Temperature dependence of the deactivation of electronically excited indazoles in solution

The T-T spectra, the triplet quantum yield φ_T and the fluorescence quantum yield φ_F of 2-tert. butyl-4-methylindazole and 1,3-dimethylindazole have been measured in the temperature range of +25° to −196°C in solvents of different viscosities. It could be shown that in all cases where photochemical reactions from the first excited singlet state are absent, the sum φ_F + φ_T equals unity within the limits of error.     

Mechanisms in the quantum yield of Cypridina bioluminescence

Abstract— –The influence of temperature, pH, salts, and reactant concentrations on the biolumin-escent oxidation of Cypridina luciferin catalyzed by Cypridina luciferase indicates a highest quantum yield φ (einsteins per mole of luciferin oxidized) of 0.31 in H₂O, or 0.33 in 99% D₂O. With the aid of data on fluorescence of the light-emitting oxyluciferin-luciferase complex, and of oxyluciferin in diglyme, partial explanations are suggested for the observed variations in φ, including the relatively low φ, of 0.03 for chemiluminescence of luciferin in organic solvents, wherein a different pathway of luciferin degradation, as indicated by chromatographic evidence, results in much less population of the excited state.     

THE EFFECTS OF HINDERED AMINES ON METHYL METHACRYLATE POLYMERIZATION

The effects of hindered amines, such as TMP, TMPM, TUV-770, on MMA and St homopoly-merization have been studied. It was found that in the MMA polymerization initiated with BPO orAIBN, the presence of TMPM, TUV-770 prolonged the induction period and a little change on R_p wasobserved. However, in the presence of TMP R_p increased slightly. The activation energy of poly-merization and polymerization rate equation were determined.     

The dynamics of formation of vibrationally excited HF in reactions of O (21 D2) atoms with partially fluorinated alkanes

The nascent vibrational energy distributions of the HF? formed in the reactions of a series of partially fluorinated alkanes (R_FH; R_F = CH₂F, CHF₂, CF₃, C₂F₅, C₃F₇, and C₇F₁₅) with electronically excited oxygen atoms O(2¹D₂) have been determined by measuring the appearance times of stimulated emissions from various vibration–rotation transitions in a grating-tuned optical cavity. The vibrational energy contents of the HF formed in these reactions were found to be considerably greater than statistically expected. These reactions are believed to occur via vibrationally excited short-lived α;-fluorinated alcohols (R_FOH?), formed by insertion of the O(2¹D₂) atoms into C? H bonds. The observation of nonstatistical energy partitioning in the above reactions is in clear contrast to the result obtained from the O(2¹D₂) + CF₃CH₃ reaction that produces the β-fluorinated alcohol CF₃CH₂OH, from which the HF product carries a near statistical vibrational energy distribution. A mechanism for HF? formation in these very exothermic reactions is presented.     

PHOTOSTATIONARY FLUORESCENCE EMISSION AND TIME RESOLVED SPECTROSCOPY OF SYMMETRICALLY DISUBSTITUTED ANTHRACENES ON THE meso AND SIDE RINGS: THE UNUSUAL BEHAVIOR OF THE 1,4 DERIVATIVE*

Abstract— Steady state and time resolved fluorescence emission properties of symmetrical dialkoxy-anthracenes (especially substituted on the side rings) 1-X, Y were studied in methylcylohexane. At room temperature, the fluorescence spectra of 1-X, Y show bands in the region of 380–550 nm and quantum yields (φ_F) in the range of 0.2–1. The fluorescence emission decays were found to be single exponential. The determination of the intersystem crossing quantum yields (φ_isc) for the weakly fluorescent compounds (1–1,5, 1–1,8 and 1–2,3) demonstrates that internal conversion is negligible compared with fluorescence emission and intersystem crossing, as previously observed for other anthracene derivatives. The fluorescence emission efficiency of compounds 1-X,Y is controlled by the relative mutual positions of the second triplet T₂ (whose energy varies significantly with substitution) and the first excited singlet S₁ states, respectively. An unusual solvatochromism was found for compound 1–1,4 which has a very weak permanent dipole moment in the ground state. This behavior was assigned to strong changes in the electronic densities between the excited singlet state and the ground state.     

