Intervalence transitions in the mixed-valence monocations of bis(triarylamines) linked with vinylene and phenylene-vinylene bridges

Article abstract of DOI:10.1021/ja054136e

(E)-4,4′-Bis{bis(4-methoxyphenyl)amino}stilbene, 1, (E,E)-1,4-bis[4-{bis(4-methoxyphenyl)-amino}styryl]benzene, 2, and two longer homologues, (E,E,E)-4,4′-bis[4-{bis(4-methoxyphenyl)amino}-styryl] stilbene, 3, and (E,E,E,E)-1,4-bis(4-[4-{bis(4-methoxyphen

Full text of DOI:10.1021/ja054136e

Published on Web 11/04/2005
Intervalence Transitions in the Mixed-Valence Monocations of
Bis(triarylamines) Linked with Vinylene and
Phenylene-Vinylene Bridges
Stephen Barlow,*,†,‡ Chad Risko,† Sung-Jae Chung,†,‡ Neil M. Tucker,‡
Veaceslav Coropceanu,† Simon C. Jones,† Zerubba Levi,† Jean-Luc Bre´das,† and
Seth R. Marder†,‡
Contribution from the Center for Organic Photonics and Electronics and School of Chemistry
and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and
Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721
Received June 22, 2005; E-mail: stephen.barlow@chemistry.gatech.edu
Abstract: (E)-4,4′-Bis{bis(4-methoxyphenyl)amino}stilbene, 1, (E,E)-1,4-bis[4-{bis(4-methoxyphenyl)-
amino}styryl]benzene, 2, and two longer homologues, (E,E,E)-4,4′-bis[4-{bis(4-methoxyphenyl)amino}-
styryl]stilbene, 3, and (E,E,E,E)-1,4-bis(4-[4-{bis(4-methoxyphenyl)amino}styryl]styryl)benzene, 4, have been
oxidized to their mono- and dications using tris(4-bromophenyl)aminium hexachloroantimonate. The
intervalence charge-transfer (IVCT) band of 1+ is narrow and asymmetric and exhibits only weak
solvatochromism. Analysis of this band indicates that 1+ is a class-III or class-II/III borderline mixed-valence
species. In contrast, a broad, strongly solvatochromic IVCT band is observed for 2+, indicating that this
species is a class-II mixed-valence species. The assignment of 1+ and 2+ as symmetric class-III and
unsymmetric class-II species, respectively, is also supported by AM1 calculations. Hush analysis of the
IVCT bands of both 1+ and 2+ gives larger electronic couplings, V, than for their analogues in which the
double bonds are replaced with triple bonds. The diabatic electron-transfer distance, R, in 1+ can be
estimated by comparison of the V estimated by Hush analysis and from the IVCT maximum; it is considerably
less than the geometric N-N separation, a result supported by quantum-chemical estimates of R for 1+-
4+. In 3+ and 4+, the IVCT is largely obscured by an intense absorption similar to a band seen in the
corresponding dications and to that observed in the monocation of a model compound, (E,E,E)-1-{bis(4-
methoxyphenyl)amino}-4-[4-{4-(4-tert-butylstyryl)styryl}styryl]benzene, 5, containing only one nitrogen redox
center; we attribute this band to a bridge-to-N+ transition. The corresponding dications 12+-42+ show a
complementary trend in the coupling between redox centers: the shortest species is diamagnetic, while
the dication with the longest bridge behaves as two essentially noninteracting radical centers.
dioxaborines,7 nitro groups,8,9 and perchlorotriphenylmethyl
centers10,11 in anionic organic MV systems, and hydrazines,12,13
Introduction
Mixed-valence (MV) compounds are comprised of two or
more linked redox centers in different formal oxidation states.1,2
Since Hush first developed the theory linking optical electron
transfer with the Marcus theory of thermal electron transfer,3
there has been considerable interest in transition-metal MV
systems as model systems for understanding electron-transfer
processes. More recently, organic MV systems have attracted
increasing interest; these materials tend to exhibit stronger
intersite coupling than their transition-metal-based analogues,4
and, in some cases, they provide insight into the behavior of
organic electronic and optical materials. Many different redox
centers have been investigated, including quinones and imides,5,6
1,4-dialkoxybenzenes,14,15 and various alkylamines16,17 in cat-
ionic systems. Triarylamine MV species have been the focus
of a number of studies;18-33 triarylamines may be readily
(7) Risko, C.; Barlow, S.; Coropceanu, V.; Halik, M.; Bre´das, J.-L.; Marder,
S. R. Chem. Commun. 2003, 194.
(8) Nelsen, S. F.; Konradsson, A. E.; Weaver, M. N.; Telo, J. P. J. Am. Chem.
Soc. 2003, 125, 12493.
(9) Nelsen, S. F.; Weaver, M. N.; Zink, J. I. J. Am. Chem. Soc. 2005, 127,
10611.
(10) Bonvoisin, J.; Launay, J.-P.; Rovira, C.; Veciana, J. Angew. Chem., Int.
Ed. Engl. 1994, 33, 2106.
(11) Sedo´, J.; Ruiz, D.; Vidal-Gancedo, J.; Rovira, C.; Bonvoisin, J.; Launay,
J.-P.; Veciana, J. Synth. Met. 1997, 85, 1651.
(12) Nelsen, S. F.; Chang, H.; Wolff, J. J.; Adamus, J. J. Am. Chem. Soc. 1993,
115, 12276.
(13) Nelsen, S. F.; Ismagilov, R. F.; Powell, D. R. J. Am. Chem. Soc. 1996,
118, 6313.
† Georgia Institute of Technology.
‡ University of Arizona.
(14) Rosokha, S. V.; Sun, D.-L.; Kochi, J. K. J. Phys. Chem. A 2002, 106,
2283.
(1) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(2) Allen, G. C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357.
(3) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391.
(4) Nelsen, S. F.; Tran, H. Q.; Nagy, M. A. J. Am. Chem. Soc. 1998, 120,
298.
(15) Lindeman, S. V.; Rosokha, S. V.; Sun, D.; Kochi, J. K. J. Am. Chem. Soc.
2002, 124, 843.
(16) Nelsen, S. F.; Tran, H. Q. J. Phys. Chem. A 1999, 103, 8139.
(17) Bailey, S. E.; Zink, J. I.; Nelsen, S. F. J. Am. Chem. Soc. 2003, 125, 5939.
(18) Bonvoisin, J.; Launay, J.-P.; Van der Auweraer, M.; De Schryver, F. C. J.
Phys. Chem. 1994, 98, 5052.
(5) Jozefiak, T. H.; Miller, L. L. J. Am. Chem. Soc. 1987, 109, 6560.
(6) Rak, S. F.; Miller, L. L. J. Am. Chem. Soc. 1992, 114, 1388.
9
16900
J. AM. CHEM. SOC. 2005, 127, 16900-16911
10.1021/ja054136e CCC: $30.25 © 2005 American Chemical Society

