Distinction of the delayed fluorescences S1 → S0 And S2 → S0 of pyrene by selective quenching Of S1 → S0

In the spectrum of the delayed fluorescence (DF) of pyrene, caused by triplet-triplet annihilation T₁ + T₁ → S_n + S_o (n = 1,2), a strong DF S₁ → S_o and a very weak DF S₂ → s₀ are observed. The DF S₁→ S_o is quenched selectively by compounds like N-diethylanine or triethylamine which do not quench T₁ of pyrene.     

THE S0 AND Ti STATES OF ISOMERIC CANTHAXANTHIN AS COMPARED WITH THOSE OF β-CAROTENE: EFFECT OF THE TERMINAL CARBONYL GROUPS DETECTED BY RAMAN SPECTROSCOPY

The S₀ (ground singlet-state) Raman spectra of the all-trans, 9-cis, 13-cis, 9,9-cis, 9, 13-cis, 9,13′-cis and 13,13′-cis isomers of canthaxanthin as well as the T₁ (lowest-excited triplet-state) Raman spectra of the all-trans, 9-cis, 13-cis and 9,13-cis isomers were recorded. In order to reveal the effect of the carbonyl groups at both ends on the carbon-carbon conjugated system in-between (in both the S₀ and T, states), the carbon-carbon stretching frequencies were compared between isomeric canthaxanthin and p-carotene: in the S₀ state, the C=C stretching frequencies of canthaxanthin were lower by3–10 cm^?1 than those of β-carotene, indicating increased conjugation in the former. In the T, state, the “C=C” stretching frequencies of canthaxanthin were lower by12–15 cm^?1, indicating a large decrease in the “C=C” bond order in the central part of canthaxanthin. Further, the relations of the C=C (S₀) and “C=C” (T₁) stretching frequencies vs the number of carbon-carbon double-bonds were examined for the above two and additional five carotenoids. The result indicated that the terminal carbonyl groups of canthaxanthin are incorporated in the carbon-carbon conjugated system in the T, state, but that they are almost independent of it in the S₀ state. Both observations support the idea that the “triplet-excited region” of canthaxanthin is extended over the entire double-bond system.     

The deexcitation of the S1 state of aminoanthraquinones: A steadystate and timeresolved study

The non-radiative processes of deactivation from the lowest singlet excited state of aminoanthraquinones have been studied using steady-state and time-resolved methods. The fluorescence decay rate constant, k_f correlates well with the solvent polarity parameter, E_T(30), in nonhydrogen bonding solvents. Large deuterium isotope effects in fluorescence lifetimes (τ_f) and quantum yields (ϕ_f) are observed in the case of 1-amino (AAQ) and 1-methylaminoanthraquinones (MAQ), where the S₁ state is mainly deactivated through internal conversion to the ground state. The temperature-dependence of the fluorescence quantum yields of various aminoanthraquinones was also investigated. The ϕ_f and τ_f exhibited strong temperature-dependence in the case of 1-acetylaminoanthraquinone (ACAQ). In the case of ACAQ, the intersystem crossing to the triplet state is a major deactivation channel from the S₁ and in this derivative a close-lying T₂ state seems to be responsible for the high k_isc rate. The fluorescence properties of 1,5-diaminoanthraquinone (DAQ) are affected by intermolecular hydrogen bonding with alcohols. Increasingn-alkyl chain length in the case of l-(n-alkyl)aminoanthraquinones from methyl to butyl does not produce any change in the fluorescence properties, whereas a hydroxypropyl substitution results in a small decrease of ϕ_f and τ_f in these compounds, indicating an interaction of the hydroxyl group with the carbonyl group of the aminoanthraquinones.     

