Inorg. Chem. 1986, 25, 227-234
227 Contribution from the Department of Chemistry, Tulane University, New Orleans, Louisiana 701 18
Independent Control of Charge-Transfer and Metal-Centered Excited States in Mixed-Ligand Polypyridine Ruthenium(11) Complexes via Specific Ligand Design William F. Wacholtz, Roy A. Auerbach,* and Russell H. Schmehl* Received April 1 , 1985 A series of tris(polypyridine)ruthenium(II) complexes [ R ~ ( d m b ) ~ ][Ru(dmb),(decb)12+, ~+, [Ru(dmb)(de~b)~]~+, and [Ru(decb),12+ have been prepared, where dmb is 4,4'-dimethyl-2,2'-bipyridine and decb is 4,4'-bis(ethylcarboxy)-2,2'-bipyridine. Absorption + ~ ( d e c b ) ~>] ~[Ru(dmb)(de~b)~]~+ + > [R~(dmb)~(decb)]~+ and and emission energies decrease in the order [ R ~ ( d m b ) ~>] ~[ R are linearly related to AEO, the difference between the first oxidation and reduction potentials of the complex. Temperaturedependent emission quantum yields and lifetimes in CH2CI, yield activation barriers, AE', for nonradiative decay from the 3MLCT ] ~ +M'values are 2750 and 1180 cm-I, respectively, and represent the thermal barrier state. For [Ru(dmb),12+and [ R ~ ( d e c b ) ~the to population of a metal-centered excited state. Upon photolysis in the presence of CI- in CH2CI2,substitution occurs, resulting in the cis chloro complex. The mixed-ligand complexes exhibit much smaller activation barriers for nonradiative decay and do not undergo anation upon prolonged photolysis in the presence of Cl-. The nonradiative decay and photosubstitution results are discussed in terms of the energetic separation between the )MLCT and 3MC states. Introduction The photochemistry of transition-metal complexes having metal to ligand charge-transfer (MLCT) transitions as the lowest energy excited state has been a topic of great interest in inorganic photochemistry.' The focus of much research in this area has involved an examination of the relationship between the M L C T energies of a series of complexes and the photochemical lability of the systems;2 data available suggest photosubstitution results from population of low-energy ligand field states that are not spectroscopically observed. For series of complexes that have been examined, such as [ R U ( N H ~ ) ~ ( ~ ~ X[Fe(CN),(pyX)I3-, )]~+, and [R~(T$C,H,)(CO),(~~X)]?-~ where p y x is a series of substituted pyridines, the quantum yield for ligand loss decreases as substituents are added that decrease the energy of the M L C T state. The decrease in the substitution yield occurs as the M L C T state becomes isolated from communication with the ligand field states responsible for photosubstitution. The photophysical and photochemical properties of numerous ruthenium(I1) and osmium(I1) polypyridyl complexes have been thoroughly examined. For ruthenium, the M L C T state exhibits emission in the red that allows the characterization of the decay. Scheme I outlines the photophysical decay pathways common to many ruthenium bipyridyl complexes. Excitation is followed by rapid intersystem crossing from the initially formed 'MLCT to a 3MLCT state6,' and is believed to occur with nearly unit efficiency for [ R ~ ( b p y ) ~ ] ~The + . ~triplet charge-transfer state decays via both temperature-dependent and temperature-independent p r o c e s ~ e s ~ and - ' ~ the thermally activated decay mode has been (1) See for example: (a) 'Inorganic and Organometallic Photochemistry"; Wrighton, M. S.,Ed.; American Chemical Society: Washington, DC, 1978;Adv. Chem. Ser. No. 168. (b) Whitten, D. G. Acc. Chem. Res. 1980, 13, 83. (c) Meyer, T.J. Prog. Inorg. Chem. 1983, 39, 389. (d) Seddon, K. R. Coord. Chem. Rev. 1982,41, 79. (e) Balzani, V.;Boletta, F.; Gandolfi, M. T.; Maestri, M. Top. Curr. Chem. 1978, 71, 1. (2) For two recent reviews see: (a) Ford, P. C.; Wink, D.; Dibenedetto, J. Prog. Inorg. Chem. 1983, 30, 213. (b) Ford, P. C. Rev. Chem. Intermed. 1978, 2, 267. (3) Malouf, G.; Ford, P. C. J . Am. Chem. Soc. 1977, 99, 7213. (4) Figard, J. E.; Petersen, J. D. Inorg. Chem. 1978, 17, 1059. (5) Giordano, P. J.; Wrighton, M. S . Inorg. Chem. 1977, 16, 160. (6) (a) Porter, G. B.; Schlafer, H. L. Ber. Bunsenges. Phys. Chem. 1964, 68,316. (b) Crosby, G.A.; Perkins, W. G.; Klassen, D. M. J . Chem. Phys. 1965, 43, 1498. (c) Lytle, F. E.; Hercules, D. M. J . Am. Chem. SOC.1969, 91,253.
