The effect of Cu(I)-doping on the photoinduced electron transfer from aqueous CdS quantum dots.

The doping of CdS quantum dots (QDs) with Cu(I) disrupts electron-hole correlation due to hole trapping by the dopant ion, post-photoexcitation. The present paper examines the effect of such disruption on the rate of photoinduced electron transfer (PET) from the QDs to methyl viologen (MV2+), with implications in their photocatalytic activity. A significantly greater efficiency of PL quenching by MV2+ is observed for the doped QDs than for the undoped ones. Interestingly, the Stern-Volmer plots constructed using PL intensities exhibit an upward curvature for both the cases, while the PL lifetimes remain unaffected. This observation is rationalized by considering the adsorption of the quencher on the surface of the QDs and ultrafast PET post-photoexcitation. Ultrafast transient absorption experiments confirm a faster electron transfer for the doped QDs. It is also realized that the transient absorption experiment yields a more accurate estimate of the binding constant of the quencher with the QDs, than the PL experiment.

Competition among recombination pathways in single FAPbBr3 nanocrystals.

Single particle level microscopy of immobilized FAPbBr3 nanocrystals (NCs) has elucidated the involvement of different processes in their photoluminescence (PL) intermittency. Four different blinking patterns are observed in the data from more than 100 NCs. The dependence of PL decays on PL intensities brought out in fluorescence lifetime intensity distribution (FLID) plots is rationalized by the interplay of exciton- and trion-mediated recombinations along with hot carrier (HC) trapping. The high intensity-long lifetime component is attributed to neutral exciton recombination, the low intensity-short lifetime component is attributed to trion assisted recombination, and the low intensity-long lifetime component is attributed to hot carrier recombination. Change-point analysis (CPA) of the PL blinking data reveals the involvement of multiple intermediate states. Truncated power law distribution is found to be more appropriate than power law and lognormal distribution for on and off events. Probability distributions of PL trajectories of single NCs are obtained for two different excitation fluences and wavelengths (λex = 400, 440 nm). Trapping rate (kT) prevails at higher power densities for both excitation wavelengths. From a careful analysis of the FLID and probability distributions, it is concluded that there is competition between the HC and trion assisted blinking pathways and that the contribution of these mechanisms varies with excitation wavelength as well as fluence.

Unique Metal-Ligand Interplay in Directing Discrete and Polymeric Derivatives of Isomeric Azole-Carboxylate. Varying Electronic Form, C-C Coupling, and Receptor Feature.

This article dealt with the ruthenium and osmium derivatives of isomeric 1H-indazole-3-carboxylic acid/2H-indazole-3-carboxylic acid (H2L1) and 1H-benzimidazole-2-carboxylic acid (H2L2) along with the π-acidic bpy (bpy = 2,2'-bipyridine) and pap (pap = 2-phenylazopyridine) co-ligands. It thus extended structurally authenticated monomeric ([(bpy)2RuII(HL1-)]ClO4 [1]ClO4, (pap)2RuII(L12-) 2, (bpy)2OsII(L12-) 3, (pap)2OsII(L12-) 4, (bpy)2RuII(L22-) 5, (bpy)2OsII(L22-) 8, and (pap)2OsII(L22-) 9) and dimeric ([(bpy)2RuII(µ-L22-)RuII(bpy)2](ClO4)2 [6](ClO4)2) complexes. It also described modified L2'2- (L2'2- = 2,2'-bisbenzimidazolate)-bridged [(pap)2RuII(µ-L2'2-)RuII(pap)2](ClO4)2 [7](ClO4)2, where L2'2- was developed selectively with the {Ru(pap)2} metal fragment via in situ intermolecular C-C coupling of the two units of decarboxylated benzimidazolate. Moreover, chemical oxidation (OsII to OsIII) of (bpy)2OsII(L12-) 3 (E0 = 0.11 V versus SCE) and (bpy)2OsII(L22-) 8 (E0 = 0.12 V versus SCE) by AgClO4 yielded unprecedented OsIII-AgI derived polymeric {[(bpy)2OsIII-L12--AgI(CH3CN)](ClO4)2}n {[10](ClO4)2}n and dimeric [(bpy)2OsIII-L22--AgI(CH3CN)](ClO4)2 [11](ClO4)2 complexes as a function of trans and cis orientations of the active N2 donor with special reference to the carboxylate O2 of L2-, respectively. Microscopic (FE-SEM, TEM-EDX, and AFM) and DLS experiments suggested a homogeneously dispersed hollow spherical shaped morphology of {[10](ClO4)2}n with an average particle size of 200-400 nm as well as its non-dissociative feature in the aprotic medium. Experimental (structure, spectroscopy, and electrochemistry) and theoretical (DFT/TD-DFT) explorations revealed a redox non-innocent feature of L2- in the present coordination situations and the selective anion sensing (X = F-, CN-, and OAc-) event of [1]ClO4 involving a free NH group at the backface of HL1-, which proceeded via the NH···X hydrogen bonding interaction.

