J . Phys. Chem. 1987,91, 4599-4602 volumeI4 and low ionic charge of the thiocyanate ion, water molecules around the ion, being subjected to weak electrostriction, form probably a bulky hydration structure through hydrogen bonding with the ion. Thus, when the outer-sphere complex forms, dehydration from the anion produces volume contraction. For the purpose of examining the present results as a whole, the total volume change, AVl, of complexation was evaluated from the relationI3 AV, = AV12 K23(l K23)-'AV23. The data in Table I1 yield AV, = 4.4 cm3 mol-', which seems reasonable compared with the value 4.0 cm3 mol-' obtained previously for the zinc(I1) monothiocyanate complex4 and also with the AVs found for the monoglycinate complexes of bivalent metals.I5 Thus, not only the concentration dependences of the relaxation parameters but also the obtained values of the reaction parameters are wholly consistent with the Eigen-Tamm mechanism. That is, the complex formation of cadmium( 11) monothiocyanate proceeds via the stepwise mechanism even a t the high ionic strength of 3 M, and the high ionic concentration does not reduce the outer-sphere association constant to very small values, against
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(14) Millero, F. J. Chem. Reu. 1971, 71, 147. (15) Grant, M. W. J. Chem. SOC.,Faraday Trans. 1, 1973, 69, 560.
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Spiro's expectation.s The present study demonstrates that ultrasonic absorption measurements over a wide concentration range are very effective for determining the reaction mechanism involved and also that the high ionic strength needed for this experimental condition does not necessarily alter the reaction mechanism. The present investigation together with the previous one reveals different kinetic behavior from the chemically similar systems of zinc(I1) and cadmium(I1) thiocyanate. We have also noticed that ultrasonc absorption data obtained on aqueous cadmium(I1) halides16 are inconsistent with the reaction mechamism proposed for zinc(I1) halides." Elucidation of these points requires further studies. Acknowledgment. I thank Dr. Shoji Harada of Hiroshima University and Professor Z. A. Schelly of the University of Texas at Arlington for reading the manuscript and offering helpful comments. Registry No. Cd, 7440-43-9; SCN-, 302-04-5; cadmium(I1) thiocyanate, 865-38-3. (16) Tamura, K.; Harada, S.; Yasunaga, T., unpublished results. (17) Tamura, K. J. Phys. Chem. 1977, 81, 820.
Effect of Solvent on the Rate Constant for the Radiative Deactivation of Singlet Molecular Oxygen ('Ago,) Rodger D. Scurlock and Peter R. Ogilby* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 871 31 (Received: February 9, 1987; I n Final Form: April 3, 1987)
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Relative rate constants for the radiative deactivation (k,) of singlet molecular oxygen (3Z;02 lLigo2) have been determined in 15 solvents. A substantial solvent effect is observed. Changes in the value of k, can exceed a factor of 20. A reasonably good correlation exists between the solvent polarizability, defined as a function of the solvent refractive index, and the radiative rate constant. We suggest that our data support a model in which 'Ago2 is perturbed through the formation of a discrete oxygen-solvent collision complex.
Introduction In recent years, it has become evident that singlet molecular oxygen (IA,O2) is an ideal model system for the study of a solvent-induced Since the transition 32;02 '$0, is fobidden as an electric dipole p r o ~ e s s ,the ~ , ~radiative lifetime in the absence of collisions (e.g., the upper atmosphere) of 'Ago2 1 h).5 However, in the presence of is quite long ( 7 , = l/k, transition proba perturbing environment, the 32-02 'Ago2 ability increases dramatically.'s2*b The total rate constant for IA,02 deactivation ( k A )in solution is determined by two components: a radiative (k,) and a nonradiative (knr)channel. In general, for any molecule, the rate constant for a radiative process will depend intrinsically on the refractive index of the ~olvent."~
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(1) Wayne, R. P. In Singlet Oxygen; Frimer, A. A., Ed.; CRC: Boca Raton, FL, 1985; Vol. I, p 81 and references cited therein. (2) Monroe,8. M. Reference 1, p 177 and references cited therein. (3) Herzberg, G. Molecular Spectra and Molecular Structure. I. Spectra ofDiatomic Molecules; Van Nostrand Reinhold; New York, 1950; p 279. (4) Herzberg, L.;Herzberg, G. Astrophys. J. 1947, 105, 353-359. ( 5 ) Badger, R. M.; Wright, A. C.; Whitlock, R. F. J. Chem. Phys. 1965, 43, 4345-4350. (6) This is also observed, as a change in the absorption cross section, for the procws 'Ago2 3Z-02 (ref 7, 8). In order to observe an absorption signal, however, these exkriments were performed at elevated pressures. Due to the danger of combustion, the number of solvents accessible for study was restricted. (7) Long, C.; Kearns, D. R. J. Chem. Phys. 1973, 59, 5729-5736. (8) Cho, C. W.; Allin, E. J.; Welsh, H. L. Can J. Phys. 1963, 41, 1991-2002. (9) Andrews, J. R.; Hudson, B. S. J. Chem. Phys. 1978,68,4587-4594.
