Synthesis of a triethylene glycol-capped benzo[1,2-c:4,5-c']bis[2]benzopyran-5,12-dione: A highly soluble dilactone-bridged p-terphenyl with a crankshaft architecture

Article abstract of DOI:10.1016/j.tetlet.2020.152429

3,10-Bis(triethylene glycol)benzo[1,2-c:4,5-c']bis[2]benzopyran-5,12-dione has been synthesized as an example of a dilactone-bridged p-terphenyl with a C_2h crankshaft architecture that exhibits significant fluorescence. Lactone-bridged rotation

Full text of DOI:10.1016/j.tetlet.2020.152429

Tetrahedron Letters 61 (2020) 152429
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Tetrahedron Letters
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Synthesis of a triethylene glycol-capped benzo[1,2-c:4,5-c’]bis[2]
benzopyran-5,12-dione: A highly soluble dilactone-bridged p-terphenyl
with a crankshaft architecture
⇑
Justin J. Dressler, Eva M. Charlesworth-Seiler, Bart J. Dahl
Department of Chemistry and Biochemistry, University of Wisconsin Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54702, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2020
Revised 25 August 2020
Accepted 30 August 2020
Available online 12 September 2020
3,10-Bis(triethylene glycol)benzo[1,2-c:4,5-c’]bis[2]benzopyran-5,12-dione has been synthesized as an
example of a dilactone-bridged p-terphenyl with a C2h crankshaft architecture that exhibits significant
fluorescence. Lactone-bridged rotationally restricted p-terphenyl compounds are relatively unexplored,
with the few known examples having very poor solubility and are therefore difficult to characterize
and study. This superior TEG-solubilized analog has been shown to have much improved solubility,
which should improve processability, and has attractive optical properties including efficient absorption
in the near UV and significant emission of blue-violet light. Studies enabled by this TEG-solubilized
crankshaft shaped molecule establishes that compounds with this architecture could be useful as pH-dri-
ven molecular switches with limited switching cycles or as ‘‘turn-off” sensors with instant fluorescence
and UV absorption attenuation at high pH values.
Keywords:
Highly conjugated materials
Fluorescent compounds
Molecular switch
Rotational restriction
Ó 2020 Elsevier Ltd. All rights reserved.
Introduction
p-conjugation between the arene units. As an example of this
effect (Fig. 1), biphenyl (dihedral angle in solution is 30-40°) [3]
is a fluorescent molecule with a UF = 0.18 [4]. When biphenyl is
bridged by a single methylene unit as in 9H-fluorene, restricting
the aryl-aryl torsions, it becomes especially fluorescent
Biphenyl, p-terphenyl, and other linear oligophenyl-containing
conjugated materials have long been studied for their interesting
optical properties (Fig. 1) [1]. These properties are intimately con-
nected to certain structural characteristics of the phenyl-contain-
ing molecules, specifically the number of individual conjugated
arene units and their torsional angle relative to one another [2].
It is known that increasing the number of conjugated arene units
in unsubstituted oligo(p-phenylenes) causes a bathochromic shift
in both absorbance and emission spectra, coupled with an increase
in fluorescence quantum yield (UF) [2b]. However, there is a gen-
eral convergence observed for these types of systems after which
additional benzene units (typically around 4 total units) have min-
imal effect on these properties. In oligophenylenes with terminal
donor–acceptor (D-A) units this convergence is often realized more
rapidly, sometimes even resulting in a hypsochromic shift in
absorption spectra, as the cumulative phenyl–phenyl torsions
become increasingly dominant, thus disrupting electronic commu-
nication between the D-A end groups [2c,2d]. A solution to this
issue is to engineer bridging units between the individual arenes
that will restrict aryl-aryl torsional motion, allowing for efficient
(UF = 0.80) [4]. Because of these enhanced optical properties,
bridged oligo(p-phenylene) chromophores, such as indeno[1,2-b]
fluorene and derivatives, and larger poly(p-phenylene) chro-
mophores have found utility in organic electronic devices such as
polymer lasers, organic light-emitting diodes(OLEDs), and solar
cells [5]. One of the key areas of interest for the oligomeric com-
pounds is in the creation of stable blue light emitters for OLED
applications [6]. However, a major drawback of both bridged and
non-bridged oligo(p-phenylenes) is their poor solubility due to
strong intermolecular
p-p stacking interactions. This leads to
aggregation which alters key optical properties and causes difficult
characterization and processing [7]. Most significantly, aggregation
often diminishes light emission, which can be attributed to non-
radiative relaxation via energy transfer between aggregated spe-
cies. The issue of aggregation is most conveniently addressed in
methylene bridged oligo(p-phenylene) or poly(p-phenylene) struc-
tures by the incorporation of alkyl or aryl solubilizing groups along
the methylene backbone, allowing for the facile synthesis of a myr-
iad of interesting variations on these types of structures in recent
years [8].
