Comparison of rhenium–porphyrin dyads for CO₂ photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy

We report a study of the photocatalytic reduction of CO₂ to CO by zinc porphyrins covalently linked to [ReI(2,2′-bipyridine)(CO)₃L]⁺/⁰ moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C₆H₄NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C₆H₄NHC(O)C₆H₃(OMe)NHC(O), while Dyad 3 has a saturated bridge C₆H₄NHC(O)CH₂; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5 : 1) is approximately an order of magnitude better than [Dyad 1 pic]PF₆ or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re–Br units is very similar to that of the dyads with [Re–pic]⁺ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO₂into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH₂Cl₂) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of 3MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium

Molecular tweezers with freely rotating linker and porphyrin moieties

Molecular tweezers were synthesised by using a microwave accelerated alkene plus cyclobutane epoxide reaction between norbornyl appended porphyrin moieties and a diepoxide functionalised phenyl diimide spacer. The tweezers contain several rotational degrees of freedom; about the porphyrin with respect to the norbornyl linker, and between the two norbornyl backbone sections. The ability of Zn(super)II metallated tweezer 1 to complex 1,4-diazabicyclo[2.2.2]octane (DABCO) was studied by UV/Vis and ¹H NMR spectroscopy and multivariate global spectral analysis. The system was found to form a strong 1:1 intramolecular complex (1:DABCO) with an association constant of K₁₁ = 8.1 × 10⁷ M⁻¹, transforming to a 1:2 open complex [1:(DABCO)₂] with K₁₂ = 2.7 × 10⁹ M⁻² at high concentrations of DABCO.Rhys B. Murphy, Duc-Truc Pham, Stephen F. Lincoln, and Martin R. Johnsto

Localization and Photodynamic Efficacy of Two Cationic Porphyrins Varying in Charge Distribution

Two meso-tetraphenylporphyrin derivatives bearing adjacent: 5,10-di[4-(N-trimethylaminophenyl)-15,20-diphenylporphyrin (DADP-a) or opposite: 5,15-di[4-(N-trimethylaminophenyl)-10,20-diphenylporphyrin (DADP-o) cationic-N-(CH3)3+ groups on two of the para-phenyl positions were examined with regard to photodynamic properties as a function of charge distribution. The two adjacent positive charges in the DADP-a structure result in a molecular distortion (asymmetry), likely from electrostatic repulsion. This could be responsible for the unusual interaction of this compound with some solvents and detergent micelles. In contrast, DADP-o is a much more symmetric molecule. In a cellular environment, fluorescence spectra of the two agents were essentially identical. Subcellular localization played a major role in photodynamic efficacy. DADP-a localized in mitochondria, and irradiation of photosensitized cells (640-650 nm) resulted in a rapid loss of the mitochondrial membrane potential (ΔΨm), usually a prelude to apoptotic cell death. In contrast, DADP-o localized in lysosomes, and extensive lysosomal photodamage was observed after irradiation. Both steady-state accumulation levels and absorbance spectra favored DADP-o, but the light dose required for a 90% cell kill was two-fold greater for DADP-o than for DADP-a, at a constant extracellular sensitizer concentration. These data indicate that, on a photons/cell basis, DADP-a was five-fold more efficacious. Fluorescence emission spectra in different solvents and detergents demonstrated a tendency for DADP-a association. We interpret these results to indicate partition of both drugs to membrane loci, with mitochondria being the more lethal site for photodamage

Synthesis of novel carboranylchlorins with dual application in boron neutron capture therapy (BNCT) and photodynamic therapy (PDT)

Total synthesis of carboranylchlorins 3 and 4, from readily available starting materials, are described and the molecular structures of two key intermediates are presented. Chlorins 3 and 4 show similar spectroscopic behavior but differ considerably in their solubility properties; whereas closo-carboranylchlorin 3 is completely insoluble in water, its nido derivative 4 has good water-solubility. Carboranylchlorin 3 absorbs in the red region of the optical spectrum (at λmax=642nm) six times more strongly than porphyrin 1, and displays a fluorescence emission band at λmax=651nm, upon excitation at 642nm. The water-soluble carboranylchlorin 4 also displays intense absorption and emission bands at λmax=642 and 651nm, respectively, in ethanol solution. It is concluded that carboranylchlorins 3 and 4 have higher promise for the dual application in PDT and BNCT than do comparable porphyrins. © 2004 Elsevier Ltd. All rights reserved

Synthesis of a porphyrin-labelled carboranyl phosphate diester: A potential new drug for boron neutron capture therapy of cancer

A boron-rich, water-soluble porphyrin conjugate was synthesized by coupling of two carboranyl alcohols with 2-chlorophenoxyphosphorus dichloride, followed by conjugation to an amine-functionalized tetraphenyl-porphyrin via an amide linkage

Synthesis and cellular studies of an octa-anionic 5,10,15,20-tetra[3,5- (nido-carboranylmethyl)phenyl]porphyrin (H\u3csub\u3e2\u3c/sub\u3eOCP) for application in BNCT

The total synthesis of 5,10,15,20-tetra[3,5-(carboranylmethyl)phenyl] porphyrins 2-5 containing 36-43% boron by weight are reported. All compounds were characterized by spectroscopic methods and, in the case of 2, by X-ray crystallography. The water-soluble nido-carboranylporphyrin 5 (H2OCP) was found to have low dark toxicity toward V79 lung fibroblasts (CS 50 ≥ 250 μM), to be readily taken up by human glioblastoma T98G cells in culture and to localize subcellularly preferentially in the cell lysosomes. In comparison with a known tetra(nido-carboranyl)porphyrin (6), H2OCP (5) is taken up slower and to a lower extent by T98G cells, possibly as a result of its higher hydrophilic character. The metal-free H 2OCP (5) was also found to accumulate to a higher extent in T98G cells compared with its zinc(II) complex analog 4. Our studies show that carboranylporphyrins bearing eight nido-carborane cages can still accumulate intracellularly and have low dark toxicity toward cells in culture, and therefore might have promise for application in BNCT. © 2004 Elsevier Ltd. All rights reserved

Synthesis and cellular studies of a carboranylchlorin for the PDT and BNCT of tumors

The syntheses of closo- and nido-carboranylchlorins 4 and 5 from a known carboranylporphyrin are described. Water-soluble nido-carboranylporphyrin 5 was found to have very low dark cytotoxicity (IC50 \u3e 500 μM using a MTT-based assay) but to be toxic in the presence of red light (IC50 = 80 μM at 0.55 J/cm2 light dose). Under the same experimental conditions, carboranylchlorin 5 was taken up by human glioma T98G cells to a significantly higher extent than chlorin e6, a chlorophyll degradation product. The preferred sites of subcellular localization of carboranylchlorin 5 were found to be the cell lysosomes. Our results suggest that carboranylchlorin 5 is a promising new dual sensitizer for the PDT and BNCT treatment of tumors. © 2006 Elsevier Ltd. All rights reserved