The Reaction of Oxygen with Electronically Excited Acriflavine¹

Abstract

INTERACTION OF OXYGEN AND ACRIFLAVINE 3257tion that 1 0 is almost temperature independent if the phosphorescence is observed following a photostationary illumination with a tungsten lamp. For this condition steady-state kinetics leads to equation 5 for the triplet populationwhere Iabs is the quantal rate of light absorption during the photostationary state. The population of triplets is seen to be dependent on the rate both of the S' + T process and also the reverse T -+ S' process. The temperature dependence of the two just about cancel each other. The energy of activation for a-phosphorescence, Eg, was estimated in two ways. ks was estimated from kobsdj the reciprocal of the mean life as tabulated in Table 11, by use of equation 3. k~, ranges from 0 a t -100" and below, to 5.3 set.-' at +20°.The resulting values of k6 gave a linear semi-log vs. 1/T plot, with E6 = 6 kcal. Because of errors inherent in the several subtractions required for this procedure, the following method was used.The ratio of a-to ,&phosphorescence intensities a t any time is proportional to kzkc/k6ks'. The proportionality constant is independent of temperature because of constant geometrical and optical factors and invariant band shapes. Also, kz and k5 are independent of temperature. Thus, the plot in Fig. 2 represents the temperature dependence of kslks'. At the lower temperatures, where ks'is independent of temperature, the observed slope leads to a value of 8 kcal. for Eg. This method is believed to be more reliable than the use of kg values obtained from the decay rates. The hightemperature break in the Fig. 2 curve is attributed to the increase in ks' and to the slight temperature dependence of the green band shape.TO k41abd(ks'kT -kakd (5)The energy difference between radiation from the S' and T states, obtained from the peak wave lengths of the bands, is about 5 kcal. The difference in activation energies for the T + . S' and S' -+ T steps, E6 -Eq, is 1 to 3 kcal. short of the spectroscopic difference in the levels. This is not surprising if the T state radiates to a higher vibrational level of S than does the S' state. This would correspond to a greater nuclear displacement in the T state than in the S' state.Internal Conversion.-The assumption was made above that k3 = 0. Semiquantitative estimates of the ratio of steady-state fluorescence to a-phosphorescence at room temperature indicate that k3iks' does not exceed 0.1. We tried to estimate the effectiveness of the direct radiationless depopulation of the triplet state, reaction vii. From the experimental comparison of the low temperature ,!%phosphorescence yield with the room temperature a-phosphorescence, we estimate that k 6 / k T is a t least 0.25. We thus cannot exclude the participation of step (vi;) in depopulating the triplet state on the basis of our data. Reactions of type (vii) occur very rapidly in fluid media. Apparently the degrees of freedom responsible for this type of quenching are not completely frozen out in adsorbates.Triplet-Triplet Interactions.-The observed...

Christopher Foote's Discovery of the Role of Singlet Oxygen [1O2 (1Δg)] in Photosensitized Oxidation Reactions

“…Opinions in the papers on photooxidation in the following 2 decades (∼1940−1963) were one-sided. The paradigm in vogue at the time attributed O 2 uptake in photooxidation reactions to an excited sensitizer−oxygen adduct. − Individuals working in this field, many of whom were highly opinionated, discounted the contribution of 1 O 2 ( 1 Δ g ). Blum surveyed the literature in a selective manner and concluded that a sensitizer−oxygen complex is the key reactive intermediate .…”

The Participation of Excited Oxygen in a Chemiluminescent Reaction†

“…(16) Hydrogen, helium, and oxygen are much less efficient in promoting vibrational relaxation between the two lowest levels for 0, than was CO, in the experiment of Lipscomb et al cited above. Our own evidence for the specific effect of CO, in inhibiting the oxygen-induced enhancement can thus be taken as strong evidence that 0," formed by the quenching act of oxygen is vibrationally excited and that this 02* participates in the luminescence-yielding process (1).…”

“…Since 0," is formed at the expense of dye excitation energy, two processes may be expected to occur in oxygen with opposite observable effects, one the quenching of phosphorescence (formation of O,*) and one the stimulation of luminescence, Process (1). Evidence for this was found by examining the decay of luminescence very quickly following flash excitation of a sample equilibrated with oxygen in the dark.…”

“…Evidence for this was found by examining the decay of luminescence very quickly following flash excitation of a sample equilibrated with oxygen in the dark. If the enhancement of luminescence is kinetically of second order in photo-products as (1) indicates, it should be especially noticeable at very high triplet concentrations. We therefore examined the very early portions of the luminescence decay curve, particularly from 0.6 to 5 msec following a strong microflash (about 50psec).…”

Oxygen quenching of acriflavine phosphorescence

“…Rosenberg and Shombert indeed found that quenching does not occur below -182°because the 02* cannot escape during the natural lifetime of the dye triplet. 23 A discussion of the nature of 02* has been given elsewhere. 19 There is a respectable body of evidence in the literature that 02 quenches triplet states of aromatic substances in fluid solution by a collisional mechanism.…”