A pH stable fluoran-triphenylamine photosensitizer with efficient type I and type II ROS generation

Photosensitizers (PSs) with robust pH stability and the ability to generate both type I and type II reactive oxygen species (ROS) have gained significant attention due to their versatility in various applications. In this study, we employed an electron donor-acceptor engineering strategy to design and synthesize a fluoran-triphenylamine photosensitizer (Fl-TPA), using an ester-protected ring-opened fluoran cation as the electron acceptor and triphenylamine (TPA) as the electron donor. Compared to fluoran with a spirolactone structure, Fl-TPA exhibits a significant redshift in absorption, with good light capture capabilities in the 300-600 nm range. In comparison with the reference compound Fl-H, which lacks the TPA group, Fl-TPA shows a substantial decrease in fluorescence intensity. Transient fluorescence measurements reveal biexponential decay characteristics for both compounds. Specifically, Fl-TPA shows τ₁ = 0.21 ns (41%) and τ₂ = 2.92 ns (59%), while Fl-H shows τ₁ = 0.14 ns (93%) and τ₂ = 2.23 ns (7%). The longer-lived component in Fl-TPA is more pronounced, suggesting the presence of additional non-radiative decay pathways, as further supported by the steady-state fluorescence analysis. Additionally, Fl-TPA exhibits a significant Stokes shift in solvents of varying polarity. Time-dependent density functional theory (TD-DFT) calculations reveal that the introduction of the strong electron-donating TPA group reduces the ΔE_S-T of Fl-TPA to 1.25 eV, which is significantly lower than that of Fl-H (1.46 eV), facilitating intersystem crossing (ISC). Thus, in the ROS generation experiment, it can be observed that Fl-H produces almost no ROS. In contrast, Fl-TPA not only exhibits high type I ROS generation capability, but also demonstrates excellent type II and total ROS generation capabilities, with performance far superior to the clinically approved near-infrared PS, indocyanine green (ICG). Moreover, Fl-TPA exhibits excellent pH stability compared to the non-esterified fluoran. The results of this study present a new photosensitizer with strong ROS generation capability and good stability across a wide pH range, providing a theoretical foundation for the design of PSs.