Zum Mechanismus der photochemischen Umwandlung von 2-Alkyl-indazolen in 1-Alkyl-benzimidazole. II. Photophysikalische und photochemische Primärprozesse

Mechanistic studies on the photoisomerization of 2-alkyl-indazoles into 1-alkyl-benzimidazoles. II. Primary photochemical processes and photophysical deactivation. In the previous paper [1] the structure of the intermediate in the photochemical indazole-benzimidazole-isomerization was discussed ( 3 in Scheme 1). In this communication experiments concerning the photochemical primary processes and photophysical deactivation of 2-alkyl-indazoles ( 1 ) are described. The quantum yield of the rearrangement 1 → 2 (Φ_R) decreases with decreasing temperature while the fluorescence quantum yield (Φ_F) increases and finally reaches a constant value ( ≠ 1) (Fig.10). This behaviour is inconsistent with the mechanism shown in Scheme 2. Photoreaction and fluorescence are both quenched, but not to the same extent, by freon 113 (Fig. 2). In addition the Stern-Volmer-plots are not linear. These observations are best explained by assuming the existence of two excited states in equilibrium (Scheme 3). The mechanism in Scheme 3 correctly explains the quenching experiments and the temperature dependence of Φ_R and Φ_F if the Arrhenius law holds for the two rate constants k_sx and k_R. However, for a quantitative calculation of Φ_R, an additional branching of the reaction pathway must be postulated (Scheme 4). Two-dimensional drawings of hypothetical potential energy surfaces of the ground state and the first excited singlet state yielding a qualitative picture of the reaction and deactivation pathways of the discussed molecule are given in Fig. 15 a and b.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—VIII. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRUM OF THE BACTERIOPHEOPHORBIDE a ANION RADICAL

Ab initio configuration interaction wavefunctions and energies are reported for 16 doublet states of the anion radical of ethyl bacteriopheophorbide a (Et-BPheo a^-˙), and are employed in an analysis of the electronic absorption spectrum. The lowest excited doublet state D₁ is predicted to lie 8601 cm^-1 above the ground state D₀; the D₁← D₀ transition is nearly forbidden, with a computed oscillator strength f= 0.002. The visible absorption spectrum is shown to consist of transitions to three ²(π, π*) states, D₂, D₃, and D₄. The D₄← D₀ transition (y-polarized, f= 0.91) appears to account for observed intense absorption at 15 800 cm^-1. The Soret band of Et-BPheo a^-˙ is shown to consist of transitions to several ²(π,π*) states, D₇-D₁₅. Transitions of particularly high intensity include D₇← D₀ (y-polarized, f= 0.72), D₁₀← D₀ (y-polarized, f= 1.1), D₁₂← D₀ (xy-polarized, f= 0.86) and D₁₅← D₀ (y-polarized, f= 0.83). Spin density data and plots are used to describe and compare the general features of the unpaired spin distributions in D₀ and D₁, which are in reasonable agreement with other reported calculated values and available experimental data for D₀.     

