J. Phys. Chem. 1986, 90, 6425-6430
6425
Phosphorescence and Zero-Field Optically Detected Magnetic Resonance Studies of the Lowest Excited Triplet States of Organometallic Diimine Complexes. 1. [Rh(bpy),13+ and [Rh(phen),l3+ Yasuo Komada, Seigo Yamauchi,* and Noboru Hirota* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, 606, Japan (Received: May 20, 1986)
We have made spectroscopic and optically detected magnetic resonance studies on 37r7r* [Rh(bpy),I3+ and [Rh(phen),] )+ at liquid helium temperature. We have observed a well-resolved phosphorescence spectrum of [Rh(phen),] (BF.,), that resembles that of ],lo-phenanthroline except for changes in some vibrational frequencies. The triplet lifetimes of the Rh complexes were compared with those of the Cd and Ir complexes in the light of the effect of d r * states. Zero-field (zf) ODMR signals of the ,UT* Rh complexes were observed, and triplet sublevel properties, such as zf splitting (zfs) and radiative and nonradiative decay rate constants, were obtained. Very interestingly, small zfs (3-5 GHz) were observed, contrary to previous estimates. Rationalization of this observation is given. On the basis of the obtained data we discuss the structures, zf schemes, and radiative and nonradiative mechanisms in the ,TU* Rh complexes.
Introduction Among the various kinds of organometallic complexes, those of nd6 configuration have attracted much attention because of their activity in charge-transfer reactions and possible utility for solar energy conversion. Different combinations of coordination metals and aromatic ligands give rise to different characters of the lowest excited states depending on the ligand-field parameters and magnitudes of oxidation potentials of the metal ions. The lowest excited UP*,d t * , and dd* states have been found for different complexes of well-known diimine ligands such as 1,lOphenanthroline (phen) and 2,2'-bipyridine (bpy). The Rh(II1) and Ir(II1) tris diimine complexes are believed to possess 3 ~ type lowest excited triplet (TI) states while the corresponding Ru and Os complexes are considered to have TI (dr*) states. There are, however, many unresolved problems concerning the spectroscopic and magnetic properties of the T1 states of these organometallic complexes. In a series of reports we will present the results of our optically detected magnetic resonance (ODMR) and spectroscopic studies on these diimine complexes at liquid helium temperature in which attention is focused on the properties of the triplet spin sublevels. We begin with the less perturbed 3 ~ systems. ~ * Rh complexes such as [Rh(phen)J3+ and [Rh(bpy),I3+ have been investigated extensively as prototypes of the least perturbed 3 ~ 7 r *organoFrom these invesmetallic complexes of d6 configuration.'-'' tigations it now seems to be established that excitation io localized on ligands, especially on a single ligand molecule, to produce 3 ~ states of ligands that are only slightly perturbed by chelating Rh(II1) ions. The localized character of the excitation was nicely demonstrated by an experiment for mixed-ligand complexes, [Rh(bpy),(phen)13+ and [ R h ( b p ~ ) ( p h e n ) ~ ] in ~ + yhich , dual emissions, one due to bpy and the other due to phen, were observed2with the spectra very similar to those of the tris complexes. The interpretation of the other triplet properties is, however, less certain. The triplet lifetimes are very short, being 1/30 to 1/100 of those of free phen and bpy. It was also suggested that the zero-field splitting (zfs) parameters of the T, state increase remarkably by complex formation to the order of 10-100 cm-I.'O (1) (2) (3) (4) (5) (6)
DeArmond, M. K.; Hillis, J. E. J . Chem. Phys. 1971, 54, 2247. Halper, W.; DeArmond, M. K. J . Lumin. 1972, 5, 225. Carstens, D. H. W.; Crosby, G. A. J . Mol. Spectrosc. 1970, 34, 113. Crosby, G. A. Acc. Chem. Res. 1975, 8, 231. Crosby, G. A.; Elfring, W. H., Jr. J . Phys. Chem. 1976, 82, 2206. Watts, R. J.; Van Houten, J. J. Am. Chem. Soc. 1978, 100, 1718. (7) DeArmond, M. K.; Carlin, C. M.; Huang, W. L. Inorg. Chem. 1980, 19, 62. (8) DeArmond, M. K.; Carlin, C. M. Coord. Chem. Reu. 1981, 36, 325. (9) Hillis, J. E.; DeArmond, M. K. J. Lumin. 1971, 4, 273. (10) Halper, W.; DeArmond, M. K. Chem. Phys. Lett. 1974, 24, 114. (1 1) Nishizawa, M.; Suzuki, T. M.; Sprouse, S.; Watts, R.J.; Ford, P. C. Inorg. Chem. 1984, 23, 1837.
