Pinning Down\nSmall Populations of Photoinduced Intermediates\nUsing Transient Absorption Spectroscopy and Time-Dependent Density\nFunctional Theory Difference Spectra to Provide Mechanistic Insight\ninto Controlling Pyridine Azo Dynamics with Proton

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