Hydrogen bond assisted photoinduced intramolecular electron transfer and proton coupled electron transfer in an ultrafast time domain using a ruthenium-anthraquinone dyad.

Quinones play a significant role as primary electron acceptors in the natural photosynthetic system of photosystem II, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, a ruthenium(ii) polypyridyl complex 1 coupled to an anthraquinone (AQ) functionality through a rigid imidazole (Im) spacer has been synthesized to examine the effect of H-bonding on both the thermal and photoinduced electron transfer reactions. The anthraquinone moiety of complex 1 is fused to a benzi-imidazole system bearing C[double bond, length as m-dash]OAQHNIm based H-bonding at one side of the anthraquinone moiety so that intramolecular hydrogen bonding from the imidazole group to the nearby quinone carbonyl can occur. The hydrogen bond formation involving the semiquinone radical anion produced through the photoinduced reduction process in Ru-im-AQ and the imidazole proton in complex 1 resulted in a significant positive shift of one electron reduction potential of complex 1. The kinetics for the formation of the charge-separated states was explored by using femtosecond transient absorption spectroscopy. Hydrogen bonding between water and the reduced anthraquinone accounted for thermodynamic and kinetic stabilization of these charge-separated states. An attempt has been made to assess the relative importance of the driving force and solvent polarity, in the rates of photoinduced electron transfer in complex 1. The 490 nm transient absorption band of anthraquinone radical anions (AQ˙-) and a broad absorption in the 580-750 nm region having maxima at ∼690 nm have been observed and this is attributed to the generation of a transient Ru3+-species of the corresponding complex 1. Addition of water entails an acceleration of electron transfer rates by a factor of 3.33. The system investigated may serve as a model for the mechanistic diversity of PCET reactions in general with water as a primary proton donor. Furthermore, our studies are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as a variant form of PCET.

Impact of "half-crown/two carbonyl"-Ca2+ metal ion interactions of a low molecular weight gelator (LMWG) on its fiber to nanosphere morphology transformation with a gel-to-sol phase transition.

We report here a smart functional low molecular weight gelator (LMWG) L, containing an unusual metal ion coordination site, i.e. "half-crown/two carbonyl". The gelator L shows excellent gelation behavior with typical fibrillar morphology in acetonitrile, methanol and ethanol media. Upon Ca2+ ion binding with its "half-crown/two carbonyl" coordination site, the acetonitrile gel of L exhibits a fiber to nanosphere morphology transformation along with a gel-to-sol phase transition as confirmed by microscopic investigation and by direct naked eye visualization, respectively. The mechanism involved in this morphology transformation and gel-to-sol phase transition process was studied thoroughly with the help of computational calculations and various spectroscopic experiments and discussed.

Chiral Discrimination through 1 H†NMR and Luminescence Spectroscopy: Dynamic Processes and Solid Strip for Chiral Recognition.

The appropriate choice of the host molecules with well-defined optical activity (S-H/R-H) helps in the differentiation between two secondary ammonium ion-derivative guest molecules with different optical activities (R-G/S-G) based on the fluorescence resonance energy transfer (FRET)-based luminescence responses. Crown ether-based host molecules with opposite chiral configurations (R-H, S-H) have been derived from 1,1'-bi-2-naphthol (BINOL) derivatives that have axially chiral biaryl centers. These chiral crown ethers form host-guest complexes (i.e., [2]pseudorotaxanes) with chiral secondary ammonium ion derivatives (R-G, S-G). NMR spectroscopic studies show that the complexes are in a dynamic equilibrium in solution. Results of the 1 H NMR and fluorescence spectroscopic studies indicate a head-on orientation of the host and guest in the [2]pseudorotaxanes. The difference in the efficiency in the FRET-based responses between anthracene and the BINOL derivatives allow efficient chiral discrimination of the guests. Isothermal titration calorimetry and NMR investigations reveal that inclusion complexes between hosts and guests of the same chirality (R-H⋅R-G, S-H⋅S-G) are more stable relative to those of opposite chirality (R-H⋅S-G, S-H⋅R-G). However, FRET-based energy-transfer efficiency is higher for R-H⋅S-G and S-H⋅R-G complexes. NMR spectroscopic studies show that the relative orientation of the guest in the host cavity is significantly different when the host binds a guest of the same or opposite chirality; furthermore, the latter is more favorable for FRET, thus enabling discrimination between enantiomers. Interestingly, chiral discrimination of guest ions could also be achieved by using silica surfaces modified with chiral host molecules.

Proton-Coupled Electron-Transfer Processes in Ultrafast Time Domain: Evidence for Effects of Hydrogen-Bond Stabilization on Photoinduced Electron Transfer.

