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      Volume 130, Issue 10

      October 2018

    • Editorial:Special Issue on Photochemistry, Photophysics and Photobiology


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    • Table of Contents

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    • Remarkable photochemical HER activity of semiconducting 2H MoSe2 and MoS2 covalently linked to layers of 2D structures and of the stable metallic 1T phases prepared solvo- or hydro-thermally


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      MoS2 and MoSe2 in the stable semiconducting 2H form show negligible photocatalytic activity for the hydrogen evolution reaction (HER). By linking the layers of the dichalcogenide with layers of other 2D materials such as carbon-rich borocarbonitride (BC7N), one can enhance the photochemical HER activitysignificantly. Interestingly, such nanocomposites, MoSe2–MoSe2 and MoSe2–BCN show high photocatalytic activity even though the dichalcogenide itself is in the 2H form. This study shows the important role played by covalent cross-linking of layered compounds. Photocatalytic activity of covalently cross-linked layer of 2H-MoSe2 is higher than that of 2H-MoS2. Unlike the 2H forms, the metallic 1T forms of MoS2 and MoSe2 prepared by lithium intercalation followed by exfoliation, exhibit high photocatalytic HER activity. Unfortunately, materials prepared by lithium intercalation are unstable. The 1T forms of MoSe2 and MoS2 prepared by solvothermal or hydrothermal methods are, however, quite stable and exhibit good photochemical activity for HER. The 1T forms are generally superior to the covalently linked 2H forms. The present study shows how MoSe2 and MoS2 in both 2H and 1T forms can be exploited for photochemical HER activity by appropriate chemical manipulation.

    • Modification of fatty acid vesicle using an imidazolium-based surface active ionic liquid: a detailed study on its modified properties using spectroscopy and microscopy techniques


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      Fatty acid vesicles have attracted views as model protocell membranes in understanding the emergence of life, but their properties can be further modified in the presence of some external molecules. In this work, we have investigated the spontaneous formation of large unilamellar vesicles (LUVs) of oleic acidin aqueous medium in presence of a popular imidazolium-based cationic surface active ionic liquid (SAIL) [C16mim] Cl and studied the micelle–vesicle transition of aqueous [C16mim] Cl solution in presence of different molar fractions ( f ) of oleic acid. This newly formed oleic acid [C16mim] Cl vesicles exhibit some modifiedproperties compared to the pure fatty acid vesicles. Unlike pure fatty acid vesicles, these vesicles are stable in the pH range of 2 to 11.2. We have observed the fusion process of these oleic acid/SAIL vesicles to form giantunilamellar vesicles (GUVs) in presence of low concentration of NaCl solution. To investigate the dynamics of different oleic acid/[16] Cl self-assemblies, we have used fluorescence correlation spectroscopy (FCS). The translational diffusion behavior of three different dyes, Rhodamine 6G, DCM and Pyrromethene 597, which are non-covalently bound to the different regions of the oleic acid/SAIL self-assemblies, have been determined using FCS during the micelle–vesicle transition and upon varying the pH of the vesicular solution.

    • Synthesis and in vitro photobiological studies of porphyrin capped gold nanoparticles


