2068
J. Phys. Chem. 1980, 84, 2068-2074
Charge-Transfer Excited States of Osmium(I1) Complexes. 2. Quantum-Yield and Decay-Time Measurements'a D. E. Lacky,Ib B. J. Pankuch, and G. A. Crosby*lc Deparfment of Chemistry and Chemical Physics Program, Washington State University. Pullman, Washington 99 164 (Received: January 9, 1980) Publication costs assisted by the Air Force Office of Sclentific Research
Computer analysis of the temperature dependence (-4-77 K) of the intensity and decay time of the photoluminescence observed from five tris osmium(I1) complexes containing 7r-conjugated ligands has yielded energy-levelgaps and both radiative and radiationless decay constants for the lowest emitting levels. The derived empirical parameters have been rationalized on the basis of an ion-parent coupling model previously invoked to describe the optical behavior of the analogous ruthenium(I1)complexes. The three emitting levels have been assigned to a d-a*(a2) charge-transfer configuration with Al, E, and A2 symmetries in the group D3.
Assignments of the excited states responsible for the visible absorption and luminescence spectra of osmium(I1) complexes containing r-conjugated ligands were presented in the previous article of this series.2 In that study, heavy reliance was placed on the model for charge-transfer-toligand (CTTL) excited states developed previously for rationalizing the spectra of analogous ruthenium(I1) compounds. A pillar of the argument to extend the theoretical framework to encompass osmium(I1) spectra was the assignment of the symmetries of the lowest excited (luminescing) states of the latter compounds. In the present article we describe the detailed spectroscopic evidence for making these initial assignments and present additional arguments supporting the fundamental role played by spin-orbit coupling in controlling the nature and disposition of low-lying CTTL states in (nd)6(n = 4, 5 ) complexes of this type.
Experimental Section Syntheses, Purification of Compounds, and Sample Preparation, Procedures for preparing all the compounds described herein were given in the previous article. For luminescence, lifetime, and intensity measurements at temperatures below 77 K, the compounds were dissolved in rigid poly(methy1methacrylate) (PMM) matrices. The single exception was [ 0 ~ ( 4 , 4 ' - M e ~ b p y )for ~ ] Iwhich ~ all measurements were carried out on rigid 4:l ethanolmethanol glasses. The plastic samples were prepared by dissolving 15 0.25411. cubes of PMM in 100 mL of CHC13 and then adding compound until the desired visual color density was obtained. Excess solvent was removed by pumping for 48 h. Usually a clear, colored plastic film remained that could be easily peeled off the beaker. When, infrequently, a cloudy specimen containing particulates was produced, it was redissolved in an excess of CHC13and subjected to the pumping procedure again; usually two cycles produced an acceptable, optically clear sample. Spectra. The method of measurement of absorption spectra at room temperature and at 77 K in the ethanolmethanol solvent (4:1, v/v) was described earlier.2 Also included there is the procedure for recording emission spectra at 77 K. Quantum- Yield Measurements. Quantum-yield determinations were made relative to fluorescein purified by the method of Orndoff and Hemmer3 and contained in 0.1 M NaOH. The Cary 14 spectrophotometer was adjusted to zero absorbance at 435.8 nm with matched 10-cm cells containing 0.1 M NaOH in both sample and reference 0022-3654/80/2084-2068$0 1.OO/O
beams. Fluorescein was added directly to the sample cell, and the absorbance measured. This solution was quantitatively diluted to a concentration producing an absorbance less than 0.005 cm-l for the subsequent yield measurements. Next a 2-cm cell filled with 4:l (v/v) ethanol-methanol at room temperature was placed in the reference beam, and a partially silvered Dewar that contained a glass of the same solvent in a 1.76-cm Pyrex cylindrical tube at 77 K was interposed in the sample beam. At 720 nm the spectrophotometer was optically balanced, and a base line was established down to 400 nm. Then a 4:l (v/v) ethanol-methanol glass containing the desired osmium compound was placed in the sample beam. The instrument was again optically balanced at 720 nm, and the absorbance was recorded at 435.8 nm. The osmium sample was warmed to room temperature and then diluted quantitatively to produce an absorbance less than 0.005 cm-l. Emission intensities of the fluorescein standard and the osmium complex were measured on the near-infrared recording spectrofluorimeter by the procedure already described.2 The setup for 435.8-nm excitation was used. A substitutional method was employed. The luminescence intensity of fluorescein at its band maximum was first recorded. The sample, at room temperature, was excited in a Pyrex cell within an optical Dewar. Immediately afterward, a precooled sample of the complex (77 K)was inserted into the Dewar that was now filled with liquid nitrogen to just below the optical path. The luminescence intensity at the emission band maximum was recorded. By use of integrated areas from previously recorded emission spectra normalized to the band maxima, the relative quantum yields of the Os(I1) complexes were determined. A value of 0.90 was assumed for the quantum yield of fluorescein. Determinations were made in triplicate. For a full discussion of the method used in the laboratory for yield measurements, we refer the reader to published description^.