Advances in Energy‐Level Systems of Organic Lasers

Organic gain materials, with the merit of abundant excited‐state processes with large stimulated emission cross‐section, show considerable potential in lasers. The abundant excited‐state processes provide excellent platform for the construction of various forms of energy‐level systems, based on which the efficient population inversion and the tailorable gain region are supported. Herein, the development of organic lasers with distinctive energy‐level systems is summarized according to the classification of excited‐state gain processes including singlet quasi‐four‐level transition, energy transfer, excimer, charge transfer (CT), excited‐state intramolecular proton transfer (ESIPT), and some other novel photophysical processes, giving deep insight into the inherent relationship between energy‐level systems and laser action. Finally, the challenges and perspectives for the future development are presented, hopefully to offer valuable enlightenment for the further optimization of energy‐level systems toward high‐gain organic lasers including electrically pumped organic lasers. The energy‐level systems support the population inversion necessary for lasers. For organic gain materials, the abundant excited‐state processes provide a platform for various forms of energy‐level systems. This review is a systematical summary of organic lasers with distinctive energy‐level systems according to the classification of excited‐state gain processes, providing inspiration for the rational design of efficient energy‐level systems.