A Visual Understanding of Circular Dichroism Spectroscopy: The Case of Interacting Chromophores

Abstract

Exciton coupling shapes the photophysical and chiroptical properties of molecular aggregates, but a microscopic, real-space understanding of how intermolecular interactions generate circular dichroism (CD) remains incomplete. Here, we computationally investigate the microscopic origins of CD in coupled molecular dimers to separate intermolecular (topological) from intramolecular (intrinsic) chirality. A combination of time-dependent density functional theory and transition chiral tensor (TCT) analysis provides a quantitative, visually intuitive framework showing how electronic coupling strength, intermolecular separation, and relative angular orientation govern chiroptical response. We find that pairs of individually achiral chromophores exhibit robust, topological chiroptical signals which give rise to the conventional Cotton effect, a standard interpretation of derivative features in circular dichroism spectra. We next establish that intrinsically chiral chromophores, on the other hand, disrupt the Cotton effect due to the competition between intrinsic chirality and topological chirality. This work establishes a foundation for interpreting exciton-coupled CD spectra in molecular aggregates, supramolecular assemblies, and nanostructures using the TCT approach, which offers an attractive tool for analyzing chiroptical responses in complex multi-chromophore systems relevant to emerging quantum technologies.