This work demonstrates the colorimetric and ratiometric fluorogenic detection of harmful organic peroxides (OPs) by using the S–S annulated perylene monoimide (PMISS) mediated emergence of a unique solid-state red-emitting perylene monoimide (PMI) derivative. PMISS dye has been successfully used for the detection of hazardous and toxic peroxides present in THF and 1,4-dioxane solvents via exhibiting a drastic visible color change from blue to red accompanied by a turn-on fluorescence response, thus offering a much-needed method to detect the presence of high-risk OPs. The chemical reaction between PMISS and peroxide present in THF yielded us two new PMISSO and PMISSO₂ derivatives, which were purified, and their structural and optical properties were subsequently investigated using ¹H-NMR, UV-vis absorption, and fluorescence spectroscopy, single-crystal X-ray diffraction analysis (PMISSO₂ only), and calorimetry. Among both species, PMISSO₂ exhibits unique fluorescence behavior in solution and, more notably, in the solid-state, while PMISSO displayed weak fluorescence in solution and inactivity in the solid-state. Single crystal X-ray analysis of PMISSO₂ shows that steric congestion is caused by the molecular motion of one bulky phenoxy-alkyl substituent in the bay position, and this leads to considerable twisting of the PMI π-scaffold supported by C–H⋯O hydrogen bonds between the SO₂ moieties and the 1,6-bay substituents. This type of molecular flexibility, as observed in the bay-substituent, sterically prohibits PMI aggregation in the solid-state, which gives rise to defined vibronic progressions in the solid-state absorption spectra and also enables it to emit bright red fluorescence. Time dependent density functional theory (TD-DFT) calculations obtained for PMISSO₂ of the crystal geometry nicely support the experimental observations made in both bulk and solution states. This outcome has added a novel dimension to studies of fundamental structure–property relationships of relevance in PMI-based organic electronics.