6990
J. Phys. Chem. 1991,95,6990-6996
revealed a concentration-independent blue shift of the first absorption band that indicated the existence of methylene blue dimers in the monolayer. A simple model of methylene blue dimer adsorption which fits the SHG orientation data was suggested. Finally, it has been demonstrated that the inclusion of multiple elements of 8, monolayer dielectric constants, and phase mea-
surements all contribute to the improvement of the SHG orientation calculation.
Acknowledgment. We gratefully acknowledge the support of l+x"mion in these studies. the National S 5 m ~ Registry No. Silica, 60676-86-0 methylene blue, 61-73-4.
Reflectance Spectroscopic Studies of the Cation Radical and the Triplet of Pyrene on Alumina Surapol PIIIlkasem and J. Kerry Thomas* Department of Chemistry and Biochemistry. University of Notre Dame, Notre Dame, Indiana 46556 (Received: November 20, 1990; In Final Form: February 7, 1991)
The triplet state and the cation radical of pyrene are produced on excitation of pyrene adsorbed on y-alumina. The characteristic absorption spectra of these two alumina bound transients are similar to those in solution and exhibit absorption bands at 415 and 450 nm for the triplet and the cation, respectively. The triplet state decays exponentially with a lifetime of 5.1 ms, while the cation decays in a more complicated fashion which can be described by a Gaussian distribution of the logarithm of the rate constant at various adsorption sites. Movement of ferrocene to the triplet state on the surface quenches the triplet via energy transfer. The bimolecular quenching rate constants of pyrene triplet by ferrocene are 3.31 X IO", 2.63 X lo", and 2.20 X 10" m2 mol-' s-l for alumina at pretreatment temperaturesof 130, 250, and 350 'C, respectively. Pretreatment of the alumina surface at high temperature decreases the number of surface hydroxyl groups. These bind adsorbed molecules less strongly than the acidic sites formed at the high pretreatment temperature. Hydration of the surface by small amounts of wadsorbed water increases the number of surface hydroxyl groups and enhances the degree of dynamic quenching. The formation of the cation radical of pyrene on alumina is a two-photon process, and the cation radical is quenched dynamically by ferrocene. The studies clearly distinguish the role played by heat activation of alumina on the diffusion and rates of photoinduced process on the surfaces. A comparison with similar results on silica shows that adsorbed molecules are less mobile on alumina.
Introduction Molecular dynamics of adsorbed m o l d e s on metal oxides have received attention for many decades. The technique of fluorescence probing has been used to investigate the diffusion of adsorbed fluorescing polyaromatic hydrocarbons on silica and alumina.'+ The diffuse reflectance laser photolysis techniqueS enables one to monitor triplet-triplet absorption of adsorbed hydrocarbons on these oxides.68 The literature indicates some disagreement regarding the effect of oxide pretreatment temperature on the diffusion processes on silica surfaces. A triplet-triplet annihilation study of acridine adsorbed on silica indicated that higher pretreatment temperatures cause the acridine to diffuse more slowly as the number of silanol groups is reduced. It was also observed that acridine does not move on an alumina surface on the time scale of the experiments! On the other hand, fluorescence photobleaching studies of tetracene9 showed that the diffusion increased when the silica was pretreated at high temperatures. This paper presents the transient reflectance laser photolysis study of pyrene adsorbed on y-alumina. Photolysis of adsorbed pyrene yields the triplet state and the cation radical. With lowtemperature pretreated alumina surfam, the energy transfer from the triplet state of pyrene to ferrocene, Le., the triplet quenching, is dynamic in nature. It is believed that this is the first report on diffusion on alumina by diffuse reflectance laser photolysis. The bimolecular quenching rate constants on alumina surfaces a t different pretreatment temperatures indicate that the mobility of the adsorbed species decreases with increasing pretreatment temperature. The difference in mobility is attributed to the relative strength of solute binding to the hydroxyl layer on the surface compared to binding at the Lewis acid sites. Higher pretreatment temperatures increases the surface Lewis acid sites which leads *To whom corrsrpondena rhould be a d d d . OO22-3654/91/2095-6990$02.50/0
surf. area,
Lewis acid sites! cm-2 130 200 0.8 x 1014 250 200 7.8 X I O l 4 1.6 X IO" 350 200 6.2 x 1014 2.1 x 1014 4.6 X IO1' 2.4 X IO" 450 200 750 200 1.3 x 1014 3.0 x 1014 OThe numbers were taken from ref 26. bCalculated form the e amount of the cation radical of N,N,N',N'-tetramethylbenzidinproduced similar to that described in ref 12.
T.. OC
m2/R
OH Rroups.@ 11.5 x 1014
~~
to increasing yield of ferrocenium ions. The quenching process on the surfaces of high pretreatment temperatures therefore switches from dynamic to static in nature.
Experimental Section Matedda Pyrene (Aldrich Chemical Co.) was purified by silica gel/cyclohexane column chromatography. Ferrocene (Aldrich Chemical Co.) was purified by vacuum sublimation. Cyclohexane ( I ) Bauer, R. K.; Borenstein, R.; Mayo, P.de; Odaka, K.; Rafalska, M.; Ware, W. R.; Wu, K. C. J. Am. Chem. Soc. 1982, 104,4635. (2) Uhl, S.; Krabichler,G.; Rempfer. K.; Oelkrug, D. J. Mol. Srrucr. 1% 143, 279.