SIMPLE EXPLANATION OF THE MISSES IN THE COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION

Abstract —In the model of Forbush et at. (1971) the observed damping of the flash yield sequence of photosynthetic O₂ evolution was related to a certain percentage of ‘misses’ (α; i.e. centers not converted). The possibility of a miss was supposed to be equal for all states S_0.1,2,3. We propose a new model and a new recurrence law that gives better quantitative agreement with the O₂ yield oscillations observed in Chlorella during a sequence of flashes. We find a better fit with all experimental results by assuming very unequal misses; the misses occur nearly exclusively on S₂ (and also sometimes on S₃). In the simpler case of only one miss on one state, half of S₂ exists as an inactive form S₂₊^- because it is in apparent equilibrium with pool A. The active form of S₂ is converted to S₃ in a flash and the unchanged inactive form S₂₊^- explains the miss: S ₁ hv→S₂₊^-=S₂hv→S₃ (S₂₊^- is a transition state between S₁ to S₂ associated with Q^-). In the dark, the apparent equilibrium constant K_A between pool A and Q (i.e. S₀, S₁ in the dark) is very large; this explains why there is no miss on these states. In light, the experimental value of K_A between pool A and Q (i.e. S₂, S₃ in the light) is 1, and this explains why the misses are large for states S₂, S₃; i.e., S₂₊^-/S2- 1 and sometimes S₃₊^-/S₃?0–1. This new model predicts that the total number of active states ΣS_i=S₀+S₁+S₂+S₃ is an oscillating function of the flash number. This sum 2S, is also the number of trapping centers for excitons. As fluorescence is proportional to excitons that are not trapped, our model explains why the fluorescence oscillates as a function of the flash number. We find also that the initial rates of O₂ evolution after (n - 1) flashes vs the 02 yield of the n^th flash are not exactly on a straight line, which also favors our model.     

Oxidation of Chiral 1,4-Cyclohexadienes: A Comparison of Chemically and Photochemically Generated Singlet Oxygen1O2(1Δg)

Chiral alkyl-substituted 2,5-cyclohexadiene-l-carboxyIic acids la-c have been oxidized in water and in methanol with singlet oxygen, ¹O₂ (¹Δ_g), generated either photochemically or chemically from the catalytic system hydrogen peroxide/sodium molybdate. These methods were compared in terms of chemo-, regio- and diastereoselec-tivities and the chemical (k_T) and physical (k_q) quenching rate constants of ¹O₂ were determined. The ratio of the cis and trans isomers of the hydroperoxides 2a-c is not influenced by the source of ¹O₂ but, on the other hand, it depends slightly on the solvent and greatly on the steric hindrance of the substituents linked to the chiral carbon. The results may be interpreted on the basis of the successive formation of an exciplex and a perepoxide that evolves either by giving the final allylic hydroperoxide or by dissociating into the starting substrate and singlet or triplet oxygen.     

Effect of Preferential Solvation on the Thermodynamic Properties of Antidepressant Drug Trazodone in Aqueous Ethanol: Linear Free‐Energy Relationships

Dissociation constants (pK_a) of trazodone hydrochloride (TZD⋅HCl) in EtOH/H₂O media containing 0, 10, 20, 30, 40, 50, 60, 70, and 80% (v/v) EtOH at 288.15, 298.15, 308.15, and 318.15 K were determined by potentiometric techniques. At any temperature, pK_a decreased as the solvent was enriched with EtOH. The dissociation and transfer thermodynamic parameters were calculated, and the results showed that a non‐spontaneous free‐energy change (Δ_dissG^o>0) and unfavorable enthalpy (Δ_dissH^o>0) and entropy (Δ_dissS^o<0) changes occurred on dissociation of trazodone hydrochloride. The free‐energy change or pK_a varied nonlinearly with the reciprocal dielectric constant, indicating the inadequacy of the electrostatic approach. The dissociation equilibria are discussed on the basis of the standard thermodynamics of transfer, solvent basicity, and solute‐solvent interactions. The values of Δ_transG^o and Δ_transH^o increased negatively with increasing EtOH content, revealing a favorable transfer of trazodone hydrochloride from H₂O to EtOH/H₂O mixtures and preferential solvation of H⁺ and trazodone (TZD). Also, Δ_transS^o values were negative and reached a minimum, in the H₂O‐rich zone that has frequently been related to the initial promotion and subsequent collapse of the lattice structure of water. The pK_a or Δ_dissG^o values correlated well with the Dimroth‐Reichardt polarity parameter E_T(30), indicating that the physicochemical properties of the solute in binary H₂O/organic solvent mixtures are better correlated with a microscopic parameter than the macroscopic one. Also, it is suggested that preferential solvation plays a significant role in influencing the solvent dependence of dissociation of trazodone hydrochloride. The solute‐solvent interactions were clarified on the basis of the linear free‐energy relationships of Kamlet and Taft. The best multiparametric fit to the Kamlet‐Taft equation was evaluated for each thermodynamic parameter. Therefore, these parameters in any EtOH/H₂O mixture up to 80% were accurately derived by means of the obtained equations.     

Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr₂Nb₂O₇ and calcium niobate Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form C_pm = 248.0 + 0.04350T − 3.948 × 10⁶/T² J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and C_pm = 257.2 + 0.03621T − 4.434 × 10⁶/T² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, S_m°(298.15 K) = 238.5 ± 1.3 J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and S_m°(298.15 K) = 212.4 ± 1.2 J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇, were evaluated from the low-temperature heat capacity measurements.     

Delayed fluorescence from the lowest 1B+3u state of anthracene,due to hetero-triplet—triplet annihilation of 3anthracene* and 3xanthone*

The P-type delayed fluorescence (DF) S_i→S_o of aromatic compounds results from the population of excited singlet states S_i by triplet—triplet annihillation (TTA) of molecules in their lowest and metastable triplet state T₁ : T₁ + T₁

S_i + S_o; S_i may be any excited singlet state whose excitation energy E(S_i ? 2 E(T₁). TTA of unlike molecules A and B (hetero-TTA) may lead to excited singlet states either of A or of B. In particular, if E(T_A¹) < E(T₁^B), hetero-TTA may lead to excited singlet states S_k^A which are not accessible by TTA of 2 T₁^A. In the present paper we report the first example of the detection of the DF from a very short-lived upper excited singlet state S_k^A which has been populated by hetero-TTA. The systems investigated are liquid solutions of A = anthracene-h₁₀ or anthracene-d₁₀ or 9,10-dimethylanthracene and B = xanthone in 1,1,2-trichlorotrifluoroethane at 243 K. S_k^A is the lowest ¹B_3U⁺ state (B_b state) of anthracene.     

Determination of heat capacities of PbCrO4(s), Pb2CrO5(s), and Pb5CrO8(s)

Abstract  

Heat capacities of PbCrO₄(s), Pb₂CrO₅(s), and Pb₅CrO₈(s) were measured by differential scanning calorimetry. The measured heat capacities as a function of temperature are expressed as C _p <PbCrO₄> J K⁻¹ mol⁻¹ = 150.37 + 27.74 × 10⁻³ T − 2.80 × 10⁶ T ⁻² (T = 300–750 K), C _p <Pb₂CrO₅> J K⁻¹ mol⁻¹ = 194.55 + 76.09 × 10⁻³ T − 4.64 × 10⁶ T ⁻² (T = 300–700 K), and C _p  <Pb₅CrO₈> J K⁻¹ mol⁻¹ = 323.35 + 184.80 × 10⁻³ T − 5.48 × 10⁶ T ⁻² (T = 300–600 K). From the measured heat capacity data, thermodynamic functions such as enthalpy increments, entropies, and Gibbs energy functions were derived.     

