Computation of the D1 Excited-State Formation Mechanism in Radical-Based Organic Light-Emitting Diodes

Radical-based organic light-emitting diodes (OLEDs) emit from a doublet excited-state, D1, enabling spin-allowed radiative decay without the singlet-triplet exciton constraints of closed-shell emitters. However, the charge-mediated pathways by which D1 is populated under electrical excitation from D0 remain unclear. Here we investigate, by means of quantum-chemical calculations and Marcus-type theoretical modeling, how competing charge-mediated pathways populate the emissive D1 state in three TTM-based radical emitters: TTM-1Cz, TTM-3PCz, and TTM-3NCz. We compare anion-mediated, cation-mediated, charge-charge annihilation (CCA), and excited-charge pathways within a minimal CBP-host model. The calculated energetics and rates suggest that D1 formation probably proceeds via an S0 anion intermediate, D0 → [S0]- → D1, while excited-state anion and CCA pathways may also contribute. In contrast, cation-mediated routes are generally disfavored, and undesirable relaxation pathways returning charged species to D0 are very far in the Marcus inverted regime and therefore kinetically unfavorable. Overall, this work provides a computational framework for determining the D1 formation mechanism in radical OLED emitters and highlights anion-mediated pathways as the most plausible route to emissive doublet state population.