Intervalence Transitions in Bis(triarylamine) Monocations
A R T I C L E S
Figure 1. Structures of bis(triarylamines) considered in this work. The MV cations of L0, L0′, L1, and L2 have previously been studied by Lambert and
No¨ll,22 while those of 1-4 are the focus of the present work. The structure of the monoamine model compound, 5, is also shown.
combined with a wide range of bridging groups and, with
appropriate substitution patterns, can be converted to rather
stable radical cations at only moderate oxidizing potentials.34,35
Moreover, bis(triarylamine) derivatives are used as hole-
transport agents in organic electronics applications,36-39 the
charge-transfer process involving electron hopping between
neutral molecules and the corresponding MV radical cation.
Lambert and No¨ll have studied the MV properties of a variety
of bis(triarylamine) systems in which the terminal aryl groups
have methoxy groups in the 4-position, including those with
phenylene, biphenyl, and phenylene-ethynylene bridging
groups;22,25,32 additionally, several computational analyses have
been published on these compounds.24,27,29 The phenylene-
ethynylene-bridged species L1+ and L2+ (Figure 1) appear to
be Robin-Day1 class-II (localized) MV compounds. The inter-
valence (ICVT) absorptions of shorter species with phenylene
(L0+,40 Figure 1) or biphenyl (L0′+) bridges show unusual
markedly unsymmetrical profiles; it was originally suggested
that these line shapes indicated localized species close to the
class-II/III borderline,22,41-43 but subsequent studies show these
profiles are consistent with class-III systems exhibiting strong
coupling of the electron transfer to symmetrical vibrations.27
Comparison of UV-photoelectron and IVCT data allows one
to estimate very small barriers for intramolecular electron
transfer in these species, confirming them to be at least very
close to the class II/III borderline, or possibly to belong to class
III.31 Recently, crystalline salts of two examples of these cations
s 1,4-bis(diphenylamino)benzene44 and 4,4′-bis[phenyl-(2,4-
dimethylphenyl)amino]biphenyl30 s have been isolated and
shown using X-ray crystallography to have symmetrical struc-
tures consistent with assignment to class III. Vibrational studies
confirm that these two species and the 4,4′-bis[phenyl-(2,4-
dimethylphenyl)amino]biphenyl cation (TPD+) belong to class
III.30,44,45 More recently still, we have reported the first example
of a MV bis(triarylamine) with a vinylene bridge, 1+, which
(19) Stickley, K. R.; Blackstock, S. C. Tetrahedron Lett. 1995, 36, 1585.
(20) Lambert, C.; No¨ll, G.; Schma¨lzlin, E.; Meerholz, K.; Bra¨uchle, C. Chem.
Eur. J. 1998, 4, 2129.
(21) Lambert, C.; No¨ll, G. Angew. Chem., Int. Ed. 1998, 37, 2107.
(22) Lambert, C.; No¨ll, G. J. Am. Chem. Soc. 1999, 121, 8434.
(23) Lambert, C.; No¨ll, G.; Hampel, F. J. Phys. Chem. A 2001, 105, 7751.
(24) Coropceanu, V.; Malagoli, M.; Andre´, J. M.; Bre´das, J. L. J. Chem. Phys.
2001, 115, 10409.
(25) Lambert, C.; No¨ll, G.; Schelter, J. Nat. Mater. 2002, 1, 69.
(26) Lambert, C.; No¨ll, G. Chem. Eur. J. 2002, 8, 3467.
(27) Coropceanu, V.; Malagoli, M.; Andre´, J. M.; Bre´das, J. L. J. Am. Chem.
Soc. 2002, 124, 10519.
(28) Yano, M.; Ishida, Y.; Aoyama, K.; Tatsumi, M.; Sato, K.; Shiomi, D.;
Ichimura, A.; Takui, T. Synth. Met. 2003, 137, 1275.
(29) Coropceanu, V.; Lambert, C.; No¨ll, G.; Bre´das, J. L. Chem. Phys. Lett.
2003, 373, 153.
(30) Low, P. J.; Paterson, M. A. J.; Puschmann, H.; Goeta, A. E.; Howard, J.
A. K.; Lambert, C.; Cherryman, J. C.; Tackley, D. R.; Leeming, S.; Brown,
B. Chem.sEur. J. 2004, 10, 83.
(31) Coropceanu, V.; Gruhn, N. E.; Barlow, S.; Lambert, C.; Durivage, J. C.;
Bill, T. G.; No¨ll, G.; Marder, S. R.; Bre´das, J.-L. J. Am. Chem. Soc. 2004,
126, 2727.
(32) Lambert, C.; Amthor, S.; Schelter, J. J. Phys. Chem. A 2004, 108, 6474.
(33) Lambert, C.; Risko, C.; Coropceanu, V.; Schelter, J.; Amthor, S.; Gruhn,
N. E.; Durivage, J. C.; Bre´das, J. L. J. Am. Chem. Soc. 2005, 127, 8508.
(34) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, D. W.;
Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3498.
(35) Dapperheld, S.; Steckhan, E.; Grosse-Brinkhaus, K.-H.; Esch, T. Chem.
Ber. 1991, 124, 2557.
(40) For the first synthesis of L0+ and its isolation as a hexafluorophosphate
salt, see: Selby, T. D.; Blackstock, S. C. J. Am. Chem. Soc. 1998, 120,
12155.
(36) Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(37) Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332.
(38) Bulovic, V.; Gu, G.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Nature
1996, 380, 29.
(39) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for Xerography;
Marcel Dekker: New York, 1998.
(41) Nelsen, S. F. Chem. Eur. J. 2000, 2000, 581.
(42) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. ReV. 2001, 101,
2655.
(43) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. ReV. 2002, 31, 168-
184.
(44) Szeghalmi, A. V. et al. J. Am. Chem. Soc. 2004, 126, 7834.
9
J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005 16901

A R T I C L E S
Barlow et al.
Scheme 1. Synthesis of Compounds 1-4
also has a markedly unsymmetrical IVCT band, and shown,
using X-ray crystallography and vibrational spectroscopy, that
it also belongs to class III, in contrast to its alkyne analogue,
L1+.46 Thus, it appears that bis(triarylamine)s with strongly
asymmetric IVCT bands generally belong to class III.
Here we compare the spectroscopic and computational results
for 1+ with those for three additional longer phenylene-
vinylene-bridged MV species (2+-4+) (Figure 1). We show
that 2+ is a class-II MV compound with stronger electronic
coupling between the two redox centers, V, than in its alkyne
analogue, L2+. In the case of 3+ and 4+, strong NIR bands are
observed. At first sight, one might attribute these absorptions
to IVCT bands and deduce rather large V in these species;
however, we show, by comparison with similar features found
in the spectra of the corresponding dications and of the
monocation of a model compound containing a single amine
redox center, 5+ (Figure 1), that these features have a different
origin.
as shown in Scheme 1. (The synthesis of 1 is described
elsewhere;46 a very similar synthesis of 2 has recently been
published,53 and independent syntheses of several close ana-
logues of 354 and 455 have recently appeared.) UV-vis data
for the neutral compounds are tabulated in Table 1, along with
electrochemical data and with spectroscopic data for the
corresponding monocations and dications (vide infra). UV-
vis spectra for the neutral species are very similar to those
reported for other triarylamine-terminated phenylene-vinylene
oligomers with the same chain lengths: for example, 4 shows
λmax ) 435 nm (ꢀmax ) 113 000 M-1 cm-1); similar values of
λmax ) 437 nm (ꢀmax ) 99 000 M-1 cm-1) have been reported
for an analogue with unsubstituted phenyl end groups and with
n-octyloxy groups on the central phenylene ring of the bridge.
To better understand the spectra of 3+ and 4+ (vide infra), we
were interested in a model compound in which only one
triarylamine group was attached to a phenylene-vinylene chain.
Hence, we also synthesized compound 5, using the synthetic
route shown in Scheme 2; data for 5 and 5+ are also included
in Table 1.
Results and Discussion
Synthesis and Characterization of 1-5. Compounds 1-4
were synthesized using a combination of palladium-catalyzed
C-N coupling,47,48 Horner-Emmons,49,50 and Heck51,52 reactions,
Electrochemistry. Cyclic voltammetry (CV; Table 1) shows
that two electrons can be removed readily and reversibly from
each of the compounds 1-4. In the case of the stilbene species,
1, two distinct potentials are resolvable corresponding to
(45) Littleford, R. E.; Paterson, M. A. J.; Low, P. J.; Tackley, D. R.; Jayes, L.;
Dent, G.; Cherryman, J. C.; Brown, B.; Smith, W. E. Phys. Chem. Chem.
Phys. 2004, 6, 3257.
oxidation to monocation (E1/2+/0) and dication (E1/22+/+); the
+/0
separation between these potentials, ∆E1/2 ) E1/22+/+ - E1/2
,
(46) Barlow, S. et al. Chem. Commun. 2005, 764.
was more accurately determined using differential pulse volta-
mmetry (DPV) to be 134 mV. The more facile first oxidation
(47) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7217.
(48) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118,
7215.
(49) Horner, L. Chem. Ber. 1958, 83, 733.
(50) Wadsworth, W. S.; Emmons, W. D. J. Am. Chem. Soc. 1961, 83, 1733.
(51) Heck, R. F. J. Am. Chem. Soc. 1968, 90, 5518.
(52) de Meijere, A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379.
(53) Kauffman, J. M.; Moyna, G. J. Org. Chem. 2003, 68, 839.
(54) Li, C.-L.; Shieh, S.-J.; Lin, S.-C.; Liu, R.-S. Org. Lett. 2003, 5, 1131.
(55) Detert, H.; Sadovski, O. Synth. Met. 2003, 138, 185.
9
16902 J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005