Recent Advances in Small Molecule-Based Intracellular pH Probes.

Intracellular pH plays an important role in many biological and pathological processes. Small-molecule based pH probes are found to be the most effective for pH sensing because of ease of preparation, high sensitivity, and quick response. They have many advantages such as small perturbation to the functions of the target, functional adaptability, cellular component-specific localization, etc. The present review highlights the flurry of recent activity in the development of such probes. The probes are categorized based on the type of fluorophore used like quinoline, coumarin, BODIPY, rhodamine, indolium, naphthalimide, etc., and their analytical performance is discussed.

Photoluminescent Silica Nanostructures and Nanohybrids.

The complicated photophysics of wide variety of defects existing in silica (SiO2 ) layer of nanometer thickness determines widespread photoluminescence bands of Si/SiO2 based system as well as SiO2 nanoparticles (NPs) for their applications in photovoltaic and optoelectronic devices. This review attempts to summarize different photophysical processes in pure SiO2 NPs. Moreover, these NPs also act as scaffolds for various guest molecules to perform their specific functions. Guest fluorophore molecules when trapped inside pores of SiO2 NPs exhibit a different photodynamics than free state, which opens up several important applications of hybrid SiO2 NPs in artificial photosynthesis, sensing, biology and optical fiber.

Interplay of conformational relaxation and hydrogen bond dynamics in the excited states of fluorescent Schiff base anions.

Time resolved fluorescence spectroscopic investigation of four Schiff base anions has established that their excited state dynamics is governed by several solvent properties: polarity, viscosity and hydrogen bond donating ability. With viscous protic solvents like glycerol, fluorescence lifetimes of anions have been found to be markedly longer than those in ethanol, implying that conformational relaxation of molecules plays a key role in their nonradiative relaxation. Surprisingly, the lifetimes in less viscous aprotic solvents, like acetonitrile, are found to be even longer. The only plausible rationalization of this observation is in the light of hydrogen bond-assisted nonradiative phenomena that are operative in protic solvents. This contention draws support from a time evolution of the emission in the red end of the spectrum in low to moderately hydrogen bond donating protic solvents, with regard to an absence of such a rise time in aprotic solvents and strongly hydrogen bond donating solvents, viz., 2,2,2-trifluoroethanol. Rudimentary quantum chemical calculations provide a preliminary idea about the nature of excited state hydrogen bond redistribution involved in the process.

Role of Solvent in Electron-Phonon Relaxation Dynamics in Core-Shell Au-SiO2 Nanoparticles.

Relaxation dynamics of plasmons in Au-SiO2 core-shell nanoparticles have been followed by femtosecond pump-probe technique. The effect of excitation pump energy and surrounding medium on the time constants associated with the hot electron relaxation has been elucidated. A gradual increase in the electron-phonon relaxation time with pump energy is observed and can be attributed to the higher perturbation of the electron distribution in AuNPs at higher pump energy. Variation in time constants for the electron-phonon relaxation in different solvents is rationalized on the basis of their thermal conductivities, which govern the rate of dissipation of heat of photoexcited electrons in the nanoparticles. On the other hand, phonon-phonon relaxation is found to be much less effective than electron-phonon relaxation for the dissipation of energy of the excited electron and the time constants associated with it remain unaffected by thermal conductivity of the solvent.

