Cascaded Excited-State Intramolecular Proton Transfer Towards Near-Infrared Organic Lasers Beyond 850 nm.

Near-infrared (NIR) organic solid-state lasers play an essential role in applications ranging from laser communication to infrared night vision, but progress in this area is restricted by the lack of effective excited-state gain processes. Herein, we originally proposed and demonstrated the cascaded occurrence of excited-state intramolecular proton transfer for constructing the completely new energy-level systems. Cascading by the first ultrafast proton transfer of <430 fs and the subsequent irreversible second proton transfer of ca. 1.6 ps, the stepwise proton transfer process favors the true six-level photophysical cycle, which supports efficient population inversion and thus NIR single-mode lasing at 854 nm. This work realizes longest wavelength beyond 850 nm of organic single-crystal lasing to date and originally exploits the cascaded excited-state molecular proton transfer energy-level systems for organic solid-state lasers.