The disclosed technology is directed to the utilization of supramolecular complexes to generate exciplexes that can be are utilized as efficient photosensitizers. More particularly the technology is directed to supramolecular porous organic nanocomposites for heterogeneous photocatalysis.
Considerable interest has been devoted towards the photo-oxidation of the sulfur mustard (SM) and 2-chloroethyl ethyl sulfide (CEES) using 1O2. The latter is a mild oxidant and photocatalysis has been proven to involve faster kinetics and also to be more selective when the less harmful sulfoxide derivative, 2-chloroethyl ethyl sulfoxide (CEESO), is formed as a major product with the 2-chloroethyl ethyl sulfone derivative (CEESO2), as a minor product. Several porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), with photosynthesizing properties have been utilized for heterogenous photocatalysis of SM or CEES since the large surface areas of these porous materials facilitate the accessibility of the reactants to the photoactive sites. Processability of these crystalline powder materials towards military protective equipment (MPE), however, remains challenging. Recently, Karwacki et al.12 reported efficient photocatalysis of CEES to CEESO using 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) photosensitizers doped into organic polymeric matrices. Nevertheless, the weak interactions through dispersive forces of the BODIPY photosensitizers to the polymer matrices hamper development of viable MPEs because of the leaching of the photosensitizer under catalytic conditions. In addition, a large amount of photocatalyst is required to decrease the conversion lifetime to < 1 min. As a result, there is a need for photocatalysts capable of photooxidizing SM and other harmful compound.
The present technology is directed to inexpensive, photostable, easily processed, and environment friendly porous organic polymeric nanocomposites and photocatalysts for removing contaminants or reactive substrates, such as chemical warfare agents, from the environment. The nanocomposites comprise an admixture of a polymeric matrix and a macrocyle. The composites are microporous and efficiently absorb electron rich molecules such as polyaromatic hydrocarbons. In some embodiments, the macrocycle is a cationic cyclophane, such as tetracationic ExBox4+ or Ex2.2Box4+. In some embodiments, the polymeric matrix is an anionic polymer, such as polystyrene sulfonate (PSS).
The present technology is further directed to the generation of host/guest complex between the macrocycle and a polyaromatic guest which is dispersed within the polymeric matrix to form a photocatalyst. In some embodiments, the polyaromatic guest comprises 1,3,5,8-tetrabropyrene (TBP).
Under photoexcitation, the host-guest complexes shown efficient singlet oxygen (1O2) generation and therefore is an effective photocatalyst for a selective removal of contaminants from the environment. The efficient photocatalysis is associated with efficient intersystem crossing and the existence of a manifold of newly excited states that lead to the population of a locally excited state on the polyaromatic guest. Efficient intersystem crossing occurs because of a combination of spin-orbit coupling and a charge transfer between the guest and the host molecules.
These nanocomposites and photocatalysts are insoluble in organic and aqueous solvents which make them desirable for device fabrication. Accordingly, fibers, fabrics, or nanoparticles may be formed from any of the nanocomposites or photocatalysts described herein.
Another aspect of the technology is a method for photocatalytic oxidation of a reactive substrate. The method may comprise contacting any of the photocatalysts or the nanocomposites described herein with a reactive substrate and irradiating the photocatalyst or the nanocomposite in the presence of the reactive substrate, thereby oxidizing the reactive substrate. Suitably, the reactive substrate is a thioether or an organophosphorous compound, including without limitation chemical warfare agents.
Another aspect of the technology is a method for the generation of singlet oxygen (1O2). The method may comprise irradiating any of the photocatalysts or the nanocomposites described herein in the presence of an oxygen source. Suitably, the oxygen source is triplet oxygen (3O2).
Another aspect of the technology is a method for sequestering an environmental contaminant. The method may comprise contacting any of the photocatalysts or the nanocomposites described herein with the environmental contaminant under conditions suitable to the adsorption of the environmental contaminant. Suitably, the environmental contaminate is a polyaromatic compound.
Another aspect of the technology is methods for preparing nanocomposites or photocatalysts. The method may comprise providing a first macrocycle solution comprising a macrocycle, a macrocycle solvent, and a first counterion, preparing a second macrocycle solution comprising the macrocycle, the solvent, and a second counterion, wherein the second counterion is different than the first counterion, providing a polymer solution comprising a polymer and a polymer solvent, mixing the second macrocycle solution and the polymer solution, thereby precipitating the nanocomposite or the photocatalyst from solution. In some embodiments, the first macrocycle solution and/or the second macrocycle solution comprises a host-guest complex comprising the macrocycle and a polyaromatic guest. In some embodiments, the second macrocycle solution is prepared by ion exchange between the first counterion and the second counterion.
These and other aspects of the technology will be further described herein.
Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.
The present technology is directed to supramolecular porous organic nanocomposites for heterogenous photocatalysis as well as methods of making and using the same. The photocatalysts are inexpensive, photostable, easy to process, and environmentally friendly porous organic polymeric nanocomposites for removing contaminants from the environment, including photocatalysis of chemical warfare agents. The nanocomposites described herein comprise an admixture of a polymeric matrix and a macrocycle. The nanocomposites may form host-guest complexes between the macrocycle host and a guest that is dispersed within the polymeric matrix. These composites are insoluble in organic and aqueous solvents which make them desirable for device fabrication. In addition, these composites are microporous, therefore can adsorb polyaromatic pollutants. Under photoexcitation, host-guest supramolecular photocatalysts have shown efficient singlet oxygen (1O2) generation and therefore effective photocatalyst for a selective removal of contaminants from the environment. The photocatalysts show high photostability and reusability. These novel photocatalytic materials can be utilized as gels, powders, membranes, coatings, paints, filters, fibers, fabrics or textiles, personal protective equipment (such as masks), or materials for photodynamic therapy.
The present technology possesses a number of advantages that improves or differentiates it from photocatalysts known in the art. The nanocomposites are microporous and the macrocycles possess a permanent cavity which is efficient for hosting electron-rich compounds such as polyaromatic hydrocarbons. The composites may be transparent; therefore, more material can be utilized for photocatalysis. The nanocomposites and photocatalysts described herein are highly stable materials under photocatalytic process, biocompatible, environmentally friendly, easily processed for the fabrication of gels, porous membranes, coatings, and fibers. These amorphous polymeric nanocomposites materials are easy to process and can be included with textile polymers to develop protective clothes and equipment against chemical warfare agents or develop water purification filters and antimicrobial materials.
One aspect of the invention is a novel method for the preparation of the nanocomposites and photocatalysts described herein via counterion exchange between a macrocycle or porous cyclophane and polymer matrices. The strategies utilized so far for the preparation of porous materials are the covalent organic frameworks (COFs), metal-organic frameworks (MOFs), porous organic polymers (POPs) and polymer of intrinsic microporosity (PIMs). COFs and the MOFs often are difficult to prepare in large scale and are very complicated to process for device fabrication. In addition, COFs and MOFs are crystalline and it is difficult to control their structural integrity when incorporated within devices and other polymeric materials. Other COFs and MOFs structures collapse upon removal of solvent. POPs, requires extensive organic synthesis and required expensive rare-earth metal catalysts. All these materials are difficult to process for the development of large-scale equipment for water purification, or filtrations, and protective equipment against chemical warfare agents.
In contrast, the nanocomposites described herein are easily tunable to be (or not) soluble in water by changing the ratios of polymer to macrocycle. In addition, all these composites are amorphous which bypass the crystallinity problems often encountered in COFs and MOFs when applied into materials and devices. In addition, the composites can be incorporated into other polymers and fibers for the development of several porous materials.
In this context, development of sustainable photosensitizing organic materials for the heterogenous catalysis requires that the material fulfill these four main requirements — (i) it is porous and increases the photoactive surface area and facilitates the diffusion of reactant and products, (ii) 1O2 generation is efficient, (iii) the material is stable under photocatalytic conditions, and (iv) its preparation needs to be easy, inexpensive, scalable, and capable of being incorporated into products, such as MPE.