^^^ Measurement of Temperature-Dependent Phenomena. For all three types of measurements involving temperature changes, an Andonian Associates Model 0.24/7M-H liquid helium Dewar was used. The geometry of the optical setup was always maintained such that luminescence was monitored at 90° to the exciting light. Each sample, a PMM square containing the appropriate compound and held in place by a sapphire plate on a milled copper block, was oriented at a 45O angle with respect to both entrance and exit optical axes. The block contained two thermal sensors, 0 1980 American Chemical Society
Charge-Transfer Excited States of Os(I1) Complexes
a germanium resistor that monitored temperatures below 40 K, and a chrome1 P-constantan thermocouple that registered temperatures above 20 K. Temperature was controlled to within 0.1 K by a dynamic balance between the cooling of the probe by ambient helium gas, controlled manually, and the heating of the sample block by a 7-W resistance heater, To achieve temperatures below 4.2 K, we first allowed liquid helium to fill the bottom of the optical chamber that was directly connected to a highcapacity pump. By controlling the pump rate, we were able to maintain selected temperatures between 1.6 and 4.2 K. (a) Luminescence Spectra. After passing through 5 cm of CuS04 (100 g CuS04.5H20/L),a Corning 7-60 glass filter, and a quartz liens, light from a 1OOO-W Hg-Xe lamp was focussed on the sample. The optical detection system was the same as described earlier for recording emission spectra at 77 K.2 Spectra were recorded at several fixed temperatures in the 4.2-77 K range. ( b ) Decay Times. For recording luminescence decay times two different experimental setups were employed. For the compound dissolved in a rigid glass the excitation source was an EG and G FX-12 flashlamp, which had a decay time of about 1.5 hs. Light from the flash was focussed directly on the sample. The emitted radiation passed through 2 cm of a solution of tris(2,2'-bipyridine)iron(III) dichloride (0.6 g/L) in ethanol and a Corning 2-58 glass fiilter before falling on an EM1 9558QC red sensitive photomultiplier. For exciting samples embedded in PMM matrices 337.1-nm light from a nitrogen-pulsed laser passed through 5 cm of CuS04solution (200 g CuS04*5H20/L)and a 7-60 Corning glass filter before hitting the sample. Sample emission was filtered by two 3-69 and one 7-63 Corning glass filters and a concentrated solution (1cm) filter of K N 0 2 in water befiore impinging on the phototube. In both variations the detector viewed at right angles to the excitation axis and t:he signal was displayed on a Tektronix 535A oscilloscope, along with time marks from a Tektronix Model 184 time mark generator. The photographed traces were manually digitized, transferred to the computer, and fit to the equation Iln I = Itt C by a least-squares program. Within the precision of the measurements all decays were strictly exponential. During the latter phases of the work a digital electronic device constructed in the laboratory was used to monitor the transients. This instrument displayed the mean decay times directly. The transients were frequently photographed, manually evaluated, and displayed graphically to check for nonexponential decays, especially at low temperatures. (c) Relative Intensities. The luminescence intensity of each complex was measured between 77 and 4.2 K. An electronic method, described earlier, of comparing the luminescence intensity of the sample l,o the strength of the exciting light was The exciting light from the 1000-W Hg-Xe lamp was passed through 5 cm of CuS04 (200 g CuS046H20/L),a Corning 7-59 glass filter, and an Optics Technology 450 redpass filter. This system transmitted between 420 and 480 nm, a region well-suited for exciting all the complexes. A small portion of the exciting ray was deflected by a quartz beam splitter onto an EM1 9558QC photomultiplier to provide a reference signal for the ratio module. Emitted light from the sample passed through two Corning 3-69 glass filters and a 1.0 OD neutral density filter before falling on an RCA C31034 photomultiplier. This combination of filters and phototube yielded a near flat quantum response over the emission bands. The sample signal, first preamplified by a Keithley
+
The Journal of Physical Chemistry, Vol. 84, No.
18
14
14
IO
cm-'
x
IS, 1980 2068
IO
10-3
Figure 1. Luminescence spectra of osmium(I1) complexes in PMM; (---) 77 K; (..) 7 K; (-) 4.2 K: (a) [Os(bpy),]I,; (b) [Os(4,4'Me,bpy),]I, (measured in 4: 1 (v/v) ethanol-methanol glass); (c) [Os(phen),]I,; (d) [Os(5,6-Me2phen),]12; (e) [Os(4,7-Meghen),]12; (f) [OS(tPY),l12.
