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The interaction of rose bengal (RB) aggregates with silver nanoparticles (AgNPs) is investigated to understand the factors that contribute toward metal nanopaticle enhanced fluorescence (MEF), such as reproducibility, spectral shift, and distortion. Various shapes and sizes of RB aggregates (spherical, rods, and fibrils) are formed upon preparing films from their solution in solvent with different polarities. These molecular aggregates are disrupted in the presence of AgNPs, resulting in different enhancement factors, not only because of MEF but also due to hindrance to aggregation-caused quenching. Microspectroscopic studies provide valuable insights into the microheterogeneity of these mixed aggregates. Interestingly, the excited state decay pathways remain the same at the nanosecond time scale for different emission wavelengths. Additionally, the lifetime distribution is very narrow due to the interaction of RB deaggregates with the plasmonic AgNPs. [ABSTRACT FROM AUTHOR]

Ag-(I)-doped CdSe nanotetrapods (NTPs) have been prepared using the post-synthesis cation exchange method. The shape and crystal structure of NTPs are found to be conserved after doping. The photoluminescence (PL) spectrum is Stokes shifted by 110 nm (0.33 eV) in doped NTPs at the highest dopant content (0.6%). The PL quantum yield increases to 11% from a value of 1.6% in undoped NTPs, with the emergence of a new, significantly longer component with a lifetime of 143 ns, at the cost of the fastest, nanosecond or lower component in the undoped NTPs. The dynamics of hole capture, which is the primary photoprocess that brings about the PL enhancement, has been elucidated by ultrafast transient absorption spectroscopy. Hence, Ag-doped colloidal three-dimensional NTPs are shown to be promising photoluminescent materials, which are less likely to suffer from the issue of the reabsorption of emitted photons due to the large Stokes shift emission. There are immense possibilities for their application in colloidal nanocrystal-based optoelectronic and photonic devices like light-emitting diodes and luminescent solar concentrators. [ABSTRACT FROM AUTHOR]

A combination of fluorescence spectroscopy and quantum chemical calculations is used to study the extent of fluorescence enhancement of an epicocconone in different surfactant assemblies, which are found to be very different from each other. The analysis reveals that the depth-dependent microviscosity highly affects the fluorescence quantum that further proves the involvement of fluorophore as a dual emitting marker for fluorescence microscopy.

The effect of microenvironment of a Nafion membrane on the excited-state proton transfer (ESPT) of 2-(2'-pyridyl)benzimidazole (2PBI) was investigated by steady-state fluorescence spectroscopy. The results suggest that mechanism of ESPT depends on the water content of the membrane, but is slowed down by cation exchange due to dehydration of the membrane, the less acidic environment around 2PBI molecules, and slowing down of the proton-transfer process.

The aggregation/deaggregation of chlorin p(super 6) with the surfactants CTAB, SDS, and TX 100 are studied using absorption, florescence, and light scattering techniques. Surfactants with ionic as well as nonionic headgroups are found to be capable of inducing aggregation of chlorin p6 at submicellar concentrations.

Excited-state proton transfer (ESPT) of 2-(2'-pyridyl)benzimidazole (2PBI) in reverse micelles is studied by steady-state and time-resolved fluorescence spectroscopy. The requirement of ESPT to occur in microheterogenous media is that the medium should be a ternary system comprised of water and a hydrophobic phase separated by a negatively charged interface.

The study describes the synthesis, structures, spectroscopic and electrochemical aspects of the nitrosyl complexes. The effectiveness of L(super 1-4) in the (trpy)Ru-No core specifically toward the electrophilicity and stability of the coordinated NO function with special reference to reported ancillary ligands is described.

The excited-state intramolecular H-atom transfer reactions of hypocrellins B and A are compared with those of calphostin C. The results of transient absorption measurements and ab initio quantum mechanical calculations conclude that large-amplitude conformational changes are coupled to the H-atom transfer in calphostin C, just as they are in hypocrellins B and A.

