White Light Emission from Zn(II) and DMSO-Induced Copper Nanocluster Assembly.

An assembly of metal nanoclusters driven by appropriate surface ligands and solvent environment may engender entirely new photoluminescence (PL). Herein, we first synthesize histidine (His) stabilized copper nanoparticles (CuNPs) and, subsequently, copper nanoclusters (CuNCs) from it using 3-mercaptopropionic acid (MPA) as an etchant. The CuNCs originally emit bluish-green (λem=470 nm) PL with a low quantum yield (QY∼1.8%). However, it transformed into a dual-emissive nanocluster assembly (Zn-CuNCs) in the presence of Zn(II) salt, having a distinct blue emission band (λem = 420 nm) and a red emission band (λem = 615 nm) with eight times QY (∼9.1%) enhancement. Adding dimethyl sulfoxide (DMSO) further modifies the emission intensities; the red band was amplified four times, while the blue band was diminished by 2.5 times. The transmission electron microscopy (TEM) images unveiled that the Zn-CuNCs are a large assembly of tiny nanoclusters, which become more compact in DMSO. The blue emission possesses steady-state fluorescence anisotropy, while the red emission shows no anisotropy. Further, near-perfect white light emission(WLE) was rendered with CIE coordinates of (0.33, 0.32) by combining the dual emission of the Zn-CuNCs with the original green emission of the CuNCs.

Reversible CsPbBr3 ↔ CsPb2Br5 Transformation via Reverse Micellar Aqueous Solution.

Lead halide perovskites suffer from water and moisture instability due to the highly ionic nature of the crystal structures, though a few groups took advantage of it for chemical transformation via water-assisted strategy. However, direct exposure of the perovskite to bulk water leads to uncontrolled chemical transformation. Here, we report a controlled chemical transformation of CsPbBr3 to CsPb2Br5 triggered by nanoconfined water by placing CsPbBr3 in the nonpolar phase within a reverse micelle. The chemical transformation reaction is probed by using steady-state and time-resolved optical spectroscopy. We observe absorption and photoluminescence in the UV region stemming clearly from the CsPb2Br5 phase upon interaction with the reverse micellar aqueous solution. Transmission electron microscopy and X-ray diffraction measurements further provided the structure and morphology. Our results direct the formation of CsPbBr3-CsPb2Br5 nanocomposite under dry conditions while the chemically transformed CsPb2Br5 phase exists only in moist conditions, which we explain via the CsBr-stripping mechanism.

Effect of salt addition on a triblock copolymer-zwitterionic surfactant assembly: insight from excited-state proton transfer.

Copolymer-surfactant assemblies are frequently utilized across various fields, from medicine to nanotechnology. Understanding the organization of the mixed assemblies in a saline environment will further expand their application horizons, especially under physiological conditions. Excited-state proton transfer (ESPT) can provide insight into the hydration nature and organization of the non-toxic assembly of a triblock copolymer F127 (poly-(ethylene oxide)101 (PEO101)-poly(propylene oxide)56 (PPO56)-PEO101)) and a zwitterionic sulfobetaine surfactant N-dodecyl-N,N-dimethyl-3-ammoniopropane sulfonate (SB12). Here, we present a comprehensive investigation of the compactness and hydration nature of the F127-SB12 mixed assemblies at different salt concentrations using the ESPT of 8-hydroxy pyrene-1,3,6-trisulfonate (HPTS). In the absence of salts, gradual SB12 addition to a premicellar (0.4 mM) or a post-micellar (4 mM) F127 solution leads to an anomalous modulation of the protonated and deprotonated emission bands. The emission intensity ratio (protonated/deprotonated) first increases to a maximum at a particular SB12 concentration (6 mM and 35 mM for the premicellar and post-micellar F127 assemblies, respectively), and then the ratio decreases with a further increase in the surfactant concentration. Since the intensity ratio is an indicator of the retardation of the ESPT process, the mixed micellar configuration displaying a maximum intensity ratio represents the most compact and least hydrated state. Salt addition to this configuration lowers the intensity ratio, signifying an enhanced ESPT process. Dynamic light scattering (DLS) results indicate that the size of the mixed assembly remains almost unaltered with the addition of salts. Thus, salinity enhances the ESPT process inside the F127-SB12 mixed assemblies without significantly altering the hydrodynamic radius.