⇑
Corresponding author.
E-mail address: dahlbj@uwec.edu (B.J. Dahl).
https://doi.org/10.1016/j.tetlet.2020.152429
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.

J.J. Dressler et al.
Tetrahedron Letters 61 (2020) 152429
Fig. 1. Reported solution-phase electronic absorption and emission efficiency for small bridged and unbridged p-phenylenes in cyclohexane [4].
We have been interested in a exploring a unique bridging group
to restrict aryl-aryl rotation and therefore augment -conjugation
p
in biphenyl and oligophenylene molecules: the lactone unit [9]. We
have demonstrated that donor–acceptor functionalized biphenyls
with a lactone bridge have attractive optical properties which are
enhanced relative to non-bridged analogs [9a]. Additionally, unlike
other common bridges, we have shown that lactone bridges have
the unique potential to function as pH-driven aryl-aryl dihedral
angle molecular switches through the reversible base-promoted
cleavage and acid-promoted reformation of the lactone bridge,
reversibly affecting their optical properties [9a,9b]. Although com-
pounds containing a biphenyl unit with a lactone bridge, i.e. benzo-
coumarins, are fairly common [10], applying multiple lactone
bridges to larger oligophenyl-containing compounds is quite rare.
In previous work, we have reported the synthesis of some of the
only known examples (Fig. 2) of dilactone-bridged p-terphenyls
with C2h symmetric or ‘‘crankshaft” architectures [9b] (benzo
[1,2-c:4,5-c’]bis[2]benzopyran-5,12-diones 2 and 3, and benzo
[1,2-c:4,5-c’]bis[1]benzopyran-6,13-dione 5) and trilactone-
bridged 1,3,5-triphenylbenzenes with C3h symmetric or
‘‘triskelion” architectures [9c] (benzo[c]diisochromeno[3,4-f:3’,4’-
h]chromene-6,12,18-triones 6 and 7). Beyond these studies, there
are only a few known examples of crankshaft shaped dilactone-
bridged p-terphenyls [11] and no examples of triskelion shaped
lactone-bridged 1,3,5-triphenylbenzenes. Unsubstituted dilac-
tone-bridged p-terphenyl compounds benzo[1,2-c:4,5-c’]bis[2]
benzopyran-5,12-dione 1 [11a] and benzo[1,2-c:4,5-c’]bis[2]ben-
zopyran-6,13-dione 4 [11b], isomeric via the flipping of the two
lactone bridging units, are known but have very poor solubility.
Unsubstituted trilactone-bridged 1,3,5-triphenylbenzene (6H-
benzo[c]diisochromeno[3,4-f:3’,4’-h]chromene-6,12,18-trione) is
currently unknown, although we had previously attempted its syn-
thesis to find that it was completely insoluble in all solvents and
were therefore unable to fully characterize this compound [9c].
We attempted to address the solubility issues caused by the
Benzo[1,2-c:4,5-c']bis[2]benzopyran-5,12-diones:
Di-Lactone Bridged p-Terphenyl Crankshafts
O
O
O
O
O
O
O
O
O
O
O
O
3
1
2
limited
solubility
Benzo[1,2-c:4,5-c']bis[1]benzopyran-6,13-diones:
Di-Lactone Bridge-Flipped p-Terphenyl Crankshafts
O
O
O
O
O
O
O
O
limited
solubility
5
4
Benzo[c]diisochromeno[3,4-f:3',4'-h]chromene-6,12,18-triones:
Tri-Lactone Bridged 1,3,5-Triphenylbenzene Triskelions
intense p-p intermolecular forces of these compounds by installing
t-butyl [9b,9c] (2, 5, 6), 3,5-di-t-butylphenyl [9b] (3), and 1,1,4,4-
tetramethylbutylene [9c] (7) groups. Each of these groups are com-
monly employed to solubilize planar conjugated materials [12].