Another aspect of the disclosed technology is the use of host-guest supramolecular donor-acceptor dyads to enhance the photosensitizing performance. A polyaromatic guest may be utilized as an electron donor with macrocycle as the electron acceptor in order to form a host-guest D-A supramolecular complex. This complex promotes the S-T exciton transformation between the two excited states of the two components (
Although compounds, such as TBP which is used in the Examples, may have a low-lying triplet state (T1, 1.89 eV) close in energy to the molecular oxygen (1.63 eV) facilitating the energy transfer to generate the singlet oxygen, the inefficient intersystem crossing and internal conversion may hamper the population of the T1 triplet state. Host-guest complexes may have a manifold of excited states involving, locally excited states, charge transfer states, and hybrid locally charge transfer states that enhance not only the intersystem crossing mechanism but also the decay from the upper states through internal conversion mechanisms. As demonstrated in the Examples, the photosensitizers prepared according to the presently disclosed technology efficiently generate singlet oxygen. Previous studies, in contrast, have been limited into the development of intramolecular donor-acceptor dyads as efficient photosensitizers. Another aspect of the invention is the incorporation of the host-guest photosensitizer into polymer matrices for the preparation of heterogenous photocatalysts. Although, non-porous polymers have been used to prepare singlet oxygen photosensitizer thin films, leaching of photosensitizer, the lack of porosity in the materials prepared in this fashion, and aggregation of photosensitizers inhibit development of these materials for use protective equipment against chemical agents, such as sulfur mustard.
The nanocomposites and photocatalysts described herein comprise an admixture of a polymeric matrix and a macrocycle. “Macrocycle” refers to a cyclic macromolecular or a macromolecular cyclic portion of a macromolecule. “Macromolecule” refers to a molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass. In some embodiments, the macrocycle is cyclophane. The incorporation of the macrocycle into the polymeric matrix allows for the material to have an intrinsic porosity.
“Cyclophane” refers to compounds having (i) mancude-ring systems, or assemblies of mancude-ring systems, and (ii) atoms and/or saturated or unsaturated chains as alternate components of a large ring. “Mancude-ring systems” refers to rings having (formally) the maximum number of noncumulative double bonds, e.g. benzene, indene, indole, 4H-1,3-dioxine. Exemplary cyclophanes include ExBox4+ or Ex2.2Box4+.
“Polymeric matrix” refers to a polymer capable of surrounding the macrocycle and interacting with the macrocycle to prepare nanocomposites. In some embodiments, the polymeric matrix may non-covalently interact (for example via electrostatic, van der Waals, π-π interaction, or the like) with the macrocycle to prepare stable nanocomposites where neither the polymeric matrix nor macrocycle substantially leeches into solution when immersed into a solvent. In some embodiments, the polymeric matrix may be transparent or substantially transparent in a desired spectral window.
Exemplary polymeric matrixes include anionic polymers, biopolymers or natural polymers, polymers suitable for 3D printing of plastics. For example, anionic polymers may include sulfate polysaccharides (heparin, mannan sulfate, dextran sulfate and chondroitin sulfate) and starch with carboxylic substitutions. Biopolymers or natural polymers may include, for example, proteins, polynucleic acids, poly lactic acid, polyglyconic acid, poly-3- hydroxybutyrate, cellulose, chitosan, guar gum, starch, tannin and sodium alginate for the development of composites for biomedical applications. Polymers suitable for 3D printing of plastics may include polylactic acid or polyethylene terephthalate. Other exemplary polymers for use with the present technology include, without limitation, polystyrene sulfonate (PSS), cellulose acetate (CA), polyamide (PA), polyvinylidene fluoride (PVDF), polysulfone (PSF), polyethersulfone (PES), polyvinyl chloride (PVC), polyimide (Pl), polyacrylonitrile (PAN), polyethylene glycol (PEG), polyvinyl alcohol (PVA), poly(methacrylic acid) (PMAA), poly(arylene ether ketone) (PAEK), poly(ether imide) (PEI), polyaniline nanoparticles (PANI), sulfonated poly(arylene ether sulfone) (SPAES), and the like.
The macrocycles of the present invention may be charged to allow for electrostatic interactions with the polymeric matrix. Suitably the macrocycles are cationic such as those that may be prepared from pyridinium subunits but other cationic or anionic subunits may also be employed to prepare the macrocycle.
In some embodiments, the nanocomposite comprises a photosensitizer. “Photosensitization” referrers to a process by which a photochemical or photophysical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called a photosensitizer. Suitably, the photosensitizer is not consumed in the reaction.
The photosensitizer may comprise a host-guest complex comprising the macrocycle and a guest. The guest suitably absorbs the same or substantially similar wavelength as the host macrocycle and has efficient intersystem crossing between the guest and host. In some embodiments, the guest is insoluble to avoid leaching of the guest from the macrocycle and the nanocomposite.
Suitably the guest is a polyaromatic guest. Polyaromatic guests comprise two or more fused aromatic rings. In some embodiments, the polyaromatic guest comprises one or more heavy atoms. As used herein, a heavy atom may include any atom heavier than carbon or, in some embodiments, heavier than fluorine or chlorine. Exemplarily polyaromatic guests include, without limitation, tetrabromopyrene (TBP), naphathalene diimide, or perylene diimide. Utilization of two organic dyads of similar excited state energies (similar exciton energies) leads to the increase of the efficiency of the intersystem crossing by combining both the spin-orbit charge transfer-intersystem crossing (SOCT-ISC) and spin orbit coupling (SOC) associated with the heavy atoms between the excited states. Therefore, the host-guest complex is a more efficient photo-synthesizer comparing to the performance of the individual component. This strategy offers great advantages since it does not require significant organic synthesis to prepare a donor-acceptor dyad. The present technology is versatile and macrocycles absorbing visible light or near-lR light can be used in combination with other guest molecules absorbing similar wavelengths to form a supramolecular Donor-Acceptor dyad.
The present technology may be used in a number of different applications. In one embodiment the nanocomposites and photocatalysts disclosed herein may be used to prepare gels, powders, membranes, coatings, paints, filters, fibers, fabrics or textiles, personal protective equipment (such as masks), or materials for photodynamic therapy. Suitably, the nanocomposites and photocatalysts can be incorporated into woven and nonwoven fibrous materials, such as wool felt, fiberglass paper, polypropylene, and so forth, or into polymers, such as polyester, polyamide, wool, and so forth. These materials can be used to develop textiles, clothes, masks, filters with photosensitizing properties for sequestering environmental contaminants, catalytically degrading contaminants, or killing or inhibiting the proliferation of microbes.
In another embodiment, the nanocomposites and photocatalysts disclosed herein may be used for photocatalytic oxidation of reactive substrates. The method may comprise contacting any of the nanocomposites and photocatalysts disclosed herein with a reactive substrate and irradiating the nanocomposite or photocatalyst in the presence of the reactive substrate, thereby oxidizing the reactive substrate. Suitably the reactive substrate may be a thioether or organophosphorous compound, which may be generally recognized as being a chemical warfare agents such as a sulfur mustard. Exemplary chemical warfare agents include, without limitation, VX, Soman, Sarin, Tabun, cyclosarin, mustard, and the like.
In another embodiments, the nanocomposites and photocatalysts disclosed herein may be used for the generation of singlet oxygen (1O2). The method may comprise irradiating any of the nanocomposites and photocatalysts disclosed herein in the presence of an oxygen source. Suitably, the oxygen source is triplet oxygen (3O2) but other sources of oxygen may also be used. Suitably, the singlet oxygen may be used for photodynamic therapy. In another use, the singlet oxygen may be used to kill or inhibit the proliferation of microbes, such as viruses and bacteria, suitable for the purification of water or other liquids.
In another embodiments, the nanocomposites and photocatalysts disclosed herein may be used for sequestering an environmental contaminant. The method may comprise contacting any of the nanocomposites and photocatalysts disclosed herein with the environmental contaminant under conditions suitable to the absorption of the environmental contaminant. Suitably, the environmental contaminant is a polyaromatic compound, but other compounds may also be sequestered. Suitably, the environmental contaminant may be reversibly released by changing, for example, the redox state of the macrocycle. This allows for the preparation of recyclable materials for trapping or filtering environmental contaminants, such as polyaromatic compounds.
