J. Phys. Chem. 1983,87,5057-5059
from our assumed potential models and in this regard it
will be interesting to see if our assumptions concerning the X-H and X-0 potentials are justified by quantum mechanical studies. The fact that the H atom turns out to be clathrated in part justifies a postiori our neglect of polarization effects since the net dipolar field at the center of an ideal clathrate cage is ~ m a 1 l . l ~ (15) Davidson, D. W. In "Water a Comprehensive Treatise"; Franks, F., Ed.; Plenum: New York, 1973; Vol. 2, Chapter 3, p 115.
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For the future it remains to not only justify our empirical potential functions but also to explore the role played by quantum effects7 in determining the effect on the cavity size when H is replaced by Mu. Acknowledgment. M.L.K. thanks Gren Patey and David Walker for a simulating discussion that prompted this investigation, and Ian McDonald for help in setting up the calculation. Registry No. Hydrogen atom, 12385-13-6; muonium, 1258765-4.
Photophysics of a Macrobicyclic Hexaminechromium(II I ) Complex and the Mechanism for Excited-State Relaxation in Chromium( I I1)-Amine Complexes' T. Ramasami, John
F. Endlcott," and George R. Brubaker
Department of Chemistry, Wayne State Univefsity,Detroit, Michigan 48202 (Received: August 8, 1983)
The macrobicyclic complex, chromium (S)-1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane (sep), has been synthesized with a view to testing the proposition that direct chemical reaction determines the lifetime of the lowest energy doublet excited state. Even though chemical reactions are sterically blocked in the (2E)Cr(sep)3+ complex, its lifetime is only slightly longer than that of the (2E)Cr(en)33+ parent compound in DMF at 0 "C. Alternatives to the direct reaction model for doublet-state deactivation are considered. The most plausible of these is deemed to be vibrationally promoted surface crossing to a lower energy species with chemical reactions occurring after entry into the deactivation channel.
There has been a continuing controversy regarding the pathway for relaxation of the lowest-energy excited states (2Ein octahedral microsymmetry) of simple chromium(II1) complexes.2-s The two popular models for this process are (A) quenching of the excited doublet state by direct chemical reaction2i6and (B) back-intersystem crossing to a very reactive, higher energy, electronically excited quartet ~tate.~-~ A third model (C) would account for the behavior of the 2Estate in terms of a surface crossing to a reactive ground-state intermediate, with the partitioning between reaction and nonradiative relaxation being a function of the configuration coordinates at the surface crossing and the vibrational relaxation trajectory on the ground-state surface.s (1) Partial support of this research by the National Science Foundation (Grant CHE 80-05497) is gratefully acknowledged. (2) (a) Gutierrez, A. R.; Adamson, A. W. J. Phys. Chem. 1978,82, 902. (b) Walters, R. T.; Adamson, A. W. Acta Chem. Scand., Sect. A , 1979, 33, 53. (3) (a) Kirk, A. D. Coord. Chem. Reu. 1981,39,225. (b) J. Phys. Chem. 1981,85, 3205. (4) (a) Shipley, N. J.; Linck, R. G. J. Phys. Chem. 1980,84, 2490. (b) Cimolino, M.; Linck, R. G. Inorg. Chem. 1981,20, 3499. (5) Linck, N. J.; Berens, S. J.; Magde, D.; Linck, R. G. J.Phys. Chem. 1983,87, 1733. (6) Kang, Y. S.; Castelli, F.; Forster, L. S. J. Phys. Chem. 1979, 83, 2368. (7) Allsopp, S. R.; Cox, A.; Kemp, T. J.; Reed, W. J. J . Chem. SOC., Faraday Trans. I 1980, 76, 162. (8) (a) Endicott, J. F.; Ferraudi, G. J. J.Phys. Chem. 1976,80, 949. (b) Endicott, J. F. J. Chem. Ed., in press. (c) Endicott, J. F.; Ramasami, T.; Tamilarasan, R.; Brubaker, G. R. "Structure-Function Relationships in Inorganic Chemistry"; Hodgson, D. J.,Ed.; Academic Press: New York, in press.
