7961
J . Phys. Chem. 1989, 93, 7961-7966
Solvent and Temperature Dependence of Intramolecular Energy Transfer in the Complex [(dmb),Ru (b-b) Ru(dmb) (CN)
,I2+
Chong Kul Ryu and Russell H. Schmehl* Department of Chemistry, Tulane University, New Orleans, Louisiana 701 18 (Received: March 1 , 1989)
The complex [(dmb),R~(b-b)Ru(dmb)(CN),]~+, 1, was prepared (dmb = 4,4'-dimethyL2,2'-bipyridine and b-b = 1,4bis[2-(4'-methyl-2,2'-bipyridyl-4-yl)ethyl]benzene), and the redox and photophysical properties of the complex were examined in several solvents. Luminescence is observed from the Ru dmb metal to ligand charge-transfer (MLCT) states of both the [(dmb),Ru(b-b)] and [(b-b)R~(dmb)(CN)~] chromophores. The lifetime of the higher energy [(dmb),Ru(b-b)] center of 1 is less than 450 ns in each solvent studied at room temperature, indicating significant quenching of the complex. Excitation spectra of the [(b-b)R~(dmb)(CN)~] emission of complex 1 indicate that energy transfer from the [(dmb),Ru(b-b)] center is contributing to the emission. From the emission energy of the [(dmb),Ru(b-b)] center and redox data, photoinduced electron-transfer quenching of the donor is shown to be endergonic. The energy-transfer process exhibits both solvent dependence and temperature dependence. The temperature dependence of the energy-transfer rate between 200 and 300 K indicates the existence of more than one energy-transfer path. The data are analyzed in terms of combined Coulombic and electron-exchange energy-transfer mechanisms.
-
Introduction The investigation of energy-transfer processes in transitionmetal-complex systems has been the subject of numerous theoretical and mechanistic studies. Work with organic systems'-3 established that energy transfer can occur via a Coulombic (dipole-dipole, dipole-quadrupole, etc.) interaction of an electronically excited donor with an acceptor in the ground state. This Coulombic or resonance energy transfer, described by Forster,2 follows selection rules corresponding the electric dipole transitions (Le., absorption and emission of light). Since electronic transitions in transition-metal complexes are frequently symmetry and spin forbidden, Coulombic energy transfer is usually unimportant in describing energy-transfer processes in these systems4 In such systems, Dexter showed that energy-transfer reactions can be described by an electron-exchange mechanism in which orbital overlap of the donor and acceptor is r e q ~ i r e d .Dexter's ~ approach allows for both Coulombic and exchange interactions; however, exchange interactions are generally important only when the energy-transfer process involves a spin change in the individual transitions. The electrostatic and exchange mechanisms are typically distinguished by evaluating the dependence of donoracceptor separation on the rate of energy transfer; an rd distance dependence is expected for a dipoledipole coupling process while an exponential distance dependence is obtained in exchange energy transfer. More recently, rates for exchange energy-transfer processes have been described by theories similar to those used to describe outer-sphere electron-transfer proces~es.~-~Balzani and coworkers have applied classical electron-transfer models to energy transfer in stilbenes and ketone^.^,^ Endicott's group has extended this approach to include quantum effects in which both nuclear
and electronic tunneling are In this description, the rate constant for energy transfer within the donor-acceptor encounter complex (eq la-c) depends upon the electronic coupling D
hu
D*
