The synthesis of lactone-bridged 1,3,5-triphenylbenzene derivatives as pi-expanded coumarin triskelions

Article abstract of DOI:10.1016/j.tetlet.2017.11.010

Two triply lactone-bridged 1,3,5-triphenylbenzene derivatives with solubilizing moieties have been synthesized in five and six steps from commercially available starting materials. Compounds containing the 1,3,5-triphenylbenzene core with two atom bridges are relatively unknown. This new class of pi-expanded coumarins contain triskelion architectures and X-ray crystallographic studies of one of the triskelions indicates that the 1,3,5-triphenylbenzene core adopts a near-planar geometry. This is the only known example of a two atom-bridged 1,3,5-triphenylbenzene derivative to adopt a planar structure.

Full text of DOI:10.1016/j.tetlet.2017.11.010

Accepted Manuscript
The synthesis of lactone-bridged 1,3,5-triphenylbenzene derivatives as pi-ex-
panded coumarin triskelions
Heather A. Hintz, Nicholas J. Sortedahl, Samantha M. Meyer, Daniel A. Decato,
Bart J. Dahl
PII:
S0040-4039(17)31404-1
https://doi.org/10.1016/j.tetlet.2017.11.010
TETL 49453
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 September 2017
26 October 2017
5 November 2017
Please cite this article as: Hintz, H.A., Sortedahl, N.J., Meyer, S.M., Decato, D.A., Dahl, B.J., The synthesis of
lactone-bridged 1,3,5-triphenylbenzene derivatives as pi-expanded coumarin triskelions, Tetrahedron Letters
(2017), doi: https://doi.org/10.1016/j.tetlet.2017.11.010
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The synthesis of lactone-bridged 1,3,5-triphenylbenzene derivatives as pi-expanded
coumarin triskelions
Heather A. Hintz,† Nicholas J. Sortedahl, † Samantha M. Meyer, † Daniel A. Decato,‡ and Bart J.
Dahl*,†
†Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI, 54702, United
States
‡Department of Chemistry and Biochemistry, Macromolecular X-ray Diffraction Facility,
University of Montana, Missoula, MT, 59812, United States
*Corresponding author. email: dahlbj@uwec.edu
Abstract
Two triply lactone-bridged 1,3,5-triphenylbenzene derivatives with solubilizing moieties have
been synthesized in five and six steps from commercially available starting materials.
Compounds containing the 1,3,5-triphenylbenzene core with two atom bridges are relatively
unknown. This new class of pi-expanded coumarins contain triskelion architectures and X-ray
crystallographic studies of one of the triskelions indicates that the 1,3,5-triphenylbenzene core
adopts a near-planar geometry. This is the only known example of a two atom-bridged 1,3,5-
triphenylbenzene derivative to adopt a planar structure.
Keywords
branched molecules; coumarins; extended conjugation; planar geometry; triskelions
Introduction
Large highly conjugated organic molecules with restricted bond rotation have long
fascinated chemists because of their unique and attractive optical and electronic properties. It is
known that enforcing rigidity and planarity in conjugated pi-systems typically results in more
efficient absorption and emission of light1 as well as improved electronic conductance.2 Biphenyl
is the simplest conjugated system to contain multiple arene units connected by a single bond.
However, biphenyl is not a rigid or planar molecule, with solution-phase dihedral angles
measured to be between 30-40º, depending on the solvent.3 Our group has been interested in
bridging aryl-aryl bonds with lactone units, in the form of new pi-expanded coumarins,4 to force
rigidity and planarity in the aryl-aryl molecular structures.5 Our previous efforts in this area have
been to study the effects of restricting aryl-aryl bond rotation in various biphenyls5a and C2h
symmetric p-terphenyl crankshaft-shaped systems5b with one and two lactone bridges,
respectively, resulting in new pi-expanded coumarin structures. We found that the presence of
lactone bridging units greatly enhances both optical absorption and emission relative to their
lactone-cleaved analogs.
Branched pi-conjugated small molecules and dendrimers with arene cores have been
previously synthesized and studied for possible application in organic solar cells, field-effect
transistors, light-emitting diodes, and non-linear optical devices.6 Within this class of molecules,
1