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      We describe the synthesis and characterization of a thiol-functionalized porphyrin derivative 2 and its gold nanoparticle conjugates. The porphyrin 2 exhibited its characteristic intense Soret absorption at 420 nm with a molar extinction coefficient value of 3.6 × 10 5 M−1cm−1 and good fluorescence in the region of 650–660 nm. The porphyrin-capped gold nanoparticles (POPNPs) were synthesized from the citrate-capped gold nanoparticles by the ligand exchange method and characterized by spectroscopic and morphological analyses(UV–Vis, DLS and TEM). The broadening of the absorption spectrum and quenching of the fluorescence intensity for the porphyrin gold nanoconjugates suggest efficient incorporation of the porphyrin moiety onto the gold surface. The results of DLS and TEM analyses indicate that the nanoconjugate POPNPs are uniformly spherical in shape with a size of ca. 25±5 nm and exhibits a negative zeta potential value of −16.0±2 mV. The singlet oxygen generation efficiency of the porphyrin 2 and POPNPs was calculated and are found to be ca. 0.53 ±0.02 and 0.43 ± 0.03, respectively. The in vitro photobiological studies revealed that POPNPs exhibited enhanced photodynamic activity compared to their parent porphyrin derivative 2 with an IC50 value of 5 μM in MDA MB 231 cell lines. The mechanism of the cell destruction was studied by Annexin-FITC and confirmedthrough TMRM assay. We observed the increase in the percentage of cell population corresponding to the late apoptotic stage ca. 37.7% and 51.2% for 5 and 10 μM of POPNPs, respectively, thereby demonstrating their apoptotic-mediated cell destruction and use in PDT applications

    • Self-assembly and photoinduced electron transfer in a donor β-cyclodextrin-acceptor supramolecular system


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      Equimolar amounts of native β-cyclodextrin (β-CD), pyrene-linked adamantane (PYAD) and tert-butylpyromellitic diimide (PMDI) when dissolved in water self-assembled to form the supramolecular donor-acceptor system PYAD⊐β-CD>PMDI. The high affinity of adamantane derivatives for inclusion bindingin the β-CD cavity and the propensity of PMDI to undergo rim-binding at the narrow rim of β-CD led to the formation of PYAD⊐β-CD>PMDI. The ternary complex PYAD⊐β-CD>PMDI was thoroughly characterized using various spectroscopic techniques. β-CD performs three functions in the self-assembled complex: (1) encapsulate the adamantane unit and keep the pyrene (PY) moiety above the secondary rim, (2) rim-bind PMDI and keep it at the primary rim, and (3) act as a spacer between pyrene and PMDI. Thus, the ternary complex canfunction as a donor-spacer-acceptor system capable of undergoing photoinduced electron transfer (PET). Upon excitation of the pyrene moiety in PYAD⊐β-CD>PMDI an electron is transferred from the excited pyrene tothe PMDI ground state. Steady state and time resolved fluorescence experiments were carried out to study the PET in PYAD⊐β-CD>PMDI. Existence of the ternary system and PET processes taking place within it are further supported by laser flash photolysis experiments.

    • Rational synthesis of a polymerizable fullerene–aniline derivative: study of photophysical, morphological and photovoltaic properties


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      This paper describes the synthesis of a polymerizable, aniline appended fullerene derivative, 3-aminobenzyl-phenyl-C61-butyrate (PCBAn) and its corresponding polymer (P-PCBAn), and detailed photophysical and morphological analysis towards application as an acceptor in polymer solar cells (BHJ-PSCs). The poly-3-aminobenzyl-phenyl-C61-butyrate (P-PCBAn), having a substituted polyaniline (PANI) skeletal structure, was synthesized via FeCl3 oxidative polymerisation of PCBA in its non-conducting leucoemaraldinestate. HOMO and LUMO energies estimated using optical and electrochemical techniques revealed upshifted LUMO levels for PCBAn (−3.68 eV, ∆E=0.1 eV) and P-PCBAn (−3.66 eV, ∆E=0.12 eV) compared to the parental fullerene derivative, PCBM (−3.78 eV). The morphologies of PCBAn and P-PCBAn individually and in polymer blends with P3HT were investigated using AFM and TEM analysis, which showed nanoflake-like aggregates for P3HT/PCBAn and a favourable interconnected nanonetwork structure for P3HT/P-PCBAn.The wide angle X-ray scattering (WAXS) studies of PCBAn films drop-cast from THF/water (3:7) mixture and P-PCBAn films drop-cast from 1,2-dichlorobenzene exhibited plane reflections of lamellar mesophases with dspacing of 3.4 nm and 3 nm for PCBAn and P-PCBAn, respectively. The fluorescence quenching experiments with P3HT indicated efficient electron transfer from P3HT to P-PCBAn when compared to PCBAn. The fabrication of an inverted BHJ-PSC device using PCBAn and P-PCBAn as an acceptor in combination withP3HT showed PCE of 0.9% and 1.1%, respectively, showing considerable enhancement in the case of the polymeric acceptor. The polymeric acceptor and the rational design strategy used here could open up new opportunities in the PSC device fabrication.

