Scalable synthesis of 5,11-diethynylated indeno[1,2-b]fluorene-6,12-diones and exploration of their solid state packing

Article abstract of DOI:10.3762/bjoc.10.219

We report a new synthetic route to 5,11-disubstituted indeno[1,2-b]fluorene-6,12-diones that is amenable to larger scale reactions, allowing for the preparation of gram amounts of material. With this new methodology, we explored the effects on crystal pac

Full text of DOI:10.3762/bjoc.10.219

Scalable synthesis of 5,11-diethynylated
indeno[1,2-b]fluorene-6,12-diones and exploration
of their solid state packing
Bradley D. Rose1, Peter J. Santa Maria1, Aaron G. Fix1, Chris L. Vonnegut1,
Lev N. Zakharov2, Sean R. Parkin3 and Michael M. Haley*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 2122–2130.
1Department of Chemistry & Biochemistry and the Materials Science
Institute, University of Oregon, Eugene, Oregon 97403-1253, USA,
2CAMCOR, University of Oregon, 1443 East 13th Avenue, Eugene,
Oregon 97403, USA and 3Department of Chemistry, University of
Kentucky, Lexington, Kentucky 40506-0055, USA
doi:10.3762/bjoc.10.219
Received: 14 July 2014
Accepted: 15 August 2014
Published: 05 September 2014
Email:
Associate Editor: P. R. Schreiner
Michael M. Haley* - haley@uoregon.edu
© 2014 Rose et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
crystal packing; electron accepting; indenofluorenes; organic
electronics; polycyclic conjugated hydrocarbons
Abstract
We report a new synthetic route to 5,11-disubstituted indeno[1,2-b]fluorene-6,12-diones that is amenable to larger scale reactions,
allowing for the preparation of gram amounts of material. With this new methodology, we explored the effects on crystal packing
morphology for the indeno[1,2-b]fluorene-6,12-diones by varying the substituents on the silylethynyl groups.
Introduction
Polycyclic conjugated hydrocarbons (PCHs) have been studied properly, these must include materials that conduct holes (elec-
extensively due to the wide variety of physical properties that tron donating) and conduct electrons (electron accepting) [8].
can be accessed by appropriate manipulation or “tuning” of a While there are many systems that display high hole mobilities,
molecular scaffold (e.g., installation of donor/acceptor groups, there are far fewer that exhibit high electron mobilities.
inclusion of heteroatoms, etc.) [1-3]. Recently there has been
resurging interest in PCHs for use as active materials in organic Our laboratory has been exploring a new class of PCHs based
electronic devices. Some popular examples of devices under- on the five structural isomers of indenofluorene [9]. In particu-
going extensive exploration are organic field effect transistors lar, the indeno[1,2-b]fluorene (IF, 1, Figure 1) skeleton is
(OFET) [4,5], organic photovoltaics (OPV) [6], and organic similar to linear oligoacenes, with the notable exception that the
light emitting devices (OLED) [7]. For such devices to operate molecule contains two five-membered carbocycles. This modest
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Beilstein J. Org. Chem. 2014, 10, 2122–2130.
Figure 1: Previously reported indeno[1,2-b]fluorenes and related indeno[1,2-b]fluorene-6,12-diones.
alteration imparts an inherent propensity of the IF scaffold to be Notably, 6 showed balanced hole and electron mobilities when
electron accepting [10,11]. A simple explanation for the high vapor or solution processed. Molecule/polymer solubility is
electron affinity of the IF is that to make all five rings formally desirable because it offers the benefit of being solution process-
aromatic two electrons must be added to the system, effectively able, which could allow for the inexpensive large area printing
creating two cyclopentadiene anions [12]. The result of the IFs of electronic devices.
high electron affinity is nearly balanced ambipolar charge trans-
port in OFETs [11,13].
