Proximity effects in the excited state ordering and photophysics of thienyl-pyridyl ketones

Romani, A.; Ortica, F.; Favaro, Giovanna

doi:10.1016/s0301-0104(98)00264-x

Cited by 10 publications

(14 citation statements)

References 55 publications

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“…It appears that the phosphorescence emission of 4-MBP is associated with the n,π* triplet state character, in accordance with previous observations on related aryl ketones. 17, 22 The decrease of the 4-MBP intersystem crossing quantum yield with increasing solvent polarity can be due to charge transfer deactivation of the singlet excited state (Table 1). Similar observations have been reported for other benzophenone derivatives such as 4-ABP or p-hydroxybenzophenone.…”

Section: Discussionmentioning

confidence: 99%

Inversion of 4-methoxybenzophenone triplet in aqueous solutions

Boscá

Cosa

Miranda

et al. 2002

Photochem Photobiol Sci

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The triplet state of 4-methoxybenzophenone (4-MBP) has been investigated by laser flash photolysis and emission techniques in several solvents. In non-polar cyclohexane, 4-MBP triplet has an (n,pi*) configuration with the typical triplet-triplet absorption spectrum of benzophenone (lambda(max) ca. 525 nm). However, due to the proximity of the two lowest triplet states of different configuration, some unusual features are observed in polar solvents. Thus, 4-MBP shows in aqueous solutions a transient absorption spectrum with lambda(max) at 450 and 680 nm, which can be attributed to a T1 (pi,pi*) state. Further, transient absorption spectra due to T1 (n,pi*) and T2 (pi,pi*) being simultaneously populated are observed upon laser excitation of 4-MBP in polar solvents such as acetonitrile or methanol. The triplet state inversion (n,pi* to pi,pi*) is also detected by the measurement of triplet quenching rate constants by 1,4-cyclohexadiene (a good hydrogen donor) in acetonitrile and water (kH ca. 2 x 10(8) and 5 x 10(5) M(-1) s(-1), respectively) and by the determination of room-temperature phosphorescence (the emission quantum yield at room-temperature decreases from 0.004 in acetonitrile to less than 1 x 10(-6) in water). Further, the energy of the 4-MBP triplet state is ca. 288 kJ mol(-1) both in polar and non-polar organic solvents, while in water it drops to 275 kJ mol(-1). The photophysical properties of 4-MBP are compared with those of 4-aminobenzophenone (4-ABP), which also possesses an electron-donating group. In polar organic solvents such as acetonitrile, the transient absorption spectrum and the quenching rate constant of hydrogen abstraction for triplet 4-ABP are practically the same as those obtained for 4-MBP in aqueous solutions. On the other hand, a small T2 (pi,pi*) contribution is observed in the triplet-triplet absorption spectrum of 4-ABP in cyclohexane.

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Section: Discussionmentioning

confidence: 99%

Inversion of 4-methoxybenzophenone triplet in aqueous solutions

Boscá

Cosa

Miranda

et al. 2002

Photochem Photobiol Sci

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“…Also included in the table are the weighted average lifetimes for each ketone, which were shown to range from ∼2 and ∼5 ms in the cases of ketone 2 and 3 , respectively, to almost 1 s in the case of ketones 4 and 5 . The results in Table indicate that the T 1 states of all of the derivatives studied here have some (π, π*) character or that states with the two configurations are close in energy. − The dominant nature of the short-lived (n, π*) state in the case of ketones 2 and 3 is interesting considering that they do have strong electron-donating substituents in their aromatic rings. This observation may be the result of a relatively poor conjugation in their corresponding aromatic carbonyls, which have dihedral angles of 81.6° and 85.2°, respectively.…”

Section: Resultsmentioning

confidence: 85%

Mechanistic Studies of Adamantylacetophenones with Competing Reaction Pathways in Solution and in the Crystalline Solid State

