Beyond the Hammett Effect: Using Strain to Alter the Landscape of Electrochemical Potentials

Article abstract of DOI:10.1002/cphc.201700607

The substitution of sterically bulky groups at precise locations along the periphery of fused-ring aromatic systems is demonstrated to increase electrochemical oxidation potentials by preventing relaxation events in the oxidized state. Phenothiazines, whi

Full text of DOI:10.1002/cphc.201700607

DOI: 10.1002/cphc.201700607
Communications
Beyond the Hammett Effect: Using Strain to Alter the
Landscape of Electrochemical Potentials
Matthew D. Casselman,[a] Corrine F. Elliott,[a] Subrahmanyam Modekrutti,[a] Peter L. Zhang,[a]
Sean R. Parkin,[a] Chad Risko,*[a, b] and Susan A. Odom*[a]
We dedicate this manuscript to Professor John E. Anthony in honor of his 50th birthday.
The substitution of sterically bulky groups at precise locations
along the periphery of fused-ring aromatic systems is demon-
strated to increase electrochemical oxidation potentials by pre-
venting relaxation events in the oxidized state. Phenothiazines,
which undergo significant geometric relaxation upon oxida-
tion, are used as fused-ring models to showcase that electron-
donating methyl groups, which would generally be expected
to lower oxidation potential, can lead to increased oxidation
potentials when used as the steric drivers. Reduction events
remain inaccessible through this molecular design route, a criti-
cal characteristic for electrochemical systems where high oxi-
dation potentials are required and in which reductive decom-
position must be prevented, as in high-voltage lithium-ion bat-
teries. This study reveals a new avenue to alter the redox char-
acteristics of fused-ring systems that find wide use as electro-
active elements across a number of developing technologies.
corporated at the 2, 2’, 6, and/or 6’ positions increase the twist
angle between the phenyl groups, and oligothiophenes, where
similar tactics are employed with substituents placed at the 3
and/or 4 positions.[3] Notably, these considerations have been
widely used in the development of p-conjugated polymers for
electronics and solar cell applications. Strain has also been
used to impose curvature on p-conjugated networks to alter
optical and/or redox characteristics, for example, in ful-
lerenes,[4] cyclized stilbenes,[5] and twisted acenes (so-called
“twistacenes”).[6] In each of these latter systems, the p-conju-
gated networks are strained in both the ground (neutral) and
ionized (oxidized or reduced) electronic states. Taking these
studies as inspiration, we demonstrate molecular design princi-
ples to dictate the relaxation processes of p-conjugated mole-
cules through strain solely in their ionized state, while impart-
ing minimal change to the ground-state characteristics, as an
effective way to synthetically control redox potentials.
Our interest in this idea stemmed from the study of a series
of phenothiazine derivatives, in which the geometry of the oxi-
dized (radical-cation) state tends to be planar, while the neutral
ground state is bent.[7] In particular, for N-substituted pheno-
thiazines (Figure 1, left), we observed a correlation between
The electrochemical redox potentials of p-conjugated organic
molecules and polymers influence a material’s performance in
applications such as organic electronics, energy collection,
energy storage, and catalysis, to name but a few. In these sys-
tems, the general prescriptions to control redox potentials in-
volve substitution with electron-donating and/or electron-with-
drawing groups, making use of the well-known Hammett con-
stants as predictors of the extent of the change in redox po-
tentials,[1] and/or modifying the length of the p-conjugation
pathway, as in the transition from ethylene to butadiene to
hexatriene and beyond. Another route to modify molecular
redox potentials exploits strain-induced disruptions of the p-
conjugated framework.[2] Such chemical modifications typically
use so-called “bulky” substituents to manipulate dihedral tor-
sions among the p-conjugated moieties that make up the mo-
lecular systems. Prime examples include biphenyl (or more
generally oligo- or polyphenylenes), in which substituents in-
Figure 1. Representation of the structures of various phenothiazine deriva-
tives.
the degree of molecular bending (referred to as the “butterfly
angle”) of the radical-cation geometry and the ease of oxida-
tion: From ethyl to iso-propyl to tert-butyl (EtPT, iPrPT, tBuPT),
the increased substituent size led to a larger radical-cation but-
terfly angle and higher-potential oxidation events (Table 1). Im-
portantly, this effect is opposite to what one would predict
based on the Hammett constants of the substituents, suggest-
ing that preventing planarization in radical cations may raise
oxidation potentials, regardless of the substituent’s Hammett
constant.
