Journal of the American Chemical Society
1944
Cotion Induced Changes inRate Constants
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100:6
/ March 15, 1978
and to the National Science Foundation (Grant No. CHE76-05175) for partial support of this work.
References and Notes
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Figure 1. Changes i n rate constants o f excited-state processes of crowns 1,2, and 3 caused by K+ (a light cation) and Cs+ (a heavy cation). The arrows are to scale and represent the difference from unity of the ratio of the perturbed and unperturbed rate constants (larger rate constant taken as the numerator). A n arrow pointing up indicates that the cation perturber increases the rate constant relative to that of free crown, and vice versa.
(1) 1,5-Naphtho-22-crown-6(1) (mp 55-56 'C) was synthesized by reaction of 1,5-bis(bromomethyl)naphthalene with pentaethylene glycol in tetrahydrofuran with potassium fed-butoxide. (2) L. R. Sousa and J. M. Larson, presented in part at the 174th National Meeting of the American Chemical Society, Chicago, Ill.,Aug 1977. (3) (a) L. R. Sousa and J. M. Larson. J. Am. Chem. Soc., 99, 307 (1977). (b) A detailed description of results with 2 and 3 has been submitted for publication. (4) Quantum yields and phosphorescence lifetimes were measured in uncracked 95 % ethanol glasses at 77 K with crown generally at 1.O X F. Fluorescence lifetimes and 77 K UV absorption spectra were measured in absolute ethanol-methanol (4: 1) glasses. (5) Results with NaCI, NaBr. Nal, CsCI, and CsBr show anion independance. (6) The values of kpand kdl are calculated assuming that # f t #isc = 1.O: (a) V. L. Ermolaev and E. B. Sueshnikova. Opt. Spectrosc., 16, 320 (1964); (b) A. R. Horrocks and F. Wilkinson. Proc. R. SOC.London, Ser. A., 306, 257 (1968); (c) R. H. Fleming, F. H. Fleming, F. H. Quina, and G. S. Hammond, J. Chem. Phys., 59, 3433 (1973). (7) With Rb+ and Cs+ perturbation, the kf values (which also affect knr)are low estimates based on a comparison of the integrated intensities of the 77 K UV absorption spectra. The approximate integrated intensities of the Lb absorptions of Ifully complexed with Na+, K+, Rb+. and Cs+ relative to that of uncomplexed 1 are, respectively, 1.9:2.0:1.7:2.0:1.0. (8) Attachment of the crown ring to naphthalene in the case of 1 causes relatively large changes in most rate constants compared with naphthalene whichhask,= 1.4X106,k,,=3.3X106,kk,=0.017,andkd,=0.38s-' under the conditions of Table I. However, complexation of Cs+ by 1 causes changes in the rate constants which are clearly larger than those due to the crown substituent effect. (9) (a) M. Kasha, Discuss. Faraday SOC., No. 9, 14 (1950); (b) M. R. Wright, R. P. Frosch, and G. W. Robinson, J. Chem. Phys., 33, 934 (1960); (c) S. P. McGlynn, T. A. Zumi, and M. Kinoshita, "Molecular Spectroscopy of the Triplet State", Prentice-Hall. Englewood Cliffs, N.J., 1969, p 284. (IO) For a report of exterml heavy-atom perturbation by CsCI, see L. K. Patterson and S. J. Rzad, Chem. Phys. Lett., 31, 254 (1975). (11) An x-ray crystal structure of 2 plus KSCN (D. L. Ward, H. S. Brown, and L. R. Sousa, Acta Crystallogr., Sect. 13, 33, 3537 (1977)) and a partial ~ ~ ~structure of 3 plus the same salt show the cations to be on C, axes in the plane of the naphthalene K system. (12) Unpublished work by James M. Larson. (13) G. G. Giachino and D. R. Kearns, J. Chem. Phys., 52, 2964 (1970). (14) S. Siege1 and H. S. Judeikis, J. Chem. Phys.. 42, 3060 (1965). (15) K. B. Eisenthal and M. A. El-Sayed, J. Chem. Phys.. 42, 794 (1965). (16) (a) S. P. McGlynn, J. J. Reynolds, G. W. Daigre, and N. D. Christodoyleas, J. Phys. Chem., 66, 2499 (1962); (b) S. P. McGlynn, J. Daigre, and F. J. Smith, J. Chem. Phys., 39, 675 (1963). (17) The order of rate constant susceptibilities is the same as that seen when naphthalene is perturbed by charge-transfer complexers (even without heavy atoms); however, there is no indication of a charge-transfer component in the emission spectra in this case. See ref 1Oc. p 322, and N. Christodouleas and S. P. McGlynn, J. Chem. Phys., 40, 166 (1964). (18) The difference in the perturbing effectiveness of Cs+ complexed by 1as compared with 2 and 3 might in principle be rationalized by a superposition of a distance effect and some type of angular dependence (perhaps with small effects when Cs+ is on the nodal plane of the K system). These results do not provide a separation of effects, however, since 1 allows (but does not require) Cs+ to contact the K system while models (Corey, Pauling, Koltun) suggest that 2 and 3 hold Cs+ so that 1.6 and 2.0 A, respectively, separate the edge of the K system from the Cs' van der Waals radius.
