Modulation of the proton transfer rate by excitation photons

The effect of the exciting photon energy on the excited state proton transfer in a dye with dual fluorescence—FET (4′-diethylamino)-3-hydroxyflavone)—is studied. The steady-state fluorescence spectra are studied upon selective excitation by photons with different energies in the region of the main absorption band, as well as at its long-wavelength wing, in the temperature range of 2–30°C. It is found that, at all temperatures, the ratio of the integral emission of the normal and tautomeric forms, which are observed at 480 and 570 nm, respectively, depends on the excitation wavelength; namely, this ratio noticeably decreases with increasing excitation wavelength in the region of the main absorption band and its long-wavelength wing at 390–440 nm, and the rate of this decrease depends on temperature. In the same region, the long-wavelength excitation effect, which is atypical for inviscid solvents at room temperature, is observed; i.e., a short-wavelength emission band is bathochromically shifted by 6–15 nm depending on temperature. This spectral shift is directly related to the inhomogeneous broadening of the electronic spectra of the normal FET form, which is very large due to a considerable (>10 D) difference in the dye dipole moments. Most probably, the excitation creates the possibility of emission from nonrelaxed nonequilibrium orientational sublevels because their lifetime becomes shorter due to the proton transfer reaction, the rate of which in acetonitrile is comparable with the rate of intermolecular orientational relaxation. It is proposed to explain these dependences using energy diagram taking into account the dependence of the free energy on the orientational polarization of the solvent.     

Excitation-wavelength Dependent Fluorescence of Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

The excitation wavelength dependence of the steady-state and time-resolved emission spectra of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in tetrahydrofuran (THF) at room temperature has been examined. It is found that the ratio of the fluorescence intensity of the long-wavelength and short-wavelength fluorescence bands strongly depends on the excitation wavelength, whereas the wavelengths of the fluorescence excitation and fluorescence bands maxima are independent on the observation/excitation wavelengths. The dynamic Stokes shift of fluorophore in locally excited (LE) and intramolecular charge transfer (ICT) states has been studied with a time resolution about 30 ps. The difference between Stokes shift in the LE and ICT states was attributed to the solvent response to the large photoinduced dipole moment of EAADCy in the fluorescent charge transfer state. On this base we can state that, the relaxation of the polar solvent molecules around the fluorophore was observed.     

Modulation of charge transfer rate in (N,N′-dimethylamino)benzonitrile liquid solutions

We studied the properties of the emission, absorption and excitation of dual fluorescence of (N,N′?dimethylamino)benzonitrile in a polar aprotic solvent acetonitrile under selective irradiation of solutions by light with different energies of quanta to elucidate mechanisms of dual fluorescence arising in this solvent at different temperatures in the range 274–313 K. In all cases, dual fluorescence of the solute in acetonitrile was observed, which is caused by emission from locally excited Franck-Condon and charge-transfer states. A change in the energy of excitation quanta has a weak effect on the position of the fluorescence bands; however, the intensity ratio between the bands noticeably changes in favour of the intensity of the long-wavelength band at excitation in the range of the long-wavelength absorption band. An interesting and unusual fact is that solution heating is accompanied by essential growth of quantum yield of dual fluorescence at all wavelengths of the excitation. To explain the observed effects, the same dependences were measured and analysed for DMABN in neutral solvent n-hexane in the same conditions. We involve also the data of quantum-mechanical calculations, which show that there is a considerable probability of occurrence in solutions of DMABN rotational isomers with differing orientation of the dimethylamino group with respect to the benzonitrile. In the excited state, these have different charge-transfer rates, resulting in a modulation in the intensity ratio of the observed fluorescence bands with change excitation energy quanta on the red wing of the absorption band, doi: 10.1134/S0030400X12050219.     

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.     

