Phosphorescence and ODMR studies of isotopically mixed monoclinic and triclinic 1,4-dichlorobenzene

Phosphorescence and optically detected magnetic resonance (ODMR) studies were performed on the perprotonated triplet traps in isotopically mixed (h₄ in d₄) 1,4-dichlorobenzene at 1.75 K. We find evidence for two distinct crystalline phases of the d₄ host crystal which are stable in temperature ranges near room temperature, each of which induces a distinct set of ODMR parameters on the trap species being monitored. Both cw and pulsed ODMR are used to probe for evidence of trap-to-trap triplet energy transfer at several trap concentrations. We find that at low concentrations ODMR signals are strong, and the excitations are definitely localized. At concentrations above 9%, both coherent and cw ODMR signals become hard to detect. The results are discussed in terms of likely mechanisms for energy exchange among the triplet traps.     

Dihydrosilaanthracenylidene and dihydroanthracenylidene

Carbenes generated from 10-diazo-9,9-dimethyl-9,10-dihydro-9-silaanthracene and 10-diazo-9,9-dimethyl-9,10-dihydroanthracene gave hydrogen abstraction products by the triplet nature of these carbenes. The triplet dihydrosilaanthracenylidene was shown by the electron spin resonance study.     

Thermal analysis of a polymorphic azo dye derived from 2-amino-5-nitrothiazole

An azo dye, derived from 2-amino-5-nitrothiazole and a substituted N,N-dimethyl aniline, was studied by various techniques. Two sets of proton signals were obtained by NMR in CDCl₃ solution, suggesting that two polymorphs coexist, however, only one set of signals is observed in DMSO solution. Differential scanning calorimetry and thermogravimetric analysis were used to confirm the existence of two forms. The events registered during thermal treatment of a sample were assigned to a glass transition, recrystallization and fusion of crystalline compounds. Hot-stage microscopy was used to obtain images of the dye samples at various stages of the heating program and these observations support the interpretation of the calorimetric results.     

Formation and decay of triplet excimers as revealed by their absorption spectra: Intramolecular excimers of α,α-dinaphthylalkanes

Transient absorptions, which can be assigned to the triplet—triplet absorptions of excimers, have been observed from fluid solutions of α,α-dinaphthylalkanes. The temporal characteristics of the absorptions are identical to those of the emission previously assigned to excimer phosphorescence. Analysis of the monomer and the excimer absorptions indicate that the kinetics of formation and decay of the triplet excimer can be represented by a simple consecutive reaction scheme: ³M* → ³D* → ¹M.     

Reaction of 3-Diazocamphor with Silver Ion - Formation of Tricyclanone

The usual method of formation of tricyclanone (1) involves thermolysis of an intimate mixture of 3-diazocamphor (3) and copper powder and is thought to involve ketocarbene (5).¹ Recently this method has been supplemented² by one which involves reaction of 3,3-dibromocamphor with diethylzinc in benzene where α-elimination to give ketocarbene (5) is mediated by 3-bromocamphor carbanion whose role is indicated by trapping as 3-bromocamphor; this material accounts for ca. 10% of product even after prolonged reaction times.     

Reconstituted LH2 in multilayer membranes induced by poly-l-lysine: Structure of supramolecular and electronic states

To explore the effect of cell membrane stacking on the function of light-harvesting complex 2 (LH2) in purple nonsulfur photosynthetic bacteria, LH2 from Rhodobacter sphaeroides 2.4.1 (R. sph 2.4.1) was reconstituted into lipid bilayer vesicles (LH2@liposome) and further formed multi-layer structure by electrostatic interaction with poly-l-lysine (LH2@liposome/PLL), which was characterized by cryo-electron microscopy (cryo-EM) and TEM. When embedded in liposomes and additionally in multi-layer liposomes, the absorption band, zero-crossing point of CD signals and fluorescence emission of LH2 B850 excitons were observed to uniformly have 1–2 nm red-shifting. Combining with the corresponding fluorescence quench and the generation of shorter-living fluorescence species, a new excitonic species generated through B850 structural splitting was proposed. By FT-Raman and triplet carotenoid dynamics, the structural mechanism was deduced and discussed. Briefly, all environmental changes, including LH2 aggregating and multi-layer membrane stacking, eventually applied forces on B850 exciton molecules mainly through the hydrogen bonding between the C3-acetyl carbonyl groups of B850 BChls and Tyr44 and 45 residues at C-terminus of α-polypeptides. The effect of multi-layer structure on LH2 could be assigned as a kind of photoprotection.     

