Through-space (cofacial) π-delocalization among multiple aromatic centers: Toroidal conjugation in hexaphenylbenzene-like radical cations

Article abstract of DOI:10.1002/anie.200501005

(Chemical Equation Presented) Pi and doughnuts: Highly effective through-space conjugation among cofacial (aromatic) redox centers is established in stable hexaanilinylbenzene radical cations (1^.+; π conjugation shown by toroid). The radical ca

Full text of DOI:10.1002/anie.200501005

Angewandte
Chemie
Conjugation
DOI: 10.1002/anie.200501005
Through-Space (Cofacial) p-Delocalization
among Multiple Aromatic Centers: Toroidal
Conjugation in Hexaphenylbenzene-like Radical
Cations**
Duoli Sun, Sergiy V. Rosokha, and Jay K. Kochi*
Strong (nonbonded) p-to-p interactions were first recognized
by the appearance of characteristic near-IR absorption bands
and doubled hyperfine lines in the electronic and ESR spectra
of various intermolecular dimer cations formed upon electron
+
ejection from stacked aromatic donors,as in I!IC + eÀ.[1,2]
Equivalent spectral changes are also observed in the corre-
sponding intramolecular assembly derived from tethering the
same aromatic dyad,as in II (Do = electron donating func-
tional group).[3,4] Indeed,such a spectral (NIR,ESR)
equivalence relates to the common p-electronic transition
from the filled HOMO to the singly occupied LUMO that are
[*] Dr. D. Sun, Dr. S. V. Rosokha, Prof. Dr. J. K. Kochi
Chemistry Department
University of Houston
Houston, TX 77204 (USA)
Fax: (+1)713-743-2709
E-mail: jkochi@uh.edu
[**] We thank I. S. Neretin for computational assistance and the R. A.
Welch Foundation for financial support.
Supporting information for this article (synthesis, characterization,
and X-ray data for the hexaarylbenzene donors 1–3, spectral (Vis-
NIR) characterization of their radical cations, and Mulliken–Hush
(six-state) analysis of the radical cation of 1) is available on the
WWW under http://www.angewandte.org or from the author.
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more or less equally disposed in I and II. This HOMO–
latter case,the blue polycrystalline solid is isolated in
+
LUMO or charge-transfer (intervalence) transition is well
accommodated by Mulliken–Hush theory to provide the
important connection between the experimental NIR spec-
trum and the electronic coupling energy (HDA) that binds such
cofacial moieties.[5]
excellent yields and quantitatively analyzed as 1C BÀ.
The second unusual redox property of 1 is detected in the
electronic spectrum of 1C+,generated by either method. This
spectrum shows the highly distinctive low-energy absorption
(Figure 2) that stretches continuously from the visible (red)
In broader context,the further applicability of Mulliken–
Hush theory to extended aromatic (stacking) arrays that are
inherent to molecular conductors and wires for organic
materials science,[6] depends first on the stepwise extension
of the dyad structure II to the closed-loop hexad structure
III,[7] in which extensive p-electron delocalization would
result in complete toroidal conjugation IV in the limit of
Figure 2. Electronic spectrum of hexadonor radical cation 1C+ in com-
parison with its bidonor 2C+ and monodonor 3C+ models.
strong interactions among the six (Do-phenyl) centers.[8] To
this end,we now report the synthesis and structural character-
ization of the hexadonor 1 (see the Supporting Information
for structure) which is an analogue of III containing six
electron-rich anilinyl groups with the six donor groups Do
being N,N-diethylamino or N-methyl-N-ethylamino. Com-
pound 1 has highly unusual redox properties in two important
ways: First,the linear-sweep voltammogram of the hexaani-
linylbenzene 1 (Figure 1A) shows a reversible one-electron-
region all the way to the infrared region beyond 4000 nm
(with lmax = 2570 nm, e = 3300 mÀ1 cmÀ1). By comparison,the
+
bidonor analogue 2C merely exhibits a single near-Gaussian[9]
band in the NIR region with lmax = 2300 nm (e =
+
2800mÀ1 cmÀ1); and the monodonor 3C is completely trans-
parent in the region beyond 1000 nm. Indeed,such character-
istic low-energy optical behaviors in 1C+ and 2C+ are diagnostic
of intervalence transitions as observed in both organic and
inorganic mixed-valence systems.[3,5,10]
To account for the spectral and electrochemical properties
+
of the radical cation 1C ,we extend the Mulliken–Hush (two-
state) model to the redox system with six equivalent (anilinyl)
centers. The diabatic (non-interacting) states yaÀyf with the
hole localized on different centers in the hexagonal array are
þ
presented pictorially in Scheme 1 (hole = ꢀ) and the adia-
+
batic wave functions (Y1ÀY6) of 1C are given in Equation (1).
