surfaces were generated with GaussView 5.0. CCDC 1041229 (1,5-
A(5,5)), 1041230 (1,5-N(6,6)), and 1041231 (2,6-A(CH3) endoperox-
ide) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
13.1 ppm; IR: n˜ =2976 (CÀH), 1702 (C=O), 1597, 1473, 1404 cmÀ1
;
HRMS: m/z calcd for C28H24O4: 447.1566 [M+Na]; found: 447.1549.
1
2d: Light yellow powder (45%, 0.51 g); H NMR (400 MHz, CDCl3;
observed two rotamers): d=8.10–8.00 (m, 4H), 7.86–7.84 (m, 2H),
7.71–7.66 (m, 2H), 7.62–7.59 (m, 2H), 7.56–7.42 (m, 4H), 7.28–7.25
(m, 2H), 3.82–3.68 (m, 4H), 1.00–0.46 ppm (m, 6H); IR: n˜ =2979
(CÀH), 1705 (C=O), 1597, 1442 cmÀ1; HRMS: m/z calcd for C32H26O4:
475.1909 [M+H]; found: 475.1915.
Synthesis of BINs and BIAs: A solution of 4-bromotoluene
(0.68 mL, 5.6 mmol) in THF (10 mL) was cooled to À788C and
nBuLi (2.21 mL of 2.5m solution, 5.5 mmol) was added. After stir-
ring at À788C for 1 h, a solution of 2a–2d (0.56 mmol) in THF
(10 mL) was added slowly and the reaction mixture was then
warmed to room temperature and stirred for 1 h before being
quenched with water. The organic layer was extracted with ethyl
acetate, dried over MgSO4, and concentrated in vacuo. The crude
product was washed with hexane and used directly in the cycliza-
tion step. Trifluoroacetic acid (5 mL) was added to the flask con-
taining intermediates 3a–3d (0.20 mmol) and the mixture was
stirred at room temperature for 2 h. The resultant solution was
poured into water (100 mL). The mixture was extracted with di-
chloromethane and the combined organic extracts were dried over
MgSO4 and concentrated. The residues were purified by flash silica
gel column chromatography (1:1 hexanes/CH2Cl2) followed by
washing with hexanes.
2,6-N-CH3: White powder (230 mg, 60%); 1H NMR (400 MHz,
CDCl3): d=7.95 (d, J=8.4 Hz, 2H), 7.78 (d, J=8.8 Hz, 2H), 7.70 (d,
J=7.2 Hz, 2H), 7.41 (d, J=7.6 Hz, 2H), 7.32 (dd, 1J=7.2, 0.8 Hz,
2H), 7.23 (d, J=8.4 Hz, 8H), 7.18 (dd, J=7.2, 1.2 Hz, 2H), 7.05 (d,
J=8.4 Hz, 8H), 2.30 ppm (s, 12H); 13C NMR (125 MHz, CDCl3): d=
155.8, 147.6, 140.2, 140.0 137.3, 136.1, 130.4, 129.2, 128.8, 127.6,
127.1, 126.9, 125.0, 119.4, 118.7, 65.8, 21.0 ppm; IR: n˜ =2970 (CÀH),
1508, 1460, 1286, 1186, 1020, 915 cmÀ1; UV/Vis (CHCl3): lmax (e)=
285 (43400), 341 (18500), 358 (19000), 372sh nm (1400mÀ1 cmÀ1);
HRMS: m/z calcd for C52H40: 665.3203 [M+H]; found: 665.3191.
Syntheses
All reactions were performed under an inert atmosphere (N2).
Synthesis of 2a–2d: A dioxane solution of 2-ethoxycarbonylphe-
nylboronic acid (1.16 g, 6.0 mmol), K3PO4 (4.07 g, 19.2 mmol),
Pd(PPh3)4 (69 mg, 0.06 mmol), and 1a–1d (2.4 mmol) was heated
at reflux at 1008C for 24 h. After cooling to 208C, a saturated aque-
ous solution of NH4Cl was added. The organic layer was extracted
with ethyl acetate and dried (MgSO4), filtered, and the filtrate was
concentrated in vacuo. The residue was purified by silica column
chromatography (1:3 hexane/CH2Cl2) to yield the desired com-
pounds.
