4211
J. Phys. Chem. 1993,97, 42774283
Excited-State Decay Processes of Binuclear Rhodium(I) Isocyanide Complexes+ Vincent M. Miskowski,' Steven F. Rice, and Harry B. Gray' Arthur Amos Noyes Laboratory, California Institute of Technology, Pasadena, California 91 I25
Steven J. Milder*9* Department of Chemistry, Brookhaven National Laboratory, Upton, New York 1 1 973 Received: November 18, 1992; In Final Form: January 22, I993
Emission lifetimes, quantum yields, and polarized excitation spectra of Rh2bd2+and R ~ z ( T M B ) ~(b~=+ 1,3diisocyanopropane; T M B = 2,5-diisocyano-2,5-dimethylhexane) have been determined. The singlet and triplet da* - p ~ ( ~ , ~ A 2 excited ,) states are luminescent with radiative rates of ca. lo8and lo4S-', respectively, consistent with values obtained from Strickler-Berg calculations based on the corresponding absorption bands. Both singlet and triplet upper excited states ( d a p a and metal-to-metal charge transfer, E, symmetry) undergo nonradiative decay primarily to 3A2u,bypassing IAz,, and the branching ratios for decay through several intermediate states have been estimated. The temperature dependences of the lifetimes of the lA2, and 3A2u states of Rh2(TMB)d2+ are interpreted in terms of a model in which potential-surface crossings with the 3B2u (da* dX2-9) state facilitate nonradiative decay.
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The complexes Rh2b42+and Rh2(TMB)2+ (b = 1,3-diisocyanopropane; TMB = 2,5-diisocyano-2,5-dimethylhexane)l~Zpossess highly emissive singlet and triplet Az, (da* pa) excited state3that are derived from monomer dZ2 pz excitations. Both the fluorescent ('Az,) and phosphorescent ()A2,) states are strongly stabilized (relative to monomer states) in the d 8 4 8 systems, whereas other excited states, notably a Ip3E,, pair corresponding to monomer dxzyz pz excitation, are much less perturbed.' The energetic effects have been interpreted in terms of a valence-bond model:4 half of the d848 states correlate to metal-to-metal charge transfer, and hence are at considerably higher energy; the remainder correlate to monomer states, but most of them are very weakly metal-metal bound, like the ground state. The "excimer-like" Ig3Azustates are exceptions (to date, the only well-characterized ones) in that they are much more strongly metal-metal bound than the ground state, and accordingly are strongly stabilized relative to the monomer dz2 pz excited statesa5 The Iv3AZustates of these complexes are separated by about 3500 cm-' . 4 5 Since there is no direct spin-orbit coupling between them (and because they are well separated from other states), nonradiative lAzuto 3AZudecay is relatively slow. Importantly, the differences in emission lifetimes and wavelengths of lAZuand 3A2uhave made it possible to investigate the photophysics of both states; in particular, these differences have allowed us to study the selective population of lAzuand 3A2ufollowing excitation into higher-lying excited states.
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Experimental Section The compounds [Rh2b4](03SCF3)2,[Rh2(TMB)4](03SCF&, and [Rh2(TMB)4](PF6)2were prepared by literature methods.' The equipment and procedures employed for measurements of absorption and emission spectra,2aand nanosecond (and longer) emission lifetimes: have been described. Subnanosecondemission lifetimes were measured employing equipment at Brookhaven National Laboratory.8 Corrected emission excitation spectra were determined on a Perkin-Elmer MPF-66 spectrofluorimeter. Polarized spectra at 17 K employed samples held in quartz electron paramagnetic resonance (EPR) tubes and immersed in an N2-liquid-filledfinger
' Contribution No. 8759 from the Arthur Amos Noyes Laboratory. 1 Present
address: The Evergreen State College, Olympia, WA 98505.
dewar. A matched pair of double Glan-Taylor air-spaced calcite polarizers in rotatable mounts was placed in front of the emission and excitation ports of the spectrometer. Polarization data were corrected for instrumental polarization bias in the usual way699 by measuring emission for all of the four possible combinations of the polarizers involving vertically or horizontally polarized light. We present polarized low-temperature data only for Rhz(TMB)42+,as our results for Rh2b42+were of much lower quality because of poor solubility of the available salts. Polarization data are presented in the form of the polarization ratio N = 111/11 as a function of excitation wavelength, where we monitored emission at the uncorrected instrumental fluorescence or phosphorescenceintensity maximum. Signal/noise ratios were low for the absorption features at A < 300 nm because of low excitation lamp output, but control measurements of the emission spectra for 250-nm excitation showed that scattered light contributions to thesignal werenot significant at this wavelength. Since our instrumental polarization correction proved to be nearly wavelength-independent,we can also calculate isotropic excitation spectra at 77 K according to I ( X ) = ( I I I ( X ) + 211(A))/3.9
Emission Spectra Emission spectra for the binuclear Rh(1) complexes at room temperature are summarized in Table I (Figure 1 shows spectra for Rh2b42+at room and low temperatures). Our spectra for Rh2(TMB)42+are very similar to those reported previously.lc In each case, two distinct emissions are seen, separated by about 3500 cm-1, which is comparable to the lv3AZu splitting established by absorption s t ~ d i e s .Since ~ the Stokes shifts are very similar for the lA2, and 3AZustates, their distortions from the ground state must also be similar. The lower energy Rh2(TMB)42+ emission is very weak at room temperature, but its existence is established by observation in time-resolved experiments of a substantial component of relatively long-lived emission near 800 nm whose intensity and lifetime increase dramatically as the temperature is lowered.'c Neither lAzu 'Al, emission nor 'AI, ]Azu absorption maxima vary significantlywith changesin the solvent;for example, the maxima agree within 1000 cm-l for the solvents H20 and CH2C12. These results, which facilitate comparisons between solutions in different solvents, imply that the ground and excited states do not interact strongly with solvent.