Intervalence Transitions in Bis(triarylamine) Monocations
A R T I C L E S
Table 1. Electrochemical (0.1 M [nBu4N]+[PF6]-/CH2Cl2) and UV-vis-NIR Spectroscopic (CH2Cl2) Data for 1-5 and Their Cations and
Dications
1
E1 2/Va
ν¯max
(
ꢀmax)/103 cm-1 (103 M-1 cm-
)
/
+/0
2
+/
+
3+/2+
4+/3+
neutral
monocationb
dicationb
1
2
3
4
5
+0.08
+0.22
-
-
-
-
25.1 (54), 32.7 (27)
23.8 (76), 33.3 (36)
23.3 (100), 32.7 (37)
23.0 (113), 32.8 (31)
24.0 (88), 33.0 (26)
6.1 (39), 16.2 (34)
11.1 (129), 15.9 (13), 20.0 (10)
9.3 (63), 14.3 (28)
8.6 (30), 14.2 (29), 16.3 (25)
8.5 (34), 14.2 (29)
-
+0.20c
+0.20c
+0.18c
+0.81
+0.69
+0.65
-
6.1 (16), 10.2 (6), 14.6 (23)
8.7 (11), 14.2 (14), 16.0 (13)
8.5 (15), 14.3 (14), 17.9 (12)
8.5 (17), 14.3 (14), 15.7 (14)
+0.77
+0.21
+0.66
-
+/0
a Electrochemical half-wave potentials corresponding to successive oxidations of the molecule, referenced to FeCp2
. )
b The high-energy (>24 000 cm-1
parts of the spectra of the cationic species are not reported due to overlap with the absorptions of excess neutral species (in the case of monocations) and
with the tris(4-bromophenyl)amine side product. c No separation resolved between first and second oxidations.
Scheme 2. Synthesis of Compound 5
observed in 1 versus its longer homologues presumably reflects
greater stabilization of this cation through delocalization. For
2-4, ∆E1/2 is too small to be evident in the CV and was also
undetectable by DPV; the CVs are characteristic of species
undergoing two independent one-electron redox processes at
very similar potentials. Lambert and No¨ll obtained redox
splittings of 150 and 60 mV through digital simulations of the
CVs for L1 and L2, respectively, in the same electrolyte
medium.22 In addition, they reported a linear correlation between
∆E1/2 for their compounds and the electronic coupling, V, in
the corresponding MV monocations, as determined by Hush
analysis of their IVCT absorptions.22 However, it should be
noted that ∆E1/2 is affected by many factors besides electronic
coupling, and such a correlation is not necessarily expected.56
In the species with longer phenylene-vinylene bridges, 2-4,
and in the monoamine model compound, 5, additional less
reversible (reduced Ired/Iox, increased Eox - Ered) redox features
are observable at more strongly oxidizing potentials. These
additional features presumably correspond to bridge-based
oxidations and, accordingly, become increasingly facile with
increased length of the bridging group; this behavior of increased
ease of oxidation with increased chain length is seen in other
oligo(phenylenevinylene) derivatives (for example, see ref 57).
Electronic Spectra of the Monocations and Dications of
1-5. Tris(4-bromophenyl)aminium hexachloroantimonate was
chosen as an oxidizing agent for the generation of the mono-
cations and dications of the bis(triarylamine) species; the salt
is easily weighed out stoichiometrically and readily able to effect
oxidation of all of our bis(triarylamine)s to both mono- and
dications (E1/2 ) +0.70 V vs ferrocenium/ferrocene in dichlo-
romethane58), and the side product of oxidation, tris(4-bro-
mophenyl)amine, shows neither a vis-NIR absorption59 nor any
electron spin resonance (ESR) signal. Solutions containing the
monocations were obtained by the addition of tris(4-bromo-
phenyl)aminium hexachloroantimonate solution to large excesses
(>10 equiv) of the neutral bis(triarylamine) solutions, while the
dications were generated by the addition of 2 equiv of oxidizing
agent to the amines. Similar spectra were obtained using
solutions of isolated crystalline salts of the mono-46 and
dications60 for 1. These solutions were then investigated using
vis-NIR (Figure 2) and ESR spectroscopy.
The MV monocations of the bis(triarylamines) all show
intense absorptions in the NIR. In the cases of 1+ and 2+, these
features are observed at considerably lower energy than the
lowest energy absorption of the corresponding dications and
can be attributed to the IVCT bands.61 Some parameters
characterizing these bands are collected in Table 2 along with
those for their alkyne analogues, L1+ and L2+. Lambert and
No¨ll previously reported the corresponding parameters for L1+
and L2+ electrochemically generated in 0.1 M [nBu4N]+[PF6]-/
CH2Cl2,22 and we have reported line-shape data for 1+
chemically generated in 0.1 M [nBu4N]+[PF6]-/CH2Cl2;46 the
line-shape parameters depend only slightly on the presence or
absence of electrolyte, although the absorption maxima are red-
(56) For examples of strong solvent and counterion dependence of ∆E1/2, see:
(a) Barriere, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U. T.;
Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262. For an example of a pair
of isoelectronic mixed-valence species where estimating the electronic
coupling from ∆E1/2 would lead to the opposite conclusion to that indicated
by UV-vis-NIR spectroscopy, compare the dicobalt and dimanganese
species of: (b) Manriquez, J. M.; Ward, M. D.; Reiff, W. M.; Calabrese,
J. C.; Jones, N. L.; Carroll, P. J.; Bunel, E. E.; Miller, J. S. J. Am. Chem.
Soc. 1995, 117, 6182. (c) Jones, S. C.; Hascall, T.; Barlow, S.; O’Hare, D.
J. Am. Chem. Soc. 2002, 124, 11610, respectively.
(58) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
(59) The absorption maximum for the neutral tris(4-bromophenyl)amine molecule
is at 312 nm (Schmidt, W.; Steckhan, E. Chem. Ber. 1980, 113, 577).
(60) Zheng, S. et al. Submitted.
(61) The spectra for 12+ and 22+ can also be compared to those of the dications
of stilbene and 1,4-bis(styryl)benzene which have low-energy maxima at
8850 and 8720 cm-1, respectively (Spangler, C. W.; Hall, T. J. Synth. Met.
1991, 44, 85. Spangler, C. W.; Liu, P.-K.; Nickel, E. G. AIP Conference
Proceedings 1992, 262, 28). The maximum for the 1,4-bis(4-methylstyryl)-
benzene monocation is at 8330 cm-1 (Sakamoto, A.; Furukawa, Y.; Tasumi,
M. J. Phys. Chem. B 1997, 101, 1726).
(57) Peeters, E.; van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.;
Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. B 2000, 104, 10174.
9
J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005 16903

A R T I C L E S
Barlow et al.
Figure 2. Spectra in dichloromethane of the mono- (solid lines) and dications (broken lines) of 1-4, along with the spectrum of the monocation of 5 (see
Experimental Section for details). Note each graph has a different vertical scale.
shifted somewhat in the presence of electrolyte (least so in the
case of 1+, consistent with its weaker solvatochromism; vide
infra).
comparison of the observed widths (or widths on the high-energy
side) with the width expected from Hush theory for class-II
compounds, νj1/2[Hush], given in cm-1 by eq 1:
The asymmetry of the IVCT bands is characterized in Table
2 by νj1/2[high]/νj1/2[low], where νj1/2[high] and νj1/2[low] are twice
the half-widths on the high- and low-energy side of the band,
respectively;22 for 1+, the asymmetry is stronger than that for
its alkyne analogue, L1+. The width of the bands can be
compared directly through the values of νj1/2[obs], or through
νj1/2[Hush] ) 2310 × νj
where νjmax is also in cm-1. In the case of L1+ in CH2Cl2, our
data show that twice the width on the high-energy side,
νj1/2[high], somewhat exceeds the Hush limit, while νj1/2[obs] is
(1)
x
max
9
16904 J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005

