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In this work the preferential accumulation of long-lived acridone derivatives in mitochondria were evaluated, based on a new concept of non-protonable and non-charged group carriers. In this sense, the thiophene ring has been proved to act, for the first time, as mitochondria targeting group. Moreover, the unique long lifetime of the dyes has allowed the determination of the polarity of the organelles by fluorescence lifetime imaging microscopy (FLIM), without causing remarkable cytotoxicity. The neutral nature of the vectoring group favors the synthetic process and prevents the modification of different organelle parameters as membrane potential, making these probes excellent candidates to organelle tracking. Image 1 • Thiophene ring acts as unreported non-charged mitochondrial targeting group. • Long-lived fluorescent probes allow polarity determination inside mitochondria and lysosomes. • Co-localization in organelles has been analyzed with FLIM microscopy taking advantage of time-gated technique. [ABSTRACT FROM AUTHOR]

In this work, we have performed an in-depth study of the photophysics and solvatofluorochromism of a red-emitting Si-xanthene dye, an analog of Tokyo Magenta (TM) historically developed by Egawa et al. (Chem. Commun. 2011 , 47 , 4162–4164). The results show a strong dependency of the emission properties of 2-Me-4-OMe-TM on the polarity of the solvent. For instance, the dye exhibited an increase in its fluorescence lifetime with the decrease in solvent polarity. Therefore, in this work, this spectral behavior has been used as a new approach for determining the intracellular microenvironment polarity through the measurement of its fluorescence lifetime by Fluorescence Lifetime Imaging Microscopy (FLIM). Our experiments confirmed the ability of the dye to detect changes in polarity between different intracellular compartments. Image 1 • A silicon-substituted xanthene dye was synthesized and its photophysics studied. • The dye shows a strong solvatochromic behavior involving two emission bands. • A strong dependency on the solvent's acidity was observed in the red-shifted band. • The dye works as an intracellular polarity fluorescence lifetime sensor. [ABSTRACT FROM AUTHOR]