Influence of the chloro substituent position on the triplet reactivity of benzophenone derivatives: a time‐resolved resonance Raman and density functional theory study

Abstract

A nanosecond time‐resolved resonance Raman (ns‐TR3) spectroscopic investigation of the photoreduction reactions and ability of several chloro‐substituted benzophenone (Cl‐BP) triplets is described. The TR3 results show that the 3‐chlorobenzophenone (3‐Cl‐BP), 4‐chlorobenzophenone (4‐Cl‐BP) and 4,4′‐dichlorobenzophenone (4,4′‐dichloro‐BP) triplets exhibit similar hydrogen abstraction ability with the parent BP triplet. In 2‐propanol, the 3‐Cl‐, 4‐Cl‐ and 4,4′‐dichloro‐diphenylketyl (DPK) radicals were observed and they appear to react with dimethylketyl radicals at the para‐position to form a light absorption transient species. These transient species were characterized with TR3 spectra, and identified with the help of results from density functional theory calculations. In an acetontitrile/water (MeCN:H2O) 1:1 mixed solvent, these DPK radicals were also observed but with slower formation rates. However, the 2‐Cl‐DPK radical was observed to form with a lower yield and a significantly slower formation rate than the other chloro‐substituted benzophenones examined here in 2‐propanol under the same experimental conditions. These results reveal that the 2‐chloro substituent reduces the hydrogen abstraction ability of the substituted BP triplet, which was not as expected based on the assumption that the electron‐withdrawing group could increase its photoreduction ability. This unusual ortho effect of the chlorine substitution is briefly discussed. Copyright © 2011 John Wiley & Sons, Ltd.

Recent advances in linear and nonlinear Raman spectroscopy. Part VII

“…Their results reveal that the 2‐chloro substituent reduces the hydrogen abstraction ability of the substituted BP triplet, which was not as expected on the basis of the assumption that the electron‐withdrawing group could increase its photoreduction ability. This unusual ortho effect of the chlorine substitution is briefly discussed …”

Photocross-linking Kinetics Study of Benzophenone Containing Zwitterionic Copolymers

“…Electron donors increase the electron density at the carbonyl, making it less electrophilic and therefore less reactive, inhibiting H-abstraction. Electron-withdrawing groups on BPs have an opposite effect, increasing the reactivity of the n−π* radical. ,, Other factors that affect the kinetics of BP include stereoelectronic control, regioselectivity, and heterologous behavior. ,, …”

“…Electronwithdrawing groups on BPs have an opposite effect, increasing the reactivity of the n−π* radical. 18,22,23 Other factors that affect the kinetics of BP include stereoelectronic control, regioselectivity, and heterologous behavior. 15,24,25 As BP is widely used in the polymer modification, the reactivity and kinetics of BP in the polymer matrix also attracted lots of attention.…”

Effect of chlorine substitution on triplet state structure of thioxanthone: A time‐resolved resonance Raman study

“…Recently, Wen Li et al . have carried out time‐resolved resonance Raman (TR3) study on various chloro derivatives of benzophenone and concluded that photoreduction reactivity of benzophenone is not significantly affected by the chlorine substitution, except 2‐chloro derivative, which shows unusual reduced photoreduction activity . Neumann et al .…”