Rotational reorientation dynamics of Rhodamine 700 in different excited states

The rotational reorientation dynamics of rhodamine 700 (LD700) in the first (S₁) and the fifth (S₅) excited state in three aprotic polar solvents have been investigated using femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP TD) spectroscopy. In both excited states, the overall rotational relaxation of LD700 occurs on a time scale of 40-230 ps depending on the solvent, and a quantitative analysis of this time constant has been performed using the Stokes-Einstein-Debye (SED) hydrodynamic theory combined with the extended charge distribution model developed by Alavi and Waldeck. The experimentally measured reorientation times for LD700 in S₅ are smaller than those in S₁, which is in accord with the predictions by the SED theory. In addition, for LD700 in S₅, a rapid initial decrease on the time scale less than 0.5 ps has been found. According to our analysis, this fast component may account for the rapid internal conversion from S₅ to S₁, and the rate of internal conversion was found to be sensitive to the solvent polarity.     

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.     

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.     

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.     

Singlet and Triplet State Properties and Singlet Oxygen Yield of an Amino-Acyl-Quinoxalinone Yellow Dye

Amino-acyl-quinoxalinone yellow dyes are cyclised analogues of the yellow azomethine dyes developed for, and still used in, silver halide colour photography. Unlike image azomethine dyes, which are rapidly deactivated in their excited states by torsion about the azomethine bond, amino-acyl-quinoxalinone dyes have an interesting photophysics because torsion is not possible due to their cyclised structure. We report results from studies on singlet and triplet state properties, and singlet oxygen yields, of the yellow dye, 7-diethylamino-3-(2,2-dimethyl-propionyl)-5-methyl-1-phenyl-1H-quinoxalin-2-one, in polar and nonpolar solvents. The dye photophysics is characterised by a weak fluorescence, with a solvent dependent emission yield (Φ_F?≈?0.002–0.004), and short singlet state lifetime (τ_expt?≈?20–50 ps), both increasing by a factor of ≈2 in going from polar acetonitrile to non-polar dioxane as solvent. DFT ZINDO calculations show a transition involving significant electron transfer from the diethyl-amino group into the carbonyl region of the molecule. In solution, in the presence of oxygen, the triplet state decays almost exclusively by oxygen quenching, and singlet oxygen is produced in high yield (Φ_??≈?0.5–0.55). The triplet state absorbs across the 450–750 nm region with maxima around 480 and 650 nm, and moderate molar absorption coefficients (ca. 6000–8000 M^?1 cm^?1). In a glass at 77 K, triplet decay gives a red phosphorescence, with λ_max?≈?640–650 nm, and a ?≈?0.25 s lifetime. If singlet oxygen yields are a good indication of triplet yields, then internal conversion and intersystem crossing occur with roughly equal efficiency.

     

Ab initio SCF CI study of the electronic spectrum of s-tetrazine

Configuration interaction (CI) studies of ground, n→ π* and π→ π* electronically excited states are reported for s-tetrazine. The first n→ π* singlet excited state (¹ B _3u ), which is responsible for the purple-red colour of the molecule, is calculated at 2·80 eV, compared to the experimental transition energy of 2·22–2·70 eV. The singlet-triplet split of the first n→ π* states (¹ B _3u and ³ B _3u states) is calculated to be 0·76 eV.

The interaction of nitrogen lone pair orbitals (n-orbitals) is studied in terms of the ordering of the n π* excited states and found that the SCF orbital ordering is qualitatively in accord with the ordering of the n π* excited states in the CI level.

The first π→ π* excited state (¹ B _2u ) is calculated at 5·99 eV, slightly above the observed range of absorption. Numerous other high-lying singlet states as well as the triplet states have been calculated and they are used to verify several proposals relating to the excited state dynamics in the photo-physical studies of s-tetrazine.     

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.     

Low Temperature Spectra of the ortho-POPOP Molecule: Additional Arguments of Its Flattening in the Excited State

Low temperature absorption and fluorescence spectra were measured for 1,2-bis-(5-phenyl-oxazolyl-2)-benzene (ortho-POPOP), a sterically hindered molecule, substantially non-planar in its ground state. Quantum-chemical calculations with the optimization of molecular geometry were made using the semiempirical AM1 method. The observed spectral changes, together with the obtained theoretical results, evidenced in favor of our previously made assumption about the considerable flattening of ortho-POPOP in the lowest singlet excited state. The data of spin-orbit coupling between singlet and triplet terms, which has been estimated in CNDO/S scheme, were used to calculate the efficiency of intersystem crossing in the studied molecule.     

