Triplet of cyclooctatetraene: reactivity and properties

Das, Tomi Nath; Priyadarshini, K. Indira

doi:10.1039/ft9949000963

“…The positive offset is understood because the detachment of electron from COT − yields both the singlet and triplet states of COT, as evident in the PE spectrum. The offset is flat within 4 ps (Figure 3) and up to 12 ps (not shown), and this is consistent with a contribution from a long‐lived triplet state (≈100 μs) 23…”

supporting

confidence: 72%

The Transition State of Thermal Organic Reactions: Direct Observation in Real Time

Paik

¹

,

Yang

²

,

Lee

³

et al. 2004

Angewandte Chemie

3

0

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The motion of atoms during chemical reactions occurs on the femtosecond time scale, and only with this resolution can the transition state be frozen in time. Numerous examples of femtochemistry studies have been reported for reactions on excited surfaces and in all phases of matter (see, for example, recent references [1] and [2]). Combining the femtosecond time resolution and negative-ion-beam techniques provides a unique way to study the nuclear motions of neutral molecules in the electronic ground state, and this has been successful for Ag 3 and Ag 4 . [3][4][5][6][7] For stable (positive electron affinity) negative ions prepared in the gas phase the electronic states are lower in energy than the neutral ground state. Thus, the ground state of the neutral reaction can be prepared with well-defined nuclear configuration (coherent wave packet) by photodetaching the electron from the anion with a femtosecond laser pulse. Because the geometries of the anion and the neutral molecule are different, the vertical transition results in significant nuclear motion toward the equilibrium geometry of the ground state. This motion can be monitored in real time by using a second femtosecond laser pulse, with variable time delay, through ionization and detection with mass spectrometry.

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“…[7,8,[26][27][28] Several experimental observations related to large structural changes in the excited states, when compared to the S 0 state, have been reported for COT and a number of COT derivatives. [9,29,30] With regard to dibenzannelated heterocycles, dibenz[b,f]oxepin displays a large Stokes' shift and well-defined vibrational fine structure in the fluorescence spectrum, evidences that a change from a V-shaped to a planar conformation occurs in the S 1 state (Figure 2), [31] and similar findings were made for dibenz[b,f] thiepin. Interestingly, dibenz[b,f]oxepin shows an increased photostability when compared to its 10,11-dihydrogenated analogue, [31] a feature that could be connected to the gain in S 1 state aromaticity of a cyclic system with 4n π-electrons.…”

Section: Introductionmentioning

confidence: 53%

“…More recently, it was possible to obtain an experimental assessment of excited state aromatic stabilization in a chiral COT derivative. [29] The racemization enthalpy determined through time-resolved CD spectroscopy of the isolated enantiomer revealed its excited state aromatic stabilization to be 21-22 kcal/mol in both the T 1 and S 1 states.…”

Section: Introductionmentioning

confidence: 98%

Is Excited‐State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π‐Electron Heterocycles?

Toldo

¹

,

Bakouri

²

,

Solà

³

et al. 2019

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Compounds with dibenzannelated heterocycles with eight π‐electrons are found in a range of applications. These molecules often adopt a bent structure in the ground state (S0) but can become planar in the first excited states (S1 and T1) because of the cyclically conjugated 4nπ central ring, which fulfils the requirements for excited state aromaticity. We report on a quantum chemical investigation of the aromatic character in the S1 and T1 states of dibenzannelated seven‐ and six‐membered heterocycles with one, two, or three heteroatoms in the 8π‐electron ring. These states could have ππ* or nπ* character. We find that compounds with one or two heteroatoms in the central ring have ππ* states as their S1 and T1 states. They are to a significant degree influenced by excited state aromaticity, and their optimal structures are planar or nearly planar. Among the heteroatoms, nitrogen provides for the strongest excited state aromaticity whereas oxygen provides for the weakest, following the established trend of the S0 state. Yet, dibenzannelated seven‐membered‐ring compounds with N=N bonds have non‐aromatic nπ* states with strongly puckered structures as their S1 and T1 states.

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“…The offset is flat within 4 ps (Figure 3) and up to 12 ps (not shown), and this is consistent with a contribution from a longlived triplet state (% 100 ms). [23] The oscillatory behavior is reminiscent of the transient behavior observed by our research group for the photoisomerization of cis-stilbene. [24] The transient fit shown in Figure 3 was obtained using the molecular-response function with proper convolution given in Equation (2), where g is the…”

Section: Methodsmentioning

confidence: 90%

The Transition State of Thermal Organic Reactions: Direct Observation in Real Time

Paik¹,

Yang²,

Lee³

et al. 2004

View full text Add to dashboard Cite

The motion of atoms during chemical reactions occurs on the femtosecond time scale, and only with this resolution can the transition state be frozen in time. Numerous examples of femtochemistry studies have been reported for reactions on excited surfaces and in all phases of matter (see, for example, recent references [1] and [2]). Combining the femtosecond time resolution and negative-ion-beam techniques provides a unique way to study the nuclear motions of neutral molecules in the electronic ground state, and this has been successful for Ag 3 and Ag 4 . [3][4][5][6][7] For stable (positive electron affinity) negative ions prepared in the gas phase the electronic states are lower in energy than the neutral ground state. Thus, the ground state of the neutral reaction can be prepared with well-defined nuclear configuration (coherent wave packet) by photodetaching the electron from the anion with a femtosecond laser pulse. Because the geometries of the anion and the neutral molecule are different, the vertical transition results in significant nuclear motion toward the equilibrium geometry of the ground state. This motion can be monitored in real time by using a second femtosecond laser pulse, with variable time delay, through ionization and detection with mass spectrometry.

show abstract

Triplet of cyclooctatetraene: reactivity and properties

Cited by 30 publications

References 19 publications

The Transition State of Thermal Organic Reactions: Direct Observation in Real Time

The Transition State of Thermal Organic Reactions: Direct Observation in Real Time

Is Excited‐State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π‐Electron Heterocycles?

The Transition State of Thermal Organic Reactions: Direct Observation in Real Time

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