Spectroscopy, Electrochemistry and Photochemistry of Phenylazonaphthalene Dyes

The spectroscopy, electrochemistry and photochemistry of seven 2-(4-R-phenylazo)naphthalene-4,8-disulfonate dyes (R-Span dyes), were studied to report on their structure and reactivity in water. The dyes differ in their structure by only the R substituent attached to the phenyl ring, representing a range of electron-donating and electron-withdrawing substituents; most studies were carried out on OH-Span.

A combination of UV/Visible, NMR, Raman and IR spectroscopy together with DFT calculations has shown that the dyes are planar in their stable trans-isomer form. The spectra were found to be sensitive to the R substituent and generally they show good correlations with Hammett σp substituent constants. The structures of OH-Span (pKa = 7.98) and NH2-Span (pKa = 2.88) are pH dependent, and none of the dyes show aggregation at ≤ 3 * 10^-2 mol dm^-3.

Spectroelectrochemistry and controlled potential electrolysis studies showed that OH-Span undergoes an irreversible four electron reduction process, where detailed product analysis showed that naphthyl and phenyl fragments of the dye are produced due to azo bond scission; similar results were observed for the other R-Span dyes. Dyes containing electron-donating R substituents are more resistant to reduction, whereas dyes containing electron-withdrawing R substituents are more resistant to oxidation.

The stability of the R-Span dyes was assessed by reaction with photoinitiator generated 2-hydroxy-2-propyl radicals to study reductive fading. Time-resolved studies were carried out on OH-Span:photoinitiator solutions and rate constants for electron transfer to produce the dye radical anion and subsequent disproportionation were found to be 6.00 * 10^9 and 5.00 * 10^8 dm^3 mol^-1 s^-1, respectively. The detailed product analysis identified the naphthyl fragment of the dye, which was also observed for other R-Span dyes, indicating that the reduction mechanism occurs via disproportionation resulting in azo bond scission.

The R-Span dyes show trans to cis photoisomerisation, and NH2-, OH-, OMe- and NHAc-Span showed complete cis to trans thermal back reactions within ca. 30 ms, 35 μs, 11 days and 1 day, respectively.