Introducing 3D conjugated acceptors with intense red absorption: homoleptic metal(II) complexes of di(phenylacetylene) azadipyrromethene

A novel strategy for the design and synthesis of functional materials with excellent acceptor properties is presented. The materials are based on homoleptic metal(II) complexes of azadipyrromethene (aza-DIPY) derivatives, and exhibit intense red absorption and high electron affinity. Their strong accepting properties were demonstrated by fluorescence quenching experiments using poly(3-hexylthiophene) as the donor. DFT calculations showed that the homoleptic metal(II) complexes of 2,8-di(4-tert-butylphenylacetylene)-1,3,5,7-tetraphenylazadipyrromethene had a similar distorted tetrahedral geometry to the complexes of 1,3,5,7-tetraphenylazadipyrromethene, but with additional conjugated 'arms' extending in 3 dimensions (3D). A unique feature of these complexes is that the two aza-DIPY ligands are p-stacked with each other, with frontier molecular orbitals delocalized over the two ligands. These complexes can therefore easily accept electrons, delocalize the negative charge over a large conjugated structure and have the potential of transporting charges in 3D. These properties make them attractive alternatives to fullerene derivatives for use as acceptors in organic solar cells, photo-detectors and other optoelectronic applications.