J. Phys. Chem. 1993,97, 12705-12709
12705
Highly Resolved Emission of [Os(bpy-h&,(bpy-&)~~]~+ (n = 0-3): Evidence for Electronic Delocalization Peter Huber and Hartmut Yersin' Institut f i r Physikalische und Theoretische Chemie, Universitiit Regensburg, 0-93040 Regensburg, Germany Received: July 22, 1993; I n Final Form: September 28, 1993'
Highly resolved emission spectra of the title compounds doped into single-crystal [Zn(bpy-h&] (C104)* have been recorded at 1.3 K. The partially deuterated dopants (n = 1, 2) occupy different sites in the host matrix. It is possible to excite site selectively the dominant site (designated B) for both compounds. Properties of the lowest excited electronic states of 3MLCT character of these sites of the partially deuterated dopants (n = 1, 2) are compared to the properties of the fully protonated (n = 3) and the fully deuterated (n = 0) species. The electronic origins lie at 14 223 cm-l (n = 3), 14 238 cm-1 (site B,n = 2), 14 233 cm-l (site B,n = l), and 14 256 cm-1 (n = 0). Within the experimental error o f f 1 cm-l they are found at exactly the same energies in emission and excitation. The emission spectra of the four compounds exhibit rich vibrational satellite structures, being connected with these origins. A comparison of the satellite structures reveals that in the vibrational energy range of the metal-ligand modes the metal couples the different ligands, while in the energy range of the ligand modes (above -500 cm-l) the modes can be attributed to the individual ligands. This leads to the interesting result that the partially deuterated complexes exhibit (to the same electronic origin) vibrational lines, which occur also in the fully protonated and the fully deuterated compounds. For a given compound, the emission lifetimes are exactly the same, when measured at the energy of the origin and at any vibrational line, respectively. The decay is strictly monoexponential for every compound. The lifetimes increase upon deuteration [ T values: n = 3,22ps;n = 2 (site B),26 ps;n = 1 (site B), 3 1 ps;n = 0,46ps]. These results lead to the conclusion-contrary to currently accepted models-that the excited electronic state is delocalized over all three ligands.
1. Introduction During the past decades there has been enormous activity in studying the photochemistry and spectroscopy of [Ru(bpy)3]2+ and the related [0s(bpy)3l2+ complexes (with bpy = 2,2' bipyridine).'" Since the properties of the lower excited states of these compounds determine largely the photochemical and photophysical behavior, many different spectroscopictechniques have been applied to develop a deeper understanding. It is agreed in the literature that the lowest excited states are classified as 3MLCT states. More details about these electronic states are found especially from emission spectra, which exhibit highly resolved vibrational structures. Recently, such emission spectra could be registered from the fully protonated [Os(bpy-h~)~]2+ and the fully deuterated [Os(bpy-d&12+ doped into different matrices. A detailed comparison of the spectroscopic properties ofthedifferent compounds is worked out in refs 6-9. In particular, it is possible to correlate many of the vibrational modes of the perprotonated with those of the perdeuterated complexes. It is the subject of this paper to investigate also the vibrational satellite structures of the partially deuterated complexes (with n = 1,2), since it is of high interest to introduce an isotopic marking of one (or two) of the ligands. By this it seems possible to answer the fundamentalbut still controversiallydiscussed questionof whether or not the electronic charge distribution in the lowest excited state(s) is localized toward one particular bpy ligand. Simply investigating the energy range of the electronic origins might lead toan inconclusiveinterpretation(as presented for [Ru(bpy)3](PF6)2") dueto theoccurrenceofdifferent crysta1lographicsites.l' Thus, the present paper will answer this question by examining the electronicorigins, the vibrational satellite structures, and the luminescence decay properties of the title compounds. 69'
2. Experimental Section
Deuterated 2,2'-bipyridine (bpy-da) was prepared as is described in ref 12. The final degree of deuteration was better than 98%, Abstract published in Advance ACS Absrrucrs, November 1, 1993.
