Electrochemical, Spectroscopic, and¹O₂Sensitization Characteristics of Synthetically Accessible Linear Tetrapyrrole Complexes of Palladium and Platinum

Abstract

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethyl-5,15-bis(pentafluorophenyl)-biladiene (DMBil1) linear tetrapyrrole supporting PdII or PtII centers is presented. Both of these nonmacrocyclic tetrapyrrole platforms are robust and easily prepared via modular routes. X-ray diffraction experiments reveal that the Pd[DMBil1] and Pt-[DMBil1] complexes adopt similar structures and incorporate a single PdII and PtII center, respectively. Additionally, electrochemical experiments revealed that both Pd[DMBil1] and Pt[DMBil1] can undergo two discrete oxidation and reduction processes. Spectroscopic experiments carried out for Pd[DMBil1] and Pt[DMBil1] provide further understanding of the electronic structure of these systems. Both complexes strongly absorb light in the UV–visible region, especially in the 350–600 nm range. Both Pd[DMBil1] and Pt[DMBil1] are luminescent under a nitrogen atmosphere. Upon photoexcitation of Pd[DMBil1], two emission bands are observed; fluorescence is detected from ~500–700 nm and phosphorescence from ~700–875 nm. Photoexcitation of Pt[DMBil1] leads only to phosphorescence, presumably due to enhanced intersystem crossing imparted by the heavier PtII center. Phosphorescence from both complexes is quenched under air due to energy transfer from the excited triplet state to ground state oxygen. Accordingly, irradiation with light of λ ≥ 500 nm prompts Pd[DMBil1] and Pt[DMBil1] to photosensitize the generation of 1O2 (singlet oxygen) with impressive quantum yields of 80% and 78%, respectively. The synthetic accessibility of these complexes coupled with their ability to efficiently photosensitize 1O2 may make them attractive platforms for development of new agents for photodynamic therapy.

Photochemotherapeutic Properties of a Linear Tetrapyrrole Palladium(II) Complex displaying an Exceptionally High Phototoxicity Index

“…The emission properties of the Pd [ DMBil1 ] complex in nitrogen-saturated methanol are also largely unaffected by PEGylation. As has been previously observed for Pd- [ DMBil1 ], 15 excitation of Pd [ DMBil1 ] -PEG 750 with 500 nm light elicits weak fluorescence from 500 to 700 nm as well as phosphorescence from 700 to 850 nm (Figure 2). Table 1 shows that, while the fluorescence emission maximum of Pd [ DMBil1 ] -PEG 750 in methanol is red-shifted by 11 nm compared with that of Pd [ DMBil1 ], the fluorescence quantum yield of the PEGylated derivative ( Φ fl = 1.4 × 10 −4 ) is essentially identical to that of the parent compound ( Φ fl = 1.3 × 10 −4 ).…”

“…Pd [ DMBil1 ] -PEG 750 is readily accessible from unfunctionalized Pd [ DMBil1 ], which is readily prepared (Scheme 2) in four steps using methodology previously developed for the synthesis of biladienes 15,20 and related tetrapyrroles containing sp 3 hybridized meso -carbons. 21–23 PEGylation of the parent Pd [ DMBil1 ] architecture was facilitated by the presence of the two pentafluorophenyl substituents at the 5- and 15-positions, which can be derivatized via nucleophilic aromatic substitution of the para -fluorine substituents.…”

DFT Investigation of the Molecular Properties of the Dimethylglyoximato Complexes [M(Hdmg)2] (M = Ni, Pd, Pt)

“…The calculated HOMO-LUMO energy gaps are 6.302 and 6.413 eV for the Ni and Pd complexes, respectively and reduced to 6.062 eV for the Pt complex, which is caused by a marked stabilisation of the LUMO (Figure 3). This series Pd > Ni > Pt agrees quite well with the experimental binding energies and electrochemical or optical HOMO-LUMO gaps of comparable series of Pt, Pd, and Ni complexes [63][64][65]67,77].…”

“…A shorter The calculated M-N bond lengths increase from 1.912 to 2.015 Å along the series Ni << Pt < Pd which reflects the size of the central atom (Tables S2-S4, Supplementary). In many experimental and theoretical studies, Pd II has been found slightly larger than Pt II [63][64][65][66][77][78][79][80][81], in keeping with the effects of the lanthanide contraction [82]. While for the Ni complex (Table S2) the two calculated M-N bonds show identical length, for Pd the M-N6(oximate) bond is slightly shorter than M-N3(oxime) (Table S3).…”

Evaluating Nanoshells and a Potent Biladiene Photosensitizer for Dual Photothermal and Photodynamic Therapy of Triple Negative Breast Cancer Cells

“…Pd[DMBil1]-PEG 750 was synthesized using methods described previously [ 28 , 38 , 39 , 40 , 41 ]. Reactions requiring an inert atmosphere were carried out in oven-dried round bottomed flasks fitted with Suba-seal rubber septa purchased from Chemglass and maintained under positive nitrogen pressure using standard schlenk techniques.…”