A time-resolved study of singlet molecular oxygen (1.DELTA.gO2) formation in a solution-phase photosensitized reaction: a new experimental technique to examine the dynamics of quenching by oxygen

Abstract

charges on the porphyrin periphery do not exert the same effect as a point charge or a uniformly distributed charge (see, e.g., ref 22a). Moreover, the reference self-exchange rates listed in Table III were calculated in most cases by disregarding the work terms. Because of all these reasons it was decided to present only the results obtained without accounting for work terms (Table III).From the rate of reduction of IrCl61 2" we calculate a very low value of fc(S02/S02'~), in disagreement with the other values.The reason for the difference may lie in the fact that Kn in this case is very high, so that the rate of electron transfer kn may be low due to the effect predicted by the "inverted region" of the Marcus relationship, i.e. when further increase in driving force results in slowing down the reaction.A previous estimate of the self-exchange rate of S02/S02*" of 3 X 102 M"1 11*s'1 was derived40 from rate constants with two cobalt complexes.9 On the other hand, from reactions with viologens13 there was an indication of a very high self-exchange rate for S02/S02'" as is found here with methylviologen and the porphyrins. Taking all these results into account, we found k-(S02/S02'~) to vary over many orders of magnitude. The variations in fc(S02/S02*~) are reminescent of the wide variations found in the self-exchange rates for 02/02'~.47 This is not surprising since the behavior of S02*~w as suggested to parallel that of 02-.30 Such variations may be a general characteristic of redox pairs composed of a small number of atoms and one form of which is neutral, so that solvation of the two species varies greatly.47 Similar but smaller variations were noticed also in the reactions of C102 with different types of compounds.48 Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. We thank Dr. P. Hambright for many helpful discussions.

Is Singlet Oxygen Formation Dominant in Triplet Quenching Processes? Measurement of the Quantum Yield of Singlet Oxygen Formation by the Time‐resolved Thermal Lens Method

“…Here, we assume that the triplet state of the sensitizer is completely quenched by oxygen and that the radia-tive transition processes from the triplet state of the sensitizer and from the IA, state of oxygen can be neglected. These assumptions are considered reasonable based on the fact that the intrinsic lifetimes of the triplet state of the organic molecules which we examine here are in the range of milliseconds to seconds, and the quenching process due to oxygen is an order of sub-microsecond under the air saturated conditions (Andrews and Abrahamson, 1971;Merkel and Kearns, 1973;Garner and Wilkinson, 1977;Iu and Ogilby, 1987). Moreover, the quantum yield of phosphorescence from the IAs state of oxygen is known to be very low (-4.4x10-3 in CCl,) (Krasnovsky, 1981).…”

“…First, since the energy gap between the S1 and T1 states of quinoxaline (5200 cm-l) (Wilkinson, 1975) is smaller than that between the 'Ag and 3C.-g states of oxygen (7870 cm-l), singlet oxygen cannot be produced by quenching of this sensitizer (Iu and Ogilby, 1987). Nevertheless, a concentration dependence of QA is observed for this sensitizer (Fig.…”

Effect of solvent on the rate constant for the radiative deactivation of singlet molecular oxygen (1.DELTA.gO2)

“…The experimental apparatus and approach used to monitor the time-resolved 1 802 phosphorescence have been described in earlier reports. [24][25][26][27] In separate experiments, a pulsed laser (fwhm 5 ns) was used to excite the ! 802 photosensitizers acridine (355 nm, 0.25 mJ/pulse), rose bengal (532 nm, 2.5 mJ/pulse), and rubrene (532 nm, 0.65 mJ/pulse). '…”

Degradation of organic pollutants in/on snow and ice by singlet molecular oxygen (1O*2) and an organic triplet excited state

“…Because O 2 is the dominant sink for 3 RB* in our air-saturated samples, eliminating O 2 will increase the steady-state concentration of 3 RB*, making it a more important sink for the pollutant. Based on past work, [30][31][32] the lifetime for the RB triplet state is approximately 3 ms in air-saturated solution (where O 2 is the dominant sink) and 130 ms in degassed solution (where unimolecular decay to the ground state is the dominant sink). Thus the steady-state concentration of 3 RB* will be higher by a factor of approximately 40 in the degassed samples compared to in the air-saturated samples (ESI Section S3 †).…”