    • Charge carrier dynamics in CdTe/ZnTe core/shell nanocrystals for photovoltaic applications


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      CdTe/ZnTe type-II core/shell nanocrystals were synthesized and characterized by the red-shift in the UV-Vis absorption and emission spectra along with the increase in both emission quantum yield and lifetime. The charge carrier dynamics was investigated through ultrafast transient absorption spectroscopy revealing the excited state carrier distribution and the dynamics through which the carriers decay. Upon laser pulse excitation the transient absorption spectrum was characterized by a broad ground state bleach signal in the core/shell nanocrystals in accord with the absorption spectra. Slower electron cooling was found in the core/shell nanocrystals compared to the CdTe core due to the type-II band-alignment that decouples the electron from hole preventing Auger-assisted electron cooling process. The recombination was found to be much slower in thecore/shell nanocrystals due to the better surface passivation provided by the ZnTe shell eliminating the surface trapping process. The low band-gap CdTe based nanocrystals with a charge separated state are a viable candidatefor photovoltaic applications and finally, we have investigated the potentiality of the synthesized nanocrystals as sensitizers in quantum dot solar cells.

    • Viable access to the triplet excited state in peryleneimide based palladium complex


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      Triplet excited state in organic chromophores is ubiquitously significant owing to its utility in light harvesting and photovoltaic device applications. Herein, we report the enhancement in the triplet character of an innately triplet deficient peryleneimide chromophore via incorporation of a heavy atom. Palladium incorporated perylenemonoimide (PMI-Pd) was synthesized via oxidative addition of PMI-Br into Pd(0) under inert experimental conditions. The structural sanctity of the PMI-Pd and the model derivative PMI was characterized via single crystal X-ray diffraction and the close-packing was examined employing Hirshfeld surface analysis. The steady-state spectroscopic measurements of PMI-Pd in chloroform reveal an apparent perturbation in the UV-Vis absorption, fluorescence emission and lifetime characteristics. A much higher perturbation is observed inthe ultrafast photoexcited processes of PMI-Pd in chloroform as envisaged via nanosecond transient absorption (nTA) measurements. The nTA measurements of PMI-Pd in chloroform reveal a significant enhancement in the triplet character of PMI-Pd as compared to the model derivative PMI. Spin-orbit coupling (SOC) mediated triplet enhancement in PMI-Pd suggest heavy atom incorporation as a viable route for accessing the triplet excited states in triplet deficient aromatic chromophores. SOC mediated triplet enhancement in innately triplet deficient organic chromophores can revive the utility of these materials for novel photovoltaic and energy storage applications.

    • Probing the bilayer-monolayer switching of capping agents on Au nanorods and its interaction with guest molecules


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      The anisotropic features of Au nanorods make them an attractive nanoscale precursor for the design of higher order nanostructured materials. However, the mode of interaction of various molecular systems on Au nanorods is not well-understood. In the present study, we have employed isothermal titration calorimetry and surface-enhanced Raman scattering for understanding various types of interactions of functional molecules on the surface of gold nanorods. The binding of thiol-bearing analyte molecules is effective with the surface of goldnanorods in acetonitrile-rich solvents and found to be weak in an aqueous medium. The effective interaction of thiol-bearing analyte molecules on nanorods is facilitated by the breakdown of cetyltrimethylammonium bromide bilayer to a monolayer in organic-rich solvent systems, thereby resulting in appreciable signals in isothermal titration calorimetry and surface-enhanced Raman spectra. The electrostatic interaction of analyte molecule is mainly driven by the charge reversal on the surface of Au nanorods on switching the solvent from aqueous to organic medium. Thus, based on isothermal titration calorimetry and surface-enhanced Raman scattering investigations, it is established that the microheterogeneous environment around the Au nanorods plays a crucial role in driving the interaction of analyte molecules.

