Unraveling the Microscopic Origin of Triplet Lasing from Organic Solids
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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.
Accepted manuscript version. Published version available at https://doi.org/10.1021/acs.jpclett.8b02191.