Stereochemical studies of the photochemical cycloaddition reaction of alkenes with N-benzoylindole and N-carboethoxyindole; evidence for biradical intermediacy

1991 ◽  
Vol 69 (7) ◽  
pp. 1171-1181 ◽  
Author(s):  
David J. Hastings ◽  
Alan C. Weedon
Keyword(s):  
Excited State ◽  
Rate Constant ◽  
Ground State ◽  
Addition Reaction ◽  
Cycloaddition Reaction ◽  
Adduct Formation ◽  
Quantum Yields ◽  
Triplet Excited State ◽  
Marked Contrast ◽  
Cis And Trans

The stereochemistries of the 2 + 2 cycloaddition products obtained from the photochemical addition reaction between N-benzoylindole or N-carboethoxyindole and the alkenes cyclopentene, cyclohexene, cycloheptene, cis- and trans-2-butene, and cis- and trans-4-octene are examined. The structures of the products are shown to be consistent with a photo-cycloaddition mechanism involving the intermediacy of triplet 1,4-biradical species. The quantum yields of adduct formation between N-benzoylindole and both cis- and trans-octene were measured as a function of alkene concentration. The results suggest that cis-octene reacts with the indole derivative's triplet excited state with a rate constant of (1.7 ± 0.3) × 107 M−1 s−1. The results are also consistent with the immediate products of this reaction being 1,4-biradicals, 98% of which revert to the ground state indole derivative and alkene, and only 2% of which proceed to cycloadduct. In marked contrast, the same treatment suggests that trans-octene reacts with the triplet excited state of N-benzoylindole with a rate constant estimated to be in the range of 1 × 106 and 6 × 105 M−1 s−1, and it appears that the 1,4-biradicals formed revert much less efficiently to the starting materials; it is estimated that between 67 and 100% of the 1,4-biradicals proceed to cycloadducts. In the reaction with cis-octene biradical reversion leads to the formation of trans-octene ("Schenk isomerization"); the quantum yield of this process is determined to be 0.074 ± 0.004, which may imply that approximately 75% of the biradicals collapse to cis-alkene and 25% collapse to the trans isomer. Key words: indole, photocycloaddition, 1,4-biradicals.

The mechanism of the photochemical cycloaddition reaction of N-benzoylindole with cyclopentene

1990 ◽  
Vol 68 (10) ◽  
pp. 1685-1692 ◽  
Author(s):  
Bimsara W. Disanayaka ◽  
Alan C. Weedon
Keyword(s):  
Excited State ◽  
Quantum Yield ◽  
Ground State ◽  
Mechanistic Model ◽  
Cycloaddition Reaction ◽  
Intersystem Crossing ◽  
Triplet Excited State ◽  
Excited State Lifetime ◽  
Yield Data ◽  
Counting Procedure

The mechanism of the photochemical cycloaddition reaction between N-benzoylindole, 1, and cyclopentene to give cyclobutane adducts 2 and 3 has been examined. The triplet excited state lifetime and quantum yield of intersystem crossing were determined for 1 as (2.8 ± 0.3) × 10−8 s and 0.39 ± 0.01, respectively, using the triplet counting procedure. In addition, the dependence of the quantum yield of cycloadduct formation upon the concentration of cyclopentene and upon the concentration of excited state quenchers has been determined. The results are used to propose a mechanistic model in which the triplet excited state of 1 reacts with cyclopentene to give a triplet 1,4-biradical intermediate. Following spin inversion the biradical intermediate reverts to the ground state starting materials or proceeds to the products 2 and 3; this partitioning, along with the quantum yield of intersystem crossing, gives rise to a limiting quantum yield of cycloaddition at infinite alkene concentration of 0.061. It is calculated that 84% of the biradical intermediates revert to the starting materials and 16% proceed to cycloadducts. The quantum yield data are also used to calculate two independent values of the rate constant for reaction of the triplet excited 1 with alkene; the values are (1.8 ± 0.1) × 107M−1 s−1 and (4.0 ± 0.8) × 106 M−1 s−1'. Some evidence for self quenching of the triplet excited state of 1 by ground state 1 was also observed. The quantum yield of intersystem crossing and the triplet excited state lifetime of 1 were found to vary with the solvent used; this is discussed in terms of the possible existence of a charge transfer triplet excited state. Keywords: indole, photocycloaddition, mechanism.

