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 pi-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.