Exploring the Physical Properties of Lipid Membranes with Polyhydroxy Oxanorbornane Head Group Using NBD-Conjugated and DPH Fluorescent Probes.

The present study focuses on exploring the physical properties of lipid membranes based on the polyhydroxy oxanorbornane (PH-ONB) headgroup, designed as synthetic analogues of naturally occurring archaeal lipid membranes. Specifically, we study two variants of PH-ONB headgroup-based lipids differing in the number of hydroxy groups present in the headgroup, with one having two hydroxy groups (ONB-2OH) and the other having three (ONB-3OH). These lipids form stable bilayer membranes. The study begins with a comprehensive analysis of the fluorescence characteristics of nitrobenzoxadiazole (NBD)-tagged ONB-based lipids in different solvent environments and within a model lipid membrane 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Subsequently, the physical properties of the ONB-based membranes were examined by using an NBD-tagged ONB-based probe and a commonly used extrinsic 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probe. The steady-state and time-resolved fluorescence properties of the NBD-tagged ONB-based probe and DPH were used to compare the physical properties of the ONB-based membranes, including polarity, fluidity, phase transition, order, hydration, location, heterogeneity, and rotational diffusion. The solid gel to liquid crystalline phase transition temperatures of ONB-2OH and ONB-3OH lipid membranes are found to be (68 ± 1) °C and (74 ± 1) °C, respectively. The variation in organization (size), fluidity, and phase transition temperature of ONB-based lipid membranes is explained by the extent of hydrogen bonding interactions between lipid head groups. ONB-based membranes exhibit characteristics similar to those of phospholipid membranes and possess a notably high phase transition temperature. These properties make them a promising and cost-effective synthetic alternative to archaeal lipid membranes with a wide range of potential applications.

One-step synthesis of orange-red emissive carbon dots: photophysical insight into their excitation wavelength-independent and dependent luminescence.

A low-temperature method was developed to synthesize orange-red luminescence phosphor-doped carbon dots (CDs) without complicated purification procedures. These CDs showed excitation wavelength-independent narrow emission (photo-luminescence quantum yield, Φf ∼ 12 to 22%) with single exponential time-resolved decay in weakly polar/non-polar solvents, indicating the presence of one kind of chromophore. In contrast, the same CDs showed excitation wavelength-dependent broad emission (Φf ∼ 1 to 8%) with multi-exponential fluorescence decay in polar solvents. These CDs exhibited poor solubility in polar solvents, resulting in CD aggregates contributed by excitation wavelength-dependent weak luminescence. The CDs embedded in polymethyl methacrylate (PMMA) polymer film displayed bright orange-red fluorescence under UV 365 nm illumination, indicating their potential application in solid-state luminescence. Further, an analytical method was developed for the naked-eye detection of trifluoracetic acid (red emission) and triethylamine (green emission) under UV 365 nm illumination with reversible two switch-mode luminescence. Additionally, this efficient orange-red luminescence of CDs was utilized for possible bioimaging applications with negligible cytotoxicity in 3T3 mouse fibroblast cells.

Probing the heterogeneity of molecular level organization of ionic liquids: a comparative study using neutral Nile red and cationic Nile blue sulfate as fluorescent probes for butyrolactam-based protic ionic liquids.

