839
J . Phys. Chem. 1991,95, 839-844
Coveragehpendent Electronic Absorption Spectrum of Phenanthrene on Ai203(0001) and Butane Multilayer Surfaces D. R. Haynes, K. R. Helwig, N. J. Tro: and S. M. George* Department of Chemistry, Stanford University, Stanford, California 94305 (Received: April 16, 1990; In Final Form: July 25, 1990)
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The electronic absorption spectrum of phenanthrene was studied as a function of coverage in disordered adlayers at 20 K on both AI2O3(0001)and butane multilayer surfaces. The absorption maximum of the S, S2electronic transition red-shifted from X = 293 nm at 0 = 7 A to X = 301 nm at 0 2 100 A on A1203(0001). On the butane multilayer surfaces, the So S2absorption maximum red-shifted from X = 290 nm at 0 = 7 A to X = 301 nm at 0 1 100 A. These observed frequency red shifts versus coverage were consistent with dispersion interactions. Given the magnitude of the frequency shifts relative to the gas-phase transition frequency, the Bayliss equation was employed to calculate the refractive indices in the phenanthrene adlayer as a function of coverage. Refractive indices versus coverage were also determined from the Lorentz-Lorenz equation after calculating the effective local density within an interaction sphere centered about each phenanthrene admolecule. The sphere radius of r = 22 A obtained by fitting the calculationsto the experimental measurementswas in agreement with dispersion interactions.
Introduction The photophysical behavior of molecules on surfaces is currently a topic of great interest.'+ Although spectroscopic characteristics in molecular crystal, solution, and gas phases have been welldefined, relatively few investigations have explored the electronic absorption spectra of surface adlayers. The basic mechanisms of surface photochemistry, photodesorption, and photocell sensitization may be elucidated if the spectral and photophysical properties of surface adlayers can be carefully defined. Absorption is the initial and most fundamental of the surface photophysical processes. Absorption studies can reveal both the composition and structure of surface adlayers.*-17 Previous investigations have also shown that the characteristics of electronic absorption spectra on surfaces are dependent upon adlayer coverage."-'3*'6J7 However, a generalized model has not been developed to explain the coverage-dependent frequency shifts observed in surface electronic absorption spectra. DispersionI7 and exciton splitting" interactions may give rise to coverage-dependent frequency shifts in surface electronic absorption spectra. Frequency shifts caused by dispersion interactions occur when the electronically excited admolecule creates a polarized reaction field in its surrounding dielectric This external reaction field is polarized opposite in sign to that of the electronic transition dipole. Consequently, the reaction field results in a stabilization of the electronic dipole and a red shift of the absorption frequency relative to the gas-phase transition frequency. In contrast, exciton splitting involves the resonant interaction of an electronically excited dipole with neighboring dipoles.2' When these resonant interactions are present, the excitation will be delocalized over the interacting molecules and the resultant transition dipole is a vectorial sum of the neighboring dipoles. Therefore, the direction and magnitude of frequency shifts induced by exciton splitting are dependent on both the relative orientation and oscillator strength of the interacting dipoles. In this study, the electronic absorption spectra of disordered phenanthrene adlayers were examined a t 20 K under ultrahigh vacuum (UHV)for adlayer thicknesses ranging from 0 = 7-200 A (0 = 1-30 monolayers) on A1203(OOOl)and butane multilayer surfaces. The frequency of the So S2absorption maximum was observed to red-shift as a function of coverage on both surfaces. Dispersion and exciton splitting models were compared with the frequency shifts of the S, S2absorption spectra versus coverage. An analysis of the electronic absorption spectral data indicated that dispersion interactions were in agreement with the coverage-dependent frequency shifts.
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This investigation builds on our previous examinations of the spectroscopy and photophysics of organic adlayers on oxide surfaces. These earlier studies have explored the disorder-order transition and energy-transfer processes," the crystallization kinetics,I0 and the desorption kinetics22 for phenanthrene adlayers on A1203(1 120). The desorption kinetics? excimer formation kinetics? and coverage-dependent electronic absorption spectral7 have also been investigated for pyrene adlayers on A1203(1 120). In addition, a complementary investigation of the fluorescence spectra and the fluorescence lifetimes for disordered phenanthrene adlayers on both A1203(OOOl)and butane multilayer surfaces has been performed using laser-induced fluorescence (LIF) technique~.~~
Experimental Section Chamber and Sample Preparation. A schematic diagram of the UHV chamber and experimental setup used for this study has been described previously.*-22 Briefly, the UHV chamber was (1) Grassian, V. H.; Pimentel, G. C. J . Chem. Phys. 1988, 88, 4484. (2) Roop, B.; Lloyd, K. G.; Costello, S.A,; Campion, A.; White, J. M. J. Chem. Phys. 1989,91,5103. (3) Hanley, L.; Guo, X.; Yates, J. T., Jr. J. Chem. Phys. 1989,91,7220. (4)Marsh, E. P.; Gilton, T. L.;Meier, W.; Schneider, M. R.; Cowin, J. P. Phys. Rev. Leu. 1988,61, 2725. (5) Cho, C.-C.;Polanyi, J. C.; Stanners, C. D. J. Chem. Phys. 1988.90, 598. (6)Domen. K.: Chuann. T. J. J. Chem. Phvs. 1989. 90. 3318. (7j Alivasatos,'A. P.; &ndt, M. F.; Efrima: S.;Waldeck, D. H.; Harris, C . E. J. Chem. Phys. 1987,86,6540. (8) Tro, N. J.; Nishimura, A. M.; George, S.M. J. Phys. Chem. 1989.93, 1776 _-. -.
