The Journal of
Physical Chemistry VOLUME 99, NUMBER 46, NOVEMBER 16,1995
0 Copyright 1995 by the American Chemical Society
LETTERS Deuteration Effects in the Lowest-Excited 3MLCT States of [Ru(bpy)3I2+ in [WbPY)31(c104)2 Hans Riesen,* Lynne Wallace, and Elmars Krausz Research School of Chemistry, The Australian National University, Canberra, ACT 0200 Australia Received: June 6, 1995; In Final Form: August 11, 1995@
Recently, we have reported luminescence and excitation spectra of the series [Ru(bpy)3-,(bpy-d~),]~+(x = 0-3) in [Zn(bpy)3](C104)2. Two sets of electronic 3MLCT origins were observed for the x = 1 and x = 2 systems. Our assignments of these two sets as transitions involving either the bpy-h8 or the bpy-d8 ligands have been challenged. We have extended our study of deuteration effects and found clear confirmation of (x = 0, 2, 6, 8), and our previous assignments. One set of origins is observed in the series [R~(bpy-d.J3]~+ the origins shift gradually to higher energy with increasing deuteration degree. The [Ru(bp~)(bpy-d&]~+ complex was further deuterated or protonated to [Ru(bpy-d2)(bpy-d~)~]~+ and [R~(bpy>(bpy-d&]~+, respectively. Transitions involving the bpy-d2 and the bpy-d6 ligand shift to higher and lower energy, respectively, whereas transitions involving the bpy-d8 or the bpy-h8 ligand remain at the same energy as in the [Ru(bpy)(bpyd&I2+complex.
that the lowest-excited triplet metalIt is widely to-ligand charge transfer (3MLCT) states in [Ru(bpy)3I2+ in solutions and frozen glasses are localized (Le., the transferred electron resides on one ligand at a time). By using Stark, Zeeman, and laser spectroscopies, we have established that the lowest-excited 3MLCT states in [Ru(bpy)312+ doped into crystalline [Zn(bpy)3](C10.& are also l o c a l i ~ e d . ~ - ' ~ Confirmation of the localized nature of these states has been obtained in the spectroscopy of the series [R~(bpy)3-~(phen),]~+ ( x = 0-3) in the same host." In this system the electronic origins involving the bpy or the phen ligands are separated by -125 cm-I. Racemic [Zn(bpy)3](C104)2 crystallizes in the monoclinic space group C2/c with four formula units per unit ce11.I2 All cations are equivalent, but within a single cation one bidentate ligand lies about the symmetry axis while the other two ligands are equivalent, being related by the symmetry axis (crystal b axis). As a consequence of the C2/c structure, the charge ~~
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Abstract published in Advance ACS Abstracts, October 15, 1995.
0022-3654/95/2099-16807$09.OO/O
transfer transitions involving the distinct ligand (on the crystal b axis) are shifted to higher energy by a few hundred wavenumbers. This energy difference can be directly probed by dopants like [Ru(bpy)~(3,3'-bipyridazine)]~+. An energy difference for the lowest-excited 3MLCT state involving the 3,3'bipyridazine ligand in the two possible positions of %360 cm-' is 0b~erved.I~ The electronic origins of the three lowest-excited 3MLCT states of [Ru(bpy)3I2+in [Zn(bpy)3](C104)2can be observed in luminescence and e x ~ i t a t i o n . ~ - ' These ~ - ' ~ three lowest-energy origins are usually referred to as I, 11, and III. The energy separations are hE(1-11) = 8.7 cm-' and hE(I-III) = 62.1 cm-l. Origin I is W ? O times weaker than origin 11, and thus the luminescence spectrum is dominated by emission from level II above 4.2 K. Origin 111 is e 3 times more intense than origin II. Upon perdeuteration I, 11, and I11 shift to higher energy by 37 cm-I. The spectroscopy of the series [Ru(bpy)3-,(bpy-ds),12+ (x = 0-3) in [Zn(bpy)3](C10& provided direct evidence for the localized nature of the lowest-excited states.I4 Significantly,
0 1995 American Chemical Society
Letters
16808 J. Phys. Chem., Vol. 99, No. 46,1995 I
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Figure 1. Luminescence spectra of the series [R~(bpy)3-~(bpy-d~),]~+ ( x = 0-3) in [Zn(bpy)l](C104)2at 6 K. The inserts show the orientation of the [R~(bpy)(bpy-d&~+complex in the lattice with respect to the crystal b axis for deuterated (-6)and protonated ( - h ) emission. Arcs with arrowheads indicate the direction of intramolecular excitation energy transfer. Asterisks denote the ligands from which emission can occur.
