Basaric, NikolaThomas, Suma S.Bregovic, Vesna BlazekCindro, NikolaBohne, Cornelia2020-08-212020-08-2120152015Basaric, N., Thomas, S. S., Bregovic, V. B., Cindro, N., & Bohne, C. (2015). Phototautomerization in Pyrrolylphenylpyridine Terphenyl Systems. Journal of Organic Chemistry, 80(9), 4430-4442. https://doi.org/10.1021/acs.joc.5b00275.https://doi.org/10.1021/acs.joc.5b00275http://hdl.handle.net/1828/12014[4-(2-Pyrrolyl)phenyl]pyridines 2–4 were synthesized and their photophysical properties and reactivity in phototautomerization reactions investigated by fluorescence spectroscopy and laser flash photolysis (LFP). The pKa for the protonation of the pyridine nitrogen in 2–4 was determined by UV–vis and fluorescence titration (pKa = 5.5 for 4). On excitation in polar protic solvents, 2–4 populate charge-transfer states leading to an enhanced basicity of the pyridine (pKa* ≈ 12) and enhanced acidity of pyrrole (pKa* ≈ 8–9) enabling excited-state proton transfer (ESPT). ESPT gives rise to phototautomers and significantly quenches the fluorescence of 2–4. Phototautomers 2-T and 4-T were detected by LFP with strong transient absorption maxima at 390 nm. Phototautomers 2-T and 4-T decayed by competing uni- and bimolecular reactions. However, at pH 11 the decay of 4-T followed exponential kinetics with a rate constant of 4.2 × 106 s–1. The pyridinium salt 4H+ 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 pKa for 4H+ (pKa = 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.enLight absorptionReaction mechanismsPyridinesFluorescencePyrrolesPostprintDepartment of Chemistry