J. Phys. Chem. 1903, 87,1733-1737
1733
Doublet Excited States in Chromium(I 11)-Amine Complexes: Does Back Intersystem Crossing Determine Their Lifetimes? Nancy Jo Llnck,' Sylvia J. Berms,' Douglas Magde,'? and R. G. Linck" Department of Chemistiy, Universiv of California, San Diego, La Jolla, California 92093, and Deparfment of Chemistiy, Smith College, Northampton, Massachusetts 01063 (Received: May 21, 1982; In Final Form: November 8, 1982)
The lifetimes for emission from a number of chromium(II1)complexes, most of which contain four amine ligands in the coordination shell, have been measured at room temperature in aqueous solution by time-correlated single-photon detection. The lifetimes varied from 4.4 ps for trans-Cr(en)z(NCS)z+to less than 2 ns for trans-Cr(en)zNCS(Br)+, cis-Cr(tn)zC1(Hz0)2+, Cr(Hz0)63+, and C r ( ~ r e a ) ~These ~ + . data are interpreted with a model that estimates the relative back-intersystem-crossing rates and assumes this process is the dominant mode of decay of the excited state of Cr"'-amine complexes.
Introduction Despite substantial study, the primary photophysical and photochemical processes that take place within complexes of Cr"' remain p ~ z z l i n g . ~ -There ~ are two spin manifolds involved, but the degree of mixing has not been e ~ t a b l i s h e d . ~The ~ ~ relative importance of the various states during and after vibrational relaxation to configurations distorted relative to the ground state is unknown. It will not be surprising if features of the photophysics and the photochemistry are determined by different processes under different conditions even in a given species, and differ substantially for different species. Nevertheless, some regularities do exist in the photochemist#$ and can be expected in the photophysics. In this report, we shall demonstrate that a pattern does appear in the photophysics of Cr"'-amine complexes, and shall advance a model that explains the lifetime controlling process for the decay of phosphorescence from the lowest-lying doublet state as back intersystem crossing. We will admit that some exceptions to the fit of our model exist, but we believe the general concordance between the data and the predictions of our model is satisfactory. In order to study a lifetime-controlling process, one must monitor lifetimes directly. Inferences from indirect measurements of quantum yield of emission or of chemical reaction are liable to fail without warning. In order to identify a lifetime controlling process one must characterize the relevant energy barrier. Two strategies are available for investigation: one may attempt to hold the barrier constant while varying the energy available to the molecule by, for example, changing the temperature, or one may try to keep the available energy constant while adjusting the barrier by, for example, making small controlled changes in the molecular structure that one claims one can understand. Neither procedure is entirely free of problems. In condensed phases, at least, changing temperature affects the solvent environment surrounding the molecule, and (1) Smith College. (2)University of California, San Diego. (3) Porter, G. B. In "Concepts of Inorganic Photochemistry"; Adamson, A. W., Fleischauer, P. D., Ed., Wiley: New York, 1975; p 37. (4)Zinato, E.In "Concepts of Inorganic Photochemistry"; Adamson, A. W., Fleischauer, P. D., Ed., Wiley: New York, 1975; p 143. (5)Kirk, A. D.; Porter, G. B. J. Phys. Chem. 1980,84,887. (6)Kane-Maquire, N.A. P.; Phifer, J. G.; Toney, C. J. Inorg. Chem. 1976.5-.,593. (7)Sandrini, D.; Gandolfi, M. T.; Moggi, L.; Balzani, V. J.Am. Chem. SOC.1978,100, 1463. ( 8 ) Kirk, A. D. Coord. Chem. Rev. 1981,39, 225. ~~
- v
~~~
this influences barriers to an extent which may range from undetectable to p r o f ~ u n d . On ~ the other hand, changing molecular structure as a means of varying barriers is suitable only to the extent that one can calculate or measure the change in the barrier and argue that factors not explicitly considered do in fact remain constant throughout the series of perturbations imposed. It is important to test the correlation between barrier and lifetime over as wide a range as possible. This increases the problem of undesired side effects, effects not easily recognized, but for just this reason the correlation that survives over a large range of perturbations becomes all the more convincing. In this study, we have chosen to keep the experimental conditions (temperature, solvent) reasonably constant while changing the energy barrier to back intersystem crossing in a controlled fashion. We were able to vary the phosphorescent lifetimes over more than three orders of magnitude by making changes in the structure that we believe can be modeled reliably. We use these results to test a model that explains the lifetimes in terms of a correlation with "calculated" barriers to back intersystem crossing.