Unless otherwise specified or indicated by context, the terms “a”, “an”, and “the” mean “one or more.” For example, “a molecule” should be interpreted to mean “one or more molecules.”
As used herein, “about”, “approximately,” “substantially,” and “significantly” will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which they are used. If there are uses of the term which are not clear to persons of ordinary skill in the art given the context in which it is used, “about” and “approximately” will mean plus or minus ≤10% of the particular term and “substantially” and “significantly” will mean plus or minus >10% of the particular term.
As used herein, the terms “include” and “including” have the same meaning as the terms “comprise” and “comprising.” The terms “comprise” and “comprising” should be interpreted as being “open” transitional terms that permit the inclusion of additional components further to those components recited in the claims. The terms “consist” and “consisting of” should be interpreted as being “closed” transitional terms that do not permit the inclusion additional components other than the components recited in the claims. The term “consisting essentially of” should be interpreted to be partially closed and allowing the inclusion only of additional components that do not fundamentally alter the nature of the claimed subject matter.
All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
Preferred aspects of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred aspects may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect a person having ordinary skill in the art to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
Molecular oxygen may be applied in the detoxification of Chemical Warfare’s Agents (CWAs), such as Sulfur Mustard (SM). Efficient heterogenous photosensitizing materials need to present both large accessible surface areas and excitons of suitable energies and with well-defined spin structures. Confinement of the tetracationic cyclophane (ExBox4+) within a non-porous anionic polystyrene sulfonate (PSS) matrix leads to a surface area increase of up to 225 m2.g-1 in ExBox•PSS. Our approach to enhancing the intersystem crossing (ISC) involves combining (i) efficient spin-orbit coupling (SOC) associated to lone-pair electrons of heavy atoms (Br atoms) in the 1,3,5,8-tetrabropyrene (TBP), and (ii) photoinduced electron transfer in a TBP⊂Exbox4+ supramolecular donor-acceptor (D-A) dyad to trigger a spin-orbit charge transfer ISC. The TBP⊂Exbox4+ complex displays a charge transfer band at 450 nm and an exciplex emission at 520 nm (λex = 380 nm, (ΦF < 3%) with a short life time (< 1 ns) in both solution and in the solid state, indicating the formation of new mixed-electronic states between the D and A. Time-dependent DFT calculations have revealed that the efficient singlet-triplet (S-T) transformation is the result of the formation of a hybrid locally charge transfer (HLCT) excited state in the D-A complex and the close energy levels with the same transition configurations. The lowest triplet state (T1, 1.89 eV) is a locally excited (LE) state on the TBP and close in energy with the charge separated state (CT, 2.14 eV). Transient absorption spectroscopy exciting the HLCT state at 414 nm shows the population of an emissive CT state followed by recombination to a long-lived triplet state (> 1.5 µs). The photocatalytic activities of the TBP⊂Exbox•4Cl and TBP⊂Exbox•PSS in homogenous and heterogenous media respectively for the conversion of a sulfur mustard simulant to its non-toxic sulfoxide analogue, has proved to be significantly more efficient than TBP and
ExBox4+, confirming the importance of the newly formed excited-state manifold in TBP⊂Exbox4+ for the population of low-lying T1state. The high stability, inexpensiveness, facile preparation, and high performance of the TBP⊂ExBox•PSS complex augur well the future development of new supramolecular heterogenous photosensitizers using host-guest chemistry.
Here we describe (Scheme 1) the preparation of supramolecular porous organic composites using anionic polymeric matrices such as Polystyrene Sodium Sulfonate (Na•PSS) and extended tetracationic cyclophanes such as ExBox4+ and Ex2.2Box4+. The rigidly defined cavities of these cyclophanes, when assembled within a polymeric matrix relying electrostatic interactions offers porous properties that are necessary in order to optimize the diffusion of reactants and products within them and increase the active surface area for 1O2 generation. Furthermore, tetracationic cyclophanes such as ExBox4+ are attractive candidates for ultrafast intermolecular CT from an electron-rich guest,13 intramolecular through-bond CT from thep-xylylene bridges to the extended bipyridinium units14 and multielectron accumulation,15 leading to an array16 of accessible mixed-valence states, and energy transfer from ExBox4+. Other investigations reported17 that the close interaction and the significant orbital overlap between the PDI (perylene diimide) as a guest and Exbox4+ acting as a host, enables ultrafast energy transfer to proceed by the electron exchange Dexter mechanism.18 In addition, incorporation of heavy atoms into the cyclophane leads to an efficient quenching of the fluorescence as a result of efficient spin-orbit ISC pathways leading to the generation of the triplet state on the PDI guest.16
Scheme 1. (top) The structural formals of building blocks utilized in the design of supramolecular photosynthesizing porous organic polymer for the a heterogenous photocatalysis. (a) ExBox4+, (b) Ex2.2Box4+, (c) 1,3,6,8-tetrabromopyrene (TBP) (d) Sodium Polystytene Sulfonate (Na•PSS). (e) Synthesis of the TBP⊂ExBox•4PF6, TBP⊂ExBox•4Cl and TBP⊂ExBox•PSS composites.
1,3,6,8-tetrabromopyrene (TBP) is utilized as an electron donor with ExBox4+ as the electron acceptor in order to form a host-guest D-A supramolecular complex (TBP⊂ExBox4+). This complex promotes the S-T exciton transformation between the two excited states of the two components (
From a practical perspective, the very low solubility of the TBP in organic and aqueous media at ambient temperatures is necessary in order to enhance the stability of the supramolecular photocatalyst since host-guest formation is not in equilibrium. It was previously reported21 that the water soluble cobalt(III) tetrahedral coordination capsules exhibit non-equilibrium guest binding properties because of the hydrophobic effect which is associated with the low solubility of the guest molecules in aqueous media. Finally, incorporation of the tetracationic TBP⊂ExBox4+ photosensitizer within the anionic matrix of PSS leads to the formation of a stable and porous composite for the heterogenous photocatalysis of CEES. All compounds have been characterized in solution and in the solid state by absorption, diffuse reflectance and fluorescence spectroscopies. Furthermore, the electronic properties of the host-guest complex have been unraveled using transient absorption spectroscopy and time-dependent DFT calculations. Finally, we have investigated the photocatalytic performance of TBP⊂ExBox4+•4PF6, and TBP⊂ExBox4+•PSS for the elimination of the sulfur mustard simulant (CEES) in both homogenous and heterogenous media.
Preparation and Characterization. The ExBox•4PF6 and Ex2.2Box•4PF6 cyclophanes were synthesized following the protocols already reported in literature22. Although TBP is insoluble in the most common organic solvents, at high temperatures it becomes soluble in PhMe to afford a pale yellow solution. The host-guest complex TBP⊂ExBox•4PF6 can be formed (Scheme le) by dropwise addition of TBP, solubilized in hot PhMe into a solution of ExBox•4PF6 in hot dimethyl formamide (DMF). After heating the mixture for 24 h at 80° C., an intense yellow/orange colored solution is formed. After evaporation of the solvent and solubilization of TBP⊂ExBox•4PF6 in MeCN, the insoluble excess of TBP can be removed by filtration. Tetrabutylammonium chloride was added to the MeCN solution containing TBP⊂ExBox•4PF6 in order to exchange the PF6- to Cl- anions, a process that renders the cyclophanes soluble in aqueous media. After isolation of the TBP⊂ExBox•4Cl complex as a yellow powder, it was dissolved in H2O and Na•PSS was added dropwise under strong agitation to form (Scheme le) a precipitate of TBP⊂ExBox•PSS composite of 5/3 w/w ratio. The very low solubility of the TBP, combined with the trapping of TBP⊂ExBox4+ within the PSS polymer matrix as a result of electrostatic interactions, is essential in order to enhance the stability of the composite in both aqueous and organic media with efficient heterogenous photocatalysis.