as was determined by mass spectrometry. No preferred position of deuteration occurred as checked by recording the NMR spectrum of 2,2'-bipyridine-d8 (e.g. see ref 13). [Os(bpy-d&C121,14 [Os(bp~-ha)sl(C104)2,~~ and [Zn(bp~)3l(C104)2~ were prepared following the cited procedures. [Os(bpy-h&(bpy-d8)](C104)rHzO and [Os(bpy-h~)(bpy-d~)~] (C104)&0 were prepared as follows: cis- [Os(bpy-h&Cl2] and cis- [OS(bpy-d&C12], respectively, and a small excess of (bpy-da) and (bpy-ha), respectively, were heated under reflux (200 "C) in 30 mL of ethylene glycol for 3 h. The solvent was distilled off, and the remaining residue was dissolved in 100 mL of ethanol/acetone (1:l). After filtration, hydrolized AgClO4 was added until no further AgCl precipitated. AgCl was removed by vacuum filtration, and diethyl ether was added slowly to precipitate the compounds. The products were washed with ether and dried in a vacuum at 80 "C. Elemental analysis of C, H, and N (H and D were determined simultaneously) gave the following. Anal. C, 40.77; H, 2.97; Calcd for [Os(bpy-h&(bpy-d~)](C104)~H~O: N, 9.51. Found: C, 40.68; H, 2.80; N, 9.53. Anal. Calcd for [O~(bpy-hs)(bpy-da)~](ClO~)~.H~O: C, 40.40; H, 2.94; N, 9.42. Found: C, 40.40; H, 3.12; N, 9.42. The utilized substances 2,2'-bipyridine (Aldrich), Zn(C104)2*6H20(Alpha), OsCl3.3H20 (Degussa), and AgC104xHlO (Janssen) were obtained commercially and used as purchased. The applied solvents, also obtained commercially, were used as reagent grade without further purification. Single crystals of [Zn(bpy-h&](C104)2 doped with Os complexes were grown from a solvent containing the dissolved matrix material and the chromophore by slow evaporation of bidistilled water (molar ratio in solution 0s:Zn 0.002). The emission measurementswere camed out withsinglecrystals placed on a copper tongue and introduced into a helium bath cryostate (Leybold), which could be cooled to 1.3 K by pumping off liquid helium. For excitation, we used an argon ion laser (A = 5 14.5 nm, Spectra Physics 164AC),for site-selectiveexcitation, an argon ion laser pumped cw dye laser (Spectra Physics 375, line half-width 2 cm-I), and for decay measurements, an excimer
0022-365419312097-12705%04.00/0 0 1993 American Chemical Society
12706 The Journal of Physical Chemistry. Vol. 97, No. 49, 1993
Huber and Yersin
The lowest excited electronic state is generally classified as a component of a 3MLCT state of Os(Sd, *)-bpy(r*) character, while the ground state is a ~ i n g l e t . ~ The * ~ Jforbidden" ~~~ of this transition is manifested by the relatively long emission lifetime (Figure 2, Table I). Interestingly, the decay curves are strictly matrix n=3 n=2 n=l n=O monoexponential for more than four lifetimes for each compound A 14 220 (25) A 14 218 (b) andeachsite. Thedecay timesvarywiththedegreeofdeuteration. [Zn(bpy-h&]- 14 223 (22) B 14 238 (26) B 14 233 (31) 14 256 (46) They are larger by more than a factor of 2 for the predeuterated C 14 258 (35) C 14 256 (36) (C104)z compound (T = 46 f 1 ps) than for the perprotonated one (T = 0 The electronic origins were identified in emission and excitation 22 f 1 ps; Figure 2). This is a known effect. By deuteration, spectra. Emission lifetimes (M) are given in parentheses. For n = 1,2 the nonradiativedeactivationrates are reduced,giving an increase one finds different sites; they are designated A, B, and C, respectively. of decay times and emission intensities.812' (Experimentalerror = *1 cm-1 and 1 ps). b Could not be measured due to the extremely low emission intensity. Further, the decay times were measured at the energies of the electronic origins and at the positions of the different vibrational components, respectively. For the same species, one always laser (Radiant Dyes EXC 150)pumped dye laser (Lambda Physik obtains the same decay time. This shows that the components Fl 2000, line half-width 0.1 5 cm-1) with pyridine 1 in ethylene at the low-energy side of the origin are directly related to this glycol/propylenecarbonate (4: 1) as laser dye. The optical setup specificorigin. (See also section 3.3.) Especially, the same result of the detection system is described in ref 16. The signals of the is found for the partially deuterated compounds. For example, photomultiplier (RCA 7164R, cooled to -30 OC by a Joule[O~(bpy-h~)(bpy-d~)2]~+ exhibits exactly the same decay time of Thomson cooling system, Seefelder Messtechnik, D-82229 T = 31 f 1 ps when measured at 14 233 cm-l (origin site B), at Seefeld) were processed by a computerized photon