[a] Dr. M. D. Casselman, C. F. Elliott, Dr. S. Modekrutti, P. L. Zhang,
Dr. S. R. Parkin, Prof. Dr. C. Risko, Prof. Dr. S. A. Odom
Department of Chemistry, University of Kentucky
Lexington, KY, 40506 (USA)
E-mail: susan.odom@uky.edu
chad.risko@uky.edu
[b] Prof. Dr. C. Risko
Center for Applied Energy Research
University of Kentucky, Lexington, KY, 40511 (USA)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cphc.201700607.
The possibility of raising phenothiazine oxidation potentials
without incorporating electron-withdrawing groups intrigued
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groups can be traced back to their having reduction potentials
Table 1. Adiabatic ionization potentials (IP), half-wave first oxidation po-
tentials (E1/2+/0) versus Cp2Fe+/0, and neutral and radical cation butterfly
angles.
>0 V vs. Li+/0 [7a,10e]
Dimethoxybenzene derivatives containing
.
strongly electron-withdrawing groups exhibit short-lived over-
charge protection when incorporated into lithium-ion cells
containing graphite anodes,[10a–d] perhaps as a result of reduc-
tive decomposition at the anode/electrolyte interface. This hy-
pothesis is supported by reports of the redox shuttle 1,4-di-
tert-butyl-2,5-bis(2,2,2-trifluoroethoxy)benzene surviving 2ꢁ
longer in cells containing the Li4Ti5O12 anode when compared
to LIBs with either the highly reducing graphitic or lithium-
metal anodes.[10b] As such, the incorporation of strongly elec-
tron-withdrawing groups is a problematic route for molecules
that require stability at both high and low redox potentials.
Thus, a new approach for designing stable, high-voltage redox
shuttles is needed.
+/0
Compound
IP
[eV][a]
E1/2 [V]
Butterfly angles [8][a]
[b]
Neutral
Radical
Cation
165.2 (177.8)
171.4 (173.8)
160.7
148.3
180.0
171.1
171.1
(164.5)[g]
156.6
MPT
EPT
iPrPT
tBuPT
PhPT
6.58
6.48
6.52
6.67
6.34
0.31[e]
0.27
0.33[e]
0.53[e]
0.26[e]
0.13[f]
0.61[f]
143.4 (143.7)[d]
138.7 (136.8)[d]
142.6 (137.9)[e]
134.2 (135.0)[e]
149.5 (162.3)[e]
138.8 (149.3)[f]
139.7 (144.5–
152.1)[g]
3,7-DMeEPT 6.24
3,7-BCF3EPT 7.06
1,9-DMeEPT 6.68
1,9-BCF3EPT 7.21
0.55
–
143.1 (146.5)
141.8
159.0
Inspired by studies of molecular strain, we sought to address
whether the strategic placement of substituents on the pheno-
thiazine core could be used to tune molecular redox character-
istics through geometric constraints, with minimal impact by
the substituent electronic effects. Our hypothesis was the fol-
lowing: Deliberate incorporation of substituents around the
periphery of the phenothiazine core would disrupt the relaxa-
tion of the radical-cation state, thereby increasing the oxida-
tion potential compared to the unsubstituted system. This hy-
pothesis led us to evaluate derivatives of N-substituted pheno-
thiazines where substituents were incorporated at positions
ortho to the nitrogen atom (1 and 9 positions), which we envi-
sioned would prevent planarization of the oxidized species
through steric interactions with the N-alkyl group. We com-
pared the electrochemical characteristics of these derivatives
to a parent compound with only an N substituent, and to de-
rivatives in which the same substituents are incorporated at
positions para to the nitrogen atom (3 and 7 positions) so that
planarization in the oxidized state remains possible and the
full electronic effects of the substituents are play. For the sake
of clarity, we will focus our discussion on the N-ethyl deriva-
tives, where EPT is the parent; 1,9-DMeEPT the crowded deriv-
ative; and 3,7-DMeEPT the uncrowded analogue (Figure 1).
