A Conformationally Stable Contorted Hexabenzoovalene

Article abstract of DOI:10.1002/anie.201607740

Contorted two-dimensional aromatic molecules are fascinating synthetic targets because they are molecular “cutouts” of nonplanar graphene structures, fullerenes, or carbon nanotubes. In most cases, the curvature is introduced by the implementation of eith

Full text of DOI:10.1002/anie.201607740

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International Edition: DOI: 10.1002/anie.201607740
German Edition: DOI: 10.1002/ange.201607740
Polycyclic Aromatic Hydrocarbons
A Conformationally Stable Contorted Hexabenzoovalene
Kevin Baumgꢀrtner, Ana Lucia Meza Chincha, Andreas Dreuw, Frank Rominger, and
Michael Mastalerz*
Abstract: Contorted two-dimensional aromatic molecules are
fascinating synthetic targets because they are molecular “cut-
outs” of nonplanar graphene structures, fullerenes, or carbon
nanotubes. In most cases, the curvature is introduced by the
implementation of either five-, seven-, or eight-membered rings
into the fused aromatic plane. Curvature can also be generated
for two-dimensional systems consisting of six-membered rings
exclusively, by the introduction of cove or fjord regions. The
synthesis of a polycyclic aromatic hydrocarbon (PAH) that
contains two peripheral triptycene units and six tert-butyl
substituents is described. As a result of steric repulsion, the
structure is highly contorted with two phenylene blades of the
peripheral triptycene units oriented almost coplanar with
respect to each other at a distance of 16 ꢁ, as has been verified
by single crystal X-ray diffraction. The conformation is stable
in solution even at a temperature of 1508C. Additionally,
internal tert-butyl groups could be selectively removed, allow-
ing a UV/Vis-spectroscopic comparison of two structures with
the same p-system, but different degrees of contortion.
contorted fused p-systems consisting of six-membered rings
exclusively, have been realized by implementation of one or
more cove and/or fjord regions.[7] Further bending can be
induced by placing bulky groups in bay, cove, or fjord
positions.[8]
As a result of twisted p-planes, contorted PAHs are
generally more soluble than planar analogues, simply because
they cannot pack efficiently by p–p stacking. Recently, we
found that triptycene end-capped fused p-systems with eleven
rectilinear condensed six-membered aromatic rings are highly
soluble, showing at the same time a pronounced tendency to
form single crystals.[9] We were interested in introducing
triptycene end-caps to larger 2D-fused PAHs to increase
solubility, while maintaining the tendency to form single-
crystals for the purposes of performing structural investiga-
tion in the solid state. The first target was PAH 1, a hexa-cata-
condensed hexabenzoovalene derivative (HBO), an aromatic
scaffold hitherto not described. Interestingly, a PM3-model of
one possible conformer (Figure 1) shows a large curvature of
the fused p-system, resulting in an almost coplanar arrange-
ment of two triptycene phenylene units with a distance of
approximately 16 ꢀ, which makes it a fascinating synthetic
target for fused cyclic structures by formal dimerization.
The synthesis of HBO 1 and related model compound 5
starts with Diels–Alder reaction of triptycene TMS triflate 2
and cyclopentadienones 6 or 3 (Scheme 1), which have been
T
wo-dimensional polycyclic aromatic hydrocarbons (PAHs)
are a fascinating class of compounds, holding promise for
organic electronic applications.[1] Among PAHs, contorted or
curved structures are attractive synthetic targets because they
represent cutouts of fullerenes,[2] carbon-nanotubes,[3] or
other interesting three-dimensional aromatic allotropes,
such as the Mackay crystals.[4] To induce curvature into 2D
p-systems, basically three different strategies have been
pursued. One possibility is the incorporation of pyrene and
similar larger structural scaffolds into strained pyrenophanes
and related structures.[5] The most common approach to cause
curvature is likely by introduction of either five-, seven-, or
eight-membered rings into the fused system.[6] Alternatively,
[*] K. Baumgꢀrtner, A. L. Meza Chincha, Dr. F. Rominger,
Prof. M. Mastalerz
Figure 1. Molecular structure of triptycene end-capped hexabenzo-
ovalene 1 (left) and a PM3-optimized model of a contorted conformer.
