Enhanced Acidity, Photophysical Properties and Liposome Binding of Perfluoroalkylated Phthalocyanines Lacking C-H Bonds

Photoelectrochemical Behavior of Phthalocyanine-Sensitized TiO2 in the Presence of Electron-Shuttling Mediators

Dye-sensitized TiO₂ has found many applications for dye-sensitized solar cells (DSSC), solar-to-chemical energy conversion, water/air purification systems, and (electro)chemical sensors. We report an electrochemical system for testing dye-sensitized materials that can be utilized in photoelectrochemical (PEC) sensors and energy conversion. Unlike related systems, the reported system does not require a direct electron transfer from semiconductors to electrodes. Rather, it relies on electron shuttling by redox mediators. A range of model photocatalytic materials were prepared using three different TiO₂ materials (P25, P90, and PC500) and three sterically hindered phthalocyanines (Pcs) with electron-rich tert-butyl substituents (t-Bu₄PcZn, t-Bu₄PcAlCl, and t-Bu₄PcH₂). The materials were compared with previously developed TiO₂ modified by electron-deficient, also sterically hindered fluorinated phthalocyanine F₆₄PcZn, a singlet oxygen (¹O₂) producer, as well as its metal-free derivative, F₆₄PcH₂. The PEC activity depended on the redox mediator, as well as the type of TiO₂ and Pc. By comparing the responses of one-electron shuttles, such as K₄Fe(CN)₄, and ¹O₂-reactive electron shuttles, such as phenol, it is possible to reveal the action mechanism of the supported photosensitizers, while the overall activity can be assessed using hydroquinone. t-Bu₄PcAlCl showed significantly lower blank responses and higher specific responses toward chlorophenols compared to t-Bu₄PcZn due to the electron-withdrawing effect of the Al³⁺ metal center. The combination of reactivity insights and the need for only microgram amounts of sensing materials renders the reported system advantageous for practical applications.

Correlation between the Fluorination Degree of Perfluorinated Zinc Phthalocyanines, Their Singlet Oxygen Generation Ability, and Their Photoelectrochemical Response for Phenol Sensing

Electron-withdrawing perfluoroalkyl peripheral groups grafted on phthalocyanine (Pc) macrocycles improve their single-site isolation, solubility, and resistance to self-oxidation, all beneficial features for catalytic applications. A high degree of fluorination also enhances the reducibility of Pcs and could alter their singlet oxygen (¹O₂) photoproduction. The ethanol/toluene 20:80 vol % solvent mixture was found to dissolve perfluorinated F_nPcZn complexes, n = 16, 52, and 64, and minimize the aggregation of the sterically unencumbered F₁₆PcZn. The ¹O₂ production ability of F_nPcZn complexes was examined using 9,10-dimethylanthracene (DMA) and 2,2,6,6-tetramethylpiperidine (TEMP) in combination with UV-vis and electron paramagnetic resonance (EPR) spectroscopy, respectively. While the photoreduction of F₅₂PcZn and F₆₄PcZn in the presence of redox-active TEMP lowered ¹O₂ production, DMA was a suitable ¹O₂ trap for ranking the complexes. The solution reactivity was complemented by solid-state studies via the construction of photoelectrochemical sensors based on TiO₂-supported F_nPcZn, F_nPcZn|TiO₂. Phenol photo-oxidation by ¹O₂, followed by its electrochemical reduction, defines a redox cycle, the ¹O₂ production having been found to depend on the value of n and structural features of the supported complexes. Consistent with solution studies, F₅₂PcZn was found to be the most efficient ¹O₂ generator. The insights on reactivity testing and structural-activity relationships obtained may be useful for designing efficient and robust sensors and for other ¹O₂-related applications of F_nPcZn.

Fluoroalkyl phthalocyanines: Bioinspired catalytic materials

The design of self oxidation-resistant catalytic materials based on organic molecules, although advantageous due to the ability to control their structures, is limited by the presence of labile C–H bonds. This mini review summarizes recent work aimed at first-row transition metal complexes of a new class of coordinating ligands, fluoroalkyl-substituted fluorophthalocyanines, R[Formula: see text]Pcs, ligands in which all, or the majority of their C–H bonds are replaced by a combination of fluoro- and perfluoroalkyl groups yielding porphyrin-bioinspired catalyst models. In the case of homogeneous systems, cobalt(II) complexes catalyze the aerobic oxidation of thiols to disulfides, a reaction of both biological significance and industrial importance. Zinc(II) complexes photo-generate excited state singlet oxygen, [Formula: see text]O[Formula: see text], resulting in both the incorporation of O[Formula: see text] in C–H bonds or, depending on the reaction parameters, oxidation of dyes, model pollutants. Catalyst heterogenization using oxidic and other supports yields stable, active hybrid materials. Functionalized R[Formula: see text]Pcs with acidic (–COOH) or basic (–NH[Formula: see text]R[Formula: see text], [Formula: see text] 2) groups exhibit scaffolds that afford both conjugation with biological vectors for theranostic applications as well as solid-supported materials with superior stability. Electrodes modified with hybrid R[Formula: see text]Pc-containing supports have also been used in photo-oxidations, replacing enzymes and H[Formula: see text]O[Formula: see text] associated reagents with a combination of light and air. An analytical device employed for the nano-level detection of environmentally deleterious antibiotics has been constructed.