Experimental Section Materials. The following compounds were prepared as indicated in the literature references: Cr(urea),3+,1° trans-Cr(en)z(NCS)2+,11,12 t ~ a n s - C r ( e n ) ~ F and ~ + , 'trans~ Cr(en)2F(H20)2+.14 The preparation of trans-Cr(en)zNCS(F)+ and trans-Cr (en)zNCS(Br)+ was by the procedure of Wirth et aLl5 from the corresponding fluoroaquo'* and bromoaquo complexes.16 The solid complexes containing 1,3-propanediamine, trans-Cr(tn)2(NCS)2+and cis-Cr(tn)z(NCS)z+,were prepared by procedures (to be described in more detail17)that are essentially minor modifications of the preparations of the cor(9)Caatelli, F.; Forater, L. S. J. Am. Chem. SOC.1973,95, 7223. (10) Hein, F.;Herzog, S. In "Handbook of Preparative Inorganic Chemistry"; Brauer, G., Ed.; Academic Press: New York, 1965;Vol. 2, p 1359. (11)Bifano, C.;Linck, R. G. Inorg. Chem. 1974,13, 609. (12)House, D. A. J. Inorg. Nuclear Chem. 1973,35, 3103. (13)Pyke, S. C.; Linck, R. G. Inorg. Chem. 1971,10, 2445. (14)Vaughan, J. W.; Stvan, 0. V.; Magnuson, V. E. Inorg. Chem. 1968, 7, 736. (15)Wirth, G.;Bifano, C.; Walters, R. T.; Linck, R. G. Inorg. Chem. 1973,12, 1955. (16)Fee, W. W.; MacHarrowfield, J. N.; Jackson, W. G. J. Chem. SOC. A 1970,2612. (17)Linck, N. J.; Linck, R. G. To be submitted for publication.
0022-365418312087-1733$0 1.5010 0 1983 American Chemical Society
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responding ethylenediamine complexes."J2 The purity of all solid complexes was established by chemical analysis and spectrophotometric study. Solutions of trans-Cr(en)2NCS(H20)2+, cis-Cr(en)2NCS(H20)2+, ~is-Cr(en)~(H20)23+,trans-Cr(tn)2NCS(HzO)2+,cis-Cr(tn)2NCS(H20)2+,t r ~ n s - C r ( t n ) ~ ( H ~ O and ) ~~is-Cr(tn),(H,O),~+ ~+, were prepared by ion-exchange chromatographic separation of the mixture of products generated by mercuric ion induced aquation of the corresponding diisothiocyanoto complexes. A solution of cis-Cr(tn)2C1(H20)2+ was obtained by ion-exchange separation from other components of a mixture obtained by room temperature aquation of cis-Cr(tn)2C12+ prepared by the literature procedure.18 All complexes isolated only in solution were established as pure by comparison of the spectrophotometric parameters with values in the literature. The Cr(H20),3+solution was made by dissolving the corresponding perchlorate salt which in turn was prepared by formic acid reduction of Cr03 followed by recrystallization of the salt from perchloric acid. Techniques. All lifetime data were collected a t 25 f 2 " C in a solution M in perchloric acid. Those complexes isolated from ion-exchange columns were in solutions containing sodium ion as well. Measurements were performed in 1-cm2 fused silica fluorometer cells. In general, solutions were not degassed, although two complexes were measured both in aerated solution and after degassing by multiple freezethaw cycles. For those complexes which were thermally or photochemically unstable, appropriate special attention was given to verify that there was no significant effect of light or heat; measurements of the intensity of phosphorescence were substantially unchanged between initiation and complettion of each experimental run. The photochemically sensitive materials were stirred manually at frequent intervals during measurement. The apparatus for the lifetime measurements is based on time-correlated single-photon detection. A very weak pulse excites the sample and also starts a clock. The clock is stopped when a photon of luminescence is detected. As long as the chance of detecting even one photon is very low, less than 1% , a histogram of photon counts plotted against clock interval is a reconstruction of the luminescence decay. This procedure, which is attractive even with conventional excitation sources, becomes much more powerful when mode-locked lasers are incorporated. Several laboratories have developed such instruments; our own version evolved following discussions with Spears.lg Outstanding attributes of this approach include extreme sensitivity for weak emission; amplitude and time linearity over several orders of magnitude, so that multiple decays may be recognized and distinguished; and time resolution below 100 ps. For the experiments reported here, characteristics of the apparatus were as follows: The excitation source was a mode-locked argon ion laser operating at 514.5 nm to generate a train of picosecond pulses of 150-ps duration at a repetition rate of 82 MHz. A PIN photodiode monitored the laser. Clock triggers a t such a high rate paralyze part of the electronics, so a divide-by-four circuit was built by using emitter-coupled logic. Luminescence was observed by an RCA C31034 photomultiplier. A solution of dichromate ion acted as a filter in front of an ISA H-20 monochromator; the latter determined the detection wavelength within a bandpass of 10 nm. The same instrument operates very well in a scanning mode, so we can (18) Pedersen, E. Acta Chem. Scand. 1970, 24, 3362. (19) Spears, K. G.; Cramer, L. E.; Hoffland, L. D. Reo. Sci. Instrum. 1978, 49, 255.
