Abstract:

Phenothiazines (PTZs), have received a lot of attention for many optoelectronic applications, such as hole-transporting layers, functioning as host materials for organic light-emitting diodes; dye sensitizers in dye-sensitized solar cells; and hole-transporting materials for perovskite solar cells. However, studies on benzophenothiazine materials are limited. In this study, we synthesize three isomeric bis-benzophenothiazine compounds (D-PTZa, D-PTZb, and D-PTZc), all bearing an aromatic ring at the 1, 2-, 2, 3-, and 3, 4-positions, respectively. Next, we systematically investigate the relationship between their structures and properties and compare them with bis-phenothiazine compounds (D-PTZ). The highest occupied molecular orbital (HOMO) distributions for D-PTZb and D-PTZc are dispersed over benzophenothiazine moities, whereas the lowest unoccupied molecular orbitals (LUMOs) are localized at the middle phenyl- and naphthyl-groups, which are similar frontier orbital distribuitions to the D-PTZ case. For D-PTZa, the steric hindrance between the phenyl groups at the 1, 2- and middle positions increases, significantly distorting its spatial structure. Therefore, its HOMO and LUMO distributions differ from those of D-PTZb and D-PTZc. Notably, the HOMOs in D-PTZa are dispersed over the middle phenyl group and nitrogen atom, whereas the LUMOs are localized at the naphthyl group. The hole/electron excitation and frontier orbital analyses demonstrate that strong local π → π* transition mixing with weak charge transfer transition is responsible for the luminescence of D-PTZb and D-PTZc. Interestingly, the ultraviolet–visible absorption spectra of all samples exhibit strong π → π* transition absorption and weak n → π* transition absorption. Furthermore, the conjugated length of the molecule can be effectively increased with the introduction of an aromatic ring, resulting in a red-shift in the maximum absorption wavelength. Compared to D-PTZ, D-PTZa emits yellow-green light with a photoluminescence quantum efficiency (PLQE) of 14%. In addition, the introduction of a phenyl group at the 2, 3-position effectively stabilizes the HOMO energy level, slightly increasing its π → π* transition gap, while also emitting blue light with a PLQE of 1.7%. For D-PTZc, the introduction of a phenyl group at the 3, 4-position better linearizes the LUMO distribution, thereby stabilizing the LUMO energy level and reducing its π → π* transition gap. The maximum emission peak is observed at 520 nm, emitting yellow-green light with a PLQE of 13%. Overall, our molecular design and results on structure–property relationships can provide fundamental guidance for the design of phenothiazine derivatives with specific photoelectric performance.

Key words: Benzophenothiazine, Organic optoelectronic materials, Density functional theory, Hole-electron analysis, Frontier molecular orbitals, Photophysical property