Electron-Transfer-Induced Self-Assembly of a Molecular Tweezer Platform

Article abstract of DOI:10.1002/chem.202001919

The design and synthesis of a new molecular tweezer (T-tmp) with electron-rich pincers are reported. The stable monocationic radicals and self-assembled dimeric radicals of this molecular tweezer platform were prepared by chemical oxidative titration. With the aid of DFT calculations, it was found that the dimeric radicals with syn-syn-syn conformer has the most stable structure, with the hole primarily delocalized between parallel stacked pyrenyl groups.

Full text of DOI:10.1002/chem.202001919

Accepted Article
Accepted Article
Title: Electron transfer induced self-assembly of a molecular tweezer
platform
Authors: Khushabu Thakur, Denan Wang, Saber Mirzaei, and
Rajendra Rathore
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To be cited as: Chem. Eur. J. 10.1002/chem.202001919
Link to VoR: https://doi.org/10.1002/chem.202001919
01/2020

10.1002/chem.202001919
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Electron transfer induced self-assembly of a molecular tweezer
platform
Khushabu Thakur, [a] Denan Wang, *[a] Saber Mirzaei [a] and Rajendra Rathore [a]‡
[a]
Dr. K. Thakur, Dr. D. Wang, S. Mirzaei and Prof. R. Rathore
Department of Chemistry
Marquette University
Milwaukee, WI, United State, 53201-1881
E-mail: denan.wang@marquette.edu
Deceased February 16, 2018.
‡
Supporting information for this article is given via a link at the end of the document.
Abstract: We report the design and synthesis of a new molecular
tweezer (T-tmp) with electron-rich pincers. The stable monocationic
radicals and self-assembled dimeric radicals of this molecular tweezer
platform were prepared by chemical-oxidative titration. With the aid of
DFT calculations, we found that the dimeric radicals with syn-syn-syn
conformer has the most stable structure, with the hole primarily
delocalized between parallel stacked pyrenyl groups.
pincers, which indicated that the generated cationic radical was
unstable on electrochemical time scale.
Stable dimeric cation radicals are critical for organic
materials science, as they constitute the smallest intermolecular
unit that carries a delocalized positive charge, and therefore
provide a foundation for the further development of novel organic
materials with diverse properties, such as conductivity,
ferromagnetism, and long range electron/charge transport.[1] Thus
far, several dimeric cation radicals involving naphthalene and
derivatives have been characterized by electronic spectra in
solution and X-ray crystallography in solid state.[2] The binding
constants for the formation of dimeric cation radicals (i.e. D2+•) in
solution are generally small, and higher order aggregation (i.e.
D3+• or D4+•) is rarely observed in solution.[1a, 3] The poor stability
of dimeric cation radicals at room temperature, derived from
single aromatic plane, has prevented their further applications.
In contrast to the spontaneous formation of cation radicals
in solution, disfavoured on entropic grounds, molecular tweezers
provide a rigid platform with two aromatic pincer arms to trap or
bind aromatic acceptor guest via π-π interaction or electron
donor-acceptor (EDA) interaction. Representing the earliest
example, Zimmerman type molecular tweezers (Fig. 1a) showed
facile binding of an aromatic acceptor guest, with the distance
between the two aromatic donor pincers alike 7 Å.[4] Furthermore,
those molecular tweezers also hold potential to be assembled into
dimeric cation radical structures when prompted by one-electron
oxidation.
Fig. 1． Left. Zimmerman’s tweezer platform; Right. Our recently reported
molecular tweezer (T-Py) and X-ray structure of 1:1 EDA complex of T-Py∩
DDQ together with a solvent molecule (i.e., CH2Cl2).
In order to improve the stability of the cation radical of
molecular tweezer, here we report the development of a tweezer
platform based on protected pyrene pincers, which incorporate
three methyl groups at 3,6,8 positions of pyrene. It is shown that
this modification improves the solubility of molecular tweezers in
common organic solvents. Redox titration and X-ray diffraction
experiments of cation radical of 1,3,6,8-tetramethylpyrene
disclose that the stability of the one electron oxidation product is
significantly enhanced in comparison with our prior platform,[6]
and thus the present system holds great potential to study the
self-assembly of dimeric cationic radicals via one electron
oxidation.
This manuscript describes the design and synthesis of a
doubly annulated m-terphenyl-based molecular tweezers
containing 3,6,8-trimethylpyrenyl (tmp) aromatic pincers
(Scheme 1). This molecular tweezers was characterized with 1H,
13C -NMR and MALDI-TOF mass spectrometry. With the aid of
electrochemistry and absorption spectroscopy techniques, we
found that the new molecular tweezers T-tmp undergoes an
efficient dimerization when prompted by one electron oxidation.
