Very fast electron migrations within p-doped aromatic cofacial arrays leading to three-dimensional (toroidal) π-delocalization

Article abstract of DOI:10.1021/ja060393n

The charge-resonance phenomenon originally identified by Badger and Brocklehurst lies at the core of the basic understanding of electron movement and delocalization that is possible within p-doped aromatic (face-to-face) arrays. To this end, we now utiliz

Full text of DOI:10.1021/ja060393n

Published on Web 07/04/2006
Very Fast Electron Migrations within p-Doped Aromatic
Cofacial Arrays Leading to Three-Dimensional (Toroidal)
π-Delocalization
Sergiy V. Rosokha, Ivan S. Neretin, Duoli Sun, and Jay K. Kochi*
Contribution from the Department of Chemistry, UniVersity of Houston,
Houston, Texas 77204-5003
Received January 19, 2006; E-mail: jkochi@uh.edu
Abstract: The charge-resonance phenomenon originally identified by Badger and Brocklehurst lies at the
core of the basic understanding of electron movement and delocalization that is possible within p-doped
aromatic (face-to-face) arrays. To this end, we now utilize a series of different aryl-donor groups (Ar) around
a central platform to precisely evaluate the intramolecular electron movement among these tethered redox
centers. As such, the unique charge-resonance (intervalence) absorption bands observed upon the one-
electron oxidation or p-doping of various hexaarylbenzenoid arrays (Ar6C6) provide quantitative measures
of the reorganization energy (λ) and the electronic coupling element (Hab) that are required for the evaluation
of the activation barrier (∆GET*) for electron-transfer self-exchange according to Marcus-Hush theory. The
extensive search for viable redox centers is considerably aided by the application of a voltammetric criterion
that has led in this study to Ar ) N,N-dialkyl-p-anilinyl, in which exceptionally low barriers are shown to lie
in the range ∆GET* ) 0.3-0.7 kcal mol-1 for very fast electron hopping or peregrination around the hexagonal
circuit among six equivalent Ar sites. Therefore, at transition temperatures Tt > 0.5/R or roughly -20 °C,
the electron-transfer dynamics become essentially barrierless since the whizzing occurs beyond the
continuum of states and effectively achieves complete π-delocalization.
Chart 1
Introduction
The efficient (three-dimensional) delocalization of electrons
within extended organic arrays represents an emerging research
frontier for various novel applications of fullerenes, nanotubes,
and Mobius systems.1 Electron delocalizations in such aromatic
structures mostly occur in relatively thin surface layers, and the
development of new topologies that allow broad three-
dimensional delocalization constitutes a fascinating synthetic
challenge.2 Another promising but conceptually different ap-
proach to this problem derives from through-space electron
delocalization in open-shell (p-doped) systems consisting of
planar aromatic donors (ArH) and their cofacial cation-radical
counterparts (ArH•+), as schematically depicted in Chart 1 (for
ArH ) C6H6), where the blue arrows represent the odd-electron
(or positive charge) movement between the juxtaposed π-faces.3
so that strong coupling results in complete π-delocalization of
positive charge, and a weaker coupling leads to electron hopping
between the intermolecular aromatic sites.4 As such, π-delo-
calization has been experimentally observed in dimeric cation-
radicals of naphthalene, octamethylbiphenylene, phenalenyl,
etc.,5 whereas electron hopping has been reported for the dimeric
cation-radicals of dimethoxybenzenes, phenothiazines, and
tetrathiafulvalene,6 as well as in dimeric anion-radicals of
tetracyanoethylene, chloranil, and dinitrobenzene.7 From the
•+
Such an electron interchange within the cationic (ArH)2
complex is modulated by the electronic coupling element (Hab),
(4) The potential-energy surface for an electron-hopping process (via localized
adiabatic states) is characterized by the presence of two potential minima
with an activation barrier ∆GET* for the electron-transfer rate, whereas no
such barrier exists for π-delocalization. Experimentally, fast electron
hopping can sometimes (but not always) be distinguished by time-resolved
spectroscopy s as in the use of temperature-dependent ESR line-broadening
techniques with micro- to nanosecond resolution.
(5) (a) Kochi, J. K.; Rathore, R.; Le Magueres, P. J. Org. Chem. 2000, 65,
6826. (b) LeMagueres, P.; Lindeman, S. V.; Kochi, J. K. Org. Lett. 2000,
2, 3562. (c) Le Magueres, P.; Lindeman, S.; Kochi, J. K. J. Chem. Soc.,
Perkin Trans. 2 2001, 1180. (d) Small, D.; Zaitsev, V.; Jung, Y.; Rosokha,
S. V.; Head-Gordon, M.; Kochi, J. K. J. Am. Chem. Soc. 2004, 126, 13850.
(6) (a) Sun, D.; Rosokha, S. V.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem.
Soc. 2003, 125, 15950. (b) Sun, D.; Rosokha, S. V.; Kochi, J. K. J. Am.
Chem. Soc. 2004, 126, 1388. (c) Rosokha, S. V.; Kochi, J. K. Submitted
for publication.
(1) For a recent thematic issue on σ- and π-delocalization, see: Schleyer, P.
v. R., Guest Ed. Chem. ReV. 2005, 105, 3433.
(2) (a) Chen, Z.; King, R. B. Chem. ReV. 2005, 105, 3613. (b) Lu, X.; Chen,
Z. Chem. ReV. 2005, 105, 3643. (c) Rzepa, H. S. Chem. ReV. 2005, 105,
3697.
(3) (a) This represents the prototypical first (dyad) member of a wide variety
of p-doped aromatic arrays. For some specific examples of p-doped cofacial
dyads, see: (b) Meot-Ner, M.; Hamlet, P.; Hunter, E. P.; Field, F. H. J.
Am. Chem. Soc. 1978, 100, 5466. (c) Meot-Ner, M. J. Phys. Chem. 1980,
84, 2724. (d) Meot-Ner, M.; El-Shall, M. S. J. Am. Chem. Soc. 1986, 108,
4386. (e) Masnovi, J. M.; Kochi, J. K. J. Phys. Chem. 1987, 91, 1878. (f)
Rathore, R.; Abdelwahed, S. H.; Guzei, I. A. J. Am. Chem. Soc. 2003,
125, 8712. (g) For more general examples of p-doping, see: Cox P. A.
The Electronic Structure and Chemistry of Solids; Oxford University
Press: New York, 1987.
9
9394
J. AM. CHEM. SOC. 2006, 128, 9394-9407
10.1021/ja060393n CCC: $33.50 © 2006 American Chemical Society

Fast Electron Migration in p-Doped Aromatic Arrays
A R T I C L E S
standpoint of the classification of classical mixed-valence
complexes,8,9 π-delocalization and electron hopping are distin-
guished via the Robin-Day paradigm10 as Class III (delocalized)
versus Class II (localized), respectively, such that Class III
systems have Hab > λ/2 and Class II systems have Hab < λ/2
(where λ is defined as the Marcus reorganization energy).11
In solution, the intermolecular π-electron interchange can be
experimentally recognized via the distinctive appearance of near-
IR absorptions and doubled ESR spectra in dimeric (paramag-
netic) π-complexes of p-doped aromatic dyads.12,13 In the solid
state, analogous π-electron interchanges are observed in ex-
tended (p-doped) face-to-face arrays containing a large number
of π-stacked aromatic donor units as potential molecular “wires”
(Chart 2.) These are characterized via their distinctive X-ray
of extensive (multiple) interchange in which electron or hole
migration could be unambiguously demonstrated to take place
over more than the requisite minimum number of two aromatic
centers in Chart 1.
Basically, the experimental problem of quantitatiVely assess-
ing π-delocalization versus electron hopping between sites in
extended systems depends on the proper evaluation of the
discrete pairwise dyad as in Chart 1, since the theoretical
formulation by Marcus and Sutin11 has been shown to be
applicable to such electron-transfer (donor/acceptor) self-
exchange in Robin-Day Class II systems.6,7,17 To this end, we
recognize that a multiple array of aryl-donor groups all tethered
around a central platform corresponds to the discrete version
of stacked aromatic sites in Chart 2. As such, we examine how
hexaarylbenzene structures18-20 can serve as the prototypical
bridge between the discrete dyad (Chart 1) and infinite solid-
state (Chart 2) systems in at least two ways: First, when all six
aryl-donor groups (D) are equivalent, the electron hopping
between hexad sites describes a “closed” loop migration of
electronic charge around the central C6-core, as in I (Chart 3).
Chart 2
crystallography,14 and such novel electron-carriers constitute the
molecular basis for the construction of unusual organic conduct-
ing and magnetic materials.15 Since the Robin-Day classifica-
tion has been shown to be applicable to intermolecular (through-
space) as well as to intramolecular (through-bond) electron
exchange in p-doped aromatic dyads,6,7 let us now consider
discrete examples showing extensive electron interchange
involving multiple (yet a finite number of) redox centers that
would constitute the intermediate bridge between (a) the
pairwise interaction in a well-defined [1:1] cation-radical
π-complex, as in Chart 1, and (b) the polycentric interactions
in an essentially infinite p-doped solid-state stack, as in Chart
2. Indeed, until very recently,16 there were no extant examples
Chart 3
By the same reasoning, the replacement of a single electron
donor with an electron-acceptor group (A) interrupts the closed
circuit, and electronic migration will then constitute an “open”
representative of an abbreviated or finite pentad of cofacial
aromatic donor units, as in II.
In hexaphenylbenzene and derivatives, the nearly perpen-
dicular arrangement of the six planar substituents relative to
the central platform effectively eliminates any through-bridge
interactions between these tethered phenyl (aryl) groups.18b,21
(7) (a) Ganesan, V.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003,
125, 2559. (b) Rosokha, S. V.; Lu, J.-M.; Newton, M. D.; Kochi, J. K. J.
Am. Chem. Soc. 2005, 127, 7411. (c) Rosokha, S. V.; Newton, M. D.; Head-
Gordon, M.; Kochi, J. K. Chem. Phys. 2006, 324, 117.
(8) (a) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (b) Demadis, K. D.;
Hartshorn, C. M.; Meyer, T. J. Chem. ReV. 2001, 101, 2655. (c) Nishiumi,
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(9) (a) Lambert, C.; Noll, G. J. Am. Chem. Soc. 1999, 121, 8434. (b) Nelsen,
S. F. Chem. Eur. J. 2000, 6, 581.
(10) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(11) (a) Marcus, R. A. Discuss. Faraday Soc. 1960, 29, 21. (b) Marcus, R. A.
J. Phys. Chem. 1963, 67, 853. (c) Marcus, R. A. J. Chem. Phys. 1965, 43,
679. (d) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.
(e) Sutin, N. Prog. Inorg. Chem. 1983, 30, 441.
(12) (a) Badger, B.; Brocklehurst, B. Nature 1968, 219, 263. (b) Badger, B.;
Brocklehurst, B. Trans. Faraday Soc. 1969, 65, 2582; 1970, 66, 2939.
[These were the first to identify the NIR charge-resonance bands.] (c) Lewis,
I. C.; Singer, L. S. Chem. Phys. 1965, 43, 2712. (d) Howarth, O. W.;
Fraenkel, G. K. J. Am. Chem. Soc. 1966, 88, 4514.
(13) (a) Fritz, H. P.; Gebauer, H.; Friedrich, P.; Ecker, P.; Artes, R.; Schubert,
V. Z. Naturforsch. 1978, 336, 498. (b) Fritz, H. P.; Gebauer, H.; Friederich,
F.; Schubert, V. Angew. Chem. 1978, 90, 305. (c) Enkelmann, V.; Morra,
B. S.; Kro¨hnke, C.; Wegner, G. Chem. Phys. 1982, 66, 303. (d) Enkelmann,
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V.; Schmelz, D.; Mu¨llen, K. Chem. Eur. J. 2000, 6, 1834. (e) Chi, X.;
Itkis, M. E.; Reed, R. W.; Oakley, R. T.; Cordes, A. W.; Haddon, R. C. J.
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L.; Gutierrez, A.; Perpinan, M. F.; Rico, S.; Azcondo, M. T.; Bellito, C.
Inorg. Chem. 1999, 38, 4430.