In order to ascertain the role of the host-guest D-A complex in the photocatalytic performances, the ExBox•PSS and Ex2.2Box•PSS composites have also been prepared (Scheme 2 and 3) quantitatively following similar protocols. After ExBox•4Cl and Ex2.2Box•4Cl have been dissolved in H2O and Na•PSS was added dropwise to form the ExBox•PSS and Ex2.2Box•PSS composites at 1/1 and 3/2 w/w ratios, respectively. These composites are insoluble in both aqueous and non-aqueous media. In order to study the optical properties of the composites in aqueous solutions, we prepared the ExBox•PSS and Ex2.2Box•PSS composites at ⅓ and 1:1 w/w ratios, respectively.
Scheme 2: Preparation of ExBox•PSS composite
Scheme 3: Preparation of ExBox2.2•PSS composite
Sorption and Morphological Investigations. The CO2 adsorption on the ExBox•PSS and Ex2.2Box•PSS composites has been performed and compared to the adsorption isotherm of the pristine Na•PSS in order to confirm the role of the tetracationic cyclophanes in forming the porous nature of these composites. Furthermore, the surface area and the porosity of the ExBox•PSS were measured at 195 K and 295 K (
In order to test the diffusion of larger molecules, we investigated the adsorption of the tetrathiafulvalene (TTF) inside the ExBox·PSS composite. Previous studies have revealed25 that TTF has a relatively strong affinity for the tetracationic cyclophanes, forming dark green host-guest complexes. Incorporation of the ExBox•PSS composite within a solution of the TTF of concentration of 10-5M led to the absorption of the TTF molecules, affording (
Solution Studies: Steady-State Spectroscopy: Absorption and fluorescence investigations have been carried out in order to unravel the electronic properties of the host-guest complex (TBP⊂ExBox•PF6) in solution and the polymer composites in the solid-state. Na•PSS is colorless in H2O and the UV-Vis absorption profile is characterized by the existence of two absorption bands at 223 and 252 nm, while fluorescence spectroscopy has shown that excitation at 254 nm offers a single emission band at 308 nm. ExBox•4Cl in H2O displays excitation and emission bands at 358 and 383 nm, respectively, arising from the lowest singlet excited state. The Exbox•PSS composite of 1:3 w:w ratio is soluble in H2O and displays the characteristic absorption features of ExBox4+ and PSS. The emission of this composite in aqueous solution exhibits a slight bathochromic shift of 47 nm to become centered at 430 nm as a consequence of the change in the polarity and viscosity of the media. Time-resolved photoluminescence decay was monitored at 430 nm, using 374 nm as the excitation wavelength. The decay curve was fitted to the double-exponential function, resulting in a slow component (τ1 = 1.43 ns) and a fast one (τ2 = 0.47 ns). In PhMe, TBP is weakly soluble and the absorption profile of TBP shows several absorption bands at 378, 359, 341 and 293 nm characteristic of the π→π* and n→π* transitions. The diffuse reflectance of TBP reveals (
Transient Absorption Spectroscopy: The electronic properties of the TBP⊂ExBox•4PF6 complex have also been investigated with transient absorption spectroscopy. Exciting at either 414 or 450 nm, the kinetics of the charge separation and recombination for TBP⊂ExBox4+ were obtained. See
Nanosecond transient absorption measurements leads to the observation at λex = 414 nm of long-lived triplet of >1.5 µs, implying that excitation of the upper 1CT and 1LE states (S2, S3, S4 states, vide infra) populates the T1 state of the TBP following charge recombination, while excitation of the 1CT states at 450 nm, does not lead to a detectable triplet population. The lack of triplet formation, following 450 nm excitation, is associated with the lower amount of triplet character in the CT state populated by absorption, which is also consistent with the discrepancy in the decay time constants at different excitation wavelengths. Whilst excitation at 414 nm offers shorter time-constants, associated with the more efficient SOCT-ISC between the upper states (S2→T6, S3→T6 and S4→T8 for example,
Solid-State Studies: Diffuse reflectance measurements on solid films of the ExBox•PSS composite exhibit (
Time-Dependent DFT (TD-DFT). In order to understand better the electronic properties of the TBP⊂ExBox4+ complex and have an estimation of the singlet-triplet energy gap (ΔEST), we utilized both the APFD and the B3LYP functionals in conjunction with the 6-31G(d) basis set to calculate molecular geometries. Optimization of the superstructure of the TBP⊂ExBox4+ at the B3LYP/6-31G(d) energy level leads to a larger interplanar distance between the TBP and the Exbipy2+ units (~4.2 Å), while utilization of the APFD functional offers a superstructure with interplanar distances between the TBP and Exbipy2+ of 3.5 Å, similar to those reported22 from the crystals structures of polyaromatic compounds inside the ExBox4+. The discrepancy between these optimized superstructures is a result of incorporation of an empirical dispersion correction term within the APFD formalism, while dispersion interactions are neglected within the B3LYP fuctional.31 In addition, these two geometries offer the possibility to determine the energy of the LE states of the TBP and ExBox4+ and, hence, unravel the role of the orbital overlap between the D-A into the formation of mixed excited states. Satisfied by the presence of zero negative frequencies, gas-phase TD-DFT calculations have been subsequently, carried out at the B3LYP/6-31G(d) level of theory using Gaussian16 software.32 Here we discuss the electronic properties of TBP⊂ExBox4+ derived from the APFD/6-31G(d) optimized structure (
The singlet and triplet excited states of the TBP⊂ExBox4+ complex consist of (
The photocatalytic performance of the TBP⊂ExBox4+ D-A dyad is high in the excitation range 380-420 nm (λmax = 395 nm, 3.13 eV) and so, the photocatalytic properties arise from the 1HLCT states, S2, S3 and S4 states (band at 387 nm,
In order to decipher further the contribution of the LE, CT and HLCT states to the overall ISC process in the TBP⊂ExBox4+ complex, natural transition orbital (NTO) analysis, based on the singular value decomposition of 1-particle transition density matrix, was performed. NTOs give a compact representation of the orbital transformation composition for a given transition. The highest occupied natural transition orbital (HONTO) and the lowest unoccupied natural transition orbital (LUNTO) orbitals represent any one electron property associated with the electronic transition and excitation amplitude is always the most significant for any particular excited state, as a result of its dominating role in determining the one electronic transition for the generation of the corresponding excited state from the ground state (S0).35 The HONTOs and LUNTOs of all the hybridized singlet (S2, S3 and S4) and triplet states (T6, Ts, T9, and T10) were investigated. Within the singlet/triplet excited state pairs that can undergo exciton transformation (
Photocatalytic Activity. Generation of 1O2 by stable microporous organic photocatalysts in both aqueous and organic media provide countless opportunities, not only for the development of environmentally and economically viable materials for the elimination of SM stockpiles, but also in the design MPEs. Compared to other oxidants, the reaction of 1O2 with CEES leads to the selective formation of less toxic CEESO as a major product, while CEESO2 is formed as a minor product (
Homogenous Photocatalysis. The photocatalysis of CEES with 1% mol catalyst of ExBox•4Cl, Ex2.2Box•4Cl or TBP⊂ExBox•4Cl has been carried out (
Heterogenous Photocatalysis. The design of protective equipment against chemical warfare agents such as SM requires the development of efficient heterogeneous photocatalyst. The fulfilment of this goal requires taking into account multiple parameters namely— (i) the polymer matrix needs to be transparent in order for the photocatalyst to be able to absorb a maximum of light irradiation, (ii) the polymer is porous to allow a facile transport of species to and from the active sites, (iii) the photosensitizer needs to be photostable in relation to photobleaching, (iv) the stabilization of specific transition state is required in order to optimize the selectivity, and finally (v) the different components need to be insoluble to avoid chemical leaching. Blending cationic cyclophanes with commercially available an anionic polymer matrix leads to the formation of insoluble composites with relatively large surface areas, a characteristic that can help the transport of reactant (3O2, CEES) and products (1O2 and CEESO) to and from the photocatalytic sites. The heterogenous catalytic reactions have been achieved with 1 mol% catalyst of ExBox•PSS, Ex2.2Box•PSS, and TBP⊂ExBox•PSS under photoirradiation at 395 nm (
The Na•PSS did not show significant photocatalytic performance except for a slight conversion because of the decomposition of CEES to MeOEES in MeOH (
Supramolecular porous organic composites based of tetracationic cyclophanes (ExBox4+ and Ex2.2box4+) and an anionic polymer matrix such as polystyrene sulfonate (PSS) have been prepared. These materials were found to be microporous as evidenced by CO2 adsorption isotherms. In addition, larger molecules such as TTF can diffuse inside the polymer confirming the possibility for larger molecules to diffuse in/out of the ExBox•PSS composite. While the photocatalysis of CEES by ExBox•4Cl in solution is fast and selective, in the solid state the conversion of the CEES to CEESO is very slow as the result of stabilization of the singlet excited state. Other cyclophanes, such as Ex2.2Box•4Cl are not stable under photoirradiation and the photocatalysis of CEES is slow and not selective. Notably, Ex2.2Box•PSS is stable under photoirradiation and the conversion of CEES to CEESO is 100% selective. Although the lowest triplet state (T1) of the 1,3,5,8-tetrabropyrene (TBP) is very low in energy, it is inaccessible on account of the large energy barrier separating the T1 states from the S1 and T2 states. The efficiency of the singlet to triplet (S-T) transformation in the TBP⊂ExBox4+ host-guest complex is associated with a combination of both a large spin-orbit coupling of the Br atoms, with spin-orbit charge transfer intersystem crossing of the D-A dyad. In addition, DFT calculations revealed the existence of a manifold of excited states that can enhance the internal conversion (IC) of the upper triplet states to populate the low lying T1 excited state. This efficient S-T transformation and IC played a central role in the enhancement of the 1O2 generation and subsequently increase in the photocatalytic performances. The high stability, facile preparation, processability and high performance of the TBP⊂ExBox•PSS composite augur well the future development of the supramolecular heterogenous photosensitizer using host-guest chemistry. More broadly, these results reveal a number of other opportunities for facile fine-tuning the S-T transformation in D-A dyads using host-guest chemistry which can unleash several fundamental and technological advances for future design of triplet excited state chromophores.
