scholarly journals Communication: The dark singlet state as a doorway state in the ultrafast and efficient intersystem crossing dynamics in 2-thiothymine and 2-thiouracil

2014 ◽  
Vol 140 (7) ◽  
pp. 071101 ◽  
Author(s):  
Marvin Pollum ◽  
Carlos E. Crespo-Hernández
Keyword(s):  
Singlet State ◽  
Intersystem Crossing ◽  
Doorway State

Photolysis of ketene. I

1968 ◽  
Vol 46 (14) ◽  
pp. 2353-2360 ◽  
Author(s):  
A. N. Strachan ◽  
D. E. Thornton
Keyword(s):  
Carbon Monoxide ◽  
Quantum Yield ◽  
Triplet State ◽  
Singlet State ◽  
Inert Gases ◽  
Inert Gas ◽  
Excited Singlet State ◽  
Intersystem Crossing ◽  
Excited Singlet ◽  
Increasing Temperature

Ketene has been photolyzed at 3660 and 3130 Å both alone and in the presence of the inert gases C4F8 and SF6. The quantum yield of carbon monoxide has been determined at both wavelengths as a function of pressure and temperature. At 3660 Å the quantum yield decreases with increasing pressure but increases with increasing temperature. At 3130 Å the quantum yield with ketene alone remains 2.0 at both 37 and 100 °C at pressures up to 250 mm. At higher pressures of ketene or with added inert gas the quantum yield decreases with increasing pressure. The results are interpreted in terms of a mechanism in which intersystem crossing from the excited singlet state to the triplet state occurs at both wavelengths, and collisional deactivation of the excited singlet state by ketene is single stage at 3660 Å but multistage at 3130 Å.

Scanning Electron Microscope Induced Cathodoluminescence

1972 ◽  
Vol 30 ◽  
pp. 368-369 ◽  
Author(s):  
Richard H. Falk
Keyword(s):  
Singlet State ◽  
Electron Gas ◽  
Exciting Radiation ◽  
Intersystem Crossing ◽  
Chemical Bonds ◽  
Excited Singlet ◽  
Accelerated Electrons ◽  
Radiationless Energy Transfer ◽  
Bound Electrons ◽  
Quantized Levels

Luminescence is a process in which radiation is emitted by molecules or atoms that have been excited by the absorption of radiation. Accelerated electrons have energies very much greater than the quantized levels of atoms and molecules and in passing through a substance, detach bound electrons. A plasma (ion-electron gas) is formed and as recombinations occur, light quanta are emitted. Thus, if one uses accelerated electrons as the source of exciting radiation, observations of cathodoluminescence (CL) may often be made.Much of the energy absorbed is lost in processes other than luminescence. Losses by internal conversions may occur which involve intersystem crossing from the excited singlet state to the triplet state (phosphorescence or heat) or through predissociation (disruption of chemical bonds in the compound). Energy loss by external conversion (quenching) may also occur through collisions with other molecules (radiationless energy transfer or heat).

Fluorescence of chloropentafluoroacetone

1970 ◽  
Vol 48 (16) ◽  
pp. 2611-2616 ◽  
Author(s):  
H. S. Samant ◽  
A. J. Yarwood
Keyword(s):  
Rate Constant ◽  
High Pressures ◽  
Singlet State ◽  
Radiative Lifetime ◽  
Heavy Atom ◽  
Excited Singlet State ◽  
Intersystem Crossing ◽  
Excited Singlet ◽  
Gaseous State ◽  
Fluorescent State

The fluorescence of chloropentafluoroacetone in the gaseous state at room temperature is reported. The fluorescence extends from 337 nm to greater than 560 nm with a maximum intensity at 420 nm and is unaffected by low pressures of oxygen. The energy of the first excited singlet state is estimated as about 83 kcal and the fluorescence data for 313 and "290" nm excitation indicate that fluorescence enhancement occurs as the pressure increases. The fluorescent yield at high pressures [Formula: see text] is independent of the nature of the gas used to attain the high pressure. The lifetime of the fluorescent state is 35.1 ± 0.3 ns.A simple mechanism correlates the experimental observations and the values of the rate constants are considered. The reciprocal of the natural radiative lifetime is calculated to be 6.6 × 105 s−1 and the rate constant assigned to intersystem crossing to the triplet is 2.8 × 107 s−1. A comparison with the data in the literature for hexafluoroacetone shows the effect of substitution by a heavy atom in the fluorinated ketone. It about doubles the value of the rate constant associated with intersystem crossing from the first excited singlet state in the gas phase.

