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

TL;DR

This study uses computational methods to elucidate how structural defects and layer arrangements in carbon dots influence their spectroscopic properties, providing insights for optimizing their use in bioimaging and optoelectronics.

Contribution

It systematically investigates the effects of various defects and structural conformations on the spectroscopic behavior of complex carbon dots using DFT and TD-DFT.

Findings

Strong oxidizing defects cause redshift in spectra.

Less oxidizing defects have minimal impact unless interacting with oxidizing defects.

Surface layer conformations significantly alter excitation energies and characters.

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.