Small-Molecule Fluorophores with Large Stokes Shifts: 9-Iminopyronin Analogues as Clickable Tags

The design, synthesis, and both experimental and theoretical studies of several novel 9-(acylimino)- and 9-(sulfonylimino)pyronin derivatives containing either an oxygen or a silicon atom at position 10 are reported. These compounds, especially the Si analogues, exhibit remarkably large Stokes shifts (around 200 nm) while still possessing a high fluorophore brightness, absorption bands in the near-UV and visible part of the spectrum, and high thermal and photochemical stabilities in protic solvents. The reason for the observed large Stokes shifts is an intramolecular charge-transfer excitation of an electron from the HOMO to the LUMO of the chromophore, accompanied by elongation of the C9-N bond and considerable solvent reorganization due to hydrogen bonding to the solvent. Due to the photophysical properties of the studied compounds and their facile and high-yielding synthesis, as well as a simple protocol for their bioorthogonal ligation to a model saccharide using a Huisgen alkyne-azide cycloaddition, they represent excellent candidates for biochemical and biological applications as fluorescent tags and indicators for multichannel imaging. 9-(Acylimino)pyronins alter their optical properties upon protonation and may also be used as pH sensors.