Imparting Stability to Organic Photovoltaic Components through Molecular Engineering: Mitigating Reactions with Singlet Oxygen.

One key challenge in the development of viable organic photovoltaic devices is to design component molecules that do not degrade during combined exposure to oxygen and light. Such molecules should thus remain comparatively unreactive towards singlet molecular oxygen and not act as photosensitizers for the generation of this undesirable species. Here, novel redox-active chromophores that combine these two properties are presented. By functionalizing indenofluorene-extended tetrathiafulvalenes (IF-TTFs) with cyano groups at the indenofluorene core using Pd-catalyzed cyanation reactions, we find that the reactivity of the exocyclic fulvene carbon-carbon double bonds towards singlet oxygen is considerably reduced. The new cyano-functionalized IF-TTFs were tested in non-fullerene acceptor based organic photovoltaic proof-of-principle devices, revealing enhanced device stability.

Perturbed and Activated Decay: The Lifetime of Singlet Oxygen in Liquid Organic Solvents.

Singlet oxygen, O2(a1Δg), the lowest excited electronic state of molecular oxygen, plays an important role in a range of chemical and biological processes. In liquid solvents, the reactions of singlet oxygen with a solute kinetically compete with solvent-mediated deactivation that yields the ground electronic state of oxygen, O2(X3Σg-). In this regard, the key parameter is the solvent-mediated lifetime of singlet oxygen, which embodies fundamental physical principles ranging from intermolecular interactions that perturb the forbidden O2(a1Δg) → O2(X3Σg-) transition to the transfer of oxygen's excitation energy into the vibrational modes of a solvent molecule M. Extensive research performed by the global community on this oxygen-related issue over the past ∼50 years reflects its significance. Unfortunately, a satisfactory quantitative understanding of this unique solvent effect has remained elusive thus far. In temperature-dependent studies, we have quantified the singlet oxygen lifetime in common aromatic and aliphatic organic solvents, including partially deuterated molecules that exploit the H/D solvent isotope effect on the lifetime. We now account for experimental data, including previously intractable data, using a model that exploits both weak and strong coupling in the M-O2 complex to accommodate the roles that M plays to (1) induce the forbidden O2(a1Δg) → O2(X3Σg-) transition and (2) accept the excitation energy of O2(a1Δg). As such, our approach brings us appreciably closer to an accurate and predictive ab initio solution for the long-standing oxygen-dependent problem that, in turn, should be relevant for a host of other molecular systems.

Biological Evaluation of Photodynamic Effect Mediated by Nanoparticles with Embedded Porphyrin Photosensitizer.

Clinically approved photodynamic therapy (PDT) is a minimally invasive treatment procedure that uses three key components: photosensitization, a light source, and tissue oxygen. However, the photodynamic effect is limited by both the photophysical properties of photosensitizers as well as their low selectivity, leading to damage to adjacent normal tissue and/or inadequate biodistribution. Nanoparticles (NPs) represent a new option for PDT that can overcome most of the limitations of conventional photosensitizers and can also promote photosensitizer accumulation in target cells through enhanced permeation and retention effects. In this in vitro study, the photodynamic effect of TPP photosensitizers embedded in polystyrene nanoparticles was observed on the non-tumor NIH3T3 cell line and HeLa and G361 tumor cell lines. The efficacy was evaluated by viability assay, while reactive oxygen species production, changes in membrane mitochondrial potential, and morphological changes before and after treatment were imaged by atomic force microscopy. The tested nanoparticles with embedded TPP were found to become cytotoxic only after activation by blue light (414 nm) due to the production of reactive oxygen species. The photodynamic effect observed in this evaluation was significantly higher in both tumor lines than the effect observed in the non-tumor line, and the resulting phototoxicity depended on the concentration of photosensitizer and irradiation time.

Photodynamic effect of TPP encapsulated in polystyrene nanoparticles toward multi-resistant pathogenic bacterial strains: AFM evaluation.

