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.