counting 12 700 cm-1 (1533-cm-' mode), and at 12 669 cm-' (1564-cm-1 system. The emission decay was registered with a fast multimode) (Figure IC). This result leads to important consequences, channel analyzer with a minimum dwell time of 5 ns/channel which will be discussed in section 3.4. (CMTE 7885, FAST, D-82041 Oberhaching). The monochromator (Spex 1401) readout was calibrated with a neon lowThe electronictransition energy of the perdeuterated complex pressure lamp. Thus, the experimentalerror of the absoluteenergy is blue-shiftedby 32 f 2 cm-l relative to that of the perprotonated positions of the sharp lines is = f l cm-'. The emission spectra complex. This effect has already been discussed in detail and were corrected for the spectral response of the detection system. ascribed to very small differences in the decrease of the zeropoint vibrational energies in the electronicground state compared 3. Results and Discussion to the excited state upon deuteration (very small differences in force constant^).^-^ The absolute energies of the origins of the 3.1. Different Sites. The title compounds diluted in a partially deuterated compounds at the B site are also blue-shifted crystalline [Zn(bpy-h~)3](C104)2matrix exhibit, at low temperrelative to the energy of the perprotonated compound. But, as ature, highly resolved spectra. These are very complicated for is manifested by the occurrence of different sites, an additional the partially deuterated complexes ( n = 1,2) when, for example, energy shift is present, which results from an individual chroan excitationwavelength of 5 14.5 nm is used. However, applying mophore-site interaction. the techniques of the site-selective excitation and emission 3.3. Vibrational SatelliteStructures. The lines in the emission spectroscopy, it is possible to show (compare ref 7) that the spectra found at the low-energy side of the electronic transition partially deuterated compounds are found at three different sites correspond to vibrational modes which are involved in the in the [Zn(bpy)s](C104)2 matrix.17 On the other hand, the fully electronic deactivation paths. An analysis of the satellite protonated and the fully deuterated Os complexes, respectively, structures leads to a seriesof important results also about electronic occupy only one site (compare refs 6 and 8). We determined the propertiesof [Os(bpy)g]2+. For an examinationof the vibrational energy of the lowest electronic origin (line I; see below) for each satellite structures it is convenient to discussthree different energy of the title compounds and for the different sites of the partially ranges. deuterated compounds (Table I). But is was not yet possible to (1) The peaks, which appear in the low-energy range up to register selectively excited emission spectra of every individual -100 cm-I from the origin, are assigned to result from lattice site. However, it is obvious that the spectra correspondingto the modes (e.g. 33,48,67,76 cm-l; Figure 1). The energies of these different sites of the same compound exhibit some differences are independentof the investigatedchromophoresince the matrix with respect to the emission intensities, the vibrational satellite is not changed. These lattice modes couple also to several highstructures, and the emission decay times. (The decay times are energy vibrational modes [e.g. Figure l a (767 +33, +48, +67, included in Table I.) Fortunately, one site (designated site B, +76) cm-1, (1491 +33, +48) cm-l; Figure Id (731 +33, +48, Table I) exhibits a more intense emission than the others, and +67, +76) cm-l, (1429 +33, +48) ~ m - ~ (Thesecombinations ] . ~ further, it is possible to obtain selectively excited and well-resolved are clearly seen in the figures, but they are not designated.) spectra for this site B of both partially deuterated compounds. Therefore, we will focus, in this contribution, on the properties (2) In the adjacent energy range up to =500 cm-l one finds of these dominant sites B. the vibrational peaks, which correspond to metal-ligand (M-L) 3.2. Electronic Origins. Figure 1 shows the emission spectra modes which, however, may also includeappreciablecontributions (T = 1.3 K) of the lowest excited states of [Os(bpy-hs),,(bpyfrom ligand deformations. This is concluded from a comparison d8)>J2+, with n = 0-3, doped into crystalline (Zn(bpy-h&]of the vibrational energies reported for [Ru(bpy)#+ 599~22and (C104)2. The spectra with n = 1, 2 are site-selectively excited. the ones determined in this investigationfor the