Rhodamine molecules are setting benchmarks in fluorescence microscopy. Herein, we report the deuterium (d12) congeners of tetramethyl(silicon)rhodamine, obtained by isotopic labelling of the four methyl groups, which improves photophysical parameters...
Two four‐coordinate organoboron N,C‐chelate complexes with different functional terminals on the PEG chains are studied with respect to their photophysical properties within human MCF‐7 cells. Their excited‐state properties are characterized by time‐resolved pump‐probe spectroscopy and fluorescence lifetime microscopy. The excited‐state relaxation dynamics of the two complexes are similar when studied in DMSO. Aggregation of the complexes with the carboxylate terminal group is observed in water. When studying the light‐driven excited‐state dynamics of both complexes in cellulo, i. e., after being taken up into human MCF‐7 cells, both complexes show different features depending on the nature of the anchoring PEG chains. The lifetime of a characteristic intramolecular charge‐transfer state is significantly shorter when studied in cellulo (360±170 ps) as compared to in DMSO (∼960 ps) at 600 nm for the complexes with an amino group. However, the kinetics of the complexes with the carboxylate group are in line with those recorded in DMSO. On the other hand, the lifetimes of the fluorescent state are almost identical for both complexes in cellulo. These findings underline the importance to evaluate the excited‐state properties of fluorophores in a complex biological environment in order to fully account for intra‐ and intermolecular effects governing the light‐induced processes in functional dyes.
Optoacoustic imaging, also known as photoacoustic imaging, promises micron-resolution noninvasive imaging in biology at much deeper penetration (>cm) depths than e.g. fluorescence. However, the loud, photostable, NIR-absorbing molecular contrast agents which would be needed for optoacoustic imaging of enzyme activity remain unknown: most organic molecular contrast agents are simply repurposed fluorophores, with severe shortcomings of photoinstability or phototoxicity under optoacoustic imaging conditions, which are consequences of their slow S1→S0 electronic relaxation rates. We now disclose that known fluorophores can be rationally modified to reach ultrafast S1→S0 rates, without much extra molecular complexity, simply by merging them with molecular switches. Here, we merge azobenzene switches to cyanine dyes to give ultrafast relaxation (<10 ps, >100-fold faster). Even without adapting instrument settings, these azohemicyanine optoacoustic imaging agents deliver outstanding improvements in signal longevity (>1000-fold increase of photostability) and signal loudness (here: >3-fold even at time zero). We show why this still-unexplored design strategy can offer even stronger performance in the future, as a simple method that will also increase the spatial resolution and the quantitative linearity of photoacoustic response even over extended longitudinal imaging. By bringing the world of molecular switches and rotors to bear on unsolved problems that have faced optoacoustic agents, this practical strategy may be a crucial step towards unleashing the full potential, in fundamental studies and in translational uses, of optoacoustic imaging.
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