Pressure effect on the quenching of singlet oxygen by amines in different solvents

Abstract

AbstractRate constants kq of the quenching of singlet oxygen in its a1Δg‐state by five amines (piperazine (P), N,N′‐dimethylpiperazine (DMP), N‐methylpiperidine (MPP), quinuclidine (Q), N,N,N′,N′‐tetramethylphenylene‐1,4‐diamine (TMPD)) have been determined in solvents with different polarity in dependence on pressure up to 1200 or 1600 bar using time‐resolved phosphorescence techniques. In the case of the efficient O2(a1Δg) quencher TMPD in o‐dichlorobenzene and in benzonitrile at about 700 bar the transition is observed from the reaction‐controlled rate to the diffusion‐controlled rate limit. For all amines the highest negative value of the experimental activation volume was found in n‐hexane (−33.0 ≥ ΔV ≥ −42.0 cm3 · mol−1) and the lowest negative value in benzonitrile (−9.0 ≥ ΔV ≥ −19.0 cm3 mol−1). These results are consistent with the assumption that quenching of O2(a1Δg) by amines occurs via polar exciplexes with singlet and triplet multiplicity, respectively. The data were analysed taking into account contact complex formation. The reaction volume ΔVc of the formation of the contact complex was evaluated using the statistical model developed by Yoshimura and Nakahara. On the basis of Kirkwoods electrostriction model the dipole moments of the singlet exciplexes were calculated in the range between 5.7 (P) and 11.5 Debye (TMPD), indicating that only a partial charge transfer takes place in the quenching process.