(7) (a) Harrigan, R. W.; Crosby, G. A. J. Chem. Phys. 1973,59,3468. (b) Hipps, K. W.; Crosby, G. A. J. Am. Chem. SOC.1975, 97, 7042. (c) Crosby, G. A. Acc. Chem. Res. 1975,8,231and references therein. (d) Felix, F.; Ferguson, J.; Gudel, H. C.; Ludi, A. J . Am. Chem. Soc. 1980, 102,4096. (8) (a) Boletta, F.; Juris, A.; Maestri, M.; Sandrini, D. Inorg. Chem. Acta 1980, 44, L175. (b) Demas, J. N.; Taylor, D. G. Inorg. Chem. 1979, 18, 3177. (9) (a) Van Houten, J.; Watts, R. J. Inorg. Chem. 1978,17, 3381. (b) Van Houten, J.; Watts, R. J. J . Am. Chem. SOC.1976, 98,4853. (10) Allsopp, S. R.; Cox, A.; Kemp, T. J.; Reed, W. J. J . Chem. SOC., Faraday Trans. 1 1978, 1275.
0020-1669/86/ 1325-0227$01.50/0
Scheme I tRu(L-L'133
nr
IRu(L-L')31''
CRU(L-L')~I~~
CRu(L- L'133'+
C R U( L - L' I 3 13'
tRu(L-L'133
t products
ascribed to internal conversion to a triplet metal-centered state, 3MC. In nonpolar aprotic solvents the ruthenium complexes undergo efficient photoanation, and the photoreactivity appears to occur via population of the 3MC ~ t a t e . ~ ~ ' ~If~the ' ~ photolabile -'~ state is indeed metal-centered, and this state is populated principally by internal conversion from the 3MLCT, it should be possible to design complexes that are inert to photosubstitution. The energy of the 3MC state will be influenced principally by the u-donating strength of the ligands and the M L C T state energy depends principally on the energy of the lowest ligand A* orbitals. Preparation of complexes having one bipyridine with a low-energy A* level and more basic bipyridines filling the remaining coordination sites may result in ruthenium polypyridine complexes having 3MC states thermally inaccessible from the 3MLCT. Since most polypyridyl complexes of ruthenium(I1) luminesce in fluid solution a t room temperature, it is possible to monitor the 3MLCT to 3MC internal conversion as a function of systematic structural variations associated with the ligands. Very recently several groups have presented results of photophysical studies of several ruthenium polypyridyl complexes that suggest such separation of 3MLCT and 3ML states. In examining mixed-ligand complexes containing both bpy and 2,2'-biquinoline,14 Barigelletti and co-workers have observed only a slight temperature dependence of luminescence lifetimes between 77 K and room temperature. Such an observation is expected when thermal activation to a 3MC state is no longer feasible energetically. (11) Cherry, W. R.;Henderson, L. J. Inorg. Chem. 1984, 23,983. (12) (a) Caspar, J. V.; Sullivan, B. P.; Kober, E. M.; Meyer, T. J. Chem. Phys. Lett. 1982, 91,91. (b) Caspar, J. V.;Meyer, T.J . Inorg. Chem. 1983, 22,2444. (c) Caspar, J. V.; Meyer, T. J. J . Am. Chem. SOC.1983, 105, 5583. (13) Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J . Am. Chem. SOC.1982, 104,4803. (14) Barigelletti, F.;Juris, A,; Balzani, V.; Belser, P.; Zelewsky, A. V. Inorg. Chem. 1983, 22, 3335. (15) Fasano, R.;Hoggard, P. E. Inorg. Chem. 1983, 22, 567. (16) Jones, R. F.; Cole-Hamilton, D. J. Inorg. Chim. Acta 1981, 53, L3. (17) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J . Am. Chem. SOC.1984, 106, 2613. (18) Crutchley, R. J.; Lever, A. B. P. Inorg. Chem. 1982, 21, 2276. (19) Pinnick, D. V.;Durham, B. Inorg. Chem. 1984, 23, 1440.
