Photoluminescence Reshaping via Structural Confinement and Local Density of Optical States Control in Ir(III)‐Doped Biodegradable Metasurfaces

ABSTRACT Metasurfaces incorporating phosphorescent emitters offer a route to engineer radiative and vibronic processes beyond chemical design. Yet extending their control to phosphorescent triplet emitters remains challenging. In Ir(III) complexes, the spin–orbit‐mixed MLCT manifold and the strong coupling between structural relaxation and radiative decay make the interplay between LDOS, matrix rigidification, and emission dynamics intrinsically non‐trivial and difficult to disentangle. Here we realize a phosphorescent metasurface on a biodegradable ethyl cellulose matrix that enables cooperative structural and electromagnetic control of an embedded Ir(III) complex. Matrix‐induced confinement suppresses deeply relaxed MLCT configurations, increasing the lifetime from 55 ns in solution to 270 ns in film (+390%). The metasurface then imposes a wavelength‐selective LDOS that filters low‐ and high‐energy vibronic channels, yielding an additional +52% lifetime increase (410 ns total, +650% vs. solution) together with a 233% PL enhancement from film to metasurface. A point‐by‐point anti‐correlation between the measured and the isotropic Purcell factor provides direct evidence of radiative‐rate redistribution across the manifold. This establishes a design principle for integrating molecular emitters with scalable, biodegradable nanophotonic platforms and is extendable to systems where vibronic relaxation competes with radiative DOS, including thermally activated delayed fluorescence (TADF) emitters and bio‐derived luminophores.