Photophysical properties and pH sensing applications of luminescent salicylaldehyde derivatives

The photophysical properties, density functional theory calculations, and pH sensing applications of a series of salicylaldehyde derivatives that contain only one small pi-conjugated benzene ring system are reported. The steric, electronic and pi-extended effects on their photophysical properties are fully investigated. The presence of a donor-acceptor (DA) system ensures not only blue, green, and yellow emission, but also high fluorescence quantum yields up to 0.35 in dimethylformamide (DMF) at room temperature. The absorption and fluorescence properties of the salicylaldehyde derivatives are highly dependent on the pH value of the solution, which realize their applications in pH sensing and living cell imaging. The pK (a) values of these compounds ranging from 4.6 to 9.6 can be well-tuned by their structures. Moreover, in order to achieve good solubility in water, a sodium sulfonato group was introduced into the salicylaldehyde derivatives, which are very useful for tuning pK (a) values and detecting pH values in water.