Ionic liquids (ILs) are liquid salts composed of cations and anions, known for their significant local heterogeneity at the molecular level. To understand the microheterogeneity with regard to their local polarity and local viscosity, we have used two structurally similar but chemically distinguishable fluorescent probes: Nile red (NR), a neutral molecule, and Nile blue sulfate (NBS), a charged molecule. A comparative study of the response of the two probes to the molecular level heterogeneity of ILs is expected to provide a better clarity of understanding regarding the charged polar domain and the uncharged hydrophobic domain of ILs. Towards this, we synthesized two butyrolactam-based protic ionic liquids (PILs), i.e., BTF and BTD, with the same ionic headgroup ([BT]+) and different alkyl tails ([RCOO]-), where {R = H, C11H23}. BTF has no significant hydrophobic domain, whereas BTD has a larger hydrophobic domain. Temperature-dependent fluorescence parameters such as fluorescence intensity, lifetime, and anisotropy were measured for both NR and NBS molecules. The use of a pair of structurally similar but ionically different probes enables differential estimation of parameters like the microviscosity of a domain using the fluorescence anisotropy parameter (r). The absorption and emission spectra of both probe molecules are observed to be blue shifted upon going from BTF to BTD. NR showed a significant blue shift in absorption and emission band maxima. Conversely, NBS exhibited a small wavelength shift, possibly influenced by the preferred location of their charged head group domain. Temperature-dependent rotational relaxation time (θ) of NR in BTD is smaller than that of NBS by 60-70%, indicating that stronger charge-charge interactions exist between the polar domain of BTD and NBS. Moreover, it is observed that the local viscosity of the BTF IL around both probes is similar, whereas there is a considerable difference for the BTD IL. These results are an indication that NBS being charged prefers to locate itself in the charged head group region of the IL, whereas NR being neutral tends to reside both in the hydrophobic domain and in the head group but is predominantly located in the hydrophobic domain.

Phenylethynylanthracene based push-pull molecular systems: tuning the photophysics through para-substituents on the phenyl ring.

Organic push-pull molecules contain donor and acceptor moieties connected via π-linkages through which intramolecular electron charge transfer (ICT) can occur in the ground and excited states; giving these molecules interesting photophysical properties. The molecules chosen in this work are some basic phenylethynylanthracene derivatives to show that with just a change of substituents this class of small molecules can show dramatic changes in their photophysical properties. The emission properties and ICT abilities of these molecules are compared with regards to various electron donating and withdrawing substituents. Substituents such as cyano and methoxycarbonyl groups do not induce any ICT character whereas substituents like aldehyde, N,N-dimethylamino and nitro groups cause appreciable ICT character in this class of molecules and their emission spectra extend almost throughout the whole visible region. The comparative ICT character was correlated with the results of electron density difference calculations. Computational studies show that the molecules are planar in their ground as well as excited states; except the nitro group containing molecule, which has an orthogonally twisted structure in the excited state. The emission properties of this molecule led to its inclusion into a class of nitroaromatics which shows maximum emission intensity in moderately polar solvents and the emission is quenched drastically by either decreasing or increasing solvent polarity. Fluorescence anisotropy studies show very good sensitivity of these compounds towards microviscosity of their immediate molecular environment. A white light emitting (WLE) gel was prepared using 4-(anthracen-9-ylethynyl)benzonitrile (AnPCN) and 4-(anthracen-9-ylethynyl)-N,N-dimethylaniline (AnPNMe2) by taking polyvinyl alcohol (PVA) as the gelator and the resulting gel exhibited very good CIE (0.31, 0.33) with CCT (6598 K) and CRI (87). As an example, the use of the gel was also demonstrated by applying it to a commercial UV LED which showed satisfactory results. AnPNMe2 was used to sense polar solvent vapors in TLC plates and Whatman paper due to its good solvatochromic behavior.

Effects of donor and acceptor substituents on the photophysics of 4-ethynyl-2,1,3-benzothiadiazole derivatives.