(3) Oelkrug, D.; Fleming, W.; Fulleman, R.; Gunther, R.; Honnen, W.; Krabichler, G.; Schafer, M.;Uhl, S. Pure Appl. Chrm. 1986, 58, 1207. (4) Wilkinson, F.; Willrher, C. J.; Johnston, L.J.; Casal, H. L.;Scaiano,
J. C. Can. J. Chrm. 1986,64, 539. ( 5 ) Kcasler, R. W.; Wilkimn, F. J . Chrm. Soc., Faraday Trans. I 1981, 17, 309. (6) Turro, N.J.; Zimmt, M. B.;Gould, I. R. J . Am. Chem. Soc. 1985,107, 5826. (7) Oelkrug, D.; Uhl, S.; Wilkinson, F.; Willsher, C. J. J. Phys. Chcm. 1989, 93, 4551. (8) Beck, G.; Thoman, J. K. Chem. Phys. Len. 1983, 94, 553. (9) Bjarnwon, D. W.; Peterson, N.0.J. Am. Chem. Soc. 1990,112,988.
Ca 1991 American Chemical Society
Reflectance Spectroscopic Studies of Pyrene on Alumina
30
t !i
HB programmable digitizer (response time of 0.4 ns) and finally transferred to a Zenith 2-200,IBM PC-AT compatible computer. Data Treatment. The concept of light absorption by uniformly distributed solid materials has been discussed in detail.13 Two major goals of the transient investigation are (1) to obtain the absorption spectrum of the transient and (2) to obtain kinetics of processes involving the transient. In general, the concentration of species adsorbed on the opaque material can be described by the Kubelka-Munk function, F(R,).14 This function can be defined as follows
-
'"1 k" 0
10
0
The Journal of Physical Chemistry, Vol. 95, NO. 18, 1991 6991
Ta=140'C
Ta 550% E
20
30
40
50
60
Figure 1. Adsorption isotherms for pyrene on y-alumina at two different pretreatment temperatures.
(Aldrich, HPLC grade), methanol (Mallinckrodt, spectrograde), and carbon tetrachloride (Fisher, technical grade) were used as received. Ferricenium chloride was prepared by irradiation of solutions of ferrocene in carbon tetrachloride with a Xe lamp for a half-hour. The green precipitate was washed several times with carbon tetrachloride and then dried in air. y-Alumina (Versa1 GH) was obtained from LaRoche Chemical. It has the surface area of 200 m2/g as determined by BET (nitrogen adsorption), the density of 0.59 g/cm3, and the particle size of 45 Hm. From room temperature to 750 OC, there is no significant change in the structure of the so1id.l" The only changes on the surface of this alumina with pretreatment temperature are the numbers of hydroxyl groups and Lewis acid sites which have been described by a number of publications, for example ref 1 1 . However, the number of Lewis acid sites of this alumina at various pretreatment temperatures was determined spectroscopically from the amount of the cation radical of N,N,N',N'-tetramethylbenzidine produced.I2 Specific properties of y-alumina used in this study are summarized in Table I. Method. ?Alumina was activated at particular temperatures in an oven for 20 h; the solid was then allowed to cool in a desiccator. Cyclohexane solutions of pyrene with or without quencher were then added to the solid, and the mixture was stirred and allowed to equilibrate for at least 2 h. The solvent was then removed under vacuum at room temperature because heat may remove ferrocene from the surface, and the sample was kept under vacuum for at least 2 h to eliminate ox gen. For all experiments the concentration of pyrene was 2 X 10- mol/g, which corresponds to 1.5%monolayer. The examples of adsorption isotherms for pyrene adsorbed on alumina at two different pretreatment temperatures are shown in Figure 1 . The monolayer coverage were determined to be 27 and 29 mg/g for T, = 140 and 550 OC, respectively. Diffuse reflectance laser photolysis was performed in the following manner: the samples were excited with a pulsed laser (Lambda-Physik Model EMG 100 N2-He excimer laser with 337.1 nm, 8 mJ/pulse, and 10 ns pulse width) at 30' to the sample front surface. An Oriel 450-W Xe lamp is placed in front of the sample surface at 45O to the exciting beam. The analyzing light was focused onto the surface at the same position as the exciting beam. The resultant light was captured by an Oriel fiber-optic bundle and focused by two pairs of lens before it was transferred to a Bausch and Lomb diffraction monochromator. The signal was then passed to a RCA IP 120 photomultiplier. The electrical signal was amplified and then captured with a Tektronix 791 2
Y
(IO) (a) Direct information from LaRoche technical service. (b) Misra, C. Industrial Alumina Chemicak ACS Monograph 184;American Chemical Society: Washington, DC,1986; p 78. (1 I ) Knbzinger, H.; Ratnasamy, P. Catal. Reo.-Sci. Eng. 1981,17, 31. (12)Pankawm, S.;Thomas, J. K. J. Phys. Chem., submitted for publication.
