Mechanisms of hydrogen atom abstraction reactions by triplet state p-chloranil (3CA) from durene (DH) were studied by picosecond and nanosecond laser photolysis and transient photoconductivity measurements. 3CA was quenched by DH through diffusional encounter to form a triplet ion pair (IP) between CA and DH, p-chloranil semiquinone radical (CAH·), and 2,4,5-trimethylbenzyl radical (D·). Ionic dissociation of IP was observed in 1,2-dichloroethane (DCE) as well as in acetonitrile. However, no transient species was observed by direct excitation of a charge-transfer (CT) band of the electron donor-acceptor (EDA) complex between CA and DH. The H-atom transfer leading to production of CAH· was found to proceed through two distinct mechanisms; H-atom transfer via IP (Mechanism I) and a more rapid transfer competing with IP formation (Mechanism II). The quantum yields of CAH· produced by Mechanisms I and II and the first-order rate constants for proton transfer, ionic dissociation, and intersystem crossing competing with one another in the IP state were estimated to be (0.1 and 0.2) and (2, 5, and 13)×106 s−1, respectively, in DCE at room temperature.
The behavior of electronically excited chloranil interacting with the titled vinyl compounds and solvents has been studied by means of the nanosecond laser photolysis technique. Semiquinone radicals and/or chloranil anions have been detected as transients except in the system containing styrene. Careful observation and analysis of the kinetic behavior of the transient absorptions reveal that these transients are brought about by the interaction of excited triplet state of chloranil with vinyl monomers and/or solvents. The logarithms of the quenching rate constants and rate constants of the formation of transients are linearly related to the ionization potentials of the vinyl monomers and solvents used. The mechanisms of the triplet quenching and transient formation can reasonably be interpreted by taking the relaxed triplet-state complex with a charge-transfer character into account as the precursor leading to the production of semiquinone radicals, chloranil anions, and other intermediate compounds.
A comparative study on decay dynamics of triplet ion pairs has been performed for the chloranil (CA) and acenaphthene (ACN) system in both the nonpolar solvent benzene (BZ) and the moderately polar 1,2-dichloroethane (DCE) by means of nanosecond laser photolysis. Triplet chloranil (3CA) is quenched by ACN to yield the (1:1)triplet ion pair (IP1), 3(CA\ewdot, ACN\underset.+), in BZ, while the (1:2) ion pair (IP2), 3(CA\ewdot, ACN2\underset.+), in DCE as well as IP1 in the concentration range less than 1 M (1 M=1 mol dm−3) of ACN. Temperature dependence of the transient spectra indicates that IP2 is energetically lower than IP1 in DCE. In BZ, IP1 decays through both back electron transfer (back ET) to the ground state of the donor-acceptor pair and intra-ion-pair proton transfer (PT) leading to formation of the chloranil semiquinone radical (CAH·). In DCE, both IP1 and IP2 disappear through the back ET and ionic dissociation (ID) to free anion and cation radicals, while no PT is observed. The efficiency of ID is greater in IP2(0.70) than in IP1(0.33). From temperature dependence on decay profiles, Arrhenius parameters are obtained for each decay process of the ion pairs. The PT process in BZ requires a considerably high activation energy (30 kJ mol−1), while the ID process in DCE proceeds with practically no activation energy for both IP1 and IP2. The activation barrier for the back ET of IP1 in BZ is obtained to be 13 kJ mol−1 which is significantly greater than that (∼0 KJ Jmol−1) in DCE. This solvent effect may be attributed to the difference in the ion pair states: i.e., a contact type in BZ and solvent-separated type in DCE.
β-Cyclodextrin added to the fluorescence quenching system of the title compounds has two effect, depression on dynamic quenching and binding between the fluorescer and quencher causing rapid quenching.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.