Naslov
Phototautomerization in pyrrolylphenylpyridine terphenyl systems

Autori
Basarić, Nikola ; Thomas, S. Suma ; Blažek Bregović, Vesna ; Cindro, Nikola ; Bohne, Cornelia

Izvornik
Journal of organic chemistry (0022-3263) 80 (2015), 9; 4430-4442

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
excited state proton transfer (ESPT); pyridine; pyrrole; laser flash photolysis; inclusion complexes; cucurbit[7]uril

Sažetak
[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 (CT) 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.

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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