Hipwell

2019

J. Org. Chem.

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Photochemical reactions in crystals occur under conditions of highly restricted molecular mobility such that only one product is generally obtained, even when there are many others that can be observed in the gas phase or in solution. A series of 2-(1-adamantyl)-o-alkyl-acetophenones with γ-hydrogen atoms on both the adamantyl and ortho aromatic groups was selected to determine whether one can engineer and observe competing Norrish type II reaction pathways in the crystalline state. It was shown that excited state competition for hydrogen abstraction between secondary adamantyl and benzylic hydrogens is affected not only by the relative bond dissociation energies but also by the molecular conformation in the crystal. The subsequent fate of the resulting biradical species is determined by competition between radical recombination to form the photoproduct and reverse hydrogen atom transfer to regenerate the starting ketone. Crystallographic information, photoproduct distributions in solution and in the solid state, and the results of multiple mechanistic experiments, including transient absorption spectroscopy in acetonitrile and with nanocrystals suspended in water, are reported. The results demonstrate that it is possible to engineer competing reactions in crystals and that consideration of all of the aforementioned factors is necessary to account for the observed photoproduct selectivity.

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“…Pinacol formation through photoreaction with an alkene has been reported for 2,2′-DTK, 9 while 2-TPhK was found to react at the ring position with olefins to give an unstable photoadduct. 10 Recently, we investigated the photophysical behavior of the six n,n′-TPK isomers, 11,12 2,2′-DTK and 2-TPhK. 13 The lowest S 0 f S 1 transitions of all these molecules were assigned to n,π* excitation, based on several pieces of evidence (spectral region, vibronic structure, a low molar absorption coefficient, and solvent effect).…”

Section: Introductionmentioning

confidence: 99%

“…Intersystem crossing yields are close to unity. The orbital character of the triplet was shown by the lifetime and vibronic structure of the phosphorescence spectra in a rigid matrix at 77 K. 12 The triplet configuration, n,π* or π,π*, is dictated by the position (2 or 3) of the thienyl sulfur atom relative to the carbonyl. Generally, when the thienyl group is in the 3 position, the lowest triplet state is of n,π* character the 2 position, the lowest triplet state is of π,π* configuration.…”

Section: Introductionmentioning

confidence: 99%

“…Laser flash photolysis experiments gave further information on the triplet properties and triplet dynamic behavior in mono-and bi-molecular interactions. 12,13 A matter of great significance from our earlier studies is the dependence of the photophysics of these molecules on the close proximity of states of different configuration. Even though the energy difference between the lowest triplets is small enough to allow thermal equilibration at room temperature, we proposed that, in some cases, the lowest triplets are independently populated by singlet states.…”

Section: Introductionmentioning

confidence: 99%

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Light-Induced Hydrogen Abstraction from Isobutanol by Thienyl Phenyl, Dithienyl, and Thienyl Pyridyl Ketones

1999

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In this paper the bi-molecular interactions of some heteroaromatic (thienyl phenyl, dithienyl, and thienyl pyridyl) ketones with isobutanol were studied by using nanosecond laser flash photolysis in trifluoroethanol at room temperature. The absorption spectra of the triplets and ketyl radicals and their decay dynamics were investigated. Depending on the thienyl ring position (2 or 3), two types of photobehavior were found: (1) the 2-thienyl ketones, which have lowest triplets of π,π* character, abstract hydrogen at a slow rate (k H < 2 × 10 4 dm 3 mol -1 s -1 ) and ( 2) the 3-thienyl ketones, which have lowest triplets of n,π* character, abstract hydrogen at a faster rate (k H ∼ 3 × 10 5 dm 3 mol -1 s -1 ). For most of the latter compounds, both n,π* and π,π* triplets are spectrally and kinetically observed. While in 2-thienyl ketones the upper reactive n,π* triplet is thermally populated by the lowest π,π* one, in 3-thienyl ketones the two states are independent populated from the singlet state and vibronic mixing between them is not efficient.

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Proximity effects in the excited state ordering and photophysics of thienyl-pyridyl ketones

Cited by 10 publications

References 55 publications

Inversion of 4-methoxybenzophenone triplet in aqueous solutions

Inversion of 4-methoxybenzophenone triplet in aqueous solutions

Mechanistic Studies of Adamantylacetophenones with Competing Reaction Pathways in Solution and in the Crystalline Solid State

Light-Induced Hydrogen Abstraction from Isobutanol by Thienyl Phenyl, Dithienyl, and Thienyl Pyridyl Ketones

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