Singlet-states-mediated highly efficient two-photon fluorescence in organic dyes

This study reports a brand-new continuous-wave-excited (CW-excited) two-photon fluorescence emission mechanism in indocyanine green (ICG), an organic fluorescent dye widely used in clinical practice. This mechanism is based on the excited state absorption (ESA) process of the first singlet excited state. Intramolecular electrons sequentially absorb two photons to reach a high-energy singlet excited state, followed by direct radiative transition to the ground state to generate fluorescence. The entire process is exclusively mediated by singlet states. We further summarize the essential requirements for organic dyes to realize this luminescence mechanism. First, the dye must possess at least two well-separated singlet excited states with distinct energies, corresponding to two absorption peaks at different wavelengths in the absorption spectrum. The wavelength of the high-energy singlet excited state is about half that of the first singlet excited state. Second, the peak in the absorption spectrum corresponding to the transition from the ground state to the first singlet excited state has a sufficiently large molar extinction coefficient. Third, the first singlet excited state exhibits the capability of ESA. Fourth, electrons at the high-energy singlet excited state can directly transit to the ground state and emit fluorescence. We validated this mechanism in a variety of organic dyes satisfying the above conditions, confirming its universality. Using CW laser as the excitation source, we achieved two-photon fluorescence imaging of mouse cerebral blood vessels at a depth of 400 μm, which clearly resolves three-dimensional vascular networks with high resolution. We also performed two-photon fluorescence imaging on human gastric cancer tissue samples at a depth of around 150 μm, which provides a low-cost strategy for clinicians to rapidly acquire high-contrast tumor tissue images.