(17) Strictly speaking, two-state theory requires diabatic states in which through-
space (aromatic) interchange will involve little π-orbital overlap. The extent
of the latter in Ar/Ar•+ dyads is roughly limited by the π-separation, and
previous studies showed that, at the van der Waals distance of rvdW ≈ 3.4
Å, the π-overlap can be neglected in the semiquantitative application of
the theoretical model.6,7 Moreover, Mulliken-Hush theory based on the
same two-state model leads to values of the electronic coupling element
(Hab) which are close to those calculated by ab initio molecular orbital
6c,7c,17b
analysis.
(b) See ref 6c and Huang, J.-S.; Kertesz, M. J. Chem.
Phys. 2005, 122, 234707.
(18) (a) Lambert, C.; No¨ll, G. Angew. Chem., Int. Ed. 1998, 37, 2107. (b)
Lambert, C.; No¨ll, G. Chem. Eur. J. 2002, 8, 3467. (c) Lambert, C. Angew.
Chem., Int. Ed. 2005, 44, 7337.
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Chem., Int. Ed. 1999, 38, 3483. (b) Bauer, R. E.; Enkelmann, V.; Wiesler,
U. M.; Berresheim, A. J.; Mu¨llen, K. Chem. Eur. J. 2002, 8, 3858. (c)
Kobayashi, K.; Sato, A.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc.
2003, 125, 3035.
(20) (a) Dijkstra, H. P.; Steenwinkel, P.; Grove, D. M.; Lutz, M.; Spek, A. L.;
van Koten, G. Angew. Chem., Int. Ed. 1999, 38, 2186. (b) Rathore, R.;
Burns, C. L.; Deselnicu, M. I. Org. Lett. 2001, 3, 2887. (c) Traber, B.;
Wolff, J. J.; Rominger, F.; Oeser, T.; Gleiter, R.; Goebel, M.; Wortmann,
R. Chem. Eur. J. 2004, 10, 1227. (d) Shen, X.; Ho, D. M.; Pascal, R. A.,
Jr. J. Am. Chem. Soc. 2004, 126, 5798. (e) Rathore, R.; Burns, C. L.;
Abdelwahed, S. A. 2004, 6, 1689. (f) Bottari, G.; Torres, T. Chem. Comm.
2004, 2668. (g) Ito, S.; Ando, M.; Nomura, A.; Morita, N.; Kabuto, C.;
Mukai, H.; Ohta, K.; Kawakami, J.; Yoshizawa, A.; Tajiri, A. J. Org. Chem.
2005, 70, 3939.
(15) (a) Williams, J. M. Organic Superconductors: Synthesis, Structure,
Properties and Theory; Prentice Hall: Englewood Cliffs, NJ, 1992. (b)
Farges, J.-P., Ed. Organic Conductors: Fundamentals and Applications;
Marcel Dekker: New York, 1994. (c) Miller, J. S., Drillon, M., Eds.
Magnetism: Molecules to Materials; Wiley-VCH: Weinheim, 2001. (d)
See also: Batail. P., Guest Ed. Chem. ReV. 2004, 104, 4887 (thematic issue).
(16) For a preliminary (in part) report, see: Sun, D.; Rosokha, S. V.; Kochi, J.
K. Angew. Chem. 2005, 44, 5133.
(21) Compare: Rosokha, S. V.; Sun, D.; Kochi, J. K. J. Phys. Chem. A 2002,
106, 2283.
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A R T I C L E S
Rosokha et al.
Chart 6
As a result, the optical transitions and electron transfers that
may characterize any p-doped hexaarylbenzene and derivatives
in Chart 3 will derive from through-space interactions that are
equivalent to those extant intermolecularly between neighboring
aromatic moieties in Charts 1 and 2.22 Most importantly, the
six aryl groups (with Ar ) D-C6H4 in I) are sufficient to make
this closed hexad array structurally and electronically akin to
open vertical stacks of six D-C6H5 donors, thereby eliminating
the boundary conditions and alleviating the computational task.
Accordingly, we first directed our attention to the synthesis of
a series of hexaphenylbenzene derivatives, 1a-d, in which the
same nitrogen-centered donor substituent occupies every para
position, as shown in Chart 4. In addition to these N,N-dialkyl-
amine-based hexanuclear donors in Chart 4 were prepared via
the Co2(CO)8-induced trimerization of bis(N,N-dialkylanilinyl)-
acetylene24,25 that was previously prepared by the coupling of
the appropriate iodoaniline with acetylene in the presence of
palladium catalyst.26 The heterocyclic analogues in Chart 5 were
synthesized according to modified literature procedures23 by the
reaction of hexafluorobenzene with the potassium salts of either
carbazole or pyrrole. The pentad donor 6 was prepared from
hexaanilinylbenzene 1d by monomethylation with methyl tri-
flate. The corresponding dinuclear dyad (5) and mononuclear
model (4) were synthesized via the Diels-Alder addition of
tetraphenylcyclopentadienone to the appropriate dianilinyl- or
phenylanilinylacetylene, followed by the elimination of CO.27
The details of the synthetic procedures and donor characteriza-
tions are presented in the Experimental Section.
Chart 4
B. Structural Characterization of Hexanuclear Donors.
Pale yellow crystals of hexaanilinyl donors 1c and 1d suitable
for the X-ray crystallographic analysis were isolated by slow
removal of dichloromethane from an equimolar mixture with
acetonitrile. X-ray structural analysis in Figure 1A,B shows the
and N,N-diarylanilinyl derivatives, we also examined the all-
heterocyclic analogues, hexakis(N-3,6-di-n-butoxylcarbazolyl)-
benzene (2) and hexakis(N-pyrrolyl)benzene (3),18b,26 shown in
Chart 5, to complete the series of hexad donors.
Chart 5
Figure 1. Molecular (ORTEP) structures of hexanuclear donors 1c (A),
1d (B), and 2 (C), with hydrogen atoms and solvent molecules, and butyl
substituents in 2, omitted for clarity.
six spoke-like anilinyl groups in a propeller conformation, tilting
72 ( 10° relative to the C6-platform in both hexanuclear donors.
All anilinyl moieties retain their benzenoid structure, with ring
(C-C) bond lengths of 1.39 ( 0.01 Å and mean planar
deviations of less than 0.01 Å. The interplanar dihedral angles
between adjoining donor planes are all ∼60°. The distances
between adjacent nitrogen centers are uniformly ∼7 Å, and the
distance between anilinyl centroids is 4.3 ( 0.3 Å.28 The central
benzenoid ring remains essentially planar and symmetrical, with
the aromatic C-C bond length of 1.405 ( 0.005 Å and mean
deviation from the plane of 0.007 Å. In a similar way, the X-ray
crystal structure of the carbazolyl-based donor 2 (Figure 1C)
shows the heteroaromatic planes to be arranged at an angle of
62° relative to the central benzene ring, with distances between
nitrogen centers of 2.83 ( 0.03 Å.
To verify the theoretical analysis (vide infra), we also tested
the electronic effects arising from decreasing numbers of
interacting donor groups and prepared the monoanilinyl parent
4 and the ortho-dianilinyl dyad 5, together with the open pentad
analogue 6 (Chart 6), in which N-methylation effectively creates
a single ammonium-acceptor center that effectively inhibits full
toroidal conjugation.
Results
I. Synthesis and Structural Characterization of Hexaaryl-
benzenoid Donors. A. Synthetic Methodologies. The aryl-
(22) (a) Although the dihedral angle between neighboring (cofacial) aromatic
groups in hexaarylbenzenes subtends a ∼60° angle, the difference between
orbital overlap with that in the cofacial orientation is insufficient to
significantly affect the intervalence band.6
(23) Biemans, H. A. M.; Zhang, C.; Smith, P.; Kooijman, H.; Smeets, W. J. J.;
Spek, A. L.; Meijer, E. W. J. Org. Chem. 1996, 61, 9012.
(24) (a) Berresheim, A.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99, 1747.
(b) Watson, M. D.; Fechtenkotter, A.; Mu¨llen, K. Chem. ReV. 2001, 101,
1267.
II. Voltammetric Characterization of p-Doped Hexaaryl-
benzenes. The p-doping of the hexaarylbenzenes depicted in
(27) Tugcu, N.; Park, S. K.; Moore, J. A.; Cramer, S. M. Ind. Eng. Chem. Res.
2002, 41, 6482.
(28) At this separation, partial solvent penetration at the anilinyl periphery could
moderate the electron-transfer process.
(25) Vollhardt, K. P. C. Acc. Chem. Res. 1977, 10, 1.
(26) (a) Pal, M.; Kandu, N. G. J. Chem. Soc., Perkin Trans. 1 1996, 5, 449. (b)
Fieser, L. F. Organic Experiment, 1st ed.; Heath: Boston, 1964; p 307.
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Fast Electron Migration in p-Doped Aromatic Arrays
A R T I C L E S
Figure 2. CV (dashed lines) and OSWV (solid lines) voltammograms of the mononuclear donor 4 (a), dinuclear donor 5 (b), pentanuclear donor 6 (c), and
hexanuclear donor 1c (d), showing progressive splittings and potential shifts of the anodic waves with increasing numbers of anilinyl redox centers.
Table 1. Oxidation Potentials of Polynuclear Donors
0.51 V was clearly separated from its subsequent oxidation at
E1/2 ) 0.64 V to the dication and thence to the trication at E1/2
E1
,
2
∆
E,
δE,
/
donor
V vs SCE
mVa
mVb
) 0.74 V; the fourth anodic wave corresponded to an overall
3e- process at E1/2 ) 0.86V (eq 1). Essentially the same CV
1a
1b
1c
1d
2
0.62 (1e-), 0.75 (1e-), 0.84 (4e-)
1.11 (6e-)
130
0
130
120
0
120c
d
-
-
-
-
-
3e
0.51 (1e-), 0.64 (1e-), 0.74 (1e-), 0.86 (3e-)
0.57 (1e-), 0.69 (1e-), 0.77 (1e-), 0.86 (3e-)
0.94 (2e-), 1.11 (1e-), 1.34 (1e-), 1.46 (1e-),
1.59 (1e-)
230
1e
1e
1e
1c y
z 1c•+ y
z 1c2+ y
z 1c•3+ y
z 1c6+
170c
(0.51 V)
(0.64 V)
(0.74 V)
(0.86 V)
d
(1)
-
d
behavior was found for donor 1d (Table 1). Furthermore, the
CV and OSWV behaviors of the hexapiperidinyl donor (1a)
were similar in shape to those of 1c and 1d, but incompletely
resolved (see Supporting Information, Figure S1), and their
computer simulation indicated that the first and second waves
corresponded to 1e- processes and the third wave to an overall
4e- process (see the relevant oxidation potentials in Table 1).
CV and OSWV measurements of the pentaanilinyl donor 6
revealed two reversible waves at 0.67 and 0.77 V and a broad
reversible (composite) wave at 0.87 V vs SCE (Figure 1C).
Computer simulation indicated an approximately 1:1:3 intensity
ratio, which suggested that these corresponded to successive
1e-/1e-/3e- processes.
In comparison to the voltammetric behavior of the dialkyl-
anilinyl donors 1a, 1c, and 1d, showing significant splittings
of their oxidation waves, the cyclic voltammogram of the bis-
(p-bromophenyl)amino derivative 1b revealed only a single
reversible anodic wave at E1/2 ) 1.11 V, corresponding to an
unresolved 6e- process (Figure S1B).31 This oxidation potential
coincided with E1/2 ) 1.11 V for the reversible 1e- oxidation
of tris(p-bromophenyl)amine to the corresponding cation-radical.
The hexacarbazole donor 2 showed four reversible oxidation
waves at E1/2 ) 0.94, 1.11, 1.34, and 1.46 V, in which the first
wave corresponded to an unresolved 2e- process and each of
the other three waves corresponded to 1e- processes (Figure
S1C). The fifth irreVersible wave for the final 1e- process, to
afford the hexacation, was observed in the OSW voltammogram
at E1/2 ) 1.59 V.32 Finally, the hexapyrrole donor 3 showed
only unresolved irreversible oxidation waves in its voltammo-
grams.
3
4
5
6
∼ 1.3 (irrev)
-
0
190
100
-
0.74 (1e-)
0
110
70
0.63 (1e-), 0.82 (1e-)
0.67 (1e-), 0.77 (1e-), 0.87 (3e-)
a Potential difference between the first and second anodic waves.
b Difference between the potential of the first oxidation wave and that of
the mononuclear donor. c Relative to N-Me,Et-substituted mononuclear
donor. d Mononuclear analogue unavailable.
