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Thin microcrystals of pyrene have been recrystallized by using a surfactant. With a size-range of several hundred nm to a few micrometers, the crystals have been characterized by using standard techniques of X-ray and Raman spectroscopy besides the atomic force and scanning electron microscopy. The fluorescence microscopy of single microcrystal shows fluorescence due to monomer as well as the red-shifted broad, structureless excimer-type emission. The observed decay profiles of the red-shifted emission reveal existence of dimer and higher aggregates in the crystalline state when excited at the red-edge of the absorption band. This is in contrast to the excimer emission observed at higher energy excitation range corresponding to the monomer absorption. Distinctly different nature of time-resolved decay profiles differentiates the two mechanisms. The observations have been used to propose existence of different sites of crystal packing under two different excitations (370 nm and 408 nm). The structural anisotropy of these crystals makes them useful as a probe in crystal structure based molecular and biological sensing. • Pyrene microcrystals have been synthesized in the presence of a surfactant to control their thickness. • The excimer, dimer and higher oligomers formation are distinguished using time-resolved studies. • There is no evidence of excimer formation at the red edge excitation. [ABSTRACT FROM AUTHOR]

The present article deals with the interaction of octadecylamine (ODA) capped cadmium selenide quantum dots (CdSe QDs) and two ferroelectric liquid crystals (FLCs) having different values of spontaneous polarization (Ps). The pristine FLCs and FLC-QDs composites have been investigated by using steady-state and time-resolved fluorescence spectroscopic techniques. Stokes shift and the preferred alignment of FLC molecules in the presence of QDs are found to depend on the secondary order parameter of FLCs (i.e. Ps). The secondary fluorescence emissions have been strongly influenced by the concentration of QDs. The Förster resonance energy Transfer (FRET) was absent in both the composites. This article discloses the role of various factors, like relative orientation, dipolar interaction between donor and acceptor and the proximity of QDs (>10 nm), which might be responsible for the absence of FRET. FLC-QDs composites do not form a new class of luminescent material though the fluorescence properties of host FLCs are tailored in the presence of QDs by means of the change in molecular alignment. The change in lifetimes of FLC-QDs composites has not been observed. [ABSTRACT FROM AUTHOR]

Two common and popular deep eutectic solvents (DESs) composed of the salt choline chloride and H-bond donors glycerol and urea in 1:2 mol ratio named glyceline and reline, respectively, are investigated for the analysis of polycyclic aromatic hydrocarbons (PAHs) using quenching of both steady-state and time-resolved fluorescence of ten different PAHs by nitromethane at 30 °C. Based on their quenching efficiencies, the PAHs are divided into two groups – group 1 is constituted of the five PAHs whose fluorescence are quenched less effectively by nitromethane whereas the other five exhibiting high quenching efficiency are associated to group 2. Quenching of steady-state fluorescence of group 1 PAHs by nitromethane, albeit not very significant, follow a simple Stern-Volmer behavior. The excited-state emission intensity decay of these PAHs, in both absence and presence of nitromethane, fit best to a single exponential model with small but monotonic decrease in lifetimes. The decrease in lifetime also follows Stern-Volmer behavior, however, the quenching constants ( K D ) are lower than those obtained from steady-state fluorescence ( K SV ). This is ascribed to the possible formation of charge-transfer complex between the PAH and the nitromethane. Steady-state fluorescence quenching of group 2 PAHs exhibit distinct upward curvature from linear Stern-Volmer behavior implying highly efficient quenching. The intensity decay fits best to a double exponential decay model with longer of the decay times following simple Stern-Volmer behavior. Formation of a complex or the presence of nitromethane within the quenching sphere of action of the PAH having short decay time is proposed. Quenching behavior was found to be similar irrespective of the identity of the DES. A representative group 2 PAH, pyrene, is employed to investigate diffusion dynamics within aqueous mixtures of the two DESs. The bimolecular quenching rate constant ( k q ) is found to increase linearly with the mole fraction of water ( x w ) in aqueous mixtures of both the DESs. Stokes-Einstein relationship is not obeyed within the DES aqueous mixtures and is attributed to be due to the inherent heterogeneity of the mixtures. Diffusion dynamics of pyrene-nitromethane pair is compared with that of a tracer fluorescein isothiocyanate obtained using fluorescence correlation spectroscopy. Good correlation between the two highlights generality of diffusion dynamics with aqueous DES mixtures as far as solutes are concerned. [ABSTRACT FROM AUTHOR]