where R, is the reflectance of the sample, and k and s are the absorption and scattering coefficients of the sample, respectively. In order to fulfill the first goal, many functions such as R,, log R,, l/R,, 1 - R,, and -log R, can be used instead of the Kubelka-Munk function to simulate the absorption spectra of transients on solids. However, kinetic analysis of the transient requires quantitative treatment of the concentration change. The function (1 - RT)has been introduced by Oelkrug et al. to evaluate the decay profile of the transient on the opaque materials.15 The term RT is the relative reflectance defined by the ratio of the reflectance of the sample at the analyzing wavelength to the reflectance of the background. In the case where the absorption of the transient at t = 0 is less than 596, Le., [ 1 - RT(t=O)]< 0.05, it has been shown that the concentration of the transient will be directly proportional to 1 - RF In this study, the function (1 RT) was used to both simulate the spectra of the transient and treat the kinetics of the transients. The maximum value of (1 - RT)never exceeded 0.06 in any study due to the small surface coverage, 2 X lo-' mol/g. Samples were also checked with the Kubelka-Munk function by using MgO as a standard. Decay parameters obtained from both functions are identical.
Results Absorption of the Transients. Transient reflectance spectra of pyrene on alumina at low pretreatment temperature (T, = 130 "C) are shown in Figure 2. Figure 2a,the spectra of the transients under vacuum, shows three main absorption bands at 415,450, and 515 nm. When 9 mbar of oxygen was added to the system, the 41 5- and 5 15-nmbands disappear while the 450-nmband still remains as shown in Figure 2b. Molecular oxygen quenches the triplet state of pyrene, PT,very efficiently in homogeneous solution.I6 Thus, the absorption bands at 415 and 515 nm can be assigned to the triplet-triplet absorption of pyrene and the 450-nm band to the absorption of cation radical. The triplet-triplet absorption maxima are very similar to those found in homogeneous solution and can be assigned to 3Ag 3B2u+and 3Bl - 3B2u+ transitions, re~pectively.~' The absorption spectrum of the triplet of pyrene on alumina, which is obtained by subtracting the spectrum of the cation radical from the transient spectrum in the vacuum, is shown in Figure 2c along with the spectrum of the cation radical. Figure 3 shows the decays of two different transients at the two different wavelengths. The triplet state which was detected at 415 nm (vacuum) decays exponentially, while the species absorbing at 450 nm (in the presence of oxygen) has an initial fast falloff followed by a slow decay. The lifetime of the triplet state at T, = 130 OC was found to be 5.1 ms. It is found
-
-
(1 3) (a) Kortllm, G. ReflectanceSpcrroscopy; Springer-Verlag: Berlin, 1969. (b) Wendlandt, W. W.; Hecht, H. G. ReflecranceSmctrosconv: . _ .Interscimce: New York, 1966. (14) (a) Kubelka, P.;Munk, F. Z . Tech. Phys. 1931.12,593. (b) Kubelka. P. J. Opt. SOC.Am. 1948,38,448. (15) Oelkrug, D.;Honnen, W.; Wilkinson, F.;Willsher, C. J. J. Chem., Faraday Trans. 1987,83.2081 and references therein. (16)Thomas, J. K.; Richards, J. T.; West, G. J . Phys. Chem. 1970,74, 4137. (17) (a) Labhardt. H.;Heinzelmann, W. In Organic Molecular Physics; Birks, J. B.,Ed.; Wiley: Chichester, 1973;Vol. I, p 297. (b) Windwr, M.;
Novak, J. R. The Triplet State. In Proceedings of an International Symposium Held at the American Unlocrsityof &irut, Lebanon; Zahlan, A. B., Ed.; Cambridge University Res: N e w York, 1967;p 229. (c) Lavalette, D. J. Chim. Phys. 1%9,66, 1845.
6992 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991
Pankasem and Thomas
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Figure 3. Dccay profiles of the triplet state and the cation radical of pyrene on alumina (T, = 130 "C):solid line, triplet state monitored at 450 nm; dashed line, the cation radical monitored at 415 nm in the presence of oxygen. 1.00
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Figure 4. Decay of the triplet state of pyrcne adsorbed on alumina (T, = 130 "C)as a function of total concentration of adsorbed ferrocene: 0, 2.15 X 4.30 X lW7, 6.45 X lW7,and 8.6 X lW7 mol/g.
