Fluorescence-quenching studies and temperature dependence of fluorescence quantum yield, decay time and intersystem crossing activation energy of TPB

Fluorescence quenching of 1, 1, 4, 4-tetraphenyl-1, 3-butadiene (TPB) by aniline has been carried out at room temperature (298 K) to understand the role of quenching mechanisms. The study has been carried out by both steady state (in different solvents) and by transient method (in cyclohexane). The Stern-Volmer plot has been found to be linear for all the solvents studied. The probability of quenching per encounter ‘p’ is determined in all the solvents and is found to be less than unity. It is found that, the activation energy E_a (E′_a) is greater than the activation energy of diffusion, E_d. The results obtained by the transient method infer that the thermally assisted intersystem crossing, a non-radiative deactivation process from S₁ to T₂ is responsible for observed decrease in quantum yield and lifetime. Hence, from both the methods it can be concluded that quenching mechanism is not solely due to the material diffusion, but there is also contribution from the activation energy.     

Use of the SPP scale for the analysis of molecular systems with dual emissions resulting from the solvent polarity

The solvent polarity/polarizability (SPP) scale is shown to be highly useful for analyzing the solvatochromic behavior of molecular systems emitting dual fluorescence by virtue of the solvent polarity. Stokes' shift data are presented on a test set of seven solutes in 58 solvents correlating with the SPP (single parameter polarity/polarizability index). SPP values for seven further solvents are given, extending the previously available list.     

Influence of interaction of chromophores, linked by the unconjugated chain, on the luminescence properties of biscyanine dyes

Deactivation of electronically excited state of chemically bound dimers – biscyanines with two chromophores linked by unconjugated chains and corresponding monomer dye was investigated. It was found that the quantum yield of dimer fluorescence is lower than that of a monomer dye. The amount of quantum yield of fluorescence decreases with the increase of chromophores interaction degree (the decrease of the isolating polymethylene bridge length). It is shown on the basis of the external heavy-atom effect studies that the decrease of biscyanines fluorescence ability is connected with the enhancment of singlet–triplet intersystem crossing (S₁T₁). It has been established that the probability of the triplet states population in biscyanines with chromophores in parallel arrangement is considerably higher than that in similar compounds with an angular arrangement of chromophores. The delayed fluorescence was observed in the case of dyes with parallel arrangment of chromophores.     

Nature of dual fluorescence in 2-(quinolin-2-yl)-3-hydroxychromone: Tuning between concurrent H-bond directions and ESIPT pathways

Synthesis of the quinoline-substituted 3-hydroxychromone is reported, spectral and fluorescent properties of the title compound are investigated in different solvents. The question of the possibility of the participation of the substituent heterocycle’s nitrogen atom in H-bond formation as well as in the ESIPT process is discussed. Quantum-chemical modeling provides evidence that the structure with an alternative H-bond to nitrogen is more energetically favorable and strongly prevails in solutions. However, our calculations together with the experimental data reveal that the excited species with the intramolecular N…HO hydrogen bond type undergoes rapid radiationless deactivation. This leads to a conclusion that the low-intensity dual-band fluorescence of the title compound in solutions originates from the traces of the conformation with the traditional “flavonol-like” H-bond to the carbonyl oxygen atom. A possibility to reversibly tune the direction of the intramolecular H-bond formation and thus regulate the ESIPT process pathway is demonstrated by the example of protonation/deprotonation of the title compound in a methylene chloride solution.     

Excited state intramolecular proton transfer in amino 2-(2′-hydroxyphenyl)benzazole derivatives: Effects of the solvent and the amino group position

The excited-state intramolecular proton transfer (ESIPT) mechanism in six amino 2-(2′-hydroxyphenyl)benzazole derivatives were investigated in different solvents by means of UV-vis absorption and steady-state fluorescence. The amino benzazoles are fluorescent in the blue-orange region under UV radiation. Changes in the absorption, emission and excitation spectra were analyzed and correlated to the position of the amino group and the solvent polarity. The equilibrium between the conformers in solution in the ground state, confirmed by the solvatochromic effect, reflects the dual fluorescence emission presented by these dyes.     

界面电子转移对量子点荧光闪烁行为的影响

利用激光扫描共聚焦显微系统分别测量了CdSe/ZnS量子点在SiO2玻片表面、铟锡氧化物(ITO)纳米粒子表面和聚甲基丙烯酸甲酯(PMMA)薄膜表面上的荧光闪烁行为.研究发现,不同界面环境中量子点的亮态发光持续时间的概率密度都服从指数修正的幂律分布P(t)∝t-αexp(-t/μ).与处于SiO2玻片表面的情况相比,在ITO表面上的单量子点具有非常短暂的亮态发光持续时间,而在PMMA表面的单量子点亮态发光持续时间最长.这种荧光闪烁行为的不同主要归因于量子点与三种材料之间的界面电子转移特性.     

