This study introduces a new nanocrystallization method assisted by ultrasounds that produces needles or nanocubes of CMONS, a stilbene dye, with an excellent control over the polymorphism, and with a narrow size distribution. Owing to the production of radicals from dissolved dioxygen by high-intensity ultrasounds, trans-to-cis isomerization was observed in the absence of nitrogen bubbling, with the formation of two distinct crystalline phases for the different diastereomers. The crystallinity of CMONS needles was probed by various techniques, including X-ray and electron diffractions, fluorescence spectroscopy, and dynamic nuclear polarization (DNP) enhanced solid-state nuclear magnetic resonance. The latter was used to hyperpolarize 1 H nuclei and to record 1 H-13 C and 1 H-15 N CPMAS NMR spectra at natural isotopic abundance with very high signal-to-noise ratio. With such sensitivity, one can easily discriminate between cis and trans-I forms of CMONS, detect the presence of multiple polymorphic phases (even with minor contributions) and check the absence of amorphous phase. Finally, the mechanism involved in the formation of CMONS needles was ascertained after stabilizing intermediate nanocubes against Ostwald ripening and ordered aggregation mechanisms using the CTAB surfactant.