Here we followed a synthetic strategy recently developed in
the Rathore laboratory which affords a doubly-annulated m-
terphenyl tweezers platform (1a in Scheme 1) in excellent yield,
by attaching the pincer arms in one synthetic step through
standard Suzuki coupling reaction.[5] First, the boronic ester
derivative of trimethylpyrene (Bpin-tmp) was obtained in good
yield from pyrene as shown in Scheme 1. A mixture of pyrene
Recently, part of us reported an efficient synthesis of a new
molecular tweezer platform containing a doubly-annulated m-
terphenyl structure, which can be readily transformed to a variety
of tweezers with different pincers in one synthetic step.[5] It was
shown that molecular tweezers (T-Py) with pyrene incorporated
in the pincer arms displayed efficient binding ability to a single
DDQ molecule as guest and formed a stable sandwich mode
complex which was characterized with X-ray crystallography (Fig.
1b). However, electrochemical characterization revealed an
irreversible cyclic voltammetry (CV) upon the oxidation of pyrene
1
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and aqueous hydrobromic acid (48%) in a 1:1 mixture of methanol
and diethyl ether was treated with aqueous hydrogen peroxide
(30%) dropwise and the resulting mixture was stirred at room
temperature for 24 h to afford a nearly quantitative yield of an
isomeric mixture of dibromopyrenes.[7] This mixture was then
treated with one equivalent of bromine in nitrobenzene at ambient
temperature to afford a good yield of tribromopyrene after
recrystallization in toluene.[8] It is noted that the use of more than
one equivalent of liquid bromine is to be avoided as it leads to
formation of tetrabromopyrene, which cannot be easily separated
from the tribromopyrene. The desired tribromopyrene product
was quantitatively converted to the corresponding trimethylpyrene
(tmp) by a palladium catalyzed methylation reaction using
trimethylaluminum-DABCO complex as a methylation agent.[9]
Next, the bromotrimethylpyrene (Br-tmp) was obtained by a
reaction of tmp with NBS in acetonitrile with the presence of
ammonium nitrate as the catalyst (Scheme 1).[10] Subsequently,
the bromotrimethylpyrene was easily converted to pincer
precursor by Miyaura-borylation reaction.[11]
ferrocene (Eox = 0.45 V vs SCE) as an internal standard. The
cyclic and square wave voltammograms of T-tmp and TMP are
illustrated in the Fig. 2. The reversible oxidation potential of model
compound TMP was found at Eox = 0.55 V (vs. ferrocene). It is
noted that under these conditions, model TMP does not undergo
a second electrochemical oxidation in the readily accessible
potential range (i.e. <1.2
V
vs ferrocene).
The cyclic
voltammogram of tweezer molecular T-tmp (Fig. 2) shows two
closely spaced reversible oxidation waves which correspond to
the two sets of one electron oxidation events occurring at the two
pyrene groups with oxidation potentials of 0.50 and 0.66 V. The
observed difference (∼50 mV) in the first oxidation potentials of
T-tmp when compared to the TMP can be attributed to energy
compensation for the formation of the weak stacking interactions
(or the through space interaction), which are expected to stabilize
the positive charge. On the other hand, the second oxidation is
governed by the on-site Coulombic repulsion interaction. As a
result, this weak interaction renders the second oxidation to occur
at a higher potential at 0.66 V.
The electrochemical reversibility and low oxidation potential
of both TMP and T-tmp allowed us to generate their cation
radicals by chemical oxidation using the stable aromatic cation
-
radical salt THEO+•SbCl6 (Ered = 0.67 V vs. ferrocene) as a 1-
electron oxidant. [12] As shown in Fig. 3a and Fig. S3, the addition
of the first equivalent of neutral tweezer T-tmp to a solution of
-
THEO+•SbCl6 showed the appearance of several intense
absorption features in the visible range (495, 577 and 750 nm)
and a weak broad NIR transition (>1400 nm). Sequentially adding
neutral tweezer T-tmp until 3.5 eq, we found that the visible bands
(495, 577 and 750 nm) slowly decreased, however, the broad NIR
band (>1400 nm and max = 2620 nm) increased until 2.0 eq were
added and remained constant until the end of titration (3.5 eq).
