3276
J . Phys. Chem. 1989, 93, 3276-3282
Dkorder-Order Transition and Energy Transfer in Phenanthrene Adlayers on AI,O,( 11%) N. J. Tro, A. M. Nishimura? and S. M. George* Department of Chemistry, Stanford University, Stanford, California 94305 (Received: August 12, 1988)
The disorder-order transition and energy transfer in phenanthrene adlayers on A1203(l120) at coverages from IO to 125 monolayers were studied by using laser-induced fluorescence and electronic absorption spectroscopy. After adsorption at 90 K, the absorption spectra of phenanthrene adlayers displayed a broad absorption peak at 304 nm. The corresponding fluorescence spectra were broad and featureless with a maximum at 395 nm and a lifetime of 55 ns. The absorption spectra were consistent with a disordered, glasslike adlayer, and both the featureless fluorescence spectra and the relatively long 55-11s fluorescence decay times were consistent with excimer emission. The absorption spectra changed dramatically and resembled the crystalline absorption spectra when the adlayers were annealed to 200 K. After annealing above 200 K, the fluorescence spectra also revealed vibrational structure originating at 354 nm that was characteristic of the crystalline fluorescence spectra and a fluorescence lifetime of 18 ns. The sudden changes in the absorption and fluorescence spectra at 195-200 K were interpreted as a disorder-to-order transition in the phenanthrene adlayer on A1,03(1 120). During this transition, the phenanthrene adlayer evolves from a glasslike to a crystalline state. Polarized absorption spectra indicated that the crystalline phenanthrene adlayer was oriented with the ab plane of the crystal aligned parallel to the A1203(1120) surface. Energy-transfer studies revealed that excimer sites were very effective traps for excited electronic energy. Energy-transfer studies performed with anthracene impurities in the phenanthrene adlayer also demonstrated that electronic energy was immobilized at excimer traps in the disordered film and was very mobile in the ordered crystalline adlayer.
1. Introduction
Energy transfer in organic adlayers on surfaces has recently been the subject of many experimentall-I1 and theoreticaIl2J3 studies. These investigations are important for an understanding of photophysics and photochemistry in thin films on surfaces. Moreover, these studies are relevant to the mechanisms of photocell s e n ~ i t i z a t i o n ~and ~ - ~to * electronic energy percolation in two dim e n s i o n ~ . ' Unfortunately, ~~~~ a detailed understanding of energy transfer in surface adlayers has not been achieved. Thick evaporated films of various aromatic compounds have been studied previously with electronic absorption and fluorescence ~ p e c t r o s c o p y . ~ ' -These ~ ~ studies have shown that organic films are glasslike when evaporated onto a cold substrate. Many of these aromatic systems have displayed a broad, featureless fluorescence emission from the glassy film that has been attributed to excimer e m i ~ s i o n . ~ 'Likewise, -~~ these glassy films have been shown to form microcrystals after substrate annealing.29-32 Previous studies of molecular crystals of aromatic compounds have also demonstrated that crystal defects are influenced by the degree of order in the ~ r y s t a I . ~These ~ - ~ ~defects can act as traps for electronic excitation en erg^.^^-^^ Likewise, chemical impurities in extremely small quantities can also lead to electronic energy Electronic energy trapping by defects and impurities can result in the blockage of electronic excitation transport. As demonstrated by previous studies of molecular crystals, an understanding of electronic energy transfer in adlayers requires a precise knowledge of the adlayer order and adlayer composition. Despite the importance of adlayer order for energy transfer, few studies have explored the effect of order on electronic energy transfer in adlayers. Likewisc, the effect of adlayer composition on electronic energy transfer has not been studied because of the difficulty in preparing and characterizing adlayers of coadsorbates. In this paper, the disorder-to-order transition and energy transfer in phenanthrene adlayers adsorbed on A1203(1 120) were examined in ultrahigh vacuum (UHV) at coverages of 10-125 monolayers."' The coverage, composition, temperature, and order of the adlayer were established with precise control using UHV surface science techniques. The phenanthrene adlayers on A1203(1120) at 90 K were found to have vastly different properties than the bulk phenanthrene crystal. These differences were caused primarily by the random adsorption of phenanthrene molecules onto a substrate at low temperature. 'Permanent address: Department of Chemistry, Westmont College, Santa Barbara, CA 93108.