Intervalence Transitions in Bis(triarylamine) Monocations
A R T I C L E S
Table 2. Parameters Relating to the IVCT Absorptions of 1+, 2+, L1+, and L2+ Acquired Using Chemical Oxidation with
[(4-BrC6H4)3N]+[SbCl6]-
CH2Cl2
MeCN
1
1
1
1
1
ν¯
max/cm-
ꢀmax/M-1 cm-
ν¯1/
2[obs]/cm-
ν¯1/
2[Hush]a/cm-
ν¯1/
2[high]/ν1/
¯
2[low]b
ν¯1/
2[high]/ν1/
¯
2[Hush]c
µged/D
ν¯
max/cm-
1+e
6080e
6130
5760
7780
39 300
15 510
21 800
5260
2760
4310f
3460
5180f
3750
3760
3650
4240
1.40
1f
0.86
13.5
10.3
11.3
5.85
7010
8200
7930
9910
2+
1.15f
1.04
1.22f
L1+
L2+g
1.23
1f
a Calculated according to eq 1. b Ratio of bandwidth on high-energy side to bandwidth on low-energy side. c Ratio of twice the bandwidth on the high-
energy side to the bandwidth predicted by eq 1. d Transition dipole moment calculated from the IVCT band using eq 3 (vide infra). e Similar values (νjmax
) 6150 cm-1 and µge ) 13.0 D) were given for 1+ in CH2Cl2 in a footnote to ref 66. f Some overlap with another transition on high-energy side; band was
fitted acceptably to a symmetrical Gaussian, and so νj1/2[high] was assumed to be equal to νj1/2
µge ) 6.2 D) for L2+ generated in CH2Cl2 by oxidation with SbCl5.
.
g Reference 25 gives similar values (νjmax ) 8060 cm-1 and
marginally less than the Hush limit; Lambert and No¨ll showed
that, in 0.1 M [nBu4N]+[PF6]-/CH2Cl2, both νj1/2[high] and
νj1/2[obs] exceed the Hush limit.22 However, for 1+, both νj1/2[obs]
and νj1/2[high] are considerably less than νj1/2[Hush] with or
without electrolyte present; indeed, the νj1/2[obs] values and the
ratios of νj1/2[high]/νj1/2[Hush] are less than that for any of the
species reported by Lambert and No¨ll in ref 22 (line-shape data
for 1+ in electrolyte solution are given in ref 46). Moreover,
the IVCT band of 1+ is considerably less solvatochromic than
that of L1+; that of 1+ is blue-shifted by some 930 cm-1
between dichloromethane and acetonitrile, whereas the corre-
sponding blue-shift seen for L1+ is ca. 2170 cm-1 (more
extensive solvatochromic data for 1+ and L1+ are given in the
Supporting Information of ref 46). The solvatochromism of 1+
is similar in magnitude to that seen for the bis(diphenylamino)-
benzene cation (860 cm-1 shift from dichloromethane to
acetonitrile), which has been shown to belong to class III using
crystallographic and vibrational data.44 The differences in the
asymmetry, width, and solvatochromism of the IVCT bands of
1+ and L1+ are consistent with their assignment to classes III
and II, respectively, an assignment also supported by vibrational
data (the IR spectrum of a solution of L1+ in dichloromethane,
generated in the same way as the samples for electronic
spectroscopy, shows an alkyne stretch, clearly indicating a
broken symmetry structure).46
The IVCT band of 2+ is qualitatively similar to that of L2+
in terms of symmetry (both bands were fitted with symmetrical
Gaussians to deconvolute them from other overlapping bands,
but the IVCT band shown for 2+ in Figure 2 is clearly at least
approximately symmetrical), width versus the Hush limit, and
solvatochromism, all of which are consistent with assignment
of both species to class II. The main differences are that the
transition for 2+ is at lower energy, indicating a smaller
reorganization energy, and has a much higher transition dipole
moment than that for L2+.
side than those of 32+ or 5+; presumably, this is due to overlap
between the B f D+ transition and a weaker lower-energy
IVCT band. In contrast, the line shapes of the low-energy band
of 4+ are very similar to those of 42+, suggesting the IVCT in
this species is much weaker than in 3+. The situation of 3+ and
4+ strongly resembles that of [Ar(p-C6H4)4Ar]+ {Ar ) 2,5-
dimethoxy-4-methylphen-1-yl}, which has its lowest energy
electronic absorption at 7810 cm-1, while the corresponding
dication absorbs at 7750 cm-1 with ca. 1.5 times the absorptivity,
both bands being assigned to B f D+ transitions.14 Moreover,
a transition in L2+ seen at ca. 12 000 cm-1 has also been
attributed to a B f D+ transition; the transition becomes red-
shifted when the central benzene ring is replaced by anthracene
or dimethoxybenzene,25,32,62 or by trans-Pt(PEt3)2.63 A transition
at 10 200 cm-1 in the spectrum of 2+ (in dichloromethane; the
band is not seen in acetonitrile, presumably due to overlap with
the blue-shifted IVCT) is possibly due to a similar B f D+
transition; the band energy (i.e., higher than that for 3+ and
4+ and lower than that for L2+) is certainly broadly consis-
tent.
Higher energy transitions are seen at ca. 14 000-16 500 cm-1
for all compounds. Similar bands are seen for Lambert and
No¨ll’s compounds and were assigned to localized transitions
within the Ar2N+ units;22 the mononuclear triarylaminium
species [(4-MeOC6H4)3N]+ and [(4-MeC6H4)3N]+ absorb at ca.
14 000 and 15 000 cm-1, respectively, in acetonitrile.64 Sig-
nificantly, for 1+, this band is considerably blue-shifted from
those of simple triarylamine radical cations; this has also seen
in the case of other class-III bis(triarylamine) derivatives, such
as L0′+ and other bis(diarylamino)biphenyl cations;22,30 TD-
DFT calculations suggest that in these systems the transitions
can be regarded as transitions from the HOMO (using the orbital
label from the neutral molecule) to a bridge-based LUMO.30 In
the species with longer π-bridges, and in L1+ and L2+, the
energies of these bands more closely approach those of the
parent triarylaminium species; moreover, the band is seen at
similar energies in mono- and dications, and in 3+ and 4+, the
intensity in the dication is approximately twice that in the
monocation. These observations are entirely consistent with 1+
having more delocalized structures than L1+ and than the species
with longer bridges.65
In the cases of 3+ and 4+, rather intense (ꢀmax ) ca. 104 M-1
cm-1) bands are observed in the NIR (ca. 9000 cm-1). The
unwary might assign these near-IR bands to IVCT and deduce
rather strong electronic coupling between the two redox centers
using Hush analysis. However, absorptions at similar energy,
with approximately twice the absorptivity, are seen in 32+ and
4
2+, and a band with similar energy and absorptivity is also
seen in 5+, a compound in which there is only one triarylamine
center and where, therefore, a triarylamine-to-triarylaminium
IVCT transition is not possible. We, therefore, assign these
transitions to charge transfer from the highest bridge-based
orbital to the terminal donor radical cation (B f D+). The NIR
band of 3+ is, however, considerably broader on the low-energy
(62) A similar assignment has been made for the low-energy band of a bis-
(hydrazine) MV species with an anthracene bridge (Nelsen, S. F.; Ismagilov,
R. F.; Powell, D. R. J. Am. Chem. Soc. 1998, 120, 1924).
(63) Jones, S. C.; Coropceanu, V.; Barlow, S.; Kinnibrugh, T.; Timofeeva, T.;
Bre´das, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2004, 126, 11782.
(64) Walter, R. I. J. Am. Chem. Soc. 1966, 88, 1923.
(65) The differences in profile and similarity in intensity for the band at ca.
14 000 cm-1 in 2+ and 22+ suggest 2+, although still belonging to class II,
is more delocalized than L2+, 3+, and 4+.
9
J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005 16905

A R T I C L E S
Barlow et al.
Figure 3. Left: First derivative ESR spectrum of 42+ generated in situ in CH2Cl2. Right: Plot showing numbers of spin-1/2 centers per molecule determined
by integration of the ESR signals for mono- (open circles) and dications (solid squares) of 1-4, normalized to unity for 1+.
3
2+, and 42+, respectively. In combination with the diamagnetic
Electron-Spin Resonance Spectra of Mono- and Dications.
behavior of 12+, these data indicate a gradual transition from
strongly coupled redox centers to essentially noninteracting
radical centers in the longest species (Figure 3), thus presenting
a similar trend to that seen in the monocation NIR spectra.
Optimized Geometry of Neutral and Cationic Species. The
AM1-optimized geometries for neutral 1 and 2 are shown in
Figure 4. The phenylene-vinylene bridges of neutral 1-4 are
somewhat twisted in a helical fashion, with dihedral angles of
ca. 20° between the phenyl rings and the vinylene units;
however, planarization of the bridges only costs energy of the
order of kT. The amines are slightly pyramidal; the angle sum
around the nitrogen atoms is ca. 357°. DFT calculations produce
very similar results for the neutral species with the main
difference being a less pronounced twist in the phenylene-
vinylene bridges and nonpyramidal amines.
Figure 4 also shows the AM1/CI structures for 1+ and 2+; in
the cases of all four species studied, there are significant
geometric changes relative to the neutral species (details of the
bond length changes are tabulated in the Supporting Informa-
tion). 1+ maintains a symmetric structure and is planarized both
at nitrogen and in the bridge. The N-Cstilbene bonds decrease
by 0.046 Å on oxidation, and the C-C bonds in the stilbene
bridge change toward a quinoid-like arrangement. The net
charge is symmetrically distributed over the whole structure.
In contrast, the AM1/CI structures of 2+-4+ show definitively
broken symmetry; in each case, one of the nitrogen centers and
a stilbene unit become planarized with a similar bond length
pattern to 1+, while the remainder of the system maintains a
twisted neutral-like structure. The excess charge is localized
solely within the planarized section of the structure.
Room-temperature ESR spectra were acquired for the in situ
generated dichloromethane solutions of 2+-5+ and 22+-42+
.
We have previously reported the spectrum of a solution of
1+[SbF6]- and of in situ generated L1+.46 All the monocations
show spectra centered at g ) 2.004 (Ph3N+ is reported at g )
2.00367). The monocation spectra are poorly resolved and, hence,
not particularly useful in giving insight into the delocalization
in the monocations. While the ESR spectra of several tris(aryl)-
aminium radical cations, such as [(4-MeOC6H4)3N]+, show very
well-resolved 1H coupling, the lack of resolution in the present
cases is not particularly surprising given the number of
inequivalent protons; for example, the poor resolution of the
spectrum of the tris(biphenyl-4-yl)aminium cation has been
attributed to the large number of inequivalent protons.68 In the
case of 1+ and L1+, we were able to resolve coupling; we
interpret this as arising from two equivalent I ) 1 centers with
a coupling constant, AN, of ca. 4.3 G, indicating these cations
are either class-III or class-II with a room-temperature inter-
molecular exchange rate in excess of 107 s-1. The coupling
constant is consistent with these radicals being triarylaminium-
centered; AN ) 8.97 G for [(4-MeOC6H4)3N]+ in MeCN.34
Our model mononuclear triarylaminium species, 5+, shows a
poorly resolved triplet characterized by AN ) 8.6 G, indicating
the oxidation in this species is also triarylamine-centered.
The ESR spectra of the dications afford more interesting
information; 22+-42+ all show structureless features at g )
2.004 (as an example, the ESR spectrum of 42+ is shown in
Figure 3), while 12+ shows only a very weak signal that we
attribute to an impurity. Spin concentrations for solutions of
these dications were estimated by double integration of the first-
derivative spectra of solutions of known concentration and
comparison with those from monocation solutions at comparable
Electron Transfer Distance and Electronic Coupling. Hush
showed that the electronic coupling, V, between redox centers
in a MV species can be derived from the IVCT band according
to
known concentrations. These integrals correspond to values
1
equivalent to 1.2, 1.8, and 2.1 ((0.1) S ) /2 centers for 22+
,
µgeνjmax
eR
(66) Heckmann, A.; Lambert, C.; Goebel, M.; Wortmann, R. Angew. Chem.,
Int. Ed. 2004, 43, 5851.
V )
(2)
(67) Bamberger, S.; Hellwinkel, D.; Neugebauer, F. A. Chem. Ber. 1975, 108,
2416.
(68) Pearson, G. A.; Rocek, M.; Walter, R. I. J. Phys. Chem. 1978, 82, 1185.
where R is the effective separation between donor and acceptor
9
16906 J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005