Yi ¼ ai ya þ bi ya þ ci yc þ di yd þ ei ye þ fi yf
ð1Þ
Figure 1. CV (dotted line) and Osteryoung square-wave voltamme-
try (OSWV; solid line) in V versus SCE of A) hexadonor 1, B) bidonor
2, and C) monodonor 3.
Scheme 1.
oxidation wave at E1/2 = 0.51 V (versus saturated calomel
electrode (SCE)) which is strongly (negative) shifted relative
to those of the bis and monoanilinyl analogues 2 and 3
(Figure 1B and C). Indeed,the voltammetry of 1 shows four
separate anodic waves at 0.51,0.64,0.74,and 0.86 V; and
controlled-potential coulommetry reveals each of the first
three waves to correspond to reversible one-electron changes
and the fourth a composite three-electron process.
To evaluate the adiabatic states,we follow Lambert–
Noll[7b] and take into account only equivalent interactions
(HDA) between neighboring groups (neglecting other inter-
actions),and set the diabatic state ya with relaxed nuclear
geometry of all six centers as the zero point of reaction
coordinate (i.e., Haa = 0 at X = 0). As such,the energy of the
other diabatic states at the same point on the nuclear reaction
coordinate (Franck–Condon excitation) are equal to the
reorganization energy l; and the secular determinant is
expressed as Equation (2).
The wide separation of the potentials of the redox events
+
in 1 allow the radical cation (1C ) to be quantitatively prepared
either by controlled-potential anodic oxidation or by chem-
ical oxidation with dodecamethylcarboranyl (BC).[3b] In the
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Chemie
Mulliken–Hush (six-state) model agree with those obtained
by deconvolution of the experimental (intervalence transfer)
spectrum.
In conclusion,the compounds 1–3,which are closely
related to III,have been prepared and studied. The Mulliken–
Hush (MH) quantification of l and HDA confirms for the first
time the substantial stabilization allowed in the one-electron
oxidized hexadonor system III since the resonance energy of
DGr = À2H2DA/l ꢁ À3.5 kcalmolÀ1 shows the extensive intra-
The resulting energy levels of the adiabatic states
(truncated to the second order) and their pictorial represen-
tations are shown in Scheme 2 (exact eigenvalues and
+
annular (electron) delocalization in 1C relative to the
localized 3C+. Furthermore,the MH treatment of the exper-
imental spectral behavior correctly predicts the potential shift
of DEox = 0.21 V (for the first anodic wave of 1 relative to 3)
which is close to the experimental value of DEox = 0.23 V
when the entropy contribution is taken into account. Finally,
+
the attractive interaction in 1C with HDA = 5.3 kcalmolÀ1 is
almost sufficient to offset the reorganization penalty of l/2 =
8.0 kcalmolÀ1 and predicts a low barrier for hole (electron)
hopping between six (contiguous) anilinyl sites (Scheme 1).
Indeed,the value of DG° = 0.5 kcalmolÀ1 evaluated from HDA
and l[11] is at the threshold of the thermal barrier,and implies
Scheme 2.
coefficients aiÀfi in terms of HDA and l are given in the
+
Supporting Information). In contrast to bidonor 2C with one
+
intervalence transition (i.e., nIV = l),the absorption band of
that 1C has borderline Robin–Day Class II/III behavior.[9,10]
+
1C contains five optical transitions with energies determined
As such,we hope that our search for other redox systems will
reveal complete toroidal delocalization (i.e.,Robin–Day
Class III behavior) as in IV—of practical interest for the
new design of molecular “motors.”
as differences between ground state Y1 and excited states Y2
to Y6. The intensities of the transitions are related to the
corresponding transition moments which can be expressed
(according to Mulliken–Hush theory[5]) as Equation (3),in
which i = 2 to 6,and ma to mf are moments of diabatic
states a to f within the arbitrarily chosen coordinate system.