At the outset of the investigation, we made use of density
functional theory (DFT) calculations at the B3LYP/6-311G(d,p)
level to predict molecular geometries in the neutral and radi-
cal-cation states, as well as the adiabatic and vertical ionization
potentials (IPs), of various substituted phenothiazines. The neu-
tral geometries of EPT, 3,7-DMeEPT, and 1,9-DMeEPT are quite
similar, with the butterfly angles showing little variation (139–
1438). However, as radical cations, EPT and 3,7-DMeEPT are sig-
nificantly more planar (1718) than 1,9-DMeEPT (1578) (Table 1);
note that 1808 represents a fully planar phenothiazine. Similar
trends are noted for the other 3,7- and 1,9-substituted pheno-
thiazines in the series.
1,9-
DMeiPrPT
1,9-
6.76
0.68
132.5 (134.1)
147.5
6.85
0.86[c]
132.4 (134.9)
137.2
DMePhPT
[a] DFT calculations at the B3LYP/6-311G(d,p) level of theory; X-ray crystal-
lographic values in parentheses. [b] CV performed with 1.6 mm in 0.1m
nBu4NPF6/DCM at 100 mVsꢀ1. [c] Oxidation was irreversible. [d] Ref. [11].
[e] Ref. [7b]. [f] Ref. [9d]. [g] Multiple molecules in asymmetric unit,
Ref. [12].
us due to our work in overcharge protection of lithium-ion
batteries (LIBs). Maximizing both the energy density and the
durability of a LIB requires charging to a precise voltage:
Below that potential, the full capacity of the cell is not utilizied;
at higher potentials, a cell enters overcharge, a condition that
can seriously degrade LIBs and lead to hazardous operating
conditions.[8] Molecular redox shuttles can be used to mitigate
excess current in overcharging batteries by spatially ferrying
charge via a series of electron-transfer reactions, oxidizing to
their radical-cation form at the cathode and reducing to their
neutral form at the anode. For molecules designed to mitigate
overcharge in LIBs, chemical substitutions are made to adjust
the oxidation potentials with respect to the reduction potential
of the cathode to ensure that the shuttles become redox
active just after a cell is fully charged.[8] Extensive overcharge
protection has been demonstated for lower-voltage LiFePO4
cathodes,[8a,9] which requires shuttles that oxidize at potentials
of 3.8 to 3.9 V vs. Li+/0. However, the protection of higher-volt-
age cathodes (e.g. LiCoO2, LiMnO2, and LiNi1/3Mn1/3Co1/3O2) re-
quire oxidation potentials of 4.2 V or higher, and no shuttle
has provided extensive protection in cells containing graphitic
anodes.
The road to shuttles with higher oxidation potential via the
incorporation of strong electron-withdrawing groups on viable
low-voltage shuttles can introduce perils to the framework of
LIBs. Electron-withdrawing substituents shift both the oxida-
tion and reduction potentials to more-positive values, produc-
ing compounds that are more susceptible to decomposition
via reactions that take place following reduction to the radical
anion at the anode.[7a,10] Premature failure of phenothiazine
redox shuttles containing chlorine, bromine, cyano, or nitro
DFT calculations predict 3,7-DMeEPT to have an adiabatic IP
0.24 eV smaller than EPT, consistent with measured oxidation
potentials[9c] and expectations based on the electron-donating
capability of a methyl group. However, 1,9-DMeEPT has an IP
0.20 eV larger than EPT (Table 1). Thus, for the two DMeEPT
constitutional isomers, we observe a difference in IP of 0.44 eV.
