J. Phys. Chem. 1982, 86, 3781-3789
3781
Surface Photochemistry. The Photophysics of Pyrene Adsorbed on Silica Gel, Alumina, and Calcium Fluoride‘ Richard K. Bauer,* Paul de Mayo,’ Wllllam R. Ware,* and Kam C. Wu3 photochemistry Unn, Depertment of Chemlstv, The University of Western Ontario, London, Ontario, Canada N6A 587 (Received: hfarch 24, 7982)
A detailed study of the emission, excitation spectra and fluorescence decay of pyrene adsorbed on silica gel, porous Vycor, alumina, and calcium fluoride is reported. Monomer emission and a longer-wavelength broad-band emission (neither decaying with a single exponential) was observed on silica. The excitation spectrum of the broad-band emitter was red shifted in comparison with the absorption responsible for the monomer emission. These,and other, results are interpreted as indicating the existence of bimolecular ground-statepyrene association. The effect of coadsorbates (glycerol, 1-decanol)was examined. At high coverage the monomer emission decay approaches a single exponential and the long-wavelength emission can be deconvoluted with a double exponential with a negative preexponential term indicating true excimer formation. Adamantanol did not have this effect. A model for the pyrene-silica interaction is proposed to rationalize these and other observations.
Introduction The occurrence of inter- and intragranular motion of aromatic hydrocarbons on dry silica gel, on time scales set by either singlet or triplet excited-state lifetimes, has been demonstrated recently in our laboratories! Evidence was drawn from photodimerization studies, fluorescence quenching observations, and emission and excitation spectroscopy. These studies provided the first semiquantitative data for diffusion rates on silica gel surfaces. Photochemistry and photophysics are, in general, more complex in inhomogeneous systems, such as micelles or porous materials, than in those that are homdgeneous. Only a few systematic studies have been reported involving a solid substrate.”1° We have, therefore, undertaken to investigate the photophysical properties of adsorbed molecules and the molecular interaction between adsorbate and adsorbent and between coadsorbed molecules on a solid surface with the aid of the accumulated photophysical information obtained in homogeneous systems. In this paper we report a detailed photophysical study of pyrene adsorbed on silica gel, alumina, calcium fluoride, and porous Vycor. It is well-known that pyrene forms excimers in solution as well as in the crystalline state.11J2 It also shows prominent 0-0 band enhancement, in both electronic absorption and fluorescence, in polar s o l ~ e n t s , the ~ ~ -so~~ (1) Publication No. 280 from the Photochemistry Unit, Department of Chemistry, University of Western Ontario, London, Canada. (2) On Sabbatical leave from N. Copernicus University, Torun, Poland. (3) Present address: 3M Co. Ltd., Oxford St. East, London, Canada. (4) Bauer, R. K.; Barenstein, R.; de Mayo, P.; Okada, K.; Rafalska, M.; Ware, W. R.; Wu, K. C., submitted for publication. (5) Ishida, H.; Takahahi, H.; Sato, H.; Tsubomuro, H. J . Am. Chem. SOC.1970, 92, 275. (6) Ishida, H.; Takahashi, H.; Tsubomura, H. Bull. Chem. SOC. Jpn. 1970,43,3130. (7)Ishida, H.; Tsubomura, H. J . Photochem. 1973/74,2, 285. (8) Oelkrug, D.; Radjaipour, M.; Erbser, H. Z. Phys. Chem. (Frankfurt am Main) 1974, 88, 23. (9) Oelkrug,D.; Erbse, H.; Plauschinat, M. Z. Phys. Chem. (Frankfurt am Main) 1975,96, 283. (10) Oelkrug, D.; Schrem, G.; Andra, I. Z. Phys. Chem. (Frankfurt am Main) 1977, 106, 197. (11) Birka, J. B. ‘Photophysics of Aromatic Molecules”;Wiley-Interscience: New York, 1970. (12) Stevens, B. “Advances in Photochemistry”; Pith, J. N., Jr., Hammond, G. S., Noyes, W. A., Jr., Eds.; Interscience: New York, 1971; VOl. 8. 0022-3654/82/2086-3781$01.25/0