0022-365418912093- 3216$0 1.50/0
The degree of order in the phenanthrene adlayer was then increased slowly by annealing the film to progressively elevated
( I ) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1986, 90, 5094. (2) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1987, 91, 1423. (3) Kemnitz, K.; Murao, T.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1983, 101, 337. (4) Willig, F.; Blumen, A.; Zumofen, G. Chem. Phys. Lett. 1984,108,222. (5) Liang, Y.; Moy, P. F.; Poole, J . A,; Ponte Goncalves, A. M. J . Phys. Chem. 1984, 88, 2451. (6) Liang, Y.; Ponte Goncalves, A. M.; Negus, D. K. J . Phys. Chem. 1986, 90, 5094. (7) Hara, K.; DeMayo, P.; Ware, W. R.; Weedon, A. C.; Wong, G. S . K.; Wu, K. C. Chem. Phys. Lett. 1980, 69, 105. (8) Bauer, R. K.; DeMayo, P.; Ware, W. R.; Wu, K. C. J . Phys. Chem. 1982,86, 3781. (9) Anfinrud, B.; Crackel, R. L.; Struve, W. S . J . Phys. Chem. 1984,88, 5873. ( I O ) Anfinrud, P. A,; Causgrove, T. P.; Struve, W. S. J. Phys. Chem. 1986, 90, 5887. ( 1 1) Alivisatos, A. P.; Arndt, M. F.; Efrima, S.; Waldeck, D. H.; Harris, C. B. J . Chem. Phys. 1987, 86, 6540. (12) Loring, R. F.; Fayer, M. D. Chem. Phys. 1982, 70, 139. (1 3) Nakashima, N.; Yoshihara, K.; Willig, F. J . Chem. Phys. 1980, 73, 3553. (14) Gerischer, H.; Willig, F. In Physical and Chemical Applications of Dyestuffs; Topics in Current Chemistry, Vol. 61; Springer-Verlag: Berlin, 1976; p 31. (1 5) Honda, K. In Photochemical Conversion and Solar Energy; Rabari, J., Ed.; Weizmann Science Press: Jerusalem, 1982; p 75. (16) Kamat, P. V.; Fox, M. A. J . Electrochem. Soc. 1984, 131, 1032. (17) Kamat, P. V.; Fox, M. A.; Fatiadi, A. J. J . Am. Chem. Soc. 1984, 106, 1191. (18) Hohman, J. R.; Fox, M. A. J . A m . Chem. Soc. 1982, 104, 401. (19) Kopelman, R.; Monberg, E. M.; Octis, F. W.; Prasad, P. N. Phys. Rev. Lett. 1975, 34, 1506; J . Chem. Phys. 1975, 62, 292. (20) Colson, S . D.; George, S . M.; Keyes, T.; Vaida, V. J . Chem. Phys. 1977, 67, 4941. (21) Arden, W.; Peter, L. M.; Vaubel, G. J . Luminesc. 1974, 9, 257. (22) Takahashi, Y.; Uchida, K.; Tomura, M. J . Luminesc. 1977, 15, 293. (23) Muller, H.; Baessler, H.; Vaubel, G. Chem. Phys. Lett. 1974, 29, 102. (24) Hofmann, J.; Seefeld, K. P.; Hofberger, W.; Baessler, H. Mol. Phys. 1979, 37, 973. (25) Seefeld, K. P.; Muller, H.; Baessler, H. J . Luminesc. 1978, 16, 395. (26) Peter, G.; Baessler, H . Chem. Phys. 1980, 49, 9. (27) Peter, G.; Ries, B.; Baessler, H. Chem. Phys. 1983, 80, 289. (28) Hesse, R.; Hofberger, W.; Baessler, H . Chem. Phys. 1980, 49, 201. (29) Seki, H.; Itoh, U. J . Chem. Phys. 1980, 72, 2166. (30) Kamura, Y.; Seki, K.; Inokuchi, H. Chem. Phys. Lett. 1975,30, 35.
0 1989 American Chemical Society
Energy Transfer in Phenanthrene on A1203(1120) VIEWPORT
V
LASER
SPECTROMETER MONOCHROMATOR
I
FAST GATED INTEGRATOR
1
COMPUTER
I
I
Figure 1. Schematic of the experimental apparatus for electronic absorption spectroscopy and laser-induced fluorescence studies on singlecrystal oxide surfaces in ultrahigh vacuum.
temperatures. Concurrently, the degree of order in the phenanthrene adlayer and the effects of order on electronic energy transfer were studied with electronic absorption spectroscopy and laserinduced fluorescence. These studies revealed a dramatic disorder-to-order transition at 195-200 K and demonstrated that electronic energy transfer in phenanthrene adlayers on A1203(1 1 SO) was extremely sensitive to adlayer order and adlayer composition. 11. Experimental Section A schematic diagram of the UHV chamber used for these experiments is shown in Figure l.42 The A1203 sample was mounted at the end of a cold finger. This cold finger was at the end of a double-vacuum-jacketed dewar mounted on a rotary f e e d t h r ~ u g h . ~This ~ dewar was capable of holding either liquid helium or liquid nitrogen. The UHV chamber was pumped by a 190 L/s Balzers turbomolecular pump that was backed by another 50 L/s Balzers turbomolecular pump. After a bake, this tandem turbomolecular Torr. pumped UHV chamber obtained base pressures of 8 X The predominant residual gas in the chamber at these pressures was hydrogen. The UHV chamber was also equipped with a UTI lOOC quadrupole mass spectrometer with a 1-300 amu mass range. The mass spectrometer was used for background gas analysis during sample dosing and for temperature-programmed desorption (TPD) studies. (31) Maruyama, Y.; Iwaki, T.; Kajiwara, T.; Shirotani, I.; Inokuchi, H. Bull. Chem. SOC.Jpn. 1970, 43, 1259. (32) Kamura, Y.; Shirotani, I.; Ohno, K.; Seki, K.; Inokuchi, H. Bull. Chem. SOC.Jpn. 1976, 49, 418. (33) Maruyama, Y.; Iwasaki, N. Chem. Phys. Lett. 1974, 24, 26. (34) Inokuchi, H.; Kurcda, H.; Akamatu, H. Bull. Chem. SOC.Jpn. 1961, 34, 749. (35) Fielding, P. E.; Jarnagin, R. C.J . Chem. Phys. 1967, 47, 247. (36) Helfrich, W.; Lipsett, F. R. J . Chem. Phys. 1963, 43, 4368. (37) Auweter, H.; Schmid, D.; Wolf, H. C. Chem. Phys. 1974, 5 , 382. (38) Powell, R. C.; Soos, Z . G. J. Luminesc. 1975, 11, I . (39) Wolf, H. C. In Advances in Atomic and Molecular Physics; Bates, D. R., Estermann, I., Eds.; Academic Press: New York, 1967; Vol. 3. (40) Birks, J. B. Photophysics ofAromaric Molecules; Wiley-Interscience: London, 1970. (41) Tro, N . J.; Nishimura, A . M.; George, S. M. J. Vac. Sci. Technol. 1988, Ab, 852. (42) Tro, N . J.; George, S. M. Sur/. Sei. 1988, 197, L246. (43) George, S. M. J . Vac. Sci. Technol. 1986, A4, 2394.