温度对半花菁LB多层膜的光致荧光特性的影响

利用稳态荧光谱和时间分辨荧光技术研究了温度对半花菁Langmuir-Blodgett(LB)多层膜光致荧光特性的影响.半花菁分子在半花菁/花生酸、半花菁/花生酸镉交替及纯半花菁Z-型LB多层膜中均形成了H-聚集体,加热能使聚集体部分离解.由于Z-型膜中没有花生酸或花生酸镉的屏蔽,分子间具有较强的偶极相互作用,加热使聚集体的离解的程度较小.镉离子的加入对半花菁LB多层膜的稳态荧光谱也有一定的影响.     

Influence of lipid composition and membrane curvature on fluorescence and solvent relaxation kinetics in unilamellar vesicles

Time-resolved fluorescence on unilamellar vesicles shows that increasing amounts of anionic, natural lipid lead to a larger increase in polarity close to the headgroups than in the hydrophobic core of the bilayer. The region close to the headgroups is less polar in vesicles containing phosphatic acid rather than phosphatidylserine. A greater membrane curvature increases the mobility of the hydrated headgroups.     

Spectroscopic study on the 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine and its metal complexes

Multitopic ligand, 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine (pyterpy), has attracted growing attention because of its unique structural features, optical and electrochemical properties. Here, we report spectroscopic studies of pyterpy and its metal complexes in methanol solution. For the pure pyterpy, the ligand emission intensity increased with its concentration in the dilute solution, but decreased when its concentration was over 1.3×10⁻⁵ mol/l due to the concentration quenching. No significant influence on the ligand luminescence was observed for the Zn²⁺-pyterpy complex but strong luminescence quenching was observed for the electroactive Fe²⁺- and Co²⁺-pyterpy complexes. The lanthanide (Sm³⁺, Eu³⁺ and Tb³⁺) complexes of the pyterpy showed both ligand and lanthanide ion emissions, especially for the Tb³⁺-pyterpy complex, suggesting that the excited energy of pyterpy ligand could be efficiently transferred to the central Tb³⁺ ions. The luminescence was pH sensitive with the strongest emission in the neutral solution. The results indicated that the multitopic ligand of pyterpy could not only act as linkers for the metal-directed building blocks, but also act as optical materials with its own emission at about 364 nm and as light antenna for the lanthanide ions.     

Influence of intramolecular hydrogen bond formation sites on fluorescence mechanism

The fluorescence mechanism of HBT-HBZ is investigated in this work. A fluorescent probe is used to detect HClO content in living cells and tap water, and its structure after oxidation by HClO (HBT-ClO) is discussed based on the density functional theory (DFT) and time-dependent density functional theory (TDDFT). At the same time, the influence of the probe conformation and the proton transfer site within the excited state molecule on the fluorescence mechanism are revealed. Combined with infrared vibrational spectra and atoms-in-molecules theory, the strength of intramolecular hydrogen bonds in HBT-HBZ and HBT-ClO and their isomers are demonstrated qualitatively. The relationship between the strength of intramolecular hydrogen bonds and dipole moments is discussed. The potential energy curves demonstrate the feasibility of intramolecular proton transfer. The weak fluorescence phenomenon of HBT-HBZ in solution is quantitatively explained by analyzing the frontier molecular orbital and hole electron caused by charge separation. Moreover, when strong cyan fluorescence occurs in solution, the corresponding molecular structure should be HBT-ClO(T). The influence of the intramolecular hydrogen bond formation site on the molecule as a whole is also investigated by electrostatic potential analysis.     

Effect of graded triple delta-doped sheets on the performance of GaAs based dual channel pseudomorphic high electron mobility transistors

The characteristics of InGaP/InGaAs/GaAs dual channel pseudomorphic high electron mobility transistors (DCPHEMTs) with different graded triple delta-doped sheets are investigated and experimentally demonstrated. Based on a two-dimensional simulator of ATLAS, the band diagrams, electron densities and DC characteristics of studied devices are comprehensively analyzed. Due to the use of properly graded triple delta-doped sheets, good pinch-off and saturation characteristics, improved transport properties and wide current swing are obtained. For comparison, a practical DCPHEMT with good device performances is fabricated as well. It is found that the simulated data are in good agreement with experimental results.     

