J . Phys. Chem. 1994,98, 398-400
398
Widely Different Luminescence Lifetimes of the ARRR, ASSS and the ARRS, ASSR Diastereomers of f'c-Tris[ (8-q~nolyl)phenylmethylsilyl]iridium(III): Exciplex Formation with Solvents by Distinct a-Donor and .rr-Acceptor Binding Mechanisms Peter I. Djurovich,+ Wendy Cook,Radhika Joshi, and Richard J. Watts' Department of Chemistry, University of California, Santa Barbara, California 93106 Received: September 28, 1993'
Luminescence lifetimes (7m) of the u-bond-to-ligand charge-transfer (SBLCT) excited states of two diastereomers of fuc-tris[(8-quinolyl)phenylmethylsilyl]iridium(III) differ by about a factor of 2 and are strongly solvent dependent. The 7,,, values of the more symmetric ARRR, ASSS diastereomer (A) are generally longer than those of the less symmetric ARRS, ASSRdiastereomer (B);7m'S of both diastereomers are substantially shortened relative to their values in aliphatic hydrocarbons by exciplex formation with a variety of weakly coordinating solvents including aromatic hydrocarbons, olefins, ethers, ketones, alcohols, and nitriles. Quenching constants (k,) due to exciplex formation are found to be much larger for B than they are for A in the u-donor solvents (cyclic ethers, ketones, alcohols, and nitriles); however, k, values of B are slightly smaller than those of A in *-acceptor solvents (aromatic hydrocarbons, olefins). The results suggest that u-donor solvents form exciplexes by binding a t the metal center, whereas s-acceptor solvents bind a t a quinolyl radical anion ligand site. A and B may prove useful as luminescent environmental probes which can distinguish between a-donor and s-acceptor binding sites.
Introduction The ARRR, ASSS diastereomer (A) as well as the ARRS, ASSRdiastereomer(B)offac-tris[ (8-quinolyl)phenylmethylsilyl]iridium(II1) (Figure 1) have recently been isolated and characterized.' The luminescencelifetimes (7,) of bothdiastereomers are found to be strongly dependent upon the solvent medium due to quenching via exciplex formation (k,); however, in each case the T~ value for the less symmetric B diastereomer is found to substantially shorter than that of the A diastereomer in fluid solvents at room temperature (Table 1). Estimates of k, indicate two distinct mechanisms for exciplex quenching. Aromatic hydrocarbons (*-acceptors, class 1) display k, values which are generally slightly smaller for the B isomer than they are for the A isomer; on the other hand, k, values of the cyclic ethers, ketones, alcohols, and nitriles (u-donors, class 2) are generally 2-3 times greater for the B isomer than is the case for the A isomer. The emission spectroscopy of several fac-tris[(8-quinolyl)diorganosilyl]iridium(111) complexes (diorgano = dimethyl, phenylmethyl, or diphenyl) indicates luminescence from excited states formed by transfer of charge from an I r S i bond to a **-orbital of the quinolyl ring system ((Si-Ir)ab Lr* or SBLCT);- prior studies indicate similar luminescencefrom Snand Ge-bonded Re(1) c o m p l e ~ e s .The ~ ~ strong dependence of the luminescence lifetimes of the methyl silyls upon the solvent in fluid solutions has been noted, and this has been attributed to associative quenching (exciplex formation) of the excited state by the s ~ l v e n t .The ~ phenylmethyl derivatives can occur as four distinct diastereomers, and transient absorption studies of the ARRR, ASSS diastereomer of fac-tris[(6-isopropyl-S-quinolyl)phenylmethylsilyl]iridium(III) in toluene at 298 K indicate that exciplex formation leads to a transient intermediate which persists after decay of the absorption signal of the quinolyl radical anion.10
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Results and Discussion In 2-methyltetrahydrofuran (2-MeTHF) glasses at 77 K, A and B are found to have nearly identical luminescence lifetimes (7,(A) = 62 ps; r,(B) = 64 ps) and emission profiles (Amx t Current address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104. Abstract published in Aduunce ACS Abstracts, January 1 , 1994.
0022-3654/94/2098-0398$04.50/0
Figure 1. Representations of the structures of the ARRR, ASSS (A) and the ARRS, ASSR (B) diastereomers offac-tris[(8-quinolyl)phenylmethylsilyl]iridium(III).
