Photophysical properties, laser activity and photoreactivity of a heteroaryl chalcone: A model of solvatochromic fluorophore

The absorption and fluorescence characteristics of 3-(4′-dimethylaminophenyl)-1-(2-thienyl)prop-2-en-1-one (DMATP) have been investigated in different solvents. DMATP dye exhibits a large red shift in both absorption and emission spectra as solvent polarity increases, indicating a large change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMATP gave an excimer like emission at 570 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B-type class of Steven's classification. A dye solution ca. 10⁻³ mol dm⁻³ in CHCl₃ gave a good laser emission in the range 480–560 nm with emission maximum at 530 nm upon pumping by nitrogen laser (λ_ex=337.1 nm). The excitation energy transfer from 7-dimethylamino-4-methyl coumarine (DMC) to DMATP has been also studied in CHCl₃ and the values of energy transfer rate constant and critical transfer distance indicate a Főrster-type mechanism. The photoreactivity and net photochemical quantum yield of DMATP in chloromethane solvents are also determined. We applied semiempirical MO calculation using AMI and ZINO/S calculation to understand the geometric and electronic structure of DMATP molecule in both ground and excited states.     

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.     

Structure and Luminescent Properties of the 4-Arylidene-2-Aryl-5-Oxazolones(Azlactones)In Solution and Crystalline State

Experimental and theoretical evaluations have proven that very low fluorescence quantum yields of azlactones in solution are not caused by an efficient inter system crossing from S₁*(ππ) to T*(nπ) states, but rather from solvation and steric effects, that result in non-planarity of the molecular system. High fluorescence quantum yields in the solid state are attributed to the planarity of the azlactone molecule upon packing into the crystal lattice. Supporting evidence was found upon observation of the excited state proton transfer (ESIPT) bands of fluorescence emissions of o-hydroxyarylidene derivatives. The photoinstability of azlactones in liquid states are attributed to photochemical E-Z isomerization and cleavage of the hetero ring α to the carbonyl group.     

Relaxation Processes at High Levels of Vibrational Excitation of Polyatomic Molecules in the Ground Electronic State

By the spectral and kinetic characteristics of the luminescence of vapors of polyatomic molecules (anthracene, anthraquinone, fluorenone) initiated by selective IR multiphoton excitation (IR MPE) of molecules in the ground electronic state S ₀ the relaxation processes proceeding under vibrational excitation of molecules to energies exceeding the energies of the lower excited electronic states have been investigated. The changes in the spectral and kinetic characteristics with increasing CO₂ laser energy density and vapor P _v and foreign gas pressure P _FG are analyzed. They are similar to the characteristics obtained for normal fluorescence of these molecules with changing vibrational energy E _vib content. On the basis of experimental data and model calculations it has been concluded that at the laser radiation densities used in the case of IR MPE the molecules reach energies considerably exceeding the energies of the electronic levels. It is shown that a nonadiabatic connection between the electronic states leads to the population of mixed electronic states isoenergetic to the vibrational levels of the ground electronic state and to emission of delayed luminescence spectrally identical to the normal luminescence of these molecules. It has been found that when high vibrational levels are populated, new relaxation channels, such as reverse electron relaxation, emission from high vibrational levels of the ground electronic state, and multiquantum vibrational energy transfer at collisions leading to a rapid establishment of vibrational equilibrium become important.     

Electronic relaxation processes of all-trans-2,4,6,8-decatetraene vapour revealed by the measurements of the S1 and S2 fluorescence

Fluorescence and excitation spectra of all-trans-2,4,6,8-decatetraene vapour have been measured at different buffer gas pressures from 0 to 1.07 × 10⁵ Pa (800 Torr). The quantum yield of the S₁ fluorescence (Φ_F(S₁)) increases with increasing the buffer gas pressure, while that of the S₂ fluorescence (Φ_F(S₂)) decreases slightly. Φ_F(S₁) decreases significantly with increasing the excitation energy under the collision free condition, while Φ_F(S₂) decreases only slightly. The pressure dependence of the fluorescence yields is interpreted in terms of the relaxation model involving the reversible internal conversion between S₁ and S₂ and the vibrational relaxation in the S₁ manifold. The reverse S₁ → S₂ internal conversion rate is shown to be very slow compared with the forward S₂ → S₁ internal conversion rate.     

