J. Phys. Chem. 1988, 92, 265-267
265
Interaction of Singlet Molecular Oxygen with Disubstituted Olefins. Evidence for a Physical Quenching Induced by the Hydrocarbon Chain Charles Tanielian* and Robert Mechin Laboratoire de Photochimie, Ecole Europeenne des Hautes Etudes des Industries Chimiques de Strasbourg, 1 rue Blaise Pascal BP 296 R8, 67008 Strasbourg- Cedex, France (Received: September 21, 1987)
Besides the reactive quenching of singlet molecular oxygen by disubstituted olefins (rate constants kR),a physical quenching is observed and the corresponding rate constants k , are determined. It is shown that this physical quenching does not result from an interaction of 0 2 ( l A J with the double bond itself, but mainly with CH3 and CHI groups, and it is concluded that this process is of the same nature as the O2(IA8) deexcitation by a solvent. However, the value of kQ ((0.3-1.5) X lo4 M-' s-') cannot be always neglected with respect to kR, particularly for trans olefins or long-chain cis olefins.
Introduction Several classes of compounds A are known both as reactive singlet oxygen acceptors and physical quenchers: OZ(lAg) + A
kR
A02
Many kinetic methods provide only a measurement of the overall interaction of A with O2(l$), kQ kp1-7whereas a more restricted number of methods allows separation of kq and kR.8-10 These techniques have been used to demonstrate the Occurrence of reaction and quenching and to evaluate the corresponding rate constants for various substrates such as amino acids," amines,I2 phenols,I3 and conjugated dienes?*I4 By contrast, it has been reported that unconjugated olefins do not quench O2(IAg) significantly, which has been established for 2-methyl-2-butene,I5 2-methyl-2-pentene,I6 tetrameth~lethylene,~' and ~qua1ene.I~ In particular, we demonstrated in recent work that k , < 0.05 kR for the three latter acceptors." This result justifies many previous reports in which kQ + kR measurements are used to estimate olefin reactivities at least for tetra- and trisubstituted double bonds for which kR falls in the range 105-5 X lo7 M-' s-l.l8 However, one might wonder whether the inequality k ,
