Towards the rational design of novel charge-transfer materials: Biaryls with a dihedral angle-independent hole delocalization mechanism

Article abstract of DOI:10.1039/c8cc02595a

Biaryl cation radicals are important electroactive materials, which show two mechanisms of hole delocalization: static delocalization at small interplanar dihedral angles and dynamic hopping at larger angles, reflecting the interplay between electronic coupling and structural reorganization. Herein, we describe the rational design of biaryls possessing an invariant hole delocalization mechanism.

Full text of DOI:10.1039/c8cc02595a

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Towards the Rational Design of Novel Charge-Transfer Materials:
Biaryls with A Dihedral Angle-Independent Hole Delocalization
Mechanism
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
DOI: 10.1039/x0xx00000x
Lena V. Ivanova,* Tushar S. Navale, Denan Wang, Sergey Lindeman, Maxim V. Ivanov* and
a,b
Rajendra Rathore
www.rsc.org/
Biaryl cation radicals are important electroactive materials, which where dynamic hopping occurs even for the smallest
show two mechanisms of hole delocalization: static delocalization interplanar dihedral angles?
at small interplanar dihedral angles and dynamic hopping at larger
angles, reflecting the interplay between electronic coupling and
structural reorganization. Herein, we describe the rational design
of biaryls possessing an invariant hole delocalization mechanism.
Biaryls are widely used to explore the fundamental properties
1
-3
of charge transfer in π-conjugated molecular wires. For
example, in case of a coherent charge transfer without
oxidation of the biaryl, the rate of the electron transfer scales
as the square of the electronic coupling. However, at the
steady-state condition that involves complete oxidation of the
4
-6
biaryl, two scenarios are possible: the hole can be (statically)
delocalized over both aryls, or can (dynamically) hop between
them. The favored mechanism depends on the interplay
Fig. 1 Superimposed structures and schematic representation of HOMO of biaryls with
varied length of n-methylene linker and aryl groups based on (A) veratrole and (B)
hydroquinone ether moieties.
between electronic coupling (H ), determined by orbital
ab
overlap, and reorganization energy (λ), which involves slow
modes of solvent reorientation and fast bond vibrations
In
Vn series the favorable nodal arrangement of HOMO
7
associated with charge transfer. As the charge transfer in the promotes efficient orbital overlap (Fig. 1A) and large electronic
long molecular wires is often dominated by an incoherent coupling that is reflected in the extensive hole delocalization
8
-11
hopping mechanism,
assemblies exhibiting hole delocalization via (steady-state) positions alters the arrangement of HOMO lobes,
dynamic hopping is of particular importance.
expected to reduce the orbital overlap and lead to a dynamic
rational design of molecular up to ϕ = 45°. Moving the methoxy groups from 1,2 to 2,5
1
2,13
and is
A recent study of a set of biaryl cation radicals based on hopping mechanism even at smaller interplanar angles. In this
the 1,2-dimethoxy aryl (i.e., veratrole) groups with varied communication we will show that in biaryls based on
number of methylenes in the linker (Vn, n = 1-6, Fig. 1A, Chart hydroquinone ether with n-methylene linker (HEn, n = 1-6, Fig.
S1 in the ESI) showed that the hole is statically delocalized for 1B, Chart S1 in the ESI), the dynamic hopping mechanism
+•
interplanar dihedral angles (ϕ) in the 0°-45° range and begins from HE1 (ϕ = 24°), as opposed to
4
dynamically hops in biaryls with larger angles. Building on this switchover occurs for
V
n series where the
+•
V4 (ϕ = 45°). Such a contrasting angular
work, in the context of a rational design of materials for long- dependence of the hole delocalization mechanism in two
range charge transfer we ask, can one design a model biaryl similar biaryls underscores an importance of the analysis of the
frontier molecular orbitals in the rational design of novel
charge-transfer materials, where the mechanism of hole
delocalization can be tailored to achieve desired charge-
transfer characteristics.