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3277 Single crystals of AlzO3( 1120) with dimensions of 1 in. by 1 in. and a thickness of 0.031 in. were purchased from Saphikon. Four 0.070-in.diameter holes were ground in the sample near each corner, and four stainless steel screws were used to clamp the sample to two gold-plated copper posts. The copper posts were attached to the end of the cold finger using a sapphire plate for electrical isolation. The gold plating was used to maximize heat transfer at the pressed junctions.u A film of tantalum with a thickness of 6000 8,was evaporated onto the back side of the A1203(1120) sample. By masking the sample during tantalum evaporation, a clear circular aperture with a 0.25-in. diameter was formed at the center of the sample. The A1203sample was heated resistively by passing current through the tantalum film. A 0.020-in. hole centered at the top of the sample allowed a 0.003-in.-diameter chromel-alumel thermocouple to be attached with Aremco 569 ceramic adhesive. The temperature was maintained by a temperature contfoller that determined the current output of a HP 6264B programmable power supply. The temperature controller could maintain temperatures to better than f0.1 K and could also produce linear temperature ramps. With liquid helium cooling, a temperature range from 20 to 700 K was obtainable. Liquid nitrogen cooling raised the lower limit to 90 K. Each sample was cleaned in an ultrasonic oscillator using acetone and rinsed with methanol before mounting in the UHV chamber. Once in the UHV chamber, the A1203(1120) sample was cleaned by heating to 373 K and exposing the sample to an oxygen plasma discharge."5 Auger analysis has shown that this procedure produces clean A1203surfaces.45 Phenanthrene was obtained commercially, purified with maleic anhydride, and ~one-refined.~~ Large organic molecules, such as phenanthrene, have low vapor pressures at room temperature, and elevated temperatures are required for sample dosing. Consequently, the phenanthrene was placed in a small stainless steel tube. This tube was then attached to the inlet of a variable leak valve, and the valve outlet was connected to the sample doser. The stainless steel tube was Torr was achieved. The pumped out until a pressure of 1 X tube, leak valve, and sample doser were then all heated to 430 K to increase the vapor pressure and prevent phenanthrene condensation. Resistive heating of nichrome wire wrapped around the sample doser line inside the UHV chamber was required to maintain the sample doser at 430 K. The doser was a 0.1 25-in.-diameter stainless steel tube that was attached at the end to a 0.50-in. stainless steel tube with a 1-in. length. The end of this doser assembly was positioned approximately 0.50-in. from the A1,03 crystal during dosing. The phenanthrene doses were monitored and verified by using the mass spectrometer. Absorption measurements were performed with an Oriel 75-W Xe arc lamp. The ultraviolet light was passed initially through a Spex Model 1681B 0.22-m f/4 monochromator for wavelength selection and then focused onto the A1203sample. The radiation was incident on the A1203(1120) surface at normal incidence. Light from the Xe arc lamp could be polarized by using a Glan Taylor polarizing prism. The excitation source for the fluorescence experiments was a Molectron UV-300 nitrogen laser producing IO-ns pulses (fwhm) at 337 nm with 350 FJ per pulse. The light was collimated and then focused by a lens with a focal length of 20 cm onto the clear aperture at the center of the A1203sample. The laser pulses were incident on the A120,(11~0)surface at an angle of 4 5 O relative to the surface normal as shown in Figure 1. The fluorescence was collected by f/0.95 optics positioned in the chamber in a stainless steel holder. The collected fluorescence was collimated and then focused withf-matching optics onto the (44) White, G. K. Experimental Techniques in Low-Temperature Physics; Clarendon Press: Oxford, 1979. (45) Poppa, H.; Moorhead, D.; Heinemann, K. Thin Solid Films 1985, 128, 252. (46) McArdie, B. J.; Sherwood, J. N . J. Crysf. Growth 1974, 22, 193.
3278 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989
Tro et al.
I
I 320
340
360
Wavelength (nm) 0.1
'
b)
X.351
nm
0.08 0.06.
