Long-Lived Photoluminescence of Molecular Group 14 Compounds through Thermally Activated Delayed Fluorescence

Photoluminescent molecules exploiting the sizablespin-orbit coupling constants of main group metals and metalloidsto access long-lived triplet excited states are relatively rarecompared to phosphorescent transition metal complexes. Herewe report the synthesis of three air- and moisture-stable group 14compounds E(MePDPPh)2,whereE=Si,Ge,orSnand[MePDPPh]2-is the doubly deprotonated form of 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine. In solution, all threemolecules exhibit exceptionally long-lived triplet excited stateswith lifetimes in the millisecond range and show highly efficientphotoluminescence (Phi <= 0.49) due to competing promptfluorescence and thermally activated delayedfluorescence at andaround room temperature. Temperature-dependent steady-stateemission spectra and photoluminescent lifetime measurementsprovided conclusive evidence for the two distinct emission pathways. Picosecond transient absorption spectroscopy allowed furtheranalysis of the intersystem crossing (ISC) between singlet and triplet manifolds (Tau ISC= 0.25-3.1 ns) and confirmed the expectedtrend of increased ISC rates for the heavier elements in otherwise isostructural compounds