While these solubilizing groups did successfully allow for solu-
tion-phase characterization of these compounds, their solubility
was still very poor and limited to only a few solvents, sometimes
requiring heat to dissolve: CH2Cl2, CHCl3, toluene, and THF (in
some cases) [9b,9c]. While some of these compounds had very
attractive optical properties, their poor solubility would be an issue
for the processing of these compounds for potential application in
optical devices. Another issue caused by their poor solubility is that
the pH-driven molecular switch potential of these larger lactone-
bridged compounds could not be examined fully due to the precip-
O
O
O
O
O
O
O
O
O
O
O
O
6
limited
solubility
7
Fig. 2. Examples of lactone-bridged oligo(p-phenylene)s with limited solubility.
2

J.J. Dressler et al.
Tetrahedron Letters 61 (2020) 152429
itation that occurred after the introduction of switching stimuli
[9b,9c].
found that the TEG groups in 15 to be the perfect compromise in
that it solubilized the compound and was also amenable to stan-
dard inexpensive purification techniques.
We sought to incorporate a new solubilizing group into these
rare classes of compounds which would allow for more favorable
processing for optical applications and the demonstration of their
pH-driven switch capabilities. Of all the compounds in Fig. 2,
crankshaft 2 and 3 had the most desirable optical properties, so
we prioritized the synthesis of a soluble analog of those com-
pounds. Oligoethylene glycol solubilizing groups are well-known
for their ability to solubilize conjugated aromatics in polar solvents
[13]. Other common solubilizing groups employed for these pur-
poses include carboxylates, sulfonates and quaternary amines
[14]. However, unlike neutral oligoethylene glycol groups, these
groups are charged and could interfere with potential pH-driven
switching studies. We therefore set out to synthesize (Fig. 3) and
study triethylene-glycol (TEG) substituted crankshaft terphenyl-
dilactone (15). We concurrently performed the synthesis of analo-
gous compounds with shorter mono and diethylene glycol solubi-
lizing units. The shorter units did not afford as significant
improvements in solubility and potential units longer than TEG
were anticipated to result in troublesome purifications due to the
highly polar nature of the longer oligoethylene glycol groups. We
Results and discussion
Crankshaft dilactone 15 with TEG solubilizing groups was syn-
thesized in 10 steps (Fig. 3) from commercial starting materials. 2-
Bromo-5-methoxybenzoic acid was isomerized to 10 in two steps
in a single pot with a BBr3-promoted methyl ether cleavage, fol-
lowed by a Fischer esterification. Attempts at a more cost-effective
methyl ether cleavage on
a larger scale using a commonly
employed method of heating in HBr and AcOH resulted in bromine
migration. Bromine migrations under these conditions are rare,
however, aromatic compounds with a bromine atom situated ortho
to a carboxylic acid group and either ortho or para to a potential
hydroxy substituent are documented to be susceptible [15]. TEG
tosylate 8 was synthesized as reported [16] and subsequently sub-
stituted onto phenol 10 to achieve TEG-substituted 11, which was
then converted into boronic ester 12 under Miyaura borylation
conditions in good yields. Double Suzuki coupling of 12 with
Fig. 3. Synthesis of lactone-bridged terphenyl 15 containing triethylene glycol solubilizing groups.
3

J.J. Dressler et al.