Thermodynamic properties of BaCeO3 and BaZrO3 at low temperatures

Specific heat capacities (C_p) of polycrystalline samples of BaCeO₃ and BaZrO₃ have been measured from about 1.6 K up to room temperature by means of adiabatic calorimetry. We provide corrected experimental data for the heat capacity of BaCeO₃ in the range T < 10 K and, for the first time, contribute experimental data below 53 K for BaZrO₃. Applying Debye's T³-law for T → 0 K, thermodynamic functions as molar entropy and enthalpy are derived by integration. We obtain C_p = 114.8 (±1.0) J mol⁻¹ K⁻¹, S° = 145.8 (±0.7) J mol⁻¹ K⁻¹ for BaCeO₃ and C_p = 107.0 (±1.0) J mol⁻¹ K⁻¹, S° = 125.5 (±0.6) J mol⁻¹ K⁻¹ for BaZrO₃ at 298.15 K. These results are in overall agreement with previously reported studies but slightly deviating, in both cases. Evaluations of C_p(T) yield Debye temperatures and identify deviations from the simple Debye-theory due to extra vibrational modes as well as anharmonicity. The anharmonicity turns out to be more pronounced at elevated temperatures for BaCeO₃. The characteristic Debye temperatures determined at T = 0 K are Θ₀ = 365 (±6) K for BaCeO₃ and Θ₀ = 402 (±9) K for BaZrO₃.     

Réactions chimiques oscillantes. II. Comportement de la «période d'induction» dans les systémes BrO/Ce4+/Cyclohexanone et BrO/Ce4+/Cyclopentanone

Oscillating Chemical Reactions. 11. Behaviour of the “induction period” in the BrO /Ce⁴⁺/Cyclohexanon and BrO /Ce⁴⁺/Cyclopentanon systems

• (1) The addition of α-monobromoketone, one of the products of reaction of the BrO₃^?/Ce⁴⁺/Cyclohexanon (S1) and BrO₃^?/Ce⁴⁺/Cyclopentanon (S2) oscillating systems, decreases and even suppresses the induction period (τ_ind.) in the case of S2. Such is not the case with S1: τ_ind. increases and the oscillations can even be completely inhibited.
• (2) The order of addition of the reagents and the time lapse (t_adj.) preceding the addition of the last of them influences τ_ind., particularly when the last reagent added is Ce⁴⁺.
• (3) In our experimental conditions, the inhibition of the oscillatory phenomenon by Cl^? ions is definite only for | Cl^? | ≥ 5,0 · 10^?2M (S1) and |Cl^?| > 2,5 · 10^?3M (S2); for lower concentrations τ_ind. increases with | Cl^?|.

     

Determination of Chain Transfer Constant for Solvent When the Viscosity of the Polymerizing System Is Considered Important

Abstract

The chain transfer constant of the polymethyl methacrylate radical for N,N-dimethylaniline was determined in two solvents, benzene and dimethyl phthalate. Plots were made using1/P_n=k_t°R_p/k_p ²[M]²η + C_S1 [S₁]/[M] + C_S2 [S₂]/[M] +C_M where η=viscosity of monomer-solvents mixture, k_t°=rate coefficient of termination when η=1 cP, S₁=benzene or dimethyl phthalate, S₂=N,N-dimethylaniline, and other symbols have their usual meanings. The plots agreed well for the two solvents. If the plots were made without considering the viscosity term, two separate lines resulted for the two solvents. Thus it is essential to consider the viscosity of the polymerizing system in the analysis of chain transfer reactions when the termination reaction is diffusion-controlled and the viscosities of the monomer and solvent differ markedly.     

1H, 13C and 7Li nuclear magnetic resonance study of the lithium chloride–N,N-dimethylacetamide system

The complex solvent obtained by dissolving 5-10% of lithium chloride in N,N-dimethylacetamide (DMA) presents a good method for dissolving highly insoluble polymers, such as cellulose. ¹H, ¹³C and ⁷Li NMR spectroscopy have been used, together with viscosity and conductivity measurements, for the study of this complex solvent. The ¹H and ¹³C chemical shift variations of DMA, on increasing the lithium chloride concentration, are found to be in opposite directions. The T₁ relaxation times show a large decrease in the mobility of DMA in the presence of lithium chloride. Methyl-β-D -glucopyranoside has been used as a model for cellulose in order to investigate the mechanism of solution of this polymer. It was found that each hydroxy group of the solute interacts with one lithium chloride molecule in solution.     