1
280
I
I
I
300
320
340
0.04
I
360
Wavelength ( n m ) Figure 2.- Electronic absorption spectra of 35 ML of phenanthrene on A1203(1 120) at 90 K taken at normal incidence as a function of annealing temperature. The vibrational structure designated on the 200 K spectrum is given in wavenumbers (cm-I). slits of the Spex Model 1681B monochromator equipped with a Hamamatsu R928 photomultiplier tube. The signal was then sent to a Stanford Research Systems Model SR250 gated integrator and boxcar averager interfaced to an IBM PC. The response function for this instrumentation had a rise time of approximately 8 ns. In a typical experiment, the A1203crystal was heated to 500 K in order to desorb any background gases from the surface and then cooled to 90 K. Phenanthrene was dosed immediately after cooling while the dose was monitored with the mass spectrometer. Typical doses lasted between 15 and 200 s. Phenanthrene surface coverages were determined by electronic absorption spectroscopy. The coverage in monolayers was related to the absorbance by 0 = 2.303A/(uNML), where NMLis the number of molecules per square centimeter in one monolayer and u is the absorption cross section. For phenanthrene, N M L = 3.8 X I O l 4 molecules/cm2 was derived from the number density in the ab crystal plane.47s48 Likewise, an absorption cross section of u = 5.0 X lo-'* cm2 at 351 nm was obtained by averaging the absorption intensities for the a and b crystal axes.49 Annealing was accomplished by raising the temperature of the A1203crystal to the annealing temperature and maintaining that temperature for 60 s before cooling back to 90 K. Annealing temperatures could be reached in approximately 15 s. Likewise, the AI2O3sample could cool back to 90 K from the annealing temperature in approximately 20 s. 111. Results
The results of temperature-programmed desorption (TPD) and temperature-programmed spectroscopy studies of phenanthrene from A1203(1 120) have been reported earlier.42 Briefly, the TPD peak did not saturate with increasing exposure, and phenanthrene desorption was characteristic of zero-order desorption from a multilayer. The temperature-programmed studies gave a desorption activation barrier of Ed = 22 kcal/mol and a zero-order desorption preexponential of uo = 1 X molecules/(cm2 s ) . ~ ~ The electronic absorption spectra at 90 K for 35 monolayers (ML) of phenanthrene on A1203(1120) as a function of annealing temperatures are shown in Figure 2. These spectra were taken at a normal angle of incidence. After adsorption at 90 K, there (47) Mason, R. Mol. Phys. 1961, 4 , 413. (48) Trotter, J. Acta Crystallogr. 1963, 16, 605. (49) Craig, D. P.; Gordon, R. D. Proc. R. SOC.London 1965, A288, 69.
.'b
.
0,02[
01 0
20
40
60
80
I
-
Polarization Angle (degrees)
Figure 3. (a) I, and I,, absorption spectra for the So S, transition. The spectra were taken at 90 K with 125 ML of phenanthrene on A1203(llZO) after annealing at 230 K. (b) Absorbance at 351 nm as a function of the polarization angle relative to the plane of incidence.
-
is a broad absorption peak at 304 nm that corresponds to the So S2electronic transition in In addition, the absorption intensity of the So SI electronic transition is much weaker and originates at 351 nm. The absorption spectra were independent of coverage for all measured coverages between 20 and 125 ML. Likewise, the absorption spectra were independent of annealing temperatures for temperatures from 90 to 190 K. Figure 2 shows that the absorption spectrum undergoes a dramatic transformation when the adlayer is annealed at temperatures greater than 195 K. The total change in the absorption spectrum occurs over a 5 K range from 195 to 200 K. During this change, the absorption intensity of the So S2 transition plummets. Concurrently, the absorption intensity of the So SI transition increases and resembles the crystalline s p e c t r ~ m . ~ ~ . ~ ~ The vibrational structure at 90 K after annealing to 200 K is designated according to previous molecular crystal assignments and So S2s4transitions at 77 K. Annealing for the So to higher temperatures did not result in any further changes in the electronic absorption spectrum. Figure 3 shows the I, and Ill absorption spectra for the So SI transition with 125 ML of phenanthrene on A1203(1120). The spectra were taken at 90 K after annealing the adlayer at 230 K for 60 s. In these experiments, the incoming light was incident at 65' relative to the surface normal. In Figure 3a, I, represents the spectrum for light polarized perpendicular to the plane of incidence. Illrepresents the spectrum for light polarized parallel to the plane of incidence. For clarity in presentation, the I, and Illspectra are offset slightly. Figure 3a reveals that light polarized perpendicular to the plane of incidence, Le. parallel to the surface, gives a much higher absorbance for the So SI transition. Figure 3b displays the absorbance at 351 nm for the same adlayer for light incident at 65' relative to the surface normal versus polarization angle relative to the plane of incidence. In this figure, Oo corresponds to light polarized parallel to the plane of incidence, i.e. largely perpendicular to the surface. Figure 3b
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-
-
-
-
-
-
(SO) Dick, B.; Nickel, B. Chem. Phys. 1986, 110, 131. (51) Jones, R. N.; Spinner, E. Specfrochim. Acra 1960, 16, 1060. (52) Berlman, I . B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York, 1971. (53) Lendray, E.; Hornyak, I. J . Luminesc. 1974, 9, 18. (54) Gordon, R. D. Mol. Cryst. 1966, I , 441.