Linck et
al.
identify the relatively sharp doublet phosphorescence emission and be confident that our lifetime measurements referred specifically, in all but the cases so noted, to that feature. Signals from the laser monitor and the photomultiplier were fed to a constant-fraction discriminator (EGG-Ortec 934) and thence to a time-to-amplitude converter (Canberra 2044). Output from the latter was accumulated as a histogram on a pulse height analyzer (Norland 5300). The time decays were analyzed as the best fit to a sum of exponentials. For times greater than 300 ps all data could be fit by a single exponential function, using as a criterion for the fit both the numerical value of the reduced x 2 statistic and a visual examination of the residuals themselves. (Luminescence behavior of Cr"' complexes at times shorter than 300 ps will constitute the subject of a forthcoming paper.) The effect of repetitive excitation was included by calculating the convolution of the trial decay function with an infinitely repeated repetition of the measured 12.2-11s instrument response. For the faster decays deconvolution from the instrument response was essential; the deconvolution algorithm was, in fact, used in all cases. Deconvolution was accomplished iteratively by varying the parameters in a forward convolution, using our own version of the nonlinear least-squares algorithm of Marquardt.20 The instrument response needed for deconvolution was obtained by measuring the apparent time course of the Raman signal from the hydrogen stretch of water at 622 nm. For materials with decay times of longer than 50 ns we changed the excitation source from the mode-locked laser to a home-built spark relaxation oscillator filtered by a Corning 5-61 glass filter. Data aquisition times then increased from a few minutes to a few hours, despite the fact that the slowly decaying samples have a much higher quantum yield for phosphorescence.
Results and Discussion The observed data are collected in Table I for all the compounds that we have studied. In that table are listed the observed phosphorescence peak position, when such a peak was observed in the scanning mode, and the observed lifetime with errors defined by the least-squares fit. In the case of Cr(H20),3+we could not observe a peak corresponding to phosphorescence and assume in this case that the observed emission is delayed fluorescence. This was supported by a failure to see a wavelength dependence of the lifetime of this peak over a range from 650 to 750 nm. For trans-Cr(en)zNCS(Br)+repeated attempts to isolate by recrystallization or ion-exchange chromatography a material completely free from contamination with an impurity failed. We believe that trans-Cr(en),(NCS),+ is the impurity in the solutions of trans-Cr(en),NCS(Br)+ because of the position and the lifetime of the observed phosphorescence peak. (See ref 15 for a discussion of why trans-Cr(en)z(NCS)z+is a likely impurity in the synthesis of trans-Cr(en),NCS(Br)+.) However, when a carefully recrystallized sample of trans-Cr(en)zNCS(Br)+is used, in addition to a rather long-lived component, which can be assigned to tran~-Cr(en),(NCS)~+, there is also a short-lived component that is not present in a sample of pure trans-Cr(en),(NCS),+. It is the lifetime of this short-lived component that we assign to the doublet of trans-Cr(en),NCS(Br)+. For Cr(urea)63+,we could observe no emission nor measure any lifetime distinguishable from the instrument response; weak fluorescence is known to occur over a wide range of wavelengths a t lower temper(20)Marquardt, D. J . SOC.Ind. Appl. Math. 1963, 11, 431
Doublet Excited States in Cr"'-Amine Complexes
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
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TABLE I : Doublet State Lifetimes in Cr" Complexes compd
1 2 3
4 5 6 7 8
9 10 11 12 13 14 15 16
trans-Cr(en ) ,(NCS ) ,+ trans-Cr( tn),(NCS),+ trans-Cr(en),F ,+ trans-Cr(en),NCS( F)' cis-Cr(tn),NCS(H,0)2+ trans-Cr(en),F( H,O)Z+ cis-Cr(en),NCS(H , 0 ) 2 + trans-Cr(en),NCS( H 2 0 ) 2 + trans-Cr(tn),NCS( H 2 0 ) 2 + cis-Cr( tn),(H,0),3+ trans -Cr(t n ),(H 0 )*3+ trans-Cr( NH,),NCS(H,0)2+ ~is-Cr(en),(H,O),~+ W H , O )6 3+ cis-Cr ( tn ) C1(H ,0),+ trans-Cr(en),NCS(Br)r
,
phosphorescence peak, nm 730 725 b 723 696 770 703 705 699 675 672 694 680 688
lifetime, ns 4400 * 1500k 1280* 325* 30 * 25* 20 * 12*
300 100 100 30 6 6 4 2 lo* 2 3.5 * 0.5 3.0 f 0.3 2.5 f 0.5 2.4 i 0.4 1.8 f. 0.3 1.3 * 0.2 1.9 f 0.3
10- 3~(4x),a cm-' 19.78 19.63 19.03 19.44 19.24 18.98 19.40 19.30 19.15 18.80 18.67 18.98 18.96 15.83 18.10 18.20
This is the difference between the ground state and the calculated energy of the lowest-lying quartet at ground-state internuclear distances. Broad-band emission.
