7-Azabicyclo[2.2.1]heptane as a unique and effective dialkylamino auxochrome moiety: demonstration in a fluorescent rhodamine dye.

A new type of a highly fluorescent sulforhodamine dye, 221SR, was designed and synthesized using 7-azabicylco[2.2.1]heptyl moieties as the electron donating auxochrome groups. Using the prototypical dye tetramethylsulforhodamine, TMSR, as a benchmark, we show this new dye has higher fluorescent quantum yields, Phi, (Phi = 0.95 vs Phi = 0.65 at 20 degrees C), emission efficiencies that are invariant in the 20--> 60 degrees C temperature range (Phi = 0.95 vs Phi = 0.38 at 60 degrees C), and fluorescence lifetimes that increase with a rise in temperature (20 --> 60 degrees C) as compared to a decrease for the benchmark dye (3.8 --> 3.9 vs 2.8 --> 1.7 ns). Importantly, photostability studies found the azabicyclic rhodamine to be many times more stable than its tetramethyl analogue. To the best our knowledge this is the first report of the use of an apex-N-substituted azabicycloalkane as an electron donor group in any class of donor-acceptor dye. Thus, the concept of using an apex-substitutes bicyclic amine as a donor moiety constitutes a new paradigm for simultaneously inducing remarkable beneficial effects on both emission efficiency and photostability in a donor-acceptor fluorophore.