Model 410 microammeter, was fed to the ratio module, whose output was recorded directly. The degree of confidence we place in this method of studying intensities from these kinds of molecules has been discussed previously.6
Results Absorption spectra, both at room temperature and at liquid-nitrogen temperature, and emission spectra in an ethanol-methanol glass (4:1, v/v) at 77 K are displayed in the preceding article.2 Their remarkably uniform appearances and the obvious relationship between the emission and absorption bands have been discussed there. Additional information on the excited states responsible for the emission is evident in the emission spectra taken in PMM at 77 and 4 K, respectively (Figure 1). The spectrum of a complex at 77 K in PMM, in every case, resembles very closely that measured in the glass a t the same temperature. The band envelope broadens considerably as the temperature is lowered to 4.2 K and undergoes a substantial red shift. Spectra taken a t intermediate temperatures reveal that the leading edge of the strong first peak seen at 77 K slowly loses intensity with respect to the rest of the band, thus producing the red shift. The second prominent peak in the spectrum gains intensity as the temperature is lowered until, at 4 K, it carries a substantial fraction of the intensity of the emitted light. The intermediate case (7 K) for [ O ~ ( p h e n ) ~is] I ~ included in Figure IC. Exception to this described spectral behavior is displayed by the 0~(CN)~(bpy), complex. For this molecule neither the position of the maximum nor the band shape changes substantially as the temperature is dropped from 77 to 4 K. Absolute quantum yields at 77 K for Os(CN)z(bpy)z, [ O ~ ( t p y ) ~and ] I ~ the , five tris species are reported in Table I. Also included are the measured decay times at the two fixed points, 4.2 and 77 K, for those emissions bright enough to monitor. The absolute quantum yields were measured in an ethanol-methanol (4:1, v/v) glass, the matrix system chosen for most of the measurements carried out in this laboratory. For monitoring the temperature dependence of both the decay times and the relative
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The Journal of Physical Chemistry, Vol. 84, No. 16, 1980
Lacky et ai.
TABLE I : Photoluminescence Quantum Yields and Lifetimes of Charge-Transfer Excited States of Osmium(I1) Complexes comPd @(77K)“ ~ ( 7 K),b 7 ps ~ ( 4 . K),6 2 ps ~ ( 7 7K)/@(77K) [OS(bPY),1 I, 0.038 1.8 10.8 47 (0.035) (0.9) [Os(4,4’-Me2bpy 11, 0.020 0.7 9.4 35 0.120 2.8 32.2 23 [Os(phen), 11, (0.126) (2.4) [ Os(5,6-Me2phen),]I, 0.178 2.3 31.4 13 [ Os(4,7-Me2phen),]I, 0.144 3.4 35.0 24 0.016 Os(CN)*(bPY1, 0.112 3.9 9.5 35 [ WtPY 1 2 1 I, (0.124) (3.9) a Measured in ethanol-methanol glass (4:1, v/v); quantum yield in () reported by Demas and Crosby, ref 5. Measured in PMM matrix except [ Os(4,4’-Me,bpy),]I,, which was measured in ethanol-methanol glass (4:1, v/v); values in ( ) estimated by Demas and Crosby, ref 5, by use of the method of moments on data from flashlamp experiments.
quantum yields, however, most of the compounds (except [0~(4,4’-Me~bpy)~]I~) were incorporated in PMM for experimental convenience. In order to convert the relative yields to absolute quantities, we measured the intensity from a sample in PMM at 77 K and normalized it to the absolute quantum yield in the glass at the same temperature. Thus, for all except the [0~(4,4’-Me~bpy)~]~+ cation we made the explicit assumption that the quantum yield is matrix independent. This assumption can be checked by measurement of the decay time both in PMM and in the ethanol-methanol glass. At the fiducial temperatures (4.2 and 77 K) the respective decay times were indeed matrix invariant within our limits of error. For all the compounds at all temperatures the decay curves exhibited a simple exponential form. The temperature dependences of both the decay times and the quantum yields of the five tris complexes and [Os(tpy),]12 are displayed in Figure 2. For all the complexes the same general behavior is observed. The decay time monotonically rises as the temperature is lowered below 77 K, finally reaching a plateau somewhat below 10 K (20 K for the D M species). In all the D3 cases the relative quantum yield rises slightly as the temperature is lowered, goes through a broad maximum between -25 and 40 K, falls off substantially between 20 and 10 K, and approaches a constant value at the lowest temperatures. Considerable scatter is apparent in the data for both the [Os(bpy),lz+ and [ O ~ ( p h e n ) ~ions. ] ~ + These were the first species studied and reflect the substantial difficulty we initially encountered in our attempts to control and monitor the sample temperature. Nonetheless, treatment of the data on the two molecules in a variety of ways always led to similar excited-state parameters. Neither the lifetimes nor the relative intensities of any of the compounds undergo large changes with temperature, thus dictating the exercise of considerable experimental finesse by the investigators. Discussion For all the spectroscopic work on Os(I1) complexes containing 7-conjugated ligands, we employed both the experimental methods and the theoretical models developed for the corresponding Ru(I1) species.- Indeed, the main thrust of the current studies is that the similarities between the low-lying excited states of the Ru(I1) (4d)6 complexes and those of the analogous Os(I1) (5d)6species are far more important than the differences. Differences show up, but they do not, in our view, dictate any fundamental change in viewpoint concerning the origin and behavior of the low-lying electronic excited states, nor do they require any essential changes in the methods used to extract information from the observed spectroscopic data. Thus, in what follows, we rely heavily on the previous studies of Ru(I1).