called “Ham-band” phenomenon. Furthermore, the vibrational structure of the pyrene emission is dependent on solvent polarity. Pyrene molecules also have a very long excited-singlet lifetime as well as a relatively high fluorescence quantum efficiency. Thus, pyrene and its derivatives are valuable probes with which to study heterogeneous systems. In a previous paper,16 it was found from time-resolved fluorescence and excitation spectra that both pyrene and naphthalene adsorbed on silica gel show “excimer-like” emission and that associated complexes of adsorbates in the ground state are responsible for some of the excimer-like fluorescence. This is a remarkable observation since, in solution, a ground-state bimolecular complex of pyrene is, of course, dissociative. It was, thus, of interest to investigate further the nature of the interaction between pyrene and silica gel which leads to the forming of such bimolecular ground-state associations (RGSAs). The silica gel surface is made complex by its irregularity with respect to both topography and the disposition of the groups (silanol, siloxane, water) active in adsorption. It is frequently postulated that the adsorption of organic molecules on this substrate is primarily through hydrogen bon$ing. We have studied the interaction between adsorbed pyrene and silica gel by changing the number of silanol groups and water molecules on the surface and by modifying the surface by preferentially associating the silanol groups with an “inert” molecule. Experimental Section Fluorescence lifetimes were measured with a PRA (Photochemical Research Associates, London, Ontario) System 3000 nanosecond fluorimeter. Monochromators in both the excitation and emission light paths were employed with typical bandwidths of 5 nm. Hydrogen was used in the flash lamp which gave an instrument response function of between 1.6 and 2.0 ns (full width half-maximum). Deconvolution was performed on a DEC MINK-11 computer with PRA software which employs iterative (13) Nakijama, A. Bull. Chem. SOC.Jpn. 1973, 46, 2602. (14) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. SOC.1977,99, 2039. (15) Hara, K.; Ware, W. R. Chem. Phys. 1980, 51, 61. (16) Hara, K.; de Mayo, P.; Ware, W. R.; Weedon, A. C.; Wong, G. S. K.; Wu, K. C. Chem. Phys. Lett. 1980, 69, 105.
0 1982 American Chemical Society
3782
The Journal of Physical Chemistty, Voi. 86, No. 19, 1982
Bauer et al.
TABLE I: Properties of Adsorbed Pyrene type of surface
silica gel
surface coverage for e = l%,a mol/g 4.8X surface occupied for e = l,a % 0.76 mean distance between centers of molecules, m 1 4 x 10-9 surface area, m'/g 560 a The surface coverage is found from isotherms. The actual surface occupied by ing that one pyrene molecule occupies a surface equal to 150 A '.
porous glass
3.7
X
alumina 11X
1.37 1 2 x 10-9
0.65
250
150
CaF, 0.7 X
1 5 x 10-9
pyrene molecules is calculated by assum-
least-squares deconvolution."J8 In general, it was necessary to fit to two components, as in eq 1. with the reIF = Ale-t/'l + A2e-t/'z (1) covery of four parameters. The average lifetime, 'i, was calculated from eq 2. For both steady-state and A1712 A2712
+
t=
+ A272 fluorescencedecay measurements, front-face excitation and observation were used. Steady-state measurements were made with a Perkin-Elmer MPF-4 spectrofluorimeter with a specially designed cell holder, or with a homemade spectrofluorimeter which allowed one to use horizontal samples. Typical bandwidths were 0.75 nm. Samples were prepared by adsorbing pyrene from cyclohexane solutions. The solvent was then removed by evaporation at room temperature at a pressure of approximately torr. The samples were degassed at a pressure below torr for about 1 h. The adsorbents used were Merck's silica gel (Merck 60, 35-75 mesh), neutral alumina (Woelm, 50-90 mesh), calcium fluoride (Aldrich, 99.999% pure), and Corning porous Vycor glass 7390 (100 mesh). For most experiments, adsorbents were first heated for 8 h at 500 "C, kept 24 h a t room temperature and humidity, and then stored at 170 "C. Quartz cuvettes (1 cm X 0.2 mm) were employed which were fitted with a graded seal shank to permit the outgassing and sealing off of samples. The dimensions of the cell were ideally suited for front-face illumination and observation. From isotherm curves19the concentrations required for 100% surface coverage were established to be 97,75,22, and 13 mg/g for pyrene adsorbing on silica gel, Vycor, alumina, and calcium fluoride, respectively. Some of the adsorbate properties are listed in Table I. The degree of surface coverage in percentage will be denoted by 8. Ai71