Wavelength, nm Figure 2. (a) Transient diffuse reflectance spectra of pyrene adsorbed on y-alumina under vacuum at various time (T,= I30 O C ) : ( 0 )immediately after the pulse; (+) 0.5 ms, ( 0 )1.0 nu,and (A)3.0 ms after the pulse. (b) Transient diffuse reflectance spectra of pyrene adsorbed on y-alumina in the presence of 9 mbar of oxygen at various times (T, = 130 " C ) : (0)immediately after the pulse; (+) 0.5 ms, (0) 1.0 ms, and (A)3.0 ms after the pulse. (c) Diffuse reflectance spectra of the triplet and the cation radical of pyrene adsorbed on alumina at T,= 130 O C : (0) the triplet and (+) the cation radical. that O2quenches the triplet of pyrene on alumina with a quenching rate constant of 4.82 X 1 0 , Torr18-I compared to 3 X 104 TO& s-l for quenching of the triplet-state anthracene.8 Quenching of Triplet Pyrene by Ferrocene. Ferrocene, due to its low triplet energy (ET = 1.74 eV)I8 compared to that of pyrene (2.09 eV),I9 is a triplet quencher. The decay of the pyrene triplet (18) (a) Hcrbtroeter, W. 0. J . Am. Chcm. Soc. 1975, 97, 4161. (b) Armstrong. A. T.; Smith, F.;Elder, E.; McGlynn,S.P.J. Chem. Phys. 1967, 46,4321. (c) Scott, D.R.;Becker, R.S.J. Orgat~omct.Chcm. 1%5,4,409. (19) McClure. D. S . J. Chcm. fhys. 1949, 1 7 , 9 0 5 .
-
0 0
2
4
6
8
1
0
[Ferrocene], 10 - 7 moVg Figure 5. Observed decay rate constant for the triplet of pyrene on alumina ( T , = 130 "C)as a function of total concentrationof adsorbed
ferrocene.
TABLE II: Quenching Rate Comtu~ts,k, of tbe Triplet of Pyrene by Ferroceoe 8s IFunction of Pretreatment Temperature (T,) k,, 10" T,, O C 130 250
m2 mo1-Is-I
T,. "C
k,,, 10" m2 mol-' s-1
3.31 2.63
350 130 + water
2.40 5.58
with different concentrations of coadsorbed ferrocene is shown in Figure 4. All the decay profiles were fitted by a single-exponential model. Figure 5 shows the values of observed rate
The Journal of Physical Chemistry, Vol. 95, NO.18, 1991 6993
Reflectance Spectroscopic Studies of Pyrene on Alumina l
-
in
W
8ooi
7
'
6
0
600
400Y
0
I
0
0
2oou '1 00
2
4
6
[Ferrocene], 10
6
1
0
I
-'mol/g
0
0
Figm 6. Observed decay rate constant of the triplet of pyrene adsorbed
on alumina (T,= 350 'C) as a function of total concentration of adsorbed ferrocene.
constants, kh, as a function of total concentration of adsorbed ferrocene. A linear relationship of kob versus concentration was found which ave a quenching rate constant of 3.33 X 10" m2 mol-' s-l for P with ferrocene on alumina at T,= 130 O C . Table I1 shows the bimolecular quenching rate constants of pyrene triplet by ferrocene as a function of pretreatment temperature. Because of the low ionization potential of ferrocene, 6.81 eV,ZOwhen the pretreatment temperature is increased over 350 OC, ferrocene is oxidized by the Lewis acid site which is formed by dehydroxylation of the surface. The diffuse reflectance spectra of adsorbed ferrocene on these highly preactivated alumina surfaces show two new absorption bands at 400 and 620 nm which can be ascribed to the absorption of the ferricenium ion.21 The value of the quenching rate constant at T, = 350 O C was therefore calculated from the high concentration region of ferrocene where the Lewis acid sites are completely occupied. Figure 6 shows the observed decay rate constant of pyrene triplet as a function of total concentration of adsorbed ferrocene at T, = 350 O C . At low total concentrations of ferrocene, the decay of triplet pyrene does not change significantly with increasing concentration of ferrocene. However, at higher concentrations, the lifetime increases linearly with the concentration of ferrocene. As described earlier, at this TIthe ferricenium ion is formed. For lower concentrations of adsorbed ferrocene, there are sufficient Lewis acid sites on the surface to completely oxidize the ferrocene. If the Lewis acid sites are all occupied by ferricenium ion, then an additional amount of ferrocene would remain in the reduced form and quench the triplet state of pyrene dynamically. At this point, it is pertinent to ask the question whether or not ferrocenium ion can quench the triplet of pyrene dynamically on alumina. This question was answmd by studying first the reaction between the triplet of pyrene and ferricenium chloride in 1:l water/methanol. Ferricenium ion exhibits dynamic quenching of triplet pyrene in homogeneous solution with a quenching rate constant of 1.05 X lo9 M-'s-I, a value which is close to diffusion-controlled. Ferrocene interacts strongly with the Lewis acid site via electron transfer forming ferricenium ions; the latter do not quench PT. At T,= 450 O C , a greater number of Lewis acid sites are produced so that the greater amounts of ferricenium ion are formed. The dynamic interaction between the triplet of pyrene and absorbed ferricenium ion still could not be detected. These data show that ferricenium ion does not quench PT under conditions where ferrocene quenches efficiently. Hence, quenching of PTis the result of diffusion of ferrocene to PT,which is immobile over the time range of the study. Effect of Codsorbed Water. Coadmrbed water decreases the lifetime of triplet pyrene. Addition of 7.25 X 10-l2 mol/m2 of water to the alumina surface decreases the lifetime of the triplet from 5.1 to 1 m. The quenching rate constant of PTby ferrocene in the presence of axidsorbed H20 is almost twice as large as that
+
(20) (21)
Moet-Ner, M.J . Am. Chcm. Soc. 1989, 1 1 1 , 2830. Traverclo, 0.;Scandola, F. Inorg. Chfm.Acra 1970, 4, 493.