An interaction of the boron clusters closo -borates K 2 [B 10 H 10 ], K 2 [B 12 H 12 ] and their functionalized derivatives with serum proteins human (HSA) and bovine (BSA) albumins and immonoglobulin IgG as well as globular proteins β-lactoglobulin and lysozyme was characterized. The steady state and time resolved protein fluorescence quenching studies point on the binding of the closo -borate arylamine derivatives to serum albumins and discrimination of other proteins. The mechanism of the albumin fluorescence quenching by the closo -borate arylamine derivatives was proposed. The complex formation between albumin and the closo -borate molecules has been confirmed by isothermal titration calorimetry (ITC). The compound (K 2 [B 10 H 10 ]) and its arylamine derivative both interact with HSA, have close values of K a (1.4 and 1.2×10 3 M −1 respectively) and Gibbs energy (−17.9 and −17.5 kJ/mol respectively). However, the arylamine derivative forms complex with the higher guest/host binding ratio (4:1) comparing to the parent closo -borate (2:1). [ABSTRACT FROM AUTHOR]

The interaction of Eu-1,4,7,10-tetraazacyclododecane (Cyclen) complex by using 4,4,4 trifluoro-1-(2-naphthyl)1,3-butanedione (TNB) as antenna with some nucleosides (guanosine, adenosine, cytidine and inosine), nucleotides (AMP, GMP, CMP, ATP and IMP) and DNA is studied using fluorescence technique. Two detection modes are employed one is the time-resolved mode, and the other is the normal luminescence mode. The time-resolved mode is more sensing than the normal luminescence mode in the present study. By using Benesi–Hildebrand equation binding constants were determined at various temperatures. Thermodynamic parameters showed that the reaction is spontaneous through the obtained negative values of free energy change Δ G . The enthalpy Δ H and the entropy Δ S of reactions were all determined. [ABSTRACT FROM AUTHOR]

The compound 4-aminophthalimide (4-AP) is a well-known dye used as an environment polarity sensitive probe e.g. in solvation studies. This paper presents the effect of temperature on 4-aminophthalimides steady-state and time-resolved fluorescence in five n-alcohols. It is shown that the hydrogen bonding ability of n-alcohols affects the shifts of steady-state absorption and fluorescence spectra of 4-aminophthalimide at room temperature, and the shifts of fluorescence also at temperatures from the range 180 to 323 K. Temperature is shown to affect the change in hydrogen bond energy that follows 4-AP excitation, in a way dependent on the n-alcohol alkyl chain length. On the other hand, time-resolved results indicate that the temperature dependence of 4-AP deactivation follows mainly from the energy-gap dependent non-radiative deactivation rate. Fluorescence transition dipole moments at room temperature have been found to be slightly dependent on the solvent, but nothing proves that these changes are connected to different hydrogen bonding character of each n-alcohol. Therefore, while the steady-state results provide clear evidence of hydrogen bonding between 4-AP and n-alcohols, the time-resolved results do not show any evident sign of hydrogen bonding, besides the influence of the position of fluorescence emission on the radiative and non-radiative rates. [ABSTRACT FROM AUTHOR]

The interaction of coumarin 120 (C120) with CdS nanoparticles (CdS NPs) in aqueous suspension was examined by using UV–vis absorption, steady-state, time-resolved fluorescence, and electron paramagnetic resonance (EPR) spectroscopy techniques. The fluorescence intensity of C120 was quenched with increasing the amount of CdS NPs in the aqueous suspension. The spectroscopic data revealed that the C120 molecules adsorbed on CdS NPs via electrostatic interactions. The apparent association constant ( K app ) and the degree of association (α) for C120/CdS NPs were determined as 130.3 M −1 and 0.51 for 4 nm CdS NPs and 624.3 M −1 and 0.71 for 8 nm CdS NPs, respectively. The photoinduced EPR studies exhibited that no electron transfers between CdS and C120 taking place. The results revealed that the fluorescence quenching of C120 with different CdS NPs is due to the formation of a non-fluorescent complex. [ABSTRACT FROM AUTHOR]

