We report the synthesis and photophysical characterization of novel halogenated dipyrrolonaphthyridine-diones (X 2 -DPNDs, X = Cl, Br, and I), as candidates for photodynamic therapy (PDT) application. Apart from the heavy atom-induced spinorbit coupling (SOC) dynamics in the investigated X 2 -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 O 2 to 1 O 2 using X 2 -DPND correlates with the rate of triplet population, k ISC >1.6 × 10 8 s −1 for I 2 -DPND vs k ISC >2.9 × 10 9 s −1 for Cl 2 -DPND and Br 2 -DPND (where τ ISC = 343 AE 3 ps for I 2 -DPND and τ ISC = 5-6 ns for Cl 2 -DPND and Br 2 -DPND are the lowest time constants/values for ISC). Furthermore, the heavy atom-induced SOC in Cl 2 -DPND and Br 2 -DPND did not lead to a reduction of the corresponding fluorescence (ca 75% vs 67% for the parent DPND). The attractive photophysical characteristics of Cl 2 /Br 2 -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.
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