J. Phys. Chem. 1980, 84, 3485-3486 (10) The spin densities were calculated by uslng the atomic parameters of ref 8a. If the older parameters of ref 8b are employed, the total spin density on arsenic in As#@+ increases to 1.26. I t should be noted, however, that the choice of atomic parameters affects the derived bond anglles by less than 1'.
(11) (a) J. R. Preeir, F. D. Tsay, and H. 8. Gray, J. Am. Chem. Soc., 94, 1875 (1972); (b) A. R. Lyons and M. C. R. Symons, /bid., 95, 3483 (1973). (12) R. W. Fessentden and R. H. Schuler, A&. Radlat. Chem., 2, 1 (1970).
COMMUNICATIONS TO THE EDITOR Quenching of Methylene Blue (S,) by Fe(1II)
Sir: We recently reported evidence for reductive quenching of singlet excited methylene blue, MB+(SI),by Fe(II).l In tho course of an investigation of the mechanism of quenching of 3ME!H2+and 3MB+by oxidative quenchers, we have found that quenching of MB+(S1) by Fe"'(H20),3+in 0.01 M aqueous nitric acid, 1.5 M in KNOB, occurs with the formation of the half-oxidized product, MB2+,. The efficiency of net electron transfer per quenching event has been estimated to be less than 1% . Cqtalysis of intersystem crossing to the triplet manifold is not significant. The Q-switched ruby laser (1.0 J per flash) and monitoring system are ddescribed Absorbance by 3MBH2+and MB2+vwere monitored at 7102and 5203nm, respectively. Fluorescence was measured with a PerkinElmer MPF 44A spectrofluorimeter with excitation a t 635 nm and emission monitored at 710 nm. Methylene blue chloride trihydrate was Fluka puriss. Ferric nitrate, potassium nitrate, and nitric acid were reagent grade. Laboratory distilled water was passed through a Millipore deionizer and filter. Deaeration was by purging with deoxygenated nitrogen. Figure l a shows adherence of the data to the form of the Stern-Volmer equation with a slope of 6.6 M-l. Together with the intrinsic lifetime of MEP(Sl),365 i 21 ps: this corresponds to kp,SL= 1.7 X 1O1O M-l s-l, a value about 5 times the specific rate of encounter of large molecules of all charge types in water at 23 "C with p 1 1 M, 3.2 X loBM-l s- la5Accordingly, static quenching is the principal process. The data do not discriminate between rapid Forster transfer of excitation energy among molecules of MB+ to statistical M13+-Fe1"(H20)63+pairs and groundstate association.6 If clrlly the latter is involved, the slope of Figure 1A corresponds to K, = 6.6 M-l. Flash photolysis of I O pM MB+ gave the following: (1) The pseudo-first-order rate of decay of absorbance by 3MBH2+at 710 nm increased linearly with increasing [Fe111(Hz0)63+], k q ~=l 1.4 X lo6 M-' s-' . (2) The absorbance intensity by bMBH2+extrapolated to the time of the laser flash decreased with increasing [I~eeI"(H20)63+]. (3) Absorbance by MB2+.a t 520 nm was almost fully developed by the end of the flash and increased very little during decay of 3MBH2+. (4) The intensity of prompt absorbance by MB2+-increased less than linearly with increase in [Fe1n(H20),3+]from 0.01 to 0.12 M. As shown in Figure lB, 1/[MB2+.]( c ~ ~ * +=' 58000 M-l cm-l)" varied linearly with 1/ [Fe111(]H20)63+], consistent with formal association of ground-slate dye with qulencher and corresponding to 1/[MB2+.] = (l/[MB+]$l')(l + l/K,[Fe"'(H20)63+]0),where [XIorepresents the initial concentration of X and F,' is the apparent efficiency of electron transfer (see below). The resulting values of K, and [MB+]$,' are 0022-3654/80/2084-3405$0 1.OOf 0
Figure 1. (A) Stern-Volmer plot of data for quenching of fluorescence of MB+(S,) by FeyH20):+ in 0.01 M aqueous nitric acld, 1.5 M in KNO,. (8)Double reciprocal plot of dependence of yields of ME2+-on [Fe(III)] under same conditions as for A.
12 M-l and 6.9 pM,respectively, and Flf is 0.69. The intrinsic lifetime of MB+(S1),0.365 ns: is very short compared to the flash length, -40 ns at half-height for a 1.0-J flash. Thus, many excitation-decay cycles will occur during a single flash unless the long-lived (intrinsic 7 = 4.5 ps2) and slowly quenched triplet is generated. Since kq,T1is less than lo9 times the specific rate of encounter and prompt yields of 3MBH2+vary inversely with [FelI1(HzO)63+], efficient catalysis of intersystem crossing from S to T is ruled out. The lifetime of MB+(S1)presumably decreases with increasing [Fen1(H20),3+]so that the number of excitation-decay cycles per flash and the value of Fl' increase. This increase in F,' may account for the value of K, from flash data (12 M-l) being greater than ik, value from fluorescence quenching (6.6 M-l). The number of excitation-decay cycles during a flash is >lo2. Since F,' = Fl(Con(l - F,)") != Fl(l + n),where Fl is the average efficiency of net electron transfer per decay and n is the number of excitation-decay cycles per flash, Fl is