Compounds HE1-HE6 were synthesized following literature
1 13
and characterized by H/ C NMR and MALDI
14, 15
procedures
spectroscopies and X-ray crystallography of representative
molecules (see ESI for full details). Crystal structures of neutral
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+
•
HE6 in CH
2
Cl
2
at 22°C. The positions (nm) of the lowest-energy band with molar
HEn showed that the interplanar dihedral angles between two
-
1
-1
absorptivity (M cm ) in parenthesis are indicated.
aryl groups lie in the 41°-92° range (Fig. 2). Unfortunately,
+
•
repeated attempts to isolate crystal structure of HE
n
were
The electrochemical stability of HEn prompted us to
16
generate their cation radicals via quantitative redox titrations
unsuccessful.
1
7
using robust aromatic oxidants (see ESI for details). The
cation radicals exhibit a characteristic near-IR band (Fig. 3B),
which shifts red from 1620 to 2190 nm in the absorption
+
•
+•
+•
spectra of HE1 , HE
3
and HE4 , accompanied by a minor
-
1
-1
reduction in molar absorptivity from 3500 to 2400 M cm .
Importantly, the position of the near-IR band remains invariant
+•
in HE4 –HE6 , appearing at 2190, 2200, and 2200 nm,
+•
respectively, with similar molar absorptivities of 2400, 2500,
-
1
-1
200 M cm , respectively. Surprisingly, the near-IR band in
2
+
2 demonstrated an unusual blue shift to
•
the spectrum of HE
-
90 nm and a reduction in molar absorptivity (1700 M cm ).
1
-1
8
+•
We compare these findings to the Vn series, where the near-
IR band shifts by some 1000 nm from 1098 to 2050 nm with
the intensity of the band dramatically decreasing by some
-
1
-1
4
1
0,000 M cm (Table S3 in the ESI).
In order to rationalize the disparate evolution with
increasing n of the redox and optical properties of HEn and n,
Fig. 2 ORTEP diagrams (50% probability) of HEn. Hydrogens are omitted for clarity.
V
Compounds HEn showed reversible cyclic voltammograms
we carried out DFT calculations using a customized B1LYP-40
functional that has been parameterized to accurately
(
CVs), with an oxidation potential that varied within 200 mV
Fig. 3A), in contrast to the 470 mV range observed for the
(
V
n
reproduce the electronic structure of π-conjugated cation
18,19
series. Furthermore, oxidation potentials (E ) of HEn decrease
ox
radicals.
The DFT calculations at B1LYP-40/6-
with increasing number of methylenes going from E = 0.82 V
ox
3
1G(d)+PCM(CH Cl ) level of theory well reproduced the
2 2
+
(
HE1, ϕ = 41°) to 0.68 V vs Fc/Fc (HE4, ϕ = 52°), and remain
available X-ray structures of neutral HEn, and showed that
upon oxidation, the interplanar dihedral angle decreases, with
the most pronounced changes corresponding to the biaryls
+
invariant up to HE5 and HE6 (0.62 and 0.63 V vs Fc/Fc ,
respectively), although the dihedral angle ϕ continues to
increase (ϕ = 68°
contrast with the evolution of Eox in
and 92
°
, respectively). This is in sharp
n series, where
with a longer methylene linker (Table S4 in the ESI). The
+•
V
resulting equilibrium dihedral angles of the optimized HE
n
increasing the number of methylenes leads to an increase in
the Eox due to the decreasing interchromophoric electronic
span a very narrow range of values (ϕCR = 24°-49°, Fig. 4A).
4
coupling (Table S3 in the ESI).
+•
+•
Fig. 4 Calculated structures and spin-density plots of (A) HEn and (B) Vn series.
Analysis of the calculated bond length changes in
2 2
Fig. 3 (A) Cyclic voltammograms (CVs) and indicated Eox1 of 2 mM HE1-HE6 in CH Cl
+•
-
1
+•
(
0.1 m n-Bu
4 6
NPF ) at a scan rate of 200 mVs and 22°C. (B) Absorption spectra of HE1
-
HE
n→
HE
n
transformations showed that the oxidation-
2
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induced structural reorganization follows a quinoidal distortion equilibrium structures of HE
COMMUNICATION
+
•
n
revealed that the aryl groups
of each aromatic ring (Table S7 in the ESI). Importantly, the are not equivalent across the series, as the varied length of the
distribution of the structural reorganization is uneven with methylene linker changes the orientation of the ether group
most of the reorganization localized on a single aryl group for with respect to aromatic plane. For example, upon
+
•
20
all HEn . The natural population analysis (NPA) of the charge incorporation of additional methylenes, the dihedral angle C-
+•
density in HE
n
further confirmed that most (i.e., 0.9/0.8) of O-Car-Car varies in the α1/2 = 180°-2° range (Table S4 in the ESI).