many triply-bridged triskelion-shaped C3h symmetric compounds containing the 1,3,5-
triphenylbenzene core unit have also been synthesized (Figure 1).7,8,11 Derivatives of 1,3,5-
triphenylbenzene containing one atom bridges, such as truxene and its analogs,7 have been
known for many years and are widely studied. Many derivatives of these structures have also
been recently synthesized, many with promising optical and electronic properties.8,9
Interestingly, despite containing the fluorene moiety (fluorescence quantum yield (ΦF) = 0.80),10
truxene itself is only weakly fluorescent (ΦF = 0.07).8m However, appending arene units to the
basic truxene structure has resulted in highly fluorescent compounds with two-photon absorption
properties, non-linear optical properties and potential for application in organic light-emitting
diodes (OLEDs).8g-k,9 Despite its early synthesis, truxenone7b,c and many of its derivatives had
been poorly studied because to its poor solubility8e due to planarity and aggregation. However it
has been incorporated as a starting point for the synthesis of many interesting molecules with
potential use as photovoltaic devices, NLO materials, and semiconductors.8b,d-f Trioxatruxene7d
and its derivatives,8l having only been synthesized in very small yields, are currently less well
studied. Triazatruxene has been synthesized7e and its derivatives studied as the core unit for
organic solar cells.8a Unlike the numerous single-atom bridged systems described above,
synthesis of triskelion shaped 1,3,5-triphenylbenzene compounds with two atom bridges (C-O,
CH2-CH2, and CH=CH are known) have been quite rare and some only with limited
characterization studies.11 The large majority of these structures are based on the twisted
benzo[c]naphtho[2,1-p]chrysene motif with CH=CH bridges.11a,e-i None are known to be planar
compounds.
Figure 1. Examples of known triply-bridged 1,3,5-triphenylbenzene triskelions.
We were interested in further exploring branched variations of the lactone-bridged
oligophenyls previously studied by our group.5 We desired to expand our knowledge of these
unique structures by synthesizing and analyzing two-dimensional triskelion-shaped analogs of
the linear lactone-bridged biphenyls and terphenyls (Figure 2). Given the dearth of 1,3,5-
triphenylbenzene derivatives with two atom bridges and complete lack of those with known
planar structures, the fundamental knowledge obtained from the synthesis and study of these new
compounds is important. Two potential “bridge-flipped” isomeric pi-expanded coumarins
containing 1,3,5-triphenylbenzene cores could be envisioned with C3h symmetry, neither known
2

to date. Compound 1 and its derivatives were chosen as the synthetic targets over the more
sterically congested “bridge-flipped” isomers in this study due to their predicted planarity
relative to the isomeric form and more accessible syntheses. In the “bridge-flipped” isomer of 1
it would be predicted that the carbonyl unit would have unfavorable steric interactions with the
adjacent arene C-H, thus affording a twisted structure. In the proposed triskelion 1, the carbonyl
unit is away from the adjacent ring and the oxygen should be small enough to avoid significant
steric interactions with the adjacent arene C-H.
Previous work: linear biphenyls and p-terphenyl crankshafts with lactone bridges
O
O
O
O
O
O
R
R
R
R
R
R
O
O
O
O
This work: 1,3,5-triphenylbenzene triskelions with lactone bridges
O
O
O
O
O
O
O
O
O
O
O
1
O
1
"Bridge-flipped" isomer of
not synthesized
insoluble trilactone bridged
1,3,5-triphenylbenzene
O
O
O
O
O
O
O
O
O
O
O
2
3
O
synthesized derivatives of 1 with solubilizing groups
Figure 2. C3h symmetric trilactone bridged 1,3,5-triphenylbenzene triskelions
Results and discussion
We initially embarked to synthesize the most basic variation of a triskelion-shaped pi-
expanded coumarin: previously unexplored C3h symmetric tri-lactone bridged 1,3,5-
triphenylbenzene derivative 1 containing no additional substituents (Scheme 1). To synthesize 1,
it was ascertained that benzylic oxidation of known triether 6 should result in 1 in only three
steps from commercially available starting materials. Attempts to achieve 6 replicating a known
procedure11d of triple ether formation from phloroglucinol followed by intramolecular Heck
coupling were met with unacceptable yields in our hands. This was optimized by replacing
phloroglucinol with triacetoxybenzene (4) to avoid C-alkylation during the triple ether
formation.12 The addition of tricylohexylphosphine13 in the subsequent triple Heck coupling to
achieve 6 also led to improved yields. Oxidation of 6 with excess PCC14 afforded a white solid
but, to our disappointment, was completely insoluble in all organic solvents, including DMSO,
diethyl ether, dichloromethane, chloroform, and toluene. By contrast, the immediate precursor to
1, previously reported triskelion-shaped triether 6, had excellent solubility in many organic
solvents. The poor solubility of 1, likely due to the planarity and rigidity of the structure inducing
3