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0022-3654/93/2091-4211$04.00/00 1993 American Chemical Society
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4278
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993
TABLE I: Corrected Emission-Band Parameters for Binuclear Rh(1) Complexes in CHJCN Solution at Room Temperature comDlex transition ,A (nm) vmlr (cm-ll fwhm" (cm-') Rh2bd2+
IAzu 'AI, 3A2u4 'AI, Rhz(TMB)d2+ 'Azu 'AI, +
3A2u 'AI, +
a
656 865 614 -780'
15 240 11 560 16 300 -12 800
1600 1650 1800 -2000
*
fwhm = full width at half maximum. Weak shoulder.
v(cm-1) Figure 1. Corrected emission spectra for Rh2bd2+ in 2:l 2-methyltetrahydrofuran/propionitrile solution at 295 (- -) and 77 K (-).
-
Photophysical parameters are summarized in Table 11. The lifetime data are extracted from direct emission decay measurements, while the quantum yields derive from excitation into the 'AI, IAzutransition (A,, = 500-550 nm). Our results are in good agreement with previously reported values,l~lO~ll where available, including lifetime measurements performed by transient absorption decay.'03l1 An exception is that the quantum yields reported earlier1, for Rh2(TMB)42+are a factor of 2-3 smaller than the ones we have obtained in the present investigation. The quantum-yield and lifetime data allow us to estimate radiative rate constants (k,), according to @ F / T F (fluorescence) or ( @ p / ~ p ( 1- @F)-~ (phosphorescence) (Table 111). Thek,values for fluorescence and phosphorescence differ by a factor of about 103, but are similar for the two diisocyanidecomplexes, and appear to be insensitive to temperature. It may be noted that the spinorbit splitting of the 3A2, state of Rh2bd2+has been determinedI2 to be only 11 cm-I, which is small with respect to kT at 77 K . The experimentally derived values of k,may be compared with those estimated from the absorption spectrum and emission maximum of Rh2b42+ using the Strickler-Berg theory; these estimates, 4.3 X lo7s-1 for IAlg- IA2, and 3.8 X lo4s-I for ]Al, 3A2u,13 are in excellent agreement with the experimentalvalues. We would not expect such agreement to hold for the 3A2ustate if there were significant nonradiative decay processes of the lA2, state (which is the state directly excited in the luminescence experiments) that bypassed the 3A2, state, in addition to the 5-10% radiative losses (Table 11) from IAz,,, since the value of @p would reflect such losses. We therefore conclude that the IA2, state does not undergo nonradiative decay to the ground state to a significant extent, which is consistent with the estimate @ISC(1A2u-3A2u) > 0.8 obtained for Rh2bd2+on the basis of transient absorption measurements,Ia as well as with estimates of @ ~ S C from more recent transient absorption studiesI0J1 of Rh2(TMB)42+.