We report herein the preparation of a variety of diethynylated
IF-diones 8a–j that are readily soluble in common organic
The synthetic precursors to 1, the indeno[1,2-b]fluorene-6,12- solvents, and the exploration of their packing in the solid-state
diones (IF-diones, 2, Figure 1) have also been explored as an by X-ray crystallography. The prototypical molecule that served
active layer in OFETs. The first reported IF-dione OFET as inspiration for our studies was pentacene, as it, along with
utilized 3 – while the solid-state structure of 3 showed several numerous other acene derivatives, has been substituted with
sub-van der Waals contact distances, the n-type mobility of the trialkylsilylethynyl groups of varying size to study the effect on
OFET was very low (2 × 10−5 cm2 V−1 s−1) [14]. On the other the solid state packing in single crystals [17]. This was shown to
hand, an OFET utilizing 4 (X = F) had measured electron have a large effect on the OFET performance as slight changes
mobilities of 0.17 cm2 V−1 s−1, and its X-ray crystal structure in the geometry can dramatically alter the intermolecular elec-
revealed 1-dimensional π-stacking with a close interplanar dis- tronic coupling, which is what ultimately dictates performance
tance of 3.30 Å [15]. Due to the inherent insolubility of com- of the device [18,19].
pounds 3 and 4, however, they needed to be vapor deposited
under vacuum. More recently IF-diones 5–7 were reported Results and Discussion
(along with polymeric and other derivatives) with 5 and 6 ex- Synthesis. Our initial studies [20] toward 8 (Scheme 1) focused
hibiting both n- and p-channel behavior in OFETs [16]. on the Sonogashira cross-coupling of known diiodo intermedi-
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Scheme 1: Transannular cyclization route to diethynyl-IF-diones 8.
ate 9, which was prepared by double transannular cyclization
of 10 using elemental iodine under air [21]. Dehydro-
benzo[12]annulene 10 in turn was synthesized via Glaser homo-
coupling of 1,2-diethynylbenzene [21-23]. While in theory this
route permitted relatively easy access to diethynyldiones 8, in
practice it was fraught with problems: (1) the formation of 10
was very sensitive to the reaction conditions and thus typically
gave low yields (approx. 10%) upon scale-up; (2) the reaction
must be run in very dilute solution to minimize the formation of
larger cyclooligomers as well as polymer; (3) pure 10 in the
solid state is reported to be shock sensitive [22], a fact that we
can readily reaffirm; and (4) the iodine atoms on 9 are quite
labile as we often observed formation of elemental iodine if
solutions of 9 were exposed to heat or sunlight. If we wanted to
obtain quantities of diones 8 beyond 20–30 milligrams at a
time, we had to overcome the synthetic roadblock that
Scheme 1 represented.
The improved synthetic route to 8 arises from a retrosynthetic
analysis of the current method to prepare IF derivatives
[9-11,13]. The needed modification must include halogens at
the 5 and 11 positions for subsequent functionalization, such as
the more robust bromines in 11, yet avoid annulene transan-
nular synthesis [24]. Instead, the route we chose involved key
precursor 12, which surprisingly is an unknown compound.
Starting with commercially available 2,5-dibromo-p-xylene (13)
(Scheme 2), iodination using the method reported by Kitamura
gave tetrahalide 12 in good yield on >10 g scale [25]. Suzuki
cross-coupling with 12 furnished p-terphenyl 14, followed by
oxidation of the methyl groups to produce diacid 15. Intramole-
cular Friedel–Crafts acylation then afforded 5,11-dibromo-IF-
dione 11. The yields for the Sonogashira cross-coupling of a
variety of trialkylsilylacetylenes to either 9 or 11 were modest
to very good (Table 1) but were not optimized.
Scheme 2: Suzuki/Friedel-Crafts route to diethynyl-IF-diones 8.
Table 1: Diethynyl-IF-diones synthesized and yields for Sonogashira
cross-coupling.
isolated yield
from 9
from 11
trialkylsilyl group
R1
R2
R3
8a
8b
8c
8d
8e
8f
Me
Et
Me
Et
Me
Et
40
8
–
27
72
48
–
n-Pr
iPr
n-Pr
iPr
n-Pr
iPr
–
61
7
iBu
Ph
iBu
Ph
iBu
Ph
15
–
–
8g
8h
8i
Me
Me
Me
Me
Me
Me
Me
Me
CF3(CH2)2
iBu
30
35
–
Optical and electronic properties. Shown in Figure 2 are the
UV–vis spectrum and the cyclic voltammogram of 8c, data that
are representative of all the 5,11-diethynyl-IF-diones. As antici-
pated, altering the trialkylsilyl group has very little effect on the
–
t-Bu
Ph
17
–
8j
24
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Figure 2: UV–vis spectrum (left) and cyclic voltammogram (right) of dione 8c.
