A common example of quenching is observed with the collision of an excited state fluorophore and another (non-fluorescent) molecule in solution, resulting in deactivation of the fluorophore and return to the ground state. In most cases, neither of the molecules is chemically altered in the collisional quenching process. A wide variety of simple elements and compounds behave as collisional quenching agents, including oxygen, halogens, amines, and many electron-deficient organic molecules. Collisional quenching can reveal the presence of localized quencher molecules or moieties, which via diffusion or conformational change, may collide with the fluorophore during the excited state lifetime. The mechanisms for collisional quenching include electron transfer, spin-orbit coupling, and intersystem crossing to the excited triplet state. Other terms that are often utilized interchangeably with collisional quenching are internal conversion and dynamic quenching.

The adherent culture of COS-7 African green monkey kidney fibroblast cells featured in the digital image above was immunofluorescently labeled with primary anti-tubulin mouse monoclonal antibodies followed by goat anti-mouse Fab fragments conjugated to Cy3, targeting microtubules. The cells were simultaneously stained for the cytoskeletal filamentous actin network with Alexa Fluor 488 conjugated to phalloidin, and for DNA with DAPI. Images were recorded in grayscale with a QImaging Retiga Fast-EXi camera system coupled to an Olympus BX-51 microscope equipped with bandpass emission fluorescence filter optical blocks provided by Omega Optical. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.