All chemicals and reagents were purchased from commercial suppliers (Aldrich and TCI chemicals) and used without further purification. Exbox•4PF6, ExBox•4Cl and Ex2.2Box•4PF6 were prepared according to previous literature procedures.1 Column chromatography was carried out on silica gel 60F (Merck 9385, 0.040-0.063 mm). 1H and 13C Nuclear magnetic resonance (1H and 13C NMR) spectra were recorded on a Bruker Avance 500 with working frequencies of 500 MHz. Chemical shifts are reported in ppm relative to the signals corresponding to the residual non-deuterated solvents (CD3CN: δ = 1.94 ppm, D2O: δ = 4.79, CD3OD δ= 3.34). Gas Chromatography GC-FID measurements were carried out on an Agilent Technologies 7820A GC system equipped with an Agilent J&W GC HP-5 capillary column (30 m × 320 µm × 0.25 µm film thickness). Heterogenous samples were filtered and diluted with CH2Cl2 prior to injection. Starting temperature: 70° C., Hold: 0.5 min, Ramp: 30° C./min, Time: 1 min, Ramp: 75° C./min, End temperature: 250° C. The disappearance of the reactant was calculated relative to a 0 min time point.
a) ExBox·PSS with 1/1 w/w ratio. ExBox•4Cl (35 mg, 0.043 mmol) and sodium polystyrene sulfinate (Na-PSS) (36 mg, 0.172 mmol) were dissolved separately in H2O (5 ml). The number of moles is determined according to the repeating unit (C8H7SO3Na) of molecular weight of 206.19 g.mol-1. Therefore, to achieve a full exchange of the Cl- anions of the ExBox•4C1, four equiv of the (C8H7SO3Na) are required.The amount of Na·PSS utilized was calculated according to the number negative charges. Therefore, four equivalents of Na·PSS unit are needed to exchange the four chloride ions of the Exbox•4Cl. Upon dropwise addition of Na·PSS into an aqueous solution of the ExBox•4Cl, a light-yellow pale precipitate is formed immediately. After stirring the mixture for 1 h, the solid was isolated by centrifugation and was washed three times with H2O to remove the NaCl. Yield: 55 mg.
b) ExBox·PSS with ⅓ w/w ratio. ExBox•4Cl (32 mg, 0.04 mmol) have been solubilized in MeCN (5ml) while sodium polystyrene sulfinate (Na•PSS) (98 mg, 0.47 mmol) have been solubilized in H2O (5 ml). To increase the solubility of the composite in water, we utilized 12 equiv of the (C8H7SO3Na) unit of the Na·PSS polymer. Upon dropwise addition of an aqueous solution of Na·PSS into an aqueous solution of the ExBox•4Cl, a light-yellow pale precipitate is formed immediately. After stirring the mixture for 1 h, the solid was isolated by centrifugation and the solid was washed three times with H2O to remove NaCl. Yield: 60 mg.
B. Preparation of Ex2.2Box•PSS composite at 3/2 w/w ratio: Ex2.2Box•4PF6 (30 mg, 0.022 mmol) was solubilized in MeCN (5 mL) to afford a pale-yellow solution. Sodium polystyrene sulfinate (Na-PSS) (19 mg, 0.091 mmol) was solubilized in H2O (5 mL). The number of moles of PSS utilized corresponds to the number of negative charges required to exchange all the PF6 counterions of the Ex2.2box4+. Upon dropwise addition of the solution of the Na·PSS into the solution of Ex2.2Box•4PF6, a dark yellow precipitate formed immediately. After stirring the mixture for 1 h, the solid was isolated by centrifugation and washed three times with H2O to remove the Na•PF6. Yield: 28 mg, 77%. The number of moles is determined according to the repeating unit (C8H7SO3Na) of molecular weight of 206.19 g.mol-1. Therefore, to achieve a full exchange of the (PF6)- anions of the Ex2.2Box•4PF6, four equiv of the (C8H7SO3Na) are required.
Ex2.2Box•4PF6 (17 mg, 0.012 mmol) was solubilized in MeCN (5ml) to afford a pale-yellow solution. Sodium polystyrene sulfinate (Na•PSS) (18 mg, 0.092 mmol) was solubilized in 5 ml of water. The number of moles of PSS utilized corresponds to the number of negative charges required to exchange all the PF6 counterions of the Ex2.2box4+ Upon dropwise addition of the solution of the Na·PSS into the solution of the Ex2.2Box•4PF6, a bright yellow colored solution formed immediately in an excess of H2O. After stirring the mixture for one hour, the solvent was evaporated and the isolated solid was washed three times with acetonitrile to remove the Na•PF6. The number of moles is determined according to the repeating unit (C8H7SO3Na) of molecular weight of 206.19 g.mol-1. Therefore, to achieve a full exchange of the (PF6)- anions of the Ex2.2Box•4PF6, four equiv of the (C8H7SO3Na) are required. Yield: 19 mg.
ExBox•4PF6 (30 mg, 0.024 mmol) was solubilized in dimethylformamide (5 mL) to afford a colorless solution. Excess of 1,3,6,8-tetrabromopyrene (TBP) (37 mg, 0.071 mmol) was dissolved in hot PhMe to afford a pale yellowish solution which was added dropwise to the solution of ExBox•4PF6 at 80° C. The mixture was kept warmed at 80° C. for 24 h leading to the evaporation of the PhMe and offering a dark yellowish solution of TBP⊂ExBox•4PF6 in DMF. After complete evaporation of the solvent, a crude yellow powder of TBP⊂ExBox•4PF6 contaminated with an excess of TBP was isolated. MeCN was added to the crude product in order to solubilize the TBP⊂ExBox•4PF6 complex and remove the insoluble excess of TBP by filtration. After drying the yellow solution, TBP⊂ExBox•4PF6 was isolated as a bright yellow powder. Yield: 40 mg, 94%.
