Carbon dots as photosensitizers: unraveling their ultrafast charge transfer, challenges, and future prospects

Carbon dots (C-dots) have emerged as highly promising light-harvesting materials, particularly as photosensitizers, due to their eco-friendliness, biocompatibility, and cost-efficiency. Their adaptability as photosensitizers has garnered widespread attention, marking them as pivotal materials for future technological innovations. One of the key attributes of C-dots is their dual functionality in charge transfer, enabling them to serve as both electron donors and acceptors. This charge transfer process between C-dots and small organic molecules as quenchers plays a critical role in diverse applications such as photocatalysis, sensing, and optoelectronics. In this perspective, we have discussed the capability of C-dots in confined environments, doped C-dots, C-dots/molecular hybrids, and perovskite/C-dots composites as photosensitizers. This perspective includes the origin of fluorescence and carrier dynamics in full colour light-emitting C-dots, followed by a novel way to control the photosensitizer capability of C-dots <i>via</i> the transfer of electrons and holes in hybrids, composites, and doped C-dots, and the effects of the core and surface in the electron transfer process. The photosensitizer capability of C-dots was investigated <i>via</i> exploring the charge transfer dynamics using various advanced optical techniques like steady-state and time-resolved photoluminescence and ultrafast transient absorption (TA). This perspective also focuses on understanding the ultrafast dynamics of C-dots, such as charge transfer, charge transport, and charge recombination, in various environments, composites, and hybrid systems, with an emphasis on their development as effective photosensitizers. The extensive range of reported electron donor-acceptor systems underscores the versatility of C-dots as photosensitizers, with their tuneable electronic properties tailored to address the demands of emerging technological challenges.