Aggregation-Induced Phosphorescence of a Trans-Bis(iminomethylpyrolato)Platinum Complex Bearing a Polymethylene Vaulted Structure: Chain Length-Dependent Solid-State Emissions.

Aggregation-induced emission (AIE) has recently attracted considerable attention. Although phosphorescent transition-metal complexes exhibit significant potential as aggregation-induced emission luminogens (AIEgens), their exploration and utilization have remained limited, partly due to the insufficient understanding of the underlying mechanisms. Here, we report a new series of platinum(II) complexes, trans-bis(2-iminomethylpyrrolato)platinum derivatives (1a-d), which feature vaulted polymethylene chains of varying lengths (n = 9-12) that bridge over the metal atom. Notably, racemic complexes 1b and 1d (with chain lengths of 10 and 12) exhibit strong yellow emission in the crystalline state (quantum yields of 24% and 33%, respectively), despite showing no emission in solution. X-ray diffraction analysis revealed face-to-face dimer packing in these emissive crystals, suggesting that dimer formation contributes to their enhanced emission. In contrast, complexes with odd-numbered chain lengths (1a and 1c) showed no such emission enhancement. Density functional theory (DFT) calculations revealed that the minimum energy crossing point (MECP) between the excited triplet and ground state lies at significantly higher energy in the dimeric structure than in the monomer, suggesting suppression of nonradiative decay in the crystalline state. This work provides a design strategy for inducing solid-state phosphorescence by controlling the 3D molecular structure to facilitate favorable molecular stacking while suppressing nonradiative decay pathways.