Modulating the Photophysical and Photothermal Response of Porphyrin and Chlorin Hybrids: The Impact of Graphene Oxide Reduction Degree

The performance of graphene-based nanohybrids in combined photodynamic and photothermal therapy depends strongly on the balance between photosensitizer binding, excited-state deactivation, and heat generation. In this work, we examine how the degree of graphene oxide reduction influences the photophysical and photothermal properties of nanohybrids formed with 5,10,15,20-tetrakis(3-hydroxyphenyl)porphyrin (m-THPP) and its chlorin analogue (m-THPC). Graphene oxide (GO) and partially reduced graphene oxide (RGO) were functionalized noncovalently with both macrocycles, and the effect of reduction on their interactions was evaluated by steady-state and time-resolved spectroscopy. Absorption measurements indicate ground-state complex formation driven by π-π interactions, with RGO showing higher adsorption capacity than GO. Fluorescence studies reveal predominantly static quenching with little effect on excited-state lifetimes. Femtosecond transient absorption measurements demonstrate ultrafast photoinduced electron transfer from the excited photosensitizers to the graphene-based materials, leading to decreased triplet-state population and lower singlet oxygen generation. At the same time, RGO exhibits substantially higher photothermal efficiency under 808 nm irradiation than GO. These results show that increasing the degree of reduction enhances photothermal performance but also strengthens excited-state quenching. This provides guidelines for the rational design of dual-function nanohybrids, emphasizing that balancing the reduction level with reduced material loading is essential for optimized therapeutic performance.