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Emission Excitation Spectra Corrected emission excitation spectra for both the fluorescence and phosphorescence of R h ~ b 4 ~in+ room-temperature fluid solution are shown in Figure 2a. The phosphorescence excitation spectrum agrees well with the absorption spectrum,' but as has been noted previously,I4 the fluorescenceexcitation spectrum does not. The ratio of these two excitation spectra, normalized to the
Miskowski et al. IAlg- IA2, maximum a t 554 nm, is shown in Figure 2b. Clearly, there is a large decrease in the fluorescence efficiency upon excitation at wavelengths less than -450 nm. The well-defined absorption bands corresponding to the upper excited states E,(IE,) and E,(3E,) at 318 and 344 nm show phosphorescence/ fluorescence efficiency ratios of 6.0 and 7.9, respectively. We summarize the derived excited-state conversion yields in Figure 3. Conversion yields from any given state are defined so as to sum to 1.O. Those from the l,3Euexcited states represent a composite, in that there are a number of other excited states near them that are extremely weak in a b s o r p t i ~ nhence ,~ not directly detectable in excitation spectra. Decay through tHese states might be involved in IJE, decay, and hence may affect the net Is3A2,,yields. However, since the phosphorescence excitation spectrum agrees well with the absorption spectrum, we exclude the possibility that significant decay of the upper excited states occurs directly to the IAl,ground state. This finding also requires @ I = 0. However, the observation (Figure 2b) that the fluorescence yield from ]E, excitation is slightly larger than that from )E, excitation does require 0 1 to be slightly greater than zero. A detailed fit gives the yields displayed in Figure 3. We have assumed that @p6 = 1 - @F, that is, that the nonradiative decay ofthe IA2,statedirectly to thegroundstateisnotsignificant. The calculated values of 91 through @4 are not very sensitive to thevalueof @,5, however, since thefluorescenceexcitation spectrum demands that and a3be small. The values found for O1 through @4 are easily understood on the basis of spin-orbit coupling. Thus, the transition to3E, gains the considerable intensity it shows in absorption through spinorbit coupling of the E,(3E,) component with the nearby ]E, state. This is clearly shown by the spin-orbit calculation of Shimizu et al.I5 for valence-isoelectronic Pt2(PzOsH2)$-. Rapid and efficient decay of 'E, to 3E,is thereforereasonable. Similarly, 3E, is spin-orbit-coupled to the E, component of 3 A ~ uand , this accounts for the large value of @4. Indeed, when viewed in this way, @3 seems surprisingly high. However, as already alluded to, there area number of other electronicstates in thegap between 3E, and I,3A2u,and decay through these states may increase the net decay to IA2,. The fluorescence excitation spectrum of R ~ z ( T M B )(Figure ~~+ 4) is very similar to that of Rh2b42+.While a phosphorescence excitation spectrum of this complex could not be determined at room temperature because of the extreme weakness of this emission under these conditions (steady-state phosphorescence intensity is comparable to that of the long-wavelength tail of the fluorescence), comparison of the fluorescenceexcitation spectrum to the absorption spectrum, under the assumption that there is no direct upper excited-state deactivation to the ground state, gives conversion yields nearly identical with those of Figure 3. We have performed related measurements for [Rh(CN-cycloh e ~ y I ) ~ ]as ~ ~well + , as for Ir(1) complexes such as Ir2(TMB)42+ and Ir2(p-pyra~olyl)2(CO)~,4~J~ and found similar reductions of relative fluorescence quantum yields upon excitation into upper excited states. The effect may thus be general for complexes of this type. We suspect that rapid intersystem crossing that is competitive with internal conversion occurs in the upper excited states of many if not most second- and third-row transition-metal complexes; however, it is difficult to confirm this, as fluorescence is only very rarely observed for such complexes. We note that an upper limit for the lifetime of the IE, state of Rh2bd2+has been estimated ( ~ 1 O - s),14 l ~ on the basis of the lack of observable emission from this state, while the extreme narrowness (500-700 cm-I) of the 1,3EUstates in low-temperature a b s o r p t i ~ nplaces ~ , ~ a lower limit (via the uncertainty principle) on their lifetimes of -lO-I4 s. Thus, fairly narrow bounds can be placed on the rates of the upper excited-state nonradiative decay processes of this complex, lOI3-1014 s-1.
Binuclear Rhodium(1) Isocyanide Complexes
The Journal of Physical Chemistry, Vol. 97, NO.17. 1993 4219
TABLE 11: Photophysical Parameters for Solutions of Binuclear Rh(1) Complexes. room temperature 7~ (ns) 1.3(l)b 0.9(l)kd
complex Rh2bd2+ Rh2(TMB)d2+
*F
TP
17 K
(rs)
@P
-
TF (ns)
*F
7P
(rs)
@P
0.08 12.5( 10) 0.6 0.055 0.03( 1) 1.4(1)'.d 0.09 20.q 10) 0.5 The solvent is 2:l 2-methyltetrahydrofuran/propionitrileexcept as indicated. The subscripts F and P refer to fluorescence and phosphorescence, respectively. CH3CN solution. 4:l C2H50H/CHjOH solution. For a PMMA film of Rh2(TMB)d2+, T F was determined to be 0.9 and 1.8 ns at room temperature and 77 K, respectively. e Not measured, see text. 0.07
8.3(5)
TABLE 111: Derived Values of k,and k., (s-1) for the Rh2L4*+Complexes fluorescence
77 K
kr k,, k, knr
5 X lo7
7
X
lo8
6X 1X 6X 6X
e
1
E,
02 =
phosphorescence
TMB
b
300K
0.32 10-3
lo7 lo9 lo7 lo8
b
TMB
4.1 X lo4 7.9 X lo4 5 X lo4 3 X lo4
- 4 X lo4
0 1= 0.05
3 X lo7 2.7 x 104 2.2 x 104
Figure3. Excited-state conversion yields for Rh2bd2+at room temperature (see text for discussion).
1 'VI
2iO
360
3iO
460
4iO
560
540
6d0
'
Vnm) Figure2. (a) Corrected excitation spectra for Rhzb42+in CH3CN solution at room temperature, monitoring phosphorescence (810 nm, -) and fluorescence (650 nm, -). Spectra have been normalized at 550 nm, with a slight misnormalization for clarity. (b) Ratio of the spectra of (a) exactly normalized at 550 nm.
h (nm) Figure 4. Fluorescence excitation spectrum (monitored at 650 nm) for Rh2(TMB)d2+ in CH3CN solution a t room temperature. A portion of the spectrum is shown on an expanded sensitivity scale.
