Anionic Ones Research Articles

Influence of chemical speciation on photochemical transformation of three fluoroquinolones (FQs) in water: Kinetics, mechanism, and toxicity of photolysis products

This study investigated the contribution of direct, indirect, and self-sensitized photolysis to the photochemical fate of three model fluoroquinolones (FQs), i.e., lomefloxacin (LOM), norfloxacin (NOR), and ofloxacin (OFL), and demonstrated the influence of chemical speciation on their photodegradation behavior, a topic that has received relatively little attention. Results suggest that these FQs in water transformed mainly via direct photolysis, while hydroxyl radical played a key role in their indirect and self-sensitized photolysis. Chemical speciation of such zwitterionic compounds significantly affected the kinetics of their phototransformation, with the quantum yields of photodegradation decreased in the order of zwitterionic (FQsH) > anionic (FQs−) > cationic (FQsH2+). The photodegradation pathways of FQs depended on both their structures and chemical speciation. Defluorination for LOM in C-8 and NOR in C-6 was more significant when they were present in zwitterionic form than in the other forms. Cationic FQs underwent direct piperazinyl ring cleavage, and zwitterionic ones underwent piperazinyl ring oxidation, while the degradation pathway of piperazinyl ring for FQs in anionic form was structure dependent. Decarboxylation for zwitterionic FQs occurred more slowly compared to both cationic and anionic ones, and the FQs bearing electron-donating groups in C-8 position degraded more easily in cationic form than the anionic ones, while the opposite was true for the FQs without such a group in C-8 position. Results of Vibrio fischeri bioluminescence inhibition tests showed the toxicity of zwitterionic NOR and OFL significantly decreased after photodegradation, while the degradation products of LOM exhibited greater toxicity. These findings indicate that chemical speciation of zwitterionic compounds could affect the kinetics and pathways of their photochemical transformation, and thus have important implications on their fate and risk in aquatic environment.

Why are the anionic porphyrins so efficient to induce plant cell death? A structure-activity relationship study to solve the puzzle

Previous work showed that compared to cationic porphyrins under light irradiation, the anionic ones were the strongest tobacco bright yellow cell death inducers. As this result was unexpected in light to literature, it was of importance to try to understand why anionic porphyrins were able to efficiently induce cell detah in tobacco cell suspension. Thus we decided to investigate structure-activity relationships of a serie of anionic porphyrins toward Tobacco Bright Yellow - 2 (TBY-2) cells, namely 5,10,15,20-tetrakis(carboxyphenyl) porphyrin TPPC, its zinc analogue Zn-TPPC, 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin TPPS, and its zinc analogue Zn-TPPS, 5,10,15,20-tetrakis(4-phosphonatophenyl) porphyrin TPPP. In particular, their charge states as a function of the pH medium, their abilities to generate reactive oxygen species as well as their spectroscopic and photophysical properties have been investigated and correlated to their biological effect.

Synthesis and Characterization of a Novel Green Cationic Polyfluorene and Its Potential Use as a Fluorescent Membrane Probe.

In the present work, we have synthesized a novel green-emitter conjugated polyelectrolyte Copoly-{[9,9-bis(6′-N,N,N-trimethylammonium)hexyl]-2,7-(fluorene)-alt-4,7-(2-(phenyl) benzo[d] [1,2,3] triazole)} bromide (HTMA-PFBT) by microwave-assisted Suzuki coupling reaction. Its fluorescent properties have been studied in aqueous media and in presence of model membranes of different composition, in order to explore its ability to be used as a green fluorescent membrane probe. The polyelectrolyte was bound with high affinity to the membrane surface, where it exhibited high fluorescence efficiency and stability. HTMA-PFBT showed lower affinity to zwitterionic membranes as compared to anionic ones, as well as a more external location, near the membrane-aqueous interface. Fluorescence microscopy studies confirmed the interaction of HTMA-PFBT with the model membranes, labelling the lipid bilayer without perturbing its morphology and showing a clear preference towards anionic systems. In addition, the polyelectrolyte was able to label the membrane of bacteria and living mammalian cells, separately. Finally, we explored if the polyelectrolyte can function also as a sensitive probe able of detecting lipid-phase transitions. All these results suggest the potential use of HTMA-PFBT as a green membrane marker for bioimaging and selective recognition of bacteria cell over mammalian ones and as a tool to monitor changes in physical state of lipid membranes.

