Phototautomerization in Pyrrolylphenylpyridine Terphenyl Systems

摘要

[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 (pK(a) = 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* approximate to 12) and enhanced acidity of pyrrole (pKa* approximate to 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 x 10(6) s(-1). The pyridinium salt 4H(+) forms a stable complex with cucurbit[7]uril (CB[7]) with 1:1 stoichiometry (beta(11) = (1.0 +/- 0.2) x 10(5) M-1, [Na+] = 39 mM). Complexation to CB[7] increased the pK(a) for 4H(+) (pK(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.