The primary process in the photolysis of hexafluoroacetone vapour III. The efficiency of intersystem crossing

1968 ◽  
Vol 306 (1487) ◽  
pp. 529-536 ◽  
Keyword(s):  
Quantum Yield ◽  
Lower Limit ◽  
Singlet State ◽  
Excited Singlet State ◽  
Intersystem Crossing ◽  
Primary Process ◽  
Excited Singlet ◽  
A Value ◽  
Low Concentrations

The value of k 6 /( k 4 + k 5 + k 6 ) (mechanism of part I) was determined by two techniques, namely the sensitization of the isomerization of cis - to trans -butene-2 and the sensitization of the phosphorescence of biacetyl. Both techniques yielded a value for the ratio of 0⋅9 ± 0⋅1. The value obtained by the isomerization technique is a lower limit because the quantum yield for the isomerization did not attain a value independent of [ cis -butene-2], but reached a maximum at low concentrations of olefine and then decreased. A similar scavenging of the excited singlet state of hexafluoroacetone was observed when biacetyl was present as an addend. However, in both cases the measurements were made with sufficiently low concentrations of addend that the singlet scavenging should have introduced less, than 10% error. It is concluded that reaction (5) of the mechanism cannot be an important mode of disappearance of excited ketone molecules.

Activation energy barrier for intersystem crossing in platinum phthalocyanine

1976 ◽  
Vol 54 (6) ◽  
pp. 633-637 ◽  
Author(s):  
Tzer-Hsiang Huang ◽  
Klaus E. Rieckhoff ◽  
Eva-Maria Voigt
Keyword(s):  
Triplet State ◽  
Ground State ◽  
Energy Barrier ◽  
Singlet State ◽  
Quantum Yields ◽  
Intersystem Crossing ◽  
Singlet Ground State ◽  
Activation Energy Barrier ◽  
Excited Singlet ◽  
Platinum Phthalocyanine

Phosphorescence quantum yields and lifetimes of platinum phthalocyanine in α-chloronaphthalene solution are reported for temperatures from 1.3 K to 300 K. Activation energies of intersystem crossing are deduced and found to be Ea = 17 ± 3 cm−1 for crossing from the first excited singlet state to the lowest lying triplet state and Eap = 8 ± 1 cm−1 for crossing from the lowest lying triplet state to the singlet ground state.

scholarly journals Phosphorescent oxygen-sensing and singlet oxygen production by a biosynthetic silk

RSC Advances ◽  
2016 ◽  
Vol 6 (46) ◽  
pp. 39530-39533 ◽  
Author(s):  
Conor C. Horgan ◽  
Yong-Shen Han ◽  
Holly Trueman ◽  
Colin J. Jackson ◽  
Tara D. Sutherland ◽  
...  
Keyword(s):  
Heavy Metal ◽  
Visible Light ◽  
Triplet State ◽  
Singlet Oxygen ◽  
Singlet State ◽  
Oxygen Sensing ◽  
Coiled Coil ◽  
Intersystem Crossing ◽  
Oxygen Production

A recombinant coiled-coil silk was utilised to immobilise heavy-metal-macrocycles which are known to undergo efficient intersystem crossing from the singlet state to the triplet state following excitation with visible light.

scholarly journals Theoretical Investigation of Oxidative Cleavage of Cholesterol by Dual O2 Activation and Sulfide Reduction

2016 ◽  
Vol 69 (9) ◽  
pp. 933 ◽  
Author(s):  
Richmond Lee ◽  
Michelle L. Coote
Keyword(s):  
Singlet State ◽  
Theoretical Calculations ◽  
Methionine Sulfoxide ◽  
Oxidative Cleavage ◽  
Amyloid Peptide ◽  
Intersystem Crossing ◽  
Aβ Amyloid ◽  
Plausible Mechanism ◽  
Non Covalent Interactions ◽  
Covalent Interactions

Theoretical calculations are used to explore a plausible mechanism for oxidative cleavage of cholesterol mediated by two ground-state O2 molecules. It is shown that cholesterol can form a stable pre-complex with the two triplet dioxygen molecules, which could be further stabilized in an enzyme environment by methionine (modelled here as Me2S). Triplet O2 can then react to form a metastable biradical species that is then further stabilized by reaction with a second triplet O2, resulting in an intermediate that undergoes an intersystem crossing to form a diperoxy intermediate. This in turn is reduced to the final cholesterol secosterol aldehyde product by the same methionine, which may provide an explanation for the presence of methionine sulfoxide fractions in Aβ amyloid peptide. The mechanistic theozyme model predicts an energetically viable pathway that is unusual in that triplet oxygen is normally considered to be unreactive in this context unless first excited to the singlet state. Although we show that the same reaction can also proceed via photosensitization of the complex if an appropriate cofactor is available, the energetics for the triplet oxygen reaction are competitive. Reactivity studies revealed that the reaction can also occur with other unsaturated substrates, with the lowest barriers occurring with more nucleophilic alkenes, or by rendering the 3O2 more electrophilic via non-covalent interactions with Me2S.

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