Tetrahedron Letters 61 (2020) 152429
Table 2
Electronic absorption and emission data for 2, 3, 5, and 15.
dibromoarene 9 [9b] resulted in terphenyl 13. Terphenyl 13, con-
taining two TEG units, was very challenging to separate from the
excess 12 (and its deborylated side product) employed in the
Suzuki coupling because of its highly polar nature and strong inter-
actions between the ethylene glycol units of the product and side
products. However, after numerous trials, we found that column
chromatography with THF in hexane immediately followed by a
subsequent column with EtOAc in CH2Cl2 was essential to achieve
this separation. All other eluent conditions tested resulted in co-
elution. In previous studies to achieve crankshafts 2, 3, and 5 we
found that direct demethylation of terphenyls analogous to 13
with BBr3 conveniently achieved dilactone-bridged final products
in a single step. In this current study, attempts to selectively cleave
the methyl ethers on 13 using BBr3 unfortunately resulted in
decomposition, likely due to competing cleavage of the TEG
groups, even with careful stoichiometric addition at low tempera-
tures. This necessitated an alternative route consisting of a ceric
ammonium nitrate (CAN) oxidative cleavage of the p-dimethoxy
groups to obtain quinone 14. Quinone 14 was subsequently
reduced with NaBH4 and then underwent acid-promoted lac-
tonization in a single pot to achieve solid TEG solubilized crank-
shaft dilactone 15 in good overall yields.
Unlike previously studied lactone-bridged oligophenyls 1–7
(Fig. 2) [9b,9c,11a,11b], TEG-containing 15 displays solubility in a
wide variety of solvents (Table 1). Previously reported crankshaft
compounds containing the ‘‘bridge-flipped” lactone orientation
(unsubstituted 4 [11b] and tert-butyl solubilized 5 [9b]) show
slightly better solubility than isomeric crankshafts 1-3 [9b,11a]
or triskelions 6 and 7 [9c]. Compound 1 [11a] has only been shown
to dissolve in concentrated H2SO4 and compounds 2 [9b], 3 [9b], 6
[9c], and 7 [9c] are sparingly soluble in THF, CH2Cl2, CHCl3, and
compd
lowest E abs kmax[nm] (
e
[MÀ1cmÀ1])a
em kbmax[nm]
UFc
2
3
5
15
364 (36832)
384 (65080)
335 (37983)
374 (51400)
385
394
439
405
0.23
0.92
0.24
0.33
aUV–vis spectral data from 2, 3, and 5 dissolved in CH2Cl2 and 15 dissolved in
CH3CN. bEmission spectral data from 2, 3, and 5 dissolved in CH2Cl2 and 15 dis-
solved in CH3CN and excited at 350 nm. cCalculated relative to a quinine sulfate
standard dissolved in 0.5 M H2SO4 and excited at 350 nm (values corrected for
analyte and standard solvents) [21].
blue light with Stokes shifts ranging from 10 to 104 nm and rela-
tive UF between 0.23 and 0.92. However, these studies were again
limited by the poor solubility of these compounds and all spectro-
scopic studies were required to be done in CH2Cl2. Indeed, lactone-
bridged terphenyl crankshaft 15 was found to have very favorable
optical properties (Table 2), similar to the previously studied 2, 3,
and 5. The primary advantage of TEG-solubilized 15 over 2/3/5 is
that spectroscopic studies could now be performed in a variety
of solvents, but most importantly, a polar solvent. UV–vis studies
revealed that 15 had a kmax = 374 nm with very strong molar
absorptivity (e
= 51400 MÀ1cmÀ1) in acetonitrile. Crankshaft 15
also displayed significant blue-violet fluorescence at kmax = 405 nm
in acetonitrile with a UF = 0.33.
Compounds 2, 3, and 5 were originally envisioned as possible
pH-driven molecular switches [9b] and TEG-solubilized 15 is also
a potential pH-driven switch with switch states similar to those
proposed for 2 and 3 (Fig. 4). Unlike simple unsubstituted p-ter-
phenyl, the lactone-bridges engineered in terphenyls 2, 3, and 15
allows for these compounds to adopt a rigid conformation with
toluene, often requiring heat to dissolve. The strong
p- p inter-
molecular interactions of crankshafts 1–3 make it difficult to solu-
bilize these compounds, despite 2 and 3 containing tert-butyl and
3,5-di-tert-butylphenyl solubilizing groups. The strength of their
intermolecular interactions can be probed indirectly by simple
melting point analysis. Compounds 1 [11a], 2 [9b], and 3 [9b] have
very high reported melting points of 420–422 °C, 366 °C, and
440 °C, respectively. Triskelions 6 [9c] and 7 [9c] also display
restricted rotation, resulting in a high degree of
p-orbital overlap
between the individual arenes. Although X-ray quality crystals of
these compounds were elusive in our hands, evidence of the rigid-
ity of 2, 3 and 15 can be observed in the significant downfield
chemical shifts of the sterically-compressed aromatic H’s [17] in
the 1H NMR spectra of these compounds relative to their non-
bridged precursors (see previous studies [9b] and supplementary
data). This rigidity leads to these compounds being very efficient
at absorbing near-UV/visible light and emitting blue-violet light.