The proton-coupled electron-transfer (PCET) reaction is investigated for a newly synthesized imidazole-anthraquinone biomimetic model with a photoactive RuII -polypyridyl moiety that is covalently coupled to the imidazole fragment. Intramolecular H-bonding interactions between imidazole and anthraquinone moieties favor the PCET process; this can be correlated to an appreciable positive shift in the one-electron reduction potential of the coordinated anthraquinone moiety functionalized with the imidazole fragment. This can also be attributed to the low luminescence quantum yield of the RuII -polypyridyl complex used. The dynamics of the intramolecular electron-transfer (ET) and PCET processes are studied by using femtosecond transient absorption spectroscopy. The steady-state spectroscopic studies and the results of the time-resolved absorption studies confirm that H-bonded water molecules play a major role in both ET and PCET dynamics as a proton relay in the excited state. The electron-transfer process is followed by a change in the H-bonding equilibrium between AQ and imidazole in acetonitrile solvent, and protonation of AQ.- by water leads to PCET in the presence of water. A slower forward and backward electron-transfer rate is observed in the presence of D2 O compared with that in H2 O. These results provide further experimental support for a detailed understanding of the PCET process.

[2]Pseudorotaxane Formation with FRET Based Luminescence Response: Demonstration of Boolean Operations through Self-Sorting on Solid Surface.

Binary pseudorotaxane formation between an aza crown derivative as host (H) and two different imidazolium derivatives as guests (G1 and G2) have been studied in detail by NMR (1H NMR, 2D NOESY), optical (steady state electronic and emission spectroscopy), and mass spectroscopy. Binding stoichiometry (1:1), association constant for the respective [2]pseudorotaxane formation (KaH.G1 = (2.61 ± 0.015) × 103 M-1 and KaH.G2 = (1.27 ± 0.16) × 103 M-1), and associated thermodynamic parameters are also evaluated based on isothermal titration calorimetric (ITC) studies. FRET based luminescence ON responses are observed on formation of the binary pseudorotaxane (H.G1 and H.G2) in a nonpolar medium like dichloromethane. The thermodynamic feasibility of such an energy transfer process is also examined. The higher affinity of H and 18-crown-6 toward K+, as compared to those toward G1 or G2, and the reversibility in the host-guest binding process are utilized in demonstrating the self-sorting phenomena with associated changes in luminescence responses that could be correlated for Boolean operators like YES, INHIBIT, OR, and AND gates.

Exploiting hot electrons from a plasmon nanohybrid system for the photoelectroreduction of CO2.

Plasmonic materials convert light into hot carriers and heat to mediate catalytic transformation. The participation of hot carriers (photocatalysis) remains a subject of vigorous debate, often argued on the basis that carriers have ultrashort lifetime incompatible with drive photochemical processes. This study utilises plasmon hot electrons directly in the photoelectrocatalytic reduction of CO2 to CO via a Ppasmonic nanohybrid. Through the deliberate construction of a plasmonic nanohybrid system comprising NiO/Au/ReI(phen-NH2)(CO)3Cl (phen-NH2 = 1,10-Phenanthrolin-5-amine) that is unstable above 580 K; it was possible to demonstrate hot electrons are the main culprit in CO2 reduction. The engagement of hot electrons in the catalytic process is derived from many approaches that cover the processes in real-time, from ultrafast charge generation and separation to catalysis occurring on the minute scale. Unbiased in situ FTIR spectroscopy confirmed the stepwise reduction of the catalytic system. This, coupled with the low thermal stability of the ReI(phen-NH2)(CO)3Cl complex, explicitly establishes plasmonic hot carriers as the primary contributors to the process. Therefore, mediating catalytic reactions by plasmon hot carriers is feasible and holds promise for further exploration. Plasmonic nanohybrid systems can leverage plasmon's unique photophysics and capabilities because they expedite the carrier's lifetime.

Hydrogen evolution with hot electrons on a plasmonic-molecular catalyst hybrid system.

Plasmonic systems convert light into electrical charges and heat, mediating catalytic transformations. However, there is ongoing controversy regarding the involvement of hot carriers in the catalytic process. In this study, we demonstrate the direct utilisation of plasmon hot electrons in the hydrogen evolution reaction with visible light. We intentionally assemble a plasmonic nanohybrid system comprising NiO/Au/[Co(1,10-Phenanthrolin-5-amine)2(H2O)2], which is unstable at water thermolysis temperatures. This assembly limits the plasmon thermal contribution while ensuring that hot carriers are the primary contributors to the catalytic process. By combining photoelectrocatalysis with advanced in situ spectroscopies, we can substantiate a reaction mechanism in which plasmon-induced hot electrons play a crucial role. These plasmonic hot electrons are directed into phenanthroline ligands, facilitating the rapid, concerted proton-electron transfer steps essential for hydrogen generation. The catalytic response to light modulation aligns with the distinctive profile of a hot carrier-mediated process, featuring a positive, though non-essential, heat contribution.

