The Unexpected Selectivity Switching from Mitochondria to Lysosome in a D-π-A Cyanine Dye.

Two interesting benzothizolium-based D-π-A type hemicyanine dyes ( 3a - 3b ) with a diphenylamine (-NPh 2 ) donor group were evaluated for fluorescence confocal microscopy imaging ability in live cells (MO3.13, NHLF). In sharp contrast to previously reported D-π-A dyes with alkyl amine donor (-NR 2 ) groups ( 1 ), 3a and 3b exhibited significantly different photophysical properties and organelle selectivity. Probes 3a and 3b were nearly non-fluorescent in many polar and non-polar solvents but exhibited a bright red fluorescence (λ em ≈ 630-640 nm) in stained MO3.13 and NHLF with very low probe concentrations (i.e., 200 nM). Fluorescence confocal microscopy-based co-localization studies revealed excellent lysosome selectivity from the probes 3a - 3b , which is in sharp contrast to previously reported D-π-A type benzothiazolium dyes ( 1 ) with an alkyl amine donor group (-NR 2 ) (exhibiting selectivity towards cellular mitochondria). The photostability of probe 3 was found to be dependent on the substituent (R') attached to the quaternary nitrogen atom in the cyanine dye structure. The observed donor-dependent selectivity switching phenomenon can be highly useful in designing novel organelle-targeted fluorescent probes for live-cell imaging applications.