The potential of 2, 3, and 15 as molecular switches lies in the abil-
ity of their lactone bridges being readily cleaved by excess tetra-
butylammonium hydroxide (TBAOH), which results in non-planar
strong intermolecular
p- p interactions with melting points in
excess of 340 °C. In contrast, TEG-solubilized 15 has a much lower
melting point of 160–161 °C, despite having a much greater molar
mass than 1 and 2, and a similar molar mass to 3. This suggests that
the conformational flexibility of the TEG-groups can better over-
come
p-p interactions and allow for solute–solvent interactions
anions 2a, 3a, and 15a with weakened
p-overlap. The individual
to compete more favorably compared to 1–3. The higher solubility
of 15 makes it more tractable to characterize, process, and poten-
tially more useful in optical applications.
The spectroscopic properties of lactone-bridged terphenyls 2, 3,
and 5 were studied previously by us (Table 2) [9b]. All three com-
pounds were found to absorb strongly in the near UV to visible
arenes are now no longer rotationally restricted and both absorp-
tion and emission of light should be greatly attenuated. The lactone
can then be re-formed by addition of trifluoroacetic acid (TFA), and
thus the desirable optical properties should return. However, it is
known that hydroquinone dianions are highly susceptible to oxida-
tion and, under strongly basic conditions like those required to
generate 2a, 3a, and 15a, the resulting benzoquinones could read-
ily decompose [18]. Thus, the switching of these compounds could
be limited. In our previous studies [9b] the drastic difference in sol-
ubility between 2/3 and 2a/3a meant that studies probing reversi-
ble switching could not be done, since 2a/3a were found to
precipitate upon addition of TBAOH. However, newly synthesized
TEG-solubilized 15 can overcome these challenges because both
15 and 15a should each be at least partially soluble in polar organic
solvents.
region with large molar extinction coefficients (e) all greater than
35,000 MÀ1cmÀ1. The restricted rotational motion caused by the
lactone bridges between the individual arene units of 2, 3, and 5
also resulted in significant visible fluorescence, emitting violet-
Table 1
General solubility of 15 in various solvents.
A. Readily
B. Dissolves with heat
C. Dissolves with
D.
dissolves at rt
and remains in solution heat but ppt after Insoluble
Since 15 was soluble in acetonitrile, studies could be performed
to probe the capabilities of 15 as a molecular switch. We found that
the intensity of absorption and emission were drastically reduced
when a drop of TBAOH was added to samples of 15 (Fig. 5), sug-
gesting that lactone cleavage was occurring resulting in non-rigid
without heating after cooling to rt
cooling to rt
at all T
THF, CH2Cl2,
CHCl3
dioxane, DMSO,
acetone, acetonitrile,
EtOAc, DMF, toluene
iPrOH, EtOH,
MeOH
Hexane,
H2O,
diethyl
ether
4

J.J. Dressler et al.
Tetrahedron Letters 61 (2020) 152429
Fig. 4. Proposed pH-driven switching process for 2, 3 and 15 and possible oxidation and decomposition.
in a return to higher absorption and emission, indicating the re-
forming of 15, however both absorption and emission were atten-
uated relative to the original sample of 15 (Fig. 5), indicating some
decomposition was occurring during the TBAOH induced ring-
opening. Spontaneous oxidation of aryl lactones with characteris-
tics similar to 15 have been reported upon base-promoted lactone
cleavage resulting in immediate oxidation to quinone formation
[19]; simple 1,4-benzoquinone is known to be sensitive toward
strong bases resulting in spontaneous decomposition [18]. These
studies enabled by TEG-solubilized 15 establish that terphenyl-
dilactone crankshaft shaped compounds like 15 having the poten-
tial to be pH-driven molecular switches, albeit with limited switch-
ing cycles, or as ‘‘turn-off” fluorescent sensors [20] of high pH.