lime, ms
Figure 7. Logarithm of (1 - RT)for daay of the cation radical of pyrene adsorbed on alumina (T,= 130 "C) as a function of time at various total concentrationsof adsorbed ferrocene: 0,2.15 X lW1,4.30 X lW1,6.45 X and 8.6 X IW7 mol/g.
found in the absence of water (see Table 11). Decay of the Cation Radical of Pyrew. Due to the overlap between the absorption of the triplet and the cation radical at 450 nm, a small amount of pure oxygen ( 5 mbar) was added to eliminate the pyrene triplet. Figure 7 shows a plot of the logarithm of the cation radical absorption produced on alumina (at T, = 130 OC, monitored at 450 nm) with time at various concentrations of adsorbed ferrocene. The nonlinearity of logarithmic plots indicates that the decay of cation radical of pyrene does not follow a simple exponential; however, the cation decay increases with increasing concentration of ferrocene. There have been a number of models describing charge recombination of the ion pairs, but the general model has the form -dc/dt = kt-"
(2)
where c is the pair concentration. The value of n has been ranged from 1 to 1S.8*22-24 However, an attempt to use this function to explain the decay of the cation radical on alumina was not successful* The deviation of the cation decay from monoexponential kinetics can be ascribed to ion pairs at different surface adsorption sites. A Gaussian distribution kinetic model, which was developed by Albery basad on a distribution in the free energy of the reaction,25 was used to describe this situation. The model has been employed successfully for the time-resolved fluorescence studies of pyrene on silicaz6and alumina surfaces.12 According to the model, the time-dependent concentration profile of the transient can be represented by
J - m
-
where
c,
and co are the concentrations of the transient at time t and t
= 0, rwpectively, k is the average rate constant, and y is the width
of the distribution. (22) Dcbye, P.;Edwards, J. 0. J. Chcm. Phys. 1952,20, 236. (23) Tachiya, M.; Mozumber, A. Chcm. Phys. k t r . 1975, 31, 77. (24) Abcll, G. C.; Mozumder, A. J. Chcm. Phys. 1972, 56,4079. (25) Albcry. W. J.; Bartlett, P.N.;Wilde, C. P.;Darwent, J. R. J. Am. Chcm. Soc. 1905, 107, 1854. (26) (a) Krasnansky, R.;Koike, K.; Thomas, J. K.J. Phys. Chcm. 1990, 94,4521. (b) Hite. P.;Krasnansky, R.;Thomas, J. K.J. Phys. Chcm. 1986, 90,5765.
6994 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991
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Pankasem and Thomas
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1
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1
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Figure 8. Decays of the cation radical of pyrene on alumina (T,= 130 "C)at various temperatures: from top to bottom 21, 28, and 40 "C. 200,
,
,
150
r 0
Figure 10. Observed decay rate constant of the pyrene cation as a function of ferrocene concentration (7'. = 130 "'2). TABLE I V QwDelbg Rate Coarbnt, k,', of the cltiol R d k d of Pyrene by Fcrroeeae as a F u " of FWmtmut Temperature k,,', 10" k,', 10" T., "C m2mol-' S-I T., "C m2 mol-' s-l 130 3.39 350 2.35 250 2.71 130 + water 4.18
It is also noted that the quenching rate constants of the cation radical of pyrene by ferrocene increase slightly when a small amount of water (7.25 X mol/m2) is coadsorbed on the surface.
Y CI
mollg
100
m
Y i-4 C
50.0
0 0
2.0
1.0
3.0 t i m e / ms
5.0
4.0
Figure 9. Decays of the cation radical of pyrene at various concentrations of adsorbed ferrocene (T,= 130 "C)with the Gaussian distribution fit to data: 0, 2.15 X lW7,4.30 X lW7,6.45X 1W7, and 8.6 X lO-'mol/g.
TABLE Ilk Decay Panmeten of the CItb R a w of Pyreoe AQorkd 011 A h " 8t T, 1% "c at VldOlg ConcCahrtiOaS Of Fcrrormc Determined from tbe Gawsira Distribution Kinetics
[ferrocene], 10-7 mol/g 0.00 2.15 4.30
Lh 62.1 265 695
y 4.53 2.49 2.14
[ferrocene], IO-' mol/g 6.45 8.60 10.75
L* 1190 1460 1970
y
1.34 1.45 1.68
Figure 8 shows the decay of pyrene cation radicals on alumina (T, = 130 "C)at different temperatures. At high temperatures the cation radical of pyrene alone decays faster, which indicates that temperature increases the rate of diffusion of the ions on the surface. Figure 9 presents the decay profiles for pyrene cation radicals on alumina at T, = 130 "C at various concentrations of ferrocene together with a Gaussian distribution fit to the data. The values of k& and y parameter obtained from the fitting procedure at various concentrations of ferrocene are given in Table 111. The relationship between the observed decay rate constants, k&, obtained from the fit and total concentration of adsorbed ferrocene at T, = 130 O C is shown in Figure 10 and can be represented by Lob = k,, + k([quencher] (5) where k; is the bimolecular quenching rate constant and Lo is the average rate constant in the absence of quencher. The data suggest that quenching of the pyrene cation radical is by diffusion of ferrocene along the surface to the cation. Several quenching rate constants at various pretreatment temperatures are summarized in Table IV.