of the donor and acceptor, E , the thermally averaged FranckCondon factor, N , and the preequilibrium constant for association of the donor and acceptor, KA(=kd/kd) (eq 2). Extensive studies ken = KA*E.N
(2)
of the Endicott group have led to the evaluation of four separate cases, depending on the magnitude of the energy gap and the degree of displacement of the donor and acceptor potential surfaces. These individual cases have been discussed for a variety of bimolecular energy-transfer processes, involving Cr( III)11~le16 and Ru(I1) donors and Co(II1) and Cr(II1) acceptors,I3 respectively. From eq 1, the experimental energy-transfer rate constant, kob, is given by eq 3 as long as the acceptor decay rate is greater than kd [D] Since, for most exergonic energy-transfer processes, ken>> k,,, the rate constant depends principally on kd, k4, and ken. When ken>> k d . the process becomes diffusion limited (kok kd (3) kobs = k-d k-en .495
1+-+-
ken
( I ) General reviews: (a) Baltrop, J. A.; Coyle, J. D. Principles of Photochemistry; Wiley: New York, 1978; Chapter 4. (b) Yardley, J. T. Introduction to Molecular Energy Transfer; Academic Press: New York, 1980; Chapter 8. (c) Birks, J. B. Phorophysics of Aromatic Molecules; Wiley: New York, 1970. (2) (a) Forster, T. in Modern Quantum Chemistry; Sinanoglu, O . , Ed.; Academic Press: New York, 1966, pp 93-137. (b) Forster, T. Discuss. Faraday SOC.1959, 27, 7-17. (3) Dexter, D. L. J . Chem. Phys. 1953, 21, 836-50. (4) Scandola, F.; Balzani, V. J. Chem. Educ. 1983, 60, 814-23. (5) (a) Balzani, V.;Bolletta, F.; Scandola, F. J. Am. Chem. Soc. 1980,102, 453-63. (b) Balzani, V.;Bolletta, F. J. Am. Chem. Soc. 1978, 100, 7404-6. (6) (a) See: Sutin, N. In Prog. Inorg. Chem., Vol. 30, Lippard, S. J., Ed.; 1983, pp 442-98. (b) Sutin, N. Acc. Chem. Res. 1968, 1, 225-30. (7) Hush, N. S. Coord. Chem. Reu. 1985,64, 135-58. ( 8 ) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta., 1985,811,265-322. (9) Meyer, T. J. In Progress in Inorganic Chemistry; Lippard, S . J., Ed.; Wiley: New York, 1983; pp 389-441.
(la)
ken
= k d ) . In any event, determination of ken from bimolecular quenching studies necessarily depends on evaluation of kd and k 4 (or KA and k d ) . (IO) Endicott, J. F. Acc. Chem. Res. 1988, 21, 59-66. (11) Endicott, J. F. Coord. Chem. Reu. 1985, 64, 293-310. (12) Endicott, J. F.; Tamilarasan, R.; Brubaker, G. R. J . Am. Chem. SOC. 1986, 108, 5193-201. (13) Tamilarasan, R.;Endicott, J. F. J . Phys. Chem. 1986, 90, 1027-33. (14) Endicott, J. F. A C S S y m p . Ser. 1982, 198, 227. (15) Endicott, J. F.; Heeg, M. J.; Gaswick, D. C.; Pyke, S. C. J . Phys. Chem. 1981, 85, 1771-9. (16) Endicott, J. F.; Ramasami, T.; Geswich, D. C.; Tamilarasan, R.; Heeg, M. J.; Brubaker, G. R.; Pyke, S. C. J . Am. Chem. SOC.1983, 105, 5301-10.
0022-365418912093-7961$01.50/00 1989 American Chemical Society
7962 The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 Recently, bridging ligands have been used to covalently link transition-metal-complex chromophores to other metal complexes that may serve as energy or electron acceptors, thus eliminating the need to evaluate the association equilibrium constant, KA.17-32 A few examples exist for which intramolecular energy transfer clearly occurs;25-27,29-31 however, there has been no systematic examination of free energy relationships in covalently linked complexes of this type. For donor-acceptor series having relatively small energy gaps, A E (E,,(donor) - E,,(acceptor)), Endicott et al. have found two limiting ~ a s e s . ~Where ~ J ~ there is a large displacement between reactant ((D*,A]) and product ((D,A*]), potential surfaces energy transfer is thermally activated. The activation energy, E,, can be formulated from a classical model? assuming simple harmonic potential surfaces as E,