Investigation of reactions from the highest excited states of molecules by fluorescent spectroscopy methods

The effect of the highest excited states on the yield of photoproducts that are usually formed upon excitation of the first singlet electronic state of polyatomic molecules is discussed. It is shown that the excitation of molecular objects through the highest singlet states can, in some cases, increase the yield of reaction products. This allows one to estimate the probabilities of reactions from the corresponding states. The consideration concerns a wide range of primary photoreactions, including the electronic density redistribution (the intramolecular electron transfer) in the excited state, the protolytic reactions, the intramolecular proton transfer (the phototautomerization), the hydrogen bond formation, and the formation of excimers and exciplexes. The relations obtained are used to analyze the experimental fluorescence spectra of 3-hydroxyflavone solutions, excited by electromagnetic radiation with different wavelengths in the region of the S ₁, S ₂, and S ₃ absorption bands. The analysis fulfilled shows that the highest singlet states play an important role in the formation of tautomers in 3-hydroxyflavone due to the intramolecular proton transfer.     

Excitation transfer between extended band states and N-related localized states in with x up to 1%

We present time-resolved photoluminescence (PL) results of a series of GaP_1-xN_x samples with x up to 0.01. The temperature dependence, the concentration dependence as well as the temporal behavior indicate that the PL is dominated by excitation transfer processes between the extended band states and localized N-related states (such as the isolated N-impurity, various N-pair states and higher N-clusters) as well as by excitation transfer between the various localized N-related states themselves. The excitation transfer processes in conjunction with the concentration-dependent statistics of the various N-related states alone are sufficient to explain the observed red-shift of the luminescence of GaP_1-xN_x with increasing x as well as the spectral dependence of the PL decay times. However, this implies that the PL data alone do not give any conclusive evidence that a hard transformation from an indirect to a direct-gap semiconductor takes place in Ga(N,P) with increasing N up to 2% as often stated.     

Charged Porphyrin – dopa Melanin Interaction at Varied pH: Fluorescence Lifetime and Photothermal Studies

The influence of pH on the spectral forms of negatively and positively charged porphyrin dyes: tetrakis(4-sulfonatophenyl)porphyrin (TPPS₄) and tetra(4-N, N, N, N-trimethylanilinium) porphyrin (TAP) in the presence of dopa melanin is investigated. The interaction between dyes and dopa melanin is shown to be dependent on the kind of dye spectral forms and on environmental pH. The creation of different forms, deactivation of their electronic excited states, and their interaction with dopa melanin have been monitored by absorption, fluorescence, and photoacoustic spectroscopy in the pH range of 2–9. This article demonstrates that the TAP–dopa melanin complex is destroyed at the lowest pH because of competition between positively charged peripheral groups of TAP and protons, which interact with negative centers of dopa melanin. Otherwise, dopa melanin affects photothermal and fluorescence properties of monomeric dications (TAP and TPPS₄) rather weakly. It has also been suggested that the aggregated dicationic of TPPS₄ can serve as an acceptor in the energy transfer from dopa melanin. Presented results seem to be applicable to a photodynamic therapy of cancer.     

Steady-State and Time-Resolved Fluorescence Polarization Behavior of Acenaphthene

Steady-state and time-resolved fluorescence polarization studies have been carried out on acenaphthene (ACE) in low-temperature glass solutions and at room temperature. In the low-temperature glass the fluorescence polarization values vary considerably with both emission and excitation wavelength. There is a time dependence (on the nanosecond time scale) of the fluorescence anisotropy, r(t), at 77 K, which has a strong dependence upon the excitation and emission wavelengths. Under these conditions, the time-dependent decay of the anisotropy is not attributable to chromophoric motion. The observations are consistent with emission from two closely lying and interconverting excited states. Rate constants for the photophysical processes involved have been determined by fitting the data using a model proposed by Fleming et. al. The results are discussed with particular reference to the care required in using dynamic fluorescence polarization measurements to determine energy transfer rates in systems containing this chromophore.     

Density functional theory study on a fluorescent chemosensor device of aza‐crown ether

Three derivatives of alkyl anthracene covalently bonded to aza‐18‐crown‐6 at the nitrogen position, anthracene(CH₂)_n, (n = 1–3) which act as an on–off fluorogenic photoswitch have been theoretically studied using a computational strategy based on density functional theory at B3LYP/6‐31 + G(d,p) method. The fully optimized geometries have been performed with real frequencies which indicate the minima states. The binding energies, enthalpies and Gibbs free energies have been calculated for aza‐18‐crown‐6 ( L ) and their metal complexes. The natural bond orbital analysis is used to explore the interaction of host–guest molecules. The absorption spectra differences between L and their metal ligands, the excitation energies and absorption wavelength for their excited states have been studied by time‐dependent density functional theory with the basis set 6‐31 + G(d,p). These fluorescent sensors and switchers based on photoinduced electron transfer mechanism have been investigated. The PET process from aza‐crown ether to fluorophore can be suppressed or completely blocked by the entry of alkali metal cations into the aza‐crown ether‐based receptor. Such a suppression of the PET process means that fluorescence intensity is enhanced. The binding selectivity studies of the aza‐crown ether part of L indicate that the presence of the alkali metal cations Li⁺, Na⁺ and K⁺ play an important role in determining the internal charge transfer and the fluorescence properties of the complexes. In addition, the solvent effect has been investigated. Copyright © 2014 John Wiley & Sons, Ltd.     