Photosensitization by Norfloxacin is a Function of pH

Abstract— Norfloxacin is a fluoroquinolone (FQ) antibiotic that has been reported to cause cutaneous photosensitivity in animals and occasionally in humans. We have studied the fluorescence and singlet oxygen (¹O₂)-generating properties of norfloxacin. Upon UV excitation the drug fluoresces in water, and the relative intensities of two major fluorescence bands at ca 420 and 450 nm are affected by pH. The overall quantum yield of fluorescence (φ_F) is also strongly pH dependent: φ_F is low in 0.2 N HC1 solution (0.2), increasing steeply to 0.12 at pH 4, then gradually decreasing to 0.01 at pH 10. The changes in φ_F are accompanied by changes in fluorescence lifetime from 0.6 ns at pH 1 to 1.8 ns at pH 4. Norfloxacin exhibits phosphorescence in low temperature glasses. The formation of a triplet state at room temperature is also suggested by ¹O₂ phosphorescence in aerobic D₂O. This phosphorescence is “self-quenched” by norfloxacin itself with an efficiency that is pH dependent: k_q is 7.9 ×10⁶M^?1s^?1 at pD 4, decreases to 1.9 × 10⁶M^?1 s^?1 at pD 7.5 but then increases about 20-fold in alkaline D₂O solutions. This quenching causes the observed ¹O₂ production by norfloxacin (0.1 mM) to show a maximum at around pH 8–9. However, after correction for self-quenching, the quantum yield of ¹O₂ production (φ_so), measured by using perinaphthenone as a standard, yielded the following values: φ_so is about 0.07 in 0.2 N DCl solution, 0.08 at pH 7.5 and then increases smoothly to ～ 0.2 in 0.1 M NaOD solution. The relatively high, unquenched ¹O₂ production at physiological pH 7.4 (φ_so～ 0.08) suggests that ¹O₂ reactions may play an important role in the cutaneous phototoxicity of norfloxacin and other FQ antibiotics.     

ACTION SPECTRA FOR THE PHOTOLYSIS OF 5-IODODEOXYURIDINE IN DNA AND RELATED MODEL SYSTEMS: EVIDENCE FOR SHORT-RANGE ENERGY TRANSFER*

Abstract— A variety of polynucleotides containing 5-iodouracil residues were irradiated in aqueous solution with wavelengths between 240 and 313 nm. From the rate of deiodination the photochemical cross sections (a_B) were determined as a function of the irradiation wavelength (A). The expression was used to relate the observed values of _B to the intrinsic quantum yield, φ_INT, and to the absorption cross sections, and for the iodinated and noniodinated residues, respectively. φ_INT is the probability an excited iodouracil residue will deiodinate, while the parameter b is a measure of the number of noniodinated bases which contribute their excitation energy to the deiodination process. For IdUrd and poly(5-iodouridylic acid), the average values of φ_INT calculated from the experimental _B values were 0.0202 and 0.0188, respectively, for irradiation in air. In native, denatured, and depurinated DNA in which IdUrd was substituted for 10% of the Thd, the average φ_INT values were 0.0069, 0.0088, and 0.0153, respectively, indicating an enhancement in φ_INT upon decreasing the order of the polynucleotide. In contrast, the average values of b bor the same set of compounds were found to be 4 , 2 and 0.4, respectively, indicating a decrease in b with decreasing polynucleotide order, i.e. a loss of base stacking decreases the extent of energy transfer. A value of b= 4 for native DNA is assumed to mean that the extent of energy transfer in native DNA is limited to four base donors per iodouracil residue serving as an energy trap.     

THE PHOTOPHYSICS OF BONELLIN: A CHLORIN FOUND IN MARINE ANIMALS

The photophysical properties of bonellin, a free-base chlorin, were studied in ethanolic solution. For the singlet excited state the following data were determined: an energy level, E^B_S= 187 ± 2kJ mol^-1, a lifetime, τ_f= 6.3± 0.1ns at 298 K, and fluorescence quantum yields, φ_r= 0.07 ± 0.02 (298 K) and 0.20 ± 0.04 (77 K). The S₁→ T intersystem crossing quantum yield was φ_isc= 0.85 ± 0.1. No phosphorescence was observed at 298 K and 77 K. Based on quenching experiments the triplet state energy level was determined to be E^B_T= 180 ± 20 kJ mol^-1. A unimolecular decay rate constant, k₁= (2.3 ± 0.5)· 10³ s^-1 at room temperature, and a molar absorption coefficient, ε^T₄₄₃= 9500 ± 500 M^-1 cm^-1, were obtained for the triplet state. This species was quenched by O₂ with ko₂= (1.7 ±0.3)· 10⁸M^-1 s^-1, and by benzoquinone with k_q= (5.2 ± 0.3)-10⁹M^-1 s^-1. The latter value, as well as the high value determined for the triplet annihilation rate constant, k₂= (2 ± 0.5)· 10⁹M^-1 s^-1, might reflect an electron transfer mechanism. Copper bonellin had a shorter triplet lifetime (>20 ns), which offers a possible explanation for its lack of photodynamic action.     

