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. In contrast to solution-phase emission, this red-shifted fluorescence is characteristic of 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.