Photodynamic inactivation of Bovine herpesvirus type 1 (BoHV-1) by porphyrins

In this work, the photodynamic efficiency of anionic meso-tetrakis sulfonophenyl (TPPS4), cationic meso-tetrakis methylpyridiniumyl (TMPyP) and their zinc complexes (ZnTPPS4 and ZnTMPyP) in the inactivation of Bovine herpesvirus type 1 (BoHV-1) was evaluated. At a non-cytotoxic concentration, all porphyrins showed significant antiviral activity after irradiation using a halogen lamp. The efficiency of the cationic porphyrins was higher than that of the anionic ones. Porphyrin complexation with zinc increases its lipophilicity and the number of absorbed photons, dramatically reducing the time for complete virus inactivation. The high superposition of the compound optical absorption and light source emission spectra played a key role in the virus inactivation efficiency. The results demonstrated the high effectivity of the photodynamic inactivation of BoHV-1. This method can be used as an auxiliary in the treatment of disorders attributed to BoHV-1 infection, and the porphyrins are promising photosensitizers for this application.

Preparation of positively charged composite nanofiltration membranes by quaternization crosslinking for precise molecular and ionic separations

Developing nanofiltration (NF) membranes with highly efficient and precise separation ability is of great significance for the molecular and ionic separations. In this work, positively charged composite NF membranes were engineered via soaking of polysulfone (PSF) ultrafiltration membranes in poly(N-vinyl imidazole) (PVI) solutions followed by a quaternization crosslinking step. The PVI was firmly attached to the PSF membrane by this method and acted as an active separation layer of the composite NF membrane. The obtained composite NF membrane featured a high rejection (83%) to vitamin B12 (molecular radius: 0.74 nm) but a low rejection (24.6%) to vitamin B2 (molecular radius: 0.47 nm), exhibiting a great potential in precisely molecular separation. Furthermore, the ionic separation ability of the composite NF membrane was confirmed with a rejection order of Na2SO4 < MgSO4 < NaCl < MgCl2 and the MgCl2 rejection reached up to 90.1%. Compared to conventional polyamide NF membranes, the developed PVI/PSF composite NF membranes were characterized with high separation precision to organic molecules, higher rejection over cationic ions than over anionic ones, better chlorine resistance and stability in long-term operation. In addition, the membrane fabrication process is convenient and easily scaled up in industry. This work offers a novel alternative of NF membranes for high precision in molecular and ionic separations.

Sulfonated Hollow Covalent Organic Polymer: Highly‐Selective Adsorption toward Cationic Organic Dyes over Anionic Ones in Aqueous Solution

AbstractA sulfonated hollow covalent organic polymer (sh‐COP‐P) was prepared by post sulfonation of hollow covalent organic polymer (h‐COP‐P) synthesized through poly‐condensation of tetrabiphenylporphyrin (TBPP). In comparison with h‐COP‐P, sh‐COP‐P exhibits significantly enhanced adsorption capacity of organic cationic dyes in aqueous solutions accompanied with notably reduced adsorption capacity of anionic dyes. This gives sh‐COP‐P a satisfactory performance in selectively separating cationic organic dyes from anionic ones, mainly attributed to the electrostatic interaction between polymer backbone and the guest molecules.

Regulation of clay particles charge for design of protective electrokinetic barriers

Coupled electrokinetic protective reactive barriers (PRB) are considered as a perspective technology for the treatment of contaminated groundwater. Design of PRB is directly connected with a problem of barrier material choice. Clays can be considered as an appropriate material due to high adsorptive properties and relative cheapness. The barrier internals are formed by clay surface charge properties. We revealed that acidic and alkaline treatment of clay is an effective way to affect its protective properties so that clay can be used to treat various pollutants. Surface charge and electrokinetic properties of clays were characterized by point of zero charge (p.z.c.), point of zero net proton charge (p.z.n.p.c.) and ζ-potential at different pH. Suspensions of 3 main clay types were studied by microelectrophoresis and potentiometric titration methods. At pH > p.z.n.p.c. clayey barrier adsorbs predominantly cationic toxicants and at pH < p.z.c. – anionic ones. The barrier is seemed to be the least effective in pH range between p.z.c. and p.z.n.p.c. Given the physicochemical and electrokinetic parameters, the most efficient clays for barrier design are Cambrian illite and all montmorillonite clays.