Results Pyrene Adsorbed on Silica Gel. The fluorescence and excitation spectra of the highest concentration of pyrene used on silica gel (6 = 130) are shown in Figure 1along with those obtained in solution and from a polycrystalline sample. The excimer-like emission of adsorbed pyrene has a band maximum at 460 nm, which is red shifted in comparison to the band position in the crystal or in ethanol solution. Thus, one may be sure that the emission from the adsorbed pyrene on silica gel is not from bulk crystalline pyrene, even at very high coverages. In Figure 2 are shown the fluorescence spectra of pyrene adsorbed on silica gel at various coverages. It should be noted that even for a concentration far below monolayer coverage (0 = 0.2) the broad excimer-like emission is observed and that for 0 = 3 the integrated intensities of the monomer and ex(17) Lewis, C.; Ware, W. R.; Duemeny, L. J.; Nemzek, T. L. Reu. Sci. Instrum. 1973, 44, 107. (18) Ware, W. R.; Andre, J. C.; O'Connor, D. V. J.Phys. Chem. 1979, 83. ~. , 1333. ~ - - (19) Weis, L. D.; Evans,T. R.; Leermakers, P. A. J. Am. Chem. SOC.
1968,90,6109.
WAVELENGTH inm
Flgure 1. Fluorescence and excltation spectra of pyrene: (-) adsorbed on silica gel with 0 = 130, (- - -) crystal powder, and (. .) ethanol solution. Excitation wavelength: 318 nm; wavelength of observation for excitation spectra: 475 nm.
-
WAVELENGTH / nm
Flgure 2. Emission spectra of pyrene adsorbed on silica gel: (a) surface coverage 0 = 3, excitation wavelength = 342 nm; (b) surface coverage 0 = 1, excitation wavelength = 342 nm; (c) surface coverage 0 = 0.2, excitation wavelength = 342 nm; (d) surface coverage 0 = 0.2, excitation wavelength = 331 nm.
cimer-like emission are practically equal. The so-called excimer-like emission is definitely not an emission from a typical excimer since the excitation spectra for the monomer emission (observed at 390 nm) and the broad-band emission (observed at 475-480 nm) are different. In Figure 3 two excitation spectra of a sample with 8 = 1 are presented; the wavelengths of observation were 390 and 475-480 nm. The excitation spectrum of the BGSA is shifted to the red in comparison with the adsorption responsible for the monomer-like emission. The absorption bands are quite sharp, narrow, and structured. The excitation spectral bands (fluorescence observed at 390 and 480 nm) depend only weakly on surface coverage, even up to 0 = 10. With increasing 0 values one observes an increasing bandwidth, as can be seen from Figures 3 and 4, which may be attributed to higher optical density
The Jownal of phvslcal Chemistry, Vol. 86, No. 19, 1982 3783
Surface Photochemistry
-
TABLE 11: Decay of the Emission of Pyrene Adsorbed on Silica Gel
330nm
345nm
surface coverage excitation: 331 nm A, emission: 390nm 7 , , m A2
2:F, m (7),
excitation: 342 nm A, emission: 460 nm r , , ns
A* 72,
A2IAl (T),
0
300
ns
3%
1%
0.12 288.0 0.04 86.5 0.37 268.2 0.01 192.4 0.17 58.3 14.3 85.2
0.07 346.2 0.07 170.3 0.91 290.0 0.01 233.5 0.08 59.5 9.1 106.5
0.2% 0.08 310.1 0.05 140.7 0.65 271.7 0.05 168.1 0.17 49.2 3.5 108.1
0
350 WAVELENGTH (nm)
A,, 331 nm
Flgw 3. Excitation and emission spectra of pyrene on dice gel: (-) excitation (observed at 390 nm) and emlssion (excited wlth 331 nm for a 0 = 1 swface coverage sample; (..a) excitation (0 = 1, observed at 480 nm) and emission (8 = 3, excited with 345 nm).
