A fluorescent sensor for pH based on rhodamine fluorophore

In this study, an 'Off-On'-type acidic pH fluorescent chemosensor, 3',6'-Bis (ethylamino)-2',7'-dimethyl-2-(2-oxoethylideneamino)spiro[isoindoline-1,9'-xanthen]-3-one (RG1) has been designed, synthesized, and characterized by high-resolution mass spectrometry (HRMS), X-ray crystallography, infrared spectroscopy (IR), and H-1 NMR and C-13 NMR spectroscopy. The design tactics for the sensor was based on the change in structure between spirocyclic (non-fluorescent) and ring-open (fluorescent) forms of rhodamine dyes. Fluorescence "off-on" behaviors of RG1 were investigated on the basis of variable acid concentrations. The pH titrations showed a ca. 581-fold increase in fluorescence intensity within the pH range of 7.5 to 1.1 with a pK(a) value of 2.32 in acetonitrile-water (1:1, v/v) solution when excited at 510 nm. The fluorescence change of RG1 was fully reversible and took place mainly within the pH range from 1.0 to 4.0, which was valuable for pH researches in acidic environments. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.