Charts 4-6 was first examined by transient electrochemical
techniques since their distinctive anodic behaviors were strongly
dependent on the number as well as the type of redox center.
For example, the initial positive-scan cyclic voltammogram (CV)
of parent donor 4, containing only a single anilinyl redox center,
showed a reversible one-electron (1e-) oxidation wave at 0.74
V vs SCE (Figure 2a), which is essentially that of N,N-
diethyltoluidine itself, with E1/2 ) 0.76 V. By comparison, the
dinuclear ortho-dianilinyl dyad 5 showed reversible anodic
waves at E1/2 ) 0.63 and 0.82 V (Figure 2b), with each wave
corresponding to a 1e- process. Such a pair of oxidation waves
is well known for different types of mixed-valence systems,8,9,29
and the magnitude of the splitting (∆E, as listed in Table 1,
second column) is directly related to the degree of electronic
interaction between the oxidized and neutral donor centers in
Chart 1.30
The pentaanilinyl and hexaanilinyl donors 6 and 1c,d showed
further splittings of the oxidation waves. Thus, the cyclic
voltammogram of 1c revealed four reversible anodic waves at
0.51, 0.64, 0.74, and 0.86 V; controlled-potential coulometry
(see Experimental Section) indicated the first, second, and third
waves to correspond to 1e- processes and the fourth wave to
an unresolved 3e- process (Table 1). The peak potential and
relative (peak-current) intensity of the oxidation waves for this
polynuclear donor were confirmed by Osteryoung square-wave
voltammetry (OSWV), as shown in Figure 2d. As such, the
Most importantly, the first anodic wave of each of the five
N,N-dialkyl-substituted anilinyl donors in Table 1, that cor-
responded to the production of the p-doped cation-radicals 1a•+,
1c•+, 1d•+, 5•+, and 6•+, all occurred at potentials significantly
negative-shifted by an amount δE relative to that of the N,N-
initial production of the p-doped cation-radical 1c•+ at E1/2
)
(29) (a) Lahlil, K.; Moradpour, A.; Bowlas, C.; Menou, F.; Cassoux, P.;
Bonvoisin, J.; Launay, J.-P.; Dive, G.; Dehareng, D. J. Am. Chem. Soc.
1995, 117, 9995. (b) Nelsen, S. F.; Ismagilov, R. F.; Powell, D. R. J. Am.
Chem. Soc. 1997, 119, 10213. (c) Lindeman, S. V.; Rosokha, S. V.; Sun,
D.-L.; Kochi, J. K. J. Am. Chem. Soc. 2002, 124, 843. (d) Risko, C.; Barlow,
S.; Coropceanu, V.; Halik, M.; Bre´das, J.-L.; Marder, S. R. Chem. Commun.
2003, 194.
(31) A similar single broad oxidation wave (simulated as six 1e- waves with
close potentials) was previously obtained for hexakis[4-(N,N-(dimethoxy-
phenyl)amino)phenyl]benzene.18b
(32) (a) The related 1,2,4,5-tetracarbazolyl-3,6-difluorobenzene shows three
oxidation waves at E1/2 ) 1.08 (2e-), 1.23 (1e-), and 1.38 V (1e-) vs SCE
(Figure S1D). (b) Note that the cyclic voltammogram of the hexakiscar-
bazolylbenzene donor 2 in Figure S1C was arbitrary truncated at 1.5 V
because the oxidation to the hexacation was chemically irreversible. By
comparison, square-wave voltammetry does not limit the potential sweep.
(30) Evans, C. E. B.; Naklicki, M. L.; Rezvani, A. R.; White, C. A.; Kondratiev,
V. V.; Crutchley, R. J. J. Am. Chem. Soc. 1998, 120, 13096.
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9397

A R T I C L E S
Rosokha et al.
dialkylaniline, as listed in the last column of Table 1. These
electrochemical studies also indicated that only the polyanilinyl
donors 1c, 1d, 4, 5, and 6 showed cleanly separated first
oxidation states; this unique voltammetric feature allowed us
to isolate and characterize the corresponding p-doped mono-
cations and identify the comparative effects of varying numbers
and types of redox centers on the p-doping properties, as follows.
III. Isolation of p-Doped Hexaarylbenzene Cation-Radical
Salts. Although the facile generation of the p-doped hexaaryl-
benzenes listed in Table 1 revealed their persistent character,
the various electrochemical methodologies were not readily
applicable to the isolation of the cation-radical salts owing to
the experimental difficulties in effectively separating the sup-
porting electrolytes. To circumvent the latter, we identified the
dodecamethylcarboranyl radical CB• as the viable uncharged
oxidant33 owing to its oxidation potential of Eox° ) 1.41 V vs
SCE, together with its blue color (λmax ) 905 nm, ꢀ ) 550
M-1 cm-1) in dichloromethane solutions. As such, the p-doped
cation-radicals 4•+ and 5•+ of the mono- and dinuclear donors
were readily prepared in the one-electron oxidation of the
corresponding neutral donors in dichloromethane at -28 °C with
a stoichiometric (1:1) amount of CB•.The paramagnetic salts
4•+ CB- and 5•+ CB- were separated in nearly quantitative yield
simply by addition of hexane, and their purity was found by
spectral titration to be >98% (see Experimental Section). In a
similar manner, the desirable p-doped hexanuclear cation-radical
salts 1c•+ CB- and 1d•+ CB- were also isolated as dark blue
powders in essentially quantitative yields with high (>96%)
purity, i.e.
Figure 3. Electronic spectra of the p-doped mononuclear 4•+ (a), dinuclear
5•+ (b), pentanuclear 6•+ (c), and hexanuclear 1c•+ (d) cation-radicals, with
D ) Et2N and A ) MeEt2N+ substituents.
Table 2. Electronic Spectral Data for p-Doped
Polyanilinylbenzenes
1
cation-radicals
λ
max, nm (ꢀmax, cm-1 M-
)
1a•+
1c•+
1d•+
4•+
657 (3700), 1000-3300 (br)
657 (3900), 2570 (br, 3300)
656 (3900), 2570 (br)
732 (3000)
596 (1600), 683 (1600), 2300 (2800)
650 (3500), 2400 (3000)
5•+
6•+
amounts of multiply charged cations inhibited the clean growth
of single crystals of 1c•+, 1d•+, and 5•+ suitable for X-ray
analysis, despite our various attempts at recrystallization.
IV. Spectroscopic Characterizations of p-Doped Hexaaryl-
benzene Cation-Radicals. A. Electronic and ESR Spectra
of Mono- and Dinuclear Cation-Radicals 4•+ and 5•+. The
electronic spectrum (Figure 3a) of the mononuclear cation-
radical 4•+ in dichloromethane solution consisted of only a single
absorption band at λmax ≈ 732 nm, which was readily assigned
to the local transition in the redox center.34 The temperature-
independent ESR spectrum of 4•+ was characterized by partially
resolved hyperfine splittings arising from one nitrogen and
hydrogens of the alkyl substituents and benzene ring (Figure
S2).
In strong contrast, the electronic spectrum of the dinuclear
cation-radical 5•+ showed twin absorption bands in the visible
region (λmax ) 596 and 683 nm, Table 2). More importantly,
this dinuclear cation-radical exhibited a new strong absorption
band (Figure 3b) in the near-IR region, with λmax ) 2300 nm,
which was absent in the mononuclear analogue 4•+ (Table 2).
As such, this NIR band was assigned to the intervalence
transition (characteristic of mixed-valence compounds)6,8,9,35
resulting from interaction of the oxidized and the neutral redox
centers. The ESR spectrum of the dinuclear cation-radical 5•+
consisted of a single (unresolved) broad band at all temperatures
and concentrations studied.
Reduction of the blue solid with zinc dust in dichloromethane
solution led to the quantitative recovery of more than 96% of
the neutral hexaarylbenzene donor (1c).
Moreover, treatment of the pentanuclear donor 6 (containing
a triflate counterion) with 1 equiv of carboranyl radical led to
a similar 1e- oxidation, to yield the p-doped dication-radical
6•+, the complex mixed (OTf-)(CB-) salt of which was not
isolated in solid form (vide infra).
It is important to emphasize that the significant difference
between the first and second oxidation potentials of the pertinent
donors s 1c, 1d, and 4-6 s excluded the disproportionation
of these p-doped cation-radicals into the corresponding neutral
donor and the dication as a complicating factor since the
disproportionation (equilibrium) constants based on ln KD
)
F∆E were predicted to be KD < 10-2 for ∆E > 100 mV in
(34) Shida, T.; Nosaka, Y.; Kato, T. J. Phys. Chem. 1978, 82, 695.
(35) (a) Coropceanu, V.; Malagoli, M.; Andre, J. M.; Bre´das, J. L. J. Am. Chem.
Soc. 2002, 124, 10519. (b) Barlow, S.; Risko, C.; Chung, S.-J.; Tucker, N.
M.; Coropceanu, V.; Jones, S. C.; Levi, Z.; Bredas, J.-L.; Marder, S. R. J.
Am. Chem. Soc. 2005, 127, 16900.
Table 1. Nonetheless, the unavoidable admixture of small
(33) For the preparation and characterization of CB•, see: King, B. T.; Noll, B.
C.; McKinley, A. J.; Michl, J. J. Am. Chem. Soc. 1996, 118, 10902.
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Fast Electron Migration in p-Doped Aromatic Arrays
A R T I C L E S
5). Further incremental additions of oxidant were accompanied
by the rise in intensity of these absorptions, but their shapes
(hereinafter referred to as spectrum A) remained essentially
unchanged until the concentration of CB• became equal to the
concentration of the donor. As more oxidant was added (from
1:1 to 2:1 equiv), the shape of the spectrum changed, and the
intensity of another absorption at 1530 nm grew significantly,
whereas the absorption at 2570 nm decreased with the con-
comitant appearance of an isosbestic point at 2300 nm. The
intensity of the 657-nm band decreased, and new bands appeared
at 545 and 450 nm.36 As a result, a new intense absorption band
(spectrum B) with narrow bandwidth was apparent at λmax
)
1530 nm and optimized at the oxidant/donor ratio of 2:1. Further
increases in oxidant concentration led to an intensity decrease
since the multiply charged cations (3+ to 6+) did not show
clear absorption in the NIR region (see Figure S3 in the
Supporting Information).
Figure 4. Equilibrium concentrations of the different oxidation states
formed upon the incremental addition of CB• to a 1 mM solution of donor
1c.
Quantitative spectral comparison at different oxidant/donor
ratios with the corresponding concentrations of the different
oxidation states in Figure 5 indicated that spectra A and B were
directly related to the monocation (1d•+) and the dication (1d2+),
respectively. In particular, the experimental spectra measured
in the titration window between the 0:1 and 2:1 ratios of oxidant
to donor were well simulated as the simple superposition of
the individual spectra of monocation and dication with each
multiplied by its relative concentration taken from Figure 4.36
This procedure allowed the unambiguous assignment of the
spectrum A (recorded at 1:1 oxidant/donor ratio in Figure 5,
bold curve) to 1d•+. The highly distinctive (and unprecedented)
electronic transitions extant in this monocation are underscored
by the exceptionally broad low-energy intervalence envelope
(containing partially resolved components) that stretches con-
tinuously from the visible (red) region all the way down to the
infrared region beyond 4000 nm (with a nominal maximum at
λmax ) 2570 nm, ꢀ ) 3300 M-1 cm-1) s certainly by
comparison to the simple near-Gaussian band of the dyad
analogue 5•+ in Figure 3b.37 Moreover, the oxidative titration
of the hexanuclear donor 1c also led to essentially the same
spectral changes, and the complete (UV-NIR) spectrum of the
p-doped monocation 1c•+ is shown in Figure 3d.38 Most
importantly, the dissolution of the dark blue powder isolated
as the 1c•+ CB- salt in eq 2 produced the same electronic
spectrum as that obtained by the spectrochemical titration at
the 1:1 oxidant/donor ratio. The ESR spectrum of 1c•+ showed
only one broad, unresolved peak at g ) 2.0026 in the
temperature range from +20 to -88 °C.
Figure 5. Typical spectral changes upon the addition (in 0.2 M increments)
of oxidant to a 1 mM solution of hexanuclear donor 1d, showing the rise
of spectrum A (black lines) in 0:1 to 1:1 oxidant-to-donor ratios and its
transformation to spectrum B (gray lines) with further additions of oxidant.