Phosphorescence- and delayed-fluorescence-detected magnetic resonance at zero-field on mobile and localized triplet states in anthracene

Optically-detected magnetic resonance (ODMR)-experiments at zero field were performed on the lowest triplet state of anthracene single crystals monitoring the delayed fluorescence (DF) as well as the phosphorescence (P) at 1.2 K. At low RF-power levels the P-ODMR and DF-ODMR signals are almost identical and can be attributed to at leat two X-traps with the following ZFS-parameters: D₁ = 0.06967(3), |E₁| = 0.00793(3), D₂ = 0.06940(3) and |E₂| = 0.00808(3). At RF-levels exceeding the threshold of the trap signals by about 5 orders of magnitude, DF-ODMR signals from excitons are observed. Their ZFS-parameters show clearly, that the motion is mainly between translationally non-equivalent oriented anthracene molecules [D^* = ?0.004256(1) cm ^?1 and E^* = 0.03311(1) cm ^?1]. ODMR-linewidth analysis for optical excitation directly into one of the two Davydov components of the first triplet exciton band indicate, that radiationless interbranch transitions between both components can populate k ≠ 0 states in the lower band with band energies higher than kT. The thermalization time within this band can be estimated to be about 10 ^?7 s.     

Quenching of triplet states by molecular oxygen and the role of charge-transfer interactions

The rate constants for oxygen quenching in benzene solution of the triplet states of several organic compounds with relatively high triplet energies have been measured in laser photolysis and pulse radiolysis experiments. The previously observed trend for aromatic hydrocarbons where the quenching rate constants decrease from a limiting value of about one ninth of that expected for a diffusion controlled reaction to lower values for triplet states with increasing triplet energy was not observed for the triplet states of certain aromatic ketones and amines. The higher rate constants observed, e.g. oxygen quenching of triplet N-methyl indole has k_Q = 1.4 × 10¹⁰ dm³ mol^?1 s^?1, are interpreted as being due to the presence of low lying triplet charge-transfer states which enhance the efficiency of quenching.     

Conformations of the triplet state of aromatic olefins. Observation of the transoid triplet states of 3,3-dimethyl-1-(2-naphthyl)-1-butene and 3,4-dihydrophenanthrene as a rigid planar triplet model

The observed triplet—triplet absorption of 3,3-dimethyl-1-(2-naphthyl)-1-butene is assigned to the planar transoid geometry (³t*) from its spectral similarity to the 3,4-dihydrophenanthrene triplet, a rigid planar model, as well as from the measured rate constants for its quenching by azulene and oxygen. The results are in contrast the 1,2-diarylethylene triplets which favor the perpendicularly twisted geometry (³p*).     

Synthesis of a 1,3,4,5-tetrahydrobenzindole β-ketoester

Pd(OAc)₂-catalyzed decomposition of ethyl 2-diazo-4-(4-indolyl)-3-oxobutanoate leads to a tricyclic tetrahydrobenzindole compound formed by a formal C-H insertion reaction. This tricyclic indole rearranges to a novel and thermodynamically more stable naphthalene derivative. Treatment of N-tosyl-protected ethyl 2-diazo-4-(4-indolyl)-3-oxobutanoate with a base induces facile pyrazole formation.     

Time-resolved resonance raman spectra and decay kinetics of triplet anthracene in fluid media

Reported here are the time-resolved resonance Raman spectra and decay kinetics of the lowest triplet state (³B_2u⁺) of anthracene-h₁₀ and anthracene-d₁₀ molecules in fluid media at room temperature. The triplet population (≈3 × 10^?5 M) is observed to decay at microsecond times by triplet—triplet annihilation. Vibrational assignments for the observed Raman bands are proposed.     

