Pinning Down Small Populations of Photoinduced Intermediates Using Transient Absorption Spectroscopy and Time-Dependent Density Functional Theory Difference Spectra to Provide Mechanistic Insight into Controlling Pyridine Azo Dynamics with Protons

In this work, the impact of protonation on the photoisomerization (<i>trans</i> → <i>cis</i>) and reversion (<i>cis</i> → <i>trans</i>) of three pyridine-based azo dyes (<b>PyrN</b>) is investigated by using a combination of transient absorption spectroscopy and time-dependent density functional theory computed difference spectra. The photophysical behaviors of the <b>PyrN</b> dyes are altered by the addition of one or two protons. Protonation of basic pyridine nitrogens results in an ultrafast accelerated reversion mechanism after photoisomerization, while protonation of azo bond nitrogens restricts <i>cis</i> isomer formation entirely. Computed difference spectra provide spectral signatures that are critical for the assignment of low-population long-lived states, providing direct evidence of the accelerated reversion mechanism. Thus, the addition of organic acids can selectively control the photophysics of azo dyes for a wide range of applications, including materials design and pharmaceuticals.