The present work explores the photophysical, electrochemical, and fluorescence polarization properties of a group of π-conjugated phenylethynyl-2,1,3-benzothiadiazole derivatives (BTDs) bearing different electron-donating (ED) or electron-withdrawing (EW) substituents at the para position of the phenylethynyl moiety. The BTDs were synthesized through the Sonogashira cross-coupling reaction between 4-bromo-2,1,3-benzothiadiazole and the respective para-substituted phenylethynyl derivatives. The BTDs with the EW-substituents show relatively weak solvatochromic behavior, while the BTDs with the strong ED-substituents like methoxy and N,N-dimethylamino-based substituents (BTDPhOMe and BTDPhNMe2) exhibit a pronounced solvatochromic behavior. The change in dipole moments in the excited states of the derivatives was calculated using Lippert-Mataga plots. The conclusions drawn on the spectral behavior of the molecules could be rationalized by TD-DFT calculations involving electron density difference (EDD) maps that correlate with the ICT characteristics of the molecules. The experimental and theoretical calculations reveal that the BTDs with the strong ED-substituents (strong push-pull type BTDs) have a strong ICT character in the excited state. These strong push-pull type BTDs show high fluorescence quantum yield (ΦF) in apolar solvents and low ΦF in polar solvents. In contrast, the BTDs with the weak ED-substituents (weak push-pull type BTDs) and EW-substituents (pull-pull type BTDs) have a weaker ICT character with low ΦF in apolar and high ΦF in polar solvent media. There is good a agreement among the HOMO-LUMO band gaps obtained from absorption spectroscopy and electrochemical studies and theoretical calculations. The fluorescence anisotropy measurement in the glycerol medium shows that the studied BTDs generally exhibit higher sensitivity towards microviscosity than the traditional DPH fluorescence anisotropy probe.

Curcumin-Induced Membrane Property Changes in DMPC Multilamellar Vesicles and the Effects of Membrane-Destabilizing Molecules on Curcumin-Loaded Multilamellar Vesicles.

Curcumin (CUR) is the major bioactive component of turmeric (Curcuma longa), commonly used as a spice and traditional medicine in India. CUR possesses a wide range of pharmacological benefits, including antioxidant, anticarcinogenic, antimutagenic, anti-inflammatory, anti-Alzheimer, and anti-Parkinson effects. The CUR-membrane interaction is believed to be the reason for such biological activity of CUR. Several research groups have modeled the interaction of CUR with artificial model lipid membranes using various techniques such as nuclear magnetic resonance (NMR), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). However, the mechanism of its action is still unclear. A fluorescent-probe-based technique could be advantageous to study the CUR-lipid membrane interaction due to its sensitivity toward the local environment and its multiparametric nature. In this work, we have used the intrinsic fluorescence properties of CUR to investigate CUR-induced physical property changes in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) multilamellar vesicles (MLVs) at various CUR concentrations. By rationalizing the results of steady-state fluorescence intensity, fluorescence anisotropy, temperature-dependent fluorescence intensity, temperature-dependent fluorescence anisotropy, and quenching experiments, we have proposed a model showing concentration-dependent effects of CUR on the DMPC bilayer membrane. We suggest that at low concentrations (≤1 mol %), CUR is homogeneously distributed in the DMPC bilayer membrane in both the solid gel (SG) and liquid crystalline (LC) phases. At high concentrations (>1 mol %), CUR molecules form segregated domains that fluidize both membrane phases. However, the CUR-induced fluidization is less pronounced in the LC phase as some CUR molecules from the domain partition into the bilayer core. Further, the effects of membrane-destabilizing molecules such as bile salts, capsaicin (CAP), and piperine (PIP) on CUR-loaded DMPC multilamellar vesicles were studied. Our work also shows that CUR has a stabilizing effect on the DMPC membrane at high concentrations.

Interaction of Dopamine with Zwitterionic DMPC and Anionic DMPS Multilamellar Vesicle Membranes.