Exploring cationic polyelectrolyte-micelle interaction via excited-state proton transfer. Signatures of probe transfer.

Excited-state proton transfer (ESPT) is a sensitive tool for the delicate monitoring of structural reorganization, hydration level, and confinement within surfactant and polymer assemblies. Here, we investigate the interaction of a cationic polyelectrolyte, poly(diallyl dimethylammonium chloride) (PDADMAC), with micelles of differently charged surfactants using 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) as an ESPT probe. We used three surfactants: anionic sodium dodecyl sulfate (SDS), cationic dodecyl trimethylammonium bromide (DTAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12), possessing the same alkyl (dodecyl) chain but varying headgroup charges. The fluorescence of HPTS residing initially within the micellar medium modulates differently in the presence of PDADMAC. For the anionic SDS and cationic DTAB micelles, the emission spectrum of HPTS does not alter significantly; however, for SB12 micelles, the emission spectrum undergoes a strong modulation upon adding the polyelectrolyte. The emission intensities quench strongly at a low concentration of PDADMAC but recover at a higher concentration. The emission intensity ratio of the two emission bands also changes significantly, implying strong modulation of the ESPT process with varying PDADMAC concentrations. The time-resolved area normalized emission spectra (TRANES) disclose single isoemissive points in the SB12 micelle at low and high concentrations of PDADMAC but two different isoemissive points (one characteristic of the SB12 micelle at 500 nm and another characteristic of the PDADMAC interface at 480 nm) in the mixed assembly at an intermediate concentration. Detailed analysis suggests that the polyelectrolyte can enforce the transfer of the anionic probe HPTS from the zwitterionic micelle to the PDADMAC assembly above a specific PDADMAC concentration. The transfer of the molecular probe between two assemblies resembles a drug sequestration event, and the study reveals necessary emission signatures.

Hit Multiple Targets with One Arrow: Pb2+ and ClO- Detection by Edge Functionalized Graphene Quantum Dots and Their Applications in Living Cells.

Recently, multimodal detection of analytes through a single nanoprobe has become an eminent approach for researchers. Herein a fluorescent nanoprobe, functionalized-GQD (F-GQD), has been designed through edge functionalization of graphene quantum dots (GQDs) by 2,6-diaminopyridine molecules. The fluorescence of F-GQD is quite sensitive to medium pH, making it a suitable pH sensor within the pH range 2-6. Interestingly, F-GQD shows dual sensing of Pb2+ and ClO- by entirely different pathways; Pb2+ exhibits fluorescence turn-on performance while ClO- triggers turn-off fluorescence quenching. The fluorescence enhancement may originate from the Pb2+-induced aggregation of the nanodots. The limit of detection (LOD) was also impressive, 1.2 µM and 12.6 nM for Pb2+ and ClO-, respectively. The detailed mechanistic investigations reveal that both dynamic and static quenching effects operate together in the F-GQD-ClO- system. The dynamic quenching was attributed to the energy migration from F-GQD to ClO- through hydrogen bonding interaction (static quenching) between the amine group at the F-GQD surface and ClO-. The F-GQD nanodot reveals excellent sensitivity toward the detection of ClO- in real samples. Moreover, the F-GQDs also serve as multicolor fluorescent probes for cell imaging; the probe can easily penetrate the cell membrane and successfully detect intracellular ClO-.

Multifunctional N-Doped Carbon Dots for Bimodal Detection of Bilirubin and Vitamin B12, Living Cell Imaging, and Fluorescent Ink.

