Naphthalimide-Appended Squaraine Rotaxanes Exhibit Enhanced FRET Efficiency

Fluorescent 1,8-naphthalimide dyes are often used as chemosensors and imaging agents, but there is a need for modified molecular designs that exploit Förster resonance energy transfer (FRET) to produce red-shifted emission wavelengths. This study compared the capacity of multicomponent 1,8-naphthalimide-squaraine and 1,8-naphthalimide-squaraine rotaxane conjugates to act as FRET pairs and produce a very large pseudo-Stokes shift. Evidence for FRET from a 1,8-naphthalimide donor to a squaraine acceptor included diagnostic changes in the fluorescence emission profile and a shorter 1,8-naphthalimide excited-state lifetime. FRET was more efficient when the conjugate's squaraine component was encapsulated as a rotaxane. Notably, the FRET efficiency for a 1,8-naphthalimide-squaraine rotaxane pentad increased in more polar solvents. To demonstrate potential as a practically useful chemosensor, a boronate-based analogue was synthesized and used to detect the presence of hydrogen peroxide in living cells by producing a ratiometric change in deep-red fluorescence with a pseudo-Stokes shift of 254 nm.