optoelectronic properties of the conjugated scaffold (Table 2). should not be visible in the UV–vis spectrum (Table 3). To see
All molecules have two strong absorptions around 310 and if this was a computational artifact, the same calculations were
330 nm, with a much weaker, broad absorption in the performed for fluorenone and benzophenone, where it has
450–550 nm range. Electrochemistry shows two reversible previously been established that the S0→S1 transition corre-
reductions with potentials of −0.78 to −0.84 V for the first sponds to π→π* and n→π*, respectively [29,30]. The calcula-
reduction and −1.18 to −1.26 V for the second reduction. The tions correctly predict the ordering of the states for fluorenone
small differences in the absorbance and cyclic voltammetry and benzophenone. To validate this experimentally, UV–vis
essentially fall within experimental error.
spectra were gathered in solvents of differing polarity. We
anticipated that if the S0→S1 transition corresponds to a n→π*
Interestingly these compounds have a low energy S0→S1 tran- transition, the energy separating the S0 and S1 states would be
sition at ca. 500–525 nm which has previously and incorrectly measurably different in polar solvents when compared to non-
been described as an n→π* transition [16,20]; however, polar solvents, thus leading to an energy shift of this transition.
TD-DFT calculations predict this to be π→π* (Figure 3) Likewise if the S0→S1 transition was a π→π* there should be
[27,28]. The n→π* transition was calculated to have a slightly essentially no change in the transition energy when changing
higher energy transition with an oscillator strength of 0; thus, it solvent polarity. We found that the shift in the spectrum upon
Table 2: Electrochemical and optical data for ID-diones 8a–j.
electrochemicala
ELUMO (eV)b
optical
compd
Ered1 (V)
Ered2 (V)
EHOMO (eV)c
λabs (nm)
gap (eV)d
8a
8b
8c
8d
8e
8f
−0.80
−0.79
−0.83
−0.82
−0.84
−0.78
−0.77
−0.78
−0.81
−0.78
−1.21
−1.23
−1.24
−1.24
−1.26
−1.18
−1.18
−1.21
−1.25
−1.20
−3.89
−3.90
−3.86
−3.87
−3.85
−3.90
−3.87
−3.86
−3.87
−3.86
−6.26
−6.28
−6.23
−6.23
−6.22
−6.29
−6.27
−6.24
−6.23
−6.24
310, 330, 524
311, 332, 522
312, 332, 524
313, 333, 525
314, 333, 524
312, 333, 520
308, 330, 516
310, 331, 521
312, 331, 526
310, 331, 521
2.37
2.38
2.37
2.36
2.37
2.39
2.40
2.38
2.36
2.38
8g
8h
8i
8j
aCV recorded using 1–5 mM of analyte in 0.1 M solution of either Bu4NOTf (8a, 8b, 8d–f, 8i) or Bu4NBF4 (8c, 8g, 8h, 8j) in HPLC-grade CH2Cl2.
Values reported as the half-wave potential (vs SCE) using the Fc/Fc+ couple (0.46 V) as an internal standard. See Supporting Information File 1 for
details. bLUMO energy levels were approximated using SCE = −4.68 eV vs vacuum [26]. cEstimated by subtracting the optical bandgap from ELUMO.
dEstimated from the λmax of the lowest energy UV–vis peak.
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Figure 3: Kohn–Sham HOMO (left) and LUMO (right) plots of 8a.
Table 3: Calculated transitions for 8a, showing only the main contribution to each excitation. Calculated using TD-B3LYP/6-311+G(d,p)//B3LYP/6-
31G(d).
contribution to
excitation, %
transition
molecular orbitals
oscillator strength
energy, eV (nm)
S0→S1
S0→S2
S0→S3
S0→S4
HOMO→LUMO
HOMO-2→LUMO
HOMO-3→LUMO
HOMO-1→LUMO
97
89
80
88
0.0295
0.0000
0.0000
0.0486
2.35 (527)
2.71 (458)
2.86 (434)
2.95 (420)
changing solvent polarity in going from n-hexane to acetone lizing groups, and π-stacking is observed in all but the largest
was 0.02 eV, supporting our hypothesis that the 450–550 nm (8f, SiPh3), but there are significant differences in the nature of
absorption is indeed a π→π* transition as indicated by calcula- the π-stacking of the other nine compounds. The smallest (8a,
tions.
