Interaction of the Triplet State of Modified Porphyrin Dye with Nitroxyl Radical

Porphyrins and related compounds are widely used as nanomaterials [1], in organic solar batteries [2], for medical treatment of various skin deceases [3], as well as photosensitizers for photodynamic therapy (PDT) [4, 5]. A series of compounds already have been approved for clinical practice [6]. Chemical modification of the initial molecules is one of the methods for enhancing the efficacy of molecules (photosensitizers) used in PDT. This modification is directed to improvement of the photodynamic properties of the molecules, to an increase in their bioavailability and a decrease in their system (dark) toxicity. In this work, we studied tetraphenylporphyrin with a carboranylsuccinimide substituent in the β-position of the porphyrin macrocycle. Earlier, it was shown that carborane derivatives of porphyrins exhibited higher antitumor activity than their nonborylated analogs [7]. The interaction of photosensitizers with stable radicals is of interest because of intensification of intersystem crossing by a paramagnetic particle (nitroxyl radical). The conjugate of naphthalenediimide derivative with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) was studied in [8]. As a result of the covalent bonding of the chromophore with the stable radical, the properties of this compound exhibits an effect similar to the heavy atom effect. It is of interest to explore the interaction of tetrapyrrole molecules and stable radicals for possible design of novel promising photosensitizers. In this study we investigated the interaction of the triplet state of 2-{3-[(o-carboran-1'-yl)thio]pyrrolidine-2,5-dion-1-yl}-5,10,15,20-tetraphenylporphyrin (P), synthesized according to the procedure described in [9], with the stable nitroxyl radical (2R) 4hydroxy-2,2,6,6tetramethylpiperidin-1-yl)oxyl (TEMPOL) (Sigma-Aldrich) by pulse photolysis technique. Absorption spectra were recorded on a Shimadzu UV-3101PC spectrophotometer in quartz cells (1 × 1 cm). The decay of the triplet states and the triplet–triplet absorption spectra of P were obtained on a pulse photolysis setup with the use of a quartz cell with an optical pathlength of 20 cm and a Xe lamp with a flash energy of 80 J (15 μs) [10] in ethanol and toluene solutions. Before photoexcitation, oxygen was removed by evacuation under reduced pressure. PhoN NH N