Solvent Polarity and Excitation Wavelength Dependence of the Dual Fluorescence in N,N-Diethyl-4-Nitrosoaniline

Dual fluorescence in N,N-Diethyl-4-nitrosoaniline (DENA) has been studied employing absorption, excitation and emission spectroscopic techniques and computational methods. The absorption and fluorescence spectra of DENA were measured in solvents of various polarities at room temperature. The emission spectra of DENA were found to exhibit a single emission band in non polar solvent (cyclohexane) and in a highly polar solvent (acetonitrile). In the contrary, two emission bands were observed in medium polar solvents (tetrahydrofuran, 1,2-dichloroethane and dichloromethane) whereby the short (local excited; LE) and long (charge transfer; CT) emission maxima correspond to the emission maxima of the compound observed in cyclohexane and acetonitrile solutions, respectively. Moreover, the two emission bands have shown strong excitation wavelength dependence, and area normalization resulted in an iso-emissive point. The two emission maxima were in addition found to correspond to two excitation maxima in 3D fluorescence spectra. Further, two minima were obtained in potential energy surface calculation of DENA. From the experimental and computational results it was concluded that the dual fluorescence may be attributed to the presence of two different ground state structural conformers of DENA in equilibrium that are stabilized through solute-solvent interaction.     

Inhomogeneous spectral broadening and the decay kinetics of the luminescence spectra of prodan

The selectively excited steady-state luminescence spectra, as well as the decay time characteristics of the luminescence, of prodan in polar solvents excited by picosecond radiation are studied. The steady-state luminescence spectra exhibit a strong inhomogeneous broadening, which is the most pronounced at elevated solution viscosities. The temporal characteristics of the luminescence decay in different spectral ranges and the instantaneous spectra suggest the presence of relaxation processes resulting in a long-wavelength shift of the emission spectrum during the lifetime of the excited state. A relation between the relaxation shift of the emission spectrum and the intermolecular orientational relaxation of solvate molecules is established.     

Inhomogeneous Broadening of the Electronic Spectra of Laurdan

The excitation spectra and dependence of the luminescence spectra on the energy of excitation quanta were investigated for a laurdan molecule in glycerin. The most appreciable displacement of the emission spectra has been recorded for the most longwave luminescence band with a maximum in the region 500–510 nm. The relationship between the luminescence components of two emission bands with maxima at 425 and 500 nm depends strongly on the excitation energy. The dependences obtained are explained by the simultaneous existence of LE (localexcited) and TICT (twisted internal charge transfer) states and by the influence of the properties of the solvation sheath of laurdan, which invariably causes inhomogeneous broadening of the spectra of the polar solution molecules.     

An anomalous emission excited near the absorption edge of pyrene crystals

Emission spectra and decay times of the fluorescence excited at the absorption edge region in pyrene crystals were measured. At liquid nitrogen temperature, the fluorescence under the excitation at 390 nm is considered as the excimer emission and its band peak shifts a little to the short wavelength in comparison with that of the excimer emission under the excitation at 360 nm. The emission decay times under the excitation at the 390 nm and 360 nm are about 155 ns and 180 ns, respectively, at liquid nitrogen temperature. The former decay time changes its value abruptly near 127 K. This abrupt change of the decay time may be due to the phase transition in pyrene crystals.     

High Sensitivity,Quantitative Measurements of Polyphosphate Using a New DAPI-Based Approach

Polyphosphate (poly-P) is an important metabolite and signaling molecule in prokaryotes and eukaryotes. DAPI (4′,6-diamidino-2-phenylindole), a widely used fluorescent label for DNA, also interacts with polyphosphate. Binding of poly-P to DAPI, shifts its peak emission wavelength from 475 to 525 nm (excitation at 360 nm), allowing use of DAPI for detection of poly-P in vitro, and in live poly-P accumulating organisms. This approach, which relies on detection of a shift in fluorescence emission, allows use of DAPI only for qualitative detection of relatively high concentrations of poly-P, in the μg/ml range. Here, we report that long-wavelength excitation (≥400 nm) of the DAPI-poly-P complex provides a dramatic increase in the sensitivity of poly-P detection. Using excitation at 415 nm, fluorescence of the DAPI-poly-P complex can be detected at a higher wavelength (550 nm) for as little as 25 ng/ml of poly-P. Fluorescence emission from free DAPI and DAPI-DNA are minimal at this wavelength, making the DAPI-poly-P signal highly specific and essentially independent of the presence of DNA. In addition, we demonstrate the use of this protocol to measure the activity of poly-P hydrolyzing enzyme, polyphosphatase and demonstrate a similar signal from the mitochondrial region of cultured neurons.     