Energy Transfer in Phenanthrene on A1203(1120)
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3279
I
1
1 440 520 Wavelength ( n m ) Figure 4. Laser-induced fluorescence spectra for 10 ML of phenanthrene on Al,03(1 120) at 90 K as a function of annealing temperature. The intensities for the various spectra have been scaled for presentation. 360
reveals that the absorbance at 351 nm increases as the component of light polarized parallel to the surface increases. This indicates that the phenanthrene molecules are oriented with their So SI transition dipole moments aligned primarily parallel to the A1203(1 120) surface. The fluorescence spectra of 10 ML of phenanthrene on A1203(1 120) as a function of annealing temperature are shown in Figure 4. The unannealed adlayer at 90 K has a fluorescence spectrum that is broad and featureless with a maximum at 400 nm. This spectrum is in marked contrast to the crystalline spectrum, which is highly structured with an origin at 354 nm1.55*56 The broad, red-shifted emission is similar to the fluorescent emission observed from other aromatic hydrocarbons in evaporated films. This broad, red-shifted emission has been attributed to excimer Figure 4 reveals that the fluorescence spectra start to show vibrational structure when annealing occurs at temperatures above 195 K. This vibrational structure sharpens with higher annealing temperatures. The spectrum after annealing to 240 K closely resembles the fluorescence spectra of phenanthrene molecular crystal^.^^^^^ Note that the changes in the fluorescence spectra occur over a 40 K range beginning at approximately 195 K. On the other hand, the changes in the absorption spectra occur over only a 5 K range between 195 and 200 K. Figure 5 displays the fluorescence decays at 90 K for 10 ML of phenanthrene versus annealing temperature. The fluorescence decay of the unannealed adlayer at 90 K measured at the fluorescence peak at 395 nm was fit to a single-exponential decay with a long lifetime of 55 ns. The fluorescence decay was independent of wavelength in the unannealed adlayer at 90 K. After annealing at temperatures above 195 K, Figure 5 shows that the fluorescence decays are reduced considerably. After annealing to 235 K, the fluorescence decay at the fluorescence peak at 375 nm was fit to an exponential decay with a shorter lifetime of 18 ns. At intermediate stages of annealing, the fluorescence decay was composed of both long and short lifetime components.
0
200
100
300
Time (nsec) Figure 5. Laser-induced fluorescence decays for 10 ML of phenanthrene on AI,03(1120) at 90 K as a function of annealing temperature.
I '
I
I
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(55) McClure, D. S.J . Chem. Phys. 1956, 25, 481. (56) Misra, T. N.; Lacey, A. R.; Lyons, L. E. Aust. J . Chem. 1966, 19, 415.
I" I 1
360
I
I
440
520
Wavelength
(nm)
Figure 6. Time-resolved laser-inducedfluorescence spectra at 90 K for 10 ML of phenanthrene on AI,O,( 1120) annealed to 197 K. The time delays are relative to the same t = 0 shown in Figure 5 .
Time-resolved fluorescence spectra at 90 K for 10 ML of phenanthrene on Al,03(1 120) annealed to 197 K are shown in Figure 6 . At early times in the fluorescence decay, the spectrum is structured and resembles the spectrum of the annealed adlayer. At later times, the fluorescence spectrum is broad and resembles the spectrum of the unannealed adlayer. These spectra were taken by scanning the time delay of the gated integrator with a 2-11s window relative to the initial trigger pulse provided by the laser pulse. The time delays are relative to the same t = 0 shown in Figure 5. In order to probe electronic energy transfer in phenanthrene adlayers, anthracene was doped into the phenanthrene adlayer at a ratio of 1:(5 X 10") anthracene:phenanthrene. Figure 7 shows
Tro et al.
3280 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 I
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Wavelength (nm) Figure 7. Laser-induced fluorescence spectra at 90 K for 10 ML of phenanthrene doped with anthracene on AI2O3(1120) before and after annealing at 240 K. the fluorescence spectrum at 90 K for I O ML of phenanthrene doped with anthracene on A1203(1 120) before and after annealing at 240 K. Before annealing, the fluorescence spectrum is identical with the undoped fluorescence spectrum. In contrast, anthracene fluorescence emission becomes prominent after annealing the phenanthrene adlayer at 240 K.
IV. Discussion The electronic absorption spectra of phenanthrene adlayers on A1203(1 120) changed dramatically after the adlayer was annealed at temperatures greater than 195 K as shown in Figure 2. The absorption spectra prior to annealing above 195 K were similar to the absorption spectra of phenanthrene dispersed at low con'-~~ the So S2and centration in a glassSoor s o l ~ t i o n . ~ However, So SI transitions were red-shifted by approximately 7 and 4 nm, respectively, relative to the low-concentration glass or solution spectra.5b53 These red-shifts are attributed to the different dispersion for phenanthrene in solution and in a phenanthrene g l a s ~ . ~ ' ~Likewise, ~* the absorption spectrum after annealing to 200 K closely resembles the crystalline s p e c t r ~ m ! ~The ~ ~ changes ~ in the absorption spectra are consistent with a disorder-to-order or glass-to-crystalline transition in the phenanthrene adlayer on A1203(1 120). The most dramatic alteration in the phenanthrene absorption spectra is the decrease of the absorption intensity of the So S2 electronic transition. This intensity decrease is caused by strong dipole interactions in the phenanthrene crystal. The So S2 transition is polarized along the long molecular axis in phenanthrene as shown in Figure 8a!9*54 In the type A crystal structure for phenanthrene, the So S2 dipole moments of neighboring molecules are aligned parallel to each other.47@ This configuration results in strong interactions between neighboring dipoles upon crystallization that perturb and split the So S2transition. In similarity to the long-axis polarized So S 3 transition in ant h r a ~ e n ea, ~large ~ fraction of the So S2oscillator strength is blue-shifted into the deep ultraviolet.60s61 The polarized absorption spectra for the So SI transition shown in Figure 3 reveal that the phenanthrene crystalline adlayer is oriented with respect to the Al,03( 1120) surface. In order to determine the crystalline orientation, the absorbance was calcu-
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(57) Bayliss, N . S . J . Chem. Phys. 1956, 18, 292. (58) Weigang, 0. E., Jr. J . Chem. Phys. 1960, 33, 892. (59) Craig, D. P.; Hobbins, P. C. J . Chem. SOC.1955, 539. (60) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (61) Philpott, M R., private communication
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Figure 8. (a) Picture of a phenanthrene molecule showing the correspondence between the short and long molecular axes and the So SI and So S2 transitions, respectively. (b) Projection onto the A1203( 1 120) surface of phenanthrene molecules in a crystalline adlayer with the ab crystal plane parallel to the surface.
