Heavy atom enhanced generation of singlet oxygen in novel indenofluorene-based two-photon absorbing chromophores for photodynamic therapy

A series of indenofluorene-based two-photon absorbing chromophores (FLO, FL-2BrN, FL-1IN, FL-2IN, and FL-2IS) have been designed and synthesized for photodynamic therapy (PDT). Two-photon absorption (TPA) properties of these chromophores are determined and their structure-property relationship has been established. By incorporating electron withdrawing halogen atoms (such as Br, I) into the pi-conjugation backbone of the chromophores, enhanced TPA cross-section values up to 2707 GM were achieved due to the formation of a donor-acceptor-donor (D-A-D) motif. At the same time, upon excitation at 430 nm, 2.2-4.1 times of enhancement in the singlet oxygen generation yield was observed for the photosensitizers with the heavy atoms incorporated. The photosensitizers with heavy atoms also show increased singlet oxygen generation efficiencies upon two-photon excitation at 808 nm, compared to those photosensitizers without heavy atoms. Photocytotoxicity of FL-2IN against cancer cells via two-photon excitation (808 nm) demonstrated that the indenofluorene-based chromophores are effective singlet oxygen donors for two-photon PDT. (C) 2015 Elsevier Ltd. All rights reserved.