White luminescence of a system based on poly(9,9-di(ethylhexyl)fluorene) activated by fluorescent dyes

Luminescence has been studied for poly(9,9-di(ethylhexyl)fluorene) containing laser dyes which fluoresce in the green and red spectral regions. It has been shown that white luminescence can be obtained as a result of singlet-singlet energy transfer from the polymer matrix to the dyes at certain concentrations of the latter. Two systems are proposed that make it possible to obtain equi-energy white emission and white emission, taking into account the characteristics of the spectral sensitivity of the human eye. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 37–41, January–February, 2007.     

Micellar effects on the molecular aggregation and fluorescence properties of benzazole-derived push-pull butadienes

The absorption and fluorescence spectral behaviour of 1-(2-benzoxazolyl)-4-(p-dimethylaminophenyl)buta-1,3-diene and its benzothiazolyl analogue (abbreviated as BODB and BTDB, respectively) have been investigated in dioxane-water mixtures and micellar environments using steady-state techniques. In water, water-rich mixtures or premicellar solutions, BODB and BTDB tend to form molecular aggregates labelled as H-aggregates. These aggregates dissociate on adding surfactants forming micellized monomers. In all micellar media the fluorescence quantum yield is greatly enhanced along with a large hypsochromic shift. Also, in TX-100, CTAB and SDS both dienes show dual emission from the locally excited (LE) and intramolecular charge transfer (ICT) states. For BTDB in TX-100 and DODB in all solutions, the LE fluorescence predominates, while for BTDB in CTAB and SDS, the ICT fluorescence is the predominant. The fluorescence shifts suggest that the fluorescing molecules penetrate the core of the micellar unit in TX-100, whereas in CTAB and SDS they occupy the interfacial region. The binding constants and the micelle properties (such as polarity and CMC) have been determined using both dienes as probes.     

Investigation on fluorescence quenching of dyes by graphite oxide and graphene

Rhodamine B (Rh B), eosin (E) and methylene blue (MB) were used as a probe to investigate the molecular structure and charge of the dyes on the sensitized efficiency of graphite oxide (GO) and graphene (G). The structure of the prepared GO and G were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. To study the electron transfer between dyes and GO or G, UV-vis absorption spectra (UV-vis), steady state fluorescence spectra (FL) and time resolved fluorescence spectra have been determined. It has been found that the electron transfer from the excited dyes to G was more efficient than to GO, and the transfer from excited MB to G was easier than to Rh B and E, because of the different electrostatic attraction between the dye and G.     

Spectral fluorescence and polarization characteristics of <Emphasis Type="Italic">Z,Z</Emphasis>-bilirubin IXα

We have studied the spectral fluorescence and polarization characteristics of Z,Z-bilirubin IXα, at room temperature in chloroform and in aqueous buffer medium, within an equilibrium complex with human serum albumin (HSA), and also under low temperature conditions (T = −100°C) in isobutyl alcohol. We have observed a bathochromic shift of the fluorescence spectra, which is most pronounced for the bilirubin-albumin complex. The following are considered as possible reasons for the observed dependence of the position of the fluorescence (fluorescence excitation) spectra on the excitation (detection) wavelength: structural and spectral differences between the chromophores making up the bilirubin molecule; conformational heterogeneity of the pigment in solution; a contribution to the fluorescence from molecules which have not completed the vibrational relaxation process; inhomogeneous orientational broadening of the levels; heterogeneity of the microenvironment of the chromophores in the protein matrix. We show that polarized fluorescence of bilirubin occurs at room temperature, due to the anomalously short fluorescence lifetime τ (picosecond or subpicosecond ranges). Despite such a short τ, the absorption and emission polarization spectra suggest the presence of intramolecular nonradiative singlet-singlet energy transfer when bilirubin is excited to high vibrational sublevels of the S₁ state (degree of polarization p = 0.11–0.12). When fluorescence is excited on the long-wavelength slope of the absorption band, no transfer occurs: the degree of polarization (p = 0.46−0.47) is close to the limiting value (p = 0.50). We discuss the question of the role played by exciton interactions between chromophores in the bilirubin molecule when it is excited. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 108–119, January–February, 2007.     

Effect of the polymethine chain length, the polarity and temperature of the medium on the spectroscopic properties of merocyanine dyes

We have studied the luminescent spectral properties and calculated the quadratic polarizability of dimethine, tetramethine, and hexamethine merocyanines based on 1,3-dihydro-1,3,3-trimethyl-(2H)-indol-2-ylidene and malonodinitrile in solutions and polymer films at room temperature and 77 K. We have shown that increasing the polarity of the solvent and lengthening the polymethine chain in the studied merocyanines lead to an increase in fluorescence quantum yields and narrowing of the fluorescence bands. This is explained by the decrease in bond length alternation and weakening of vibronic interactions in the singlet-excited fluorescent state. The mirror symmetry in the structured absorption and fluorescence spectra of merocyanine dyes in ethanol at 77 K, observed here for the first time, is due to enhancement of electrostatic interactions of the merocyanine and solvent molecules. Increased stiffness of the medium considerably decreases the probability of nonradiative processes, and the major channel for deactivation of the electronic excitation energy at 77 K is fluorescence. The higher values of the quadratic polarizability (6.0·10⁻²⁸ esu in ethanol and 3.3·10⁻²⁸ esu in chloroform) for hexamethine merocyanine, estimated by the solvatochromic method, lets us consider this compound as a promising material for designing nonlinear optical converters for harmonics of laser radiation. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 26–35, January–February, 2006.     

