Curved polycyclic aromatic molecules that are π-isoelectronic to hexabenzocoronene

Article abstract of DOI:10.1021/ja3054354

Reported here are two types of curved π-molecules that are π-isoelectronic to planar hexabenzocoronene (HBC) but are forced out of planarity either by an embedded seven-membered ring or by atom crowding at the fjord region. Embedding a heptagon in HBC leads to a novel saddle-shaped molecule 1, whose π-backbone is slightly less curved than the previously reported [7]circulene in terms of the average Gauss curvature, but surprisingly much more rigid than [7]circulene. Overcrowded fjord regions in novel derivatives of hexabenzoperylene (HBP) 2a,b lead to both chiral twisted and antifolded conformers. The successful synthesis of 1 and 2a,b is related to introducing alkoxyl groups to unprecedented positions of hexaphenylbenzenes. It is found that the red twisted isomer of 2b isomerizes at elevated temperature to the yellow anti-folded conformer. This finding along with the study on the thermodynamics and kinetics of the thermal isomerization has improved the early understandings on the conformation of HBP. In the crystals, 1 lacks π-π interactions between neighboring molecules, while twisted-2a exhibits both face-to-face and edge-to-face π-π interactions. Twisted-2b is found to function as a p-type semiconductor in thin film transistors, but the thin films of 1 appear insulating presumably due to lacking π-π interactions. By exploring three different types of curvatures in 1 and the two isomers of 2b, this study has revealed that the curvature of π-face plays a role in determining the frontier molecular orbital energy levels and π-π interactions and thus needs to be considered when one designs new organic semiconductors.

Full text of DOI:10.1021/ja3054354

Article
pubs.acs.org/JACS
Curved Polycyclic Aromatic Molecules That Are π‑Isoelectronic to
Hexabenzocoronene
Jiye Luo,† Xiaomin Xu,† Renxin Mao,† and Qian Miao*,†,‡
†Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
‡Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee), Hong Kong, China
S
* Supporting Information
ABSTRACT: Reported here are two types of curved π-molecules that are π-isoelectronic to planar hexabenzocoronene (HBC)
but are forced out of planarity either by an embedded seven-membered ring or by atom crowding at the fjord region. Embedding
a heptagon in HBC leads to a novel saddle-shaped molecule 1, whose π-backbone is slightly less curved than the previously
reported [7]circulene in terms of the average Gauss curvature, but surprisingly much more rigid than [7]circulene. Overcrowded
fjord regions in novel derivatives of hexabenzoperylene (HBP) 2a,b lead to both chiral twisted and antifolded conformers. The
successful synthesis of 1 and 2a,b is related to introducing alkoxyl groups to unprecedented positions of hexaphenylbenzenes. It
is found that the red twisted isomer of 2b isomerizes at elevated temperature to the yellow anti-folded conformer. This finding
along with the study on the thermodynamics and kinetics of the thermal isomerization has improved the early understandings on
the conformation of HBP. In the crystals, 1 lacks π−π interactions between neighboring molecules, while twisted-2a exhibits both
face-to-face and edge-to-face π−π interactions. Twisted-2b is found to function as a p-type semiconductor in thin film transistors,
but the thin films of 1 appear insulating presumably due to lacking π−π interactions. By exploring three different types of
curvatures in 1 and the two isomers of 2b, this study has revealed that the curvature of π-face plays a role in determining the
frontier molecular orbital energy levels and π−π interactions and thus needs to be considered when one designs new organic
semiconductors.
nanotubes containing both heptagons and pentagons.13 To
experimentally approach these interesting negatively curved
INTRODUCTION
■
Polycyclic aromatic hydrocarbon (PAHs) molecules can be
carbon structures, heptagon-embedded PAHs can serve as
forced into nonplanar structures commonly either by
embedded nonhexagonal rings1 or by steric strain from atom
segments, models and, in principle, synthetic precursors.
crowding.2 As shown in Figure 1a, reported here are curved
However, saddle-shaped PAHs that are embedded with
seven-membered rings are very rare. To the best of our
large polycyclic aromatic compounds that are π-isoelectronic to
knowledge, [7]circulene14 (shown in Figure 1a) and [7.7]-
the well-known planar hexa-peri-hexabenzocoronene (HBC)3
circulene15 are the only known examples of such negatively
but are forced out of planarity by the two strategies.
