1758
J. Phys. Chem. 1982, 86, 1758-1760
Ligand Effects on the Dynamics of Ligand Field Excited States. Photosubstitutlon Reactions of the Rhodium( I I I ) Complexes CIS-and trans-Rh(NH,),Br,+ in 298 K Aqueous Solution D. A. Sexton," L. H. Sklbsted,lb D. Magde," and P. C. Ford*'' Depaflment of Chemistry, University of California, Santa Barbara, California 93 106; Department of Chemlstty, University of California, San D W 92093; and the Royal Veterfnery and Agricuttural University, Copenhagen V, Denmark (Received: September 14, 198 1; I n Final Form: December 3, 1981)
The emission lifetimes of cis- and trans-Rh(NHJ4Br2+in 298 K aqueous solution have been measured by pulse laser techniques to be 1.0 and 1.5118, respectively. From these values and the ligand photosubstitution quantum yields, reaction rate constants of the ligand field excited states were evaluated by assuming that = 1 and that all deactivation processes occur from the same triplet excited state. These indicate a much faster rate for Br- aquation from the excited states of both the cis- and trans-dibromo complexes and for NH3 photolabilization from the cis complex relative to the analogous excited-state reactions of Rh(NH3)6Br2+.Given that ~ j ~ ~ , ( R h ( b ! H ~ )> ~ BCr~~N+~) ~ ( C ~ S - R ~ ( these N H ~ experiments ) ~ B ~ ~ + )illustrate , the marked advantage of using dynamics techniques for evaluating excited-state reactivities.
Introduction The ligand field (LF) excitation of hexacoordinate d6 metal ion complexes generally leads to labilization of coordinated ligands (e.g., eq 1) with quantum yields deRbINH3'5 h2a3'
Rh(Nh3 5 X 2 -
*
h2G
5
= (ZkJ-' (3) $i under analogous conditions allows evaluation of each individual rate constant, ki. For the systematic examination of a series of complexes (or conditions) the k;s are a better measure of reactivity differences than are the 4;s given that the latter values reflect simultaneous perturbations of more than one parameter. For the rhodium(II1) ammine complexes, we have previously examined the effects on such kinetics parameters of ammine perdeuteration3 and ~olvents.~ Here we report differences resulting from minor changes in the coordination sphere which dramatically emphasize the advantages of comparing k i rather than r j values ~ ~ for the respective systems. T
t X-
Rh(NH3i4(Hz0)X2'
excited-state rate constants for aquation of NH3, aquation of X-, radiative deactivation, and nonradiative deactivation, respectively. Thus,given the definition of the lifetime T (eq 3), measurement of T and of all the quantum yields
(1) t Nh3
pendent on the nature of the central metal, of the ligand labilized, of the balance of the first coordination sphere, and of the reaction medium.2 As a model for such photoreactions, we have been investigating the ligand field photochemistry of rhodium(II1) ammine complexes with a focus not simply on measuring quantum yields (I$J but more specifically upon using combined photophysical/ photochemical techniques to evaluate the rate constants for reactions from the LF excited state^.^^^ Previous studies utilizing wavelength dependence and sensitization techniquess concluded for the halopentaExperimental Section a"inerhodium(II1) ions Rh(NH3)6X2+that initial LF The room-temperature luminescence lifetimes of the excitation is followed by efficient internal conversion/inions Rh(NH3)J39+(Aar), cis-Rh(NH,),Br,+ (cis-Br,), and tersystem crossing (Cjisc = 1) to a common set of states t r ~ n s - R h ( N H ~ ) ~ B(trans-Br2) r,+ (bromide salts in each responsible for the bulk of the photoreaction.6 These case) were measured in dilute M) aqueous solutions states were concluded