Organisch-Chemisches Institut, Ruprecht-Karls-Universitꢀt
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: michael.mastalerz@oci.uni-heidelberg.de
synthesized in multiple steps from rather simple building
blocks (Supporting Information). Diels–Alder reactions
between 2 and 3 have been performed in acetonitrile/
dichloromethane in the presence of CsF to generate the
aryne from 2 in situ,[10] giving 4 in 87% yield. The subsequent
cyclodehydrogenation of 4 to 5 was achieved with DDQ as
oxidant in the presence of methanesulfonic acid in dichloro-
Prof. A. Dreuw
Interdisziplinꢀres Zentrum fꢁr Wissenschaftliches Rechnen
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 205, 69120 Heidelberg (Germany)
A. L. Meza Chincha
Present address: Institut fꢁr Organische Chemie
Universitꢀt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
methane, giving model compound
5
in 61% yield.[11]
Supporting information for this article, including details of the
synthesis, characterization, NMR spectra, optical properties, inter-
conversion energy calculations, computational studies, and X-ray
data for 1, 3–6, 7a, 7b, 8, and S6, can be found under:
http://dx.doi.org/10.1002/anie.201607740.
Although the aryne precursor 2 has been added in excess to
biscyclopentadienone 6, the larger compound 7 was isolated
in significantly lower yields (36%). However, for the cyclo-
dehydrogenation of 7 to HBO 1 the protocol needed to be
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by X-ray crystallography to be a chloroform solvate of
conformer 7b. After three days, yellow polyhedral crystals of
conformer 7a were obtained from the same solution, again as
chloroform solvate. The most significant difference between
the two structures is the relative orientation of the central
peri-fused rings of the pyrene unit (Figure 2; highlighted in
orange). In 7a these are anti-oriented and in 7b they are syn-
oriented (bent about 498 out of plane). As a consequence, in
7a the rectilinear annulated aromatic backbone is twisted
twice with the triptycene units oriented periplanar with
respect to each other.[7b,8a,b,d] In 7b the twist is perpendicular to
the aromatic backbone, forming a curved structure where
a maximum angle of contortion between the blue marked
planes of 528 occurs and two of the triptycene phenylene rings
adopt an almost coplanar orientation (deviation from co-
planarity ca. 78). The formation of a third energy-minimized
conformer (7g; Figure 2c) with only one twist along the
aromatic backbone (with a twisting angle of 788) should in
principle be possible and has been calculated with DFT
methods (B3LYP, G-311G*). Comparison of the heat of
formation of geometry optimized molecular structures of all
three conformers reveals that indeed the U-shaped conformer
7b, which crystallized first, is with DHf = 1118.7 kJmolÀ1
about 6–7 kJmolÀ1 energetically favored over 7a (DHf =
1125.4 kJmolÀ1) and 7g (DHf = 1125.1 kJmolÀ1). As men-
tioned above, both polymorphs crystallized as solvates with
enclathrated chloroform. The small differences in steric
energies is outweighed by the packing energies (7a:
À763.3 kJmolÀ1 and 7b: À735.4 kJmolÀ1), explaining the
formation of both polymorphs. In contrast to the solid state,
by 1H NMR spectroscopy at room temperature only one
dataset could be detected, suggesting that the barrier of the
interconversion of conformers is rather low and even at 178 K
no broadening or splitting of peaks could be detected.
Scheme 1. Synthesis of PAH 1, 5, and 8.
slightly varied to ensure full cyclization at all targeted
positions. Here the combination of DDQ and FeCl3 as
reactants gave compound 1 in 90% yield.[7a] By addition of
an etheric chlorohydride solution to 1 in chloroform, selective
ipso-substitution of the centered tert-butyl groups by protons
occurred, giving HBO 8 in 59% yield.[12]
For both the Diels–Alder products 4 and 7, as well as the
cyclodehydrogenated PAHs 5, 1, and 8 single crystals have
been grown and analyzed by X-ray diffraction. For fused
compound 7, the formation of two polymorphic crystals (7a
and 7b; Figure 2) was observed, when grown by diffusion of
methanol into a chloroform solution of 7. After 30 minutes
fine colorless needles started to grow, which were confirmed
Single crystals of 5 (Figure 3a,b) suitable for X-ray
diffraction have been grown from chloroform/methanol.