(9)Tro, N.J.; Haynes, D. R.; Nishimura. A. M.; George, S.M. J. Chem. Phys. 1989,91,5778. (10)Tro,N. J.; Nishimura, A. M.; Haynes, D. R.; George, S.M. Surf. Sci. 1989,207, L961. (1 1) Kemnitz, K.; Tamai, N.; Yamazaki, 1.; Nakashima, N.; Ycshihara, K. J. Phys. Chem. 1986, 90,5094. (12) Garoff, S.;Stevens, B. B.; Hanson, C. D.; Sorenson, G. K. Op?.
Commun. 1982,41,257. (13) Peterson, E. S.;Harris, C. E. J . Chem. Phys. 1989,91. 2683. (14)Kamura, Y.; Shirotani, 1.; Ohno, K.; Seki, K.; Inokuchi, H. Bull. Chem. Soc. Jpn. 1976,49,418. (15) Seki, H.;Itob, U. J. Chem. Phys. 1980,72, 2166. (16)Kamura, Y.; Seki, K.; Inokuchi, H. Chem. Phys. Let?.1975,30,35. (17)Tro,N . J.; Haynes, D. R.; Nishimura. A. M.; George. S. M. Chem. Phys. Lett. 1989,159,599. (18) Bell, R. P. Trans. Faraday Soc. 1931, 27, 797. (19)Onsager, L. J. A m . Chem. Soc. 1936,58,1486. (20)Bottcher, C. J. F. Theory . of- Electric Polarization; Elsevier: Amsterdam, 1973. (21)Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, I ,
*.I.
11, JI1.
'Permanent address: Department of Chemistry, Wcstmont College, Santa Barbara, CA 93108.
(22)Tro. N.J.; George, S.M. Surf.Sci. 1988, 197, L246. (23)Haynes, D. R.; Helwig, K. R.; Tro, N. J.; George, S.M. J. Chem. Phys. 1990, 93,2836.
0022-3654191 /2095-0839S02.50/0 0- 1991 American Chemical Societv , I
,
840 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 pumped by a 190 L/s Balzers turbomolecular pump that was backed by another Balzers 50 L/s turbomolecular pump. This tandem turbomolecular pump system yielded a base pressure of 5 X 1O-Io Torr as measured by a Bayard-Alpert ion gauge. Additionally, the chamber was equipped with a UTI quadrupole mass spectrometer with a 1-300 amu mass range and 300 A/Torr sensitivity. The mass spectrometer was used for background gas analysis and temperature-programmed desorption. Single crystals of A1203(OO01)were purchased from Insaco. A crystal sample was cleaned with acetone in an ultrasonic oscillator and rinsed with methanol before mounting at the end of a cold finger. The cold finger was at the end of a double vacuum jacketed dewar capable of holding liquid helium or liquid nitrogen. This cold finger was mounted to the UHV chamber using a differentially pumped rotary f e e d t h r o ~ g h . ~ ~ A schematic diagram of the A1203(OOOl)sample mounting technique has been presented previously? The A1203(OOOl)crystal was cleaned inside the UHV chamber by heating the A1203 to approximately 400 K with simultaneous exposure to an oxygen plasma discharge lasting at least 45 This plasma process has been shown to produce a clean A1203surface.2s However, recent laser-induced thermal desorption (LITD) studies in a separate vacuum chamber have indicated that A1203surfaces still contain hydroxyl groups after this cleaning procedure.26 A film of tantalum with a thickness of 5000 A was sputtered on the back side of the A1203(OOOl)sample. A clear window with a diameter of 0.25 in. remained at the center of each crystal. This arrangement allowed the crystal to be resistively heated by passing current through the tantalum film. A 0.020-in. hole was ground into the top center of each sample, and a 0.003-in.-diameter chromel-alumel thermocouple was attached by means of Ceramabond 569 high-temperature adhesive. Accurate crystal temperature measurement could be achieved with the thermocouple that was directly attached. The temperature was maintained by a temperature controller that determined the current output of a H P 6264B programmable power supply. This temperature controller could maintain temperatures to within f0.5 K. With liquid helium cooling, a temperature range from 20 to 700 K was obtainable. Liquid nitrogen cooling raised the lower limit to 86 K. Sample Dosing and Thickness Calibration. Phenanthrene was commercially obtained and subsequently zone-refined. To dose the phenanthrene molecules onto the substrate, the phenanthrene was placed in a small stainless steel tube attached to the inlet of a variable leak valve. The outlet of the variable leak valve was attached to a sample doser. The tube, leak valve, and sample doser were then all resistively heated with nichrome wire to approximately 430 K to increase the vapor pressure of the phenanthrene. The sample doser was a 0.125-in.