Picosecond time-resolved Stokes shift is used in studying solvation of 4-aminophthalimide (4-AP) in water-in-oil microemulsion of Triton X-100 in mixed solvents with 30% benzene and 70% n-hexane. Triton is a neutral surfactant which does not use sodium ions in the solvation of microemulsions. Results show the steady-state spectrum of 4-AP in microemulsion is very different from that in TX reverse micelle or in water. This suggests that 4-AP molecules stay in the water pool and the micropolarity of the water pool is different from ordinary water.

A study was conducted to analyze solvation dynamics in coumarin 480 in dimyristoylphosphatidylcholine vesicles in water using picosecond time-resolved Stokes shift. The response time of the experimental setup was about 50 ps while the wavelength of excitation for the time-resolved studies was 300 nm. Experimental results indicated that coumarin 480 is a sensitive fluorescent probe the microenvironment of the vesicles.

The specific binding of two model drugs for photodynamic therapy, namely chlorin [p.sub.6] and purpurin 18 in the vicinity of Sudlow's Site I of HSA is examined by monitoring the intrinsic fluorescence of single tryptophanyl residue and by competitive binding with warfarin. Time-resolved fluorescence results of competitive binding with warfarin have confirmed that they bind to the warfarin binding site.

The binding of serum albumin and lipoprotein with chlorin p6 and purpurin 18, two structurally related chlorins, is studied to understand the role for these proteins as endogenous carriers for these drugs. Low-density lipoproteins (LDLs) are found to be the preferred carrier for both the compounds, particularly at a slightly acidic pH, resembling the situation in tumor cells.

The photophysics of the fluorescent probe Lucifer yellow CH is investigated using fluorescence spectroscopic and computational techniques. Photophysical investigation in solvent mixtures and in deuterated solvents clearly indicates the predominance of the solvent-mediated intramolecular proton transfer in the excited state of the fluorophore in protic solvents.

The effect of micellar environment on the excited state p5roton transfer (ESPT) of 2-(2'-pyridyl )benzimidazole (2PBI) is investigated by steady state and time resolved fluorescence spectroscopy. Three factors namely diffusion controlled protonation of the normal form of 2PBI, slow and possibly incomplete solvation of the transition state, leading to a slowing down of the proton transfer process and a similar slow dynamics of the tautomeric excited state is explored.

The synthesis and structure of paramagnetic diruthenium complexes [{(acac)2RuIII}2(u-OC2H5)2] and [{(acac)2RuIII}2(u-L)](CIO4)2 is described. The reaction of binucleating bridging ligand with the monomeric metal precursor unit under aerobic conditions is produced for this purpose.

In the presence of an acidic Ce(IV) solution, [RuII(trpy)(L)H2O](CIO4)2 exhibits dual luminescence and [RuII(trpy)(L)H2O](CIO4)2 undergoes 2e-/2H+ oxidation to the corresponding Ru(IV). Stepwise synthesis of [RuII(trpy)(L)(X)]n+ is also presented.

Effectsof annealing have been studied on the chemical structure,water uptake, and acidity of the cast Nafion thin film of thickness∼ 6 μm using a fluorescent probe, 2-(3′-pyridyl)benzimidazole(3PBI), and attenuated total reflactance infrared (ATR-IR) spectroscopy.Nonannealed films and thick Nafion 117 membrane have been used asreference materials, in order to develop a complete understandingof the effect. Annealing has been found to cause a decrease in acidityof otherwise highly acidic ionomer, as sensed by the fluorescent probeand loss of water, as reflected in the ATR-IR spectrum. This observationis surprising and cannot be explained in the light of previous reportsof physical changes. Our ATR-IR study has revealed changes in thechemical structure of the hydrophilic part of the ionomer, leadingto the formation of sulfonic acid anhydrides. This phenomenon canrationalize the decreased acidity reported in our fluorescence study.Interestingly, acid treatment of the annealed film restores the acidityof the unannealed films. This cannot be rationalized simply by a greaterproton uptake from the solution, as the film has to be electroneutral.It appears that the anhydrides formed during the annealing processundergo acid hydrolysis, leading to an increase in the number of SO3–groups and, consequently, an increasein the number of H3O+ions in the water channels.Besides, the films can be hydrated to an extent that is much greaterthan Nafion membranes, but the water uptake of acid treated annealedfilm is slightly less than that of nonannealed films. Hence, we concludethat, along with annealing, acid treatment, a procedure that is generallynot performed on the cast films, is an important pretreatment procedureto improve the acidity and hence the transport properties of the castfilm. [ABSTRACT FROM AUTHOR]