curved π-molecules. Here we report a new saddle-shaped π-
Embedding five-membered rings into an otherwise flat PAH
molecule (1 shown in Figure 1a), which has a heptagon
molecule can lead to curved PAH molecules known as aromatic
bowls,4 (or buckybowls6), which not only serve as segments
embedded in HBC. Unlike [7]circulene and [7.7]circulene, 1
has an sp3 carbon atom in its seven-membered ring. This is
similar to the bowl-shaped sumanene, which has an sp3 carbon
atom in each of its three five-membered rings.16
and model compounds for fullerene structure and reactivity4−6
but also are used as synthetic precursors for synthesis of
isomerically pure fullerenes7 and carbon nanotubes.8 In
contrast to pentagon-embedded PAHs containing positive
curvature, heptagon-embedded PAHs can give rise to negative
curvature.9,10 The interests on sp2 carbon structures containing
negative curvature can be traced back to theoretical11 and
experimental studies12 on negatively curved graphitic carbon
networks containing heptagonal defects nearly twenty years
ago, and have been recently extended to toroidal carbon
Another type of curved π-molecules explored here are
alkoxylated hexabenzoperylenes 2a,b as shown in Figure 1a.
Like 1, 2a,b has the same number of Clar’s aromatic sextets as
HBC. Similar to hexabenzotriphenylene,17 hexabenzoperylene
Received: June 5, 2012
Published: July 25, 2012
© 2012 American Chemical Society
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a
Scheme 1. Synthesis of Heptagon-Embedded HBC 1
Figure 1. (a) Structures of curved large polycyclic aromatic
compounds and related PAHs; (b) molecular models of twisted-
HBP and anti-HBP as optimized at the B3LYP level of DFT with the
6-31G(d,p) basis set.
can be formally regarded as a partially hydrogenated HBC with
opened bonds, and its curvature arises from overcrowding at its
two fjord regions. They are even more curved than the earlier
reported contorted hexabenzocoronene18 because of the
severer steric strains at the fjord regions. Unsubstituted
hexabenzoperylene (HBP) was first synthesized by Clar et al.
four decades ago19 and reinvestigated by Ohshima et al. in
2004.20 The recent computational study by Agranat et al.21
indicates that HBP has two energy-minimum conformations,
the chiral twisted conformation (twisted-HBP) at the global
minimum and the anti-folded conformation (anti-HBP) at the
local minimum as shown in Figure 1b. The D2 symmetric
twisted-HBP is calculated as more stable than the C2h
symmetric anti-HBP by 20.5 kJ/mol. Because the conversion
between the two conformers is through a twisted-folded
transition state, which is calculated higher than anti-HBP in
energy by 114.7 kJ/mol, anti-HBP can in principle exist at room
temperature. However, the kinetic stability of anti-HBP was not
recognized in the previous studies,20,21 and it was concluded
that “only the twisted conformation of HBP exists a room
temperature in solution”.21 The convenient synthesis of 2a,b
has led us to a comprehensive investigation on the twisted and
anti-folded conformations of 2a,b and their thermal isomer-
ization as detailed below. Moreover, with suitable energy level
of highest occupied molecular orbital (HOMO) and π−π
stacking, 2b is found to function as a p-type semiconductor in
organic thin film transistors (OTFTs).
a
Reagents and conditions: (a) DCC, DMAP, CH2Cl2; (b) 1,2-
diphenylethane-1,2-dione, KOH, C2H5OH, reflux; (c) (CH3)3COK,
ether; (d) BBr3, CH2Cl2; (e) C6H13Br, K2CO3; (f) FeCl3, CH3NO2,
CH2Cl2, 3 h.