(EA + AE)2/4Ex
(4)
where EA is directly related to the excited-state distortion given by the spectroscopic Stokes shift4t5(vide infra). In cases where both the energy gap and the degree of distortion between reactant and product potential surfaces is small in magnitude, the reactant potential surface is nested within the product surface.I’ In this case, the energy-transfer rate constant decreases with increasing AE and can be described by the same energy gap relationship used to describe excited-state decay. Examples of the latter case have been reported by Endicott’s group for bimolecular reactions, including energy transfer from [(bpy),Ru(CN),] to a variety of Cr(ll1) complex acceptors (bpy = 2,2’-bipyridine).I3 We have recently prepared the coupled donor-acceptor system [(dmb),R~(b-b)Ru(dmb)(CN)~]~+ where b-b is a bis(bipyridine) ligand in which the chelating ligands are linked by a saturated bridge (below)
b - b. R = p - CHZC~H~CHZ
and dmb is 4,4’-di1nethyl-2,2’-bipyridine.~’ This work presents the preparation of the complex and its spectroscopic characterization in a variety of solvents. The emission energy of the donor, [(dmb)zRu(b-b)]2+,is relatively unaffected by changes in solvent while that of the acceptor, [(b-b)Ru(dmb)(CN),], is strongly solvatochromic. This behavior allows examination of the energy-transfer rate as a function of the energy gap, AE, by varying ( I 7) Cooky, L. F.; Headford, C. E. L.; Elliott, C. M.; Kelley, D. F. J . Am. Chem. SOC.1988, 1 IO, 6673-82. (18) Elliott, C. M.; Freitag, R. A.; Blaney, D. D. J . Am. Chem. SOC.1985, 107. 4647-55. (19) Elliott, C. M.; Freitag, R. A. J . Chem. Soc., Chem. Commun. 1985, 156-7. (20) Westmoreland, T. D.; LeBozec, H.; Murray, R. W.; Meyer, T. J. J . Am. Chem. Soc. 1983, 105, 5952-4. (21) Matsuo, T.; Skamoto, T.; Takuma, K.; Sakura, K.; Ohsako, T. J . Phys. Chem. 1981, 85, 1277-9. (22) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S.;Anton, D.; Neveux, P. E., Jr.; Meyer, T. J. Inorg. Chem. 1987, 26, 1 116-26. (23) Schanze, K . S . ; Neyhart, G. A.; Meyer, T. J. J . Phys. Chem. 1986, 90, 2182-93. (24) (a) Curtis, J. C.; Bernstein, J. S.;Schmehl, R. H.; Meyer, T. J. Chem. Phys. Lett. 1981, 81, 48-54. (b) Curtis, J. C.; Bernstein, J. S.; Meyer, T. J. Inore. Chem. 1985. .., 24. -385-90. -(25) Wacholtz, W. F.; Auerbach, R. A,; Schmehl, R. H. Inorg. Chem. 1985. 25. 227-14 . .- -, ., __ - .. (26) Schmehl, R. H.; Auerbach, R. A,; Wacholtz, W. F. J . Phys. Chem. 1988, 92, 6202-6. (27) Schmehl, R. H.; Auerbach, R. A.; Wacholtz. W. F.: Elliot. C. M.; Freitag, R. A.; Merkert, J. W. Inorg. Chem. 1986, 25, 2440-5. (28) Nishizawa, M.; Ford, P. C. Inorg. Chem. 1981, 20, 2016-20. (29) Moore, K. J.; Lee, L.; Figard, J. E.; Gelroth, J. A.; Stinson, A. J.; Wholens, H . D.;Petersen, J. D. J . Am. Chem. SOC.1983, 105, 2274-9. (30) Gelroth, J. A.; Figard, J. E.; Petersen, J. D. J . Am. Chem. SOC.1979, 101, 3649-51. (31) Petersen, J . D.; Murphy, W. R.; Sahai, R.; Brewer, K. J.; Ruminski, R. R. Coord. Chem. Reo. 1985, 64, 261-72. (32) Ruminski, R. R.; Petersen, J. D. Inorg. Chem. 1982, 21, 3706-8. ~
-
0
~
~
.
Ryu and Schmehl solvent. In this system, AE varies between 800 cm-’ (2.3 kcal/mol) and 1800 cm-l (5.1 kcal/mol). For this complex, kenincreases with increasing AE, and for a given solvent, the energy-transfer rate is temperature dependent. The results are analyzed by several approaches, including Coulombic and electron-exchange mechanisms.