It has been established that orientation of the methoxy
the spin/charge is localized on the same aryl (Table S8 in the
ESI), which can be visually seen from the spin-density plots of groups relative to the aromatic plane in various HE derivatives
23
has a dramatic impact on their oxidation potentials. For
+•
HEn
(Fig. 4A).
+•
n
In contrast, the
V
series displays a mechanism of hole example, a computational relaxed potential scan of a model
delocalization that is extremely sensitive to the value of the 2,5-dimethoxy-p-xylene (DMX), where dihedral angle C-O-Car
4
interplanar dihedral angle (Fig. 4B). Starting from planar
-
+•
1 ,
V
C
ar is varied from completely planar (α = 0°) to a perpendicular
where the hole is statically delocalized over both aryl groups, (α = 90°) arrangement, showed that the relative energy of
+
•
increasing the interplanar angle changes the mechanism of DMX dramatically increases by 0.4 eV (Fig. S13 in the ESI).
hole distribution to the dynamic hopping, as evidenced by the Thus, when the number of methylenes in the linker of HEn is
change in the slopes of the oxidation potentials (E ) and small, the orientation of the ether groups is forced to adopt a
ox
cation radical excitation energies (νmax) against cos(ϕ ) as more energetically demanding out-of-plane arrangement,
CR
shown in Figs. 5A and 5B, respectively.
leading to higher oxidation potentials. Upon addition of more
methylenes, the linker flexibility increases and the ether
groups can adopt a more energetically favorable in-plane
arrangement, lowering the oxidation potential.
In hindsight, the choice of linking two aryl groups via a
methylene bridge involving the ether group is not ideal, as the
reorganization energy and electronic coupling parameters may
be influenced by the linker in an unpredictable manner. For
example, compound HE2 is an outlier in the electronic
+
•
absorption spectra of HE
n series, with its spectrum displaying
a blue-shifted absorption band in contrast with the general
trend of the absorption band shifting to the longer
wavelengths (Fig. 3). An improved series of HE-based biaryls
would have ether groups that are rigidly held in the aromatic
plane by additional ethylene bridges (i.e., pHEn, Fig. 6A).
Indeed, preliminary calculations show that hole is largely
Fig. 5 Plots of experimental (A) Eox1 and (B) νmax against cos(ϕCR) for HEn (magenta) and
Vn (blue).
The contrasting evolution in the mechanism of hole
delocalization with respect to the varied interplanar angle in
+•
localized on a single aryl group in the entire pHE
n
series as in
+
HEn series , yet their oxidation potentials and cation radical
•
+
•
+•
V
n
and HE
n
must arise from differences in electronic
) parameters.
excitation energies are weakly dependent on the interplanar
dihedral angle ϕ and show the expected weak increase with
increasing ϕ (Fig. 6B).
coupling (Hab) and structural reorganization (λ
According to the Marcus-Hush theory, the characteristic
interplanar dihedral angle at which the mechanism of hole
We have demonstrated a rational approach to the design
of novel charge-transfer molecular assemblies in which one
simultaneously considers both geometrical and electronic
delocalization changes corresponds to the case where 2Hab
=
+•
λ
must be less than the half of the reorganization energy (Hab
. This fact implies that in HE
n
series, the electronic coupling
<
structure properties on the example of model biaryls (
V
n, HE
n
λ/2). Indeed, molecular orbital analysis showed that the
electronic coupling in HEn lies in the 0.1-0.3 eV range (Fig. S9 in
+•
and pHEn series). In Vn two mechanisms of hole distribution
2
1
exist that depend on the value of the interplanar dihedral
angle, i.e. static delocalization when ϕ ≤ 45° and dynamic
hopping when ϕ > 45°. A modification in position of methoxy
the ESI), while TD-DFT calculations showed
that the
reorganization energy is significantly higher, 0.8-1.2 eV (Table
S4 in the ESI), and therefore both parameters satisfy the Hab
<
+
•
groups from Vn to HEn changes the nodal arrangement of
λ
occur on the picosecond timescale as estimated by the Marcus
/2 criteria. The dynamic hole hopping in HE
n
is estimated to
HOMO and reduces the orbital overlap at the biaryl linkage,
2
equation (Table S9 in the ESI).
