Intramolecular proton transfer from the highest singlet states in 3-hydroxyflavone

It is found that the excitation spectra of the dual fluorescence of 3-hydroxyflavone are different for different recording wavelengths and that the intensity ratio of the emission of the normal and tautomeric (with intramolecular proton transfer) forms upon selective UV excitation in the regions of the S ₁, S ₂, and S ₃ singlet absorption bands strongly depends on the excitation wavelength. The results obtained directly point to the existence of an additional channel of population of the excited state of the tautomeric form and are explained by the intramolecular proton transfer through the S ₂ and S ₃ excited singlet states of fluorophore molecules. The constants of this transfer are estimated using analytical relations for the steady-state fluorescence excitation.     

Studying reactions that proceed from highest excited states of molecules using fluorescence quenching

The steady-state monochromatic excitation of a luminophore that has fluorescing products is considered. The effect of dynamic quenching of highest excited states on the fluorescence of singlet states under its excitation via singlet S ₁ and S _n (n ≥ 2) states is discussed. It is shown that the use of the method of fluorescence dynamic quenching by foreign impurities opens new possibilities for studying photoreactions that proceed via S _n singlet states. A large number of primary photoprocesses are considered which include the electron density redistribution (the internal electron transfer) in the excited state, protolytic reactions, intramolecular proton transfer (phototautomerization), hydrogen bonding, and formation of excimers and exciplexes. It is shown that, upon dynamic quenching, the bimolecular quenching constant of an excited level depends on the amount of thermal energy released in the luminophore before the occurrence of the light emission event. Based on the experimental measurements of the fluorescence spectra at different quencher contents, the calculation of the Stern-Volmer constant for reaction products is considered in detail. It is shown that this constant can be most reliably determined from the dependence of the fluorescence intensity ratio of the initial reagents and the quencher product rather than from the dependence of the fluorescence intensity of the products on the concentration of the quencher. The relations determined are used in analysis of the experimental fluorescence spectra of solutions of 3-hydroxyflavone excited by radiation with different wavelengths lying in the range of the S ₁ and S ₂ absorption bands. The temperature behavior of the Stern-Volmer constant for different fluorescence bands of 3-hydroxyflavone is considered. It is shown that, if these constants for the normal and tautomeric forms are correctly determined, their temperature dependences are similar.     

Direct observation of proton transfer from the excited <Emphasis Type="Italic">S</Emphasis><Subscript>2</Subscript> state in the 3-hydroxyflavone molecule

The spectra of dual fluorescence of 3-hydroxyflavone molecules excited by 44-ps pulses in the region of the S ₁ and S ₂ absorption bands are measured with a picosecond resolution. The dynamics of the spectra directly demonstrates the time development of the proton transfer from the carboxyl to the carbonyl group of the molecule. Upon excitation into the main absorption band, the transfer process occurs for about 210 ps. The excitation into the region of the S ₂ band results in a faster (～170 ps) process, and the relative contribution made to the total spectrum by the long-wavelength band, which belongs to the proton-transfer state, is higher in this case for all the time ranges of luminescence recording. The data obtained directly point to an additional channel of proton transfer via the S ₂ state. The probability of this process is estimated to be 0.84 × 10¹² s^?1.     

Fluorescence quenching of vaporous polycyclic aromatic hydrocarbons by oxygen

The fluorescence quenching by oxygen of vapors of nine polycyclic aromatic hydrocarbons with strongly different oxidation potentials 0.44 eV < E _ox < 1.61 eV (anthracene, 9-methylanthracene, 2-aminoanthracene, 9,10-dibromanthracene, pyrene, chrysene, phenanthrene, fluoranthene, and carbazole) is studied. From the dependences of the fluorescence decay rates and intensities on the oxygen pressure P _O2, the quenching rate constants k _S ^O2 for the excited singlet states S ₁ and the fraction f _S ^O2 of the S ₁ states quenched by oxygen are estimated. At P _O2 = 5 Torr, the k _S ^O2 constants vary from 1.2 × 10⁷ to 3.0 × 10⁵ s^?1 Torr^?1, while the fraction of the quenched excited singlet states changes from 0.1 (fluoranthene) to 0.7 (chrysene) and 0.8 (pyrene). The dependences of k _S ^O2 on the photophysical and electron-donor characteristics of the fluorescing compounds are analyzed. It is shown that, in the gas phase of anthracene and its derivatives, the magnitudes of k _S ^O2 are limited by the rate constants of gas-kinetic collisions k _gk and do not depend on the electron-donor characteristics of fluorophores, while the fraction of quenched states f _S ^O2 changes with the oxidation potential. For compounds with k _S ^O2 < k gk, both the rate constants k _S ^O2 and the fraction of quenched states f _S ^O2 depend on the E _ox of sensitizers, which demonstrates an important role played by the charge-transfer interactions in quenching of the S ₁ states. The dependence of the rate constants k _S ^O2 on the free energy of electron transfer ΔG _et is considered.     

