Why is luminescence often more efficient in gases and crystals than in liquids?

Luminescence efficiency is generally higher in gases and crystals than in liquids because of how these states handle the dissipation of excitation energy.

In condensed phases like liquids, excited electrons frequently lose their energy through nonradiative processes. Because the particles in a liquid are relatively free to move and interact, excitation energy is easily dissipated as heat through molecular collisions or by being converted into rotational and vibrational energy.12

In contrast, crystals provide a more rigid environment. The strong binding forces between the ions or atoms in a crystal lattice restrict the movement of particles compared to a liquid. This structural stability makes it more difficult for electron-excitation energy to be transformed into vibrational energy, thereby preserving more of that energy for radiative processes (the emission of light).1

In the gas phase, interactions between atoms or molecules are even smaller, especially at low pressures, which minimizes collisional deactivation and allows for high luminescence efficiency. In all cases, the primary challenge to efficiency is the competition between radiative emission and the loss of energy to the environment as thermal, nonradiative energy.123