The mechanism responsible for the emission of light from luminescent materials in photochemistry is called photoluminescence. Photoluminescence occurs when a material absorbs photons light energy and then re-emits the energy as photons of lower energy. The process can be divided into two main types: fluorescence and phosphorescence.1. Fluorescence: In fluorescence, the absorbed photon excites an electron from the ground state to a higher energy level excited state . The excited electron then rapidly relaxes to a lower vibrational level within the excited state through non-radiative processes such as internal conversion or vibrational relaxation. Finally, the electron returns to the ground state by emitting a photon, which has lower energy longer wavelength than the absorbed photon. The entire process occurs on a timescale of nanoseconds.2. Phosphorescence: Phosphorescence is similar to fluorescence, but the excited electron undergoes a transition to a triplet state intersystem crossing before returning to the ground state. The triplet state has a longer lifetime than the singlet excited state, so the emission of light occurs on a timescale of microseconds to seconds. This results in a delayed emission of light, which can persist even after the excitation source is removed.Factors affecting the intensity and wavelength of the emitted light:1. Quantum yield: The quantum yield is the ratio of the number of emitted photons to the number of absorbed photons. A higher quantum yield indicates a more efficient luminescent material. Factors affecting the quantum yield include the efficiency of the radiative and non-radiative processes, the presence of impurities, and the molecular structure of the material.2. Absorption and emission spectra: The absorption and emission spectra of a luminescent material determine the range of wavelengths that can be absorbed and emitted. The Stokes shift, which is the difference between the peak wavelengths of the absorption and emission spectra, affects the wavelength of the emitted light.3. Concentration and aggregation: The concentration of the luminescent material can affect the intensity of the emitted light. At high concentrations, self-absorption and concentration quenching can occur, reducing the intensity of the emitted light. Aggregation of luminescent molecules can also lead to quenching or changes in the emission spectrum.4. Temperature: The temperature of the luminescent material can affect the intensity and wavelength of the emitted light. Higher temperatures can cause thermal quenching, where non-radiative processes become more dominant, reducing the intensity of the emitted light. Temperature can also affect the vibrational relaxation processes, leading to changes in the emission spectrum.5. External factors: The presence of external factors, such as other molecules or ions, can affect the intensity and wavelength of the emitted light. These factors can interact with the luminescent material, causing changes in the molecular structure, energy levels, or radiative and non-radiative processes.