Phototautomerization in Pyrrolylphenylpyridine Terphenyl Systems

[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 × 10(6) 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) × 10(5) 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.