Absorptivity and fluorescence properties of 9,10-diiodoanthracene in solutions

The molar extinction coefficient, oscillator strength, natural fluorescence lifetime, and fluorescence quantum yield have been determined for 9,10-diiodoanthracene in ethanol at 20°C. The temperature effect on the quantum yield was studied in the range 120–300 K. The fluorescence lifetime was measured at 77 K. During glassification of ethanol, the fluorescence intensity of 9,10-diiodoanthracene increases by more than 50 times due to the activation nature of the intersystem crossing from the S₁(¹B_2u⁺) state. The activation energy and pre-exponential factor for the probability of the intersystem {ie319-01}-crossing are found. The long-wavelength shift of the absorption spectrum with increasing bulk polarizability of the solvent is interpreted as evidence of changes in the nonspecific interactions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 304–308, May–June, 2008.     

Influence of ultrasonic amplitude,temperature, time and solvent concentration on bioactive compounds extraction from propolis

An ultrasound assisted method was investigated to extract bioactive compounds from propolis. This method was based on a simple ultrasound treatment using ethanol as an extraction medium to facilitate the disruption of the propolis cells. Four different variables were chosen for determining the influence on the extraction efficiency: ultrasonic amplitude, ethanol concentration, temperature and time; the variables were selected by Box-Behnken design experiments. These parameters were optimised in order to obtain the highest yield, and the results exhibited the optimum conditions for achieving the goal as 100% amplitude of ultrasonic treatment, 70% solvent concentration, 58 °C and 30 min. The extraction yield under modified optimum extraction conditions was, as follows: 459.92 mg GAE/g of TPC, 220.62 mg QE/g of TFC and 1.95% of balsam content. The results showed that the ultrasound assisted extraction was suitable for bioactive compounds extraction from propolis. The most abundant phenolic compound was kaempferol (228.8 mg/g propolis) followed by myricetin (115.5 mg/g propolis), luteolin (27.2 mg/g propolis) and quercetin (25.2 mg/g propolis).     

Deactivation of an excited charge-transfer complex I. Influence of solvent polarity on radiative and radiationless transitions

The influence of solvent polarity on the fluorescence of charge-transfer complexes of pyromellitic dianhydride with some methylated benzenes has been investigated. Quantum yields and decay times of fluorescence, in mixed solvents of different polarities, have been measured. It was shown that with increasing solvent polarity the quantum yield of fluorescence is mainly decreasing owing to the decrease of the radiative transition probability k_f is due to the increasing degree of solvent-induced changes in the electronic and geometrical structure of the equilibrium excited state and increasing solvation of the components of the complex. Emissive properties of charge-transfer complexes are compared with emissive properties of heteroexcimers.     

Absorption,fluorescence, and fluorescence excitation spectra of free molecules of indole and its derivatives

We have obtained and analyzed the absorption, fluorescence, and fluorescence excitation spectra of indole vapor, N-acetyl-L-tryptophan vapor, and 3-indole aldehyde vapor. From analysis of the dependence of the fluorescence spectrum of the free indole molecules on the wavelength of the exciting radiation λ_ex, it follows that emission of fluorescence occurs when the molecules undergo a transition from the one electronically excited state ¹L_b. The fluorescence spectra of the studied compounds are insignificantly different, suggesting a major role for the indole chromophore in formation of the compounds. The absorption spectrum of N-acetyl-L-tryptophan, in which the group of atoms is added to the indole ring through a-C-C bond, is similar to the spectrum of indole, while the spectrum of 3-indole aldehyde is significantly different from the indole spectrum due to the effect of the C=O group conjugated with the indole ring. The fluorescence excitation spectra are considerably different from the absorption spectra. This is associated with the strong dependence of the quantum yield for the free molecules on λ_ex. Qualitatively, they are mirror-symmetric to the fluorescence spectra of the stodied compounds. Analysis of the data obtained provides a basis for assuming that in the case of free molecules of indole and its derivatives, the ¹L_a absorption in the extreme long-wavelength region of the spectrum does not overlap ¹L_b absorption. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 218–222, March–April, 2007.