Influence of Isotope Substitution on Lattice and Spin‐Peierls‐Type Transition Features in One‐Dimensional Nickel Bis‐dithiolene Spin Systems

Four new 1D spin‐Peierls‐type compounds, [D₅]1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate ([D₅]R‐Py; R=F, I, CH₃, and NO₂), were synthesized and characterized structurally and magnetically. These 1D compounds are isostructural with the corresponding non‐deuterated compounds, 1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate (R‐Py; R=F, I, CH₃, and NO₂). Compounds [D₅]R‐Py and R‐Py (R=F, I, CH₃, and NO₂) crystallize in the monoclinic space group P2₁/c with uniform stacks of anions and cations in the high‐temperature phase and triclinic space group P$\bar 1$ with dimerized stacks of anions and cations in the low‐temperature phase. Similar to the non‐deuterated R‐Py compounds, a spin‐Peierls‐type transition occurs at a critical temperature for each [D₅]R‐Py compound; the magnetic character of the 1D S=1/2 ferromagnetic chain for [D₅]F‐Py and the 1D S=1/2 Heisenberg antiferromagnetic chain for others appear above the transition temperature. Spin‐gap magnetic behavior was observed for all of these compounds below the transition temperature. In comparison to the corresponding R‐Py compound, the cell volume is almost unchanged for [D₅]F‐Py and shows slight expansion for [D₅]R‐Py (R=I, CH₃, and NO₂) as well as an increase in the spin‐Peierls‐type transition temperature for all of these 1D compounds in the order of F>I≈CH₃≈NO₂. The large isotopic effect of nonmagnetic countercations on the spin‐Peierls‐type transition critical temperature, T_C, can be attributed to the change in ω₀ with isotope substitution.     

The mercury-sensitized oxidation of carbon monoxide

The mercury-photosensitized oxidation of CO was studied at 275°C over a wide range of [O₂]/[CO] ratios in the absence and presence of the oxygen atom scavenger 2-trifluoromethylpropene (TMP) and at 25°C at low [O₂]/[CO] ratios in the presence of TMP. By following the quantum yield of CO₂ production, Φ {CO₂}, as a function of the [O₂]/[CO] ratio, the reactions of vibrationally excited CO (v υ 9) and electronically excited O₂, probably in the c¹Σ^?_u state, were studied. At low [O₂]/[CO] ratios the predominant reactions are of vibrationally excited CO (v υ 9). Relative rate constants for chemical reaction versus deactivation of CO (v υ 9) were obtained. At higher [O₂]/[CO] ratios, the principal reactions are of electronically excited O₂. Relative rate constants for chemical reactions and deactivation of this electronically excited O₂ with CO, O₂, and TMP were obtained. From the effect of total pressure on Φ {CO₂}, it is proposed that an intermediate CO₃ is formed in the reaction of electronically excited O₂ with CO.     