I
( c )I
0.14
173 cm-I
20
0.12
8
0.109
IO
20
30
40
50
60
70
O K
Figure 2. Temperature dependence of the lifetimes and quantum yiekls of osmlum(I1) complexes in PMM and computer-generated parameters for each luminescing charge-transfer manifold: (a) [Os(bpy),]I,; (b) [Os(4,4’-Me,bpy),]12 (measured In 4: 1 ( v h ) ethanol-methanol glass); (c) [Os(phen),lI,; (d) [Os(5,6-Me2phen),l12; (e) [Os(4,7-Me2phenhlIZ; (f) [Os(tPY),lI,.
A Multistate Phenomenological Model. Three cardinal characteristics of the spectroscopic data on these Os(I1)
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980 2071
Charge-Transfer Excited States of Os(I1) Complexes k 3 = k3 r + k3q
T I
'i k L = klr + k Figure 3. Proposed energy-level scheme for emltting charge-transfer states of osmium(I1) arid ruthenium(I1) complexes. Right: definition of radiative, k,, and radietionless, k,, rate constants and energy gaps, Aq, Left: splittings of the lowest d-?r*(a,) charge-transfer manifold in terms of theoretical coupling parameters. Assigned symmetries in D3.
complexes that point to the appropriateness of a thermally equilibrated multiple-state model for the origin of luminescence are the following: (a) the decay times are exponential over the entire temperature range, (b) the spectra show subtle changes with temperature, and (c) the measured limiting life at 4.2 K exceeds the ~ ( 7 7 ) / 4 ( 7 7 ) ratio for three of the substances. For a single emitting level or a cluster o€degenleratelevels the ~(77)/4(77)ratio equals the intrinsic radiative life, i.e., the limiting maximum decay time for the system in the absence of any quenching. Since the measured decay times, which moreover include quenching, obviously exceed this value for several cases, a single-state model for the emission must be abandoned. The changes in the spectra with temperature strongly point toward multiple-state emission also. The apparent red shift is due to the dlisappearance of the first high energy peak edge and the concomitant relative increase in a second peak lying at sliightly lower energy. The intermediate case (Figure IC)clearly shows that the red shift should be attributed to the change in relative emission intensities of at least two states and not to the simple displacement of a single level by environmental effects. That the emitting states are in thermal equilibrium we infer from the exponential behavilor observed throughout. A model incorporating these same features was developed to describe the1 lowest excited states of the analogous Ru(I1) complexes? We adopt it here. The assumed energy level manifold is presented in Figure 3. The physical assumptions underlying the model are that (a) the CT luminescence in thebee D3 complexes is a superposition of the emissions from three closely spaced electronic states, (b) each of the sublevels is capable of coupling with the ground state either radiatively or radiationlessly, and these pathways are controlled by first-order kinetics with temperature-independent rate constants, k, and k,, (c) Boltzmann equilibrium is established and maintained in time domains much shorter than the characteristic lifetimes of any of the sublevels, and (d) the manifold of luminescing states is populated from higher excited states with near unit efficiency at all temperatures, and this efficiency is independent of excitation wavelength. From a mathematical analysis of this model one can derive expressions for the temperature dependences of both the decay time and the quantum yield? The results are eq 1 and 2.
k + 2kZre-Af1/kTf k3,e-A€2/kT 4(T) = -2 kl + 2k2e-AdkT + k3eiA4kT
(2)
These two expressions completely define the properties of the luminescing manifold at all temperatures. In
principle a simultaneous computer fit of both T ( T )and 4(T) to the measured data points displayed in Figure 2 should yield all the empirical parameters defied in Figure 3. The resultant level schemes are included in Figure 2. In none of the cases could a two-level fit of the data be generated; three levels were both necessary and sufficient. Support for a three-level model and the inclusion of the twofold degeneracy factor in the second level appearing in eq 1and 2 comes from the theoretical model discussed below. State Assignments from Ion-Parent Model. A theoretical model to rationalize the existence of three closely spaced excited levels responsible for CT luminescence from da complexes of this type (D3)was first proposed on group theoretical grounds and later quantified mathematically.7-8 The construction of the model is outlined in the previous article? At this time we focus attention on the three lowest levels predicted by the scheme. The mathematics relates the energy separations of three levels to simple functions of three parameters, two exchange integrals, K(al,az) and K(e+,az),and a third parameter, kt,that is related to the degree of spin-orbit coupling manifested in the Os(I1) d6 core when the promoted electron is on the ligand (Figure 3). This model predicts a cluster of three low-lying