Enhanced basicity of 1,3-dialkoxy-substituted benzenes: cyclophane derivatives

1994 ◽  
Vol 72 (12) ◽  
pp. 2388-2395 ◽  
Author(s):  
Guangzhong Zhang ◽  
Yijian Shi ◽  
Renée Mosi ◽  
Thao Ho ◽  
Peter Wan
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Structural Constraints ◽  
Excited Singlet ◽  
Deuterium Incorporation ◽  
Stable Conformation ◽  
Substituted Benzenes ◽  
Singlet States ◽  
Excited Singlet States

The photoprotonation of four dialkoxy-substituted benzenes in their excited singlet states has been studied. The parent systems 4 and 5 are regioselectively photoprotonated at the 2-position, with significant quantum efficiencies for deuterium incorporation at acidities greater than pH 2. The structurally related cyclophane derivatives 6 and 7 did not show any deuterium exchange over the same acidity range but fluorescence quenching by proton (in aqueous solution) and laser flash photolysis studies (in 1,1,1,3,3,3-hexafluoro-2-propanol) indicate that photoprotonation does take place with these compounds. Due to structural constraints imposed by the most stable conformation adopted by these molecules (6 and 7), the attacking proton (deuteron) is also the proton (deuteron) that is deprotonated from the cyclohexadienyl cation (2,6-dialkoxybenzenonium ion) intermediate, resulting in the absence of incorporation of deuterium.

Nanosecond Flash Photolysis and the Absorption Spectra of Excited Singlet States

Nature ◽  
1968 ◽  
Vol 220 (5173) ◽  
pp. 1228-1229 ◽  
Author(s):  
GEORGE PORTER ◽  
MICHAEL R. TOPP
Keyword(s):  
Absorption Spectra ◽  
Flash Photolysis ◽  
Excited Singlet ◽  
Singlet States ◽  
Excited Singlet States

Steady state and time-resolved spectroscopic studies of the photochemistry of 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1136-1147 ◽  
Author(s):  
William J Leigh ◽  
Rabah Boukherroub ◽  
Christine J Bradaric ◽  
Christine C Cserti ◽  
Jennifer M Schmeisser
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Singlet State ◽  
Fluorescence Emission ◽  
Laser Flash ◽  
Excited Singlet State ◽  
Acetonitrile Solution ◽  
Quantum Yields ◽  
Excited Singlet

Direct photolysis of 1-phenylsilacyclobutane and 1-phenyl-, 1-(2-phenylethynyl)-, and 1-(4'-biphenylyl)-1-methylsilacyclobutane in hexane solution leads to the formation of ethylene and the corresponding 1-arylsilenes, which have been trapped by photolysis in the presence of methanol. Quantum yields for photolysis of the three methyl-substituted compounds have been determined to be 0.04, 0.26, and 0.29, respectively, using the photolysis of 1,1-diphenylsilacyclobutane Φsilene = 0.21) as the actinometer. The corresponding silenes have been detected by laser flash photolysis; they have lifetimes of several microseconds, exhibit UV absorption maxima ranging from 315 to 330 nm, and react with methanol with rate constants on the order of (2-5) × 109 M-1 s-1 in hexane. Absolute rate constants for reaction of 1-phenylsilene and 1-methyl-1-phenylsilene with water, methanol, tert-butanol, and acetic acid in acetonitrile solution have been determined, and are compared to those of 1,1-diphenylsilene under the same conditions. With the phenylethynyl- and biphenyl-substituted methylsilacyclobutanes, the triplet states can also be detected by laser flash photolysis, and are shown to not be involved in silene formation on the basis of triplet sensitization and (or) quenching experiments. Fluorescence emission spectra and singlet lifetimes have been determined for the three 1-aryl-1-methylsilacyclobutanes, 1,1-diphenylsilacyclobutane, and a series of acyclic arylmethylsilane model compounds. These data, along with the reaction quantum yields, allow estimates to be made of the rate constants for the excited singlet state reaction responsible for silene formation. 1-Methyl-1-phenylsilacyclobutane undergoes reaction from its lowest excited singlet state with a rate constant 10-80 times lower than those of the other three derivatives. The results are consistent with a stepwise mechanism for silene formation, involving a 1,4-biradicaloid intermediate that partitions between product and starting material.Key words: silene, silacyclobutane, photochemistry, biradical.