Scheme 4: Preparation of the TBP⊂ExBox•4PF6 complex.
TBP⊂ExBox•4PF6 (20 mg, 1.5 × 10-5 mmol) was dissolved in MeCN (5 mL). Tetrabutylammonium chloride (50 mg, 0.18 mmol) is added to exchange the PF6 anions with chloride anions. After centrifugation and several washes with MeCN, a yellow powder was obtained which was dried under vacuum for 24 h. Yield: 12 mg, 80%.
Scheme 5: Preparation of ExBox•4Cl composite
TBP⊂ExBox•4Cl (10 mg, 0.0075 mmol) and Na·PSS (6 mg, 0.031 mmol) were dissolved separately in H2O (5 mL). The amount of Na·PSS utilized was calculated according to the number negative charges.The number of moles is determined according to the repeating unit (C8H7SO3Na) of molecular weight of 206.19 g.mol-1. Therefore, to achieve a full exchange of the Cl- anions of the TBP⊂ExBox•4Cl, 4 equiv of the (C8H7SO3Na) are needed. Thus, 4 equiv of sodium styrene sulfonate units are required to fully exchange the 4 Cl- atoms of the TBP⊂ExBox•4Cl. Upon addition dropwise of the solution of the Na·PSS into the solution of the TBP⊂ExBox•4Cl, a bright yellow pale precipitate of TBP⊂ExBox•PSS is formed immediately. After stirring the mixture for 1 h, the solid was isolated by centrifugation and washed two times with H2O. Yield: 11 mg, 73%.
Scheme 6: Preparation of TBP⊂ExBox•PSS composite
Powder X-ray diffractions were conducted on a STOE-STADI MP powder diffractometer equipped with an asymmetric curved Germanium monochromator (CuKα1 radiation, λ = 1.54056 Å) and a one-dimension silicon strip detector (MYTHEN2 1 K from DECTRIS). The line focused Cu X-ray tube was operated at 40 kV and 40 mA. Samples for structural analysis were measured at room temperature in transmission geometry.
Adsorption Isotherms. The CO2 adsorption isotherms of ExBox·PSS were measured at 278 K and 195 K using a Micromeritics ASAP 2020 instrument. Pore-size distributions were estimated using 2D-NLDFT (N2-carbon finite pores, As = 6) method with a non-negative regularization of zero. The CO2 adsorption isotherms of Na·PSS and Ex2.2Box•PSS were measured at 195 K using a Micromeritics ASAP 2020 instrument. The activation of Na·PSS, ExBox·PSS and Ex2.2Box•PSS was achieved by a supercritical-drying process using a TousimisTM Samdri® PVT-3D critical point dryer (Tousimis, Rockville, MD, USA) in which liquid CO2 was used to exchange the CO2 five times over the course of 10 h. The materials were heated to above the critical point of CO2 (T = 31° C., P = 73 atm) and the instrument was bled at a rate of ~0.5 sccm. Finally, samples were degassed at 35° C. for 6 h under high vacuum on a Smart Vacprep from Micromeritics. Around 30-50 mg of sample was used in each measurement, and the BET surface areas were calculated in the region P/Po = 0.005-0.05.
The SEM images and map scans were collected on a Hitachi SU8030 FE-SEM (Dallas, TX) microscope at Northwestern University’s S-9 EPIC/NUANCE facility. Samples were activated and coated with OSO4 to ~ 9 nm thickness in a Denton Desk III TSC Sputter Coater (Moorestown, NJ) before imaging.
Solution UV/Vis absorption spectra were recorded using a UV-3600plus Shimadzu spectrophotometer. The fluorescence spectra are collected using the Horiba Fluoromax-4 Spectrophotometer. Preparation of thin films for solid-state investigations was carried out by drop-casting on quartz slide. After solvent evaporation, thin films are formed.
Diffused reflectance spectra for the solid samples were measured using a JASCO V-670 UV-Vis-NIR Spectrophotometer equipped with a 60 mm BaSO4-coated integrating sphere and a PMT//PbS detector. Steady-state emission and excitation-emission mapping spectra were recorded at room temp using an Edinburgh Instruments FS5 spectrofluorimeter. Samples for spectroscopic measurements were packed inside a quartz capillary tube (ID = 3 mm), charged with degassed MeTHF solvent, and then sealed inside the glovebox: the samples were then soaked overnight. The spectra were collected in the front-face configuration using a 1.4 nm excitation and 0.4 nm emission slit widths and corrected by using the instrumental correction functions for the excitation light source as well as detector response. The absolute quantum yields (QYs) were measured using a 150 mm integrating sphere. QY values were calculated with EI F980 software that accounts for the diminished intensity (photon counts) of the incident excitation beam over the increased intensity (photon counts) of fluorescence, based on the manually selected respective integration range. Fluorescence lifetime emission decay profiles were recorded using an Edinburgh Lifespec II Picosecond Time-Correlated Single Photon Counting Spectrophotometer equipped with a Hamamatsu H10720-01 detector and a 405 nm picosecond pulsed diode laser as TCSPC source (IRF ≈180 ps). An iterative deconvolution procedure with exponential fitting was used within the EI F980 software to extract lifetime data.
The excitation-wavelength dependent fluorescence (Red-edge effect phenomena)2 is related to a slow solvation dynamic in relation to the time scale of the fluorescence.
The setup for transient absorption measurements has been described elsewhere.3 Photoexcitation pulses at 414 nm were obtained through a β-barium borate (BBO) crystal doubling the fundamental, and the 450 nm pulses were generated with a commercial non-collinear optical parametric amplifier (TOPAS-White, Light-Conversion, LLC). The pulse energy for photoexcitation was attenuated to ~1 µJ/pulse using neutral density filters. The pump polarization was randomized employing a commercial depolarizer (DPU-25-A, Thorlabs, Inc.) to eliminate any orientational dynamics contributions from the experiment. All the spectra were collected on a commercial spectrometer (Ultrafast Systems, LLC Helios and EOS spectrometers, for fsTA and nsTA, respectively). All samples were stirred to avoid localized heating or degradation effects. The optical density was maintained around 0.5 for all samples.
The Ex2.2Box•PSS has yellow pale color in H2O and the UV-vis absorption profile is characterized by the existence of two broad absorption bands at 360 and 390 nm, while fluorescence spectroscopy has shown that excitation at 380 nm offers a single emission band at 430 nm with ΦF = 35% (λem = 430 nm, τ1 = 0.31 ns, τ2 = 1.23 ns). In the solid state, although excitation at different wavelengths (390 and 414 nm) offers a similar emission broad band at 490 nm), the singlet excited state display excitation-dependence behavior similar to the ExBox·PSS composite.
The photostability of the Ex2.2Box•4PF6 was monitored using absorption and 1H NMR spectroscopies under photoirradiation at 395 nm in MeOH. After 1 h irradiation, both the UV-vis absorption and 1H NMR spectra undergo significant changes indicating the decomposition of the Ex2.2Box•4PF6.
The structure of 1,3,6,8-tetrabromopyrene (TBP) was optimized at the B3LYP/6-31G(d) level. Time-dependent DFT (TD-DFT) calculations were carried out on the singlet (S) and triplet (T) states on the optimized gas-phase geometry using Gaussian 16 package,4 considering a total of 20 excited states. Three singlet state transitions were determined with oscillator strengths (ƒ) > 0.05 and these are tabulated in Table 8. The energy levels of the S and T states possess a possible intersystem crossing channel between the S1 and T2 state. The T1 state is very low in energy (1.91 eV) and cannot be populated by ISC from the S1 (S1→T1) state or by internal conversion from the T2 state (T2→T1). These results are consistent with the optical studies showing the absence of phosphorescence in TBP at 77 K.
