F. M. Pfeffer et al. / Tetrahedron Letters 46 (2005) 6579–6584
6583
Acknowledgements
We thank IRCSET (basic research grant scheme) Dea-
kin University and Trinity College Dublin for their
financial support, Dr. Gail Dyson for assistance with
NMR and Sarah Tonkin for assistance with molecular
modelling.
References and notes
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Figure 7. Tꢀhe proposed structure of the 1:1 adduct formed between 3
and H2PO4 showing H-bonded pairs.
hydrogꢀen bond donors. Despite this, the binding of
H2PO4 to 3 is stronger than to the simple aromatic
thiourea 1 and only the more electron withdrawing
trifluoromethyl substituted aromatic thiourea 2 (logb =
4.0) is more powerful. This clearly demonstrates the
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H2PO4 to the amine gave rise to large changes in the
1H NMR, no significant luminescence changes were ob-
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most likely due to the longer spacer used in 3 compared
to that in 1 and 2, which reduces the PET quenching effi-
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as electron rich as the aromatic analogue used in 1 and
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In conclusion, several specific features of 3 and those
of the anions it binds may be used to explain the
results observed from 1H NMR studies. Firstly, the
complementarity of the ꢀY-shapedꢁ AcOꢀ anion and
the thiourea receptor dictates the preference of AcOꢀ
for this site. The naphthalimide N–H in 3 appears,
therefore, to be incapable of cooperatively participating
in this interaction on steric grounds and this is sup-
ported by the molecular modelling study. However,
ꢀ
the tetrahedral H2PO4 anion has two further oxygen
atoms available for hydrogen bonding in addition to
the two that interact with the thiourea moiety. The
increased degree of freedom and the relaxed steric
restrictions associated with this allow 3 to use the
naphthalimide N–H to participate cooperatively in
binding.
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In summary, we have shown that naphthalimide N–H
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H2PO4 anion and give rise to significant enhancement
in binding affinity in 3 when compared to 1 and 2. We
are currently evaluating frameworks that can elicit a
distinctive visible or luminescent response following
efficient binding of anions using receptors based
upon 3.
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