Angewandte
Communications
Chemie
Mechanochemistry
Probing Force with Mechanobase-Induced Chemiluminescence
Jess M. Clough, Abidin Balan, Tom L. J. van Daal, and Rint P. Sijbesma*
Abstract: Mechanophores capable of releasing N-heterocyclic
carbene (NHC), a strong base, are combined with triggerable
chemiluminescent substrates to give a novel system for
mechanically induced chemiluminescence. The mechano-
phores are palladium bis-NHC complexes, centrally incorpo-
rated in poly(tetrahydrofuran) (pTHF). Chemiluminescence is
induced from two substrates, adamantyl phenol dioxetane
lower forces may be characterized. However, direct synthetic
modification of the dioxetane structure is far from trivial:
thermally more labile dioxetanes are generally more difficult
to synthesize and may not necessarily be mechanically more
labile.
We set out to develop an alternative approach to
mechanoluminescence that obviates the need for complex
synthetic modification of the dioxetane mechanophore. The
rich array of existing substrates for highly efficient, chemically
triggerable chemiluminescence served as our source of
(
APD) and a coumaranone derivative, upon sonication of
dilute solutions of the polymer complex and either APD or the
coumaranone. Control experiments with a low molecular
weight Pd complex showed no significant activation and the
molecular weight dependence of the coumaranone emission
supports the mechanical origin of the activation. The develop-
ment of this system is a first step towards mechanolumines-
cence at lower force thresholds and catalytic mechanolumi-
nescence.
[16]
inspiration.
We envisaged that it would be possible to
activate such substrates mechanochemically. In the course of
our research, complexes of transition metals and polymers
end-functionalized with N-heterocyclic carbene ligands have
[4,5,17]
proven to be excellent latent sources of carbenes,
which
are both highly basic and nucleophilic. In the present work we
make use of their high basicity to deprotonate a substrate. The
primary advantage of such a system, in which the mechano-
phore is decoupled from the chemiluminescent emission, is
that transition-metal complexes with thermal stabilities
similar to weak covalent bonds are expected to have lower
stability under force by virtue of the longer CÀM bond with
D
eveloping molecular force probes for the high-sensitivity
detection of small forces would enable the study of a broad
array of mechanical phenomena, particularly in soft materials
and biological systems. In the past decade, polymer mecha-
nochemistry has opened up many new mechanoresponsive
[
1–5]
[6,7]
[18]
behaviors,
of which mechanically induced fluorescence
a wider potential well. This prediction can be rationalized
[
8]
and mechanoluminescence have emerged as valuable indi-
cators for stress in polymeric materials. Whilst the former is
now a highly established force-sensitive transformation,
by representing force with a “tilted potential energy surface”,
on which the barrier for scission of the bond with the wider
potential well decreases more with applied force. Addition-
ally, much higher quantum yields are achievable with the
precursor chemiluminescent substrates than with alkyl-sub-
[7,9–11]
mechanoluminescence is preferred in situations where sensi-
tivity and time resolution are critical, as no excitation signal is
required to visualize the signal and light is emitted directly on
bond scission. The analytical scope of mechanoluminescence
now spans a wide range of polymers, from common engineer-
ing materials such as polyacrylates and thermoplastic elas-
[16,19]
stituted dioxetanes such as the bis(adamantyl)dioxetane,
boosting sensitivity.
To test the generality of the concept, we selected two
different chemiluminescent probes (Scheme 1): a spiroada-
mantyl-substituted phenolic 1,2-dioxetane, 3-(4-methoxy-
[
12]
tomers to novel designer systems, including multiple inter-
[13]
penetrating network elastomers
cross-linked gels.
and supramolecularly
spiro[1,2-dioxetane-3,2’-tricyclo[3.3.1.13,7]decan]-4-yl)phe-
[
14]
[20,21]
nol 1,
and a 2-coumaranone derivative, ethyl (5-fluoro-2-
[22,23]
Despite this progress, only one luminescent mechano-
phore has been developed to date, bis(adamantyl)dioxetane,
and its mechanical reactivity is ultimately determined by the
strength of the dioxetane bond, which has an activation
energy barrier to decomposition of approximately
oxo-2,3-dihydrobenzofuran-3-yl) carbamate 2.
When
exposed to a chemical base as stimulus, both substrates
form a high-energy intermediate which gives out light upon
decomposition under ambient conditions. In the case of 1,
deprotonation of the phenol group affords an unstable
À1
[15]
[24]
1
50 kJmol (at zero force).
It would be desirable to
intermediate dioxetane
that decomposes within minutes
lower the force threshold at which mechanoluminescence is
activated, in part so that mechanical processes operative at
(by comparison, the TBDMS-protected derivative of the
[25]
phenol has a half-life of 4 years)
with the concomitant
emission of blue light with a high excitation yield (total
[26]
quantum yield 12% in acetonitrile). Chemiluminescence of
the other probe, coumaranone 2, is triggered by deprotona-
tion of the lactone at the a-position. Subsequent reaction with
oxygen generates an unstable dioxetanone intermediate in
[
*] J. M. Clough, Dr. A. Balan, T. L. J. van Daal, Prof. Dr. R. P. Sijbesma
Laboratory of Macromolecular and Organic Chemistry and the
Institute for Complex Molecular Systems,
Eindhoven University of Technology
P.O. Box 513, 5600 MB, Eindhoven (The Netherlands)
E-mail: r.p.sijbesma@tue.nl
[27]
situ, which decomposes to give out bright blue light.
Metal NHC complexes are well-known as thermally latent
[28]
catalysts and bases. Our group has reported on the use of
silver(I) and ruthenium(II) bis-NHC complexes centrally
Angew. Chem. Int. Ed. 2016, 55, 1445 –1449
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1445
Clough, Jess M.