The superstructure of TBP⊂Exbox4+ was optimized using three levels of theory: (i) B3LYP/3-21G (ii) B3LYP/6-31 G(d) (iii) APFD/6-31 G(d). The B3LYP functional does not provide a correct description of dispersion forces leading therefore to an overestimation of the distances (> 4 Å) between the TBP and the Exbipy2+ units of the ExBox4+. This large distance between the D and the A, will decrease the orbital overlap between the two moieties, offering a possibility to estimate the contribution of the locally excited (LE) states into transitions of similar energies. While in the case of APFD functional set, the dispersion forces are included, and the molecular optimized geometry is consistent with the crystal structures1 of polyaromatic hydrocarbons incorporated into ExBox4+. In the APFD/6-31G(d) optimized molecular structure, the interplanar distance between the TBP and the Exbipy2+ unit are of 3.5 Å, corresponding to the Van Der Waal radii for [C···C] contact. A side-by-side comparison between these geometries illustrates how the extent of orbital overlap governs the formation of mixed states between the D and the A and the oscillator strength of the CT transitions.
The superstructure of TBP⊂Exbox4+ was optimized at the B3LYP/3-21G level and time-dependent DFT (TD-DFT) calculations were carried out at the B3LYP/LACV3P*+ level on the optimized gas-phase geometry using Jaguar,5 considering a total of 130 excited states reaching into the upper end of the absorption spectrum (242.3 nm). Good agreement) was achieved between the calculated and experimental spectra. Ten low energy transitions were found with oscillator strengths > 0.001 which are tabulated in Table 10. Notably, there is one very weak transition at 435.64 nm with an oscillator strength that is 74 times less than the transition at 339.19 nm. The transitions at 384 and 374 nm which are relevant to the catalytic wavelength range (375-420 nm) are tabulated in Tables 11-15. All these transitions involve orbitals from both TBP and Exbox4+ components.
Time-dependent DFT calculations were carried out on the optimized gas-phase geometry of TBP⊂Exbox4+ at the B3LYP/6-31G(d) level of theory using the gaussian 16 package.4 These calculations were performed in order to investigate the singlet/triplet exciton transformation. The excited singlet (Sn) and triplet (Tn) states were investigated by time-dependent DFT (TD-DFT) on the optimized ground-state geometry using the same level of theory to investigate the vertical excitation energies. In order to gain insight into mixed transitions, natural transition orbitals (NTOs) were calculated to give a compact orbital representation for the electronic transformation within each state. Orbital overlap was calculated using the multi-wavefunction analysis software Multiwfn version 6.06.
The singlet and triplet excited states of TBP⊂ExBox4+ consist of (Table 17) locally excited (LE) states residing either on the TBP or ExBox4+ components, charge transfer (CT) excited states and hybrid local charge-transfer (HLCT) excited states which are mixed states intermediate between a locally excited (LE) state and a charge-transfer (CT) state.7 The formation of mixed excited-states is consistent with the emission profile of the TBP⊂ExBox4+ complex which revealed the emergence of a lower energy band (520 nm) arising from exciton relaxation in the TBP⊂ExBox4+ complex (S1, Table 16). The S0→S1 transition (Table 16) possesses a very weak oscillator strength and involves a pure CT transition from the TBP guest to the Exbox4+ host. Noteworthy, the lowest T1 state (1.92 eV) is exclusively a LE state in TBP guest, while the S0→T2 (2.11 eV) and S0-T3 (2.13 eV) transitions are identical to the S0→S1 transitions having a pure CT transition from the host to the guest in the TBP⊂ExBox4+ complex. The extent of the HOMO to LUMO overlap is very small (7.9%) because of a larger interplanar distance (~ 4.2 Å) between the TBP and Exbipy2+ units (Van der Waal radii dC-C = 3.5 Å). It was proposed8 previously that the minimum requirement for realizing exciton transformation is a matching of energy levels between two states, based on a thermal equilibrium between the singlet and triplet excited states. Although the exciton transformation channels S1→T2 and S2→T3 have a very small ΔEST (~ 0 eV), the weak molar absorption coefficient (small f) of the CT transitions in the TBP⊂ExBox4+ leads to a weak photosensitizing efficiency at λex = 450 nm. The lowest singlet excited state with non-negligible oscillator strength is the S2 state (2.85 eV, ƒ= 0.0016) with pure CT character between HOMO (H) and LUMO+2 (L+2) (99%).
From Table 16, the S0→T6 transition configuration is very similar to that of S0→S2, both containing high HOMO→LUMO+2 components. The energy gap between the S2 and T6 states is very small (ΔEST = 0.0018 eV) and implies a facile S2 →T6 exciton transformation. The weak oscillator strength of the CT band, however, hampers efficient 1O2 generation at λex = 450 nm. The S3 and S4 excited states have non-negligible oscillator strengths (Table 16). Both the S0→S3 (3.20 eV, 387 nm, f = 0.07) and S0→S4 (3.25 eV, 380 nm, f = 0.03) are HLCT excited states involving both TBP→TBP, ExBox4+→ExBox4+ and TBP↔ExBox4+ transitions. Nevertheless, as a result of a large interplanar distance between TBP and the Exbipy2+ units, the extent of the orbital overlap is very small, and therefore the mixing of the orbitals in the host-guest complex is less significant compared to the APFT/6-31G(d) optimized molecular structure (
The HONTOs and LUNTOs of all the hybridized singlet (S3) and triplet states (T7, T8, T9, T12 and T13) were investigated for the TBP⊂ExBox4+ complex. The singlet/triplet excited state pairs that have energy levels conducive to exciton transformation according to the energy gap law (|ΔEST| < 0.37 eV), have very similar HONTO and LUNTO distributions with the HONTO residing on the (TBP) donor moiety and the LUNTO residing on the (ExBox4+) acceptor moiety. A very small overlap between the HONTOs and LUNTOs were observed for these transitions. It is noteworthy that the low orbital overlaps between the D and A leads to LE and CT states while increasing orbital overlap between states of similar energies leads to HLCT states (
Natural transition orbital (NTO) analysis was performed on the mixed excited states that involve components from both the TBP and ExBox4+ to elucidate the orbital migration in the singlet/triplet excited states.
The S0→T8 transition is a pure LE state involving the TBP guest exclusively. The S0→T9 transition is a pure CT state involving the TBP⊂ExBox4+ host-guest complex.
The S0→T12 transition is a pure CT state involving the TBP⊂ExBox4+ host-guest complex.
The S0→T13 transition is a LE state residing on the ExBox4+ host.
The S0→T14 and S0→T15 transitions are characteristic of LE behavior involving the ExBox4+ host exclusively, indicating the possibility of charge recombination in the p-xylene+•-Exbipy3+• complex, as it was already investigated experimentally.3
The optimized molecular structure of the TBP⊂Exbox4+ using APFD/6-31G(d) basis set gave interplanar distances between the TBP host and Exbipy2+ guest of 3.5 Å. TD-DFT calculations were performed on this geometry using the B3LYP/6-31G(d) level of theory to investigate the singlet/triplet exciton transformation using the Gaussian 16 package.4 Good agreement was achieved between the positions of the calculated and experimental profiles. Natural transition orbital (NTO) analysis was performed to give a compact orbital representation for the electronic transformations to the excited states. The calculated UV-Vis absorption spectrum was plotted. The absorption band at 420 nm is associated to a CT transition between TBP and ExBox4+ while the band centered at 387 nm involves HLCT transitions for the S2, S3 and S4 states.
The excited states S0→S2 and S0→T1 involves the same transition configuration (L→L+2 and L→L+3). The NTOs show that the T1 is predominantly a LE transition while the S2 is a HLCT transition. Although the orbital overlap between the HONTO and LUNTO for the S2 and T1 states are 38% and 90% respectively, the ΔEST (1.1 eV) is significantly larger than the limit of 0.37 eV for efficient intersystem crossing. Therefore, population of the T1 state is more likely to arise from internal conversion mechanism from the upper Tn states. Particularly the 1CT→3CT transformation (S1→T2 and S1→T3) display (
DFT calculations on the geometry optimized structure of the Ex2.2box4+ have been performed at the RB3LYP/6-31G*+ theory level using Jaguar.5 Both the HOMO and the LUMO are localized on the Ex2.2bipy2+ units, therefore the photosensitizing properties originate from the locally excited triplet state of the Ex2.2bipy2+ units.