Unique photoluminescence response of MoS2quantum dots over a wide range of As (III) in aqueous media.

We report the solvothermal synthesis of MoS2based quantum dots (QDs) and the performance evaluation of bare QDs for the detection of aqueous As (III) oxidative state at room temperature and neutral pH over a vast range (0.1-1000 ppb). Concentration-dependent photoluminescence (PL) of the QDs enhances up to 50 ppb and then suppresses till 1000 ppb. It shows two distinctive slopes for enhancement and suppression. The enhancement is possibly due to the passivation of trap states or defects. The formation of tiny glassy As2S3particles on the QD surface may be the possible reason for suppression. The pattern of optical absorption of QDs follows the similar patterns of PL. Still, it shows an enhanced absorbance in the near UV range below ≤300 nm, which increases with As (III) concentration up to 50 ppb and then decreases following the PL pattern. The MoS2QDs were characterized by using transmission electron microscopy, x-ray diffraction, UV-Vis, and PL spectroscopy. The enhancement and suppression results were excellently fitted with the modified Stern-Volmer equation. The detection of arsenic is possible using these linear fit equations as calibration curves.

L-cysteine functionalized graphene quantum dots for sub-ppb detection of As (III).

Here, we report functionalized graphene quantum dots (GQDs) for the optical detection of arsenic at room temperature. GQDs with the fluorescence of three fundamental colors (red, green, and blue) were synthesized and functionally capped with L-cysteine (L-cys) to impart selectively towards As (III) by exploiting the affinity of L-cys towards arsenite. The optical characterization of GQDs was carried out using UV-vis absorption spectroscopy, Fourier transform infrared spectroscopy, and fluorescence spectrometry, and the structural characterizations were performed using transmission electron microscopy. The fluorescence results showed instantaneous quenching in intensity when the GQDs came in contact with As (III) for all test concentrations over a range from 0.025 to 25 ppb, which covers the permissible limit of arsenic in drinking water. The experimental results suggested excellent sensitivity and selectivity towards As (III).

From fluorogens to fluorophores by elucidation and suppression of ultrafast excited state processes of a Schiff base.

Strategies have been explored for developing strongly fluorescent species out of a weakly fluorescent Schiff base, 2-(((pyridin-2-ylmethyl)imino)methyl)phenol (salampy). The locally excited enolic state of salampy undergoes an intramolecular proton transfer with a time constant of ca. 200 fs. The emissive cis keto state thus formed decays completely within 50 ps. Its fast decay and miniscule fluorescence quantum yield are attributed to efficient non-radiative channels associated with conformational relaxation. The anionic form, salampy-, has a significantly longer fluorescence lifetime of 800 ps. Its emissive state evolves in tens of picoseconds, from the locally excited state, by solvent and conformational relaxation. Both the neutral and anionic forms have a fluorescence lifetime of about 6 ns at 77 K, a temperature at which all activated nonradiative channels are blocked. This lifetime is similar to that obtained at room temperature, upon rigidification of the anion by complexation with Zn2+. Two such complexes have been studied. The first is binuclear, with acetate bridge between the two Zn2+ ions. The second, with ClO4- as the counterion, is mononuclear with two salampy ligands ligating the metal ion. Unlike a previous report on a different Schiff base, in which the ligands are π-stacked in its dimeric Zn2+ complex, no additional nonradiative deactivation pathway opens up in the Zn complexes of salampy, which are devoid of such stacking. The complex of salampy with Al3+ has an even longer fluorescence lifetime of 9 ns, indicating a greater degree of rigidification and consequent suppression of nonradiative processes.

Design and Expeditious Synthesis of Quinoline-Pyrene-Based Ratiometric Fluorescent Probes for Targeting Lysosomal pH.