--
Polarized Excitation Spectra Figures 5a and 6a show, respectively, the fluorescence and phosphorescence excitation spectra of Rh2(TMB)42+in a methanol/ethanol glass at 77 K. In both cases, these spectra have been corrected for anisotropic effects resulting from photoselection9 in the rigid matrices. Comparison to the roomtemperature data (Figures 2 and 4) shows that upper excitedstate conversion efficiencies to the 193A2ustates are very similar at both temperatures. Figures 5band 6b show, respectively,theexcitation polarization ratios N(X) for the fluorescence and phosphorescence. For D4 or D4h symmetry there are three limiting theoretical values9 of N: 3, if both the emission and absorption are polarized along the unique (z)axis; 4/3, if both emission and absorption are polarized perpendicular to z; '12, if emission and absorption are polarized, respectively, parallel and perpendicular to z (or vice versa). In practice, even after correction for nonideal spectrometer effects,6,9 sample imperfections always result in some loss of polarization.
Employing numerous test molecules, we observe experimental values of 1.9-2.4, 1.2-1.3, and -0.6, respectively, for the three cases listed above. Our data in Figures 5b and 6b are in excellent agreement with expectations for the z-polarized IA2" ]AIgfluorescence and the x,y-polarized E,(3A2,) IA1, phosphorescence, with the -530-nm ]AI, lA2, and -340- and -315-nm lAl, E,(3E,,,1E,,)absorptions showing the expected polarization ratios in both sets of spectra. The absorption assignments have been established previously by single-crystal polarized spectroscopy,5 and are supported by magnetic circular dichroism (MCD) spectra.4a We note that the polarization information presented here is equally unambiguous in its interpretation and it is acquired with relatively little experimental effort. The emission polarizations also are unambiguously established, and this new information eliminates the possibility that either relaxed excited state distorts significantly from uniaxial symmetry. We now consider two types of relatively weak electronic transitions that appear in the emission excitation spectra. One of these is exemplified by the very weak band at -375 nm in Figure 4. As discussed in more detail e l s e ~ h e r e , ~ athe , ~ Jmost ~
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Miskowski et al.
4280 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993
,
9.1
al
log k- j b
0.003
I
B
7
0.005
0.007
0.009
1/T Figure 7. log of the reciprocal fluorescence lifetime versus the reciprocal temperature for Rh2(TMB)d2+in ( 0 )4: 1 C2H50H/CH,OH and (A)4: 1 C*HsOD/CH3OD.
because N for fluorescence is much less than the expected value for z-polarized absorption. This result is consistent with assignments (IAl, I,,EU(d,, p,), metal-to-metal charge transfer transitions) that we have suggested4, on the basis of absorption and MCD spectra. These states appear to show internal conversion yields to the lAzu fluorescent state that are higher than those from the Il3Euexcited states (Figure 2), but direct deactivation to the 3A2uphosphorescent state still dominates.
-
N
(nm)
-
Temperature Dependence of
Figure 5. (a) Corrected fluorescence excitation spectrum (corrected to isotropic) for Rh2(TMB)42+in 3:l CH,OH/CH,CH20H glass at 77 K (monitored at 612 nm). (b) Polarization ratio N for the data shown in Figure 5a. a
!
lAzu Decay
The emission lifetimes of the IAzustates of Rh2b42+and Rh2(TMB)42+were determined to be 1.3 and 0.9 ns, respectively, in CH$N at room temperature (Table 11). The low-temperature solubility of available Rh2bd2+salts was poor in all glassing solvent systems that were tested, and because of instrumental sensitivity limitations, so far we have been unable to determine accurately the 77 K fluorescence lifetime of this complex. However, in view of the very small temperature dependence of ap~for this complex (Table 11),it is likely that the lA2" lifetime is nearly temperatureindependent. In contrast, @F and T F for Rh2(TMB)d2+were both found to increase significantly at low temperature (Table 11). Measurements of the temperature dependence of the IA2,, lifetime were therefore undertaken to allow a comparison with previously reported studies of the temperature dependences of the ,Azu lifetimelc-' of Rh2(TMB)42+and the lA2" lifetime8 of Pt2(P205H2)4&*
3.0
The change in singlet lifetime with temperature was studied for Rh2(TMB)42+in EtOH/MeOH (4:1), EtOD/MeOD (4:1), and poly(methy1 methacrylate) (PMMA). The results for the two mixed-solvent systems are presented in Figure 7, where the logarithm of the observed decay rate is plotted against the reciprocal of the temperature. These data have been fitted to an equation of the form
2.4 1.8
N 1.2
0.6 250
300
350
400
450
500
550
600
(nm) Figure 6. (a) Corrected phosphorescenceexcitation spectrum (corrected to isotropic) for R ~ z ( T M B ) ~ in~3:l + CH,OH/CH3CH2OH glass at 77 K (monitored at 772 nm). (b) Polarization ratio N for the data shown in Figure 6a.