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TABLE I
cryst plane
p,
PY
pz
ab bc ac
0.453
0.871
0.190
0.871
0.190
0.453
0.190
0.453
0.871
lated as a function of polarization angle for three probable crystalline orientations on the surface. The calculations considered the ab, ac, and bc crystal planes oriented parallel to the A1203(1 120) surface. These calculations required a decomposition of the electric field and the molecular transition dipole moment onto the same x , y , z coordinate system. The components of the electric field incident on the surface arc E , = E sin 0
(1)
Ey = E cos 0 cos
(2)
E, = E cos 0 sin @
(3)
In these expressions E , and Ey are the components of the electric field polarized in the plane of the A1203surface and E, is the component polarized perpendicular to the surface. Likewise, 0 is the angle of the polarized electric field with respect to the plane of incidence, and @ is the angle of incidence with respect to the surface normal. The corresponding components of the transition dipole moment are P,, Py, and P,. P, and P y are the components of the So SI transition dipole moment polarized in the plane of the AI2O3 surface, and P, is the component of the transition dipole moment polarized perpendicular the surface. P,, Py,and P, are tabulated in Table I for the three different crystalline orientations on the surface.62 With these definitions, the relative absorbance is given by
-
A
0:
(Ex2+ E;)P,,,Z
+ E>P>
(4)
where Pay,is defined by
The results of these absorbance calculations are shown in Figure 9. The orientation with the ab crystalline plane parallel to the (62) Bree, A.; Solven, F. G.; Vilkos, V. V. B.J . Mol. Spectrosc. 1972, 44, 208.
Energy Transfer in Phenanthrene on A1203(1120)
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3281
of 18 and 55 ns. On the other hand, fluorescence decays monitored outside the excimer emission bandwidth at 354 nm show only a fluorescence lifetime of 18 ns. Successive changes were observed in the fluorescence spectra displayed in Figure 4 after annealing at progressively higher temperatures above 195 K. Notice that the fluorescence spectrum still shows a large amount of excimer emission after the adlayer is annealed to 200 K. In order to remove the excimer emission completely, annealing to 240 K was required. This behavior is in marked contrast to the absorption spectrum, where the changes occur suddenly and completely between 195 and 200 K. Absorption studies measure the majority species. On the other hand, fluorescence studies are sensitive to electronic energy traps. The absorption spectrum reveals that a majority of the phenan0 20 40 60 80 threne molecules exist in a crystalline form after annealing to 200 K. In contrast, the presence of a large amount of excimer emission Polarization Angle (degrees) after annealing to 200 K indicates that a small minority of excimer Figure 9. Normalized absorbance versus polarization angle calculated sites at defects in the crystalline adlayer can significantly affect for the ab, ac, and bc crystal planes parallel to the AI,O,( 117.0) surface. the fluorescence emission. These observations illustrate the importance of combined absorption and fluorescence spectroscopic A1203(1 130) surface closely reproduces the experimental data. investigations. A projection of the phenanthrene molecules in the first layer of Substantial excimer emission from a small minority of excimer this crystalline orientation onto the A1203(l130) surface is shown sites occurs because excimers are red-shifted considerably from in Figure 8b.4749,62.63In this orientation, the long molecular axis the monomer electronic energy leveL2' Consequently, minority is 15O relative to the normal to the ab plane. excimer sites are effective energy traps for migrating electronic Crystal orientations with their ab crystal planes parallel to the energy. Excimer formation is possible at defects in the phenansubstrate interface have also been observed for anthracene on threne crystalline adlayer where the geometrical constraints imquartz,29perylene and coronene on quartz,32additional aromatics posed by the type A crystal lattice are relaxed.65 on glass3' and LiF,30and naphthalene on Pt( 11l).64 Moreover, The time-resolved fluorescence spectra for adlayers annealed the ab plane is the cleavage plane of the phenanthrene molecular to 197 K are consistent with electronic energy migration to micrystaL4* Orientation of the cleavage plane parallel to the surface nority excimer traps. The fluorescence spectra shown in Figure is expected to be the minimum energy configuration. Thus the 6 display predominantly fluorescence emission typical for the crystalline ab plane aligned parallel to the A1203(1120) surface phenanthrene molecular crystal at early delay times. At longer is consistent with previous observations and expectations. delay times, the fluorescence spectra show progressively less Excimer emission can occur from molecular crystals of aromatic monomer emission. molecules when pairs of molecules align in parallel.65 This At early times, the fluorescence emission is derived from structure is found in type B crystals, such as pyrene, where excimer crystalline monomer sites because the excitation energy is absorbed emission is facile because the crystal structure allows for the initially by exciton transitions in