Dopamine (DA), a naturally occurring neurotransmitter, plays a crucial role in the function of the mammalian nervous system. DA-lipid-membrane interaction is inevitable during the neurotransmission process. In this report, we have studied the interaction of DA with anionic 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine (DMPS), neutral (zwitterionic) 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and synaptic membrane-mimicking mixed DMPC/DMPS (3:1 molar ratio) model multilamellar vesicle (MLV) membranes. Differential scanning calorimetry (DSC) studies suggest a strong specific interaction of DA with the anionic DMPS membrane, a weak interaction with the zwitterionic DMPC membrane, and a moderate interaction with the mixed DMPC/DMPS (3:1) membrane. The intrinsic fluorescence of DA was used as a new approach to gain a molecular-level understanding of DA-lipid-membrane interaction. Toward this end, a detailed photophysical study of DA, including its steady-state fluorescence anisotropy and fluorescence lifetime, was undertaken for the first time. The partition coefficient, location, and distribution of DA in the DMPS and DMPC model membranes were studied by employing intrinsic fluorescence. The effect of DA on the phase transition of the model membranes was also examined using the intrinsic fluorescence of DA. Zeta potential studies suggest a strong electrostatic interaction of DA with the anionic DMPS membrane and a nonspecific, relatively weak interaction of DA with the zwitterionic DMPC membrane. In addition, we observed cholesterol-induced DA expulsion from both DMPS and DMPC membranes. We believe that this work will provide a more in-depth understanding of DA-membrane interaction at a molecular level.

Nature of Saccharide-Induced F127 Micellar Dehydration: An Insight with FDAPT (2-Formyl-5-(4'-N,N-dimethylaminophenyl)thiophene), a Multiparametric Fluorescent Probe.

FDAPT (2-formyl-5-(4'-N,N-dimethylaminophenyl)thiophene) is an efficient environment-sensitive fluorescent probe, which senses the alteration of its microenvironment with six different fluorescent parameters, namely, emission intensity, wavelength, fluorescence anisotropy, and corresponding three time-dependent parameters fluorescence lifetime, time-resolved emission spectrum, and anisotropy decay. In the present work, the nature of saccharide-induced dehydration of a F127 polymeric micelle is investigated in detail with FDAPT emission. Using a multiparametric fluorescence approach, it is observed that the saccharide molecules not only decrease the critical micellization temperature of the F127 solution but also strongly alter the physical properties inside the micellar structures. The local polarity and fluidity significantly decrease in the saccharide-induced micelle as compared to the normal F127 micelle. The probe solvation dynamics study reveals that the water content in the core as well as corona domain diminishes significantly in the saccharide-induced micelle as compared to the normal micelle. More precisely, dehydration occurs more in the core region than in the corona region. Also, the saccharide-induced dehydration alters the relative size of the core and corona regions. The extent of dehydration varies with different saccharide molecules. It is also found that the dehydration efficiency order is trisaccharide (raffinose) > disaccharide (sucrose) > monosaccharide (glucose and fructose).

A Multiparametric Fluorescence Probe to Understand the Physicochemical Properties of Small Unilamellar Lipid Vesicles in Poly(ethylene glycol)-Water Medium.

FDAPT (2-formyl-5-(4'-N,N-dimethylaminophenyl)thiophene) efficiently senses the minimum alteration of lipid bilayer microenvironment with all six different fluorescence parameters namely emission wavelength, fluorescence intensity, steady-state anisotropy, and their corresponding time-dependent parameters (Sahu et al., J. Phys. Chem. B 2018, 122, 7308-7318). In the present work, the effect of poly(ethylene glycol) on the small unilamellar vesicle is demonstrated with the emission behavior of the FDAPT probe. A medium and a high molecular weight PEG were chosen to perturb the lipid vesicles. The alteration of the bilayer polarity, water content inside bilayer, lipid packing density in the perturbed vesicles reflect significant changes in different fluorescence parameters of FDAPT probe. The effect of PEG on the unilamellar vesicle was rationalized with the alteration of the emission behavior, fluorescence lifetime, steady-state anisotropy and anisotropy decay of the probe. The simple and convenient fluorescence measurements provide new insights into the effect of PEG on the packing density, water volume, micro polarity, and microviscosity of the small unilamellar vesicle. The physiological understanding was extended to rationalize the cryoprotecting behavior of PEG.

Introducing Tween-curcumin niosomes: preparation, characterization and microenvironment study.

