GROUND-STATE DIMERS IN EXCIMER-FORMING BICHROMOPHORIC MOLECULES .1. BIS(PYRENYLCARBOXY)ALKANES

1H NMR spectra of the l,n-bis(l-pyrenylcarboxy)alkanes (lPC(n)lPC) and 1,n-bis(2-pyrenylcarboxy)alkanes (2PC(n)2PC) with n= 1-16, 22, and 32 were measured in chloroform-d at room temperature. From the changes in chemical shift of the aromatic protons in 2PC(n)2PC, as compared to the hexyl ester of 2-pyrenecarboxylic acid, it is concluded that an intramolecular ground-state dimer is present for n = 3-16, 22, and 32, a sandwich dimer for n > 8, and a shifted symmetric dimer for n = 3-8. With lPC(n)lPC, dimer formation is found for n = 3-16: predominantly an asymmetric dimer, next to a symmetric one (especially for n = 3-7). The solvent dependence of the dimer formation (2PC(5)2PC) can be correlated with the Hildebrand solubility parameter. Variation of temperature has practically no influence (1PC(5)1PC). The conclusions based on the NMR data are confirmed by time-resolved excimer fluorescence measurements. In the case of 2PC(3)2PC, the presence of a ground-state dimer is seen from the dependence of its fluorescence spectrum on the excitation wavelength. In the series of 1PC(n)1PC and 2PC(n)2PC as well as with the dipyrenylalkanes lPy(n)lPy and 2Py(n)2Py mutual through-bond interactions of the aromatic end groups are detected up ton = 8. © 1990, American Chemical Society. All rights reserved.