Geometry‐Driven Whispering‐Gallery Modes in Self‐Assembled Phosphorescent Organic Microtubes

Whispering‐gallery resonances provide a unique mechanism for light confinement through continuous rotational circulation, yet their realization in organic microcavities has been largely hindered by the thermodynamic tendency of small molecules to form solid faceted crystals. Here, we demonstrate that the phosphorescent complex tris(2‐phenylpyridine)iridium(III) (Ir(ppy) 3 ) can be kinetically directed to self‐assemble into hollow hexagonal microtubes where the internal air void functions as a high‐contrast dielectric boundary for rotational optical confinement. These microtubes sustain robust whispering‐gallery modes (WGMs), confirmed via polarization‐resolved and spatially mapped photoluminescence to exhibit both azimuthal circulation and longitudinal propagation along the longitudinal axis. Unlike conventional Fabry–Pérot resonances that suffer from axial dissipation, the WGM coherence in these hollow architectures is preserved over extended propagation distances exceeding tens of micrometers. This persistence demonstrates that the tubular geometry enables complex 3D mode trajectories that effectively couple tight confinement with long‐range transport. Our results establish a lithography‐free strategy for engineering mode‐selective organic photonic devices, offering a versatile platform for coherent triplet‐exciton harvesting and high‐sensitivity optical sensing.