0022-3654/86/2090-642S$Ol . S O / O
~
~
Although these two properties were explained by the vibronic and/or spin-orbit couplings between the TI and ls3dn* states, convincing evidence for these arguments is lacking. In particular, no definitive evidence for the large zfs has been given. Therefore, further studies are needed to understand these systems satisfactorily. Broadness of the phosphorescence spectra is another feature of these systems that has prevented detailed discussion about the structure of the TI state. In order to answer these unresolved problems and to clarify the uncertain properties of 3 ~ [Rh~ * (phen),I3+ and [Rh(bpy),I3+, we have made various spectroscopic studies at liquid helium temperature. * The most important feature of our work is the observation of the zf ODMR signals, which represents the first successful ODMR experiment on Rh complexes. The observed small zfs unambiguously deny the previously suggested possibility of very large zfs for these systems.I0 The other important result is the first observation of a well-resolved phosphorescence spectrum for the 3 ~ Rh complexes, which enables us to analyze the vibrational structure. We have also observed the phosphorescence excitation spectra and examined the triplet properties of the 3 m * Ir(II1) and Cd(I1) tris diimine complexes at 4.2 K so as to ascertain the effect of the d r * states. From these data we discuss the structures of the Tl states, the importance of the interaction with the d r * states, the small zfs, and the symmetries and ordering of the spin sublevels. Last we analyze the radiative and nonradiative mechanisms of the triplet sublevels of these Rh complexes on the *basis of the kinetic data obtained by ODMR and time-resolved electron paramagnetic resonance (TREPR) experiments.
~
Experimental Section [Rh(bpy)JCI3 and [Rh(phen),]Cl, were prepared by the method of Harris and Mckenzie,I2 [Rh(bpy),](BF,), and [Rh(phen),] (BF,), were made from [Rh(bpy),]Cl, and [Rh(phen),]Cl,, respectively, by treating with NaBF4. [ I r ( b ~ y ) ~ ] (NO,), was prepared by the method of Flynn et al.I3 Phosphorescence spectra were normally taken at 4.2 K with a Spex Model 1704 1-m monochromator and a 900-W xenon arc lamp as an excitation source. Excitation spectra were obtained by exciting samples with light from the Xe lamp monochromatized by the the same monochromator and by monitoring the total phosphorescence emission at 4.2 K. The zf ODMR experiment was performed at 1.4 K. Our zf ODMR setup used in the present work is essentially the same as that described in a previous pape.r.I4 In order to enhance the signal-to-noise ratio of the ODMR signal, the following device was made for a sample container. A microwave helix containing a quartz tubing was inserted into (12) Harris, C. M.; Mckenzie, E. D. J. Inorg. Nucl. Chem. 1963, 25, 171. (13) Flynn, C. M., Jr.; Demas, J. N. J . Am. Chem. SOC.1974, 96, 1959. (14) Cheng, T. H.; Hirota, N. J . Chem. Phys. 1972, 56, 5019.
0 1986 American Chemical Society
*
6426
Komada et al.