B. Electronic and ESR Spectra of p-Doped Hexaarylben-
zene Cation-Radicals. Owing to the multiple oxidation states
that are readily available to the hexaarylbenzene donors listed
in Table 1, it was necessary to carefully deconvolute the spectral
changes accompanying p-doping into the component spectra of
the mono-, di-, tri-, etc. cationic species. For example, as a result
of the presence of six redox centers, the oxidation of hexanuclear
donor 1c led to a complex set of equilibria among seven
oxidation states (from 0 to 6+), the relative concentrations of
which were established by the differences in the redox potentials
according to the Nernstian relationship. As such, the values of
the oxidation potentials in Table 1 allowed us to calculate the
equilibrium concentrations of the different oxidation states when
CB• was incrementally added to the donor; the quantitative
computer simulation (see Experimental Section for details) of
the redox equilibria leading to typical equilibrium dependences
of the different oxidation states is presented in Figure 4.
To quantitatively ascertain the spectral properties of the
p-doped hexaarylbenzene (1c•+ and higher oxidation states), we
first carried out the spectrochemical titration by the stepwise
addition of the strong (uncharged) oxidant CB• to a solution of
the hexaanilinylbenzene in dichloromethane under conditions
in which the 1 mM solution of neutral donor 1d showed no
absorption in the vis-NIR region (Figure 5). First, the addition
of a small amount (0.2 mM) of CB• immediately led to the
appearance of a new sharp band at λmax ) 657 nm, together
with a broad absorption in the 1000-3000 nm range (Figure
C. Spectral Characterization of the p-Doped Pentanuclear
Dication-Radical 6•+. The calculated distributions of the various
(36) As the oxidant/donor ratio approached 2:1, the spectra deviated from this
isosbestic point due to intrusion of the trication 1c3+ (Figure 5); thus, the
spectra could not be reliably simulated as the linear superposition of the
mono- and dication spectra.
(37) The abrupt drop (cutoff) of the low-energy half of the intervalence
absorption of 5•+ (under conditions in which the high-energy half is
Gaussian, as shown in Figure S10) also indicates that the value of Hab is
significant as compared to λ/2.9 Such a mixed-valence cation-radical at
the Class II/III border has been appropriately described by Nelsen as “almost
delocalized”.9b
(38) (a) Addition of an excess of the neutral donor 1c to the solution containing
a 1:1 donor/oxidant ratio did not materially change the electronic spectrum
in the vis-NIR range, and this supports its assignment to the cation-radical
1c•+. (b) To compensate the molecular vibration components, the solution
of the neutral donor at the same concentration was used as the reference
for baseline measurements in the region λ ) 3000-4000 nm (see Figure
S9 in the Supporting Information).
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9399

A R T I C L E S
Rosokha et al.
oxidation states in the solution of the pentanuclear donor 6 upon
successive additions of the carborane radical in the 0:2 oxidant-
to-donor ratios were similar to those of the hexaanilinylbenzenes
1c and 1d (since the differences between the first, second, and
third oxidation waves were roughly the same in both systems).
In fact, the addition of small amounts of the carborane radical
to the solution of 6 led to the appearance of the band at 650
nm, corresponding to the local absorption of the pentaanilinyl
cation-radical, together with its broad intervalence band with
maximum around 2400 nm in the NIR region. The intensities
of both of these bands increased more or less linearly until the
oxidant/donor ratio attained 1:1. Further additions of carborane
radical led to the decrease in the intensity of the low-energy
half of the NIR band, an increase of the absorption around 1050
nm, and a shift of the absorption band at 650 nm. As a result,
at the 2:1 oxidant/donor ratio, quite a different spectrum was
observed, with maxima at 558 and 1050 nm (Figure S4). As
such, the NIR spectrum at oxidant/donor ratios less than or equal
to 1:1 can be readily assigned to that of the cation-radical, as
shown in Figure 3c alongside the cation-radical spectra of other
aniline-substituted benzenes. ESR measurements revealed that
the cation-radical 6•+ was characterized by a broad, unresolved
signal similar to that of either 1c•+ or 1d•+.
those included in Chart 1 are identified experimentally by their
unique optical transitions (usually occurring in the near-IR
region) that Hush41 characterized as the intervalence absorption
bands (νIV) on the basis of the earlier Mulliken charge-transfer
formulation for organic donor/acceptor systems.42 According
to the Mulliken-Hush two-state analysis,43 the adiabatic ground-
and excited-state wave functions (Ψ1 and Ψ2) of the p-doped
dinuclear donor (5•+) are expressed via the diabatic initial and
final states (ψa and ψb) as Ψ1 ) aψa + bψb and Ψ2 ) bψb -
aψa. The solution of the corresponding secular 2 × 2 determi-
nant leads to the energies of adiabatic states (E1, E2) and optical
transition (νIV), expressed as44
2 1/2
Haa + Hbb ((Hbb - Haa)2 + 4Hab
)
E2,1
)
(
(4)
(5)
2
2
νΙV ) E2 - E1 ) ((Hbb - Haa)2 + 4Hab
)
2 1/2
where Haa and Hbb are the energies of the initial and final states
and Hab ) ∫ψaHψb is the electronic coupling element. For
“localized” Robin-Day Class II mixed-valence systems10
(which are characterized by relatively small values of the
electronic coupling with 0 < Hab < λ/2), the maximum of the
intervalence transition (νΙV) corresponds to the reorganization
energy, i.e., λ ) νΙV. The coupling element Hab is evaluated
experimentally via the Mulliken-Hush formalism:4545
Discussion
The characteristic but highly unusual redox behaviors of the
hexanuclear donors in Figure 2 lead to the distinctive hexa-
nuclear cation-radicals 1c•+ and 1d•+, showing unique electronic
absorption bands stretching continuously from the visible (red)
to well beyond 4000 nm in the infrared spectral region, and
clearly consisting of multiple (partially) resolved components.
Such a complex intervalence transition in the p-doped hexa-
nuclear donors 1c•+ and 1d•+ (Figure 3d) strongly contrasts with
the more usual intervalence band shown by a dinuclear analogue
such as 5•+, in which the electronic transition (see Figure 3b)
relates to a single Gaussian envelope truncated at the low-energy
edge (Figure S10) in a manner that was previously characterized
for Robin-Day Class II mixed-valence systems at or near the
Class III border.9 As such, let us first consider whether the
p-doped dinuclear cation-radical 5•+ can be theoretically treated
as such a mixed-valence system and the electron movements
within the p-doped stacked array assessed in a quantitative
manner by the application of Marcus electron-transfer theory.6,11,39
The simplest aromatic array consisting of the p-doped dyad
depicted in Chart 1 is to be considered the self-exchange between
bridged Ar/Ar•+ redox centers; within the context of the later
Marcus-Sutin development,40 the rate of such an electron
movement is governed by the reorganization energy λ and the
coupling element Hab, so the activation energy for the dynamic
electron-transfer process is
0.0206(νΙV ꢀ(ν) dν)1/2
∫
MH
Hab
)
(6)
rDA
where ∫ꢀ(ν) dν is the integrated intensity (oscillator strength)
of the intervalence absorption band and rab is the separation
(Å) between the redox centers.46 Application of eq 6 to the
intervalence absorption of cation-radical 5•+ (with νΙV ) 4.35
× 103 cm-1 and ∫ꢀ(ν) dν ) 5.52 × 106), together with rDA
)
7.0 Å (taken as the distance between the nitrogen centers), leads
to HabMH ) 460 cm-1, which is much less than λ ) 4350 cm-1
,
in agreement with the assignment of 5•+ as a Robin-Day Class
II mixed-valence monocation.
(41) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391.
(42) (a) Mulliken, R. S.; Person, W. B. Molecular Complexes; Wiley: New
York, 1969. (b) It is important to emphasize that Hush interValence
transitions are a special case of the more general Mulliken charge-transfer
transitions.10a,35 Furthermore, the charge-resonance transition also represents
a special case of charge-transfer transitions.3h All three phenomena relate
to intermolecular optical transitions associated with the electronic interaction
between the HOMO of an electron-rich donor and the LUMO of an electron-
poor acceptor.
(43) (a) Creutz, C.; Newton, M. D.; Sutin, N. J. Photochem. Photobiol. A: Chem.
1994, 82, 47. (b) Brunschwig, B. S.; Sutin, N. In Electron Transfer in
Chemistry, Vol. 2; Balzani, V., Ed.; Wiley: New York, 2001; p 583.
(44) For 5•+, the diabatic states represent hypothetical non-interacting states with
the hole localized on one redox center. The energies of the diabatic states
show quadratic dependence on the reaction coordinate with minima located
at X ) 0 and 1; i.e., Haa ) λX2 and Hbb ) λ(X - 1)2,40 see Figure 6.
(45) Significant deviation of the intervalence band of 5•+ from a Gaussian shape37
∆GET* ) (λ - 2Hab)2/4λ
(3)
MH
(Figure S10) prevented the use of the common expression based on Hab
) 0.0206(νIV ∆νꢀ)1/2/rDA
.
I. Evaluation of the Reorganization Energy and Electronic
Coupling Element for the p-Doped Dinuclear Dyad (5•+).
The critical values of Hab for strongly coupled dyads such as
(46) (a) The application of eq 6 often suffers from difficulties associated with
the proper choice of rab, especially for organic species with significant
charge delocalization.6,9,29 (b) Nelsen, S. F.; Newton, M. D. J. Phys. Chem.
A 2000, 104, 10023. (c) Coropceanu, V.; Gruhn, N. E.; Barlow, S.; Lambert,
C.; Durivage, J. C.; Bill, T. G.; Noell, G.; Marder, S. R.; Bredas, J.-L. J.
Am. Chem. Soc. 2004, 126, 2727. (d) In addition, as pointed out by a
reviewer, charge delocalization over the phenyl groups with center-to-center
separation of 4.3 Å results in significant shortening of the effective charge-
transfer distance rab and a corresponding increase of the calculated coupling
element.
(39) For recent reviews, see: (a) Newton M. D. Electron Transfer in Chemistry,
Vol. 1; Balzani, V., Ed.; Wiley-VCH: New York, 2001; p 3. (b) Newton,
M. D. Coord. Chem. ReV. 2003, 238-239, 167.
(40) (a) Brunschwig, B. S.; Sutin, N. Coord. Chem. ReV. 1999, 187, 233. (b)
Sutin, N. AdV. Chem. Phys. 1999, 106, 7.
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Fast Electron Migration in p-Doped Aromatic Arrays
A R T I C L E S
Importantly, the electronic interaction between redox sites
results in the stabilization of the Ar/Ar•+ dyad relative to the
isolated Ar and Ar•+ centers, and the free energy of stabilization
is related to the electronic coupling and the reorganization
energy as ∆Gr ) -Hab2/λ.30,40 Such a stabilization is the major
factor determining the splittings in the electrochemical oxidation
of 5 into two waves in Figure 1b, so ∆E ≈ 2∆Gr/F.40
Accordingly, the value of the electronic coupling element can
be estimated from the electrochemical data, and the splitting of
∆E ) 190 mV in Table 1 leads to Hab ) 1800 cm-1. This
CV
estimate confirms the assignment of 5•+ to Class II, but the
CV
magnitude of Hab is significantly higher than that estimated
from the spectral data (HabMH). To reconcile this discrepancy,
let us consider both methods more critically.
Figure 6. Energy diagrams for electron transfer within the p-doped
dinuclear donor with the diabatic reactant and product states of 5•+ shown
as dashed lines, and the adiabatic ground state and excited state shown as
red (solid) lines.