Intervalence Transitions in Bis(triarylamine) Monocations
A R T I C L E S
Figure 4. Ball-and-stick model of geometries of 1 (top left), 2 (top right), 1+ (lower left), and 2+ (lower right) according to AM1 (neutral) and AM1/CI
(radical cation) levels of theory.
Table 3. Excitation Energies, E, and Transition Dipole Moments,
(diabatic states), and µge is the transition dipole moment
associated with the transition and given, in Debye, by
for Compounds 1+-4+ According to ZINDO/CIS Calculations at
the Neutral AM1 Geometries, along with the Geometric N-N
Distance (AM1, neutral), Diabatic Electron-Transfer Distances, R,
and Estimated Adiabatic Electron-Transfer Distance, R12
ꢀ(νj)dνj
∫
E/eV
µ±/D
RNNa/Å
R[eq6]/Å
R[eq7]/Å
R12/Å
µge ) 0.09584
(3)
νjmax
x
1+
0.389
0.287
0.249
0.043
0.436
21.0
27.6
32.5
52.4
21.5
12.23
18.78
25.32
31.79
12.48b
8.78
11.53
13.58
21.98
8.98
5.92
8.72
-
1.88
7.58
10.68
22.22
-
2+
where νjmax and ꢀ(νj) are in cm-1 and M-1 cm-1, respectively.
In the case of symmetric Gaussian IVCT bands, eqs 2 and 3
are often combined to give eq 4:
3+
4+
-
L1+
-
a For 1+-4+, we have used the AM1 neutral geometries since the
parameter R[eq6] is also derived from a calculation at neutral geometry; the
value taken for 1 is very close to that found in the crystal structure (12.24
Å).46 However, AM1/CI optimization of the cation geometries gives
extremely similar values (12.16, 18.77, 25.31, and 31.77 Å for 1+, 2+, 3+,
and 4+, respectively), and so the couplings obtained from eq 2 (Table 4)
are rather insensitive to whether neutral or cation values are used. b From
ref 22; calculated using AM1/UHF for the cation.
ꢀ
maxνjmaxνj1/2
x
V ) 0.0206
(4)
R
where R is in Å.3,69 A problem in applying Hush’s equation is
in determining the appropriate value of the diabatic electron-
transfer distance, R. In traditional inorganic MV systems, the
metal-metal geometric separation is often used as an ap-
proximation, but in organic systems, there is more difficulty in
defining the appropriate center of the charge-bearing unit.
Moreover, in both organic and inorganic cases, any delocaliza-
tion of the charge away from the formal charge-bearing center
toward the bridging group (for the diabatic states) will lead to
reduced R. For a class-III system, the coupling can also be
obtained directly from the absorption maximum:
surfaces at the transition (crossing point of diabatic surfaces)
geometry. An alternative estimate is given by eq 7:
∆µ212 + 4µ2ge
x
R )
(7)
e
where µge is the (experimental or calculated) transition dipole
moment between the two adiabatic surfaces at the minimum-
energy geometry, and ∆µ12 is the difference in the static dipole
moments of the two adiabatic states.70,71
2V ) νjmax
(5)
To estimate R according to eq 6, ZINDO/CIS calculations
were performed for 1+-4+ using the neutral DFT geometry
(chosen since it is symmetrical in each case); values of µ( are
given in Table 3, along with the derived values of R, the
calculated transition energies, and configuration descriptions of
the transitions. In each case, the transition between ground and
first excited states can be well-described as a HOMO-1 f
HOMO one-electron transition (using the molecular orbital
labels from the neutral electronic configuration). The HOMO
and HOMO-1 of 1 and 2 are shown in Figure 5 and consist of
different symmetry combinations of nitrogen p orbitals, with
some contribution from other atoms, particularly from the
π-bridging unit in the HOMO.
Thus, for a class-III system, one can input a value of V from eq
5 into eq 2 and solve for R (rather than estimating V from an
assumed value of R); for 1+, we obtain a value of R ) 5.67 Å,
considerably less than the geometric N-N distance of 12.23
Å, suggesting that the diabatic states cannot be regarded as
centered on the nitrogen atoms, but are centered somewhat into
the bridge. Unfortunately, analogous estimates of the appropriate
value of R cannot be made for the class-II species 2+-4+.
However, R can be estimated in several other ways from
computational and/or spectroscopic data. One possible method
is to use eq 6:27
|µ(| ) |eR/2|
(6)
We also estimated R according to eq 7 using experimental
where µ( is the calculated transition dipole between adiabatic
values of µge (see Table 2) and values of ∆µ12 obtained from
(69) Creutz, C.; Newton, M.; Sutin, N. J. Photochem. Photobiol. 1994, A82,
(70) Nelsen, S. F.; Newton, M. D. J. Phys. Chem. A 2000, 104, 10023.
(71) Johnson, R. C.; Hupp, J. T. J. Am. Chem. Soc. 2001, 123, 2053.
47.
9
J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005 16907

A R T I C L E S
Barlow et al.
Figure 5. Contour plots of the HOMO and HOMO-1 for 1 and 2 obtained at the AM1 level.
Table 4. Electronic Coupling, V (in cm-1), Calculated According to
Equation 2 (µge, νjmax, and R values are taken from Table 3), and
According to Equation 5 (for class-III species). All Data Refer to
CH2Cl2 or, in Parentheses, to CH2Cl2/0.1 M [nBu4N]+[PF6]-
AM-CI calculations70,71 of the monocation state dipole mo-
ments.72 These values are also given in Table 3; AM1 values
of RNN and of the adiabatic electron-transfer distance, R12 (∆µ12/
e), are also given. Estimates of the diabatic electron-transfer
distance, R, from either eq 6 or 7, are considerably smaller than
the geometric N-N separations, RNN, reflecting the bridge
contributions to the diabatic states (see also Figure 3). In 2+,
consistent with its assignment to class-II, the adiabatic and
diabatic electron-transfer distances are similar to one another.
In the more delocalized 1+, R12 is much smaller than R.
However, the small nonzero value of R12 of this system is more
consistent with a species on the class-II/III borderline than a
fully delocalized class-III cation. TD-DFT estimates of µ( give
qualitatively similar results (i.e, R < RNN) but somewhat smaller
values of µ( and, hence, of R are obtained; we have previously
reported a TD-DFT value of R[eq6] for L1+ to be 6.89 Å.27
We calculated values of the electronic coupling, V, for 1+
and 2+ from eq 2 using values of R from Table 3 and
experimental values of µge and νjmax from Table 2. These values
are collected in Table 4 and compared with the corresponding
values for L0+, L0′+, L1+, and L2+. The species with double
bonds in the bridge clearly show stronger electronic coupling
than the analogous triple-bonded species, consistent with
previous studies of MV systems with ferrocenyl,73,74 octahedral
Ru,75 and dimethoxybenzene14 redox centers.76 AM1/UHF
calculations for bis(dimethylamino)polyene and polyyne cations
also predict stronger coupling in the former class.4 However,
V[eq2]
R[eq6]
class
a
assumed
RNN
R[eq7]
V[eq5]
1+
III
II
III
III
II
1400
1950
2890 3020 (3140)
2+
700
1140
-
1510
-
-
L0+
L0′+
L1+
L2+
(3250b)
(1550b)
(4765b)
(3180b)
-
(1990d)
-
1080 (1200b) 1500 (1665d)
-
II
490c (500b)
-
1000e
-
a RNN values used for all except 1+ and 2+ were from AM1/UHF
b
optimizations of the cations and are taken from ref 22. µge and νjmax data
taken from ref 22. c A value of 540 cm-1 is obtained using the µge and
νjmax values from ref 25. d µge and νjmax data taken from ref 22; ZINDO/CIS
value of R[eq6] taken from ref 29. e From ref 32.
as we have previously noted,46 1+ and L1+ are, to the best of
our knowledge, the first case where an alkene/alkyne pair of
MV species fall on different sides of the class-II/III borderline.
Interestingly, the stilbene-bridged species, 1+, shows very similar
coupling to its biphenyl-bridged analogue, L0′+; presumably,
this is due to a canceling resulting from the interplay of opposing
effects, perhaps associated with different bridge lengths and
planarities. Literature precedents show that such canceling effect
is not necessarily expected; in an example with bis(dialkoxy-
benzene) redox centers, a stilbene-bridged species showed a
coupling ca. 1.4 times that of its biphenyl analogue,14 whereas,
in a ruthenium example, a species with a 4,4′-bipyridyl bridge
shows a coupling ca. 1.2 times that of a 1,2-bis(4-pyridyl)ethene
bridge.75
(72) Experimental values of ∆µ12 are, in principle, accessible through Stark
spectroscopy. Indeed, several studies have focused on the determination
of ∆µ12 for inorganic MV compounds; for example, see: Oh, D. H.; Sano,
M.; Boxer, S. G. J. Am. Chem. Soc. 1991, 113, 6880.
(73) Le Vanda, C.; Bechgaard, K.; Cowan, D. O. J. Org. Chem. 1976, 41, 2700.
(74) Ribou, A. C.; Launay, J.-P.; Sachtleben, M. L.; Li, H.; Spangler, C. W.
Inorg. Chem. 1996, 35, 3735.
We were also interested in comparing the trends in electronic
coupling obtained from various theoretical estimates. Table 5
shows couplings estimated from Koopmans’ theorem (KT),77
that is, from taking half the energy difference between the
HOMO and HOMO-1, using various computational techniques.
(75) Sutton, J. E.; Taube, H. Inorg. Chem. 1981, 20, 3125.
(76) Other donor-acceptor interactions have also been found to be more strongly
mediated through alkenes; for example, (E)-FcCHdCHB(mes)2 {Fc )
ferrocenyl, mes ) 2,4,6-trimethylphenyl} shows a lower-energy and more
intense donor-acceptor CT transition than (E)-FcCtCB(mes)2 (Yuan, Z.;
Stringer, G.; Jobe, I. R.; Kreller, D.; Scott, K.; Koch, L.; Taylor, N. J.;
Marder, T. B. J. Organomet. Chem. 1993, 452, 115).
(77) Koopmans, T. Physica 1933, 1, 104.
9
16908 J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005

