Excited-State Intramolecular Proton Transfer Parent Core Engineering for Six-Level System Lasing Toward 900 nm

Organic molecules which can undergo excited-state intramolecular proton transfer (ESIPT) process have been considered as ideal gain materials for near-infrared organic lasers owing to their effective four-level systems. However, extending lasing wavelength beyond 800 nm with present ESIPT-active gain materials is still in challenge. Herein, we established a molecular design strategy that operates via extending the π-conjugated system of the ESIPT parent core to enhance the cascaded double ESIPT process and thus to achieve the red-shifted six-level system lasing. Concretely, a model molecule with 1,9-dihydroxyanthracene as the ESIPT parent core was designed and synthesized, which was proved to undergo twice cascaded ESIPT processes while the 1,8-dihydroxynaphthalene-based analogue can only undergo once ESIPT process based on DFT calculations and ultrafast dynamics analyses. Finally, a six-level system lasing toward 900 nm was achieved with a low threshold of 27.4 μJ cm^-2 .