Unveiling the Effect of Symmetry-Breaking Charge Transfer on Intersystem Crossing in Group 14 TADF Emitters

Abstract

Three organotetrel compounds (^MePDP^Ph)EMe₂ (E = Si, Ge, Sn) have been synthesized by the reaction of 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine (H₂^MePDP^Ph) with EMe₂Cl₂. All three molecules show photoluminescence (Φ_PL = 0.66–0.75) through a combination of prompt fluorescence (PF) and thermally activated delayed fluorescence (TADF). Increased contributions from TADF for the heavier tetrel species imply more facile intersystem crossing due to intramolecular heavy-atom effects. Computational studies confirm that the lowest-energy singlet and triplet excited states in (^MePDP^Ph)EMe₂ result from localized transitions within the [^MePDP^Ph]^2– ligand, with only minor contributions from the tetrel atom. In contrast, the previously reported bis-PDP tetrel compounds E(^MePDP^Ph)₂ (E = Si, Ge, Sn), for which the lowest-energy singlet excited state was computed to be a symmetry-broken ligand-to-ligand charge transfer state, exhibit increased contributions from TADF compared to (^MePDP^Ph)EMe₂. Kinetic analysis of the TADF emission for all six compounds supports increased intersystem crossing and reverse intersystem crossing rate constants (k_ISC/rISC) and quantum yields (Φ_ISC/rISC) for E(^MePDP^Ph)₂ compared to (^MePDP^Ph)EMe₂. These results highlight the importance of symmetry-breaking charge transfer in facilitating intersystem crossing and provide a blueprint for the design of molecules that can access long-lived triplet states in the absence of heavy elements.