Experimental Section Materials. Unless otherwise specified, all solvents used were obtained from Aldrich Chemical Co. Hexamethylphosphoramide (HMPA) was vacuum distilled prior to use. N,N’-Dimethylformamide (DMF) and N-methylformamide (NMF) were freshly distilled from BaO before use. Pyridine (PY) was distilled from KOH and stored over KOH. Acetonitrile (AN) (Burdick and Jackson) was distilled from CaHz before use. N,N’-Dimethylacetamide (DMA), propylene carbonate (PC), and dimethyl sulfoxide (DMSO) were obtained as spectroscopic grade solvents and used without further purification. Ethanol (EtOH) (Pharamacia) was dried by distillation from Na and stored over 5-A molecular sieves (Linde). The ligand (b-b) and the complex [(dmb)2Ru(b-b)](PF,)2were prepared as described earlier.27 [(dmb)Ru(DMSO),(Cl,)] and [(dmb)2Ru(CN),] were prepared by the methods of Evans33and D e m a ~ , respectively. ,~ Other solvents and materials were reagent grade and obtained from local distributors. S)mthesis Of [ (Dm6)2Ru(6-6)Ru(dmb)(C m 2 ](PF6)2 ( 1 ) . [(dmb)zRu(b-b)](PF6)z(0.20 g, 1.63 X mol) dissolved in ethanol (140 mL) was added dropwise to a refluxing solution of [ R U ( D M S O ) ~ ( ~ ~ ~(0.10 ) C Ig,~ 1.95 ] X lo4 mol) in ethanol (20 mL) in a 200-mL two-necked round-bottom flask equipped with a dropping funnel and condenser and blanketed with Ar. After the addition (2 h), the solution was refluxed for 22 h. The solvent was removed under reduced pressure and the solid was washed with water (20 mL) and ethano1:ether ( 1 5 v/v) and air dried. TLC of the product on neutral alumina (activity I) using CH,CN/toluene (1:l) as eluent indicated that no [(dmb),Ru(b-b)](PF,), remained. The solid and an excess of NaCN (0.4 g) were dissolved in ethanol/H,O (1:l; 50 mL), and the mixture was refluxed for 4.5 h under an argon blanket. The solution was removed by rotary evaporation, and the solid obtained was filtered and washed with H 2 0 (10 mL) followed by ether/ethanol (5:l v/v; 10 mL) and dried in vacuo. The collected product was chromatographed on neutral alumina (activity I) with use of pure CH3CN first to remove any unreacted [R~(dmb)~(b-b)](PF,),. The eluent was changed to CH3CN/MeOH/Hz0 (1:l:l V/V/V) to remove the product as a weakly luminescent orange band. Removal of CH3CN and MeOH from the eluted product by rotary evaporation left a precipitate that was collected by filtration and dried. TLC revealed a strongly emitting impurity. Further purification was achieved by chromatography using silica gel (60-200 mesh) and eluting first with CH3CN followed by CH3CN/ MeOH/ 10% KNO,(aq) (1 :1:1 v/v/v). The broad orange middle band was collected. As above, the organic solvents were removed by rotary evaporation. The solid was taken up in H 2 0 . Addition of saturated aqueous NH4PF, (1 mL) to the aqueous solution resulted in formation of an orange precipitate that was collected by vacuum filtration, washed with H 2 0 and ether/ethanol (.5:1), and dried in vacuo. UV in pyridine ,A, = 466 f 2 ( t = 20 800 M-I cm-I) and 512 f 2 nm (sh, 12000 M-’ cm-’); vCrN (KBr) = 2072 and 2062 cm-l. Anal. Calcd for C70H66N1zR~ZP2F12~ 2Hz0: C, 52.43; H, 4.40; N, 10.48. Found: C, 52.40; H, 3.95; N, 10.29. Apparatus. Absorption spectra were recorded on a H P Model 845 1 diode array spectrophotometer. Luminescence spectra were measured with a Spex Model 11 1 C fluorimeter equipped with ~
~~~~
(33) Evans, I. P.; Spencer, A,; Wilkinson, G. J . Chem. Soc.,Perkins Trans. 2 1973, 2, 204-9. (34) Demas, J. N.; Turner, T. F.; Crosby, G. A. Inorg. Chem. 1969, 8,
674-5. (35) Wacholtz, W. F.; Auerbach, R. A,; Schmehl, R. H. Inorg. Chem. 1986, 25, 227-3 1.
