Chemical and Conformational Control of the Spectroscopic Properties of Multi-Layer and Multi-Defect Carbon Dots

TL;DR

This work addresses how multi-layer and multi-defect carbon dots (CDs) translate complex structure into optical spectra. Using density functional theory and time-dependent DFT on three-layer CD models, the authors dissect the roles of defect type, oxidation state, protonation, size, and interlayer motion, including solvent effects, in shaping $S_0 \rightarrow S_1$ excitations and emission. They identify two defect classes—dominant oxidizing defects that drive redshifts and excitations with charge-transfer character, and spectator defects that largely preserve the baseline spectra—while showing that higher oxygen content correlates with narrower optical gaps. Crucially, they demonstrate that twisting, sliding, and folding of surface-functionalized layers can significantly alter excitation energies, characters, and oscillator strengths, offering a mechanism for polarization fluctuations and emission intermittency observed experimentally. Overall, the study provides actionable structure–property relationships to tailor CDs for bioimaging, photocatalysis, and optoelectronic applications, aligning computational predictions with experimental trends.

Abstract

Carbon dots (CDs) are renowned for their bright and tunable photoluminescence (PL), stability, and biocompatibility, yet it remains challenging to link their heterogeneous structures to their spectroscopic properties. This study utilizes density functional theory (DFT) and time-dependent DFT (TD-DFT) to systematically investigate how the spectroscopic properties of complex CDs with multiple layers and multiple defects are determined by their structures and compositions. Calculations reveal that strongly oxidizing defects, such as carbonyl and carbonyl acetate, significantly redshift absorption and emission spectra. In contrast, less oxidizing defects, such as hydroxyl, behave as spectators with minimal impact on absorption and emission, except when they interact strongly with more oxidizing defects. We find that not only the excitation energy but also the excitation character itself is impacted by the presence of specific defects, and the pH-dependence of the spectroscopic properties can be attributed to their protonation state-dependent excitation character. We show that the twisting, sliding, and linker-mediated folding of surface-functionalized layers in CDs markedly alter excitation energies and characters, offering a molecular explanation for experimentally observed emission intermittency and polarization fluctuations. These insights provide strategies for optimizing CDs for various applications, including bioimaging, photocatalysis, and optoelectronic devices.

Figures (8)

• Figure 1: (a) The top and the side view of the model (idealized) three-layered carbon dot system used to study the effect of oxygen and nitrogen-containing surface defects. The outer layers are 3.42 Å apart from the central layer, as measured by the distance between planes passing through the atoms of each outer layer and the centroid position (red sphere) of the central layer. Surface defects are introduced by substituting the –CH groups (circled in black) of the fused aromatic ring in the central layer with common functional groups (R). The position marked using blue circles are substituted with oxygen atoms or –NH groups to obtain s-/o-dipyran or s-diazinane defects (b) Calculated vertical excitation (S$\textsubscript{0}$→ S$\textsubscript{1}$ transition) and emission energies of the model CD system decorated with different surface defects. The optimized coordinates of the systems are provided in the Supporting Information.
• Figure 2: Absorption (a) and emission (b) energies (eV) of pristine and functionalized carbon dot models with varying numbers of hydroxyl groups. The presence of dominant surface defects reduces the influence of additional hydroxyl groups on the vertical transition energies.
• Figure 3: (a) Relationship between excitation energy and oxygen content (%) for CD models with different surface defects. Increasing oxygen content generally lowers the excitation energy, corresponding to a smaller optical gap and a red-shift in absorption. (b) Ground-state Mulliken partial atomic charge distributions for representative CD models containing various surface defects. Atoms are color-coded by charge: green denotes near-neutral atoms, red indicates positive charge accumulation, and blue indicates negative charge accumulation. (c) Correlation between excitation energy (eV) and the ground-state charge (e) on the most perturbed aromatic carbon atom for CDs functionalized with different surface defects. Each point represents a model with a single defect type, illustrating how local charge perturbations modulate excitation energy.
• Figure 4: Natural transition orbitals (NTOs) for the lowest vertical singlet S$\textsubscript{0}$→ S$\textsubscript{1}$ excitation of (a) a pristine model and a model functionalized with (b) hydroxyl and (c) carbonyl functional groups. The plots use an isovalue of 0.03 au.
• Figure 5: Absorption (a) and emission (b) energies (eV) of variously sized CD models containing oxygen-bearing defects reveal that increasing the size leads to a redshift in the spectrum. However, the size of the functionalized layer plays a crucial role, while additional layers contribute negligibly.