efficient pi-stacking and aggregation, meant that it could not be chromatographically purified or
characterized using solution-phase techniques, primarily NMR spectroscopy. Since compounds
containing the basic structure of 1 were completely unknown, we set out to produce analogs with
more tractable properties.
Br
O
O
AcO
OAc
HO
OH
Ac2O
Br
4
O
pyridine
microwave
NaH, H2O
DMF
63%
OAc
4
OH
Br
Br Br
87%
5
O
O
O
O
O
O
PCC
Pd(OAc)2
celite
CH2Cl2
PCy3 HBF4
K2CO3
DMA
O
O
O
1
6
76%
Insoluble: could not adequately purify or characterize
Scheme 1. Attempted Synthesis of Unsubstituted Triskelion 1.
Given the known utility of t-butyl groups for improving solubility of polycyclic aromatic
hydrocarbons in our own work5b and others,15 triskelion 2 was designated as the next target
(Scheme 2). The placement of the t-butyl group meta to the aryl-aryl bond in 2 was largely due
to synthetic convenience: 4-tert-butyltoluene is inexpensive and widely available commercially
whereas 3-tert-butyltoluene is not. Following known protocols16 4-tert-butyltoluene was
brominated sequentially on both the aromatic ring and the benzyl carbon with sodium bromate
and sodium bisulfite though a simple change in solvent, resulting in 7. Triple-ether formation of
7 with phloroglucinol gave 8 in reasonable yields, albeit requiring a 48 h reaction time. Triple
Heck coupling and subsequent benzylic oxidation with excess PCC and celite resulted in
triskelion pi-expanded coumarin 2 in five total synthetic steps. Triskelion 2 had only slightly
improved solubility compared with 1 but, fortunately, it could be adequately characterized using
solution phase techniques. However, after testing numerous solvents it was found that 2 was only
very slightly soluble in dichloromethane and chloroform, and was effectively insoluble in all
other solvents thus making it only a minor improvement compared to 1. Other attempts by our
group to synthesize analogous triskelion-shaped compounds containing other commonly used
solubilizing groups, triisopropylsilylacetylene or oligoethylene glycol, were unfortunately
fruitless. We discovered that both of these groups were incompatible with the harsh PCC
oxidation step, resulting in decomposition. This prompted a synthesis of triskelion 3 which was
anticipated to be more robust to reaction conditions, retain the same structural rigidity in its
1,3,5-triphenylbenzene core, and more amenable to characterization and further studies.
Although less common than t-butyl groups, 1,1,4,4-tetramethylbutylene fused-ring based
moieties are also known17 to greatly help solubilize polycyclic aromatic hydrocarbons, and
should also be inert to the strongly oxidizing conditions used in the synthesis of 2. To our
excitement, triskelion pi-expanded coumarin 3 was readily synthesized in six steps from
commonly available starting materials (Scheme 2). Arene 10 was prepared in in two steps
4