In this work, we report unusual niosomes (non-ionic surfactant based vesicles), prepared using non-ionic surfactant Tween 80 (T80) as well as Tween 20 (T20) and curcumin. Conventional niosomes consist of non-ionic surfactant and cholesterol. We found that, despite being a probiotic, curcumin plays a similar role to cholesterol in the formation and stabilization of niosomes. The prepared Tween-curcumin niosomes were characterised using Dynamic Light Scattering (DLS), zeta potential, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and Transmission Electron Microscopy (TEM) techniques. The curcumin-induced micelle to vesicle transition in the Tween surfactants was investigated by DLS, zeta potential, fluorescence anisotropy, and fluorescence lifetime studies. At room temperature (298 K), the prepared niosomes were found to be stable; however, at a higher temperature (333 K), the niosomes degrade gradually and irreversibly to form micelles. The temperature-dependent vesicle to micelle degradation was monitored using fluorescence anisotropy, absorption, DLS and Differential Scanning Calorimetry (DSC) measurements. Further, the Tween-curcumin niosomes show a controlled release of curcumin, which could open up the possibility of multidrug therapy.

Total Emission Time Resolved Decay: a Method for Measurement and Resolution of Broad-Band Emission.

This article reports a time-resolved fluorescence data acquisition technique termed as "Total Emission Time Resolved Decay" (TETRD). TETRD is recorded by using zero-order diffraction of emission grating in TCSPC instrument. TETRD decay curve has entire wavelength dependent decay information buried in it. Cut-off filters are used to avoid scattering contamination. Two existing approaches are used for analysing the interconnected TETRD data. (i) First, global analysis: for discretely decaying multiple components, TETRD dataset is analyzed using global analysis. The normalized pre-exponentials (αi) and relative amplitudes (fi) recovered from global analysis reflect the individual component emission more faithfully and resembles with steady-state spectral data as well. (ii) Second, stretched exponential fitting (StrEF): for continuous lifetime distribution systems, StrEF (I(t) = I0 exp[-(t/τ)1/h]) has been used to analyse TETRD data. The average lifetime (τ) of StrEF matches well with the average lifetime of multi-exponential fitting, the heterogeneity factor (h) of StrEF is an additional parameter, which informs about local heterogeneity in the system. It is shown that the lifetimes obtained with TETRD matches well with the lifetimes obtained using conventional time resolved emission spectra (TRES). TETRD holds advantage in rapid data acquisition and facilitates inclusion of another variable (like concentration, solvent composition, pH, excitation wavelength etc.) into experimental design. Further, with the use of an appropriate data analysis tool, the multi-component decay profiles can be resolved conveniently.

Stercobilin and Urobilin in Aqueous Media: Existence of Specific H-Aggregates and Nonspecific Higher Aggregates at Different Concentrations.

Fecal matter is considered to be one of the primary sources of water pollution. Understanding the aggregation behavior of the fecal pigments (FPs) could play a critical role in their detection and analysis. This work shows that in aqueous media, the fluorescence of FPs indicates the presence of multiple emitting species, which have been assigned to monomers, lower-order H-aggregates (dimers), and higher-order H-aggregates. Steady-state absorbance, fluorescence and time-resolved fluorescence decay studies conclude that the emission of FPs in aqueous medium indicates H-type of aggregation, even up to nanomolar and sub-nanomolar concentrations. Four sets of independent experiments involving the variation of (i) concentration of FPs, (ii) temperature, (iii) pH, and (iv) ethanol/water composition as solvent media suggest the presence of monomer (540 nm), dimer (516 nm), and higher-order aggregates (500 nm) of FPs in aqueous solutions. The dimeric FP species appear to be present in the entire concentration range of 1 pM to 1 µM. Fluorescence lifetimes of H-aggregates are relatively longer as compared to the corresponding monomers. Hydrogen bonding appears to play an important role in forming H-aggregates in the aqueous phase of FPs as observed in the IR spectra of the FPs in dichloromethane. Density functional theory (DFT) calculations using the B3LYP functional and the LANL2DZ basis set show the contributions of π-π stacking and hydrogen-bonding interactions toward the formation of H-aggregated dimer of FPs in aqueous media.

