Dual Emission from Rhenium(I) Complexes Induced by an Interligand Aromatic Interaction

A series of rhenium(I) diimine complexes cis,trans‐[Re(dmb)(CO)2(PR1R2R3)(PR4R5R6)]+ (dmb=4,4′‐dimethyl‐2,2′‐bipyridine, Rn=phenyl or alkyl), each of which bears two phosphine ligands with various numbers of phenyl groups, has been synthesized by using the photochemical ligand‐substitution reaction. Detailed studies of the structural features, not only in the crystal but also in solution, indicate that the number of phenyl groups is a crucial factor in controlling the rotational conformation of the phosphine ligands, which in turn determines the extent of the π–π interaction between the aromatic diimine ligand and the phenyl group(s). The π–π interaction strongly affected both electrochemical and photophysical properties: 1) the oxidation power of the Re complex became stronger, 2) the lifetime of the excited state became longer, and 3) the Stokes shift between the 1MLCT absorption band and emission from the corresponding 3MLCT excited state became smaller. In particular, the diphenyl and triphenyl phosphine had much greater influence on the properties than the monophenyl phosphine ligand. Dual emission was observed from the different rotational conformers of the complexes with an intermediate number of phenyl groups in the phosphine ligands. A group effort: Several rhenium(I) diimine complexes that bear two phosphine ligands with various numbers of phenyl groups exhibited dual emission modulated by the π–π interaction between the aromatic diimine ligand and the phenyl group(s) (see figure). The number of phenyl groups is crucial in controlling the rotational conformation of the phosphine ligands.