to be the lowest energy LF triplet (pH 1, HC104) by exciting the complexes with 458-nm line excited state and those in thermal equilibrium with the of a Spectra Physics Model 171 mode-locked argon ion lowest energy excited ~ t a t e Similar . ~ ~ ~ conclusions have laser. been made for analogous Ir(II1) amine complexes6and may also apply to the photochemical properties of C O ( C N ) ~ ~ . ' ~ This laser was operated at approximately 82 MHz giving mode-locked pulses each of about 200-ps duration. The With this model, the quantum yield for an excited state resulting luminescence was detected near the emission process (e.g., eq 1)can be described in terms of a ratio of maxima of the complexes (see Table I) with an ISA H20 rate constants (eq 21, where kNHs,kx-, k,, and k , are the monochromator for wavelength selection. Decay curves 4: = ki/(kNHB+ kx- + kr + k,) (2) were determined by using single-photon correlation with computer deconvolution. Key components included an RCA 31034 photomultiplier, an EGG discriminator, Can(1) (a) University of California, Santa Barbara. (b) Royal Veterinary and Agricultural University, Copenhagen. (c) University of California, berra time-to-amplitude converter, and a Norland 5300 San Diego. multichannel analyzer. The "start" pulse was generated (2) Ford, P. C.; Petersen, J. D.; Hintze, R. E. Coord. Chem. Reo. 1974, with a fast pin photodiode monitoring the laser beam. The 14, 67. (3) Bergkamp, M. A.; Brannon, J.; Magde, D.; Watts, R. J.; Ford, P. laser beam was attenuated by use of neutral density filters C. J. Am. Chem. SOC.1979,101,4549. such that a sampling time of 200 s could be used without (4) Bergkamp, M. A.; Watts, R. J.; Ford, P. C. J.Am. Chem. SOC.1980, excessive photolysis. The resulting intensity vs. time data 102,2627. (5) Kelly, T. L.; Endicott, J. F. J. Phys. Chem. 1972, 76, 1937. were transfered to a computer for deconvolution via a (6) Talebinasab-Savari, M.; Zanella, A. W.; Ford, P. C. Inorg. Chem. routine based on the Marquardt algorithm. The instru1980,19, 1835. mental response function was obtained by measuring the (7) Nishizawa, M.; Ford, P. C. Inorg. Chen. 1981, 20, 294. (8)Scandola, M. A.; Scandola, F. J. Am. Chem. SOC.1972, 94, 1805. Raman scattered signal from pure water. For such weak 0022-3654/82/2086-1758$01.25/0
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 10, 1982 1759
Dynamics of Ligand Field Excited States
TABLE I: Photochemical and Photophysical Data for Rhodium( 111) Bromoammine Complexes complex
ns
T,O
Rh(NH,),Br'' cis-Rh(NH,),Br,+ trans-Rh(NH, ),Br,+
12.4
f
1.2e
1.0 f 0 . 1 1.5 f 0.2
1 0 3 , b cm-'
1 0 - 3 ~ ~ ,cm-' ~,b
~NH,'
@JBr-d
14.8Yf 12.95 * 0.05 13.39 f 0.08
2.745 2.14 f 0.04 2.48 t 0.01
0.18 f O . O l e
90.02e 0.24 f 0.01
0.064 GO.002
f
0.005
0.10
i
0.01
Determined in 4 : l MeOH/H,O glasses a t 77 K (D. a Measured in dilute aqueous solution a t room temperature, -298 K. Sexton, unpublished observations). Quantum yield for NH, photosubstitution in moles/einstein. Reference 9. Quant u m yield for Br' photosubstitution in moles/einstein. Reference 9. e Reference 3. Reference 16. TABLE 11: Calculated Rate Constants for Br- and NH, Labilization and Nonradiative Deactivation (in s-') from t h e Lowest E n e r m Ligand Field Excited States complex
kBr-O
~ N H a,
kn
Rh(NH,),BrZ+ cis-Rh(NH,),Br, trans-Rh(NH,),Br,+
9 1 . 6 X lo6 (2.4 * 0.3) x lo8 (6.7 t 1 . 2 ) x 107
(1.5 t 0.1) x 10' ( 6 t 1)x l o 7 91.3 X lo6
(6.5 f 0 . 2 ) x 107 (7 * 1)x l o 8 (6.0 r 0 . 6 ) x 10'
Calculated according t o k i = @i/r; limits estimated according to method of propagation of errors. cussed in ref 11; error limits were calculated in the same manner.
