Polyazacrown ethers containing one, two, and four appended naphthyl or anthracenyl units have been synthesized. The absorption spectra and photophysical properties of the novel compounds have been investigated in CH2Cl2 solution. Acid titration of the amine nitrogens of the polyazacrown ethers causes strong changes in the absorption and fluorescence spectra. In the unprotonated compounds of the naphthalene family, the naphthalene-type emission is completely quenched and a weak, unstructured, and broad fluorescence band with maximum at 440 nm is observed. Upon addition of trifluoroacetic acid, the absorption maximum is displaced to the red by a few nanometers, and a revival of the strong naphthalene-type emission at 340 nm is observed. These results are accounted for by the change in the nature of the lowest excited state (CT to pi-pi*) upon protonation. In the compounds of the anthracene family, the deprotonated forms of the species containing the polyazacrown groups show an absorption around 300 nm and a long wavelength tail at lower energies, neither of which are present in the 9-[(methylamino)methyl]anthracene. In all cases a weak, anthracene-type fluorescence is present, increasing in intensity on protonation. Furthermore, the emission spectra of the compounds with two or four anthracene moieties show an excitation-dependent, broad emission band at lower energies, which almost disappears upon protonation. This behaviour can be accounted for by the quenching of the anthracene-type emission caused by lower lying charge-transfer levels and, in the case of the compounds containing two and four anthracene moieties, of conformers where an anthracene-anthracene interaction is present. In all cases, each equivalent of added acid causes protonation of one equivalent of crown nitrogen. However, in the azacrown with four appended naphthyl units the revival of the naphthalene-type fluorescence does not parallel the number of added protons. This shows that the higher energy pi-pi* levels of the protonated units are quenched by the lower energy CT levels of the units involving the not yet protonated crown nitrogens.
|Titolo:||Synthesis and photophysical properties of polyazacrown ethers with appended naphthyl or anthracenyl units|
MANFREDI, AMEDEA GIUSEPPINA (Secondo)
|Parole Chiave:||Charge transfer; Luminescence; Polyazacrown ethers; Protonations; Supramolecular chemistry|
|Settore Scientifico Disciplinare:||Settore CHIM/06 - Chimica Organica|
|Data di pubblicazione:||giu-2000|
|Appare nelle tipologie:||01 - Articolo su periodico|