Energy Transfer in [(dmb),R~(b-b)Ru(dmb)(CN)~]~+
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7963
TABLE I: Absorption and Emission Characteristics of Complexes in Various Solvents at Room Temperature [(dmb)zRu(Wzl [(dmb),Ru(b-b)] 2t [ (dmb),Ru( b-b)Ru(dmb) (CN),] 2t
solvent HMPA DMA
PY DMF PC AN DMSO EtOH NMF
A,
nm 528 516 514 510 496 496 502 466 466
E,,, cm-l 13 850 14 120 14310 14 180 14330 14280 14280 15 170 14880
SS! cm-l
5090 5260 5140 5430 5830 5880 5640 6290 6580
A,,
nm 460 460 462 462 462 460 462 458 462
E,,, cm-I 15600 15620 15770 15 580 15 750 15 800 15 500 15950 15670
h" nm
@em
0.066 0.065 0.090 0.064 0.077 0.066 0.072 0.069 0.067
466 464 466 464 464 462 466 460 462
E,,, cm-I 14290 14 350 14390 14 500 14640 14 560 14600 15 400 15 400
"Absorption maxima, f2 nm;emission maxima, f l O O cm-l; emission quantum yields, f0.005. *Observed Stokes shift determined from EabD Em.
a 450-W Xe arc lamp and a cooled PMT (Hammamatsu Model R928) housing. The PMT wavelength response was calibrated with an Epley NBS calibrated tungsten source (500-900 nm). Cyclic voltammograms were measured with a Princeton Applied Research system described earlier.35 The supporting electrolyte for electrochemical measurements was 0.1 M tetraethylammonium perchlorate, and an aqueous sodium saturated calomel electrode was used as reference. Luminescence quantum yields were measured for [(dmb),Ru(b-b)(PF,),] and [(dmb)2Ru(CN)2]in freeze-pump thaw degassed solutions of each solvent with use of [Ru(bpy),]C12 in H 2 0 as the standard (&, = 0.042).36 All measurements were made at room temperature (298 K) with the integrated intensity (I,) over the entire emission envelope to 850 nm. Spectra were corrected for PMT response and for differences in the solvent refractive indices (n)by eq 5. Solutions were absorbance matched at the excitation wavelength (460 nm) to an absorbance IO0 ns or a PAR Model 162 boxcar averager. Data acquisition and analysis were managed with an HP Model 9826 microcomputer. Solutions were freeze-pump-thaw degassed to a final pressure of 10-5 torr. Variable-temperature measurements were made with an Air Products Displex cryostat. Samples were freeze-pump-thaw degassed in 2-mm cylindrical tubes, sealed under vacuum and mounted to the Displex with a home-built copper sample holder with windows for excitation and emission. Luminescence decays were analyzed as either single or double exponentials by using a Marquadt algorithm for least-squares minimi~ation.~'
Results Three complexes are required to provide the information needed to analyze the spectroscopic behavior of complex 1. Along with 1, the mononuclear components making up 1, [Ru(dmb),(bb)](PFa)225and [ R ~ ( d m b ) ~ ( C N ,were ) l ~ ~prepared by literature methods. The coupled dimer 1 was prepared in two steps from [Ru(dmb),(b-b)(PF,),. The bridging ligand was first coupled to (36)Van Houten, J.; Watts, R.J. J . Am. Chem. Soc. 1976,98,4853-8. (37)Bevington, P.R.Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969. (38)Fung, E. Y.; Chua, A. C. M.; Curtis, J. C. Inorg. Chem. 1988,27, 1294-6. (39)Kitamura, N.; Sato, M.; Kim, H.-B.; Obata, R.; Tazukes, S. Inorg. Chem. 1988,27, 651-8.