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Figure 2. Luminescence spectra of [Ru(bpy-3,3’-d2)3l2’ and [Ru(bpy4,4’,5,5’,6,6’-d6)~I2+ in [Zn(bpy)l](C104)2at (a) 6 K and (b) 1. 8 K in comparison with spectra of the perprotonated and perdeuterated complexes.
to 3MLCT origins involving the bpy-h8 ligand to shift to higher two sets of electronic origins I, 11, and I11 are observed in energy upon further deuteration. Correspondingly, transitions excitation for the x = 1 and x = 2 systems. From their energies involving the bpy-d8 ligand will shift to lower energy upon and relative intensities, these two sets are identified as indeprotonation. pendent 3MLCT origins I, 11, and I11 involving the bpy-h8 or It is known that selective deuteration or protonation of the the bpy-d8 ligands. Luminescence occurs from both the bpy3,3’-position of the bpy ligand occurs in alkaline solution^.'^,^^ d8 and the bpy-h8 ligand in the n = 2 system as the complex The complexes used in the following section were obtained by can enter the host with the bpy-d8 ligands in the crystallographiapplying the method outlined in ref 16. The starting materials cally equivalent positions. l 4 In this case luminescence involving the bpy-d8 ligand occurs because the deuteration shift is only { [Ru(bpy)3I2’, [Ru(bpy-d~hI~+, and [ R U ( ~ P Y ) ( ~ P Y - ~were S)~I~+~ synthesized as is described e1~ewhere.l~ They were added to a ~ 3 cm-’ 7 whereas the bpy-h8 ligand in the crystallographically 0.05 M solution of sodium methoxided3 in DMSO-ddmethanoldistinct position is a few hundred wavenumbers higher in energy. d4 or to a 0.05 M solution of sodium methoxide-h3 in DMSOFast intramolecular excitation energy transfer ensures that in hdmethanol-h4 (12: 1 v/v) for deuteration or protonation, rethe x = 1 system emission occurs only from the bpy-hs ligand.I4 spectively. The solutions were kept at 40 “C. NMR spectroscopy In Figure 1 we summarize luminescence spectra measured (300 MHz) was used to closely monitor the progress of the at 6 K in the region of the electronic origins I and I1 of the selective deuteratiodprotonation process of the 3,3’-position. entire series. The luminescence is dominated by emission from the second lowest-excited level I1 at this temperat~re.’-’~%’~ Under these conditions, deuterationlprotonation of the 3,3’position is complete after 1-2 h. Furthermore, we observe Recently, Braun et al. have ~hallenged’~ our interpretati~n’~ of deuteratiodprotonation of other positions, although at a lower the spectroscopy of the [Ru(bpy)s-,(bpy-d8)J2+ series in [Znrate (zl/lO). Complexes obtained by this procedure contain (bpy)3](C104)2 and suggested that the two sets of origins in the ( < 5 % ) species with f l deuteron. x = 1 and x = 2 systems are due to two crystallographically In Figure 2, luminescence spectra of [Ru(bpy-3,3’-d2)3I2+and inequivalent sites. This hypothesis can be refuted on the basis of the observed intensity ratios in luminescence, absorption, and [Ru(bpy-4,4’,5,5’,6,6’-d6)3I2+ (prepared by deuteratiodprotonation of the perprotonated/perdeuterated complexes) in [Zn(bpy)3]excitation and systematic observations made in luminescence (c104)2 are shown in comparison with the parent spectra at (a) and excitation line narrowing experiment^.^,'^.'^ Furthermore, 6 K and (b) 1.8 K. At 6 and 1.8 K luminescence is dominated we observe a rise in deuterated emission in comparison with by emission from level I1 and I, respectively. A linear shift of protonated emission in the x = 2 system when increasing the origins I and I1 to higher energy is observed as a function of temperature. This arises at higher temperature as energy can the deuteration degree. This arises because the deuteration be transferred back to the higher-lying bpy-d8 ligand in the degree of all three ligands is the same. A gradual shift of the crystallographically equivalent position for the complexes which enter the lattice by substituting one of the equivalent ligands origins to higher energy would also result in the series [Ruby bpy-h8. The x = 1 system shows also emission from the (bpy)3-,(bpy-d8),12+ (x = 0-3) if the lowest-excited states were bpy-d8 ligand at higher temperature for the same reason. delocalized over all three ligands. In Figure 3 the luminescence spectra in the region of the Our assignments can be directly tested by further deuteration/ electronic origins I and I1 of the complexes [Ru(bpy-hs)(bpyprotonation of the [R~(bpy)3-,(bpy-d~)~]*+ complexes. Deuteratiodprotonation can only occur on the bpy-h&py-ds ligands, 4,4’,5,5’,6,6’-d6)2I2+ and [Ru(bpy-3,3’-dz)(bpy-ds)z12+ (prepared by protonation/deuteration of [Ru(bpy)(bpy-d&I2+ ) are shown respectively. We expect transitions which have been assignedI4
Letters
J. Phys. Chem., Vol. 99, No. 46,I995 16809 they involve the bpy-h8 ligand. Correspondingly, the lowerenergy origins shift to higher energy in the [Ru(bpy-3,3’-d2)(bpyds)2I2+complex as they involve the bpy-3,3‘-d2 ligand whereas the higher-energy origins stay at the same energies as they involve the bpy-dg ligand. These results confirm the validity of assignments given in ref 14. The lowest-excited 3MLCT levels in [Ru(bpy)3I2+systems appear invariably localized.
