Heteroatom-Containing Organic Molecule for Two-Photon Fluorescence Lifetime Imaging and Photodynamic Therapy

Two-photon fluorescence photosensitizers (TP-FPSs) with long fluorescence lifetime (t_F) and high singlet oxygen quantum yield (κ_δ) have attracted ever-growing attention as novel theranostic materials. However, to date, TP-FPSs with long t_F and high κ_t have been rarely reported. Toward achieving this goal, we report in this study a systematic investigation of excitation dynamics in a heteroatom-containing organic molecule (3-aminophthalic acid, abbreviated as 3-APA). Our study demonstrates that 3-APA has an n-π∗ absorption band and two-photon absorption. Derived from n-π∗ transition feature, 3-APA exhibits not only a considerably long t_F (11.5 ns) but also an efficient singlet-to-triplet intersystem crossing (19.2%), which leads to a high _δ (18.4%). With these properties, we successfully used 3-APA as a novel TP-FPS for the proof-of-concept application in two-photon fluorescence lifetime imaging and two-photon photodynamic therapy. Notably, this work gives a better understanding of the excitation process and mechanism involved in TP-FPSs, which should be a valuable guide for constructing novel high-performing TP-FPSs.