Theoretical elucidation of the effect of the linkage and orientation of carbazole and naphthalenediimide on the TADF and RTP propensities.

Thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) are most promising processes for harvesting triplet excitons in organic light-emitting diodes. In this work, the effect of the linkage between the carbazole (Cz) donor (D) and the naphthalenediimide (NDI) acceptor (A) on the TADF and RTP propensities is elucidated using density functional theory computations employing D-A, D-A-D, D-π-A, and D-π-A-π-D structural designs. The effects of the dihedral angle between the donor and acceptor units on the energy difference between the singlet and triplet excited states (Δ), the spin-orbit coupling (SOC) constants, and radiative (), intersystem crossing () and reverse intersystem crossing () rates are unravelled. The molecules possessing a direct linkage between Cz and NDI exhibit large Δ values due to substantial overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, the insertion of a phenyl spacer between the Cz and NDI units led to disjoint HOMO and LUMO and consequently resulted in a small Δ. Furthermore, the presence of two donors with and without a phenyl spacer on NDI resulted in a high-lying triplet state (T) that is energetically lower than the lowest singlet excited state (S), hence providing additional channels to the TADF and RTP processes. Also, the orientation of Cz and NDI in the -positions of the phenyl unit resulted in a T state with dominant LE character which led to moderate spin-orbit coupling constants and highest rates compared to the analogous - and -linked derivatives. Thus, the -derivatives possessed small Δ, charge transfer dominated S, joint holes and electrons for the T state, characteristic of local excitation, high SOC, and promising rISC and rates. Overall, the phenyl linked derivatives possess TADF characteristics, while the directly linked analogues show RTP propensity.