Exciton Isolation in Cross-Pentacene Architecture.

Abstract

Null aggregates are an elusive, emergent class of molecular assembly categorized as spectroscopically uncoupled molecules. Orthogonally stacked chromophoric arrays are considered as a highlighted architecture for null aggregates. Herein, we unveil the null exciton character in a series of crystalline Greek cross (+)-assembly of 6,13-bisaryl-substituted pentacene derivatives. Quantum chemical computations suggest that the synergistic perpendicular orientation and significant interchromophoric separations realize negligible long-range Coulombic and short-range charge-transfer-mediated couplings in the null aggregate. The Greek cross (+)-orientation of pentacene dimers exhibits a selectively higher electron-transfer coupling with near-zero hole-transfer coupling and thereby contributes to the lowering of charge-transfer-mediated coupling even at shorter interchromophoric distances. Additional investigations on the nature of excitonic states of pentacene dimers proved that any deviation from a 90° cross-stacked orientation results in the emergence of delocalized Frenkel/mixed-Frenkel-CT character and the consequent loss of null exciton/monomer-like properties. The retention of exciton isolation even at a short-range coupling regime reassures the universality of null excitonic character in perpendicularly cross-stacked pentacene systems. The null-excitonic character was experimentally verified by the observation of similar spectral characteristics in the crystalline and monomeric solution state for 6,13-bisaryl-substituted pentacene derivatives. The partitioned influence of aryl and pentacene fragments on interchromophoric noncovalent interactions and photophysical properties, respectively, resulted in the emergence of pentacene centric Kasha's ideal null exciton, providing novel insights toward design strategies for cross-stacked chromophoric assemblies. Identifying the Greek cross (+)-stacked architecture-mediated null excitons with a charge-filtering phenomenon for the first time in the ever-versatile pentacene chromophoric systems can offer an extensive ground for the engineering of functional materials with advanced optoelectronic properties.