A N-doped carbon dot (NCD) has been synthesized via a simplistic one-step hydrothermal technique using l-aspartic acid and 3,6-diaminoacridine hydrochloride. The NCDs exhibit a high quantum yield (22.7%) and excellent optical stability in aqueous media. Additionally, NCDs display good solid-state yellowish-green emission and are suitable for security ink applications. The remarkable fluorescence (FL) properties of NCDs are further applied to develop a multifunctional sensor for bilirubin (BR) and vitamin B12 (VB12) via fluorescence quenching. We have systematically studied the FL quenching mechanisms of the two analytes. The primary quenching mechanism of BR is via the Förster resonant energy transfer (FRET) pathway facilitated by the H-bonding network between the hydrophilic moieties existing at the surface of BR and NCDs. In contrast, the inner filter effect (IFE) is mainly responsible for the recognition of VB12. The practicability of the nanoprobe NCDs is further tested in real-sample analysis for BR (human serum and urine samples) and VB12 (VB12 tablets, human serum, and energy drink) with a satisfactory outcome. The in vitro competency is also verified in the human cervical cancer cell line (HeLa cell) with negligible cytotoxicity and significant biocompatibility. This result facilitates the application of NCDs for bioimaging and recognition of VB12 in a living organism.

Effect of Photoacid Strength on Fluorescence Modulation of 2-Naphthol Derivatives inside ß-Cyclodextrin Cavity: Insights from Fluorescence, Isothermal Calorimetry, and Molecular Dynamics Simulations.

Fluorescence response of a photoacid inside a confined environment often differs markedly from the bulk response. Is there any correlation between the extent of fluorescence modulation and the strength of the photoacid? Here, we used three photoacids: 2-naphthol (2OH, pKa* = 3.3), 6-sulfonate-2-naphthol (6SO3-2OH, pKa* = 3.06), and 6-cyano-2-naphthol (6CN-2OH, pKa* = 0.6) with remarkably different excited-state acidities to investigate fluorescence modulation inside the nanocavity of ß-cyclodextrin (ß-CD). Interestingly, we found strong fluorescence modulation for 2OH and 6SO3-2OH but almost none for 6CN-2OH. Isothermal calorimetry measurements showed that all three fluorophores form 1:1 inclusion complex with comparable binding constants (285, 420, and 580 M-1 for 2OH, 6SO3-2OH, and 6CN-2OH, respectively). Molecular dynamics simulation further revealed that binding modes are quite similar, and the distribution of water molecules around the proton-donating hydroxyl group of the photoacids are also comparable. Consequently, the difference in the fluorescence response should be accounted solely to the difference in the photoacidity strengths.

Anomalous Variation of Excited-State Proton Transfer Dynamics inside a Triblock Copolymer-Cationic Surfactant Mixed Micelle.

Mixed micelles formed by triblock copolymers [poly(ethylene oxide)m (EOm)-poly(propylene oxide)n (POn)-EOm] with various surfactants have widespread applications. Molecular-level understanding of the composition, interfacial organization, and hydration of the copolymer-surfactant mixed micelle is greatly necessary from application perspectives. Here, we applied 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) to probe the mixed micelle of a triblock copolymer F127 (EO101-PO56-EO101) and a cationic surfactant dodecyltrimethylammonium bromide (DTAB) at various compositions. The emission spectrum of HPTS modulates anomalously with the variation of DTAB concentration, displaying at least four regimes. The ratio of the emission intensities of the two bands (protonated/deprotonated) (1) first increases steeply in the low-concentration range (0.1-6 mM), (2) remains almost steady at the intermediate concentration (8-20 mM), (3) decreases at high concentration (20-80 mM), and (4) finally, remains almost constant at a very high concentration (100-400 mM) of DTAB. Time-resolved measurements confirm that excited-state proton transfer dynamics varies unusually with the concentration of DTAB in the mixed micelle; substantial retardation is observed up to ∼12 mM, but after that, the dynamics becomes somewhat faster upon further addition. The rotational dynamics of a methoxy analogue of HPTS, 8-methoxypyrene-1,3,6-trisulfonate, becomes slower up to ∼12 mM DTAB and after that becomes faster at higher concentration. Moreover, dynamic light scattering measurements showed that the size of the mixed micelle decreases sharply in the low-concentration region (<20 mM DTAB) but decreases moderately at high concentration. Thus, the nature of the mixed micelle is very different at low and high concentrations of DTAB. At low concentration, the incorporation of DTAB results in a more compact and less hydrated mixed micelle, whereas a more hydrated and less organized assembly is formed at high concentration of DTAB.

