Switch-Over in Photochemical Reaction Mechanism from Hydrogen Abstraction to Exciplex-Induced Quenching:  Interaction of Triplet-Excited versus Singlet-Excited Acetone versus Cumyloxyl Radicals with Amines

Abstract

The fluorescence and phosphorescence quenching of acetone by 13 aliphatic amines has been investigated. The bimolecular rate constants lie in the range of 10(8)-10(9) M(-1) s(-1) for singlet-excited acetone and 10(6)-10(8) M(-1) s(-1) for the triplet case. The rate data indicate that a direct hydrogen abstraction process dominates for triplet acetone, while a charge-transfer mechanism, namely, exciplex-induced quenching, becomes important for singlet-excited acetone. Pronounced stereoelectronic effects toward H abstraction, e.g., for 1,4-diazabicyclo[2.2.2]octane (DABCO), and significant steric hindrance effects, e.g., for N,N-diisopropyl-3-pentylamine, are observed. A negative activation energy (E(a) = -0.9 +/- 0.2 kcal mol(-1) for triethylamine and DABCO) and the absence of a significant solvent effect on the fluorescence quenching of acetone are indicative of the involvement of exciplexes. Full electron transfer can be ruled out on the basis of the low reduction potential of acetone, which was found to lie below -3.0 V versus SCE. The participation of H abstraction for triplet acetone is corroborated by the respective quenching rate constants, which resemble the reaction rate constants for cumyloxyl radicals. The latter were measured for all 13 amines and showed also a dependence on the electron donor properties of the amines. It is suggested that the H abstraction proceeds directly and not through an exciplex or ion pair. Further, abstraction from N-H bonds in addition to alpha C-H bonds has been corroborated as a significant pathway for excited acetone. Product studies and quantum yields for photoreduction of singlet- and triplet-excited acetone by triethylamine (8% for S(1) versus 24% for T(1)) are in line with the suggested mechanisms of quenching through an exciplex and photoreduction through direct H abstraction.

Scope and limitations of the TEMPO/EPR method for singlet oxygen detection: the misleading role of electron transfer

“…Being a secondary amine, TEMP (E ox = 1.0 V vs SCE) [45] could be oxidized by PET if the process is thermodynamically allowed. According to Rehm-Weller [32], this requirement is fulfilled for all the investigated photosensitizers.…”

Absolute Rate Constants for Hydrogen Atom Transfer from Tertiary Amides to the Cumyloxyl Radical: Evaluating the Role of Stereoelectronic Effects

“…Stereoelectronic effects have been studied in detail for HAT from the α-C−H bonds of amines and ethers to alkoxyl radicals. [12][13][14][25][26][27]42 These studies reveal that the reaction is most rapid when the α-C−H bond being broken is eclipsed with the heteroatom lone pair. This eclipsing allows for the optimal orbital overlap required to impart the greatest amount of stabilization through the delocalization of the developing excess spin on the carbon center from which the hydrogen atom is transferred.…”

Functionalization of polymeric surfaces by simple photoactivation of CH bonds

“…[29][30][31][32][33][34][35][36] have also been reported. 36 The results showed that formaldehyde, acetone, and cyclohexanone have a high grafting efficiency, especially acetone which gives the highest grafting efficiency.…”