Alkynyl-naphthalimide Fluorophores: Gold Coordination Chemistry and Cellular Imaging Applications

A range of fluorescent alkynyl-naphthalimide
fluorophores has been synthesized and their photophysical properties
examined. The fluorescent ligands are based upon a 4-substituted 1,8-naphthalimide
core and incorporate structural variations (at the 4-position) to
tune the amphiphilic character: chloro (L1), 4-[2-(2-aminoethoxy)­ethanol]
(L2), 4-[2-(2-methoxyethoxy)­ethylamino] (L3), piperidine (L4), morpholine (L5), 4-methylpiperidine
(L6), and 4-piperidone ethylene ketal (L7) variants. The amino-substituted species (L2–L7) are fluorescent in the visible region at around 517–535
nm through a naphthalimide-localized intramolecular charge transfer
(ICT), with appreciable Stokes’ shifts of ca. 6500 cm–1 and lifetimes up to 10.4 ns. Corresponding two-coordinate Au­(I)
complexes [Au­(L)­(PPh3)] were isolated, with X-ray structural
studies revealing the expected coordination mode via the alkyne donor.
The Au­(I) complexes retain the visible fluorescence associated with
the coordinated alkynyl-naphthalimide ligand. The ligands and complexes
were investigated for their cytotoxicity across a range of cell lines
(LOVO, MCF-7, A549, PC3, HEK) and their potential as cell imaging
agents for HEK (human embryonic kidney) cells and Spironucleus
vortens using confocal fluorescence microscopy. The
images reveal that these fluorophores are highly compatible with fluorescence
microscopy and show some clear intracellular localization patterns
that are dependent upon the specific nature of the naphthalimide substituent.