2306
J. Phys. Chem. 1980, 84, 2306-2310
Synthesis and Spectroscopic Properties of Tris( 2,2'-bipyridine)ruthenium( 11) in Zeolite Y Witty DeWilde, Guldo Peeters, and Jack H. Lunsford" Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: February 1 I, 1980)
Tris(2,2'-bipyridine)ruthenium(II) complexes have been synthesized within the large cavities of dehydrated Y-type zeolites by allowing bipyridine to react with a Ru(NH,),-Y form of the zeolite. The resulting Ru(bpy)t+ complexes are characterized by absorption and emission bands similar to those found in aqueous solutions. The relatively high concentration of the Ru(bpy)32+complexes resulted in concentration quenching. As the addition of water to the samples approached saturation,the luminescence was quenched; however, small amounts of water increased the luminescence in a sample which contained 2.8 complexes per unit cell. Emission was also quenched upon addition of O2 to the zeolite, with the effect being more pronounced in samples having a smaller concentration of complexes.
Introduction Excited states of tris(2,2'-bipyridine)ruthenium(II) having relatively long lifetimes may be readily formed by irradiation with light of X C700 nm. In the presence of suitable reagents the excited state of the complex may serve either as an electron donor or accept0r.l" The resulting Ru+ and Ru3+complex ions are capable of reducing and oxidizing water, respectively, thus resulting in the It should be noted, dissociation of water into H2and 02.536 however, that both reactions have not been observed simultaneously in the same system. The photoefficiency of these processes are often small because of reverse electron transfer and energy transfer reactions which leave Ru( b ~ y ) complexes ~~+ in the ground state. A number of schemes have been devised in order to prevent these reverse reactions, including the deposition of monolayer assemblies of complexes in which the ligand is an ester derivative of bipyridine. Earlier results by Whitten and co-workers' suggested that this system was effective in the photochemical dissociation of water; however, more recently it was reported that highly purified samples of the surfactant-ruthenium complex were inactive in the photochemical reaction.8 In an attempt to provide a suitable environment in which the electron transfer reactions may occur we have synthesized Ru(bpy),2+ complexes within the large cavities of a Y-type zeolite. Restricted motion of these complexes excludes collisional deactivation, yet the open structure of the zeolites provides access to gas phase molecules such as water and oxygen. In the present study we report the synthesis of the complexes in the zeolite and the quenching effect of water, oxygen, and other Ru(bpy)?+ complexes (concentration quenching). Iron,geuropium? copper,l"and silverll have been previously explored in zeolites by use of luminescence techniques, but no studies of metal complexes in zeolites have been reported. Experimental Section Zeolite Y, obtained from Linde (lot no. 373856), was exchanged with Ru(NH3),3+for 24 h in an aqueous slurry which contained from 1to 5 g of zeolite/L. The initial and final pH of the aqueous phase was 5.2 and 6.5, respectively. The zeolites were filtered from the solution, washed three times in deionized water, and dried at 25 "C. The ruthenium-ammine complex was prepared as Ru(NH3),Br3, according to the method of Fergusson and Love.12 The ruthenium-bipyridine complex was prepared by allowing R U ~ ~ ' ( N H ~ )t~o- react - Y with a 4:l mole ratio of
TABLE I: Cation and Complex Content of the Zeolite Samples %
com-
sample
a
Na'l u c" 55.3b 53.3 45.0 45.0 44.1b 40.1 28 28
uc denotes unit cell.
Na'
Ru3'/ uc 0.25 0.90 3.45 3.45 3.97 5.30 9.15 9.15
+ NH,'
Ruplex (bpy),'+/ foruc mation 0.19 76 0.65 72 1.3 37 2.8 81 3.2 81 4.2 78 2.7 29 6.9 76 3.3
content.