Experimental Section
Solvents and chemicals were prepared and handled as described
earlier.[3] Hexaanilinylbenzene donors (1) were obtained by trimeri-
zation of bis(N,N-diethyl or methylethylanilinyl)acetylene with
dicobalt octacarbonyl catalyst.[7b,12a] Bidonor (2) and monodonor (3)
models were synthesized by Diels–Alder addition of tetraphenylcy-
clopentadienone to the appropriate anilinylacetylene and subsequent
decarbonylation.[12b] Electronic spectra were measured on Varian
Cary 5 spectrometer,IR spectra measured with a Nexus 470 FT-IR
(Thermo Nicolet),and structural studies were carried out with
Siemens SMART APEX diffractometer. Cyclic voltammetry (CV)
and Osteryoung square wave voltammetry (OSWV) were performed
Z
Z
moi
¼
Y1mYi ¼ ða1ya þ . . . þ f1yfÞmðaiya þ . . . þ fiyfÞ ꢁ
ð3Þ
ða1aima þ . . . þ f1fimfÞ
When the coupling element is small (HDA ꢁ 0),all optical
transitions (Y1!Yi, i = 2–6) are essentially degenerate (i.e.,
nIV ꢁ l),however significant electronic coupling between the
redox sites leads to broadening of the intervalence absorption
and splitting into individual components with the energy
[3]
on BAS100A Electrochemical Analyzer,as described previously.
For full experimental and analytical details see the Supporting
Information.
difference between the highest and lowest transition given by
pffiffi
n5Àn1 = 2HDA 3. Indeed,the broad intervalence NIR absorp-
+
Received: March 18,2005
Published online: July 12,2005
tion of 1C in Figure 2 (with several partially resolved
components) points to a significant value of HDA,and digital
deconvolution into Gaussian bands (see Supporting Informa-
Keywords: arenes · charge transfer · conjugation ·
Mulliken–Hush theory · radical ions
tion Figure S2) leads to l = 5400 ꢂ 600 cmÀ1 and HDA
=
.
1600 ꢂ 400 cmÀ1 (see Supporting Information for details).
The coefficients (a1 to f6) and transition moments are
calculated from l and HDA (see Supporting Information
Table S2),and the results in Table 1 demonstrate how the
energies and intensities of the optical transitions based on the
[1] a) B. Badger,B. Brocklehurst, Nature 1968, 219,263; b) B.
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Badger,B. Brocklehurst, Trans.
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+
Table 1: Energies[a] and relative intensities[b] of intervalence optical transitions in radical cation 1C .
c) I. C. Lewis,L. S. Singer, Chem.
Phys. 1965, 43,2712; d) O. W.
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Chem. Soc. 1966, 88,4514.
Y1!Y2
Y1!Y3
Y1!Y4
Y1!Y5
Y1!Y6
Theoretical (MH)[c]
Spectral[d]
3.7(0.30)
3.7(0.25)
4.8(0.50)
4.8(0.52)
7.5(0.07)
7.4(0.10)
8.6(0.12)
8.6(0.13)
10.2(ꢁ0)
10.1(0.01)
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[a] In 103 cmÀ1. [b] In parenthesis, normalized intensities. [c] MH=Mulliken–Hush model. Calculated
according to MH with l=5500 cmÀ1 and HDA =1900 cmÀ1. [d] From Gaussian deconvolution of the
intervalence absorption.
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Communications
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[8] a) Toroidal conjugation occurring through p-faces is to be
distinguished from the usual planar conjugation occurring at p-
edges of aromatic moieties; b) A reviewer has kindly pointed out
an earlier study[8c] of hexaiodobenzene dication as a stable s-
delocalized or (Hꢀckel) s-aromatic species. Such extensive
conjugation is essentially circular since it occurs among six iodo
atoms in a single plane. This contrasts with electron delocaliza-
tion among six anilinyl faces (each involving multiple p-centers),
which is thus enveloped by a toroid-shaped surface. As such,the
concept of toroidal conjugation also extends to enlarged systems
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polyacenes (in general),as well as to more flexible perarylated
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[11] a) Calculated as DG° = (lÀ2HDA)2/(4l) based on two-state
model.[11b] Preliminary results indicate that the six-state model
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