levels for the emitting manifold as long as excitation to a vacant ?r* orbital of a2 symmetry delocalized over all three ligands is assumed. If a delocalized ?r* orbital of e symmetry is used (the only other possible case), then a cluster of five levels is predicted. Since we only require three levels to fit our data, we opt for d-r*(a2) excitation. Once this choice of d-?r*(az) is made, the mathematics also requires the state of AI symmetry to lie lowest. The disposition of the nearby E and Az levels depends, however, on the relative magnitudes of the three parameters. For all the o(;l(II)complexes described here, the radiative lives (Table 11)clearly show a long-lived state lying lowest, followed by two states of progressively increasing allowedness. We therefore confidently assign the lowest level to AI symmetry, which is dipole forbidden in D3.We also tentatively assign the next two levels as E and Az in analogy with previous work on Ru(I1) complexes. The lowest is (x,y);the latter is z allowed. Experiment confirms that these two levels do radiate substantially faster than the lowest one (Table 11). The data presented here do not entirely exclude the reverse assignment, however (vide infra). The group theoretic labeling of the upper two levels of this first low-lying manifold does not affect, however, the assignmenta of the absorption bands made in the previous paper.2 In fact, once the general validity of the proposed coupling model is accepted, and a d-?r*(az) configuration is ascribed to these lower emitting levels, the model leads inexorably to the clusters of states depicted in Figure 5 of the preceding article2that we directly correlate to the observed longer wavelength absorption bands. Methods of Data Reduction. The methods of extracting energy-level splittings and rate constants from the T vs. T and 4 vs. T curves displayed in Figure 2 were the same as those used to extract these parameters from the analogous curves for Ru(II).69' A multiparameter least-squares curve-fitting program based on the Newton-Raphson method with internally approximated gradients was employed.1° The program is able to handle data in terms of functional forms that contain up to 20 parameters in five independent variables. Initially the lifetime data were analyzed by using the functional form given in eq 1. This analysis produced a total rate constant for each level and the two energy gaps. Next, the quantum yield data were
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The Journal of Physical Chemistry, Vol. 84, No. 16, 1980
Lacky et
81.
TABLE 11: Experimental Parameters for the Lowest d-n*(a,) CTTL Excited States of Osmium(I1) Complexes in PMM compd [ WbPY ) 3 11,
[Os(5,6-Me,phen),] I,
symmetry
Ti, M s
2
0.32 5.05
A,
2 3 A,
[ Os(4,7-Me2phen),']I,
A, a
0.55 5.13 32.8 0.34 1.47 31.6 0.26 2.17 35.5
Tin
Ps
8.36 121.9 333.6 1.16 48.5 591.8 4.19 40.5 486.4 2.25 7.48 276.7 2.49 13.7 405.3
Tiq, Ps
@i
0.33 5.27 11.4 0.024 1.04 9.46 0.627 5.87 35.2 0.40 1.84 35.6 0.29 2.58 38.9
0.038 0.041 0.033 0.020 0.021 0.016 0.130 0.127 0.068 0.152 0.197 0.114 0.104 0.158 0.088
Measured in ethanol-methanol glass (4:1, v/v),
analyzed by using the same computer program. In this case five parameters derived from the lifetime data by use of eq 1 were inserted into eq 2, and the three additional radiative rate constants were determined by a least-squares analysis of the quantum yield data. Simple arithmetic produced the individual level quenching parameters and quantum yields listed in Table 11. For the [0s(tpy),]I2 molecule possessing DZdsymmetry, the group theoretic labels from the ion-parent coupling model for D, symmetry do nbt apply. The phenomenological model does apply, however, and a set of rate constants and energy gaps can be obtained by using eq 1and 2. It is not entirely clear, however, that three levels are needed; two may be sufficient for this substance. The symmetry labeling of these levels awaits additional theoretical developments. The question that naturally arises concerns the degree of confidence one has in the final results. We have discussed this issue previously and have found no definitive method to assess the accuracy of the numbers produced by our curve-fitting procedures.' We point out, however, that a high degree of self-consistency is required of the parameters. The same energy-level gaps and set of total rate constants that are produced from the analysis of a decay curve must also satisfy the quantum-yield expression. This is not a trivial restriction. In spite of the uncertainties inherent in our procedures we assert that the trends in the energy levels and rate constants do indeed reflect the realities of the situation and base our remarks about these Os(I1) systems and the comparisons with the results obtained from our published studies of Ru(I1) analogues on these trends. Since we have no definitive way to assess the accuracy of these numbers, we have chosen to report them in the figures and tables exactly as they were generated by the computer.