0 1986 American Chemical Society
228 Inorganic Chemistry, Vola25, No. 2, 1986
Wacholtz e t al.
Table I. Redox Properties of Complexes Examined“ complex [R~(dmb)~],+ [Ru(dmb),(decb)12+ [Ru(dmb)(decb),12+ [Ru(decb),] 2+ [Ru(bpy)312+
oxidn E0(3+/2+), V 1.10 1.30 1.44 1.55 1.28
reductions
W+/O), v
E0(2+/+), V -1.46 -1.03’ -0.96’ -0.91 -1.32
E O ( O / - ) ,v -1.86 -1 .725 -1.64 -1.27
-1.63 -1.53 -1.16 -1.05 -1.52
Potentials vs. SSCE reference in C H X N with tetrabutvlammonium hexafluorophosphate as supporting electrolyte; ’ difference between the first oxidation and first reduction.
Results of this type have also been made by Cherry, examining temperature-dependent luminescence in water of mixed-ligand complexes of Ru(I1) having both bpy and 4,4’-dicarboxy-2,2’bipyridine.” Further, Meyer and co-workers have thoroughly examined complexes having bpy and either bipyrazine, bipyrimidine, or 4,4’-dicarbamoyl-2,2’-bipyridine. 17,20 We report here the examination of a series of mixed bipyridyl complexes of ruthenium(I1) having 4,4’-dimethyl-2,2’-bipyridine (dmb) and 4,4’-bis(ethylcarboxy)-2,2’-bipyridine (decb) as ligands. COOEt
COOEI
AEo,b V 2.56 2.33 2.40 2.46 2.60 = 200 mV/s.
L!
Potential
Table 11. Absorption and Emission Maxima of Ruthenium(I1) Polypyridyl Complexes in Methylene Chloride emissn max,b cm-‘
abs max: cm-’ com p1ex
d+rl*
K+K*
,+
[Ru(bpy),l 34500 [Ru(dmb),12+ 34700 [ R ~ ( d m b ) ~ ( d e c b ) ] ~35000 + [ R u ( d m b ) ( d e ~ b ) ~ ] ~32 + 500 [Ru(de~b)~]~+ 32500
d*X2*
22100 21800 20300 20 700 21413
23 300 22 600
298 K
77 K
16560 16180 14390 15200 15900
17200 16850 15580 15830 16480
“Maxima & l o 0 cm-I. *Maxima A50 cm-’. ,
dm b
decb
The complexes prepared are [Ru(dmb),Jz+ (l), [Ru(dmb)2(decb)J2+(2), [Ru(decb),(dmb)12+ (3), and [Ru(decb),12+ (4). In this work the energetic relation between ground and chargetransfer states for this series is described on the basis of the electrochemical and spectroscopic properties of the complexes. Temperature-dependent emission quantum yields and luminescence lifetimes are also reported; activation barriers obtained are related to normal-region electron transfer between the ,MLCT and ,MC states.Ic The temperature-dependent photosubstitution of complexes 1 and 4 by C1- in CHzC12has also been examined. Results are discussed in terms of the energetic relationship between the ,MLCT and ,MC states, relating AE’,the activation barrier for internal conversion between the states, to the nature of the equilibrium of the process.
Results Electrochemistry. The Eo values for the complexes, determined by cyclic voltammetry, are listed in Table I. The redox behavior for both oxidation and reduction of all four complexes indicates electrochemical reversibility (AEp = 60-80 mV) and chemical stability of the redox products (ip,a/ip,c= 1) on the time scale of the experiment. Sequential replacement of the dmb ligands of 1 with decb results in increases in the RU,+/~+ potential, from 1.10 V for 1 to 1.55 V for 4. Similar results have been obtained by Rillema et al. for related mixed ligand complexes.21 Reduction of ruthenium bipyridyl complexes has been shown to be ligandlocalized on the basis of UV-visz2 and ESRZ3spectra. In these complexes, the first reduction is observed a t -1.46 V for 1 and a t -0.91 V for 4. The mixed-ligand derivatives, 2 and 3, both exhibit first reductions indicating reduction of coordinated decb. The second reduction of 3 occurs a t -1.16 V compared to -1.53 V for 2, reflecting differences in the nature of the second reducible ligand in the two complexes. The potentials for reduction of [Ru(decb),] agree well with those reported by Elliott.22 Absorption and Emission Spectra. The absorption spectra of all of the complexes exhibit UV transitions corresponding to intraligand n+n* transitions between 290 and 3 10 nm (Table 11). For complexes 1 and 4,a single maximum in the 400-500-nm (20) Neveux, P. E.; Meyer, T. J., private communication. (21) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem. 1983, 22, 1617. (22) Elliot, C. M.; Hershenhart, E. J. J . Am. Chem. SOC.1982, 104, 7519. (23) DeArmond, M. K.; Carlin, C. M. Coord. Chem. Rev. 1981, 36, 325. (24) Klassen, D.M. Chem. Phys. Lett. 1982, 93, 383.