starting from 2,5-dimethyl-2,5-hexanediol following a literature protocol.18 Benzylic
bromination from NaBrO3/NaHSO3 then resulted in 11. Triskelion 3 was then afforded as
previously described for 1 via a triple ether formation and triple Heck coupling in six total
synthetic steps. The addition of 1,1,4,4-tetramethylbutylene solubilizing groups improved
solubility with 3 being more soluble in dichloromethane, chloroform, and toluene.
Scheme 2. Synthesis of triskelions 2 and 3.
Unlike 1 and 2, Triskelion 3 was easily characterized by 1H NMR spectroscopy.
Comparison of the chemical shifts of the aromatic H signals of 3 and its immediate precursor 13
clearly shows the increased rigidity that the newly added carbonyl groups impart. The aromatic
H signals are significantly downfield in 3 (9.54 ppm and 8.45 ppm) compared to 13 (8.26 ppm
and 7.18 ppm) due to steric compression induced by planarization of the ring-system. Less
soluble triskelion 2 (9.57, 8.31, and 7.79 ppm) also displays a similar downfield shift in aromatic
H signals relative to precursor 9 (8.41, 7.38, and 7.23 ppm). Similar steric compression effects
have been observed in our previous studies on crankshaft terphenyl dilactones5b and other
congested highly-conjugated arene systems.19
To obtain more direct evidence of the planarity-inducing effects of the carbonyl group on
these systems, X-ray crystal studies were desirable. After numerous attempts, 2 could not be
sufficiently crystallized in our hands due to poor solubility. However, the improved solubility of
3 allowed for preparation of X-ray quality crystals, grown successfully though very slow
evaporation in dichloromethane. To compare the effect of ether and lactone bridges on the
structures of triskelion shaped 1,3,5-triphenylbenzene, single-crystal X-ray diffraction was
performed on both ether-bridged precursor 13 and lactone-bridged triskelion 3 (see
supplementary data). The effect of the lactone bridging unit on flattening the structure of 1,3,5-
5

