A phototoxic substance is a chemical compound which becomes toxic only when exposed to light.

Phototoxicity is a phenomenon known in live-cell live cell fluorescence microscopy, where illuminating a fluorescent molecule (the fluorescently active site is called a fluorophore) causes the selective death of the cells expressing it. While not completely understood, it seems to be clear that the main cause for phototoxicity is the formation of oxygen radicals due to non-radiative energy transfer. It has been suggested that this article or section be merged into microscope. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ... A fluorophore is a component of a molecule which causes a molecule to be fluorescent. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... In chemistry, radicals (often referred to as free radicals) are atomic or molecular species with unpaired electrons on an otherwise open shell configuration. ...

Typically in fluorescence, photons of a certain wavelength excite electrons of the illuminated fluorophore to higher energy states. When these excited electrons return to a lower energy state, they emit a photon with a lower energy level thus causing the emission of light of a longer wavelength. This principle of fluorescence is also known as Stokes shift. In physics, the photon (from Greek φως, phōs, meaning light) is the quantum of the electromagnetic field; for instance, light. ... The wavelength is the distance between repeating units of a wave pattern. ... Stokes shift Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and luminescence spectra of the same electronic transition. ...

Unfortunately for microscopists, in many cases some of the energy is not used for this radiative energy transfer but is transferred to oxygen causing the formation of oxygen radicals. These radicals are highly toxic to living cells, sometimes killing cells in seconds.

Phototoxicity in live cells depends strongly on the kind of fluorescent molecule used. The isolation and characterization of fluorescent proteins such as green fluorescent protein (GFP) has provided biologists with fluorochromes which show a much weaker phototoxic effect compared to most smaller chemically synthesized fluorescent molecules such as FITC or rhodamine. Still, the energy level of excitation light as well as the duration of illumination must be minimized to ensure long-term survival of living cells during fluorescent imaging. GFP ribbon diagram from PDB database The green fluorescent protein (GFP) is a protein from the jellyfish Aequorea victoria that fluoresces green when exposed to blue light. ... fluorescein isothiocyanate Fluorescein is a fluorophore commonly used in microscopy, in a type of dye laser as the gain medium, and in forensics and serology to detect latent blood stains. ... Rhodamine is a family of related chemical compounds. ...