-diameter stainless steel tube that had a 0.50-in.4iameter stainless steel tube with a 1-in. length attached at the end. The end of this doser assembly was positioned approximately 0.50 in. from the A1203 crystal during dosing. Phenanthrene doses were monitored and verified by the mass spectrometer. A butane gas lecture bottle was purchased from Matheson and connected directly to a gas handling line. The butane was dosed onto the substrate by means of another variable leak valve. The outlet of the leak valve was connected to a doser assembly identical with the sample doser for phenanthrene described above. The multilayer film thickness of butane was determined by using He-Ne optical i n t e r f e r e n ~ e . ~This ~ , ~ ~technique allows a film thickness to be measured in situ for any nonindex matched multilayer-substrate system. The reflected laser light from the vacuum-multilayer and multilayer-A1203 interfaces combine and form an interference pattern that is measured by a photodiode. ~
~~
(24) George, S. M. J . Vac. Sci. Technol. A 1986. 4, 2394. (25) Poppa, H.; Moorhead, D.; Heinemann, K. Thin Solid Films 1985, 128, 251.
(26) Arthur, D. A,; Meixner, D. L.; George, S. M. Unpublished results. (27) Rosetti, R.; Brus. L. E. J . Chem. Phys. 1980. 73, 572. (28) Tro, N. J.; Arthur, D. A.; George, S. M. J . Chem. Phys. 1989,90, 3389.
Haynes et al. The film thickness may then be determined by monitoring this interference pattern versus exposure time using Snell’s law and geometrical relationships. A butane multilayer film thickness of 500 A was employed in this study. Electronic Absorption Spectroscopy. Experiments were performed on disordered phenanthrene adlayers on either A1203(OOOl) or butane multilayer surfaces at 20 K. Phenanthrene coverages were calculated by electronic absorption spectroscopy. The coverage in monolayers was related to the absorbance, A, by B = 2.303A/(uNML). In this expression, NML is the number of molecules per cm2 in one monolayer, u is the absorption cross section of the electronic transition, and A = -log (Illo)where I and Io are the transmitted and incident light intensities, respectively. In similarity to the solution spectra of phenanthrene, the absorption spectra of the disordered phenanthrene adlayers showed a broad absorption peak centered at approximately X = 301 nm. This absorption feature corresponds to the So S2 electronic The So SI electronic transition was much tran~ition.*.~~-~’ weaker and originated at X = 351 nm.893*32 The absorption cross section, u, for the So S2 transition in disordered phenanthrene adlayers at high coverages was calculated by first experimentally determining A for the disordered adlayer at X = 301 nm. The adlayer was then annealed to 230 K for 60 s. Previous worksJo has shown that this annealing process results in a disorder-order transition in which the adlayer crystallizes and the intensity of the So S2absorption decreases dramatically because of strong dipole interactions in the molecular crystal. Likewise, the absorbance of the So SItransition increases and resembles the crystalline s p e ~ t r u m . ~Polarized , ~ ~ ~ ~absorption ~ studies have also shown that this ordering transition aligns the ab plane of the phenanthrene crystal parallel to the A 1 2 0 3 surface! The coverage of the ordered adlayer was calculated by using values of NML = 3.8 X lOI4 molecules cm2 obtained from the number density in the ab crystal planedvY and u = 5.0 X cm2 for the So SI transition determined by averaging the absorption intensities for the a and b crystal axes.32 This coverage of the ordered adlayer was then used together with the experimental value of A for the disordered adlayer to determine the absorption cross section in the disordered adlayer. This procedure cm2 for the So S2 transition in the yielded u = 4.0 X disordered adlayer a t high coverages. A number density in the disordered adlayer of N M=~2.2 X 1014molecules/cm2 was derived from the number density of solid ~ h e n a n t h r e n e . Coverages ~~ for the amorphous phenanthrene adlayer were converted to units of Angstroms by using 1 ML = 6.7 A as determined from the number density of solid phenanthrene. Absorption measurements were performed using an Oriel 75-W Xe arc lamp. The ultraviolet light was passed initially through an Instruments SA Made1 H-10 monochromator for wavelength selection and then focused onto the surface at normal incidence. The transmitted light was recollimated by a second lens and focused into a Hamamatsu R928 photomultiplier tube. The photomultiplier signal was digitized and stored in a computer for subsequent data reduction.
