Null Exciton Splitting in Chromophoric Greek Cross (+) Aggregate.

Exciton interactions in molecular aggregates play a crucial role in tailoring the optical behaviour of pi-conjugated materials. Though vital for optoelectronic applications, ideal Greek cross-dipole (alpha=90 degrees) stacking of chromophores remains elusive. We report a novel Greek cross (+) assembly of 1,7-dibromoperylene-3,4,9,10-tetracarboxylic tetrabutytester (PTE-Br-2) which exhibits null exciton coupling mediated monomer-like optical characteristics in the crystalline state. In contrast, nonzero exciton coupling in X-type (alpha=70.2 degrees, PTE-Br-0) and J-type (alpha=0 degrees, theta=48.4 degrees, PTE-Br-4) assemblies have perturbed optical properties. Additionally, the semi-classical Marcus theory of charge-transfer rates predicts a selective hole transport phenomenon in the orthogonally stacked PTE-Br-2. Precise rotation angle dependent optoelectronic properties in crystalline PTE-Br-2 can have consequences in the rational design of novel pi-conjugated materials for photonic and molecular electronic applications.