Heat capacity of MnAs0.88P0.12 from 10 to 500 K: Thermodynamic properties and transitions

The heat capacity of MnAs_0.88P_0.12 has been measured by adiabatic shield calorimetry from 10 to 500 K. It is shown that very small energy changes are connected with two magnetic order-order transitions, indicating that these can be regarded as mainly “noncoupled” magnetic transitions. At higher temperatures contributions to the excess heat capacity arises from a magnetic order-disorder transition, a conversion from low- to high-spin state for manganese, and a MnP- to NiAs-type structural transition. The observed heat capacity is resolved into contributions from the different physical phenomena, and the character of the transitions is discussed. In particular it is substantiated that the dilational contribution, which includes magnetoelastic and magnetovolume terms as well as normal anharmonicity terms, plays a major role in MnAs_0.88P_0.12. The entropy of the magnetic order-disorder transition is smaller than should be expected from a complete randomization of the spins, assuming a purely magnetic transition. Thermodynamic functions have been evaluated and the respective values of C_p, {S^O_m(T) - S^O_m(0)}, and -{G^O_m(T) - H^O_m(0)}/T at 298.15 K are 68.74, 72.09, and 32.30 J K⁻¹ mole⁻¹, and at 500 K 56.05, 108.12, and 56.64 J K⁻¹ mole⁻¹.     

Thermodynamics of the dissociations of glycine in 50 mass % aqueous monoglyme from 5 to 55°C

Electromotive-force measurements on cells without liquid junction have been used to determine the pK ₁ and pK ₂ values of glycine in 50 mass % aqueous monoglyme at 11 temperatures from 5 to 55°C. The change in the first dissociation constant is given as a function of the thermodynamic temperatureT by the equation pK ₁=–2058.6/T+15.421–0.019169T, whereas that for the second dissociation constant is given by the equation pK ₂=1200.5/T+6.7211–0.0042897T. At 25°C, the pK ₁ is 2.806 in the mixed solvent, as compared with 2.350 in water; hence, protonated glycine becomes a weaker acid in the mixed solvent. The pK ₂ is 9.453 in the mixed solvent, whereas that in water is 9.780, suggesting that the second dissociation process becomes stronger in terms of acidity. The thermodynamic quantities G ^o, H ^o, S ^o, and C _p ^o have been calculated, and the results have been discussed with respect to preferential solvation and also compared with similar data for the same two processes in 50 mass % methanol.     

EFFECT OF THE BROMINE ATOM UPON THE DISSOCIATION CONSTANTS OF BROMOPYRIDINES,BROMOQUINOLINES AND 4-BROMOISOQUINOLINE IN THEIR LOWEST EXCITED SINGLET AND TRIPLET STATES*

Abstract— The lowest excited singlet-state dissociation constants (pK^S_a) of bromosubstituted pyridines, quinolines, and isoquinolines were determined from the pH-dependent shifts in their electronic absorption spectra. The lowest excited triplet-state dissociation constants (pK^T_a) of bromosubstituted quinolines and 4-bromoisoquinoline were obtained from the shifts of the 0–0 phosphorescence bands measured in rigid aqueous solution at 77 K. The pK^S_a values indicate that the basicity of these brominated nitrogen heterocycles is increased in the lowest excited singlet state by 2 to 10 orders of magnitude as compared with the ground state. The pK^T_a values are found to be significantly different from the corresponding ground-state pK_a values, indicating that the basicity of bromoquinolines is increased in the lowest excited triplet state by 1.7 to 3.0 pK units. The enhancement of the excited singlet-and triplet-state basicity of brominated nitrogen heterocycle derivatives as compared with the unsuhstituted parent compounds is attributed to the increased electron-donor conjugative interactions of the bromine atom pπ orbitals with π orbitals in the lowest excited singlet and triplet state.     