SiMe3, vol. ~130 Å3) is monoclinic, P21/c, with a unit cell
elongated along b. The molecules form 1-D π-stacks (inter-
X-ray crystallography. We explored the solid-state packing planar spacing 3.446(5) Å) parallel to a, with adjacent stacks
geometries resulting from altering the substitution on the silyl having the opposite tilt by virtue of the c-glide, leading to a
groups. The high crystallinity of the majority of the compounds herringbone motif in which inter-stack interactions are solely
examined permitted facile growth of large single crystals, of between the SiMe3 groups. This is the only structure of this type
approximately several millimeters, from hexanes solution. in the group of ten compounds. The next largest (8g,
Single crystals for 8g and 8j were grown from chlorobenzene SiMe2PrF3, vol. ~198 Å3) is orthorhombic, Pca21, with a squat
due to the low solubility and crystallinity of these compounds. unit cell (short b axis). Molecules form 1-D π-stacks parallel to
The molecular structures were then elucidated using x-ray the b-axis (interplanar spacing 3.358(3) Å), while adjacent
diffraction.
stacks interact via C-H···π contacts to form herringbone-
patterned layers (interstack molecular tilt 66.85(2)°) in the ab
The ten compounds 8a–j exhibit several different packing plane, separated by layers of fluorinated ‘grease’. Compounds
motifs (Figure 4). In a broad sense, the progression in packing 8b, 8i and 8h, which have essentially the same volume solubi-
follows a trend with the volume of the substituted silyl groups lizing groups (~204 Å3), exhibit very similar overall packing
[31,32]. Segregation of the IF-dione backbone and the solubi- motifs. They are monoclinic, P21/c, with squat unit cells (short
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Beilstein J. Org. Chem. 2014, 10, 2122–2130.
Figure 4: Views perpendicular to the average plane of the π stack. 1st row left to right – 8a, 8b, 8c; 2nd row – 8d, 8d, 8e; 3rd row – 8f, 8f, 8g; 4th row
8h, 8i, 8j. Hydrogen atoms omitted for clarity; ellipsoids drawn at the 30% probability; individual molecules were colored the same to identify overlap
easier.
b axis), and form 1-D π-stacks parallel to b (interplanar spac- contacts, and are related by the c-glide operation to form the
ings of 3.454(2) Å, 3.340(2) Å and 3.385(3) Å, respectively, for common herringbone motif (interstack molecular tilts of
8b, 8i and 8h). Adjacent stacks along c interact via C–H···π- 64.88(2)°, 64.45(2)° and 68.99(3)°, respectively, for 8b, 8i and
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8h). These bc layers are separated by layers of trialkylsilyl thus, a system with maximum overlap would have θ = 0°, while
groups. Compound 8j has a slightly larger (~210 Å3) solubi- θ = 90° would result in no π orbital overlap. Using these two
lizing group, but is triclinic,
. The difference in crystal parameters a crude model for examining the possible intermole-
symmetry compared to 8b, 8i, 8h (and to 8g), however, belies cular electronic coupling can be developed, which is pictorially
the similarities. The unit cell is a squat skewed box (short represented in Figure 5.
a-axis, though easily transformed to a short b setting that makes
the similarity to 8b, 8i, 8h clearer). There are two half mole-
cules per asymmetric unit, each sitting on inversion centers.
Crystallographically identical molecules form 1-D π-stacks
along the short a-axis (cf short b-axis in 8b, 8i, 8h, 8g), leading
to two independent molecular π-stacks, which have slightly
different interlayer spacings (3.385(2) Å and 3.397(2) Å).
Between adjacent stacks, the crystallographically independent
molecules are tilted relative to each other by 66.88(2)°, giving a
variant of the common herringbone motif, despite the lack of a
crystallographic glide plane. In addition to the IF-dione
π-stacking, inversion-related phenyl groups on the solubilizing
groups are paired by π–π interactions. The stacking in com-
Figure 5: Schematic of the parameters used for comparing X-ray
crystal structures, view is parallel to the molecular plane. Black lines
represent the molecules with circles denoting the centroid.
pounds 8c and 8d (Si(n-Pr)3 and Si(iPr)3, vol. ~278 Å3) are Looking at Table 4 the only visible trend in the series appears to
similar at first glance. In 8c, each end of the IF-dione backbone be within the n-alkyl symmetric. The n-alkyl symmetric series
overlaps by different amounts with adjacent molecules (inter- has both the d and θ follow the trend of the alkyl group’s radius.