Fluorescence studies of retinol and retinyl acetate

The polarized emission study of all-trans retinol and retinyl acetate in solid films at room temperature has been carried out. The shape of the absorption and fluorescence is typical of Franck-Condon forbidden bands. The high positive polarization of the fluorescence excitation and the emission (when excited into main absorption band due to 1_Bu ← 1_Ag transition) suggests that the lowest excited singlet state of retinols has a large amount of 1_Bu character. The explanation for the high positive polarization involving strong vibronic interactions suggesting 1_Ag - (a very weakly allowed state) being the lowest excited singlet has also been considered.     

Photodynamics of intramolecular proton transfer in polar and nonpolar biflavonoid solutions

Using methods of steady state luminescence and femtosecond spectroscopy, we have studied the mechanism of intramolecular proton transfer in synthesized 3,7-dihydroxy-2,8-di(4-methoxyphenyl)-4H,6H-pyrano[3,2-g]chromen-4,6-dion in polar and nonpolar solutions, films, and polycrystals at 293 and 77 K. In an excited singlet state, intramolecular proton transfer occurs in two stages. At the first stage, a tautomer with one transferred proton (OTP tautomer) is formed from the Franck-Condon state within ??₁ = 0.6 ps. At the second stage, the second proton is transferred within ??₂ = 3.1 ps and a tautomer with two transferred protons (TTP tautomer) is formed, which fluoresces in toluene at 293 K with a high quantum yield, ?? _f = 0.66, and the fluorescence spectrum of which is characterized by a large Stokes shift, 9900 cm^?1. At 293 K, polar solvents (dimethylformamide, dimethyl sulfoxide, ethanol, etc.) solvate the BFV molecule in the ground state, while, in the excited state, an OTP tautomer is mainly formed. In polar ethanol at 77 K, a dual fluorescence spectrum is observed, which is caused by the fluorescence emission of polysolvates with ?? _max ^f = 460 nm and TTP phototautomers at ?? _max ^f = 610 nm.     

Spectroscopic investigations of a novel push-pull azo compound embedded in rigid polymer

A new heterocyclic push-pull azo compound-in-poly(methymethacrylate) (PMMA) film has been made by means of the spin-coating method. The spectroscopic properties of the films have been investigated with the steady-state absorption spectra, and steady-state fluorescence and femtosecond time-resolved fluorescence spectra in the first time, which is an important characteristic for the application of the film. The excited singlet (S₁) state lifetimes for trans and cis isomers of the film at room temperature have been measured. The excited triplet (T₁) state lifetime of cis isomer of the film has been obtained. The electronic structure of the film has been explained. The results show that the aggregate state of the azo molecules greatly influences its absorption spectra.     

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.     

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.     

Vibronic coupling and intersystem crossing in aromatic amines

The fluorescence and phosphorescence spectra of the aromatic amines acridan, iminobibenzyl, and carbazole have been measured in Shpolskii matrices at 10 K. Under these conditions the emission exhibits a detailed vibrational structure which has been analyzed. The change of the polarization degree observed within the fluorescence spectra at 77 K, particularly pronounced in acridan and iminobibenzyl, is attributed to vibronic interaction between the closely lying S₁(¹A₁) and S₂(¹B₁) excited states. This process activates a b₁ vibration with a frequency of 1200 cm⁻¹ in the ground state. The appearance of a long-axis (b₁) polarized vibration (700 cm⁻¹) following the out-of-plane polarized 0-0 band of the phosphorescence of these amines at 77 K is suggested to arise from vibronic interactions in the triplet manifold. This second-order spin-orbit coupling (soc) process is superimposed upon the dominant first-order electronic soc mechanism, which couples the lowest π, π^* triplet with high-energy (σ, π)^* singlet states.     

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.     

Special Features of Quenching of Fluorescence in the 1-Naphthol and Coumarin 1 Molecules in Binary Mixtures

The work studies the effect of intermolecular interactions on the relation between the rate constants of radiative processes and internal intercombination conversion in particlular molecule - solvent systems. The 1-naphthol complexes in binary mixtures with hydrogen bonding and those of coumarin 1 with nitrogen-nitrogen bonding are examined. The Stern - Volmer curves for the complexes with additions of nitromethane are shown to deviate from linearity.The interaction of coumarin 1 and 1-naphthol with nitromethane was simulated by the INDO/SP method to show that the lower singlet state is associated with nitromethane. The experimentally observed fluorescence band of the coumarin 1 and 1-naphthol molecules belongs to the third singlet state. The quantum yield of fluorescence from the S₃ state is decreased by an order of magnitude for the complexes under study. The quantum yield from the lower singlet state in the 1-naphthol - nitromethane complex is decreased by an order of magnitude, while in the coumarin 1 - nitromethane complex - by two orders. It is found that there is a possibility of photoprotonation in the S₁ and S₂ states for the 1-naphthol molecule along with the foregoing processes of quenching of fluorescence.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 17–23, January, 2005.     

