Rapid internal conversion in a symmetric molecule LD 700 studied by means of femtosecond fluorescence depletion

The rapid internal conversion dynamics at room temperature is determined by using the femtosecond time-resolved fluorescence depletion measurements of a complex solvated molecule of LD 700 (rhodamine 700) combined with steady-state absorption and fluorescence spectroscopy, as well as quantum chemical calculation. The molecule is excited by a 50fs laser pulse at 400nm which directly populated the highly excited singlet state, the rapid internal conversions (ICs) are observed, which leads to the directional changes of the emission transition moment following photoexcitation to the highly excited singlet state S₅ of LD 700.     

关于PDT光敏剂敏化效应光谱及其量子产率的研究

从实验的角度研究了HA光敏剂激发态的敏化特性,建立了单态氧量子产率在近红外区的荧光光谱测量系统,测定了HA光敏剂的单态氧量子产率为0．78。结果表明,该类光敏剂具有较强的敏化效应,是一类有前途的光动力治疗(PDT)候选光敏剂。     

Influence of Complexing and Excitation Energy on Spectral and Luminescent Properties of 2-Amino-4-Methylphenol

Energy level diagrams of 2-amino-4-methylphenol and its complexes with water are calculated by the method of intermediate neglect of differential overlap (INDO). It is demonstrated that the substitution by the amino group results in the dependence of the quantum fluorescence yield on the excitation energy. The decrease of the quantum fluorescence yield of 2-amino-4-methylphenol in going from hexane to water is explained. Complexing of the 2-amino-4-methylphenol molecule with water with the formation of the H-bond reduces the quantum fluorescence yield compared to the isolated molecule due to the increased efficiency of the S₁ → T₄ conversion.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 71–76, March, 2005.     

Studies on Curcumin and Curcuminoids. XLVI. Photophysical Properties of Dimethoxycurcumin and Bis-dehydroxycurcumin

The steady-state absorption and fluorescence, as well as the time-resolved fluorescence properties of dimethoxycurcumin and bis-dehydroxycurcumin dissolved in several solvents differing in polarity and H-bonding capability are presented. The singlet oxygen generation efficiency of the two compounds relative to curcumin is estimated. The photodegradation quantum yield of the former compound in acetonitrile and methanol is determined. The dimethoxycurcumin and bis-dehydroxycurcumin decay mechanisms from the S ₁ state are discussed and compared with those of curcumin, dicinnamoylmethane and bis-demethoxycurcumin.     

Correspondence of Fluorescing States of Naphthols and Naphtholate Anions and its Effect on the Calculation of pKa∗ from Spectral Shifts

The validity of dissociation constants of electronically excited acids and bases, calculated from spectral shifts^1,2 accompanying protolytic dissociation, especially where shifts of fluorescence spectra are employed, is dependent, among other factors, upon fluorescence orginating from excited states of the same electronic configuration in both acid and conjugate base.³ In some of the molecules whose excited state acid-base properties have been most extensively studied, notably some derivatives of naphthalene, energy level reversals have been shown to accompany dissociation in the lowest excited singlet state in aqueous solutions.⁴ The spectroscopic properties and excited state dissociation phenomena of the naphthols have been studied extensively from several points of view.^5-9 While the calculation of the excited state dissociation properties of β-naphthol, in which fluorescence occurs from the ¹L_b state in both anion and neutral molecule, presents no particular problem with regard to correspondence of the lowest excited singlet states of conjugate acid and base, it appeared that in α-naphthol, by analogy with α-naphthylamine⁴, fluoescence might occur from the ¹L_a state in the conjugate base and from the ¹L_b state in the conjugate acid. Thus it seemed that a careful consideration of the states from which the fluorescences of the conjugate species derived from α-naphthol in fluid aqueous solutions orginated, would be useful.     

Excited state absorption dynamics of fast relaxing dye no. 5

The excited state absorption dynamics of the fast mode-locking dye No. 5 for Nd-glass lasers is investigated. The excited state absorption cross-section σ_ex for 1.053 μm picosecond pulse excitation is determined from energy transmission measurements. The excited state absorption populates a higher lying singlet state S₄. The population lifetime τ₄ of this state is determined from two-step excited fluorescence quantum yield measurements. The obtained values are σ_ex= (5.5±1)×10^-17 cm² and τ₄=60±20 fs.     

