Synthesis and spectroscopic characterisation of heptamethincyanine NIR dyes for their use in optochemical sensors

A set of near-infrared absorbing tricarbocyanine dyes has been synthesised and characterised for their future application as chromoionophores in optochemical sensors. Their absorption maxima are localised in the NIR region, where matrix interferences are minimal and the use of suitable and inexpensive optical communication components gives great advantages. The acid form of the synthesised dyes in pure ethanol spans the region from 675 to 815 nm and the molar absorptivities are up to 3.3 × 105 l/mol cm. The calculated pKa values in ethanol lie between 11.4 and more than 13. The effect of the introduction of substituents on the pKa values and on the spectroscopic characteristics of the dyes is also discussed. Moreover, pH-sensitive aggregation processes have been observed in aqueous solution.