A triphenylamine double-decker: From a delocalized radical cation to a diradical dication with an excited triplet state

Article abstract of DOI:10.1002/anie.201204106

The redox properties and electronic structures of polycationic species were examined using a triphenylamine double-decker species. The double-decker has a strong electron-donating ability, and the spin in the radical cation is delocalized over the whole caged skeleton, despite no direct transannular π-π interaction between two TPA decks. Moreover, the diradical dication has spin-singlet character, despite the meta-phenylene linkage.

Full text of DOI:10.1002/anie.201204106

Angewandte
Chemie
DOI: 10.1002/anie.201204106
Radical Ions
A Triphenylamine Double-Decker: From a Delocalized Radical Cation
to a Diradical Dication with an Excited Triplet State**
Yuichiro Yokoyama, Daisuke Sakamaki, Akihiro Ito,* Kazuyoshi Tanaka, and Motoo Shiro
Triphenylaminium radical cations are well-known for their
remarkable stability; furthermore, the oxidation potential is
adjustable by the introduction of a series of substituents, in
particular, at the para positions. Thus, they are widely used
not only as one-electron oxidative reagents but also as
catalysts in organic chemistry.[1] Besides such an application
as versatile chemical reagents, the ambient stable radical
cations generated from triphenylamine (TPA) derivatives
could be utilized as spin sources in molecule-based magnetic
materials. In fact, a considerable number of TPA-based
molecular multi-spin systems have appeared in the literature.
However, from the viewpoint of assemblage of the spin
sources into the architecture of the reported molecular
systems, the spin–spin exchange coupling pathway has been
confined to one- and two-dimensionalities,[2] and accordingly,
TPA-based multispin systems with three-dimensional (3D)
connectivity are rare.
action between two nitrogen lone pairs.[4] In spite of its
sensitivity to oxidation and the formation of persistent radical
cation, no detailed information was available concerning the
electronic structure. Such a cage-like oligo(triphenylamine)
can be regarded as a potent building block to afford the
opportunity furnishing new TPA-based macromolecular sys-
tems with genuine 3D connectivity. Although neither the
parent 1 nor its derivatives have been reported since then,[5]
an analogue 2 in which two TPA moieties are connected at
para positions of all phenyl groups by three meta-phenyl-
enediamine pillars, was successfully prepared in our labora-
tory.
Slow evaporation of a dilute mixed solution (CH3OH and
CH2Cl2) of a pale yellow product 2, which was obtained by
using Buchwald–Hartwig cross-coupling reaction[6] between
4,4’,4’’-tribromo-TPA as a deck and meta-phenylenediamine
as a pillar in a ratio of 2:3,[7] gave yellow block crystals suitable
for X-ray structure analysis.[21] The crystal belongs to the
space group P21, and contains two molecules of 2 per unit cell
in which there exist three sites for CH2Cl2 molecules so as to
fill spaces between the double-deckers (Supporting Informa-
tion, Table S1).
In 1985, Neugebauer and co-workers reported the first
synthesis of a TPA double-decker 1 (Scheme 1)[3] in which two
TPA moieties are directly connected by threefold o,o’-link-
age, in association with an interest in intramolecular inter-
The X-ray structure of 2 has almost C3h symmetrical
trichetria-like[8] shape when all the methoxy substituents are
ignored, and the upper and lower TPA decks adopt almost
planar propeller-like conformation (the tilted angles of
phenyl rings range from 428 to 538), and moreover, are totally
eclipsed (Figure 1). Three meta-phenylenediamine planes are
perpendicular to the TPA planes, and therefore, the distance
between two TPA decks are estimated to be about 4.9 ꢀ at
the peripheral three nitrogen pairs and 5.8 ꢀ at the central
nitrogen pair, indicating the direct transannular p–p inter-
action between two TPA decks is virtually ignorable. The sp2
planes of the nitrogen atoms in the meta-phenylenediamine as
a pillar are found to be at angles of about 608 to the meta-
phenylene plane.
Scheme 1. The first reported TPA double-decker 1[3] and meta-phenyl-
enediamine-bridged TPA double-decker 2.