Excited state proton transfer based fluorescent molecular probes and their application in studying lipid bilayer membranes.

Molecules with ionizable protons, with different proton transfer efficiencies in the excited and ground states, show excited state prototropism (ESPT). In suitable proton donating/accepting environments, ESPT of a molecule can result in the observation of emissions from different prototropic species, each characterized by different emission spectra and different emission lifetimes. In condensed media, the immediate environment around the ESPT molecule can significantly influence the emission spectral parameters of different prototropic species. This forms the basis of ESPT based fluorescence sensing. The concept of ESPT has been widely used for probing dynamical and structural information of micro-heterogeneous media like micelles, polymers, lipid bilayer membranes, etc. ESPT molecules like naphthol and intra-molecular ESPT (ESIPT) molecules like hydroxyflavones etc., are said to be good multi-state fluorescent molecular probes if (i) the partitioning of these molecules to a micro-heterogeneous organized medium is more efficient, and (ii) the molecules possess distinct excitation and emission wavelengths corresponding to their different prototropic forms. The fluorescence of different prototropic forms shows a sensitive response towards the change in the local environment around the micro-heterogeneous organized medium concerning the physical properties, local structure, and dynamics. This review mainly comprises the work carried out on ESPT fluorescence molecular probing of biomimetic liposomes/lipid bilayer membranes from 1990 onwards.

Study on the dependence of fluorescence intensity on optical density of solutions: the use of fluorescence observation field for inner filter effect corrections.

In this study, we report the identification of absorbance value of an analyte at the excitation wavelength that corresponds to the maximum of the observed fluorescence intensity obtainable for a certain instrument operating with right-angle fluorescence measurement (). The value depends on the fluorescence observation field (FOF) dimensions of the concerned spectrofluorometer. As the FOF varies from instrument to instrument, this study presents a simple method for obtaining FOF dimensions. Using the knowledge of FOF, absorbance of analyte at the excitation wavelength (Aλex) and emission wavelength (Aλem), we deduced a derived absorbance spectral parameter (Dabs). The observed fluorescence intensity of an analyte is proportional to the Dabs. While differentiating Dabs w.r.t. Aλex, the value of for the concentred spectrofluorometer was obtained and subsequently could be used for maximizing fluorescence sensitivity. It was observed that when the FOF was a point at the centre of a 1 cm path-length cuvette, the value was 0.87 with a progressive widening of FOF, the value increased gradually till ∼1.0. The proposed methodology was established using two well-known inner filter effect (IFE) correction models (Parker and Lakowicz model). The Dabs obtained from the Parker model corresponded well with the observed fluorescence data; however, the Dabs obtained using the Lakowicz model overestimated the loss of fluorescence because of IFE. Using equations derived from the Parker model, the correction of observed fluorescence intensity for IFE could be achieved. Furthermore, it is demonstrated that the commonly used the Lakowicz model loses its correction efficiency at absorbance values of ≥0.7.

Quantification of micropolarity and microviscosity of aggregation and salt-induced gelation of sodium deoxycholate (NaDC) using Nile red fluorescence.

This study reports the utility of the hydrophobic probe Nile Red (NR) to understand the concentration induced microenvironmental changes of sodium deoxycholate (NaDC) bile salt from the premicellar to postmicellar range. The spectroscopic properties like absorbance value, fluorescence intensity and fluorescence lifetime of NR are significantly sensitive towards different states of aggregation of NaDC bile salt. The critical aggregation concentrations of different states (dimer to primary micellar aggregates (1.0 mM), secondary micellar aggregates (7.0 mM), and higher micellar aggregates (14 mM)) have been determined from the absorbance value and fluorescence intensity measurements. The ET(30) polarity parameter values suggest a considerable decrease in the micropolarity with an increase in NaDC concentrations. Furthermore, the spectroscopic properties of NR are also sensitive towards the NaCl induced gelation process of NaDC bile salt. Changes in the micropolarity and microviscosity of the NaDC + NaCl mixed system have been estimated using the emission maximum value (cm-1) and fluorescence lifetime values of NR with an increase in the NaCl concentration. Microviscosity of the medium increases from ∼19 mPa s to ∼26 mPa s from the sol phase to the gel phase. Temperature dependence of both size and phase changes of the NaDC + NaCl (30 mM + 1 M) gel network has been studied using differential scanning calorimetry and dynamic light scattering studies. Temperature induced polarity and microviscosity changes of the NaDC + NaCl (30 mM + 1 M) gel network have also been studied using the fluorescence properties of NR.

