Newly Designed Quasi-intrinsic Photosensitizers for Fluorescence Image-Guided Two-Photon Photodynamic Therapy with Type I/II Photoreactions.

In this work, a set of quasi-intrinsic photosensitizers are theoretically proposed based on the 2-amino-8-(1'-β-d-2'-deoxyribofuranosyl)-imidazo[1,2-α]-1,3,5-triazin-4(8H)-one (P), which could pair with the 6-amino-5-nitro-3-(1'-β-d-2'-deoxyribofuranosyl)-2(1H)-pyridone (Z) and keep the essential structural characters of nucleic acid. It is revealed that the ring expansion and electron-donating/electron-withdrawing substitution bring enhanced two-photon absorption and bright photoluminescence of these monomers, thereby facilitating the selective excitation and tumor localization through fluorescence imaging. However, instead of undergoing radiative transition (S → S), the base pairing induced fluorescence quenching and rapid intersystem crossing (S → T) are observed and characterized by the reduced singlet-triplet energy gaps and large spin-orbit coupling values. To ensure the phototherapeutic properties of the considered base pairs in long-lived T state, we examined the vertical electron affinity as well as vertical ionization potential for production of superoxide anions via Type I photoreaction, and their required T energy (0.98 eV) to generate singlet oxygen O via Type II mechanism.