Crystal Structure and Fluorescence Spectra of A Complex of Europium Picrate with N,N,N′,N′-tetraphenyl-3,6,9-trioxaundecanediamide

The title complex Eu(Pic)₃(TTD) crystallizes in space group Pcab with a = 23.714(4), b = 29.245(6), c = 32.035(8)Å, V = 22216(7)ų, Z = 16, D_c = 1.63 g cm^-1, w = 12.5 cm^-1, F(000) = 10976 and its structure was solved and refined to R = 0.083. Two crystallographically different Eu(Pic)₃(TTD) units, in which the coordination numbers of two central ions (Eu1 and Eu2) are nine and ten, respectively, are present in the asymmetric unit. In the ⁷F₀ → ⁵D₀ excitation spectrum of the complex, there are two sharp peaks at 578.5 and 579.0 nm indicating that the complex has two Eu(III) sites. Two different patterns in the ⁵D₀ → ⁷F_j (j = 1-4) luminescence spectrum have been observed (selectively excited ⁵D₀), which further reveals the existence of two Eu(III) sites compatible with the result of the structure determination.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—IX. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRUM OF THE BACTERIOCHLOROPHYLLIDE a ANION RADICAL

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for 19 doublet states of the anion radical of ethyl bacteriochlorophyllide a (Et-BChl a˙), and are employed in a resolution of the electronic absorption spectrum, as well as in a comparison with a previously reported study of the electronic states and spectrum of the anion radical of ethyl bacteriopheophorbide a (Et-BPheo a˙). The lowest two excited doublet states, D₁ and D₂, of Et-BChl a˙ are found to be approximately degenerate and are predicted to contribute to the experimentally observed absorption at 10000 cm^?1. In contrast, the D₂←D₀ transition in Et-BPheo a˙ is predicted to contribute to the 11000 cm^?1 absorption, while the D₁←D₀ transition appears at approximately 8600 cm^?1 with a low oscillator strength (f= 0.002). The prominent visible absorption at ～15700 cm^?1 in both molecules is found to be due to the D₄← D₀ transition. Another difference between the predicted spectra of the two molecules appears in the low-energy shoulder of the Soret band. Here, two intense transitions, D₁₀←D₀ and D₁₁←D₀, are predicted for Et-BChl a˙, as opposed to three fairly intense transitions, D₇←D₀, D₈←D₀ and D₉←D₀, for Et-BPheo a˙, differences which may provide a means of distinguishing between the two molecules using resonance Raman spectroscopy. The remainder of the Soret band of Et-BChl a˙ above 26000 cm^?1 consists of a number of closely-spaced transitions to states D₁₂←D₁₈. The intense transitions D₁₂←D₀, D₁₃←D₀, D₁₄←D₀ are predicted to contribute to the Soret maximum near 30000 cm^?1. The ground state spin densities of the two molecules are similar, with the minor difference of somewhat less spin density located on the methine carbon atoms of Et-BChl a˙ compared with Et-BPheo a˙.     

MECHANISM OF THE PHOTOSENSITIZED REDOX REACTIONS OF ACRIDINE ORANGE IN AQUEOUS SOLUTIONS-A SYSTEM OF INTEREST IN THE PHOTOCHEMICAL STORAGE OF SOLAR ENERGY†