The IP trends are similar for 3,7-BCF3EPT and 1,9-BCF3EPT, with
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1,9-BCF3EPT having a larger IP, though the IP range is smaller
(0.15 V). These results showcase the considerable impact that
steric crowding of the radical-cation state can have on increas-
ing the IP.
Similar trends are observed for the vertical IPs (Table S1). No-
tably, the relaxation energy (l) in the radical-cation potential
energy surface is smaller for 1,9-DMeEPT (at 0.27 eV) than for
EPT and 3,7-DMeEPT (both at 0.40 eV), signifying the impact of
the methyl groups at the 1 and 9 positions in limiting radical
cation relaxation to a more planar configuration. Moreover, the
vertical IPs for EPT and 1,9-DMeEPT are quite similar (differing
by 0.07 eV), while that of 3,7-DMeEPT is much smaller than EPT
(by 0.23 eV), revealing the differences in the electronic impact
of methyl substituents in the 1 and 9 vs. 3 and 7 positions. Po-
tential energy surfaces displaying these differences in terms of
the adiabatic and vertical IPs and relaxation energies are repre-
sented in Figure 2.
Scheme 1. Synthesis of 1,9-DMeEPT, 1,9-DMeiPrPT, and 1,9-DMePhPT: i) urea,
Pd(dba)2, tBu3·HBF4, NaOtBu, dioxane, reflux, o/n, ii) S, cat. I2, ODCB, 1808C,
8 h, iii) 1. NaH, DMF, THF, rt, 10 min, 2. EtBr, 608C, o/n, iv) 1. NaH, DMF, THF,
08C, 5 min, 2. iPrBr, rt, o/n, v) PhBr, Pd(dba)2, tBu3·HBF4, NaOtBu, PhMe,
reflux, o/n.
Figure 3. Cyclic voltammograms of EPT, 3,7-DMeEPT, 1,9-DMeEPT, 1,9-
Figure 2. Representations of the potential energy surfaces for the oxidation
of EPT, 3,7-DMeEPT, and 1,9-DMeEPT. The gray arrow with a dotted line de-
picts the vertical ionization process from ground-state, neutral species.
Shaded regions represent relaxation energies in the radical-cation potential
energy surfaces. The change in IP is given by DIP.
DMeiPrPT, and 1,9-DMePhPT at 1.6 mm in 0.1m nBu4NPF6 in DCM. Voltammo-
grams are calibrated to Cp2Fe+/0 at 0 V, and recorded at 100 mVsꢀ1
.
thiazines, the oxidation potentials further increase. Of the
three 1,9-dimethyl-substituted derivatives, 1,9-DMePhPT has
the highest oxidation potential. Notably, this trend does not
follow that of the solely N-substituted equivalents (iPrPT is
harder to oxidize than both EPT and PhPT),[7b] though within
each series of three compounds, the oxidation potentials
follow the same trends as the calculated adiabatic IPs and radi-
cal-cation butterfly angles. Furthermore, scans of the full elec-
trochemical window (Figure S6) show that reduction events
remain inaccessible throughout the series, with solvent reduc-
tion occurring before a reduction event could be observed.
UV/Vis absorption spectroscopy was used to explore elec-
tronic transitions in the neutral and radical-cation states. While
the absorption profiles of neutral EPT and 3,7-DMeEPT are
nearly identical, the absorption onsets of the 1,9-dimethyl-sub-
stituted derivatives are slightly blue-shifted (Figure 4a). These
changes in energy, although small, are consistent with trends
in the energy of the S0!S1 transition determined by time-de-
pendent DFT (TD-DFT) calculations (Table S1). UV/Vis spectra of
the radical cations, generated by chemical oxidation, further
With computational results in hand that supported our hy-
pothesis, we prepared 1,9-DMeEPT and related N-iso-propyl
(1,9-DMeiPrPT) and N-phenyl (1,9-DMePhPT) derivatives, as
shown in Scheme 1. Bis(o-tolyl)amine was synthesized from o-
bromotoluene in a palladium-catalyzed amination with urea.
