Organic Room-Temperature near-IR Phosphorescence Harvested by Intramolecular Through-Space Sensitization in Composite Molecules

A family of coordination compounds with short intramolecular spatial separation between an organic chromophore and a metal center was studied. The specific geometry was realized by means of anthracene-functionalized tertiary aryl phosphanes. Their silver and gold complexes (1, 2) operate as conventional fluorophores, with photophysical behavior defined by anthracene-localized allowed transitions. In contrast, bichromophoric species, containing phenyl bipyridine- (3, 5, 6, 8) or terpyridine- (4, 7) derived platinum(II) fragments, demonstrate fast intersystem crossing to the triplet state associated with the pincer metal component. Theoretical results corroborated that the short intramolecular distance between the platinum constituent and the adjacent anthracene facilitates subsequent through-space triplet (T2, pincer fragment)-> triplet (T1, anthracene) energy transfer. This process occurs at a rate of similar to 1011 s-1, surpassing the rates of T2 -> S0 relaxation. This prevents visible phosphorescence from the platinum(II) motifs but enables near-IR organic phosphorescence in the solid state, including dyes with very inefficient intersystem-crossing (ISC). Thus, the composite molecules 3-8 illustrate a feasible approach to the tunable sensitization of organic dyes and the design of low-energy triplet emitters.