Position Over Composition: Positional Isomerism Enables Encapsulation‐Independent Intrinsic Stability in a Conjugated Polymer

ABSTRACT Conjugated polymers derive their optoelectronic function from π‐delocalization. However, this feature renders them intrinsically susceptible to photochemical degradation. Here, we show that intrinsic stability can be altered without changing chemical composition by repositioning side chains. In polydiphenylacetylene derivatives, shifting substituents from the para to the meta position increases photostability by more than fivefold. This stability hierarchy, driven by positional isomerism rather than substituent type, persists across chemical modification and in solution and film states. Composition‐invariant solvent titration reveals that the enhanced stability of the meta isomer correlates with position‐induced conformational changes, specifically increased rigidity and reduced kink formation. Beyond photostability, the meta isomer exhibits greater resistance to high‐energy ion‐beam irradiation. As a functional demonstration, an unencapsulated surface‐contact fluorescent viscosity sensor based on the meta isomer shows a 6.6‐fold longer operational lifetime than its para analogue, defined by the time for a 10% loss in sensitivity (T 90 ), and retains approximately 75% of its initial response slope under prolonged irradiation conditions that nearly eliminate the sensing response of the para isomer. These results establish chain conformation as a key determinant of intrinsic stability and identify positional isomerism as a composition‐invariant molecular design rule for improving the reliability of surface‐exposed functional conjugated materials.