Coupled Regulation of the Triplet‐State Manifold and Host–Guest Confinement Enables Efficient Room‐Temperature Phosphorescence of Dibenzofuran Derivatives in PVA

Efficient organic room‐temperature phosphorescence requires both efficient triplet formation and suppression of nonradiative deactivation, but these factors are often difficult to distinguish when structurally unrelated emitters are compared. Here, a series of 2‐substituted dibenzofuran (DBF) derivatives with a common molecular framework was used to examine how substitution affects both excited‐state properties and interactions with poly(vinyl alcohol) (PVA). Spectroscopic measurements and theoretical calculations show that substitution changes the energies and orbital characters of the excited states, thereby altering intersystem crossing pathways, while also modulating guest–PVA interactions that stabilize triplet excitons under ambient conditions. The carboxylic acid derivative (DBF‐CA) combines favorable singlet–triplet coupling with strong host–guest interactions, giving blue phosphorescence at 438 nm with a lifetime of 3.55 s and a phosphorescence quantum yield of 25.0%. The hydroxy derivative exhibits excitation‐dependent blue‐to‐green afterglow arising mainly from isolated and aggregated species, while DBF‐CA further sensitizes fluorescent dyes to generate multicolor delayed emission. These results show that peripheral substitution within a common molecular framework provides complementary control over triplet formation and host‐mediated triplet stabilization.