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.