A theoretical investigation of the effect of water on the structure and fluorescence spectra of L-tryptophan

Fluorescence spectra of two long-wavelength electron transitions S₀ ← ¹L_b and S₀ ← ¹L_a in uncharged and zwitterionic forms of L-tryptophan (Trp) in aqueous solution and in the complex of Trp with water molecule were calculated using the Frank–Condon approximation. Geometric parameters of Trp in electronically excited states were determined, and the vibrational structure of vibronic spectra was analyzed. It was shown that the relative position of structural fragments of alanine (R-Ala) and indole (R-In) could have a determining effect on the fluorescence and formation of the vibrational structure of electronic spectra. The increase of the rotation angle between the R-Ala and R-In, which depends on the Trp environment, results in the Trp fluorescence originating only from the singlet excited state ¹L_a.     

Determination of the stern-volmer constant in quantum systems with dual fluorescence

The mathematical relations describing the properties of the steady-state spontaneous emission of quantum systems with dual fluorescence under conditions of dynamic quenching of excited states by foreign impurities are analyzed. The direct dependence of the intensity and yield of the photoproduct fluorescence on the quencher concentration is not simple and cannot serve as a convenient base for determining the Stern-Volmer constant. It is shown that, in the case of a kinetic character of product formation, the fluorescence intensity ratio of the initial dye and its photoproduct linearly increases with the quencher concentration. The relation obtained can be used to determine the constant of bimolecular quenching of the excited states of reaction products. This conclusion is based on the analysis of the experimental fluorescence spectra of 3-hydroxyflavone, obtained upon excitation in the region of the S ₁ absorption band under conditions of dynamic quenching by potassium iodide. This analysis can be applied to a wide range of luminophores with photoreactions accompanied by dual fluorescence (charge transfer, proton transfer, phosphorescence, complexation, etc.).     

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.     

Retardation of the relaxation over quantum-well energy levels in CdSe/ZnS quantum dots with increasing number of excited carriers

The differential transmission spectra of CdSe/ZnS quantum dots are investigated. It is revealed that the differential transmission spectra measured upon resonant excitation of electrons into the first excited state 1P(e) exhibit a number of specific features, such as a decrease in transmission at the pump frequency, bleaching in the course of the pump pulse at frequencies corresponding to the fundamental optical transition 1S _3/2(h)-1S(e) and transitions between excited hole states and the 1S(e) electron ground state, and retardation of this process with an increase in the energy of the pump pulse. The observed specific features can be explained by the following factors: (i) the absence of a “phonon bottleneck” for electrons due to the energy transfer from hot electrons to rapidly relaxing holes, (ii) relaxation through intermediate quantum-well energy levels of holes, and (iii) retardation of relaxation with increasing number of excited charge carriers in a quantum dot.     

Probabilities of radiative dipole transitions of parahelium in an electric field

Quadratic Stark corrections to the wave functions, matrix elements, and probabilities of transitions between the singlet states ¹ S ₀ and ¹ P ₁ of helium atoms are calculated. The coefficients of the polynomials that depend on the effective principal quantum number of the upper level v _f and that approximate the numerical values of the polarizabilities, the quadratic corrections to the wave functions, and the probabilities of transitions to highly excited Rydberg states with large v _f are determined. The results of calculations testify that the probabilities of all σ transitions n _i ¹ S ₀ → n _f ¹ P ₁ and π transitions to the states with n _f > n _i /2 are decreased with increasing electric field strength, except for the transition 2¹ S ₀ → 2¹ P ₁, whose probability increases both for σ and for π transitions.     

On estimating probability of excited-state intramolecular proton transfer

Higher singlet states can play an important role in various intramolecular processes. Recent investigations of the time-resolved (with a picosecond resolution) spectra of the dual fluorescence of 3-hydroxyflavone molecules excited in the region of the S ₁ and S ₂ absorption bands by pulses with a duration of ∼44 ps have directly shown the occurrence of the proton transfer from the carboxyl to the carbonyl group of the molecule upon excitation into the second singlet absorption band. The reaction times estimated from the emission characteristics are comparable with the electronic level lifetime (several picoseconds), as a result of which the direct measurements are rather difficult. The proton transfer through the S ₂ state is also recorded in the steady-state fluorescence excitation spectra. In this study, it is shown how the reaction rate can be estimated from these data.     