Photodynamic inactivation (PDI) is a promising approach for the efficient killing of pathogenic microbes. In this study, the photodynamic effect of sulfonated polystyrene nanoparticles with encapsulated hydrophobic 5,10,15,20-tetraphenylporphyrin (TPP-NP) photosensitizers on Gram-positive (including multi-resistant) and Gram-negative bacterial strains was investigated. The cell viability was determined by the colony forming unit method. The results showed no dark cytotoxicity but high phototoxicity within the tested conditions. Gram-positive bacteria were more sensitive to TPP-NPs than Gram-negative bacteria. Atomic force microscopy was used to detect changes in the morphological properties of bacteria before and after the PDI treatment.

Multifunctional Photosensitizing and Biotinylated Polystyrene Nanofiber Membranes/Composites for Binding of Biologically Active Compounds.

A three-step postprocessing functionalization of pristine electrospun polystyrene nanofiber membranes was used for the preparation of nanostructured biotinylated materials with an externally bonded porphyrin photosensitizer. Subsequently, the material was able to strongly bind biologically active streptavidin derivatives while keeping its photosensitizing and antibacterial properties due to the generation of singlet oxygen under the exclusive control of visible light. The resulting multifunctional materials functionalized by a streptavidin-horseradish peroxidase conjugate as a model bioactive compound preserved its enzymatic activity even in the presence of a porphyrin photosensitizer with some quenching effect on the activity of the photosensitizer. Prolonged kinetics of both singlet oxygen luminescence and singlet oxygen-sensitized delayed fluorescence (SODF) were found after irradiation by visible light. The above results reflected less effective quenching of the porphyrin photosensitizer triplet state by ground state oxygen and indicated hindered oxygen transport (diffusion) due to surface functionalization. We found that SODF could be used as a valuable tool for optimizing photosensitizing efficiency as well as a tool for confirming surface functionalization. Full photosensitizing and enzyme activity could be achieved by a space separation of photosensitizers and enzyme/biomolecules in the nanofiber composites consisting of two layers. The upper layer contained a photosensitizer that generated antibacterial singlet oxygen upon irradiation by light, and the bottom layer retained enzymatic activity for biochemical reactions.

Polystyrene and Poly(ethylene glycol)-b-Poly(ε-caprolactone) Nanoparticles with Porphyrins: Structure, Size, and Photooxidation Properties.

The transport of a photosensitizer to target biological structures followed by the release of singlet oxygen is a critical step in photodynamic therapy. We compared the (photo)physical properties of polystyrene nanoparticles (TPP@PS) of different sizes and self-assembled poly(ethylene glycol)-b-poly(ε-caprolactone) core/shell nanoparticles (TPP@PEG-PCL) with different lengths of copolymer blocks, both suitable for the transport of the tetraphenylporphyrin (TPP) photosensitizer. The singlet oxygen was formed inside both nanoparticles after irradiation with visible light. Its kinetics was controlled by the size of TPP@PS; its lifetime (τΔ) increased with increasing nanoparticle size (from 6.5 to 16 µs) because of hindered diffusion into the external aqueous environment, where it was quickly deactivated. Accordingly, the prolongation of the singlet oxygen-sensitized delayed fluorescence kinetics was found for TPP@PS of high size. The TPP@PEG-PCL self-assemblies allowed for enhanced oxygen diffusion, and the estimated low values of τΔ ≈ 3.7 µs were independent of the size of building blocks. The delayed fluorescence in oxygen-free conditions originating from triplet-triplet annihilation indicated a high mobility of TPP in the PCL core in comparison with fixed molecules in the PS matrix. Photooxidation of uric acid revealed the highest efficacy for TPP@PS of small sizes, whereas the largest TPP@PS exhibited the lowest activity, and the efficacy of TPP@PEG-PCL remained independent of the sizes of the building blocks.

The effect of iodide and temperature on enhancing antibacterial properties of nanoparticles with an encapsulated photosensitizer.