On one hand, the splitting of the oxidation waves of the Ar/
Ar•+ dyad is affected not only by resonance stabilization but
also by some additional entropy, electrostatic, and other
factors.30 Therefore, the CV-based value represents an upper
limit of the electronic coupling (i.e., Hab < HabCV). On the other
hand, recent studies show that the proper separation parameter
rab in eq 8, in many cases, is only 50-60% of the value
determined from the molecular geometry.46 Since any decrease
of rab increases HabMH, the value calculated with rab ) 7 Å
should be considered as a lower limit of electronic coupling
element (i.e., Hab > HabMH). We thus conclude that the spectral
and electrochemical data provide upper and lower limits for
the electronic coupling element, and the optical and thermal
electron transfer in the dinuclear cation-radical 5•+ can be
consistently described within the framework of the two-state
analysis using the reorganization energy, λ ≈ 4400 cm-1, and
nuclear reaction coordinate (Franck-Condon excitation) are
equal to the reorganization energy, λ.47b The solution of the
corresponding secular determinant (Chart S3) with the aid of
the Mathematica 4.1 program48 yields the exact eigenenergies
(Ei) as a complex function of Hab and λ (Chart S4). However,
for illustrative purposes, Table 3 shows semiquantitatively how
the eigenenergies vary as a function of Hab and λ by arbitrarily
truncating the exact solution at second order s likewise for the
coefficients of the adiabatic wave functions at third order.49 The
latter reflect the relative odd-electron population on each redox
site, and Chart 7 qualitatively depicts the relative electron
population on each redox site for the adiabatic wave functions
Ψ1, ..., Ψ6.
the electronic coupling, Hab ≈ 1000-1700 cm-1 37
.
The results in Table 3 thus predict that the intervalence
absorption of hexanuclear cation-radicals 1•+ potentially50
The extent to which the value of Hab approaches the
reorganization energy, the activation barrier for electron move-
ment, is significantly lowered so that, within the p-doped Ar/
Ar•+ dyad with λ ≈ 4400 cm-1 and Hab ≈ 1300 cm-1, the value
of ∆GET* according to eq 5 is only ∼0.5 kcal mol-1, relative
to the barrier of 3.1 kcal mol-1 in the absence of electronic
coupling, as illustrated in Figure 6.
(47) (a) That is, the geometry at the center “a” corresponds to that of the cationic
center and all others to the neutral anilinyl group. (b) Indeed, the terms
Haa, ..., Hff refer to the energies of diabatic (non-interacting) states ψa, ...,
ψf, which correspond to six isolated non-interacting redox sites with the
hole localized on the sites a-f, respectively, as illustrated in Chart S1 in
the Supporting Information. Furthermore, the diabatic state ψa, with relaxed
nuclear geometry of all six centers (i.e., relaxed geometry of cationic center
a and relaxed geometries of neutral centers b-f), was set as the zero point
II. Electron Delocalization in p-Doped Hexanuclear Do-
nors. Owing to the presence of six equivalent redox centers,
the adiabatic ground-state and excited-state wave functions (Ψ1,
..., Ψ6) of the p-doped hexanuclear donors (1c•+ and 1d•+) are
expressed via six diabatic initial and final states, ψa, ..., ψf:
of the reaction coordinate, i.e., at X ) 0, Haa ) EaC + EbN + EcN + EdN
+
EeN + EfN ) 0, where EaC represents the energy of the cationic center in
the relaxed geometry and EbN, ..., EfN are the energies of the neutral centers
in the relaxed geometries. At the same point of X ) 0, the nuclear
geometries of diabatic states ψb, ..., ψf are the same as the relaxed geometry
of state ψa, but the hole is transferred from site a to one of the sites b-f,
respectively. Thus, at X ) 0, the energy of the diabatic state ψb is Hbb
)
E
aN* + EbC* + EcN + EdN + EeN + EfN, where EaN* is the energy of the
Ψ1 ) a1ψa + b1ψb + c1ψc + d1ψd + e1ψe + f1ψf
(7a)
neutral center a with the nuclear geometry of the (relaxed) cation and EbC
*
is the energy of the cationic center b with the geometry of the neutral center.
Since all the redox centers are equivalent and they do not interact in the
diabatic states, the notation a-f can be omitted, and the expression can be
rewritten as Hbb ) EN* + EC* + 4EN. Addition and subtraction of the
term EC + EN leads to Hbb ) (EN* + EC* - EC - EN) + (EC + 5EN). The
term in the first set of parentheses represents the reorganization energy, λ,
and the term in the second set of parentheses is Haa ) 0. Therefore, it
follows that Hbb ) λ. Furthermore, if the relaxed nuclear geometry of the
diabatic state ψa is frozen and the hole is transferred to one of the (non-
l
Ψ6 ) a6ψa + b6ψb + c6ψc + d6ψd + e6ψe + f6ψf
(7f)
[For the visualization of the noninteracting (diabatic) states with
the hole localized on different redox centers, see Chart S1 in
the Supporting Information.] To simplify the secular 6 × 6
determinant, we explicitly take into account only interactions
(Hab) between neighboring anilinyl groups (neglecting the
expectedly weak interactions between meta and para substit-
uents).18b Furthermore, we assign the zero points of the reaction
coordinate and the energy to the diabatic state with the electron
on the redox center “a” and relaxed nuclear geometries for all
six centers47a (i.e., Haa ) 0 at X ) 0). Under such conditions,
the energies of all other diabatic states at the same point of the
interacting) sites c-f, the same procedure results in Hcc ) Hdd ) Hee
Hff ) λ.
)
(48) Mathematica, Version 4.1; Wolfram Research, Inc.: Champaign, IL 61820-
7237.
(49) In Table 3, the higher-order Hab/λ terms were neglected. However, the
numerical values of eigenenergies Ei and coefficients ai-fi listed in Table
S1 were obtained by the exact solution of the 6 × 6 determinant in Chart
S3 with Hab ) 1900 cm-1 and λ ) 5500 cm-1 via Mathematica 4.1 using
the N[Eigenvalues[...]] and N[Eigenvectors[...]] options;48 these Ei and ai-
fi values were used for the calculation of transition energies νiTHEO and
their relative intensities for the hexad 1c•+ in Table 4. In a similar way,
the exact solution of the 5 × 5 determinant in Chart S5 produces the values
of Ei and ai-ei listed in Table S2, which lead to the transition energies
νiTHEO and their relative intensities for the pentad 6•+ in Table 6.
(50) Some of the transitions could be symmetry-forbidden, vide infra.
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9401

A R T I C L E S
Rosokha et al.
Table 3. Energies (Ei) and Coefficients (ai-fi) of the Adiabatic States, Approximated in Terms of the Reorganization Energy λ and Coupling
49
Element Hab
coefficientsa
i
energy
a
b
c
d
e
f
1
2
3
4
5
6
-Hab2/λ
1
Hab/λ
-1
-1
-1
1
Hab2/λ2
-x3
-1
2Hab3/λ3
Hab2/λ2
Hab/λ
-1
1
-1
-1
-1
λ - Habx3 + Hab2/3λ
λ - Hab
2Hab/λ
0
2Hab/λ
0
2Hab/λ
-2
-x3
0
1
0
-2
1
λ + 4Hab2/3λ
λ + Hab
-2Hab/3λ
-1
-2Hab/3λ
1
x3
λ + Habx3 + Hab2/λ
-1
x3
a For simplicity of representation, the eigenvectors are not normalized.
Chart 7
Table 4. Calculated and Experimental Energiesa and Relative
Intensitiesb of Intervalence Optical Transitions in the Hexanuclear
Cation-Radical 1c•+
Ψ1 f Ψ2
3.7 (0.30) 4.8 (0.50) 7.5 (0.07) 8.6 (0.12) 10.1 (∼0)
specd 3.7 (0.25) 4.7 (0.52) 7.4 (0.10) 8.6 (0.13) 10.1 (0.01)
Ψ1 f Ψ3
Ψ1 f Ψ4
Ψ1 f Ψ5
Ψ1 f Ψ6
theoc
contains five optical transitions with energies νi ) E(i+1) - E1,
where i ) 1-5, in sharp contrast to the dinuclear cation-radical
5•+, which is characterized by a single electronic transition (νIV
) λ). Notably, all optical transitions are characterized by
essentially the same energy (ν ≈ λ) if the coupling element is
small (Hab , λ). However, any significant coupling between
redox sites leads to the noticeable separation of electronic
transitions (e.g., the energy difference between the highest and
lowest transitions is ν5 - ν1 ≈ 2Habx3). This results in the
considerable broadening of the intervalence spectrum and its
ultimate resolution into individual (electronic) components.
Indeed, the experimental electronic spectrum of the hexanuclear
cation-radical 1c•+ in Figure 3d shows quite a broad intervalence
NIR absorption band (containing several components) and
indicates that significant coupling occurs between multiple redox
centers.51
To experimentally evaluate λ and Hab from the intervalence
absorption, the coupling element is first estimated as Hab ) 1590
cm-1 from the difference between the low-energy maximum
(∼3500 cm-1) and the high-energy half (∼9000 cm-1), given
as ν5 - ν1 ≈ 2Habx3. Next, the reorganization energy is
estimated as λ ) 5500 cm-1 from the lowest-energy maximum,
given as ν1 ) E2 - E1 ≈ λ - Habx3 + 2Hab2/λ, as listed in
Table 3. The solution of the determinant in Chart S3 with these
values of Hab and λ leads to the transition energies νiTHEO, which
are used as the starting point for the numerical fitting (decon-
volution) of the intervalence absorption to five Gaussian
functions. The transition energies νiSPEC resulting from this
fitting are used for the first optimization of λ, Hab, and modified
νiTHEO. Subsequent iterations carried out by further (multiple)
fittings to the experimental absorption envelope lead to the final
optimized values of λ ) 5500 cm-1 and Hab ) 1900 cm-1 and
to the corresponding νiTHEO, listed in Table 4. The pictorial
results of the digital simulation (see Table S3 for numerical
parameters) are shown in Figure 7, which presents the five
Gaussian components (in green), together with the fit of the
composite spectrum (in red) to the experimental intervalence
a In 103 cm-1 b In parentheses, normalized intensities. c Calculated
.
according to the six-state analysis with λ ) 5500 cm-1 and Hab ) 1900
cm-1 30 d From the Gaussian deconvolution of the intervalence absorption
.
band (Table S3 in the Supporting Information).
envelope (in black). The energies of the Gaussian components
νiSPEC and their normalized oscillator strengths are listed in Table
4.
It is important to note that, although the spectral fittings into
five Gaussian bands provides the close correspondence (R2 >
0.999) of the experimental and simulated spectra of 1c•+ and
1d•+ within a relatively wide range of Hab ) 1600 ( 400 cm-1
and λ ) 5400 ( 600 cm-1 52
, we now apply two additional and
independent criteria, i.e., (i) the band intensities and (ii) the redox
potentials, to further validate the analysis, as follows.
(i) The intensities of the charge-transfer (intervalence) optical
transitions are directly related to the corresponding transition
moments, which can be expressed as µoi ) ∫Ψ1µΨi ) ∫(a1ψa
+ ... + f1ψf)µ(aiψa + ... + fiψf) ≈ (a1aiµa + ... + f1fiµf), where
i ) 2-6 and µa-µf are the moments of the diabatic states a-f
within the arbitrarily chosen coordinate system. By setting the
zero point of the coordinate system at the center of symmetry
of the hexanuclear donor (i.e., in the center of the benzene ring),
all vectors µa through µf can be expressed (based on symmetry
reasons) as the product of the scalar constant µ and the unity
vectors directed at 0, 60, 120, 180, 240, and 300° relative to
the x-axes (taken as the direction from the benzene center to
redox center “a”).53 Together with the coefficients (ai-fi)
calculated with λ ) 5500 cm-1 and Hab ) 1900 cm-1 (see Table
S2), this leads to the striking correspondence between the
(52) Although it may seem, from the incompletely resolved spectra in Figure
7, that the digital deconvolutions are somewhat “overdetermined”, we find
that the three independent lines of unequivocal evidence provide the
requisite (confirmative) information to more than offset any ambiguities.
First, the unmistakable coincidences of the deconvolution results in Tables
4 and 6 with all the calculated energies as well as spectral intensities provide
many (∼20) fixed points to strongly support the validity of multistate
analysis. Second, the calculated ground-state stabilizations based on these
deconvolutions quantitatively accord with the unique electrochemical data
presented in Table 5. Third, the multistate analysis carried out by digital
deconvolution yields results that are equally applicable to aromatic arrays
that are different, such as the dyad (5•+), the pentad (6•+), and the hexads
(1c•+ and 1d•+).
(53) The calculated values of the transition moments (and therefore band
intensities) do not depend on the choice of the coordinate system. For
example, setting the zero point of the coordinate system at redox center a,
the transition moment can be presented as µoi′ ≈ a1aiµa′ + b1biµb′ + ... +
f1fiµf′ ) a1ai(µa - µa) + b1bi(µb - µa) + ... + f1fi(µf - µa) ) (a1aiµa +
b1biµb + ... + f1fiµf) - (a1ai + b1bi + ... + f1fi)µa ) µoi (the sum in the
second set of parentheses is zero due to the orthogonality of states).
