Covalent Carbon Nanodot-Azobenzene Hybrid Photoswitches: The Role of Meta/Para Connectivity and sp3 Spacer in Photophysical Properties

Abstract

The covalent surface functionalization of carbon nanodots (CNDs) can facilitate the design and development of nanocarbon hybrids with photoswitching properties, which can be applied in a wide range of applications, including sensing, optoelectronics, and even bio-applications. This study underscores the potential utilization of these hybrids as photoresponsive molecules, for potential application in optostimulation. To this aim, it is imperative to comprehend the resulting properties that emerge from the combination of CNDs with photoresponsive functionalities, which will provide a foundation for the rational design of future nanocarbon hybrids. In this study, we examine the characteristics of covalent azobenzene-functionalized CNDs, with a particular emphasis on the impact of connectivity and the additional introduction of a spacer. The CND synthesis process comprises a bottom-up microwave condensation of ethylenediamine and citric acid. The successful amide coupling to azobenzenes is confirmed through NMR diffusion-ordered spectroscopy and corresponding diffusion decay analysis. A comprehensive investigation is conducted into the size and optical properties of the resulting hybrids. A theoretical analysis of the absorbance characteristics is presented, employing time-dependent density functional theory calculations to facilitate a deeper understanding of their behaviour. Furthermore, advanced optical characterisations are utilised to examine energy/charge transfer between the constituents. Finally, an in-depth study of the switching properties, fatigue resistance, and half-life of the hybrids is conducted to evaluate their performance for prospective applications like in optostimulation.