Aqueous dispersions of sulfonated polystyrene nanoparticles (average diameter: 30 ± 14 nm) with encapsulated 5,10,15,20-tetraphenylporphyrin (TPP) are promising candidates for antibacterial treatments due to the photogeneration of cytotoxic singlet oxygen species O2(1Δg) under physiological conditions using visible light. The antibacterial effect on gram-negative Escherichia coli was significantly enhanced after the addition of nontoxic potassium iodide (0.001-0.01 M) because photogenerated O2(1Δg) oxidized iodide to I2/I3-, which is another antibacterial species. The improved antibacterial properties were predicted using luminescence measurements of O2(1Δg), transient absorption of TPP triplets and singlet oxygen-sensitized delayed fluorescence (SODF). In contrast to a solution of free photosensitizers, the aqueous dispersion of photoactive nanoparticles did not exhibit any quenching of the excited states after the addition of iodide or any tendency toward aggregation and/or I3--induced photo-aggregation. We also observed a decrease in the lifetime of O2(1Δg) and a significant increase in SODF intensity at higher temperatures, due to the increased oxygen diffusion coefficient in nanoparticles and aqueous surroundings. This effect corresponds with the significantly stronger antibacterial effect of nanoparticles at physiological temperature (37 °C) in comparison with that at room temperature (25 °C).

Antibacterial nitric oxide- and singlet oxygen-releasing polystyrene nanoparticles responsive to light and temperature triggers.

Novel therapies to prevent bacterial infections are of utmost importance in biomedical research due to the emergence of multidrug-resistant strains of bacteria. Herein, we report the preparation, characterization and antibacterial evaluation of sulfonated polystyrene nanoparticles simultaneously releasing two antibacterial species, nitric oxide (NO) and singlet oxygen (O2(1Δg)), upon irradiation with visible light. The nanoparticles were prepared by simple and scalable processes from nanofiber membranes with an encapsulated NO photodonor and/or ionically entangled tetracationic porphyrin/phthalocyanine photosensitizers. The release of NO and O2(1Δg) from the polystyrene nanoparticles is controlled by light wavelength and dose, as well as by temperature, which influences the diffusion coefficient and solubility of both species in the polystyrene matrix. The concentrations of NO and O2(1Δg) were measured by amperometric and time-resolved spectroscopic techniques and by chemical analysis. Due to the efficient photogeneration of both species at physiological temperature and resultant strong antibacterial action observed on Escherichia coli, the nanoparticles are a promising material for antibacterial applications triggered/modulated by light and temperature.

Nanoparticles with Embedded Porphyrin Photosensitizers for Photooxidation Reactions and Continuous Oxygen Sensing.

We report the synthesis and characterization of sulfonated polystyrene nanoparticles (average diameter 30 ± 14 nm) with encapsulated 5,10,15,20-tetraphenylporphyrin or ionically entangled tetracationic 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin, their photooxidation properties, and the application of singlet oxygen-sensitized delayed fluorescence (SODF) in oxygen sensing. Both types of nanoparticles effectively photogenerated singlet oxygen, O2(1Δg). The O2(1Δg) phosphorescence, transient absorption of the porphyrin triplet states, and SODF signals were monitored using time-resolved spectroscopic techniques. The SODF intensity depended on the concentration of the porphyrin photosensitizer and dissolved oxygen and on the temperature. After an initial period (a few microseconds), the kinetics of the SODF process can be approximated as a monoexponential function, and the apparent SODF lifetimes can be correlated with the oxygen concentration. The oxygen sensing based on SODF allowed measurement of the dissolved oxygen in aqueous media in the broad range of oxygen concentrations (0.2-38 mg L-1). The ability of both types of nanoparticles to photooxidize external substrates was predicted by the SODF measurements and proven by chemical tests. The relative photooxidation efficacy was highest at a low porphyrin concentration, as indicated by the highest fluorescence quantum yield (ΦF), and it corresponds with negligible inner filter and self-quenching effects. The photooxidation abilities were sensitive to the influence of temperature on the diffusion and solubility of oxygen in both polystyrene and water media and to the rate constant of the O2(1Δg) reaction with a substrate. Due to their efficient photogeneration of cytotoxic O2(1Δg) at physiological temperatures and their oxygen sensing via SODF, both types of nanoparticles are promising candidates for biomedical applications.

Antibacterial, Antiviral, and Oxygen-Sensing Nanoparticles Prepared from Electrospun Materials.

A simple nanoprecipitation method was used for preparation of stable photoactive polystyrene nanoparticles (NPs, diameter 30 ± 10 nm) from sulfonated electrospun polystyrene nanofiber membranes with encapsulated 5,10,15,20-tetraphenylporphyrin (TPP) or platinum octaethylporphyrin (Pt-OEP). The NPs prepared with TPP have strong antibacterial and antiviral properties and can be applied to the photooxidation of external substrates based on photogenerated singlet oxygen. In contrast to nanofiber membranes, which have limited photooxidation ability near the surface, NPs are able to travel toward target species/structures. NPs with Pt-OEP were used for oxygen sensing in aqueous media, and they presented strong linear responses to a broad range of oxygen concentrations. The nanofiber membranes can be applied not only as a source of NPs but also as an effective filter for their removal from solution.