Gas-phase photoreactions of anthracene, 2-aminoanthracene,and pyrene with oxygen and water vapors

Excited singlet (S ₁) and triplet (T ₁) state quenching by O₂ and by (O₂ + H₂O) gas-vapor mixtures was studied in the gas phase for polycyclic aromatic hydrocarbons (PAHs, anthracene, 2-aminoanthracene, pyrene). Addition of water vapor is shown not to influence quenching of both fluorescence and delayed fluorescence of PAHs by oxygen. The role of complexes stabilized by charge transfer and hydrogen bonds in quenching the excited states of PAHs by atmospheric gases was analyzed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 342–348, May–June, 2009.     

Ab initio Calculation of Normal Vibrations of Chlorobenzene in the First Singlet Electron-Excitebd State 1 B 2

An analysis of experimental assignments of the frequencies of normal vibrations of chlorobenzene in the first singlet electron-excited state ¹ B ₂ has been performed with the use of two quantum-mechanical prediction methods — the method of frequency shifts and the method of transfer of scaling factors. Based on the data of this analysis, a new assignment of a number of vibration frequencies has been made. Normal vibrations of the chlorobenzene molecule in the excited state have been calculated by the CIS method with a 6-311 + G ^** basic set. An algorithm of autoscaling in dependent natural coordinates has been proposed. The force field of chlorobenzene has been scaled in dependent natural coordinates. It is shown that the scaling factors of benzene in the electron state ¹ B _2u can be used for calculating the frequencies of normal vibrations of chlorobenzene in the first singlet excited state ¹ B ₂.     

Transient absorption spectroscopy of a monofullerene C<Subscript>60</Subscript>-bis-(pyropheophorbide <Emphasis Type="Italic">a</Emphasis>) molecular system in polar and nonpolar environments

The population dynamics of the excited and ground states of the monofullerene-bis (pyropheophorbide a) complex (FP1) were studied in polar (DMF) and nonpolar (toluene) solvents using picosecond transient absorption techniques. A strong quenching of the fluorescence signal of FP1 was observed in both solvents, in comparison to the fluorescence of bis (pyropheophorbide a) (P2). This quenching is due to an intramolecular photoinduced electron transfer from the pyropheophorbide a (pyroPheo) moiety to the fullerene C₆₀ monoadduct. In DMF the charge-separated (CS) state of FP1 has a lifetime of 0.32 ns and undergoes a direct transition to the ground state, resulting in a very low value of photosensitised singlet oxygen generation. In toluene, energy transfer from the first excited triplet state of pyroPheo, which has been populated via relaxation of the CS state, generates a considerable amount of singlet oxygen. The lifetime of the CS state in the nonpolar solvent was estimated to be 0.29 ns. It was also shown that in both DMF and toluene the first excited singlet state as well as the triplet state of the fullerene moiety in FP1 are not occupied. PACS 31.70.Dk; 31.70.Hq; 33.50.-j; 34.70.+e     

Qualitative assessment of ultra-fast non-Grotthuss proton dynamics in S<Subscript>1</Subscript> excited state of liquid H<Subscript>2</Subscript>O from ab initio time-dependent density functional theory

We study qualitatively ultra-fast proton transfer (PT) in the first singlet (S₁) state of liquid water (absorption onset) through excited-state dynamics by means of time-dependent density functional theory and ab initio Born-Oppenheimer molecular dynamics. We find that after the initial excitation, a PT occurs in S₁ in form of a rapid jump to a neighboring water molecule, on which the proton either may rest for a relatively long period of time (as a consequence of possible defect in the hydrogen bond network) followed by back and forth hops to its neighboring water molecule or from which it further moves to the next water molecule accompanied by back and forth movements. In this way, the proton may become delocalized over a long water wire branch, followed again by back and forth jumps or short localization on a water molecule for some femtoseconds. As a result, the mechanism of PT in S₁ is in most cases highly non-Grotthuss-like, delayed and discrete. Furthermore, upon PT an excess charge is ejected to the solvent trap, the so-called solvated electron. The spatial extent of the ejected solvated electron is mainly localized within one solvent shell with overlappings on the nearest neighbor water molecules and delocalizing (diffuse) tails extending beyond the first solvent sphere. During the entire ultra-short excited-state dynamics the remaining OH radical from the initially excited water molecule exhibits an extremely low mobility and is non-reactive.