[*] Y. Yokoyama, D. Sakamaki, Dr. A. Ito, Prof. Dr. K. Tanaka
Department of Molecular Engineering
Graduate School of Engineering, Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: aito@scl.kyoto-u.ac.jp
Dr. M. Shiro
Rigaku Corporation, X-ray Research Laboratory
Matsubaracho 3-9-12, Akishima, Tokyo, 196-8666 (Japan)
[**] The present work was supported by Grant-in-Aid for Scientific
Research (B) (20350065) from the Japan Society for the Promotion
of Science (JSPS). D.S. thanks the JSPS Research Fellowship for
Young Scientists.
Figure 1. Two ORTEP views of 2. Ellipsoids are set at 50% probability;
solvent molecules of crystallization, methoxy groups, and hydrogen
atoms are omitted for clarity. Nitrogen atoms are colored in black.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204106.
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In accordance with the X-ray structure, DFT calculations
(B3LYP/6-31G* level of theory) also demonstrated that the
C3h-symmetric structure was adopted for a model compound
2’ in which all the methoxy groups are replaced by hydrogen
atoms. As is apparent from the HOMO (Figure 2a), the spin
Figure 3. Cyclic voltammogram (CV) and differential pulse voltammo-
gram (DPV) of 2, measured in CH2Cl2 containing 0.1m nBu4NBF4 at
298 K (scan rate 100 mVsꢀ1).
hexaaza[16]paracyclophane (Eox1 = ꢀ0.28 V)[9] and the second
generation of dendritic triphenylamines (Eox1 = ꢀ0.30 V).[10]
Taking this into account, 2 is considered to be a good electron
donor, and indeed it afforded the charge-transfer (CT)
complex with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ)
in the ratio of 1:1 (Supporting Information, Figure S3). More
2
1
interestingly, the splitting of redox potentials (Eox ꢀEox
=
170 mV) indicates a relatively strong electronic coupling
between two TPA decks, irrespective of meta-phenylene
linkage, and therefore, the intervalence state between the two
redox-active TPA decks is expected to appear in radical
cation of 2, as has been elaborately been investigated for
TPA-based organic mixed-valence compounds.[11] This is also
supported by comparison with the first oxidation potential
(Eox1 = ꢀ0.16 V (1e)) for the reference compound 3[10] corre-
sponding to a single TPA deck (Scheme 2). Furthermore, two
charged radical centers generated by the second oxidation
process of 2 are located separately on the upper and lower
TPA decks to alleviate the energetically unfavorable Cou-
lombic repulsion, and as a consequence, magnetic interaction
takes place between the two-spin system.
Figure 2. a) HOMO and HOMOꢀ1 (B3LYP/6-31G*) of 2’; b) spin-
density distribution of 2’+ (black: positive spin, white: negative spin;
spin isosurface value=0.0003 electron auꢀ3).
On the basis of the electrochemical studies, we measured
the optical absorption spectral change of double-decker 2 in
CH2Cl2 during the course of the oxidation going from neutral
to tetracation 24+ by using optically transparent thin-layer
electrochemical cell (Supporting Information, Figure S4). To
density distribution in 2’+ revealed delocalization over the
entire molecule (Figure 2b). Although the C3h symmetric
structure is retained on going from 2’ to 2’4+, the quinoid-type
deformation in TPA decks was predicted to be remarkable
with increase in the degree of oxidation (Supporting Infor-
mation, Table S2).
Double-decker 2 exhibited multiple redox behavior owing
to contribution from the eight TPA redox centers. The
observed five oxidation waves are chemically reversible, and
the oxidation potentials [Eoxn vs. Fc0/+ (ne)] are determined to
be Eox1 = ꢀ0.33 V (1e), Eox2 = ꢀ0.16 V (1e), Eox3 =+ 0.09 V
(2e), Eox4 =+ 0.46 V (1e), and Eox5 =+ 0.61 V (1e) on the basis
of cyclic voltammetry (CV) and differential pulse voltamme-
try (DPV) (Figure 3). It should be noted that although meta-
phenylene linkage is included in connectivity of 2, the first
oxidation potential of Eox1 = ꢀ0.33 V is more negative than
that of the related all-para-phenylene-linked TPA oligomers:
Scheme 2. Reference compound 3 for a single TPA deck.