2,2'-bipyridine (K&K Laboratories) to ruthenium. A mixture of the zeolite and 2,2'-bipyridine was kept under vacuum 4 h at 25 "C, and then heated to 200 "C for 24 h in a closed system. The sample was subsequently evacuated at 200 "C for 24 h. In one experiment zeolite Y was impregnated with tris(2,2'-bipyridine)ruthenium(II) chloride obtained from the G. Frederick Smith Co. The complex was dissolved in water and stirred with the zeolite slurry for 4 h, after which the sample was dried under vacuum. In experiments to be subsequently described water vapor was added back to the zeolite. The amount of water added was determined by placing the sample prepared for luminescence measurements in a closed system with a second sample which was attached to a quartz spring. Water vapor was admitted to the system, and after allowing several days for equilibration the amount of water in the zeolites was gravimetrically determined. The zeolites were analyzed for sodium and uncomplexed ruthenium by partially dissolving the sample in HC1. The undissolved amorphous silicate was centrifuged out and washed. The solution and the washings were combined and analyzed by either atomic absorption or flame emission spectroscopy. The zeolites which contained Ru(bpy)t+ were partially dissolved in citric acid. After removing the undissolved silicates the spectrum of the solution was recorded between 350 and 600 nm. The concentration of the complex was calculated from the absorption band at 453 nm, using an extinction coefficient of 14335 M-l cm-l. Exchange levels and complex concentrations are given in
0022-3654/80/2084-2306$0 1.0010 0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 18, 1980 2307
Ru(bpy)$+ Complexes in Y-Type Zeolites
290
I
I
285
280
Binding Energy, eV
u
Figure 1. ESCP, spectral of (A) Ru(bpy),-Y(5.9) and (6)a Na-Y zeolite impregnated wkh 3.3 niolecules of Ru(bpy);+/unit cell.
Table I. The samples are designated by the complex, the zeolite, and the complex concentration, which is designated by the number in parentheses. Diffuse reflectance spectra were recorded by use of a Cary Model L4 spectrophotometer with a type I1 reflectance attachment. /spectra were obtained over the range 300-700 nm. Sodium-Y zeolite was used as a reference standard. A Baird-Atomic SF-100 Fluorospecinstrument was used to obtain the luminescence spectra. The sample at 25 "C was contained in a 6-mm 0.d. Pyrex tube, Since the position of the sample holder in the spectrometer significantly influenced the measured emission intensity, a spinning sample holder was constructed. The error in reproducibility of intensities was less than 5%. The 366-nm line of a 100-W Hg lamp was the excitation source. The emission spectrum, recorded in the 450-700-nm region, was not corrected for photomultiplier response. The X-ray photoelectron spectra were obtained by using a Hewlett-Packard 5950A ESCA spectrometer with A1 Ka,,, radiation. A carbon 1s line at 285 eV was used as an internal standarc! for binding energy determinations.
Results and Discussion Location of Complexes. From the analytical data presented in 'Table I it is evident that under favorable conditions from 72 to 81% of the ruthenium reacted with bipyridine to form Ru(bpy)?+ complexes. In the two cases where the conversion was less, R~I'(bpy)~-Y(1.3)and Ru"(bpy),-Y (2.71, the zeolite was only briefly degassed before heating the reactants in the closed cell. At maximum loading there was slightly less than one complex per large cavity. The size of the complex and the ability of transition metal ions to migrate during complex formation made it necessary for us to demonstrate that R ~ ( b p y ) , ~was + actually formed within the large cavities rather than on the external surfaces of the zeolite crystallites. Although, to our knowledge, no X-ray diffraction studies have been copper complex has reported for R ~ ( b p y ) ~the ~ +analogous , a maximum dimension of 1.21 nm1.13Since Cu2+and Ru2+ have the same ionic radii of 0.069 nm, one would expect that the overd size of the complex would be similar. Thus, the Ru(bpy),2'' complex is sufficiently small to fit in the large cavities of the 'IT-type zeolite, which have a free diameter of ca. 1.3 nm. More direct evidence for the location of the complex within the zeolite comes from ESCA data. Since the escape
I
400
SO0
800
IO
wavelength ,nm
Flgure 2. Diffuse reflectance spectra of Ru(bpy),-Y zeolites and of Na-Y and NaNH,-Y zeolites containing bpy: (a) R~(bpy)~-Y(S.9); (b) Ru(bpy),-Y(4.2); (c) Ru(bpy),Y(3.2); (d) Ru(bpy),-Y(O.19); (e) bipyralne absorbed in NaNH4-Y; ( f ) bipyrldine absorbed in Na-Y.
