Molecular Engineering of Xanthene Dyes with 3D Multimodal‐Imaging Ability to Guide Photothermal Therapy

Abstract Phototheranostics integrates light‐based diagnostic techniques with therapeutic interventions, offering a non‐invasive, precise, and swift approach for both disease detection and treatment. The efficacy of this approach hinges on the multimodal imaging potential and photothermal conversion efficiency (PCE) of phototheranostic agents (PTAs). Despite the promise, crafting multifunctional phototheranostic organic small molecules brims with challenges. In this research, four innovative xanthene‐derived PTAs are synthesized by fine‐tuning the donor‐ π ‐acceptor (D‐ π ‐A) system to strike a balance between radiative and nonradiative decay. The inherent robust photostability and intense fluorescence of the traditional xanthene core are preserved, meanwhile the addition of highly electron‐withdrawing groups boosts the non‐radiative decay rate to enhance PCE and photoacoustic imaging capabilities. Remarkably, one of the PTAs, DMBA, demonstrates an exceptional absolute fluorescence quantum yield of 2.46% in PBS, and when encapsulated into nanoparticles, it achieves a high PCE of 79.5%. Consequently, DMBA nanoparticles (DMBA‐NPs) are effectively employed in fluorescence, 3D photoacoustic, and photothermal imaging‐guiding tumor photothermal therapy. This represents the first instance of a multimodal phototheranostic xanthene agent achieving synergistic fluorescence and photoacoustic imaging for diagnostic purposes. Furthermore, this work paves the way for leveraging xanthene fluorophores as versatile tools in the development of multifunctional reagents.