Compound 15 was also shown to have striking visible fluorescence
in both the solid state and in acetonitrile solution (Fig. 6), emitting
bright blue-violet light. The pH-driven switching of 15 could be
visibly observed as fluorescence was greatly attenuated upon addi-
tion of TBAOH and returned upon addition of TFA.
In summary, TEG-solubilized terphenyl 15 with a crankshaft
architecture was synthesized and its optical properties were stud-
ied. Lactone-bridged rotationally restricted oligoarenes are a rare
and unusual class of molecules with attractive optical properties,
and only a few known examples to date. However, they are
Fig. 5. (A) UV–vis absorption of 15 1.01Â10À5 M in CH3CN (blue), after addition of 1
drop of TBA-OH (green), and after re-acidification with TFA (red). (B) Fluorescence
emission scan (excited at 350 nm) of 15 1.01Â10À5 M in CH3CN (blue), after
addition of 1 drop of TBA-OH (green), and after re-acidification with TFA (red).
15a. The absorption spectra of 15 is typical of polyaromatics with
restricted rotational freedom in that multiple defined peaks are
observed due the multiple vibrational modes of the
p ?
p⁄ transi-
tion, whereas the broad absorption spectra of 15a is typical of a
compound with a higher degree of bond rotational freedom. The
identity of 15a as the product of ring opening was further sup-
ported by 1H NMR studies (see supplementary data) showing a sig-
nificant upfield chemical shift of 15a relative to 15 due to electron
donation from the resultant anions as well as a relief from the
steric compression of aryl H’s induced by adopting a less rigid
geometry. Addition of TFA to the ring-opened sample 15a resulted
Fig. 6. (A) Solid sample of 15 irradiated by UV light. (B) 15 in CH3CN (1.0Â10À3 M)
irradiated by UV light. (C) Sample from (B) after addition of 1 drop of TBA-OH. (D)
Sample from (C) after re-acidification with TFA.
5

J.J. Dressler et al.
Tetrahedron Letters 61 (2020) 152429
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plagued by poor solubility, even when appended with commonly
utilized alkyl and aryl solubilizing groups which would limit their
processability. The much-improved solubility of 15 in a wide vari-
ety of solvents compared to previously studied analogs has allowed
for detailed optical studies and has enhanced its potential for
applications. Crankshaft 15 was found to have highly desirable
optical properties, strongly absorbing in the near-UV/visible and
emitting bright blue-violet light in both the solid state and in solu-
tion. The improved solubility of 15 has also enabled detailed stud-
ies of its, and by extension previously studied analogs, potential for
pH-driven conformational switching. It was discovered that 15 is
capable of limited conformational switching via ring-opening in
base and ring-closing in acid. The base-promoted ring-opening of
15 resulted in severe attenuation of light absorption and emission
and the subsequent acid-promoted ring-closing resulted in a
return to the previous state, although weakened relative to the ini-
tial sample. The ring-opened state 15a was found to be only semi-
stable, likely through oxidative decomposition, and thus iterative
switching is restricted by this factor. However, 15 has been shown
to be, at minimum, a ‘‘turn-off” fluorescent sensor at high pH. From
the switching studies enabled by 15 we propose to explore the pos-
sibility of a lactone ‘‘bridge-flipped” analog of 15 (e.g. TEG-solubi-
lized 5) which should be less sensitive to oxidative degradation
and might function better as a pH-driven switch. TEG-solubilized
15 has been shown to be much more tractable than previous stud-
ied crankshaft dilactones while maintaining desirable optical prop-
erties. Applying TEG solubilizing groups to lactone bridged
oligophenylenes as a general synthetic strategy should result in
facile processing for potential optical devices.
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Declaration of Competing Interest
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The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
Acknowledgements
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Acknowledgment is made to the Research Corporation for
Science Advancement Cottrell College Science Award (ID: 22480)
for support of this research. We are also grateful for the Student
Blugold Commitment Differential Tuition (BCDT) funds through
the University of Wisconsin-Eau Claire Faculty/Student Research
Collaboration Grants program and the Wisconsin Louis Stokes Alli-
ance for Minority Participation (WiscAMP) for financial support.
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