Discussion Excitation of adsorbed pyrene on alumina surfaces by 337.1-nm laser pulses produces the pyrene triplet state and the cation radical. This indicates that intersystem crossing and photoionization take place as deactivation channels for the singlet excited state. In the presence of oxygen, a distinction between these two transients can be made. In a way similar to that in homogeneous solutions, molecular oxygen quenches the triplet state of pyrene efficiently on alumina surface, while cation radical remains unchanged. Oxygen can enhance the formation of the cation radicals of polyaromatic compounds in BF3/trifluoroaceticacid by accelerating the conversion of protonated aromatic compounds to their cation radicals.*' In general, the simple expression describing a deactivation process is -dc,/dt = k,c, + k2(c,)* (6) where c, is the concentration of the transients, which is triplet state in this case. In this study, only the unimolecular component contributes to the deactivation of triplet states for following reactions: (1) the very low loading of the probe, 2 X lo-' mol/& was used, (2) the exciting beam was not focused so that only small concentrations of the triplet state were produced; (3) a change in the intensity of the exciting beam did not affect the decay profile of the triplet. For the above reasons, it is unlikely that the bimolecular component would contribute to the deactivation of the triplet. The single-exponentialmodel was therefore used to fit the triplet decay. Oxygen quenches the triplet state of pyrene on alumina surface with a bimolecular rate constant of 4.82 X lo4 Torr-' s-', which is about 160 times slower than that of quenching of the singlet excited state of pyrene on the same surface (7.9 X lo6 Torr-' s-I). In solution the relative rate of quenching of the singlet and the triplet of pyrene by oxygen is 9: 1. The much larger difference of 160-fold observed on alumina and 30 on silica is primarily due to the lack of a cage effect on the solid surfaces compared to the case in solution. The singletoxygen reaction is diffusion-controlled in solution while that of the triplet is 1/9 of diffusion-controlled due to the spin selection exhibited by the reaction.n The solvent (27) Aalbmberg, W.Ij.; Hoijtink, G. J.; Mackor, E. L.; J . Chcm. Soc. 1959, 3049, 3055.
Weijland, W. P.
Reflectance Spectroscopic Studies of Pyrene on Alumina cage ensures that many collisions of the triplet and oxygen occur on one encounter of the two reactants. On a solid surface no cage exists and simple collision of the triplet and oxygen occurs. As the reaction is not diffusion-controlled, the rate is reduced, compared to quenching by oxygen of excited pyrene singlet. Contacting the surface with liquid cyclohexane provides a solvent cage for the reaction, and the ratio of oxygen quenching of the singlet and the triplet is 1O:l for silica and 12:l for alumina which approaches that observed in solution. Quenching of the pyrene triplet by coadsorbed ferrocene on alumina at low pretreatment temperatures is dynamic in nature. In an earlier study,I2 the excimer formation of pyrene was not detected on this surface. This indicates that pyrene does not move appreciably on the surface during the lifetime of the excited singlet state (1 20 ns). There are two possible processes that may contribute to the dynamic energy transfer to ferrocene: (1) triplet energy transfer at a distance with no molecular diffusion on the time scale of pyrene triplet lifetime and (2) ferrocene surface diffusion and collision-induced energy transfer. The transfer at a distance mechanism cannot contribute to the rate because of the very low loading was used, and the average separation of pyrene and ferrocene would be 100 A,29 which is too great for this mechanism to take place. The only possible process for dynamic energy transfer between adsorbed pyrene and ferrocene is the diffusion of ferrocene along the surface. The fact that ferrocenium ion quenches the triplet of pyrene dynamically in solution but not on the surfaces also supports the idea that ferrocene is the species that translates on the surface. Wilkinson studied triplet-triplet annihilation of acridine on silica and alumina surfaces.' It was reported that acridine diffused on the silica surface with a diffusion rate of 8 X 10" m2 mol-' s-l but was immobilized on alumina. The acridine molecule possesses an amino group, and the lonepaired electron in this group interacts strongly with the Lewis acid site on the alumina surface, which tends to prohibit solute diffusion. The effect of increasing pretreatment temperature on the silica is considered as an inhibition of the mobility of the adsorbed molecule due to a decrease in the number of silanol groups and, at the same time, an increase in the number of siloxane groups. Fluorescence photobleaching techniques have been recently used to measure the diffusion of tetracene on silica surfaces? This method claims that the effect of heat pretreatment of the silica surface is to increase the surface diffusion. These two opposing results may be a result of using different techniques and probes. On the alumina surface, the effect of pretreatment by heating the surface not only decreases the number of hydroxyl groups but also increases the number of coordinatively unsaturated (CUS) cations and anions." The interaction between the adsorbed species and CUS cation involves charge transfer which decreases the ability of the species to move on the surface. The present study demonstrates that the number of mobile sites (the physically adsorbed sites where adsorbed species interact with the surface through the hydroxyl layers) is decreased by increasing the pretreatment temperature. This strong interaction prevents the diffusion-induced energy transfer from triplet pyrene to ferricenium ion, while in homogeneous solutions ferricenium ion quenches the triplet at a diffusion-controlled rate. The number of Lewis acid sites increases with increasing pretreatment temperature. The bimolecular quenching rate constants for PT by ferrocene are 3.33 X 10". 2.63 X loll, and 2.40 x 10" m2 mol-' s-l at the pretreatment temperature of 130, 250, and 350 OC, respectively. For the temperatures above, the number of hydroxyl groups would be 11.5 X IOI4, 7.8 x IO", and 6.2 x I O i 4 groups cm-2 which correspond to 90%, 63%, and 50% of a monolayer, respectively.' I Coadsorption of water on the alumina surface at T,= 130 OC increases the efficiencyof diffusion on the surface. Water tends to bind to the Lewis acid sites on alumina surfaces,=' which leads (28) Gijzman. 0. L. J.; Kaufman, F.; Porter, G. J . Chcm. Sa., Furuduy
Trans. 2 1973, 69,108.