Effectiveness of charge separation from the long-lived second excited state of donors

In molecular zinc-porphyrin-based donor–acceptor systems, the electron transfer from the second singlet excited state S₂ is accompanied by ultrafast recombination into the first excited state, resulting in a low quantum yield of the thermalized charge-separated state (20%). It is demonstrated that the quantum yield of ultrafast charge separation in donor–acceptor triads D–A₁–A₂ can be close to 100% in molecular systems with lifetimes of the S₂ state longer than 150 ps. As prototypes of such systems, donor–acceptor diads D–A₁ and triads D–A₁–A₂ are considered, wherein the xanthione molecule plays the role of a donor. The ranges of the model parameters are determined in which the efficiency of charge separation is high. The twostage photoinduced charge transfer is studied within the framework of a multichannel stochastic model that takes into account the reorganization of a polar solvent and a high-frequency intramolecular vibrational mode.     

Effect of cyclopalladation on the spectroscopic properties of a Coumarin dye

The cyclopalladated [Pd(C6)Cl]₂ and [Pd(C6)En]PF₆ complexes based on Coumarin 6 are studied by ¹H NMR spectroscopy and electronic absorption and emission spectroscopy. It is shown that cyclopalladation leads both to a long-wavelength shift of the spin-allowed intraligand absorption bands of coumarin and to the formation of a characteristic metal-ligand absorption band. It is found that, in comparison with coumarin, the complexes are characterized by a bathochromic shift and by a decrease in the fluorescence quantum yield due to more efficient intersystem crossing to the lower excited triplet state responsible for the low-temperature (77 K) phosphorescence of the complexes.     

B-type delayed fluorescence of rubreneperoxide in solution.

B-type of delayed fluorescence was observed for the first time for rubreneperoxide. Rubreneperoxide molecules were excited in a two step process. In the first step an excited singlet S₁ is created, which undergoes intersystem crossing to T₁; then T-T absorption creates an excited triplet rubreneperoxide molecule, which returns to the first excited singlet level by intersystem crossing. The recreated first excited singlet of rubreneperoxide decays back to the ground state by emitting B-type of delayed fluorescence.     

High Resolution B-Type Delayed Fluorescence of Dibenzanthracene in PMMA

B-Type delayed fluorescence of 1,2,3,4-dibenzanthracene in PMMA was experimentally observed for the first time. Dibenzanthracene molecules were exerted in a two-step process. In the first step, an excited singlet S₁ is created, which undergoes intersystem crossing to T₁ then T-T absorption creates an excited triplet dibenzanthracene molecule, which returns to the first exerted singlet level by intersystem crossing. The recreated first excited singlet of dibenzanthracene decays back to the ground state by emitting this new type of delayed fluorescence.     

Multiple fluorescence and the electronic relaxation processes of coronene vapor: The fluorescence from the S1, S2, and S3 states

Fluorescence and excitation spectra of coronene vapor have been measured under different conditions. Weak emission which can be regarded as the fluorescence from the third excited singlet state, S₃(¹E_1u), was observed in addition to the S₁(¹B_2u) and S₂(¹B_1u) fluorescence. The observed S₂ and S₃ fluorescence are substantially different from those reported previously for coronene vapor. Addition of oxygen resulted in significant decrease of the S₁ fluorescence intensity, but did not affect the S₂ fluorescence intensity, indicating the faster decay rate of the S₂ state than that of S₁. Excitation energy dependence of the S₁, S₂ and S₃ fluorescence quantum yields (Φ_F(S₁), Φ_F(S₂) and Φ_F(S₃), respectively) revealed that Φ_F(S₁) decreases with increasing excitation energy, while Φ_F(S₂) and Φ_F(S₃) increase significantly. The quantum yield ratios, Φ_F(S₂)/Φ_F(S₁) and Φ_F(S₃)/Φ_F(S₂), obtained as a function of excitation energy are correlated with the ratios of the relative internal conversion rates.     

Vapor-phase fluorescence spectra from the second excited singlet state of pyrene and its derivatives

Vapor-phase fluorescence spectra have been measured for pyrene and its simple derivatives, i.e., pyrene-d₁₀, 1-methylpyrene, and 4-methylpyrene. Each of these derivatives shows a weak fluorescence emission which is similar to the fluorescence from the second excited singlet state (S₂) of pyrene and is to be assigned to the S₂-fluorescence. The methyl substitution causes frequency shifts (Δν) in both absorption and fluorescence, and the Δν values for absorption and fluorescence transitions that are associated with the same excited state, i.e., the first excited singlet state (S₁) or the second excited singlet state (S₂), are approximately equal to each other. When the excitation energy increases, the S₂-fluorescence shifts gradually to the red in almost the same way as the S₁-fluorescence. The S₂-fluorescence spectrum has a sort of mirror-image relation to the S₂-absorption. A comparison of S₂-emissions of pyrene and pyrene-d₁₀ suggests that the ratio between the quantum yields of S₂- and S₁-fluorescence may be related to a ratio $\text{?}{}_2\text{?}{}_1$, where ?₁ and ?₂ are the densities of vibrational states in S₁ and S₂ at the energy of excitation.     

