Unusual Through-Space Interactions between Oxygen Atoms Mediate Inverse Morphochromism of an AIE Luminogen

We have studied the photophysics of tetrafurylethene, an aggregation-induced emission luminogen that has exceptionally short intramolecular O-O distances of 2.80 Å and shows a significant redshifted morphochromism (27 nm) when going from the aggregate to the crystal. The short O-O distances, which are substantially smaller than the sum of the van der Waals radii (3.04 Å), are due to the fact that the oxygen atoms act as an electronic bridge connecting the furan rings on opposite ends of the central double bond, giving rise to a circular delocalization of the π electron density across the rings. In the excited state the O-O distance is further reduced to 2.70 Å, and the increased O-O interaction causes a narrowing of the HOMO-LUMO gap that results in the red morphochromism of the emission. Our results establish that the structural origin of the red-shifted emission lies in the close O-O contacts, and pave the way for understanding the clusteroluminescence of oxygen-rich, non-conjugated systems that emit visible light. Introduction.