and knr)are increased by complexed Cs+ for all three crowns, and are increased relative to the values seen with K+ perturbation. The postulation of Cs+ enhanced spin-orbit c o ~ p l i n g ~ in 1, 2, and 3 seems reasonable. A major feature of the results is the very much larger effect of Cs+ on k,, k,, and k d t of 1 compared with 2 and 3. Apparently, the enhancement of spin-orbit coupling is very much greater when the Cs+ heavy atom is held near the face as opposed to the end or side" of the a system. For comparison, changes in the rate constants of 1,2, and 3 caused by 20% (v/v) ethyl bromide in ethanol-methanol (4:l v/v)I2 at 77 K are similar to each other, and similar to those caused by the complexed Rb+ (except that ethyl bromide increases k , of 1 three times more than Rb+ does). A second feature is that the order of susceptibility of the rate constants of 1 to Cs+ (knr > k , > k d t ) is different from that found in studies of the effects of external heavy-atom perturbation of unsubstituted naphthalene. Greater susceptibility of k , compared with k d t was seen in work by Kearns,13 SieJames M. Larson, Lynn R. Sousa* ge1,I4 and El-Sayed,I5 but not that of McGlynn.16 However, Department o f Chemistry, Michigan State University the observation that k,, of 1 is the most perturbed rate constant East Lansing, Michigan 48824 is consistent with the work on naphthalene by McGlynn,l6>l7 Receiued October 25, 1977 but not that of Siegel.14 The order is also different from that observed when crowns 1,2, or 3 are perturbed by ethyl bromide.I2 In those cases k , is increased about twice as much as k d t which is only slightly more affected than knr. It is not yet clear whether the difference between Cs+ perturbation of 1 and that of unoriented alkyl halidesI2-l6 is due to the added presStructural Diversity in Iron Porphyrins: ence of a positive charge, the precise orientation18 required by Solution Characterization of a High-Spin 1, different thresholds of k, k,, and k d t to the spin-orbit Six-Coordinate Ferric Complex perturbation, or some property of cesium. Sir: Acknowledgments. We thank David Carr for technical assistance, Houston s. Brown for computer programming, The detailed characterization of solid-state structures and Professor Peter J. Wagner for heavy use of his Hitachi Permagnetic susceptibilities of iron porphyrin complexes in various kin-Elmer spectrophotofluorometer, Professor Harry A. Eick ligation, oxidation, and spin states has produced what is genfor the use of his PDP-8 computer, and Professor M. Ashraf erally accepted as a predictable correlation between stereoEl-Bayoumi for the use of his single photo counting apparatus. chemistry and spin state that is directly applicable toward Acknowledgment is made to the donors of the Petroleum Reunderstanding certain structure-function relationships in hesearch Fund, administered by the American Chemical Society, mog1obins.l The cornerstone of this correlation is that high-spin 0002-7863/78/1500-1944$01.00/0
0 1978 American Chemical Society
1945
Communications to the Editor C
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Chemical S h i f t .
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Figure 1. 'H NMR trace at -72 "C of a 3:l C2HC13/C2H2C12solution 12 m M in total porphyrin, i.e., [d20-PFeI]o = 12 m M ,and 40 m M in total dimethyl sulfoxide, ;.e., [L]o = 40 mM. Peaks a and b a r e the pyrrole H for d2vPFeI and d20-PFeLn+I-, respectively; c and d are the residual solvent peaks for CHC13 and CH2 Cl2, respectively, and e is the peak for free L at its diamagnetic position (slow exchange). Comparison of areas of e to a t b under slow (