Intracellular pH plays a significant role in many pathological and physiological processes. A series of quinoline-pyrene probes were synthesized in one-step fashion through an oxonium-ion-triggered alkyne carboamination sequence involving C-C, C-O and C-N bond formation for intracellular pH sensing. The quinoline-pyrenes showed significant red shifts at low pH. Fluorescence lifetime decay measurements of the probes showed decreases in lifetime at pH 4. The probes showed excellent selectivity in the presence of various potential interfering agents such as amino acids and cations/anions. Furthermore, the probes were found to show completely reversible emission behaviour in the window between pH 4 and 7. A morpholine-substituted quinoline-pyrene probe efficiently stained lysosomes with high Pearson correlation coefficients (0.86) with Lysotracker Deep Red DND-99 as a reference. A co-localization study of the probe with Lysotracker DND-99 showed selective intracellular targeting and a shift in fluorescence emission due to acidic lysosomal pH.

Release of Warfarin from Human Serum Albumin by Water-soluble CdSe Nanotetrapods.

Water soluble CdSe nanotetrapods are prepared by ligand exchange from organic-soluble ones. In order to achieve this, oleic acid, which is the capping agent for as prepared water-insoluble nanotetrapods, is exchanged with mercaptopropanoic acid (MPA). Shape and size of nanotetrapods are conserved upon ligand exchange. Ground state bleach recovery dynamics remain the same as well. However, they are completely non-emissive. These water-soluble nanotetrapods disrupt the secondary structure of Human Serum Albumin (HSA) and consequently, release warfarin bound to the protein.

Enhancement of the band edge emission of CdSe nano-tetrapods by suppression of surface trapping.

Preparation of CdSe nano-tetrapods in a controlled environment, using a glove box for the preparation of the Se precursor, was found to eliminate radiative surface trap emission to a large extent. This manifested in an increase in the band edge photoluminescence quantum yield by a factor of 20 and a concomitant decrease in the surface trap emission by a factor of 8. Interestingly, the time for recovery of ground state bleaching was more or less the same in the two kinds of nano-tetrapods, even though the photoluminescence dynamics are significantly slower in the more emissive ones. In single emissive nano-tetrapods, the kinetic traces for ground state bleaching recovery became gradually faster upon increasing the pump power. This is likely to be due to the effect of multiexciton relaxation and nonradiative Auger recombination.

Femtosecond Hydration Map of Intrinsically Disordered α-Synuclein.

Protein hydration water plays a fundamentally important role in protein folding, binding, assembly, and function. Little is known about the hydration water in intrinsically disordered proteins that challenge the conventional sequence-structure-function paradigm. Here, by combining experiments and simulations, we show the existence of dynamical heterogeneity of hydration water in an intrinsically disordered presynaptic protein, namely α-synuclein, implicated in Parkinson's disease. We took advantage of nonoccurrence of cysteine in the sequence and incorporated a number of cysteine residues at the N-terminal segment, the central amyloidogenic nonamyloid-ß component (NAC) domain, and the C-terminal end of α-synuclein. We then labeled these cysteine variants using environment-sensitive thiol-active fluorophore and monitored the solvation dynamics using femtosecond time-resolved fluorescence. The site-specific femtosecond time-resolved experiments allowed us to construct the hydration map of α-synuclein. Our results show the presence of three dynamically distinct types of water: bulk, hydration, and confined water. The amyloidogenic NAC domain contains dynamically restrained water molecules that are strikingly different from the water molecules present in the other two domains. Atomistic molecular dynamics simulations revealed longer residence times for water molecules near the NAC domain and supported our experimental observations. Additionally, our simulations allowed us to decipher the molecular origin of the dynamical heterogeneity of water in α-synuclein. These simulations captured the quasi-bound water molecules within the NAC domain originating from a complex interplay between the local chain compaction and the sequence composition. Our findings from this synergistic experimental simulation approach suggest longer trapping of interfacial water molecules near the amyloidogenic hotspot that triggers the pathological conversion into amyloids via chain sequestration, chain desolvation, and entropic liberation of ordered water molecules.

Neat Protein-Polymer Surfactant Bioconjugates as Universal Solvents.