Abstract: A novel molecular triad (AQ-(H 3 ) 2 ) based on tritolylcorrole and anthraquinone having azomethine-bridge at the pyrrole-β position has been designed and synthesized by following a facile one step reaction. The molecular system, AQ-(H 3 ) 2 is characterized by elemental analysis, MALDI-MS, 1H-NMR, UV–Visible and fluorescence spectroscopy (steady-state and time-resolved) as well as electrochemical methods. In absorption spectra, prominent changes such as red-shift (~7nm) and broadening of the both Soret and Q-bands with respect to their monomer units were observed. The present study supported by density functional theory calculations manifest that there exists a negligible electronic communication in the ground state between the donor tritolylcorrole and acceptor anthraquinone of the triad. However, interestingly, in the triad AQ-(H 3 ) 2 , fluorescence emission of the tritolylcorrole quenched significantly (17–80%) compared to their monomeric units. The emission quenching is attributed to the excited state intramolecular photoinduced electron transfer from donor tritolylcorrole to acceptor anthraquinone and the electron transfer rates (k ET) are found in the range 4.1×108 to 2.4×109 s−1 and are found to be solvent dependent. [Copyright &y& Elsevier]

The combined dynamic process of Förster resonance energy transfer and transient quenching is quantified in the time-resolved fluorescence of cationic polyfluorene/fluorescein-labelled single-stranded DNA complex. We found that the radiation boundary condition fails to predict transient quenching due to a single quenching rate at the encounter distance between a fluorophore and a quencher; however, the predictions of the micellar kinetics model were in good agreement with the measured time-resolved fluorescence as an alternative to the complicated distance-dependent quenching model. The combined dynamics model enables the separation of the rate of Förster resonance energy transfer from that of transient quenching, by which we obtained an accurate estimation of the donor–acceptor intermolecular distance (41±1.6Å) in comparison with the Förster distance (43Å). [ABSTRACT FROM AUTHOR]

Abstract: Interaction of the nonionic surfactant Hecameg® with the plasma protein Bovine Serum Albumin (BSA), and its effect on protein conformation, has been studied using spectroscopic techniques such as steady-state and time-resolved fluorescence and circular dichroism. A weak interaction of the surfactant with BSA is reflected by changes in the intrinsic fluorescence of BSA in either steady-state or time-resolved measurements. The fluorescence intensity data allowed us to determine the corresponding binding curve, which suggests a sequential binding mechanism, in which the surfactant first occupies the hydrophobic sites of the inner protein cavity and then, condenses onto the surface hydrophobic sites of BSA via a cooperative mechanism. Additional fluorescence data obtained by synchronous, three-dimensional and anisotropy experiments show that the surfactant mainly interacts with the tryptophan residues of BSA, which seem to experience motional restriction as a result of this interaction. Time-resolved fluorescence data, which were analyzed using the modified Stern–Volmer equation, also support the above mechanism. Finally, far-UV circular dichroism studies indicated that the secondary structure of the protein remains almost unaltered even for BSA to surfactant molar ratio as high as 1 to 100. [Copyright &y& Elsevier]

Abstract: The characterization of aggregates of an anionic surfactant, sodium dodecyl sulphate (SDS) with bovine serum albumin (BSA) in various regions of binding isotherm of SDS to BSA with increasing concentration of the former have been done by exploring the twisted intramolecular charge transfer (TICT) fluorescence properties of a probe, trans-2-[4-(dimethylamino)styryl] benzothiazole (DMASBT). The TICT fluorescence, steady-state fluorescence anisotropy and time-resolved fluorescence of DMASBT, and the fluorescence resonance energy transfer (FRET) study reveal the characteristics of the native protein as well as the protein–surfactant aggregates viz., micropolarity, microviscosity, locations of probe, denaturation of protein in various regions of binding isotherm, and also the validation of necklace-bead model. The changes in the polarity and the viscosity of the microenvironment around the probe from one binding region of SDS to other have been reflected in the highly sensitive fluorescence properties of DMASBT. The study of FRET between the DMASBT and the tryptophan residue (Trp) of BSA has identified the locations of the probe molecule in the native protein as well as that in various BSA–SDS aggregates. The energy transfer efficiency decreases, whereas the distance between the DMASBT and the Trp residue increases with increasing concentration of SDS. The significant change in the conformations of protein molecules during the non-cooperative binding region of SDS is evidenced by the fluorescence anisotropic behavior of DMASBT in the same region. [Copyright &y& Elsevier]