(29) The distance was catimated from the assumption that both donor and acceptor molecules are uniformly distributed on the surface. (30) Hindin, S.0.;Weller, S. W. J . Phys. Chrm. 1956, 60, 1501.
The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 6995
-E 0
P
0.03
1
0 0 . 0.0
e
0
0
0
0
0
0
0
0.01 ".VV
.
0
20
, 60
40
.
. 80
-
, 100
Relative Laser Intensity
Triplet 0
Cation
0.04
O.O1
1 0
O@"O
20
40
60
80
100
120
Square of Intensity (Rel.)
Figure 11. (a) Relative yield of the triplet and the cation radical of pyrene on alumina at 7'. = 130 O C (loading, 2 X lo-' mol/g; excitation, 337 nm) as a function of the relative pulse intensity. (b) Relative yield (same as above) as a function of the square of the pulse intensity.
to a decrease in the binding ability of these sites toward the adsorbates. The interaction of water with the surface active sites can be represented by the following:
/
Ak
'0
AI;'
Thus, the fraction of adsorbate adsorbed to the physisorption sites, the mobile sites, is increased, as the CUS sites are blocked. The bimolecular diffusion rate constant of adsorbed aromatic compounds on silica varies from one study to another: triplet quenching of acenaphthylene by ferrocene,' 7 X 10') m2 mol-' PI; triplet quenching of benzophenone by naphthalene! (7-8) X 10') m2 mol-' s-I; triplet-triplet annihilation of acridine,' 8 X 10" m2 mol-' s-I. The different rate constants are due to different pretreatments of the absorbents and to the different extents of binding of adsorbates to the surfaces. The numbers obtained in this study demonstrate a stronger binding force between adsorbates and alumina than in silica. Excitation of pyrene on alumina also produces the cation radical of pyrene via photoionization. The mechanism of photoionization on the solid can take place via two possible processes: (1) monophotonic process P(ad) A P*(ad) (2) biphotonic process P(ad) -!L P*(ad)
-+
P+(ad) + e-(ad)
Pl*(ad)
-
P+(ad)
+ e-(ad)
(7) (8)
The monophotonic process usually occurs when the energy of the exciting light is greater than the ionization potential of the probe. The biphotonic mechanism takes place in the system where ionization potential of the probe is greater than the exciting beam, (31) Tamele, M. W. Dfscuss. Furuduy Soc. 1950,8, 270.
6996
J. Phys. Chem. 1991, 95,6996-7002
and it involves an intermediate PI* which may be a triplet or a higher singlet state. The ionization potential of pyrene is 7.55 eV in the gas phase.32 Thus, photoionization requires more than just one photon of the exciting energy (3.68 eV). Although the ionization potential of pyrene adsorbed on alumina surfaces may be smaller than that in the gas phase, the linear relationship between the amount of cation radicals produced and the square of the laser pulse intensity suggests that the mechanism of photoionization is a two-photon process (see Figure 11). By contrast, the yield of the triplet increases linearly with the pulse energy as expected for a monophotonic process. The decay profile of the cation radical exhibits a initial fast falloff followed by a slow decay which extends to a time scale of seconds. In the previous study,I2 the results from fluorescence study have indicated that there are a variety of adsorption sites for pyrene on the y-alumina surface. These adsorption sites are the combination of the physisorption sites and the Lewis acid sites. Thus, the decay of the pyrene cation radical is described by the distribution of pyrene cation produced at different active sites. The interaction between the Lewis acid site and pyrene is most likely a charge-transfer type, which leads to cation radical. The cation radicals produced from pyrene adsorbed at the Lewis acid sites may decay faster because the radical ion pairs are in close proximity to each other. Conversely, the cation radicals that are produced on the physically adsorbed sites are well separated from the electrons by the hydroxyl groups and, accordingly, live longer. The combination of cation radicals produced at these different adsorption sites is well represented by a Gaussian distribution from which the average decay rate constant of the pyrene cation radical can be calculated. The increase in the rate of decay of P+ with concentration of ferrocene enables a measurement of the diffusion quenching rate constant. The quenching rate constants of the (32) (a) Birke, J. B.;Slifkin, M.A. Nature 1%1,191,761. (b) Briegleb, 0.:Czekalla, J. Z . Elektrochrm. 1959, 63, 6.
pyrene cation radical by ferrocene are comparable with those for the pyrene triplet, supporting the idea of diffusional process by ferrocene along the surface, and not PT or P+. The effect of pretreatment temperature can be explained in the same way.