Ultrafast Infrared-to-Visible Photon Upconversion on Plasmon/TiO2 Solid Films.

Optical upconversion via a multiphoton absorption process converts incoherent low-energy photons to shorter wavelengths. In this contribution, we report a solid-state thin film for infrared-to-visible upconversion composed of plasmonic/TiO2 interfaces. When excited at λ = 800 nm, three photons are absorbed, leading to the excitation of TiO2 trap states into an emissive state in the visible domain. The plasmonic nanoparticle enhances the light absorption capabilities of the semiconductor, increasing emission efficiency by 20 times. We demonstrate that the plasmonic nanoparticle only changes the optical absorption of the semiconductor; i.e., the process is purely photonic. The process occurs in the ultrafast domain (<10 ps), contrasting with molecular triplet-triplet exciton annihilation, the commonly used method in photon upconversion, in the nano- to microsecond time scales. The process utilizes pre-existing trap states within the semiconductor bandgap and involves three-photon absorption.

A combination therapy strategy for treating antibiotic resistant biofilm infection using a guanidinium derivative and nanoparticulate Ag(0) derived hybrid gel conjugate.

Bacteria organized in biofilms show significant tolerance to conventional antibiotics compared to their planktonic counterparts and form the basis for chronic infections. Biofilms are composites of different types of extracellular polymeric substances that help in resisting several host-defense measures, including phagocytosis. These are increasingly being recognized as a passive virulence factor that enables many infectious diseases to proliferate and an essential contributing facet to anti-microbial resistance. Thus, inhibition and dispersion of biofilms are linked to addressing the issues associated with therapeutic challenges imposed by biofilms. This report is to address this complex issue using a self-assembled guanidinium-Ag(0) nanoparticle (AD-L@Ag(0)) hybrid gel composite for executing a combination therapy strategy for six difficult to treat biofilm-forming and multidrug-resistant bacteria. Improved efficacy was achieved primarily through effective biofilm inhibition and dispersion by the cationic guanidinium ion derivative, while Ag(0) contributes to the subsequent bactericidal activity on planktonic bacteria. Minimum Inhibitory Concentration (MIC) of the AD-L@Ag(0) formulation was tested against Acinetobacter baumannii (25 µg mL-1), Pseudomonas aeruginosa (0.78 µg mL-1), Staphylococcus aureus (0.19 µg mL-1), Klebsiella pneumoniae (0.78 µg mL-1), Escherichia coli (clinical isolate (6.25 µg mL-1)), Klebsiella pneumoniae (clinical isolate (50 µg mL-1)), Shigella flexneri (clinical isolate (0.39 µg mL-1)) and Streptococcus pneumoniae (6.25 µg mL-1). Minimum bactericidal concentration, and MBIC50 and MBIC90 (Minimum Biofilm Inhibitory Concentration at 50% and 90% reduction, respectively) were evaluated for these pathogens. All these results confirmed the efficacy of the formulation AD-L@Ag(0). Minimum Biofilm Eradication Concentration (MBEC) for the respective pathogens was examined by following the exopolysaccharide quantification method to establish its potency in inhibition of biofilm formation, as well as eradication of mature biofilms. These effects were attributed to the bactericidal effect of AD-L@Ag(0) on biofilm mass-associated bacteria. The observed efficacy of this non-cytotoxic therapeutic combination (AD-L@Ag(0)) was found to be better than that reported in the existing literature for treating extremely drug-resistant bacterial strains, as well as for reducing the bacterial infection load at a surgical site in a small animal BALB/c model. Thus, AD-L@Ag(0) could be a promising candidate for anti-microbial coatings on surgical instruments, wound dressing, tissue engineering, and medical implants.

Self-assembled cationic organic nanosheets: role of positional isomers in a guanidinium-core for efficient lithium-ion conduction.