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Results The electronic absorption spectra for disordered phenanthrene adlayers at 20 K as a function of adlayer thickness on A1203(OOOl) are shown in Figure 1. One can convert adlayer thickness to monolayer units using 1 ML = 6.7 A. The broad absorption band S2 electronic transition of phenancorresponds to the So threne.a-2”31 Figure 1 clearly reveals a spectral red shift for the absorption maximum of the So S2transition as coverage in-
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(29) Gordon, R. D. Mol. Crysr. 1966, I , 441. (30) Jones, R. N.; Spinner, E. Spctrochim. Acta 1960, 16, 1060. (31) Berlman, 1. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York. 1971. (32) Craig, D. P.; Gordon, R. D. Proc. R . SOC.London 1965, A288,69. (33) Mason, R. Mol. Phys. 1961, 4, 413. (34) Trotter, J. Acta Crystallogr. 1963, 16. 605. (35) CRC Handbook of Chemistry and Physics, 60th ed.; Weast R. C., Astle, M. J., Eds.; CRC Press: Boca Raton, FL, 1980.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 841
Phenanthrene on AI2O3(0001)and Butane Surfaces 0.06
c ' go
g 5
a
0.06
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0.04
-
0.02
-
g
g
n
a
0.04
F-----l
0.08
0.02
0
gm
:
a
9
0 -
I ' 286
295
305
Wavelength (nm)
I'
I
315
285
-
295
Wavelength (nm)
Figure 1. Electronic absorption spectra of the So S2transition for disordered phenanthrene adlayers on Al2O3(00O1)at 20 K for several adlayer thicknesses. :
2
0
3 Y
298
-
294-
0
60
100
150
I 200
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(A) S2absorption maximum for disordered phenanthrene adlayers on AI2O3(0001)at 20 K versus adlayer thickness.
290
.. . . r'
creases from 8 = 7 A to fl = 90 A. Figure 2 displays the frequency of the absorption maxima for the So S2 transition as a function of phenanthrene thickness on A1203(OOOl). An absorption maximum of A = 293 nm was observed at 0 = 7 A. A red shift to longer wavelengths was observed versus coverage until an absorption maxima of A = 301 nm was measured for e L 100 A. The So S2 electronic absorption spectra for disordered phenanthrene adlayers at 20 K versus coverage on a butane multilayer surface are shown in Figure 3. The thickness of the butane multilayer surface was approximately 500 A. Figure 3 reveals a spectral red shift of the absorption maxima as a function of coverage. This red shift versus coverage on the butane multilayer surface is very similar to the red shift versus coverage observed for phenanthrene on the A1203(OOOl)surface shown in Figure 1. Figure 4 reveals that the frequency of the So S2absorption maximum shifts from A = 290 nm at 8 = 7 A to A = 301 nm for 0 1 100 A on a butane multilayer surface. The frequency shift at low coverages was somewhat greater on the butane multilayer surface compared with the Al2O3(O001) surface. This behavior suggests that the magnitude of the interactions responsible for the observed frequency shifts versus coverage may be somewhat different on the A1203(OO01)and butane multilayer surfaces.
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Discussion Mechanism for Coverage-Dependent Frequency Shifts . Chemical interactions with the A1203(0001) surface could cause the coveragedependent frequency shifts observed in the absorption spectrum of phenanthrene on A1203(0001). On the other hand, strong interactions between phenanthrene and the butane multilayer surface would not be anticipated. Figures 1-4 reveal that coverage-dependent frequency red shifts are very similar on both
4
.9
9
0
Adsorption on Butane
50
Phenanthrene Thickness
Figure 2. Wavelength of the So
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.
.
E
Adsorption on Al,O,(OOOl)
315
Figure 3. Electronic absorption spectra of the So S2transition for disordered phenanthrene adlayers on a butane multilayer surface at 20 K for several adlayer thicknesses.