This synthesis was adapted from the well-known Mullen’s
̈
synthesis of substituted HBCs3 including Diels−Alder cyclo-
addition and Scholl-type oxidative cyclodehydrogenation as the
key steps. The seven-membered ring was introduced from 10-
bromo-5H-dibenzo[a,d]cycloheptene (4),23 which generated a
strained alkyne in situ as the dienophile in the Diels−Alder
cycloaddition with cyclopentadienone 3. After changing the
methyl groups to hexyl groups to increase solubility of the final
product, the resulting heptagon-containing hexaphenylbenzene
5 was then subjected to oxidative cyclodehydrogenation with
FeCl3 as both Lewis acid and oxidant. In this reaction, both the
position of alkoxyl groups and the reaction time were found
playing important roles. It was necessary to equip the heptagon-
containing hexaphenylbenzene with alkoxyl groups ortho or
para to the reaction sites since the Scholl reaction was reported
to preferably form C−C bonds ortho or para to activating
substituents.24 Unlike 5, the analogous precursors with alkoxyl
groups meta to the reaction sites, 6 and 7 (shown in Chart 1),
at the same condition yielded complex mixtures, from which
the corresponding heptagon-embedded HBCs could not be
isolated. It was found that the products of oxidative
cyclodehydrogenation were highly dependent on the reaction
time. When the reaction time was less than 2.5 h, an
RESULTS AND DISCUSSION
■
Synthesis. Shown in Scheme 1 is the synthesis of 1 starting
from commercially available 3,5-dimethoxyphenylacetic acid.22
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a
Chart 1. Precursors That Failed to Yield Heptagon-
Embedded HBCs
Scheme 2. Synthesis of Hexbenzoperylene 2a,b
incompletely cyclized product 8 (shown in Chart 2) was found
in the crude product and was difficultly removed from 1 by
Chart 2. Byproducts from Oxidative Cyclodehydrogenation
of 5 with FeCl3
column chromatography or recrystallization. When the reaction
time was prolonged to 3 h, the reaction yielded 1 (62%) along
with a monochlorinated product 9 (37%) as shown in Chart 2.
The exact position of chlorine in 9 was not determined based
on its complicated 1H NMR spectrum. Further prolongation of
reaction time led to unknown products with lower yield of 1.
Moreover, the recently developed DDQ/CH3SO3H25 was also
tested in the oxidative cyclodehydrogenation, but only yielded
the incompletely cyclized product 8 (67%).
a
Reagents and conditions: (a) diphenyl ether, reflux; (b) DDQ,
CH3SO3H, CH2Cl2; (c) i. BBr3, CH2Cl2; ii. K2CO3, C6H13Br, DMF.
Shown in Scheme 2 is the synthesis of 2a,b starting from
cyclopentadienone 3 following Mullen’s strategy for synthesiz-
̈
3
ing substituted HBCs. Interestingly, the Scholl-type oxidative
cyclodehydrogenation of tetraalkoxy-hexabenzobenzenes 10a
and 10b with the recently developed DDQ/CH3SO3H25 did
not yield the corresponding HBCs but 2a and 2b as red solids,
respectively. The possible reason for this incomplete cyclo-
dehydrogenation is that the Scholl reaction is directed by the
activating alkoxyl groups to form C−C bonds ortho or para to
the activating substituents24 first leading to the hexabenzoper-
ylene backbone, which is too crowded at the fjord regions to
allow two benzene rings getting close enough to form a C−C
bond. In agreement with this hypothesis, 2a and 2b appeared
inert toward oxidation by DDQ/CH3SO3H or FeCl3. Due to its
lower solubility, 2a was more difficultly purified by column
chromatography on silica gel leading to the lower isolated yield.
Structures and Properties of Heptagon-Embedded
HBC 1. Heptagon-embedded HBC 1 is soluble in common
organic solvents resulting in a yellow solution with green
fluorescence when irradiated with UV light. Shown in Figure 2
(top) are the absorption and fluorescence spectra of 1 in
CH2Cl2. In comparison to HBC and alkylated HBCs,26
although 1 exhibits absorptions at very similar wavelengths,
its absorption in the visible light region is much more intensive.
The molar extinction coefficient of 1 at the longest-wavelength
absorption (λmax at 472 nm) is higher than that of HBC (λmax at
463 nm) by 2 orders of magnitude.27 Such enhanced
absorption can be attributed to the lower symmetry of 1
Figure 2. Absorption and fluorescence (excited at 410 and 440 nm,
respectively) spectra of 1 (top) and 8 (bottom) in CH2Cl2 (5 × 10−6
mol/L).