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 a
TABLE I: Vibrational Analyses of Phosphorescence Spectra of Neat Crvstals of IRh(phen)J(BFd)*and Free 1.10-Phenanthroline at 4.2 K
[Rh(phen)3l(BFd3 b/cm-' intensity 0 (21 864)
vs
interpretation 0-0
161
VW
283
m
fund.*
343 436
W
Rh-N
S
fund.
604 739 784 876
Vw
1210 1308
w VW
W
m
fund.
1579
W
1624
m m
2332 2521
2670 2765 2903
X
fund.
S
1895 2069 2190
+ 343
W
1455
1750
436 436
2
fund. 436 436
+ 1210 + 1308
W
+ 1455 436 X 2 + 1308 436 X 2 + 1455 436 X 3 + 1210 436 X 3 + 1455
W
1455 X 2
m W VW VW
vw vw
436
phen intensity &/cm-' 0 (22 157) 25 132 248
vs
414
vs
546 607
W
713 827
W
1
m W W
W S
959 1057 1146
vw
1194 1251
VW
1299 1407 1450 1506
m
vw W W
460
460
500
WAVELENGTH /nm Figure 1. Phosphorescence spectra of the neat crystals of (a) [Rh(phen),](BF,), and (b) 1,lO-phenanthrolineat 4.2 K. "*
m W
1
W
1557
W
1616 1646 1709
W
1817
m
1862 1920 2038
W
2226 2439
W
S W
a
W
m
W
Fundamental. another quartz tubing, making the space between the two quartz tubings a sample compartment. This produces better transmission of the exciting light and increased efficiency of microwave irradiation. To determine the triplet sublevel properties, total decay rate constants k, ( i = x, y , z ) , and relative radiative decay rate constants k{(rel), we used the fast-passage ODMR originally developed by Winscom and Maki.Is The experiments were performed at the 0-0 bands of the phosphorescence at 1.4 K. For the TREPR experiments Rh(II1) complexes were excited at 308 nm by a Lumonics T E 861 M excimer laser with a XeCl fill. Transient EPR signals were detected with a JEOL-FE3X EPR spectrometer without field modulation. The other apparatus were the same as described elsewhere.I6
Results We have observed the phosphorescence spectra of various Rh(II1) complexes and the free ligands bpy and phen at 4.2 K. The observed spectra are the same as those reported previously. However, for the first time we observed a well-resolved phosphorescence spectrum in the neat crystal of [Rh(phen),] (BF,), as shown in Figure la. The vibrational analysis of this spectrum is listed in Table I together with that of phen. The vibrational structure of the complex is very similar to that of the free ligand as seen in Figure 1, though some of the vibrational frequencies of the bands are changed by complex formation (Table I). We have observed the phosphorescence excitation spectra of the Rh(II1) complexes at 4.2 K. A typical spectrum is shown in Figure 2a. The positions of the 0-0 bands and the triplet lifetimes are summarized in Table 11. In order to see the effect of the d r * state we also examined the T, properties of the Cd and Ir com(15) Winscom, C. J.; Maki, A. H. Chem. Phys. Lett. 1972, 12, 264. (16) Terazima, M.: Yamauchi, S.; Hirota, N. Chem. Phys. Lett. 1983, 98, 145.
4 50
350
WAVELENGTH /nm Figure 2. Phosphorescence excitation spectra of (a) [Rh(b~y)~]Cl, and (b) [Ir(bpy)3](N03)3 in ethanol at 4.2 K. The dotted line indicates an absorption spectrum of the same system at 77 K. TABLE 11: Positions of the 0-0 Bands of Phosphorescence and Triplet Lifetimes system 0-0 band/nm i/ms bpy (durene) 432 819 [Cd(bPY)SI(NO,),(neat) 433 321 [Rh(bPY)3lC13 (neat) 452 3.6
[Ir(bpy),l(NO3)3
450
[Rh(phen),]CI, (neat)
45 1 457
(4:l methanol-water) phen (neat)
0.049 1400 30.6
plexes (Figure 2 and Table 11). A typical ODMR signal is shown in Figure 3. The time dependence of the signal under the fast passage condition is given by AZO(?)= A exp(-k,?) - B exp(-k,?) where A and B are quantities related to the radiative decay rate Therefore, k,' and k,' were determined constants by AIB = k;/k;. from the analysis of the obtained fast-passage signals. The sublevel schemes were determined from the resonance frequencies as discussed in the next section. The results of the ODMR exper-
ODMR Studies of [Rh(bpy),I3+ and [Rh(phen),I3+
>-J
t u) z
W
+
z_
I 0
40
20
60
Time,"
Figure 3. A Fast-passage O D M R signal of a neat crystal of [Rh(bpy)3]C1, observed a t 1.4 K by sweeping the microwave through the resonance frequency of 2.03 GHz.