Calculated as dis-
indicate that the introduction of an additional Br- into the coordination sphere markedly accelerates Br- labilization from the excited state regardless of the stereochemical position relative to the leaving Br-. Similar rate accelerations are seen for the ground-state substitution reactions of Co(II1) and Rh(II1) amine complexes with the parallel observation that the effect is much larger when the second Results and Discussion halide is in the cis rather than the trans site.12 Notably, Emission lifetimes 7 and quantum yields 4 y 8and 4 ~ ~ -the magnitude of the kBr-increases is closer to the types of effects seen for Co(II1) complexes (concluded to undergo for NH, and Br- photoaquati~n~ are recorded in Table I interchange dissociative thermal aquations) and much for the complex ions A& cis-Br2,and trans-Br2. Notably larger than those seen with similar thermal reactions of the lifetime for each of the dibromo complexes is markedly Rh(II1) complexes (concluded to react via interchange shorter than that for the pentaammine complex. Prelimassociative mechanisms)13despite the fact that the excitinary studies in these laboratories1° indicate similar short ed-state reactions are many orders of magnitude faster lifetimes for other dihalo- and haloaquotetraamminethan the analogous ground-state reactions. Another difrhodium(II1) ions in ambient temperature solutions. The ference between ground-state and excited-state substitucalculated rate constants kNH3, kBr-, and k, for the excittion reactions for Rh(II1) ammines is that the former are ed-state reactions and nonradiative deactivation processes stereoretentive while the latter are stere~mobile.~ The are summarized in Table II.ll stereochemical differences have been rationalized on the The values listed in Table I1 indicate that the rate basis of the excited-state reactions proceeding via a limiting constants for Br- labilization from the respective LF exdissociative me~hanism;~ however, there have been no cited state follow the order cis-Br2> trans-Br2 >> A5Br direct observations of pentacoordinate intermediates in the while the kNH3 values follow the order cis-Br2 > A5Br > photosubstitution reactions of d6 Werner-type complexes. trans-Br2. Interpretation of these rate constants suffers In contrast to the analogous parameters for Br- photofrom the complications of changes in the nature and extent aquation, kNH and &H do not even qualitatively parallel of excited-state distortions as a function of the different each other. $he 4 N H 3 !or cis-Br2is smaller than that for symmetries of the complexes. Nonetheless, the kBr-values A5Br but kNHg for the former is a factor of four larger, nicely illustratmg the roles of all the deactivation pathways (9) Skibsted, L. H.; Strauss, D.; Ford, P. C. Znorg. Chem. 1979, 18, in determining $NH3. The lower $NH3 for cis-Br, is the 3171. consequence of the much faster deactivation of the excited (10) Sexton, D.A., work in progress. (11) (a) As noted in the Introduction,these calculations first require state via the k, and kBr- pathways which decreases the the assumption that & = 1for all cases. Given that limiting quantum relative importance of NH, aquation as an excited-state yields for NHs and Br- labilization from Rh(NH3)@+ utilizing biacetyl deactivation pathway (see eq 2). The fact that kNH3 is triplet sensitization are equivalent to thoee found for direct excitation of singlet absorption bands (ref 5) and supportive observations in the pholarger for the cis-Br2 than for A5Br parallels the kBr-intoreaction and photoluminescenceproperties of other Rh(II1)and Ir(II1) creases for the same complexes and again can be attributed complexes at ambient temperatures,this assumption seems quite valid to greater stabilization of the ligand labilization pathway (P. C. Ford, Coord. Chem. Reo., in press). However, it is also likely that transition state by Br- in the coordination sphere relative a amall fraction of the photochemistry occurs from higher excited states competitive with internal conversion/intersystem crossing to the lowest to NH, in the same site. We consider this result to be more ligand field excited state. This may be evidenced by the report that a consistent with an excited-state reaction of largely dissofraction of the photochemical reactions from halopentanmminerhodiumciative character than with the alternative associative (111) complexes is not quenched when triplet emission is quenched by hydroxide in aqueous solution (Adamaon,A. W.; Fukuda, R.; Larson, M.; mechanism. Mlcke, H.; Piuaux, J. P. Znorg. Chim. Acto 1980, 44, L13). Thus the Predictions can be made regarding the identity of the calculated rate constants in Table I1 represents upper limits for reaction ligand(s) labilized from each of these complexes by using rates from the lowest energy ligand field excited state itself, the actual value being somewhat smaller if a fraction of the photoreaction occurs the excited-state bond index model of Vanquickenborne from higher energy states. (b) The k,, value is calculated from the and Ceulemans14which implictly assumes a dissociative equation emitters as these complexes, a small blank correction is required. This was accomplished by subtracting counts in the multichannel analyzer for a sample of dilute perchloric acid run under identical conditions (including the appropriate scaling for any inner-filter effect on the excitation beam).
k,
9
k, + k, = (1- Qsr- 6mJ7-l
given the very weak emissions, thus the valid assumption that k,