    • In search of stable visible light absorbing photocatalysts: gold nanoclusters


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      Photochemical reactions are beginning to play an important role in the synthesis of complex organic molecules. The impetus to employ light initiated photoreactions as a synthetic tool derives from the fact that these reactions require no reagents except light. The abundance of sunlight has prompted the chemists to search for the visible light absorbing sensitizers (catalysts) to initiate the desired reactions. Our goal in this contextis to develop stable and readily available catalysts that would function under sunlight. In this manuscript, wepresent the results of our experiments with gold nanoclusters (AuNCs) as a visible light absorbing catalysts. AuNCs absorb and emit in the visible region, soluble in water and transfers electron to suitable acceptors. Employing a series of acceptors we found that excited AuNC can transfer one electron to any acceptor whose reduction potential is above −1.1 eV. In the excited state, AuNC does not accept electrons. Also, it did not serve as an energy transfer sensitizer even with molecular oxygen. We are optimistic that AuNP and AuNC could be developed into a stable and water-soluble visible light absorbing photocatalysts to perform useful photoreactions.

    • Influence of silyloxy substitution on the photochromic properties of diarylbenzo- and naphthopyrans


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      Diarylbenzopyrans and their annulated derivatives are a class of photochromic compounds, which have been extensively investigated for photochromism. In fact, some naphthopyrans are employed industrially in ophthalmic lens applications. Photoirradiation of 2,2-diphenylbenzopyran leads to the formation of coloredo-quinonoid intermediates. The latter revert too rapidly to allow their observation only at low temperatures (173–263 K). Annulation and other strategies are exploited to observe the colored o-quinonoid intermediates that persist for a few minutes at room temperature. We have examined photochromism of a set of silyloxysubstituted 2,2-diphenylbenzo- and naphthopyrans to explore how the mesomeric effects transmitted by the strongly e-donating silyloxy group substituted in the ring and at phenyl groups manifest in the spectrokineticproperties of their photogenerated colored o-quinonoid intermediates. We show that silyloxy substitution in the benzopyran ring leads to remarkable stabilization of the colored intermediates to enable their persistence at 283 K for a few minutes. In contrast, similar substitution in the C2-phenyl rings destabilizes the colored intermediates.

    • Self-assembly of tetraphenylethylene-based dendron into blue fluorescent nanoparticles with aggregation induced enhanced emission


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      Luminescent organic nanoassemblies have received great attention in recent years due to their potential applications in material science and bioimaging. Since most of the fluorophores undergo aggregationcaused quenching in the solid state, their technological applications are limited. Hence, there is a high demandfor the design of fluorophores which show enhanced emission in the aggregated and solid states. Herein we report the design and synthesis of a tetraphenylethylene based dendron through multistep organic reactions.Very weak emission is observed for the dendron in the molecularly dissolved state, which is attributed to the rotational relaxation of intramolecular C(sp2)–(sp2) bond rotation in the excited state. The aggregates of thedendron are prepared with the addition of a ‘poor’ solvent into a solution of the dendron in a ‘good’ solvent. A huge enhancement in fluorescence is observed in the aggregated state, which is attributed to the restriction of intramolecular C(sp2)–C(sp2) bond rotation and makes the radiative decay as the main decay channel for the aggregated state. The induced self-assembly of the dendron with the addition of ‘poor’ solvent results in the formation of emissive nanoparticles. We hope the blue emissive nanoparticles may find applications in materialscience and bioimaging.

