ABSTRACT
Noncovalent interactions of anions with electron-deficient aromatic rings that have been studied so far involve non-heteroaromatic or nitrogen-based heteroaromatic systems. Here we report the first case of an organic oxygenated aromatic system, in particular the tri-aryl-pyrylium tetrafluoroborate system, for which noncovalent anion-π interactions of the pyrylium cation with the tetrafluoroborate anion have been experimentally detected and demonstrated by means of (19)F NMR spectroscopy in solution. A series of pyrylium tetrafluoroborate salts were synthesized in the presence of BF3·Et2O, by direct reaction of 4-substituted benzaldehydes with 4-substituted acetophenones or via the previously obtained chalcone of the less reactive ketone. Correlations of (19)F NMR chemical shifts of tetrafluoroborate anion for most of the synthesized tri-arylpyrylium tetrafluoroborate complexes with both the pyrylium cation molecular weight and the standard substituent Hammett constants, demonstrate anion-π(+) interaction to act between the polyatomic anion BF4(-) and the pyrylium aromatic system. DFT calculations reveal that an additional (C-H)(+)-anion hydrogen bond involving the H(5) of pyrylium ring exists for these fluorescent dyes that show a tunable cup-to-cap shape cavity. The strong fluorescence emission observed for some representative pyrylium tetrafluoroborates described herein, makes them a promising class of tunable emission wavelength dyes for laser technology applications.
Subject(s)
Heterocyclic Compounds, 3-Ring/chemistry , Models, Chemical , Models, Molecular , Anions/chemistry , Computer Simulation , Electromagnetic Fields , Heterocyclic Compounds, 3-Ring/radiation effects , Materials Testing , Molecular ConformationABSTRACT
The current article presents the photobleaching properties of a group of pyrylium salts under ultrashort pulsed illumination. These pyrylium salts have the same basic chemical structure and differ only by a specific substituent. It is proven experimentally that two different mechanisms are simultaneously present to the photobleaching of all molecules studied (independently of their specific chemical structure). However, the particular parameters of each mechanism are influenced significantly by the substituent change. The experimental investigation of these parameters showed the presence of multiphoton interactions in the photobleaching of pyrylium salts depending essentially on the specific substituent.