-We have carried out a very detailed study, using fluorescence and optical flash photolysis techniques, of the photoreduction of methyl viologen (MV²⁺) by the electron donor ethylene diamine tetraacetic acid (EDTA) in aqueous solution sensitized by the dye acridine orange (AOH⁺). A complete mechanism has been proposed which accounts for virtually all of the known observations on this reaction. This reaction is novel in that both the triplet and the singlet state of AOH⁺ appear to be active photochemically. We have shown that mechanisms previously proposed for this reaction are probably incorrect due to an artifact. At pH 7 the fluorescence quantum yield φ_s of AOH⁺ is 0.26 ± 0.02 and the fluorescence lifetime is 1.8 ± 0.2 ns. φ_s is pH dependent and reaches a maximum of 0.56 at pH 4. The fluorescence of AOH⁺ is quenched by MV²⁺ at concentrations above 1 mM and the quenching obeys Stern-Volmer kinetics with a quenching rate constant of (1.0 ± 0.1) × 10¹⁰M^?1 s^?1. The quenching of the AOH⁺ excited singlet state by MV²⁺ almost certainly returns the AOH⁺ to its ground state with no photochemistry occurring. EDTA also quenches the fluorescence of AOH· with Stern-Volmer kinetics but with a smaller rate constant (6.4 ± 0.5) × 10⁸M^?1s^?1 at pH 7. In this case the quenching is reactive resulting in the formation of semireduced AOH. In the presence of MV²⁺, flash irradiation of AOH⁺ does result in the reversible formation of the semireduced MV? which absorbs at 603 nm. We attribute this to a photochemical reaction of the triplet state of AOH⁺ with MV²⁺. The initial quantum yield for formation of MV? (φ_MV:)₀ was found to be constant at 0.10 ± 0.05 for [MV²⁺] from 5 × 10^?5 to 1.0 × 10^?3 with [AOH⁺] = 8 × 10^?6M. Previous workers had found that (φ_MV:)₀ appears to decrease with decreasing [AOH⁺]; however, on careful investigation, we found this was most probably due to quenching of the triplet state of AOH⁺ by trace amounts of oxygen. When EDTA is added to a mixture of AOH ⁺ and MV²⁺ at pH 7, the photochemical formation of MV? becomes irreversible as the [EDTA] is increased. The quantum yield for the irreversible formation of MV? exceeds 0.10 becoming as large as 0.16 for [EDTA] = 0.014M. This fact requires that an alternative photochemical process must be operative and we present evidence that this is a reaction of EDTA with the excited singlet state of AOH⁺ to produce the semi-reduced AOH- which then reacts with MV²⁺ to produce MV?. The full kinetic scheme was tested by computer simulation and found to be totally consistent. This also enabled the processing of a full set of rate constants. When colloidal PtO₂ was added to the optimal mixture [EDTA] = 3.4 × 10^?2M; [MV²⁺] = 5 × 10^?4M; [AOH⁺] = 4 × 10^?5M; pH6 H₂ gas was produced at a rate of 0.2μmol H₂h^?1. Thus, acridine orange should serve as an effective sensitizer in reactions designed to use solar energy to photolyze water.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—VII. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRA OF CHLOROPHYLLIDE a AND BACTERIOCHLOROPHYLLIDE a CATION RADICALS

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for 29 doublet states and three quartet states of the cation radicals of ethyl chlorophyllide a (Et-Chl a⁺) and ethyl bacteriochlorophyllide a (Et-BChl a⁺). In Et-Chl a.⁺ I the lowest excited doublet state D₁ is estimated to lie 5220 cm^-1 above the ground state D₀, with a negligibly small D₁← D₀ transition probability. The lowest quartet state, Q₁, is estimated to lie 7980 cm^-1 above D₁. The absorption spectrum up to 20,000cm^-1 is shown to consist primarily of numerous low-intensity ‘background’ transitions, with transitions to D₅ and D₁₁ accounting for the observed peaks at 12,200cm^-1 and 17,500cm^-1, respectively. The large intense band at 25,000cm^-1 is due primarily to transitions to D₂₂ and D₂₃, with numerous lower-intensity transitions to neighboring states. In Et-BChl a.⁺ D₁ is estimated to lie 7112 cm-¹ above D₀, and Q, is approximately 5725 cm^-1 above D. A pair of states, D₃ and D₅, account for the absorption at 11,000 cm^-1, while another pair of states, D₁₃ and D₁₄, are associated with the broad, weak absorption near 20,000 cm^-1. The two prominent intense peaks at 23,700 cm^-1 and 27,700 cm^-1 are assigned to D₂₃ and D₂₈, respectively, while the shoulder located at 25,500cm^-1 is attributed to transitions to D₂₄ and D₂₆. As in Et-Chl at, numerous background transitions are found throughout the spectrum. The π spin density distribution in D₀ of both molecules is similar, with spin density found predominantly on the α-carbon atoms. In both systems, approximately 65% of the π spin density in D₁ is found on the methine carbon atoms, with the remainder found largely on the nitrogen atoms.     

