Solvent Effect on Reaction Mechanism and Kinetics of Triplet Ion Pairs between Chloranil and Acenaphthene

Abstract

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.

Influence of Solvent on Photoinduced Electron-Transfer Reaction: Time-Resolved Resonance Raman Study

“…The effect of solvent polarity on the mechanism of photoinduced electron-transfer reaction in the case of FL is quite different from that of p-chloranil. 9,10,17 Transient absorption and TR3 studies on photoinduced electron-transfer reaction between chloranil and aromatic donors suggest that in nonpolar solvents such as 1,2-dichloroethane (dielectric constant of 10.3) 80% of the collison undergoes electron-transfer reaction to form the ion pair, whereas 20% of the collison complex undergoes H-transfer reaction. 10 In contrast to this, in our present study the effect of nonpolar media in 4:1 (v/v) CCl 4 -CH 3 CN (dielectric constant ) 13.4) results in formation of ketyl radical without the presence of ion-pair intermediate even though the polarity is slightly higher than that of 1,2-dichloroethane.…”

“…Transient absorption studies have shown that, in nonpolar media, CIP between chloranil and acenaphthene decays through intra-ion-pair proton transfer and, in polar solvents, the nature of ion pair is SSIP, which favors ionic dissociation into free anion and cation radicals. 9 Chloranil durene system in 1,2-dichloromethane revealed the presence of both CIP and SSIP in equilibrium. 10 But in such studies reported thus far, there is no structural evidence to differentiate the nature of ion pair formed in different solvent polarity.…”

“…Competition between the stabilization gained from Coulombic interaction of the ions in an ion pair and the solvation of the ions has important consequence on the nature of the ion pair, which can be either a contact ion pair (CIP) or a solvent-separated ion pair (SSIP). In this respect, solvent polarity has revealed interesting features in the distribution of these intermediates. − In polar solvents, triplet ion pairs, commonly observable in the nanosecond time domain from bimolecular photoinduced electron-transfer reactions, are mainly SSIP, whereas in less polar solvents CIPs are expected in the same timescale. − In highly polar solvents such as acetonitrile, the SSIP may dissociate into solvated ions. Transient absorption studies have shown that, in nonpolar media, CIP between chloranil and acenaphthene decays through intra-ion-pair proton transfer and, in polar solvents, the nature of ion pair is SSIP, which favors ionic dissociation into free anion and cation radicals .…”

“…In this respect, solvent polarity has revealed interesting features in the distribution of these intermediates. − In polar solvents, triplet ion pairs, commonly observable in the nanosecond time domain from bimolecular photoinduced electron-transfer reactions, are mainly SSIP, whereas in less polar solvents CIPs are expected in the same timescale. − In highly polar solvents such as acetonitrile, the SSIP may dissociate into solvated ions. Transient absorption studies have shown that, in nonpolar media, CIP between chloranil and acenaphthene decays through intra-ion-pair proton transfer and, in polar solvents, the nature of ion pair is SSIP, which favors ionic dissociation into free anion and cation radicals . Chloranil durene system in 1,2-dichloromethane revealed the presence of both CIP and SSIP in equilibrium .…”

Factors that Impact Photochemical Cage Escape Yields

“…Kobashi and coauthors used tetrachloro-1,4-benzoquinone (chloranil) as the photosensitizer and naphthalene derivatives as quenchers that form 1:2 chloranil/naphthalene adducts in the ground state. 337,356 As discussed throughout this review, the formation of ground-state adducts can have a considerable effect on the cage escape yields. However, in this system, the authors noted that the barrier for dissociation from the adduct Benzophenone as a photosensitizer was studied with a much wider range of substituted quenchers, primarily phenol derivatives, some of which include heavy atom substituents.…”