Photocatalysts (0.002 mmol) 1 mol% were weighted in 17 mm × 83 mm glass microwave vials with a magnetic stir bar and sealed tightly by a crimper. Anhydrous MeOH or CD3OD were utilized in order to monitor the catalysis kinetics using GC-FID and NMR spectroscopy, respectively. Solvent (1 mL) was injected through rubber cap and mixture was bubbled with O2 gas for 20 min. Vials were left under 1 atm O2 atmosphere. An internal standard, 1-bromo-3,5-difluorobenzene (10 µL, 0.08 mmol), and 2-chloroethyl ethyl sulfide (CEES) (23 µL, 0.2 mmol) were introduced through rubber cap by using a 50 micro liter syringe. Heterogeneous mixtures were sonicated for 10 sec before irradiation. Microwave vials were placed between two UV light-emitting diodes (LEDs) (max@395 nm, 500 mW.cm-2) over a magnetic stirrer and stirring was started at 700 rpm. Catalysis data points were collected using a 1 mL syringe at the beginning and after each irradiation time intervals. MeOH aliquots taken from the vials were transferred to a GC vial with dilution of CH2Cl2 (0.8 mL) and MeOH/ CD3OD aliquots were diluted with CD3OD (0.3 mL) in an NMR tube. Accordingly, 1H and 13C NMR or GC-FID analysis of the samples were performed.
ExBox•4Cl photocatalyst (1 mol%) was solubilized in CD3OD in a microwave vial and sealed tightly with microwave vial cap with a septum by using a crimper. The reaction solution was bubbled with O2 for 20 min. The reaction vial was left under O2 atmosphere (P = 1 Atm) after O2 purging. Internal standard (10 µL) and sulfur mustard simulant (23 µL) was added to the solution successively by using 50 µL syringe. Photo-irradiation at λmax = 395 nm was achieved using two LEDs with power of 500 mW•cm-2 while stirring with a small magnetic bar at 700 ppm. After 16 min photo-irradiation, the CEES is fully and selectively converted to the CEESO. The 13C NMR spectra confirms the formation of CEESO and the disappearance of the peaks of the CEES at 13.8, 25.4, 32.4 and 42.7 ppm and appearance of the peaks of the CEESO at 13.8, 25.4, 33.4, and 42.7.
Very similar to preparation of ExBox•4Cl reaction mixture as described above, a mixture of IS, CEES and Ex2.2Box•4Cl photocatalyst (1 mol%) were prepared in CD3OD and the solution and the reaction was carried out under O2 atmosphere (P = 1 Atm). Photoirradiation at λmax = 395 nm was achieved using same LEDs and the photo-conversion of CEES was monitored by 1H and 13C NMR spectroscopies. The overall conversion of the CEES was very slow and only reaches (
On account of to the decomposition of Ex2.2Box4+ with photo-irradiation, formation of MeOCEES increased significantly under UV light. Since photosensitization was also available during decomposition, some mono-oxidized version of MeOCEES (MeOCEESO) appeared during the reaction. In order to confirm the role of the photoirradiation in the increase of the reaction of the MeOH with CEES, we collected the 1H NMR spectrum of CEES left in a solution of Ex2.2Box4+ in MeOH for 7 h (See spectrum labelled 7 h (no irradiation). CEES is relatively stable in methanol and confirms the role of the instability of the Ex2.2Box4+ in the formation of MeOEES and MeOEESO.
Photocatalytic oxidation of CEES using TBP⊂ExBox•4Cl was realized in the same fashion as with the former homogeneous photocatalysts. After 9 min photoirradiation, the CEES is fully and selectively converted to the CEESO. Seemingly, the rate of conversion of the CEES with the supramolecular photocatalyst TBP⊂ExBox•4Cl is 50% faster than ExBox•4Cl, indicative of the efficiency of the singlet to triplet transformation in such supramolecular complexes. 1H NMR and 13C NMR confirmed the oxidation of CEES to CEESO.
ExBox·PSS, Ex2.2Box•PSS and TBP⊂ExBox•PSS photocatalyst composites (1 mol%) were suspended in a solution of MeOH in a similar way to that explained above for the homogenous catalysts. The solution was saturated with O2 and the reaction was carried out under O2 atmosphere (P = 1 bar). Dispersions were sonicated for 10 sec and then photoirradiation at λmax = 395 nm was conducted using LEDs while stirring at 700 rpm. All samples were filtrated with 10 µm pore sized PFFE syringe filter using CH2Cl2 (0.8 ml) and filtrate was collected in GC vials and GC-FID analysis of the samples were conducted. Additional control experiments with Na·PSS, TBP and TBP•PSS have been carried out using similar reaction conditions to unravel the contribution of the PSS anionic matrix as well the TBP to the photocatalytic performances.
The photocatalysis of CEES with ExBox·PSS composite was carried out for 60 min under 395 nm photoirradiation. The conversion of the CEES was observed to be only 50% at 60 min indicating the weak photosensitizing character of the ExBox4+ in the solid state as the result of the stabilization of the singlet state evident with the increase in the singlet lifetime. Both 1H and 13C NMR spectra show the presence of both CEES and CEESO. Despite of low conversions, the reaction was highly selective towards the sulfoxide product. No harmful sulfone product was observed.
The photocatalysis of CEES with the Ex2.2Box•PSS composite was carried out (
The conversion kinetics of the CEES to CEESO was monitored by GC-FID. The photocatalysis of CEES with the TBP⊂ExBox•PSS composite was carried out for 60 min under 395 nm photoirradiation. The conversion of the CEES was 100% completed after 18 min photoirradiation. 13C NMR spectra show that the formation of CEESO is 100% selective. It is noteworthy that over irradiation up to 60 min does not lead to the formation of the sulfone derivative (CEESO2) confirming the selective nature of the TBP⊂ExBox•PSS composite.
The kinetic of the conversion of the CEES to CEESO was monitored by GC-FID. The photocatalysis of CEES with the TBP⊂ExBox•PSS composite was carried out for 35 min under 450 nm photoirradiation. The conversion of the CEES is very slow (18%) (
After preparation catalysis reaction in MeOH (1 mL) using 1 mol% of the TBP, which is not soluble in MeOH, as described above, photoirradiation of TBP resulted in two major products, CEESO and MeOEES in 60 min. Among the products, the sulfoxide version of MeOEES (MEOEESO) was also observed. 13C NMR spectra of the products at 60 min confirms the GC-FID results. Overall selectivity (
Photocatalytic reaction of the TBP•PSS (1 mol%) was performed (
Na·PSS was used as a photocatalyst to oxidize CEES. According to 13C NMR spectra of reaction solution at 60 min, no CEESO formation was observed. According to GC-FID and NMR analysis, however, 10% CEES was converted (
The leach test was conducted (
a Solubilized in MePh
2,1ΔEST
1,2ΔEST
1,3ΔEST
3,4ΔEST
3,5ΔEST
3,6ΔEST
4,8ΔEST
4,9ΔEST
3,10ΔEST
aH and L represents respectively HOMOs and LUMOs. b The most similar and energy close ΔEST are highlighted with the same color.
3,1ΔEST
1,2ΔEST
1,3ΔEST
4,4ΔEST
4,5ΔEST
2,6ΔEST
5,7ΔEST
5,8ΔEST
4,9ΔEsT
4,12ΔES T
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This application claims benefit of priority to U.S. Pat. Application Ser. No. 63/012,642, filed Apr. 20, 2020, the contents of which are incorporated by reference in its entirety.
Filing Document | Filing Date | Country | Kind |
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PCT/US2021/070431 | 4/20/2021 | WO |
Number | Date | Country | |
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63012642 | Apr 2020 | US |