Electronically tunable anion−π interactions in pyrylium complexes: experimental and theoretical studiesElectronic supplementary information (ESI) available: Experimental details, chemical shifts and Hammett correlations, UV-visible and fluorescence spectra, characterization data including 1H and 13C NMR spectra of all new compounds, 19F NMR spectra of studied compounds and cartesian coordinates for some representative complexes. See DOI: 10.1039/c4cp01977f

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 BF 3 ·Et 2 O, 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 BF 4 − 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. The first organic oxygenated aromatic system, tri-aryl-pyrylium tetrafluoroborate fluorescent dyes, for which noncovalent π + /(C-H) + -anion interactions have been demonstrated, is reported.