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0022-3654/87/2091-4599$01.50/0
This relationship appears in the Einstein coefficients and in Planck's blackbody radiation law." However, even for a substantial change in the solvent refractive index, the magnitude of this effect on k, does not exceed a factor of about 1.5.9Jo Consequently, other models have been presented to account for large solvent effects on the radiative rate constant of a solute molec ~ l e . ~ For J ~ Jexample, ~ the formation of a discrete solute-solvent complex and, by definition, a new set of molecular eigenfunctions, may allow a process forbidden in the isolated solute to "steal intensity" from an allowed t r a n ~ i t i o n . ~ ~ ' ~For - ' * the particular example of molecular oxygen, the perturbation offered by the solvent could, therefore, make the transition 3z;02 '4,02more probable. This, in turn, would be manifested as an increase in the value of k,. In solution, the nonradiative channel dominates deactivation process ( k A= k, + k,,, with k,, >> the total 'Ago2 k,). This results in an extremely small quantum efficiency for '$0, phosphorescence ($ = 10-3).19 Under these circumstances,
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(10) Giniger, R.; Amirav, A. Chem. Phys. Lett. 1986, 127, 387-391. (1 1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; pp 87-88. (12) Birks, J. B. Z . Phys. Chem. 1976, 101, 91-104. (13) Olmsted, J. Chem. Phys. Left. 1976, 38, 287-292. (14) Robinson, G. W. J. Chem. Phys. 1967, 46, 572-585. (15) Hoytink, G. J. Arc. Chem. Res. 1969, 2, 114-120. (16) Dijkgraaf, L.; Sitters, R.; Hoytink, G. J. Mol. Phys. 1962, 5, 643-644. (17) Rettschnick, R. P. H.; Hoytink, G. J. Chem. Phys. Lett. 1967, I , 145-148. (18) Ogilby, P. R.; Foote, C. S. J. Am. Chem. SOC.1983, 105, 3423-3430.
0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
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efficient coupling to a plyatomic solvent molecule not only induces the transition 3Z;02 'A8O2but provides a sink for the 'A,O2 excitation energy (22.5 kcal mol-', 7874 cm-I). It is reasonable, therefore, that recent attempts to quantify the effect of solvent on the observed '$0, lifetime ( T A = l / k A )have focused on solvent parameters which principally influence the rate of a radiationless transition. 18,20-22 It is now possible to observe the phosphorescence of solutionphase '$0, (3Zg-02(u= 0) '$O,(u = 0); 1270 nm, 7874 cm-') in both ~ t e a d y - s t a t eand ~ ~ time-resolved experimenh2 We have used this experimental technique to examine the effect of solvent on k, at atmospheric pressure.6 In this paper, we report relative rate constants for the radiative deactivation of '$0, in 15 common organic solvents. Our results provide an important experimental reference which is necessary to understand the mechanism by which a solvent molecule can influence the transition 32,-02
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]Ago2.
