Rational Design of Organic Probes for Turn-On Two-Photon Excited Fluorescence Imaging and Photodynamic Therapy

The rational design of two-photon absorption (2PA) organic dyes with 2PA cross-section (δ) values tunable by mild stimuli under physiological conditions remains a challenge. Using a simulation-assisted structure screening method, we have designed an acetal terminated distyrylbenzene derivative (Ace-DSB), which can be readily converted into aldehyde terminated molecules (Ald-DSB) to exhibit a significant increase in δ values at 760 nm. The conversion reaction, which dramatically turns on the 2PA absorption capability of the molecules, can occur in an intracellular environment. Furthermore, Ald-DSB exhibits 40 times higher efficiency of two-photon generation of singlet oxygen than Ace-DSB. These turn-on features allow enhanced two-photon laser confocal scanning microscopic imaging and two-photon excited photodynamic therapy (2PE-PDT) for MCF-7 cancer cells and melanoma tumors. This work opens a new avenue toward the design of turn-on-type two-photon absorption molecules for both in vitro and in vivo tumor imaging and 2PA-PDT.