Excimer Disaggregation Enhanced Emission: A Fluorescence “Turn-On” Approach to Oxoanion Recognition

A new approach to anion sensing that
involves excimer disaggregation induced emission (EDIE) is reported.
It involves the anion-mediated disaggregation of the excimer formed
from a cationic macrocycle. This leads to an increase in the observed
fluorescence intensity. The macrocycle in question, cyclo[1]N2,N6-dimethyl-N2,N6-bis­(6-(1H-imidazolium-1-yl)­pyridin-2-yl)­pyridine-2,6-diamine­[1]­1,4-dimethylbenzene
(12+; prepared as its PF6– salt), is obtained in ca. 70% yield via a simple cyclization. X-ray
diffraction analyses of single crystals revealed that, as prepared,
this macrocycle exists in a supramolecular polymeric form in the solid
state. Macrocycle 12+ is weakly fluorescent
in acetonitrile. The emission intensity is concentration dependent,
with the maximum intensity being observed at [12+] ≈ 0.020 mM. This finding is ascribed to formation of an
excimer, followed possibly by higher order aggregates as the concentration
of 12+ is increased. Addition of tetrabutylammonium
pyrophosphate (HP2O73–) to 12+ (0.020 mM in acetonitrile) produces a ca. 200-fold
enhancement in the emission intensity (λex = 334
nm; λem = 390–650 nm). These findings are
rationalized in terms of the HP2O73– serving to break up essentially non-fluorescent excited-state dimers
of 12+ through formation of a highly fluorescent
anion-bound monomeric complex, 12+·HP2O73–. A turn-on in the fluorescence
intensity is also seen for H2PO4– and, to a lesser extent, HCO3–. Little
(HSO4–, NO3–) or essentially no (N3–, SCN–, F–, Cl–, Br– and I–) response is seen for other anions. Solid-state
structural analysis of single crystals obtained after treating 12+ with HP2O73– in the presence of water revealed a salt form wherein a H2P2O72– anion sits above the
cone-like macrocycle.