Ping–Pong Energy Transfer in a Heteroleptic Mixed‐Chromophore Coordination Cage

Gaining control over chromophore‐functionalized multicomponent architectures sets the stage to construct modular libraries of tunable emitters, light‐harvesting complexes, and photo‐redox catalysts. Based on a “Shape Complementary Assembly” (SCA) approach, we report two luminescent heteroleptic coordination cages built from Pd(II) cations, a diketopyrrolopyrrole ligand ( L A ) and matching counter ligands with either a fluorenone ( L 1 ) or carbazole ( L 2 ) π‐system. Transient absorption (TA) spectroscopy reveals an energy transfer from either L 1 or L 2 to L A , however, only for cage Pd 2 L A 2 L 1 2 do the TA data show a subsequent (re)population of L 1 ‐centered (triplet) states, which we attribute to an intra‐assembly excitation back‐transfer from L A to L 1 , accompanied by a prolonged overall excited‐state lifetime. Computations suggest that different extents of vibrational relaxation of the donors’ S 1 states enable excitation back‐transfer in only one of the two cages and propose a Förster‐type mechanism for it, indicating that the energy‐transfer pathway can be selected through the choice of the co‐ligand. As a nanosized cage, this system allows guest molecules to be closely bound within an outer shell of photoactive ligands. Hence, the encapsulation of a chiral guest bestows both cages with circular dichroism (CD) and circularly polarized luminescence (CPL).