Derived Absorbance Spectral Parameter as a Tool for Sensitive Fluorescence Measurements of Optically Dense Systems.

The present study proposes an approach to use the derived absorbance spectral parameter (Dabs) as a tool for sensitive fluorescence measurement of optically dense multifluorophoric systems. Mishra and group have demonstrated the use of Dabs in obtaining optimum offset wavelength for sensitive fluorescence measurement in synchronous fluorescence spectra (SFS) (Anal. Chim. Acta 2016, 940, 113-119; Anal. Chim. Acta 2008, 630 (1), 47-56). The Dabs is obtained with the knowledge of UV-vis absorbance spectral data of the system under study and fluorescence observation filed (FOF) parameters of the instrument. Dabs is proportional to the observed fluorescence intensity of the system. Excitation of an optically dense sample at Dabs maximum leads to maximum fluorescence intensity obtainable for that system even in the presence of considerable inner filter effect (IFE). The earlier studies were applicable to SFS at right-angle fluorescence measurement geometry and for samples showing only primary IFE. In the present study, the work has been expanded to general fluorescence measurements, even when both primary and secondary IFE is present and for different fluorescence measurement geometries (right-angle, 45° and front-face). The procedure for obtaining Dabs is simple and can be applied to other analyte-specific/unconventional fluorescence measurement strategies reported in the literature.

Photophysical Impact of Diacetylenic Conjugation on Classical Donor-Acceptor Electronic Energy Pair.

Organic fluorophores with extended π-conjugation are important for their widespread applications. The present work provides photophysical insights into a diacetylene bridged classical donor-acceptor electronic energy pair, naphthalene-pyrene, in comparison with its constituents' molecular structures, naphthyl and pyrenyl acetylenes, as well as parent naphthalene and pyrene chromophores. The diacetylenic dye loses the individual spectral identities of the donor and acceptor fluorophores exhibiting a locally excited (LE) emission (∼411 nm) from the overall molecular entity with high fluorescence quantum yields (0.55-0.84) in nonaqueous media. In contrast to the parent pyrene, the hybrid derivative shows a strongly allowed S0 → S1 transition. In mixed-aqueous media, the dye forms aggregates displaying a new red-shifted absorption (∼425 nm) as well as emission (∼510 nm) band. Unlike the hybrid dye, the naphthyl and pyrenyl acetylenes do not form aggregates. In the aggregate state of the hybrid fluorophore, electronic energy transfer takes place from the naphthyl moiety to pyrenyl ring. The excited-state photophysical properties of the dye are exploited in vapor sensing in the solid state.

Effect of Indole-3-Carbinol on Dimyristoylphosphatidylcholine Multilamellar Vesicles.

This study reports the interaction of indole-3-carbinol (I3C), which is a chemopreventive reagent, with an artificial model membrane {(dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles (MLVs)}, using the intrinsic fluorescence properties of I3C, extrinsic fluorescence properties of Nile Red (NR), differential scanning calorimetry (DSC), dynamic light scattering (DLS), and confocal microscopy. The intrinsic fluorescence of I3C helps to provide information about its location, partitioning ability, and sensitivity toward the phase-transition temperature of liposomes, confirmed by cetylpyridinium chloride (CPC) quenching study, partition coefficient values {(4.60 ± 0.1) × 105 (solid gel phase) and (7.29 ± 0.1) × 105 M-1 (liquid crystalline phase)} and temperature-dependent emission behavior of I3C. I3C perturbs the DMPC MLVs above 15 mol %, as observed using the fluorescence properties of NR, DSC, and DLS data. This perturbation occurs as a consequence of interfacial hydration of the DMPC MLVs, which was clearly indicated by the fluorescence properties (emission intensity, fluorescence lifetime, and nonextensive distribution analysis) of NR.