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To a significant extent the direct reaction model A has evolved from the contrasting large (-0.5) and small ( quantum yields for ligand photosubstitution found for trans-CrI"(AM),XY and trans-Cr"'([14]aneN4)XY (AM = NH, or 1,2-diaminopropane/2; [14]aneN4 = 1,4,8,11-tetraazacyclotetradecane) complexes, respectively, following population of their lowest-energy doublet state^.^^^^ It has been presumed that the steric constraints of the coordinated macrocyclic [ 141aneN, ligand interfere with doublet-state reactivity, thus resulting in the small quantum yields. As a corollary to this line of reasoning, these same complexes have been presumed to have relatively long-lived doublet s t a t e ~ . ~ s Indeed, ~J~ the observation that the photoinert trans-Cr([ 14]aneN4)(CN)2+ complex is exceptionally long lived could be taken as support for model A." Actually, the lifetimes of transCr"'( [ 14]aneN4)XYcomplexes are not exceptionally long; rather they correlate well with the lifetimes of other chromium(II1)-amine complexes based on inferences from model C.8b,c In view of these various considerations, we have synthesized a macrobicyclic chlathrochelate complex, chromium (S)-1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicos~e (sep), which is formed by encapsulation of C r ( e ~ ~ ) ~In ~+.l~ (9) (a) Kutal, C.; Adamson, A. W. Inorg. Chem. 1973, 12, 1454. (b) Kutal, C.; Adamson, A. W. J. Am. Chem. SOC.1971,93,5581. (c) Pribush, R. A.; Poon, C. K.; Bruce, C. M.; Adamson, A. W. Ibid. 1974, 96, 3027. (10) Adamson, A. W.; Macke, H.; Puaux, J. P.; Zinato, E.; Ricierri, R.; Poon, C. K. 'Proceedings, XXI International Conference on Coordination Chemistry; Tolouse, France, July, 1980; p 270. (11)Miller, P. K.; Crippen, W. S.; Kane-Maguire, N. A. P. Inorg. Chem. 1983,22, 696.
0022-3654/83/2087-5057$01.50/00 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87,
TABLE I:
No. 25, 1983
Letters
G r o u n d - S t a t e a n d Excited-State Properties of S o m e Chromium(II1)-Hexamine Complexes property
Cr( ~ e p ) ~ +
Cr( e n ) 1 3 +
Cr(NH,),3+
--
Cr(ND,),3+
--
-1.15a 1 1 -1.09 i 0.03 4 6 0 i 2 (100 i 5 ) 456 ( 7 2 ) 459 (45) 459 345 i 2 (102 i 5) 353 ( 6 3 ) 351 (38) 351 E('Eo), p M - ' 1.50 1.50 1.52 1.52 est E(4T,o)- .??('Eo), p M - ' ( k J m o l - ' ) 0.43 ( 5 3 ) 0.44 ( 5 4 ) 0.45 ( 5 5 ) 0.45 (55) T( 'E) a t 0 "C in D M F , 1 s 1oi 1 6.2 f 0.5 17i 2 36 i 3 Ea,b k J m o l - ' 5 1 . 0 f 0.8 4 6 . 9 0.8 4 5 . 2 i 0.8 4 5 . 2 i 0.8 q u a n t u m yield ( w a t e r , 25 " C ) -20 "C and temperature independent at lower temperatures in the fluid DMF solutions. The apparent activation energies in the temperature-dependent region were 46.9 f 0.8 and 51.0 0.8 kJ mol-' respectively for Cr(er~),~+ and C r ( ~ e p ) ~These +. values are 2-7 kJ mol-I smaller than the estimated excited-state quartet-doublet energy gaps13J4(see Table I). The C r ( ~ e p ) complex ~+ is electronically very similar to its Cr(er~),~+ parent compound, very much as has been reported12for the cobalt analogues. The similarity between these chromium complexes extends to their emission spectra: both complexes show an intense 0-0 band at 1.50 pM-l and qualitatively similar low-intensity vibronic structure (Figure 1). There are some small differences in the vibronic structure of these compounds, consistent with small differences in metal-ligand vibrational frequencies and again similar to observations on the cobalt(111) ana10gues.l~ The correlation of (2E)Cr(III)lifetimes with the variations in electronic structures of a family of Cr(II1) complexes has been noted by Adamson,2band has been variously cited as evidence for the back-intersystem crossing mechanism (model B)5or for a deactivation mechanism which involves strong coupling of the doublet state to a ground-state reaction intermediate (model C).8bic Since the lifetime-energy gap correlation can be extended to trans-Cr( [14]aneN4)(CN)2+,11 for which E(4T20) - E('E0) 110 kJ M-l, which should make back-intersystem crossing very inefficient, model B cannot be the only mechanism for excited-state deactivation in these complexes. Furthermore, an assumption of model B is that there are no activation barriers to either of the surface crossings, 2Eo 4T2or 4T20 4A2;i.e., the only energy barrier to 2E deactivation is presumed to be the .W4TZ0) - E(2Eo)energy gap. Thus the 4T20 state must in model B be presumed to be an unbound state, either because of