2
resulting in the dynamic hopping mechanism starting from n =
+•
+
•
1, i.e., HE1 . One may further refine the biaryl structure by
forcing the orientation of the ether groups into the aromatic
plane via additional methylene bridges (i.e., pHEn) in order to
achieve a near-invariance of the oxidation potentials and
cation radical excitation energies with respect to the
interplanar dihedral angle. This work thus highlights the
importance of frontier molecular orbital analysis for the
The dynamic hole hopping mechanism evidenced in HE
n
should lead to a near-invariance of Eox with interplanar angle,
comparable to the modest ~110 mV increase of Eox in n series
for ϕCR = 45°-90°. However, electrochemical analysis of HE
V
n
showed that Eox decreases by 200 mV over a significantly
smaller range of interplanar angles (ϕCR = 24°-49°). A further
analysis of the X-ray structures of HEn and calculated
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1
0
=
00 K, 2948 reflections measured, 2654 unique reflections, Rint
=
.0217, 222 parameters refined, R(all) = 0.0395, wR(all) = 0.0861, S
rational design of novel long-range charge-transfer materials
with tailored redox and optoelectronic properties.
1.018 (CCDC 1830467). Crystal structure data for HE5 [C H O ]
2
1
26
4
(
raj10j): FW = 342.42, P-1, a = 10.9041(3) A
̊
̊
c =
1
=
5.3920 (4) A ̊, α = 67.718(1), β = 79.863(1), γ = 89.142(1), Z = 4, V
3
-
3
1803.04(8) A ̊ , D = 1.261 g cm , T = 100 K, 5910 reflections
measured, 5574 unique reflections, Rint = 0.0143, 460 parameters
refined, R(all) = 0.0332, wR(all) = 0.0868, S = 1.006 (CCDC 1830464).
Crystal structure data for HE6 [C H O ] (raj10i): FW = 356.44, P65,
22
28
4
a = 9.0951(1) A
γ = 120, Z = 6, V =2781.31(8) A
reflections measured, 2718 unique reflections, Rint = 0.0162, 249
parameters refined, R(all) = 0.0274, wR(all) = 0.2722, S = 1.016
̊, b = 9.0951(1) A ̊, c = 38.8243(9) A ̊, α = 90, β = 90,
3
-
3
̊
, D = 1.277 g cm , T = 100 K, 2722
(
CCDC 1830462).
1
2
3
D. M. Guldi, H. Nishihara and L. Venkataraman, Chem. Soc.
Rev., 2015, 44, 842.
L. Venkataraman, J. E. Klare, C. Nuckolls, M. S. Hybertsen and
M. L. Steigerwald, Nature., 2006, 442, 904.
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Wandlowski and M. Mayor, Angew. Chem. Int. Ed., 2009, 48
886.
M. R. Talipov, T. S. Navale, M. M. Hossain, R. Shukla, M. V.
Ivanov and R. Rathore, Angew. Chem. Int. Ed., 2017, 129
72.
A. Heckmann and C. Lambert, Angew. Chem. Int. Ed. Engl.,
012, 51, 326.
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B. S. Brunschwig, C. Creutz and N. Sutin, Chem. Soc. Rev.,
2002, 31, 168.
+
•
Fig. 6 (A) Structures and spin density plots of pHEn . Plots of (B) Eox1 and (C) νmax of Vn
blue) and pHEn (green) against cos(ϕCR). Values for Vn are experimental, while values
8
9
C. E. Smith, S. O. Odoh, S. Ghosh, L. Gagliardi, C. J. Cramer
and C. D. Frisbie, J. Am. Chem. Soc., 2015, 137, 15732.
G. Li, N. Govind, M. A. Ratner, C. J. Cramer and L. Gagliardi, J.
(
for pHEn were obtained using DFT and scaled to experimental data using the linear
correlation in Fig. S14.
Phys. Chem. Lett., 2015,
0 C. Lambert, G. Nöll and J. Schelter, Nat. Mater., 2002,
1 M. E. Walther and O. S. Wenger, Chemphyschem., 2009, 10
203.