Complexation in two-component chlortetracycline-melanin solutions

The spectra and kinetics of fluorescence of two-component solutions of the chlortetracycline (CHTC)-DOPA-melanin (melanin or ME) system in water have been investigated. The data obtained have been compared to similar data for solutions of CHTC-melanosome from bull eye (MB), which contains natural melanin, in K-phosphate buffer at pH 7.4. The overall results indicate the occurrence of complexation between molecules of CHTC and ME as they are being excited. The studies of complexation in the solution of CHTC-MB in the buffer are complicated by the formation of a CHTC-buffer complex. The effect of optical radiation in the range 330–750 nm on the CHTC-ME complex shows selectivity: the greatest change in the spectrum occurs when the wavelength of the exciting radiation coincides with the long-wavelength band maximum of the fluorescence excitation spectrum of the CHTC-ME complex in aqueous solution. In this range, CHTC and especially ME show high photochemical stability. The nature of the radiation effect on the studied compounds in the hard UV range (λ < 330 nm) differs greatly from that in the range 330–750 nm. It is apparently accompanied by significant photochemical transmutations of all system components. By comparing the characteristics of the CHTC-ME systems with those of the related drug doxycycline (DC-ME), the conclusion has been made that the chlorine atom plays a vital role in formation of the short-wavelength band in the fluorescence spectrum of the CHTC-ME complex. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 52–62, January–February, 2008.     

Analysis of branched oligophenylene by absorption and fluorescence spectra

A branched oligophenylene has been synthesized based on 1,3,5-tri(4′-bromophenyl)benzene. Absorption and fluorescence spectra were studied and fluorescence quantum yields and lifetimes were measured for the compound in solution. It is demonstrated that the absorption spectrum is a superposition of p-quaterphenyl, p-terphenyl, and biphenyl chromophore absorption bands in a 1:2:1 ratio. The oligomer fluorescence spectrum is found to depend on the excitation wavelength. It is shown that the oligomer fluorescence is determined by two fluorochromic groups, namely fragments with branched p-terphenyl and p-quaterphenyl units. The main fluorescence maxima for these fluorochromic groups coincide with each other and lie in the vicinity of λ = 360 nm. A very weak fluorescence band found in the region 380–440 nm is excited by light with a wavelength lying beyond the oligomer self-absorption region. The reasons for a decrease in fluorescence quantum yields of branched models and the studied oligophenylene as compared with those of linear p-polyphenylene chromophores are discussed.     

Influence of excitation light wavelength on the photoluminescence properties for ZnO films prepared by magnetron sputtering

Highly orientated polycrystalline ZnO films were deposited on sapphire, silicon and quartz substrates at room temperature by r.f. magnetron sputtering. Different photoluminescence (PL) spectra were observed when excited with different wavelength light. A UV emission peak (356 nm) and a blue peak (446 nm) were generated for the films on sapphire, silicon and quartz substrates, and only the 446 nm blue emission appeared for the films on glass substrates when the wavelength of the excitation light was 270 nm. With increasing the wavelength of the excitation light up to 300 and 320 nm, the UV emission disappeared for films on various substrates and the wavelength of the PL peaks increased up to 488 and 516 nm, respectively. When the wavelength of the excitation light increased to 398 nm, the PL spectrum becomes a wide band that is consistent with three emission peaks.     