since the very weak longest-wavelength absorption of HBC is
known due to the strictly forbidden 0−0 transition in D6 h
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symmetry.26,28 Unlike the substituted HBCs of lower
symmetry, which have less than six alkyl or alkoxyl chains
attached to the D6h symmetric chromophore,26,28 1 has a
curved π-backbone of Cs symmetry, leading to the stronger
longest-wavelength absorption. The fluorescence spectra of 1
exhibits a small Stokes shift from 472 to 480 nm. Unlike 1, the
incompletely cyclized molecule 8 exhibits broad absorption
bands without fine structures (vibrational details) and a much
larger Stokes shift from 444 to 506 nm as shown in Figure 2
(bottom), both suggesting that 8 is more flexible than 1.29 This
is in accordance with the fact that the incompletely cyclized
structure of 8 can allow greater movement. The cyclic
voltammogram of 1 in CH2Cl2 exhibits two reversible oxidation
waves with half-wave oxidation potentials of 0.41 and 0.64 V vs
ferrocenium/ferrocene. It does not exhibit any reduction waves
in the testing window. From the first half-wave oxidation
potential, the HOMO energy level of 1 is estimated as −5.21
eV.30
ring was excluded for the purpose of calculating curvature of
the π-face of 1 as it is not a part of the sp2 network. The total
Gauss curvature for the π-face of 1 is calculated as −0.6483 sr,
which is more negative than that of [7]circulene (−0.4857
sr).32 The average Gauss curvature for the π-face of 1 is
calculated as −0.01477 sr·Å−2, which is slightly less negative
than that of [7]circulene (−0.01736 sr·Å−2) because 1 has a
larger π-face. As highlighted in Figure 3b, the C−C bonds
shown in blue have bond lengths close to those of the C−C
single bonds shown in green. This can be attributed to the
localization of π-bonds in the seven-membered ring. Moreover,
the bonds shown in magenta are longer than the corresponding
bonds in HBC by 0.013 to 0.022 Å. Such lengthened bonds
may be related to distortion arising from the embedment of a
heptagon in an originally flat π-molecule. Shown in Figure 3c is
the molecular packing of 1, which interestingly does not exhibit
π−π interactions between the two neighboring curved π-faces.
The distance between the two curved π-faces is 8.72 Å as
measured from the distance between the two least-squares
planes of central benzene rings. The space between the two
curved π-faces is occupied by alkyl side chains. The absence of
π-interactions between molecules of 1 in principle prevents its
application as an organic semiconductor in the solid state.
The π-backbone of 1 is in principle able to invert itself in a
way similar to the saddle-to-saddle inversion of [7]circulene,
which is through a planar transition state with a small activation
energy of about 36 kJ/mol as indicated by an early
computational study.33 As 1 has two chemically different
protons in its seven-membered ring and the two protons
interconvert during the inversion process, variable temperature
1H NMR spectroscopy can be conveniently used to monitor
the saddle-to-saddle inversion of 1 in solution. In this study, the
1H NMR spectra of 1 in CDCl3 from 25 to 55 °C and in
DMSO-d6 from 90 to 150 °C were recorded due to the low
solubility of 1 in DMSO-d6 at lower temperature. In the two
temperature ranges, the benzylic protons in the seven-
membered ring of 1 exhibited a pair of well-resolved doublet
signals (at 3.72 and 4.09 ppm in CDCl3, and at 3.58 and 4.22
ppm in DMSO-d6) without movement and broadening of
signals as shown in the Supporting Information. This indicates
that the saddle-to-saddle inversion of 1 is a slow process
compared to the NMR time scale or does not occur in the
temperature range tested here. Taking an assumption that the
two signals reached coalescence at 150 °C, an exchange rate
could be estimated as 329 S−1 using k = πΔν/√2, where Δν
=148 Hz is the frequency separation of the two signals under
the condition of slow exchange as measured in CDCl3. This
exchange rate would lead to an activation free energy of 84 kJ/
mol at 150 °C.34 Therefore the activation free energy for the
saddle-to-saddle inversion of 1 must be higher than 84 kJ/mol,
indicating that 1 has a rigid π-backbone, which inverts itself
much more difficultly than [7]circulene does. This conclusion
is also in agreement with the finding that 1 shows fine
structures in the UV−vis absorption spectrum with a small
Stokes shift. In contrast, the benzylic protons in the seven-
membered ring of 8 exhibited a pair of well-resolved doublet
signals at −20 °C but two broad peaks at room temperature.
This is in agreement with the finding that the incompletely
cyclized structure of 8 is more flexible.