Carbon dioxide/water foams stabilized with a zwitterionic surfactant at temperatures up to 150 °C in high salinity brine

Stabilization of CO2-in-water (C/W) foams with surfactants at high temperature and high salinity conditions is challenging given limitations in the solubility and thermal stability of surfactants, particularly for surfactants other than anionic ones. Here we show that a single zwitterionic surfactant, cetyl betaine (CH3(CH2)15N+(CH3)2CH2COO−), stabilizes viscous C/W foam (>15 cP) in crushed limestone (76-Darcy) at a temperature up to 150 °C and over a wide range in salinity up to 22% total dissolved solids (TDS) at superficial velocity of 940 ft/day. According to liquid chromatography mass spectrometry (LC/MS), cetyl betaine has high thermal stability with negligible chemical degradation after incubation at 135 °C for 30 days. Interfacial tension (IFT) at the C/W interface was reduced significantly from ∼37 mN/m to <5 mN/m with the addition of 0.08 mmol/L of cetyl betaine. Relative to ionic surfactants, the relatively weak electrostatic repulsion between the local charges in the headgroup is shown to produce strong adsorption at the C/W interface (∼134 Å2/molecule), resulting in generation of viscous foam (∼6 cP) even in pure water without added salt. At a low superficial velocity of 6 ft/day (shear rate 45 s−1) in a beadpack, foam was still formed with cetyl betaine. Static adsorption of cetyl betaine on carbonates was on the order of 1 mg/m2, and core flood test also proved that cetyl betaine could stabilize viscous C/W foam (>20 cP) within 6 pore volumes of injection at 120 °C. The surfactant stabilized viscous C/W foam at low oil fractions and broke in the presence of large fractions of oil, both of which are desirable in CO2 mobility control and oil displacement. The ability to form foams at high temperatures and over a wide range in salinity is important for various practical applications including CO2 enhanced oil recovery, fracturing with energized fluids and CO2 sequestration.

Size-selective adsorption of methyl orange using a novel nano-composite by encapsulating HKUST-1 in hyper-crosslinked polystyrene networks

A novel nano-composite adsorbent, NDA88-Cu, was successfully prepared by immobilization of HKUST-1 (a typical MOF) into hyper-crosslinked polystyrene networks NDA88 (loading ratio: 350 mg of MOFs with particle size in the range of 10–100 nm per gram of polystyrene networks). The adsorbent demonstrated advantages of improved stability without leakage of metal ions during the adsorption assays compared to HKUST-1 itself, and faster solid-liquid separability than NDA88 or HKUST-1, which could effectively avoid the shortcomings of MOFs like potential secondary pollution and poor performance in separation. When applied for the adsorption of five typical dyes with various charges and molecular sizes (cationic: methylene blue (MB) and rhodamine-B (RhB); anionic: methyl orange (MO), methyl red (MR) and acid green (AG25)), NDA88-Cu worked for the removal of anionic ones but showed almost no adsorption capacity towards cationic ones because of Donnan membrane effect exerted by the host NDA88. For the three anionic dyes, NDA88-Cu displayed not only heightened adsorption capacity towards MO (398.8 mg/g), but also size-selective uptake of MO from solution with mixed dyes. The adsorption capacity remained high levels after ten adsorption-desorption cycles. Mechanism investigations, including macroscopic physico-chemical analyses, instrumental investigation and density functional theoretical calculations, gave direct evidences of the size selectivity of NDA88-Cu with hierarchical pore structure. Coordination effect of Cu-MO predominated, while π-π interactions and electrostatic attraction also took place, in the fixing of MO onto the adsorbent. The current work provides a useful idea for the separation of contaminants with similar structure from water, and acts as a good case for the application of nanotechnology in wastewater treatment and resource recovery.