title compounds. The dominant peak at the blue side of every individual emission Several of these modes are designated in the spectra. In general, spectrum is assigned as the electronic origin. This is manifested one observes a red shift, when comparing the mode energies of mainly by two facts. First, theemission and theexcitation spectra the fully deuterated to those of the fully protonated compounds show lines at exactly the same energy (experimental error A1 (Figure 1a,d; see also refs 6-9). However, an obvious and gradual cm-I). Second, the satellites in the emission spectra measured shift is not observed, when the partially deuterated compounds relative to the correspondingorigins fit well to known vibrational are included in this comparison. One also finds blue shifts and energies (see section 3.3). This behavior has already been energy splittings which make a direct comparison very difficult discussed in detail for [Os(bpy-h8)#+ 6,7 and [Os(bpy-d~)3]~+,~ (Figure 1). For example, the vibrations at 442 cm-' of [Osrespectively. (bpy-h~)~]2+ and of [0s(bpy-d8)#+ are accidentally at the same TABLE I: Energy Positions (cm-l) of the Electronic Origins Of [Os(bpy-lg),(bp (a 0-3) Do@ into SingleCrystal (Zn[bpy-lg)~](ClO~)r(T= 1.3 K)'
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The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12707
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0 LOO 800 1200 Icm"I 1600 Figure 1. Emission spectra at T = 1.3 K of [Os(bpy-hn),(bpy-dn)~I2+doped into single-crystal [Zn(bpy-h8)3](C10,)2. Concentrations of the dopants: -0.2%. For a better comparison, the electronic origins (0-0) are set to zero on a wavenumber scale, but the absolute energies (AI cm-1) of the origin lines are given in the figures. The emission intensities at the origins are normalized. The excitation wavelengths A+,, were 514.5 nm (a, d), 692.1 nm (b), and 691.2 nm (c). The asterisk in part b designates the origin of site A, which could not be separated by site-selective excitation. 10'
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0 20 4 0 60 80 100 120 1 1 4 160 Figure 2. Decay curves for [0s(bpy-h&l2+, [Os(bpy-hn)(bpy-ds)zlz+ (site B), and [Os(bpy-d&]Z+ doped into single-crystal [Zn(bpy-hs)+ (CIO&(T= 1.3K).Excitationwavelength&= 337.1 nm. Thedecay time for aspecificcompounddoesnotdependon thedetectionwavelength. The monoexponential decay of [Os(b~y-hn)2(bpy-d8)]~+ with 7 = 26 ps (see Table I) is not reproduced.
energy. This situation results in the appearance of new lines in the spectra of the partially deuterated complexes. These lines can be explained, assuming resonances of 5-10 cm-I between M-L vibrations of the protonated and deuterated ligands (dotted line in Figure 1). These results reflect the fact that in the metalligand energy range the metal ion couples different ligands
vibrationally (e& see refs 23 and 24). A more detailed analysis would require a complete normal coordinate analysis. (3) In the energy range between 4 0 0 and 1700 cm-1 one observes the ligand modes of the complexes, as is concluded from an inspection of the potential energy distributions (PED)given for [ R ~ ( b p y ) 3 ] ~ +Upon . ~ deuteration, every ligand mode is red-~hifted.'-~Interestingly, the spectra of the partially deuterated compounds exhibit the modes of both the protonated and the deuterated ligands. Moreover, the vibrational energies are not altered within the limits of experimental error in a comparison of the four complexes. The broken lines in Figure 1 demonstrate these effects. This result can be explained by assuming that the vibrational energies in this energy region are largely determined by the force constants of the individual ligands (see also refs 5,23,24). The heavy metal ion in the center serves as a buffer, which vibrationally decouples the ligands from each other in the high-frequency region, simply because the massive metal ion cannot follow the vibrations of the light ligand atoms.23 Furthermore, a direct vibrational coupling between two neighboring bpy moieties is expected to be very weak. Experimental support is provided by the low-temperature emission spectrum of [Rh(th~y)~(bpy)]+ (thpy = ortho-C-deprotonated form of 2,2'thienylpyridine).25 The lowest electronic transition in this complex is ligand-centered on one thpy ligand, and the emission spectrum exhibits, in the energy range of the ligand vibrations, only thpy modes but no bpy modes, as would be expected for a situation of a vibrational coupling between the ligands.