Interestingly, the curvature of the molecule fits half of the
structural model of HBO 1, proposing that the inherent cove
region is mainly responsible for the contortion in the solid
state. The 1H NMR spectrum (Supporting Information)
showed two broad peaks at d = 7.05 and 7.52 ppm for the
phenylene blades of the triptycene scaffold, suggesting that
there is a dynamic interconversion of two indistinguishable
contorted structures through
a planar transition state.
Figure 2. Conformers of 7. Conformers a) 7a and b) 7b have been found in the crystalline state (data are derived by single-crystal X-ray
diffraction). The optimized geometry of c) 7g was calculated by DFT methods (B3LYP, G-311G*). Semi-empirical method PM3 has been used to
calculated differences in steric energies of the conformers. Solvent and hydrogen atoms are omitted for clarity.
2
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U-shaped structure (Figure 3c). Unfortunately, as a conse-
quence of a systematic disorder the quality of the X-ray data
was not high enough to analyze bond lengths and angles
accurately. Similarly, as observed in polymorph 7b, two of the
triptycene phenylene rings of HBO 1 are almost arranged
coplanar (deviation from coplanarity: 7.58). The curvature of
the p-backbone is obvious upon comparison of the relative
arrangements of the blue marked planes with respect to each
other (angle of 55.78). The centered, tert-butyl substituted, six-
membered rings of the pyrene core (marked orange) are
nonplanar, forming an envelope-type intrinsic conformation
(torsion angle ca. 7–148), which is accompanied by a loss of
aromaticity if compared to HBO 8 (see discussion below).
It is worth mentioning that Mꢁllen et al. synthesized
a related larger structure, for which no single-crystal X-ray
diffraction data could be obtained at that time, therefore no
clear assumption could be made for the assignment of the
conformation (syn- or anti-isomer) exclusively on the basis of
the NMR spectrum and theoretical calculations.[12]
Comparison of the 1H NMR spectra of HBO 1 relative to
HBO 8, shows broad signals for the aromatic triptycene
protons of the latter (d = 7.04 and 7.53 ppm; Figure 4b),
revealing a rapid interconversion of conformers on the NMR
timescale. By temperature dependent 1H NMR measure-
ments a coalescence temperature of TC = 253 K was deter-
mined, corresponding to DG° = 49.37 kJmolÀ1 and kC =
337.73 sÀ1, which are comparable to the values for 5.
Conformational analysis of HBO 8 (Supporting Information,
Figure S42) suggests that two isomers are of nearly the same
minimum energy; a U-shaped conformer, structurally similar
to that of HBO 1, and an undulated S-shaped conformer.
Single crystals of HBO 8 grown from toluene/methanol gave
the S-shaped conformer in the solid state. The centrosym-
metric structure can be viewed as a conformational dimer of
structure 5 sharing one central biphenyl unit (orange rings).
Figure 3. Crystal structures of a,b) 5, c) 1, and d) 8. Solvent and
hydrogen atoms are omitted for clarity.
By variable temperature 1H NMR measurements,
a coalescence of the triptycene signals was found
at TC = 258 K, which corresponds to DG° =
50.55 kJmolÀ1 for the interconversion with a rate
constant of kC = 314.86 sÀ1.
In contrast to 5, the 1H NMR spectrum of
cyclodehydrogenated HBO 1 shows four sharp
and distinct signals at d = 6.87, 7.30, 7.34, and
7.79 ppm for the protons of the peripheral tripty-
cene phenylene blades (Figure 4a). Even at 423 K
in [D6]DMSO no coalescence or significant peak
broadening was observed, which revealed that by
cyclodehydrogenation of 7 one thermodynami-
cally stable conformer of HBO 1 formed exclu-
sively. Analysis of 14 possible energy-minimized
conformers—seven with the tert-butyl groups
attached at the pyrene in syn- and seven with
those in anti-conformation (Supporting Informa-
tion)—revealed that the most stable structure is
the C2v-symmetric syn-conformer, as depicted in
Figure 1. Indeed, single crystals of HBO 1 grown
1
Figure 4. Comparison of H NMR spectra recorded at room temperature of a) 1
measured in CD2Cl2 and b) 8 measured in CD2Cl2/CS2 (5:1). Only the aromatic
regions are depicted. The asterisk indicates benzene from CS2. See the Supporting
from chloroform/n-pentane confirmed the Information for full spectra.
