Excitonic Origin of Long-Wavelength Fluorescence in Crystalline DAST

Long-wavelength photoluminescence in crystalline 4-N,N-dimethylamino-4′-N′-methylstilbazolium tosylate (DAST) has been repeatedly observed, yet its physical origin has remained unresolved. Unlike solution-phase emission, this red-shifted fluorescence appears exclusively in the crystalline state, indicating an intrinsic lattice-related mechanism. Here, experimental spectroscopy is combined with a structure-derived excitonic model based on the crystallographic arrangement of DAST chromophores. The model reveals a low-energy collective exciton state near 1.7 eV, in quantitative agreement with the observed Form III emission. The results demonstrate that structural non-equivalence within the unit cell plays a decisive role in stabilizing low-energy emissive excitonic states in organic ionic crystals.