The mutual influence of two different dyes on their sensitized fluorescence (cofluorescence) in nanoparticles from complexes

We have studied the fluorescence sensitization and quenching for pairs of different dyes simultaneously incorporated into nanoparticles from complexes M(diketone)₃phen, where M(III) is La(III), Lu(III), or Sc(III); diketone is p-phenylbenzoyltrifluoroacetone (PhBTA) or naphthoyltrifluoroacetone (NTA); and phen is 1,10-phenanthroline. We have shown that, upon formation of nanoparticles in the solution in the presence of two dyes the concentrations of which are either comparable with or lower than the concentration of nanoparticles (<20 nM), the intensities of the sensitized fluorescence of dyes in nanoparticles in binary solutions and in solutions of either of the dyes coincide. We have found that the intensity of sensitized fluorescence of small (<20 nM) concentrations of rhodamine 6G (R6G) or Nile blue (NB) increases by an order of magnitude upon simultaneous introduction into nanoparticles of 1 μM of coumarin 30 (C30), while the intensity of fluorescence of C30 sensitized by complexes decreases by an order of magnitude. The same effect is observed as 1 μM of R6G are introduced into nanoparticles with NB ([NB] ≤ 20 nM). The increase in the fluorescence of dye molecules upon their incorporation from the solution into nanoparticles from complexes is noticeably lower than that expected from the proposed ratio of concentrations of complexes and dyes in nanoparticles. Analysis of the obtained data indicates that the introduction of large concentrations of C30 or R6G dyes into nanoparticles makes it possible to prevent large energy losses due to impurities or upon transition to a triplet state that arises during the migration of the excitation energy over S ₁ levels of complexes. Energy accumulated by these dyes is efficiently transferred to another dye that is present in the solution at lower concentrations and that has a lower-lying S ₁ level, which makes it possible to increase its fluorescence by an order of magnitude upon its incorporation into nanoparticles.     

Particular features of manifestation of energy migration toward impurity in nanoparticles of metal complexes

We measure the fluorescence quantum yields (q _fl) of complexes of Al, Sc, Y, In, Lu, and Gd with dibenzoylmethane in aqueous and isopropanol solutions at different concentration ratios of ions and dike-tone. We reveal that, for the examined solutions, qfl of complexes varies more than by two orders of magnitude under the influence of the heavy atom. It is found that a considerable decrease in q _fl and τ_fl of ligands of complexes of listed ions caused by the influence of heavy atoms weakly affects the intensity of sensitized fluorescence of Nile red and rhodamine 6G molecules introduced into nanoparticles from these complexes in aqueous solutions. The revealed result is explained by the comparability of the singlet exciton free path length and the dimension of nanoparticles under study. We show that a lower fluorescence intensity of heavy metal complexes makes it possible to decrease its contribution in the range of the cofluorescence maximum of rhodamine 6G and to monitor the occurrence of the dye in the aqueous solution down to the concentration of 0.05 nM. We show that, in nanoparticles from Eu complexes, further fluorescence quenching of dibenzoylmethane is observed, as well as the appearance of cofluorescence of rhodamine 6G, the intensity of which is comparable with its intensity in nanoparticles of other complexes. The appearance of this cofluorescence cannot be explained by the existence of S-S energy migration.     

Kinetics of the b{su1}Σ{sk{su\s+}/{in{itg}}→\ga{su1}\gD{in{itg}} fluorescence of molecular oxygen in solutionsfluorescence of molecular oxygen in solutions

The kinetics of sensitized fluorescence of O₂ has been recorded for the first time in solutions at transition from the second singlet excited state b{su1}Σ{sk{su\s+}/{in{itg}} to the first excited state \ga{su1}\gD{in{itg}} (b→a. We measured the quantum yield of the b→a-fluorescence at γ≈1.93 μm and assessed its probability with respect to the a→ X-phosphorescence at 1.27 μm. We show that the lifetime of the b→a-fluorescence (105 nsec for CCl₄ and 140 nsec for CS₂) becomes much shorter on addition of solvents with large frequencies of intramolecular vibrations. The measured radiative velocity constant of the b→a-transition K_b→a is by three orders of magnitude larger than for the phosphorescence (≈1.27 μm) of the a→X-transition from the first singlet to the ground triplet state. The method of recording IR fluorescence is promising for determining O₂ in solutions and biosystems. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp 5–8, January–February, 1998.     

