Unraveling the Microscopic Origin of Triplet Lasing from Organic Solids

We present a heuristic mechanism for the origin of the unusual triplet lasing from (E)-3-(((4-nitrophenyl)imino)methyl)-2H-thiochroman-4-olate·BF₂.We demonstrate that whereas the moderate lifetime (1.03 μs) of the first triplet state (T₁) prohibits triplet-triplet annihilation, the relatively faster S₁ → T₁ intersystem crossing and the 10⁴ times smaller reverse intersystem crossing effectively help achieve population inversion in the T₁ state. Furthermore, the triplet lasing wavelength (675 nm) for the tetramer does not overlap with the triplet-triplet absorptions wavelength, indicating that the spin-forbidden emission cross section is very large. Additionally, the almost complete absence of a vibrational progression in the vibronic phosphorescence spectrum of the monomer plays an important role in ensuring efficient triplet-state lasing from this organic material. Our results show that controlling the triplet-state lifetimes combined with lowering of the triplet-triplet absorption in the emission region and small vibronic coupling will be the key steps when designing novel organic triplet-lasing materials.