Photoluminescence

Photoluminescence is luminescence produced by the absorption of photons by a material followed by emission of light. The absorbed energy promotes electrons to higher electronic states, after which the excited system returns to lower energy levels by radiative and non-radiative processes. If the emitted photon energy is lower than the excitation energy, the process exhibits a Stokes shift.

Fluorescence is a fast form of photoluminescence in which emission occurs from the initially excited singlet state and typically lasts from picoseconds to nanoseconds. Phosphorescence involves emission from a triplet state and is slower due to spin-forbidden transitions, ranging from microseconds to seconds or longer. Some materials show delayed fluorescence, such as thermally activated delayed fluorescence.

The efficiency of photoluminescence is described by quantum yield, the ratio of emitted to absorbed photons, and is influenced by nonradiative losses and environmental factors. Emission spectra reveal the energy structure of the material and can be tuned by size, composition, and crystal field effects. Common photoluminescent materials include organic dyes, inorganic phosphors, quantum dots, and perovskites.

Applications span lighting and displays, where photoluminescent compounds convert absorbed energy to visible light; bioimaging and sensing, where fluorescence signals report on chemical or biological processes; and materials characterization, where luminescence lifetimes and spectra provide information about electronic structure and interactions.

Measurement typically involves excitation with a light source such as a lamp or laser and detection of emitted photons as a function of wavelength and time. Factors such as concentration quenching, temperature, and solvent can affect intensity and spectra.