Excited-State Properties Although we have relied throughout on the previous analyses of Ru(I1) spectra and employed the same model for the CTTL excited states of the analogous Os(I1) (5dY and Ru(I1) (4d)6complexes, many of the detailed features of the excited states responsible for the emissions observed from the two sets of moleucles are unique to each ion. In the following sections we focus on both their similarities and their differences and attempt to relate the excitedstate properties to current conceptions of bonding in these interesting materials. In what follows we explicitly assume a d-3r*(a2) configuration for the low-lying excited states in both sets of luminescent molecules, adopt the group theoretical ( D J assignments of Al, E, and A2 in order of
increasing energy for the emitting levels, and rely on the semiquantitative validity of the ion-parent coupling model for the emitting manifold as depicted in Figure 3. Rate Constants and Energy Gaps. A comparison of the energy-level schemes reproduced in Figure 2 and the rate constants in Table I1 with those obtained previously for the Ru(I1) analogues9shows the essential similarities between the two sets of complexes. In all cases a long-lived level lies lowest, a shorter-lived one lies nearby, and a still faster decaying level at higher energy completes the set. This trend in measured lives is uniform in both the 0411) and Ru(I1) data. Without exception it is reflected in the radiative lives of the states also. In fact, although the measured lives of the Al levels in Os(I1) complexes are substantially shorter than the corresponding ones of Ru(II),the radiative lives of the corresponding levels on both systems of molecules are comparable. Indeed, the real differences between the two sets of molecules show up in the quenching parameters, riq. The lower luminescence yields of the Os(I1) compared to the Ru(I1) complexes are reflected in the very low values of the quenching lives for the former set. The predominance of radiationless processes in the Os(1I) species is also reflected in the quantum yields from the individual levels. They are substantially smaller than those for Ru(I1). In fact, only in the quantum yields from individual levels does one discern any differences in trend between the results from the complexes of the two elements. Whereas for all Ru(I1)molecules, except one, the quantum yields increase in step with the energies of the levels, for Os(I1)several minor reversals occur. This result is a consequence of the slow falloff of the relative luminescenceyields from 40 to 77 K of the latter molecules; the Ru(I1) species, however, usually tend toward a maximum yield as the temperature rises toward 77 K. We conclude, therefore, that the clusters of emitting levels in both sets of substances can be reliably described as possessing essentially the same radiative properties. Thus, the same symmetry assignments are dictated for both sets of levels. The big differences are attributable entirely to enhanced radiationless rates in the (5d)6systems. For both sets of complexes, however, there is a parallelism between the radiative and radiationless rate constants for the individual levels. We infer that the matrix element responsible for the radiative decay of a given level is related to those controlling its radiationless relaxation to the ground state. This is a result that must be factored into any fundamental discussion of radiationless processes in these materials. When one compares the rate constants and quantum yields within a particular series, a dependence on molecular
Charge-Transfer Excited States of Os(I1) Complexes
structure becomes apparent. Just as was found for Ru(I1) complexes, the Os(I1) entities with the 2,2’-bipyridine backbone undergo more quenching than those containing the 1,lO-phenanthroline moiety. The stiffness and forced planarity of the latter ligands evidently slow down the radiationless processes somewhat. A comparison of‘ the energy-level gaps presented in Figure 2 with thosle for Ru(I1) (ref’ 9, Figure 2) reveals essentially the same pattern; a smaller gap separates the first two levels than separates the second and third states. The pattern is repeated for every molecule. For Ru(I1) species the A1-E gap is remarkably constant. For the Os(I1) analogues two salient differences show up: the A,-E energy gap is substantially greater than the corresponding Ru(I1) split (-30 vs. -10 cm-l), and the entire level pattern is highly sensitive to substitution on the ligands. A peculiar finding is that the two parent complexes, [0s(bpy),l2+ and [ Os(phen),12+, have clusters that are substantially compressed in comparison with the substituted analogues. This situation does not occur for ruthenium. We do not believe this result can be attributed to the obviously poorer data that we have for these two systems compared with the others. We have, however, no explanation for it. The comparable magnitudes of the radiative rate constants found experimentally for the lowest levels of both Os(I1) and Ru(I1) species leads one to expect comparable oscillator strengths for the inverse absorption processes. For Os(I1) the corresponding absorption peak is distinct, and its area leads to a value for the radiative life of the collection of upper states constituting it that agrees well with the rate constants obtained from the analysis of decay time and intensity data., Yet, no obvious peak overlapping the emission exists for Ru(I1) complexes. This apparent inconsistency is independent of the model assumed for the excited states; it is based on the validity of the fundamental equation connecting radiative lives to absorption intensity, a relationship we fundamentally accept. The absence of any definite peak overlapping the luminescence for Ru(I1) complexes can be traced, we believe, to the degree of vibronic coupling manifested in the transitions. Apparently, for Os(I1) complexes, a substantial fraction of the intensity is c(arriedby the 0-0 transitions of the E and A, states; wherieas, for Ru(I1) species, the intensities are smeared out over a broad diffuse band envelope. Exchange Integrals, Spin-Orbit Coupling, and d-Orbital Expansion, 14s mentioned above, the ion-parent coupling model leadls to a description of the splitting5 for the lowest cluster of excited states in terms of two exchange integrals and a mixing parameter. The relevant expressions are derived in ref 8 and are included for convenience on Figure 3. In these formulas K(al,a2)and K(e+,a2)are measures of the nonclassical Coulombic interactions in the excited configuration between the promoted electron residing in the lowest .*(a2) orbital on the ligand system and the electrons remaining in the a1 and e+ metal orbitals, respectively, on the d5 core. The al metal orbital extends along the unique axis of the D3 complex, and the e+ orbital is directed perpendicular to the principal axis, i.e., out into the 7r-conjugated ligands. The parameter, AI2, is dependent on the degree of spin-orbit coupling within the d5 core remaining after excitation. It becomes unity in the limit of vanishing spin-orbit interaction. Since we have two measurable energy gaps and three parameters, the system is obviously overdetermined. Within the context of the ion-parent coupling model the value of k12 can be obtained from spin-resonance measurements on the ground states of the corresponding