Single crystals of 1 were grown by slowly evaporating
solvents from solutions in CHCl3. Shown in Figure 3 is the
Figure 3. Crystal structure of 1. (a) Side view and (b) top view of the
π-backbone of 1 with the hexyloxyl groups removed and carbon atom
positions shown as 50% probability ellipsoids; (c) molecular packing
of 1 as viewed along the b axis of the unit cell with hydrogen atoms
removed for clarity. (Carbon, oxygen, and hydrogen atoms are shown
in gray, red, and white, respectively.).
crystal structure of 1, which reveals that the π-backbone of 1
adopts a saddle shape of negative curvature. To quantify the
curvature of the π-face of 1, its Gauss curvature was calculated
using the method detailed in the Supporting Information from
the space coordinates of carbon atoms in the crystal structure.31
Particularly, the benzylic sp3 carbon in the seven-membered
Conformation and Isomerization of Hexabenzoper-
ylenes 2a and 2b. Although anti-HBP is less stable than
twisted-HBP by 20.5 kJ/mol as calculated by Agranat et al.,21 it
can be kinetically stable because the conversion between the
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two conformers is through a twisted-folded transition state,
which is higher than anti-HBP in energy by 114.7 kJ/mol as
calculated. The early synthesized HBP was reported as twisted-
HBP,20 while anti-HBP was not experimentally achieved. With
the red crystals of hexbenzoperylenes 2a,b in hand, we became
interested in the following two questions: Do 2a,b exist as the
twisted or the anti-folded conformation? Can 2a,b convert to
their isomers by changing the conformation of their π-
backbone at elevated temperature?
To answer the first question, crystals of 2a,b were grown
from solutions by slow evaporation of solvents, but only the red
crystals of 2a grown from CH2Cl2 were qualified for single-
crystal X-ray crystallography, which revealed the chiral twisted
conformation of 2a. The unit cell of this crystal contains two
pairs of enantiomers of twisted-2a together with crystallized
solvent molecules as shown in Figure 4a. In the crystal of
twisted-2a•CH2Cl2, the geometry of twisted-2a is essentially
the same as the DFT energy-minimized model except that it
slightly deviates from the D2 symmetry by having unequal
torsion angles in the two fjord regions possibly due to the
experimental errors. As shown in Figure 4b, the torsion angles
as defined by the three magenta bonds in the fjord regions are
42.1° and 39.2°, which are in good agreement with the torsion
angle of 40.9° as found in the DFT energy minimized model.
As indicated by the red and blue arrows in Figure 4c,
neighboring molecules of twisted-2a interact with each other in
both face-to-face and edge-to-face modes.35 A neighboring pair
of enantiomers of twisted-2a stack in a face-to-face arrangement
with the two curved π-faces separated by about 3.6 Å as
measured from two parallel least-squares planes of benzene
rings. Two neighboring twisted-2a molecules of the same
handedness interact with each other in an edge-to-face
arrangement with three intermolecular carbon−carbon contacts
within the range of 3.61 to 3.85 Å.36 Because the red
compounds 2a and 2b exhibit almost identical UV−vis
absorption spectra and 1H NMR spectra in the aromatic
range, it is concluded that the as-synthesized 2b is also the
twisted conformer (twisted-2b).
To test whether twisted-2a and twisted-2b can isomerize to
their anti-folded conformers, we then optimized the twisted and
anti-folded conformers of 2a and HBP at the B3LYP level of
DFT with the 6-31G(d,p) basis set and calculated their highest
occupied molecular orbitals (HOMOs) and lowest unoccupied
molecular orbitals (LUMOs) with the 6-311++G(d,p) basis set.
As summarized in Table 1, the calculated energy for the twisted
Table 1. Relative Energy and Energy Levels of HOMO and
LUMO for Twisted and anti-Folded Conformers of 2a and
a
HBP as Calculated at the B3LYP Level of DFT
twisted-
2a
twisted-
HBP
anti-
HBP
anti-2a
b
relative energy (kJ/mol)
HOMO (eV)
0
0.67
−4.92
−1.76
3.16
0
19.32
−5.46
−2.16
3.30
−4.78
−2.05
2.73
−5.34
−2.41
2.93
LUMO (eV)
HOMO−LUMO gap (eV)
a
The relative energy was optimized with the 6-31G(d,p) basis set, and
the energy levels of HOMO and LUMO were calculated with the 6-
b
311++G(d,p) basis set. The energy of twisted-2a and twisted-HBP is
set as 0.
