Solvatochromic behavior of some α-styrylpyridinium dyes

• Study of solvatochromism of some α-styrylpyridinium dyes in 21 solvents of various polarities.
• Sensing solvent polarity change from non-polar to dipolar aprotic to polar protic using the dyes.
• Correlation of solvatochromic shifts of the dyes with substituent effects.
• Classification of the substrate into four groups based on the substituent-type.
• Ortho-effect in dyes proposed to be the chief cause of efficient solvent sensing ability.

The influences of solvent polarity and substituent on the electronic transition of six different N-butyl-α-styrylpyridinium dyes have been investigated in 21 solvents. Reichardt’s ET(30) scale has been used to propose a quantitative approach towards the relative stability of the electronic ground and excited state species. The solvents have been classified into three types and the dyes have been classified into four groups based on the contribution of field and inductive effects of the substituents towards the change in their absorption maxima values. Instead of a steady solvatochromism, all the dyes except p-nitro substituted one, suffer reversals in solvatochromism at ET(30) values of ∼37 and ∼48. The extents of contribution of non-polar and polar protic solvents towards the solvation of the excited states of the dye molecules have been determined to be 30–40% more than that towards the stability of their ground states by the dipolar aprotic solvents. The ortho effect shown by this class of dyes in contrast to their corresponding γ-isomers might have been responsible for their better solvent polarity sensing capability.

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