Environment-sensitive dyes change their properties (intensity, half-life, and excitation and emission spectra) depending on the polarity (hydrophobicity and charge) of their environments. Examples include: Indole, Cascade Yellow, prodan, Dansyl, Dapoxyl, NBD, PyMPO, Pyrene and diethylaminocumarin.<br />
This change is most pronounced when electron-donating and electron-withdrawing groups are placed at opposite ends of an aromatic ring system, as this results in a large change in dipole moment when excited.
When a fluorophore is excited, it generally has a larger dipole moment (μ) than in the ground state (μ). Absorption of a photon by a fluorophore takes a few picoseconds. Before this energy is released (emission: 1–10 ns), the solvent molecules surrounding the fluorophore reorient (10–100 ps) due to the change in polarity in the excited singlet state; this process is called solvent relaxation. As a result of this relaxation, the energy of the excited state of the fluorophore is lowered (longer wavelength), hence fluorophores that have a large change in dipole moment have larger stokes shift changes in different solvents. The difference between the energy levels can be roughly determined with the Lipper-Mataga equation.
A hydrophobic dye is a dye which is insoluble in water, a property independent of solvatochromism. <br>Additionally, The term environment-sensitive in chemistry actually describes changes due to one of a variety of different environmental factors, such as pH or temperature, not just polarity; however, in biochemistry environment-sensitive fluorphore and solvatochromic fluorophore are used interchangeably: this convention is so widespread that suppliers describe them as environment-sensitive over solvatochromic.