Computational mechanistic study of methanol and molecular oxygen reaction on the triplet and singlet potential energy surfaces

The reaction of CH₃OH with the O₂ on the triplet and singlet potential energy surfaces (PES) was carried out using the B3LYP, MP2, and CCSD(T)//B3LYP theoretical approaches in connection with the 6-311++G(3df–3pd) basis set. Three pre-reactive complexes, ¹C1, ¹C2, and ³C1, on the singlet and triplet PES were formed between methanol and molecular oxygen. From a variety of the complexes, seven types of products are obtained, of which four types are found to be thermodynamically stable. Results reveal that there exists one intersystem crossing between triplet and singlet PES. For P4 adduct that is the main and kinetically the most favorable product, the rate constants are calculated in the temperature range of 200–1,000 K in the reliable pathway.     

Quantitative estimation of trans-1,4 and cis-1,4 isomers in polychloroprene by high-resolution NMR

In the ¹H-NMR spectrum of polychloroprene dissolved in C₆D₆, the ?CH proton signal was separated into two triplet peaks. These triplet signals were assigned to the ?CH proton in the trans-1,4 and cis-1,4 isomers by measurement of ¹H-NMR spectra of 3-chloro-1-butene and a mixture of trans- and cis-2-chloro-2-butene as model compounds for the 1,2, trans-1,4 and cis-1,4 isomers. In ¹H-NMR spectra (220 Mcps) of polychloroprene dissolved in C₆D₆, two triplet signals were separated completely from which the relative concentrations of trans-1,4 and cis-1,4 isomers could be obtained quantitatively.     

Vibrational and electronic excitation in p-benzoquinone by electron impact

Vibrational and electronic excitation by electron impact in p-benzoquinone was studied using a trochoidal electron spectrometer. Two distinct patterns of vibrational excitation were observed. First, low quanta of a few selected vibrations are specifically excited at incident electron energies corresponding to shape resonances. Some resonances excite mainly the CO stretch, others the CH stretch vibration, and this selectivity is used in the discussion of the assignment of the resonances. A second pattern is an unspecific excitation of a quasi-continuum where no structure due to individual vibrational levels can be discerned. This feature peaks at threshold, large amounts of vibrational energy can be deposited in the molecule, and the excitation also proceeds via shape resonances. Electronic excitation spectra in the valence and Rydberg regions are also presented and discussed. A band observed at 4.37 eV with low residual energies has been tentatively assigned to the second π — π* triplet state ³B_3g.     

Fluorescence decay kinetics of acetone vapour at low pressures

Acetone-h₆ and -d₆ were excited by a short UV laser pulse to the nπ* state. Using pressures of 10^?4-10 ^?3 Torr, two distinct decay components were observed - the faster with a decay time of less than 20 ns and the slower of about 5 μs. Increasing the pressure leads to the appearance of two longer-lived decay components, which are apparently absent in the case of isolated molecules. Based on the deuteration effect, excitation wavelength dependence, quenching kinetics and analogy with other molecules, the four decay components are assigned as follows. The fastest component is due to dephasing of the initially excited state, forming a quasi-stationary eigenstate. The second component is due to the radiative decay of the latter states. The third, to decay of triplet states not directly coupled to the initially excited singlet states, and the last to the thermalized triplet state.     

Chemo- and stereoselective six-membered oxonium ylide formation–[2,3]-sigmatropic rearrangement of 2-diazo-3-ketoesters with dirhodium(II) catalyst and its application to the synthesis of (+)-tanikolide

Dirhodium(II)-catalyzed oxonium ylide formation–[2,3]-sigmatropic rearrangement of 6-allyloxy-2-diazo-3-ketoesters possessing a C-6 substituent is described. The reaction of 6-alkyl- or 6-aryl-substituted 6-allyloxy-2-diazo-3-ketoesters with a catalytic amount of Rh₂(S-PTTL)₄ proceeded in a chemoselective and stereoselective manner to provide 6-substituted 2-allyl-3-oxotetrahydropyran-2-carboxylates in good yields and with high diastereoselectivities. To demonstrate the utility of this sequential reaction, we conducted the total synthesis of (+)-tanikolide, in which the construction of the δ-lactone skeleton was achieved by employing a 2-iodobenzamide-catalyzed oxidative cleavage of tetrahydropyran-2-methanol.     

An ab initioCI calculation of the triplet–triplet absorption spectrum of naphthalene

The triplet–triplet absorption spectrum of naphthalene (³B_1g ← ³B_2u; ³A_g ← ³B_2u) was calculated using an ab initio method with a STO -3G basis set and moderately large configuration interaction. Eight bands were found in the region 18,000–45,000 cm^?1 and compared with those of the experimental spectrum. The strongest band observed at about 44,000 cm^?1 was assigned to a ³A_g ← ³B_2u tansition. The excited triplet and singlet ground-state electronic charge distributions are compared.     

