This paper focuses on the fundamental question how hydrogen bonding affects the energy dissipation processes of excited molecules. The photophysical properties of two excited aromatic carbonyl compounds, Nile Red and N-(4-pyridyl)-1,2-naphthalimide were studied in the presence and the absence of the strong hydrogen bond donor, hexafluoro-2-propanol. Hydrogen bonding of Nile Red with alcohols can occour both in the ground and the excited states. The fluorescence lifetime and the quantum yield of fluorescence are found to diminish significantly with increasing hydrogen-bond donating power of the medium, whereas dielectric solvent-solute interactions do not accelerate the relaxation of the excited state. The observed deuterium effect clearly shows that the deactivation in alcohols takes place via the vibrations associated with hydrogen bonding. Fluorescence lifetime and quantum yield of N-(4-pyridyl)-1,2-naphthalimide exhibit opposite change on addition of hexafluoro-2-propanol compared with that observed for Nile Red. Both quantities increase significantly upon hydrogen bonding with the 4-pyridyl moiety. This effect is due to the increase of the energy gap between S1 and S 2 singlet excited states which leads to deceleration of the radiationless deactivation processes. In the excited state the electron density on the oxygen of naphthalimide moiety is much lower than on the carbonyl group of Nile Red therefore, hexafluoro-2-propanol barely causes fluorescence quenching for the former compound.
|Number of pages||5|
|Journal||Magyar Kemiai Folyoirat, Kemiai Kozlemenyek|
|Publication status||Published - Mar 1 2004|
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