Based on the oxidation potential of tweezer T-tmp (0.5 and 0.66
V), we expect to generate both dication and monocation radicals
at the beginning of the titration. The observed weak NIR transition
can be attributed to the weak interaction of two pyrenyl
groups in tweezer T-tmp, which is consistent with the
electrochemical results (Fig. 2). The dimer cation radical was
sequentially generated by adding neutral T-tmp to its monocation
radical solution which correspond to the rising of the intense NIR
band (>1400 nm and max = 2620 nm). A similar titration
experiment was performed on model compound TMP. The
increment of visible feature of (492, 595 and 690 nm) was found,
on the other hand, only a very weak NIR band was observed in
the beginning of titration.
Scheme 1. Synthesis of tweezer molecular T-tmp and a model compound TMP.
a. i) HBr/ H2O2, diethyl ether/methanol (1:1), 74%; ii) Br2, 1 eq., nitrobenzene,
82%; b. DABAL-Me3, X-Phos, Pd2(dba)3 ,THF, 85%; c. NBS, NH4NO3, CH3CN,
88%; d. bis(pinacolato)diboron, KOAc, PdCl2(dppf), 1,4-dioxane, 82%; e.
Pd(PPh3)4 (2 mol%), 2M Na2CO3, toluene, reflux, 16h, 71%; f. Br2, PhNO2, 4 h,
120°C, 95%.
1,3,6,8-tetramethylpyrene (TMP) was synthesized as a
mono pyrene model compound, generated from pyrene through
bromination with bromine water in nitrobenzene solution,
following a palladium catalyzed methylation reaction using
trimethylaluminum-DABCO complex as a methylation agent.[9]
Both model compound (TMP) and molecular tweezers (T-tmp)
were characterized by 1H, 13C -NMR and MALDI-TOF mass
spectrometry; experimental details and characterization data are
provided in the Supporting Information (SI).
The electron donor strength and the initial indication of the
cation radical stability of the tweezer T-tmp and its model
compound TMP were evaluated by electrochemical oxidation at a
platinum electrode as a 2×10-3 M solution in dichloromethane
containing 0.1 M n-Bu4NPF6 as the supporting electrolyte.
Furthermore, all oxidation potentials were calibrated with added
2
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Fig. 2. Cyclic voltammograms (CV, —) and Square wave voltammograms (SWV,
…) of 2 mM tetramethylpyrene (TMP) and tweezer T-tmp in CH2Cl2 containing
0.1 M n-Bu4NPF6 at scan rate of 100 mV s-1 at 22 oC.
The resulted spectra also provide valuable information on
the electronic structure. In the case of T-tmp2+ (Fig. 4a), the most
intense local absorption band at 745 nm was observed but no NIR
absorption, which can be attributed to Coulombic repulsion
interaction between two positive charged pyrene groups.
However, the extinction coefficient of local absorption band in T-
tmp+• was reduced to about half comparing to T-tmp2+,
meanwhile a weak and broad NIR absorption was presented. This
result indicated that the only positive charged pyrene has weak
through space interaction with the other neutral pyrene group
resulted weak NIR absorption, which was also supported by our
electrochemistry results at (Fig 2). On the other side, the
generated dimerized radicals (TMP)2+• displayed slightly decrease
of the band at 745 nm, and the NIR absorption increases
significantly; this indicates that dimerization help to delocalize the
positive charge in multiple pyrene groups.
The dimer cation radical can be produced by adding neutral
TMP to its cation radical solution, leading to production of the NIR
absorption (>1400 and max = 1965 nm). However, the titration is
not completed with 5.3 equivalent of TMP which implies a low
dimerization binding constant. In order to accurately obtain the
physical properties (e.g. absorption spectra), numerical
deconvolution of the absorption spectrum at each increment
produced mole fractions of THEO+•, T-tmp+•, T-tmp2+ and (T-
+•
tmp)2 against the added equivalents of T-tmp (Fig. 3c).
According to the deconvolution results, we are able to obtain the
absorption spectrum (Fig. 4a) for all proposed species and
sequentially perform the non-linear simulation[13] and derive the
binding constants in dimerization. Remarkably, the derived
binding constant 5.0 (±1.5) × 105 M-1 which, to the best of our
knowledge, is the largest dimerization binding constant in
molecular tweezer. In contrast, the dimerization constant for
model compound TMP is two order of magnitude difference, 3.9
(±1.2) × 103 M-1.
The dimerization product was further characterized by
electrospray ionization mass spectrometry (ESI-MS) (Fig. 4b).
The product solution of titration experiment was directly subject to
ESI-MS, and intense feature at m/z = 1702.80 was observed
+•
which is nicely match with dimeric product
(T-tmp
)
2
.