conformer of 2a (twisted-2a) is only slightly lower than that of
anti-folded conformer (anti-2a) by 0.67 kJ/mol, while the
calculated energy difference between twisted-HBP and anti-
HBP is 19.32 kJ/mol, which is the same as that calculated by
Agranat et al. when using the same basis set.21 This large shift
of relative stability of the twisted isomer upon going from
unsubstituted HBP to 2a is presumably related to a steric
repulsion between the oxygen atom and the neighboring
aromatic hydrogen atom as shown in the Supporting
Information. As measured from energy-minimized models,
such steric repulsion involves an H-to-O distance of 2.07 Å in
twisted-2a and 2.16 Å in anti-2a. These distances are shorter
than the sum of van der Waals radii by about 0.5 Å and suggest
that twisted-2a is destabilized by the steric repulsion in a larger
degree. The most interesting finding from Table 1 is that the
twisted conformers have a smaller HOMO−LUMO gap than
the corresponding anti-folded conformers, predicting that anti-
2a and anti-2b would have blue-shifted absorption in
comparison with their twisted isomers. The different
HOMO−LUMO gaps of twisted and anti-folded conformers
may be related to the different distributions of electrons on the
central benzene ring, which is more distorted in the twisted
conformer.21 A similar effect was observed earlier in some
Figure 4. Crystal structure of twisted-2a•CH2Cl2: (a) a unit cell of
twisted-2a viewed along the a axis showing the crystallized CH2Cl2
molecules; (b) π-backbone of twisted-2b with the methoxyl groups
removed and carbon atom positions shown as 50% probability
ellipsoids; (c) π−π interactions between neighboring molecules of
twisted-2a with hydrogen atoms removed for clarity. (Carbon, oxygen,
hydrogen, and chloroine atoms are shown in gray, red, white, and
green, respectively.).
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overcrowded bistricyclic aromatic enes, which exist as a
colorless or yellow anti-folded conformer at room temperature
and convert to a deep-blue or deep-green twisted conformer at
higher temperature.37 Another finding from Table 1 is that
substitution with alkoxyl groups reduces the HOMO−LUMO
gap of hexbenzoperylene as the electron-donating substituents
raise HOMO by a larger degree than raising LUMO.
indicates that anti-2b would have blue-shifted absorption in
comparison with twisted-2b, we assign the anti-folded
conformation to the yellow isomer of twisted-2b. From the
longest-wavelength absorption, the optical gap of anti-2b is
determined as 2.80 eV, which is 0.39 eV greater than that of
twisted-2b. Such difference agrees quantitatively with the DFT
calculation that the HOMO−LUMO gaps of anti-2a and anti-
HBP are greater than those of their twisted isomers by about
0.4 eV.
Not only the UV−vis absorption but also the electro-
chemistry of the twisted and anti-folded isomers of 2b exhibit
apparent difference. The cyclic voltammogram of twisted-2b in
CH2Cl2 (shown in the Supporting Information) exhibits one
reversible reduction wave, which has the half-wave potential at
−2.04 V vs ferrocenium/ferrocene, and two reversible oxidation
waves, which have the half-wave potential at 0.22 and 0.50 V vs
ferrocenium/ferrocene. From the first oxidation potential and
reduction potential, the HOMO and LUMO energy levels of
twisted-2b are estimated as −5.02 and −2.76 eV, respectively.30
In contrast, the cyclic voltammogram of anti-2b exhibits no
reduction waves in the testing window but only two reversible
oxidation waves, which have the half-wave potential at 0.34 and
0.57 V vs ferrocenium/ferrocene. From the first oxidation
potential, the HOMO energy level of anti-2b is estimated as
−5.14 eV.30 The higher oxidation potentials and absence of
reduction wave in the cyclic voltammogram of anti-2b are in
agreement with the DFT calculation that the anti-folded isomer
has higher HOMO and LUMO energy levels than the twisted
isomer as shown in Table 1.
As suggested by the transition state connecting twisted-HBP
and anti-HBP calculated by Agranat et al.,21 and the small
energy difference between twisted-2a and anti-2a calculated
above, heating the twisted isomer of 2a,b would result in a
mixture of the twisted isomer and the anti-folded isomer.
Following this prediction, we heated twisted-2a and twisted-2b
in toluene. Heating twisted-2a led to a new compound together
with twisted-2a as found from the thin layer chromatography.
