Synthesis and fluorescence properties of dioxa-, dithia-, and diselena-[3.3](1,3)pyrenophanes

Abstract

[3.3](1,3)Pyrenophanes tethered by oxygen (1), sulfur (2) and selenium (3) atoms were synthesized and the structural and physical properties of these substances were determined. The absorption maxima of the [3.3](1,3)pyrenophanes were observed to shift to longer wavelengths in the order of 1 < 2 < 3. The fluorescence spectra of 1–3 contained both monomer and intramolecular excimer emissions, which correspond to anti and syn conformers, respectively. The ratios of the intensities of intramolecular excimer to monomer emission were observed to increase with the increasing solvent polarity. The intensity ratios also depend on temperature. For example, an increase in temperature results in an increase of the ratio of intensities of the intramolecular excimer to monomer fluorescence of 2. The results of ¹H NMR spectroscopic investigations show that resonances for the methylene and aromatic hydrogens in these substances coalesce at low temperatures with coalescence temperatures (T_c) that decrease in the order of 1 > 2 > 3. The results of geometry optimization studies using B3LYP/6-31G(d,p) demonstrate that the syn conformers of 1–3 have lower enthalpies than their anti counterparts, but the syn conformer of 1 and the anti conformers of 2–3 are entropically more favorable. These findings suggest that an equilibrium exists between the syn and anti conformers of the [3.3](1,3)pyrenophanes and that the conformer ratios are dependent on both the solvent polarity and temperature in a manner that can be explained in terms of a combination of enthalpies, dipole moments and entropies. The combined results show that the pyrenophanes are interesting substances that emit different fluorescence colors in a manner that is controlled by the surrounding environment.