Intervalence Transitions in Bis(triarylamine) Monocations
A R T I C L E S
In 3+, the IVCT overlaps with another band, which we
attribute to a bridge-to-cation (B f D+) charge transfer, while
that of 4+ is sufficiently weak that it cannot be detected under
the B f D+ band. It is worth noting that the B f D+ bands
seen in 3+ and 4+ could be easily be mis-assigned as IVCT
bands since they occur at rather long wavelength (ca. 1200 nm)
and are rather broad and somewhat solvatochromic; only by
comparison with the spectra of the corresponding dications or
with the mononuclear model compound, 5+, can their origin
be clarified.
Table 5. Estimates of V from Koopmans’ Theorem, Calculated at
Neutral Geometry Using Three Different Methods
1
V
KT/cm-
AM1 at B3LYP/6-31G*
geometry
AM1
B3LYP/6-31G*
1+
2+
3+
4+
1180
550
260
110
1840
1170
720
1390
800
430
230
440
In general, our experimental and computational data show
that the electronic coupling decreases as one moves from 1+ to
4+. The decrease in coupling in the monocations is paralleled
by a decrease in the coupling between the two triarylaminium
centers in the corresponding dications: 1+ appears to be a class-
III species and 12+ is diamagnetic; 4+ has a barely detectable
IVCT band and so can be regarded as approaching the class-I
limit, while the ESR signal intensity for 42+ is consistent with
a system with minimal coupling between two independent S )
1/2 centers. Thus, we have mapped the transition from strongly
coupled and delocalized to very weakly coupled and rather
localized behavior in both mono- and dications of these bis-
(diarylamino)phenylenevinylenes as a function of chain length.
The couplings according to the KT calculations show similar
trends irrespective of the method used; the electronic coupling
decreases more-or-less exponentially with RNN. However, the
values of the couplings are rather sensitive to the details of the
method employed, as we have found before,27 as, to a lesser
extent, are the decay constants. Interestingly, it should be noted
that the KT estimates of couplings for 1+ are somewhat lower
than those obtained for the alkyne analogue L1+,27 in contradic-
tion to the results indicated by the optical data. However, if the
KT calculation is carried out using AM1 using the symmetrical
DFT cation geometry for 1+, a coupling of 2270 cm-1 is
obtained, greater than the 1730 cm-1 obtained for L1+ using
the symmetric DFT cation geometry.27 These differences are
presumably related to the nonplanar geometry of neutral 1;
greater coupling relative to L1+ is only calculated when the
planarized cation geometry is used. We have previously seen a
similar effect, in which using KT at neutral geometry for 4,4′-
bis(di-p-anisylamino)biphenyl gives a smaller coupling than for
L1+,27 again in contradiction to optical data; the biphenyl bridge
of the former species is also twisted in the neutral molecule
and planarized in the cation.22
Experimental Details
General. Electrochemical measurements were carried out under
argon on dry (distilled from CaH2) deoxygenated dichloromethane
solutions ca. 10-4 M in analyte and 0.1 M in tetra-n-butylammonium
hexafluorophosphate using a BAS potentiostat, a glassy carbon working
electrode, a platinum auxiliary electrode, and, as a pseudo-reference
electrode, a silver wire anodized in 1 M aqueous potassium chloride.
Potentials were referenced to ferrocenium/ferrocene by using cobalto-
cenium hexafluorophosphate78 as an internal reference (since many of
the redox events of interest were at similar potentials to that of
ferrocene). Cyclic voltammograms were recorded at a scan rate of 50
mV s-1; differential pulse voltammograms were run at 20 mV s-1, with
a pulse height of 50 mV, on the same samples, to afford better resolution
of the overlapping peaks in 1 (no additional features were resolved for
2-5). UV-vis-NIR spectra were recorded in 1 cm cells using a Varian
Cary 5E spectrometer. Room-temperature ESR spectra were acquired
using a Bruker EMX spectrometer. Compound 1 was prepared as
described previously,46 while compounds L1 and L2 were prepared
by the literature procedures;22 other chemicals were either obtained
commercially or prepared as described below.
(E,E)-1,4-Bis[4-{bis(4-methoxyphenyl)amino}styryl]benzene, 2.
This compound was synthesized in 75% yield from 4-{bis(4-
methoxyphenyl)amino}benzaldehyde,79 essentially as recently described
elsewhere,53 but using tetramethyl 1,4-xylene-R,R′-diyl diphosphonate80
in place of the tetraethyl phosphonate, running the reaction in THF for
2 h (rather than in DMF overnight) and purifying the product using
column chromatography (silica gel, 4:1 hexanes/ethyl acetate). The 1H
NMR spectrum was in agreement with the published data.53
Discussion and Summary
The narrowness, asymmetry, and solvatochromism of the
IVCT band of the 4,4′-bis(di-p-anisylamino)stilbene MV cation,
1+, indicates that this species falls into a similar re´gime of
delocalization as bis(diarylamino)biphenyl cations. Several
recent crystallographic and vibrational studies,30,44 including one
of 1+, indicate that these species characterized by narrow
asymmetric IVCT bands do belong to Robin and Day’s class
III.46 In contrast, the triple-bonded analogue of 1+, L1+, appears
to belong to class II, consistent with both a previous NIR study22
and with vibrational data.46 While there is ample precedent for
stronger coupling in double-bonded versus triple-bonded sys-
tems, it is interesting that the increased coupling (ca. 30%
according to Hush analysis) in the former case is sufficient in
this case to place double- and triple-bonded systems into
different re´gimes of delocalization. Since 1+ belongs to class
III, we can estimate the coupling both from Hush theory and
from setting the coupling to half the IVCT transition energy;
comparison of these values indicates that the diabatic electron-
transfer distance in 1+ is considerably less than the geometric
N-N separation, a result consistent with quantum-chemical
estimates of the diabatic electron-transfer distance for 1+ and
also for the longer class-II homologues, 2+-4+. The 1,4-bis-
[4-(di-p-anisylamino)styryl]benzene cation, 2+, also shows
stronger coupling (ca. 40% higher) relative to its triple-bonded
analogue, L2+, although the coupling is sufficiently low that
both species belong to class II. AM1 calculations also support
the assignment of 1+ and 2+ to classes III and II, respectively.
(78) We measured the cobaltocenium/cobaltocene couple at -1320 mV versus
ferrocenium/ferrocene in dichloromethane; previous studies give values of
-1350 mV (Koelle, U.; Khouzami, F., Angew. Chem., Int. Ed. Engl. 1980,
19, 640), -1330 mV (ref 58), and -1320 mV (Barlow, S. Inorg. Chem.
2001, 40, 7047).
(79) 4-Bis(4-methoxyphenyl)aminobenzaldehyde has previously be obtained by
a variety of syntheses; we obtained it in 79% yield through the Vilsmeier
formylation of N,N-bis(4-methoxyphenyl)-N-phenylamine, which was itself
obtained using Pd-catalyzed coupling chemistry from bis(4-methoxyphenyl)-
amine and bromobenzene.
(80) This compound was synthesized essentially according to the literature
(Tewari, R. S.; Kumari, N.; Kendurkar, P. S. Ind. J. Chem. 1977, 15B,
753), but we did not find it necessary to distill the product after removing
excess trimethyl phosphite and dimethyl methylphosphonate under reduced
pressure.
9
J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005 16909

