Twisting Rhodamine─Design of Bright Dyes for Circularly Polarized Fluorescence

Helicenes with chiral conjugated π-systems are prime candidates for molecular dyes with the ability to emit circularly polarized luminescence (CPL). In this work, we present the first chiral dye that embeds a rhodamine chromophore into a configurationally stable helicene framework. Rhodamine dyes are brightly fluorescent amino-substituted xanthenium derivatives that excel as fluorescent labels and probes in biological imaging and assays. We developed an original synthesis pathway to successfully yield three new cationic [4]helicenes. These helicenes were designed with different substitution patterns of electron-donating groups at a chromenoxanthenium [4]helicene scaffold. This way, we aimed to control the lowest-energy transitions and consequently improve the fluorescence and chiroptical properties, with the final helicene being highly absorbent with outstanding quantum yields and an increased dissymmetry factor. The conscious design of the chromophore in the helical scaffold led to the CPL brightness values being up to 4 times higher than the average of previous cationic [4]helicenes. Moreover, a last step introduces an N-acyl handle, enabling further functionalization and potential future bioapplication.