(51) Note that, generally speaking, the vibrational splittings of a single electronic
transition could also lead to distinct components in the intervalence
absorption spectrum. However, the intervalence spectra of cation-radicals
1c•+ and 1d•+ are much broader (and the separation between components
larger) than in the spectrum for dinuclear cation-radical 5•+, as previously
observed in the NIR intervalence spectra of other dinuclear organic mixed-
valence compounds showing vibrational splittings.22c,d Therefore, the
vibrational splitting alone is unlikely to be sufficient to explain such a
broadening of the spectra of 1c•+ and 1d•+
.
9
9402 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Fast Electron Migration in p-Doped Aromatic Arrays
A R T I C L E S
Figure 7. Spectral deconvolution of the NIR absorption bands for the hexanuclear cation-radicals 1c•+ (a) and 1d•+ (b) and the pentanuclear analogue 6•+
(c), as described in the text.
Table 5. Resonance Stabilization (∆Gr) of Cation-Radicals in
Comparison with the Shift (δE)b of the First Oxidation Potential of
Di- and Hexanuclear Donors Relative to That of Mononuclear
Model 4
Table 6. Calculated and Experimental Energiesa and Relative
Intensitiesb of Intervalence Optical Transitions in the Pentanuclear
Cation-Radical 6•+
Ψ1 f Ψ2
Ψ1 f Ψ3
Ψ1 f Ψ4
Ψ1 f Ψ5
cation-radical
∆
Gr, eVa
δ
E, Vb
theoc
specd
4.7 (0.40)
4.1 (0.39)
5.4 (0.31)
5.8 (0.28)
8.5 (0.23)
7.7 (0.26)
9.0 (0.06)
9.5 (0.07)
5•+
0.08
0.15
0.11
0.23
1c•+
a In 103 cm-1 b In parentheses, normalized intensities. c Calculated
.
a Calculated according to the two-state (5•+) or six-state (1c•+) analysis
with λ ) 5500 cm-1 and Hab ) 1900 cm-1 b See Table 1.
according to the five-state analysis with λ ) 5500 cm-1 and Hab ) 1900
.
.
cm-1 d From the Gaussian deconvolution of the intervalence absorption
band.
calculated transition moments and the relative (normalized)
intensities of each of the five transitions from the experimental
(deconvoluted) values listed in Table 4.
combination of five diabatic wave functions. [The corresponding
secular 5 × 5 determinant is presented in Chart S5 in the
Supporting Information.] In this case, the electronic interaction
of the methylated center with the neighboring anilinyl group
must be excluded, and therefore the energetics of the system
depends substantially on the placement of the hole. As such,
we chose the placement of the hole on the group opposite to
the methylated one (i.e., 0 should be in the center of the
determinant to produce the lowest-energy ground state). The
(ii) Voltammetric criteria provide further support for the
validity of our analysis. Thus, Table 5 presents the ground-
state resonance stabilization (∆Gr) of the cation-radicals of the
dinuclear and hexanuclear donors, 5•+, 1c•+, and 1d•+, relative
to the mononuclear cation-radical 4•+. The values in column 2
indicate that the resonance (ground-state) stabilization of
hexanuclear cation-radicals is twice that of the dinuclear
analogue (calculated with the same values of the reorganization
energy λ and the coupling element Hab).
solution of this determinant, using the same values of Hab
)
1900 and λ ) 5500 cm-1 as evaluated for the hexanuclear
cation-radical, results in five adiabatic energy levels, with the
corresponding coefficients describing the wave functions (Table
S2). The solution leads to four electronic transitions with the
energies and relative intensities presented in Table 6.
Notably, such a stabilization represents the major component
in the difference δE between the first oxidation potential of
polynuclear donors and that of the mononuclear counterpart in
column 3. (Note that δE > ∆Gr arises from the nonresonance
contributions, such as entropy, electrostatic interactions, etc.30,40
)
The main features of the theoretical prediction, i.e., the notable
blue shift and the narrowing of the spectrum relative to that of
the p-doped hexanuclear donor, agree with the experimental
observations. Importantly, the parameters resulting from the
digital deconvolution of the experimental spectrum (Figure 7c)
also agree reasonably well with the calculated spectrum in Table
6, and these together provide the further important validation
of the multistate analysis.
IV. Mechanism for Electron Migration in Various p-
Doped Aromatic (Face-to-Face) Arrays. The diagnostic
electronic spectra of the p-doped polynuclear donors presented
in Figure 3 reveal two striking characteristics of the distinctive
intervalence bands. First, the partially resolved near-IR bands
all exhibit clear spectral maxima that are essentially invariant,
with only slight (but regular) red shifts from the dinuclear 5•+,
to the pentanuclear 6•+, and thence to the hexanuclear 1c•+ and
1d•+. Second, the nominal bandwidths (at half-maximum) of
the same series of polynuclear cation-radicals show strong
variations of 1500, 4000, and 5500 cm-1, consistent with their
digital deconvolutions into one,37 four, and five Gaussian
Most importantly, the experimental (electrochemical) measure-
ments of the oxidation potentials leads to δE ) 110 mV for
the dinuclear donor 5, which is close to half that for the
hexanuclear donor 1c (230 mV). Thus, the observed doubling
of the potential shift accords with changes in ∆Gr and, indeed,
verifies the theoretical predictions of the multistate analysis.
III. Partial Electron Delocalization in the p-Doped Pen-
tanuclear Donor. To further test the validity of the multistate
analysis, let us also consider the spectral properties of the
p-doped pentanuclear donor. Thus, the qualitative spectral
comparison in Figure 3 shows that the spectrum of 6•+ is
intermediate between those of the dinuclear and the hexanuclear
counterparts, being much broader and more red-shifted relative
to the spectrum of 5•+, but somewhat narrower and more blue-
shifted relative to that of 1•+. To apply the same multistate
analysis, we take into account the fact that one of the six anilinyl
groups (i.e., the methylated center) in cation-radical 6•+ is
excluded from the electron/hole delocalization. As such, the
adiabatic wave functions must be now presented as the linear
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9403

A R T I C L E S
Rosokha et al.
Chart 8
components, respectively, in Figure 7. It is thus notable that
the multistate analysis of these three types of intevalence
absorption bands yields values of the reorganization energy and
electronic coupling element that are the same within experi-
mental error, with λ ) 5400 ( 600 cm-1 and Hab ) 1600 (
400 cm-1, irrespectiVe of the number of redox centers. As such,
we now address the importance of the valid (theoretical)
extension of the two-state analysis for the dyad 5•+ to the
corresponding five-state analysis of the pentad 6•+ in Table 6
and to the six-state analysis of the hexad 1c•+ in Table 4. The
simplest and most direct conclusion we can draw from the rather
invariant values of λ and Hab for all these p-doped polynuclear
donors is that they share a common mechanism for electron
migration that involves site-to-site electron hopping, the pro-
totypical example of which is depicted by the potential-energy
surface for the two-state dyad in Figure 6. As applied to the
p-doped hexanuclear donors 1c and 1d, the evaluation of the
potential energy surface for electron migration requires the
order to further lower the barrier and increase the electron-
transfer rate. If so, at the upper limit of Class II systems at or
close to the Class III border with Hab ) λ/2, the separate
concepts of electron hopping versus π-delocalization are merged,
and their distinction becomes obscure s an ambiguous situation
aptly described by Nelsen as “almost delocalized”.9b Indeed,
such a conundrum is common in chemistry, and there are
continuing polemics regarding the classical concepts of equi-
librium versus resonance, valence-bond versus molecular-orbital,
etc.55
extension of the single-point energy calculation (at X ) 047a
)
along the electron-transfer reaction coordinate. It should be
stressed, however, that the traditional parabolic dependence of
the diabatic (state) energy on the reaction coordinate is valid
here only in limited segments of this multidimensional space.
Therefore, as a first approximation, we consider electron transfer
between two neighboring sites, a and b, as determined primarily
by the parabolic dependence of the diabatic energies Haa and
The ultimate achievement of Class III and π-delocalization
can be visualized with a single ground state for the three types
of p-doped polynuclear aromatic arrays discussed in this study,
as shown in Chart 9. In Chart 9, the blue-shaded areas represent
Chart 9
Hbb on the nuclear coordinates of these two sites (as in the two-
state analysis). We then assign X ) 0 as described above, and
X ) 1 to the state with the hole on site b (with relaxed cationic
geometry of this site and relaxed neutral geometry of site a).
The electron hopping thus corresponds to movement from X )
0 to X ) 1, with the transition state located at X ) 0.5.
Importantly, all other sites remain in the fixed nuclear geometry,
and the energies of the other diabatic states are equal to λ at all
points along this reaction coordinate. Accordingly, we determine
the ground-state energy at different points from X ) 0 to X )
1 for the six-state system in which the secular determinants
three-dimensional envelopes within which complete electron or
charge delocalization occurs, i.e., (i) between both Ar redox
centers in the dyad•+, (ii) among five contiguous redox centers
in the open-pentad•+, and (iii) completely encompassing all
redox centers of the hexad•+. Since such through-space interac-
tions derive from juxtaposed face-to-face benzenoid rings,6,28
π-delocalization is best described by (i) a short toroidal segment
(dyad•+), (ii) an open toroid (open-pentad•+), or (iii) a full
(donut-shaped) toroid (closed-hexad•+), as in Chart 9.16 Indeed,
they all represent the various extents of toroidal delocalization/
conjugation that is consonant with three-dimensional (multi-
centered) interactions of cofacial aromatic moieties. The struc-
tural depiction of toroidal π-delocalization can also be applied
in quantitative measure to intermolecular cofacial stacks of
p-doped aromatic donors (as first presented in Chart 2 above)
when these are considered as a linear combination of the diabatic
reactant/product states. However, before proceeding much
further, we emphasize the caveat that the theoretical treatment
of electron transfer derived from the Mulliken-Hush analysis
is semiempirical, and our quantitative evaluation of λ and Hab
in this study may not fully withstand more rigorous computa-
tions based on ab initio quantum-mechanical methodologies.
include Haa ) λX 2 and Hbb ) λ(1 - X)2, together with Hcc
)
Hdd ) Hee ) Hff ) λ. The same procedure is applied to electron
movement from site b to site c, etc. As a result, such a six-state
analysis predicts the electron hopping along a hexagonal circuit,
the closed potential-energy surface of which consists of six
minima separated by the activation barriers54 ∆GET* as graphi-
cally depicted in the “roller coaster” topology shown in Chart
8.
V. Future Prospectives for Electron Interchange in p-
Doped Cofacial Aromatic Arrays. For the N,N-diethyl- and
methylethylanilinyl redox centers of principal focus in this study,
the relevant values of λ ) 5400 ( 600 cm-1 and Hab ) 1600
( 400 cm-1 lead to the (averaged) activation barrier of ∆GET
*
) 0.55 ( 0.25 kcal mol-1 for electron hopping from site to
site.54 To lower this barrier, we can imagine the synthesis of
other multinuclear aromatic cation-radicals to modulate the
relative electronic coupling and reorganization contributions in
(54) (a) Calculated with the six-state model as the difference between the
minimum and the maximum ground-state energies on the electron-transfer
reaction pathway from X ) 0 to X ) 1. (b) Thus, the multistate (temporal)
events can be considered as a sequence of two-site processes in which the
latter has been modified by the presence of four neighboring states. (c) It
is interesting to speculate that the rate of such an electron (hole) migration
in the manner of a “ring current” may be externally modulated by applied
magnetic and/or electrostatic fields.
(55) (a) Hoffmann, R.; Shaik, S.; Hiberty P. S. Acc. Chem. Res. 2003, 36, 750.
(b) Roberts, J. D. Acc. Chem. Res. 2004, 37, 417. (c) Streitwieser, A. Acc.
Chem. Res. 2004, 37, 419.
9
9404 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Fast Electron Migration in p-Doped Aromatic Arrays
A R T I C L E S
(CDCl3) δ 145.2, 141.0, 140.8, 140.7, 140.5, 140.2, 140.0, 132.1, 131.5,
131.4, 128.1, 126.4, 124.9, 124.7, 111.5, 44.1, 12.1. Anal. Calcd for
C46H39N: C, 91.24; H, 6.45. Found: C, 91.22; H, 6.48.