depth of the photoelectrons is about 30 A, complexes on the external surface of a zeolite crystallite give rise to a much greater signal than complexes which are uniformly dispersed through the crystal, assuming a constant average concentration. Upon comparing in Figure 1 the signal intensity of the Ru 3d5/, line at 280.8 eV for the synthesized sample (curve A) and the zeolite impregnated with Ru(bpy),2+(curve B), it is evident that the impregnated sample had a greater surface concentration of the complex, even though the average concentration of ruthenium in the synthesized sample was three times as large (Table I). Diffuse Reflectance and Emission Spectra. Diffuse reflectance spectra of the orange-colored Ru"(bpy),-Y zeolites at four concentration levels are depicted in Figure 2. Although these spectra were recorded with the sample in air, essentially the same spectrum was observed for Ru"(bpy),-Y (2.8) under vacuum. The spectra are characterized by a maximum at 460 nm with shoulders at 430 and 540 nm. I t is important to note that the band at 540 nm did not decrease proportionally with the other two bands as the concentration of the complex decreased. Two bands in the 425-454-nm region have been reported by several investigators for the Ru(bpy),,+ complex in different media.'"l' These bands have been ascribed to metal-to-ligand (t2-n*) transitions. Lytle and Herculesl'j also reported an absorption around 525-550 nm which was explained as a singlet-to-triplet transition. Such a transition is normally forbidden by spin selection rules; however, it may occur in this species because of strong spinorbit coupling. Determining the origin of the band at 540 nm in the zeolite is not straightforward since Na-Y and NaNH4-Y heated with bipyridine exhibit a spectrum having a maximum at 530-540 nm (curve e). In addition, we have observed that Fe(bpy)32+in a Y-type zeolite has a maximum at 520 nm with a shoulder at 490 nm. We tentatively assign the band at 540 nm to F e ( b ~ y ) , ~com+ plexes formed from Fe(I1) ions which are present as impurities in the zeolites. The greater intensity of the band observed in the NaNH4-Y zeolites may be attributed to the reduction of Fe(II1) to Fe(I1) by ammonia.
2308
The Journal of Physical Chemistry, Vol. 84, No. 18, 1980
DeWilde, Peeters, and Lunsford 2
.-clcn a
ZI
.-e e
em
F 1
D
[Ru(bpy)3rTcomplexes2/unitcell 660
550
600
650
700
Figure 4. Dependence of the reciprgcal emission intensity upon the square of the concentration of Ru(bpy),*+ complexes in zeolite Y.
was 0.77 M for the Ru"(bpy),-Y(6.9) sample. The extremities of the complexes are almost in contact with one another through the 12-membered rings which separate the large cavities. Resonance transfer can occur with high probability between centers separated by tens of angstroms if absorption and emission bands of the centers overlap appreciably, and if the transitions are allowed.26 The transfer probability per unit of time, wtr, is proportional to 1/R6, where R is the distance between two absorbing molecules, and in turn 1/R6 0: (concentration)2 (Figure 4). Thus if thermal quenching is neglected, the emission intensity is related to wtr and w,, the radiation probability, according to wr
+ wtr
or 1 Wtr -al+-
I
and if wtr/w,
'4
>> 1 1 Wtr - a - a Wr
I
1
R6
(3)
When the absorption and emission bands for Ru(bpy)32+ are compared it is evident that the requirement for overlap is met in the region from 550 to 575 nm. By contrast, Seefield et a1.26found that for monolayer assemblies of ruthenium-bipyridine complexes (donors) and suitable acceptor molecules resonance energy transfer (exciton migration) was not a significant factor in quenching. This was demonstrated by the observation that dilution of the donor molecules by inert matrix molecules did not affect electron transfer. The differences between this observation and the results reported here for zeolites may be due both to the nature and the concentration of the acceptor sites, as well as to the mode of quenching, i.e., electron vs. energy transfer. In the monolayer assemblies significant quenching was observed only at average distances between acceptor molecules of
