4662
J . Phys. Chem. 1988, 92, 4662-4666
flow of electrons, and therefore it may present an entrance channel barrier to the reactions. In Table V we show the energies of the reactant LUMOs. These numbers are the differences between the total energy of the neutral molecule at the optimized geometry and the anion frozen a t the same configuration. As is shown in Table V, the LUMOs of CF4 and SiF4 are significantly higher in energy than the LUMO of GeF4 or of any other species. It seems reasonable to suggest that the low reactivity of CF4 and SiF4 is due to this feature. The LUMOs of the other species are much lower and do not reveal any patterns that correlate with the observed reactivities. We have examined the bond strengths of the reactant molecules for possible hints on reactivity. The available bond dissociation energies do not show a pattern consistent with the measured reactivities. One data point, however, may be explained by energy considerations. As was mentioned earlier, tabulations suggest that the reactions of boron with SiC14 may be slighly endoergic by perhaps 3 kcal/mol. This may be the reason for the out-of-sequence low reactivity of SiC14. In ref 1, a model based on MNDO calculations was postulated that successfully explained the pattern of reactivity for the
chloromethanes and chlorofluoromethanes. The model suggested that the reactivity was governed by the separation of charges between the carbon and chlorine atoms in the collision complex. This model, however, failed to explain the results obtained for the bromofluoromethanes, and likewise it fails to explain the present results. For these compounds the model also predicts a decreased reactivity with fluorine substitution. As was discussed in ref 2, this failure does not necessarily invalidate the model. The problem may be due to the structure of the M N D O program, which does not contain d-orbitals. The occupied d-orbitals can back-donate electron density to the central atom. The omission of d-orbitals from the M N D O program is likely to overestimate the charge separation between the central atom and the attached halogen, yielding an excessive exit barrier.
Acknowledgment. This work was supported by the National Science Foundation, Grant No. CPE-83 10767. We gratefully acknowledge their assistance. Registry No. B, 7440-42-8;SiF,, 7783-61-1;SiClF3, 14049-36-6; SiCI,, 10026-04-7; SiBr4, 7789-66-4;GeF4, 7783-58-6;GeCl,, 1003898-9;GeBr,. 13450-92-5.
Formation of Singlet Molecular Oxygen ('Ago2) in a Solution-Phase Photosensltized Reaction. 2. A Comment on Static Quenching Kai-Kong Iu and Peter R. Ogilby* Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 87131 (Received: December 10, 1987)
The solution-phase, time-resolved phosphorescence of singlet molecular oxygen ('Ago2), created by energy transfer from a sensitizer, has been examined by using a near-infrared detection system capable of resolving events that result in the formation By monitoring the rate of 'Agoz formation as a function of the ground-state oxygen (32;02) concentration, we of 'Agoz. have determined bimolecular rate constants for 32;02quenching of six organic molecule triplet states. Our results compare quite well with those independently determined from flash absorption studies of the sensitizer triplet state. In an earlier report, we suggested that data obtained from I$O2 formation measurements could be interpreted to indicate that ?Z;OTinduced deactivation of some organic triplet states may include a diffusion-independent,static quenching component. In this report, however, evidence is presented to indicate that, even at the limit of an oxygen-saturated solution, triplet-state deactivation by oxygen is independent of a static quenching component. Rather, the quenching process may be characterized solely by the encounter, through diffusion, of solvated triplet states and 32,02.
