Edge excitation red shift and energy migration in quinine bisulphate dication

Photoinduced excited state dynamical processes in quinine sulphate dication (QSD) have been studied over a wide range of solute concentrations using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The edge excitation red shift (EERS) of emission maximum, emission wavelength dependence of fluorescence lifetimes and the time dependence of emission maximum are known to occur due to the solvent relaxation process. With increase in solute concentration, the emission spectrum shifts towards the lower frequencies accompanied with decrease in fluorescence intensity, however, absorption spectrum remains unchanged. A decrease in EERS, fluorescence lifetimes, time dependent fluorescence Stokes shift (TDFSS), fluorescence polarization and the solvent relaxation time (τ_r) is observed with the increase in solute concentration. The process of energy migration among the QSD ions along with solvent relaxation has been found responsible for the above experimental findings.     

Edge excitation red-shift and excited-state proton association in acridine

Acridine in methanol shows an edge excitation red-shift. On shorter-wavelength excitation the emission from acridine is observed, while on red edge excitation the emission corresponds to acridinium. At room temperature the shift is abrupt with no effect of viscosity and solvent deuteration; however, at 80 K a slightly less abrupt shift is observed. The excited-state proton association is found to be wavelength-dependent. It seems that at room temperature with red edge excitation, protonation in the excited state is fast compared to the lifetime of the excited state, while the excess energy with shorter wavelength excitation may lead to non-promoting modes such that the proton transfer does not take place.     

Red edge excitation and proton association in the excited state of acridine

A comprehensive study of acridine spectra with variation of pH, wave length of excitation, deuteration of the solvent, etc., has been made. The excited state protonation of acridine is found extra-ordinarily excitation wavelength sensitive near the red edge of the first absorption band. The proton association takes place very fast (K _PT ~ 10¹⁰ sec^-1) on excitation at the red edge of the first absorption band (ree) and acridinium emission is observed while it is slow on short wavelength excitation (swe). The reaction rate slows down at lower temperature which is indicated by a delay in the initiation of the effect by ~ 8 nm on ree. The acridinium type emission with ree at 80 K shows that proton tunnelling is the chief mechanism of proton transfer. The quantum yields are also found wavelength dependent. Contrary to previous observations acridinium ion also shows a ree shift at 80 K.     

Red-edge effect in excited state reaction in β-naphthylamine

Alkaline aqueous solutions of β-naphthylamine show the edge effect; a decrease in the relative intensity or emission from the anion (RNH^?) and the neutral molecule (RNH₂) on red-edge excitation (REE) is observed. The nature of quenching of the neutral molecule by OH^? ions also changes on REE. It is suggested that on REE, participation of some non-promoting out-of-plane modes slows down the proton-transfer rate in the excited state.     

Cryogenic studies, site selectivity and discrete fluorescence in salicylic acid dimer

Cryogenic effects (10–293 K) on the photophysics of salicylic acid (SA) dimer have been using steady state and time-resolved spectroscopic techniques. SA dimer shows two emissions at approximately 390 nm (dimer, D) and approximately 430 nm (tautomer, T), formed by cyclic double proton transfer plus fast excited state intramolecular proton transfer (ESIPT), at low temperatures; a vibrational structure also develops which is due to C = O and OH stretches. On red edge excitation (REE), only the dimer-type (UV) emission is observed, which shifts with excitation energy resembling emission due to site selectivity. Due to the asymmetry of the double potential energy curves of D and T, all dimers can be trapped in the D minimum. The UV emission of the dimer is accompanied by the Stokes' shifted tautomer emission on excitation at 1050 cm⁻¹ higher than the (0,0) band of D, which is interpreted as the barrier height of the double potential energy curves of D and T. Time-resolved studies at various temperatures have helped to clarify the photophysics of crystalline SA.     

Dipolar relaxation within the protein matrix of the green fluorescent protein: a red edge excitation shift study

The fluorophore in green fluorescent protein (GFP) is localized in a highly constrained environment, protected from the bulk solvent by the barrel-shaped protein matrix. We have used the wavelength-selective fluorescence approach (red edge excitation shift, REES) to monitor solvent (environment) dynamics around the fluorophore in enhanced green fluorescent protein (EGFP) under various conditions. Our results show that EGFP displays REES in buffer and glycerol, i.e., the fluorescence emission maxima exhibit a progressive shift toward the red edge, as the excitation wavelength is shifted toward the red edge of the absorption spectrum. Interestingly, EGFP displays REES when incorporated in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT), independent of the hydration state. We interpret the observed REES to the constrained environment experienced by the EGFP fluorophore in the rigid protein matrix, rather than to the dynamics of the bulk solvent. These results are supported by the temperature dependence of REES and characteristic wavelength-dependent changes in fluorescence anisotropy.     