After
successful generation of the dimer species, we found that the
+•
dimer cation radical
(T-tmp
)
2
has excellent stability at room
temperature. In the absence of water and oxygen, its absorption
spectrum remains unchanged at least for three days. We are
actively pursuing the crystallographic characterization of mono
+•
cation radical T-tmp+• and dimeric self-assembled of
(T-tmp
)
2
.
After repeated attempts to crystallize T-tmp+• and (T-tmp)2+•, thus
far no success has been achieved. Interestingly, no obviously
formation of excimer dimer were observed in both T-tmp and TMP,
based on the results of concentration dependent fluorescent
spectra measurement experiment. As we know the formation of
excimer dimer requires a perfect sandwich-like arrangement,[14]
methyl substituted groups and m-terphenyl based linker
significantly increase the steric effect and prevent the formation of
excimer dimer in the solution.
Fig. 3. a) Spectral changes and c) corresponding evolution of mole fractions of
+•
oxidant and oxidized product cation radicals (T-tmp+•, T-tmp2+ and (T-tmp)2
)
on the representative titration of tweezer T-tmp with 51 μM THEO+•. b) Spectral
changes and d) corresponding evolution of mole fractions of oxidant and (TMP+•
and (TMP)2+•) on the representative titration of model TMP with 82 μM THEO+•
(both in CH2Cl2 at 22 °C).
3
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dimeric cation radical with syn-syn-syn conformer has the most
stable structure, furthermore, the TD-DFT calculation indicated
electron density is mainly delocalized within the pyrene groups in
Fig. 4. a) Deconvolution absorption spectra from titration. B) ESI-MS spectrum
+●
of dimer cation radical (T-tmp)2
.
(T-tmp)
+•. The investigation of novel molecular tweezers with
2
more pyrene units are under progress in our lab.
In order to further study the structure and electronic
properties of the dimerization products, we performed (TD-)DFT
calculations using CAM-B3LYP-D3BJ/6-31G*+PCM(CH2Cl2)
level of theory (for computational details see the SI). The results
show that the syn-syn-syn structure has the lowest energy (Fig.
5a), which is about 1.5 kcal/mol more stable than the next lowest
structure. The possible structures and corresponding relative
energies are summarized in the SI.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The dimerization energy for the formation of (T-tmp)2+• is
about 40 kcal/mol. It should be noted that this value for the
The authors would like to thank Prof. Scott A. Reid (Marquette
University) for helpful discussion and the National Science
Foundation (CHE-1508677) for financial support. The
computational calculations were performed on the high-
performance computing cluster Pere at Marquette University.
+•
formation of (TMP)2 is around 12 kcal/mol. The distance
between pyrene layers are 3.4, 3.3 and 3.5 Å in (T-tmp)2+•, which
is slightly shorter than model compound TMP with 3.5 Å (Fig. 5b).
The TD-DFT calculation results indicated the electron density is
mainly delocalized within the pyrene groups and the spectrum
simulation of (TMP)2+• show the intense NIR absorption (>1500
nm, max = 2430 nm) corresponding to HOMO to LOMO transition,
which is in good agreement with our experiment results. In case
of (TMP)2+•, the predicted NIR absorption (>1300 nm,max = 2000
nm) is also well matched with experiment results (>1400 nm, max
= 1965 nm). Also, the calculated near-IR transition shows lower
Keywords: tweezers • cation radical • dimer • charge transfer •
trimethylpyrene
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0.13, respectively; Fig. 5c and Fig. 5d) which agrees with the
experimental data (Fig. 3).
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In summary, we report the design and synthesis of a new
molecular tweezer (T-tmp) with electron-rich pincers. The
electrochemical properties indicated the two pyrenyl groups have
a weak interaction after one electron oxidation. Furthermore,
monocationic radicals and self-assembled dimer radicals were
successfully prepared by chemical-oxidative titration. The
dimerization binding constant was determined to be 5.0 (±1.5) ×
105 M-1. With the aid of DFT calculations, we found that the
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[12] M. R. Talipov and R. Rathore in Robust aromatic cation radicals as redox
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5
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Entry for the Table of Contents
Insert graphic for Table of Contents here.
It has been established to efficiently install a modified molecular tweezer in one synthetic step. The electrochemical oxidation of T-
tmp revealed two pyrene groups hold weak π-π interaction after pumping out one electron. The resulted mono cation radical can carry
out self-assembly with the neutral tweezer to form dimeric cation radical with remarkably binding constant.
6
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