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980 2073
one-electron oxidized complexes. For the Ru(I1) species a value of kI2 = 0.84 was derived from published measurements and inserted into the energy-level splitting equations. The observed energy-level separations led to average values of K(al,a2) 35 cm-l and K(e+,a2) 60 cm-l for the s e r i e ~ .For ~ the analogous Os(I1) complexes investigated here no independent measurement of k12is at hand. Because of the structural similarities between the two sets of molecules we assume that K(al,a2)/K(e+,a2) ratio to be identical in the two series (-0.60). The experimental energy then leads to a unique value for k12 for each of the complexes. The parameter ranges from 0.70 to 0.81. We adopt the mean value of 0.75. By insertion of this value into the splitting formulas we calculate average values of K(al,a2) -70 cm-l and K(e+,a2) 115 cm-l for the Os(I1) series. Thus, within the context of the coupling model used to describe the low-lying CTTL excited states of these two sets of complexes, the experimental results point to a higher degree of spin-orbit coupling on the metal core [kI2(Os)< k12(Ru)]and larger exchange interactions for the (5d)6systems. The first result is intuitively reasonable, since the spin-orbit coupling constant for the Os(I1) ion is at least a factor of 2 greater than that of Ru(I1). The second result, the higher exchange terms, we attribute to the greater extension of the 5d orbitals into the ligand system of a complex compared to the 4d set. The extension of the d orbitals of the central metal ion into the ligands in both the Ru(I1) and the Os(I1) complexes determines the properties of these substances in many ways. The very existence of intense charge-transfer transitions signals substantial overlap between ground and excited orbitals. Furthermore, the extensive a backbonding postulated for these systems can be realistically described as a substantial mixing of excited d-a* configurations into their ground states, a feature that translates into considerable d-electron diffusion onto the ligands. Another manifestation of this orbital extension onto the ligands is the spectroscopic requirement, for Ru(I1) and Os(I1)complexes, that the excited a* orbital be delocalized over the entire ligand system. For d6 systems where this is not required spectroscopically,such as Rh3+complexes, no low-lying charge-transfer states occur either.11J2 The greatly enhanced quenching rate constants for Os(I1) complexes over those for Ru(I1) species also point toward considerably more 5d over 4d extension onto the ligands. Radiationless transitions require coupling to vibrational modes and extension of the excited system over more nuclei intuitively leads to greater coupling also. A final spectroscopic indicator of the increased diffusion of the 5d electrons of Os(I1) onto the a-conjugated ligands over that of the Ru(I1) electrons is the substantial gap that appears between the onset of the strong absorption bands in both Os(I1) and Ru(I1) complexes and the emission bands. In our view this gap is a measure of the ligandligand interaction modulated through the d orbitals of the metal atom (see ref 2). It is 25% larger in Os(I1) complexes than in those of Ru(I1).
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Extension to Other (nd)6Systems We have introduced a model for CTTL excited states of these complexes that offers a new description of such transitions. According to this view spin-orbit coupling on the (nd)5core plays an important role in controlling the disposition of the low-lying (nd)6excited states. These states are separated into clusters by a combination of spin-orbit coupling, interelectronic interactions, and the ligand field; each cluster is further split a small amount by the weak electrostatic interactions between the electron
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J. Phys. Chem. 1980, 84, 2074-2078
promoted onto the ligand system and those still remaining on the ( r ~ dcore. ) ~ The final observed electrostatic splitting of a given cluster of states is -100 cm-', whereas the contribution of spin-orbit coupling in the Hamiltonian of the (ndI6 system that separates the clusters is an order of magnitude larger than this. This view is just the reverse of the usual descriptions of excited states in which the gross splittings that produce the states from a given configuration are electrostatic and spin-orbit coupling plays a minor role in determining energies.13 We have used this ion-parent coupling model successfully to rationalize the existence, splittings, and behaviors of the low-lying CTTL excited states of Ru(I1) and Os(I1) complexes. Initially, because of the enormous increase in spin-orbit coupling that occurs when one switches from Ru(I1) to Os(II), we expected the coupling model to be far better suited for complexes of the heavier atom. Surprisingly, this did not happen. The expected dominance of spin-orbit coupling in Os(I1) complexes was mitigated by the greater extension of the 5d electrons onto the ligand system over that of the 4d set. This brings us to the central question: how does one choose between a singlet-triplet and an ion-parent coupling language to describe CTTL excited states? For a given molecule, experiment will dictate in the end, but some assertions can be made. A critical factor is the formal oxidation state of the central metal ion; a high formal oxidation state favors the coupling model. Important also is the magnitude of S; the spin-orbit coupling constant of the metal ion; the higher this constant is, the more appropriate becomes the ion-parent coupling description for the CTTL states. Thus, for complexes of this type involving the (5d)6 ions Pt(IV), Ir(III), Os(II), Re(I), and W(O), we expect the coupling model to work best for Pt(IV) but to fail as a first-order description for W(0) molecules. The CTTL excited states of these latter species are probably better described by the usual singlet-triplet formalism. For Re(1) we expect an intermediate situation to prevail.