O
W
m
25000
E
15000
5oa)
o
--
325
i
KO
:
375
:
:
400
: : : I 4% 475 500 WAVELENGTH (nm)
425
: 525
; 5%
: 575
Figure 1. Visible absorption spectra in CH,CI, for [ R ~ ( d m b ) ~ (] l~)+, [Ru(decb)312+(4), [Ru(dmb),(decb)12+ (2) and [Ru(decb),(dmb)]*+ (3).
region is observed (Figure 1) and has been assigned as a metal to ligand charge-transfer (MLCT) t r a n s i t i ~ n . ~ .The ’ ~ complexes having both of the bipyridine ligands, 2 and 3, exhibit two maxima between 400 and 500 nm, indicative of two independent MLCT transitions. Here, the towest energy M L C T maximum of both 2 and 3 is red-shifted relative to that of 4. The red shift in the n* transition is related to both the reduction in symmetry d of the mixed-ligand complexes as well as the cumulative inductive effect of the u-donating and n-withdrawing ligand orbitals. Room-temperature emission spectra of 1-4, corrected for photomultiplier response, are shown in Figure 2. Emission maxima in CH2Cl, a t room temperature and 77 K are listed in Table 11. Relative energies of absorption and emission maxima for 1-4 and values of AEO, the potential difference between the first oxidation and first reduction for the complexes (Table I), have the same ordering: 1 > 4 > 3 > 2. The result indicates that -+
Inorganic Chemistry, Vol. 25, No. 2, 1986 229
Mixed-Ligand Polypyridine Ru(I1) Complexes
Table 111. Activation Parameters for the Decay of the 'MLCT State from Temperature-Dependent Emission Quantum Yields and Lifetimes" A m
complex Ru(bov),2t . Ru(dmb) j2+ Ru(dmb)2(decb)2+ Ru(dmb)(decb)p R~(decb)~*' r,,>
AE', cm-' 3070 f 250 3166 f 418 948 f 552 945 f 436 2802 f 152
kd, s-l 4 f 2 X 10l2 2 f 2 X 10l2 2 2 X lo' 2 f 4 X 10' 7 f 9 X 10"
*
T
W k l , s-l 3.03 0.10 4.81 f 0.11 10.10 f 0.46 5.26 f 0.45 3.48 f 0.03
*
iO-57ickr,s-I 1.09 f 0.1 1.33 f 0.2 0.77 f 0.1 0.66 f 0.1 1.34 f 0.2
U',cm-'
kd, s-]
3068 f 300 2741 f 622 447 f 140 612 f 134 1177 f 514
3f 1 3f4 4f1 5f1 3f4
l W k l , s-l 3.77 f 0.15 5.28 f 0.18 6.45 f 1.05 4.09 f 0.28 3.30 f 0.10
X 10l2 X 10" X lo6 X lo6 X lo7
"The error margins indicated represent 2a, from the standard deviations of each point. [Ru(bpy)J2+is included with the data for these complexes for comparative purposes. The method of analysis yields AE' and kd values somewhat smaller than those reported elsewhere. Table IV. Room-Temperature Excited-State Decay and Photoanation Parameters complex T, ns Vi.6 h l
WbPY)' Ru(dmb), R~(dmb),(decb)~ Ru(dmb)(decb), Ru(decb)'
576 f 0.8 931 f 25 853 f 6 1415 f 9 2230 f 56
0.84 f 0.06 0.50 f 0.10 0.047 f 0.02 0.42 f 0.04 0.26 f 0.10
"
0.06 f 0.01 0.12 f 0.02 0.07 f 0.01 0.10 f 0.02 0.30 f 0.03
@aobsd
4,dd
0.049 f 0.010 0.005 i 0.002