Detection and chemistry of the triplet state of thymine

1970 ◽  
Vol 48 (4) ◽  
pp. 694-696 ◽  
Author(s):  
A. G. Szabo ◽  
W. D. Rlddell ◽  
R. W. Yip
Keyword(s):  
Excited State ◽  
Triplet State ◽  
Rate Constant ◽  
Triplet Excited State

The transient produced on flash excitation of degassed solutions of thymine in acetonitrile has been characterized as the triplet excited state of thymine. This triplet state has a lifetime of 14 ± 1 μs and the associated dimerization rate was found to be 5.3 ± 0.3 × 108 M−1 s−1. The triplet state of thymine could be quenched by 2,4-hexadien-1-ol with a rate constant of 8.1 ± 0.6 × 109 M−1 s−1.

Article

1999 ◽  
Vol 77 (5-6) ◽  
pp. 868-874
Author(s):  
Maike Fischer ◽  
Yijian Shi ◽  
Bao-ping Zhao ◽  
Victor Snieckus ◽  
Peter Wan
Keyword(s):  
Aqueous Solution ◽  
Vinyl Ether ◽  
Hydroxy Group ◽  
Ground State ◽  
Cycloaddition Reaction ◽  
Quinone Methide ◽  
Quantum Yields ◽  
Ethyl Vinyl Ether ◽  
High Chemical ◽  
Ethyl Vinyl

The photosolvolysis of 1- and 2-hydroxy-9-fluorenols 4-6 has been studied in aqueous solution. All of these 9-fluorenols photosolvolyze efficiently in 1:1 H2O-CH3OH, to give the corresponding methyl ether products in high chemical and quantum yields. Whereas the photosolvolysis of the parent 9-fluorenol (2, R = H) is known to proceed via the very short-lived and formally ground-state antiaromatic 9-fluorenyl cation (1, R = H), the photosolvolysis of 1-hydroxy-9-fluorenol (4) proceeds via a much longer-lived (approximately = 5-10 s) fluorenyl quinone methide 9, which is trappable by ethyl vinyl ether via a [4+2] cycloaddition reaction to give a chroman derivative. Interestingly, 2-hydroxy-9-fluorenol (5) photosolvolyzes via a very short-lived intermediate with similar lifetimes as observed for the 9-fluorenyl cation (1, R = H), although a corresponding fluorenyl quinone methide intermediate is accessible for this compound. This study demonstrates that the mechanism of photosolvolysis of these types of compounds can be dramatically altered when an aryl hydroxy group is present.Key words: photosolvolysis, 9-fluorenyl cation, quinone methide, carbocation, antiaromatic.

Forbidden Electronic Transitions between the Singlet Ground State and the Triplet Excited State of Pt(II) Complexes

1998 ◽  
Vol 37 (14) ◽  
pp. 3588-3592 ◽  
Author(s):  
Greg Y. Zheng ◽  
D. Paul Rillema ◽  
Jeff DePriest ◽  
Clifton Woods
Keyword(s):  
Excited State ◽  
Ground State ◽  
Electronic Transitions ◽  
Singlet Ground State ◽  
Triplet Excited State

Photolysis of thiophene vapor

1969 ◽  
Vol 47 (16) ◽  
pp. 2965-2979 ◽  
Author(s):  
H. A. Wiebe ◽  
Julian Heicklen
Keyword(s):  
Excited State ◽  
Total Energy ◽  
Rate Constant ◽  
Energy Content ◽  
Oxidation Products ◽  
Quantum Yields ◽  
Rate Dependent ◽  
Light Intensities ◽  
Total Energy Content

