The utilization of stable tris(2,4,6-trichlorophenyl)methyl radical (TTM) doublet emitters in OLEDs is promising, as light-emitting radicals can overcome spin-statistical efficiency limitations that exist for conventional closed shell emitters. Donor-functionalized TTM derivatives show quite long fluorescence lifetimes, which are in part caused by structural reorganization of the excited state, in which the donor plane becomes more perpendicular with regards to the TTM unit.
Restricting this change in the dihedral angle between donor and acceptor moieties is expected to shorten the fluorescence lifetime, potentially opening up other new applications of light-emitting radicals, for example in the field of organic lasers.
I will show the data to support my claims and discuss possible pathways for non-radiative relaxation as well as strategies for further improvement of photoluminescence lifetimes and quantum yields.
While open-shell luminophores with doublet spin properties are considered highly promising for next generation organic-light emitting devices, most radicals are associated with poor photostability and photoluminescence quantum yield (PLQYs). We establish certain structure-performance interrelations to improve the optical properties of radicals specifically. Two series of trityl radicals functionalized with one to three 2,7-disubstituted carbazole units are studied, carrying either nitriles or bromines as substituents. The electron-withdrawing substituents in 2,7-position induce a blue-shift of emission and exceptional PLQYs up to 87 %. Quantum mechanical calculations further elucidate the electronic and steric properties of the molecules responsible for the outstanding optical performance.
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