Segmentation Driven Peeling for Visual Analysis of Electronic Transitions

TL;DR

This work addresses the challenge of deciphering electronic transitions in molecules by identifying donor/acceptor subgroups using a bivariate visualization framework based on hole NTO $\phi_h$ and particle NTO $\phi_p$. It introduces a CSP-based analysis with fiber surfaces and a segmentation-driven CSP peeling operator that links range-space patterns to spatial subgroups, enabling donor/acceptor classification and differentiation between local and charge-transfer excitations. The approach is demonstrated on Thiophene-Quinoxaline and copper complexes, showing its ability to reveal how donor-acceptor roles vary with conformation and ligand environment, and to compare transitions across states. The method relies on Gaussian-derived NTOs and topology-based CSP computations (Tierny), suggesting a practical, visually driven tool for theoretical chemists seeking intuitive insight into electronic transitions; future work will explore alternative segmentation strategies and quantitative metrics for peeled CSPs.

Abstract

Electronic transitions in molecules due to absorption or emission of light is a complex quantum mechanical process. Their study plays an important role in the design of novel materials. A common yet challenging task in the study is to determine the nature of those electronic transitions, i.e. which subgroups of the molecule are involved in the transition by donating or accepting electrons, followed by an investigation of the variation in the donor-acceptor behavior for different transitions or conformations of the molecules. In this paper, we present a novel approach towards the study of electronic transitions based on the visual analysis of a bivariate field, namely the electron density in the hole and particle Natural Transition Orbital (NTO). The visual analysis focuses on the continuous scatter plots (CSPs) of the bivariate field linked to their spatial domain. The method supports selections in the CSP visualized as fiber surfaces in the spatial domain, the grouping of atoms, and segmentation of the density fields to peel the CSP. This peeling operator is central to the visual analysis process and helps identify donors and acceptors. We study different molecular systems, identifying local excitation and charge transfer excitations to demonstrate the utility of the method.

Figures (4)

• Figure 1: Visual analysis workflow. Molecular structure, electron density fields, and subgroup descriptions are available as input. A weighted Voronoi segmentation identifies atomic regions. The segment corresponding to a molecular subgroup is computed as the union of atomic regions. The CSP of the bivariate field may be explored either using fiber surfaces or peeled on demand based on individual segments corresponding to the subgroups. Peeled CSPs are visually analyzed side-by-side and in comparison with the CSP of the bivariate field for subgroup classification and donor-acceptor strength comparisons.
• Figure 2: CSP peeling for analyzing Thiophene-Quinoxaline (green and orange planes, respectively) for varying dihedral angle based conformations of state 1. In the $60^{\circ}$ and $90^{\circ}$ conformations, the peeled CSP for Thiophene covers a small region suggesting local excitation within Quinoxaline. $90^{\circ}$ exhibits least charge transfer from Thiophene to Quinoxaline. Thiophene behaves as a donor within other molecular conformations.
• Figure 3: CSP peeling for analyzing copper complexes with different ligand configurations, all in state 1. Copper behaves as a donor and PHE behaves as an acceptor within all complexes. The charge transfer from Cu is symmetric in Cu-PHE-PHE.
• Figure 4: CSP peeling for comparing different excitation states. For Cu-PHE-PHEOME, we observe charge transfer in state 9 and local excitation within PHE in state 10. For Cu-PHE-XANT, in state 3 we note charge transfer and local excitation within XANT in state 10.