triphenylbenzene is dramatic. The average aryl-aryl dihedral angle between each peripheral arene
ring and the central benzene core found in the X-ray structure of 3 is 5.9°. Precursor 13 is
considerably less planar with average aryl-aryl dihedral angles of 24.7°, resulting in a significant
twist. The sp2 hybridized carbonyl in the bridges of 3 flattens the aryl-aryl units much more
effectively than the sp3 hybridized methylene in the bridges of 13. Additionally, triskelion 3 also
has the opportunity to become planar relative to the known two-atom bridged (CH=CH)
compound benzo[c]naphtho[2,1-p]chrysene (Figure 1), because the oxygen in the bridge of 3 is
much smaller than the arene CH unit in the latter. In benzo[c]naphtho[2,1-p]chrysene, the arene
CH has a significant steric interaction with the adjacent ring, forcing it to twist, whereas the O on
3 is much smaller and allows the adjacent ring to lay flat. Comparing 3 to the known crystal
structure of unbridged 1,3,5-triphenylbenzene20 is also significantly twisted with average aryl-
aryl dihedral angles of 28.3°. While the ether bridges in 13 will undoubtedly restrict aryl-aryl
rotation relative to no bridge at all, they have little effect on the overall geometry of the 1,3,5-
triphenylbenzene core. The lactone bridges in 3 also restrict aryl-aryl rotation but, by contrast,
are much more effective at flattening the structure of 1,3,5-triphenylbenzene. Triskelion 3 has a
very unique structure in that it is the only two atom-bridged 1,3,5-triphenylbenzene derivative
shown to have a near-planar core synthesized to date.
To determine how the introduction of the carbonyl group affects the optical properties of
triskelions 2 and 3, UV-vis and fluorescence spectroscopy studies were done. Comparing the UV
absorption spectra of the compounds in this study (Figure 3a and Table 1), it is seen that
introducing lactone units (2 and 3) results in a significant bathochromic shift (roughly 30 nm in
both cases) relative to the compounds containing only ether bridging units (9 and 13). This is the
result of the presence of the more electron withdrawing carbonyl unit and also 2 and 3 being
more planar than 9 and 13, leading to more efficient conjugation between the outer arenes and
central arene core. Both bridging units result in a bathochromic shift relative to unbridged 1,3,5-
triphenylbenzene with a known absorption λmax = 250 nm in cyclohexane.21
Diminished fluorescence quantum yields of meta-branched oligophenylenes compared to
linear para-oligophenylenes is known.10 This is due to the cross conjugated meta orientation of
the phenyl groups in the branched systems leading to attenuated total conjugation across the
entire system, thus making the branched systems behave more like biphenyls, rather than
terphenyls. Thus, the addition of two or three phenyl rings in meta positions on a central arene
(i.e. m-terphenyl and 1,3,5-triphenylbenzene) has more of an inductive effect on the electronics
of the entire system rather than a conjugative effect. For the branched oligophenylene systems
containing 1,3,5-triphenylbenzene cores in this study, a similar effect was observed (Figure 3b
and Table 1).
Unsubstituted 1,3,5-triphenylbenzene has a known literature emission λmax = 354 nm (ΦF
= 0.10) in cyclohexane.21 The effect of bridging the aryl-aryl bonds with ether units (9 and 13)
increases the emission λmax (Table 1). An increase in ΦF for 13 is also observed but no increase is
observed for 9. The addition of lactone bridges (2 and 3) results in a further bathochromic shift
of the emission λmax and also the absorption λmax. However, the lactone bridges have a dramatic
effect on the fluorescence quantum yields as the ΦF of both 2 and 3 plummets to near zero,
6

despite the lactone bridge enforcing planarity in the structure. The attenuation in ΦF could be a
result of the known carbonyl-induced fluorescence quenching10 or the ability of the near-planar
lactone systems efficiently pi-stacking, resulting in aggregation induced quenching.22
Remarkably, upon addition of a small amount of trifluoroacetic acid (TFA) to samples of
2 and 3 in dichloromethane, the ΦF increases approximately 10-fold and a further small
bathochromic shift in emission λmax occurs (Figure 3b and Table 1). The small bathochromic
shift is likely due to partial protonation of the carbonyl units. The enhancement in fluorescence
was even more pronounced in toluene as the addition of TFA to dilute samples of 3 in toluene
led to visible fluorescence under UV irradiation (Figure 4). This result also points to solubility
and aggregation caused self-quenching likely playing a role in attenuation of fluorescence of 2
and 3, since 9 and 13 are both much more soluble than 2 and 3. It was also observed that
solubility of 2 and 3 increases appreciably upon addition of TFA, again, potentially due to
decreased aggregation. Partial protonation of the lactone carbonyls by the added TFA could
promote the halochromic characteristics of 2 and 3. Both 9 and 13 were found to decompose in
the presence of added TFA, rapidly affording bright red solutions, likely due to the facile
formation of benzylic carbocations under the acidic conditions. The addition of TFA also results
in a small bathochromic shift in the UV-absorption spectra of 2 and 3 (Figure 3a and Table 1).
Figure 3. (a) UV-vis spectra for compounds dissolved in CH2Cl2 or CH2Cl2 with added TFA. (b) Fluorescence
emission spectra for compounds dissolved in CH2Cl2 or CH2Cl2 with added TFA and excited at 325 nm.
Table 1. Electronic absorption and emission data for bridged 1,3,5-triphenylbenzene systems.
c
compd
lowest E abs λmax
[nm] (ε [M-1cm-1])
320 (28554)
323 (34638)
327 (27624)
330 (28507)
em λmax
[nm]
ΦF
2a
2b
3a
3b
393
403
400
410
0.01
0.08
0.01
0.10
7

