Table 1 Energy (cm21), bandwidth (cm21) and transition dipole
moment (D) for the IVCT for some MV bis(triarylamine) species, with
estimated electronic couplings, V, (cm21; see text for definitions)a
possibilities of a class II species with only slight asymmetry, or in
which asymmetry is masked by static or dynamic disorder.
Both [3]+ and [4]+ appear delocalised on the EPR timescale,{ but
the IR spectrum of a CH2Cl2 solution of [3]+ shows a CMC stretch
at 2135 cm21, indicating a lack of inversion symmetry (no CMC
stretch is seen for centrosymmetric 3), i.e. that [3]+ belongs to class
II. For [4]+, such a simple vibrational probe is not available;
however, observed IR and Raman frequencies are in good
agreement with those calculated by DFT for a symmetric [4]+
structure. Moreover, the lack of coincidences in the IR and Raman
spectra{ indicates a centrosymmetric, or effectively centrosym-
metric, structure for [4]+.
b
1/2
v
v
¯
v
1/2[Hush]c Asymmetryd mge
¯
VH
VclassIII
¯
max
[2a]+e 6360 3170
[3]+e 6190 3960
[4]+ 6280 2900
3830
3780
3810
1.45
1.20
1.34
11.6 1550
11.6f 1200f
13.5 1400
3180
—
3140
a
All in CH2Cl2/0.1 M [nBu4N]+[PF6]2, apart from mge and VH
values for [4]+ which are in CH2Cl2. Observed width at half-height.
b
c
d
Bandwidth according to the relation v 5 !(2310vmax).10 Ratio
of width on high-energy side to the low-energy side. Data from ref.
¯
¯
1/2
e
f
2
We find 11.3 D and 1080 cm21 in CH2Cl2.
The near-IR IVCT bands of of [3]+ and [4]+, generated by in situ
chemical oxidation in CH2Cl2, are compared in Fig. 2.9 The
solvatochromism of the IVCT band of [4]+ (blue shift of 930 cm21
between CH2Cl2 and MeCN) is much closer to that of class-III [1]+
cation4 (blue shift of 860 cm21) than to that of [3]+ (blue shift of
2170 cm21), suggesting [4]+ to be more delocalised than [3]+.{
Table 1 compares the lineshapes of some IVCT bands.9 [3]+ shows
a fairly symmetric band with a bandwidth close to that predicted
by Hush theory for a class II system.10 In contrast, [4]+ shows a
narrower and less symmetric band, very similar to those of [2a]+
and [2b]+ (which vibrational data indicate is class III),5b supporting
assignment to class III. Vibronic coupling simulations of the IVCT
of [4]+ indicate significant coupling of the electron transfer to
symmetric vibrations, as was the case for [2a]+.3a More evidence
for the similarity of [2a]+ and [4]+ comes from DFT calculations of
the reorganisation energies associated with the symmetrical modes;
the respective estimates of 1380 and 1490 cm21 are nearly identical.
The electronic coupling between two redox centers can be
evaluated according to the Hush equation:10
again indicate very similar degrees of delocalisation in the class-III
species [2a]+ and [4]+ (the discrepancy between VH and VclassIII
values indicating that the appropriate value of R is considerably
less than the N–N distance). This similarity suggests the effects of a
longer N–N separation in [4]+ on the electronic coupling are more-
or-less compensated by the effects of greater planarity.11
The observation of stronger coupling through CHLCH vs. CMC
is not unexpected; previous studies have shown this for both
inorganic12 and organic13 MV systems. However, the current
system is unprecedented in that the alkene and alkyne species
clearly fall into very different re´gimes of delocalisation, i.e. class III
and II, respectively. Moreover, [4]+ has the longest bridging group
of any class-III bis(triarylamine) studied to date, and allows us to
map the class II/III borderline more completely for this type of
compound.
This material is based upon work supported in part by the STC
Program of the National Science Foundation under Agreement
Number DMR-0120967. We also thank Liqun Wang for help with
Raman measurements.
VH 5 mgevmax/(eR)
¯
(1)
Stephen Barlow,*ab Chad Risko,a Veaceslav Coropceanu,a
Neil M. Tucker,b Simon C. Jones,a Zerubba Levi,a Viktor N. Khrustalev,c
Mikhail Yu. Antipin,c Tiffany L. Kinnibrugh,d Tatiana Timofeeva,d
Seth R. Marderab and Jean-Luc Bre´dasa
where R is the donor–acceptor separation (diabatic states), e is the
electronic charge, and the transition dipole moment, mge, of the
IVCT band is determined by integration of the band. A problem
with using eqn. (1) is in defining the appropriate value of R (i.e. in
defining the appropriate center of the charge-bearing unit). The
values of VH given in the Table are estimates obtained by equating
R to the geometric N–N distances. Whilst these values for [2a]+
and [4]+ are not directly comparable due to the different media and
due to the questionable validity of equating R to the N–N distance,
these values do support the idea of comparable delocalisation in
the biphenyl and stilbene species, with weaker coupling in the
aSchool of Chemistry and Biochemistry and Center for Organic
Photonics and Electronics, Georgia Institute of Technology, Atlanta,
GA 30332-0400, USA. E-mail: stephen.barlow@chemistry.gatech.edu
bDepartment of Chemistry, University of Arizona, Tucson, AZ 85721,
USA
cInstitute of Organoelement Compounds, Russian Academy of Sciences,
Moscow, Russia
dDepartment of Natural Sciences, New Mexico Highlands University,
Las Vegas, NM 87701, USA
tolane. For class III systems, V is also equal to vmax/2; this would
¯
Notes and references
{ Crystal data: for 4 (from hexane): C42H38N2O4, M 5 634.74, monoclinic,
˚
a 5 10.788(3), b 5 12.557(3), 12.734(3) A, b 5 105.886(6)u, U 5
3
˚
1659.1(7) A , T 5 120 K, space group P21/c, Z 5 2, m(Mo–Ka) 5
0.081 mm21, 12024 reflections measured, 4027 unique (Rint 5 0.0408),
R(F2) 5 0.0575 for 1907 data with I . 2s(I), wR(F2) 5 0.1205 (all unique
data). For [4]+[SbF6]2 (from CH2Cl2/Et2O layering): C42H38F6N2O4Sb,
˚
M 5 870.49, monoclinic, a 5 10.5478(10), b 5 35.449(3), 11.1632(11) A,
3
˚
b 5 116.442(2)u, U 5 3737.3(6) A , T 5 120 K, space group P21/n, Z 5 4,
m(Mo–Ka) 5 0.814 mm21, 43844 reflections measured, 10983 unique
(Rint 5 0.0329). R(F2) 5 0.0613 for 8265 data with I . s(I),
wR(F2) 5 0.1520 (all unique data).CCDC reference numbers 252359 and
graphic data in .cif or other electronic format.
Fig. 2 Intervalence bands of [3]+ (broken line) and [4]+ (solid line)
generated in CH2Cl2 by in situ oxidation with [(p-BrC6H4)3N]+[SbCl6]2.
1 J. Bonvoisin, J.-P. Launay, M. van der Auweraer and F. C. De
Schryver, J. Phys. Chem., 1994, 98, 5052.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 764–766 | 765