Steady state and time-resolved spectroscopic studies of 7-hydroxyquinoline in various polymeric matrices

The photophysical behavior of 7-hydroxyquinoline (7HQ) is studied in four polymer matrices, viz. polymethyl methacrylate (PMMA), cellulose acetate (CA), polyvinyl alcohol (PVA) and Nafion-117, at ambient temperature using both steady state and time-domain measurements. The study reveals both ground as well as excited state tautomerization in the case of PVA. In PMMA and CA, the intermolecular hydrogen bond to the host polymer results in proton transfer. Edge excitation red shift (EERS) and excitation and emission wavelength dependent decays are observed for normal emission in PMMA and CA matrices. These results are attributed to the trapping of a normal molecule in different geometries. In Nafion, the results show the presence of 7HQ cationic species trapped in the polymer.     

A study of the time-resolved fluorescence spectrum and red edge effect of ANF in a room-temperature ionic liquid

In a recent article, we have analyzed using molecular dynamics simulations the steady-state red edge effect (REE) observed by Samanta and co-workers when the fluorescent probe 2-amino-7-nitrofluorene (ANF) is photoexcited at different wavelengths in 1-butyl-3-methylimidazolium ([BMIM+]) hexafluorophosphate ([PF6-]). In this letter, we predict the time- and wavelength-dependent emission spectra of ANF in the same ionic solvent. From the analysis of our simulated data, we are able to derive an approximate time scale for reorganization of the solvent around the solute probe. The effect that slow varying local liquid environments have on the overall time-dependent signal is also discussed.     

A femtosecond study of excitation wavelength dependence of solvation dynamics in a PEO-PPO-PEO triblock copolymer micelle

Excitation wavelength (lambdaex) dependence of solvation dynamics of coumarin 480 (C480) in the micellar core of a water soluble triblock copolymer, PEO20-PPO70-PEO20 (Pluronic P123), is studied by femtosecond and picosecond time resolved emission spectroscopies. In the P123 micelle, the width of the emission spectrum of C480 is found to be much larger than that in bulk water. This suggests that the P123 micelle is more heterogeneous than bulk water. The steady state emission maximum of C480 in P123 micelle shows a significant red edge excitation shift by 25 nm from 453 nm at lambdaex=345 nm to 478 nm at lambdaex=435 nm. The solvation dynamics in the interior of the triblock copolymer micelle is found to depend strongly on the excitation wavelength. The excitation wavelength dependence is ascribed to a wide distribution of locations of C480 molecules in the P123 micelle with two extreme environments-a bulklike peripheral region with very fast solvent response and a very slow core region. With increase in lambdaex, contribution of the bulklike region having an ultrafast component (< or =2 ps) increases from 7% at lambdaex=375 nm to 78% at lambda(ex)=425 nm while the contribution of the ultraslow component (4500 ps) decreases from 79% to 17%.     

Fluorescence properties of Ca2+-independent discharged obelin and its application prospects

Discharged obelin, a complex of coelenteramide and polypeptide, is a fluorescent protein produced from the photoprotein obelin, which is responsible for bioluminescence of the marine hydroid Obelia longissima. Discharged obelin is stable and nontoxic and its spectra are variable, and this is why it can be used as a fluorescent biomarker of variable color in vivo and in vitro. Here we examined light-induced fluorescence of Ca²⁺-independent discharged obelin (obtained without addition of Ca²⁺). Its emission and excitation spectra were analyzed under variation of the excitation wavelength (260–390 nm) and the emission wavelength (400–700 nm), as well as the 40 °C exposure time. The emission spectra obtained with excitation at 260–300 nm (tryptophan absorption region) included three peaks with maxima at 355, 498, and 660 nm, corresponding to fluorescence of tryptophan, polypeptide-bound coelenteramide, and a hypothetical indole–coelenteramide exciplex, respectively. The emission spectra obtained with excitation at 310–380 nm (coelenteramide absorption region) did not include the 660-nm maximum. The peak in the red spectral region (λ _max?=?660 nm) has not been previously reported. Exposure to 40 °C under excitation at 310–380 nm shifted the obelin fluorescence spectra to the blue, whereas excitation at 260–300 nm shifted them to the red. Hence, red emission and variation of the excitation wavelength form a basis for development of new medical techniques involving obelin as a colored biomarker. The addition of red color to the battery of known (violet to yellow) colors increases the potential of application of obelin.     