The growing energy demand with the widespread use of smart portable electronics, as well as an exponential increase in demand for smart batteries for electric vehicles, entails the development of efficient portable batteries with high energy density and safe power storage systems. Li-ion batteries arguably have superior energy density to all other traditional batteries. Developing mechanically robust solid-state electrolytes (SSEs) for lithium-ion conduction for an efficient portable energy storage unit is vital to empower this technology and overcome the safety constraints of liquid electrolytes. Herein, we report the formation of self-assembled organic nanosheets (SONs) utilizing positional isomers of small organic molecules (AM-2 and AM-3) for use as SSEs for lithium-ion conduction. Solvent-assisted exfoliation of the bulk powder yielded SONs having near-atomic thickness (∼4.5 nm) with lateral dimensions in the micrometer range. In contrast, self-assembly in the DMF/water solvent system produced a distinct flower-like morphology. Thermodynamic parameters, crystallinity, elemental composition, and nature of H-bonding for two positional isomers are established through various spectroscopic and microscopic studies. The efficiency of the lithium-ion conducting properties is correlated with factors like nanostructure morphology, ionic scaffold, and locus of the functional group responsible for forming the directional channel through H-bonding in the positional isomer. Amongst the three different morphologies studied, SONs display higher ion conductivity. In between the cationic and zwitterionic forms of the monomer, integration of the cationic scaffold in the SON framework led to higher conductivity. Amongst the two positional isomers, the meta-substituted carboxyl group forms a more rigid directional channel through H-bonding to favor ionic mobility and accounts for the highest ion conductivity of 3.42 × 10-4 S cm-1 with a lithium-ion transference number of 0.49 at room temperature. Presumably, this is the first demonstration that signifies the importance of the cationic scaffold, positional isomers, and nanostructure morphologies in improving ionic conductivity. The ion-conducting properties of such SONs having a guanidinium-core may have significance for other interdisciplinary energy-related applications.

Proton-Coupled Electron Transfer for Photoinduced Generation of Two-Electron Reduced Species of Quinone.

Purpose-built molecules that follow the fundamental process of photosynthesis have significance in developing better insight into the natural photosynthesis process. Quinones have a significant role as electron acceptors in natural photosynthesis, and their reduction is assisted through H-bond donation or protonation. The major challenge in such studies is to couple the multielectron and proton-transfer process and to achieve a reasonably stable charge-separated state for the elucidation of the mechanistic pathway. We have tried to address this issue through the design of a donor-acceptor-donor molecular triad (2RuAQ) derived from two equivalent [Ru(bpy)3]2+ derivatives and a bridging anthraquinone moiety (AQ). Photoinduced proton-coupled electron transfer (PCET) for this molecular triad was systematically investigated in the absence and presence of hexafluoroisopropanol and p-toluenesulfonic acid (PTSA) using time-resolved absorption spectroscopy in the ultrafast time domain. Results reveal the generation of a relatively long-lived charge-separated state in this multi-electron transfer reaction, and we could confirm the generation of AQ2- and RuIII as the transient intermediates. We could rationalize the mechanistic pathway and the dynamics associated with photoinduced processes and the role of H-bonding in stabilizing charge-separated states. Transient absorption spectroscopic studies reveal that the rates of intramolecular electron transfer and the mechanistic pathways associated with the PCET process are significantly different in different solvent compositions having different polarities. In acetonitrile, a concerted PCET mechanism prevails, whereas the stepwise PCET reaction process is observed in the presence of PTSA. The results of the present study represent a unique model for the mechanistic diversity of PCET reactions.

The design and development of short peptide-based novel smart materials to prevent fouling by the formation of non-toxic and biocompatible coatings.

Biofouling refers to the undesirable process that leads to the accumulation of microorganisms such as bacteria or fungi on substrates. This is one of the major concerns associated with several components of our regular life such as food, health, water and energy. In the healthcare sector, biofouling on medical devices is known to cause infections, which are often resistant to conventional antibiotics and lead to increase in the number of hospital and surgery-related deaths. One of the better ways to tackle the problem of biofouling is the development of smart antifouling materials that can produce a biocompatible, non-toxic, eco-friendly and functional coating and maintain a biological environment without any adverse effect. To this end, in the present study, we have reported the design and synthesis of two simple chemically modified peptides, namely, PA1 (PFB-VVD) and PA2 (PFB-LLE). The design as well as the amino acid sequence of the peptides contains three basic components that enable their ability to (i) self-assemble into functional coatings, (ii) bind with the desired surface via the bi-dentate coordination of dicarboxylate groups and (iii) exhibit antifouling activity and generate a non-toxic biocompatible supramolecular coating on the desired surface. PA1 having aspartic acid as the anchoring moiety exhibits better antifouling activity compared to PA2 that has glutamic acid as the anchoring moiety. This is probably due to the greater adhesive force or binding affinity of aspartic acid to the examined surface compared to that of glutamic acid, as confirmed by force measurement studies using AFM. Most importantly, the simple drop-coating method promises great advantages due to its ease of operation, which leads to a reduction in the production cost and increase in the scope of commercialization. To the best of our knowledge, this is the first attempt to develop an ultra-short peptide-based smart antifouling material with a dicarboxylate group as the surface binding moiety. Furthermore, these findings promise to provide further insights into antifouling mechanisms in the future by the development of a smart material using a dicarboxylate group as an anchoring moiety.