Photo-induced Electron Transfer or Proton-Coupled Electron Transfer in Methylbipyridine/Phenol Complexes: A Time-Dependent Density Functional Theory Investigation.

It is often difficult to assign the nature of an excited-state process unambiguously based on a limited number of experimental evidence. The methylbipyridine/phenol complex is a classic example, where experimental observations support a proton-coupled electron transfer (PCET) or a photo-induced electron transfer (PET) process. Here, we implemented time-dependent density functional theory calculation to elucidate the nature of the process. We found that PCET is possible only when mediated by a H-bond between methylbipyridine and phenol. However, a conventional PET can occur through π-π stacking interaction between the donor and the acceptor. Thus, the photophysical process in the complex is indeed governed by competition of H-bonding versus π-π interaction. Our calculations including the solvent model based on density (SMD) suggest that π-π stacking is more favorable than H-bonding, and hence, conventional PET is a more favorable excited-state process for the methylbipyridine/methoxyphenol complex than PCET.

Coumarin-Annelated Regioisomeric Heptahelicenes: Influence of Helicity on Excited-State Properties and Chiroptical Properties.

Two regioisomeric pairs of heptahelical mono- and biscoumarins that are differentiated by "inward" and "outward" disposition of the pyran-2-one moiety have been synthesized and investigated to understand the influence of helicity on excited-state and chiroptical properties. A slight variation in the helicities is found to manifest in contrasting excited-state properties of coumarin-annelated heptahelicenes; in addition to the intramolecular charge transfer, structural relaxation in the excited state is shown from theoretical calculations to cause decrease in the fluorescence quantum yield for a system with higher helicity. The optically pure enantiomers of heptahelical coumarins exhibit helicity-dependent chiroptical properties, namely, specific rotations, molar ellipticities, Cotton effects, and anisotropic dissymmetry factors. Theoretical calculations point to factors that are not readily explicable.

Spectroscopic Studies of Asparaginyl-tRNA Synthetase from Entamoeba histolytica.

BACKGROUND: Aminoacyl-tRNA synthetases play an important role in catalyzing the first step in protein synthesis by attaching the appropriate amino acid to its cognate tRNA which then transported to the growing polypeptide chain. Asparaginyl-tRNA Synthetase (AsnRS) from Brugia malayi, Leishmania major, Thermus thermophilus, Trypanosoma brucei have been shown to play an important role in survival and pathogenesis. Entamoeba histolytica (Ehis) is an anaerobic eukaryotic pathogen that infects the large intestines of humans. It is a major cause of dysentery and has the potential to cause life-threatening abscesses in the liver and other organs making it the second leading cause of parasitic death after malaria. Ehis-AsnRS has not been studied in detail, except the crystal structure determined at 3 Å resolution showing that it is primarily α-helical and dimeric. It is a homodimer, with each 52 kDa monomer consisting of 451 amino acids. It has a relatively short N-terminal as compared to its human and yeast counterparts. OBJECTIVE: Our study focusses to understand certain structural characteristics of Ehis-AsnRS using biophysical tools to decipher the thermodynamics of unfolding and its binding properties. METHODS: Ehis-AsnRS was cloned and expressed in E. coli BL21DE3 cells. Protein purification was performed using Ni-NTA affinity chromatography, following which the protein was used for biophysical studies. Various techniques such as steady-state fluorescence, quenching, circular dichroism, differential scanning fluorimetry, isothermal calorimetry and fluorescence lifetime studies were employed for the conformational characterization of Ehis-AsnRS. Protein concentration for far-UV and near-UV circular dichroism experiments was 8 µM and 20 µM respectively, while 4 µM protein was used for the rest of the experiments. RESULTS: The present study revealed that Ehis-AsnRS undergoes unfolding when subjected to increasing concentration of GdnHCl and the process is reversible. With increasing temperature, it retains its structural compactness up to 45ºC before it unfolds. Steady-state fluorescence, circular dichroism and hydrophobic dye binding experiments cumulatively suggest that Ehis-AsnRS undergoes a two-state transition during unfolding. Shifting of the transition mid-point with increasing protein concentration further illustrate that dissociation and unfolding processes are coupled indicating the absence of any detectable folded monomer. CONCLUSION: This article indicates that GdnHCl induced denaturation of Ehis-AsnRS is a two - state process and does not involve any intermediate; unfolding occurs directly from native dimer to unfolded monomer. The solvent exposure of the tryptophan residues is biphasic, indicating selective quenching. Ehis-AsnRS also exhibits a structural as well as functional stability over a wide range of pH.