Trans to cis photoisomerization of N,N′-disubstituted indigo dyes via excited singlet states; a laser flash photolysis and steady state irradiation study

1987 ◽  
Vol 65 (4) ◽  
pp. 708-717 ◽  
Author(s):  
Helmut Görner ◽  
Joseph Pouliquen ◽  
Jean Kossanyi
Keyword(s):  
Steady State ◽  
Triplet State ◽  
Room Temperature ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Excited Singlet ◽  
Triplet Energy ◽  
Direct Excitation ◽  
Indigo Dyes

The relaxation pathways of excited trans-N,N′-diacylindigo dyes (trans-1–trans-4) and of the rigid trans (5) and cis (6) indigo analogues have been studied by nanosecond flash photolysis and steady state measurements. In fluid solution the trans to cis photoisomerization of 1–4 occurs via a singlet excited state mechanism on direct excitation, and the triplet state appears to be reached only by sensitized reaction. The lower quantum yield for trans → cis isomerization of 2–4 at room temperature, as compared to 1, is due to a thermally activated internal conversion step leading to the trans ground state. This pathway competes with fluorescence and twisting of the molecule around the central double bond in the excited singlet state. Increasing the viscosity of the medium decreases the twisting of the flexible molecules and, as a consequence, increases the population of the lowest triplet state. A triplet transient, the intensity of which increases with the viscosity of the medium, could be characterized at low temperature under direct excitation. A transient species, which can be assigned to a triplet state on the basis of quenching experiments and on the fact that the same transient could be found under sensitized conditions, also has been observed at room temperature for the rigid 5 and 6 molecules. Quenching measurements of triplet sensitizers by 5, trans-1, and trans-2 enables the localization of the triplet energy at around 40 kcal/mol.

Kinetic study of the reactions of methoxy-substituted phenacyl radicals

1995 ◽  
Vol 73 (2) ◽  
pp. 223-231 ◽  
Author(s):  
S.V. Jovanovic ◽  
J. Renaud ◽  
A.B. Berinstain ◽  
J.C. Scaiano
Keyword(s):  
Rate Constant ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Chemical Reactivity ◽  
Laser Flash ◽  
Carbonyl Oxygen ◽  
Peroxyl Radicals ◽  
Quantum Yields ◽  
Triplet States ◽  
Singlet States

The photochemistry of various mono- and dimethoxy-substituted α-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-α-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have π,π* character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 × 105 M−1 s−1, while for addition to the double bond in 1,1-diphenylethylene the rate constant is 9.4 × 107 M−1 s−1. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k ≤ 105 M−1 s−1). In contrast, reaction with oxygen occurs with kq = 2.5 × 109 M−1 s−1. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin. Keywords: methoxyphenacyl radicals, α-bromoacetophenones, laser flash photolysis.

Nanosecond flash photolysis

1970 ◽  
Vol 315 (1521) ◽  
pp. 163-184 ◽  
Keyword(s):  
Flash Photolysis ◽  
Pulsed Laser ◽  
Triplet States ◽  
Excited Singlet ◽  
Excited Triplet ◽  
Singlet States ◽  
Time Region ◽  
Excited Singlet States ◽  
Chemical Events ◽  
Excited Triplet States

A flash photolysis system, using a pulsed laser as source, has been designed and used to study events having a duration of a few nanoseconds; an improvement over conventional flash techniques by a factor of a thousand. The apparatus incorporates both spectrographic and photoelectric monitoring techniques which are easily interchangeable and, apart from the laser itself, it is readily constructed from standard components. Its applications to the observation of the absorption spectra of excited singlet states, short-lived excited triplet states and chemical events in the nanosecond time region are described.

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