1,2-Bis(p-N,N-diethylanilinyl)tetraphenylbenzene (5) was synthe-
sized in a similar way: yield 25%; mp > 250 °C; 1H NMR (CDCl3) δ
7.80 (m, 20H), 6.63 (d, J ) 8.7 Hz, 4H), 6.27 (d, J ) 8.7 Hz, 4H),
3.15 (q, J ) 6.9 Hz, 8H), 0.97 (t, J ) 6.9 Hz, 12H); 13C NMR (CDCl3)
δ 145.1, 141.3, 141.0, 140.8, 140.3, 139.5, 132.3, 131.6, 131.5, 128.9,
126.4, 124.8, 124.6, 111.8, 111.4, 44.3, 12.1. Anal. Calcd for
C50H48N2: C, 88.76; H, 7.10. Found: C, 88.74; H, 7.15.
Nonetheless, we believe the near coincidences of the experi-
mental and theoretical results, as presented in Tables 4-6, all
point strongly to the essential correctness of our principal
conclusions regarding electron movement in various p-doped
aromatic (cofacial) arrays. As such, we will continue our design
of other new redox centers that will further facilitate electron
interchange and eventually lead to Robin-Day Class III
systems.
Experimental Section
Hexakis(p-N,N-diethylanilinyl)benzene (1c). A 2.4 g portion of
1,2-bis(p-N,N-diethylanilinyl)acetylene and 100 mg of dicobalt octac-
arbonyl were dissolved in 30 mL of dioxane, and the mixture was heated
at reflux for 2 days. Solvent was then removed, and the mixture was
purified by column chromatography to yield 1.32 g of 1c: yield 55%;
mp > 250 °C; 1H NMR (CDCl3) δ 6.62 (d, J ) 8.4 Hz, 12H), 6.28 (d,
J ) 8.4 Hz, 12H), 3.12 (q, J ) 6.9 Hz, 24H), 0.95 (t, J ) 6.9 Hz,
36H); 13C NMR (CDCl3) δ 144.7, 140.2, 132.6, 130.5, 112.3, 44.6,
12.1. Anal. Calcd for C66H84N6: C, 82.50; H, 8.75. Found: C, 82.34;
H, 8.67.
Materials. N,N-Diethylaniline, potassium iodide, potassium iodate,
phenylacetylene, acetylene, palladium chloride, triphenylphosphine,
cuprous iodine, dicobalt octacarbonyl, lead dioxide, and tetraphenyl-
cyclopentadienone (from Aldrich, Acros, or Alfa) were used as received.
N-Methyl-N-ethylaniline and N-piperidinylaniline were synthesized
according to the literature procedure,56 and 4-iodo-N,N-dialkylaniline
was prepared by iodination of N,N-dialkylaniline.26 The dodecameth-
ylcarborane radical was produced by oxidation of the cesium salt of
decamethylcarboranate with PbO2 in acetonitrile, followed by extraction
with hexane.33 Dichloromethane, toluene, diethylamine, dioxane, and
tetrahydrofuran were purified according to published procedures.57
Syntheses. 1,2-Bis(p-N,N-diethylanilinyl)acetylene. Acetylene was
bubbled into a stirred mixture of 4-iodo-N,N-diethylaniline (3.84 g, 14
mmol), PdCl2 (30 mg), PPh3 (90 mg), CuI (104 mg), and 60 mL of
diethylamine at 40 °C. After 3 h, GC analysis showed complete
consumption of 4-iodo-N,N-diethylaniline. The crude product, after
removal of solvent, was purified by column chromatography (silica
gel, hexane), and the light yellow crystalline 1,2-bis(p-N,N-diethyl-
Hexanuclear donors 1a, 1b, and 1d were obtained by the same
procedure, starting with the corresponding acetylene precursors.
Hexakis-1,2-bis(p-N-piperidinylanilinyl)benzene (1a): yield 50%; mp
1
> 250 °C; H NMR (CDCl3) δ 6.70 (d, J ) 8.4 Hz, 12H), 6.54 (d, J
) 6.9 Hz, 12H), 3.03 (t, J ) 5.1 Hz, 24H), 1.69 (m, 24H), 1.59 (m,
12H); 13C NMR (CDCl3) δ 149.3, 140.7, 133.8, 131.6, 114.6, 51.4,
26.1, 24.8. Anal. Calcd for C72H84N6: C, 83.72; H, 8.14. Found: C,
83.88; H, 8.14. Hexakis(p-N,N-di-p-bromophenylanilinyl)benzene
(1b): yield 15%; mp > 250 °C; 1H NMR (CDCl3) δ 7.27 (d, J ) 7.8
Hz, 24H), 6.77 (m, 48H); 13C NMR (CDCl3) δ 146.4, 144.6, 140.1
132.8, 132.5, 125.4, 123.6, 115.5. Hexakis(p-N-methyl-N-ethylanilinyl)-
benzene (1d): yield 58%; mp > 250 °C; 1H NMR (CDCl3) δ 6.62 (d,
J ) 8.4 Hz, 12H), 6.28 (d, J ) 8.4 Hz, 12H), 3.18 (q, J ) 6.9 Hz,
12H), 2.69 (s, 18H), 0.93 (t, J ) 6.9 Hz, 18H); 13C NMR (CDCl3) δ
146.2, 140.5, 132.7, 122.5, 112.2, 47.5, 37.7, 10.5. Anal. Calcd for
C66H84N6: C, 82.19; H, 8.225. Found: C, 82.22; H, 8.25.
Hexakis(N-3,6-di-n-butoxylcarbazolyl)benzene (2). 3,6-Di-n-bu-
toxylcarbazole (504 mg, 1.74 mmol) was added to 84 mg (1.75 mmol)
of NaH (50%) and stirred under argon at room temperature in 5 mL of
DMF. After 30 min, 53.9 mg (0.29 mmol) of hexafluorobenzene was
added, and the mixture was stirred for an additional 3 h. The mixture
was poured into 100 mL of water, and the white precipitate (290 mg)
was filtered and dried; its X-ray crystallographic analysis (Figure S5
in the Supporting Information) revealed that it is 1,2,4,5-(N-3,6-di-n-
butoxylcarbazolyl)-3,6-fluorobenzene. This precipitate was added to
the potassium salt of carbazole (prepared by the addition of 28 mg of
potassium metal to 223 mg of 3,6-di-n-butoxylcarbazole) in 5 mL of
HMPA, and the mixture was stirred at 90 °C overnight. After cooling,
the mixture was poured into 50 mL of water and extracted with
dichloromethane (3 × 30 mL). The organic extract was washed with
water (3 × 30 mL) and dried over MgSO4. The solvent was removed,
and the residue was recrystallized from CH2Cl2/MeOH to give 218 mg
of 2 as a white crystalline product (Figure 1C): 1H NMR (CDCl3) δ
6.88 (d, J ) 7.8 Hz, 12H), 6.72 (d, J ) 2.1 Hz, 12H), 6.17 (dd, J )
7.8 Hz, 2.1 Hz, 12H), 3.73 (t, J ) 6.6 Hz, 24H), 1.67 (m, J ) 7.2 Hz,
24H), 1.39 (m, J ) 7.2 Hz, 24H), 0.92 (t, J ) 7.2 Hz, 36H); 13C NMR
(CDCl3) δ 153.0, 135.8, 133.8, 124.0, 113.6, 111.3, 103.6, 68.4, 31.4,
19.2, 13.9.
1
anilinyl)acetylene was obtained in 94% yield: mp 156-157 °C; H
NMR (CDCl3) δ 7.35 (d, J ) 8.7 Hz, 4H), 6.60 (d, J ) 8.7 Hz, 4H),
3.36 (q, J ) 6.9 Hz, 8H), 1.16 (t, J ) 6.9 Hz, 12H); 13C NMR (CDCl3)
δ 146.9, 132.5, 111.2, 109.9, 87.8, 44.2, 12.5; MS (m/e) 321 (M+ + 1,
40), 320 (M+, 100), 305 (71), 281 (10), 276 (18), 261 (40). Anal. Calcd
for C22H28N2: C, 82.50; H, 8.75. Found: C, 82.36; H, 8.96.
1,2-Bis(p-N-methyl-N-ethylanilinyl)acetylene, 1-(p-N,N-diethyla-
nilinyl)-2-phenylacetylene, and 1,2-bis(p-N-piperidinyl)acetylene
were synthesized in a similar way. 1,2-Bis(p-N-methyl-N-ethylanilinyl)-
acetylene: mp 151-152 °C; 1H NMR (CDCl3) δ 7.35 (d, J ) 8.7 Hz,
4H), 6.63 (d, J ) 8.7 Hz, 4H), 3.41 (q, J ) 6.9 Hz, 4H), 2.91 (s, 6H),
1.13 (t, J ) 6.9 Hz, 6H); 13C NMR (CDCl3) δ 146.9, 132.6, 111.9,
110.9, 88.1, 46.8, 37.6, 11.4; MS (m/e) 293 (M+ + 1, 35), 292 (M+,
100), 305 (71), 281 (10), 276 (18), 261 (40). Anal. Calcd for
C22H28N2: C, 82.19; H, 8.22. Found: C, 82.10; H, 8.25. 1,2-Bis(p-N-
1
piperidinyl)acetylene: mp 157-158 °C; H NMR (CDCl3) δ 7.42 (d,
J ) 8.7 Hz, 4H), 6.95 (d, J ) 8.4 Hz, 4H), 3.31 (t, J ) 5.1 Hz, 8H),
1.72 (m, 8H), 1.66 (m, 4H); 13C NMR (CDCl3) δ 133.7, 131.4, 116.6,
114.8, 88.4, 50.1, 26.0, 24.7. Anal. Calcd for C24H28N2: C, 83.72; H,
8.14. Found: C, 83.77; H, 8.24. 1-(p-N,N-Diethylanilinyl)-2-phenyl-
1
acetylene: yield 85%; mp 143-144 °C; H NMR (CDCl3) δ 7.35 (d,
J ) 8.7 Hz, 4H), 6.60 (d, J ) 8.7 Hz, 4H), 3.36 (q, J ) 6.9 Hz, 8H),
1.16 (t, J ) 6.9 Hz, 12H); MS (m/e) 250 (M+ + 1, 13), 249 (M+, 75),
234 (100), 205 (17), 176 (15). Anal. Calcd for C18H14N: C, 86.56; H,
7.63. Found: C, 88.68; H, 7.66.
(p-N,N-Diethylanilinyl)pentaphenylbenzene (4). A mixture of 1.25
g (5 mmol) of 1,2-bis(p-N,N-diethylanilinyl)acetylene and 1.92 g (5
mmol) of tetraphenylcyclopentadienone was sealed in a glass tube and
heated at 200 °C for 24 h. The reaction mixture was purified by column
chromatography (silica gel, hexane/CH2Cl2, 80/20) to give 2.3 g of
Pentanuclear Donor (N,N-Dimethyl-N-ethylammonium-4′-phen-
yl)-penta(N-ethyl-N-methylanilinyl)benzene Triflate Salt (6‚OTf).
A 48 µL aliquot of methyl triflate (0.43 mmol) was added to 745 mg
(0.85 mmol) of 1b in 20 mL of dichloromethane, and the solution was
stirred at room temperature overnight. The reaction mixture was purified
by column chromatography over silica gel with CH2Cl2/acetone (20:1)
to recover 400 mg of 1b, and then it was eluted with acetone to obtain
330 mg of 6‚OTf (as triflic acid solvate) as a white solid: yield 80%
1
white crystalline 4: yield 76%; mp > 250 °C; H NMR (CDCl3) δ
6.86-6.83 (m, 25H), 6.59 (d, J ) 8.4 Hz, 2H), 6.22 (d, J ) 8.4 Hz,
2H), 3.13 (q, J ) 6.9 Hz, 2H), 0.96 (t, J ) 6.9 Hz, 2H); 13C NMR
(56) Adimurthy, S.; Ramachandraiash, G.; Ghosh, P. K.; Bedekar, A. V.
Tetrahedron Lett. 2003, 44, 5099.
(57) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory
Chemicals, 2nd ed.; Pergamon: New York, 1980.
9
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A R T I C L E S
Rosokha et al.