Protein-activated transformation of silver nanoparticles into blue and red-emitting nanoclusters.

Proteins are very effective capping agents to synthesize biocompatible metal nanomaterials in situ. Reduction of metal salts in the presence of a protein generates very different types of nanomaterials (nanoparticles or nanoclusters) at different pH. Can a simple pH jump trigger a transformation between the nanomaterials? This has been realized through the conversion of silver nanoparticles (AgNPs) into highly fluorescent silver nanoclusters (AgNCs) via a pH-induced activation with bovine serum albumin (BSA) capping. The BSA-capped AgNPs, stable at neutral pH, undergo rapid dissolution upon a pH jump to 11.5, followed by the generation of blue-emitting Ag8NCs under prolonged incubation (∼9 days). The AgNPs can be transformed quickly (within 1 hour) into red-emitting Ag13NCs by adding sodium borohydride during the dissolution period. The BSA-capping exerts both oxidizing and reducing properties in the basic solution; it first oxidizes AgNPs into Ag+ and then reduces the Ag+ ions into AgNCs.

Anomalous Spectral Modulation of 4-Aminophthalimide inside Acetonitrile/AOT/ n-Heptane Microemulsion: New Insights on Reverse Micelle to Bicontinuous Microemulsion Transition.

The behavior of acetonitrile/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/ n-heptane microemulsion, whether it remains as reverse micelle (RM) or bicontinuous microemulsion (BMC), has been controversial and even termed as a "problem system". Herein, we investigate the microemulsion using spectral and dynamical responses of a hydrophilic solvatochromic fluorophore 4-aminophthalimide (4-AP) at different ws values (=[acetonitrile]/[AOT]). Interestingly, we found that emission parameters of 4-AP within the microemulsion vary differently at low and high ws regimes. The quantum yield (ϕf) and lifetime (τf) of 4-AP first increase up to ws = ∼1 and, thereafter, decrease upon a further increase in the ws values. The emission maximum of 4-AP significantly shifts to a higher wavelength from 445 nm at ws = 0 to 475 nm at ws = 8. Interestingly, unlike aqueous RMs, the emission maximum at ws = 1 matches with the emission maximum in neat acetonitrile and the emission maximum shifts to even longer wavelength at a higher ws. Steady-state anisotropy also shows a break around ws = 1; anisotropy decreases very sharply from ws = 0 to 1 and, thereafter, remains nearly constant. Solvation dynamics becomes progressively faster with an increase in the acetonitrile content only in the low ws regimes but remains almost independent of ws after ws > 2. All of the results collectively indicate that the morphology of the microemulsion may change at an intermediate ws (∼1); below this, the system behaves like reverse micelles, and above this, the system may remain as BMC. The conjecture was further supported by dynamic light scattering measurements, where we observed a gradual increment of the average size at the low acetonitrile content (up to ws = 1) but, thereafter, the size distribution becomes multimodal and sizes cannot be estimated correctly.

A Ratio-Analysis Method for the Dynamics of Excited State Proton Transfer: Pyranine in Water and Micelles.