iments are summarized in Figures 4 and 5. The TREPR spectrum of [ R h ( b ~ y ) ~ ] C in lethanol ~ observed at 4 K and at 0.4 ps after the laser pulse is shown in Figure 6, where each stationary field is indicated in the figure. The polarities of the signals at the stationary fields are E,E,A, E,A,A (EEA/ EAA) from the low field to the high field, where E and A denote emission and absorption of the microwaves, respectively.
Discussion Phosphorescence Emission and Excitation Spectra. Although the phosphorescence spectra of Rh( 111) complexes have been reported for many systems, all the spectra showed several peaks with broad bandwidths. Therefore, there have been no detailed discussions of the vibrational structures in their spectra. As seen from Figure 1 the main feature of the phosphorescence spectrum of [Rh(phen),](BF,), is very similar to that of free phen, indicating as in free phen. However, that the TI state of the complex is the position of the 0-0 band and the vibrational frequencies of the main bands are considerably different from those of free phen: 414, 1299, and 1604 cm-' vibrations in free phen change to 436, 1210, and 1455 cm-I in the complex, respectively (Table I). The 436-cm-' vibration is assigned to a ring-bending vibration, while the other two are to C=C(N) stretching vibrations. Combined effects of the changes in force constants and effective masses due to complex formation presumably produce these changes. Another feature of the structured spectrum is the appearance of a vibronic band of 343-cm-' vibration. This vibration is assigned to the Rh-N stretching vibration by referring to the assignment by Kong and Lindner on the basis of IR spectra of the bpy complexes [M(bpy)J"+ (M = Cr, V, Ti).I7 The main features in the phosphorescence spectrum of [Rh(bpy),] (BF,,), are also similar to that of bpy in durene, but the spectrum is much broader with a red shift of the 0-0 band by 1100 cm-I and changes in the vibrational frequencies of the main bands. Summarizing these results, we can conclude that the phosphorescence spectra of Rh(II1) complexes of %a* character are very similar to those of the uncoordinated ligands phen and bpy, except that the appearance of the Rh-N stretching bands and considerable changes in vibrational frequencies were observed by chelation with Rh(II1). In the phosphorescence excitation spectra observed at 4.2 K we tried to find locations of the 331da*states. However, we could not observe any evidence for the presence of the 'da* state below the Ira* state as shown in Figure 2a. In Ru(I1) and Ir(II1) complexes 's3da* states are easily observed in the phosphorescence excitation spectra as typically shown in Figure 2b. Furthermore, the absorption spectra of the Rh(II1) complexes observed at 77 K (denoted by a dotted line in Figure 2) and room temperature3s5 do not show any peak corresponding to the Ida* state ( e > 500 M-' cm- L). Therefore, we conclude that the Id** states are located above the lowest singlet ax* states in all of these Rh(II1) complexes. Slight increases in the phosphorescence intensity