A
e
I
305
150
100
-
Phenanthrene Thickness
Figure 4. Wavelength of the So
200
6)
S2absorption maximum for disordered phenanthrene adlayers on a butane multilayer surface at 20 K versus adlayer thickness. the A1203(0001) and butane multilayer surfaces. This similarity argues that frequency shifts on A1203(OO01)do not result from a strong chemical interaction between phenanthrene and the A1203(0001)surface. The desorption kinetics of phenanthrene are also consistent with only physisorption interactions between phenanthrene and the A1203surface.22 Exciton splitting interactions"V2' may be responsible for the coverage-dependent frequency shifts of the So S2 absorption maxima for phenanthrene on AI2O3(0001)and butane multilayer surfaces. Phenanthrene molecules crystallize in the type A crystal structure according to the definitions of Stevens.36 In this structure, the So S2 transition dipoles are aligned parallel to one another. This crystalline orientation results in very strong exciton splitting interactions for the So S2transition. Because of these strong interactions, much of the So S2oscillator strength in the phenanthrene molecular crystal is blue-shifted into the far-ultraviolet regi~n.~J'*~* At adsorption temperatures below 195 K, phenanthrene adlayers on AI2O3(0001)do not have the thermal energy required to orient in a crystalline configuration on the surface. Instead, phenanthrene molecules adsorb on the surface in a glasslike amorphous film. Previous electronic absorption studies of phenanthrene on AI2O3(1130) indicate that this glasslike orientation dramatically S2 transition.*JO decreases the exciton splitting for the So Consequently, a strong So S2 absorption transition occurs at X = 300 nm in similarity to the electronic absorption spectra of phenanthrene in ~olution.~' Figures 1-4 clearly reveal that the frequency of the So S2 absorption maximum red shifts versus coverage on both A1203-
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(36)Stevens, B. Spectrochim. Acta 1962, 18, 439. (37)Craig, D.P.; Hobbins, P. C. J . Chem. Soc. 1955, 539. (38) Philpott, M.R. Private communication.
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Haynes et al.
842 The Journal of Physical Chemistry. Vol. 95, NO.2, 1991
(000 1) and butane multilayer surfaces. Very similar frequency red shifts versus coverage were also observed for pyrene adsorbed on A1203(ll2O).I7 For pyrene on A1203(l120), the magnitude of the frequency shift was too large to be accounted for by exciton splitting.” This large red shift suggested that dispersion interactions must dominate the coverage-dependent frequency shifts, and a simple dispersion model was consistent with the data.” For phenanthrene on A1203(O001),an exciton splitting mechanism for the frequency red shift versus coverage cannot be as easily discounted. However, the similarity of pyrene on A1203( 1 l2O)I’ and phenanthrene on A1203(OOOl)is striking for both the magnitude and functionality of the frequency red shift versus coverage. This close resemblance argues that a dispersion interaction mechanism may explain the coverage-dependent frequency shifts for phenanthrene on A1203(OOOl). Dispersion interactions are also consistent with the gas-tosurface frequency red shifts measured for phenanthrene on AI203(0001) and butane multilayer surfaces. An isolated phenanthrene admolecule in the low coverage limit experiences a dielectric environment that is comprised of partial contributions from both the surface and the vacuum. The refractive index of butane at n = 1.35 is less than the refractive index of Alz03.at n = 1.78. Consequently, the local dielectric that interacts with the phenanthrene admolecules is less polarizable for a butane multilayer surface compared with an A1203(OO01)surface. Thus, dispersion interactions would predict a larger red shift on A1203(OOOl) relative to the butane multilayer surface at low phenanthrene coverages as observed in Figures 1-4. Dispersion Interaction Model. Dispersion interactions occur when the dielectric environment of an electric dipole is polarized by the electric field of the dipole.20 This polarization field is opposite in sign to that of the electric dipole and has a stabilization effect on the electric dipole. The polarization of surrounding dielectric media by a dipole was initially described by Bell1*and Onsager19in terms of a reaction field, R, encompassing the dipole. The reaction field is described as R = (ex/a3)(nz - 1)/(2n2 + 1) (1)
were ex is the electric dipole moment, a is the radius of the cavity formed by the oscillator, and n is the refractive index of the media. This reaction field was subsequently used to explain the frequency shifts of electronic absorption spectra observed in solvents with different refractive indices. Following the analysis by B a y l i ~ sthe , ~ ~classical dispersion equation for a vibrating electron is given by m(d2x/dt2) + mg(dx/dr) + wo2mx= eE (2) In this equation, E is the electric field, g is the damping coefficient, and wo is the natural frequency of oscillation. Likewise, m is the mass of the electron, e is the electron charge, and x is the oscillation coordinate of the vibrating electron. When the vibrating electron is placed in a dielectric media, the dipole induces the reaction field, R, in the dielectric given by eq 1. This reaction field must then be added to the classical electric field in eq 2, creating a new field E’ = E R. Because this induced reaction field scales with x, the frequency of oscillation for the vibrating electron will be red-shifted and becomes w = wo
+
- Aw.