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gain insights into the charge distribution of the oxidized
double-decker, only the spectral change in the lowest energy
transitions of 2+ to 24+ will be discussed (Figure 4). The lowest
that the charged species of 2+ to 24+ are in delocalized
intervalence states. In particular, the lowest-energy band for
2+ is assignable to a transition from doubly degenerate b
((HO-1)MOs) to b (LUMO), which corresponds to the
charge resonance between the meta-phenylenediamine moi-
eties as pillars and the core TPA decks. Moreover, the
intervalence band observed for 2+ exhibited no noticeable
solvatochromism in CH2Cl2 (e = 8.93) and benzonitrile (e =
25.59), thus supporting a charge-delocalized character for 2+
(Supporting Information, Figure S7).
Treatment of 2 with 1 equiv of tris(4-bromophenyl)ami-
nium hexachloroantimonate (Magic Blue)[1c] in CH2Cl2 at
195 K gave a green solution for 2+. However, the solution
ESR spectrum showed a single broad line with no hyperfine
structures (g = 2.0024; DHpp (peak-to-peak linewidth) =
1.05 mT), thus affording no information on the spin distribu-
tion over the molecular skeleton (Supporting Information,
Figure S8). On the other hand, the evidence of a diradical
dication for the oxidized species treated by two equiv of the
Magic Blue was confirmed by the fine-structured ESR
spectrum characteristic for spin-triplet species in the g ꢁ 2
region, as well as by the weak signal corresponding to the
forbidden transition in the g ꢁ 4 region (Figure 5). From the
Figure 4. Vis-NIR absorption spectra of the stepwise electrochemical
oxidation of 2 to tetracation 24+ in CH2Cl2/0.1m nBu4NBF4 at 298 K.
a 2+, g 22+, c 24+. The black and white sticks designate the
TD-DFT-calculated lowest-energy transition energies and their relative
oscillator strengths for 2’+, 2’2+, and 2’4+ at B3LYP/6-31G* and B1LYP-
(a=0.35)/SVP with CPCM solvent model for CH2Cl2.
energy band at 1332 nm (0.93 eV) observed for 2+ is blue-
shifted to 1190 nm (1.04 eV) when oxidized into 22+, and the
shifted band becomes twice as intense as that for 2+. Further
oxidation to 24+ resulted in a far more intense band with
a further blue-shift (1053 nm (1.18 eV)).[12] For comparison,
the spectroelectrochemical measurements were also per-
formed for the reference compound 3 (Supporting Informa-
tion, Figure S5).[12] As a consequence, the observed spectrum
for 3+ was found to be in good accordance with that for 22+.
This result strongly suggests that two charged radical centers
in 22+ are located separately on the upper and lower TPA
decks. Furthermore, both spectra observed for 24+ and 32+
were obviously similar, although the maximum wavelength
for the lowest energy band for 22+ remained unchanged
(1190 nm (1.04 eV)). Excitation energies for the oxidized
species, 2’+, 2’2+, and 2’4+, were estimated by the time-
dependent DFT (TD-DFT) calculations[13] (Supporting Infor-
mation, Figure S2 and Table S3). As shown in Figure 4, the
characteristics of the observed spectral change is reasonably
reproduced by the theoretical results for the model compound
2’. Recently, Kaupp and co-workers pointed out the fact that
the frequently used B3LYP hybrid functional may lead to
erroneous results in organic mixed-valence compounds.[14]
Thus, in addition to the B3LYP calculations, we have also
performed the DFT optimizations and TD-DFT calculations
for 2’+, 2’2+, and 2’4+ by using the B1LYP hybrid functional
with 35% exact exchange and the SVP basis sets,[15] as has
been recommended by Kaupp and co-workers.[14] Further-
more, solvents effects have been included by the CPCM
polarizable conductor calculation model for CH2Cl2 (e =
8.93).[16] These calculations resulted in the same trend as
B3LYP ones, although the transition energies showed small
blue-shifts (Figure 4; Supporting Information, Table S4).