Solvents, particularly those having low volatility, are of great interest for the biocatalytic synthesis of utility chemicals and fuels. We show novel and universal solvent-like properties of a neat and water-less polymer surfactant-bovine serum albumin (BSA) conjugated material (WL-PS pBSA). This highly viscous, nonvolatile material behaves as a liquid above its solid-liquid transition temperature (∼25-27 °C) and can be used as a solvent for variety of completely dry solutes of different sizes and surface chemistries. We show using a combination of optical microscopy and steady -state and time-resolved fluorescence spectroscopy that dry and powdered dyes (hydrophobic Coumarin 153 (C153)), enzymes (α-chymotrypsin (α-Chy)), or even 1 µm microparticles (diffusion coefficient ca. three orders slower than C153), can be solubilized and completely dispersed in the WL-PS pBSA solvent above 25 °C. While C153, irrespective of its mode of addition to WL-PS pBSA, binds similarly to BSA, α-Chy can be stabilized and activated to perform its hydrolysis function, even at 100 °C. This work therefore provides insights into the form of universal solvent characteristic property for this new class of nonaqueous, nonvolatile, biodegradable protein-polymer surfactant-based conjugated materials and suggests potential new avenues that can have a huge impact on biocatalysis, bionanotechnology, drug delivery, and other applications.

Temperature dependent excited state dynamics in dual emissive CdSe nano-tetrapods.

Time resolved spectroscopic investigation has been performed on nano-tetrapods, which are exotic nanocrystals with zinc blende type core structure and four arms with wurtzite structure. Dual emission is observed in these nanostructures. A band-edge emission occurs at 500-600 nm and a broad surface state emission occurs in the 600-900 nm region. The band-edge emission decays almost completely in a few ps, indicating the operation of an efficient trapping process. Incomplete recovery kinetics of ground state bleach from transient absorption experiments signifies the existence of a long-lived excited state. The lifetime of the surface state emission is in tens of nanoseconds. At liquid nitrogen temperature, surface state emission is enhanced to a greater degree than band edge emission, indicating suppression of various deactivation pathways at this temperature. Thus, an idea of excited state dynamics of these systems is developed, with a view of future tuning of photoluminescence properties by playing with the different radiative and nonradiative pathways involved.

Solvation and hydrogen bonding aided efficient non-radiative deactivation of polar excited state of 5-aminoquinoline.

The fluorescence of 5-aminoquinoline (5AQ) is quenched strongly in alcoholic solvents, in stark contrast to the enhancement in fluorescence observed earlier for its positional isomer, 3-aminoquinoline (3AQ). This phenomenon has been explained by the involvement of a highly dipolar excited state, which is manifested in the solvatochromism of 5AQ. A marked wavelength dependence of fluorescence decays of 5AQ in alcoholic solvents is ascribed to the ultrafast solvation of the highly dipolar excited state in these solvents. The resultant stabilization of this state leads to a decrease in the gap between its energy and lower lying triplet as well as ground singlet states, resulting in an efficient non-radiative relaxation and consequently, fluorescence quenching. Solvation dynamics reported for 5AQ is somewhat slower than earlier reports with coumarin probes, due to the involvement of solute-solvent hydrogen bonds, especially in the excited state of the solute. At lower temperatures, solvation is slowed down. An extreme case of this phenomenon is the absence of solvent relaxation at liquid nitrogen temperature. The fluorescence lifetime increases from tens of picoseconds at room temperature to tens of nanoseconds at cryogenic temperature, lending credence to the proposed model.

Enhanced fluorescence with nanosecond dynamics in the solid state of metal ion complexes of alkoxy salophens.