Anionic high-temperature conduction in single crystals of nonstoichiometric phases R1 ? y M y F3 ? y (R = La-Lu, M = Ca, Sr, Ba) with the tysonite (LaF3) structure

Electrophysical properties of single crystals of nonstoichiometric phases R_{1 − y} M _y F_{3 − y} , where R = La-Lu, M = Ca, Sr, or Ba, with the tysonite (LaF₃) structure, which are present in a metastable state after being grown and cooled, are measured in the temperature interval extending from 300 to 1073 K. It is discovered that, during a sufficiently long high-temperature investigation, solid solutions R_{1 − y} Ca _y F_{3 − y} , where R = Tb, Dy, or Ho, undergo irreversible variations in the phase composition in the temperature region 723 to 823 K. This level of temperatures, which correspond to partial decomposition of phases R_{1 − y} Ca _y F_{3 − y} with the rare-earth elements of the end of the period, lies above the temperatures to which the fluoride solid electrolytes are usually heated when used in solid-state electrochemical devices. The temperature and concentration dependences of the phases’ electroconduction are explained in the framework of the vacancy mechanism of anionic transport. Original Russian Text ? N.I. Sorokin, B.P. Sobolev, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 4, pp. 420–431. The paper is dedicated to the memory of Prof. M.W. Breiter, formerly of the Vienna Technical University, Austria.     

Equilibrium Geometry of Symmetrical Polymethine Dyes in the Solitonic Excited State

The optimized geometry in the excited state of the ions of polymethine dyes R^±—(CH=CH) _n —CH=R with the pyridinium residue and its carbo- and heteroanalogs in the excited state was calculated by quantum-chemical methods in the AM1 approximation. It was found that in addition to the symmetrical solution of the Hamiltonian there is an unsymmetrical soliton state. Significant alternation of the CC bond lengths at one end of the chain in the solitonic state explains the marked decrease in the quantum yields of fluorescence in the long molecules as a result of the decreased barriers of the conformational transformations.     

Competitive study of the reactions Br + RFI → IBr + RF and determination of bond dissociation energies D(RF−I) where RF = CF3, C2F5, n-;C3F7, i-C3F7, and n-C4F9

The overall photobromination reactions have been studied using a competitive technique. Relative Arrhenius parameters were obtained for the rate-determining step These were placed on an absolute basis using previous-absolute values of A and E for R_FI=CF₃I. The activation energies were used to calculate bond dissociation energies D(R? I) with the following results:

QUANTUM YIELD RATIO OF THE FORWARD and BACK LIGHT REACTIONS OF BACTERIORHODOPSIN T LOW TEMPERATURE and PHOTOSTEADY-STATE CONCENTRATION OF THE BATHOPRODUCT K*

The maximum photosteady state fraction of K, x_K^max, and the ratio of the quantum yields of the forward and back light reactions, trans-bacteriorhodopsin (bR) hArr; K, φ_bR/φ_K, were obtained by measuring the absorption changes produced by illumination of frozen water-glycerol (1:2) suspensions of light-adapted purple membrane at different wavelengths at -165°C. An independent method based on the second derivative of the absorption spectrum in the region of the β-bands was also used. It was found that The quantum yield ratio (0.66 ± 0.06) was found to be independent of excitation wavelength within experimental error in the range510–610 nm. The calculated absorption spectrum of K has its maximum at603–606 nm and an extinction 0.85 ± 0.03 that of bR. At shorter wavelengths there are P-bands at 410, 354 and 336 rim. Using the data of Hurley et al. (Nature 270,540–542, 1977) on relative rates of rhodopsin bleaching and K formation, the quantum yield of K formation was determined to be 0.66 ± 0.04 at low temperature. The quantum efficiency of the back reaction was estimated to be 0.93 ± 0.07. These values of quantum efficiencies of the forward and back light reactions of bR at - 165°C coincide with those recently obtained at room temperature. This indicates that the quantum efficiencies of both forward and back light reactions of bacteriorhodopsin are temperature independent down to -165°C.