Naphthalene diimides as tunable fluorophores suitable for single molecule applications

Abstract

The photophysics of two new substituted aminopropenyl naphthalene diimide (SANDI) dyes are reported. The molecules exhibit many of the photophysical properties required for fluorescence labeling applications including high photostability and high fluorescence quantum yields (> 0.5) in the visible region of the spectrum. Furthermore, the emission is sensitive to the number of substituents attached to the aromatic core, and to the surrounding environment. For example, in toluene as solvent, the mono-allyl SANDI has an emission maximum at 550 nm, whereas the di-allyl SANDI emits at 630 nm. The fluorescence decay times are in the range of ~8 - 12 ns and the Forster critical distance for fluorescence resonance energy transfer (FRET) between the mono- and di-allyl SANDI derivatives is 4.1 nm for a random donor-acceptor orientation. Single molecules of the di-allyl SANDI embedded in poly(methyl methacrylate) films show very low yields of photobleaching and very few fluorescence intermittencies or "blinks". These compounds are ideal candidates for applications at the single molecule level, for example, as FRET labels.