On the mechanisms of formation of excited hydrogen atoms in a decaying helium-hydrogen plasma

We spectroscopically studied the population of the excited hydrogen atomic states with the principal quantum numbers n=3 and 4 in a decaying plasma produced by a pulsed discharge in a mixture of helium (p=40.4 Torr) with a small amount of hydrogen ([H₂]≈10¹² cm^?3). Experiments on recording the response of the spectral line intensities to a short-duration electron temperature perturbation revealed the contribution of electron-ion recombination to the population of the H*(n=3) states in the early afterglow. The ions produced by collisions of hydrogen molecules with metastable He(2³ S ₁) atoms, whose density decreases relatively rapidly with time in the decaying plasma, were assumed to be involved in this process. No population of the H*(n=4) atomic levels due to electron-ion recombination was found. Our experimental results are consistent with the conclusions of previous studies that excitation transfer during collisions of metastable helium molecules with hydrogen molecules plays a major role in the population of the excited hydrogen atomic states both with n=3 and with n=4 during most of the afterglow.     

Spin-selective low temperature spectroscopy on single molecules with a triplet-triplet optical transition: Application to the NV defect center in diamond

The spin-selective photokinetics of a single matrix-isolated impurity molecule with a triplet-triplet optical transition, T ₀–T ₁, is considered and the manifestations of the photokinetics in the fluorescence excitation spectra and intensity autocorrelation functions g ⁽²⁾(τ) of the molecule undergoing narrow-band optical excitation is studied to resolve the fine structure of the transition. The rates of intersystem crossings (ISCs) T ₁→S→T ₀ to and from a nonradiating singlet state S of the molecule and the rate of population relaxation among the ground (T ₀) state sublevels can be obtained from the spectra and g ⁽²⁾(τ) using the analytical expressions obtained. New experiments on an individual NV defect center in nanocrystals of diamond, where, for the first time, the fine structure of its triplet-triplet ³ A-³ E zero-phonon optical transition (~637 nm) at 1.4 K was resolved, are interpreted. It is concluded that the rate of the ISC transition from the m _S =0 sublevel of the excited ³ E state to the singlet ¹ A state (~1 kHz) is much slower than the rates from the m _S =±1 substates, while the rates of ISC transitions to different m _S substates of the ground ³ A state are close to each other (~1 Hz). As a result, only the optical transition between m _S =0 sublevels in the ³ A-³ E manifold contributes strongly to the fluorescence. The experimentally observed double-exponential decay of the g ⁽²⁾(τ) function is explained by the two pathways available to the center for it to leave the S state: (i) the S→ T ₀(m _S )=0) transition and (ii) the S→T ⁰(m _S =±1) transitions followed by the slow spin-lattice relaxation T ₀(m _S =±1)→T ₀(m _S =0) (rate ~0.1 Hz). The work is important for studies where the NV center is used as a single photon source or for quantum information processing.     

Effect of temperature and dynamic fluorescence quenching on proton transfer in 3-hydroxyflavone

The effect of temperature and a fluorescence quencher on the properties of the excited states of 3-hydroxyflavone is considered. The absorption spectra and the spectra of dual fluorescence exited by electromagnetic radiation in the region of the S ₁ absorption band of 3-hydroxyflavone in acetonitrile are measured and analyzed in the temperature range of 20–80°C. The fluorescence lifetimes are also measured at different temperatures. As a quencher of excited states we used the TEMPO spin quencher. The analysis of the fluorescence parameters shows that the heating of the solution to 60°C leads to a considerable (by a factor of 1.24) increase in the proton-transfer rate for the first absorption band. The introduction of a quencher decreases the yield of the two fluorescence bands by the diffusion mechanism and increases the proton-transfer rate with respect to the rate in the pure solvent by a factor of 1.16 at room temperature and 1.65 at T = 80°C.     

Spectroscopy of charge transfer states in Mg<Subscript>1 – <Emphasis Type="Italic">x</Emphasis> </Subscript>Ni<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>O

Photoluminescence and photoluminescence excitation spectra of solid solution Mg_1–xNi_xO (x = 0.008) have been analyzed. The contributions of charge transfer electronic states and nonradiative Auger relaxation to the formation of the photoluminescence spectrum are discussed.     