The emission spectrum of a fluorophore undergoing excited state proton transfer (ESPT) often exhibits two distinct bands each representing emissions from protonated and deprotonated forms. The relative contribution of the two bands, best represented by an emission intensity ratio ( R) (intensity maximum of the protonated band/intensity maximum of the deprotonated band), is an important parameter which usually denotes feasibility or promptness of the ESPT process. However, the use of a ratio is only limited to the interpretation of steady-state fluorescence spectra. Here, for the first time, we exploit the time dependence of the ratio ( R( t)), calculated from time-resolved emission spectra (TRES) at different times, to analyze ESPT dynamics. TRES at different times were fitted with a sum of two log-normal functions representing each peak, and then, the peak intensity ratio, R( t), was calculated and further fitted with an analytical function. Recently, a time-resolved area-normalized emission spectra (TRANES)-based analysis was presented where the decay of protonated emission or the rise of deprotonated emission intensity conveniently accounts for the ESPT dynamics. We show that these two methods are equivalent but the new method provides more insights on the nature of the ESPT process.

A New Phase Transfer Strategy to Convert Protein-Capped Nanomaterials into Uniform Fluorescent Nanoclusters in Reverse Micellar Phase.

A new phase transfer strategy to convert aqueous phase protein-protected nanomaterials into fluorescent nanoclusters in the reverse micellar environment is introduced using bovine serum albumin (BSA)-protected silver nanoclusters (AgNCs) and nanoparticles (AgNPs) as an example. The basic pH employed in the fabrication of protein-protected nanoclusters induces the the protein capping to be negatively charged and facilitates the transfer process of the nanomaterials from aqueous phase to a cationic gemini surfactant (16-2-16)/hexane/hexanol/water reverse micelle (RM) phase. The original fluorescence characteristics of the seed nanocluster is retained after the transfer process. Strikingly, when both the nanomaterials (AgNCs and AgNPs) coexist in the aqueous feed solution, they are exclusively converted into uniform nanoclusters in the RM extract with enhanced fluorescence intensity.

New Insights on Hydrogen-Bond-Induced Fluorescence Quenching Mechanism of C102-Phenol Complex via Proton Coupled Electron Transfer.

The H-bonded coumarin 102 (C102)-phenol complex has been a model system usually used to understand the influence of H-bonding on photophysical processes. Zhao and Han first showed that significant H-bond strengthening occurs in the excited state and proposed the possibility of fluorescence quenching in the complex via internal conversion from a locally excited (LE) state to a low-lying charge transfer (CT) state. Later, we experimentally confirmed fluorescence quenching of C102-phenol complex in a nonpolar solvent (cyclohexane). However, we also found that the existence of the low-lying CT state is ambiguous. Here, we proposed an alternative mechanism for the fluorescence quenching in the H-bonded complex. For this, we evaluate the excited state potential energy surface considering complete H atom-transfer from phenol to C102 along the H-bonding coordinate. Surprisingly, we observed two distinct minima separated by a low-energy barrier. One minimum corresponds to the complex with shortening of H-bond consistent with that of Zhao and Han. On the other hand, the second minimum, which has even lower energy than the first minimum, is likely to be arising from the proton-coupled electron transfer (PCET) process. The nature of the lowest excited state alters from LE to CT type at the second minimum, which may account for the fluorescence quenching phenomena in the system.

How do the interfacial properties of zwitterionic sulfobetaine micelles differ from those of cationic alkyl quaternary ammonium micelles? An excited state proton transfer study.

Zwitterionic surfactants (e.g. sulfobetaine), comprising both positive and negative groups in their headgroups, are essentially electro-neutral as monomers, but their micelles preferentially uptake anions similar to cationic surfactant micelles. How do the interfacial properties (e.g. interfacial hydration or headgroup packing) of a zwitterionic micelle differ from those of a cationic micelle? For this, we investigated excited-state proton transfer (ESPT) and fluorescence anisotropy decay of an interface-localized negative fluorophore, 8-hydroxypyrene-1,3,6-trisulphonate (HPTS), and its methyl analogue, 8-methoxypyrene-1,3,6-trisulfonate (MPTS), within the micellar interfaces. Two sulfobetaine surfactants (SB-12 and SB-16) and two cationic surfactants (DTAB and CTAB) with matching alkyl tails were selected for an effective comparison. The ESPT dynamics was observed to be significantly suppressed inside the zwitterionic micelle interface compared to that of the cationic micelle. This is attributed to a less hydrated interface of the zwitterionic micelles compared to that of the cationic one. Fluorescence anisotropy decay inside the zwitterionic micellar interface was also slower compared to that of the cationic one indicating a better packing of the zwitterionic surfactants at the interface.

