Hydroxymethylaniline Photocages for Carboxylic Acids and Alcohols

<i>ortho</i>-, <i>meta</i>- and <i>para</i>-Hydroxymethylaniline methyl ethers <b>3–5-OM</b>e and acetyl derivatives <b>3–5-OAc</b> were investigated as potential photocages for alcohols and carboxylic acids, respectively. The measurements of photohydrolysis efficiency showed that the decaging from <i>ortho</i>- and <i>meta</i>-derivatives takes place efficiently in aqueous solution, but not for the <i>para</i>-derivatives. Contrary to previous reports, we show that the <i>meta</i>-derivatives are better photocages for alcohols, whereas <i>ortho</i>-derivatives are better protective groups for carboxylic acids. The observed differences were fully disclosed by mechanistic studies involving fluorescence measurements and laser flash photolysis (LFP). Photoheterolysis for the <i>para</i>-derivatives does not take place, whereas both <i>meta</i>- and <i>ortho</i>-derivatives undergo heterolysis and afford the corresponding carbocations <b>3-C</b> and <b>4-C</b>. The <i>ortho</i>-carbocation <b>4-</b><i><b>o</b></i><b>-C</b> was detected by LFP in aqueous solution (λ_max = 410 nm, τ ≈ 90 μs). Moreover, spectroscopic measurements for the <i>meta</i>-acetyl derivative <b>3-</b><i><b>m</b></i><b>-OAC</b> indicated the formation of cation in the excited state. The application of an <i>ortho-</i>aniline derivative as a protective group was demonstrated by synthesizing several derivatives of carboxylic acids. In all cases, the photochemical deprotection was accomplished in high yields (>80%). This mechanistic study fully rationalized the photochemistry of aniline photocages which is important for the design of new photocages and has potential for synthetic, biological, and medicinal applications

Phototautomerization in Pyrrolylphenylpyridine Terphenyl Systems

[4-(2-Pyrrolyl)­phenyl]­pyridines <b>2</b>–<b>4</b> were synthesized and their photophysical properties and reactivity in phototautomerization reactions investigated by fluorescence spectroscopy and laser flash photolysis (LFP). The p<i>K</i>_a for the protonation of the pyridine nitrogen in <b>2</b>–<b>4</b> was determined by UV–vis and fluorescence titration (p<i>K</i>_a = 5.5 for <b>4</b>). On excitation in polar protic solvents, <b>2</b>–<b>4</b> populate charge-transfer states leading to an enhanced basicity of the pyridine (p<i>K</i>_a* ≈ 12) and enhanced acidity of pyrrole (p<i>K</i>_a* ≈ 8–9) enabling excited-state proton transfer (ESPT). ESPT gives rise to phototautomers and significantly quenches the fluorescence of <b>2</b>–<b>4</b>. Phototautomers <b>2-T</b> and <b>4-T</b> were detected by LFP with strong transient absorption maxima at 390 nm. Phototautomers <b>2-T</b> and <b>4-T</b> decayed by competing uni- and bimolecular reactions. However, at pH 11 the decay of <b>4-T</b> followed exponential kinetics with a rate constant of 4.2 × 10⁶ s^–1. The pyridinium salt <b>4H</b>^<b>+</b> forms a stable complex with cucurbit[7]­uril (CB[7]) with 1:1 stoichiometry (β₁₁ = (1.0 ± 0.2) × 10⁵ M^–1, [Na⁺] = 39 mM). Complexation to CB[7] increased the p<i>K</i>_a for <b>4H</b>^<b>+</b> (p<i>K</i>_a = 6.9) and changed its photochemical reactivity. Homolytic cleavage of the pyrrole NH leads to the formation of an N-radical because of the decreased acidity of the pyrrole in the inclusion complex