the crystalline adlayer. As a excimer dimer pairs. On the other hand, molecules that form type function of delay time, this electronic energy dissipates by A crystals have nearest neighbors that are nearly orthogonal as fluorescence, intersystem crossing, or internal conversion at the shown in Figure 8b. Consequently, the requisite parallel dimer pairs cannot form, and excimer emission is not ~ b s e r v e d . ~ ' , ~ ~crystalline monomer sites.40 In addition, Figure 6 reveals that a fraction of the electronic energy is quickly trapped at minority Phenanthrene forms type A crystals and does not exhibit exexcimer sites. This electronic energy transfer to excimer traps ~ . ~ ~the crystal, an adlayer cimer emission in the ~ r y s t a l . ~Unlike occurs within the experimental time resolution of approximately deposited at low temperatures is not in the most stable thermo10 ns. At long delay time, all the electronic energy has either dynamic configuration. Consequently, molecules can exist in a been dissipated at crystalline monomer sites or trapped at excimer variety of configurations allowing for excimer emission. The broad, sites and only excimer emission is observed. featureless fluorescence peak with a maximum red-shifted from The fluorescence spectra from the doped phenanthrene adlayers the crystalline origin by 3250 cm-' is characteristic of excimer shown in Figure 7 also demonstrate that electronic energy is emission.21 trapped easily at excimer sites. The unannealed spectrum disThe relatively long fluorescence decay time of 55 ns after played in Figure 7 shows only excimer emission with no anthracene adsorption at 90 K also supports excimer emission. Fluorescence emission. The excimer emission indicates that electronic energy lifetimes much longer than the lifetime in molecular crystals are is trapped readily at excimer sites in the adlayer and cannot typical for excimer emission.2' In contrast, the phenanthrene migrate to anthracene traps. This behavior is consistent with the adlayer annealed to 240 K displays a much more rapid fluoreslow mobility of the excimer exciton with D C lo-' cm2/s40@and cence lifetime of 18 ns. This lifetime is shorter than the the large energy depth of the excimer trap. fluorescence lifetime in macroscopic phenanthrene molecular After annealing to 240 K, the excimer traps are removed and crystals.66 However, nonradiative relaxation to traps at the A1203 the exciton can migrate in the adlayer. Excitons are highly mobile surface or exciton quenching at the phenanthrene/vacuum or in molecular crystals, and diffusion coefficients as high as D = phenanthrene/AI2O3 interface67 may reduce this fluorescence to cm2/s are t y p i ~ a l . ~ Consequently, *,~~ the exciton is lifetime while leaving the crystalline fluorescence spectra unmobile enough to encounter anthracene minority traps in the changed. phenanthrene adlayer, and Figure 7 reveals that a substantial The fluorescence decays observed as a function of annealing fluorescent emission from anthracene is observed. are consistent with excimer emission that is removed progressively as a function of annealing temperature. Fluorescence decays at V. Conclusions intermediate stages of annealing show a two-component decay The disorder-order transition and energy transfer in phenanat 400 nm. These fluorescence decays can be fit with lifetimes threne adlayers on A1203(l120) at coverages from 10-125 monolayers were studied with electronic absorption spectroscopy and laser-induced fluorescence. After adsorption at 90 K, the So (63) Schettino, V.; Neto, N.; Califano, S. J . Chem. Phys. 1966, 44, 2724. (64) Firment, L. E.; Somarjai, G. A. Surf.Sci. 1976, 55, 413. S2absorption spectra of phenanthrene adlayers revealed a broad (65) Stevens, B. Spectrochim. Acta 1962, 18, 439. (66) Heidersdorf, C. P. Mol. Cryst. Liq. Crysr. 1974, 27, 141. (67) Birks, J. B. Proc. Phys. SOC.,London 1962, 79, 494.
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(68) Baessler, H. Phys. SfafusSolidi B 1981, 107, 9.
J . Phys. Chem. 1989, 93, 3282-3286
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peak at 304 nm, and the corresponding fluorescence spectra were broad and featureless with a maximum at 395 nm and a lifetime of 55 ns. The absorption spectra were consistent with a disordered, glasslike adlayer, and the broad featureless fluorescence spectra were attributed to excimer emission. The relatively long 55-11s fluorescence decay times were also consistent with excimer emission. The absorption spectra changed abruptly and resembled the crystalline absorption spectra when the adlayer was annealed to 200 K. The main feature of this change was a dramatic reduction of the absorbance of the So S2 electronic transition. After annealing above 195 K, the fluorescence spectra also revealed vibrational structure characteristic of the crystalline fluorescence spectra originating at 354 nm. The sudden changes in the absorption and fluorescence spectra at 200 K were interpreted as a disorder-to-order transition in the phenanthrene adlayer on A1203(1120). During this transition, the phenanthrene film evolves from a glasslike to a crystalline state. Polarized absorption spectra indicated that this crystalline phenanthrene adlayer was oriented with the ab plane of the phenanthrene crystal aligned parallel to the AI,O,( 1 120) surface.