Experimental Section The experimental apparatus and approach used to monitor the phosphorescence have been described in earlier time-resolved '$0, report^.^^-^^ In separate experiments, a pulsed laser (fwhm 5 ns) was used to excite the 'A,02 photosensitizers acridine (355 nm, 0.25 mJ/pulse), rose bengal (532 nm, 2.5 mJ/pulse), and rubrene (532 nm, 0.65 mJ/pulse). 'A,02 phosphorescence (fwhm 30 nm)Is was isolated by an interference filter centered at 1270 nm.24 The transmission bandwidth of this filter (fwhm = 50 nm) was large enough to ensure that slight solvent-dependent changes in the emission ,,A did not affect our data.28 In addition, although the responsivity of the germanium p-n junction detector the used in this study is wavelength dependent at 1270 nm3,27 solvent effect on the 'A,02 phosphorescence A-, is small enoughB to ensure that any error introduced is negligible when compared to other sources of experimental uncertainty (vide infra) A standard flash absorption spectrometer was used to monitor the bleaching of 1,3-diphenylisobenzofuran(DPBF) per pulse of ~ -steady~~ laser radiation incident on the 'A,02 ~ e n s i t i z e r . ~ A state, 150-W xenon lamp was used as the probe source. The lamp current was regulated through an optical feedback system thereby minimizing lamp intensity fluctuations. By placing a monochromator between the lamp and the sample cell, the DPBF/ sensitizer solution was illuminated with a relatively narrow spectral bandwidth of visible radiation from the probe beam (- 15 nm band-pass at 415 nm). A second monochromator was positioned between the sample cell and our optical detector (silicon photodiode).24 Although the sensitizers used did not absorb light from the probe beam under our conditions, use of the first monochromator ensured that self-sensitized DPBF bleaching did not contribute to a change in DPBF concentration in the time period of our measurement. In addition, a shutter was used to shield the sample cell from the probe lamp up to th'e time the photolysis
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(19) Kfasnovskii, A. A. Chem. Phys. Lett. 1981, 81, 443-445. (20) Lin, S.-H., Ed. Radiationless Transitions; Academic: New York, 1980. (21) Hurst, J. R.; Schuster, G. B. J. Am. Chem. SOC.1983, 105, 5756-5760. (22) Rcdgers, M. A. J. J. Am. Chem. SOC.1983, 105, 6201-6205. (23) Khan, A. U. In Singlet Oxygen; Frimer, A. A., Ed.; CRC: Boca Raton, FL, 1985; Vol. I, p 39 and references therein cited therein. (24) Scurlock, R. D.; Ogilby, P. R. J . Phys. Chem., submitted for publication. (25) Iu, K.-K.; Ogilby, P. R. J . Phys. Chem., submitted for publication. (26) Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1987, 91, 1611-1617. (27) Iu, K.-K.; Scurlock, R. D.; Ogilby, P. R. J. Photochem. 1987, 37, 19-32. (28) For a variety of solvents, Foote and co-workers have established that the change in 'Ago2 phosphorescence A,, is no greater than -5 nm. Professor C. s. Foote, private communication. (Also see ref 52.) (29) Adams, D. R.; Wilkinson, F.J. Chem. SOC., Faraday Trans. 2 1972, 68, 586-593. (30) Young, R. H.; Brewer, D.; Keller, R. A. J . Am. Chem. SOC.1973, 95, 375-3 79. (31) Kearns, D. R.; Merkel, P. B. J . Am. Chem. SOC.1972,94,7244-7253. (32) Gorman, A. A,; Lovering, G.; Rodgers, M. A. J. J . Am. Chem. SOC. 1978, 100, 4521-4532.
Scurlock and Ogilby laser irradiated the sensitizer. As a result, the monitored change produced by pulsed in DPBF concentration was due solely to '$0, laser irradiation of the sensitizer. DPBF extinction coefficients were established for each solvent on a Beckman model DU-40 spectrophotometer. Identical coefficients were obtained by using the absorption spectrometer built for the bleaching study (vide supra). Products of DPBF p h ~ t o o x y g e n a t i o n did ~ ~ -not ~ ~ perturb our absorption measurements at 415 nm. For each numerical determination in a particular solvent, three M, ref air-saturated cuvettes were prepared ( [ O , ]z 2.0 X 26). Cuvettes one and two contained an identical solution of DPBF ((0.5-3.0) X M) and the sensitizer (either acridine at 1 X IO4 M, rose bengal at -2.0 X M, or rubrene at -1.0 X lo4). The first cuvette was used in the DPBF bleaching experiment, and the second was used in the determination of kQ and 1, (all parameters are defined in the section Results and Discussion). The third cuvette, distinguished only by the lack of DPBF, was used to determine kAand I . Since all samples were prepared from the same stock solution, the sensitizer concentration in each cuvette was identical. Data were collected from a single laser shot. After exposure, the solution was mixed and the experiment repeated. The solution in each cuvette was discarded after exposure to four laser pulses. For each pulse, under all conditions, the change in [DPBF] in the volume element irradiated was
