pH-Dependent Optical Properties of Nanographenes

Area of Science:

• Materials Science
• Nanotechnology
• Physical Chemistry

Background:

• Nanographenes (NGs) are carbon nanomaterials with unique optical properties.
• Their optical characteristics can be influenced by environmental factors like pH.

Purpose of the Study:

• To investigate the pH-dependent optical properties of nanographenes.
• To elucidate the mechanisms behind their halochromic behavior in visible and near-infrared (NIR) regions.

Main Methods:

• Size-separated nanographenes were analyzed using UV-vis and photoluminescence (PL) spectroscopy.
• Experiments were conducted across a range of pH levels.
• Computational calculations were performed on a deprotonated nanographene model.

Main Results:

• Nanographenes exhibited pH-dependent halochromic behavior in both visible and NIR regions.
• Absorption spectra showed a two-step change related to carboxy group protonation/deprotonation and aggregation/disaggregation.
• Minimal impact on electronic structures was observed, suggesting localized effects at edge moieties.

Conclusions:

• The halochromic behavior of nanographenes is attributed to the protonation/deprotonation of carboxy groups, influencing edge moiety electronic transitions.
• These transitions (edge-to-edge and edge-to-surface π-π*) in the visible and NIR regions are responsible for the observed color changes.
• The findings provide insight into the optical responsiveness of nanographenes to pH variations.