Theoretical Elucidation of the Effect of Linkage and Orientation of Carbazole on the Naphthalenediimide and the TADF and RTP Propensity

Thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) are most promising technologies for harvesting triplet excitons in organic emitters. In this work, the effect of connection modes between carbazole (Cz) donor (D) and naphthalene diimide (NDI) acceptor (A) on the TADF and RTP propensities are elucidated using the density functional theoretical computations using D-A, D-A-D, D-π-A, and D-π-A-π-D structural designs. The relationship of the dihedral angle between donor and acceptor units, with the spin orbit coupling (SOC) values, the energy difference between the singlet-triplet excited states (ΔEST), radiative (kr), inter-system crossing (kISC) and reverse inters system crossing (kRISC) rates are obtained. The molecules possessing direct linkage between Cz and NDI exhibit large energy difference between the lowest singlet and triplet excited states (ΔEST) due to substantial overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, the incorporation of phenyl spacer between the Cz and NDI units led to the separation of HOMO and LUMO and consequently resulted in small ΔEST. Furthermore, the presence of two donors with and without a phenyl spacer on NDI results in high-lying triplet states that are energetically lower than the lowest singlet excited state, hence providing additional channels to the TADF and RTP process. Additionally, the orientation of Cz and NDI in ortho positions of the phenyl unit resulted in a T1 state with dominant LE character which led to large spin-orbit coupling constants and kr than the analogous meta and para-linked derivatives. Overall, the compounds containing phenyl linkage are expected to have high TADF propensity than the directly linked derivatives.