The frequency red shift relative to the free vibrating electron, Aw, may be derived by solving the differential equation given by eq 2 and including the reaction field, E‘ = E R. This frequency
+
shift is given by Bayliss as39 Aw = k(n2 - 1)/(2n2 + 1)
(3) In this equation, k is a constant that is related to the oscillator strength and polarizability of the molecule. MacRae subsequently extended the Bayliss treatment to include the effects of static electric dipoles on the frequency shift.@ Static electric dipoles could be present at the A1203(0001) surface. (39) Bayliss, N. S.J . Chem. Phys. 1956, 18, 292. (40) McRae, E. G. J . Phys. Chi“ 1957, 61, 562.
1.O
50
0
100
150
200
Phenanthrene Thlckness (A) Figure 5. Refractive indices for disordered phenanthrene adlayers on AI2O3(0001)at 20 K versus adlayer thickness. Solid line shows the theoretical fit of the local density model assuming dispersion interactions.
Adsorption on Butane
1.01 0
50
Dispersion Model
100
150
1
200
Phenanthrene Thickness (A) Figure 6. Refractive indices for disordered phenanthrene adlayers on a butane multilayer surface at 20 K versus adlayer thickness. Solid line shows the theoretical fit of the local density model assuming dispersion interactions.
However, static electric dipoles would not exist on butane multilayer surfaces. Because a similar frequency red shift is observed on both AI2O3(OOO1)and butane multilayer surfaces, static dipole effects were not considered in the present analysis. This dispersion interaction model for the red shift of the transition frequency is classical and assumes a dielectric continuum surrounding the electric dipole. Alternative dielectric models have been employed to describe solvent frequency shifts observed in electronic absorption spectra?’ Microscopic quantum mechanical theories for solvent effects on electronic spectra have also been derivede4* The underlying physics is similar, and a red shift of the transition frequency is predicted by both the dielectric continuum and quantum mechanical models. Values for Aw were experimentally determined from the absorption maxima at each coverage displayed in Figures 2 and 4. The So Sz transition frequency for phenanthrene in the gas phase was taken as A = 282.6 nm.43*44 A value of k was a p proximated by fitting eq 3 to the frequency shifts observed for phenanthrene in a cyclohexane solution” and in a phenanthrene molecular solid.29 The fitting procedure yielded a value of k = 8500 cm-I. This proportionality constant, k, was then used with the experimental Aw values to calculate the refractive index experienced by phenanthrene molecules versus coverage on A1203(OOOl)and butane multilayer surfaces. The refractive indices versus phen-
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~
~~
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(41) Sullivan, D. E.; Deutch, J . M. 1. Chcm. Phys. 1976,65, 5315. (42) Schweizer, K. S.;Chandler, D. J . Chcm. Phys. 1983, 78, 4118. (43) Ohta, N.; Baba, H. Mol. Phys. 1986, 59, 921. (44) Amirav, A.; Sonncschen, M.; Jortner, J. J . Phys. Chem. 1984, 88, 5593.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 043
Phenanthrene on A1203(OOOI)and Butane Surfaces anthrene merage on A1203(OOOl)are depicted by the data points in Figure 5. The refractive indices versus phenanthrene coverage on butane multilayer surfaces are shown in Figure 6. Local Density Model. The experimentally determined refractive indices for phenanthreneon A1203(OOOl)and butane multilayer surfaces were then compared with refractive indices obtained from a local density model. The local density model employed the Lorentz-Lorenz equation:20 CplW = (n2 - I ) / ( n 2
+ 2)
-
= W44CViPi i
(5)
In this expression, Vi is the volume within the sphere taken up by the ith species, r is the radius of the sphere, and pi is the density of that species. To calculate the effective local density, the local densities of butane and A1203are needed in the same units as the phenanthrene density. The effective local density at the lowest phenanthrene coverage on A1203(OOOl)was first obtained from eq 4 using the experimentally measured refractive index for phenanthrene on A1203(OOOl)at 8 = 7 A, i.e. n = 1.28. The effective local density at 8 = 7 A was then expressed as a sum of volume fractions with different local densities using eq 5. After iteratively determining a spherical radius of r = 22 A, the local density, pAK) = 0.68 g/cm3, was determined from eq 5 using the known Pph, vph, and vAl0 values at e = 7 A. The local density of butane, pbuc,was obtained by using a similar procedure. In this case, the experimentally measured refractive index for phenanthrene on a butane multilayer surface at 8 = 7 A was employed, i.e., n = 1.I 8. This refractive index was related to a corresponding effective local density using eq 4. Subsequently, the local density, ph, = 0.29 g/cm3, was determined from eq 5 usin the known Pph, Vph, and vbU, values at 8 = 7 A when r = 22
f.
