In conclusion, we demonstrated that the 1,3-diploar
cycloadducts 2, 3 and 5 are metallochromic. While in the case
of 2 the triazole nitrogen must be the coordinating site, we
conclude from our studies of 3 and 5 that in these the pyridine
units are the primary sites of complexation. We deem the
structural motif of 3 particularly attractive, as it represents a
modification of the bipyridine units often employed as metal
coordinating sites. The surprising photophysics of 3 and 5a
can be best explained by assuming that their electron density is
significantly influenced by the resonance structures I and II,
not accessible for 2 or 4.
The authors thank the Petroleum Research Funds for
generous financial support. The authors also wish to thank
Fig. 5 Titration of 3 with TFA in emission (top) and absorption
Jan Schonhaber and Anthony J. Zucchero who provided
¨
3,5-bistrifluoromethylphenylacetylene and 4-ethynylpyridine.
(bottom).
Notes and references
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Fig. 6 Titration of 4 with TFA in absorption.
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differentiation in metallochromic behavior. If the resonance
structure I already contributes in the ground state, its con-
tribution would be expected to be even higher in the excited
state, leading to an enhanced excited state basicity of 3 and 5a.
Indeed, examining Fig. 5, one can qualitatively see that
changes in emission precede those in absorption upon addition
of TFA to a solution of 3 or 5a (ESIw). The 2- or 4-pyridine
group is important. There is a large bathochromic shift upon
addition of metals, both in 3 and in 5a but not in 2 or 4,
suggesting that the metal ions are primarily bound to the
pyridine unit as shown in II and III for 3.
However, in the absence of the pyridine groups the triazole in 2
can work as a metal ligating entity, but the metal cation
induced changes in absorption are small.
12 B. C. Englert, S. Bakbak and U. H. F. Bunz, Macromolecules,
2005, 38, 5868.
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2203–2205 | 2205