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All compounds have been investigated by UV/Vis and
fluorescence spectroscopy (Figure 5; Table 1; Supporting
Information, Figures S29–36). Both UV/Vis spectra obtained
for 4 and 7 are comparable with broad maxima at labs = 340
and 332 nm (two shoulders are associated with each absorp-
tion maximum, labs = 364 and 382 nm for the former, and 366
and 342 nm, for the latter), which most likely can be assigned
inner rings are more aromatic for HBO 8 than in HBO
1 (Figure 6).
In summary, we have introduced the synthesis of con-
formationally highly stable contorted hexabenzoovalene 1,
which is end-capped with triptycene units, thereby improving
the solubility of the compound (10 mmolmLÀ1 CHCl3) and at
the same time rendering it crystallizable. A selective deal-
kylation of inner tert-butyl groups gave hexabenzoovalene 8,
which has the same p-electron system but a different degree
of contortion that is reflected in the equivalent shape and
structure of the UV/Vis absorption peaks (just hypsochromi-
cally shifted). DFT calculations revealed that this most likely
can be assigned to the different
to S0 to S1 transitions. Compounds 4 and 7 emit at lem
=
434 nm (F = 15%) and lem = 454 nm (F = 13%). In compar-
ison to 4 and 7, the spectra of the corresponding cyclo-
dehydrogenated PAHs 5 and 1 show a bathochromic shift of
absorption maxima to lmax = 428 and 547 nm, respectively.
degree of p-curvature. As a conse-
quence of conformational stability
and geometry, HBO 1 is an ideal
unit to construct carbon nanotubes
with a zigzag conformation, which
is currently under investigation in
our laboratory.
Keywords: aromaticity · ovalenes ·
polycyclic aromatic hydrocarbons ·
triptycene · X-ray diffraction
Figure 5. UV/Vis absorption (solid lines, normalized) and emission spectra (broken lines, normalized
to lmax) of Diels–Alder products 4, 7 and PAHs 1, 5, and 8 in chloroform at room temperature.
Table 1: Absorption and emission data.
Compound labs [nm][a,b] Eg(opt) [eV][c] lem [nm][a,d] lS [cmÀ1
]
F [%]
4
7
5
1
8
382
366
428
547
517
3.1
3.1
2.8
2.1
2.3
439
434
491
580
527
3398
4281
2998
1040
3670
15
13
4
54
55
[a] Measured in CHCl3 at room temperature. [b] Absorption maximum at
the longest wavelength. [c] Estimated from absorption onset. [d] Emis-
sion maximum (lex =highest intensity of absorption plus 10 nm).
Figure 6. NICS(0) values (negative) of compounds 1 and 8 (B3LYP/6-
31G* level). Shaded rings show aromatic character.
The peaks are more structured, which is most likely related to
a rigidification of the molecular backbone. HBO 1 emits with
a bright orange color with a maximum at lem = 580 nm and
a high quantum yield of F = 54%. Although HBO 8 has the
same p-backbone as HBO 1, all peaks are hypsochromically
shifted. The S0 to S1 transition is found at labs = 517 nm and
has the same vibronic fine structure. HBO 8 shows a bright
green emission (lem = 527 nm, F = 55%), which is also
hypsochromically shifted in comparison to that of 1. The
bathochromic shift of the two absorption maxima of HBOs
1 and 8 has also been calculated by TD-DFT at the B3LYP-
level (Supporting Information, Table S1) and most likely is
generated by the different degree of curvature of the p-system
than by the induction of electron-density by the internal two
tert-butyl groups.[13] Calculations of nuclear independent
chemical shifts (NICS) confirm that the pyrene derived
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Received: August 9, 2016
Published online: && &&, &&&&
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Communications
Polycyclic Aromatic Hydrocarbons
K. Baumgꢁrtner, A. L. Meza Chincha,
A. Dreuw, F. Rominger,
M. Mastalerz*
&&&& — &&&&
A Conformationally Stable Contorted
Hexabenzoovalene
Same but different: Two hexabenzo-
ovalenes with the same p-backbone show
significant differences in conformational
stability and emission in response to the
presence or absence of two tert-butyl
groups.
6
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