a second Ru center with [Ru(dmb)(DMSO),Cl,] prepared by a previously reported method33 (eq 6a). The cis chloro product
+ [R~(dmb)(DMS0)~C1,1
[(dmb)2Ru(b-b)]2+
-+
[ (dmb),Ru( b-b) Ru(dmb)C12]2+ (6a)
NaCN EtOH/HZO [ (dm b),Ru( b- b ) R ~ ( d m b ) ( C N )2+~ ] (6b)
[ (dmb)2Ru(b-b)Ru(dmb)C12]2+
was converted to the cyanide, 1, by reaction with NaCN (eq 6b). Purification of the complex was achieved by repeated chromatography using both alumina and silica gel stationary phases. Analytical data for complex 1 is reported in the experimental section. Absorption and emission spectra of [(dmb)2Ru(b-b)](PF6)2, [(dmb)2Ru(CN)2],and 1 in nine solvents were measured, and maxima for the lowest energy absorption and emission are reported in Table I. Emission spectra are corrected for photomultiplier response. Emission quantum yields for the donor, [(dmb),Ru(b-b)I2+, are also reported for freeze-thaw deaerated solutions of the complex in each solvent. Absorption spectra of 1 exhibit a broad metal to ligand charge-transfer (MLCT) transition centered at approximately 460 nm, and in all solvents except ethanol and N-methylformamide, a pronounced shoulder is observed at approximately 500 nm. The absorption spectrum of 1 in any of the solvents studied could be generated by summing spectra ( t vs A) of [(dmb)2Ru(b-b)]2+and [(dmb)2Ru(CN),]. The steady-state emission maxima of 1 fall between the maxima of [(dmb),Ru(b-b)I2+ and [(dmb),Ru(CN),] upon excitation at the absorption maximum. Further, the emission of 1 in all solvents is very broad (fwhm > 3000 cm-I). Excitation of 1 on the red edge of the absorption (>500 nm) results in a red shift in the observed emission maximum since a greater fraction of the light is absorbed at the cyano complex center of 1. Time-resolved emission spectra of 1 in pyridine (exciting at 337 nm) indicate that the steady-state emission consists of emission from both a short-lived component from the [(dmb)2Ru(b-b)] center of 1 and a longer lived component from the [(dmb)(CN),Ru(b-b)] center of the dimer. The luminescence lifetimes of [(dmb)2Ru(b-b)]2+and 1 are reported in Table 11. The decay rate of the [(dmb),Ru(b-b)] emitting center of 1 was determined from emission on the blue edge of the spectrum of 1 (590 nm). In most cases, this emission was a single exponential. Temperature dependence of the luminescence decay rate of the short-lived emission of 1 and [(dmb),Ru(b-b)I2' was examined in DMF, propylene carbonate, and pyridine. Figure 1 shows 1/ T vs 1/ T data for both complexes in DMF and propylene carbonate. Visible absorption and emission spectra of the three complexes (Table I) are characteristic of this class of compoundsw2 for which metal to ligand charge-transfer transitions (MLCT) predominate. Discussion
Emission Spectra. The most obvious feature of the absorption and emission maxima is the solvatochromism of [(dmb),Ru-
1964
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989
Ryu and Schmehl 1.65
A
w
1.BO--
-..
...
c
10.0 5
Tm
s
E.o
.
I
0
t \4
~
I
4
B.
4.01
900 --
4
0.0 3.00
3.50
4.00
1/T
x10
4.50
K
5.00
-'
TABLE 11: Emission Lifetimes and Energy-Transfer Rate Constants for Complex 1 in Different Solvents
AN"
AE,b cm-'
HMPA DMA
10.6 13.6 14.2 16.0
1750 1500 1460 1400 1420 1530 1220 780 790
PY DMF
PC AN DMSO EtOH M FA
18.3 19.3 19.3 37.1 32.1
r?,: ns 800 800 924 808 920 894 804 805 800
rib:
-5
Figure 1. Luminescence decay rates of [(dmb)2Ru(b-b)]2+(0)and complex 1 ( A ) , in D M F (A) and propylene carbonate (B) at different temperatures above the freezing point of the solvent.
solvent
700
ns
63 47 35 53 78 48 114 205 408
k!.,d IO6
SKI
14.6 20.0 27.3 17.7 11.7 19.7 7.5 3.6 1.2
"Solvent acceptor number. See ref 46. *E,,(donor) - E,,(acceptor). From Table I . 'Luminescence lifetime,
