Formation of Excimers in Isoquinolinyl Pyrazolate Pt(II) Complexes: Role of Cooperativity Effects

The interplay between noncovalent interactions involving metal complexes may lead to the formation of aggregates (i.e., ground state dimers, trimers, n-mers, etc.), and this is often linked to dramatic changes in their physical and chemical properties as compared to the original properties of the isolated units. Dimers and trimers can also be formed in the excited state potential energy surfaces, i.e., excimers. Excimers are short-lived but are also often characterized by different optical properties from those of the isolated units. Understanding the nature of noncovalent interactions and the presence or not of cooperativity effects in both aggregates and excimers is thus extremely important to rationalize these variations. In this study, we present computational investigations on isoquinolinyl pyrazolate Pt(II) complexes. Our results highlight that cooperativity effects between noncovalent interactions, which are modulated by sterically demanding substituents and metallophilic Pt···Pt interactions, are present only on certain investigated excimers. We use density functional theory (DFT) calculations to examine the cooperativity effects and the changes in the photophysical properties. Different descriptors of cooperativity effects between noncovalent interactions, including the synergetic, genuine nonadditive, and total interaction energies, were evaluated for a series of Pt(II) aggregates and excimers. In addition, energy decomposition analysis (EDA) calculations were performed to rationalize the origins of the cooperative effects. The cooperative effects in trimer excimers (in their lowest triplet excited state, i.e., T₁) led to shortened Pt···Pt contacts as compared to the trimer aggregates. Furthermore, this synergy between noncovalent interactions is ultimately responsible for the formation of the excimers and the striking changes in the measured photophysical properties. More in detail, we report a change in the character of the lowest-lying triplet excited state when going from dimer excimers (i.e., of mixed triplet ligand-centered and triplet metal-to-ligand charge transfer (³LC/³MLCT) character) to trimer excimers (i.e., of triplet metal-metal-to-ligand charge transfer (³MMLCT) character). The EDA reveals that the total interaction energy on trimer excimers is subtly controlled by the electrostatic and dispersion terms.