Abstract: The steady-state absorption and emission as well as the time-resolved fluorescence spectra of coumarin 102 (C102) in both aprotic and alcoholic solvents have been used to study the effect of excited-state hydrogen bond on the dynamic fluorescence of C102 chromophore in various solutions. The dual fluorescence of C102 in alcohols, which is dependent on the hydrogen-bonded donation ability of the solvent, has been assigned to the distribution of free C102 and a hydrogen-bonded complex. Furthermore, a shift of the fluorescence spectra induced by excited-state hydrogen bond has been demonstrated to take place within hundreds of picoseconds by the performance of the time-resolved fluorescence spectra with the time-correlated single-photon-counting (TCSPC) technique. Moreover, the time-dependent density functional theory (TDDFT) has been used to calculate the hydrogen-bonded equilibrium constant pK HB in different electronic states. It has been demonstrated for the first time that the hydrogen bond strengthening in electronic excited states could decrease the free energy of the hydrogen-bonded complex due to its stronger binding energy. Therefore, the hydrogen-bonded equilibrium will become markedly in favor of the hydrogen-bonded forms in electronic excited states by comparison with the case in the ground state. [Copyright &y& Elsevier]

Abstract: Single microcrystals of pyrene have been studied by steady state and time resolved fluorescence microscopy. The fluorescence spectra of microcrystals exhibit vibrational structure unlike the broad spectrum observed in pyrene excimer. A risetime is observed in the decay curves of the concentrated solutions, indicating the excimer formation. In contrast, the fluorescence decay profiles of the single microcrystals are nonexponential in nature and the decay times vary with their size and the wavelength of emission. This behaviour has been explained mainly by considering the pyrene dimer stabilized in the ground state. [Copyright &y& Elsevier]

Abstract: Interaction of acridinedione dyes with model transport proteins, bovine serum albumin (BSA) in aqueous solution were investigated by fluorescence spectral studies. A fluorescence enhancement was observed on the addition of BSA to photoinduced electron transfer (PET) based acridinedione dyes, which posses C6H4(p-OCH3) in the 9th position of the basic acridinedione ring. On the contrary, the addition of BSA to non-PET based acridinedione dyes with methyl or phenyl substitution in the 9th position does not result in any fluorescence enhancement. The enhancement in the fluorescence intensity is attributed to the suppression of PET process through space between –OCH3 group and the acridinedione moiety is elucidated by steady state fluorescence measurements. The fluorescence anisotropy value (r) of 0.40 reveals that the motion of the dye molecule is highly constrained and is largely confined to the rigid microenvironment of the protein molecule. The binding constant (K) was found to be in the order of 6.0×103 [M]−1, which implies the existence of hydrophobic interaction between the PET based dye and BSA. Time resolved fluorescence lifetime measurements reveal that the PET based acridinedione dye preferably binds in the hydrophobic interior of BSA. [Copyright &y& Elsevier]

Abstract: A versatile method to take femtosecond spectral snapshots of fluorescence has been developed based on a double gating technique in the combination of an optical Kerr gate and an image intensifier as an electrically driven gate set in front of a charge-coupled device detector. The application of a conventional optical-Kerr-gate method is limited to molecules with the short fluorescence lifetime up to a few hundred picoseconds, because long-lifetime fluorescence itself behaves as a source of the background signal due to insufficiency of the extinction ratio of polarizers employed for the Kerr gate. By using the image intensifier with the gate time of 200ps, we have successfully suppressed the background signal and overcome the application limit of optical-Kerr-gate method. The system performance has been demonstrated by measuring time-resolved fluorescence spectra for laser dye solution and the riboflavin solution as a typical sample of biomolecule. [Copyright &y& Elsevier]

Abstract: The intra- and inter-molecular vibrational relaxation in the electronic excited state (1Bu +) of spheroidene derivative (the number of conjugated double bonds, ) has been investigated at room temperature by means of femtosecond time-resolved fluorescence spectroscopy based on an optical-Kerr-gate technique. Depending on the photo-excitation either to the or vibronic level, remarkable differences were observed in hot luminescence spectra related to the vibrational relaxation process of high- and low-frequency modes. Under the excitation to the state hot luminescence from the state was observed as a dominant feature of the time-resolved spectra while the dynamic Stokes shift originating from the low-frequency-modes dynamics was clearly observed under the excitation to the . These observations of the excitation energy dependence of time-resolved fluorescence spectra were discussed by analyzing the Franck–Condon factors of transitions from , and 2 levels of high-frequency modes. [Copyright &y& Elsevier]