References and Notes
k I
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Figure 3. Luminescence spectra of the complexes (k) [Ru(bpy-hs)(bpy-4,4’,5,5’,6,6’-d6)2]*+ and ti) [Ru(bpy-3,3’-d~)(bpy-ds)z12Cin comcomplex in parison with the spectrum of the (i) [Ru(bpy)(bp~-dg)2]~+ [Zn(bpy)J(ClOa)z at (a) 6 K and (b) 1.8 K.
in comparison with the spectrum of the starting complex in [Zn(bpy)3](ClO&. The higher-energy origins I and I1 in the [Ru(bpy)(bpy-4,4’,5,5’,6,6’-d6)2]2+ complex are shifted to lower energy in comparison with the corresponding origins ( I d , 11-d) in the [Ru(bp~)(bpy-d&]~+ complex as they involve the bpyd6 ligand. In contrast, the lower-energy transitions I and II stay at the same energies in comparison with the corresponding transitions (I-h, 11-h) in the [Ru(bp~)(bpy-d8)2]~+ complex as
(1) Bradley, P. G.; Kress, H.; Homberger, B. A.; Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. SOC. 1981, 103, 7441. (2) Smothers, W. K.; Wrighton, M. S. J. Am. Chem. SOC. 1983, 105, 1067. (3) Juris, A,; Barigelleti, F.; Balzani, V.; Belser, P.; Zelewsky, A. Inorg. Chem. 1985, 24, 202. (4) Caroll, P. J.; BNS, L. E. J. Am. Chem. SOC. 1987, 109, 7613. (5) Kato, M.; Yamauchi, S.; Hirota, N. J. Phys. Chem. 1989, 93, 3422. (6) Striplin, D. R.; Crosby, G. A. Chem. Phys. Lett. 1994, 221, 426. (7) Riesen, H.; Rae, A. D.; Krausz, E. J. Lumin. 1994, 62, 123. (8) Riesen, H.; Krausz, E. Chem. Phys. Lett. 1994, 214, 613. (9) Riesen, H.; Gao, Y.; Krausz, E. Chem. Phys. Lett. 1994,228, 610. (10) Riesen, H.; Krausz, E. Chem. Phys. Lett. 1993, 212, 347. (1 1) Riesen, H.; Wallace, L.; Krausz, E. Chem. Phys. Lett. 1994, 228, 610. (12) (a) Harrowfield, J. M.; Sobolev, A. N. Aust. J. Chem. 1994, 47, 763. (b) Krausz, E.; Riesen, H.; Rae, A. D. Aust. J. Chem. 1995, 48, 929. (13) Riesen, H.; Wallace, L.; Krausz, E. Chem. Phys. 1995, 198, 269. This work presents also an analysis of the vibrational sidelines of the entire series [R~(bpy),-~(bpy-d~).J~+ in the [Zn(bpy)3](C104)2 host. (14) Riesen, H.; Krausz, E. J. Chem. Phys. 1993, 99, 7614. (15) Braun, D.; Huber, P.; Wudy, J.; Yersin, H. J. Phys. Chem. 1994, 98, 8044. (16) Constable, E. C.; Seddon, K. R. J. Chem. Soc., Chem. Commun. 1982, 34. (17) Strommen, D. P.; Mallick, P. K.; Danzer, G. D.; Lumpkin, R. S.; Kincaid, J. R. J. Phys. Chem. 1990, 94, 1357.
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