Diaminodibenzoxanthenium based near infrared pH probes with lower pKa

• One step synthesis of diaminodibenzoxanthenium skeleton.
• The PET type pH probes with lower pKa.
• Long wavelength emission in near-infrared region.

The fluorescent pH probes, 3,11-diamino-7-(4-(diethylamino)-phenyl)- dibenzo[c,h]xanthen-14-ium trifluoromethanesulfonate (1a) and 3,11-bis-(diethylamino)-7-(4-(diethylamino)phenyl)dibenzo[c,h]xanthen-14-ium trifluoromethanesulfonate (1b), were reported as pH probes worked in strong acidic conditions. Probes 1a–b were prepared by one-step condensation with 4-diethylaminobenzaldehyde and aminonaphthalen-1-ol derivatives, their absorption maxima gave slight changes between neutral and acidic conditions, and they exhibited pH-dependent near-infrared OFF–ON emission in the range of pH 2.0–5.0 with increasing fluorescent intensities of 272 and 114 folds, the calculated pKa values were 3.28 and 3.24 for probe 1a and probe 1b, respectively. The probes also showed excellent selectivity toward proton over other interferents in the selectivity and competition assays. The photoinduced electron transfer effect from diethylaminophenyl group to dibenzoxanthenium skeleton was proposed and further verified by theoretical calculation. The work indicates that dibenzoxanthenium would be a near-infrared fluorophore for probe design.

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