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likely assignment of this feature is to a 'Al, lA1, (d,z pz) transition, in which the excited-state Rh2 bond length remains similar to the ground-state value (in accord with a valence-bond model).4a This transition, which is expected to be allowed only by a vibronic mechanism, is too weak for the determination of reliable excitation polarization. However, the fact that these bands appear in the excited spectra eliminates the possibility that the weak absorption features are due to an impurity. A second set of weak bands (two shoulders) falls in the 220250-nm region (Figures 2 and 4). Figure 5 strongly suggests that absorption in this region is predominantly molecular x,y-polarized,
kobs = k, + ( A / ( ~ T ) ' " ) exp(-Ea/kT) (1) This equation models a two-channel decay in which one channel represents temperature-independent weak coupling and the other channel represents the high-temperature limit of quantummechanical strong coupling.s Nonlinear least-squares fits, which are presented as the lines in Figure 7, give ko values of 7.2 X 108 and 7.0 X lo8s-I, A values of 1.0 X 1012 and 1.6 X 1012 s-I cml/2, and E, values of 1070 and 1110 cm-1, for the complex in EtOH/ MeOH and EtOD/MeOD, respectively. We note that fits of these data to the simple Arrhenius expression (eq 2) that was
employed in previous studies of emission decaylcpffor this complex yield E, values that arevery similar to those given. Values of A derived from eq 1 can also be compared roughly to those from eq 2 after dividing them by (kZ')1/2 (which has a value of 14.3 cm1/2a t room temperature).
Binuclear Rhodium( I) Isocyanide Complexes
3
5
5 t
2
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4281
w \
2.4
2.8 3.2 d(Rh2) (A)
3.6
4
Figure 8. Harmonic potential surfaces along the metal-metal stretching coordinate for the ground state ( IAlg)and low-lying excited states ( Iq3AZu and 1.3Eu)of a d 8 d Scomplex. Assumed force constants, equilibrium bond distances, and excited-state energies are experimentally derived values5 for Rh2b42+.
It is striking that the data and parameters are very nearly identical for the two solvent systems, implying that solvent deuteration has little effect on the rate associated with either of the two nonradiative decay channels. This contrasts to the results for the valence-isoelectroniccomplex Pt2(P205H2)4G,8 for which deuteration of the solvent led to a decrease in the value of ko to l / 3 of its value in protonated solvents, and hence a tripling of the low-temperaturelifetime. The methyl groups of the TMB ligands do not block the axial positions of R ~ Z ( T M B ) ~as~many + , strongly bound axial adducts of the Rh(I1) species Rh2(TMB)44+ have been prepared and characterized.” The temperature dependence of the lAzu emission of Rhz( T M B ) P in a PMMA film was well fit by the same two-channel kinetic model. In this solvent environment the values of the parameters obtained are 5.6 X lo8s-I for ko, 1.6 X 10l2s-I cm1/2 for A, and 1110 cm-I for E,.
Nature of the Nonradiative Decay Processes
-
The only significant geometry changes in the low-lying d p excited states of the binuclear Rh(1) isocyanide complexes involve metal-metal c o n t r a c t i ~ n . ~The . ~ ~ IJAzu excited states are known to possess similar geometries, with, in the case of Rh2b42+, a Rh2 bond distance that shrinks from 3.25 A (ground state) to about 2.93 A in the IJAzu state^.^ The low-lying IJEu states are, in contrast, little distorted from the ground state? as evidenced by the narrow ]Al, Iq3Euabsorption bands. Calculated potential surfaces are shown in Figure 8. Since the lJEu surfaces have low-energy crossings with the 133A2u surfaces, it is not surprising that the Iq3Eunonradiative decay rates are large, 1013-1014 s-I, as estimated earlier. Figure 8 suggests that there are small potential barriers to these processes, but there are a number of additional states (not shown in the figure) in the gap between the Iq3Euand Is3Azustates that may mitigate these barrier^.^,,^ In contrast, the 133A2upotential surfaces are “nested”, both mutually and with respect to the ‘AI, ground state. The latter results occurs, despite a large metal-metal distortion, because the lJA2uexcited states possess a metal-metal force constant that is larger than that of the ground state by over a factor of 2.18 Thus, direct nonradiative decay among these three states is
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-
1
\
expected to occur in the weak-coupling limit,I9and weak coupling will dominate the observed low-temperature nonradiative rate. (The spin-orbit splitting of the 3A2ustate is too smallhJ2 for thermal equilibrium among the spin-orbit components to be significantly temperature-dependent at 77 K and above.) The important matrix elements coupling them are those of the spinorbit operator; none of these three states have any spin-orbit components of common symmetry, so there are no “special” couplings. For similar state energies we would thus expect decay rates to scale with the square of the spin-orbit constant, and, with {= 1000 cm-I for Rh and {= 3000 cm-1 for Pt and Ir, we predict rates about 1 order of magnitude larger for 5d than for 4d complexes. For P t ~ ( P 2 0 ~ H 2 )the 4 ~low-temperature , limiting nonradiative decay rates are not, in fact, very different from those found here for Rh(1) dimers; for thePt(I1) complex in HzO/glycerol solution, ko values are 1.5 X lo9 and 5 X lo4s-I for ]Azu 3A2uand jAZu lAlg, respectively.2b*8However, both the 1A2u/3A2u splitting (5200 cm-I) and the energy of the 3A2ustate (22 200 cm-I) are substantially larger for Pt2(Pz05H2)44- than for the Rh(1) complexes,2Jand the rates of nonradiative processes connecting these states should therefore be decreased accordingto the energygap law that applies in the weak-coupling limit.8J9,20 Another comparison is to the complex I ~ Z ( T M B ) ~ ~values +:~,, (which are temperature-independent between 77 K and room temperature) are 9.5 X lo9 and 5 X lo6 s-l for IAzu jA2” and 3A2u IAI,, respe~tively.