Anionic iridium(III) complexes and their conjugated polymer soft salts for time-resolved luminescent detection of intracellular oxygen levels

It is well known that hypoxia is a critical parameter with respect to several pathologies, like solid tumors. Hence, it is necessary to realize highly sensitive and selective monitoring of oxygen levels in biological systems. Phosphorescent iridium(III) complexes are an excellent class of oxygen probe through the energy transfer from triplet excitons to oxygen molecules. Current research mainly focuses on the neutral and cationic complexes, while the anionic ones received few attentions. Herein, we designed a series of anionic iridium(III) complexes with different C^N ligands and investigated their oxygen-sensing capability in solution and living cells. Importantly, based on the metathesis reactions of the anionic iridium(III) complexes with oxygen-insensitive cationic polyfluorene, water-soluble polymer soft salt P1 was synthesized successfully. Thus, ratiometric luminescent and reversible polymer hypoxia probe can be obtained because P1 can give dual emission with oxygen-sensitive phosphorescence from the iridium(III) complexes and reference fluorescence from polyfluorene, respectively. Furthermore, the photoluminescence lifetime imaging at different oxygen levels exhibited that P1 is an excellent oxygen probe utilizing the sensitive phosphorescent lifetime to oxygen level. To our knowledge, this is the first report about the application of anionic iridium(III) complexes and their conjugated polymer soft salts in biosensing and bioimaging fields.

Investigation of the Adsorption of Amphipathic macroRAFT Agents onto Montmorillonite Clay.

Recently, there has been significant interest in the use of the reversible addition-fragmentation chain-transfer (RAFT) technique to generate a variety of organic/inorganic colloidal composite particles in aqueous dispersed media using the so-called macroRAFT-assisted encapsulating emulsion polymerization (REEP) strategy. In this process, special attention should be paid to the adsorption of the macromolecular RAFT (macroRAFT) agent onto the inorganic particles, as it determines the final particle morphology and can also influence latex stability. In this work, different amphipathic macroRAFT agents were synthesized by RAFT, and their adsorption onto commercial Montmorillonite clay Cloisite Na+ (MMT) was studied by means of adsorption isotherms. Three types of macroRAFT agents were considered: a nonionic one based on poly(ethylene glycol) methyl ether acrylate (PEGA) and n-butyl acrylate (BA), anionic ones, including a block copolymer and random copolymers, based on acrylic acid (AA), BA and PEGA, and cationic ones based on 2-(dimethylamino)ethyl methacrylate (DMAEMA), BA and PEGA. Six adsorption isotherm models (Langmuir, Freundlich, Tempkin, Redlich-Peterson, Sips, and Brunauer-Emmett-Teller) were adjusted to the experimental isotherms. The nonionic macroRAFT agent formed a monolayer on the clay surface with a maximum adsorption capacity of 400 mg g-1 at pH 8, as determined from the Sips adsorption model. Adsorption of the AA-based macroRAFT agents onto MMT was moderate at alkaline pH due to electrostatic repulsions, but increased with decreasing pH. The DMAEMA-based macroRAFT agents displayed a much stronger interaction with the oppositely charged MMT surface at acidic pH due to electrostatic interactions, and the concentration of adsorbed macroRAFT agent reached values as high as 800 mg g-1. The BET model fitted the experimental data relatively well indicating multilayer adsorption promoted by the presence of the hydrophobic BA units. In addition, the cationic macroRAFT agents afforded stable MMT/macroRAFT agent complexes as evaluated by dynamic light scattering and zeta potential analyses.

Theoretically obtained insight into the mechanism and dioxetanone species responsible for the singlet chemiexcitation of Coelenterazine

Coelenterazine is a widespread bioluminescent substrate for a diverse set of marine species. Moreover, its imidazopyrazinone core is present in eight phyla of bioluminescent organisms. Given their very attractive intrinsic properties, these bioluminescent systems have been used in bioimaging, photodynamic therapy of cancer, as gene reporter and in sensing applications, among others. While it is known that bioluminescence results from the thermolysis of high-energy dioxetanones, the mechanism and dioxetanone species responsible for the singlet chemiexcitation of Coelenterazine are not fully understood. The theoretical characterization of the reactions of model Coelenterazine dioxetanones showed that efficient chemiexcitation is caused by a neutral dioxetanone with limited electron and charge transfer, by accessing a region of the PES where ground and excited states are nearly-degenerated. This finding was supported by calculation of equilibrium constants, which showed that only neutral dioxetanone is present in conditions associated with bioluminescence. Moreover, while cationic amino-acids easily protonate amide dioxetanone, anionic ones cannot deprotonate the neutral species. These results indicate that, contrary to existent theories, efficient chemiexcitation can occur with significant electron and/or charge transfer. In fact, these processes can be prejudicial to chemiexcitation, as anionic dioxetanones showed a less efficient chemiexcitation despite the occurrence of significant electron and charge transfer.