planar spacings of 3.422(3) Å and 3.446(4) Å) to form a 2-D When examining symmetric and asymmetric series there is not
π-stacked brickwork motif. In 8d, the superficially similar an easily interpretable trend between the radii and d or θ. Picto-
π-stacking motif is better described as a 1-D π-stack. There is rially the overlap between the molecular planes is clearly visible
significant overlap between adjacent molecules on only one side in Figure 5. For the n-alkyl symmetric series visual inspection
(interlayer spacing = 3.404(3) Å), while on the other side, any reveals that the overlap between the planes of the molecules is
'overlap' amounts to only the mutual superposition of C10 over greatest for the smallest group, 8a, and least for the largest
the C5–C10 bond of the neighboring molecule. Moreover, a group, 8c. Yet again, there is no clear visual trend in the overlap
rudimentary superposition of the LUMO plots (Figure 3) for the between the symmetric and asymmetric series. Considering that
relative positions of this molecular pair arrangement suggests derivatives of 8 are most likely an n-channel material in OFETs,
no favorable orbital interactions. In 8e (Si(iBu)3, vol. ~353 Å3), the density and phase of the LUMO should be the most impor-
the structure is monoclinic, P21/n. The IF-dione molecules form tant since in a molecular orbital picture of charge transport these
1-D π-stacks (interplanar spacing = 3.394(2) Å) that are are the orbitals that the extra electron would occupy. The
completely segregated from neighboring stacks by the bulky Kohn–Sham LUMO density in Figure 3 is predominately
Si(iBu)3 groups. The largest group, SiPh3 (vol. ~372 Å3) in 8f, located on the indacene moiety and the oxygen. From this
effectively suppresses overlap of the IF-dione backbones perspective the best candidates for n-channel OFET materials
between adjacent molecules.
would have large overlap between the indacene moiety and
oxygen. In 8a–j the oxygen is pointing away from the indacene
There are three distinct substitution patterns in the array of moiety in the neighboring molecules along the stack and there is
IF-diones synthesized – (1) the three groups are n-alkyl chains little to no overlap between the indacene moieties.
with symmetry (three mirror symmetry planes about the
silicon), (2) bulky alkyl groups with symmetry, and (3) Conclusion
dimethyl-substituted possessing only one mirror plane of We have described an improved synthetic route to 5,11-
symmetry about the silicon. When looking further for trends, diethynyl-functionalized indeno[1,2-b]fluorene-6,12-diones that
we compared two parameters to see if any of them ultimately permits a scalable synthesis of larger amounts of material. We
yielded packing motifs with close contacts between the carbons explored the solid state packing motifs that result from altering
of the conjugated system. One parameter examined is the dis- the bulkiness as well as directionality of the trialkylsilyl groups.
tance between the centroid of the planar system and its next Altering the substitution on the silyl group had little, if any,
nearest neighbor along the one-dimensional π stack. The other effect on the electronic properties of 8, which are dominated by
parameter is the angle between the centroids of the nearest the conjugated core; however, there were marked differences in
neighbor molecules and the normal to the plane of a molecule; the solid-state packing of single crystals of these compounds.
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Table 4: Sizes of trisubstituted-silylethynyl groups in 8.
intermolecular close contacts
radius
(Å)a
radius
(Å)b
contact
distance (Å)
dc
θd
n-alkyl symmetric
8a
8b
8c
8a
8b
8d
8c
2.69
4.06
5.26
2.69
4.06
4.09
5.26
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
6.366
6.439
57.2
57.5
8.239
65.4
symmetric
6.366
57.2
6.439
57.5
9.550, 7.419e
8.239
69.6, 62.7e
65.4
C9···C2
C6···C3
C6···C6
3.407
3.562
3.524
8e
5.28
–
6.134
56.4
C10···C10
C17···C4
3.188
3.369
8f
8i
5.89
2.73
2.68
–
10.984
6.356
5.976
74.7
58.0
55.3
asymmetric
3.99
5.26
C7···C3
3.348
C9···C2
C10···C3
3.437
3.409
8h
C6···C10
C3···C17
3.361
3.361
8g
8j
2.67
2.68
5.74
5.94
6.096
6.292
56.5
57.3
–
–
aSi···X distance where X is the farthest atom from Si with the covalent radii of X added to the distance [30]. bRadius for other axis of lower symmetry
trialkylsilanes. cDistance between the centroid of two molecules in the 1-D stack. dAngle between the centroid of each of two molecules of the π stack
and the normal to the average plane. eThere are two symmetrically independent 1-D stacks.
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We thank the National Science Foundation (CHE-1013032 &
CHE-1301485) for support of this research. B.D.R. acknowl-
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