~~J The 1A2u/3A2usplitting of this complex is identical with that of Rh2(TMB)42+,4aand the orderof-magnitude increase in the intersystem crossing rate for the Ir(1) complex fits the spin-orbit model fairly well. The much larger (2 orders of magnitude) increase in k,, for 3A2u---c IAl, of the Ir(1) complex is largely attributable to the energy-gap law; the 3A2uexcited-state energy of I ~ Z ( T M B )is~about ~ + 3000 cm-1 less than that of Rhz(TMB)42+.4a A final interesting aspect of the low-temperature limiting nonradiative decay rates is the absence, for Rh2(TMB)42+,of any significant effect of solvent deuteration on the rate of either lAzu 3A2uor 3Azu-.* lAlg.lc In view of the absence of significant solvent effects on any other photophysical properties of this complex, this is not surprising. Neither the ground state nor the Iq3AZuexcited states appear to undergo specific interactions with the solvent. The large solvent deuteration effects on lAzu 3A2u nonradiativedecay that wereobserved8for P t ~ ( P ~ O ~ H 2 ) 4 ~ r e m a i n to be explained. It is obvious that the large negative charge of the Pt(I1) complex should, in a general way, enhance interactions with protic solvents;more specifically,the polar P205H22ligands can hydrogen-bond to the solvent. In view of the Rh2(TMB)42+ results, the low-temperature interaction of P ~ ~ ( P ~ O S Hwith ~)~’ H20 or CH3OH is likely not solvent 0 atom coordination to the metal. The absence of H / D isotope effects for R ~ Z ( T M B ) ~ ~ + excited-state decay processes may reflect the absence of bridging ligand hydrogen-bonding interactions with protic solvents. It is attractive to postulate that this interaction may be related to the room-temperature hydrogen-abstraction excited-state reactivity that has been establishedzdJ2for Pt~(P205H&+-. A combination of protic-solvent hydrogen (deuterium) bonding to the P ~ O S H ~ ~ ligands and weak axial interaction of hydrogen (deuterium) atoms of the “docked” solvent molecules with excited-state Pt atoms may be invoked.22 Strikingly, both the lAqu and 3A2uexcited states of Rh2(TMB)42+undergo thermally activated nonradiative decay. The only other d8-d8system for which thermally activated IAzudecay has been establisheds is Pt2(P205H2)4+. Importantly, a ligandfield excited state (3B2urdu* dX2-,,!--y2) had been experimentally located for the Pt(I1) complex just above the lA2ustate,2band it was possible to identify coupling to this state as the likely source of the thermally activated decay channel.8 We believe that the 3BZuexcited state can similarly account
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-
-
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4282 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 30
/ /
a
Fro+
W
Miskowski et al. larger than that of R ~ z ( T M B ) ~ If ~ +E,. for the 3A2uthermally activated decay channel is increased by 1700cm-I, the exponential term of eq 1 or 2 is decreased by 4 orders of magnitude, and assuming similar preexponential terms, the thermally activated channel would be negligibly small with respect to ko at room temperature. Similarly, the fact that the 133A2ustates of Ir2(TMB)42+are 3000 cm-l lower in energy that those of Rh2(TMB)42+may account for the absence of thermally activated decay processes for these d8-d8Ir(1) We finally consider the interesting case of Rh2b42+. The temperature dependence of the 3A2ulifetime of this complex is much smaller than that observed for R ~ z ( T M B ) (Table ~ ~ + 11), and it has been suggestedlethat the rigidity of the Rh2b unit (in contrast to the flexible RhzTMB unit) might be a factor. Large nontotally symmetric distortions have been invoked in the nonradiative decay of d p excited states of mononuclear d8 complexes.25However, a key observation for these latter systems is that the excited-state lifetimes shorten dramatically as the medium becomes In contrast, it has been found that thermal activation parameters for decay of both the 3A2u1c,d and lAaustates of Rh2(TMB)42+are qualitatively similar in fluid and rigid (PMMA) environments.26 An alternative explanation arises from the fact that the 193A2u states of Rh2bd2+are about 1200 cm-l lower in energy than those of R ~ z ( T M B ) ~ ~We + . expect the 3B2ustates to have similar energies for the two binuclear Rh(1) complexes. Assuming E, to be larger for R h ~ b 4 ~by+ about 1200 cm-I, the exponential term of eq 1 should be smaller than that for R ~ z ( T M B ) at ~~+ room temperature by a factor of several hundred. This effect by itself is sufficient to account for the observed differences in nonradiative decay rates, within the broad parameter allowances of the crude theory employed. In conclusion, all of the available results for nonradiative decay of the 1,3A2u excited states of face-to-face d848 complexes can be accounted for on the basis of a two-channel model. One channel is temperature-independentweakcoupling,and the rate is sensitive to the magnitude of spin-orbit coupling and the energy gap ( IAzu/ or 3A2u/'A~,).The other channel involves strong coupling of 1A2uor 3A2uto a 3B2uligand-field excited state, and the roomtemperature rates involving this channel are enormously sensitive to the relative energies of the 133A2u and 3B2ustates. -+
2L+ - 2 -
1
0
1
2
3
4
5
Q (arbitrary units)
-
Figure 9. Harmonic potential surfaces along a metal-ligand coordinate for the ground state (]A,*) and the 1,3A2u(du* pu) and 3B2u(ligandfield) excited states of a ds-ds complex. See text for discussion.