Extraction and Characterization of Surfactants from Atmospheric Aerosols.

Surface-active compounds, or surfactants, present in atmospheric aerosols are expected to play important roles in the formation of liquid water clouds in the Earth's atmosphere, a central process in meteorology, hydrology, and for the climate system. But because specific extraction and characterization of these compounds have been lacking for decades, very little is known on their identity, properties, mode of action and origins, thus preventing the full understanding of cloud formation and its potential links with the Earth's ecosystems.In this paper we present recently developed methods for 1) the targeted extraction of all the surfactants from atmospheric aerosol samples and for the determination of 2) their absolute concentrations in the aerosol phase and 3) their static surface tension curves in water, including their Critical Micelle Concentration (CMC). These methods have been validated with 9 references surfactants, including anionic, cationic and non-ionic ones. Examples of results are presented for surfactants found in fine aerosol particles (diameter <1 μm) collected at a coastal site in Croatia and suggestions for future improvements and other characterizations than those presented are discussed.

Extraction and Characterization of Surfactants from Atmospheric Aerosols

Surface-active compounds, or surfactants, present in atmospheric aerosols are expected to play important roles in the formation of liquid water clouds in the Earth's atmosphere, a central process in meteorology, hydrology, and for the climate system. But because specific extraction and characterization of these compounds have been lacking for decades, very little is known on their identity, properties, mode of action and origins, thus preventing the full understanding of cloud formation and its potential links with the Earth's ecosystems. In this paper we present recently developed methods for 1) the targeted extraction of all the surfactants from atmospheric aerosol samples and for the determination of 2) their absolute concentrations in the aerosol phase and 3) their static surface tension curves in water, including their Critical Micelle Concentration (CMC). These methods have been validated with 9 references surfactants, including anionic, cationic and non-ionic ones. Examples of results are presented for surfactants found in fine aerosol particles (diameter <1 μm) collected at a coastal site in Croatia and suggestions for future improvements and other characterizations than those presented are discussed.

Protein Immobilization onto Cationic Spherical Polyelectrolyte Brushes Studied by Small Angle X-ray Scattering.

The immobilization of bovine serum albumins (BSA) onto cationic spherical polyelectrolyte brushes (SPB) consisting of a solid polystyrene (PS) core and a densely grafted poly(2-aminoethyl methacrylate hydrochloride) (PAEMH) shell was studied by small-angle X-ray scattering (SAXS). The observed dynamics of adsorption of BSA onto SPB by time-resolved SAXS can be divided into two stages. In the first stage (tens of milliseconds), the added proteins as in-between bridge instantaneously caused the aggregation of SPB. Then BSA penetrated into the brush layer driven by electrostatic attractions, and reached equilibrium in the second stage (tens of seconds). The amount of BSA immobilized onto brush layer reached the maximum when pH was increased to about 6.1 and BSA concentration to 10 g/L. The cationic SPB were confirmed to provide stronger adsorption capacity for BSA compared to anionic ones.

Synthesis and Characterization of Halloysite–Cyclodextrin Nanosponges for Enhanced Dyes Adsorption

Inorganic–organic nanosponge hybrids based on halloysite clay and organic cyclodextrin derivatives (HNT-CDs) were developed by means of microwave irradiations in solvent-free conditions. The HNT-CDs nanomaterials characterized by FT-IR, TGA, BET, TEM, SEM, DLS, and ζ-potential have showed a hyper-reticulated network which possesses both HNT and cyclodextrin peculiarities. The new HNT-CDs nanosponge hybrids were employed as nanoadsorbents, first choosing Rhodamine B as the dye model, and furthermore for the removal of some cationic and anionic dyes, under different pH values (1.0, 4.54, and 7.4). The collected results showed that the pH solution as well as the electrostatic interactions affect the adsorption process. Factors controlling the adsorption process were discussed. The experimental adsorption equilibrium and kinetic data were best described by the Freundlich isotherm model. Excellent adsorption efficiency for cationic dyes were observed with respect to anionic ones. The results suggest that HNT-C...

