The role of zero-field splitting and π-stacking interaction of different nitrogen-donor ligands on the optical properties of luminescent rhenium tricarbonyl complexes

Rhenium tricarbonyl complexes are among the most important classes of coordination compounds in inorganic chemistry. Exploring their luminescent excited states, the lowest singlet (S1) and lowest triplet (T1), is an important topic to understand their interesting photophysical behavior and potential applications in organic light-emitting diodes (OLEDs). In this work, a systematic evaluation of the role of zero-field splitting (ZFS) and the geometric arrangement of different nitrogen-donor ligands, including π-stacking interactions, in five selected rhenium luminescent complexes was performed. The optical properties have been deeply discussed using relativistic quantum mechanics methods and other theoretical approaches. Spin–orbit coupling TDDFT (SOC-TDDFT) calculations showed an excellent prediction of emission properties. The zero-field splitting (ZFS) values were relatively large, verifying the presence of the 3MLCT state in the transition character of T1 and efficient spin–orbit coupling (SOC). The introduction of two nitrogen-donor ligands introduced π-stacking interactions that produced a synergistic effect between the ligands and the metal center, promoting blueshift emissions and higher rate constants for T1–S0 transition mediated by SOC.