Crystalline Free-Standing Two-Dimensional Zwitterionic Organic Nanosheets for Efficient Conduction of Lithium Ions.

Crystalline two-dimensional organic nanosheets (2D-ONs) having atomic or near-atomic thickness with infinite lateral dimensions are of crucial significance for their possible application as a material for energy storage. The presence of nanofluidic channels with a designed array of molecular interlayers in such 2D-ONs, for a favorable lithium-ion transport, has special significance for improving the efficacy of lithium-ion batteries. However, the rational design of crystalline 2D-ONs remains a challenge because of the lack of appropriate monomers and convenient preparation methods. Herein, we report a unique lithium-ion conducting behavior of zwitterionic 2D-ONs, formed through self-assembly of a small organic molecule AM-1. Different microscopic studies confirm the near-atomic thickness (∼3.5 nm) of these 2D-ONs. Results of the single-crystal X-ray diffraction studies confirm the presence of a one-dimensional (1D) channel in crystalline 2D-ONs, which was generated during the self-assembly process of the zwitterionic monomer scaffold. The presence of immobilized ionic centers with well-defined directional channels in the 2D-ONs favors the transportation of lithium ions with a room-temperature lithium-ion conductivity of 5.14 × 10-5 S cm-1, which is rather unique for self-assembled 2D-ONs.

Two-dimensional lanthanide coordination polymer nanosheets for detection of FOX-7.

Despite the recent surge of interest in two-dimensional (2D) inorganic nanosheets derived from photoactive coordination polymers of lanthanide ions having interesting optical properties, research in this area is still in its infancy. Luminescent lanthanide ions, Eu(iii) or/and Tb(iii), as well as a bis-terpyridine ligand (L), were used in this study as the building blocks for the synthesis of the archetypical layered structure of coordination polymers (CPs) (L·Eu/L·Tb). 2D-nanosheets were obtained through exfoliation of the layered precursor of CPs in a suitable solvent system following a sonication-assisted strategy. These nanosheets exhibit lateral sizes on the micrometer scale (0.3-1 µm) and an ultrathin thickness of 2-6.5 nm. 1,1-Diamino-2,2-dinitroethene or FOX-7 is an insensitive high explosive; in a binder mixture, it exhibits a slightly superior detonation velocity of 8870 m s-1 in comparison to RDX. The insensitive nature of FOX-7 makes it a key component for the development of low vulnerable high explosive compositions for further application in weaponry. The growing demand for FOX-7, for use as a suitable replacement of conventional explosives, is of serious concern to human security. Achieving rapid and efficient detection of this unexplored explosive is a challenging task. In the present study, the developed luminescent nanosheets were used for the first time for micromolar level detection of FOX-7 both in solution and in the solid state. A visually distinct color change of the nanosheets from red (L·Eu) and green (L·Tb) to colorless was witnessed upon UV light irradiation during the detection process. Notably, the solid-state detection technique could be exploited for developing a commercial spray kit for quick onsite screening of this important explosive.

Water induced morphological transformation of a poly(aryl ether) dendron amphiphile: helical fibers to nanorods, as light-harvesting antenna systems.

Self-assembly of suitable molecular building blocks is an efficient and convenient approach to generate nanomaterials with various morphologies and functions. Moreover, understanding the nature of molecules and controlling factors of their self-assembly process is crucial in fundamental aspects of molecular self-assembly which provide insights into the design of new assemblies with functional nano-architectures. To this end, the present study reports water induced self-assembled multifaceted morphology formation and the plausible pathway of the morphology transformation of a single poly(aryl ether) dendron amphiphile 1(D). In THF, 1(D) self-assembles into helical fibers. However, with an increase in the water fraction in its THF solution, the morphology changes to nanorods through an intermediate scroll-up pathway of exfoliated fibers. The nanorod formation and transformation of 1(D) are investigated using various microscopy and spectroscopy techniques, which indicate that it has highly ordered multilayered arrays of 1(D) molecules. Finally, these multilayered arrays of 1(D) nanorods are exploited for constructing a model light-harvesting system via the incorporation of small quantities of two newly designed BODIPY based molecules as energy acceptors and 1(D) as an antenna chromophore.