2a: White powder (63%, 0.64 g); 1H NMR (400 MHz, CDCl3): d=
7.90 (d, J=7.2 Hz, 2H), 7.87 (d, J=8.4 Hz, 2H), 7.88 (s, 2H), 7.59–
7.55 (m, 2H), 7.49–7.43 (m, 6H), 4.09 (q, J=7.2 Hz, 4H), 0.92 ppm
(t, J=7.2 Hz, 6H); 13C NMR (125 MHz, CDCl3): d=168.8, 142.3,
139.2, 132.3, 131.4, 131.3, 131.0, 129.9, 127.5, 127.5, 127.3, 126.7,
61.0, 13.7 ppm; IR: n˜ =2981 (CÀH), 1710 (C=O), 1596, 1444,
1
2,6-N-CF3: White powder (260 mg, 53%); H NMR (400 MHz, CDCl3):
d=7.95 (d, J=8.4 Hz, 2H), 7.77 (d, J=8.8 Hz, 2H), 7.70 (d, J=
7.6 Hz, 2H), 7.41 (d, J=7.6 Hz, 2H), 7.31 (d, J=7.6 Hz, 2H), 7.23 (d,
J=8.0 Hz, 8H), 7.19 (d, J=7.6 Hz, 2H), 7.04 ppm (d, J=8.0 Hz, 8H);
13C NMR (125 MHz, CDCl3): d=153.8, 146.4, 146.0, 139.8, 138.0,
129.9, 129.3, 129.1, 128.3, 128.1, 126.8, 125.3, 124.9, 122.8, 120.0,
119.5, 65.9 ppm; IR: n˜ =1616, 1408, 1323, 1163, 1069, 919 cmÀ1
;
UV/Vis (CHCl3): lmax (e)=289 (47200), 324 (12400), 338 (19200),
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352 nm (20200mÀ1 cmÀ1); HRMS: m/z calcd for C52H28F12: 880.2005
[M+]; found: 880.1998.
Acknowledgements
1
2,6-A-CH3: Light yellow powder (220 mg, 57%); H NMR (400 MHz,
This work was supported by NSERC and NanoQuebec. M.R.R.
acknowledges the PBEEE postdoctoral fellowship from FQRNT.
CDCl3): d=8.35 (s, 2H), 7.88 (d, J=8.4 Hz, 2H), 7.85 (d, J=8.4 Hz,
2H), 7.79 (d, J=7.6 Hz, 2H), 7.47 (d, J=7.6 Hz, 2H), 7.36 (td, J=
7.2 Hz, J=0.8 Hz, 2H), 7.26 (d, J=8.4 Hz, 8H), 7.23 (d, J=7.62 Hz,
1
2
2H), 7.02 (d, J=8.0 Hz, 8H), 7.27 ppm (s, 12H); 13C NMR (125 MHz,
CDCl3): d=156.2, 146.0, 140.4, 140.0, 137.4, 136.2, 132.3, 130.6,
129.1, 128.8, 127.5, 127.2, 127.1, 125.8, 124.9, 119.4, 118.6, 65.7,
21.0 ppm; IR: n˜ =2961 (CÀH), 1508, 1441, 1258, 1186, 1016,
915 cmÀ1; UV/Vis (CHCl3): lmax (e)=305 (70400), 322 (138600), 373
(6000), 395 (8100), 407 (8700), 432 nm (6600mÀ1 cmÀ1); HRMS: m/z
calcd for C56H43: 715.3365 [M+H]; found: 715.3388.
Keywords: acenes
indenofluorenes · organic electronics
· conjugation · fused-ring systems ·
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1
2,6-A-CF3: Light yellow powder (290 mg, 56%); H NMR (400 MHz,
CDCl3): d=8.18 (s, 2H), 7.96 (d, J=8.8 Hz, 2H), 7.92 (d, J=8.4 Hz,
2H), 7.87 (d, J=7.6 Hz, 2H), 7.51 (d, J=8.8 Hz, 8H), 7.47 (m, 10H),
7.43 (d, J=7.2 Hz, 8H), 7.32 ppm (dd, J=7.6, 0.8 Hz, 2H); 13C NMR
(125 MHz, CDCl3): d=154.3, 146.3, 144.0, 140.2, 138.3, 132.3, 131.3,
129.4, 129.3, 129.1, 128.1, 126.8, 125.38, 125.35, 125.30, 124.8,
120.1, 119.1, 65.6 ppm; IR: n˜ =1614, 1411, 1322, 1169, 1109,
920 cmÀ1; UV/Vis (CHCl3): lmax (e)=307 (69600), 321 (140000), 375
(5800), 395 (8000), 408 (8600), 430 nm (6740mÀ1 cmÀ1); HRMS: m/z
calcd for C56H31F12: 931.2229 [M+H]; found: 931.2210.
1,5-N(6,6): White powder (120 mg, 32%); 1H NMR (500 MHz,
CDCl3): d=8.02 (m, 4H), 7.35 (t, J=7.0 Hz, 2H), 7.24 (t, J=7.5 Hz,
2H), 7.15 (m, 4H), 7.00 (d, J=8.0 Hz, 8H), 6.87 (d, J=8.5 Hz, 8H),
2.31 ppm (s, 12H); 13C NMR (125 MHz, CDCl3): d=144.9, 142.8,
142.1, 135.7, 132.8, 131.1, 130.3, 128.6, 128.5, 128.3, 127.6, 126.9,
126.8, 123.7, 119.5, 59.3, 20.9 ppm; IR: n˜ =2917 (CÀH), 1574, 1449,
1397, 1195, 1021, 912 cmÀ1; UV/Vis (CHCl3): lmax (e)=272 (12200),
345 (11500), 368 (19600), 388 nm (18900mÀ1 cmÀ1); HRMS: m/z
calcd for C52H40: 665.3208 [M+H]; found: 665.3203.