Therefore, the present TD-DFT calculations strongly suggest
Figure 5. ESR spectrum of 22+ in CH2Cl2/n-butyronitrile (3:1) at 123 K.
Inset: forbidden resonance observed at 123 K.
zero-field splitting parameters (D = 4.9 mTand Effi0 mT), the
average distance between the radical centers[17] was estimated
to be 8.3 ꢀ. The deviation from the face-to-face distance
between the two TPA decks (5.8 ꢀ) determined by X-ray
analysis strongly suggests that spin-density distributions on
each TPA decks are to some extent delocalized over meta-
phenylenediamine pillars. Figure 6 displays the temperature
dependence of the ESR signal intensity for 22+, and the
ground state of 22+ turned out to be in spin-singlet state,
apparently from decrease in intensity with decreasing temper-
ature. The energy gap between the singlet and triplet states
(DES-T) was estimated to be ꢀ0.18 kcalmolꢀ1 as a result of
curve-fitting with the Bleaney–Bowers singlet–triplet model
equation [Eq. (1)].[18]
C
T
1
ꢀ
ꢁ
I ¼
ð1Þ
DESꢀT
3 þ exp ꢀ
kB
T
On the other hand, as shown in Figure 2a (see also the
Supporting Information, Figure S1), the HOMO and
HOMOꢀ1 of 2’ were found to be of the nondisjoint (or
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Keywords: arylamines · cyclophanes · electrochemistry ·
.
EPR spectroscopy · radical ions
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reflected in the orbital patterns over meta-phenylenediamine
moieties, and moreover, the quasi degeneracy and nondisjoint
character suggested the high-spin preference for 2’2+ (DES–T
=
5.3 kcalmolꢀ1). The discrepancy between the DFT-calculated
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present DFT calculations, and therefore, the present calcu-
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ground state in diradicals.[20] In contrast, tetracation 24+,
generated by treatment of 2 with four equiv of the Magic
Blue, was ESR silent (Supporting Information, Figure S9), in
good accordance with the theoretical consideration.
In conclusion, we have described the electronic structures
for the polycationic states of a double-layered TPA linked by
three meta-phenylenediamine pillars, that is, TPA double-
decker. As exemplified by the X-ray analysis, no direct
through-space interaction between two nitrogen lone pairs on
both TPA decks can be inferred. However, it was clarified that
the generated spin in 2+ is delocalized over the whole
molecular skeleton, which is probably due to through-bond
interaction through three-dimensional connectivity in 2.
Furthermore, although the dication of 2 revealed the diradical
character originating from the nondisjoint quasi doubly
degenerate NBMOs owing to three meta-phenylenediamine
pillars, its spin preference was contrary to the simple B3LYP
prediction. The outcome of the present study demonstrates
that it is not straightforward to endow three-dimensionally
connected TPA-based macromolecular systems with desired
electronic properties. To put it the other way around,
however, intriguing electronic features can be emerged from
this class of molecular systems. The extension to multiple-
deckers and the construction of the other related TPA-based
architectures with complicated connectivity are currently
underway in our laboratory.
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Received: May 26, 2012
Revised: July 10, 2012
Published online: && &&, &&&&
4
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Radical Ions
The redox properties and electronic
structures of polycationic species were
examined using a triphenylamine double-
decker species. The double-decker has
a strong electron-donating ability, and the
spin in the radical cation is delocalized
over the whole caged skeleton, despite no
direct transannular p–p interaction
between two TPA decks. Moreover, the
diradical dication has spin-singlet char-
acter, despite the meta-phenylene linkage.
Y. Yokoyama, D. Sakamaki, A. Ito,*
K. Tanaka, M. Shiro
&&&& — &&&&
A Triphenylamine Double-Decker: From
a Delocalized Radical Cation to
a Diradical Dication with an Excited
Triplet State
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!