3
300
350 40 0 WAVELENGTH (nm)
450
Figure 5. Excitation and emission spectra of pyrene on CaF,: (. * e) excitation (observed at 392 nm), 0 = 5; (-) excitatlon (observed at 392 nm) and emisslon (excited with 331 nm), 8 = 0.2. I
360
400
440
480
520
56C
WAVELENGTH (nm) Flgurs 4. Emission spectra of pyrene adsorbed on siica gel (6 = 1, excitation wavelength = 342 nm) vs. svface activatbn temperature: (-) 950 "C, 20 h; (..a) 500 "C, 2 h; and (---) 800 "C, 3 h.
and increasing inhomogeneity. This anisotropic interaction of pyrene with the surface is evident, also, if the excitation bands are compared with the narrower absorption bands of pyrene in cyclohexane. To check the influence of the activity of the silica gel surface on the efficiency of bimolecular ground-state association with pyrene, we prepared three differently activated silica gel surfaces. Sample I was degassed under vacuum a t 500 "C for 2 h prior to the pyrene adsorption (8 = 1);samples I1 and I11 were heated in air at 800 "C for 3 h and a t 900 "C for 24 h, respectively. These last two samples were cooled down in the oven to about 200 "C, and then the adsorption procedure was performed immediately. The emission spectra of these samples are shown in Figure 4. The ratio, R, of the broad-band emission intensity measured at 460 nm to the monomer fluorescence intensity measured a t 390 nm is a measure of the efficiency of pyrene BGSA formation. This ratio rises from 0.12 for the sample prepared with silica gel stored at 170 "C to 0.24, 0.48,and 2.76 for the samples, 11, I, and 111, respectively. It is, therefore, evident that the formation of a stable BGSA of pyrene is most probable on the most activated surface (sample 111). The decay curves of pyrene on silica gel were measured for different surface coverages as well as for different ex-
citation and emission wavelengths. The intrinsic decay curves were deconvoluted assuming a two-exponential decay, although actually the decay of the pyrene monomer emission as well as of the excimer-like emission was always found to follow this law only approximately. The ratios of amplitudes of the two decay components (longer/ shorter) is higher the more single-exponential the decay becomes. The average decay time of the monomer emission of pyrene on surfaces is shorter than in cyclohexane solution, indicating that the surfaceadsorbate interaction increases the nonradiative rate constant. The excimer-like emission decays in a much shorter time than that associated with the monomer. The results are presented in Table 11. Pyrene Adsorbed on Alumina and Calcium Fluoride. These two adsorbenta were chosen because, according to Oelkrug and co-workers,mthey are more active than silica gel. However, the least active porous material appears to be calcium fluoride. In spite of this fact, the adsorbed pyrene-which was photostable on silica gel and alumina surfaces-appeared to be photosensitive and heat sensitive at temperatures higher than 30 "C on CaFP This behavior seems to indicate that some photodecomposition occurs on the CaFz surface. The excitation and emission spectra are presented in Figure 5. The emission spectra of pyrene on CaFz are more structured than those on alumina, but leas structured than those obtained on silica gel. This seems to be in good agreement with the results of Oelkrug.m These spectra do not depend on excitation wavelength, even for a surface (20) Oelkrug, D.;Schrem, G.;Andra, I. 2.Phys. Chem. (Frankfurt am Main) 1977,106, 197.