Abstract: The nonlinear optical properties and the ultrafast dynamics for the polymers with the donor–acceptor structure were measured by the femtosecond laser spectroscopic techniques. Two polymers show intense two-photon absorption properties due to the unique tribranched structure. The ultrafast dynamics of the excited states were measured by two-color femtosecond pump–probe method, and the relaxation properties of the fluorescence emission state were detected by time-resolved fluorescence (TRFL) technique. We suggest that the fast decay component in the TRFL dynamics comes from the exciton migration process, and the slow decay component is caused by the recombination process of the excitons. In two-color femtosecond pump–probe and TRFL dynamics, both decay processes were accelerated when the solvent was changed from CHCl3 to DMSO, due to the fact that the dipole moment of DMSO is larger than that of CHCl3. [Copyright &y& Elsevier]

Fluorescent properties of 1,2,3,4-tetrachloro-11H-isoindolo-[2,1-a]-benzimidazol-11-one (TCIB) in various organic solutions are described. Detailed investigation of the fluorescence from the solutions revealed that it was ascribable to the electronic transition from the lowest singlet excited state of an isolated molecule to its ground state, though the fluorescence spectrum was broad and structureless and the Stokes’ shift was about 1 eV. The absorption peak of TCIB was relatively insensitive to change in solvent polarity, whereas its fluorescence peak shifted to the red with an increase in the polarity. This finding suggests that the dipole moment of the molecule in the ground state is almost zero, and that the excited state has a non-zero dipole moment, which coincides with the predicted semiempirical molecular orbital calculation. Fluorescence quantum efficiency decreased with increase of solvent polarity. The efficiency was reduced by a factor of 39 in going from n-hexane solution to acetonitrile solution. The radiative rate constant also decreased with increase of solvent polarity. However, its reduction was very moderate; the reduction factor was only 2.5 for acetonitrile solution as compared with n-hexane solution. This finding indicates that the emitting state of the title compound is influenced by a solvent-dependent non-radiative mechanism, for which solvent-sensitive intersystem-crossing deactivation is tentatively proposed. [Copyright &y& Elsevier]

Molecular imprinting of synthetic polymers provides a powerful approach to control electronic and optical properties through nanoscale modification of molecular design of the material. The functional monomers were co-polymerised in the presence of a target/imprint molecule, which acts as a molecular template. Subsequent removal of the template molecules left behind functionalised cavities that are able to recognise the template molecule. The steady-state and time-resolved fluorescence spectroscopy studies are presented as methods for monitoring of the specificity and selectivity of binding of the template on the functionalised cavities. [Copyright &y& Elsevier]

A new cryptate incorporating an unsymmetrical arch as 6-methyl-2-[3-(methyl)pyrazol-1′-yl]pyridine has been obtained in a good yield. The effect of the dissymmetry introduced by the pyrazole moiety on the corresponding Eu(III), Tb(III) and Sm(III) complex fluorescence behavior is discussed. [Copyright &y& Elsevier]

The time dependence of Sm2+ fluorescence in orthorhombic BaCl2 was investigated between 77 and 300 K. The thermal quenching mechanism of the 4f–4f emissions of Sm2+ was examined. The position of the lowest level of Sm2+ 4f55d states was calculated from the temperature dependence of 5D1 lifetime by the two-step quenching model and was estimated in good approximation from the emission and excitation peaks by the electron–phonon coupling theory. A growing process of 5D0–7F0 emission and a double-decay process of 5d–4f emission were observed in the time-resolved fluorescence. They show clearly the population transfer among the Sm2+ excited states via thermal transition. [Copyright &y& Elsevier]

We report time-resolved second-order fluorescence between Stark levels of 1D 2 and 3H 4 in Pr3+:YPO 4 nanocrystals with different crystal phase. Both intensity ratios of transition peaks between Stark levels and fluorescence lifetimes are modulated by phase composition and dressing effect. We have found that the maximum excitation peak shifts from yellow to orange-red region with continuous phase transformation from hexagonal phase to tetragonal phase, which is attributed to different multi-phonon relaxation rate induced by O–H group in hexagonal phase. The fluorescence lifetimes and intensity ratios of splitting peaks in time domain have also been effectively modulated by choosing excitation frequencies to resonate with different Stark levels. Both wavelength and time tunable luminescence has potential applications in biological labeling, imaging and solid state display devices. • The time-resolved second-order fluorescence of Pr3+:YPO 4 nanocrystals. • High signal-to-noise ratio are obtained by dressing effect and choosing delay time. • The maximum emission peak shifts with continuous phase transformation. • The fluorescence lifetime is modulated by choosing resonate excitation frequency. [ABSTRACT FROM AUTHOR]