FULL PAPER
charge-transfer complex between the viologen and the elec-
tron-rich second guest. As a result of these properties,
CB[8] has been used in a diverse range of applications, in-
cluding molecular machines,[26] polymeric materials,[27,28] and
other molecular assemblies.[23] Moreover, the CB[8]·MV
complex has been shown to bind selectively to tryptophan
(Trp), phenylalanine, and tyrosine, and to peptides contain-
ing Trp, with preference for the N-terminal position.[29,30]
Unfortunately, MV has a number of drawbacks for certain
applications. It is toxic, susceptible to demethylation, and a
strong oxidizer (E1/2 =À0.7 V versus saturated calomel elec-
trode; SCE).[31–33] Although the latter feature of MV enables
applications involving reversible “switching” behav-
ior,[23,26,34–37] it can lead to incompatibilities with many
redox-active systems, synthetic protocols, and experimental
conditions.[31,38–43] Hence, there is great interest in finding al-
ternatives to MV,[38,44] particularly those which facilitate
tuning of recognition and optical properties, while minimiz-
ing potential toxicity and unwanted reactivity. Here we ex-
plore the use of tetramethylbenzobis(imidazolium) (MBBI)
as an available alternative for MV.
As shown in Figure 1, MBBI is slightly shorter (11.0
versus 13.0 ꢂ) and wider (8.1 versus 6.6 ꢂ) than MV. The
positively charged nitrogen atoms of MBBI are separated
by 5.1 versus 7.2 ꢂ in MV. These distances compare favora-
bly to a separation of 6.1 ꢂ between carbonyl oxygens on
opposite portals of CB[8]. However, while the portal–portal
separation is identical among the CB[n] homologues, the
cavity sizes depend on the number of glycoluril units in the
ring. As such, MBBI was expected to fit well inside the
8.8 ꢂ inner diameter cavity of CB[8] but less stably in the
7.3 ꢂ inner diameter cavity of CB[7]. Moreover, while
MBBI and MV share many similar structural and physical
characteristics (Table 1), the former offers numerous differ-
ences that are potentially advantageous, including intrinsic
fluorescence, high chemical and electrochemical stability,
and multiple options for modulating structural and optoelec-
tronic properties. Collectively, these features should effec-
tively expand and enrich applications of CB[n] complexes.
Figure 1. Structures of MBBI (top left), MV (top right), and CB[8]
(bottom) including space-filling models. Guest structures show total dis-
tances (including van der Waals radii, in ꢂ) to the right and below each
structure, and nitrogen-to-nitrogen internuclear distances to the left of
each structure. The O–O interportal distance is shown for CB[8].
architecture is ideally suited for such applications partly be-
cause it consists of two imidazolium moieties annulated to a
common arene linker[4] and may be finely tuned by synthetic
modification at multiple, independent sites.[5] Primary, sec-
ondary, tertiary and aryl groups have been installed as N-
substituents, and the C2 position can be functionalized with
aryl or heterocyclic groups, resulting in an extensive reper-
toire of molecules.[6] BBIs are also chemically and thermally
stable with decomposition temperatures typically between
270–3408C.[7] As a result of this high modularity and stabili-
ty, they have found use as phase-tunable fluorophores[5,8]
(lem = 329–561 nm, Ffs up to 0.91), crosslinkers for well-de-
fined block ionomers,[9] and repeat units for polyelectro-
lytes,[10] in addition to their well-documented use as precur-
sors for bis(N-heterocyclic carbene)s[4] and corresponding
organometallic and other complexes.[11–17] Orthogonal to
these myriad applications, we demonstrate here that BBIs
are also well suited for applications in supramolecular
chemistry, particularly in combination with the cucurbit[-
n]uril (CB[n]) family of synthetic macrocycles.[18,19]
CB[n]s have received considerable attention due to their
abilities to bind selectively to a wide variety of substrates
over an exceptionally large range of affinities, with equilibri-
um association constants (Ka) as high as 1016 mÀ1 in aqueous
media.[19–21] The repeating methylene-bridged glycoluril units
present a ring of ureido carbonyl groups at both constricted
entrances to the nonpolar cavity, thus selecting for guests
that can include a nonpolar component inside the CB[n]
cavity and position a cationic group proximal to the carbon-
yl oxygens. Moreover, CB[8] and larger homologues can
bind two guests simultaneously.[18,21–25] For example, not only
is CB[8] known to bind strongly to methyl viologen (MV)
(Ka =106 mÀ1), but the resulting CB[8]·MV complex is also
capable of binding to a range of aromatic second guests, in-
cluding naphthalene, catechol, and indole.[22,23] Binding to
indole and naphthalene is accompanied by the quenching of
their fluorescence, as well as the formation of a visible
Results and Discussion
Binary complexes of MBBI: Our efforts toward exploring
the abilities of MBBI to bind to CB[7] and CB[8] began
with a series of isothermal titration calorimetry (ITC) as
well as 1H NMR spectroscopy, fluorescence, and electro-
spray mass spectrometry (ESI-MS) experiments. As shown
in Figure 2A, the ITC experiments (10 mm sodium phos-
phate, pH 7.0) revealed that, similar to MV, MBBI binds to
both hosts in a 1:1 stoichiometry, a result confirmed by ESI-
MS (see Figures S4 and S5), and with favorable enthalpic
and entropic binding contributions. MV has a high affinity
for both hosts, with a 7.8-fold preference for CB[7] (Ka =
6.6ꢀ106 mÀ1)[43] over CB[8] (8.5ꢀ105 mÀ1).[29] By contrast,
MBBI has a strong affinity for only CB[8] (Ka =5.7ꢀ
105 mÀ1) with an 86-fold selectivity over CB[7] (Ka =6.6ꢀ
Chem. Eur. J. 2010, 16, 13716 – 13722
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13717