Molecular design of dual-emission rhodamine analogs

Abstract

Rhodamine derivatives are one of the most important classes of fluorophores, exhibiting a unique equilibrium between a fluorescent open form and a (nearly) non-emissive closed form. The closed form has been understudied, due to its short wavelength and low quantum yield. In this work, through detailed quantum chemical calculations, we showed that closed-form rhodamines are non-fluorescent, because a weakly emissive charge transfer (CT) state is more stable than the virtually emissive locally excited (LE) state. We also proposed a rapid design method to activate the fluorescence of rhodamine analogs in the closed form, via a judicious choice of three molecular fragments with matched frontier molecular orbitals to stabilize the LE state. We foresee that the resulting dual emissions (from both the closed and open forms of these analogs) can be used for constructing ratiometric probes and dual-channel fluorescent labels for advanced bioimaging and biosensing applications.