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 6427 (Figure 2a) just below the 'aa* state might be due to the Idd*, 3dd*, or ,da* states, though they do not show any peaks. Importance of the da* State. As we could not determine the locations of the dn* states, we were not able to show the importance of the d a * states in determining the triplet properties of the Rh(II1) complexes only from the spectroscopic data. We try to compare the lifetimes among the different transition-metal complexes where the locations of the d a * states are expected to be very different and show the importance of the effect of the da* state on the TI properties of the Rh complexes. In [ C d ( b ~ y ) ~ ] ~ + , where Cd(I1) has a configuration of dIo, the d a * states are expected to be located very high in energy due to the fully occupied d orbitals. The phosphorescence excitation spectrum of [Cd(bpy)12+certainly resembles that of free bpy. In contrast, in the case of [Ir(bpy),](NO,), in ethanol we could recognize a peak below the Ian* state that could be assigned to the 3 J d r * state (Figure 2b).I8 Therefore, energies of the ,,Ida* states of the Cd(II), Rh(III), and Ir(II1) complexes decrease in this order. The lifetimes ( T ) of these 37r7r* complexes are -321 ms, 3.6 ms, and 49 ps, indicating a qualitative correlation between T and AEST (=ElIda*] - E(3aa*]).Judging from the results of our previous work on the external heavy atom effect, we expect the ordinary external heavy atom effect to reduce the triplet lifetimes of the aromatic molecules to the order of 100 ms in the Rh and Cd complexes and to 10 ms in the Ir complexes, re~pective1y.l~ Therefore, it seems certain that the short lifetimes of the 3aa* Rh and Ir complexes are due to interactions with the da* states. zfs and the Structure of the TI State. Halper and DeArmond tried to observe the triplet-state EPR of the Rh complexes, but they failed to detect EPR signals at both X and K bands. On the basis of this result they concluded that the zfs parameters D for these complexes are 1-2 orders of magnitude greater than those of the free ligands.I0 Our obtained zfs, however, deny their conclusion and establish the fact that the D values are of the same order of magnitude as those of the free ligands. Therefore, zfs do not vary much with coordination to metals, which in other words means that the contribution of the spin-orbit coupling to zfs is very small in the Rh complexes. The changes in zfs observed for the systems with different counterions seem to reflect the changes in the spinspin couplings because there is no correlation between the changes in zfs and the lifetimes. In any case zfs of the Rh complexes are also similar to those of the free ligands just as the vibrational structures of the phosphorescence spectra are. A negligible effect of the spin-orbit couplings to zfs can be easily rationalized in the following way. We first make an order of magnitude estimate of the spin-orbit coupling matrix element between the Tl(aa*) and the Ida* state from the radiative decay rate constant k,' of the T I state. k,' is generally given by
-
(17) Kong, E.; Lindner, E. Spectrochim. Acta, Part A 1972, 28A, 1393.
where k i is the radiative decay rate constant of the perturbing singlet state SI and the other symbols have their usual meanings. We first assume that the Ida* state is situated 10000 cm-I (>E("*) - E(,aa*) 8500 cm-') above the TI state and k i of Id** is the same as that of [Ru(bpy),](BF,), ( k i lo8 s-I) estimated from the absorption spectrum.*O Combining these values with k,' = 90 s - I , ~ ' we obtain ( H s o ) 16 cm-l. The second-order effect of the spin-orbit coupling to the zf sublevel is given by
-
-
-
(18) Watts, R. J.; Harrington, J. S.;Van Houten, J. J . Am. Chem. SOC. 1977, 99, 2179. (19) Komada, Y.; Yamauchi, S.; Hirota, N. J . Chem. Phys. 1985, 82, 1651. (20) Yamauchi, S.; Komada, Y.; Hirota, N. Chem. Phys. Lett. 1986,129, 197. (21) k,' = 1 / X~ 9,~(-0.29 and a,,, = 1 for [Rh(bpy)J'+).
6428
Komada et al.