9a
290 (56535)
293 (59883)
387
381
0.10
0.21
13a
aUV-vis spectral data from compounds dissolved in CH2Cl2;
emission spectral data from compounds dissolved in CH2Cl2
b
and excited at 325 nm. UV-vis spectral data from compounds
dissolved in 5% TFA in CH2Cl2; emission spectral data from
compounds dissolved in 5% TFA in CH2Cl2 and excited at 325
c
nm. Calculated relative to quinine sulfate dissolved in 0.5 M
H2SO4 and excited at 325 nm.23
Figure 4. Photographs of samples under irradiation with a UV lamp: (a) 10 mg of triskelion 3 in toluene. (b) 10 mg
of triskelion 3 in 5% TFA in toluene. (c) 5% TFA in toluene with no other solute.
To further analyze the effect on TFA on the electronics of 2 and 3, 1H and 13C NMR
spectra were taken in CD2Cl2 with added TFA-D (see supplementary data). No decomposition
was noted in the NMR spectra of 2 and 3 in the presence of TFA-D. However, a small downfield
chemical shift was observed for the aryl H signal nearest to the carbonyl in the 1H NMR
spectrum for the sample in CD2Cl2 (8.45 ppm) compared with CD2Cl2 + TFA-D (8.53 ppm). A
similar small downfield shift was observed for the carbonyl C signal in the 13C spectrum in
CD2Cl2 (160.15 ppm) and with added TFA-D (163.33 ppm). This general downfield shift of
signals from atoms near the carbonyl upon addition of TFA-D is consistent with partial
deuteration of the carbonyl units. Thus the partial protonation likely alters the electronic
characteristic of 2 and 3 and could also reduce aggregation due to the protonated forms of 2 and
3 repelling each other, leading to improved emission of light.
In summary, two new pi-expanded coumarins (2 and 3) have been synthesized in five and
six steps. These unique molecules are the first planar triskelion-shaped 1,3,5-triphenylbenzene
derivatives containing lactone bridging units and their structures confirmed by X-ray
crystallography. The addition of the carbonyl moiety into the structure of these systems has a
dramatic effect on their geometries and optical properties, when comparing triskelions 2 and 3
with 9 and 13. This new class of pi-expanded coumarins are the first examples of planar two-
8

atom bridged compounds containing the 1,3,5-triphenylbenzene core. Despite the addition of
solubilizing groups, both triskelion-shaped compounds were found to have relatively poor
solubility in most solvents likely due to pi-pi stacking and aggregation. Future studies will be
performed to enhance the solubility and optical properties of these promising systems and to
explore their ability to act as molecular geometry switches.
Acknowledgements
Acknowledgment is made to the Research Corporation for Science Advancement Cottrell
College Science Award (ID: 22480) and to the donors of the American Chemical Society
Petroleum Research Fund (ACS PRF 51259-UNI1) for support of this research. We are also
grateful for the Student Blugold Commitment Differential Tuition funds through the University
of Wisconsin-Eau Claire Faculty/Student Research Collaboration Grants program for financial
support. X-ray crystallographic studies performed at the University of Montana were supported
by the National Science Foundation (NSF)-MRI (CHE - 1337908) and National Institutes of
Health (CoBRE NIGMS P20GM103546).
Supplementary Data.
For detailed synthetic procedures for known (4-7, and 10) and new (2, 3, 8, 9, and 11-13)
compounds, copies of 1H and 13C NMR spectra for new compounds (2, 3, 8, 9, and 11-13), 1H
NMR spectra of 3 in CD2Cl2 and TFA-D, and crystallographic data for 13 and 3 see the
supporting information.
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Highlights
• The first synthesis of a trilactone-bridged 1,3,5-triphenylbenzene.
• The compound adopts a triskelion shape and has a near planar configuration.
• This is the first example of any 1,3,5-triphenylbenzene derivative with 2-atom bridges
that adopts a near-planar configuration.
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Graphical Abstract