Helicity-Dependent Regiodifferentiation in the Excited-State Quenching and Chiroptical Properties of Inward/Outward Helical Coumarins.

Influence of helicity on the excited-state as well as chiroptical properties of two sets of regiohelical coumarins that are differentiated by "inward" and "outward" disposition of the pyran-2-one ring has been investigated. A subtle difference in the helicities manifests in divergent excited-state properties and significant differences in the dipole moments. The latter permit heretofore unprecedented regiodifferentiation in the O-H⋅⋅⋅O hydrogen-bond assisted electron-transfer quenching by phenols. Furthermore, the enantiopure hexahelical coumarins exhibit strong Cotton effects and lend themselves to a very high differentiation in the specific rotations and anisotropic dissymmetry factors. The specific rotation observed for 6-in turns out being the highest of the values reported for all hexahelicenes reported so far.

Coupling of Molecular Transition with the Surface Plasmon Resonance of Silver Nanoparticles inside the Restricted Environment of Reverse Micelles.

Interaction of molecular transitions of two fluorophores-fluorescein (FL) and safranin O (SAF)-with the surface plasmon resonance (SPR) of silver nanoparticles (AgNPs) inside a water/sodium dioctylsulfosuccinate (AOT)/n-heptane reverse micelle (RM) has been studied using ultraviolet-visible and fluorescence spectroscopies. Here, we exploit the natural capacity of a RM to act simultaneously as a template for nanoparticle formation and host the fluorophores. The fluorophores and reducing agent were loaded together into the water pool; thereafter, silver salt was added, and subsequently, spectral modification and size evolution were monitored by steady-state and time-resolved optical spectroscopy. In the FL-AgNP composite, the SPR band of AgNPs undergoes a strong red shift. Moreover, significant modifications of both the fluorescence intensity and lifetime of FL were found when AgNPs formed inside the RM core. On the contrary, in the SAF-AgNP composite, no such effect was noticed, and the composite system retains the original optical characteristics of their constituents (i.e., both the position of molecular transitions and SPR maximum remain unchanged). This differential effect has been rationalized by the dissimilar plasmon-fluorophore coupling in the two systems, controlled by a combination of spatial distribution and spectral detuning of the molecular absorption maxima of the dyes (455 and 530 nm for FL and SAF, respectively) from the SPR band maximum (∼400 nm) of AgNPs.

Elucidating the H-Bonding Environment of Coumarin 102 in a Phenol-Cyclohexane Mixture by Molecular Dynamics Simulation: Implications for H-Bond-Guided Photoinduced Electron Transfer.

Recently, we have experimentally demonstrated that the fluorescence intensity of coumarin 102 (C102) modulates anomalously upon hydrogen bonding to phenol in a nonpolar solvent: cyclohexane. The fluorescence intensity is first quenched gradually up to a particular mole fraction (XPH ≈ 0.013) but thereafter increases with further increases in the phenol mole fraction. These studies speculate about the importance of C102-phenol H-bonding to induce photoinduced electron transfer (PET) and propose a competition between the C102-phenol and phenol-phenol H-bonding to account for the anomalous fluorescence modulation. In this work, we investigate the exact H-bonding environment around the acceptor C102 at various compositions by molecular dynamics simulation and correlate the H-bonding environment to the observed fluorescence quenching. In addition to the 1:1 C102-phenol complex, 1:2 C102-(phenol)2 complexes with two different types of geometries were also found. Furthermore, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were carried out to understand the H-bonding in these complexes in the ground state and in the excited state and their possible contribution to the observed fluorescence quenching.