560 nm)." These emission profiles red shift in fluid 2-MeTHF at room temperature but remain nearly identical to each other (Am,, = 610 nm). The positions of the emission maxima for A and B are solvent dependent, but the two continue to display emission profiles nearly identical to each other in a variety of solvents at room temperature. Luminescence quantum yields (@PI)for each diastereomer in nitrogen-saturated toluene at room temperature are @I = 0.013 (A) and @I = 0.006 (Bisomer);'2-'5 estimatesof radiativelifetimes ( T = ~ T,/@I) in toluene are 76 ps (A) and 92 ps (e). These values suggest that, within limits of experimentaluncertaintiesassociated with thequantum yield determinations,'s theradiativedecayrates of A and B (1/ T ~ are ) nearly identical and that the 91values at 77 K arc near unity. Assuming that solvent effects upon 7f are negligible leads to nonradiative decay rates (k, = 1 / - ~l / r r~) compiled in Table 1. The 7, values for the two diastereomers change but little in a variety of aliphatic hydrocarbons; however, the T~ values of the A diastereomer are substantially longer than those of the B diastereomerin each of these solvents. Since quenching via solvent association to form an exciplex is unlikely in aliphatic hydrocarbons, the k, values in these solvents are taken to be the contributions to nonradiative decay in the absence of exciplex formation. The added quenching due to exciplex formation, k,, can be estimated from the measured 7,,, and ' T values ~ and the k, value taken from aliphatic hydrocarbons (kn0)as k, = 7,-l - 7;l - kn0 (Table 1); for these estimates, kn0 was taken to be the Q 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 399
Letters
TABLE 1: Lumiwclcence Lifetimes (T,,), Nonradiative Decay Rates (k,,), and Exciplex Quenching Rates (4,) of Ir(Pmsip)S Diastereomers in Nitrogen-Saturated Solvents at 298 K
aliphatic hydrocarbons cyclohexane methylcyclohexane ?err-butylcyclohexane 2,2,4-trimethylpentane aromatic hydrocarbons benzene toluene o-xylene mesitylene ?err-butylbenzene alkenes 1-hcxene 1-octcne ethers tetrahydrofuran 2-methyltetrahydrofuran hexyl ether ketones acetone 3-pentanone 2-pentanone alcohols 2-butanol 2-methyl-2-propanol nitriles but yronitrile propionitrile acetonitrile
2.69 2.52 2.39 2.86
0.359 0.383 0.405
1.00
0.877 0.746 0.850
0.337
0.787 0.987 0.970 1.02 1.25
1.02 0.970 0.786
1.17 1.49
0.887
0.988 1.13
1.33 1.16
0.648 0.600 0.416
0.492 0.549 0.555 0.737 0.688
2.02 1.81 1.35 1.44
0.659 0.637 0.193 0.289
0.842 0.658
0.471 0.288
0.699 0.700
1.43 1.42
0.270 0.265
0.296 0.418 1.22
3.36 2.38
2.99
0.165 0.226 0.678
6.06 4.41 1.46
4.91 3.26
0.136 0.171 0.219
7.36 5.83 4.54
6.99
0.059 0.086 0.093
16.9
5.46 4.18
11.6 10.7
15.8 10.5 9.59
0.269
3.71
0.097 0.133
9.14
3.30
3.34 2.94
10.3
0.301
7.48
6.33
9.89
0.041 0.029 0.028
24.4 34.7 36.0
0.0973 0.0580 0.0465
1.26 1.00
0.809
10.3 17.2
21.5
average of the values in the aliphatic hydrocarbons for each of the diastereomers (kno(A) = 0.371 ps-I; kno(B) = 1.15 ps-l). A model for added lifetime quenching effects due to exciplex formation (kq) is outlined below. A*
According to this scheme, excitation of the complex leads to an excited state, A* (or B*), which may return to the ground state by radiative (k,)or nonradiative (kn) processes in aliphatic hydrocarbons. In addition, formation of the exciplex, A*S (or B*S) leads to quenching in associative solvents (S);a similar scheme has been used in studies of solvent quenching of MLCT luminescence of d* complexes of C U ( I ) . ' ~ IIf~the rate of exciplex formation is slow while the rate of exciplex decay to the ground state ( k i ) is fast, k, is a measure of the pseudo-first-order rate constant for exciplex formation, k,. Alternatively, A*S may be formed in a rapid equilibrium with equilibrium constant Kc4 followed by a slow rate of exciplex decay; in that case, k, is a measure of K&'. The solvent-dependent quenching component, k,, varies widely, as anticipated for quenching due to exciplex formation; however, there are two distinct classes of solvent association indicated by the k, values compiled in Table 1. One class of solvents consists of the aromatic and olefinic hydrocarbons. Within this class, k, values are generally slightly smaller for the B isomer than is the case for the A isomer; it is also generally seen that k,valuesdecrease as the number and size of ring substituents increase for the aromatic hydrocarbons which have been studied. A second class of solvents consists of cyclic ethers, ketones, alcohols, and nitriles. For this class, kq values of the B diastereomer are generally 2-3 times as large as those of the A isomer, and a strong dependence of kq on the steric bulk of the solvent is again noted, particularly for the strongly a-donating nitriles.