Synthesis and Crystal Structure of 3-3’-Diaminomethyldipropylammonium Hexachlorobismuthate (III)

Synthesis and crystal structure of 3-3’-diamino-N-methyldipropylammonuim hexachlorobismuthate (III) are reported. The compound crystallizes in the triclinic system with space group P1. The unit cell dimensions are: a = 7.5580(5), b =7.8710(6), c = 8.3709(7) Å with Z=1. The crystal is built up of separated [BiCl6]3- octahedral anions and 3-3’-diamino-N- methyldipropylammonium cations. The organic layers are arranged in sandwich between the anionic ones. The crystal packing is governed by means of the ionic N–H···Cl hydrogen bonds, forming a three dimensional network.

Analysis of Invertebrate and Protist N-Glycans.

N-glycans from invertebrates and protists have often unusual structures which present analytical challenges. Both core and antennal modifications can be quite different from the more familiar vertebrate glycan motifs; thereby, contrary to the concept that "simple" organisms have "simple" N-glycans, rather complex oligosaccharides structures, including zwitterionic and anionic ones, have been found in a range of species. Thus, to facilitate the optimized elucidation of the maximal possible range of structures, the analytical workflow for glycomics of these organisms should include sequential release and fractionation steps. Peptide:N-glycosidase F is sufficient to isolate N-glycans from fungi and some protists, but in most invertebrates core α1,3-fucose is present, so release of the glycans from glycopeptides with peptide:N-glycosidases A is required. Subsequent solid-phase extraction with graphitized carbon and reversed phase resins enables different classes of N-glycans to be separated prior to high-pressure liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Depending on the types and numbers of glycans present, either reversed- or normal-phase HPLC (or both in series) enable even single isomeric or isobaric structures to be separated prior to MALDI-TOF MS and MS/MS. The use of enzymatic or chemical treatments allows further insights to be gained, although some glycan modifications (especially methylation) are resistant. Using a battery of methods, sometimes up to 100 structures from a single organism can be assigned, a complexity which raises evolutionary questions regarding the function of these glycans.

Enhancing the charge-storage performance of N-doped reduced graphene oxide aerogel supercapacitors by adsorption of the cationic electrolytes with single-stand deoxyribonucleic acid

Single-stand deoxyribonucleic acid/N-doped reduced graphene oxide aerogel (ssDNA/N-doped rGO) composites were synthesized and used as the electrode materials for supercapacitors. The mass change of the electrolytes adsorbed on the ssDNA/N-doped rGO electrode during a charge/discharge process determined by an electrochemical quartz crystal microbalance (EQCM) at a cathodic potential range (−0.5 to −0.1 V vs. Ag/AgCl) is 10-fold higher than that at an anodic potential range (−0.1–0.3 V vs. Ag/AgCl). This is because the ssDNA having negatively charged phosphate and nitrogen-containing nucleobase groups prefers cationic electrolytes to anionic ones. An as-fabricated supercapacitor of the ssDNA/N-doped rGO with a coin-cell size (CR2016) provides 644 F/g at 0.5 A/g, which is 1.3-fold higher than that of the N-doped rGO device. The specific power and specific energy of the as-fabricated ssDNA/N-doped rGO device are ca. 30 Wh/kg and 4500 W/kg, respectively with a capacity retention of 94.1% over 2500 cycles. Two as-fabricated supercapacitor cells connected in series can supply electricity to the red LED over 10 min. The composite materials in this work may also be used in other applications where the functional groups of ssDNA can play an important role to the adsorption of the cationic electrolytes.

Interstate Crossing-Induced Chemiexcitation Mechanism as the Basis for Imidazopyrazinone Bioluminescence

Imidazopyrazinone dioxetanone is a central molecule in bioluminescence, as it is the scaffold responsible for light emission in many marine species. Herein, we have theoretically studied the thermolysis of its neutral and anionic forms. Our results allowed us to explain imidazopyrazinone bioluminescence with the Interstate Crossing-Induced Chemiexcitation (ICIC) mechanism, and explain why neutral forms emit light efficiently to the contrary of anionic ones. Both forms decompose via a stepwise-biradical pathway, but the biradical is formed either via an electron transfer (anion) or due to homolytic bond cleavage (neutral species). Absence of electron transfer leads to an entropic trap, a flat region where infinite possibilities for chemiexcitation exist. However, the resulting products can be in either triplet or singlet excited states. The triplet to singlet ratio is determined by the electron spin density distribution, which controls the rates of intersystem crossing.