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(12) (a) Creaser, I. I.; Harrowfield, J. MacB.; Herit, A. J.; Sargeson, A. M.; Springborg, J.; Geue, R. J.; Snow, M. R. J.Am. Chem. SOC.1977,99, 3181. (b) Creaser, I. I.; Geue, R. J.; Harrowfield, J. MacB.; Herit, A. J.; Sargeson, A. M.; Snow, M. R.; Springborg, J. Ibid. 1982, 104, 6016. (13) Based on the shifts of the 4Az 4T2band maxima and the difference in excited-state energies estimated for Cr(NHJG3+,14 a review of information about the excited states of this complex is contained in ref 8c. (14)Wilson, R. B.; Solomon, E. I. Inorg. Chem. 1978, 17,1729. (15) Endicott, J. F.; Brubaker, G. R.; Ramasami, T.; Kumar, K.; Dwarakanath, K.; Cassel, J.; Johnson, D. Inorg. Chem., in press.
-
2.
6
z I-
t
Flgure 1. Emission spectra of ('E)Cr(~ep)~+(a) and (2E)Cr(en)33+(b). Spectra obtained at -30 "C in DMF solutions. The relative signal amplifications are approximately 1: 12:35 for Cr(~ep)~+ and 1:2080 for ~r(en),~+.
barrierless ligand dissociation or due to very strong coupling with the ground state. The low-temperature, single-crystal spectroscopy gives no hint to the required strong coupling, but indicates that at 5 K both of the lowest-energy excited states are bound.14 The macrobicyclic ligand ensures that ligand dissociation will have a large activation barrier, even in a-metal-centered electronic excited state. Consequently, neither model A nor model B adequately accounts for the photophysical similarities of C r ( ~ e p ) ~ + , Cr(en)33+,and Cr(NH3)63+.A vibrationally promoted (2E) excited state to (4A2)ground state surface crossing model (such as model C) seems a realistic alternative. Indeed, only this alternative readily accounts for the increases of #E) with N-H deuteration (Table I), an effect which implies a nuclear tunneling contribution, and therefore an effect which requires a significant Franck-Condon barrier to electronic relaxation. Furthermore, model C is more consistent with contemporary ideas about the relaxation
J. Phys. Chem. 1983,87,5059-5061
of molecular excited states.16 Experimental Synthesis and Characterization of Cr(sep)2+. Anhydrous chromium sulfate was mixed with concentrated ethylenediamine and heated in a water bath for 4 h. A solution formed by mixing equal volumes of formalin and concentrated NH40H was added dropwise to the yellow brown residue. During these additions, which included occasional ethylenediamine and continued over a period of 3-4 h, the mixture was constantly heated on a steam bath, and eventually a solid residue was obtained. This residue was dissolved in a formalin solution and NH3 gas was passed through the solution (15-20 "C). The resulting mixture was filtered and the pH of the filtrate adjusted to -5 with HC1. Following rotary evaporation and removal of the hexamethylenetetraamine side product, a crude product was obtained. This material was redissolved in aqueous HC1 a t pH 4.5 and purified with a Sephadex LH-20 ion-exchange column (preswollen in aqueous methanol; 15:85 ratio). Column separation resulted in three bands; the middle band was the desired product. Rotary evaporation of the aqueous methanolic eluate gave a solid product which was recrystallized from aqueous NaC1O4. The yield was -10%. The purified Cr(sep)(ClO,), exhibited the lower energy N-H and C-H stretching vibrations and the -2880-cm-l capping methylene stretch which are characteristic of these compounds.15 Voltammetry. Differential pulsed voltammetry was performed with a PAR Model 174 polarographic analyzer. (16) (a) Engleman, R.; Jortner, J. Mol. Phys. 1970,18, 145. (b) Freed, K. F.; Jortner, J. J. Chem. Phys. 1976, 52,6272.