Photolysis of acylphosphine oxides II: The influence of methyl substitution in benzoyldiphenylphosphine oxides

Argon-saturated dilute solutions of 2-methyl, 4-methyl-, 2,4,6-trimethyl- and non-substituted benzoyldiphenylphosphine oxide (OTDPO, PTDPO, TMDPO and BDPO respectively) in various solvents were irradiated with flashes (duration, 20 ns) of 347 nm light. All compounds underwent α scission in times much shorter than the flash time: φ(α) = 0.5 – 0.6 (OTDPO, TMDPO and BDPO) and φ(α) - 1.0 (PTDPO). Diphenylphosphonyl radicals generated in this way characteristically absorb light around 330 nm. In the case of OTDPO, photoenolization competes with α scission, and transient spectra were observed which were assigned to the excited triplet (k = 3 × 10⁷ s⁻¹) and to the ground state (k = (3 × 10⁵) - (1.5 × 10⁴) s⁻¹) of the enols. TMDPO did not undergo enolization. It is concluded that enolization involves triplets and that a triplet lifetime longer than a few nanoseconds is a prerequisite for its occurrence.     

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. II. Theory of the positions of double resonances near a singlet-triplet anticrossing. Application to the precise measurement of singlet-triplet coupling and of fine and hyperfine structure parameters of the triplet of glyoxal

We set the theory for the position of double resonance signals near an anticrossing point. We use this theory to interpret our experimental results for anticrossings between the (K ≈ 6, J = 13) rotational level of the vibrationless 0° ¹A_u state of glyoxal and five sample rovibrational triplet levels, leading to very precise (up to a few 10^?4 on relative value) measurements of singlet-triplet coupling and fine and hyperfine structure parameters. The zero field singlet-triplet coupling matrix elements measured vary from 11.4±0.14 to 300.0±0.8 MHz. The hyperfine structure constant (82 MHz) is constant to within ±2% for the five triplet levels. The two fine structure parameters to the contrary vary by a factor of roughly two.     

Inverse electron demand asymmetric cycloadditions of cyclic carbonyl ylides catalyzed by chiral Lewis acids—Scope and limitations of diazo and olefinic substrates

High enantioselectivities (94-96% ee) were obtained for the inverse electron-demand 1,3-dipolar cycloadditions between cyclohexyl vinyl ether and 2-benzopyrylium-4-olate generated via Rh₂(OAc)₄-catalyzed decomposition of o-methoxycarbonyl-α-diazoacetophenone. The reactions were effectively catalyzed by Eu(OTf)₃, Ho(OTf)₃, or Gd(OTf)₃ complexes (10 mol %) of chiral 2,6-bis[(4S,5S)-4,5-diphenyl-2-oxazolinyl]pyridine. The reactions with the other electron-rich dipolarophiles such as allyl alcohol, 2,3-dihydrofuran, and butyl-tert-butyldimethylsilylketene acetal showed moderate enanantioselectivities (60-73% ee). Good to high enantioselectivities (73-97% ee) were also obtained for the cycloadditions between 3-acyl-2-benzopyrylium-4-olates, generated from methyl 2-(2-diazo-1,3-dioxoalkyl)benzoates and butyl or cyclohexyl vinyl ethers, in the presence of binaphthyldiimine (BINIM)-Ni(II) complexes (10 mol %). Under similar conditions, the reaction between methyl 2-(2-diazo-1,3-dioxohexyl)benzoate and 2,3-dihydrofuran was highly endo-selective, and moderately enantioselective (70% ee). For the BINIM-Ni(II)-catalyzed reactions of cyclohexyl vinyl ether, the use of an epoxyindanone as the 3-acyl-2-benzopyrylium-4-olate precursor revealed that the chiral Lewis acid can function as a catalyst for asymmetric induction. The scope of the cyclic carbonyl ylides was extended to those generated from 1-diazo-2,5-pentanedione derivatives, which were reacted with butyl or TBS vinyl ether and catalyzed using the (4S,5S)-Pybox-4,5-Ph₂-Lu(OTf)₃ complex to give good levels of asymmetric inductions (75-84% ee).