Large Stokes shift fluorophores from meta-substituted zwitterions

Many commonly used dye classes suffer from strong overlap of their absorption and emission spectra, favoring reabsorption and hampering the combination of several dyes for multi-analyte sensing with single wavelength excitation. This can be overcome by increasing the energy difference between absorption and emission using donor-acceptor dyes with charge-transfer processes. A neglected concept to finetune the Stokes shift is meta-substitution, which we exploited to design a single-benzene fluorophore exhibiting the largest Stokes shift of a zwitterionic compound. The meta-substitution of permanently-charged donor and acceptor groups provides a Stokes shift of >10 000 cm^-1 (1.24 eV), absorbing light in the UV-region at 375 nm and emitting yellow-orange light at 605 nm. Relative to para-substitution, the orbitals are primed for more effective intramolecular charge-transfer and more energy is dissipated by structural reorganisation upon excitation, stemming from greater excited-state antiaromaticity. The large Stokes shift is retained by π-extended derivatives, thus meta-substitution of zwitterionic groups is a general way to design organic fluorophores with small spectral overlap.