An important manifestation of the degree of spin-orbit coupling in CTTL excited states of Ru(I1) and Os(I1) complexes of the type reported here, and also those of Ir(III),14is the rapid relaxation that occurs among the levels in the excited state manifold. All our decay-time measurements on these systems produced single exponentials even at temperatures of less than 2 K. Our conclusion is that equilibrium among the levels is maintained at all temperatures at all times. This behavior contrasts sharply with that displayed by organic systems, for which equilibrium is not maintained below -7 K.I3 In our view the substantial spin-orbit interactions in the complexes effectively tie the spins to the molecular framework and provide a mechanism for facile vibrational-electronic relaxation. Note Added in Proof. See Note Added in Proof of paper 1. References and Notes (1) (a) Research sponsored by the Air Force Offlce of Scientific Research, Air Force Systems Command, USAF, under Grant No. AFOSR 762932; (b) abstracted in part from the dissertation of D. E. Lacky submitted to the Graduate School of Washington State University In partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1975; (c) US. Senior Scientist (Humboldt Awardee), Unlversltat Hohenheim, West Germany, 1978-9. (2) B. J. Pankuch, D. E. Lacky, G. A. Crosby, J. phys. Chem.,preceding paper. (3) W. R. Ordorff and A. J. Hemmer, J . Am. Chem. Soc., 49, 1272 (1927). (4) J. N. Demas and G. A. Crosby, J. Am. Chem. Soc., 92, 7262 (1970). (5) J. N. DemasandG. A. Crosby, J. Am. Chem. Soc., 93, 2841 1971). (6) G. D. H aw and G. A. Crosby, J. Am. Chem. Soc., 97, 7031 (1975). (7) R. W. Harrigan, 0. D. Hager, G. A. Crosby, Chem. Phys. Lett., 21, 487 (1973); R. W. Harrigan and G. A. Crosby, J . Chem. Phys., 59, 3468 (1973). (8) K. W. Hipps and G. A. Crosby, J. Am. Chem. Soc.,97, 7042 (1975). (9) G. D. Hager, R. J. Watts, 0. A. Crosby, J . Am. Chem. Soc., 97, 7037 (1975). (10) M. J. D. Powell, Comput. J., 7, 303 (1965). (11) G. A. Crosbyand W. H. E M g , Jr., J. phys. Chem., 80, 2206 (1976). (12) M. K. DeArmond and J. E. Hlllis, J. Chem. Phys., 54, 2247 (1971). (13) M. A. El-Sayed, Acc. Chem. Res., 1, 8 (1968). (14) R. J. Watts and 0. A. Crosby, unpublished work, this laboratory.
Nonradiative Relaxation Process of the Higher Excited States of Meso-Substituted Anthracenes Kumao Hamanoue, Satoshi Hirayama,+ Toshlhiro Nakayama, and Hlroshl Teranlshi Deparlment of Chemistty, Faculty of Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan (Received: December 31, 1979)
Using pico- and nanosecond spectroscopic methods, we have measured the time-resolved absorption spectra for 9-nitroanthracene, 9-benzoyl-lO-nitroanthracene,and 9-cyano-10-nitroanthracene.Rather long buildup times of the triplet-triplet absorptions (72-86 ps) lead us to the conclusion that the observed buildup times do not reflect the lifetimes of the singlet states but might represent the rates of the internal conversion in the triplet manifold and that the indirect intersystem crossing S1(m*) Tn(nr*) Tl(m*) is the most important process to populate T1 in accordance with El-Sayed's rule.
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Introduction Recently, the study of the nonradiative process of the fluorescing state in anthracene derivatives has received considerable attention from both experimental and t h o retical points of view.' For example, Bennett and 'Faculty of Textile Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan. 0022-3654/80/2084-2074$01.00/0
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McCartin2 have concluded that, in those anthracene derivatives which show a significant variation of fluorescence yield with temperature, radiationless deactivation proceeds entirely by intersystem crossing (isc). The rate of isc is proportional to the vibrational overlap factor whose magnitude increases rapidly with decreasing energy gap between the two interacting states.3 As a result, the number and the order of the higher triplet state(s) lying 0 1980 American Chemical Society