Introduction The quenching of organic molecule excited electronic states, specifically the triplet state, by molecular oxygen (32;0,)has been a focus of scientific study for many The quenching channel which results in energy transfer to form singlet molecular oxygen (lA,Oz) has been of particular interest to both photophysicists and photochemists In a solution-phase photosensitized experiment, we have recently been able to detect the '$02: time-resolved near-IR phosphorescence of ' $ 0 2 (3Z;02 1270 nm, 7884 cm-I) under conditions in which the rate of 'Ago2 formation can be re~olved.~As a method for examining oxygen quenching reactions, our technique complements standard flash absorption studies of the sensitizer excited ~ t a t e . ~ - ~ . ' ~Unlike -'' existing absorption techniques, however, our approach (1) is uniquely sensitive to rate-limiting steps which might exist in the process of energy transfer to form '$02,(2) may be useful under conditions where the triplet-state absorption cross section is small and where sensitizer fluorescence may obscure the triplet absorption signal, and (3) is expected to be particularly useful at
-
* Author to whom correspondence should be addressed. 0022-3654/88/2092-4662.$01.50/0
higher oxygen concentrations where triplet absorption signals are often difficult to detect. In our first report: we suggested that data on the rate of '$0, formation could be interpreted to indicate that 3Z,02-induced deactivation of some organic triplet states may include a diffusion-independent, static quenching component. In the present (1)Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970;p 509,and reference8 cited therein. (2)Birks, J. B. Organic Molecular Photophysics, Wiley: New York, 1973; Vol. I, 1975;Vol. 11, and references cited therein. (3) Turro, N. J. Modern Molecular Photochemistry; BenjaminfCummings, Menlo Park, 1978;p 589, and references cited therein. (4)Gijzeman, 0. L.J.; Kaufman, F.; Porter, G. J . Chem. Soc., Faraday Trans. 2. 1973. 69. 708-720. (5)Gijzeman, 0.L.J.; Kaufman, F. Ibid. 1973, 69, 721-726. (6)Reference 3, p 583 ff. (7) Wasserman, H.H.;Murray, R. W., Eds. Singlet Oxygen; Academic: New York, 1979,and references cited therein. (8) Frimer, A. A,, Ed. Singlet Oxygen; CRC Press: Boca Raton, 1985; Vol. I-IV, and references cited therein. (9)Iu, K.-K.; Ogilby, P. R. J . Phys. Chem. 1987, 91, 1611-1617. (10)Patterson, L.K.; Porter, G.; Topp, .. M. R. Chem. Phys. Lett. 1970, 7, 612-614. (1 1) Bensasson, R. V.;Land, E. J.; Truscott, T. G. Flash Photolysis and Pulse Radiolysis; Pergamon: Oxford, 1983,and references cited therein.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 16, I988 4663
Formation of Singlet Molecular Oxygen
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T I ME ( m 1 c r o s e c o n d s ) Figure 1. Example of the time-resolved 'Ago2 phosphorescence signal observed at 1270 nm subsequent to the pulsed excitation of phenazine in benzene. The oxygen concentration was 5.5 X lo4 M. The interfering luminescence signal coincident with the laser pulse is clearly visible. The fitting function is a convolution of eq 5 and the IR detector response.
work, we have examined the rate of 'Ago2 formation as a function sensitizers. By using this of 32;02concentration for six 'Ago2 data, we have determined bimolecular rate constants for 32;02 quenching of the organic molecule triplet state and have compared our results to those independently determined in flash absorption studies of the triplet sensitizer. The experiments described in this report provide evidence to indicate that, even at the limit of an oxygen @-O,) saturated solution, triplet-state deactivation by 32;02is independent of a static quenching component. Rather, for the six molecules examined, the quenching process may be characterized solely by the encounter, through diffusion, of a solvated triplet state and 3Z;02.
Experimental Section With the exception of points noted below, the experimental approach used to monitor 0, phosphorescence, under conditions in which the rate of 'Ago2ormation can be resolved, is similar to that used in our earlier study.g lAg02was created by irradiating an oxygenated solution of the sensitizer with a pulsed laser (Quanta-Ray DCR-2A Nd:YAG). With the exception of mesoporphyrin IX dimethyl ester (MP9), all of the sensitizers were irradiated at 266 nm (fourth harmonic of Nd:YAG fundamental wavelength). MP9 was irradiated at 355 nm (third harmonic of Nd:YAG). At the laser energies (