1
1,5-N(5,6): White powder (80 mg, 21%); H NMR (500 MHz, CDCl3):
d=8.75 (d, J=8.5 Hz, 1H), 8.35 (d, J=7.5 Hz, 1H), 8.16 (d, J=
7.5 Hz, 1H), 8.11 (d, J=8.0 Hz, 1H), 7.70 (t, J=7.5 Hz, 1H), 7.47 (m,
3H), 7.38 (t, J=7.0 Hz, 1H), 7.31 (d, J=7.5 Hz, 1H), 7.21 (t, J=
7.5 Hz, 1H), 7.04 (d, J=8.0 Hz, 4H), 6.99 (d, J=8.0 Hz, 4H), 6.92 (m,
5H), 6.80 ppm (d, J=8.5 Hz, 4H); 13C NMR (125 MHz, CDCl3): d=
152.4, 150.1, 145.0, 142.5, 142.4, 142.1, 141.2, 136.2, 135.5, 133.1,
132.8, 131.4, 131.1, 130.2, 129.6, 128.8, 128.1, 128.1, 127.6, 127.5,
127.2, 127.0, 126.8, 126.7, 126.4, 126.1, 124.3, 123.9, 123.2, 119.5,
64.9, 59.7, 21.0, 20.9 ppm; IR: n˜ =2916 (CÀH), 1575, 1445, 1395,
1261, 1185, 1018, 913 cmÀ1; UV/Vis (CHCl3): lmax (e)=273 (2100),
349 (11200), 363 (16900), 381 nm (17000mÀ1 cmÀ1); HRMS: m/z
calcd for C52H40: 665.3208 [M+H]; found: 665.3203.
1
1,5-A(5,5): Light yellow powder (210 mg, 54%); H NMR (400 MHz,
C2D2Cl4): d=9.36 (s, 2H), 8.55 (d, J=7.0 Hz, 2H), 8.14 (d, J=7.5 Hz,
2H), 7.62 (d, 2H), 7.54 (m, 4H), 7.36 (t, J=8.5 Hz, 2H), 7.14 (d, J=
8.5 Hz, 8H), 7.04 (d, J=8.5 Hz, 8H), 2.31 ppm (s, 12H); 13C NMR
(125 MHz, C2D2Cl4): d=153.0, 150.0, 141.6, 141.2, 136.3, 133.7,
132.3, 129.3, 128.9, 128.1, 127.4, 127.3, 126.7, 125.8, 123.9, 122.9,
120.1, 64.7, 20.9 ppm; IR: n˜ =2919 (CÀH), 1596, 1508, 1446, 1407,
1182, 1160, 1037, 914 cmÀ1; UV/Vis (C2H2Cl4): lmax (e)=388 (14100),
411 (26900), 436 nm (28200mÀ1 cmÀ1); HRMS: m/z calcd for C56H43:
715.3365 [M+H]; found: 715.3385.
1,5-A(6,6): Light yellow powder (3 mg, 1%); 1H NMR (400 MHz,
CDCl3): d=7.78 (d, J=8.2 Hz, 1H), 7.69 (m, 4H), 7.23 (t, J=8.0 Hz,
2H), 7.13 (d, J=8.2 Hz, 2H), 7.08 (d, J=7.6 Hz, 8H), 7.00 (m, 10H),
6.86 (dd, J=8.2, 1.2 Hz, 1H), 2.20 ppm (s, 8H); IR: n˜ =2917 (CÀH),
1506, 1446, 1259, 1194, 1160, 1019, 916 cmÀ1; UV/Vis (CHCl3): lmax
(e)=408 (22600), 432 (27900), 460 nm (28000mÀ1 cmÀ1); HRMS:
m/z calcd for C56H43: 715.3365 [M+H]; found: 715.3384.
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Received: December 26, 2014
Published online on && &&, 0000
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
9
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
An extended family: A series of aro-
matic indenofluorene (IF) analogues
with naphthalene and anthracene cores
have been synthesized (see figure; 1,5-
N(5,6) is an isomer of a naphthalene de-
rivative). Compared with the parent IF,
the extended acene core endows red-
shifted absorption and emission spectra
and lowered oxidation potentials.
&
Indenofluorenes
M. R. Rao, A. Desmecht, D. F. Perepichka*
&& – &&
p-Extended Indenofluorenes
&
&
Chem. Eur. J. 2015, 21, 1 – 10
www.chemeurj.org
10
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!