Polystyrene Nanofiber Materials for Visible-Light-Driven Dual Antibacterial Action via Simultaneous Photogeneration of NO and O2((1)Δg).

This contribution reports on the preparation, characterization, and biological evaluation of electrospun polystyrene nanofiber materials engineered with a covalently grafted NO photodonor and ionically entangled tetracationic porphyrin and phthalocyanine photosensitizers. These photofunctional materials exhibit an effective and simultaneous photogeneration of two antibacterial species such as nitric oxide (NO) and singlet oxygen, O2((1)Δg) under illumination with visible light, as demonstrated by their direct detection using amperometric and time-resolved spectroscopic techniques. Dual-mode photoantibacterial action is demonstrated by antibacterial tests carried out on Escherichia coli.

Superhydrophilic polystyrene nanofiber materials generating O2((1)Δ(g)): postprocessing surface modifications toward efficient antibacterial effect.

The surfaces of electrospun polystyrene (PS) nanofiber materials with encapsulated 1% w/w 5,10,15,20-tetraphenylporphyrin (TPP) photosensitizer were modified through sulfonation, radio frequency (RF) oxygen plasma treatment, and polydopamine coating. The nanofiber materials exhibited efficient photogeneration of singlet oxygen. The postprocessing modifications strongly increased the wettability of the pristine hydrophobic PS nanofibers without causing damage to the nanofibers, leakage of the photosensitizer, or any substantial change in the oxygen permeability of the inner bulk of the polymer nanofiber. The increase in the surface wettability yielded a significant increase in the photo-oxidation of external polar substrates and in the antibacterial activity of the nanofibers in aqueous surroundings. The results reveal the crucial role played by surface hydrophilicity/wettability in achieving the efficient photo-oxidation of a chemical substrate/biological target at the surface of a material generating O2((1)Δg) with a short diffusion length.

Effect of temperature on photophysical properties of polymeric nanofiber materials with porphyrin photosensitizers.

Electrospun nanofibers possess large surface to volume ratios, high porosity, and good mechanical properties that are necessary for biological applications. We prepared different types of photoactive polymeric nanofiber materials with encapsulated or externally bound porphyrin photosensitizers. The kinetics of formation and the decay of both singlet oxygen O2((1)Δg) and porphyrin triplet states that are generated by irradiation of nanofiber materials in an air atmosphere or in an air-saturated aqueous solution were measured and evaluated by luminescence and transient absorption spectroscopy in the temperature range between 5 and 60 °C. We found shortening of the O2((1)Δg) lifetime and a significant increase in singlet oxygen-sensitized delayed fluorescence at higher temperatures. These photophysical data show an increase in the diffusion coefficient for O2((1)Δg) with temperature, and they are consistent with a stronger antibacterial effect of the nanofiber material on Escherichia coli at higher temperature.

Polystyrene nanofiber materials modified with an externally bound porphyrin photosensitizer.

Polystyrene ion-exchange nanofiber materials with large surface areas and adsorption capacities were prepared by electrospinning followed by the sulfonation and adsorption of a cationic 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP) photosensitizer on the nanofiber surfaces. The morphology, structure, and photophysical properties of these nanofiber materials were characterized by microscopic methods and steady-state and time-resolved fluorescence and absorption spectroscopies. The externally bound TMPyP can be excited by visible light to form triplet states and singlet oxygen O2((1)Δg) and singlet oxygen-sensitized delayed fluorescence (SODF). The photophysical properties of the nanofibers were strongly dependent on the amount of bound TMPyP molecules and their organization on the nanofiber surfaces. The nanofibers demonstrated photooxidative activity toward inorganic and organic molecules and antibacterial activity against E. coli due to the sensitized formation of O2((1)Δg) that is an effective oxidation/cytotoxic agent. The nanofiber materials also adsorbed heavy metal cations (Pb(2+)) and removed them from the water environment.