Dependence of the properties of the dual luminescence of 3-hydroxyflavone on the excitation wavelength

The luminescence and luminescence excitation spectra of 3-hydroxyflavone in acetonitrile obtained at different excitation/recording wavelengths are studied. The dependences of the position of the normal luminescence band maximum and of the intensity ratio of the normal and proton-transfer bands, on the excitation wavelength are found and studied for the first time. It is found that the spectral contour of dual luminescence also depends on the excitation wavelength and the blue emission band is cut off at a sufficiently long-wavelength excitation in the region of 390 nm. In the luminescence excitation spectrum, an additional wide band is observed in the proton-transfer region at 200–260 nm. Excitation in the region of 380–440 nm allowed us to reveal a wide structureless band near 470 nm (with the maximum of its excitation near 420 nm) belonging to the anionic form of 3-hydroxyflavone. Addition of water at a concentration of ～2.2 M quenches this band almost completely.     

The influence of excitation radiation parameters on photosensitized generation of singlet oxygen in water

Photosensitized generation of singlet oxygen with the aid of Radahlorin® photosensitizer has been investigated. The dependences of the intensity of singlet oxygen phosphorescence and photosensitizer fluorescence on the excitation radiation wavelength in the range of 350–440 nm and on the irradiation dose have been obtained. The dependence of the ratio of the sensitizer fluorescence intensity at about 670 nm to the singlet oxygen phosphorescence intensity at a wavelength of 1270 nm on the excitation radiation wavelength is found to be nonmonotonic and have a minimum near the center of the absorption band on its red wing. The results obtained can be used to monitor the singlet oxygen concentration in solutions.     

Investigation of excited-state proton transfer in 3-hydroxyflavone molecules

The dual fluorescence spectra of 3-hydroxyflavone molecules excited by electromagnetic radiation in the region of the S ₁ and S ₂ absorption bands in the temperature region of 20–80°C are studied using the dynamic quenching of the excited state. An analysis of the fluorescence parameters shows that heating the solution from room temperature to 60°C increases the proton transfer rate by a factor of 1.24 in the case of standard excitation into the main absorption band and even stronger (by a factor of 6.9) in the case of excitation into the second absorption band. The presence of a quencher reduces the yield of the two emission bands and noticeably increases the proton transfer rate, by a factor of 1.16 at room temperature and by a factor of 1.25 at 80°C. Upon excitation into the second singlet band, the transfer rate increases even more (especially at higher temperatures), by a factors of 1.24 and 3.5 for the same temperatures. The temperature dependences of the transfer rate constant allowed us to estimate the activation energies of the proton transfer reaction under different physical conditions and reach conclusions about the mechanism by which this reaction proceeds. It is found that the proton transfer activation energy decreases from 500 to 360 cm⁻¹ when measured in temperature ranges of 20–40 and 20–60°C. The introduction of a quencher with a concentration of 5 × 10⁻³ M increases the activation barrier to 534 and 471 cm⁻¹ in the same temperature ranges.     

Directed energy transfer due to orientational broadening of energy levels in photosynthetic pigment solutions

The directed non-radiative energy transfer through monomeric molecules of chlorophyll “a” and pheophytin “a” at high concentrations (c~10^-2 M) in a rigid matrix of polyvinylbutyral has been found by using the nanosecond laser spectrofluorimeter. The phenomenon is caused by orientational broadening of pigment molecular spectra owing to its interaction with a solvent. The observed temporal shift of the luminescence spectrum to the red region in a nanosecond time scale as well as the red shift of the time integrated spectrum at a high concentration of pigment molecules and the monotonic growth of the luminescence lifetime with a shift to the red region of the spectrum served as indications of the directed energy transfer in the sample. The non-radiative energy transfer from monomeric molecules towards aggregates is also directly demonstrated by the deformation of instantaneous luminescence spectra in the long-wavelength range (λ>700 nm). The role and the possibility of the directed energy transfer between molecules with orientationally broadened spectra in the biological systens are discussed.     

紫细菌捕光天线LH2中的超快光物理研究

采用飞秒泵浦探测技术研究了紫细菌外周捕光天线LH2中的超快光动力学过程.从B800蓝侧的激发态动力学中观察到B800到B850的能量传递时间,实验结果与理论计算结果的差别说明激发B800时可能引起B850上激子带的直接激发,或存在由B800到B850上激子态的能量传递通道.在B800红侧激发的动力学过程中,漂白信号前端存在的一个快速光吸收信号主要来源于B850上激子带的直接激发.在天然RS601和突变体GM309的LH2中,800 nm激发时的动力学过程都表现为一个类似的光漂白过程,动力学曲线的衰减时间常量在天然LH2中明显快于突变体中,说明在GM309中B800到B850的激发能传递速率有所降低.而在845 nm激发下两个样品中的快过程类似,但慢过程在GM309中有所增快,激发态中的能量重新分布包括逆向的能量传递也受到类胡萝卜素微结构的影响.     