    • Fe(III)-grafted K-doped g-C3N4/rGO composite photocatalyst with efficient activity towards the degradation of organic pollutants


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      The quick recombination of charge carriers and the low oxidizing ability of VB holes of g-C3N4 limits its practical application. Herein, we have synthesized a visible-light sensitive Fe(III)-grafted K-doped g-C3N4/rGO heterostructure photocatalyst to tackle the above problem so as to achieve enhanced photocatalytic activity. The prepared samples were characterized by standard analytical techniques such as XRD, FT-IR, EPR, SEM, TEM, DRS and PL spectroscopy. EPR analysis reveals that the grafted iron exists in +3 oxidation state in the composite material. As compared to pristine g-C3N4, surface grafted nanocomposite photocatalyst exhibits excellent photocatalytic performance for the degradation of Rhodamine B (RhB), Methylene Blue (MB) and Methyl Orange (MO) under visible light irradiation. The enhanced activity could be attributed to the creation of defect levels in the band gap, shift of absorption band towards the visible region and intimate interfacial interactions between graphene and g-C3N4, effectively promoting separation of photoinduced charge carriers. The idea to tune the bandgap of graphitic carbon nitride by introducing alkali metal along with its composite formation with graphene could present a new concept to effectively steer the photocatalytic activity.

    • Glutathione-capped gold nanoclusters: photoinduced energy transfer and singlet oxygen generation


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      Glutathione-capped gold clusters prepared in an aqueous medium are known to exhibit excellent photosensitizing properties. We have now successfully transferred these gold clusters in an organic medium while retaining all the characteristic excited state properties. These gold clusters can be further modified with organic ligands such as 2-Phenylethanethiol (PET). The gold clusters in organic medium exhibit enhanced emission yield (Φf=0.15) compared to that in an aqueous medium (Φf=0.08). The excited state lifetimesof 3.7 μs (untreated) and 1.5 μs (PET treated) in toluene are also greater than the lifetime observed in aqueous solution (0.77 μs). By employing laser flash photolysis we are able to induce triplet energy transfer to β-carotene and oxygen. A singlet oxygen generation with the efficiency of 13% was observed in these experiments. The excited state properties of glutathione-capped gold clusters further shows its importance as a photosensitizer in light energy conversion and biomedical applications.

    • Spectrally resolved photon-echo spectroscopy of CdSe quantum dots at far from resonance excitation condition


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      Spectrally resolved photon echo spectroscopy in the off-resonance condition is reported for the first time to study the coherence and population dynamics of CdSe quantum dots. In this case, the information related to the system dynamics can be inferred indirectly. This is especially useful when such dynamical information might be hidden under the absorption maxima of the sample. We observe that a substantial intensity of the photon echo signal was obtained in two different CdSe quantum dot samples (CdSe 610 and CdSe 640), whichhave absorption maxima at 620 nm and 590 nm, respectively. Due to the difference in sizes of these two quantum dots, a small change is observed in the molecular dynamics of these two quantum-dot samples. Specifically, thespectral diffusion of CdSe 640 occurs within the first 50 fs, whereas that for CdSe 610 occurs at about 100 fs timescale. The integrated plots of the photon echo signal, as a function of population time, result in two decay constants. The faster among the two decay components is pulse width limited and is in between 30 and 40 fs at different fixed coherence times for both samples. The slower decay component for the CdSe 610 sample is found to be in the range of 75–85 fs, while that for CdSe 640, it is between 82 and 92 fs at different fixed coherencetimes.

