Turn-on fluorescence sensor for mono- and di-phosphonic acid derivatives using anthracene-based diamidine and its detection of amidinium-phosphonate and amidinium formation

The fluorescence detection of di-phosphonic acid and mono-phosphonic acid derivatives using the anthracene-based diamidine 1 has been investigated. The diamidine 1 forms 1:1 and 1:2 complexes with the di-phosphonic acid and mono-phosphonic acid derivatives, respectively, and showed a blue fluorescence (λ_em = 432–442 nm) in a DMSO solution. The formation of amidinium-phosphonate (complex formation) and dissociated amidinum (λ_em = 468 nm as a broad band) were distinguished by the difference in the fluorescence wavelength, and confirmed by DOSY NMR spectroscopy and TD-DFT calculations. The formation of a 1:2 complex with diamidine 1 and methylphosphonic acid having additional intermolecular hydrogen-bonding between the methylphosphonic acids is proposed.