1
The H NMR spectrum of the crude product suggested that
this new compound might be an isomer of twisted-2a, which
unfortunately could not be isolated from the crude product by
column chromatography or recrystallization for full character-
ization. In contrast, refluxing a solution of twisted-2b in toluene
under an atmosphere of nitrogen for 1 h led to a new yellow
compound together with twisted-2b. This yellow compound
was conveniently isolated from the mixture by chromatography
on silica gel and was determined as an isomer of twisted-2b
38
1
based on the H NMR and mass spectra. Shown in Figure 5
To better understand the stability of the two isomers of 2b,
we studied the thermodynamics and kinetics of the thermal
1
isomerization using H NMR spectroscopy. Heating a solution
of either pure twisted-2b or pure anti-2b in toluene at 110 °C
for 1 h led to the same equilibrium mixture of twisted-2b and
anti-2b in a ratio of 1: 0.35, which was easily determined from
1
the integrations in the H NMR spectra of the mixture as
shown in Figure 6a. From this ratio, the free energy change ΔG
for the twisted-to-anti isomerization is calculated as 3.3 kJ/mol,
which is qualitatively in agreement with the calculated results
that the anti-folded isomer is less stable. Because anti-2b is less
soluble than twisted-2b, the most effective way to prepare anti-
2b is to boil a solution of twisted-2b in toluene without a
condenser allowing part of the solvent to evaporate. In this way,
precipitation of anti-2b as yellow powders shifts the equilibrium
leading to a higher yield of anti-2b. The thermal isomerization
of twisted-2b is a reversible unimolecular reaction, and its rate
constants can be estimated by using the equation ln([x]e/([x]e
− [x])) = (kf + kr)t or its equivalent form, −ln(1 − [x]/[x]e) =
(kf + kr)t, where [x] is the concentration of twisted-2b that has
been depleted at a certain time t representing the extent of
reaction, [x]e is defined as [x] at equilibrium, kf and kr are the
rate constants for the forward and reverse reactions,
respectively.40 In the kinetic experiment, a solution of
twisted-2b in toluene-d8 was heated at 110 °C in a sealed
NMR tube, and the progress of twisted-to-anti isomerization
was monitored with 1H NMR spectroscopy. The integrations of
the two sets of peaks were used to calculate the ratio of [x]e/
[x]. Ploting −ln(1 − [x]/[x]e) versus time leads to a slope of
1.48 × 10−3 s−1 as shown in Figure 6b. From this slope and the
reaction equilibrium constant K = kf/kr = 0.35, the rate
constants kf and kr at 110 °C are determined as 3.8 × 10−4 s−1
and 1.1 × 10−3 s−1, respectively. Using the Eyring equation k =
κ(kBT/h)exp(−ΔG‡/RT) and assuming a value of unity for the
Figure 5. Absorption and fluorescence (excited at 500 and 430 nm,
respectively) spectra of twisted-2b (top) and anti-2b (bottom) in
CH2Cl2 (5 × 10−6 mol/L).
are the absorption and fluorescence spectra of twisted-2b and
its yellow isomer in CH2Cl2. Compared with the reported
spectrum of twisted-HBP, the absorption of twisted-2b has
almost the same shape but shifts to red by about 50 nm.39 Such
red shift is in agreement with DFT calculation that the electron-
donating alkoxyl substituents reduce the HOMO−LUMO gap.
On the other hand, the yellow isomer of twisted-2b exhibits
absorption bands of different shape. The longest-wavelength
absorption of this yellow isomer shifts to blue by about 70 nm
relative to that of twisted-2b. Because the DFT calculation
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Article
possibly arises from the large number of grain boundaries
between small crystallites. On the other hand, the films of 1
behaved as an insulator, which can be attributed to the absence
of π−π interactions in its solid state.
CONCLUSION
■
In summary, this study has explored how the properties of
curved π-molecules depend on their curvature by synthesizing
and characterizing two types of curved π-molecules that are π-
isoelectronic to the planar HBC. They are forced out of
planarity either by an embedded seven-membered ring or by
atom crowding at the fjord region. The successful synthesis of
these compounds is related to introducing alkoxyl groups to
unprecedented positions of hexaphenylbenzenes. Embedding a
heptagon in HBC leads to a novel saddle-shaped molecule 1,
whose π-backbone is slightly less curved than the previously
reported [7]circulene in terms of the average Gauss curvature,
but surprisingly much more rigid than [7]circulene. Over-
crowded fjord regions in novel derivatives of hexabenzoper-
ylene (HBP) 2a,b lead to chiral twisted and anti-folded
conformers, which exhibit different photophysical and electro-
chemical properties. Red twisted-2b converts at elevated
temperature to its yellow isomer, anti-2b, with an activation
free energy of 120 kJ/mol. The red-shifted absorption of
twisted conformer may be attributed to the severer distortion of
its central benzene ring, which leads to redistribution of
electrons in HOMO and LUMO. These findings have
improved the early understandings on the conformation of
HBP. Unlike 1, which lacks π−π interactions in the solid state,
2a exhibits both face-to-face and edge-to-face π−π interactions.