A R T I C L E S
Barlow et al.
N,N-Bis(4-methoxyphenyl)-N-(4-vinylphenyl)amine, S1.81 Potas-
sium tert-butoxide (10 mL of a 1.6 M solution in THF) was added to
4-{bis(4-methoxyphenyl)amino}benzaldehyde79 (2.00 g, 6.00 mmol)
and methyl triphenylphosphonium iodide (3.15 g, 7.80 mmol) in THF
(50 mL) at room temperature, and the reaction was stirred for 3 h. The
mixture was poured into distilled water and extracted into ethyl acetate.
The pure compound (1.73 g, 89%) was obtained after silica gel
chromatography, eluting with dichloromethane/hexane (1:1). 1H NMR
(CD2Cl2, 300 MHz): δ 7.23 (d, J ) 8.7 Hz, 2H), 7.08 (d, J ) 8.7 Hz,
4H), 6.91 (d, J ) 8.7 Hz, 2H), 6.85 (d, J ) 8.7 Hz, 4H), 6.68 (dd, J
) 17.7, 10.8 Hz, 1H), 5.58 (d, J ) 17.7 Hz, 1H), 5.10 (d, J ) 10.5
Hz, 1H), 3.81 (s, 6H). 13C NMR (CD2Cl2, 75 MHz): δ 155.75, 148.32,
140.73, 136.23, 129.83, 126.79, 126.43, 120.35, 114.57, 110.99, 55.35.
GC/MS m/z 331 (M+).
stirred for 2 h. The mixture was then poured into water and extracted
by ethyl acetate. After removing the solvent under vacuum, the crude
product was purified by crystallization from methanol. The yield of
product was 1.70 g (79%). 1H NMR (CDCl3, 300 MHz): δ 7.45 (d, J
) 8.1 Hz, 2H), 7.43 (d, J ) 7.8 Hz, 2H), 7.56 (d, J ) 8.7 Hz, 2H),
7.35 (d, J ) 8.4 Hz, 2H), 7.07 (d, J ) 16.2 Hz, 1H), 6.97 (d, J ) 16.2
Hz, 1H), 1.32 (s, 9H). 13C NMR (CDCl3, 75 MHz): δ 151.32, 136.36,
134.06, 131.61, 129.11, 127.77, 126.51, 126.20, 125.59, 120.96, 34.72,
31.34. GC/MS m/z 314 (M+). Anal. Calcd for C18H19Br: C, 68.58; H,
6.07. Found: C, 68.39; H, 6.18.
(E)-4-Formyl-4′-tert-butylstilbene, S4. To a solution of S3 (2.0 g,
6.3 mmol) in dry THF (40 mL) was added n-butyllithium (4 mL of a
2.5 M solution in hexane, 10.0 mmol) and stirred at -78 °C for 30
min. DMF (0.7 g, 10 mmol) was then added dropwise at -78 °C, and
the reaction was stirred for 2 h at room temperature. The resulting
solution was poured into water (50 mL) and extracted with ethyl acetate.
After removing the solvent under vacuum, the crude product was
purified by column chromatography, eluting with hexane, to give 1.0
(E,E,E)-4,4′-Bis[4-{bis(4-methoxyphenyl)amino}styryl]stilbene, 3.
(E)-4,4′-dibromostilbene46,82 (0.77 g, 2.27 mmol) and S1 (1.50 g, 45.0
mmol) were dissolved in DMF (10 mL) under nitrogen, and tri-o-
tolylphosphine (0.05 g, 0.16 mmol), triethylamine (3 mL), and
palladium(II) acetate (0.02 mg, 0.09 mmol) were added. The reaction
mixture was heated to 90 °C for 24 h. After the mixture was cooled to
room temperature, it was poured into ethanol. The precipitate formed
was collected on a filter and washed thoroughly with ethanol. The crude
product was purified by column chromatography, eluting with hexane/
1
g (60%) of a pale yellow powder. H NMR (CDCl3, 300 MHz): δ
9.97 (s, 1H), 7.84 (d, J ) 8.1 Hz, 2H), 7.63 (d, J ) 7.2 Hz, 2H), 7.48
(d, J ) 8.4 Hz, 2H), 7.40 (d, J ) 8.1 Hz, 2H), 7.24 (d, J ) 16.2 Hz,
1H), 7.09 (d, J ) 16.2 Hz, 1H), 1.32 (s, 9H). 13C NMR (CDCl3, 75
MHz): δ 191.46, 151.68, 143.54, 134.99, 133.63, 131.93, 130.13,
126.56, 125.85, 125.45, 115.51, 34.79, 31.31. GC/MS m/z 264 (M+).
Anal. Calcd for C19H20O: C, 86.32; H, 7.63. Found: C, 86.47; H, 7.55.
(E,E)-1-(4-Bromostyryl)-4-(4-tert-butylstyryl)benzene, S5. To a
solution of S4 (1.0 g, 3.8 mmol) and diethyl 4-bromobenzylphosphonate
(1.3 g, 4.5 mmol) in THF (30 mL) was added potassium tert-butoxide
(0.73 g, 7.6 mmol) at room temperature and stirred for 2 h. The mixture
was poured into methanol and filtered, and then washed with methanol.
The crude product was purified by crystallization from hot methanol
1
ethyl acetate (8:1), to afford an orange solid (1.70 g, 89%). H NMR
(CD2Cl2, 500 MHz): δ 7.55 (d, J ) 9.0 Hz, 4H), 7.52 (d, J ) 8.5 Hz,
4H), 7.38 (d, J ) 8.5 Hz, 4H), 7.17 (s, 2H), 7.12 (d, J ) 16.0 Hz, 2H),
7.11 (d, J ) 9.0 Hz, 8H), 7.00 (d, J ) 16.0 Hz, 2H), 6.92 (d, J ) 8.5
Hz, 4H), 6.89 (d, J ) 9.0 Hz, 8H), 3.78 (s, 12H). 13C NMR (CD2Cl2,
125 MHz): δ 156.57, 148.87, 140.93, 137.60, 136.57, 129.68, 128.58,
128.15, 127.54, 127.16, 127.12, 126.81, 125.66, 120.38, 115.03, 55.80.
HRMS (EI) calcd for C58H50N2O4: 838.3771. Found: 838.3736. Anal.
Calcd for C58H50N2O4: C, 83.03; H, 6.01; N, 3.34. Found: C, 83.38;
H, 6.03; N, 3.00.
1
to give 1.0 g (63%) of a pale yellow powder. H NMR (CD2Cl2, 500
MHz): δ 7.54 (s, 4H), 7.49 (app. d, app. J ) 7.5 Hz, 4H), 7.42 (m,
4H), 7.15 (m, 4H), 1.38 (s, 9H). 13C NMR (CD2Cl2, 125 MHz): δ
151.34, 137.65, 136.74, 136.50, 134.91, 132.24, 129.46, 129.04, 128.36,
127.82, 127.53, 127.29, 127.20, 126.69, 126.07, 121.69, 35.06 31.70.
HRMS (EI) calcd for C26H25Br: 416.1140. Found: 416.1132. Anal.
Calcd for C26H25Br: C, 74.82; H, 6.04. Found: C, 74.77; H, 6.11.
(E,E,E)-1-{Bis(4-methoxyphenyl)amino}-4-[4-{4-(4-tert-butylstyryl)-
styryl}styryl]benzene, 5. S5 (0.20 g, 0.55 mmol) and S1 (0.18 g, 0.55
mmol) were dissolved in DMF (20 mL) under nitrogen, and tri-o-
tolylphosphine (2 mg, 0.065 mmol), triethylamine (1 mL), and
palladium(II) acetate (1.00 mg, 0.044 mmol) were added. The reaction
mixture was heated to 110 °C for 24 h. After the mixture was cooled
to room temperature, it was poured into methanol. The precipitate
formed was collected on a filter and washed thoroughly with ethanol.
The crude product was purified by column chromatography on silica
gel, eluting with hexane and dichloromethane (10/1), to give 0.17 g
(E,E)-1,4-Bis(4-bromostyryl)benzene, S2.83 Potassium tert-butoxide
(2.0 g, 16 mmol) was added to a solution of 4-bromobenzaldehyde
(2.46 g, 13.0 mmol) and tetraethyl 1,4-xylene-R,R′-diyl diphosphonate
(2.00 g, 5.00 mmol) in THF (30 mL) over 2 h, during which time the
product precipitated out as a yellow solid. The mixture was stirred at
room temperature for an additional 1 h. The solid was filtered and
washed several times with ethanol. The yield of product was 2.90 g
(50% assuming it to be pure material). This compound was used in
the next reaction without further characterization due to low solubility.
(E,E,E,E)-1,4-Bis(4-[4-{bis(4-methoxyphenyl)amino}styryl]styryl)-
benzene, 4. This compound was prepared in the same way as 3 from
S2 (0.44 g, 1.00 mmol assuming pure material), S1 (0.70 g, 2.00 mmol),
tri-o-tolylphosphine (0.025 g, 0.082 mmol), triethylamine (2 mL), and
palladium(II) acetate (0.01 g, 0.045 mmol). The crude product was
purified by column chromatography, eluting with dichloromethane, to
afford an orange solid (0.78 g, 41%). 1H NMR (CDCl3, 300 MHz): δ
7.49 (s, 4H), 7.46 (s, 8H), 7.31 (d, J ) 8.1 Hz, 4H), 7.10 (s, 4H), 7.05
(d, J ) 9.3 Hz, 8H), 7.02 (d, J ) 16.2 Hz, 2H), 6.92 (d, J ) 16.2 Hz,
2H), 6.89 (d, J ) 9.0 Hz, 4H), 6.83 (d, J ) 9.0 Hz, 8H), 3.79 (s, 12H).
13C NMR (CD2Cl2, 125 MHz): δ 156.61, 140.92, 137.72, 137.17,
136.53, 128.66, 128.52, 128.08, 127.57, 127.20, 126.84, 125.66, 120.39,
115.05 (the compound is poorly soluble, and we were unable to observe
the remaining four expected aromatic/vinylic 13C peaks), 55.83. HRMS
(EI) calcd for C66H56N2O4: 940.4240. Found: 940.4241. Anal. Calcd
for C66H56N2O4: C, 84.23; H, 6.00; N, 2.98. Found: C, 83.91; H, 6.17;
N, 3.05.
1
(45%) as a yellow powder. H NMR (CD2Cl2, 500 MHz): δ 7.57 (s,
4H), 7.55 (d, J ) 7.0 Hz, 2H), 7.52 (d, J ) 8.5 Hz, 2H), 7.44 (d, J )
8.0 Hz, 2H), 7.39 (d, J ) 9.0 Hz, 2H), 7.19 (s, 2H), 7.16 (d, J ) 9.5
Hz, 2H), 7.20-7.10 (m, 3H), 7.11 (d, J ) 8.5 Hz, 4H), 7.02 (d, J )
16.2 Hz, 1H), 6.91 (d, J ) 8.7 Hz, 2H), 6.89 (d, J ) 7.0 Hz, 4H), 3.84
(s, 6H), 1.33 (s, 9H). 13C NMR (CD2Cl2, 125 MHz): δ 156.61, 151.37,
148.82, 140.95, 137.70, 137.28, 137.02, 136.55, 134.89, 128.69, 128.61,
128.43, 128.11, 127.68, 127.56, 127.20, 127.15, 127.10, 126.83, 126.56,
126.05, 125.66, 120.39, 115.06 (the remaining two expected aromatic/
vinylic 13C peaks not observed, presumably due to overlap), 55.83,
34.92, 31.40. MS (EI) m/z 667 ([M]+, 100%), 652 ([M - CH3]+, 15%),
637 ([M - 2CH3]+, 13%), 565 (10%). Anal. Calcd for C48H45NO2: C,
86.32; H, 6.79; N, 2.10. Found: C, 85.96; H, 6.89; N, 1.86.
(E)-4-Bromo-4′-tert-butylstilbene, S3. To a solution of 4-tert-
butylbenzaldehyde (1.1 g, 6.8 mmol) and diethyl 4-bromobenzylphos-
phonate (2.0 g, 6.8 mmol) in THF (40 mL) was added potassium tert-
butoxide (1.3 g, 13.6 mmol) at room temperature; the reaction was
Chemical Oxidation of 1-5, L1, and L2. Monocations and
dications of the bis(triarylamine) species and 5+ were generated in
solution by addition of appropriate amounts of tris(4-bromophenyl)-
aminium hexafluoroantimonate (Aldrich) in dry solvents (<0.1 equiv
for monocations, 2 equiv for dications). Monocations were generated
(81) Ferrar, W. T.; Jin, X.; Sorriero, L. J.; Weiss, D. S. Eur. Patent Application,
2004.
(82) Baumgarten, M.; Yuksel, T. Phys. Chem. Chem. Phys. 1999, 1, 1699.
9
16910 J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005

