Colorimetric calcium-response of β-lactosylated μ-oxo-bis-[5,15-meso-diphenylporphyrinatoiron(III)]

β-Lactosylated 5,15-meso-diphenylporphyrinatoiron(III) chloride was prepared by ironization of the corresponding free base porphyrin having acetylated lactoside-units followed by deacetylation with ammonia in a water–methanol mixture. The resultant 5,15-meso-bis(β-lactosylphenyl)porphyrinatoiron(III) chloride showed unique colorimetric response to calcium cation. This colorimetric response is calcium-specific and no other cations, such as sodium, potassium, or magnesium ions induced such colorimetric response. Lines of evidence including UV–vis spectra under different conditions and TEM images strongly indicate that interdigitations of the corresponding μ-oxo-dimers are responsible for this colorimetric change.