Thiophene vapor was photoexcited with 2139 and 2288 Å radiation as well as by mercury sensitization at 2537 Å. Experiments were done at 25, 200, and 305 °C at various pressures and light intensities as well as in the presence of CO2, C2H4, butenes, and O2. In all cases the products were C2H2, CH2CCH2, CH3CCH, CS2, CH2CHCCH, and polymer. When O2 was present CO2, COS, SO2, and CO were also produced. The product quantum yields dropped as the pressure was raised. A mechanism was deduced and is given by the lettered reactions in the text. The initially formed excited state, Th* (where Th stands for thiophene), as well as the excited intermediate leading to CH2CCH2 production, I*, decomposed at a rate dependent only on the total energy content, regardless of whether thermal or photolytic. The efficiencies for energy removal from both Th* and I* decrease along the series: thiophene, C2H4, CO2, O2. Appropriate rate constant ratios for all the gases were obtained and are listed in Table IX. Experiments in the presence of olefins showed that sulfur atoms were not intermediates. The experiments with O2 further eliminated a number of possibilities. The hydrocarbones are all produced in primary processes with additional CH2CHCCH and C2H2 coming respectively from the postulated intermediates •CHCHCHCHS• and C2H2S. Along with the C3 hydrocarbons, excited CS species are produced. They can either react with the above intermediates to produce CS2 or be deactivated to eventually form polymer (or oxidation products in the presence of O2).

The high-order optical nonlinearity of 2,11,20,29-tetrabromo-2,3-naphthalocyanine iron

2018 ◽  
Vol 22 (09n10) ◽  
pp. 863-867
Author(s):  
Wang Zhang ◽  
Chunying He ◽  
Lining Zhang ◽  
Li Jiang ◽  
Yijun Yuan ◽  
...  
Keyword(s):  
Excited State ◽  
Cross Sections ◽  
Ground State ◽  
Nonlinear Refraction ◽  
Optical Nonlinearity ◽  
High Order ◽  
Saturable Absorption ◽  
Reverse Saturable Absorption ◽  
Triplet Excited State ◽  
First Excited State

A novel naphthalocyanine 2,11,20,29-tetrabromo-2,3-naphthalocyanine iron was synthetized. Its optical nonlinearity was investigated using the Z-scan technique. Reverse saturable absorption and high-order optical nonlinear refraction were detected. The absorption cross sections of the ground state, the singlet first excited state and the triplet first excited state were fitted to be 3.2 × 10[Formula: see text] cm[Formula: see text], 6.2 × 10[Formula: see text] cm[Formula: see text] and 4.6 × 10[Formula: see text] cm[Formula: see text], respectively. Fits also gave 1.17 × 10[Formula: see text] cm[Formula: see text] for the refractive volume of the ground state, 0.6 for the ratio of the refractive volume of the singlet first excited state to the ground state and 2.7 for the ratio of refractive volume of the first triplet excited state to the ground state.

Temperature effects on xanthone–β-cyclodextrin binding dynamics

2011 ◽  
Vol 89 (3) ◽  
pp. 395-401 ◽  
Author(s):  
Tamara C. S. Pace ◽  
Cornelia Bohne
Keyword(s):  
Excited State ◽  
Complex Formation ◽  
Ground State ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Equilibrium Constants ◽  
Dissociation Rate ◽  
Activation Entropy ◽  
Triplet Excited State ◽  
Dissociation Rate Constants