Singlet oxygen-based electrosensing by molecular photosensitizers

Enzyme-based electrochemical biosensors are an inspiration for the development of (bio)analytical techniques. However, the instability and reproducibility of the reactivity of enzymes, combined with the need for chemical reagents for sensing remain challenges for the construction of useful devices. Here we present a sensing strategy inspired by the advantages of enzymes and photoelectrochemical sensing, namely the integration of aerobic photocatalysis and electrochemical analysis. The photosensitizer, a bioinspired perfluorinated Zn phthalocyanine, generates singlet-oxygen from air under visible light illumination and oxidizes analytes, yielding electrochemically-detectable products while resisting the oxidizing species it produces. Compared with enzymatic detection methods, the proposed strategy uses air instead of internally added reactive reagents, features intrinsic baseline correction via on/off light switching and shows C-F bonds-type enhanced stability. It also affords selectivity imparted by the catalytic process and nano-level detection, such as 20 nM amoxicillin in μl sample volumes.

Application of enzyme-based sensors is usually affected by costs, enzyme stability and immobilization and use of additional chemicals. Here, the authors show a cost-effective and robust photoelectrochemical detection system that can mimic enzymatic sensors using only air and light.

Photoreduction and light-induced triplet-state formation in a single-site fluoroalkylated zinc phthalocyanine

Anaerobic red-light illumination leads to reduction of perfluoroisopropyl-substituted zinc(ii) phthalocyanine in ethanol, while low power UV illumination favours the formation of a triplet excited state.

Photophysical and photochemical properties of novel phthalocyanines bearing non-peripherally substituted mercaptoquinoline moiety

The synthesis, photophysical and photochemical properties of nonperipherally (α) mercaptoquinoline substituted Zn(II) , TiO(IV) and Mg(II) and quaternized Zn(II) phthalocyanines are described for the first time. These complexes (2 to 5) and their precursor are characterized by elemental analysis, FT-IR, 1H NMR, electronic spectroscopy as well as mass spectroscopy. Complexes 2, 4 and 5 have good solubility in organic solvents such as CHCl3 , DCM, DMSO, DMF, THF and toluene and are not aggregated in all solvents within a wide concentration range. Complex 3 showed very good solubility in water as well as DMSO and DMF. General trends are described for singlet oxygen, photodegradation and fluorescence quantum yields of these complexes in DMSO and DMF. While complex 2 has higher singlet oxygen and fluorescence quantum yields than 3, 4 and 5, complex 4 has higher fluorescence quantum yields in DMF and DMSO than 2, 3 and 5. The effect of the solvents and metal on the photophysical and photochemical parameters of the metallophthalocyanines are also reported.

Aggregation of sulfonated free-base phthalocyanine on gold as a function of solution pH

Aggregates of tetrakis(4-sulfonatophenyl)phthalocyanine were deposited on Au(111) from solutions of varying pH. The resulting surface species were studied by scanning tunneling microscopy (STM) under ambient and ultra high vacuum conditions. The pH of the solution from which the TSPc is deposited has profound effects on the adsorbed structures. We observed a diversity of surface structures. At low pH web like aggregates composed of vertically stacked coherent columnar assemblies averaging five molecules per stack were observed. Basic solutions yielded dense molecular monolayers.

Protonation and deprotonation of nitrogens in tetrapyrazino-porphyrazine macrocycles

Acid-base properties of zinc phthalocyanine (Pc) and zinc and metal-free tetrapyrazinoporphyrazine (TPPz) macrocycles with eight peripheral tert-butylsulfanyl or diethylamino substituents are investigated in this work by means of UV-vis spectroscopy. The Pc and TPPz are protonated on the azomethine nitrogens in acidic media. The monoprotonated form was found using trifluoroacetic acid and lower amounts of H 2 SO 4. The higher concentrations of sulfuric acid lead to the appearance of the diprotonated form, and possibly also the triprotonated form in the case of Pc. It was found that the pyrazine nitrogens in the TPPz macrocycle undergo protonation at approx. 20-30% of sulfuric acid in THF solution. TPPz with diethylamino peripheral substituents are protonated preferably on these peripheral tertiary amines. Metal-free TPPz can readily lose the hydrogens on central nitrogens in strong basic media (e.g. tetrabutylammonium hydroxide) to directly form a dianion, with no presence of important or detectable amounts of the monoanion species. A different form of deprotonated macrocycle was observed in pyridine suggesting the formation of only a proton transfer complex, not a true ionic form. The acidity of the central nitrogens is dependent on the strength of the electron-donating effect of peripheral substituents. TPPz with tert-butylsulfanyl substituents is a stronger acid than the one with diethylamino substituents.

STRUCTURE AND PROPERTIES OF PERFLUOROALKYLATED PHTHALOCYANINES: A THEORETICAL STUDY

Density functional theory (DFT) calculations are performed on a series of peripherally substituted free-base and fluorinated zinc phthalocyanines, namely F n Pc M (M = 2 H , Zn ; n = 16, 32, 48, 64). F 16 Pc M, F 32 Pc M, and F 48 Pc M all prefer a planar skeletal structure of the metallomacrocycle core. In the case of F 64 Pc , F 64 PcZn remains planar but F 64 PcH 2 is domed. The occurrence of a dome distortion appears to be related, at least in part, to the orientation of the isopropyl - C 3 F 7 substituents as well as to the intermolecular (packing) interactions. The trends in the calculated results of the Pc 2-– M 2+ binding energy, polarizabilities, and electron excitation energies in H 16 Pc M, F 16 Pc M, and F 64 Pc M are in agreement with the experimental observations, as are the geometrical features. It is further confirmed that the simpler F 32 Pc M or F 48 Pc M mimic the basic electronic properties of F64 Pc M .