The conductivity of polyaniline (PANI)is ascribed to its emeraldinesalt (PANI-ES), which is formed by protonation of its emeraldine base(PANI-EB) by acids. Generally, mineral acids are used for this purpose,but the use of dopants and additives to maintain the required acidityprovides an alternative method to the preparation of PANI-ES. Thepresent work attempts to achieve the protonation by the use of a weakorganic acid, namely, 6-cyano-2-naphthol (6CN2), which is generallyused as a superphotoacid, as its excited state pKais significantly smaller than its ground state pKa. The question here is if the protonation ofthe aniline moieties in PANI takes place and if it does, whether ittakes place by dissociation of the ground state or the excited stateof 6CN2. Room temperature conductance measurements were carried outto see the effect of doping. The formation of PANI-ES from PANI-EBhas been monitored by UV–vis spectrophotometry. When a polarcounterion is inserted into the polymer matrix, it changes the environmentof the nearby chains by introducing defects, reorganization of chargesas a result of interaction with the polymer. Morphological investigationwas done using optical microscopy, field emission gun scanning electronmicroscopy (FEGSEM), and field emission gun transmission electronmicroscopy FEGTEM. The influence of 6CN2 on the crystallinity of thepolymer was determined by X-ray diffraction (XRD). [ABSTRACT FROM AUTHOR]

Thebehavior of lamellar structures of Aerosol OT (AOT) as hosts,vis-à-vis the flexible normal micelles and rigid nanochannelsof Nafion membranes, has been investigated with two different fluorophores,[2,2′-bipyridyl]-3,3′-diol (BP(OH)2) andcoumarin 102 (C102). Surprisingly, for BP(OH)2, a risetime is observed at intermediate emission wavelengths and not in thered edge of the fluorescence spectrum. A shoulder at 525 nm is observedin time resolved emission spectra (TRES) at initial times of BP(OH)2in AOT lamellar structures. This feature is the signatureof the monoketo (MK) tautomer, observed for the first time in a microheterogeneousmedium. Also, the usually ultrafast single proton transfer in BP(OH)2is retarded to an considerable extent in lamellar structures.The potential of this medium in promoting unusual intermediates isthus highlighted. This property may be ascribed to the rigidity oflamellar structures, compared to hosts such as regular micelles. However,studies using another fluorophore, coumarin 102 (C102), brings outthe fact that these structures are significantly different from therigid host, Nafion, as well. The absence of excited state proton transfer(ESPT) in this molecule in AOT lamellar structures indicates thatit is not protonated, unlike in Nafion. Thus, the interfacial pH oflamellar structures is found to be significantly greater than thatof Nafion nanochannels. From the time dependent Stokes shift (TDSS)of the emission spectra of C102, the relaxation time (0.85 ns) ofinterfacial water in lamellar structures is found to be an order ofmagnitude faster than that observed in Nafion nanochannels, in whichH3O誉 have been substituted by differentcations. Hence, this study demonstrates that AOT lamellar structuresare rather unique hosts and that they behave very differently fromconventional rigid and flexible hosts such as normal micelles andNafion, respectively. [ABSTRACT FROM AUTHOR]