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Solid-state
Energy-transfer studies revealed that excimer defect sites in the adlayer were very effective traps for excited electronic energy. Energy-transfer studies performed with anthracene impurities in the phenanthrene adlayer demonstrated that electronic energy was immobilized at excimer traps in the disordered film. In contrast, the electronic energy was observed to be mobile in the ordered crystalline adlayer.
Acknowledgment. This research was supported by the Office of Naval Research Contract No. N00014-86-K-737. Some of the equipment utilized in this work was provided by the NSF-MRL program through the Center for Materials Research at Stanford University. We are grateful to Prof. R. N . Zare for providing the nitrogen laser and to Prof. M. D. Fayer for supplying the purified phenanthrene. We also thank Dr. M. R. Philpott for numerous useful discussions. N.J.T. gratefully acknowledges the National Scientific Foundation for support through a graduate fellowship. A.M.N. acknowledges the National Science Foundation through Grant CHE-85 14103 for a Research Opportunity Award. Registry No. AIZO3,1344-28-1; phenanthrene, 85-01-8.
M R Spectroscopy of Benzene Adsorbed on q-AI,O, Charles F. Tirendi,+G. Alex Mills, and Cecil R. Dybowski* Departments of Chemistry and Chemical Engineering and the Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 1971 6 (Received: August 24, 1988)
The temperature dependence of proton NMR spectra of benzene adsorbed on ?-A1203is affected by the pretreatment of the alumina. Between approximately 123 and 185 K, a composite line shape is observed, which can be divided into a broad component associated with benzene in large pores and a narrow component associated with benzene in small pores. Desorption at 298 K selectively removes molecules associated with the broad resonance, until at monolayer coverage only a narrow resonance is observed.
Introduction Physical adsorption of benzene is known to occur on metal oxides such as AI2O3,S O 2 , and TiOz because of the interaction of surface sites with benzene's a-electrons.' Depending on the pore distribution created by pretreatment and the amount of gas adsorbed, benzene molecules may occupy a variety of environments. Tetrahedrally and octahedrally coordinated AI3+ sites, as well as Bronsted acid and surface hydroxyl sites, probably participate in the adsorption process. The binding strengths differ, with the Lewis acid sites having the greatest affinity for benzene and the hydroxyl groups the weakest.* The distribution of the various sites and their occupancy at a given coverage depend on the pretreatment to which the oxide has been subjected. One general trend is noticed: As the severity of calcination is increased, the percentage of Lewis acid sites (AI3') increases? Besides direct interaction with binding sites, Mikhai13 suggests that benzene physisorbed in narrow pores of a silica gel should be oriented with the normal to the benzene plane lying parallel to the surface, whereas molecules in larger pores lie with the normal perpendicular to the surface. N M R spectroscopy provides information on the state of the alumina through its effect on the spin states of nuclei in species at the surface. For example, hydroxyl protons exhibit a second moment of 2.8 G 2on alumina calcined at 773 K, 2.6 G2of which arises from heteronuclear dipolar coupling between the protons *To whom correspondence should be addressed. 'Present address: UCSD Medical Center, Department of Radiology H756, University of California, San Diego. San Diego, CA 92103.
0022-3654/89/2093-3282$01.50/0
and aluminum nuclei of the surface. The remainder originates from proton-proton interactions, indicating that the protons are widely separated (>3 A) and i m m ~ b i l e . ~ Adsorption of benzene has been shown to induce mobility in the surface hydroxyl groups on alumina, as indicated by a narrowing of the hydroxyl N M R r e ~ o n a n c e . ~The same effect is observed in infrared spectra of these materials.'V6 Even though there is interaction between benzene molecules and surface hydroxyl groups, infrared spectroscopy indicates that physisorbed benzene is weakly bound and that this interaction does not reduce benzene's symmetry from DSh.laA previous NMR investigation' indicates that benzene physisorbed on ?-alumina gives a single resonance at 298 K. The shape and position of this resonance do not change under the eight-pulse dipole-dipole suppression sequence. In this case, the physisorbed benzene molecules must be moving freely and relatively isotropically on the surface, since (1) (a) Haaland, D. M. Surf. Sci. 1981, 102,405. (b) Pfeifer, H.; Winkler, H.; Freude D. Bull. 14th Coll. Ampere Ljubljana 1966, 1145. (c) Michel, D. Z . Naturforsch. 1968, 230, 823. (2) (a) Peri, J. B. J . Phys. Chem. 1965, 69, 211. (b) Peri, J. B. J . Phys. Chem. 1965,69, 220. (3) Mikhail, R. In Surface Area Determinafion; Everett, D. H., Otwell, R. H., Eds.; Butterworths: London, 1970; p 133. (4) Schreiber, L. B.; Vaughan, R. W. J . Catal. 1975, 40, 226. (5) Pfeifer, H. Tagung Hochfrequenzspektroskopie Leipzig. 1967, 222. (6) (a) Kiselev, A. V.; Uvarov, A. V. Surf. Sci. 1967, 6, 389. (b) Sempels, R. E.; Rouxhet, P. G.J . Colloid Interface Sci. 1967, 55, 263. ( 7 ) Tirendi, C. F.; Mills, G. A,; Dybowski, C. R. J . Phys. Chem. 1984, 88, 5165.
0 1989 American Chemical Society
