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

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 3 O2 to 1 O2 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.