Probing the temperature-dependent changes of the interfacial hydration and viscosity of Tween20 : cholesterol (1 : 1) niosome membrane using fisetin as a fluorescent molecular probe.

A detailed photophysical study of fisetin in a Tween20 : cholesterol (1 : 1) niosome membrane has been carried out. Fisetin is found to partition well into the Tween20 : cholesterol (1 : 1) niosome membrane at low temperature (Kp = 2.7 × 104 M-1 at 10 °C). Cetylpyridinium chloride quenching study confirms the location of fisetin molecules in the interfacial domain of Tween20 : cholesterol (1 : 1) niosome membrane. The emission from the prototropic forms of fisetin (neutral form, excited state anion, ground state anion and phototautomer form) is found to sensitively reflect the local heterogeneities in Tween20 : cholesterol (1 : 1) niosome membrane. The shift in anionic emission maximum with variation in temperature shows the sensitivity of fisetin towards water accessibility at the interfacial domain of Tween20 : cholesterol (1 : 1) niosome membrane. Zeta potential value confirms that there is no role of surface charge in the multiple prototropism of fisetin in Tween20 : cholesterol (1 : 1) niosome membrane. The microviscosity changes with temperature, as reflected in fluorescence anisotropy values of fisetin phototautomeric species FT*, give information about the temperature-induced changes in the motional resistance offered by the interfacial domain of the niosomal membrane to small molecules. A temperature-dependent fluorescence lifetime study confirms the distribution of FT* in the two different sites of niosomal interfacial domain, i.e. water-deficient inner site and water-accessible outer site. This heterogeneity in distribution of FT* is further confirmed through time-resolved fluorescence anisotropy decay resulting in two different rotational time constants (faster component of ∼1.04 ns originates from water-accessible outer site and slower component of ∼16.50 ns originates from water-deficient inner site). The interfacial location of fisetin in Tween20 : cholesterol (1 : 1) niosome membrane has an important implication with regards to antioxidant activity as confirmed from a DPPH radical scavenging study.

Photophysics and peripheral ring size dependent aggregate emission of cross-conjugated enediynes: applications to white light emission and vapor sensing.

Photophysical understanding of organic fluorophores with π-conjugated scaffolds is crucial as such dyes are central to optoelectronic applications. This work presents a detailed photophysical investigation of a class of cross-conjugated homo- and hetero-enediynes (Y-shaped) peripherally attached to common aromatic moieties such as benzene, naphthalene, and anthracene. The cross-communicated electronic communication among the three aromatic units located at the tri-poles of the Y-shaped enediynes results in a broad S0 → S1 absorption band and locally excited (LE) emission signals. In addition to the LE emission band, a red-shifted aggregate emission is observed for some of the dyes in non-aqueous solvents where a clear size dependence of the peripheral aromatic rings is noted for the appearance of the aggregate fluorescence. The aggregates are static in nature as is evident from ground-state absorption spectral changes and the absence of rise-time in the time-resolved fluorescence decay studies, which are substantiated further through nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction experiments. Molecular orbital calculations support the local nature of the dominant electronic transition. The optimized ground state geometries of the dyes from partially to fully propeller shaped structures confirm the ring-size dependence of the aggregates. The LE and aggregate state emissions are judiciously exploited to generate single-component white light emission in binary solvent mixtures. The excited state photophysics are further applied toward polar aprotic vapor sensing in the solid state.