Intertrimer and intratrimer metallophilic and excimeric bonding in the ground and phosphorescent states of trinuclear coinage metal pyrazolates: A computational study

The interactions present in cyclic trinuclear coinage metal pyrazolates were studied computationally. Cuprophilic interaction was found to bind the singlet ground state of the dimer of trimers {[Cu(Pz)](3)}(2), overcoming electrostatic repulsion. The large variation in intertrimer separations found in the literature for coinage metal pyrazolates is consistent with the relatively weak metallophilic interaction. The emissive triplet excited-state geometry of {[M(Pz)](3)}(2) is predicted by density functional calculations to show major geometric distortion caused by Jahn-Teller instability and excimeric M-M bonding. Large calculated Stokes' shifts, which are also observed for experimental models, are consistent with significant excited-state distortions for these materials. The major finding derived from the present study is that the intertrimer (MM)-M-... contraction in the emissive T-1 state is much more than the intratrimer contraction in all {[M(Pz)](3)}(2) models, giving rise to a lower T-1 -> S-0 phosphorescence energy in these models than in analogous monomer-of-trimer models. The observations made here point to a great potential for rationally tuning the emission properties of trinuclear coinage metal complexes through choice of the metal and ligands.