The rates of reaction of singlet oxygen with 25 hydrazines were determined by following the emission of singlet oxygen at 1270 nm as a function of time. These data are utilized to discuss various options for the quenching mechanism including electron transfer, electronic to vibrational energy transfer, and a contact charge-transfer process.It is now well established that singlet oxygen is responsible for photodynamic destruction of both biologically and commercially important molecules. As a consequence, it is of some practical importance to search for and to investigate the properties of molecules capable of physically quenching1 this reactive species.Molecules that have been identified as physical quenchers of singlet oxygen include amines,2 ~ulfides,~ carotenoids: metal chelates,lb nitroxides,Ib phenols,lb inorganic anions,Ib and nitroso compounds.Ib It has been recognized that physical quenching of IO2 occurs by four distinctly different mechanisms:1a (an energy-transfer mechanism (for example, carotenoids have low-lying excited states ( I ) (a) Foote, C. 7 R , = R , = M e ; R 3 -R 4 -1 9 u 8. R, = R 2 = Me; R 3 = R 4 = necPe 9. R, .Rz. R,= Me; R , = iBu I O . R~-R , = R , = M~. R4-necPe 12. R, = R,. Me: R, = R, = necPe 11. R , = R 3 = M e . R,-R,=ISu 13. R, = R z = R , = R , = C & 2 2 2 3 2 4 2 5capable of accepting the energy from the IAg state of 02); (2) a reactive quenching mechanism that involves covalent bonding of IO2 to a substrate followed by decomposition to triplet oxygen 0002-7863/90/ 151 2-5080$02.50/0 Q