Energy transfer in Er:Eu:Yb co-doped tellurite glasses: Yb as enhancer and quencher

Energy transfer has been studied from Er³⁺ to Eu³⁺ ions on excitation with NIR photons (796 and 980 nm) with and without Yb³⁺ ions. It is found that in one case the presence of Yb³⁺ enhances the fluorescence yield (980 nm excitation) whereas in the other case it quenches (796 nm excitation). Energy transfer from Er³⁺ ion's levels ⁴S_3/2 and ²H_11/2 is verified by decay curve analysis in both the cases. The nature of interaction between the donor (Er) and the acceptor (Eu) ions is found to be dipole-dipole. The energy transfer parameters viz. transfer probability, critical distance etc. have been calculated.     

The effect of annulation of benzene rings on the photophysics and electronic structure of tetraazachlorin molecules

The photophysics and electronic structure of tribenzotetraazachlorins (H₂, Zn, and Mg), which are novel analogues of phtalocyanines, have been studied experimentally and theoretically. At 293 K, the electronic absorption, fluorescence, and fluorescence excitation spectra are recorded and the fluorescence quantum yield and lifetime, as well as the quantum yield of singlet oxygen generation, are measured; at 77 K, the fluorescence, fluorescence excitation, and fluorescence polarization spectra are recorded and the fluorescence lifetime values are measured. The dependences of the absorption spectra and photophysical parameters on the structure variation are analyzed in detail. Quantum-chemical calculations of the electronic structure and absorption spectra of tribenzotetraazachlorins (H₂, Mg) are performed using the INDO/Sm method (modified INDO/S method) based on molecular-geometry optimization by the DFT PBE/TZVP method. The results of quantum-chemical calculations of the electronic absorption spectra are in very good agreement with the experimental data for the transitions to two lower electronic states.     

Emission and laser-excitation spectra of rotationally cooled methylcyanodiacetylene cation (A˜2EāX˜2 E) and photoelectron—photon

The A?²EāX?² E band system of rotationally cooled methylcyanodiacetylene cation has been studied by emission and excitation spectroscopies. Spectra were obtained by electron-impact ionisation of a seeded helium supersonic free jet and by Penning ionisation followed by laser excitation. These data enable the vibrational frequencies of most of the A₁ fundamentals to be inferred for the X?²E (to within ±2 cm^?1) andX?²E (to within ±4 cm?1) states. The fluorescence quantum yields and lifetimes of the cation prepared in selected levels of theA?²E andB?²A₁, excited states have been determined by photoelectron—photon coincidence measurements and yield a quantitative insight into the radiationless-decay pathways.     

Purine Scaffold Effect on Fluorescence Properties of Purine-Hydroxyquinolinone Bisheterocycles

The fluorescence properties of bisheterocyclic compounds that contain purine and the 3-hydroxyquinolin-4(1H)-one skeleton connected with an aliphatic spacer of a different length/structure (3HQP) were examined. It was found that the introducing of the spacer-purine scaffold led in the comparison to 3HQs themselves to (1) the possibility of the effectual excitation in the wider range of excitation wavelengths, moreover, some derivatives can be excited at relatively high wavelengths around 400 nm, (2) the lowering of the quantum yield and (3) the slight longer wavelength shift of the dual emission spectra. Tested organic solvents did not affect significantly the 3HQP fluorescence properties. The characters of emission spectra as well as the quantum yields of 3HQPs were notably influenced by the ratio of water and DMSO in their composed mixture applied as a solvent. With increasing water content in the mixture both I₁/I₂ and the quantum yield decreased.     

Temperature dependence of photoluminescence of semiconductor quantum dots upon indirect excitation in a SiO2 dielectric matrix

The processes of excitation and relaxation of confined excitons in semiconductor quantum dots upon indirect high-energy excitation have been considered. The temperature behavior of photoluminescence of quantum dots in a SiO₂ dielectric matrix has been described using a model accounting for the process of population of quantum-dot triplet states upon excitation transfer through mobile excitons of the matrix. Analytical expressions that take into account the two-stage and three-stage schemes of relaxation transitions have been obtained. The applicability of these expressions for analyzing fluorescence properties of semiconductor quantum dots has been demonstrated using the example of silicon and carbon nanoparticles in the thin-film SiO₂ matrix. It has been shown that the complex character of the temperature dependences of the photoluminescence upon indirect excitation can be an indication of a multistage relaxation of excited states of the matrix and quantum dots. The model concepts developed in this study allow one to predict the form of temperature dependences of the photoluminescence for different schemes of indirect excitation of quantum dots.