多孔氧化铝薄膜的制备和光学特性研究

采用阳极氧化法制备了二维有序纳米孔氧化铝膜.研究了工艺参数对多孔薄膜有序性、孔径、膜厚度等的影响，测量了多孔氧化铝有序膜的光透过、光吸收和光发射等光学特性.结果表明，在波长360 nm附近多孔氧化铝有序膜的光透过谱线和光吸收谱线发生突变，波长大于360 nm时，光透过增强；波长小于360 nm时，光吸收增强.多孔氧化铝有序膜的光致发光强度和峰位与激发光波长有关，光致发光谱范围在340~600 nm.     

Origin of Tryptophan Fluorescence Lifetimes Part 1. Fluorescence Lifetimes Origin of Tryptophan Free in Solution

Fluorescence intensity decays of L-tryptophan free in polar, hydrophobic and mixture of polar-hydrophobic solvents were recorded along the emission spectrum (310–410 nm). Analysis of the data show that emission of tryptophan occurs with two lifetimes in 100 % polar and hydrophobic environments. The values of the two lifetimes are not the same in both environments while their populations (pre-exponentials values) are identical. Fluorescence lifetimes and pre-exponentials values do not change with the excitation wavelength and thus are independent of excitation energy. Our results indicate that tryptophan emission occurs from two specific sub-structures existing in the excited state. These sub-structures differ from those present in the ground states and characterize an internal property and/or organization of the tryptophan structure in the excited state. By sub-substructure, we mean here tryptophan backbone and its electronic cloud. In ethanol, three fluorescence lifetimes were measured; two lifetimes are very close to those observed in water (0.4–0.5 ns and 2–4 ns). Presence of a third lifetime for tryptophan in ethanol results from the interaction of both hydrophobic and hydrophilic dipoles or chemical functions of ethanol with the fluorophore.     

Fluorescence and picosecond induced absorption from the lowest singlet excited states of quercetin in solutions and polymer films

The spectroscopic and photophysical properties of the biologically important plant antioxidant quercetin in organic solvents, polymer films of polyvinyl alcohol, and a buffer solution at pH 7.0 are studied by stationary luminescence and femtosecond laser spectroscopy at room temperature and 77 K. The large magnitude of the dipole moment of the quercetin molecule in the excited Franck–Condon state μ _e ^FC = 52.8 C m indicates the dipolar nature of quercetin in this excited state. The transient induced absorption spectra S ₁→S _n in all solvents are characterized by a short-wave band at λ _abs ^max = 460 nm with exponential decay times in the range of 10.0–20.0 ps. In the entire spectral range at times of >100 ps, no residual induced absorption was observed that could be attributed to the triplet–triplet transitions Т ₁ → Т _k in quercetin. In polar solvents, two-band fluorescence was also recorded at room temperature, which is due to the luminescence of the initial enol form of quercetin (~415 nm) and its keto form with a transferred proton (550 nm). The short-wave band is absent in nonpolar 2-methyltetrahydrofuran (2-MTHF). The spectra of fluorescence and fluorescence excitation exhibit a low dependence on the wavelength of excitation and detection, which may be related to the solvation and conformational changes in the quercetin molecule. Decreasing the temperature of a glassy-like freezing quercetin solution in ethanol and 2-MTHF to 77 K leads to a strong increase in the intensity (by a factor of ~100) of both bands. The energy circuits for the proton transfer process are proposed depending on the polarity of the medium. The main channel for the exchange of electronic excitation energy in the quercetin molecule at room temperature is the internal conversion S ₁ ? S ₀, induced by the state with a proton transfer.