Time-resolved fluorescence study of 4-dimethylaminobenzonitrile in nonhydrogen-bonding polymers,using picosecond dye laser pulses as excitation source

It is shown that the time-resolved fluorescence (TRF) spectra and the red edge effect (REE) phenomena are useful tools to observe the twisted intramolecular charge transfer (TICT) band, of 4-dimethylaminobenzonitrile (DMABN) in nonhydrogen-bonding polymers. The TRF results show that the relative intensity of the TICT band increases with the increase of time after excitation in all polymers used. The highest TICT relative intensity is observed in poly(butyl methacrylate) (PBMA) polymer matrix, which has a medium polarity and medium free volume compared with either poly(methylmethacrylate) (PHMA), which is more polar and more rigid, or poly(hexylmethacrylate) (PHMA), which is less polar and less rigid than PBMA polymer matrix. The lifetimes are rationalized to the opposite roles of local polarity and local free volume of polymer sites on TICT emission. © 1995 John Wiley & Sons, Inc.     

Large edge-excitation red-shift of the fluorescence of an ethanol solution of 4-amino-4'-nitrodiphenyl (and). The case of a solute with a dipole moment increase in the excited state

The observed large edge-excitation red-shift of the fluorescence of 4-amino-4'-nitrodiphenyl (AND) in a rigid ethanol medium and the red-shift of the absorption band as the temperature is lowered are explained in terms of a microscopic solvent heterogeneity. The AND results where the dipole moment is increased upon excitation are compared with the merocyanine dye results where the reverse is true.     

Ultrafast photoinduced relaxation dynamics of the indoline dye D149 in organic solvents

The relaxation dynamics of the indoline dye D149, a well-known sensitizer for photoelectrochemical solar cells, have been extensively characterized in various organic solvents by combining results from ultrafast pump-supercontinuum probe (PSCP) spectroscopy, transient UV-pump VIS-probe spectroscopy, time-correlated single-photon counting (TCSPC) measurements as well as steady-state absorption and fluorescence. In the steady-state spectra, the position of the absorption maximum shows only a weak solvent dependence, whereas the fluorescence Stokes shift Δν?(F) correlates with solvent polarity. Photoexcitation at around 480 nm provides access to the S(1) state of D149 which exhibits solvation dynamics on characteristic timescales, as monitored by a red-shift of the stimulated emission and spectral development of the excited-state absorption in the transient PSCP spectra. In all cases, the spectral dynamics can be modeled by a global kinetic analysis using a time-dependent S(1) spectrum. The lifetime τ(1) of the S(1) state roughly correlates with polarity [acetonitrile (280 ps) < acetone (540 ps) < THF (720 ps) < chloroform (800 ps)], yet in alcohols it is much shorter [methanol (99 ps) < ethanol (178 ps) < acetonitrile (280 ps)], suggesting an appreciable influence of hydrogen bonding on the dynamics. A minor component with a characteristic time constant in the range 19-30 ps, readily observed in the PSCP spectra of D149 in acetonitrile and THF, is likely due to removal of vibrational excess energy from the S(1) state by collisions with solvent molecules. Additional weak fluorescence in the range 390-500 nm is observed upon excitation in the S(0)→S(2) band, which contains short-lived S(2)→S(0) emission of D149. Transient absorption signals after excitation at 377.5 nm yield an additional time constant in the subpicosecond range, representing the lifetime of the S(2) state. S(2) excitation also produces photoproducts.     

New highly fluorescent ketocyanine polarity probes

The syntheses and spectral properties of three new and highly fluorescent solvent polarity probes are described. They are found to be extremely sensitive to solvent polarity in that spectral red shifts in both absorption and fluorescence spectra occur upon increasing solvent polarity. Excitation and emission data of the dyes in a set of different polar solvents are given. The emission data are compared with the standard ET^N values of solvent polarity and a linear correlation is obtained over a wide range. The origin of the unusual solvatochromic properties is discussed in terms of the resonance structures of this new group of molecular probes. Their outstanding features include high spectral sensitivity to polarity, high molar absorptivities, high fluorescence quantum yields, longwave excitation and emission, insignificant quenching by oxygen, and a sufficient stability in aqueous solution. Therefore, the new probes are considered to be advantageous over other polarity probes used so far in probing biochemical and biological systems.     

RED EDGE EXCITATION EFFECT IN INTACT EYE LENS

Shift in the wavelength of emission upon shift in the excitation wavelength towards the red edge of the absorption band is termed Red Edge Excitation Shift (REES). This effect is observed only in situation where the fluorophore mobility with respect to the surrounding matrix is considerably reduced. We have observed such red edge excitation effect in the intact eye lens. The REES observed for a normal lens is different from that seen in a photodamaged lens and hence appears to be a potential tool to monitor the changes in the state of the lens. Photodamage experiments with tryptophan in polyethylene glycol (PEG) and intact eye lens indicate that the red edge photon can also cause photodamage.     