Intervalence Transitions in Bis(triarylamine) Monocations
A R T I C L E S
by addition of [(4-BrC6H4)3N]+[SbCl6]- to large excesses of the neutral
species; absorptivities were calculated assuming all of the oxidizing
agent added results in formation of monocation and that dispropor-
tionation is negligible at these concentration ratios. Consistent with
these assumptions, we do not observe unreacted oxidizing agent in the
optical spectra, nor do we see evidence for disproportionation for 1+
and 2+, where the dication absorbs at very different energy to the
monocation. In the case of 3+ and 4+, where mono- and dication spectra
are closely overlapping, we established that we were in a re´gime of
negligible disproportionation by showing that the spectra obtained were
independent of the exact ratio of oxidizing agent to neutral molecule
used. Dications were generated by addition of 2 equiv of [(4-
BrC6H4)3N]+[SbCl6]- to the neutral species, and the electron-transfer
reaction was again assumed to be complete in calculating absorptivities.
Neither the side product, (4-BrC6H4)3N, nor neutral 1-5 absorb in the
energy range plotted in Figure 2. Optical and ESR spectra for 12+ were
also recorded using the isolated bis(hexafluoroantimonate) salt described
elsewhere.60
Computational Methods. Geometry optimizations for neutral
molecules were performed using both the semiempirical Hartree-Fock
Austin Model 1 (AM1) and Density Functional Theory (DFT) methods.
The DFT calculations were carried out using the B3LYP functional,
in which Becke’s three-parameter hybrid exchange functional is
combined with the Lee-Yang-Parr correlation functional,84-86 with
a 6-31G* split valence plus polarization basis set. Correlated semi-
empirical AM1/CI and AM1-UHF methods were utilized for the
investigations of the radical cations. While AM1-UHF predicts asym-
metric geometries for each of the molecular systems investigated, the
technique suffered from high degrees of spin contamination ( S2
∼
5). Therefore, only the AM1/CI results are reported. The configuration-
interaction (CI) space for the coupled AM1/CI method was restricted
to the orbitals corresponding to the HOMO-1 and HOMO in the neutral
species for each molecular system. In addition, UB3LYP/6-31G* was
used for optimization of the geometry of 1+.
Excitation energies and transition dipole moments for 1+-4+ were
calculated with the correlated semiempirical Zerner’s intermediate
neglect of differential overlap (ZINDO/CIS) method.87 The calculations
were executed using the radical cation electronic configuration with
the neutral geometry obtained from AM1. The use of the optimized
radical cation geometries led to large degrees of spin contamination
and, hence, unreliable results. All AM1-based methods were carried
out using the implementation in the AMPAC88 software package, while
the DFT and ZINDO calculations were performed with Gaussian98.89
Acknowledgment. This material is based upon work sup-
ported in part by the STC Program of the National Science
Foundation under Agreement Number DMR-0120967 and by
the NSF Grant CHE-0342321.
Supporting Information Available: Calculated geometric
parameters for 1-4 and 1+-4+; full atomic coordinates and
absolute energies for computed geometries; complete citations
for refs 44, 46, 60, and 89. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA054136E
(83) Hu, Q.-S.; Vitharana, D.; Liu, G.-Y.; Jain, V.; Wagaman, M. W.; Zhang,
L.; Lee, T. R.; Pu, L. Macromolecules 1996, 29, 1082.
(84) Becke, A. D. Phys. ReV. 1988, A38, 3098.
(87) Zerner, M. C.; Loew, G. H.; Kichner, R. F.; Mueller-Westerhoff, U. T. J.
Am. Chem. Soc. 1980, 102, 589.
(88) AMPAC 6.55 User’s Manual; Semichem: 7128 Summit, Shawnee, KS
66216, 1997.
(85) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(86) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. 1988, B37, 785.
(89) Frisch, M. J. et al. Gaussian 98; Gaussian Inc.: Pittsburgh, PA, 1995.
9
J. AM. CHEM. SOC. VOL. 127, NO. 48, 2005 16911