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 phen (ethanol)
(neat)
1
[ Z n ( b p y ) 3 1( N 0 3 1 2
(
[ R h ( p h e n )3 1 ( B F q )
[Rh ( p h e n ) 31 C13
[ R h ( p h e n ),lC13
ki/s-l
(neat)
ki/s-l
kr (0-0:r e l )
kr (0-0I r e l )
16
14 2 . 0 GHz
2 . 0 8 GHz
2 . 2 GHz
2 . 9 1 GHz
4 . 4 GHz
60
30
2 . 9 2 GHz
1.0
x39,6 ms
Tp=1.4 s
(ref
45
0.90
30.6 ms
2 6 . 7 ms
.24I
Figure 4. Zero-field splittings and dynamic properties of triplet [Rh(phen)J3+. The data are compared with those of free phexZ4 bPY (durene)
0.84 GHz
1
1 . 0 5 GHz
3 . 5 5 GHz
0.2 1 . 3 4 GHz
100
3 . 2 9 GHz
3 . 8 0 GHz
3 . 0 4 GHz
1
580
0.59
Y-i-
z-
2 . 0 3 GHz
I t
I
3 . 4 2 GHz
2 . 0 8 GHz 170
xx-
1.0 x-
I
t p = 8 1 9 ms 3 2 1 ms 3.6 ms 3.1 ms Figure 5. Zero-field splittings and dynamic properties of triplet [Rh(bpy)J3+, [Cd(b~y)~]*+, and free bpy. Bpy in durene is in a trans conformer.z4
Hml"
ABS
2000
3000 MAGNETIC
4000
FIELD / G
Figure 6. Time-resolved EPR spectrum of triplet [Rh(b~y)~]Cl, in ethanol observed at 4 K and 0.8 1 s after the laser pulse.
- -
-
where AETTdenotes the energy difference between the T,(mr*) and 3da* states. With AEsr 10000 cm-, AETT 8000 cm-', cm-l (-0.2 and ( B i ) = 16 cm-', eq I1 gives A€,, 6.4 X GHz). This estimate is of course very crude, but it clearly shows that the contribution of the spin-orbit coupling to zfs is negligibility small. Assignments of T I Symmetry and Sublevel Ordering. An important conclusion derived from the work on the mixed-ligand complexes is that the excitation is localized in the single ligand (bpy or phen) even in the tris complexes. All our results are consistent with their conclusion as described above. Therefore, we adopt the monochelated complex model developed by Ceulemans and Vanquickenborne22and Kober and M e ~ e to r ~analyze ~ our results. The symmetry of the monochelated complexes is considered to belong to the C2Lpoint group, which is the same (22) Ceulemans, A.; Vanquickenborne, L. G. J . Am. Chem. SOC.1981, 103, 2238. (23) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1984, 23, 3877.
Figure 7. Two kinds of molecular axes of Rh(II1) complexes. See the text for the definition. as those of the free ligands (bpy and phen) themselves.
We first discuss symmetries of the T I ( m * ) states of [Rh-
hen)^]^' and [ R h ( b ~ y ) ~ ]where ~ + , molecular axes are taken as shown in Figure 7. As described in the previous section the phosphorescence emission and excitation spectra as well as the absorption spectra3 of the Rh(II1) complexes resemble closely those of the free ligands. The maximum zfs ([Dl + IEI) of TI [ [ Rh(phen),13+ and [ R h ( b ~ y ) ~ ]which ~ + , are 4.4-5.5GHz and 3.1-3.4 GHz, respectively, are also very close to the values of free phen (4.6 GH)24 and bpy (3.4 GHz). The 0-0 bands of the phosphorescence (Table 11) do not vary much with coordination to Rh(II1) metal. On the basis of these results we conclude that the (24) Gondo, Y.; Maki, A . H. J . Phys. Chem. 1968, 72. 3215.
ODMR Studies of [Rh(bpy),13+ and [ R h ( ~ h e n ) ~ ] ~ +
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 6429 SCHEME I: Radiative Mechanisms for the 3B2(nbl~*,2) State
[Rh(~hen)~I~+
3B2(nb17r*a2) ki(0-0:rel)
2.0 GHz
+
kf (0-0:rel)
1-
o'2
4 -7-
2 . 4 GHz
SO
'See text.