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We used a 10 mV s-l scan rate and a modulation amplitude of 25 mV (peak-to-peak). We were able to obtain good quality quasireversible cathodic and complementary anodic waves (Epa- E,, = 80 mV) in several media, for C r ( ~ e p ) ~in+ 0.1 ; M aqueous NaCF3S03,El - 1.09 f 0.03 V vs. NHE. Under similar conditions, we o6.7 Lamed a single, broad, cathodic voltammogram for Cr(e111,~' (Epc= -1.15 V vs. NHE); the anodic wave is absent for this complex owing to the very rapid equilibration of Cr(en)32+. Emission Spectra and Lifetimes. We employed a Molectron UV-1000 pumped Molectron DL-14 tuneable dye laser for excitation at 446 nm. Spectroscopic studies employed a Princeton Applied Research optical multichannel analyzer (OMA) with a silicon-intensified detector. Gating was accomplished by using an OMA signal, generated at the start of each spectral scan, as an external trigger for the laser. We accumulated 100-300 scans for each spectroscopic determination. The first- and second-order scatterings of the excitation light were used to calibrate the wavelength scale. For the lifetime studies we passed the sample emission through a Jobin-Yvon H-100 spectrometer to remove scattered laser frequencies. An RCA 7102 photomultiplier in a Products for Research housing was used for signal detection, and signals were stored in a Nicolet Explorer I11 digital oscilloscope. Samples were thermostated to f l "C in a PRA cell holder. Qualitatively similar observations were obtained in aqueous and DMF media, but the more limited aqueous temperature range precluded acquisition of good spectroscopic data.
Correlation between the Symmetry Factor of the Electrode Reaction and the Band Shape of the Photoelectron Spectrum for Alkylamine Masao Takahashi," Iwao Watanabe, and Shigero Ikeda Deparfment of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: August 9, 1983)
The width of the first band in the UV photoelectron spectrum of a free molecule, along with information on its reorganization in an oxidation reaction in vacuo, is compared with the symmetry factor, one of the parameters concerning the kinetics at an electrode in solution. There exists a correlation between them for a series of alkylamines. The correlation can be interpreted by means of potential energy diagrams for the amine and amine cation.
Introduction The purpose of this work is to examine the UV photoelectron spectrum (PES) of a free molecule for any information which can be connected with the kinetics of the electrochemical reaction in solution. A certain molecule deforms so significantly after ionization of the HOMO electron that the equilibrium or ground-state molecular geometries are quite different for the reactant and the product cation. For such a molecule we expect (1)the first band of the PES to have a large difference between the adiabatic and the vertical IPS,or no observable adiabatic IP and only a broad structureless band, and (2) the electrochemical oxidation tQ behave totally irreversibly because of the large activation energy needed to rearrange the molecular geometry for the oxidation to occur through a thermal electron transfer process. This implies that, if the 0022-3654/83/2087-5059$0 1.50/0
intramolecular reorganization energy contributes most of the total reorganization energy for the oxidation reaction, the band width in the PES should be correlated with the parameter for electron transfer kinetics for the irreversible electrode reaction. Alkylamines were selected for the present investigation. They undergo a large geometrical change from pyramidal to planar after ionization of the nitrogen lone-pair electron (HOMO),I and give a broad PES lone-pair band, isolated enough from the other bands to allow an exact measurement of the band width, and an irreversible oxidation wave on a cyclic v o l t a r n m ~ g r a m . ~ We ~ ~have found that there (1) (a) Aue, D. H.; Webb, H. M.; Bowers, M. T.J. Am. Chem. SOC. 1975,97,4136. (b) Ibid. 1976,98, 311. (c) Potts, A. W.; Price, W. C. Proc. R . SOC.London, Ser. A 1951, 326,181.
0 1983 American Chemical Society
