Influence of Intermolecular Interactions on the Photophysical Properties of Carbazolyl Phosphorescent Molecules

The photophysical properties of three carbazolyl phosphorescent molecules with similar structures-5-(9H-carbazole-9-group) nicotinitrile (P35N), 5-(9H-carbazole-9-group) nicotinamide (P35M) and 2-(9H-carbazole-9-group) isonicotinamide (P25M)-are investigated theoretically. The influence on luminescence is primarily elucidated through an analysis of geometry, electronic structure, and the dynamic processes occurring within the excited state. Structural analysis indicates that P35M exhibits the most effective luminescence due to minimal structural changes, reduced separation between the electron and hole, and enhanced intermolecular interactions within the crystal. Excited state dynamics and recombination energy analysis indicate that the higher triplet exciton population in the P35M molecule contributes to its long-lived phosphorescence. Furthermore, the intersystem crossing process for the three molecules is predominantly governed by low-frequency torsional rotation, while bond stretching vibrations are the primary factor leading to the inactivation of non-radiative processes. The distinct vibrational channels associated with reverse intersystem crossing and non-radiative processes present opportunities for further enhancement of the phosphorescent characteristics of these molecules.