Two-photon absorbing 8-hydroxy-benzo[g]coumarins with giant Stokes shifts: an environment-insensitive dye platform for probing biomolecules.

Fluorescent compounds with distinct photophysical properties are essential for the development of optical probes for chemical, biological, and environmental species, in addition to optoelectronic devices. In this context, we synthesized a series of 3-substituted-8-hydroxybenzo[g]coumarin derivatives and characterized their photophysical and cellular imaging properties. Being dipolar π-extended coumarin analogues, they have intramolecular charge-transfer character and good two-photon imaging capability, as shown for two selected dyes. Most of the dyes emit in a wavelength range of 530-580 nm in aqueous media and show large Stokes shifts as high as 197 nm. In spite of its dipolar nature, the 3-pyridinium-substituted derivative 5h notably shows insignificant solvatochromism as well as viscosity- and polarity-insensitive emission intensity, offering an ideal dye platform for probing biological targets. As a demonstration, we prepared an esterase probe based on it, which shows ratiometric sensing behavior.