GHz
2.4
0.9
1.0
T~ & 1A2(dalB*,2) diplc.forbiddcn T, Eu- no dn* state' 7 ' E a IAl(da2n*,2) So
SCHEME I 1 Radiative Mechanisms for the 3 B 2 ( ~ p 2 ~ *State bl) 0.9
3B2(*aZ~*bl)
2.0 GHz
1
0.2
Ty T, %L, T, u !-
no dn* state" a-nd dipole-forbidden 'Bl(d,,**b,) So 'A,(db,T*b,) so
5
See text.
where the metal-fixed x', y', and z'axes are taken as shown in , Figure 7. As we consider the two kinds of A* orbitals, A * ~ and ?T*~ six~ kinds , of d?r* states are involved in our systems. They are B2(db,A*az),Al(da2r*a2),A2(dalr*a2)3Al(dblA*b,),B2(da2A*bl) orbital character of the TI state of each Rh(II1) complex is the and B l ( d , , ~ * b ,states. ) The TI states are Tl(phen) = a(,B2same as that of the corresponding free ligand. The symmetry of (rbla*az)) + b(3Bz(*a2~*b,)) (a > b) and Ti(bpy) = 3B2(Aa2a*bl) both TI phen and bpy belongs to ,B2(7rr*), though the orbital as discussed in the previous section. The radiative mechanisms symmetries are somewhat different.25-27 In phen two configufor the 3 B 2 ( ~ b l ~ *and a 2 )3 B 2 ( ~ a 2 r * states b l ) are summarized in rations are involved in the TI state, in contrast to the case of bpy Schemes I and 11, respectively, where we consider that a spin-orbit with a dominant single configuration.26 These are represented coupling operator, H,,, is a one-electron operator. The fact that by the forbidden character of Ty is apparently realized (k,'(O-0;rel) = 0.1-0.2) in both the phen and bpy complexes indicates that the Tl(phen) = a(3pb1r*a2)+ b(3*a2r*bl) (a > b ) (1) excited properties of these complexes are satisfactorily explained and by the C,, symmetry group as analogous to the cases of the free ligand molecules. We first discuss the case of [Rh(bpy),13+ Tl(bpy) 3Aa2A*bl (2) (Scheme 11). The T, and T, sublevels involve the matrix elements of spin-orbit coupling of (ra211,1daJ)and ( ra#,ldb1), respectively. The mixed configuration of phen is due to the close-lying ra2 and We can easily obtained the equation (razll,ldal)= ( r a 2 ~ l x ~ dbyb l ) Abl, and A * , ~and r*bi orbitals. using eq 3a and 3c and the relations 1, = 1/21/2(-lyr+ l,,) and If we attend to the 3B2symmetry of TI and the ODMR results I, = li. Therefore, the difference in the radiative activities between we can determine the ordering of the spin sublevels. The ODMR T, and T, is attributed to that in the oscillator strengths of the results provide two possible pathways, I and 11, for [ R h ( ~ h e n ) ~ ] ~ + singlet intermediate states. The 'Al(db,a*b,)state is expected to 8. First we assign the Ty[A2] sublevel as the as shown in Figure possess a larger radiative decay rate than the 'Bl(d,,r*b,) state 0.2) because of its dileast radiative sublevel (k,'(O-O;rel) owing to the larger overlap between the db, and a * b , orbitals, and pole-forbidden character. Next we refer to the zf scheme of free k,'(C-O) > kl(0-O) is expected. This is consistent with the result phen obtained by Gondo and Maki,24 in which T, is the top k,'(O-0) / k,'( 0-0) = 0.59. sublevel, and to the estimate of the changes of zfs due to spin-orbit In [Rh(phen),I3+ two configurations, 3Ab1r*a2 and 3 ~ a 2 x *are pl, couplings in the previous section (
