GROUND-STATE DIMERS IN EXCIMER-FORMING BICHROMOPHORIC MOLECULES - NMR AND SINGLE-PHOTON-COUNTING DATA .2. RACEMIC AND MESO DIPYRENYLPENTANES AND DIPYRENYLALKANES

1H NMR spectra of the racemic and meso diastereomers of 2,4-di(1-pyrenyl)pentane (1DPP) in toluene-d8 and of the 1,n-di(1-pyrenyl)alkanes (1Py(n)1Py with n = 0-10 and 13) and 1,n-di(2-pyrenyl)alkanes (2Py(n)2Py with n = 1-10 and 14) in chloroform-d were measured at room temperature. The conformer distribution in both 1DPP molecules was determined from an analysis of the vicinal coupling constants between the methylene and methine protons. In meso-1DPP, an intramolecular sandwichlike ground-state dimer was detected, whereas in rac-1DPP a dimer only overlapping at the aromatic protons H9 and H10 is formed. The presence of both dimers was confirmed by time-resolved fluorescence measurements (single-photon counting (SPC)). With the dipyrenylalkanes, an analysis of the chemical shifts of the aromatic protons shows that sandwich dimers are not present. Only in the case of 1Py(3)1Py is a partial-overlap dimer detected. These findings are supported by results from SPC measurements. © 1990 American Chemical Society.