Telecom‐Wavelength Organic Single‐Crystal Lasers Triggered by the Molecular Conformation‐Dependent Cascaded Proton Transfer Processes

Organic semiconductor molecules possess chemical tunability and large stimulated emission cross section, representing a promising candidate for laser gain medium. However, for the pursuit of telecom‐wavelength organic lasers, one of the major obstacles is the lack of effective energy‐level systems with high optical gain to compensate the exciton deactivation losses. Herein, the effects of molecular conformation‐dependent distinct cascaded proton‐transfer six‐level energy gain systems on lasing emission properties are systematically investigated based on organic polymorphs, proving that an energy‐level gain system without reversible transition channels is more conducive to the formation of efficient population inversion and high optical gain. Notably, the one‐way irreversible six‐level energy system of β‐phase polymorph supports more favorable population inversions than the two‐way reversible six‐level system of α‐phase polymorph due to the irreversible excited‐state second proton transfer and the irreversible ground‐state recovery, achieving the amplified spontaneous emission at telecom‐wavelength of ≈850 nm. This study provides useful guidelines for constructing efficient energy‐level gain systems, promoting the exploration of high‐gain organic semiconductor laser materials from visible to near‐infrared region. The one‐way irreversible six‐level energy system of β‐phase polymorph supports more favorable population inversions than the two‐way reversible six‐level system of α‐phase polymorph due to the irreversible excited‐state second proton transfer and the irreversible ground‐state recovery, which thus supports the lasing emission at telecom‐wavelength of ≈850 nm.