2-(4′-Pyridyl)benzimidazole (4PBI) has been usedto investigate the microheterogeneity of water nanochannels of Nafionmembranes at two different hydration levels. Native as well as cation-exchangedNafion membranes are found to protonate one of the two monoprotonatedforms of 4PBI selectively. In native membranes and in those in whichH3O+ions are replaced by (CH3)4N+ions, the pyridyl nitrogen is protonated preferentially.In Na+-exchanged membranes, however, the benzimidazolenitrogen is protonated selectively. Unlike other fluorescent probesused in earlier studies, 4PBI can differentiate between the two differentcation-exchanged membranes at lower as well as higher hydration levels. [ABSTRACT FROM AUTHOR]

(2,2′-Bipyridyl)-3-3′-diol (BP(OH)2) undergoesexcited state intramolecular double proton transfer (ESIDPT). Thephotophysics of this molecule has been investigated, in the highlyacidic water channels inside nafion membrane. The dianionic enolateform of the dye, which is formed only in alkaline conditions in neataqueous solutions, is found to be present in its excited state innative nafion membrane at low water content. This surprising phenomenonhas been explained in the light of loss of protons from the fluorophoreto the medium, due to increased electrostatic interactions with thependant sulfonate groups of nafion, at the lower water content. Thisobservation fortifies the model of microscopic interactions that wehave proposed in recent past. Further, the ground state of the diketoform, observed only in neat aqueous solutions so far, is found tobe present in Na+exchanged membranes at lower levels ofhydration. The steady-state spectral features of the dye is differentin (CH3)4N+exchanged membranes thanthat of Na+exchanged membranes, unlike in our earlierstudies with coumarin 102 and 2-(2′-pyridyl)benzimidazole,here we observed formation of an unusual ground state in Na+exchanged membranes, while no such feature was observed in (CH3)4N+exchanged membranes. [ABSTRACT FROM AUTHOR]

A molecular level understanding of the mobility of cations in the Nafion membrane has been attempted, using the excited state proton transfer (ESPT) process in the fluorescent probe Coumarin 102. ESPT is hindered significantly upon decreasing the water content. Using TRANES (time-resolved area normalized emission spectroscopy), the evolution of the ESPT state is clearly observed over hundreds of picoseconds in lower water content, implying that ESPT is hindered even in the nanovolume probed by the dye. Most remarkably, in the partially dried membrane, the predominant fluorescent species is the zwitterionic form, generated by excited state deprotonation of the cationic form. This implies that the molecule loses a proton from its nitrogen center in the excited state, as usual, but cannot recapture it readily at the oxygen center, at low water contents. This phenomenon is rationalized in light of an increased electrostatic attraction that is experienced by cations upon drying. [ABSTRACT FROM AUTHOR]

We have assessed the potential of salophen, a tetradentate Schiff base, in fluorescence sensing of Al3+ ions. While performing this investigation, we have noticed conflicting literature reports on the fluorescence spectral maximum and quantum yield of salophen. So, the compound has been purified by repeated crystallization. Fluorescence studies have been performed on samples in which the absorption and excitation spectra are completely superimposable. The purified compound exhibits a feeble fluorescence at 545 nm, associated with an ultrafast fluorescence decay. This is rationalized by excited state proton transfer and torsional motions within the molecule, which provide efficient nonradiative channels of deactivation of its excited state. The fluorescence quantum yield increases upon complexation of salophen with Zn2+ as well as Al3+. The increase is significantly more upon complexation with Al3+. However, fluorescence maxima are similar for the two complexes. This indicates that fluorescence intensity may not be a good parameter for Al3+ sensing by salophen, in the presence of a large excess of Zn2+. This problem can be circumvented if fluorescence lifetime is used as the sensing parameter, as the lifetime of the Al3+ complex is in the nanosecond time regime while that of the Zn2+ complex is in tens of picoseconds. The significant difference in the fluorescence quantum yield and lifetime between the two complexes is explained as follows: the Al3+ complex is monomeric, but the Zn2+ complex is dimeric. Quantum chemical calculations indicate a higher density of states near the locally excited state for the dimeric complex. This may lead to more efficient nonradiative pathways. [ABSTRACT FROM AUTHOR]