Photophysical Insights of Halogenated Dipyrrolonaphthyridine-Diones as Potential Photodynamic Therapy Agents†

Abstract

We report the synthesis and photophysical characterization of novel halogenated dipyrrolonaphthyridine-diones (X2–DPNDs, X = Cl, Br, and I), as candidates for photodynamic therapy (PDT) application. Apart from the heavy atom-induced spin-orbit coupling (SOC) dynamics in the investigated X2–DPNDs, it was found that the position of the halogen atom (relative to the nitrogen of the pyrrole ring) also influenced the triplet excited state behavior. Interestingly, the faster/efficiency sensitization of 3O2 to 1O2 using X2–DPND correlates with the rate of triplet population, kISC >1.6 × 108 s−1 for I2–DPND vs kISC >2.9 × 109 s−1 for Cl2–DPND and Br2–DPND (where τISC = 343 ± 3 ps for I2–DPND and τISC = 5–6 ns for Cl2–DPND and Br2–DPND are the lowest time constants/values for ISC). Furthermore, the heavy atom-induced SOC in Cl2–DPND and Br2–DPND did not lead to a reduction of the corresponding fluorescence (ca 75% vs 67% for the parent DPND). The attractive photophysical characteristics of Cl2/Br2–DPND put them on the landscape as not only promising PDT agents but also as fluorescence probes. The present study is a stepping stone in the development of novel organic photosystems for synergistic photomedicinal applications.