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Ed. Engl., 2015, 54, 14468.
6, 4889.
1
1
1, 69.
We thank the NSF (CHE-1508677) and NIH (R01-HL112639-
,
0
4) for financial support, and Prof. Scott Reid for helpful
1
discussions. The calculations were performed on the high-
performance computing cluster Père at Marquette University
and XSEDE.
1
1
3 M. V. Ivanov, V. J. Chebny, M. R. Talipov and R. Rathore, J.
Am. Chem. Soc., 2017, 139, 4334.
1
1
1
4 R. Rathore and J. K. Kochi, J. Org. Chem., 1995, 60, 7479.
5 L. Zhai, R. Shukla and R. Rathore, Org. Lett., 2009, 11, 3474.
6 M. R. Talipov, A. Boddeda, M. M. Hossain and R. Rathore, J.
Phys. Org. Chem., 2015, 29, 227.
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Properties, and Applications, John Wiley & Sons, Hoboken,
NJ, 2015.
8 M. R. Talipov, A. Boddeda, Q. K. Timerghazin and R. Rathore,
J. Phys. Chem. C, 2014, 118, 21400.
9 M. V. Ivanov, M. R. Talipov, T. S. Navale and R. Rathore, J.
Phys. Chem. C, 2018, 122, 2539.
0 A. E. Reed, R. B. Weinstock and F. Weinhold, J. Chem. Phys.,
Conflicts of interest
There are no conflicts to declare.
1
Notes and references
1
1
2
2
‡
Crystal structure data for HE1 [C17
18 4
H O ] (raj10f): FW = 286.31,
C2/c, a = 11.1202(3) A
̊
̊
̊
α = 90,
3
-3
β = 99.472(2), γ = 90, Z = 4, V = 1390.21(6) A
1
0
=
00 K, 1202 reflections measured, 1126 unique reflections, Rint
.0220, 133 parameters refined, R(all) = 0.0291, wR(all) = 0.0778, S
0.980 (CCDC 1830466). Crystal structure data for HE2 [C18
=
1
985, 83, 735.
1 Note that in the case of dynamic hopping mechanism
excitation energy of the biaryl cation radical equals
reorganization energy λ, see ref. 7.
H
20
O
4
]
(
raj10d): FW = 286.31, Pca21, a = 48.377(2) A
̊
̊
3
2
1
1.0775(10) A ,
̊
α = 90, β = 90, γ = 90, Z = 4, V = 7711.2(6) A
, D =
-3
2
2
2 R. A. Marcus and N. Sutin, Biochim. Biophis. Acta., 1985, 811
65.
3 M. V. Ivanov, D. Wang, S. H. Wadumethridge and R. Rathore,
J. Phys. Chem. Lett., 2017, , 4226.
,
.294 g cm , T = 100 K, 13208 reflections measured, 12390 unique
2
reflections, Rint = 0.0334, 1013 parameters refined, R(all) = 0.0573,
wR(all) = 0.1411, S = 1.019 (CCDC 1830465). Crystal structure data
8
for HE3 [C19
H
22
O
4
] (raj10m): FW = 314.37, Pbcn, a = 7.4984(2) A
̊, b =
1
=
0.1051 (2) A
̊
, c = 20.5483(4) A , α = 90, β = 90, γ = 90, Z = 4, V
̊
-3
3
1556.99(5) A ̊ , D = 1.341 g cm , T = 100 K, 1365 reflections
measured, 1303 unique reflections, Rint = 0.0206, 150 parameters
refined, R(all) = 0.0295, wR(all) = 0.0846, S = 0.976 (CCDC 1830463).
Crystal structure data for HE4 [C20
24 4
H O ] (raj0z): FW = 328.39, C2/c,
a = 40.9654(10) A , b = 8.6677(2) A ,
̊
̊
c = 9.9042(2) A ,
̊
α = 90, β =
3
-3
1
02.185(1), γ = 90, Z = 8, V =3437.51(13) A
̊
, D = 1.269 g cm , T =
4
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Sentence highlighting the novelty of the work:
Herein, we demonstrate a rational design of novel materials using FMO analysis
on the example of biaryls possessing an interplanar dihedral angle-invariant
mechanism of hole delocalization.