The complexation dynamics of the triplet excited state of xanthone with β-cyclodextrin were studied at various temperatures between 10 and 50 °C. Association and dissociation rate constants were determined using the laser flash photolysis quenching methodology with Cu2+ as a quencher. The rate constants for the association and dissociation of triplet xanthone with β-cyclodextrin increased with temperature, while the equilibrium constant for the triplet excited state remained relatively constant. Equilibrium constants for the ground-state complexation of xanthone with β-cyclodextrin were determined from fluorescence studies at various temperatures. The ground-state binding efficiency decreased with temperature and was markedly greater than that of the triplet excited state at all temperatures. The enthalpy and entropy for the β-cyclodextrin complex formation of the ground and triplet excited states fall on the enthalpy–entropy compensation relationship previously established for cyclodextrin complexes. The activation enthalpies for the association and dissociation rate constants for triplet xanthone are similar. The activation entropy is favorable for the association process, whereas a negative activation entropy was measured for the dissociation process, suggesting that solvation plays a key role in the complex formation between xanthone and β-cyclodextrin.

scholarly journals Cis and trans linkage of spin frustrated copper triangles creating Cu6 clusters

2020 ◽  
Author(s):  
Basharat Ali ◽  
Frédéric Gendron ◽  
Xiao-Lei Li ◽  
Boris Le Guennic ◽  
Jinkui Tang
Keyword(s):  
Experimental Data ◽  
Excited State ◽  
Ground State ◽  
Magnetic Data ◽  
Antiferromagnetic Coupling ◽  
First Excited State ◽  
B3lyp Calculations ◽  
Frustrated Systems ◽  
Cis And Trans ◽  
Good Agreement

Two C3-symmetric guanidine-based copper triangles bridged by acetates in a cis manner and by chloride anions in a trans manner, respectively gave rise to two antiferromagnetically coupled hexanuclear CuII compounds, namely [Cu6L2Cl(μ-OAc)(DMF)3]·DMF (Cu6) and [Cu6L2(μ-Cl)2(DMF)4] (Cu6Cl)(where L stands for fully deprotonated tris (2-hydroxybenzylidene) triaminoguanidinium chloride, H5L). The experimental magnetic data of the two compounds were analyzed theoretically. A relatively good agreement with the experimental data was obtained when using the wavefunction theory (CASSCF) in combination with DFT (B3LYP) calculations for the very strong antiferromagnetic coupling within the Cu3 triangles (Javg = − 300 cm−1 for Cu6 and Javg = − 250 cm−1 for Cu6Cl), leading to spin-frustrated systems. It is worth mentioning that the electronic structure of each CuII center remains very similar in each complex with a Kramers ground state well separated from the first excited state (over 12000 cm−1) and weakly anisotropic (g∥ ≈ 2.40 and g⊥ ≈ 2.10).

Synthesis, photophysicochemical studies of adjacently tetrasubstituted binaphthalo-phthalocyanines

2005 ◽  
Vol 09 (05) ◽  
pp. 316-325 ◽  
Author(s):  
Itumeleng Seotsanyana-Mokhosi ◽  
Ji-Yao Chen ◽  
Tebello Nyokong
Keyword(s):  
Excited State ◽  
Quantum Yield ◽  
Triplet State ◽  
Singlet Oxygen ◽  
Photophysical Properties ◽  
Zinc Phthalocyanine ◽  
Quantum Yields ◽  
Triplet Excited State ◽  
General Increase ◽  
Photochemical Behavior

Adjacent binaphthalo-phthalocyanines tetra-substituted with phenoxy (4a), 4-carboxyphenoxy (4b) and 4-t-butylphenoxy (4c) groups, as well as the di-substituted 4-carboxyphenoxy (5b) have been synthesized and characterized. The photophysical and photochemical behavior of 4a-c, were compared with those of the corresponding di-substituted derivatives, (5a-c). The secondary substituents on the phenoxy ring have an influence on the aggregation of the molecules and hence on their photophysical properties. All of the complexes exhibit a relatively good conversion of energy from the triplet-excited state to the singlet oxygen. The less aggregated molecule (4c), has the highest singlet oxygen quantum yield. For all the molecules, fluorescence yields are low and they all have relatively shorter triplet lifetimes compared with the unsubstituted zinc phthalocyanine. Increasing the number of ring substituents on these rigid MPc complexes (from complexes 5 to 4) showed a general increase in the triplet state lifetimes and singlet oxygen quantum yields, and a decrease in stability.

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