for the thermally activated processes seen for Rh2(TMB)42+.A qualitative set of potential surfaces is shown in Figure 9. The distortion coordinate Q here is not the metal-metal coordinate of Figure 8, but instead a metal-ligand coordinate, along which the 133A2uexcited states are not significantly distorted from the ground ~ t a t e . ~ ~Harmonic - ~ ~ , ~ surfacesz3 -~ were assumed; the distortion coordinate is in arbitrary units, and parameters were chosen so as to produce crossings of the 3B2usurface with the lAzu and 3A2usurfaces at energy differences AE = E(3B2u)- E(1(3)A2u) corresponding roughly to the observed E, values for thermally activated nonradiative decay. It can be seen that a smaller energy difference for the intersection of lAzuwith 3B2uthan for the )A2,,/ 3Bzuintersection arises very simply from the model as long as the various potential surfaces do not have very different curvatures. If the force constants k for the harmonic potential surfaces are identical, then AI3 (as defined above) can be expressed as
AI3 = (Bo + X)2/4X
(3) where AEo is the energy difference between the minima of the two potential surfaces, and X = k(AQ)2/2 is the reorganization energy. AE can furthermore be roughly equated with the experimental parameter E, for E, >> kT.I9 Spectroscopic data indicate that X is in the 200&2500-~m-~range for the ligandfield excited states of square-planar d8 ~omplexes.2~Since A is similar in magnitude to the experimental values of E,, AEo must also (eq 3) be of similar magnitude to E, (because the state crossings then occur near the upper state minimum). From Figure 9 we predict that the 'AI, 3B2uvertical absorption transition of the binuclear Rh(1) isocyanide complexes should occur near 21000 cm-I. Absorption bands of this type have not been l ~ c a t e dfor ~,R ~ ~ z ( T M B ) or ~ ~Rh2b42+, + but since they are expected to be both very broad (because of the large excited-state metal-ligand distortion) and very weak (because they are spin-forbidden),they may be obscured by other absorption bands in the vicinity. The absence of thermally activated nonradiative decay for the 3A2ustate of &2(P205H2)4" 2b may be explained if the shapes of the various excited-state potential surfaces of this complex are similar to those of Rh2(TMB)d2+. The activation parameters for IA2, decay are similar for the two complexes, suggesting similar energy differences between the IAzu and 3B2,,states, while the 1JA2usplitting of Pt2(P20sH2)46is 5200 cm-I,Za,b,8 1700 cm-l
-
Acknowledgment. We thank David Smith for helpful discussions. This work was supported by National Science Foundation Grant CHE-8922067. Workat Brookhaven National Laboratory was performed under Contract DE-AC02-76CH00016 with the US. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. References and Notes (1) (a) Miskowski, V. M.; Nobinger, G. L.; Kliger, D. S.; Hammond, G. S.;Lewis, N. S.; Mann, K. R.; Gray, H. B. J. Am. Chem. SOC.1978,100,485. (b) Mann, K. R.; Thich, J. A.; Bell, R. A.; Coyle, C. L.; Gray, H. B. Inorg. Chem. 1980, 19,2462. (c) Milder, S. J. Inorg. Chem. 1985, 24, 3376. (d) Marshall, J. L.; Stobart, S. R.; Gray, H. B.; J. Am. Chem. SOC.1984, 106, 3027. (e) Rice, S. F.; Milder, S. J.; Gray, H. B.; Goldbeck, R. A,; Kliger, D. S. Coord. Chem. Rev. 1982, 43, 349. (2) Related Pt(I1) compounds: (a) Rice, S. F.; Gray, H. B. J . Am. Chem.