1
(based on methyl triflate added); mp > 250 °C; H NMR (acetone) δ
7.60 (d, J ) 7.8 Hz, 12H), 6.72 (d, J ) 2.1 Hz, 12H), 6.17 (dd, J )
7.8 Hz, 2.1 Hz, 12H), 3.73 (t, J ) 6.6 Hz, 24H), 1.67 (m, J ) 7.2 Hz,
24H), 1.39 (m, J ) 7.2 Hz, 24H), 0.92 (t, J ) 7.2 Hz, 36H); 13C NMR
(CDCl3) δ 153.0, 135.8, 133.8, 124.0, 113.6, 111.3, 103.6, 68.4, 31.4,
19.2, 13.9. Anal. Calcd for C62H75F3N6O3S‚0.85CF3SO3H: C, 64.39;
H, 6.54. Found: C, 64.50; H, 6.43.
where K n is the equilibrium constant determined by the redox potential
differences,
K n ) S exp((Ered - E1/2n)F/RT)
(11)
n
Ered is the reduction potential of the carborane radical, E1/2 is the
oxidation potential of the (n - 1)+ to n+ oxidation states of the
hexanuclear donors (Table 1), and S is the entropy-related factor.59 On
the basis of eq 10 (with the equilibrium constants calculated via eq
11), and taking into account the mass balance, i.e., ∑[1n+] ) [1]o, and
the charge balance, i.e., ∑n[1n+] ) [CB•]o + [CB-] (n ) 1-6), the
equilibrium concentrations of all the components were numerically
calculated with the Mathematica program.48 The dependence of the
concentration of each oxidation state on the concentration of the oxidant
added to the solution is presented in Figure 4.
Spectral Titration of Polynuclear Donors. The UV-vis-NIR
absorption spectra were recorded at -20 °C in a Dewar equipped with
a quartz lens, using a quartz spectroscopic cell with a 1-mm path length
and equipped with a Teflon valve fitted with Viton O-rings. Typically,
a 1 mM solution of the neutral donor 1 (or pentanuclear cation 6) was
prepared in a Schlenk tube and transferred under argon into the
spectroscopic cell. The oxidant CB• was added to this solution so that
the ratio of concentrations of carborane radical to donor varied from
0.1:1 to 10:1. [To ensure reproducibility, fresh solutions of 1 or 6 were
used in each measurement.] Upon the addition of oxidant, the colorless
solution immediately turned blue, and the spectrum was recorded.
Spectra of the solutions with oxidant/donor ratios varying from 0:1 to
2:1 are shown in Figure 5 and described in the text. Further increases
of oxidant (beyond a 2:1 ratio) led to a decrease of the intensity of the
band at λmax ) 1530 nm, which indicated that higher oxidation states
did not absorb significantly in the near-IR range (see Figure S3 in the
Supporting Information).
Isolation of Cation-Radical Salts of p-Doped Polynuclear Donors.
Typically, to prepare a p-doped cation-radical salt, 3.2 mL of a 15.6
mM solution of CB• was added to a stirred solution of 48 mg (0.05
mmol) of hexanuclear donor 1c in 2 mL of dichloromethane at -60
°C. The dark-blue solution was stirred for 30 min, and then 6 mL of
hexane was added. The resulting dark-blue precipitate was filtered,
washed with hexane (2 × 2 mL), and dried in vacuo. The isolated
solid powder (57 mg) corresponded to a 90% yield of 1c•+ CB-, and
its purity was established to be 97% by spectral titration with p-chloranil
anion-radical (oxidation potential of ∼0 V vs SCE) as follows. A 1
mM solution of NPr4+ p-CA•- in dichloromethane was added incre-
mentally to the stirred solution of 1c•+ CB- (13 mg, 0.01 mmol) in 1
mL of dichloromethane until the solution became colorless. In two
separate experiments, 9.8 and 10.1 mL amounts of the solution of
p-chloranil were consumed until the blue color of 1c•+ disappeared,
which corresponded to 98% and 101% purity of the dark-blue solid
1c•+ CB-. In the same manner, the solid dark-blue powders of 4•+ CB-
and 5•+ CB- were prepared in 88% and 80% yields, respectively,
starting with the corresponding mono- and dinuclear donors 4 and 5;
the purities of the salts were found by titration with p-chloranil to be
99% and 98%, respectively. Similarly, 5 mL of a 2 mM solution of
CB• was added to 10.4 mg of 6‚OTf dissolved in 1 mL of dichlo-
romethane at -68 °C. The titration of the resulting dark-blue solution
with the p-chloranil anion-radical indicated 90% yield of the dication-
radical 6•+
.
Instrumentation. The 1H NMR spectra were recorded on a General
Electric QE-300 NMR spectrometer, and the chemical shifts are reported
in ppm downfield from internal tetramethylsilane. UV-NIR spectra
were measured with a Varian Cary 500 spectrometer, and infrared
spectra were recorded on a Nicolet 10 DX FT spectrometer. GC-MS
analyses were carried out on a Hewlett-Packard 6890A (G1530A)
chromatograph with a HP5973 Network mass-selective detector. Cyclic
voltammetry (CV) and Osteryoung square-wave voltammetry (OSWV)
were performed on a BAS 100A electrochemical analyzer, and the ESR
spectra were measured with a Varian E-line Century Series ESR
spectrometer as described previously.6 X-ray crystallographic measure-
ments were performed with a Bruker SMART diffractometer equipped
with a 1K CCD detector using Mo KR radiation (λ ) 0.71073 Å) at
-150 °C. The structures were solved by direct methods and refined
by the full-matrix least-squares procedure;58 the X-ray structure details
are on deposit and can be obtained from the Cambridge Crystallographic
Database.
Among the various hexaarylbenzenes examined, the N,N-dialkyla-
nilinyl analogues showed the most significant splitting of the CV waves,
with a clear separation of the first oxidation wave and the broadest
NIR absorption. By comparison, the oxidation of hexakis(p-bromophe-
nylanilinyl)benzene 1b with CB• contained only a small fraction of
cation-radical 1b•+ (within a single CV wave, the distribution among
oxidation states is determined by statistical factors). The electronic
spectrum of this solution showed a local band (characteristic of the
mononuclear analogue), together with a NIR absorption with λmax
)
1500 nm (Figure S6), that could be related to a mixture of mono- and
dications (since the 3+ to 6+ oxidation states of donors 1c, 1d, and 6
have relatively weak absorptions in the NIR range). The ESR spectrum
of this solution consisted of 13 lines, with a hyperfine splitting constant
of aN ) 1.7G (Figure S7) and g ) 2.0030. Comparison with the ESR
spectrum of the mononuclear triphenylamine cation-radical (character-
ized by hyperfine nitrogen splittings of ∼10 G60) suggested that this
splitting was related to fast spin delocalization over six nitrogen centers.
Thus, the CV data point to a relatively weak interaction between the
redox centers in 1b•+, which may be related to partial charge
delocalization onto the p-bromophenyl substituents. However, the
interaction is probably still sufficient to ensure fast (on the ESR time
scale) charge migration between the redox sites.
Likewise, the oxidation of the carbazole-based hexanuclear donor 2
led to the appearance of the local absorption in the visible region and
an additional NIR band with λmax ) 1310 nm. Essentially the same
spectral shape was obtained when the tetracarbazole derivative was
oxidized. This indicated that the NIR absorptions in such polynuclear
cations derived from only the pairwise interaction of redox centers
(Figure S8). The ESR spectrum consisted of one broad, unresolved
peak with g ) 2.0028, and was independent of the temperature (+22
to -88 °C). The spectral (UV-vis-NIR and ESR) characterization of
Calculation of the Equilibrium Concentrations of Different
Oxidation States of Polynuclear Donors. The redox equilibria in
solutions of hexanuclear donors 1 upon addition of CB• can be presented
as
1
(n-1)+ + CB• a 1n+ + CB-
(9)
where n ) 1-6. The equilibrium concentrations of 1n+, CB•, and CB-
are related via the set of equations
K n ) [1n+][CB-]/[1(n-1)+][CB•]
(10)
(58) (a) Sheldrick, G. M. SADABS, Version 2.03; Bruker/Siemens Area Detector
Absorption and Other Corrections, 2000. (b) Sheldrick, G. M. SHELXS
97, Program for Crystal Structure Solutions; University of Go¨ttingen:
Go¨ttingen, Germany, 1997. (c) Sheldrick, G. M. SHELXL 97, Program for
Crystal Structure Refinement; University of Go¨ttingen: Go¨ttingen, Ger-
many, 1997.
(59) S ) 1, 6, 15, 20, 15, 6, and 1 for the 0 to +6 oxidation states, respectively.
(60) Van Willigen, H. J. Am. Chem. Soc. 1967, 89, 2229.
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9406 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Fast Electron Migration in p-Doped Aromatic Arrays
A R T I C L E S
p-doped hexapyrrole donor 3 suffered from its cation-radical instabili-
ties, even at low temperature, since the solution of this donor bleached
immediately upon the addition of carborane oxidant, even at -78 °C.
Computational methodology for the Mulliken-Hush analysis of
the pentanuclear cation-radical 6•+ and the hexanuclear cation-radicals
1c•+ and 1d•+ is described in the Supporting Information.
Acknowledgment. We thank the National Science Foundation
and R.A. Welch Foundation for financial support.
X-ray Crystallography of Hexanuclear Donors. Single crystals
were prepared by slow evaporation of solvent from the solutions of
hexanuclear donors in a CH2Cl2/CH3CN mixture. 1c: formula,
C66H84N6‚0.09CH2Cl2; MW, 968.36; triclinic, space group P1h; a )
9.3616(6), b ) 18.615(1), and c ) 18.893(1) Å; R ) 63.621(1), â )
84.051(1), and γ ) 78.052(1)°; V ) 2885.4(3) Å3; Z ) 2; F ) 1.115
g cm-3. The total number of reflections measured at 123 K was 26 797,
of which 14 246 were symmetrically nonequivalent. Final residuals were
R1 ) 0.0984 and wR2 ) 0.2957 for 10 392 reflections with I > 2σ(I).
1d: formula, C60H72N6‚2CH2Cl2; MW, 1047.09; triclinic, space group
P1h; a ) 9.750(4), b ) 15.374(6), and c ) 19.369(11) Å; R ) 96.84(3),
â ) 95.38(4), and γ ) 90.80(3)°; V ) 2869(2) Å3; Z ) 2; F ) 1.212
g cm-3. The total number of reflections measured at 173 K was 42 791,
of which 16 456 were symmetrically nonequivalent. Final residuals were
R1 ) 0.0782 and wR2 ) 0.2304 for 5945 reflections with I > 2σ(I).
2: formula, C126H135N6O12‚4C2H3N; MW, 2089.62; monoclinic, space
group C2/c; a ) 39.292(2), b ) 116.3439(9), and c ) 40.408(2)(11)
Å; â ) 111.055(2)°; V ) 24217(2) Å3; Z ) 8; F ) 1.146 g cm-3. The
total number of reflections measured at 123 K was 130 050, of which
21 554 were symmetrically nonequivalent. Final residuals were R1 )
0.0626 and wR2 ) 0.177 for 8870 reflections with I > 2σ(I).
Supporting Information Available: CV and OSWV of donors
1a, 1b, and 2 and the tetracarbazolyldifluorobenzene derivative
(Figure S1); ESR spectrum of monoanilinyl donor 4•+ (Figure
S2); electronic spectra of the higher oxidation states of hexa-
nuclear donor 1c (Figure S3); electronic spectra of the higher
oxidation states of pentanuclear donor 6 (Figure S4); ORTEP
diagram of tetracarbazolyldifluorobenzene (Figure S5); elec-
tronic spectrum of oxidation products of donor 1b (Figure S6);
ESR spectrum of 1b•+ (Figure S7); electronic spectra of the
oxidation products of hexa- and tetracarbazolylbenzene deriva-
tives (Figure S8); electronic spectra of cation-radicals in the
NIR and IR range (Figure S9); electronic spectrum of the cation-
radical 5•+ in the vis-NIR range (Figure S10); solution of the
six-state determinant (Charts S1-S4 and Table S1); solution
of the five-state determinant (Chart S5 and Table S2); parameters
for the Gaussian deconvolution for 1c•+ and 6•+ (Table S3);
and details of the multistate analysis. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA060393N
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