0.630
2.01 0.439
16.9 21.1
1.79
0.869
0.311
23.2 33.6
34.8
The results indicate that two distinct mechanisms for exciplex quenching are operative. Aromatic and olefinic hydrocarbons bind metal centers through a combination of a-donation and a-accepting properties, while the solvents in class 2 are noted primarily as a-donor ligands. The SBLCT excited states of A and B may be formally described as consisting of an Ir(II1) bonded to a quinolyl radical anion and to a Si atom through a 2c-le bond. This bond, which is embedded deeply within the coordination sphere, represents a site which may be attacked by an incoming a-donor ligand. On the other hand, the quinolyl radical anion provides an electron-rich site where a-acceptors might bind on the periphery of the coordination sphere. It is therefore hypothesized that the class 1 aromatic and olefinic hydrocarbons form exciplexes due to binding at a quinolyl radical anion site in the SBLCT excited state. The class 2 a-donor solvents, on the other hand, are believed to bind the SBLCT excited state in the region of the weakened I r S i bond. This requires substantial penetration of the solvent into the coordination sphere, and it may be the case that larger cavities in the ligand environment enable more penetration of a-donors into the I r S i bonding region in the B isomer than is possible in the A isomer. The apparent sensitivity of k, to the stereochemistry as well as the bulk of the solvent is illustrated by 3-pentanone and 2-pentanone. Although these two isomeric ketones are expected to havevery similar overall dimensions and a-donor properties, k, is smaller for 2-pentanone for both A and B. This suggests that exciplex formation requires a specific orientation of the incoming solvent within a cavity in the coordination sphere; the more symmetric 3-pentanone is apparently more able to become oriented for exciplex formation than the less symmetric 2-pentanone. Although hexyl ether, which formally displays class 1 behavior, is noted as a a-donor rather than a a-acceptor, it is a far bulkier ligand than the cyclic ethers, which show class 2 behavior. It may be the case that this bulky ether is so large that it cannot penetrate the ligand coordination sphere of either diastereomer. A further understanding of the mechanism for exciplex formation with this type of ligand will require additional studies of comparably bulky solvating ligands with other functional groups.
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The Journal of Physical Chemistry, Vol. 98, No. 2, 1994
The sensitivity of the luminescence lifetimes of A and B to the u-donor and r-acceptor ability of the solvent medium via exciplex formation suggests that these may be useful environmental luminescent probes.19 It should be possible to distinguish a-donor and r-acceptor binding sites via measurements of kq(A) and kq(B); furthermore, each isomer is a potentially sensitive probe of the steric bulk, functionality, and orientation of u-donor sites in particular. The specific sensitivity of these probes may be further adjusted through the use of different combinations of organic groups bonded to the Si center for u-donor sensitivity or by quinolyl ring substituentsto influence the r-acceptor sensitivity.
Acknowledgment. This work was supported by the Office of Basic Energy Sciences, Department of Energy, Project DE-FG0388ER13842. References and Notes (1) Unpublished results, this laboratory. (2) Djurovich, P. I.;Safir,A.; Keder,N.; Watts, R. J. Coord. Chem.Reu. 1991,111,201. (3) Djurovich, P. I.; Safir, A.; Keder, N.; Watts, R. J. Inorg. Chem. 1992, 31, 3195. (4) Djurovich, P. I.; Watts, R. J. Inorg. Chem., in press. ( 5 ) Luong, J. C.; Faltynek, R. A,; Wrighton, M. S . J . Am. Chem. SOC. 1979,101, 1597.
Letters (6) Luong, J. C.; Faltynek, R. A.; Wrighton, M. S . J . Am. Chem. Soc.
1980, 102,7892.
(7) Andrea, R. R.; de Lange, W. 0.J.; Stufkens, D. J.; Oskam, A. Inorg, Chim. Acta 1988, 149,77. ( 8 ) Andrea, R. R.; de Lange,W. G. J.; Stufkens, D. J.; Oskam, A. Inorg. Chem. 1989,28, 318. (9) Stufiens, D. J.; Nieuwenhuis, H.A.; Oskam, A.; RoslKnaar, B. D.; Stor, G. J. Tenth International Symposium of the Phorophyslcs and Photochemistry of CoordinationCompounds, 1993, Sendai, Japan; Abstract 0-15, p 23. (10) Djurovich, P. I.; Watts, R. J. J . Phys. Chrm., preceding paper in this issue. (1 1) Methodsusedfordetenninationsoflumincscenctlifetimmandspectra have been described in prior publications. See, for example: Wilde, A. P.; King, K. A,; Watts, R. J. J . Phys. Chem. 1991,95,629. (12) Luminescence quantum yields were measured by a modified Parker Rees method relative to Ru(bipy)>z+in aqueous solution at room temperature (*I = 0.042; see: Van Houten, J.; Watts, R.J. J . Am. Chem. Soc. 1975, 97, 3843). (13) Parker, C. A.; Rtes, W. T. Analysr (London) 1960, 85, 587. (14) Parker, C. A. Photoluminescence ofSolurions; Elsevier: New York, 1968. (15) Demas, J. N.; Crcwby, G. A. J . Phys. Chem. 1971, 75,991. (16) McMillin, D.; Kirchhoff, J. R.; Goodwin, K. V. Coord. Chem. Rev. 1985. 64, 83. (17) Goodwin, K. V.; McMillin, D. R. Inorg. Chrm. 1987, 26, 875. (18) Crane, D. R.; DiBenedetto, J.; Palmer, C. E. A.; McMillen, D. R.; Ford, P. C. Inorg. Chem. 1988, 27, 3698. (19) Demas. J. N.; DeGraff, B. A. Anal. Chem. 1991,63, 829A.