Elemental sulfur was used to form the phenothiazine ring. De-
protonation of the amine followed by treatment with bromo-
ethane or 2-bromopropane yielded 1,9-DMeEPT or 1,9-
DMeiPrPT, respectively. A palladium-catalyzed reaction with
bromobenzene yielded 1,9-DMePhPT.
Solid-state butterfly angles show good agreement with DFT
calculations (Table 1). Importantly, the trends in oxidation po-
tentials from CV experiments (Figure 3, polarographic conven-
tions) validated our hypothesis and computational results.
Unlike 3,7-DMeEPT, which is more easily oxidized than EPT (by
0.14 V), 1,9-DMeEPT is harder to oxidize than EPT (by 0.28 V).
Moreover, as the size of the radical-cation butterfly angle in-
creases within the series of 1,9-dimethyl-substituted pheno-
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a strain-induced modulation of electrochemical properties or-
thogonal to conventional tuning by substituent character. By
disrupting relaxation pathways in fused-ring systems, we hope
to identify new organic materials with higher oxidation poten-
tials while limiting access to reduction events.
Acknowledgements
This work was funded by the National Science Foundation’s Divi-
sion of Chemistry (Award 1300653) and EPSCoR Program (Award
1355438). CFE thanks the Division of Organic Chemistry of the
American Chemical Society for a Summer Undergraduate Re-
search Fellowship. SAO and CR thank the University of Kentucky
for start-up funding.
Conflict of interest
Figure 4. UV/Vis absorption spectra of EPT, 3,7-DMeEPT, 1,9-DMeEPT, 1,9-
DMeiPrPT, and 1,9-DMePhPT in their neutral (a) and radical cation (b) forms
at 14 mm and 0.1 mm, respectively, in DCM. Note: No spectrum is reported
for the 1,9-DMePhPT radical cation due to its irreversible first oxidation
event.
The authors declare no conflict of interest.
Keywords: conjugation
·
electronic structure
·
redox
chemistry · strained molecules · substituent effects
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reflect differences in the oxidized species (Figure 4b). The ab-
sorption spectra of the radical cations of EPT and 3,7-DMeEPT
show profiles consistent with the radical cations of several
phenothiazine derivatives prepared in our laboratory: intense
absorption features between 500–600 nm and low-energy,
low-intensity bands between 650–950 nm.[7b,13] By contrast, the
analogous absorption features for the 1,9-disubstituted deriva-
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sults suggest that variation in placement of methyl groups af-
fects the electronic gap between the HOMO-1 and HOMO
(using the nomenclature of the neutral species), which in turn
influences the D1!D2 transition of the radical cation that is
predominately HOMO-1!SOMO (singly-occupied molecular or-
bital) in nature. From DFT calculations, the HOMO of EPT and
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is energetically destabilized; this observation is consistent with
the trends of the computed vertical IPs, confirming the validity
of a Koopmans’ theorem[14] (Janak’s theorem[15] in the context
of DFT) estimate of the vertical IPs based on the HOMO ener-
gies. However, the HOMO-1 of 3,7-DMeEPT and 1,9-DMeEPT are
of similar energy and energetically destabilized when com-
pared to EPT, revealing the comparable electronic impact of
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group’s location on the phenothiazine. The combination of
these effects results in a smaller HOMO-1 to HOMO energy
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Manuscript received: June 6, 2017
Accepted manuscript online: June 7, 2017
Version of record online: && &&, 0000
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M. D. Casselman, C. F. Elliott,
S. Modekrutti, P. L. Zhang, S. R. Parkin,
C. Risko,* S. A. Odom*
Through the strategic placement of
otherwise electron-donating methyl
groups at the positions ortho to nitro-
gen, planarization of the phenothiazine
radical cation is prevented, thus raising
the oxidation potential of this com-
pound (blue) compared to its para-sub-
stituted isomer (green) and parent com-
pound (black).
&& – &&
Beyond the Hammett Effect: Using
Strain to Alter the Landscape of
Electrochemical Potentials
&
ChemPhysChem 2017, 18, 1 – 6
www.chemphyschem.org
6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!