S0~.1983,105,4571.(b)Stiegman,A.E.;Rice,S.F.;Gray,H.B.;Miskowski, V .M. Znorg. Chem. 1987,26,1112. (c) King, C.;Auerbach, R. A.; Fronczek, F. R.; Roundhill, D. M. J . Am. Chem. SOC.1986,108,5626. (d) Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989, 22, 55. (3) ,We use D 4 b symmetry labels in this paper. The lower D4 symmetry appropriateIbfor Rh>(TMB)4*+has no significant effect upon selection rules
for the electronic transitions of interest. (4) (a) Smith, D. C.; Miskowski, V. M.; Mason, W. R.; Gray, H. B. J. Am. Chem. SOC.1990, 112, 3759. (b) Smith, D. C.; Miskowski, V. M. Unpublished work. (c) Smith, D. C. Ph.D. Thesis, California Institute of Technology, 1989. ( 5 ) Rice, S. F.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1988, 27, 4704.
Binuclear Rhodium(I) Isocyanide Complexes (6) Miskowski, V. M.; Gray, H. B.; Hopkins, M. D. Inorg. Chem. 1992, 31, 2085. (7) Nocera, D. G.; Winkler, J. R.; Yocom, K. M.; Bordignon, E.; Gray, H. B. J. Am. Chem. SOC.1984, 106, 5145. (8) Milder, S.J.; Brunschwig, B. S.J . Phys. Chem. 1992, 96, 2189. (9) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (10) Winkler, J. R.; Marshall, J. L.; Netzel, T. L.; Gray, H. B. J. Am. Chem. SOC.1986, 108, 2263. ( 1 1) Milder, S. J.; Kliger, D. S.;Butler, L. G.; Gray, H. B. J . Phys. Chem. 1986,90, 5567. (12) Oberneder, S.; Gliemann, G. J . Phys. Chem. 1989, 93, 4487. (13) The IA2" value is from ref la. That for 'A2" was calculated by us from the single-crystal absorption data of ref 5, taking account of both the
anisotropy of the crystal spectrum and the spin degeneracy. For the theory involved, see: Strickler, S.J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814. (14) Milder, S.J.; Kliger, D. S. J . Phys. Chem. 1985,89, 4170. (15) Shimizu, Y.;Tanaka, Y.; Azumi, T. J. Phys. Chem. 1984,88,2423. (16) Marshall, J. L.; Hopkins, M. D.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1992, 31, 5034. (17) Miskowski, V. M.; Smith, T. P.; Loehr, T. M.; Gray, H. B. J . Am. Chem.'Soc. 1985, 107, 7925. (18) (a) Dallinger, R. F.; Miskowski, V. M.; Gray, H. B.; Woodruff, W. H. J. Am. Chem. Soc. 1981. 103. 1595. (b) Doorn. S. K.: Gordon. K. C.: Dyer, R. B.; Woodruff, W. H. Inorg. Chem: 1992, 31, 2284. (19) (a) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. (b) Freed, K. F.; Jortner, J. J . Chem. Phys. 1970, 52, 6272.
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4283 (20) Caspar, J. V.;Sullivan, B. P.; Kober, E. M.; Meyer, T. J. Chem. Phys. Lett. 1982, 91, 91. (21) The lAzUlifetime of I ~ Z ( T M B )has ~ ~not + been directly measured.
The quoted value for k,, of of this complex4cis based upon a StricklerBerg estimate of k, from the data of ref 4a. (22) Sweeney, R. J.; Harvey, E. L.;Gray, H. B. Coord. Chem. Rev.1990, 105, 23. (23) Two complications neglected in Figure 9 are that there should be an
avoided crossing between the E, spin-orbit components of the 3Aluand 3B2" states, and that the distortion coordinate may not be a totally symmetric one,lE in which case the 3B2upotential surface should actually be a double well with a barrier centered at the ground-state minimum. (24). Values of A (estimated as half of the Stokes shift between absorption and emission maxima) for the lowest-energy ligand-field excited state ()(dVb dxz.p)) of P t X P are the following: X = CI, A = 2375; X = Br, A = 2025 cm-I. Data are from the following: Preston, D. M.; Guentner, W.; Lechner, A.; Gliemann, G.; Zink, J. I. J. Am. Chem. SOC.1988, 110, 5628. Yersin, H.; Otto, H.; Zink, J. I.; Gliemann, G. J . Am. Chem. SOC.1980, 102, 951. Kroening, R. F.; Rush, R. M.; Martin, D. S.,Jr.; Clardy, J. C. Inorg. Chem.
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1974, 13, 1366. (25) (a) Geoffroy, G. L.; Wrighton, M. S.; Hammond, G. S.;Gray, H. B. J . Am. Chem. SOC.1974, 96, 3105. (b) Andrews, L. J. J . Phys. Chem. 1979,83, 3203. (26) There is some medium sensitivity for E,, which has been found to
. ~ significanceof this Barclayfollow a linear correlation of loa(A)with E ; , : ' cthe Butler (Barclay, I. M.; Butle