In situ FLUORESCENCE SPECTROSCOPIC STUDIES ON BOVINE CORNEA

Abstract— The cornea is a transparent ocular tissue and its transparency is thought to be a result of intramolecular interactions and the supramolecular organization of its protein constituents. We have studied the intrinsic fluorescence properties of intact bovine corneas and compared these with that of the opaque sclera. It was observed that with increasing excitation wavelengths the emission maxima shifted toward the red edge exhibiting the phenomenon of red edge excitation shift, which is indicative of immobilization of the constituent fluorophores. The magnitude of the shift increased after photodamage by irradiation at 295 nm. Many of the spectral characteristics of the cornea are shown to be due to its proteoglycans, which show surprisingly significant red edge excitation shift in solution.     

Photophysical processes of some benzimidazole derivatives

The photophysical properties of N-(alpha-naphthyl)-benzimidazole (alpha-NABI), N-(beta-naphthyl)-benzimidazole (beta-NABI) and N-(alpha-pyridyl)-benzimidazole (alpha-PYBI) were studied and alpha-NYBI exhibit intramolecular charge transfer fluorescence in polar solvents. The fluorescence of benzimidazoles can be quenched by acetic acid and the existence of exciplexes was observed between the benzimidazole derivatives and acetic acid. Particularly, the maximum emission peak of solution of alpha-PYBI in mixed solvent, ether and acetic acid, presents obvious red-shift with the increase of concentration of acetic acid in the mixed solvent.     

The Preparation of (8‐Hydroxyquinolinato)Bis(2‐Phenylpyridyl)Iridium Complexes and Their Photophysical Properties

Four derivatives of the titled compounds, (8‐hydroxyquinoline)bis(2‐phenylpyridyl)iridium ( IrQ(ppy)₂ ), were prepared. Two of them were confirmed by single crystal X‐ray diffraction analyses, in which solvent molecules were found to be incorporated in the crystal lattices. Their emission spectra display separated dual bands in de‐aerated solutions at about 515 and 645 nm upon excitation. These green and red emissions are attributed to the triplet metal‐to‐ligand charge transfer (³MLCT) and triplet ligand centered (³LC) transitions in Ir(ppy)₂ and IrQ, respectively. It is suggested that such a multiple emission is feasible by nearly orthogonal orientation between the ppy and quinoline ligands in the mixed‐ligand Ir‐compounds which prohibits energy transfer between the two different ligands. The electroluminescence (EL) of these compounds was examined by the fabrication of light‐emitting diodes (LEDs). Unlike the spectra in solutions, their EL spectra displayed only the red emission band. Devices displaying white light can be obtained by mixing the red emission of IrQ(ppy)₂ with a compatible blue emitter (NPB) in separated layers.     

Sol–gel synthesis of Eu<Superscript>3+</Superscript>: Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> nanophosphor

Eu (0.1, 0.5 and 1.0 mol%) doped Tb₃Al₅O₁₂ (TAG) was prepared by sol–gel technique through nitrate-citrate route followed by sintering in air (1,100 °C maximum temperature). XRD analysis showed that Eu³⁺ enters the TAG lattice substitutionally replacing the Tb³⁺ ion. Both XRD as well as FTIR investigation showed improvement in crystalline phase with the increase in the sintering temperature. SEM and TEM analysis showed that the powder contains the particles in 5–20 nm size with almost spherical morphology. The excitation spectrum recorded in 300–500 nm showed dominant absorption due to Tb³⁺ while the emission spectra recorded with 380 nm excitation had strong red emission characteristic of Eu³⁺. The intensity of this emission increases with the increase of the Eu concentration from 0.1 to 0.5 mol%. However, the emission intensity decreased on further increase in Eu concentration to 1.0 mol%. This intensity variation with dopant concentration is attributed to well-known “concentration quenching” observed in rare-earth doped materials. Reasonably strong red emission due to Eu was observed when excited with the blue (480 nm) radiation of a Xe lamp indicating the usefulness of the material for the realization of white light LED.     

A novel strategy for selective detection of Ag+ based on the red-shift of emission wavelength of quantum dots

Selective determination of Ag(I) ion was accomplished based on the red-shift of the emission band of quantum dots (QDs). Under optimal conditions, a linear relationship does exist between the red-shift of the emission and the concentration of Ag(I) in the range from 1.0?×?10^?7 to 1.5?×?10^?5 mol L^?1, with a detection limit of 5.0?×?10^?8 mol L^?1. The method has been successfully applied to the determination of Ag(I) ion in water samples. The possible reaction mechanism was investigated by ultraviolet–visible absorption, fluorescence, Raman spectroscopies and by high resolution transmission electron microscopy. The results suggest that the red-shift in emission be attributed to the stabilization of a charge-transfer state, but not due to the aggregation induced by AgI(I) ion.