    • Stimuli-responsive, protein hydrogels for potential applications in enzymology and drug delivery


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      Enhancing the stability of enzymes for sensing or biocatalysis applications is still an unmet challenge. Ordinary paper is a very attractive support for anchoring enzymes but enzyme attachment to cellulose without surface activation is still another challenge. To make progress toward these goals, we developed a simple method to prepare highly active and stable enzyme-hydrogels within the mesh of the cellulose fibers of paper. A mixture of the desired enzyme, bovine serum albumin (BSA) and arginine were reacted with carbodiimide to formstable hydrogels. A set of critical concentrations (BSA([BSA]0)≥1 mM),[carbodiimide] 0≥100 mM and [amino acid]≥100 mM) were required to form transparent hydrogels. The thermal reversibility of gelation proved that the gels are stabilized by non-covalent bonding interactions between the BSA oligomers that were formed via covalent interactions. Both dynamic light scattering and SDS-PAGE studies, under pre-gelation conditions, support idea that one BSA oligomeric unit contained 40–70 protein molecules. Scanning electron micrographs, thermogravimetry and swelling studies suggest that the formation of water cavities inside the cross-linked gel matrix, where the water mass was 7–8 times higher than that of the protein and the free amino acid used as a linker/spacer. Due to the higher water content and benign gelation conditions, active enzymes could be incorporated into the gel structure during the synthesis. Hydrogels, thus, embedded with glucose oxidase (GOx) and horseradish peroxide (HRP) showed catalytic activity towards glucose, where efficientchanneling of hydrogen peroxide from GOx to HRP was observed (70% efficiency in initial rate compared to free enzymes in solution). Moreover, the enzymes retained their activity after pasting the hydrogel onto ordinary paper, which was demonstrated as a glucose sensing platform with a detection limit of 5 mM glucose. Trypsin embedded in the gel showed temperature dependent self-degradation by utilizing optimum protease activity at 37 C. The temperature-triggered degradation of the gel can be used as a drug delivery vehicle, which was demonstrated using a reporter dye. The hydrogel made of a completely proteinaceous material that releases drugs at body temperature but bound to the matrix at room temperature (25C) is useful for noninvasive drug delivery platforms. The biocompatibility and non-thermal synthetic route for the hydrogel makes it a superior material for incorporation of temperature sensitive enzymes, drug molecules or nucleic acids, for a diverse set of applications.

    • Photoinduced electron transfer processes of (E)-9-(4-nitrostyryl)anthracene in non-polar solvent medium: generation of long-lived charge-separated states


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      In the present study, photoinduced electron transfer (PET) dynamics between N,N-diethylaniline (DEA) and (E)-9-(4-nitrostyryl)anthracene (An-NO2) in a non-polar solvent medium {methylcyclohexane (MCH)}, has been investigated. The rate constant of back electron transfer (kKBT) for the An-NO2 – DEA pair was 3.8×105s -1 which is ca. 2 orders of magnitude less compared to the anthracene (An)-DEA (control) system. The results indicate that long-lived charge separated species can be generated using the design strategy used herein by achieving resonance stabilization of the excited state (acceptor) radical via conjugation.

    • Atomically precise cluster-based white light emitters


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      Materials emitting white luminescence are receiving increasing attention due to their potential applications in electroluminescent devices, information displays and fluorescent sensors. To produce white light, one must have either three primary colors, blue, green and red or two colors, blue and orange.In this paper, we have used thiol/phosphine protected red luminescent silver nanoclusters (Ag NCs), [Ag29(BDT)12(PPh3)4]3- (BDT=1,3-benzenedithiol),[AuxAg29-x-(BDT)12(PPh3)4]3-and Ag29(LA)12 (LA=lipoic acid) as one of the fluorophores for white light emission. These clusters are mixed with blue luminescent silicon nanoparticles (Si NPs) and green luminescent fluorescein isothiocyanate (FITC). The mixtures show white luminescence with CIE coordinates of (0.31, 0.34), (0.33, 0.35) and (0.29, 0.31) which are in good agreement with pure white light (0.33, 0.33). The other clusters with yellow, blue, orange, etc.,luminescence can also be used to make white light. This work provides a prospective pathway for white light emission based on atomically precise noble metal NCs.

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