Excitons in monoclinic zinc diphosphide: The <Emphasis Type="Italic">A</Emphasis>-exciton series and Fano effect

The A-exciton series in the absorption spectra of β-ZnP₂ monoclinic zinc diphosphide samples is investigated at different directions of the wave vector and different polarization states of radiation. It is shown that the oscillator strengths determined for the observed transitions are adequately described by the relationship F _n ∝n^?3 characteristic of S-type exciton states. The assumption is made that the A-exciton series is associated with the partially allowed dipole transitions to nS states of the orthoexciton with Γ ₂ ^? (x) symmetry at m _s =0. These states are mixed, to a first approximation, with nS states of the Γ ₂ ^? (z) singlet exciton due to the spin-orbit 2 interaction and are split off by the long-range (nonanalytical) part of the exchange interaction. The Fano antiresonances arise in the absorption spectra at resonances of the A-exciton series when the radiation vector E (or the induction vector D) has a component along the crystallographic axis c. These antiresonances are induced by the configurational interaction of discrete exciton states of the A series with the continuum of the exciton-phonon spectrum due to indirect transitions to the 1S band of the singlet exciton with phonon emission.     

Precision measurements of optical excitation functions for the mercury atom

We describe a computer-based facility for studying the excitation of atoms by ultramonochromatic electrons and give optical excitation functions for the 12 mercury spectral lines that originate from the n ¹ S ₀, n ¹ P ₁, n ¹ D ₂, n ³ S ₁, n ³ P _j , and n ³ D _j levels. We detected about 100 features in the energy dependences measured from the excitation threshold to 15.5 eV. The previously found positions of the features on the energy scale are in good agreement with our results. Most of the resonant features are shown to be mainly attributable to the decay of short-lived states of the negative mercury ion. We detected a postcollision interaction effect in the optical excitation functions of the lines that originate from the n ¹ S ₀ levels at energies of about 11 eV.     

A nonradiative intersystem crossing <Emphasis Type="Italic">S</Emphasis>(ππ*) ↝ <Emphasis Type="Italic">T</Emphasis>(ππ*) transition in dibenzo-<Emphasis Type="Italic">p</Emphasis>-dioxin

The probability of the nonradiative S-T intersystem crossing in dibenzo-p-dioxin is theoretically studied using a model for the vibronically induced spin-orbit coupling between electronic states and taking into account all out-of-plane vibrational modes. Several symmetry variants for the lowest S ₁(ππ*) singlet state are considered. In the case of g symmetry of this state, a provision is made for the possibility of its vibronic coupling with the nearest dipole-active singlet ¹ B _2u ππ* state. The rate constants K _ST of the S ₁ ? T(ππ*) transitions to the T ₁(³ B _3g ) state are estimated taking into account several intermediate triplet T _m (ππ*) states of g and u symmetry. For different symmetry types of the S ₁ state, the effect of K _ST on the fluorescence quantum yield ?_fl is discussed. The ¹ B _3g symmetry state is found to be the lowest S ₁ state. It is found that the main contribution to K _ST is made by the S ₁(¹ B _3g ) ? T ₄(³ A _g ) transition.     

Spin-orbit interactions in heterocyclic analogues of fluorene

For carbazole, dibenzofuran, and dibenzothiophene—heterocyclic analogues of fluorene containing N-H, O, and S groups, respectively—the transition dipole moments P ₀₀ ⁱ for the transitions ³ B ₂→S ₀ and ³ A ₁→S ₀ from the sublevels i=z, y, x of the triplet electronic ππ* states, which are caused by intramolecular spin-orbit (SO) interactions, are calculated. The effect that the SO coupling between the S ₀ state and highest triplet states has on the calculation results is considered. The effects exerted on the value of P ₀₀ ⁱ by such specific features of the molecular structure as the position of a heteroatom on the symmetry axis, its valence, and different constants of SO coupling in heteroatoms are discussed. The reason for the weak influence of the quantity ?_HA on the rate constant of radiative deactivation of the lowest T state is ascertained.     

Excitation of phosphorescence of pyrene implanted into a photoconductive polymer

Methods of populating the lowest excited triplet state T ₁ of pyrene implanted into polystyrene (PS) or photoconductive polymer materials polymethylphenylsilane (PMPS), poly-N-epoxypropylcarbazole (PEPC), and poly-N-vinylcarbazole (PVK) are studied. Photoluminescence was excited in the first electron transition band of pyrene and the above photoconductive polymers. It is found that, at concentrations of 0.05–0.50 mol l^?1, pyrene is effective in quenching the fluorescence of PMPS, PEPC, and PVK but has only a slight effect on the photoemission efficiency of geminate electron-hole pairs. As a result, the phosphorescence of pyrene in photoconductive polymers is excited both by the intersystem crossing from the first excited singlet state (S ₁ ? T ₁) and by the capture of triplet excitons created in the recombination of charge pairs. In addition, in PEPC and PVK, the phosphorescence of pyrene is excited by recombination of a captured hole with an electron. For this reason, the ratio of the quantum yields of phosphorescence and fluorescence of pyrene in photoconductive polymers is much larger than that in PS, wherein the T ₁ state of pyrene is populated by intersystem crossing S ₁ ? T ₁ only.