Theoretical and experimental investigations of photophysics of the nile red molecule and its protonated structures

Results of quantum-chemical studies of the nile red (NR) molecule and its protonated structures by the INDO/S method are presented. It is demonstrated that the best agreement between the calculated and experimental data is obtained for the flat molecule in the ground electron state. Energies of the strongest singlet and triplet electronic states, molecular nature of these states, transition oscillator force, dipole moments in the ground and excited states, electron density distribution around atoms and molecular fragments in the S₀ and S₁ states, and rate constants of radiative, internal, and intercombination conversion are presented for the NR molecule and its protonated structures. The most probable NR protonation centers are analyzed using the molecular electrostatic potential (MESP) method. It is established that the reaction of proton addition to the NR molecule proceeds at the cyclic nitrogen atom. As demonstrated the results of quantum-chemical calculations, low fluorescent properties of the protonated NR structures (with a quantum yield of 4%) are due to a high rate of the S₁ – T₄ intercombination conversion.     

Fluorescence of 5-hydroxy-6-methyl-(1-thietanyl-3)-pyrimidine-2,4(1 H,3 H)-dione at the transition with the second singlet-excited S 2 level to the ground level in acetonitrile solutions

The influence of the excitation wavelength on the fluorescence spectra of 5-hydroxy-6-methyl-(1-thietanyl-3)pyrimidine-2,4(1H,3H)-dione (TOMU) in acetonitrile solutions has been studied. It is found that, upon excitation of the singlet S ₂ state of TOMU luminescence, occurs from not only the first excited S ₁ level (??_max. = 350 nm, quantum yield ??(S ₁ ?? S ₀) = (4.5 ± 0.5) × 10^?3), but also at the transition from the second S ₂ level to the ground level (??_max = 305 nm, ??(S ₂ ?? S ₀) = (1.0 ± 0.1) × 10^?3).     

Luminescence of water-soluble rh(III) porphyrins

The luminescent properties of complexes of rhodium(III) with three water-soluble porphyrins— meso-tetrakis(4-N-methyl pyridyl) porphyrin, meso-tetrakis(4-N,N,N-trimethyl aminophenyl) porphyrin, and meso-tetrakis(4-sulfonate phenyl) porphyrin—were studied. All three complexes were found to phosphoresce both at 77 K and in deaerated solutions at room temperature, with their fluorescence being very weak. The rate constants of radiative (k _p=40-60s^-1) and nonradiative deactivation of a triplet excited state were determined. It was ascertained that, in aqueous solutions, the phosphorescence is quenched by molecular oxygen via an energy-transfer mechanism that is accompanied by formation of singlet oxygen. The quantum yields of formation of singlet oxygen for all the metalloporphyrins studied were found to be close to unity, which is the quantum yield of formation of their triplet 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.     

Intra- and intermolecular proton transfer in methyl-2-hydroxynicotinate

Spectral characteristics of methyl 2-hydroxynicotinate (MEHNA) have been studied using absorption, fluorescence excitation and fluorescence spectroscopy, as well as, using single photon counting nanosecond spectrofluorimeter. MEHNA is present as enol in less polar solvents and keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to phototautomer, formed by excited state intramolecular proton transfer (ESIPT), whereas fluorescence is only observed from keto form in polar solvents. In aqueous and polar solvents monocation (MC) is formed by protonating the exo carbonyl oxygen atom in the ground state (S₀) and in the first excited singlet state (S₁), MC is obtained by protonating carbonyl oxygen atom of the ester. It is formed by ESIPT from exo carbonyl proton to carbonyl oxygen atom of the ester. Dication is formed by protonating both the oxygen atoms. Two kinds of monoanions formed by deprotonating phenolic proton or >N-H proton of keto suggest the presence of enol and keto in aqueous solution. In cyclohexane MC is formed by protonating carbonyl oxygen in both S₀ and S₁ states. The electronic structure calculations were performed on each species using semi-empirical quantum mechanical AM1 method and density functional theory B3LYP with 6-31G^** basis set using Gaussian 98 program, along with potential energy mapping, to characterize the particular species.