P I
(4)
In the Lorentz-Lorenz equation, C is a constant, plot represents the local density, and n is the refractive index. To employ the Lorentz-Lorenz equation, values for C and pk must be determined. The value of C can be calculated from eq 4 in the limit of large phenanthrene multilayer coverages of 8 1 100 A. In this limit, the refractive index of solid phenanthrene is n = 1.59 at X = 300 nm and the bulk density of solid phenanthrene is plW= 0.98 g/cm3. Employing these parameters for the refractive index and the local density, eq 4 produced a value of C = 0.35 cm3/g. The refractive index of phenanthrene can be calculated at X = 300 nm using $ = I($ + 2)Il2l2. The real part of the refractive index, 7, was taken at the sodium D line. The imaginary part, K, was calculated from the absorption cross section of u = 4.0 X IO-” cm2for the S, S2transition in the disordered phenanthrene adlayer at X = 301 nm. This calculation revealed that the imaginary portion of the refractive index was negligible. Consequently, the local density model as described by eq 4 was affected only by real contributions to the refractive index. Values for n and pbc can be defined easily at large phenanthrene coverages. In the limit of a thick phenanthrene multilayer, the phenanthrene adlayer approximates a single-component system. At smaller coverages, the effective values of n and plot will be influenced by a combination of the phenanthrene coverage, the nearby vacuum, and the Al2O3(OOO1)or butane multilayer surfaces. In general, the effective local density, pk, can be calculated by considering a phenanthrene molecule centered in an interaction sphere with a radius of r. The effective local density that this central phenanthrene molecule experiences will be determined by the densities for the vacuum, phenanthrene, and AI203 or butane species contained within the sphere: Ploc
Vacuum
The Lorentz-Lorenz equation relates the local density to the refractive index and can be employed for single- and multiplecomponent systems.20 For a composite system, we have assumed that the local density is a weighted sum of the local densities of each species. However, the application of the Lorentz-Lorenz
Figure 7. Depiction of two phenanthrene admolecules in a phenanthrene adlayer on AI20,(0001). At different distances from the AI2O3(0001) surface, the admolecules have different local densities within their interaction spheres.
equation is not straightforward when actual densities are utilized because different C constants should be used for each component.m We circumvented these problems by retaining the constant C value determined from the refractive index for thick phenanthrene multilayers and using local densities for butane and A1203in terms of phenanthrene local density units. Application of Local Density Model. The effective local density that a particular phenanthrene molecule experiences will change versus both coverage and the distance of the phenanthrene molecule from the A1203(OO01) or butane multilayer surface. These changes occur because the contributions of vacuum, phenanthrene, and A1203or butane densities within the interaction sphere of each phenanthrene admolecule will evolve versus coverage and distance from the surface. Different effective local densities for two phenanthreneadmolecules at different distances from the A1203(OO01)surface are depicted in Figure 7. A refractive index at a given distance from the A1203(OOOl) surface can be calculated after substituting eq 5 into eq 4. (n; - I ) / ( $
+ 2) = C(~/~T?)CI/,P, i
(6)
In this equation, nj is the refractive index experienced by a phenanthrene molecule at a given distance, d, from the surface. The volume fractions, Vi, are calculated according to the distance, d, and the radius of the interaction sphere centered on the phenanthrene molecule. The average refractive index for a given coverage was determined by taking the average of the refractive indices at the different distances that characterize the coverage: N
(7)
N is the number of increments calculated for a given coverage. In this local density model, the refractive indices were calculated at 1-A increments. Therefore, N was the total coverage in Angstroms. By varying the interaction sphere radius, r, in eq 5, these calculated average refractive indices were fit to the experimental values for the refractive indices measured versus phenanthrene coverage. The solid lines in Figures 5 and 6 show the experimental fits to the refractive indices versus coverage on A1203(OOOI)and butane multilayer surfaces, respectively. The spherical interaction radius that produced the best fit to both sets of experimental data was r = 22 A. This interaction radius indicates that a phenanthrene admolecule is influenced by its surrounding environment within a 22-A spherical radius. This radial distance is consistent with dispersion interactions that have a range of several molecular diameters. Previous work derived a smaller dispersive interaction radius for pyrene on A1203(1 120) of r = 8 A.” The dispersion model
J . Phys. Chem. 1991, 95, 844-848
844
employed to determine this interaction radius considered only the dispersion forces experienced by pyrene admolecules in the first monolayer on A1203(1120). The refractive indices contributed by pyrene admolecules above the first monolayer were not taken into account. In contrast, the present model for the average refractive index considers equal contributions from all phenanthrene molecules in the adlayer. The dispersion interaction radius is also dependent on accurate absolute coverage calibrations. The previous work for pyrene on AI20,( 1 120) employed a calibrated desorption analysis to determine the pyrene coverages1' and utilized the measured electronic absorption cross section for pyrene in cyclohexane?' Because the line width of the So S2transition is larger on A1203(Wl)than in cyclohexane, the cross section for pyrene on A1203(OOOl)was overestimated by a factor of 3.8. Using the revised absorption cross section and the present model, the previous measurements for pyrene on A1203(1120) yielded a similar dispersive interaction radius of r = 18 A.