3784 The Jownal of Physical Chemistry, Vol. 86, No. 19, 1982 TABLE 111: Decay of the Emission of Pyrene Adsorbed on CaF," surface coverage AI 71,
ns
A2 7 2 , ns AJAI (T), ns
Bauer et al.
TABLE IV: Decay of the Emission of Pyrene Adsorbed in Alumina
5%
1%
0.2%
0.044 125.0 0.064 222.9 1.45 195.6
0.091 100.7 0.140 226.6 1.54 198.4
0.035 88.2 0.100 228.9 2.86 212.2
surface coverage excitation: 334 nm A , emission: 374 nm T , , ns A2
2$, (7)
Excitation: 331 nm; emission: 392 nm.
excitation: 344 nm A , emission: 460 nm T , , ns A2
?;E1
5%. h,,460nrr -0 2 % hOk390nrr
(7)
3
300
350 400 WAVELENGTH (nm)
4 50
5 3
-0 8. Exdtatkn and emission spectra of pyrene on alumina: (...) excitatlon and emlssion spectra of B = 5 sample, with 460- and 344-nm observation and excitation wavelengths, respectively; (-) excitatbn and emlssion spectra of B = 0.2 sample with 390- and 334-nm observation and excitation wavelengths, respectively.
coverage of B = 5. For this coverage a very weak band appears at the red edge of the emission. The excitation spectrum of pyrene on CaF2, although broadened, is not shifted for higher coverages and does not depend on the wavelength of observation. The average decay time of the monomer pyrene on CaF2 (0 = 0.2) is longer than that observed on alumina but shorter than the lifetime on silica gel. The main feature of the luminescence of pyrene on CaF2 is, however, the complete lack of fluorescence from any kind of complex. Nevertheless, despite the absence of an emitting complex, the decay of fluorescence cannot be represented by a singleexponential function (Table In). Pyrene was also adsorbed on alumina. Three samples with 5%, 1%, and 0.2% surface coverage were prepared. The values for 100% surface coverage and the surface area of alumina are 22 mg/g (pyrene/alumina) and 150 m2/g, respectively. Both are considerably small compared to these quantities for silica gel. The mean distance between pyrene molecules, however, appears to be only slightly shorter. The results of emission and excitation spectral measurements are given in Figure 6. Comparing these spectra with those obtained for silica gel as adsorbent, one sees that (a) they are broader and one vibrational peak of the emission spectrum at about 378 nm disappears, (b) both emission and excitation spectra are shifted to the red by about 2 nm, (c) for the B = 0.2 sample, there is no dependence of the emission spectrum on the excitation wavelength or the excitation spectrum on the observation wavelength, (d) the excitation spectra broaden substant i d y with coverage, and (e) the most striking fact is again the very low fluorescence intensity at 460 nm even for the sample with B = 5. The results of decay time measurements are shown in Table IV. The mean decay times of pyrene on alumina are shorter as compared with those for pyrene on silica gel
5%
1%
0.2%
0.079 124.0 0.173 228.0 2.2 207.4 0.061 186.9 0.179 57.122 2.9 125.5
0.110 127.3 0.154 228.7 1.4 199.9
0.142 129.2 0.134 229.2 0.94 191.8
and CaF2, indicating that stronger interaction with the surface increases the fluorescence decay constant. The decay times of the monomer emission also become shorter with decreasing surface coverage: this probably means that at low coverages a site selection occurs with stronger adsorbent-adsorbate interaction. Pyrene Adsorbed on Porous Vycor Glass. Porous glass is manufactured from a borosilicate glass. Chemical extraction of this product causes the alkali and boric acid to be removed almost completely from the glassy material. What remains is a silica skeleton of 97% silica gel and about 3% B203,with a very uniform pore size (average pore diameter of 40 A). Porous glass is, therefore, very similar to silica gel except that it has an amorphous structure. According to the resulta of Ron and co-workers21 and their interpretation of Leermakers' even the adsorption sites are similar and are formed from silanol groups. The porous glass was treated as described in the Experimental Section, and a sample with coverage of 0 = 3 was prepared. This sample was degassed at 90 OC for 2 h a t a pressure of