Salophen (N,N'-bis-(salicylidene)-o-phenylenediamine) is a Schiff base, which is weakly emissive in solution, but strongly emissive in the solid state. The trend is reversed in its aluminium complex (SalAl+), which is strongly emissive in solution, but feebly emissive in the solid state. Thus, SalAl+ is not attractive for application as a solid state emitter. In the present study, salophen derivatives containing -OCH3 and OC2H5 groups have been studied with the intent to perturb the structure and control the emission properties. The newly synthesized salophen derivatives are feebly emissive with ultrafast lifetime in solution. The mutiexponential nature of their fluorescence decay is ascribed to multiple ground state conformers of the enol and keto tautomers. In the solid phase, the ligands exhibit an order of magnitude increase in emission. Complexation with Al3+ (and Zn2+) does not bring about any significant increase in emissivity in solution, possibly because rotation/vibration of the alkoxy groups (with a contribution from axial water molecules) now provides the major nonradiative deactivation pathway. Such pathways are blocked in the solid state, leading to an increase in emission of up to 50 times in the aluminium complex. Thus, introduction of small functionalities is found to have profound effects on the suitability of a molecule as a solid state emitter.

Excited-State Proton Transfer and Conformational Relaxation of 2-(4'-Pyridyl)benzimidazole in Nafion Films.

The effect of annealing on the acidity and water uptake of Nafion films has been studied by using the acidity sensing fluorophore 2-(4'-pyridyl)benzimidazole (4PBI). The difference in acidity and the microenvironment of the fluorophore in annealed and nonannealed films is brought out in this study. The annealed film is found to have less water uptake than nonannealed films. The amount of water uptake increases upon acid treatment of the films, as all the steady-state and time-resolved behaviour of the molecule in nonannealed films is restored. These observations are rationalised by the formation of anhydrides upon annealing and their hydrolysis to sulfonic acid groups upon acid treatment. Interestingly, the acidity of annealed films is found to be even less than that of Na+ exchanged films, indicating that annealing removes more protons from the Nafion films than cation exchange can.

Quantum Chemical Investigation of Light-Activated Spin State Change in Pyrene Coupled to Oxoverdazyl Radical Center.

Low-spin ground states and low-lying excited states of higher spin were investigated for four pyrene oxoverdazyl monoradicals 1-4 and eight pyrene dioxoverdazyl diradicals 5-12. The ground states for quartet and quintet spin symmetries that are in reality excited states were found in the region of 565-775 nm above the respective electronic ground states. We calculated the "adiabatic" magnetic exchange coupling constant in the electronic ground state of each isolated biradical (5-12) by unrestricted density functional theory. A number of hybrid functionals such as B3LYP, PBE0, M06, and M06-2X were used. We also used range-separated functionals such as LC-ωPBE and ωB97XD to compare their effects on the coupling constant and the relative energy of the high-spin state. Molecular geometries were optimized for the doublet and quartet spin states of every monoradical (1-4), and the broken symmetry and triplet solutions were optimized for every biradical (5-12), by systematically using 6-311G, 6-311G(d,p), and 6-311++G(d,p) basis sets with each functional. The geometry of each quintet diradical (5-12) was optimized using 6-311G basis set. B3LYP produced the best spin values. The excited state (quartet or quintet)-ground state energy difference (ΔE) increases in the presence of para-phenylene connectors. These energy differences were predicted here. The nature of spin coupling and consequently the ground state spin agree with spin alternation rule and the calculated atomic spin population. The adiabatic coupling constants were predicted for the biradicals (5-12) in their electronic ground states. Electron paramagnetic resonance parameters were determined at 6-311++G** level for the ground state and the quartet state of 1 and compared with the available experimental data. Low-lying excited states were found for the radical center (oxoverdazyl), pyrene, molecule 1, and diradical 5 by time-dependent density functional theory (TDDFT) method using B3LYP hybrid, 6-311++G(d,p) basis set, and the molecular geometry in the electronic ground state. Data from these calculations were used to discuss possible mechanisms for the achievement of the high-spin (excited) states in 1 and 5 and to predict a similar outcome for radicals 2-4 and 6-12 upon excitation. A comprehensive mechanism for the first excitation is proposed here. In particular, we show that the initial excitation of 1 involves large contributions from mixed transitions between pyrene and oxoverdazyl moieties, whereas the initial excitation of 5 is basically that of only the pyrene fragment. Subsequent internal conversion and intersystem crossing are likely to lead to the high-spin states of lower energy. Sample spin-flip TDDFT calculations were also done to confirm the energetic location and composition of the quartet state of 1 and the quintet state of 5.