-
COnel~iOns The electronic absorption spectrum of phenanthrene was studied as a function of coverage in disordered adlayers at 20 K on A1203(OOOl)and butane multilayer surfaces. The absorption maximum of the So S2 electronic transition shifted from X = 293 nm at e = 7 A to A = 301 nm at e 1 100 A on A1203(OO01). On the butane multilayer surfaces, the absorption maximum
-
red-shifted from X = 290 nm at 0 = 7 A to A = 301 nm a t 0 L 100 A. The observed frequency red shifts versus coverage were consistent with dispersion interactions in the phenanthrene adlayer. Given the magnitude of the frequency shifts relative to the gas-phase transition frequency, the Bayliss equation was employed to calculate the refractive indices in the phenanthrene adlayer as a function of coverage. Refractive indices versus coverage were also determined from the Lorentz-Lorenz equation after calculating the average local density within an interaction sphere centered about each phenanthrene admolecule. The sphere radius of r = 22 8, obtained by fitting the calculations to the experimental measurements was in agreement with dispersion interactions. Acknowledgment. This work was supported by the Office of Naval Research under Contract No. NO00 14-86-K-737. Some of the equipment used in this work was provided by the NSF-MRL program through the Center for Materials Research at Stanford University. D.R.H. is grateful to the E.K. and Lillian F. Bishop Foundation for partial support. K.R.H. thanks the Natural Sciences and Engineering Research Council of Canada for partial support. N.J.T. thanks the National Science Foundation for a graduate fellowship. S.M.G. acknowledges the National Science Foundation for a Presidential Young Investigator Award and the A.P. Sloan Foundation for a Sloan Research Fellowship. Registry No. Al2O3, 1344-28-1; phenanthrene, 85-01-8; butane, 106-97-8.
Electrostatic Potentials on the Molecular Surfaces of Cyclic Ureides Jane S. Murray, Pat Lane, Tore Brinck, Peter Politzer,* Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
and Per Sjoberg Nobel Chemicals, Nobel Industries Sweden, S-691 85 Karskoga, Sweden (Received: April 23, 1990; In Final Form: July 23, 1990) Ab initio SCF-MO electrostatic potentials have been computed at the STO-SG//STO-3G level on the molecular surfaces of a group of cyclic ureides, in order to assess their relative reactivities toward nucleophiles, as in hydrolysis. The surfaces were defined by the 0.002 electron/bohP contour of the molecular electronic density. The relative hydrolytic stabilities within a series of NO2 and NF2derivatives are predicted, on the basis of the magnitudes of the potentials above the acyl carbons. The surface electrostatic potentials of the polycarbonyl systems parabanic acid and alloxan are shown to be fully consistent with unusually short intermolecular distances that have been observed in crystallographic studies of these compounds.
Introduction Hydrolytic stability is a very important consideration in evaluating both existing and proposed energetic materials. For example, tetranitroglycoluril (Sorguyl, I) shows the desired fea02YN
0 4 N 0 , "
,NO2
NO2
XNb0 N.
I
NO2
o = C " XNN b o 02N0N
LI
tures of high measured density (2.01 g/cm3) and detonation velocity (91 50 m/s)' but suffers from being readily decomposed by hydrolysis.' A related compound, dinitroglycoluril (DINGU, [I), is stable toward neutral or acidic hydrolysis but easily decomposes under alkaline conditions.' For molecules of the general type RC(=O)Z (111), susceptibility to hydrolysis has been found to correlate with the reactivity of the acyl group, reflecting the resonance structures IIIA-C.2 (1)
Meyer, R. Explosfues;VCH Publishen: Weinheim, FRG, 1987. 0022-3654/91/2095-0844$02.50/0
u
m
IIII;
The greater the contribution of IIIB, the more positive is the acyl carbon, and thus the more reactive it is toward nucleophilic attack, e.g. hydrolysis. We have shown that an effective indicator of the positive nature of the acyl carbon is the electrostatic potential on the surface of the m~lecule.~For example, the surface potentials of acetyl fluoride (IV) and acetamide (V) reveal the acyl carbon H3C-