     

Excitation transfer into bound and continuum states investigated by optical and electron spectroscopy

We present energy spectra of electrons formed in the reaction of He(2³ S, 2¹ S) with NO₂ which have significantly improved counting statistics and resolution compared to earlier work. Further, we show spectra of the fluorescence light emitted in these reactions. The data are recorded in the same molecular beam apparatus as the electron spectra. For the metastable singlet state He(2¹ S) the spectra have not been measured before. We find that in addition to ionization excitation transfer takes place into Rydberg states of NO₂**. Subsequently, the highly excited NO₂** molecules dissociate into NO and atomic O* Rydberg atoms.     

Excitation Wavelength Dependence of Dual Fluorescence of DMABN in Polar Solvents

Steady-state absorption, fluorescence excitation and emission spectra of 4-(N,N-dimethylamino)benzonitrile (DMABN) have been measured at room temperature in cyclohexane, 1,4-dioxane, dichloromethane, and acetonitrile solutions. The fluorescence spectra of DMABN are found to exhibit dual emission in 1,4-dioxane, dichloromethane, and acetonitrile solutions and single emission in cyclohexane solution. The effect of solvent polarity and excitation wavelength on the emission spectra has also been studied. The fluorescence excitation spectra of DMABN monitored at the emission bands are different. The presence of two different conformations of the same molecule in the ground state has lead to two close lying excited states; local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the compound. The experimental studies were supported by ab initio density functional theory (DFT) calculations performed at the B3LYP/6-31Gd level of theory. On the basis of the experimental results and our theoretical calculations, we suggest that there are two conformers of DMABN, which are stable in the ground state, equilibrated in solution at room temperature that give rise dual fluorescence upon excitation.     

Laser study of the photophysical properties of UF6

The purpose of this paper is to review the most relevant theoretical and experimental results reported in recent years, on the photophysical properties of the uranium hexafluoride molecule in the gas phase. Details of the structure of the molecular orbitals of UF₆ are discussed with reference to theoretical calculations of the electronic states. Tentative assignments of the electronic transitions in the experimental visible-uv absorption spectrum are also considered. The fluorescence properties of the excited states of UF₆ are critically reviewed. The use of laser beams to excite the fluorescence emission of UF₆ is shown to be a fruitful experimental technique. The measurement of fluorescence parameters (i.e. the emission and excitation spectra, the decay time and quantum yield) is a particularly valuable and sensitive way of studying the dynamics and structure of the excited electronic states. The different mechanisms of fluorescence quenching that have been proposed are also critically reviewed.     

Laser study of the photophysical properties of UF6

The purpose of this paper is to review the most relevant theoretical and experimental results reported in recent years, on the photophysical properties of the uranium hexafluoride molecule in the gas phase. Details of the structure of the molecular orbitals of UF₆ are discussed with reference to theoretical calculations of the electronic states. Tentative assignments of the electronic transitions in the experimental visible-uv absorption spectrum are also considered. The fluorescence properties of the excited states of UF₆ are critically reviewed. The use of laser beams to excite the fluorescence emission of UF₆ is shown to be a fruitful experimental technique. The measurement of fluorescence parameters (i.e. the emission and excitation spectra, the decay time and quantum yield) is a particularly valuable and sensitive way of studying the dynamics and structure of the excited electronic states. The different mechanisms of fluorescence quenching that have been proposed are also critically reviewed.     

Surveying the nuclear caloric curve

The 4π array INDRA was used to detect nearly all charged products emitted in Ar + Ni collisions between 52 and 95 MeV/u. The charge, mass and excitation energy E^* of the quasi-projectiles have been reconstructed event by event. Excitation energies up to 25 MeV per nucleon are reached. Apparent temperatures obtained from several double isotopic yield ratios Tr⁰ show different dependences uponE^* . T⁰_6Li7Li-3Heα yields the highest values, as well as the high energy slopesTs of the kinetic energy spectra. Two statistical models, sequential evaporation and gas in complete equilibrium, taking into account side feeding and discrete excited states population, show that the data can be explained by a steady increase of the initial temperature with excitation energy without evidence for a liquid-gas phase transition.