Synthesis of new ketocyanine dyes for the development of optical sensors: Optical characterisation and solvatochromic behaviour

Some new symmetrical and unsymmetrical near-infrared absorbing acidochromic dyes are synthesised by a simple one-step reaction. Such indicators belong to the ketocyanine dyes type and as well as their full characterisation as the relationship between their structure and optical properties are discussed. They are mainly tested for their future application as pH or solvent polarity sensors. The absorption maxima of the long-wavelength absorbing acidic form span the region from 715 to 750 nm, nearly matching the emission wavelength of the 780 nm laser diode. pKa values in ethanol cover the range between 1.7 and 4.3, and reversible absorption spectra are observed while pH is changed forth and back. As merocyanine dyes, the synthesised ketocyanine dyes show a positive solvatochromism. Since one of the requirements for a solvatochromic molecule to be useful to establish a solvent polarity scale is its stability, the photostability of the dyes is tested in ethanolic solution as well.