atures.2l That emission at ambient temperature is so weak demands either that the lifetime is very short, less than 200 ps, or that the compound has an anomalously low oscillator strength compared to, for instance, Cr(H20),3+. Both trans-Cr(en)2( NCS)2+and trans-Cr(~ I I ) ~ N C S were ~+ measured under deaerated as well as aerated conditions. In the absence of oxygen the values of the lifetime were 5.3 f 0.3 and 1.6 f 0.1 ,us, respectively. From a comparison of these values with those in Table I we conclude that the effect of oxygen-induced relaxation is insignificant in most cases and not rate limiting in the compounds of interest here, but could become important in longer-lived complexes. Our results are in reasonable agreement with those in the literature, where available, considering differences in temperature and m e d i ~ m . ~ ~ ~ ~ ~ ~ Some general comments can be made about these data. In comparing a change in the amine ligands in the trans-diisothiocyanoto complexes, the 1,3-propanediamine (tn) complex has a slightly smaller lifetime than the en complex, but in both the cis- and trans-thiocyanotoaquo complexes, there is little difference in lifetimes. In the sole case of an ammine complex studied, there is a slight decrease in lifetime with the replacement of the chelating ligands by ammonia. There is, however, a rather consistent pattern of trans complexes having shorter lifetimes than the corresponding cis complexes. As we shall discuss below, this last result, and the slight trend of increasing rate of decomposition of the complexes as the amines are changed from ethylenediamine to l,&propanediamine to ammonia, is consistent with the model we use to explain the general trend of the variations observed. We have not made quantitative measurements of the intensity of phosphorescence as the compounds are changed, but qualitative observations clearly indicate that the rough trend is that longer-lived complexes have more intense emission. This result is consistent with the radiative rate constant being more or less constant as the complex is varied, but with some other process, the major pathway for decay, increasing in rate as one moves down the column of complexes in Table I. A similar conclusion has been reached .5 In agreement with others* we rule out direct nonradiative decay of the doublet to the ground (21) Klassen, D. M.; Schlafer, H. L. Eer. Bumenges. Phys. Chem. 1968, 72, 663. (22) Walters, R. T.; Adamson, A. D. Acta Chem. Scand., Sect. A 1979, 33, 53. (23) Shipley, N. J.; Linck, R. G. J. Phys. Chem. 1980, 84, 2490.
Flgure 1. Schematic representation of the model used to estimate the energy of back intersystem crossing. I t is assumed that changes in E,, are reflected by changes in E . ' G is the symbol for the ground state.
