Effects of <i>ortho</i>-Linkages on the Molecular Stability of Organic Light-Emitting Diode Materials

Abstract

ortho-Linkages are effective in improving the performance of organic light-emitting diode (OLED) materials but may weaken the local chemical bonds and even incur chemical degradation due to bond cleavage reactions in operating OLEDs, especially for blue-emitting materials. Here, we conducted a systematic theoretical study on a series of carbazole (Cz)-based molecules with typical donor (D)−π−acceptor (A) structures to investigate the influence of ortho-substituents on the bond dissociation energies (BDEf) of their fragile D−π bonds (i.e., C–N bonds), where “fragile” means that the bonds have the minimum or a comparable-to-the-minimum BDE in the molecule. Through calculations, it was found that different substituents or even the same substituent on different sites of the molecule could bring variable effects on the polar D−π bond. Specifically, in the π-group, the effects of a substituent on BDEf mainly depend on the steric hindrance that it brings to the D−π bond. Thus, ortho-CH3 groups hardly affect BDEf, while ortho-phenyl and Cz groups both decrease the BDEf by more than 0.3 eV. ortho-CH3 and phenyl groups scarcely affect the singlet excitation energies [E(S1)] of the molecule, while ortho-Cz groups decrease E(S1). Non-ortho-substituents on the π-group can freely tune E(S1) with little effect on BDEf. On the donor group (Cz), substituents at 1,8-positions usually decrease BDEf. On 3,6-positions, electron-donating groups can decrease both the BDEf and E(S1), while electron-withdrawing groups can increase both. Importantly, a positive correlation between the BDEf and the Hirshfeld charge of the N atom (qN) of the C–N bond is found, which provides a convenient way to predict the BDEf by the easily acquired qN. According to these results, several feasible strategies are proposed to manage BDEf and E(S1) for the design of robust OLED materials.