Directional control of π-conjugation enabled by distortion of the donor plane in diarylaminoanthracenes: A photophysical study

We designed and synthesized a series of diarylaminoanthracenes in which the planarity of the diarylamine moiety is controlled by methylene- or ethylene- bridges. The X-ray crystallographic structures confirm that the methylene- and ethylene bridges gradually decrease the disorder of the diarylamine planes. To quantitatively analyze the photophysical properties and underlying electronic structures of these compounds, we carried out UV-vis and fluorescence spectroscopy, fluorescence quantum yield, and fluorescence lifetime measurements. The results reveal that enhanced planarity of the diarylamine moiety optically forbids the charge-transfer transition between the diarylamine and anthracene moieties. Although it is generally accepted that a planar π-conjugated system favors electron delocalization, our results indicate that distortion of the donor plane induces interchromophoric conjugation rather than conjugation within the local structure. Density functional theory calculations further illustrate that the frontier orbitals of diarylamine and anthracene interpenetrate as the donor plane is distorted. Additionally, natural bonding orbital analyses reveal that distortion of the donor plane changes the directionality of the π-conjugation of the nitrogen n-orbital from intrachromophoric to interchromophoric.