Pyrazinoindole-Based Lewis-Acid/Base Assembly: Intriguing Intramolecular Charge-Transfer Switching through the Dual-Sensing of Fluoride and Acid

Pyrazinoindole-based
Lewis-acid/base assemblies are prepared through
the use of regioselective formal [3 + 3] cycloaddition reactions,
and their intriguing photophysical properties are described. The assemblies
exhibit strong emissions in THF solution, which are attributed to
through-space intramolecular charge-transfer (ICT) transitions between
the branched Lewis-acid/base moieties. Furthermore, these show ratiometric
color-change responses in PL titration experiments, which give rise
to new colors through turn-on emissions ascribable to ICT transitions
that alternate between the pyrazinoindole units and each triarylboryl
or amino moiety, a consequence of the binding of the fluoride or acid.
Pieces of filter paper covered by these assemblies exhibited blue-shifted
color changes when immersed in aqueous acidic solutions, suggesting
that these are promising candidate indicators that detect acid through
emissive color. Computational data for these assemblies and their
corresponding adducts verify the existence of ICT transitions that
alternate through fluoride or acid binding.