Conclusion Unlike silica surfaces, the alumina surface provides a variety of adsorption sites for adsorbed species: hydroxy physical adsorption sites and Lewis acid chemisorption sites. Organic molecules interact differently with these two adsorption sites. Diffusion process can be distinguished by the dynamic energy transfer from the triplet state of pyrene to coadsorbed ferrocene. Pretreatment of the surface at high temperature not only d e m w the number of hydroxyl groups but also produces CUS cations which are known to be electron acceptors. A decrease in the bimolecular quenching rate constants on the surface at high T, indicates a stronger interaction between adsorbed species and Lewis acid sites than on the physisorption sites. Hydration of the surfaces by coadsorbed water decreases the number of Lewis acid sites and enhances the dynamics of quenching. The bimolecular quenching rate constants obtained in the study are about 2 orders of magnitude smaller than those obtained on the silica surface, which indicates stronger bonding of organic molecules to the alumina surface. Formation of the cation radical of pyrene on alumina take places through a biphotonic mechanism which may involve the higher singlet excited states. Since the pyrene cation radicals are produced at different adsorption sites, the deactivation process of the cation radicals is explained in terms of diffusional recombination of the ion at different adsorption sites along the surface. Adsorbed ferrocene also quenches the cation radical in a dynamic fashion.
Acknowledgment. This work was supported by the Environmental Protection Agency via Grant EPA-R-815953-01-0. Registry No. Pyrene, 129-00-0; pyrene radical ion, 34506-93-9; yalumina, 1344-28-1; ferrocene, 102-54-5.
Nuclear Magnetic Resonance Spectroscopic Study of Spin-Lattice Relaxation of Quadrupoiar Nuclei in Zeolites J. Haase, K. D.Park, K. Guo, H. K. C,Timken,+ and Eric Oldfield* Department of Chemistry, University of Illinois at Urbana-Champaign, 505 South Mathews Ave., Urbana, Illinois 61801 (Received: November 20, 1990; In Final Form: April 1, 1991)
We have obtained the temperature-dependent nuclear magnetic resonance spin-lattice relaxation times of several nonintegral *a, nAl, and "Ga) in a series of hydrated zeolites(NaA, NaX, Nay, NaGaY, and NH,-ZSM-5) spin quadmpolar nuclei ("0, at 8.45 T. There is no correlation between TIand the static part of the electric field gradient, which gives rise to the observc. i nuclear quadrupole coupling constants ($qQ/h). In all cases, however, we find that relaxation is overwhelmingly dominated by modulation of the time-dependentelectric field gradient at the nuclear site due to motion of water molecules and hydrated cations in the zeolite pores. The relaxation times are a function of Si/AI ratio as well as hydration. Our results provide estimates of the Sternheimer antishielding factors (1 - y O ) in moderate agreement with generally accepted literature valuca and should be-applicable to analysis of the relaxation behavior of other nonintegral spin quadrupdar nuclei in zeolites, e.&, 7Li, IlB, 39K,*7Rb,and 133Cs.
Introduction Recent developments in solid-state nuclear magnetic resonance (NMR) spectroecopy have permitted the determination of nuclear quadrupolecoupling constants in a wide range of inorganic solids, such as zeolites,1*2so it is now natural to progress further by interpreting spin-spin' and spin-lattice relaxation in such systems. Preliminary results from this group'"' showed that 170spin-lattice Present addrw: Mobil Research and Development Corporation, Paulsboro Research Laboratory, Billingsport Road, Paulsboro, NJ 08066.
0022-3654/91/2095-6996$02.50/0
relaxation in the zeolites Linde A and ZSM-5 was rather inefficient when compared with e.g. "AI NMR relaxation, even though static ''0nuclear quadrupole coupling constants ($qQ/h) were ( I ) Timken, H. K. C.; Janes, N.;Turner, G. L.; Gilson, J. P.;Welsh, L. 8.;Oldfield, E. J. Am. Chem. Soc. 1986, 108,7231. (2) Timken, H. K.C.; Janes, N.; Turner, G. L.; Lambert, S. L.; Welsh, L. B.;Oldfield, E. J. Am. Chem. Soc. 1986, 108,1326. ( 3 ) Haase, J.; Oldfield, E. Submitted for publication. (4) Timken, H. K. C. Ph.D. Thesis, University of Illinois at UrbanaChampaign, 1987, unpublished.
Q 1991 American Chemical Society
