Hydrogen bond formation between 4-(dimethylamino)pyridine and aliphatic alcohols.
The photophysics of 4-(dimethylamino)pyridine (DMAP) has been investigated in different solvents in the presence of aliphatic and fluorinated aliphatic alcohols, respectively. For most systems, consecutive two-step hydrogen-bonded complex formation is observed in the presence of alcohols. Equilibrium constants are determined from UV spectroscopic results for the formation of singly and doubly complexed species. The resolved absorption and fluorescence spectra for the singly and doubly complexed DMAP are derived by means of the equilibrium constants. Exceptionally large hydrogen bond basicity values are found for the ground and singlet excited DMAP molecules. In n-hexane, as a consequence of complex formation, the intramolecular charge transfer (ICT) emission becomes dominant over of the locally excited fluorescence; the fluorescence and triplet yields increase considerably with complexation. In polar solvents, both the fluorescence and triplet yields of the complex are much smaller than that of the uncomplexed DMAP. The dipole moments derived for the singly complexed species from the Lippert-Mataga analysis are much larger than those of the uncomplexed molecules. However, for the relaxed ICT excited-state one obtains different dipole moments in apolar and polar solvents. This may be explained by a conformational change of the molecule in the ICT excited state from planar geometry in apolar solvent to the perpendicular structure (characterized with bigger dipole moment) in polar solvent.
Demeter
Attila
A
Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri u. 59-67, Hungary. demeter@chemres.hu
Mile
Viktória
V
Bérces
Tibor
T