The doorstop proton: acid-controlled photoisomerization in pyridine-based azo dyes

Abstract

The photochemical properties and straightforward synthesis of a wide range of azo dyes has led to their use in a wide range of applications. Despite their broad use, the fundamental structure-function relationships of many variants are unexplored. Here, azo dyes with a pyridine moiety are systematically investigated. The pyridine affords a range of electronic effects due to the position of the nitrogen heteroatom relative to the azo bond and serves as a second protonatable site that provides a way to alter the isomerization yield of the dye through inductive effects and protonation. Photometric titrations, visible-light photoisomerization, and density functional theory (DFT) calculations reveal and rationalize the unexpected loss of photoisomerization that occurs upon protonation of the pyridine N (the first protonation site on the pyridine-based azo dyes). The result is particularly surprising as this site is not adjacent to or on the azo bond and yet it completely shuts downs bulk photoisomerization. The first protonation of the azo dyes onto the pyridine N results in a red shift of the spectra by 132 nm, 64 nm and 106 nm for Pyr2, Pyr3, and Pyr4, respectively. The second protonation onto the azo bond blueshifts the spectra by 102 nm, 19 nm, and 89 nm, respectively. pK_a values of the pyridine N are 14.9, 14.6 and 15.3, while the pK_a values of the azo bond N are 11.3, 11.4, and 13.8, for Pyr2, Pyr3, and Pyr4, respectively. DFT reveals that the loss of photoisomerization arises from both a reduction in the generated cis-isomer upon photoexcitation and an accelerated cis to trans reversion process on the ground-state potential energy surface.