state as the path responsible for the lifetime of Cr"' complexes at room temperature: There is ample evidence that a process with a low activation energy occurs at low temp e r a t u r e ~ this ; ~ ~pathway, ~~~ surely nonradiative decay, becomes inefficient at room temperature because some new path with a larger activation energy appears and overwhelms the contribution from nonradiative decay. (See also ref 26.) The question crucial to an understanding of the photophysics and photochemistry of Cr"' complexes at room temperature thus reduces to a choice of two options: Is it chemical r e a c t i ~ n ~or~back s ~ ~ intersystem ~ r o s s i n that g ~ dictates ~ ~ ~ ~the ~ ~lifetime ~ of the doublet in Cr"' complexes? Figure 1 illustrates in a schematic way how we plan to estimate the energy cost for the back-intersystem-crossing process in an attempt to test which of the two options dictates the lifetimes of Crm complexes. The fundamental (24) Gutierrez, A. R.; Adamson, A. W. J. Phys. Chem. 1978,82, 902. (25) Kang,Y. S.; Caatelli, F.; Forster, L. S. J. Phys. Chem. 1979, 83, 2368. (26) Allsopp, S. R.; Cox, A.; Kemp, T. J.; Reed, W. J.; Sostero, S.; Traverso, 0. J. Chem. SOC.,Faraday Trans. 2 1980, 76, 162. (27) Cimolino, M.; Linck, R. G. Inorg. Chem. 1981,20, 3499. (28) Kirk, A. D. J. Phys. Chem. 1981,85, 3205.
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postulate is that the back-intersystem-crossing rate should be inversely proportional to the energy difference between the doublet state and the crossing point of the potential surface of the doublet and that of the lowest-lying quartet state. In our admittedly simplified model, this difference is related to the energy difference between the doublet and the lowest-lying excited quarted u t the position in coordinate space defined by the minimum in the doublet surface. We are able to obtain an estimate of this Franck-Condon energy difference between the doublet and the lowest-lying excited quartet from spectroscopic data. To do this we have solved the matriciesZ9for the energy levels with configuration interaction for the various complexes studied. For the trans-disubstituted and the trans-XY complexes this has been done by using tetragonal or by assuming pseudo-tetragonal symmetry; that is, the trans-Cr(en)zXYn+complexes have been approximated as trans-Cr(en),ZZn+where Z is a ligand with average parameters of X and Y.30 The ~ i s - c r ( N ) ~ Xcomplexes ,~+ were assumed to have pseudo-D,,, symmetry of he type t r ~ n s - C r ( N ) ~ Zwhere ~ ~ + Z, is a ligand of field intermediate between that of an amine nitrogen and X. The cis-Cr(N)4XYn+complexes were treated as if they possessed D% symmetry'?' and the matricies with electrostatic repulsion were set up for this symmetry32and solved. All calculations were done with the orbital overlap parameters for the non-amine ligands given by Vanquickenbourne and C e ~ l e m a n sand , ~ ~a B value of 0.70 X lo3 cm-l was estimated. The values of the orbital overlap parameter of the amines was estimated from data on the 4B1g 4B2gtranand from the first sition in tetragonal quartet transition in hexamine complexes.18 In all comparisons between complexes with identical non-amine ligands the ethylenediamine species absorb at greater energy than do the 1,3-propanediamine complexes, which in turn absorb at greater energy than do the ammine complexes. We estimate the value of u for these amines as 7.33 x lo3, 7.22 X lo3, and 7.09 X lo3 cm-l, respectively. In all cases the lowest level quartet was selected. (This was the 4Egstate in tetragonal or pseudo-tetragonal symmetry of the trans complexes, 4B in the pseudo-tetragonal symmetry of the cis-CrN4Xli+ complexes, and the 4B1gstate for the cis-CrN,XY"+ complexes.) Our model assumes that changes in the energy difference between the vibrationally relaxed doublet and the intersection of the potential energy surfaces of doublet and lowest-excited energy quartet state are mimicked by changes in the vertical energy difference between the ground state and this lowest-lying excited quartet energy state. This relationship makes the vertical energy difference the assessable parameter of interest. It is shown plotted against the observed lifetimes in Figure 2, where data points from the literaturez2are included. Within the energy region from about 18.8 X lo3 to 19.8 X IO3 cm-' there is an evident correlation between the observed lifetime and the energy difference; larger values of the latter lead to smaller rate constants for relaxation of the excited doublet. This model nicely explains why trans complexes generally have shorter lifetimes than cis ones do: The effect of both of the weaker field ligands is concentrated along one axis in the former, which generates -+
(29) Perumareddi, J. R. Coord. Chem. Rev. 1969, 4 , 73. (30) Vanquickenborne, Id.G.; Ceulemans, A. J. Am. Chem. SOC.1977, 99, 2408. (31) Vanquirkenborne, L. G.; Ceulemans, A. Inorg. Chem. 1981, 20, 110, (32) Griffith, J. S. "The Theory of Transition Metal Ions"; Cambridge
University Press: Cambridge, 1961. (33) Couldwell, M. C.; House, D. A,; Powell, H. K. J. Inor
