Rose Bengal Sodium | CAS 632-69-9 | Dye Content ≥80%

Rose Bengal Sodium (Rose Bengal disodium salt, C.I. 45440, Acid Red 94, disodium 4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein) is an anionic xanthene dye and one of the most heavily halogenated members of the fluorescein family. Rose Bengal carries the unusual combination of four chlorine substituents on the benzoate ring and four iodine substituents on the xanthene chromophore, which is what sets it apart from other halogenated fluoresceins: the heavy iodine atoms drive efficient intersystem crossing through the internal heavy-atom effect, while the full tetrachloro/tetraiodo framework gives Rose Bengal its strongly visible-absorbing, red-shifted xanthene-dye profile relative to fluorescein, eosin Y, and erythrosine B. The compound is supplied as a dark red to brown powder and is handled as a water-compatible anionic xanthene dye; solvent choice and concentration should be matched to the intended staining, photochemical, or analytical workflow. In aqueous solution it absorbs strongly in the green-yellow region with an absorption maximum near 548 nm and emits in the orange-red near 567 nm, and decomposes above 300 °C without a discrete melting point. Excitation populates a long-lived triplet excited state that transfers energy to ground-state molecular oxygen to generate singlet oxygen (¹O₂). Rose Bengal sodium is used as a reference photosensitizer in photochemical and photophysical research, as a metal-free visible-light organophotocatalyst in synthesis, and as a versatile biological stain in microbiology, marine biology, and analytical applications.

Singlet Oxygen Generation and Reference Photosensitizer

Use Rose Bengal sodium as a reference photosensitizer for singlet-oxygen (¹O₂) generation under visible-light irradiation in the green region of the spectrum. Excitation near 548 nm populates the singlet excited state, which undergoes efficient intersystem crossing to the triplet state through the heavy-atom effect of the four ring-bound iodine atoms; triplet-triplet energy transfer to ground-state O₂ then delivers ¹O₂. Under suitable oxygenated conditions, Rose Bengal is a high-yield singlet-oxygen photosensitizer, with commonly cited Φ(¹O₂) values around 0.75 in water depending on measurement conditions, solvent, aggregation state, and matrix. This places Rose Bengal among the standard benchmark photosensitizers — alongside methylene blue and selected porphyrins — for calibration of new photosensitizing agents, determination of ¹O₂ quantum yields by competitive trapping or near-infrared phosphorescence detection at 1270 nm, and comparison of photosensitizer performance across solvents and matrices. Use Rose Bengal sodium as the singlet-oxygen source in mechanistic studies of [4+2] cycloaddition with dienes (endoperoxide formation), ene-type reactions with alkenes (allylic hydroperoxide formation), and oxidation of sulfides, phenols, and electron-rich heteroaromatic substrates. As with other photosensitizing dyes, solutions are light-sensitive.

Visible-Light Organophotocatalysis

Rose Bengal sodium is one of the established metal-free organophotocatalysts in visible-light-mediated synthetic transformations. Under visible-light or green-LED irradiation, the long-lived Rose Bengal triplet is generated by intersystem crossing from the singlet excited state and is the reactive species in the catalytic cycle. Rose Bengal-mediated transformations include aerobic oxidative hydroxylation of organoboron compounds, oxidative C–H functionalisation of tertiary amines via α-amino radical intermediates, selected sulfonylation reactions, and thiol-ene radical additions. Depending on substrate redox potentials, oxygen, additives, and solvent, these reactions can proceed through oxidative or reductive quenching, energy transfer, singlet oxygen, superoxide, or related radical pathways. Its water compatibility, low cost, metal-free composition, and green-region absorption make it a useful organic-dye alternative in reactions where the redox window and operating conditions match the Rose Bengal excited state, complementing rather than universally replacing ruthenium and iridium polypyridyl photocatalysts whose absorption profiles, excited-state lifetimes, and redox potentials cover different chemical territory.

Selective Microbiological and Marine Biology Staining

Rose Bengal sodium is a long-established biological stain with two canonical applications. In microbiology, Rose Bengal is incorporated into Rose Bengal–Chloramphenicol Agar for selective enumeration of yeasts and moulds in food, dairy, environmental, and industrial quality-control samples: Rose Bengal restricts mould colony size and spreading and is taken up by fungi to aid enumeration, while chloramphenicol provides antibacterial selectivity. In marine biology and paleoecology, Rose Bengal is a traditional protoplasm stain for cytoplasm-bearing benthic foraminifera in sediment samples, helping separate stained cytoplasm-containing tests from empty tests; interpretation requires caution because recently dead or preserved cytoplasm can also retain stain, so Rose Bengal staining is not on its own a definitive proof of living status at collection. The compound also appears as a contrast component in selected composite histological staining methods, but its primary biological-stain reputation rests on the mycology and foraminiferal applications.

Model Anionic Dye for Adsorption and Photodegradation Studies

In materials and catalysis research, Rose Bengal can serve as a model anionic dye for adsorption, membrane-filtration, ion-exchange, and photocatalytic-degradation studies. Its visible absorption enables concentration tracking by UV–vis spectroscopy, while its anionic xanthene structure makes it useful for evaluating dye binding on activated carbons, clays, polymeric resins, metal-organic frameworks, and related adsorbent platforms. In semiconductor-photocatalyst studies, Rose Bengal is one of several model dye substrates used to monitor decolourisation kinetics under UV or visible-light irradiation, alongside more conventional benchmarks such as methylene blue, rhodamine B, and methyl orange. Note the inherent tension in this use: Rose Bengal is itself a high-yield singlet-oxygen photosensitizer, so its degradation kinetics under visible light reflect both external photocatalyst activity and self-sensitised photodegradation, which makes it a well-characterised but mechanistically informative test substrate rather than a passive contaminant model.

Other Applications

• Photosensitizer for singlet-oxygen trapping studies using chemical probes such as ABDA or related anthracene derivatives, with probe selection matched to solvent and detection method
• Photosensitizer component in visible-light photoinitiating systems for radical photopolymerisation, including thiol-ene and acrylate formulations
• Model sensitizer in studies of dye-sensitised photocatalysis on TiO₂, ZnO, silica-supported systems, and related semiconductor or oxide surfaces
• Component of selective culture media in mycology and food microbiology beyond Rose Bengal–Chloramphenicol Agar
• Reference compound in spectroscopic and photophysical method development where a green-absorbing, heavy-atom-substituted xanthene dye is required