Effect of Solvents on the Photoenolization ofo-Methylanthrone at Low Temperatures. Evidence for H-Atom Tunneling from Nonequilibrating Triplets

Abstract

The reactivity of 1,4-dimethylanthrone (1,4-MAT) and 1,4-dimethylanthrone-d 8 (1,4-DMAT) toward hydrogen transfer was investigated in methylcyclohexane (MCH), ethanol (EtOH), and 2,2,2-trifluoroethanol (TFE) by comparing their triplet-state emission spectra and lifetimes with those of compounds without the o-methyl substituent. A predominantly n,π* lowest triplet state in nonpolar solvents such as MCH changes toward a less reactive π,π* configuration in polar solvents such as EtOH and TFE. In EtOH, 1,4-DMAT exhibits a temperature-dependent dual emission while the corresponding protio compound (1,4-MAT) decays by reaction at all temperatures analyzed (ca. 15−90 K). Although compounds without the o-methyl group also presented dual emission, their relative intensities and lifetimes were shown to be temperature-independent, indicating the lack of thermal equilibration between the two triplets below 90 K. In the case of 1,4-DMAT in EtOH, an Arrhenius plot based on the data from the short-lived component (mainly n,π*) and detection of the enol between 19 and 80 K revealed that deuterium transfer occurs by quantum-mechanical tunneling (QMT). In agreement with results in MCH where a short-lived n,π* state predominates, the D-tunneling rate constant was estimated to be ca. (1.5−7.5) × 103 s-1. The phosphorescence of anthrone derivatives in TFE was long-lived, with a strong π,π* character, and highly heterogeneous, but not dual. A large isotope effect on phosphorescence is assigned to the isotope-dependent tunneling reaction from a predominant π,π* state.

Kinetic isotope effects in chemical and biochemical reactions: physical basis and theoretical methods of calculation

“…This is the case of the photoenolization of 1,4‐dimethylanthrone (1,4‐MAT) and 1,4‐dimethylanthrone‐ d 8 (1,4‐DMAT) in several solvents. Garcia‐Garibay and coworkers have experimentally shown that deuterium transfer of the triplet ketone 3 K in 1,4‐DMAT at very low temperatures (ca. 15–90 K) is slow enough to compete with radiative and thermal decay, so that phosphorescence emission was detected.…”

Rate Acceleration below 20 K in the H-Atom Tunneling of Triplet ortho-Methyltetralones

“…All triplet decays were relatively complex but could be fit reasonably to a double exponential, with rates k 1 and k 2 and preexponential factors I 1 and I 2 . Although solvation effects, slightly different conformations, and mixing of nearby π,π* states may contribute to the observed nonexponentiality, , the greatest deviations were observed at the lowest temperatures. Given the complexity of the experimental data, to carry out a qualitative but insightful analysis of deuteurium transfer using eq 1, the average decay rates of 1a − c and 2a − c were calculated according to eq 2, with the contribution of each component obtained from the ratios I 1 / k 1 and I 2 / k 2 2d…”

“…Our interest in reactions that occur under highly restricted molecular motion in crystalline and amorphous solids has led us to analyze transformations that occur at extremely low temperatures. Recent examples include the photoenolization of conformationally restricted ortho -methyl aryl ketones, which are illustrated in Scheme with 5,8-dimethyltetralone ( 1a ) .…”

“…Recent examples include the photoenolization of conformationally restricted ortho -methyl aryl ketones, which are illustrated in Scheme with 5,8-dimethyltetralone ( 1a ) . The reaction transforms an ortho -methyl aryl ketone ( K ) into an ortho -quinodimethane ( E ) in a process that occurs at ultralow temperatures by quantum mechanical tunneling (QMT) in the triplet excited state. ,,

1

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“…Experimentally, deuterium-transfer rates can be obtained by considering that the rates of triplet decay ( k dec ) are the sum of thermal and radiative rates ( k TS + k P ) plus the rate of deuterium transfer ( k D , eq 1), and by assuming that the first two rates ( k TS + k P ) may be obtained from model compounds with similar chromophores but with no reaction possible (e.g., 2a − c ) …”