Exciton Isolation in Cross-Pentacene Architecture

Abstract

Null aggregates are 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 synergistic perpendicular orientation and significant inter-chromophoric separations realize, negligible long-range Coulombic and short-range charge transfer mediated couplings in the null aggregate. The Greek cross (+) orientation of pentacene dimers exhibit a selectively higher electron transfer coupling with near-zero hole transfer coupling and thereby contribute to the lowering of charge transfer mediated coupling even at shorter inter-chromophoric distances. Additional investigations on the nature of excitonic states of pentacene dimers proved that any deviation from 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 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. 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 in-sights towards the design strategies for cross-stacked chromophoric assemblies. Identifying Greek cross-stacked architecture mediated null excitons with 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.