S1 (n,π*), T1 (n,π*) and S2(n,π*) emissions in 3,6-diphenyl-s-tetrazine

Simultaneous emissions from S₁(n,π^*) and S₂(n,π^*) states in 3,6-diphenyl-s-tetrazinc (DPT) have been observed along with weak luminescence from T₁ (n,π^*). The occurrence of the S₂(n,π^*) fluorescence has been justified on the basis of the slow S₂ XXX S₁ internal conversion resulting from the large energy gap between the two states. This is the first case of dual fluorescence where both the emitting states are of (n,π^*) nature.     

Thermal properties of the pyrochlore, Y2Ti2O7

The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y₂Ti₂O_7, have been determined over the temperature range 2 K?T?300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ_D, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for C_p−C_v. From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15 K)−H₀=34.69 kJ mol⁻¹ and S(T=298.15 K)−S₀=211.2 J K⁻¹ mol⁻¹. The thermal conductivity shows a peak at ca. θ_D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y₂Ti₂O₇ is 2.8 W m⁻¹ K⁻¹, close to the calculated theoretical thermal conductivity, κ_min, for fully coupled phonons at high temperatures.     

Fluorescence from the second excited singlet of aromatic thioketones in solution

The spectral characteristics and the quantum yield of the fluorescence from the second excited singlet state S₂ of the aromatic thioketone molecules xanthione (XS) and thioxanthione (TXS) have been determined in solution at room temperature and 77 K. In 3-methylpentane, the measured quantum yields are φ_f (295 K) = 5.1 × 10^?3 and φ_f(77 K) = 1.0 × 10^?2 for XS, and φ_f (295 K) = 1.5 × 10^?3 and φ_f (77 K) = 2.5 × 10^?3 for TXS. Using the Strickler-Berg expression for the radiative lifetime, the decay rate of S₂ is derived. It is concluded that internal conversion S₂ ? S₁ is the dominating deactivation channel of S₂ with k^{77 K}_nr(S₂ ? S₁) = 1.0 × 10¹⁰ s^?1 for XS and k^{77 K}_nr (S₂→S₁) = 2.2 × 10¹⁰ s^?1 for TXS. Between 295 and 77 K, φ_f increases by a factor of about 2 following an Arrhenius type expression. This temperature dependence of φ_f is considered to be intramolecular in nature and is attributed to a temperature sensitive rate constant k_nr(S₂?S₁) with an activation energy of 190 ± 20 cm^?1 and a frequency factor k′_nr = 3 × 10¹⁰ s^?1 for the XS molecule in 3-methylpentane.     

Pulsed NMR of cis-polyisoprene solutions T1 and T2 relaxations,free volume,viscosity relationships

Previous pulsed NMR studies of polyisoprene have largely been concerned with entangled or crosslinked networks. This paper deals with (i) the relaxation of high molecular weight entangled; (ii) cross-linked; (iii) monodisperse low molecular weight; and (iv) high molecular weight polymer in the presence of tetrachloroethylene which, by increasing molecular mobility, can be expected to influence the NMR relaxation. For all four types of polyisoprene, the spin-lattice T₁, relaxation shows a minimum with position depending only on the free volume, as influenced by changes in temperature T and polymer concentration v₁,. For monodisperse polyisoprene of molecular weight 7200, insufficient to form an entangled network, the spin-spin relaxation decay constant T_2L is quantitatively related to the free volume ₁ by two parameters A′ and B″ when the free volume is altered by a change in temperature, or in polymer concentration (10–100/). This can also be expressed in the form where the parameter T_∞ at 100% concentration agrees with the value used to describe rheological properties. At other concentrations of polymer, T_∞ and B′ can be derived quantitatively from the coefficients of volume expansion of polymer and solvent. The variation of T_2L with molecular weight (T_2L ∝ M^?0.5) occurs via the A′ parameter. It is concluded that T_2L can be quantitatively related to the free volume available for molecular motion (as influenced by temperature and solvent concentration) as well as to molecular weight. Furthermore T_2L is simply related to viscosity n, over a wide range of temperatures and concentrations. T₂ can be used to analyse the molecular motions involved in theology.