12708 The Journal of Physical Chemistry, Vol. 97, No. 49, I993
3.4. Electronic Localization versus Delocalizationin the Lowest Excited State. The question is whether the electronic charge distribution in the lowest excited state of MLCT character of [Os(bpy)#+ extends over the spatial area of the metal and one ligand or the metal and three ligands. The answer is displayed in the vibrational satellite structures. These reflect spatial changes of electronic charge distributions according to an electronic transition.26J7 In particular, the vibrational coupling behavior of (high energy) ligand modes displays properties of MLCT transitions. For example, the emission spectrum of [Ru(bpy)~(bpz)]2+ (bpz = 2,2’-bipyrazine) represents an interesting illustration of this spectroscopic behavior. The lowest excited electronic state of this complex is commonly assigned to be of 3MLCT character of the type Ru(4d, r)-bpz(r*), which means that the excited electron is localized on the bpz ligand.Z8-31 As expected, theemission spectrumof [Ru(bpy)2(bpz)12+doped into crystalline [Zn(bpy),] (C104)2 exhibits, in the energy range of the ligand modes, only bpz ligand modes (e.g. 1563 and 1581 cm-I), but bpy ligand modes are not d e t e ~ t a b l e .Only ~ ~ those vibrational lines (high-energymodes) correspondingto the ligand involved in the electronic transition appear in the emission spectrum. In contrast to the described behavior of this heteroleptic Ru complex, the emission spectra of the partially deuterated Os complexes show the ligand modes of both the protonated and the deuterated ligands being connected to the same electronicorigin. This allows the conclusion that the excited electronic charge distribution is delocalized over all three (bpy) ligands. An alternative interpretation should be taken into account. Let us assume a statistical superposition of emission spectra resulting from complexes where the excited electron is localized on a protonated ligand (type a; leading to the occurrence of bpyha modes) and from other complexes (type b) with localization on a deuterated ligand (giving the bpy-d8 modes). However, this model can be excluded due to a series of arguments. The two emitting types of complexes (a and b) would not be in a thermal equilibrium due to the low concentrations of the dopants. Further, a and b should have two different electronic origins according to the different amounts of blue shifts in the two situati0ns.Q’ This is not the case. The observed different vibrational satellites belong to the same electronic origin. A supposed accidental coincidenceof the origins corresponding to types a and b is extremely improbable, since this should occur for [Os(bpy-h~)(byp-d&]~+ as well as for [Os(bpy- h&( bpy-d8)]2+ and for every site. Moreover, a (statistical) superposition of type a and type b emission spectra should lead to a biexponential decay or to two different decay times, when measured on a vibrational satellite corresponding to a bpy-d8 ligand and to a bpy-ha ligand, respectively. This is expected since the isotope effect should be of different importance in the two situations a and b. But the experiment yields only one decay time, lying intermediatebetween the one of the fully deuterated and the one of the fully protonated species. This decay time is exactly the same for every vibrational satellite (and the origin). Further, the decay is strictly monoexponential (Figure 2, section 3.2). It is of interest to discuss a further alternative model, which assumesa localizationof the excitationon one of the three ligands of a partially deuterated complex, however, allowing the occurrence of hopping or tunneling processes. For this discussion it is appropriate to distinguishthree time ranges:27 (1) Suppose the hopping time t h between different ligands is of the order of the emission decay time T . Then one would observe two distinct decay times and two different electronic origins according to the involvement of the bpy-ha and the bpy-d8 ligands, respectively. This is not seen experimentally. (2) For the time range T >> t h >> t,, where tn is time needed for the nuclear relaxation accompanyinga localization process,27 e.g. t, = 10-12s, one would
Huber and Yersin observe a monoexponential and mean decay time but twodifferent electronic origins according to the different isotope-induced blue shifts.9 Experimentally, one observes only one origin. (3) A very fast hopping with th
