Dihedral Angles and Photoluminescence Quantum Yields: An NMR Analysis

The synthesis of two different series of donor-acceptor (D-A) molecules is reported, consisting of a series of four structurally related donors and two different acceptors. The subtle differences in the electron density of these D-A-D and D-A compounds are clearly reflected in the different chemical shifts of certain donor protons in the ¹H NMR spectra. These shifts show a cosine squared correlation of the dihedral angle between the donor units and the neighbouring phenyl unit of the acceptor. This correlation is also related to optical properties such as the photoluminescence quantum yield, which shows a similar trend due to the different degree of charge transfer during excitation and relaxation processes. In this way, it is possible to directly correlate a molecular structural parameter with a material property on a purely experimental basis, which should be applicable to many donor-acceptor systems.