It is found that twisted-2b functions as a p-type semiconductor
in thin film transistors, while the thin films of 1 appear
insulating presumably due to lacking π−π interactions. In view
of the three different types of curvatures in 1 and the two
isomers of 2b, we come to the conclusion that the curvature of
π-face plays a role in determining the frontier molecular orbital
energy levels and π−π interactions and thus needs to be
considered when one designs new organic semiconductors.
1
Figure 6. Isomerization of 2b as monitored by H NMR (400 MHz)
1
spectroscopy: (a) selected H NMR spectra of twisted-2b and anti-2b
during the progress of twisted-to-anti isomerization in toluene-d8 at
110 °C; (b) a plot of −ln(1 − [x]e/[x]) for twisted-2b versus time.
transmission coefficient (κ),40 the activation free energy ΔG‡ is
calculated as 120 kJ/mol for the twisted-to-anti isomerization
and 116 kJ/mol for the anti-to-twisted isomerization, which is
in agreement with the calculated energy barrier for anti-HBP
(114.7 kJ/mol).21
Semiconductor Properties. To explore the potential of
the curved π-molecules as semiconductors, we tested 1, twisted-
2b, and anti-2b in thin film transistors. Because of the thermal
isomerization of twisted-2b and anti-2b, thin films of these
compounds were fabricated not by thermal evaporation, which
requires high temperature, but by solution process at much
lower temperature. To fabricate thin film transistors, a solution
of 1, twisted-2b, or anti-2b in chlorobenzene was drop-cast
onto a silicon wafer, which had its SiO2 surface pretreated with
a self-assembled monolayer of hexamethyldisilazane (HMDS)
and had its predeposited bottom-contact gold source and drain
electrodes modified with a self-assembled monolayer of
pentafluorobenzenethiol (PFBT).41 Under the drop-casting
condition, the isomerization of twisted-2b or anti-2b was
negligible because the solution was heated at 60 °C within only
30 s before the solvent evaporated completely. This was
ASSOCIATED CONTENT
* Supporting Information
■
S
Details of synthesis and characterization, the crystallographic
information files (CIF) for 1 and twisted-2a, fabrication and
characterization of organic thin film transistors, NMR spectra,
and calculation of the Gauss curvature. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
miaoqian@cuhk.edu.hk
■
1
supported by the H NMR spectra from the solutions of these
Notes
films, from which the corresponding isomers were not
observed. The drop-cast films of 1 and twisted-2b were
composed of connected crystallites, while those of anti-2b only
contained isolated crystallites of larger size. Because the isolated
crystallites of anti-2b were not able to form continuous pathway
for charge transport, the films of anti-2b appeared insulating.
Unlike its anti-folded isomer, twisted-2b performed as a p-type
semiconductor in the thin films with a field effect mobility of 5
× 10−5 to 2 × 10−4 cm2 V−1 s−1. This semiconductor property is
in agreement with the suitable HOMO energy level of twisted-
2b and a sufficient π−π stacking as suggested by the crystal
structure of twisted-2a. The low mobility of twisted-2b very
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We sincerely thank Dr. Yi Liu (Department of Mathematics,
University of California, Berkeley) for the calculation on Gauss
curvature and Ms. Hoi Shan Chan (Department of Chemistry,
the Chinese University of Hong Kong) for the single crystal
crystallography. This work was supported by the University
Grants Committee of Hong Kong (project number AoE/P-03/
08) and the Research Grants Council of Hong Kong (project
number GRF402412).
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(27) The molar extinction coefficient of 1 at 472 nm is 4.22 × 104 L
mol−1 cm−1, while the molar extinction coefficient of 1 at 463 nm is
220 L mol−1 cm−1 as reported in ref 26.
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NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was published ASAP on August 8, 2012. Changes
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